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MODERN 

FISHING    GEAR   OF 
THE    WORLD 

Edited  by 

HIILMAR      KR  1ST  JONS  SON 

Chief,  Fishing  Gear  Section,  Fisheries  Division, 

Food    and    Agriculture     Organization    of    the 

United  Nations 


THIS  volume  represents  a  further  important 
stage  in  the  work  being  done  by  the  Fisheries 
Division,  F.A.O.  to  develop  and  expand  the 
fish  production  of  the  world.  Just  as  the  Con- 
ferences of  1953  at  Paris  (France)  and  Miami 
(U.S.A.)  concentrated  on  the  design  and  operation 
of  fishing  boats  and  craft,  so  the  Congress  held 
at  Hamburg  in  September  1957,  had  as  its  theme 
the  comprehensive  investigation  and  survey  of 
substantially  all  advanced  and  commercially  im- 
portant gear  and  equipment  used  for  catching  fish. 

Over  50O  representatives  from  all  countries  of 
the  globe  interested  in  the  commercial  fisheries 
of  both  salt  and  fresh  water  then  attended  to 
hear  and  discuss  more  than  one  hundred  papers 
contributed  by  scientists,  technologists  and  manu- 
facturers on  the  gear  and  equipment  devised  for 
the  better  catching  of  fish. 

On  the  foundation  of  those  papers  and  dis- 
cussions, this  volume  has  been  built  by  the  editor 
and  technologists  of  the  Fishing  Gear  Section  of 
the  Fisheries  Division.  The  papers  have  been 
collated  into  some  thirteen  logical  sections, 
amplified  where  necessary,  and  supplemented  as 
required  to  round  out  as  full  and  authoritative  a 
presentation  as  possible  on  all  aspects  of  modern 
fish  catching  equipment,  from  the  use  of  man- 
made  fibres  in  various  lines,  nets  and  trawls  to 
electronic  aids  in  fish  location  and  detection,  and 
to  the  latest  techniques  in  electric  fishing. 

The  editorial  work  necessary  to  avoid  duplica- 
tion and  repetition  was  tremendous  and  because 
of  the  need  frequently  of  translation  and  close 
co-operation  with  the  authors  of  papers  in  widely 

Continued  on  inside  back  cover 


PRICE    £7  5s.  Od. 

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MODERN    FISHING    GEAR    OF    THE    WORLD 


MODERN  FISHING  GEAR 

OF     THE 

WORLD 


Edited  by 
HILMAR      KRJSTJONSSON 

Chief,  Fishing  Gear  Section,  Fisheries  Division,  Food  and  Agriculture 
Organization  of  the  United  Nations,  Rome,  Italy 


Published  by 

FISHING  NEWS  (BOOKS)  LTD. 
LUDGATE   HOUSE,   110  FLEET  STREET,   LONDON,   E.C.4,   ENGLAND 

APRIL,  1959 
REPRINTED  SI  PTfcMBfcR  1968 


COPYRIGHT,   1959 

by 

FOOD   AND   AGRICULTURE   ORGANIZATION 
OF  THE   UNITED   NATIONS 

All  rights  reserved 


The  views  expressed  in  the  papers  and  discussions  are  those 
of  the  contributors  and  not  necessarily  those  of  the  Food  and 
Agriculture  Organization  of  the  United  Nations 


MADE       AND       PRINTED       IN      GREAT       BRITAIN 


REPRODUCED  BY  LITHOGRAPHY  AT  THF  WHITLFRIARS  PRESS  LTD.,  LONDON  AND  TONBRIDCE 


CONTENTS 


General  Illustrations 
List  of  Contributors 
Abbreviations 
Preface 
Introduction 


Page 

No. 

XV 
XVII 
XX11 
XXIII 
XXV 


Section  1.  MATERIALS,  TERMINOLOGY 
AND   NUMBERING   SYSTEMS 

Terminology  and  count  of  synthetic  fibre  twines 

for  fishing  purposes                       Hans  Stutz  1 

Names  of  fibres      ......  1 

Specific  weight  and  tenacity        ....  1 

Numbering  systems            .....  3 

Construction  and  numbering  of  synthetic  net  twines 

G.  A.  Hayhurst  and  A.  Robinson  4 

Yarns 4 

Twines          .......  4 

Terminology  and  numbering  systems  used  in  Japan 

Shigene  Takayama  6 

Specifications  of  fishing  materials  in  Japan        .           .  6 

Yarn  count  or  numbering  systems 

British  Standards  Institution  8 

Tex  system               ......  8 

Direct  systems         ......  8 

Indirect  systems      ......  8 

Conversion              ......  8 

Discussion  on  terminology  and  numbering  systems  10 

Yarn  size      .......  10 

Twine  construction            .           .           .           .           .  10 

Numbering  systems            .           .           .           -           -  11 

Conversion  factors             .           .           .           -           -  11 

Need  for  uniformity          .           .           .           .           .  12 


Section  2.    CHARACTERISTICS  OF 
FISHING  TWINES  AND  THEIR  TESTING 


Man-made  fibres  R.  Arzano 

Properties  ..... 

Density      ....... 

Strength  ...... 

Knot  strength      ...... 

Loop  strength      ...... 

Elastic  properties  ..... 

Toughness  ...... 

Stiffness     ....... 

Moisture  content  ..... 

Brief  definitions  of  mechanical  properties 


13 
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15 
15 
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Page 

No. 

Synthetic  polymer  fibres     .           .           .           .           .  15 

Polyamides               .           .           .           .           .           .  15 

Polyamide  66  (Nylon)               .           .            .           .  16 

Polyamide  6  (Perlon)     .           .           .           .           .  16 

Polyamide  11  (Rilsan)               .           .           .           .  16 

Polyester  fibres 17 

Vinyl  fibres 17 

Polyvinyl  chloride  fibres            .           .            .           .  17 

Vinylidene  chloride  fibres         .           .            .           .  17 

Polyethylene  fibres          .           .           .           .           .  17 

Polyvinyl  alcohol  fibres             .           .            .            .  18 
Man-made  fibres  and  finishes  in  protective  clothing  for 

fishermen          .           -           -           .           .           .  18 

Characteristics  of  synthetic  twines  used  for  fishing 

nets  and  ropes  in  Japan        Yoshinori  Shimozaki  1 9 

How  to  indicate  the  structure  and  size  of  net  twine       .  19 

Thickness  of  netting  twines         ....  20 

Weight  of  net  twines  in  the  air  and  in  water      .           .  21 

Sinking  speed           -           .           .           .           .           .  21 

Tensile  strength  of  net  twine  and  the  knot  strength       .  22 

Abrasive  resistance             .           .           .           .           .  25 

Other  characteristics           .....  28 

Nylon  in  fishing  nets                        ,/.  E.  Lonsdale  30 

Specification  for  ideal  fish  net  fibre           ...  30 

Yarn  strength           .           .           .           .           .           -  31 

Twine  strength         .            .           .           .           .           .  31 

Knot  strength          .           .           .           .           .           .  32 

Wetting 33 

In  use           .......  33 

The  technological   characteristics  of  Perlon  for 

fishing  equipment    Per  Ion-  Warenzeichen  verband  34 

What  is  Perlon? 34 

Forms  of  Perlon     ......  34 

Processing  into  net  twines,  cordage  and  ropes              .  35 

Properties                 .           .           .           .           .           .  37 

Use  of  Perlon  in  various  types  of  fishing  gear    .           .  41 

Trawls        .......  42 

Purse  seines          ......  42 

Bottom-set  gillnets          .....  42 

Stationary  fishing  gear              ....  42 

Whaling  ropes  and  cordage     ....  42 

Tarpaulins  and  protective  covers        ...  42 

"Terylene"  polyester  fibre  and  its  relation  to  the 

fishing  industry  Imperial  Chemical  Industries  Ltd.  43 

Types  of  yarn          ......  44 

Physical  properties             .....  45 

Knot  slippage         ......  46 

Dyeing          .......  47 

Mesh  retention        ......  48 

Sunlight  exposure               .....  48 


[v] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


CHARACTERISTICS  OF  FISHING  TWINES 
AND  THEIR  TESTING  "TERYLENE"  POLY- 
ESTER FIBRE  (continued) 

Durability     ....... 

Ease  of  handling  ..... 

Cost 

Construction  ...... 

Cords  and  twines  ..... 

Webbing 

Nets 

Gillnets  

Trawl  nets  ...... 

Purse  seines         ...... 

Other  applications  ..... 

Teviron  fishing  nets  The  Teikoku  Rayon  Co.  Ltd. 
Characteristics  of  Teviron  .... 

Teviron  fishing  nets          ..... 

Krehalon  fishing  nets  and  ropes 

Ktireha  Kasei  Co.  Ltd. 

Some  physical  properties  of  manila  rope  J.  Renter 
Influence  of  the  means  of  manufacture 
Influence  of  the  lay  ..... 

The  manufacture  and  testing  of  synthetic  yarns  and 
fibres  used  in  Japanese  fishing  gear 

Japan  Chemical  Fibres  Association 
Production  of  fishing  nets  and  lines  in  Japan  1956-57 

(Table) 

Scope  and  definition  of  testing  methods 
Sampling  and  preparation  .... 

Test  items     ....... 

Methods  of  testing  ..... 

The  physical  properties  of  netting  and  twines  suitable 
for  use  in  commercial  fishing  gear 

P.  J.  G.  Car  rot  hers 
Materials  tested       ...... 

Properties  and  test  procedures 

The  physical  properties  of  some  common  fishing  twines 

(Table) 

Definitions  and  dimensions  .... 

Testing  methods  for  net  twines  and  nets,  especially 
those  manufactured  from  synthetic  materials 

J.  K.  van  Wijngaarden 
Methods  of  testing  yarns  and  twines  . 

Yarn  number       ...... 

Twist          ....... 

Strength  and  extensibility         .... 

Shrinkage  ...... 

Stiffness     ....... 

Methods  of  testing  knots  .... 

Knot  strength      ...... 

Tip-over  resistance         ..... 

Knot-slip  resistance       ..... 

Open-up  resistance  ..... 
Methods  of  testing  meshes  .... 

Testing  of  materials  used  in  fishing  J.  Reuter 
Examination  of  hemp  twine  .... 
Testing  of  cotton  webbing  .... 


Page 
No. 


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Testing  of  silk  yarns  ..... 

Testing  synthetic  fibre  webbing     .... 
Testing  of  trawl  twine        ..... 

Lateral  strength  and  knot-firmness  of  synthetic 
twines  for  fishing  purposes  H.  Stutz 

Testing  methods  ..... 

Tensile  strength,  load-extension    .... 

Knot  strength          ...... 

Knot  firmness          ...... 

Discussion  on  properties  of  twines  and  testing 

methods  

Methods  of  testing  ..... 

Properties  ...... 

Standardization  of  tests      ..... 

Report  of  the  working  groups  on  terminology  and 
numbering  systems,  and  testing  the  properties  of 
twines 


Section  3.    NET  MAKING 

Practical  considerations  of  the  use  of  synthetic 
fibres  in  twines  and  nets    G.  A.  Hayhurst  and 

A.  Robinson 
Impregnation  ...... 

Knots  ....... 

Shrinkage  in  water  ..... 

Ropes  ....... 

Some  considerations  on  net  making     P.  A.  Lusyne 
Strains  and  stresses  ..... 

Hanging  of  webbing          ..... 
Joining  of  webbing  ..... 

Tailoring  of  webbing         ..... 

The  knotless  net     The  Nippon  Seimo  Co.  Ltd. 
Special  advantages  ..... 

Mending  of  knotless  nets  .... 

Method  of  manufacture    ..... 

Discussion  on  rational  manufacture  of  fishing  gear 

Rationalization  and  its  problems 

Conversion  from  natural  fibres  to  synthetic  fibres 

Standardization  of  terminology 

Knotless  nets  ...... 


Page 

No. 
84 
85 
86 

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92 

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94 
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98 


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111 


Section  4.    NET  PRESERVATION 

Development  of  fishing  net  and  rope  preservation 
in  Japan 

Shigene  Takayama  and  Yoshinori  Shimozaki  1 1 3 

Preservation  by  sterilization         .           .           .           .  113 

Sunlight  disinfection      .           .           .           .           .  113 

Sunlight  disinfection  for  non-dyed  nets        .           .  113 

Preservation  by  copper  sulphate        .          .          .  114 

Copper  naphthcnate  preservation       .          .          .  117 

Tannin  preservation  .  .  .  .  .120 

Bichromate  treatment  of  tannin  preservation     .          .  1 20 

Coal-tar  dyeing 120 

Preservation  by  cyanoethylation             .          .          .  121 

Summary 122 


[VI] 


CONTENTS 


Page 
No. 
NET  PRESERVATION  (continued) 

Rot-resistant  fishing  nets  by  the  "Arigal"  process 

A.  Rupert i        123 

Application  of  acetylated   cellulose  for   fishing 
gear  Sandoz  Ltd.        125 

History 125 

Properties  .  .          .          .          .          .126 

Tensile  strength  and  cost  .  .  .  .126 

Test  reports  .          .          .          .          .  1 26 

Application  of  partially  acetylated  cellulose  fibres     .         127 

Evaluation  of  rot-retarding  net  preservatives 

J.Zaucha        128 

"Static"  method 129 

"Fishing**  method  .          .  .  .  .129 

Evaluation  tests  of  protection  methods  .  .          131 

Method  of  testing  resistance  of  net  materials  to 

micro-organisms                              A.  v.  Brandt  133 

Testing  methods      .          .          .          .          .  .  133 

Determination  of  the  rotting  action  of  the  water  .  1 34 

Change  of  the  method  with  heavy  rotting  activity  .  1 34 

Preparation  of  samples  and  number  of  samples  -  135 

Description  of  experiment              .           .           .  .  135 

Evaluation  of  the  results               .           .           .  .  135 

Evaluation  of  the  test-figure          .           -           .  .  1 36 

Discussion  on  net  preservation  .  .  •  137 

Need  for  preservation  treatment  .  .  .  137 

Importance  of  Cu  and  tannin  content  .  .  .  137 

Testing  methods      .          .          .  .  .  .  137 

Chemically  modified  natural  fibres  .  -  .  137 


Section  5.    RELATIVE  EFFICIENCIES  OF 

NETS  MADE  OF  DIFFERENT 

MATERIALS 


The  efficiency  of  synthetic  fibres  in  fishing, 
especially  in  Germany  C.  Klust 

Proof  against  rot     ...... 

Types  of  gear,  with  regard  to  the  material  used 
Evaluation  of  the  most  important  properties 

Breaking  strength  in  knotted,  wet  condition  - 

Extensibility  and  elasticity         .... 

What  significance  does  this  difference  in  behaviour 
have  for  fishing  gear?  .... 

Abrasion  resistance        ..... 

Resistance  to  weathering          .... 
Choice  of  synthetic  fibres  for  various  fishing  gear 

Synthetic  fibres  in  the  fishing  industry 

E.  I.  Du  Pont  de  Nemours  and  Co. 
Fibre  properties      ...... 

Gillnets         ....... 

Purse  seines  ...... 

Trap  nets      ....... 

Trawls          ....... 

Fish  net  ropes         ...... 

New  developments  ..... 


139 
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141 

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149 


Page 
No. 
The  features  and  use  of  "Amilan"  fishing  nets 

M.Amano       150 

Salmon  and  trout  gillncts  in  the  Northern  Pacific  .  150 
Sanmai  net  (trammel  net)  .  .  .  .  151 
Purse  seine  net  .  .  .  .  .  151 
Drag  net 151 

Development  of  synthetic  netting  and  its  effect 
on  the  fishing  industry 

Momoi  Fishing  Net  Mfg.  Co.  Ltd.         152 
Natural  fibres          .  .  .  .  .  .          152 

Synthetic  fibres        .  .  .  .  .  .152 

Different  fibres  for  different  fishing  purposes     .  .         152 

Deep  water  gill  nets  and  seines      .  .  .  .153 

Test  with  nylon  fishing  tackle  in  Swedish  inland 

fisheries  Goxta  Molin  156 

The  results  of  comparative  fishing  tests  .  .  157 

The  scope  of  application  of  nylon  nets    .  .  .  157 

F.xperience  in  the  use  of  nylon  nets          -  .  .  157 

Experience  with  synthetic  materials  in  the 
Norwegian  fisheries  A'.  Mugaas       159 

On  the  fishing  power  of  nylon  gillncts  G.  Saetersdal        \  6 1 

Specifications  of  the  net     .  .  .  .  .          161 

Fishing  results         -  .  .  .  .  .162 

Discussion  on  relative  efficiencies  of  nets  made 
of  different  materials  -         -         .164 

Selectivity  .  .  .  .  .  .164 

Comparative  fishing  .  .  .  .  .165 


Section  6.    RATIONAL  DESIGN: 

ENGINEERING    THEORY 
AND  MODEL  TESTING 

The  use  of  model  nets  as  a  method  of  developing 

trawling  gear  W.  Dickson  166 
Comparison  between  an  eighth  scale  model  and  the 

full-sized  gear  .  .          .  .  .167 

Modelling  rules 168 

A  method  of  making  specification  drawings       .           .  1 70 

Some  underwater  observations     .           .           .           .  171 

Reactions  of  fish  to  small  nets      •          •           •          •  173 

Development  of  mechanical  studies  of  fishing  gear 

Tasae  Kawakami  1 75 

The  hydrodynamic  force  acting  on  a  twine  .  .  175 
The  equilibrium  configuration  and  tension  of  a  flexible 

twine  in  a  uniform  flow  -  .  •  1 76 

Estimation  of  the  fishing  depth  of  a  towed  gear  .  1 77 

The  resistance  of  plane  netting  in  a  current  .  .  177 

Equilibrium  configuration  of  webbing  in  a  current  -  1 78 

Analytical  studies  on  towed  gear  .  .  •  1 79 
Practical  procedures  of  model  experiments  (general 

rules) 180 

Some  model  tests  of  fishing  gear  .  .  .  1 82 

Full  scale  tests — using  underwater  measuring  instruments  1 82 

Reference  literature  .  .  -  •  •  183 


[VII] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


Page 
No. 

RATIONAL  DESIGN:  ENGINEERING 
THEORY  AND  MODEL  TESTING  (continued) 

Increasing  the  opening  height  of  a  trawl  net  by 

means  of  a  kite   5.  Takayama  and  T.  Koyama  185 

Model  experiments  .          .  .  .          .  185 

Construction  and  specification  of  the  nets      -  -  1 86 

Comparison  of  height  of  net  mouth     .          .          •  1 87 

Comparison  of  net  shapes         .          •          •          .  1 88 

Conversion  of  the  kite  from  model  to  full  scale       .  189 

Estimation  of  hydraulic  resistance       -  •          .  1 89 

The  kite 189 

False  headline  189 

The  connecting  legs  .          .          .          .190 

Field  experiments  in  Tokyo  Bay  .          .          .  191 

Field  experiments  in  the  Yellow  Sea        .          .          .  193 

The  headline,  the  footrope  and  their  influence  on 
the  vertical  opening  of  the  mouth  of  the  trawl 

S.  Okonski  and  S.  Sadowski  196 

The  Polish  herring  trawl  net  28/23  for  cutters  .  .  196 

The  Polish  herring  trawl  net  22/24/72  ft.  for  big  deep 
sea  trawlers         .  .  .  .  .  .198 

The  mouth  of  the  trawl  Jack  Phillips  200 

Raising  the  headline 200 

The  forces  involved  .....  200 

Speeds  of  tow — underwater  observations  .          .  202 

Further  research— tank  tests         ....  202 

Latest  developments          .....  203 

A  large-sized  experimental  tank  of  twin  symmetric 
elliptical  circuits 

Yoshikazu  Narasako  and  Masaji  Kanamori  205 

Construction  of  the  circulating  tank        .          .          .  205 

The  general  performance  of  the  circulating  tank          .  206 


Section?.  RATIONAL  DESIGN: 

USE   OF    MEASURING   INSTRUMENTS 
AND  UNDERWATER  OBSERVATIONS 


Midwater  trawl  design  by  underwater  observations 

R.  F.  Sand 

Underwater  television       ..... 
Diving  sled  and  camera  observations    . 
Discussion  of  results         ..... 

Study  of  the  Mediterranean  trawl  net 

Menachem   Ben-Yami 
The  trawler  ...... 

The  gear  (other  than  net)  .... 

The  net 

Test  results  ....... 

Further  developments       ..... 

Factors  affecting  the  efficiency  of  dredges 

R.  H.  Baird 
Effect  of  diving  plates      ..... 

The  effect  of  teeth 

Absolute  efficiency  ..... 


209 
209 
210 
210 


213 
213 
213 
214 
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220 


222 
222 
223 
224 


Trawl  gear  measurements  obtained  by  underwater 
instruments  P.A.de  Boer 

Description  of  the  instruments    .... 

Spread  meter       ...... 

Clinometer  ...... 

Net  height  meter  ..... 

Angle  of  attack  meter  .... 

Dynamometer      ...... 

Results          ....... 

Spread       ....... 

Tilt 

Height  of  the  net          ..... 

Angle  of  attack  ..... 

Influence  of  the  instruments  on  the  gear     . 

Influence  of  the  length  of  the  warps 

Ratio  between  warp  length  and  depth 

Influence  of  legs  ..... 

Influence  of  different  types  of  floats  on  the  opening 
height 

Pull  on  the  legs  and  amount  of  catch 

Studies  on  two-boat  trawls  and  otter  trawls  by 
means  of  measuring  instruments 

Chikamasa  Hamuro  and  Kenji  hhii 
The  automatic  net  height  meter    .... 
The  automatic  footrope  indicator 
Two-boat  trawls      ...... 

Opening  height    ...... 

Curvature  of  the  footrope        .... 

Otter  trawl  ...... 

Comparison  of  both  methods 

The  use  of  echo-sounding  as  a  means  of  observing 
the  performance  of  trawling  gear         /.  Schdrfe 
Depth  of  the  net  and  opening  height 
Position  of  the  lastriches  .... 

Position  of  the  otter  boards        .... 

Slope  of  the  legs 

Position  of  the  net  ..... 

Experiments  to  decrease  the  towing  resistance  of 
trawl  gear  J.  Schdrfe 

Results          ....... 

The  angle  of  attack       ..... 

The  size  of  the  net  opening 

The  opening  height       ..... 

The  angle  of  attack  of  the  kites 

Towing  speed      ...... 

The  towing  resistance    ..... 


On  the  relation 
towing  power 


between  otter 


trawl  gear 

H.  Miyamoto 


Engine  power  and  size  of  otter  boards 

Otter  boards  and  nets       ..... 

Engine  power  and  size  of  net     . 

Weight  of  the  otter  boards         .... 

Warp  length  and  water  depth     .... 

Studies  to  improve  the  efficiency  of  otter  doors  and 
trawl  floats  Luigi  Catasta 

Otter  boards  ...... 

Floats  ....... 

Lifting  device  for  trawl  nets       .... 


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[vm  ] 


CONTENTS 


Page 
No. 


RATIONAL  DESIGN:  USE  OF  MEASURING 
INSTRUMENTS  AND  UNDERWATER 
OBSERVATIONS  (continued) 

Simple   devices   for   studying   the   geometry   of 
various  gears  and  for  relating  some  commercial 
fishing    operations    to     the    existing    water 
movements  J.  N.  Carruthers 

Gillnetting  ...... 

Longlining  ...... 

Geometry  of  fishing  gear,  particularly  trawls 

A  research  on  set  nets  Masaji  Kanamori 

Characteristics  and  possibilities  for  improvement 
Classification  of  types       ..... 
The  effect  of  direction  and  speed  of  current 


254 
254 
255 
255 

256 
256 

257 
258 


Section  8.    RATIONAL  DESIGN: 
METHODS  OF  SPECIFYING  GEAR 

Specification  of  fishing  gear  Albert  Percier  260 

Type  and  size  of  twine       .....  260 

Webbing 260 

Shape 261 

Assembly  of  the  net  .  .  -  .  26 1 

Floats  and  sinkers  .....  262 

Size  of  fishing  gear  .....  263 

Gillnets 263 

Roundhaul  nets       ......  263 

Trawl  nets    .......  263 

The  size  specification  of  trawl  nets  in  Poland 

M.  Szatybelko  264 

Discussion  on  rational  design  of  fishing  gear  266 

Value  of  engineering  theories  and  methods        .  .  266 

Model  tests 266 

Full  scale  testing  269 

The  opening  height  of  trawl  nets  .          .          .  270 

Significance  of  the  weight  of  net  material  .          .  271 

Specification  and  standardization  of  trawl  nets  .  272 

Performance  of  floats  at  speed      ....  272 


Section  9.     FISHING  GEAR  AND  ITS 
OPERATION 


Classification  of  fishing  gear 

Fishing  without  gear 

Wounding  gear 

Stupefying  gear 

Line  fishing 

Fish  traps     . 

Traps  for  jumping  fish 

Bagnets  with  fixed  mouths 

Dragged  gear 

Seine  nets     . 

Surrounding  net 

Dip  or  lift  nets 

Falling  nets 

Gillnets  and  tangle  nets 


A.  von  Brandt 


274 
276 
276 
278 
279 
284 
287 
288 
289 
291 
292 
292 
293 
294 


Trawling  gear  //.  N.  Binns 

Setting  of  otter  trawl  gear  .... 

The  otter  trawl  net  ..... 

Opening  width        ...... 

Opening  height        ...... 

Other  gear  components     ..... 

Herring  trawls         ...... 

Midwater  trawls      ...... 

Synthetic  net  materials      ..... 

German  cutter  trawling  gear  J.  Scharfe 

General         ....... 

The  vessels          ...... 

The  trawling  gear          ..... 
The  flatfish  and  roundfish  gear  .... 
The  herring  otter  trawling  gear 
The  herring  pair-fishing  gear       .... 

Shrimp  trawling  gear  as  used  in  the  Gulf  of  Mexico 

John  S.  Robas 
Try  net         ....... 

Doors  and  warps  ..... 

Blocks  and  booms  ..... 

Nets 

Production    ....... 

Trends  in  trawling  methods  and  gear  on  the  West 
Coast  of  the  United  States    Dayton  L.  Alverson 
Electronic  aids        ...... 

Depth  recorders      ...... 

Loran  ....... 

Radar 

Fishfinders    ....... 

Deep  water  trawling          ..... 

Nets 

Cable  meter  ...... 

Drum  trawlers         ...... 


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297 
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298 
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300 
300 
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305 
308 


311 
312 
312 
314 
314 
316 


317 
317 
318 
318 
318 
319 
319 
320 
320 
320 


Stern  trawling  versus  side  trawling      C.  Birkhoff  321 

Power  requirements  for  deep  sea  trawling 

G.  C.  Eddie  325 

Effect  of  preservation  methods  on  the  speed      -          .  325 

Trawling  power       ......  326 

Fleet  operation  of  trawlers  with  a  mothership 

C.  Birkhoff  329 

Transfer  of  the  catch 329 

Operation  of  trawler  fleets          ....  330 

The  Pacific  Explorer 330 

Hie  Morska  Wola 331 

The  Russian  flotillas         .  .  •          .          .331 

Recent  and  future  developments            -          -           .  332 

Midwater  trawls  and  their  operation    B.  B.  Parrish  333 

Factors  governing  design,  operation  and  efficiency  of 

midwater  trawls             .....  333 

Regulation  of  fishing  depth     ....  333 

Importance  of  biological  factors        .          -          .  335 

General  features  of  design  and  operation       .          .  335 


[IX] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


FISHING   GEAR  AND  ITS   OPERATION— 

MIDWATER  TRAWLS  (continued) 

Types  of  midwatcr  trawls  .... 

The  Larsen  two-boat  trawl        .... 

The  net     ....... 

The  bridles          ...... 

Operation  ...... 

One-boat  midwater  trawls  .... 

The  nets  ...... 

Arrangements  of  rones,  bridles  and  otter  boards 

Otter  boards        ...... 

Danlenos  ....... 

Floats,  elevators  and  depressors 

Operational  features       ..... 

Scandinavian  experience  with  midwater  trawling 

Karl- Hugo  Larsson 


Midwatcr  trawl  types  in  use 
The  Thames  floating  sprat  trawl 


Page 
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337 
337 
338 
338 
339 
340 
341 
341 
342 
342 
343 


344 
346 


H.  S.  Noel        348 


The  development  of  a  new  herring  trawl  for  use  in 
midwater  or  on  the  bottom 

W.  E.  Barraclough  and  A.  W.  H.  Needier  351 

Design  and  testing  of  a  midwater  trawl,  1954-55      .  351 

The  net               352 

Otter  boards,  floats  and  depressors               .          .  353 

Bridles 354 

Operation  of  the  trawl  .          .          .  .354 

Fishing  tests 354 

Adaptation  of  the  trawl  for  use  close  to  the  bottom     .  354 
Modification  of  the  gear          .           -           .           .354 

Fishing  tests        ......  355 

Commercial  use  .          .          .          .          .355 

Development  of  a  dual-purpose  otter  board           .  355 
Further  developments        .          .          .          .          .356 

Midwater  trawl  for  small  vessels          •          -          .  356 

Midwater  trawls  for  faster-swimming  fish                 .  356 

On  the  use  of  midwater  trawls  for  anchovy  in  the 

Black  Sea                                Erdogan  F.  Akyuz  357 

Experimental  hauls                      -          •          •          •  357 

Discussion               ......  358 

Otter  boards  for  Pelagic  trawling    F.  Suberkrtib  359 

Considerations  on  a  new  type  of  midwater  trawl 

Y.  Grouselle  361 

The  Exocet  kite 362 

Exocet  stabilizers              .....  362 

A  practical  depth  telemeter  for  midwater  trawls 

R.  L.  McNeely  363 

Description  of  the  electrical  depth  telemeter              .  363 

Sea  tests  and  trials           .....  367 

Advantages  and  disadvantages               -          .          .  367 


Olympic  trawl  cable  meters    Carlyle  A.  Crecelius        369 


Page 
No. 

Ocean    cables    and    trawlers-  the    problem    of 

compatibility    R.  A .  Goodman  and  C.  S.  Law  ton  37 1 

Cost  of  cable  interruptions  .  .  .  .371 

Theories  as  to  how  fouling  occurs           .           .           .  372 

Efforts  to  minimise  damage          ....  372 

Disregard  of  regulations               ....  373 
Need  for  closer  co-operation       .           .           .           -373 

Hopes  for  ihc  future         .....  374 

The  use  of  the  Danish  seine  net           W.  Dickson  375 

Danish  seining  versus  otter  trawling        .           .           .  375 

Anchor  seining  versus  fly  dragging           .           .           .  376 

Anchor  seining    ......  376 

Fly  dragging         ......  376 

The  development  of  Danish  seining         .           .           .  376 

Boats 378 

Winches 378 

Gear 380 

General  description            .           .           .           .           .  383 

Anchor  seining  operation             ....  385 

Fly  drugging  operation                 ....  386 

Comparison  of  starboard  and  port  side  operation 

for  (Danish)  seining                        Ishiro  Saito  388 

Fishing  with  South  African  pursed  lampara 

C  G.  Du  Plessis  391 

Const  ruction  of  gear  .  .  .  .  .391 

Operation                ......  392 

Menhaden  purse  seining                   John  S.  Robas  394 

Vessels 394 

Operation                 ......  395 

Purse  boats             ......  396 

Seine  net       .......  396 

Fish  pumps              ......  398 

Radio-telephone  equipment          ....  398 

Aircraft  spotting     ......  399 

The  Puretic  power  block  and  its  effect  on  modern 

purse  seining                      Peter  G.  Schmidt,  Jr.  400 

The  conception  of  the  power  block          .           .           .  400 

Advantages  of  the  power  block  method          .           .  401 

Description  and  application  of  the  power  block        -  401 

Present  models  of  power  block              .           .           .  403 

Selection  of  proper  size  and  type          -          -          .  404 

Basic  systems  of  hauling  with  the  power  block             .  405 
Adaptation  of  the  power  block  to  different  methods  of 

purse  seining       ......  406 

Power  block  principle  used  for  gillnets  and  drift  nets  411 

Effect  of  power  block  on  purse  seining  methods      .  411 

Effect  of  the  power  block  on  the  design  of  fishing  vessels  4 1 2 

The  use  of  fish  pumps  in  the  U.S.A.  D.  W.  Burgoon  414 

The  boat-to-dock  system    .          .          .          .          .  414 

The  ocean-to-boat  system           .          .          .          .  417 

The  development  of  the  Philippine  bag  net  (basnig) 

S.  B.  Rasalan  418 

The  development  of  the  gear      .          .          .          .  418 

The  modern  basnig           .....  420 


[x] 


FISHING    GEAR    AND    ITS    OPERATION 

(continued) 

Some  improvements  in  the  stick-held  dipnet  for 
saury  fishing  Akira  Fukuhara 

Present  stick-held  dip  net  and  its  defects 
Particulars  of  newly  devised  stick-held  dip  net  . 
Construction  and  operation     .... 

The  methods  and  gear  used  for  mackerel  fishing 
in  Japan  Kaishun  Mihara 

Handlining  ...... 

Construction  of  the  gear         .... 

Operation  ...... 

Pole  and  line  fishing  ..... 

Construction        ...... 

Operation  ...... 

A  new  method  of  handling  longline  gear  A  des- 
cription of  POFI  "Tub"  gear  Herbert  J.  Mann 
Description  of  the  gear  ..... 
Operation  of  the  gear  ..... 

Drifting  lines  for  large  pelagic  fish  M.  Sanchez  Roig 
The  fishing  gear      ...... 

General  information          ..... 

Fishing  operation  ..... 

Power  hauler  for  longlines  Jzuilron  Works  Co.  Ltd. 
Construction  ...... 

Operation  ...... 

Automatic  steering  system  for  ocean-going  fishing- 
boats  Suiyo-Kai 
Characteristics  of  the  M.C.P.  .... 
Installation  ...... 

Discussion  on  choice  of  fishing  gear  and  some  new 
methods 

Factors  affecting  choice  .... 

Difficulties  of  experimenting  during  commercial  fishing 
Hanging  of  trawls  and  seine  nets 
Off-the-bottom  trawling     ..... 
Effect  of  fish  behaviour  .... 

Fishing  in  fresh  water       ..... 
Trial  and  error  ..... 

Discussion  on  efficient  handling  of  fishing  gear 

Purse  seining  ...... 

Towing  power  for  trawling          .... 

Towing  resistance  as  indication  of  catch 

Buoylines  for  traps  ..... 

Effect  on  quality  of  the  catch     .... 

Ocean  cables  ...... 

Decca  for  small  boats       ..... 

Safety  on  trawlers  ..... 


CONTENTS 

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422 
422 
424 
424 


426 
426 
426 
427 
428 
428 
429 


430 
430 
431 

433 
433 
434 
435 

436 
436 

437 

438 
438 
439 

440 
440 
441 
443 
443 
444 
445 
446 

447 
449 
450 
450 
451 
451 
452 
452 
452 


Section  10.   THE  LOCATION  OF  FISH 

Locating    fish    concentrations    by   thermometric 

methods     G.  Dietrich.  D.  Sahrhage,  K.  Schubert  453 

General  basis  for  fish  finding  by  thermomctric  methods  453 

Results  in  selected  hydrographical  regions         .          .  454 

Zones  of  convergence    .....  454 


Zones  of  divergence       ..... 

Zones  of  local  differences  in  vertical  turbulence  . 
Concentration  of  haddock  in  relation  to  bottom 

temperature         ...... 

Concentration  of  herring  in  relation  to  temperature 

distribution  ...... 

Radio    direction    finders    and    radar    used    by 
Japanese  fishing  vessels  Sw'yo-Kai 

Radio  direction  finding  ..... 
Small  marine  radars  ..... 
Application  to  fishing  operations 

Radio     communication     apparatus     for     fishing 
boats  in  Japan  A'.  Kodaira 

Radio  facilities  on  board  the  vessels     - 
Frequency  bands     ...... 

Coast  stations         ...... 

Equipment    ....... 

Radio  telegraphy  ..... 

MHF  Radio  telephony  .... 

HF  Radio  telephony  .... 

Radio  receivers  ..... 

Radio  buoys        ...... 

General  tendencies  of  improvement 

The  Decca  Navigator  system  and  its  application 
to  fishing  The  Decca  Navigator  Co.  Ltd. 

Description  of  operation  ..... 
Coverage  and  range  ..... 

Application  to  fishing  ..... 
The  automatic  plotter  ..... 

Discussion  on  fish  location 

Location,  detection    definition 
Means  of  locating  fish      ..... 
Plankton  sampling  ..... 

Hydrographic  observations  .... 

Fishing  charts          ...... 


Page 
No. 
455 
456 

457 
458 

462 
462 
463 
463 


465 
465 
465 
466 
466 
466 
466 
466 
466 
466 
466 

467 
467 
469 
469 
469 

471 
471 
471 
472 
472 
473 


Section  11.    DETECTION  OF  FISH 

Echo  sounding  and  fish  detection         R.  E.  Craig  474 

A  dilemma  in  echo  sounding      .  .  .  .474 

Fish  detection    demersal  fish                  -           •           •  474 

Fish  detection— pelagic  fish          ....  475 

Stabilizing  the  beam          .....  475 

Assessment  of  the  seabed            -  475 

The  problem  of  presentation       ....  476 

Sonar            .......  477 

Studies  on  the  behaviour  of  ultrasound  in  sea  water 

Tomiju  Hashimoto  and  Yoshimutsu  Kikuchi        478 

Characteristics  of  horizontal  propagation  .         478 

Characteristics  of  vertical  propagation  .  .         480 

Reflection  loss  at  the  sea-bottom  .  .  -         480 

Reflection  loss  at  the  fish  body  .  .  -481 

Experiments  on  fish  detection  with  high  frequencies          481 

Sounding  with  200  kc.  .  -  -  .481 

Horizontal  ranging  with  PP1  Sonar  .  .         482 

Effect  of  air  bubbles  on  ultrasound  of  different  frequencies        482 

Laboratory  experiment  ....         482 

Propeller  wake  .....         482 


[XI  1 


MODERN    FISHING    GEAR     OF    THE    WORLD 


DETECTION  OF  FISH  (continued) 

Reliability    of   bottom    topography  obtained   by 
ultrasonic  echo  sounding 

T.  Hashimoto,  M.  Nishimwa  and  Y.  Maniwa 
Effect  of  wide  beam  ..... 
Method  and  apparatus  ..... 
Results  ....... 

On  the  difference  between  24  kc.  and  200  kc. 
ultra  sound  for  fish-finding 

T.  Hashimoto  and  Y.  Maniwa 
Experimental  arrangement  .... 

Results          ....... 

Recent  trends  and  development  in  echo  fishing 

R.   W.  Woodgate 

General   comparison   of  paper-recorder   and  C.R.T. 
display       ....... 

The  Pye  fish-finder  ..... 

Depth  gauge  for  midwatcr  trawls 

Horizontal  ranger  ..... 

Fish  counter  ...... 

Outlook         ....... 

The  development  of  echo  sounding  and  echo  ranging 

Commander  R.  G.  Haines 
Bottom  fish  ...... 

Comparison  of  C.R.T.  and  paper  recording 
White  line  recording      ..... 

Relation    between    quantity    of   fish    echoes    and 
amount  of  catch        ..... 

Horizontal  fish  detection  .... 

Limitations  of  echo  ranging 

Training  of  operators  .... 

Fish-finder  for  bottom  fish 

Tomiju  Hashimoto  and  Yoshinobu  Maniwa 
Resolving  power  of  common  echo-recording  and  C.R.T. 

display  for  bottom  fish  .... 

Comparison  of  200  kc.  recorder  and  50  kc.  recorder 

with  suppressor  circuit  .... 

Improved  echo  sounding  equipment  for  the  detection 

of  shoals  of  bottom  fish  Hans  Kietz 

Significance  of  length,  shape  and  frequency  of  the  sound 

pulse         ....... 

Influence  of  ship  movements        .... 

Grey-black  recording         ..... 

Some  electro-technical  improvements  in  echo 
sounders  Dr.  Fahrentholz 

Improvement  of  bottom  fish  representation     . 
Cathode  ray  tube  representation 
Simple  device  for  horizontal  ranging    . 

Fish  detection  by  asdic  and  echo  sounder  G  Vestnes 
Detection  of  herring  by  asdic     .... 

Winter  season      ...... 

Summer  season  ..... 


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No. 


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485 


486 
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488 

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489 
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492 


493 
493 
494 
495 

495 
495 
496 
497 


499 
499 
500 

501 

502 
502 
503 


504 
504 
505 
506 

507 
507 
507 
508 


Experiments  with  echo  sounders 
Improvement  of  sensitivity 
Identification  of  bottom  fish    . 
Bottom  blocking 
Fish  surveys 

Horizontal  echo  ranging 

Miniature  Lodar     . 
Lodar  equipment 


Karl  Feher 


Correlation  of  midwater  trawl  catches  with  echo 
recordings  from  the  North-Eastern  Pacific 

E.  A.  Schaefers  and  /).  E.  Powell 
Gear  and  equipment  used  .... 

Fishing  results         ...... 

Interpretation  of  "Sea  Scanar"  traces 

Fish  finding  on  the  salmon  fishing  grounds  in  the 
North  Pacific  Ocean 

T.  Hashimoto  and  Y.  Maniwa 

The  quantitative  use  of  the  echo  sounder  for  fish 
surveys  D.  H.  Cushing 

Method 

The  relation  between  catch  and  signal 
The  use  of  the  method  on  survey 

Locating  herring  schools  on  the  Icelandic  North 
Coast  fishing  grounds  Jakob  Jakobsson 

Aerial  scouting        ...... 

Asdic  surveying       ...... 

The  two  methods  complement  each  other 

Discussion  on  fish  detection 

Aerial  scouting        ...... 

Technical  requirements  in  finders 

Cathode  ray  lube  display  .... 

Paper  recording      ...... 

Representation  of  bottom  fish  traces     • 
Resolving  power  and  quality  of  sound  transmission 
Problems  of  horizontal  ranging 
Echo  sounders  for  small  craft     .... 

Detection  of  wrecks          ..... 

Incorporating  latest  improvements  in  old  models 
Echo  sounding  and  efficiency  of  trawling 


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No. 
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509 
510 
510 
510 

512 
513 
514 


517 
518 
519 
520 


523 


525 
525 
526 
526 


528 
529 
530 
531 

532 
532 
532 
532 
533 
533 
533 
533 
535 
536 
536 
537 


Section  12.    ATTRACTION  OF  FISH 

Summary  of  experiments  on  the  response  of  tuna 

to  stimuli                                    Albert  L.  Tester  538 

Establishing  tuna  in  captivity      ....  538 

Response  to  visual  stimuli          ....  539 

Response  to  auditory  stimuli      ....  539 

Response  to  chemical  stimuli      ....  540 

Response  to  edible  and  inedible  lures  .           .           •  541 
Response  to  electrical  stimuli      .           .           .           .541 

Discussion                ......  542 


XII  ] 


CONTENTS 


ATTRACTION  OF  FISH  (continued) 

Fundamental  studies  on  the  visual  sense  in  fish 

Tamotsu  Tamura 
Form  perception     ...... 

Resolving  power  of  lens  and  retina  - 

Accommodation  and  visual  axis 
Some  aspects  of  the  vision  of  fish 

Diameter  of  nylon  twine  that  fish  can  recognize 

Food  searching  of  Lateolabrax  japonicus     . 

Light  fishing        ...... 

Optimal  intensity  of  illumination 


On  the  behaviour  of  fish  schools  in  relation  to 
gillnets  Masatsume  Nomura 

Depth  of  net  and  amount  of  catch 
Reaction  of  fish  approaching  a  net 
Influence  of  light    ...... 

Colour  of  the  net  . 

Value  of  echo  sounding    ..... 


The  significance  of  the  quality  of  light  for  the 
attraction  of  fish    Nobu  Yuki  Kawamoto,  D.Sc. 
The  wave  length     ..... 
The  relation  between  wave  length  and  radiant  energy 
Diurnal  rhythm  in  phototaxis      .... 
The  significance  of  light  gradient  for  attraction 
Influence  of  moonlight      ..... 


The  use  of  light  attraction  for  traps  and  setnets 

Tadayoshi  Sasaki 

Effect  of  a  directional  fish  attraction  lamp 
A  string  of  lamps  with  a  setnet 


The  basic  principles  of  fishing  for  the  Caspian 
"Kilka"  by  underwater  light       /.  V.  Nikonorov 
The  development  of  the  fishery     .... 

Theoretical  premises  of  submerged  light  attraction 
Observations  on  the  behaviour  of  kilka  in  an  illuminated 
zone          ....... 

Investigations  of  pump-fishing  with  light  attraction 


Fishing  jigs  in  Japan  with  special  reference  to  an 
artificial  bait  made  of  latex  sponge  rubber 

Takeo  Koyama 

Jigs  for  trolling  and  angling       .... 
Artificial  bait  for  longline  fishery 

Discussion  on  fish  attraction 

Fish  response  to  stimuli  .... 

Artificial  lures         ...... 

Reaction  to  light  ..... 

Underwater  lamp  ..... 

Pump  fishing  ...... 


Page 
No. 


543 
543 
543 
544 
544 
544 
545 
546 
546 


Attraction  of  fish  by  the  use  of  light  F.  J.  Verheyen       548 


550 
550 
551 
551 
551 
552 


553 
553 
554 
555 
555 
555 


556 
556 

557 


559 
559 
560 

561 
562 


567 
567 
568 

571 
571 
572 
572 
573 
574 


Page 
No. 

Section  13.    ELECTRICAL  FISHING 

The  effect  of  pulsating  electric  current  on  fish 

E.  Halsband  575 

The  effect  of  variations  in  the  pulse  type       .  -  576 

The  effect  of  variations  in  the  pulse  rate        -  .  576 

The  effect  of  variations  in  the  impulse  period  .  577 

The  influence  of  the  intensity  of  metabolism  .  579 

Electrical  fishing  in  Japan  T.  Kuroki        581 

Electric  screen         .  .  .  .  .  .581 

Fundamental  investigations          -          .  .  .581 

Trials  with  electric  harpoons,  hooks,  screens  and  trawls         582 
Future  possibilities  .....         582 

Electro  fishing                              Juergen  Dethloff  583 

Possibilities  of  application           ....  583 

Established  or  successfully  tested  methods         .           .  583 

Electric  tuna  hook          .....  583 

Application  to  purse  seining      ....  583 

Narcotising  sardines       -  -  .  .  .584 

The  gun-pump  method            ....  584 

Fencing     .......  584 

Future  possibilities             .....  584 

The  electro-trawl            .....  584 

Fish  magnet         ......  585 

Fencing     .......  585 

Whaling              585 

Technique  and  construction  of  pulse  devices              .  585 
Limitations              .           .           -           .           .           .585 

Electro  fishing  in  Lake  Huleh 

O.  H.  Oren  andZ.  Fried       586 

Boat  and  electric  equipment        ....         587 
Operation  .          .          .  .          .  .587 

Dangers  and  precautions  in  the  electrical  fishery 

A.  Hosl  589 

Significance  of  electrical  accidents          .           .           -  589 

Particular  dangers  of  electrical  fishing              .           .  590 

Precautions              ......  590 

Discussion  on  electrical  fishing  592 

Significance  of  impulse  current              -           .           .  592 

Killing  effect 593 

Influence  on  fish  quality              ....  593 

Frightening  effect    ......  593 

Working  range  of  electric  gear              .           .           .  594 

Safety  requirements           .....  594 

Commercial  application  in  freshwater              .           .  594 

Developments  in  Japan     .....  594 


Section   14.    CLOSING   REMARKS 

Future  developments         .....  596 

The  economic  value  of  fisheries  .  .  .  597 

The  need  for  research       .....  597 

Spreading  the  Gospel        .....  597 

International  collaboration          ....  598 

Detailed  Index 601 


[  XHI  ] 


NOTICE   TO    THE    READER 

Containing  the  papers  and  proceedings  of  the  International  Fishing  Gear  Congress  convened 
by  F.A.O.  in  Hamburg,  October  1957,  this  book  has  been  edited  in  the  Fishing  Gear  Section  of  the 
Fisheries  Division,  F.A.O.,  where  a  very  big  share  of  the  work  was  done  by  Mr.  P.  Lusyne  and 
Dr.  J.  Scharfe,  Gear  Technologists. 

Due  to  its  origin  the  text  is  divided  into  more  than  a  hundred  articles  written  by  different  authors. 
During  editing  we  have  attempted  to  minimize  repetition,  unify  terminology,  etc.,  and  also  arranged 
the  contributions  into  sections  according  to  subject  matter.  It  is,  however,  inevitable  that  informa- 
tion on  one  subject  is  often  found  in  many  widely  separate  parts  of  the  book,  but  it  is  hoped  that 
the  detailed  list  of  Contents — and  particularly  the  Index  which  appears  at  the  end  of  the  literary 
contents  of  the  book  will  be  of  help  when  using  this  volume  as  a  reference  book. 

No  attempt  was  made  to  include  material  ably  covered  in  well  known  and  accessible  books  on 
basic  net  making,  knots  and  splices,  nor  on  the  English  ground  trawls  already  fully  described  by 
Hodson,  Garner  and  other  authors.  For  such  general  references  the  readers  are  referred  to  the 
Annotcd  Bibliography  of  Fishing  Gear  and  Methods  available  from  F.A.O.,  Rome. 

HILMAR  KRISTJONSSON 
Editor 


XIV 


GENERAL    ILLUSTRATIONS 

To  supplement  the  specific  diagrams  and  illustrations  in  the  various  papers  a  number 
of  general  illustrations  covering  fishing  craft  and  activities  in  various  parts  of  the  world  have  been 
included  on  the  pages  listed  below. 

Page 
Making  of  twine  fibres   .....  ....  5 

Main  operation  stages  in  net  manufacture       ........  7 

Throwing  cast  nets  on  a  lake  in  Indonesia        ........  9 

Woman  spinning  a  yarn  from  sun-hemp  in  Ceylon  ......          29 

Menhaden  purse  seine  dories  with  the  catch  "dried  up"  in  the  bunt      ....          33 

Repairing  drift  nets  in  Ceylon  .........          56 

Liftnet  fishing  in  Jakarta  harbour      .........          58 

Brailing  herring  from  a  purse  seine  on  the  West  Coast  of  Canada        ....          68 

Log-rafts  returning  from  drift  net  fishing,  Ceylon  ......          74 

Hand-hauling  a  Marlon  pursed  lampara  seine  in  South  Africa  .  .  .  .81 

A  canoe  fisherman  mending  his  net  on  the  Indian  Malabar  coast  ....          86 

The  883  tons  distant-water  trawler  Portia  of  Hull  ......          99 

Washing  fishing  nets  in  Pakistan        .  .  .  .  .  .  .  .  .101 

Net  sections  being  braided  by  hand    .........        106 

Stacking  a  beach  seine  after  a  haul  in  Ceylon        .......        109 

Drying  herring  drift  nets  at  Great  Yarmouth  .  .  .  .  .  .  .132 

Two  canoes  fishing  with  a  midwater  bag  net  on  the  Malabar  coast  of  India  .  .136 

Testing  the  breaking  strength  and  extensibility  of  twine  (left:)  138 

Hand  and  foot  operated  net  weaving  loom  (right:)  .  .  .  .  .  .138 

Hand  spinning  of  cotton  yarn  in  an  Indian  fishing  village  .....        146 

Bottom-set  cod  gillnets  of  nylon  hauled  mechanically  off  Iceland  .  .  .  .149 

Handling  heavy  ground  rope  with  steel  bobbins  on  an  Arctic  trawler    ....        151 

Underwater  television  photo  of  monofilament  and  cotton  net        .  .  .  .  .158 

Underwater  photo  of  a  trawl  headline  with  spherical  floats          .....       204 

Trawl  toads  and  wingdoor  of  hydrodynamical  design          ......        208 

Skindiver  team  preparing  for  trawl  observations      .  .  .  .  .  .  .212 

Hydrofoil  otter  board  with  a  recording  angle-of-attack  meter  (left)        ....       224 

An  oval  otter  board  with  an  open  angle-of-attack  meter  (right)  ....        244 

Surface  dynamometer  for  measuring  towing  pull  (left)  :    Simultaneous  calibration  of  4  under- 
water and  2  surface  dynamometers  (right)  .......        247 

Modern  Mediterranean  stern  trawling  vessel  .......       250 

Hauling  salmon  gillnets  on  a  power  drum  .......        263 

200  tons  of  herring  taken  in  one  set  in  Norway     .......        273 

Deckload  of  cod  on  an  Arctic  trawler  ........       299 

Small  inshore  shrimp  trawler  on  the  USA  Gulf  Coast 316 

Hauling  the  codend  over  the  stern  chute  of  a  factory  trawler     .....        324 

The  Nekrasov,  one  of  the  Russian  factory  trawlers  ......       330 

Mothership  loading  catches  from  fishing  vessels  on  the  high  seas  ....       332 

[XV] 


MODERN     FISHING     GEAR     OF     THE     WORLD 

Page 

Underwater  photo  of  a  midwater  trawl  model         .......  350 

Hydrofoil  otter  board  with  recording  dynamometer             ......  356 

Echo  trace  of  a  pelagic  sardine  school          ........  360 

A  3-drum-trawl  and  anchor  winch  on  a  Swedish  cutter     ......  368 

The  Janet  Helen,  a  73  ft.  modern  small  trawler,  U.K.     ......  370 

An  oval  otter  board  for  bottom  trawling      ........  374 

Beach  seining  live  bait  for  tuna  fishing,  Haiti         .......  390 

Indian  fishermen  hauling  lampara  type  bagnet         .......  393 

A  big  set  with  a  two-boat  purse  seine  on  the  Norwegian  West  Coast               .            .            .  399 
Conventional  hand  hauling  of  a  Norwegian  purse  seine     .            .            .            .            .            .413 

South  African  fishing  vessels  unloading  their  catch  by  fishpump               ....  417 

Taking  the  catch  out  of  a  big  tuna  trap  in  Lybian  waters           .....  421 

Hauling  up  the  bag  of  a  large  Japanese  setnet       .......  425 

Shark  longline  hauled  mechanically  on  a  22  ft.  FAO  motor  boat,  India           .            .            .  435 

"  Chinese  "  liftnet  in  Cochin,  India      .........  446 

Modern  Japanese  tuna  longline  vessel  for  high  seas  fishing          .....  473 

C.R.T.  echo  display  of  herring  and  redfish  near  the  bottom        .....  477 

Echogram  of  stratified  bottom              .........  485 

Echogram  showing  fish  concentrations  in  a  bottom  depression      .....  506 

Brailing  herring  from  a  Norwegian  purse  seine       .  .  .  .  .  .  .511 

Echo  traces  of  tuna  in  the  North  Sea           ........  522 

Conical  liftnets  used  for  light  fishing  in  the  Caspian  Sea              .....  547 

Echogram  showing  the  reaction  of  freshwater  fish  to  artificial  light       ....  549 

Mediterranean  light  boat  with  three  big  gas  lamps  for  attracting  fish               .            .            .  566 

Electro-fishing  with  a  battery  powered  impulse  gear  in  fresh  water        ....  580 

Electrical  fish  barrier    ...........  591 

Electrotaxic  concentration  of  anchovy  around  pump-hose  opening            ....  595 


XVI  ] 


LIST  OF  CONTRIBUTORS 


No. 
\AGAARD,   O.  .  .  .  .  .111 

Organization    of    Norwegian    Fishing    Gear    Manufacturers, 
Christiania  Seildugsfabrik  Tordenskioldsgt.   ]    Oslo,  Norway. 

\KYUZ,  E.  F. 357 

Chief,  Demersal  Fish  Laboratory,   Fishery  Research  Centre 
Meat  and  Fish  Office,  Istanbul,  Turkey. 

\LBRECHTSON,  Gunnar        .  .  269,  270,  271,  272 

Technical  Manager  of  Jul  Albrechtson  and  Co.  A.B.,  Kom- 
mendorsgatan  11,  Goteborg  V,  Sweden. 


ALVERSON,  D.  L. 


.  317,446,537 


Fishery  Biologist,  Washington  Department  of  Fisheries,  4015. 
30th  Avenue  West,  Seattle  66,  Washington,  U.S.A. 


AMANO,  M. 


150 


Head  of  Synthetic  Fibres  Export  Section,  Toyo  Rayon  Co.  Ltd., 
No.  5,  3-chome,  Nakanoshima,  Kitaku,  Osaka,  Japan. 

ANDERSON,  A.  W 596,  598 

Assistant  Director.  Bureau  of  Commercial  Fisheries,  U.S.  Fish 
and  Wildlife  Service,  Washington  25,  D.C.,  U.S.A. 

ARZANO,  R.    .          .          .          .          .          .13 

Chairman,  Industrial  Uses  Sub-Committee  of  International 
Rayon  and  Synthetic  Fibres  Committee,  c/o  Societa  Rhodiatoce, 
via  Rugabella  15,  Milano,  Italy. 


BAIRD,  R.  H. 


222 


Senior  Experimental  Officer,  Ministry  of  Agriculture,  Fisheries 
and  Food  Fisheries  Experiment  Station,  Castle  Bank,  Conway, 
Caernarvonshire,  U.K. 


BARRACLOUGH,  W.  E. 


351 


Associate    Scientist,    Fisheries    Research    Board    of    Canada, 
Biological  Station,  Nanaimo,  B.C.,  Canada. 


BEN-YAMI,  M. 


165,213,270 


Master  Fisherman,   Kibbutz  "Sa'ar",   D.N.   Gallil   Ma'aravi 
Israel. 


BlNDLOSS,  E.  C 


536 


Commander,  Assistant  Technical  Manager,  Marconi  International 
Marine  Communication  Co.  Ltd.,  Marconi  House,  Chelmsford, 
Essex,  U.K. 


BINNS,  H.  N. 


297 


Managing  Director,  The  Great  Grimsby  Coal,  Salt  and  Tanning 
Co.  Ltd.,  Fish  Dock  Road,  Grimsby,  Lines.,  U.K. 


BlRKHOFF,  C. 


321,  329 


Naval  Architect,  Rickmers  Werft,  Lloydstrasse  34,  Brcmerhaven, 
M.,  Germany. 


DE  BOER,  P.  A. 


.    225,  451,  534,  597 


Deputy  Inspector  of  Fisheries,  Wasscnaarscweg  18,  The  Hague, 
Netherlands. 


Page 
No. 

VON  BRANDT,  Prof.  Dr.  A.    11,  95,  97,  98,  1 12,  133,  274 

598 

Director,  Institut  fiir  Netz —  und  Materialforschung, 
Bundesforschungsanstalt  fur  Fischcrei,  Palmaillc  9,  Hamburg— 
Altona,  Germany. 

BRITISH  STANDARDS  INSTITUTION     ...          8 

2,  Park  Street,  London,  W.I,  U.K. 

BUCHAN,  Jf       .  .  .  .  .  .112 

British  Nylon  Spinners  Ltd.,  Pontypool,  Mon.,  U.K. 

BURGOON,  D.  W.  .  .  .  .       414 

President,  Yeomans  Brothers  Co.,   1999,  North  Ruby  Street, 
Melrose  Park,  III..  U.S.A. 


CARROTHERS,  P.  J.  G. 


.  69,  95 


Technological  Station,   Fisheries  Research  Board  of  Canada, 
898,  Richards  Street,  Vancouver  B.C.,  Canada 


CARRUTHIiRS,  Dr.  J.  N. 


254 


National  Instutute  of  Oceanography,  Wormlcy,  Nr.  Godalming, 
Surrey,    U.K. 


CARVALHO,  M.  Melo 


442 


Commander,  Gabinete  de  Estudos  das  Pcsca,  Avenue  da  Liber- 
dade  211-4,  Lisbon,  Portugal. 


CATASTA,  L. 


251 


Costruttore  Navale,  Via  Monte  S.   Michele,  S.  Benedetto  del 
Tronto,  Italy. 


CRAIG,  R.  E. 


474 


Scottish  Home  Department,  Marine  Laboratory,  Victoria  Road, 
Torry,  Aberdeen,  U.K. 


CRECELIUS,  C.  A. 


369 


Manager,  Olympic  Instrument  Laboratories,  Vashon,  Washing- 
ton, U.S.A. 


GUSHING,  Dr.  D.  H. 


525,  537 


Principal  Naturalist,  Fisheries  Laboratory,  Lowestoft,  Suffolk, 
U.K. 


DECCA  NAVIGATOR  Co.  LTD.,  THE 


459 


Marine  Sales  Dept.  9,  Albert  Embankment,  London,  S.E.ll 
U.K. 

DETHLOFF,  J.  .....       583 

Director.  Dethloff-Electronic  Lottestrasse  52,  Hamburg-Lokstedt, 
Germany. 

DICKSON,  W.  .  .166,  268,  375,  440,  443 

Senior   Scientific   Officer,  Marine  Laboratory,  Victory  Road, 
Torry,  Aberdeen,  U.K. 

DIETRICH,  Prof.  Dr.  G.  .  .  .453 

Oceanographer,  Dcutschcs  Hydrographisches  Institut,  Bernhard 
Nocht  Strassc  78,  Hamburg  11,  Germany. 


[xvu] 


A2 


MODERN     FISHING     GEAR     OF    THE    WORLD 


No. 

DROST,  H.  S.  ....          270,  598 

Inspector  of  Fisheries,  Wassenaarseweg  1 8,  The  Hague,  Nether- 
lands. 


EDDIE,  G.  C. 


325 


Food  Investigation  Organisation,  Department  of  Scientific  and 
Industrial  Research,  Tony  Research  Station,  Aberdeen,  U.K. 

EINSELE,  Dr.  W 96 

Director,  Bundesinstirut  fur  Gewasserforschung  und 
Fischereiwirtschaft,  Scharfling  am   Mondsee,  Austria. 

FAHRENTHOLZ,  Dr.  S.  504,  535,  536 

Manager,  Echolote  und  Signalgeratc,  Graswcg2-6,  Kiel,  Germany 

FEHER,  Dr.  K. 512 

F.lectroacustic  G.m.b.H.,  Westring  425-429,   Kiel,  Germany. 

FINLAYSON,  Capt.  I.  R.         .  .  .  .451 

Engineering  Dept.,  Sub-marine  Branch,  British  Post  Office, 
Leith  House,  Gresham  Street,  London,  E.C.2,  U.K. 

FRIED,  Z 586 

Senior   Technologist,  Sea   Fisheries   Research   Station,  Haifa, 
Israel. 

FUKUHARA,  A.  .....         422 

Hokkaido  Fisheries  Experimental  Station,  Yoichishi,  Hokkaido 
Japan. 

GERHARDSEN,  Th.      .....       536 

Simonsen  Radio  A/S,  Ensj0veien  20,  Oslo,  Norway. 

GEROULT,  E.  R 450 

Naval  Architect,  26,  rue  Danielle  Casanova,  Paris  (II),  France 

GOODMAN,  R.  A.  .  .  .          371,  597 

European  Plant  Engineer,  The  Western  Union  Telegraph  Co., 
22,  Great  Winchester  Street,  London,   fc.C.2.,   U.K. 

GROUSELLE,  Y.          .          .          .          .         271,361 

24,  Boulevard  Chateaubriand,  Saint- Mai o  (Ils-et-Vilaine),  France. 

GUNDRY,  E.  F.  .  .  .  .  .96 

Director,  Joseph  Gundry  and  Co.  Ltd.,  Bridport,  Dorset,  U.K. 

HAGENBUCII,  Dr.  W.  .          .          .          .137 

Sandoz  A.G.,  Basel,  Switzerland. 

HAINES,  R.  G 493,  534,  536 

Commander,  Technical  Officer,  Kelvin  and  Hughes  (Marine)  Ltd. 
99,  Fenchurch  Street,  London,  E.C.3,  U.K. 

HALAIN,  C.  P 96,  596 

Commissaire  General  du  Fonds  du  Roi  Gouverneur  de  Province 
Honoraire,  B.P.  3119,  Leopoldville,  Congo  Beige. 

HALME,  Prof.  Dr.  E. 
Bureau  for  Fishery  Investigations,  Verkkosaari,  Helsinki,  Finland 

HALSBAND,  Dr.  E.     .          .          .          .          .       575 

Institut  fur  Kusten-  und  Binnenfischerci,  Palmaille  9,  Hamburg — 
Altona,  Germany. 


HALSBAND,  Dr.  H. 


594 


Institut  fur  Kusten-  und  Binnenfischerei,  Palmaille  9,  Hamburg — 
Altona,  Germany. 

[ 


No. 
HAMURO,  Prof.  C.  ....       234 

Fishing    Boat    Laboratory,    Fisheries    Agency,    1,    2-Chome, 
Kasumigaseki,  Chiyoda-ku,  Tokyo,  Japan. 

HASHIMOTO,  Prof.  T.  .       478,  483,  486,  499,  523 

Chief,  Instrument  Section  Fishing  Boat  Laboratory,  Fisheries 
Agency,  1,  2-Chomc,  Kasumigaseki,  Chiyoda-ku,  Tokyo,  Japan. 

HAYHURSI,  G.  A.       ....       4,96,100 
William  Kenyon  and  Sons  Ltd.,  Dukinfield,  Cheshire,  U.K. 


HENNINGS,  Dr.  Ch.    . 


.       451 


Chief.   Physico-Chemical   Laboratory,   Institut  fur  Fischverar- 
bcitung,  Palmaille  9,   Hamburg— Altona,  Germany. 


HESS,  Dr.  E. 


Ill 


Formerly  Chief,  Fisheries  Technology  Branch  FAO,  now: 
Fisheries  Adviser,  Ministry  of  Agriculture,  West  Indies  Federa- 
tion, Federal  House,  Port-of-Spain,  Trinidad,  West  Indies. 


HODSON,  Captain  A. 


270,  445,   536 


Instructor,  Grimsby  College  of  Further  Education,  80,  Clee 
Road,  Cleethorncs,  Lines.,  U.K. 


HOLT,  S. 


165,  471 


Chief,  Research  Program  Section,  Fisheries  Biology  Branch, 
Fisheries  Division   FAO. 


HOSL,  Dr.  Ing.  A. 


589 


Elektro-Beratung  Bayern  G.m.b.H.  Seidl-Strasse  25,  Miinchen  2, 
Germany. 


ILES,  T.  D. 


95 


Fisheries  Research  Officer,  Joint  Fisheries  Research  Organization, 
P.O.  Nkata  Bay,  Nyasaland. 

IMPERIAL  CHEMICAL  INDUSTRIES  LTD.        .          .        43 

Fibres  Division,  Hookstone  Road,  Harrogate,  Yorks.,  U.K. 


ISHH,  Prof.  K. 


234 


Fishing    Boat    Laboratory,    Fisheries    Agency,    1,    2-Chome 
Kasumigaseki,  Chiyoda-ku,  Tokyo,  Japan. 


Izui  IRON  WORKS  Co.  LTD. 

Muroto-cho,  Kochi-kcn,  Japan. 
JAKOBSSON,  Jakob 


.       436 


.  443,  472,  528,  536 


Fishery  Biologist,  Atvinnudeild  Hask61ans,  Fiskideild,  Borgartun 
7,  Reykjavik.  Iceland. 

JAPAN  CHEMICAL  FIBRES  ASSOCIATION       .          .        62 

3,  3-Chome  Muromachi,  Nihonbashi  Chuo-ku,  Tokyo,  Japan. 
KANAMORI,  Prof.  M.  .          .          .         205,  256 

Faculty  of  Fisheries,  Kagoshima  University,  470,  Shimo-Arata- 
Machi,  Kagoshima  City,  Japan. 


KAWAKAMI,  M. 
XVIH  ] 


597 

Chief,  Trawling  Department,  Taiyo   Fishing  Co.   Ltd.,   1-4 
Marunouchi,  Chiyoda-ku,  Tokyo,  Japan. 


LIST    OF    CONTRIBUTORS 


Page 
No. 

KAWAKAMI,  Dr.  T.    .          .          .          .          .175 

Department  of  Fisheries,  Faculty  of  Agriculture,  Kyoto  Uni- 
versity, Maizuru,  Kyoto  Prefecture,  Japan. 


KAWAMOTO,  Prof.  Dr.  N.  Y. 


553 


Faculty  of  Fisheries,  Prefectural  University  of  Mie,  Oya  Mach 
Tsu  City,  Mie  Prefecture  (Japan),  Lecturer,  Department  of 
Fisheries,  Kyoto  University,  Japan. 


KELLER,  Chem.  Ing.  H. 
Fibron  S.A.,  Domat/Ems,  Switzerland. 


12 


KIETZ,  Dr.  H 501,  536 

Chief,  Electro-Acoustic  Laboratory,  Atlas-Werke  AG,  Postfach  9, 
Bremen,  Germany. 


KlKUCHl,  Y. 


478 


Research    Institute    of    Electrical    Communication,    Tohoku 
University,  Sendai,   Japan. 


KLUST,  Dr.  G. 


93,  97,   139 


Senior  Gear  Technologist,  Institut  fiir  Net/-  und  Materialfors- 
chung,  Bundcsforschungsanstalt  fur  Fischerei,  Palmaillc  9, 
Hamburg  -  Altona,  Germany. 


KOBAYASIII,  H. 

Nippon  Seimo  Co.  Ltd.,  6,  1-chome  Ginza,  Chuo-ku,  Tokyo, 
Japan. 

KODAIRA,  K. 465 

Anritsu  Electric  Co.  Ltd.,  39,  Azabu  Fijimi-cho,  Minato-ku, 
Tokyo,  Japan. 

KOYAMA,  T 185,567 

Fishing  Methods  Division,  Tokai  Regional  Fisheries  Research 
Laboratory,  Fisheries  Agency,  Tsukishima,  Chuo-ku,  Tokyo, 
Japan. 


KRAMER,  K.  M. 


96 


Chief,  Sea  Fisheries  Section,  Ministry  of  Agriculture,  Hakirya, 
Tel-Aviv,  Israel. 

KRISUONSSON,  Hilmar          12,  447,  451,  535,  536,  573 

Chief,    Fishing   Gear   Section,    Fisheries   Technology   Branch, 
Fisheries  Division,  FAO. 


KROHN-HANSEN,  S. 


12 


F.A.    Campbell-Andersens    Fnke    A/S,    Postbox    11,    Bergen, 
Norway. 


KUREHA  KASEI  Co.  LTD. 

Tokyo,  Japan. 

KUROKI,  Prof.  Dr.  T. 
Faculty  of  Fisheries,  Hokkaido  University. 

KUTSCH,  A. 
Netmaker,  Fishereihafen,  Hamburg,  Germany. 


57 


581 


597 


LARSSON,  K.  H 271,  344,  444 

Naval  Architect,  Stadsgarden  10,  Stockholm,  Sweden. 


LAWTON,  C.  S. 


371 


The  Western  Union  Telegraph  Co.,  60  Hudson  Street,  New 
York  13,  N.Y.,  U.S.A. 


LENIER,  J.  R. 


Page 
No. 

452,  594 


Directeur-G6neraI,  S.A.  "Comptoir  Radio  Naval"  11  bis,  rue 
Saint- August  in,  Asnieres  (Seine),  France. 

LEVY,  H 138 

Port  de  Peche,  Safi,  Morocco. 

LoNSDALt,  J.  E.         .  .  .  10,  12,  30,  95 

British  Nylon  Spinners  Ltd.,  Pontypool,  Monmouthshire,  U.K. 

LUSYNE,  P.  A.  .  .  .  .  .102 

Gear  Technologist,  Fishing  Gear  Section,  Fisheries  Division, 
FAO. 

MANIWA,  Y.  ...  483,  486,  499,  523 

Fishing    Boat    Laboratory,    Fisheries    Agency,    1,    2-Chome, 
Kasumigaseki,  Chiyoda-ku,  Tokyo,  Japan. 


MANN,  H.  J. 


430 


Gear    Technologist,    Pacific    Oceanic    Fishery     Investigations, 
Honolulu,  Hawaii,  U.S.A. 


McKEE,  D. 


no 


The  Linen  Thread  Co.  Ltd.,  95  Bothwell  Street,  Glasgow,  C.2. 
U.K. 


-11'112  MCNEELY,  R.  L. 


363 

Flcctronic  Scientist,  North  Pacific  Exploration  and  Gear 
Research  Bureau  of  Commercial  Fisheries,  Seattle,  Washington, 
U.S.A. 

Mi  YFR-WAARDEN,  Prof.  Dr.  P.  F.  .  .       592 

Director.  Institut  fiir  K  listen-  und  Binnenfischerei  Bundes- 
forschungsanstalt  fiir  Fischerei,  Palmaille  9,  Hamburg — Altona, 
Germany. 

MIHARA,  K.     .  .  .  .  .  .       426 

Chiba  Prefectural  Fisheries  Experimental  Station,  1510Tateyama, 
Tatcyama-shi  Chiba-Ken,  Japan. 


MIYAMOTO,  Dr.  H. 


248 


FAO  Fishing  Gear  Technologist,  Central  Fisheries  Technological 
Station,  XXI/29,  Kochangadi  Cochin  5,  Kerala,  South  India. 

MOLIN,  G 156 

Fiskmaslarc,  Sotvattenslaboratoriet,  Drottningholm,  Sweden. 

MOMOI  FISHING  NET  MFG.  Co.  LTD.         .  .152 

Ako,  Hyogo-Ken,  Japan. 

MUG  A  AS,  N. 159 

Engineer,  Statens  Fiskeredskapsimport  Kadstuplass  10,  Bergen, 
N  01  way. 

NARASAKO,  Dept.  Prof.  Y.  .  .  .205 

Faculty  of  Fisheries,  Kagoshima  University,  470  Shimo-Arata- 
Machi,  Kagoshima  City,  Japan. 

NLHJUR,  Dr.  A.  W.  H.       .      272,  351,  442,  471,  572 

Director,  Fisheries  Biological  Station,  Nanaimo,  B.C.,  Canada. 

NIKONOKOV.  I.  V 574,  559 

Director,  Caspian  Institute  of  Marine  Fisheries  and  Oceano- 
graphy, Astrakhan,   U.S.S.R. 


NlLSSEN,  E.  A. 

Bergens  Notforretning,  P.O.  Box  870,  Bergen.  Norway. 


96 


[XIX  ] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


Page 

No. 

NIPPON  SEIMO  Co.  LTD.,  THE         .          •  .107 

6,  l-Chome  Ginza,  Chuo-ku,  Tokyo,  Japan. 

NlSHlMURA,  M.  .  .  .  .  .483 

Fishing    Boat    Laboratory,    Fisheries    Agency,    1,    2-Chome, 
Kasumigaseki,  Chiyoda-ku,  Tokyo,  Japan. 


NOEL,  H.  S. 


348,  535 


Staff  Writer   "World    Fishing",   Temple   Chambers,   Temple 
Avenue,  London,  E.C.4,  U.K. 


NOMURA,  M. 


550 


Gear  Technologist,  Fishing  Gear  and  Methods  Section,  Tokai 
Regional  Fisheries  Research  Laboratory,  Fisheries  Agency, 
Tsukishima,  Chuo-ku,  Tokyo,  Japan. 


OCRAN,  R. 


97,445 


Fishing  Company  Owner,  Mankoadze  Stores,  P.O.  Box  1492, 
Accra,  Ghana. 

O'GRADY,  A.  .  .  .  .   12,  442,  451 

Technical  Adviser  (Fishing),  Commonwealth  Fisheries  Office, 
Fisheries  Division,  Dept.  of  Primary  Industry,  Barton,  Canberra, 
Australia. 

OKONSKI,  S.    .          .          .          .          .          .196 

Sea  Fisheries  Institute,  Al.  Zjednoczenia  1,  Gdynia,  Poland. 

OLAFSSON,  David       .  .  .  .  .12 

Director  of  Fisheries,  The  Fisheries  Association  of  Iceland, 
Reykjavik.  Iceland. 

OREN,  O.  H. 586 

Deputy  Director,  Sea  Fisheries  Research  Station,  Haifa,  Israel. 

PARRISH,  B.  B.  .  .  .   164,  333,  444,  536 

Principal  Scientific  Officer,  Marine  Laboratory,  Victoria  Road, 
Torry,  Aberdeen.  U.K. 


VAN  PEL,  H.    . 


574 


Fisheries  Officer,  South  Pacific  Commission,   Noumea,   New 
Caledonia. 


PERCIKR,  A.     . 


12,  260 


Institut  Scientifique  et  Technique  des  Peches  Maritimes, 
Laboratoire  dc  Biarritz,  Centre  Scientifiquc  dc  PAtalaye, 
Biarritz,  France. 

PERLON-WARENZKICHEN  VERBAND  E.  V.     .          .        34 

Westendstrasse  41,  Frankfurt  am  Main,  Germany. 

PHILLIPS,  J.  W 200,  272 

Managing  Director,  Phillips  Trawl  Products  Ltd.,  Grimsby, 
Lines.,  U.K. 


DU  PLESSIS,  C.  G. 


391 


Assistant  Director,  Division  of  Fisheries,  Department  of  Com- 
merce and  Industries,  Beach  Road,  Sea  Point,  Cape  Town, 
Union  of  South  Africa. 

E.  I.  DU  PONT  DE  NEMOURS  AND  Co.  INC.  .       147 

Wilmington  98,  Delaware,  U.S.A. 

POWELL,  D.  E.          .          .          .          .          .517 

Exploratory  Fishing  and  Gear  Research  Branch,  Bureau  of 
Commercial  Fisheries,  U.S.  Fish  and  Wildlife  Service,  Washing- 
ton 25,  D.C.,  U.S.A. 


Page 
No. 

RACK,  R.  S 11,95,445,597 

Fishery  Advisor,  Government  of  Northern  Rhodesia,  P.O.  Box  1, 
Chilanga,  Lusaka,  Northern  Rhodesia. 


RASALAN,  S.  B. 


418 


Chief  Division  of  Commerical  Fisheries,  Bureau  of  Fisheries, 
Department  of  Agriculture  and  Natural  Resources,  Manila, 
Philippines. 

REUTER,  Dr.  J.  59,  82,  96,  137 

Director,  Het  Nederlandsche  Visserij-Procfstation  en  Labora- 
torium  voor  Material-Onderzoek,  Maliebaan  103,  Utrecht, 
Netherlands. 

RICHARDSON,  I.         .          .          .          .         272, 446 

Senior  Naturalist,  Fisheries  Laboratory,  Lowestoft,  Suffolk,  U.K. 

ROBAS,  J.  S 311,394 

Commercial  Fisheries  Consultant,  Fernandina  Beach,  Florida, 
U.S.A. 


ROBERTS,  D. 


12,  269,  441,  536 


Captain  of  deep  sea  trawler,  Derwent  Trawlers  Ltd.,  Hutton 
Road,  Grimsby  Fish  Docks,  Grimsby,  Lines.,  U.K. 


ROBINSON,  A. 


.  4,  96,  97,  100,  111 


Technical  Officer,  William  Kenyon  and  Sons  Ltd.,  Chapelsfield 
Works,  Dukinfield,  Cheshire,  U.K. 


Ruivo,  Dr.  M. 


473,  571 


Sub-Director,  Institute  de  Biologia  Marinha,  Cais  do  Sodre, 
Lisbon,  Portugal. 


RLIPERTI,  A. 


123,  138 


Ciba  Aktiengesellschaft,  Kunststoffabteilung,  Klybeckstr.   141, 
Basle,  Switzerland. 


SAETERSDAL,  G. 


161 


Fishery  Biologist,  Fiskeridirektoratets,  Havforskningsinstitutt, 
Postboks  189,  Bergen,  Norway. 

SADOWSKI,  S.  .  .  .  .  .196 

Sea  Fisheries  Institute,  Al.  Zjednoczenia,  Gdynia,  Poland. 

SAHRHAGE,  Dr.  D.     .  .  .  .          453, 472 

Marine  Biologist,  Institut  fiir  Seefischerei,  Bundcsforschungsan- 
stall  fiir  Fischerei,  Palmaille  9,  Hamburg — Altona,  Germany. 

SAITO,  Prof.  Dr.  1 388 

Faculty  of  Fisheries,  Hokkaido  University,  253  Minato-Machi, 
Hakodate,  Japan. 

SANCHEZ  ROIG,  Dr.  M.  .          .          .      433 

Institute  Nacional  de  la  Pesca,  Habana,  Cuba. 

SAND,  R.  F 209 

Chief,  Great  Lakes  Fisheries  Exploration  and  Gear  Research, 
Bureau  of  Commercial  Fisheries,  U.S.  Fish  and  Wildlife  Service, 
P.O.  Box  640,  Ann  Harbour,  Michigan,  U.S.A. 

SANDOZ  LTD.  .          .          .          .          .125 

Department  for  Chemical  Fibre  Modification,  Basle,  Switzerland. 

SASAKI,  Prof.  T 269,  559 

Tokyo  University  of  Fisheries  and  Chief  Research  Fellow  of 
the  Scientific  Research  Institute,  6-chome  Kaigandori,  Minato-ku 
Tokyo,  Japan. 


[XX] 


LIST    OF    CONTRIBUTORS 

Page 

No. 

SCHARFE,  Dr.  J.         .  241,  245,  269,  300,  444,  532,  537          TESTER,  A.  L. 


Gear  Technologist,  Fishing  Gear  Section,  Fisheries  Division. 
FAO. 

SCHAEFERS,  E.  A.  .  .  .  .517 

Acting  Chief,  Branch  of  Exploratory  Fishing  and  Gear  Research, 
Bureau  of  Commercial  Fisheries,  U.S.  Fish  and  Wildlife  Service, 
Washington  25,  D.C.,  U.S.A. 


Sf  HEFOLD,  K. 


594 


Vizeprasidcnl,  Osterreichische,  Hschereigescllschaft, Elisabethstr. 
22,  Wien  I,  Austria. 


SCHMIDT,  P.  G. 


400,  449 


President,  Marine  Construction  and  Design  Co.,  2300  Commo- 
dore Way,  Seattle  99,  Washington,  U.S.A. 

S(  HUBERT,  Dr.  K.  .  .  .  .       453 

Marine  Biologist,  Institut  fur  Seefischerci,  Bundesforschungsan- 
stalt  fur  f  ischerei,  Palmaille  9.  Hamburg— Altona,  W.  Germany. 


Sen v ML,  M. 


534 


Sales  Manager,  Flektroakustik  G.m.b.H.,  Westring  425-429, 
Kiel,  Germany. 

SHIMOZAKI,  Y.  .  .  .  .  19,  113 

Gear  Technologist,  Fishing  Gear  Division,  Tokai  Regional 
Fisheries  Research  Laboratory,  Tsukishima,  Chuo-ku,  Tokyo, 
Japan. 


SMI  ni,  H.  C. 


11 


Managing  Director,  Apeldoornse  Nettenfabriek  von  Zeppelin 
and  Co.   N.V.,  Apeldoorn,  Netherlands. 


SOUBI  IN,  L. 


441,  452 


President,  Federation  des  syndicats  d'armuleurs  a  la  peche, 
59,  rue  des  Mathurins,  Paris  8e,  France. 

SPRINGER,  S.  .  .  .  .97,  165,  445 

Chief,  Exploratory  Fishing  Section,  Bureau  of  Commercia' 
Fisheries,  U.S.  Fish  and  Wildlife  Service,  Washington  25. 
D.C.,  U.S.A. 

Siurz,  Dipl.  Ing.  H.  .  .  .  .    1,  87 

Farbwerke  Hocchst  AG,  Bobingen  Mills,  Germany. 

StJBERKRUB,  F.  .....         359 

Naval  Architect,  Chile  Haus  C  VII,  Hamburg  1,  Germany. 

SUIYO-KAI 438,  462 

c/o  Anritsu  Electric  Co.  Ltd.,  39,  Azabu-Fijimi-cho,  Minato-ku, 
Tokyo,  Japan. 

SVETINA,  M 445 

Vice-President  du  C.S.P.  de  Slovenie,  Poljanska  20B,  Ljubljana, 
Yugoslavia. 

SZATYBELKO,  M.  .  .  .  .         264 

Sea  Fisheries  Institute,  Al.  Zjednoczenia  1,  Gdynia,  Poland. 

TAKAYAMA,  Prof.  S.  .  6,  111,  113,  185,  269,  594 

Chief,   Fishing  Gear  and  Methods  Section,  Tokai   Regional 
Fisheries  Research  Laboratory,  Tsukishima,  Chuo-ku,  Tokyo 
Japan. 

TAMURA,  Prof.  T 543 

Fisheries  Institute,  Faculty  of  Agriculture,  Nagoya  University, 
Anzyo,  Aiti-Prcfecture,  Japan. 


No. 
538 

Director,  Pacific  Oceanic  Fishery  Investigations,  Honolulu 
Hawaii,  U.S.A. 

TEIKOKU  RAYON  Co.  LTD.,  Tin-      ...        55 

Teviron  Department.  Hdobori-Mmamidori.  Nishiku,  Osaka, 
Japan. 

THOMAS,  Dr.  H.  A.  .  .  .  97,  271 

President  de  la  Commission  Technologique,  Comite  Inter- 
national de  la  Rayonne  el  des  Fibres  Synthetiques,  c/o  Court  - 
aulds  Ltd.,  15  Cross  Street,  Manchester  2,  Lanes.  U.K. 

TRAUNG,  J.  O.  .  .  .   266,  269,  270,  574 

Chief,  Fishing  Boat  Section,  Fisheries  Division,  FAO. 


TRESCHEV,  A.  I. 


450 


Chief,    Fishing   Technique   Laboratory,    U.S.S.R.    Institute   of 
Marine    Fisheries    and    Oceanography    (VNIRO),     Moscow 
U.S.S.R. 

TSUDA,  K.  K.  .  .  .  .  .       535 

Japan  Radio  Co.  Ltd.,  930  Kamirenjaku,  Mitaka,  Tokyo,  Japan. 

Vi  RHEYEN,  F .  J.          .  .  .  .  548,  572 

Laboratorium     voor    Vergehjkende,     Physiologic     der     Rijks 
UnivcrsiteiU  Jan  van  Galenstraat  17,  Utrecht,  Netherlands. 


VESINES,  G. 


507 


Fiskcridirektoratets      Havforskningsinstitutt.      Postbox       189 
Bergen,  Norway. 


WARNCKI.,  H. 


1,  110 


Director,  Itzehoer  Net/fabrik  AG,  Brunnenstr.  2-10,  It/cnhoc 
Germany. 


WENT,  Dr.  A.  E.  J.    . 


165 


Inspector  and  Scientific  Advisor,  Fisheries  Branch,  Department 
of  Agriculture,  3,  Cathal  Brugha  Street,  Dublin,  Ireland. 

WESTROP,  K.  J.          .  .  .  .  .97 

Fibres  Division,  Imperial  Chemical  Industries  Ltd.,  Hookstone 
Road,  Harrogate,  Yorks.,  U.K. 


VAN  WlJNC.AARI>LN,  J.  K. 


.  12,  75 


N.V.  Research  Laboratory  and  Affiliated  Companies.  Velperweg 
76,  Arnhem,  Netherlands. 


DE  WIT,  J.  G. 


450 


Naval  Architect,  P.C.  Hooftlaan  38,  Dnejuis  (Gemccntc  Velsen), 
Netherlands. 


WOLMARANS,  B.  J. 


95 


Cooper,  Wolmarans  and  Co.  Ltd.,  P.O.  Box  799,  Cape  Town, 
Union  of  South  Africa. 

WOODGATE,  R.  W 488 

Pye  Marine  Ltd.,  Oulton  Works,  Lowestoft,  Suffolk,  U.K. 

ZAUCHA,  J.      .  .  .  .  .  .128 

Sea  Fisheries  Institute,  Al.  Zjednoczenia  1,  Gdynia,  Poland. 

ZEBROWSKI,  Z 272,  452 

Reedcrei  "Dalmor",  Gdynia,  Poland. 

ZEI,  Prof.  Dr.  M 537 

Director,  Institute  of  Marine  Biology,  Roving,  Yugoslavia. 


[XXI] 


ABBREVIATIONS 


A  -  Ampere 

°C.  --  E>egrees  Centigrade 

cm.  -  Centimetre(s) 

sq.  cm.    —  Square  Centimetre(s) 

cu.  cm.    -=--  Cubic  Centimetre(s) 

col.  =  Column 

cu.  =  Cubic 

diarn.  ==  Diameter 

°F.  —  Degrees  Fahrenheit 

fm.  —  Fathom(s) 

ft.  Feel 

g.  Gram(s) 

gal.  -^  American  gallon(s) 

ha.  ~  Hectare(s) 

h.p.  —  Horsepower 

hr.  -=-  Hour 

hrs.  ~  Hours 

in.  =  Inch(es) 

kg.  --  Kilogram(s) 

km.  ---  Kilometre(s) 

kw.  Kilowatt 

1.  =  L.itre(s) 


Ib.  ^-  Pound(s)  avoirdupois 

m.  —  Metre(s) 

sq.  m.  —  Square  metre(s) 

cu.  m.  —  Cubic  metre(s) 

mg.  —  Milligram(s) 

min.  =--'  Minute(s) 

ml.  =-  Millilitre(s) 

mm.  —  Millimetre(s) 

sq.  mm.  ---=  Square  millimetre(s) 

cu.  mm.  ~  Cubic  millimetre(s) 

oz.  —  Ounce(s)  avoirdupois 

p.  =  Page 

pp.  —  Pages 

p. p.m.  -  Parts  per  million 

r.p.m.  Revolutions  p>er  minute 

sec.  --  Sccond(s) 

sp.  —  Species 

spp.  -  Plural  of  species 

V  Volts 

Vol.  -=  Volume 

W  —  Watt(s) 

wt.  =  Weight 


XXII   ] 


PREFACE 

THE  International  Fishing  Gear  Congress,  which  was  held  in  Hamburg,  Germany  in  October, 
1957,  was  the  first  meeting  of  its  kind,  and  it  marked  an  important  step  forward  in  international 
cooperation  in  dealing  with  some  of  the  many  problems  concerned  with  the  development 
of  fisheries  throughout  the  world.     This  book  contains  the  edited  versions  of  more  than  one  hundred 
technical  papers  presented  to  the  Congress,  as  well  as  the  gist  of  the  discussions  which  took  place. 

Broadly  speaking,  FAQ's  function  is  to  promote  food  production,  especially  through 
international  cooperation.  One  part  of  the  work  of  the  Organisation  is  to  provide,  at  the  request 
of  any  of  the  77  Member  Governments,  direct  technical  assistance  by  assigning  specialists  to  deal 
with  specific  fisheries  problems. 

The  other  part  of  FAO's  function  is  to  act  as  a  clearing  house  for  technical  information, 
stimulate  the  exchange  of  ideas  and  experience,  and  to  fogus  attention  of  governments  on  key 
problems,  particularly  those  which  need  to  be  dealt  with  through  international  cooperation.  The 
International  Fishing  Gear  Congress  provides  an  example  of  this  function  of  the  Organisation. 

While  a  great  deal  of  FAO's  work  concerns  the  fisheries  in  the  underdeveloped  countries, 
it  is  also  of  value  to  technically  developed  countries.  This  was  shown  at  the  Congress  where  a  major 
part  of  the  540  participants  were  from  countries  in  the  front  rank  of  fishing  nations.  These  people 
found,  for  example,  that  the  papers  and  discussions  were  most  useful  in  focusing  attention  on  the 
problems  concerned  with  the  development  of  fishing  gear  technology.  This  is  a  new  concept  in  the 
fishing  world  which  has  emerged  in  recent  decades  because  of  the  growing  complexity  of  fishing. 

Literature  in  this  field  is  scattered  and  incomplete  and  this  book  is  the  first  comprehensive 
reference  work  covering  net  materials,  rational  gear  design,  description  of  modern  fishing 
gear  and  its  operation  as  well  as  the  strategy  and  tactics  of  finding  and  attracting  fish.  Although 
the  profession  of  gear  technology  is  still  in  the  development  stage,  and  gear  research  facilities  are, 
as  yet,  available  only  in  a  few  countries,  much  progress  has  been  made  towards  evolving  the  method- 
ology and  tools  of  the  trade.  But  experimenting  with  fishing  gear  is  expensive  and,  in  view  of  the 
economic  pressure  under  which  the  fishing  industry  generally  works  and  the  need  to  increase  gear 
efficiency,  this  is  clearly  a  field  for  governmental  action.  If  rapid  progress  is  to  be  made,  then  Govern- 
ments must  set  up  and/or  support  gear  research  institutes.  Such  institutes  are  as  essential  as  the 
already  well  established  biological  and  fish  processing  stations  now  found  in  all  fishing  countries. 
The  need  for  such  institutes  is  underlined  by  the  fact  that,  despite  the  progress  made,  fishing  is, 
broadly  speaking,  still  a  very  inefficient  operation.  For  example,  in  the  most  highly  developed  fishing 
countries  each  fisherman  produces  over  80  tons  of  fish  per  year  but  there  are  more  than  two  million 
fishermen  in  the  less  advanced  countries  who  produce  only  about  one  ton  offish  per  man  per  year,  i.e.  a 
little  over  1  per  cent,  of  the  former.  This  illustrates  the  great  need  to  spread  existing  knowledge  from 
those  countries  with  the  technical  "know-how'*  to  regions  where  it  is  lacking. 

In  conclusion,  I  must  put  on  record  the  gratitude  of  FAO  to  the  Government 
of  the  German  Federal  Republic  and  the  Senate  of  the  Frei  und  Hansastadt,  Hamburg,  for  so 
generously  providing  the  facilities  for  holding  the  Congress.  Many  individuals,  too,  from  a  great 
number  of  countries  did  much  to  assist  us  in  preparing  and  conducting  the  Congress.  There  is  no 
space  for  me  to  mention  them  all  so  that  1  must  limit  myself  to  naming  those  who  were  particularly 
closely  associated  with  the  Congress.  Much  of  the  burden  of  making  local  arrangements  was  carried 
by  Prof.  Dr.  A.  von  Brandt,  Director  of  the  Netz  und  Material  Forschungsinstitut,  Hamburg,  and 
his  staff,  and  we  are  especially  grateful  to  them  for  all  their  help.  Special  thanks  are  also  due  to  Dr. 
G.  Meseck,  Director  of  Fisheries,  Germany,  and  Mr.  A.  W.  Anderson,  Assistant  Director,  Bureau 
of  Commercial  Fisheries,  United  States  Fish  and  Wildlife  Service,  who  acted  respectively  as  Honorary 
Chairman  and  General  Chairman  of  the  Congress. 

D.   B.   FINN 
Rome,  Italy,  January,   1959 

[  xxui  ] 


INTRODUCTION  — MODERN    TRENDS    IN    FISHING 

*HE  oceans,  seas  and  other  waters  cover  more  than  70  per  cent,  of  the  earth  surface.  They  occupy 
about  90  million  sq.  miles  but  produce  less  than  10  per  cent,  of  humanity's  food.  How  much 
food  could  eventually  be  taken  by  man  from  the  sea  cannot  be  assessed. 

From  time  to  time  estimates  are  made  of  how  much  fish  we  might  expect  to  catch. 
Only  a  few  years  ago,  it  was  thought  the  sustained  catch  would  hardly  exceed  some  25  million  metric 
tons  per  year.  But  already  fish  production  nears  30  million  metric  tons  (1957).  Biologists  and  other 
experts  now  suggest  this  catch  might  be  increased  to  some  60  million  tons  a  year  from  known  stocks, 
without  taking  into  account  the  possible  discovery  of  new  fisheries. 

In  a  recent  statement  summing  up  reasons  for  the  increase  in  world  fish  catch  since  the  start 
of  the  twentieth  century,  Dr.  D.  B.  Finn,  Director  of  the  Fisheries  Division,  F.A.O.  declared  that 
progress  in  fisheries  has  been  greater  in  the  past  30  years  than  in  the  previous  three  thousand ! 

Modern  fishing  has  developed  through  three  main  technological  revolutions. 

The  first  one,  mechanization,  began  late  in  the  19th  century  with  the  use  of  steam  propulsion 
in  fishing  vessels — later  followed  by  steam-driven  winches.  Semi-diesel  and  diesel  engines  began  to 
make  their  mark  at  the  turn  of  the  century  and  have  been  gaining  ground  steadily  till  now  steam  is 
almost  displaced  except  in  North  Atlantic  long  distance  trawlers.  But  even  there  steam  is  gradually 
yielding  to  the  diesel  and  such  innovations  as  diesel -electric  drives  and  turbine  propulsion  are  finding 
limited  application-  and  perhaps  atomic  energy  is  not  far  beyond  the  horizon.  Mechanical  propul- 
sion of  the  craft  is  important  in  itself,  but  power  handling  of  the  gear  greatly  extends  the  trend  towards 
fully  mechanised  fishing — a  trend  which  still  offers  great  scope  for  development. 

The  second  revolution  to  influence  modern  fishing  is  the  use  of  electronic  echo  sounding 
and  echo  ranging  equipment.  Echo  sounders  had  already  become  standard  equipment  in  big  North 
European  distant  water  trawlers  before  1939,  but  were  only  used  for  depth  sounding.  The  improve- 
ment of  acoustical  underwater  equipment  during  the  Second  World  War  paved  the  way  for  electronic 
fish  detection  and  after  the  war  recording  echo  sounders  were  speedily  adopted  in  all  medium  and 
large  fishing  vessels  in  the  developed  fishing  countries.  This  can  be  likened  to  giving  a  blind  man  his 
sight.  Fishermen  once  accustomed  to  these  facilities  feel  they  are  groping  in  the  dark  if  deprived  of 
them. 

The  third  major  revolution  in  modern  fishing  is  the  advent  of  synthetic  fibres.  Nylon  in 
various  forms  was  the  first  of  the  man-made  fibres  to  be  widely  applied  in  fishing  nets,  but  in  recent 
years,  several  other  synthetics  have  also  become  important,  especially  in  Japan,  which  leads  the  world 
in  using  synthetic  fibres  for  fishing,  both  as  regards  quantities  and  variety.  Nylon  is  for  instance 
stimulating  an  important  renaissance  in  gillnetting  the  world  over,  giving  this  age-old,  simple  but 
really  basic  form  of  fish  net,  a  new  lease  of  life. 

In  the  economically  developed  countries  fishing  has  met  ever  keener  competition  from 
land  industries  which  offer  steady  and  relatively  comfortable  employment.  To  meet  this  challenge, 
the  fishing  industry  has  been  forced  to  increase  the  size  of  fishing  units,  put  more  horse  power  behind 
each  man  on  the  sea,  and  back  him  with  expensive  shore  installations  for  processing  his  catch.  This 
capitalization  necessitates  operation  at  an  even  and  high  efficiency  to  pay  dividends.  Thus  economic 
pressure  relentlessly  forces  further  technological  development. 

Every  fishing  method  has  been  affected  by  this  development.  Even  handlining  which 
was  well  on  the  way  to  becoming  extinct  in  the  Western  World  has  gained  new  importance  through 
the  use  of  nylon  monofilament,  artificial  lures  and  hand-powered  reels. 

Longlining,  both  bottom  set  and  floating,  is  still  an  important  method  in  the  North  Atlantic 
cod  fisheries,  the  North  Pacific  halibut  fishery  and  the  Japanese  high  seas  tuna  fishery,  but  technique 

f  xxv  ] 


MODERN     FISHING     GEAR     OF     THE     WORLD 

of  handling  the  gear  has  greatly  improved.  The  lines  are  now  streamed  out  at  7  to  9  knots  and 
hauled  in  on  mechanically  or  hydraulically  driven  gurdies  at  a  rate  of  2  to  5  miles/hr.  Longlining  is, 
however,  still  laborious,  and  formidable  quantities  of  bait  are  also  consumed.  For  each  day's  fishing 
an  Icelandic  longline  boat  for  instance  will  use  700  to  1,000  Ibs.  of  frozen  herring  which  is  caught  in 
another  season  and  stored  frozen  for  several  months.  It  is  therefore  tempting  to  contemplate  the 
savings  in  bait  and  labour  which  could  be  achieved  if  an  effective  artificial  bait  were  developed — such 
as  possibly  pieces  of  spongy  material,  suitably  shaped  and  coloured,  permanently  attached  on  each 
hook.  The  line  might  then  be  immersed  in,  doused  with,  or  drawn  through  a  tank  with  a  liquified  fish, 
fish  oil  or  other  natural  or  artificial  liquid  attractant.  The  testing  of  such  sponge  baits,  suitably 
treated  is  certainly  one  of  many  interesting  problems  awaiting  solution. 


— — 2pr  'XPC^**ir  '^^^^^B£?w^^^^^  '^?SliaiBiSS^888S 


.-.  \i»fl!Vy   *'v •/' 


Ov^r  two  hundred  thousand  fishermen  in  India  and  Ceylon  fish  from  log  rafts  with  primitive  gear,  landing  on  the  average  only 

half  to  one  ton  offish  per  man  per  yevr.  Photo:  FAO 


Gillnetting  has  been  more  affected  by  the  man-made  fibres  than  any  other  fishing 
method  and,  as  already  mentioned,  is  stimulating  a  renaissance  in  this  technique.  The  elastic  pro- 
perties of  nylon,  its  flexibility,  softness,  high  tensile  strength  and  other  characteristics  have  so  pro- 
foundly enhanced  the  catching  power  of  gillnets  that  the  name  itself  is  hardly  appropriate  any  longer 
except  where  school  fish  of  a  uniform  size  are  truly  gilled.  In  most  other  forms,  loosely  hung  gillnets 
of  nylon  act  more  as  tangle  nets,  catching  fish  in  a  wide  range  of  size  and  shape. 

One  attractive  feature  of  the  gill  net  is  that  it  can  be  used  even  from  primitive  unpowered 
craft  —a  fact  of  great  importance  in  countries  with  underdeveloped  fisheries.  In  F.A.O*s  Technical 
Assistance  work  in  tropical  countries,  the  introduction  of  nylon  gillnets,  both  bottom-set  and  drifting, 
has  invariably  met  with  almost  immediate  success  and  acceptance  by  the  local  fishermen  who  are 
quick  to  realise  the  advantages.  The  non-rotting  character  of  synthetics  is,  of  course,  of  the  greatest 
importance  in  hot  climates.  Indian  fishermen  operating  log  rafts  (see  above)  increase  their  catches  5 

[xxvi] 


INTRODUCTION 

to  10  times  when  equipped  with  nylon  gill  nets,  thus  raising  their  income  and  eventually  enabling  them 
to  replace  their  primitive  craft  with  small  mechanized  boats. 

When  selecting  synthetic  fibre  twines  for  fishing,  due  attention  should  be  paid  to  the  fact 
that  nylon,  for  instance,  is  drawn  to  different  degrees  during  manufacture  to  give  either  high  tenacity 
and  relatively  low  extensibility  or  else  lower  tensile  strength  and  high  extensibility.  For  certain  uses, 
the  latter  may  be  preferable. 

Initial  high  price  of  nylon  has  come  down  to  a  competitive  level,  and  the  early  difficulties 
of  knot  slippage,  etc.,  have  now  been  largely  overcome  with  bonded  twines  and/or  heat-setting  of 
knots.  This  is  still  not  fully  realized  in  some  countries,  nor  is  the  fishing  industry  everywhere  fully 


In  sharp  contrast  to  the  facing  photo,  modern  purse  seining  exemplifies  the  high  level  of  efficiency  of  modern  fishing  units  as 
this  one  on  the  British  Columbia  coast ;  where  hundreds  of  tons  of  herring  are  frequently  caught  in  one  set. 

Photo:  Info.  Serv.t  Dcpt.  of  Fisheries,  Ottawa 


informed  of  the  development  and  rapidly  increasing  use  of  other  synthetic  fibres  such  as  the  polyvinyl 
alcohol  and  polyester  fibres.  While  practically  all  new  gillnets  used  in  the  North  Atlantic  cod 
fisheries,  the  Pacific  salmon  fisheries  and  in  inland  waters,  even  in  the  great  lakes  of  Africa,  are  now 
made  of  synthetics,  man-made  fibres  have  not  yet  been  adopted  in  European  herring  drift  net  fisheries. 
It  seems,  however,  likely  that  this  is  only  a  matter  of  time,  despite  initial  difficulties  with  thin  nylon 
twine  which  cut  herring  and  make  it  difficult  to  shake  out  of  the  nets.  The  Japanese  in  their  drift 
netting  for  herring,  sardines,  mackerel  and  salmon  already  use  man-made  fibres  extensively. 

In  world  fisheries,  gillnets,  bottom-set  and  drifting,  rank  next  after  trawls  and  purse  seines 
in  importance  in  terms  of  total  catch.  Growing  awareness  of  the  danger  of  depleting  fish  stocks 
which,  in  some  cases,  has  led  to  the  closing  of  coastal  waters  to  trawling,  is  another  factor  in  favour 
of  such  methods  as  gillnetting  and  longlining. 

[  xxvii  ] 


MODERN     FISHING     GEAR     OF     THE     WORLD 

At  the  turn  of  the  century  purse  seining  came  into  general  use,  having  evolved 
basically  from  Mediterranean  roundhaul  nets.  American  two-boat  menhaden  purse  seines  were  first 
introduced  into  Icelandic  and  Norwegian  herring  fisheries  in  1903-4,  and  in  a  few  years  largely 
replaced  the  big  locknets  used  previously  for  fencing  in  large  schools  of  herring  in  bays  and  fjords. 
Until  the  mid-thirties,  two-boat  purse  seine  dories  were  rowed,  but  became  mechanised  soon  after. 
While  pursing  is  normally  done  with  a  power  driven  winch,  the  net  itself  is  still  hand  hauled  so  this 
method  of  fishing  requires  a  crew  of  sixteen  to  thirty  men — an  increasingly  serious  handicap  to-day. 

On  the  other  hand  significant  progress  has  been  made  in  the  power  handling  of  one-boat 
purse  seines,  on  the  Pacific  Coast  of  America.  The  Puretic  power  block  is  the  latest  example  of 


(j  i line t ting  for  thread/in  ("  Dara")  north  oj  Bombay  where  engines  have  been  installed  in  almost  a  thousand  boats,  but  the 
nets  are  still  hauled  by  hand.   With  a  power  driven  net  hauler  costing  only  about  S/00,  this  boat  could  easily  fish  twice  as 

many  nets  with  a  smaller  crew.  Photo  :  \*  AO. 

this.  There  is  great  scope  for  saving  labour  and  otherwise  improving  efficiency  by  applying  mechanical 
power  to  other  types  of  purse  seines  and  roundhaul  nets  elsewhere. 

Trawling  has  come  a  long  way  since  sailing  smacks  drifted  downwind  with  beam  trawls. 
Mechanization  and  the  use  of  otter  boards,  amplified  later  by  Vigneron-Dahl  gear,  led  to  the  develop- 
ment of  many  widely  different  trawl  types  for  various  operating  conditions,  both  on  and  off  the  bottom. 
While  midwater  trawling  has  attracted  considerable  attention  throughout  the  world,  other  important 
developments  have  gone  almost  unnoticed  outside  the  areas  where  they  are  used.  This  is  true  for 
instance  of  the  high-opening  trawls  now  widely  used  in  Sweden,  Denmark,  Germany,  Netherlands 
and  Belgium. 

Before  the  war  large  German  herring  trawlers  had  fished  for  years  with  moderately 
high-opening  bottom  trawls  where  the  headline  was  lifted  by  kites  and  the  fish  schools  depressed  by 

[  xxviii  ] 


INTRODUCTION 

kites  riding  on  false  headlines.  It  was,  however,  mainly  after  the  war  that  high-opening  trawls  for 
vessels  in  the  20  to  200  ton  class  were  developed  and  came  into  general  use.  These  trawls  are  of 
many  different  types  but  in  general  they  have  certain  main  features  in  common,  i.e.  they  are  lightly 
built  of  thin  twine,  have  short  but  high  wings  and  the  pull  of  the  net  is  transmitted  to  the  towing 
bridles  along  the  lastrich  lines  or  side  seams  in  order  not  to  drag  back  the  headline  but  release  it  to 
rise  high.  These  trawls  do  not  depend  on  kites  or  hydrodynamic  floats  to  secure  a  high  vertical 
opening  but  are  so  proportioned  that  they  billow  out — somewhat  like  a  parachute. 

Measurements  have  shown  that  such  light  trails  for  relatively  low-powered  vessels  (100 
to  300  h.p.)  frequently  open  as  high  as  20  to  30  ft.  (6  to  9  m.)  headline  height.      They  arc  therefore, 


Bottom  set  cod  gillnets  of  nvlon  hauled  on  a  hydraulicaily  driven  gurdy  off  Iceland.  2,000  to  2,500  fms,  of  nets  are  operated 

by  each  motor  boat  and  the  average  catch  is  6  to  10  tons  per  day.  Photo:  FAO 


particularly  efficient  for  catching  semi-pelagic  and  pelagic  species  schooling  at  some  distance  above 
the  bottom,  such  as  herring,  sprat,  mackerel,  etc. 

During  the  last  decade,  general  adoption  of  this  type  of  trawl  has  had  a  profound  effect 
in  certain  fisheries.  In  Denmark,  for  instance,  the  fish  catch  is  now  five  times  greater  than  it  was 
before  the  war,  i.e.  has  risen  from  about  100,000  tons  (1938)  to  about  530,000  tons  (1957),  largely 
through  changing  over  from  quality  fishing  with  Danish  seines  and  low  opening  trawls  to  quantity 
fishing  with  high-opening  and  midwater  trawls. 

Certainly  there  is  scope  for  such  nets  in  many  other  parts  of  the  world  and  it  is  hoped  the 
information  contained  in  this  book  will  help  accelerate  this. 

Another  rather  recent  development  not  widely  appreciated  is  the  extension  of  trawling 
into  deeper  water,  even  with  relatively  small  boats  of  modest  power.  Both  on  the  U.S.  Pacific  Coast 

[  xxix  1 


MODERN     FISHING     GEAR     OF     THE     WORLD 

and  in  the  Mediterranean  30  to  80  ton  vessels  of  150  to  250  h.p.  commonly  trawl  down  to  350  fms. 
which  is  as  deep  as  the  deepest  grounds  fished  now  by  the  biggest  trawlers  in  Northern  Europe  of 
over  1,000  h.p.  While  the  Pacific  Coast  deep  trawlers  use  relatively  very  short  warps  and  heavy 
doors,  the  Mediterranean  boats  commonly  use  about  3:1  warp/depth  ratio  and  rather  light  trawl 
doors  with  bracket  adjustment  giving  an  outward  tilt  and  fitted  with  broad  mud  skis  to  avoid  wasting 
towing  power  on  ploughing  deep  ditches  in  soft  mud  bottoms.  To  fish  such  depths,  an  echosounder 
is  indispensable  for  staying  on  bottom  contour  lines. 

In  both  these  regions  all  boats  use  stern  trawling  which,  with  wheelhouse  forward  and  clear 
deck  aft,  facilitates  mechanical  handling  of  the  gear  and  simplifies  manoeuvring  during  hauling 
and  shooting  light  trawl  gear  (without  bobbins)  as  compared  with  side  trawling. 

Yet  Mediterranean  trawl  boats  normally  carry  crews  of  6  to  9,  while  comparable  boats  in 
the  Gulf  of  Mexico  shrimp  fishery  or  Danish  side  trawlers,  carry  only  3  or  4.  Growing  demands 
for  better  income  among  Mediterranean  fishermen  will  force  streamlining  and  reduce  the  number 
of  the  crew.  A  similar  need  for  rationalization  of  working  methods  and  equipment  exists  in  most  of 
the  world's  fisheries  and  there  is  certainly  very  great  scope  for  applying  work  study  techniques  on 
board  fishing  vessels  of  all  types  to  accelerate  this  development  just  as  is  done  in  industry  on  land. 

A  trend  in  modern  trawling  which  must  be  mentioned  is  the  big  stern  trawlers  pioneered 
by  the  Fairtry.  The  Russians  now  have  a  sizeable  fleet  of  such  factory  ships,  and  three  distant  water 
trawlers  of  a  similar  design,  but  smaller,  have  also  been  built  for  German  owners.  All  these  vessels 
are  built  to  fish  in  distant  waters,  normally  more  than  a  thousand  miles  from  their  home  port.  Despite 
rational  methods  of  handling  the  gear,  most  of  these  factory  ships  carry  very  large  crews  for  filleting, 
freezing  and  otherwise  processing  the  catch.  Such  elaboration  of  the  catch  at  sea  is  inherently  expen- 
sive, and  perhaps  the  future  trend  may  be  towards  further  simplifying  the  handling  of  the  catch  so 
that  distant  water  fishing  vessels  could  carry  a  small  crew,  provided  with  good  accommodation  and 
labour-easing  as  well  as  labour-saving  appliances.  A  similar  development  is  long  overdue  on  the 
conventional  deep  sea  trawlers. 

The  use  of  bigger  and  more  powerful  vessels  has  led  to  higher  trawling  speeds.  This  gives 
rise  to  various  problems  in  relation  to  the  hydrodynamical  properties  of  floats  and  trawl  boards 
and  it  also  brings  up  the  choice  between  towing  a  relatively  small  net  fast  or  using  a  much  bigger 
net  at  a  slower  speed.  Here  the  decisive  factors  are,  firstly,  the  fish  behaviour  and,  secondly,  the 
economical  or  critical  speed  of  the  gear,  beyond  which  a  doubling  or  trebling  of  the  power  expen- 
diture results  in  only  insignificant  increase  in  towing  speed.  A  careful  study  of  these  little-known 
factors  may  well  lead  to  improved  fishing  efficiency  and  saving  of  costly  fuel. 

Hydrographic  and  biological  observations,  coupled  with  organized  fish  searching  are 
now  beginning  to  supplement  the  empirical  knowledge  and  intuition  of  experienced  fishing  skippers 
in  locating  fish  concentrations.  A  good  example  of  this  is  the  service  rendered  to  the  Norwegian 
herring  industry  by  Asdic  equipped  research  vessels  which  track  the  annual  migration  of  the  herring 
during  its  feeding  run  in  the  summer  in  the  Norwegian  Sea  and  the  winter  spawning  run  to  the 
Norwegian  west  coast.  Other  examples  lie  in  the  large  fleets  of  Russian  herring  fishing  vessels  which, 
serviced  by  mother  ships,  follow  these  same  herring  stocks  throughout  the  year  in  the  sea  between 
Norway,  Iceland  and  Scotland;  also  the  Japanese  high  seas  fishery  for  salmon  in  the  North  Pacific 
and  tuna  longlining  in  equatorial  waters  around  the  world. 

The  echo  sounding  equipment  developed  for  fish  detection  after  the  last  war  has  had  a  pro- 
found effect  on  most  modern  methods  of  fishing.  The  latest  technical  developments  overcome  some 
of  the  limitations  of  conventional  models  and  open  up  new  horizons;  for  instance,  the  considerable 
increase  in  sound  output,  enlarged  recording  of  small  depth  segments  and  suppression  of  the  bottom 
echo  greatly  improve  the  detection  of  fish  near  the  bottom  and  in  great  depths.  Some  of  these  new 
features  can  be  built  into  older  models.  Fishing  Asdic  units  are  coming  into  use  in  the  North 
Atlantic  herring  fisheries  and  in  whaling. 

Apart  from  finding  fish  the  new  echo  sounders  can  sometimes  indicate  whether  to  use  a 
trawl  of  high  opening  (and  limited  spread)  or  of  the  widest  possible  spread  (and  small  gape). 

Echo  fish  detection  made  midwater  trawling  possible.  An  absolute  prerequisite  is,  of 
course,  to  find  the  fish  and  its  depth.  The  next  step  is  to  regulate  and  determine  accurately  the  fishing 

[  xxx  1 


INTRODUCTION 

depth  of  the  trawl,  and  this  has  presented  difficulties  in  the  past.  At  the  time  of  this  writing  encour- 
aging progress  is,  however,  being  made  towards  evolving  and  introducing  a  simple,  workable  and 
inexpensive  arrangement  of  using  an  echo  sounder  transducer  attached  to  the  headline  or  the  footrope 
of  midwatcr  trawls  as  outlined  on  p.  491  in  this  book.  Not  only  does  this  clearly  show  the  exact 
depth  of  the  trawl  but  the  vertical  opening  is  also  indicated  and  the  fish  entering  the  trawl  mouth  or 
evading  below  or  above  it.  This  may  well  lead  to  a  much  wider  application  of  midwater  trawling 
which  is  still  very  limited. 

The  use  of  light  for  attracting  fish  is  an  important  method  in  Japan,  Philippines  and  the 
Mediterranean  region,  but  amazingly  enough,  it  is  still  largely  neglected  elsewhere.  One  notable 
exception  is,  however,  the  novel  use  of  conical  lift  nets  and,  more  recently,  of  pumping  in  conjunction 
with  light  attraction  in  the  Caspian  Sea. 

Attracting  fish  by  light  is  of  special  value  where  sardine,  anchovy,  mackerel,  saury  or  other 
light  seeking  species  arc  present,  but  do  not  spontaneously  form  dense  schools  that  arc  easily 
located.  Definitely  the  use  of  lights  should  be  thoroughly  tested  not  only  throughout  the  tropics,  but 
in  the  temperate  and  cold-temperate  zones.  When  experimenting  with  light  attraction,  negative 
initial  results  should  not  be  taken  as  conclusive,  as  a  great  deal  of  patience  is  often  needed  to  develop 
a  suitable  technique  for  each  species  under  each  set  of  circumstances. 

While  electrical  fishing  is  already  used  to  some  extent  in  fresh  water,  serious  problems 
are  met  with  in  applying  it  to  saltwater  fishing,  due  to  the  progressive  increase  in  power  requirements 
as  range  increases.  Several  years  ago,  optimistic  reports  were  published  heralding  bright  future 
prospects  for  revolutionary  development,  but  this  news,  unfortunately,  proved  to  be  premature  and 
interest  in  electrical  fishing  waned.  Full-scale  experimentation  at  sea  is  expensive,  and  while  some  tests 
have  been  made  in  recent  years,  much  more  work  needs  to  be  done  in  this  field  to  assess  the  practic- 
ability of  using  electrofishing  in  salt  water,  and  particularly  in  conjunction  with  conventional  fishing 
methods,  including  the  use  of  light  which  might  possibly  help  to  bring  fish  within  range  of  the  electric 
fields.  Spontaneously  formed  dense  schools  of  herring  and  other  pelagic  fish  also  appear  to  offer 
prospects  in  this  connection. 

The  International  Fishing  Clear  Congress  dealt  mainly  with  the  latest  types  of  commer- 
cially important  fishing  gear  and  with  current  thought  and  experiment  concerned  with  making  it  more 
efficient  and  operating  it  more  effectively.  This  book  therefore  emphasizes  the  recent  developments 
rather  than  describing  traditional  types  of  gear  which  have  long  been  used  in  fishing  in  various 
countries. 

In  the  past  almost  all  fishing  gear  has  been  evolved  entirely  by  trial  and  error,  but  in  the 
last  few  years  a  small  beginning  has  been  made  towards  supplementing  the  empirical  approach  by 
theoretical  calculations — as  is  clearly  evidenced  by  Section  6  of  this  book.  When  dealing  with  living 
nature  the  use  of  mathematical  theory  and  other  forms  of  pure  logic  is  often  severely  limited  by  our 
generally  incomplete  knowledge  of  the  complex  factors  affecting  the  problems  under  study  and 
this  applies  particularly  to  fishing  where,  undoubtedly,  many  of  these  factors  are  still  unidentified 
while  others  have  not  been  properly  evaluated.  In  this  respect  fishing  is  in  the  same  boat  as  many 
fields  of  engineering,  i.e.  that  theoretical  solutions  must  be  carefully  checked  against  empirical 
knowledge  and  tested  in  the  field  before  they  can  be  trusted.  Thanks  to  recent  technological  advances 
we  are,  however,  rapidly  learning  more  about  the  behaviour  of  fishing  gear  under  water  and  the 
reaction  of  fish  to  it,  and  it  seems  safe  to  say  that  we  stand  at  the  threshold  of  a  new  era  where 
systematic  gear  research  will  be  increasingly  fruitful. 

Mention  has  been  made  of  three  major  revolutionary  changes  which  have  altered  the  scope, 
nature  and  effectiveness  of  fishing,  namely  mechanization,  echo  fish  finding  and  synthetic  fibres.  A 
fourth  one  is  perhaps  just  around  the  corner — and  may  be  brought  about  by  applying  engineering 
theory  and  rational  methods  to  the  development  of  fishing  gear  and  its  operation. 

HILMAR  KRISTJONSSON 


[  xxxi  ] 


Section  1 :  Materials — Terminology  and  Numbering  Systems. 


TERMINOLOGY   AND   COUNT   OF  SYNTHETIC   FIBRE  TWINES 

FOR   FISHING   PURPOSES 

by 
HANS  STUTZ 

Farbwcrkc  Hoechst  A.G.,  Bobingen  Mills,  Germany 

Abstract 

The  author,  speaking  as  a  manufacturer,  tries  to  bring  some  sort  of  order  into  the  present  day  chaos  which  surrounds  textile  fibres. 
He  gives  a  comprehensive  classification  of  all  kinds  of  fibres  pointing  out  the  difficulties  of  using  different  numbering  systems  as  well  as 
different  units  of  measurement  (cm.,  g.,  lb.,  and  inch). 


Resume 


La  terniinologic  et  le  numerotage  des  fils  dc  fibres  synthetiques  pour  la  peche 


L'auteur,  parlant  en  tant  que  fabricanl,  cssaie  de  mettre  de  1'ordre  dans  le  chaos  qui  regne  actucllcment  dans  le  domaine  ties  fibres 
textiles.  II  donne  unc  classification  comprehensive  de  toutes  les  sortcs  de  fibres  en  faisant  ressortir  les  difficultcs  a  ('utilisation  de  diffc rents 
systemes  de  numerotage  et  dc  diflcrcntcs  unites  de  mcsurc  (cm.,  g.,  et  pouce). 

Tcrminologia  y  numeracion  de  los  hi  los  sinteticos  para  artes  de  pesca 
Extracto 

El  autor.  hablando  como  fabricante,  trata  de  ordenar  el  caos  en  quc  se  encuentran  las  fibras  textiles.  Para  este  objeto  clasifica 
detalladamcnte  todos  los  tipos  cxistentes,  senalando  las  dificultades  que  ofrece  el  uso  de  diverse*  sistemas  de  numeruci6n  y  unidades  de 
mcdida  (cm.,  g.t  lb.,  y  pgda.). 


EXTK  of  agreement  on  the    definition    of   technical 
terms  creates  difficulties  in  translating  papers  from 
one  language  to  another.  Indeed,  it  is  not  always 
easy  for  experts  speaking  the  same  language  to  understand 
each   other   as   a    particular    term    may   have    several 
meanings. 

NAMES  OF  FIBRES 

Consider,  for  instance,  the  large  number  of  names  for 
the  various  fibres  which  arc  on  the  market  today.  At 
first  sight  they  may  not  seem  to  have  any  relationship 
with  each  other  but  anyone  who  makes  an  effort  to 
introduce  some  kind  of  order  with  these  fibres  will 
recognise  that  they  are  connected  with  each  other 


Editor's  Note:  In  editing  the  papers  and  discussion  for  uniformity 
of  terms  and  expression  it  was  found  necessary  to  adopt  the  following 
standard  terminology  for  "yarn",  "twine",  "extension*',  "elongation", 
etc.: 

1'wine  Twisting  Stages: 

1st  step:     fibres  spun  or  twisted  into  yarn 
2nd  step:    yarns  twisted  into  strand  (or  twine) 
3rd  step:     strands  twisted  into  twine  (or  rope) 
4th  step:     twines  twisted  into  cord  (or  cable) 

Thus  nets  arc  normally  made  of  "twine". 

When  describing  the  various  properties  of  net  materials,  total 
extension  is  the  increase  in  length  under  load.  This  may  consist  of 
elastic  extension  (recoverable  on  release  of  stress)  and  permanent 
elongation. 


in  certain  ways.  Table  1  shows  one  way  of  placing  the 
variety  of  textile  fibres  into  a  system. 

Most  of  the  fibres  used  in  the  manufacture  of  cordage 
and  nets  are  included,  although  the  list  is  by  no  means 
complete  as  far  as  trade  names  arc  concerned. 

Two  of  the  problems  of  terminology  which  concern 
manufacturers  are: 

(1)  Tenacity  data 

(2)  Determining    the    diameter    or    thickness    of 
filaments,  threads,  etc. 

SPECIFIC  WEIGHT  AND  TENACITY 

It  is  customary  to  measure  and  indicate  the  Breaking 
Load  of  a  yarn,  filament  or  thread  in  kilograms.  For 
the  purpose  of  comparing  yarns,  etc.,  of  different 
thicknesses  (yarn  count  or  diameter),  a  mathematical 
factor,  the  "Specific  Breaking  Load"  (G),  is  introduced 
and  the  "Break  ing  Length  "(R)  is  known  from  the  spin- 
ning and  processing  of  natural  fibres. 

The  following  relations  exist  : 

R       Nm  v  Pj                                Nc  Yarn  count  in  Mnglish 

Nc  *-  P»  number 

Nm         metric 

P3  number 

1                                             Pj  Breaking-load  in    kg. 

pn                                       P.»  -  ,.          „       „    Ibs. 

(g/dcn.)  PV~         ", er- 

T  -  Titer  in  denier  or  tex. 


"Tdcn 


The  specific  weight  of  the  great  variety  of  natural  and 


MODERN    FISHING    GEAR    OF    THE    WORLD 

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[2] 


SYNTHETIC    FIBRE    TWINES    FOR    FISHING    PURPOSES 


man-made  fibres  existing  today  varies  very  much,  as  is 
shown  in  the  following  Table: 


Natural 
Fibre 

Y 

(I/O 

Cotton 
Wool 
Hemp 
Flax 
Jute 
Ramie 
Natural  Silk 

•47 
•31- 
48 
43- 
•43- 
50- 
25- 

56 

•32 

50 
•48 
•52 

•37 

12- 

15 

38 

40 

13- 

20 

•35- 

•72 

31 

60 

0  92 

Chemical  Fibre 


Polyamids 

Polyester 

Polyacrylinitrile 

Poly  vi  ny  Ichloridc 

Polyvinylalcohol 

Polyethylene 

Regenerated  Cellulose    1   50-1  60 


The  following  relations  exist  : 

n   -    P  x  Nm  --,  y(kg./mm.2)     P  --    Breaking  load  in  kg. 
--  Rkm  x  v(kg./mm.2)          Nm—  Yarn  count  metric 
—  9  x  G  /  y(kg./mm.2)          y   ~  specific  weight  (g/cm.3). 

With  these  very  different  specific  weights,  the  substanc; 
cross-sections  of  yarns  of  equal  weight  are  also  different 
the  cross-sectional  areas  are  inversely  proportional  to 
the  specific  weights.  When,  on  the  other  hand,  tenacity 
values  arc  compared,  only  the  Substance  Cross  Sectional 
Area  is  decisive.  Hence  in  the  case  of  two  yarns,  twines, 
etc.,  made  from  fibres  of  very  different  specific  weights, 
far  more  accurate  and  comparable  values  arc  obtained 
by  the  term  "Specific  Tenacity"**,  which  has  long  been 
applied  in  testing  metals  and  plastics  and  which  represents 
a  force  or  tension  related  to  the  cross-sectional  area. 

NUMBERING  SYSTEMS 

The  second  problem,  namely  the  indication  of  the  dia- 
meter or  thickness  of  the  yarn,  twines,  etc.,  is  considerably 
more  difficult  because  of  deep  rooted  and  varying  customs 
and  traditions  in  the  various  industries  and  among 
processors  and  users. 

To  achieve  mutual  understanding,  measures  and 
descriptions  should  as  far  as  possible  characterize  un- 
equivocally the  thickness  of  yarn,  twines,  ropes  and 
cordage.  Experience  shows  that  the  thinner  the  filament 
the  greater  the  difficulty  in  measuring  the  diameter. 
Therefore  the  indirect  course  was  adopted  for  the  fine 
yarns  and  filaments.  The  length  and  weight  of  a  particular 
piece  of  filament  was  determined  in  a  relatively  simple 
and  unequivocal  manner.  Mathematical  calculations 
made  it  possible  to  obtain  from  these  quantities  the 
count,  the  length  per  weight,  and  the  titre,  etc. 

Unfortunately  various  systems  of  measurements  (cm., 
g.,  lb.,  inch,  etc.)  still  exist  so  that  even  in  such  a  small 
sector  of  the  textile  industry  as  throwsters,  rope,  cordage 


and  net  manufacturers  there  are  a  multitude  of  co- 
existing and  partly  confusing  count  systems  in  use  as, 
for  example: 

Nme,  Nee,  NCL,  Nt,  Schokker-Nr,  m-weight,  denier, 
tex,  diameter,  circumference  (the  latter  two  even  in  mm. 
and  inches). 

it  is  obvious  that  these  numerous  units  of  measure  for 
the  same  term  are  confusing,  and  are  contrary  to  uniform- 
ity and  rationalization.  However,  each  industry  is 
very  reluctant  to  give  up  old  customs  so  that  the  efforts 
made  to  introduce  standards  make  little  headway. 

The  man-made  fibres  industry  has  always  used  the 
weight  count  in  accordance  with  the  formula: 
f-+ 

Titre:    Td       --(den.);  G  -  Weight  (g) 


*-* 

Tt  -  j-(tex); 


L,     Length 
(9000  m.) 

L     Length  (km.) 


The  weight  count  has  certain  advantages,  e.g.  for 
calculating  more  accurately  the  twist  count  of  coarser 
threads.  This  is  also  shown  clearly  by  the  recommendation 
of  Committee  No.  38  (ISO  Textiles  of  the  9th  July  1951) 
which  states  that  in  the  future  a  weight  count  with  the 
units  "tex"  should  be  applied  and  that  it  would  be  sensi- 
ble to  adjust  this  system  to  metric  units  of  length. 
The  formula  for  conversion  of  this  new  unit  of  measure 
into  the  units  hitherto  used  is  as  follows: 


Conversion  Formula 


tx 


1000 


590,541 
NeB 


Nm 
=  0,111  Td  = 


1653,52 

NCL  " 
10 

Schokker  No7 


1 
Nt 


Another  peculiarity  in  establishing  the  count  of  net 
yarns  and  twines,  although  in  general  use,  is  contrary 
to  the  standards  applicable  in  Germany,  DIN  60900. 

A  commonly  used  net  twine  or  cord,  is  for  example, 
"Nm  20/45".  On  further  consideration,  however,  it  is 
found  that  this  means  a  multiple  twist  (twine  or  cord) 
which  actually  ought  to  be  expressed  as  follows: 

Nm  20/5/3/3 
Using  the  tex  unit  this  would  be  expressed  as  follows: 

50  tx   x   5    x   3    x   3, 
which,  by  simple  multiplication,  give  the  total 

2250  tx  (without  loss  of  length  during 
twisting) 


3] 


CONSTRUCTION  AND  NUMBERING   OF  SYNTHETIC   NET  TWINES 

by 
G.  A.  HAYHURST  and  A.  ROBINSON 

William  Kenyon  &  Sons  Ltd.,  Dukinfield,  Cheshire,  U.K. 

Abstract 

This  paper  is  written  from  the  twine-maker's  point  of  view.     The  construction  of  twines  is  described  and  the  merits  of  2,  3  or  A 
stranded  twines  are  mentioned.     There  is  also  a  review  of  twine  si/e  descriptions. 


Resume 


Fabrication  et  numerotage  des  ills  a  filets  synthetiques 


Ccttc  etude  est  ccntc  du  point  dc  vue  du  fabricant  de  fil.     La  fabrication  des  fils  cst  dccritc  ct  il  est  fait  mem  ion  des  a  vantages  de* 
tils  a  2,  3  on  4  brins.     Lcs  descriptions  des  dimensions  des  fils  sont  aassi  passees  en  revue. 

Fabrication  y  numeracion  de  los  hilos  sinteticos  para  redes  de  pesca 
Kxtracto 

En  este  trabajo,  desde  el  punto  de  vista  del  fabncante,  se  describe  en  detatle  la  manufactura  de  los  hilos  de  2.  3  o  4  hcbras,  as 
como  sus  meritos.     Tambien  se  pasa  revista  a  las  descripciones  de  la  numeracion  de  los  hilos. 


YARNS 

It  is  sufficient  to  mention  the  two  main  types — 

(a)    Continuous  Filament  Yarns 

These  have  a  shiny,  lustrous  appearance  and  the 
size  or  length/weight  ratio  is  usually  described  by 
denier  measurement.  Tensile  strength  varies  from 
5  grams  per  denier  up  to  8*8  grams  per  denier  when 
tested  in  the  dry  state  on  a  straight  pull, 

(/>)    Staple  Yarns 

Appearance  is  similar  to  cotton  because  the 
filaments  are  cut  and  spun  to  form  a  continuous 
yarn.  Size  may  be  described  by  reference  to  French 
metric  count  or  English  cotton  count.  Tensile 
strength  varies  between  2'3  and  3*3  grams  per 
denier  according  to  type  of  fibre,  staple  length, 
method  of  spinning,  and  similar  considerations. 


the  advantage  that  the  strands  cannot  become 
displaced  due  to  one  strand  "riding"  over  another. 
On  the  other  hand,  if  one  strand  is  slacker  than  the 
other,  the  load  falls  almost  entirely  on  only  half  the 
total  number  of  yarns  in  the  cord.  In  all  two  strand 
twines  the  interstices  are  wide  and  the  angle  of  lay 
is  also  wide,  so  that  the  twine  does  not  have  a 
round  appearance. 

The  three  strand  twine  is  a  construction  which  is 
exceptionally  stable  and  free  from  distortion,  the 
reason  being  that  the  sectional  view  is  in  the  form 
of  a  triangle  which  is  not  easily  pushed  out  of  shape. 


TWINES 

Basically  the  manufacture  of  twines  from  single  yarns 
consists  of  two  twisting  operations,  first  twisting  together 
two  or  more  single  yarns  to  form  a  strand,  and  then 
twisting  two  or  more  strands  together  to  form  a  twine. 
This  appears  deceptively  simple  and  the  following  points 
merit  attention: 

(a)    Constructions 

Nearly  all  fish-net  twines  are  made  up  of  2  strands, 
3  strands  or  4  strands.    The  two  strand  twine  has 


In  spite  of  the  precautions  taken  in  the  design  of 
modern  machinery,  one  strand  occasionally  rides 
over  another  and  thus  creates  a  fault  in  the  twine. 
Such  faults  are  infrequent  in  a  good  quality  product 
and  where  they  do  occur  the  load  is  borne  by  at 
least  66$  per  cent,  of  the  total  yarns  in  the  twine.  The 
angle  of  lay  is  more  acute  (compared  with  2  strand 
twines),  the  interstices  are  narrower,  and  the  twine 
has  a  rounder  appearance. 

Four  strand  twines  of  synthetic  fibres  give  an 
exceptionally  round  formation,  which  some  users 


[4] 


SYNTHETIC    FIBRE    TWINES     f  OR    FISHING    PURPOSES 


seem  to  prefer,  but  in  fact  the  lay  is  easily  distorted 
like  this: 


This  causes  uneven  stresses,  and  there  is  a  tendency  for 
"riding"  strand  faults  to  occur  frequently. 

(/>)    Twist  Constants 

The  suitability  of  the  twine  for  a  particular  use 
is  influenced  by  the  amount  of  twist  inserted  into  the 
strand  and  twine.  However,  mass  production 
methods  require  that  one  standard  be  adopted  for  all 
netting  twines  manufactured  in  a  factory.  The  twines 
are  therefore  a  compromise  of  the  characteristics 
most  generally  in  demand  and  the  economic  pro- 
duction possibilities. 

Additional  twist  has  four  effects: 
(i)       Breaking  strength  is  reduced 
(ii)      Extension  at  break  is  increased 
(iii)     Length  per  weight  is  reduced 
(iv)     Resistance  to  abrasion  and  general  wear 

is  improved. 

Every  twine  manufacturer  has  to  compromise 
between  these  factors.  It  is  impossible  to  have  the 
advantage  of  item  (iv)  without  sacrificing  something 
as  regards  items  (i)  and  (iii). 

(r)    Twine  Si/e  Descriptions 

The  most  usual  description  for  cotton  fish-net 
twines  is  to  state  the  counts  of  the  single  yarn  and 
the  total  number  of  yarns,  e.g.  10s/9  means  10s. 
cotton  count,  3  yarns  in  each  strand,  3  strand 
construction. 

Unfortunately,  there  is  no  universally  adopted 
description  method  for  synthetic  fibre  twines,  and 


confusion  is  the  result.  Some  Japanese  manu- 
facturers have  used  systems  similar  to  that  for 
cotton  twines,  so  that  a  twine  described  as  210 
Den/60  will  probably  be  210  denier,  20  yarns  per 
strand,  3  strand  construction.  Note  that  the  des- 
cription is  not  quite  explicit,  for  the  construction 
might  possibly  be  210  denier,  30  yarns  per  strand, 

2  strands  or  even  210  denier,  15  yarns  per  strand, 
4  strand.  Yet  another  interpretation  would  be  a 
cabled  strand  construction  giving  210  denier,   15 
yarns,  folded  2  ply,  2  strands  in  cord,  or  possibly 
210  denier,  10  yarns,  2  ply,  3  strands  in  cord. 

The  Fisheries  Research  Board  of  Canada  has 
adopted  a  system  which  calls  for  definition  by  yarn 
denier,  number  of  yarns  in  strand  and  number  of 
strands  in  twine. 

Thus  210-33  means  210  denier,  3  ends  in  each 
strand,  3  strand  construction.  Similarly  210-63 
means  210  denier,  6  yarns  in  each  strand,  3  strand 
construction.  No  doubt  this  system  is  excellent 
where  all  persons  understand  fully  that  the  Fisheries 
Research  Board  system  is  being  used.  It  will  be 
appreciated  that  in  the  Japanese  system  210-63 
could  mean  21  yarns  per  strand,  3  strand  construc- 
tion. Many  other  systems  could  be  mentioned  as 
examples. 

We  would  suggest  that  a  simple  yet  effective  and 
easily  understood  size  description  system  would  be 
to  state  — 

(i)       Yarn  denier 

(ii)      Number  of  yarns  in  strand 

(iii)     Number  of  strands  in  twine 

On   this   basis  210  denier,   6  yarns  per  strand, 

3  strand  would  be  described  as  210/6/3,  and  it  is 
quite  certain  that  everybody  would  understand  this 
system,   without  any   additional   explanation   and 
without   the   slightest   doubt   as   to   the   size   and 
construction  required.  A  further  advantage  is  that 
if  the  strands  are  sub-divided  into  2  or  3  plies  this 
can    be   clearly  indicated,   e.g.    210/10/2/3   would 
mean  210  denier,  10  yarns,  2  ply  in  each  strand, 
3  strands  in  twine. 


MIXING 

CARDING 

SPINNING 

I—  1  BREAKING-  H 

L 

1 

DRAWING 

Conversion   of  fibres   into   twine. 


[5] 


TERMINOLOGY  AND  NUMBERING  SYSTEMS  USED  IN  JAPAN 

by 

PROFESSOR  SHIGENE  TAKAYAMA 

Chief,  Fishing  Gear  and  Methods  Section,  Tokai  Regional  Fisheries  Research  Laboratory,  Tokyo,  Japan 

Abstract 

Technical  terms  and  numbering  systems  differ  from  country  to  country  and  there  exists  today  no  standard  for  comparison.  Fisheries 
technologists  find  it  difficult  to  compare  the  characteristics  of  materials  and  gear  when  studying  foreign  publications,  and  placing  orders  for 
gear  is  a  problem  to  the  fishermen.  It  would  be  beneficial  to  the  whole  industry  if  an  international  standard  of  terms  and  measurements 
could  be  agreed  upon.  As  an  example,  a  description  is  given  of  the  specification  of  fishing  twines  now  used  in  Japan. 


Resume 


Terminologie  et  Systemes  de  Numerotage  au  Japan  Utilises 


Les  termes  techniques  et  les  syst ernes  de  numerotage  different  cTun  pays  a  1'autrc,  et  actuel lenient  il  n'existe  aucun  talon  de  comparai- 
son.  Pour  les  specialistes  des  peches  il  est  difficile  de  comparer  les  caracteristiques  des  materiaux  et  des  engins  quand  ils  etudient  les  publica- 
tions etrangeres.  De  meme  c'est  un  probleme  pour  les  pecheurs  quand  il  s'agit  de  commander  des  engins.  II  serait  bon  que  toute 
1'industric  se  mette  d'accord  sur  des  termes  et  des  mesures  standard  ct  internationaux.  Comme  exemple,  il  est  donn£  une  description  dc  la 
specification  des  fits  de  peche  utilises  uctue I  lenient  au  Japon. 

TerminoloRia  y  Sistemas  de  Numeration  Usados  en  el  Japon 
Extracto 

Los  lerminos  tccnicos  y  sistemas  de  numeration  dificren  de  un  pais  a  oiro,  no  exist  iendo  en  la  actual!  dad  norm  as  de  comparacion. 
For  estos  motives,  los  tecnicos  pcsqucros  tropiezan  con  dificultades  al  cotejar  las  caracteristicas  de  los  materiales  o  de  los  artes  que  se 
dcscribcn  en  publicaciones  extranjeras.  Estas  circunstancias  tambicn  son  un  prohlema  para  el  pescador  cuando  desca  adquirir  dicho  equipo. 

Seria  beneficioso  para  toda  la  industria  acordar  la  normalization  intcrnacional  de  terminos  y  medidas.  Como  ejemplo  de  esto  se 
dcscriben  las  esnecificaciones  de  los  hilos  para  artes  de  pcsca  que  se  usan  actualmentfc  en  el  Japon. 


TECHNICAL  terms  and  numbering  systems  related  to 
fishing  gear  and  its  operation  often  differ  from  area  to 
area,  depending  upon  customary  practices  of  fishermen. 
Thus,  a  standardi/ation  of  the  terms  and  system  is  not  easy 
even  in  a  single  country  and  still  more  difficult  on  an  inter- 
national scale.  In  Japan,  British  measurements  are  used  side 
by  side  with  the  metric  system,  despite  the  fact  that  the  latter 
has  been  encouraged  by  the  Government  for  years.  However, 
fishermen  of  the  country  prefer  their  own  traditional  denom- 
inations, taking  little  heed  of  the  other  systems. 

It  is  felt,  nevertheless,  that  simplification  of  the  terminology 
and  the  numbering  system,  once  established  on  an  inter- 
national scale,  would  make  an  invaluable  contribution  to 
advancement  of  fishing  techniques.  It  would  benefit  fishing 
gear  technologists  reading  foreign  publications,  and  fishermen 
and  others  in  related  industries.  Standardized  measurements 
would  also  eliminate  many  difficulties  between  fishermen, 
experts,  gear  manufacturers,  their  export  agencies,  and  so 
forth  in  technical  assistance  programmes. 

We  would  like  to  make  the  following  suggestions: 

1.  That  Member  Governments  and  their  qualified  personnel 
be  requested  to  report  to  FAO  all  the  technical  terms  that 
are  used  in  major  types  of  commercial  fisheries  in  their 
countries,   with   descriptive  explanations  and/or  illus- 
trations. 

2.  That   Member  Governments   be  requested   to  aim  at 


adjusting  or  simplifying  technical  terms  as  much  a 
possible  in  accordance  with  a  numbering  system  or  on 
any  other  rational  basis.  It  would  be  desirous  to  set  out 
the  relationships  between  their  own  systems  with  those 
existing  in  other  countries. 

3.  That  the  Secretariat  of  FAO  compile  and  publish,  on  the 
basis  of  the  reported  data,  regional  classifications  of  the 
terms,  numbering  systems,  conversion  tables  and  any 
other  pertinent  reference,  for  distribution  among  the 
countries  interested. 

It  is  suggested  that  FAO  endeavour  to  extend  advice  and 
assistance  wherever  needed  on  how  to  indicate  standards  for 
fishing  materials  and  equipment. 

Furthermore,  we  hope  that  Fisheries  Division  of  FAO  will 
encourage  the  Member  Governments  to  promote  standard- 
ization in  the  various  methods  and  procedures  now  employed 
in  the  different  fisheries  research  organizations  in  experiments 
on  fishing  materials,  equipment  and  methods,  so  that  the 
results  can  be  brought  to  a  comparable  basis. 

Such  action  would  be  in  accord  with  the  work  of  FAO  in 
promoting  the  use  of  modern  fishing  gear  and  techniques 
throughout  the  world. 

SPECIFICATIONS  OF  FISHING  MATERIALS  IN 
JAPAN 

So  far  as  Japan  is  concerned,  this  is  how  we  specify  various 
items  of  fishing  gear  materials: 


[6] 


TERMINOLOGY    AND    NUMBERING 


2. 


4. 


5. 


6. 


Raw  Material:  the  raw  materials  used  for  fishing  nets 
and  ropes  are: 

1-1.    Cotton  (American,  Indian  or  Egyptian  cotton) 
1-2.  Flax    1-3.  Hemp       1-4.  Ramie       1-5.  Manila 
1-6.  Sisal    1-7.  Maguey    1-8.  Coir          1-9.  Rice  straw 
1-10.  Silk    1-11.  Synthetic  fibre  (Nylon,  Vinylon, 

Vinylidene,  etc.) 
1-12.  Others 

Yarn  Number:  different  counts  of  yarn  are  used  according 
to  kinds  of  raw  material: 

2-1.     For  Cotton:  English  count  system  is  used,  in  which 

840  yards   X  N 

Count  number  N  = 

1  pound 

2-2.     For  ramie  and  flax: 

300  yards   x  N 


Count  number  N  = 
2-3. 


2-4. 


Denier  number  N  — 


1  pound 

For  silk  and  synthetic  fibre:  Denier  system  is  used, 
in  which 

For  synthetic  spun  yarns:  English  cotton  numbering 
is  used. 

450  metres  of  a  yarn 

0.05  gr.    ~<~N 

Direction  of  Twisting:  Left  twist  is  indicated  by  *Z* 
right  twist  by  4S\ 

Degree  of  Twisting:  A  degree  of  twist  is  indicated  by  the 
number  of  twists  in  a  unit  length  (0-25  metre).  It  is 
ordinarily  denoted  by  terms:  soft,  medium,  hard,  or 
extra  hard. 

Number  of  Strands:  is  specified  as  2  strands,  3  strands, 
4  strands,  and  so  forth.  The  term  "strand"  is  used  as 
defined  in  the  next  paragraph. 

Denotation  of  Thread,  Twine,  Rope  and  Cord:  The 
following  formula  may  help  one  understand  the  relation 
between  various  composites  of  a  thread  and  so  forth. 
Fibre   1 

„       [r     Yarn        '] 

„     J          „  J-     Strand       1        f  Thread 

J          „  !•     J}  Twine 

J        I  Rope 

As  seen  above,  a  number  of  fibres  are  twisted  into  a  yarn; 
a  number  of  yarns  into  a  strand,  and  again  a  number  of 


strands  into  a  thread,  twine  or  rope.  Usually,  the  term 
thread  is  used  for  small  sized  material,  twine  for  medium 
size,  and  rope  for  material  large  in  size. 

7.  Size  of  Thread,  Twine  and  Rope: 

7-1  Diameter  system  is  usually  employed  for  indicating 
the  size  of  thread  and  so  forth. 

7-2  Size  of  twine  and  rope  is  sometimes  indicated  by  the 
weight  for  a  unit  length  as  **monme"/5  ft.  (One 
"monme"  approximates  0*13  ounce  or  3*75  grams.) 

7-3  Numbering  system  of  cotton  thread  is  also  used  by 
counting  the  number  of  yarns  in  one  strand.  Cotton 
No.  1  has  two  strands,  one  strand  consisting  of  two 
yarns,  but  the  other  numbers  of  cotton  thread  have 
3  strands. 

8.  Denotation  of  Net  Material:  By  use  of  the  above  indica- 
tions, a  piece  of  net  material  which  consists  of  a  thread, 
for  example,  count  20,  2  yarns  twisted,  3  strands,  may  be 
expressed  as  20S/2/3  or  20/2/3S.  Another  example,  210 
denier,  6  yarns,  3  strands  left  twisted  thread  210d/6/3  left 

ply. 

9.  Conversion  Table:  of  various  items  pertaining  to  fishing 
gear  whose  indications  may  vary  from  case  to  case,  the 
following  has  been  prepared  as  an  example. 

Conversion  Table  for  Size  of  Yarn,  etc. 
(Denier  to  English  Count) 


Denier 

English  Count 

Denier 

F-nglish  Count 

10 

531-5 

90 

59  1 

20 

265*2 

100 

53-2 

30 

177-8 

120 

44'3 

40 

132-9 

150 

35-4 

50 

106-3 

200 

26-6 

60 

88*6 

250 

21-3 

70 

75-9 

300 

17-7 

80 

66*4 

500 

10-6 

1000 

5'3 

5315 

Correlation  Formulae:  English 

count 

denier 

5315 

Denier  — 


Eng.  Count 


Main  operation  stages  in  net  manufacture. 


[71 


YARN  COUNT  OR  NUMBERING  SYSTEMS 

by 

BRITISH  STANDARDS  INSTITUTION 

London,  U.K. 

Abstract 

There  are  many  different  ways  of  designating  yarn  count  or  number,  and  these  can  be  classified  as  either  a  direct  system,  in  which 
the  yarn  count  or  number  is  expressed  in  terms  of  its  mass  per  unit  length  or  an  indirect  one,  in  which  the  count  or  number  is  stated  in  terms 
of  its  length  per  unit  mass.  It  has  been  recommended  that  the  Tex  system  should  be  universally  used  and,  as  it  will  take  some  time  to 
implement  this  recommendation,  it  is  suggested  that  whenever  the  traditional  yarn  count  or  number  is  used,  the  Tex  equivalent  should  also 
be  quoted.  Conversion  tables  are  given  for  changing  from  one  system  to  another. 


Rfeumt 


Syst&mes  dc  numcrotage  dcs  fils 


11  existe  un  grand  nombre  de  types  de  numerotage  des  tils.  On  peut  les  classer  suivant  deux  met  nodes:  le  systeme  direct  dans  lequel 
le  numdrotage  du  fil  est  exprim*  d'apres  son  poids  par  unit6  de  longueur,  ou  le  systeme  indirect,  dans  lequel  il  est  exprim6  d'apres  sa  longueur 
par  unite  de  poids.  On  a  recommend^  1'adoption  universelle  du  systeme  TEX  et,  en  raison  du  delai  n&essaire  £  sa  mise  en  application,  il 
est  suggerc  de  mentionncr  cgalement  I'equivalent  d'apres  le  systeme  TEX,  chaquc  fois  que  Ton  utilise  le  numcrotage  trad  it  ionnc  I  des  fils. 
On  trouve  dans  cet  ouvrage  des  tables  de  conversion  pcrmcttant  de  passer  d'un  systeme  a  un  autre. 

Sistema  de  numeracion  de  los  hilos 
Extracto 

Las  sistemas  de  numeracion  de  hilos  pueden  clasificarse  en  directi»s,  o  sea,  expresando  el  niimero  en  terminos  de  su  peso  por  unidad 
dc  longitud  o,  indirectos,  considcrando  la  longitud  por  unidad  de  peso.  Se  ha  recomendado  adoptar  universalmente  el  sistema  "Tex"  (peso 
de  1  Km.  de  hilo  expresado  en  gramos;  pero  como  esto  tomar£  tiempo,  se  sugiere  que  al  utilizar  el  sistema  de  numeracion  tambien  se  dc  cl 
cquivalente  en  unidades  "Tex". 

En  el  trabajo  tambien  se  incluyen  tahlas  de  conversi6n  para  los  diversos  sistemas. 


THE  many  ways  of  designating  yarn  count  or 
number  may  be  classified  into  two  systems,  (a)  the 
Direct  System  in  which  the  yarn  count  or  number 
is  expressed  in  terms  of  its  mass*  per  unit  length,  and 
(b)  the  Indirect  System  in  which  the  yarn  count  or 
number  is  expressed  in  terms  of  its  length  per  unit  mass. 

With  the  increasing  use  of  yarns  containing  more  than 
one  kind  of  fibre  and  of  fabrics  containing  yarns  made 
from  different  fibres,  it  has  become  evident  that  the 
adoption  of  a  single  yarn  count  or  number  system  would 
avoid  confusion  and  save  time. 

The  International  Standards  Organization  conference 
held  at  Southport  in  1956  recommended  that  a  universal 
direct  system  should  be  adopted  by  all  member  nations 
and  that  it  should  be  the  Tex  system  described  below. 

Inevitably,  some  time  will  be  required  to  implement 
this  recommendation.  It  is  therefore  suggested  that 
wherever  traditional  yarn  count  or  number  is  mentioned, 
the  equivalent  Tex  value  should  he  given  in  brackets. 

TEX  SYSTEM 

The  yarn  number  in  the  Tex  system  is  obtained  by  divid- 
ing the  weight  of  a  given  length  of  yarn  (expressed  in 

*  The  term  mass  is  normally  used  to  describe  the  quantity  of  matter 
in  a  body:  the  term  weight  describes  the  force  exerted  by  gravity 
on  a  body.  To  continue  to  use  the  word  weight  when  one  means  mass 
leads  to  confusion  when  the  yarn  number  is  used  in  derived  quantities 
such  as  tenacity. 

Based  on  B.S.  947  Yarn  Count  Systems  and  their  conversions. 


grams)  by  its  length  in  kilometres;  the  unit  in  this  system 
is  the  Tex  multiples  and  submultiplcs  recommended  in 
preference  to  other  possible  combinations  are  mg.  per 
km.,  named  millitex,  and  g  per  m  or  kg.  per  km.,  named 
kilotex.  Where  a  symbol  is  needed  (e.g.  in  formulae)  to 
represent  the  yarn  number  in  Tex,  it  is  recommended  that 
this  symbol  should  be  the  capital  letter  N. 

DIRECT  SYSTEMS 

System  Unit  of  Mass          Unit  of  Length 

Tex  Gram  Kilometre 

Denier  gram  9,000  metres 

Linen  (dry  spun),  pound  14,400  yards 
Hemp,  Jute  (spindle) 

INDIRECT  SYSTEMS 


Unit  of  Length       Unit  of  Mas\ 

840  yards  (hank)    pound 
1,000  metres  i  kilogram 


System 

Cotton  (British) 
Cotton 

(Continental) 
Linen  (wet  spun)      300  yards  (lea)       pound 
Metric  kilometre  kilogram 

CONVERSION 

Within  either  direct  or  indirect  systems,  conversion  from 
one  yarn  number  to  another  is  done  by  means  of  multi- 


[81 


NUMBERING    SYSTEMS 


plying  factors.  These  factors  can  be  conveniently  arranged 
in  tabular  form. 

Table  I  gives  the  multiplying  factors  for  converting 
from  one  to  another  of  selected  direct  systems. 


For  conversion  from  an  indirect  system  to  a  direct 
system,  and  vice  versa,  a  constant  into  which  the  known 
yarn  number  is  divided  is  necessary.  Commonly  required 
constants  are  set  out  in  Table  111. 


TABLE  i 

Multiplying  Factor  for  Converting  from  one  Direct  System 
to  another 

Multiplying  factor  to  give  yarn  numher 


Known  yarn 
number  in 

Tex 

Tex 

Denier 
Linen  (dry  spun). 
Hemp,  Jute 

1 
0111    1 

34-45 

Denier 


y.ooo 

I 
310.0 


Linen  (dry  spun) 
Hemp*    Jute 

0  29    03 
0-003  225 

1 


Example:  The  equivalent  of  yarn  number  10  in  the  linen  (dry 
spun)  system  is  the  yarn  number  10  ,-  34 '45  or  344"5  in  the  Tex 
system. 

Table  II  gives  the  multiplying  factors  for  converting 
from  one  to  another  of  selected  indirect  systems. 

TABLL  II 

Multiplying  Factors  for  Converting  from  one  Indirect  System 
to  another 


Multiplying  factors  to  give  equivalent  count  in 


Known  count  in 


Cotton  and  spun 

silk 

Linen  (wet  spun) 
Metric 


Cotton  ami 
Spun  Silk 


0-357   I 
0-590  5 


Linen 
(wet  xpun) 

2.800  0 
1 
P653  5 


Metric 
Count 


1-693  4 
0*604  8 

1 


TABLE  III 

Selected   Constants  for  Converting  from   Direct  to  Indirect 
Systems  and  Vice  Versa 


Constant  into   which  the  known  yarn  count  is 
divided  in  order  to  obtain  the  equivalent  yarn 
numher  in  the  other  systems 


Linen  (dry  spun) 

Tex 

Denier 

Hemp,  Jute 

Cotton  and  spun 
silk 

590-5 

5  315 

17-14 

Linen  (wet  spun) 

I  654*0 

14  880 

48'00 

Metric 

1  000  0 

9  000 

29-03 

For  the  conversions  from  British  units  to  metric  units 
and  vice  versa  the  following  equivalents  have  been  used: 

1  yard  --  0-91440  metres 
1  Ib.          453-592  grams 


Throwing  ca.\t  nets  on  a  lake  in  Indonesia. 
[9] 


Photo  FAO 


TERMINOLOGY  AND   NUMBERING   SYSTEMS  —  DISCUSSION 


Mr.  J.  E.  Lonsdale  (U.K.)  Rapporteur  :  Just  as  in  this  room 
we  speak  many  languages,  so  do  we  have  many  ways  of 
describing  fishing  gear,  fishing  methods  and  the  properties  of 
fishing  materials.  There  is  a  r*~ed  for  all  interested  in  fishing 
to  know  what  other  people  *  an  when  they  talk  about  these 
subjects,  particularly  when  .ey  must  compare  the  properties 
of  different  types  of  net.  This  concerns  not  only  the  fishermen, 
who  want  to  compare  the  various  fibres  and  textile  materials 
from  different  countries,  but  also  scientists  and  manufacturers. 

The  first  difficulty  is  that  of  translation.  It  is  often  impossible 
to  find  correct  translations  for  technical  words  which  carry 
exactly  the  same  meaning.  These  papers  make  no  specific 
references  to  the  difficulties  of  translation  but  a  good  deal  of 
work  on  this  problem  has  been  carried  out  by  the  Institut 
fur  Netz-und  Materialforschung,  Hamburg;  their  Paper 
No.  77  deals  with  fishing  gear  nomenclature.  The  next 
difficulty  is  in  the  use  of  trade  names,  variations  occurring 
even  in  one  class  of  textile  fibre,  from  country  to  country  and 
from  manufacturer  to  manufacturer. 

The  papers  before  us  give  us  two  main  ways  of  describing 
the  properties  of  textiles.  The  first  is  the  strength  of  the  net  or 
twine,  and  the  second  is  the  weight  of  the  net  or  twine  which 
is,  of  course,  of  importance  commercially.  But  firstly  the  size 
of  the  yarn,  and  then  the  size  of  the  twines  must  be  described. 

To  clarify  the  terms  "twine"  and  "yarn"  I  will  adopt  the 
definition  given  in  Dr.  van  Wijngaarden's  paper  (No.  6). 
A  "yarn1*  is  a  continuous  strand  of  fibres  and/or  filaments 
from  which  all  twist,  if  any,  can  be  removed  in  one  untwisting 
operation,  and  "twine"  is  the  product  of  twisting  together 
two  or  more  yarns.  The  basic  yarn  may  be  a  continuous 
monofilament,  a  continuous  multifilament,  or  a  yarn  spun 
from  short  lengths  of  fibre. 

Yarn  Size.  The  size  of  yarns  cannot  easily  be  described  by 
thickness  and  diameter  because  of  the  practical  problems 
involved.  It  is  more  common  to  describe  size  by  the  weight 
method  either  directly,  that  is  by  giving  the  weight  for  a  fixed 
length,  or  indirectly,  by  comparing  it  to  a  yarn  of  standard  size. 

The  main  systems  in  use  throughout  the  world  are: 

(1)  The  Tex  System.  Tex  is  a  theoretically  attractive  unit 
but  is  not  yet  in  practical  use.  It  is  attractive  because  it  is  a 
c.g.s.  unit,  a  metric  unit  based  on  centimetres  and  grams.  The 
tex  is  the  weight  in  grams  of  one  kilometre  of  the  yam. 

(2)  The    Metric    System  (Nm.)   giving  the  number  of 
kilometres  of  the  yarn  which  weigh  one  kilogram.  It  is 
therefore  a  reciprocal  of  tex  multiplied  by  1,000.  You  will 
notice  that  in  the  case  of  tex  the  heavier  the  yarn  the  greater 
the  number,  but  in  the  case  of  Nm.  the  heavier  the  yarn  the 
smaller  the  number. 

(3)  The  Denier  System  (den.)  is  in  very  widespread  use. 
This  is  the  weight  in  grams  of  9,000  metres  of  the  yarn.  This 
is  also  a  c.g.s.  unit,  but  introduces  a  factor  of  9. 


(4)  The  English  Cotton  Count  (N.e.)  which  is  an  indirec 
non-c.g.s.  system  and  is  the  number  of  hanks,  each  of  840 
yards  which  weigh  one  pound. 

Mr.  Shimozaki  (Paper  No.  75)  suggests  relating  twine 
thickness  to  a  cotton  yarn  of  20*s  cotton  count — a  size  in 
common  use.  The  size  will  then  be  called  the  C20  equivalent. 

The  papers  give  some  25  other  methods  of  describing  size, 
but  fortunately  not  all  of  these  are  used  in  fishing  technology. 
This  does  raise  the  important  question  of  whether  there  should 
be  one  or  possibly  two  international  methods  of  measuring 
size,  and  what  attempts  should  be  made  to  rationalize  systems 
of  size  numbering,  particularly  in  view  of  the  international 
nature  of  fishing.  There  is  a  natural  reluctance  on  the  part  of 
manufacturers  to  change  from  their  old  standard  of  measure- 
ment. The  paper  that  puts  forward  the  advantages  of  a  tex 
system  (52)  suggests  that  it  should  first  be  introduced  as  an 
addition  to  the  older  systems.  This  subject  of  standardization 
has  been  worked  on  by  textile  bodies  for  a  number  of  years 
and  among  those  who  have  made  recommendations  on  the 
subject  are  the  Bureau  Internationa]  pour  la  Standardisation 
de  la  Rayonne  et  des  fibres  synthetiqucs,  the  American 
Society  for  Testing  Materials,  BISFA,  the  British  Standards 
Institute,  the  Textile  Institute  and  a  number  of  other  bodies. 
Suggestions  for  standardization  come  mainly  from  technolo- 
gists and  fibre  manufacturers.  The  papers  before  us  do  not 
reveal  what  the  fishermen  think  about  this  subject. 

Construction.  A  number  of  methods  exist  to  describe  the 
construction  of  the  twine  from  individual  yarn.  Examples  of 
methods  for  describing  the  same  fishing  twine  are  as  follows: 

(1)  100  Tex  Z  200  X  2  S  300  x  3  Z  400 

The  term  'Z'  and  4S'  refer  to  direction  of  twist,  and  fortunately 
are  now  fairly  standard.  The  numbers  200,  300  and  400  refer 
to  the  degree  of  twist  in  turns  per  metre. 

(2)  210  X  2  X  3 

This  is  similar  to  the  first  example,  but  omits  twisting  descrip- 
tions. 

(3)  210  x  23  denier 

This  is  referred  to  as  size  23. 

(4)  1,260  denier 
This  being  the  total  denier. 

(5)  126 — obtained  by  dividing  the  gross  denier  by  10. 

From  the  papers  it  would  appear  that  the  lengthy  and 
accurate  way  serves  a  different  purpose  to  the  shorter  way, 
but  obviously  the  quick  method  (which  appeals  to  fishermen) 
can  lead  to  confusion.  Perhaps  there  is  a  need  to  standardize 
on  two  methods,  a  longer  one  for  manufacturing  purposes  and 
a  short  one  for  commercial  use. 


[10] 


DISCUSSION    ON     FIBRES    AND     SYSTEMS 


An  important  part  of  the  papers  is  devoted  to  the  measure- 
ment  of  strength.  The  strength  of  a  twine  is  expressed  as  the 
breaking  load  in  pounds  or  kilos.  To  compare  different  yarns 
or  twines,  it  is  necessary  to  express  the  load  divided  by  the 
size  of  the  yarn  or  twine.  Usually  size  is  measured  by  the 
weight  system  just  mentioned.  However,  it  is  also  an  advantage 
to  choose  other  expressions  so  as  to  compare  different  textile 
fibres  more  easily.  These  points  are  very  fully  discussed  in  the 
papers,  particularly  by  Stutz  (79),  Arzano  (80),  and  Carrothers 
(16).  Two  useful  units  for  strength  measurement  often 
mentioned  in  the  papers  are,  first  the  length  of  twine  whose 
weight  equals  the  breaking  strength  of  that  twine,  i.e.  breaking 
length.  Second,  there  is  the  unit  which  is  related  to  the 
breaking  length  by  specific  gravity,  so  as  to  compare  fibres  of 
different  specific  gravities.  This  measures  the  load  per  cross- 
sectional  area.  All  these  strength  measurements  are  further 
complicated  by  whether  c.g.s.  units  or  other  units  are  adopted. 

Apart  from  the  papers  listed  in  the  programme,  Papers 
Nos.  16,  80,  95  and  104  also  have  some  relevance  to  this 
subject. 

Dr.  A.  von  Brandt  (Germany):  In  Germany  there  are  three 
main  numbering  systems  for  materials:  the  English  cotton 
number;  the  metric  number;  and  the  denier  system.  Sometimes 
other  numbers  are  invented  by  the  net  makers  themselves. 
Three  papers  propose  the  tex  number.  One  proposes  that 
the  tex  number  should  be  in  addition  to  the  national  numbers. 
That  is,  1  believe,  a  good  idea  and  could  help  us  to  understand 
each  other.  On  this  subject,  one  should  hear  not  only  the 
textile  people  but  also  — and  more  important — the  fishermen 
and  net  makers. 

Mr.  H.  Warncke  (Germany):  My  firm  is  a  net  manufacturing 
company.  We  have  found  no  major  difficulty  in  interpreting 
any  definition  given  either  by  the  English,  metric  or  denier 
systems.  However,  a  designation  of  international  importance 
should  be  useful,  and  as  a  net  maker  1  feel  that  the  system  used 
should  be  followed  by  a  tex  number  in  order  to  accustom  the 
fishermen  to  the  new  system,  and  thus  make  way  for  the 
eventual  use  of  the  tex  system  alone. 

We  use  the  metric  system  for  the  Eederal  Republic  of 
Germany  and  inside  the  factory,  and  the  English  system  in  the 
trade  with  foreign  countries.  In  the  net  industry  the  composi- 
tion of  twine  is  mainly  given  by  one  size  only,  and  this 
complicates  the  use  of  the  tex  system.  For  instance,  the 
English  term  9-ply,  which  is  always  3  times  3  strands,  is 
merely  designated  by  *9",  without  specifying  the  composition 
of  the  twine.  This,  however,  has  been  found  sufficient  to 
meet  our  requirements. 

Mr.  H.  C.  Smith  (Netherlands):  As  a  net  maker,  1  fully  agree 
with  Mr.  Warncke.  Great  confusion  is  caused  by  these 
numbering  systems  and  we  feel  it  would  be  in  the  best  interest 
of  the  fisherman  if  all  the  net  makers  adopted  the  tex  system. 
I  am  not  in  favour  of  using  two  systems;  i.e.  the  old  system 
followed  by  the  tex  system.  We  have  to  get  the  fishermen  used 
to  one  system  and  1  think  it  should  be  the  tex  system.  It  will 
certainly  take  some  time,  maybe  2  or  3  years,  before  the 
public  will  become  accustomed  to  it,  but  it  will  contribute 
to  a  better  understanding  of  the  numbering  of  yarns. 

STATEMENT  FROM  NETHERLANDS 


out  that  four  classification  systems  arc  officially  and  uni- 
versally applied  to  yarn. 

1.  The  English  yarn  number  (Ne,)  for  the  classification  of 
cotton  yarns  and  spun  yarns,  manufactured  from  synthetic 
fibres. 

2.  The  English  yarn  number  (Nc2)  for  the  designation  of 
linen  and  hemp  yarns. 

3.  The  metric  yarn  number  (Nm)  for  the  designation  of 
cotton  yarns  and  spun  yarns  from  synthetic  fibres. 

4.  The  denier  system  for  the  designation  of  yarns  from 
continuous  filament  synthetic  fibres,  such  as  nylon. 

It  is  difficult  even  for  experts  in  the  industry  to  convert  one 
numbering  system  to  another  to  compare  yarns.  As  far  as 
synthetic  yarns  are  concerned,  it  is  often  impossible  to  make 
a  comparison  on  the  basis  of  the  number  indication  only.  The 
composition  of  a  yarn  must  be  established  by  a  test.  How 
confusing  therefore  must  the  numbering  be  for  the  fisherman 
who  has  to  be  certain  he  buys  the  right  yarn. 

It  would  be  logical  and  most  profitable  to  adopt  the  Tex 
system,  rather  than  add  it  to  the  existing  systems. 

The  Tex  numbers  should  also  indicate  how  the  yarn  has 
been  composed. 

This  can  be  affected  by  indicating  the  twined  yarns  by  num- 
bers and/or  figures,  connected  by  an  X-mark  and  preceded  by 
the  indication  Tex,  Millitex  and  Kilotex  The  numbers  in 
order  of  sequence  could  show: 

1st:     which  yarn  has  been  used  for  the  construction  of  the 
finished  yarn  in  units  of  the  Tex  system. 

2nd:  how  many  threads  are  twisted  for  the  first  twining. 

3rd:    whether  the  second  twining  is  2,  3  or  4-ply. 

4th:    the  direction  of  the  twist,  indicated  by  the  letters  s 

and  z  behind  each  number. 

This  system  of  yarn  numbering,  if  practised  universally  for 
twined  yarns,  would  enable  the  manufacturer  and  the 
trader  to  use  the  same  numbering  and  would  make  a  compari- 
son between  various  yarns  and  different  raw  materials  much 
easier  because  all  numbers  would  be  indicated  in  Tex  units. 

Some  examples  by  way  of  illustration 

Old  system 

Ne,  20/12  cotton  3-ply  z-z-s. 

Nc,  20  =  Tex  29,4,  rounded  off  to  e.g.  Tex  30. 

Twist  of  yarn  z,  1st  twining  z,  second  twining  s. 


New  system 

Tex  30z       4z 
Tex  30    ; :  4 

Old  system 

Nylon  Td  210 


3s  cotton  or 
3  z-z-s  cotton. 


24  3-ply  z-s. 


Apeldoornse    Nettenfabriek    (Netherlands):    in    a    written 
statement  on  a  plea  for  a  universal  numbering  system.  He  set 


New  system 

Tex  23  ;<  8z     ,  3s  nylon  or 
Tex  23   x  8     X  3  z-s  nylon. 

Mr.  H.  Kobayashi  (Japan) :  As  far  as  Japan  is  concerned,  the 
denier  system  is  preferred  for  monofilamcnts  as  well  as 
continuous  multifilaments,  while  English  count  is  used  for 
spun  nylon  yarn  and  also  for  cotton  yarn. 

Mr.  Rack  (Northern  Rhodesia):  Our  interest  is  in  gillnet 
fishing  and  our  fishermen  are  just  emerging  from  very 
primitive  methods.  We  need  a  system  which  will  be  uniform. 


in  i 


MODERN    FISHING    GEAR    OF    THE    WORLD 


Our  fishermen  arc  primarily  interested  in  the  following:  the 
diameter,  expressed  in  a  constant  term;  (this  is  important  to 
the  gillnet  fishermen  who  are  buying  yarn  for  making  and 
mending  nets);  the  runnage;  i.e.  the  amount  of  twine  he  will 
get  to  his  given  weight;  and  the  breaking  strength.  At  present 
large  quantities  of  twine  are  sold  without  even  a  written 
guarantee  that  they  are  made  of  a  certain  synthetic  fibre  and 
not  a  blend  of  fibres,  and  it  would  be  our  suggestion  to  intro- 
duce Merchandise  Mark  Acts  in  which  we  might  invite  the 
manufacturer  to  state  the  specification.  This  would,  we  hope 
be  one  which  our  government  officers  could  check  in  our  own 
laboratories. 

Mr.  S.  Krohn-Hansen  (Norway):  It  might  be  better  to  use  the 
metric  system  than  the  tex  system,  because  then  the  fishermen 
know  the  runnage — how  many  metres  there  are  to  a  kilo. 
The  metric  system  is  in  use  in  Norway  with  other  generally 
used  numbering  systems,  and  1  would  suggest  it  as  an  inter- 
national standard  system. 

Mr.  A.  Percier  (France) :  I  would  support  the  adoption  of  the 
metric  system  which  is  the  most  widely  used  and  because  it 
is  easier  for  the  fishermen  to  learn  and  would  generally  be  more 
useful.  The  Tex  and  the  Tex  and  Denier  systems  are  more 
complicated. 

Mr.  J.  K.  van  Wijngaarden  (Netherlands):  In  the  textile 
industry  and  in  other  scientific  and  technological  fields  attempts 
are  being  made  to  standardize  measuring  systems  based  on 
the  c.g.s.  system.  The  tex  system  does  not  only  have  a  following 
in  the  textile  industry  but  also  throughout  the  scientific  and 
technological  fields. 

Mr.  A.  O'Grady  (Australia):  We  import  our  netting  material, 
and  the  fishermen  are  accustomed  to  using  the  English  yarn 
counts  number.  With  the  introduction  and  growing  popularity 
of  the  synthetic  fibres,  plus  the  added  problem  of  import 
restrictions,  we  find  that  a  fisherman  will  deal  with  a  particular 
firm,  changing  to  another  only  when  that  firm  is  out  of  stock 
of  what  he  requires.  One  firm  may  perhaps  be  using  the 
nylon  numbering,  210/3;  the  alternative  firm  may  be  using 
only,  say,  No.  6  nylon  thread  number.  But  instead  of  making 
the  purchase  easier  for  the  fisherman,  this  simple  numbering 
leads  to  confusion.  Irrespective  of  what  system  is  decided 
upon,  we  should  have  one  only.  Initially  we  might,  however, 
retain  the  old  measuring  system  which  the  fisherman  knows 
and  also  print  the  new  equivalent  on  the  label  of  the  bundle 
of  netting.  Eventually  the  fisherman  will  get  used  to  the  new 
system  and  adopt  it. 

Mr.  H.  Keller  (Switzerland):  I  am  of  the  opinion  that  the 


metric  numbering  system  is  the  best  and  most  practical  for 
use  between  the  producer  and  the  fisherman.  It  is  almost 
impossible  to  devise  a  system  to  cover  all  the  characteristics 
of  certain  materials. 

Captain  D.  Roberts  (U.K.):  I  am  skipper  of  a  Grimsby 
trawler.  Last  week  I  was  fishing  at  Iceland  and  next  week  I 
shall  be  fishing  at  Iceland.  1  like  to  read  publications  from 
Holland,  Norway,  Germany,  besides  from  my  own  country, 
and  it  takes  me  a  long  time  to  work  out  the  difference  of  these 
measurements.  From  a  practical  fisherman's  point  of  view, 
I  do  not  mind  what  system  is  used  as  long  as  it  is  one  system 
alone ! 

Mr.  D.  Olafsson  (Iceland):  I  would  like  to  support  what  has 
been  said  by  various  delegates  here,  on  the  necessity  of  agree- 
ing on  some  unification  of  the  numbering  systems.  I  know  the 
fishermen  in  Iceland  have  great  difficulties  in  choosing  net 
materials  because  of  the  many  different  numbering  systems 
used.  There  is  one  question  I  would  like  to  ask:  are  these 
discussions  going  to  be  followed  up  by  some  action  by  FAO 
or  by  some  other  institution,  or  are  we  going  to  wait  indefinitely 
before  we  get  something  practical  done?  I  know  that  many 
of  the  people  here  would  like  to  sec  something  practical 
coming  out  of  these  discussions. 

Mr.  H.  Kristjonsson  (FAO),  General  Secretary:  We  have  in 
mind  to  appoint  today  a  small  Working  Group  to  study  this 
complex  problem,  and  report  back  to  the  Congress  on 
Saturday.  What  action  we  can  take  thereafter  depends  on 
the  findings  of  that  Group. 

This  Congress  itself  cannot  pass  any  binding  resolutions: 
it  will  be  the  aim  of  the  Working  Group*  to  indicate  a 
solution  to  the  problem.  Various  international  bodies  are 
directly  concerned  with  the  unification  of  standards,  and  FAO 
will  collaborate  with  them. 

Mr.  J.  E.  Lonsdale  (U.K.) — Rapporteur:  To  summarise  this 
discussion,  the  fishermen  with  one  voice  have  said  "let  there 
be  a  simple  and  uniform  method  of  numbering". 

The  fibre  scientists  and  technologists  have  suggested  the 
tex  system,  which  is  not  yet  in  practical  use.  Between  the 
fibre  manufacturers  and  the  fishermen  there  are  the  twine 
and  net  manufacturers,  and  any  simplification  of  numbering 
systems  inevitably  start  in  that  section. 


*  Later  in  the  day  a  Working  Group  on  Terminology  and  Numbering 
Systems  was  formed,  consisting  of  representatives  from  fibre  manu- 
facturers, net  makers  and  gear  technologists.  Mr.  Lonsdale  was 
appointed  Chairman. 


[12] 


Section  2:  Materials — Characteristics  of  Fishing  Twines  and  Their  Testing. 


MAN-MADE  FIBRES 

The  Synthetic  Polymer  Fibres  and  Filaments  ;  their  general  characteristics,  chemical  and  biological  properties,  with 

special  reference  to  their  use  in  Fishing  Gear 

by 

R.  ARZANO 

Chairman  of  the  Industrial  Uses  Sub-Committee  of  International  Rayon  and  Synthetic  Fibres  Committee. 


Abstract 

This  paper  deals  with  the  following  synthetic  fibres:  Polyumides  (Nylon,  Perlon  and  Rilsan);  Polyesters  (Terylene);  Vinyl  Fibres 
(Saran,  Polyvinyl  chloride,  Courlene  and  Courlenc  X3,  and  Vinylon).  Basic  information  is  given  about  these  fibres,  with  special  reference  to 
iheir  uses  in  fishing  gear  and  also  their  employment  (mixed  with  cotton  or  wool)  in  the  manufacture  of  fishermen's  protective  clothing.  The 
traditional  seaman's  jersey  can  be  made  of  a  blend  of  50  per  cent,  viscose  staple  and  50  per  cent,  wool,  or  15  per  cent,  nylon,  35  per  cent. 
viscose  rayon  staple  and  50  per  cent,  wool,  anil  (hese  mixtures  can  also  be  used  for  very  serviceable  sea-boot  stockings. 


Resume 


1/es  fibres  synfhetiques 


Cettc  communication  traite  des  fibres  synlhetiques  suivantes:  Polyamides  (Nylon,  Perlon  et  Rilsan);  Polyesters  (T^ryldnc);  Fibres 
vinyliques  (Saran,  chlorurc  de  Polyvinyl,  Courlene  et  Courlene  X3,  et  Vinylon).  On  donnc  des  renseignements  fondamentaux  sur  ccs  fibres, 
en  particulier  sur  leur  cmploi  dans  les  engins  de  peche  et  aussi  (melangecs  au  coton  ou  a  la  laine)  pour  la  fabrication  de  vgtecnents  pro- 
tecteurs  destines  aux  pecheurs.  Le  chandail  traditionnel  du  marin  peut  etre  fait  avec  un  melange  de  50  pour  cent,  de  fibres  de  viscose  et  50  pour 
cent,  de  lainc,  ou  15  pour  cent,  de  nylon,  35  pour  cent  de  fibres  de  viscose  rayonne  et  50  pour  cent,  de  lainc,  et  ces  melanges  peuvcnt  aussi 
servir  i\  faire  des  bas  qui  sont  d'une  grande  utility  dans  les  bottes  de  mer. 

Fibres  artiflciales 
Extracto 

Hste  trabajo  trata  de  las  fibras  sinteticas  de  los  siguientes  materiales:  Poliamidas  (nylon,  perlon  y  rilsan);  poliesteres  (terileno); 
I'inilo  (saran,  cloruro  de  polivinilo,  "courlene",  "courlcne  X3"  y  vini!6n).  Tambien  conticne  informacion  basica  sobre  estas  fibras  y  su 
cmpleo  (mezcladas  con  algoddn  o  lana)  en  la  confeccion  de  prendas  de  vcstir  para  proteger  a  los  Pescadores.  El  tradicional  "sweater" 
marinero  puede  tejerse  con  una  mezcla  de  50  por  cent  de  hilado  de  viscosa,  y  50  por  cent,  dc  lana,  o  15  por  cent,  dc  nyl6n,  35  por  cent  dc 
hilado  de  rayon  con  viscosa  y  50  por  cent  de  lana.  hstas  mezclan  tambien  se  utilizan  para  tejer  calcctas  muy  durables  que  se  usan  con  botas 
de  mar. 


GENERAL 

HIG  H  polymer  chemistry,  called  also  macromolecular 
chemistry,  is  a  very  young  science,  dating  back  only 
35  years.  The  term  "macromolecule"  was  first  in- 
troduced in  1922  by  Staudinger,  who  used  it  to  describe 
the  high  molecule  hydrocarbon  obtained  by  the  hydro- 
genation  of  natural  rubber.  Since  then  a  great  deal  of 
research  has  been  carried  out  and  eventually,  as  a  result  of 
work  started  in  1928,  W.  H.  Carothers,  discovered  a  fibre- 
forming  synthetic  condensation  polymer,  for  which  the 
name  "nylon"  was  coined.  The  discovery  was  made  public 
in  1938. 

The  researches  of  W.  H.  Carothers  and  his  associates 
led,  within  a  relatively  short  time,  to  further  important 
developments  in  the  field  of  polymers,  and  to  the 
discovery,  by  Whinfield  and  Dickson,  of  polyester  fibres 
from  polyethylene  therephthalate  (1939-41). 

During  the  period  1939-1940  a  great  expansion  took 
place  in  high  polymer  research.  Some  old  polymers  were 
carefully  reinvestigated  and  many  new  ones  discovered. 
But  the  effect  of  the  discoveries  and  researches  made 


during  the  1930s  and  the  1940s  have  not  yet  been  fully 
investigated,  and  their  impact  is  not  yet  fully  realised. 

Besides  polyamides  and  polyesters,  the  production  of 
many  new  synthetic  fibres  -polyethylene,  polyacryloni- 
trile,  polyvinyl  chloride,  polyvinylidene  chloride,  poly- 
urethanes  and  various  vinyl  copolymers  was  also 
started  and  reached  the  commercial  stage. 

The  chemical  industry  was  thus  given  the  opportunity 
of  producing  a  wide  variety  of  new  fibres,  the  character- 
istics of  which  could  be,  to  a  point,  modified  in  order  to 
make  them  suitable  for  specific  needs  and  end-uses.  It 
has  been  rightly  stated  by  Harold  DeWitt  Smith1  that 
"mankind  has  initiated  a  second  great  textile  project, 
namely,  the  creation  of  textile  fibres".  This  implied, 
however,  extensive  research  for  improving,  on  one  hand, 
the  characteristics  of  the  fibres  for  the  various  applica- 
tions and  developing,  on  the  other,  new  uses  or  expanding 
already  accepted  ones. 

At  the  same  time  the  vast  industrial  expansion  of  the 
post-war  years  called  for  ever  increasing  quantities  of 
those  specialized  products  known  as  "industrial  textiles" 


f  131 


MODERN    FISHING    GEAR    OF    THE    WORLD 


and  imposed  more  and  more  exacting  requirements. 
The  new  synthetic  fibres  have  enabled  the  textile  industry 
to  keep  abreast  of  the  increased  demand  and  to  meet 
successfully  the  particular  needs  of  processes  and  trades 
using  textiles. 

The  industrial  use  of  man-made  fibres  has  increased 
very  rapidly.  In  the  United  States,  industrial  uses  took  a 
substantial  proportion  of  man-made  fibres  in  the  early 
1950s;  in  England,  34  per  cent,  of  all  continuous  filament 
man-made  fibre  yarn  went  to  industrial  uses  in  1953  and 
19542. 

Industrial  textiles  comprise,  however,  a  very  wide 
range  of  products,  from  cloth  for  filter  presses  to  con- 
veyor belts,  from  tyre  cords  to  tarpaulins,  from  ropes 
and  cordages  to  boat  sails,  from  cigarette  filter  tips  to 
fishing  nets.  Each  of  these  applications  requires  efficient 
and  lasting  performance,  under  the  respective  conditions 
of  use,  and  the  textile  technologist  has  to  meet  very 
different  needs  and  stringent  specifications.  In  the  case 
of  industrial  textiles,  quality  is  of  paramount  importance 
and  where  these  products  are  part  of  a  process,  a  break- 
down in  performance  will  at  least  cost  money  and  may 
even  cost  lives.  It  is  imperative,  therefore,  that  quality 
and  performance  characteristics  be  of  the  highest  possible 
standard. 

Not  so  many  years  ago,  we  could  hardly  speak  of  a 
"fishing  industry".  For  thousands  of  years,  fishing  had 
been  a  handicraft  and  down  the  ages  the  methods  and 
equipment  showed  little  progress.  Now  the  picture  has 
changed,  and  is  changing,  rapidly.  To  keep  pace  with 
development,  scientific  and  application  research  is 
essential;  the  more  the  properties  of  the  various  fibres  are 
known,  the  better  we  shall  be  able  to  employ  them  in  the 
most  useful  and  appropriate  way. 

PROPERTIES 

The  following  main  properties  of  fibres  are  to  be  specially 
considered  for  use  in  the  fishing  industry: 

Density 
Tenacity 
Tensile  strength 
Knot  strength 
Loop  strength 
Elastic  properties 
Toughness 
Stiffness 
Water  absorption 

Effect  of:  heat 
age 

sunlight 
chemicals 
sea-water 

Resistance  to  bacteria,  mildew  and  insects 

Density.  According  to  J.  T.  Marsh3  "the  density  of  a 
substance  is  the  quantity  of  matter  contained  in  unit 
volume".  The  relative  density  is  the  ratio  of  comparison 
between  a  given  substance  and  a  standard  one  (usually 
water). 

When  the  C.G.S.  System  is  used  (unit  of  volume  :  c.c., 
unit  of  mass  :  gram-standard  substance  :  water  at  4  deg. 
C.)  the  values  of  absolute  and  relative  densities  are 


identical.  Specific  gravity  and  specific  volume  are 
reciprocally  proportional,  so  that  the  lower  the  density, 
the  higher  the  bulk  and  vice  versa. 

Strength  is  a  general  name  for  defining  both  tenacity  and 
tensile  strength  of  fibres.  The  difference  between  these 
two  terms  is  clearly  stated  by  Marsh3  as  follows:  'Ten- 
acity is  the  breaking  force  in  terms  of  the  fibre  or  yarn 
denier,  whereas  tensile  strength  is  the  breaking  force  in 
terms  of  the  unit  area."  The  first  is  expressed  in  terms  of 
grams  per  denier,  the  latter  in  terms  of  grams  per  square 
millimetre.  In  this  respect,  it  must  be  pointed  out  that 
values  for  tenacity  are  not  directly  comparable,  as  they 
do  not  take  into  account  the  density  of  the  material; 
values  for  tensile  strength,  on  the  other  hand,  are  directly 
comparable. 

The  breaking  force  is  often  expressed  as  "breaking 
length"  which  according  to  "Textile  Terms  and  Defini- 
tions"4 is:  "the  length  of  a  specimen  whose  weight  is 
equal  to  the  breaking  load" 

The  conversion  formulae  are: 

— breaking  length  (in  Km.)  =  9   •  tenacity  (in  g/den.) 

— tensile  strength  (kg./mm.2)  — breaking  length 

(in  Km.)  **  specific  gravity. 

Knot  strength  is  the  tenacity  of  a  fibre  or  a  yarn  in  which 
a  plain  knot  has  been  tied.  As  Prof.  Vivianr*  points  out 
"the  knotting  test  may  give  an  idea  of  the  transverse 
strength  of  the  fibre".  The  ratio  of  the  knot  strength  to 
the  single  fibre  or  yarn  strength  gives  the  effect  of  bending 
due  to  the  tying  of  the  knot. 

Loop  strength.  To  show  to  what  extent  a  fibre  or  a  yarn 
is  affected  by  bending,  the  "loop  strength  ratio"  is  a  very 
useful  figure.  To  determine  this  value  the  free  ends  of 
linked  loops  of  fibre  or  yarn  are  secured  in  the  grips  of 
the  testing  machine,  and  the  load  required  to  break  them 
is  found. 

Elastic  properties,  i.e.  extensibility  and  elastic  recovery 
of  a  fibre  are  as  important  as  strength.  Too  much  stress 
has  been  placed  on  tenacity  as  the  most  valuable  property 
of  textile  materials  and,  although  high  breaking  load  is  of 
significance,  extensibility  is  at  least  of  equal  importance. 
An  inextensible  fibre,  or  a  fibre  possessing  very  poor 
extensibility — even  when  combined  with  high  tenacity  —is 
of  little  actual  value,  if  the  extension  at  break  is  con- 
sidered3. For  the  evaluation  of  a  fibre,  however,  the 
extension  at  break  is  not  the  only  point  to  be  considered; 
from  the  practical  point  of  view,  the  behaviour  of  the 
fibre  when  stressed  or  extended  to  a  degree  not  reaching 
the  breaking  point  is  of  great  value. 

The  elastic  behaviour  of  a  fibre  is  referred  to  as  Young's 
Modulus,  which  is  the  relationship  between  stress  and 
strain  at  loads  below  the  elastic  limit.  It  is  expressed  in 
grams  of  stretching  force  per  denier  of  fibre  (g/den.)  and 
corresponds  to  the  tension  required  to  produce  an 
extension  of  1  per  cent.  The  values  thus  expressed  are 
based  on  the  denier  of  the  fibre  and  not  on  the  cross- 
sectional  area  and  may  be  converted  into  kilograms  per 
square  millimetre  by  multiplying  by  9  times  the  specific 
gravity  in  grams  per  cubic  centimetre. 

It  must  be  borne  in  mind,  in  this  respect,  that  textile 


[14] 


SYNTHETIC    FIBRE    TWINES    FOR    FISHING    PURPOSES 


fibres  do  not  obey  the  physical  law  (Hooke's  Law), 
according  to  which  strain  is  proportional  to  stress  or,  to 
be  more  accurate,  they  obey  this  law  up  to  a  point,  called 
"yield  point"  beyond  which  the  fibre  exhibits  a  'Viscous 
or  plastic  flow".  Consequently  Young's  Moduli  are 
useful  for  comparison,  but  only  in  the  region  where 
Hooke's  Law  applies,  when  they  are  "constants". 

Elasticity  is  the  power  of  recovery  from  strain  or 
deformation.  The  total  extension  of  a  fibre  is  formed  by 
two  components:  an  elastic  extension,  which  is  recoverable 
on  release  of  stress,  and  a  permanent  (or  plastic)  elonga- 
tion which  is  not  recoverable.  In  its  turn  elastic  extension 
comprises  partly  "immediate  recovery"  and  partly 
"delayed  recovery",  so  that  the  time  factor  has  to  be 
taken  into  account  in  determining  it. 

According  to  Kornreich6  "the  elastic  limit  is  reached 
when  stress  results  in  a  permanent  extension,  known  as 
'elongation'".  The  ability  of  a  fibre  to  recover  from 
strain  is  of  importance  for  several  end-uses. 

Toughness.  Of  practical  importance  also  is  the  "tough- 
ness", "work  of  rupture",  or  "energy  absorption"  of  a 
fibre.  According  to  Kaswell7  "the  load  is  a  force,  the 
extension  is  a  distance  and  the  area  under  the  load- 
extension  (stress-strain  or  stress-elongation)  diagram  is 
the  product  of  force  and  distance,  or  work  (energy)". 
The  area  therefore  depicts  the  fibre's  ability  to  have  work 
done  upon  itself,  i.e.  to  absorb  energy.  The  "toughness 
index"  may  be  just  the  same  for  a  fibre  of  high  tenacity 
and  low  extensibility  as  for  fibre  of  low  tenacity  and  high 
extensibility.  In  general,  the  higher  the  "toughness 
index",  the  better  is  the  fibre  from  the  standpoint  of  use, 
as  work  of  rupture  shows  the  ability  of  a  fibre  not  only 
to  absorb  energy,  but  also  to  withstand  a  sudden  load, 
(i.e.  a  "live-load"). 

Stiffness  is  defined  by  Harold  DeWitt  Smith1  as  resistance 
to  deformation.  Average  stiffness  is  the  ratio  of  breaking 
stress  to  breaking  strain;  elastic  stiffness  the  ratio  of 
stress  to  strain  at  the  yield  point. 

Moisture  content  affects  the  physical  properties  of  fibres 
and,  in  particular,  their  tenacity,  extensibility,  rigidity 
and  swelling.  In  this  respect  the  hygroscopic  nature  of 
fibres,  i.e.  their  power  to  absorb  and  desorb  water,  is  of 
importance,  as  water  acts  as  a  plasticizer  to  a  higher 
degree  for  hydrophilic  fibres  and  to  a  lesser  degree,  or 
scarcely  at  all,  for  hydrophobic  ones. 

Water  influences  the  fibres  in  various  forms,  e.g.  as 
relative  humidity  (in  the  atmosphere),  as  liquid  water 
(water  or  imbibition),  and  as  steam;  furthermore,  the 
action  of  cold  water  is  different  from  that  of  hot  water. 

The  amount  of  atmospheric  moisture  that  a  fibre  is  able 
to  absorb  when  exposed,  fully  dried,  to  surrounding 
air  is  referred  to  as  "regain"  and  is  expressed  as  a 
percentage  of  the  dry  weight  of  the  fibres.  To  ensure  that 
the  regain  of  different  fibres  be  comparative,  tests  are 
conducted  in  a  standard  atmosphere.  "Standard  regain" 
is  the  amount  of  moisture  a  fibre  absorbs  at  65  per  cent, 
relative  humidity  at  a  temperature  of  20  deg.  C.  (inter- 
national standards).  The  standard  testing  atmosphere 
allows  a  tolerance  of  ±  2  per  cent.  R.H.  and  ±  2  deg.  C. 

The  moisture  content  of  the  fibres  affects  their  tenacity 


in  different  degrees.  Natural  cellulosic  fibres  (cotton, 
ramie,  linen)  show  an  increase  in  tenacity  from  the  dry 
to  the  wet  state;  regenerated  cellulosic  fibres  are,  on  the 
contrary,  stronger  in  the  oven-dry  state  than  moist; 
synthetic  fibres  show  no  significant  variation  in  strength 
from  the  dry  to  the  wet  state. 

Liquid  water,  entering  the  fibre  structure,  causes 
swelling  according  to  the  water  imbibition  properties 
which  are  related  inversely  to  wet-strength.  Hot  or 
boiling  water  affects  the  plastic  properties  of  the  fibres. 
For  most  of  the  synthetic  fibres  it  causes  shrinkage  and, 
for  some  types,  even  degradation. 

The  action  of  steam  is  dependent  upon  its  temperature 
and  saturation.  It  has  a  great  influence  on  the  plasticity 
of  the  fibres,  and  it  is  accompanied  by  shrinkage,  more 
pronounced  with  synthetic  than  with  other  fibres. 

The  property  of  synthetic  fibres  to  shrink  under  the 
action  of  heat  (wet  or  dry)  is  utilized  by  a  special  treat- 
ment (thermo-setting)  to  impart  to  fabrics  and  articles  a 
dimensional  stability,  which  is  permanent  as  long  as  they 
are  not  treated  at  higher  temperature  than  that  of  setting. 

Brief  definitions  of  mechanical  properties.  It  might  be 
useful  to  sum  up  briefly  the  basic  mechanical  properties 
of  textile  fibre  materials  to  be  evaluated  under  the 
influence  of  tensile  forces  as  defined  by  Harold  De  Witt 
Smith1: 

strength    — the  ability  to  support  a  load 
elongation— the  deformation  produced  by  a  load,  along 

the  line  of  action  of  the  load 
stiffness       -resistance  to  deformation 
toughness  -    the  ability  to  absorb  work 
elasticity    — the  ability  to  return  to  original  shape  and 

dimensions  upon  cessation  of  deforming 

force 
resilience  — the    ability    to    absorb    work    without 

suffering  permanent  deformation. 

SYNTHETIC  POLYMER  FIBRES 

The  physical  and  chemical  properties  of  some  synthetic 

polymer  fibres,  all  satisfactorily  resistant  to  sea  water  and 

fresh  water,  are  given  below: 

Polyamides:  The  principal  commercial  polyamide  fibres 

are: 

polyamide  66  (Nylon)  made  from  polyhexamethylene 

adipamide 

polyamide  6  (Perlon)  made  from  polycaproiactam 
polyamide  1 1  (Rilsan)  obtained  by  auto-condensation 

of  o-aminoudecanoic  acid  (based  on  castor  oil) 

They  are  produced  both  in  the  form  of  continuous 
filament  yarn  and  staple  fibre,  in  a  wide  range  of  deniers. 

As  already  mentioned,  the  discovery  of  polyamide  66 
is  due  to  Dr.  Wallace  Hume  Carothers  (b.  Burlington, 
Iowa,  1896— d.  New  York,  1937),  who  since  1928  had 
been  entrusted  with  research  in  the  laboratories  of  E.I. 
Du  Pont  de  Nemours  Co. 

It  should  be  mentioned,  incidentally,  that  many 
suppositions  have  been  made  and  several  explanations 
(often  fantastic)  given  about  the  origin  of  the  name 
"nylon".  In  a  publication  of  E.L  Du  Pont  de  Nemours 
Co.8  it  is  stated,  however,  that  "the  name  given  to  the 
new  product  was  'nylon' — a  term  just  as  synthetic  as 


15] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


nylon  itself.   It  was  chosen  because  it  was  short,  catchy, 
pleasant  to  the  ear  and  not  easily  mispronounced". 

Polyamide  66  (nylon)  and  polyamide  6  (Perlon)  yarns 
and  fibres  are  available  in  normal  and  high-tenacity 
types.  The  characteristics  of  each  quality  and  type  are 
as  follows: 

(A)  Polyamide  66  (Nylon) 

Normal  tenacity     High  tenacity 

(1)  Density  .  .  M4 

(2)  Tenacity  dry  (g/den.)  4'5  —  6  6'5  —  8'5 
breaking  length:  dry  (Km.)     40         54  58  —  76 

wet  (%of  dry)     85—90  90 

(3)  Tensile  strength  (Kg./mm2)     46   —62  67        87 

(4)  Knot  strength  (%  of  tenacity)  85-90 

(5)  Loop  strength  (%  of  tenacity)  84-87 

(6)  Extension  at  Break  %:  dry      26  —  32  15  -    20 

wet          30  —  37  18  —  28 

(7)  Elastic  recovery  .  100%  100% 

up  to  8%  at  4% 

(8)  Toughness         .  .  T08  0'77 

(9)  Stiffness  (g.p.d.)          .18  32 

(10)  Water  absorption  at    65%  R.H.  4'5% 

at  100%  R.H.  7—9% 

(11)  Effect  of  heat  melting  point  250°C 

softening  point  235°C 

Effect  of  age.    Virtually  none. 

Effect  of  sunlight.  Loss  of  strength  on  prolonged  exposure. 

Effect  of  chemicals.  Concentrated  hydrochloric  acid,  con- 
centrated nitric  acid,  sulphuric  acid  cause  rapid  dis- 
integration of  the  fibre.  Phosphoric  acid  degrades  the 
fibre,  at  elevated  temperatures.  Formic  acid  in  concen- 
trations above  80  per  cent,  will  dissolve  the  fibre. 

Acetic  acid,  oxalic  acid,  glycollic  acid,  lactic  acid  in  high 
concentration  have  similar  effect  to  that  of  formic  acid. 
Other  organic  acids  have  little  or  no  effect  on  the  fibre. 

The  resistance  to  alkalis  is  outstanding. 

Bleaching  agents  containing  available  chlorine  will 
cause  degradation  on  the  fibre;  hydrogen  peroxide  too 
causes  degration  though  less  rapid. 

Solution  of  neutral  inorganic  salts  have  no  effect  on  the 
fibre,  at  room  temperature.  At  100  dcg.  C  concen- 
trated solutions  of  magnesium  perchlorate,  lithium 
bromide  or  lithium  iodide  will  dissolve  fibre.  The 
fibre  is  inert  towards  liquid  ammonia,  sulphur  diox- 
ide and  cuprammonium  solutions. 

Petrol,  mineral  oils,  benzene,  xylene,  ethers,  esters, 
ketones,  alkyl  halidcs  and  mercaptans  have  no  effect, 
even  at  elevated  temperatures. 

Alcohols  (except  benzyl  alcohol),  glycols,  and  alde- 
hydes (except  formaldehyde,  glyoxal,  and  chloral  under 
certain  conditions)  have  only  a  little  effect. 

Benzyl  alcohol,  nitrobenzene,  chlorhydrins,  chloral 
hydrate,  nitroalcohols,  and  adiponitrile  are  solvents  for 
the  fibre  at  high  temperatures. 

Phenols,  particularly  phenol  itself  (i.e.  carbolic  acid), 
meta-cresol,  and  cresylic  acid  are  solvents  for  the  fibre. 
Resistance  to  bacteria  and  fungi.  Untreated  fibres  are  immune 
to  attack  from  all  micro-organisms,  whether  fungi  or 
bacteria  (i.e.  air  borne).  Yarns  and  cords  show  extreme 
resistance  to  attack  by  marine  algae. 

Resistance  to  insects.  There  is  no  known  instance  of  insects 
or  moth  larvae  deriving  sustenance  from  polyamide  66 
fibres. 

Resistance  to  abrasion.  In  the  whole  field  of  textile  fibres, 
polyamide  fibres  have  the  highest  resistance  to  abrasion. 


Polyamide  66  fibres  are  also  produced  in  the  form  of 
staple  fibres;  the  characteristics  of  these  are  the  same  as  for 
continuous  filament,  except  for  the  following  items: 

Tenacity:  dry  (g/dcn.)        .  4'1  —  5 

wet  (%  of  dry)  . 
Breaking  length  :  (Km.)     . 
Tensile  strength:  (Kg/mm2) 
Total  extension:  %  dry 
wet 
Stiffness  (g.p.d.)       .  11 


85 
37 
42 
37 
42 


90 
45 
51 
40 
46 


(b)  Polyamide  6  (Perlon) 

Normal  tenacity     High  tenacity 

(1)  Density  ...  1'14 

(2)  Tenacity  dry  (g/den.)  4'1         5'8  6'5  —  8 
breaking  length:  dry  (Km.)     37        52  58  --    72 

wet(%  of  dry)  85    90 

(3)  Tensile  strength  (Vi%.lmm*)     42        60  67        82 

(4)  Knot  strength  (%  of  tenacity)  85-  90 

(5)  Loop  strength  (%  of  tenacity)  85-  90 

(6)  Extension  at  Break  :  dry         24        30  16        24 

wet  .  27        34  19  —  23 

(7)  Elastic  recovery  .  .     100%  100% 

up  to  8%  at  4% 

(8)  Toughness  .  .     0'67  0'68 

(9)  Water  absorption  at    65%  R.H.  48% 

at  100%  R.H.  7—9% 

(10)  Effect  oj  heat    melting  point  217°C 

softening  point  170  -180°C 

Effect  of  age.     Virtually  none. 

Effect  of  sunlight.     Lx>ss  of  strength  on  prolonged  exposure 

Effect  of  chemicals. 

Resistance  to  bacteria 


Similar  to  ^amide  66  fibre 


o  insects 

Resistance  to  abrasion       J 

Polyamide  6  fibres  are  available  also  in  the  form  of  staple, 
which  have  as  in  the  case  of  polyamide  66,  a  lower  tenacity 
and  a  higher  extension  than  continuous  filament. 

(c)  Polyamide  11  (Rilsan) 

Polyamide  11  (Rilsan)  fibres  are  relatively  new,  as  their 
production  was  started  only  a  few  years  ago.  For  this  reason, 
the  data  about  the  properties  and  characteristics  of  these 
fibres  are  not  yet  complete. 

(1)  Density  ...  1  04 

(2)  Tenacity  dry  (g/den.)  .  4'7  —  55 
breaking  length:  dry  (Km.)  42  -—  50 

wct(%  of  dry)  100% 

(3)  Tensile  strength  (Kg.lmm2)    .  44-51 

(4)  Extension  at  Break  %:  dry  and  wcl  25  —  40 

(5)  Elastic  recovery        '    .      "     .  100%   up  to  8% 

(6)  Water  absorption  &\  65%  R.H.  1'2% 

at  100%  R.H.  3% 

(7)  Effect  of  heat   melting  point  189°C 

softening  point  178°C 

(8)  Effect   of  chemicals.    The   resistance   to   chemicals   (acids) 

alkalis,  organic  solvents,  mineral  salts  and  oxidising  agents, 
is  slightly  superior  to  that  for  polyamide  66  and  6. 

In  respect  to  ageing,  effect  of  sunlight,  resistance  to  bacteria, 
fungi,  insects  and  to  abrasion  there  is  no  substantial  difference 
between  Rilsan,  nylon  and  Perlon. 

Yarns,  cords  and  ropes  made  from  poly  am  ides  (66  and  6) 
are  already  well  established  in  the  field  of  fishing  gear, 
because  of  their  special  characteristics,  namely  high  strength 
combined  with  extensibility,  low  absorption  of  moisture  and 
water,  quick  drying,  outstanding  resistance  to  abrasion, 
bacteria  and  fungi  and  seawater  and  algae. 


SYNTHETIC    FIBRE    TWINES    FOR    FISHING    PURPOSES 


POLYESTER  FIBRES 

The  principal  polyester  fibre  is  made  from  polyethylene 
tercphthalate,  which  was  first  synthesized  by  Whinfield  and 
Dickson  in  the  laboratories  of  the  Calico  Printers  Association, 
in  Great  Britain. 

Mr.  Whinfield  designated  the  fibre  "Terylene"  for  conven- 
ience in  writing  and  was  surprised  that  the  name  has  survived. 

We  report  below  the  data  regarding  polyester  filament  yarn ; 

Normal  tenacity    High  tenacity 
1-38 


4  

36  — 


5'5 
50 


100% 


6.0  —  7 
54  —  63 


74  —  87 
80 
70 

15    -  7 
100%  at  2% 
90%  at  8% 
0-5 
51 

0-4% 

0-5% 

260°C 

230--240"C 


(1)  Density 

(2)  Tenacity  dry  (g/den.) 
breaking  length  (Km.) 

wet  (%  of  dry) 

(3)  Tensile  strength  (Kg. /mm2)      50  —  68 

(4)  Loop  strength  (%  of  tenacity)       90 

(5)  Knot  strength  (%  of  tenacity)       70 

(6)  Extension  at  Break  dry  and  wet  27  —  17 

(7)  Elastic  recovery  .  97%  at  2% 

80%  at  8% 

(8)  Toughness         .  .  .     0*78 

(9)  Stiffness  (g.p.d.)  .  .   23 

( 10)  Water  absorption  at  65%  R.H. 

at  100%  R.H. 

(11)  Effect  of  heat  melting  point 

softening  point 

Effect  of  age.     Virtually  none. 

Effect  of  sunlight.  Only  slight  loss  of  strength  on  prolonged 
exposure. 

Effect  of  chemicals.  Resistance  of  a  high  order  to  chemical 
attack;  outstanding  resistance  to  mineral  and  organic 
acids  (expecially  hydrofluoric  acid).  Only  fair  resistance 
to  alkalis  but  adequate  for  most  purposes.  Resistant  to 
oxidising  agents  and  to  organic  solvents  (except  at  the 
boiling  point).  Most  phenols  dissolve  the  fibre. 
Resistant  to  bacteria,  fungi  and  insects. 
Resistance  to  abrasion:  ranks  second  to  polyamide  fibres 

only. 

Polyester  staple  fibre  is  also  commercially  available;  the 

characteristics  are  the  same  as  those  of  continuous  filament, 

except  for  tenacity  and  extension,  which  are  respectively 

lower  and  higher  than  for  filament  yarn. 

Vinyl  fibres 

The  first  vinyl  fibre  was  described  in  the  literature  nearly 
fifty  years  ago.  Polymer  chemistry  had  first  of  all  to  prepare 
polymers  possessing  suitable  properties  for  spinning  and  as  a 
result  of  a  great  deal  of  research  and  experiments,  fibres 
having  satisfactory  physical  and  chemical  properties  were 
obtained.  From  the  standpoint  of  utilization  in  the  fishing 
industry,  the  following  vinyl  fibres  are  to  be  mentioned: 
poly  vinyl  chloride,  vinylidene  chloride  (Saran),  polyethylene 
(Courlene  and  Courlene  X  3)  and  poly  vinyl  alcohol  (Vinylon). 
We  summarize  below  the  main  properties  of  these  fibres: 


(a)  Pol>  vinyl  chloride  fibres 

(1)  Density 

(2)  Tenacity  dry  (g/den.) 

breaking  length  (Km.) 
wet  (%  of  dry) 

(3)  Tensile  strength  (Kg./mm2) 

(4)  Loop  strength  (%  of  tenacity) 

(5)  Knot  strength  (%  of  tenacity) 

(6)  Extension  at  Break  dry  and  wet 

(7)  Elastic  recovery 

(8)  Water  absorption 

(9)  Effect  of  heat 


1'39 

2'7  —  3-7 
24  —  33 

100% 
33  —  46 
70% 
70% 

13  —  30 
80  to  85%  at  3% 

O'l  at  95%  R.H. 
softens  at  110  —  120°C 
shrinkage  starts  at  60°  to  70°C. 


Effect  of  age.      Virtually  none. 

Effect  of  sunlight.  Substantially  unaffected  after  prolonged 
exposure. 

Effect  of  chemicals.  Resistant  to  acids,  including  aqua  rcgia 
and  to  concentrated  (caustic)  alkalis.  Dissolves  or  swells 
in  some  aromatics,  chlorinated  hydrocarbons,  ketones, 
esters.  Generally  good  resistance  to  other  chemicals. 

Resistant  to  bacteria,  fungi,  insects  and  algae  and  sea 
water. 


(b)    Vinylidene  chloride  fibres 

(1)  Density 

(2)  Tenacity  dry  (g/den.) 

breaking  length:     dry  (Km.) 

wet  (%  of  dry) 

(3)  Tensile  strength  (Kg./mm2)    . 

(4)  I  Atop  strength  (%  of  tenacity) 

(5)  Knot  strength  (%  of  tenacity) 

(6)  Extension  at  Break  (%) 

dry  and  wet 

(7)  Elastic  recovery 

(8)  Water  absorption 

(9)  Effect  of  heat  melting  point 


170 

1-5  —  2*6 
13*5        23 
100°0 
23        40 
70  —  80 
70        80 


18  —  33 

98  in  100%  at  5% 
0*1%  at  95%  R.H. 

150    -   I60°C 


Effect  of  age.     Virtually  none. 

Effect    of  sunlight.    Slight    discoloration    after    prolonged 
exposure. 

Effect  of  chemicals.  Unaffected  by  most  acids,  including  aqua 
regia.  Affected  by  some  alkalis,  including  concentrated 
ammonium  hydroxide,  and  sodium  hydroxide.  Sub- 
stantially inert  to  organic  solvents.  Generally  good 
resistance  to  other  chemicals. 
Resistant  to  bacteria,  fungi,  insects,  moth  larvae,  algae 

and  sea  water. 

(c)    Polyethylene  fibres 

(The  data  refer  to  polyethylene  yarns  produced  by 
Courtaulds  Ltd.  under  the  registered  trade-names  of  "Cour- 
lene" and  "Courlene  X  3".) 

C 'ourlene  Com  lene  X  3 

(1)  Density  .  .  .     (V93  096 

(i.e.  the  lowest  density  of  all  the 
synthetic  polymer  fibres— giving 
good  flotation  properties). 
.      1  —  1-5  4-0  —  6-0 

)     9    -    13-5  36  —  54 

100% 
.   8'5     -  12*5  34-5-    52 

25     -  50  20        40 

90  to  95%  at  5% 
practically  none 
Courlene  softens  above  90°C; 
melts  between  110'  and  120"C. 

Courlene  X  3  (High  tenacity) 
softens  at  120  C;  melts  at  135'C. 

Effect  of  age.      Virtually  none. 

Effect  of  sunlight.  Loss  of  strength  after  prolonged  exposure. 
Effect  of  chemicals.  Remarkable  resistance  to  attack  by  alkali, 
acids,  solvents,  organic  salts,  unequalled  in  this  respect 
by  other  fibres. 

Resistant  to  bacteria,  fungi,  insects,  moth  larvae,  algae 
and  sea  water. 

The  abrasion  and  electrical  resistance  of  Courlene  X  3  is 
very  good  and  both  Courlene  and  Courlene  X  3  are  flexible 
at  very  low  temperatures  such  as  70  deg.  C. 


(2)  Tenacity  dry  (g/den.) 

break  i  ng  lengt h :  d ry  ( K m 
wet  (%  of  dry) 

( 3 )  Tensile  strength  ( Kg./mm2) 

(4)  Extension  at  Break  (%) 

dry  and  wet 

(5)  Elastic  recovery 

(6)  Water  absorption 

(7)  Effect  of  heat 


[17] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


(d)  Poly  vinyl  alcohol  fibres 

Vinylon  is  the  generic  term  for  polyvinyl  alcohol  fibres: 
these  are  produced  in  staple  and  in  filament. 

Staple  filament 

Normal          High  Normal          High 

Tenacity      Tenacity  Tenacity      Tenacity 

(1)  Density  1  -26—1  30  1  •  2(v-l  - 30 

(2)  Tenacity  dry 

(g/den)  .  4-2-6-0    6-7—8-0    3-5-   45    77-92 

breaking  length: 

dry  (Km.)       .     38—54        60-72        32-41         69—83 
wet  (°0  of  dry)     77~85°0     80— 85°0    8O-90°0    80    90';; 

(3)  Tensile  strength 

(Kg./mm)        .     50    70        78-94        41—53        90     110 

(4)  Loop  strength 

(°0  of  tenacity)     35    43%    35— 40°0    90— 95  °n    58    65  "0 

(5)  Knot  strength 

( %  of  tenacity )    60  -67  "  ;    64-  70  ° ;    75—80  °  u    39—52 

(6)  Extension  at  break 

(?0)  dry          .      17-26         13-  16         14—19          9—20 
Wet         .      19—30         14—17         14    22         12-22 

(7)  Elastic  recovery       75  -80 ^    78     82        70— 90 °0     85—98 

at     3°0      at     3°0      at     3°0      at     3°0 

(8)  Water  absorption 

(at65°r,R.H.)    4-5— 5'0°0  4-5     5'0°0  3'5    4V\  3'0— 5'0 
(at95°0R.H.)  10  -12'0°0 

(9)  Effect  of  heat 

melting  point  220    225  C 

shrinkage        .  starts  at  abt.  200  C 

Effect  of  age.    Virtually  none. 

Effect  of  sunlight.  Loss  of  strength  after  prolonged  exposure. 

Effect  of  chemicals.  Concentrated  sulphuric,  hydrochloric, 
formic  acids  cause  decompositions  or  swelling.  Strong 
alkalis  cause  yellowing  but  do  not  affect  strength.  Good 
resistance  to  organic  solvents.  Soluble  in  hot  pyridine, 
phenol,  cresol.  Resistant  to  oils. 
Resistance  to  bacteria,  fungi,  insects:  not  affected. 

Polyvinyl  chloride,  polyvinyl  alcohol  and  polyvinyl- 
idene  chloride  fibres  are  being  used  in  making  set  nets, 
trawl  nets  and  seine  nets.  Polyethylene  is  the  only  textile 
fibre  that  has  a  specific  gravity  lower  than  that  of  water. 
Owing  to  this  low  density,  the  diameters  of  Courlenc 
filaments  are  greater  than  those  of  other  textile  fibres  of 
the  same  denier.  In  the  case  of  125  denier  monofilaments, 
for  instance,  the  diameters  in  l/1000ths/inch  are  Cour- 
lene  5'4,  nylon  4*9,  acetate  4*5,  viscose  4'2. 

Courlene  and  Courlene  X3  are  used  in  making  ropes 
of  all  kinds  for  use  on  board  ship.  The  tensile  strength  is 
sufficient  to  make  it  suitable  for  most  applications, 
while  improvements  being  made  may  increase  the  tensile 
strength  from  4  to  6  g/den.  to  8  g/den.  The  yarn  is  spun- 
dyed,  which  makes  it  easy  to  produce  coloured  ropes; 
it  does  not  absorb  water  and  therefore  does  not  swell; 
ice  does  not  form  inside  it,  and  snow,  etc.  can  easily  be 
brushed  or  shaken  off.  Its  specific  gravity  is  low  and  ropes 
made  from  it  will  float  on  water.  The  yarns,  being  inert, 
will  not  support  fungal  or  weed  growth. 

The  fibre  is  useful  where  tensile  strength  and  buoyancy 
are  important,  as  in  trawl  nets  where  the  upper  net  will 
tend  to  rise  of  its  own  volition ;  it  is  claimed  that  con- 
sequently fewer  floats  are  required.  The  fibre  has  been 
used  both  for  the  main-lines  and  snoods  in  longline 
fishing,  and  for  weaving  canvases  which  do  not  become 


hard  and  are  relatively  easy  to  handle  in  wet,  cold 
conditions.  Insoles  made  from  Courlene  provide  thermal 
insulation  in  the  fisherman's  boots. 

MAN-MADE  FIBRES  AND  FINISHES  IN  PROTEC- 
TIVE CLOTHING  FOR  FISHERMEN 

The  fishermen  are  exposed  to  wet,  cold  climatic  con- 
ditions and  must,  to  work  efficiently,  receive  adequate 
protection.  In  the  past  they  wore  oilskins,  made  of  a 
heavy  cotton,  flax  or  similar  fabric,  coated  with  many 
layers  of  linseed  oil.  Today  many  of  the  basic  fabrics  are 
made  from  100  per  cent,  viscose  rayon  staple  or  blends 
with  cotton.  The  fabrics  are  produced  with  good  tear  and 
tensile  strength  and  a  surface  smooth  enough  to  allow 
even  coatings,  now  generally  of  p.v.c.  All  types  of  gar- 
ments, such  as  coats,  sou'westers  and  leggings,  are  made 
from  them. 

One  of  nylon's  chief  contributions  is  as  a  lightweight 
base  for  waterproof  clothing.  Fabric  of  1^  to  2J  ozs./sq. 
yd.  can  be  successfully  proofed  with  p.v.c.  neoprene  and 
polyurethane  resins.  These  cloths  have  the  advantages 
of  lightness,  flexibility  and  a  smooth  p.v.c.  coating  which 
is  of  special  advantage  in  collar  fittings  where  the  fabric 
may  be  flat  against  the  wearer's  face. 

Another  innovation  is  the  double  textured  rainproof 
jackets,  coats  and  leggings.  These  are  made  from  two 
similar  fabrics  laminated  with  rubber.  The  textile  fabric 
provides  resistance  to  abrasion  and  the  rubber  lamination 
provides  waterproofing. 

The  traditional  seamen's  jersey  can  be  made  in  a  blend 
of  50  per  cent,  viscose  staple,  50  per  cent,  wool  or  15  per 
cent,  nylon,  35  per  cent,  viscose  rayon  staple,  50  per  cent, 
wool.  The  latter  blend  has  good  resistance  to  abrasion 
and  can  be  made  to  resist  shrinkage.  The  same  yarns 
have  also  been  used  in  heavyweight  half-hose  and  seamen's 
stockings,  comfortable  in  use  and  hard  wearing. 

Yarns  made  from  100  per  cent,  viscose  rayon  staple  and 
viscose  rayon/staple  cotton  blends  can  be  knitted  on 
interlock  machines,  and  lend  themselves  to  coating  with 
vinyl  compounds,  rubber  or  p.v.c.  Gloves  are  now  made 
of  this  type  of  fabric  and  also  rubber  leggings  and 
waders.  The  fabrics  can  be  raised  to  give  a  fleecy  lining. 


REFERENCES 

1  Harold    De    Witt    Smith — "Textile    Fibres — An    Engineering 

Approach   to   their   Properties   and    Utilization",  ASTM 
Proceedings,    1944. 

2  Douglas  C.  Hague — "The  Economics  of  Man-Made  Fibres", 

London,  1957. 

3  J.  T.  Marsh— "Textile  Science*1,  London,  1949. 

4  "Textiles  Terms  and  Definitions"-  The  Textile  Institute,  Man- 

chester. 3rd  Edition,  1957. 

8  E.  Viviani  and  L.  Graziano — "Fibre  Artificial!  e  Sintetiche", 
Turin,  1952. 

6  E.  Kornreich — "Introduction  to  Fibres  and  Fabrics",  London, 

1952 

7  E.   R.   Kaswell— "Textile  Fibres,  Yarns  and   Fabrics".   New 

York,  1953. 

8  E.  I.  Du  Pont  de  Nemours  Co.— "Nylon  Textile  Fibres  in  In- 

dustry", Wilmington,  1947. 


18] 


CHARACTERISTICS    OF    SYNTHETIC    TWINES    USED    FOR    FISHING 

NETS  AND  ROPES   IN  JAPAN 

by 
YOSHINOR1  SHIMOZAKI 

Tokai  Regional  Fisheries  Research  Laboratory,  Tokyo,  Japan 

Abstract 

The  main  physical  properties  of  synthetic  netting  twine  used  for  fishing  gear  in  Japan  are  discussed  in  this  paper.  The  items  con- 
sidered include  the  sinking  speed,  breaking  strength,  knot  strength,  friction  resistance,  resistance  to  sunlight,  and  seawatcr,  etc.  and  reference 
is  also  made  to  treatments  by  tar  and  resin  for  improving  the  characteristics  of  nets  and  ropes. 


Resume 


Caracteristiqucs  des  ficelles  synthetiqucs  utilisees  pour  les  filets  et  les  cordes  de  peche  au  Japon 


Cet  article  examine  les  principals  proprietes  physiques  des  ficelles  a  filets  synthctiqucs  utilisees  pour  les  engins  de  peche  au  Japon. 
Les  sujets  traites  comprcnnent  la  vitesse  de  plongec,  la  resistance  A  la  rupture,  la  resistance  des  noeuds,  la  resistance  a  la  friction,  Ja  resistance 
a  la  lumiere  solaire  et  a  Peau  de  mer,  etc.  el  il  est  aussi  fait  mention  des  traitements  par  le  goudron  et  la  resine  pour  amdliorcr  les  caracter- 
istiques  des  filets  et  des  cordes. 


Extracto 


Caracteristicas  de  los  hilos  sinteticos  usados  en  la  fabricaci6n  de  redes  e  hilos  en  el  Japon 


En  cste  trabajo  se  cstudian  las  principales  propiedades  fisicas  de  los  hilos  sintciicos  usados  en  el  Japon  para  fabricar  artes  de  pest  a. 
Entrc  los  puntos  considerados  figuran  la  velocidad  de  inmcrsidn,  resistencia  a  la  rupture  y  de  los  nudos,  asi  como  al  roce,  lu/.  solar  agua 
salada,  etc.  Tambien  se  mcncionan  los  tratamientos  con  alquitran  de  hulla  y  cxtractos  curtientes  para  mejorar  las  caracteristicas  de  las 
redes  e  hilos. 


1.     Introduction 

FISHING  gear  is  dependent  on  several  factors,  such 
as  the  fishing  techniques  to  be  employed,  the  fish  to 
be  caught,  the  material  for  nets  and  ropes,  and  so 
on.  With  regard  to  the  latter,  there  are  requirements 
which  largely  influence  characteristics  and  efficiency  of 
the  gear.  They  are: 

the  thickness  or  diameter  of  the  netting  twine; 

the  weight  of  the  twine  in  air  and  in  water; 

sinking  speed; 

strength  and  extension  up  to  the  breaking  point; 

strength  and  extension  knotted; 

friction  resistance; 

resistance  to  sunlight  and  seawater; 

knot  fastness; 

resistance  to  factors  such  as  shock,  heat,  chemicals  and 

fatigue; 
elasticity; 

susceptibility  to  dye  and  dye  fastness; 
stiffness  or  handiness; 
plying  and  fabricating  capacity. 

In  evaluating  these  properties  different  methods  and 
means  would  give  different  values.  Therefore  when 
selecting  one  kind  of  material  out  of  a  group  each 
attribute  must  be  evaluated  with  a  uniform  method 


which  should  approximate  the  actual  fishing  conditions 
as  far  as  possible. 

The  efficiency  of  a  fishing  net  is  partly  predictable 
from  the  quality  of  the  twine,  therefore  the  results  of 
twine  testing  will  be  first  described  in  regard  to  two  of 
the  factors  (1)  the  nature  of  the  fibre,  and  (2)  the  number 
of  twists  given  to  the  yarn  as  well  as  the  strand. 

2.     How  to  Indicate  the  Structure  and  Size  of  Net  Twines 

One  group  of  synthetic  fibres  made  of  spun  yarn  just  as 
with  cotton  or  hemp  includes  Kuralon  (Manryo) 
and  Mulon  yarns.  As  in  cotton  the  strands  made  by 
twisting  the  yarn  are  then  again  plied  to  form  twine. 
Most  of  the  synthetic  yarns  used  for  fishing  nets  are  made 
equivalent  to  English  20's  of  cotton  yarn  in  size,  and  are 
usually  of  2  or  3  plies.  The  size  is  indicated  by  the 
number  of  yarns  used  for  a  strand  with  the  number  of 
strands  (e.g.  No.  5  with  3  plies),  or  by  the  total  number  of 
yarns  contained  in  a  twine  (e.g.  15  yarns  in  3  plies). 
The  latter  is  more  often  used. 

In  another  group  the  continuous  filaments  are  plied 
into  strands  or  threads.  Amilan  (nylon),  Saran,  Krehalon, 
Teviron,  Envilon,  Kyokurin  and  Kuralon  No.  5  (Manryo 
No.  5)  generally  belong  to  this  group.  In  this  case, 
however,  the  strand  may  be  formed  by  a  single  filament 
(monofilament)  or  by  yarns  consisting  of  several  fila- 


[19] 


MODERN    FISHING     GEAR    OF    THE    WORLD 


TABLE  I.    Construction  of  Some  Kinds  of  Netting  Twines 


Material 
Amilan 


Chemical 
name 


Construction  of  yarn  and/or 
strand  and  twine 


Polyamide  (250D/15F.-Y)  x  n,-S,  S  x  n2-T 
(210D/15F1-Y)  x  n,-S,  S  x  n2=T 
(110D/30F1-Y)  x  n.-S,  S  x  n2=T 
(  60D/20Ft-Y)  x  nx-S,  S  x  n,=T 

Kuralon  No.  5  Polyvinyl    (500D-F2)  x  n3-S,  S    >    n2-T 
(Manryo  No.  5)  alcohol 

Saran     Polyvinylidene         720D/6Fj  x  n0    S,  S  x  n2--T 
Polyvinyl  chloride   lOSOD^FjXnr -S,  S  x  n2-T 
(360D-F2)xn3-S,  S    >:  n,-T 
(1000D-F.,)  xha-S,  S   s   n,-T 


Krchalon 

Teviron 
Envilon 


1080D/6F1xn4-S,  S  x 
(360D-  F2)xn~-S,  S  x  n2-T 
(IOOOD    F2)  A  n3-S,  S  x  n2    T 

<300D/30Fr  Y)xni-S,  S  x  n2 
(450D-  F2)xn3-S.  S   x   n2-T 


Cotton  not  applicable          (20's-Y)  x  n,-S,  S  x  n2--T 
Kuralon  Polyvinyl  alcohol  (20's-  Y)xn1-S,  S   x   n2~T 

D  :    denier. 

multifilaments. 

monofilaments. 

yarn. 

number  of  yarns  contained  in  a  strand.  Fach  yarn  of  this  type 
comprises  multifilaments. 

strand. 

number  of  strands  (usually  3  but  sometimes  2  or  4)  construct- 
ing a  twine. 

twine. 

number  of  monofilament  contained  in  a  strand.  The  thickness 
of  a  strand  varies  depending  on  n3. 

number  of  multifilaments,  usually  one.  or  two  bundles. 


ments  (multifilament).  Amilan,  Teviron,  and  Kyokurin, 
and  small  diameter  twines  of  Saran,  Krehalon  have 
strands  made  of  multifilament  yarns.  Kuralon  No.  5  and 
other  large  diameter  twines,  such  as  those  of  Saran  and 


Krehalon,  have  strands  made  of  monofilament.  Table  I 
illustrates  how  these  yarns,  strands  and  twines  arc 
constructed. 

In  the  Table,  250/15F  x  nA  of  (nylon)  Amilan,  for 
instance,  means  that  a  strand  is  made  by  twisting  a 
number  of  yarns  of  250  total  denier,  each  of  which  is 
constructed  in  15  filaments.  The  thickness  of  a  filament 
in  that  yarn  is  16*6  denier  approximately.  As  a  (nylon) 
Amilan  yarn  of  210  denier  corresponds  in  thickness  to 
cotton-20-counts  (20's),  the  number  of  yarn  of  any  other 
denier  in  this  twine,  equivalent  to  cotton  20's  (hereinafter 
called  C20-equivalent),  is  converted  by  dividing  the  total 
denier  of  the  twine  by  210  denier.  The  twines  consist  of  3 
and  sometimes  4  strands,  e.g.  for  some  salmon  gillncts. 
Similar  calculations  can  be  made  for  other  synthetic 
fibres  produced  in  Japan. 

In  all  these  synthetic  products,  the  numbers  of  yarn 
for  C20-equivalent  twine  are  obtained  by  dividing  the 
total  denier  of  the  twines  with  the  thickness  of  the 
C20-equivalent.  It  should  be  remembered,  however,  that 
the  thickness  of  C20-equivalent  determined  for  these 
twines  does  not  necessarily  represent,  in  a  strictly  physical 
sense,  the  true  thickness  of  cotton  20's. 

3.    Thickness  of  Netting  Twines 

No  matter  how  similar  may  be  the  indicated  thickness 
of  different  kinds  of  twines,  the  indication  does  not 
warrant  that  they  are  all  alike  in  diameter,  circumference 
or  weight  per  unit  length  (Tables  III  and  IV).  Naturally, 
the  real  thickness  of  a  twine,  viz.  the  whole  area  of 
the  horizontal  section  of  all  the  filaments  of  the  twine, 
varies  according  to  the  kind  of  twine,  making  it  impossible 
to  assess  the  characteristics  of  twines  on  a  comparable 
basis.  As,  however,  no  other  standard  than  the  conversion 
into  C20-equivalent  has  become  available  for  the 
purpose,  characteristics  of  both  synthetic  and  natural 
twines  will  be  compared  with  the  help  of  this  conventional 
method. 

The  thickness  of  net  twines  has  been  measured 
(Table  III)  in  a  wet  state.  The  twines  were  immersed  in 
water  for  24  hours  and  hung  until  the  water  slopped 
dripping  from  them.  The  twines  used  are  generally 


TABLE  If. 

Number  of  Twist  per  30  em.  of  Twines 

Number  of  yarn 

4 

6 

9 

12 

15 

18 

21          27          36 

45 

54          60 

75 

84 

Amilan 

A         132 
H          198 

123 
189 

111 
167 

— 

72 
112 

69 
124 

64          57 
131         117 

51 
104 

43 
78 

35 
70 

Kuralon  (Manryo) 

A 
B           ~ 

96 
241 

81 
189 

— 

59 
115 

50 
92 

44          38 
78          62 

36 

57 

33 

52 

29 
49 

Criran 

A 

76 

68 

64 

59 

57 

53          45 

33 

*25 

(66)  23 

daidn       .... 

B           ~~ 

135 

123 

110 

98 

94 

83          75 

60 

45 

41 

Krehalon 

A 
B 

68 
104 

63 
100 

— 

56 
93 

51 
90 

48          46 
87          85 

44 
81 

39 

75 

36 
66 

33 
60 

Teviron 

A          138 
B         246 

90 
202 

% 
158 

84 
148 

- 

60 
123 

*51  (24)                48 
114             "         102 

42 
84 

Cotton 

A 
B 

102 
405 

95 
336 

— 

75 
264 

66 
212 

60          51 

~~         171         135 

42 
90 

38 
60 

34 
53 

Kyokurin 

A           81 
B          126 

68 
114 

57 
95 

51 
81 

— 

— 

42                       31 

65          '  *          52 

— 

A:  twist  for 

twine. 

B:  twist  for  strand. 

*  The  number  within 

parentheses  is 

the  number  of  yarn  equivalent 

in  the  thickness  to  cotton  20's,  and  the 

number  outside  parentheses 

is  the  number  of  twist  of  that  twine. 

[20] 


JAPANESE    SYNTHETIC    TWINES 


TABLE  III.    Diameter  of  Wet  Netting  Twines 


(Unit: 

mm.) 

PlylNo.  of  yarn 
of  CW-equivalent                             3/6 

3/12      : 

5/18         : 

1/24         : 

*/30         : 

1/36 

3/42 

3/48 

3/54 

3/60 

3/66 

372 

Amilan      .           .                      .0-68 

0-92 

•15 

-37 

•56 

1-76 

1-94 

2-11 

2-26 

2-41 

2-54 

2  78 

Kuralon  (Manryo)                    .     0-83 

1-10 

•36 

•60 

•83        : 

!-08 

2-23 

2-42 

2-59 

2-75 

2-90 

3-04 

Saran         .                                     0  66 

0-86 

-03 

-22 

-37 

-52 

1-66 

1-77 

1  88 

2-00 

2-09 

2-18 

Krehalon  .                                 .0-66 

0-88 

•05 

•24 

•40 

-55 

1-69 

1-83 

1-95 

2-06 

2-16 

2-25 

Teviron                                     .     0-70 

0-96 

-21 

•43 

•64 

•84 

2-03 

2-21 

2-37 

2-03 

2-66 

2-79 

Kyokurin                                   .     0-67 

0-90 

•08 

•28 

•46 

•63 

1-78 

1-93 

2-06 

2-18 

2-28 

2-40 

Cotton       .                                .0-78 

1-06 

-32 

•56 

•77 

-98 

2-18 

2-36 

2-53 

2  70 

2-83 

2-97 

considered  to  be  of  medium  twist  (Table  II).  Roughly 
speaking,  the  twist  below  that  range  has  about  3  to  5  per 
cent,  greater  thickness. 

4.    Weight  of  Net  Twines  in  the  Air  and  in  Water 

(1)  Weight  in  the  air:  In  the  air,  the  weight  of  fishing 
nets  has  a  close  relation,  particularly  when  they  are  wet, 
with  the  loading  capacity  of  small  boats  as  well  as  with 
the  working  conditions  for  fishermen.  Although  weight 
is  largely  governed  by  the  thickness  of  the  twine,  mesh 
size  and  type  of  knot,  a  table  of  weights  prepared  by 
actually  weighing  nets  in  use  is  useful  reference.  In 
preparing  such  a  table,  the  weight  per  metre  of  dry  twine 
and  the  rate  of  weight  increase  for  wet  twine  may  be 
taken  as  the  basis  for  estimating  the  wet  weight  of  various 
types  of  nets. 

The  wet  weight  of  a  net  in  air  can  be  obtained  by  multi- 
plying the  known  dry  weight  of  the  net  with  the  ratio 
Ww/Wd,  as  in  Table  IV. 

The  situation  is  somewhat  different  with  nets  processed 
with  resins.  The  amount  of  water  that  sticks  to  the  nets 
differs  according  to  the  kind  of  fibre  and  thickness  of  the 
resin  coating  as  well  as  the  water  absorption  or  repellency 
of  the  resin,  so  that  it  is  difficult  to  estimate  the  wet 
weight  of  a  net.  However,  an  experiment  has  proved  that 
resin-processed  nets  gain  in  weight  over  those  not 
processed  by  30  to  80  per  cent,  which  includes  the 
weight  of  the  resin  adhering  to  the  material. 

Coal  tar  treated  nets  in  dry  state  are  70  to  150  per  cent, 
heavier  than  non-treated  nets.  If  the  coal  tar  is  diluted 
with  creosote  or  gasoline,  the  weight  gain  is  approxi- 
mately from  50  to  100  per  cent.  Use  of  a  centrifugal 
machine  when  dyeing  can  reduce  the  amount  of  tar  on 
the  net  and  bring  the  weight  gain  down  to  60  to  80  per 
cent. 


(2)  Underwater  weight:  Increase  in  underwater  weight 
of  twines  implies  a  quicker  sinking  capacity  and  in  some 
cases  a  better  shape  of  the  net.  This  is  an  important 
quality  for  fixed  nets,  purse  seines  and  stickheld  dip  nets. 
Table  V  presents  the  underwater  weight  of  various  kinds 
of  netting  twines  on  the  basis  of  the  unit  length  of  one 
metre  measured  in  air.  In  the  case  of  webbing,  the  same 
relation  may  be  expected  between  synthetic  twines  and 
cotton  (Wo/Woe)  and  between  tarred  synthetic  and 
tarred  twines  (Wt/Wtc.) 

5.    Sinking  Speed 

Sinking  velocity,  an  important  factor  especially  for  such 
types  of  nets  as  purse  seines,  has  a  close  connection  with 
the  thickness  and  specific  gravity  of  the  raw  material, 
is  affected  by  the  degree  of  twist  given  to  the  yarn, 
strand  and  twine,  and  by  the  smoothness  of  the  twine 
surface.  In  a  sinking  speed  test,  a  piece  of  twine,  2  cm. 
long  with  each  end  glued,  was  immersed  in  water  for 
24  hours.  The  air  bubbles  were  then  removed  from 
the  surface  of  the  twine.  The  terminal  velocity  shown 
by  the  test  piece  in  sinking  straight  down  in  a  salt 
solution  (specific  gravity  1-020)  was  then  determined 
(fig.  1).  The  solid  line  indicates  the  sinking  speed  of 
non-treated  twine,  and  the  dotted  line  the  sinking  speed 
of  the  tarred  twine.  It  is  obvious  that  the  velocity  differs 
even  between  the  same  kind  and  quality  of  twines 
depending  upon  the  thickness  of  the  twines  as  well 
as  on  the  nature  of  the  raw  material,  the  number  of 
twists,  twisting  technique,  dyeing  and/or  heating. 

Table  VI  shows  comparative  sinking  speeds  for  various 
twines.  The  coal  tar  treatment,  which  was  first  used  for 
cotton  twines  to  increase  the  sinking  speed  by  preventing 
water  absorption,  has  been  found  still  mote  effective 


Items 

Wd  (mg/m) 
Ww  (mg/m) 
Ww 

Wd 


Amilan 

25-0   X   n 
34-0   x   n 

1-36    x    n 


TABLL  IV.     Weight  of  Dry  or  Wei  Netting  Twines 


Kuralon 

36-0   X   n 
64-8    x   n 

1 • 80   x   n 


Saran 

46-6   x   n 
50-5    x   n 

1-08    -v   n 


Krehalon 


45-3 
50-3 

1-11 


Teviron 

38-0    x    n 
50-0    -    n 

1-32    \   n 


Kyokurin 
35-0    •    n 


35-0   ->•    n 
57-5    >    n 

1-64   x   n 


Ww :     wet  weight. 
Wd  :    dry  weight, 
n    :    number  of  yarn  equivalent  in  the  thickness  to  cotton  20's. 


[21] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


TABLE  V.    Underwater  weight  of  Netting  Twines 


Kuralon 

Items 

A  mi  Ian             (Manryo)             Saran 

Krchalon 

Teviron 

Kyokurin 

Cotton 

Manila  Hemp 

Wo  (mg/m) 

3-33    x   n          6-70   x   n          17-7   x   n 

17-2   x   n          8-35   x   n 

11-8   x  n          12 

•8   x  n 

10-8   x 

m 

Wo/Woe 

0     S                   0-52                    1-38 

1-34 

0  65 

0-92 

1-00 

0-84 

Wt  (mg/m) 

8-0        n          13-2   >    n          24-2   x   n 

23-7   x   n 

15-7   x   n 

25 

•4  x   n 

21-8   x 

n 

Wt/Wtc 

0-32                    0-52                    0-96 

0*94 

0-62 

1-00 

0-86 

Wt/Woc 

0-62                    1-03                    1-89 

1-85 

1-24 

— 

1-98 

1-70 

Wt—  Wo 

v  inn 

(%)     140                      97                        37 

38 

88 

98 

102 

Wo 

Wo 

the  weight  of  non-treated  netting  twine. 

Woe 

the  weight  of  non-treated  cotton  twine. 

Wt 

the  weight  of  tarred  twine. 

Wtc 

the  weight  of  tarred  cotton  twine. 

n 

number  of  yarn  equivalent  in  the  thickness  to  cotton  20's. 

m 

monme.    One  monme  equals  3  •  75  gr.    The 

number  of  monme 

per  151-5 

cm.  m  abaca  twine  are 

used  as  the 

unit  of 

thickness  in  Japan.   One  monme  of  abaca  twine  is  equivalent  to  66 

yarns  of  cotton  20's. 

when  applied  to  some  synthetics.  For  this  reason,  coal 
tar  and  similar  products  are  widely  used  in  Japan. 

About  80  per  cent,  tar  adhesion  results  in  the  quickest 
sinking  speed1  while,  according  to  our  experiments, 
twines  over  or  undercharged  with  tar  sank  slowly. 

6.     Tensile  Strength  of  Net  Twine,  and  the  Knot  Strength 

(1)  Differences  in  strength:  Tensile  strength  of  twine 
depends  upon  that  of  the  raw  material  used,  whether 


TABLF    VI 

Comparison   of  Sinking  Speed   between   Non-treated   Twines 
and  Tarred  Twines 


Material 

Items 

3  plies, 
1  5  yarns 

Amilan 

Vo/Vc 
Vt/Vo 
Vt/Vtc 

0-57 
1-68 
0-55 

Kuralon 
(Manryo) 

Vo/Vc 
Vt/Vo 

Vt/Vtc 

0-74 
1-88 
0-75 

Saran 

Vo/Vc 
Vt/Vo 
Vt/Vtc 

1-77 
1-09 
1-20 

Krehalon   . 

Vo/Vc 
Vt/Vo 

Vt/Vtc 

1-65 
1-14 
1-17 

Teviron 

Vo/Vc 
Vt/Vo 
Vt/Vtc 

1-43 
1  20 
0-93 

Kyokurin  . 

Vo/Vc 
Vt/Vo 
Vt/Vtc 

1-37 

Cotton 

Vo/Vc 
Vt/Vo 
Vt/Vtc 

1-00 
1   63 
1-00 

3  plies, 
30  yarns 

3  plies, 
60  yarns 

0-54 
1-65 
0-60 

0  51 
1-75 
0  66 

0-73 
1-55 
0-76 

0-66 
1-53 

0-77 

1-65 
1-08 
1-18 

1-50 
1-07 
1-17 

1   56 
1    10 
1-15 

1-43 
1-10 
1-14 

1-21 
1-14 
0-92 

1-17 
1    10 
0-92 

1-27 

1  21 

1-00 
1-50 
1-00 

1-00 
1-38 
1-00 

The  number  of  yarn  is  equivalent  in  the  thickness  to  cotton  20's. 


Vo 
Vc 
Vt 
Vtc 


sinking  speed  (cm. /sec.)  of  non-treated  twine. 

non-treated  cotton  twine. 
„          „  „  tarred  twine. 

tarred  cotton  twine. 


the  twine  consists  of  short  or  long  fibres,  and  on  the 
number  of  twists  given  to  the  strand  and  twine.  The 
balance  between  the  primary  twist  for  making  the  strands 
from  the  yarns  and  the  secondary  twist  to  make  the  twine 
from  strands  is  also  very  important  because,  while  an 
increased  twist  augments  the  tensile  strength  up  to  an 
optimal  point,  an  excessive  increase  in  the  twist  produces 
the  opposite  effect2.  This  also  applies  to  the  strength  of 
a  fishing  net  itself. 

Among  twines  with  various  twists  used  for  fishing  in 
Japan  there  is  a  difference  of  about  15  to  20  per  cent,  in 


Name  of 
twine 

Amilan 
Manryo 

(Kuralon) 
Saran 
Krehalon 
Teviron 
Kyokurin 
Cotton 
Abaca 


Tarred     Non 
twine       treated 


C  c 

D  d 

K  e 

F  f 

G  g 

H  (i 


Fig.  1.  Relation  between  the  sinking  velocity  and  thickness  of 
netting  twines  of  various  kinds.  Dotted  lines  show  the  sinking 
velocity  of  tarred  twines,  and  solid  lines  non-treated  twines. 


[22] 


JAPANESE    SYNTHETIC    TWINES 


50 
# 

g  45 

fc 

•P 

•> 

a 
g40 

H 

35 
32 


30     l.S      ^O  30  40  J>0 

Number  of  twist  for  twine  per  30  cm. 


60 


fi#.  2-a.    Relation  between  the  breaking  strength  and  number  of 
twists  given  to  twines.  A,  B  and  C,  JOOO  /),  .?  plies,  24  mono- 
filaments,  produced  by  different  makers. 

X 

a   17.5 


17.0 


16.5 


g  ie.c 
H 


15.5 


15.0 


in     15      20  30  40  50 

Number  of  twist  for  strand  per  30  cm. 


60 


Fig.  2-b.  Relation  between  the  breaking  strength  and  the  number 
of  twists  given  to  a  strand  of  the  twines  as  of  fig.  10. 

tensile  strength,  that  is,  an  overtwisted  and  weaker  twine 
may  be  found  side  by  side  with  a  correctly  twisted 
stronger  one,  as  shown  in  fig.  2.  Kondo  and  Koizumi3"6 
pointed  out  the  same  in  regard  to  both  cotton  and 
synthetic  twines.  Even  with  identical  twines,  and 
whether  the  balance  is  good  or  not,  there  are  differences 
of  10  to  15  per  cent,  in  the  tension  resistance  because  of 


TABLE    VII 

Coefficients  a.  and  «2  for  Breaking  Strength  of  Wet  or  Dry 
Twines 


Wet 


Dry 


Amilan 

0  89 

0-94 

Kuralon  (Manryo) 

0-78 

0-% 

Saran 

0-70 

0-68 

Krehalon 

0-60 

0-56 

Teviron 

0-72 

0-68 

Kyokurin 

0-76 

0-81 

Cotton 

0-59 

0-47 

Envilon 

0-76 

0-74 

the  different  numbers  of  twists.  That  is  why  careful 
attention  must  be  paid  to  the  amount  of  the  twist  in  the 
twine  when  buying  fishing  nets  and  ropes7. 

In  regard  to  the  difference  in  the  wet  and  dry  strengths 
of  netting  twines,  an  experiment  proved  that  natural 
fibre  twine  is  about  10  to  20  per  cent,  stronger  wet  than 
dry,  but  the  contrary  is  true  with  Amilan  and  Kuralon 
(Table  VII).  With  Teviron,  Envilon,  Saran  and  Krehalon, 
the  wet  twines  are  slightly  (3-5  per  cent.)  stronger  than 
dry  ones8.  Similar  results  applied  to  the  tensile  strength 
of  knots  of  fishing  nets9. 

Temperature  is  another  influencing  factor,  the  tensile 
strength  of  nets  decreasing  by  10  to  20  per  cent,  in 
temperatures  between  30  deg.  C.  and  0  deg.  C.,  and  the 
strength  of  twines  by  about  5  to  10  per  cent,  according  to 
the  kind  of  twine10.  The  effect  of  temperature  on  the 
tensile  strength  of  synthetic  twines  and  knots  is  greater 
than  on  natural  twines. 

There  is  a  disparity  in  the  breaking  strength  between 
twines  of  less  than  50  cm.;  the  longer  the  stronger  they 
are,  and  vice  versa,  whereas  between  twines  of  a  same 
material  and  thinner  than  60  yarns  of  C20-equivalent, 
there  is  little  difference  in  the  breaking  strength11. 
An  approximate  breaking  strain  for  each  twine  specified 
in  Table  II  can  be  obtained  by  multiplying  u\  or  n2  in 
Table  VII  with  number  of  C20-equivalent  yarn  (fig.  3). 

(2)  Strength  at  knot:  A  series  of  tests  have  been  con- 
ducted at  temperature  18-0  deg.  C.-f  1  -5deg.C.  for  tensile 
strength  at  knot  with  30  cm.  long  wet  pieces  of  twine. 
In  one  test,  two  pairs  of  the  legs  (AB  and  CD)  of  an 
English  knot  were  pulled  asunder  (figs.  4  to  6).  Although 
not  convincing  as  a  proof  of  the  tensile  strength  of  a 


Fig.  3.   Wet  tensile  strength  of  twines  of  various  kinds,  each  with 
the  number  of  yarns  equivalent  in  thickness  to  cotton  20\\. 


[23] 


MODERN     FISHING    GEAR     OF    THE    WORLD 

TABLE  VIII 
The  Constant  in  Proportion  to  Tensile  Strength  at  the  Knots  of  Various  Kinds  of  Twines 


Kind  of  knot 
Drawing  direction  * 
P    and  Y 


English  knot 
AB-CD  AC  — 

Pi  Yi  P2 


Amilan 
Kuralon 


18 
21 


Saran 
Krehalon 
Teviron 
Kyokurin 
Cotton 

Kuralon  No.  5 
(Manryo  No.  5) 


1-25 
0-86 
0-80 
0-78 
0-7C 
0-84 
0-93 
0-97 


30 
45 
49 
43 
35 
34 
37 
18 
39 


1-28 
0-93 
0-86 
0-83 
0-79 
0-87 
0-97 
0-99 


Y2 

28 
40 
45 
41 
32 
32 
33 
16 
37 


AG 


1-25 
0-93 
0-90 
0-78 
0-74 
0-81 

0-96 


Flat 

knot 

CD 

AC  — 

BD 

Y3 

V* 

f* 

30 

1-23 

31 

40 

0-77 

5i 

42 

0-74 

53 

43 

0-70 

50 

36 

0-68 

41 

37 

0-71 

45 

19 


0-86        27 


*  Drawing  direction  of  test  twines  is  shown  in  fig.  4. 

PI,  P«.  P3,  and  p4  are  coefficient  of  proportion  of  tensile  strength  at  knots  of  various  kind  and  in  every  direction. 
"it  Yjj,  Ys«  and  YI  are  ratios  of  decrease  in  tensile  strength  of  these  twines. 


(b) 


English  knot  ("  Kaerumata" 
in  Japan).  This  type  is  most 
popular  all  over  the  world. 


Reef  knot  or  Square  knot 
("Homme"*   in   Japan). 


(c) 


(d) 


Double    English    knot    (*'Niju 
kaermala'"  in  Japan). 


Lock  Knot 


fishing  net,  such  a  test  may  still  be  significant  enough  for 
comparing  the  strength  of  various  types  of  nets  webbed 
by  the  same  knotting  technique.  The  strength  of  the 
knots  differs  considerably  according  to  the  type  of  knot 
and  the  directions  in  which  they  are  drawn12.  Figs.  7-A 
and  -B  show  the  strength  of  the  various  types  of  knots 
(a  and  b  in  fig.  4)  when  their  legs  were  strained  in  either 
direction,  AB  apart  from  CD  (lengthwise)  or  AC  apart 
from  BD  (crosswise). 

Breaking  strength  of  knotted  twine  can  be  regarded 
to  be  nearly  in  proportion  to  /i,  the  number  of  yarns  of 
C20-equivalent  of  a  twine  in  which  the  knot  is  constructed; 
coefficients  of  proportion  (i^  //.,,  ft^  and  />'.,  are  shown 
in  Table  VIII.  Decreasing  percentage  y,,  y.,,  y3,  and  7',  in 
knot  strengths  are  represented  by 


Pi 


x    100 


Fig.  4.     Type  of  Knot. 


where  a,  is  derived  from  Table  VII,  and  ft\  represents  any 
one  of  /?,  to  fi4. 

In  fig.  4,  knots  c  and  d  are  the  types  required  for 
repairing  broken  nets  and  making  gillnets,  and  nets  in 
which  the  knots  should  never  work  loose.  Knot  c  is 
called  the  double  English  knot,  and  Knot  d  the  Lock 
knot.  However,  the  tests  showed  no  noticeable  differences 
in  strength  between  the  English  knot  and  those  stated 
above13.  A  study  is  now  under  way  in  connection  with 
the  strength  of  a  knotless  net. 


TABLE  IX 
Breaking  Extension  of  Wet  Netting  Twines 


(Unit  %) 


Thickness 
of  twines 

3-plies 
4-9  yarns 

3  plies 
12-18  yarns 

Amilan 
Kuralon  (Manryo 
Saran 
Krehalon 
Teviron 
Cotton 
Kyokurin 

30 
)             21 
23 
22 
18 
20 
21 

30 
25 
25 
25 
20 
22 
22 

3  plies 
21-30  yarns 

3  plies 
33-45  yarn* 

30 
24 
26 
26 
24 
24 
26 

32 
24 
28 
28 
24 
29 
28 

3  plies 
48-60  yarns 

35 
32 
28 
30 
26 
30 
30 


3  plies 
over  63  yarns 

40 
37 
30 
36 
26 
32 


[24] 


JAPANESE    SYNTHETIC    TWINES 


Number  of  yarn  4-15 

Kind  of  knots      English  knot    Flat      knot 


Drawing* 
direction 


Amilan 
Kuralon 

(Manryo) 
Saran 
Krehalon  . 
Teviron 
Cotton 

Manila  hemp    . 
Kyokurin 
Kuralon  No.  5 
Manryo  No.  5) 


AB      AC 


CD 

20 
21 

17 
18 
13 

22 

17 


BD 

21 

22 

18 
18 
13 
23 

18 


AB       AC 
CD       BD 


21 
22 

18 
18 
12 
21 


20 
21 

15 
16 
II 
20 


TABLE  X 
Breaking   Elasticity  at  Wet  Knot 


(Unit     °0) 


18   -24 
English  knot     Flat     knot 


AB 


20 
22 

19 
17 
19 

27 

20 


AC 

A  B 

AC 

* 

t 

4- 

± 

1 

i 

BD 

CD 

BD 

22 

21 

21 

24 

25 

24 

19 

19 

18 

18 

19 

17 

18 

18 

17 

28 

26 

24 

23 


27—45 
English  knot     Flat      knot 


AB       AC       AB 


CD 

25 


18 
20 
19 
30 
12 
20 


BD 


26 


19 
20 
19 
31 
13 
23 


_CD_ 

24 


19 
21 
18 
30 
13 


AC 

t 

4 

BD 

23 


17 
19 
18 
28 
11 


48  or  more 

English 

knot 

Flat 

knot 

AB 

AC 

AB 

AC 

* 

t 

* 

« 

^ 

4 

, 

+ 

CD 

BD 

CD 

BD 

30 

30 

28 

21 

30 

30 

26 

26 

18 

19 

16 

17 

23 

22 

24 

22 

20 

20 

19 

19 

34 

34 

32 

30 

14 

15 

15 

13 

24-26    25-27 


*  Sec  Fig.  4  for  drawing  direction. 


(3)  Extension  of  twine  and  knots:  The  breaking 
extension  of  various  kinds  of  twine  and  their  knots  is 
affected  by  the  heating  and  stretching  procedures  during 
twisting  (see  Tables  IX  and  X).  Most  of  the  twines  tested 
were  not  elongated,  except  Amilan  which  was  extended 
10  per  cent. 


7.     Abrasive  Resistance 

( I )  The  relation  of  twist  to  frictional  pressure  and  strength: 
Generally  speaking,  there  are  two  forms  of  friction  which 
wear  down  fishing  nets;  one  between  net  twines  at  the 
seam  and  the  knots,  and  the  other  against  comparatively 


2100 
20' a 

L'lOD  of  Arotli 
120D  of  SB  ran 
3  WU> 
120Dx  3 
1ROD  x  2 
20- b 


Fig.  5.     Relation  between  wet  strength  of  an  English  knot  and 

the  number  of  yarns  of  C20-equivalent  less  than  24.    In  the  test, 

two  pairs  of  the  legs  of  the  knot,  AB  and  C7>,  as  in  Jig.  4  were 

pulled  asunder. 


VA  30  36  42  48  M  60  66  7 


Fig.  6.    Relation  between  wet  strength  of  an  English  knot  and 

the  number  of  yarns  of  C20-equivalent  more  than  24.     The 

pulling  direction    was   the  same  as  in  fig.   4. 


[25] 


Kind  of 
products 

A 
B 
C 


MODERN    FISHING     GEAR    OF    THE    WORLD 


TABLE  XI 

Number  of  Twist  of  Amilan  Netting  Twine  used  for  the  Wear  Test 

(Indicated  on  the  basis  of  length  30  cm.) 


Low  Twist  (L) 
For  twine    For  strand  For  yarn 


Medium  Twist  (M) 
For  twine    For  strand  For  yarn 


56-4 
70-0 
54-0 


39-2 
46-3 
32-3 


56-3 
66-0 

47-3 


67-7 
65-8 
51-5 


65-7 
50-4 

35-7 


89-7 
65-6 
53-0 


A,  B  and  C  are  the  names  of  the  twine  manufacturers. 


Hard  Twist  (//) 
For  twine    For  strand  For  yarn 


77-1 
78-5 
64-1 


68-0 
50  8 
55-4 


87-0 
69-9 

71-7 


hard  substances,  such  as  the  sea  bottom,  hull  of  the  boat, 
and  net  or  line  haulers.  The  friction  between  the  net 
twines  and  hard  objects  was  examined  by  use  of  a  device 
shown  in  fig.  8.  In  order  to  keep  the  temperature 
constant,  water  was  made  to  drip  on  the  sample  of  twine 
under  test.  The  twine  stretched  across  an  oil-stone  C 
fixed  on  a  block.  The  block  was  kept  moving  back  and 
forth  between  E  and  F  at  80  oscillations  per  minute,  and 
the  number  of  rubs  counted. 
The  results  of  these  tests  with  Amilan  twines  of  various 


twists  are  seen  in  fig.  9.  The  number  of  twists  in  the 
twines  is  shown  in  Table  XI.  Of  the  twists,  those  of  the 
strand  affect  the  abrasive  resistance  most,  as  they  seem 
to  play  the  greatest  role  in  increasing  the  rigidity  of  the 
twine.  Judging  from  the  results,  it  seems  that  hard  twisted 
twine  should  be  used  in  that  part  of  the  net  which  comes 
into  friction  with  hard  objects.  The  utmost  care  should, 
however,  be  taken  to  ensure  that  the  number  of  twists  in 
the  strand  should  not  be  too  much  to  the  detriment  of 
tensile  strength  for  the  sake  of  friction  resistance. 


TABLE  Xll 

Decreasing  Percentage  of  Strength  of  the  Twines  subjected  to  the  seawater  and  the  Sunlight 

(Unit    %) 


Kind 

of 

test 


Submerged 
in  the 
Sea 


Exposed 
to  the 
Sunlight 


Name 

of 

twines 

Amilan 
Kuralon 

(Manryo) 
Saran 
Teviron 
Cotton 

Amilan 
Kuralon 

( Manryo) 
Saran 
Teviron 
Cotton 

(ditched) 


Non-treated 

conk  except 

cotton 

8 
5 

3 

0 

100 

16 

24 

10 
6 
8 


Tarred  group  A 

a 

b 

c 

8 

5 

12 

26 

0 

31 

16 

6 

12 

14 

6 

19 

0 

0 

0 

21 

9 

20 

9 

7 

18 

10 

8 

12 

1! 

7 

19 

4 

10 

4 

Tarred  group  B 
a          b          c 


8 
17 

9 
17 


18 
17 

6 

13 

8 


15 
10 

4 
6 
0 


10 
30 

10 
16 


23 
27 

10 
18 


Refined  coal  tar  used  in  treating  samples  in  group  A  was  different  in  component  from 
the  one  used  for  group  B.  Both  were  used  with  or  without  other  ingredients  as  follows : 
Sample 'a'  was  treated  only  with  the  tar,  'b'  with  emulsion  of  alkaline-soap-water  (30%) 
and  coal  tar  (70°;,),  and  V  with  water  (30%)  and  coal  tar  (70%). 

TABLE  XIII 
Characteristics  of  Various  Kinds  of  Synthetic  Twine  Comparable  with  Cotton  Twine* 


Items 

Susceptibility  to 

dye  and  dye  fastness 
Head  resistance 
Rigidity  when  wet 
Elasticity 

Resistance  to  chemicals 
Shock  resistance 


Kuralon 
Amilan     (Manryo)    Saran  Krehalon  Teviron    Envilon    Cotton 


0 

I  2 

2 

i  2 


2 

!  1 

2 

0 

0 

+  1 


-1 
i  2 
-2 
•1-1 
-1 


1 

-{   1 

+  i 
o 


-  2 

1 

'  1 
-1 


1 

f-3 
-2 


0 
0 
0 
0 
0 
0 


Obtained  on  the  basis  of  cotton  twine  regarded  as  standard  (0). 


[26] 


JAPANESE    SYNTHETIC    TWINES 


AC  -  BD  In  Fig.  3{a) 

AB  -  CD  in  Pig.  3(«) 

AB  -  CD  In  Pig.  3  (b) 

AC  -  BD  in  Fig.  3  (b) 


Rural on   Tovlron    Saran    Krahnlon  Kyokurln  Cotton 


Fig.  7  A-B.     Relation  between  the  knot  strength  and  tyres  of  knot 

of  various  mines.  In  text  /4,  twines  consisting  of  6  yarns,  and  in  B 

12  yarns,  of  C2()-equi  relent  were  used.  The  pulling  direction  was  the, 

same  as  in  Jtg.  4. 

The  size  of*  net  twines  is  not  always  in  conformity 
even  when  they  have  the  same  number  of  yarns,  con- 
sequently, it  is  not  feasible  to  compare  the  abrasive 
resistance  of  one  type  of  fishing  net  with  another  that 
has  different  characteristics.  However,  for  general 
purposes,  comparison  between  various  net  twines  made 
of  C20-equivalent  yarns  is  given  in  fig.  10.  The  method 
and  the  device  employed  for  the  test  were  identical  with 
those  described  in  the  preceding  paragraph.  For  this 
test  the  twines  were  of  medium  twist,  having  the  same 
twist  range  as  given  in  Table  11. 

Twine  made  of  spun  yarns,  including  cotton  and  some 

synthetics,    have    a    comparatively    greater    resistance 

(figs.  9  and  10).  In  fig.  10  the  solid  lines  show  the  abra- 

v     Pouring  into  // 

r    K  ••] 


M  'JiV'i'ii  I  !''M  Pouring 


Draining  our 


AB 

c 

D 
EF 
G 


Teic  piece  H 

Oil  scon*  (Frictmg  object)  I 

Weight  j 

Reciprocating  direction  K 

Stopper  for  testpiece  L 


Counting  apparatus 

Crank 

Motor 

Water  tank  for  pouring 

Water  tank  for  draining 


Fig.  8.    A  device  used  for  testing  the  wear  resistance  of  twines. 
The  velocity  of  the  reciprocative  abrasion  is  80  per  minute  both 
back  and  forth. 


TABLE  XIV 

Contraction  in  Length  of  Netting  Twine  by  Heating  in  Water 

(Unit    %) 


Heating 
temperature 

Amilan 
Rural  on 

( Manryo) 
Saran 
Krehalon 
Teviron 

Cotton 


40  C        60°C        70 'C        80  C        90°C      1(XTC 


0 

5 

0 
0 

5 

(3) 
6 


0 

7 

6 

1 

15 
(8) 

7 


0 

8 

1 

3 

25 
(15) 

8 


1 
11 

3 

5 

30 
(20) 

8 


-1 
12 

5 

7 

45 
(30) 

8 


-1 
15 

6 
11 
56 
(35) 

9 


Numerals  in  parentheses  show  the  percentage  of  contraction 
the  length  of  twine  heated  before  test. 


sive  resistance  of  tarred  twine,  and  one  may  see  that  tar 
treatment  enhances  the  abrasive  resistance  of  net  twine. 
As  a  result  of  tarring  long  fibre  twines  resist  friction 
better  than  others  which  have  the  same  number  of 
C20-equivalent  yarn. 

With  resin  treatment  the  increase  in  abrasive  resistance 
depends  upon  the  thickness  of  the  coating.  A  3  to  5  per 


50,000  - 


10,000 


10 


•00   1.000 


3,000 


Hg.  9.  Friction  resistance  of  Amilan  netting  twines,  A.  B  and  C\ 
each  produced  by  different  makers  with  different  twists.  The 
test  temperature  was  kept  at  18 'Ci-  /"C.  The  notation  AH, 
for  instance,  indicates  the  twine  made  by  A  maker  with  hard 
twist.  See  Table  XI  for  further  information. 


[27] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


210D.   3/15 
20- •.  3/li 

1/9  (210D  of  AmUM  *  2) 
120D  of  9« 

.  s/u 

1200x9.  J/U 
ISODi  2.  3/1B 
20' •.  3/18 


i  may  double  or  quadruple  the  strength, 
mid  be  washed  off  during  fishing  operati 


But, 


Fig.  10.  Relation  between  the  friction  resistance  of  netting  twines 
and  the  fricative  load  increased  at  the  place  D  of  fig.  8.      The 
test  temperature  was  the  same  as  in  fig.  9. 


cent,  adhesion : 

as  the  resin  would  be  washed  oft  during  fishing  operations 
a  more  practical  method  of  fixing  it  must  be  developed 
as  a  counter  measure  to  dilution. 

(2)  Interfriction  between  twines:  The  results  obtained 
by  Miyamoto  and  Mori15  may  be  summarized  as  follows: 
When  synthetic  fibre  twines  are  rubbed  against  another, 
abrasion  between  hard  quality  twines  results  in  quick 
snapping;  when  a  hard  twine  is  rubbed  against  a  soft 
one  the  latter  breaks  first;  if  both  are  soft  the  abrasive 
resistance  of  both  seems  to  be  increased.  Hardness  has 
a  big  influence  on  the  abrasive  strength.  For  example, 
when  cotton  twine  is  rubbed  against  Kuralon  twine  it 
needs  260  rubbings  to  snap  the  latter,  yet  Kuralon  can 
withstand  up  to  530  rubbings  against  the  cotton.  When 
Kuralon  is  rubbed  against  Saran  it  snaps  after  only  60 
rubbings  and  the  same  applies  when  cotton  is  rubbed 
against  Teviron  and  Kyokurin.  Between  twines  of  the 
same  kind,  the  number  of  rubs  needed  to  break  them  is 
mostly  as  many  as,  but  sometimes  a  little  more  than, 
the  number  required  to  break  cotton. 

8.    Other  Characteristics 

(1)  Knot  fastness:  With  natural  fibre  twines,  knot 
slippage  hardly  constitutes  a  problem  as  compared  to 
synthetic  fibre  twines.  In  Japan,  the  synthetic  twines 
which  lend  themselves  best  to  knotting  are  Kuralon, 
Teviron,  Envilon,  Krehalon,  Saran,  Kuralon  No.  5, 
Kyokurin  and  Amilan  in  the  order  named.  For  fixing 
the  knots,  various  types  of  knots  as  shown  in  fig.  4  are 


TABLE  XV.     Breaking  Strength  of  Various  Kinds  of  Ropes  in  Dry  State 


Thickness 

Manila  hemp 

Kuralon 

Kuralon  No.  5 

Diameter    Circum 

-        (/IS) 

Amilan 

(Manryo) 

(Manryo  No.  5)        Saran 

Krehalon               Envilon                 Teviron 

mm 

inch 

inch 

w 

BS 

W 

BS 

W 

BS 

H 

BS 

W 

BS 

W 

BS            W        BS            W        BS 

3 

__  ' 

™ 

2-74 

0  13 

2-54 

0-085 

__ 

_ 

_1!" 

___.—__ 

4 

i 

5-2 

0-13 

4-4 

0-210 

4-24 

0  145 



-_. 

0-130 

—                         55     0  130 

5 

i 



6-6 

0-310 

6-8 

0-235 

„ 

_ 





.___ 

—                         -             8-6    0  195 

6 

* 

1 

117 

0-27 

10  0 

0-470 

10-2 

0-335 

10-5 

0-3S7 

14 

0-250 

14 

0240       10740270       11-2     0-270 

7 

__ 

— 

13-2 

0-630 

13-6 

0-455 

14  4 

0  490 

18 

0  325 

18 

0-310       15  34  0-350       15-7    0  360 

8 

A 

10 

20  7 

0  46 

16  6 

0-790 

17-0 

0-580 

19-2 

0-653 

24 

0-430 

24 

0-410       19  94  0  450       19  8     0  450 

9 

i 

1* 

26  2 

0  57 

21-6 

1-030 

21-6 

0-730 

24-0 

0  816 

30 

0  540 

30 

0  510       26-08  0-590       25-1     0-570 

10 

tt 

U 

32-4 

0  70 

26-4 

1-270 

25-6 

0-920 

30-0 

0-970 

36 

0-640 

36 

0610       33-8     0720       31   4     0-715 

11 

— 

31-4 

1-500 

— 

— 

36-0 

1    160 

— 

— 

.- 

—           39-8     0-850        — 

12 

ti 

U 

46-6 

0  98 

39-6 

1-900 

39-2 

1-360 

42-0 

1-380 

54        0-970 

54 

0-930      46-0     1  000       440     1-000 

14 

* 

l* 

63-4 

1-30 

52-8 

2-530 

51  2 

1-850 

57-0 

1-870 

72 

1-300 

72 

1-240       61-4     1-350       62-2     1-380 

16 

ft 

2-0 

82-9 

1-67 

66-0 

3-170 

64-8 

2-400 

66-0 

2-460 

96 

1-730 

96 

1-650       82-8     1-830       80-5     1-830 

18 

tt 

2i 

105  0 

2-08 

87-8 

4-230 

85-4 

2-850 

92-0 

3-130 

120 

2-160 

120 

2  100     104-8     2  300     102-5     2-380 

20 

« 

2* 

129-0 

2-53 

109-8 

5-290 

108-2 

3-520 

116-0 

3-800 

150 

2-700 

150 

2-570     128-8     2-850     125-5     2-860 

22 

* 

21 

157-0 

3-02 

131   8 

6-350 

131-0 

4-260 

140  0 

4  600 

180 

3-240 

180 

3-100     156-4     3-480     151-0     3-430 

24 

tt 

3-0 

186-0 

3-55 

151-4 

7-300 

157-2 

5  080 

162-0 

5-510 

216 

3-890 

216 

3-700     183  6    4000     175         4-000 

26 

1A 

3* 

219-0 

4-12 

177-8 

8-570 

184-4 

5-960 

192-0 

6-530 

252 

4-540 

252 

4-350     214-2     4-750     207         4-720 

28 

U 

3* 

254-0 

4-73 

210-8 

10-160 

211-8 

7-000 

222-0 

7-560 

294 

5-300 

294 

5-100     2440     5-000     245         5-570 

30 

1* 

31 

291-0 

5-38 

237-2 

11-430 

247-6 

7-940 

258-0 

8-780 

336 

6  050 

336 

5-800     287-6     6-450     277         6-280 

32 

U 

4-0 

331-0 

6  06 

270-6 

13  040 

281-8 

9-030 

294-0 

10-000 

3-242     7-300 

34 

1 

4* 

374-0 

6-79 

298  0 

14-390 

315-6 

10-000 

330-0 

11-300 

36 

1A 

4* 

419-0 

7-55 

336  0 

16-190 

372-0 

12-600 

38 

U 

4| 

467  0 

8-34 

374-0 

17-800 

40 

i» 

5-0 

518-0 

9  18 

410-0 

19-800 

42 

if* 

5i 

571  0 

10-05 

458-0 

22  000 

45 

1H 

58 

655-0 

11-43 

522-0 

25-200 

JIS 

Japanese  Industrial 

Standard. 

50 

2-0 

6i 

809-0 

13-90 

644-0 

31-000 

W 

Weight 

in  pounds  per  200  metres. 

55 

2ft 

6* 

979-0 

16-60 

784-0 

37-800 

BS 

Breaking  strength  in  metric  tons 

60 

2* 

7| 

1165-0 

19-52 

932-0 

45-000 

65 

2* 

8-0 

1367-0 

22-65 

1092-0 

52-500 

[28] 


JAPANESE    SYNTHETIC    TWINES 


used  by  Japanese  fishermen.  Most  synthetic  fibre  nets 
except  Kuralon  and  Mulon  need  heat  treatment  to  fix  the 
knots. 

Heating  with  hot  water,  gas,  or  air,  is  widely  applied 
to  synthetic  fibre  nets  for  knot  setting,  as  the  treatment 
can  reduce  the  amount  of  complicated  knotting,  which 
means  that  the  finished  product  weighs  less.  Amilan 
nets  may  be  heated  at  the  temperature  100  deg.  C.  or 
higher,  while  the  others,  except  Kuralon,  can  be  treated 
at  lower  than  72  deg.  C. 

(2)  Resistance  to  sunlight  and  seawater:  Table  XII 
shows  the  results  obtained  from  one  group  of  tarred  net 
twines  immersed  in  the  sea  and  another  exposed  to  the 
sunlight,  both  for  one  year.  From  the  table  it  appears 
best  to  use  coal  tar  with  low  isolated  acid  content  or  to 
neutralize  the  tar  with  alkali  before  use16. 

(3)  Characteristics  of  ropes:  For  comparison,  the  break- 
ing strain  of  manila  and  synthetic  ropes  is  reproduced  in 
Table  XV   by  courtesy  of  the  Tokyo   Seiko  Kaisha, 
Limited. 

REFERENCES 

1  Shimozaki,   Y.:   Unpublished. 

2  Tauti,  M.:  Studies  of  Netting  Cords— IV.     Strength  of  Cord 

in   Relation  to  the  Twist.     Vol.  25,  No.  2.    Jour.  Imp. 
Fish.  Inst.  pp.  31-41,  (1929). 

8  Shimozaki,  Y.  and  K.  Mori:  Studies  on  the  Breaking  Strength 
of  Netting  Cords  Made  with  Various  Twisting  Machines— I. 
Bull.  Tokai  Reg.  Fish.  Res.  Lab.,  No.  13,  pp.  61-72  (1956). 


Kondo,  J.:  Influence  of  the  Difference  in  Size  of  Cotton  Yarns 

Composing  the  Netting  Cord  on  the  Breaking  Strength  and 

the  Flexibility  of  the  Cord.    Bull.  Japan.  Soc.  Sci.  Fish. 

Vol.  4,  No.  5. 
Koizuma,  T.  :  The  Tensile  Strength  of  Saran  Netting  Cord  of 

Various  Twist.  Bull.  Japan.  Soc.  Sci.  Fish.  Vol.  20,  No.  7 

pp.  569-570  (1954). 
The  Tensile  Strength  of  Amilan  Netting  Cord  of  Various 

Twist,  Ibid.  Vol.  21,  No.  3,  pp.  139-140  (1955). 
Shimozaki,    Y.     and    K.    Mori  :    Studies    on    the    Breaking 

Strength  of  Netting  Cords  Made  with  Various  Twisting 

Machines— I;  Bull.  Tokai  Reg.  Fish,  Res.  Lab.,  No.  13, 

pp.  68-70,  (1956). 
Shimozaki,    Y.  :   Synthetic    Fishing    Nets    and    Ropes.    Fish. 

Tochnol.  Scries.  No.  3,  pp.  68-69,  Assoc.  Fish.  Material 

(1951). 

Unpublished. 

On  the  Change  in  Strength  of  Netting  Cords  Immersed 

in  the  Sea— I.  Relation  between  the  Strength  of  Synthetic 
Netting  Cords  and  the  Temperature,  and  Comparison  of 
Power  of  Various  Kind  of  Cords  Changing  when  Immersed 
in  the  Sea.  Bull.  Japan.  Soc.  Sci.  Fish.  Vol.  20,  No.  5  (1954). 

Honda,  K.:   Influence  on  the  Length  of  Test  Piece  on  the 

Breaking  Strength  and  Elongation  of  Netting  Cord.  Bull. 

Japan.  Soc.  Sci.  Fish.,  Vol.  22,  No.  6  (1956). 
Shimozaki,    Y.:    Synthetic    Fishing    Net    and    Ropes.    Fish. 

Technol.  Series.  No.  3,  pp.  77-79,  Assoc.  Fish.  Material 

(1957) 

Ibid,  pp.  79-81. 

Unpublished.  Lectures  delivered  at  Japan  Soc.  Sci.  Fish. 

April  (1956)  and  (1957). 

Miyamoto,  H.  and  K.  Mori:  How  Netting  Threads  Wear 
Down  Due  to  Interdiction.  "TFICH"  No.  12,  Nippon 
Set  Net  Fish.  Soc.  pp.  9-12  (1957). 

Shimozaki,  Y.  :  Synthetic  Fishing  Nets  and  Ropes.  Fish. 
Technol.  Series.  No.  3,  pp.  94-95  (1957). 


Although  synthetic  fibres  are  being  used  in  ever  increasing  quantities  even  in  those  countries  where  the  fishing  industry  is  not  highly  developed 
technically,  the  majority  of  fishing  nets  are  still  being  made  of  natural  fibres.  The  woman  in  the  photo  is  spinning  a  yarn  from  sun-hemp  in 

Ceylon.  Photo    FAO. 

[29] 


NYLON   IN   FISHING   NETS 

by 
J.  E.  LONSDALE 

British  Nylon  Spinners  Ltd.,  Pontypool,  Monmouthshire,  Great  Britain 

Abstract 

In  this  paper,  the  author  gives  an  outline  of  the  desired  characteristics  of  the  ideal  fibre  to  be  used  for  fishing  nets,  and  then  compares 
this  specification  with  the  properties  of  nylon  66.  The  physical  properties  of  this  material  are  discussed  in  detail — yarn  strength,  twine  strength, 
knot  strength  and  the  comparison  between  the  strengths  of  wet  and  dry  twines  — and  in  general,  nylon  nets  are  able  to  withstand  attacks  by 
chemicals,  oils,  insects,  vermin,  bacteria  and  moulds,  and  their  ability  to  stand  up  to  considerable  flexing  and  abrasion  is  very  good.  Since 
the  nets  do  not  rot,  it  is  not  necessary  to  dry  them  in  the  sun,  and  in  certain  African  fisheries  it  is  the  habit  to  store  gillnets  in  the  water  when 
they  are  not  being  fished. 


Resume 


Le  nylon  dans  les  filets  de  peche 


L'auteur  expose  dans  cette  etude  les  caracteristiqucs  de  la  fibre  idcule  destmee  a  la  confection  des  filets  de  peche,  et  les  compare 
avec  les  proprietes  du  nylon  66.  II  fait  un  examen  approfondi  des  proprietes  physiques  de  ce  materiau:  resistance  des  files,  des  fils,  des 
noeuds,  et  resistance  comparee  des  fils  sees  et  mouilles.  En  general,  les  filets  de  nylon  sont  insensibles  aux  produits  chimiques,  aux  huiles, 
aux  inscctcs,  £  la  vermine,  aux  bacteries  et  aux  moisissures.  et  la  resistance  A  la  flexion  et  &  Pabrasion  est  excellence.  Comme  les  filets  ne 
pourrissent  pas,  il  n'est  pas  neccssaire  dc  les  fairc  secher  au  soleil,  et  dans  certaines  entreprises  africaines  de  peche  on  a  coutumc  dc  conserver 
les  filets  maillants  dans  1'eau  quand  ils  ne  sont  pas  utilises  pour  la  peche. 

El  nylon  en  las  redes  de  pesca 
Extracto 

En  este  trabajo  el  autor  hace  una  rcsena  de  las  caracteristicas  que  debe  tener  una  fihru  ideal  para  red  de  pesca  y  las  cornpara  con  las 
propiedades  del  nylon  66.  Tambien  analiza,  en  detalle,  las  propiedadcs  de  cste  material—  rcsistencia  de  las  fibras,  hilos  y  nudos,  com  panic  ion 
de  la  resistcncia  de  los  hilos  humedos  y  secos — y  expresa  que,  generalmente,  las  redes  de  dicha  fibra  resisten  cl  ataque  dc  compucstos  quimicos, 
aceites  minerales,  insectos,  gusanos,  bactcrias  y  mohos.  Ademas,  dicha  fibra  ofrece  bastante  resistencia  a  la  flexion  y  al  desgaste  causado  por 
el  roce. 

Como  las  redes  de  ny!6n  no  se  pudren,  es  innecesario  secarlas  al  sol,  y  en  algunas  pesquerias  africanas  existe  la  costumbre  de 
mantcner  los  artes  de  enmalle  en  el  agua  cuando  no  se  usan. 


SPECIFICATION  FOR  IDEAL  FISH  NET  FIBRE 

(1)  The  basic  cost  should  be  low. 

(2)  Processing  into  net  form  should  be  cheap,  easy  and 
efficient.     For  example,  shrinkage  on  setting  should 
be  low. 

(3)  In  both  wet  and  dry  states  it  should  have  high 
strength,  by  which  is  meant: 

(a)  high  tensile  strength; 

(b)  good  ability  to  withstand  repeated  shocks; 

(c)  good  flex  strength  or  fatigue  resistance; 

(d)  high  knotting  efficiency  and 

(e)  good  resistance  to  abrasion 

which  leads  to  fine,  long-lasting  nets,  capable  of 
holding  large  catches. 

(4)  It  should  maintain  its  strength  in  use. 

(5)  It   should   have   good   dimensional    stability  and 
should  not  distort  in  size  or  shape  during  use. 

(6)  It  should  have  low  moisture  absorption  so  that  the 
increase  in  weight  is  small  when  a  net  becomes  wet 
and  handling  is  consequently  easier. 

(7)  The  fibre  should  have  a  low  specific  gravity,  since 


this  allows  a  greater  length  of  netting  for  a  given 
weight  of  yarn,  and  may  permit  lighter  fittings  and 
savings  in  power  and  manhandling.  On  the  other 
hand,  a  low  specific  gravity  may  not  be  desirable 
where  quick  sinking  of  the  net  is  required. 

(8)  It  should  be  resistant  to  damage  and  attack  by 
chemicals,  oils,  moulds,  bacteria,  insects  and  vermin 
in  order  that  treatments  and  routine  maintenance 
can  be  kept  to  a  minimum. 

(9)  The  performance  of  the  fibre  should  remain  constant 
at  extremes  of  temperature. 


TABLE   I 
Specific   Gravity   of  Fibres 


Nylon  66  and  nylon  6 

Polyvinyl  alcohol 

Polyester  fibre     . 

Hemp     . 

Flax 

Cotton 

Polyvinylidene  chloride 


14 
30 
38 
•48 
•50 
•52 
•72 


[30] 


NYLON    IN     FISHING    NETS 


(10)  The  net  should  hold  the  fish  firmly  when  caught,  yet 
not  damage  them. 

(11)  Some  types  of  fishing  nets  may  demand  other 
requirements,  such  as  translucency  for  gill  nets    or 
a  lower  initial  elastic  modulus  as  is  required  for 
salmon  gillnets. 

The  initial  modulus  (calculated  by  measuring  the 
load  to  produce  a  5  per  cent,  extension  under  con- 
ditions of  70  deg.  :i-  4  deg.  F.  and  67  ±  2  per  cent. 
R.H.)  for  nylon  yarns  lies  in  the  range  20  to  40  grams/ 
denier,  with  nylon  66  high  tenacity  yarns  at  the  top 
end  of  the  bracket,  while,  for  comparison,  the  initial 
modulus  of  high  tenacity  polyester  fibre  lies  in  the 
range  90  to  100  grams/denier. 

The  basis  of  the  specification  can  be  extended  to 
methods  of  fishing  which  do  not  employ  nets.  It 
could  include  the  principles  involved  in  the  choice 
of  fibre  for  long-lines  or  whaling  foregoers,  etc. 
There  is  now  a  wide  range  of  fibres  available  to 
satisfy  the  diverse  needs  of  the  fishing  industry. 
None  meets,  nor  could  meet,  the  specification 
completely. 

Even  when  one  fibre  is  isolated  and  examined 
alone,  it  can  be  seen  that  there  is  ample  variety  of 
choice.  Thus,  nylon  can  be  divided  into  classes 
according  to  the  type  of  polymer  used,  and  whether 
the  yarn  is  assembled  from  continuous  filament 


or  spun  fibre;  within  each  of  these  classes  there  is 
a  range  of  yarns  with  varying  properties. 

The  task  facing  a  supplier  of  continuous  filament 
nylon  for  the  production  of  fishing  nets  is  to  provide 
a  yarn  which  measures  well  in  comparison  with  the 
specification.  Some  of  the  conditions  are  met 
automatically  (e.g.  low  specific  gravity,  rot  resist- 
ance, high  strength,  good  stability  and  small 
temperature  influence),  while  others  have  to  be  met 
as  nearly  as  possible.  One  of  the  chief  ways  in  which 
a  nylon  yarn  can  be  upgraded  is  by  increasing  its 
strength. 

YARN  STRENGTH 

The  dry  tensile  strength  of  all  nylon  yarns— 66  and  6 — 
lies  in  the  range  4-  5  to  9-0  grams/denier.  The  yarns  at  the 
top  end  of  the  bracket  are  stronger  than  any  other 
commercially  available  fibre.  The  equivalent  range  of 
extension  at  break  is  25  to  12  per  cent.  Generally  speaking 
a  higher  tenacity  implies  a  lower  extension  at  break. 

Fig.  1  shows  typical  load-extension  curves  for  three 
yarns  of  B.N.S.  nylon  66  (measured  under  a  rate  of  load 
application  of  0-5  grams/den ier/sec.),  and  illustrates  the 
different  forms  these  may  take.  The  energy  absorption  of 
each,  as  measured  by  the  area  under  the  curve,  is  shown 
in  Table  II,  together  with  values  for  tenacity  and  extension 
at  break. 


TABLt  II 

Load/Extension  Properties  of  Nylon  66  Yams 


Tvpt     500 


-  NM.OM    T>P£    IOO 


Fig.  L   Load/ Extension  Curves  of  BNS  nylon  66  yarns. 


Yarn  Reference 


B.N.S.  nylon  205  denier 

type  IOO 
B.N.S.  nylon  210  denier 

type  300 
B.N.S.  nylon  840  denier 

type  600 


Tenacity 
(gramsldeni€'r) 


4-5 

7-4 
8-8 


Extension        Energy 
at  (break      Absorption 
(%)       (inch  Ibs./inch) 


22 
15 
13 


0*27 
0-31 
1-11 


It  will  be  seen  that  the  energy  absorption  of  210  denier 
type  300  on  a  weight  for  weight  basis  is  higher  than  the 
other  two  yarns.  This  very  high  energy  absorption  has 
been  a  factor  in  its  ready  acceptance  for  fishing  nets. 

TWINE  STRENGTH 

As  in  the  case  of  all  fibres,  nylon  twines  are  made  with 
twist  in  order  to  bind  the  yarns  together.  The  degree  of 
twist  and  the  construction  control  the  feel  of  the  twine. 
Increasing  the  degree  of  twist  in  a  nylon  yarn  lowers  the 
tensile  strength.  The  ratio  of  twine  strength  to  aggregate 
yarn  strength  is  known  as  the  doubling  efficiency.  Most 
nylon  twines  are  made  with  a  doubling  efficiency  of  95 
per  cent,  or  more.  This  can  be  affected,  not  only  by 
twist,  but  also  by  conditions  of  setting  and  heat  stretching. 
Twine  strength  is  commonly  expressed  in  Ibs.  or  other 
units  of  weight.  Another  method  is  to  refer  to  specific 
strength  (or  breaking  length),  this  being  defined  as  the 
greatest  length  of  twine  which  can  be  supported  by  a 


[31  ] 


MODERN     FISHING    GEAR     OF    THE     WORLD 


TABLE  III 
Average  Specific  Strength  of  Fibres1 


Nylon  66 
Nylon  6 
Polyester  fibre 
Linen 


Twine 


Drv 


Mesh 
Single  Knot        Double  Knot 


Wet       Dry       Wet        Dry       Wet 


(yd.)  (yd.)  (yd.)  (yd.)  (yd.)  (yd). 

63700  56000  30800  28100  34300  29900 

39100  35200  24400  22000  26600  23100 

48600  50200  20000  20700  24100  23500 

44000  58800  19900  31600 


The  investigation  was  continued  by  studying  the  effect 
of  change  of  molecular  orientation  on  shear,  torsion  and 
compression.  No  appreciable  effect  on  shear  and  torsion 
could  be  detected,  but  compressive  force,  as  represented 
by  loop  tenacity,  described  a  parabola.  From  these 
facts,  it  was  deduced  that  compressive  and  tensile  forces 
act  in  opposition  as  orientation  is  changed  (fig.  3). 

The  effect  of  compressive  forces  was  subsequently 
shown  experimentally  by  knotting  unstretched  polythene 
and  examining  it  visually,  and  subjecting  broken  nylon 
knots  to  cross-polarised  light.  Tests  on  various  knots 


single   piece  of  that  twine  without   breaking  it.   The 
supporting  piece  may  be  dry,  wet  or  knotted. 

Specific  strength  (or  breaking  length)  (in   yards) 
Twine  strength  (Ibs)  ,-   twine  weight  (yds./lb.) 
This   unit   is  of  value   in   comparing  different   fibres. 
Results  on  nets  tested  in  Canada  appear  in  Table  III, 
which  lists  the  specific  strength  of  dry  and  wet  twine  and 
knots.  Nylon  66  will  be  similar  to  type  300. 


o 


O 


KNOT  STRENGTH 

When  a  knot  is  tied  in  a  twine,  it  constitutes  a  place  of 
weakness,  and  reduces  the  effective  strength  of  the  twine. 
The  term  "knotting  efficiency"  can  be  used  to  describe 
the  ratio  of  dry  knot  strength  to  dry  twine  strength  (or 
alternatively  wet  knot  to  wet  twine  strength).  The  knot- 
ting efficiency,  measured  either  way,  of  nets  made  from 
nylon  66  yarn  is  of  the  order  of  40  to  50  per  cent,  for 
single  knots  and  50  to  60  per  cent,  for  double  knots. 

An  investigation2  has  been  carried  out  into  the  factors 
influencing  the  loss  of  strength  on  knotting.  This  has 
resulted  in  a  clearer  appreciation  of  the  mechanism  of 
knotting  and  the  effect  of  yarn  properties  on  knotting 
efficiency. 

Practical  tests  have  shown  that  every  type  of  knot  has 
a  different  knotting  efficiency,  and  that  their  order  of 
efficiency  approximates  to  the  same  for  all  fibres  examined 
(nylon  66,  nylon  6,  polyester  fibre,  and  polyvinylidene 
chloride).  The  differing  configurations  of  various  knots  is 
responsible  for  the  variations;  it  has  been  shown  that  a 
decrease  in  the  angle  through  which  the  loop  is  formed 
decreased  the  loop  strength;  increasing  the  number  of 
loops  or  hitches  in  a  knot  increased  the  knot  strength. 
The  effect  of  molecular  orientation  on  knotting  efficiency 
was  investigated  to  study  the  nature  of  the  rupturing 
forces  more  closely.  Nylon  66  yarn  of  the  same  nominal 
denier  and  number  of  filaments  was  used  to  tie  three 
types  of  knots  —  the  overhand,  warpers  and  double 
weavers.  It  was  shown  that  with  increasing  molecular 
orientation  the  yarn  tenacity  increased  linearly,  knotting 
efficiency  decreased  linearly,  whilst  knot  strength  assumed 
a  parabolic  curve  (fig.  2).  The  shape  of  the  latter  could 
be  confirmed  by  calculation  from  the  two  straight  line 
relationships.  The  degree  of  orientation  of  the  yarn 
exhibiting  maximum  knot  strength  is  of  obvious  practical 
importance.  The  manufacturer  of  nylon  yarn,  having  the 
opportunity  to  tailor-make  his  fibre,  should  attach 
proper  importance  to  this  effect. 


MOL.ECAJL.AC 


u 
z 

UJ 

y 
C 

Lu 
UJ 

O 

z 


MOLECLUL.AQ 


t 
U 


7. 

ot 


MOLECUI.AQ 
Fig.  2.    Individual  Effect  of  Molecular  Orientation. 


[32] 


NYLON       IN     FISHING     NETS 


M  oc  e  w_  in.  AW       Oft  i  e.  r«j  -r  >»»  T  IOINI 
•/#.  .?.     Combined  Effect  of  Moleculw 


showed  thai  the  break  always  occurred  under  the  looped 
section.  In  symmetrical  knots,  such  as  the  Blood,  two 
positions  of  stress  concentration  corresponding  to  the 
two  looped  sections  were  observed. 

WETTING 

When  nylon  twines  are  wetted  lhe>  lose  strength  but 
gain  in  extensibility.  The  loss  in  strength  (of  both  type 
66  and  6)  whether  in  yarn,  twine  or  net  form  is  of  the 
order  of  10  to  15  per  cent.  The  increase  in  extension  at 
break  of  twines  made  from  nylon  66  is  of  the  order  of 
15  to  20  per  cent,  according  lo  size  and  construction.  The 
importance  of  this  effect  is  to  counterbalance  the  loss  of 
tensile  strength  when  assessing  the  change  in  energy 
absorption.  It  is  this  latter  property  which  is  of  critical 
importance  in  deciding  whether  a  net  will  break  or  not 
under  dynamic  conditions.  Nylon's  present  place  in  fish- 
ing nets  is  largely  due  to  its  comparatively  high  energy 
absorption  under  dry  and,  even  more  so,  wet  conditions. 
A  typical  3/3/210  denier  nylon  66  twine  had  the  pro- 
perties shown  in  Table  IV.  By  measuring  the  area  under 


JABLL   IV 

Comparison  of  Dry  and  Wet  Twine  Properties  for  a  Typical  3/3/210 
Denier  B.M.S.  Nylon  66  Twine 


Breaking  load 
Extension  at  break 
hnergy  absorption 


Dry 


32-4lbs. 
21 -0°n 
3-36in.lb./in 


Wet 


28-6lbs. 


Change  on 
Wetting 


3-96  in.lb./m. 


« 
18% 


the  load/extension  curves,  dry  and  wet,  it  was  found  that 
the  energy  absorption  actually  increased  when  wet  by 
18  per  cent. 

IN  USE 

Nylon  nets,  in  common  with  those  made  from  all  other 
fibres,  slowly  Jose  strength  in  use.  In  certain  African 
fisheries  it  is  now  the  habit  to  store  gillnets  in  the  water 
when  they  are  not  being  fished. 

The  abrasion  resistance,  dry  and  wet,  of  nylon  66  is 
extremely  good.  This  can  be  varied  by  choice  of  filament 
denier.  All  nylon  fishing  twines  are  based  on  a  filament 
denier  of  about  6  (as  opposed  to  1  to  3  for  most  apparel 
uses),  thus  providing  a  happy  compromise  between 
good  abrasion  resistance  and  flexibility  (which  decreases 
with  increased  filament  denier). 

ACKNOWLEDGMENTS 

The  author  wishes  to  thank  the  Directors  of  British 
Nylon  Spinners  Limited,  Pontypool,  Monmouthshire, 
Great  Britain,  for  permission  to  present  this  paper,  and 
his  colleagues  for  help  given  in  its  preparation. 

REFERENCES. 

J*The  Selection  and  Care  of  Nylon  Gillnets  for  Salmon." 
P.  J.  G.  Ca  rot  hers.  Fisheries  Research  Board  of  Canada 

-"Knots,  Their  Effects,  and  the  Forces  Involved/'  R.  J.  Harrison, 
Associateship  Thesis,  Royal  College  of  Science  and  Tech- 
nology, Glasgow,  June 


Menhaden  purse  seine  dories  with  the  catch  "dried  up"  in  the  hunt. 

[33] 


Photo  FAO. 


THE  TECHNOLOGICAL   CHARACTERISTICS   OF   PERLON   FOR 

FISHING  EQUIPMENT 

by 

PERLON-WARENZEICHENVERBAND  F.V. 
Frankfurt  Am  Main,  Westendstrasse  41 

Abstract 

Perlon  is  a  synthetic  fibre  which,  like  nylon,  is  one  of  the  polyamides.  It  is  made  as  Monofilament,  Continuous  Filament  and 
Staple  Fibre  and  this  comprehensive  paper  deals  with  its  chemical  and  physical  properties  and  discusses  its  suitability  for  trawls,  seines, 
setnets,  whaling  ropes  and  cordage,  also  for  tarpaulins  and  protective  coverings. 


Resume 


caractfrristiques  technologiques  de  Perlon  pour  le  material  de  pcche 


Comme  nylon,  Perlon  est  unc  fibre  synthetique  du  groupe  des  polyamides.  On  le  fabrique  en  monofilament,  fibres  continues 
et  fibres  filees  et  cette  communication  detaillee  traite  de  ses  proprietds  chimiques  et  physiques  et  examine  sfil  convient  pour  la  confection  de 
chaluts,  de  sennes,  dc  filets  fixes,  de  cables  pour  la  chassc  a  la  baleine  et  de  cordes.  et  uussi  pour  faire  des  baches  et  des  enveloppcs  pro- 
tec  trices. 

Caracteristicas  tecnoldgicas  del  perlon  para  equipo  de  pesca 
Kxtracto 

En  este  trabajo  se  estudian  las  propiedadcs  quimicas  y  fisicas  del  perlon  qtie,  como  el  ny!6n,  cs  una  poliamida,  y  se  analizan  sus 
condiciones  para  usarlo  en  la  fabricaci6n  de  redes  de  arrastre,  fijas  y  dc  cerco,  estachas  para  arpones,  cordeleria,  encerados  y  cubiertas 
protectoras.  Esta  fibra  sinletica  se  fabrica  en  forma  de  hilo  y  filamento  continuos  o  como  hilado. 


I.    WHAT  IS  PERLON  ? 

PERLON,*  Jike  nylon,  is  a  synthetic  fibre  which 
belongs  to  the  polyamides'  group.  Like  all  fibre- 
forming  materials,  it  consists  of  large  elongated 
molecules.  It  results  from  the  assembly  of  a  large  number 
of  molecules  of  a  homogeneous  material,  caprolactam. 

Perlon: 

<CH,)S 


n.  f-  ..HN(CH2)6COHN(CH2)5CO.. 


NHOC 

E — Aminocaprolactam 

Both  fibres  are  manufactured  in  the  United  States, 
where  the  fibre  developed  by  Du  Pont  is  called  Nylon  66, 
as  opposed  to  Nylon  6  for  Perlon,  a  fibre  developed  by 
the  former  IG-Farbenindustrie  combine  in  Germany. 
In  their  technological  properties  Perlon  and  nylon  are 
largely  identical,  except  for  the  melting  point  which,  in 
the  case  of  nylon,  is  approximately  30  deg.  C.  higher. 

Both  fibres  are  manufactured  by  the  extrusion  process. 

*  The  name  Perlon  and  its  emblem  are  registered  trademarks  of 
the  PERLON-Waren/eichenverband  e.V.  (PERLON  Trade  Mark 
Association),  Frankfurt/Main,  Germany. 


As  a  result  the  fibres  are  cylindrical  and  have  a  smooth 
featureless  surface. 

By  adding  titanium  dioxide  or  other  suitable  substances 
during  the  polymerization  process,  a  matt  fibre  can  be 
extruded.  This  does  not  affect  the  physical  properties  of 
the  fibre  except  resistance  to  sunlight  which  is  consider- 
ably reduced.  Matt  fibres  should  not  be  chosen  for  the 
manufacture  of  nets. 

II.    FORMS  OF  PERLON 

Perlon  is  available  in  3  basic  forms  for  the  fishing  industry: 
Monofil 

Continuous  Filament  and 
Staple  Fibre. 

(a)  Monofil 

Monofil  has  the  advantage  that  it  can  be  used  for  the 
manufacture  of  nets  without  having  to  undergo  further 
processing  (e.g.  twisting  or  plaiting). 

Monofils  are  continuous  transparent  wires,  with  a 
diameter  of  between  0- 1  and  2  mm.  (0-004  in.  to  0-08 
in.).  Table  I  gives  average  values  of  breaking  load  and 
length  per  unit  weight. 

(b)  Continuous  Filament 

Continuous  filament  has  greater  strength  and  less 
extensibility  than  staple  fibre  and  is  lighter  in  weight  than 


[34] 


CHARACTERISTICS    OF     PERLON 


TABLE 
Mono 

1 
fils 

Diameter 
In  mm. 

Breaking 
Load 
In  kg. 

Length  per 
Unit  Weight 
mlkg. 

Nm 
(Metric 
Number) 

0-10 

0-5 

app.  92,000 

92-0 

0-15 

1-1 

43,000 

43-0 

0-17 

32,200 

33-0 

0-20 

1-8 

25,000 

25-0 

0  25 

2-7 

15-16,000 

15-16-0 

0-30 

3-7 

10-12,000 

10-12-0 

0-35 

5-1 

8,600 

8-6 

0-40 

6-5 

6.600 

6-6 

0-45 

8-4 

5,300 

5-3 

0-50 

10-0 

4,300 

4-3 

0-55 

12-2 

3,500 

3-5 

0-60 

14-5 

3,000 

3-0 

0-65 

16-5 

2,600 

2-6 

0-70 

19-0 

2,200 

2-2 

0-75 

22-0 

,900 

•9 

0-80 

25-0 

,700 

•7 

0-85 

28-0 

,500 

•5 

0-90 

31-5 

,300 

•3 

0-95 

33-5 

,200 

•2 

•00 

36-0 

,100 

•1 

•10 

42-0 

900 

0-9 

•20 

50-0 

760 

0-76 

•30 

57-0 

650 

0-65 

•35 

600 

0-60 

•40 

66-0 

560 

0-j6 

•50 

76-0 

490 

0-49 

•60 

87-0 

430 

0-43 

•70 

98-0 

380 

0-38 

•80 

110-0 

340 

0-34 

•90 

125-0 

300 

0-30 

2-00 

140-0 

270 

0  27 

TABLE    111 
Perlon  Continuous  Filaments  in  Heavy  Deniers 


the  equivalent  cotton  product.  It  is  supplied  in  fine  and 
coarse  qualities. 


TABLE  II 
Perlon  Continuous  Filaments  in  Fine  Deniers 


Denier 

Metric 
Number 
Nm. 

Length  per 
Unit  Weight 
mlkg. 

Number  of 
Filaments 

210 

43-0 

42.860 

35 

630 

14-0 

14,290 

105 

750 

12-0 

12,000 

125 

840 

10-7 

10,710 

140 

Heavier  deniers  ("cables")  is  used  as  the  raw  material 
for  net  twines  subject  to  severe  stress,  and  for  the 
manufacture  of  ropes  (Table  HI). 

The  choice  of  6  and  20  filament  denier  allows  for 
different  degrees  of  stiffness  required,  the  stiffness  of 
Perlon  increasing  with  the  denier. 

Consequently  Perlon  made  up  to  20  denier  filaments 


Denier 

Metric 
Number 

Nm. 

Length  per 
Unit  1  Weight 
mlkg. 

f-'i  lament 
Denier 

Number  of 
Filaments 

1,050 

8-6 

8,570 

6 

175 

1,260 

7-1 

7,140 

6 

210 

2,500 

3-6 

3,600 

20 

125 

3,000 

3-0 

3,000 

6;  20 

500; 

150 

3,210 

2-8 

2,800 

20 

160 

4,500 

2-0 

2,000 

20 

225 

5,000 

1-8 

1,800 

20 

250 

6,200 

1-5 

1,500 

6 

1,035 

7,500 

1-2 

1,200 

6;  20 

1,250; 

375 

9,300 

1-0 

1,000 

6 

1,550 

10,000 

0-9 

900 

20 

500 

11,250 

0-8 

800 

6 

1,875 

12,500 

0-7 

700 

20 

625 

15,000 

0-6 

600 

6;   20 

2,600; 

750 

30,000 

03 

300 

20 

1,500 

should  be  used  where  the  end  product  is  subjected  to  a 
sustained  process  of  kneading  and  flexing. 

However,  where  a  highly  flexible  product  is  required 
6  denier  filaments  are  more  suitable. 

(c)    Staple  Fibre 

Spun  staple  fibre  twines  are  particularly  suitable  for 
inland  and  coastal  waters.  The  yarns  are  spun  like  cotton 
yarns  to  attain  the  highest  possible  degree  of  strength  or 
produced  from  a  spinning  tow  and  then  spun  into  yarn 
by  means  of  a  modified  schappe  spinning  process  after 
cutting. 

The  following  types  of  staple  fibre  are  available  in 
yarn  numbers  up  to  Nm.  50: 

Cotton  spinning  process: 

2  den.,  cut  length  60  mm. 
27  den.,  cut  length  60  mm. 

Schappe  spinning  process: 

2,  7  den.  average  length  of  staple  100  mm. 

The  optimum  tensile  strength  of  spun  staple  fibre 
yarn  should  be  a  breaking  length  of  30  km.  in  a  Nm. 
34/1  yarn. 

Where  net  yarns  must  be  stiffened  with  black  varnish, 
spun  staple  fibre  yarn  is  superior  to  continuous  filament 
yarn,  since  the  staple  fibre  yam  absorbs  the  stiffening 
preparation  more  thoroughly. 

III.  PROCESSING  INTO  NET  TWINES,  CORDAGE 
AND  ROPES 

(a)    Perlon  Monofils 

Table  IV  gives  the  appropriate  monofil  strength  for 
gillnets  to  replace  cotton  net  twines. 

For  mesh  sizes  in  excess  of  30  mm.  at  least  0-20  mm. 
diameter  should  be  chosen  even  for  the  finest  types  of 
nets. 

The  monofils  are  available  as  fishing  lines  in  diameters 
between  0-1  and  2  mm.  The  rope  making  industry  is 


[35] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


TABLE  IV 
Monoflls  suitable  for  Gillnets 


TABLE  VI 


Cotton  Net 

renon 

Length  pet- 

A//M 

twine 

Nm. 

Diameter 

Unit  Weight 

/  Tffl. 

(Metric  Number) 

270/6        15 

•0  or  0  20 

a  pp.     43,000  resp. 

43-0  resp.  25-0 

240/6 

25,000 

200/6 

0  20 

25,000 

25-0 

160/6 

0-20 

25,000 

25-0 

140/6 

0-20 

25,000 

25-0 

120/6 

0  25 

16,000 

16-0 

100/6 

0  25 

16,000 

16  0 

100/9 

0-25 

16,000 

16-0 

85/6 

0-30 

12,000 

12-0 

85/9 

0-30 

12,000 

12-0 

using  Perlon  monofils  as  raw  material  for  the  manu- 
facture of  braided  and  twisted  cordage. 


TABLF  V 

Comparison  in  length  per  unit  weight  of  coarser  monofils  with 
cotton  twine  of  equal  wet  strength 


Cotton  Net  twine 


Perlon  Monofils 


Cotton 

Length  per 

Metric  Number 

Unit  Weight 

Nm. 

mfkp. 

50/9 

5,000 

50/12 

3,600 

50/15 

2,900 

50/18 

2,400 

20/6 

2,900 

20/9 

1,950 

20/12 

1,400 

20/15 

IJOO 

20/18 

940 

20/21 

830 

20/24 

680 

Mean 

Diameter 

Length  pel- 

mm. 

Unit  Weight 

mlkg. 

0-35 

8,600 

0-40 

6,600 

0-45 

5,300 

0-50 

4,300 

0-45 

5,300 

0-50 

4,300 

0-60 

3,000 

0-70 

2,200 

0-70 

2,200 

0-70 

2,200 

0-80 

1,700 

Mff 

Length 

Length 

/Vt  1 

per  Unit 

Diameter 

Net  twine 

per  Unit 

Diameter 

t  wine 

Weight 

mm. 

den. 

Weight 

mm. 

iten. 

m.fkg. 

m/kg. 

210/2 

20,000 

0-28 

210/18 

2,100 

0-88 

210/3 

13,000 

0-30 

210/21 

1,850 

0-95 

210/6 

6,400 

0-48 

210/24 

1,600 

1-00 

210/9 

4,300 

0-60 

210/27 

1,450 

1-10 

210/12 

3,200 

0-70 

210/30 

1,320 

1-15 

210/15 

2,480 

0-80 

Continuous  filament  twines  are  quite  smooth  and 
much  less  dirt  adheres  to  nets  made  of  such  material  than 
to  nets  made  of  cotton  or  spun  staple  fibre  twines. 
Continuous  filament  or  Perlon  monofil  is  particularly 
suitable  for  heavily  polluted  waters.  Braided  or  twisted 
twines  of  840  denier,  1,059  denier,  1,260  denier,  and  3,000 
denier  are  especially  suitable  for  equipment  subject  to 
severe  strain  such  as  bottom  trawls.  Perlon  continuous 
filament  twisted  twines  are  considerably  stronger  than 
manila  twines.  This  makes  it  possible  to  manufacture 
lighter  weight  nets  to  reduce  the  towing  drag  and  allowing 
the  use  of  less  power. 


Manila-Extra 

Length  per      Breaking 
Unit  Weight  (kg.) 

mjkg.         dry 

186          120 
248  95 


375 


65 


TABLE  VI  1 

Twisted 

Twines 

Perlon  Continuous 

Filament 

'  Load 

Length  per 

Breaking  Load 

Unit  Weight 

(kg.) 

wet 

mlkg. 

dry 

wet 

127 

192 

260 

221 



250 

208 

177 

100 

256 

195 

165 

— 

312 

167 

142 

_ 

315 

173 

147 

68 

385 

130 

110 

— 

1,248 

43 

37 

(b)  Twines  of  Perlon  Continuous  Filament 

Twines  for  equipment  exposed  to  little  or  moderate 
strain   differ  from   twines  for  nets   subject   to   severe 
stresses. 
Net  twines  are  produced  from  the  following  materials: 

Denier  .  .  210  630  750  840 

Metric  Number 

Nm.  .  .  43-0  14-0  12-0  10-7 

Length  per  Unit  Weight 

m/kg.         .  .  42,860       14,290       12,000       10,710 

Table  VI  gives  examples  of  approximate  lengths  per 
unit  weight  and  diameters  of  net  twines  made  of  210 
denier  filaments. 

Only  the  finest  deniers  listed  in  Table  VI  are  suitable 
for  bottom-set  gill  nets;  the  others,  including  deniers 
heavier  than  those  listed  can  be  used  for  line  fishing, 
baskets,  trap  nets,  bagnets,  trawls,  seines,  purse  seines, 
etc. 

Given  an  equal  wet  knot  strength  the  weight  of  nets 
made  of  these  twines  are  less  than  that  of  the  equivalent 
cotton  product. 


Table  VIII  gives  examples  of  Perlon  continuous  filament 
braided  twines: 

TABLE  Vlll 
Perlon  Continuous  Filament  Braided  Twines 


Length  per 

Unit  Weight 

Breaking  Load 

mlkg. 

(kg.) 

_QPP- 

dry 

wet 

1,440 

40 

34 

1,270 

51 

43 

1,040 

55 

47 

840 

74 

62 

690 

83 

71 

650 

101 

81 

500 

110 

94 

450 

130 

110 

402 

151 

128 

350 

160 

136 

300 

190 

162 

265 

200 

170 

190 

280 

238 

36 


CHARACTERISTICS    OF    PERLON 


(c)    Cords  and  Ropes  of  Continuous  Filament 

Manila,  sisal,  hemp,  and  cotton  have  up  to  now  been 
used  as  raw  materials  for  the  cords  and  ropes  used  in  the 
fishing  industry. 

Cords  and  ropes  of  natural  fibres  shrink  and  swell 
which  renders  wet  ropes  hard  and  stiff.  Natural  fibres 
rot  in  the  water.  Coarser  qualities  dry  very  slowly  and 
when  stored  are  destroyed  by  mould.  Synthetic  fibre 
ropes  should  therefore  be  used  with  synthetic  fibre  nets. 

Allowance  must  be  made  for  the  extensibility  of  Perlon 
which  is  many  times  that  of  the  natural  fibres.  To  prevent 
the  ropes  from  untwisting  or  developing  kinks,  it  is 
necessary  to  adjust  the  angle  and  direction  of  the  twist 
so  that  the  individual  yarn  constructions  reinforce  each 
other  within  the  body  of  the  rope. 

Cords  and  ropes  are  made  of  Perlon  continuous 
filament  yarn  in  the  heavier  deniers  as  listed  in  Table  III. 


TABLE  IX 
Comparison  of  Weight  and  Breaking  Load 


Hemp*  Manila  ami  Sisal 
itntarred 

Weight 
Dia-  Cirenm-  Break- 


C  'ontinuous  Filament 

Perlon 

Breaking  Load 
in  kg. 

m 


meter  ference                    ing 
in         in        Weight  Load  in 
mm.   inches       *'„  m         kg 

Haw- 
ser 
laid 

Cable-      Hawser-       Cable- 
laid           laid        laid  and 
Twisted 

1 

2- 

6 

230 

1-5 

410 

6 

I 

3- 

6 

380 

2-25 

580 

8 

1 

5- 

8 

572 

3-7 

1,000 

10 

U 

7- 

4 

763 

5-9 

1,500 

12 

M 

11- 

5 

1,143 

8-7 

2,250 

14 

l? 

14 

4 

1,525 

12 

3,100 

16 

18-8 

1,970 

14-5 

3,800 

18 

2| 

23- 

7 

2,460 

18-5 

4,900 

20 

2* 

29- 

5 

3,000 

25  0 

5,700 

22 

35- 

5 

3,580 

31 

6,950 

24 

1 

42- 

3 

4,210 

36 

8,400 

26 

3] 

49 

6 

4,870 

42 

9,750 

28 

3J 

57- 

5 

5,580 

49 

11,200 

30 

31 

66- 

6,310 

57 

52 

13,000 

11,500 

32 

4 

75- 

1 

7,090 

65 

59 

14,800 

12,200 

36 

4* 

95 

8,720 

82 

74- 

5       18,700 

15,700 

40 

117- 

5 

10,500 

101 

92 

5    (23,000) 

19,500 

44 

5  A 

142 

12,500 

124 

111 

(27,800) 

23,900 

48 

6 

169 

14,700 

145 

132 

(33,000) 

29,000 

52 

6* 

198 

17,000 

170 

155 

(39,000) 

34,600 

56 

7 

230 

19,400 

197 

180 

(45,000) 

40,800 

60 

7A 

264 

21,900 

227 

208 

(52,000) 

47,500 

64 

8 

301 

24,600 

252 

236 

(59,000) 

55,000 

72 

9 

380 

30,100 

315 

299 

(75,000) 

69,000 

80 

10 

470 

36,000 

388 

367 

(92,000) 

84,000 

90 

llj 

570 

43,000 

493 

467 

(116,000) 

106,00 

100 

124 

710 

50,000 

608 

577 

(144,000) 

130,000 

The  values 

in  brackets  merely  serve  purposes  of  comparison  - 

With 

these 

sizes  it  is 

advisable  to 

use  cable-laid 

ropes,  a 

construction 

which  has  proved  particularly  efficient. 

(d)    Perlon  Staple  Fibre  Twines 

Two  yarn  counts:  Nm.  50  and  Nm.  20  are  mainly  used 
for  net  twines;  details  of  strength  are  given  in  Table  X. 


TABIF   X 

Cotton  and  Perlon  Staple  Fibre 

Net  twines  of  Kqual  Wet  Strength 

Cotton 

Net  twine 

Perlon  Staple  Fibre 

and  Net  twine 

Metric 

Length  per 

Metric 

Length  per 

Number 

Unit  Weight 

Number 

Unit  Weight 

Nm. 

mlkg. 

Nm. 

ml  kg. 

50/9 

5,000 

50/6 

7,500 

50/12 

3,600 

50/9 

5,000 

50/15 

2,900 

50/12 

3,600 

50/18 

2,400 

50/15 

2.900 

20/6 

2,900 

50/9 

MXK) 

20/9 

1,950 

50/12 

3,600 

20/12 

1,400 

20/6 

2,900 

20/15 

1,100 

20/9 

1,950 

20/18 

940 

20/12 

1,400 

20/21 

830 

20/12 

1,400 

20/24 

680 

20/15 

1,100 

20/27 

620 

20/18 

940 

20/30 

540 

20/18 

940 

20/32  to  20/36 

460 

20/24 

680 

IV.     PROPERTIES 

1.    Specific  Gravity 

Perlon  has  a  lower  specific  gravity  than  natural  fibres: 
Perlon  1-14  g/cu.  cm. 


Cotton 
Hemp,  jute 
Ramie 


1-54 
1-48-1-50 
I   51 


g/cu.  cm. 
g/cu.  cm. 
g/cu.  cm. 


Coupled  with  a  high  degree  of  strength,  this  property 
enables  the  industry  to  make  very  light  nets. 

2.  Rot  Resistance 

Perlon  does  not  rot  as  it  is  not  attacked  by  bacteria  or 
fungi,  nor  does  it  need  preservative  treatment. 

Perlon  net  twines  continuously  immersed  in  the  brackish 
water  of  a  North  Sea  harbour  for  a  period  of  3J  years 
have  shown  no  more  than  a  10  per  cent,  loss  of  strength. 

3.  Tensile  Strength 

The  specific  tenacity  of  monofilaments  is  highest  with 
a  small  diameter.  The  breaking  length  varies  between 
37  and  47  kilometres.  The  tenacity  of  water-saturated 
Perlon  is  between  80  per  cent,  and  90  per  cent,  of  its 
air-dry  tenacity.  The  knot  strength  shows  an  equally 
close  relation  to  the  diameter: 

(in   °(',   of  tenacity) 

70— 80 "0  for  small  diameter  (0  10— 0  30  mm.) 
60-70%  „  medium  „  (0-35— 0-70  mm.) 
50— <*)%  „  large  „  (  over  ) 

The  strength  of  the  filaments  is  determined  by  the 
degree  to  which  they  have  been  stretched  or  "drawn" 
during  manufacture,  as  shown  in  an  example  of  Perlon 
continuous  filament  of  1,060  denier: 

Special 

Ordinary  Pre-stretch 

km        55    '  71 

km        47  61 

"0         20  17 

%        22  19 

km       44  41 

km        41  36 


Breaking  Length  air-dry 

Breaking  Length  wet 

Break  air-dry 

Break  wet 

Extension  at  Knot  Strength  air-dry 

Extension  at  Knot  Strength  wet 


[37  1 


MODERN     FISHING     GEAR    OF    THE    WORLD 


A  high  degree  of  stretching  results  in  a  considerable 
increase  in  strength  coupled  with  an  appreciably  reduced 
extension  at  break.  However,  as  filament  strength  is 
raised,  there  occurs  a  reduction  in  knot  strength,  parti- 
cularly when  water-saturated. 

This  phenomenon  is  particularly  important  when  the 
fibre  is  subjected  to  a  specially  high  pre-stretch  as  for 
fishing  net  twines.  High  strength  and  a  low  extension  at 
break  are  desirable,  but  can  only  be  achieved  at  the 
price  of  low  knot  strength. 

A  comparison  in  knot  strength  of  filament  composition 
10,000  denier,  single  filament  denier  20,  and  of  monofils 
with  a  hemp  string  of  equal  strength: 


Strength  dry  kg/sq.  mm. 
Strength  wet  kg/sq.  mm. 
Knot  Strength  dry  kg/sq.  mm. 
Knot  Strength  wet  kg/sq.  mm. 
Relative  Knot  Strength  dry  % 
Relative  Knot  Strength  wet  % 

Experience  has  shown  the  knot  strength  of  all  fibres 
to  be  below  their  ordinary  strength. 

4.    Extensibility  and  Elastic  Properties 

The  extension  at  break  of  Perlon  monofil  varies  between 
20  and  35  per  cent. 

Other  characteristic  values  for  the  extensibility  and 
elasticity  of  Perlon  monofils  are: 

Elastic  extension  75-90%  \  of  the  total  extension  at 

Permanent  elongation       10-25%  /     80%  of  breaking  load 


Details  of  the  relationship  between  load  and  total 
extension  are  shown  in  the  graphs  of  Illustration  I. 

These  diagrams  show  that  the  load  extension  curves 
of  Perlon  climb  at  a  more  obtuse  angle  than  those  of 
natural  fibres. 

This  divergence  becomes  conspicuous  in  a  comparison 
of  hemp  and  Perlon  ropes  with  a  circumference  of  1J  in. 


Perlon 

Perlon 

Hemp 
35 

Filament 
59 

Monofil 

52 

38 

52 

42 

23 

26 

26 

23 

24 

20 

66 

44 

50 

66 

41 

48 

Italian  hemp 

Manilla 

~  .    Siva! 

• »    Perlon  accordjnjt  to  construction  and  stabilization 

Illustration  11. 

Loadj Extension  curves  of  Ropes  in  Natural  Fibres  ami  Perlon, 
circumference  \\  in. 


Illustration  I.  %  extension 

Load/ Extension  cur  vex  of  Perlon  Products  compared  with  Cotton  ami 
Manila  twines. 


—     total  extension 

— permanent  elongation 

.  —       elastic  extension 

//lustration  III. 

Elasticity  Graph  of  Perlon  Continuous  Filament  in  Denier  7,500 
(Single  Filament  Denier  6). 


[381 


CHARACTERISTICS     OF     PERLON 


The  graph  shows  not  only  the  greater  strength  of  Perlon 
rope  but  also  its  higher  working  capacity,  which  enables 
it  to  absorb  shocks  like  a  spring;  when  stretched  it 
recovers  its  original  length  very  soon  except  for  a  slight, 
but  permanent  elongation  of  approximately  10  per  cent. 

Illustration  I II  gives  some  idea  of  this  elasticity  as 
shown  by  a  composition  of  7,500  denier,  single  filament 
denier  6,  with  a  tensile  strength  of  64  kg.  sq.  mm.  Natural 
fibres  can  achieve  similar  values  only  through  special 
construction.  There  is  complete  elasticity  up  to  approxi- 
mately 5  per  cent,  of  the  breaking  load  i.e.  the  permanent 
elongation  is  nil.  Above  this  load  a  permanent  deforma- 
tion occurs  which  becomes  relatively  less  as  the  extension 
increases.  As  a  result  the  difference  between  total 
extension  and  permanent  elongation  increases  further. 

The  relation  of  elastic  extension  to  total  extension  gives 
the  elastic  ratio,  which  can  be  said  to  hold  good  for 
Perlon  proportionate  to  the  magnitude  involved. 
Illustration  IV  compares  the  elastic  ratios  of  Perlon  and 
cotton  yarns. 

The  load/extension  curve  has  already  shown  up 
fundamental  differences,  which  occur  once  again  in  a 
comparison  of  elasticity.  Whilst  Perlon  shows  a  high 
proportion  of  elasticity  within  the  total  extension,  with 
cotton  fibres  the  permanent  elongation  predominates  and 
leads  to  a  much  steeper  fall  of  the  elasticity  graph. 


<Tnted>ntrl»  o«»i  i 


Illustration    \'. 

Extensibility  and   Elasticity   of  Perlon    Continuous   hi  lament 

(Denier  7,500,  Single  Filament  Denier  6)  in  Relation  to  Time 

and  Load. 

5.     Flexibility 

Net  twines  made  of  continuous  filament  and  staple  fibre 
are  softer  than  those  made  from  natural  fibres.  This 
applies  particularly  to  their  wet  state  as  shown  in  tables. 

Hardness,  dry  and  wet  of  Net  Twines  of  2*3  mm.  Diameter 


Raw  Material 

Manila 

Hemp 

Cotton 

Perlon  Filament 


Dry 

700 
120 
42 
44 


Wet 

410 
110 
140 
21 


The  higher  the  figure  the  harder  the  net  twine. 

For  some  fishing  nets,  in  particular  fine  gillnets, 
flexibility  is  a  very  desirable  quality;  for  others  it  is  of 
minor  importance.  This  can  be  achieved  by  applying 
a  stiffening  preparation  which  at  the  same  time  gives  the 
nets  greater  resistance  to  abrasion  and  sunlight. 


PorJon  continuous  fila men's 


Illustration 


Elasticity  Graph  of  Perlon  Continuous  Filament  in  Denier  7,500 
(Single  Filament  Denier  6)  and  of  a  Cotton  Yarn. 


From  Illustration  V  it  appears  that  immediately  the 
load  has  been  applied  a  high  degree  of  extension  ensues, 
which  regains  its  equilibrium  within  the  next  15  and 
30  minutes  respectively.  The  extension  is  not  proportional 
to  the  increase  in  the  load,  but  is  relatively  greater  at 
low  and  medium  loads  than  at  high  loads. 

This  gives  Perlon  a  springy  quality  which  enables  it 
to  absorb  kinetic  energy  as  shown  below. 

Material  Dry  Wet 


Manila 
Italian  Hemp 
Perlon 


100 
160 
700 


60 
HO 
500 


6.     Abrasion  Resistance 

Perlon  has  a  high  resistance  to  abrasion  which  together 
with  non-rotting  quality  determine  the  useful  life  of  nets. 
Wet  abrasion  tests  of  net  twines  showed  the  following 
results  related  to  their  weight  per  metre: 


Net 
Twine 
Weight 

Kim 

1-5 

2-5 

3-0 

4-0 

4-5 


Wet  Abrasion  Rubs 


Hemp 

420 

630 

940 

1.210 

1,340 


Manila 

380 
400 
540 
580 


Perlon 
Filament 

660 

930 

1,510 

2,120 

2,430 


Staple  fibre  net  twines  show  a  lower  abrasion  resistance 
than  those  made  of  continuous  filament  but  still  show 
considerably  greater  resistance  than  those  made  of 
cotton  fibres.  A  staple  fibre  net  twine  Nm.  20/24  has  a 
wet  abrasion  resistance  of  270  whereas  a  resistance  of 
1 50  was  shown  by  a  cotton  net  twine  of  the  same  strength. 


39] 


MODHRN     FISHING     GEAR     OF    THE    WORLD 


Illustration    1 7. 

Manila  net  twine  ami  Perlon  continuous  filament  braided  cord 
after  500  wet  abrasion  double  rubs. 

Photo:  Must  1957 


A  breaking  extension  test  showed  a  62  per  cent,  loss 
of  tensile  strength  for  the  manila  cord,  compared  with 
19  per  cent,  for  the  Perlon  braided  cord. 


Where  Perlon  twines  are  exposed  to  intense  sunlight 
for  long  periods,  as  for  example,  in  the  case  of  fyke  nets, 
it  is  advisable  to  dye  them.  They  can  easily  be  dyed  with 
Perliton  dyes,  which  dyes  must  be  selected  with  a  view 
to  fastness  and  with  acid  dyes — Telon — light  and  fast 
dyes  respectively,  which  are  remarkable  for  their  fastness 
in  water.  Treatment  with  a  mixture  of  tannic  acid  and 
antimony  potassium  tartrate  is  particularly  durable  in 
seawater.  Treatment  with  a  catechu  solution  has  proved 
an  excellent  protection  against  sunlight. 

8.     Hygroscopic  Behaviour 

Perlon  has  a  low  moisture  regain.  An  examination  of 
moisture  content  in  air-dry  conditions  (65  per  cent, 
relative  air  humidity,  20  deg.  C.),  and  swelling  ratio 
(degree  of  saturation)  shows  the  following  results: 


/ton 
Material 

Cotton 
Bast  Fibres 
Perlon 


Moisture 

Content  in 

A  ir-  Dry-  Condition* 


1   13 
4-2 


Swelling 
Ratio  °,, 

45 

100 -110 
12-14 


7.     Weather  Resistance 

Both  synthetic  and  natural  fibres,  are  weakened  b> 
exposure  to  sunlight,  but  monofils  display  a  high  degree 
of  resistance  to  sunlight  and  weather  conditions,  superior 
to  that  of  vegetable  fibres,  and  close  to  the  immunity 
of  poly-acrylonitrile  fibres. 

Net  twines  of  staple  fibre  or  continuous  filament  lose 
more  strength  than  natural  fibre  twines  when  exposed  to 
intense  sunlight,  but  because  of  their  high  initial  strength 
they  remain  in  the  last  analysis  superior  to  natural  fibres. 
The  bigger  the  diameter  the  less  noticeable  the  photo- 
degradation,  which  is  insignificant  for  thick  ropes  as  the 
layers  below  are  protected  by  the  degraded  outer  layer 
and  which  is  probably  no  deeper  than  I  mm.  to  which 
ultraviolet  rays  can  penetrate. 

decrease    of    strength     , 


crlo~!     t./isted    and    dyed 


hemp,    twisted 


Illustration   VII. 

Influence  of  weather  exposure  on  ropes  made  of  Perlon  continuous 
filament  and  hemp  respectively.   Rope  circumference:    1|  in. 
Weather  exposure  at  6.230  ft.  above  sea  level. 


relative   air   humidity 


—      cotton 

lute 

Perlon 


Illustration   VIII. 
Absorption  and  Desorption  of  cotton,  lute  and  Per/on. 

For  Perlon,  absorption  and  desorption  are  of  almost 
identical  magnitude  in  contrast  with  the  far  more 
marked  "swelling  hysteresis"  of  natural  fibres  and  it 
therefore  dries  appreciably  faster. 

The  level  of  moisture  regain  is  known  to  be  closely 
related  to  the  lateral  and  longitudinal  swelling  caused  by 
wetting.  Cotton  net  twines  receiving  a  first  wetting  of  24 
hours,  showed  lateral  swellings  of  between  5  and  15  per 
cent.;  bast  fibre  net  twines  showed  an  increase  of  20  to 
40  per  cent,  in  their  cross-sectional  area  whereas  Perlon 
continuous  filament  net  twines  contract  rather  than 
swell.  The  following  comparison  in  thickness  of  a  Perlon 
cable  Nm.  0-9,  single  filament  denier  20,  with  a  hemp 
cord  of  equal  size  may  serve  as  an  example: 

Hemp  Perlon 

Diameter  dry         .  .  mm  1-65  1-42 

Diameter  wet         .  .  mm  2«02  1*38 

Variation  in  Cross-section  %  +22  —3 


[40 


CHARACTERISTICS    OF    PERLON 


Net  twines  made  of  natural  fibres  swell  considerably 
and  shrink  as  a  result.  It  amounts  to  approximately  6  to 
9  per  cent,  for  cotton  net  twines,  and  to  approximately 
2  to  8  per  cent,  for  hard  bast  fibres. 

Continuous  filament  twine  behaves  quite  differently. 
An  immersion  in  water  gives  rise  to  an  extension  pro- 
portionate to  the  cross-sectional  shrinkage,  in  the  region 
of  1  to  3  per  cent.  The  changes  in  length  and  cross-section 
are  only  slight  and  consequently  assure  a  constant  mesh 
size. 

9.    Chemical  Resistance 

Perlon  is  resistant  to  rot  in  both  fresh  and  seawater. 
Solvents  which  do  not  affect  it  and  may  be  used  in 
preparations  to  increase  resistance  to  sunlight  or  stiffness 
are  listed  below: 


Methyl  ethyl  ketone 

perchlorcthylenc 

acetone 

trichlorelhylcne 

carbon  disulphide 

dimethyl  formamide 

benzine 

cyclohcxanone 

chloroform 

carbon  tetrachloride 

formamide 

(base)  alcohols 

ethyl  oxide 


(  I  60°  C); 

<  +  20°C); 
(  I  56°  C); 

<  +  80°C); 
I  20°  C); 
1  20°  C); 
I  6<r  C); 
{  80"  C); 
f20°C); 
I  60°  C); 
T  20°  C); 

.  35C  C); 


Perlon  solvents  include  concentrated  solutions  of 
formic,  hydrochloric  and  sulphuric  acid,  as  well  as 
phenol  (carbolic  acid),  cresylic  acid  and  resorcin. 

Products  with  a  phenol  content,  such  as  tars,  can  in 
certain  cases  cause  swelling  and  consequently  a  loss  of 
strength.  Specially  treated  tars  contain  only  small 
quantities  of  phenol  derivatives  and  of  phenol  itself.  In 
crude  tar,  however,  these  substances,  "acid  oils",  may 
be  present  in  fairly  large  quantities,  although  there  is  a 
sleep  variation  in  the  percentage.  It  seems  advisable  to 
draw  special  attention  to  this  aspect,  since  tars  are  also 
used  as  stiffeners. 

10.  Thermal  Properties 

Perlon  has  a  high  resistance  to  cold  and  retains  its 
elasticity  even  when  frozen.  There  is,  in  fact,  an  increase 
in  strength  and  conversely  a  loss  of  extensibility  down 
to  a  temperature  of  —40  deg.  C.  The  temperature  needed 
in  dyeing  processes  does  not  damage  the  nets,  but  a 
certain  amount  of  shrinkage  must  be  expected  and 
therefore  ascertained  by  preliminary  tests  on  small 
samples. 

11.  Visibility 

Perlon  net  twines  can  be  very  fine  and  accordingly 
inconspicuous  in  the  Water. 

The  translucent  monofil  is  almost  invisible  under 
water  and  this  property  has  increased  the  catches  of 
monofil  setnets  and  fyke  nets. 

12.  Processing 

Staple  fibre  net  twines  can  be  processed  into  netting 
without  difficulty,  manually  or  by  machinery.  They  can 
be  tied  in  non-slip  knots  in  the  same  way  as  cotton  net 
twines.  With  monofil  and  continuous  filament  twine, 


however,  slip-proof  knots  cannot  be  assured  because  of 
the  smoothness  of  the  fibre.  Tests  are  about  to  be  com- 
pleted which  aim  at  giving  monofils  a  rough  surface  while 
preserving  the  inherent  strength  to  increase  the  knot 
fastness. 

Continuous  filament  net  twines  can  be  roughened  by 
treating  them,  preferably  while  they  are  being  twisted, 
with  a  preparation  insoluble  in  water,  which  gives  them 
adhesive  properties  (bonding). 

The  resistance  to  slippage  of  Perlon  continuous  fila- 
merU  twine  has  been  improved  by  blending  spun  staple 
fibres. 

Another  possibility  is  the  heat-setting  of  the  knots 
under  tension,  at  temperatures  of  150  to  180  deg.  C. 
usually  applied  for  short  periods  only. 

With  nets  it  is  essential  to  maintain  an  even  tension  and 
twist  in  the  Perlon  material.  Hawser-laid  or  cable  laid 
ropes  must  be  heat-set  if  necessary.  No  stabilization  is 
needed,  however,  if  the  individual  twists  reinforce  each 
other.  With  such  a  construction  heat-setting  is  only 
needed  where  splices  have  to  be  made. 

Cordage  can  be  set  by  hot  air  treatment,  saturated 
steam  treatment  or  boiling.  Boiling  is  preferable  for 
heavier  types  since  neither  hot  air  nor  saturated  steam 
can  penetrate  evenly  enough  through  the  rope. 

In  continuous  filament  too  hard  a  twist  should  be 
avoided  as  this  would  reduce  strength  and  increase 
extension,  particularly  that  proportion  which  is  per- 
manent. Braided  twines  for  fishing  nets  should  be  braided 
with  a  medium  degree  of  hardness.  On  no  account 
should  a  high  degree  of  hardness  be  employed. 

13.  Storage 

Perlon  nets  and  ropes  should  not  be  left  exposed  to  the 
sun.  They  are  best  kept  in  dark  rooms  and  can  be  stowed 
while  still  wet.  Nets  which  are  freshly  treated  with  pre- 
servatives should  be  handled  in  the  same  way  as  natural 
fibre  nets:  they  should  only  be  stored  after  having  been 
used  at  least  once.  This  applies  particularly  when 
stiffening  preparations  have  been  used. 

14.  Attack  by  Micro-Organisms 

Perlon  is  immune  to  attacks  by  micro-organisms  such  as 
bacteria  or  fungi,  nor  do  molluscs,  barnacles  and  other 
organisms  harm  it.  However,  the  larvae  of  the  mayfly 
which  settle  particularly  on  stationary  fishing  gear  in 
inland  waters  can  cause  considerable  damage  to  nets; 
synthetic  fibres  are  as  much  affected  as  cotton  nets. 
Treatment  with  pesticides  (Arkotine,  Dieldrin)  ensures 
a  high  degree  of  protection. 

In  running  waters  which  contain  organic  effluents 
thick  clusters  of  "sewage  fungi"  occur  which  soon  cover 
the  nets.  Frequent  cleaning  is  needed,  or  catches  are  lost. 
Rhine  fishermen  are  particularly  affected  by  this  fouling 
of  their  stow  nets.  Nets  made  of  Perlon  filament  braided 
twine  are  less  affected  because  of  their  smooth  surface 
and  can  be  cleaned  easily  and  quickly. 

V.     USE   OF  PERLON   IN   VARIOUS  TYPES   OF 
FISHING  GEAR 

The  economics  of  fishing  gear  depend,  generally  speaking, 
on  initial  costs,  useful  life,  costs  of  preparation,  main- 
tenance and  efficiency. 


[41  ] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


The  initial  cost  for  a  given  weight  of  nets  is  higher  for 
Perlon  than  for  cotton,  hemp  and  especially  maniia, 
but  the  useful  life  of  Perlon  is  considerably  longer  because 
of  the  high  degree  of  resistance  to  rot  and  abrasion. 
While  these  nets  need  little  or  no  preservative  treatment 
nets  of  cotton,  hemp  and  flax  must  be  treated  frequently. 
The  expenditure  in  money  and  time  involved  represents 
a  severe  burden. 

Maintenance  largely  consists  in  preventing  unnecessary 
exposure  to  sunlight  and  with  monofil  even  this  protective 
measure  becomes  unnecessary. 

The  following  paragraphs  outline  the  use  of  Perlon  for 
some  of  the  more  important  types  of  equipment. 

1.  Trawls 

At  the  Fishery  Industry  Fair  1957  in  Copenhagen  was  a 
still  usable  bottom  trawl  made  of  Perlon  braided  twine 
used  by  a  German  trawler  to  catch  128,000  cwt.  of 
herrings  having  a  performance  and  useful  life  between 
8  to  10  times  as  great  as  that  of  the  customary  manila 
trawls.  Although  Perlon  trawls  cost  two  or  three  times 
as  much  as  manila  trawls,  their  economic  advantages  are 
beyond  question.  Their  low  weight,  flexibility  and  smooth- 
ness make  them  easier  to  handle  and  the  finer  net  twines 
noticeably  reduce  the  tow  drag. 

Probably  the  most  important  advantage  is  the  capacity 
of  Perlon  to  absorb  kinetic  energy.  The  nets  also  ensure 
the  safe  landing  of  big  catches  on  deck  as  shown  by  the 
big  catches  of  Norway  haddock  when  Perlon  codends 
were  used.  About  half  the  German  cutter  fishermen  use 
Perlon  for  the  codends  in  drag-nets  for  catching  herring. 
The  material  in  this  case  was  spun  staple  fibre  twine  in 
counts  Nm.  20/15  to  20/21. 

2.  Purse  Seines 

This  type  of  equipment  plays  an  important  part  in  the 
fishing  industry  of  many  countries,  although  not  in 
Germany.  Its  traditional  net  material  is  cotton  twine  of 
medium  strength.  In  the  Portuguese  fishing  industry, 
for  example,  cotton  nets  treated  with  preservatives  may 
have  a  useful  life  of  between  400  and  500  fishing  days. 

Large  pieces  of  Perlon  spun  staple  fibre  webbing  have 
been  employed  in  Portuguese  purse  seines  for  1,300 
fishing  days  without  becoming  unusable.  These  nets 
were  not  dyed  or  prepared  in  any  way.  For  purse  seines 
Perlon  has  the  following  advantages:  reduction  in  the 
total  weight  of  fishing  equipment  (essential  considering 
the  size  of  the  equipment  involved);  almost  no  expendi- 
ture on  net  preservatives;  no  necessity  to  keep  one  set  of 
nets  on  land  for  preservation  purposes;  less  labour 
required  to  handle  the  gear;  water-saturated  nets  can  be 
safely  stowed  and  a  useful  life  at  least  double  compared 
with  cotton  nets. 

Perlon  continuous  filament  rope  of  30  mm.  in  diameter 
has  been  used  for  1,164  days  as  a  purse  line.  It  should 
have  five  times  the  useful  life  of  a  sisal  rope  to  justify  its 


higher  price;  it  has  already  had  nineteen  times  the  life 
of  a  sisal  rope. 

3.  Bottom-Set  Gillnets 

Set  and  floating  nets  used  as  fine  gillnets  are  highly 
selective,  helping  to  conserve  young  stock  and  supplying 
fish  of  high  quality.  Bottom-set  nets  are  passive  rather 
than  active  and  must  be  as  fine  and  as  soft  as  possible 
having  the  least  degree  of  visibility,  requirements  met 
by  net  twines  of  Perlon  continuous  filament  yarn  in  a 
denier  even  finer  than  that  quoted  in  Illb  (100  denier  and 
finer).  Monofils  are  particularly  suitable  for  set  gillnets 
due  to  their  extremely  low  visibility  in  water.  Now  that 
the  problem  of  providing  non-slip  knots  has  been  solved 
and  mechanically  produced  netting  can  be  obtained,  set 
gillnets  of  monofil  assumed  considerable  importance  for 
fishing  in  inland  waters. 

4.  Stationary  Fishing  Gear 

Perlon  products  have  proved  their  efficiency  and  economy 
when  used  for  fyke  and  stow  nets  and  traps. 

A  cotton  eel  trap  such  as  those  used  by  fishermen  in 
the  North  German  inland  lakes,  costs  about  DM  24.00 
and  lasts  4  years  with  preservative  treatment.  A  Perlon 
spun  staple  fibre  eel  trap  costs  DM  17.08,  needs  no 
additional  expenditure  and  remains  completely  efficient 
for  approximately  four  years. 

Monofils  proved  even  more  suitable  for  eel  traps; 
owing  to  their  translucence  they  catch  more  eels. 

Cotton  stow  nets  have  a  useful  life  of  about  two  years 
if  treated  between  7  and  10  times  with  hot  tar  to  preserve 
and  stiffen  them.  A  stow  net  of  Perlon  staple  fibre  twine 
has  been  in  use  for  six  years  now  and  has  been  stiffened 
twice,  once  with  black  varnish  and  once  with  tar.  Total 
costs  for  this  stow  net,  which  is  still  in  full  use,  amounts 
to  DM  1,625.00.  During  the  six  years,  at  least  3  cotton 
stow  nets  would  have  had  to  be  purchased,  costing 
DM  3,270.00. 

5.  Whaling  Ropes  and  Cordage 

In  antarctic  whaling,  ropes  of  Perlon  continuous  filament 
between  100  and  120  metres  in  length  used  with  the  70 
kilogram  explosive  harpoons  have  proved  their  worth 
as  "foregoers". 

The  strength  of  the  fibres  makes  it  possible  to  use 
ropes  with  a  diameter  of  33  to  34  mm.,  as  compared  to 
38  mm.  for  manila  ropes.  Perlon  ropes  do  not  stiffen  in 
water  and  hardly  ever  ice  up.  The  elasticity  can  absorb 
the  high  shock  loads  involved  and  greatly  reduces  the 
danger  of  a  rope  breaking. 

5.    Tarpaulins  and  Protective  Covers 

Tissues  of  Perlon  continuous  filament  yarns,  PVC-- 
Polyvinyl  Chloride  coated  on  both  sides  are  used  for 
tarpaulins,  lifeboat  covers,  etc.  They  can  be  folded 
quickly  and  require  little  storage  space,  They  are  rot- 
proof,  watertight,  tough  and  can  be  fireproof. 


42] 


"TERYLENE"   POLYESTER  FIBRE  AND   ITS   RELATION 
TO  THE   FISHING  INDUSTRY 

by 

IMPERIAL  CHEMICAL  INDUSTRIES  LTD. 

(Fibres  Division),  Harrogate,  U.K. 

Abstract 

"Terylene"  polyester  fibre  is  a  new  synthetic  fibre  of  British  manufacture  which,  although  being  of  considerable  importance  in  the 
textile  trade,  is  comparatively  new  to  the  fishing  industry.  It  has  physical  properties  that  make  it  suitable  for  fishing  twines  and  nets.  It 
has  high  tensile  strength  and  a  high  wet  knot  strength.  It  is  rot -proof  and  has  good  resistance  to  sunlight,  and,  under  wet  conditions  has  also 
good  resistance  to  abrasion.  Like  other  synthetics,  knot  slippage  is  a  problem,  but  this  can  be  overcome  by  bonding  agents  either  before  or 
after  the  weaving  of  the  net.  Dyeing  is  preferably  done  during  the  manufacture  of  the  nets  and  it  is  not  recommended  that  the  fishermen 
should  do  it  themselves  because  of  difficulties  in  temperature  control.  Terylene  is  used  with  success  for  gillnets  in  many  parts  of  the  world, 
mainly  for  catching  "hard"  fish  such  as  salmon  and  cod.  For  "soft"  fish  like  the  herring,  it  has  not  yet  been  fully  accepted  because  of  the 
damage  done  to  the  fish  when  hauling  the  nets.  Trawls,  however,  have  proved  very  successful,  for  on  test  they  have  lasted  for  9  trips  instead 
of  the  usual  single  trip  with  the  conventional  trawl.  The  comparatively  high  cost  of  "Terylene"  is  to  some  extent  offset  by  the  length  of  its  life. 


Resume 


La  fibre  polyester  "Terylene"  et  ses  applications  dans  1'industrie  des  Pftches 


fibre  polyester  "Terylene"  est  une  fibre  synth&iqiic  nouvellc  de  fabrication  britannique;  bien  qu'clle  soit  ires  rcpandue  dans  le 
commerce  textile,  son  emploi  dans  rindustrie  des  pdches  est  relativement  recent.  Elle  possede  des  proprigtes  physiques  qui  font  qu'elle 
convient  a  la  confection  de  fils  et  de  filets  de  peche.  La  resistance  A  la  traction  des  fibres  et  des  noeuds  mouiltes  est  tres  dlevee.  Elle  est 
insensible  £  la  pourriture  et  possede  une  bonne  resistance  £  P action  du  soleil;  a  1'gtat  humide  elle  possede  egalement  une  bonne  resistance 
a  1'abrasion.  Les  noeuds  executes  avec  des  fils  dc  Terylene,  comme  avec  les  fils  constitues  par  d'autres  fibres  synth&iques,  ont  tendance  a 
glisser,  mais  on  peut  r&soudre  ce  probleme  en  trait  ant  les  fils  avec  des  adh&ifs  soit  avant,  soit  aprds  la  confection  du  filet.  II  est  preferable 
de  proddcr  a  la  teinture  au  cours  de  la  confection  du  filet  et  il  est  deconseilte  aux  pecheurs  de  F6xecuter  aux-memes  en  raison  des  difficultes 
qu'entraine  le  contrdle  tres  exact  de  la  temperature  pendant  I'op6ration.  Le  T6rylcne  est  utilise  avec  succes  dans  de  nombreux  pays  pour 
la  fabrication  des  filets  maillants,  principalement  pour  la  capture  de  poissons  "durs"  comme  le  saumon  et  la  morue.  Pour  des  poissons  "mous" 
comme  le  hareng,  il  n*a  pas  encore  6te  universellement  adoptd  car  il  endommagc  les  poissons  lorsque  Ton  embarque  les  filets.  Mais  11  a 
donn£  d'excellents  resultats  pour  la  confection  de  chaluts  qui,  a  la  suite  d'essais,  ont  tenu  neuf  campagnes  au  lieu  d'une  seule  avec  le  chalut 
convent ionncl.  Lc  coQt  relativement  £levc  du  Terylene  est  compensd  dans  une  certaine  mesure  par  sa  plus  longue  duree. 

La  fibra  de  "terileno"  y  su  relacion  con  la  industria  pesquera 
Extracto 

La  fibra  sint&ica  llamada  "terileno"  es  un  nuevo  tipo  de  poliester  manufacturado  en  Gran  Bretafta,  que  tienc  gran  importancia  en  la 
industria  textil  pero  es  comparativementc  nuevo  en  las  faenas  de  pesca.  Sus  propiedades  fisicas  se  prestan  para  fabricar  hilos  y  redes,  a 
causa  de  su  gran  resistencia  a  la  trace  ion  aun  cuando  esta  anudada  y  humeda;  ademds  no  se  pudre,  sufre  bien  la  acci6n  de  la  luz  solar  y 
ofrece  bastante  resistencia  al  desgaste  cuando  esti  mojada.  Como  en  otros  productos  sint£ticos,  los  nudos  presentan  el  problema  de  correrse 
pcro  esto  se  soluciona  mediante  agcntes  de  uni6n  que  se  aplican  antes  o  despu£s  de  tejer  el  artc.  Lucgo  de  confeccionar  la  red  es  p refer ible 
proccder  a  su  entintadura,  no  recomendandose  que  la  haga  el  pescador  a  causa  de  las  dificultades  que  presenta  la  regulaci6n  de  la  temperatura. 
HI  "terileno"  se  usa  con  dxito  en  muchas  partes  del  mundo,  principalmente  para  capturar  peces  **duros**  como  el  salmdn  y  el  bacalao,  pero 
no  ha  sido  aceptado  del  todp  para  las  especics  "blandas"  como  cl  arenque,  a  causa  del  dano  que  produce  al  pescado  cuando  se  recogen  las 
redes.  No  obstante,  ha  tenido  ^xito  en  la  const rucci6n  de  artes  de  arrastre,  por  haher  resistido  durante  las  prucbas  nuevc  vez  de  una  marea- 
que  dura  el  material  corriente.  El  costo  relativamentc  alto  del  "terilcno"  sc  compcnsa,  en  parte,  con  su  mayor  duracion. 


GENERAL  INTRODUCTION 

TERYLENE  polyester  fibre  is  a  new  synthetic  fibre 
which  has  already  made  a  considerable  impact  on 
the  textile  industry  and  has  significantly  affected 
certain  fields  which  have  hitherto  been  dominated  by 
natural  fibres.  Apart  from  its  merits  in  the  manufacture 
of  wearing  apparel  and  for  industrial  use,  the  special 
properties  of  Terylene  make  it  suitable  for  employment 
in  the  fishing  industry.    It  has  been  used  not  only  for 
netting  twines,  ropes  and  lines,  but  also  for  lifeboat  and 
hatch  covers,  sails  and  tarpaulins. 

The  fibre  is  a  British  discovery  made  by  J.  R.  Whin- 
field  and  J.  T.  Dickson  in  the  Laboratories  of  the  Calico 


Printers'  Association  Ltd.,  between  1939  and  1941.  Later 
the  world  patent  rights,  with  the  exception  of  the  USA 
were  acquired  by  ICI  Ltd.,  and  the  name  "Terylene" 
became  a  registered  trade  mark,  the  property  of  ICI  Ltd. 
Recently  several  European  companies  have  been  licensed 
to  manufacture  the  fibre  under  their  own  trade  names, 
and  it  is  also  being  made  in  Canada  by  Canadian 
Industries  Ltd. 

A  new  factory  at  Wilton,  Middlesbrough,  Yorkshire, 
produces  over  25  million  pounds  per  annum,  and  larger 
outputs  are  envisaged. 


[43] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


TYPES  OF  YARN 


Both  continuous  filament  yarn  and  staple  fibre  are 
manufactured.  The  continuous  yarn  can  be  divided  into 
two  categories.  The  first  is  a  yarn  with  a  tenacity  of  6  to 
7  grams  per  denier  and  a  corresponding  extension  at 


break  of  12£  to  7£  per  cent.  This  yarn,  which  is  intended 
primarily  for  industrial  uses,  is  being  manufactured 
currently  in  125  denier/24  filaments  and  250  denier/48 
filaments.  It  enables  very  fine  and  strong  twines  to  be 
produced.  The  development  of  a  high  tenacity  extra  heavy 
denier  yarn  is  under  way  which  will  enable  heavier  nets, 


TABLE  I 
Physical  Properties  of  Fishnet  Twines-  Natural  Fibres 


Construction 

Count 

Twist  t.p.i.  twine 
strand 
yarn 

Runnage  (yd./lb.) 

(Breaking  Load  (I  h.) 
DRY  «{  Extension  (%) 

[Tenacity  (g.p.d.)     . 

f  Breaking  Load  (Ib.) 
WET  1  Extension  (%) 

L Tenacity  (g.p.d.)      . 

fKnot  B/L  (Ib.) 
DRY  <{  Extension  (%) 

L  Tenacity  (g.p.d.) 

fKnot  B/L  (Ib.) 
WET  J  Extension  (%) 

(  Tenacity  (g.p.d.)      . 

Abrasion  dry  revs, 
wet  revs. 


Hemp 
7-85/3      6-3.9/3 

6-9^/3 

Flax 
105/3 

3-35 

6-6S 

6-7Z 
1006 

13-25/3 

!         8-85/3/3 

Cotton 

305/7/3 

1-4 
8-3S 
9-5Z 
1  4Z 
1170 

295/5/3 

1-96 
8-6S 
12-4Z 
1-9Z 
1646 

2-6 

5*7 
5-7 
780 

2-1 

6-0 
6-OS 
630 

2-6 

3-7Z 
6-OS 
780 

4-4 

3-9Z 
6-2S 
1318 

1-1 
5-5Z 
18-7S 
1-1Z 

922 

38-3 
4-6 
3-0 

37-8 
5-8 
2-4 

28-3 
6-5 
2-3 

27-8 
6  25 
2-8 

23-8 
5-4 

3-2 

14-8 
16-0 
1-4 

15-0 
11-5 
1-8 

10-5 
14-2 
1-8 

44-8 
8-9 
3-6 

49.4 
10-0 
3-2 

45-5 
9-7 
3-6 

36-8 
7-8 
3-8 

29-4 
9-1 
3-9 

19-9 
24-2 
1-9 

19-1 
27-8 
2-3 

13-1 
27-2 
2-2 

15-7 
11-5 
1-2 

22-2 

n 

19 
9-0 
1-5 

14-8 
10-0 
1-5 

12-0 
10-0 
1-6 

8  21 
17-0 
0-77 

8-5 
10-1 
1-0 

5-5 
13-0 
0-92 

19  5 
22-4 
1-6 

18-2 
15-5 
1-J 

19  1 

'?:? 

12  8 
8-1 
13 

13-0 
10-0 
1-7 

12-6 
24-1 
1-1 

11-9 
28-9 
1-4 

8-3 
24-5 
1-4 

48 

54 

124 

60 

71 

1195 

898 

1183 

178 

201 

125 

123 

158 

208 

169 

154 

TABLE  I  (continued) 
Physical  Properties  of  Fishnet  Twines— Synthetic  Fibres 


Construction 

Denier 
Twist    t.p.i.    strand 
yarn 
Runnage  (yds./lb.) 

250/8/3 

Terylene 
250/6/3 

250/4/3 

3342 
7-1Z 
14-9S 
1336 

210/8/3 

Amilan 
210/6/3 

210/5/3 

210/8/3 

Nylon 
210/6/3 

4160 
5-1  Z 
12-2S 
1073 

210/4/3 

6662 
6-3Z 
10-7  S 
672 

4980 
5  4Z 

12-2S 
896 

5247 
5-8Z 
10-6S 
850 

4034 
6-OZ 
1I-5S 
1106 

3323 
6-5  Z 
11-7S 
1343 

5547 
4 
11 
805 

•6Z 
•8S 

2784 
7-OZ 
15-OS 
1603 

DRY 

(Breaking  Load  (Ib.) 
*'  Extension  (%) 
L  Tenacity  (g.p.d.) 

78-0 
31-1 
5-3 

58-5 
29-4 

5-33 

38-3 
27-5 
5-2 

50-7 
56-0 
4.4 

39-3 
47-6 
4-4 

31-7       !       63 
45-0              45 
4-3                5 

•8 
•0 
-2 

52-8 
42-6 
5-8 

33-8 
41-1 

5-5 

WET 

f  Break  ing  Load  (Ib.) 
•!  Extension  (%) 
L  Tenacity  (g.p.d.) 

78-4 
30-9 
5-3 

58-9 
27-6 
5-4 

38-1        1       45-9 
20-5              55-7 
5-2                4-0 

31-0 
48-9 
3-5 

29-0 
46-2 
4-0 

60 

4l 

•9 
•2 
•0 

45-2 
43-7 
4-9 

27-1 
41-8 
4-4 

DRY 

fKnot  B/L  (Ib.) 
<  Extension  (%) 
(..Tenacity    (g.p.d.) 

27-0 
11-1 
1-8 

17-6 
9-1 
1-6 

12-2              26-4 
9-2       :       24-6 
1-7       ;         2-3 

18-3 
24-3 
2-1 

16-6 
23-3 
2-3 

26 
18 
2 

•8 

•7 
•2 

21-4 
17-9 
2-3 

11-6 
17-6 
1-9 

WET 

fKnot  B/L  (Ib.) 
1  Extension  (%) 
^  Tenacity  (g.p.d.) 

26-0 
9-6 
1-8 

20-8 
9-6 
1-9 

14-3              21-7 
10  5        ,       26-7 
1-9                1-9 

16-4 
25-2 
1-8 

13-1 
22-5 
1-8 

20 

f 

•3 

•7 
•7 

17-9 
21-7 
1-9 

12-1 
21-4 
2-0 

Abrasion  dry  revs. 

1412 

1921 

2090 

5949 

8093 

10024 

3277 

6817 

8974 

„        wet  revs. 

1098 

1429 

1785 

1891 

1095 

1561 

1649 

1968 

2622 

[44] 


VALUE     AND     USE      OF    TERYLENE 


e.g.  trawls,  to  be  produced  more  economically.  All  the 
high  tenacity  yarns  are  bright. 

The  second  category  comprises  a  yarn  with  a  tenacity 
of  4J  to  5J  grams  per  denier  and  a  corresponding  exten- 
sion at  break  of  25  to  15  per  cent.  It  is  intended  mainly 
for  wearing  apparel  and  is  produced  in  deniers  ranging 
from  25  to  150,  most  of  which  can  be  obtained  in  either 
bright  or  delustred  forms. 

A  staple  fibre,  with  a  tenacity  of  3£  to  4  grams  per 
denier  and  a  corresponding  extension  at  break  of  40  to 
25  per  cent.,  is  currently  being  manufactured,  ranging  in 
staple  length  from  Ij  to  6 in.  and  from  Ik  to  6  denier 
(that  is  U  in. staple,  14  denier)  for  processing  on  the  cotton 
system,  up  to  6  in.  staple,  6  denier  for  processing  on  the 
flax  system.  All  staple  fibre  has  a  heat  stabilized  crimp 
and  is  available  with  a  dull-lustre.  Filament  yarn  is 
twisted  to  three-quarters  of  a  turn  per  inch  and  supplied 
to  the  trade  on  bobbins. 

The  fibre  is  made  from  polyethylene  terephthalate,  a 
condensation  product  of  terephthalic  acid  and  ethylene 
glycol,  both  of  which  are  derived  through  various 
chemical  processes  from  the  products  of  mineral  oil 
cracking.  The  polymer  is  chipped,  and  the  fibre  produced 
by  a  melt  spinning  process.  Molten  polymer  is  pumped 
through  a  spinneret  and  the  spun  yarn  is  subsequently 
mechanically  stretched  to  develop  fibre-like  properties. 
The  filament  yarn  has  been  exported  overseas  to  be  made 
into  twines  and  nets.  In  certain  countries,  where  limited 
doubling  facilities  are  available,  twines  have  been  ex- 
ported by  manufacturers  in  the  United  Kingdom. 
Complete  nets  have  also  been  supplied  in  certain  instances. 

The  fishing  industry  is  mainly  interested  in  the  high 
tenacity  filament  yarn  on  the  grounds  of  high  strength 
and  low  extensibility,  although  the  staple  fibre  is  used 
for  specialized  purposes,  such  as  net  mounting  ropes. 

PHYSICAL  PROPERTIES 

The  most  important  physical  properties  which  this  yarn 
can  offer  for  use  as  twines  or  netting  are: 

(i)  High  tensile  strength  which  is  unaffected  by 
wetting.  In  particular,  it  has  a  high  wet  knot 
strength. 

(ii)      Low  extensibility  and  high  modulus, 
(iii)     Rot  proof  and  not  weakened  by  mildew, 
(iv)     Good  resistance  to  sunlight, 
(v)      Stability  on  water  immersion  coupled  with  low 

moisture  absorption. 

(vi)     Good  resistance  to  abrasion   under  wet  con- 
ditions, 
(vii)    The  smooth   nature  and  transparency  of  the 

material. 

Other  fibres  mainly  used  for  netting  twines  are  cotton, 
flax,  manila,  sisal  and  poly  vinyl  alcohol  such  as  Kuralon, 
and  the  polyamides  Perlon  and  Nylon.  The  general 
physical  properties  of  a  range  of  Terylene  fishnet  twines 
in  comparison  with  twines  made  from  the  fibres  men- 
tioned above  are  given  in  Table  I. 

A  study  of  this  table  demonstrates  the  advantages 
which  the  fibre  has  to  offer.  A  loss  in  strength  on  knotting 
is  common  to  all  fibres,  and  synthetic  fibres  may  lose 
relatively  more  strength  on  knotting  than  natural  fibres, 
such  as  cotton  and  linen,  but  as  the  initial  strength  of 


O«Y 


K«OT 

TCNACITV 


WET 


19O/»/»         J9O/4/9 


Single  Knot  Tenacity  of  Terylene  Twines. 


Terylene  is  very  much  higher,  its  actual  knotting  strength 
remains  above  the  level  of  natural  fibres. 

In  the  dry  state  the  twines  may  have  a  slightly  lower 
dry  knot  tenacity  than  comparable  nylon  twines.  The 
wet  knot  tenacities  of  the  two  are  not  significantly 
different  (see  figs.  1  and  2).  The  wet  knot  strength  of  a 
fishnet  twine  is  obviously  of  more  importance  than  its 
dry  knot  strength.  The  strength  of  a  twine  depends  not 
only  on  the  fibre  from  which  it  is  made,  but  also  on  its 
construction  (twist,  number  of  strands,  etc.).  The  tables 
also  show  the  low  extensibility  of  Terylene  and  its  resis- 
tance to  stretch.  This  facilitates  the  manufacture  of  nets 
with  mesh  sizes  which  conform  to  the  required  specifica- 
tion and  which  resist  distortion  in  use. 

Terylene  twines  do  not  shrink  when  immersed  in  water 
at  ambient  temperature,  but  they  will  shrink  at  elevated 
temperatures.  If  the  nets  are  to  be  subjected  to  various 
heat  treatments,  e.g.  dyeing,  prior  to  use,  and  the  mesh 
size  is  critical,  an  allowance  should  be  made  for  shrinkage. 


DRY 


KNOT 

TENACITY 


!«/«/«  HO/4/1 


Fig.  2 
Single  Knot  Tenacity  of  Nylon  Twines. 


[45 


MODERN     FISHING     GEAR     OF    THE    WORLD 


The  abrasion  resistance  of  the  twines  is  superior  to 
those  made  from  cotton  and  flax.  Unlike  most  synthetic 
fibres,  its  abrasion  resistance  is  not  appreciably  different 
under  wet  conditions.  The  data  above  were  obtained 
by  rubbing  both  wet  and  dry  twines  over  a  3  mm.  thick 
hardened  carbide  steel  bar.  The  abrasion  resistance  of 
nylon  twines  is  significantly  better  than  that  of  Terylene 
twines,  but  the  difference  under  wet  conditions  is  not 
nearly  so  marked.  In  practice  Terylene  fishnets  can  be 
expected  to  show  good  wearing  qualities  because  of  their 
good  wet  abrasion  resistance.  Mention  has  already  been 
made  of  the  low  moisture  absorption  of  the  fibre  (Table  J) 
and  this  is  clearly  shown  in  the  graph  (fig.  3).  This  means 
that  on  immersion  in  water  the  twines  do  not  swell,  the 
mesh  sizes  of  the  nets  remain  intact,  little  change  in 
effective  net  weight  occurs,  and  the  twines  dry  quickly. 

The  smoothness,  fineness  and  transparency  of  the 
material  eliminate  air  bubbles  and  contribute  to  a  low 
order  of  visibility. 

The  fibre  has  very  good  resistance  to  chemical  attack, 
in  particular  to  acids  and  oxidizing  agents.  It  is  thus 
resistant  to  sea  water  attack  and  unaffected  by  contact 


THE  TWINU  we»I  IMMIRUD  IN  TAP  MATCH   FOR  >*  HRI,   LEFT  TO 

DR»   FOR    IS  MINI     TMCN  WEIGHED  AT   HOURLY     INTERVALS 


fig.  3 
The  rate  of  drying  of  Manila,  Sisal,  Nylon  and  Terylene  Twine\. 


with  oils,  cutch  and  tar.  This  has  been  demonstrated  in 
certain  areas  where  traditional  nets  were  affected  by 
chemical  contamination  which  had  no  effect  on  Terylene 
nets. 

In  connection  with  the  use  of  Terylene  in  twines  and 
nets,  the  following  points  are  worthy  of  mention: 

KNOT  SLIPPAGE 

In  the  manufacture  of  fishing  nets,  two  types  of  knots 
are  commonly  used,  the  single  sheet  bend  (single  weaver's 
knot)  and  double  sheet  bend  (double  weaver's  knot). 
The  latter  type  does  not  slip  during  use  and  double 
knotted  twines  have  a  knot  strength  about  10  per  cent, 
higher  than  single  knotted  twines.  Both  Terylene  and 
nylon  single  knotted  nets  are  prone  to  knot  slippage,  but 
the  tendency  is  less  with  Terylene.  Tests  carried  out  on 
samples  of  Terylene,  nylon,  cotton  and  linen  single 
knotted  gill  nets  showed  that  three  out  of  every  four 
knots  slipped  in  the  case  of  nylon  when  dry  and  one  in 
four  when  wet.  One  out  of  every  four  Terylene  knots 
slipped  when  dry,  but  none  when  wet.  No  knot  slippage 
occurred  with  the  cotton  net  and  only  occasionally  with 
the  linen  net.  However,  the  resistance  of  Terylene  to 
single  knot  slippage  is  not  considered  to  be  good  enough 
and  as  more  single  knotted  fish  netting  is  produced  than 
any  other,  knot  slippage  constitutes  a  problem.  It  is 
said  that  the  somewhat  slow  rate  of  production  of 
double  knotting  machines  has  been  improved  and  is  now 
about  the  same  as  the  single  knotting  machine. 

Knot  slippage  is  a  result  of  the  smoothness  of  synthetic 
filament  yarns,  and  an  obvious  answer  is  to  increase 
the  coefficient  of  friction  of  the  twines  by  the  application 
of  a  surface  coating.  The  anti-slip  agent  may  be  applied 
to  the  twine  during  manufacture  of  the  net,  or  to  the 
finished  net  to  fix  the  knots.  Some  net  manufacturers 
consider  both  pre-  and  post-treatments  are  necessary. 
The  general  view  seems  to  be  that  the  nets  should  be 
made  from  bonded  twines  to  enable  an  undistorted 
net  to  be  taken  off  the  machine  and  safely  transported 
to  a  stretching  frame  to  tighten  and  fix  the  knots.  In 
certain  cases  it  is  possible  to  avoid  the  use  of  prebonding 
agents  by  the  application  of  very  high  tensions  at  the 
back  of  the  loom.  These  consolidate  the  knots  sufficiently 
for  net  handling  prior  to  the  stretching  treatment. 

The  bonding  agents  recommended  for  Terylene  fishnet 
twine  are  Colophony  resin  and  Bedesol  76.  Colophony 
resin  is  applied  from  a  solution  in  methylated  spirits  or 
aqueous  ammonia.  Bedesol  76  is  applied  from  a  solution 
in  64  deg.  over  proof  methylated  spirits.  Both  bonding 
agents  can  be  applied  either  by  single-end  gumming  at  a 
speed  of  100  to  150  yards  per  minute,  or  by  hank  dipping 
for  15  minutes,  draining,  and,  in  the  case  of  the  ammonia 
solution  of  Colophony  resin,  drying  for  one  hour  at 
80  deg.  C.  The  percentage  solids  of  resin  required  on  the 
twines  depend  on  the  net  making  machine  used.  For  a 
Seriville  machine,  the  required  pick-ups  of  Colophony 
resin  and  Bedesol  76  are  1  -0  per  cent,  and  2  •  5  to  3  •  5  per 
cent,  respectively.  The  former  is  achieved  using  a  5  per 
cent,  solution  and  the  latter  a  6  per  cent,  solution.  For 
the  Zang  machine  the  respective  pick-ups  are  2  to  3 
per  cent,  and  5  to  7  per  cent.  In  the  first  case  a  7  to 
8  per  cent,  solution  is  required  and  in  the  latter  a  12  to 


[46] 


VALUE    AND     USE    OF    TERYLENH 


15  per  cent,  solution.  As  twines  bonded  from  methy- 
lated spirit  solution  may  be  slightly  sticky  and  as  the 
solvent  is  inflammable,  bonding  from  aqueous  ammonia 
should  be  followed  by  drying  at  80  deg.  C.  It  also  has 
the  advantage  of  keying  the  resin  to  the  fibre.  The 
differences  in  optimum  pick-up  of  the  bonding  agent  for 
the  Zang  and  Seriville  machines  are  a  feature  of  the  more 
critical  operating  conditions  for  the  Seriville  machine. 

As  already  mentioned,  the  net  itself  can  be  treated 
with  bonding  agents  to  prevent  knot  slippage.  Both 
Colophony  resin  and  Bedesol  76  have  been  satisfactorily 
used  in  concentrations  akin  to  those  for  bonding  twines. 
The  net  is  opened  out  as  much  as  possible  to  ensure  that 
all  of  it  comes  into  contact  with  the  solutions.  It  is 
immersed  for  a  minimum  period  of  10  minutes,  with 
stirring  or  agitation  to  ensure  an  even  application  of 
solids.  The  net  is  then  removed  and  dried.  A  more  even 
application  of  the  solids  is  obtained  if  the  net  is  cen- 
trifuged  to  remove  surplus  solution  before  drying. 
Drying  at  a  temperature  of  80  deg.  C.  is  preferable  to 
drying  in  air  at  room  temperature,  since  the  latter 
process  is  very  much  slower,  and  leaves  a  net  slightly 
sticky.  About  4  per  cent,  allowance  should  be  made  for 
the  shrinkage  in  drying  at  80  deg.  C. 

During  manufacture  of  the  net  it  is  important  to  ensure 
that  the  tensions  applied  to  the  twine  are  sufficient 
to  pull  the  knots  tight.  In  addition  to  being  stretched, 
nets  can  also  be  given  a  steam  or  hot  air  treatment  to 
consolidate  the  knots.  Steaming  the  nets  on  a  frame  at 
150  deg.  C.  for  15  minutes  has  been  found  to  be  most 
satisfactory. 

As  an  alternative  method  P.V.C  bonded  twines  have 
been  used  (about  5  per  cent,  pick  up).  The  success  of  nets 


produced  from  such  twines  has  been  reported  from 
Sweden.  As  a  means  of  avoiding  the  use  of  bonding 
agents,  Terylene/cotton  and  Terylene/spun  acetate 
mixture  twines  have  been  produced.  Single  knots  pro- 
duced from  such  twines  do  not  slip.  It  is  also  noteworthy 
that  Terylene  staple  twines  can  be  single  knotted  without 
slippage  occurring. 

DYEING 

In  many  instances  dyeing  has  been  shown  to  give 
increased  catches  (e.g.  blue-grey  nets  for  Norwegian 
lake  trout),  but  whether  this  applies  generally  to  dyed 
synthetic  nets  has  yet  to  be  confirmed.  However,  fisher- 
men usually  demand  nets  dyed  to  a  wide  variety  of 
shades,  from  reddish-brown  to  green  and  blue,  and  in 
many  cases  they  prefer  to  dye  their  own  nets. 

No  difficulties  are  to  be  expected  in  nets  made  from 
dyed  Terylcne  twines.  The  twist  set  yarns  may  be  dyed 
by  using  the  appropriate  equipment  under  normal 
dyeing  techniques,  i.e.  with  disperse  dyestuffs  for  90 
minutes  at  the  boil  (with  or  without  carrier),  or  at  higher 
temperatures  under  superatmospheric  pressures.  If, 
however,  the  fisherman  wishes  to  dye  his  own  nets  the 
matter  is  not  so  simple,  because  the  necessary  equipment 
is  often  not  available  for  dyeing  at  the  boil,  although 
suitable  dyestuflf  packages  are  available  to  enable  the 
fisherman  to  dye  his  nets  satisfactorily  to  a  variety  of 
shades.  As  mentioned  previously,  an  allowance  for 
shrinkage  must  be  made  if  the  mesh  size  of  the  net  is 
critical. 

Tinting  of  bonded  twines  at  ambient  temperature  has 
been  successfully  carried  out  and  it  is  possible  also  to  tint 


4OO  41*  5OO 

HOUR*  OF        SUNiHINt 


F/V.  4.     Weather  Tendering  of  Terylene  and  other  fibres. 
[47] 


MODERN     FISHING     GEAR     OF    THE    WORLD 

TABIF  II 
Average  variation  from  nominal  mesh  size  of  different  nets  when  tested  dry  and  after  immersion  in  water  for  15  minutes  and  24  hours 


Linen 
Cotton 
Nylon 
Terylene 


Dry 

1-92",,  to    I  1-92°;, 

-1-16%  to  H  1-16% 

2-35%  to  +2-59% 

1-16%  to    tO-58% 


After  immersion 
for  15  minutes 


2-88';0  to      ()-%°(, 

-- -.VS-V.'u  to  Nominal 

-2-22%  to    }  2-00"0 

-1-16%  to    i  2-32";, 


After  immersion 
for  24  hours 

2-88"u  to  0-96% 
4-05%  to  Nominal 
2-06%  to  !  2-34°;, 
0-58%  to  i  2-32% 


(Data  supplied  by  Dominium  Textile  Company  Limited  prepared  for  Canadian  Government  Specification  Board). 


the  unbonded  twine  or  net  at  ambient  temperature  by 
using  disperse  dyestuffs  dissolved  in  chlorinated  hydro 
carbons,  but  some  shrinkage  of  the  net  may  occur. 
Asphalt  based  solution  can  be  used  to  stain  and  stiffen 
the  net  if  required. 

MESH  RETENTION 

Some  tests  have  been  carried  out  on  the  mesh  stability 
of  various  types  of  net.  The  mesh  was  measured  dry  at 
room  temperature,  before  being  immersed  in  water,  and 
also  after  the  nets  had  been  immersed  for  15  minutes  and 
24  hours  respectively.  A  summary  of  the  results  is 
given  in  Table  II,  and  the  superiority  of  the  fibre  in 
respect  of  mesh  size  retention  is  clearly  illustrated.  It 
is  interesting  to  note  that  after  immersion  for  24  hours 
cotton  shows  the  greatest  variation  with  an  average 
shrinkage  of  4-05  per  cent.,  followed  by  linen,  nylon  and 
Terylene  in  that  order.  It  is  submitted  that  a  tolerance  of 
3  per  cent,  above  or  below  the  nominal  mesh  size  is  a 
realistic  approach.  This  excellent  mesh  size  retention  of 
Terylene  is  of  obvious  importance  with  respect  to  gill- 
netting. 


SUNLIGHT  EXPOSURE 

The  fibre  has  about  the  same  resistance  to  daylight  and 
weather  as  the  best  of  the  natural  fibres  (see  fig.  4),  but 
because  it  has  a  higher  strength  premium  and  is  rot 
proof  it  has  a  longer  useful  life.  Trials  have  been 
carried  out  with  Terylene  and  nylon  twines  exposed  to 
daylight  and  weather  in  the  United  Kingdom,  and  the 
results  are  given  in  Table  III  and  fig.  5.  Even  though 
the  overall  time  of  exposure  is  comparatively  short  the 
superiority  of  Terylene  is  clearly  demonstrated.  Terylene 
nets  and  others  made  from  natural  and  synthetic  fibres 
arc  now  being  exposed  in  several  countries,  such  as 
Canada,  Kenya,  India  and  Scandinavia,  but  it  is  too 
early  to  draw  conclusions  from  these  tests. 


DURABILITY 

The  general  toughness  of  the  fibre,  its  very  good  mechan- 
ical properties,  complete  resistance  to  rotting  and  good 
resistance  to  sunlight  and  weather,  ensure  a  long  life  for 
Terylene  nets. 


TABLE  III 
Comparison  of  Terylene  and  Nylon  Twines  after  Weather  Kxposure  taken  at  Monthly  Intervals 

Constructional  details  of  Twines 

Terylene        .         Resultant  denier  4,190        Structure  250/5/3 
Nylon  .         Resultant  denier  3,570        Structure  210/5/3 


Total                                    TER  YLENE                                                          ,V  YLON 

Exposure  Dates 

Sun  Hours 

H,!!n-i           Breaking     Extension     Tenacity     °;,  loss  in      Break  inK     Extension     Tenacity     %  loss  in 

Loaddb.)         "0            (*»!./</.)      Bk.Load     Loaddb.)         %          (*/ff./rf.)       Bk.  Load 

Control 

477             23-6               52               —                 42-5             30-6               5-4 

1st  July    —31st  July 

137-63 

137-63            37-4             18-0              4-1             21-6              33-3            27-4              4-2            21-6 

1st  Aug.  —31st  Aug. 

117-2 

254-83            32-0            15-9              3-5            33-2              28-4            24-8              3-6            33-2 

1st  Sept.  —30th  Sept. 

108-7 

363-53            29-6            14-9              3-2            37-9              24  1            22-5              3-1            43-2 

1st  Oct.   —31st  Oct. 

127-9 

491-13            29-7            18-6              3-2            37-7              22-6            21-6              2-9            46-8 

1st  Nov.  —30th  Nov. 

55-6 

547-03            27-0             14-0              2-9            43-4              22-1             21-3              2-8            48-0 

1st  Dec.  —31st  Dec. 

13-1 

560-13            26-0            15-0              2-8            45-8              16-2            19-8              21            61-9 

1st  Jan.   —31st  Jan. 

30-69 

59082            26-4            14-9              2-9            446              18-7            21-1              2-4            56-0 

1st  Feb.  —28th  Feb. 

73-3 

664-12            27-0             14-7              2-9            43-4               17-0            18-6              2-2            60-0 

1st  Mar.  —31st  Mar. 

80-2 

744-32            27-1             16-5              2-9            43-2              14-8            195              1-9            68-2 

1st  April  —30th  April 

133-0 

877-32            25-7            16-9              2-8            46-1               15-3            22-1               1-9            64-0 

48] 


VALUE    AND     USE    OF    TERYLENE 


NYLON    HO/SJl 


1O        4O        «0 


<\nnpanwn  of  250  5  3  Tetvlenc  ami  210:5/3  Nylon  T\\ines  after  Exposure 


EASE  OF  HANDLING 

Nets  made  of  the  fibre  are  easy  to  handle  because  thinner 
twines  can  be  used  to  give  lighter  nets.  Trawl  nets,  tor 
example,  can  be  towed  more  easily.  This  weight  saving 
factor  makes  it  possible  to  use  larger  nets  or,  alternative!), 
a  small  vessel  can  be  used  to  handle  a  net.  The  low 
moisture  uptake  means  little  effective  change  in  weight, 
and  the  net  is  less  prone  to  freeze  under  icy  conditions. 
The  somewhat  high  specific  gravity  of  the  fibre  permits 
a  net  to  sink  more  rapidly,  while  the  high  resistance  to 
stretch  is  favourable  to  easy  hauling. 

COST 

The  nets  are  more  expensive  than  natural  fibre  nets  but 
they  provide  considerable  savings  on  a  price/life  basis. 
In  fairness  it  must  be  pointed  out  that  the  initial  cost  of 
the  netting  is  sometimes  very  considerable  and  may  be 
more  than  some  fishermen  can  afford.  The  risk  of 
accidental  damage  and  loss  must  also  be  borne  in 
mind.  The  difference  in  price  between  Terylcne  and  the 
natural  fibres  becomes  less  marked  in  nets  because  twines 
of  greater  runnage  can  be  used.  The  price  of  Terylene 
compares  favourably  with  that  of  nylon  in  terms  of 
pence  per  pound,  but,  because  of  its  greater  specific 
gravity,  the  twines  have  less  runnage  than  nylon  twines 
of  equal  thickness.  Weight  for  weight,  however,  the 
twines  have  identical  runnage. 


CONSTRUCTION : 

Cords  and  Twines 

The  fishnet  twines  are  normally  produced  from  125  and 
250  denier  high  tenacity  filament  yarn  with  plied  and 
cabled  constructions.  In  general,  twist  is  inserted  in 
the  singles  yarns  in  twines  having  plied  constructions. 
The  ply  twist  required  to  produce  a  ''balanced"  twine 
can  be  obtained  from  the  formula: 


t.p.i.  strand 


—  t.p.i.  singles 
A  No.  of  strands 


The  negative  sign  indicates  opposite  twist.  No  twist  is 
inserted  in  the  singles  yarn  of  twines  having  cable 
constructions.  The  twine  twist  required  to  produce  a 
balanced  cord  is  given  by  the  formula: 


t.p.i.  twine 


—  t.p.i.  strands 
v  No.  of  twines 


The  amount  of  twist  inserted  at  each  stage  in  tne  pro- 
duction of  the  fishnet  twines  depends  on  the  hardness 
of  "handle"  required  and  most  manufacturers  have 
certain  twist  factors  which  enable  them  to  calculate  the 
twist  required.  In  order  to  produce  a  twine  from  Terylene 
filament  yarn  which  has  a  balanced  construction  and  a 
good  yarn  to  twine  strength  conversion  efficiency,  twist 
factors  in  the  range  3-6  to  4-6  are  suggested. 


[49] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


14 


12 


TPI     TWINC      =z    T  Rl.  STRAND 


/ 


No   OF  TWlHf 


X 


yl8 

§ 


»-6 


10  12  14 

T.PI        STRAND 


16 


18 


2O 


22 


24 


Note:  Twist  factor  ~ 


t.p.i.  x  V  denier 

73 


The  construction  for  a  range  of  the  filament  twines  are 
given  in  Table  IV. 


The  information  given  in  this  table  has  been  interpreted 
graphically  (see  figs.  6  and  7)  and  from  these  the  twist 
required  for  the  production  of  a  range  of  twines  having 
stranded  constructions  may  be  obtained.  The  use  of 
balanced  twists  gives  twines  which  are  completely  dead. 


TABLE  IV.    Terylene  Fish-Net  Twine  Constructions 


Breaking 
load 
(lb.) 


Twist  Factor 
3-6  (/./>./.) 


Strand 
Twist 


Twine 
Twist 


Twist  Factor 
4  0 


Strand 
Twist 


Twine 
Twist 


Twist  Factor 
4-6  (t.p.i.) 


Strand 
Twist 


Twine 
Twist 


125/2/3       . 

100        !         16-5 

9-5                  18-2 

10  5 

21-0 

12-2 

125/3/3       . 

15-0                  14-0 

8-1                  16-8 

9-75 

18-0 

10-4 

125/4/3       . 

19-8         '         11-5 

6-7 

13-0 

7-5 

14-7 

8-5 

125/5/3      . 

24-4 

10-5 

6-1 

11-9 

6-9 

13-5 

7-8 

125/6/3       . 

29-7         j           9-5 

5-5         ;         10-75 

6-2 

12-1 

7-0 

125/7/3       . 

35-0 

9-0 

5-2 

9.9 

5-7 

11-4 

6-6 

125/8/3       . 

39-6                   8-5 

4-8 

9-1 

5-25       i         10-7 

6-2 

125/9/3       . 

44-5         '           8-3 

4-6 

8-7 

5-0        !         10-2 

5-9 

125/10/3     . 

49-5 

7-8 

4-3 

8-25 

4-75 

9-5 

5-5 

250/2/3       . 

19-8 

11-5 

6-7 

13-0 

7-5 

14-7 

8-5 

250/3/3       . 

29-7 

9-5 

5-5 

10-75 

6-2 

12-1 

7-0 

250/4/3 

39-6 

8-5 

4-8 

9-1 

5-25 

10-7 

6-2 

250/5/3       . 

49-5 

7-8 

4-3 

8-25 

4-75 

9-5 

5-5 

250/6/3       . 

59-5 

6-9 

3-9 

7-5 

4-3 

8-7 

5-0 

250/7/3       . 

69-3 

6-3 

3-7 

7-0 

4-0 

8-1 

4-7 

250/8/3       . 

79-3 

6-0 

3-4 

6-5 

3-75 

7-4 

4-3 

250/9/3       . 

89-0 

5-7 

3-2 

6-25 

3-6 

7-1 

4-1 

250/10/3     . 

99-0 

5-3 

3-0 

5-9 

3-4 

6-6 

3-8 

50] 


VALUE     AND     USE     OF    TERYLENE 


TWIST  FACTOR  =  T.P.I.  X  VOENIER 


73 


.ISO/ft/l 


\ 


X 


.   -    2SO/S/1 


\ 


\ 


-   250/1/3 


.   »  7  SO /2/1 


TWINE   TWIST    (T.P.I.) 

/•'iff.   7.     twine  Twists  foi   Twines  with  Twist  /actors  of  4  ft,  4-0,  3 -ft. 


TARLF  V.     1  cry  lene/I  ilament  Acetate  Twines 


Afo.  3A» 


Aw.  4 


r>.  6 


,V«».  7 


Construction 

5/3 

6/3 

6/3 

6/3 

10/3 

7/3 

3/3 

4/3 

3 

250  T 

4       250  T 

3       250  T 

3       250  T 

3       250  T 

4       250  1 

2   250  T 

2       250  T 

2 

.'   200  Ac 

2       200  Ac 

3       200  Ac 

3       200  Ac 

4    -   200  Ac 

3        200  Ac 

1    200  Ac 

2       200  Ac 

Nominal  denier 

3450 

3700 

4050 

4050 

6900 

4800 

2100 

2700 

Resultant  denier 

3574 

4380 

4216 

4368 

7348 

5066 

3172 

2801 

T.p.i.    *'S"  strand 

8-0 

7-5 

7-1 

6-7 

6-7 

6-9 

9-7 

8-2 

"Z"  twine 

3-5 

2-5 

2-2 

6-0 

2-5 

2-8 

4-5 

3-3 

Strength 

33-5 

44-2 

35-4 

35-2 

64-0 

44.9 

23-0 

23-8 

Extension 

15-0 

15-8 

16  0 

17-0 

25-1 

22-6 

14-7 

15-6 

Tenacity 

4-25 

4-58 

3-81 

3-65 

3-9 

4-0 

4-8 

3-8 

Knot    Strength 

16-8 

20-6 

19-4 

18-0 

33-2 

23-4 

10  8 

12-4 

Extension 

9-1 

8-3 

9-1 

10-0 

10-9 

8-8 

7-3 

9-1 

Tenacity 

2-13 

2-14 

2-08 

1-87 

2-1 

21 

2-3 

2-0 

Wet  Strength    . 

34-4 

44-0 

34-2 

33*0 

64-8 

44-8 

21-9 

21-9 

Extension 

14-6 

15-4 

15-0 

16-8 

24-4 

15-4 

14-7 

14-5 

Tenacity    . 

4-37 

4-56 

3-68 

3-43 

4-0 

4-0 

4-6 

3-5 

Wet  Knot  Strength 

16-4 

19-1 

17-2 

15-9 

31-7 

20-6 

9-9 

12-1 

Extension 

11-3 

8-8 

10-3 

12-0 

12-7 

10-7 

9-3 

10-3 

Tenacity    . 

2-08 

1-98 

1-85 

1-65 

19 

1-8 

2-1 

2-0 

Dry  Knot  Slippage 

None 

None 

None 

1  in  5 

None 

None 

None 

None 

Wet    „ 

2  in  5 

None 

None 

None 

None 

None 

1  in  5 

None 

151  ] 


MODERN     FISHING     GEAR     OF    THE    WORLD 


show  no  tendency  to  snarl  and  have  maximum  strength. 
Some  net  manufacturers  prefer  to  use  twines  with  un- 
balanced twists  and  these  must  be  twist  set  before  net 
manufacture  to  consolidate  the  twist  and  prevent  twine 
liveliness.  The  twist  setting  process  is,  in  essence,  one  of 
free  shrinkage  and  as  such  the  properties  of  the  twine 
alter.  The  general  effect  is  to  increase  the  denier  and 
extension  at  break  and  to  decrease  tenacity,  shrinkage 
potential  and  initial  modulus  of  elasticity,  the  breaking 
load  remaining  unchanged.  Such  changes  may  detract 
from  the  performance  of  the  netting. 

Webbing 

Terylene  filament  twines  can  be  handled  on  traditional 
net  making  machines  and  the  nets  are  said  to  be  easier 
to  make  than  those  of  other  synthetic  fibres,  because  of 
the  twine's  high  resistance  to  stretch.  The  twines  have  been 
processed  satisfactorily  on  single  knot  machines,  such 
as  the  Zang  and  the  rather  more  critical  Seriville  machine. 
In  using  bonded  twines  care  must  be  taken  to  ensure  that 
the  correct  percentage  pick-up  of  bonding  agents  has 
been  achieved  so  that  the  net  making  machine  may 
function  at  optimum  efficiency.  Certain  machine  adjust- 
ments are  also  necessary  to  take  this  into  account.  In 
particular,  it  has  been  found  advantageous  to  alter  the 
number  of  turns  of  twine  round  the  emery  beam  and  also 
round  the  drag  rod  on  the  shuttle  holder.  Terylenc 
twines  have  been  used  successfully  on  double  knotting 
machines. 

NETS: 
Gillnets 

The  gillnet  market  overseas  is  at  present  the  largest 
consumer  of  synthetic  twines  and  Terylene  gillnetting 
has  been  used  successfully  in  many  parts  of  the  world, 


Skill  ami  care  ore  concentrated  on  this  stage  in  the  manufacture 
of  Tcrvlcne  ffillnetx. 


TABLt  VI.      "Terylene'VAcefate    Spun    Twines 


Construction 


Nominal  denier 
Resultant  denier 
T.p.i.    yarns  Acetate 
strand  "S" 
twine  "Z" 

Strength  (Ibs.) 
Extension  (%) 
Tenacity  (g.p.d.)  . 

Knot  strength 
Knot  extension 
Knot  tenacity 

Wet  strength 
Wet  extension 
Wet  tenacity 

Wet  knot  strength 
Wet  knot  extension 
Wet  knot  tenacity 

Knot  slippage 


No.  2 

6/3 

250  Terylene 
1/22's  Acetate 
4346 
4678 
12-5 
5-8 
5-3 

43-8 
16-0 
4-25 

16-7 
10-3 
1-62 


45-5 
15-7 
4-41 

19*6 
10-6 
1-90 

None 


No.  3 

6/3 

3   -'  250  Terylene          6 
3    *    1/22's  Acetate        4 
4419 
4492 
13-0 
5-4 
4-1 

36-9 
16-7 

3-73 

16-8 
10-0 
1-70 

36-3 
15-2 
3-67 

18-0 
10-0 
1-82 

None 


No.  4 

10/3 

250  Terylene 
I  /22's  Acetate 
7392 
7732 
12-0 
69 
3-1 

66-4 
23-9 
3-90 

29-8 
12  9 
1-75 

65-1 
22-3 
3-82 

30-0 

12-6 

1-81 

None 


4   « 
3  > 


No.  5 

7/3 

250  Terylene 
1/22's  Acetate 
5169 
5329 
12-0 
5-5 
3-7 

46-9 
16-0 
4-0 

20-8 
14-0 

1-77 


46-4 
15-2 
3-95 

21-8 
10  0 
1-86 

None 


[52] 


VALUE    AND    USE    OF    TERYLENE 


i\  \httwn  a 


in  the  manufacture  of  Terylene  gillnets. 


principally  for  catching  salmon  and  cod.  It  has  been  used 
in  Canada  for  fishing  salmon  oft*  the  West  Coast,  the 
nets  showing  up  to  particular  advantage  in  fast  moving 
water  or  ocean  currents,  where  lively  fish  can  dive  straight 
through  a  net  or,  when  caught  by  the  gills,  escape  by 
expanding  the  net  mesh.  The  small  diameter  of  the  twines 
may  lead  to  lower  visibility,  but  it  is  thought  that  this 
very  good  performance  can  be  attributed  essentially  to 
the  high  resistance  to  stretch. 

In  certain  types  of  synthetic  fibre  gillnetting  it  is  now 
becoming  customary  to  pull  out  the  fish  head  foremost 
which  is  speedier  than  the  normal  practice,  i.e.  com- 
pressing the  gills  and  pulling  the  fish  out  backwards. 
In  such  cases  the  high  resistance  to  stretch  is  somewhat  of 
a  disadvantage,  as  it  is  not  always  possible  to  clear  the 
nets  in  such  a  manner. 

The  gillnets  have  been  used  successfully  in  Kenya 
lakes  for  fishing  borus  and  tilapia,  while  cod  gillnets 
have  been  very  successfully  used  in  Scandinavian  waters. 
The  gillnets  are  being  increasingly  used  in  India. 

Such  gillnets  have  to  date  been  found  more  suitable 
for  catching  hard  rather  than  soft  fish.  The  twines  used 
have  been  fine  and  hard  and  have  cut  into  soft  fish,  such 
as  herrings,  to  such  an  extent  that  the  fish  have  become 
bruised  and  damaged.  When  removed  from  the  net  by 
shaking  the  fish  may  be  decapitated.  Chiefly  for  this 
reason,  Terylene  has  not  yet  been  extensively  used  for 
British  home  water  drift  netting.  However,  such  drift 
netting  is  being  developed  and  twine  constructions  have 
been  revised  to  give  thicker  and  more  suitable  twines. 
Further  trials  are  in  progress  to  develop  this  market. 


A  mixed  high  tenacity  Terylene  filament/spun  acetate 
netting  twine  is  being  developed,  principally  for  pilchard 
nets.  It  will  not  bruise  or  damage  the  fish  and  will  not  slip 
when  single  knotted.  Tables  V  and  VI  give  the  principal 
physical  properties  of  such  twines.  The  use  of  Terylene 
core  spun  cotton  yarns  is  also  being  considered  for  similar 
reasons,  but,  because  of  the  cotton  present,  it  could  not 
be  expected  to  be  so  rot  resistant  as  a  mixed  filament/spun 
acetate  yarn. 

Trawl  Nets 

The  filament  twine  has  been  used  with  success  for  bottom 
trawls.  Trials  were  carried  out  some  time  ago,  principally 
for  the  codend,  because  of  the  shortage  of  twine,  although 
some  complete  trawls  were  also  made  up.  These  nets 
were  made  by  the  Great  Grimsby  Coal,  Salt  and  Tanning 
Co.  Ltd.,  of  Grimsby,  and  the  tests  were  carried  out  by 
distant  water  trawlers  on  various  fishing  grounds.  It  was 
found  that  each  trawl  net  lasted  on  average  about  9  trips 
and,  in  one  case,  15  trips  were  made  before  the  net  was 
lost.  The  normal  trawl  net  is  generally  good  for  an 
average  of  one  trip.  It  was  noted  that  the  Terylene  nets 
had  good  mesh  stability  and  their  resistance  to  abrasion 
reduced  any  chance  of  the  codend  bursting  as  it  was 
hauled  in.  The  nets  were  much  easier  to  tow  through  the 
water  and  being  completely  rot-proof,  drying  was  un- 
necessary. It  is  understood  that  cleaner  catches  were 
obtained.  The  ship's  crew  reported  that  the  nets  were  more 
pleasant  to  handle  than  the  usual  nets.  The  lower 
moisture  uptake  was  particularly  advantageous  in  the 


153] 


MODERN     FISHING     GEAR     OF    THE     WORLD 


icy  operating  conditions  in  Northern  waters.  Another 
advantage  was  that  unloading  of  the  codend  was  some- 
times done  in  fewer  operations  because  of  its  greater 
strength. 

Risk  of  loss  or  accidental  damage  of  the  net  is  now 
being  greatly  reduced  by  introduction  of  Decca  equip- 
ment, which  is  of  special  advantage  when  expensive 
trawls  are  used.  The  longer  life  and  greater  security 
offered  by  such  nets  outweigh  the  occasional  losses. 
Already  a  number  of  trawler  companies  have  started 
their  own  trials  for  near,  middle  and  distant  water  fishing 
with  Terylene  trawl  nets. 

There  is  perhaps  a  need  for  the  use  of  thinner  twines 
to  reduce  the  price  of  the  net  and  without  doubt  the 
introduction  of  an  extra  heavy  denier  yarn  will  enable 
nets  to  be  produced  more  economically. 

Appreciable  quantities  of  the  fibre  are  now  being  used 
in  Sweden  for  mid-water  trawls  and  trials  with  such 
trawls  arc  also  being  conducted  in  the  United  Kingdom 
by  the  Ministry  of  Agriculture,  Fisheries  and  Food 

Purse  Seine  Nets 

Until  comparatively  recently,  little  has  been  done  with 
Teiylene  purse  seine  nets,  principally  because  of  their 
cost,  which  may  be  as  much  as  £5,000. 

Trials  have  been  carried  out  in  Canada  with  drum 
seines  near  Deep  Water  Bay  off  the  West  Coast.  The 
netting,  whilst  only  about  a  quarter  of  the  weight  of 
corresponding  tarred  cotton  netting  and  with  twines  a 
third  as  fine,  was  about  10  per  cenf.  stronger  in  the  wet 
mesh.  This  means  that  smaller  vessels  can  be  used  or 
alternatively  outsize  seines  can  be  carried  by  vessels 
which  normally  work  with  the  smaller  seine  nets. 

The  nets  wound  easily  on  to  the  drum,  but  some 
difficulty  was  experienced  in  playing  out,  the  loose  bights 
of  netting  tending  to  get  entrapped.  The  bunt  end  of 
the  seine  was  said  to  be  easily  held.  There  was  less  drag 
on  the  net  so  that  it  could  be  closed  much  more  quickly 
than  can  normal  purse  seines.  This  is  a  real  asset, 
ensuring  a  quicker  and  more  efficient  fishing  operation. 
The  nets  were  held  more  easily  against  the  tide,  which 
means,  firstly,  they  can  be  used  in  faster  waters,  thus 
enabling  fishermen  to  operate  in  more  fishing  grounds 
and,  secondly,  fishing  time  can  be  extended.  The  nets 
tended  to  become  entangled,  when  fish  could  only  be 


extracted  with  difficulty,  a  disadvantage  arising  from 
their  high  order  of  flexibility.  This  could  be  overcome  by 
using  coarser  twines  or  applying  suitable  coating  agents. 

Despite  the  disadvantages  the  preliminary  results  were 
most  encouraging  and,  with  the  suggested  modifications, 
it  should  be  possible  to  use  such  purse  seines  with  great 
profit.  It  is  worthy  of  note  that  the  somewhat  higher 
specific  gravity  of  the  fibre  compared  with  other  synthetic 
fibres  was  an  advantage,  since  it  enabled  the  net  to  sink 
more  easily.  The  lightness  in  weight  of  the  netting  is 
particularly  useful  for  table  seines. 

More  extensive  trials  of  these  purse  seines  nets  are  now 
being  made. 

Purse/Lampara  Nets 

Many  standard  purse/lampara  seine  nets  used  in  Walvis 
Bay,  South  West  Africa,  have  failed  prematurely,  due  it 
is  thought,  to  chemical  contamination,  but  a  preliminary 
test  of  Terylene  netting  for  use  in  these  waters  has 
proved  most  encouraging.  Detailed  trials  are  now  in 
progress. 

Seine   Nets 

Trials  with  Terylene  seme  nets  arc  to  be  carried  out 
shortly  in  the  United  Kingdom. 

OTHER    APPLICATIONS 

Other  applications  include  lines  and  snoods.  A  fairly 
substantial  market  is  developing  in  Norway,  where  the 
use  of  the  twines  has  resulted  in  increased  catches.  The 
high  resistance  to  stretch  facilitates  pulling  in  the  line 
and  it  is  easier  to  tell  when  the  fish  are  hooked  as  a  more 
definite  response  is  obtained.  Strength  stability  on  wetting 
and  the  quick  drying  properties  arc  also  important.  The 
lines  are  slightly  more  expensive  but  this  is  offset  by 
fishing  performance. 

The  twines  are  being  tried  for  lobster  pots  in  the  United 
Kingdom,  chiefly  because  of  their  rot  resistance  and 
general  toughness. 

Fishnet  mounting  ropes  of  spun  Terylene  (flax 
system)  cordage,  which  are  used  for  supporting  the  nets, 
have  been  tried.  The  staple  yarn  is  preferred  to  the 
continuous  filament  yarn  because  its  hairy  nature 
minimises  slippage  of  the  net  and  corks.  Headlines  of 
the  fibre  are  also  being  tested. 


54 


TEVIRON   FISHING  NETS 

by 
THE  TEIKOKU  RAYON  CO.  LTD. 

Edobori— Minamidori,  Nishiku,  Osaka,  Japan 

Abstract 

The  Teikoku  Rayon  Co.  Ltd.  introduced  in  October  1956  the  new  synthetic  fibre  Teviron,  made  from  poly  vinyl  chloride,  which  can 
be  obtained  either  as  filament  or  staple  fibre,  the  former  having  a  silk-like  appearance,  the  latter  resembling  wool. 

This  paper  deals  with  the  use  of  this  material  in  the  manufacturing  of  fishing  gear  and  it  is  pointed  out  that,  in  competition  with 
other  yarns,  it  ranks  next  to  cotton  in  price.  Other  advantages  are  explained  and  tests  have  shown  that  Teviron  can  compete  with  Nylon 
in  the  Salmon  and  Trout  fisheries,  although  Nylon  has  long  been  rated  the  best  material  for  those  fisheries. 


Resume 


Filets    de    peche    en    Teviron 


The  Teikoku  Rayon  Co.,  Edobori— Minamidori,  Nishiku,  Osaka,  Japon,  a  mis  sur  le  march£  en  octobre  1956  une  nouvelle  fibre 
synthetique,  le  Teviron,  obtenue  £  partie  du  chlorurc  de  polyvinylc  qui  se  pr£sente  sous  Paspecl  soit  de  filaments  soyeux,soitdefibresressemblant 
a  la  laine. 

Ce  document  est  consacr£  £  1' utilisation  dc  ce  material  pour  la  fabrication  d'engins  de  peche  et  on  fait  remarquer  que,  par  comparaison 
avcc  d'autres  filets,  son  prix  le  place  juste  apres  le  coton.  D'autres  avantages  de  cette  fibre  sont  enumdres  et  des  essais  ont  montrg  que  le 
teviron  peut  rivaliser  avec  le  nylon  pour  la  peche  au  saumon  et  A  la  truite,  alsor  qu'on  a  longtemps  consider^  que  le  nylon  dtait  le  meilleur 
materiel  dans  ce  cas. 

Redes  de  pesca  de  Teviron 
Extracto 

En  octubrc  de  1956  la  'Teikoku  Rayon  Co.  Ltd./1  Edobori- Minamidori,  Nishiku -Osaka,  Janon,  Ianz6  al  mercado  una  nueva  fibra 
sintetica  llamada  "teviron",  hccha  a  base  dc  cloruro  de  polivinilo,  la  cual  puede  obtenerse  en  forma  dc  hilo  continuo  o  como  fibra  para  hilar. 
El  rrimero  tiene  el  aspect  o  de  seda  y  la  segunda  de  lana. 

Este  trabajo  se  reficrc  al  uso  del  "teviron**  en  la  tcjeduria  de  artes  dc  pesca,  senalandosc  que,  en  compctencia  de  precio  con  otras 
fibras,  figura  a  continuaci6n  del  algod6n.  Tambien  se  dan  u  conocer  otras  ventajas,  demos trando  las  pruebas  efectuadas  que  puede  competir 
con  el  nyI6n  en  las  nesqucrias  de  salm6n  y  truchu,  no  obstantc  considerarse  desdc  hace  tiempo  que  este  ultimo  material  es  el  mas  apropiado 
para  la  pesca  de  dichas  especies. 


CHARACTERISTICS  OF  TEVIRON 

TEVIRON,  a  polyvinyl  chloride  synthetic  fibre  first 
introduced    in    October    1956,    is    produced    by 
Teikoku  Rayon.  The  yarn  is  available  both  as 
filament  and  as  a  staple  fibre:  the  former  has  a  silk-like 
appearance  and  touch,  while  the  latter  resembles  wool. 
This  fibre  is  suitable  for  making  fishing  nets  and  rope. 


Filament 


Staple  Fibre 


300  den.  (20-60  fil.) 
1-39 
3-0-3-7g/den. 

2-15  den. 
1-39 
2-0-3-0  g/den. 

t}         14-25% 
2-0-2-7g/den. 

9-17% 

50-75% 
1-5-2  -5  g/den.. 
50-70% 

Denier 

Specific  Gravity 
Normal  Strength 
Extensibility  (unknotted) 

(independent  of  humidity) 
Knot  strength 
Extensibility  (knotted) 
Ratio  of  Knot  Strength  to 

Normal  Strength  .  70-75%  80-85% 

Young's  Modulus  .  800-900  kg./sq.  mm.  200-300  kg./sq.  mm 

Elasticity  at  3%  .  80-85%  80-85% 

Temperature  at  beginning  of 

Shrinkage  .          .        60-70  deg.  C.         100  deg.  C. 

Resistance  to  friction      .  Great  (esp.  in  water)  Great 

Resistance  to  Acid  and  Alkali     Great  Great 

Resistance  to  sunlight     .         very  great  very  great 


TEVIRON   FISHING   NETS 
Cost 

A  Teviron  net  costs  less  than  a  net  of  any  other 
synthetic  fibre  yarn,  and  is  only  30  per  cent,  more  than 
a  cotton  net. 

Ease  of  handling 

(a)  Owing  to  the  great  resistance  to  rot  (see  figs.  1 
and  2),  little  work  is  needed  for  drying  or  re-dyeing 
Teviron  nets  or  for  other  maintenance  services. 

(b)  As  the  net  does  not  absorb  water,  it  is  very  light 
and  can  be  handled  by  a  smaller  crew. 

The  table  below  gives  the  results  of  an  investi- 
gation into  labour  and  time  factors.  The  com- 
parison was  made  between  two  large  fixed  nets 
of  approximately  similar  size,  one  of  Teviron,  the 
other  of  manila: 


Teviron  net 
Manila  net 


Number  of 
Workers 

.       44 
.       60 


Time  required  for  pulling  up 

the  net  completely  from  the  sea 

(Minutes) 

25 
45 


155] 


MODERN     FISHING     GEAR     OF     THE     WORLD 


23456789 
TiB»  of  iomroion  (month.) 


10       11        12 


Fig.    I.      Results  obtained  from  immersion  in  sea-water. 


012        34        5        6        789      10      11      12 
Tine  of  liBMraion     (Months) 

Fig.  2.     Results  obtained  from  outdoor  exposure. 


The  Teviron  net  could  be  pulled  up  when  the 
current  was  rapid;  the  manila  net  could  not  be 
moved. 


Suitability  for  various  gear 

(a)  Gillnets  :  Teviron  twine  is  flexible,  and  a 
minimum  of  shrinkage  assures  stability  of  mesh 
size.  A  test  of  Teviron  and  nylon  drift  nets  for 
salmon  and  trout  in  northern  seas  showed  that 


the  Teviron  net  is  not  inferior  to  nylon  net: 
A  verage  number  offish  caught  for  each  operation 
Teviron  net       .  .2-27 

Nylon  net         .  .2-22 

(b)  Purse  seine  nets:  Teviron  needs  no  drying,  and 
little  repair  and  other  maintenance  work,  allowing 
more  fishing  time  per  day,  and  longer  trips.  More 
fish  can  be  caught  because  the  net  sinks  rapidly. 

(c)  Fixed  nets:  The  fibre  does  not  decay  even  in  the 
warmest  season  of  the  year,  and  has  proved  a 
strong  and  reliable  material  in  rough  water. 


Repairing  drift  nets  in  Ceylon. 

[56] 


Photo  FAO. 


KREHALON   FISHING   NETS   AND   ROPES 

by 

KUREHA  KASEI  CO.  LTD. 

Tokyo,  Japan 


Abstract 

Krehalon  is  a  vinylidenc  chloride  filament  which  has  a  higher  specific  gravity  (1-7)  than  other  synthetic  fibres.  It  is  less  affected  by 
currents,  sinks  faster  and  drains  water  faster.  The  pliability  of  the  fibre  gives  more  strength  to  the  knots  and  resistance  to  impact  and  friction. 
These  characteristics  make  it  particularly  suitable  for  setnets.  Big  setnets  of  Krehalon  have  been  in  use  since  1952  and  are  said  to  show  no 
signs  of  wear. 

Krehalon  is  also  used  for  stick-held  dipnets,  surrounding  nets,  gillnets,  longlines  and  trawlnets.  it  is  available  both  as  monofilament 
and  continuous-multifilament  yarn;  nets  arc  either  knotless  or  made  with  English  knot.  Lines  and  ropes  are  also  made  of  Krehalon.  This 
fibre  is  sensitive  to  heating  and  should  be  kept  off  sandy  beaches  or  cobblestones  in  midsummer. 


Resume 


Filets  de  peche  et  cordages  en  Krehalon 


Le  K  rehalon  est  un  filament  de  chlorurc  dc  vinylidenc,  qui  possede  un  poids  specifique  plus  eleve  (1,7)  que  les  autres  fibres  synthc- 
tiques.  11  est  moins  affect  6  par  les  courants,  plonge  plus  rapidement  et  1'eau  s'en  egoutte  plus  vite.  La  souplesse  de  la  fibre  donne  plus  de 
resistance  aux  noeuds,  augmente  la  resistance  aux  impacts  et  au  frottement.  Ces  caracteristiques  le  font  convenir  part  icu  I  Bremen  t  bien  pour 
les  filets  fixes.  Dcpuis  1952  on  utilise  des  grands  filets  fixes  de  Krehalon  et  on  declare  qu'ils  ne  montrent  pas  de  signes  d*usure. 

On  utilise  aussi  le  Krehalon  pour  les  carrelets  monies  sur  des  perches,  les  filets-pares,  les  filets  maillants,  les  palangres  et  les  chaluts. 
Lc  Krehalon  est  prod  nil  en  fil  monofilament  et  en  fil  multi-filaments  continus;  les  filets  sont  soit  sans  noeuds,  soit  noues.  On  fait  aussi  des 
lignes  et  des  cordages  de  Krehalon.  Ccttc  fibre  est  sensible  a  la  chaleur  et  doit  fctre  maintenue  a  1'ecart  des  plages  de  sable  ou  des  galets 
pendant  la  saison  chaudc. 

Redes  de  y  cuerdas  de  "Krehalon" 
Extracto 

Desde  1952  sc  han  comenzado  a  utilizar  grandes  artes  de  "krehalon"  filamento  de  cloruro  de  yinilideno.  Este  material  tiene  mayor 
peso  especifico  (1,7)  que  el  resto  dc  las  fihras  sinteticas  y  sufre  en  menor  grado  la  influencia  de  la  corriente;  ademas  se  hunde  y  deja  escurrir 
cl  agua  con  mayor  rapidez.  La  flexibilidad  dc  las  fibras  impartc  una  mayor  firmeza  a  los  nudos  a  la  vez  que  ofrece  mas  resistencia  al  impacto 
y  a  la  fricci6n,  haciendolo  especialmente  apropiado  para  la  fabricacidn  de  redes  fijas. 

Esta  fibra  tambien  se  usa  para  salabardos  provistos  de  mango,  artes  de  enmallc  y  arrastre,  palangres  y  espineles. 

Las  redes  de  este  material  pueden  fabricarse  sin  nudos  o  empleando  el  nudo  de  tejedor.  El  "krehalon"  tambien  sirve  para  fabricar 
cabos  y  cuerdas,  pero  es  muv  sensible  al  calentamiento  y  no  debc  extenderse  sobre  piedras  o  playas  de  arena  durante  el  verano. 


GENERAL 

THIS  vinylidene  chloride  filament  has  the  greatest 
specific  gravity  (J  -7)  of  any  synthetic  fibre.  It  is 
less  water  absorbent  and  drains  water  faster.  Nets 
made  with  this  fibre  are  pliable  and  strong  and  retain 
their  shape  even  in  turbulent  waters.  They  sink  quickly. 
The  tensile  strength  of  the  fibre  is  120  to  130  per  cent, 
higher  than  that  of  comparable  cotton  yarn.  The  flex- 
ibility of  the  fibre  gives  higher  knot  strength  and  greater 
resistance  to  impact  and  friction. 

The  fibre  is  particularly  suitable  for  constructing 
setnets,  as  fish  of  any  size  are  unlikely  to  be  injured  when 
trapped  because  of  the  pliability  of  the  fibre.  Some  set- 
nets  made  of  Krehalon  (see  fig.  1)  have  been  in  use 
since  1 952  and  show  no  signs  of  wear  ( 1 957).  Fishermen 
say  they  can  withstand  the  buffeting  of  the  severest 
typhoons.  Stick-held  dipnets  (see  fig.  2),  surrounding 
nets,  gillnets,  longlines  and  trawlnets  made  of  this 
fibre  show  similar  durability. 

The  mesh  size  of  Krehalon  nets  is  usually  made 
slightly  smaller  than  that  of  nets  of  natural  fibres  as  it 


does  not  shrink  in  usage.  The  fibres  can  be  dyed  freely 
in  any  colour. 

Sizes:  In  multi-filament  twines  several  180  denier 
filaments  are  usually  put  together  to  form  yarn,  twine  or 
line.  Mono-filaments  are  composed  of  one  large 


L&rg*   Mtiwt  BKd*  of 
Kmhalon 

1.  l«*d«r  nat 

2.  *nt«-oliMb«r 

3.  funml 

4.  b*« 

5.  floats 

6.  anchoring  atom*  or 


Fig.  I. 


[57] 


MODERN     FISHING     GEAR     OF    THE    WORLD 


A  •tlok-h»ld  dipiwt 
of 


t.  2. 


filament  (i.e.  as  1,000  denier).  360  denier  is  equivalent 
to  cotton  yarn  of  count  20's.  Thickness  and  tensile 
strength  have  been  taken  into  consideration.  Multi- 
filament  nets  are  more  pliable  than  mono-filament  nets. 

Nets:  The  nets  are  of  the  same  specifications  as  the 
conventional  cotton  yarn  or  manila  twine  nets  available 
in  knotless  or  English  knot.  Smallest  are  720  D  :;  3, 
mesh  size  25  knots  per  6  inches. 

Ropes:  Ropes,  lead  line,  twisted  twines,  longlines, 
cord,  etc.,  are  manufactured  according  to  specifications. 

Cautions  regarding  use:  Long  exposure  to  high  temper- 
ature can  cause  a  chemical  change  in  the  filament.  For 
this  reason,  a  sandy  beach  or  cobblestones  are  best 
avoided  in  midsummer. 

Tarring:  Krehalon  nets  do  not  normally  require 
tarring,  but,  if  necessary,  put  one  part  of  refined  tar 
into  two  parts  of  5  per  cent,  solution  of  neutral  soap. 
Tar  at  a  temperature  less  than  40  deg.  C. 

Use  a  hot  iron  knife  when  cutting  a  filament.  This 
obviates  the  danger  of  fraying. 


Lifinet  fishing  in  Jakarta  harbour. 
[58] 


Photo  FAO 


SOME  PHYSICAL  PROPERTIES  OF  MANILA   ROPE 

by 
J.  REUTER 

Nederlandsche  Visscherij-Proefstation  en  Laboralorium  voor  Materialen-Onderzoek  Utrecht,  Netherlands 


Abstract 

Very  little  has  been  published  to  date  on  the  properties  of  rope,  in  spile  of  its  vast  importance  to  the  fishing  industries  of  the  world. 
People  have  learned  about  rope  by  experience  but  definite  conclusions  based  on  exact  and  concrete  data  are  not  always  available.  In  this 
paper  the  author  attempts  to  bring  out  some  of  the  properties  of  hard-laid,  medium-laid  and  soft-laid  ropes  and  tables  showing  the  physical 
characteristics  in  both  the  wet  and  dry  state  arc  given. 


Resume 


Quelques  proprietes  physiques  des  cordcs  de  Manillc 


Jusqu'ii  present  tres  pen  d 'articles  ont  etc  publics  au  sujet  des  proprietes  des  cordes  malgre  leur  grande  importance  pour  les  industries 
des  peches  dans  le  mo  rule.  On  a  appris  empiriquement  a  connaitrc  Ics  cordes  mats  on  nc  dispose  pas  toujours  dc  conclusions  definitives 
hashes  sur  des  donnees  exactes  et  concretes.  Dans  cet  article,  Pauteur  essaie  de  faire  ressortir  quelques-unes  des  proprietes  des  cordes  com- 
mises  lache.  moyennement  et  serre,  ct  donne  des  tableaux  montrant  Ics  caractcristiques  physiques  £  l'6lat  sec  et  a  Petal  mouille. 

Algunas  propiedades  fisicas  de  la  cucrda  de  abaca 
Extracto 

Sc  ha  publicado  muy  poca  informacion  sobre  las  propiedades  de  las  cucrdas  de  abaca  o  manila  no  obstante  la  gran  importancia  que 
ncnen  en  la  industria  pcsquera  de  todo  el  mundo.  Se  sabe  mucho  acerca  de  cuerdas  por  experiencia,  pero  no  siempre  se  dispone  de  con- 
clusiones  definitivas  basadas  en  datos  exaclos  y  concrelos.  En  cste  trabajo  el  amor  trata  de  dar  a  conocer  algunas  caracteristicas  de  las 
cuerdas  poco,  medianamenlc  o  muy  retorcidas,  e  incluyc  tablas  con  las  propiedades  fisicas  de  ellas  tanto  humedas  como  secas. 


KNOWLEDGE  of  the  various  intrinsic  properties 
of  ropes  is  indispensable  for  the  right  choice  and 
economic    use    of   them.     Although    experience 
has   resulted   in   some  general   knowledge,   no  definite 
conclusions  based  on  exact  and  concrete  data  are  avail- 
able and  very  little  has  been  published  on  this  matter. 
The  following  article  is  an  effort  to  contribute  some 
information  for  fishermen  on  this  subject. 

Influence  of  basic  manufacturing  procedures 

During  the  process  the  fibres  are  twisted  successively  in 
opposite  directions  into  yarns,  strands,  ropes  and  cable. 
This  results  in  a  decrease  in  breaking  load.  A  remarkable 
fact  is  that  even  when  the  same  basic  fibre  material  is 
used,  the  breaking  strength  of  "Z"  yarn  is  different 
from  that  of  "S"  yarn.  It  is  obvious,  therefore,  that  the 
actual  twisting  process  itself  exercises  a  very  considerable 
influence  on  the  breaking  strength. 

Trial  I — Influence  of  the  means  of  manufacture 

Rope  was  formerly  made  exclusively  on  the  rope  walk 
but  the  process  has  now  been  mechanized.  Theoretically, 
it  makes  no  difference  how  rope  is  made,  although  the 
finger-tips  of  an  experienced  ropemaker  possess  certain 
qualities  which  cannot,  except  with  great  difficulty,  be 
duplicated  in  a  machine. 
To  obtain  more  concrete  information  on  this  aspect, 


a  ropemaker  was  found  who  agreed  to  make  rope  in 
three  different  types  of  machine.  He  was  requested  to  use 
different  ways  from  a  given  yarn,  viz.  on  the  rope  walk 
and  on  two  different  types  of  machine.  He  was  requested 
to  do  his  utmost  to  ensure  that  the  ropes  were  as  identical 
as  possible.  Little  difference  was  apparent  in  the  three 
ropes  at  first  sight,  yet  very  marked  dissimilarities  were 
disclosed  by  investigation  (see  Table  I).  These  differences 
were  not  apparent  from  the  slight  variations  in  circum- 
ference, which  is  generally  assumed  to  be  an  indication 


TABLL   1 

Rope 

Rope 
closing 
method 

A  verage1 
weight  of 
\  metre  of 
rope  in 
grammes 

Total 
number  of 
turns  per 
strand  per 
metre 

Average- 
circum- 
ference 
in  mm. 

Average^ 
breaking 
strength 
in  kg. 

Breaking 
length  in 
km. 

Rope  walk      90 
Machine  1      95 

25-0 

27-3 

38-6 
38-8 

1284 
929 

14-27 
9.78 

Machine 

2      95 

26-0 

37-7 

1028 

10-82 

1  Average  for  16  determinations. 

2  Average  for  80  determinations. 


59] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


of  strength  when  comparing  ropes  made  of  the  same 
material. 

This  should  not  be  interpreted  as  an  endeavour  to 
expound  a  theory  that  rope  made  on  the  rope  walk  is 
"good"  and  machine-made  rope  *4bad".  Such  a 
sweeping  statement  could  never  be  made  on  the  basis  of 
a  single  trial;  the  only  purpose  was  to  prove  that  the  way 
in  which  a  rope  is  made  can  result  in  significant  differ- 
ences in  the  breaking  strength  even  when  the  same  fibre 
gradings  are  used. 

Trial  II— Influence  of  the  lay 

Procedure.  Three  types  of  rope  were  made  on  the  rope 
walk.  The  conditions,  i.e.  basic  material,  equipment,  etc., 
under  which  the  experiments  were  carried  out  were 
identical  in  all  cases.  The  only  intentional  deviation 
was  that  a  number  of  different  lays  was  selected,  so  as 
to  produce  ropes  of  hard,  medium  and  soft  lay.  This 
was  done  not  only  to  ascertain  possible  modifications 
in  the  properties  of  the  rope,  attributable  to  the  different 
lays,  but  also  because  these  modifications  are  important 
in  some  instances  in  commercial  fishing. 

The  three  different  ropes  were  all  based  on  20  yarns 
per  strand,  made  from  the  same  fibre  mixture. 


Test  Methods.  During  the  closing  of  the  rope,  20 
bobbins  (£rd  of  the  total  number  used)  were  marked. 
The  yarn  was  subsequently  examined  to  establish  its 
various  properties.  Five  lengths  were  cut  off  simultan- 
eously from  the  rope  to  be  tested  for  breaking  strength, 
etc.  The  first  and  third  lengths  (3-5  metres  long)  were 
examined  dry,  the  second  and  fourth  (4  metres  long) 
were  examined  wet,  while  the  fifth  (1  metre  long)  was 
untwisted  to  enable  the  yarns  to  be  examined  dry. 

The  lengths  intended  for  use  in  testing  the  breaking 
strength  at  the  splice,  with  overhand  knot  and  as  a  sling, 
were  then  cut  off  successively. 

Breaking  strength  tests  were  carried  out  on  a  hydraulic 
breaking-strength  machine  fitted  with  clamps.  The 
distance  between  clamps  was  ISO  cm.  and  the  rate  of 
movement  of  the  straining  head  approximately  12  J  cm. 
per  minute. 

The  ropes  examined  "wet"  were  immersed  in  fresh 
water  for  at  least  16  hours.  Their  weight  was  determined 
after  all  non-absorbed  water  had  run  off  for  30  minutes. 
The  yarn  tensile  strength  iest  was  carried  out  with 
100  cm.  between  clamps,  the  rate  of  movement  of  the 
straining  head  being  approximately  25  cm./min. 

Results.  In  the  following  the  results  of  this  experiment 


TABLE  II 
Soft-laid  Rope 


TABL£   HI 

Normal-Laid  Rope 


Experiment 


Type 


dry 


3-strand, 
plain-laid 


wet 


Total  number  of  yarns    3  x  20  —  60 
Weight  per  metre  in 


grammes 


322-4 


Average  number  of  turns 
per  strand  per  metre       13-78 

Average  circumference 
in  mm.  .        74-85 

Average  breaking 
strength  in  kg.        .        4930 


100 


100 


100 


100 


Average  breaking  length 
in  km.  (based  on  dry 
weight)        .  .         15-30  100 

Average  breaking  length 
in  km.  (based  on 
wet  weight)  .          — 

Average  duration  of 
test,  after  application 
of  initial  load         .  6-425ximin.    100 


3-strand, 
plain-laid 

3x20-60 

521-5 

161-7 

13-86 

100-6 

84-70 

113-2 

5205 

105-6 

16-14 


9-98 


105-5 


65 


Experiment  dry 

Type  .  .  .     3-strand, 

plain-laid 

Total  number  of  yarns     3  x  20  -60 
333-3 


Weight  per  metre  in 
grammes 


8-125ximin.  126-5 


Average  number  of  turns 
per  strand  per  metre       15-15 

Average  circumference 
in  mm.  .        72-45 

Average  breaking 
strength  in  kg.       .        4513 

Average  breaking  length 
in  km.  (based  on  dry 
weight)        .  .         13-54  100 

Average  breaking  length 
in  km.  (based  on 
wet  weight)  .          — 

Average  duration  of 
test,  after  application 
of  initial  load         .6-75xjmin.      100 


3-strand, 
plain-laid 

3x20    60 

100 

494-2 

148-3 

100 

15-02 

99-1 

100 

80-1 

110-6 

100 

4357 

96-5 

13-06 


8-82 


96-5 


65-1 


8-05  •  imin.  119-3 


Breaking  strength  short 
splice  in  kg.  .        4295  87-1 

Breaking  strength  with 
overhand  knot  in  kg.       2033  41-2 

Breaking  strength  with 
sling  around  rods  7344  149 

10  cm.  in  diameter 


Breaking  strength  short 
4389  84-3  splice  in  kg.  3652  80-9 

Breaking  strength  with 
2333  47-3  overhand  knot  in  kg.        1978  43-8 

Breaking  strength  with 

—  —  sling  around  rods  6789          150-4 

10  cm.  in  diameter 


3700 
2322 


94-9 
51-5 


[60] 


PROPERTIES    OF     MANILA     ROPE 


are  interpreted  according  to  a  system  developed  at  the 
Nederlandsche  Visscherij-Proefstation. 

(a)    Dry  Rope 

The  increase  of  number  of  turns  per  unit  length  (lay) 
results  in  : 

(1)  increase  of  weight  per  unit  length  (see  below) 

relation  of  weight  per  unit  length 
yarns  (not  twisted)  rope 

Soft  laid  .  .  1  :  1-256 

Medium  laid  -  1  :  1-298 

Hard  laid      ...  1  :  1-332 

(2)  decrease  in  diameter  (that  means  more  material 
per  unit  diameter) 

(3)  decrease  in  breaking  strength  (that  means  more 
material  for  unit  breaking  strength) 

In  the  present  example  the  remaining  breaking  strength 
of  one  yarn  in  the  rope  in  the  three  different  types 
(1/60  x  rope  strength)  is: 

In  soft  laid  rope          .     82-17  kg.       (62-6%) 
In  medium  laid  rope        75-22  kg.       (57-34%) 
In  hard  laid  rope         .     56-67  kg.        (43  -20%) 

The  breaking  strength  of  the  yarn,  prior  to  closing  the 
rope,  was  131  •  18  kg.  (100  per  cent.)  (see  Table  V). 

(4)  This  means  that  the  breaking  length,  the  only 


accurate  tensile  strength  criterion  for  rope,  cannot  be 
used  for  the  comparison  of  ropes  of  different  lays. 

(5)  increase  in  knot  strength. 

(6)  increase  in  strength  of  a  loop. 

The  total  extension  at  break  is  maximal  in  normal 
and  less  in  hard  and  soft  laid  ropes  (see  Table  VI). 

(b)  Wet  Rope 

Increase  in  number  of  turns  per  unit  length  (lay) 
results  in: 

(1)  less  water  absorption  (less  increase  in  weight). 

(2)  less  increase  of  diameter  due  to  water  absorption. 
Contrary  to  the  dry  rope,  the  total  extension  at  break 

of  the  wet  rope  is  higher  with  hard  and  soft  lay  than 
with  normal  lay  (see  Table  VI).  With  a  tension  of  2,000 
kg.  or  less  the  soft  laid  rope  has  the  highest  extension  and 
the  hard  laid  rope  the  lowest.  Immersion  has  no 
remarkable  influence  on  the  number  of  turns  per  unit 
length.  There  is  a  certain  influence  of  immersion  on  the 
breaking  strength  but  the  results  available  at  present  are 
not  sufficient  to  draw  reliable  conclusions. 

(c)  Influence  of  rope  manufacturing  on  the  yarns 

in  order  to  examine  the  effect  of  manufacturing,  samples 
of  the  three  types  of  rope  were  untwisted  and  the  single 
yarns  tested.  It  was  found  that  whilst  soft  and  normal 
lay  has  only  very  little  influence  (loss)  on  the  breaking 


TABLE  IV 
Hard-laid    Rope 

strength 
decrease 

of  the  single  yarn,  hard 
(see  Table  V). 

lay  results  in  a 

certain 

Experiment                      dry               %                wet               % 

TABLE  V 
Yarn 

Type  .           .           .     3-strand,                       3-strand, 
plain-laid                         plain-hiid 

Total  number  of  yarns    3  x  20  --  60                 3  x  20  =-  60 

Untwisted    Untwisted    Untwisted 
Before          from            from            from 
closing        soft-laid    normal-laid  hard-laid 
rope             rope             rope 

Weight  per  metre  in 
grammes                 .       340-5              100          473-7            139-  1 

Average  number  of  turns 
per  strand  per  metre       16-81            100            16-71            99-4 

Average  circumference 
in  mm.                    .         65-8               100            69-9             106  2 

Average  breaking 
strength  in  kg.        .         3484              100            3284              94-3 

Average  breaking  length 
in  km  (based  on  drv 

Average  breaking 
strength  in  kg.        .       131    18       130-76         128-38         121-11 

No.  of  determinations    200            100              100              100 

Average  weight  per 
1  00  metres  in  grammes  428            434  -5          427-4          424-7 

Average  breaking 
length  in  km.          .         3065        30-09          3010          28-45 

Percentage  of  breaking 
length          .           .       100             98-2            98-2           92-8 

weight)        .           .         10-22            100             9-65            94-4 

Average  breaking  length 

TABLE  VI 

in  km.  (based  on 
wet  weight)           .                                               6-94            67-9 

Total  Extension 

Average  duration  of 
test,  after  application 
of  initial  load         .      5-85ximin.    100        6-70^  £  min.  114-5 

Tension 
kg. 

soft 
dry 

rope 
wet 

normal 
dry 

rope 
wet 

|        hard 
|      dry 

rope 
wet 

250     - 

*U' 

(1 

10-8% 

5-3% 

7*9% 

1     4-0% 

6-4°, 

Breaking  strength  short 
splice  in  kg.            .         3407            97-7            2944              89-6 

500 

* 

13-8 

7-4 

11 

!     5-6 

9-8 

1000 

10-9 

15-8 

11-1 

13-5 

:   8-4 

13-0 

Breaking  strength  with 
overhand  knot  in  kg.        1778            51-0            1878              53-9 

2000     ! 
3000 

13-2 
14-8 

18-0 
19-6 

14-8 
16-7 

16-1 
17-8 

12-8 
15-0 

16-8 
18 

Breaking  strength  with 
sling  around  rods           5878          1  68  •  7 

4000 

16-1 

20-8 

17-7 

19-2 

16-2 

0 

10  cm.  in  diameter    . 


Not  determined. 


61  ] 


THE   MANUFACTURE  AND   TESTING   OF   SYNTHETIC   YARNS 
AND   FIBRES   USED   IN   JAPANESE   FISHING   GEAR 


by 

JAPAN  CHEMICAL  FIBRES  ASSOCIATION 

Tokyo,  Japan 

Abstract 

Synthetic  fibre  has  been  used  for  fishing  gear  in  Japan  since  1932  when  it  was  tried  out  as  fishing  gut,  and  after  1948  an  effort  was 
made  to  increase  its  use,  and  now  all  kinds  of  nets  and  gear  are  made  from  synthetic  materials.  This  paper  gives  in  tabular  form  the  physical 
properties  and  uses  of  the  various  fibres  and  also  the  quantity  of  nets  and  lines  manufactured  from  both  natural  and  synthetic  fibres  in  1956. 
The  weight  of  nets  exported  is  also  given.  Then  follow  the  rules  which  have  been  designed  to  standardize  the  testing  of  spun  vinylon, 
filament  nylon,  filament  vinylidcnc  chloride  and  filament  vinyl  chloride. 


Resume 


Fabrication  et  essai  des  fils  et  fibres  synthetiques  entrant  dans  la  construction  dcs  engins  de  peche  japonais 


Les  fibres  synth£tiques  sont  utilisees  au  Japon  pour  la  confection  des  engins  dc  peche  dcpuis  1932,  epoque  a  laquclle  clles  ont  etc 
cssayees  en  remplacement  du  crin,  et  Ton  s'effprce  depuis  1948  de  developper  leur  cmploi  en  sorte  qifactuellement  les  filets  et  engins  de  tous 
types  sont  confectionnes  en  materiaux  synthctiqucs.  Ce  document  donne  sous  forme  de  tableaux  les  proprietes  physiques  ct  les  applications 
des  diflercntes  fibres  ainsi  que  les  quantites  de  filets  et  de  lignes  fabriquees  d'une  part  en  fibres  nature-lies,  et  de  Paul  re  en  fibres  syntheliques 
en  1956.  Le  poids  des  filets  cxportes  cst  egalcmcnt  indiqu£  suivant  les  regies  adoptees  pour  la  normalisation  des  essais  des  fils  de  brins  de 
vinylon  ainsi  que  dcs  filaments  de  nylon,  de  chlorure  de  vinylidene  et  de  chlorurc  dc  vinyl. 


Extracto 


La  manufactura  y  ensayo  de  las  fibres  e  hilos  sinteticos  usados  en  los  artes  de  pesca  japoneses 


A  partir  de  1932  la  industria  pcsqucra  japonesa  comenzo  a  usar  fibras  sinteticas  en  los  artcs  de  nesca  cuando  las  ensay6  como  sedales, 
pero  s6lo  dcspues  dc  1948  se  hicieron  esfuerzos  para  aumentar  su  uso  y,  en  la  actualidad,  todos  los  tipos  dc  redes  y  artes  son  confeccionados 
con  este  material.  En  el  trabajo  matcria  de  cste  extracto  sc  compendian  en  tablas  las  propiedadcs  fisicas  y  usos  de  las  diversas  fibras,  asi 
como  las  cantidades  de  redes,  palangres,  etc.  fabricados  con  fibras  naturales  y  sinteticas.  Tamhien  se  mcluyc  el  peso  de  las  redes  exportadas 
y  las  disposiciones  que  se  proyectaron  para  norrmilizar  los  ensayos  dc  vinilon  hilado,  nylon  y  cloruros  dc  vinilidcno  y  de  \  inilo  en  hilos  dc 
una  sola  hebra. 


SYNTHETIC    fibre    was    first    used    for    fishing  in 
Japan  in  1932,  as  fishing  gut.  Since  about  1948 
experiments  have  been  carried  out  on  the  adapt- 
ability of  synthetic  fibre  for  fishing  gear  through  co- 
operation of  the  Fisheries  Agency,  Fisheries  College, 
Fisheries  Research  Institute,  fibre  makers,  fishing  net 
makers,  fishermen,  etc. 


As  a  result  of  improvement  in  quality,  advance  in 
net-making  techniques  and  the  reduction  in  cost  by  mass 
production  in  1953,  the  demand  for  synthetic  fibre  has 
considerably  increased  for  seine  and  setnets,  and  salmon 
and  trout  gillnets  (see  Tables  I,  11,  Ml). 

The  export  of  fishing  nets  has  gradually  increased 
since  1951,  about  1  -2  million  Ibs.  being  exported  in  1956. 


TABLE   I 
Production  of  Synthetic  Fibre  Nets  by  Netting  Types  for  1956 


Unit:  Pounds 


Fibre 

Nylon 

Vinylon 

Poly  vinylidene  Chloride  Fibre 

Polyvinyl  Chloride  Fibre 

Two  Fibres  Plied 

Total      .... 


Type 

English  Knot       Reef  Knot         Knotle.\s  Net 


4,489,213 
1,893.088 

598,034 
70,619 

631,900 

7,682,854 


50 

3,028,940 
6,454 


3,035,444 


82,287 

810,478 

1,546,712 

104,181 


2,543,658 


Moji  Net 
183,394 


Total 


4,571,550 

5,915,900 

2,151,200 

174,800 

631,900 


183,394          13,445,350 


[62] 


SYNTHETIC     FIBRES    JN     JAPAN 


TABLE  II 
Production  of  Fishing  Nets  and  Lines  in  1956  (or  1957) 


Unit:  1,000  Pounds 


Type 


Month 
1          2          3          4          5          6          7          8          9         10        11         12     Total  1955 


Fishing  Net\ 

Cotton           .... 

1,152 

1,478    1,215 

915 

730 

641 

586 

731 

774 

702 

708 

673  10,305  14,845 

Silk                ... 

0 

0          0 

0 

0 

0 

0 

0 

0 

0 

0 

0          0        12 

Manila  Hemp 

HI 

117        71 

71 

43 

53 

60 

111 

168 

108 

115 

112    1,110    1,273 

Other  Hard  Fibres 

3 

23         19 

18 

11 

11 

49 

3 

3 

2 

2 

3       147      437 

Synthetic  Fibres 

713 

1,082    1,120 

1,090 

920 

783 

829 

898 

969 

1,290 

1,101 

1,095    11,8906,923 

Sub-Total 

1,949 

2,700   2,425 

2,094 

1,704 

1,488 

1,524 

1,743 

1,914 

2,102 

1,926 

1,88323,45223,490 

Fishing  Lines 

Cotton           .... 

826 

991       895 

888 

805 

885 

703 

844 

833 

775 

797 

863  10,105  12,092 

Silk                .... 

0 

0          0 

0 

0 

0 

0 

0 

0 

0 

2 

025 

Manila  Hemp 

(~\t\+t*r   WurH    T^iKr^c 

13 

8         17 

13 

15 

7 
2 

0 

•j 

9 
i 

17 

12 

17 

13       141         51 

01  A          IJA 

Synthetic  Fibres 

152 

229      287 

274 

234 

275 

289 

373 

330 

422 

427 

498   3,790    1,511 

Sub-Total 

994 

1,235    1,203 

1,182 

1,057 

1,169 

999 

1,227 

1,180 

1,209 

1,243 

1,37414,07213,805 

Total 

2.943 

3,935    3,628 

3,276 

2,761 

2,6^7 

2,523 

2,970 

3,094 

3,311 

3,169 

3,25737,52437,295 

TABLL  111 
Production  Capacity  of  Fishing  Nets  and  Lines  in  1956  (or  1957) 


Hand  Machine 


Power  Machine 


Twister 


Machines 

Machines  for 

Operable* 

Synthetic  fibres 

Reef  knoi 

8,258 

6,437 

English  knot 

74 

— 

Moji  net 

6 

Reef  knot 

87 

42 

English  knot 

2,199 

2,469 

Moji  net 

426 

— 

Knot  less  net 

57 

56 

Hari  twister 

48,038 

n.  a. 

Ring  twister 

181,522 

„ 

Companies  concerned 


111 


64 


Machines 
Operated* 

3,898 
62 

2 


51 

1,398 
181 
44 


35,835 
163,889 

101 


Monthly  Capacity 
per  Machine 

150  Ibs. 


370 
540 

2,940 


Machines  for  natural  fibre  nets  and  lines  included. 


Owing  to  the  rapid  development  of  different  synthetic 
fishing   gears,    efforts   are   being   made   to   determine 
suitable  testing  methods  for  synthetic  yarns. 
The  following  methods  and  standards  are  proposed. 

Method  of  Testing  Twines  for  Fishing  Nets 

A — Spun   Vinylon.  B — Filament   Nylon. 

C — Filament  Vinylidene  Chloride  and  Filament 
Vinyl  Chloride. 

In  the  following  text,  the  materials  to  which  each 
paragraph  refers  are  shown  by  the  letters  A,  B  and  C. 

1.    Scope 

These  standards  shall  cover  the  methods  of  testing  twines 
of  the  materials  shown  above. 


2.    Definition 

2-1.  Standard  Condition  in  Testing  Room  (A.  B.C.) 
Temperature  at  20  f  2  deg.  C.,  and  relative 
humidity  at  65  ±  2  per  cent. 
Remark:  For  determining  temperature  and 
humidity,  the  Assman's  Aspiration  Psychro- 
meter  shall  be  employed,  and  the  relative 
humidity  obtained  from  the  humidity  table  by 
Sprung's  formula. 

2-2.  Standard  Condition  of  Test  Sample  (A.  B.  C.)— 
is  when  the  sample  left  in  a  testing  room  under 
standard  conditions  (2-1)  has  reached  moisture 
equilibrium  (2-3). 

2-3.  Moisture  Equilibrium.  (A.  B.)  After  pre-drying  a 
test  sample  at  a  temperature  of  40  to  50  deg.  C. 


[63] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


it  is  left  in  the  laboratory  under  standard 
conditions.  When  the  sample  has  been  brought 
to  the  constant  weight  (2-5),  being  steady  and 
uniform  in  hygroscopic  state,  it  shall  be  con- 
sidered to  be  in  moisture  equilibrium. 

2-3.  Constant  Weight  (C.)  Weigh  the  test  sample 
twice  successively  at  intervals  of  one  hour  or 
more  at  the  time  of  drying.  The  constant  weight 
shall  be  considered  as  that  when  the  difference 
between  the  two  weighings  is  under  0-03  per 
cent. 

2-4.  Absolute  Dry  Condition  (A.  B.)  Is  reached  when 
the  weight  of  a  test  sample  becomes  constant 
(2-5)  due  to  being  left  in  a  drying  oven  at 
105  to  110  deg.  C. 

2-4.  Absolute  Dry  Weight  (C.)  Is  the  weight  under 
standard  conditions  of  test  sample,  also  called 
the  bone  weight. 

Remark:  Moisture  regain  under  Standard  con- 
ditions is  almost  nil. 

2-5.  Constant  Weight  (A.  B.)  Weigh  the  test  sample 
at  intervals  of  one  hour  or  more  for  the  moisture 
equilibrium,  and  at  10  minutes  or  more  for  the 
absolute  dry  condition.  Constant  weight  is 
reached  when  the  difference  is  within  0-05  per 
cent,  of  the  last  respective  weights. 

2-6.  Commercial  Moisture  Regain.  (A.)  5  per  cent, 
of  the  absolute  dry  weight  (B.)  4-5  per  cent. 
(C.)  0  per  cent. 

2-7.  Denier  (A.  B.  C.)  Denier  is  a  unit  of  fineness, 
the  yarn  having  a  weight*  of  0-05  gr.  per  450 
metre  length.  The  denier  is  equal  numerically 
to  the  number  of  grams  per  9,000  metres. 

2-8.     Yarn  Count  (A).  Yarn  count  to  be  expressed  by 


840  yards) 


the  number  of  hanks  (One  hank 
per  pound  in  weight*. 

Indication  of  Yarn  Count  (A.) 

Yarn  count  to  be  expressed  as  follows. 

Single  Yarn    20  yarn  count     ........  20s 

Twine  20  yarn  count  2  ply       .....  20/2  s 

3  strands  of  20  yarn  count  2  ply    20/2/3* 

Indication  of  Denier  (B.) 

Denier  to  be  expressed  as  follows: 

Twine  3  strands  of  15  filament  2  10  denier  5  ply 

2IOD/I5f      5 

Indication  of  Denier  (C.) 
Denier  to  be  expressed  as  follows: 

Twine  3  strands  of  1  filament  1000  denier  8  ply  . 

1000i>/lf  >   8    -. 
3  strands  of  10  filament  1500  denier  6  ply 

ISOOo/lOf  .-63 

2-10.  Indication  for  Direction  of  Twist  (A.  B.  C.) 

The  direction  of  twist  to  be  expressed  by  S  and 
Z  as  shown  in  fig.  1  : 


2-9. 


2-9. 


2-9. 


3 


3 


The  direction  of  upper  twist,  middle  twist  and 
lower  twist  to  be  expressed  as  follows: 

UPPER   TWIST   DIRECTION          MIDDLE 
TWIST    DIRECTION  LOWER    TWIST 

DIRECTION. 


The  weight  includes  the  commercial  moisture  regain. 


TABLfc   IV 

Export  of  Natural  and  Synthetic  Fibre  Fishing  Gear  in  1956  (or  1957) 


Unit:  Pound 


Type 

Natural  Fibres 
Cotton 
Silk 

Manila  Hemp 
Sisal 
Flax 
Ramie 
Coil  Yarn      . 

Sub-Total 


1 


3 


Month 

1  8 


10 


II 


12 


Total 


363,037    532,585    528,811    448,219   494,895   401,129   412,297    361,712  424,347   426,674   351,561    473,724   5,218,991 


18.706 


20 

159 

31 



_,_ 

2 

13,345 

42,629 

2,507 

14,170 

40,159 

9,164 

32,894 

55,099 

15,815 

47,146 

25,187 

54,945 

1,230 

2,110 

-- 

1,183 

1,933 

749 

10 

2,000 

.._ 

825 

— 

--- 

7,572 

894 
375 


5 

99 

4 



320 

21,434 

10,445 

91,330 

194,436 

459,319 

29,710 

44,826 

— 

20,867 

326,489 

285 

2,363 

595 

10,823 

207 

440 

5 

3,487 

... 

--- 

7,572 

381,743  580,084  630,808  466,572  557,394  476,805  477,157  362,981  475,703  482,329  445,698  689,627  6,026,901 


Synthetic  Fibres 

Nylon 

Vinylon 

Polyvinylidene 

Chloride  Fibre 
Polyvinyl 

Chloride  Fibre 
Two  Fibres  Plied 

Sub-Total 
Total      . 


17,140     36,570     49,072     63,955     62,795     65,009     38,818     55,464     59,264 
4,797       6,877      14,148       8,776       8,217      12,789       4,060      16,938      17,616 


18,841 


850 


720 

2,672 

6,373      8,692 


992      16,047 


19 


951 


261 


622       6,698       9,004      13,872      16,871 


40,778     44,297     72,265     82,143     72,626  100,543      51,901      87,225     94,012 
422,521    624,381    703,073   548,715   630,020   577,348   529,058   450,206   569,715 


67,519     74,017     93,309 
12,6%     97,335      17,195 


682,932 
221,444 


254     44,034          316       63,594 


7  442         3,121 

24,050  27,624  31,534      165,031 

104,526  243,010  142,7%  1,136,122 

586,855  688,708  832,423  7,163,023 


[64] 


SYNTHETIC     FIBRES     IN     JAPAN 


2-11.  Indication  for  Number  oj  Twist  (A.  B.  C.) 

The  number  of  twist  will  be  indicated  by  the 
numerical  value  of  turns  per  metre  (t./m.)  or 
turns  per  inch  (t./in.).  Jn  the  case  of  twine  the 
number  of  upper  twist,  middle  twist  and  lower 
twist  to  be  indicated  as  follows: 

LOWER    TWIST    NUMBER  MIDDLE 

TWIST  NUMBER  UPPER  TWIST  NUM- 
BER. 

Example:  Upper  twist  Z  120  turns  per  metre, 
middle  twist  S  5  turns  per  inch  and  lower  twist  Z 
10  turns  per  inch-  Z  10  t./in.  -  S  5  t./in.  x 
Z  120  t./m. 

2-12.  Standard  Initial  Tension  (A.) 

Standard  initial  tension  is  the  first  tension  in 
which  the  yarn  is  suspended  without  any  exten- 
sion. In  cases  where  tension  affects  mean 
"thickness",  "ply  number'1,  "apparent  yarn 
count",  "twist",  "tensile  strength  and  extensibil- 
ity", "knot  strength",  "clastic  recovery"  and 
"shrinkage  in  water",  the  initial  tension  is  given 
to  a  yarn  of  20"  with  a  load  of  5  grams.  An 
initial  tension  other  than  the  standard  one  shall 
be  indicated. 

2-12.   Standard  Initial  Tension  (B.  C.) 

In  cases  where  tension  affects  mean  "thickness", 
"ply  number  and  filament  denier",  "denier", 
"twist",  "tensile  strength  and  extensibility", 
"knot  strength",  "clastic  recovery"  and  "shrink- 
age in  boiling  water"  the  initial  tension  used  is 
1/30  g.  of  the  nominal  denier  (filament  denier 
-  ply  number).  An  initial  tension  other  than  the 
standard  one  shall  be  noted. 

3.  Sampling  and  Preparation  (A.  B.  C.) 

The  test  sample  is  taken  by  cutting  off  5  m.  from  the  end 
of  the  yarn.  Care  must  be  exercised  to  prevent  change  in 
twist,  and  no  tension  given.  In  case  of  testing  for  "knot 
strength"  and  "elastic  recovery",  the  test  sample  will  be 
left  in  the  testing  room  under  ordinary  conditions  until 
it  reaches  the  constant  weight.  When  the  testing  room 
cannot  be  kept  in  standard  condition,  the  lest  sample  will 
be  put  in  a  closed  vessel  (of  36  per  cent,  sulphuric  acid) 
and  the  temperature  be  kept  at  the  constant  degree 
(20  deg.  C.). 

4.  Test  Items  (A.  B.  C.) 

(1)  Corrected  weight 

(2)  Moisture  regain 

(3)  Standard  weight 

(4)  Thickness 

(5)  Ply  number 

(6)  Yarn  count  or  denier 

(7)  Twist 

1.  Upper  twist  number 

2.  Middle  twist  number 

3.  Lower  twist  number 

4.  Twist  shrinkage 

(8)  Twist  setting 

(9)  Tensile  strength  and  extensibility 

1.    Dry  tensile  strength  and  extensibility 


2.     Wet  tensile  strength  and  extensibility 

(10)  Knot  strength 

1.  Wet  strength  of  reef  knot 

2.  Wet  strength  of  English  knot 

(11)  Elastic  recovery 

(12)  (a)  Shrinkage  in  cold  water;  (b)  in  boiling  water 

(13)  Moisture  absorption 

(14)  Sinking  speed 

(15)  Weathering  resistance 

5.     Methods  of  Testing 

The  test  of  "standard  weight",  "tensile  strength  and 
extensibility",  "knot  strength"  and  "elastic  recovery", 
will  be  carried  out  in  a  testing  room  under  standard 
conditions.  When  the  testing  room  cannot  be  kept  at  the 
standard  temperature,  the  temperature  at  the  time  of  the 
test  will  be  noted. 


5-1. 


5-2. 


5-3. 


5-4. 


5-5. 


Corrected   \\  'eigh  t 

Find  out  the  weight  of  gross  and  tare  of  two 

samples  and  get  the  corrected  weight  from  the 

following    formula    and    indicate    the    average 

number. 


Corrected  weight 
\\here:  W 
R 


W 


100  H    Rt 
100   *    R 

weight  of  the  test  sample  (gross  weight 


(are  weight) 

moisture  regain  measured  (per  cent.) 
Rt       commercial   moisture   regain   (A        5  per 
cent.,  B      45  per  cent.,  C       0  per  cent.) 

Moisture  Regain  (A.  B.)  or  Absorbed  Moisture  (C) 
Weigh  two  test  samples  both  before  and  after 
absolute  dry  condition.  The  average  moisture 
regain  is  obtained  from  the  following  formula 
to  (one  place  of  decimal): 

W  __  \\'d 

Moisture  Regain  (per  cent.)  Wi         *   10° 

where:        weight  before  drying  the  test  sample. 
Wd       weight  of  absolute  dr>  test  sample. 

Standard  Weight  (A.  B.  C.) 
Suspend  a  test  sample  2  m.  or  more  in  length  in 
a    perpendicular   position.   Then    find   out   the 
weight  of  a  standard  length   and   indicate   the 
average  weight. 

Thickness  (A.   B.  C.) 

Take  five  test  samples  and  wind  them  20  times 
closely,  parallel  to  each  other,  and  with  a 
standard  initial  tension,  around  a  cylinder 
about  5  cm.  in  diameter.  Measure  the  breadth 
and  divide  by  20.  The  average  number  is  indi- 
cated in  millimetres  (to  one  place  of  decimal)  as 
shown  in  fig.  2. 


Plv    Number   (A.)    Ply    Number   and   Filament 
Denier  (B.  C.) 

After  untwisting  the  twine  the  ply  number  and 
filament  denier  are  measured. 


65 


MODERN     FISHING     GEAR     OF    THE    WORLD 


5-6.     Yarn  Count  (A.)  or  Denier  (A.  B.  C.) 

Give  the  testing  sample  the  standard  initial 
tension  and  cut  it  into  ten  lengths  of  30  to  90  cm. 
After  weighting  the  yarn  count  or  denier  is 
obtained  from  the  following  formula: 


VV 


Yam  count  (s)  --  0-5906  x 

W 

Denier  (D)  -  9000  x  -    -    - 
L  s 

where:  W   -  weight  of  test  sample  (g.) 

L        total  length  of  the  test  sample  (m.) 

Remark:  The  corrected  denier  shall  be  calculated 
by  the  following  formula: 


100  +  4-5 


100 


5-7. 


Corrected  denier  (B)  ---  D  x  '^ — ^  ;  (C)  -  D  x  - 

lOU    .    K.  11JU    t    K. 

where:  D  ^  denier  measured 

R  —  moisture  regain  measured  (per  cent.) 

Twist  (A.  B.  C.) 

Apply  a  standard  initial  tension  to  the  yarn, 
with  25  cm.  (10  inches)  between  the  clamps, 
with  a  yarn  twist  tester,  and  test  for  twist  as 
follows,  taking  an  average  of  ten  or  more  tests 

5-7-1  Twist  Direction  (A.  B.  C.) 

After  untwisting  the  test  sample,  the  twist 
directions  are  examined  from  upper  twist, 
middle  twist,  and  lower  twist. 

5-7-2  Upper  Twist  Number  (A.  B.  C.) 

Untwisting  the  upper  twist  thoroughly,  the 
untwisted  number  is  converted  into  the  number 
corresponding  to  one  meter  or  one  inch,  and 
this  figure  is  indicated  as  the  upper  twist  number. 

5-7-3  Middle  Twist  Number  (A.  B.  C.) 

All  but  one  strand  of  the  thoroughly  untwisted 
strands  of  the  upper  twist  are  cut  out,  and  then 
untwisted.  This  untwisted  number  is  converted 
into  the  number  corresponding  to  one  metre  or 
one  inch,  and  this  figure  is  indicated  as  the  middle 
twist  number. 

5-7-4  Lower  Twist  Number  (A.  B.  C.) 

All  but  one  yarn  of  the  thoroughly  untwisted 
strand  of  middle  twist  are  cut  out,  and  then 
untwisted.  This  untwisted  number  is  converted 
into  the  number  corresponding  to  one  metre  or 
one  inch,  and  this  figure  is  indicated  as  the  lower 
twist  number. 

5-7  -5  Twisting  Shrinkage  (A.  B.  C.) 

After  untwisting  the  test  sample,  measure  the 
length  of  the  yarn.  The  shrinkage  percentage  is 
measured  from  the  following  formula: 

j^i L 

Twisting  Shrinkage  (per  cent.)= x  100 

!_/ 

where:  L    —  Length  of  the  test  sample 
L1  =-  Length  after  untwisting 

5-8      Twist  Setting  (A.  B.  C.) 

Take  ten  pieces  of  any  test  specimen  each  one 
metre  long,  pick  up  both  ends  and  put  them 
together  and  count  their  twisting  number. 

5-9      Tensile  Strength  and  Extensibility 

5-9- 1  Dry  Tensile  Strength  and  Extensibility  (A.  B.  C.) 
Employing  a  suitable  "Tensile  Strength  Tester", 
and  exercising  care  not  to  untwist  the  test 


specimen,  grip  one  end  in  the  upper  clamp,  and 
after  applying  a  standard  initial  tension,  grip  the 
other  end  in  the  lower  clamp,  the  clamps  being 
25  cm.  apart,  and  tension  speed  being  30  cm./ 
min.  Then  measure  the  tensile  strength  and 
extensibility  (kg.  and  per  cent.)  at  the  time  of 
breaking.  Take  the  mean  of  ten  or  more  tests. 
(Carry  to  three  figures.) 

5-9  •  2  Wet  Tensile  Strength  and  Extensibility  (A.  B.  C.) 
The  test  specimen  is  immersed  in  water  at  room 
temperature,*  and  after  it  has  thoroughly 
absorbed  water,  the  wet  tensile  strength  and 
extensibility  is  measured  in  a  similar  manner  as 
described  in  5  —  9-1. 

5-10    Knot  Strength 

5-10-1     Reef  Knot     Wet  ( A.  B.  C.) 

The  standard  initial  tension  is  applied  to  the  test 
specimen,  and  a  reef  knot  is  made  as  shown  in 
figure  3.  Then  it  is  immersed*.  After  the  test 
specimen  has  thoroughly  absorbed  water,  it  is 
gripped  in  the  clamps,  keeping  the  knot  in  the 
middle,  and  the  wet  strength  (kg.)  is  measured  as 
in  5  to  9-1. 
The  average  of  ten  tests  is  taken. 

5-10-2  English  Knot— Wet  (A.  B.  C.) 

A  standard  initial  tension  is  applied  to  the  test 
specimen  and  an  English  knot  breaking  strength 
is  measured  as  in  5  —  10-1.  The  average  of 
ten  tests  is  taken. 


Fig.  4. 


5-11    Elastic  Recovery  ( A.  B.  C.) 

Employing  a  suitable  "Tensile  Strength  Tester'*, 
the  test  specimen  is  extended  to  12-5  mm.  (5  per 
cent,  of  the  original  length).  Then  the  load  is 
removed  for  two  minutes,  and  again  the 
standard  initial  tension  is  applied  and  the  re- 
maining elongation  is  measured.  The  elastic 
recovery  is  measured  from  the  following  formula: 

Hlastic  recovery  (per  cent.)  —  — ----- —  x  100 

Where:  L  --  remaining  elongation  (mm.) 
The  average  of  ten  tests  or  more  is  taken. 

5-12    Shrinkage  in   Water  (A.) 

A  standard  initial  tension  is  applied  to  the  test 
specimen,  and  a  section  one  meter  long  is 
marked.  Then  a  loop  is  made  by  tying  both  ends 
together  outside  the  marks.  Immerse  as  for  5  — 
9-2  and  after  the  specimen  has  thoroughly 


*  The  time  for  immersion  is  twelve  hours. 
Remark:  Tests  in  which  the  specimens  break  at  the  clamp  should 
be  rejected. 


[66) 


SYNTHETIC     FIBRES     IN     JAPAN 

TABLE    V 
Synthetic  Fibres  Produced  and  Used  for  Fishing  Gear  in  Japan 


June ,  7957 


y'nylon                                         Nyhn                                 Vinvlulene          Polvvinyl  Chloride  Fibre 
Staple                                      Filament 

Filament                        Filament 

Regular       High  Tenacity         Regular        High  Tenacity 

Tensile  Strength                 Std.       4  2  to  6-0      6-5  to  7-0            5-0  to  6-0      6-4  to  7-7                  1-5  to  2-6                  2-7  to  3  7 

(gram  per  den.)              Wet       3-2  to  4-8      5-2  to  5-8            4-2  to  5-0      5*4  to  6-7                  I  -5  to  2-6                  2-7  to  3-7 

Std.  Loop       l-5to2-6      3-0  to  3-4            4-7to5-5      5-4to6-5                  1-0to2-0 

Std.  Knot       2-5  to  4-0      4-0  to  4-5            4-5  to  5-4      5-0  to  6-3                   1-0  to  20                  1-8  to  2-7 

Elongation                          Std.       17  to  26          14  to  18 

23  to  36        18  to  24                        18  to  33          !            13  to  30 

(%)                                 Wet       19  to  30          15  to  19 

38  to  48        21  to  28                        18  to  33                      13  to  30 

Elastic  Recovery  (%)             .        75  to  88          Up  to  81 

98  at  3%                       98  to  100  at  5%      '            80  to  85  at  3% 

at  2%              at  2% 

60  to  70          65  to  70 

at  5%               at  5% 

Specific  Gravity                                       1  -26  to  1  -30                                     1-14                                     1  -70                                1-39 

Moisture  Regain                            5-0°;  at  standard  condition    4-5%  at  standard  condition               None                             None 

Moisture  Absorption 

120%   at  95%   R.H.                 8-5%   at   95%    R.H.         0-1%  at  95%  R.H.     0-1%    at   95%    R.H. 

Effect  of  Heat                        .     \  Softens  at  220  to  225  deg.  C.     Softens  at  180  deg.  C.  melts           Softens  at           Softens  at  1  10  to  120  deg. 

j  Shrinkage  starts  at  about             at    215    deg.    C.           >        140    to    160                          C. 

:              200  deg.  C.                  Yellows  slightly  at  150  deg. 

deg.  C.             Shrinkage  starts  at  60  to 

when  held  for  5  hours.                    Self-                          70  deg.  C. 

extinguishing. 

Effects  of  Acids 

Concentrated  sulphuric,        Hydrochloric  and  Sulphuric        Unaffected  by       Unaffected  by  most  acids 

hydrochloric,  formic  acids     acids    cause     degradation.            most  acids             including  aqua  regia. 

decompose   or   swell.          Benzoic  and   Oxalic   acids  \          including 

will  cause  loss  in  tenacity  '.        aqua  regiu. 

and  elongation  depending  ! 

upon  time  and  concent  ra-  j 

;                                                                        lions.                     ! 

Effect  of  Alkalis                      .     |       Strong   alkalis   cause 

Substantially  inert.            Unaffected  bv  most      Not  affected  by  con- 

yellowing  but  not  affect 

alkalis  with  the            centratcd   alkalis. 

strength.                  '                                                         exception  of 

concentrated 

ammonium 

hydroxide  and 

sodium  hydroxide. 

Effect  of  Organic  Solvents                    Good  resistance. 

Generally  insoluble,  soluble          Substantially          Dissolves  or  swells  in 

in  some  phenolic  compounds                inert.                 some  aromat  ics,  chlori- 

and  in  concentrated  formic                                          naled  hydrocarbons, 

acid. 

ketonej>,  esters. 

Effect  of  Other  Chemicals      . 

Soluble    in    hot     pyridinc               Generally   good 

Generally  good 

Generally  good 

phenol,  cresol.             .                 resistance. 

resistance. 

resistance 

Resistant    to   oils. 

Effect  of  Sunlight 

Loses  tensile  strength 
after  prolonged  exposure. 

Loses  strength  on  prolonged 
exposure.  No  discoloration. 

Darkens  slightly 
after 

Substantially  unaffected 
after  prolonged  exposure. 

Bright  yarn  is  more  resist- 

prolonged 

ant  than  semi-dull. 

exposure. 

Remarks 

Vi  nylon  is  the  generic  term 

Vinylidene  is 

of  polyvinylalcohol  fibres. 

an  abbreviation 

of  Vinyl-chloridc- 

Vinylidene-chloride 

copolymer  fibres 

in  Japan. 

Producers  and  Trade  Names 

Kurashiki  Rayon  Co.  Ltd. 

Toyo    Rayon    Co.    Ltd. 

Asahi-Dow   Ltd. 

Teikoku  Rayon  Co.  Ltd. 

"Kuralon" 

"Amilan" 

"Saran" 

"Teviron" 

Dainippon  Boseki  Co.  Ltd. 
"Mewlon" 

Nippon   Rayon  Co.   Ltd. 
"Grilon" 

Kureha  Kasei 
Co.  Ltd. 

Toyo  Kagaku  Co.  Ltd. 
"Fnvilon" 

Kanegafuchi  Boseki  Co.  Ltd. 

"Krehalon" 

"Kanebian" 

End  Use 

Seine  net          Setnet 

Gillnct                  Seine   net 

Setnet 

Setnet            Trawl  net 

Longline          Trawl  net 

Seine  net 

Lift  net 

Trawl  net 

Notes:      For  making  fishing  gears,  these  fibres  arc  used  generally  in  filaments,  excepting  vinylon  which  is  not  produced  in  filament  form. 
In  vinylon  the  high  tenacity  staple  is  employed  for  this  purpose. 

[671 


MODERN     FISHING     GEAR     OF    THE    WORLD 


absorbed  water,  dry  it  in  the  air.  Apply  an 
initial  tension  again  and  then  measure  the  length 
of  the  marked  section.  The  shrinkage  in  water  is 
obtained  from  the  following  formula  (to  one 
decimal): 

.    .   ,  1000        L 

Shrinkage  (per  cent.) 


5-12 


5-13 


5-14 


1000 


100 


where:  L 


length  (mm.)  of  the  air  dry  test  specimen 
after   treatment. 

Shrinkage  in  Hoi/ing  Water  (B.) 
Take  the  test  sample  of  1  m.  or  more  in  length, 
fix  both  ends  of  the  yarn  together  to  overlap 
two-fold,  and  then  measure  a  length  of  50  cm., 
marking  the  two  end  points.  Apply  the  weight  to 
give  the  standard  initial  tension.  Remove  the 
weight,  immerse  the  test  specimen  in  boiling 
water  for  30  minutes,  and  then  take  out  of  water 
to  allow  to  dry  in  the  air.  Applying  the  same 
weight  again,  measure  the  length  of  the  air  dried 
sample.  Calculate  the  shrinkage  according  to 
the  following  formula  and  take  mean  value  of 
5  tests  or  more  (to  one  place  of  decimal). 

Shrinkage  in  boiling  water  (per  cent.)       "        -         •    100 

where:  L        length  of  air-dried  test  specimen  after 

immersion  (mm.) 

Moisture  Absorption  (A.  B.  C.) 
Take  about  2  m.  length  (if  this  weighs  less  than 
2  g.,  take  about  2  g.)  of  the  test  specimen,  and 
after  measuring  the  weight  in  air  dry  condition, 
immerse  it  in  water  at  room  temperature*.  Then 
take  the  specimen  out  of  the  water  and  allow  the 
water  to  drip  for  two  minutes;  take  the  weight, 
and  calculate  the  moisture  absorption  from  the 
following  formula: 

w1     w 

Water  absorption  (per  cent.)  - 


W 


100 


where: 


W 

W1 


air  dry  weight  of  test  specimen 

water  absorption  weight  of  test  specimen 


Sinking  Speed  (A.  B.  C.) 

Take  a  piece  of  twine  of  2  cm.  in  length  with  a 


5-15 


knot  in  the  middle,  let  it  sink  from  the  surface  of 
water  at  20  deg.  ;!_  5  deg.  C.  contained  in  a 
glass  beaker  (see  fig.  5).  Measure  the  speed 
per  second  from  AB  to  CD,  a  distance  of  50  cm. 
The  test  specimen  should  be  previously  de- 
aerated  and  immersed  in  clear  water*.  Take  the 
average  of  three  or  more  tests. 

Remark:  Tests  in  which  the  specimen  sank  in  a  diagonal 
way  or  sank  close  to  the  wall  of  the  vessel  should  be 
rejected. 


Weathering  Resistance  (A.  B.  C.) 
Take  two  test  specimens  at  random,  fix  them  to  a 
textile  testing  board,  and  measure  their  strength 
(ten  times  2)  by  exposing  them  under  the 
condition  mentioned  below  for  20  hours  with 
the  weathering  resistance  tester  made  in  the  form 
of  the  weather-O-mctcr.  This  strength  is  com- 
pared with  that  of  the  control  sample  and  the 
average  value  is  taken  in  per  cent. 

Arc  130  to  145  V.  50  to  00  c/s  15  to  I7A  to  2  sets 
Carbon  for  Arc  "  70  (Solid)  and  "  20  (Core),  or  other 

corresponding  types. 


Temperature  in  the  icslcr 

Revolving  Speed 

Exposing  time 

Spraying  time 

Pressure  of  spraying  water 

Water  requirement  for  spraying 


40  to  50°  C. 

once  per  min. 

102  mm. 

18  min. 

25  to  30  Ibs./in.- 

20  10  30  gal./hr. 


*  The  time  for  immersion  is  twelve  hours. 

Remark:  Tests  in  which  the  specimens  break  at  the  clamp  should 
be  rejected. 


Broiling  herring  from  a  purse  seine  on  the  West  Coast  of  Canada.     Photo  Inf.  Serv.  Dept.,  Fish.,  Ottawa. 

|68] 


THE   PHYSICAL   PROPERTIES   OF   NETTING  AND   TWINES 
SUITABLE  FOR   USE    IN   COMMERCIAL    FISHING   GEAR 

by 
P.  J.  G.  CARROTHERS 

Fisheries  Research  Board  of  Canada,  Technological  Station,  Vancouver,  B.C.,  Canada 

Abstract 

Many  new  materials  Kaxc  recently  been  introduced  inio  commercial  fishing  pear,  hut  many  oiher  useful  materials  have  been  rejected 
because  they  have  not  been  tried  to  the  best  advantage.  Often  the  tensile  strength  of  the  dry,  straight  twine  is  used  as  the  basis  foi  the 
substitution  of  new  materials  f  >r  more  conventional  ones,  but  this  basis  can  result  in  serious  error  because  the  twines  are  invariably  knotted 
and  wet  while  fishing,  and  kr.c  t  ing  and  wetting  affect  dillcrcnt  materials  in  different  ways.  Because  pertinent,  reliable  and  comparable  data 
arc  often  not  available  for  new  materials,  original  tests  have  had  to  be  made,  and  the  results  thereof  are  presented  herewith. 

The  materials  tested,  in  forms  generally  suitable  for  use  on  the  coa*t  of  British  Columbia,  include  cotton,  linen,  ramie,  hemp,  Manryo, 
nylon  and  Tcrylcne. 

The  test  procedures  used  to  measure  net  weight  in  water,  change  in  length  due  to  wetting,  linear  twine  density,  twine  diameter, 
twine  stretch  under  load,  and  tensile  strength  are  briefly  described. 

The  properties  of  the  twine  and  netting  are  divided  into  two  groups.  The  first  group  includes  the  "parameters"  (properties  which 
have  the  same  numerical  value  for  all  twine  si/es  of  the  same  material  and  style),  and.  where  available,  quantitative  data  for  these  properties 
arc  reported  in  tabular  form.  The  second  group  includes  properties  whose  values  vary  with  and  are  dependent  on  the  twine  si/e.  and 
formulas  are  presented  for  estimating  values  of  this  second  group  of  properties  from  data  for  the  first.  Definitions  and  unit  equivalents  for 
all  properties  are  quoted. 

Proprieties  physiques  de  filet  et  des  fils  pouvant  vtrt*  utilises  dans  les  engins  de  pt-chc  eommereiaux 
Resume 

On  a  reccmment  adopte  pour  la  fabrication  des  engins  de  pcchc  commcrciaux  un  grand  nombre  de  matenaux  nouveaux  mais 
beaucoup  d'autres  materiaux  utilcs  ont  etc  rcjetes  simplcment  parce  qu'ils  n'ont  pas  etc  essayes  dans  les  mcilleures  conditions.  On  manque 
ires  souvent  pour  les  nouveaux  materiaux  de  donnees  pert  monies,  dignes  de  foi  et  compatibles,  et  il  a  fallu  fa  ire  des  essais  origmaux  dont 
y  on  trouvera  les  resultats  ci-dessous. 

Les  malcriaux,  essayes  sous  des  formes  convenant  gencralenvjnt  a  P utilisation  sur  la  cote  de  la  Colombie  britannique,  comprennent 
le  lin,  la  ramie,  le  chanvrc,  le  manryo.  le  nylon  et  le  terylene.  On  trouvera  une  breve  description  dc*  techniques  utilisces  pour  ccs  essais  qui 
consistaient  a  mesurer  le  poids  net  dans  1'eau,  les  variations  de  longueur  subies  par  le  fil  mouille.  la  dcn><ie  lineaire  du  fil,  le  'Jiamctrc  du  fil, 
Pallongcmcnt  du  fil  sous  Pmfluence  d'un  poids  et  la  icsistance  a  la  rupture.  On  a  reparti  les  proprieics  des  fils  en  deux  categories,  les 
"parametres"  (proprietes  ayant  la  meme  valeur  numcriquc  pour  loules  les  dimension.**  du  fil  fabrique  avcc  le  memc  materiel  et  de  mcme  facon) 
et  les  proprictcs  dont  les  valours  sont  fonction  de  la  dimension  du  fil  et  on  trouvera  des  formules  pour  calculer  la  valeur  des  propnctes  dc 
cctte  seconde  categoric  a  partir  des  donnees  relatives  a  la  premiere.  On  donne  des  definitions  et  des  equivalents  unitaires  pour  Unites  les 
propriety's. 

Propiedades  fisicas  de  las  redes  e  hilos  adecuados  para  artes  de  pesea  comercial 
Fxtracto 

Recientemente  ha  comenzado  a  usarse  gran  numero  de  nuevos  matcriales  en  las  artes  de  pesca  comercial,  pero  tambicn  se  han 
rechazado  muchos  utiles  a  causa  de  no  haber  sido  onsayados  de  manera  que  puedan  utili/arse  con  ventaja.  Como  a  menudo  no  se  dispone 
de  dates  seguros  quo  permitan  comprobar  nuevos  productos  textiles,  en  cste  trabajo  se  dan  a  conocer  las  prucbas  originales  que  debicron 
hacerse  para  este  objeto,  asi  como  los  resultados  obtemdos. 

Los  materiales  ensayados  en  las  formas  como  generalmente  sc  usan  a  lo  largo  de  la  costa  de  Colombia  Britanica  mcluycn:  algodon, 
lino,  ramio,  canamo,  "manryo"  ("vinylon"),  "nylon"  y  "terylene".  En  el  trabajo  tambicn  sc  describen,  en  forma  sucinta,  los  procedimientos 
usados  en  las  pruebas  de  materiales  para  medir  su  peso  ncto  en  el  agua,  cambio  de  longitud  al  humcdecerlo,  peso  lineal  y  diametro,  alarga- 
miento  del  hilo  con  el  peso  y  resistencia  a  la  tensi6n. 

Las  propiedades  de  los  hilos  y  de  las  redes  se  dividen  en  dos  griipos:  los  "parametros"  (propiedades  con  el  mismo  valor  numenco 
para  los  hilos  del  mismo  tipo  y  material)  y  las  propiedades  cuyos  valores  varian  y  dependen  del  diametro  del  hilo.  Tambien  se  dan  formulas 
para  estimar  los  valores  de  este  segundo  grupo  de  propiedades  basandosc  en  datos  del  primcro.  definiciones  y  cquivalencias  para  todas  las 
propiedades. 


INTRODUCTION 

DURING    the    past    decade,   many  new  materials,  the    data    describing    the    new    and    the    conventional 

particularly  synthetic  fibres,  have  been  introduced  materials  need  only  be  relative.  But  where  these  materials 

into  commercial  fishing,  usually  being  presented  arc  to  be  used  in  new  applications,  then  the  physical 

as  substitutes  for  other  materials.  If  these  new  materials  properties  should  be  fully  described.  It  is  the  author's 

are  to  be  substituted  rationally,  certain  physical  proper-  opinion  that  many  new   materials  have  been  rejected 

ties  should  be  determined  quantitatively  a  priori,  although  as  unsatisfactory,  through  improper  use  of  the  material 

[69] 


MODERN     FISHING     GEAR     OF    THE    WORLD 


during  initial  trials,  a  result  of  complete  neglect  of 
quantitative  test  data  or  because  only  irrelevant  physical 
properties  have  been  considered. 

The  misuse  of  test  results  has  been  illustrated  in  the 
selection  of  twine  sizes  for  salmon  gillnets.  Because 
fish  must  be  caught  from  water,  wet  strength  is  more 
important  than  dry  strength  even  though,  for  convenience, 
many  people  measure  only  the  latter.  At  one  time  all 
salmon  gillnets  on  the  British  Columbia  coast  were 
made  of  premium-grade  linen  which  increases  about 
50  per  cent,  in  strength  when  wet.  In  contrast,  nylon 
loses  about  15  per  cent,  strength  in  water.  Therefore,  if 
a  nylon  twine  is  chosen  to  give  the  same  dry  strength 
as  the  linen  it  is  to  replace,  the  wet  strength  of  the  nylon 
net  will  be  little  more  than  half  that  of  the  linen  net. 

The  manufacturers  of  nylon  gillnets  were  aware  of 
this  and,  for  the  first  experimental  nets,  selected  twine 
sizes  of  sufficient  wet  strength  to  carry  normal  fishing 
loads.  The  results  were  satisfactory.  Nevertheless 
some  net  men  and  fishermen  still  select  their  nylon  gill- 
nets  on  the  basis  of  hand  tests  applied  to  dry  netting, 
with  the  result  that  nets  are  chosen  too  light  for  their 
loads.  They  are  torn  more  easily,  and  their  owners 
erroneously  claim  that  the  quality  of  nylon  is  becoming 
poorer.  Because  wetting  affects  the  strength  of  different 
materials  in  different  ways,  wet  strength  tests  are  much 
more  significant  to  fishing  gear  design  than  are  dry 
strength  tests. 

Another  example  of  the  improper  use  of  test  results 
is  in  the  choice  of  the  physical  property  which  the  test 
evaluates.  Because  twine  must  be  knotted  to  form  netting 
the  strength  of  the  knot  or  the  mesh  is  more  important 
than  the  strength  of  the  straight  twine.  Soon  after  the 
introduction  of  nylon  66  multifilament  gillnets  into 
the  British  Columbia  salmon  fishery,  nylon  6  multifila- 
ment gillnets  began  to  appear.  On  one  occasion  at  least, 
the  tensile  strength  of  twine  from  the  latter  nets  was  found 
to  be  about  40  per  cent,  weaker  than  nylon  66  twine  of  the 
same  weight,  and  nylon  6  was  rejected  as  being  unsatisfac- 
tory for  salmon  gillnets.  However,  because  these  two 
nylons  react  differently  to  knotting,  the  mesh  of  a  nylon  6 
is  only  about  20  per  cent,  weaker  than  that  of  a  nylon  66 
net  of  the  same  weight.  Considering  their  lower  price 
nylon  6  nets  do  have  a  place  in  the  British  Columbia 
salmon  fishery  in  competition  with  the  nylon  66  nets. 
Because  knotting  affects  the  strength  of  different  materials 
in  different  ways,  knot  strength  or  mesh  strength  tests 
are  much  more  significant  to  fishing  gear  design  than 
are  tensile  tests  on  the  straight  twine. 

Many  new  materials  have  been  made  available, 
unaccompanied  by  specific  test  data  describing  their 
physical  properties.  Inquiries  addressed  to  the  suppliers 
often  elicit  no  further  information,  or  bring  data  which 
have  little  significance  to  fishing  gear  applications.  It 
has  therefore  been  necessary  for  the  Fisheries  Research 
Board  of  Canada  to  perform  its  own  tests  prior  to  recom- 
mending how  these  materials  may  be  used  to  greatest 
advantage.  Obtaining  our  own  test  data  has  the  three- 
fold advantage  that:  (1)  pertinent  properties  may  be 
measured;  (2)  the  test  data  are  reliable  to  the  best  of 
our  ability;  and  (3)  by  using  consistent  test  procedures, 
data  on  all  materials,  both  conventional  and  new,  may 
be  compared. 

This  paper  presents  the  results  of  our  many  tests  on 


seven  different  fibres  in  eighteen  different  forms  suitable 
for  use  in  commercial  gear  for  the  British  Columbia 
fishery.  Because  different  materials  were  designed  for 
different  applications,  not  all  properties  of  all  materials 
were  measured,  and  the  accompanying  table  is  not 
complete.  However,  except  for  the  mesh  strength  of 
hemp  and  medium-laid  cotton,  test  procedures  were 
consistent  throughout  and  the  test  data  presented  are 
comparable  between  different  materials.  All  test  results 
have  been  reduced  to  "parametric"  form,  that  is,  to 
properties  which  are  reasonably  constant  over  the  com- 
plete twine-size  range  of  a  given  material  and  style. 
Sometimes  corresponding  materials  from  different  manu- 
facturers have  properties  in  which  measured  differences 
are  statistically  significant,  but  these  have  been  grouped 
into  the  overall  averages  presented  in  Table  I. 

MATERIALS  TESTED 

The  cotton  netting  and  twine,  the  Manryo  twine,  and 
the  twisted,  spun,  nylon  66  twine,  were  of  "cable" 
construction:  i.e.,  the  twine  contains  three  or  four 
plies  and  each  ply  contains  several  yarns.  The  yarn 
style  is  identified  by  the  British  system  of  hanks  ( 1  hank— 
840  yards  -  768  metres)  per  pound  (453-6  grams).  The 
twine  size  is  identified  by  two  numbers,  the  first  being 
the  number  of  yarns  per  ply  and  the  second  the  number 
of  plies  in  the  twine.  The  total  number  of  yarns  in  the 
twine  is  the  product  of  these  two  numbers.  In  the  medium- 
laid  twines  the  helix  angle  of  the  plies  was  about  54  deg., 
in  the  soft-laid  twines  about  28  deg.,  and  in  the  extra- 
soft-laid  twines  about  22  deg. 

The  linen,  ramie,  and  hemp  twines  were  of  plied 
construction,  i.e.,  the  yarns  were  doubled  directly  into 
the  twine.  The  linen  yarn  is  identified  according  to  an 
arbitrary  commercial  numbering  system,  but  the  approxi- 
mate number  of  leas  (1  lea  300  yards  =  274-3  metres) 
per  pound  (453-6  grams)  is  quoted  in  parenthesis.  The 
ramie  yarn  is  identified  according  to  this  same  leas-per- 
pound  system,  and  the  hemp  yarn  is  identified  by  the 
number  of  pounds  (1  pound  =  453-6  grams)  per  spindle 
(14,400  yards  =  13,167  metres).  In  all  cases  the  twine 
size  is  identified  by  the  number  of  yarns  which  have  been 
twisted  into  the  twine. 

The  monofilament  twine  is  identified  by  its  nominal 
diameter. 

The  continuous  multifilament  yarns  (nylon  and 
Tcrylene)  are  identified  both  as  to  weight  or  total  denier 
per  yarn  and  as  to  the  number  of  filaments  per  yarn. 
The  twisted  twines  were  of  "cable"  construction  and  are 
described  by  two  numbers  in  the  same  way  as  are  the 
cotton  twines. 

The  braided  twines  are  described  by  the  total  number 
of  yarns  in  the  braid. 

PROPERTIES  AND  TEST  PROCEDURES 

The  nett  weight  (gravitational  force  less  buoyant  force) 
of  the  netting  in  the  water  is  an  important  factor  in 
determining  how  it  will  lie  or  move  in  the  water  and  how 
much  lead  and  what  float  capacity  is  required  for  a 
given  piece  of  equipment.  For  example,  the  low  density 
of  nylon  contributes  toward  the  fishing  efficiency  of 


[70] 


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1 


MODERN     FISHING    GEAR    OF    THE    WORLD 


nylon  gillnets  but  creates  handling  problems  in  nylon 
purse  seines.  The  nett  weight  was  measured  by  weighing 
an  air-dry  sample  of  netting  in  the  air,  by  soaking  the 
sample  overnight  in  distilled  water  at  70  dcg.  F.  (21  -1 
deg.  C.)  and  then  reweighing  while  still  immersed  in  the 
water.  The  weight  in  water  was  then  expressed  as  a 
per  cent,  of  the  air-dry  weight.  This  weight  per  cent,  is 
numerically  equal  to  the  weight  in  water  in  pounds 
(or  kilograms)  per  hundred  pounds  (or  kilograms^  air-dry 
in  air.  The  specific  gravity  was  calculated  from  the 
weight  per  cent,  in  water.  The  nett  weight  (per  cent.)  of 
the  netting  in  salt  water  or  in  fresh  water  and  at  any 
temperature  may  be  estimated  by  the  relation: 

Weight  (%)  |   _  F    __  specific  gravity  of  sea  water  ~1 
in  sea  water  f       L       specific  gravity  of  material  J 

The  change  in  length  due  to  wetting  must  be  known 
where  the  mesh  length  is  specified  only  for  dry  netting. 
In  many  types  of  fishing  gear,  such  as  gillnets  and  trawls, 
the  mesh  length  of  the  web  in  the  water  is  an  important 
factor  in  determining  the  fishing  efficiency  of  the  gear 
or  in  releasing  under-size  fish  for  conservation  purposes. 
Different  materials  react  differently  to  wetting,  e.g.,  most 
natural  fibres  shrink  and  most  synthetic  fibres  extend, 
so  that  a  quantitative  measure  of  this  reaction  is  needed 
before  dry  mesh  length  can  be  properly  specified  for  a 
given  application.  The  change  in  length  (per  cent.)  due 
to  wetting  was  determined  by  measuring  the  length  of 
meshes  in  the  sample  when  air-dry,  by  soaking  the  sample 
overnight  in  cold  tap  water,  by  measuring  the  length  of 
meshes  in  the  same  sample  while  wet,  and  by  expressing 
the  change  in  mesh  length  which  occurred  during 
wetting  as  a  per  cent,  of  the  average  mesh  length  of  the 
dry  sample.  Shrinkages  are  prefixed  by  the  negative  ( — ) 
sign  and  extensions  are  prefixed  by  the  positive  ( + ). 

The  linear  density  is  a  convenient  measure  of  the  size 
of  the  twine.  Often  twine  must  be  purchased  by  weight 
but  is  required  for  use  by  length.  A  knowledge  of  linear 
density  therefore  permits  estimation  of  the  quantity 
(weight)  of  twine  required  to  construct  a  given  piece  of 
equipment.  In  keeping  with  common  Canadian  practice, 
the  indirect  system  (length  per  unit  weight)  is  used.  Ten 
one-yard  (1  yard  -----  0-9144  metres)  pieces  of  air-dry 
twine  were  cut  under  four  ounces  (113-4  grams)  tension 
and  were  weighed  collectively  to  the  nearest  milligram. 
The  linear  density  of  the  twine  (1  yard  per  pound  ----- 
2-016  metres  per  kilogram)  was  calculated  by  dividing 
the  weight  of  the  ten-yard  sample  in  grams  into  4536. 
The  effective  linear  density  per  yarn  was  then  calculated 
by  multiplying  the  linear  density  of  the  twine  by  the 
total  number  of  yarns  in  the  twine.  Further  division  by 
1,000  reduced  the  number  of  integers  required  for  the 
table  and  changed  the  length  unit  from  yards  to  kiloyards 
(1  kiloyard  —  1,000  yards  -  914-4  metres). 

The  diameter  of  the  twine  is  a  major  factor  in  deter- 
mining how  strongly  moving  water  pulls  the  netting, 
how  easily  swimming  fish  can  move  the  netting,  and 
what  power  is  required  or  speed  attained  while  towing 
a  given  piece  of  equipment.  The  diameter  was  measured 
by  placing  four  parallel  strands  of  air-dry  twine  under 
the  1  -25  inch  (3-15  centimetres)  diameter  circular  foot 
of  a  dial  gauge.  The  total  force  exerted  diametrically  on 
the  twine  by  the  foot  during  measurement  was  6  ounces 
(170-1  grams).  The  separation  of  the  foot  from  the 


supporting  anvil  caused  by  the  interposed  twine  was 
indicated  by  the  dial  gauge  in  mils  (1  mil  -  0-001  inches 
—  0-0254  millimetres)  and  was  reported  as  the  twine 
diameter.  This  method  of  measurement  has  certain 
defects  when  used  to  determine  the  diameter  of  twine 
for  fishing  gear.  Twines  made  from  staple  fibres  have 
loops  and  fibre  ends  protruding  from  the  twine  which 
increase  the  resistance  to  movement  of  the  twine  through 
water  but  which  are  easily  pressed  to  the  twine  during 
diameter-measurement;  and  soft-laid  twines  are  pressed 
out  of  round  by  the  measuring  device  more  than  are 
harder  laid  twines.  However,  when  interpreted  in  the 
light  of  these  effects,  twine  diameters  measured  in  this 
way  can  be  useful. 

The  effective  cross-section  area  per  yarn  was  calculated 
as  a  dimension  which  is  a  function  of  the  diameter  and 
yet  is  reasonably  constant  over  all  twine  sizes  for  each 
yarn  style  and  twist  hardness.  The  effective  cross-section 
area  of  the  twine  in  circular  mils  (1  circular  mil  ^ 
0-0005067  square  millimetres)  is  the  square  of  the  twine 
diameter  in  mils  (1  mil  0-001  inches  0-0254  milli- 
metres). Further  division  by  the  total  number  of  yarns 
in  the  twine  gives  the  effective  cross-section  area  of  each 
yarn.  The  diameter  of  any  size  twine  of  the  same  style 
may  then  be  estimated  by  multiplying  the  effective  yarn 
area  by  the  number  of  yarns  in  the  twine  and  by  deriving 
the  square  root  of  this  product. 

Total  extension  and  breaking  load  of  twines  were 
measured  simultaneously  on  a  Scott  Model  J2  recording 
tester  of  100  pounds  and  400  pounds  maximum  capacities, 
using  type  X-l  clamps.  The  tester  was  set  so  that  10 
inches  (25-4  centimetres)  of  untensioned  twine  lay 
between  the  nips  of  the  clamps  prior  to  test.  For  knot 
strength  tests,  the  twine  was  cut  and  the  appropriate 
knot,  single  weavers'  knot  or  double  weavers'  knot,  was 
tied  in  the  twine  by  hand. 

For  testing  the  mesh  strength  of  netting  for  seines 
and  trawls  (medium-laid  cotton  and  hemp),  small  panels 
of  netting  were  cut  from  the  sample,  all  the  meshes 
along  two  opposite  sides  of  the  panel  were  threaded 
respectively  onto  \  inch  (6-35  millimetres)  diameter  rods 
on  the  tester,  and  the  sample  was  stressed  to  rupture 
through  separation  of  the  rods  by  the  tester.  The  mesh 
strength  was  then  calculated  by  dividing  the  panel 
strength  by  the  number  of  meshes  in  the  width  of  the 
panel.  The  netting  was  tested  in  both  directions  relative 
to  the  selvedge  and  the  overall  average  is  reflected  in 
the  data  quoted  here.  Netting  in  seines  and  trawls  is 
stressed  in  use  over  several  meshes  at  a  time  rather  than 
on  single  meshes;  therefore,  by  testing  panels  as  above, 
irregularities  in  the  physical  dimensions  of  the  netting 
result  in  lowered  mesh  strength  as  is  experienced  in  use. 
Netting  in  gillnets  is  more  commonly  stressed  on  indivi- 
dual meshes  so  that  these  materials  (linen,  ramie,  nylon, 
and  Terylene)  were  tested  one  mesh  at  a  time.  The  mesh 
was  placed  around  two  one-inch-diameter  (2-54  centi- 
metres) drums  so  that  no  knots  touched  either  drum, 
and  the  drums  were  separated  by  the  tester  until  the 
mesh  broke.  For  dry  strength,  the  materials  were  tested 
in  their  natural,  air-dry  state,  and  for  wet  strength,  the 
materials  were  soaked  overnight  in  cold  tap  water. 

Specific  strength  is  dimensionally  identical  with  tena- 
city, the  only  difference  being  in  the  units  employed. 
Arithmetically  it  is  the  test  strength  per  twine  (i.e., 


I  721 


SUITABLE    TWINES     FOR     FISHING     GEAR 


twine  strength,  knot  strength,  or  half  the  mesh  strength) 
in  pounds  (1  pound  -  453-6  grams)  multiplied  by  the 
linear  density  of  the  air-dry  twine  in  kiloyards  per  pcund 
(1  kiloyard-=  1,000  yards^914-4  metres).  This  specific 
strength 'may  be  converted  into  tenacity  in  grams  per 
denier  by  further  multiplying  by  0-1016.  For  specific 
strength  units,  the  weight  or  gravitational  force  was 
cancelled  against  the  strength  or  breaking  force,  leaving 
length  as  the  unit  for  specific  strength.  This  permits  an 
easier  understanding  of  the  physical  significance  of 
specific  strength  or  tenacity  than  is  possible  with  the 
metric  units,  grams  per  denier.  Specific  strength  (and 
tenacity)  is  the  length  of  twine  the  weight  of  which  equals 
the  breaking  strength  of  that  same  twine.  For  the  same 
strength-quality,  an  increase  in  weight  per  unit  length 
causes  an  identical  increase  in  strength,  leaving  the 
specific  strength  (and  tenacity),  which  could  well  be 
called  breaking  length,  constant.  For  a  given  material 
of  a  given  quality  the  specific  strength  is  reasonably 
constant  over  all  twine  sizes;  and  the  breaking  strength 
of  a  particular  twine  may  be  estimated  from  the  specific 
strength  by  dividing  this  latter  by  the  linear  density  of 
the  twine.  Specific  strength  figures  may  be  used  directly 
to  compare  the  strength  of  different  materials  on  an 
equal  weight  basis,  e.g.,  high  tenacity  nylon  66  when  wet 
is  seen  to  be  slightly  weaker  than  wet  linen,  a  relation 
often  obscured  by  the  fact  that  nylon  66  and  linen 
twines  are  not  made  in  identical  weights.  Specific  strength 
may  also  be  used  as  a  measure  of  strength-quality,  the 
specific  strength  of  different  specimens  being  directly 
comparable  even  though  the  yarns  may  be  spun  to 
slightly  different  weights.  Finally,  the  tensile  strength 
(breaking  force  per  unit  area  normal  to  the  direction 
of  the  force)  of  the  fibre  itself  is  related  to  the  specific 
strength  through  the  fibre  density  so  that  the  usable 
tensile  strength  of  the  fibre  in  pounds  per  square  inch 
(1  pound  per  square  inch  70-307  grams  per  square 
centimetre)  may  be  estimated  from  the  relation: 

Tensile   Strength  1,300    x    Specific   Gravity    x 

Specific  Strength.  Comparison  of  this  usable  tensile 
strength  with  data  available  elsewhere  for  the  virgin 
fibre  gives  an  idea  of  the  manufacturing  and  geometrical 
efficiencies  realised  in  these  fishing  gear  materials. 

The  load  extension  characteristics  on  initial  loading 
of  each  twine,  both  dry  and  wet,  were  drawn  autographic- 
ally  by  the  Scott  tensile  tester.  The  average  often  replicate 
test  plots  was  redrawn  showing  the  per  cent,  extension 
(increase  in  length  under  load  expressed  as  a  per  cent,  of 
the  unstressed  length)  as  a  function  of  specific  load. 
Specific  load,  analogous  to  specific  strength,  is  the  actual 
load  in  pounds  (a  pound  =  453-6  grams)  multiplied  by 
the  linear  density  of  the  air-dry  twine  in  kiloyards  per 
pound  (1  kiloyard  -^  1,000  yards  914-4  metres).  All 
these  average  curves  for  all  twine  sizes  of  a  given  material 
and  style  were  drawn  against  the  same  reference  axes 
and  an  average  of  the  curves  was  then  drawn.  The 
stretch  data  given  in  Table  I  were  read  from  this  last 
average  plot  against  the  corresponding  specific  strengths 
of  each  material  in  each  state.  These  data  do  not  include 
elongations  consequent  upon  tightening  of  knots,  which 
are  functions  of  the  initial  knot  tightness;  but  they  do 
include  irreversible  elongations  in  the  twine  itself. 
Undoubtedly,  the  load-extension  characteristics  under 


cyclic  loading  are  more  significant  to  fishing  gear  design 
than  are  initial  loading  characteristics,  but  the  former 
cannot  be  obtained  with  our  Scott  tensile  tester. 

Data  describing  the  tensile  strength  of  the  dry,  straight 
twine  often  have  little  application  to  fishing  gear  design; 
in  fact,  they  can  lead  to  serious  error.  Even  so,  such  data 
are  frequently  all  that  are  available.  Th  •  change  (increase 
or  decrease)  in  tensile  strength  which  results  from  wetting 
the  materials  is  reported  as  a  per  cent,  of  the  dry  strength 
to  assist  in  estimating  the  strength  in  water  where  only 
dry  strength  data  are  available.  Similarly,  the  per  cent, 
of  the  twine  strength  which  is-  retained  in  knotted 
structures  is  reported  to  assist  in  estimating  the  knot 
or  mesh  strengths  where  only  twine  strength  data  are 
available.  These  per  cent,  changes  in  strength  to  wetting 
and  per  cent,  strength  efficiencies  of  knots  and  meshes 
were  calculated  from  the  specific  strength  data. 

The  tensile  strength  of  the  fibrous  materials  used  in 
fishing  gear  can  be  measured  relatively  easily,  so  this 
property  is  often  the  only  one  evaluated,  and  is  sometimes 
blindly  adopted  as  the  only  measure  of  quality  and 
performance.  Where  the  material  is  to  be  subjected 
only  to  dead  loads,  tensile  strength  is  an  adequate 
measure  of  quality.  However,  fighting  fish  are  not  dead 
loads,  and  it  is  more  important  that  the  material  be  able 
to  absorb  the  energy  inflicted  on  it  by  the  fish  than 
merely  carry  the  weight  of  the  fish.  A  dramatic  illus- 
tration of  this  is  that  nylon  gillnets  for  salmon  need  not 
be  so  strong  as  linen  gillnets  to  hold  the  same  fish, 
because  the  greater  elastic  extension  of  the  nylon  imparts 
greater  ability  to  absorb  the  energy  expended  by  the 
salmon.  The  "toughness"  used  here  is  a  rough  measure 
of  the  energy  required  per  unit  length  of  twine  to  break 
that  twine,  whether  it  be  straight,  knotted,  or  in  a  mesh. 
Arithmetically  it  is  one-half  the  product  of  the  breaking 
load  and  the  fractional  extension  from  unstressed  length 
to  rupture.  This  estimate  is  in  error  to  the  extent  that 
the  load-extension  curve  is  assumed  to  be  linear;  but, 
whereas  most  fishing  gear  materials  deviate  from  linearity 
toward  the  extension  axis,  the  error  is  in  the  same  direc- 
tion in  most  cases  and  the  figures  are  reasonably  com- 
parable. A  further  loss  of  correlation  between  this 
calculated  "toughness"  and  performance  toughness 
originates  from  use  of  the  load-extension  relation  under 
initial  load  -permanent  elongation  is  included.  However, 
despite  approximations  in  its  estimation,  "toughness" 
is  more  important  in  the  design  of  some  types  of  fishing 
gear  than  is  tensile  strength.  "Toughness"  is  reduced  to 
the  parametric  form,  viz.,  toughness  index,  by  multiplying 
it  by  the  linear  density  of  the  air-dry  twine.  This  toughness 
index  is  thus  the  energy  required  per  unit  weight  of 
twine  to  break  that  twine,  and  is  reasonably  constant 
for  all  twine  sizes  of  the  same  material  in  the  same  style. 

DEFINITIONS  AND  DIMENSIONS 

A.  Parametric  properties,  quantitatively  reported  in 
the  table,  which  are  reasonably  constant  for  all 
twine  sizes  of  the  same  material  and  style. 

I.  Weight  (per  cent.)  in  water  is  the  nett  weight 
(gravitational  force  less  buoyancy)  of  the  material 
when  immersed  in  distilled  water  at  70  deg.  F. 
(21-1  deg.  C),  expressed  as  a  per  cent,  of  the 
air-dry  weight. 


[73] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


«,  •  ^   /„,  x  -        * 
Weight  (%)  in  water  - 


Nett  weight  in  water 

--    —  ~  --  —  -  -  -- 


<    100 


--    —  ~  --  —  -  -  --  r 
Air-dry  weight  in  air 

2.  Specific   gravity  is  the  density  of  the   material 
relative  to  the  density  of  pure  water  al  4  deg.  C. 
39  2deg.  F.). 

Specific  gravity  ~  }  ^^^^^^^^ 

3.  Change  in  length  (per  cent.)  to  wetting  is  the 
increase  (+)  or  the  decrease  (-)  in  length  caused 
by  soaking  the  material  in  water,  expressed  as  a 
per  cent,  of  the  air-dry  length. 

Change  in  length  ^  Wet  length  —  Dry  length 
(%)  to  wetting  Dry  length 

4.  Linear   density   is   the   length   per   unit   weight 
(indirect  system). 


100 


Linear  density 
per  yarn 
(kyd./lb.) 


Linear  density  of , 


Total    num- 

•      ,  j  in.  v     x  her  of  yarns 
twine  (yd./lb.)        in  (he  (winc 

2-016  m./kg. 


1000 
1  yd./lb. 

1  kyd./lb.-  1000  yd./lb. 
-  2-016m./g. 

5.  Effective  cross-section  area   is  the  area  of  the 
circle    whose    diameter    equals    the    measured 
diameter  of  the  twine. 

Effective  cross-section  area  _  (Twine  diameter  (mils)  )2 
per  yarn  (circular  mils)         Number  of  yarns  in  twine 
1  circular  mil  -  area  of  circle  1  mil  (0-001  in.    0  0254 

mm.)  in  diameter. 
=  0  -0005067  sq.  mm. 

6.  Total  extension  (per  cent.)  at  break  is  the  increase 
in  the  length  of  the  twine  caused  by  the  load 
required  to  break  the  twine  (straight,  knotted,  or 
mesh)  expressed  as  a  per  cent,  of  the  unstressed 
length. 

Total  extension     Twine  length  at  break  -  Unstressed  length 
at  break  Unstressed  length 

7.  Specific  strength  (kyd.)  is  the  effective  tenacity. 

Specific  strength         j         Linear  density       Measured 
(kyd.)  —      -x      of  air  dry       ?    strength 

1000       twmc  (yd./lb.)  (Ib.) 

1  kyd.    -    1000  yd. 

-  0  1016  g./den. 


100 


100 


x    100 


.      Total  extension  (%)  at  break      Specific 
ft  *  ™-  x  strength 


8.  Strength  change  (per  cent.)  to  wetting  in  the  in- 
crease (4-)  or  decrease  (-)  in  tensile  strength 
caused  by  soaking  the  material  in  water,  expressed 
as  a  per  cent,  of  the  dry  strength. 

Change  in  strength  _  Wet  strength—Dry  strength 
to  wetting  Dry  strength  X 

9.  Strength  efficiency  (per  cent.)  is  the  per  cent,  of  the 
straight    twine    strength    which    is    retained    in 
knotted  structures. 

Strength  efficiency  Knot  strength 

of  Knotted  twine  Straight  Twine  strength 

Strength  efficiency  Mesh  strength 

of  mesh  2x  Straight  twine  strength 

10.  Toughness  index  (kyd.  lb./lb.)  is  an  approximate 
estimate  of  the  energy  per  unit  weight  of  twine 
required  to  break  the  twine. 

Toughness 
index  juu  (Ryd) 

1  kyd.  -  lb./lb.    -     3000  ft.  -  lb./lb. 
-  91440  g.  -  cm./g. 

B.  Specific  properties,  variable  with  twine  size,  which 
may  be  estimated  from  the  values  of  the  parametric 
properties  given  in  the  table. 

Linear  density  per  yarn 
Number  of  yarns  per  twine 
1  yd./lb.  -  2 -01 6m. /kg. 

2.  Twine  diameter      /Effective  cross-section  Number  of  \  \ 
(mils).                  y        area  per  yarn  twine     / 

1  mil.  —  0-001  in. 

0-0254  mm. 

Specific  strength  of  twine  x  no.  of  yarns 

Linear  density  per  yarn 
1   Ib.         453-6  g. 

4.  Mesh  strength      2x  Specific  strength  of  mesh  x  no.  of  yarns 
(Ib.)  Linear  density  per  yarn 

1  Ib.  -  453-6  g. 

5'  T(ft^1b/ft  )       TouShncss  index   x   Number  of  yarns 

Linear  density  per  yarn 
1  ft.-lb./ft.  -  453-6  g.  -cm./g. 


I.  Twine  weight  (yd./lb.)  -  1000 


3.  Twine  strength  (Ib.) 


Log-rafts  returning  from  drift  net  fishing,  Ceylon. 
[74] 


Photo  FAO. 


TESTING   METHODS  FOR   NET  TWINES  AND   NETS,   ESPECIALLY 
THOSE  MANUFACTURED   FROM   SYNTHETIC   MATERIALS 

by 

J.  K.  VAN  WIJNGAARDEN 

Research  Laboratory,  A.K.U.  and  Affiliated  Companies,  Arnhem,  Netherlands 

Abstract 

This  paper  describes  a  number  of  tests  for  yarns  and  cords  used  in  the  fishing  industry,  and  the  author  points  out  that,  as  a  certain 
amount  of  standardization  has  already  been  achieved  in  the  textile  industry,  the  International  Fishing  Gear  Congress  can  help  in  fostering 
a  uniformity  of  measuring  methods  in  the  various  branches  of  the  fishing  gear  industry. 

He  recommends  that  "TEX",  the  unit  for  yarn  numbering,  representing  the  mass  in  grams  per  kilometre  of  yarn,  be  used  for  net 
yarns  and  cords,  and  further  that  "twist",  indicated  by  Z  or  S  according  to  the  direction  of  the  turn,  should  be  expressed  as  the  number  of 
turns  per  metre  (t/m). 

The  methods  for  testing  for  stretching,  shrinkage  and  stiffness  of  yarns  and  cords  are  fully  described,  and  the  question  of  knot- 
strength  is  dealt  with  in  detail.  He  discusses  the  different  ways  in  which  a  knot  can  break  down  when  tested  in  the  clamps  of  a  dynamometer, 
such  as  tip-over,  breaking  before  and  after  slip  and  breaking  after  tip-over,  etc.,  and  then  he  tests  for  the  "opening-up"  of  knots  in  synthetic 
materials,  by  putting  knotted  cords  in  a  container  revolving  at  60  r.p.m.  and  finding  the  number  of  revolutions  necessary  to  loosen  the  knots. 
Finally,  the  methods  used  for  testing  knots  are  applied  to  the  testing  of  meshes,  and  mesh-strength,  i.e.  the  load  at  which  a  mesh  breaks,  is 
surely  one  of  the  most  important  factors  affecting  the  performance  of  a  fishing  net. 

Mtthodes  d'essai  pour  les  conies  des  filets  et  les  filets,  en  particulier  pour  les  articles  fabriques  a  partir  de  fibres  synthetiques 
Rfeume 

I /Industrie  textile  etant  deja  arrivec  £  un  certain  degre  dc  normalisation,  le  Congrcs  international  des  engins  de  peche  peut  contribucr 
£  1'adoption  de  me th odes  uni formes  de  mesure  dans  les  di verses  branches  de  P Industrie  des  engins  de  pechc.  L'auteur  recommande  d'utiliser 
pour  les  filets  et  les  cordes  destines  aux  filets,  le  "TtX"  unite  pour  le  numcrotagc  des  fils  rcpresentant  la  masse  en  grammes  par  kilometre  de 
fil  et  de  plus  d'exprimer  la  torsion,  indiquee  par  Z  ou  S  selon  le  sens,  en  nombre  de  tours  par  metre  (t/m). 

Les  mcthodcs  servant  i\  determiner  1'extension,  le  raccourcissement  et  la  rigidite  des  files  et  des  cordes  sont  decrites  en  detail,  de 
meme  que  la  question  de  la  resistance  des  noeuds.  L'auteur  examine  les  differentes  manicres  dont  un  noeud  peut  se  rompre  au  cours  d'essai 
au  dynamometre,  et  il  verifie  Touverture  des  noeuds  des  materiaux  synthetiques  en  placant  des  cordes  nouees  un  recipient  tournant  £  60  t/m 
et  en  cherchant  le  nombre  de  revolutions  nccessaires  pour  relacher  les  noeuds.  Hnfin,  les  methodes  utilisees  pour  verifier  les  noeuds  sont 
appliquees  &  la  verification  des  mail  les  et  la  resistance  des  mailles. 

Metodos  para  ensayar  cordajes  y  redes,  especialmente  de  fibres  sinteticas 
Kxtracto 

Como  ya  se  ha  logrado  cierto  grado  de  normalizacion  en  la  industria  tcxtil,  el  Congreso  International  de  Artes  de  Pesca  podria 
ayudar  a  uniformar  los  metodos  de  medida  en  las  diversas  ramas  de  la  industria  de  artes  de  pesca.  El  autor  recomienda  el  uso  dc  la  unidad 
'TEX**  para  numerar  los  hilos,  que  representa  cl  peso  en  gramos  por  ki!6metro  de  kilo  o  cordaje  empleados  en  la  fabricaci6n  de  redes,  e 
insinua  expresar  la  "torsi6n"  o  colchadura,  indicada  per  Z  o  S  segun  el  sentido  en  que  se  practique,  en  numcro  de  vueltas  por  metro  (vxm). 

Tambicn  describe,  en  detallc,  los  metodos  usados  para  determinar  el  alargamiento,  encogimiento  y  flexibilidad  de  los  hilos  y  cordeles, 
asi  como  el  problema  dc  la  resistencia  de  los  nudos.  Ademas  analiza  las  diversas  maneras  en  que  puede  romperse  un  nudo  al  probarlo  con 
las  pinzas  de  un  dinam6metro,  como  la  rotura  antes  y  despues  de  correrse,  etc.  y  las  pruebas  para  determinar  su  aflojamiento  cuando  se 
usan  materiales  sinteticos,  poniendo  las  cuerdas  anudadas  en  un  recipiente  que  gira  a  60  r.p.m.  a  fin  de  encontrar  el  numero  de  revoluciones 
necesarias  para  que  se  aflojen  dichos  nudos.  Por  ultimo,  los  metodos  usados  en  las  pruebas  dc  nudos  se  han  aplicado  a  los  ensayos  de  las 
mallas  y  a  su  resistencia. 


1.     INTRODUCTION 

THE  use  of  synthetic  yarns  during  the  last  few  years 
as  material  for  nets  has  caused  a  growing  interest 
in  testing  methods  for  twines  and  nets.    This  is 
expressed  by  more  exact  measuring  and  by  development 
of  new  measuring  methods  to  deal  with  the  special 
problems  which  have  arisen. 

The  "Bureau  International  pour  la  Standardisation  de 
la  Rayonne  et  des  Fibres  Synthetiques"  (BISFA)  with  a 
membership  of  all  the  main  North  and  West  European 
producers  of  man-made  fibres,  and  connected  with  the 
International  Organisation  of  Standardisation  (ISO), 
is  engaged  in  standardizing  the  different  testing  methods 


for  textile  and  tyre  yarns,  and  for  this  purpose  publishes 
the  "BISFA-rules".  In  the  USA  similar  work  is  done  by 
the  American  Society  for  Testing  Materials  (ASTM). 

This  contribution  discusses  various  testing  methods 
for  net  yarns  and  twines,  and  nets.  Some  of  these  methods 
are  already  known  in  the  textile  industry,  some,  of  specific 
interest  to  the  fishing  industry,  relate  to  testing  methods 
developed  in  our  own  laboratories.  The  BISFA  rules 
are  quoted  with  regard  to  the  first  group,  but  proposals 
are  made  for  the  second  primarily  from  the  point  of  view 
of  the  yarn  producer. 

The  term  "yarn"  is  understood  to  mean  a  continuous 


(75) 


MODERN     FISHING     GEAR     OF    THE    WORLD 


strand  of  fibres  and/or  filaments,  from  which  twist,  if  any, 
can  be  removed  in  one  operation;  strand  is  the  product 
obtained  by  twisting  together  two  or  more  twisted  yarns, 
twine  is  the  product  obtained  by  twisting  two  or  more 
strands  together. 

2.   METHODS  OF  TESTING  YARNS  AND  TWINES 

21  Yarn  Number 

Several  units  are  used  for  the  numbering  of  net  yarns 
and  twines.  The  ISO  Conference,  May  1956,  recom- 
mended the  use  of  the  combination  of  grammes  per 
kilometre  of  yarn,  called  "tex*\  as  a  unit  for  yarn 
numbering  and  we  strongly  advocated  the  use  of  the  same 
unit  for  net  yarns  and  twines.  Table  I  shows  the  num- 
bers in  tex,  denier  and  m/kg.  for  some  yarns. 

TABLE    I 

m/kg.  100        500      1,000      5,000      10,000     50,000  1,000,000 

Td 

(g./9000 

m.)         90,000  18,000     9,000    18,000          900  180  90 

Tex 

(g./IOOO 

m.)         10,000    2,000      1,000        200  100  20  10 


2  2  Twist 

Since  the  determination  of  the  twist  of  net  yarns  and 
twines  is  the  same  as  that  of  textile  yarns  only  a  few 
points  will  be  mentioned.  The  direction  of  twist  is 
normally  indicated  by  the  letters  Z  or  S,  as  shown  in  fig.  1 . 
In  order  to  maintain  the  metric  system  we  recommend 
that  the  twist  shall  be  expressed  as  the  number  of  turns 
per  metre  of  twisted  yarn  or  twine  (t/m.). 

Below  is  illustrated  the  indication  of  the  twist  con- 
struction of  a  twine: 


Z- twist     S- twist 
fig.  1.     Indication  of  the  twist  direction. 

100  tex  Z  400  x  2  S  300  X  3  Z  100  means  a  twine 
composed  of  6  yarns:  every  yarn  is  first  twisted  to  400 
t/m.  in  Z-direction,  then  two  of  these  twisted  yarns  are 
twisted  together  to  300  t/m.  in  S-direction,  and  finally  3 
of  these  strands  are  twisted  in  Z-direction  to  100  t/m. 

2-3  Strength  and  Extensibility 

The  stress-strain  curve,  and  the  strength  and  total 
extension  at  break,  are  determined  by  the  load-extension 
tester  or  dynamometer.  The  many  types  of  dynamometer 
can  be  classified  according  to  the  time  conditions  under 
which  the  determination  of  strength  and  extension  are 
performed.  The  time  factor  plays  an  important  part  in 
this  determination  for  the  materials  used  in  fishing  nets. 
The  velocity  and  the  way  of  increasing  the  strain  have 


an  influence  on  the  shape  of  the  stress-strain  curve  and 
the  values  of  strength  and  extension  at  break.  Efforts 
have  been  made  to  realise  one  of  the  following  principles 
in  making  the  dynamometers: 

1 .  the  length  of  the  test  object  increases  in  proportion 
to  the  time;  thus  the  rate  of  extension  is  constant; 

2.  the  force  on  the  test  object  increases  in  proportion 
to  time;  constant  rating  of  loading. 

The  electronic  recording  dynamometers  which  are 
characterized  by  their  stability,  accuracy  and  versatility, 
are  of  recent  design. 

One  clamp  is  moved  at  constant  speed  while  the  force 
is  measured  by  the  other  clamp,  connected  with  an 
electric  clement  for  measuring  the  force  (fig.  2). 

These  meters  have  some  specific  properties,  viz.,  the 
possibility  of  recording  both  a  decrease  and  an  increase 
in  stress,  and  the  many  and  wide  measuring  ranges  that 
can  be  covered  by  a  single  instrument.  These  make  them 
particularly  suitable  for  testing  twines  and  nets. 

In  practice,  the  wet  strength  and  total  extension  at  the 
breaking  point  of  the  twines  are  of  primary  importance. 
As  regards  modern,  synthetic  materials,  such  as  e.g. 
nylon  6,  nylon  66  and  polyesters,  the  wet  stress-strain 
properties  do  not  differ  very  much  from  those  measured 
in  dry  condition,  so  it  sufliccs  to  determine  the  latter 
only  for  routine  measurement. 

The  BISFA  rules  describe  the  measuring  conditions 
as  "the  length  of  the  test  specimen  between  the  jaws  shall 
be  50  cm.,  and  the  mean  time  to  break  shall  be  20  i  2 


Fig.  2.     A  KU  electronic  dynamometer. 


[76] 


METHODS    OF    TESTING    TWINES 


seconds".  From  a  scientific  point  of  view  this  may  not 
be  the  best  possibility,  but  this  prescription  of  the  time 
was  the  only  solution,  owing  to  the  many  different 
working  principles  of  the  dynamometers  which  are  in 
use  in  the  various  laboratories. 

In  consequence  of  the  growing  use  of  the  electronic 
dynamometer,  however,  more  and  more  laboratories 
have  begun  to  apply  a  constant  rate  of  extension, 
generally  1  per  cent,  of  the  original  length  between  the 
clamps  per  second. 

To  avoid  slipping  or  breaking  of  the  test  specimen  in 
the  clamps,  it  is  advisable  to  use  clamps  of  such  a  shape 
that  a  very  sharp  kink  is  avoided  (fig.  3).  Usually  only 


(c)  hot  water  shrinkage,  measured  in  wet  condition. 

(d)  hot  water  shrinkage,  measured  after  conditioning. 

In  figs.  5  and  6,  these  4  values  arc  given  for  some  twine 
samples  of  synthetic  material  as  a  function  of  the 
immersion  time  in  water  (t). 


Water      2  K -F* ""^~^ZZ_~ 

Shrinkage 


1 

1  10  (30) 

-^Immersing  timp  (min.  ) 

•    :Measured  after  conditioning  (65  %  H.  H.  20    C.  ) 
A    :Mt-asured  in  wcl  condition 


Twine  type: 

I  Fnkalon 
(nylon  6) 

(93  -|-  23) 
tcx  y  3 

II  Hnkanylon 
(nylon  66), 
70  tcx  ,-  4 


Fig.  5.     Influence  of  the  Immersing  time  on  the  cold  water 
shrinkage. 


Callaway  clamp. 


the  strength  and  total  extension  at  break  are  given  of  the 
stress-strain  properties.  In  practice,  the  shape  of  the 
stress-strain  curve,  or  the  modulus,  can,  however,  be  of 
great  importance  to  the  behaviour  of  the  twine.  It  is 
Therefore  advisable  to  determine  some  points  on  this 
curve,  for  example,  the  extension  at  25  per  cent,  of  the 
breaking  load. 

24  Shrinkage 

The  amount  of  shrinkage,  either  in  hot  or  in  cold  water, 
can  be  important  as  it  may  effect  a  change  in  size  of  the 
mesh  when  the  nets  receive  a  final  treatment  in  hot  water 
or  when  they  are  dyed. 

The  method  we  have  used  is  as  follows:  5  knots  were 
laid  in  a  twine  at  distances  of  about  1  m.,  these  distances 
being  measured  to  an  accuracy  of  1  mm.  ( lfl)  on  an  appar- 
atus as  shown  in  fig.  4.  The  pre-tensions  were  taken 


Knot 


Cord 


Knot 


Pre-tension 


Fig.  4.     Apparatus  for  measuring  the  shrinkage. 

according  to  the  BISFA  rules.  The  sample  was  then 
placed  in  cold  (20  deg.  C.)  or  in  boiling  water,  after  which 
the  distance  between  the  knots  was  measured  again  (1 ,), 
wet  or  after  conditioning  in  a  standard  atmosphere 
according  to  the  BISFA  rules :  temperature  20  deg.  ri  2 
dcg.  C.,  relative  humidity  65  per  cent,  and  to  the  ASTM 
standards 

70deg.   I  2  F.,  65  per  cent.  The  value    °~   l  x  100  then 

gives  the  shrinkage  in  per  cent. 

In  this  way  we  can  determine  4  values  of  the  test 
sample:. 

(a)  cold  water  shrinkage,  measured  in  wet  condition. 

(b)  cold  water  shrinkage,  measured  after  conditioning. 


1 2 
10 
8 
ti 
4 
2 
0 


Cord  type    I  . 
I- 

1  I 

-  •  '  Measured  after  condidoniriQ 

A     Measured  in  wd  condition    , 


UK , 2o*C  ) j      I 

I 


1  10  (30) 

Immersing  time  in  hot  wat*»r  (min) 


Fig.  6.  Influence  of  the  immersing  time  on  me  hot  water  shrinkage. 

It  can  be  seen  that  especially  in  the  first  minute  of  t. 
the  shrinkage  strongly  increases,  and  that  for  t.  =  30  min. 
a  nearly  constant  value  is  reached  for  all  the  samples. 
An  immersing  time  of  30  min.  should  be  accepted  as  the 
standard  immersing  time  for  routine  measurements. 

25  Stiffness 

For    measuring   the   stiffness   of  twines   we    used    the 
following  method: 

Around  a  rod,  4  cm.  in  diameter,  we  wound  20  turns 
of  the  twine  close  to  each  other,  held  together  by  a  narrow 
strip  of  adhesive  tape  at  the  turns.  The  coil  thus  formed 
is  taken  off  the  rod  (fig.  7a). 


Connected  to 
at  r»-fls- gauging 


_-_.—»    — a,y»trnv  -     _.  .  — -  - 

|  L      of  thr         _!/   '  T] 

{ J    dynamometer  I  J 

'  '•      *      > 


?f)  turns  of  tWliu* 


T"^           T            I  '    "inectcd  to 

I /_. L--— -   moving  damp 


b)  At  thr  beginning  cf 
the  ti-Bt 


r)  At  the  rod  of 


Fig.  7.    Stiffness  test  of  twines. 


77] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


Two  flat  plates  of  about  10  cm.  diameter  are  attached 
to  the  clamps  of  the  electronic  dynamometer,  the 
distance  between  the  two  plates  in  the  highest  position 
of  the  moving  clamp,  being  2  •  5  cm.  The  moving  clamp 
is  not  lowered  a  few  centimetres  and  the  cord  cylinder 
is  placed  on  the  lower  flat  plate  (fig.  7b).  The  lower 
clamp  is  then  moved  upwards  until  the  extreme  position 
is  reached  (fig.  7c),  when  the  cylinder  becomes  an  ellipse 
with  a  short  axis  of  2-5  cm.  The  force  needed  for  this 
is  taken  as  a  measure  of  the  stiffness. 

3.  METHODS  OF  TESTING  KNOTS 
31     General 

The  different  methods  of   test    on    knots,  dealt  with 
further  on,  refer  to  the  single-knot,  as  drawn  in  fig.  8.  In 


Fig.  8.    Single  knot. 

the  following  the  knot-ends  will  be  as  indicated  according 
to  the  letters  given  in  this  figure. 

There  are  various  methods  of  loading  a  knot  according 
to  the  twine  ends  which  are  clamped  on  the  dynamo- 
meter. Table  II  gives  these  different  possibilities  of 
clamping,  with  the  results  obtained  on  loading  the  knots 
with  an  increasing  force. 


Clamped 
Twine-ends 

A  and  B 
AandC 


A  and  D 
Band  C 
B  and  D 
CandD 


Result 

tip-over  of  the 
knot 


TABLE   U 

Clamped 
Twine-ends 

A  and  C4  D 


knot  breakage,  B  and  C  f  D 

in  some  cases 

after  slip 

CandA  +  B 
D  and  A  IB 
knot  breakage 
knot  breakage 

in  some  cases 

after  tip-over 

of  the  knot. 


Result 

slip  of  AB  through 
CD  or  knot  break- 
age 


knot  breakage 


|-*~  To  force  measuring 
system 


3*—  Moving  clamp 
1_ 

Fig.  9.    Determination  of  the  knot  strength. 

(b)  the  velocity  of  the  moving  clamp  (range:  1  per  cent, 
to  4  per  cent,  rate  of  elongation). 

(c)  the  waiting  time,  i.e.  the  time  between  the  tightening 
of  the  knots  and  the  beginning  of  the  test  (range: 
6  seconds  to  24  hours). 

The  factors  were  not  found  to  influence  significantly 
the  knot  strength. 

To  find  agreement  between  the  values  for  the  knot 
strength,  an  exact  description  of  the  circumstances  under 
which  the  test  is  performed  is  therefore  unnecessary. 

33    Tip-Over  Resistance 

Generally,  there  are  two  causes  of  changes  in  the  size  of 
the  meshes,  viz.  the  tip-over,  and  the  slip  of  the  knots.  In 
the  tip-over  the  structure  of  the  knot  changes,  after 
which  the  twine  AB  can  easily  shift  through  the  knot. 
The  testing  method  is  performed  as  follows  (fig.  10): 


—  To    force    measuring  -^ 

system  of  the 
A  dynamometer 


Moving  clamp 
Moving  c 

Normal  knot(uidc  view)  Tin-over  *"»««  /'---*  view) 

Fig.  10.     Determination  of  the  tip-over  resistance. 


-b- 
Tin-ovpr  Ur» 


3-2  Knot  Strength 

The  knot  strength,  and  especially  the  wet  knot  strength 
determines  the  strength  of  the  nets.  The  twine  ends  B  and 
D  are  clamped  in  on  the  dynamometer  to  determine  the 
knot  strength  (fig.  9). 

The  stress  at  which  the  knot  breaks — expressed  in  kg. 
or  in  per  cent,  of  the  breaking  load  of  the  twine — is 
called  the  (wet  or  dry)  knot  strength. 

We  investigated  the  influence  of: 

(a)  the  load  with  which  the  knots  were  tightened 
initially  (in  the  range  from  30  per  cent,  to  60  per 
cent,  of  the  twine  strength). 


the  twine  ends  A  and  B  are  clamped  on  the  dynamometer 
and  the  force  at  which  the  tip-over  of  the  knot  takes 
place  is  determined.  This  force,  in  kg.  or  in  per  cent,  of 
the  knot  strength,  is  called  the  tip-over  resistance. 

When  the  tip-over  begins,  the  stress-strain  curve 
mostly  shows  that  for  a  short  time  the  force  is  constant 
or  decreases  somewhat. 

In  some  cases,  e.g.  in  research  on  bonded  twines,  it 
can  be  of  importance  to  the  producer  to  determine  the 
tip-over  resistance  of  the  knots  without  making  a  net. 
The  knots  must  be  tied  manually  and  tightened  in 
accordance  with  what  happens  on  the  stretching  appara- 
tus. Subsequently,  whether  or  not  after  fixation,  the 


[78] 


METHODS    OF    TESTING     TWINES 


knots  are  measured.  In  fig.   11  a,  b  and  c  the  tip-over 
resistance  is  given  as  a  function  of  the  three  above- 


Tip -over 

resistance 

(Kg.) 


Tightening  force 
(tf>  of  knot  strength) 


Tip-over      fi    " 

-  —     -  - 

- 

resistance 

I 

(Kg    ) 

4    - 

1  ,  —  —  ••    -in            1 

-It- 

2 



Hi 

0 

_.  1- 

F--H- 


120 


"ml  ^  60 

Tightening  time  (sec.  ) 

-b- 


Twine  type : 

I  Enkalon(93  -  23)  ic\  .<  3; 
bonded 

II  Enkalon  93  lex   /  2x3: 
unbonded 

III  fcnkanylon  70  lex     <    4  ; 
unbonded 


3-4  Knot-Slip  Resistance 

The  slippage  of  the  knots,   while  retaining  the   knot 
structure  also  causes  a  change  in  mesh  size. 

To  determine  the  resistance  to  this  slippage,  the  knots 
are  clamped  as  drawn  in  fig.  12.  The  force,  in  kg.  or  in 


The  force  measuring 
•ystem 


Fig.  12.     Determination  of 
the  knot-slip  resistance. 


—  Moving  clam. 

per  cent,  of  the  knot-strength,  at  which  the  slip  begins  is  a 
measure  of  knot-slip  resistance.  The  slippage  can  occur 
cither  suddenly  or  gradually,  depending  on  the  specimen 
type.  Fig.  1 3  shows  the  stress-strain  curve  which  can  be 
obtained,  where  S  in  the  knot-slip  resistance. 

Curves  a,  b  and  c  show  a  sort  of  slip-stick  effect, 
curve  d  shows  the  slippage  which  takes  place  gradually. 
Sometimes  only  a  slight  slippage  occurs  (see  arrows  in 
fig.  13,  c  and  d).  In  this  connection  we  decided  to 


Fig.  11.   Influence  of  (a)  tightening  force,  range  30-90  per  cent. 

of  the  knot  strength;  (b)  tightening  time,  range  1-60  seconds 

(r)  waiting  time,  range  5-120  min.  on  the  tip-over  resistance. 


mentioned  factors,  and  the  analysis  of  variance  is  given 
in  Table  III  for  twine  sample  II. 


t       5 


Source  of  variation 


A  (tightening  force) 
B  (tightening  time) 
C  (waiting  time) 


AB 
AC 
BC 
ABC 

Residual 

Total 


TABLE  HI 

Sum  of     Degrees  of 
squares       freedom 

Main  Effect  : 

.    1334  2 

.       12  1 

9  I 

Interaction  : 
2  2 


Fig.  13. 


Stress-strain  curves  which  can  be  obtained  with  the 
determination  of  the  knot-slip  resistance. 


Mean      Variance 
square        ratio 


21 

1 

3 

469 

1851 


2 
1 

2 
108 

119 


667-00 

12-00 

9-00 


1-00 
10-50 
1-00 
1-50 
4-34 


154 
2-76 
2-07 


0-23 
2-42 
0-23 
0-34 


It  appears  that  in  the  applied  ranges  of  the  factors  only 
the  tightening  force  has  a  significant  influence. 

We  have  chosen  for  the  tightening  force  a  fairly 
arbitrary  value,  viz.  20  per  cent,  of  the  knot  strength. 
For  the  rest  the  standard  conditions  were: 

rate  of  extension  :        1  per  cent,  per  second 
tightening  time:  10  sec. 

waiting  time:  5  min. 

As  a  chemical  preparation  is  applied  to  synthetic 
materials  to  prevent  the  knots  slipping,  it  is  advisable  to 
determine  both  the  wet  and  the  dry  tip-over  resistance. 


take  only  that  force  as  knot-slip  resistance,  in  which  the 
slippage  occurs  for  longer  than  1  second  under  the 
measuring  conditions  mentioned  below. 

We  investigated  the  influence  of  various  factors  on  the 
knot-slip  resistance.  First  the  influence  of  the  angle  (a), 
under  which  the  twine  ends  c  and  d  can  be  clamped. 
We  determined  the  knot-slip  resistance  at  a=  0  deg.,  60  deg. 
and  120  deg.  In  all  cases  it  was  lowest  at  a=0deg.  Further, 
just  as  for  the  tip-over  resistance  (see  3-3),  the  influence 
of  tightening  force,  tightening  time  and  waiting  time 
were  investigated.  The  results  are  shown  in  fig.  14,  and 


T 
i       1 

\  \ 

f'H 

: 

I 

13579 
— -  Tightening  fore*  (Kg) 


Knot  -  slip    6 
resistance 
(Kg) 


0.1  120 

-  Waitin?  time(min) 


1  60       . 
-Tightening  time  (aec) 


Fig.  14.  Influence  of  (a)  tight- 
ening force,  range  1-9  kg. 

(b)  tightening  time,  range 
1-60  seconds; 

(c)  waiting    time,     range 
0,  1-120  min.  on  the  knot- 
slip  resistance. 


[79] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


the  analysis  of  variance  is  given  for  one  of  the  samples  in 
Table  IV. 

TABLE  IV 


Source  of  variation 

Sum  of     Decrees  of        Mean     Variance 
squares      freedom          square 

Main  Effect  : 

A  (tightening  force) 
B  (tightening  time) 
C  (waiting  time) 

.37773 
2 
4 

2 
1 

1 

18886 
2-00 
4-00 

176 
0-02 
0-02 

Interaction 

AB 

.       22 

2 

11-00 

0-10 

AC 

.     401 

2 

200-50 

1-87 

BC 

.      154 

1 

154-00 

1-44 

ABC 

.     384 

2 

192-00 

1-79 

Residual 

.11571 

108 

107-14 

Total 


50311 


119 


In  all  cases  the  influence  of  the  tightening  force  is 
significant,  and  that  of  the  tightening  time  is  not.  In  the 
range  investigated,  the  influence  of  waiting  time  is  only 
significant  for  some  samples  (see  e.g.  sample  I,  fig.  14,  c), 
so  the  influence  of  this  factor  was  investigated  on  a 
somewhat  larger  scale.  The  results  are  shown  in  fig.  15, 


Knot-slip 

resistance 

(Kg' 


Twine  type  : 

I  Enfcalon  (93+23)tex  x  3  .unbonded 
II  Enkmlon  (93+23Hex  x  3  ; bonded 
III  Enkanylon  70  tex  x  4      ; unbonded 


0,  1       1         10       100    1000 
—  Waiting  time  (min   > 

Fig.  15.  Influence  of  the  waiting  time  on  the  knot- slip  resistance 
range  0,  1-1440. 


brought  together  and  a  weight  is  attached  to  them.  The 
knots  are  then  tightened  by  dropping  the  weight  over  a 
distance  of  about  20  cm.  The  weight  is  taken  according 
to  the  yarn  number  and  amounts  to  about  1  g.  per  5  tex. 
Five  knots  are  tested  with  an  apparatus  as  shown  in 
fig.  16.  The  knots  are  placed  in  a  wooden  box,  which  is 
rotated  with  a  velocity  of  60  r.p.m. 


20  cm 
Hox((iO  R.  P.  M 


Motcr— 
/ 


.  30  cm 


20  cm 


Fig.   16.     Apparatus  for  determining  the  open-up  resistance. 

This  process  is  regularly  interrupted  to  check  on  how 
many,  if  any,  of  the  knots  have  loosened. 

From  the  values  found,  i.e.  the  number  of  revolutions 
after  which  each  of  the  knots  had  loosened,  we  deduced 
that  the  frequency  distributions  of  these  values  approxi- 
mately correspond  to  those  given  in  fig.  17. 


Sample1 


Number  of  revolutions 


Fig.  17.     Frequency  distribution  of  the  number  of  revolution*, 
after  which  the  knots  have  loosened. 


from  which  it  appears  that  for  a  waiting  time  exceeding 
5  min.  there  is  no  further  influence  on  the  knot-slip 
resistance. 

On  the  basis  of  the  above-mentioned  results,  we  have 
chosen  the  following  standard  conditions  for  routine 
measurements: 

angle  between  the  clamped  specimen  ends  C  and  D: 

Odeg.; 

rate  of  elongation :  1  per  cent,  per  second, 

and,  if  the  knots  are  tied  manually: 

tightening  force:    20  per  cent,  of  the  knot-strength 
tightening  time:      10  seconds 
waiting  time:          5  minutes 

For  some  twine  types  the  knot  slip  resistance  appears 
to  be  higher  and  for  other  types  lower  than  the  tip-over 
resistance.  Hence,  for  a  complete  evaluation  of  a  twine, 
both  measurements  must  be  carried  out. 

35  Open-Up  Resistance 

In  the  use  of  synthetic  materials,  one  difficulty  which 
may  arise  is  that  the  knots  loosen  just  after  they  have  been 
made  on  the  netting  machine.  To  evaluate  a  net  twine  in 
this  respect,  the  following  testing  method  has  been 
developed:  in  two  twines  (length  about  20  cm.)  a  knot  is 
tied  and  just  tightened.  Two  of  the  twine  ends  are 


In  such  a  frequency  distribution,  the  most  character- 
istic value  when  taking  a  random  sample  is  the  middle  or 
so-called  median  value.  In  this  case,  for  example,  if  the 
number  of  revolutions  at  which  the  five  knots  loosen,  is 
arranged  in  order  of  size:  X,,  X2,  X3,  X4  and  X&,  then  X3 
is  taken  as  the  determining  value,  called  the  open-up 
resistance.  Generally,  the  measurements  are  repeated 
several  times  to  find  an  average  value  of  X3,  (X3),  and 
a  measure  of  the  spread  (standard  deviation  sX3). 

In  fig.  18  the^3-values  are  given  for  some  samples  of 


Yarn  type:  EnkaJon, 
93  tex  x2  x3 

d3=Standard  deviation 


Concentration 
bonding  bath: 

Fig.  18.  Some  values  obtained  with  measurements  of  the  open-up 
resistance. 


the  same  twine,  each  dipped  into  a  bath  of  different 
concentration  and  containing  a  bonding  preparation. 
Tests  were  made  of  6  x  5  knots  of  each  sample.  It  appears 


[80] 


IV 


III 


To  force  measuring  syete 
of  the  dynamometer 


METHODS    OF    TESTING     TWINES 

42  Mesh  Strength 

The  meshes  are  clamped  as  shown  in  fig.  19.  The  load  ai 
which  the  mesh  breaks  is  called  the  mesh  strength,  and  is 
given  in  kg.  or  in  per  cent,  of  twice  the  twine  strength 
(because,  apart  from  the  four  knots,  there  are  two  ends 
between  the  clamps).  It  appears  that  mostly  knot  II  or 
III  (fig.  19)  breaks,  and  sometimes  knot  IV.  As  the 
weakest  of  these  3  knots  will  determine  the  mesh  strength, 
the  latter  will  be  somewhat  lower  than  twice  the  knot 
strength. 


Moving  hook 

front  view  aide  view 

riff.  J9.    Determination  <>J  the  me\h  strength. 

that  the  difference  between  the  X:i  values  arc  relatively 
large  as  compared  with  the  spread  sx:j. 

4.     METHODS  OF  TESTING  MESHES 

41    General 

In  principle,  some  of  the  methods  mentioned  above  for 
testing  knots  can  be  applied  also  to  testing  the  meshes  for 
mesh  strength  and  tip-over  resistance  of  the  mesh.  As  the 
remarks,  made  in  3-2,  and  3-3  also  apply  partly  to  these 
two  qualities,  only  a  few  points  of  the  testing  methods 
will  he  dealt  with. 


4-3    Tip-Over  Resistance  in  the  Mesh 

For  determining  tip-over  resistance  the  mesh  is  clamped 
as  in  the  mesh  strength  measurement  (fig.  19).  There  is 
usually  a  tip-over  of  knot  1  (fig.  19)  before  the  mesh 
breaks.  The  load  at  which  this  happens  is  called  the 
tip-over  resistance  of  the  mesh  and  is  given  in  kg.  or  in 
per  cent,  of  the  mesh  strength.  The  value  is  about  twice 
as  high  as  the  corresponding  tip-over  resistance  of  the 
knot. 


5.     CONCLUSION 

We  have  not,  of  course,  given  a  complete  survey  of 
testing  methods  for  yarns,  twines,  cords,  knots  and 
nets,  but  have  only  recorded  some  of  the  experience 
gained  with  the  various  testing  methods  normally  used 
in  our  laboratory  for  evaluating  yarns  and  net  t\vines. 


hauling  a  Marlon  pursed  lampara  se'ne  in  South  Africa. 

[81  ] 


Pholo  FAO. 


TESTING   OF  MATERIALS   USED   IN   FISHING 

by 

J.  REUTER 

Nederiandsche  Visscherij-Proefstation  en  Laboratorium  voor  Malerialen-Onderzoek  Utrecht,  Netherlands 

Abstract 

This  paper  points  out  the  necessity  for  testing  under  fishing  conditions  materials  used  for  fishing,  i.e.  in  the  wet  state  instead  of 
the  dry.  It  shows  how  it  is  possible  to  be  misled  by  taking  only  dry  breaking  strength  as  a  criterion  instead  of  testing  materials  wet  and 
knotted.  In  the  case  of  hemp,  it  was  found  that  the  lower  grades  yarn,  i.e.  those  with  low  dry  breaking  length,  possessed  a  superior  wet, 
knotted  breaking  strength.  Tables  showing  the  characteristics  of  hemp,  cotton  (Egyptian  and  American),  silk  and  synthetic  fibres  are  included. 

Essais  de  filet  et  de  til  a  filet 
Resume 

Cette  communication  fait  ressortir  la  necessite  d'cssayar  les  matcriaux  utilises  pout  la  peche  dans  les  conditions  d'emploi,  c'est-a-dire 
mouilles  au  lieu  de  sees.  Elle  montre  comment  il  est  possible  de  se  tromper  en  prcnant  comme  seul  critere  la  resistance  a  la  rupture  a  P6tat 
sec  au  lieu  d' essay er  les  materiaux  mouiltes  et  noues.  On  a  trouve,  dans  le  cas  du  chanvre,  que  les  fils  dc  basse  qualitd,  c*est-a-dire  a  faible 
resistance  a  la  rupture  a  sec,  avaient  une  plus  forte  resistance  a  la  rupture  quand  ils  Itaicnt  mouilles  et  noues.  Des  tableaux  donnent  les 
caraotlristiques  des  fibres  de  chanvre,  de  colon  (£gyptien  et  am£ricain),  de  soie  et  synth&iques. 

Ensayo  de  redes  e  hilos  empleados  en  su  fabricacidn 
Extracto 

En  este  trabajo  se  expone  la  necesidad  de  ensayar  los  materialcs  de  pesca  en  las  condiciones  como  trabajan,  e.i.  humcdos  en  vez  de 
secos.  Sc  ha  demostrado  que  es  posible  inc^rrir  en  cquivocaciones  utili/ando  como  criterio  de  comparaci6n  la  resistcncia  a  la  tracci6n  del 
material  seco,  en  vez  de  humedo  y  amidado.  fcn  el  caso  del  caftamo,  sc  cncontr6  que  los  hilos  de  material  de  calidad  inferior,  o  sea  aquellos 
con  un  bajo  m6dulo  dc  ruptura  cuando  estan  sccos,  presentan  una  resistenua  a  la  rolura  muy  superior  cuando  se  hallan  humedos  y  anudados. 
Se  incluyen  tablas  con  las  caracteristicas  de  las  fibras  dc  caftamo,  algod6n  (egipcio  y  amcricano),  scda  y  materialcs  sintcticos. 


IN  most  places  where  twine  is  bought  or  sold  for 
making  webbing  and  nets,  an  attempt  is  invariably 
made  to  appraise  the  quality.  This  is  done  in  many 
ways,  and  attempts  are  usually  made  to  define  the 
various  properties  possessed  by  the  material,  but  there 
is  one  property  that  is  invariably  assessed,  viz.  the  strength 
of  the  twine.  The  average  fisherman  tests  the  strength 
by  trying  to  break  the  twine  with  his  hands;  the  more 
well-to-do  arrange  for  it  to  be  examined  at  a  research 
laboratory  under  ideal  test  conditions  (20  deg.  C.  ±  2deg. 
and  65  per  cent,  relative  humidity). 

The  greatest  value  is  attached  to  the  breaking  strength 
and  it  is  taken  for  granted  that  strong  twine  makes 
strong  webbing.  When  various  twines  are  to  be  compared 
qualitatively,  this  same  criterion  is  adopted,  so  that  the 
prospective  purchaser  inevitably  assumes  that  the 
strongest  twine  is  most  advantageous. 

A  fundamental  misconception  underlies  this  appraisal 
method  which  frequently  leads  to  faulty  conclusions, 
with  all  the  attendant  disadvantages  to  the  fisherman, 
because  the  strongest  twine  is  not  always  synonymous 
to  the  strongest  net. 

Twine  intended  to  be  worked  up  into  webbing  is 
usually  tested  dry  to  determine  its  tensile  strength.  The 
way  in  which  this  is  actually  done  is  of  secondary 
importance,  as  the  process  is  used  simply  to  obtain  data 
on  the  dry  breaking  strength  or  dry  breaking  length  of 
the  twine.  But  the  only  correct  basis  for  examining  a 


twine's  properties  is  to  test  it  under  wet  conditions,  i.e. 
wet  and  knotted,  since  it  will  be  used  in  wet  condition, 
and  the  properties  when  wet  are  decisive  when  it  comes 
to  comparing  various  twines  for  webbing. 

It  would  be  of  great  help  if  both  national  and  inter- 
national standards  of  testing  could  be  established  and 
the  results  passed  on  to  the  fishermen;  this  would  be  of 
practical  value  to  them  when  buying  materials. 

As  a  supplement,  a  list  of  tests  are  given  here  which 
have  been  carried  out  at  the  Netherlands  Fishery 
Experimental  Laboratory  as  a  routine  investigation  of 
the  qualities  of  the  materials  bought  and  for  comparing 
properties  of  materials.  It  is  not  considered  that  these  are 
the  only  suitable  tests,  but  they  are  based  on  experience 
and  have  met  requirements  so  far. 

The  figures  quoted  for  the  breaking  strength  are  the 
mean  averages  of  10  test  samples,  as  customary  in 
routine  investigations.  The  usual  sample  lengths  and 
rates  of  traverse  were  adopted.  The  material  for  the 
"wet"  tests  was  previously  immersed  in  water  for  at 
least  12  hours. 


EXAMINATION  OF  HEMP  TWINE 

The  tests  were  carried  out  in  accordance  with  the 
normal  textile  method  (Table  I).  The  only  possible 
conclusion  under  these  conditions  was  that  Quotation  I 


[821 


TESTING     FISHING     MATERIALS 


TABLE  I 
Hemp  Net  Twine 


Theor-  Theor- 


Actual  Actual 


Dry      Break- 


Quota-   Analy-  Make   etical  etical  7,  '    f    "  /  breaking   ing 
tion        sis      lays    No.  of  wight  ™;  £'  ""*  ™  strength  length 
No.         No.      (1)     ™tres  ingr.™tr"ingrm       °f      mkm' 

perk8'purn' 


perkg.perm. 


twine 


(2) 


, 

24 

100 

340 

2 

941 

309 

3 

240 

65-2 

20 

•6 

1 

23 

130 

442 

2 

•262 

412 

2 

•427 

53-6 

22-1 

I 

22 

150 

510 

1 

•961 

478 

2-090 

58-0 

27 

•3 

I 

21 

180 

612 

1 

•634 

512 

1 

•952 

51-2 

26 

•2 

II 

25 

100 

340 

2 

941 

378 

2 

646 

59-8 

22 

6 

II 

26 

130 

442 

2 

262 

357 

2- 

801 

52-5 

18 

•7 

II 

27 

150 

510 

1 

961 

467 

2- 

141 

48-1 

22- 

•5 

II 

28 

180 

612 

1 

•634 

526 

1 

901 

42-9 

22- 

6 

(1)  Lay -No.   of   1-70  m   lengths   together   weighing  exactly 

500  grammes. 

(2)  Dry  breaking  strength  divided  by  dry  weight  per  metre. 


TABLE  II 
Hemp  Net  Twine 


W  ith  over- 


With  overhand 


No. 


No. 


24 
23 
22 
21 

25 
26 
27 
28 


strength 


66-8 

39-4 

46-2 

23-2 

53-6 

26-8 

53-2 

28-8 

53-1 

44-4 

47-7 

41-9 

39-1 

36-9 

38-4 

33-2 

58-9 
50-2 
51-3 
54-0 

76-1 
87-8 
94-4 
86-5 


12-2 

9-6 

12-8 

14-7 

15-27 
14-96 
17-46 
16-67 


Breaking  strength  (wet)  divided  by  dry  weight  per  metre. 


was  the  best,  as  the  breaking  length  of  this  material  was 
considerably  superior  to  that  of  Quotation  II. 

As  a  considerable  quantity  of  sample  material  remained 
unused  at  the  end  of  the  tests,  curiosity  prompted  us  to 
carry  out  the  same  breaking  strength  tests,  using  wet 
twine  instead,  incorporating  an  overhand  knot. 

The  results  were  surprising  for  we  suddenly  found 
ourselves  confronted  with  the  fact  that  this  "fishery 
method"  (wet,  knotted  breaking  strength)  yielded 
results  diametrically  opposed  to  those  obtained  by  the 
textile  method.  Quotation  II  was  now  found  to  be  the 
best  (Table  II). 

When  the  data  in  Tables  I  and  II  are  compared,  a 
striking  fact  emerges  in  connection  with  these  hemp 
twines,  which  were  of  special  hard  twist,  viz.  the  wet, 
knotted  breaking  strength  of  the  "higher  grade"  twines 
deteriorates  by  a  much  greater  percentage  than  that  of 
the  "lower  grade"  twines. 

The  wet,  knotted  breaking  length  leaves  no  doubt 
that  one  type  of  twine  was  far  superior  to  the  other,  and 
that  there  was  thus  every  justification  for  purchasing 
the  twine  which  would  inevitably  have  been  rejected, 
had  the  dry  evaluation  method  been  employed. 

As  the  hemp  samples  (both  hard  twisted)  came  from 
different  manufacturers,  samples  (normal  twist)  from 
yet  another  manufacturer  were  tested.  The  results  of 
this  third  series  of  tests  confirmed  our  findings  in  the  two 
previous  tests  (Table  III).  Here,  too,  it  was  found  that: 

1.  twines   characterized    by    a    good    dry   breaking 
length  were  considerably  inferior  in  wet,  knotted 
breaking  strength  (webbing  as  used  under  fishing 
conditions),  and  that  lower  grade  twines  (low  dry 
breaking  length)  possess  a  superior  wet,  knotted 
breaking  strength. 

2.  as  this  transition  is  fairly  rapid  (between  3/36  and 
3/30),   it   cannot    be  attributed   to   the  twist  or 
diameter  of  the  twine,  but  to  the  intrinsic  properties 
of  the  raw  material  itself. 

A  happy  coincidence  also  made  it  possible  to  carry 
out  this  series  of  tests  with  twines  made  of  spun  yarns. 
As  they  were  not  the  same  as  the  net  twines  originally 
tested,  the  results  do  not  correspond  exactly,  but  the 
general  principles,  defined  in  1  and  2  above,  still  hold 
good. 


TABLE  III 
Hemp  Net  Twine 


Analysis 
No. 


Make 

of 
product 


Actual 
No.  of 
metres 
per  kg. 


Dry 

breaking 
strength 
in  kg. 


Dry 

breaking 
in  km. 


Wet 

breaking 
strength 
in  kg. 


With  over- 
hand, knot 
breaking 
strength 
in  kg.  (wet) 


36 
37 
38 
39 
40 
41 
42 
43 


3/60 
3/42 
3/36 
3/30 

3/24 
3/18 
3/15 
3/12 


1990 

1263 

1101 

964 

755 

766 

553 

371 


8-6 
15-1 
16-0 
22-9 
31*2 
33-5 
41-1 
66-1 


100 
100 
100 
100 
100 
100 
100 
100 


17-1 
19-1 
17-6 
22-1 
23-5 
25-6 
22-7 
24-5 


12-1 
20-7 
23-8 
27-0 
32-9 
36-6 
45-3 
59-0 


140 
136 
149 
118 
106 
109 
110 
89 


10-7 
16-7 
19-3 
19-7 
27-9 
28-6 
35-1 
47-9 


130 
110 
120 
88 
89 
86 
86 
73 


[83] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


TABLE  HlA 


Make  A 

Our  mark  .  .  .21  blue 

Runnage  (metres/kg. )  .385-0 

Breaking  strength  in  kg. 

dry         ... 

wet 

wel  overhand  knot 

Breaking  length  in  km. 

Wet  overhand  knot  breaking 
length  in  km. 

Breaking  strength   in  grammes 
per  tex 

dry          ...        17-48  (100",,) 
wet          ...        17-56 
wet  overhand  knot       .       14-82 


22  blue 
383-0 


45-2  (100%) 
45-6 
38-6 
17-44  (100%) 

37-8  (83-5%,) 
35-9 
34-6 
14-74(84-3%) 

14-82(100%) 

13-32(90%) 

14-47  (82 
13-75(78 
13-25  (89 


8% 

9",, 
4% 


A  preliminary  investigation  indicated  that  in  the  case 
of  flax,  made  from  fibres  of  various  lengths,  the  results 
differ  to  a  certain  extent,  depending  on  whether  the 
textile  or  fishery  test  method  was  used. 

The  long  staple  fibres  of  this  material  were  separated 
from  their  short  counterparts  and  both  were  used  to 
produce  the  same  type  of  netting  twine  with  precisely 
the  same  lay  and  twist. 

The  results  arc  summarized  in  Table  H!A. 

Considered  from  the  dry  breaking  length  point  of 
view,  it  appears  that  sample  A  was  the  best,  the  ratio 
being  100  :  84-5  on  the  basis  of  breaking  length.  When 
the  we/,  knotted  breaking  length  is  taken  as  criterion, 
the  corresponding  ratio  is  100  :  90. 

Here,  too,  there  is  yet  another  indication  that  the 
"poorer"  yarn  (shorter  fibres)  stands  up  better  to  knotting. 

TESTING  OF  COTTON  NETTING 

The  same  characteristic  deviations  between  the  dry  twine 
and  the  wet  knotted  breaking  length  are  also  found  in 
the  case  of  cotton  twines. 

To  ascertain   whether  these  data  could  be  used  as 


quality  indications,  the  figures  of  the  wet  breaking 
strength  were  measured  at  the  mesh,  the  results  being 
expressed  in  the  number  of  grams  breaking  strength  per 
fibre  quantity  employed.  An  abbreviation,  "tex"  was 
used  for  this  latter  unit  ( 1  tex  the  quantity  of  material 
involved  when  1,000  m.  weigh  exactly  1  g.). 

When  the  quality  of  the  various  net  samples  is  evaluated 
by  this  method  (see  Table  IV),  it  will  be  found  that: 

1.  the  unit  "g.  tex"  provides  a  serviceable  standard 
for  comparing  the  strength  of  different  net  twines. 

2.  webbing    made    from    Egyptian    cottons    is    less 
strong  than  that  made  from  American  cottons. 

3.  the  breaking  length  of  dry  twine  is  not  a  suitable 
criterion    for    evaluating    the    strength    of    wet 
netting.  The  dry  twine  breaking  length  of  sample  h, 
for  example,  is  very  low,  whereas  calculated  on  the 
basis  of  g./tex,  based  on  the  wet  mesh  breaking 
strength,  this  sample  is  the  strongest.     The  dry 
breaking  lengths  of  twine  samples  />,  d  and  /  are 
almost  identical,  yet  they  show  a  difference  in  the 
number  of  g./tex  calculated  on  the  basis  of  the  wet 
mesh  breaking  strength. 

4.  Twine  sample  c  is  stronger  than  a  and  b\  the  dry 
breaking  length   is  also   better.  On  the  basis  of 
g./tex,   however,  calculated  from   the  wet   mesh 
strength,  a  and  b  arc  stronger  than  c. 

TESTING  OF  SILK  YARNS 

Similar  deviations  in  results  when  both  the  textile  and 
fishery  methods  were  employed  are  also  found  when 
silk  yarns  were  tested  to  ascertain  the  number  of  grammes 
wet,  knotted  breaking  strength  per  unit  of  weight  The 
unit  of  weight  "denier"  was  adopted  (1  denier  the 
quantity  involved  when  9,000m.  of  yarn  weighs  exactly 
Ig.). 

Two  different  lots  ol  material  were  tested,  and  here 
again  it  was  very  obvious  that  this  "fishery  test"  method 
brought  to  light  qualities  which  would  not  have  been 


'lABIL    IV 

Cotton    Webbing 


30/12 

30/12 

30/12 

30/12 

36/12 

36/12 

<  arded 

carded 

carded 

carded 

carded 

carded 

Type 

American 

American 

American 

Egyptian 

American 

/•ffyptian 

cotton 

cotton 

cotton 

cotton 

cotton 

cotton 

Ply 

4*3   -   12 

4,-  3         12 

4x3         12 

4x3         12 

4-3         12 

4  -'3         12 

Twist  direction 

zzs 

ZZS 

ZZS 

ZZS 

ZZS 

ZZS 

Runnage  (m./kg.) 

3851 

3824 

3943 

3935 

4796 

4570 

Breaking  strength  (dry) 

5-21 

5-15 

5-94 

4-84 

4-48 

4-19 

Percentage 

100 

100 

too 

UK) 

100 

100 

Breaking  length 

20-06 

19-69 

23-42 

19-05 

21-49 

19-15 

Breaking  strength  (wet) 

5-70 

5-73 

6-81 

5-60 

5-18 

4-46 

Percentage  (in  terms  of  dry  twine) 

109 

111 

115 

116 

116 

106 

Breaking  strength  at  mesh,  dry 

7-20 

7-60 

7-29 

6-64 

5-93 

5-70 

Percentage  (in  terms  of  dry  twine) 

138 

148 

123 

137 

132 

136 

Breaking  strength  at  mesh,  wet 

8-15 

8-68 

8-00 

7-59 

7-10 

6-24 

Percentage  (in  terms  of  dry  twine) 

156 

169 

135 

157 

158 

149 

Wet  breaking  strength  at  mesh, 

grammcs/tex. 

31-382 

33-257 

31-152 

28-870 

34-053 

28-519 

[841 

TESTING     FISHING     MATERIALS 

TABLL    V 
Silk  Net  Twine 


Supplier 


No. 

80/3  >  3 

80/3  -  3 

80/3  /  4 

80/3  <5 

80/3  •  6 

80/3  -  8 

Runnage  (m./kg.) 

82.17 

8177 

5783 

5015 

4139 

3247 

Total  denier 

1012-5 

1012-5 

1350-0 

1687-5 

2025-0 

2700-0 

Dry  breaking  strength  (kg.) 

3  -58 

3  29 

4-60 

5-93 

7-35 

8-76 

Breaking  length  (dry) 

29-49 

26-90 

26-61 

29-74 

30-42 

28-44 

Dry  breaking  strength  (g./denier) 

3-538 

3-259 

3-407 

3-514 

3-620 

3-244 

Wet  breaking  strength  (kg.) 

2  64 

2-44 

3-47 

4-37 

5-37 

6-61 

Wet  breaking  strength  (g./denier) 

2-607 

2-410 

2-570 

2-548 

2-652 

2-448 

Wet  breaking  strength,  with 

overhand  knot  (kg.) 

1-57 

1-70 

2-09 

2-70 

3-45 

4-30 

Wet  breaking  strength,  with 

overhand  knot  (g./denier) 

1   551 

1-679 

1-548 

1   600 

1-709 

1   593 

shown  by  the  dry  test  method.  As  this  material  is 
intended  solely  for  use  as  wet  webbing,  the  number  of 
g./dcnicr  calculated  on  the  basis  of  the  wet,  knotted 
breaking  strength,  is  the  only  correct  criterion  to  adopt 
when  evaluating  the  quality  of  the  yarns  for  fishery 
purposes  (Table  V). 

The  breaking  strength  in  g.  denier,  calculated  on  the 
basis  of  the  dry  yarn  breaking  strength,  runs  parallel  with 
the  dry  breaking  length,  as  this  is,  in  point  of  fact,  the 
same  unit  expressed  in  dilTercnl  terms.  When  the  break- 
ing strength  of  wet,  knotted  yarn,  is  determined,  the  data 
in  respect  of  the  various  yarns  will  he  found  to  differ 
entirely. 

TESTING  SYNTHETIC  FIBRE  WEBBING 

The  term  "g./denier"  is  very  frequently  used  lu  express 
the  breaking  strength  of  synthetic  twines.  This,  however, 
invariably  refers  to  dry  twines.  The  current  trend  is  more 
and  more  in  favour  of  the  wet  breaking  strength.  The 


fact  that  some  manufacturers  of  synthetic  fibre  twines 
regularly  indicate  the  wet,  knotted  breaking  strength,  is 
certainly  a  step  in  the  right  direction. 

Net  manufacturers  treat  the  twines  (continuous 
filaments)  which  have  previously  been  worked  up  into 
webbing,  in  various  ways  in  order  to  "fix"  the  knots.  The 
determination  of  the  breaking  strength  in  g./ denier, 
calculated  on  the  basis  of  the  wet,  mesh  breaking  strength, 
also  enables  the  investigator  to  ascertain  whether  this 
treatment  has  damaged  the  t\\ine  in  the  knot.  This 
determination  is  far  more  logical  than  simply  testing  the 
twine  prior  to  the  knot  being  fixed. 

Table  VI  summarizes  certain  data  in  respect  of  various 
samples,  all  expressed  in  terms  of  the  wet,  knotted 
breaking  strengths  of  both  twines  and  webbing. 

One  cannot  help  being  amazed  by  the  great  changes 
which  nylon  6  undergoes  (1  was  unable  to  examine  any 
webbing  made  from  nylon  66),  when  made  into  \\ebbing 
with  specially  treated  knots. 


TAUI  i    VI 
Synthetic  Net  Twine  and  Webbing 


Denier 
Material 

Filament 
Product 


_!:   726     :;_    10425        |    2520 
nvlon 


nylon 
'6 


nylon 
*6 


.:    4788 

poly- 
phony 1 
alcohol 


6300  5670 


contin- 
uous 

contin- 
uous 

contin- 
uous 

fibre 

new 
twine 

new 
twine 

new 
twine 

new 
twine 

nylon 
'6 

contin- 
uous 


m  Ion 
'6 


contin- 
uous 


5760 


nylon 
6 


contin- 
uous 


6930 


nylon 
6 


contin- 
uous 


I  0080 


contin- 
uous 


webbing      nebbing 


new  new  new 

webbing      \\cbbing      webbing 


Dry  breaking  strength  (kg.) 
Dry  breaking  strength  (g.;  denier) 
Wet  breaking  strength  (kg.) 
Wet  breaking  strength  (g./dcnicr) 

Wet  breaking  strength,  with 
overhand  knot  (kg.) 

Wet  breaking  strength,  with 
overhand  knot  (g./denier) 


4-78 

69-0 

9-41 

19-2 

43-06 

33-14 

6-579 

6-610 

3-734 

4-008 

6-835 

5-845 

4-27 

59-2 

8-30 

14-6 

39-95 

29-67 

5-851 

5-679 

3-532 

3-048 

6-341 

5-233 

2-62          25-6  5-80 

3-607          2-456          2-302 


6-70 


30-86  41-4  52-8 

5-445  5-973  5-23 

27-35  30-30  42-5 

4-824  4-372  4-216 


[85  1 


MODERN     FISHING    GEAR    OF    THE    WORLD 


In  the  case  of  a  polyalcohol  yarn,  the  dry  breaking 
strength  or  other  evaluation  figures  based  on  such  a  test 
would  prove  misleading  as  a  basis  for  the  comparison. 

This  fact  has  quite  rightly  been  publicized  by  the 
manufacturers,  and  it  is  indeed  encouraging  to  note  that 
the  wet,  knotted  breaking  strength  of  this  twine  is  now 
being  advocated  as  a  quality  criterion. 

TESTING  OF  TRAWL  TWINE 

A  single  test  was  carried  out  with  trawl  twine,  and  here, 
too,  differences  came  to  light  in  respect  of  the  wet, 
knotted  breaking  strength,  despite  the  fact  that  the  manila 
twines  were  of  the  same  runnage.  This  was  fully  in 
accordance  with  the  results  of  our  investigations  on  other 
yarns. 

The  investigation  had  to  be  interrupted,  however,  as  it 
proved  impossible  to  resolve  the  problem  of  the  manila 
fibre  gradings  employed. 

CONCLUSIONS 

1.  The  most  widespread  method  in  use  at  present  for 
the  comparison  of  net  twines  of  different  quality 
is  the  dry  breaking  strength  or  other  criteria  based 
on  it,  e.g.  breaking  length  or  tenacity  (the  breaking 
strength  in  denier  or  tex). 


2.  As  the  twine  is  used  wet  and  knotted  in  the  form  of 
webbing,  it  is  more  logical  to  employ  an  evaluation 
standard  based  on  the  wet,  knotted  condition  of 
the  twine. 

3.  The  wet,  breaking  strength  of  knotted  twine,  or 
the  wet  mesh  breaking  strength,  is  therefore  the 
most  appropriate  criterion.    Evaluation   figures, 
derived  from  the  wet,  knotted,  breaking  length  or 
the  wet,  knotted  breaking  strength  in  denier  or 
tex,  can  also  be  used  for  this  purpose. 

4.  The  evaluation  figures,  calculated  according  to 
1  and  3  above,  do  not  yield  parallel  results.  In 
fact,  in  some  cases,  they  are  diametrically  opposed, 
as  a  higher  dry  breaking  length,  for  instance,  is 
attended  by  a  lower  wet,  knotted  breaking  strength. 
Thus,  some  twines,  otherwise  very  strong,  are 
found  to  be  highly  sensitive  to  strong  flexion, 
while  less  strong  twines  suffer  to  a  smaller  degree 
when  wet  and  flexed,  i.e.,  knotted. 

5.  For  webbing,  the  wet  mesh  breaking  strength  is 
the  only   logical    strength   criterion,   because   it 
corresponds  more  accurately  with  the  conditions 
under  which  the  webbing  is  used,  and  permits  a 
more  accurate  comparison  to  be   made  of  the 
various  materials  used. 


A  canoe  fisherman  mendinp  his  net  on  the  Indian  Malabar  coast. 

[86] 


Photo  FAO. 


LATERAL  STRENGTH  AND   KNOT-FIRMNESS  OF   SYNTHETIC 
TWINES  FOR   FISHING  PURPOSES 

by 

HANS  STUTZ 

Farbwerke  Hoechst  A.G.,  Bobingen  Mills,  Germany 

Abstract 

The  author  describes  tests  for  determining  lateral  strength  and  knot -firm  ness  of  netting  twines  and  shows  how  knot-firmness  can  be 
improved  by  treating  the  twines  with  bonding  agents.     The  paper  is  illustrated  by  numerous  tables  and  diagrams. 


Rfeume 


Resistance  aux  efforts  lattraux  et  tenue  des  noeuds  des  flls  syntbttiques  pour  la  ptehe 


L'autcur  d6crit  les  essais  pour  la  determination  de  la  resistance  aux  efforts  lateraux  et  la  tenuc  des  noeuds  des  fils  a  filets.  II  montrc 
comment  on  peut  ameliorer  la  tcnue  des  noeuds  en  traitant  les  fils  par  impregnation  de  divers  produits.  L'article  est  illustrl  par  de  nombreux 
tableaux  et  diagrammcs. 

Resistencia  lateral  y  firmeza  de  los  nudos  de  hilos  sinttticos  empleados  en  los  artcs  de  pesca 
Extracto 

El  autor  describe  los  ensayos  hcchos  para  determmar  la  resistencia  lateral  y  la  firmeza  de  los  nudos  hechos  con  hilos  de  fibras 
sint6ticas,  y  la  manera  como  esta  ultima  puede  aumentar  tratando  el  material  con  fijativos.  hi  trabajo  tambi&n  contienc  numerosas  tablas 
y  esqucmas. 


IN  spite  of  all  well-known  lest  results  published  on  the 
subject  of  general,  physical  and  chemical  properties 
of  synthetic  fibres,  time  and  again  two  problems 
emerge  which  cause  a  certain  amount  of  trouble  in  the 
manufacture   and    use    of  nets,    twines   and    cordage: 
"lateral  strength"  and  "knot-firmness". 

Lateral  strength  means  the  resistance  which  the 
material  offers  to  any  stress  differing  from  the  normal 
tensile  stress  operating  in  the  longitudinal  direction  of 
the  fibre  axis.  Well-known  examples  are  the  strength  in  a 
knot  and  loop,  alternating  bending  strength,  etc.  Since 
in  these  cases  there  is  an  additional  stress  on  the  material 
the  normal  tensile  strength  is,  as  a  rule,  thereby  reduced. 
Various  materials  react  to  this  additional  stress  in  very 
different  ways  and,  in  this  connection,  the  type  of 
material  (e.g.  Perlon  or  hemp),  its  special  properties 
(depending  on  the  type),  the  structure  of  the  yarn  or 
twine  and  other  factors  often  play  an  important  part. 

Knot-firmness  concerns  the  reaction  of  a  knot  to 
forces  which  cause  shifting  or  loosening  and  thereby  a 
change  in  the  size  of  mesh. 

TESTING  METHODS 

Our  investigations  were  limited  to  the  two  synthetic 
fibres  which  are  at  present  most  important  for  the 
fishing  industry,  at  least  in  Europe:  polyamides  (Perlon, 
nylon)  and  polyesters  (Terylene,  Trcvira,  Dacron).  The 
material  was  tested  in  many  different  forms:  filament 
yarns,  staple  fibre  yarns,  twisted  twines,  and  braided 
twine. 


The  object  of  the  investigations  was  to  find  ways  and 
means  of  reducing  the  loss  in  tenacity  caused  by  knotting 
and  to  improve  the  firmness  of  knots,  particularly  in 
very  smooth  materials  made  from  continuous  filament. 

These  efforts  were  successful,  at  least  in  the  laboratory 
but  the  results  will,  of  course,  have  to  be  confirmed  in 


\\      // 


B 


Types  of  knots  used  in  the  tests. 
= Fishermen's  knot;  B  -  Overhand  knot ;  C- Double  overhand  knot 


[87] 


MODERN     FISHING     GEAR     OF    THE     WORLD 


Apparatus  used  for  testing  knot -firmness. 

practice.    In  our  experiments,  we  determined  the  lateral 
strength  of  the  various  materials  and  structures  examined 
by  testing  the  strength  of  normal  knots  and  customary 
net  knots  both  in  dry  and  wet  conditions  (see  fig.  1). 
Knot-firmness  is  more  difficult  to  determine  bv  tests. 


T 


10      12     1 
Total  extena 


4      Ik      Ik 
ion     % 


22     24       1       28% 


Fig.  3 
Load-extension  properties  of  different  materials  (dry). 

7.  TREVIRA  840  den.  (10,300  ml  kg.) 

2.  PERLON  840  den.  high  tenacity  (70,550  ml  kg.). 

3.  PERLON  7,750  den.  (8,080  m/kg.). 

4.  TREVIRA  Nm  20  (79,650  mlkg.). 

5.  PERLON  Nm  20  (19,500  m/kg.). 


Here  we  also  tried  two  ways  which,  although  not 
corresponding  exactly  to  the  stress  on  the  knot  of  the 
net  in  use,  nevertheless  allowed  a  comparison  of  the 
knot  firmness.  Since  the  problem  of  maximum  resistance 
in  a  knot  to  displacement  is  obviously  closely  connected 
with  maximum  roughness  of  the  surface  of  filaments, 
yarns  and  twines,  we  first  tested  the  yarns  in  'loop  knot" 
or  "double  overhand  knot"  forms  (see  fig.  1,  C)  in  a 
normal  tensile  strength  testing  device.  The  knot  was 
tightened  by  means  of  a  constant  weight.  When  a  load 
was  applied  to  the  knotted  yarn,  the  knot  loosened 
(meaning  poor  firmness),  or  held  fast  while  the  yarn  or 
twine  broke  in  the  knot  or  in  some  other  place. 

In  the  second  method,  we  desisted  from  any  tensile 
stress  on  the  knotted  yarns  since  in  the  net  the  knots  will, 
as  a  rule,  shift  only  when  they  are  not  loaded.  Instead, 
we  placed  a  number  of  short  pieces  of  twine  connected 
by  the  fishermen's  knot  (fig.  1,  A)  in  a  rubber-lined 
box  (fig.  2).  A  rubber  roller  was  also  put  in  the  box. 
The  closed  box  was  rotated  for  30  minutes  at  about 
60  r.p.m.,  i.e.,  a  total  of  about  1,800  revolutions.  During 
this  process,  a  number  of  knots  will  loosen,  depending 
on  material  and  previous  treatment.  This  number  is 
expressed  as  percentage.  By  this  method,  which  corres- 
ponds approximately  to  the  I.C.I,  pilling  tester,  we 
were  able  to  find  very  marked  differences. 

TENSILE  STRENGTH,  LOAD-EXTENSION 

Before  giving  the  results  of  the  investigations,  let  us 
refer  again  to  the  influence  of  the  raw  materials  on  the 
lateral  strength  of  the  net  twines.  The  main  facts  can  be 
seen  in  the  load-extension-diagrams  (fig.  3).  This  shows 
a  comparison  between  the  two  materials,  Perlon  and 


Fig.  4 

Load-extension  properties  of  different  twines  (dry  and  wet). 
I.     PERLON  840       3  den.  (3,160  m/kg.)  dry 
•?•  ..  ,.  .,  ,.  wet 

3.  TREVIK.t  S40  ,   3  den.  (.1,000  m/kg.)  dry 

4.  „  „  .,  „  wet 

5.  PERLON  Nm  20/2,'J  (3,110  m/kg.)  dry 

6.  „  .,  ,.  „  wet 

7.  TREVIRA  Nm  20/2/3  (3,040  m/kg.)  dry 
*•  ..  ..  „  ,.  wet 


[88  ] 


STRENGTH     AND     KNOT-FIRMNESS    OF    SYNTHETICS 


70. 

460 

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0 

Per 

Ion'                                                 Perlon 

Perlon                                                Pcrlon 

1                  840  den                                             1150  den 

Nm  50                                               Nm  20 

11)550  m/Kg                                       8080  m/Kg 

4B400  m/Kg                                     19500  m/KR 

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Pcrlon                     Perlon                   Perlon                    Perlon 

Perlon 

Pcrlon 

Perlon                    Pcrlon                  Perlon                   Perlon 

840  x  3  den              840x2x3                 1150x3                1150x2x3 
3.  160  m/Kg          1520  m/Kg          2190  m/Kg            1050  m/Kg 

1150 
219U 

X3C 
m/Kg 

1150x2x3  D 
1050  m/Kg 

Nm  50/3           Nm  50/2/3            Nm  20/3            Nm  20/2/? 
15500  m/kg         7300  m/kg           6080  m/kg           3110  m/kg 

Comparison  of  the  relative  knot  strength  of  different  yarns  and  of  twines  made  thereof.     A  -  tensile  strength*  B 
strength  with  overhand  knot,  I)     knot  strength  with  fishermen's  knot.  -  --dry,  . 


total  extension,  C -kno 
wet. 


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x 

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"jj               Pcrlon                   Perfon                  Pcrlon                   Perlon 
g               Nm  20                 Nm  20/3             Nm  20/2/3           Nm  '20/3/3 
^           19500  m/Kg          60BO  m/kg           3110  m/Kg           1980  m/Kj- 

Perlon 
Nm  20/4/3 
1380  m/Kg 

Pcrlon                    Pcrlon                   Perlon                    Pet  Ion                    Pcrlon 
Nm  50/3               Nri  50/3            Nm  50/2/3           Nm  50/3/3           Nm  50/4/3 
4R400  m/Kg         15500  m/Kg          7300  m/Kg           bo  10  m/Kg           3«8C  rn/Kg 

. 

Relative  krux  strength  of  Perlon  staple  twines.    A     tensile  strength,  R    total  extension.  C    knot  strength  with  overhand  knot,  D    knot  strength: 

with  fishermen's  knot.     ---dry —wet. 

[89] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


g                        —                             -                               A 

JFK 

A 

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8 

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5  0_ 

*                Nylon                    Nylon                     Pcrlon                  Pcrlon                    Perlon                   Perlon                   Perlon                    Pcrlon                  Perlon                   Perlon 

g            210  x  3  den          210  x  6  den          840  x  3  den         840x2x3  den        1.150x3  den         840x10x3  den      1160x3  den  C           1.150x2x3         840x3x8  den         840x2x8  den 

13.400  m/Kg         6.700  rn/kg           3160  m/Kg          1520  m/Kg         2.  190  m/Kg          314  m/Kg          2.  190  m/Kg                den  D                 braided                 braided 

1.050  m/Kg            391   m/kg             488  j^g 

Relative  knot  strength  of  Perlon  continuous  filament    twines.     A^  tensile  strength,   B^  total  extension,   C~knot  strength   with   overhand 

knot.  D  —  knot  strength  with  Jishe rmens  knot. —  dry,     ....  ^wet. 


Trevira,  in  their  various  forms,  i.e.,  as  continuous  filaments 
and  as  spun  yarn.  The  different  values  for  breaking 
strength  and  extension  are  clearly  seen  as  well  as  the  very 
different  behaviour  under  small  to  medium  loads,  i.e., 
the  most  important  in  practical  use. 

The  polyester  yarns  are  distinguished  by  a  curve  which 
is  relatively  steep  at  small  loads.  Their  extension  is  thus 
relatively  small  in  this  region,  which  is  advantageous  in 
nets.  This  material,  when  twisted,  often  shows  very 
different  characteristics,  depending  both  on  the  material 
used  and  on  the  structure. 

Dr.  Klust  shows  in  his  comprehensive  report  "Perlon 


Twines",  the  important  influence  of  doubling,  twisting, 
braiding,  etc.,  on  the  physical  properties  of  net  twines, 
so  that  it  is  enough  to  mention  this  in  passing.  For 
instance,  Perlon  twines  made  of  continuous  filament 
usually  have  higher  tenacity  and  lower  extension  than 
those  made  of  spun  yarn. 

In  net  twines  wet  tenacity  is  of  particular  interest. 
Fig.  4  shows  some  comparisons  between  Perlon  and 
Trevira  (Terylene),  the  twines  being  of  similar  thick- 
ness, although  some  are  of  filament  and  others  of  spun 
yarns.  Trevira  is  distinguished  by  a  very  good  wet 
tenacity. 


§70. 


5  60. 

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Perlon 
Nm  20/3 

Trevira                 Perlon                  Trevira                 Pcrlon                  Trevira                 Perlon                Trevira 
Nm  20/3            Nm  20/2/3            Nm  20/2/3           Nm  20/4/3           Nm  20/4/3           Nm  50/2/3          Nm  30/2/3 

H             6080  m/Kg            6350  m/Kg           3110  m/Kg           3040  m/Kg           1380  m/Kg           1500  m/Kg            7300  m/Kg          7650  m/Kg 

Fig.  8 

Comparison  of  Perlon  and  Trevira  staple  twines  in  regard  to  their  tensile  strength  (A)t  total  extension  (B)  and  knot  strength  with  overhand 

knot  (C)  and  fishermen's  knot  (/)). =dry ^wet. 

[90] 


STRENGTH     AND     KNOT-FIRMNESS    OF    SYNTHETICS 


••* 

s                         ^ 

j-x 

A 

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n 

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<J 

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§10. 

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rrra 

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o>  0, 

"2                 Petlon                  Trevira                Perlon                 Trevira                  Perlon                 Trevira 

g             840x2x3  den         340x2x3  den       840x10x3  den       3000x3x3  den        840x3x8  den         914x3x8  den 

H               1520  m/Kg           1545  m/Kg          314  m/Kg              312  m/Kg               braided                braided 

319  m/Kg          385  m/Kg 

Comparison  oj  Perlon  and  Trevira  continuous  filament  twines  in  regard  to  their  tensile  strength  M),  total  extension  (B),  knot  strength  with 

overhand  knot  (C)  and  fishermen's  knot  (D).  ~dry —wet. 


KNOT  STRENGTH 

Wet  tenacity,  and  above  all,  knot  strength  is  decisive  in 
judging  the  usefulness  of  the  material  in  nets  and  of 
course  knot  strength  in  a  wet  condition  is  of  special 
interest. 

It  becomes  obvious  that  high  normal  tensile  strength, 
does  not  indicate  equally  good  knot  strength.  Fig.  5 
shows  these  values  for  Perlon  filament,  staple  fibre  yarns 
and  twines  of  different  structures.  The  higher  tensile 
strength  of  the  continuous  material  can  be  clearly  seen 
along  with  its  higher  sensitivity  to  knotting  and  plying. 
The  initial  differences  in  the  knots  still  exist,  though  no 


longer  in  the  same  relation.  In  multiple  twists,  the 
knot  strength  of  the  continuous  material  is,  under 
certain  circumstances  equal  to,  or  below,  that  of  twines 
of  spun  material.  Braided  twines,  however,  usually  show 
a  better  result. 

This  fact  does  not  question  the  other  advantages  of 
twines  of  continuous  material  as  far  as  smoothness, 
lower  thickness,  lower  towing  drag,  etc.,  are  concerned. 
Trcvira  gives  slightly  better  results,  especially  in  regard 
to  the  wet  knot  strength  of  spun  material. 

Twines  made  of  Perlon  (spun  yarn  as  well  as  filament) 
of  various  structures  are  compared  in  figs.  6  and  7  where 
the  influence  of  the  structure  emerges  clearly.  It  is  well 


100. 


BCD 


•0. 
TO. 

A 

A 

* 

2: 

Ida 

C 

201 

- 

S 

— 

10. 

0» 

P 

Pcflon  840x3  iten 
-ifio  m/kg 

BCD 


Prrlon  H4(K2x:i  Urn 


A 

I     IB 


r-,  A 


B    C 


Aj 


'D 


lon  ll&O  x  2  «  :<  P 
lo.SU  m/Kg 


Trcvira   840  x   i 
J2%   m/KK 


Fig.  JO 

Knot  firmness  of  different  Perlon  and  Trevira  staple  twines  according  to  1  double  overhand  knot  and  II  fishermen's  knot  testing  method. 
A—untreated,  B^treatment  x,  C—  treatment  y,  D^treatment  z.     Results  given  in  per  cent,  loosened  knots.  =*dry ^wet. 

[91  ] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


known  that  the  "critical  twist"  in  continuous  filament 
twines  is  reached  much  earlier  than  in  spun  material. 
This  is  also  true  of  corresponding  twist  structures  made 
of  Trevira.  In  this  case,  too,  the  lower  extension  and  the 
universally  high  wet  strength  are  evident. 

In  figs.  8  and  9  you  see  pairs  of  twines,  each  of  which 
contains  a  strand  made  of  Perlon  and  the  other  of 
Trevira,  directly  comparable  or,  at  least,  very  similar  in 
their  total  thickness  (length  per  weight).  It  is  tempting 
in  this  comparison  to  examine  in  detail  the  influence  of 
the  twist,  but  Dr.  Klust  has  dealt  with  this  subject  very 
extensively  in  his  above  mentioned  report. 

The  purpose  of  this  comparison-test  was  merely  to 
point  out  the  chances  open  to  the  processors  in  selecting 
material  and  structure  and  to  warn  against  jumping  to 
quick  conclusions  regarding  the  suitability  of  a  particular 
material  before  all  factors  have  been  taken  into  account. 

KNOT  FIRMNESS 

The  next  point  concerns  the  problem  of  knot  firmness. 
It  is  well  known,  and  seems  to  be  confirmed  in  practice, 
that  net  twines  of  spun  yarn  show  sufficient  firmness  of 
knots.  This  is  probably  directly  connected  with  the  con- 
siderably rougher  surface  produced  by  the  many  pro- 
jecting fibre  ends  and  the  position  of  the  capillary  fibres. 
This  fact  is  also  very  clearly  reflected  by  the  values  found 
by  means  of  the  two  test  methods  already  described. 
Furthermore,  it  was  found  that  untreated  twines,  wetted 
for  24  hours  and  then  dried,  had  a  better  knot  firmness 
than  unwetted  twines. 

Subsequently,  we  tried  to  improve  the  knot  firmness 
by  treating  the  twines  with  various  agents.  The  results 

T>  -D 

100. 


90- 


80. 


0. 


obtained  with  three  different  products  are  given  below: 

X    means  an  adhesive  which  is  almost  water-insoluble 

and  was  dissolved  in  a  solvent; 
Y    is  a  product  on  poly  vinyl  acetate  basis,  likewise 

almost  water-insoluble; 
Z    is  a  product  on  silicate  basis,  likewise  having  a 

roughening  effect  on  the  surface. 

The  influence  of  these  agents  on  the  firmness  of  the 
knots  is  shown  by  both  test  methods  so  that  the  stability 
in  twines  of  spun  yarns  may  be  considered  to  be  practically 
100  per  cent,  there  being  only  slight  differences  between 
the  various  products. 

The  result  of  the  treatment  of  twines  of  continuous 
filaments  is,  of  course,  clearer  (fig.  10).  Although  the 
various  materials  and  structures  show  different  reactions 
to  the  agents,  the  knot  firmness  of  the  Terylene  is 
striking. 

In  fig.  1 1  the  results  of  a  braided  twine  of  Perlon  and 
of  Terylene  compared  with  Perlon  monofilament  are 
shown.  The  braided  twines  are  distinguished  because  of 
their  structure  by  a  better  knot  firmness.  The  results  have 
been  further  improved  by  the  above  mentioned  trca  tment. 
The  position  of  monolilaments  is  somewhat  different. 
Although  knot  firmness  was  improved,  loo,  by  the 
treatment,  the  results  were  not  uniform.  However, 
certain  other  possibilities  are  emerging  for  monofilaments, 
but  it  would  be  premature  to  report  on  them  now. 

In  conclusion,  it  must  be  pointed  out  that  the  examina- 
tion of  this  very  interesting  subject,  has  by  no  means  been 
concluded.  Further  tests,  as  well  as  a  free  exchange  of 
experience  between  all  concerned,  will  have  to  follow 
if  optimum  results  are  to  be  achieved. 


A  B 


BCD 


D 


Perlon  -  Monofilamem    Type 
K  0  0,30  mm  105^0  m/kg 


Trevira  914x3x8,   braided 
385  m/kg 


Perlon  840x3x8.   braided 

391  m/kg 
FIR.  11 

Knot  firmness  of  Perlon  monofilament  and  Perlon  and  Trevira  braided  twines  according  to  /.  double  overhand  knot  and  II.  Fishermen's  knot  testing 
method.  A  —untreated,  B-  treatment  x,  C-^  treatment  y%D^  treatment  z.  Results  given  in  per  cent,  unloosened  knot. — dry wet. 

92 


DISCUSSION   ON   THE   PROPERTIES   OF  TWINES  AND 

TESTING  METHODS 


Dr.  G.  Klust  (Germany)  Rapporteur:  This  subject  may  be 
divided  into  two  sections:  Testing  Methods  and  the  Properties 
of  Twines.  Among  testing  methods,  those  of  prime  importance 
here  refer  to  twines,  nets  or  ropes. 

Renter,  Shimo/aki  and  Arzano  list  tests  for  net  twines. 
These  tests  take  into  account:  Construction  of  the  twine; 
thickness  of  twine,  dry  and  wet;  weight  in  air  and  in  water; 
length-weight  unit;  sinking  speed;  shrinkage;  breaking 
strength,  dry  and  wet;  breaking  length;  breaking  strength  per 
weight  unit  (tcnacit\);  knot  strength;  breaking  extension; 
load-extension  curve;  elastic  extension  and  permanent 
elongation;  toughness  or  the  ability  to  absorb  work;  abrasion 
resistance;  rcsisfar.ee  to  light,  weathering  and  to  water; 
stiffness  or  handiness;  knot-slippage  or  knot  firmness; 
resistance  to  bacteria,  mildew  and  insects. 

The^e  tests  were  carried  out  under  conditions  as  close  as 
possible  to  those  of  actual  fishing,  therefore  they  will  not 
always  correspond  with  ihc  methods  generally  used  by  textile 
industries. 

Different  methods  often  give  different  values  and  van 
Wijngaardcn  in  his  paper  urged  that  there  should  be  a  certain 
uniformity  in  the  measuring  methods  and  the  conditions 
under  which  they  are  performed;  this,  I  believe,  is  necessary. 
This  Congress  will  of  course  not  be  able  to  engage  in  such 
special  problems,  but  it  would  be  a  considerable  practical 
success  if  the  papers  and  discussions  helped  promote  closer 
co-operation  between  those  who  make  such  tests. 

Of  the  testing  methods  specified  in  these  papers,  a  few, 
which  are  not  very  well  known  at  present,  may  be  cited. 

Japan  Chemical  Fibres  Association  gives  a  simple  method 
of  measuring  the  thickness  of  twines.  Pieces  of  test  samples  arc 
wound  twenty  times  closely  in  parallel  around  a  cylinder  of 
about  5  cm.  in  diameter  with  a  standard  initial  tension.  Then 
the  breadth  is  measured  and  divided  by  twenty.  The  average 
number  is  indicated  in  millimetres. 

A  method  of  testing  the  sinking  speed  is  given  in  the  same 
paper.  A  cylindrical  glass  vessel  is  used,  having  two  marks  at 
a  distance  of  50  cm.  A  piece  of  twine,  2  cm.  long,  with  a  knot 
in  the  middle,  is  put  into  the  glass  tilled  with  water,  and  the 
time  taken  by  the  twine  to  sink  from  the  top  to  the  bottom 
mark  is  recorded. 

There  is  no  need  to  mention  such  important  testing  methods 
as  those  for  breaking  strength  and  extension  because  they  are 
very  well  known  and  are  dependent  on  the  machines  at  the 
disposal  of  the  examiner.  In  these  tests  too  there  should  of 
course,  be  conformity  in  the  methods  in  order  to  produce 
comparable  results. 

Knot  slippage  or  knot  firmness  has  become  of  great  im- 
portance. We  know  that  the  smooth  surface  of  continuous 
filament  twines  does  not  assure  absolutely  slip-proof  knots. 
Van  Wijngaarden  and  Slutz  describe  methods  for  determining 
the  resistance  of  knots  to  slippage.  The  load-extension  curves 


of  knotted  twines  are  drawn  by  a  tensile  tester.  When  the 
knot  is  slipping,  the  arc  of  the  curve  is  interrupted  by  the 
appearance  of  jags  or  teeth.  Slippage  can  occur  suddenly  or 
gradually.  The  force  in  kilograms  or  per  centage  of  the  knot 
strength  at  which  the  slip  begins  is  the  measure  of  the 
resistance. 

Knots  in  smooth  twines  often  shift  without  a  load.  To  find 
differences  in  the  behaviour  of  twines  made  of  different  fibres, 
Stutz  placed  a  number  of  short  pieces  of  yarn,  connected  by 
the  usual  net  knots,  in  a  rubber-lined  box.  A  rubber  roller 
was  also  put  into  the  box  which  was  then  rotated  for  30 
minutes.  During  this  process  a  larger  or  smaller  number  of 
knots  will  loosen,  depending  on  material  and  previous 
treatment.  The  number  is  expressed  as  a  percentage.  A  similar 
method  is  described  by  van  Wijngaarden. 

Van  Wijngaarden  also  determines  the  moment  when  the 
loaded  knot  is  changing  its  structure  so  that  the  twine  slips 
through  the  knot.  The  force  in  kg.  or  percentage  of  the  knot 
strength  at  which  this  takes  place  is  called  the  "tip-over 
resistance." 

I  or  measuring  the  stiffness  of  twines,  van  Wijngaardcn 
winds  20  turns  of  the  twine  round  a  rod  4  cm.  in  diameter. 
The  coil  is  taken  off  the  rod  and  placed  between  one  of  the 
scales  of  a  balance  and  a  fixed  metal  plate.  Weights  are  added 
to  the  other  scale-pan  until  the  coil  is  in  the  form  of  an  ellipse, 
the  short  axis  of  which  is  2-5  cm. 

In  this  connection  there  is  von  Brandt's  method,  described 
in  his  book  "Arbcitsmethoden  dcr  Net/forschung". 

As  expected,  nearly  all  papers  deal  with  synthetic  fibres. 
They  are  now  of  great  importance  in  most  of  the  leading  fish- 
ing countries  and  are  more  and  more  replacing  natural  fibres. 
Needham  states  in  his  publication  on  the  use  of  nylon  in 
the  fishing  industry:  "They  bring  to  one  of  man's  oldest 
occupations  the  miracle  of  science  and,  in  doing  so,  provide 
easier  living  for  the  fisherman."  For  instance,  the  miracle 
can  be  noted  in  the  rot-proof  quality  of  the  synthetic  fibres. 
1  remember  the  great  astonishment  and  the  scepticism  of  our 
inland  fishermen  when  they  first  received  gear  made  of  PeCe. 
This  rot-proof  quality  is  most  important  in  fishing  gear  as  it 
ensures  greater  durability  and  eliminates  the  work  and  expense 
of  preservation.  It  also  enables  the  gear  to  remain  in  water  for 
an  unlimited  time  and  eliminates  the  need  to  dry  it. 

This  rot-proof  quality  is  now  taken  for  granted,  and  the 
developments  in  synthetic  fibres  have  considerably  increased 
the  demand  by  fishermen  or  these  materials. 

Lonsdale  outlines  the  desired  properties  of  the  ideal  fibre 
for  use  in  fishing  gear,  but  adds  that  this  fibre,  of  course,  does 
not  exist. 

Teviron  seems  to  be  the  cheapest  of  synthetic  fibres,  and 
the  price  of  Teviron  fishing  net  is  only  30  per  cent,  higher 
than  that  of  cotton.  Its  resistance  to  weathering  is  very  high. 
The  same  holds  true  for  Rhovyl  and  PCD. 


931 


MODERN     FISHING    GEAR    OF    THE    WORLD 


Polyethylene  fibres  have  the  lowest  density  of  synthetic 
fibres,  lower  than  that  of  water,  therefore  ropes  made  of  this 
fibre  will  float  and,  in  trawling,  the  upper  net  will  tend  to  rise 
of  its  own  buoyancy. 

The  fibres  with  the  greatest  specific  gravity  are  those  made 
of  polyvinylidcne  chloride,  e.g.,  Krchalon.  This  property 
enables  the  nets  to  sink  faster.  It  makes  the  fibre  particularly 
suitable  for  setnets. 

Ter>lene  has  a  high  tensile  strength  which  is  unaffected  by 
wetting.  The  most  important  characteristic  seems  to  be  its 
very  low  extensibility.  Gillnets  of  Terylene  have  been  used 
successfully,  principally  for  catching  salmon  and  cod,  but 
net  herring.  This  soft  fish  becomes  damaged  by  the  fine 
twine.  Trawls  made  of  Terylene  have  proved  very  successful. 

The  most  important  characteristic  of  the  polyacrylonitrile 
fibres  is  a  very  good  resistance  of  weathering,  but  the  fibre  does 
not  seem  to  be  used  in  fishery  and  none  of  the  papers  mention 
it  as  a  net  material. 

Twines  made  of  polyamide  filament  or  staple  fibre  have  a 
relatively  low  resistance  to  weathering,  less  than  that  of  cotton. 
This  refers  especially  to  types  of  polyamides  which  have  been 
delustred,  and  which  are  not  suitable  for  use  in  the  fishing 
industry.  For  most  fishing  gear  this  sensitivity  to  light  is  not, 
however,  an  important  disadvantage.  Practical  experience 
gained  by  Swedish  freshwater  fishermen  has  proved  that 
nylon  nets  discarded  after  3  to  4  years  have  become  unservice- 
able because  of  damage  by  tearing  and  not  because  of  the 
effect  of  sunlight. 

In  Portugal,  purse  seine  nets  made  of  perlon  staple,  not 
dyed  or  prepared  in  any  way,  have  been  in  use  1,300  fishing 
days  and  are  still  effective.  Cotton  nets  frequently  treated 
with  preservatives,  have  a  durability  of  400  to  500  fishing 
days.  The  excellent  physical  properties  of  polyamide  twines 
are  of  much  greater  importance  than  their  resistance  to 
light. 

But  there  are  cases  where  the  resistance  to  light  will  be 
insufficient,  as,  for  example,  when  part  of  the  fishing  gear 
remains  above  the  water  level.  Colouring  the  net  may  help, 
or  twines  made  of  polyester,  polyvinyl  chloride,  polyviny- 
lonitrile  or  Saran  may  be  used.  Terylene  polyester  fibre 
has  a  better  resistance  to  light  and  weathering  than  the 
polyamides  and  is  equal  to  the  best  of  the  natural  fibres,  such 
as  cotton,  although  the  polyvinyl  alcohol  fibres  arc  better. 
These  fibres,  such  as  the  polyacrylonitrile  (Orion,  Dralon 
and  PAN)  and  especially  the  unchlorinated  polyvinyl  chloride 
(Rhovyl,  PCU  and  probably  the  new  Japanese  fibre  Teviron) 
have  the  highest  resistance  to  weathering. 

Resistance  to  abrasion  is  very  important  but  we  do  not 
know  exactly  the  abrasion  resistance  of  nets  made  of  different 
types  of  fibres.  Test  methods  never  give  absolute  values, 
but  only  relative  ones.  They  are  in  a  high  degree  dependent 
on  the  method  employed. 

A  few  papers  give  examples  for  assessing  the  abrasion 
resistance.  Shimozaki  has  chafed  twined  across  an  oil  stone 
and  a  figure  shows  that  the  resistance  of  non-treated  twines 
increases  in  the  following  order:  Saran,  Krchalon,  Teviron, 
Cotton,  Kyokurin  (a  mixed  twine  made  of  Amilan  and  Saran), 
Kuralon  and  polyamide  Amilan.  In  tests  made  by  Imperial 
Chemical  Industries  Ltd.  twines  were  abrazed  over  a  hardened 
carbide  steel  bar.  The  abrasion  resistance  of  Terylene  twines 
was  superior  to  that  of  cotton  and  flax,  but  less  than  that  of 
nylon  twines.  There  is  a  good  conformity  between  these  data 
and  those  of  Klust.  Examples  of  the  wet  abrasion  resistance 
of  Perlon  filament  tissue  in  comparison  with  those  of  hemp  and 


manila  are  to  be  found  in  the  Perlon- Warenzeichenverband 
paper. 

It  seems  certain  that  the  polyamides  have  an  especially  high 
abrasion  resistance,  not  equalled  by  any  other  fibre.  The 
durability  of  fishing  gear  made  of  such  fibres,  such  as  trawl 
nets,  is  probably  primarily  due  to  their  abrasion  resistance. 

What  types  of  fishing  gear  ought  to  be  made  of  fibre  of  the 
highest  strength?  First,  those  subjected  to  the  heaviest  stress 
and  strain,  such  as  trawl  nets,  cxpecially  those  used  by  large 
fishing  boats.  But  it  is  also  profitable  to  use  strong  fibres  for 
fine  gillnets.  The  stronger  the  fibre  the  finer  the  twine  and  the 
finer  the  twine  the  greater  the  quantity  of  fish  caught. 

The  test  records  given  in  the  papers  show  that  two  types  of 
synthetic  fibres  arc  superior  to  others;  polyamide  and  poly- 
ester. But  here  we  have  to  distinguish  between  fibres  of  normal 
or  ordinary  tenacity  and  fibres  of  high  tenacity.  The  factor 
which  determines  the  strength  of  synthetic  fibres  is  the  degree 
to  which  they  have  been  drawn  out  during  manufacture.  A 
high  degree  of  drawing  cr  pre-stretching  results  in  a  consider- 
able increase  in  tensile  strength,  which  is  connected  with  a 
reduced  breaking  extension.  But  high  tenacity  leads  to  loss  in 
lateral  strength.  It  is  important  to  know  the  decrease  in 
strength  caused  by  knotting  and  by  high  tenacity.  Tests  of 
knot  strength  arc  much  more  significant  than  those  of  the 
straight  twine.  In  this  connection  it  is  interesting  to  note 
the  physical  properties  of  twines  in  Japanese  knotless  nets. 

Twine  and  knot  strengths  of  polyamide  fibres  arc  dependent 
on  the  degree  of  stretching,  not  on  the  type  of  fibre.  In  principle 
nylon  and  Perlon,  for  example,  do  not  differ  in  strength  when 
they  are  made  with  the  same  degree  of  drawing.  (Thai  may 
be  seen  in  the  paper  by  Arzano).  Can-others  has  tested  twines 
of  nylon  66  and  nylon  6  or  Perlon.  The  twine  made  of  the 
latter  was  found  to  be  about 40  per  cent,  weaker  than  nylon  66 
of  the  same  weight,  but  the  two  twines  reacted  differently  to 
knotting.  The  mesh  of  nylon  6  or  Perlon  was  only  about 
20  per  cent,  weaker  than  the  mesh  of  nylon  66  and  not  40 
per  cent,  weaker  as  in  the  case  of  the  twine.  Carrothers 
has  not,  1  think,  compared  twines  with  the  same  degree  of 
tenacity,  but  a  twine  made  of  a  highly  drawn  nylon  with  a 
twine  made  of  normal  Perlon.  Lonsdale  has  found  that  every 
type  of  knot  has  a  different  knotting  efficiency.  A  decrease  in 
the  angle  through  which  the  loop  is  formed  weakens  the 
strength.  If  the  number  of  loops  in  a  knot  increases,  the  knot 
strength  increases  too. 

As  Carrothers  states,  tensile  strength  is  often  adopted  as 
the  only  measure  of  quality  because  it  can  be  measured 
relatively  easily.  But  it  is  important,  too,  that  the  material  is 
able  to  absorb  kinetic  energy.  The  degree  of  extension 
(elastic  and  permanent)  is  sometimes  of  the  same  or  of  a 
greater  importance  than  tensile  strength.  Statements  of 
breaking  extension  of  the  twine,  mostly  given  by  manu- 
facturers, do  not  provide  possibilities  of  evaluation.  In 
synthetic  nets,  the  meshes  of  which  are  formed  by  knots, 
knot-strength  reaches  only  about  50  to  60  per  cent,  of  the 
breaking  load.  Breaking  extension  of  twines  made  of  different 
fibres  may  be  similar,  but  the  shape  of  the  load-extension 
curves  may  be  different.  This  can  be  of  great  importance  to  the 
behaviour  of  the  twine  under  working  conditions.  Van 
Wijngaarden  proposes  to  determine  extension  under  lower 
load,  for  example,  extension  at  25  per  cent,  of  the  breaking 
load. 

Net  twine  has  a  great  capacity  for  working  if  total  and 
elastic  extensions  are  great.  It  will  be  able  to  absorb  kinetic 
energy  and  will  stand  shock  loads  better  than  twine  of  lower 


[94] 


DISCUSSION:     PROPERTIES    AND    TESTING    OF    TWINES 


extensibility.  The  demands  on  net  materials  differ  very  much 
between  different  types  of  gear,  but  for  all  gear  it  is  advan- 
tageous to  use  fibres  of  high  tensile  strength.  On  the  other 
hand,  one  type  of  gear  may  require  a  low  extension  while 
another  may  require  a  higher  extension.  It  is  always  an 
advantage  for  a  fibre  to  possess  a  high  degree  of  elasticity 
which  guarantees  a  good  consistency  of  mesh  size.  There  is 
no  ideal  fibre  for  fishing  gear,  but  the  various  types  of  syn- 
thetic fibres  each  having  different  properties,  provide  a  range 
of  choice  from  which  to  select  the  best  for  each  type  of  gear. 
So  we  can  say  with  Amano  "Modern  fishing  means  the  use  of 
synthetic  fibre  nets". 

Mr.  J.  E.  Lonsdale  (U.K.).  One  problem  before  synthetic 
fibre  makers  is  to  determine  the  exact  properties  required  in  a 
fishing  net  fibre  and  on  this  point  fibre  producers  can  learn 
a  great  deal  from  the  fishermen  and  from  the  fisheries 
technologists.  As  synthetic  fibres  are  man-made,  they  can  be 
tailored  to  produce  the  required  properties  but  until  the 
manufacturers  know  what  the  fishermen  want,  they  cannot 
produce  the  best  possible  fibre.  Even  for  a  material  such  as 
nylon,  which  has  been  in  existence  for  some  twenty  years, 
there  is  great  room  for  exact  assessment  of  the  properties 
required  by  the  fishermen.  One  major  complication  is  to 
differentiate  between  the  properties  required  in  different 
types  of  fishing  and  different  types  of  nets  in  different  parts 
of  the  world. 

There  has  so  far  been  no  real  attempt  to  try  to  rationuli/c 
these  properties  but  once  this  has  been  done,  and  an  attempt 
is  made  to  find  out  exactly  what  the  fishermen  want,  advances 
will  be  made  in  the  properties  of  man-made  fibres. 

Dr.  Avon  Brandt  (Germany)  (Chairman).  The  polyester  and 
polyamide  fibres  arc  used  mainly  in  Europe  and  in  America. 
The  group  of  polyvinyl  alcohol  fibres  is  most  important  in 
Japan. 

Mr.  P.  J.  G.  Carrothers  (Canada).  The  results  of  my  tests 
on  Manryo  were  published  in  the  Progress  Report  of  the 
Fisheries  Research  Board  of  Canada,  and  copies  may  be 
obtained  from  the  Vancouver  Technological  Station. 

Manryo  twine  is  similar  to  cotton  in  general  appearance. 
Generally  speaking,  we  found  the  properties  of  the  new 
Manryo  (dry  twine)  to  be  considerably  better  than  cotton, 
hut  when  knotted  and  wetted,  the  properties  of  both  twines 
were  similar. 

So  many  of  the  tests  have  been  on  (he  dry  twine,  but  wet 
mesh  and  wet  knot  tests  are  of  far  greater  importance  than  a 
straight,  dry  twine  test. 

1  would  also  like  to  say  a  word  about  the  need  for  standard- 
ization of  t£st  procedures.  So  far,  these  have  been  primarily 
developed  from  the  manufacturers'  point  of  view,  but  nets 
are  used  to  catch  fish,  so  the  test  methods  are  better  designed 
from  the  fisherman's  rather  than  from  the  manufacturers' 
point  of  view.  Pertinent  properties  which  relate  to  the  fishing 
function  should  be  measured,  such  as  handling  characteristics, 
the  wet  mesh  strength,  probably  also  toughness,  etc. 

Test  procedures  in  most  countries  are  fairly  well  standardized 
up  to  the  stage  of  the  dry  twine,  i.e.,  the  ASTM  in  the  States. 
Could  not  this  Congress  try  to  encourage  some  of  these 
groups  to  consider  also  standard  tests  for  netting?  Many  of 
the  methods  available  have  been  developed  solely  for  research 
purposes.  There  is  a  need  to  develop  these  methods  further  so 
that  they  can  also  be  used  for  routine  control  purpose*.  It 
would  then  be  possible  for  netting  manufacturers  to  state 


how  strong  their  netting  is  without  fear  of,  say,  a  government 
laboratory  or  standards  laboratory  testing  the  netting  by 
another  procedure,  and  saying  that  the  nets  are  not  what  they 
are  claimed  to  be. 

Mr.  B.  F.  Wolmarans  (Union  of  South  Africa).  About  2 
years  ago,  the  first  blend  of  Japanese  synthetic  netting  was 
put  into  use  in  South  Africa.  We  find  this  is  vastly  superior 
to  straightforward  synthetics.  Other  types  have  been  imported 
from  U.K.,  Germany  and  Holland,  but  we  find  Marlon  very 
much  rftorc  resistant  to  the  peculiar  water  and  weather 
conditions  we  have  in  South  West  Africa.  Firstly,  nets 
manufactured  of  this  material  are  reputed  to  sink  very  much 
faster;  they  are  abrasion-resistant  to  a  very  great  degree.  The 
price  factor  also  comes  into  consideration.  After  much 
experimentation  our  industry  has  concentrated  on  Marlon 
particularly  in  the  Walvis  Bay  area  where  conditions  are  very 
bad.  Kuralon  is  unsatisfactory,  although  it  has  been  put  into 
use  in  the  areas  further  south  where  the  water  is  clearer  and 
where  damage  is  not  so  noticeable. 

Mr.  R.  S.  Rack  (Northern  Rhodesia).  A  very  few  years 
ago  our  local  fishermen  were  taking  motor  car  tyres  to  pieces, 
in  order  to  get  threads  for  their  fishing  nets.  Subsequently  they 
have  come  to  use  nylon,  which  has  superseded  cotton.  They 
are  very  simple  people;  they  know  cotton  and  nylon;  it  might 
be  a  blow  to  their  vanity  if  I  say  they  know  very  little  of  any 
other.  In  fact,  I  believe  that  if  I  were  to  say  "this  is  not  nylon, 
this  is  Marlon",  they  would  say  "yes,  you  mean  Marlon- 
nylon."  There  is  a  danger  in  this  wide  use  of  trade  names  which 
are  being  interpreted  as  common  names.  A  worthless  fishing 
twine  may  be  sold  under  a  high  sounding  name.  I  would 
therefore  suggest  that  the  trade  name  always  be  accompanied 
by  a  specification  of  performance.  I  would  also  suggest  that 
the  testing  methods  be  brought  under  two  headings:  Practical 
forms  of  test  which  could  be  carried  out  on  behalf  of  the 
fishermen  to  ensure  that  the  nets  sold  are  adequate,  and  more 
exhaustive  tests  which  could  be  carried  out  for  the  satisfaction 
of  the  industry. 

Could  not  this  matter  be  referred  to  the  various  standard 
institutions  who,  in  cooperation  with  users  and  manu- 
facturers, might  be  able  to  provide  tests  of  this  nature? 

With  regard  to  the  value  of  synthetic  fibres  under  tropical 
conditions  in  my  part  of  the  world  we  use  practically  all 
nylon.  Our  primary  aim  is  to  reduce  the  cost  as  much  as 
possible.  Many  of  our  fishermen  make  their  own  nets,  and 
therefore  a  nylon  which  is  non-slipping  and  which  could  be 
knotted  with  a  single  knot,  would  be  of  great  advantage. 
Abrasion  is  also  a  problem.  However,  the  physical  character- 
istics are  not  so  vitally  important  to  us.  When  1  tell  you  that 
we  lose  a  good  many  nets  to  crocodiles  you  will  understand 
this  point  of  view! 

Mr.  T.  D.  lies  (Nyasaland).  I  am  concerned  more  with  the 
biological  side,  the  fisheries  research  in  general,  rather  than 
with  test  of  net  characteristics,  but  1  have  had  an  opportunity 
on  Lake  Nyasa,  where  we  arc  investigating  a  deep  water 
gillnet  fishery,  in  comparing  cotton,  flax  and  nylon  nets. 
Under  our  local  conditions,  the  physical  characteristics  of  the 
nets  such  as  tensile  strength,  do  not  give  such  a  great  advan- 
tage as  far  as  the  fishermen  are  concerned.  A  problem  in  this 
particular  fishery  concerns  a  fresh  water  crab,  which  causes 
a  great  amount  of  physical  damage  and,  in  fact,  whereas  a 
flax  net  will  give  up  tc  25  sets,  a  nylon  net  may  only  give  up 
to  40,  despite  the  fact  that  the  tensile  strength  of  the  nylon  is 


[95] 


MODERN     FISHING     GEAR    OF    THE     WORLD 


very  high,  far  higher  than  is  really  needed  for  the  fish  that 
are  being  caught.  Such  problems,  under  these  conditions  at 
least,  are  far  more  important  than  physical  characteristics 
of  the  net. 

Mr.  C.  P.  Halain  (Belgian  Congo).  Nylon  fishing  twine 
— to  some  extent  imposed  upon  us,  owing  to  difficulties  in 
finding  fishing  twines — was  first  used  in  the  Belgian  Congo  in 
1943.  The  fishermen  now  use  nylon  almost  solely  and  cotton 
or  linen  yarn  is  seldom  sold,  but  we  arc  still  looking  for  a 
better  synthetic  fibre. 

Incidentally,  some  80,000  to  100,000  tons  of  river  and  lake 
fish  are  marketed  in  the  Congo  every  year. 

Mr.  M.  K.  Kramer  (Israel).  We  have  carried  out  three 
experiments  with  synthetic  fibres.  The  first,  with  trawling, 
was  unsuccessful,  and  we  are  now  waiting  for  other  and 
better  fibres.  Good  results  were  obtained,  however,  with  the 
fibre  net  in  Lake  Tiberias.  It  was  found  that  a  white  fibre 
net  doubled  catches  and  these  were  tripled  last  year  by  using  a 
red  fibre  net.  Today  there  is  not  one  gillnet  in  the  Lake  which 
is  not  of  synthetic  fibre. 

We  have  now  begun  experiments  for  the  change-over  of  sea 
purse-seine  nets.  We  have  begun  by  using  fibres  from  various 
countries — three  kinds  from  Germany,  two  from  the  U.K., 
and  two  from  Japan.  In  1958  we  hope  to  experiment  with 
Italian  fibres  also,  and  so  arrive  at  some  final  result. 

Dr.  W.  Einsele  (Austria).  Monofilament  nets  have 
revolutionized  our  fisheries.  There  is  very  little  plankton  in 
our  lakes  and  the  light  penetrates  very  deeply.  Catches  were 
exceedingly  low,  especially  in  the  summer,  not  because  there 
were  no  fish  but  because  we  were  unable  to  catch  them.  This 
has  changed  entirely  since  the  introduction  of  monofil  nets 
and  this  summer,  for  the  first  time,  enough  fish  has  been 
caught  to  supply  the  hotels,  restaurants  and  villages  in  the 
Lake  region. 

Mr.  E.  F.  Gundry  (U.K.).  There  is  a  tendency,  especially 
at  the  government  fishery  officer  level,  to  assume  that  man- 
made  fibres  make  the  best  nets  to  have.  The  biggest  production 
of  nets  today  is  still  from  the  orthodox  vegetable  fibres,  and 
therefore  the  advantages  of  the  "old-fashioned"  material 
must  not  be  overlooked.  The  great  advantage  of  sisal  or  cotton 
over  other  fibres,  is  their  lower  cost,  and,  in  an  industry  such 
as  fishing,  which  is  having  economic  difficulties,  the  lower  the 
cost  of  the  nets,  the  more  attractive  they  are.  The  advantages 
of  the  man-made  fibre  will  increase  as  their  price  decrease. 
Another  characteristic  of  cctton  nets  is  in  the  extensibility 
of  the  material.  Herring  constitutes  a  very  big  part  of  the 
fishing  catch  both  for  man  and  for  animal  feeding  stuff,  and 
there  are  many  types  of  herring  nets  and  herring  gillncts.  The 
man-made  fibre  herring  gillncts  are  not  entirely  successful, 
the  fish  becoming  so  deeply  enmeshed,  that  ths  catch  can 
be  extracted  only  with  a  great  amount  of  labour  and  the  fish 
often  being  damaged.  Also  in  the  trawler  field  the  advantages 
of  synthetic  nets  are  not  so  apparent,  because  the  physical 
conditions  under  which  the  net  is  operated  do  not  permit 
it  to  last  as  long  as  their  qualities  might  promise.  The  rock  or 
the  wreck  on  the  bottom  of  the  sea  is  capable  of  tearing  and 
pulling  loose  an  expensive  synthetic  net  as  easily  as  the  one 
made  of  the  less  expensive  natural  fibre.  It  would  be  a  mistake 
to  think  that  there  is  no  future  for  the  orthodox  vegetable  fibre 
materials  and  those  in  authority  and  responsible  for  advising 
fishermen  should  inform  them  that  good  service  can  be 


obtained  from  the  vegetable  fibres  as  long  as  the  nets  are 
kept  clean  and  maintenance  is  good. 

Dr.  J.  Reuter  (Holland).  When  a  fisherman  has  to  make  his 
choice  of  material,  the  more  figures  he  sees  the  less  able  he  is 
to  make  his  choice.  This  information  about  testing  was 
started  by  scientists.  Fishermen  are  not  scientifically  trained, 
so  we  should  adopt  test  methods  that  are  scientifically  correct 
and  at  the  same  time  be  easily  understood  by  the  fishermen. 
Take  for  example  the  breaking  strength  of  twine;  every  textile 
man  has  this  as  his  main  goal,  because  the  stronger  the  twine 
the  better  it  is,  but  he  forgets  that  the  fisherman  tests  his  net 
wet  by  pulling  on  the  mesh  and  not  the  straight  twine.  There 
are  fibres  that  are  strong  in  the  dry  condition  but  become 
weak  when  they  are  wet  and  knotted. 

Messrs.  G.  A.  Hayhurst  and  A.  Robinson  (U.K.)  in  a 
written  statement:  When  checking  twines  for  breaking 
strength  and  extension  it  is  essential  to  avoid  distorting  them. 
If  simply  secured  between  two  grips  there  may  be  a  severe 
bend  in  the  twine  which,  when  subjected  to  the  test  force, 
will  cause  a  stress  concentration  and  give  a  most  unreliable 
result.  It  is  therefore  recommended  that  quarter-circle 
bollards  be  used  to  lead  the  twine  to  the  grip,  as  shown  in  the 
sketch. 


When  checking  extension  under  load,  the  test  sample  must 
be  prc-tcnsioned  to  a  slight  but  known  extent.  We  always 
make  such  measurements  at  1  per  cent,  of  breaking  stress. 

Mr.  E.  A.  Nilssen  (Norway).  Synthetic  fibres  were  intro- 
duced about  three  years  ago  in  Norway.  1  think  that  at 
present  only  synthetic  fibres  are  used  for  the  gillnets.  Such 
nets  bring  in  bigger  catches,  they  are  stronger,  easier  to 
handle,  lighter,  need  not  be  dried  and  need  no  preservation. 
The  fishermen  in  Norway  use  sisal  and  manila  for  the 
mounting,  but  they  would  also  prefer  synthetics  in  the 
mounting  as  they  could  then  just  leave  the  nets  without  any 
drying.  We  are  now  trying  to  find  the  right  material  for  these 
mounting  lines.  We  have  tried  monofilament  nets,  but  without 
much  success.  This  is  rather  astonishing,  when  one  thinks  of 
Sweden,  where  they  arc  using  so  many  of  these  nets.  As 
for  the  breaking  strength,  we  use  single-knot  and  double-knot 
nets.  The  double-knot  nets  were  the  best  at  the  beginning 
because  there  was  no  knot  slippage,  but  the  single-knot  nets 
have  improved  much  in  the  last  two  years.  The  double-knot 


[961 


DISCUSSION:     PROPERTIES     AND    TESTING     OF    TWINES 


nets  give  somewhat  higher  strength.  In  Norway  there  is  a 
State  import  monopoly  for  nets  and  raw  materials;  the 
Government  also  uses  test  machines.  As  most  Norwegian 
manufacturers  send  the  nets,  the  twine,  etc.  to  this  laboratory, 
tests  are  therefore  uniform.  The  most  important  test  is  that 
of  the  knot  strength.  The  fishermen  normally  pull  the  net 
in  a  wet  and  in  a  dry  condition,  we  must  find  a  twine  that 
gives  good  results  in  both  conditions.  Synthetic  fibre  has 
also  been  tried  for  the  big  purse  seines  (about  180  fathoms 
long)  in  Norway.  This  has  resulted  in  an  increasing  demand 
for  the  nets.  As  soon  as  the  price  of  synthetic  fibres  comes  down 
more  or  less  to  the  level  of  cotton  and  hemp,  it  is  possible 
that  only  synthetic  fibres  will  be  used. 

Mr.  R.  Ocran  (Ghana).  In  Ghana,  we  are  trying  to  develop 
line  fishing.  We  catch  heavy  fish  and  the  breaking  strength  of 
cotton  and  hemp  cord  is  not  strong  enough.  How  can  synthetic 
fibre  be  applied  to  line  fishing? 

Dr.  H.  A.  Thomas  (U.K.).  Some  of  the  first  trials  with 
Courlcne  (a  polyethylene  material)  were  in  fact  with  line 
fishing  using  a  large  number  of  snoods  in  the  Irish  Sea  from 
Fleet  wood,  and  remarkable  catches  were  achieved.  The 
fishermen  spoke  very  highly  of  the  knot  strength  at  the  point 
where  the  hooks  were  running  out  on  the  snood  from  the 
main  line.  Some  purse  seine  nets  of  this  yarn  were  then 
produced,  in  cooperation  with  the  Flectwood  fishermen.  It 
was  found  that  the  knot  with  the  polyethylene  yarn—  Courlcne 
— did  not  have  to  be  heat-scaled;  if  they  were  tied  with  the 
right  technique,  they  obtained  satisfactory  results.  We  have  a 
large  number  of  individual  reports  on  these  fishing  trials, 
which  we  shall  be  pleased  to  give  in  detail  to  anyone  who  is 
interested. 

It  has  not  been  possible  at  this  Conference  to  include  a 
full-si/e  paper  dealing  with  materials  suitable  for  protective 
clothing  for  fishermen.  1  would,  however,  suggest  that 
perhaps  at  another  Conference  it  might  be  possible  to  include 
one  session  on  the  clothing  and  protective  equipment. 

The  last  pages  in  Dr.  Arzano's  paper  give  some  details  of 
the  use  of  man-made  fibres  in  protective  equipment.  There  is 
a  development  which  started  in  Norway,  namely  the  string 
vest.  The  object  of  this  vest  is  to  maintain  a  layer  of  air  next 
to  the  skin  to  prevent  clothing  from  becoming  moist  from 
perspiration,  this  also  being  a  very  good  heat  insulant  and 
thereby  adding  to  the  warmth  of  the  body.  The  older  esta- 
blished man-made  fibres  (blend  or  a  mixture  of  viscose 
staple  and  acetate  staple)  can  be  used  for  the  string  vest.  To 
maintain  the  warmth  of  the  feet  it  is  of  advantage  to  have  an 
insole  or  interlining  between  the  feet  and  the  rubber  boot  or 
the  gumboot.  For  gloves,  blends  of  viscose  staple  with  nylon 
staple  and  with  wool  seem  to  achieve  the  best  results.  Viscose- 
staple  and  filament  coated  with  polyvinylchloride  or  with 
linseed  oil  and  rubber  are  used  for  such  things  as  aprons, 
gumboots,  overcoats  and  sou'westers. 

Mr.  A.  Robinson  (U.K.).  We  have  some  experience  with  long- 
line  fishing  and  particularly  with  snoods  of  nylon  yarn.  These 
have  been  found  to  be  very  successful  indeed  in  various  respects, 
not  only  as  to  durability  but  also  for  increasing  the  catch. 

Where  the  speed  of  replacing  snoods  is  of  great  importance, 
nylon  snoods  have  been  particularly  advantageous.  Very  few 
snoods  indeed  have  broken  when  the  fish  has  been  ripped 
from  the  hook,  and  very  little  time  has  been  lost  in  repairs. 
The  line  itself  is  usually  made  of  staple  nylon  in  order  that 
the  snoods  can  be  attached  to  it  securely  and  do  not  slide  on 


the  line.  The  life  of  the  staple  nylon  longline  is  very  good  but, 
of  course,  the  price  is  high. 

Mr.  S.  Springer  (United  States  of  America).  In  the  Gulf 
of  Mexico  longline  tuna  fishery,  we  have  been  making  use  of 
synthetic  lines — both  mainlines  and  branch  lines.  Here,  the 
advantage  of  the  synthetic  fibre  is  in  its  great  strength—- 
possibly double  that  of  the  natural  fibre  lines.  The  nylon  line 
that  has  been  most  successful  in  this  fishery-  is  a  heat-stabilized 
line,  which  has  less  tendency  to  twist. 

Mr.  K.  J.  Westrop  (U.K.).  There  is  a  need  for  standard- 
ization of  testing  methods.  Different  methods  lead  to  different 
actual  figures,  which  do  however  serve  usefully  for  purely 
relative  testing  of  the  fibres.  For  instance,  the  abrasing 
resistance  described  by  Dr.  Klust  shows  the  superiority  of  the 
polyamidc  and  the  polyester  fibres  over  the  natural  fibres, 
though  just  what  these  relative  figures  would  mean  when 
you  are  concerned  with  the  actual  nets,  of  course,  is  another 
story.  As  yarn  producers,  we  can  evaluate  our  yarn  as  a 
textile  material,  but  the  fisherman  himself  must  obviously 
be  the  man  to  decide  what  is  required  of  a  net.  A  practical 
trial,  to  be  valid,  must  be  made  with  many  nets  to  cover  the 
whole  range  of  different  types  and  the  different  conditions 
under  which  the  nets  are  used. 

Presumably  something  will  come  out  of  the  Congress  and 
these  discussions  as  to  what  should  be  done  about  surveying 
such  testing  methods.  1  would  like  to  add  that  the  Imperial 
Chemical  Industries  could  probably  contribute  to  any  future 
conference  if  a  paper  of  this  sort  is  required. 

Dr.  A.  von  Brandt  (Germany).  I  would  propose  that,  in 
addition  to  the  Working  Party  on  Terminology  and  Number- 
ing Systems,  we  form  a  Working  Party  on  Testing  Methods*, 
and  that  the  following  gentlemen  be  appointed  as  members: 
Mr.  Reutcr  (Netherlands);  Mr.  Carrothers  (Canada);  Mr. 
Takayama  (Japan);  Mr.  Percier  (France).  The  standardization 
of  testing  methods  should  be  considered  from  the  fisherman's 
point  of  view,  and  not  from  that  of  the  textile  industry. 

Dr.  G.  Klust  (Germany)  Rapporteur.  I  should  like  to  give  a 
brief  summary  of  what  has  been  said  during  the  discussion. 
Synthetic  fibres  have  given  good  results  from  northern  waters 
to  the  tropics.  For  instance,  in  the  Belgian  Congo,  the  inland 
fishermen  almost  exclusively  fish  with  nylon  nets.  Several 
speakers  stressed  the  extremely  good  results  that  had  been 
obtained  with  monofilament  nets.  Mr.  Gundry  defended 
natural  fibres,  especially  cotton  which  still  is  being  used 
extensively  in  the  fishing  industry,  the  lower  price  being  a  vcr\ 
important  factor. 

The  necessity  of  uniformity  in  methods  of  testing  net 
materials  has  been  stressed.  These  test  methods,  it  was  pointed 
out,  must  be  brought  into  relation  with  experience  gained  in 
practical  fishing.  A  close  relationship  between  the  manu- 
facturers of  synthetic  fibres  and  the  fishing  industry  could  give 
extraordinarily  good  results,  because  synthetic  fibres  could 
be  produced  in  such  a  way  as  to  meet  the  requirements  of  the 
consumers. 

If  the  manufacturers  know  the  requirements  of  the  fishing 
industry  they  are  in  a  better  position  to  manufacture  what  is 
actually  wanted. 


*  Editor's  note. — This  Working  Party  has  been  active  since  the 
Congress.  Under  Dr.  von  Brandt's  leadership,  considerable 
progress  has  been  made  towards  reaching  an  agreement  on  uniform 
standard  testing  methods.  \  Sec  preliminary  report  on  pages  <>8and99. , 


[97] 


REPORT  OF  THE  WORKING  GROUPS  ON  "TERMINOLOGY  AND 
NUMBERING  SYSTEMS"  AND  "TESTING  THE  PROPERTIES 

OF  TWINES" 


Prof,  von  Brandt  (Germany)*:  The  two  working  parties 
have  met  and  started  to  deal  with  the  problem  of  standard- 
ization. Due  to  the  little  spare  time  left  by  the  packed 
programme  of  the  Congress,  no  definite  results  can  be  expected 
yet.  The  difficulties  which  such  an  attempt  of  standardization 
meets  have  been  discussed  and  are  well  known  by  everybody 
working  in  this  field.  The  quality  of  twines  for  the  fishery 
or  particularly  the  standardization  of  their  denotation  depends 
not  only  on  the  basic  material  which  is  presently  often 
camouflaged  under  a  big  number  of  trade  names,  and  the 
manufacture  of  this  material  into  fibres  but  also  on  the 
way  how  the  fibres  are  processed  to  the  twine.  The  final 
goal  of  the  Working  Party  for  Standardization  of  the  Num- 
bering System  dealing  with  these  questions  is  to  find  out 
and  establish  a  simple  and  unmistakable  system  for  defining 
the  twines  used  in  fishery.  This  system  is  meant  to  enable 
the  fishermen  all  over  the  world  to  know  what  really  is 
offered  to  them  and  what  will  best  suit  their  purpose  when 
substituting  one  twine  or  material  for  another.  The  Working 
Group  consists  of  fibre  producers,  twine  makers  and  net 
makers.  Mr.  J.  E.  Lonsdale,  U.K.,  has  been  appointed 
chairman.  After  having  discussed  the  matter  repeated  I  \ 
also  with  many  people  at  this  Congress,  the  present  situation 
is  characterized  as  follows: 

1.  The  presently  used  numbering  systems  are  too  com- 
plicated and  too  confusing  for  use  in  the  fishery. 

2.  The  numbering  complications  are  caused  mainly  b> 
the  twine  and  net  makers  who  often  create  own  systems. 

3.  It  is  necessary  for  the  fishing  industry  to  use  always 
a  common  numbering  system  at  least  in  addition  to 
the   national  or  manufacturers'   systems.    Tt   may   in 
time  replace  those. 

4.  As  basic  units  for  this  common  system  cither  Denier 
or  Tex  are  suggested.    The  former  is  a  non-decimal 
unit  generally  used  in  the  textile  industry  and  already 
introduced  to  a  certain  extent  into  the  fishery.    The 
latter  one  is  a  newer  decimal  system  which  is  strong!} 
recommended  for  international  use. 

The  Working  Party,  having  accepted  English  as  working 
language,  is  undertaking  to  survey  the  numbering  systems 
currently  used  throughout  the  world  for  fisheries  purposes, 
devise  methods  of  unification  and  make  recommendations. 
In  addition  to  the  inquiries  to  be  made  to  this  effect,  everybody 
who  wishes  to  bring  his  view  to  the  attention  of  this  Working 
Party  is  requested  to  write  to  Mr.  J.  E.  Lonsdale  of  the 
British  Nylon  Spinners,  Ltd.,  Pontypool,  England. 


* Editor's  M?/r.--This  report  was  given  on  the  last  day  of  the 
Congress. 


It  is  the  intention  of  this  Working  Party,  while  endea\ourmg 
to  bring  order  in  the  systems  of  counts  and  numbering, 
to  take  into  account  what  has  been  customary  in  the  fishing 
industry  as  to  date,  and  the  specific  demands  of  this  industry 
The  future  count  system,  therefore,  should  at  the  same  time 
indicate  the  main  qualities  important  for  the  various  fishing 
purposes.  The  following  list  of  twine  and  net  qualities  has 
been  set  up  and  may  be  useful  as  a  base  for  discussion 

(a)  tensile  strength 

(b)  knot  strength 

(c)  knot  slippage 

(d)  mesh  si/e 

(e)  permanent  elongation 
(0     elastic  extension 

(g)    softness 

(h)    resistance  to  abrasion 

( i )     weight 

(j)     water  absorption 

(k)    resistance  to  rotting 

(I)     resistance  to  weathering 

(m)   resistance  to  chemicals 

If  applicable,  these  qualities  should  be  tested  in  v\et  con- 
ditions. For  nets  the  knot  strength  of  the  twine  is  of  more 
importance  than  the  unknottcd  tensile  strength.  The  mesh 
si/e  often  has  to  be  considered  in  regard  to  economic  and 
biological  problems  as  marketing  and  overfishing.  The 
permanent  elongation  may  be  of  significance  for  the  propei 
mesh  si/e.  Elastic  extension  or  elast icily  may  be  very  effec- 
tive when  dealing  with  shock  loads.  Softness  comes  in 
particularly  with  gillncts.  Abrasion  may  be  caused  in 
different  ways  during  handling  or  when  towing  a  trawl  net 
over  the  sea  bed.  The  weight  is  always  considered  as  the 
weight  in  water  not  only  for  easy  handling  but  also  in  regard 
to  operation  performance  for  instance  in  bottom  trav\K 
when  a  good  and  continuous  contact  with  the  bottom  is 
wanted  or  in  purse  seining  where  the  sinking  speed  can  be 
of  great  importance.  Water  absorption  has  an  obvious 
relation  to  these  weight  problems.  Resistance  to  rotting 
is  such  a  general  problem  in  fisheries  that  no  further  comments 
are  needed.  Resistance  to  weathering  is  of  particular  impor- 
tance in  tropical  areas  with  bright  sunlight.  The  means  of 
improving  the  insufficient  resistance  to  weathering  of  certain 
synthetic  materials  have  been  discussed.  They  lead  to  the 
question  of  resistance  to  chemicals  of  which  the  different 
dying  materials  consist. 

The  proper  selection  of  the  optimal  material  for  the  fishing 
purpose  in  question  is  strongly  affected  by  the  mutual  inter- 
ference between  these  qualities.  With  a  stronger  material, 
for  instance,  the  twine  diameter  can  be  reduced  saving 


[98] 


RHPORT     OF     WORKING     GROUPS 


twine,  and  towing  or  current  resistance.  But,  with  thinner 
material  there  might  be  more  abrasion  or,  in  giilnetting,  the 
fish  might  be  damaged,  or  it  might  be  inconvenient  to  handle. 
The  choice,  therefore,  must  be  based  on  the  main  qualities 
required  tor  the  particular  purpose.  This  working  party 
has  been  limited  to  manufacturers  of  twine  and  webbing 
because  they  are  closest  to  the  practical  fishermen. 

It  is  the  intention  to  submit  the  information  compiled 
and  the  conclusions  drawn  in  the  form  of  a  report  to  FAO  next 
vear.  FAO  will  then  distribute  this  report  to  all  people 
interested  in  this  matter.  The  collaboration  which  is  hoped 
to  be  created  hereby,  and  the  comments  and  discussions 
resulting,  are  expected  to  finally  result  in  terms  and  a  system 
of  numbering  and  defining  twine  and  net  qualities  which 
is  acceptable  and  most  suitable  for  all  concerned  with  the 
fishing  industry. 

To  be  comparable,  the  qualities  listed  above  must,  of 
course,  not  only  have  uniform  units  but  must  also  be  deter- 
mined in  a  uniform  way.  The  standardization  of  the  testing 
methods  is  the  subject  of  the  second  working  party  having 
heen  established  at  the  beginning  of  this  Congress.  These 
methods  also  must  apply  to  the  demands  of  the  fishing 
mdustry  rather  than  to  the  textile  industry,  which  mainly 
is  concerned  with  clothing,  stockings,  etc.  In  the  short 
lime  available  this  working  party  has  prepared  the  following 
«>hort  report. 

"The  working  group  was  formed  to  find  a  common 
agreement  on  test  methods  which  will  be  adapted  to 
test  material  under  similar  conditions  as  it  is  used  in 
practice.  The  group  has  appointed  Prof.  A.  von  Brandt 
as  Chairman.  The  various  testing  methods  \\ere  dis- 


cussed and   a   preliminary   division   agreed   upon: 

( 1 )  Test  methods  outside  the  scope  of  the  working  group, 
such  as  determination  of  the  specific  gravity  of  fibres, 
etc.   which  are  not  entirely  adapted  to  fishing  con- 
ditions but  absolutely  necessary  for  manufacture  and 
research. 

(2)  Test  methods  which  have  already  been  accepted  and 
standardised  nationa  I  ly  or  internat  tonally,  as  for  instance 
French  elongation  test  etc.  These  test  methods  are 
also  not  entirely  adapted    to   fishing  conditions  but 
absolutely  necessary. 

(3)  Test  methods  of  particular  interest   for  fishery  pur- 
poses on  which  a  common  agreement  has  still  to  be 
found    for  instance,  sinking  speed  of  the  net,  visibility 
in  the  water,  mesh  knot  and  mesh  breaking  strength  etc. 
The  last  group  of  test  methods  will  be  the  main  task 
of  the  working  group.    Prof.  A.  von  Brandt  will  write 
to  people  interested  in  the  subject  who  he  thinks  can 
contribute  to  the  unification  of  this  subject.    He  will 
be  glad  to  receive  contributions  from  any  participant 
to  the  present  Congress.   A  full  report  will  be  written 
next  year." 

The  working  party  has  decided  to  compile  the  existing 
results  of  tests,  discuss  and  evaluate  them,  after  which  a 
programme  of  work  will  be  set  up.  Testing  results  often 
depend  on  the  type  of  test  carried  out,  and  we  must  determine 
how  we  can  work  in  agreement  since  so  many  people  through- 
out the  world  are  carrying  out  tests,  there  is  need  for  some 
unity  in  this  field.  FAO  has  also  emphasized  to  the  different 
governments  that  they  should  help  in  this  matter. 


The  883  tons  distant-water  trawler  Portia  of  HulL     Her  diesel-electric  engines,   which  can  be  controlled  directly  from 

the  bridge,  give  her  a  speed  of  nearly  16  knots. 

[991 


Section  3 :  Net  Making. 

PRACTICAL   CONSIDERATIONS  OF  THE   USE  OF   SYNTHETIC 

FIBRES   IN  TWINES  AND   NETS 

by 
G.  A.  HAYHURST  and  A.  ROBINSON 

William   Kenyon  and  Sons  Ltd.,   Dukinfield,  Cheshire,   U.K. 

Abstract 

From  the  l wine-maker's  poinl  of  view  ihe  characteristics  of  synthetic  fibres  are  discussed  and  a  comparison  made  of  the  results 
obtained  from  nylon  nets  with  those  from  cotton  nets.  The  heat-setting  of  nylon  twine  is  discussed,  and  also  the  question  of  the  use  of 
bonded  twines  for  single  knot  nets.  Brief  notes  are  given  on  testing  methods  for  twines. 


Ktaime 


Considerations  pratiques  sur  ('utilisation  dcs  fibres  synthctiques  dans  les  His  et  filets 


Cettc  etude  est  ecritc  du  point  de  vue  du  fabricant  de  fil  de  peche.  Les  aiiteurs  examinent  les  caractcristiqucs  des  fibres  synihctiqucs 
et  comparent  les  resultats  obtenus  avec  des  filets  dc  nylon  et  des  filets  de  colon.  La  stabilisation  a  chaud  du  fil  de  nylon  est  etudice  ainsi  quc 
la  question  de  1'cmploi  de  fils  impregnes  pour  les  filets  a  noeuds  simples.  L.es  methodes  d'essai  des  fils  sont  exposees  brievement. 

Consideraciones  practicas  sobre  el  uso  de  fibras  sinteticas  en  los  hilos  y  redes  de  pesca 

Extracto 

Desde  el  punto  de  vista  del  fabricante  de  hilos  se  analizan  y  comparan  los  resultados  obtcnidos  con  redes  dc  nylon  y  de  algod6n. 
Tambien  se  consideran  el  tratamiento  termico  de  los  hilos  de  nylon  para  dismmuir  el  resbalamiento  de  los  nudes  y  el  problema  de  la  fijaci6n 
de  los  hilos  en  redes  de  nudos  simples.  El  trabajo  tambicn  contiene  breves  descripcioncs  de  los  metodos  usados  en  los  ensayos  dc  los  hilos 


CONTINUOUS  filament  yarns  of  nylon  and 
Terylene  are  extremely  "lively"  when  twisted  so 
that  when  twine  is  cut  the  yarns  promptly  untwist. 
Heat  setting  of  the  twine  during  manufacture  reduces  this 
liveliness  by  altering  the  linear  molecular  structure  of 
the  fibre.  If  this  process  is  carried  out  when  the  twine  is  in 
relaxed  state  contraction  takes  place,  and  influences  the 
length/weight  ratio  and  the  extensibility.  The  extent  of 
the  contraction  depends  upon  the  temperature  and  time 
the  twine  is  heated.  The  nylon  and  Terylene  twines  are 
treated  at  or  below  100  deg.  C.  for  up  to  10  minutes. 

If  the  twine  is  permitted  to  contract  during  this  process 
a  greater  length  will  be  necessary  to  make  any  particular 
specification  of  net  and  the  raw  material  cost  is  therefore 
increased.  The  elasticity  of  the  twine  is  affected  and  if  it 
were  allowed  to  contract  during  the  heat  setting  process, 
extension  under  load  would  be  increased;  much  depends 
on  the  temperature  and  time.  For  this  reason  the  Kenlon 
brand  twines  are  heat-set  by  methods,  devised  and 
developed  within  the  Organization  and  contraction 
during  heat-setting  is  avoided  so  that  the  length/weight 
ratio  is  improved.  These  twines  show  about  18  per  cent, 
up  to  25  per  cent,  extension  at  break  whereas  twines 
which  have  been  heat-set  in  a  relaxed  state  show  about 
21  per  cent,  up  to  30  per  cent,  extension  at  break.  The 
difference  in  length/weight  ratio  may  be  anything  from 
5  per  cent,  up  to  10  per  cent,  in  the  case  of  nylon  66 
of  210  denier. 


IMPREGNATION 

The  low  coefficient  of  friction  against  itself  caused  some 
difficulty  when  synthetic  fibre  nets  were  first  introduced 
and  necessitated  the  use  of  double  knots  to  prevent  knot 
slippage.  If  suitable  impregnating  materials  and  methods 
are  employed,  it  is  possible  to  increase  the  coefficient  of 
friction  so  that  secure  single  knots  may  be  achieved  with 
both  machine-made  or  hand-made  nets.  It  goes  without 
saying  that  coating  of  the  twine  with  materials  which  are 
incompatible  with  nylon  or  Terylene  does  not  give 
satisfactory  results.  Reasonable  resistance  to  flaking  oft 
during  rubbing  or  surface  abrasion  and  rapid  leaching 
out  in  water  are  two  of  the  points  to  be  considered  in  this 
respect. 


KNOTS 

Where  double  knots  are  made,  satisfactory  results  ma> 
be  obtained  with  thermoset  type  twines,  but  where  single 
knots  arc  made  it  is  advisable  to  use  bonded  type  twines. 
In  either  case  heat  setting  of  the  net  is  usual  and  this 
must  be  done  at  a  temperature  higher  than  that  used  for 
the  heat  setting  of  the  twine.  It  is  therefore  an  advantage 
if  the  twine  is  heat  set  at  a  temperature  less  than  100  deg. 
C.,  so  that  boiling  water  or  low  pressure  steam  may  be 
used  for  heat  setting  the  net.  Where  possible,  the  net 
should  be  kept  on  tension  during  heat  setting,  otherwise 


100 


HEAT-SETTING 

some  contraction  will  take  place,  thus  affecting  mesh  size. 
To  further  avoid  knot  slippage  the  net  can  be  im- 
pregnated with  suitable  materials.  When  nets  are  so 
treated  with  a  recent  development  of  our  own  it  is 
practically  impossible  to  move  the  knot  in  any  way, 
under  any  conditions,  wet  or  dry. 

SHRINKAGE  IN  WATER 

Where  an  exact  match  of  mesh  size  is  required  a  check 
should  be  made  when  both  natural  fibre  and  synthetic 
fibre  nets  are  saturated  in  water.  This  is  necessary  because 
of  the  different  water  absorption  characteristics. 


AND     BONDING 
ROPES 

From  the  remarks  above  regarding  rot-proofing,  it 
will  be  obvious  that,  in  order  to  gain  the  maximum 
advantage,  synthetic  fibre  mounting  ropes  should  be 
used  where  possible.  It  is  sometimes  found  that  ropes 
made  from  staple  fibre  yarns  (instead  of  continuous 
filament  yarns)  are  the  most  suitable  as  this  avoids 
any  possibility  of  the  tying  cords  gliding  on  the  head 
rope.  On  the  other  hand,  ropes  made  from  continuous 
filament  yarn  are  much  stronger  and  this  consideration 
may  have  a  bearing  on  the  size  of  rope  required  and 
therefore  on  the  cost. 


•ro-rr*^.^    ..    :.-^^,^.    -_-   ./,.. 
^_...    .:-  -;,v;*     ^  -    -    ,£. 

^'^^'^    •---.•.         '^v^ 


^;^^4'^^.^;'-  ,     , 
l**^3^^^^^-^ -:'t  -r   .,- 
S*£  -»»5?^.- .  ^%-^X/^^ ,      ^ 


•«??-*s^:^a^T.^:.- 


Washing  fishing  nets  in  Pakistan. 

1 101  j 


Photo  FAO 


SOME   CONSIDERATIONS   ON   NETMAKING 

by 

P.   A.    LUSYNE 

Technology  Branch,  FAO  Fisheries  Division 


Abstract 

When  designing  a  new  net  the  netmaker  endeavours  to  obtain  the  largest  catching  potential  for  the  smallest  cost  in  material  and  the 
least  power  output.  He  has  to  take  into  account  many  factors  whose  effect  only  become  apparent  when  the  gear  is  in  action,  a  condition  he 
cannot  observe  and  has  to  guess  at.  In  most  cases,  however,  they  produce  deformation  of  meshes  in  the  netting  or  leave  other  signs  which 
become  apparent  during  operation  or  when  the  net  has  been  used  for  some  time.  The  stress-strain  reaction  in  webbing  is  dealt  with  and  it  is 
pointed  out  that  very  often  a  stress  can  be  neutralised  by  adjusting  the  hanging  at  the  stress  point.  The  effect  on  the  net  of  hanging  co- 
efficients, webbing  joints  and  tailoring  are  also  discussed. 


Resume 


Quelques  Considerations  sur  la  Fabrication  des  filets 


Quand  il  dessine  un  nouveau  filet,  le  fabricant  de  filets  s'eflbrcc  d'obtenir  la  plus  grandc  puissance  de  capture  pour  Ic  coin  le  plus 
bas  de  matidre  et  la  moindre  puissance  mise  en  jeu.  11  doit  tenir  compte  de  beaucoup  de  facteurs  dont  les  effets  se  manifestent  seulement 
quand  Tengin  est  en  action,  une  condition  qu'il  ne  peut  pas  observer  et  qu'ij  faut  qu'il  suppose.  Cependant,  il  sc  produit  des  deformations 
des  mailles  dans  le  filet  qui  a  etc  utilise  pendant  quelque  temps.  L'auteur  traite  de  la  reaction  £  1'effort  dc  tension  dans  le  filet  et  il  fait  ressortir 
que  tres  souvent  un  effort  peut  etre  neutralist  en  ajustant  le  montage  au  point  soumis  £  I'effbrt.  II  examine  aussi  reflet  sur  le  filet  des  co- 
cffcients  de  montage,  des  assemblages  des  parties  et  de  la  coupe. 


Consideraciones  sobre  la  Fabrication  de  redes 
Exfraeto 

Al  proyectar  una  nueva  red  el  fabricante  trata  de  darle  el  mayor  potencial  de  pesca  posible  con  el  menor  costo  dc  material  y  el 
consumo  de  fuerza  motriz  mas  pequefto.  Ha  de  tomar  en  consideracibn  muchos  factorcs  cuyo  efecto  no  se  hace  evidente  hasta  que  el  arte 
esta  en  funcionamiento,  situaci6n  que,  por  no  poder  observarla,  ha  de  tratar  de  adivinarla.  Pero  en  la  mayoria  de  los  casos  esos  factores 
causan  deformaciones  de  las  ma  lias  de  los  panos  o  dejan  otras  senales  que  s61o  se  hacen  evidentes  cuando  el  arte  esta  trabajando  o  cuando 
se  ha  usado  la  red  durante  aigun  tiempo.  Trata  el  autor  de  la  reaccibn  a  la  tensi6n  y  tracci6n  a  que  esta  sometida  la  red  y  subraya  que,  con 
mucha  frecuencia,  una  tensibn  puede  neutralizarse  ajustando  el  montaje  en  el  punto  donde  ocurre.  Menciona  tambien  el  efecto  que  el  co- 
cfkiente  de  colgadura,  las  costuras  de  los  panos  y  la  manera  de  cortarlos  ejercen  en  el  arte. 


A  FEW  decades  ago  the  construction  of  fishing 
nets,  from  the  braiding  of  the  webbing  to  the 
assembly  of  the  whole  gear  was  the  task  of  the 
fishermen  themselves.  The  boat  owner  was  only  interested 
in  the  cost  and,  in  many  cases,  the  fishermen  had  to 
help  pay  for  the  gear  or  even  owned  a  part  of  it.  In 
the  driftnet  and  line  fisheries,  for  instance,  each  fisherman 
owned  a  certain  number  of  the  nets  that  went  to  make 
up  the  set.  The  cost  of  materials  was  high  in  relation 
to  earnings  from  the  catch  and  very  often  more  con- 
sideration was  given  to  the  price  and  durability  of  the 
material  than  to  its  catchability  and  efficiency  as  a  part 
of  the  gear. 

The  operating  range  of  the  vessels  was  small  and  the 
nets  used  were  largely  the  same  for  each  home  port 
as  the  gear  was  adjusted  to  the  fishing  conditions  pre- 
vailing on  the  grounds  within  reach  of  that  port.  Large 
differences  were,  however,  to  be  noted  between  different 
fishing  areas,  in  boats,  in  gear  and  even  in  the  type 
of  operation. 

The  use  of  larger  vessels,  engine  power  and  better 
material  has  brought  many  changes;  fishing  gear  is 


now  constructed  to  give  a  high  degree  of  efficiency  in 
a  certain  fishing  method  over  a  much  greater  range  of 
grounds.  Higher  speeds  of  operation,  with  resulting 
higher  strains  on  the  nets,  and  the  use  of  new  materials 
and  improved  methods  of  netmaking,  have  brought 
changes  to  net  manufacture.  While  the  cost  of  material 
in  relation  to  returns  from  the  catch  has  decreased, 
the  cost  of  labour  has  increased  and  the  new  non- 
rotting  fibres  have  made  preservative  treatment  practic- 
ally unnecessary,  so  that  the  conditions  governing  the 
cost  of  fishing  gear  are  now  totally  different.  Today  the 
nets  are  increasingly  planned  and  constructed  ashore 
and  the  fishermen  are  relying  more  and  more  on  the 
netmaker  to  make  and  improve  his  gear. 

Although  some  observations  of  gear  in  action  have 
been  made  by  frogmen  and  results  are  being  obtained 
by  underwater  television,  the  netmaker  himself  does 
not  normally  see  how  the  net  he  has  made,  functions. 
He  still  has  to  evaluate  the  performance  of  the  net  by 
interpreting  various  telltale  signs  which  are  only  apparent 
when  the  net  is  back  on  board,  such  as  localized  tighten- 
ing of  knots,  deformation  of  meshes,  gilling  offish,  wear 


[102] 


NETMAKING 


on  knots,  etc.  Many  of  these  signs  only  become  apparent 
after  some  time  so  that  the  fisherman  must  still,  as  of 
old,  keep  his  gear  under  close  observation.  His  criticisms 
and  suggestions,  based  on  such  observations,  are  of  the 
greatest  help  to  the  netmaker. 

GENERAL 

When  making  a  new  design,  the  aim  of  the  netmaker 
is  to  produce  a  net  which  will  give  the  largest  catching 
potential  for  the  smallest  cost  in  materials  and  the  least 
energy  output.  Depending  on  the  type  of  gear,  many 
different  factors  have  to  be  taken  into  account,  such 
as  resilience  of  webbing,  strength  and  elasticity,  resistance 
to  flow  of  water,  weight  and  bulk,  speed  of  operation, 
cost  of  materials,  condition  of  fishing  ground,  etc. 
However,  in  most  cases,  the  most  important  factor  is 
ihe  shape  the  completed  net  will  take  when  in  action, 

Gillnets  and  trammel  nets  need  no  fashioning  as  their 
shape  in  operation  is  determined  by  the  looseness  given 
to  the  webbing  when  it  is  hung  between  the  float-  and 
leadlines.  This  not  only  determines  the  opening  of  the 
meshes  but  also  the  resilience  and  the  bulge  the  webbing 
can  take  during  certain  types  of  operation.  The  amount 
of  resilience  given  to  these  nets  differs  according  to  the 
type  of  operation,  the  net  height  and  fish  species  to  be 
caught.  It  is  regulated  by  the  amount  of,  and  relation, 
between,  the  buoyancy  and  weight  of  floats  and  sinkers. 

A  certain  amount  of  shaping  is  necessary  during  the 
construction  of  roundhaul  nets  to  give  the  webbing 
its  bulge  during  operation  and  to  take  up  the  various 
strains  on  the  webbing  during  shooting  and  hauling. 
This  is  achieved  partly  by  using  the  appropriate  hang-in 
coefficient  but  mainly  by  adjustment  of  take-up  and/or 
tuck-in  between  the  strips  of  webbing  that  go  to  compose 
the  complete  net.  With  trawl  nets  and  other  types  of 
bagnets,  the  netmaker  has  to  tailor  the  webbing  to  make 
individual  net  sections  fit  the  curve  of  the  lines  to  which 
they  are  hung  and  to  obtain  the  required  bagshape. 
The  problem  here  is  made  more  difficult  by  the  fact  that 
in  some  cases  the  operational  conditions  are  liable  to 
change;  variation  in  resistance,  or  drag  due  to  change 
in  the  flow  of  water  through  the  net,  influence  the 
performance  of  the  whole  gear.  Points  and  direction 
of  forces  acting  on  the  net  may  change,  with  a  resulting 
change  in  the  shape  of  the  net  in  operation. 

The  strains  on  the  net  must  be  spread  as  evenly  as 
possible  and,  in  general,  should  be  across  the  knots. 
The  twine  must  everywhere  be  strong  enough  to  withstand 
the  strains  acting  upon  it,  yet,  on  the  other  hand,  its 
weight,  resistance  to  flow  of  water  and  cost  have  to  be 
taken  into  account. 

In  the  case  of  gillnets,  the  mesh  size  must  be  appro- 
priate for  gilling  and/or  entangling  the  particular  fish 
species  caught;  with  other  nets,  it  must  prevent  such 
gilling.  When  deciding  on  the  mesh  size  for  bagnets, 
the  netmaker  has  to  take  into  consideration,  apart  from 
the  resistance,  the  water  release  so  as  prevent  "whirling" 
or  backwash  within  the  net. 

Apart  from  the  above  considerations,  which  are 
connected  with  the  catchability  of  the  net,  the  netmaker 
has  to  give  thought  to  the  handling  of  the  net  during 
the  shooting  and  hauling  operations.  For  instance, 
very  fine  twines  may  make  hauling  difficult  and  too  much 


loose  webbing  may  slow  down  the  speed  of  operation 
as  in  the  case  of  roundhaul  nets. 

There  is  no  such  thing  as  an  ideal  net.  All  nets  are 
a  compromise  to  fit  a  certain  set  or  collection  of  fishing 
conditions;  all  are  subject  to  controversy,  being  praised 
by  some  fishermen  and  condemned  by  others.  Much 
depends,  of  course,  on  how  the  net  is  used,  both  as  to 
assembly  of  the  net  to  other  gear  parts  and  the  skill 
with  which  it  is  operated.  A  well  constructed  net  may 
give  unsatisfactory  results  because  of  defective  auxiliary 
gear,  such  as  floats  and  kites,  sinkers  and  anchors, 
trawl  boards,  etc.  All  such  gear  must  be  of  the  right 
type  and  fitted  in  the  correct  position;  they  help  the 
net  to  take  its  correct  shape  in  operation  and  are  there- 
fore as  important  as  the  net  itself.  The  fisherman  must 
realize  that  he  can  only  get  maximum  efficiency  from  a 
given  net  if  he  uses  it  under  the  conditions  and  in  the 
manner  for  which  it  was  constructed.  Finally,  much 
can  be  done  to  adapt  a  net  to  the  conditions  prevailing, 
if  the  netmaker  is  made  aware  of  those  conditions. 

STRAINS   AND  STRESSES 

Although  certain  studies  have  been  made  on  the  forces 
acting  on  the  webbing  of  a  net,  most  netmakers  still 
use  empirical  values  when  planning  and  constructing 
new  nets.  There  is  a  complex  stress-strain  reaction  in 
webbing,  very  much  like  that  in  an  clastic  fabric,  i.e. 
it  elongates  in  the  direction  and  at  the  point  of  stress, 
while  contracting  in  the  lateral  direction.  In  fact,  the 
webbing  takes  up  a  part  of  the  stress  by  yielding  to  it. 

The  deformation  of  webbing  due  to  a  stress  acting 
on  it  depends  on  the  position  of  the  stress  point  in 
relation  to  the  whole  net  and  the  magnitude  and  the 
direction  of  the  stress.  Normally  the  stress  acts  "across 
the  knots"  and  in  that  direction  a  small  strain  will  be 
taken  up  completely  by  the  partial  closing  of  the  meshes 
directly  below  the  point  of  stress.  Larger  stresses  will 
cause  an  increased  strain  reaction  along  the  "halvers" 
until  the  selvedge  is  reached.  Further  increase  of  strain 
will  result  in  a  lateral  closing  of  the  webbing  section. 

The  amount  of  reaction  to  a  given  stress  in  influenced 
by  the  hang-in  coefficient  and  the  mesh  size,  increasing 
as  these  values  increase.  In  general,  individual  stresses 
cause  positive  strain  reaction  in  the  area  of  webbing 
enclosed  by  those  halvers  which  run  out  from  the  point 
of  stress;  the  meshes  along  these  lines  are  opened  and 
the  knots  tend  to  slip,  while  directly  under  the  point 
of  stress  the  meshes  are  closed  and  elongated.  Above 
the  "halver"  lines,  the  webbing  is  loose  and  the  area  of 
loose  webbing  is  in  proportion  to  half  the  area  under 
stress. 

When  considering  the  effect  of  the  direction  of  strains 


•light  strain  full  strain 

Fig.  1.     Effect  of  stre.\A  on  webbing 


103  ] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


and  forces  acting  on  webbing,  it  can  be  said  that  the 
more  the  strain  acts  out  of  the  vertical  (across  the  knots), 
the  easier  the  webbing  will  give  to  the  strain,  but  the 
more  deformation  it  will  cause.  The  onJy  remedy  for 
diagonal  stresses  is  the  use  of  tension  lines  to  take  up 
the  strain.  An  important  fact  is  that,  by  proper  adjust- 
ment of  the  hanging  in  stress  points,  the  stress  can  be 
neutralized  to  a  certain  extent.  This  is  done  by  allowing 
the  meshes  to  elongate  by  close  hanging  at  the  point 
of  stress,  thereby  spreading  the  strain. 

For  the  rational  design  of  fishing  nets  and,  in  par- 
ticular, of  trawl  nets,  more  accurate  knowledge  of  the 
stress-strain  behaviour  in  webbing  is  needed  to  counter- 
act the  deformation  it  causes  to  the  shape  of  the  net  and 
weaknesses  which  arises  from  it.  This  could  well  be  a 
subject  of  practical  research  at  one  of  the  Research 
Institutes. 

Because  of  this  yielding  characteristic  of  webbing  it 
is  difficult  to  foresee  the  actual  shape  of  a  new  designed 
net.  However,  to  the  experienced  netmakcr,  the  con- 
dition of  a  used  net  gives  much  indication  on  its  behaviour 
in  operation;  the  tightening  of  knots  below  stress  points, 
the  swelling  of  twine  in  slack  parts,  the  deformation  of 
meshes,  accumulation  of  scrap,  wear  on  the  knots, 
etc.,  all  help  to  form  a  picture  of  the  net's  performance 
and  its  shape  in  action. 

HANGING  OF  WEBBING 

The  correct  hanging  of  the  webbing  to  the  framing  or 
supporting  lines  is  an  important  factor  in  all  nets.  At 
present  there  are  two  methods  in  use  for  expressing  the 
hanging  coefficient  and  this  causes  confusion  when 
constructing  nets  from  a  plan.  The  first  is  to  express 
the  length  of  the  line,  to  which  the  webbing  is  hung, 
in  a  percentage  of  the  total  stretched  webbing: 

length  of  line        100 

—  per  cent,  of  hanging 

length  of  stretched  webbing 

The  second  method  is  to  express  the  amount  of  excess 
or  loose  webbing  (total  webbing  minus  line  length) 
as  a  percentage  of  the  total  webbing: 

excess  webbing  ,-    100 

—  per  cent,  of  hang-in 

total  webbing 

Both  are  reciprocal  values,  but  as  the  second  expression 
gives  a  direct  proportion  of  the  looseness  or  resilience 
in  the  net,  this  would  appear  to  be  the  most  logical 
for  expressing  the  hanging  coefficient  of  nets.  Taking 
the  stretched  mesh  as  unity,  the  following  table  gives  the 
theoretical  data  on  the  meshes  for  different  hang-in 
coefficients.  It  is  clear  that  the  mesh  height,  for  instance, 
will  depend  to  a  certain  extent  on  the  strains  acting 
on  the  webbing. 

The  resilience  of  a  section  of  webbing  is  determined 
by  the  forces  acting  on  it.  With  low  strains,  however, 
the  hang-in  coefficient  has  a  big  influence  so  that  when 
hung  squared,  webbing  has  its  greatest  area  but  least 
flexibility.  With  increasing  hang-in,  it  becomes  looser 
in  the  lateral  direction,  whereas  with  less  than  28  per  cent, 
of  hang-in  the  webbing  has  a  certain  vertical  springiness. 

In  gillnets  and  entangle  nets  the  hang-in  determines 
the  looseness  of  the  net,  true  gillnets  being  usually 
hung  somewhat  tighter  than  entangle  nets.  The  resilience 


Hang-in 
or 
Looseness 
per  cent. 

Ratio  of 
line  to 
webbing 
per  cent. 

Angle 
of 
mesh 

Height 
of 
mesh 

of' 

3$% 

10 

90 

128  20' 

0-436 

0-90 

0-785 

12 

88 

123  20' 

0-475 

0-88 

0-836 

14 

86 

118  40' 

0-510 

0-86 

0-877 

16 

84 

114  20' 

0-542 

0-84 

0-911 

18 

82 

110  10' 

0-572 

0-82 

0-938 

20 

80 

106  20' 

0-599 

0-80 

0-958 

22 

78 

102  40' 

0-626 

0-78 

0-975 

24 

76 

99  - 

0-649 

0-76 

0-986 

26 

74 

95  30 

0-672 

0-74 

0-991 

28 

72 

92  10' 

0-693 

0-72 

0-998 

30 

70 

89   - 

0-713 

0-70 

0-998 

32 

68 

85  40' 

0-733 

0-68 

0-997 

34 

66 

82  40 

0-751 

0  66 

0-991 

36 

64 

79  30' 

0-768 

0-64 

0-983 

38 

62 

76  40' 

0-784 

0-62 

0-972 

40 

60 

73  40 

0-801 

0-60 

0-961 

42 

58 

70  50' 

0-815 

0-58 

0-945 

44 

56 

66  10 

0-829 

0  56 

0-928 

46 

54 

65  20' 

0-842 

0-54 

0-909 

48 

52 

62  40' 

0-854 

0-52 

0-888 

50 

50 

60 

0-866 

0  50 

0-866 

52 

48 

57  20' 

0-877 

0  48 

0  842 

54 

46 

54  50' 

0-888 

0-46 

0-817 

56 

44 

52  10 

0-898 

0  44 

0-790 

58 

42 

49  40' 

0-907 

0-42 

0-762 

60 

40 

47   10 

0-916 

0-40 

0-733 

62 

38 

44  40' 

0-925 

0  38 

0-703 

64 

36 

42  10' 

0-933 

0-36 

0-672 

66 

34 

39  50' 

0-940 

0-34 

0-639 

68 

32 

17  20' 

0-947 

0-32 

0-606 

70 

30 

34  50' 

0-954 

0-30 

0-572 

and  fishing  height  of  these  nets  is,  of  course,  largely 
influenced  by  the  amount  of  sinker  weight  on  buoyed-up 
nets,  and  the  amount  of  buoyancy  on  bottom  operating 
nets,  which  produces  the  strain  in  the  webbing. 

In  roundhaul  nets,  the  hang-in  is  adjusted  to  give  depth 
and  bulge  during  operation;  in  the  wings,  however,  it 
is  often  curtailed  as  too  much  webbing  affects  the  speed 
of  operation. 

With  trawl  nets,  the  hang-in  to  be  given  to  wings, 
quarters  and  bosom,  depends  on  the  general  shape  and 
the  cut  of  the  sections  of  the  net.  It  is  difficult  to  deter- 
mine accurately,  and  is  usually  based  on  previous 
experience  and  experimenting.  It  is,  therefore,  most 
important  that  the  amount  of  hang-in  on  wings,  quarters 
and  bosom  is  denoted  on  trawlnet  plans. 

The  fastening  of  the  webbing  to  the  lines  is  done  in 
different  ways  according  to  the  gear.  The  main  require- 
ment is  to  ensure  that  the  hanging  cannot  shift  over  the 
line  or  become  loose  once  the  net  is  hung.  The  net-line 
hitch  and  rolling  hitch  are  about  the  best  knots  to  use 
when  making  hangings  to  carry  two  or  more  meshes. 
The  clove  hitch  is  to  be  discouraged  unless  locked  by 
an  additional  half  hitch,  as  it  tends  to  become  loose. 
In  most  cases,  it  is  better  to  use  a  rather  heavy  twine  for 
the  hangings  as  it  reduces  wear  on,  and  cutting  of,  the 
selvedge  meshes.  When  the  nets  are  hauled  mechanically, 
it  is  best  to  keep  the  hangings  rather  short  to  avoid 
snagging  while  in  operation. 

The  backhand  marline  hitch  is  still  the  most  secure 
for  rigid  fixing  of  the  webbing  to  the  lines,  but  is  also 
the  most  time-consuming  to  make.  It  has  the  further 


[104] 


NETMAK1NG 


disadvantage  of  requiring  renewal  every  time  the  selvedge 
meshes  need  repairing. 

JOINING  OF  WEBBING 

When  sections  of  webbing  of  the  same  width  are  joined 
together,  the  angle  of  the  meshes  in  both  sections  is 
the  same.  When  the  width  of  the  sections  differs,  the 
angle  of  the  meshes  in  the  widest  section  will  be  smaller, 
so  that  this  piece  will  have  more  hang-in  and,  by  the  same 
token,  less  lateral  strain.  Several  combinations  are 
possible  and  each  is  applied  according  to  the  purpose 
of  the  joint: 

(a)  the  sections  have  the  same  width  but  different 

mesh  size 

(b)  the  sections  have  the  same  mesh  size  but  differ 

in  width 

(c)  the  sections  differ  both  in  width  and  mesh  size 

The  first  is  a  simple  joint  between  sections  of  different 
mesh  si/e  and  the  take-up  meshes  are  inserted  at  suitable 
intervals.  It  is,  however,  advisable  to  allow  the  large 
strip  a  little  extra  width  to  enable  it  to  expand  fully 
during  operation. 

In  the  cases  of  (b)  and  (c)  the  greater  width  of  one 
of  the  sections  is  given  to  allow  the  webbing  freedom 
to  bulge  out  laterally,  as  in  the  case  of  roundhaul  nets 
and  some  types  of  bagnets.  Under  (c)  is  included  the 
special  case  where,  although  both  width  and  mesh  size 
differ,  the  sections  are  joined  mesh  to  mesh,  which  is 
very  common  in  trawl  nets.  It  must  be  borne  in  mind 
that  in  the  above  case  the  effect  is  more  a  narrowing 
of  the  wide  section  instead  of  bulging.  It  results  in 
the  wider  webbing  section  reacting  as  if  it  were  baited 
at  a  faster  rate;  this,  at  the  same  time,  helps  to  shift 
the  stress  towards  the  side  or  selvedge  of  the  complete 
section. 

In  small-mesh  webbing,  it  is  better  to  make  the  take- 
ups  by  creases  rather  than  by  baitings,  especially  in 
the  case  of  joining  two  sections  of  equal  width,  as  the 
baitings  tend  to  form  a  contraction  in  the  row  they 
are  made,  which  causes  slipping  of  knots. 

Where  a  tuck-in  is  needed  in  the  lacing  of  vertical 
strips  of  webbing  because  of  difference  in  depth  or 
mesh  size,  the  tucks  should  be  made  by  half  meshes  or 
legs,  rather  than  by  full  meshes.  Full  mesh  tucks  form 
strain  points  in  the  webbing,  which  soon  become  holes. 

In  calculating  the  width  of  the  sections  to  be  joined, 
the  take-up  or  tuck-in  rate  should  be  rounded  off  to 
allow  for  an  easy  sequence.  This  not  only  facilitates  the 
actual  work  of  joining  but  allows  the  sequence  to  be 
expressed  in  a  simple  fraction  form  such  as  5/6,  for 
example,  meaning  6  meshes  are  joined  to  5,  or  one 
take-up  every  fifth  mesh.  Simple  take-up  sequences 
are,  furthermore,  easy  to  replace  in  mending  and  when 
new  webbing  sections  have  to  be  inserted  during  over- 
hauling of  nets. 

TAILORING  OF  WEBBING 

All  bagnets  need  fashioning,  either  by  braiding  them 
in  the  round,  with  baitings  inserted  at  appropriate 
intervals,  or  by  assembly  of  shaped  webbing  sections. 
With  the  latter  method,  the  sections  can  be  braided  to 
shape  or  they  can  be  cut  to  shape  from  machine-made 


strips  of  webbing.  Theoretically,  it  is  possible  to  cut 
or  braid  webbing  to  any  shape;  in  practice,  however, 
all  shapes  are  obtained  by  decreasing  the  width  of  sections 
at  certain  baiting  rates,  as  curves  give  rise  to  many 
difficulties  both  in  construction  and  in  mounting  the 
webbing  to  its  supporting  lines.  Curves,  therefore, 
are  always  made  by  a  series  of  straight  lines,  each  at  a 
different  angle  to  the  vertical  or  "across  the  knots" 
direction. 

When  drawing  plans  of  nets,  it  is  best  to  use  a  "1  to  2" 
ratio,  i.e.  all  width  measurements  are  drawn  in  as  half 
values  while  heights  are  drawn  to  full  value.  It  would 
be  helpful  when  comparing  net  designs,  if  such  a  ratio 
could  be  adopted  by  all  nctmakers,  as  a  comparable 
picture  of  the  general  net  shape  would  then  be  apparent 
at  a  glance.  Apart  from  representing  a  fair  mesh  opening, 
53  degrees  or  about  56  per  cent,  hang-in,  this  ratio 
gives  a  fair  picture  of  the  net  shape  and  has  the  advantage 
of  being  easy  to  work  with  on  the  drawing  board. 
Furthermore,  with  this  ratio  the  baiting  rates  can  be 
taken  directly  from  the  drawings  as  they  are  equivalent 
to  the  cotangents  of  their  angle  of  slope  to  the  vertical. 
The  angle  of  these  slopes  increases  with  the  baiting 
ratio  and  fig.  2  shows  the  slopes  of  the  most  frequently 
used  baiting  ratios  and  illustrates  the  equivalent  cutting 
rates. 

In  some  net  sections,  a  faster  slope  is  necessary  to 
fit  the  slope  of  curving  supporting  lines  as,  for  instance, 
at  the  quarterpoints  where  the  wings  join  the  bosom  in 
trawlnets.  in  such  cases,  the  slope  can  be  increased  by 
inserting  baitings  under  the  selvedge  of  the  wing  which, 
by  reason  of  its  fly  meshes,  already  has  a  slope  of  1  :  2. 


•VM7  7  row* 
5  poiat*  4  bar* 


•Tory  8  ram 
3  point*  ?  bar* 


•vary  1O  i 

2  point*  1   bar 


Fig.  2.    Slope  ami  cutting  ratios  of  most  frequently  used  baiting 
rates. 


[105] 


MODERN     FISHING     GEAR     OF    THE     WORLD 


The  additional  baitings  cause  a  •  localized  closing  of 
the  meshes  and  a  consequently  greater  height,  which 
allows  the  selvedge  to  stand  further  off. 

Where  desired,  the  meshes  can  be  kept  lower  down  at 
the  normal  opening  by  inserting  creases  to  equalize 
the  number  of  meshes. 

When  considering  the  dimensions  of  new  nets,  it  is 
always  better  to  overestimate  the  width  the  net  will 
have  in  action;  this  is  especially  true  of  all  types  of  trawl 
nets.  Too  high  assessment  of  the  opening  width  of  a 
trawlnet  may  result  in  the  use  of  too  much  webbing, 
but  the  net  will  still  operate  properly  and  catch  fish. 
Underestimating  the  width  at  the  trawl  mouth,  however, 
will  cause  a  change  in  the  whole  shape  of  the  net.  When 
the  wings  are  pulled  further  open  than  calculated,  the 
bosom  is  pulled  forward  while  the  sideseams  fall  back; 
this  causes  a  contraction  in  the  webbing  of  the  throat 
with  resultant  bad  water  release.  It  may  even  lead  to 
the  cod-end  turning  over. 

This  desirability  to  over-  rather  than  under-estimate 
net  width  does  not  mean  that  it  is  better  to  choose  a 
bigger  net,  but  that  careful  consideration  should  be 
given  to  the  amount  of  webbing  to  be  allowed  to  a 
headline.  Indeed,  where  it  is  a  question  of  the  size  of 
net  to  be  used  by  a  certain  vessel  the  contrary  is  true: 
it  is  always  better  to  under-estimate. 

fig.  3.     Double  baiting  at  the  quarter  points. 


Net  sections  being  braided  by  hand. 
[106] 


THE  KNOTLESS  NET 

by 

THE  NIPPON  SEIMO  CO.  LTD. 

Tokyo,  Japan 

Abstract 

This  method  of  making  nets  was  invented  in  1922  by  the  Nippon  Seimo  Co.  Ltd.,  in  Japan  and  it  is  becoming  increasingly  popular 
in  the  fisheries  of  that  country.  It  has  many  advantages;  the  meshes  are  not  distorted  under  strain,  and  because  of  the  absence  of  knots, 
less  material  is  needed  with  a  consequent  saving  of  weight  and  bulk. 

The  paper  describes  the  manufacture  of  these  nets  and  shows  how  easily  they  can  be  repaired. 

Le  filet  sans  noeuds 
Kcsum£ 

Cette  in  genie  use  met  node  dc  fabrication  des  filets  a  etc  mventee  en  1922  par  la  Nippon  Seimo  Co.  Ltd.,  au  Japon,  et  ce  filet  connait 
une  vogue  de  plus  en  plus  grande  dans  les  peches  de  ce  pays.  11  comporte  de  nombreux  a  vantages;  les  mailles  ne  se  d&brmcnt  pas  sous  la 
tension  ct,  du  fait  de  Pabsence  de  noeuds,  la  fabrication  exige  moins  de  matiere  premiere,  d'ou  economic  de  poids  el  moindre  encombrement. 

I. 'article  decrit  la  fabrication  dc  ces  filets  et  montre  combien  leur  reparation  est  facile. 

l^a  red  sin  nudos 
Extracto 

F.n  las  pesquerias  del  Japon  va  aumentando  la  popularidad  de  un  ingenioso  metodo  para  fabricar  redes,  idcado  en  1922  por  la  Nippon 
Seimo  Co.  Ltd.,  de  ese  pais.  Entre  las  diversas  ventajas  de  este  procedimiento  figuran  el  hecho  de  que  las  ma  I  las  no  se  deforman  con  la 
tensi6n  y  la  ausencia  de  nudos;  ademas,  se  utiliza  menos  material  con  la  consiguiente  economia  de  peso  y  volumen. 

En  el  trabajo  original  se  describe  la  fabricacidn  de  estras  redes  y  demuestra  la  facilidad  con  que  pucden  reparar.se. 


THE  knotless  net  was  invented  in  1922  by  the  Nippon 
Seimo  Co.  Ltd.  (Japan  Fishing  Net  Manufacturing 
Co.).  In  this  type  of  webbing  the  twines  are  joined 
at  the  mesh  corners  by  an  interlacing  of  the  two  twine 
strands.  Knotless  nets  did  not  come  into  general  use 
while  only  natural  fibres  were  available,  but  followed 
with  the  development  of  rotproof  synthetic  fibres  and 
their  increased  use  for  fishing  nets.  Now  this  type  of 
webbing  is  rapidly  becoming  popular  for  several  kinds 
of  nets. 

SPECIAL  ADVANTAGES 

The  rapid  change-over  to  knotless  nets  in  Japan  is 
chiefly  due  to  the  following  special  features,  of  which  the 
most  important  are: 

Less  weight  and  bulk 

Less  twine  is  used  to  make  the  meshes  which,  in  some 
cases,  can  mean  a  saving  of  as  much  as  SO  per  cent,  of 
the  raw  material.  As  there  are  no  knots,  the  bulk  of  the 
net  is  greatly  reduced. 

Higher  strength 

When   knotted,   twines   of  natural   fibres   lose   about 
1 8  to  20  per  cent,  in  strength.    The  loss  is  often  higher 
with  synthetic  fibres  and  may  reach  30  to  40  per  cent. 
As  the  fibres  undergo  practically  no  sharp  bending  in 


knotless  nets,  there  is  no  reduction  in  strength  so  that  a 
correspondingly  lighter  twine  can  be  used. 

Less  resistance  in  water 

The  aggregate  resistance  of  the  knots  in  a  traditional 
net  is  considerable  and  becomes  an  important  factor  in 
the  use  of  nets  which  are  towed  or  set  in  a  current.  The 
resistance  of  knotless  nets  is  very  much  less. 

Easier  to  handle— less  friction 

As  there  is  no  knot  friction,  the  net  can  be  hauled  over 
the  ship's  side  with  less  effort,  so  that  the  net  can  be 
lifted  even  during  a  change  of  current  without  danger  of 
becoming  fouled.  During  shooting,  the  net  runs  out 
much  smoother  as  there  is  no  inter-knot  friction. 

Less  labour  and  smaller  tackle  required 

As  the  whole  net  is  lighter  and  less  bulky,  time  and 
labour  is  saved  and  tackle  can  be  lighter,  factors  leading 
to  a  saving  in  manpower. 

No  wearing  away  of  knots 

The  damage  caused  by  abrasion  of  knots  in  the  belly  of 
trawls,  and  other  nets  which  are  dragged  over  the  sea 
bottom,  is  well  known.  In  other  types  of  nets  the  knots 
are  worn  away  during  operation  by  rubbing  against  the 
ship's  side,  net  rollers  or  other  gear  parts. 


[107] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


Fig.  I.     Starting. 


Fig.  3.     Pick-up. 


Fig.  2.     Joining. 


With  synthetic  fibres,  which  are  rotproof,  knotwear  is 
very  important  as  it  shortens  the  useful  life  of  the  net. 

Knotless  nets,  having  no  such  "points**  of  wear, 
should  last  longer  than  knotted  nets. 

No  damage  to  catch 

When  fish  are  collected  in  codends  and  bags  of  purse 
seines,  many  are  damaged  by  rubbing  and  friction  against 
the  meshes.  With  knotless  nets  such  damage  is  consider- 
ably reduced,  which  affects  the  quality  of  the  catch. 

Constant  mesh  size 

Because  there  is  no  tightening  of  knots,  as  in  knotted 
nets,  the  meshes  undergo  no  change  so  that  the  mesh 
size  of  a  knotless  net  is  almost  100  per  cent,  constant 
throughout  its  life. 

Easy  to  dye 

Having  no  knots,  the  nets  can  be  dyed  more  easily  and 
completely.  The  absence  of  knots  and  the  smaller  bulk 
also  means  that  less  dyeing  material  is  used  and  the  nets 
dry  quicker. 

Less  liable  to  fouling 

There  can  be  no  deposit  of  dirt  and  micro-organisms 
between  the  interstices  of  knots,  so  that  knotless  nets  are 
much  less  fouled  and  need  less  washing. 

MENDING  OF  KNOTLESS  NETS 

When  knotless  nets  were  first  introduced,  the  fishermen 
were  anxious  about  the  mending  of  them.  In  fact,  torn 


parts  are  mended  as  easily  as  in  knotted  nets.  The 
only  difference  is  that,  when  starting,  the  mending  twine 
should  be  attached  from  one  mesh  outside  and  all  ends 
of  torn  meshes  should  be  bent  back  within  the  mending 
knot,  as  shown  in  figs.  1,  2  and  3. 


Fig.  4.     Knotless  net. 


[108] 


JAPANESE     KNOTLESS     NETS 


METHOD  OF  MANUFACTURE 


The  knotless  net  is  manufactured  in  two  stages.  In  the 
first  stage  the  twine  is  prepared  by  doubling  the  appro- 
priate number  of  single  yarns  together  and  then  twisting 
two  or  more  of  such  yarns  into  strands  with  a  "Z"  twist. 
The  strand  thus  obtained  is  wound  on  the  bobbins  of 
the  net  making  machine. 

In  the  net  making  stage  two  such  bobbins  are  attached 
to  both  sides  of  a  special  flat  spindle.  Turns  of  the  spindle 
insert  in  the  twine  a  second  twisting  operation  in  an  "S" 
direction.  When  the  necessary  number  of  twists  are  made, 
the  two  bobbins  move  and  exchange  places  with  two 
neighbouring  bobbins  and  the  point  of  intersection  of 
twine,  which  corresponds  to  a  "knot"  in  the  knotted  net, 
is  made  accordingly. 

The  construction  of  a  "knot"  is  shown  in  the  diagram 
(fig-  4). 

Strands  of  both  twines  run  across  and  through  each 
other,  and  are  twisted  in  opposite  directions  as  shown. 
Thus  a  "knot"  or  intersection  is  made. 


The  twine  in  the  knotless  net  has  an  "S"  twist  of  two 
strands. 

The  webbing  is  examined,  flaws  and/or  irregular 
twists  are  corrected  and  each  web  section  is  then  com- 
pleted by  weaving  on  the  top  and  bottom  selvedges. 

Any  size  of  web  section  can  be  made  to  suit  different 
types  of  gear. 

The  nets  are  heat-set  so  that  the  twist  of  the  strands 
is  fixed  permanently.  During  this  operation  the  tensile 
strength  of  the  twine  increases  by  about  15  per  cent. 
By  adjusting  the  process,  the  flexibility  of  the  twines  can 
be  modified  to  suit  particular  purposes.  Hard  fibres  can 
be  made  softer  and  soft  fibres  stiffened. 

The  heat  setting  process  is  necessary  for  nets  of  filament 
yarns  such  as  nylon,  Krehalon  and  Saran  (vinylidene), 
Teviron  (polyvinylchloride)  and  Kuralon  5  but  not  for 
cotton,  Kuralon  (vinylon)  and  the  other  spun  yarns. 

With  the  exception  of  materials  belonging  to  the 
polyvinylchloride  group,  such  as  Krehalon  and  Saran, 
the  nets  made  of  nylon,  Kuralon,  etc.  can  be  dyed  with 
commercial  dyes,  pigment  colours  or  coal  tar.  Heat 
setting  is  not  generally  required  when  coal  tar  is  used. 


Slacking  a  beach  seine  after  a  haul  in  Ceylon. 
I  109  I 


Photo  FAO. 


DISCUSSION   ON    RATIONAL   MANUFACTURING  OF  FISHING   GEAR 


Mr.  H.  Warncke  (Germany)  Rapporteur:  Our  problem 
is  now  to  produce  machine-made  webbing  on  a  more  rational 
basis  and  so  reduce  costs.  This  aim  could  be  achieved  by: 

1.  Greater  uniformity  and  standardization  of  net  types, 
and    of   the    assembling   components,    which    would 
gradually  bring  stock-carrying  risks  of  manufacturers 
and  dealers  to  a  minimum; 

2.  A  reasonable  limitation  of  types  of  synthetic  materials; 

3.  A  general  exchange  of  information   on   net-making 
problems. 

Some  progress  has  been  achieved  towards  such  a  standard- 
ization by  the  fishermen  and  gear  technicians.  This  is  illus- 
trated by  Barraclough  and  Needier  in  their  description  of 
the  construction  of  a  midwater  trawl  for  a  62  ft.  175  h.p. 
trawler  equipped  with  typical  British  Columbia  gear  and 
deck  layout,  and  by  du  Plcssis  who  describes  the  South 
African  Pursed  Lampara.  These  two  examples  show  that 
by  careful  consideration  of  the  many  factors,  fishing  nets 
can  be  restricted  to  a  few  types.  Standardization  must, 
however,  be  based  on  scientific  study  and  practical  experience. 

The  papers  presented  indicate  that  synthetic  materials 
are  becoming  increasingly  important  to  the  fishing  industry. 
It  appears,  however,  that  a  fibre  well  suited  for  some  types 
of  net  and  gear  lacks  certain  qualities  for  others.  The  net 
manufacturer  must  therefore  produce  and  stock  many  types 
of  gear  in  order  to  satisfy  the  requirements  of  the  fishermen. 
A  limitation  in  this  ever  increasing  variety  would  enable  the 
net  manufacturer  to  reduce  his  stock -carry  ing  overheads  to  a 
reasonable  level,  and  so  reduce  production  costs. 

It  was  recommended  that  net  manufacturers  should 
exchange  information  so  as  to  arrive  at  more  rational  pro- 
duction. However,  owing  to  the  big  differences  in  production 
conditions  between  countries,  created  by  subsidies,  import 
rules,  etc.,  most  manufacturers  would  at  present  be  reluctant 
to  agree  to  such  exchange  as  it  would  interfere  with  business 
competition. 

It  would  seem,  therefore,  that  the  most  likely  methods 
of  achieving  rationalization  arc: 

(a)  Limitation  of  mesh-sizes  to  a  standard  set  of  sizes, 
together   with   a  standard    method   of  measurement. 
This  would  reduce  the  need  for  frequent  adjustments 
to  the  net-making  machines  and  allow  for  simplifi- 
cations in  their  construction; 

(b)  limitation  of  sizes  and  twists  of  twines.    For  instance, 
the  denier  system  counts  that  are  at  present  in  pro- 
duction, should  be  sufficient  to  supply  all  the  required 
sizes  of  twines.    The  number  of  different  twists  and 
plies  could  probably  be  reduced  without  disadvantage 
to  the  fishing  industry 

Such  rationalization  could  be  discussed  by  working 
groups  formed  of  representatives  of  all  parties  concerned. 

Problems  of  more  general  interest  which  might  be  con- 
sidered are: 


(a)  Does  dyeing  really  provide  sufficient  protection  to  the 
synthetic  fibres  against   decay   through  exposure    to 
ultra-violet   rays,   as   seems   to   be  widely   accepted? 
Recommendations  concerning  the  appropriate  methods 
of  dying  synthetics  are  often  difficult  to   interpret. 
Such  recommendations  should  be  consistent  with   the 
limited  dyeing  facilities  of  the  fishermen. 

(b)  Which    is   the   appropriate   size   of  synthetic   twines 
used  in  place  of  natural  fibres?   To  replace  210/6  ply 
and   coarser   twines,    synthetic   fibres,    25    per   cent, 
lighter  in  weight,  are  generally  chosen.  For  finer  twines, 
the  criterion  is  the  same  running  length  as  the  natural 
fibre.   This  would  mean  that,  in  some  cases,  the  main 
advantage  of  synthetics,  i.e.  their  higher  tensile  strength, 
is  not  fully  utilized  and  unnecessarily  strong  and  heavy 
twines  are  used. 

The  Nippon  Seimo  Co.  Ltd.  deal  with  the  construction 
of  knotless  nets  designed  and  developed  in  Japan.  They 
report  that  such  nets,  constructed  of  2  strand  twine,  suffer 
no  loss  in  strength,  whereas  natural  fibres  lose  from  1 8  per  cent, 
to  20  per  cent,  and  synthetic  fibres  from  30  per  cent,  to  40 
per  cent,  when  knotted. 

As  the  selvedges  have  to  be  woven  separately,  causing 
additional  expense,  it  would  be  interesting  to  know  if  the 
initial  financial  advantage  of  lower  weight  is  thus  reduced? 
The  relation  between  cost  of  production  and  price  of  material 
can  perhaps  explain  why  knotless  webbing,  which  was  not 
in  demand  when  made  of  natural  fibres,  has  increased  in 
sales  since  it  has  been  made  of  synthetic  fibres. 

Mr.  D.  McKee  (Scotland):  With  regard  to  catering  to 
the  wishes  of  the  fishermen,  it  is  necessary  for  the  manu- 
facturers first  to  obtain,  either  from  the  fishermen  or  from 
fisheries  management,  information  as  to  the  thickness  or 
weight  by  runnage  of  the  twines  or  lines  required. 

A  good  illustration  of  this  point  is  the  fishing  line  situation. 
In  inshore  haddock  line  fishing,  a  man  uses  a  line  weighing 
approximately  1  lb./60  fm.  Now  he  wants  a  synthetic  line. 
We  find  the  breaking  strength  of  such  a  line  in  cotton  or 
hemp  is,  say,  20  Ib.  The  problem  is — does  he  want  a  line 
of  this  strength  or  does  he  want  the  same  thickness?  Generally, 
in  the  case  of  light  lines,  the  man  wants  the  same  body  or 
feel  in  the  new  line,  rather  than  the  same  strength.  My  firm 
which  is  mostly  concerned  with  inshore  herring  fishing, 
gillnetting  and  that  type  of  gear,  has  published  a  booklet 
for  the  industry,  on  the  makeup  of  synthetic  twines.  This 
gives  the  dry  and  wet  breaking  strength  of  a  cotton  twine  and 
recommended  equivalent  size  in  nylon  or  other  synthetic 
twines  in  yds./lb.,  and  m./kg.  One  omission  is  the  breaking 
strength  of  knotted  webbing,  but  information  on  this  point 
can  be  obtained  directly  from  our  works.  There  is  a  certain 
risk  in  declaring  breaking  strengths  and  giving  yardages  as 
the  figures  depend  on  the  testing  method  used.  We,  therefore, 
would  claim  at  least  10  per  cent,  margin  either  way. 


110] 


DISCUSSION:     RATIONAL     NET     MANUFACTURING 


On  the  question  of  economy,  I  would  Jike  to  quote  an 
example.  Recently,  while  abroad,  1  was  confronted  with 
general  complaints  about  the  economic  failure  of  the  fishing. 
Until  a  few  years  ago,  the  fishermen  were  using  natural 
fibres  and  then  changed  to  synthetics  which  cost  double 
the  price.  When  natural  fibres  were  in  use  and  the  net  got 
damaged,  it  was  thrown  overboard.  I  tried  to  find  out 
how  much  more  service  they  obtained  from  a  synthetic  net. 
I  got  the  amazing  answer — little,  if  any.  It  appeared  that 
when  the  synthetic  fibre  net  became  damaged  the  same  pro- 
cedure was  followed,  and  the  net  was  thrown  aside  and  not 
used  again.  It  is  not  the  part  of  the  manufacturer  to  work 
down  to  e  -onomy  level  of  that  type,  although  successful 
manufacturing  depends  on  the  success  of  fishing. 

There  are  many  dfferent  types  of  mesh  measurement. 
In  Canada,  for  fine  gillnets,  you  measure  between  knots; 
yet,  in  some  parts  of  Canada,  heavier  nets  are  measured 
from  the  inside  of  one  knot  to  the  outside  of  another  knot. 
The  best  suggestion  I  can  make  to  the  fisherman  is  to  rel> 
on  the  netting  manufacturers,  but  to  have  a  precise  idea 
of  requirements. 

Vlr.  A.  Robinson  (U.K.):  1  full}  agree  with  the  comments 
made  so  far,  particularly  with  those  of  Mr.  McKcc.  Mr. 
Warncke  also  has  made  a  very  strong  point  regarding  the 
difficulties  which  arise  due  to  the  large  stocks  which  arc 
necessary  at  present.  My  firm  is  essentially  in  the  twine  trade, 
but  if  the  variety  of  sizes  and  special  requirements  can  trouble 
us,  how  much  more  will  they  trouble  the  net  manufacturer! 
Of  course,  these  are  difficulties  inside  the  trade,  perhaps  not 
fully  realised  by  the  fishermen,  but  they  have  a  very  strong 
bearing  indeed  on  the  price  that  the  fisherman  is  eventually 
asked  to  pay. 

I  also  feel  that  the  twine  maker,  and  I  am  sure  also  the  not 
maker,  will,  if  only  he  is  given  full  information,  do  all  he 
can  to  help  the  fisherman.  Net  and  twine  factories  get,  daily, 
numerous  inquiries  and  orders  for  materials,  from  which  it 
is  difficult  to  determine  exactly  what  is  required.  Yet  this  is 
necessary  because  the  customer  docs  not  have  sufficient  tech- 
nical knowledge  or  information.  However,  if  he  states  his 
problem  exactly  he  will  get  reliable  information  in  exchange. 

With  regard  to  the  dyeing  of  synthetic  fibres,  nothing  very 
definite  has  been  proved  so  far  as  we  are  aware.  Our  tests 
have  failed  to  prove  that  dyeing  does  give  protection  against 
deterioration  due  to  sunlight. 

Most  firms  would,  I  think,  be  willing  to  issue  comparative 
lists  showing  the  conversion  from  natural  to  synthetic  fibres. 
We,  ourselves,  give  such  information  quite  freely  and  I 
believe  the  wet  knot  strength  should  be  the  criterion  for  such 
material.  A  comparison  on  any  other  basis,  certainly  so 
far  as  net  twine  is  concerned,  is  apt  to  be  misleading.  At  the 
same  time,  there  are  circumstances  where  other  characteristics 
of  synthetic  fibres  can  be  taken  into  consideration  as.  for 
instance,  the  greater  elasticity  which  improves  the  energy 
absorption  of  the  twine,  quite  apart  from  its  merits  as  to 
breaking  strength.  It  has  been  mentioned  that  synthetics 
may  be  as  much  as  25  per  cent,  lighter  than  natural  fibres. 
On  the  basis  of  wet  knot  strength,  and  also  taking  into  con- 
sideration the  high  tenacity  yarns  available  today,  a  fairly 
reliable  changeover  to  cotton  can  be  made  on  the  basis  of 
a  synthetic  twine  giving  about  50  per  cent,  greater  length 
per  unit  weight. 

I  would  like  to  raise  one  further  point,  i.e.  that  of  mutual 
understanding  between  fisheries  workers  all  over  the  world. 


Apart  from  using  the  same  standards  and  tests  in  our  work, 
we  should  also  be  able  to  understand  one  another's  writings 
Translation  of  fisheries  literature  is  extremely  difficult 
because  of  the  different  meanings  given  to  the  terms  used 
in  different  countries  and  areas. 

Dr.  E.  Hess  (FAO):  As  you  probabl>  know,  FAO  has  for 
the  last  8  years  been  publishing  the  World  Fisheries  Abstracts 
in  English,  French  and  Spanish.  We  have,  during  these 
years  accumulated  quite  a  lot  of  terms  and  we  are  now  in 
the  process  of  preparing  this  material  for  publication  as  a 
dictionary  in  the  three  official  FAO  languages,  English, 
French  and  Spanish,  at  least  to  begin  with.  The  form  of 
these  dictionaries  was  agreed  upon  with  UNESCO,  who  has 
done  much  work  along  similar  lines. 

The  English  dictionary  will  form  the  base  and  each  term 
will  have  a  definition.  Each  term  is  given  a  number  and  its 
meaning  in  another  language  can  be  found  by  simply  referring 
to  that  same  number  in  the  other  language  part  or  book. 

The  whole  field  of  fisheries  technology  will  be  covered, 
not  just  gear.  The  first  section,  on  fish  curing,  is  ready  now 
The  section  on  fishing  gear  and  boats  we  hoped  to  have  read> 
for  this  Congress,  but  I  am  sorry  to  sa>  we  did  not  get  that 
far.  We  have  another  section  on  refrigeration  also  in  an 
advanced  stage.  Before  long,  we  hope  to  have  this  published, 
but  will  first  issue  a  mimeographed  draft  and  send  it  to  as 
many  people  as  we  can  think  of  in  various  countries  for  their 
comments.  After  study  of  these  comments  and  necessary 
adjustments  we  will  then  publish  in  book  form. 

Mr.  O.  Aagaard  (Norway):  Much  has  been  said  about 
standardization  as  seen  from  the  manufacturer's  point 
of  view.  In  Norwa>,  a  Standardization  Committee  was 
formed  in  1930  at  the  request  of  the  Fishermen's  Organiza- 
tion. However,  when  the  same  fishermen  discovered  that 
standardization  would  mean  that  their  individual  taste 
and  wishes  as  to  twine  and  rope  sizes,  mesh  sizes,  etc.  would 
be  interfered  with,  the  whole  thing  was  dropped. 

Before  going  over  to  an>  kind  of  standardization,  it  may 
be  useful  to  hear  the  fishermen's  opinion. 

Prof.  S.  Takayama  (Japan):  About  half  the  knotlcss  nets 
used  at  present  in  Japan,  i.e.  3  million  Ib.  year,  are  made 
of  synthetic  fibres.  All  Japanese  knot  less  nets  are  made  of 
two  strand  twines*. 

There  are  two  types  of  nets  based  on  practically  the  same 
system  of  connecting.  In  the  first  type  the  twines  run  diagon- 
ally through  the  webbing,  whilst  in  the  second  type  they  run 
in  a  zigzag  line.  These  different  directions  depend  on  how 
many  times  the  tyyo  twines  arc  threaded  through  each  other 


Fig.  I      Three  types  of  joinings  used  in  A  not  less  nets. 

*  Editors  note.  Knot  less  nets  made  of  braided  twine  are  noyv 
being  manufactured  and  tested  for  instance  in  U.S.A.  and  Belgium 


MODERN    FISHING     GEAR    OF    THE   WORLD 


Fig.  2.     Diagonally  constructed  knotless  net. 

If  they  pass  through  only  once,  the  direction  of  the  twines 
is  diagonal;  if  they  arc  threaded  twice,  the  zigzag  line  is 
obtained;  if  they  pass  through  three  times,  a  diagonal  is 
obtained,  and  four  times  again  a  zigzag,  i.e.  uneven  numbers 
of  interweaving  results  in  diagonal  twine  direction  and  even 
numbers  in  zigzag.  The  number  of  the  interweavings  can 
be  built  up  until  a  quite  unusual  mesh  form  results  which 
could  be  called  the  tortoise  type. 

Knotless  nets  can  be  adapted  for  practically  all  types  of 
gear,  but  until  now,  they  are  mainly  used  for  big  setnets, 
leader  nets  and  gillnets. 

Mr.  H.  Kobayashi  (Japan).  As  regards  mending  tears  or 
connecting  pieces  of  webbing,  there  is  no  remarkable  differ- 
ence between  knotted  and  knotless  nets,  and  consequently, 
there  are  practically  no  additional  costs  with  regard  to 
selvedges.  If  double  selvedges  arc  needed,  they  must  in 
any  case  be  hand  braided.  Double  selvedges  can,  however, 
be  avoided  simply  by  taking  up  two  meshes  instead  of  one, 
when  hanging  a  net  to  lines.  This  is  no  longer  a  problem 
in  the  commercial  fishery  in  Japan  as  the  remarkable  increase 
in  the  use  of  knotless  nets  shows. 

Mr.  J.  Buchan  (U.K.).  It  seems  that  there  are  two  basic 
types  of  knotless  nets,  the  "diagonal"  and  the  "zigzag". 

The  twines  in  a  webbing  made  with  conventional  double 
English  knot,  run  in  a  zigzag  line.  There  arc  two  ways  of 
testing  this  knot,  either  by  pulling  the  two  bars  belonging 
to  one  continuing  twine  against  the  two  bars  belonging  to 
the  other  twine,  or  by  testing  the  two  bars  belonging  to 
different  twines  against  the  opposite  pair.  The  same  two 
testing  directions  can  be  applied  for  both  types  of  knotless  net. 

We  have  found  that  the  conventional  double  English 
knot  will  give  an  efficiency  of  54-6  per  cent.,  that  is  the 
breaking  load  of  the  knot  is  54  per  cent,  of  the  total  breaking 
load  of  the  two  twines  in  the  first  direction,  and  51  per  cent, 
in  the  second.  In  the  first  type  of  knotless  net,  that  is  the 


°2  &2  C2          d? 

Fig.  .?.    Zigzag  constructed  knotless  net. 

straight  cross,  you  get  75  per  cent,  and  57  per  cent.,  but  the 
knot  will  break  at  the  weakest  point,  i.e.  57  per  cent. 

With  the  second  type  of  knotless  nets  (that  made  by  double 
inter-weaving  of  twine),  we  obtained  91  per  cent,  and  51 
per  cent.,  showing  that  the  actual  lowest  efficiency  of  the 
knotless  connection  is  not,  in  fact,  better  than  the  conven- 
tional double  English  knot. 

Knotless  nets  are  made  in  a  two- fold  construction,  if 
one  strand  breaks,  the  twine  can  untwist,  causing  break 
in  the  net  running,  which  will  not  happen  with  an  ordinary 
webbing  braided  with  double  English  knot. 

Prof.  A.  von  Brandt  (Germany).  The  fact  that  not  only 
Japan  but  also  Russia  use  knotless  nets  to  a  large  extent 
indicates  to  me  that  there  must  be  considerable  advantages. 
Furthermore,  the  gillnctting  experiments  we  have  carried 
out  with  Japanese  nets  in  Germany  gave  good  results.  We 
had  no  difficulties  with  "knot"  slippage  and  repair,  for  which 
our  fishermen  found  an  even  simpler  way  than  the  one 
recommended  by  the  Japanese. 

In  addition  to  what  we  have  been  told  until  now,  I  believe 
that  knotless  nets  should  have  great  value  for  trawl  nets. 
The  limiting  factor  for  size  of  the  gear  or  towing  speed  is 
the  relation  between  towing  power  and  gear  resistance. 
Dr.  Schiirfe  has  shown  how  the  towing  resistance  can  be 
decreased  by  using  hydrofoil  otter  boards  and  thinner 
synthetic  twine.  The  smaller  area  of  knotless  nets  should 
further  decrease  the  towing  resistance.  This  would  be  a 
valuable  gain  in  addition  to  the  savings  in  material  resulting 
in  lower  weight. 


[112] 


Section  4:  Net  Preservation. 


DEVELOPMENT  OF   FISHING  NET   AND   ROPE 
PRESERVATION   IN   JAPAN 

by 
SH1GENE  TAKAYAMA  and  YOSHINORI  SHIMOZAKI 

Tokai  Regional  Fisheries  Research  Laboratory,  Tokyo,  Japan 

Abstract 

This  paper  deals  with  the  methods  used  in  Japan  for  preserving  different  kinds  of  fishing  gear.  The  methods  which  arc  described 
are  as  follows:  (a)  Sunlight  disinfection;  (b)  Preservation  by  Copper  Sulphate:  (c)  Copper  Naphthenate  preservation;  (d)  Tannin  preservation: 
(e)  Bichromate  treatment  after  tannin;  (f)  Coal  tar  and  (g)  Preservation  by  Cyanoethylation.  These  processes  are  described  in  detail  and  a 
comparison  of  their  relatixc  efficiencies  is  shown  in  tabular  form.  The  materials  concerned  in  the  investigation  arc  Cotton  and  Abaca. 


Resume 


La  preservation  des  cables  ct   filets  de  peche  au  Japon 


Les  auieiirs  exposent  les  mcihodcs  appliquees  au  Japon  pour  la  preservation  des  dirferents  types  d'cngins  dc  peche.  Ces  methodes 
sont  les  suivantes:  (a)  desmfection  par  exposition  jinx  rayons  du  solcil;  (b)  preservation  par  traitemeni  an  sulfate  de  cuivre;  (c)  preservation 
par  traitcmcnt  au  naphtenate  de  cuivre;  (d)  preservation  par  traitement  au  tanin;  (e)  traitement  au  bichromate  apres  tannage;  (e)  traitement 
au  goudron;  et  (g)  traitement  par  cyanoethylalion.  Les  auteurs  font  unc  description  detaillec  de  ces  procedes  ct  comparent  sous  forme  de 
tableaux  leurs  eflicacites  respect ives  I  es  essais  ont  porte  sur  k*  colon  et  r abaca. 

Dcsarrollo  de  la  red  de  arrastre  >  preservation  de  cuerdas  en  el  Japon 
Ivx  tract  o 

Lstc  tr.ibajo  irata  de  los  mcthodos  usados  en  el  Japon  para  preservar  diversos  tipos  de  urtcs  de  pcsca,  a  saber:  (a)  desmfcccion  con 
lu/  solar;  (b)  preservation  con  sulfato  de  cobrc;  (c)  preservation  con  naftenato  dc  cobre;  (d)  preservacion  con  tanino:  (e)  tratamiento  con 
iiicromatos  despucs  dc  la  entmtadura  mediante  cxtractos  curticntcs:  (f )  alquitrande  hulla,  y  (g)  preservation  mediante  cianoetilacion.  T.stos 
tratammetos  sc  dcscriben  en  detalle  y  se  compendia  en  tablas  su  eficacia  relativa.  Fn  la  investigation  efectuada  se  estudiaron  materiales 
dc  algodon  v  abaca 


INTRODUCTION 

THE  various  preservation  methods  of  fishing  nels 
and  ropes  may  he  grouped,  according  to  then 
respective  functions,  into  three  categories: 

1.  Sterilization  to  destroy  putrefactive  bacteria,  either 
by   sunlight-drying   after   boiling,    copper   sulphate 
bathing,  or  copper  nuphthcnate  bathing. 

2.  Protection   from   bacterial   putrefaction    by   coating 
the  natural  fibres  with  a  film  of  cither  tannin,  coal-tar 
or  tannin  and  coal-tar. 

3.  Combination  of  these  two  methods,  using  copper 
naphthenate  and  coal-tar  dyeing. 

Most  of  these  methods,  while  they  are  distinguished 
from  the  sun-drying  in  which  no  preservatives  are  used, 
allow  the  materials  to  be  dried  in  the  sunlight  after 
treatment.  However,  the  materials  treated  in  copper 
sulphate  bathing  and  the  solution  itself  must  not  be 
exposed  directly  to  the  sunlight  or  air.  The  copper 
naphthenate  treatment  or  its  combination  with  coal-tar 
coating  recently  introduced  in  Japan  is  still  in  the 
experimental  stage. 

When  a  fisherman  chooses  one  of  these  procedures, 
he  has  to  consider  not  only  the  preserving  effect  which 


differs  for  each  method  bin  also  the  most  suitable  one 
for  his  particular  type  of  fishing. 

PRESERVATION  BY  STERILIZATION 

Sunlight  disinfection:  Infectious  micro-organisms  cannot 
survive  against  heat  or  dryness,  and  sunlight  disinfection 
of  fishing  nets,  therefore,  is  practised  all  over  the  world, 
though  only  applicable  as  a  supplementary  method  at 
certain  intervals  between  fishing  operations  or  seasons 
(Table  I). 

Sunlight  disinfection  for  non-dyed  nets:  Since  colour 
is  inseparable  from  dyeing  for  some  fishing  gear  in  Japan, 
non-dyed  cotton  nets  are  preferred  i.e.,  the  boat  seine 
("bacchi-ami")  in  Ise  Bay,  the  purse  seine  for  post  larval 
anchovy,  and  the  small  trawl  used  from  sailing  boats 
("utase-ami")  in  the  Seto  Inland  Sea.  The  nets  are  simph 
dried  in  the  sun  or  in  the  shade  after  every  operation, 
mostly  one-day  trips.  In  addition,  the  nels  are  sterilized 
by  periodic  boiling.  As  this  method  is  not  very  effective, 
the  nets  are  liable  to  decay  in  a  short  period.  The  only 
alternative  is  to  adopt  white  synthetic  fibres  and  for  these 
reasons  Vinylon  is  now  being  used  to  some  extent. 


[  113  1 


MODERN     FISHING     GEAR     OF    THE     WORLD 

TABLt     I 

Specifications  for  Various  Preservations  of  Fishing  Nets  and  Ropes 


Kind  of  preservation 


Type  of  gear 


Kind  of  Formula  of  Bath       Time  neede<l  Redyeing 

fibres  preservatives  temperature      for  hath         fishing  season 

ro 


inc'™e 
1   "' 


treated 
{years) 


1 .  Copper  sulphate 


Small  round  hauls  Cotton 


2.  Copper  naphthenatc 


3.  Sunlight 

disinfection 

4.  Tannin  coating 


5.  Improved  tannin 

coating 

6.  Coal  tar  coat  ing 


7.  Tannin  and  coal  tar 

coating 

8.  Resin  and  coal  tar 

coating' 


Float  lines, 
lead  lines  and 
ropes 


A  haca 


Boat  seines,  and      Cotton 
sailing  trawl  nets 


Selnets 

Round  hauls  and 
stick-held  nets 


Cotton 

and 

abaca 
Cotton 


0*  1%  for  dyeing  bath  Normal  Several  hrs. 
(0-3°,,  if  sea  water  is 

used). 
0-01%forsterili7ing 

bath 
(0-03",,  if  sea  water  is 

used) 


Initial  onl> 

12  hrs.  between  trips     S 


I5'0°u  copper 
naphthcnate 
0-2"o  sterilizer 
84 1>0  solvent 
0-8",,  miscellaneous 


3",,  cutch  or  tannin 
extract  solution 
Same  as  above 


Setnets,  round 
hauls  and  stick- 
held  nets 

Cotton        Apply  No.  4,  then 
and              fix  in  1%  K2Cr2O7 
abaca          or  Na2Cr2O7  bath 

Trawl  nets 
Setnets 

Abaca                       — 
Cotton  and 
abaca 

Setnets 


Setnets 


Same  as  4  plus  6 


10-15  min.     Once  a  season  12 


Between  operations      0 


Boil  ing  for  2  hrs.  then 
cooling  for  12  hrs. 

For  abaca,  normally 
impregnate  only  12 
hours 

Normal       1-2  hrs. 


30    40  C  5  10  mm. 
30°  40X7.  5- 10  min. 


Same  us  4  Same  as  4 
plus  6          plus  6 


Once  two  weeks  with  3-5 
weekly  sun  drying 

Once  two  weeks  3-5 

with  daily  sun 
drying 

Once  4  weeks  for        3-5 
each  with  sun  drying 
fortnightly 

Once  three  months      130 
Once  three  months      130 

with  sun  drying 

monthly. 


After  treating  with  a     Normal 
synthetic  resin,  treat 
with  a  specific  coal  tar 


Once  5  months 
with  tar  dyeing 

Same  as  abo\e 


150 


1 50 


3-4 
3-4 

1-2 


2-3 
2-3 

3-5 


2 
3-4 


4-f> 
5-6 


*  Commercial   dye   H, 


H2,  for  which  see  footnote  of  Table  II. 


Preservation  by  copper  sulphate:  In  the  Shizuoka  and 
Wakayama  Prefectures  preservation  of  fishing  nets  by 
copper  sulphate  is  more  prevalent  than  in  any  other 
fishing  community  in  Japan.  With  this  treatment  a  life 
of  three  to  four  years  is  obtained.  The  nets  are  mostly 
made  of  cotton.  They  are  used  for  sardine,  anchovy, 
(and  their  post  larvae  called  "shirasu"),  mackerel  and 
jack  mackerel. 

(1)  Dyeing  bath:  Repeated  treatment  is  necessary  for 
sufficient  preservation  by  copper  sulphate  solution.  The 
dyeing  bath  needs  the  strongest  concentration  with  0- 1 
per  cent,  in  the  case  of  fresh  water  and  0-3  per  cent,  for 
sea  water1. 

The  net  is  soaked  for  several  hours.  In  sea  water,  the 
copper  sulphate  turns  to  basic  copper  chloride.  If  left 
exposed  long  to  the  sunlight  and  air,  this  basic  copper 
chloride  would  deteriorate  the  net.  The  net,  therefore, 
has  to  be  kept  wet  after  the  dyeing  bath  and  put  into 
use  as  soon  as  possible. 


(2)  Sterilizing  baths:  Following  the  dyeing  bath,  the 
net  is  sterilized  after  each  fishing  operation.  The  con- 
centration should  be  0-01  per  cent,  in  fresh  water  and 
0-03  per  cent,  in  sea  water.  In  practice,  the  solution  for 
the   sterilizing   baths   may   be   prepared    by   using   the 
remaining  solution  of  the  dyeing  bath.   Copper  sulphate 
is  added  according  to  the  amount  of  water  needed  to 
replace  that  consumed.  A  simple  method  for  determining 
the  concentration  of  CuSO4  in  the  serving  solution  has 
been  established  by  Kanna  and  Matsumoto2.     Soaking 
should  last  for  at  least  several  hours.  The  optimal  length 
of  time  has  not  been  ascertained. 

(3)  Caution:  The  net  should  always  be  kept  wet.  Do 
not  heat  the  bath  solution  as  in  cutch  dyeing.  The  solution 
can  be  prepared  at  ordinary  temperatures  in  any  season. 

(4)  Storage:   Keep  a  treated   net   under  sterilizing 
solution  (0-005  to  0-001  per  cent.)  and  cover  it  with  a 
straw  mat  or  canvas.  When  the  net  has  to  be  stored  in 


[1141 


JAPANESE     METHODS     OF     PRESERVATION 


0.1   - 


20 


40 


60 


80 

Days 


100 


120 


14O 


160 


f!*  I-     CluiHgi'  in  breaking  \trcnaih  <>/  the  cotton  twitu^  MthwergctJ  in  \eu  waiei .     The  test  \va\  commencctf  in  Dcicmhcr  1955. 

7  .      OrizuHil   breaking   strength. 

niv   \trentfth  ttf'ter  Mibnierg'ni*  in  the  sett. 

f  115  ] 


MODERN     FISHING     GFAR     OF     THE     WORLD 


0.1  - 


20 


40       60 


80 
Days 


100      120      140      160 


Fig.  2.     Change  in  breaking  strength  of  the  man  I  la  twines  submerged  in  \ea  \\-atet . 

Tt .     Original  breaking   strength. 

T.     Breaking  strength  after  submerging  in  the  sea. 


116  ] 


JAPANESE    METHODS    OF    PRESERVATION 


TABLI  II 
Specification  of  the  Treatments  shown  in  FIRS.  1  and  2 


First  bath 


Second  bath 


Third  bath 


Cutch  (3",,) 


HI 
I, 

B 
C 

H, 
Coal  tar 

Coal  tar  <8()"()) 
Gasoline  (20",,) 

D, 
B 


K2Cr.2O7  or  Na2CY2<>7  (I",,) 

Coal  Tar 
Coal  tar (80%) 
Gasoline  (20",, 
Coal  tar 

Coal  tar  (80"0) 
Gasoline  (20°,.) 

H., 


7 
X 

10 
11 
12 
13 
14 

Gasoline  (20",,) 

15  D,  IX 

16  Ji £°^LliM 

H,    contains  melanin  resin  etc.;  H2  a  kind  of  coal  tar  dye. 

B      contains  copper  naphlhcnate,  synthetic  disinfectant  3DM,  etc., 

to  he  used  in  dyeing  with  vacuum  compressors. 
C1      has  the  same  formula  as  B  to  be  mixed  with  mineral  oil  when 

the  twine  is  twisted. 
D,     mainly  consists  of  cutch;  and  D.,,  CuSO4,  alum,  K.,Cr.,O7,  or 

Na2Cr2O7,  etc. 
l-j     contains  cutch,  tannin,  and  persimmon  tannin;  F.,.  Nu2Cr2O7, 

SiO2,  etc.,  E3,  coal  tar  and  resins. 

The  trade  names  of  commercial  dyes  produced  h\  different 

companies.    Their  addresses  arc  in  possession  of  the  authors 


the  air,  wash  out  the  dye  residue  with  fresh  water,  if 
available,  or  otherwise  with  sea  water.  After  drying  in 
the  shade,  store  it  in  a  cool  dark,  airy  place.  The  value 
of  sprinkling  malt  over  the  net,  as  is  sometimes  done  for 
storing,  has  not  yet  been  confirmed. 

(5)  Life  of  the  treated  net:    The  net  treated  as  above 
may  last  for  more  than  three  years,  serving  in  every 
fishing  season.  However,  one  drawback  is  that  the  treat- 
ment is  not  applicable  to  those  nets  in  continuous  use  in 
the  sea  or  in  high-sea  operations. 

(6)  Dyeing  and  preserving  mechanisms:    It  is  under- 
stood that  CuSO4  in  the  net  reacts  to  sea  water  on  and 
between  the  net  fibres.  It  first  turns  into  a  blue  hydrophile 
colloid  precipitation,  viz.,  basic  copper  carbonate,  which 
again  reacting  to  sea  water,  gradually  changes  into  basic 
copper  chloride,  a  light-greenish  hydrophobic  crystal, 
which  is  precipitated  on  the  fibres.  Since  the  precipitation 
is  not  so  effective  against  micro-organisms  as  CuSO4, 
the  net  has  to  be  subjected  to  the  sterilizing  bath  imme- 
diately after  every  operation. 

COPPER   NAPHTHENATE   PRESERVATION 

Due  to  its  high  cost  and  the  progress  made  in  other 
types  of  dyestuffs,  it  was  not  until  1951  that  copper 
naphthenate,  as  a  preservative  of  fishing  nets,  attracted 


the  attention  of  technologists  and  consumers  in  Japan. 
Excellent  preserving  qualities  are  obtained  when  copper 
naphthenate  is  mixed  with  some  other  sterilizers,  at  least 
in  respect  of  abaca3.  This  dyestuff  can  be  applied  in  a 
quick  and  simple  way  without  affecting  the  properties 
of  the  treated  net. 

(1)  Treatment:      More   than    90    per   cent,   of  the 
copper  naphthenate  dye  consumption  in  Japan  consists 
of  three  different  products — called  A,   B,  C,*  for  the 
purposes  of  this  paper. 

Notwithstanding  an  inferior  preserving  effect  upon 
cotton  or  ramie  twines,  they  are  used  mostly  in  dyeing 
fishing  ropes  made  of  Manila  and  Sisal,  such  as  float 
lines  for  driftnets.  Three  different  treatments  are  used 
to  apply  these  dyes  for  nets  and  ropes. 

When  a  fisherman  is  about  to  dye  his  material  with 
either  A  or  B,  the  preservative  is  diluted  at  a  normal 
temperature  with  kerosene,  as  specified  for  the  product. 
After  soaking  for  15  to  20  minutes,  the  material  is  dried 
in  the  sun  or  in  the  shade. 

In  the  case  of  net  dyers  and  net  makers,  the  material 
may  be  treated  in  an  autoclave.  After  vacuumizing,  the 
solution  is  put  into  the  kettle,  and  the  material  is  then 
treated  at  about  15  to  20  Ib./sq.  in.  of  pressure  for  a  few 
minutes  before  being  taken  out  for  drying. 

In  a  rope  factory,  copper  naphthenate  solution 
prepared  as  above  with  the  mineral  oil,  is  usually  applied 
to  the  strands  of  twines  or  ropes,  while  they  are  being 
twisted;  thus,  the  rope  is  dyed  at  the  same  time  as  it  is 
manufactured. 

(2)  Preserving  effect:  Vacuum-compressing  is  likely 
to  produce  the  best  results  of  any  of  these  methods 
(samples   10  and    1 1    in   fig.   2).  The   first   method   is, 
however,  as  effective  as  the  second,  if  the  soaking  time 
is  more  than  five  hours4.    When  compared  with  cutch 
treatment  which  is  most  commonly  used  in  Japan,  the 
effect  of  this  preservative  has  been  found  superior,  when 
applied  to  abaca,  but  inferior  when  applied  to  cotton  or 
ramie  (fig.  4). 

(3)  Caution:   Since   these  preservatives   are   inflam- 
mable due  to  the  kerosene  used  as  the  solvent,  special 
care  is  needed.  Such  nets  are,  however,  less  likely  to 
suffer  damage  by  rats  or  animals  during  storage  than  nets 
dyed   by   other  means.      It   is   recommended   that   the 
concentration  of  the  dye  should  secure  more  than  0-5 
per  cent,  of  copper  to  fix  on  the  material. 

(4)  Dyeing    and     preserving    mechanisms:    In    per- 
forming its  function,  copper  naphthenate  is  not  supposed 
to  require  chemical  combination  with  any  other  clement. 
Instead,  it  is  precipitated  on  the  surface  of  the  fibres 
and   penetrates   between  and   inside  them,   destroying 
infectious  microbes  that  are  mainly  on  the  surface  of  the 
material. 

(5)  Trend  in  technological  studies  of  the  preservation: 

Synthetic  fibres,  although  used  increasingly  by  Japanese 
fishermen,  will  not  take  the  place  of  natural  fibres  for 


A:    "Kanadem  42",  composed  of  copper  naphthenate.  synthetic 

sterilizer,  and  synthetic  resin  acid. 
B:     "Ropelife",    composed    of  copper    naphthenate,    organic 

preservative,  and  water-proofer. 
C:     "Shin  Asanoha",  mineral  oil  mixed  with  A  or  B. 


[117] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


1.0 


0.9   _ 


0.8 


0.7   - 


0.6   - 


0.5      - 


0.4   - 


0.3   - 


0.2   ~ 


0.1   - 


20       40 


8%) 


60      80 

Days 


100      120      140 


/'if.  3.     Change  in  breaking  strength  of  the  manila  twine*  treated  with  different  concentrations  oj  copper  nti/>hthcn<ite. 

Tf,.     Original  breaking  strength. 

T.     Breaking  strength  after  submerging  in  the  sea. 


[118) 


JAPANESE     METHODS     OF     PRESERVATION 


1.0 


0.9  . 


0.8 


0.7  _ 
I 


0.6[ 


b 
c 
d 


0.5 


10 


20 


30  40 

Days 


60 


70 


Fig.  4.     (Change  in  break i up  strength  ol  the  manila  twines  treated  with  copper  naph thenate  dyes •/«••*/*<// tv/  //•<»/;/    \   eaih  <>/  \\liich  has 

quantities  of  fungicidal  agent. 

r  .     Onyinal  breaking  strength. 

T.     Breaking  st  length  after  submerging  in  I  he  sea. 


some  time,  on  account  of  the  great  difference  in  price, 
particularly  between  synthetic  fibres  and  abaca,  the 
material  used  for  ropes  and  twines.  For  this  reason,  the 
adoption  of  synthetic  fibres  for  fishing  ropes  has  not  yet 
made  great  progress.  Further  studies  should  be  made  on 
the  technological  aspects  of  copper  naphthenate  preserva- 
tion, its  application  to  abaca  described  above  being  an 
example  of  the  work  already  done  (samples  10,  11,  figs.  2, 
3  and  4). 

In  a  recent  study,  twines  treated  in  the  various  con- 
centrations were  submerged  in  sea  water.  The  results 
revealed  that  the  preserving  effect  depends  on  the 
concentrations  as  shown  in  fig.  3,  in  which  the  numerals 
enclosed  in  parentheses  indicated  the  amount  of  pre- 
cipitant, Cu. 

Among  the  sample  twines,  each  weighing  approximately 
48-7  g./m.  1,  2,  and  3  were  washed  in  running  water  for 
5  hours,  dried,  then  soaked  for  12  hours  in  40  per  cent., 
30  per  cent,  and  20  per  cent,  solutions  of  A  respectively; 
sample  4,  without  washing,  was  soaked  with  20  per  cent, 
solution  of  A.  The  samples  were  then  submerged  in  sea 
water  for  about  20  days  before  testing  the  remaining 
strength.  As  shown  in  fig.  3,  the  preservative  has  been 
found  effective  in  the  decreasing  order  of  concentration 
of  40  per  cent.,  30  per  cent.,  and  20  per  cent,  with  the 
precipitant  on  the  samples  rating  0-8  per  cent.,  0-5  per 
cent.,  0-3  per  cent,  and  0-2  per  cent,  of  the  copper. 
Certainly  this  is  suggestive  of  correlationship  between  the 


concentrations  and  the  amounts  of  the  precipitant. 
Despite  the  same  concentration  20  per  cent,  applied  to 
both  samples  3  and  4,  the  preserving  effect  was  much 
greater  in  the  washed  sample  3  than  in  4;  while  no 
appreciable  difference  was  found  in  the  effect  between 
samples  1  and  3,  sample  3  according  to  microscopy,  had 
the  dyeing  solution  soaked  much  deeper  into  the  fibres 
than  sample  4,  which  was  most  likely  responsible  for 
making  sample  3  better  than  sample  4. 

Experiments  to  establish  an  optimal  mixing  rate  for 
securing  the  preserving  at  a  reasonable  cost  by  minimising 
the  amount  of  expensive  sterilizers  to  be  mixed  with  copper 
naphthenate,  were  also  conducted.  Sample  twines,  each 
weighing  approximately  48-7  g./m.  washed  in  running 
water  for  5  hours,  were  subjected  for  another  5  hours  to 
copper  naphthenate  baths,  which  were  prepared  by 
mixing  48  per  cent,  copper  naphthenate,  less  than  0-05 
per  cent,  synthetic  resin,  and  either  2,  1-5,  1,  or  0  per 
cent,  synthetic  sterilizer  3  DM,  and  diluting  to  30  per 
cent,  with  kerosene.  The  sample  twines  a,  b,  c,  and  d, 
were  made  to  correspond  to  the  different  concentrations 
of  the  synthetic  sterilizer  in  that  order.  After  drying, 
the  samples  were  submerged  in  sea  water  and  the 
remaining  breaking  strength  tested  at  intervals  of  about 
three  weeks.  Fig.  4  is  indicative  of  efficiency  of  the 
synthetic  sterilizer  used,  since  all  the  samples  but  d  had 
fairly  good  results.  Comparing  a,  b,  and  c,  one  may 
notice  that  the  first  two  with  higher  mixing  rates  were 


119  ] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


better  than  c  until  the  42nd  day  of  the  submergence, 
while  little  or  no  difference  was  apparent  from  the  67th 
day  onward.  In  other  words,  after  about  60  days  of 
immersion,  the  precipitation  of  the  synthetic  sterilizer 
decreases  nearly  to  the  same  extent  among  the  three 
samples,  and  this  in  turn  depletes  their  strength  in  the 
same  degree.  These  findings  may  warrant  the  necessity  of 
carrying  out  redyeing  of  fishing  gear  after  about  that 
period  of  use. 

In  respect  of  the  relationship  between  the  preserving 
effect  and  the  length  of  time  for  immersion,  preliminary 
experiments  showed  no  discrepancies  between  sample 
ropes  kept  for  various  durations  over  five  hours  in  the 
dyeing  bath  and  those  treated  in  the  vacuum-cofnpressor. 
More  detailed  experiments  are  under  way. 

TANNIN  PRESERVATION 

The  commonest  types  of  cutch  used  in  Japan  are  dis- 
tinguished from  each  other  by  their  trade  names,  B,  R, 
and  T.  In  addition,  considerable  amounts  of  tannin 
products,  such  as  wattle  and  quebracho  extracts,  arc 
used,  though,  according  to  experiments  by  Kanna, 
there  is  little  difference  in  the  preserving  effect1"1. 

1 I )  Dyeing  bath:    The  cotton  nets  are  first  boiled  for 
one  to  two  hours,  washed  in  fresh  water  and  dried. 
They  arc  then  boiled  with  3  to  4  per  cent,  solution  of 
culch  in  fresh  or  sea  water  for  some  2  hours,  left  in  the 
bath  overnight  for  cooling,  then  dried  in  the  sun  or  in 
the  shade.  Upon  drying,  the  net  is  soaked  once  more  in 
the  same  solution  at  ordinary  temperature  for  one  night. 
A  4  per  cent,  concentration  of  culch  solution  is  the  best 
for  net  preservatives". 

When  dyeing  hemp  twine,  the  same  procedure  as  for 
cotton  materials  can  be  employed,  but  heating  should 
be  avoided. 

(2)  Redyeing  for  various  nets:    The  first  dyeing  bath 
described  above  is  applicable  to  various  small  types  of 
fishing  nets.   In  the  case  of  small-sized  trap  nets  they  arc 
disinfected  by  sun-drying  part  by  part  after  one  week's 
use.   In  addition  to  that  they  are  redyed  with  3  per  cent, 
solution  every  three  weeks  during  the  period  in  use. 

For  other  types  of  net,  such  as  small  purse  seines, 
stick-held  dip-nets  and  beach  seines,  it  is  recommended 
that  they  receive  sunlight  disinfection  after  each  day's 
use  and  redyeing  at  intervals  of  a  week  to  ten  days  in 
3  to  4  per  cent,  solution.  The  life  of  a  small  trap  thus 
treated  may  be  two  or  three  years,  and  of  the  other 
nets,  three  or  four  years. 

BICHROMATE    TREATMENT    OF    TANNIN 
PRESERVATION 

By  impregnating  with  tannin  products  and  consecutive 
sun-drying,  a  thin  film  of  tannic  acid  is  built  up  of  the 
fibres.  This  film  prevents  infectious  bacteria  from  getting 
into  the  fibres.  Under  conditions  in  the  coastal  waters  of 
Japan,  however,  the  acid  film  would  remain  on  a  net  no 
longer  than  one  week,  if  the  net  is  in  continuous  use. 
Therefore,  frequent  re-dyeing  is  needed  to  maintain  the 
strength  of  the  net  which  otherwise  would  quickly 
deteriorate.  Since  one  of  the  remedies,  coal-tar  coating, 
worked  out  in  an  attempt  to  prolong  the  life  of  big  set- 


nets  and  such  gear,  has  not  yet  been  made  free  from 
certain  disadvantages,  another  improvement  has  come 
into  being  in  the  form  of  either  potassium  bichromate 
or  of  sodium  bichromate. 

(1)  Treatment:    A  net  thoroughly  impregnated  with 
tannin  solution  in  the  same  way  as  described  above,  is 
again  soaked  in  1  per  cent,  of  solution  potassium  bichro- 
mate or  sodium  bichromate  at  ordinary  temperature  for 
one  or  two  hours.  The  chemicals  may  be  dissolved  by 
heating,  but  the  dyeing  bath  must  not  be  heated.  After 
the  two-hour  immersion  and  subsequent  washing,  the  net 
is  ready  for  use  or  it  can  be  dried  for  storing7. 

A  defect  of  this  treatment  is  that  it  needs  a  considerable 
amount  of  labour,  and  is  costly  and  time  consuming,  but 
this  may  be  offset  by  the  prolonged  life  of  the  net,  as 
tannic  acid  fixed  by  the  bichromate  makes  it  possible  for 
the  net  to  be  used  for  twice  as  long  as  a  net  dyed  in  a 
conventional  tannin  bath  (samples  1  and  2,  figs.  1  and  2). 

(2)  Commercial  dyes  with  tannin  fixation:     Among 
several  makes  of  commercial  dycstuffs  developed  from 
the  principle  of  tannin  fixation,  D*  and  E+  showed  the 
best  results  (samples  8  and- 15,  fig.  2). 

The  product  D,  good  for  preserving  abaca  materials, 
has  been  adopted  with  success  for  such  gear  as  sctnets 
which  remain  in  the  sea  for  prolonged  periods.  However, 
a  net  must  be  put  into  service  soon  after  D  treatment; 
if  not,  it  is  liable  to  suffer  from  the  quantities  of  copper 
sulphate  contained  along  with  the  sodium  bichromate 
in  the  product.  Product  E,  comprising  fine  grains  of 
silicic  acid  in  the  fixative,  gave  excellent  results,  parti- 
cularly when  applied  to  cotton  twine  (footnote  of 
Table  II). 

COAIXIAR   DYEING    AND  ITS  MODIFICATION 

(1)  Evaluation:  Coal-tar  treatment  shows  greater 
preservability  than  most  of  the  other  dyes  reported 
above  samples  (13  and  14),  fig.  1,  due  to  the  heavy 
coating  on  the  fibre.  A  disadvantage  is  that  the  net 
becomes  uncomfortably  sticky  and  sometimes  too 
heavy  for  efilcient  manipulation.  In  addition,  this  would 
often  result  in  deteriorating  the  structure  of  fibre  to  a 
critical  extent,  and  increasing  the  weight  so  much  that 
nets  such  as  set  nets,  run  a  risk  of  being  lost  in  an 
abnormal  current  or  rough  sea. 

(2)  Treatment:  Formerly  coal-tar  immersion  was 
carried  out  at  100  deg.  C.  or  higher  for  5  to  10  minutes. 
However,  recent  experiments  have  established  that  the 
temperature  need  not  be  higher  than  about  60  deg.  C. 
for  cotton  twine,  or  30  to  40  deg.  C.  for  abaca  nets  and 
ropes,  the  time  required  being  around  10  minutes  for  both. 
The  fractional  distillation  of  coal  tar  can  now  be  con- 
tinued further  than  before,  making  possible  the  utiliza- 
tion of  the  resulting  volatile  preservatives;  this  would 
evaporate  if  heated  to  a  higher  temperature. 

When  either  creosote  or  heavy  oil  is  used  as  diluent  to 
minimize  the  disadvantages,  the  former  tends  to  acceler- 


*  "Horyo  Senryo"',  requiring  3  per  cent,  cutch  solution  for  the 
first  bath,  contains  K2  Cr2  O7  copper  sulphate,  and  alum  for 
the  second  bath. 

t  "Aritoku  Tannin'*,  requiring  3  per  cent,  cutch  tannic  acid 
solution  for  the  first  bath,  contains  K2  Cr2  O7,  silicic  acid  and 
tartar  emetic  for  the  second  bath. 


[1201 


Treatment 


JAPANESE    METHODS    OF    PRESERVATION 

0 


Ratio  of  fouling  (%) 
10 
1 


20 


30 


Copper  naphthenate  dye  A      A 
Coal  -  tar 


Manila  twine 
Cotton  twine 


Copper  naphthenate  dye  B 
Coal  -  tar 


Cutch 

Potassium  Bichromate 
Coal  -  tar 


Cutch 


Coal  -  tar 


SSSKKSS^^ 


Fig.  -5       Ratio  of  average  weigh/  of  marine  plants  grown  on  the  treated  twines  during  60  day**  sea  submerging  to  the  weight  of  the  new  twines 


ate  the  tar  deterioration,  while  the  latter  would  prevent 
the  net  from  drying  up. 

(3)  Modified  Coal-Tar  Treatment:     Further  experi- 
ments in  coal-tar  preservation  showed  that  re-treatment 
of  a  cutch-dyed  set  net  with  coal-tar  or  diluted  coal-tar 
could  extend  the  life  of  the  net  from  three  to  four  years  to 
four  to  six  years  (samples  3  to  7  and  16,  figs.  1  and  2). 

Commercial  dyes  D  and  E  mentioned  above,  are 
successful  applications  born  respectively  from  these 
ideas  along  with  their  special  formulae.  Tables  1  and  II 
give  the  relative  merits,  as  preservatives  of  coal-tar, 
diluted  coal  tar  and  some  other  dyes. 

(4)  Maintenance:      Creosote   or   heavy-oil   dilution 
containing  40  to  80  per  cent,  coal-tar  is  commonly  used  for 
the  re-dyeing  bath  which  a  setnet,  for  instance,  requires 
only  every  80  days.     Meanwhile,  sunlight  disinfection 
should  be  applied  to  the  net  two  to  three  times  between 
the  baths.  Re-dyeing  for  a  trawl  net  is  required  after  two 
or  three  operations  covering  60  to  90  days;  for  round 
haul  nets,  it  is  only  necessary  after  each  fishing  sesson. 

The  ideal  storehouse  for  the  treated  net  must  have  an 
elevated  floor  in  order  to  secure  enough  ventilation  with 
the  lowest  degree  of  humidity.  Salt  sprinkled  between  the 
layers  of  a  folded  net  would  partially  check  growth  of 
micro-organisms  and  the  deterioration  by  tar. 

(5)  Trends  in  technological  research:    Technological 
research  in  coal-tar  preservation  aims  at  development  or 
improvement    of  (1)    the   diluted   coal-tar   treatment; 

(2)  the  protection  of  nets  against  foul  organisms,  and 

(3)  a  single  dyeing  bath  treatment.   In  the  case  of  cotton 
twine,  experiments  have  shown  more  or  less  satisfactory 
results  with  regard  to  the  first  two  questions.  However, 
much  remains  to  be  learnt  in  connection  with  abaca 
material  as  well  as  the  single  dyeing  bath8  (Table  II; 
samples  4,  6,  9,  16,  figs.  1,  2;  samples  5,  fig.  3;  fig.  5). 

It  has  also  been  found  that  coal-tar  treatment  following 
the  copper  naphthenate  bathing  of  cotton  and  abaca 


materials  is  effective  against  foul  organisms.  The  twine 
samples  were  submerged  in  the  sea  and  controls  were 
carried  out  at  20-day  intervals  to  dry  and  assess  the 
amount  of  foul  organisms,  animals  by  number  and 
plants  by  weight.  Fig.  5  shows  the  average  percentage 
increase  of  weight  comparable  by  group  of  samples,  on 
the  basis  of  the  total  weight  of  plants  found  at  the  end 
of  about  a  two-month  period.  The  results  obtained  were 
nearly  the  same  for  both  the  plants  and  the  animals. 
Although  the  treatment  was  also  found  effective  for 
preserving  the  net  (sample  16,  fig.  2),  further  research 
is  under  way  to  obtain  a  similar  result  by  a  single  bath 
instead  of  the  double  bath  normally  required. 

These  procedures  are  based  on  sterilization,  and  pre- 
clusion of  micro-organisms  and  any  technological 
advancement  of  fishing  gear  preservation  is  expected  to 
proceed  in  these  directions. 

PRESERVATION    BY    CYANOETHYLATION 

In  regard  to  the  preservation  of  salmon  gillnets  made 
of  ramie,  twine  treated  with  acrylonitrile  was  compared 
to  those  treated  with  cutch  and  sodium  bichromate,  by 
submerging  the  samples  in  the  sea  for  50  days.  The 
breaking  strength,  tested  at  10-day  intervals,  proved  that 
the  former,  although  it  had  a  lower  initial  strength 
because  of  cyanoethyli/ing,  can  be  used  for  a  much 
longer  period  (Table  IIT). 

Usually,  the  extent  of  cyanoethylation  is  indicated  by 
the  amount  of  nitrogen  contained  in  the  twine,  though 
it  is  difficult  to  determine  exactly  to  what  extent  it 
really  is  cyanoethylated. 

If  the  nitrogen  content  is  2  •  5  per  cent.,  as  in  the  present 
experiment,  the  treatment  would  cost  nearly  twice  as 
much  as  the  cutch  treatment  or  about  0-22S  U.S.  1  Ib. 
of  the  material.  However,  treatment  in  bulk  reduces  the 
cost  to  a  level  within  the  means  of  the  fishermen. 

Caution:     In  using  acrylonitrile,  workers  should  be 


[121  ] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


TABLE  III 

Decrease  in  Breaking  Strength  of  Ramie  Thread  (17  counts,  5  plies ) 
(Submerged  in  the  sea  for  10  to  50  days) 


~,-  _  t-.t-Mi       Breaking  strength  (kg),  assessed  at  10  days  intervals 
i  wine  tested  -        10  days   2Q  days  3Q  days  ^  days  5Q  days 


1-0 


25-4        11-9        8-1 


Non-treated 

Treated  with 

cyanoethylene        18-6        19-8      21-7      21-5       17-4       10-5 

Treated  with  cutch 

3  times,  then  with 

sodium 

bichromate  206         19-2       19  6        9-0 


protected  from  the  hazards  detrimental  to  their  health. 

SUMMARY 

1.  Disinfection  by  sun-drying  and  boiling  is  applied  to 
certain  types  of  nets  which  are  used  without  dyeing 
because  of  reasons  pertinent  to  their  operation. 

2.  Preservation  of  nets  and  ropes  by  copper  suJphate 
is  recommended  for  fisheries  using  sardine  purse  seines 
and  beach  seines,  which  are  based  near  the  shore  and  the 
gear  can  therefore  be  retreated  daily.    The  first  dyeing 
bath  requires  several  hours  immersion  in  0-1  per  cent, 
copper  sulphate  solution  in  case  of  fresh  water  or  0-3 
per  cent,  solution  in  case  of  sea  water.    After  a  day's 
operation,  the  net  has  to  be  sterilized  in  0-01  per  cent, 
solution  (fresh  water),  or  0-03  per  cent,  solution  (sea 
water).  A  suitable  amount  of  the  solution  should  be 
added  to  the  re-dyeing  bath  from  time  to  time  to  secure 
a  constant  concentration.     Tn  no  case  should  a  net 
treated  with  copper  sulphate  be  exposed  to  the  sunlight. 

3.  Copper   naphthenate   has   a   greater   preserving 
effect  when  mixed  with  organic  sterilizers  and  similar 
dyes.  The  dye  is  not  so  effective  for  cotton  or  ramie 
materials  as  for  abaca.     There  should  be  more  than 
0-5  per  cent,  precipitation  of  copper  on  the  materials 
in  all  cases. 

4.  The  optional  concentration  of  cutch  is  3  to  4 
per  cent.    Application  of  fixative  such  as  K2Cr2O7  or 


Na2CrX)7  can  double  the  life  of  the  cutch-treated 
materials.  Among  several  commercial  dyes  produced  on 
the  principle  of  the  tannin  fixation  bichromates,  those 
containing  copper  sulphate  as  a  fixative  are  good  for 
preserving  abaca  materials,  and  those  containing  tartar 
emetic  or  silicic  acid  powder  are  suitable  for  cotton. 
No  appreciable  efficiency  has  been  noticed  with  resin 
dyes. 

5.  Despite  its  remarkable  qualities  as  a  preservative, 
coal  tar  has  the  disadvantage  of  making  nets  sticky, 
heavy  and  fragile.     Diluted  coal-tar  treatment,  which 
could    lessen    these   disadvantages,    still    needs   to    be 
improved.  Experiments  show  that  gasoline  is  a  better 
diluent  than  creosote  or  heavy  oil.  The  more  coal  tar  is 
diluted,  the  lower  its  value  as  a  preservative.    However, 
pretreatment  by  formulae  1,2,  8,  or  12  in  Table  II  has 
been  found  to  improve  greatly  the  preservation  of  a  net 
when  treated  v\ith  diluted  coal  tar. 

6.  It  is  most  important  to  preclude  foul  organisms, 
such  as  seaweeds  and  shells,  from  growing  on  setnets, 
etc.  Nets  treated  first  with  copper  naphthenate  and  then 
with  coal  tar  were  proved  to  be  markedly  free  from 
infectious  microbes  and  foul  organisms.   Research  to 
secure  a  similar  effect  by  a  single  dyeing  bath  is  no* 
under  way. 

7.  Ramie   netting  twines  treated   with   i»crylonitrile 
agent  and  submerged  in  sea  water  indicated  that  the 
treatment    made    the    material    strongly    resistant    to 
micro-organisms,    without    affecting    other    properties 
required  for  fishing. 

REFERENCES 

1  Matsumoto,  J.  :     Unpublished. 

~  Migita,  M.,  Matsumoto,  J.  and  Karma,  K.  :  Studies  on  Fish 
Net  Preservation  by  Copper  Sulphate  I;  A  simple  determination 
of  Copper  Sulphate. 

Bull.  Japan.  Soc.  Sci.  Fish.,  Vol.  18  No.  2  pp.  78-84  (1952). 

3  Shimpzaki,    Y.  :     On     the     Net     Preservation    and     some 
Characteristics  of  Synthetic    Netting   Cords.     "Teich"    No     10 
(1956). 

4  Shimozaki,     V.  :     Unpublished. 

5  Kanna,  K.  and  Migita,  M.  :  Unpublished. 

*  Migita,  M.  :  Fixing  of  Tanned  Nets;  Journal  Fish.  Exp 
St.,  No.  13,  (1943). 

7  Migita,    M.  :     Tanning    Net    in    Sea    Water;   Bull.  Japan 
Soc.  Sci.  Fish..  Vol.  15,  No.  12.  pp.  781-786.  (1950). 

8  Shimozaki.  Y.  :     Unpublished. 


[122] 


ROT-RESISTANT   FISHING  NETS  BY   THE   "ARIGAL"   PROCESS 

by 

A.  RUPERTI 

CIBA  Limited,  Basle,  Switzerland 

Abstract 

Micro-organisms  living  in  water  attack  cellulose,  which  is  the  reason  why  unprotected  cotton  nets  rot.  Synthetic  fibres  are  rot-proof, 
this  being  one  of  the  reasons  for  their  increasing  use  in  net  manufacture.  Impregnation  of  nets  as  carried  out  by  fishermen  does  not  render 
them  durably  rot  proof;  the  treatment  has  to  be  repeated  frequently.  In  the  U.S.A.,  processes  have  been  worked  out  to  produce  rot-proof 
cotton  by  chemical  treatments.  Acetylatcd  and  cyanoethylated  cotton  are  durably  rot-proof,  but  the  chemical  treatments  are  complicated 
and  costly.  CIBA's  Arigal  proofing  process  is  not  based  on  chemical  treatment,  but  on  deposition  of  a  synthetic  resin  in  the  fibre.  The 
process  consists  of  impregnation  with  an  aqueous  Arigal  solution  and  fixation,  without  intermediate  drying.  The  Arigal  is  not  applied  to 
the  finished  nets  by  the  fishermen,  but  to  the  cotton  yarn  in  a  textile  mill.  The  process  is  simple  and  gives  a  degree  of  conservation  superior 
to  that  of  the  chemical  treatments,  and  the  properties  of  the  fibre  are  not  adversely  affected.  Cotton  treated  with  Arigal  exhibits  outstanding 
weather  resistance.  It  may  thus  be  expected  that  the  Arigal  proofing  process  will  find  use  in  the  field  of  net  making,  and  that,  in  all  respects, 
nets  proofed  by  this  process  will  meet  the  stringent  requirements  placed  on  them. 


Resume 


Des  filets  de  peche  imputrescibles  par  le  precede  "Arigal"  de  CIBA 


L'eau  rcnferme  des  micro-organismcs  qui  attaquent  la  cellulose;  c'est  pourquoi  les  filets  de  coton  qui  n'ont  pas  subi  un  traitement  de 
protection,  pourrissenL  Les  fibres  synth&iques  sont  imputrescibles;  c'cst  une  des  raisons  pour  lesquelles  elles  sont  de  plus  en  plus  employees 
dans  la  fabrication  des  filets.  1 /impregnation  telle  qu'clle  est  pratiquee  par  les  pecheurs  nc  confere  pas  aux  filets  une  protection  durable 
contre  la  pourriture,  et  doit  etre  r6pclec  fr&juemment.  On  a  mis  au  point  aux  Etats-Unis  des  traitemcnts  chimiqucs  rendant  le  coton  insensible 
a  la  pourriture.  Le  coton  ac£tyl£  ou  cyano&yle  est  protege  d'unc  (agon  durable,  mais  les  traitements  chimiques  sont  complexes  et  couteux. 
Le  precede  Arigal  de  CIBA  n'est  pas  un  traitement  chimiquc;  il  consiste  &  impregner  la  fibre  de  rcsinc  synthetiquc  au  moyen  d'une  solution 
aqucuse  d* Arigal  ct  a  la  fixer  sans  sechage  intermediate.  L'Arigal  n'est  pas  applique  par  les  pecheurs  aux  filets  fabriques,  mais  sur  Ic  fil  de 
coton  dans  la  filature.  Le  procede  est  saimple  et  assure  un  dcgrt  de  conservation  sup£rieur  a  celui  confer^  par  les  traitements  chimiques; 
en  outre,  il  n'exercc  pas  d'eflel  nuisible  sur  les  proprietes  de  la  fibre.  Le  coton  traite  a  P  Arigal  posscde  une  resistance  remarquable  aux 
conditions  atmosph6riques.  On  peut  done  prevoir  que  le  procedc  Arigal  sera  adopt6  dans  le  domaine  de  la  fabrication  des  filets  et  que  les 
engins  ainsi  trailed  rcsisteront  an  dur  service  qui  leur  est  impose. 

Redes  de  pesca  resistentes  a  la  pudricion  mediante  el  procedimiento  "Arigal"  de  CIBA 
Lxtracto 

La  resistcncia  dc  las  fibras  sinteticas  es  una  de  las  razones  que  han  influido  sobre  la  popularidad  de  us  uso  en  la  manufactua  de 
urtes  de  pesca,  ya  que  los  microorganismos  prescntcs  en  el  agua  atacan  a  la  cclulosa  causando  la  pudricidn  de  las  redes  de  algod6n  sin  tratar. 
La  cniintadura  hecha  por  los  pcscadoras  impide  el  detcrioro  durante  relativamcnte  poco  tiempo,  dcbiendo  rcpetirse  con  frecuencia. 

Ln  los  E.U.A.  sc  han  ideado  procedimientos  quimicos  para  obtener  algodoncs,  acetilados  y  cianoetilados,  que  resistcn  durante 
largo  tiempo  los  efectos  de  las  pudrici6n,  pcro  lienan  el  inconveniente  de  ser  complicados  y  caros.  El  metado  "Arigal"  ideado  por  CIBA. 
no  se  basa  en  la  aplicacion  de  un  proceso  quimico,  si  no  en  el  deposito  de  resinas  sinteticas  en  las  fibras  impregnandolas  con  una  soluci6n  acuosa 
dc  "Arigal"  que  se  fija  sin  necesidad  de  recurrir  a  una  desecaci6n  previa.  Hste  procedimincto,  ademas  de  ser  secnillo,  cs  aplicado  en  la  hilanderia 
a  las  fibras  y  no  por  el  pescador,  a  las  redes,  da  una  preseryaci6n  superior  a  la  obenida  con  procesos  quimicos,  no  afccta  adversamente  a 
.is  propiedades  dc  las  fibreas  y  connmion  al  algod6n  gran  resistencia  a  los  agentes  climAticos. 


A  THOUGH,   in   the    main,   fishing    nets    are    still 
made  from  natural  fibres,  such  as  cotton,  synthetic 
fibres  have  in  recent  years  been  used  in  increasing 
quantities.  Synthetic  fibres  are  very  strong  and  light, 
absorb  very  little  water,  ase  rot-proof  and  have  good 
catching  properties.  Knot  slippage  and  deterioration  by 
weathering  are  for   the   present  still   disadvantageous 
but  efforts  are  being  made  to  counteract  them. 

The  advantages  of  synthetic  fibres  greatly  outweigh 
iheir  disadvantages,  as  is  shown  by  the  enormous 
increase  in  use  of  synthetic  fibres.  According  to  Prof, 
von  Brandt,  more  than  nine  million  pounds  of  synthetic 
fibres,  primarily  vinylon,  were  used  for  net  manufacture 
in  Japan  in  1955,  compared  with  only  110,000  pounds 
used  5  years  ago. 
The  high  price  of  synthetic  fibres  is  a  drawback,  but 


natural  fibres,  although  cheaper,  are  not  rot-proof,  the 
cellulose  being  attacked  by  the  micro-organisms  living  in 
water,  so  that  they  have  to  be  protected.  This  protection 
increases  costs,  thus  reducing  the  price  advantage  which 
cotton  has  over  synthetics  and,  in  some  cases,  making  it 
quite  illusory.  Hitherto,  preservation  has  been  carried 
out  by  the  fishermen  who  use  copper  preparations, 
chrome  compounds,  linseed  oil,  tar  oils,  etc.,  some 
treatments  being  carried  out  at  the  boil.  None  of  these 
treatments  is  very  durable  and  most  have  to  be  repeated 
periodically,  which  adversely  affects  the  nets  and  causes 
shrinkage. 

Methods  for  making  cotton  durably  rot-proof  have 
been  studied  very  intensively  in  recent  years  in  the  U.S.A., 
the  world's  greatest  producer  of  cotton,  and  considerable 
efforts  have  been  made  to  produce  a  rot-resistant 


123] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


cellulose  fibre.  Theoretically,  there  are  two  ways  of 
protecting  the  cotton  fibre  from  micro-organisms.  First, 
the  fibre  can  be  loaded  with  toxic  substances  which  kill 
the  organisms.  This  system  of  active  preservation  is  used 
by  fishermen  e.g.  when  they  impregnate  their  nets  with 
copper  compounds,  but  it  does  not  offer  durable  preserva- 
tion. In  order  to  be  effective,  an  active  toxic  agent  should 
be  able  to  penetrate  into  the  interior  of  micro-organisms, 
and,  to  do  this,  it  should  be  soluble  to  a  certain  degree. 
The  drawback,  is  however,  that  even  partly  soluble 
substances,  are  eventually  washed  out  by  prolonged 
immersion  in  water.  The  second,  and  more  promising 
method,  is  that  of  passive  preservation,  this  being  the 
method  chosen  by  American  research  workers.  Here  the 
fibre  is  chemically  converted  into  a  cellulose  derivative  by 
csterification  or  etherification  and  thus  made  resistant  to 
attack.  The  so-called  chemical  finishing  of  cotton  by 
acetylation  or  cyanoethylation  has  been  carried  out  on 
pilot  plant  scale  in  the  U.S.A.  and  has  been  the  subject 
of  much  discussion  in  recent  years  in  interested  circles. 

The  great  disadvantage  of  chemical  treatments  is  their 
complexity  and  high  cost.  Processes  involving  the  use  of 
volatile  and,  in  some  cases,  toxic  chemicals,  necessitating 
the  use  of  enclosed  equipment  made  of  acid-resistant 
material,  can  be  neither  simple  nor  cheap.  It  is  therefore 
very  doubtful  whether  chemically-treated  cotton  would 
offer  an  appreciable  price  advantage  over  synthetic 
fibres.  This  is  the  reason  why  the  chemical  finishing  of 
cotton  has  not  been  able  to  stem  the  advance  of  the  rot- 
proof  synthetic  fibres. 

C1BA  has  attempted  to  solve  the  problem  of  rot-proof 
cotton  by  a  different  and  considerably  simpler  means. 
By  CIBA's  Arigal  process  [see  A.  Ruperti,  Mell. 
Textilberichte  37,  1419-1421  (1956)],  passive  protection 
is  given  by  depositing  a  synthetic  resin  in  the  fibre,  a 
process  which  gives  permanent  protection  against  attack. 

The  resin  is  fixed  by  a  wet  treatment  which  has  been 
patented  by  CIBA.  The  Arigal  is  converted  into  a  com- 
pletely insoluble  condensation  product  within  the  fibre, 
thus  rendering  the  cotton  rot-proof.  This  is  achieved  with- 
out any  adverse  affect  on  the  mechanical  properties  of 
the  fibre  and  without  the  fibre  being  degraded  in  any  way 
whatever. 

The  method  consists  in  impregnating  the  cotton  with 
an  aqueous  solution  of  Arigal  C  in  the  presence  of 
a  catalyst  (Arigal  Catalyst  C)  and  then  fixing  the 
preserving  agent  by  a  heat  treatment,  preferably  under 
pressure,  the  material  not  being  dried  between  impregna- 
tion and  fixation.  This  process  is  simple  and  can  be 
carried  out  in  the  conventional  equipment  available  in 
the  majority  of  textile  works.  It  is  only  the  wet  fixation 
treatment  which  does  not  conform  to  the  usual  type  of 
operation,  since  hitherto  it  has  not  been  the  practice  to 
subject  resin-treated  goods  to  a  heat  treatment  without 
previous  drying.  However,  the  wet  fixation,  too,  can  be 
carried  out  quite  satisfactorily,  using  equipment  generally 
to  be  found  in  dyeworks. 

In  order  to  ensure  the  maximum  degree  of  preservation 
by  this  process  it  is  essential  that  the  Arigal  be  uniformly 


deposited  throughout  the  material,  the  knots  being 
adequately  penetrated.  The  materials  should  therefore 
be  treated  at  the  textile  mill,  where  best  results  are  ob- 
tained by  treating  the  untwisted  yarn. 

Calculated  on  the  weight  of  the  treated  goods,  10  per 
cent,  of  Arigal  C,  when  uniformly  deposited,  gives  a 
very  high  degree  of  protection  against  micro-organisms. 
Cotton  net  thread  treated  with  Arigal  showed  absolutely 
no  reduction  in  tensile  strength  after  two  years  immersion 
in  a  particularly  muddy  part  of  the  Rhine. 

Untreated  cotton  yarn  immersed  at  the  same  place 
was  completely  destroyed  in  a  matter  of  weeks. 

Comparison  of  tests  made  with  chemically  treated 
cotton  from  America  showed  that  the  Arigal  treatment 
gave  greater  durability  (A.  Ruperti  loc.  cit.).  The  Arigal 
cotton  showed  no  reduction  in  tensile  strength  after  a 
4-week  immersion  in  the  Rhine  followed  by  burial  in 
compost  earth  for  24  weeks  at  30  deg.  C.  The  chemically 
treated  cotton,  on  the  other  hand,  was  destroyed. 

Cotton  treated  with  Arigal  is  also  very  resistant  to 
weathering.  It  is  well  known  that  the  tensile  strength  of 
textile  fibres  is  reduced  by  outdoor  exposure.  Cotton  is 
one  of  the  more  resistant  of  the  textile  fibres  in  this 
respect,  being  far  superior,  for  example,  to  nylon  and 
Orion  42.  Chemical  treatment  reduces  the  weather 
resistance  of  cotton,  whereas  Arigal  C  treatment  con- 
siderably increases  this  resistance.  After  a  12-month 
weathering  test  carried  out  at  CIBA,  a  boiled  out, 
untreated  cotton  fabric  had  retained  21-7  per  cent,  of  its 
original  tensile  strength,  and  Dynel  23  per  cent.  The 
same  quality  cotton  fabric  proofed  with  10  per  cent. 
Arigal  C  retained  47-8  per  cent,  of  its  tensile  strength, 
while  Orion  42  lost  all  strength  after  1 2  months'  exposure, 
and  nylon  after  only  6  months. 

Jute  and  ramie  can  also  be  treated  by  the  Arigal  C 
process  with  very  good  results.  However,  the  degree  of 
proofing  obtained  on  hemp  and  linen  leaves  much  to  be 
desired,  and  treatment  with  Arigal  alone  cannot  be 
recommended  for  these  two  fibres. 

Nets  made  with  Arigal  treated  cotton  do  not  require 
the  usual  cutch  treatment  since  such  a  treatment  would 
not  give  added  protection.  However,  if  cutching  should 
be  desirable  for  colouring  purposes,  there  is  no  reason 
why  this  should  not  be  done.  Even  after  prolonged 
immersion,  cutch  is  not  washed  off  Arigal  treated  nets 
and  they  do  not  have  to  be  re-treated. 

The  Arigal  process  is  still  young  and  has  yet  to  be 
proved  in  bulk  application  and  practice  before  final 
judgment  can  be  passed  as  to  its  possibilities.  But  even 
now  its  advantages  are  evident  the  simplicity  with 
which  it  can  be  applied,  its  relatively  low  cost,  the 
complete  absence  of  any  fibre  degradation,  the  improve- 
ment in  the  weather  resistance  of  the  cotton  fibre  and, 
finally,  the  outstanding  durability  of  the  rot-proofing 
itself. 

Note: 

Arigal— Trade-mark  of  CIBA  Limited,  Basle,  Switzerland. 
Dynel— Trade-mark  of  Carbide  &  Carbon  Chemicals  Co.,  U.S.A. 
Orion— Trade-mark  of  E.  J.  DuPont  de  Nemours  &  Co.,  U.S.A. 


[124] 


APPLICATION   OF   ACETYLATED   CELLULOSE  FOR   FISHING   GEAR 

by 
SANDOZ  LTD. 

Basle,  Switzerland 

Abstract 

The  partial  acetylation  of  cotton,  with  an  acetic  acid  content  of  25-30  per  cent,  leaves  the  mechanical  and  technological  properties 
of  I  he  yarn  unaffected.  There  is  no  change  in  appearance  or  colour  of  the  yarn  and  it  is  odourless,  non-poisonous,  and  insoluble  in  water 
or  solvents  and.  moreover,  it  has  an  extremely  high  resistance  to  micro-organisms.  Besides  cotton,  fibres  such  as  jute,  hemp,  linen,  sisal,  etc. 
Ciin  also  be  immunised  to  attack  by  bacteria  without  impairing  their  tensile  strength.  Tests  on  materials  buried  in  compost  with  30-40  per 
cent,  humidity  at  20-25  deg.  C.  showed  that  untreated  cotton  and  jute  yarns  rotted  away  in  14  days  while  the  acetylated  yarns  retained  their 
original  tensile  strength  after  6  months.  Comparative  tests  in  sea  water  showed  th.-it  cotton  lines  were  very  weak  after  two  months  but  the 
acetylaled  lines  were  still  strong  after  seven  months,  and  untreated  sisal  twine  lost  50  per  cent,  of  its  tensile  strength  within  one  week,  while 
acetylated  sisal  took  nine  weeks  to  lose  the  same  amount.  Lines,  nets,  and  material  for  making  sandbags  are  all  improved  by  treatment 
and  the  fact  that  the  present  cost  of  manufacture  is  relatively  high  is  offset  to  a  large  extent  by  the  fact  that  partially  acetylated  cellulose  fibres 
last  50-fcO  times  longer  than  the  untreated  matoitals. 

Application  de  la  cellulose  acctylee  aux  engins  dc  peche 
Resume 

l.'acetylation  particllc  du  colon,  avec  une  tencur  de  25  a  30  pourcent  d'acide  aceticiue  laisse  intacie  les  proprietes  mccaniques  et 
chimiques  du  fil.  II  ne  sc  produit  aucun  changernent  de  I'aspect  on  de  la  couleur  du  fil:  le  produit  est  inodore,  non  toxiquc,  insoluble  dans 
I'eau  et  les  solvants,  et  possede  unc  icsistance  cxtrememcnt  elevee  aux  microorganismes.  En  dehors  du  colon,  on  pcut  immuniser  d'autres 
!iorc%s  telles  quc  lc  jute,  le  chanvre,  le  lin,  le  sisal,  etc.,  contre  Pattaque  des  bacteries  sans  affcctcr  leur  resistance  a  la  traction.  Des  essais 
cffectiics  sur  des  lils  enfouis  dans  un  compost  rcnfermant  30  a  40  pourcent  d'humiditc  et  maintenu  sa  une  temperature  de  20  a  25  deg.  C.  ont 
montrc  que  des  lils  de  colon  et  de  jute  non  traites  pourrissaient  complctement  en  14  jours  tandis  que  des  fils  acclyles  conservaient  leur  resistance 
primitive  a  la  iraclion  au  bout  de  6  mois.  DCS  essais  comparatifs  executes  dans  de  I'eau  de  mer  ont  montre  que  les  ligncs  de  colon  ctaient 
devcnucs  tres  fragiles  au  bout  de  2  mois,  mais  que  les  lignes  acetylees  etaient  encore  solides  apres  sept  mois;  des  fils  de  sisal  ont  perdu  50 
pourcent  de  leur  resistance  en  une  scmaine  tandis  qif  il  a  fallu  neuf  semaines  aux  fils  de  sisal  acetylcs  pour  arnver  au  memc  resultat.  Les 
I'gncs.  les  filets  et  les  matcriaux  utilises  a  la  confection  des  sacs  de  sable  ont  tons  etc  ameliores  par  le  traitcment.  el  lc  cout  relativcment  eleve 
de  fabrication  actuel  est  largement  compcnse  par  le  fait  que  les  fibres  de  cellulose  partiellement  acetylees  durent  50  a  60  fois  plus  que  les 
memes  matemux  non  traites. 

l;sos  de  la  celulosa  acetilada  en  los  artes   de  pesca 
Kxtracto 

La  acetilacion  del  algod6n  con  25  -  10  per  ceni  de  acido  acetico  no  afecta  a  las  propiedades  mecanicas  y  tecnologicas.  ni  produce 
cambios  de  aspecto  o  color  en  la  libra  que  cs  inodora,  inocua  c  insoluble  en  agua  o  disolventes;  adcmas  le  comunica  una  gran  resistancia  a 
los  microorganismos.  Fucra  dc  las  fibras  dc  algodon  tambien  es  posiblc  inmunizar  las  de  yule,  canamo.  lino,  sisal,  etc.  contra  el  ataquede  las 
hactcrias  sin  disminuir  su  rcsistcnciu  a  la  traccion.  Los  ensayos  dc  matenales  enterrados  en  abono  vegetal  con  30  a  40  per  ceni.  de  humcdad, 
cuya  tcmperaiura  fluctuaba  entrc  20  deg.  y  25  deg.  C..  dcmostraron  que  las  fibras  dc  yute  y  de  algodon  sin  tratar  sc  pudren  en  14  dias  y  las 
.icetiladas  conscrvan  su  resistencia  a  la  traccion  luego  dc  6  mcscs. 

Los  ensayos  comparativos  en  agua  salada  permiticron  observar  que  la  resistencia  de  los  hilos  de  algodon  dismmuyc  bastante  despue* 
dc  2  scmanas,  peio  en  los  acctilados  aunicnta  despucs  dc  7  mcscs.  Los  hilos  dc  sisal  picrdcn  un  50  per  cent,  dc  su  resistencia  en  1  scmana, 
mientras  que  en  los  acctilados  csta  nusma  disminucion  sc  logra  al  cabo  dc  9  meses.  I  os  hilos,  redes,  y  material  para  fabricur  sacos  de  arena 
mejoran  con  cstc  tratamiento  >  su  costo  dc  manufactura,  relativamenie  elevado.  sc  compensa  en  gran  pane  por  el  hccho  de  que  las  fibras 
pnrcialmente  acctiladas  duran  unas  50  ;»  60  \cccs  mas  que  los  matenales  sin  tratar. 


HISTORY 

IN  1920  C.  Dorce  (Biochem.  J.  14,  709-14  (1920))  men- 
tioned for  the  first  time  the  excellent  resistance  to  micro- 
organisms of  acetate  silk  and  acetylated  cotton  (the 
latter  without  change  of  fibrous  structure).  A  year  later,  a 
process  of  partial  esterification  of  cotton  fabrics  to  give 
resistance  to  bacterial  attack  was  patented  in  U.S.A.  by 
Wolcott  and  Jennison.  (U.S.A.  Patent  1.474.574). 

In  1926  A.  Rheiner  (Sandoz  Ltd.,  Basle,  Switzerland), 
succeeded  in  developing  a  much  improved  method  of 
partial  acetylation  of  cellulose  (ref.  DRP  525*084  and 
530'395  and  A.  Rheiner  Angew.  Ch.  46,  675  (1933) 
"Ueber  niedrig  acetylierte  Fasercellulosen")  which  made 
bulk  production  possible.  The  commercial  production  of 


partially  acetylated  cellulose  without  change  of  structure, 
according  to  this  process,  was  taken  up  by  Sandoz  Ltd. 
in  1927  and  by  their  affiliated  company  Cotopa  Ltd.  in 
Horsforth  (England)  in  1929  and  the  acetylated  products 
have  since  been  marketed  under  the  registered  trade 
marks  "Passivgarn",  "Kristallgarn",  "Cotopa",  "Cres- 
tol",  "Crestine",  "Crestic"  and  "Crestose". 

After  the  second  World  War  the  Southern  Utilization 
Research  Branch,  New  Orleans  (U.S.  Department  of 
Agriculture)  carried  out  an  extensive  programme  of 
research  work  in  connection  with  partially  acetylated 
cotton.  Now,  two  U.S.  firms  are  said  to  manufacture 
partially  acetylated  cotton  in  bulk. 


[125] 


PROPERTIES 


MODERN     FISHING    GEAR    OF    THE    WORLD 

TEST  REPORTS 


For  the  production  of  cellulose  derivatives,  and  in 
particular,  cellulose  esters,  it  is  sufficient  to  modify  only 
the  amorphous  part  of  the  fibre  to  obtain  the  maximum 
resistance  to  micro-organisms.  This  corresponds  on  an 
average  to  the  substitution  of  one  hydroxyl  group  for 
one  glucose  anhydride  unit.  By  this  means,  the  swelling 
and  water  absorption  capacity  of  the  fibre  are  reduced 
to  such  an  extent  that  bacteria  and  fungi  have  great 
difficulties  in  decomposing  it.  The  crystalline  part  which 
has  not  been  modified  is  not  easily  attacked  by  micro- 
organisms, as  was  proved  by  P.  Karrer  (Kolloid-7,  52, 
304  (1930)). 

Practical  experience  has  shown  that  for  partial 
acetylation  in  bulk,  an  acetic  acid  content  of  25  to  30 
per  cent,  should  be  aimed  at,  to  leave  the  physical 
properties  of  the  yarn  unaffected.  A  modified  cellulose 
with  an  extremely  high  resistance  to  micro-organisms  is 
obtained  by  this  method. 

The  P.A.  cotton  does  not  show  any  change  in  appear- 
ance or  colour.  It  is  odourless,  non-poisonous  and  in- 
soluble in  water  or  solvents.  It  can  be  knotted  easily, 
causes  no  skin  irritation  or  corrosion  and  shows  consider- 
ably better  resistance  to  heat  than  untreated  material. 
Tensile  strength  remains  the  same,  but  it  is  not  immune  to 
attacks  by  termites. 

Other  fibres,  such  as  jute,  hemp,  linen,  sisal,  ramie, 
etc.,  can  be  similarly  immunised  to  micro-organisms. 
Acetylated  bast  fibres  are  of  particular  interest  when  the 
highest  tensile  strength  is  wanted.  It  is,  however,  ques- 
tionable whether  the  higher  priced  cellulose  fibres  (such 
as  ramie,  which  shows  a  better  natural  resistance  to 
micro-organisms  than  cotton)  can  stand  the  price 
increase  imposed  by  the  acetylation  process. 

TENSILE  STRENGTH  AND  COSTS 

Acetylation  increases  the  weight  of  material  by  15  to 
25  per  cent.,  and  this  has  to  be  considered  in  the  total 
manufacturing  costs.  The  absolute  tensile  strength 
remains  unchanged  or  decreases  only  slightly  (according 
to  the  yarn  quality  0  to  7  per  cent.),  and  the  breaking 
length  is  reduced  by  20  to  30  per  cent.  This  may  have 
to  be  taken  into  account  by  selecting  suitable  raw 
materials  and  by  paying  special  attention  to  the  spinning 
and  twisting  of  the  yarn. 

A  considerably  increased  tensile  strength  can  be 
obtained  by  stretching  acetylated  yarn  in  superheated 
steam.  Combinations  of  acetylated  and  synthetic  fibres 
may  be  used.  For  instance,  acetylated  cotton  is  spun 
round  polyamide  filament  or  yarns  made  of  polyester 
and  acetylated  fibres  are  twisted  together,  or  synthetic 
staple  fibres  arc  spun  with  acetylated  cotton  fibres. 

The  partial  acetylation  of  cellulose  is  dearer  than  most 
impregnation  methods  hitherto  used  to  protect  cellulose 
fibres  against  mildew,  sea  water  and  other  forms  of 
bacteria.  It  can  only  be  used  generally  for  this  purpose 
if  ways  are  found  to  reduce  the  manufacturing  costs. 
But  as  the  fibres  last  50  to  60  times  longer  than  untreated 
material  there  should  be  many  possibilities  for  using 
them  in  spite  of  their  relatively  high  price. 


The  following  reports  give  a  vivid  picture  of  the  remark- 
able resistance  of  treated  cotton  to  micro-organisms. 

1.  Resistance  to  rot  and  bacteria  (see  Table  I)  in  compost 
with  30  to  40  per  cent,  humidity  at  20  to  25  deg.  C.  The 
yarns  were  wound  on  glass  tubes  of  6  cm.  exterior 
diameter.  After  the  tests,  the  yarns  were  washed  and 
aired  and  the  tensile  strength  was  established.  The  cotton 
and  jute  yarn  rotted  away  completely  after  two  weeks 
but  the  acetylated  yarn  still  showed  the  original  tensile 
strength  after  26J  weeks. 

2.  Comparative  mildew  resistance  tests  carried  out  by 
the  Manchester  Chamber  of  Commerce  Testing  House 
and  Laboratory.  Entry  No.  342934  Prog.  No.  380124 
of  15.10.1956  (see  Table  II). 

Hank  No.  1  marked  3/10's  Cotopa  XL  (acetylated 
cotton  yarn)  Howard  Green  (raw  cotton,  soap  scoured 
and  vat  dyed,  followed  by  acetylation). 

Hank  No.  2  marked  3/1 0's  Howard  Green  raw  cotton 
treated  with  Copper  Naphthenate  (raw  cotton,  soap 
scoured  and  vat  dyed  and  impregnated  with  Copper 
Naphthenate  in  accordance  with  BSS  2087). 

These  samples  were  tested  for  resistance  to  mildew 
according  to  the  method  prescribed  in  the  Standards 
Association  of  Australia  Interim  Specification  S.A.A. 
Int.  88  Sect.  3. 

Specimens  from  each  sample  were  washed  in  a  water 
spray  for  7  days  and  other  specimens  were  treated 
for  5  days  at  70  deg.  C.  :L  2  deg.  C.  in  an  oven  with  a 
forced  air  circulation.  Then,  with  some  grey  cotton 
yarn  as  control,  they  were  sterilized  and  pressed  on  to 
nutrient  agar  medium  in  sterile,  covered,  glass  vessels, 
inoculated  with  a  spore  suspension  of  Menmoniella 
echinata,  and  incubated  for  14  days  at  a  temperature  of 
30  deg.  C.  2  deg.  C.  and  95  to  100  per  cent,  relative 
humidity. 

They  were  then  removed,  washed,  aired,  and  their 
tensile  strengths  determined  with  the  following  results: 

Single  thread  strength  on  Goodbrand  machine. 
Distance  between  jaws  12  inches.  Speed  of  traverse  of 


I  \bl  i      I 


1  ensile  im'iiKth  \a\  />ei  <  em.  of  original)  after  interment 


raw  cotton  w 
( Egyptian) 


W  ""d 
< onoii  van, 

r.gvptian) 


Jtue 


boiled 


nun-       (icervlaieil       mm-      acetvlated    ra\\\  non-  , 

„«•/>>-          28  "„          mm-        28  -.         ,,,-fly-     "< 
luted       ineticm'id      la  led    acetic  acid      la  ted 


acetic  acid 


100 

100 

100 

100 

100 

100 

3      25 

129 

27  A 

116 

~ 

"7          ty 

128 

23 

124 

41 

15l> 

14       0 

113 

0 

121 

n 

112 

28 

105 

124 

3 

112 

56 

117 

123 

<) 

137 

117 

104 

^. 

101 

112 

185 

108 

— 

88J 

— 

104 

126  ] 


ACETYLATION     OF    COTTON 


TABLE  II. 

U.Npiu>om>  GREY  COMROL 

A\ 

Received 

After 

/ncubatitiK 

7-9 

8-0 

01 

01 

8-7 

8-2 

0-5 

0 

8-9 

7-7 

02 

04 

7-9 

84 

nil 

nil 

8-0 

8-1 

nil 

0-6 

82 

91 

01 

0-4 

8-0 

8-2 

0-2 

0-1 

H-7 

8-7 

06 

01 

8-0 

8-6 

ml 

04 

X-9 

«-3 

nil 

0-6 

8-3  Ihs. 

1  oss,  per  cent. 


0  2  Ihs. 

97  A 


Marked:  "3/10  Coiopu  XI    Hovturd  Green" 


4\  Received 
Ib. 

After 
Incubating 
Ib. 

After    Washing 
and   Incubating 
Ib. 

After  Heating 
and  1m  abating 
Ib. 

5-7 

46 

5-1 

5-3 

5   3 

4-6 

4  9 

44 

4-9 

5-2 

53 

5-1 

4   7 

4-7 

51 

5-2 

5-5 

4-7 

5-4 

4-9 

49 

5-7 

45 

5-3 

5-4 

51 

50 

4-7 

^  -2 

5-6 

49 

4-7 

5-9 

5   1 

51 

4    1 

5-3 

5-3 

4   7 

4-4 

4-8 

45 

49 

<   *> 

^   -» 

5-8 

51 

5-3 

^  -8 

51 

5-7 

42 

53 

5  2 

52 

56 

5-7 

5-1 

5-^ 

*  •  1 

4-7 

5-2 

51 

5-5 

4-7 

49 

5-1 

52 

S  4 

41* 

5  7 

5-2 

4-6 
5-1 

49 

5-5 

s  5 

*•  •  1 

5 

50 

43 
5 

5-f> 
•0 

.V 

1 

2 

I  ossf  per  cent. 


nil 


nil 


Marked:  "3,10  Howard  Green  Ra\\  Cotton  treated  \\ith  Copper 
Naphthenatc" 


Is  Reiened 
Ib. 

After 
Im  nbating 
Ib. 

After   Hashing 
and  IncnbafitiK 
Ib. 

Atlef  Heating 
and  Incubating 
Ib. 

5-7 

5 

•8 

4-2 

6 

0 

2 

4 

^ 

4 

3 

•4 

42 

6-0 

V 

•  2 

5-7 

5 

•7 

9 

0 

•7 

3 

0 

33 

^•8 

6 

•  2 

4-9 

5 

•8 

4 

1 

4 

4 

•4 

3-2 

(vl 

5 

•2 

5-7 

5 

-8 

1 

-»  . 

4 

3 

6 

42 

4-9 

5. 

•1 

5-9 

5 

•4 

l 

•4 

0 

9 

3 

2 

2-9 

~    . 

1 

6 

3 

•4 

4-7 

S   6 

5 

6 

56 

5 

1 

0 

i) 

1 

•j 

3 

2 

35 

S-2 

5 

1 

5-5 

S 

6 

2 

4 

0 

6 

3 

.  7 

\  -7 

^•8 

5- 

7 

5  -ft 

5 

7 

0 

-7 

•> 

(» 

4 

•  2 

\   2 

5-3 

5 

(> 

51 

s 

6 

0 

X 

"> 

3 

2' 

^ 

3  -1 

5  6 


Loss,  per  cent. 


3  6 


73 


lower  grip  18  inches  per  minute.  Machine  capacity  10  Ib 

It  will  be  seen  that  the  "3/10  Cotopa  XL  Howard 
Green"  sample  was  not  affected  by  the  incubation  either 
before  or  after  washing  or  heating.  The  4k3/10  Howard 
Green  Raw  Cotton  treated  with  Copper  Naphthenate" 
sample,  however,  lost  73  per  cent,  of  its  strength  after 
washing  and  incubating  and  36  per  cent,  after  heating 
and  incubating. 

These  figures  clearly  show  the  superiority  of  partially 
acetylated  cotton. 


3.     Resistance  to  Sea  Water 

(a)  Comparative  tests  on  fishing  lines  in  tidal  waters 
showed  that  cotton  lines  were  very  weak  after  two 
months,  but  the  acetylated  lines  were  still  strong 
after  seven  months. 

(b)  Similar  results  were  obtained  in  testing  untreated 
and   acetylated    sisal   twine   in    sea   water.   The 
tensile  strength  of  the  untreated  twine  dropped 
within  a  week  to  50  per  cent.;  the  acetylated 
sisal  material  lost  50  per  cent,  after  nine  weeks. 

Application  of  partially  acetylated  cellulose  fibres 

The  extraordinary  resistance  of  partially  acetylated 
fibres  to  long  immersion  in  water  makes  them  most 
suitable  for  fishing  lines,  ropes  and  seines.  Fishing  nets 
made  of  acetylated  fibres  are  more  flexible,  remain 
cleaner  and  are  easier  to  handle  than  normal  nets  which 
are  impregnated  and  must  undergo  regular  cutch  treat- 
ment, which  increases  costs  in  time  and  money.  Nets  of 
acetylated  fibres  or  mixtures  of  synthetic  fibres  and 
acetylated  cellulose  do  not  have  to  be  treated  again. 
Another  advantage  is  that  acetylated  fibres  facilitate 
knotting. 

Tests  in  Scotland  with  herring  nets  made  of  partiallv 
acetylated  cotton  cord  have  shown  excellent  results  to 
date,  but  more  tests  will  be  made  before  a  conclusive 
report  is  issued  in  such  mixtures. 

Tests  of  fishing  nets  made  of  acctylated  cotton  twist 
are  being  made  in  Germany,  supervised  by  the  official 
German  testing  station  (Bundesforschungsanstalt  fiir 
Fischerci,  Hamburg). 


[127] 


EVALUATION   OF   ROT-RETARDING   NET   PRESERVATIVES 


by 

J.  ZAUCHA 

Sea  Fisheries  Institute,  Gdynia,  Poland 

Abstract 

The  most  important  methods  of  examining  the  effectiveness  of  rot-resistant  proteclants  are  (1)  the  static  method,  (2)  the  aquaria  I 
method,  and  (3)  the  tishing  method.  The  author  describes  these  in  detail  and  points  out  some  of  their  defects.  In  the  second  part  of  the 
paper  Dr.  Klust's  tests  on  impregnated  fish  netting  are  discussed  and  certain  amendments  are  suggested. 


Resume 


Evaluation  des  agents  de  preservation  retardant  la  poupriture  des  filets 


Les  plus  importantes  meihodes  cTexamen  de  I'efficacite  des  agents  protegeant  la  resistance  a  la  pourriturc  sent:  (1)  la  m&hodc 
statique  par  enfouissement  dans  le  sol,  (2)  Iam£thodc  par  immersion  dans  un  bac  et  (3)  la  m£thode  realisant  les  conditions  de  peche.  L'autcur 
decrit  ces  methodcs  en  detail  et  fait  ressortir  quclques-uns  de  leurs  defauts.  Dans  la  secondc  partic  de  la  communication,  1'auteur  examine 
les  essais  du  Dr.  Klust  sur  les  filets  impregnes  et  suggcrc  certaines  modifications. 


Extracto 


Evaluation  del  efecto  de  los  preservatives  que  retardan  la  pudricion  de  las  redes 


El  autor  describe,  en  detail  t,  los  metodos  mas  importantes  para  determinar  la  eficacia  de  los  preservatives  que  reiardan  la  pudrici6n 
de  las  redes  de  pesca.,  a  saber:  (I)  el  entierro  del  material  tratado,  (2)  la  inmersi6n  en  agua,  y  (3)  el  uso  en  la  pesca,  y  senala  sus  defect os. 
En  la  segunda  parte  del  trabajo,  se  analizan  los  ensayos  del  Dr.  Klust  con  redes  de  pesca  entmtadas  y  se  sugicren  ulgunus  modificaciones. 


A  PROPER  and  generally  recognized  method  of 
examination  of  preservatives  used  for  cellulose 
fabrics  should  be  introduced.  An  ever-increasing 
number  of  preservatives  of  different  rot-resistant 
strengths  are  appearing  on  the  market  and  the  indications 
of  their  resistant  properties,  as  given  by  the  manufacturers 
or  by  the  research  laboratories,  are  not  comparable  and 
can  result  in  misuse. 

Two  important  factors  causing  deterioration  of  textile 
fabrics  must  be  considered  when  evaluating  the  preserva- 
tive: (a)  the  decomposing  effect  on  cellulose  of  cellulose- 
producing  organisms,  and  (b)  the  rinsing  properties  of 
water,  its  mechanical  destruction  of  the  protective  layer 
and  its  slow  chemical  effect  on  the  cellulose  of  the  fabric. 
The  first  of  those  factors  is  generally  taken  into  considera- 
tion, whereas  the  second  is  often  practically  neglected. 
Even  a  mild  preservative  which  is  not  very  soluble  in 
water  is  more  effective  in  the  first  stage,  than  later  on 
when  it  has  been  partly  rinsed  out  and  a  local  bacterial 
invasion  has  taken  place,  causing  partial  rot.  If  this 
bacterial  invasion  spreads,  the  fabric  is  regarded  as 
useless,  even  though  the  chemical  indicators  of  the 
preservative  remain  unchanged. 

So  far,  no  suitable  laboratory  methods  have  been 
established  which  can  constitute  a  proper  examination 
of  preserved  textile  fabrics.  There  is  no  method  which 
enables  one  to  determine  and  to  estimate  the  suitability 
of  various  preservatives  for  fishing  purposes.  On  the 
one  hand,  such  a  method  would  have  to  ensure  a  proper 
estimation  of  the  intensity  of  bacterial  degradation  and  the 


effect  of  water  on  the  sample  and,  on  the  other,  it  would 
have  to  enable  the  regulation  and  determination  in  each 
case — of  the  principal  factors.  It  is  no  less  important 
that  other  parameters,  effecting  the  final  results,  should 
likewise  be  guaranteed. 

The  most  important  method**  of  examining  the 
effectiveness  of  rot-resistant  protcctants  are: 

(1)  the  static  method 

(2)  the  "aquarial"  method 

(3)  the  "fishing"  method 

(1)  The  static  method  consists  of  burying  the  samples 
in  soil.  The  soil  is  watered  systematically  at  a  temperature 
of  18  to  20  deg.  C.  thus  causing  the  cellulose-destroying 
bacteria  which  live  in  the  mouldy  soil  to  increase  abund- 
antly, and  bring  about  a  rapid  decomposition  of  cellulose 
fabric. 

A  disadvantage  of  this  method  is  that  there  is  scarcely 
any  rinsing  out  of  the  preservative  by  the  water.  Another 
drawback  is  that  the  results  of  tests  from  different  parts 
of  the  soil  are  always  found  to  differ,  which  is  particularly 
obvious  in  the  first  stage  of  decay,  i.e.,  after  the  same 
period  of  time  similar  tests  show  different  losses  in 
initial  strength.  A  number  of  factors,  such  as  a  lesser 
contact  of  the  fabric  with  the  soil,  an  unequal  access  of 
oxygen,  etc.,  seem  to  be  responsible. 

There  are  certain  modifications,  such  as  inoculating 
the  impregnated  cellulose  fabric  with  certain  bacterial 
cultures  or  fungi  causing  decomposition  of  the  cellulose, 
and  then  storing  the  samples  in  thermohygrostats. 


M28] 


EVALUATION     OF     PRESERVATIVES 


These  methods  have  no  practical  use  in  fishing  gear 
research  except  in  certain  experiments  connected  with 
storage  of  fishing  net. 

(2)  With    the   "aquarial"   method  the   samples   arc 
immersed  in  an  aqueous  solution  of  mineral  salts — the 
nutritive    components    essential    for    the    bacteria.    A 
necessary  amount  of  cellulotytic  bacteria  suspension  is 
added  to  the  solution.  If  therophilic  bacteria  are  used,  it 
is  possible  to  evaluate  the  rot-resistance  of  the  sample 
within  two  days.     If,  however,  mezophilic  or  psychro- 
philic  bacteria  are  applied,  no  results  are  apparent  under 
a  fortnight.     In  laboratory  practice,  pure  cultures  of 
cellulolytic  bacteria  are  not  always  applied,  and  frequently 
river  or  sea  water  is  used  for  the  experiment  with  mud- 
suspension — a  rich  source  of  cellulose-destroying  organ- 
isms. This  experiment  is,  however,  difficult  to  repeat  and 
the  results  of  the  examination  are  generally  incomparable. 
Considering,  however,  the  simplicity  of  the  procedure, 
it  is,  in  spite  of  its  drawbacks,  often  applied  to  evaluate 
various  kinds  of  netting  protectants,  although  the  rotting 
period   of  the  examined   fabrics,   especially   when   sea 
water  is  used,  is  relatively  very  long. 

One  of  the  main  defects  of  the  "aquarial"  method  is 
that  the  impregnation  is  not  rinsed  out  as  much  as  in 
natural  conditions.  If  the  water  in  the  aquarium  tank 
is  not  changed  at  all,  the  surroundings  undergo  a  gradual 
intoxication  by  the  substances  rinsed  out  from  the 
samples,  which  invariably  stops  the  normal  activity  of 
the  organisms  and  causes  manifold  biological  changes 
in  the  medium  used.  A  frequent  changing  of  water,  on 
the  other  hand,  causes  a  change  of  physical  conditions 
effecting  the  intensity  of  the  bacterial  growth.  The 
results  obtained  by  means  of  this  method  can,  therefore, 
only  be  treated  as  material  for  further  research. 

(3)  The  "fishing"  method  consists  in  laboratory  exam- 
ination of  cellulosic  cotton  fabric  in  conditions  approxi- 
mating those  normal  to  fishing  gear  when  in  use.  This 
method,  developed  by  Dr.  Mcseck,  seems,  therefore,  to 
be  generally  recognised  as  the  most  suitable,  especially 
for  industrial  practice. 

The  impregnated  twine  samples  arc  fixed  to  frames  and 
immersed  in  deep  water.  At  certain  intervals  the  samples 
are  taken  out  of  the  frames  and  tested  for  mechanical 
resistance.  To  evaluate  the  surrounding  medium,  the 
temperature,  oxygen  content,  pH  value,  geographical 
conditions  and  the  presence  of  hydrogensulphide  at  the 
bottom  of  the  water,  etc.,  are  repeatedly  tested. 

Russian  scientists  do  not,  as  a  rule,  use  twine  in  their 
experiments  but  samples  of  fish  netting.  These  samples 
are  either  tested  apart  or  are  sewn  into  different  parts 
of  the  fishing  gear  working  on  industrial  scale.  Sometimes 
both  methods  are  applied  simultaneously. 

From  the  above  survey  of  practical  methods  of 
examination  we  see  that  each  affects  the  deteriorating 
factors  in  a  different  way.  Whilst  the  static  method 
does  not  lay  much  stress  on  the  "rinsibility"  of  the 
preservative,  the  fishing  method  guarantees  this  condition 
sufficiently  but  does  not  give  any  possibility  of  interfering 
with  the  parameters  influencing  the  intensity  of  cellulose 
decomposition.  The  results  obtained  by  the  two  men- 
tioned methods  must  necessarily  be  quite  different  and 
misleading  in  assessing  the  actual  rot-resistance  of  the 
given  fabric. 


In  1952,  the  laboratories  of  the  Polish  textile  industries 
started  to  promote  a  very  practical  and  cheap  method  of 
impregnation  of  cotton  twine  by  means  of  copper- 
treatment,  the  process  consisting  in  emulsive  coating  of 
the  fabric  with  a  copper  naphthenate.  At  the  same  time 
the  laboratory  of  the  Sea  Fisheries  Institute  rejected  this 
method  for  fish  netting  purposes.  The  two  contrary 
conclusions  were  the  result  of  two  different  methods 
applied  to  evaluate  the  same  rot-resistant  protectant. 

'Tests  were  then  carried  out  with  copper-treatment  of 
fish  netting  by  two  methods  the  "static"  and  the 
"fishing"  method. 

"Static"  method 

Samples  of  treated  and  untreated  twine  were  buried 
in  compost  soil  10  cm.  below  the  surface.  The  soil  was 
sprayed  regularly  every  day  with  distilled  water  in  order 
to  ensure  its  proper  water  content.  The  temperature  was 
kept  within  the  range  18  deg.  to  20  deg.  C.  The  average 
moisture  of  the  medium  was  70  per  cent.  The  raw 
samples  were  removed  approximately  every  5  days 
and  their  mechanical  resistance  determined.  Every  10 
days  fresh  samples  of  raw  twine  were  deposited  and  their 
loss  in  strength  was  determined  for  the  purpose  of 
ascertaining  whether  the  intensity  of  the  cellulose- 
decomposition  differed  in  the  course  of  the  tests.  The 
treated  samples  were  removed  and  tested  for  mechanical 
resistance  at  longer  intervals. 

"Fishing"  method 

Samples  of  twine  wound  into  strands  with  32  to  34 
strings  each  were  fixed  on  frames  by  means  of  grips  and 
immersed  in  water  in  a  tank  specially  erected  for  testing 
fish  nets.  The  depth  of  the  immersion  was  about  0-6  m. 
below  the  water  level.  The  distance  from  the  bottom  was 
about  1  m.  The  samples  were  removed  from  the  water 
every  10  days  and  dried  in  the  air,  then  tested  for  mech- 
ical  resistance  by  means  of  the  Schopper  dynamometer 
(distance  between  grips  500  mm,  breaking-time  10  sees.). 
During  a  test  that  lasted  from  6.9.55  to  26.10.55  the 
parameters  of  water  medium  were  on  an  average  as 
follows: 


Measured  parameters 
Temperature  of  water 


Value 

September  16  deg. 

October  1 2  deg. 
Salt  content  7-2  to  7-6  per  cent. 

pH  8-2  to  8-4 

For  both  tests,  cotton  twine  with  the  following 
properties  was  used: 

Metric  number  40/3     3 

Initial  breaking  strength  5-640  kg. 

Length  increase  11-4  per  cent. 

The  impregnating  solution  was  a  water  emulsion  of 
copper-naphthenate,  10  g.  of  CuSO4.  5H2O  diluted  in 
100  ml.  of  water  warm.  After  cooling,  a  quantity  of 
25  ml.  of  5  per  cent,  ammonia  solution  and  water  up  to 
500  ml.  were  added.  A  separate  sample  of  20  g.  of 
naphthenic  acids  was  neutralized  with  10  per  cent, 
ammonia  solution  and  diluted  with  water  up  to  500  ml. 
The  two  solutions  were  mixed  and  a  "ready-for-use" 
impregnated  mixture  was  obtained  in  which  the  cotton- 


[129] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


UNTREATED  TWINE 
TREATED  TWINE 


20  1.11.55 

Figs.  1  and  2. 


day* 


twine  was  copper-treated.  The  samples  were  immersed 
for  15  minutes,  the  surplus  drained  off  and  the  twines 
dried  at  80  deg.  C.;  after  drying,  the  samples  had  a  light 
green  colour.  The  copper  content  in  samples  calculated 
as  dry  values  was  0-65  per  cent.  Other  properties  were: 


Metric  number 

Initial  breaking  strength 

Length  increase 


4(1/3x3 
5-690  kg. 
13*5  per  cent. 


Figs.  1  and  2  illustrate  the  results  obtained  during  the 
experiment.  Fig.  1  shows  the  loss  in  strength  as 
evaluated  by  the  "static"  method,  whilst  fig.  2  indicates 
the  results  obtained  by  applying  the  "fishing"  method. 
Both  figures  are  drawn  to  an  equal  scale  (of  time  and 
strength);  they  also  show  the  loss  in  strength  of  untreated 
twine  samples,  in  order  to  demonstrate  the  intensity  of 
bacterial  degradation  of  the  examined  medium.  We  see 
a  distinct  difference  in  the  process  of  loss  in  strength  of 
treated  samples  examined  by  "static"  and  "fishing" 
methods,  although  the  degree  of  rotting,  on  the  whole, 
is  almost  the  same.  The  almost  identical  loss  in  strength 
of  untreated  cotton  twine  in  both  experiments  can  be 
seen  from  the  parallel  curves  in  figs.  1  and  2,  whilst  the 
results  with  treated  cotton  twine  differ  widely.  Hence,  the 


evaluation  of  fish  netting  twine  of  the  same  dimensions, 
treated  in  the  same  way,  differs  considerably  according 
to  the  test  method  applied.  The  numbers  which  showed 
how  many  samples  were  decomposed  up  to  50  per  cent, 
of  their  initial  strength,  during  the  time  that  the  treated 
twine  sample  had  lost  50  per  cent,  of  its  initial  strength, 
served  as  comparable  indicators  for  both  method  of 
protection,  the  condition  being  that  each  successive 
untreated  twine  sample  was  not  immersed  in  the  medium 
before  the  preceding  one  had  lost  50  per  cent,  of  its 
initial  strength. 

For  the  treated  twine  samples  evaluated  by  means  of 
the  "static"  method,  this  number  amounted  to  "PT" 
(Protection  Test)  =-  9,  whilst  for  the  twine  samples 
evaluated  by  "fishing"  method  it  was  "PT"  —  3.  The 
mere  comparison  of  the  two  numbers  proves  sufficiently 
how  different  the  two  methods  are.  The  obvious  con- 
clusion is  that  for  fishing  purposes  the  so-called  "fishing" 
method  is  the  most  suitable  one,  provided  it  is  carried 
put  under  actual  fishing  conditions.  Although,  hitherto, 
it  has  yielded  non-comparable  results  (which  could  be 
prevented  by  a  greater  number  of  experiments),  yet  it 
seems  to  be  by  far  the  most  reliable  and  appropriate  for 
industrial  practice. 


[130] 


EVALUATION    OF    PRESERVATIVES 


Evaluation  Tests  of  Protection  Methods 

A  great  disadvantage  of  all  the  methods  for  testing  the 
quality  of  impregnated  fish  nettings,  either  described  in 
literature  or  known  in  laboratory  practice,  is  that  none 
of  them  are  practically  repeatable,  and  their  results  are 
therefore  difficult  to  compare  and  to  transfer  to  any  other 
conditions  or  surroundings.  Consequently,  various 
research  laboratories,  unable  to  compare  the  results  of 
their  experiments,  were  forced  to  carry  out  the  same 
investigations  relating  to  an  identical  or  practically 
identical  protection  method. 

The  first  efforts  to  compare  the  value  of  different 
protection  methods  were  undertaken  by  Dr.  Klust  and 
put  into  practice  in  1952.  His  tests  consisted  of  deter- 
mining the  so-called  "rotting  value"  of  the  medium  in 
which  the  experiments  were  carried  out.  Accordingly, 
the  "rotting  value"  of  the  medium  is  equal  to  the  total 
losses  in  strength,  expressed  as  percentage  values  of  the 
initial  strength  of  the  successively  examined  samples  of 
untreated  twine  Nm  50/15,  so  that  the  test  of  Dr.  Klust, 
as  such,  stands  for  the  sum  of  "rotting  values"  of  the 
medium  during  the  time  in  which  the  examined  sample 
of  treated  twine  has  lost  50  per  cent,  of  its  initial 
strength. 

Dr.  Klust  has  examined  a  number  of  twine  preserva- 
tives and  has  given  them  his  own  symbols  expressed  in 
numbers,  the  higher  the  number  the  better  the  quality 
of  the  preservative. 

From  our  own  experiments,  in  completely  different 
conditions,  we  have  obtained  quite  different  numbers  for 
certain  kinds  of  preservatives,  probably  resulting  from 
the  fact  that  the  "rotting  value"  of  our  medium  was 
2  to  2-5  lower.  Another  cause  was  the  very  strong 
rinsing  effect  of  the  water  currents  and  its  mechanical 
destruction  of  the  protecting  coaling  of  the  twine 
threads.  Other  parameters  involved  in  the  experiments 
did  not  seem  to  have  such  a  great  effect. 

In  his  tests  Dr.  Klust  has  given  a  very  exact  method  for 
determining  the  main  destroying  factor  of  the  cellulosic 
fabric,  i.e.,  the  activity  of  the  micro-organisms,  thus 
enabling  a  proper  comparison  of  the  deteriorative  effect 
of  microbial  activity  when  examining  the  fish  netting 
fabric.  Dr.  Klust  has,  however,  disregarded  the 
influence  of  the  second  important  factor,  viz.:  the 
rinsing  effect  of  water  and  the  mechanical  destruction  of 
the  membranous  structure  of  the  protection  layer  of  each 
fibre.  The  importance  of  this  factor  is  evident  when 
applying  certain  excellent  preservatives  almost  insoluble 
in  water;  the  smallest  local  destruction  of  the  protective 
layer  then  causes  an  immediate  invasion  of  bacteria  and 
a  complete  destruction  of  the  thread.  The  case  is  different 
with  preservatives  more  soluble  in  water.  A  mechanical 
destruction  of  the  layer  does  not,  at  first,  provoke  a 
general  attack  of  bacteria,  as  the  exposed  area  is  pro- 
tected by  the  field  of  diffusion  of  toxic  bactericidal 
components  of  the  protectant.  After  some  time,  however, 
the  process  of  diffusion  slows  down,  its  field  diminishes 
and  the  bacterial  activity  becomes  more  effective.  There- 
fore, when  comparing  different  kinds  of  protectants,  the 
influence  of  non-biological  factors  of  water  on  the 
impregnation  of  fish  netting  fabric  must  be  considered. 

Another  minor  disadvantage  of  Dr.  Klust's  tests  is  that 
they  have  no  physical  value  and  are  merely  symbol 


numbers  comparable  with  each  other.  The  way  of 
obtaining  them  seems  also  rather  questionable.  It  is 
not  quite  clear  to  us  why  Dr.  Klust  has  taken  as  a  basis 
for  the  treated  twine  the  50  per  cent,  of  its  loss  in  strength 
and  has  not  applied  the  same  principle  to  the  untreated 
twine.  Presumably,  in  this  case  Dr.  Klust  wanted  to 
avoid  any  extrapolation  in  time — of  his  results. 
Though,  when  taking  into  consideration  the  typical  loss 
in  strength  of  the  untreated  twine  which,  in  the  first 
stage  and  for  quite  a  long  period  practically  retains  its 
initial  strength,  then  begins  to  lose  it  rapidly,  it  may  be 
concluded  that  the  loss  in  strength  taken  in  regard  to 
time  is  quite  different  in  the  range  0  to  30  per  cent,  and 
50  to  80  per  cent.  Therefore  a  lesser  mistake  would  be, 
in  our  opinion,  to  extrapolate— in  time  the  results  of 
the  loss  in  strength  of  the  examined  twines  down  to  a 
certain  point  of  their  strength,  than  to  apply  the  principle 
of  summing  up  their  actual  losses  in  strength.  The  extra- 
polation would  have  to  be  possibly  small  in  time,  seeing 
that  it  affects  the  exactness  of  the  calculation  of  the 
protection  tests.  In  our  opinion,  both  treated  and 
untreated  twine  samples  should  be  brought  down  to 
50  per  cent,  of  loss  of  their  initial  strength. 
We  would  suggest  a  modification  of  the  test  as  follows: 
The  protection  test  of  the  examined  treated  twine 
Nm  50/15  stands  for  a  certain  number  of  the  same 
untreated  twine  samples  which  successively  have  lost 
50  per  cent,  of  their  initial  strength  in  the  time  in  which 
the  sample  has  also  lost  50  per  cent,  of  its  initial  strength, 
Due  to  such  a  definition  the  tests  acquire  a  real  physical 
sense  and  the  reader  can  see  at  once  how  many  times 
stronger  is  the  treated  twine  than  the  untreated  one. 

Such  a  definition  does  not  alter  the  principal  idea  of 
Dr.  Klust  but  modifies  it  to  a  certain  extent  having 
eliminated  the  so-called  "rotting  value"  of  the  medium. 
But,  in  spite  of  this  definition,  the  test  does  not  illustrate 
the  effects  of  the  second  factor,  viz.:  the  non-biological 
parameters,  the  intensity  of  water  current  in  the  research 
station,  etc.  Considering  the  changeability  of  all  these 
non-biological  factors,  it  is  difficult  to  determine  any  of 
them  or  to  choose  one  or  more  to  introduce  in  some  form 
into  the  test.  To  a  certain  extent,  the  time  in  which  the 
experiment  is  carried  out  gives  an  approximate  idea  of 
the  resultant  activity  of  the  above  mentioned  non- 
biological  factors,  and,  although  it  does  not  help  us 
with  any  indications  it  lets  us  have  a  general  idea  of  their 
activity  and  gives  us  indications  as  to  the  nature  of  the 
medium  in  which  the  experiment  has  been  carried  out. 
The  factor  of  time  enables  us  to  judge  the  degree  of 
resistance  of  various  protectants  to  the  bacterial  de- 
composition, particularly  in  highly  infected  mediums 
for,  even  an  ineffective  protectant  will  initially  keep  the 
twine  from  rot,  until  after  a  certain  time  when  mechanical 
damage  has  taken  place  the  micro-organisms  will  in- 
variably cause  complete  decomposition.  If  such  a 
treatment  process  were  only  to  be  evaluated  according  to 
Dr.  Klust,  a  very  high  number-  denoting  efficiency- 
would  be  obtained  in  a  highly  infected  medium.  The 
same  treatment  process  evaluated  in  a  medium  with 
smaller  bacterial  activity  (a  low  rotting  number  of  the 
medium)  would  yield  a  different  result — a  relatively  low 
number.  This  example  shows  that  the  duration  of  the 
experiment  has  a  great  influence  on  the  results. 


[131] 


MODERN     FISHING    GEAR     OF    THE    WORLD 


Therefore,  we  would  suggest  that  to  the  test  of  Dr. 
Klust  be  added  a  number  to  indicate  the  duration  of 
time  (months)  in  which  the  experiment  had  been  carried 
out.  Maybe  such  a  test  would  be  less  comparable, 
but  much  more  useful,  for  it  would  help  to  avoid  greater 
mistakes  which  undoubtedly  occur  in  the  simple  and 
attractive  interpretation  of  Dr.  Klust. 

Hence,  "PT"  20/5  would  signify  that  in  the  course  of 
5  months  the  treated  sample  of  cotton  twine  Nm  50/15, 
had  lost  50  per  cent,  of  its  initial  strength  and  that  in  the 
same  period  20  successive  untreated  twine  samples  had 
also  lost  50  per  cent,  of  their  initial  strength. 


In  spite  of  this  modification,  the  k%PT"  test  will  go  on 
being  a  non-comparable  number,  as  long  as  there  is  no 
better  method  to  determine  and  to  regulate  the  rotting 
processes  and  the  "rinsibility"  of  the  preservative. 
These  demands,  however,  are  in  a  technical  way  still 
difficult  to  answer.  At  present,  there  is  no  practical 
answer  except  to  continue  applying  the  classical  method 
of  Mescck  for  fishing  purpose.  Results  obtained  by  any 
other  method  should  be  considered  with  great  care. 
They  can  only  be  reliable  if  confirmed  by  the  "fishing" 
method  the  results  of  which,  in  practice,  have  not  been 
reported  as  contradicting  each  other. 


Herring  nets  used  in  the  East  Anglian  fishery  being  dried  at  Ct.  Yarmouth,  U.K. 

1132] 


METHOD   OF   TESTING   RESISTANCE   OF   NET   MATERIALS 

TO   MICRO-ORGANISMS 

by 

A.  v.  BRANDT 
Prof,  und  Dir.,  Institut  fur  Nctz-  und  Matcrialforschung,  Hamburg,  Germany 

Abstract 

This  paper  describes  a  method  of  assessing  the  efficiency  of  net  preservatives  independent  of  the  rotting  powers  of  the  waters  in 
which  the  nets  are  used. 

A  numerical  value  has  been  found  for  the  test  figure  T,  which  is  the  amount  of  rotting  activity  necessary  to  reduce  the  initial  breaking 
strength  of  a  material  by  50  per  cent,  and  the  following  criterion  is  used  to  assess  the  value  of  preservatives: 

T  *~     200  or  less    ---     poor  efficiency 
-     200-       500          low 
-=     500  —  1000          medium 
1000        2000  —     good 
over         2000          very  good    „ 
In  this  way  it  is  possible  to  make  comparisons  which  are  quite  independent  of  the  type  of  water  and  time  of  year 


Kfeum* 


Technique  d'essais  de  la  resistance  aux  micro-organismcs  des  mattriaux  employes  pour  la  fabrication  des  filets 


Ce  document  deem  une  methodc  permettant  d'evaluer  1'efficacilc  des  agents  de  preservation  des  filets,  mdependamment  du  pouvoir 
de  putrefaction  dc  1'eau  dans  laquelle  ceux-ci  sont  utilises. 

On  a  trouv&  pour  le  test  de  la  Figure  T  une  valeur  numeric) uc  qui  represente  le  pouvoir  de  putrefaction  necessaire  pour  reduire  de 
50  pour  cent,  la  resistance  initiate  A  la  rupture  des  materiaux  et  1'echelle  suivante  est  utilises  pour  evaluer  rintcrdt  des  agents  de  preservation: 


T 


200  ou  moins 

200  a     500 

500  a  KKK) 

1000  a  2000 

plus  de  2000 


mauvais 
mediocre 
passable 
bon 
tres  bon 


II  est  ainsi  possible  de  faire  des  comparisons  tout  a  fait  independantes  du  type  de  I'eau  et  de  la  saison  de  1'annee. 

Metodo  para  determinar  la  resistcncia  a  los  microorganismos  de  los  materiales  empleados  en  la  fabricacidn  de  redes 
Kxtracto 

En  este  trabajo  se  describe  un  metodo  para  cvaluar  la  eficacia  de  los  agentes  empleados  en  la  preservation  de  redes,  excluyendo  el 
efecto  putrefactivo  de  las  aguas  donde  se  calan  los  artes. 

Al  determinar  los  valores  (T)  obtenidos  en  las  pruebas,  sc  han  encontrado  cifras  que  reprcscntan  la  cantidad  de  acci6n  putrefactiva 
necesaria  para  reducir  en  un  50  por  cent,  la  resistencia  a  la  ruptura  que  inicialmente  tenia  el  material.  lin  la  evaluation  del  valor  de  los  pre- 
servatives se  uso  el  siguiente  criterio: 

T          200  o  menos  eficacia  muy  baja 

-     200  500    -  „        baja 

500    —     1000  „        mediana 

1000  2000    -  „        buena 

mas   de    2000  „        muy  buena 

De  esta  manera  es  posible  hacer  comparaciones  con  independencia  del  tipo  de  agua  y  de  la  epoea  del  ano. 


NATURAL  fibres  tend  lo  rot  when  they  are  exposed 
to  humidity  and  unfavourable  temperatures.  The 
destruction  is  caused  by  micro-organisms,  bac- 
teria, and  even  by  fungi,  which  attack  the  cellulose  and 
protein.  Numerous  methods  of  preserving  the  nets  have 
been  developed,  but  their  efficiency  varies  considerably. 
An  adequate  method  in  one  kind  of  water  may  fail  in 
another,  not  only  because  of  the  varying  strains  on 
fishing  gear,  but  also  because  of  the  variable  activity  of 
these  organisms.  This  activity  varies  in  different  fishing 
areas,  and  in  a  characteristic  rhythm  during  the  year 
(Diagram  1-8). 


These  varying  factors  make  it  difficult  to  compare  the 
efficiency  of  preservative  methods,  and  so  an  attempt 
has  been  made  to  find  a  method  of  testing  net  preserva- 
tives irrespective  of  place  and  season. 

TESTING  METHODS 

The  growth  of  fungi  and  the  amount  of  destruction  in 
textiles  are  usually  tested  by  the  accelerated  fungal 
inoculation  method,  or  the  soil  burial  method.  Standards 
have  been  established  in  several  countries  for  carrying 
out  these  tests. 


[133] 


MODERN     FISHING     GEAR    OF    THE     WORLD 


100 

80 
60 

40 


Figs.   1-8. 

(1)  Monthly  rotting  activity  us  compared  with  cotton  twines  in  the 
course  of  a  year  in  a  eutrophic  lake  (l^owentien  lake.  East 
Prussia,  average  1938/1942). 

(2)  Monthly  rotting  activity  as  compared  with  cotton  twines  in  the 
course  of  a  year  in  an  oligotrophic  lake  ( Wigry  lake  near  SuwaJki. 
average  1940/1942). 

(J)  Monthly  rotting  activity  as  compared  with  cotton  twines  in  the 
course  of  a  year  in  the  North  Sea  (light  vessel  Elbe  /,  average 
1948/1956). 

(4)  Monthly  rotting  activity  as  compared  with  cotton  twines  in  the 
course  of  a  year  in  the  Baltic  (light  vessel  Kiel,  average  1952/1956). 

(5)  Rotting   activity   (fortnight-figure)    as    compared   with    cotton 
twines  in  a  non-polluted  running  water  (Schlitz  river  near  Schlitz- 
Hessen,  average  19511  1956). 

(6)  Rotting  activity  (fortnight-figure)    as    compared  with    cotton 
twines  in  r  heavily  polluted  running  water  (Elbe  estuary  neat 
Cuxhaven,  average  1947/1956). 

(7)  Rotting  activity  (fortnight-figure)  as  compared  with  silk  twines 
in  a  non-polluted  running  woter  (Schlitz  river  near  Schlitz- Hessen, 
average  1955/1956). 

(8)  Rotting  activity  (fortnight-figure)  as  compared  with  silk  twines 
in  a  heavily  polluted  running  water  (Elbe  estuary  near  Cuxhaven, 
average  1953/1956). 


These  methods  cannot  simpl>  be  adopted  for  fishing 
gear  because  fishing  gear  is  subject  to  continuous 
rinsing  by  water.  Tests  therefore  should  be  carried  out 
in  water. 

In  testing  under  natural  conditions,  net  twines,  lines 
or  ropes  are  freely  immersed  in  water.  Their  breaking 
strengths  are  ascertained  before  and  during  the  experi- 
ment. The  smaller  the  loss  in  breaking  strength  after  a 
certain  period,  the  greater  the  resistance  to  rotting.  The 
rotting  action  of  the  water  is  determined  separately  and 
related  to  the  loss  in  breaking  strength.  Rotting  activity 
and  breaking  strength  indicate  the  preservation  test 
figure. 


DETERMINATION    OK 
OF  THE  WATER 


THE    ROTTING    ACTION 


The  monthly  rotting  degree  of  the  test  water  is  deter- 
mined by  immersing  test-twines,  cotton  twines  Nm  50/15 
(Ne  30/15),  on  the  first  day  of  each  month.  Before 
immersion,  the  twines  are  boiled  in  distilled  water,  and 
their  initial  breaking  strength  is  determined  in  wet 
condition.  With  new  cotton  twines  Nm  50/15  it  amounts 
to  about  6-0  to  6-5  kg.  Four  bunches  of  10  twines, 
each  twine  30  cm.  long,  are  used  in  the  experiment. 
After  one,  two  or  three  weeks,  and  on  the  last  day  of  the 
month,  one  bunch  is  taken  from  the  water  to  determine 
the  average  loss  in  breaking  strength  of  the  twines. 

The  loss  in  breaking  strength  of  the  sample  remaining 
in  the  water  throughout  the  month  indicates  the  rotting 
activity  for  that  month.  By  dividing  it  by  the  number  of 
days  the  rotting  activity  per  day  is  determined. 

CHANGE  OF  THE  METHOD  WITH   HEAVY 
ROTTING  ACTIVITY 

In  heavil}  rotting  waters,  i.e..  in  running  waters,  a 
100  per  cent,  loss  of  breaking  strength  may  occur  before 
the  end  of  the  month,  possibly  already  after  a  week,  and 
in  that  case  it  is  impossible  to  determine  the  rotting 
activity  as  described  above,  so  the  samples  are  replaced 
by  new  ones  when  they  have  lost  approximately  three- 
quarters  of  their  initial  breaking  strength.  The  losses  in 
breaking  strength  of  all  samples  are  added  up  to  establish 
the  monthly  rotting  activity.  The  loss  of  breaking 
strength  shown  in  a  graph  has  the  form  of  an  S-curve. 
The  monthly  figure  may  therefore  only  be  computed 
from  the  figures,  for  instance,  of  a  fortnight,  if  the  sample 
has  lost  at  least  three-quarters  of  its  strength  (but  not 
more  that  90  per  cent.)  after  14  days.  Frequently,  that 
date  cannot  be  foreseen,  so  it  is  suggested  that  four 
samples  should  be  put  in  the  water  on  the  first  day  of 
the  month,  as  described  before,  and  subjected  to  a 
weekly  control.  This  guarantees  that  the  samples  rotted 
up  to  three-quarters  of  their  initial  strengths  are  recorded 
and  renewed. 

Table  I  is  an  example  of  recording  the  monthly  rotting 
activity  in  a  year  in  per  cent,  loss  of  breaking  strength. 

In  Table  1  the  first  four  months  show  the  loss  in 
breaking  strength  of  the  cotton  test  twines  after  1,  2  and 
3  weeks  and  1  month.  In  the  fifth  month  the  samples 
rotted  within  two  weeks  and  had  to  be  replaced.  (The 
sign  "*"  in  the  table  means  that  the  samples  had  been 


1341 


TESTING     RESISTANCE     AGAINST     ROT 


TABIF  I 


Test 
month 

Loss  in  strength  after 

I                           3 

4i 

m 

*, 

Value 
per 
month 

Value 
per 
day 

January 
February 
March 
April 
May 
June 

(0 
(0 
(0 
(0 
(40 

(88* 

0 
0 

5 
9 
93 

V 

20 
20 
40 
70 
')       (50 
(88*) 

54*) 
54*) 
78*) 
100*) 
100*) 
(73*) 

i  period 

54 
54 
78 
100 
193 
249 

1 
1 
2 
3 
6 
8 

•7 
9 
5 
3 

•3 

luly 

(88*)     (100*) 

(84 

*) 

|  10  days    272 

8 

•8 

August 

(84* 

)       (81*) 

(90 

*  V 

not  1 

255 

8 

2 

September 

(80* 

)       (75*) 

(74 

*) 

|  week 

229 

7 

6 

October 

(76* 

)       (75*) 

(65 

*) 

I 

218 

7 

0 

November 

(10 

40 

76*) 

(36 

*) 

112 

3 

7 

December 

(10 

40 

76*) 

(18 

*) 

94 

3 

0 

Value  p.  year  :        1908 


renewed.)  In  the  following  five  months  the  samples 
lost  about  three-quarters  of  their  breaking  strengths 
after  ten  days,  and  had  to  be  replaced  twice.  In  the 
last  two  months  the  test-twines  had  to  be  replaced  after 
three  weeks. 

PREPARATION  OF  SAMPLES  AND  NUMBER  OF 
SAMPLES 

For  tests  of  preservatives  a  uniform  twine  Number  as 
cotton  Nm  50/15  (Ne  30/15)  is  recommended.  The 
material  should  not  be  too  strong,  or  the  test  will  take 
too  long,  but  it  should  not  be  too  fine  because  the 
differences  of  the  preservation  should  be  distinctly 
demonstrated. 

The  samples  are  cut  into  pieces  of  twine  60  cm.  long 
and  treated  with  the  preservative.  The  increase  in  weight 
by  the  treatment  may  be  determined,  the  stiffness 
measured  and  the  content  of  certain  substances  deter- 
mined, etc.  In  any  case,  the  wet  strength  after  the 
treatment  has  to  be  determined. 

The  twines  are  then  folded  together  and  bound  into 
bunches  30  cm.  long.  It  is  important  not  to  use  binding 
material  which  may  rot  and  influence  the  preservative, 
or  which  may  even  be  destroyed  by  it.  The  bunches 
would  get  loose  and  be  lost. 

The  free  end  of  each  twine  is  knotted  to  prevent  the 
bunch  from  becoming  matted.  The  bunches  are  sufficient 
for  6  tests  of  10  twines  each  or  12  tests  of  5  twines  each. 
Thicker  bunches  are  not  recommended  as  they  might 
influence  decay  of  breaking  strength. 

It  is  not  necessary  to  rinse  the  bunches.  As  with 
fishing  nets,  rinsing  occurs  automatically  during  the 
first  days.  Differently  treated  samples  should  not  be 
exposed  together  as  they  may  have  a  reciprocal  influence. 

DESCRIPTION  OF  EXPERIMENT 

The  bunches  are  fastened  to  a  cord  of  Perlon.  nylon  or 
similar  material,  the  distance  between  them  being  about 
10  cm.  They  are  suspended  so  that  they  neither  emerge  nor 
touch  the  bottom  or  piles,  etc.  In  tidal  waters  the  samples 
must  be  suspended  so  deeply  that  they  do  not  get  dry. 
Tests  are  carried  out  to  obtain  comparative  values. 


It  is  suggested  that  the  preservatives  should  be  tested 
several  times,  in  order  to  obtain  a  sure  average  value  of 
the  method  chosen  as  a  standard.  For  instance,  the 
following  are  used  as  standard  preservatives: 

Cutch  |  Testalin. 

The  same  with  Carbolineum. 

Cutch,  Potassium- Bichromate  (or  copper  vitriol  with 

ammonium), 
as  above,  with  Carbolineum. 

Others  may,  of  course,  be  used  for  these  experiments. 

All  samples  are  simultaneously  put  into  the  test  water. 
Considering  the  climatic  conditions  in  Europe,  it  is 
thought  best  to  remove  the  first  sample  of  5  or  10 
twines  after  one  month,  and  if  no  decrease  in  the  breaking 
strength  is  found,  the  next  samples  may  be  removed  at 
intervals  of  two  or  even  three  months.  A  test  should  be 
made  every  two  months  in  summer  and  every  three 
months  in  winter  if  the  initial  decay  does  not  indicate 
tighter  control. 

Removed  twines  must  be  cleaned  of  weeds,  etc.,  and 
their  breaking  strengths  tested  immediately  in  the  wet 
condition.  If  it  is  impossible  to  do  this  at  once,  the  twines 
should  be  dried  and  kept  for  a  later  test  of  wet-strength. 
Further  decay  may  be  prevented  by  using  disinfectants. 
The  wet-strength  test  is  made  with  conventional  testing 
instruments  used  in  textile  research. 

EVALUATION  OF  THE  RESULTS 

The  initial  breaking  strength  (A)  of  the  material  to  be 
tested  is  expressed  in  kg.  (wet).  The  retained  strength 
(Blf  B2,  etc.  Bx)  is  tested  after  certain  periods  and  is 
computed  as  percentage  figures  of  the  initial  strength : 

Retained  strength  per  cent.    =- 


(1) 


(2)     Loss  in  strength  per  cent. 


,oo-L°°-xB* 

A 


Net  twines  are  considered  to  be  unserviceable  when 
the  loss  in  breaking  strength  exceeds  50  per  cent,  of  the 
initial  strength.  This  loss  therefore  is  considered  the 
value-limit. 

Similarly,  the  loss  of  50  per  cent,  of  the  initial  strength 
of  treated  material  determines  the  evaluation  of  a 
preservative.  This  will  be  explained  by  an  example: 

In  Table  II  the  wet  breaking  strengths  of  4  net  twine 
samples  preserved  by  methods  A,  B,  C  and  D  are  given 
in  kg.  for  certain  days.  The  test  No.  1  is  the  initial 
strength. 

TABLE  II 


Test  Number 
Date 

1 
1.1. 

2 
15.1V. 

3 
20.VI. 

4 
2I.VIII. 

5 
I5.XI. 

A 

6-0 

4-6 

4-1 

3-4 

l-6kg 

B 

5-9 

4.4 

3-2 

1-7 

0-3 

C 

5-8 

4-2 

2-9 

1-7 

0-3 

D 

6-4 

4.9 

1-8 

01 

— 

Table  III  gives  the  several  retained  breaking  strengths 
in  per  cent. 


M35] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


TABLF  III 


Test  Number 
Date 

1 
I.I. 

2 
15.1V. 

A 

100 

77 

B 

100 

75 

C 

100 

72 

D        . 

100 

77 

3 
20.  VI. 

4 
2I.VIII. 

5 

15.X1. 

68 

57 

27 

54 

29 

5 

50 

30 

5 

28 

2 

Sample  A  lost  50  per  cent,  of  its  strength  between  the 
fourth  and  fifth  tests,  sample  B  between  the  third  and 
fourth,  sample  C  in  the  third  and  sample  D  between 
the  second  and  third  test.  It  is  assumed  that  the  rotting 
activities  mentioned  in  Table  I  were  existent  throughout 
the  experiment.  The  rotting  activity  required  to  reduce  the 
initial  strengths  of  the  test-material  by  50  per  cent,  test 
figure  T,  is  computed  according  to  the  following  formula: 


(3)    T-- 


50) 
—     i    t. 


tx)  x  (R, 
R,       R2 

In  this  formula  ta  means  the  rotting  activity  which 
became  effective  when  the  test-material  had  lost  more 
than  50  per  cent,  and  tj  means  the  rotting  activity  before 
that  state  was  reached.  The  desired  test-figure  T  must  lie 
between  these  two  figures.  Rl  is  the  breaking  strength 
before  and  R2  the  breaking  strength  after  the  said  state. 

The  rotting  activities,  as  explained  in  the  above 
example,  result  according  to  Table  I  in  the  following: 

Period  1.1.  to  15.1V.:  January  54 

February  54 

March  78 

until     April  1 5th  50  (15  x  3-3) 

236 

Rotting  activity  is  computed  from  the  daily  figures  for 
months  which  have  not  been  completely  tested. 


Period  I.I.  to  20.VI.:  Jan.  to  May 
until  June  20th 


Period  I.I.  to  21.VIII.  Jan.  to  June 


479 
166 

645 
1000 


until  August  21st     172 


Period  I.I.  to  I5.XI.      Jan.  to  Oct. 
until  Nov.  15th 


1172 

1702 
56 

1758 


Medium  A     T 


Medium  B     T 


1172 


I    1172       1309—1310 


4  645 


'   645       729-730 


If  the  obtained  values  are  inserted  into  the  formula 
(3),  the  following  values  are  obtained  as  test-figures  for 
the  preservatives  A  to  D: 

(1758       1172)  x  (57  -•  50) 
(57-    27) 

4102 
30 

(1172-645)  v  (54      50) 
54       29 

T       21°8 

25 

Medium  C  :   As  the  date  was  exactly  the  20th  June,  T    645 

(645  -    236)  *'  (77         50) 
(77       28) 

T       M™3    •   236      461-460 

The  greater  the  test-figure,  the  greater  the  performance 
of  the  preservative  method. 

EVALUATION  OF  THE  TEST-FIGURE 

The  efficiency  of  the  several  methods  of  preservation 
can  be  summed  up  as  follows: 

Test  figure  up  to  200:     poor  effect. 

200  to  500:    minor  effect. 
500  to  1000:    medium  effect. 
1000  10  2000:     good  effect. 
above  2000:     very  good  effect. 


Medium  D     T 


!-  236 


Two  boat  midwaler  trawling  on  the  Malabar  coast  oj  India — an  age  old  method  ! 

[136] 


DISCUSSION   ON   NET  PRESERVATION 


Dr.  Reuter,  Rapporteur.  When  using  natural  fibres  the 
fisherman  must  periodically  treat  his  nets  with  preservatives 
against  rotting,  as  he  knows  from  experience  that  if  this  is 
not  done  regularly  and  properly  his  nets  will  deteriorate  very 
quickly.  His  knowledge  of  preservation  is  based  mainly  on 
practical  experience.  The  available  data  from  preservation 
experiments  with  textile  yarns  are  not  directly  comparable 
as  these  arc  invariably  conducted  on  materials  in  dry  condition, 
whereas  fishing  nets  are  always  used  in  wet  condition.  Such 
tests  can  therefore  only  be  used  as  a  guide.  The  fishermen  in 
various  countries  and  even  in  different  areas  of  the  same 
country  use  different  preservation  methods,  each  being  con- 
vinced that  his  method  is  the  best.  Some  only  dry  the  nets, 
others  use  coaltar  while  a  third  may  use  cutch;  others  ma> 
use  linseed  oil,  copper  sulphate  or  a  combination  of  two  or 
three  products.  Very  rarely  can  they  give  reasons  for  their 
choice  and  they  are  normally  very  reluctant  to  change  the 
custom. 

In  his  paper,  Takayama  considers  the  different  types  of 
preservatives  and  their  action,  showing  that  some  kill  the 
bacteria  while  others  make  it  physically  impossible  for  the 
bacteria  to  attack  the  fibre. 

The  decisive  factors  are  the  Q/-contenl  in  copper  com- 
pounds and  the  tannin  content  in  cutch  or  catechu.  When 
buying  such  preservatives  the  fisherman  is,  therefore,  only 
interested  in  the  amount  of  these  active  agents,  i.e.  the  per- 
centage of  Cu  or  tannin — but  not  in  the  bulk  alone.  Gear 
Research  laboratories  can  render  valuable  service  in  analysing 
the  various  brands  of  preservatives,  determine  the  percentage 
of  active  ingredients  and  advise  on  proper  measures  to  prevent 
leaching  out  of  the  preservatives  by  fixing  them  chemically 
or  by  waterproofing. 

When  conducting  preservation  experiments,  one  of  the 
difficulties  is  to  obtain  results  that  can  be  compared  with 
those  obtained  at  a  different  season  or  in  a  different  area. 
Dr.  von  Brandt  gives  a  solution  to  this  problem  in  his  paper; 
his  method  not  only  gives  a  measure  of  the  "rotting  value" 
of  the  medium  in  which  the  experiment  is  conducted,  but 
also  the  value  of  the  preservation  agent  used.  The  values 
obtained  are  expressed  in  numbers  by  which  the  rotting 
value  of  different  waters  and  the  preserving  power  of  different 
preservation  methods  can  be  expressed. 

The  way  in  which  this  is  done  is  as  follows:  While  untreated 
cotton  yarns  are  subjected  to  rot  the  loss  of  breaking  strength 
is  determined  regularly  and  the  percentage  of  loss  added. 
This  number  gives  the  rotting  power  of  that  period.  A  treated 
yarn  is  tested  in  the  same  way,  but  the  experiment  is  stopped 
as  soon  as  the  breaking  strength  has  decreased  by  50  per  cent. 
The  total  percentage  of  white  yarn  decrease  of  breaking 
strength  during  this  period  is  an  indication  of  the  "preserva- 
tive power.'* 

All  natural  fibres  are  open  to  attack  by  small  organisms, 
so  that  all  have  to  be  protected  against  rot  and  loss  of 
strength.  In  the  face  of  growing  competition  of  the  non- 


rotting  synthetic  fibres,  the  producers  of  natural  fibres  arc 
trying  to  find  new  ways  of  immunizing  them  against  micro- 
organisms, and  whereas  all  older  methods  are  based  on 
preventing  the  organisms  from  reaching  the  fibre  by  giving 
it  a  protective  ^coating,  new  methods  are  being  considered 
which  chemically  change  the  fibres  so  that  they  become 
indigestible  to  the  micro  organisms. 

The  papers  by  Sandoz  and  Ciba  Ltd.,  both  describe  such 
methods  which  are  claimed  to  give  good  and  long  lasting 
protection  without  causing  major  changes  in  the  physical 
properties  of  the  fibre.  This  last  is  very  important  as  most 
protective  preservation  methods  do  change  some  of  the 
physical  properties. 

Although  preservatives  are  mainly  used  for  natural  fibres, 
there  are  circumstances  where  they  arc  useful  for  the  synthetic 
fibres  too.  Vinylon  nets  and  lines  are  generally  tarred  in 
Japan.  Other  synthetics  are  sometimes  coal-tarred  to  give 
them  more  weight  or  stiffness,  to  give  protection  against 
abrasion,  knot  slippage,  etc.  In  some  cases  synthetic  fibres 
are  dyed  or  otherwise  treated  to  prevent  deterioration  through 
exposure  to  sunlight. 

Dr.  W.  Hagenbuch  (Switzerland).  1  am  representing  the 
chemical  firm  Sandoz  in  Basle,  Switzerland.  Dr.  Renter  has 
mentioned  modified  natural  fibres  which,  of  course,  are 
something  different  from  synthetic  or  man-made  fibres.  1 
would  like  to  make  a  few  additional  remarks  on  acctylated 
fibres  such  as  cotton,  man i la,  sisal,  etc. 

Acetylated  cotton  has  been  produced  on  an  industrial  scale 
for  about  25  years  according  to  a  process  which  has  been 
worked  out  by  us  in  Basle,  and  consists  in  a  reaction  of  the 
hydroxile  group  of  cellulose  with  acetic  acid  or  acetic  anhy- 
dride, forming  a  new  chemical  compound.  The  result  actually 
is  a  new  fibre  and  the  process  is  not  comparable  with  a  coating 
or  a  preservative.  It  is  not  cotton  any  more.  Until  now, 
acetylated  fibres  have  been  used  mainly  in  the  textile  field 
because  of  their  special  properties  in  regard  to  dyeing. 
Acctylated  cotton  does  not  take  up  a  dye-stuff  as  normal 
cotton  does.  This  field,  of  course,  is  of  no  interest  here,  and 
the  remark  was  made  only  to  show  how  and  why  this  has 
developed.  Since  about  10  years  it  has  been  used  in  the 
laundry  industry  because  of  its  exceptional  heat  resistance, 
which  is  about  six  to  seven  times  higher  than  with  normal 
cotton. 

In  recent  times  we  have  started  studying  the  rot  resistance 
of  this  acctylated  material  which  proved  to  be  very  good  in 
sea  water.  This  rot  resistance  is  permanent  and  the  treatment 
does  not  smell  or  discolour  the  fibre  and  is  not  poisonous. 
Acetylated  cotton  looks  exactly  like  normal  cotton  and  can 
only  be  determined  by  chemical  methods. 

Under  the  name  of  Otoba  it  has  been  produced  in  England 
since  1927.  At  the  present  experiments  are  going  on  in  Scotland 
with  herring  nets  and  so  far  in  li  seasons  these  nets  have 
been  behaving  exceptionally  well.  The  price  is  at  the  moment 


137] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


10  to  30  per  cent,  less  than  for  synthetic  fibres,  but  it  can  be 
expected  that  with  increased  production  a  considerable 
decrease  of  price  will  take  place,  and  then  I  think  acctylated 
cotton  will  become  a  very  interesting  new  fibre  for  the 
fishing  industry. 

Mr.  A.  Rupert i  (Switzerland):  I  am  representing  the  firm 
Ciba,  Basle.  We  also  have  elaborated  a  procedure  to  prevent 
rotting  by  converting  cellulose  without  chemically  modifying 
the  fibre  itself.  With  this  socalled  "Arigal"  procedure  we 
also  achieve  permanent  protection,  and  this  is  what  really 
counts  in  fishery. 


Mr.  H.  Levy  (Morocco):  For  12  years  past  I  have  been 
engaged  in  preserving  fishing  gear  made  of  natural  fibres, 
especially  cotton.  In  Morocco  we  have  three  preservation 
methods,  i.e.  tarring  and  tanning  with  two  different  materials. 


Tarring  is  used  only  for  twines  that  are  constantly  submitted 
to  abrasion,  i.e.  certain  parts  of  the  big  trap  nets  (madrague), 
the  lower  parts  of  trawl  nets  and  especially  the  bags  of  sardine 
seines.  Tanning  with  catechu  gives  satisfactory  results, 
i.e.  for  the  sardine  net.  The  third  method  consists  in  tanning 
with  ground  pine  bark  which  is  very  rich  in  tannin  and,  by 
the  way,  is  abundant  in  North  Africa.  This  gives  really 
excellent  results.  But  in  recent  years  the  Moroccan  fishermen 
have  found  a  combination  of  half  catechu  and  half  ground 
pine  bark  that  gives  results  one  would  not  even  have  dared  to 
hope  for.  1  would  recommend  strongly  to  all  fishermen  to 
give  this  new  tanning  method  a  test.  The  effect  of  tanning  is 
improved  by  subsequent  tarring  to  retard  the  tannin  being 
washed  out.  Due  to  these  preservation  treatments  in  Mor- 
occo lines  and  ropes  are  worn  out  rather  than  deteriorated 
by  putrefaction.  But  it  must  be  said  that  this  preservation 
effect  is  only  obtained  if  the  fishing  gear  is  dried  once  every 
week,  or  at  least  twice  a  month. 


Left:   Operator  i.\  testing  the  breaking  strength  and  extensibility  of  fishing  twine.      Right:  With  hand  and  fool  operated  net  weaving  looms 
such  as  the  above,  nets  can  he  knitted  three  or  four  times  faster  than  by  hand  braiding. 


(  138  ] 


Section  5 :  Relative  Efficiencies  of  Nets  Made  of  Different  Materials. 

THE   EFFICIENCY   OF  SYNTHETIC  FIBRES   IN   FISHING, 
ESPECIALLY   IN   GERMANY 

by 
GERHARD  KLUST 

Institut  fur  Netz-  und  Materialforschung,  Hamburg,  Germany 

Abstract 

Gear  is  divided  into  three  groups,  according  to  strain  on  net  material  during  operation.  An  attempt  is  (hen  made  to  classify  various 
fibres  (polyvinyl  chloride,  polyvinyl  alcohol,  polyester,  polyamidc,  polyethylene,  polypropylene)  by  referring  them  to  these  groups. 

Net  twines  of  these  synthetic  fibres,  and  also  net  twines  of  vegetable  fibres,  are  compared  with  regard  to  breaking  strength  (knotted 
and  wet),  extensibility  and  elasticity,  and  resistance  to  weather  and  abrasion.  An  estimate  is  then  made  of  the  suitability  of  the  different 
fibres  to  various  types  of  fishing  gear. 

For  the  fine  gillnets,  polyamide  monofi lament  and  twines  of  either  polyamide  high  tenacity  filament  or  of  polyester  filament  are 
most  suitable.  For  herring  and  salmon  drift  nets,  twines  of  polyamide  or  polyester  filament,  and  also  twines  of  polyvinyl  alcohol,  are  recom- 
mended. Polyamide  staple  should  not  be  used. 

For  standing  gear,  such  as  trap  nets  (especially  when  much  exposed  to  sunlight),  twines  of  polyvinyl  chloride  fibres  (PCU,  Rhovyl, 
Feviron),  may  be  used.  Plaited  twine  made  of  polyamide  filament  (Perlon,  nylon),  which  has  very  high  tensile  strength,  abrasion  resistance 
md  elasticity,  is  considered  the  best  net  material  for  very  heavily  strained  gear,  such  as  bottom  trawl  nets. 

L'efficaeite  des  fibres  synthctiques  dans  la  peche  specialemcnt  en  Allemagne 
Resume 

Les  engins  sont  divises  en  3  groupes  d'apres  TerTort  exerce  sur  la  ma  tic  re  du  filet  pendant  la  peche.  L'auteur  a  fait  unessaide 
classification  des  diverses  fibres  (chlorure  de  polyvinyle,  polyacrilonitrile,  alcool  de  polyvinyle,  polyester,  polyamide)  en  les  rapportant  a  ces 
groupes. 

Les  fits  a  filet  de  ces  fibres  synthetiques.  et  aussi  les  tils  a  filet  dc  fibres  vegetales,  sont  compares  en  ce  qui  concerne  la  resistance  a 
la  rupture  (noues  et  mouill6s),  1'extensibilite  et  Telasticite,  et  la  resistance  aux  intempenes  et  a  1'anrasion.  L'auteur  cstime  cnsuite  a  quel 
degr6  les  differentes  fibres  conviennenl  aux  divers  types  d'engins  de  peche. 

Pour  les  filets  maillants  fins,  le  polyamide  monofilament  ct  les  tils  dc  polyamide  a  tenacite  elevee  on  de  filaments  de  polyester  con 
vienncnt  le  mieux.  Pour  les  filets  denvunts  a  harengs  et  a  saumons  on  recommande  les  files  de  filaments  de  polyamide  on  de  polyester,  et 
aussi  les  fils  d 'alcool  de  polyvinyle.  II  ne  faut  pas  utiliser  de  polyamide  tresse. 

Pour  les  engins  fixes  comme  les  filets-trappes  (specialcment  quand  ils  sont  beaucoup  exposes  a  la  lumiere  solairc),  les  fils  de  fibres  de 
chlorure  de  polyvinyle  (PCU,  Rhovyl,  Teviron),  pcuvent  etre  utilises.  Le  fil  tresse  de  filaments  de  polyamide  (Perlon,  nylon),  qui  possede 
unc  tres  grande  resistance  a  la  traction,  a  Tabrasion  ct  une  grande  clasticite  est  considere  comme  etant  le  meilleur  material!  pour  fabriquer 
des  engins  supportant  de  grands  efforts  comme  les  chaluts  de  fond. 

La  eficacia  de  las  fibras  sinteticas  en  la  pesca,  espmalmenle  en  Aleniiinia 
Kxtracto 

Como  los  artes  de  pesca  puedcn  dividirsc  en  ires  grupos  segun  el  esfuerzo  a  quc  se  somete  el  material,  sc  ha  tratado  dc  clasificai 
las  diversas  fibras  (cloruro  de  potmnilo,  policrilomtrilo,  alcohol  de  polivinilo,  poliester,  poliamida)  en  cada  uno  de  ellos. 

Para  determmar  la  rcsistencia  a  la  ruptura  (anudados  y  hiimedos),  alargamiento,  clasticidad  y  resistencia  a  los  agcnies  climaticos 
y  al  roce,  sc  procedio  a  evaluar  las  caracteristicas  de  los  hi  los  dc  estos  materiales  smteticos  con  los  de  origen  vegetal  empleados  en  la  fabricaci6n 
de  redes.  Luego  se  valoraron  las  maneras  como  se  prestan  las  caracteristicas  de  las  diversas  fibras  para  tejer  los  artes. 

F.n  el  caso  dc  redes  de  enmalle  finas  son  mas  adecuados  los  hilos  de  poliamida  de  una  sola  hebra  o  de  vanos  filamentos  con  gran 
resistencia,  ya  sea  dc  este  material  o  de  poliesteres,  asi  como  tambicn  los  de  alcohol  de  polivinilo.  Para  las  redes  dc  dcriva  que  se  utilizan  en 
la  pesca  de  a  re  n  que  y  salmon  se  recomiendan  los  hilos  de  poliamida  o  poliester  y  tambicn  las  de  alcohol  de  polivinilo.  Fn  la  fabricacion  de 
estos  no  conviene  usar  material  preparado  a  base  dc  poliamida. 

Para  los  artes  fijos,  como  las  redes  trampas  ( especial mente  cuando  quedan  expuestas  a  la  luz  durante  largo  tiempo,  deben  usarse 
hilos  formados  por  fibras  dc  cloruro  de  polivinilo  ("PCU",  "rhovyl",  "teviron")  al  tratarse  de  artes  sujetas  a  grandes  esfuer/.os  como  las 
redes  de  arrastre  de  fondo,  es  mas  convementc  utilizar  hilo  de  hebras  de  poliamida  (perlon,  nylon),  tren/adas  que  poseen  gran  elastic  id  ad, 
resistencia  a  la  tension  y  al  roce. 


I.  PROOF  AGAINST  ROT  -THE  MOST  IMPORT- 
ANT PROPERTY  OF  SYNTHETIC  FIBRES  IN 
FISHING 

THE    most    important    raw    materials    for    nets   in  Factors  affecting  the  durability  of  the  net  of  vegetable 

European  fisheries  have  been  vegetable  fibres  fibres,  and  of  more  importance  than  this  resistance  are 
cotton,    flax,    hemp,    sisal    and    manila.    When  1.    Duration   of  immersion   in   water:    Fishing  gear 

immersed  in  water,  they  are  exposed  to  cellulose-digesting  left  in  water  for  a  long  time  arc  more  liable  to  rotting 

micro-organisms,  especially  bacteria.  The  resistance  of  than  those  used  only  temporarily.  Rotting  is  stopped 

vegetable  fibres  increases  in  the  following  order:  flax,  only  when  the  nets  are  dried  completely,  also  inside  the 

hemp,  ramie,  jute,  cotton,  sisal,  manila,  coir  (Klust  1961).  knots. 

[139] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


2.  Water  temperature:  The  warmer  the  water,  the 
quicker  the  rate  of  rotting1. 

3.  Kind  of  water:  Due  to  intensive  rotting4  nets  will 
be  destroyed  considerably  sooner  in  eutrophic  than  in 
oligotrophic  or  dystrophic  waters-'  3. 

Jn  middle  European  eutrophic  water,  with  higher 
temperature,  unpreserved  cotton  nets  may  become  useless 
after  7  to  10  days.  Only  very  intensive  preservation 
methods  can  prolong  their  durability5-  fi.  Preservation 
is  most  effective  for  cotton  nets  whereas  efficiency  is  poor 
with  bast  fibre  or  hard  fibre  nets. 

The  durability  of  fishing  gear  is  primarily  affected  by 
rotting.  The  original  cost  is  increased  disproportionately 
owing  to  frequent  renewals  and  preservation  expenses. 
The  smaller  the  business,  the  greater  are  the  relative 
expenses  for  nets. 

In  a  report  on  nylon  as  net  material,  Needham  writes7: 
"Nylon  brings  to  one  of  man's  oldest  occupations  the 
miracle  of  science  and,  in  doing  so,  provides  easier  living 
for  the  fisherman  ..."  This  statement  can  apply  to  all 
synthetic  fibres,  for  this  miracle  is  apparent  in  their  proof 
against  rot.  The  first  synthetic  fibre,  known  as  PeCe 
(polyvinyl  chloride),  was  developed  by  the  former 
German  I.G.  Farbenindustrie,  in  1931.  Some  years  later, 
twines  made  of  these  fibres  began  to  be  used  for  smaller 
gears  in  German  inland  fisheries  8-  y,  and  its  rot-proof 
quality  became  apparent.  This  meant  much  greater 
durability  of  gears  (fyke  nets  made  of  PeCe,  have  been 
in  continuous  use  for  over  15  years),  less  work  and  lower 
preservation  expenses,  obviating  the  time  consuming 
drying  of  nets  and  thus  ensuring  longer  catching  periods. 
Apart  from  this  rot-proofness  which  is  the  most  import- 
ant characteristic  of  synthetic  fibres,  other  important 
properties  are:  breaking  strength,  extensibility,  elasticity, 
abrasion  resistance,  diameter,  weight,  stiffness,  resistance 
to  weathering,  knot  stability  and  behaviour  in  water, 
including  change  of  length,  sinking  speed  and  visibility. 

II.  TYPES  OF  FISHING  GEAR,  WITH  REGARD  TO 
THE  MATERIAL  AND  KINDS  OF  SYNTHETIC 
FIBRES  USED 

A  great  variety  of  gear  is  used  in  fishing,  differing  very 
much  in  size,  structure,  method  and  purpose  of  use. 
They  have  been  classified  by  v.  Brandt10.  According  to 
strain  on  the  net-material  German  fishing  gear  can  be 
divided  into  three  groups11: 

1.  Low  Strain 

Fine  gillnets  belong  to  this  first  group.  The  traditional 
material  is  fine  cotton  twine  of  metric  counts  270/6  to 
70/9  (English  counts  160/6  to  40/9)  or  similar  thickness, 
with  a  diameter  of  less  than  0-40  mm.  and  a  breaking 
load  (wet)  not  higher  than  about  3  kg. 

2.  Medium  Strain 

This  group  is  formed  by  the  various  fishing  gears  of 
the  German  inland,  coast  and  deep-sea  fisheries  which 
formerly  were  made  of  cotton  twines,  of  metric  counts 
20  and  50  (English  counts  12  and  30).  Fine  twines  of 
flax  or  hemp  were  occasionally  used.  This  includes: 

most  fishing  lines 

baskets,  also  trap  nets  and  box  nets 


scoops 

gape  nets  (without  those  mentioned  above) 

dragged  gears  also  bottom  trawls  of  smaller  vessels 

floating  trawls  of  cutters 

seines  and  purse  seines 

dip  nets  and  lift  nets 

drift  nets 

3.    Heavy  Strain 

Bottom  trawls  of  large  vessels  (trawlers  and  cutters) 
and  gape  nets  set  in  fast  running  rivers  belong  to  this 
group.  They  were  made  of  thick  manila,  sisal  or  hemp 
twines,  with  diameters  up  to  3  -9  mm.  and  breaking  loads 
up  to  nearly  200  kg. 

Table  I  gives  the  characteristics  relevant  to  fisheries 
purposes  for  the  most  important  synthetic  fibres,  and 
shows  for  which  of  the  three  groups  these  fibres  could  be 
used.  In  the  column  "trade  names",  only  the  best-known 
are  listed.  Only  nylon  and  Perlon  are  given  for  the 


TABU    1 
Fibre          Trade  Nantf\  Characteri\ttc\ 


Suitable 
for  u\e  in 


Polyvinyl         F:  Rhovyl  medium  breaking  strength. 
Chloride          G:  PCU,  Rhovyl   medium  abrasion  resist.,          group  2 

J:  F.nvilon.  very   good    resistance   to 

'I  eviron  weathering 

Polyvinyl         J:    Vinylon,  low  price,  medium  breaking 

Alcohol  Manryo,  strength,  medium  abrasion      group  2 

Kuralon,  resistance,  good  resistance 

Trawlon,  to  weathering 

Cremona, 

Mcwlon 
G:  PVA 


Polyester 


GB:  Terylene 
USA:  Dacron 
G:  Diolen, 
Trcvira 
1:  Tergal 
I:    Tcrital 
J :   Tetoron 


very  high  breaking  strength, 
low  extensibility,  groups   1 

medium  abrasion  ricsistance,    and  2, 
relatively  good  resistance        group  3? 
to  weathering 


Polyethylene  GB:  CourleneX3,  high  breaking  strength, 

Drylene         very  high  abrasion 
N:  Nymplcx  resistance,  wiry, 

G:  Polyathylen-     swimming  in  water 
Hoechst 


group  2, 
group  3? 


Polypropy- 
lene 


Polyamide 
mono- 
filament 


Polyamide 
staple 


Polyamide 
continuous 
filament 


I'    Meraklon 
USA 


Nylon-monofil., 
Perlon-monofil., 
Platil 

Nylon — staple, 
Perlon-   staple 


low  price,  medium  to 

high  breaking  strength,  low  group  2. 

resistance  to  weathering,  gr.la.3? 
swimming  in  water 

transparency  (little  visible-  group   I, 

ness  in  water),  very  good  group  2: 

resistance  to  weathering,  wiry  fyke  nets 


high  breaking  strength, 

high  abrasion  resistance, 

good  knot  stability,  group  2 

very  high  extensibility, 

low  resistance  to  weathering 


Nylon — filament    very  high  breaking  strength,    all    3 
Perlon — filament  very  high  abrasion  resistance,  groups 
high  elasticity,  low  of  gear 

resistance  to  weathering 


F=France,   GB=Great  Britain,   G— Germany,   I  — Italy,   J-Japan, 
N  = Netherlands. 


f  1401 


EFFICIENCY     OF    SYNTHETICS     FOR     FISHING 


polyamides,  although  fibres  of  this  group  and  of  equal 
value  to  fishing  gear  are  produced  in  different  countries 
under  various  names,  such  as:  Amilan,  Anzalon,  Dayan, 
Dederon,  Ducilon,  Enkalon,  Fcfcsa,  horlion,  Grilon, 
Kapron,  Kenlon,  Knoxlock,  Nylock,  Rilsan,  Silon, 
Steelon  and  Tynex. 

III.  EVALUATION  OF  THE  MOST  IMPORTANT 
PROPERTIES  OF  SYNTHETIC  FIBRES  USED  IN 
FISHING 

Properties  of  primary  importance  in  the  evaluation  of 
net  twine  are  rot-proofncss,  breaking  strength,  extensi- 
bility and  abrasion  resistance.  Resistance  to  weathering 
may  be  of  primary  importance  for  fishing  gears,  which 
arc  used  just  below  the  surface  or  partially  out  of  water. 
Based  on  experience  in  German  fisheries  and  on 
investigations  by  the  Institut  fur  Netz-und  Material- 
forschung,  an  attempt  is  made  below  to  evaluate  these 
special  properties. 

1.     Breaking  strength,  in  knotted,  wet  condition 

Twines  of  cotton,  hemp  and  manila  arc  strongest  in 
wet  condition.  With  synthetic  fibres  there  are  groups 
which  have  the  same  or  nearly  the  same  strength  wet  or 
dry:  Polyvinyl  chloride  (PeCc,  PCU,  Rhovyl),  Polyester 
(Terylcne,  Dacron,  Trcvira,  Diolen),  polyethylene  poly- 
propylene and  some  copolymcrs  (Saran,  Vinyon,  Dyncl, 
Acrylan).  Other  groups  show  a  loss  of  strength  when 
wet,  such  as:  polyamide  (Nylon,  Perlon),  polyacryloni- 
trile  (Orion,  PAN),  polyvinyl  alcohol  (PVA,  Manryo, 
Kuralon,  Vinylon).  Diminution  of  strength  occurs  as 
soon  as  the  net  twines  come  in  contact  with  water.  After 
a  24  hour  immersion  the  following  approximate  losses  of 
strength  were  observed:  twines  made  from  polyamide 
staple  and  cont.  filament  J3  to  17  per  cent.,  polyamide 
monofilament  nearly  20  per  cent.,  twines  made  from 
polyvinyl  alcohol  staple  about  25  per  cent. 

During  prolonged  immersion  in  water  extending  over 
several  months,  a  further  gradual  decrease  of  strength 
takes  place,12  though  by  no  means  comparable  to  that 
of  even  well-preserved  net  twines  of  vegetable  fibres. 

It  is  of  importance  to  know  the  decrease  in  strength 
caused  by  knotting.  This  is  connected  closely  with  the 
properties  of  the  fibre  substance.  Continuous  filaments  of 
polyamide  and  polyester  are  of  high  tenacity  (highly 
stretched)  in  order  to  obtain  as  high  a  strength  as  possible, 
and  have  a  lower  extensibility.  Unfortunately,  they  show 
a  diminution  of  knot  strength  too.  Losses  of  strength 
of  wet  twines  by  knotting  (in  the  average): 

Twines  made  from  losses 

in  per  cent 

manila,  medium  fine  tw.  30 

manila,  thick  twines  40 

hemp  25 

cotton  30 

Perlon  staple  35 

Nylon  contin.  fil.,  fine  tw.  33 

Nylon  contin.  fil.,  medium  fine  tw.  43 

Nylon  contin.  fil.,  thick  tw.  50 

Perlon  contin.,  twisted      (trawl  twines)  50 

Perlon  contin.,  braided     (trawl  twines)  42 

Peilon  monofilament     (trawl  twines)  M) 

Polyester  contin.,          (trawl  twines)  50 

Polyethylene  36 

Net  material,  therefore,  has  to  be  evaluated  according 

to  its  strength  in  knotted  and  wet  condition.  Table  II  gives 


TABLE  II 
Breaking  strength  of  net  twines,  single  knot  and  wet 


Twines  matlejrom 


Break  ing  Length  Tensile  Strength 
km.  kg. /  mm.2 


Polyvinyl  chloride 

Rhovyl-Hbre  (staple) 

6-4 

9-0 

Rhovyl,  continuous  filament 

7-6 

10  6 

PCU,  staple 

8-2 

11-5 

Polyvinyl  alcohol,  staple 

10-2 

LV3 

Polyacrylonitrile,  continuous  filament 

14-5 

17-0 

Polyester,  continuous  filament 

26-3 

36-3 

Polyamide,  staple 

15-7 

17-9 

Polyamide,  continuous  filament     . 

30-1 

34-3 

Cotton           .... 

12-2 

18-5 

Hemp 

28-9 

42-8 

such  an  evaluation  for  net  twines  from  group  2.  Based 
on  tests  of  a  great  number  of  net  twines  of  different 
thickness,  the  breaking  length  in  km.  and  tensile  strength 
have  been  calculated.  For  calculating  the  tensile  strength 
(kg./mm.2)  the  following  specific  gravities  (which  are 
mostly  taken  from  the  list  of  Grunsteidl  and  Preussler16) 
have  been  used: 

Polyvinyl  chloride  -40  g./cm. 

Polyacrylonitrilc  -17 

Polyvinyl  alcohol  -30 

Polyester  -38 

Polyamide  -14 

Cotton  -52 

Hemp  1-48 

Hence  of  the  synthetic  net  twines  considered  here, 
those  of  continuous  polyamide  and  polyester  are  by 
far  the  strongest.  Their  wet  knot-strength  is  twice  that  of 
cotton  twines. 

2.     Extensibility  and  Elasticity 

The  extensibility  of  net  twines  consists  of  several  com- 
ponents: that  of  the  fibre,  the  single  yarn,  and  finally 
extensibility  of  the  end  product  or  twine.  The  inherent 
extensibility  of  synthetic  fibres  varies,  being  large  for  the 
polyamide  group  and  comparatively  small  for  the 
polyester  group.  Short  staple  fibres  produce  a  higher 
extensibility  than  continuous  filaments.  Hard  twisted 
yarn  is  more  extensible  than  a  soft  twisted  one.  The 
manufacturing  process  of  the  final  products  is  also  of 
great  importance.  The  more  the  twine  is  twisted  or  the 
tighter  the  braid  is  plaited,  the  greater  the  extensibility. 
To  obtain  a  high  extensibility,  it  is  better  to  use  fibres  with 
a  large  original  extensibility  than  to  use  hard  twisting  of 
yarn  and  twine  to  attain  this  high  extensibility. 

Total  extensibility  comprises  a  part  of  elastic  extension 
and  the  permanent  elongation.  Apart  from  the  nature  of 
the  fibre  and  the  manufacturing  process,  their  value  is 
dependent  on  the  applied  load.  Where  the  total  extension 
results  mainly  from  the  manufacturing  process,  i.e.,  from 
twist  of  yarn  and  twine,  elasticity  will  be  small,  and  the 
permanent  elongation,  which  remains  after  unloading, 
will  be  comparatively  great. 

Net  twine  has  a  greater  efficiency,  if  total  extension  and 
elasticity  are  high.  It  will  be  able  to  absorb  kinetic 


[141  ] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


Fig.  /.    Load-extension  curves. 

energy  and  will  take  up  shock  loads  better  than  net  twines 
of  small  extensibility.  This  problem  was  first  examined 
thoroughly  with  nylon  ropes17.  It  is  of  particular 
importance  for  gear  under  heavy  strain  where  one  has  to 
reckon  with  sudden  strong  loads l*. 

It  is  difficult  to  evaluate  the  breaking  extension  of 
nets,  as  it  must  be  expected  that,  for  high-class  fibres, 
knot-strength  reaches  only  about  50  to  60  per  cent,  of 
the  breaking  load.  Knowledge  of  breaking  extension,  is, 
therefore,  not  very  important.  The  degree  of  extension 
with  ascending  load  which,  at  the  most,  may  amount  to 
50  per  cent,  of  the  breaking  load  is  more  important.  The 
relation  between  load  and  extension,  up  to  this  limit,  has 
to  be  observed,  and  an  example  is  represented  by  the 
load-extension  curves  of  fig.  1  for  net  twines  of  material 
group  2.  The  curves  have  been  drawn  from  average 
values  which  were  calculated  from  the  results  of  several 
tests  of  twines. 

Two  different  types  of  extensibility  are  represented  in 
fig.  1;  that  of  polyester  and  polyacrylonitrile  (both  of 
continuous  filament),  and  that  of  polyamide,  polyvinyl 
alcohol  and  cotton. 

The  total  extensibility  of  the  first  type  is  small,  and 
such  twines  offer  strong  resistance,  even  at  small  loads. 
There  is  a  steep  ascent  of  the  curves,  as  for  those  of 
manila  twines. 

The  second  type  is  affected  at  low  tension  or  pressure, 
where  the  increase  of  extension  already  exceeds  the 
increase  of  load.  AH  curves  apply  to  wet  twines.  Twines 
of  staple  fibres  are  much  more  extensible  than  those  of 


Twine. 


TABU-  HI 
Extension  in  per  cent,   (dry) 


j±r      JSST 


Immediately  after 

loading        .  .83  3-8 

1  hour  loaded  .         9-0  4-2 


Immediately  after 

unloading  .  .2-2  12 
I  hour  after 

unloading  .  .  1-7  08 

1  day  after  unloading  1-0  0-4 

Final  condition  .  0-8  0-4 


Total  extension  in 

per  cent.      .  9-0  42 

Elastic  extension  in 

percent.      .  .         8-2  38 

Permanent  elongation 

in  per  cent.  0-8  0-4 

Degree  of  elasticity 
in  per  cent.          .91  95 


22 
2-4 


14 

1-2 
1-2 
1-2 


2-4 
1-2 
12 
50 


14  2 
15-0 


8-0 

6-0 
5-0 
5-0 


15-0 

10  0 

5-0 

67 


continuous  filaments,  especially  if  the  latter,  as  in  the 
case  in  question,  are  of  high  tenacity.  When  cotton 
twines  are  wetted  they  show  a  shrinkage  of  nearly  10  per 
cent.  With  a  small  load,  the  extension  of  wet  cotton 
twines  is  very  great;  with  low  tension  pressure,  shrinkage 
will  be  compensated  by  extension.  Twines  of  fibres  which 
have  small  shrinkage  in  water,  such  as  twines  of  polyester 
or  polyamide  filament,  will  also  show  small  differences 
between  dry-extension  and  wet-extension. 

Extension  in  connection  with  elasticity  is  of  special 
interest.  Elasticity  regarding  the  stronger  net  twines, 
which  are  used  for  large  bottom  trawls,  is  shown  in 
Table  111.  The  twines  have  been  loaded  in  dry  condition 
for  one  hour  by  30  per  cent,  of  their  breaking  load. 

The  thick  cotton  twine  shows  the  highest  extension  at 
load.  After  unloading,  a  rather  high  permanent  elonga- 
tion remains.  For  manila,  the  total  extension  is  very 
small.  Therefore,  the  absolute  measurement  of  the  perm- 
anent elongation  is  also  small,  though  it  comprises 
50  per  cent,  of  the  total  extension.  Polyester  twine  is  very 
elastic,  its  extensibility,  however,  is  very  small.  Perlon 
filament  twine  has  a  proportionately  high  extensibility, 
together  with  high  elasticity. 

What  significance  does  this  difference  in  behaviour  have 
for  fishing  gear? 

A  fixed  extensibility  cannot  be  set  for  net  twines,  as  the 
requirements  differ  according  to  the  type  of  gear. 

In  drift  nets,  for  example,  the  fish  will  be  caught  in 
the  meshes.  The  mesh  sizes,  which  must  be  adapted  to 
the  size  of  the  fish,  should  not  alter,  so  the  permanent 
elongation  must  be  small.  The  twine  should  yield  to  the 
pressure  of  the  fish  caught  in  the  mesh.  Extension  and 
elasticity,  however,  must  not  be  too  great,  otherwise  the 
fish  would  be  squeezed  in  too  tight,  the  quality  of  the 


[H21 


EFFICIENCY     OF    SYNTHETICS     FOR     FISHING 


fish  would  suffer,  and  it  would  be  also  very  difficult  to 
release  it  from  the  net.  The  very  extensible  twines  of 
polyamide  staple  fibres  are  therefore  less  suitable  for 
drift  nets.  Twines  of  polyester  (Terylene,  Dacron, 
Trevira,  Diolen)  (see  fig.  1  and  Table  III)  would,  however, 
be  especially  qualified  for  these  gears,  since  their  perman- 
ent elongation  is  small,  but  their  relative  elasticity  great. 

The  requirements  of  gear  exposed  to  strong  tension 
or  pressure  are  very  different.  This  applies  to  trawls, 
especially  the  codends,  when  they  are  rushed  up  from  a 
great  depth  to  the  surface  or  while  heaving  a  large  catch 
on  deck.  These  nets  were  made  of  strong  manila  or 
sisal  twines,  in  order  to  stand  sudden  shock  loads.  In 
consequence  of  their  very  small  extensibility,  hard  fibres 
are  not  able  to  absorb  kinetic  energy.  Best  suited  for  large 
bottom  trawls  are  twisted  or  plaited  twines  of  polyamide 
filament  (e.g.  Perlon,  nylon).  Their  extensibility  is  great 
enough  to  stand  strong  shock  loads,  and  the  high  degree 
of  elasticity  guarantees  a  good  constancy  of  mesh  size. 
In  the  German  trawl  fishery  most  of  the  lighter  bottom 
trawls  and  floating  trawls  of  cutters  are  now  also  made 
of  Perlon  (in  this  case  from  staple  fibres)  because  its  high 
elastic  extension  gives  the  necessary  strength  for  handling, 
also  big  catches.  Perlon  staple  fibres  have  also  proved 
suitable  for  ottcrboard  stow  nets. 

As  regards  extensibility,  drift  and  trawl  nets  are 
examples  of  extreme  types  of  gear.  Between  them  there 
is  a  great  range  of  other  types  of  gear  with  different 
requirements  of  extensibility  and  elasticity.  For  synthetic 
fibres,  polyester  filament  and  polyamide  staple  fibre  are 
the  extremes  as  regards  extensibility. 

3.     Abrasion  resistance 

In  practice,  it  is  scarcely  possible  to  obtain  exact 
knowledge  of  the  relative  abrasion  resistance  of  nets 
made  of  different  kinds  of  fibres.  One  is  dependent  on 
laboratory  test  methods,  and  only  relative  and  never 
absolute  values  are  obtained  and  these,  moreover,  are 
dependent  on  the  test  method  employed. 

For  the  following  tests,  the  machine  for  testing  abrasion 
resistance  after  Sander  was  used,  in  which  the  wet 
net  twines  are  chafed  across  a  bar  of  carborundum Ml. 
The  counts  or  runnages  of  the  various  types  of  twines 
under  comparison  were  taken  as  equal  as  possible. 
Thicker  twines  for  large  trawl  nets  were  compared  with 
one  another,  and  also  twines  for  gears  of  material  group 
2.  If  in  the  first  case,  abrasion  resistance  of  thick  cotton 
twine  is  equal  to  100,  the  following  approximate  relation 
can  be  established: 

Cotton                             -  100 

Manila  130 

Hemp  280 

Polyester  cont.  filament  =  230 

Polyamide  cont.  filament  460 

These  figures  refer  to  new,  unused  and  wet  twines,  with 
a  runnage  of  above  350  m./kg.  The  abrasion  resistance 
of  manila  twines  is  very  low.  As  some  tests  have  shown, 
it  is  about  the  same  as  for  sisal  twines.  As  the  breaking 
strength  of  nets  of  natural  fibres  is  reduced  by  rotting, 
abrasion  resistance  will  also  be  reduced.  For  twines  of 
manila  and  hemp,  the  following  relations  have  been 
established'0: 


breaking  strength 
in  per  cent. 


Diminution    of 

abraxitm  resistance 
manila,  per  cent.  hemp,  per  cent . 


10 
20 
30 
40 
50 
60 
70 
80 


2 

3 

5 

(> 

15 

26 

37 

48 


5 
9 
14 
19 

23 
27 
34 
50 


Trawl  nets  are  very  much  exposed  to  abrasion,  which 
largely  explains  the  short  durability  of  manila  nets. 
Polyamide  fibres  (Perlon,  nylon)  show  the  best  abrasion 
resistance,  and  are  by  far  superior  to  those  of  natural 
fibres.  Trawl  nets  of  Perlon  filament  used  in  the  German 
trawl  fishery,  show  about  ten  times  the  durability  of 
manila  nets,  this  is  due  not  only  to  their  rot-proofness, 
but  also  to  the  greater  abrasion  resistance  of  poly  am  ides. 
Twines  of  polyester  filament  have  only  half  the  abrasion 
resistance  of  twines  of  polyamide  filament,  but  their 
resistance  is  better  than  that  of  cotton  and  manila  twines. 

Abrasion  resistance  depends  not  only  on  the  type  of 
fibres  and  on  the  diameter,  but  also  on  the  manufacturing 
process  of  the  twine18.  Hard  twisted  twines  of  polyamide 
filament  have  less  abrasion  resistance  than  soft  twisted 
twines.  Corresponding  features  apply  to  braids  of  poly- 
amide filament. 

The  finer  twines  for  gears  of  material  group  2  have  the 
following  abrasion  resistance  (wet),  when  cotton  is 
again  equal  to  100: 


cotton 

polyvinyl  chloride 

Rhovyl-staple 

PCU-staple 
polyvinyl  alcohol 

staple 

filament 
polyacrylonitrile 

cont.  filament 
polyamide 

staple 

cont.  filament 


100 

50-55 

75 

50-60 
110 

70 

170-250 
400-500 


These  relative  figures  correspond  to  twines  of  the  same 
runnage  or  count.  If  the  calculation  is  based  on  the  same 
diameter,  the  values  will  be  somewhat  displaced.  The 
third  possibility  would  be  a  comparison  of  abrasion 
resistance  of  twines  of  the  same  breaking  strength.  In 
that  case  twines  of  high  tenacity  fibres,  such  as  polyamide 
filament  and  polyester,  do  not  show  such  a  great  superior- 
ity, since  they  are  much  finer  than  twines  of  weaker 
types  of  fibres. 

4.     Resistance  to  weathering 

There  are  few  experiments  on  the  resistance  to  weather- 
ing of  nets  made  of  vegetable  fibres.  These  fibres  are 
injured  by  sunlight.  But  such  damage  is  small  compared 
with  that  caused  by  micro-organisms  in  the  water.  Re- 
sistance to  light  and  weathering  is  about  the  same  in  all 
vegetable  fibres. 
Synthetic  fibres  show  very  great  differences  in  their 


[  143  1 


MODERN     FISHING     GEAR    OF    THE    WORLD 


^ .  PCU) 


Fig.  2.     Relative  resistance  to  weathering. 


degree  of  resistance  to  light  and  weathering.  Fig.  2 
demonstrates  their  relative  resistance  to  weathering20. 
Of  the  fibres  represented  here,  delustred  poly  amide  offers 
least  resistance,  so  that  dull  polyamide  twines  are  not 
suitable  for  fish  nets,  since  their  resistance  to  light  is  too 
small. 

It  could  be  mentioned  here  that  the  USA  are  said  to 
have  succeeded  in  producing  a  dull  nylon  type  with  the 
same  resistance  to  weathering  as  bright  nylon.21  The 
resistance  of  twines  made  of  bright  polyamide  (staple  and 
filament)  is  still  below  that  of  cotton  and  hemp  twines. 
Polyamide  monofilament  and  fibres  of  polyacrylonitrile 
and  non-chlorinated  polyvinyl  chloride  have  a  very 
great  resistance  to  weathering. 

However,  the  other  excellent  physical  properties  of 
twines  made  of  polyamide  filament  and  staple  more  than 
compensate  for  their  smaller  resistance  to  light,  especially 
for  fishing  gear  of  special  economic  significance,  such  as 
trawl  nets. 

As  an  example:  in  the  Portuguese  purse  seine  fishery, 
nets  of  unprotected  (uncoloured)  Perlon-staple  twines, 
have  been  used  for  1,306  working  days  and  are  still 
serviceable,  whereas  preserved  cotton  nets  usually  last 
for  no  more  than  about  500  working  days. 

For  trap  nets  and  other  baskets,  which  stand  partly 
out  of  the  water  or  close  below  the  surface,  colouring  of 
the  material  as  a  protection  against  light  will  often  help. 
In  this  case  nets  of  polyvinyl  chloride,  polyacrylonitrile 
and  polyamide  monofilament  would  be  rather  suitable. 

IV.    CHOICE   OF   SYNTHETIC   FIBRES   FOR 
VARIOUS  FISHING  GEAR 

The  discussion  in  Section  III  may  now  serve  to  answer 
the  question  as  to  the  suitability  of  certain  synthetic 
fibres  for  certain  gear  used  in  Germany  (ref.  especially 
Table  I). 


Material  Group  1.  This  comprises  the  different  types 
of  set  gillnets,  floating  gillnets  and  trammel  nets  used  in 
inland  fisheries.  Also  fine  herring  nets  of  the  Baltic 
fishery  belong  to  this  group.  Gillnets  are  passive  gears 
into  which  migrating  fish  swim  by  accident  and  become 
stuck  in  the  meshes.  Here,  invisibility  is  of  prime  import- 
ance. The  twines  must  therefore  be  of  small  diameter  of 
just  sufficient  breaking  strength,  this  depending  on  the 
species  of  fish  to  be  captured. 

Accordingly,  it  would  be  advantageous  to  use  very 
fine  twines  from  strong  types  of  fibres.  Two  types  of 
synthetic  fibres  have  a  particularly  high  strength: 
polyester  filament  and  polyamide  filament  (Table  II).  Very 
fine  twines  made  of  the  latter  have  stood  the  test  very 
well  in  inland  fisheries,  and  also  for  herring  gillnets  in 
the  Baltic.  They  catch  more  fish  than  cotton  gillnets, 
since  they  are  finer  and  softer  and  less  visible.  The  best 
material  for  fine  gillnets  is,  undoubtedly  monofile 
palyamide-wire.  Its  translucency  makes  it  nearly  invisible 
in  water.  The  catch  of  gillnets  of  this  material  is  many 
times  that  of  cotton  nets,  as  many  fishing  countries  can 
testify. 

Material  Group  2.  German  fishing  gear  belonging  to 
this  group  were  all  formerly  made  of  cotton  twines.  For 
this,  twines  of  breaking  strength  of  3  kg.  up  to  about 
50  kg.  were  used. 

The  largest  gear  of  this  group,  the  herring  drift  nets 
of  the  North  Sea  drifter,  have  been  discussed  in  Section 
III,  (2),  showing  why  twines  of  polyester  filament  are 
regarded  as  particularly  suitable.  Extension  and  elasticity 
of  twines  of  polyamide  filament  arc  also  suitable,  but  not 
twines  of  polyamide  staple. 

For  gear  of  such  large  dimensions,  the  initial  costs  must 
especially  be  taken  into  consideration.  The  complete 
string  of  herring  drift  nets  (called  "fleet'*)  often  consists 
of  more  than  1,500  kg.  cotton  twine.  The  much  greater 
strength  of  the  synthetic  fibres  just  mentioned  cannot  be 
fully  utilized.  It  is  not  possible  to  use  twines  as  fine  as 
the  breaking  strength  would  permit,  because  of  the 
damage  that  could  be  caused  to  the  fish.  Therefore,  the 
weight  advantage  as  compared  to  cotton  nets  and, 
consequently,  the  lower  costs,  will  not  be  as  great  as  for 
other  gears.  Thus,  drift  nets  of  polyester  or  polyamide 
are  much  more  expensive  than  cotton  nets.  Perhaps  it 
would  be  better,  in  this  case,  to  use  polyvinyl  alcohol 
which  is  the  cheapest  synthetic  fibre.  Drift  nets  made  of 
that  fibre  are  scarcely  more  expensive  than  cotton  nets 
and  may  be  completely  suitable,  if  a  good  stiffening 
process  is  used.  The  production  of  knotless  drift  nets  of 
polyvinyl  alcohol  fibres  would  be  a  great  advantage. 
Satisfactory  results  have  been  obtained  in  a  first  test  with 
such  nets  made  of  Japanese  Manryo. 

During  1956  the  German  Baltic  Fishery  recorded  very 
good  results  with  salmon  drift  nets  made  of  polyamide 
filament,  and  this  material  will  probably  completely 
displace  the  traditional  hemp  nets  during  the  next  few 
years. 

Passive  gears,  such  as  baskets,  with  the  exception 
perhaps  of  the  large  trap  nets  (box  nets,  stake  nets,  fixed 
nets)  of  the  coast  fishery,  do  not  generally  require  a  very 
strong  material.  If  fibres  such  as  polyamide  and  polyester 
filament  are  used,  then  here  too,  may  be  a  chance  of 
catching  more  fish  with  a  finer  twine.  Since  all  synthetic 


[144] 


EFFICIENCY     OF    SYNTHETICS     FOR     FISHING 


fibres  are  rot-proof,  there  is  a  very  great  choice  of 
suitable  materials.  For  trap  nets,  such  as  the  Danish 
"Bundgarn",  which  stand  partly  out  of  the  water,  fibres 
of  high  resistance  to  light  would  be  recommendable, 
e.g.,  polyvinyl  chloride  fibres  PCU  and  Rhovyl  and  also 
the  new  Japanese  Teviron  or  Envilon. 

Translucency,  a  special  property  of  polyamide  mono- 
filament,  has  proved  to  be  of  primary  importance  in 
catching  more  fish,  also  with  baskets.  Eel  fyke  nets,  made 
of  monofilament,  caught  about  twice  as  much  as  those 
made  of  cotton;  that  also  would  be  true  of  othtr  baskets. 
But  this  would  not  seem  profitable,  if  the  long  leaders  and 
wings  of  trap  nets,  were  also  made  of  monofilament. 
They  would  not  lead  the  fish  into  the  trap  net  (net 
proper),  but  act  as  gillncts  and  the  fish  would  be  caught 
in  the  meshes—. 

Dip  nets,  lift  nets  and  falling  nets  (lantern  nets,  cast 
nets)  are  of  little  importance  in  the  German  fishery,  but 
what  has  been  said  about  material  for  baskets  will 
mostly  be  applicable. 

The  most  important  gears  of  German  rivers  are  gape 
nets.  According  to  the  flowing  speed  of  the  water,  they 
require  material  of  great  to  very  great  strength,  of  relative 
high  extension  and  elasticity  and  of  great  abrasion  resist- 
ance. Polyamides  have  these  requirements.  For  swing 
nets  (stow  nets)  at  anchor  (also  for  otter-board  stow 
nets),  with  the  smaller  mesh  sizes  or  such  nets  which  are  in 
rivers  with  a  flowing  speed  of  less  than  5  km./hr.,  twines 
of  Perlon-staplc  have  lasted  very  well  for  more  than  six 
years.  For  stiffening,  they  were  treated  with  "Black 
Varnish".  Gape  nets  with  large  mesh  sizes  from  rivers 
with  strong  current  (e.g.  the  Rhine),  are  made  either 
wholly,  or  in  their  most  forward  part,  of  plaited  or 
twisted  twines  of  polyamide  filament,  belonging  to 
group  3. 

Two  more  gear  types  of  material  group  2  arc  seines 
and  light  cutter  trawls. 

For  seines,  and  also  for  purse  seines,  there  arc  man> 
possibilities  of  choice  of  synthetic  twines.  During  the 
first  years  of  introduction  of  PcCc  into  the  German 
inland  fishery  there  were,  for  instance,  boat  seines  made 
of  these  fibres,  although  breaking  strength  and  abrasion 
resistance  were  less  than  those  of  cotton  twines.  Since 
the  physical  properties  of  the  non-chlorinated  polyvinyl 
chloride  fibres  are  superior,  these  can  also  be  used  for 
seines.  The  same  goes  for  polyvinyl  alcohol  twines  which 
arc  also  inexpensive.  These  fibres,  however,  do  not  have 
a  very  high  absolute  strength  in  wet  and  knotted  con- 
dition. Therefore,  one  cannot  count  on  the  gear  being 
lighter  than  a  cotton  net  of  the  same  size.  For  polyester 
and  polyamide,  however,  these  relations  are  more 
favourable.  If  the  breaking  strength,  wet  and  knotted,  is 
taken  as  a  basis,  then  gear  of  polyamide  filament  is  only 
half  as  heavy  as  that  of  cotton. 

Material  group  3.  The  requirements  of  the  material  of- 
trawls,  have  already  been  partly  indicated  in  Section  111,2. 
They  are  the  same  as  for  large  gape  nets;  very  high  break- 
ing strength  and  abrasion  resistance  and  comparatively 
great  extension  and  elasticity,  and  possibly  the  smallest 
weight.  That  goes  both  for  lighter  bottom  trawls  and 
floating  trawls  of  cutters  which  still  belong  to  material 
group  2,  and  for  the  large  bottom  trawls  of  deep  sea 
trawlers  belonging  to  material  group  3.  There  are  differ- 


ences only  in  the  degree  of  strain  and  not  in  principle. 
By  far  the  best  material  for  these  purposes  seems  to  be 
polyamides.  For  cutter  trawls,  besides  twines  of  polya- 
mide filament  of  liter  210  denier,  weaker  twines  of 
polyamide  staple  may  also  be  used.  For  heavy  and  large 
trawls,  plaited  and  laid  twines  of  polyamide  filament 
have  stood  the  test  so  excellently  that  the  German  trawl 
fishery  works  now  mostly  with  Perlon  nets  for  the  catch 
of  herring  and  of  round  fish. 

Especially  in  the  Japanese  fishery,  trawl  nets  of  poly- 
vinyl alcohol  are  frequently  used.  But  their  advantage 
seems  to  be  in  the  low  price  only,  as  rot-proofness  is 
peculiar  to  all  synthetic  fibres.  They  may  be  perfectly 
suitable  for  lighter  trawl  nets  of  smaller  fishing  boats, 
but  for  gears  of  large  trawlers,  twines  of  polyvinyl 
alcohol  fibres  do  not  seem  to  be  very  suitable.  The  test 
figures  for  one  Japanese  Manryo  twine  and  for  twines  of 
rnanila  and  Perlon-filament,  with  nearly  the  same 
runnage,  given  in  Table  IV,  may  serve  as  an  illustration. 

TABLE  IV 


Manvro  Manila 

2W SUM 'twine  NmO.  7/3 


Perlon 
Km.  3/12 


Runnage,  m/kg. 

239 

-230 

242 

Breaking  strength. 

wet,  kg. 

79  2 

~  95 

156 

Strength,  wet, 

knotted,  kg. 

35-7 

~  60 

99 

The  lighter  a  trawl  net  is,  the  easier  its  handling  and 
the  lower  its  trawling  resistance.  These  aspects  are 
particularly  important  for  large  gear.  From  Table  IV 
it  can  be  seen  that  trawl  nets  of  Manryo  become  much 
heavier  and  must  have  thicker  twines,  if  they  arc  to  be 
as  strong  as  manilu  trawl  nets.  The  great  advantages  of 
trawl  nets  of  polyamide  filament  in  thK  respect  are 
apparent. 


I  Klusl,  Ci.  iind  H.  Mann:  txperimentellc  I'nicrsuchungcn  uber 
den  Zelluloseubbau  im  Wasscr.  "Vom  Wasser"  Bd.  21,    100-109, 
1954. 

-  v.  Brandt,  A.:  Uber  den  Zelluloseabbau  in  Seen  Arch.  f. 
Hydrobiol.  Bd.  XL.  778-821,  1944. 

;i  v.  Brandt,  A.u.  Ci.  Klust:  Z.elluloseabbau  im  Wasser.  Arch. 
fur  Hydrobiol.  Bd.  XLI1I,  223-233,  1950. 

4  v.    Brandt.   A.:    Cellulosc-Abbuu    in    Fliessge\\axsern.     Ber. 
Limn.  Station.  Freudcnthal,  Bd.  4,  17-19,  1953. 

5  v.    Brandt,    A.:    Untersuchungen    zur    Konservierung    von 
Fischnetzen.-  Prot.  7.  Fischerei  lechnik,  H.7,  Nr.  14,  1952;  H.10. 
Nr,   19.   1953. 

6  Klust,  G.:  Die  Bcurtcilung  der  konservicrenden,  Wirkung  von 
Fischnetz-Konservierungsverfahren.     Mclliand  Textilber.,  Bd.  33. 
763-764,    1952. 

Klust,    G.:    Testing    methods    of   fishing    net    preservation. 
Cordage  World,  Vol.  33,  Nr.  396,  1952. 

7  Nccdham.  E.  R.:  Use  of  Nylon  yarns  in  fish-netting.  Canadian 
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8  v.  Brandt,  A.:  Bekomnun  wir  unfaulbare  Nelzgarne?  Fischerei- 
Ztg.  Bd.  42,  301-302,  1939. 

9  v.  Brandt,  A.:  Die  bisherigcn  trfahrungen  mit  PeCc-Garnen 
in  der  Fischerei.  Zeitschft.  f.  Fischerei,  Bd.  42   45-65.  1944. 

10  v.  Brandt,  A.:  Fanggerate-Systematik.  Protok.  z.  Fischerei 
technik  Bd.  2,  Nr.  13,  1952. 

II  Klust,  G.:  Zweite  Ubersicht  uber  die  Vcrwcndung  von  Perlon 
in  der  Fischerei.  Prot.  z.  Fischereitechnik,  Bd.  2,  Nr.  20,  1953 


[1451 


MODERN     FISHING    GEAR    OF    THE    WORLD 


12  Klust,  G.:  Zur  Wasserbestandigkeit  von  Netzgarnen  aus 
einigen  synthetischen  Faserstoffen.  (vorlaufige  Mittcilung)  Prot. 
z.  Fischereitechnik,  Bd.  4,  168-174,  1957. 

18  Klust,  G.:  Untersuchungen  an  Fischnetzgarnen  aus  endlosen 
Polyamidfaden.  ~ Arch.  f.  Fischereiwiss.,  Bd.  8,  H.  1/2,  1957. 

14  Can-others,  P.  C:  Tests  on  Kuralon  twine.  Western  Fisheries, 
Vol.  50,  No.  6,  1955. 

16  Klust,  G.:  PolyvinylalkohoI-Zwirne.  Melliand  Textilberichte 
-Bd.  38,  134-136,  1957. 

16  Griinsteidl,  E.  und  H.  Preussler:  Chemische  Faserstoffe  und 
ihre  Eigenschaften.  St.  Johann  i.T.,  1953. 

17  British  Ropes  Ltd.:  Ropes  made  from  Nylon.  Leigh/Fdin- 
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18  Klust,      G.:      Perlonschnure,      ihre      Herstellung      und 
Eigenschaften    unter  Beriicksichtigung  ihrer  Verwendung  in  der 
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19  Klust,  G.:  Untersuchungen  iiber  die  Scheuerfestigkeit  von 
Fischnetz-Schnttren.  Prot.  z.  Fischereitechnik,  Bd.  3, 64-68, 1954. 

20  Klust,  G.:  Zur  Wetterfestigkeit,  von  Zwirnen  aus  einigen 
synthetischen  Faserstoffen.  Textil-Praxis,  Bd.   12,  233-237,   1957 
(Hier  auch  weitcre  Literaturangaben  zu  dieser  Fragc!) 

-1  Ashton,  H.  and  J.  Boulton:  A  review  of  developments  in  the 
properties  processing  and  utilisation  of  man-made  fibres.  Journ. 
Text.  Inst.  Vol.  47,  No.  8,  p.  532-566,  1956. 

22  Schlieker,  E.:  Mehr  Perlongerate  fiir  die  Fischerei.  Deutsche 
Fischerei-Zeitung.  Bd.  3.  356-367,  1956. 


Spinning  a  cotton  fishing  yarn  in  an  Indian  fishing  village. 
[146] 


SYNTHETIC   FIBRES   IN   THE   FISHING   INDUSTRY 

by 

E.  I.  DU  PONT  DE  NEMOURS  AND  CO. 

Delaware,  U.S.A. 


Abstract 

This  is  a  general  account  of  the  use  of  nylon  and  *kDacron"  polyester  fibre  for  all  kinds  of  fishing  gear.  The  lightness,  elasticity  and 
durability  of  these  fibres  makes  them  ideal  for  gillnets,  seines,  traps  and  pots,  but  the  risk  of  total  loss  makes  them  a  doubtful  investment 
for  bottom  trawls.  An  important  recent  improvement  has  been  the  introduction  of  Type  330  nylon  yarns  which  have  better  sunlight  durability 
than  cotton  or  previous  types  of  nylon,  an  important  attribute  in  hot  and  sunny  climates.  The  use  of  "Taslan"  textured  nylon,  a  process 
which  creates  tiny  loops  in  the  individual  filaments  of  the  yarn,  means  better  knot  holding  properties  and  easier  blending  of  two  or  more 
continuous  filament  varns. 


Resume 


Lcs  fibres  synthetiques  dans   1'industrie  des   peches 


C'est  un  expose  general  sur  Femploi  du  nylon  et  du  Dacron,  fibre  de  polyester,  pour  Unites  sortes  d'engins  de  peche.  La  legerete. 
I 'elasticity  et  la  durcc  de  ces  fibres  les  rend  ideales  pour  les  filets  maillants,  les  sennes,  les  trappes.  les  casicrs  et  les  nasses,  mais  le  risque  de 
perte  totale  en  fait  un  invcstissement  douteux  pour  les  chain  Is  de  fond.  Un  important  perfectionncment  recent  est  ^'introduction  des  tils  de 
nylon  type  330  qui  a  une  meilleure  duree  sous  1'action  du  soleil  que  le  colon  ou  les  types  precedents  de  nylon,  ce  qui  est  une  qualite  sous  les 
clirnats  chauds  et  ensolciltts.  l/emploi  de  nylon  texture  "Taslan",  un  precede  qui  cree  de  pelites  boucles  dans  les  filaments  individuels  du 
HI.  signifie  une  meilleure  tenuc  des  noeuds  et  line  union  plus  facile  de  deux  ou  plusieurs  fils  du  type  filament  continu. 


Extracto 


Las  fibres  sinteticas  en  la  industria  pesquera 


Description  general  de  Jos  usos  ed  las  fibras  de  policsteres,  nylon  y  dacron.  en  todos  los  tipos  de  artes  de  pesca.  El  poco  peso, 
elasticidad  y  duraci6n  de  el  las  haccn  que  sean  ideales  para  la  confection  de  redes  de  enmalle  y  cerco.  nasas,  etc.  pero  el  riesgo  de  la  pcrdida  total 
durante  la  pesca  ponen  en  duda  su  empleo  para  fabricar  redes  de  arrustre  de  fondo.  Una  mcjora  importante  introducida  recientemente  es 
el  hilo  de  nylon  lipo  330,  mas  resist entc  a  la  action  de  la  luz  solar  que  el  algoddn  o  lipos  de  ny!6n  usados  anteriormente,  lo  cual  es  una 
propiedad  importante  en  climas  calidos  o  con  mucho  sol.  El  uso  de  "taslan"  nylon  sometido  a  un  procedimientoque  crea  pequeftos  lazos 
en  los  filamentos  individuates  -permite  obtener  nudos  no  tan  corredizos  y  un  material  que  se  retuercc  en  mejores  condiciones  al  mezclarlo 
con  dos  o  mas  filamentos  continues. 


WHILE  Du  Pont  66  nylon  was  first  tested  experi- 
mentally in  the  fishing  industry  in  the  United 
States  for  gillnets  in  1939,  military  demand  for 
nylon  during  the  war  delayed  further  work  in  the  fishing 
industry.  Since  1948,  however,  the  use  of  nylon  in  gill- 
netting  has  grown  rapidly.  Recent  estimates  show  that 
well  over  90  per  cent,  of  all  Great  Lakes'  gillnets  are  now 
made  of  nylon. 

More  recently,  nylon  has  taken  over  most  of  the  salmon 
gillnetting  in  the  Pacific.  This  rapid  acceptance  has  been 
due  primarily  to  the  great  increase  in  numbers  of  fish 
caught  by  nylon  gillnets  compared  to  the  nets  they 
replaced,  as  well  as  their  lower  maintenance  costs,  longer 
life,  and  easier  care.  Gillnets  made  of  nylon  twines  have 
reportedly  caught  from  3  to  12  times  more  fish  per  net 
than  those  used  previously,  although  the  reasons  for 
this  phenomenon  are  not  thoroughly  understood. 

In  the  heavier  types  of  netting  which  used  cotton  twines 
in  the  past,  nylon  has  also  gained  the  approval  of 
American  fishermen.  The  first  menhaden  seine  made 
entirely  of  nylon  was  placed  in  service  in  June  1954, 
and  proved  to  be  far  more  durable  than  nets  previously 
used.  In  addition  nylon  nets  have  proved  more  economical 


in  the  long  run.  Significant  growth  has  also  been  noted 
for  nylon  in  lobster  and  shrimp  gear  and  in  tuna  seining. 
Nylon  is  also  being  used  in  longlining  for  tuna  and  halibut. 
In  addition  to  the  wide,  general  use  of  nylon  twines 
for  nets,  the  use  of  ropes  of  nylon  and  "Dacron" 
polyester  fibre  for  hanging  and  purse  lines  has  become 
quite  common.  Present  industry  sales  of  nylon  in  the 
U.S.A.  for  fish  netting  and  twines  amount  to  over 
1,000,000  Ib.  annually.  It  is  expected  that  in  about  five 
years'  time  at  least  75  per  cent,  of  all  nets  used  in  this 
country  will  be  made  of  nylon. 

FIBRE   PROPERTIES 

Du  Pont  66  nylon  has  gained  acceptance  in  netting 
because  it  offers  an  outstanding  comdination  of  pro- 
perties and  price.  Of  these  properties,  high  strength, 
wet  or  dry;  abrasion  resistance,  complete  resistance  to 
rotting  caused  by  marine  organisms  and  high  impact 
strength  are  the  most  important.  Some  specialized 
applications  that  call  for  low  stretch  under  load  have 
resulted  in  the  use  of  "Dacron"  polyester  fibre  in  both 
net  and  rope  forms. 


1147] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


The  following  table  gives  some  of  the  most  important 
specification  properties  of  nylon  and  "Dacron"  used  in 
netting: 


Property 


Du  Pont  66  nylon  "Dacrori" 

Type  300  Type  700        7 'ype  5 1 


Denier 

210 

840 

220 

Filaments 

34 

140 

50 

Denier  per  filament 

6-2 

6-0 

4-3 

Tenacity  Dry  (grams/denier) 

8-0 

8-7 

6-4 

Tenacity  Wcl  (grams/denier) 

6-8 

8-1 

6-4 

Tenacity  Loop  (grams/denier) 

5-4 

7-1 

3-9 

Total  extension  dry  (°0)     . 

17 

17 

10 

Total  extension  wet  (','„)     . 

22 

24 

10 

These  properties  combine  to  produce  more  economical 
or  more  efficient  nets. 

Continuous  filament  nylon  is  the  predominant 
synthetic  type  of  raw  material  used  by  netting  and  rope 
manufacturers  in  the  U.S.A.,  because  it  offers  the  highest 
breaking  strength  and  maximum  abrasion  resistance. 
Spun  nylon  and  combination  twines  are  also  being  used 
successfully.  Early  problems  of  knot  slippage  or  knot 
loosening  have  been  largely  overcome  in  nets  made 
from  continuous  filament  twines  by  the  use  of  special 
resin-treated  twines,  special  knots,  or  heat  treatment  of 
the  finished  nets.  Spun  twines  of  100  per  cent,  nylon  or 
twines  made  of  filament  nylon  combined  with  spun 
fibres  arc  also  used  as  a  means  of  overcoming  knot 
slippage  without  the  need  for  any  special  twine  treatments. 
Combination  twines  of  filament  nylon  and  spun  fibres, 
in  addition  to  good  knot  holding  properties,  offer  the 
advantage  of  increased  bulk  at  lower  cost  for  certain 
types  of  netting  where  handling  of  fine  twines  might  be  a 
problem. 

GILLNETS 

The  high  strength  of  nylon  allows  the  use  of  thinner 
continuous  filament  twines  which  gill  fish  more  effectively. 
Thus,  the  higher  initial  cost  of  nylon  is  more  than  offset 
by  the  greater  number  offish  caught  per  net.  In  addition, 
some  fishermen  feel  that  the  elasticity  of  the  nylon  allows 
fish  to  force  the  twines  apart  and  become  caught.  This 
may  account  for  the  fact  that  larger  sized  fish  have  been 
caught  with  a  given  mesh  size  than  previously.  The 
abrasion  resistance  of  nylon  is  less  important  in  this 
type  of  netting,  but  its  rot  resistance  allows  continued 
use  of  nets  without  the  need  for  drying  and  treating. 

PURSE  SKINES 

Abrasion  resistance  and  resistance  to  rotting  due  to 
micro-organisms  in  sea  water  are  the  most  important 
requirements  for  this  type  of  net.  It  is  also  important  to 
use  twines  of  sufficient  size  to  make  handling  easy.  One 
hundred  per  cent,  filament  nylon  twines  are  being  used 
in  menhaden  purse  seines  to  give  maximum  abrasion 
resistance  as  are  combination  twines  of  filament  nylon 
and  spun  acetate,  which  give  the  required  bulk  and  still 
achieve  satisfactory  strength  and  abrasion  resistance. 
To  illustrate  the  unusually  high  abrasion  resistance  of 


nylon,  a  cotton  "bunt"  or  centre  section  of  a  menhaden 
seine  used  in  one  area  lasts  an  average  of  5,000,000  fish 
caught.  A  100  per  cent,  nylon  menhaden  net  placed  in 
service  during  June  1954  has  caught  a  total  of  50,000,000 
fish;  the  net  was  still  in  service  at  the  last  report  with  the 
original  bunt  section  intact. 

The  largest  nylon  purse  seine  ever  manufactured  in  the 
U.S.A.  was  produced  in  early  1956  for  tuna  seining. 
Because  of  the  greater  strength  of  nylon,  slightly  finer 
twines  were  used  which  resulted  in  a  weight  saving  of 
of  approximately  2,400  lb.,  as  compared  to  an  all-cotton 
tuna  seine.  The  nylon  tuna  seine  weighed  a  total  of 
10,000  lb.  and  was  410  fm.  long  and  34  fm.  deep.  When 
wet,  this  nylon  net  weighs  only  half  as  much  as  a  wet 
cotton  net  of  the  same  size. 

The  Pacific  fishermen,  after  using  this  net  for  nearly 
two  years,  are  highly  enthusiastic  about  its  light  weight 
and  easier  handling  characteristics.  Jn  addition,  the  net 
shows  little  wear  and  the  fishermen  estimate  it  will  be  in 
service  for  a  number  of  seasons  to  come. 

In  order  to  achieve  optimum  handling  characteristics 
fishermen  in  the  U.S.A.  ordinarily  dip  nylon  purse  seines 
in  a  stiffening  agent,  such  as  an  asphalt  base  tar,  to 
impart  required  stiffness.  Also,  it  has  been  found  that 
slightly  heavier  weights  should  be  used  on  the  leadlines 
to  provide  the  desired  sinking  qualities  to  the  net. 

TRAP  NETS 

The  use  of  nylon  in  trap  nets  is  growing  slowly,  although 
acceptance  in  the  sardine  industry  has  been  widespread. 
Since  these  nets  are  stationary,  rot  resistance  and  abrasion 
resistance  (particularly  in  the  bottom  section  of  the  net) 
are  important  properties.  Because  of  nylon's  natural 
rot  resistance,  fishermen  have  found  they  can  leave  their 
nylon  nets  in  the  water  almost  indefinitely  without  fear 
of  damage.  One  hundred  per  cent,  filament  nylon  or 
combination  twines  of  nylon  and  spun  acetate  are  being 
used  in  trap  netting.  In  the  sardine  fisheries  experience 
has  been  good  with  a  net  of  filament  nylon,  produced  on 
a  high  speed  knitting  machine.  This  type  of  webbing 
has  no  knots  and  the  mesh  formation  is  made  by  proper 
settings  on  the  knitting  machine. 

TRAWLS 

In  shrimp  trawls  where  the  sea  bottoms  that  are  fished 
are  moderately  level,  nylon  nets  perform  well  and  are 
being  accepted.  In  the  larger  nets  for  bottom  fish  where 
equipment  is  frequently  snagged  and  either  lost  or 
badly  torn,  the  extra  cost  of  nylon  may  often  not  be 
justified. 

FISH   NET   ROPES 

With  the  increased  use  of  synthetic  fibres  in  fish  netting, 
ropes  and  lines  with  the  same  strength,  elasticity  and  rot 
resistant  properties  as  the  net  material  itself  are  preferable 
in  most  cases.  For  this  reason,  the  use  of  ropes  of  nylon 
and  "Dacron"  polyester  fibre  has  grown  considerably 
as  more  fishermen  have  switched  to  synthetic  netting. 
Nylon  ropes  require  no  preservative  treatment  and  give 
outstanding  wear  life  compared  to  ropes  made  of 


[148] 


AMERICAN     EXPERIENCE    OF     SYNTHETIC    FIBRES 


natural  fibres.  A  tuna  fisherman  on  the  Pacific  coast,  for 
example,  has  reported  that  his  nylon  net  lines  have  been 
in  service  for  over  five  years,  and  are  still  in  good 
condition. 

A  certain  amount  of  difficulty  was  experienced  at  first 
in  hanging  nets  on  ropes  of  nylon  or  "Dacron".  This  was 
due  primarily  to  the  difference  in  elongation  and  shrink- 
age properties  of  ropes  of  these  fibres  as  compared  with 
natural  fibre  ropes.  As  a  result  of  long  experience  with 
natural  fibre  ropes,  the  number  of  meshes  of  netting  to 
hang  per  foot  of  rope  had  become  well  known  and  were 
correct  for  the  stretch  and  shrinkage  of  such  ropes. 
Fishermen  have  now  learned  that  properly  made  ropes 
of  nylon  or  "Dacron"  stretch  somewhat  more  than 
manila  but  do  not  shrink  when  wet.  They  have  modified 
their  hanging  techniques  to  compensate  for  these  differ- 
ences. 

Treated  nylon  is  being  used  in  longlining  for  ground 
fish  such  as  halibut.  Similar  lines  arc  being  used  for 
lobster  pot  warps,  as  well  as  for  headers  in  the  pots. 
In  these  cases,  the  shock  absorbency  resulting  from  the 
high  elasticity  of  nylon  is  a  definite  advantage,  particu- 
larly in  rough  weather. 


NEW  DEVELOPMENTS 

Improvement  of  the  nylon  yarn  as  produced  by  Du  Pont, 
as  well  as  improved  preparation  of  the  twine  by  the  net 
manufacturer  continues. 

An  example  is  the  recent  introduction  by  Du  Pont  of 
Type  330  nylon  yarn.  This  yarn  has  better  resistance  to 
sunlight  than  either  cotton  or  previously  available  types 
of  nylon.  This  yarn  is  now  available  to  fish  net  manu- 
facturers and  should  insure  longer  life  for  nets  when 
sunlight  is  a  significant  factor. 

A  new  development  in  the  manufacture  of  nets  of 
continuous  filament  yarn  has  been  the  use  of  "Taslan" 
textured  nylon  for  netting  twines.  The  texturing  process 
creates  tiny  loops  in  the  individual  filaments,  imparting 
new  surface  characteristics.  Thus,  good  knot  firmness 
can  be  obtained  by  the  use  of  twines  made  from  fc'Taslan" 
yarns.  In  addition,  the  texturing  process  is  useful  for 
combining  two  or  more  continuous  filament  yarns 
intimately  without  the  need  for  twisting. 

Such  developments  and  many  others  are  typical  of 
the  avenues  which  will  continue  to  be  explored  as  means 
of  improving  gear  for  the  fishing  industry. 


Bottom-set,  cod  gillnets  of  nylon  being  hauled  mechanically  off  Iceland. 
[149] 


THE   FEATURES   AND    USE   OF   "AMILAN"   FISHING    NETS 

by 

M.  AMANO 
Toyo  Rayon  Co,  Ltd..  Tokyo.  Japan 


Abstract 


"Amilan"*  is  the  registered  trade  name  for  nylon,  made  under  licence  hv  the  lo>o  Ruyon  Co  .  and  ncis  made  \vith  this  material  are 
at  present  being  used  extensively  in  Japan,  and  in  addition,  they  are  being  exported  to  the  U.S.A.,  Canada.  Africa,  etc.  The  use  of  "Amilan" 
nets  of  various  kinds  is  discussed,  and  the  rise  in  the  catch  of  Pacific  salmon  and  trout  since  the  introduction  of  these  nets  is  remarkable.  It 
is  true  that  the  initial  cost  is  high  compared  with  cotton,  but  in  spite  of  this,  fishermen  are  changing  over  more  and  more  to  the  use  of  these 
nets,  because  the  physical  properties  of  the  material  give  it  a  longer  life. 


Resume 


Les  caracteristiuues  et  1'emploi  des  filets  dt  pet  he  d*  "Ami  Ian" 


"Amilan"  est  le  nom  commercial  depose  du  nylon  fabrique  sous  licence  par  la  Toyo  Rayon  Co.  J  es  lilets  fabriques  avec  ces 
mat£riaux  sont  &  present  largemcnt  utilises  an  Japon  et  sont,  de  plus,  exportes  aux  I'.-l-.,  an  Canada,  en  Afrique.  etc.  On  examine  Temploi 
des  filets  cTAmilan  de  diflerents  types;  ('augmentation  des  captures  de  saumons  et  de  truites  du  Pacifique  dcpuis  I'introduction  de  ccs  filets  est 
remarquable.  II  est  vrai  que  le  coOt  initial  est  61ev6  par  rapport  a  celui  du  colon,  malgre  cela  les  pecheurs  se  rallient  de  plus  en  plus  &  Pemploi 
de  ces  filets  parce  que  les  proprietes  physiques  du  materiau  leur  assurcnt  une  plus  longue  diiree  d'existencc. 


Extraclo 


( 'aracteristicas  y  usos  dtr  las  redes  de  "amilan" 


por 


Ln  este  trabajo  se  estudian  los  usos  de  los  di versos  tipos  de  redes  de  "amilan"  (marca  comercial  registrada  del  nylon  manufacturado 
la  "Toyo  Ray6n  Co.")  que  se  han  difundido  ampliamente  en  el  Japon  v.  ademas,  se  exportan  a  los  L.U.A.,  Canada.  Africa,  ete.  La 
introduce! 6 n  de  estas  redes  en  las  pesquerias  de  salmon  del  Pacifico  >  trucha  ha  permit ido  notables  rendimientos.  notandosc  que,  a  pesar  de 
su  alto  precio  inicial  comparado  con  las  de  algodon.  los  Pescadores  las  prelieren  a  causa  de  su  mayor  duration  y  propiedades  fisieas  del 
material. 


SYNTHETIC  fibre  nets  are  now  popularly  used  and 
their  efficiency  is  highly  appreciated  by  fishermen 
all  over  the  world. 

Amilan  is  the  trade  name  of  nylon  produced  b>  the 
Toyo  Rayon  Co.  under  licence  and  with  the  technical 
cooperation  of  E.I.  du  Pont  de  Nemours  and  Co.. 
U.S.A.  Amilan  fishing  nets  are  very  well  known  todav 
among  fishermen  in  Japan,  and  are  also  exported  to 
many  fishing  countries. 

In  the  following  sections  the  use  of  Amilan  for  nets  in 
various  types  of  fishing  is  described. 

SALMON    AND   TROUT   GILLNKTS    IN    THF 
NORTHERN  PACIFIC 

The  Northern  Pacific  fishing  has  become  vitally  important 
to  Japan,  as  sea  products  are  indispensable  to  feed 
Japan's  ever-growing  population.  In  pre-war  days,  the 
main  practice  in  the  Northern  Pacific  was  large  scale 
drift  net  fishing  near  to  the  coast  of  Kamchatka  Peninsula, 
under  agreement  with  the  U.S.S.R.  At  present,  however, 
the  chief  fishing  is  based  on  mother-ship  operation, 
which  method  originated  around  1929. 

When  the  Pacific  War  ended,  production  of  Amilan 
was  started  and  by  1953,  fishermen  were  using  both 
conventional  ramie  and  Amilan  nets  in  the  same  quantity. 


\  sciics  ol  tests  clearly  indicated  the  superior  catching 
abihiv  ol  in  Ion  nets  which  in  addition  are  claimed  to 
stand  about  three  years  of  consecutive  use  as  compared 
to  hall  a  season  for  ramie.  At  that  time,  the  price  of 

\milan  nets  \\as  twice  as  high  as  that  of  ramie  nets,  but 
uikmu  into  account  the  durability  and  larger  catches, 
ihe>  proved,  however,  more  economical. 

The  catch  figures  illustrate  this  point.  In  1952,  the 
first  fishing  Heels  after  the  war  caught  37,000  salmon  and 
irout  in  the  Northern  Pacific.  In  1953,  the  catch  was 
tvMcc  as  much,  at  least  partly  due  to  the  increased 
can-liability  of  Amilan  nets.  Since  then,  more  and  more 
boats  have  adopted  the  nets  and  thcv  are  now  used  by 
everx  tishinu  vessel 


//!•«•/  v 

\uinhc' 
of 

CUH/l 

Catch 
)  ship 

No.  of 
Amilan    / 
nets  used 
(yds.) 

No.  of 
Sarnie  nets 
used 
(yds.) 

i9>: 

s~ 

2,ICK) 

37 

2,784 

127,200 

1953            * 

115 

7,700 

73 

174,000 

124,800 

1954            9 

210 

20,500 

100 

780,000 

0 

1955          14 

405 

64,040 

163 

2,040,000 

0 

1956          16 

506 

52,000 

J03 

3,000,000 

0 

I  150  J 


USE    OF     JAPANESE    SYNTHETIC     FIBRES 


SANMAI  NET  (TRAMMEL  NET) 

This  net  is  known  by  different  names  in  Japan,  such  as 
Jigoku-Net  (Hell  Net),  Sanzyu-Net  (Three-fold  Net),  etc., 
and  is  widely  used  for  inshore  fishing.  Before  nylon 
made  its  appearance,  the  nets  were  made  of  natural 
fibres  such  as  cotton,  ramie  and  silk.  Today,  the  nets 
are  made  of  Amilan  and  about  80  or  100  times  more  of 
such  nets  are  being  operated. 

PURSE  SEINE  NET 

The  use  of  nylon  for  the  construction  of  these  nets  has 
great  advantages  owing  to  its  increased  tensile  strength 
per  weight  unit,  as  compared  to  cotton.  When  Amilan 


is  used  the  boat  and  crew  can  work  a  much  larger  net, 
or  less  crew  is  necessary  to  operate  a  purse  seine  of  the 
size  a  cotton  net  would  require.  Amilan  is  therefore  now 
used  widely  in  the  tuna  and  bonito  fisheries  of  Japan. 

DRAG  NET 

The  strength,  lightness,  small  moisture  absorption  and 
durability  of  Amilan  trawl  nets  allow  for  increased 
trawling  speed.  During  1952-55  tests  by  Fukuoka 
Fisheries  Experiment  Station  showed  that  the  nets 
produced  higher  catches.  Some  small  fishermen,  however, 
still  hesitate  to  use  them,  because  of  the  cost,  but  it  is 
considered  that  their  advantages  outweigh  this  factor 


On  this  Arctic  trawler,  the  belly,  codend  and  headline  of  her  trawl  are  made  from  nylon  which  ably  withstands  the  strain  of  heavy  catches. 

[151   ] 


DEVELOPMENT  OF   SYNTHETIC   NETTING  AND   ITS  EFFECT 

ON  THE  FISHING  INDUSTRY 

by 

MOMOI  FISHING  NET  MFG.  CO.  LTD. 

Ako  Hyogo-Kcn,  Japan 

Abstract 

No  fibre  is  as  yet  available  which  would  suit  the  requirements  of  all  types  of  fishing  gear,  although  some  of  the  new  synthetic  fibres 
come  closer  to  an  ideal  material  than  the  natural  fibres.  Blending  two  synthetic  fibres  can  produce  twines  particularly  suitable  for  some 
specific  gears. 


Resume 


Le  dSveloppement  de  filets  des  fibres  synth&tiques  et  ses  effcts  sur  1'industrie  de  la  p€che 


11  n'existe  pas  encore  dc  fibre  satisfaisant  toutcs  les  exigences  pour  tous  les  types  d'cngins  de  pcche,  bien  que  quelqucs-uncs  dcs 
nouvelles  fibres  synthetiqucs  se  rapprochent  plus  du  materiau  ideal  que  les  fibres  nature  I  les.  Le  melange  de  deux  fibres  synthetiques  peut 
p rod u ire  des  fils  particulierement  appropries  £  la  construction  d'engins  determines. 

Perfeccionamiento  de  las  redes  de  hilos  sinteticos  y  su  influencia  sobre  la  industria  pesquera 
Extracto 

Todavia  no  se  disponc  de  ninguna  fibra  sintetica  que  satifaga  los  requisites  exigidos  por  todos  los  tipos  de  artcs  de  pcsca,  si  bien 
muchas  de  las  obtenidas  ultimamente  se  acercan  al  producto  ideal  mas  que  las  fibras  naturales.  Hs  necesario  agregar  que  la  mezcla  dc  dos 
fibras  sinteticas  permite  fabricar  hilos  especialmcntc  adccuados  para  ciertos  tipos  de  artes  de  pesca. 


THE  Japanese  fishing  net  manufacturing  industry 
has  made  considerable  progress  in  the  last  few  years, 
due  largely  to  the  development  of  synthetic  fibres, 
such  as  nylon,  vinylon,  vinylidene,  vinyl  chloride,  and 
combination  synthetics.  The  export  of  nets,  twines  and 
ropes  of  all  types  from  Japan,  including  natural  fibres, 
totalled  7,163,000  Ibs.  in  1956,  a  22  per  cent,  increase  over 
the  previous  year's  figure,  compared  with  an  increase  of 
9i  per  cent,  in  exports  of  natural  fibre  fishing  nets, 
twines,  and  ropes. 

The  following  are  the  highlights  in  a  survey  made  by  the 
Japanese  Government's  Fisheries  Agency  of  export 
trends  in  fishing  gears,  classified  according  to  natural 
and  man-made  fibres. 

NATURAL  FIBRES 

The  exports  of  cotton  fishing  gear  continue  to  lead  all 
other  kinds  of  fishing  nets,  totalling  5,220,000  Ibs.  in 
1956.  This  accounts  for  73  per  cent,  of  the  total  exports; 
460,000  Ibs.  of  manila  hemp  were  exported  in  1956; 
336,500  Ibs.  of  sisal  and  a  total  of  22,000  Ibs.  of  other 
natural  fibres  (flax,  coir,  ramie  and  silk). 

SYNTHETIC  FIBRES 

The  exports  of  nylon  nets,  twines  and  ropes  amounted 
to  683,000  Ibs.  and  increased  158  per  cent,  in  1956  over 
1955.  More  than  91  per  cent,  were  nylon  fishing  nets, 


while  8  per  cent,  were  twines;  ropes  constituted  1  per  cent, 
of  this  total.  The  export  of  other  synthetic  fibres  in  1956 
was  as  follows;  vinylon  221,000  Jbs.  (increased  2-8 
times  over  the  previous  year);  vinylidene  63,500  Ibs.; 
vinyl  chloride  (which  is  a  comparatively  new  synthetic 
fibre)  3,000  Ibs.;  combination  synthetics  165,000  Ibs. 
(increased  3-8  times  in  1956  and  are  expected  to  exceed 
420,000  Ibs.  in  1957). 

DIFFERENT  FIBRES   FOR  DIFFERENT  FISHING 
PURPOSES 

Synthetic  fibre  fishing  gear  has  contributed  materially 
toward  the  stabilization  of  the  fishing  industry,  but 
because  of  the  industry's  complexity,  no  synthetic  fibre 
developed  to  date  is  ideal  for  all  types  of  fisheries.  At 
present,  gillnet  fisheries  find  nylon  a  satisfactory  replace- 
ment for  linen  netting.  The  purse  seine  fisheries  have 
accepted  Marlon  (nylon/vinylon)  as  a  replacement  for 
cotton  netting,  while  vinylon  may  prove  an  acceptable 
replacement  for  cotton  and  manila  for  fishing  lines  and 
ropes.  For  trawl  fishing  however,  no  particular  synthetic 
stands  out  as  acceptable  to  replace  natural  fibres. 
Approximately  75  per  cent,  of  the  nylon  netting  exported 
from  Japan  is  used  in  the  gillnet  fisheries  of  various 
countries.  A  great  variety  of  nylon  types  can  be  produced 
by  changing  its  chemical  construction,  production 
method,  yarn  size,  degree  of  twist,  amount  and  method 


[152] 


USE    OF    JAPANESE    SYNTHETIC     FIBRES 


Fig,  I.     Manujactnrc  of  synthetic  twine\. 


of  relieving  the  stress  in  the  twine,  and  the  different 
processes  for  setting  the  knots. 

It  should  be  borne  in  mind  that  when  selecting  the 
appropriate  nylon  twine  to  replace  linen  for  a  gillnet, 
wet  knot  strength  of  nylon  is  less  than  that  of  first  grade 
linen.  So,  where  it  is  necessary  to  maintain  the  same 
strength  a  slightly  heavier  nylon  twine  must  be  chosen. 

Nylon  exposed  continually  to  sunlight  of  normal 
intensity  for  two  months  can  lose  as  much  as  40  per  cent, 
of  its  tensile  strength.  It  must  therefore  be  kept  away 
from  direct  sunlight. 

Nylon  is  not  weakened  by  bacteria,  but  a  nylon  net 
should  be  kept  clean,  as  acid  caused  by  fish  slime  can 
be  harmful. 

DEEP  WATER  G1LLNETS  AND  SEINES 

To  determine  the  twine  diameter  most  suitable  for  deep 
water  gillnets,  consideration  must  be  given  to: 

a.  Strength  of  the  twine 

b.  Fishability 

c.  Form  of  the  net  in  the  water  when  fishing 

d.  Durability  and 

e.  Initial  cost  of  the  net. 

Factors  a  and  b  are  opposite  in  function,  namely 
thicker  twine  means  higher  strength,  but  lower  catching 
ability,  therefore,  a  compromise  has  to  be  found. 

The  specific  gravity  of  the  material  influences  the  form 
of  the  net  when  fishing  and  it  was  found  that  pure 


nylon  nets  were  too  light.  Attempts  were  made  to  over- 
come this  deficiency  by  hanging  a  strip  of  vinylidene 
net  360d/30  ply  on  a  ratio  of  nylon  4  :  vinylidene  1  under 
the  nylon.  This  method  was  not  entirely  satisfactory. 

The  later  method,  which  proved  satisfactory,  was  to 
combine  nylon  and  vinylidene  fibre  during  manufacture 
of  the  yarn  and  twine.  This  product  called  Livlon  has 
considerable  merit  for  deep  sea  gillnetting  operations. 

It  is  especially  important  to  determine  accurately  the 
most  suitable  twine  size  for  deep  water  purse  seine 
because  the  webbing  is  subject  to  much  water  resistance 
while  sinking.  The  specific  gravity  of  a  fibre  can  be 
determined  by  the  laws  of  physics,  but  accurate  deter- 
mination of  the  resistance  is  complicated  by  various 
factors. 

For  a  comparative  study,  the  unit  of  resistance  K 
D/L  can  be  used  where  D  is  the  diameter  of  the  twine 
and  L  the  meshsize  (stretched). 

The  unit  of  resistance  of  a  standard  sardine  purse 
seine  made  of  cotton  twine  20/15  ply  with  a  mesh  size 
of  20  m/m  would  be  0-05. 

If  the  influence  of  the  surface  condition  of  the  twine 
on  the  resistance  is  called  44R",  the  numerical  values  of 
kkR"  for  different  types  of  twine  are  as  follows: 


Cotton       

Nylon        

Vinyl  Chloride 
Vinylon 


1-00 
0-75 
0-70 
1-00 


153] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


Fig.  2.     Mechanical  knitting  of  synthetic  fibre  nets. 


Fig.  3.     Inspection  oj  machine-braided  webbing. 
\  154  ] 


USE    Oh     JAPANESE    SYNTHETIC    FIBRES 


/-'if;.  4      Fishing  with  iwo-hoal  pin.\c  M'incs  <>/  Marlon  (in  Japan). 


These  factors  "K"  and  "R"  influence  the  shape  of 
the  seine  in  operation,  sinking  speed,  etc. 

It  may  be  assumed  that  a  set  is  made  with  a  cotton 
seine  20/15  ply,  20  m  m  stretched  mesh,  300  fathoms 
long  by  10  fathoms  deep,  on  a  sardine  school  of  100 
tons,  20  fathoms  in  width,  that  the  travelling  speed 
of  the  school  is  0-5  fathoms  second,  and  that  it  takes 
2  minutes  for  the  sardines  of  the  lower  part  of  (he  school 
to  arrive  at  the  skirt  of  the  net.  In  this  case,  the  seine 
must  be  pursed  within  two  minutes  to  catch  the  entire 
school,  thus  the  net  must  sink  at  a  rale  of  I  fathom  per 
second,  which  is  quite  difficult  to  accomplish  with  cotton 
netting.  However,  Livlon  seine  netting  of  smaller 
diameter  twine,  manufactured  with  a  special  twisting 
method  equal  in  specific  gravity  to  cotton,  is  able  to 
meet  the  requirements. 

Marlon  netting  was  first  put  to  commercial  use  in 
1951,  and  many  of  the  original  nets  are  still  giving  good 
service.  Its  most  notable  characteristics  are  derived  from 
the  combination  of  the  best  features  of  nylon  and 
vinylon  fibres. 

Some  of  the  advantages  of  such  a  combination  are: 
increased  specific  gravity  of  the  twine,  maintenance  of 


strength  b>  the  predominance  of  mlon  fibre  in  the  twine 
and  decreased  knot  slippage. 

Marlon  netting  is  available  in  a  \anei\  of  colours, 
twine  and  mesh  sizes,  and  complete!)  made  up  purse 
seines  of  standard  or  controlled  specifications  art- 
available. 

Like  most  synthetics,  it  requires  little  maintenance, 
but  reasonable  precaution  should  be  taken  lo  prevent 
undue  exposure  lo  sunlight. 

CONCLUSIONS 

Synthetic  fibres  are  rapidlv  replacing  natural  fibres  in 
the  construction  of  all  kinds  of  fishing  gear,  but  an 
ideal  synthetic  fibre  suitable  to  all  types  of  gears  is  noi 
yet  available  to  the  fishing  induslrx. 

At  present  the  synthetic  fibres  most  commonK  used  in 
the  main  types  of  fishing  gear  can  be  listed  as  follows 

Type  i>f  dear  Srnihefn    nn»\i  i/.\«'«/ 

Ciillncts,  trammel  nets,  tangle  nets  nylon 

purse  seines,  beach  seines.  Mai  Ion  aiul  othei  combination 

lamparas,  trap  nets  synthetics 

beam  trawl,  otter  trawl,  balloon  various  but  none  cut ircl> 

trawl  satisfactorv 

twines  and  ropes  \  my  I  on.  n\  Ion  and  combinations 


[155 


TESTS   WITH  NYLON  FISHING  TACKLE  IN   SWEDISH 

INLAND   FISHERIES 

by 

GOSTA  MOLIN 

Institute  of  Freshwater  Research,  Drottningholm,  Sweden 

Abstract 

It  has  become  quite  clear  that  nylon  is,  in  several  respects,  superior  to  cotton,  flax  or  hemp,  and  among  its  foremost  advantages  are 
its  rot-resistance  and  its  high  fishing  capacity.  Fishing  experiments  have  proved  that  continuous  multifilament  nylon  has  something  like 
twice  the  fishing  capacity  of  cotton,  and  that  the  monofi lament  nylon  has  a  fishing  capacity  about  seven  times  as  good  as  cotton.  During 
the  last  five  years  practically  every  professional  fisherman  has  changed  over  to  nylon  nets,  and  while  it  is  true  that  the  initial  cost  of  these  is 
high,  this  is  offset  by  higher  catches  of  fish  and  longer  life  for  the  gear. 

In  the  case  of  set  nets,  those  parts  which  are  constantly  above  the  surface  of  the  water  are  soon  destroyed  by  the  sun's  rays  and  have 
to  be  replaced,  so  now  these  parts  should  better  be  made  of  such  materials  as  Saran,  Kuralon  or  Tcrylene  which  arc  less  sensitive  to  ultra- 
violet rays.  The  monofilament  nylon  is  especially  good  for  fishing  in  clear  water  because  of  its  invisibility,  but  in  muddy  waters  this  advantage 
is  lost. 


Resume 


Essais  effectues  sur  des  engins  de  pftche  en  nylon  dans  les  ptehes  int&ieures  sutdoises 


II  est  devenu  Evident  que  le  nylon  est  supe>ieur,  a  plusieurs  points  de  vue,  au  coton,  au  lin  ou  au  chanvre;  ses  principaux  avantages 
6tant  son  imputrescibilite  et  sa  grande  capacite  de  peche.  Des  essais  ont  montre  que  la  capacite  de  peche  du  nylon  a  fibres  multiples  continues 
etait  deux  fois  superieure  &  celle  du  coton,  et  celle  du  nylon  monofilament,  sept  fois.  Au  cours  des  cinq  dernieres  annees,  la  presquc  totality 
des  pdcheurs  professionnels  ont  adopte  les  filets  de  nylon,  et  bien  que  leur  prix  d'achat  soit  eleve,  il  est  compense  par  leur  plus  longue  duree  et 
les  pcches  plus  abondantes  qu'ils  permettent  de  realiser. 

En  ce  qui  concerne  les  filets  fixes,  les  parties  qui  se  trouvent  constamment  au-dessus  du  niveau  de  Peau  sont  rapidement  detruites  par 
les  rayons  du  soleil  et  doivent  etre  remplacees,  en  sorte  qu'il  serai t  pr£f<6rablc  de  les  fabriqucr  en  matdriaux  tels  que  le  saran,  le  Uuralon  ou 
le  terylene  qui  sont  moins  sens ib les  a  Faction  des  rayons  ultra-violets.  Le  nylon  monofilament  est  particulierement  efficace  en  eau  claire  a 
cause  de  son  invisibilite,  mais  il  perd  cet  avantage  en  eau  trouble. 


Extracto 


Pruebas  hilo  de  nylon  para  aparejos  de  pesca  usados  en  las  pesquerias  interiores  de  Suecia 


Es  evidcnte  que  el  ny!6n  rcsulta,  dcsde  varios  puntos  de  vista,  superior  al  algod6n,  lino  o  c&ftamo,  contdndose  cntre  sus  ventajas 
mas  sobresalientes  su  resistencia  a  la  pudricion  y  alta  capacidad  de  pesca.  Los  expenmentos  hechos  han  demostrado  que  los  hilos  de  ny!6n 
formados  por  gran  numero  de  filamentos  continuos  y  los  de  una  sola  hebra,  permiten  obtencr  el  doble  y  siete  veces  mas  rendimiento  que  los 
de  algod6n.  Durante  los  ultimos  cinco  anos  practicamente  todos  los  Pescadores  profesionales  han  comenzado  a  usar  redes  de  ny!6n,  cuyo 
alto  costo  inicial  se  compensa  con  la  mayor  cantidad  de  pesca  y  duracidn  del  arte. 

En  el  caso  de  las  redes  fijas,  las  partes  que  estan  constantemenlc  sobre  la  superficie  del  agua  son  dcstruidas  con  mas  rapidez  por  la 
acci6n  de  los  rayos  solares  y  deben  ser  reemplazadas.  Para  evitar  este  inconvenicnto  se  recomienda  confeccionar  dichas  secciones  con 
materiales  como:  saran,  kura!6n  o  terrileno  que  son  menos  sensibles  a  los  rayos  ultraviolados.  £1  ny!6n  dc  una  hebra  se  presta  cspccialmente 
para  la  pesca  en  aguas  claras  a  causa  de  su  invisibilidad,  pcro  en  las  cenagosas  esta  ventaja  no  ejerce  ninguna  influencia. 


THIS  paper  gives  a  description  of  tests  which  were 
begun  in  1947  by  the  Institute  of  Freshwater 
Research  (Sotvattenslaboratoriet),  on  the  design  of 
nets  made  of  nylon. 

One  of  the  greatest  disadvantages,  particularly  of 
continuous  filament  or  monofilament  nylon,  was  the 
difficulty  of  making  knots  that  would  not  slip.  This  was 
gradually  eliminated  by  using  various  methods,  such  as 
the  treatment  e.g.  chemical  treatment  of  the  material 
before  or  after  knotting,  as  well  as  the  use  of  double 
knots.  In  cases  where  the  nets  are  exposed  to  great  stress, 
the  last-mentioned  method  is  still  the  most  generally  used. 

Certain  difficulties  are  still  encountered  in  the  manufac- 
ture of  monofilament  nylon  nets.  Apart  from  the  use  of 
double  knots,  the  chemical  treatment  has  proved  to  be 
most  effective. 


One  more  disadvantage  of  nylon  nets  for  fishing 
purposes  is  their  relatively  high  sensitivity  to  ultra-violet 
rays.  Tests  carried  out  with  both  normal  sunshine  and 
quartz-lamps  have  proved  multifilament  nylon  to  be 
most  sensitive  to  such  radiation,  whereas  the  mono- 
filament  nylon  is  more  resistant.  While  it  is  true  that 
special  impregnation  of  multifilament  nylon  with 
catechu  offers  a  certain  protection  it  cannot  prevent  the 
gradual  reduction  in  strength.  From  the  practical  point 
of  view  this  sensitivity  to  light  is  not  an  important 
disadvantage  to  the  fisherman.  If  he  knows  of  the 
destructive  influence  of  light  on  his  fishing  tackle,  he 
can  take  care  that  his  nets  and  tackle  are  not  exposed 
to  sunshine  more  than  necessary.  Practical  experience 
gained  by  Swedish  fresh-water  fishermen  has  also  proved 
that  nets  that  must  be  discarded  after  3  to  4  years  have 


[156] 


NYLON    TACKLE    JN    SWEDISH     FISHERIES 


not  become  unserviceable  due  to  the  destructive  action 
of  sunshine,  but  to  damage  through  tearing  during 
intensive  fishing. 

Quite  different  are,  on  the  other  hand,  the  conditions 
prevailing  in  connection  with  the  use  of  certain  types  of 
fixed  fishing  tackle  such  as  bottom  nets  and  fish  baskets 
(bow  nets),  when  part  of  the  tackle  often  remains  standing 
above  the  water  level.  In  such  cases  the  sensitivity  of 
nylon  to  sunshine  necessitates  frequent  replacement  of 
the  destroyed  parts.  It  would  be  practical  to  use  for 
these  parts  Terylene  or  Saran  fibres,  which  are  extremely 
resistant  to  U.V.  radiation. 

It  must  be  mentioned  here  that  monofilament  nylon 
very  easily  becomes  slimy  through  the  adherence  of 
microscopic  particles  whirling  about  in  the  water, 
particularly  when  the  wind  is  strong.  Since  the  com- 
paratively high  catching  ability  of  this  type  of  net  is 
mainly  dependent  on  its  relative  invisibility  in  water,  the 
slimy  coating  of  the  net  reduces  its  efficiency  which,  in 
certain  cases,  becomes  less  than  that  of  spun  nylon. 
This  slimishness  is  particularly  noticeable  when  the  catch 
is  checked  without  lifting  the  net  from  the  water,  and 
the  net  is  thus  left  in  a  submerged  position,  sometimes 
for  several  days.  The  thorough  rinsing  of  the  nets  is 
absolutely  necessary  to  avoid  a  reduction  of  the  catching 
ability.  This  disadvantage  is  responsible  for  the  relatively 
limited  use  of  monofilament  nets  in  many  flat-country 
lakes,  where  the  water  often  has  a  high  slime  content. 

THE  RESULTS  OF  COMPARATIVE  FISHING 
TESTS 

In  order  to  estimate  the  catching  ability  of  continuous 
multifi lament  nylon  compared  with  cotton,  fishing  tests 
were  carried  out,  using  nets  of  equal  size.  Separate  tests 
have  proved  the  catching  ability  of  nylon  nets  to  be 
twice  that  of  cotton  nets.  Other  tests  showed  that  the 
excess  catch  made  with  nylon  nets  varied  between 
1  -75  to  3  times. 

Apart  from  the  tests  carried  out  by  the  Institute, 
supervised  tests  were  made  by  professional  fishermen 
and  gave,  on  the  whole,  the  same  results.  All  tested 
nylon  nets  were  manufactured  from  continuous-multi- 
filament  twine  as  staple  nylon  twine  is  unsuitable  for 
gillnets.  The  reason  for  the  high  catching  ability  of 
nylon  is  its  low  water  absorption,  relatively  high  elasticity 
and  high  breaking  strength.  These  qualities  permit  the 
use  of  thinner  twine  than  is  possible  with  the  weaker 
cotton.  This  rule  is  valid  for  all  kinds  of  net  fishing: 
the  catching  ability  of  nets  increases  with  a  reduced  twine 
diameter.  The  choice  of  the  proper  diameter  is  naturally 
merely  a  question  of  experience.  The  twine  must  not, 
however,  be  so  fine  as  to  cause  very  quick  wear  and 
consequent  need  for  frequent  repairs. 

Since  1951  the  Institute  has  carried  out  catching  tests 
with  nets  made  from  monofilament  nylon.  This  is  an 
almost  ideal  material  for  certain  fishing  tackle  as,  due 
to  its  transparency,  it  is  practically  invisible  in  water. 
It  was  found  that  the  monofilament  gillnets  caught  not 
less  than  seven  times  as  much  fish  as  cotton  nets,  and 
approximately  four  times  as  much  as  nets  of  continuous- 
multifilament  nylon. 

The  tests,  which  were  carried  out  during  both  summer 
and  autumn,  also  proved  that  the  catching  ability  of 


nets  made  of  monofilament  nylon  are  to  a  certain  extent 
dependent  on  light  conditions.  Thus,  the  excess  catches 
were  higher  during  the  light  season  when  the  advantage 
of  transparency  is  greater.  It  has  also  proved  possible  to 
achieve  good  results  with  these  nets  in  daytime  in  lakes 
with  clear  water,  when  cotton  or  continuous-mukifilament 
nylon  nets  were  quite  unsuccessful. 

A  test  was  carried  out  during  the  same  season  in  two 
different  lakes.  The  degree  of  visibility  in  these  lakes  was 
13  yards  and  IJ  yards  respectively.  In  the  former  lake, 
the  ratio  of  the  catches  made  with  nets  of  cotton,  con- 
tinuous-multifilament  and  monofilament  nylon  was 
1:2:14.  The  corresponding  figures  for  the  lake  with 
reduced  degree  of  visibility  were  1 :2  •  5: 1 1 .  The  fact  that 
the  catches  made  with  monofilament  nets  in  the  latter 
lake  were  unexpectedly  high  indicates  that  apart  from 
the  positive  influence  of  the  invisibility  in  water  on  the 
fishing  results,  this  material  must  possess  additional 
advantages  on  account  of  its  structure. 

Continuous-mukifilament  nylon  proved  to  be  more 
efficient  than  cotton  quite  irrespective  of  the  size  of 
gillnets  used—  whether  high  or  low  (superiority  varying 
between  1  -75  and  3). 

When  higher  (18  to  22  ft.)  nets  made  of  monofilament 
nylon  came  into  use,  it  was  discovered  that  the  superiority 
of  this  material  compared  with  continuous-multifilament 
nylon  disappeared  and  that  the  catch,  in  some  cases, 
was  even  less.  Professional  fishermen  using  such  high 
nets  had  the  same  experience.  The  reason  most  probably 
lies  in  the  increased  stiffness  of  the  monofilament  nylon 
used  for  these  nets.  This  may  have  an  adverse  influence 
on  the  flexibility  of  the  net  surface,  which  is  of  importance 
when  fish  jostle  against  the  net.  Nevertheless,  singular 
positive  results  were  achieved  and  one  of  these  cases 
may  be  quoted. 

In  a  fishing  test  a  net  of  monofilament  nylon  caught 
60  per  cent,  more  white  fish  than  a  corresponding  net 
of  continuous-multifilament  nylon.  The  net  used  for  this 
species  of  fish  is  20  ft.  deep,  provided  with  extremely 
light  weights,  and  has  its  floats  on  the  water  level, 
connected  to  the  net  by  lines  by  which  the  net  position 
between  bottom  and  surface  can  be  varied. 

THE  SCOPE  OF  APPLICATION  OF  NYLON  NETS 

The  change  from  cotton  to  nylon  nets  among  professional 
fishermen,  despite  their  conservative  attitude  to 
modernization,  is  practically  100  per  cent.  They  soon 
realized  the  great  advantages  of  nylon,  thanks  to  its  high 
catching  ability  and  absolute  resistance  to  rot  as  compared 
with  cotton.  It  is  also  undeniable  that  the  introduction 
of  the  new  material  for  fishing  tackle  has  brought  about 
a  considerable  improvement  in  the  economic  situation 
of  professional  fishermen.  Today,  the  tackle  of  pro- 
fessional fishermen  is  mostly  made  of  continuous- 
multifilament  or  staple  nylon.  The  shallow  nets  used  by 
the  non-professional  and  casual  fishermen  are  made  of 
monofilament  'nylon. 

EXPERIENCE  IN  THE  USE  OF  NYLON  NETS 

In  order  to  secure  the  greatest  possible  catches  when 
using  nets  made  of  monofilament  nylon,  it  is  necessary 
to  follow  certain  rules.  Perhaps  the  choice  of  the  right 


[157] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


diameter  in  relation  both  lo  the  mesh  size  and  to  the 
species  of  fish  for  which  the  net  is  intended,  is  the  most 
important.  There  are  quite  a  number  of  different  species 
of  fish  in  the  same  lake  and  consequently  the  material 
must  be  strong  enough  for  the  strongest  species  of  fish. 
The  following  standard  diameters  are  used  for  certain 
mesh  sizes  in  Swedish  lakes:  0-12  to  0-15  mm.  for 
42  to  55  mm.  stretched  mesh;  0-15  to  0-20  mm.  for 
60  to  75  mm.;  0-20  to  0-25  mm.  for  85  to  100  mm.; 
and  0-22  to  0-30  mm.  for  1 10  to  150  mm.  In  many  lakes 
in  northern  Sweden,  where  the  catches  often  consist 
mainly  of  white  fish,  thinner  material  is  used  for  the 
respective  mesh  sizes,  as  such  fish  does  not  tear  the  nets 
to  the  same  extent  as  pike,  pike-perche,  trout  or  char. 

Unfortunately,  nets  made  of  monofilament  nylon 
earned  a  bad  reputation,  because  the  firms  delivered 
nets  where  the  material  was  too  thick  in  relation  to 
the  mesh  size,  which  resulted  in  inferior  catching  ability. 
Another  reason  was  that  with  some  of  the  nets  the  knots 
were  slipping. 

The  nets  made  of  continuous-multifilament  nylon  at 


present  on  the  market  are  almost  without  exception  ol" 
good  quality  with  good  resistance  against  slipping. 

Fishermen  using  fixed  fishing  tackle  have  to  a  great 
extent  changed  over  to  the  use  of  nets  made  of  staple 
nylon  twine,  particularly  in  lakes  where  this  kind  of 
fishing  is  carried  on  practically  the  year  round  and  where 
the  rot-problem  in  connection  with  cotton  becomes 
extremely  serious.  As  already  stated,  the  parts  of  the 
tackle  remaining  above  water  level  must  be  renewed 
now  and  then,  due  to  deterioration  by  sunshine. 

The  staple  nylon  twine  used  for  this  type  of  tackle  is 
always  treated  with  a  stiffening  agent  which  also  makes 
it  less  elastic.  As  a  rule  coal-tar  or  bituminous  varnish, 
dissolved  in  an  equal  quantity  of  ben/ol,  is  used. 

The  maintenance  of  fishing  tackle  made  of  nylon  is 
extremely  simple.  The  nets  must  not  be  exposed  to 
sunshine  for  long  periods  of  time.  The  repairs  are, 
however,  somewhat  complicated.  So  far  as  continuous- 
multifilament  twine  or  monofilament  is  concerned, 
special  knots  must  be  made  since  usual  netting  knots, 
such  as  those  used  with  cotton  twine,  arc  not  firm  enough. 


Photo  of  television  screen  showing  monofilament  (left)  and  ordinary  cotton  net  under  water.      Photo:  v.  Brandt. 

f!58] 


EXPERIENCE  WITH  SYNTHETIC  MATERIALS  IN  THE 

NORWEGIAN   FISHERIES 

by 

N.  MUGAAS 

Statens  Hskcredskapsimport,  Bergen,  Norway 

Abstract 

Tests  carried  out  by  the  Directorate  of  Fisheries,  Bergen,  showed  that  the  use  of  nylon  and  Perlon  gillnets  in  the  commercial  fisheries 
produced  from  twice  to  seven  times  the  number  of  fish  per  net  per  day  caught  by  the  traditional  cotton  and  hemp  nets,  and  as  a  result  of 
these  tests  the  import  of  synthetic  materials  has  risen  from  15  to  20  tons  in  1954/55  to  400  tons  in  1956/57.  Cod  traps  made  of  Rural  on  have 
been  in  continuous  use  for  3  to  6  months  whereas  traps  made  of  cotton  and  hemp  h;u1  to  be  renewed  every  seventh  week.  Experiments  arc 
continuing  with  other  kinds  of  gear  such  as  purse  seines  and  herring  nets. 


Kfeume 


I /experience  des  pechcs  norvegiennes  avec  les  materiaux  synth&iques 


Les  cssais  efleclues  par  la  Direction  des  Peches,  Bergen,  ont  montre  que  Pemploi  des  filets  maillants  de  nylon  ct  de  Perlon  dans  les 
pcches  industriellcs  produisait  de  deux  a  sept  fois  plus  de  poissons  par  filet  et  par  jour  dc  peche  quo  les  filets  traditionnels  de  coton  ct  de  chanvre. 
A  la  suite  de  ccs  essais  Timportation  dc  matdriaux  symhctiques  est  passec  dc  15  au  20  tonnes  en  1954,55  a  400  tonnes  en  1956/57.  Les  trappes 
a  morues  de  Kuralon  sont  employees  continucllemcnt  dc  3  a  6  mois  alors  que  celles  de  coton  ct  dc  chanvre  doivant  etre  renouvcldes  toutes 
les  sept  semaincs.  Les  experiences  se  poursuivant  avec  d'autrcs  sortes  d'engins  tcls  que  la  scnnc  tournante  et  les  filets  a  harcngs. 

Experiences  con  materiales  sinteticos  en  las  pesqucrias  noruegas 
Extracto 

Las  prucbas  hechas  por  la  Direction  de  Pesca.  en  Bergen,  han  demos trado  que  el  uso  dc  redes  de  enmallc  de  nylon  y  Pcrldn  permite 
obtener  diariamcnte  entrc  dos  y  sietc  vcccs  mas  pescados  por  artc  que  las  tradicionales  dc  algoddn  y  canamo.  Como  rcsultado  de  estas 
prucbas  ha  aumentado  la  imporlacion  de  materiales  sinteticos  desdc  15  -  20  tons  en  1954-5,  a  400  tons  en  1956-7.  Las  redes  "trampas" 
de  "kuralon"  pudicron  usarse  continuamente  durantc  3  a  6  meses,  pero  las  hechas  de  algodon  y  caftamo  debicron  reemplazarse  cada  semana. 
En  la  actual i dad  se  continiian  los  ertsayos  con  otros  tipos  dc  artes  dc  pesca,  como  redes  dc  ccrco  ce  jarcta  y  para  arenque. 


SYNTHETIC  materials  are  mainly  used  in  Norway 
for  catching  cod  and  coalfish  with  gillnets,  and  have 
mostly  replaced  the  conventional  cotton  and  hemp 
fibres.  Nylon  66,  nylon  6  and  Terylene  arc  chiefly 
used,  the  twine  being  imported  from  Great  Britain, 
Canada,  U.S.A.  and  the  continent,  although  a  small 
amount  is  produced  in  Norway.  Synthetic  materials  are 
also  used  for  snoods,  lines  and  ropes. 

The  use  of  synthetic  materials  by  Norwegian  fishermen 
has  gradually  increased  since  1955,  following  experiments 
with  gillnets  of  nylon  and  Perlon  which  were  started 
by  the  Directorate  of  Fisheries  in  1951  and  continued  in 
1952,  1953,  1954  and  1955.  On  the  condition  that 
reports  were  made  on  catchability,  etc.,  certain  fishermen 
along  the  coast  were  given  a  quantity  of  nets  of  nylon 
and  perlon  to  use  alongside  the  conventional  gillnets 
made  from  cotton  and  hemp.  The  number  of  fish  caught 
by  each  type  of  net  was  counted  in  every  haul  and  a  report 
sent  to  the  Directorate  of  Fisheries  when  fishing  was 
finished.  The  following  example  is  typical  of  the  reports 
received  (overleaf). 

As  will  be  noticed  from  the  report,  the  catchability 
of  the  nylon  nets  was  approximately  twice  that  of  the 
conventional .  nets.  This  figure  is  also  typical  as  an 
average,  although  according  to  several  reports,  catches 


were  sometimes  seven  times  higher.  The  types  of  twine 
used  in  these  experiments  were  comparable  as  to  wet 
knot  strength  and,  according  to  the  reports,  durability 
and  strength  were  satisfactory. 

When  the  results  of  these  promising  experiments  were 
published,  the  interest  in  the  synthetic  material  grew 
rapidly  among  the  fishermen  and  production  of  nylon 
gillnets  was  taken  up  by  the  Norwegian  manufacturers 
to  meet  the  increasing  demand.  The  nets  produced  in 
Norway  are  mainly  the  single  knot  type. 

The  following  figures  illustrate  the  growth  of  imports 
of  synthetic  material  for  the  fisheries,  including  twine, 
filaments  and  nets  (a  large  part  of  the  nets  are  of  the 
double  knot  type): 

1954/55  approx.   15  to  20  tons 
1955/56        „      290 
1956/57        „      400 

1954, 55  was  the  first  season  when  nylon  came  into  use. 

The  strength  of  nylon  is  often  overestimated  when  first 
used  for  fishing  gear.  The  dry  tensile  strength  of  nylon 
shows  a  great  superiority  when  compared  to  that  of 
natural  fibres,  but  it  sustains  a  great  reduction  of 
strength  when  wetted,  whereas  the  tensile  strength  of 
cotton  increases  in  wet  condition.  The  strength  of  nylon 


M59] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


1953 

65  ordinary  codnels 
(cotton  and  hemp) 

10  nylon  nets 

Date 

Total  catch     A  verage  catch 

Total  catch     A  verage  catch 

per  day 

per  net  per  day 

per  day       per  net  per  day 

19/2 

40 

0-62 

19 

1-9 

21/2 

108 

1-66 

132 

13-2 

23/2 

168 

2-58 

170 

17-0 

24/2 

29 

0-44 

17 

1-7 

26/2 

21 

0-32 

14 

1-4 

27/2 

27 

0-42 

9 

0-9 

28/2 

71 

1-09 

11 

11 

2/3 

45 

0-69 

9 

0-9 

3/3 

371 

5-70 

112 

11-2 

4/3 

87 

1-33 

26 

2-6 

5/3 

234 

3-60 

47 

4-7 

7/3 

319 

4-90 

111 

11-1 

10/3 

323 

4-96 

133 

13-3 

11/3 

235 

3-61 

93 

9-3 

12/3 

140 

2-15 

21 

2-1 

14/3 

314 

4-83 

71 

7-1 

16/3 

221 

3-40 

83 

8-3 

17/3 

307 

3-18 

93 

9-3 

18/3 

59 

0-90 

12 

1-2 

19/3 

76 

1-16 

31 

.M 

20/3 

231 

3-55 

52 

5-2 

24/3 

498 

7-66 

129 

12-9 

25/3 

87 

1-33 

14 

1-4 

26/3 

14 

0-22 

y 

09 

27/3 

73 

1-12 

18 

1-8 

28/3 

366 

5-63 

57 

5-7 

30/3 

82 

1-26 

27 

2-7 

31/3 

140 

2-17 

16 

1-6 

1/4 

270 

4-15 

28 

2-8 

Total 

1,885  nets       4,956  fish 

290  nets 

1,564  fish 

Average  - 

-2-63  fish 

Average  --5 

•38  fish 

per  net 

per  day 

per  net  per  day. 

is  further  reduced  by  knotting.  This  has  often  led  to  the 
wrong  selection  in  size  of  nylon  twines  in  replacing 
cotton  and  hemp  twines. 

For  example,  nylon  twine,  chosen  on  the  basis  of 
dry  tensile  strength,  produced  a  fine  netting  with  good 
catchability  but  resulted  in  much  damage  to  nets  in 
hauling  the  catch  in  the  cod  fishing  season  1954/55.  An 
investigation  showed  that  the  twine  was  too  fine  and  the 


wet  mesh  strength  of  the  nets  was  lower  than  that  of 
similar  nets  of  hemp  and  cotton. 

Tests,  wet  and  dry,  with  and  without  knots,  were 
carried  out  with  nylon,  hemp  and  cotton  to  find  a  rule 
for  choosing  nylon  as  a  substitute  for  cotton  and  hemp 
for  a  particular  net. 

As  a  result  of  these  tests  it  was  found  that  a  nylon 
twine  with  50  per  cent,  more  runnage  than  the  cotton 
it  would  replace,  or  which  was  50  per  cent,  longer  per 
unit  weight,  had  a  satisfactory  wet  knot  strength. 

Knot  slippage  in  single  knot  nylon  netting  has  also 
created  problems,  but  experiments  and  experience 
have  led  to  methods  being  evolved  to  overcome  this 
difficulty.  The  application  of  heat  and/or  bonding  agents 
has  made  it  possible  to  produce  a  single  knot  webbing 
which  is  satisfactory  as  regards  knot  slippage  and 
stiffness  of  the  netting. 

The  degree  of  knot  slippage  depends  on  the  construc- 
tion of  the  knot  and  double  knots  show  less  tendency 
to  slip  than  single  knots.  On  the  other  hand,  mending 
double  knot  webbing  requires  more  work. 

Polyester  fibre  nets  of  Terylene  are  used  for  catching 
cod  and  coalfish.  Their  catchability  and  durability  seem 
to  be  similar  to  those  of  nets  made  from  the  polyamide 
fibres. 

In  1954/55  experiments  with  codtraps  made  from  the 
Japanese  poly  vinyl  alcohol  fibre  Kuralon  were  carried 
out  by  the  Directorate  of  Fisheries.  As  this  type  of  gear 
is  almost  continuously  in  the  sea  until  it  is  renewed, 
traps  made  from  hemp  or  cotton  have  a  relatively  short 
life,  being  destroyed  by  micro-organisms.  A  first  report 
from  fishermen  said  that  Kuralon  traps  had  been  in  the 
sea  continuously  for  20  weeks  while,  during  this  same 
period,  traps  from  hemp  and  cotton  had  to  be  renewed 
every  seventh  week.  Up  to  April  25th,  1957,  seven  reports 
said  that  Kuralon  traps  had  been  in  the  sea  continuously 
from  3  to  6  months,  averaging  4  •  5  months,  and  there  were 
no  signs  of  reduction  in  the  strength  of  the  twine. 

Experiments  are  still  being  carried  out  with  most  types 
of  fishing  gear,  such  as  purse  seines,  herring  nets,  etc. 
made  of  synthetic  material.  Despite  the  high  cost  of 
these  materials,  there  seems  to  be  a  great  future  for  their 
use  in  the  various  types  of  fishing  gear. 


160] 


ON   THE   FISHING   POWER   OF   NYLON    GILLNETS 

by 

G.  SAETERSDAL 

Institute  of  Marine  Research,  Bergen,  Norwa> 


Abstract 

A  series  of  experiments  was  earned  out  in  Noivtay  to  ie*t  the  fishing  capacity  of  nylon  nets  as  compared  with  nets  made  of  cotton. 
In  the  mackerel  and  coalfish  fishery,  nylon  gillnets  were  used;  in  the  cod  fishery,  nylon  and  Perlon  were  used.  It  was  found  that  the  observed 
fishing  power  of  the  nets  made  of  artificial  fibres  was  from  2-5  to  4-4  times  that  of  cotton  in  the  case  of  cod.  1  -4  to  2-3  in  the  case  of  coal- 
fish,  and  1  -2  to  1-3  in  the  case  of  mackerel.  The  reason  for  this  variation  between  the  different  species  may  be  related  to  the  shoaling  habits 
of  the  fish  and  this  possibility  is  discussed  in  the  report. 


Resume 


Sur   la   capacitc  de  peche  des  filets  maillants  do  nylon 


t.n  Norxege,  on  a  effect uc  unc  serie  d'cxperiences  pour  essayer  la  capacite  de  peche  des  filets  de  nylon  computes  a  filets  de  colon 
On  a  utilise  des  filets  maillants  de  nylon  pour  la  peche  du  maquereau  et  du  colin,  et  des  filets  dc  nylon  ct  de  perlon  pour  la  peche  de  la  morue. 
La  capacitc  de  peche  qui  a  etc  observee  pour  les  filets  de  fibre  artificiellc  etait  de  2,5  £  4,4  fois  celle  du  coton  dans  le  cas  de  la  morue,  1,4 
A  2,3  fois  dans  le  cas  du  colin  et  1,2  a  1,3  fois  dans  le  cas  du  maquereau.  La  raison  de  cette  variation  entre  les  diflTcrcntcs  especes  peui 
etre  attribute  aux  habitudes  dc  vivre  en  banes  des  poissons  et  cette  possibility  cst  discutec  en  detail  dans  le  rapport. 


I. -A  capacidad  de  pesca  de  las  redes  de  enmalle  de  nylon 
Extracto 

Fn  Noruega  sc  hicieron  series  de  cxpcrimcntos  para  detcrminar  la  capacidad  de  pesca  de  las  redes  de  nylon  en  comparacion  con  las 
de  algodon.  En  las  pesquei  las  de  caballa  y  colin  se  utilizaron  artes  del  primero  de  estos  materiales  y  de  perlon,  encontrindose  que  el  rendi- 
miento  de  los  tejidos  con  fibras  artificiales  era  de  2,5  a  4,4,  1,4  a  4,4  y  1,2  a  1,3  veces  mayor  que  los  d  algod6n  en  el  caso  del  bacalao,  colin 
y  caballa,  respect  ivamentc.  fcn  el  in  for  me  tambien  se  analizan  detallaclamcntc  las  causas  de  estas  vanaciones  entre  las  diversas  especies, 
que  se  hallan  relacionadas  con  los  habitos  gregarios  de  los  peces. 


ON  the  provision  of  the  Fisheries  Director,  Bergen, 
a  few  fishing  experiments  for  cod  with  gillnets 
made    of  artificial    fibres    were    carried    out  in 
northern  Norway  in  1952  and  1953.  The  results  obtained 
were  promising  and  for  the  season  J954  a  large  series  of 
experiments  was  planned  in  order  to  ascertain  the  fishing 
power  and  the  practical  usefulness  of  such  nets  compared 
with  conventional  ones. 

The  experiments  were  planned  and  carried  out  under 
the  leadership  of  Fisheries  Consultant  M.  Halaas, 
Fisheries  Directorate,  Bergen.  An  agreement  was  made 
with  a  number  of  fishermen  from  different  parts  of  the 
coast  that  they  should  include  some  nets  of  artificial 
fibres  in  their  sets  of  conventional  nets  and  report  on  the 
results.  As  a  whole  this  arrangement  proved  to  be 
successful.  Most  of  the  fishermen  continued  this  work 
also  in  the  season  1955. 

SPECIFICATIONS  OF  THE  NETS 

The  experiments  include  the  species  cod,  coalfish  and 
mackerel.  In  the  Norwegian  gillnet  fisheries  for  cod  and 
coalfish  the  mesh  size  and  the  dimensions  of  the  nets 


\ary  in  the  various  coastal  districts,  in  order  to  obtain 
comparable  results  the  nylon  nets  were  in  each  case  made 
to  match  these  different  specifications  as  far  as  possible. 
The  following  types  of  artificial  fibre  nets  were  used: 

Mackerel:  nylon,  210/2  s  3,  600  m./kg.,  breaking  strength 

8  kg.  18  knots  per  ell  (24  inch).      . 
Cod:  nylon,  210/5x3,  2,500  m./kg.  breaking  strength 

20  kg.  6]  and  7  knots  per  ell  (24  inch). 

Perlon  No.  26,  continuous  multi-filament,  2,200 

m./kg.,  breaking  strength  19  kg.,  6J,  61  and  1\ 

knots  per  ell  (24  inch). 
Coalfish:     nylon,     210/6x3,    2,000    m./kg.    breaking, 

strength  24  kg.,  7,1,  8  and  8}  knots  per  ell  (24  inch). 

The  knots  in  the  Perlon  nets  were  single,  in  the  nylon 
nets  double. 

The  conventional  nets  used  for  comparison  were  of 
(he  following  types: 

Mackerel:  cotton,  18  knots  per  ell  (24  inch). 
Cod:  cotton,  12/12  and  12/15,  6j  and  6i  knots  per  ell 

(24  inch). 
Coalfish:  hemp,  7/3,  and  8/3,  8  and  8J  knots  per  ell 

(24   inch). 


[161  1 


MODERN    FISHING    GEAR    OF    THE    WORLD 


r  =  0,71 


4  6 

COTTON 
Fig.  L     C^-cotton.    P^Perlon. 

FISHING  RESULTS 


fish 
6  per   net 


Fig.  2.     Creation.     N-  nylon. 


Fig.  3. 


In  the  reports  from  the  fishermen  the  following  data  were 
given  for  each  lift:  number  of  conventional  nets  used, 
number  of  synthetic  fibre  nets  used,  and  total  number  of 
fish  caught  in  each  type  of  net.  The  distribution  of  the 
fish  in  the  single  nets  is  thus  not  available,  but  each  lift 
can  be  described  by  a  paired  mean. 

In  most  cases  the  numbers  of  nylon  nets  used  were 
small  compared  with  the  numbers  of  other  nets  in  the 
sets,  causing  a  large  spread  in  the  distribution  of  the  paired 
means.  This  is,  however,  partly  compensated  by  the 
large  number  of  lifts  in  most  of  the  series. 

Fig.  1  shows  the  distribution  of  the  paired  means  from 
one  of  the  experimental  series.  In  this  case  the  number  of 
both  types  of  nets  were  almost  equal.  The  regression 
lines  have  been  fitted  by  the  method  of  least  squares. 
(The  fact  that  there  was  a  small  variation  in  the  number 
of  nets  used  in  some  of  the  lifts  has  not  been  accounted 
for  in  this  calculation.) 


In  our  analysis  there  is  no  logical  reason  to  prefer  one 
of  these  regression  lines  to  the  other.  They  run  almost 
symmetrically  relative  to  the  origin,  and  it  seems  fair 
to  assume  that  the  true  relation  is  a  simple  proportionality 
The  best  description  of  this  relationship  is  therefore 
a  straight  line  through  the  origin  and  the  paired  mean 
of  all  the  observations  shown  as  the  line  P-=3-94  C  in 
fig.  1. 

Fig.  2  and  fig.  3  show  the  results  of  two  more  series  of 
cod  catches  with  regression  lines  calculated  in  the  same 
way  as  in  fig.  1 .  The  spread  is  larger,  but  the  regression 
lines  are  still  fairly  symmetrical  around  the  origin. 

Similar  cluster  diagrams  were  made  for  all  the  series  or 
smaller  groups  of  experimental  series,  and  they  all  show 
a  more  or  less  pronounced  trend  in  the  distribution  of 
the  paired  means.  In  Table  I  the  results  have  been  listed 
as  the  probable  proportional  relationship  between  the 
fishing  powers  of  the  two  types  of  nets  used,  represented 
by  the  straight  line  through  the  origin  and  the  paired 
mean  of  all  the  observations. 


f  162] 


NYLON    GILLNETS    IN    NORWAY 


TABLE  1 


Series  No.              Species           Locality—year 

Sum  nets  x  //// 

Sum  catch 
convent, 
nets 

Sum  catch 
artific. 
fibre  nets 

Probable  relation 
of 
fishing  power 

I.                    Cod              Scnjabank,  1955 

Cotton  500,  Perlon  452 

744  fish 

2,651  fish 

P 

3-94.C 

2. 

Kvaenangcn,  1954-55 

Cotton  1496,  nylon/Perlon  374 

1,835 

1,396 

N/P 

3-03.C 

3. 

BAtsfjord,  1954 

Cotton  982,  nylon/Perlon  44 

2,868 

430 

N/P 

:  3-37.C 

4. 

Vesteraien,  1953 

Cotton  477,  nylon/Perlon  95 

1,703 

1,036 

N/P 

3-03.C 

5. 

S0r0y,  1954-55 

Cotton  1926,  nylon  172 

4,830 

1,361 

N 

:3-15.C 

»»            it 

„     Perlon  127 

t 

802 

P 

:2-51.C 

6. 

Mager0y,  1954-55 

Cotton  1124,  nylon  175 

3,211 

2,189 

N 

:4-37.C 

»             »» 

„    Perlon  69 

>t 

735 

P 

:3-72.C 

7.                   Coa 

fish          Karm0y,  1954-55 

Hemp  2986,  nylon  436 

14,037 

4,744 

N 

:2-30.H 

8. 

Hcrdla,  1954-55 

Hemp  900,  nylon  150 

2,644 

888 

N 

:2-01.H 

9. 

Herdla,  1954-55 

Hemp  2128,  nylon  225 

15,253 

2,281 

N 

:  1-41.H 

10. 

Bremanger,  1954-55 

Hemp  645,  nylon  100 

2,738 

923 

N 

:2-17.H 

11. 

Stad,  1954-55 

Hemp  748,  nylon  114 

5,515 

1,211 

N 

:  1  -44.H 

12.                  Mackerel        Langesund,  1954-55 

Cotton  2749,  nylon  210 

30,841  kg. 

2,721  kg. 

N 

:  1-16.C 

13.                        „               Flekker0y,  1954 

Cotton  2850,  nylon  150 

27,357 

1.826 

N 

:  1-27.C 

nylon;    P^  Perlon;     C— cotton;    H=hcmp. 


In  the  series  numbered  2  to  6  in  this  table  both  nylon 
and  Perlon  nets  were  used  in  the  same  sets  together  with 
the  conventional  cotton  nets.  Records  of  the  catch  by 
each  of  these  types  of  artificial  fibres  arc  available  only 
from  the  series  5  and  6.  They  show  slightly  lower  values 
for  Perlon  than  for  nylon,  but  the  number  of  observations 
is  too  small  to  allow  conclusions. 

CONCLUSIONS 

In  the  case  of  the  cod  it  is  seen  that  the  observed  fishing 
power  of  the  nets  made  of  artificial  fibres  varies  from 
2-5  to  4-4  times  that  of  the  cotton  nets.  The  correspond- 
ing range  for  the  coalfish  is  from  J  -4  to  2-3  and  for  the 
mackerel  1  -2  and  1  -3. 

The  variation  "within  the  species"  may  partly  be 
chance  variation,  partly  due  to  different  experimental 
conditions  such  as  the  rigging  of  the  nets,  the  arrange- 
ment of  the  two  types  of  nets  in  the  sets,  etc.  The  variation 
"between  the  species"  is,  however,  probably  a  true  one, 
and  must  in  some  way  be  related  to  the  nature  of  the 
higher  efficiency  of  the  artificial  fibre  nets. 

The  observer  of  experiment  No.  6  remarked  in  his 
report  that  the  nylon  nets  seemed  to  catch  a  wider  range 
offish  sizes  than  usual.  A  sampling  was  planned  for  the 


season  1955/56  to  ascertain  whether  such  a  difference  in 
the  selectivity  really  existed,  but  sufficient  data  was 
unfortunately  not  obtained.  Such  a  difference  could  be 
caused  by  the  higher  elasticity  of  the  nylon  twine. 
Another  practical  experience  which  may  touch  on  this 
point  is  that  the  quality  of  the  cod  caught  on  the  nylon 
nets  seems  to  be  inferior  to  that  of  the  fish  caught  on 
cotton  nets,  presumably  because  the  fish  die  more 
quickly  in  the  nylon  nets. 

If  the  higher  fishing  power  of  the  nylon  nets  is  con- 
nected with  the  ability  of  the  nets  to  catch  a  wider  range 
of  fish  sizes,  then  this  would  offer  us  an  explanation  of 
the  different  efficiency  of  the  nylon  nets  towards  the 
different  species  found  in  these  experiments.  As  a  result 
of  the  pronounced  schooling  behaviour  of  the  coalfish, 
the  range  offish  sizes  present  in  a  concentration  of  coalfish 
is  usually  considerably  smaller  than  in  a  concentration  of 
cod.  This  phenomenon  is  even  more  striking  in  the 
mackerel.  In  agreement  with  this  hypothesis  is  v. 
Brandt's  (1955)1  report  of  only  up  to  35  percent,  increase 
of  catches  with  Perlon  nets  in  herring  drifting. 


i  Brandt,  A.  v.,  1955. 
wirtschaft,    1955,  99-101. 


REFERENCE 

Perlon-Netze  in  der  Loggertischcrei.  Fisch- 


[163) 


DISCUSSION   ON    RELATIVE    EFFICIENCIES   OF   NETS    MADE 

OF  DIFFERENT  MATERIALS 


Mr.  B.  B.  Parrish  (U.K.)  Rapporteur.  When  judging  the 
relative  efficiencies  of  gear  made  of  different  materials, 
selectivity  must  be  considered.  By  selectivity  we  mean  how 
much  will  a  certain  gear  catch,  what  species  and  sizes  of 
fish? 

All  fishery  workers  arc  concerned  with  this  question  in 
one  form  or  another.  The  fisherman  usually  wants  from  each 
of  his  operations  the  greatest  possible  catch,  but  not  necessar- 
ily the  greatest  amount  of  any  or  all  kinds  of  fish.  He  tries  to 
catch  fish  of  a  species  and  size  for  which  there  is  a  market  in 
his  particular  area.  The  technologist  sees  in  the  catch  an 
indication  of  the  efficiency  of  the  gear  used,  as  compared  to 
another. 

The  fishery  economist  looks  at  the  catch  as  an  income 
resulting  from  a  certain  expenditure. 

The  fishery  biologist  looks  at  the  catch  from  the  point  of 
view  of  its  significance  with  regard  to  the  fish  stock  from 
which  it  has  been  taken,  as  that  stock  determines  what  can, 
and  in  part  what  will,  be  taken  in  the  future.  For  him  the 
catch  is  a  proportion  of  the  stock  and  the  removal  of  this 
proportion  signifies  certain  effects  on  the  stock-changes 
in  its  size  and  other  properties. 

Let  us  first  of  all  define  what  selectivity  really  is.  We  can 
say  that  every  individual  in  a  fish  population  of  a  certain 
area  has  an  equal  chance  of  being  caught,  then  the  catch  will 
reflect  the  composition  of  the  whole  population.  When  the 
proportions  in  the  catch  show  a  different  composition,  then 
the  gear  may  be  said  to  be  selective.  This  can  be  due  to  the 
biological  characteristics  of  the  fish,  to  the  type  of  the  fishing 
operation,  to  the  behaviour  of  the  fish  in  relation  to  the 
operation  or  to  the  rig  and  design  of  the  gear  so  that  selection 
is  not  attributable  to  the  characteristics  of  the  gear  alone. 

Another  type  of  selection  will  operate  when  the  fishing 
boats  of  a  certain  area  fish  in  regions  where  certain  high 
quality  fish  are  to  be  found,  to  satisfy  the  market  demands  for 
a  particular  species. 

Perhaps  the  most  important  factor,  determining  the  extent 
to  which  this  type  of  selection  process  will  operate,  is  the 
knowledge  of  the  distribution  of  the  species  wanted.  The 
collection  of  such  knowledge  should  benefit  from  a  close 
cooperation  between  fishery  biologists  and  the  fisherman.  So 
we  can  say  that  selection  begins  as  soon  as  the  fishing  fleet 
puts  to  sea  and  concentrates  in  places  where  the  most  desirable 
catch  is  to  be  found.  This  selection  process  becomes  more 
complex  where  several  types  of  gear  operate  at  the  same  time 
to  supply  different  market  demands.  A  second  stage  of 
selection,  which  we  can  call  gear  selection,  starts  when  the 
gear  is  put  into  operation. 

Gear  selection  can  be  divided  into  two  components:  that 
due  to  the  avoidance  of  the  gear  by  certain  species  and  that 
due  to  the  ability  or  tendencies  to  escape  from  the  gear  when 
the  fish  come  into  contact  with  it.  If  the  magnitude  of  these 


components  can  be  identified  and  measured,  then  the  gear 
designer,  the  gear  manufacturer  and  fishermen  can  take 
account  of  them  in  the  construction  and  operation  of  the 
gear,  or  the  fishery  administrator  in  his  fish  conservation 
practices. 

In  gill  net  fishing,  for  instance,  the  catch  depends  on  fish 
swimming  into  and  being  meshed  or  entangled  by  a  wall  of 
net.  The  influence  which  biological  factors  may  have  in  this 
action  is  illustrated  very  clearly  by  Nomura  who  discusses 
the  factors  affecting  the  selectivity  and  fishing  ability  of  gill- 
nets  in  Japanese  waters.  This  paper  demonstrates  that  the 
behaviour  of  the  fish  in  relation  to  the  visibility  of  the  nets, 
plays  a  very  important  part.  Saetersdal  and  Mugaas  also 
mention  data  which  point  to  the  inter-action  of  the  fish 
behaviour  to,  and  the  gear  selectivity  of,  different  types  of 
material  used  for  gillnets,  as  being  important  in  determining 
the  si/e  of  catch  and  its  composition.  In  line  fishing,  too, 
there  are  data  which  demonstrate  the  importance  of  these 
factors;  fish  of  some  sizes  or  ages,  for  example,  may  avoid 
baits  of  a  particular  shape  or  si/e,  while  others  will  take  them 
A  further  stage  at  which  selection  takes  place  is  the  escape- 
ment of  fish  from  the  gear  once  they  have  contacted  it,  such 
as  through  the  meshes  of  the  net. 

This  mesh  escapement,  of  course,  is  a  process  which  is 
well  known  to  technologists,  fishermen  and  fishery  adminis- 
trators as  well  as  fishery  biologists.  It  is,  in  fact,  the  selection 
process  which  is  utili/ed  in  effecting  fishery  conservation 
practices  in  some  important  regional  fisheries. 

For  example,  we  have  learned  a  great  deal  in  recent  years 
about  the  behaviour  of  fish  in  trawl  codends  and  seine  cod- 
ends  by  the  adoption  of  television  techniques,  under- water 
observations  by  frogmen,  by  measuring  devices  attached  to 
nets,  by  controlled  experiments  with  gears  and  observations 
on  the  catches  in  different  parts  of  the  net.  This  has  shown 
that  mesh  escapement  is  effected  by  features  of  the  gear  as 
well  as  by  the  behaviour  of  the  fish.  One  of  the  most  important 
of  the  gear  factors  which  affects  escapement  is  the  material 
from  which  the  net  is  made.  The  results  of  experiments  have 
shown  conclusively  that  the  escape  of  fish  differs  between 
nets  having  the  same  mesh  size  but  made  of  different  materials. 
Some  quality  -of  the  material  which  is  being  identified  by 
many  workers  with  the  property  of  flexibility  and  stretch- 
ability  of  the  twines  is  responsible  for  this.  Mesh  escapement 
is  also  affected  by  the  size  of  the  catch  as  fish  in  the  codend 
will  tend  to  block  meshes  and  prevent  further  escapement. 

I  do  not  propose  to  go  into  the  methods  by  which  the 
selection  processes  can  be  studied  and  examined  yet  I  must 
draw  attention  to  the  method  known  as  comparative  fishing. 
Comparative  fishing  has  been  used  almost  exclusively  hitherto 
to  study  selectivity  and  other  features  of  fish  operations,  such 
as  determining  the  fishing  power  of  nets  of  different  structures, 
different  materials  and  so  on,  and  in  four  of  the  present  papers 


164  1 


DISCUSSION  -  RELATIVE     EFFICIENCIES 


there  arc  examples  of  the  use  of  the  comparative  fishing 
technique.  That  the  method  must  be  worked  under  strictly 
controlled  conditions  is  apparent  from  the  experiments 
referred  to  in  the  papers  on  the  efficiency  of  gill  nets  made  of 
natural  and  synthetic  fibres.  Whereas  these  papers  conclude- 
that  the  synthetic  fibres  are  catching  much  more  efficiently 
than  the  natural  fibres,  the  results  of  similar  experiments 
conducted  elsewhere  show  very  little  superiority  of  synthetic 
fibres.  The  higher  catches  of  synthetic  fibre  nets  may  be  due 
to  the  decreased  visibility  when  they  are  fished  in  clear  water. 
On  the  other  hand,  one  would  not  expect  to  find  a  marked 
difference  in  very  turbid  water. 

Mr.  S.  Holt  (FAO).  In  measuring  how  effective  one  gear 
is  by  comparison  with  another  slightly  different  gear,  we  use 
the  "comparative  fishing  method".  Although  it  essentially 
means  fishing  the  two  gears  side  by  side  and  comparing  the 
results,  the  difficulty  has  been  that  results  arc  very  variable 
and  therefore  rather  complex  statistical  methods  have  to  be 
used  if  we  arc  to  make  the  correct  interpretation  of  the  results 
of  such  trials.  The  trouble  is  that,  although  we  can  measure 
the  differences  between  gears,  we  do  not  know  if  that  differ- 
ence will  be  true  when  we  make  the  same  experiment  on 
another  day,  or  in  a  different  place,  a  different  season,  or  at 
a  different  time  of  day. 

Comparative  fishing  experiments  can  give  us  measurements, 
they  cannot  Icll  us  the  reasons  why.  The  problem  of  inter- 
pretation of  the  results  is  a  matter  for  both  biological  and 
technical  study.  There  are  very  complex  problems  involved, 
as  Mr.  Parrish  has  said,  but  somehow  they  have  got  to  be 
solved  if  we  are  to  try  to  devise  gears  rationally.  If  we  are 
going  to  accelerate  the  rate  at  which  we  can  improve  fishing 
gears,  we  must  do  more  than  just  make  trials  working  in  the 
dark  the  whole  time.  To  do  this  at  all  we  will  need  to  under- 
stand a  lot  more  about  the  reactions  of  fish,  particularly  to 
different  characteristics  of  fishing  gear  such  as  its  visibility, 
the  speed  at  which  it  moves,  and  so  on. 

At  a  meeting  of  biologists  in  Lisbon  this  year,  it  was 
suggested  that  this  Congress  was  the  appropriate  place  to  tell 
technologists  and  fishermen  what  kind  of  information  biolo- 
gists expect  from  them. 

Fishery  biologists  are  in  general  responsible  for  measuring 
the  properties  of  the  fish  which  determine  its  liability  to 
capture.  They  believe  it  is  the  responsibility  of  gear  tech- 
nologists to  analyse  and  measure  the  corresponding  properties 
of  the  gear.  They  believe  that  this  work  would  be  greatl> 
assisted  if  a  world  catalogue  and  description  of  fishing  gears 
could  be  compiled  and  if  a  classification  of  gears  based  on 
toxonomic  principles  could  be  devised.  They  believe  that  a 
simple  classification  of  gear  would  serve  the  following 
purposes.  Firstly,  it  would  stabilize  the  nomenclature  of 
gears  with  special  reference  to  synonymous  terms  used  in 
various  countries.  Secondly,  it  would  assist  the  population 
dynamic  worker  by  indicating  the  essential  properties  of 
gears  with  which  they  are  working,  and  thirdly  it  would 
facilitate  the  correct  comparison  of  results  obtained  with 
various  gears  in  different  circumstances. 

The  biologist  cannot  advise  on  the  correct  gear  to  use. 
unless  he  knows  what  is  the  composition  of  the  fish  stock  on 
the  ground.  If  that  changes,  then  the  choice  of  gear  must 
also  change,  i.e.  another  gear  may  be  more  economical.  The 
trouble  is,  we  cannot  wholly  measure  the  composition  of  the 
fish  on  the  ground  without  also  knowing  a  great  deal  about 


the  selectivity  of  the  gears  which  .ire  actually  being  used  by 
the  fishermen.  The  only  solution  is  by  very  close  co-operation 
between  the  fishermen,  the  gear  technologist  and  the  biologist. 

Mr.  S.  Springer  (U.S.A.).  In  the  CJulf  of  Mexico  xve  use 
shrimp  trawls  which  run  from  12  ft.  across  the  mouth  to 
125  ft.  and  all  of  these  are  used  to  a  very  great  extent 
some  on  the  same  boats.  The  smaller  nets  are  used  for  try 
nets  to  test  the  ground  and  the  larger  nets  are  used  for  pro- 
duction. The  smaller  nets  are  usually  a  lot  faster  than  the 
larger  nets.  I  presume  that  all  of  these  nets  work  well  enough 
but  one  thing  that  we  have  found  in  analysing  the  catches  of 
the  nets  of  different  sizes  is  that  the  smaller  nets  catch  smaller 
fish  than  the  larger  nets.  Yet  it  appears  that,  owing  to  the 
greater  speed  of  the  smaller  net,  this  should  not  be  so.  One 
explanation  of  why  this  may  happen  could  be  seen  in  the 
television  film  showing  haddock  inside  the  codend.  In  general, 
these  haddock  seemed  to  move  and  swim  forward  to  a  certain 
extent  before  they  actually  go  into  the  tail  of  the  codend.  It 
would  seem  to  me  that  we  might  infer  from  this  that  the  net 
has  to  have  a  certain  length  in  order  to  retain  the  large  fish 
that  is,  a  short  net  with  a  short  codend  would  catch  more  small 
fish  because  the  larger  fish,  being  faster  swimmers,  can 
escape.  I  would  be  interested  to  know  if  there  is  any  experience 
like  this  in  other  fisheries  and  whether  there  is  any  relation 
between  the  length  of  the  throat  and  the  codend  and  the  size 
composition  of  the  catch. 

Dr.  Went  (Ireland).  I  rom  a  point  raised  this  morning  it 
would  appear  that  the  fisherman  in  general  is  not  convinced 
of  the  practical  value  of  selectivity  and  comparative  fishing 
for  the  industry.  1  think  we  must  stress  the  very  great  import- 
ance of  this  subject  to  the  practical  fisherman  who,  of  course 
is  interested  in  getting  tish.  However,  if  he  is  to  continue 
obtaining  good  catches,  fish  conservation  regulations  must 
be  made  to  preserve  the  stock.  I  think  the  experiments  on 
selectivity  of  fishing  gear  and  comparative  fishing  will  give 
us  the  basis  on  which  to  draw  up  such  international  regulations 

Mr.  M.  Ben-Vami  (Israel).  1  would  like  to  emphasi/c  the 
importance  of  comparative  fishing  in  assessing  the  relative 
value  of  fishing  gear.  We  met  this  problem  in  Israel  in  con- 
nection with  the  high  opening  trawl  we  had  developed.  We 
were  aware  of  the  difficulties  in  determining  the  actual  value 
of  differences  in  catching  efficiency  between  two  types  of 
gear  accurately.  But,  for  the  commercial  fishermen  only  very 
obvious  differences  count.  I.  therefore,  think  that  accurate 
statistical  evaluation,  as  applied  in  selection  experiments,  is 
not  needed  for  such  practical  purposes.  We  actually  used  two 
techniques  of  comparative  fishing:  one  and  the  same  trawler 
fishing  with  the  new  gear  and  the  conventional  gear  alterna- 
tively, and  two  boats,  one  with  the  new  gear  and  one  with 
conventional  gear,  towing  side  by  side.  These  experiments 
were  conducted  for  several  months  in  commercial  operation 
in  various  areas.  After  eliminating  all  not  strictly  comparable 
hauls,  some  60  comparative  tows  remained,  the  evaluation  of 
which  indicated  that  the  new  gear  gave  about  20  per  cent, 
better  catches.  For  such  commercial  comparative  fishing  it  is 
advisable  to  put  a  very  experienced  and  very  conservative,  but 
good,  fisherman  in  charge  of  the  conventional  gear  and  make 
the  designer  of  the  new  gear  compete  against  him.  If  a  new 
design  passes  such  a  test  successfully,  I  think  one  can  be  sure 
and  the  fishermen  can  be  convinced,  that  something  worth 
while  has  been  achieved. 


165 


Section  6:  Rational  Design — Use  of  Engineering  Theory  and  Model  Testing. 


THE  USE  OF  MODEL  NETS  AS  A  METHOD  OF  DEVELOPING 

TRAWLING  GEAR 

by 

W.    DICKSON 

Marine  Laboratory,  Aberdeen,  U.K. 

Abstract 

This  paper  deals  with  the  theory  and  practice  of  using  models  in  connection  with  the  design  of  fishing  gear  and  the  study  of  fish 
behaviour  in  relation  to  the  gear.  A  certain  amount  of  success  has  been  achieved  by  underwater  photography  by  frogmen  but  there  arc 
hazards  and  difficulties  when  the  gear  is  towed  quickly  in  areas  of  low  visibility.  The  use  of  models  in  shallower  water  can  overcome  some  of 
these  difficulties,  and  the  author  lays  down  rules  governing  the  making  and  operation  of  small-scale  nets. 

Models  were  observed  in  action  in  a  West  Scottish  sea  loch  by  frogmen  working  in  pairs  and  hanging  on  to  a  tow-rope  instead  of 
to  the  net  itself,  and  several  photographic  records  have  been  procured  by  this  means. 

The  need  for  correlating  observations  and  measurements  of  models  with  observations  on  the  full -sized  gear  is  stressed  and  it  has 
been  found  that  the  agreement  between  the  performance  of  models  and  full-sized  gear  has  generally  been  sufficiently  close  to  conclude  that 
much  can  be  learned  from  models,  provided  that  comparisons  are  viewed  critically.  Models  can  show  fairly  accurately  the  effect  of  alternate 
methods  of  rigging  a  gear  and  can  at  the  same  time  reveal  faults,  but  they  are  not  intended  as  a  substitute  for  instrumentation  or  comparative 
fishing,  the  latter  being  regarded  as  the  final  stage  of  gear-testing.  The  paper  is  fully  illustrated. 


Rfemne 


Utilisation  des  modules  de  filets  dans  la  tnise  au  point  des  chaluts 


Cet  article  traite  dc  la  th^orie  et.de  la  pratique  des  essais  dc  modeles  en  vue  de  la  construction  des  engins  de  peche  et  de  l*6tude  du 
comportcment  du  poisson  en  fonction  des  engins.  Des  photographes  sous-marines  prises  par  des  horn  mes-grenoui  lies  ont  perm  is  d'obtcnir 
quelques  succes,  mais  Toperation  est  difficile  et  dangereuse  lorsque  1'engin  est  remorque  rapidemcnt  dans  des  zones  ou  la  visibility  est  faible. 
L* utilisation  de  modeles  dans  des  eaux  peu  profondes  permet  dc  surmonter  ccrtaines  de  ces  difticultes  et  1'auteur  pose  des  regies  applicables 
a  la  fabrication  ct  £  1'utilisation  de  filets  de  pctites  dimensions. 

Des  hommes-grenouilles  travail  lent  deux  par  deux  et  se  tenant  a  un  cable  de  remorquage  au  lieu  de  se  tenir  au  ft  let  lui-meme  ont 
observe^  des  modeles  en  action  dans  un  loch,  sur  la  cdte  ouest  de  TEcosse  et  plusieurs  enregistrements  photographiqucs  ont  et£  obtenus  dc 
cctte  maniere. 

L'auteur  souligne  la  necessity  d'£tablir  un  rapport  entre  les  observations  et  les  mesiircs  des  modeles  ct  les  observations  faites  sur  les 
filets  et  sur  les  engins  de  grandeur  naturelle  et  il  a  ct£  constate  quc  le  comportement  des  modeles  et  des  engins  de  grandeur  naturelle  a  etc 
sufhsamment  analogue  pour  permet t re  de  conclure  que  les  modeles  pcuvent  constituer  une  excel Icnte  source  d'enscignement,  sous  reserve  de 
soumettre  les  comparaisons  £  un  examen  critique.  La  construction  de  modeles  peut  montrcr  d'une  fagon  assez  sure  1'effet  des  di verses  methodes 
possibles  pour  garnir  un  engin  et  en  m£me  temps  en  montrer  les  defauts,  mais  elle  n'est  pas  destinee  £  remplacer  les  instruments  ou  la  p€che 
comparec,  cette  derniere  etant  toujours  considdree  comme  1'etape  finale  de  1'essai  des  encins. 


Extracto 


Uso  de  modelos  de  redes  de  pesca  como  metodo  para  perfeccionar  artes  de  arrastre 


En  este  trabajo,  que  cuenta  con  numerosas  ilustraciones,  se  estudia  la  teoria  y  practica  del  uso  dc  modelos  a  escala  reducida  en 
relaci6n  con  el  proyecto  de  los  artes  de  pesca  y  el  estudio  de  la  manera  como  reaccionan  los  peces  ante  las  redes.  Se  ha  logrado  cierto  exito 
mcdiante  fotografias  submarinas  tomadas  por  "hombres  ranas",  pero  este  m£todo  es  peligrose  y  ofrcce  dificultades  cuando  la  red  es  arrastrada 
rfpidamente  en  zonas  de  poca  visibilidad.  Como  el  empleo  de  modelos  en  aguas  menos  profundas  puede  evitar  algunas  de  estas  dificultades. 
el  autor  da  algunas  reglas  que  gobiernan  su  const ruccion  y  uso. 

En  una  ensenada  dc  la  costa  occidental  de  Escocia  parejas  de  buzos  que  colgaban  del  cable  de  remolque  en  vez  de  la  red,  observaron 
el  comportamiento  del  modelo  y  tomaron  varias  fotografias. 

La  neccsidad  de  cprrelacionar  las  observaciones  y  medidas  de  los  modelos  con  observaciones  de  los  artes  de  tamano  correinte  han 
permitido  encontrar  que  existe  una  concordancia  entre  el  rendimiento  de  los  modelos  a  escala  reducida  y  las  redes  normales,  lo  suficientemente 
estrecha  como  prra  llegar  a  la  conclusi6n  de  que  pueden  sacarsc  grandes  experiencias  con  el  uso  de  modelos  sicmpre  y  cuando  las  compara- 
ciones  se  analicen  con  sentido  critico.  Los  modelos  pueden  demostrar  con  bastante  certeza  el  cfecto  que  ofrece  el  empleo  de  mdtodos  alterna- 
tivos  empleados  en  la  confecci6n  de  las  redes  y,  al  mismo  tiempo,  revelar  sus  inconvenientes  pero  no  se  trata  dc  utilizarlos  para  reemplazar 
el  uso  de  instrumentos  o  la  pesca  comparativa,  ya  que  esta  ultima  es  considerada  como  la  etapa  final  de  las  pruebas  a  que  es  nccesario  someter 
un  arte. 


WORK  was  begun  on  the  observation  of  models 
underwater  in  1954  by  the  Marine  Laboratory 
in  Aberdeen.  Data  obtained  in  the  meantime, 
from  measurements  by  instruments  on  the  performance 
of  full-sized  trawling  and  Danish  seine  net  gear,  together 
with  the  underwater  films  and  measurements  already 
made,  served  as  a  comparison  against  which  the  per- 
formance of  models  could  be  judged.  The  comparison 
between    models   and   fullsized    gear   was    sufficiently 


encouraging  to  warrant  repeating  and  extending  the 
trials  in  1955  and  1956. 

There  are  three  reasons  for  observing  fishing  gear  or 
models  of  it  in  operation: 

1 .  To  form  an  impression  of  the  performance  of  the 
gear,  to  seek  defects  in  its  design  and  to  try  the 
effect  of  alterations  in  design  and  rigging. 

2.  To  make  measurements  to  obtain  a  fuller  apprecia- 
tion of  the  gear's  hydrodynamical  performance. 


M66] 


MODEL    NETS     FOR     EXPERIMENTAL    WORK 


3.     To  obtain  information  on  the  reactions  of  fish  to 
the  gear. 

Certain  rules  should  be  observed  as  closely  as  possible 
when  modelling,  and  if  the  conditions  of  water-flow 
are  to  be  similar  for  model  and  full-sized  net,  there  should 
not  be  too  great  a  change  of  Reynold's  Number.  When 
the  scale  becomes  too  small  the  rules  cannot  be  followed. 
The  purpose  will  also  dictate  how  far  the  rules  should 
be  obeyed.  Lastly,  the  choice  of  model  size  is  dependent 
on  the  towing  power  of  the  available  boat. 

When  the  purpose  is  to  look  for  bad  features  in  the 
design  of  a  net,  the  model  should  correspond  mesh  for 
mesh  with  the  original  and  the  twine  diameter  should  be 
proportionally  reduced.  Practical  considerations,  how- 
ever, often  force  a  compromise.  Expense  and  the  avail- 
ability of  suitable  materials  have  to  be  taken  into 
account  and,  if  the  setting  of  the  net  is  to  be  altered 
during  the  experiment,  labour  costs  must  be  considered. 
A  quarter  scale  model  with  a  half  scale  mesh,  or  an 
eighth  scale  model  with  a  quarter  scale  mesh,  are  reason- 
able compromises.  If  a  true  scale  model  is  required,  a 
scale  of  one  quarter  is  about  the  smallest  that  can  reason- 
ably be  used. 

Small  nets  of  some  20  ft.  headline  length  towed  by 
the  15  h.p.  coble  shown  in  fig.  1,  are  big  enough  to  be 
effective  fishing  instruments,  and  there  is  little  likelihood 
of  significant  fish  behaviour  passing  unnoticed.  For  this 


purpose  the  nets  should  be  made  with  meshes  of  the 
size  used  commercially. 

COMPARISON    BETWEEN    AN    EIGHTH    SCALE 
MODEL  AND  THE  FULL-SIZED  GEAR 

Measurements  on  models  can  be  taken  by  direct  readings 
on  measuring  sticks,  strings  and  spring  balances,  but 
to  take  corresponding  readings  on  a  full-sized  trawl 
requires  some  degree  of  instrumentation.  In  most  cases 
only  a  few  spot-checks  are  possible.  The  net  chosen  for 
the  present  comparison  is  the  small  "Aberdeen"  trawl 
used  by  most  trawlers  from  that  port.  Its  headline  length 
is  62£  ft.  The  model  is  to  one-eighth  scale  with  the  mesh- 
size  and  twine  diameter  reduced  to  approximately  one- 
quarter.  The  following  data  were  obtained  for  the  model 
and  full-sized  net  in  a  series  of  trials.  (Scaled  up  values  of 
the  model  data  are  given  in  brackets.)  The  original  and 
the  model  were  rigged  in  the  same  way  with  10  fm. 
sweep  lines,  bobbin  danlenos  and  20  ft.  spreading  wires. 


Warp  Length 
Spread  of  otter 
boards 

Headline  Height 
Drag  of  Gear 

Towing  speed 

Full  Size  Net 

100  fm. 
105ft.  (by  wire  angle 
at  surface) 

6  ft.  (by  differential 
depth  recorder) 
3-25  tons  (by  warp 
tension  recorder, 
1  -63  in  each  warp) 

3  knots 

8/A  scale  Model 

10  fm.  (80  fm.) 
11  ft.  8  in.  (93  ft.  but 
equivalent  to  97  ft. 
with  100  fm.  aft). 
9  in.  (6  ft.) 

16  Ib.  (3-7  tons  lowest 
estimate) 
40  Ib.  (9-2  tons  highest 
estimate) 
1  knot 

The  comparison  is  close  enough  to  emphasize  the 
value  of  models  and  yet  divergent  enough  to  give  the 
hint  that  significant  differences  can  arise. 

A  photograph  of  the  differential  depth  recorder  used 
to  measure  the  headline  height  is  shown  in  fig.  2.  It  is 
an  ordinary  depth  recorder,  except  that  the  pressure 
inside  is  maintained  at  headline  depth  through  the 
collapsible  rubber  tyre  attached  to  the  headline,  the 
recorder  itself  being  attached  to  the  ground  rope.  The 
instrument  used  for  recording  warp  tension  is  shown  in 


Fig.  I. 


Coble  with  cameras  ami  breather  set  on  the  foredeck  and 
winch  in  the  stern. 


Fig.  2.     Differential  depth  recorder. 


167 


MODERN     FISHING     CiF.AR     OF    THE     WORLD 


/'iff.  3.      MVwy>  tension  recordei . 


Fig.  4.     Stui  hoard  winx  and  pan  of  the  square  of  the  small  Aberdeen 

trawl.     Shot  from  the  film  "Trawls  in  action'  made  by  "Basic 

Films"  for  M.A.F.F.  Fisheries  Laboratory,  Loweito//.  in  con/unction 

with  Siehe  Gorman\  I.  id. 


tig.  3.  It  works  on  the  principle  of  one  wheel  deflecting 
the  warp  a  small  lateral  distance  between  the  two 
outside  wheels  at  a  fixed  distance  apart.  The  deflecting 
wheel  is  mounted  on  the  piston  of  an  oil-filled  cylinder 
and  the  oil  pressure  recorded.  The  instrument  can  work 
whether  the  rope  is  moving  or  stationary. 

As  the  small  "Aberdeen"  trawl  was  photographed  in 
the  M.A.F.F.  film  "Trawls  in  Action",  we  may  compare 
it  with  a  photograph  of  the  model.  Fig.  4  shows  a  vie\* 
of  the  net  and  fig.  5  one  of  the  model. 

MODELLING  RULES 

As  a  model  is  scaled  down,  all  weights  and  floats  de- 
pendent on  volume  decrease  by  the  cube  of  the  scale, 
while  all  lifts  and  drags  due  to  water-flow  and  dependent 
on  surface  areas  decrease  by  the  square  of  the  scale. 

Tand  Ware  proportional  to  /3      where  F  and  W  are  floatations 

and  weights  in  the  original  and 
where  /  is  a  unit  of  length. 

D  and  L  are  proportional  to          where  L  and   D  are  lifts  and 
72  \  -  drags  due  to  water  flow  in  the 

original  and  where  v  is  the 
towing  speed. 

where  Fs  and  Ws  are  floatations 
and  weights  in  the  model,  where 
/  is  as  before  and  s  is  the  scale. 


K  and  Ws  arc  proportional  to 
<s  /3) 

U  and  Ds  arc  proportional  to 
is  /)'-    (si  v)2 


Fin.  5. 


Model  of  small  Aberdeen  trawl. 
Mote  loose  lay t rich  line 


where  Ls  and  IX  are  lifts  and 
drags  due  to  water  flow  in  the 
model. 

L  D  F         W 

It  may  be  seen  that  for  —  =  — -  =  --  »-     -     there 

Ls  Ds  Fs        Ws 
are  several  requirements. 

1 .  All  dimensions  of  length  must  be  reduced  to  scale. 

2.  Floats  and  sinkers  must  be  of  the  same  density  in 
the  model  as  the  original,  otherwise  the  change  in 
flotation  or  weight  will  not  be  proportional  to  the 
cube  of  the  scale.  Netting  twine  should  also  be  of 
the  same  material  otherwise  there  is  a  change  in 
density,  and  here  arises  a  difficulty.  Most  big  nets 
are  made  of  man i la  twine,  which  cannot  be  obtained 
thin  enough  for  model   making,  hence  the  usual 
material  for  model  making  is  cotton  twine.  Although 
there  is  no  very  definite  agreement  about  specific 
gravities  of  twine  the  following  figures  may  be  used: 

Manila  (green  proofed  with  cuprinol)  SG  T29 
Cotton  (green  proofed  with  cuprinol)  SG  1*34 
Nylon SG  1'12 

3.  The  speed  of  towing  must  be  reduced  by  the  square 
root  of  the  model  scale.  In  operations  such  as  seining 
where  time  is  also  a  factor,  the  timing  must  also  be 
scaled  down  by  the  square  root  of  the  scale. 

If  the  normal  towing  speed  of  trawls  is  taken  as  3 
knots: 

.1  scale  towing  speed  is  3/V2  or  about  2  knots 
1  scale  towing  speed  is  3/V4  or  1 J  knots 
J  scale  towing  speed  is  3/V8  or  about  1  knot 

4.  It  is  assumed  that  the  lift  or  drag  due  to  water-flou 
changes  proportionally  with  the  square  of  the  velocity, 
but  this  will  only  be  so  over  a  limited  range  of  change 
in  scale  and  change  in  speed.  This  will  be  discussed 
more  fully  later. 


f  1681 


MODEL     NETS     KOR     FXPERI  M  ENTA 1.     WORK 


In  practice  these  rules  are  not  always  honoured 
accurately.  It  is,  therefore,  necessary  to  understand  in 
each  case  whether  and  how  they  are  being  broken  and 
the  direction  of  the  resulting  bias,  so  that  it  may  be 
allowed  for  or  measures  taken  to  overcome  it.  Consider 
only  one  of  the  probable  compromises  here,  that  on 
which  the  models  described  were  made  where  mesh 
und  twine  size  were  reduced  less  than  by  the  scale. 

I  ei    /  be  the  length  of  twine  in  the  original  (in.) 
R  be  the  runnage  of  the  twine  in  the  original 

(m./kg.) 
P  be  the  density  of  the  t\*ine  in  the  original 

(g./cm.3) 
W  be  the  weight  of  the  twine  in  the  original 

(kg.) 
d  be  the  diameter  of  the  tv\ine  in  the  original 

(mm.) 

A  be  the  projected  area  of  the  twine  in  the  original 

(mm.-) 

Similar  letters  with  the  suffix  s  refer  to  the  model 
S  be  the  scale  of  the  model 
Sm  be  the  mesh  scale,  and 
Sd  be  the  twine  diameter  scale 


Then    W 
A 




W     *  ' 


and       - 
R 


R 

M 

/s 

1         S2/ 

R> 

R\         Sm 

s2/ 

/s 

d,                     Sd 

Sm 

:                             1 

p- 

-d-approx.. 

4 

R, 

/  PR 

VK*: 


Hence  Sd 

When  the  model  scale  is  a  true  one 


(I) 
(2) 


(4) 


PN  —  d  N2  a  pprox .   ( 5 ) 
4 

(6) 


W 

•  -    

;  S      Sm      St 

R 

A 

«    /d 

ws 

S31 

1  . 

R 

A, 

-  S2 

/d 

Rut  take 

the 

simple  case: 

S 

H!   Sm      •  Sj         » 

Sd 

Vv\ 

-    — 

S2    / 

R 

As 

-   S2/d 

l\s 


R 

S2' 


P,; 


R 

Sd2 


The  area  of  twine  projected  to  the  water-flow  is 
correctly  scaled  but  the  model  is  twice  as  heavy  as  it 
should  be,  and  this  has  to  be  balanced  by  increased 
flotation.  Where  the  model  cannot  be  made  of  the  same 
material  as  the  original,  similar  effects  occur.  These 
physical  difficulties  in  constructing  an  accurate  model 
are  not  the  only  ones  to  be  considered. 

Reynold's  Number  is  a  dimensionless  number  given 


hv    R  —    —  where  \    is  the  velocity, 

n 

/  is  an  arbilar>  length  unit  such  as  say  the  diameter 
of  the  floats  or  the  twine  diameter: 


n  is  the  Kinematic  viscosity  and  n 


where 


a  is  the  viscosity  of  the  medium 
Pin  is  the  density  of  the  medium 

So  long  as  R  remains  constant  it  can  be  taken  thai 
i he  conditions  of  the  water-flow  round  the  immersed 
body  remain  the  same.  In  model  tests  in  the  sea  n 
cannot  be  altered,  and,  therefore,  for  constant  R,  the 
velocity  of  the  model  should  be  inversely  proportional 
to  the  scale  but  for  constant  flotation  to  drag  ratio,  the 
velocity  of  the  model  should  be  directly  proportional  to 
the  square  root  of  the  scale.  Both  conditions  cannot  be 
met  at  the  same  time  and  some  change  of  Reynold's 
Number  must  be  accepted.  The  question  is,  what  change 
of  drag  coefficients  does  such  a  change  in  Reynold's 
Number  bring? 

Pig.  6  shows  that  for  the  spherical  headline  floats  there 
might  be  little  change  of  drag  coefficient  between  the 
full  size  and  one-eighth  scale  at  scale  towing  speed1. 


DRAG  COEFFICIENT  OP  SPHERE 


1  DIA.MOML  N.OAT  »-OI*.  TftO 

AT  1    KNOT  AT  I    UN! 

tl»    ftCALf  'till  tltC 


-M    -1*    -10 


Fig.  6.     Relation  of  coefficient  of  drag  tor  spherical  floats  to  the  value 
of  Reynold's  h'umher. 

Possibly  in  the  full  sized  net  the  proximity  of  the  head- 
line to  the  balls  and  their  method  of  attachment  would 
encourage  the  onset  of  turbulence,  so  that  the  drag 
coefficient  might  decrease  to  values  given  in  the  sharp  dip 
in  the  graph. 

The  drag  coefficient  of  a  sphere  over  a  wide  range  of 
Reynold's  Number  has  been  thoroughly  investigated  but 
far  less  in  known  about  how  sheets  of  netting  behave 
over  a  wide  range  of  scale  and  speed.  The  indications 
are  that  at  one-eighth  scale  the  curve  of  the  drag  co- 
efficient rises  appreciably  and  would  rise  more  steeply 
at  smaller  scales.  This,  and  the  physical  difficulty  of 
making  accurate  small  scale  nets,  puts  a  lower  limit  on 
scale  size. 

Information  on  the  lift  and  drag  of  otter  boards  is 
scanty.  Gawn  gives  some  data  on  flat  boards  of  low 
aspect  ratio  at  a  Reynold's  Number  of  about  4  x  106.8. 
These  boards  (corresponding  to  about  I/  12th  scale) 
were  towed  at  speeds  up  to  5  knots  without  apparently 
much  change  in  the  lift  and  drag  coefficients.  These  data 
cover  the  model  range  up  to  one-quarter  scale  quite  well. 


[169] 


MODERN     FISHING 


GEAR    OF    THE    WORLD 

at 


ȣ  0.2 

n 


N  •-  DISTANCE  OF  CENT  HE    OF 
PRESSURE   IS  DIVIDED  BY 
WIDTH    OF  PLANE    IN 
DIRECTION  OF   MOTION. 


ASPECT  RATIO 

21      —  —  —  — 
1:2     _._ 


20  30  40 

ANCLE  OF  INCIDENCE -DECREES 


Fig.  7.    Forces  on  rectangular  and  square  plates.     From  Cawn~. 

Some  of  Gawn's  graphs  are  reproduced  in  figs.  7  and  8. 
Trawl  boards  are,  however,  in  contact  with  the  bottom, 
which  affects  their  drag  and  alters  their  lift.  Since  an 
otter  board  bears  down  on  its  heel,  the  friction  acts 
through  a  point  aft  of  the  centre  line,  bringing  the 
effective  centre  of  pressure  further  aft  than  indicated 
in  fig.  8. 

Even  where  frogmen  can  observe  large  trawl  nets  in 
action,  convenience  and  safety  of  underwater  observers 


Fig.  8.     Curves  of  centre  of  pressure  of  planes.    From  Gawri*. 

demands  reduced  towing  speed.  The  flotation  to  drag 
ratio  is  then  upset,  giving  rise  to  inaccuracies  and  prob- 
lems analogous  to  those  met  in  the  use  of  models. 

MAKING    SPECIFICATION    DRAWINGS 

There  is  no  tendency  towards  standardization  of  the 
drawings  and  specifications  of  nets.  Designers  draw  nets 
as  they  think  best.  Some  drawings  are  made  in  specula- 


/  •*"  y 

*6~       L    s'X",J 

l300"v  7~ 


100m"" 

60    ROWS  1:3"*S 

DOUBLE 


7 'SLACK  IN  i  * 

LOWER  WING  !  * 
42' 


13    S'«S  5"      )     IN  BELLY      30ROWS 

\  110m.  /  J 


HEADLINE  B21  §"(»'  •«•  1«'  •**  «''  *") 

CROUNDROPE      BO'         (JO*  *   20*  •  30') 

FISHIN6   LINES     31*  6"*  2t*  t  11"  6" 


Fig.  9.     Schematical  drawing  of  the  30  ft.  Aberdeen  trawl. 
1170] 


MODEL    NETS    FOR    EXPERIMENTAL    WORK 


>/.^"v         4:       ^2 


HEADLINE        7'lO"(2' t/ 
CROUNDROPE     10  '  (  319\  2V*  3'9") 


V.3\    N.    12        /v.l 

LINEN 

THREAD 

DOUBLE 


27    ROWS 


70.     Schematical  drawing  of  the  \th  scale  model  of  the  30  ft.  Aberdeen  trawl. 


live  fashion,  in  an  attempt  to  show  the  net  in  the  fishing 
position  with  a  few  dimensions  added  for  good  measure. 
It  is  rare  for  any  of  these  drawings  and  specifications  to 
be  complete  and  it  is  not  easy  to  tell  from  them  the 
salient  features  of  the  nets  or  how  they  compare  in  shape 
with  other  nets,  but  it  should  be  possible  to  make  such 
drawings. 

With  a  model  net  a  glance  at  the  drawings  should 
show,  when  compared  with  those  of  the  original,  whether 
or  not  it  is  a  fair  replica.  The  scales  can  be  so  chosen  that 
drawings  of  both  model  and  original  are  the  same  size 
and  can  be  checked  by  the  use  of  tracing  paper. 

The  following  rules  have  been  found  useful  in  practice: 

1.  All   lengths   are   true   lengths   with   the   netting 
stretched  but  any  lengthways  slack  is  noted  on  the 
drawing  and  indicated  by  an  irregular  line  in  the 
edge  of  the  panel  concerned. 

2.  All  widths  are  taken  by  setting  the  meshes  in  by 
the  half,  e.g.   120  meshes  of  Ij  in.  size  set  into 

120 

x  U  =  90  in. 

2 

Such  rules  give  the  drawings  a  "stepped"  appearance 
which  the  net  does  not,  of  course,  have  in  practice,  but 
they  show  directly  the  lateral  slack  and  tautness.  The 
drawings  do  not  cover  the  setting  of  the  nets  to  their 
ropes,  details  which  may  be  added  in  note  form.  Figs.  9 
and  10  show  the  drawings  of  the  small  "Aberdeen" 
trawl  and  model,  according  to  these  rules. 


Provided  that  the  same  rules  are  always  observed  the 
patterns  which  turn  out  to  be  good  and  bad  can  be 
remembered  and  take  on  their  own  significance. 

SOME  UNDERWATER   OBSERVATIONS 

Models  of  a  suitable  size  for  this  work  are  too  big  to 
be  used  in  most  testing  tanks,  consequently,  a  method 
was  devised  for  inspecting  the  models  towed  in  a  West 
Highland  sea  loch. 

The  general  requirements  of  a  location  are,  a  half  mile 
stretch  of  clean  smooth  sandy  bottom  in  depths  between 
3  and  6  fathoms,  not  too  much  tide,  a  shore  reasonably 
free  from  surf,  good  sunlight  and  an  underwater  visibility 
range  of  not  less  than  30  ft.  These  conditions  are  difficult 
to  satisfy  in  Scottish  waters  and  the  work  described 
was  done  in  conditions  which  fell  short  of  the  ideal.  A 
frogman  may  hang  on  to  a  full-sized  net  without  dis- 
torting its  shape  but  not  a  model,  nor  could  he  swim  at 
towing  speed  for  long.  He  therefore  hangs  on  to  a  towing 
rope.  The  frogmen  work  in  pairs,  one  making  measure- 
ment and  one  taking  photographs.  The  remainder  of  the 
team  of  5  comprise  the  divers'  attendant,  who  follows 
the  frogmen's  air  bubbles  in  a  rowing  boat,  and  two  men 
in  the  motor  coble  towing  models  and  frogmen. 

An  otter  board  is  shown  in  fig.  11.  The  brackets  and 
the  "normans"  to  which  the  back  strops  are  attached  are 
set  to  give  the  board  an  angle  of  attack  of  about  35 
degrees,  giving  the  maximum  lift  with  a  large  concomitant 
drag.  It  is  not  worked  at  or  near  the  angle  of  maximum 


[171  ] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


AV#.   II.     Model  otit'i   hoard  in  action. 

lift  to  drag  ratio.  Behind  the  otter  boards  come  the 
sweepwires,  the  danleno  bobbins,  the  legs  or  spreading 
wires,  and  then  the  net. 

One  of  the  main  principles  in  net  design,  and  in  avo  id- 
ing  net  distortion,  is  to  shape  the  net  as  nearly  as  possi  ble 
to  the  shape  assumed  in  its  fishing  position.  Meshes  in 
the  region  of  the  "quarter"  mesh,  where  the  inside  edge 
of  the  top  wings  join  the  square,  are  often  subject  to  high 


Fit;.    12.     Tension  behind  quarter-meshes  on  model  of   \nudl 
Aberdeen  trawl. 


%       9UARTCK 

f  \       MCSNCS        /TOP 

•A  /mm 

/    \  (a) 


\ 


tQUAMC 
COMSTftUCTION   Or 


NET   IN   fISNINt  POIITION 


SNAPC0 

Fig.  13.     Piopoxal  for  a  better  construction  of  the  \quare. 


Fig.  14.     Wing  tip  of  model   V'ingc  Trawl  In  action. 

tension  (see  fig.  12).  The  reason  for  this  is  also  seen  in 
the  backward  bend  of  the  leading  edge  of  the  square 
where  it  joins  the  wings.  It  is  as  if  the  net  were  hinged 
about  the  quarter  meshes  as  shown  in  fig.  13  (b).  There 
appears  to  be  some  argument  for  shaping  the  square  as 
shown  in  fig.  13  (c)  by  "bating"  or  tapering  meshes  in  the 
middle  of  the  net  rather  than  in  the  selvedge.  This 
improves  the  appearance  and  tends  to  give  all  parts  an 
equal  strain. 

It  is  not  really  surprising  that  a  bulge  in  the  selvedge 
at  the  join  of  top  wings  and  square  appears  (figs.  9  and 


CLE  VATIO  N 

Fig.  15.     Method  to  determine  the  proper  length  of  false  headline* 


\  172  1 


MODEL    NETS     FOR      EXPERIMENTAL     WORK 


10)  when  the  model  is  in  fishing  position  (fig.  5).  Con- 
versely, this  method  of  drawing  leads,  with  a  little 
practice,  to  a  fair  idea  of  what  shape  will  be  assumed 
underwater  by  a  given  design  of  net.  It  seems  as  if  some 
alteration  to  the  design  of  this  small  "Aberdeen"  net 
could  be  advantageous,  and  such  trials  will  be  made. 

To  increase  the  headline  height  (particularly  for 
herring  trawling)  has  long  been  the  objective  of  net 
designers.  Various  trials  have  been  made  to  this  end  with 
Vinge  trawls,  where  the  wing  ends  are  split  down  the 
selvedge.  A  model  of  this  type  was  used  with  some 
success  as  a  pilot  in  the  development  of  a  Vinge  trawl 
for  the  75  ft.  research  vessel  Clupea.  One  wing  end  of 
the  model  is  shown  in  fig.  14  where  it  is  apparent  that  the 
net  is  set  too  loosely  on  the  ropes,  both  headline  and 
footrope  being  too  short.  Models  are  particularly 
useful  for  the  correction  of  such  faults. 

The  use  of  kites  is  common  with  herring  trawls.  It  is 
difficult  to  tell  how  the  lengths  of  the  false  headlines  were 
arrived  at  in  the  first  place,  probably  on  an  empirical 
basis.  The  shape  of  a  net  in  the  towed  position  cannot 
yet  be  theoretically  determined,  but  it  was  felt  that  there 
was  sufficient  knowledge  to  estimate  certain  critical 
dimensions  and  then  calculate  the  desired  lengths  of  false 
headlines  to  bring  the  kites  to  determined  positions.  Fig. 
15  shows  the  method.  Values  must  be  assigned  to  the 
headline  height  at  its  maximum  and  at  its  wing  tips,  to 
the  spread  of  the  net  at  its  wing  tips  and  to  the  angle  of 
attack  of  the  sweep  wires.  The  agreement  between  the 
values  predicted  on  the  drawing  and  those  obtained  in 
practice  on  the  model  is  reasonable. 


Spread  of  headline 
Headline  height  H 
Height  of  1st  Kite  HK I 
Height  of  2nd  Kite  HK2 


Predicted 
41   ft. 
14ft. 

22-7  ft. 
32-7  ft. 


Observed 
54  in 
21  in. 
36  in. 
45  in. 


(36  ft.) 
(14ft.) 
(24  ft.) 
(30  ft.) 


The  appearance  of  the  kites  above  the  net  is  to  be 
seen  in  fig.  16. 

REACTIONS  OF  FISH  TO  SMALL  NETS 

The  cin6  film  is  more  useful  than  the  "still"  film  for 
recording  the  reactions  of  fish  to  the  gear. 

The  reaction  most  commonly  observed  is  that  fish  tend 
to  swim  away  perpendicularly  in  front  of  a  moving  wire 
or  rope  when  both  are  on  or  near  the  bottom.  This  is 
shown  diagrammatically  in  fig.  17  (i). 

If  the  maximum  swimming  speed  of  the  fish  be  Vf  and 
the  speed  and  angle  of  attack  of  the  wire  be  Vt  and  ET 
respectively,  then  the  angle  of  arc  (Ep)  from  which  it 


Fig-  16-     Model  herring  Irawl  kites  in  action. 

would  be  possible  to  shepherd  fish  can  be  calculated 
It  is  probably  better  to  have  ET  smaller  than  Ep,  as  in 
case  (ii)  because  the  chances  are  that  the  wire  will 
increase  the  probability  of  capture  within  its  limited  arc 
In  case  (iii)  it  is  possible  the  fish  are  already  outside  the 
arc  of  possibility  of  capture  when  near  enough  to  be 
influenced  by  the  wire.  Not  all  fish  in  the  path  of  sweeps 
and  wires  finish  up  in  the  path  of  the  net  nor,  for  that 
matter  does  every  fish  in  the  path  of  the  net  finish  up  in 
the  codend! 

A  common  line  of  escape  from  the  path  of  a  net  is 
below  the  ground  rope,  so  the  balancing  of  sinkers  and 
floats  is  critical,  if  good  headline  height  is  not  to  be 
sacrificed.  There  are  other  more  interesting  ways  of 
escape.  An  observed  escape  between  the  legs  or  spreading 
wires  is  shown  in  fig.  18,  where  the  usual  movement 
perpendicular  to  the  approaching  wires  turns  into 
escape  between  them.  Whether  this  is  due  to  vision  or 
discrimination  between  two  approaching  pressure  waves 
or  some  other  factor  is  not  known. 

Flat  fish  are  often  pinned  against  the  netting  by  the 
flow  of  water  through  it,  even  in  areas  of  the  net  where 
the  taper  does  not  appear  to  be  unduly  rapid,  as  in  fig.  19 
An  outward  turn  of  the  flow  lines  inside  the  net  might  be 
inferred.  The  after  end  of  the  lower  wings  below  the 
square  is  a  particularly  critical  region  and  if  the  meshes 
there  are  large  enough  escape  is  possible. 

Shoaling  fish  appear  to  behave  somewhat  differently 
There  is  the  shoal  reaction  as  well  as  the  individual 
reaction.  Unfortunately,  the  only  shoaling  fish  so  far 


MOVEMENT 
OF  FISH** 


(ii) 


ADVANCE 
OF  WIRE 


UNIT   DISTANCE 


('")  y\Vf 

<<X\^ 
>Ae>     \v 

,  UNIT  DISTANCE 


«F-«T  ••«'[£_ -     *'"    " 


PATH    Or 
ESCAPE  BETWEEN 
SPEADINC 
WIKES 


cos  ET 


ADVANCE  OF  NET 


/•iff.  17.     Diagram  of  the  common  reaction  offish  to  a  rope. 

\  173  1 


Fig.  18      Diagram  of  an  observed  reaction  oj 
fish  to  two  ropes 


MODERN    FISHING     GEAR    OF    THE    WORLD 


BATING 


FISH  PINNED 
ASAINSTNCTTlNt 


ALL  SANDCCLS 
THIS  REGION 
ESCAPE 


EXTENSION 
PIECE  I 

I  *l«.'>.' 

TWINE     HESH 


Fig.  79.    Diagram  of  the  way  how  a  fish  can 
become  pinned  to  the  webbing. 


Fig.  20.    Diagram  of  the  escape  of  a  school 
of  sandeel  through  the  funnel. 


Fig.  21.  Points  of  inflection  in  the  shape 
of  a  trawl  net  in  action. 


observed  in  their  reactions  to  a  net  have  been  sandeels. 
Although  the  mesh  size  presented  no  barrier  to  their 
escape,  they  kept  their  formation,  becoming  concentrated 
deep  in  the  funnel,  and  only  sought  escape  there  (though 
a  few  outriders  of  the  shoal  escaped  earlier).  The  escapes 
took  the  pattern  shown  in  fig.  20  and  there  is  presumably 
a  pressure  build  up  ahead  of  the  small  mesh  netting,  with 
a  consequent  increase  in  flow  through  the  last  piece  of 
the  larger  mesh  netting.  Shoaling  fish  are  both  easier  and 
harder  to  catch  than  other  fish:  easier,  because  any 
favourable  reaction  induced  in  the  outriders  can  be 
transferred  to  the  shoal,  and  harder  because  any  escape 
by  outriders  may  be  followed  by  the  shoal,  therefore  any 
defect  in  design  is  doubly  serious. 

The  object  of  design  must  be  to  delay  the  escape 
reaction.  All  trawls  so  far  observed  have  two  points  of 
inflexion  at  (a)  and  (b)  as  shown  in  fig.  21.  For  obvious 
reasons,  as  much  of  the  net  as  possible  should  be  in  large 
mesh9  but  a  critical  region  for  escape  lies  near  the  first  of 
these  two  points.  If  these  inflexions  are  pronounced,  the 
large  mesh  should  be  reduced  short  of  the  first  one,  while 
with  a  longer,  more  tapered  net,  in  which  the  inflexions 
are  more  gradual,  the  large  mesh  may  be  carried  farther 
aft.  (In  this  sense  a  large  mesh  is  merely  designated  as 
one  through  which  the  fish  can  pass.)  The  long  net  is 
the  common  type  in  herring  trawls,  often  having  a  fairly 
rapid  change  from  big  mesh  to  small,  escape-proof  mesh, 
so  that  there  is  not  a  large  area  in  which  the  herring 
become  meshed  as  this  is  a  nuisance  when  hauling  the 
net.  These  considerations  can  also  be  applied  to  white  fish 
trawls,  but  important  compromises  are  required  in  their 
design  because  the  groundrope  is  in  close  contact  with 
the  bottom,  which  is  often  rough.  Firstly,  the  belly  of 
the  net  should  be  kept  as  small  as  possible  to  avoid  splits, 
and,  secondly,  most  of  the  belly  should  be  made  in  large 
mesh  to  allow  as  much  bottom  rubbish  as  possible  to 
fall  through,  instead  of  passing  into  the  codend,  causing 
it  to  chafe.  The  planning  of  the  inflexions  and  mesh 
reductions  must  be  made  within  these  limits. 

This  is  a  preliminary  report  but  the  results  to  date 


suggest  that  observations  of  fish  reactions  to  small  nets 
with  full  sized  meshes  provide  ideas  for  shaping  full 
sized  nets.  These  ideas  can  next  be  applied  on  the  drawing 
board  when  designing  a  full  sized  net. 

CONCLUSIONS 

Observations  and  measurements  on  models  are  of  little 
use  unless  correlated  with  measurements  and,  where 
possible,  observations  on  a  full-sized  gear.  These 
cross-checks  are  necessary  to  establish  confidence  in  the 
value  of  modelling  as  an  auxiliary  to  the  rational  design 
of  fishing  gear.  Much  can  be  learned  from  the  observation 
of  models  but  the  agreement  between  performance  of 
models  and  full-sized  gear  is  not,  however,  close  enough 
to  be  accepted  uncritically.  Models  at  one-eighth  scale, 
for  example,  tend  to  give  a  view  of  gear  performance 
which  is  on  the  pessimistic  side. 

With  care,  models  provide  results  which  are  difficult 
to  achieve  in  other  ways,  such  as  the  effect  of  alternative 
methods  of  rigging  a  gear  and  whether  or  not  the  ad- 
vantages are  real,  and  how  errors  in  rigging  and  setting 
may  be  corrected. 

Two  other  methods  of  gear  testing  which  can  be  used 
to  compare  existing  rigs  or  help  in  their  design  and 
development,  are  instrumentation  and  comparative 
fishing.  What  is  claimed  for  modelling  is  not  that  it  can 
be  used  as  a  substitute  for  the  other  two  methods,  but 
that  it  is  complementary  to  them  lending  more  confidence 
to  the  results  obtained  and  giving  a  full  picture  of  gear 
performance.  Comparative  fishing  can  be  regarded  as  the 
final  stage  of  gear  testing — the  proof  of  the  pudding — 
but  its  results  will  be  better  and  more  confidently 
assessed  if  a  fuller  picture  of  gear  performance  is  provided 
through  the  use  of  models. 

REFERENCES 

1  Goldstein,  S.  Modern  Developments  in  Fluid  Dynamics.  Vol.  1, 

Sect.  5,  p.  16. 

2  Gawn,  R.  W.  L.     Steering  Experiments.     Trans.  Inst.  Naval 

Archt.,  1943,  pp.  35-73. 


[1741 


DEVELOPMENT  OF  MECHANICAL  STUDIES  OF  FISHING  GEAR 

by 

TASAE  KAWAKAMI 

Department  of  Fisheries,  Kyoto  University,  Maizuru,  Japan 

Abstract 

The  efficiency  of  fishing  gear  is  closely  connected  with  the  shape  it  assumes  in  the  water  under  ordinary  working  conditions,  and  while 
underwater  photography  and  television  have  helped  to  solve  some  of  the  problems,  there  still  remain  many  to  be  studied.  The  author 
suggests  experiments  with  models,  provided  that  the  law  of  similarity,  by  which  the  mechanical  relation  between  the  model  and  the  full-scale 
gear,  is  observed;  but  the  model  law  should  be  deduced  from  the  mechanical  theory  of  the  operation  of  the  gear. 

In  this  paper  the  author  gives  a  series  of  summaries  of  the  research  work  done  on  the  hydrodynamics  of  Ashing  gear — trawls,  seines, 
purse-nets,  etc.  and  their  accessories,  and  gives  the  mathematical  formulae  governing  their  optimum  performance,  in  relation  to  depth  and 
current  strength. 

It  is  stressed,  however,  that  after  trials  have  been  made  with  models  to  which  the  theory  has  been  applied,  the  final  test  should 
always  be  on  the  full-scale  gear  in  actual  fishing  operation. 


Rfeume 


Dcveloppcment  de  I 'etude  des  Engins  de  Pfehe  au  point  de  vue  mecanique 


II  existe  un  rapport  etroit  entre  I'efficacitd  d'un  engin  de  peche  et  la  forme  qu'il  prend  dans  1'eau  dans  les  conditions  d'utilisation 
ordinal  res;  la  photographic  et  la  television  sous-marines  ont  bicn  aid£  a  r&oudre  certains  problemes,  mais  il  reste  encore  de  nombreux  points  & 
ttudier.  L'auteur  propose  de  faire  des  experiences  avec  des  modules,  £  condition  de  respecter  la  loi  de  similitude  £  laquelle  doivent  ob&r  les 
rapports  m&aniques  entre  le  modele  et  Fcngin  grandeur  nalu relic;  mais  la  loi  modele  devrait  fctrc  d6duite  de  la  thforie  m6canique  du 
fonctionnement  de  1'engin. 

Dans  cct  article,  Pauteur  resume  une  serie  de  rechcrchcs  sur  I'hydrodynamique  des  engins  de  p&che  (chaluts,  sennes,  sennes  tournantes, 
etc.,  et  leurs  acccssoires)  et  indique  les  formulcs  mathcmatiques  correspondant  &  leur  meilleur  rendement  en  fonction  de  la  profondeur  et  de  la 
force  des  courants. 

II  soulignc  n&inmoins  qu'apres  les  essais  de  modules  conformes  a  la  thforie,  1'essai  final  devra  toujours  porter  sur  1'engin  grandeur 
naturelle  au  cours  d'operations  de  peche  rtelle. 


Desarrollo  de  los  estudios  mec&nicos  sobre  artes  de  pcsca 
Kxtracto 

La  cficacia  dc  un  arte  pesca  sc  halla  cstrechamentc  relacionada  con  la  forma  que  toma  en  el  agua  durante  el  trabajo  en  condiciones 
normales.  Si  bien  la  fotografia  y  televisidn  submarinas  ban  ayudado  a  resolver  algunos  de  estps  problemas,  todavia  existen  muchos  que 
debcn  ser  estudiados.  Por  este  motivo,  el  autor  sugiere  hacer  pruebas  con  modelos  siempre  que  rija  la  ley  de  la  similitud,  o  sea,  una  relation 
mecanica  entre  el  arte  a  escala  reducida  y  el  construdio  a  escala  normal;  no  obstante  la  ley  para  el  modelo  debe  deducirse  de  la  teoria  mecanica 
del  funcionamiento  del  arte. 

En  este  trabajo  el  autor  da  una  serie  de  resumenes  de  investigaciones  sobre  las  caracteristicas  hidrodinamicas  de  los  artes  de  pesca — 
redes  de  arrastre,  de  cerco,  etc.  y  sus  accessorios — ademas  de  fbrmulas  matemdticas  que  regulan  su  rcndimicnto  Optimo  en  relacibn  con  la 
profundidad  y  la  fuerza  de  la  corrcintc. 

Sin  embargo,  es  necesario  hacer  notar  que  despues  de  las  pruebas  hechas  con  modelos  a  los  cualcs  se  ha  aplicado  la  teoria,  debe 
efectuarsc  una  prueba  final  con  el  arte  a  escala  normal  empl&indolo  lances  de  pesca  reales. 


THE  efficiency  of  a  fishing  gear  is  closely  related  to 
the  shape  it  assumes  in  operation.  Until  recently,  the 
mechanical  behaviour  of  the  gear  in  action  could  only 
be  assessed  approximately  by  judging  the  tension  and 
tilt  angle  of  tow  ropes  or  manipulating  lines,  or  by  the 
actual  fishing  results  obtained  with  the  gear.  However, 
to  design  a  gear  efficiently  or  operate  it  effectively,  an 
accurate    knowledge    is    required    of    the    mechanical 
properties  of  each  part. 

In  recent  years,  underwater  television,  filming,  and 
other  submarine  devices,  have  been  used  to  observe  the 
working  pattern.  These  are  very  effective  for  determining 
shape  and  behaviour,  but  in  order  to  improve  a  fishing 
gear,  extensive  testing  is  necessary.  It  is,  however,  very 
expensive  to  make  up  a  new  full-size  net  for  each  trial, 
therefore  tests  with  models  may  be  practical.  In  this  case 
the  law  of  similarity,  by  which  the  mechanical  relation 


between  the  model  and  the  full-scale  gear  is  governed, 
must  be  observed. 

Mechanical  investigation  of  fishing  nets  in  Japan 
started  in  1915,  when  Dr.  T.  Terada  and  his  associates 
made  a  series  of  experiments  on  the  resistance  of  a  plane 
net  towed  through  the  water.  This  paper  gives  a  short 
review  of  research  since  then  from  the  theoretical  point 
of  view,  but  space  allows  only  a  summary  of  the  more 
important  investigations,  whilst  most  of  the  papers  con- 
cerned are  merely  cited  (see  Literature). 

THE  HYDRODYNAMIC  FORCE  ACTING  ON  A 
TWINE 

Fishing  gears  are  composed  mainly  of  twines  or  ropes  of 
various  sizes  (exceptions  are  weirs,  rakes,  spears,  etc.). 
Consequently  it  is  of  fundamental  importance  to  have  an 


[175] 


MODERN     FISHING     GEAR     OF    THE     WORLD 


accurate  knowledge  regarding  the  hydrodynamic  force 
of  relative  velocity  of  current  acting  upon  an  element  of 
a  twine. 

Tauti49  assumed  in  his  theory  on  the  resistance  of  a 
fishing  net  to  the  flow  of  water: 

(1)  That  the  magnitude  of  resisting  force,  Ro,  per  unit 
length  of  a  twine  varies  proportionally  with  the  sine  of 
the  angle  0  between  the  twine  and  the  current,  and  is 
directed  normal  to  the  twine  in  the  plane,  including 
directions  of  the  twines  and  the  current,  i.e. 

Ro     R  sin  0.  (1) 

where  R  is  the  drag  or  resisting  force  of  the  twine  per 
unit  length  when  it  is  supported  perpendicularly  to  the 
current. 

Another  method  is  given  below.  Resolve  the  velocity 
vector,  U,  of  the  current  into  its  normal  component  Un 
and  tangential  component  Ut  with  respect  to  the  direc- 
tion of  the  twine,  then  if  we  assume  that  both  the  velocity 
components  act  independently  with  each  other  and 
Newton's  law  for  hydro-dynamic  drag  is  tenable  to 
the  normal  component,  we  come  to  the  conclusion: 

(2)  That  R  is  proportional  to  U2  sin2  0,  i.e., 

Ro     Rsin20.  (2) 

This  assumption  is  supported  by  experimental  evi- 
dence"- 63. 

The  tangential  component,  F  is  rather  obscure.  Ex- 
periments show  that  the  tangential  force  is  much  smaller 
than  the  normal  force,  and  is  substantially  independent 
of  the  angle  until  the  twine  becomes  almost  perpendicular 
to  the  current  and  then  drops  quickly  to  zero.  Since  the 
tangential  force  is  very  small,  it  is  difficult  to  measure 
accurately  and,  hence,  the  precise  nature  of  its  variation 
with  the  angle  cannot  be  given  with  any  assurance.  It 
may  be  permissible,  for  the  sake  of  simplicity  of  calcula- 
tion, to  assume: 

(3)  That  the  tangential  force  is  constant,  F,  over  the 
whole  range  of  0. 

THE  EQUILIBRIUM  CONFIGURATION   AND 
TENSION    OF    A    FLEXIBLE    TWINE    IN    A 
UNIFORM  FLOW 

Thews  and  Landwebcr52  have  made  a  general  analysis 
of  the  equilibrium  configuration  of  a  flexible  twine  sus- 


(G-0)      Tc 

Fig.   I.     Configuration  of  a  twine  in  a  flow  of  water. 


pended  in  a  uniform  current,  under  the  assumptions  (2) 
and  (3).  Pode  3!)  has  carried  out  a  series  of  comprehensive 
numerical  computations  for  the  case  in  which  neither 
the  weight  of  the  twine  nor  the  tangential  force  of  the 
hydrodynamic  action  can  be  neglected. 

Suppose  the  form  of  a  twine  in  a  uniform  steady  flow 
of  velocity  U  (see  fig.  1).  Choose  rectangular  coordin- 
ates (x,  y),  whose  origin  is  located  at  a  point  on  the  twine 
where  the  twine  is  normal  to  or  parallel  to  the  current. 
Let  x-axis  be  directed  parallel  to  the  current  measured 
positive  up-stream  and  y-axis  be  directed  vertically  up- 
wards. Let  s  be  the  arc  length  along  the  twine  from  the 
origin  O  to  any  pont  P  (x,  y)  on  the  twine,  T  and  Tf,  be 
the  tensions  in  the  twine  at  the  points  P  and  O  respec- 
tively. The  angle  of  inclination  of  the  twine  against  the 
stream  is  measured  clockwise  from  the  direction  of  the 
current  to  the  direction  of  increasing  s.  Furthermore,  let 
W  be  the  weight  of  the  twine  of  unit  length  in  water,  and 
put  w-  W/R.  These  forces  may  be  resolved  to  and  along 
the  twine  as  shown  in  fig.  2,  then  the  equilibrium  of  the 
twine  element  ds  requires: 


dT 
ds 
do 


F     W  sin 


R  sin2  0  i  W  cos  0 


(4) 


The  solution  of  these  differential  equations  can   be 
written  in  a  parametric  form: 


T 

~ 

Jo 


Ry 

To 


rt  (0) 


(5) 


where  T,  a,  <£  and  //  are  transcendental  functions  of 
angle  0,  and  also  of  the  ratios  f  F/R  and  w-=W/R. 
Pode's  numerical  solutions  are  partly  reproduced  in 
Table  1  (reference point  at  0=n/2)  and  Table  II  (reference 
point  at  0  =  O).  In  these  tables  another  expression  of 
w  is  introduced.  Suppose  the  cord  is  simply  trailed  by 
itself,  without  any  towed  body  at  the  lower  end,  then  its 


T+dT 


—    mirrenf 


Rfiin20dS 


WdS 

Fig.  2.    Forces  acting  on  a  twine  subjected  to  flow  of  water. 


[176] 


MECHANICAL    STUDY 

configuration  may  be  a  straight  line  inclined  to  the  cur- 
rent at  such  an  angle: 

— w  »  \/w2— i 
Oc  =--  arc  cos          -  - (6) 

This  angle  Oc  is  called  the  critical  angle,  and  will  be  used 
in  place  of  w. 

ESTIMATION   OF   THE   FISHING    DEPTH   OF   A 
TOWED  GEAR 

To  illustrate  an  application  of  these  solutions  to  cable 
problems,  let  us  consider  estimating  the  depth  of  a  deep 
trolling  gear  I2.  All  parameters  related  to  the  point  where 
the  line  is  attached  to  the  depressor  and  where  it  inter- 
sects the  water  surface,  are  distinguished  by  subscrip- 
tions 1  and  2  respectively.  It  is  required  to  find  the 
depth  h(— y2  y,)  of  the  depressor  when  the  submerged 
length  1  (  -S2— S,)  of  the  tow  line  is  given.  From  the 
equations  given  in  the  preceding  section,  we  have 

R  .     r,<0»)-'i«M 


R 

T2 


! 


(7) 


where  R  is  expressed  as: 

R     A  CnpU2n 

CD  being  the  drag  coefficient  of  the  trolling  line  and  its 
numerical  value  being  approximately  equal  to  1*5  for 
ordinary  stranded  twine,  tj  the  density  of  fluid,  U  the 
velocity  of  motion  and  D  the  diameter  of  the  twine.  The 
relations  between  h/1,  0,,  0  2  and  Rl/7'2  can  be  graphically 
represented  in  a  chart,  which  greatly  facilitates  the 
estimation  of  the  working  depth  of  a  depressor.  If  the 
value  of  R  is  given,  the  ratio  h/1  can  be  read  directly 
from  this  chart  by  using  any  two  values  out  of  three 
values. 

The  tension  of  the  warp  at  the  vessel  in  trawling  is 
transmitted  to  the  gear  and  the  various  modes  of  fishing 
performance  of  the  gear  must  be  contingent  upon  the 
magnitude  and  the  direction  of  the  force  acting  on  the 
gear.  The  cable  problems  also  have  a  bearing  in  this 
connection,  i.e.  in  regard  to  mid-water  trawling.  The 
working  depth  of  these  nets  can  be  regulated  to  sonic 
extent  by  adjusting  the  length  of  the  towing  warps.  In 
these  cases,  if  the  weight  of  the  warp  in  water  is  not 
negligible  as  compared  with  its  resistance,  similar  charts 
for  various  values  of  critical  angles  must  be  prepared. 

Another  example  of  the  application  of  these  formulae 
17t  4,  is  a  simple  current-measuring  device,  the  Siomi-ito, 
which  is  in  traditional  use  on  coastal  fishing  grounds  in 
Japan.  It  consists  of  dropping  overboard  several  sinkers 
attached  to  strings  of  different  lengths.  Each  sinker  is 
subjected  to  the  current  at  a  depth  corresponding  to  the 
length  of  the  string.  The  tilt  of  the  string  at  the  surface 
will  be  determined  by  both  the  profile  of  the  current 
velocity  and  the  length  of  the  string.  If  the  configuration 
of  the  string  in  a  given  current  profile  is  known,  we  have 
a  possibility  of  determining  the  current  profile  from  the 
tilt  angles  of  several  strings  of  different  lengths  at  the 
surface.  Kawakami  and  litaka17  have  made  some  in- 
vestigations of  this  problem.  They  designed  a  new  type 


OF    FISHING     GEAR 

of  Siomi-itOt  and  proposed  a  proper  way  to  manipulate 
it. 

THE   RESISTANCE   OF   PLANE   NETTING   IN   A 
CURRENT 

The  resistance,  R,  acting  on  a  plane  net  subjected  to  a 
uniform  current  of  velocity  U,  was  first  studied  experi- 
mentally by  Terada,  Sekine  and  Nozaki52,  then  by 
Tauti,  Miura  and  Sugii47  and  by  Miyake3C.  The  ex- 
perimental method  adapted  by  these  workers  was  the 
same  in  principle.  Two  rectangular  frames,  on  which  a 
sample  webbing  was  spread  out,  were  connected  with 
each  other  along  one  side  and  spread  at  an  angle  of  2<p. 
This  pair  of  frames  was  put  in  motion  through  the  water 
under  the  action  of  constant  force  of  known  magnitude. 

Their  results  are  coincident  in  that  the  resistance  varies 
proportionally  with  the  area,  S,  of  webbing  and  also 
approximately  with  the  square  of  the  velocity  U,  i.e., 

R-kSU2  (8) 

where  k  is  a  coefficient  of  resistance  depending  upon  the 
construction  of  the  webbing  and  the  angle  of  attack  a. 
The  construction  of  the  webbing  may  be  characterised 
by  the  length,  L,  of  the  bar  of  the  mesh,  diameter,  D, 
of  the  netting  twine  and  the  angle,  2<p,  between  two 
adjacent  bars  of  the  mesh. 

Terada  and  his  collaborators  have  proposed  a  supposi- 
tion that  the  value  of  k  might  be  proportional  to  the 
area  of  webbing  projected  on  the  plane  perpendicular  to 
the  direction  of  motion.  Tauti  and  his  associates  showed 
that  k  varies  linearly  with  the  angle  a.  Miyake  deduced 
a  theory  based  on  the  supposition  proposed  by  Terada 
and  his  collaborators,  and  verified  that  the  theory  co- 
incided approximately  with  his  observed  value. 

The  most  detailed  theoretical  analysis  on  the  force 
acting  on  the  webbing  has  been  conducted  by  Tauti40 
under  the  assumption  (1).  Suppose  a  mesh  of  webbing  be 
suspended  in  a  current  of  velocity  U  (see  fig.  3),  in  which 

OA  and  OB  are  two  adjacent  bars  and  OC  the  velocity 


C(l.m,n) 


Fig.  3.     A  left-hand  system  of  rectangular  coordinates  chosen  in 
the  mesh  of  webbing. 


[177] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


vector  of  the  current.  Denote  the  angle  between  two  bars 
OA  and  OB  by  2<p.  In  a  left-hand  system  of  rectangular 
coordinates  (x,  y,  z),  let  x-axis  be  the  bisector  of  the 
angle  between  two  adjacent  bars,  y-axis  be  perpendicular 
to  x-axis  in  the  plane  of  webbing  and  z-axis  be  normal 

to  the  plane.  Denote  by  (I,  m,  n)  the  direction  cosines 

»  —  + 

of  the  velocity  vector  OC. 

Mechanical  analysis,  after  some  mathematical  pro- 
cedures, gives  a  result  that  the  x-,  y-  and  z-  com- 
ponents of  k  can  be  represented  respectively  by 

/D\2  1 

+'       '  f  ' 

\  L 
D\  /D 

mcot  9   f  (  —  1   fy,          '  (9) 


,  D\ 
—1 
L/ 


1  tan  q> 


L/ 


V 


—   I  i    I   —   i    *z,     i 

k  L  /  sin  9  cos  <p      \  L  /          J 

where  a  is  a  constant  depending  on  the  drag  co-efficient 
of  the  netting  twine  and  is  given  by 

a  =  CD  p/2 

in  ordinary  hydrodynamical  expressions,  and  second 
terms  are  correction  due  to  knots.  The  factors  fx,  fy, 
and  f7  vary  with  the  direction  of  current  relative  to  the 
coordinates  system.  These  correction  terms  would  be 
negligible  in  comparison  with  the  first  term  in  the  case 
of  large  meshes  of  fine  twine. 

When  the  direction  of  current  lies  in  the  xz  plane,  i.e., 
1-  cos  a,  n-=sin  a,  where  a  is  the  angle  of  attack  of  the 
webbing,  drag  coefficient  of  the  webbing  may  be  re- 
presented from  the  expressions  (9)  by 


k   —  a   (  —  1  cot  9  (sin2  a    j    1 ) 


(10) 


in  which  the  correction  terms  are  neglected.  This  means 
that  the  value  of  k  varies  linearly  with  sin-a. 

In  these  analyses,  it  is  assumed  that  the  knots  and 
twines  of  meshes  are  mutually  independent.  Since,  how- 
ever, the  actual  net  is  a  complicated  system  of  netting 
twine,  the  hydrodynamical  interference  between  neigh- 
bouring twines  and  knots  should  be  taken  into  account, 
when  the  net  is  kept  in  a  certain  range  of  attack-angle. 
Fujita  and  his  collaborator"6  have  investigated  this 
phenomenon. 

Miyamoto  and  his  associates31' 3G  have  made  a  series 
of  experiments  to  secure  accurate  data  for  different  types 
of  webbing.  They  measured  the  drag  of  a  piece  of  web- 
bing when  0  90  degrees  and  ^-45  degrees,  then  the 
value  of  k •/  were  determined.  The  results  are  given  by 


a  (_-  -  10*) 
\2L  / 


-\  b[ — xlO*  )2,  (kg.wt.  sec2./i 
\2L          I 


where  numerical  values  of  a  and  b  are  tabulated  below: 


Kind  of  webbing 

Knotless 
Flat-knot 
Trawler-knot 


1-72 
1-70 
1-68 


0-370 
0-433 
0-475 


The  value  of  kz  becomes  greater  with  decreasing  or  in- 
creasing of  q>  from  <p=45  degrees.  As  regards  the  material 
of  the  netting  cord,  the  value  of  k  is  somewhat  larger  in 
staple  fibres  than  in  continuous  fibres.  In  general,  it  may 
be  concluded  that  the  fibre  ends  sticking  out  of  a  twine 


made  of  short  fibres  have  a  tendency  to  catch  dirt  and 
create  more  resistance. 

EQUILIBRIUM  CONFIGURATION   OF  WEBBING 
IN  A  CURRENT 

A  stretched  piece  of  webbing  may  be  considered  as  a 
continuous  membrane,  in  the  case  where  differences  in 
physical  states  between  neighbouring  meshes  are  negli- 
gible. This  membrane  will  be  subjected  to  two  kinds  of 
external  force,  i.e.,  the  hydrodynamic  force  due  to  the 
current,  and  the  apparent  weight  in  water.  The  hydro- 
dynamic  force  is  given  by  kU2.  Let  W  be  its  apparent 
weight  of  unit  area  in  water,  (lg,  mg,  ng)  be  its  direction 
cosines  referred  to  coordinate  axes  O-x,  O-y,  and 
O-z,  which  are  chosen  in  such  a  way  that  the  principal 
curvatures  at  the  elemental  portion  of  webbing  membrane 
lie  on  the  xz-  and  yz-  planes.  Let  rx  and  ry  respectively 
be  the  radii  of  curvature  on  these  planes,  and  Ty  and 
Tx  be  the  tensions  of  the  webbing  per  unit  length  along 
the  x-  and  y-  directions  respectively.  Tautir>0  pro- 
posed the  following  differential  equations  required  to 
maintain  a  mechanical  equilibrium  of  the  webbing  mem- 
brane: 


dTx 

dx 

dTy 

dy 


kxV2  *    1BW       O, 


I   kyV2  f  mKW 

+T*.-y 

TV 


(II) 


nBW. 


In  the  analytical  treatment  of  mechanical  problems  of 
fishing  nets,  it  is  of  fundamental  importance  to  know 
exactly  the  configuration  of  a  strip  of  net  and  the  distri- 
bution of  tension  when  subjected  to  a  current.  Miya- 
moto6*, Fujita6,  and  Kawakamil!i  have  made  a  theoret- 
ical analysis  on  this  problem,  based  on  Tauti's  law. 

Suppose  that  a  long  narrow  strip  of  webbing  of  a 
constant  width  is  supported  vertically  by  both  ends,  A 


Fig. 


Configuration  of  a  rectangular  strip  of  webbing  subjected 
to  flow  of  water. 


[178] 


MECHANICAL    STUDY    OF    FISHING    GEAR 


and  B,  in  a  uniform  current  and  is  in  equilibrium  con- 
dition. A  system  of  coordinate  axes,  O-x  and  O-y,  is 
chosen  as  shown  in  fig.  4,  i.e.,  the  origin  is  located  at 
the  point  on  the  webbing  where  the  webbing  is  perpendic- 
ular to  the  current,  and  the  x-  and  y-axes  are  directed 
upstream  and  upward  respectively.  Then  let  s  be  the  arc 
length  of  the  net  from  the  origin,  O,  to  any  point  P 
(x,  y)  and  T  and  to  be  the  tensions  of  the  webbing  per 
unit  length  at  the  points  P  and  O  respectively.  Denote 
the  angle  of  the  netting  membrane  against  the  current 
at  the  point  P  by  0.  If  the  net  is  of  homogeneous  mesh  of 
2L  in  stretched  measure,  woven  with  a  netting  twine  of 
diameter  D,  and  hung  to  the  framing  lines  with  such  a 
degree  of  slack  as  to  make  an  angle  7q>  between  two 
adjacent  bars,  then  the  equilibrium  equations  (11)  take 
the  form: 


dT 

ds 


-    K  sin2  rp  cos  0   \    W  sin  0 


dO 
ds 


K  sin  0  -I   W  cos  6 


(12) 


(13) 


where  K  is  the  normal  component  of  the  resisting  force 
per  unit  area  of  the  webbing  when  the  webbing  is  per- 
pendicular to  the  current  and  W  is  the  weight  of  unit 
area  of  the  webbing  in  water. 

Let  a  be  the  area  enclosed  by  the  x-axis,  the  arc  of  the 
webbing,  and  the  straight  line  parallel  to  the  y-axis, 
x=x,  then  the  solution  of  these  equations  may  be 
written  in  a  parametric  form: 

~ 


Ks 
To 
Kx 


to 


To 


-  £„  (0), 


-    /Jn  («). 


(14) 


(0), 


where  rn,  <rn,  £n,  ^n,  and  anare  transcendental  functions 
of  angles  0  and  y,  and  the  ratio  r~K/W.  A  part  of  these 
relations  has  been  calculated  numerically  by  Miya- 
motoHH. 

If  the  weight  of  the  webbing  be  negligible  compared 
to  its  resistance,  the  solutions  will  be  simplified  to: 

rn  --  (sin  0)  -*in29, 

f  0 
on-  ]  (sin  6)-0  4-  sin-  9)  d0 

1 
sin20  *  (15) 

"0 
do 

f    7C/2 

ro 

otn   ~  '    rt  d  !; 
Jic/2 

The  numerical  solutions  have  been  calculated  by  Kawa- 
kami16. 

MECHANICS    OF    AUXILIARY    GEAR    (Floats, 
sinkers,  shearing  devices,  mooring  equipment.) 


The  buoyancy  or  specific  gravity  of  floats  and  sinkers 
is  given  by  elementary  mechanics.  The  reserve  buoyancy 
of  gillnets  has  been  discussed  by  Miyamoto31. 

These  accessories,  however,  are  subjected  to  currents 
of  various  speeds,  which  cause  the  unfavourable  deforma- 
tion of  the  gear.  It  is  essential,  therefore,  that  the  shape 
of  these  accessories  be  streamlined  to  reduce  the  drag  to 
a  minimum. 

Apart  from  floats  and  sinkers  which  work  on  the  static 
principle,  other  such  accessories  based  on  dynamic 
principles  are  used,  such  as:  otter  boards,  kites  and  de- 
pressors. 

The  action  of  the  otter  board  has,  by  some  authors, 
been  treated  as  that  of  a  flat  plate  in  a  current.  When  a 
flat  plate  of  a  length  c  is  subjected  to  a  current  at  an 
angle  of  attack  a,  to  the  direction  of  flow,  its  resistance 
force,  P,  is  approximately  normal  to  the  plane  and  the 
magnitude  is  given  by  Duchcmin's  equation: 

pp  -,4siV  .  no 

Po      1  +sm2  « 

where  PO  is  the  value  of  P  when  the  plate  is  normal  to 
the  flow.  The  distance  ft  from  the  centre  of  the  plate  to 
the  point  through  which  the  total  resisting  force  acts  is 
given  by 

S3         cos  a 

- (17) 

C       4    4    \    n  sin  a 

If  the  board  is  a  hydrofoil,  its  hydrodynamic  character- 
istics must  be  investigated  in  advance  by  a  model  ex- 
periment. Polar  diagrams  of  various  types  of  hydrofoil 
depressors  for  trolling  gear  have  been  drawn  by  Okuno36. 
To  fix  a  trap  net  at  a  point  in  the  sea,  sand  bags  or 
anchors  are  commonly  used.  The  holding  power,  H,  of 
a  sand  bag  falls  off  as  the  angle,  ft,  of  the  mooring  rope 
with  the  horizontal  at  the  bag  end  is  increased.  The 
variation  of  the  holding  power  with  this  angle  has  been 
investigated  by  Tauti47.  He  assumed  the  ordinary  fric- 
tional  resistance,  the  coefficient  of  which  is  represented 
by  /*,  between  the  under  surface  of  the  bag  and  the  sea- 
bed, and  obtained  the  following  result: 
H  ,1 


W        1   »  jz  tan  3 
where  W  is  the  weight  of  the  bag  in  water. 

With  regard  to  the  anchor,  the  circumstances  differ. 
The  holding  power  of  an  anchor  is  due  to  the  action  of 
its  fluke  which  digs  into  the  sea-bed.  Assuming  that  the 
resistance  is  proportional  to  the  total  quantity  of  bottom 
material  scooped  by  the  fluke  when  the  anchor  is  pulled, 
Tauti  has  derived  the  following  relation: 

H  -  p  tan  (  a  -  p  )  sin  (  a  -  3  )         (19) 
where  a  is  the  angle  between  the  shank  and  the  fluke 
and  p  is  the  proportionality  constant  depending  upon  the 
type  and  size  of  the  anchor,  and  also  upon  the  physical 
properties  of  the  bottom  material. 

ANALYTICAL  STUDIES  ON  TOWED  GEAR 

For  analytical  studies  of  the  mechanical  characters  of 
trawl  nets  KaWakami16  designed  a  mechanically  simple 
model.  It  had  two  wings  of  narrow  rectangular  webbing 
and  a  kind  of  codend  simple  enough  to  make  mechanical 
analysis  possible.  He  solved  a  set  of  equilibrium  equations 


[1791 


MODERN     FISHING     GEAR     OF     THE     WORLD 


for  each  portion  of  the  net  and  obtained  the  theoretical 
relation  between  the  tension  of  the  warp,  the  angle  of  its 
inclination  to  the  flow,  and  the  horizontal  spread  of  the 
wings.  This  relation  is  of  fundamental  importance  in 
designing  a  precise  assembly  of  the  net  rigging  such  as 
headline  and  otter  boards. 

As  regards  the  codend,  Taniguchi43-46  has  made  a 
series  of  studies  on  various  types  of  bagnets.  He  pointed 
out  that  the  main  factors  affecting  its  resistance  are  total 
area  of  webbing  used,  the  working  gape  of  the  bag,  and 
the  ratio  D/L  as  defined  in  the  previous  section,  and  that 
the  shape  or  mode  of  assembly  has  little  bearing  upon 
the  total  resistance.  His  further  experiments  showed  that 
the  length  of  the  bag  has  also  relatively  little  effect  on  its 
resistance. 

It  is  doubtful  whether  the  shearing  action  of  otter 
boards  is  purely  hydrodynamical  or  whether  the  plough- 
ing action  on  the  sea  bottom  contributed  in  some 
measure.  Kawakami14,  using  Duchemin's  equation,  made 
an  analytical  treatment  of  otter  boards  attached  to  the 
end  of  a  rectangular  strip  of  webbing.  Fig.  5  shows  the 
variation  of  spreading  action  of  the  otter  boards  accord- 


ing to  the  adjustment  of  the  brackets.  The  optimum 
spread  can  be  obtained  theoretically. 

A  series  of  experimental  studies  as  well  as  a  theoretical 
discussion  on  the  midwater  trawl  has  been  made  by 
Kobayashi  and  his  associates19-23.  Their  net  was  fitted 
with  specially  designed  depressors  and  otter  boards.  They 
obtained  fairly  good  working  stability.  The  mechanical 
calculation  of  the  working  depth  and  of  the  towing  re- 
sistance were  found  to  coincide  fairly  well  with  the 
results  of  full-scale  tests. 

PRACTICAL  PROCEDURES  OF  MODEL  EXPERI- 
MENTS (GENERAL  RULES) 

The  applicability  of  analytical  methods  is,  of  course, 
limited,  in  which  case  model  experiments  are  often  use- 
ful. In  making  a  model  test,  it  is  essential  to  know  what 
conditions  have  to  be  fulfilled  to  ensure  similar  mechan- 
ical and  geometrical  relations  between  the  full-scale  net 
and  the  model.  This  law  of  similarity  was  first  deduced 
by  Tauti50,  based  on  the  assumptions: 

1.  That  the  elongation  of  the  net  twine  is  negligible 
when  in  operation; 

2.  That  the  net  twines  arc  perfectly  flexible; 

3.  That  the  change  in  the  form  of  the  net  occurs  so 
slowly   that    the  external   forces   acting   on   each 
element  of  the  net  can  be  considered  to  be  in  quasi- 
equilibrium,  and 

4.  That  Newton's  law  of  hydrodynamic  force  is  valid 
for  every  portion  of  the  net,   irrespective  of  its 
Reynolds'  Number. 

Model  experiments  are  generally  conducted  in  an  ex- 
perimental tank,  using  either  one  of  two  different 
methods.  With  the  first  the  water  is  at  rest  and  the  gear 
is  towed  as  in  the  testing  of  ship  resistance.  The  net  is 
attached  to  a  carriage  moving  on  rails  and  towed  through 
the  water  by  means  of  the  towline.  This  method  is  con- 
venient for  the  trawl  net.  The  still  water  tank  is  also 
suitable  for  such  gear  as  encircling  or  lift  nets. 

The  second  method,  where  the  net  is  stationary  and 
the  water  flows,  is  comparable  to  a  wind  tunnel  in  an 
aeronautics  laboratory.  This  method  has  sometimes  an 
advantage,  especially  for  the  fixed  net,  because  changes 
in  shape  of  the  net  can  be  photographed  easily.  Fig.  6 


Fig.  5.    Showing  the  variation  of  shearing  action  of  the  otter 
board  according  to  the  adjustment  of  the  brackets. 


Fig.  6.     Model  tank  of  circulation  type. 


[180] 


MECHANICAL     STUDY     OF     FISHING     GEAR 


shows  an  experimental  water  tank  of  circulation  type. 

Practical   procedures  of  model  experiments  are  as 
follows: 

(')  following  a  value  denotes  a  full-scale  test. 
O  denotes  a  model  test. 

I.  In  the  first  place,  define  the  reduction  ratio,  J,  of 
the  model  as  large  as  the  circumstances  permit,  i.e. 


A 


(20) 


where  /  is  the  linear  dimension  of  each  section  ot 
the  net. 

2.  Next,  determine  the  diameter,  D,  of  the  net  twine 
and  the  density,  £>,  of  its  material  such  that  the  ratio: 

D"     f/'  -   p^' 

t  (21) 

D'     p'  -  pw' 

has  the  same  value  for  all  sets  of  corresponding 
portions  of  model  and  full-scale,  where  ft*  is  the 
density  of  water. 

3.  Then,  the  length,  L,  of  the  bar  of  a  mesh  should  he 
determined  so  that  the  ratio: 

IV          I 

-  M  (22) 

D          I 

has  the  same  value  throughout  all  parts  of  the  web- 
bing. Thus  the  reduction  ratio  of  mesh  is  not  neces- 
sarily the  same  as  that  of  the  net  itself. 

4.  Assemble  the  webbing  of  the  model  net  with  the 
the  same  ratio  of  take-up  or  slack  for  all  correspond- 
ing part*  of  both  nets.  This  means: 

9"  -    <p'.  (23) 

5    For  the  model  thus  made  up,  the  ratio  of  velocitv. 
V.  between  model  and  full-scale  is  given  by: 
v 

\  I  (24} 

v 

1  he  ratio  of  tension,  T,  m  the  webbing  and  the 
ratio  of  tension,  Tr,  in  the  manipulating  rope  are 
civen  respectively  by  : 

T 

-   -    \(  . 
T 

(25» 
T,  I 

--  -  A-K  I 

T, 

(>.  As  regards  the  ropes  in  the  net,  the  density  or  ol 
its  material  and  the  diameter,  Dr.  should  be  chosen 
so  as  to  hold  the  next  relations  simultaneously: 


(26) 


D, 

—  =.  A 
Or 


7.  The  size,  Da,  and  the  density,  (*u*  of  the  material 
of  the  accessories  such  as  floats  and  sinkers  should 
be  chosen  •  so  as  to  satisfy  simultaneously  the 
relations: 


10. 


II 


P"a          .-"«,  /n        l 

p-a-T."  Vn-;7x' 

rra     /„ 

D.,Vn  VV 


f 


(27) 


where  n  is  the  number  of  the  accessories  attached 
for  unit  length. 

8.  If  the  net  is  fixed  to  the  sea  bottom  by  means  of 
sand  bags,  as  in  the  case  of  fixed  or  trap  net,  the 
weight,  W,  of  the  bag  in  water,  should  be  chosen 
so  as  to  hold  the  relation: 


[W  1 


W"   k" 
W     k 


-    \2h. 


(28) 


where  k  is  the  holding  coefficient  of  the  bag. 

9.  In  the  case  where  the  accessory  is  made  of  canvas,  its 
apparent  weight  in  water  being  negligible,  the 
dimension,  /a,  is  given  by 


(2Q> 


When  the  accessor}  effects  hydrodynamic  forces  as 
in  the  case  of  otter  boards  in  a  trawl  gear,  its  size, 
/.h,  and  density,  "t>,  of  the  material  should  satisfy 
simultaneously  the  relations: 


>  b 
.''b 


(30) 


I 


Where  the  shape  of  a  net  changes  with  the  fishing 
operation,  as  in  the  case  of  a  purse  seine  or  Danish 
seine,  additional  conditions  must  be  satisfied  to 
maintain  the  mechanical  similarity.  Let  Vp  and  t  be 
the  corresponding  velocity,  and  required  lime  to 
attain  a  corresponding  stage  of  operation,  then 
the  next  relations  should  be  satisfied: 


vl 


I 


.  X'L 

If  a  net  is  worked  in  relatively  weak  currents,  as  in 
the  case  of  a  fixed  net  in  a  cove,  the  resistance  of 
the  rope  due  to  the  current  being  negligible  in  com- 
parison with  its  apparent  weight  in  water,  the  values 
of  Dr  and  yT  can  be  chosen  so  as  to  satisfy  only 
the  next  relation; 


IV V- 


-  AE. 


(32) 


13.  If  a  net  is  worked  in  relatively  strong  current,  as  in 
the  case  of  high  speed  trawling,  the  weight  of  the 
rope  being  negligible  compared  to  its  resistance, 
then  the  next  relation  holds  approximately: 


Dr 

--     --    A 
D'r 


(33) 


14.  With  regard  to  the  sinkers  or  floats,  if  the  resistance 


MODERN    FISHING    GEAR    OF    THE   WORLD 


is  negligible,  compared  to  the  apparent  weight  or 
buoyancy,  the  relations  (27)  are  simplified  to 

DV    P%  -  P'W    n* 

-._ _ __  ,-  HA.  (34) 

D'a3      p'a  —  P'W      n' 

SOME  MODEL  TESTS  OF  FISHING  GEAR 

A  number  of  model  experiments  have  been  made  for 
various  fishing  nets  since  Tauti  presented  the  law  of 
similarity  in  1934.  A  short  description  of  these  studies  is 
given  below  and  an  overall  picture  of  the  investigations 
is  given  by  the  bibliography. 

Fixed  nets 

The  fixed  net  is  one  of  the  most  important  and  popular 
of  gear  in  the  coastal  and  inshore  waters  of  Japan.  In  set- 
ting out  the  nets,  fishermen  first  place  the  frame-work  of 
ropes  with  the  sand  bags  and  buoys  in  position  and 
attach  the  bag  net  or  impounding  net  afterwards.  These 
nets  are  subjected  to  tidal  currents.  Excessive  alteration 
in  shape,  caused  by  a  strong  current,  may  hinder  the 
fish  from  entering  the  net;  a  very  strong  current  induces 
an  enormous  tension  in  the  net,  and  tends  to  drag  the 
sand  bags  or  anchors  which  serve  to  moor  it.  It  is  very 
laborious  and  often  impossible  to  haul  the  net  under 
such  conditions.  Sometimes  the  entire  net  is  swept  away. 
Figure  7  shows  a  model  test  of  a  very  popular  trap 
net  for  fishing  yellow-tail.  The  bagnet  is  seen  at  the 
right  hand  side,  the  sloping  funnel  for  entrapping  the  fish 
is  in  the  middle  and  the  impounding  portion  is  at  the  left. 
In  this  case  the  current,  the  full-scale  speed  of  which  is 
1  knot  (50  cm./sec.)  runs  from  left  to  right.  It  is  clearly 
seen  that  the  bottom  of  the  net  is  deformed  by  the  strong 
current.  This  is  an  extreme  case  which  rarely  occurs  in 
actual  practice.  Under  such  conditions,  almost  all  floats 
arc  pulled  under.  Owing  to  the  decrease  of  the  angle  // 
in  the  equation  (18),  this  increases  the  holding  power  of 
the  sand  bags  and  at  the  same  time  reduces  the  pull  they 
have  to  withstand.  A  minimum  number  of  floats  should 
therefore  be  used. 

Towed  nets 

Almost  all  types  of  towed  nets  arc  used  in  Japan.  Their 
efficiency  depends  on  their  mechanical  behaviour  in 


action,  especially  the  working  gape,  fishing  depth,  or 
degree  of  contact  of  the  footrope  with  the  sea-bed.  Model 
experiments  carried  out  on  these  problems  are  not  yet 
satisfactory. 

Figure  8  shows  an  example  of  a  model  test  of  a  shrimp 
beam  trawl  commonly  used  in  the  Seto  Inland  Sea. 

Encircling  nets 

Although  the  purse  seine  is  one  of  the  most  important 
gear  in  the  commercial  pelagic  fisheries,  few  model 
experiments  have  been  conducted.  These  nets  undergo  a 
marked  change  in  shape  during  setting  and  pursing.  The 
speed  of  this  transformation  and  the  maximum  depth  to 
which  the  bottom  margin  could  reach  are  important 
factors  in  the  fishing  capacity  of  the  net. 

When  the  purse  seine  is  operated  in  a  region  of  strong 
underwater  current,  the  lower  part  of  the  net  sometimes 
becomes  entangled.  The  solution  has  yet  to  be  found  to 
this  urgent  problem. 

Other  nets 

The  Genziki-ami  (105)  is  a  bottom  drift  net  of  a  rather 
peculiar  type.  It  consists  of  a  single  wall  of  webbing,  the 
lower  margin  of  which  is  curved  to  form  a  pouch.  The 
net  is  drifted  over  the  sea-bed  by  means  of  the  tidak 
current.  It  is  used  to  catch  shrimps  in  waters  where  the 
bottom  current  is  strong. 

FULL  SCALE  TESTS     USING  UNDERWATER 
MEASURING  INSTRUMENTS 

Model  experiments  obviously  have  their  limitations, 
mainly  due  to  the  difficulties  in  complying  accurately  with 
the  rules  of  similarity  between  the  actual  gear  and  its 
model.  Therefore,  final  tests  at  sea,  using  the  full-size 
gear,  are  indispensable.  For  this  purpose  underwater 
measuring  instruments  of  various  types  have  been  de- 
vised, such  as: 

Depth-meter  to  measure  the  fishing  depth  of  the  gear. 

Dynamometer  to  measure  the  tension  in  the  warps  and 
ropes. 

Clinometer  to  measure  the  tilt  of  a  rope  or  other 
accessorv. 


Fig.  7.     Example  of  a  model  test  of  a  set-net. 
flow  is  from  left  to  right. 


Photo  by  Honda. 
The  direction  of 


Photo  by  Tanigucht 

Fig.  8.    Example  of  a  model  test  of  a  trawl. 


[182] 


MECHANICAL    STUDY     OF    FISHING     GEAR 


Differential  manometer  to  measure  the  vertical  dis- 
tance between  two  points. 

Attack-angle  meter  to  measure  the  angle  of  attack  of 
an  otter  board. 

Spread  meter  to  measure  the  horizontal  distance  be- 
tween two  points  in  the  net. 

These  instruments  are  all  provided  with  a  self-record- 
ing appliance,  and  are  constructed  to  withstand  rough 
handling  and  high  pressure  in  deep  seas. 

Literature 

Parenthesized  papers  are  written  in  Japanese.  Asterisks  before 
the  titles  denote  that  an  English  synopsis  beside  the  Japanese  text 
is  given.  The  following  js  the  key  to  the  abbreviations  of  the  name 
of  scientific  periodicals  cited. 


Bulletin  of  the  Fisheries  Research  Board  of  Canada. 

Bulletin  of  the  Faculty  of  Fisheries,  Hokkaido  University. 

Bulletin  of  the  Faculty  of  Fisheries,  Nagasaki  University. 

Bulletin  of  the  Japanese  Society  of  Scientific  Fisheries. 

Bulletin  of  Tokai  Regional  Fisheries  Research  Laboratory. 

Deutsche  Hydrographische  Zeitschrift. 

FAO  Fisheries  Bulletin. 

Fishery  Investigation  (Supplementary  Report),  The  Im- 
perial Fisheries  Experimental  Station. 

Journal  of  Applied  Physics,  Japan. 

Journal  of  the  Imperil  I  Fisheries  Institute. 

Journal  of  the  Kagoshima  Fisheries  College. 

Journal  of  the  Oceanographic  Society  of  Japan. 

Journal  of  the  Tokyo  University  of  Fisheries. 

Memoirs  of  the  College  of  Agriculture,  Kyoto  University. 

Memoirs  of  the  Faculty  of  Fisheries,  Kagoshima  Univer- 
sity. 

Progress  Report  of  the  Pacific  Coast  Station. 

Report  of  the  U.S.  Experimental  Model  Basin. 

Report  of  the  David  W.  Taylor  Model  Basin. 

Report  of  the  Faculty  of  Fisheries,  Prefect ural  University 
of  Mie. 

Tciti-Gyogyo-Kai  (The  World  of  the  Fixed  Net  Fisheries), 

Technical  Report  of  Fishing  Boat. 


B.F.C. 

B.F.H. 

B.F.N. 

B.S.F. 

B.T.L. 

D.H.Z. 

F.F.B. 

F.l.S. 

J.A.P. 

J.F.I. 

J.K.C. 

J.O.S. 

J.T.F. 

M.A.K. 

M.F.K. 

P.P.C. 
R.E.M. 
R.T.M. 
R.U.M. 

TG.K. 
T.R.F. 

I  Carruthers,  J.  N.,  A.  J.  Wood  and  A.  J.  I  ce.  On  the  instru- 
mental measurement  of  line  shape  underwater.  D.H./.  7(1/2),  1954. 

-  Carruthcn>,  J.  N.  Some  simple  occanographical  instruments  to 
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3  dc  Boer,  P.A.  Trawl  gear  measurements  obtained  by  under- 
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4  Fukutomi,  T.   *A  mjthod  of  approximate  estimation  for  lee 
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fi  Fujitu.  H.  and  T.  Yokota.  The  drag  action  on  a  net  in  a  uniform 
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6  Fujita,  H.  The  drau  action  on  a  net  in  a  uniform  current.  II. 
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7  Hamuro,  C.  *A  study  of  Danish  seine  by  means  of  recording 
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8  Hamuro,  C.  and  K.  Ishii.  *Study  on  the  automatic  net  depth 
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9  Hamuro,  C.   *Study  of  the  automatic  net  depth  meter,  net 
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10  Hayashi,  H.    *An  investigation  of  a  spreading  device  for 
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II  Hosino,  S.  and  N.  Sato.  (An  experiment  on  the  forms  of  the 
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12  Isouti,  N.  and  T.   Kawakami.   Mechanical  studies  on  the 
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13  Kawakami,  T.  and  H.  Tubota.  On  the  configuration  and  distri- 
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14  Kawakami,  T.  Mechanical  action  of  the  otter  board  of  the 
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16  Kawakami,  T.  Equilibrium  configuration  of  a  rectangular 
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16  Kawakami,  T.  Mechanical  characters  of  the  drag  net.  M.A.K. 
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17  Kawakami,  T.  and  Y.  litaka.  On  a  simple  instrument,  Siomi- 
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18  Ketchen,  K.  S.  Preliminary  experiments  to  determine  the 
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20  Kobayashi,  K.  and  H.  Takahashi.  *A  study  on  fishing  trawl 
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21  Kobayashi,  K.  and  T.  Deguchi.  *An  experiment  of  a  beam- 
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22  Kobayashi,  K.,  H.  Takahashi  and  M.  Ucno.    *  Fundamental 
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24  Koga,  S.  *On  the  extension  of  the  wing  of  trawl  net.  B.F.N.  3 
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25  Kumakori,  T.,  C.  Hamuro  and  K.  Ishii,  O.  Furuya.  (Catch 
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27  Kumakori,  T.,  C.  Hamuro,  K.  Ishii  and  O.  Furuya.  (Catch 
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34  Nomura,    M.   and  Y.   Nozawa.    ^Resistance  of  plane  net 
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36  Nomura,  M.  and  K.  Mori.  *  Resistance  of  plane  net  against 
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37  Okuno,  H.    *  Model  experiment  on  plates  to  sink  an  end 
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38  Pode,  L.  An  experimental  investigation  of  the  hydrodynumic 
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39  Pode,  1..  Tables  for  computing  the  equilibrium  configuration 
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40  Pode.    L.    Configuration   and    tension   of  a    light   flexible 
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41  Sato,   N.   and    1.  Takayama.    *  Experiments  on    hydrofoil 
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42  Sato,  N.  and  S.  Kurita.  (An  experiment  on  the  resistance 
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43  Taniguchi,  T.  *On  the  resistance  of  various  codends  fixed 
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44  Taniguchi,   T.    *On    the    resistance    of    various    codends 
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46  Taniguchi,  T.  *On  the  resistance  of  various  codends  fixed 
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46  Taniguchi,  T.  *On  the  resistance  of  various  codends  fixed 
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47  Tauti,  M.,  T.  Miura  and  K.  Sugii.  Resistance  of  plane  net 
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48  Tauti,  M.  (Holding  power  of  anchors).  T.G.K.  77,  1930,  16. 


[183] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


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**  Thews,  J.  G.  and  L.  Landweber.  The  tension  in  a  loop  of 
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56  Honda,  K.  and  collaborators.  Model  experiments  on  fixed 
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56  Kanamori,  M.  *A  tentative  experiment  on  the  improvement 
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58  Miyamoto,  H.  The  form  of  amber-fish  keddle  net  on  the 
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69  Miyamoto,  H.  *  Model  experiments  on  sea  fixed  nets  for 
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60  Miyamoto,  H.    *  Model  experiments  on  sea  fixed  net  for 
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61  Miyamoto,  H.  *  Model  experiments  on  sea  fixed  net  for  fishing 
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62  Miyamoto,  H.  *  Model  experiments  on  sea  fixed  net  for  fishing 
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63  Miyamoto,  H.    *  Model  experiments  on  sea  fixed   net    for 
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w  Miyamoto,  H.  *  Model  experiments  on  sea  fixed  net  for  fishing 
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65  Miyamoto,  H.  *  Model  and  full  scale  experiment  on  set  net. 
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66  Miyamoto,  H.  *  Model  and  full  scale  experiment  on  set  net. 
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e7  Miyamoto,  H.  *  Model  and  full  scale  experiments  on  set  net. 
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68  Miyamoto,  H.  'Study  on  the  set-net.  B.T.I..  2,1951,  I. 

69  Miyazaki,  T.   *  Model  experiments  on  fixed  nets  for  fishing 
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70  Miyazaki,  T.  (Model  experiment  on  the  leading  net).  B.S.K 
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71  Miyazaki,  T.  (Model  experiment  on  a  leading  net.  II.)  B.S.F, 
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72  Miyazaki,  T.  (Model  experiment  on  a  fixed  net).  B.S.F.  77(3). 
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73  Mori,  Y.  *  Model  experiment  on  the  sea  fixed  net  for  fishing 
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74  Okahe,  G.  *  Model  experiments  of  a  fishing  net,  Anko-ami. 
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76  Saito,  T.  *Model  experiments  on  the  two  kinds  of  sea  bottom 
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77  Sato,  N.  *On  a  tank  experiment  with  seine  nets.  J.F.E.  .?. 
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79  Simizu,  H.  '"Model  experiments  on  the  sea  bottom  fixed  nets 
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H0  Sugano,  S.  *The  model  experiments  on  fixed  nets  for  fishing 
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81  Barraclough,  W.  E.  and  W.  W.  Johnson.    A  new  mid-water 
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82  Imamura,  Y.  (Model  experiment  of  the  British  herring  trawl) 
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83  Imamura,  Y.  (Model  experiments  on  a  trawl  net).  B.S.F.  77(2), 
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84  Koike,  A.  *  Model  experiments  on  a  Teiso-hikiami  (trawl  net). 
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86  Miyamoto,  H.  The  model  experiments  on  drifter  drag  net 
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90  Miyazaki,  T.  The  relation  between  the  mouth  height  of  the 
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01  Nomura,  M.  *  Model  experiments  on  two  boat  type  trawl  net. 
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92  Nomura,  M.  and  T.  Yasui.   *  Model  experiments  on  trawl 
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93  Ogura,  M.  *Model  experiment  on  a  Tyus(9~hikiami(\rw\  net). 
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94  Saito,  T.  and  H.  Simizu.  Model  experiments  on  *47>#//n-ij/w", 
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95  Sato,   N.     (An  experiment  on  the  use  of  a  surface  trawl  net.) 
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98  Sato,  N.  (An  experiment  with  models  of  a  surface  seine  net). 
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97  Sugano,  S.  *  Experiments  with  models  of  a  surface  trawl  net 
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(Encircling   net) 

98  litaka,  Y.   Model  experiments  on   the  sardine  purse  seine 
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9U  litaka,  Y.  Model  experiments  on  the  sardine  purse  seine 
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100  litaka,  Y.  Model  experiments  on  the  sardine  purse  seine 
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101  litaka.  Y.   Model  experiments  on  the  sardine  purse  seine 
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103  Uno,  M.  The  form  and  the  tension  on  pursing  line  of  a 
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(Other  nets) 

104  Pack,  Y.  Model  experiments  on  a  saury  blanket  net  (Boukc- 
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105  Wang,  Y.  Model  experiments  of  Genziki-anu\  a  kind  of  net 
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184 


INCREASING   THE   OPENING    HEIGHT   OF   A   TRAWL   NET   BY 

MEANS   OF   A   KITE 

by 

S.  TAKAYAMA  and  T.  KOYAMA 

Tokai  Regional  Fisheries  Research  Laboratory,  Tokyo,  Japan 

Abstract 

In  order  to  catch  fish  that  swim  close  to  the  bottom  but  not  actually  on  it,  it  is  necessary  to  have  the  opening  of  the  trawl  as  high 
as  possible,  without  unduly  distorting  the  shape  of  the  net.  To  do  this,  a  new  type  of  trawl  kite  has  been  designed  and  tested.  This  paper 
describes  the  rigging  of  a  model  trawl  and  kite,  which  can  move  freely  along  a  false  headline,  and  shows  how  the  experiences  with  the  model 
were  translated  to  a  full-size  trawl.  In  the  tests,  three  differently  rigged  trawls  were  used,  (a)  without  the  kite,  (b)  with  the  kite  but  with  no 
gussets  between  the  after  end  of  the  wing  and  the  square,  and  (c)  with  both  kite  and  gussets.  In  net  (c),  the  net  mouth  was  raised  to  twice 
the  height  of  the  control  net,  and  in  net  (b)  it  was  raised  1  '5  m.  more  than  the  control. 


Resume 


L "augmentation  de  la  Hauteur  d'ouverture  d'un  chalut  au  moyen  d"un  panneau  elevatcur 


Pour  pouvoir  capturer  Ic  poisson  qui  nage  tout  pres  du  fond  sans  le  toucher,  il  est  necessaire  que  1'ou venture  du  chalut  soit  aussi 
haute  que  possible  sans  deformcr  exagerement  le  filet.  Un  nouvcau  type  de  plateau  elevateur  a  ele  mis  au  point  et  essayg  a  cet  eflfet.  Les 
auteurs  dccrivent  le  montage  de  la  maquette  du  chalut  et  du  plateau  elevateur  qui  peut  se  deplacer  librement  le  long  d'unc  fausse  ralinguc 
sunerieure,  et  notamment  comment  les  resultats  des  essais  cflcctucs  sur  la  maquette  ont  6te  convertis  en  donnees  applicable*  a  un  chalut  de 
dimensions  normales.  Les  essais  ont  porte  sur  des  chaluts  grees  de  trois  fagons  differentes:  (a)  sans  le  plateau  eJcvateur;  (b)  avec  le  plateau 
elevateur  mais  sans  goussets  entre  I'extreniit^  posterieurc  de  Pailc  ct  le  grand  dos,  et  (c)  avcc  le  plateau  ct  les  goussets.  Avec  le  chalut  (c), 
la  hauteur  de  gucule  atteignait  le  double  de  celle  du  filet  temoin,  et  avec  le  chalut  (b),  1  m.50  de  plus  que  celle  du  temoin. 


Aumento  de  la  altura  de  la  boca  de  las  redes  de  arrastre  mediante  "pucrtas  de  clevacidn." 
fcxtracto 

A  fin  de  capturar  los  peces  que  nadan  ccrca  del  fondo  del  mar  pero  no  immediatamente  sobre  el,  es  necesario  abrir  la  boca  de  la 
red  al  maximo  sin  dcfomar  excesivamentc  su  forma.  Para  lograr  estc  objeto,  se  ha  proycetado  y  ensayado  un  nucvo  tipo  dc  **puerta  de 
elevaci6n." 

En  este  trabajo  so  describe  la  construction  de  un  modelo  dc  red  de  arrastre  con  su  **pucrta  clevadora"  que  puedc  movcrse  a  lo  largo 
de  una  falsa  relinga  superior  y  se  demuestra  la  manera  como  las  experiencias  con  el  modelos  pueden  aplicarse  a  una  red  de  arrastre  de  tamafto 
normal.  En  las  pruebas  se  usaron  tres  lipos  de  red  arrastre,  a  saber:  (a)  sin  "puerta  dc  clevaci6n'\  (b)  con  "puerta  de  elevaci6n"  pero  sin 
refuerzos  triangulares  entre  el  extremo  posterior  de  la  banda  o  pernada  y  la  visera,  y  (c)  con  "puerta  de  elevation"  y  refuer/os.  Fn  las  redes 
(c)  >  (b)  la  altura  de  la  voca  ere  2  veces  y  1.5  m.  mayor,  respect ivamente,  que  en  el  arte  usado  como  testigo. 


ONt  of  the  major  factors  influencing  the  catch  of 
a  trawler  is  the  area  of  ground  fished  by  the  net. 
But,  in  the  case  of  demersal  fish  such  as  cod,  sea 
bream,  hairtail,  and  prawn,  which  may  not  always  be 
quite  close  to  the  bottom,  the  opening  height  is  equall> 
important.  One  example  of  the  various  devices  being  used 
for  this  purpose  in  Japan  is  a  mouth  stretcher  for  the 
bull-trawl,  invented  by  Hayashi1.  In  practice  the  shape 
of  a  net  should  be  maintained  with  the  mouth  as  high  as 
possible.  But  actually  no  trawl  net  can  keep  a  constant 
shape  all  the  time  because  of  the  interfering  influences  of 
currents,  undulation  of  the  sea-bottom  and  distortion  of 
the  wings  due  to  movement  of  the  boat. 

In  order  to  overcome  these  difficulties,  a  new  type  of 
trawl  kite  has  been  developed.  This  kite  adjusts  its  posi- 
tion automatically  according  to  the  movement  of  the 
net.  After  preliminary  model  tests  in  a  tank  with  circulat- 


ing flow,  a  series  of  field  experiments  was  conducted  b> 
the  R.V.  Tenyo-Maru  and  the  Taka  Maru  of  the  Tokai 
Regional  Fisheries  Research  Laboratory  in  Tokyo  Bay 
in  September  1956,  and  more  recently  by  Taiyo-Maru 
No.  32  of  the  Taiyo  Fishing  Company  in  the  central  part 
of  the  Yellow  Sea  from  February  to  April  1957.  This 
paper  gives  the  results  obtained  from  these  experiments. 

MODEL  EXPERIMENTS 

Method 

Model  nets  with  and  without  the  new  device  were  tested 
comparatively  in  a  glass-walled  tank  with  an  effective 
area  of  65  cm.>  1  m.  The  nets  were  subjected  to  various 
velocities  of  flow.  The  shape  of  the  net  was  controlled 
and  the  elevations  of  the  net  mouth  and  the  kite  were 
measured. 


M85] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


Construction  and  Specifications  of  the  Nets 

Several  arrangements  were  made  for  constructing  model 
B  and  C,  with  kite  (Table  I  and  figs.  1,  2,  3). 


TABLE  I 
Specifications  of  Model  Net 

Name  of  part  Material     Length  or  number      Remarks 


False  head  line 

__ 

133  cm.  (total) 

Floated  part 

Cotton 

106   „ 

4  'Riding  wire"  for 

kite 

Copper 

27   .. 

Head  line 

— 

164  „  (total) 

Bo'om 

Cotton 

37   ,. 

Wings 

%< 

127   „ 

Connecting  Legs 

8.7  cm. 

Sec  fig.  3-L 

Gussets* 

Silk" 

2  pieces 

See  fig.  3-S 

Floats 

Cork 

16  „ 

Total  buoy- 

ancy 1.2  gr. 

*  Mesh  1.3  cm.,  11x11  meshes,  diagonally  cut  into  two,  one  of 
each  inserted  into  the  square  at  the  quarter  points. 


(1)  The  false  headline  is  not  fixed  to  the  kite,  but 
passes  freely  through  rings  attached  to  the  bridles  of  the 
kite,  which  consequently  can  slide  smoothly  on  the  wire 
rope  used  for  the  middle  part  of  the  false  headline.  This 
arrangement  helps  to  prevent  deformation  of  the  net. 

(2)  Each  end  of  the  false  headline  is  tied  to  either  end 


Fig.  I.     General  view  of  model  net  "C"  with  kite. 


2*9 


31 


r 


43 


26 

1 


10 

1 


20    30    40    60 


Fig.  2.     Diagram  of  the  model  kite. 


Material 
Weight 
Buoyancy 
Volume 
Specific  gravity 


Wooden  board,  cedar 

2-4  x. 

/-5  g. 

3-9  cmj 

0-62 


of  the  upper  wing  so  as  to  make  handling  of  the  net  as 
easy,  and  lift  the  wing  as  high,  as  possible. 

(3)  A  number  of  floats  are  attached  to  the  false  head- 
line to  keep  it  clear  of  the  net. 

(4)  In  type  "C"  net,  a  triangular  gusset  webbing  is  in- 
serted into  the  square  at  both  quarter  points  to  extend  the 
length  of  the  headline  in  the  bosom.  This  is  meant  to 
prevent  strain  in  the  webbing  when  the  upper  net  is 
pulled  up  by  the  kite. 

The  sizes  of  the  kite,  false  headline,  gussets,  and  so 
forth,  of  course,  have  to  be  in  correct  proportion  to  the 
rest  of  the  trawl  gear.  For  that  purpose  their  measure- 
ments have  been  determined  on  the  basis  of  Tauti's  law 
of  comparison",  by  an  experiment  using  a  model  of  a 
146  ft. -trawl.  Specifications  of  the  model  net  were  ob- 
tained as  follows: 

(1 )  The  measurements  of  the  model,  >.',  arc  reduced  to  one  thirtieth 
of  the  full  scale,  >.". 


Fig.  3.     Schematic  diagrams  of  model  nets  "A'\  "/?"  and  "C" 

",4"      Without  kite. 

"B"     Furnished  with  kite  but  without  gussets. 

"C"     Furnished  with  kite  and  gussets. 


[1861 


USING    A    KITE    TO    INCREASE    VERTICAL    OPENING 


(2)  For  the  net  twine  of  the  model,  silk  is  used  on  the  basis  of 
the  ratio 

D'  L' 

-.- 0-094 

D'  L* 

where    D'  diameter  of  twine  of  the  model  net 
D*  „          „      „        the  full  scale  net 

L'  mesh  size  of  the  model  net 
L"  „         .,         the  full  scale  net 

(3)  Using  0-094  as  the  mesh  size  ratio  between  both  nets,  1-25  as 
the  specific  gravity  of  silk,  p',  for  the  model  net's  twine  and 
I  -43  as  the  specific  gravity  of  abaca,  p",  for  the  full  scale  nets, 
one  may  obtain 


/  L'(p'-  1)  /  0-094  -  (1-25 

"     V  L"(P"  -  1)      V          1-43-1 


1) 
—        0-234 


where  V  and  V"  denote  the  current  velocities  at  which  both 
model  and  full  scale  net  have  the  same  shape. 
(4)  We  can  ignore  the  hydraulic  resistance  of  the  ropes  of  a  fishing 
net.  When  P^represents  the  diameter  of  cotton  twine  used  for 
the  ropes  of  the  model  net:  D",,  the  diameter  of  the  combination 
rope  for  the  real  trawl;  p't,  the  specific  gravity  of  cotton  (1-5), 
and  p"j,  the  specific  gravity  of  the  combination  rope  (4-6), 
then  the  ratio  between  D',  and  D'l  in  both  nets  conforming 
with  each  other  is: 


D' 


30 


4-6      I1 

•0547  •    

1-5       1 


0-115 

For  the  net  twines  made  of  the  same  material  in  both  nets,  the 
ratio  of  their  diameters  is: 


0547  x  I1    -  0  0425 


(5)  Disregarding  the  hydraulic  resistance  of  the  float,  the  ratio 
between  the  buoyancy  of  the  model  floal,  F',  and  the  full  scale 
one,  F*,  is  represented  by 

F'          />/\2 

6 


/>/\2  /V'\ 
(-)  .  (  ) 
\>.V  \V"/ 


. 

This  ratio  may  also  be  applied  in  regard  to  underwater  gravity 
of  the  foot  ropes  of  those  nets. 

After  the  model  net  and  the  kite  had  been  constructed 
according  to  the  above  proportions,  the  optimal  pro- 
portion to  obtain  maximum  height  of  the  net  mouth 
and  the  best  shape  to  the  net,  was  determined  by  repeated 
tests.  The  results  are  given  in  Table  I  and  fig.  2.  Addi- 
tional experiments  with  kites  of  different  shape,  i.e. 
rectangular,  with  the  height  greater  than  the  width, 
rectangular  with  the  width  greater  than  the  height,  and 
streamlined,  revealed  that  the  first  was  more  practical 
than  the  others,  though  somewhat  less  efficient,  because 
of  its  simple  construction  and  superior  underwater 
stability.  Best  results  were  obtained  with  an  angle  of 
attack  of  about  30  degrees. 

Comparison  of  Height  of  Net  Mouth 

The  three  types  of  model  trawls  tested  are  shown  in 
fig.  3.  They  are:  A  (without  kite),  B  (kite  but  no  gussets), 
C  (kite  and  gussets).  In  type  "B"  the  length  of  the  false 
headline  was  124  cm.  or  85  per  cent,  of  the  length  of  the 
headline.  The  specifications  of  "C"  are  given  in  Table  I 
and  figs.  1  and  2. 

The  experiments  show  that  with  a  distance  of  26  cm. 
between  the  wings  the  opening  height  of  the  type  "C" 
net  is  about  three  times  that  of  type  "A",  and  with 


Hg.  4,     Model  tests  with  trawl  gear  type  "A"  (top),  "B"  (middle} 

and  "C"  (below)  at  a  speed  of  flow  ofO  •  29  m.fsec.,  corresponding 

to  2-5  knots  with  full  scale  gear. 


53  cm.  about  twice.  Although  in  no  actual  case  would 
the  distance  between  the  wing  tips  be  as  narrow  as  7*8  m., 
i.e.  26  cm.  30,  this  was  tested  for  preliminary  informa- 
tion. With  type  "C"  net,  the  opening  height  depends 
obviously  not  only  on  the  distance  between  the  wings, 
but  also  partly  on  the  size  of  the  gussets.  This  means  that 
for  determining  the  proper  size  of  the  gussets  the  distance 
between  the  wings  must  be  considered.  Type  "B"  net 
was  found  to  come  between  "C"  and  "A".  Fig.  4  shows 


TABU  II 
(  omparison  of  the  Height  of  the  Net  mouth  of  Various  Model  Trawls  * 


Type  of 
trawl 


Speed  of 
flow 

(/W..AIT.) 


Distance  between 
wing  tips  53  cm. 


Distance  between 
wing  tips  26  cm. 


Height  of  Height  of  Height  oj  Height  of 
kite  net  mouth  kite  net  mouth 
(cm.)  (cm.)  (cm.}  (cm.) 


0  29 

5-7 

6-3 

A 

0-35 

__ 

5-3 

60 

0-41 

._. 

5-0 

5-7 

0-29 

16-0 

9-1 

17-3 

12-7 

B 

0-35 

16-0 

93 

16-0 

11-0 

0-41 

13-0 

8-3 

15  0 

10-0 

0-29 

18-3 

11-7 

23-3 

17-0 

C 

0-35 

18-3 

11-7 

22-6 

16  0 

0-41 

17-0 

11-0 

21-7 

15  3 

*  Height  measured  from  the  bottom  of  the  tank. 


187 


MODERN     FISHING     GEAR     OF    THE     WORLD 


f-'ig.  5.     Schematic  views  of  figuration  oj  model  trawls. 
D  :  Normal  figuration  of  net  "C"  (with  kite). 
E  :   Kite  adjust  ing  distortion  of  net  "C". 
F  :    Distortion  of  net  ".4"  (without  kite). 

I  he  various  types  of  model  nets  with  a  flow  speed  of 
0-29  m./sec.,  corresponding  to  2-5  knots  in  full  scale. 

Comparison  of  Net  Shapes 

During  the  tank  tests  observations  were  also  made  in 
regard  to  the  deformation  caused  by  one  wing  being 
fixed  forward  of  the  other.  The  distance  between  the 


11 


jra 
"if 


¥  ! 


IP-/--^-7          0 

r»  f  »»f:ro.n  pi«t«j   10    10    30    40    »0C1" 


/•/#.  6.     Diagram  of  the  full  scale  kite  constructed  according  t<> 
the  model. 

Material  Wooden  board,  cetlor. 

Weight  80  kg. 

Biioyancv  23  kg. 

Volume  103,000  cm* 

Specific  gravity  0  •  78 

wings  tips  was  53  cm.  and  the  speed  of  flow  was  0-3  m  / 
sec.,   0-35    m./sec.    and   0-4    m./sec.    respectively. 


with   klt« 
(Uj:j.  er   part) 


without  Kite 
(Upper  part) 


/T^-.   7.     Detailed  diagram\  uf  ihc  full  \calc  trawls   with  ami  without  kite. 

Headline  for  the  trawl  without  kite.  Ropes  and  accessories  jor  the  trawl  with  kite. 

Total  length  :  44  2  m.,  wings  :  /V-  /  /;/..  bosom  :  6-0  m  Headline,  total  length:  49- 2m..  wings  :  /V*  /  ///..  hositm:  II  V  m 

False  headline,  total  length  :     40-0  m. 

for  floated  part,  combination  rope.  2  cm.  diam.,  two  lines,  each:  16  -O  m 
"riding  wire"  for  kite,  wire  rope.  12  mm.  diam.  :  8-0  m. 
Connecting  leg.  two  lines,  tied  at  the  centre  of  the  bosom*  each  :  2-6  m 
Floats,  cylindrical,  plastic  sponge,  buoyancy  0-2  kg.  per  piece:  I00piece\ 

I  188  1 


USING     A     KITE    TO     INCREASE     VERTICAL    OPENING 


It  was  found  that  the  kite  adjusted  itself  automatically 
according  to  expectation  (see  fctE"  in  fig.  5).  Further- 
more, nets  "D"  and  "E"  had  the  same  opening  heights. 

Conversion  of  the  Kite  from  Model  to  Full  Scale 

The  full  scale  kite  is  constructed  from  the  model  accord- 
ing to  Tauti's  ratio: 


/>-'(F'S      D 

-"~V  ;r<7V-"i; 


V 

V 
where       —       --  speed  ratio  0-234 

V" 

/ 
—   reduction  scale  1/30 

p'2     --  specific  gravity  of  model  kite,  0-62 

p%     -  specific  gravity  of  full  scale  kite  (calculated 
0-78)    see   fig.    6. 

Here,  some  degree  of  error  is  to  be  expected,  partly 
because  of  a  difference  in  Reynolds  number  for  a  small- 
sized  model  and  partly  because  the  kites'  are  not  absolute- 
ly similar  geometrically. 

The  false  headline  and  connecting  leg  for  the  full  scale 
gear  were  also  dimensioned  on  the  basis  of  the  conversion 
factors  mentioned  above  (fig.  7).  For  the  scale  1/30  the 
ratio  of  the  number  of  meshes  for  the  gussets  between 
model  and  full  scale  was  computed  to  be  0-36  and  the 
ratio  of  mesh  size  0-094  (fig.  7).  On  the  basis  of  the  model 
lest  result  (Table  11)  the  heights  of  the  kite  and  net  mouth 
in  full  scale  were  estimated  for  speeds  of  flow  as  shown  in 
figs.  8  and  9. 

Estimation  of  Hydraulic  Resistance  in  various  parts  of 
the  Kite  gear 

The  extent  to  which  the  kite  could  affect  the  distance 
between  the  otter  boards,  and  thus  the  distance  between 
ihe  wing  tips,  could  not  be  determined  by  the  model 
experiments.  Although  no  reliable  value  for  the  total 
hydraulic  resistance  of  the  trawl  can  be  given,  a  ver> 
rough  estimate  may  be  of  interest.  To  simplify  matters 
the  problem  will  be  considered  as  two  dimensional,  b> 


calculating  first,  the  resistance  of  each  part  of  the  kite 
gear,  then  the  ratio  of  the  resistance  of  these  parts  to  the 
total  resistance.  Buoyancy  and  underwater  gravity  are 
neglected.  The  elevation  of  the  kite  and  the  resulting  angle 
of  the  ropes  will  be  computed  on  the  basis  of  the  data 
obtained  in  the  model  experiments. 

The  Kite 

Let  P,  (kg.)  be  the  resistance  of  the  kite  when  the  direc- 
tion of  flow  is  normal  to  its  plane,  then 

?!  ~Cx  -        v2     s 

2        g 

density  of  water  in  Kg./m.3 
acceleration  of  underwater  gravity  in  m. /sec- 
velocity  of  the  current  in  m./sec. 
plane  area  of  the  board  in  m2 
coefficient  dependent  on  function  of  Reynolds 
number. 

If  the  kite  has  an  angle  of  attack  0,  the  resistance  is  Po, 
according  to  Duchemin's  equation: 

2  sin  0 


where 


P 

8v 

S 
C\ 


Ptl 


I 


Assuming  that  the  kite  is  1-3 


sin20 
0-93  m.  in  size:  C\ 


J-13; 


0-  3T  and          104  kg.  sec'-VmA  the  resistance  at  differ  - 

g 
ent  speed  of  flow  is  given  below  (Table  III). 


T\B1>      111 


Speed  of  Hov\ 
knots 

2 
3 
4 


Resistance 

kg. 

59 

134 

236 


False  Headline 

As  the  false  headline  consists  of  wire  rope  for  the  riding 
wire  of  the  kite,  and  the  combination  rope  partly 
equipped  with  floats,  the  resistance  has  to  be  calculated 
for  each  part.  The  angle  of  attack  of  the  false  headline  is 
assumed  to  be  16  degrees,  as  was  the  case  with  the  model 


Plow  ap«»d   3,0  knots 


3,0   kn*ts 


Fig.  8.     Height  oj  kite  ami  net  mouth  with  different  speeds  of 
flow  computed  from  the  model  test  results  ( Table  //,  Distance 
between  wing  tips  53  cm.) 


189 


Fig.  9.     Height  of  kite  and  net  mouth  with  different  speed*  oj 
flow  computed  from  the  model  test  results  (Table  //,  Distance 
between  wing  tips  26  cm.). 


MODERN     FISHING     GEAR     OF    THE    WORLD 


Fig.  10.     Echoes  from  the  trawl  type  **C"  towed  at  2-5  knots. 
Date  :       Sept.  8,  7956,  0735  hours. 

TENYO-MARU 

Direction          :     S  45C1  W  (head  wind). 
Angle  between  warps  :     77° 

TAKA-MARU 

Speed  :       3-8  knots. 

Direction         :       S  44"  W. 


experiments.  The  resistance  of  a  unit  length  of  a  rope  at  an 
angle  of  attack  0  is 


PO  -   —  Cx  —          D     •     V2 

2  g 

where  D  is  the  diameter  of  the  rope  in  metres. 


sin 


When  Cx  is  1-2,  and  —  is  the  same  as  in  the  last  equation, 

g 

the  values  of  PO  for  each  length  of  the  present  type  at  false  headline 
are  tabulated  below:  (Table  IV). 


TABLL    IV 


Speed  of  Resistance  (Kg.)  of 

Flow        Combination  rope         floated  part  Wire  rope 

(knots)    D:0-021  m.  L:\6m  D:0-055  w,L:16  wD:0-012m,  L:R 


5-9 
13-2 
23-5 


15-4 
13-2 
61-6 


1-7 

3-8 
6-7 


The  Connecting  Legs 

If,  for  the  calculation,  the  same  angle  of  attack  is  used  as 
was  found  in  the  model  experiments  (44  degrees),  the 
diameter  of  the  rope  is  0-021  m.  and  its  length  5-2  m., 

c 
Cx=  1  -2  and  —  the  same  as  for  the  combination  rope,  the 

g 
results  obtained  are  given  in  Table  V. 


YAfcfc 


Fig.  II.     Echoes  from  the  trawl  type  "A"  towed  at  2-5  knots. 
Date  :       Sept.  8.  1956     1010  hours. 

TENYO-MARU 

Direction         :       N40{1  E(fair  wind). 
Angle  between  warps  :     79° 

TAKA-MARU 

Speed  :      3-0  knots 

Direction         :       N  39°  E. 


Fig.  12.     Echoes  from  the  trawl  type  "C"  towed  at  3-0  knots. 
Date  :       Sept.  5,  7956,  0770*  hours. 

TFNYO-MARU 

Direction         :       S  45"  W  (head  wind). 
Angle  between  warps  :    77° 

TAKA-MARU 

Speed  :      4-2  knots. 

Direction         :       S  44°  W. 


[190] 


USING    A    KITE    TO    INCREASE    VERTICAL    OPENING 


TABLE   V 


Flowing  velocity 

Resistance 

(knots) 

(Kg.) 

2 

4-7 

3 

10-6 

4 

18-8 

By  summing  up  the  resistance  of  the  different  parts  the 
total  resistance  of  the  complete  kite  gear  is  found  to  be 
87,  196  and  347  kg.  at  the  speed  of  flow  of  2,  3  and  4  knots 
respectively. 

For  trawl  type  "C"  an  additional  increase  in  the  resist- 
ance has  to  be  expected  from  the  modification  of  the 
net,  i.e.,  the  inserting  of  the  two  gussets.  At  present  there 
are  no  means  of  calculating  this  increase. 

The  resistance  of  a  common  146  ft.  trawl,  though  more 
or  less  variable  according  to  the  bottom  conditions,  has 
been  found  to  be  about  5  tons,  at  a  towing  speed  of 
3  knots  under  normal  conditions8.  Consequently  the 
resistance  of  the  kite  gear  of  196  kg.  can  be  considered 
to  be  of  minor  importance.  If,  in  a  liberal  estimation,  it 
is  assumed  that  the  increase  of  resistance  caused  by  the 
modification  of  the  net  is  of  similar  magnitude,  the  total 
increase  would  not  greatly  exceed  about  400  kg.  As  much 


increase  often  occurs  because  of  changes  in  bottom  con- 
ditions, warp  length  or  amount  of  catch  in  the  codend, 
it  may  reasonably  be  considered  that  the  kite  gear  and 
the  modification  of  the  net  would  not  critically  affect  the 
angle  of  attack  of  the  otter  boards. 

FIELD  EXPERIMENTS  IN  TOKYO  BAY 
Method  and  Equipment 

In  the  Tokyo  Bay  experiments  on  September  7  and  8, 
1956  two  146  ft.-trawls,  one  of  type  "C"  (with  kite  and 
gussets),  the  other  of  type  "A",  were  towed  at  various 
speeds,  and  their  characteristics  were  observed  by  means 
of  an  echo  sounder  installed  in  a  boat  following  the 
trawler.  Furthermore,  in  certain  intervals  the  position 
and  tension  of  the  otter  boards  were  controlled.  As  there 
was  no  intention  of  catching  fish,  the  codends  were  left 
open.  The  experimental  conditions  arc  given  below: 

Place:  Central  area  of  Tokyo  Bay,  depth  26  to  29  m.. 

bottom  flat  and  sandy. 

Trawler:  Tenvo-Maru.  230  tons,  430  h.p. 

Sounding  Boat:    Taka-Maru,  15  tons. 

Trawling  Speed:  2-5;  3-0;  and  3-5  knots  relative  to  the  current. 

Trawl  Gear:  Chose i  type,  similar  to  V.D.  type,  in  adjustment 
according  to  type  'A'  and  *C  described  above. 
Warp  length  60  m.  bridle  length  100  m.  Total 
length  of  net  approximately  44  m.,  including  20  m. 
wing,  14  m.  square  and  belly,  and  104  m.  codend 
(see  fig.  7).  Kite  1  -2  m.2  (see  fig.  6). 


Fig.  13.     Echoes  from  the  trawl  type  "A"  towed  at  3-0  knots. 
Date  :       Sept.  8,  7956,  0935  hours. 

TENYO-MARU 

Direction         :       S  45"  W  (head  wind). 
Angle  between  warps  :    22° 

TAKA-MARU 

Speed  :      4-5  knots. 

Direction         :       No  specific  course  was  set. 

Exact  measurements  of  the  figuration  could  not  be  taken  because 
the  boat  did  not  pass  properly  over  the  net. 


Fig.  14.     Echoes  from  the  trawl  type  "C"  towed  at  3-5  knots. 
Date  :       Sept.  8,  1956,  0640  hours. 

TENYO-MARU 

Direction         :       S  45  W°  (head  wind). 
Angle  between  warps  :    2J°. 

TAKA-MARU 

Speed  :      5-5  knots. 

Direction         :       S  43"  W. 

Note  the  net  clearing  off  the  bottom. 


[191  ] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


Echo-sounder: 
Log: 

Angle  Meter: 


Dynamometer: 


50  kc.,  50  m.  range,  half  beam  angle  30  degree. 
Current  meter  Model  CM-3,  electric,  one  for  each 
boat. 

To  determine  the  angle  between  the  warps  for 
calculating  the  approximate  distance  between  the 
otter  boards. 

Due  to  a  fault  in  the  instrument  the  measure- 
ments of  the  pull  on  the  warps  are  not  reliable. 


The  results  of  these  experiments  are  given  in  Table  VI. 
The  heights  of  the  kite  and  the  net  mouth  were  computed 
from  the  recordings  of  the  echo-sounder  by  the  following 
method:  When  the  trawler  Tenyo-Maru  tows  a  44  m.-long 
net  at  a  speed  V  (cm./sec.)  and  the  Taka-Maru  steams 
with  the  echo-sounder  at  V  (cm./sec.),  both  keeping  the 
same  direction,  the  length  of  time  required  by  Tako- 
Maru  to  pass  right  over  the  net  will  be 

4400  0-6x4400 

(sec.).  Then  (mm.) 

V  -V  V— V 

is  the  length  of  the  net  as  recorded  on  the  echo-sounder 
paper  running  at  0-6  mm./sec. 

Since  V  and  V  were  known,  and  if  Taka-Maru,  in 
following  the  track  of  her  partner,  passed  exactly  over 
the  nets,  the  calculated  value  should  agree  with  the 
observed  one.  If  this  was  not  the  case,  it  would  mean  that 
the  boat  with  the  sounder  was  not  following  right  above 


the  net.  By  selecting  some  of  those  data  which  coincided 
with  the  calculated  values,  we  have  plotted  the  heights 
of  the  kite  and  the  net  mouth  on  the  recording  papers 
(Table  VI,  figs.  10-14). 

To  find  the  distance  between  the  wing  tips,  the  distance 
between  the  otter  boards  was  first  of  all  calculated  on  the 
basis  of  the  length-angle  relationship  of  the  warps  pre- 
suming that  they  extend  straight  to  the  otter  board. 
Then,  from  the  isosceles  triangle  bounded  by  the  dis- 
tance between  the  otter  boards  as  the  base,  and  the  length 
of  the  bridles  (100  m.)  plus  the  length  of  the  net  (44  m.), 
both  supposedly  forming  a  straight  line,  the  distance 
between  the  wings  can  be  computed.  Such  a  calculation 
of  course  contains  errors,  but  we  have  used  it  because 
there  is  nothing  better  and  also  because  the  errors  would 
be  the  same  for  both  trawls. 

The  data  deduced  from  the  echo-sounder  records  and 
the  schematic  views  interpreted  from  them  are  presented 
figs.  10-14,  with  the  position  of  the  net  indicated  by  the 
legend  "mark  line",  which  was  a  buoy  rope  tied  to  the 
codend  to  simplify  the  detection  of  the  nets. 

Discussion 

The  distance  between  the  trawl  wings  ranged  from  5-6  to 
7-4  m.  (Table  VI  and  figs.  10-14).  Such  a  narrow  distance 
would  be  most  unlikely  to  occur  in  commercial  trawl 
operation.  However,  since  this  corresponds  roughly  to 


TABLE  VI 
Data  Relevant  to  Comparative  Experiments  of  a  146  ft.  Trawl  with  and  without  Kite  in  Tokyo  Bay,  September  8,  1956 


R.V.  Tenyo-Maru 
CRUISING  Angle    Revolution 


WIND 


R.V.  Taka-Maru  that  followed  the  net  by  an  echo-sounder 

CRUISING  HEIGHT  FROM  BOTTOM    Distance 


Time     Speed*  Direction  of  warp  of  engine    Direction  Force    Time    Speed*  Direction    Kite    Net  mouth  between  wings        Remarks 

(m) 

5-6 

5-6    Shown  in  rig.  10. 
5-6 

5-7     Shown  in  fig.  12. 
5-3 

The  net  rose  4  m.  above 
7  »4     the  bottom. 

The  net  rose  4  m.  above 
7-4    the  bottom  (fig.  14) 

The  net  rose  3  m.  above 
7-4    the  bottom. 


6-2  Shown  in  fig.  13 
6-2 

7-4 

7-0  Shown  in  fig.  13. 

7  •  4  The  net  r  -se  too  high 

7  •  4  The  net  rose  too  high 


[192] 


(knot} 

(knot)                    (m) 

(m) 

0800 

2-35 

S45°W 

17U         180 

SW          2        0748 

4-0 

S43°W        7-0 

5-9 

0755 

2-30 

«  0750 

2-35 

|  0735 

2-5 

0738 

3-8 

S44'W        7-0 

5-8 

t-  0730 

2-8 

^, 

0725 

2-7 

^ 

0724 

3-6 

S45'W        7-0 

5-8 

0715 

3-25 

18°        200 

5.  0710 

3-0 

0713 

4-2 

S44°W        7-0 

5-8 

,§  0705 

3-2 

,, 

16° 

0703 

4-5 

S45°W        7-0 

5-8 

|  0650 

3-6 

23°        248 

1        0648 

5-5 

S45°W        7-0 

5-6 

*  0645 

3-5 

f% 

,. 

„ 

* 

g  0640 

3-5 

0640 

5-5 

S43°W        7-0 

5-5 

**  0635 

3-5 

., 

„ 

0630 

3-4 

•• 

., 

0629 

5-6 

S43°W        7-0 

5-5 

J1020 

2-6 

N40°E 

20*         180 

SW          2 

1010 

2-5 

tf 

19° 

1013 

3-0 

N39°E 

2-0 

J8   1000 

2-6 

«*            j  » 

1008 

3-5 

N40°E 

2-0 

0945 

3-0 

S35°W 

23°        200 

0943 

5-5 

S35°W 

2-1 

g  0940 

3-1 

22J 

|  0935 

3-0 

S45°W 

I]            il        0936 

4-5 

— 

2-1 

5  0930 

2-8 

t% 

»             »» 

-   1035 

3-5 

N40°E 

23°        240 

1033 

5-5 

N40°E 

2-2 

%   1030 

3-5 

" 

22° 

£  1024 

3-4 

23° 

1027 

5-5 

N39°E        — 

2-6 

Neither  applicable  nor 

specified. 

*  Relative  speed  of 

the 

boat. 

USING     A     KITE     TO     INCREASE     VERTICAL     OPENING 


7-8  in.,  obtained  by  the  factor  30  from  26  cm.  which  was 
the  distance  between  the  model's  wings,  the  observed 
heights  of  the  net  mouths  seem  to  be  comparable  with 
the  value  calculated  from  the  model  tests  (fig.  9).  It  will 
be  recalled  that  for  the  models  the  net  mouth  of  type  4fcC" 
was  nearly  three  times  as  high  as  that  of  net  "A"  (Table 
II),  which  factor  closely  approximates  2-5  to  2-9  ob- 
tained in  the  full  scale  tests.  Furthermore,  it  can  be 
assumed  that  the  full  scale  nets  had  at  2-5  to  3  knots 
shapes  rather  similar  to  those  of  the  models.  At  3  -5  knots, 
the  full  scale  trawl  type  "C"  was  lifted  off  the  bottom 
(fig.  14),  and  also  the  type  "A"  gear  was  liable  to  leave 
the  bottom  (Table  VI).  This  is  probably  due  to  the  fact 
that  the  footropc,  which  was  a  little  too  light  and  had 
been  dried  before  use,  hardly  had  time  to  increase  its 
weight  by  absorbing  enough  water  during  the  experiments. 
On  the  other  hand,  the  model  nets  could  not  leave  the 
bottom  because  their  wings  were  fastened  to  the  bottom 
of  the  tank. 

The  height  of  the  full  scale  trawi  with  7-8  m.  distance 
between  the  wing  tips  and  at  2  to  3  knots  was  approximately 
in  accordance  with  the  values  calculated  from  the  model 
experiments.  It,  therefore,  may  be  assumed  that  with 
10  m.  distance  between  the  wing  tips  the  full  scale  type 
"C"  trawl  would  maintain  an  opening  height  nearlx 
twice  as  great  as  type  "A"  without  kite. 

The  net  can,  of  course,  easily  be  prevented  from  rising 
off  the  bottom  by  adding  some  weight  (chain)  to  the  foot- 
rope.  The  data  shown  in  Table  VI  mav  relieve  our  worries 
about  some  other  technical  difficulties  connected  with 
the  operation  of  the  kite.  For  one  thing,  the  angle  be- 
tween the  warps  does  not  change  to  such  an  extent  that 
a  considerable  decrease  of  the  distance  between  the  wing 
tips  has  to  be  apprehended. 

The  engine  revolutions  at  2  to  3  knots  gave  no  indication 
of  a  considerable  difference  in  the  resistance  of  both  Upes 
of  trawl. 

The  present  measurements  of  the  Joad  on  the  \\arps, 
unfortunately,  arc  not  reliable.  The  lack  of  differences  in 
(he  loads,  therefore,  is  not  conclusive.  Tor  observing 
directl>  the  kite  adjusting  itself  to  the  distortion  of  the 
net  modern  Jiving  techniques  arid  underwater  photo- 
graphy should  be  used.  I'rifortunatt'K.  circumstances  did 


not  permit  the  authors  to  use  such  aids.  However,  if  the 
similarity  between  the  results  of  the  model  and  field 
experiments  is  considered,  it  may  safely  be  assumed  that 
with  a  towing  speed  limited  to  2  to  3  knots,  the  kite  would 
behave  very  similar  as  it  was  observed  in  the  tank  experi- 
ments. 

During  the  present  experiments  the  kite  and  the  false 
headline  never  became  entangled  with  the  net,  and  the 
work  of  hauling  the  net  was  no  greater  than  with  an 
ordinary  trawl,  because  these  accessories  can  be  left 
afloat  on  the  water  while  only  the  codend  is  hauled 
aboard.  Of  course,  during  a  long  period  of  continuous 
operation,  the  kite  would  become  water-logged  and  lose 
its  buoyancy,  which  would  most  likely  result  in  certain 
difficulties.  Figs.  15  and  16  show  the  devices  used  in  the 
experiments. 

FIELD  EXPERIMENTS  IN  THE  YELLOW  SEA 
Method  and  Equipment 

The  second  series  of  experiments  were  conducted  by  the 
trawler  Taiyo-Maru  No.  32  (369  gross  tons,  with  700  h.p. 
diesel  engine)  on  a  fishing  ground  for  prawn  (Penaeus 
oriental!*  Kishinouye)  located  in  the  central  area  of  the 
Yellow  Sea.  In  rigging  a  trawl  (TT8-B1  type  by  the 
Taito  Fishing  Co.,  headline  128  ft.  long)  with  the  present 
device,  the  kite  was  used  as  in  the  preceding  experiments, 
while  the  gussets  and  the  '"riding  wire"  of  the  false  head- 
line were  somewhat  modified  in  length  (fig.  17).  A 
length  of  chain  was  added  to  the  ground  rope  to  keep  it 
on  the  bottom.  A  detailed  specification  of  the  gear  is 
given  in  the  footnote  of  fig.  17. 

The  height  of  the  net  mouth  was  measured  b\  a  height 
meter2  attached  to  the  centre  of  the  head  rope  All 
three  types  of  trawl  gear  have  been  tested:  "A"  (without 
kite);  kkB"  with  kite  but  without  gussets,  the  false  headline 
being  85  per  cent,  of  the  headline  length;  "(""  with  kitcand 
gussets.  The  distance  between  the  wing  tips  was  esti- 
mated from  the  angle  between  the  warps  and  the  trawling 
speed  was  determined  b\  the  "rail  log"  method. 

Results  and  Discussion 

With  a  relative  towing  speed  of  3  3  to  3  •  6  knots,  the  height 
of  the  net  mouth  was  about  25  m.  for  the  gear  type  "A'\ 


l:ig.    15.     Kite  ami  false  heatlline  used  in   the   experiments. 


If).     Kite  about  to  he  put  in  use. 


I93  ] 


MODERN 

Trawl   without  kite 
(Upper  part) 


FISHING    GEAR    OF    THE    WORLD 


Trawl  with  kite 
(Upper  part) 


Float 


10    20    30 


i i         i 


ft 


S|i 


(S|ize 
Mesh 


90 


Fig.  17.     Detailed  diagram  oj  the  128-feet-lrawl  with  kite  used  by  the  Taiyo-Maru  No.  32. 

Ropes  ami  accessories  for  the  net  with  kite  :    False  headline,  total  length  110  feet 

for  floated  part,  combination  rope,  2  cm.  diam..  two  lines,  each  50    „ 

"riding  wire"  for  kite,  wire  rope,  12  mm.  diam.  JO    „ 

Headline,  extended  by  8  feet  for  two  gussets  inserted  into  the  square  136     ,, 
Connecting  legs,  combination  wire,  2  cm.  diam.,  one  end  of  each  tied  3-1/2  feet  apart 

from  the  centre  of  the  square,  the  other  end  to  the  after  edge  of  the  kite,  each  V    „ 

Chains,  12  mm.  diam.,  for  additional  weight  of  foot  rope  40  meters 
Floats  for  false  headline,  cylimlrical  plastic  sponge,  buoyancy  200  gr.  per  piece,  60 

pieces  per  side  120  pieces 

for  bosom,  glass  ball,  24-2  cm.  diam.  10      „ 

for  wings,  glass  ball,  18  cm.  diam.  3  feet  distance,  total  number  approx.  40 


about  4  m.  for  type  "B",  and  about  5-4  m.  for  type  "C" 
(see  fig,  18).  The  angle  between  the  warps  decreased  by 
2  to  3  degrees.  The  corresponding  decrease  of  the  distance 
between  the  wing  tips  for  the  trawls  type  "B"  and  "C" 
was  approximately  3  to  4  m.  The  position  of  the  kite  would 
be  some  2  m.  above  the  headline.  The  frightening  effect 
of  the  kite  gear  may  possibly  increase  the  effective  mouth 
opening  up  to  the  level  of  the  kite. 

The  distance  between  the  wing  tips  was  found  to  be 
about  16  to  17m.  The  actual  opening  height  determined 
in  these  experiments  was  considerably  higher  than  in  the 


model  experiments  (fig.  8).  This  may  be  ascribed  partly 
to  the  net  being  smaller  than  it  ought  to  be  in  proportion 
to  the  kite,  and  partly  to  the  different  type  of  net  which 
for  the  purpose  of  catching  prawns,  is  constructed  for 
a  higher  opening  than  the  one  used  on  the  model  tests. 
The  presence  of  a  number  of  bottom-living  flat-fish  in 
the  catch  and  of  mud  of  the  footrope  was  good  evidence 
that  the  footrope  had  been  kept  on  the  bottom.  The 
number  of  engine  revolutions  varied  but  little  among  the 
different  types  of  trawls.  In  regard  to  handling  of  the 
kite  gear  very  little  difficulty  was  experienced  during  the 


1941 


USING    A     KITE    TO     INCREASE    VERTICAL    OPENING 


first  5  days.  But  later  the  kite  became  water-logged  and 
could  not  be  kept  floating  by  the  floats  on  the  false  head- 
line, thus  causing  considerable  trouble  to  the  crew. 

Problems  to  be  solved 

(1)  In  order  to  prevent  water-logging,  the  kite  should 
be  made  either  of  plastic  or  of  light  metal  alloy.  At  the 
same  time,  an  optional  weight  of  the  footrope  has  to  be 
determined  experimentally  with  regard  to  the  new  type 
of  kite,  trawling  speeds,  and  bottom  characteristics. 
(2)  The  gusset  as  used  in  the  present  net  may  not 


necessarily  be  the  best  means  of  securing  an  effective 
formation  of  the  net  mouth.  It  is  suggested  that  the  net 
mouth  should  not  only  be  raised  in  height  but  also  be  as 
round  as  possible.  One  suggestion  would  be  to  insert  a 
rhombical  piece  of  webbing  of  the  proper  size  between 
the  upper  and  lower  net  along  each  side. 

(3)  A  hydrofoil  kite  may  provide  the  same  lifting  power 
with  a  smaller  area.  In  that  case,  however,  the  underwater 
stability  and  the  lower  resistance  against  rough  treatment 
must  be  considered. 

Acknowledgements 

The  authors  wish  to  express  their  profound  gratitude  for  co- 
operation and  suggestions  rendered  in  the  course  of  the  present 
study  by  a  number  of  persons.  They  are  Captain  S.  Sakai  and 
the  crew  of  the  R.V.  Tenyo-Maru,  Captain  Oka  and  the  crew  of 
the  Taiyo  Maru  No.  32.  Messrs.  M.  Kawakami,  Chief,  Trawl 
Division  of  Taiyo  Fishing  Company,  S.  Suzuki,  Assistant  Chief 
of  the  same,  T.  Nomura,  T.  Kawakami,  fishing  technologists 
of  Taiyo  Fisheries  Laboratory,  S.  Nakamura,  H.  Taketomi, 
Mr.  Takagi,  Y.  Tahara,  and  an  English  writer,  M.  Tshida  of  the 
Regional  Fisheries  Laboratory. 

REFERENCES 

1  Hayashi,  H.     A  study  of  a  trawl  mouth  stretcher  (in  Japanese), 
Jour.  Fish.  hxp.  St.,  3,  1933. 

2  Imamura,  Y.     Model  experiment  of  the  British  herring  trawl 
(in  Japanese).   Bull.  Jap.  Soc.  Sci.  Fish.  10  (5),  1942. 

3  Nomura,  M.  and  Yasui,  T.     Model  experiment  on  various 
types  of  trawl  net  (in  Japanese),  Bull,  Jap.  Soc.  Sci.  Fish.  18  (12), 
1953. 

4  Kanda,  K.    The  notes  on  midwater  trawl.  Fish.  Tech.  1  (7), 
1954. 

•f>  Kumugori,  T.,  Hamuro,  S.  and  Ishii,  K.  Automatic  net- 
height  meter  and  ground  rope  indicator  for  trawler  with  the 
results  of  experiments  (in  Japanese),  Tech.  Rep.  Fishing  Boat, 
ft,  1955. 

6  Tauti,  M.     A  relation  between  experiments  on  model  and  on 
full  scale  of  fishing  net,  Bull,  Jap.  Soc.  Sci.  Fish.  3.  (4),  1934. 

7  Tauti,  M.    "Physics  in  fishing  technology"  Asakura,  Tokyo, 
Japan, 1949. 

8  Hayashi,  H.    The  calculated  tension  in  towing  two-boat-type 
trawl  (in  Japanese),  Fish.  Invest.  20  (1),  1925. 

9  Hayashi,  H.     Ibid.  19  (7),  1924. 

10  Sato,  H.  and  Kurita,  S.     An  experiment  on  the  resistance  and 
buoyancy  of  floats  for  the  seine  (in  Japanese),  Fish.  Invest.  Fish. 
fcx.  9,  1943. 

11  Miyamoto,    H.     "Study    on    fishing    gear    and    method," 
Kaneharu.  Tokyo,  Japan,  1957. 

12  Saito,  I.     "Trawl  fishery",  Maru/cn,  Tokyo,  Japan,  1954. 

13  Whiteleather,   R.  T.     Commercial   Fisheries   Rovieu    10  (6) 
1948.     PP.  1-6. 


/•!/».  IS.    Diagrams  of  the  net-height  recorder  giving  the  opening  height  of  the  different  types  of 
trawl  gear  tested  by  Taiyo- Maru  No.  32, 

Type  of  net 
With  or  without  kite 
With  or  without  gussets 
Headline  length,  total 
Force  of  wind 
Depth 

Relative  towing  speed 
Engine  revolution 
Warp  length 

Angle  between  the  warps 
Chains  added  to  foot 

rope 
Major  kind  of  catch 


A 

A' 

B 

C 

C 

Without 

Without 

With 

With 

With 

Without 

Without 

Without 

With 

With 

128ft. 

128  ft. 

128ft. 

136ft. 

136ft. 

I  ^  fair  wind 

1,  head  wind 

2,  fair  winti 

3  ,  fan  wind 

2,  head  wind 

70m. 

70  in. 

75  in. 

75  m. 

80  m. 

3-3  knots 

3-5  knots 

3-6  knots 

3-5  knots 

3-5  knots 

222lmin. 

226/  min. 

230/min. 

220/min. 

220fmin. 

220  in. 

220  m. 

230  m. 

230m. 

250  m. 

16 

15-5" 

13-5 

13 

13 

12  mm.  diam. 

12  mm.  diam. 

12  mm.  diam. 

Without 

Without 

40  m. 

40m. 

40  m. 

Croaker, 

Prawn  and 

Prawn  and 

Prawn,  flat- 

Prawn and 

prawn  and 

croaker 

croaker 

fish  and 

flatfish 

flatfish 

maker 

195  ] 


THE   HEADLINE,  THE  FOOTROPE  AND  THEIR   INFLUENCE  ON 
THE   VERTICAL  OPENING  OF  THE  MOUTH   OF  THE  TRAWL 

by 

S.   OKONSKI   and   S.   SADOWSKI 

Sea  Fisheries  Institute,  Gdynia,  Poland 

Abstract 

Although  trawls  have  been  changed  frequently  to  increase  their  efficiency,  designers  are  .still  without  a  complete  picture  of  how  a 
trawl  works.  Design,  so  far,  has  been  largely  determined  by  net  materials  available  and  the  power  and  size  of  trawlers.  There  should  be 
a  thorough  examination  of  the  fundamentals  in  construction  of  a  trawl  to  determine  precise  rules  for  designing  it.  This  paper  is  confined  to 
giving  a  few  examples  of  construction  problems  and  suggestions  for  overcoming  them.  It  deals  specifically  with  the  influence  of  the 
headline  and  footrope  on  the  vertical  opening  of  the  mouth  of  the  net,  and  proposes  formulae  for  determining  the  most  effective 
proportions  and  arrangements. 


Resume 


I  A  corde  de  dos,  lc  bourrclet  et  leur  influence  sur  I'ouverturc  vcrticale  de  la  gueule  du  filet 


Bien  que  Ton  ait  modifie  frequemment  les  chaluts  pour  augmcnter  leur  efticacite,  les  specialises  qui  Ics  dessinent  n'ont  pas  encore 
une  representation  complete  de  comment  fonctionnc  le  chalut.  Jusqif  ici,  le  modele  a  surtout  etc  determine  par  les  mahercs  premieres 
existantes  pour  le  filet,  et  la  puissance  et  les  dimensions  des  chalut icrs, 

On  doit  examiner  soigneusement  les  principcs  dc  base  dans  la  construction  d'un  chalut  pour  determiner  des  regies  precises  d'etablis- 
scmcnt  du  projet.  Cette  communication  donne  quelques  cxemplcs  de  problemes  de  construction  et  des  suggestions  pour  les  resoudre.  Ellc 
traite  specif iquemcnt  dc  1'influence  de  la  corde  dc  dos  et  du  bourrelet  sur  I'ouverture  vcrticale  de  la  gueule  du  filet  et  propose  des  formulas 
pour  determiner  les  proportions  el  les  dispositions  les  plus  efficaces. 

I, as  relingas  superior  e  inferior  y  su  influencia  sobre  la  altura  de  la  boca  de  las  redes  de  arrastre 
Extracto 

Aunque  a  menudo  sc  ha  modificado  la  boca  dc  las  redes  de  arrastre  para  aumentar  su  clicacia,  los  fabricantes  no  uenen  todavia 
una  idea  perfecta  de  como  funciona  el  artc.  Hasta  ahora.  la  forma  ha  sido  dctcrminada  generalmente  por  los  materiales  disponiblcs.  la 
potencia  y  tamafto  de  los  arraslreros.  Sin  embargo,  para  determmar  de  mancra  precisa  las  normas  de  construcci6n  debe  hacerse  un  cuidadoso 
examen  de  los  principles  fundamentals  quc  entran  en  la  construccion  de  una  red  arrastre. 

Esle  trabajo,  que  solo  tiene  por  objeto  dar  unos  cuantos  ejemplos  de  los  problemas  de  construccion  y  sugestiones  para  evitarloft 
trata,  especial  mente,  de  la  influencia  que  tienen  las  relingas  superior  e  inferior  sobre  la  altura  dc  la  boca  y  propone  formulas  para  determiner 
las  proporciones  y  a  r  ma  dura  mas  adecuados. 


DETAILS  of  the  construction  of  trawl  nets  shov\ 
substantial  differences  from  place  to  place.  This 
situation  appears  to  result  from  the  lack  of  a  com- 
plete picture  of  the  trawl  in  operation. 

Although  underwater  films  throw  some  light  upon  the 
subject,  they  do  not  tell  us  all  we  need  to  know;  nor  do 
the  empirical  investigations  made  by  fishermen.  What 
we  need  to  determine  are  the  precise  rules  on  which  the 
designs  of  the  trawls  should  be  based  and,  as  designing 
a  trawl  is  a  complicated  business  involving  a  diversity  of 
factors,  it  is  necessary  to  start  with  an  examination  of  the 
foundations  of  the  construction  of  the  trawl. 

The  opening  height  of  the  net  must  suit  the  particular 
fishing  conditions.  There  are,  therefore,  constructional 
differences  between  trawl  nets  used  for  catching  fish 
living  on  or  very  near  the  bottom  and  nets  for  catching 
fish  normally  found  some  distance  off  the  bottom. 

So  far,  this  problem  is  being  solved  by  using  lifting 
devices  and  different  qualities  of  net  materials,  and  by 
the  power  and  size  of  the  ship.  In  the  cutter  trawls,  made 
of  cotton  or  synthetic  fibres  which  are  extensible,  the 
vertical  opening  of  the  mouth  depends  mainly  on  the 


length  of  headline  and  footrope.  In  the  bigger  trawls 
made  of  less  extensible  materials  (sisal,  manila,  hemp), 
the  length  of  the  bosom  part  of  headline  and  footrope  is 
probably  most  important. 

A  cutter  trawl  18/23  and  a  trawl  22/24  (72  ft.)  for  big 
deep  sea  trawlers  are  used  as  examples  in  this  study  as 
they  are  common  in  the  Polish  commercial  fishery.  The 
indicated  fractions  denote  the  size  of  the  trawl  nets,  the 
numerator  indicates  the  length  of  the  headline  (in  metres) 
occupied  by  the  webbing,  and  the  denominator  half  of 
the  circumference  of  the  front  edge  of  the  belly  measured 
with  stretched  meshes. 


THE  POLISH  HERRING 
CUTTERS 


TRAWL  NET  18/23  FOR 


This  trawl  is  rigged  with  headline,  footrope  and  belly 
lines  seized  to  the  side  seams  linking  the  upper  with  the 
lower  net  (fig.  1). 

One  of  the  most  essential  points  in  the  construction  of 
such  trawls  with  sidelines  is  the  proper  length  relation 
between  the  three  elements:  headline,  footrope  and 


196 


ADJUSTING     THE     TRAWL     MOUTH     OPENING 


L__ _ _,    _ 


Fig.   I.     The  Polhlt  Herring  trawl  net  18123,  foi   cutler\. 


sidelines.  This  can  be  affected  either  by  adding  triangular 
wedges  to  the  wing  ends,  or  by  increasing  the  length 
of  the  headline  and  the  footrope  beyond  the  webbing. 

The  length  of  the  triangular  wedges  is  variable  but,  in 
practice,  it  never  exceeds  half  of  the  number  of  meshes 
of  the  wing  end  multiplied  by  the  stretched  mesh  size. 

In  fig.  2  the  top  wing  of  the  trawl  is  outlined  (A  BC  D), 
to  which  a  triangular  wedge  of  webbing  (ABE)  has 


•-*--*  \ 

^^  —  -i 

riff.  2.     The  effect  oj  lengthening  on  the  shape  of  the  headline. 


been  added.  The  wing  there  has  the  shape  A  E  D  C.  From 
point  A  and  E  the  wing  legs,  sideline  leg  and  headline 
leg  lead  to  the  danlcno.  As  both  are  of  identical  length, 
the  configuration  of  the  front  part  of  the  headline  is 
changed.  Point  F  will  take  the  place  of  point  B,  which 
in  turn  will  be  shifted  to  point  F,  and  consequently 
point  D  to  point  G.  The  position  of  points  A  and  C  on 
the  side  seam  (BE  BF-  DG)  remain  unaltered.  In  the 
case  shown  in  fig.  2  the  lengthening  of  the  headline  is 
effected  by  inserting  a  certain  amount  of  webbing.  Of 
course,  the  same  result  can  be  obtained  by  simply 
lengthening  the  headline  by  the  amount  BE.  The  foot- 
rope  can  be  treated  similarly.  In  consequence  of  lengthen- 
ing the  inner  edges  of  upper  and  lower  wing,  there  is  a 
shifting  of  the  front  part  of  the  webbing  in  relation  to 
the  body  of  the  net,  the  effect  of  which  is  more  difficult 
to  explain.  The  shape  the  headline  takes  in  action  is 
shown  in  fig.  3  for  three  coefficients  of  the  opening  width, 
viz.,  0-5,  0-6  and  0-7  of  the  headline  length.  The  curves 
are  calculated  according  to  the  method  of  Baranov 
(parabolic  equations). 

The  left-hand  part  of  the  drawing  (a)  shows  the  effect 
of  an  elongation  of  the  headline  by  1-6  m.  on  each  side, 
while  the  right-hand  part  (b)  shows  the  configuration 
with  the  original  headline  length  of  IK  m.  In  all  three 
versions  the  length  of  the  side  seams  remains  unchanged. 
In  part  4ka"  the  figure  A  B  O  C  shows  the  top  wing  with 
coefficient  0-5,  i.e.  the  distance  between  the  wing  tips 
equals  half  of  the  headline  length.  Considering  the  effect 
of  adding  the  triangular  wedges,  it  appears  as  if  a  "sur- 
plus" has  arisen,  which  may  increase  the  vertical  opening 
of  the  mouth. 

In  order  to  determine  this  "surplus"  in  the  drawing,  a 
line  is  drawn  from  point  C  parallel  with  the  line  A  E, 
which  represents  the  wing  tips. 

Point  O  denotes  the  spot  where  the  foundation  (C  O) 
of  the  wing  ends,  and  the  bosom  of  the  headline  starts. 
The  respective  points  for  the  other  versions  are  Ol  and 
O2.  The  figure  COR  gives  the  "surplus"  overlapping 
the  square  section  of  the  twine. 

With  increasing  horizontal  opening,  i.e.  equal  0*6  or 
0-7  of  the  headline  length,  the  surplus  decreases. 

If  the  horizontal  opening  equals  0-5  of  the  length  of 
the  headline,  the  surplus  of  twine  is  considerable  (C  O  R). 
At  the  opening  of  0-6  it  diminishes  (C,O,  R,)  and  at  0-7 
nothing  is  left. 

This  confirms  that  the  increase  of  the  headline  length 
ensures  correct  working  only  within  the  limits  of  the 
horizontal  opening  between  0-5  up  to  0-6  of  the  head- 
line length.  The  amount  of  surplus  obtained  allows  for 
a  higher  opening  at  0*5  than  at  the  0-6  which  is  con- 
firmed in  the  survey  of  de  Boer.  For  sufficient  surplus 
with  0-7  opening,  the  headline  should  be  further  length- 
ened by  about  0-6  m.;  otherwise  the  opening  height 
would  be  unsatisfactory  and  trawling  forces  could  cause 
a  break  up  of  the  webbing  in  the  joints  of  the  wings. 

The  otter  boards,  consequently,  have  to  be  adjusted 
according  to  the  constructional  possibilities  of  the  net. 
This,  in  practice,  has  led  to  great  caution  in  determining 
the  lengthening  of  the  headline  or  footrope,  and  it  has 
been  fixed  within  tolerant  limits  to  cover  individual 
deviations  of  the  trawl. 

Part  (b)  of  fig.  3  indicates  certain  shortcomings  in  the 


M97] 


MODERN    FISHING    GEAR     OF    THE    WORLD 


\    \ 


a 


\r. 

Fig.  3.     The  effect  of  lengthening  the  headline  on  i  he  fool  rope  on  the  wehhing*  with  three  different  ratios  oj  opening  width  to  headline  length. 

webbing  for  the  same  coefficients  discussed  above.  A 
trawl  of  such  a  construction  will  be  inefficient,  causing 
excessive  strain  in  the  webbing,  or  will  have  a  very  small 
vertical  opening. 

THE  POLISH  HERRING  TRAWL  NET  22/24/72  FT. 
FOR  BIG  DEEP  SEA  TRAWLERS 

(Fig.  4).  The  headline  length  of  this  net  is  22  m.,  i.e. 
6-7  m.  for  the  bosom  and  7-65  m.  for  each  wing.  The 
headline  is  only  slightly  longer  than  that  of  the  cutter 
irawl  (with  triangles). 

Special  attention,  however,  is  to  be  paid  to  the  char- 
acter and  significance  of  the  headline  bosom  which,  in 
our  opinion,  is  one  of  the  decisive  elements  influencing 
the  vertical  opening  of  the  mouth.  Fig.  5  shows  the  head- 
line with  the  same  three  ratios  of  opening  width  to  head- 
line length  (0-5,  0-6,  0-7).  The  headline  is  divided  into 
the  bosom  part,  to  which  the  square  is  attached,  and  the 
wing  sections.  As  the  trawl  has  no  sidelines,  the  analysis 
is  somewhat  different. 

To  examine  the  significance  of  the  bosom  for  the  three 
coefficients,  the  quarter  points  are  connected  by  the 
straight  lines  GH,  G,Hj,  G2H2,  thus  getting  figures 
GHO,  G^O,,  G?H2O2,  which  are  characteristic  for 
the  opening  coefficients  0-5,  0-6  and  0-7  respectively. 
They  differ  in  height  as  well  as  in  area.  The  headline 
enters  into  the  square  to  the  depth  indicated  by  the 
curvature  of  the  bosom,  thus  constituting  the  actual 
surplus  of  webbing  in  the  square,  which  permits  an  in- 
crease in  the  vertical  opening  of  the  trawl.  This  value 
diminishes  as  the  distance  between  the  ends  of  the  wings 
increases,  but  it  always  constitutes  a  positive,  not  a 


Fig.  4.  The  Polish  Herring  trawl  net  22 1 24-  72ft.  for  large  deep  \ea  trawler. 


\  198  I 


ADJUSTING     THE    TRAWL     MOUTH     OPENING 


A  A  ,        B    A-,       B  ,  B-, 


Fig.  .\      The  unpoitance  of  the  hosom  length  Joi   the  \cnu-al  opening  at  different  ratios  of  opening  width  it)  headline  length 


negative  factor.  Contrary  to  the  cutter  trawls,  an  adequate 
vertical  opening  can  be  obtained  by  increasing  the  length 
of  the  bosom  part  of  the  headline.  The  same  conclusion 
will,  of  course,  be  reached  when  reviewing  the  right- 
hand  part  of  fig.  5,  where  the  upper  wing  is  introduced. 

The  dimensions  have  been  defined  from  the  co- 
efficient of  the  horizontal  opening  of  the  headline.  Con- 
sequently, the  coefficient  of  the  horizontal  opening  of  the 
mesh  fixes  its  length  e.g.  the  coefficient  0  •  5  corresponds 
to  0-86,  0-6  to  0-7  and  0*7  to  0-7  (a  square). 

The  straight  lines  HC,  H^,  H2C2  correspond  to  the 
foundation  of  the  wings,  whereas  the  lines  CB,  CjB,, 
C2B2  indicate  the  seam  edge  of  the  wing.  Then  we  join 
the  centre  of  the  headline  (O,  O,  O2)  with  the  outer  end 
of  the  foundation  of  the  wing  (C  C,  C2).  The  angles  «, 
a,,  and  a2  between  the  foundations  of  the  wing  and  the 
latter  lines,  express  the  possibilities  of  extra  lift  of  the 
headline,  which  results  from  the  depth  of  the  curvature 
of  the  headline  and  the  length  of  the  bosom. 

In  our  opinion  the  value  of  these  angles  is  important 


in  the  choice  of  the  proper  length  of  the  bosom.  At 
various  opening  widths  this  value  depends  on  the  relation 
between  the  width  of  the  whole  square  (including  the 
bases  of  the  wings)  and  the  length  of  the  bosom  part 
Although  we  presume  that  the  value  of  these  angles  will 
enable  us  to  a  certain  extent  to  calculate  theoretically  the 
vertical  opening  of  the  mouth,  this  is  a  question  for  the 
future. 

The  footrope  and  the  lower  section  of  the  webbing  of 
the  belly  may  be  treated  in  a  similar  way,  mainly  to  keep 
the  footrope  to  the  ground,  which  also  may  help  in- 
directly to  increase  the  vertical  opening.  This  can  be 
obtained  by  proper  arrangement  of  the  bosom  part  of 
the  footrope  and  by  increasing  the  length  of  the  lower 
wings  in  comparison  with  the  top  wings.  The  fact  that 
with  herring  nets  the  bosom  part  of  the  lower  net  is 
shorter  than  in  the  square — which  is  opposite  to  the 
arrangement  in  the  trawls  for  demersal  fish— constitutes 
sufficient  proof  that  the  first  argument  is  right.  A  close 
contact  to  the  ground  is  not  needed  in  the  herring  trawls. 


199 


THE   MOUTH    OF   THE   TRAWL 

by 
JACK    PHILLIPS 

Managing  Director  of  Phillips  Trawl  Products  I  td.  of  Grimsby,  Fingland 

Abstract 

Since  it  is  the  amount  of  fish  in  (he  codend  that  matters  to  a  fisherman,  nothing  can  be  more  important  than  to  discover  how  best 
10  fill  the  codend  in  the  shortest  possible  time.  There  are  many  factors  concerned  in  this,  but  one  of  the  important  ones  must  be  the  size 
of  the  mouth  of  the  trawl,  and  this  paper  shows  the  necessity  for  raising  the  headline  without  its  being  restricted  by  the  lateral  pull  cf  the  doors 
etc.  The  development  and  application  of  the  Trawl-plane  for  lifting  the  headline  is  described,  and,  by  carrying  out  hydrodynamic  tests  in  a 
600  ft.  tank,  meam  of  overcoming  instability  at  lowing  speeds  greater  than  3  A  knots  were  discovered. 


Resume 


La  guele  du  Chalut 


ttant  donne  que  c  est  en  definitive  la  quant ite  de  poisson  capturee  dans  le  cul-dc-chalut  que  imcresse  le  pecheur,  le  point  Ic  plus 
important  consiste  a  trouver  le  moyen  de  remplir  ce  cul-de-chalut  dans  le  minimum  de  temps.  Ce  probleme  comporte  un  grand  nombre 
de  facteurs  mais  la  dimension  de  la  gueule  du  chalut  en  est  un  des  principaux,  et  Tautcur  demon t re  dans  cet  article  la  necessitc  de  hausscr  la 
ralingue  supcrieure  sans  que  ce  mouvement  soit  contrarid  par  la  traction  laterale  des  plateaux,  etc.  II  fait  un  expos£  dc  la  mise  au  point 
et  de  1'application  du  Trawl  plane  destine  a  soulever  la  ralingue  superieure,  ct  montre  comment  des  essais  hydrodynamiques  entrepris 
dans  un  bassin  de  180  metres  de  long  ont  permis  de  irouver  Ic  moyen  d'elimmer  les  ph£nomenes  d'mstabilite  qui  se  manifestaieni  au\ 
allures  de  chalutage  superieures  a  3.5  noeuds. 


La  boca  de  una  red  de  arrastre 
Extracto 

Como  el  interes  de  los  Pescadores  es  la  cantidad  dc  pescado  en  el  copo,  nada  liene  mayor  importancia  que  descubnr  la  manera  Je 
l.'enarlo  en  el  menor  tiempo  posible.  Aunque  hay  muchos  factores  relacionados  con  este  asunto,  uno  de  los  mas  importantes  debe  scr  el 
tiniano  de  la  boca  de  una  red  de  arrastre.  Por  este  motive,  se  demuestra  la  necesidad  de  elevar  la  relinga  de  boyas  sin  que  lo  impida  el 
tiron  lateral  de  las  puertas.  etc.  Hn  el  trabajo,  ilustrado  con  bastantes  fotografias,  tambien  se  describe  el  perfcccionamiento  y  uso  de  pianos 
de  elcvacion  para  hacer  subir  la  relinga  superior  y  los  dcscubrimicntos  hechos  mediante  ensayos  hidrodinamicos  en  un  cstanquc  de  M)0  pies 
(181  m.)  a  fin  de  evitar  su  inestabilidad  con  velocidades  de  arrastre  superiores  a  3  1/2  nudos 


IN  trawling,  there  is  an  old  and  often-quoted  saying: 
"It  all  comes  out  of  the  codend".    What,  therefore, 
can  he  of  greater  importance  than  to  discover  hou 
best  to  nil  the  codend  with  fish  in  the  shortest  possible 
lime? 

RAISING  THK  HEADLINE 

The  catch  assembled  in  the  codend  has  to  enter  lirst  at 
the  mouth  of  the  trawl  so  that  the  larger  the  area  of  the 
mouth  of  the  net,  the  more  chance  there  is  of  catching 
fish.  The  example,  given  in  figs.  1  and  2,  illustrates  this 
problem.  Fig.  1  shows  a  packet  of  20  cigarettes  open  at 
one  end,  exposing  its  contents;  fig.  2  shows  the  same 
packet,  but  because  the  top  edge  has  been  raised,  its 
capacity  has  been  almost  doubled.  Note  that  it  has  been 
possible  to  insert  no  less  than  17  additional  cigarettes, 
without  any  structural  alteration  to  the  packet  itself.  This 
must  indicate  the  importance  of  the  fact  that  the  vertical 
plane  is  more  essential  than  the  horizontal  for  high 
swimming  fish  and  the  height  of  the  headline  has  a 
more  direct  bearing  on  the  potential  catching  power 


than  the  spread  caused  by  the  trawl  doors  or  oiler  boards 
At  low  towing  speeds  the  lateral  spread  does  not  appear 
to  restrain  the  lift  of  the  headline,  but  there  is  a  tendencv 
loday  for  the  more  powerful  trawlers  to  tow  their  gear 
at  higher  speeds.  This  gives  more  spread  of  the  otter 
boards,  making  it  necessary,  in  the  case  of  the  Granton 
trawl,  to  shorten  the  side  line  in  order  to  overcome  this 
tendency.  (The  shorter  side  line  should  be  made  of  com- 
bination wire  rope  to  withstand  the  additional  strain.) 
Having  once  ensured  that  the  headline  is  free  to  rise 
to  its  maximum  extent  at  all  towing  speeds,  it  is  then  a 
question  of  selecting  the  most  suitable  method  of 
achieving  this.  There  are  two  major  factors  concerned 
with  this  problem  of  raising  the  headline: 

(i)  the  forces  involved 
(ii)  the  towing  speeds. 

THK  FORCES  INVOLVED 

Fig.  3  illustrates  these  factors  in  the  case  of  a  float.  Lift 
represents  the  total  vertical  component — being  a  force 
consisting  of  buoyancy  and  hydrodynamic  upthrust,  the 


[200] 


DEVELOPING     H  Y  DRODYN  AM  1C     TRAWL     FLOATS 

4  LIFT 


FLOAT 


DRAG 


DIRECTION 
OF   MOTION 


HEADLINE 
Fix.   •?•     Fwces  involved. 

latter  being  generated  by  the  passage  of  the  submerged 
floats  through  the  water.  Drag  is  the  horizontal  compon- 
ent of  the  force  generated  by  the  passage  of  these  floats. 
The  lower  the  drag  can  be  made  for  a  given  lift,  the  less 
the  headline  will  be  distorted  backward  and  the  greater 
the  lifting  result  will  be.  Both  these  factors  are  influ- 
enced by  the  speed  of  tow.  Hydrodynamically,  all  sub- 
merged objects  in  motion  have  critical  speeds  and  when 
the  drag  force  thus  created  overcomes  that  of  lift,  stalling 


I   ami  2.     1 1  lust  rut  ion  of  the  effect  oj  headline  Iif9 ing. 


Fix.  4.     Floats  and  kites  used  in  h\drodynwnic  tests. 
[201  1 


MODERN     FISHING     GEAR    OF    THE     WORLD 


Fig.    5.     Observation  Cabin  ami  Testing  Tank. 


is  inevitable.  This  causes  instability  and  violent  oscilla- 
tion, so  that  the  float  or  elevating  component  becomes 
useless. 

Fig.  4  illustrates  a  variety  of  experimental  buoyant  kites 
or  elevators  upon  which  liydrodynamic  tests  have  been 
made,  and  their  performances  carefully  recorded.  Such 
considerations  as  simplicity  of  manufacture,  external 
hydraulic  pressure  exerted  on  floats  when  submerged  to 
great  depths,  easy  handling  and  freedom  from  fouling 
the  net,  must  all  be  borne  in  mind.  Obviously  a  spherical 
float  is  better  able  to  withstand  external  hydraulic  press- 
ure than  floats  of  other  shapes.  Its  static  buoyancy 
depends  on  its  own  weight  subtracted  from  the  weight 
of  the  water  it  displaces. 

By  arrangement  of  suitable  and  carefully  placed 
annular  foils,  partially  or  completely  surrounding  a 
spherical  float,  a  substantial  upthrust  is  obtained,  giving 
a  high  lift/drag  ratio  which  ensures  the  headline  being 
elevated  vertically,  with  the  minimum  distortion  in  the 
backward  plane. 

SPEEDS    OF    TOW- UNDERWATER    OBSERVA- 
TION 

Photographs  and  observations  of  frogmen  watching  gear 
being  towed  have  answered  many  questions  about  the 


shape  and  behaviour  of  the  net.  These  reports  have, 
however,  proved  misleading  when  concerned  with  per- 
formances of  floats  towed  at  normal  commercial  speeds 
as,  for  obvious  reasons,  it  was  essential  for  the  gear  to 
be  towed  slowly  during  photographic  operations.  There 
is,  for  example,  the  experience  of  the  Trawl  Plane  Float. 
Action  photographs  showed  the  Trawl  Plane  to  be  stable 
and  efficient  at  that  particular  speed  of  tow  and,  thus 
encouraged,  the  Trawl  Plane  Float  was  assumed  to  be 
capable  of  functioning  efficiently  under  all  conditions. 
However,  when  the  floats  were  put  to  use  in  commercial 
fishing,  very  varied  opinions  were  expressed  by  the 
fishermen  some  skippers  criticising,  others  praising, 
while  some,  who  had  initially  been  satisfied,  became 
critical  when  they  changed  to  newer  ships. 

FURTHER  RESEARCH    TANK  TESTS 

The  Hydrodynamic  Section  of  Messrs.  Saunders  Roe,  the 
aircraft  manufacturers  of  Cowes,  Isle  of  Wight,  who  have 
facilities  for  carrying  out  hydrodynamic  tests,  assisted  in 
finding  the  solution  to  this  problem.  Their  electronically 
controlled  recording  and  observation  cabin,  and  the  rig 
used  for  testing  the  floats  in  the  600  ft.  tank,  are  shown 
in  figs.  5  and  6. 

A  programme  for  testing  many  varying  types  was  de- 


[202] 


DEVELOPING     HYDRODYNAMIC    TRAWL     FLOATS 


Fig.  6.    Testing  Rig. 


cided  upon,  and  the  first  task  was  to  clear  up  the  mystery 
of  the  inconsistent  reports  received  on  the  performance 
of  the  floats.  The  tank  tests  began  at  3  knots,  at  which 
speed  the  Trawl  Plane  Float  behaved  with  complete 
efficiency.  When  the  speed  of  tow  was  increased  to  3*1/2 
knots,  both  stability  and  efficiency  were  maintained 
during  acceleration  but,  on  reaching  the  speed,  an  in- 
dication of  instability  was  noticeable.  A  trial  at  4  knots 
revealed  that  the  behaviour  of  the  Trawl  Plane  altered 
completely;  it  became  so  unstable  that  the  test  run  had 
to  be  abandoned  to  prevent  the  rig  being  damaged.  These 
antics  and  oscillations  were  caused  by  the  drag  force 
overcoming  the  lift  force. 

LATEST  DEVELOPMENTS 

It  is  evident  that  optimum  performance  can  only  be 
achieved  from  floats  or  kites  when  a  high  lift/drag  ratio 
is  obtained.  (The  tests  on  the  original  Trawl  Plane  tended 
to  show  that  this  ratio  was  less  than  unity.) 

The  necessary  adjustments  were  then  made  to  enable 
the  Trawl  Plane  to  operate  efficiently  at  speeds  of  tow 
beyond  6  knots.  So  recent,  however,  are  these  experi- 
ments that  at  the  time  of  the  Gear  Congress,  the  modified 
float  was  not  available  for  general  commercial  use,  as 
the  tooling  up  and  die-making  involved  had  yet  to  be 
completed. 


203  ] 


MODERN     FISHING     GEAR     OF     THE    WORLD 


Fig.  7  shows  the  fast  towing  hydrofoil  float  called  the 
"Upthruster".  When  towed  at  5-1/2  knots  this  float  has 
a  lift  equal  to  ten  ordinary  spherical  floats  and  the  drag 
factor  is  only  equivalent  to  three  such  floats  and  has  now 
been  tank  tested  up  to  10  knots  with  perfect  stability. 
The  performance  of  this  new  float  has  been  much  im- 
proved by  extending  the  stabilizer  until  it  merges  with 
the  trailing  edge  of  the  flap  to  which  it  is  attached,  en- 
suring turbulent  flow  to  eliminate  a  vortex  forming  in 
the  float's  wake  as  it  passes  through  the  water,  and  is 
largely  responsible  for  the  satisfactory  drag  factor. 

A  further  feature  which  contributes  largely  to  its 
efficiency  is  the  slotted  flap  on  the  trailing  part  of  the 
foil;  this  controls  the  angle  of  incidence  preventing  the 
float  from  stalling  at  the  high  speeds  sometimes  ex- 
perienced when  shooting  the  gear. 

The  handle  attachment  is  designed  in  such  a  way  that 
a  simple  form  of  attachment  is  to  thread  a  line  or  com- 
bination rope  in  and  out  of  the  handle  forming  a  string 
of  such  floats  which  in  turn  can  be  readily  lashed  to  the 
headline  between  each  float,  see  fig.  8. 

To  ensure  that  the  float's  stabilizer  docs  its  work 
properly  and  the  float  is  free  to  function,  it  is  desirable 
to  leave  freedom  for  each  float  to  move  unhampered 
when  lashing  the  string  of  floats  to  the  headline.  An  easy 
way  of  accomplishing  this  is  after  threading  the  line 


Fig.  8.     Upthf  lister  float  a  attached  hy  \imply  threading  a  n>i>e 
in  and  out  oj  the  handles. 


through  the  handle,  pass  the  loop  completely  over  the 
float  forming  a  knot  under  the  handle,  making  it  im- 
possible for  the  float  to  slide  sideways  along  the  line 
and  giving  it  ample  freedom  to  work.  Alternatively,  when 
lashing  the  string  of  "Upthrusters"  to  the  headline  it  is 
important  to  leave  ample  play  between  the  floats  to 
enable  them  to  move  freely  and  take  up  unhampered 
their  working  position  when  in  use. 

Whilst  this  is  as  far  as  we  have  gone  at  present,  two 
additional  modifications  to  the  design  are  being  care- 
fully investigated  and  which  show  promise  of  even 
greater  efficiency  being  accomplished. 


Underwater  picture  of  the  headline  of  a  trawl  with  floats  attached,  from  the  film.  "The  Trawl  in  Action" . 

Crown  Copyright 

[204] 


A    LARGE-SIZED    EXPERIMENTAL   TANK   OF   TWIN   SYMMETRIC 

ELLIPTICAL   CIRCUITS 


YOSHIKAZU   NARASAKO   and   MASAJI    KANAMORI 

Faculty  of  Fisheries,  Kagoshima  University,  Kagoshima-City,  Japan 

Abstract 

In  this  paper,  the  authors  describe  the  general  construction  and  performance  of  the  large-sized  experiment-tank  set  in  the  Faculty 
of  Fisheries,  Kagoshima  University,  which  is  much  cheaper  and  gives  better  performance  than  the  traditional  towing  tank  for  measuring 
resistance  of  fishing  nets  and  fishing  ships  etc.  in  fluids. 

Hspccially  they  explain  the  homogeneous  speed  field  in  the  waterway  of  the  tank  which  can  be  controlled  more  easily  than  with 
the  old  circulating  tank,  by  moving  the  current  blades  hori/ontally,  the  vertically  set  wire  netting,  and  the  adjustable  lip  of  the  suppressor 
of  wave  motion. 


Resume 


I  in  grand  bassin  experimental  compose  de  circuits  elliptiques  symttriques  jumele* 


Dans  cede  communication,  les  autcurs  decrivent  la  construction  generate  et  la  fonctionnement  du  grand  bassin  d 'experiences  instalJe 
a  la  Faculte  des  Pechcs  de  I'Univcrsite  de  Kagoshima,  qui  est  bien  meilleur  march£et  donne  de  meilleurs  resultats  quo  le  bassin  actuel  a  remorqu- 
age  pour  mesurer  la  resistance  des  filets  de  peche,  bateaux  de  peche,  etc.,  dans  les  fluides. 

Us  cxpliquent  en  particulier  comment  le  champ  de  vitesse  homogene  dans  le  canal  d'essai  du  bassin  peut  etre  controle  plus  facilement 
qu'avec  1'ancien  bassin  a  circulation  en  dcplacant  horizontalement  la  lame  de  Timpulseur  et  au  moyen  des  ecrans  verticaux  de  treillagc 
metallique  et  de  la  levre  reglable  de  I'effaceur  de  vagues.  Ces  dernicrs  dispositifs  ont  etc  introduits  par  les  autcurs  aprcs  deux  ans  d'essais 
effectues  dans  le  bassin. 


Estanque  experimental  de  gran  tamano  provisto  de  los  circuitos  gemolos  simetricos  y  de  forma  eliptica 
Kxtracto 

fcn  csic  trabajo  los  autores  describen  la  construction  general  y  rendimiento  del  juego  de  estanques  experimentales  de  gran  tamano 
pertenecientes  a  la  Facultad  de  Pesca  de  la  Universidad  de  Kagoshima,  el  cual  es  mucho  mas  barato  y  permite  obtener  major  resultado  quo 
los  eslanques  de  cxpcrimentaci6n  utilizados  en  la  actualidad  para  mcdir  la  resistencia  de  las  redes  y  barcos  de  pcsca,  etc.,  en  los  liquidos. 

Los  autores  explican  especialmentc  la  manera  dc  alcanzar  un  campo  de  velocidad  uni forme  en  la  section  de  ensayos  del  estanque. 
mas  facil  de  regular  que  en  el  antiguo  estanque  moviendo  la  hoja  que  ataca  horizontalmente  corriente,  la  rcjilla  dc  alambre  dispuesta  en 
sentido  vertical  y  la  paleta  regulable  del  supresor  de  olas,  que  fueron  introducidos  por  los  autores  despues  de  dos  aftos  de  ensayos  en  dicho 
estanque. 


THE  accuracy  of  model  tests  on  fishing  nets  depends 
especially  on  the  similarity  of  twine  characteristics  of 
real  and  model  net. 

The  tank  itself  must  be  large  enough  to  allow  the  use 
of  models  having  a  construction  similar  to  the  full  size 
prototype.  In  the  present  case,  a  large-sized  tank  having 
a  two-metre  wide  waterway,  was  constructed  for  the 
experiments. 

CONSTRUCTION  OF  THE  CIRCULATING  TANK 

The  tank  consists  of  twin  symmetric  elliptical  circuits 
14-0  m.  long,  7*1  m.  wide  and  I  -0  m.  deep,  constructed 
of  ferro-concrete  (fig.  1 ). 

The  central  waterway  is  2-0  m.  wide  and  the  right  and 
left  waterways  are  1-0  m.  wide.  The  water  flows  sym- 
metrically into  the  central  section,  forming  a  straight 
waterway  for  making  tests,  then  branches  off  at  the  lower 
end  to  circulate  further. 

Paddle  wheels  of  2-5  m.  diameter  and  1  -0  m.  width, 
with  24  radial  blades  are  set  on  each  side  of  the  waterway. 


They  are  driven  by  a  vanslip  A.C.  motor  of  10  h.p. 
which  is  automatically  regulated  and  remote  controlled 
The  resultant  flow  of  water  is  symmetrical  and  easil> 
adjustable. 

The  maximum  speed  of  flow  is  0-8  m./sec.  which  can 
be  increased  by  decreasing  the  depth  of  water. 

Observation  windows,  2  -0  m.  long  and  1  -0  m.  wide,  arc 
set  in  the  right  and  left  side  walls  of  the  central  waterway 
and  measurements  and  photographs  can  be  taken  through 
them. 

The  flow  of  water  round  the  curves  of  a  circulating 
tank  is  slower  on  the  inside  and  faster  on  the  outside  of 
the  curve.  The  flow  near  the  walls  and  the  bottom  of  the 
tank  is  decreased  by  drag  due  to  the  viscosity  of  water. 
To  equalise  the  flow,  current  plates  were  fixed  at  the 
front  of  the  paddle  wheels  and  at  the  curves  of  the  water- 
way. The  plates  divide  the  flow  into  several  narrow  paths 
(figs.  2  and  3). 

There  are  also  movable  current  blades  at  the  curves 
of  the  tank,  and  these  can  control  the  horizontal  speed 
of  flow  according  to  Bernoulli's  theorem.  Fixed  current 


[2051 


MODERN     FISHING     GEAR     OF    THE    WORLD 


ffi 


*•; 


NO    NO   NO.   NO.  NQ 
~" 


12345 


6     7      ?      P     IQ 


>3    14    IS 


18    17     18     W    70 


73  ?a 


Fig.  /.     Top  view  of  the  tank  with  reference  points  for  waterflow 
measurements. 

plates  and  vertical  wire  netting  in  frames  of  25  cm. 
200  cm.  are  set  in  the  lower  stream.  The  mesh  size  of  the 
wire  netting  can  be  adjusted  to  each  speed  of  flow,  while 


-900- 


-1000  • 


Fig.  2.     Size  and  position  of  current  plates. 


.  Uifttt 


7-7^.  3.     Position  oj  fixed  ami  movable  current  plates  in  the  tank . 

an  adjustable  lip  is  set  in  the  upper  waterway  as  a  sup- 
pressor of  surface  waves  (fig.  4),  thus  controlling  surface 
motion. 

The  carriage,  consisting  of  three  parts,  moves  easily 
along  rails  on  the  top  of  the  sides  of  the  central  straight 
waterway.  One  part  can  be  moved  right  and  left,  or  up 
and  down,  and  has  a  balance  at  the  centre.  Thus  the 
point  of  resistance  measurement  is  given  three  dimen- 
sional freedom  of  movement  with  an  accuracy  of  1  mm. 
The  frames  of  the  carriage  are  all  made  of  steel  and  the 
dynamometer  (the  balance)  is  made  of  duralumin, 
according  to  Froude's  system.  Resistance  is  measured 
by  the  curve  recorded  on  a  revolving  drum. 

THE   GENERAL   PERFORMANCE   OF   THE 
CIRCULATING  TANK 

Fig.  1  shows  the  plan  of  the  tank.  The  positions  for 
measurement  of  speed  are  marked  on  the  parts  of  the 
straight  waterway.  The  speed  of  flow  can  be  examined  in 


r-Mro-rr 

is 

'*o 

% 

^« 

1* 

M 

>•* 

•wr* 

w 

w 

\t' 

Fig.  4.     Position  of  wire  netting  frames  and  wave  suppressors  in 
the   tank. 


[206] 


CIRCULATING     TYPE    TANK 


Figs.  5  and 6.  Distribution  of  flow      HK,  5  without.  Jig.  6  with,  current 
speed  in  vertical  (top)  and  hoii-      plates  and  wave  suppressors 
zontal  (below}  cross  section. 

three  dimensions,  through  6  steps  in  the  direction  of 
length,  5  steps  in  the  direction  of  breadth  (7  steps  later, 
including  two  supplementary  steps  right  and  left)  and 
5  steps  in  the  direction  of  depth. 

The  current  was  measured  with  the  "Hiroi"  current 
meter.  With  no  current  blade  and  no  suppressor  of  wave 
motion,  the  speed  is  maximum  at  about  0-7  of  the  water 
depth.  The  speed  drops  suddenly  near  the  side  wall  and 
bottom  (fig.  5).  It  can  be  seen  that  the  homogeneous 
speed  fields  in  the  central  vertical  plane  of  the  waterway 
are  symmetric.  Because  of  the  symmetry  of  the  water- 
way, a  good  natural  effect  is  obtained  as  the  water  flows 
through  the  central  section. 

When  the  current  blades  and  wave  motion  suppressor 
are  mounted,  the  standard  deviation  from  a  homogene- 
ous speed  field  is  0-04  (mean  speed  0-55  m./sec.)  (fig.  6). 
The  experiment  shows  further  that  at  a  flow  of  less  than 
0-6  m./sec.  the  water  surface  is  so  calm  that  the  wave 
motion  suppressor  is  not  necessary. 

Fig.  7  shows  that  the  speed  field  is  more  uniform  and 
that  the  deviation  from  a  homogeneous  field  is  only  0-02 
(mean  speed  0-43  m./sec.)  when  the  vertical  wire  screens 
are  also  used.  The  screens  are  of  3  cm.  and  2  cm.  mesh 
size,  made  of  1  •  1  mm.  wire,  and  are  set  vertically  in  two 
steps  under  the  surface,  from  25  cm.  to  50  cm.  and  from 
50  cm.  to  75  cm.  Wire  screens  were  used  because  control 
of  the  speed  of  flow  is  made  easy  in  a  comparatively 
shallow  tank  by  the  fluid  resistance  of  the  webbing. 

The  speed  of  flow  becomes  difficult  to  control  when 
it  increases  beyond  a  certain  limit,  because  the  resistance 
of  the  net  is  proportional  to  the  square  of  the  mesh  size. 
Therefore,  the  mesh  of  the  net  had  to  be  enlarged  when 


* -!»-£.. 

-H  (**•»*  »«| 


•/•r 


F/#.  7.  Distribution  oj  flow  speed 
in  vertical  (top)  and  horizontal 
(helow)  cross  section  when  wire 
net  frames  are  used  in  addition 
to  current  plates  and  wave 
\npprex\oi  \. 


hig.  <V.    Distribution  oj  surface 

flow  lines  at  low  speed  of  the 

paddle   wheel. 


the  mean  speed  in  the  tank  wai»  greater  than  0-4  m./sec. 
At  lower  speeds,  it  is  very  easy  to  control  the  speed  of 
flow  accurately.  As  a  rule  the  homogeneous  speed  field 
in  the  waterway  can  be  set  by  controlling  the  r.p.m.  of 
motor. 

When  the  depth  of  water  is  constant,  the  r.p.m.  of  the 
motor  is  proportional  to  the  rate  of  flow  and  the  distribu- 


(i) 


TABIF    I 
Relation  of  motor  revolutions  to  deviation  of  cqui- velocity 


distribution  when  current  plates  and  wave  suppressors  are  fixed 

500  (wit  h- 
r.p.m.         400  300  250  out 

wire  net) 


deviation 

depth 
0-05  m. 
0-225 
0-40 
0-575 
0-75 

0-43 
0-44 
0-43 
0-42 

0 
0 
0 
0 

•017 
•010 
•Oil 
•010 

Vmls 

5 

Vmls 

S 

ym/s 

00000 

S 

•033 
•033 
•044 
•034 
•041 

0-32 
0-33 
0-32 
0-31 

0 
0 
0 
0 

•017 
•010 
•Oil 
-010 

0-26 
0-27 
0-26 
0-25 
0-22 

0-020 
0-013 
0-010 
0-010 
0-027 

0-63 
0-66 
0-60 
0-52 
0  52 

(ii)  Vertical  section  of  velocity  deviation 

r.p.m. 400      300  250  250* 

AVde^ation)City  ym/s    S  Vmls    S     Vmls    S       Vml*    S 


Vertical  section 

1 

.  .         —      —      —        —  —      — 

2 

0-43  0-015   0-32  0-011    0-26  0-020  0-27  0-016 

3 

0-43  0-008   0-31  0-006  0-26  0-022  0-27  0-013 

4 

0-43  0-021    0-33  0-022  0-25  0-041  0-27  0-018 

5 

0-44  0-014   0-32  0-012   0-26  0-013  0-27  0-011 

6 

0-43  0-023   0-31  0-018   0-25  0-018  0-26  0-012 

7 

,._        —        —       —        —      —        —        — 

(iii)  Horizontal  section  of  velocity  deviation. 
r.p.m.  400  300         J250 

Average  velocity  y  /      s         ym/s    s       ym/s 
deviation  '  ' 


250  *_ 
S 


Horizontal  section 

1  0-43  0-014  0-32  0-016  0-25  0-025  0-26  0-016 

2  0-43  0-018  0-32  0-014  0-25  0-026  0-26  0-015 

3  0-43  0-019  0-32  0-013  0-26  0-022  0-27  0-013 


*ln  this  calculation  the  velocity  of  flow  at  D^O-75  m.  is  omitted. 


[207] 


MODERN     FISHING     GEAR     OF     THF    WORLD 


lion  of  water  speed  in  the  tank  is  uniform  at  all  points. 

When  the  motor  revolves  very  slowly,  the  flow  line  of 
the  surface  water  layer  deviates  about  1  per  cent,  inwards, 
being  influenced  by  the  hollows  at  the  windows  in  the 
side  walls.  The  flow  lines  are  perfectly  parallel  to  each 
other,  so  there  is  no  disturbance  to  the  model  under 
experiment  (fig.  8). 

Table  I  shows  the  relation  between  the  r.p.m.  of  the 
motor  and  the  deviation  when  the  current  blade,  wave 
suppressor  and  wire  netting  are  used. 

Table  II  shows  the  time  necessary  (17  mins.)  for  the 
speed  of  flow  to  become  constant  and  uniform  after 
starting  the  motor.  After  that,  no  change  of  speed  is 
apparent.  Jf  an  experiment  is  made  within  7  mins.  of 
starting  the  motor,  an  error  of  3  per  cent,  should  be 
allowed  on  resultant  measurements. 


TABLI    II 

Time  required  before  speed  is  constant  and  uniform  after  the 
motor   is  started. 


Point  <>/  measurement     No.  4 


Velocity 


Sec.      \  'm,  \ 


I  m< 


\o.  Iff 


Sec.      I  m 


Time  required 

1  m:n. 

18-0 

0  34 

17-  X 

0  34 

17-8 

I)  34 

12 

17-6 

0-34 

17-6 

0-34 

17-4 

0-35 

17 

17-2 

0  35 

17-2 

0  35 

17-2 

0-35 

22 

17-0 

0  36 

17-2 

0  35 

17-4 

0-35 

27 

17-0 

0-36 

17-4 

0  35 

17-0 

0-35 

Trawl  toads  and  wingdooi  designed  hy  A.  //.  Larsson  (S\\ederi)  based  on  model  ir\ts 


208 


Section  7:  Rational  Design — Use  of  Measuring  Instruments  ami  Underwater  Observation. 


MIDWATER   TRAWL   DESIGN   BY   UNDERWATER   OBSERVATIONS 

by 

R.    F.   SAND 

Bureau  of  Commercial  Fisheries,  U.S.  Fish  and  Wildlife  Service,  Washington  25  D.C.,  U.S.A. 

Abstract 

Underwater  observations  and  evaluations  of  the  I  .arsson  Phantom  Trawl  and  others  are  described.  Direct  observations  of  the  fishing 
gear  in  operation  were  obtained  by  the  use  of  underwater  television  and  divers  with  cameras.  By  means  of  modification  and  refinement  a 
practical  midwater  trawl  was  developed  for  use  by  American  fishing  vessels.  The  resulting  net  has  been  used  by  research  vessels  and  also 
commercial  fishing  boats  of  200  to  750  h.p.  and,  at  speeds  between  2  and  5  knots,  catches  ranging  from  5.000  io  20.000  pounds  have  been  taken. 

Utilisation  des  observations  sous-marines  dans  le  dessin  du  chalut  Hot  Cant 
Resume 

L'auteur  se  fondc  sur  les  observations  cffectuees  sous  1'eau  pour  evaluer  le  comportcmeru  du  chalut  I  .arsson.  type  Phantom,  airtsi  cjuc 
d'autres  types  dc  chaluts.  Des  observations  directes  des  engins  en  fonctionnement  ont  etc  obtenues  par  des  appareils  de  television  sous- 
marine  et  des  plongeurs  munis  d'appareils  cinema  tographiques.  A  la  suite  de  modifications  ct  de  perfect lonnements,  on  a  mis  au  point  un 
chalut  flottant  efficace  a  Fintention  des  navires  de  peche  amdncains.  Ce  chalut  a  ete  utilise  par  des  navires  de  recherche  et  des  bateaux  de 
pechc  d'une  puissance  de  2(H)  a  750  c.v.  et,  a  des  vitesses  de  2  a  5  noeuds  les  quantitcs  pechees  ont  attcint  de  5.000  a  20.000  Iivres. 

Uso  de  obscrvacioncs^submarinus  en  el  proyecto  de  redes  del  arrastre  polagicas 
Kxtracto 

Se  describen  las  observaciones  y  evaluaciones  submarmas  de  la  red  de  arrastre  1  arsson  y  de  otros  tipt  s  de  artes  de  pesca.  Las 
observaciones  directas  de  estas  redes  durante  el  lance  cfecutaron  c  con  aparatos  de  television  submarina  y  buzos  provistos  de  camaras  foto- 
graficas.  Mediante  diversas  modificaciones  y  perfeccionaniientos  se  logro  construir  una  red  de  arnistre  para  profundidades  intermedias, 
dcstmada  a  los  barcos  de  pesca  de  Nortcamcmcrica.  P.I  equipo  resultante  a  sido  usado  por  embaracciones  dc  investigaci6n  y  cnmercialcs  dc 
200  a  750  c.v.  a  velocidades  que  fluctuan  entre  2  y  5  nudos.  obteniendosc  redades  de  5.000  a  20.000  Ibs  (2.227  a  9,072  Kg.)  dc  pcscado 


THE  recognition  of  the  need  for  new  and  improved 
methods  in  fishing  gear  research  has  resulted  in  some 
promising  new  tools  and  techniques.  In  connection 
with  some  midwater  trawling  gear  research  studies,  the 
United  States  Fish  and  Wildlife  Service  has  demonstrated 
the  systematic  application  of  underwater  television  and 
divers    using   self-contained    breathing   apparatus   and 
cameras  as  practical  fishing  gear  research  instruments 

UNDERWATER  TELEVISION 

Since  1950,  the  U.S.  Fish  and  Wildlife  Service  has  con- 
ducted experimental  trials  with  one-  and  two-boat  mid- 
water  trawls.  Results  from  earlier  Service  work  had 
shown  economical  and  operational  advantages  in  the 
single  boat  type,  and  its  continued  development  was 
favoured  for  possible  use  by  American  fishing  vessels. 
In  May  1954  a  Service  research  vessel  carried  out  some 
preliminary  trials  with  the  Larsson  Phantom  Trawl  in 
North  Pacific  offshore  waters.  Results  of  fishing  per- 
formance were  inconclusive.  The  gear  was  then  shipped 
to  the  Services  Gear  Research  and  Development  Unit  at 
Miami,  Florida,  for  further  test  and  evaluation,  using 
underwater  television  and  other  experimental  methods. 
Here  Service  engineers  had  successfully  adapted  in- 
dustrial type,  closed-circuit  television  for  direct  observa- 


tions of  fishing  gear  m  action4.  B>  means  ot  progressive 
experiment  and  refinement  of  equipment  a  remotely  con- 
trolled submersible  vehicle  was  developed  for  the  tele- 
vision camera1. 

In  November  1954,  a  joint  cruise  of  the  Service  re- 
search vessels  Oregon  and  Pompano  made  the  first  prac- 
tical use  of  underwater  television  in  fishing  gear  research, 
with  the  Phantom  Trawl  as  the  subject  to  be  viewed. 
While  under  tow  in  clear  Gulf  Stream  waters,  the  trawl 
warps,  doors  and  net  opening  were  observed  by  a  tele- 
vision camera  streamed  between  the  Oregon  s  towing 
warps.  A  second  television  camera  was  streamed  from 
the  Pompano  to  make  simultaneous  lateral  observations 
of  the  gear.  Still  picture  and  kinescope  recordings  were 
made  of  the  entire  operation. 

The  first  tests  were  made  with  the  gear  rigged  as 
originally  received  from  Sweden:  cotton  trawl,  hydro- 
foil floats  and  depressors,  aluminium  floats,  hydrofoil 
doors  and  30  fathom  manila  towing  legs  with  danlenos. 
Previous  trials  had  established  that  this  gear  did  open 
to  an  estimated  30  feet  or  more,  but  that  it  was  unsatis- 
factory to  tow  the  trawl  at  3-0  knots  and  even  below 
this  speed.  The  heavy  drag  caused  continued  tearing  at 
sudden  speed  variations  or  full  vessel  power3.  With  50 
fathoms  of  cable  from  vessel  to  net,  the  trawl  was  now 
seen  well  below  the  propellor  wash  at  speeds  below  3 


209  ] 


MODERN     FISHING     GEAR    OF    THE     WORLD 


knots.  At  a  speed  of  2-0  knots,  the  monitor  screen  re- 
vealed the  trawl  doors,  warps,  and  net  mouth  to  be  quite 
stable  in  the  water  with  the  trawl  in  an  approximately 
20  feet  square  opening,  and  the  headline  rising  in  a  gentle 
arc.  A  close-up  camera  lens  revealed  the  terrific  stress  on 
the  twine  in  a  rigidly  appearing  trawl  mouth.  Most  of 
the  patent  trawl  floats  had  been  cleared  only  with  diffi- 
culty and  those  still  fouled  were  seen  to  produce  uneven 
stress  on  the  twine.  While  the  patent  hydrofoil  floats  on 
short  pennants  performed  well,  a  slight  tendency  to 
oscillate  suggested  turbulence  particularly  at  wing  tips. 
This  was  remedied  by  tying  off  the  floats  close-up.  Round 
aluminium  float  performance  seemed  most  satisfactory 
in  comparison.  Increases  in  speed  up  to  the  3-0  knot 
limitation  resulted  in  the  estimated  30  feet  horizontal 
spread  with,  however,  a  closing  of  vertical  opening.  This 
had  been  suspected  in  previous  sounding  tests,  but  not 
confirmed  until  registered  on  the  television  monitor. 

In  another  trial  the  30  fathom  manila  towing  legs  and 
danlenos  were  replaced  by  j}  in.  wire  cable  and  dandy-line 
gear  to  facilitate  handling.  Observers  noted  no  appreci- 
able change  in  net  performance  except  that  the  trawl 
fished  slightly  deeper  in  the  water  and  tended  to  close 
at  the  lower  wings.  This  was  remedied  by  adding  one 
fathom  of  cable  at  each  lower  towing  leg.  No  marked 
losses  of  horizontal  or  vertical  spread  could  be  attributed 
to  the  removal  of  the  danlenos.  There  followed  various 
substitutions  and  arrangements  of  floats,  leads  and  de- 
pressors, in  an  effort  to  attain  the  desired  vertical  trawl 
opening  at  2-0  to  3-0  knot  speeds.  With  the  use  of 
adequate  weights  and  depressors,  the  desired  vertical 
opening  was  obtained,  but  at  the  sacrifice  of  horizontal 
spread  in  each  case.  Although  the  hydrofoil  trawl  doors 
had  handled  and  performed  well,  they  were  replaced  with 
standard  type  trawl  doors  of  equal  and  of  larger  sizes 
in  an  attempt  to  improve  performance.  Apparently  there 
was  no  appreciable  change  in  the  trawl  opening. 

Most  noticeable  to  all  observers  during  these  trials 
was  the  evidence  of  continued  heavy  drag  and  extreme 
"sweep  back"  of  head,  breast  and  leadlines  at  all  test 
speeds.  Completely  absent  during  all  television  observa- 
tions was  any  appearance  of  the  net  acting  as  an  inflated 
body  containing  a  volume  of  water  exerting  a  pressure 
on  the  twine. 

It  was  indicated  to  observers  that,  due  to  the  excessive 
drag,  the  trawl  could  not  be  opened  to  its  maximum  de- 
signed displacement  without  some  structural  modifica- 
tion. 

DIVING  SLED  AND  CAMERA  OBSERVATIONS 

Through  1955,  experimental  trials  with  the  modified 
Phantom  Trawl  continued  in  clear  Florida  Gulf  Stream 
waters,  subject  to  observations  by  underwater  television, 
surface  water  glasses  and  divers  using  a  towed  diving 
sled  and  camera  gear.  A  diving  sled  was  made  by  con- 
verting a  tubular  steel  ambulance  litter  for  use  when 
underwater  television  observations  were  not  feasible6. 
This  device  was  equipped  with  elevator  controls  to  per- 
mit manoeuvrability.  It  provided  a  large  degree  of  com- 
fort and  protection  for  the  pilot-observer  and  camera- 
man diving  team  while  under  tow  from  the  research 
vessel.  The  diving  sled  permitted  first-hand  observations 
and  photography  of  all  aspects  of  the  trawl  performance 


from  various  angles,  to  detailed  inspections  in  actual 
contact  with  the  fishing  gear. 

During  some  20  towing  observations  at  speed  and 
depth  ranges  proximating  the  previous  television  trials, 
various  trawl  modifications  and  accessory  rigs  were  con- 
sidered separately  and  in  combination.  Since  in  the  pre- 
vious trials  the  trawl  performance  had  not  been  seriously 
affected  by  open  or  closed  codend,  emphasis  was  placed 
on  reducing  resistance  at  the  trawl  mouth.  Progressively 
the  net  wings  were  shortened,  body  length  was  reduced 
and  trawl  hanging  and  headline  increased  to  enlarge  the 
proportion  of  the  net  square  dimension  to  the  length. 
These  were  effected  with  no  real  improvement  apparent 
to  observers.  The  first  indication  that  the  gear  was  open- 
ing in  a  better  manner  was  not  obtained  until  head, 
breast-  and  leadlines  were  reduced  by  about  one-third  in 
diameter  and  the  number  of  floats  and  weights  reduced 
by  one-half. 

When  quarter  doors  or  kites  of  equal  area  were  affixed 
at  the  four  wings  in  the  same  angle  of  attack  as  the  trawl 
doors,  greater  trawl  opening  was  achieved  but  drag  and 
turbulence  on  wings  greatly  increased.  In  a  similar  man- 
ner, triangular  pieces  of  heavy  canvas  attached  to  the 
wing  extremities  failed  to  assist  trawl  opening  due  to  low 
attack  angle  made  necessary  to  reduce  drag. 

Trials  were  continued  with  the  net  modified  to  about 
two-thirds  the  original  size  and  with  trawl  doors  approxi- 
mately twice  the  area  of  the  patented  hydrofoil  doors. 
For  the  first  time  this  net  was  seen  to  open  to  an  esti- 
mated 90  per  cent,  of  the  possible  mouth  aperture  at  a 
2-0  knot  towing  speed.  At  this  speed,  a  bulge  or  lump 
in  the  top  square  was  still  seen  to  cause  some  uneven 
distribution  of  tension  on  the  body  twine.  This  was 
remedied  by  removal  of  two  centre  aluminium  floats  and 
replacement  at  wing  tips.  The  midwater  trawl  now  was 
completely  inflated  as  if  exerting  a  pressure  from  the 
centre  outwards  to  all  points  on  the  twine.  Meshes  were 
revealed  as  taking  the  desired  diamond  shape  under 
tension  from  every  angle.  That  there  was  an  apparent 
pressure  effect  within  the  trawl  was  determined  by 
physical  contact  by  the  diving  team  who  found  the  net 
rigid  yet  malleable — and  capable  of  partially  supporting 
a  50  pound  lead  weight  placed  upon  the  twine.  With 
reduced  towing  speed  below  2  -0  knots,  there  was  evidence 
that  the  net  opened,  more  as  a  result  of  resistance  to  the 
water  than  as  the  result  of  lift  by  the  floats.  This  was 
accompanied  by  some  apparent  transfer  of  towing  strain 
to  rib  lines.  Strong  evidence  had  now  been  presented 
showing  the  need  to  restrict  future  midwater  trawl 
construction  to  materials  of  higher  strength  and  lower 
drag  characteristics.  Also,  this  demonstrated  the  need 
for  float  or  lead  and  depressor  rigs  permitting  better 
trawl  opening.  Recognised  as  of  vital  importance  was  the 
extreme  need  for  symmetry  in  design,  exactness  in 
measurement,  and  careful  sewing  detail  to  ensure  perfect 
twine  balance. 

DISCUSSION  OF  RESULTS 

Within  the  frame  of  reference  provided  by  the  underwater 
determinations,  an  all-nylon  40  foot  square  opening  trawl 
was  constructed  (see  figure).  Particular  attention  was 
given  to  choice  of  materials,  twine  sizes  and  dimensions, 
geometric  configuration  and  proportions  to  permit 


[210] 


10  M 


OBSERVING     MJDWATER     TRAWLS     UNDER     WATER 

10  M 


BAR  HANG  MOUTH  TO  1/2"  COMBINATION! 
ROPE  WITH  DOUBLE  MESH 


NET  WINGS  AND  BODY  Of  NYLON 
NO.  831   110  LBS.  TEST 


DOUBLE  MESH  Al 
SEAMS  bEFORE  SEWING 


TAPER  1  PI  .  2  BAR 


NYLON  CROSS  R I bS  TOP 
BOTTOM  BAR  HUNG 


TAPER  4  PT  .  1  FAR 


12  ROUND  8   ALUMINUM 
FLOATS  ON  HEADLINE 


4  OZ.  SEINE  LEADS  ON 
LEADLINE  WITH  12   SPACING 


RIBLINES  1/2   NYLON  FULL  LENGTH 
GATHER  3  MESHES  EACH  SIDE  AND 
HANG  EVEN 


NO    TAPER 


40  //.  M'uhvater  trawl  (U.S.  hi\h  ami  M  ilit/ije  Service). 


211 


MODERN     FISHING    GEAR     OF    THE     WORLD 


performance  of  the  nei  as  a  turgid  elastic  body.  Final 
adjustments  to  trawl  and  accessory  gear  were  again 
subject  to  underwater  inspection.  The  resulting  trawl 
design  has  performed  well  in  operation  from  Service 
research  vessels  in  North  Pacific  and  North  Atlantic 
offshore  waters,  in  open  swells  and  moderate  seas  with 
25-30  m.p.h.  winds.  In  fishing  trials  from  vessels  of 
200  to  750  h.p.  at  speeds  of  between  2  and  approximately 
5  knots,  individual  trawls  have  withstood  over  100  tows 
without  serious  damage  attributed  to  excessive  drag  from 
towing  strain  in  taking  catches  varying  from  5,000  to  an 
estimated  20,000  pounds. 

There  remain  a  number  of  variables  to  be  examined 
and  the  trawls  currently  in  use  are,  of  course,  subject  to 
continued  refinement.  It  is  noted  also  that  the  genera! 
design  features  compare  closely  with  those  of  recently 
introduced  Canadian  and  European  midwater  gears1. 

For  more  than  four  years,  the  experiments— using 
conventional  gear  research  methods  were  unsuccessful, 
until  the  application  of  underwater  television  and  other 


experimental  methods  permitted  direct  observations  of 
fishing  gear  in  operation.  Continued  application  of  these 
valuable  techniques  should  assist  materially  in  removing 
a  great  barrier  to  research  in  fishing  methods  and  equip- 
ment. 

REFERENCES 

1  Barraclough.  W.  E.  and  W.  W.  Johnson.      A  New  Midwater 
Trawl  for  Herring,  Fisheries  Research  Board       Canada  Bulletin 
No.  104,  1956,  25  pp. 

2  Margets,  A.  R.    Some  Conclusions  from  Underwater  Observa- 
tions of  Trawl  Behaviour,  World  Fishing,  August  1952,  pp.  61-65. 

3  Richardson,  I.   D.  Some  Problems  in  Midwater  Trawling, 
World  Fishing,  February  1957,  pp.  26-31. 

4  Sand,  R.  F.     Use  of  Underwater  Television  in  Fishing  Geai 
Research  (Preliminary  Report)  U.S.F.W.S.,  CFR.,  Vol.  17,  No.  4. 
1955,  pp.    1-5. 

5  Sand,    R.    F.     Underwater   Television    Vehicle   for    Use   in 
Fisheries  Research,  U.S.F.W.S.,  SSR.  No.  193,  December  1956, 
15  pp. 

6  Sand,  R.  F.     New  Diving  Sled,  U.S.F.W.S..  CFR..  Vol.  18 
No.  10,  1956,  pp.  6-7. 


Skindiver  learn  of  Coral  Gables  preparing  for  Irawl  observations. 
[212] 


STUDY  OF  THE  MEDITERRANEAN  TRAWL  NET 

by 

MENACHEM    BEN -YAM  I 

Kibbutz  fc'Sa-ar",  D.N.  Gallil  Ma'aravi,  Israel 


Abstract 

This  paper  deals  with  an  attempt  to  improve  the  performance  of  the  Italian  type  of  trawl  as  used  in  the  Mediterranean  Sea.  Very 
little  progress  has  been  made  with  this  gear  because  the  fishermen  considered  that  it  represented  the  climax  of  achievement.  The  author  has 
studied  the  working  of  the  gear  by  means  of  underwater  photography  and  direct  observation  by  divers,  and  has  been  able  to  suggest  ways  and 
means  of  constructing  a  cheap,  strong,  and  efficient  trawl  suitable  not  only  for  the  Mediterranean  Sea  but  also  for  other  areas  where  the  need 
for  higher  headlines  is  of  greater  importance. 

Discussion  sur  le  chalut  mcditemmeen 

Kcsmn€ 

L'auteur  traite  d'un  essai  d 'amelioration  du  rendement  de  chalut  de  type  italien  tel  qu'il  est  utilise  dans  la  mer  Mediterranee.  Get 
engin  a  fait  tres  peu  de  progres  parce  que  les  pecheurs  consideraient  qu'il  rcprescntait  1'apogee  de  la  realisation,  mais  I'auteur  a  6tudi6  le 
fonctionnement  de  I'cngin  au  moyen  de  photographies  sous-marines  et  d'observations  directes  effect uees  par  des  plongeurs,  et  il  est  £ 
mcme  de  suggerer  des  facons  et  des  moycns  pour  construire  un  chalut  bon  marche,  robuste  et  efficace  convenant  non  seulement  pour  la  mer 
Mediterranee  mais  aussi  pour  d'autrcs  regions  oil  il  est  tres  important  d'avoir  la  corde  de  dos  plus  61evee. 

Examcn  de  la  red  de  arrastre  del  Mcditerraneo 
Rxtracto 

tste  trabajo  trata  de  las  tentativas  que  se  han  hecho  para  mejorar  el  rendimiento  de  las  redes  de  arrastre  de  tipo  italiano,  como  las 
usadas  en  le  mar  Mcditerranco.  Se  ha  logrado  mejorar  muy  poco  a  este  arte  por  considerar  los  Pescadores  que  ha  alcan/ado  el  m£ximo  de 
perfeccionamiento.  No  obstantc,  el  autor  al  estudiar  la  manera  de  trabajo  mediantc  fotografia  submarina  y  observation  direct  a  valiendose 
de  buzos,  ha  sugerido  formas  y  mdeios  para  construir  una  red  de  arrastre  barata,  resistente  y  eficaz,  apropiada  para  el  Mediterraneo  y  otras 
zonas  donde  tiene  gran  importancia  el  empleo  de  una  relinga  superior  a  bastante  altura  de  la  inferior. 


SJNCH  the  introduction  of  the  otter  trawl  into  the 
Mediterranean  Sea,  some  thirty  years  ago,  there 
has  been  no  report  whatsoever  on  the  performance 
of  the  gear. 

Attempts  by  the  fishermen  to  improve  the  net  have 
come  to  a  standstill  for,  according  to  them,  the  present 
trawl  gear  represents  the  climax  of  achievement  and  an> 
change  in  its  technical  pattern  is  unnecessary. 

To  provide  at  least  a  part  of  the  required  information, 
underwater  photographs  were  taken  of  the  Italian  type 
trawl  net  in  action,  typical  of  those  used  in  Israel  and 
southern  Italy13.  Studies  were  also  made  of  the"  Tzofia" 
type  hybrid  net,  and  the  "B"  type  hybrid  net  I-5- 6and7. 
The  observations  were  concentrated  upon  the  trawl 
net  itself,  observations  on  other  parts  of  the  gear  being 
left  to  the  future  when  financial  means  would  permit  the 
work  to  continue. 

Underwater  measuring  instruments,  as  used  by  De 
Boer"  and  Scharfe20,  were  not  available  for  this  survey, 
so  that  assumptions  and  conclusions  cannot  be  based  on 
exact  measurements.  The  numerous  photographs,  how- 
ever, and  the  direct  observations  of  the  divers  give,  at 
least,  a  general  picture  of  the  shape  and  behaviour  of 
the  nets  in  action. 


THE  TRAWLER 

The  underwater  observations  were  carried  out  in  Haifa 
Bay  at  a  depth  of  6  to  7  fathoms.  The  skin  divers  attached 
themselves  to  the  various  parts  of  the  net  or  floated 
just  above  it  on  a  diving  sled  I2«  n. 

The  observed  gear  was  towed  by  the  F.R.V.  Hatzvj,  a 
Danish  built  wooden  boat  of  Scandinavian  type  with 
120  h.p.  engine  and  converted  to  stern  trawling. 


THE  GEAR  (OTHER  THAN  NET) 

The  differences  between  the  examined  and  the  commercial 
trawl  gear  were  as  follows  : 

The  warp  was  of  10  mm.  o  steel  wire,  100  m.  long, 
and  appeared  to  be  proportional  to  the  depth  (under 
normal  conditions  250  m.  at  20  fathoms  and  200  at  15 
fathoms  are  used  in  this  area).  The  otter  boards  were 
160  \  90  cm.  and  typical  for  vessels  of  the  Hatzvi 
size  in  the  local  trawl  fishery.  The  swecplines  of  22 
mm.  o  combination  rope  were  shortened  from  210 
to  100  m.  The  mudropes,  which  are  several  pieces  of 
thick  rope  whipped  together,  5  to  7  m.  long  and  placed 
in  the  gear  between  the  sweeplines  and  the  wings 


213  ] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


h'ig.   1.     Italian  trawl  net  jor  120  h.p.  vessels. 
A  --    wing,   10-11  m.  long,   140  meshes  reduced  to  90,  JOO  mm. 

stretched. 
B  -—  upper  wedge,  5  m.  long,  80  meshes  reduced  to  20.  80  mm. 

stretched. 
C  *  -  belly \  15  m.  long,  220  meshes  in  each  part,  at  the  connection  to 

the  wing,  400  meshes  at  the  throat  connection,  54  to  50  mm. 

stretched. 
1)  -—  throat,  400  meshes  reduced  to  360  meshes,  40  mm.  stretched, 

4  m.  long. 

E  *-  codend,  5  m.  long,  400  meshes,  40  to  50  mm.  stretched. 
F  —  lower  wedge,  5  m.  long,  80  meshes  reduced  to  20,  100  mm. 

stretched. 
G  -  lower  body,  21  to  22  m.  long,  60  meshes  in  each  part,  at  the 

connection  to  the  wing,  40  to  60  meshes  at  the  codend,  50  to  60 

mm.  stretched. 

H  -     headline,  14  to  16  mm.  &  hemp  rope,  28  m. 
I        footrope,  40  mm.  0  hemp  rope,  34  m.     The  small  letters:  a, 

b,  c,  d,  show  how  the  lower  body  is  connected  to  the  rest  oj 

the  net. 

of  the  net,  form  an  integral  part  of  the  Mediterranean 
trawl  gear.  They  were  only  used  during  one  of  the 
tows.  The  divers  found  that  they  raise  a  cloud  of  mud 
which  interferes  with  visibility  to  such  a  degree  that  no 
records  could  be  taken.  As  the  divers  did  not  discover 
any  differences  in  fishing  spread  of  the  net  with  or  without 
these  ropes,  the  latter  were  removed  for  the  rest  of  the 
observations151. 

THE  NET 

The  observed  net  was  of  the  usual  Italian  type  used  in 
commercial  fishing  by  Mediterranean  trawlers  of  120  h.p. 
The  top  and  bottom  parts  of  the  Italian  net  differ 
greatly.  The  bottom  has  much  less  webbing,  made  of  a 
much  heavier  twine,  than  the  top  part.  The  parts  are 
connected  in  such  a  way  as  to  permit  a  high  degree  of 
slackness  in  the  bottom  (lacing  coefficient:  c  =  0-85  to 
0-90).  The  codend  is  one  tube-like  piece  of  webbing, 


ana  the  wing  consists  of  one  part,  also  common  for  top 
and  bottom.  The  front  edges  of  the  belly  are  completely 
connected  to  the  wings.  At  the  centre  of  the  mouth  there 
are  two  wedges,  upper  and  lower,  sewed  into  long  clefts 
in  the  top  and  bottom  of  the  belly.  Long  hangings, 
(30  to  45  cm.),  connect  the  webbing  to  headline  and 
footrope.  The  wedges  are  also  sewn  into  the  belly  by 
means  of  hangings.  The  net  parts  are  never  cut  to 
shape  but  the  whole  webbing  is  made  by  hand  and 
tapered,  to  reduce  the  size  and  number  of  meshes  towards 
the  codend.  The  Italian  trawl  net  is  much  longer  than 
most  other  types  of  trawl  nets. 

TEST  RESULTS 

The  Opening  Width 

The  method  used  in  finding  the  approximate  distance 
between  the  otter  doors  was  based  on  measuring  the 
distance  between  the  warps  at  two  particular  points  on 
board  the  vessel.  Knowing  the  exact  length  of  the  warp, 
the  spread  was  then  calculated.  This  method,  although 
not  exact,  suffices  to  obtain  a  rough  appraisal  of  the 
fishing  spread. 

Calculations  showed  that  the  distance  between  the 
boards  of  the  standard  gear  of  Hatzvi  varies  from  60  to 
70  m.,  and  in  the  examined  gear  from  40  to  50  m.11' 
To  assess  the  angular  difference  between  the  net's  wings 
in  both  cases,  a  comparison  can  be  made  between  the 
isosceles  triangles  formed  by  the  wings  and  the  sweeplines 
and  the  line  joining  the  two  otter  boards. 

(1)  Common  gear:  Distance  between  the  boards— 
60  m.  Length  of  the  arm  -220  m.  (this  includes  the 
sweepline,  the  mudrope  and  the  wing  of  the  net). 

a 
Therefore  the  sine  of — will  be  0*14  and  the  angle 

2 

itself  will  be  about  16  degrees  («  --  angle  between 
the  two  equal  arms). 

(2)  Experimental  gear:  Distance  between  the  boards 
40  m.;  the  arm—  112m.  (this  includes  the  shortened 
sweepline  and  the  wing  of  the  net).     The  sine  of 
a 

-equals  0-18  and  the  angle  itself  about  21  degrees. 
2 

The  difference  between  the  common  and  the  experi- 
mental gear,  regarding  the  angles  between  the  wings, 
calculated  in  this  simplified  way,  is  about  5  degrees.  There 
is  no  reason  to  believe  that  this  value  makes  a  great 
difference  in  practice,  causing  basic  changes  in  the  shape 
and  behaviour  of  the  net  in  action. 

All  measurements  and  observations  were  carried  out 
while  the  codend  was  empty.  The  current  opinion  of 
local  fishermen  is  that  a  big  catch  in  the  codend  increases 
the  total  resistance  of  the  net.  It  is  understandable  that 
if  the  codend  fills  with  mud,  stones  or  any  other  heavy 
load,  as  often  happens,  the  total  resistance  of  the  gear 
increases,  the  fishing  spread  decreases  and  in  some  cases 
the  towing  speed  decreases  as  well.  The  reaction  ma> 
be  quite  different  if  the  codend  fills  with  fish  only.  De 
Boer",  who  measured  the  trawl  gear  under  water,  b> 
means  of  special  instruments,  expresses  the  opinion  that 
a  big  catch  may  cause  a  decrease  instead  of  an  increase 


[214] 


THE     MEDITERRANEAN    TRAWL     NET 


of  the  pull  in  the  legs  of  a  trawl  but  a  normal  catch  would 
not  influence  the  tension. 

Influence  of  bottom  conditions 

All  observations  were  made  on  a  sandy  bottom.  Various 
beliefs  and  ideas  about  the  influence  of  the  type  of  the  sea 
bottom  on  the  behaviour  of  the  trawling  gear  exist  among 
the  fishermen.  The  most  common  opinion  is  that  on  a 
soft  muddy  bottom  the  sweeplines  dig  into  the  mud,  and 
move  mole-like  under  its  surface. 

In  spite  of  the  differences  between  the  actual  fishing 
conditions  and  the  conditions  under  which  the  study  was 
made,  the  author  believes  that  the  over-all  picture 
obtained  of  the  Italian  trawl  net  in  action  is  sufficient 
to  allow  a  preliminary  analysis  of  the  problem. 

The  Shape  of  the  trawl  net  in  action 

Some  facts  concerning  the  shape  of  the  net  in  action  may 
be  determined  from  the  photographs  and  observations 
made  by  the  divers  (fig.  2). 

(1)  The  curve  of  the  footrope  is  much  wider  than  the 
curve  of  the   headline.     This   suggests   that   the 
overhang  of  the  net  is  much  smaller  in  action  than 
it  appears  to  be  when  the  net  is  spread  on  shore. 

(2)  The  wing  forms  a  triangular  wall,  with  its  concave 
surface  inwards.    The  height  and  the  cross  section 
of  this  wall  increase  from  the  danlcno  towards  the 
junction  with  the  body. 

(3)  The  cross  section  of  the  net's  body  is  ellipse-like, 
becoming  more  and  more  circular  in  form  towards 
the  codcnd. 

(4)  The  after  part  of  the  codend  forms  a  swollen  ball, 
the  diameter  of  which  is  larger  than  the  diameter 
of  the  cylindrical  throat  (fig.  3). 

(5)  The  fishing  height,  measured  by  the  divers  at  the 
centre  of  the  headline,  is  90  to  1 20  cm. 


Fig.  2.     Combined  pliotograph-drawing  oj  the  Italian  trawl 
net  in  action. 

a   —  total  net  resistance. 

c  and  f  *="  spreading  forces. 

d  and  e  =  towing  forces. 

g   —  lifting  force  of  the  floais. 

h   =  connection  between  the  wing  ami  the  body. 


The  forces  acting  on  the  net 

Forces  acting  on  the  Atlantic  type  trawl  gear  have  already 
been  calculated  theoretically3  and  studies  of  various 
types  of  trawls  are  being  made  in  some  countries9,  lu,  '-° 
and  21.  But  up  to  the  present  no  report  has  been 
received  on  attempts  to  measure  these  forces  in  the 
Mediterranean  trawl  gear.  However,  the  experience 
of  the  fishermen  and  netmakers,  the  direct  underwater 
observations,  and  the  knowledge  of  the  construction  of 
the  Italian  trawl  net,  can  provide  an  explanation  in 
general  terms.  The  scheme  of  these  forces  is  given  in 
figs.  2  and  3. 

In  the  horizontal  plane,  three  main  forces  are  acting: 

(1)  Total  resistance  of  the  net  "a"  opposes  the  move- 
ment of  the  gear. 

(2)  Towing  farce  "e",  "d",  acts  at  an  angle  a  to  the 
direction  of  movement. 

(3)  Spreading  force   "c"   acts   perpendicular   to   the 
direction  of  the  movement  and  is  a  result  of  the 
action  of  the  water  stream  on  the  webbing. 

Although  we  cannot  deal  with  the  actual  values  of 
these  forces  because  of  the  absence  of  any  measured 
data,  it  is  clear  that  the  resulting  force  acts  along  the  net. 
parallel  to  the  movement.  This  force  (fig.  3-AB)  will  be 
called  in  the  following  "the  parallel  force" . 

The  spreading  force  acts  on  the  webbing  of  the  net 
from  inside  out  in  all  directions,  causing  the  net's  body 
to  expand  (fig.  2-c  and  f ).  The  action  of  all  these  forces 
together  with  the  lifting  force,  if  any,  of  the  spherical 
floats  (fig.  2-g),  creates  the  final  shape  of  the  Italian 
trawl  net  while  fishing.  As  whirlpools  are  formed  by 
the  net  moving  through  the  water,  it  is  possible  that 
additional  mutual  influences  exist  between  them  and  the 
shape  of  the  net. 

The  behaviour  of  the  gear  when  towing  is  started 

The  divers  found  that  when  the  net  is  released  and  the 
whole  gear  is  loose,  the  height  of  the  vertical  opening 
reaches  4  or  more  metres,  caused  by  the  lifting  force  of 
the  floats.  But  as  soon  as  towing  is  started,  the  vertical 
opening  of  the  net  decreases  rapidly  and  the  tightened 
headline  comes  down  to  a  height  of  about  1  m.  As  the 
trawl  begins  to  move,  a  stream  of  water  through  the 
net  body  presses  in  all  directions,  which  causes  the  bod> 
to  swell.  The  after-end  of  the  codend  opens  up  into  a 
ball  and,  being  constructed  from  heavy  webbing,  causes 
a  serious  resistance.  While  the  towing  speed  increases,  the 


tig.  3.     Scheme  of  forces  acting  on  an  Italian  trawl  net. 

a,  r,  e  and  d  as  above  (fig.  2). 

AB  —  parallel  force. «  —  the  angle  at  which  the  towing  forces  act. 


(215] 


MODERN     FISHING     GEAR     OF    THE    WORLD 


Fig.  4.     Top  pan  of  net  while  in  action,  under  full  towing  .strain. 

A        distorted  section  of  belly  webbing,  at  rear  connection  of  upper 

wedge. 

B        distorted  edge,  due  to  wrong  adjustment  of  hangings. 
C  and  D  -   drawing  show  mesh  angles. 

parallel  force  increases  and  the  lifting  effect  of  the 
spherical  floats  decreases.  As  the  Italian  trawl  net  has 
no  sidelines  the  elongation  of  the  body,  due  to  the  resist- 
ance of  the  codend,  is  limited  only  by  the  webbing  of  the 
upper  part.  As  the  parallel  force  increases,  the  meshes 
close  and  the  body  of  the  net  consequently  becomes 
longer.  The  result  is  that  the  net  body  narrows  and  forms 
a  flat,  long  funnel  shape. 

The  lifting  force  of  spherical  floats 

The  headline  of  the  experimental  net  was  equipped  with 
common  spherical  glass  floats.  The  lifting  force  of 
these  floats  at  usual  towing  speeds  of  about  3  knots  seems 
to  be  almost  negligible.  This  feature  of  the  spherical 
floats  has  been  described  by  various  authors  \  ni,  "* 
and  23. 

The  opening  height 

The  headline  of  the  Italian  trawl  net  lifts  at  regular 
towing-specds  to  a  height  of  90  to  1 20  cm.  The  efficiency 
of  the  spherical  floats  is  more  than  doubtful.  The 


height  of  the  wooden  danlenos  of  the  wings  is  40  to  60 
cm.  only,  and  pictures  show  that  they  move  at  an  acute 
angle  towards  the  bottom.  This  means  that  their 
effective  height  is  even  less.  It  would  seem  that  the 
water  stream,  while  expanding  the  net,  meets  the  bell> 
webbing  at  a  certain  angle  of  attack  and  lifts  the  upper 
part  of  the  net  to  the  observed  height. 

The  form  of  the  meshes  in  various  parts  of  the  experimental 
net 

Due  to  the  angle  taken  by  the  camera,  the  angles  of  the 
meshes,  as  measured  on  a  photograph,  are  not  quite 
accurate  (fig.  4).  Those  which  appeared  to  be  undis- 
torted  by  the  perspective  were  measured  but  the  results 
are  still  valid  only  for  a  rough  comparison  of  the  shape 
of  the  webbing  in  the  various  parts  of  the  net. 

The  length  of  the  mesh  bar  is  the  only  unchanging 
factor  if  the  stretching  of  the  twine  caused  by  towing 
tension  is  disregarded. 

All  other  qualities  of  the  mesh  (the  length,  the  width, 
the  mesh  angle)  change  according  to  the  action  of  outside 
forces,  so  that  trigonometric  and  geometric  formulae  can 
be  used  in  designing  nets  and  analysing  their  action.  For 
this  purpose  the  hanging  coefficients  are  convenient  and 
in  common  use  (figs.  5  and  6),  for  calculating  the 
relations  between  the  webbing  and  the  ropes.  Hanging 

b 
coefficient  cr        ,  (fig.  5),  gives  the  relation  between  the 

2a 
mesh  length  (b)  and  the  mesh  size  (2a).     The  co-efficient 

c 
c.,      —  represents  the  relation  of  the  mesh  width  to  the 

2a 

mesh  size.  The  bigger  b,  the  bigger  is  Cj,  the  smaller  c  , 
and  the  mesh  angle  (fig.  6).  The  relation  between  these 
coefficients  and  the  mesh  angle  can  be  calculated  trigono- 
metrically,  or  by  means  of  measuring  the  angles  on 
drawings  (fig.  6).  A  table  was  published  by  Baranov3 
p.  168. 

As  seen  from  the  underwater  observations  and  photo- 
graphs the  mesh  angles  in  the  Italian  trawl  net  vary  from 


><* t--'— 


/  /V.  5.     Miape  oj  a  net  mesh  in  work  ing  position. 

a  .  ,  a  ..,«...,  a  .  .  .  .       har\  of  the  mesh. 


h        mesh  length, 
c        mesh  width. 
a  —  mesh  angle. 
The  arrow  shows  the  direction  of  movement,    a  .        a  . . 

b  ( 

T/zr.     Hanging  coefficients:     q  ;     r., 

2a         ~       2a 


mesh 


Fig.  6.  delation  between  coefficient  of  hanging  r,  ami  mesh  anvle 
ad    mesh  length  at  (^    0  99;    *  -  15  degrees, 
be     mesh  length  at  q  -  0  -  91 ;      a  -  50°  degrees, 
ab  •  cd    the  difference  between  the  mesh  length  in  both  rase\ 


[2161 


THE     MEDITERRANEAN     TRAWL     NET 


-14m 


A/#.  7.     The  upper  belly  »»////  the  nedge  adiu\teil  into  the  r/eft. 

A         upper  wedge. 

B       belly. 

a         distorted  sector  of  webbing. 

wide  open  to  completely  closed.  In  the  after  part  of 
the  upper  belly,  and  in  the  throat,  the  meshes  are  almost 
completely  closed.  The  meshes  in  the  central  and  the 
front  parts  are  closed  in  the  sides  and  open  in  the 
upper  belly.  In  the  central  sectors  of  the  belly  and 
the  upper  parts  of  the  wings,  the  mesh  angle  seems  to  be 
not  less  than  40  degrees,  while  in  some  places  it  reaches  80 
degrees  (fig.  4).  The  width  of  the  webbing  strips,  the 
meshes  of  which  are  open,  is  at  least  50  rows  counting 
from  the  top  edge  downwards  (fig.  4).  Theoretically  this 
fact  should  cause  a  shortage  in  the  upper  part  of  the 
webbing  in  relation  to  the  mid  and  lower  parts. 

The  strain  in  the  upper  part  of  the  net 

According  to  observations  the  mesh  angle  in  the  lower 
parts  of  the  front  half  of  the  net  is  less  than  15  degrees 
compared  with  the  50  degrees  in  the  upper  strips.  The 
length  of  the  mesh  at  15  degrees,  is  only  1  -1  per  cent, 
shorter  than  the  mesh  size.  The  length  of  a  mesh  at 
50  degrees  is  9-4  per  cent,  shorter  than  the  mesh  size, 
(fig.  6).  The  difference  in  length  between  those  parts  of 
the  upper  and  lower  net  where  great  differences  in  the 
mesh  angle  appear,  should  be  approximately  8  per  cent, 
particularly  in  the  front  half  of  the  belly  and  in  the  wings. 

In  action,  therefore,  the  front  1  to  8  m.  of  the  upper 
belly,  should  theoretically  be  about  0-5  m.  shorter  than 
i he  lower  belly.  l-rom  actual  experience  this  is  not  found 
to  be  true.  The  stretching  of  the  twine  in  the  upper  net 
is  a  general!)  known  fact.  This  can  be  seen  when  a  new 
Italian  trawl  net  is  taken  ashore  for  the  first  time  for 
repairs  and  adjustment.  The  belly  webbing  is  found  to 
have  lost  its  original  form,  and  differences  between  the 
upper  and  lower  parts  arc  apparent,  the  former  being 
longer  than  the  latter.  The  fishermen  cut  triangular 
strips  from  the  stretched  sectors  of  the  webbing,  to  give 
the  belly  its  original  rectangular  form.  This  proves  that 
there  is  a  greater  stretching  force  acting  on  the  upper 
strips  than  on  the  rest  of  the  webbing,  and  this  causes  an 
elongation  of  the  meshes.  The  wide  opening  of  the 
meshes  is  not  due  to  a  weaker  parallel  force  acting  on  the 
top  of  the  net,  but  to  additional  spreading  forces  affecting 
the  sides  of  the  belly  (figs.  2  and  3). 

Ft  could  be  concluded  that  the  increase  in  the  angle  of 
the  meshes  in  the  upper  net  is  almost  balanced  by  the 


Fig.  ti.     Schematical  view  oj  the  belly  in  action. 

a    -    .sectors  under  distortinK  Jorces. 
f         angle  at  which  the  belly  is  broken. 

stretching.  In  action,  the  strips  with  wide  open  meshes 
would  be  about  5  to  10  per  cent,  shorter  than  the  parallel 
strips  with  closed  meshes  below.  However,  the  actual 
stretching  of  the  webbing  in  the  top  is  less  than  5  per  cent, 
so  that  these  two  phenomena  only  balance  each  other  in 
part.  It  seems,  therefore,  that  in  action  the  upper  parti 
of  the  belly  and  of  the  wings  arc  really  a  little  shorter 
than  the  rest  of  the  webbing,  although  the  total  strans 
over  the  top  is  stronger. 

Consideration  of  the  suitability  of  the  Italian  net  design 

The  upper  belly  of  the  Italian  net  is  a  trapezium  which  is 
almost  rectangular.  In  the  centre  of  its  front  part  there 
is  a  cleft  6  to  7  m.  long.  The  shape  is  achieved  by  gradually 
reducing  the  number  of  meshes  per  row,  and/or  by 
decreasing  the  mesh  size  towards  the  throat.  All  edges 
are  knitted  straight  and  no  shape  cutting  used.  A 
triangular  piece  of  webbing  is  laced  in  the  cleft  by  means 
of  thin  lines,  in  order  to  enlarge  the  net  mouth.  This  part 
is  called  the  upper  wedge  (fig.  7-A).  While  adjusting  this 
wedge  into  the  cleft  the  natural  shape  of  the  belly 
webbing  becomes  distorted.  Thus,  a  weak  constructional 
point  is  produced  near  the  posterior  connection  of  the 


Fig.  9.     \et  mouth  in  action. 

Ill  opened  meshes   in  upper  strips  of   webbinv. 

C  sector  under  distorting  Jorce\. 

E  edge  of  upper  wedge. 

/>/  hanging*  of  heaiiline 


[217  ] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


upper  wedge  (fig.  7-a).  This  place  often  tears,  and  the 
meshes  here  are  always  distorted,  so  that  fishermen  use 
various  methods  to  avoid  such  damage  but  with  relatively 
poor  results.  The  methods  used  are:  connecting  the 
wedge  to  the  belly  by  means  of  a  long  line  running  along 
the  centre  of  the  belly  and  reaching  sometimes  to  the 
throat  or  even  the  codend,  and/or  using  double  twined 
webbing  along  the  cleft. 

In  addition  to  the  distortion,  two  more  weak  points 
are  created  on  the  upper  edges  of  the  belly  where  the 
headline,  the  wedge  and  the  belly  meet  (fig.  8-a,  fig.  9-c). 
The  angle  is  clearly  visible  in  the  photograph,  although 
the  edge  here  was  constructed  straight  (fig.  9).  The 
fishermen  waste  a  lot  of  fishing  time  repairing  the  torn 
mesh  and  hangings  in  this  sector  of  the  net. 

The  form  the  belly  webbing  takes  in  action  is  recon- 
structed in  fig.  8.  Comparing  this  figure  with  the 
construction  plan,  it  is  clear  that  the  construction  does 
not  fit  the  shape  the  belly  has  when  in  action.  The  angle 
resulting  from  the  distortion  at  the  belly  edges  (fig.  8-a), 
was  calculated  as  being  approximately  20  to  30  degrees, 
depending  on  various  factors,  such  as  opening  width, 
length  of  hangings,  etc. 

The  same  constructional  defects  appear  in  the  lower 
part  of  the  net  along  the  lower  wedge  and  the  footrope. 
However,  as  there  is  less  strain  here  and  stronger  twine 
is  used,  less  damage  is  done  than  in  the  corresponding 
parts  of  the  upper  net. 

In  most  parts  of  the  net,  the  meshes  are  closed  when 
the  net  is  in  action,  so  that  the  webbing  partly  loses  its 
filtering  action.  In  the  after  part  of  the  belly,  and  in  the 
throat,  where  the  body  of  the  net  takes  the  shape  of  a 
tube,  the  meshes  are  so  near  to  closed  that  substituting  a 
non-filtering  material  for  net  webbing,  e.g.  canvas, 
would  have  only  a  slight  influence  on  the  behaviour  of 
the  gear.  The  poor  filtering  of  the  webbing  results  in 
increased  resistance  of  the  whole  net. 

The  curves  of  headline  and  footrope  in  the  sections 
connected  to  the  wedges,  are  very  broad,  especially  in 
the  footrope  (fig.  9).  The  fishermen  make  the  hangings 
of  the  wedges  shortest  in  the  centre  (less  than  5  cm.), 
and  their  length  is  gradually  increased  towards  the  side  of 
the  wedge.  The  divers  reported  that  the  difference  in 
length  between  the  central  and  the  side  hangings  is  too 
great  and  causes  increased  tension  in  the  centre  of  the 
wedge  and  some  slack,  in  its  sides. 

Possibilities  for  improvements  of  the  Italian  net 

A  differentiation  ought  to  be  made  between  the  term 
"improvements  in  the  Italian  trawl  net"  and  the  term 
"improvement  of  the  Mediterranean  trawl  net".  In  the 
former,  reference  is  made  to  small  technical  improvements 
in  the  present  net,  while  conserving  the  characteristic 
pattern  of  its  construction  and  behaviour  on  the  sea 
bottom.  In  the  latter,  this  pattern  is  rejected.  The  aim  of 
the  improvements  in  both  cases  is  the  saving  of  work 
and  material,  decreasing  the  net  resistance  to  the  water, 
increasing  the  catch  in  relation  to  the  towing  force 
invested  and  saving  the  young  specimens  of  no  commer- 
cial importance17. 

Few  changes  can  be  made  in  the  Italian  net  without 
deviating  from  the  accepted  pattern.  The  vertical 
opening  is  definitely  limited  because  the  upper  part  of 


the  body  is  10  to  15  per  cent,  shorter  than  the  bottom  part 
The  meshes  in  many  parts  of  the  net  are  closed  due  to  the 
absence  of  side  lines  along  the  body,  which  would  limit 
the  stretching  of  the  webbing  (see  page  220:  Hybrid  net). 

Reduction  of  the  amount  of  webbing 

Decreasing  the  number  of  meshes  in  those  parts  of 
the  webbing  where  the  mesh  is  closed,  without  suitable 
changes  in  the  parallel  force,  will  not  improve  the 
mesh  opening.  The  strain  acting  on  the  webbing  will  con- 
tinue to  keep  the  meshes  closed,  with  the  whole  tube  of 
the  net  body  becoming  thinner.  Since  the  shape  of  the  net 
mouth  and  its  dimensions  depend  mainly  on  the  hydro- 
dynamical  features  of  the  body,  any  change  in  the  size 
and  shape  of  the  body  in  action  must  produce  parallel 
changes  in  the  form  and  behaviour  of  the  net.  Therefore, 
only  a  limited  decrease  of  the  amount  of  webbing  in  the 
throat  and  the  codend  can  be  recommended  and  care 
must  be  taken  to  avoid  exaggeration  which  can  produce 
negative  effects. 

Reduction  of  the  twine  strength 

The  fishermen  of  the  smaller  trawlers  (up  to  120  h.p.), 
have  upper  parts  made  from  light  webbing  as  it  is  more 
efficient  despite  the  fact  that  the  thinner  the  twine  the 
weaker  and  shorter  living  the  net.  For  the  bigger  trawl 
nets,  the  light  webbing  proved  too  weak  to  resist  the 
greater  tension  of  the  higher  towing  speeds  of  the  larger 
vessels.  Substituting  the  thin  webbing  by  a  stronger 
and  heavier  one,  lowers  the  lifting  abilities  of  the  Italian 
net  and  reduces  the  catch. 

Limits  of  the  Italian  pattern 

It  can  be  said  that  in  the  three  most  important  aspects, 
namely:  (1)  saving  in  the  amount  of  webbing;  (2) 
lengthening  the  net's  life  by  use  of  stronger  twine  without 
changing  the  material,  and  (3)  increasing  the  fishing 
height,  no  full  satisfactory  results  can  be  achieved,  as 
long  as  the  main  pattern  of  the  Italian  net  remains 
unchanged. 

Experiments  should  be  carried  out  with  a  shortened 
body  and  a  flapper  in  the  throat,  and  with  synthetic 
twine,  none  of  which  has  yet  been  tried. 

Minor  improvements  of  construction 

Two  different  methods  arc  suggested  to  decrease  the 
damage  due  to  the  weak  points  in  the  upper  edges  of  the 
belly  and  around  the  posterior  connection  of  the  wedge. 
In  fig.  10  the  whole  wedge  is  made  much  longer,  and 
in  fig.  11  only  the  cleft  is  made  longer,  while  the 


present 
"  """" —  suggested 

Fig.  JO.    Longer  wedge  to  decrease  distortion  in  belly  webbing. 
A  -  wedge.  B      belly. 


12181 


THE     MEDITERRANEAN     TRAWL     NET 


Fig.  II.     Method  of  wedge  adjustment,  to  reduce  distortions  in 
the  belly   webbing. 

A    -    the  weak  points  performed. 
H  —  suggested  additional  hanging. 
C  —   wedge. 

hangings  of  both  sides  of  the  wedge  unite  behind  its 
end  and  continue  some  metres,  decreasing  their  length 
gradually.  Although  the  use  of  these  two  methods  will 
decrease  the  strain  around  the  end  of  the  wedge,  the 
distortion  in  the  edges  of  the  belly  will  be  even  more 
pronounced  than  before  (fig.  9-c.)  Therefore,  at  the  same 
time,  the  hangings  which  connect  the  headline  to  the 
belly  (fig.  9 — section  CD),  have  to  be  gradually  lengthened 
the  longest  hanging  should  be  nearest  to  the  wedge 
(fig.  9-c).  This  is  already  practised  by  some  fishermen. 
Here  the  author  would  rather  recommend,  instead  of 
making  the  hangings  in  the  critical  section  longer,  that 
the  other  hangings  along  the  whole  wing  be  shorter. 
This  method  of  hanging  was  already  successfully 
practised.  The  length  of  the  hangings  along  the  wing 
should  not  exceed  20  cm.  (instead  of  35  to  40  as  used 
generally)  but  those  along  the  critical  section  of  the 
belly  edge  should  be  gradually  lengthened  till  40  cm. 
at  the  wedge. 

The  side  hangings  connecting  the  wedges  to  the  ropes 
should  not  exceed  20  to  25  cm.  (fig.  9-t). 

A  more  radical  solution  would  be  to  cancel  the  wedges 
altogether.  For  this  purpose,  however,  the  number  of  the 
meshes  in  the  front  edge  of  the  belly  must  be  increased 
by  about  one  hundred.  For  this  purpose  a  rectangular 
part  of  webbing  (fig.  12-A)  could  be  removed  in  order  to 
form  the  mouth.  This  would  simplify  the  construction 
of  the  Italian  net  without  changing  the  principle  of  its 
action.  The  weak  point  around  the  wedge  end  disappears 
together  with  the  wedge  and  its  hangings.  After  cutting 
out  two  small  triangular  pieces  of  webbing  (fig.  12-A), 
a  form  close  to  that  appearing  in  action  is  given  to  the 
top  of  the  net  mouth. 

ft  must  be  underlined  that  such  small  technical 
adjustments  may  improve  the  economy  by  reducing  time 
and  expense  of  maintenance  but  have  nothing  to  do  with 
increasing  the  efficiency  of  the  Italian  trawl  net,  for  they 
do  not  change  its  towing  resistance  nor  the  size  of  its 
mouth.  For  more  basic  improvements  the  Italian  pattern 
in  the  construction  of  a  trawl  net  would  have  to  be 
rejected. 

Considerations  about  the  suitability  of  other  types  of  trawl 
nets 

The  long  experience  of  making,  and  fishing  with,  the 
Italian  trawl  net,  and  the  survey  described  above,  has 
convinced  the  author  that  this  net  cannot  form  a  basis 
for  the  development  of  an  improved  trawl. 


100  0 


Fig.  12.     tront  edges  of  upper  belly  cut  to  sha/)e. 

A        triangular  pieces  to  be  removed. 
B        belly. 
C-D  -     wings. 


Former  experiences 

From  publications  available  in  Israel  we  learn  that 
experiments  made  in  the  Mediterranean  proved  that  the 
Atlantic  or  northern  types  of  nets  were  in  all  cases  less 
efficient  than  the  Italian,  although  in  some  trials  they 
caught  more  pelagic  fish8. 

Experimental  trawling  with  the  Atlantic  gear  in  1957 
by  the  South  African  steam  trawler,  Drom  Africa, 
yielded  very  poor  results,  taking  into  consideration  the 
size  of  the  vessel  and  her  gear  and  the  richness  of  the 
north-east  Mediterranean  where  the  trawling  was  carried 
out18.  Some  experiments  were  made  by  the  late  Dr. 
Lissner,  Director  of  the  Sea  Fisheries  Research  Station, 
but  the  author  did  not  succeed  in  getting  enough  informa- 
tion about  them.  A  Yugoslavian  fishing  vessel,  Napredak, 
arrived  in  Israel  in  1953  to  carry  out  experimental  tuna 
fishing.  This  vessel,  when  the  expected  tuna  did  not 
appear,  fished  with  an  Atlantic  type  of  trawl  net  but, 
although  her  power  was  400  h.p.,  the  catches  were 
always  less  than  those  of  the  small  Israeli  trawlers  until 
a  net  of  Italian  pattern  was  used.  Experimental  fishing 
was  done  by  the  Sea  Fisheries  Research  Station  in  1957 
in  the  Bay  of  Tarsus  with  Portuguese  gear  on  board  the 
F/V  Lamcrchav  (240  h.p.).  The  results  were  almost  nil, 
and  the  trawling  was  stopped  after  two  tows.  At  the 
same  time  and  place,  big  catches  were  taken  by  Lamerchav 
and  other  Israeli  trawlers  fishing  with  their  standard  gear. 

The  Mediterranean  needs  an  improved  trawl  net, 
which  has  all  the  features  of  the  Italian  net,  is  economic 
to  construct  and  has  a  fishing  height  that  will  ensure 
catching  fish  swimming  some  metres  off  the  bottom. 
The  mesh  should  be  large  enough  to  avoid  catching 
young  fish  of  non-commercial  size,  as  recommended  in 
the  survey  carried  out  by  the  Sea  Fisheries  Research 
Station14. 


[219 


MODERN     FISHING     GEAR     OF     THE     WORLD 


Fig.   13.     Hybrid  net*  advanced  type,  as  n\ed  on  board  F\  I 
Shorn ria  (I /Oh. p.)  Upper  part. 

A     —  headline,  12  mm.  0  hemp,  28  m.  long. 

B     —  sideline,  12  mm.  0  hemp,  2  X  32  m.  long. 

C     — -  footrope,  40  mm.  &  hemp,  34  m.  long. 

D     ------   wing,  140  reduced  to  90  meshes,  110  rows.  100  mm.  stretched 

E     -=  square -,  40  meshes,  24  rows,  50  mm.  stretched. 

F     —  top,  170  reduced  to  120  meshes,  300  rows,  50  mm.  stretched. 

G     -  side,  115  reduced  to  80  meshes,  300  rows,  50  mm.  stretched. 

H    -    throat,  280  meshes,  60  rows,  40  mm.  stretched. 

1       -  codend*  300  meshes,  100  rows,  46  to  48  mm.  stretched. 

The  rows  are  of  full  meshes  (2  bars). 

A  flapper  is  sewn  into  the  throat. 

For  the  bottom  part  see  fig.    /--/•",   G,   /. 


The  hybrid  net 

During  1956  and  1957  four  different  vessels  fished 
commercially  with  hybrid  nets  as  well  as  with  standard 
nets,  and  the  hybrid  nets  had  at  least  the  same  catches 
as  the  standard  Italian  nets.  The  important  point  is  thai 
a  net  constructed  on  a  different  pattern  from  the  Italian 
can  achieve  satisfactory  results ll*15.  The  main  principle  of 
the  hybrid  net  is  that  the  towing  forces  are  acting  on  the 
sidelines  (seized  to  the  side-seams)  (fig.  13). 

The  top  (F)  is  connected  to  the  sidelines  with  a  hanging 
coefficient  c=0-88  to  0-90,  and  the  sides  (G)  with  a 
coefficient  c-  0-95  to  0-97.  The  Italian  pattern  of  the 
bottom  parts  below  the  sides  is  preserved.  The  loosely 
connected  top  part  can  be  lifted  by  means  of,  for  in- 
stance, Philip's  trawl-planes,  and  reaches  an  opening 
height  of  2-5  m.  (twice  that  of  the  Italian  net),  as  deter- 
mined by  the  divers1.  And  this  is  achieved  with  less 
webbing  in  the  upper  part  of  the  net. 

Fig.  13  shows  the  plan  of  a  hybrid  net  for  100  to  120 


Fig.  14.    Adjustment  o/  wing  tips  to  the  wooden  danleno  in  Ih 
hybrid-net.     The  height  of  the  danleno  /v  70  to  80  cm. 

u  -   thimble, 

h  wooden  danleno, 

<  foo  trope, 

d  —  headline. 

t'  sideline. 

/  wing  connecting  rope. 

X  wing  webbing. 

h  net  to  foot  rope  connection  ( Italian)  method, 
direct  net-to-rope  connection. 


h.p.  trawler  as  now  used  by  the  F/V  Shomria.  (See  also 
figs.  14  and  15). 

FURTHER  DEVELOPMENTS 

The  preliminary  observations  of  the  Italian  trawl  net, 
together  with  the  theory  of  net  construction3,  show  the 
way  for  further  development.  The  way  suggested  by  the 
author  is  to  remove  the  towing  strain  from  the  net 
webbing  by  the  use  of  a  rope  skeleton  sewn  into  the  net. 


f220) 


THE     MEDITERRANEAN    TRAWL    NET 


Fig  15.  Wing  adjustment  to  iron  pipe  danleno  in  hybrid-net  in  action. 

Height  of  the  danleno  80  cm. 
a         triangular  pipe  danleno. 
h        swivel, 
c    ~  jootrope. 
d    -    wing  connecting  rope, 
e    •-  sideline, 
f   =--  headline. 

g    —  spherical  glass  floats,  appearing  here  only  Jor  divert 
convenience. 

Such  constructional  improvement  will  give  back  to  the 
net  webbing  its  original  ability  to  filter  the  water  and  will 
save  a  lot  of  net  material  without  decreasing  the  net  size. 
Moreover,  it  will  enlarge  the  mouth  opening.  These 
advantages  have  been  proved  in  fishing  with  and  under- 
water observations  of  hybrid  nets.  But  the  hybrid  net  is 
only  a  first  step  to  the  future  Mediterranean  trawl  net. 
The  meshes  are  closed  in  the  after  part  of  the  body  as  in 
the  Italian  net.  Waste  of  material  and  superfluous 
resistance  are  big  in  this  net,  too.  In  the  suggested  net 
the  webbing  will  be  free  from  the  action  of  the  parallel 
force.  The  relation  between  the  webbing  and  the  ropes 
can  be  calculated  by  the  use  of  hanging  coefficients.  This 
will  enable  the  constructor  to  design  a  net  pattern 
suitable  to  the  net  shape  when  in  action.  Furthermore,  in 
such  a  net  all  the  meshes  will  be  open  and  less  webbing 
will  cover  a  particular  surface. 

According  to  calculations  made  by  the  author,  at 
least  64  per  cent,  of  the  webbing  can  be  saved  in  those 
parts  where  the  mesh  angle  does  not  exceed  18  degrees,  if 
adjusted  to  the  side  ropes  with  a  hanging  coefficient  c= 
0-87.  This  coefficient  gives  a  mesh  angle  of  60  degrees. 

The  length  of  the  whole  net  will  be  assured  by  means  of 
sidelines  preferably  made  of  thin  (12  to  14  mm.)  combina- 
tion rope,  while  the  webbing  will  control  only  the  swelling 
of  the  net.  Such  an  improvement  leads  to:  (a)  saving  of 
net  material;  (b)  decreasing  of  resistance,  and  (c)  liber- 
ating the  top  of  the  net  from  horizontal  strain,  thus 
enabling  it  to  attain  a  larger  opening  height.  It  does  not 
oppose,  otherwise,  the  Italian  principle  of  loose  bottom 
parts.  This  net  can  be  constructed  of  stronger  (heavier) 


webbing,  because  its  shape  in  action  will  be  assured  b> 
the  rope  skeleton  and  the  hydro-dynamic  floats,  and  not 
only  by  the  action  of  the  water  stream  on  the  webbing, 
as  seems  to  be  the  case  in  the  Italian  trawl  net.  The  new 
net  can  be  constructed  of  machine-made  webbing. 

CONCLUSION 

A  cheap,  strong  and  efficient  trawl  net  for  the  Mediter- 
ranean could  be  constructed  as  a  result  of  technological 
surveys  and  experiments.  The  best  way  to  develop  such 
a  net  seems  to  be  in  rejecting  the  old  pattern  and  shifting 
the  towing  strain  from  the  webbing  to  ropes. 

The  proposed  pattern  could  be  equally  applied  to  areas 
outside  the  Mediterranean  where  an  increase  in  the  fishing 
height  may  be  of  far  greater  importance. 

REFERENCES 

1  Assaf,    1.     Underwater   Observations    of  the    Hybrid    Net.. 
Fishermen's  Bulletin  No.  9,  Haifa  1956.  (In  Hebrew). 

2  Assaf,  I.     Personal  information,   1956. 

3  Baranov,  F.  I.    Theory  and  Calculations  of  Fishing  Gear 
Moscow,    1948.    (In  Russian). 

4  Bar-on,  E.   Personal  information,  1956. 

5  Ben-Yami,   M.    A  New  Trawl   Net.     Fishermen's  Bulletin 
No.  8,  Haifa,  1956.    (In  Hebrew). 

6  Bcn-Yami,   M.    The  Design  of  the  Hybrid   Net  Type  B, 
Fishermen's  Bulletin  No.  9.  Haifa,  1956.  (In  Hebrew). 

7  Ben-Yami,  M.    Preliminary  Report  on  Experimental  Fishing 
with  a  New  Trawl  Net.  General  Fisheries  Council  for  the  Mediter- 
ranean, 4th  Meeting,  Technical  Paper  No.  35,  Istanbul,  1956. 

8  Bernstein,  D.    Three  Experiments  for  increasing  the  height 
of  the  Head  Rope.   Fishermen's  Bulletin  No.  4,   Haifa   1955. 
In  Hebrew). 

9  De   Boer,   P. A.    Trawl    Gear  Measurements    obtained    by 
Underwater  Instruments.     International  Council  for  the  Explora- 
tion of  the  Sea  and  North  Sea  Sub-Committee  Comparative 
Fishing.     No.  4,  1954. 

10  De  Boer,  P.  A.      Private  correspondence,  1956. 

11  Crew  of  M.  F.  V.  Tzofta - Preliminary  Report  on  a  New 
Type  of  Trawl  Net.     Fishermen's  Bulletin  No.  8,  Haifa,   1956. 
(In   Hebrew). 

12  Fried,  Z.  and  Assaf,  I.    Aqua-lung  Tests  of  the  Italian  Trawl 
Net.     Fishermen's  Bulletin  No.  8,  Haifa,  1956.    (In  Hebrew). 

13  Fried,  Z.     Underwater  study  of  the  Italian  Type  Trawl  Gear. 

14  Gottlieb,  E.  and  Orcn,  O.  H.  Savings  Gear  Experiments  with 
Trawl  Nets  in  Israel  Waters.    General  Fisheries  Council  for  the 
Mediterranean,  Technical  Paper  No.  36,  Istanbul,  1956. 

15  Hamburger,  E.   Preliminary  Results  on  the  Operation  of  the 
Hybrid  Net,  Fishermen's  Bulletin  No.  9.  Haifa,  1956.  (In  Hebrew). 

16  Hamburger,  E.     Personal  information,  1956. 

17  Kristjonsson,  H.    The  scope  for  Technological  Development 
in  the  Mediterranean  Fisheries.   General  Fisheries  Council  for  the 
Mediterranean,  No.  2.    Technical  Paper  No.  36,  Rome,  1954. 

18  Lissner,  H.  and  Hirsh,  H.    Experimental  Fishing  in  the  Bay 
of  Alexandretta.  Bulletin  No.  1.  Sea  Fisheries  Research  Station, 
Haifa,  1948.    (In  Hebrew). 

19  Percier,  M.  A.    L'ouverture  du  Chalut  en  Hauteur.   Genera 
Fisheries  Council  for  the  Mediterranean,  Rome,  1952. 

20  Scharfe,  J.    Geraete  zur  Messung  an  Schleppnetzen.    Die 
Fischwirtschaft  Hefl  1  und  2,  1953. 

21  Scharfe,  J.  Dass  Messcn  dcr  Zugbelastung  bei  Schleppnetzen. 
Die  Fischwirtschaft,  Heft  2,  1955. 

22  Smislov,  I.  G.  Analysis  of  Action  of  Spherical  Trawl  Floats, 
V.N.I.R.O.  Works,  Vol.  XXX,  Moscow  1955.    (In  Russian). 

23  Yacovlcv,  A.  I.    Results  of  Hydrodynamical  Tests  of  Trawl 
Floats.  V.N.I.R.O  Works,  Vol.  XXX,  Moscow.  1955.  (In  Russian). 


[221 


FACTORS  AFFECTING  THE   EFFICIENCY   OF   DREDGES 

by 

R.   H.    BA1RD 

Ministry  of  Agriculture,  Fisheries  and  Food,  Fisheries  Experiment  Station,  Conway,  Caerns,  U.K. 


Abstract 

In  this  paper  the  various  factors  affecting  the  efficiency  of  the  dredge  are  discussed.  The  depressive  effect  of  diving  plates  allows  an 
increase  in  the  towing  speed,  and  the  angle  of  attack  of  the  plates  has  an  optimum  value.  The  angle  at  which  the  teeth  of  the  dredge  travel 
over  the  bottom  also  has  an  optimum  value,  about  45  degrees,  and  the  selective  action  of  the  teeth  is  a  useful  method  of  reducing  the  amount 
of  trash  taken.  Preliminary  attempts  have  been  made  to  assess  the  absolute  efficiency  of  the  standard  Manx  scallop  dredge  and  it  was  proved 
to  be  low. 


Rfeum* 


Etude  des  facteurs  conditionnant  I'efficacite  des  drogues 


L'auteur  examine  dans  cet  article  les  diflferentes  facteurs  qui  conditionnent  I'efficacite  de  la  drague..  L'eflet  d'abaissement  des 
plaques  de  plongec  pcrmet  d'accroitre  la  vitesse  de  remorquage,  ct  il  cxiste  unc  valeur  optimum  dc  Tangle  d'attaque  des  plaques.  L'angle 
selon  lequel  les  dents  de  la  drague  attaquent  le  fond,  est  optimum  £  45  degre  environ.  Une  bonne  methode  consiste  a  tirer  parti  de  1'effet 
sdectif  des  dents  pour  reduire  la  quantit6  de  matdriaux  ind&irables  ramasses  par  la  drague.  Des  essais  pr&iminaires  entrepris  pour  £  valuer 
le  rendement  absolu  de  la  drague  "Manx1*  standard  a  coquilles  St.  Jacques  ont  montre  que  Pefficacite  de  ce  modele  6tait  faible. 


Exposicidn  de  los  factorvs  que  f  nfluyen  sobrel  a  eficacia  de  los  rastros 
Extracto 

En  este  trabajo  se  analizan  los  diversos  factores  que  influyen  sobre  la  eflcacia  de  los  rastros.  HI  efecto  de  las  planchas  de  inmersion 
permitc  aumentar  la  velocidad  de  arrastre  de  acuerdo  con  su  angulo  de  ataque  hasta  alcanzar  un  valor  maximo.  For  su  parte,  el  angulo  de 
incidencia  de  los  dientes  de  estas  dragas  cuando  avanzan  por  el  fondo  del  mar  alcanza  un  valor  opt  i mo  a  45  ,  y  la  acci6n  selectiva  de  ellos 
constituye  un  metodo  util  para  reducir  la  cantidad  de  desperdicios  recogidos.  So  han  hecho  ensayos  preliminarcs  con  objeto  de  e valuer  el 
rendimiento  absolute  de  los  rastros  para  peregrinas  o  veneras  utilizados  corrientementc  en  la  isla  de  Man,  que  result 6  ser  bajo. 


THIS  paper  is  intended  for  marine  biologists  as  well 
as  for  commercial  fishermen.  Much  of  the  discus- 
sion is  of  an  elementary  nature  and  over-simplified; 
qualifications  should  follow  most  of  the  statements.  How- 
ever, as  dredges  have  evolved  very  slowly,  and  nearly 
always  in  an  empirical  manner,  at  least  in  Europe,  it  will 
perhaps  make  a  starting  point  to  discuss  in  a  general 
way  some  of  the  problems  involved. 

Underwater  observation  of  dredges  in  action  shows 
that  scallop  dredges  with  rope  warps  have  a  tendency  to 
skip  over  the  bottom1  and  oyster  dredges  to  slither.  In- 
creases in  speed  of  tow  above  a  low  level  tend  to  reduce 
catches.  This  can  be  overcome  to  some  extent  by  fitting 
depressors  or  diving  plates  to  the  dredges.  One  such 
dredge  for  scallops  has  already  been  described2.  The 
fitting  of  teeth  to  dredge  bars  improves  performance  in 
some  cases,  but  makes  the  angle  of  attack  of  the  blades 
more  critical  than  without  teeth.  Tooth  spacing  is  also 
important. 

The  shape  of  the  catenary  of  a  warp  in  the  water  will 
depend  on  the  drag  of  the  warp  which  will  vary  with  the 
material.  In  general,  a  wire  warp  will  have  a  forward  and 
downward  catenary,  the  weight  in  water  being  greater 
than  the  drag;  a  rope  warp  will  have  a  backward  and 
upward  catenary,  the  drag  being  greater  than  the  weight 
in  water. 

In  the  following  discussion,  the  weight  of  the  warp 


will  be  ignored,  although  it  can  be  seen  that  a  wire  warp 
can  increase  the  effective  weight  of  the  dredge  and  a  rope 
warp  can  exercise  considerable  lift. 

The  normally  used  warp-depth  ratio  for  dredging  is 
3:1.  The  actual  length  of  warp  required  disregarding 
the  effect  of  the  warp  itself  will  be  determined  by  the 
drag  of  the  dredge  which  is  proportional  to  the  velocity 
squared  (D--— KnVa),  and  by  its  weight  in  water.  An  in- 
crease in  speed  of  tow  necessitates  a  much  increased  ratio 
of  warp  to  depth. 

EFFECT  OF  DIVING  PLATES 

With  a  diving  plate,  another  factor — lift  —  is  introduced, 
which  can  act  upwards  (positive)  or  downwards  (nega- 
tive). In  addition  to  the  lift  from  a  diving  plate,  induced 
drag  occurs.  Thus,  there  are  two  sources  of  drag,  that 
from  the  dredge  itself,  called  parasitic  drag  (Dp),  and 
induced  drag  from  the  diving  plate  (Dj).  As  with  drag, 
lift  is  proportional  to  velocity  squared  (L— KtV2). 
From  fig.  1  it  can  be  seen  that  although  the  negative  lift 
from  a  diving  plate  increases  the  warp  angle,  increase  in 
speed  still  decreases  the  warp  angle  and  thus  some  more 
warp  is  required  for  a  given  depth.  Parasitic  drag  should 
be  kept  as  low  as  possible  by  the  use  of  as  large  a  mesh 
as  possible  and  by  avoidance  of  unnecessary  large  sur- 
faces in  the  construction. 


[222] 


EFFICIENCY    OF    DREDGES 


> 

I 


t«n«-C   -L   I 


e  I 


1 1 


vcloolty 


Fig.  1.  The  ejjevt  on  warp  angle  of  increasing  speed  of  tow  with 
diving  plate.  The  sign  of  lift  and  weight  show  direction. 

The  lift/drag  ratio  is  at  its  maximum  at  rather  small 
angles  of  attack,  the  drag  increasing,  with  increased  angle 
of  attack,  quicker  than  the  lift  which,  furthermore,  in- 
creases only  up  to  the  point  of  stall.  However,  when  the 
parasitic  drag  is  high,  the  amount  of  negative  lift  needed 
for  a  given  size  of  diving  plate  requires  a  bigger  and, 
therefore,  less  efficient  angle  of  attack. 

A  secondary  effect  of  a  diving  plate  is  to  help  material 
into  the  bag  of  the  dredge  by  deflecting  upwards  the 
waterflow  at  the  mouth  of  the  dredge.  It  has  been  ob- 
served that  if  the  angle  of  attack  of  the  diving  plate  is 
too  great,  more  trash  is  retained.  This  may  be  explained 
by  the  eddies  formed  by  the  partial  or  complete  stall  of 
the  plate  resulting  in  a  slowing  down  of  the  water  flow 
at  the  mouth  of  the  dredge,  with  a  tendency  for  trash  to 
accumulate  in  the  front  of  the  dredge  bag  (fig.  2). 

The  use  of  diving  plates  affects  the  stability  of  the 
dredge  while  it  is  in  mid-water  during  shooting.  A 
dihedral  angle  on  the  diving  plate  will  give  lateral 
stability  about  the  longitudinal  axis.  Longitudinal  stab- 
ility is  maintained  by  the  point  of  tow.  If  negative  lift  is 
maintained  on  the  plate  with  negative  dihedral  (an- 
hedral)  while  shooting,  righting  moments  occur  when  the 
dredge  is  disturbed  laterally.  If  the  plate  gives  a  positive 
lift  with  negative  dihedral  angle  the  dredge  will  be  turned 
over. 

Fig.  3  illustrates  the  effect  of  negative  dihedral  and 
negative  lift.  When  the  dredge  is  disturbed  laterally, 
weight  continues  to  act  vertically  downward  but  the 
negative  lift  is  inclined  to  the  vertical.  If  the  negative  lift 
is  split  into  its  vertical  and  horizontal  components,  it 
can  be  seen  that  the  horizontal  component  is  acting  to 
the  right,  causing  a  relative  waterflou  to  the  left,  which 


fif.  2.  Secondary  effect  of  stalled  diving  plate.  Eddies  behind 
stalled  plate  can  cause  accumulation  of  trash  in  the  dredge  bag 


r«dft«  to  right 
*  r«l»tiv«   flow 


Fig.   3.   Effect  oj  dihedral.   Temporary  displacement  of  plate 
results  in  forces  righting  moments. 

gives  positive  lift  to  the  left  plane  and  negative  lift  to 
the  right  plane,  resulting  in  righting  moments.  How- 
ever, if,  as  sometimes  happens,  when  the  tow  is  taken 
from  the  leading  edge  of  the  dredge  and  the  warp  is  run 
out  with  restraint  (fig.  4),  the  diving  plate  will  give  posi- 
tive lift,  turning  the  dredge  on  to  its  back;  the  forces 
are  illustrated  in  fig.  5. 

In  fig.  6,  a  dredge  designed  for  mussel  and  oyster 
fishing  is  shown  with  two  alternative  points  of  tow.  When 
the  forward  points  of  tow  are  used,  the  dredge  has 
positive  lift  and  is  unstable  in  mid-water  if  checked 
during  shooting.  This  had  the  disadvantage  that  when  the 
depth  suddenly  increasedjthe  dredge  left  the  bottom  and 
turned  over.  With  the  after  points,  a  negative  lift  was 
always  maintained  and  the  dredge  was  completely  stable 
and  could  be  towed  in  mid-water  without  turning  over. 

THE  EFFECT  OF  TEETH 

Scallops  normally  lie  recessed  with  the  flat  valve  approxi- 
mately in  the  plane  of  the  bottom.  On  escallop  dredges 
the  teeth  penetrate  the  bottom  and  get  below  the  edge 
of  the  shell,  thus  lifting  the  scallops  into  the  bag.  There 
are,  however,  considerable  secondary  effects.  It  has  been 
shown1  that  the  teeth  on  scallop  dredges  give  a  highly 
selective  effect,  operating  probably  more  efficiently  than 
the  selective  effect  of  the  meshes  of  the  bag.  Quantitative 
information  on  this  effect  is  limited  to  date,  but  from  my 
own  data  and  from  those  of  Mason  (1953)  the  suggestion 
is  that  the  sizes  at  which  50  per  cent,  are  selected  are 
between  20  per  cent,  and  50  per  cent,  larger  than  the  tooth 
spacing.  The  teeth,  furthermore,  are  also  sifting  out  trash, 
i.e.  smaller  organisms,  shells  and  small  stones.  This  allows 
a  much  longer  tow  with  an  increased  proportion  of  fish- 
ing time  with  the  dredge  on  the  bottom.  For  this  screen- 
ing action  to  be  effective  there  must  be  clearance  between 
the  dredge  bar  and  the  bottom.  With  a  shallow  angle  of 
attack  of  the  teeth  of  the  dredge,  excessively  long  teeth 
are  necessary  to  allow  for  sufficient  clearance  and  pene- 
tration of  the  bottom.  These  are  very  easily  damaged. 
With  an  angle  of  attack  of  the  teeth  approaching  90 
degrees,  shorter  teeth  can  be  used,  but  entry  of  the  catch 


Dirln«  plat*  «ivin*  pocltiv*  lift 


Relative 
flow      — 


Fig    4.  Diving  plate  giving  positive  lift  during  shooting  when 
tow  is  taken  from  leading  edge  and  warp  is  run  out  with  restraint. 


t  223  ] 


MODERN     FISHING     GEAR     OF    THE     WORLD 


turning 


Horisoat*!  oonponmit  of  lift  Bovine 
drod**  to  loft 


Fig.  5.  Force*  capsizing  dredge  during  shooting  \\hendi\inx  plate 
gives  positive  lift. 

into  the  bag  is  impeded.  Experiments  done  at  Conway 
with  a  model  dredge  of  about  0-5  metres  span,  with  an 
adjustable  tooth  angle  but  uniform  bottom  penetration, 
have  suggested  that,  at  varying  speeds,  entry  into  the  bag 
is  not  much  impeded  up  to  angles  of  45  degrees.  Within 
limits,  higher  speeds  of  tow  allow  steeper  tooth  angles 
to  be  used  before  passage  of  scallops  over  the  teeth  is 
impeded. 

ABSOLUTE  EFFICIENCY 

The  catching  efficiency  of  dredges  is  low  and  will  vary 
with  the  type  of  bottom.  Coral  gravel  bottoms  in  Corn- 
wall and  near  Port  Erin,  Isle  of  Man,  at  depths  of  about 
20  metres,  have  been  observed  to  be  furrowed  to  a  depth 
up  to  10  cm.  with  20  to  30  cm.  between  the  crests  of  the 
ridges.  The  passage  of  a  dredge  effectively  flattens  this 
type  of  bottom.  It  can  be  seen  that  a  dredge  will  have 
a  low  catching  efficiency  on  a  furrowed  bottom  because 
it  will  be  either  skimming  the  crests  of  the  ridges  and 
missing  many  of  the  scallops,  or  digging  into  the  ridges 
and  catching  a  lot  of  trash  which  will  soon  fill  the  dredge. 
An  increase  in  efficiency  will  occur  as  the  bottom  is 
flattened  by  working.  (It  is  often  stated  by  scallop  fisher- 
men that  catches  increase  on  some  grounds  after  some 
days  of  intensive  working.) 

Dickie3  measured  the  efficiency  of  Canadian  dredges 
by  releasing  marked  scallops  and  afterwards  dredging 
in  the  area,  assessing  efficiency  by  the  estimated  number 
encountered  per  tow  and  the  actual  number  caught.  He 
found  an  efficiency  of  from  5  per  cent,  to  12  per  cent., 
depending  on  the  ground  worked.  Walner>  assessed  the 
efficiency  of  a  hand  oyster  dredge  on  slipper  limpets 
(Crepidula  fornicata)  by  comparing  the  catch  of  the 
dredge  to  the  density  of  limpets  as  assessed  by  grab 
sampling.  He  found  an  efficiency  of  16  per  cent.  Shel- 
bourne4  measured  the  efficiency  of  a  winch-operated 
oyster  dredge  on  slipper  limpets  by  distributing  a  known 
number  of  limpets  between  poles  on  a  clean  intertidal 
area  at  low  water  and  fishing  between  the  poles  at  high 
water.  He  found  this  dredge  to  be  30  per  cent,  efficient. 
A  special  sampling  dredge  he  devised  had  an  efficiency 
of  60  per  cent.  However,  although  it  might  be  quite 
possible  to  devise  a  dredge  that  was  nearly  100  per  cent, 
efficient  for  a  short  tow  on  a  soft  bottom,  it  does  not 
follow  that  this  will  give  the  most  economical  fishing. 
This  is  clearly  demonstrated  by  Shelbourne  where  the 
ratios  of  catch  per  distance  run  of  3-3,  I  -0,  0-6  were 
obtained  for  the  survey  dredge,  the  winch-operated 
dredge  and  hand  dredge  respectively  on  oysters.  The 
ratio  of  output  of  oysters  per  man  day  was  1-0,  1-1, 
1-6  for  the  most,  medium  and  least  efficient  dredges 
respectively.  This  was  almost  entirely  due  to  the  degree 
of  selectivity,  the  least  efficient  dredge  being  most 


/ig.    ft.    Mussel  antt  oyster   dredge   showing   alternative    to\\ 

attachments.  Forward  attachment  results  in  mid- water  instability 

when  warp  is  checked.  After  points  give  complete  miil-watei 

stability. 

selective  and  resulting  in  the  highest  daily  catch  ol 
oysters  on  a  "mixed"  ground.  Where  the  organisms  to 
be  caught  are  fairly  sparse  on  the  bottom,  a  high  selec- 
tivity is  as  important  as  a  high  efficiency,  otherwise  the 
dredge  fills  too  quickly  with  unwanted  material  and  so 
has  to  be  handled  more  frequently. 

A  preliminary  attempt  to  measure  the  efficiency  of  a 
traditional  scallop  dredge  was  made  at  Port  Erin,  Isle 
of  Man,  with  the  aim  of  using  the  dredge  as  a  quantita- 
tive sampling  device  and  as  a  standard  to  which  other 
dredges  could  be  related.  Using  a  self-contained  breath- 
ing apparatus,  the  actual  density  of  scallops  was  measured 
by  collecting  all  scallops  passing  between  the  runners 
of  a  sledge  towed  slowly  over  a  known  distance.  Dredge 
hauls  of  measured  distances  were  made  in  the  same  area. 
If  only  commercial  scallops,  11-5  cm.,  and  upwards, 
are  considered,  diving  showed  these  to  be  present  at  a 
mean  density  of  1  per  40  m.-  and  the  dredge  covered 
250  m.2  for  every  one  caught,  giving  an  efficiency  of 
16  per  cent,  for  commercial  scallops.  The  mean  densitx 
of  all  sizes  of  scallops,  found  by  diving  on  the  bed  being 
investigated,  was  I  per  II  m.2  The  dredge  covered 
130  m.2  of  bottom  lor  every  scallop  caught,  giving  an 
efficiency  of  8-5  per  cent.  The  selective  effect  of  the 
dredge  will  cause  this  figure  to  vary  with  the  si^e- 
distribution  of  scallops  on  the  bed. 

It  must  be  emphasized  that  the  range  and  scatter  of 
these  results  was  such  that  the  only  inference  that  can 
safely  be  made  is  that  dredge  efficiency  is  of  a  low 
order,  probably  between  5  and  20  pzr  cent,  with  a  higher 
efficiency  when  working  on  the  larger  scallops. 

REFERENCES 

1  Baird,  R.  H.  and  Gibson,  F.  A.    "Underwater  Observations 
on  Escallop  (Pecten  maximus)  Beds."  J.  mar.  biol.  Ass.  U.K.  (1956) 
35,o  555-562. 

2  Baird,  R.  H.  "A  Preliminary  Report  on  a  New  Type  of  Com- 
mercial Escallop  Dredge".  J.  d.   Con.lnt.,  Vol.  XX,  No.  3.  1955. 

3  Dickie,  L.  M.  ''Fluctuations  in  Abundance  of  the  Giant  Scal- 
lop, Placopecten  rnaffc/lanicus  (Gmelin),  in  the  Digby  area  of  the 
Bav  of  Fundy."  J.  Fish.  Res.  Bd.  Canada,  12  (6),  1955. 

4  Shelbourne,  J.  b.  "The  1951  Oyster  Stock  in  the  Rivers  Crouch 
and  Roach,  Essex."  Fish.  Invest,.  Ser.  II,  XXI,  No.  2. 

5  Walne,  P.  R.  "The  Biology  and  Distribution  of  the  Slipper 
Limpet  (Crepidula  fornicata)  in  Fssex  Rivers.*'  Fish.  Invest.,  Ser. 
II,  XX,  No.  6. 


[2241 


TRAWL   GEAR   MEASUREMENTS   OBTAINED   BY  UNDERWATER 

INSTRUMENTS 

by 

P.   A.   de   BOER 

Fisheries  Inspectorate,  The  Hague,  Netherlands 

Abstract 

The  Netherlands  Fishery  Inspectorate  has  devised  a  series  of  instruments  for  recording  the  behaviour  of  trawl  gear,  as  follows:  1.  The 
Spread  Meter,  to  record  the  distance  between  the  after  ends  of  the  otter  boards  or,  if  the  boards  are  attached  directly  to  the  wings,  the  horizon- 
tal opening  of  the  net.  2.  The  Clinometer,  which  records  the  till  of  the  otter  boards  sideways  as  well  as  fore  and  aft.  3.  A  Net  height  meter 
for  measuring  the  vertical  opening  of  the  net.  4.  An  angle  of  attack  meter,  for  recording  the  angle  of  attack  of  the  otter  boards.  5.  An 
Hydraulic  Dynamometer,  for  measuring  the  pull  of  the  legs  between  the  boards  and  the  net,  and  also  for  recording  the  tension  in  the  warps 
on  board  the  ship. 

The  author  describes  the  evolution  of  the  instruments  and  then  gives  some  interesting  results  obtained  by  their  use,  for  instance 
the  relationship  between  warp  length,  the  spread  of  otter  boards,  their  tilt  and  angle  of  attack  and  the  opening  height  of  the  net. 

Tests  were  made  using  different  kinds  of  floats  on  the  headline,  and  it  was  found  that,  using  no  floats,  the  headline  height  was  1  -2  m.,, 
while  10  spherical  floats  raised  it  to  1  -9  m.,  5  "Siamese-twin"  floats  increased  it  to  2-3  m.,  and  10  "trawlplanes"  lifted  it  to  2-75  m. 


Resirni* 


Matures  effectives  sur  des  chaluts  au  moyen  d'appareils  sous-marins 


Le  Service  d'lnsnection  des  peches  des  Pays-Bas  a  mis  au  point  les  ap pure! Is  suivants  pour  cnrcgistrer  le  comportemcnl  du  chalut: 
(1)  un  ecartemetre  pour  cnrcgistrer  la  distance  scparant  les  extremites  posterieures  des  plateaux,  ou,  si  ceux-ci  sont  fixe  directement  aux 
ailes,  Pouverture  horizontal  de  filet.  (2)  Un  clinometre  qui  cnregistre  Pinclinaison  long*tudimilc  et  latcralc  des  plateaux.  (3)  Un  Paravane 
avec  manometre  differential  pour  mesurer  Pouvcrturc  vcrticale  du  filet;  (4)  un  appariel  pour  mesurer  l"angle  d^attaque  des  plateaux.  (5;  Un 
dynamometre  hydranlique  pour  mesurer  la  traction  des  bras  reliant  les  plateaux  au  filet,  ainsi  que  la  tension  des  funes  a  bord  du  navire. 

L'autcur  decrit  ('evolution  des  appareils  et  donne  ensuite  certains  resultats  inlercssants  obtenus  par  leur  ernploi,  par  exemple,  les 
rapports  entre  la  longueur  des  funes,  Pecartcment  des  plateaux,  leur  mclmaison  et  leur  angle  d'attaque. 

Des  essais  ont  ete  executes  avec  differents  types  de  flotteurs  sur  la  ralingue  supericure,  et  Pon  a  constate  que  la  hauteur  de  la  ralingue 
etait  de  1,20  m.  sans  flolteurs,  de  1m  90  avec  10  flotteurs  sphcriques,  dc  2m  30  avec  5  flotteurs  "freres-siamois",  et  de  2m  75  avec  10  "  trawl- 
planes". 

Mediciones  en  las  redes  de  arrastre  con  instruments  submarines 
Kxtracto 

La  Inspeccion  de  Pcsca  dc  los  Paiscs  Bajos  ha  ideado  los  siguicntes  instrumcntos  para  registrar  la  manera  como  se  comporta  una 
red  de  arrastre:  (1)  HI  medidor  de  separacion  de  las  puertas  dc  arrastre  para  registrar  la  distancia  que  media  entre  los  extremes  postcriores  de 
ellas  o,  cuando  estas  se  encucntran  unidas  directamente  a  las  bandas.  la  anchura  de  la  boca  de  la  red.  (2)  HI  clinometro  que  mide  la  inclinacion 
de  dichas  puertas,  tanto  en  sentido  lateral  como  longitudinal.  (3)  El  paravan  con  manometro  dijerencial  para  determinar  la  altura  de  la  boca 
de  la  red.  (4)  El  medidor  del  angulo  de  ataque  para  registrar  cl  angulo  de  ataque  de  las  peurtas  de  arrastre.  (5)  El  dinamometro  hidrdulico 
para  cvaluar  la  tracci6n  de  las  mulletas  y  de  los  cables  de  arrastre  a  bordo  del  barco. 

HI  autor  describe  en  el  trabajo,  muy  bien  ilustrado  con  fotografias  y  esquemas,  la  evoluci6n  de  los  inst rumen tos  y  da  a  conocer 
algunos  resultados  intcresantes  obtcnidos  mediante  su  uso;  por  ejemplo,  la  relacibn  entre  la  longitud  de  los  cables  de  arrastre,  la  separacion 
dc  las  puertas,  su  inclinaci6n  y  angulo  de  ataque.  Se  hicieron  pruebas  con  divcrsos  tipos  de  flotadores  distribuidos  a  lo  largo  de  la  relinga 
superior,  encontrandosc  que  al  usar  ninguno  la  altura  de  6sta  Ilegaba  a  1,2  m..  mientras  que  con  10  flotadores  esfericos  subia  a  1,  9  m.,  con 
5  flotadores  siameses  ("Siamese-twin")  a  2,  3  m.  y  con  10  "trawlplanes"  a  2,75  m. 


FILMS  taken  by  frogmen,  showing  trawls  and 
seines  in  action  under  water,  have  enabled  fisher- 
men for  the  first  time  to  see  the  behaviour  of 
their  nets  on  the  bottom  and  the  reaction  of  the  fish 
to  the  net. 

Although  helpful,  underwater  filming  does  not  solve 
all  the  problems.  It  is  also  necessary  to  take  measurements 
of  the  gear  in  action.  For  this  reason  the  Netherlands 
Inspection  of  Fisheries  has,  during  the  past  four  years, 
developed  several  underwater  instruments  to  record  the 
behaviour  of  the  gear  in  tow. 

Extensive  experiments  have  been  made  on  board  the 
Netherlands  F.R.V.  Antoni  van  Leeuwenhoek  (fig.  1). 
These  have  led  to  many  improvements  in  the  instruments 
which,  as  they  were  designed  for  work  in  the  Southern 


North   Sea,   only  withstand   pressures  to  a   depth  of 

lOO  mp.trp.fi 


DESCRIPTION  OF  THE  INSTRUMENTS 

The  instruments  can  be  read  to  an  accuracy  of  1  centi- 
metre, half  a  degree  or  1  kilogram. 

1.  A  spread  meter.  This  records  the  distance  between 
the  after  ends  of  the  otter  boards.  If  the  boards 
are  attached  to  the  wings  directly  (without  legs) 
this  corresponds  with  the  horizontal  opening  of 
the  net. 

2.  A  clinometer.  This  records  the  tilt  of  the  otter 
board  sideways  as  well  as  fore  and  aft. 


[225] 


MODERN     FISHING     GEAR    OF    THE     WORLD 


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Ffc.  /.  The  Netherlands  F.R.V.  Antoni  van  Leeuwenhoek. 

3.  A    net   height   meter.    This   records   the   vertical 
opening  of  the  net  (Headline  or  kite). 

4.  An  angle  of  attack  meter.  This  records  the  angle 
of  attack  of  the  otter  board. 

5.  An  hydraulic  dynamometer.  This  records  the  pull 
in  the  legs  between  the  otter  board  and  the  net 
or  the  pull  on  the  warps  on  board  ship. 

SPREAD  METER 

The  spread  between  the  otter  boards  was  first  measured 
by  reading  the  angle  between  two  rods  clipped  on  the 
warps.  At  the  same  time,  a  length  of  twine  was  attached 
to  one  of  the  otter  boards,  the  other  end  being  wound 
round  a  small  barrel  mounted  on  the  other  otter  board. 
The  barrel  was  pulled  down  by  springs  against  two 
wooden  brake  cleats.  Pull  on  the  twine  lifted  the  barrel 
from  its  brakes  so  that  twine  unreeled  according  to  the 


spread  of  the  otter  boards  and  then  braked  again.  In 
this  way,  the  maximum  spread  could  be  measured. 

However,  the  average  spread  measured  by  these  two 
methods  differed  by  about  6  metres,  so  a  much  more 
accurate  instrument  had  to  be  developed. 

First  it  was  necessary  to  determine  the  curve  in  a  steel 
wire  of  3  mm.  diameter  and  12,  16,  20  and  24  metres 
length  pulled  through  the  water  at  speeds  of  2,  3,  4  and 
5  knots,  and  carrying  longitudinal  loads  of  from  0  to 
500  kg.  Then  graphs  were  drawn  for  each  speed,  giving 
the  difference  in  lengths  at  certain  loads  between  the 
curved  wire  and  a  straight  line,  viz.  the  actual  distance 
between  the  otter  boards. 

Accepting  an  inaccuracy  of  1  per  cent,  and  with  an 
expected  spread  between  the  otter  boards  from  12  to 
16  metres,  it  was  found  necessary  to  have  a  pull  of  11 
to  15  kg.  on  the  wire  when  travelling  at  a  speed  of  3  knots. 

This  pull  is  provided  by  trawl  planes  attached  to  the 
free  end  of  the  wire.  Tests  in  the  towing  tank  in  Wagenin- 
gen  showed  that  3  trawlplanes  on  a  3  mm.  wire  produced 
a  load  of  16  kgs.  at  3  knots.  In  practice,  however,  this 
pull  was  too  much  for  the  wire,  which  had  to  be  renewed 
frequently  because  of  kinks  and  flattening  where  it 
went  over  the  guiding  reels.  So,  only  2  trawlplanes, 
which  exerted  a  load  of  about  10  kg.,  were  used  although 
this  increased  the  error  to  2  per  cent,  in  the  spread  reading. 

At  a  speed  of  5  knots,  the  pull  of  2  trawlplanes  is 
42*2  kgs.  and  for  1  trawlplane  about  20  kg.,  which  is 
double  the  amount  that  can  be  used  for  3  mm.  diameter 
wire.  The  new  spread  meter  therefore  cannot  be  used 
for  speeds  over  3  knots  without  renewing  the  wire  after 
every  haul  or  using  a  stronger  wire  and  recalculating 
all  the  data. 

The  spread  meter  works  as  follows:  a  3  mm.  diameter 
steel  wire  is  fastened  to  the  aft  part  of  one  otter  board 
and  passes  over  guiding  reels  through  a  hole  in  the  aft 
part  of  the  other  otter  board.  As  shown  in  fig.  2,  the 
wire  then  passes  (actually  in  two  turns)  over  a  wheel 
and  over  another  set  of  guiding  reels  to  the  two  trawl- 
planes (not  in  the  figure). 


Fig.  2.  The  baseplate  of  the  spread  meter  showing  arrangement 
of  wire  and  rolls. 


Fig.  3.  The  recording  device  of  the  spread  meter. 


[226 


MEASURING    TRAWL    GEAR     UNDER     WATER 


/-"iff.    4.    Clinometer,    showing    ai  fangenwnt    of  pendulum   and 
recording  device. 

In  towing,  when  the  distance  between  the  otter  boards 
increases  or  decreases,  the  wheel  turns  one  way  or  the 
other  and  so  registers  changes  in  the  spread. 

A  recording  device  is  attached  to  the  wheel  as  shown 
in  figs.  3  and  5. 

The  distance  between  the  otter  boards  is  measured 
before  shooting  and  added  to  the  distance  read  on  the 
meter,  the  total  being  the  spread  I  2  per  cent. 

CLINOMETER 

The  clinometer  mainly  consists  of  a  pendulum  arm,  free 
to  swing  in  a  vertical  plane,  and  a  clockwork  paper 
recorder.  Changes  in  the  tilt  of  the  otter  board  are  marked 
by  a  stylus  on  the  pendulum  arm,  which  carries  a  weight 
that  can  be  moved  up  or  down  to  regulate  the  sensitivity. 
Oscillations  are  damped  by  attaching  the  pendulum  to 
a  horizontal  spindle,  which  operates  a  piston  rod  in 


two  air  cylinders  (fig.  4).  Two  vent  screws  regulate  the 
escape  of  air  in  the  cylinders. 

The  apparatus  is  shown,  together  with  the  spread 
meter,  in  figs.  5  and  6,  attached  to  the  otter  board  in 
operation  position  to  determine  the  sideways  tilt  of  the 
otter  board.  By  unscrewing  the  sole  plates,  it  can  be 
fixed  in  a  position  at  right  angles  to  measure  the  fore 
and  aft  tilt  of  the  otter  board.  The  range  is  •  45  degrees. 

NET  HEIGHT  METER 

The  height  of  the  headline  is  established  by  using  a 
differential  manometer  to  measure  the  difference  in 
hydrostatic  pressure  between  the  bottom  of  the  net 
and  the  centre  of  the  headline  (fig.  7). 

A  lank  (fig.  8)  is  attached  to  the  footrope.  It  has  a 
hole  at  one  end  \\hile  the  other  is  connected  to  a  plastic 
tube,  with  an  inside  diameter  of  3  mm.  The  tube  runs 
along  the  headline  upwards  to  a  paravane  which  has 
a  buoyancy  of  4  kg.  and  is  towed  by  a  steel  wire  attached 
to  the  centre  of  the  headline.  A  second  tank  (an  ordinary 
8  in.  float  with  a  hole  at  the  bottom)  is  fastened  to  the 
middle  of  the  headline,  and  is  connected  by  a  short 
plastic  tube  to  the  paravane. 

When  the  net  is  shot,  the  water  enters  and  compresses 
the  air  in  the  tanks  and  in  the  plastic  tubes.  The  sub- 
merged capacity  of  the  tanks  is  such  that  they  only 
half  fill  with  sea  water. 

The  two  plastic  tubes  inside  the  paravane  arc  connected 
with  one  pair  of  bellows  each,  welded  to  a  steel  plate, 
which  moves  out  of  its  zero  position  if  the  pressure  in 
one  pair  of  bellows  is  more  than  in  the  other.  A  stylus 
records  the  movements.  The  paravane  is  shown  in 
fig.  9,  while  fig.  10  shows  it  with  the  head  off,  disclosing 
the  differential  manometer  and  paper  recorder. 

As  this  apparatus  measures  the  difference  between 
the  water  levels  in  the  two  tanks,  only  that  difference 
can  give  rise  to  inaccurate  readings.  If  excess  seawater 
enters  one  tank  by  accident,  the  inaccuracy  will  not  be 
more  than  5  cm.  if  the  tanks  are  lowered  carefully,  the 


6. 


Fig.  5.  Spread  meter  (top)  find  clinometer  (below)  attached  to 
the  otter  board.  Without  underwater  casings. 


Fig.  6.  Spread  meter  and  clinometer  ready  for  operation 


221} 


MODERN     FISHING     GEAR    OF    THE    WORLD 


Differential  Manomete 

< 

Rope  for  towing  the  Paravane          *-  •; 


Plaatic  tube  leading  from  the 
--er  to  thr  preaaurr  tank 
attached  to  the  headlim 


t'laatlr  tube  loading  from  th-  manomett 
to  the  pressure  tank  attached  to  the 
—  ' —  —  _      groundrope 


f  IVcsaure  tank  Attached  to  the  tfr 


Fig.  7.  Net  height  meter.  Arrangement  of  the  different  pans  at 
the  net  opening. 


maximum  error,  therefore,  will  not  he  more  than  10  cm. 

The  paravane  is  lowered  at  the  same  time  as  the  head- 
line, with  the  towing  wire  and  plastic  tubes  connected 
to  both  sides.  When  the  paravane  is  afloat  the  veering  wire 
is  disconnected  at  a  length  of  4  metres,  a  small  plastic 
float  being  attached  to  the  end.  This  can  be  picked  up 
easily  when  the  net  is  hauled  again. 

As  the  first  paravane  could  be  used  only  between 
depths  of  15  and  30  metres,  measuring  a  maximum 
height  of  5  metres,  another  was  developed  on  the  same 
principles,  designed  to  withstand  an  outside  pressure 
of  10  atmospheres. 

A  new  type  of  differential  manometer  divided  in  two 
partitions  by  a  large  brass  diaphragm  on  a  rubber  disc, 
has  been  used  with  this  paravane.  Movements  are  marked 
on  a  recording  drum  by  a  stylus  mechanism.  The  paravane 
can  be  used  at  all  depths  up  to  100  metres,  measuring 
a  maximum  height  of  10  metres. 

ANGLE  OF  ATTACK  METER 

If  a  rod  is  attached  to  an  otter  board  and  suspended  to 
move  in  horizontal  and  vertical  directions,  the  free  end 
sliding  over  the  ground  will  adopt  the  towing  direction 
of  the  otter  board. 

This  is  the  principle  underlying  the  angle  of  attack 
meter. 

A  steel  tube,  about  2  metres  long,  and  weighted  at 
the  end  with  lead,  is  connected  by  a  lever  with  a  turning 


Fig-  8.  Measuring  tank  fixed  to  the  foot  rope,  showing  the  hole 
for  entrance  of  pressure  and  the  connection  of  the  plastic  tube. 


disc  operating  in  a  sole  plate  attached  to  the  otter  board. 
Fig.    11    shows   the   registration   apparatus   mounted 
on  the  sole  plate.  The  complete  instrument  fixed  to  the 
otter  board  ready  for  operation  is  shown  in  fig.  12. 

DYNAMOMETER 

Tension  in  the  warps  varies  considerably  with  the  move- 
ment and  change  in  direction  of  the  ship,  and  with  the 
movements  of  the  otter  boards  over  uneven  ground.  For 
measuring  the  pull  on  the  warps  and  other  parts  of  the 
gear,  two  types  of  dynamometers  were  tested. 

The  principle  of  the  first  dynamometer  is  the  variation 
in  electrical  resistance  of  a  thin  wire  when  elongated. 
Fig.  13  shows  this  tension  dynamometer  fixed  between 
the  bollard  and  the  warps.  The  electronic  tension  meter 
on  which  the  pull  can  be  read  in  kg.,  is  shown  underneath 
the  dynamometer.  This  apparatus  proved  to  be  much  too 
sensitive  for  use. 

Measurements  between  the  otter  boards  and  the  net 
were  made  by  using  a  hydraulic  dynamometer  (fig.  14) 


Fig.  9.  The  paravane  of  the  net  height  meter  ready  for  operation. 


Fig.  10.     Inside  view  of  the  paravane. 


228 


MEASURING    TRAWL    GEAR     UNDER     WATER 


Fig.  11.  Angle  of  attack  meter \  inside  view. 

attached  to  the  upper  leg  between  the  otter  board  and 
the  wing  of  the  net.  This  dynamometer,  attached  to  a 
bracket  mounted  to  the  aft  side  of  the  otter  board, 
consists  of  an  oil  cylinder,  with  piston  rod  and  piston 
which  compresses  the  oil  in  the  cylinder.  The  cylinder 
is  connected  with  the  registration  apparatus  by  a  high 
pressure  rubber  tube.  A  plug  regulates  the  sensitivity. 
The  pressure,  which  is  proportional  to  the  pull,  is 
measured  by  a  spiral  hollow  tube  moving  a  stylus, 
which  is  supported  to  prevent  bad  recording  when  the 
otter  board  bumps.  The  recorder  is  mounted  in  a  vertical 
position  so  that  ink  can  be  used  instead  of  a  pencil, 
which  proved  to  be  too  unyielding. 

The  best  results  from  the  instruments  mentioned  are 
obtained  when  trawling  over  even  ground  and  when  the 
wind  force  is  not  more  than  3  to  4.  These  conditions 
apply  to  even  bigger  ships  than  the  Antoni  van  Leeu- 
wenhoek. 

RESULTS 

Instruments  1  to  4  have  been  used  together,  or  in  different 
combinations,  in  a  number  of  experiments.  The  hydraulic 


Ftp.  12.  Angle  of  attack  meter  attached  to  the  otter  hoard  in 
working  position. 

dynamometer  has  been  used  separately  in  more  recent 
trials. 

The  depth  was  noted  during  each  haul  and  the  ratio 
of  the  length  of  the  warps  to  the  depth  was  determined, 
while  the  distance  travelled  per  haul  over  the  ground  was 
calculated  to  find  out  the  strength  of  the  tide.  Wind  and 
sea  conditions  were  noted  as  well. 

A  manila  trawl,  with  a  headline  length  of  20-8  m.,  a 
foot  rope  of  25  -2  m.,  and  with  8  cm.  meshes  in  the  codend, 
was  used.  The  dimensions  of  the  otter  boards  (fig.  15) 
were  1-10  -  2- 10  m.  (weight  209  kgs.  each).  The  wings 
of  the  net  were  attached  to  the  otter  boards  without  legs, 
while  10  trawlplanes  of  8in.  diameter,  1  m.  apart,  were 
attached  to  the  headline,  5  to  each  side  of  the  middle. 
The  upper  tank,  plastic  tubes  and  towing  wire  of  the 
paravane  were  fixed  to  the  centre  of  the  headline. 

Hauls  were  made  always  with  the  running  tide  and 
with  a  constant  number  of  propeller  revolutions,  giving 
the  ship  a  fishing  speed  of  1  -7  to  3  knots  over  the  ground. 

Experiments  took  place  from  5th  May  to  4th  July 
1953,  over  smooth  bottom  between  Scheveningen  and 
Katwijk  at  a  mean  depth  of  17  metres.  Hauls  lasted 


Fig.  13.  Electronic  dynamometer  in  action. 


Fig.   14.   Hydraulic  underwater  dynamometer  attached  to  the 
otter  hoard  in  working  position. 


[229] 


MODERN     FISHING     GEAR     OF    THE     WORLD 


Tl 


Side  view 


TILT 

Fig.  17  gives  a  clinometer  record  for  one  haul.  The  first 
line  shows  an  outward  tilt  about  16-5  degrees  when  the 
otter  board  is  hanging  in  the  gallows.  When  lowering, 
the  tilt  is  unsteady,  but  becomes  steady  after  a  short 
settling  period.  The  otter  boards  take  a  more  upright 
position  as  the  warps  are  lengthened  and  finally  tilt 
inwards  and  become  unstable  when  the  warps  become 
too  long.  The  average  tilt  with  the  different  lengths  of 
warp  can  be  read  on  the  right  of  the  diagram,  while  on 
top  the  arrows  give  the  periods  of  shooting  and  hauling. 
Conclusion:  the  otter  boards  will  tilt  from  outwards 
to  inwards,  becoming  more  unstable,  as  the  warps  are 
lengthened. 


OJm 


Top  view 


Fig.  15.  Schema  of  the  otter  board  used. 

one  and  a  half  hours,  during  which  the  warps  were 
lengthened  by  veering  about  7-5  fathoms  every  15 
minutes. 

SPREAD 

A  record  of  the  spreadmeter  is  shown  in  fig.  16.  For 
convenience  the  scale  of  the  total  spread  has  been  given 
at  the  bottom  of  the  diagram.  The  distance  between 
the  otler  boards  measured  on  board  was  9-45  m.  The 
zero-position  of  the  stylus  before  lowering  the  otter 
boards  is  indicated  by  a  thick  short  arrow.  The  recording 
is  unsteady  at  first  but  becomes  nearly  constant  as  the 
gear  settles  on  the  bottom.  As  the  warps  are  lengthened 
by  about  7-5  fathoms,  the  spread  increases.  The  periods 
of  shooting  and  hauling  are  indicated  with  arrows  on 
the  left  of  the  diagram. 

Conclusion:  the  spread  will  increase  by  lengthening 
the  warp. 


HEIGHT  OF  THE  NET 

Fig.  18  shows  a  record  of  the  net  height  meter.  The 
conspicuous  peaks  indicate  that  the  height  of  the  net  is 
temporarily  increased  during  the  veering  of  the  warps. 
The  periods  of  shooting  and  hauling  are  shown  by 
arrows.  The  large  peaks  are  formed  when  the  net  hangs 
from  the  side  of  the  ship. 

Conclusion:  the  height  of  the  net  decreases  with 
increase  in  warp  length. 

ANGLE  OF  ATTACK 

A  record  of  the  angle  of  attack  meter  is  shown  in  fig.  19. 
During  the  periods  of  shooting  and  hauling,  the  position 
of  the  otter  board  is  uncontrollable.  The  periods  of 
veering  of  the  warps  are  indicated  on  the  left  of  the 
diagram.  Even  if  the  angle  of  attack  of  the  otter  boards 
for  the  different  lengths  of  the  warp  arc  rather  variable, 
it  is  possible  to  read  a  reliable  average,  indicated  on  the 
left  in  degrees. 

Conclusion:  by  lengthening  the  warps,  the  angle  of 
attack  decreases. 

The  recordings  of  the  tilt,  the  height  of  the  net  and  the 
angle  of  attack  (figs.  1 7, 18  and  19)  show  great  peaks  when 
the  warps  are  veered. 

It  appeared  that,  during  the  veering  of  the  warps,  the 
otter  boards  tilted,  on  average,  23  degrees  inwards,  while 
the  angle  of  attack  diminished  by  8  degrees  and  the  height 
of  the  net  increased  by  35  cm.  In  fig.  20  the  position  of 


Fig.  16.  A  record  of  the  spread  meter. 


Fig.   17.   A  record  of  the  clinometer. 


(  230  ] 


MEASURING     TRAWL    GEAR     UNDERWATER 


Fig.   18.  A  record  of  the  net  height  metei. 

the  otter  boards,  drawn  in  dotted  lines,  indicate  the 
situation  during  the  veering  of  the  warps,  from  which 
the  conclusion  can  be  made  that  there  is  more  slack 
in  the  headline,  which  will  be  lifted  by  the  floats. 

There  are  peaks  also  on  the  other  side  because  of  the 
sudden  braking  of  the  warps  after  veering,  in  which 
case  the  opposite  action  occurs,  but,  with  gradual  braking 
no  peaks  appear. 

INFLUENCE  OF  THE  INSTRUMENTS  ON  THE 
GEAR 

The  paravane  has  a  buoyancy  of  4  kg.,  equal  to  that 
of  a  spherical  float  of  8  in.  diameter,  while  the  clinometer 
has  a  buoyancy  of  0-8  kg.  The  new  paravane  was  given 
a  buoyancy  of  8  kg.  It  may  be  accepted  that  these 
instruments  will  not  affect  the  gear  in  any  way. 

Since  the  spread  meter,  the  angle  of  attack  meter  and 
the  clinometer  are  mounted  on  the  aft  side  of  the  otter 
boards  where,  as  the  film  "Trawls  in  Action"  shows, 
great  whirls  are  created,  the  water  resistance  of  these 
instruments  is  not  likely  to  affect  the  action  of  the  otter 
boards.  But  the  angle  of  attack  meter  weighs  9  kg.  and 
the  spread  meter  5  kg.  At  a  speed  of  3  knots,  2  trawl- 
planes  pull  with  a  force  of  10  kg.  on  the  wire  of  the  spread 
meter.  It  had,  therefore,  to  be  determined  what  influence 
these  instruments  had  on  the  gear,  separately  and  in 
combination. 

The  instruments  were  distributed  over  both  otter 
boards  (see  Table  I)  in  9  series  of  hauls. 

Before  starting  //  and  /,  the  warps  had  to  be  renewed 
and  shortened  and  since  differences  in  lengths  of  warps 


angle  of  attack 


opening  height  of  the  net 

Normal  position  of  net  and  otter  boards 
Maximal  change  opposition  when  veering 
the  warps 

Fin.  20.  Schematic  drawing,  explaining  the  behaviour  of  the  otter 
boards  and  the  net  opening  when  veering  the  warps. 

produce  different  readings,  series  h  was  made  a  repetition 
of  series  </,  so  that  //  and  i  could  be  compared. 

The  net  height  meter  and  the  clinometer  were  used 
during  all  these  hauls.  A  comparison  of  the  data  obtained 
shows  that  the  instruments  exerted  no  influence  whatso- 
ever on  the  operations  of  the  otter  boards.  Even  the 
strain  on  the  wire  of  the  spread  meter  had  no  influence 
on  the  angle  of  attack  and  the  tilt  of  the  otter  boards. 

However,  the  height  of  the  net  became  2  20  m.  when 
the  spread  meter  was  used,  compared  with  2*12  without 
it  (see  Table  IV).  This  corresponds  to  a  decrease  in 
spread  of  about  52  cm.  (calculated  from  Table  II)  on 
an  average  spread  of  12-89  (see  Table  IV),  an  error  of 
nearly  4  per  cent.,  which  must  be  added  to  the  measured 
spread.  As  already  mentioned,  by  using  2  trawlplanes, 
2  per  cent,  has  to  be  subtracted  for  the  bend  in  the  wire, 
so  the  actual  distance  between  the  aft  parts  of  the  otter 
boards  will  be  the  measured  distance  plus  2  per  cent., 
in  this  case  12-89  m.  ;  2  per  cent.  13-I5m. 

INFLUENCE  OF  THE  LENGTHS  OF  THE  WARPS 

After  determining  the  influence  of  the  instruments  on 
the  gear,  the  average  was  taken  of  the  results  obtained 


Fig.  19.  A  record  of  the  angle  of  attack  meter. 


TABU  I 

Sptead  A  Icier 

Attack-  Angle 

Clinometer 

\urnbei 

Series             on 

metei  on' 

on 

of  han/.\ 

a. 

Aft  ot  lei 

hi  on!  otter 

board 

hoard,  sidewavs 

7 

b       front  oiler 

Aft  oiler 

Front  otter 

hoard 

board 

board,  sideways 

^ 

c. 

- 

Aft  otter  hoard. 

sideways 

d 

d. 

Aft  oitei 

Aft  otter  hoard. 

hoard 

sideways 

9 

c.       Front  otter 

Aft  otter 

Aft  otter  hoard. 

hoard 

hoard 

sideways 

S 

f  . 

Front  otte»- 

hoard,  fore  and 

aft 

*> 

g. 

Aft  otter  hoard. 

fore  and  aft 

^ 

h. 

Aft  otter 

Aft  otter  hoard. 

noard 

sideways 

8 

i.           — 

Front  otter 

Front  otter 

board 

sideways 

9 

[231 


MODERN     FISHING     GEAR     OF    THE    WORLD 


Length 
of  the 
Warps 
in  m. 


Depth 
in  m. 


Ratio 

between 

Length 

of  Warps 

and 

Depth 


Angle 

of 
Attack 


TABLE  II 


Tilt  out- 
wards (o) 
or  in- 
wards (/') 


Tilt 
fore 

(/) 

or  aft 

(a) 


Spread 
in.  m. 


Height 

of  Net 

without 

Spread 

Meter 

in  m. 


Height 
of  Net 
with 
Spread 
Meter 
in  m. 


67-21 

81-05 

94-85 

108-67 

122-42 

136-17 


17-04 
17-03 
16-97 
17-01 
16-96 
16-87 


3,9 
4,8 
5,6 
6,4 
7,2 
8,  1 


35-2' 
32-7" 
28-4' 
25  -0J 
22-2' 
20  T 


12 -5°  (o) 
7-0°  (o) 
I  -0  (o) 
4-4>(i) 
8-9°  (i) 

12-5°  (i) 


TABLL  III 


3-4  (f) 
2-5  (f) 
l-7"(f) 
0-6°(f) 
0-3  (a) 
0-5°  (a) 


12-26 
12-53 
12-83 
13-14 
13-34 
13-40 


2-19 
2-17 
2-13 
2-08 
2-02 
1-97 


31 
26 
21 
15 
10 


2-05 


58-50 

17-10 

3,4 

37-0 

5-9J(o)               —                    — 

2-26                 — 

72-00 

17-00 

4,2 

34-3 

9-0   (o) 

2-19                  — 

85-50 

16-95 

5,0 

30-9LJ 

3  -3  Mo)                -                    — 

2-16 

99-00 

16-90 

5,9 

28-6IJ 

1-7   (i)                —                    — 

2-12                 — 

112-50 

17  10 

6,6 

26-2° 

5-3rj(i) 

2-10                 — 

126-00 

17-30 

7,3 

24-9° 

8-8°(i)               —                    — 

2-08 

from  series  a  to  g,  and  the  results  are  given  in  Table  II, 
while  Table  III  gives  the  averages  of  the  hauls  in  h  and  i. 
The  following  conclusions  can  be  drawn  from  Tables 
II  and  III.  By  lengthening  of  the  warps: 

1.  the  angle  of  attack  decreases; 

2.  the  otter  board  tilts  from  outward  to  inward; 

3.  the  otter  board  tilts  from  forward  to  aftward; 

4.  the  spread  increases; 

5.  the  height  of  the  net  decreases. 

We  can  also  say  that,  if  the  depth  decreases  with  a 
fixed  length  of  warp,  the  same  will  happen  as  mentioned 
above.  If  the  depth  increases,  the  opposite  will  occur. 

RATIO  BETWEEN  WARP  LENGTH  AND  DEPTH 

The  effective  spreading  surface  of  the  otter  board  is 
greatest  when  it  takes  up  a  position  perpendicular  to 
the  bottom,  i.e.  when  the  sideways  tilt  of  the  otter  board 
is  zero. 

Interpolating  the  data  in  Table  II  and  III  for  a  tilt 
of  0  degrees,  the  result  is  given  in  Table  IV. 

The  small  differences  between  the  sets  of  values  are 
caused  by  a  smaller  ratio  between  the  lengths  of  the 
warps  and  the  depth,  by  small  errors  in  the  readings  due 
to  differences  in  current,  wind  force,  swell  and  slight 
changes  in  the  depth,  and  by  small  instrument  errors. 

However,  the  results  are  sufficiently  accurate  to  show 
that,  while  fishing  at  a  depth  of  17  metres  with  an  upright 


TABLE  IV 


Length 
ofthe 


Ratio 

between 

Length 


Height      Height 

Anole  Of  Net        of  Net 

Ang-     Spread  without      with 


From     ofthe     Depth    Length         J     Spread  without  with 

TABLE      Warps    in  m.  of  Warps   A?*t,  in  m.     Spread  Spread 

inm.                     and      AnacK               Meter  Meter 

Depth                               in  m.  in  m. 


II 
III 


96-9 

94-7 


16-98 
16-92 


5,7 
5,6 


27-8° 
29-4° 


12-89 


2-12 
2-13 


2-20 


otter  board,  the  angle  of  attack  is  about  28-5  degrees 
the  height  of  the  net  just  over  2  metres  and  the  spread 
12-89  m.  -}•  2  per  cent.  -=  13-15  metres.  For  an  upright 
position  of  the  otter  board  the  length  of  the  warps  must 
be  about  5-1/2  times  the  depth. 

INFLUENCE  OF  LEGS 

Two  additional  series  of  hauls  have  been  carried  out 
with  the  same  gear  but  with  a  fixed  length  of  the  warps. 

In  the  first  7  hauls,  legs  of  3-60  m.  length  were  used 
between  the  otter  boards  and  the  net;  in  the  second 
series  of  4  hauls  the  wings  of  the  net  were  coupled  to 
the  otter  boards  directly.  The  tilt  of  the  otter  boards 
was  not  determined. 

The  average  readings  for  these  series  are  given  in 
Table  V. 

These  results  are  represented  diagrammatically  in 
fig.  21  (gear  with  the  legs  shown  by  dotted  lines). 

In  fishing  with  legs,  the  angle  of  attack  is  decreased 
and  the  spread  of  the  otter  boards  is  increased,  but  not 
the  spread  of  the  net,  so  there  is  more  slack  in  the  head- 
line, and  the  height  of  the  net  is  increased  by  41  cm. 

INFLUENCE  OF  DIFFERENT  TYPES  OF  FLOATS 
ON  THE  OPENING  HEIGHT 

With  the  same  gear  as  mentioned  above,  using  3-70  m. 
legs  between  the  otter  boards  and  the  net,  the  height  of 
the  net  without  any  floats  was  1  -20  m.,  or  10  cm.  more 


TABLE  V 


as 


Measured  Height 

Spread    of  Net 

In  m.      in  m. 


With 
Without 

95-45 
95-45 

15-2 
16-2 

6,3 
5,9 

26-0° 
31-5° 

14-54 
13-43 

2-76 
2-35 

232] 


MEASURING    TRAWL    GEAR     UNDER     WATER 


Legs 


Fig.  21.  Schematic  drawing  showing  the  influence  oj  short  legs 
on  the  net  opening. 

than  the  height  of  the  otter  board.  With  4  to  6  spherical 
8  in.  floats,  the  increase  in  height  was  not  more  than  30 
cm.,  while  with  8  floats  the  lift  was  considerably  more, 
being  about  55  cm.  (total  height  1  -75  m.).  A  kite  (with 
two  small  glass  floats  mounted  on  it)  was  used  with  the  8 
floats,  but  it  provided  no  increase  in  height. 

Comparative  trials  were  carried  out  with  10  spherical 
8  in.  floats,  5  Phillip's  Siamese-twin  floats  and  10  Phillip's 
trawlplanes,  all  having  a  diameter  of  8  in.  Strips  of  about 


Fig.  23.  Record  of  the  underwater  dynamometer  between  otter 
board  and  upper  /or,  in  a  calm  sea.  The  pull  decreases  due  to 
the  net  b»ing  choked  by  a  big  catch  of  colony-building  Hydrozoa. 

7  cm.  width  were  out  off  the  sides  of  the  half  circular 
plates  on   the  Siamese-twins,  giving  them  a  winglike 
shape,  which  reduced  resistance  and  increased  buoyancy. 
The  results  were: 


10  spherical  floats 

5  Siamese-twins 

10  trawlplanes     . 


height  of  headline  1  -90  m. 
height  of  headline  2-30  m. 
height  of  headline  2-75  m. 


Fig.  22.  Record  of  the  underwater  dynamometer  between  otter 
board  and  upper  leg,  rough  sea. 


PULL  ON  THE  LEGS  AND  AMOUNT  OF  CATCH 

The  hydraulic  dynamometer  has  only  been  used  sub- 
merged between  the  otter  boards  and  the  net,  and  only 
a  limited  number  of  hauls  have  been  made.  In  fig.  22  a 
diagram  is  given  showing  a  tension  of  about  180  kg.  in 
the  upper  leg.  During  this  haul  the  sea  was  rather  rough, 
with  a  wind  force  4. 

During  another  haul  with  very  calm  weather  (fig.  23) 
the  pull  decreased  from  250  to  1 30  kg.  The  same  happened 
in  6  consecutive  hauls  although  in  earlier  and  later 
trials  a  fairly  constant  tension  was  recorded.  The  reason 
was  found  in  an  enormous  number  of  colony-building 
Hydrozoa  which  gradually  choked  the  net  during  fishing. 
The  net  was  cleaned  before  each  haul. 

On  the  assumption  that  the  specific  gravity  of  fish 
is  practically  the  same  as  the  specific  gravity  of  the  water, 
the  strain  in  the  legs  should  be  unaffected  by  the  catch, 
especially  if  most  of  the  fish  are  swimming  with  the  net. 
But  eventually  the  codend  becomes  choked  by  the  fish 
and  starts  to  overflow.  This  may  result  in  a  decrease,  not 
an  increase,  of  the  pull.  Therefore,  a  dynamometer 
between  the  otter  board  and  the  net  may  give  an  indica- 
tion of  the  amount  of  catch. 

This  is  in  contradiction  to  a  claim  by  an  American 
manufacturer  of  dynamometers,  published  in  the 
"Atlantic  Fisherman",  Vol.  30,  No.  6,  p.  35,  July  1949, 
and  it  is  suggested  that  the  rate  of  catch  may  be  deter- 
mined by  a  decrease  in  the  tension  on  the  warps  or  the  legs. 


[233] 


STUDIES  ON  TWO-BOAT  TRAWLS  AND   OTTER   TRAWLS   BY 
MEANS  OF  MEASURING  INSTRUMENTS 

by 
CHIKAMASA   HAMURO  and   KENJI   ISH1I 

Fishing  Boat  Laboratory,  Fisheries  Agency,  Japanese  Government 

Abstract 

The  authors  have  analyzed  the  change  in  shape  of  the  net  during  shooting  and  towing  by  the  use  of  instruments  and  (his  paper 
deals  with  the  results  of  their  experiments  so  far  as  the  two-boat  trawl  and  the  otter  trawl  are  concerned.  The  automatic  net-height  meter  was 
used  to  ascertain  the  height  of  the  headline  and  the  wings  and  the  footrope  indicator  to  assess  the  shape  of  the  footrope  during  the  various 
phases  of  the  fishing  operation.  Improvements  were  made  to  the  two-boat  trawl,  as  a  result  of  the  tests  and  the  authors  claim  that  by  raising 
the  headline  by  1  -5  metres  the  catch  was  increased  by  nearly  50  per  cent.  With  the  otter  trawl  the  shape  of  the  net  was  observed  from  the 
time  it  was  put  overboard  until  it  was  on  the  sea  bottom,  and  the  time  taken  for  the  warps  and  net  to  assume  their  correct  position  was 
calculated. 


Resume 


Etude,  au  moyen  d  'instruments  du  ehalut  a  deux  bateaux  et  du  chalut  a  plateaux 


Les  auteurs  ont  etudi£  au  moyen  d'instrumcnts  de  mesure,  Ic  cnangcmcnt  subi  par  la  forme  du  filet  pendant  les  operations  de  mise 
£  1'eau  et  de  remorquage  et  exposent  dans  cet  article  les  resultats  de  leurs  experiences  obtenu  avec  des  chaluts  a  deux  bateaux  et  des  chaluts 
&  plateaux.  Us  ont  utilise  1'apparcil  automatiquc  a  mesurcr  la  hauteur  du  filet  pour  determiner  la  hauteur  de  la  corde  dc  dos  et  des  ailes,  ct 
I'indicateur  de  bourrelet  pour  mesurer  la  forme  du  bourrelet  pendant  les  differences  phases  des  operations  de  pcche.  Les  resultats  des  essais 
ont  permis  d'ameliorer  le  chalut  a  deux  bateaux,  et  les  auteurs  declarent  avoir  obtenu  une  augmentation  de  pres  de  50  pour  cent  des  quantites 
pftchdes  en  Levant  dc  1  -5  metres  Fouverture  de  gueule  du  chalut.  tin  ce  qui  concernc  le  chalut  &  plateaux,  les  auteurs  ont  6tudie  loutes  les 
positions  successives  prises  par  le  filet  depuis  le  moment  dc  sa  mise  a  1'eau  jusqu'a  celui  ou  il  a  pris  sa  forme  nornule  sur  le  fond,  et  ils  ont 
calcule  le  delai  n6cessaire  aux  funes  et  au  filet  pour  prendre  leur  position  correcte. 


Estudios  mediante  instrumentos  de  las  parejas  y  arrastreros  que  pescan  con  artes  de  puertas 
Extracto 

En  estc  trabajo  los  autores  estudian,  mediante  instrumentos  de  medida,  los  cambios  que  experimenta  la  forma  de  una  red  dc 
arras t re  durante  el  calamento  y  la  recogida,  y  analizan  los  result  a  dos  de  sus  investigaciones  con  embarcaciones  que  pescan  en  parejas  y  usand 
redes  de  arrastre  du  puertas.  Durante  las  diversas  etapas  de  estos  estudios  se  usaron  el  medidor  automatico  para  evaluar  la  altura  de  la 
relinga  superior  y  de  las  pernadas,  y  el  indicador  de  la  relinga  inferior  para  determinar  la  forma  de  este  cable  durante  las  diversas  fases  del 
lance.  En  el  caso  de  la  pesca  con  parejas.  se  pudieron  introducir  mehjoras  de  acuerdo  con  los  resultados  obtenidos  en  las  pruebas,  afirmando 
los  autores  que  al  aumentar  la  altura  de  la  boca  de  la  red,  1  -5  m.,  el  vo lumen  de  las  capturas  experimento  un  incrcmento  de  casi  un  50  per 
cent.  En  el  caso  de  la  red  dc  arrastre  de  puertas  se  estudio  cada  estado  del  arte  desde  el  momcnto  dc  lanzarlo  al  agua  hasta  que  tom6  la 
forma  detinitiva  sobre  el  fondo  del  mar.  calculandose  el  ticmpo  que  dcmoraron  los  cables  de  arrastre  y  la  red  en  tomar  la  posicion  correcta. 


THE  AUTOMATIC  NET  HEIGHT  METER 

THE  recorder  (A)  and  the  guide  part  (B)  are  both 
equipped  with  bellows  (C2  and  C,)  which  are 
connected  by  a  vinyl  pipe  (F)  filled  with  oil. 
The  antipressure  vinyl  pipe  (G)  equalises  the  air  pressure 
inside  the  recorder  and  guide  part  in  order  to  avoid 
temperature  effects.  The  recorder  is  fixed  to  the  footrope 
and  the  guide  part  to  the  point  to  be  measured.  The  two 
bellows  measure  the  difference  in  hydrostatic  pressure 
between  the  measuring  point  and  the  footrope  which 
is  recorded  in  the  usual  way  by  a  pointer  (E)  writing  on 
a  chronograph  (D)  (fig.  1). 

Three  types  of  such  net  height  meters  have  been 
developed  which  are  all  of  sturdy  construction  to  stand 
rough  handling,  and  which  are  small  and  light  enough  to 
exclude  any  influence  on  the  net  to  be  measured.  They 
have  a  working  range  of  0  to  150  m.  water  depth,  a 
measuring  range  of  0  to  7  m.  and  an  accuracy  of  5  cm. 
with  a  sensitivity  of  2  cm.  The  newest  of  these  instru- 
ments used  since  1957  is  shown  in  fig.  2. 


THE  AUTOMATIC  FOOTROPE  INDICATOR 

This  instrument  consists  of  a  handle  which  can  freel) 
turn  on  a  vertical  axle  and  which,  due  to  the  bottom 
friction  of  its  resistance  plate,  is  kept  in  towing  direction 
during  action  of  the  trawl.  The  position  of  this  handle 
is  continuously  recorded  on  a  chronograph  which 
is  attached  to  the  footrope  in  a  water  and  pressure  tight 
casing  (fig.  3).  By  attaching  several  of  such  instruments 
distributed  over  the  footrope  the  curvature  it  takes  during 
trawling  and  its  variations  can  be  determined. 

TWO-BOAT  TRAWLS 

Opening  height.  A  diagram  of  the  trawl  which  was 
tested  is  given  in  fig.  5. 

Table  I  shows  that  the  net  sinks  to  the  bottom  with 
a  speed  of  about  0-3  m./sec.  and  the  warps  are  veered  at 
about  3  •  1  to  4-2  m./sec.  It  is  also  shown  that  before  tow- 
ing really  starts  the  net  opening  is  much  higher  than 
during  trawling. 


(234] 


STUDIES     ON     TWO-BOAT     AND     OTTER     TRAWLS 


P 


Fig.    1.     Principle   of  the   recording   net   height   meter.     A 
Recorder  \  B -Guide  part;  C\     Bellows  (guide):  C^-- Bellows 
(recorder)',    D     Chronograph:     E    Pointer:    F    Liquid    pipe: 
C  —  Air    pipe. 


TABI  f  I 

Time  from 

throwing  the       -..       f 
Experiment  net  overboard  ^™gji"ut 

N°'  reaZhes'the    ^ 50  m.  warp    after  reaching 

bottom  the  sea  bottom 


Height  of 
net-entrance 
immediately 


Sea- 
depth 


} 

3  min. 

3  min. 

6-28  m. 

51  m. 

2 

3 

4 

5-55 

55 

3 

3 

3 

5-70 

52 

4 

3 

3 

6-28 

57 

5 

3 

3-5 

6-30 

51 

Fig.  2.     Newest  type  of  recording  net  height  meter. 


SEA  BOTTOM 


Re?latance 
Plate 


Foot  rope 


Casing  Handle 

Fig.    3.     Principle    of  the    recording  footrope    indicator. 

There  are  two  ways  of  shooting  this  gear  (fig.  6) 
which  have  been  found  to  influence  the  time  needed  from 
starting  towing  until  the  net  opening  acquires  its  stable 
height.  With  the  V-type  manoeuvre  it  takes  less  time 
than  with  the  U-type  where,  furthermore,  the  opening 
decreases  to  a  minimum  before  it  becomes  stable 
(Table  II). 

With  about  750  m.  warp  length,  300  to  400  m.  distance 
between  the  two  towing  boats  is  considered  to  be  con- 
venient. Fig.  7  shows  that  even  up  to  about  500  m. 
distance  no  significant  effect  to  the  opening  height 
occurred.  The  variation  is  due  to  the  changing  tidal 
current  influencing  the  actual  towing  speed. 

To  improve  the  opening  height,  the  construction  of  the 
original  net  was  changed  as  shown  in  fig.  8.  These 
changes  concern  increase  of  webbing  mainly  around 
the  net  opening,  the  relation  between  headline  and  foot- 
rope  lengths  which  shifts  the  main  pull  to  the  footrope, 
and  the  flapper  attached  in  such  a  way  that  it  can  open 
completely  during  towing.  A  comparison  of  the  values 


TABLE  II 


Ex    rimeJ'oftowT5*?"     Minimum 
N°'            S"Se'n"ng          °^nin^ 

Height  of 
stable  net 
opening 

Type  of 
setting 

1 
2 
3 

12  5  min. 
14 

8 

-43  m. 
•00 

-48 

l-82m. 
1-55 
2-14 

U 

4 

5 

1 
5 

•68 
-30 

1-68 
1   30 

V 

Fig.    4.     Recording  footrope    indicator. 


[235] 


MODERN     FISHING     GEAR    OF    THE    WORLD 

NET         _  A     *n-*fi  M  A     Boats 


300m 


18mm  wire  rope 
450m 


Fig.  5.     Diagram  of  two  boat  trawl  tested  with  indication  of  the 

measuring  points  for  the  net  height  meter  (1  to  4).     O  —  Glass 

float,  30  cm.   diameter,   O^  Glass  float,  18  cm.    diameter, 

%— Glass  float,  15  cm.  diameter;     ^— Chain. 


of  opening  height  of  the  original  design  with  the  new 
design  proves  that  a  considerable  increase  of  about 
1  -5  m.  or  75  per  cent,  could  be  obtained  (fig.  9). 

With  increasing  towing  speed  the  opening  height 
decreases  considerably  at  all  points  measured,  e.g. 
middle  of  headline  bosom,  quarter  point  and  middle  of 


0 


Net 


Net 


Fig.  6.     Two  different  types  of  shooting  a  two  boat  trawl. 

wing.  The  main  reason  for  this  is  considered  to  be  the 
decrease  in  buoyancy  of  the  floats  caused  by  their 
towing  resistance.  Without  any  floats,  the  opening  height 
was  only  0-6  to  0-8  m. 

The  graphs  in  fig.  10  show  that  the  influence  of  the 
distance  between  the  trawlers  on  the  opening  height 
increases  with  the  towing  speed.  With  a  towing  speed  of 
2-3  to  2-5  knots  the  most  satisfactory  distance  between 
the  trawlers  in  regard  to  the  opening  height  is  300  to 
350  m.  under  the  given  conditions. 

CURVATURE  OF  THE  FOOTROPE 

The  curvature  of  the  footrope  was  measured  at  five  points 
(fig.  11,  a  to  e)  by  means  of  the  footrope  indicators 


TABLE  III 


Ex. 

No. 

Distance 
between 
the  boats 

Towing  Speed 
Boat               Net 

Height 
headline  bosom                 pom 

i-r 

420  m. 

2-2  kt. 

3-3  m.                  2-15  m.                     1.15  m. 

370    „ 

3-9    „                   2-4      „                     1-5      „ 

2-2' 

300    „ 

2-5 

The  middle                   1  -9      „                    0-2      ., 

point  of  the  wing. 

1-7  in 

500    „ 

1-7    , 

2-0      ,.                     0-3       „ 

3-3' 

400    „ 

2-2    „ 

3-4 

3-0      .,                     0-4      , 

600    „ 

2-1     „ 

3-2 

2-9       .                     0-3      . 

300    „ 

2-3     „ 

3-7 

3-15 

0-55     , 

500    ,. 

3-5 

2-95 

0-55    , 

4-4' 

280    „ 

3-5     „ 

0-65 

0-35 

0-3      , 

400    „ 

3-3     „ 

0-6 

0-3 

0-3      , 

300    „ 

2-66  „ 

0-8 

0-4 

0-4      . 

5-5' 

300    „ 

1-66  „ 

5-3 

3-0 

2-3      , 

350    „ 

1-6    „ 

5-7 

3-05 

2-65     , 

6-6' 

320    „ 

1-6    „ 

5-5 

3-1 

2-4       , 

7-7' 

530    ,. 

1-66  „            0-3  kt. 

5-2 

2-95 

2-25     , 

530    .. 

1-8     ,,           0-6     „ 

5-3 

3-05 

2-25     , 

530    ., 

1-86  „            1-0    „ 

4-85 

2-80 

2-05     , 

8-8' 

380    „ 

4-3 

2-6 

1-7      , 

270    „ 

5-0 

2-85 

2-15    , 

450    „ 

2-4    „ 

3-6 

2-45 

1-15    , 

350    „ 

4-45 

2-8 

I  -65     , 

400    „ 

3-5 

2-35 

t                     *  *  *^     » 

[236] 


STUDIES    ON    TWO-BOAT    AND    OTTER    TRAWLS 


Opening 
height 


m    300  ^OTJ — — 5tfo 

Fig.  7.     Relation  between  the  distance  of  the  two  trawlers  and 


the  opening  height  of  the  net. 


G.R   HR 


I'.*  3T.4' 

s'.e-.r.ar 


Fig.  8.     Diagram  of  the  improved  net  for  a  higher  opening  with 
indication  of  measuring  points  for  di  jerent  experimental  hauls. 


Height       Wing 


C 


D 


Codend 


& 


200 


300  400  500 

Distance  between  trawl  boats 


600  m 


Fig.  10.     The  effect  of  towing  speed  on  the  influence  of  the 
distance  between  the  trawlers  on  the  opening  height. 

described  below.  An  example  of  the  recordings  is  given 
in  fig.  12. 

It  was  found  that  the  footrope  settles  to  a  stable 
curvature  in  10  to  12  minutes  with  the  U-type  of  shooting 
and  in  2-5  to  5  minutes  with  the  V-type  of  shooting, 
after  towing  is  started. 

The  angles  formed  at  the  five  measuring  points  and 
the  distance  calculated  accordingly  between  the  respec- 
tive points  for  different  distances  between  the  trawlers 
are  given  in  Tables  IV  and  V.  The  magnitude  of  the 
change  in  distance  between  the  vessels  explains  the 
influence  on  the  opening  height  discussed  above. 


TABlfc    IV 

Angle  of  the  Footrope  and  Towing  Direction 


Distance  between 

the  boats 

a 

/> 

r 

d 

e 

400m. 

26-5  l 

32-5" 

34° 

38° 

46° 

450 

35 

35 

37-5 

46 

52 

500 

32 

36 

38 

53 

55 

Start  towing 
Approach  each  other 


Parallel  towing 
Start  hauling 


Fig.  77.     Measuring  points  for 

determining  the  curvature  of  the 

footrope. 


lh 


Approach  each 
other 


Parallel 
towing 

Start  hauling 
Start  towing 


lh 


2h 


Fig.  12.     Example  for  the  recordings  of  the  footrope  indicators. 
\  237  1 


MODERN    FISHING    GEAR     OF    THE    WORLD 


TABLE  V 


Distance 
between 
the  boats 

Distance 
between 
wing  lips 

Span  of  foot- 
rope  bosom 
(6  m.  long) 

Depth  of 
footrope 
curvature 

400m. 
450 
500 

41  -1  m. 

45-6 
48-6 

5-1  m. 
5-4 
5-5 

29-3  m. 
27-8 
26-3 

Just  before  hauling  the  two  boats  approach  each  other 
and  then  tow  for  a  short  while  parallel  and  at  about 
10  m.  distance  to  force  the  whole  catch  into  the  codend. 
The  changes  in  the  curvature  of  the  footrope  during 
this  manoeuvre  is  shown  in  Table  VI  and  fig.  13.  For 
the  stages  1 ,  2,  and  3  the  distance  was  500, 450  and  400  m. 
respectively.  The  stages  4,  5,  6  and  7  refer  to  the 
approaching,  and  8,  9,  10  and  1 1  to  parallel  towing  with 
reduced  distance  (about  10  m.).  In  addition  to  the 
quantitative  data  given  it  was  found  that  after  about 
10  minutes  the  footrope  has  settled  in  a  stable  curvature 
according  to  the  reduced  distance  between  the  trawlers 
and  that  extending  the  time  of  parallel  trawling  to  get 
the  warps  parallel  would  be  useless. 

OTTER  TRAWL 

Measuring  experiments  with  this  trawl  by  means  of  net 
height  meter  and  footrope  indicator  are  being  carried 
out  in  the  Yellow  Sea  since  1953.  The  construction  of 


o         &       10         « 

metre 


Fig.  14.     One  boat  otter  trawl  net  used  during  the  measuring 

experiments.     A     Conxt ruction;  B     Measuring  points  for  the 

net  height  meter  referring  to  the  different  experiments. 

the  net  in  question  and  the  measuring  points  for  different 
experiments  are  given  in  fig.  14. 

For  measuring  the  opening  height  an  improved  meter 
was  used  with  which  two  points  of  the  net  height  could 
be  measured  simultaneously.  An  example  of  the  readings 
obtained  is  given  in  fig.  15. 

The  behaviour  of  the  trawl  and  the  variation  of  the 
distances  between  different  parts,  as  well  as  the  angles 
of  net,  sweep  lines  and  warps  during  the  shooting 
operation  have  been  studied  thoroughly.  Fig.  16  is  one 
example  of  the  configurations  drawn  according  to  the 
measurements  obtained.  It  was,  for  instance,  found 
that  the  otter  boards  reach  the  bottom  first  and  the  net 
follows  some  time  later. 

Measurements  during  trawling  have  been  made  with 


Middla  of  the  front 
of  th«  square 


^   Middi*  of  th«  aft«r 


Fig.  13.     Shape  of  the  footrope,  as  calculated  from  the  angle 
measurements,  the  trawlers  keeping  different  distances. 


of  th*  *quar« 


Fig.  15.     Example  of  the  records  of  the  net  height  meter  measur- 
ing two  points  of  the  net  height  simultaneously. 


Time 
Ex.  Mark 


b 
c 
d 
e 


Mins. 
0 

27-5° 

32-5 

34 

43-5 

46 


1 

27-5° 

34 

34 

43-5 

45 


27 -5U 

34 

34-5 

43-5 

45 


TABLK  VI 
Angle  between  footrope  ami  towing  direction 

34567 


28 
34 
35 
43 
45 


30  -Oc 


33 
35 
43 
45 


28  -0' 

32-5 

36 

43-5 

45 


27-5° 


30 
34 
42 
46 


27-0 

29-5 

32-5 

43 

46-5 


26-0° 
27*5 
32 
44-5 

47 


25-0 

27-5 

31 

43 

48 


10 

24  0° 

22-5 

28-5 

43 

49-5 


N.B.    Initial  distance  between  the  boats:    400  m. 


[238] 


STUDIES    ON    TWO-BOAT    AND    OTTER    TRAWLS 


Fig.  16.     Behaviour  of  the  trawl  during  shooting. 


a  trawl  of  the  following  dimensions  under  following 
conditions: 

Headline  38-9  m.  Footrope  53-6  m.  Sweep  lines 

90  m.;  Warp  length  250  m.  Water  depth  70  m. 

Warp  angle  to  horizontal  1 3  degrees,  Towing  speed 

2-5  knots,  Bottom  mud.  Wind  fair,  10  glass  floats 

26   cm.    diameter   on    headline    bosom,    19   glass 

floats  20  cm.  diameter  on  each  wing. 

The  results  are  given  in  Table  VII  and  fig.  17. 

The  height  of  the  wings  and  the  square  is  obviously 

unsatisfactory.  The  main  reason  for  this  is  the  fact  that 

with  these  trawls  the  main  pull  acts  on  the  headline. 

If,  by  changing  the  length  relation  between  headline  and 

footrope  the  pull  would  be  shifted  to  the  footrope,  it 


TABLE  VII 


Measuring  Points 

Wing  point  1 

Wing  point  2 

Wing  point  3 

Wing  point  4 
Middle  of  headline  bosom 
Middle  of  after  edge  of  square 
Middle  of  front  edge  of  throat 
Middle  of  front  edge  of  codend 


Vertical  distance 
from  footrope 


0-72  m. 
39m. 
82  m. 
93  m. 
2-03  m. 
•93  m. 
-65  m. 
-50m. 


Front  edge  of  square 


Cod  end 


Fojtrope 


Footrope 
Headline 


Fig.  17.     Shape  of  the  net  as  jound  hy  means  of  the  net  height 
meter. 


may  tend  to  cut  into  the  bottom.  This  could  be  overcome 
by  means  of  a  suitable  bobbin  footrope.  Furthermore, 
the  suitability  of  the  net  construction  has  to  be  checked 
including  the  flapper  which  in  the  present  form  tends 
to  restrict  the  proper  water  flow  into  the  codends. 

When  the  trawler  changes  course  the  speed  of  the  net 
and  the  horizontal  opening  decrease.  Consequently  the 
vertical  net  opening  height  increases  because  of  the 
reduced  resistance  on  the  floats  and  the  slack  in  lines 
and  webbing. 

The  influence  of  wind  direction  was  found  to  be  very 
noticeable.  The  opening  was  much  higher  with  head  wind 
than  with  fair  wind  (Table  VIII).  Furthermore,  with 
head  wind  the  opening  height  oscillated  considerably 
while  with  fair  wind  it  was  stable.  This  effect,  of  course, 
is  caused  by  the  influence  of  wind  and  water  on  the  towing 
speed  and  movements  of  the  trawler. 

As  in  the  case  of  trawling  for  flat  fish  in  Bristol  Bay, 
the  opening  height  gradually  increases  with  the  accumu- 
lated catch.  This  effect  is  due  to  the  reduction  in  towing 
speed  and  opening  width  resulting  in  an  increase  of 
resistance.  During  the  present  experiments  similar 
observations  have  been  made.  An  example  is  given  of  the 
recorded  heights  in  fig.  18.  During  this  haul  the  catch 
amounted  to  6,000  kg. 


TABLE  VIII 


The  Net  Height  when  Towing  with  Head  and  Fair  Wind 
Measuring  Point 


Average  height  Average  height 
with  head  wind    with  fair  wind 


Middle  of  headline  bosom  2-6  m.  18m. 

Middle  of  after  edge  of  square        2-4  m.  2-1  m. 

Quarter  point  2-">5  m.  1  -8  m. 


0  rain  30 


5D 


Fig.  IS.     Record  oj  net  height  meter  attached  to  the  middle  of 

the  headline  hosoni,  showing  the  influence  of  increasing  amount 

of   catch. 


I  239  ] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


500m. 
-  450m.   The  two  bo^ta  trawl 


The  ordinary 


Fig.   19.     The  wing  shaped  float. 

As  the  buoyancy  of  the  glass  floats  is  considered  to  be 
unsatisfactory  at  higher  towing  speeds,  experiments  were 
carried  out  with  the  addition  of  wing  shaped  floats 
(fig.  19).  Results  are  given  in  Table  IX. 

It  was  found  that  the  usual  net  covering  of  the  glass 
floats  is  unfavourable,  and  plastic  floats  with  a  smooth 
surface  give  better  results.  The  wing  shaped  floats  are 
superior  because  of  their  hydrodynamic  lifting  power. 
In  order  to  make  the  best  use  of  this  lifting  power, 
sufficient  surplus  of  webbing  should  be  provided  in  the 
net  mouth  to  allow  for  a  high  opening. 

COMPARISON   OF  BOTH  METHODS 

Table  X  and  fig.  20  give  a  comparison  of  two-boat  and 
one-boat  trawling.  In  two-boat  trawling  the  duration 
of  a  voyage  is  20  to  30  days.  In  the  East  China  Sea, 
otter  trawling  is  excellent  for  catching  hair  tail  or  prawn 
and  two-boat  trawling  for  guchi  (Nibea  argentata, 
Pseudosciaene  manchurica*  Nibea  nibe,  etc.). 

TABLE  IX 


With  glass 
floats  alone 


Height  of  net  opening 
Height  of  left  quarter  point 
Towing  speed 


1-60  m. 
1-35  m. 
3-7  knots 


With  glassfloats 
and  wing-shaped 
floats  in  addition 

2-95  m. 
2-20  m. 
3-7  knots 


Fig.  20.     Curvature  of  foot  rope  and  opening  width  of  the  one 

boat  otter  trawl  in  comparison  with  the  two  boat  trawl  at  different 

distances  between  the  towing  boats. 


Depth 

Warp  length 
Sweepline  length 
Footrope  length    . 
Towing  speed 
Angle  between  warps 


Two  boat  trawl 

54  m. 

750  m. 

74  m. 
2  knots 


Otter  trawl 

95  m. 
300  m. 
90  Hi. 
53  m. 
3  knots 
12-5  degrees 


Items 

Boat 
Engine 

Distance    between 
parts  on  touching 
bottom 

Wing  spread 

Height  of  net 
opening 

Towing  time 
Towing  speed 
Deck  machinery 
Fishing  areas 


TABLE  X 


Two-boat  Trawling  Otter  Trawling 


the 
the 


60  to  100  tons 

(couple) 
200  to  300  h.p. 
(diesel) 

Hawsers 
150  to  200m. 


40  to  50  m. 

Max.    Mm.   Mean 
4-Om.  1  -3m.  3-5m. 

1-5  to  2-0  hour 

T7  to  2-6  knots 

2  gurdies 

Yellow  Sea,  East 

and  South  China 

Sea 


200  to  1,000  tons 

(single) 
500  to  800  h.p. 

(diesel) 

Otter  boards 

60  to  70  m. 

25  to  30  m. 

Max.    Min.    Mean 
2-95 m.  l-4m.  1 -9m. 

3  to  4  hour 
3  to  3-6  knots 
1  winch 

Yellow  Sea,  East 

and  South  China 

Sea. 


[240] 


THE  USE  OF  ECHO-SOUNDING  AS  A  MEANS  OF  OBSERVING 
THE  PERFORMANCE  OF  TRAWLING  GEAR 

by 

J.   SCHARFE* 

Gear  Technologist,  Fisheries  Division,  FAO,  Rome 

Abstract 

Several  echograms  showing  traces  of  pelagic  trawling  gear  in  action  are  discussed.  Besides  measurements  of  the  depth  of  the  gear 
and  the  height  of  its  opening,  the  position  of  the  danlenos  and  otter  boards  in  regard  to  the  net  opening  can  be  observed,  as  well  as  the  slope 
of  the  legs  and  bridles  and  the  position  of  the  net  bag  itself.  From  these  experiments  it  is  suggested  that  echo-sounding  could  be  a  valuable 
help  in  studies  directed  towards  the  development  or  improvement  of  pelagic  trawling  gear. 

L'eraplof  du  sondage  par  ultra-sons  comme  moyen  d 'observer  le  travail  du  chalut 

Plusieurs  echogrammes  montrant  des  traces  de  chalut  pelagique  en  action  sont  examines.  Outre  les  mesures  de  la  profondeur  de 
Pengin  et  de  sa  hauteur  d'ouverture,  la  position  des  guindineaux  et  des  plateaux  par  rapport  a  1'ouverture  du  filet  pent  etre  observee,  de  m£me 
que  la  pente  des  cables  et  dcs  bras  ct  la  position  du  filet  proprcmcnt  dite.  D'aprcs  ces  cxpercnces,  il  cst  suggcrc  que  Ic  sondage  par  £cho 
pourrait  etre  une  aide  de  valcur  dans  les  recherches  pour  le  developpement  du  chalut  pelagique. 

Uso  de  los  sondeos  a  eco  como  medio  para  observer  el  rendimiento  de  las  redes  de  arrastre 
Extracto 

Se  analizan  varios  ecogramas  de  los  trazos  obtenidos  con  redes  de  arrastre  pelagicas.  Ademas  de  la  medida  de  la  profundidad  del 
arte  y  la  altura  de  la  boca,  pucde  obscrvarse  la  posicion  de  los  calones  y  puertas  con  respecto  a  la  abertura  de  la  red.  la  inclinacion  de  los 
cables  quc  unen  los  extremes  de  las  pernadas  con  las  puertas  y  los  pies  de  gallo,  asi  como  la  posicion  del  cuerpo  del  arte. 

De  estos  ensayos  se  llega  a  la  conclusi6n  quc  el  sondeo  ultrasonoro  puede  ser  de  gran  ayuda  para  los  estudios  destinados  a  desarrollar 
o  perfeccionar  las  redes  de  arrastre  pelagicas. 


RECENTLY  attempts  have  been  made  to  accelerate 
the  development  and  improvement  of  trawling 
gear  by  using  measuring  instruments.  The  prob- 
lems connected  with  this  new  approach,  such  as,  for 
instance,  identifying  those  important  characteristics  of 
a  trawl  which  should  be  measured,  and  the  different  ways 
of  doing  this  without  a  fleet  ing  the  behaviour  of  the 
gear,  need  not  be  dealt  with  here.  Instead,  only  one  of  the 
numerous  measuring  or  observing  methods  will  be 
discussed,  the  echo-sounding  method. 

To  the  author's  knowledge,  the  first,  experimental 
observations  of  a  trawl  in  action  by  means  of  an  echo- 
sounder  were  carried  out  by  Wood  and  Parrish  (Journ. 
de  Conseille  17,  1950,  pp.  25  to  36)  in  1949.  They  used 
the  sounder  from  a  rather  big  boat  which  was  towed 
by  the  trawling  vessel  and  in  this  way  operated  over  the 
gear. 

The  writer,  as  an  employee  of  the  German  "Institut 
ftir  Netz-  und  Materialforschung",  has  since  1953  used 
a  motor-driven  rubber  boat  about  16  ft.  long  and  7  ft. 
in  beam,  with  a  battery-driven  echo-sounder  (Atlas- 
Werke  AG,  Bremen,  Type  SH  37  tr,  4  AZ  42  d  tr.). 
The  main  advantage  of  this  boat  is  its  good  manoeuvr- 
ability and  the  unlimited  possibility  of  circulating 
freely.  Numerous  observations  on  trawling  gear  were 
carried  out  with  this  equipment,  mainly  to  define  the 
opening  height  of  bottom  trawls  and  their  contact  with 


•  Formerly  of  Institut  fur  Netz-  und  Materialforschung.  Hamburg. 


Fig.  1.     The  motor-driven  rubber  boat  used  by  the  "Institut 
fur  Neiz-  und  Materialforschung"  for  echo-sounding  observations. 
(Photo — Bodo  Ulrich) 


[241] 


MODERN     FISHING     CHAR     OF     THE     WORLD 


Fig.  2.     Echogram  showing  the  influence  of  the  length  of  the  \\arps  on  the  depth  of  the  gear  ami  the  height  of  the  net  opening.    I.  Headline; 
2.  Launches:     3.  f-ootrope:     4.  Otter  Board:     5.  Weight  in  front  of  the  lower  wingtip\  6.  Warp. 


the  ground,  but  also  on  the  behaviour  of  fish  coming 
near  such  gear. 

Later,  the  equipment  proved  of  even  higher  value  for 
studies  directed  towards  the  development  of  pelagic 
trawls.  The  following  echograms  were  taken  during 
pelagic  trawl  experiments  carried  out  with  the  German 
Fisheries  Research  Vessel  Anton  Dohrn  in  June  1957, 
in  the  Baltic  Sea.  Three  one-boat  pelagic  trawls  were 
tested,  all  being  equipped  with  hydrofoil  otter  boards, 
but  with  different  nets  and  different  riggings.  As  only 
the  suitability  of  the  method  will  be  discussed,  a  descrip- 
tion of  the  gear  is  not  needed. 

DEPTH  OF  THE  NET  AND  OPENING  HEIGHT 

Two  important  characteristics  of  a  pelagic  trawl  can 
be  very  easily,  and  also  exactly,  measured  by  echo- 
sounding:  the  actual  depth  of  the  net  and  the  opening 
height.  Fig.  2  shows  the  changes  in  these  characteristics 
connected  with  the  increasing  length  of  the  warps. 
With  this  gear,  under  the  conditions  in  question,  a 
lengthening  of  the  warps  by  25  m.  caused  an  increase 


in  depth  of  6  to  8  m.  Furthermore,  with  increasing  warp 
length,  the  opening  height  decreased  in  favour  of  the 
opening  width.  With  100  m.  warp  length  the  opening 
height  was  10  m.  whereas  with  175  m.  and  more  it 
decreased  to  only  8-5  m. 

POSITION  OF  THE  LASTRICHES 

Besides  the  headline  and  footrope,  the  lastriches  also 
give  good  traces.  They  appear  in  all  sections  of  fig.  2 
and  both  are  in  the  same  or  almost  the  same  depth, 
except  the  last  section,  where  a  difference  of  nearly  1  -5  m. 
is  shown.  In  this  case,  the  length  of  the  warps  was  not 
equal.  It  was  observed  that  an  inequality  of  about  one 
fathom  may  cause  the  net  to  assume  an  oblique  position, 
with  the  lastrich  connected  with  the  shorter  warp 
being  as  much  as  3  m.  higher  than  the  other  lastrich. 

POSITION  OF  THE  OTTER  BOARDS 

Furthermore,  the  last  section  of  fig.  2  shows  that  with 
this  gear,  under  the  given  conditions,  the  position  of  the 


[242[ 


STUDY    OF    TRAWL    GEAR     BY     ECHO    SOUNDING 


f< 


M  w     ^ 
petie   J 


•  •  •  '• 


•6.  i 

A,  !..,•>,• 


Fig.  3.     Echogram  of  the  same  gear  as  in  fit;-  2,  showing  the 

slope  of  the  legs.     I.  Head/ me:     2.  Last  richer;     3.  f-oo  trope; 

4.  Legs;    5.  Oner  Hoard;    6.   Wright;     7.   Warp. 


Fig.  4.     Echogram  of  another  gear  with  four  legs,  danleno  ami 

bridle.     J.  Headline;     2.  Footrope;     3.  Lastriehes;     4.  Legs; 

5.  Danleno;    6.  Bridle;    7.  Otter  Board;    8.  Warp. 


/•/#.  5.     Echogiam  of  a  /hi id  pear  with  only  t\\o  leg*,  showing 
l he  slope  of  the  legs  and  the  position  of  the  net.     I .  Headline', 
2.  f-bofrope;    3.  I^eps;    4.  Weight*    5.  Otter  Board',    6.  Warp; 
7.     .Met. 

otter  board  was  not  level  with  the  middle  of  the  net 
opening,  but  it  travelled  about  2  m.  deeper. 

SLOPE  OF  THE  LEGS 

During  another  observation  of  the  same  gear,  more  care 
was  taken  to  establish  the  slope  of  the  legs  (fig.  3). 
Coming  from  astern,  the  sounding  boat  was  steered 
accurately  over  the  legs  to  the  otter  board  and  then  up 
the  warp.  Here  the  otter  board  is  also  about  2  m.  below 
the  middle  of  the  net  opening.  Consequently,  the  legs 
have  a  down-going  tendency,  especially  the  lastrich  one. 
A  good  trace  is  given  by  the  heavy  weight,  which  is 
fixed  to  the  leg  a  short  distance  in  front  of  the  lower 
wing  tip  to  keep  it  down. 

Fig.  4  shows  traces  of  another  gear  equipped  with 
four  legs  to  each  wing,  danlcno  and  bridle.  The  record 
shows  that  the  danleno  travelled  about  1  m.  below  the 
middle  of  the  net  opening.  The  bridle  is  not  well  recorded. 
The  otter  board  obviously  had  the  same  depth  as  the 
middle  of  the  net  opening.  The  clear  reproduction  of  the 
four  legs,  in  this  case,  has  a  special  interest.  When  the 
net  was  hauled  in,  it  appeared  that  both  lastrich  legs 
of  this  side  of  the  gear  were  broken,  causing  the  net 
to  be  completely  torn.  The  echogram  proved  that  this 
damage  had  not  occurred  before  hauling  and  the  measured 
values  could  safely  be  relied  upon.  Likewise,  the  echo- 
sounding  method  can  be  successfully  used  to  check 
the  performance  of  a  gear  during  trawling  operations. 

POSITION  OF  THE  NET 

Fig.  5  shows  traces  of  the  third  gear  which  was  equipped 
with  two  legs  from  each  wing  to  the  otter  boards. 
Besides  the  depth  of  the  different  parts  of  the  gear  and 
the  height  of  the  net  opening,  the  slope  of  the  legs  and 
the  weight  keeping  down  the  lower  wing  can  easily  be 
recognised.  In  this  case,  the  influence  of  the  towing 
speed  on  the  slope  of  the  legs  is  obvious.  At  lower 
speed  (left)  they  have  a  greater  slope,  and  at  higher  speed 
(right)  they  are  almost  stretched.  The  position  of  the 


\  243  ] 


MODERN     FISHING     GEAR     OF    THE    WORLD 


otter  board  to  the  net  opening  is  naturally  influenced  by 
the  speed.  At  lower  speed,  the  board  is  almost  at  the 
same  depth  as  the  middle  of  the  opening,  but  at  greater 
speed  it  travels  about  2  m.  higher.  The  most  interesting 
feature  of  this  record  is  the  trace  of  the  net  (right). 
It  shows  that  the  net  has  no  horizontal  position  at  all. 
To  control  this,  the  sounding  boat,  coming  from  astern, 
was  steered  over  the  whole  net  from  the  codend  to  the 
opening  and  then  followed  one  pair  of  legs  to  the  otter 
board  and  up  to  the  warp.  It  showed  that,  with  this  gear, 
under  the  given  conditions,  the  codend  travelled  at 
almost  the  same  depth  as  the  footrope  and,  consequently, 
about  5  m.  below  the  middle  of  the  net  opening.  Further- 
more, this  record  shows  that  in  these  cases  the  headline 


exceeded  the  height  of  the  upper  wingtip  by  about 
1-5  m.  (right,  higher  speed)  to  about  2-5  m.  (left, 
lower  speed)  whilst  the  depth  of  the  footrope  did  not 
seem  to  differ  much  in  relation  to  that  of  the  lower 
wingtips.  The  reason  for  this  may  lie  in  the  fact  that 
heavy  weights  were  keeping  down  the  lower  wings, 
whilst  the  upper  wings  had  no  additional  lifting  device. 
Although  incomplete,  these  examples  clearly  indicate 
the  value  of  the  echo-sounding  method  for  a  quick 
test  of  the  technical  performance  of  certain  characteris- 
tics of  pelagic  trawls.  Such  a  test  provides  an  objective 
background  for  estimating  the  probable  catching 
ability  which,  as  a  second  step  has,  of  course,  to  be 
proved  by  real  fishing. 


1  I: 


Hydrofoil  otter  board  being  tested  on  a  small  research  vessel. 
The  instrument  attached  is  a  recording  angle  of  attack  meter. 
—Photo  FAO. 


Oval  otter  board  of  Russian  design  with  angle  of  attack  meter 
attached.  The  cover  is  removed  showing  the  circular  recording 
paper  and  the  stylus  in  zero  position.  Above  the  board  a  surface 
dynamometer  attached  to  the  gallows  is  used  for  measuring  the  pull 
on  the  towing  warp.  —Photo  FAO. 


f  244  J 


EXPERIMENTS  TO   DECREASE  THE  TOWING  RESISTANCE   OF 

TRAWL   GEAR 

by 

J.  SCHARFE 

Gear  Technologist,  Fisheries  Division,  FAO,  Rome 


Abstract 

In  comparison  with  a  common  German  herring  bottom  trawl,  a  gear  of  quite  the  same  size  and  characteristics  but  with  hydrofoil 
otter  boards  (Suberkrub)  and  a  lighter  net  bag  made  of  Perlon  twine  was  tested.  The  towing  resistance  of  the  latter  gear  proved  to  be  about 
30  per  cent,  lower  than  the  former.  About  80  per  cent,  of  this  decrease  was  due  to  the  boards  and  about  20  per  cent,  to  the  lighter  net. 
This  remarkable  amount  of  decrease  in  towing  resistance  obtained  in  such  a  simple  way  is  suggested  as  being  a  suitable  way  for  improving  the 
economy  of  trawling. 


Resume 


Experiences  pour  la  diminution  de  la  resistance  de  remorquage  des  chaluts 


On  a  compart  un  chalut  de  fond  ordinaire  allemand  pour  le  hareng  avec  un  engin  de  meine  taille  et  de  memes  caracteristiques  mais 
ayant  des  plateaux  a  surface  hydrodynamique  (Suberkrub)  et  un  sac  plus  leger  fait  dc  fils  de  Perlon.  La  resistance  de  remorquage  du  second 
engin  s'est  montr^e  infcrieure  d'environ  30  per  cent.  a  ccllc  du  premier.  Environ  80  per  cent  dc  ccttc  diminution  etait  due  aux  plateaux  et 
environ  20  per  cent,  au  filet  plus  leger.  Ccttc  diminution  remarquable  de  la  resistance  dc  remorquage  obtenue  d'une  maniere  simple  est  propose* 
comme  moyen  convenable  pour  amtliorer  Teconomie  du  chaluiage. 


Experimentos  para  disniinuir  la  resistenciu  que  las  redes  de  arrastre  oponen  al  remolque 
Extracto 

Se  efectuaron  diversas  pruebas  a  fin  de  comparar  una  red  dc  arrastre  de  fondo  para  arenque,  como  las  empleadas_  correintementc 
en  Alemania,  con  una  de  igual  tarmho  pero  mas  liviana  quo  tenia  puertas  con  superficies  hidroclinamicas  y  cuerpo  de  perlori. 

Este  ultimo  arte  opuso  un  30  per  cent,  msnos  de  resistencia  que  la  primera.  FJ  80  per  cent,  dc  csta  dismmuci6n  se  debio  a  las 
puertas  y  el  20  per  cent,  al  cuerpo  dc  la  red  que  era  mas  liviano.  Ksta  notable  reduccion  obtenida  dc  manera  tan  sencilla.  podria  servir 
para  aumentar  las  ceo  no  mi  as  en  la  pesca  de  arrastre. 


FOR     optimal     economy    in    trawling,    the    lowing 
resistance  of  the  trawl  gear  should  be  as  low  as 
possible.  It  is  well  known  that  the  commonly  used 
plane  otter  boards  are  very  unsatisfactory  from  a  hydro- 
dynamic  point  of  view.     Moreover,  nets  of  manila  or 
sisal,  i.e.  natural  fibres,  have  to  be  made  of  thicker  twine 
than  those  of  synthetic  fibres  (e.g.  nylon  or  Perlon).  So, 
two  simple  ways  are  open  to  decrease  the  towing  resist- 
ance of  trawl  gear: 

1.  Hydrofoil  otter  boards. 

2.  Nets  made  of  thinner  synthetic  twine. 

To  test  the  effect  of  these  changes  in  the  German 
herring  bottom  trawl,  measuring  experiments  were 
carried  out  with  the  Fisheries  Research  Vessel  Anton 
Dohrn,  during  June  1957,  in  the  Baltic  Sea. 

These  experiments  consisted  of  measurements  of: 

(a)  Towing  resistance  of  the  complete  gear. 

(b)  Towing  resistance  of  that  part  of  the  gear  behind 
the  otter  boards.    The  difference  between  (a)  and 
(b)  gives  the  share  in  total  towing  resistance  of 
warps  and  otter  boards. 


(c)  Towing  speed. 

(d)  Distance  between  the  otter  boards. 

(e)  Height  of  the  net  opening. 

(f)  Angle  of  attack  of  the  otter  boards. 

(g)  Angle  of  attack  of  the  kites. 

Weather,  course,  depth,  bottom  conditions,  propeller 
revolutions,  "cutoff"  and  boiler  pressure,  were  also 
taken  into  account. 

Of  the  total  of  27  tows,  17  had  to  be  rejected  because 
of  changes  in  weather  or  bottom  conditions,  unsatis- 
factory conformity  in  the  size  of  the  gear  opening  or 
damage  to  the  gear,  such  as  broken  lines  or  torn  net. 
Of  the  remaining  10  tows,  5  were  made  with  each  of  the 
two  following  types  of  gear: 

1.  Common     German     herring    bottom    trawl    with 
160  ft.  ground  rope,  manila  net  and  common  otter 
boards  (see  fig.   1).     (Hereafter  called  ''Common 
gear.") 

2.  German   herring  bottom  trawl  rigged  in  the  same 
way  with  a  net  of  the  same  construction  and  size 
but  made  of  thin  Perlon,  and  with  "Suberkriib" 
otter  boards  (see  fig.  2).    (Hereafter  called  "Experi- 
mental gear.") 


[245] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


Fig.   I.     Construction  drawing  of  the  common  Otter-board. 

RESULTS 

The  measurements  were  made  at  a  water  depth  of 
80  to  100  m.  on  a  rather  hard  clay  bottom  and  with 
225  fathoms  of  warps  (22  mm.  diam.). 

The  angle  of  attack  of  the  otter  boards  was  ascertained 
from  the  traces  caused  by  the  bottom  friction  on  the  iron 
shoe  plates  of  the  boards.  These  traces  give  good  average 
values  which  were  found  to  be  as  follows: 

Common  otter  boards  about  35  degrees 
"Suberkriib"  otter  boards  about    12  to  15  degree  s 

These  values  are  almost  optimal  for  both  types  of  boards 
(see  fig.  3). 

According  to  the  "Gottinger  Messungen",  and  with 
regard  to  the  influence  of  the  water  only,  the  following 
values  are  calculated  for  resistance  and  shearing  force 
at  3-8  knots  towing  speed: 

Towing 

Resistance  Shearing  Force 

Common  otter  board  0-8  tons  1  •  1  tons 

"Siiberkriib"  otter  board  0-2  tons  1-1  tons 

This  shows  a  decrease  of  resistance  with  the  "Siiberkriib" 
type  of  boards  of  1  -2  tons  (for  both  boards)  or  75  per 
cent,  compared  with  the  common  boards.  But  this 
calculation  neglects  the  influence  of  the  bottom  friction, 
which  should  be  higher  for  the  common  boards  with  their 
long  lower  edge.  The  measurements  actually  showed  a 


Fig.  2. 


Construction  drawing  of  the  "  Suberkrtib"  Otter  board 
used  in  the  experiments. 


1.6. 
1.5 
1.4 
1.3. 
1.2- 


+  14.4° 


+  12. 


+16-5° 


+37. 7C 


+34.  7* 


+39.  7^ 


1.  1 

/ 

1.0 

P*4' 

0.9 

i 

0.8. 

o.r 

1 

0.6 
0.5 
0.4. 

I 

0.3. 

1 

0.2- 

1 

0.  I 

1 
0-5° 

p+4.2°  HT+24.60 


+14.90 


Q!  1     0. 2  o'.3    0*4    0.5    0.6      o'.7    O.'s    0.9    O.lo  O.'ll    <^ 

Fig.  3.     Buoyancy  coefficients  (Ca)  ami  Resistance-coefficients 

(Cw)  for  a  common  Otterboard  ( )  and  the  "Suherkrub" 

Otter  board  ( )  uf>ed  in  the  experiments. 


difference  of  1-6  tons.  The  0-4  tons  exceeding  the 
calculated  value  is  at  least  partly  due  to  the  lower  bottom 
friction  of  the  "Sliberkrub"  boards  under  the  existing 
bottom  conditions. 

The  size  of  the  net  opening  should  be  as  similar  as 
possible  for  both  types  of  gear.  The  width  of  the  opening 
was  not  really  measured.  Instead,  the  distance  between 
the  two  warps  1  m.  behind  their  cross-over  in  the  slip- 
hooks  was  controlled.  This,  of  course,  docs  not  give 
accurate  values,  but  is,  at  least,  a  basis  for  comparing 
the  opening  width  of  equally  rigged  nets.  This  is  an  old 
fisherman's  method  for  controlling  the  behaviour  of 
the  boards.  The  following  values  were  obtained: 

Distance  of  the     Calculated  Distance 
Warps          of  the  Otter  Boards 

Common  gear  11-5 — 12 -Ocm.       about   48   m. 

Hxpcrimental  gear  13-5  —  14-0  cm.       about   55   m. 

The  slight  difference  indicates  that  the  shearing  power 
of  the  "Siiberkrub"  otter  boards  was  too  strong,  at  least 
for  the  light  Perlon  net.  As  the  most  simple  way  to 
decrease  the  shearing  power,  it  was  suggested  that  the 
angle  of  attack  should  be  made  smaller,  but  as  this  could 
lead  to  fouling  the  gear  when  shooting,  it  was  not  tried. 
It  was  not  possible  to  reduce  the  size  of  the  boards  on 
board  ship,  so  this  difference  was  accepted  as  of  no  great 
importance. 

The  opening  height  was  measured  by  means  of  an 


1246] 


REDUCING    TOWING     RESISTANCE    OF    TRAWLS 


echo-sounder,  installed  in  a  motor  driven  rubber  boat. 
The  following  values  were  obtained: 

Distance  from  the  Hottom 
Headline         1st  Kite  2nd  Kite 


Common  gear 
Experimental  gear 


3-0-3-4  m.          7-0  m.  12-0  m. 

2-5-3-5  m.  6-0    7-5m.  10-0— 12-Om. 


Because  of  the  greater  width  of  the  net  opening,  the 
false  headlines  of  the  experimental  gear  had  to  be  slightly 
lengthened.  The  conformity  thus  obtained  in  the  net 
openings  was  regarded  as  satisfactory. 

The  angle  of  attack  of  the  kites  was  measured  by  the 
jelly-bottle  method.  The  values  lay  between  2S  degrees 
and  34  degrees,  a  favourable  range. 

Despite  efforts  to  keep  the  same  towing  speed  in  all 
experiments,  a  deviation  between  3-6  and  4-1  knots 
could  not  be  avoided.  The  measured  values  of  the  towing 
resistance,  therefore,  had  to  be  converted  to  an  average 
speed  of  3-8  knots  assuming,  as  conventional,  the  resist- 
ance being  proportional  to  the  square  of  the  speed.  The 
speed  was  measured  by  meansof  a  "KempfT'resistancc  log. 


The  towing  resistance  was  measured  on  both  warps, 
close  behind  the  sliphook  (total  resistance),  as  well  as  on 
both  bridles,  close  behind  the  otter  boards  (resistance 
mainly  caused  by  the  net).  As  warps,  bridles,  danlenos, 
legs,  kites  and  false  headlines  were  the  same  for  both 
gears,  differences  between  the  measured  values  are  due 
to  the  otter  boards  and  the  net  bag.  Converted  to  a  speed 
of  3-8  knots,  the  following  average  values  were  found 
(.the  range  of  deviation  is  shown  below  in  brackets). 

This  comparison  shows  a  very  remarkable  difference, 
the  experimental  gear  offering  about  30  per  cent,  less 
resistance.  Of  this  30  per  cent,  about  24  per  cent,  was 
due  to  the  hydrofoil  shape  of  the  boards  and  about  6  per 
cent,  due  to  the  lighter  net. 

This  decrease  of  towing  resistance  means  that,  with 
the  same  engine  power,  the  experimental  gear  could  be 
towed  about  0-7  knots  faster  (i.e.  at  4-5  knots)  than  the 
common  gear  (3-8  knots),  or  its  size  could  be  increased 
by  about  30  per  cent,  or,  thirdly,  a  corresponding 
amount  of  fuel  could  be  saved. 


Common  gear 
Experimental  gear 


Propeller 

Rev.  win. 

82 

74 


Cut  off 


52-54 

50 

Main  valve 
partly  closed 


Total 


Tons 

6-7 
(6-0—7-3) 

4-7 
(4-5-5-J) 


Resistance 
Otter  boat  d  ami  warps         Net  with  line*,  danlenos 


and  warp* 


Ton\ 

2-5 
(2-1—3-0) 

(07-1-0) 


37 
19 


Tons 

4-2 
(3-9 --4-4) 

3-8 
(3-0    4-1) 


63 

Kl 


Dynamometers  for  the  determination  o)  the  lowing  resistance  oj  trawl  gear.  Left:  surface  dynamometer  in  working  position  measuring  ihc 
pull  on  one  towing  warp.  Right:  simultaneous  calibration  of  four  underwater  and  two  surface  dynamometers  against  a  master  instrument 
on  board  a  small  research  vessel.  The  necessary  pull  is  provided  by  a  titrnbuckle  handled  by  the  person  in  the  background.  Photo  F-  AO. 

[2471 


ON  THE  RELATION   BETWEEN   OTTER   TRAWL   GEAR   AND 

TOWING  POWER 

by 

H.   MIYAMOTO 

FAO  Gear  Technologist,  Central  Fisheries  Technological  Station,  Cochin,  India 

Abstract 

This  paper  deals  with  the  relation  between  trawling  gear  and  towing  power.  The  author  presents  data  he  has  gathered  on  the  subject 
in  India  and  Japan  but  stresses  that  his  deductions  are  based  on  preliminary  observations  which  need  to  be  verified  by  further  investigations. 

The  formulae  he  submits  arc  for  calculating  the  size  of  trawl  nets  and  boards,  and  the  weight  of  the  boards  in  relation  to  the  H.P. 
of  the  engine,  the  size  of  the  boards  in  the  relation  to  the  size  of  net,  and  the  'ength  of  the  warps  in  relation  to  fishing  depth. 


Rfeumt 


Sur  la  relation  entre  le  chalut  &  plateaux  et  la  puissance  remorquage 


On  dispose  de  tres  peu  de  rcnscignements  concernant  la  relation  entre  les  dimensions  dcs  plateaux  de  chalut,  la  taille  du  chalut  et 
la  puissance  de  remorquage  necessaire. 

En  s'appuyant  sur  les  observations  el  donnees  provenant  dc  different*  bateaux  de  peche,  Tautcur  a  ctabli  une  serie  de  formules  pour 
calcubr:  (a)  les  dimensions  des  plateaux  de  chalut  d'apres  la  puissance  du  moteur,  (b)  les  dimensions  du  filet  d'apres  la  puissance  du  motcur, 
(c)  le  poids  des  plateaux  d'apres  la  puissance  du  moteur,  (d)  les  dimensions  dcs  plateaux  de  chalut  d'apres  calles  du  filet,  (e)  la  longueur  des 
funes  de  chalut  d'apres  la  profondeur  de  peche. 


Eitracto 


Relacion  entre  la  red  de  arrastre  de  puertas  y  la  potcncia  emleada  en  el  arrastrc 


Se  dispone  de  muy  pocos  datos  sobre  la  relacion  quc  existe  entre  las  dimensiones  de  las  puertas  de  arrastre,  eltamano  dc  la  red  y  la 
potencia  requerida  para  el  remolque: 

Basandose  en  las  observadones  y  datos  tornados  en  diversos  barcos  pcsqueros,  el  autor  ha  dcsarrollado  una  serie  de  formulas  para 
calcular:  (a)  el  tamano  dc  las  puertas  con  relacion  a  la  potencia  del  motor,  (b)  el  tamano  dc  la  red  con  relacion  u  la  potcncia  del  motor, 
(c)  el  peso  de  las  puertas  de  arrastre  con  relacion  a  la  potencia  del  motor:  (d)  cl  tamafto  de  las  puertas  dc  arrastre  con  relacion  al  tamano 
de  la  red,  (c)  la  longitud  de  los  cables  de  arrastre  con  relacion  a  la  profundidad  dc  pesca. 


IT  Is  a  well-known  fact  that  bigger  boats  use  bigger 
trawls.  But  there  exists  no  information  about  the 
common  relation  between  size  or  engine  power  of 
the  trawler  and  the  size  of  the  trawl  gear  and  between 
the  size  of  the  trawl  net  and  the  size  of  the  otter  boards. 
Therefore,  data  has  been  collected  mainly  from  trawlers 
in  India  and  also  from  a  few  Japanese  trawlers  which 
are  presented  below. 

RELATION  BETWEEN  ENGINE  POWER  AND  SIZE 
OF  OTTER  BOARDS 

Fig.  1  shows  that  in  the  common  use  the  area  of  the  otter 
boards  is  proportionate  to  the  h.p.  of  the  engine.  If 
the  area  of  one  otter  board  is  called  S"  and  the  h.p. 
of  the  engine  P,  the  relation  found  can  be  expressed  by 
the  following  equation: 

S*  -  0-105  P  4  4 

The  ratio  between  the  length  and  the  width  of  the 
otter  boards  usually  is  2  :  1  approximately.  If  B  denotes 


the  width,  the  length  will  consequently  be  2B.  Then  B 
can  be  calculated  by  means  of  the  following  formula: 


B 


•105  P  +  4 


SIZE  RELATION  BETWEEN  OTTER  BOARDS  AND 
NET 

The  hydrodynamic  resistance  of  the  net  and  the  boards  is 
proportionate  to  their  area.  Assuming  that  the  otter 
boards  of  the  trawls  included  in  the  collection  data  are 
being  worked  at  approximately  the  same  ratio  of  lift 
to  drag,  not  only  the  lift  needed  for  keeping  the  net  mouth 
open  but  also  the  resistance  of  the  otter  boards  should 
be  proportionate  to  the  size  of  the  net. 

The  size  of  a  trawl  net  is  usually  represented  by  the 
length  of  the  headline.  For  purpose  of  comparison, 
therefore,  the  area  of  a  trawl  net  can  be  represented  by 
the  square  of  the  headline  length. 


[248] 


TRAWL    GEAR    AND    TOWING    POWER 


2- 

<D 


25 


SM 


10 


& 


20  40   60   80   100  120  140   160  180  200 


The  relation  between  the  area  of  the  otter  boards  and 
this  expression  of  the  area  of  the  nets,  is  given  in  fig.  2. 

If  S"  denotes  the  square  of  the  headline  length,  the 
values  found  can  be  expressed  by  the  following  equation: 

S'  =  415  S*—  1000 

RELATION    BETWEEN    THE    ENGINE    POWER 
(H.P.)  AND  THE  SIZE  OF  THE  NET 

Using  the  equations  given  above,  the  relation  found 
between  the  h.p.  of  the  engine  and  the  size  of  the  net  can 
be  expressed  by  the  following  equation: 

L-  A/43-6  P  H-  660 

where  L  is  the  length  of  the  headline  and  P  the  h.p.  of 
the  engine. 

WEIGHT  OF  THE  OTTER  BOARDS 

The  weight  of  the  otter  boards  was  found  to  be  propor- 
tionate to  the  h.p.  of  the  engine  and  to  the  cube  of  the 

a  -f  b 
expression       —   where  a  is  the  length  and  b  the  width 

2 
of  the  board. 

The  findings  shown  graphically  on  logarithmic  paper 


Fig.  1.     Relation  between  h.p.  of  engine  and  area  of  one  otter 
hoard. 


S« 
?5 


.n 


S 


20 


10 


A 


0   5   10   15  20   25   30   33   40   45  S" 

Fig.  2.     Relation  between  area  of  one  otter  board  and  the  area 
of  the  net. 


Relation  between  h.  p.  and  W 


Ffc.  J.     Relation  between  h.p.  of  engine  and  weight  of  one  otter 

board  and  relation  of  average  length  of  otter  board  in  feet 

and  the  weight  of  the  board. 


1249] 


MODERN     FJSHING     GEAR     OF    THE     WORLD 

in  fig.  3  can  be  expressed  by  the  following  equations:  14 

Up  to  lOOh.p.     W"=2-7  P 

1 00  to  66  h.p.    W"    65  P       400 


Up  to  4  ft.  average  size  of  board  W      3-2  (U  * 


(a  -f-  b  \  :l 
j 


where  W"  is  the  weight  of  the  board  (Ibs.),  P  the  h.p.  of 

a  +  b 
the  engine  and  the  size  of  the  board  (ft.). 


RELATION    BETWEEN    WARP    LENGTH     AND 
WATER  DEPTH 

Fig.  4  represents  the  relation  between  the  depth  of 
water  and  the  length  of  warp  for  trawling  at  different 
depths.  The  curve  is  a  hyperbola  from  which  the  follow- 
ing  approximate  equation  can  be  deduced: 


13 
12 
11 
10 
^  9 

!    8 

o 

•£     7 


0. 
j« 

b 


6 

5 
4- 
3 
2 

1 
0 


0        10       20      30     40      50      60      70     80       90    100 

F 

Dopth   of  watrir   in   fqthoma 

F/>.  4.     Re/at jt'ti  he t  ween  the  water  depth  and  the  ratio  of  \\arp 
length  to  water  depth. 


Modern  Mediterranean  stern  trawling  vessel. 

[250] 


STUDIES  TO   IMPROVE  THE  EFFICIENCY   OF  OTTER   BOARDS 

AND  TRAWL  FLOATS 

by 

LU1GI  CATASTA 

S.  Benedetto  del  Tronto,  Italy 


Abstract 

This  paper  describes  studies  made  on  models  of  otter  boards  and  trawl  floats.  Several  types  with  different  profiles  were  designed. 
Importance  was  given  to  the  smoothness  of  their  surface  and  stability.  Two  types  of  floats  were  made  and  tested,  a  hydrodynamic  plastic 
float  acting  as  kite  and  tied  to  the  headline,  and  an  elevating  hood  which  acts  directly  on  the  square  and  the  headline.  Diagrams  of  the 
resistance  and  lift  of  these  devices  arc  given. 


Rfeum* 


Recherches  pour  amlliorer  I'efficacitc  dcs  plateaux  et  flotteurs  de  chalut 


Ce  travail  relate  Ics  recherches  cflectuees  avcc  des  matjuettes  dc  plateaux  et  dc  flotteurs  de  chalut  construits  scion  le  principe  de 
similitude  m&aniquc  ct  essayes  sous  unc  pression  de  60  A  80  kg. /cm2. 

Plusieurs  types  de  plateaux  avcc  differents  profils  ont  etc  concus,  et  on  a  attache  beaucoup  d'importance  au  poli  dc  leur  surface  et  £ 
leur  stability,  [jes  types  suivants  de  flotteurs  de  plastique  ont  etc  fabriques  et  essayes:  (1)  un  flotteur  hydrodynamique  de  plastique  attache* 
A  la  corde  de  dos  par  de  courtes  chaincs  pour  eviter  rcmmelage  ct  (2)  un  systeme  e"levateur  hydrodynamique  relic  directement  au  grand  dos 
et  £  la  corde  de  dos  pour  soulever  ce  cable.  Dans  Particle  on  trouve  aussi  dcs  courbes  sur  la  resistance  de  ces  dispositifs  pendant  le  chalutage,  etc. 


Extracto 


Estudios  para  mejorar  el  rendimicnto  de  las  puertas  y  flotadores  en  la  red  de  arrastre 


En  este  trabajo  se  describcn  los  esludios  hechos  con  nuevos  tipos  de  puertas  dc  arrastre  y  flotadores  construidos  segun  la  ley  de 
similitud  mecanicu  y  sometidos  a  prucbas  dc  remolque  a  presioncs  que  vanaban  entre  60  y  80  kg.  por  cm2. 

Sc  estudiaron  varios  tipos  dc  puertas  con  peril  Ics  diferentes,  dando  gran  importancia  a  la  continuidad  de  la  superflcie  y  a  la  cstabilidad; 
adcmas  se  ensayaron  los  siguicntcs  tipos  dc  flotadores  dc  material  plastico:  (1)  un  flotador  hidrodinamico  atado  a  la  rclinga  superior  median te 
cadenas  cortas  para  evitar  que  se  enrcde  con  el  aric;  y  (2)  una  "cofia"o  dispositivo  elevador  hidrodinamico  unido  directamente  a  la  visera  y 
a  la  rclinga  superior  a  tin  dc  hacerla  subir.  l:n  el  articulo  tambien  sc  incluycn  graficas  sob  re  la  resistencia  de  estos  dispositivos  durantc  el 
arrastre,  etc. 


FISHING  gear  is  a  determining  factor  in  the  develop- 
ment of  fishing  but  little  progress  has  been  made 
in  research  and  development,  probably  because  of 
the  difficulties  to  be  overcome  in  investigating  the  under- 
water behaviour  of  nets  and  gear. 

An  old  boat  with  a  modern  net,  with  the  same  crew  and 
in  equal  conditions,  can  sometimes  compete  with  a  new 
boat  equipped  with  a  low  yield  net.  When  developing 
the  fishing  unit  as  a  whole,  one  must  improve  not  only 
the  boat  but  also  the  main  and  direct  means  of  capture, 
the  net.  This  is  indicated  in  Italy,  where  the  catch  of  fish 
remains  at  the  same  level,  despite  increased  number  and 
tonnage  of  boats  and  their  engine  power,  and  the  in- 
creased cost  of  operations. 

Fuel,  for  example,  accounts  for  about  40  per  cent,  of 
the  total  operating  expenses,  but  it  is  now  easy  to  design 
high  efficiency  hulls  using  technical  data  in  "Fishing 
Boat  Tank  Tests"  and  "Fishing  Boats  of  the  World", 
both  published  by  F.A.O.,  and  so  cut  fuel  costs.  But 
trawl  nets  and  gear  arc  still  made  by  rule  of  thumb, 


based  on  practical  experience,  and  not  on  hydrodynamic 
laws.  However,  technicians  in  many  countries  have  been 
studying  how  to  increase  the  horizontal  and  vertical 
openings  of  the  mouth  of  the  trawl  by  the  use  of  otter 
doors  and  floats. 

OTTER  BOARDS 

The  boards  are  rectangular  sections  which  have  badly 
finished  surfaces  because  of  the  frame,  reinforcements, 
nuts,  etc. 

Resistance  to  towing  at  a  determined  speed  depends 
upon  the  area  and  angle  of  attack  and  is  influenced  by 
the  finish  of  the  surface  and  the  density  of  the  media. 
But,  in  the  majority  of  cases,  these  factors  are  completely 
ignored.  The  dimensions,  weight  and  angle  of  attack 
of  the  otter  doors  vary  considerably  not  only  in  different 
countries  but  also  in  the  same  fishing  port. 

By  applying  elementary  laws  of  mechanics,  it  should- 
be  possible  to  construct  accessory  trawling  equipment  of 


[251  ] 


MODERN     FISHING    GEAR     OF    THE    WORLD 


improved  efficiency,  and  for  purposes  of  experiment  we 
built  three  models  of  otter  doors  (see  fig.  1). 

Door  "a",  similar  to  the  conventional  boards  now  in 
use,  is  made  of  wood  with  metal  reinforcements.  Door 
"b'\  designed  in  the  style  of  an  aeroplane  wing  section 
with  flat  and  convex  areas,  is  made  of  cast  or  plate  iron 


1234 
___     board  type  a 


Fig.  2.     Adjustment  of  the  angle  of  attack  on  the  otter  boards. 

and  plastic  material.  Door  "c"  is  similar  to  "b",  but 
the  shearing  area  is  concave.  Tests  with  the  models 
were  made  in  a  calm  sea — zero  Beaufort  scale — and  the 
towing  effort  was  measured  with  a  dynamometer.  The 
graph  in  fig.  1  shows  the  behaviour  of  the  three  models 
towed  at  different  speeds  of  up  to  2-5  metres  per  second. 
At  1  -54  metres  (about  3  knots)  .the  resistance  of  door 
14b"  was  6-2  kg.  while  that  of  "a"  was  9-9  kg.  which  is 
almost  60  per  cent.  more. 

Door  "c"  offered  even  less  resistance  but  the  spreading 
power  was  less  because  of  the  concave  shearing  surface 
which  made  the  door  less  efficient.  The  smaller  resistance 
of  the  new  doors  is  due  to  the  hydrodynamic  shape  of  the 
longitudinal  section  and  to  the  finish  of  the  surfaces. 
The  protuberances  on  the  conventional  doors  extend 
through  the  turbulence  layer  which  is  very  thin  because 
of  the  low  Reynolds  number.  The  results  of  the  tests 
and  the  graphs  possibly  contain  some  mistakes,  but  these 
can  only  be  eliminated  by  making  tank  tests. 

To  vary  the  shearing  power  of  the  trawl  board,  the 
angle  of  attack  may  be  changed  by  using  different  holes 
in  an  iron  plate  "d"  (fig.  2)  for  fixing  the  brackets,  or  by 
changing  the  points  of  attachment  of  the  towing  cables 
on  the  back  of  the  boards. 

We  also  studied  the  problem  presented  when  the  otter 
door  falls  flat  on  the  bottom  or  gets  wedged  when  turning 
on  end.  This  is  dangerous  on  muddy  ground  and  often 
results  in  the  loss  of  the  net.  The  fins  (fig.  2,  "e"),  are 
meant  to  act  as  stabilizers;  they  push  the  front  up  and  the 
rear  down  and  make  the  door  rest  on  its  after  end.  The 
doors  may  also  fall  flat  because  of  a  momentary  inter- 
ruption in  the  tension  of  the  towing  cables  which  may 
easily  go  slack  in  a  head  sea.  If  the  doors  fall  inwards 
the  damage  is  not  serious,  but  when  they  tilt  down 
outwards  they  get  wedged  in  the  bottom  when  the  tension 
returns  in  the  towing  cables  and  soon  the  cables  snap. 
An  important  characteristic  of  the  new  trawl  doors  is 
that  they  remain  vertical  in  all  circumstances,  except  on 
becoming  entangled  in  reefs,  etc. 


Fig.    1. 


—  —     "board  type  1> 
*"""     board  type  o 

Profiles  of  trawl  hoards  and  graph  of  performance. 

[252] 


fig.  J.     Forces  acting  on  the  oner  hoard. 


EFFICIENCY    OF    TRAWL    BOARDS    AND    FLOATS 


Ffff.  4.     Position  of  the  plastic  kite  on  the  headline. 

The  centre  of  gravity  "G"  has  been  placed  very  low  in 
the  door  (tig.  3)  while  the  centre  of  lift  "E"  has  been 
placed  very  high  so  that  the  product  of  weight  "G"  times 
the  distance  "X"  from  ihe  point  O  (tilting  moment),  is 
less  than  the  product  of  the  lift  "E"  times  the  distance 
t4Y"  from  point  O  (lifting  moment).  During  the  experi- 
ments it  was  noticed  that  each  time  the  submerged  door 
fell  flat  on  the  bottom,  it  turned  back  to  the  vertical 
position  again. 

The  new  door  models  studied  showed  the  following 
advantages: 

(1)  they  offer  smaller  resistance; 

(2)  they  do  not  fall  flat; 

(3)  they  are  less  liable  to  dig  into  the  bottom; 

(4)  they  should  last  longer. 

During  this  study  we  were  unable  to  tow  the  door 
models  at  the  speed  corresponding  to  mechanical 
similarity,  because  at  a  low  speed  they  do  not  spread  out 
well  and  advance  with  irregular  movements.  Although 
we  were  unable  to  establish  the  resistance  of  full-scale 
doors,  we  got  enough  facts  to  establish  the  marked 
differences  among  the  three  models. 

FLOATS 

Cilass  balls  are  generally  used  to  increase  the  height  of 
the  mouth  of  the  trawl  net,  chiefly  because  of  their  good 
resistance  to  pressure  and  their  low  cost.  New  types  of 
floats,  metallic  or  made  of  other  materials,  are  being 
used  in  many  countries.  They  may  be  spherical,  with 
reinforcement  rings,  or  similar  to  kites.  It  is  known  that 
spheres  have  a  high  coefficient  of  resistance  and  with 


GRAMS 

1     i      > 

!    ~"~ 

"**""" 

-r— 

n 

i 

:/ 

1 

1.200 

f 

i 
1 

1 

WOO 

\ 

f 

.-....,  j 

_ 
h-  --t-      t 

' 

i 

600 

1 

f 

600 

! 

> 

/». 

r 

4001 

i     ' 

1 
1 

! 

f 

200 

;      y 

X 

1' 

i 

1 

—  "      4- 

X 

X 

1 

,    r^" 

Fig.  5.    Elevating  hood. 


05     l      7.5     2      m/sec 

— ball  float 

•  elevating  hood 

Fig.  6 .     C  'omparison  bet  ween  to  wing  resistance  of  a  ball  float  and 
the  elevating  hood. 

increasing  speed  they  lower  the  headline.  Kites  would 
avoid  this  disadvantage  and  we  have  built  a  plastic  float 
(fig.  4)  with  a  hydrodynamic  section.  In  operation,  we 
tied  it  to  the  headline  with  chains  to  avoid  entangling. 
We  found  this  device  offered  a  very  low  resistance  to 
lowing  compared  with  the  spherical  floats  and  had  a 
fairly  good  lifting  power.  Reinforcements  of  the  same 
material  were  used  to  resist  the  hydrostatic  pressures. 
Kites  are  more  expensive  than  the  ball  type  floats,  but 
the  extra  cost  should  be  offset  by  increased  catches. 

LIFTING  DEVICE  FOR  TRAWL  NETS 

The  study  of  the  hydrodynamic  floats  revealed  certain 
disadvantages  while  trawling,  so  a  "lifting  device"  (fig.  5) 
(similar  to  a  prompter's  box)  was  built.  It  was  attached 
to  the  top  of  the  net  at  the  back  of  the  headline  to  increase 
the  opening  of  the  mouth  without  offering  excessive 
resistance. 

The  most  important  advantage  of  this  lifting  gear  is 
that  it  is  firmly  attached  to  the  trawl  net,  which  makes 
efficient  operation  possible  and  facilitates  handling. 

The  top  of  the  lifting  device  slopes  down  towards  the 
net  to  form  an  angle  of  attack  in  the  direction  of  the 
advance.  The  walls  are  also  of  hydrodynamic  profile  and 
offer  a  minimum  of  resistance.  As  in  the  case  of  the  kites, 
higher  speeds  increase  the  lifting  power  of  the  device 
and  because  of  its  smaller  coefficient  of  resistance  less 
towing  power  is  needed  than  in  the  case  of  spherical 
floats. 

The  graphs  (fig.  6)  compare  tests  of  this  lifting  device 
with  those  of  a  ball  float,  and  show  the  respective 
resistances  for  the  same  area  and  speed.  The  new  lifting 
device  withstands  oO  to  80  kg. /cm.2  hydrostatic  pressure. 


[253] 


SIMPLE  DEVICES  FOR  STUDYING  THE    GEOMETRY   OF  VARIOUS 

GEARS  AND  FOR  RELATING  SOME  COMMERCIAL  FISHING 

OPERATIONS  TO  THE  EXISTING  WATER  MOVEMENTS 

by 

J.   N.   CARRUTHERS 
National  Institute  of  Oceanography,  Wormley,  Nr.  Godalming,  Surrey,  England 

Abstract 

The  usual  method  of  studying  the  underwater  behaviour  of  fishing  gear  is  by  the  use  of  highly  technical  instruments  operated  by 
equally  highly  skilled  technicians.  In  this  paper,  the  author  describes  some  of  his  extremely  simple  ideas  for  measuring  the  speed  and  direction 
of  bottom  currents  which  undoubtedly  play  a  big  part  in  the  successful  operation  of  many  kinds  of  fishing  gear.  The  principle  upon  which  the 
indicators  are  designed  is  a  simple  one.  Hot  gelatine  is  introduced  into  suitable  containers  which  can  be  fixed  to  various  parts  of  the  fishing 
gear,  and  after  some  time,  the  lower  sea  temperature  solidifies  the  gelatine  and,  from  the  angle  of  the  "gel"  in  relation  to  its  container,  much 
information  can  be  deduced.  The  aim  of  this  work  has  been  to  produce  cheap  and  effective  instruments  which  can  be  used  by  the  fishermen 
themselves  while  they  are  actually  fishing,  and  some  typical  results  have  shown  that  a  trawl  was  not  being  towed  immediately  astem  of  the 
ship  but  slightly  to  starboard,  and  also  that  one  otter  board  fished  a  little  deeper  than  the  other. 

Dispositifs  simples  pour  £tudier  la  geomttrie  des  engins  de  p&he,  et  etablir  les  relations  entre  les  operations  dc  pfehe  industrielle  et  les 

mouvemente  des  eaux 
Rfaum* 

La  methode  generalement  adoptee  pour  1'etudc  du  comportement  des  engins  de  peche  sous  Peau  consistc  en  1'emploi  d' appareils 
trcs  complexes  manoeuvres  par  des  techniciens  hautement  qualifies.  L'auteur  expose  dans  ce  document  quelques  idees  permettant  de  mcsurcr 
d'une  facon  extremcment  simple  la  vitesse  et  la  direction  des  courants  de  fond  qui  jouent  incontestablement  un  role  important  dans  le  bon 
fonctionnement  d'un  grand  nombre  dc  types  d 'engins  de  peche.  Le  principe  d'apres  lequel  les  appareils  dc  mesure  sont  census  est  tres  simple. 
De  la  gelatine  chaude  est  versee  dans  des  recipients  de  forme  appropriec  qui  pcuvent  etre  fixes  a  difTe rents  points  de  Pengm  de  peche;  apres 
un  certain  temps,  la  gelatine  refroidie  par  I'cau  de  mer,  se  solidifie.  et  Tangle  que  forme  la  surface  solidifiee  avec  le  recipient  permet  de  deduire 
un  grand  nombre  de  renseignements.  L'auteur  a  cherche  a  realiser  des  appareils  peu  couteux  et  cfficaces  susceptibles  d'etre  utilises  par  les 
pccheurs  eux-memes  pendant  la  peche;  ils  ont  permis  entre  autres  dc  constater  qu'un  chalut  n'etait  pas  remorqui  dans  Faxc  du  navirc  mais 
legerement  sur  babord,  et  aussi  qu'un  plateau  de  chalut  ctait  un  peu  plus  enfonc6  que  1'autre. 

Dispositivos  sencillos  para  estudiar  la  geometria  de  los  artes  pesqueros  y  relacionar  algunas  operaciones  de  pesca  comcrcial  con  los 

movimientos  del  agua 
Extracto 

El  metodo  corriente  para  estudiar  la  manera  como  los  artes  de  pesca  sc  comportan  en  el  agua  consiste  en  usar  instruments  muy 
tecnicos  mancjados  por  personal  altamente  especializado.  En  este  trabajo  el  autor  describe  algunas  de  las  ideas  cxtremadamente  send  lias 
puestas  en  practica  para  medir  la  vclocidad  y  direcci6n  de  las  corrientes  junto  al  fondo  del  mar  que,  indudablemente,  juegan  un  pa  pel  dc 
gran  importancia  en  el  exito  del  funcionamiento  de  muchos  tipos  de  artes  de  pesca.  El  principio  que  rigc  a  estos  indicadores  es  sencillo: 
se  introduce  gclatina  caliente  en  envases  adccuados  que  pueden  njarse  a  las  diversas  partes  del  arte,  a  fin  de  que  despues  de  cierto  tiempo 
la  baja  temperatura  solidifique  a  la  gelatina,  lo  cual  permit c  deducir  gran  cantidad  dc  informaci6n  del  angulo  del  "gel"  en  relaci6n  con  el 
recipiente  que  lo  conticne.  El  objeto  de  estc  trabajo  es  producir  instruments  baratos  y  efectivos  que  cl  mismo  pescador  pueda  usar  durante 
sus  faenas.  Entre  algunos  resultados  tipicos  obtenidos  con  ellos  figura  la  indication  de  que  una  red  de  arrastre  no  se  cncucntra  directamente 
dctras  de  la  popa  del  barco,  sino  ligeramente  hacia  cl  costado  de  estribor  y,  tambicn,  que  una  puerta  se  halla  a  una  profundidad  ligeramente 
mayor  que  la  otra 


THERE  are  fishing  methods  and  gear  of  commercial 
importance  such  as  bottom  trawling,  gillnetting 
and  longlining,  which  are  much  affected  by  the 
strength  and  direction  of  underwater  currents  as  they 
may  influence  the  distribution  and  behaviour  of  the  fish 
and  the  operation  of  the  gear.  Fishermen  agree  that,  in 
certain  cases,  advance  knowledge  of  the  current  prevailing 
at  the  fishing  depth  would  be  a  great  advantage.  Therefore 
a  very  simple  current-meter  for  use  by  fishermen  has  been 
developed  and  constructed,  suitable  for  measuring 
underwater  currents  without  anchoring  the  fishing  vessel. 
It  can  be  used  in  any  depth  up  to  ISO  metres. 

It  is  only  necessary  to  throw  overboard  a  long  stick, 
weighted  at  both  ends.  A  thin  hauling-in  line  is  attached 


to  one  end  and  a  small  buoyant  Pyrex  bottle  tethered  at 
the  other  by  a  short  length  of  twine.  The  bottle  is  part 
filled  with  a  hot  gelatine  solution  on  which  floats  a 
circular  compass.  The  bottle  is  canted  by  any  current  and 
the  jelly  sets  solid  at  a  certain  slope,  gripping  the  compass. 
The  magnitude  of  the  slope  provides  information  on  the 
speed  and  the  compass  reading  on  the  direction  of  the 
current. 

GFLLNETTING 

The  direction  in  which  gillnets  have  fished  can  easily 
be  learnt  by  lashing  to  the  net  a  bamboo  with  a  Perspex 
tube  full  of  hot  gelatine  solution.  The  tube  is  sealed  at 


254] 


JELLY     BOTTLES     FOR    STUDYING     GEAR     SHAPE 


each  end  with  a  solid  rubber  ball  through  which  (and 
along  the  axis  of  the  tube)  runs  a  slender  brass  rod.  At 
the  outer  end  of  each  ball  is  a  washer  and  a  travelling 
wing  nut,  so  that  the  tube  can  be  sealed.  A  disc  of 
magnetised  ceramic  hangs  from  a  short  nylon  twine 
midpoint  of  the  rod  (inside  the  tube).  The  disc,  which 
carries  north  and  south  marks,  orientates  itself  resolutely 
into  the  magnetic  meridian.  It  is  easy  to  arrange  for 
the  gelatine  solution  to  remain  fluid  long  enough  in  cold 
water  by  enclosing  the  Perspex  tube  within  a  wider  tube 
of  polythene  filled  with  hot  water. 


LONGLIN1NG 

Fishermen  who  longline  for  halibut  off  the  Faroes  and  in 
Denmark  Strait  would  like  to  know  how  the  deep 
currents  strike  their  lines.  With  this  they  could  build  up 
a  body  of  knowledge  connecting  current  strength  and 
direction  with  tidal  state,  as  represented  by  hours  before 
or  after  high  water  at  a  port  of  reference.  Then  they 
would  know  when  to  fish  safely  near  the  edge  of  a  bottom 
declivity  at  places  where  uprising  water  is  held  to  produce 
good  feeding  conditions. 

There  is  a  very  easy  way  in  which  the  longline  fishermen 
can  collect  such  information.  All  that  is  necessary  is  to 
fix  a  small  yardarm  to  his  hand/anchor  line  just  above 
bottom  (one  simple  tie  of  a  lanyard)  and  hang  from  it  a 
bottle  half  filled  with  hot  gelatine  solution  and  half  with 
hot  coloured  oil.  A  brass  rod  screwed  into  the  underside 
of  the  bottle's  cap  runs  down  the  axis  of  the  bottle  and 
carries  a  directional  disc  hung  from  a  nylon  thread  at  its 
free  end.  Any  current  will  cant  the  bottle  and  a  sloped 
frozen  interface  of  known  direction  will  give  information 
about  speed  and  direction  of  the  bottom  current  when 
the  lines  are  hauled.  Sensitivity  is  increased  by  pushing 
a  tight-fitting  polythene  "cuff "on  the  bottle,  and  trailing 
a  tassel  from  it. 

A  very  simple  valve  is  used  to  avoid  bottle  breakage  if 
the  pre-heating  is  too  severe,  and  for  interior/exterior 
pressure  equalisation  when  the  hot  liquids  chill  and 
contract  deep  in  the  sea. 


GEOMETRY  OF  FISHING  GEAR,  PARTICULARLY 
TRAWLS 

Until  fully  reliable  telemetering  devices  arc  available  for 
use  from  ordinary  fishing  vessels,  it  is  perhaps  of  value 
to  make  observations  by  using  some  very  elementary 
gelatine  solution  devices  which  present  evidence  of  how 
towing  cables  sloped  during  a  tow,  their  azimuth,  the 
shape  and  height  of  a  headline,  and  so  on.  The  slope  of  a 
towing  cable  can  be  studied  by  fixing  to  it  (by  means  of 
stretched  "garters"  cut  from  a  motorcar  tyre  inner  tube) 
cylinders  of  clear  Perspex  filled  with  gelatine  solution  and 
each  containing  a  rolling  inclinometer.  This  is  contrived 
from  a  schoolboy's  celluloid  protractor  mounted  on  a 
rod  running  through  the  cylinder  and  weighted  so  as  to 
remain  always  pendulous.  A  weighted  pointer  registers 
the  slope  of  the  cylinder.  The  correct  slope  is  registered 
whatever  the  aspect  of  the  cylinder  on  the  cable — whether 
below,  above  or  to  cither  side.  The  jelly  sets  and  grips 
the  pointer  at  its  protractor  reading,  thus  making  a  record 
of  cable  slope. 

In  the  same  way,  the  shape  taken  up  by  a  trawl 
headline  during  the  tow  can  be  easily  ascertained.  We 
used  six  cylinders  fixed  at  intervals  along  the  headline 
of  a  very  big  trawl  to  establish  the  sbpe  at  each  of  the 
points.  To  measure  the  opening  height,  a  similar  tube 
was  fixed  with  one  end  to  the  middle  of  the  headline,  and 
a  length  of  rope  and  plummet  (longer  than  the  expected 
headline  height)  attached  to  its  lower  free  end.  During 
the  tow  the  line  pulls  tight  and  draws  the  tube  into  align- 
ment. The  liquid  interface  in  the  bottle  becomes  "frozen" 
so  that,  on  hauling,  one  learns  the  headline  height  by 
multiplying  a  known  length  (tube  -1-  rope)  into  the  cosine 
of  the  angle  of  slope  realized. 

A  more  ambitious  version  of  the  simple  Perspex 
cylinder  has  a  ring  aircraft  compass  in  its  pendulum  so 
that  the  direction  of  the  obliquity  as  well  as  the  slope  of  a 
net  or  cable  is  learnt. 

By  using  several  such  devices,  it  has  been  possible  to 
measure  the  angle  of  divergence  of  a  pair  of  trawl  warps 
and  something  of  the  distribution  of  direction  along  them. 
It  would  doubtless  be  easy  to  glean  such  information  of 
value  by  using  a  number  of  these  simple  instruments. 


Fig.  on  left  shows  a  jelly,  compass  bottle  as  it 
would  appear  to  a  frogman  who  viewed  it 
tethered  down  in  a  current  running  ESE  at 
/  2/5  knots.  Fig.  2  shows  the  same  bottle 
when  brought  inboard  after  use  and  stood 
upright  for  slope  measurement. 


Leaflets  issued  by  the  National  Institute  oj 
Oceanography,  Wormley*  Surrey,  Lngland. 
giving  details  of  the  simple  current  measur- 
ing apparatus  are:  for  temperate  waters 
NIO  '4795  and  for  tropical  waters  .V/O/4858. 


Fig.  I. 
[255] 


.2. 


A  RESEARCH   ON  SETNETS 

by 

MASAJI  KANAMORI 

Faculty  of  Fisheries,.  Kagoshima  University,  Kagoshima  City,  Japan 


Abstract 

Bad  weather  and  strong  tides  cause  damage  and  heavy  losses  to  the  setnets  used  in  Japanese  waters.  Using  current  meters  the 
author  carried  out  experiments  with  various  kinds  of  nets  for  the  purpose  of  reducing  these  losses  by  improving  the  shape  and  construction 
of  the  nets. 

In  this  paper  some  data  is  given  on  the  influence  of  currents  on  the  net  shape  and  the  author  claims  that  the  submerged  setnet  is 
less  influenced  than  the  floating  one.  Other  advantages  claimed  for  the  submerged  net  are  economy  in  labour  and  material,  and  higher 
catchability. 

Recherches  sur  les  filets  fixes 
Rfaim* 

Depuis  longtemps,  les  pdcheurs  japonais  subisscnt  de  lourdes  pertes  a  leurs  filets  ca!6s  causees  par  le  mauvais  temps  et  les  grandes 
marees,  et  1'auteur  essaie  de  r&luire  ces  pertes  par  I'amdlioration  de  la  construction  de  1'engin.  Dans  ce  but,  il  a  fait  des  experiences  avec 
diverses  varietes  de  filets.  Dans  ce  travail,  quelqucs  r&ultats  concernant  la  forme  des  filets  sous  1' influence  dc  la  direction  et  dc  la  vitesse 
du  courant  sont  enrcgistres,  et  1'auteur  declare  que  le  filet-trappe  fixe  immerge  est  meilleur  que  le  filet-trappe  ordinaire  £  cause  de  I'dconomie 
de  material!  et  de  main-d'oeuvre,  la  possibilite  de  le  caler  sur  les  lieux  de  peche  ou  la  vitesse  du  courant  est  relativement  elevcc,  ct  sa  plus 
grande  capacitd  dc  capture. 

fnvestgaci6n  sobre  Redes  "trampns" 
Extntcto 

Como  durante  muchos  artos  los  Pescadores  japoncscs  nan  perdido  gran  cantidad  de  redes  "trampas"  a  causa  del  mal  tiempo  y  altas 
mareas,  el  autor  trata  de  reducir  las  perdidas  mejorando  su  construcci6n.  Para  lograr  este  objcto  sc  hicieron  experimcntos  con  varios  tipos 
de  artes,  registrandose  en  estc  trabajo  algunos  resultados  sobrc  la  influencia  dc  la  direction  y  vclocidad  de  la  corriente.  El  autor  afirma  que 
las  redes  "trampas"  dc  fondo  son  mejores  que  las  comunes  a  causa  de:  la  economia  de  materiales  y  mano  de  obra,  la  posibilidad  de 
calanas  en  bancos  pesqueros  donds  la  velocidad  de  la  corriente  es  relativamente  alta  y  la  mayor  capacidad  de  pesca. 


CHARACTERISTICS    AND    POSSIBILITIES    FOR 
IMPROVEMENT 

THE  immobility  which  is  characteristic  for  setnets 
has  made  it  very  difficult  to  make  any  notable 
improvements  to  this  gear,  which  is  based  on  the 
principle  of  inducing  the  fish  to  enter  the  trap  after 
leading  them  to  it  by  obstructing  their  normal  route. 
After  more  than  thirty  years'  development  of  this  typical 
Japanese  type  of  gear,  there  is  still  scope  for  improvement. 
More  thorough  research  is  needed  on  the  behaviour  of 
the  fish,  and  on  the  physical  characteristics  of  the  net 
construction  before  really  effective  improvement  can  be 
expected.  Bad  weather  and  high  tides  have  for  many 
years  caused  severe  damage  and  loss  of  gear.  Efforts  are 
being  made  to  prevent  this  loss  of  material  and  labour 
by  improving  the  construction  of  the  gear,  by  reduction 
of  the  diameter  of  twine  and  ropes,  and  by  reducing  the 
tension  on  the  webbing  by  decreasing  the  buoyancy  of 
floats  and  the  weight  of  sinkers. 

The  most  effective  way  of  preventing  loss  and  damage  is 
to  completely  submerge  the  usual  type  of  floating  setnet 
so  that  it  becomes  a  submerged  or  bottom  setnet. 

The  fish  caught  in  the  submerged  setnet  are  mostly 
of  the  demersal  kind,  but  many  species  of  fish  which 


normally  swim  in  the  upper  layers  are  also  caught.  In 
fact,  many  so-called  pelagic  species  are  frequently  found 
near  the  sea-bottom,  especially  when  they  come  near 
to  the  coast. 

In  fact  it  is  even  possible  to  catch  such  pelagic  fish  as 
salmon,  sea-trout,  sardine  and  mackerel  with  the 
submerged  setnet. 

Some  advantages  of  the  submerged  setnet  over  the 
floating  type  are:  economy  in  material  and  labour:  less 
chance  of  damage  by  tide  and  swell,  because  of  reduced 
resistance  in  the  water.  The  reduced  resistance  at  the 
same  time  allows  the  net  to  be  used  in  stronger  currents. 
With  the  floating  type  of  net,  the  trapped  fish  often 
escape  from  the  bag  over  the  floatline  which  becomes 
submerged  in  bad  weather  or  strong  tides;  this  is  avoided 
in  the  improved  type  of  net,  where  the  fish  are  completely 
enclosed. 

When  constructing  a  floating  setnet  the  buoyancy  on 
the  floatline  is  made  as  great  as  possible  to  prevent  the 
net  being  submerged  under  the  influence  of  the  tide 
current.  On  the  other  hand,  the  submerging  of  the  net 
can  also  be  avoided  by  decreasing  its  resistance  to  water 


[256] 


RESEARCH     ON     SETNETS 


Fig.   I.     Surface  setnet  wit  It  submerged  bag  for    Yellow-Tail. 


#.  J.      Submerged  set  net  with  one  bag  in  midwater. 


currents  and  waves.  With  the  setnet  for  yellow-tail  the 
buoyancy  of  the  floatline  is  decreased  so  that  the  floats 
are  allowed  to  submerge  in  heavy  weather  or  strong 
tides;  this  is  because  the  fish  usually  stay  well  below  the 
surface.  However,  a  certain  amount  of  fish  docs  escape 
over  the  floatline  whenever  it  submerges. 

With  the  double-trapping  net,  one  of  the  recent  new 
types  of  setnets,  the  trapped  fish  is  induced  to  enter  a 
box-net  by  passing  through  a  funnel.  Once  inside  the 
box-net  the  fish  cannot  escape  as  they  are  completely 
enclosed.  This  retaining  feature  of  the  double-trapping 
net  has  been  incorporated  in  the  submerged  setnet. 

CLASSIFICATION  OF  TYPES 

The  construction  of  setnets  can  be  divided  in  two  groups. 
In  the  first  group  there  arc  two  kinds,  the  first  of  which 
has  the  ante-chamber  and  the  front  shoulder  of  the  funnel 
buoyed  up  so  that  the  floatline  is  at  the  surface,  while 
the  rear  end  of  the  funnel  and  the  entrapping  bag  are 
completely  submerged. 

Another  way   is  to   have   the   whole  net   submerged 


while  the  floatlincs  arc  held  up  partly  by  submerged 
floats  and  partly  by  surface  buoys  (fig.  3). 

In  both  cases  the  webbing  is  suspended  from  the  float- 
line  and  held  down  by  sinkers  so  the  nets  could  be  called 
surface-  and  midwater  setnets. 

In  the  second  group,  the  sinkers  keep  the  net  on  the 
sea-bottom  and  the  webbing  forming  the  walls  of  the  net, 
as  well  as  the  top  parts  of  funnels  and  bags  are  held  up 
by  the  buoyancy  of  the  floats.  The  main  difference  is 
then  that  whereas  in  the  first  group  the  net  is  hung  from 
its  floats,  in  the  second  group  the  net  is  buoyed  up  from 
its  sinkers  (fig.  4). 

The  buoys  used  at  the  surface  function  as  markers, 
or  for  lifting  the  submerged  net,  and  have  nothing  to  do 
with  the  floating  or  buoyancy  of  the  net. 

This  group  can  be  called  bottom  setnets  and  is  of 
different  construction.  In  fig.  4  the  net  has  two  wings  and 
one  bag;  the  fish  is  led  straight  to  the  entrapping  bag, 
through  the  funnel.  In  other  constructions,  as  in  figs.  5 
and  ft,  the  net  has  two  funnels  and  two  bags  but  only  one 
leader. 


Fig.  2.     Floating  setnet  with  one  surface  hag  and  one  on  the 
bottom. 


Fig.  4.     Submerged  bottom  setnet  with  one  hav  on  the  bottom. 


[257  ] 


MODERN     FISHING     GEAR     OF    THE    WORLD 


Fig.  5.     Improved  bottom  setnet  with  two  hags  for  salmon. 


Fig.  6.     Improved  submerged  salmon  and  trout  setnet  with  two 
bags  in  midwater. 


The  table  below  shows  the  relations  between  the 
buoyancy,  sinker  weight  and  the  fixing  power  of  the  nets 
classified  according  to  the  above  grouping. 

THE   EFFECT  OF  DIRECTION   AND  SPEED    OF 
CURRENT  ON  THE  SHAPE  OF  SETNETS 

The  influence  of  speed  and  direction  of  current  on  the 
webbing  structures  of  several  types  of  setncts  was 
investigated  by  the  author,  resistance  of  the  webbing  and 
the  holding  power  of  the  sandbags  were  examined  and 
measured;  some  merits  and  defects  in  the  construction 
being  ascertained. 

Almost  all  the  above  mentioned  nets,  though  there  is 
a  slight  difference,  usually  take  a  forward-bent  position 
against  the  flow  of  tide.  The  bagnet,  funnel  and  the 
ante-chamber  all  bend  inwards  on  the  up-tide  side,  while 
on  the  lee-tide  side  they  tend  to  balloon  outwards.  The 
deformation  is  more  side  pronounced  on  the  up-tide  side 
than  on  the  lee-tide  of  the  tide. 


The  lifting  movement  due  to  current  influence  of  the 
ante-chamber  floor  section  of  the  nets  in  Group  I,  was 
also  examined  and  very  little  lifting  was  observed  in  the 
case  of  the  nets  classed  under  1-a,  which  are  left  and  right 
symmetrically  constructed  nets.  However,  with  the 
increase  of  the  current  velocity  the  bag  on  the  uptide  side 
was  pushed  down  to  the  sea  bottom.  In  other  nets,  a 
lifting  movement  was  generally  caused  by  a  current 
velocity  of  13  to  38  cm. /sec.  (j  to  J  knot).  The  lifting 
occurred  in  both  tidal  directions;  when  the  tide  was 
directed  towards  the  ante-chamber  as  well  as  when  it  was 
directed  towards  the  bag. 

With  the  1-b  category  net,  the  current  directed  towards 
the  bag  had  more  effect  than  when  directed  in  the  ante- 
chamber. In  both  cases,  the  floor  of  the  net  lifts  completely 
at  a  current  rate  of  from  40  to  45  cm. /sec.  (-J  to  I 
knot). 

The  deformation  of  the  shape  of  the  net  increases  as 
the  rate  of  tide  increases  until  a  position  arises  where 
the  webbing  itself  precludes  the  fish  from  entering  the 


TABLF  I 


Category        Net 


Type  of  Net 


Buoyancy 
F.  ton 


Fixing 
Power 
B.  ton 


Sinking 

Capacity 

W.  ton 


BfF          FIW         F-W      F-WIW 


A  Yellow-tail  Middle-layer  Setnet  I  15-3  82-7  2-57 

B                      „             „                    „      II  15-3  124-0  2  81 

C                     „            „                    „      HI  11  2  850  281 

(1-a)                D                     „             „             Revised  A  type  12-2  122-0  225 

E  Improved  Salmon  and  Trout  Setnet  B  0-5  8-8  0-37 

F  Setnet  with  Bottom  Bag  13*8  65-0  3  •  83 

G  Bottom  Gourd-shaped  Net  0-63  4-6  015 

(1-b)  H  Yellow-tail  Middle-layer  Setnet 

Revised  B  type  62  60-0  2-08 


5-4 

5-95 

12-73 

4-95 

8-1 

5-44 

12-5 

4-45 

7-6 

3-98 

8-4 

2-99 

10-0 

5-42 

9-95 

4-42 

18-0 

1-32 

0-12 

0-33 

4-7 

3-6 

9*97 

2-6 

7-3 

4-2 

0-48 

3-2 

9-7        2-%        4-12 


1-98 


I 

Trapping  Type  Bottom  Setnet 

0-28 

1 

•18 

0 

71 

4-22 

0-395 

-0-43 

-0-61 

2(-a) 

J 

Ordinary  Bottom  Setnet 

0-26 

0 

-82 

0 

53 

3-16 

0-49 

-0-27 

-0-51 

K 

Bottom  Setnet  for  Cod 

0  20 

0 

•98 

0 

44 

4-9 

0-45 

0-24 

-0-55 

L 

Improved  Bottom  Setnet 

0-14 

1 

•85 

0 

16 

13-2 

0-87 

-0-02 

-0  13 

M 

Improved  Salmon  and  Trout  Set- 

(2-b) 

net    A 

0-31 

8 

•8 

0 

•37 

28-4 

0-84 

0-06 

-0-16 

N 

Improved  Salmon  Bottom  Setnet 

0-58 

5 

•25 

1 

15 

9-05 

0-50 

-0-57 

-0-5 

O 

Yellow-tail  Bottom  Setnet 

0-50 

25 

•5 

0 

64 

51-0 

0-78 

-0-14 

-0-22 

[2581 


RESEARCH     ON     SETNETS 


hag.  This  limiting  tidal  rate  depends  on  the  direction 
in  which  the  net  has  been  set. 

With  the  groupof  nets  under  1-a,  which  have  aright  and 
left  symmetrical  construction,  it  was  observed  that  when 
set  into  the  tide,  the  limiting  current  speed  was  38-6  to 
51-4  cm./sec.  (J  to  1-0  knot),  while  in  the  opposite 
direction  the  net  remained  fixed  beyond  this  rate  of  tide. 

With  the  revised  A  type  fitted  with  single  bagnet,  the 
current  velocity  needed  to  lift  the  enclosing  walls  was 
found  to  be  25-7  to  50  cm./scc.  (about  A  to  1  -0  knot), 
while  the  bag  end  was  lifted  already  at  19  to  38  cm./sec. 
(j$  to  J  knot).  Both  these  nets  are  of  the  single  bag  type. 

With  the  revised  B  type  (I-b)  the  current  speed  needed 
to  lift  the  enclosing  walls  was  found  to  be  50  cm./sec. 
(about  1  -0  knot)  and  for  the  bag  end  45  cm./sec.  (I 
knot)  respectively. 

In  the  second  group  the  2-a  category  nets  showed 
little  deformation  when  the  current  entered  at  the  ante- 
chamber, but  when  the  current  set  in  the  opposite 
direction,  they  were  easily  deformed.  In  the  first  case  the 
bagnet  began  to  be  lifted  off  the  sea-bottom  at  38  to 
51  -4  cm./scc.  (J  to  1  -0  knot),  the  entrance  of  the  net 
was  closed  and  the  height  curtailed  to  \  of  the  original. 

When  the  current  pressure  was  acting  directly  on  the 
bagnet,  the  entrance  of  the  bag  was  already  closed  at 
26  cm./sec.  (about  \  knot);  at  38  cm./scc.  (J  knot)  the 
net  was  near  to  colfapsing.  In  case  of  the  2-b  category 
nets,  strong  deformation  was  observed  over  the  whole 
net  when  set  in  up-tide  direction,  while  in  Ice-tide 
deformation  was  less,  and  even  at  a  current  velocity  of 
26  cm./sec.  (about  A  knot),  the  height  of  the  floatline  and 
the  form  of  the  net  remained  the  same,  only  being  bent 
forward;  at  38  cm./sec.  (about  J  knot)  there  was  some 
decrease  in  the  height  of  the  net;  and  at  51-4  cm./sec. 
(1  -0  knot)  the  height  decreased  to  about  one  half.  Except 
where  small  sinkers  were  used,  only  slight  lifting  up  of 
the  net  bottom  was  observed.  Fish  were  still  being  caught 
even  at  a  current  rate  of  38-6  to  51  -4  cm./sec.  (J  to 
1 -0  knot)  when  set  in  up-tide  direction,  and  at  51-4 
cm./sec.  in  the  lee-tide  direction. 

DISCUSSION  OF  RESULTS 

By  comparing  the  above  mentioned  net -constructions 
with  the  ordinary  setnet,  the  following  differences  were 
observed. 


1.  With  the  ordinary  type  net,  during  low  tidal  rates 
the  huge  surplus  buoyancy  prevents  the  floatline 
from  being  submerged;  whereas  with  the  nets  under 
category  I  even  a  small  current  velocity  sinks  the 
floatline. 

2.  With  the  ordinary  type  of  net,  the  ante-chamber 
starts  to  be  lifted  from  the  sea-bottom  at  13  to  37 
cm./sec.  (about  \  to  J  knot);  the  funnel  at  26  to 
32  cm./sec.   (about    i    to   jj    knot);    and    therefore 
the  net  is  easier  to  be  lifted  when  the  current  is 
directed  towards  the  ante-chamber  than  when  directed 
towards  the  bag.        With  Group  1  nets,  which  have 
only  one  bag,  the  net-bottom  of  the  ante-chamber 
lifted  at  the  same  current  velocity  as  that  for  the 
ordinary  one.    But  with  Group  2  nets  this  was  not 
observed. 

3.  With  the  ordinary  Setnet,     the    limit  of  "current 
velocity"  which  causes  deformation,  differs  according 
to  the  direction  of  the  current.  This  limit  was  found 
to    be    38    cm./sec.    (J    knot)    for    ante-chamber 
direction  and  26  cm. /sec.  (about  J  knot)  in  the  case  of 
bag  direction. 

The  single-bag  net  construction  showed  the  same 
tendency  as  the  ordinary  trap  net;  whereas  Group  2 
nets  can  stand  stronger  lee-tides. 


REFERENCES 

1  Miyamoto,  H.         Study  on  the  Setnet,  The  Bulletin  of  the  Toka 
Regional  Fisheries  Research  Laboratory  No.  2.  January  1951 

2  Kanamori,  M.   A  Tentative  Experiment  on  the  Improvement  of 
the  Yellow-tail  Setnet.   (Preliminary  Report).    Memoirs  of  the 
Faculty  of  Fisheries,  Kagoshima  University,  Vol.  1  (1950). 

s  Kanamori  M.  and  Enami  S.  Studies  on  the  Improvement  of  the 
Yellow-tail  Setnet  J,  On  Model  Experiments  with  a  Floating 
Setnet.  Vol.  2,  No.  1  (1952). 

4  Kanamori,  M.  and  Enami  S.  Studies  on  the  Improvement  of  the 
Yellow-tail  Setnet  II,  Model  Experiments  with  Setnets  of 
Various  Constructions.  Vol.  4  (1955). 

6  Kanamori,  M.  Studies  on  the  Improvement  of  the  Yellow-tail 
Setnet  IV,  Model  Experiment  with  a  Setnet  furnished  with 
Sea  Bottom  Bagnet.  Vol.  5  (1956). 

*  Kanamori,  M.  Studies  on  the  Improvement  of  the  Yellow-tail 
Setnet  V,  Model  Experiment  with  two  different  kinds  of 
Bottom  Setnets. 


259 


Section  8:  Rational  Design — Methods  of  Specifying  Gear 


SPECIFICATION   OF  FISHING   GEAR 

bv 
ALBERT  PERCIER 

Institut  Scientifique  et  Technique  des  Peches  Maritimcs'  Laboratoire  dc  Biarritz,  France 

Abstract 

'Io  he  complete  and  useful,  the  description  of  a  fishing  net  should  include  well-defined  data.  In  this  paper  some  general  rules  are 
proposed,  with  a  view  to  possible  unification  of  the  methods  for  measuring. 

The  twine  is  defined  by  the  material  used,  by  either  metric  number  (or  other  numeric  system)  or  useful  length  per  kilogram,  and  by 
strength.  The  direction  of  the  mesh  in  the  webbing  should  be  noted.  The  mesh  size  should  be  denoted  by  the  length  of  one  bar  (half mesh). 
The  length  of  a  piece  of  webbing  should  be  measured  on  the  stretched  webbing.  The  width  is  expressed  by  the  number  of  rows  or  meshes. 
Increase  or  decrease  should  be  denoted  by  the  gain  or  loss  of  meshes  per  row.  When  joining  webbing  of  different  mesh  si/c,  the  method  of 
joining  and  the  ratio  of  meshsize  should  be  defined.  The  length  of  foot  rope  and  headline  should  be  stated;  the  hanging  of  the  net  should  be 
expressed  by  the  percentage  of  hanging-in  and  the  amount  of  intake  between  ties.  Accessory  fillings  for  the  net  should  be  described  in 
detail,  i.e.  material  and  diameter  of  ropes,  the  buoyancy  of  floats  and  the  weight  of  sinkers. 

Les  specifications  des  engins  de  peche 
Resume 

La  description  d'un  filet  pour  <hre  complete  ct  utile  doit  comporter  une  serie  de  rcnseignements  bien  definis.  Dans  cc  travail,  I'autcur 
propose  quelques  regies  generales  qui  seraient  susceptibles  d'unificr  les  melhodes  de  mesure  existantes. 

Le  til  esl  defini  par  le  textile  utilise,  par  son  numero  metrique  (on  autres  systemes  de  quotation  equivalente)  ou  par  sa  longueur  utilc 
par  kilogramme,  ct  par  sa  resistance.  Le  sens  du  filci  doit  ctre  signale.  La  dimension  de  maille  est  donnee  par  la  mesure  du  cote  de 
maille  (entre  deux  noeuds).  La  mesure  de  la  longueur  d'une  nappe  s'effectue  sur  la  nappe  considered  ctircc.  La  largeur  J'unc  nappe  esl 
cxprimcc  par  le  nombre  de  rangs  ou  de  mailles.  Les  diminutions  ou  augmentations  sont  signalees  par  les  pertes  (ou  gains)  de  mailles  par 
rang.  Le  collage  de  n  ppes  dc  caracteristiques  differences  est  defini  par  le  procedc  de  couture  et  par  le  rapporl  des  mailles.  Le  caracterc  d'un 
filet  esl  obtcnu  surtout  an  montage.  Les  ralirigues  doivenl  etre  decnles  avcc  leurs  dimensions:  le  montage  final  du  lilet  doit  etrc  cxprime  par 
le  pourcentage  d'armement  et  par  le  nombre  dc  mailles  entre  deux  ligatures.  Les  accessoires  du  filet  dcivcnt  clre  decrits  en  detail:  matenau 
et  diametre  des  cordes,  floltabililc  des  flotteurs,  poids  du  lest. 

Especificaciones  para  artes  de  pesca 
Extracto 

Para  que  la  descripcion  de  una  red  de  pesca  sea  completa  y  util  debe  contener  datos  precisos.  Con  cstc  objcto,  en  el  trabajo 
materia  del  prcsente  extracto  se  proponen  algunas.normas  generales  con  mirus  a  la  posible  normalizacion  de  los  metodos  de  medida  e  incluyen 
algunas  definiciones. 

HI  hilo  se  designa,  segiin  el  material  usado.  mcdi:inte  la  numeracion  metrica  (u  otro  sistema  de  medida)  o  segun  la  longitud  utilizable 
(en  kilgramos)  y  de  acucrdo  con  la  resistencia.  Ademas  debc  anotarse  el  sentido  en  que  corren  las  rnallas  en  el  pafto.  La  longitud  de  estas 
se  dctcrminara  por  la  distancia  enlre  dos  nudos  contiguos  (media  malla  o  lado  del  cuadrado).  Fl  alto  y  el  ancho  del  pafio  se  cxpresaran  por 
el  numero  de  hilcras  y  corridas  dc  mallas  en  csos  scntidos.  F.I  aumento  o  disminucion  sc  dura  a  conocer  por  el  menor  numero  de  estas 
ultimas  en  las  hileras.  Tambien  es  necesario  dcscribir  los  metodos  usados  para  unir  paftos  de  mallas  distmtas,  la  relacion  que  guarda  el 
tamano  dc  ellas,  la  forma  dc  las  redes  durante  la  tcjedura,  y  la  longitud  dc  las  relmgas  superior  e  inferior.  La  manera  de  armar  el  arte  a  lo 
largo  de  estas  cuerdas  se  expresara  segiin  el  tanto  por  ciento  de  colgadura  y  el  numero  de  mallas  embebidas.  For  ultimo  es  necc^ui  io  dcscribir. 
en  detalle,  ION  accesorios  que  cntran  en  una  red  vie  pesca,  por  ej.:  materiales  y  diametro  de  las  cuerdas,  flotabilidad  y  peso  dc  los  flotadores 
y  plomos,  respectivamente. 


THE  diversity   of    terms   and   definitions    used    by 
fishermen    and    manufacturers    when    describing 
fishing  gear  causes  much  confusion,  delays  and 
expense.  In  the  French  fishing  industry  there  is  urgent 
need  for  agreement  ort  definition  of  the  terms  used  in  the 
trade  and  the  present  paper,  while  giving  some  principles 
of  gear  construction,  also  contains  proposals  for  uni- 
fication of  terms  and  definitions. 

TYPE  AND  SIZE  OF  TWINE 

These  have  been  described  in  other  papers,  but  when 
ordering  synthetic  materials  the  following  specifications 
must  be  given: 

(a)  trade  name  and/or  chemical  composition; 

(b)  whether  staple  or  continuous  multi-filament; 

(c)  whether  bonded  or  coloured. 


In  France,  the  size  of  natural  fibres  is  denoted  by  their 
metric  number,  which  gives  the  length  per  kilogram  of 
yarn  and  the  number  of  yarns  in  the  twine.  For  synthetics, 
the  manufacturers  give  the  size  of  yarn  in  denier  but  also 
usually  indicate  the  runnage  per  kilogram  ;md  the 
breaking  strength,  which  is  much  more  useful  to  the 
nclmaker.  When  ordering  a  net,  the  twine  should  be 
specified:  by  its  Nm,  Ne,  etc.,  or  by  the  runnage  per 
kilogram  and  its  breaking  strength. 

WEBBING 

Three  distinct  types  of  knots  are  commonly  used  in 
machine-made  webbing  at  present  (fig.    1): 

(a)  single  or  double  sheet-bend; 

(b)  flat  knot; 

(c)  knotless  net. 


[260 


SPECIFICATION     OF     FISHING     GEAR 


As  webbing  can  be  used  in  two  different  directions,  it  is 
always  necessary  to  indicate  the  direction  of  "lay" 
("with  or  across  the  knot").  The  sign  «  »  could  be 
used  to  indicate  the  direction  which  tightens  the  knots. 
This  is  important  in  the  case  of  seine  nets,  gillnets,  etc., 
and  particularly  with  synthetic  fibres,  where  the  knots 
will  tend  to  slip  if  the  tension  is  applied  in  the  wrong 
direction.  Mesh  size  can  be  given  in  stretched  mesh  or  in 
bar  length,  but  in  each  case  the  unit  should  be  mentioned. 
It  is  more  correct  to  measure  several  meshes  (at  least 
five)  and  divide  by  the  number  measured,  and  it  would 
seem  more  logical  to  use  the  bar  size  as  this  is  the  basic 
unit  of  webbing.  Strips  of  machine-made  webbing  are 
manufactured  with  the  lay  or  against  the  lay,  according 
to  the  type  of  machine;  in  the  first  case  the  lint  has  a 
constant  width,  while  in  the  second  case  it  has  a  constant 
depth.  As  a  mesh  is  composed  of  two  rows,  and  a  section 
of  webbing  must  be  defined  in  two  directions,  it  would  be 
advisable  to  express  width  in  the  number  of  meshes  and 
depth  in  the  number  of  rows.  To  avoid  confusion  with  the 
sign  for  "metres",  it  is  advisable  to  use  tt  as  an  abbrevia- 
tion for  "meshes".  The  length  and  width  of  sections  should 
be  expressed  in  stretched  condition,  which  will  be  the 
same  as  the  number  of  meshes  multiplied  by  the  mesh  size. 


SHAPE 

In  handmade  nets,  the  pieces  are  fashioned  by  inserting 
baitings  or  creases.  This  is  done  with  machine-made 
webbing  by  cutting  points  and  bars,  the  cutting  sequence 
could  be  indicated  as  in  the  morse  code.  For  example, 
•  could  indicate  1  bar  1  point.  However,  for  compari- 
son purposes  between  handmade  and  machine-made 


Fig.    /• 

A.  Sheet-bend. 

B.  Double  sheet-bend. 

C.  Flat  knot. 

D.  Knot  less  net. 


Decrease 


TABU  I 
Cutting  sequence  of  webbing 

Cutting 


Svmhol 


mesli  every  2  rows         All  bars                   —      -       

* 

4  bars  I  point 

4 

2  bars  I  poini 

5 

2  bars  1  point  and 

twice  1  bar  1  point 

6 

1  bar  1  point 

8 

2  bars  3  points 

2  me 

shes 

5 

8  bars  1  point 

2 

' 

7 

8  bars  3  points 

Fig.  2.      Joining  of  webbing. 

A.  With  increase  every  third  mesh. 

B.  Leaving  third  mesh  free. 

C.  Baiting  every  third  mesh. 

D.  By  threading-on. 


[261 


MODERN     FISHING     GEAR     OF    THE    WORLD 


nets,  it  appears  to  be  more  logical  to  indicate  the  shape 
by  giving  the  numbers  of  rows  per  baiting.  In  that  case, 
the  example  above  would  be  denoted  by:  1/6  or  one 
baiting  per  6  rows.  Table  I  gives  conversion  values  for 
the  three  systems. 

It  may  seem  unnecessary  to  indicate  the  baiting 
sequence  when  the  top  and  bottom  widths  and  height  of  a 
piece  of  webbing  are  indicated;  but  in  the  case  of  a  trawl 
wing  one  cannot  deduce  from  this  the  full  baiting  rate 
as  it  is  not  the  same  for  the  whole  wing  (fig.  3). 

ASSEMBLY  OF  THE  NET 

Strips  of  different  mesh  size  are  often  joined  together, 
which  can  be  done  in  different  ways  as  shown  in  fig.  2 
(for  a  take-up  of  2/3). 

Also  in  this  case  it  would  be  advantageous  to  have 
standard  ratios  of  take-up,  such  as  the  sequence  1/2, 

2/3,  2/5,  3/4,  3/5,  4/5,  5/6 9/10.  This  would  simplify 

the  work  of  the  netmaker. 

During  the  assembly  of  the  nets,  the  webbing  is  hung 
on  framing  lines  by  hangings  which  take  up  individual, 
or  a  certain  number  of,  meshes.  Before  hanging,  the 
webbing  is  normally  given  a  self-edge  for  local  strengthen- 
ing and  this  should  be  noted  on  the  plan.  In  France,  the 
size  of  leadline,  floatline,  headline,  etc.,  is  expressed  by 
the  number  of  yarns  to  the  strands  and  the  number  of 
strands  in  the  rope  or  simply  by  the  diameter.  The  hang- 
ing operation  determines  the  opening  of  the  meshes  or 
the  looseness  in  the  webbing.  If  the  webbing  is  hung  on  a 
line  which  is  73  per  cent,  of  the  stretched  web  length, 
the  meshes  will  be  squared. 

The  looseness  of  the  webbing  has  a  great  influence  on 
the  fishability  of  the  net,  and  must  be  specified  on  net 
plans. 

The   following   specification    for   a    sardine   driftnet 


illustrates  a  method  of  expressing  the  hanging  in  terms 
of  the  mesh  bars. 

Length  of  net  55  metres,  stretched 

Depth     „     „  800  meshes 

Leadline  3  meshes  per  4-1/2  mesh  bars 

Floatline  3  meshes  per  4-1/4  mesh  bars 

The  hanging  coefficient  is  here: 

4-5  :  100 

Leadline  :      75  per  cent.  (i.e.  25  per 

3    •    2  cent,  slack). 

4-25  :  100 
Floatline:      71   per  cent. 

3    *    2 

Although  this  method  is  simple  and  gives  complete 
information,  it  is  better  to  express  the  ratio  of  hanging 
by  giving  the  length  of  line  as  a  percentage  of  the  stretched 
webbing  hung  on  the  line. 

FLOATS  AND  SINKERS 

With  the  introduction  of  new  materials,  many  types  of 
floats  are  available.  Hollow  and  sponge  floats  of  plastic 
material,  and  metallic  floats,  are  now  increasingly  used 
instead  of  cork  and  glass  floats.  As  the  buoyancy  of 
these  floats  changes  with  their  si/c  and  weight,  in  the 
case  of  hollow  floats,  and  with  size  and  density,  in  the 
case  of  sponge  plastic,  size  alone  is  no  longer  an  indication 
of  the  buoyancy  of  a  float.  The  principal  characteristics 
are  given  in  Table  II  for  the  main  types  used  in  France. 
When  the  floats  are  uniformly  divided  over  the  whole 
floatline,  it  will  be  sufficient  to  specify  the  total  number. 
If,  however,  as  in  the  case  of  trawls  and  roundhaul  nets, 
the  floats  are  not  uniformly  distributed,  the  plan  should 
include  details  of  their  distribution. 

In  the  case  of  sinkers,  the  distribution  of  the  weight 


TABLE     II 
Characteristics  of  Net  Floats 


Material 


Cork      . 
Cork      . 

Cork 

Cork      . 

hxpandcd  Polystrenc 

Expanded  Polystrene 
Hxpandcd  Polystrene 

Sponge  Plastic 

Glass 

Plexiglass 

Aluminium 


Shape 

Six-sided  prism 
Cylinder 

Cylinder 
Cylinder 
Cylinder 

Sphere 
Cylinder 

Cylinder 

Sphere 
Sphere 
Sphere 


Dimensions 
in  cm. 


10  x  10  x  2 
hole  of  1  -3  cm. 
7  in  0 
4  in  height 
hole  of  2  cm. 

6  in  0 

2-5  in  height 
hole  of  1  cm. 
4-15  in  0 
2-5  in  height 
hole  of  0-8  cm. 

7  in  0 

4  in  height 
hole  of  1  -2  cm. 
7  in  0 

hole  of  1  -2  cm. 

5  -4  in  0 

2  in  height 

hole  of  0-8  cm. 

7  in  0 

3-5  in  height 

14  in  0 

10  in  0 

20  in  0 


WeiKht 
in  f?r. 

40 

27 


11-5 

6 

18 

20 
4 


25 

600 

125 

1,200 


Density 

0-20 
0-20 

0-20 
0-20 
0-12 

0-12 
0-12 

0-20 


Volume 
in  re. 


135 

60 

30 

150 

155 
30 


123 
1,400 

4.100 


Buoyancy 
in  Vr. 

150 
100—  110 

50 

25 

130 

145 
25 


100 

800 

375 

2,900 


[262] 


r 


SPECIFICATION     OF      FISHING     GFAR 

ROUNDHAUL  NETS 


7v.  -?       The  cut  of  a  Itnvei   win.if. 


along  the  line,  and  the  size  and  weight  of  sinker  used, 
should  be  indicated.  When  the  distribution  is  not 
uniform,  this  should  be  denoted  by  weight  per  metre  in 
each  section.  The  weight  and  method  of  attachment 
should  also  be  given  when  chain  is  used. 

SIZE  OF  FISHING  GEAR 

In  France,  the  si/e  of  a  net  is  usually  given  in  terms  of 
length  of  the  floatline  and,  in  the  case  of  trawls,  the 
length  of  the  headline.  This  leads  to  confusion  as  such 
lengths  provide  no  comparative  si/e  of  the  gear. 

GILLNETS 

The  size  of  these  nets  should  also  include,  apart  from  the 
length  of  the  floatline,  the  depth  in  stretched  mesh 
measure  and  the  hanging  percentage. 


French  seines  and  lamparas  are  between  120  and  250  m 
in  length.  The  depth  is  usually  given  in  fathoms  or 
metres  of  the  breastlines,  although  these  lines  alone  give 
no  information  on  the  working  depth  of  the  net.  Further- 
more, none  of  these  measures  give  an  indication  of  the 
bulge  formed  by  the  net  in  action,  although  this  is  one  ot 
the  important  factors  of  roundhaul  nets. 

Descriptions  of  roundhaul  nets  should  therefore 
include: 

Length  of  float-  and  leadline 

Length  of  breastlines 

Number  of  meshes  in  depth  and  length  of  the  main 

body  of  the  net. 
Hanging  percentages. 

TRAWL  NETS 

Usually  the  size  of  a  trawl  net  is  given  by  the  length  ot 
the  headline.  The  net  is  composed  of  top  and  lower 
wings,  the  square,  top  and  lower  belly,  throat  and  codend. 
The  lower  wings  are  longer  to  compensate  for  the  square 
in  the  top  half  of  the  net.  The  bosom  meshes  of  the 
square  are  hung  to  the  middle  of  the  headline,  with  the 
top  wings  on  each  side,  while  the  bosom  meshes  of  the 
belly  bottom  are  hung  to  the  fish  line  (bolchline),  with 
the  lower  wings  on  each  side,  and  the  fishline  is  fastened 
with  slack  to  the  footrope.  The  hanging  percentage  of 
these  net  parts  to  their  framing  lines  diflers  from  netmaker 
to  netmaker,  so  that  for  the  same  type  of  net  the  length 
of  identical  lines  can  differ  greatly.  This  means  that  a 
net  having  small  wings  and  a  large  bosom  could  have 
a  bigger  opening  than  a  net  with  large  wings  and  a  small 
bosom  mounted  on  the  same  length  of  headline.  The 
wings  are  only  an  extension  from  the  net  to  the  boards 
and  are  really  no  part  of  the  net  funnel. 

It  appears,  then,  that  the  logical  way  to  express  the 
size  of  a  trawl  would  be  in  the  dimensions  of  its  square, 
as  this  provides  a  truer  comparison  with  other  trawls. 


Hauling  nylon  salmon  gillnets  in  British  Columbia.     The  nets  are  wound  on  a  power-driven  reel. 

\  263  ] 


THE   SIZE  SPECIFICATION   OF  TRAWL   NETS   IN   POLAND 

by 

M.  SZATYBELKO 

Sea  Fisheries  Institute,  Gdynia,  Poland 


Abstract 

For  many  years  trawls  have  been  described  by  the  length  of  their  headline  or  footrope,  although  these  are  by  no  means  completely 
satisfactory  indices  of  the  characteristics  of  the  gear.  Today  there  is  a  greater  need  for  a  more  systematic  division  of  trawls  and  Poland  has 
already  adopted  a  more  rational  method  of  describing  them,  in  terms  of  the  headline  and  half  the  circumference  of  the  belly's  inlet. 


Resume 


Determination  de  la  dimension  des  chaluts  en  Polognc 


Lcs  chaluts  sont  classes  depuis  de  nombreuses  annees  d'aprcs  la  longueur  de  leur  corde  de  dos  ou  de  leur  bourrelet  bien  que  ces 
6l6ments  ne  constituent  pas  des  caracteristiqucs  satisfaisantes  de  I'cngin.  Le  besoin  se  fait  actucllement  sentir  d'une  classification  plus 
systematiquc  des  chaluts,  et  la  Pologne  a  dejd  adopte  une  methode  plus  rationnelle  fondcc  sur  la  longueur  de  la  corde  de  dos  et  la  demi- 
circonference  de  1'ouverture  du  corps  du  filet. 


Determinacidn  del  tamano  dc  las  redes  de  arrastrc  en  Polonia 
Extracto 

Durante  muchos  aftos  se  han  descnto  las  redes  de  arrastrc  por  la  longitud  dc  las  relingas  superior  o  inferior,  aunque  estas  medidas 
no  indican  en  forma  completamente  satisfactoria  las  caracteristicas  de  los  artes.  tn  la  actuatidad  es  muy  nccesurio  clasificar  en  forma 
sislcmatica  a  este  tipo  dc  redes,  y  Polonia  ya  ha  adoptado  un  metodo  mas  racional  para  describirlas  segun  la  relac.:6n  de  la  relinga  superior 
y  la  mitad  de  la  circumferencia  del  cuerpo  del  arte,  inmediatamente  detras  de  la  visera. 


CONSIDERABLE  differences  exist  in  descriptions 
of  the  size  of  trawl  nets.  Measurements  are  given 
in  feet  or  metres,  according  to  the  country  of 
origin,  and  the  size  indicators  used  may  be  the  length  of 
the  headline,  the  footrope  or  some  other  combination  of 
trawl  dimensions,  but  the  comparison  of  trawls  by  means 
of  such  indicators  as  these  is  quite  impossible  unless 
other  important  dimensions  are  known. 

In  the  early  days  of  traw)  designing  the  size  indicator 
was  more  of  a  name  for  the  trawl  than  a  true  definition 
of  its  size.  Today,  however,  there  is  a  greater  need  for  a 
systematic  specification  of  trawl  nets,  and  a  comparative 
size  indicator  is  essential  in  international  exchange  of 
ideas  as  well  as  in  technical  literature. 

The  length  of  a  headline  consists  of  the  joint  lengths  of 
both  upper  wings  and  of  the  bosom.  The  length  of  the 
bosom  is  small,  in  comparison  with  the  length  of  the 
upper  wings,  and  there  is  only  a  slight  difference  in  the 
lengths  of  the  bosoms  of  various  trawl  nets.  In  other 
words,  the  difference  in  length  of  the  headlines  is  decided 
primarily  by  the  length-difference  of  the  upper  wings. 
For  example,  two  English  trawl  nets — the  Small  and  Big 
Granton — are  marked  by  size  indicators  80  and  100  feet. 
The  length  difference  of  their  upper  wings  (jointly) 
equals  20  feet,  which  is  the  same  as  the  difference  between 


their  size  indicators.  The  bosom  and  all  other  parts  of 
these  nets  are  the  same.  Fig.  1A  shows  that  trawl  nets 
with  long  upper  wings  actually  can  be  smaller  than  those 
with  shorter  wings.  This  also  indicates  that  one  cannot 
deduce  any  closer  dependence  between  the  lengths  trawl, 
upper  wings  and  the  dimensions  of  other  parts  of  a  of  the 
One  can  consider  the  body  (belly  and  codend)  as  the 
main  part  of  a  trawl  net  which  may  be  proved  by  the 
beam-trawl,  which  has  no  upper  wings  at  all. 

The  problem  of  using  the  length  of  the  footrope  as  an 
indicator  is  similar.  The  best  evidence  is  found  in  fig.  IB 
where  two  trawl  nets  are  shown  with  similar  bodies  but 
with  very  different  lengths  of  lower  wings.  The  drawing 
proves  that  the  trawl  with  longer  lower  wings  must  not 
be  bigger,  as  one  might  expect  from  the  length  indicator 
of  the  footrope.  It  therefore  seems  misleading  to  specify 
the  size  of  the  whole  gear  from  data  characteristic  only 
for  one  part. 

The  proper  size  indicator  for  trawl  nets,  therefore, 
should  include  some  reference  to  the  dimension  of  the 
body  in  addition  to  the  numerical  value  of  one  of  the 
common  indicators.  Such  an  indicator  was  worked  out 
by  the  author  in  the  Polish  sea  fishery  in  1953. 

The  length  of  the  headline  is  used  to  define  the  part 
framing  the  mouth  of  the  trawl,  and  the  circumference  of 


[264] 


SPECIFICATION     OF    TRAWL    NETS    IN    POLAND 


cod  trawl  2P/16 


herrins  trawl  20/26  J   | 


-cod  trawl  27/23 
herring  trawl  18/23 


B 


Comparison  of  existing  trawl  net  types,  showing  insufficient  interdependence  between  the  length  of  the  upper  wings  (A)  or  the  lower 
wings  (B)  and  the  size  of  the  body.     The  examples  are  taken  from  the  Album  of  Polish  Cutter  Trawls.     The  drawings  are  based  on  a 

mesh  opening  coefficient  0,7. 


the  front  edge  of  the  belly  to  define  the  dimension  of  the 
body.  The  reason  is  that  this  is  a  dimension  closely 
connected  with  structural  calculations  of  the  net's  size, 
upon  which  depends  the  amount  of  water  filtered. 
Besides,  the  dimension  of  the  front  edge  of  the  belly  is  a 
deciding  factor  (within  certain  limits)  for  all  other 
dimensions  of  the  body,  with  the  exception  of  the  size 
of  the  meshes. 

The  following  indicator  rules  for  trawl  nets  have  been 
proposed: 

(1)  The  length  unit  is  the  metre; 

(2)  The  length  of  a  headline  is  the  length  of  the  part 
of  the  rope  to  which  webbing  is  attached; 

(3)  The  circumference  of  the  front  edge  of  the  belly 
is  measured  with  the  meshes  stretched,  or  calcu- 
lated by  multiplying  the  number  of  edge-meshes 
by  the  double  bar  length; 

(4)  The  size  indicator  is  expressed  by  a  fraction,  the 
numerator  being  the  length  of  the  headline  (m.), 


and  the  denominator  being  the  length  of  half  of 
the  circumference  of  the  belly's  front  edge.  This 
indicator  is  used  in  practice  and  fully  meets  the 
requirements  of  the  industrial  fishery  in  Poland. 

LITERATURE 

Achlynow,  I.  J. — Ustroistvo  trala  i  technika  tralovovo  leva. 

Moskwal954. 

Davis,  F.  M. — An  account  of  the  fishing  gear  of  England  and 

Wales.  Fishery  Investigations,  Ser.  II,  vol.  XV,  No.  2,  1936. 

Klimaj,  A.,   Bruski,  Z.,  Netzel,  J.     Wloki  kutrowe  i  ich 

cksploatacja.   Warszawa.    1956. 

Okoriski.  S.,  Sadowski,  S.— Album  wlokdw  kutrowych. 

Schnackenbeck,    W. — Schlcppnetze — Hanbuch    der   Seefis- 

cherei  Nordeuropas,  Band  IV,   Heft    1/2,  Stuttgart   1942. 

Wojnikanis-Mirskij,     V.     M.— Technika     promys/.lennogo 

rybolowstwa.   Moskwa    1953. 

Zebrowski,     Z.— Rybol6wstwo Przemysl     Trawlerowy. 

Fleetwood  1943. 

Zebrowski,  Z. — Rybolbwstwo  Morskie— Narzedzia  polowu. 

Gdynia  1949. 

Zebrowski,  Z. — Wloki  trawlerowc.  Warszawa  1954. 

Priiffer  S.,  Sienkiewicz,  W.f  Zebrowski,    Z.— Podstawowe 

wiadomosci  z  praktyki  sieciarskiej.  Warszawa  1954. 


[2651 


DISCUSSION   ON    RATIONAL    DESIGN   OF   FISHING    GEAR 


Mr.  J.  O.  Traung  (FAO)  Rapporteur:  The  development 
of  fishing  gear  has  been  based  simply  on  common  sense 
and  the  visual  observations  of  fishermen  and  net-makers. 

The  last  ten  years  have  seen  the  introduction  of  engineering 
theories  and  the  systematic  testing  of  gear  to  determine  the 
factors  which  influence  the  size  of  the  catch.  Test  methods 
and  the  design  of  specific  instruments  to  determine  the 
behaviour  and  quality  of  individual  parts  of  the  gear  have 
been  developed. 

The  actual  introduction  of  engineering  theories  in  fisheries 
is  not  difficult,  but  is  hampered  by  the  fisherman's  attitude. 
The  fishermen  mistrust  these  theories;  he  is  convinced  that 
as  he  docs  the  fishing,  he  knows  best  what  is  required  of  his 
gear.  The  engineer  himself  is  liable  to  disregard  the  experience 
and  the  knowledge  of  the  fisherman  and,  although  both 
want  to  improve  the  gear,  they  find  no  common  ground  of 
contact.  A  second  problem  is  the  attitude  of  biologists  towards 
technicians.  Biologists  have  long  had  an  intimate  knowledge 
of  the  fisheries  and  are  sceptical  of  engineers  who  misuse 
fisheries  terminology.  When  talking  to  fishermen  it  is  neces- 
sary to  use  simple  language.  All  too  often  engineers  use 
mathematical  formulae  and  technical  expressions  which 
are  incompiehensible  to  the  layman. 

A  few  important  points  need  mentioning  here,  such  as 
the  resistance  of  gear  to  waterflow,  which  increases  as  the 
square  of  the  speed.  Trolling  at,  say  2  knots,  the  trolling 
li  je  having  a  resistance  of  10  lb.,  may  be  found  to  work 
successfully.  If,  however,  the  speed  is  doubled  to  4  knots, 
the  resistance  of  the  line  will  increase  4  times,  to  40  lb.  The 
resistance  might  be  even  higher,  because  some  of  the  details 
in  trolling  gear,  such  as  the  sinkers,  might  change  their  posi- 
tion, causing  an  increase  more  than  by  the  square  on  the  speed 
If  the  speed  of  this  trolling  gear  is  increased  to  6  knots,  the 
resistance  will  be  9  times  higher  than  at  2  knots,  that  is, 
90  lb.,  in  which  case  the  line  may  break,  and  the  gear  be  lost. 

It  is  very  important,  therefore,  to  determine  the  speed 
exactly  when  talking  of  resistance,  because  if  the  trolling 
line  has  about  20  lb.  resistance,  this  is  in  relation  to  quite 
specific  speed.  That  speed  must  be  accurately  stated,  other- 
wise the  resistance  figure  given  may  be  misleading. 

Another  important  engineering  law  is  that  covering  the 
behaviour  of  buoyant  objects  when  moving  through  the  water. 
This  embraces  the  whole  complexity  of  hydrodynamic 
behaviour  of  trawl  boards,  depressors,  kites  and  floats, 
and  it  is  important  that  studies  of  such  bodies  should  be 
conducted  over  a  sufficient  speed  range. 

Phillips  describes  how  he  was  misled  when  designing  his 
trawl  floats  with  the  help  of  observations  by  frogmen.  The 
tests  were  carried  out  at  reduced  speed  to  protect  the  frogmen, 
but,  at  normal  operating  speed,  the  floats  behaved  quite 
differently. 

Fishing  gear  should  be  studied  over  the  full  speed  range, 
one  important  object  being  to  obtain  results  which  are  com- 
parable with  other  tests.  In  the  papers  under  consideration. 


certain  trawling  tests  were  done  at  3  knots,  some  at  3*7 
and  others  at  2  A  knots.  If  this  testing  had  been  done  over  a 
speed  range,  it  might  have  been  possible  to  pick  out  the 
resistance  at  certain  speeds  and  compare  the  different  gear. 

Furthermore,  if  a  certain  gear  or  a  certain  gadget  to  a  gear 
is  tested  within  a  certain  speed  range,  the  different  test  points 
will  not  always  lie  on  an  even  curve.  It  is  best  to  obtain 
many  test  points  to  arrive  at  reliable  average  values. 

A  few  papers  cover  tests  with  models  of  fishing  gear.  In 
this  I  want  also  to  include  tests  of  parts  of  fishing  gear 
carried  out  with  full-scale  models  as  these  produce  the  most 
accurate  results.  Such  testing  of  parts  of  the  gear  will  help 
us  in  assessing  the  efficiency  of  a  complete  gear. 

Kawakami  reports  on  a  number  of  Japanese  model  tests 
with  fishing  gear.  Miyamoto's  tests  with  different  types  of 
webbing  arc  very  interesting. 

Albrechtson  has  made  similar  tests  with  webbing  in  the 
test  tank  in  Gothenburg.  The  Japanese  have  made  tests  with 
pieces  of  nets,  to  arrive  at  figures  for  the  resistance  of  webbing 
at  different  angles  of  attack.  We  do  not  know  sufficient  of 
the  flow  characteristics  of  complete  nets.  One  thing,  however, 
is  evident:  if  one  type  of  webbing  has  less  resistance  to  water- 
flow  in  the  tank  it  will  also  have  less  resistance  when  fitted 
into  a  complicated  net,  where  the  flow  conditions  can  be 
quite  different. 

Model  tests  of  fishing  gear  can  actually  be  carried  out  in 
many  different  types  of  establishments.  There  is  the  ship 
tank,  a  long  channel  with  a  water  depth  about  half  its  width. 
It  is  equipped  with  a  carriage  which  moves  along  it  at  variable 
speeds,  towing  a  model.  A  dynamometer  registers  precisely 
the  resistance  of  what  is  being  towed,  and  it  is  easy  to  obtain 
the  exact  resistance  at  different  speeds.  Unfortunately, 
ship  model  tanks  are  usually  very  expensive  to  use,  but  in 
many  countries  there  are  University  tanks  for  the  use  of 
students  and  these  could  be  quite  useful  for  fishing  gear 
research. 

Fishing  gear  is  normally  totally  submerged  and  does  not 
set  up  surface  waves,  so  that  wave  resistance  does  not  have 
to  be  taken  into  account.  Relatively  large  models  could 
therefore  be  used  also  in  university  tanks,  although  the 
tanks  are  only  10  to  20  ft.  wide. 

Another  type  of  tank  is  that  where  the  model  is  in  fixed 
position  and  the  water  circulates  around  it.  Such  tanks, 
as  described  by  Narasako  and  Kanamori,  are  suitable  for 
observation  of  many  types  of  fishing  gear,  especially  stationary 
ones.  These  tanks  are  less  efficient  for  bottom  trawls  because 
there  is  no  friction  of  the  gear  on  the  ground.  Circulating 
water  tanks  are  also  being  used  in  testing  ship  models,  and 
there  are  to  my  knowledge  two  such  tanks  in  Europe, 
one  in  Genoa  and  one  at  the  University  of  Delft. 

Ship  model  testing  has  been  carried  on  for  about  80  years, 
but  there  are  still  very  great  difficulties  in  extrapolating  the 
results  from  the  model  to  the  full  ship.  At  the  1957  Inter- 
national Towing  Tank  Conference  in  Madrid,  it  was  found 


[266] 


DISCUSSION       RATIONAL     DESIGN 


impossible  to  reach  a  scientific  agreement  as  to  how  this 
best  could  be  done  and  a  compromise  had  to  be  found. 

The  fact  that  the  submerged  gear  is  not  rigid,  resulting 
in  varying  friction  lengths  and  friction  forms,  complicates 
the  extrapolation  of  model  results.  1  believe  that  extrapol- 
ation model  results  of  gear  will  not  be  exact  until  many  have 
been  tried  and  the  results  compared  with  full  scale  gear 
performance,  in  order  to  find  basic  data  for  extrapolation. 
Meanwhile  much  can  be  learned  from  comparative  model 
tests,  especially  on  the  behaviour  of  nets  in  the  water. 

Kawakami  and  Dickson  both  agree  that  model  tests  are 
easier  and  cheaper  than  full-scale  tests.  I  would  like  to  add 
that,  with  the  help  of  model  tests,  it  is  possible  to  test  "bad" 
designs.  By  studying  these  "bad"  designs  we  will  know 
what  to  avoid.  Nobody  would  want  to  design  a  bad  full 
scale  fishing  net.  Model  tests  have  another  advantage.  Model 
testing  work  could  be  done  in  wintertime,  when  weather 
conditions  arc  unfavourable  for  full-scale  work  with  exact 
measurements.  Gear  technologists  should  not  avoid  rough 
water,  but  under  certain  weather  conditions  it  would  be  im- 
possible to  get,  for  instance,  exact  dynamometer  readings. 

The  Japanese  have  contributed  much  in  the  use  of  complete 
models.  Kawakami's  paper  describes  the  method  in  general. 
Takayama  and  Koyama  describe  how  a  certain  trawl  was 
improved  by  the  use  of  a  1 :30  model. 

The  Japanese  use  small  models,  but  Dickson  is  more 
conservative  and  wants  large  models.  This  is  completely 
in  line  with  his  countryman,  Froude,  who,  about  100  years 
ago,  was  the  first  man  to  solve  the  way  of  testing  ship  models. 
Froude  tested  very  large  ship  models  of  about  20  ft.  (6-1  m.) 
in  length,  and  met  with  relatively  small  problems  in  extra- 
polating the  results.  With  today's  extended  knowledge  of 
laminar  and  turbulent  flow,  etc.,  a  school  using  models  as 
small  as  5  ft.  has  developed  in  shipbuilding,  but  many  people 
still  consider  these  5  ft.  models  too  small.  This  remark  may 
illustrate  the  importance  of  the  proper  model  scale  for  the 
purpose  in  question.  Dickson  and  the  Japanese  have  two 
different  ways  of  looking  upon  model  tests.  When  a  fishing 
net  is  reduced  to,  say,  1:8  scale,  it  is  very  difficult  also  to 
reduce  the  meshes  and  the  thread  diameters  correspondingly 
to  make  the  model  scale  absolutely  true  in  every  detail 
Therefore,  one  can  have  a  length  scale  1 :8  but  a  mesh  scale 
of  only  1 :4.  This  procedure  has  been  followed  both  by  the 
Japanese  and  Dickson. 

The  difference  comes  when  reducing  the  speed.  Assuming 
a  full  scale  speed  of  3  knots,  Dickson  says  "Well,  we  take  the 
length  scale  and  we  reduce  the  speed  scale  correspondingly", 
and  he  gets  1  knot.  The  Japanese  take  the  mesh  scale  and 
they  get  about  U  knots.  Naturally,  considering  that  the 
resistance  depends  on  the  square  of  the  speed,  the  I  knot 
makes  a  difference. 

In  addition  to  testing  fishing  gear  in  a  ship's  tank,  one  can 
also  use  other  means,  as,  for  instance,  Albrcchtsson  did  when 
testing  shearing  devices  and  nets  in  the  flow  of  a  creek,  from 
a  bridge.  This  is  not  only  a  very  useful  but  also  a  very  cheap 
method  even  if  speed  regulation  is  impossible  and  speed 
determination  difficult. 

There  are  several  methods  of  studying  underwater  operation 
of  fishing  gear.  First  we  have  the  sensory  method,  that  is, 
to  study  the  angle  and  inclination  of  trawl  wires,  to  feel  the 
vibrations  of  the  trawl  wire,  to  listen  to  the  engine,  to  watch  the 
r.p.m.  of  the  engine,  to  study  scratches  on  the  trawl  door 
and  gear,  and  to  study  stresses  or  tears  in  the  webbing, 


interpreting  these  observations  with  the  help  of  experience 
and  common  sense.  These  are  the  methods  commonly  used 
by  the  fishermen. 

Then  comes  underwater  observations,  either  visually 
through  clear  water  in  shallow  depths,  or  by  frogmen, 
using  cameras,  and  direct  hand  measuring.  Speed,  however, 
must  often  be  reduced.  Frogmen  arc  not  always  fishermen 
or  technicians  and  have  difficulties  getting  across  their 
observations  for  the  scientists.  British  underwater  films  are 
mentioned  in  several  papers,  and  Ben-Yami  describes  some 
very  complete  tests. 

Fishing  gear  can  also  be  studied  by  echo  sounding.  Scharfe 
describes  what  kind  of  results  can  be  obtained  by  operating 
with  a  small  sounding  boat  over  the  gear.  Television  has 
opened  up  enormous  possibilities  in  fishing  gear  research. 

Carrothcrs  describes  how  jelly  bottles  can  be  used  to 
measure  the  slope  of  trawlwarps,  headlines,  etc.  They  can 
also  be  used  for  measuring  the  shape  and  position  of  passive 
gear  such  as  longlines,  gillnets,  etc.  under  actual  fishing 
conditions  and  thus  supplement  direct  observation  by 
television  or  frogmen. 

When  making  a  study  of  the  trawl  in  action,  the  two 
main  things  to  be  determined  arc  the  speed  and  the  resistance, 
this  quite  apart  from  headline  heights  and  the  distance 
between  trawl  boards.  The  speed,  I  feel,  should  be  measured 
exactly  and  not  just  estimated.  It  should  also  be  specified 
whether  the  indicated  speed  is  that  of  the  vessel  through  the 
water  or  the  speed  of  the  trawl  over  the  ground.  This  is 
important  not  only  because  of  differences  in  current  speed 
and  direction  between  the  surface  and  near  bottom  waters, 
but  also  because  of  the  bottom  friction  of  the  gear.  For 
measuring  the  speed  over  the  ground,  Decca  navigation 
equipment  should  be  a  good  tool.  Near  the  coast,  the 
usual  means  of  terrestrial  navigation  can  be  applied. 

The  ship's  speed  through  the  water  at  the  surface  can  be 
determined  by  rail  logging  or  with  the  usual  handlog.  Better 
accuracy  is  obtained  by  towing  a  small  resistant  body, 
specially  designated  and  calibrated.  The  speed  is  then 
determined  from  the  resistance  of  that  body.  That  is  done 
in  the  Kempf  log,  used  by  Scharfe. 

Another  method  is  represented  by  pressure  logs,  such 
as  mentioned  in  Eddie's  paper. 

Finally,  the  propeller  logs  must  be  mentioned  which,  in 
high  quality  construction,  are  in  common  use  as  current 
metres.  The  ChernikofT  Log  is  one  example.  This  system 
could  also  be  used  in  the  way  biologists  do  in  connection 
with  plankton  nets,  to  measure  the  actual  speed  of  the  trawl 
net  by  attaching  the  instrument  to  the  trawl  itself  and  have 
the  measurements  recorded,  or  preferably  transmitted,  to 
the  ship  for  immediate  observation. 

With  regard  to  measuring  resistance,  Dickson  describes 
a  removable  dynamometer  of  an  interesting  type  which  can 
be  fitted  to  the  trawl  warp.  This  type  of  dynamometer  works 
on  the  principle  of  measuring  the  pressure  which  the  warp 
exerts  on  the  instrument.  Another  way  to  measure  the  pull 
without  cutting  the  warp  is  to  use  electric  strain  gauges, 
such  as  those  to  measure  strains  in  bridges  and  plating  of 
ship  hulls,  etc.  De  Boer  mentions  that  he  found  this  method 
too  sensitive  for  his  purpose.  This  method  might  be  suitable 
for  tele-measuring  the  strain  on  different  parts  of  the  gear 
in  action  and  in  that  way  study  the  forces  involved  in  a 
complete  gear. 

It  is  always  advisable  to  double-check  results,  and  1  believe 


[267] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


that,  when  determining  the  resistance  of  the  gear  itself, 
one  should  study  the  performance  of  the  boat  by  measuring 
the  propeller  thrust  or  the  engine  output.  Having  determined 
the  ship's  resistance,  the  total  resistance  of  the  gear  is  then 
obtained.  When  engine  and  gear  data  do  not  correspond, 
then  something  might  be  wrong.  This  double  check  may  also 
serve  as  a  basis  for  evaluating  the  influence  of  external 
factors,  such  as  wind,  current,  wave  action,  etc. 

Dependable  and  convenient  depth  metres  arc  most  im- 
portant for  the  study  of  trawls.  These  can  either  be  pressure 
instruments  or  echo  sounders.  The  Germans  long  ago  used 
pressure  instruments  hooked  up  to  clockwork  recorders 
to  measure  the  opening  height  of  trawl  nets  and  the  depth 
of  floating  trawls.  Such  instruments,  however,  can  only 
be  read  after  the  gear  is  hauled,  the  results  being  more 
difficult  to  interpret  than  a  direct  recording  on  board,  which 
can  be  read  immediately.  Echo  sounders  can  be  used  for 
depth  recordings  cither,  as  described  by  Scharfe,  from  a 
motor-driven  boat  operating  over  the  fishing  gear  or,  as 
described  by  Woodgate,  by  fixing  a  transducer  to  the  appro- 
priate part  of  the  gear  with  a  wire  connection  to  the  echo- 
sounder  on  board  the  vessel  where  the  soundings  arc  recorded. 

Now  comes  the  difficult  problem  of  transferring  instrument 
recorded  data  from  the  gear  to  the  vessel.  Clockwork  recor- 
ders have  the  disadvantage  of  only  supplying  the  data  after 
the  experiment;  such  instruments  are  mentioned  or  described 
in  the  contributions  of  de  Boer,  Dickson  and  Hamuro  and 
Ishii.  A  continuous  transmission  of  measured  values  during 
action  could  be  obtained  by  a  cable,  as  described  by  Wood- 
gate,  who  states  that  a  handwinch  is  sufficient  for  about 
200  fm.,  and  up  to  500  m.  cable  length  could  probably  be 
handled  with  a  special  winch.  For  wireless  transmission, 
ultrasound  waves  have  been  applied  by  the  Americans 
using  frequency  modulation,  as  described  by  Schaefers  and 
Powell,  and  by  the  Germans  using  code  signals.* 

The  main  difficulty  with  this  method  lies  in  the  propeller 
wake. 

The  most  practical  thing  would  be  to  have  an  electrical 
conductor  inside  the  trawl  wire.  McNeely's  paper  is  of 
highest  practical  value,  and  gives  hope  that  it  might  be  possible 
to  produce  such  a  wire  for  commercial  trawlers.  This  would 
give  the  skipper  the  possibility  of  fixing  small  instruments 
to  the  trawl  to  sec  how  it  behaves.  These  electrical  trawl 
wires  permit  the  use  of,  for  instance,  strain  gauges,  echo 
sounders  and  television  cameras  on  the  trawl.  It  is  important 
in  the  development  of  such  trawl  wires  to  try  to  have  many 
conductors  in  each  wire  so  as  to  hook  up  many  instruments. 
In  order  to  have  the  smallest  possible  diameter  for  these 
conductors,  a  change  of  the  voltage  for  the  passage  should 
be  considered. 

The  method  using  the  angle  of  the  trawl  wire  to  the  hori- 
zontal, to  determine  the  depth  of  a  midwatcr  trawl,  might 
work  accurately  enough  in  shallow  depths,  but  it  is  con- 
sidered insufficient  for  general  use  and  should  be  replaced 
by  the  telemetering  methods  mentioned  above.  Determining 
the  depth  of  gear  by  using  the  angle  of  the  trawl  wire  in 
connection  with  its  resistance  coefficient  to  calculate  its 
slope,  has  been  proposed  but  when  I  tried  to  do  this  in 
1949  my  recording  did  not  correspond  to  the  theoretical 
formulae.  The  explanation  probably  was  that  we  had 


vibrations  in  the  trawl  wire  due  to  the  vibrations  of  the 
engine  and  the  propeller. 

It  is  also  important  to  measure  the  distance  between 
the  trawl  boards.  De  Boer's  paper  contains  a  description  of 
his  interesting  method  of  continuous  recording.  Another 
and  simpler  way,  incidentally,  used  in  shallow  water,  is 
to  fix  a  planing  float  with  line  to  each  trawl  board  and  deter- 
mine the  distance  either  by  direct  measurement,  or  by  cal- 
culations using  the  angle  between  them  and  their  distance 
from  the  towing  vessel. 

Carruthcr's  jelly  bottle  method  can  be  of  help  in  the  study 
of  the  geometry  of  the  fishing  gear.  De  Boer  describes  a 
clinometer  and  an  angle  of  attack  meter  for  the  otter  boards. 
Hamuro  and  Ishii  describe  a  recording  ground  rope  indicator. 
Such  instruments  are  relatively  inexpensive  and,  by  using 
them  intelligently,  we  could  learn  a  lot  about  fishing  gear. 

The  following  instruments  might  also  be  of  value  although 
mainly  intended  for  research  purposes  in  connection  with 
commercial  fishing:  a  precise  log  preferably  of  the  propeller 
type;  an  instrument  to  measure  the  towing  resistance  of  the 
gear,  preferably  arranged  in  the  slip  hook  so  as  not  to  inter- 
fere with  the  operation  of  the  trawl;  an  instrument  for 
predicting  the  output  of  the  engine,  such  as  a  fuel  meter 
calibrated  in  h.p.,  and  depth  measuring  instruments.  In 
addition,  a  trawl  wire  meter  as  described  by  Crecelius  could 
be  used. 

An  instrument  indicating  the  presence  of  fish  in  the  trawl 
would  be  of  importance  in  commercial  trawling.  De  Boer 
points  out  that  the  total  resistance  of  the  trawl  decreases 
when  the  trawl  is  being  filled.  This  is  in  contradiction  to 
some  American  claims  but,  personally,  I  have  never  noticed 
additional  resistance  when  fish  are  present.  I  have  seen 
fish  on  an  echosounder  and  I  have  seen  how  the  dynamometer 
went  up  at  the  impact  of  the  fish  schools,  but  it  then  returned 
to  its  original  point.  How  to  determine  the  amount  of  catch 
would  be  a  most  interesting  point  to  discuss.  A  television 
camera  might  be  useful,  or  an  electrical  eye  counting  the 
fish.  The  electrical  conductor  equipped  trawl  wires  might  be 
of  some  help. 

Scharfe  has  shown  that  the  resistance  of  the  trawl  can 
be  reduced  by  30  per  cent,  by  applying  in  the  design  engineer- 
ing methods  aided  by  underwater  observations.  Fddie  has 
shown  that  a  trawler  used  only  600  h.p.  when  the  skipper 
believed  that  he  was  using  1,200.  This  confirms  an  observa- 
tion of  mine:  that  small  trawlers  only  use  a  fraction  of  the 
horsepower  the  skippers  believe  them  to  be  using.  Takayama 
shows  how  to  increase  the  headline  heights  from  2-5  to 
5-4  m.  with  the  help  of  model  experiments.  Hamuro 
determines  the  distance  between  pair-fishing  trawlers.  Sand 
describes  how  he  developed  an  improved  mid  water  trawl 
with  the  help  of  underwater  observation  and  television. 
But  many  problems  remain  to  be  solved,  such  as  the  resis- 
tance of  different  trawls,  and  how  this  resistance  changes 
at  different  depths. 

With  the  exception  of  getting  a  fool-proof  conducting 
trawl  wire,  there  does  not  seem  to  be  any  real  hindrance 
preventing  development  of  the  instruments  that  are  needed. 
The  problem  really  lies  with  governments,  who  should 
realize  that  gear  technologists  can  help  the  fishermen,  and 
that  fisheries  departments  should  have  such  technologists 
on  their  staff. 


*  Editor's  note.  Recently  successful  trials  using  changes  in  the 
impulse  rate  for  coding  the  measured  values  have  been  reported 
from  the  U.S.S.R. 


Mr.  W.  Dickson  (Scotland):   At  the  moment  I  think  we 
must  admit  that  the  design  of  trawl  gear  is  very  much,  as 


[268] 


DISCUSSION  -RATIONAL    DESIGN 


we  say  in  Britain,  still  by  guess  and  by  God,  and  we  should 
work  towards  the  day  when  we  can  say:  such  and  such  a 
ship  will  give  a  certain  thrust  and  have  such  and  such  trawling 
speed,  so  that  we  can  design  a  trawl  which  at  that  speed  will 
consume  that  thrust  and  then  also  fish  with  gear  of  dimensions 
which  are  predetermined.  When  we  can  do  that,  then  I 
think  we  can  say  we  will  have  made  some  real  progress  with 
design.  Those  who  design  or  build  trawl  boats  can  help 
us  by  giving  the  actual  performance  data.  They  should, 
when  they  build  new  vessels,  give  the  towing  pull  of  those 
boats  at  various  speeds  because  only  when  we  have  that 
information  will  we  be  able  to  design  nets  to  give  us  a  pre- 
determined performance. 

Captain  1).  Roberts  (U.K.):  I  am  skipper  of  a  Grimsby 
trawler. 

Those  of  you  who  are  not  fishermen,  who  are  not  directly 
connected  with  operational  fishing,  may  wonder  why  we 
fishermen  do  not  take  more  advantage  of  these  wonderful 
instruments  that  are  described  in  many  of  the  Congress 
papers.  Well,  I  must  point  out  that  my  job  is  not  just  to 
go  out  to  sea  to  get  iish.  My  job  is  to  catch  fish  that  will 
sell  for  more  than  it  is  costing  my  owner  and  I  have  not 
got  the  time  to  devote  to  such  things,  much  as  I  would  like 
to.  Once  on  the  fishing  ground  the  fisherman  is  fully  occupied 
with  making  successful  day-to-day  catches  and  assessing 
their  value  on  the  changing  market.  Experiments  are  for 
future  trips  and  slow  down  the  flow  of  the  work  under  opera- 
tion. Nevertheless,  I  do  try  things  out  in  a  small  experimental 
way,  whenever  the  circumstances  allow  it. 

This  Congress  has  shown  me  that  we  must  pool  our  know- 
ledge. I  was  not  aware  of  the  development  of  gear  in  Japan 
and  many  other  places  and  am  now  aware  that  we  fishermen 
should  keep  better  track  of  what  is  achieved  by  our  colleagues 
elsewhere.  One  thing  should  however  be  reali/ed  by  tech- 
nologists and  biologists  and  that  is  that  we  fishermen  cannot 
use  new  instruments  and  innovations  unless  they  have  reached 
the  stage  where  they  improve  the  catch.  Our  first  job  is 
to  catch  fish  that  will  sell  and  sell  for  more  than  it  costs 
us  to  catch  them. 

Mr.  G.  Albrechtson,  Chairman:  Thank  you.  Captain 
Roberts.  I  know  you  are  the  type  of  man  who  will  cooperate 
with  scientists,  boat  and  gear  designers  and  tell  them  when 
they  arc  right  and  I  assure  you  the  fisheries  workers  not 
actually  engaged  in  fishing  do  reali/e  the  difficulties  of  the 
fishermen  conducting  experiments.  On  the  other  hand  all 
innovations  must  be  tried  out  under  actual  fishing  conditions 
and  the  logical  way  is  that  the  fishermen  should  conduct  sue 
trials  themselves. 

Dr.  .1.  Scharfe  (FAO)  :  I  would  like  to  mention  one 
non-technical  reason  why  I  prefer  full  scale,  to  model  tests. 
The  Institut  fiir  Netz-  und  Materialforschung  in  Hamburg 
(Institute  for  fishing  methods  and  gear  research)  where  I 
worked  before  I  joined  FAO,  is  concerned  with  applied 
research.  We  considered  it  necessary  not  only  to  obtain 
results  of  practical  value  for  the  commercial  fishery  but  also 
to  do  all  we  can  to  make  the  fishermen  accept  what  we  have 
worked  out  for  them.  I  think  you  will  agree  that  model 
experiments,  the  results  of  which  have  to  be  interpreted 
mathematically,  give  most  people  the  impression  of  being 
basic  research  rather  than  practical  development,  and  are 


therefore  not  a  very  strong  argument  for  convincing  fishermen 
to  give  up  their  conventional  methods  or  gear.  This  psy- 
chological factor  comes  on  top  of  the  problems  of  model 
testing  of  fishing  gear  as  regards  for  instance  suitable  model 
scale  and  the  proper  method  and  the  accuracy  of  extrapolation 
to  full  scale. 

Mr.  J.  O.  Traung  (FAO)  :  1  do  not  consider  model 
testing  to  be  fundamental  research.  The  formulation  of 
friction  formulae  and  model  laws  to  permit  extrapolation 
of  results  and  development  of  suitable  instruments  might  be 
— but  not  the  evaluation  of  the  results  by  the  gear  technological 
engineer.  Model  testing  is  to  compare  small  scale  models. 
If  one  is  better  than  the  other  in  model  scale  it  predicts  that 
that  type  would  be  better  in  the  full  scale  too.  One  should  of 
course  not  go  with  the  model  test  results  to  fishermen  and  say, 
well  with  this  and  this  formula  you  will  get  that  and  that  net 
opening.  One  should  look  upon  model  tests  as  our  way  of 
testing  new  ideas.  If  the  model  test  shows  promising  results, 
there  is  less  risk  that  the  full  scale  may  turn  out  a  failure. 

Prof.  S.  Takayama  (Japan):  We  use  model  experiments 
not  only  for  qualitative  comparison  but  for  actual  extrapola- 
tion to  full  scale  fishing  gear  and  find  this  method  very  useful. 
We  are  quite  aware  of  the  problems  concerned  with  proper 
reduction  of  netting,  but  in  spite  of  the  limited  accuracy  we 
think  the  results  are  suitable  for  preparing  full  scale  operations 
and  particularly  for  cutting  down  experimental  costs. 

Dr.  J.  Scharfe  (FAO):  I  agree  with  Mr.  Traung  that  model 
experiments  can  be  useful  for  testing  certain  completely 
new  ideas  for  the  consideration  of  which  insufficient 
experience  or  knowledge  is  available.  But,  when  trying  to 
improve  existing  fishing  gear  as  for  instance  trawls  there 
usually  exists  quite  a  lot  of  experience  which  makes  it  possible 
to  foresee,  at  least  qualitatively,  what  the  results  will  be. 
If  such  improvements  are  built  up  step  by  step,  which  in 
my  opinion  is  a  sensible  way,  there  is  not  much  risk  that  the 
experiments  will  turn  out  a  complete  failure.  Further- 
more, in  such  cases  the  experiments  arc  usually  started  with 
gear  types  similar  to  the  conventional  ones,  the  value  of 
which  then  is  hardly  affected  by  the  experimental  modifications 
because  those  can  easily  be  reverted.  So,  even  in  the  worst 
case,  at  the  end  you  still  have  a  conventional  gear  left.  I, 
therefore,  think  that  Prof.  Takayama's  remark  on  the  com- 
paratively lower  costs  of  model  experiments  cannot  be 
generalized.  This  might  be  true  in  certain  cases  when  dealing 
with  extremely  big  and  expensive  gear  as  for  instance  purse 
seines  or  the  Japanese  Setnets.  But,  because  of  their  big 
size,  the  model  scale  then  has  to  be  very  small  which  leads 
to  the  well  known  extrapolation  difficulties.  To  my  opinion 
all  observations  and  experiments  possible  should  be  made 
in  full  scale.  Quite  a  lot  could  be  made  with  commercial 
gear  during  commercial  operation,  using  specially  designed 
measuring  equipment  which  docs  not  interfere  with  the 
fishermen's  work.  You  then  have  the  great  advantage  that 
you  get  the  right  picture  directly  and  under  practical  fishing 
conditions  and  there  is  no  question  of  making  assumptions 
and  figuring  out  the  proper  extrapolation  factors.  And  such 
observations  or  experiments  often  don't  cost  more  than  the 
travel  expenses. 


Prof.  T.  Sasaki  (Japan): 
Council,  Japan. 


am  representing  the  Science 


[2691 


MODERN     FISHING     GEAR    OF    THE    WORLD 


We  arc  solving  many  problems  by  using  underwater 
photography  and  television,  for  example,  in  connection  with 
trawl  Ashing  and  other  research  on  fishing  gear.  Another 
problem  we  are  studying  with  this  method  is  the  significance 
of  the  colour  of  fishing  gear.  I  would  be  interested  to  learn 
what  types  of  underwater  television  equipment  is  used  in 
other  countries  and  to  what  extent. 

Mr.  H.  S.  Drost  (Netherlands):  I  want  to  refer  to  the 
words  of  Captain  Roberts.  I  quite  agree  with  him  that 
fishermen  cannot  work  with  all  these  things  the  scientists 
are  using  but,  as  we  need  a  lot  of  data,  1  would  like  to  ask 
Mr.  Traung  if  it  would  not  be  possible  to  find  a  very  simple 
dynamometer  every  trawler  man  could  use  to  find  out  the 
resistance  of  the  trawl  net  as  measured  by  the  tension  in 
the  warps. 

Mr.  J.  O.  Traung  (FAO):  In  reply  to  Mr.  Drosf  s  question 
it  must  be  mentioned  that  when  dealing  with  the  towing 
resistance  and  its  significance  the  speed  must  be  known  too. 
1  think  that  the  resistance  of  trawls  could  most  easily  be 
recorded  without  much  work,  by  using  a  strain  gauge  built 
into  the  slip  hook,  deck  bollards,  gallows  or  attached  to 
the  warps.  With  some  simple  wiring  the  values  could  be 
transmitted  and  recorded  in  the  wheel  house.  The  speed, 
however,  is  a  much  more  complicated  matter  because  we 
must  decide  which  speed  we  want.  The  speed  over  the  ground, 
or  the  speed  through  the  surface  waters.  Many  large  trawlers 
have  already  LORAN  or  other  such  equipment  with  which 
the  speed  over  the  ground  can  easily  be  determined.  The 
speed  through  the  water  could  be  determined  with  some  type 
of  log,  but  I  am  afraid  gear  research  in  general  would  not 
gain  much  by  such  data,  collected  by  commercial  trawlers 
under  changing  conditions.  The  Captain  can,  however, 
make  use  from  even  the  comparative  values  to  know  whether 
he  is  for  instance  straining  his  net  or  his  warps  too  much 
at,  say  5,000  Ibs.  pull;  or  to  use  the  exact  speed  measurements 
to  ensure  that  he  is  not  trawling  too  fast  for  some  species 
of  flat  fish  which  he  wants  to  catch. 

Mr.  M.  Ben-Vami  (Israel):  1  am  a  commercial  fisherman 
from  Israel,  a  skipper  of  a  small  trawler,  and  I  have  also 
done  some  research  on  trawl  gear  at  the  Sea  Fishery  Research 
Station,  Haifa.  I  look  on  the  question  of  gear  research 
from  the  fisherman's  point  of  view  and  1  am  always  afraid 
that  this  research  may  become  too  concerned  with  problems 
of  no  immediate  practical  value.  We  can  take  an  experimental 
net  and  we  can  measure  the  breadth,  the  width,  the  height, 
the  length  and  the  bias  strength.  We  can  measure  until 
we  know  the  strength  of,  and  the  strain  taken  by  every  mesh 
in  this  net,  but  unless  we  use  the  information  gained  to  bring 
about  practical  improvements  of  fishing  gear,  there  is  a 
tendency  to  become  engrossed  in  some  scientific  speculation 
of  no  practical  value.  The  final  goal  of  all  such  attempts 
must,  of  course,  be  the  practical  improvement  of  the  fishing 
gear  and  1  would  like  to  stress  that  the  actual  result  can 
only  be  proved  by  comparative  fishing.  The  second  and  not 
less  important  task  is  then  to  make  the  fishermen  accept 
the  innovation.  I  think  this  cannot  be  done  by  Congress 
papers  and  also  not  by  articles  in  various  fishery  journals. 
A  fisherman  will  only  believe  what  he  sees  with  his  own  eyes 
and  here  the  comparative  fishing  comes  in  again.  If  the  new 


gear  is  operated  commercially  alongside  other  commercial 
fishing  boats  and  the  fishermen  see  that  this  gear  continuously 
catches  more  fish,  I  am  sure  that  this  will  immediately  make 
them  eager  to  adopt  the  new  design. 

I  would  now  like  to  deal  with  one  practical  problem  of 
trawl  design.  We  think  that  the  right  way  to  raise  the  headline 
to  increase  the  fishing  height  of  a  trawl  net  is  to  relieve  the 
headline  and  the  upper  net  of  strain.  But,  with  the  Italian 
type  nets  we  use  and  with  other  nets  I  have  seen  in  the  German 
harbours,  all  or  at  least  part  of  the  horizontal  strain  comes 
over  the  headline  and  the  top  part  of  the  net.  The  achievement 
of  the  proper  opening  height  is  then  effected  by  using  floating 
or  shearing  devices  against  this  strain.  It  may  be  right 
that  the  top  of  the  net  also  acts  as  a  kite  when  it  meets  the 
water  flow  at  some  angle  of  attack  but  this  effect  is  very 
limited  compared  with  the  horizontal  strain.  Therefore  this 
strain  should  be  relieved. 

To  improve  our  conventional  nets  we  have  attached  side 
lines  to  take  over  the  strain  from  headline  and  upper  net. 
These  lines  run  along  the  net  from  the  danleno  to  the  codend. 
Furthermore,  wings  and  belly  were  hung  with  some  slack 
or  coefficient  of  hanging  to  these  side  lines  to  give  the  upper 
part  sufficient  freedom.  Now  the  headline  is  longer  than 
the  side  lines  and  the  webbing  and  free  to  be  lifted  by  good 
floats.  We  found  the  trawl  plane  float  superior  to  common 
spherical  floats  and  use  them  almost  exclusively.  This 
modification  resulted  in  doubling  the  opening  height  which 
usually  is  not  more  than  about  1  m.  with  the  Italian  type 
trawl.  Such  modified  nets  are  used  in  Israel  commercially 
giving  satisfactory  results  and  they  are  slowly  coming  into 
production. 

An  additional  modification  of  the  upper  belly  was  developed 
and  successfully  tested  which  is  meant  to  give  even  more 
slack  to  the  webbing.  The  trapezium  shape  of  the  upper 
belly  can  be  obtained  in  two  different  ways.  The  usual 
method  consists  in  shape  cutting  or  braiding  so  that  the  creas- 
ings  are  at  or  at  least  near  the  side  edges.  In  this  case  the  edges 
are  longer  than  the  centre  line  which  must  result  in  some  stress 
along  the  middle  of  the  upper  net.  We  tried  the  other  way 
for  which  the  trapezium  shaped  piece  of  webbing  is  cut 
along  the  centre  line  and  the  two  pieces  sewed  together  by 
joining  the  outer  edges.  Now  the  side  edges  are  shorter 
than  the  centre  line  and  the  webbing  of  the  upper  belly 
has  more  slack.  An  experimental  trawl  has  been  made 
omitting  side  lines.  The  actual  opening  height  achieved  has 
not  been  measured  yet  but  the  practical  results  indicate  that 
it  gives  better  performance. 


Captain  A.  Hodson  (U.K.):  The  remarks  our  colleague 
from  Israel  made  about  how  to  increase  the  opening  height 
of  a  trawl  are  particularly  interesting  for  me  because  only 
last  month  I  made  some  modifications  of  this  kind.  A 
comparison  with  the  catches  of  other  vessels  fishing  in  the 
same  area  showed  that  this  modified  net  caught  more  dogfish, 
which  was  possibly  off  the  bottom. 

In  G  rims  by  we  have  the  same  length  for  headline  leg 
and  tow  leg.  The  only  thing  I  did  was  to  increase  the  head- 
line leg  a  bit.  The  effect  of  this  simple  measure  was  already 
very  noticeable  during  shooting  operation  when  steaming 
round  to  get  the  gear  away  from  the  side.  The  headline 
was  at  a  higher  arc  and  a  greater  angle  between  the  legs 
than  we  were  used  to. 


[270] 


DISCUSSION  — RATIONAL     DESIGN 


Mr.  G.  Albrechtson  (Chairman):  We  have  with  us  Mr. 
Larsson  from  Sweden.  He  is  the  inventor  of  the  Phantom 
trawl  and  has  carried  out  extensive  research  work  in  tanks 
with  models  of  nets  for  both,  pelagic  and  bottom  trawling 
including  special  designs  for  opening  the  mouth  of  the  trawl 
net. 

Mr.  K.  H.  Larsson  (Sweden):  1  have  read  with  special 
interest  Dr.  Scharfe's  paper  on  experiments  with  trawl 
gear  in  full  size  because  his  results  are  almost  exactly  the 
same  as  I  got  13  years  ago  with  tank  testing  in  Gothenburg. 
I  then  used  a  model  trawl  with  14  ft.  headline  made,  inciden- 
tally, by  our  Chairman,  Mr.  Albrechtson.  The  trawl  doors 
were  8  +  5  in.  and  we  found  that  of  the  total  resistance  of 
the  gear,  about  one  third  was  due  to  the  warps  and  the  trawl 
doors  and  the  rest  came  from  the  net.  Dr.  Schiirfe  found 
37  per  cent,  for  the  doors  and  warps.  As  the  model  tests  were 
done  on  a  very  smooth  concrete  bottom,  the  small  difference 
could  be  explained  by  the  bottom  friction  of  the  boards  in 
the  full  scale  tests. 

I  then  tested  some  different  types  of  trawl  doors  starting 
with  some  sort  of  a  paravane  which  did  not  work  at  all. 
So  I  designed  another  type  of  door  which  1  called  the 
floating  trawl  door.  I  put  some  cork  on  the  top  of  the  door 
and  lead  on  the  lower  part,  so  that,  when  floating  in  the  water, 
it  was  kept  upright.  After  experimenting  with  different 
shapes  of  doors  of  this  basic  type  I  finally  achieved  an  in- 
crease of  shearing  power  of  about  32  per  cent,  compared 
with  ordinary  boards.  As  a  result  of  this,  I  got  an  order  from 
the  Swedish  Navy  to  test  about  15  different  types  of  trawl 
doors  which  might  be  suitable  for  mine  sweeping.  That  was 
just  after  ihe  Second  World  War  when  many  mines  had  to 
be  swept  off  the  bottom  of  the  big  harbours.  As  a  matter 
of  fact,  some  mine  trawls  were  made  and  used  with  good 
results.  The  Swedish  Navy  then  made  systematic  trials 
with  half  scale  gear  in  the  open  sea  at  the  southern  end  of 
Sweden.  It  was  a  very  good  ground,  4  m.  in  depth,  with 
hard  sand  bottom.  There  we  made  observations  on  speed, 
towing  power  and  spread  of  trawls  using  the  same  method 
as  Dr.  Scharfe,  i.e.  measuring  the  distance  between  the  warps 
one  m.  from  gallows.  We  tested  doors  of  different  height 
to  length  ratio  and  also  different  cross-section  curvatures, 
these  doors  all  being  hydrofoils.  It  was  found  that  higher 
trawl  doors  gave  better  results.  These  tests  finally  led  to  a 
design  which  I  call  a  wing  door — a  door  like  an  aeroplane 
wing  standing  upright  in  the  water.  This  was  superior  to 
all  other  designs  giving  more  shear  at  much  less  resistance. 
Furthermore,  it  was  found,  by  accident,  that  this  trawl  door 
could  easily  be  made  to  go  upwards  or  downwards  in 
the  water  or  travel  steadily  at  will.  Now  it  was  possible 
to  start  work  on  the  pelagic  otter  trawl.  We  started  experi- 
ments with  a  small  trawl  net  with  these  wing  doors  and  very 
soon  found  that  the  floats  on  the  headline  and  the  weights 
on  the  footrope  could  not  keep  the  net  open  against  the 
strong  outward  pull  of  the  trawl  doors.  So  it  became  necessary 
to  design  better  means  for  securing  the  vertical  opening  of 
the  mouth.  To  be  independent  of  the  speed  this  had  to 
be  done  by  shearing  devices.  After  testing  different  models 
which  did  not  work,  we  finally  arrived  at  a  type  which  has 
been  called  the  trawl  toad.  This  is  automatically  balanced 
and  is  attached  by  one  short  strop  to  the  headline  or  the 
footrope.  There  are  two  types  of  this  kite— one  lighter 
than  water,  which  pulls  the  headline  upwards,  and  the 
other,  heavier  than  water  which  pulls  the  footrope  downwards. 


Since  the  shear  of  these  toads  increases  with  the  square 
of  the  speed  exactly  as  is  the  case  with  the  otter  doors,  the 
shape  and  size  of  the  net  opening  is  independent  of  the 
towing  speed.  This  makes  higher  speed  possible  which  I 
have  found  to  be  essential  for  getting  good  results  with 
midwater  trawls.  As  I  believe  that  also  for  bottom  trawling 
there  is  a  trend  to  increase  the  towing  speed,  the  combination 
of  shearing  devices  for  both  horizontal  and  vertical  opening 
can  be  taken  as  an  example  of  designing  for  the  future 
which,  to  my  opinion,  is  a  general  demand  for  all  gear  research. 

Mr.  Y.  Grouselle  (France):  I  have  myself  done  some  re- 
search on  mid-water  trawling  and  1  believe  that  the  problem 
of  adjusting  the  trawl  to  the  depth  wanted  can  easily  be 
solved,  provided  that  the  actual  depth  of  the  gear  during 
towing  is  recorded  by  some  suitable  measuring  device.  The 
new  type  of  mid-water  trawl  I  have  developed  and  which 
is  described  in  my  paper  apparently  achieved  the  desired 
depth  very  quickly.  But  apart  of  the  proper  working  depth 
the  optional  net  opening  has  to  be  maintained.  I  have  studied 
this  problem  in  collaboration  with  the  Institut  Sclent iliquc 
el  Technique  des  Pechcs  Maritimcs.  One  of  the  results 
of  these  studies  was  the  design  of  a  special  type  of  kite  which 
I  called  Exocet.  Contrary  to  normal  floats  this  type  of  kite 
is  rather  independent  of  the  trawling  speed  and,  to  mv 
experience,  is  well  suited  to  work  satisfactorily  under  vary- 
ing conditions. 

In  midwater  trawling  it  is  necessary  to  prevent  the  fish 
from  passing  below  the  footrope.  In  my  new  design,  there- 
fore, the  foolropc  is  in  front  of  the  headline  similar  to  a 
normal  trawl  net  with  the  square  turned  upside-down. 

Dr.  H.  A.  Thomas  (U.K.):  It  seems  that  the  question  of 
whether  a  twine  has  the  tendency  to  float  or  sink  must  play 
an  important  part  in  the  design  of  certain  nets.  Not  being 
a  practical  fisherman  myself,  I  am  putting  this  up  as  a  point 
for  discussion;  is  it  really  of  value  to  the  net  manufacturer 
and  the  fishermen  to  have  available  fibres  which  will  float 
on  water?  The  fibre  Courlene  mentioned  yesterday  with 
regard  to  materials  has  a  density  of  0*95.  Cotton  has  a 
density  of  1*5  and  will  sink  fairly  quickly  in  either  salt 
water  or  fresh  water,  while  Courlene  the  polyethylene  yarn 
will  float  quite  readily.  I  think  the  fishermen  present,  and 
all  who  have  experience  in  this  very  important  business, 
should  advise,  both  the  designers  and  the  fibre  producers 
as  to  the  value  of  yarns  of  this  type.  It  might  even  be  con- 
sidered to  make  one  part  of  the  net  of  floating  twine  and 
another  part  of  heavy  twine.  It  might  be  of  interest  to  show 
on  the  blackboard  the  densities  of  those  fibres  which  are 
now  finding  use  in  the  fishing  trade,  including  cotton: 

Courlene    ..         ..  0-95 

Nylon  or  Perlon  .  .  1-12 

Kuralon     . .         . .  1-26 

(the  Japanese  fibre) 

Terylene     . .         . .  1-36 

Cotton       . .         . .  1-5 

We  would  very  much  like  to  have  the  opinions  of  exper- 
ienced fishermen  and  designers,  whether  there  are  advantages 
to  be  derived  from  the  density  of  the  fibre  and  whether 
density  plays  an  important  part  in  the  design. 

Mr.  G.  Albrechtson,  Chairman:  The  question  of  the 
different  netting  made  out  of  synthetic  fibres  lighter  than 


[271  ] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


water  has  been  raised.  Trawl  designers  are  always  trying  to 
get  the  trawl  net  to  work  lightly  over  rough  bottoms  by  the  use 
of  bobbins  or  depressor  "toads",  so  that  the  belly  is  pulled 
up  from  the  bottom  when  sharp  obstacles  are  met  with 
which  could  damage  the  trawl.  With  the  new  polyethylene 
fibre  it  might  be  possible  to  design  and  make  nets  that  would 
stay  clear  of  such  obstacles.  I  would  be  very  interested 
to  know  if  experiments  on  rough  grounds  have  been  carried 
out  with  nets  made  of  such  more  or  less  buoyant  material. 

Mr.  Z.  Zebrowski  (Poland):  I  belong  to  the  Maritime 
Fishing  Institute  at  Gdynia.  We  have  in  the  past  experienced 
much  difficulty  in  interpreting  the  measurements  of  fishing 
gear,  especially  the  trawl,  due  to  the  different  methods  of 
specification  used.  When  studying  the  trawl  gear  used  in 
the  Baltic  in  1950  we  found  that  our  fishermen  were  using 
6  or  7  ways  of  determining  the  size  of  trawls.  Almost  every 
village  or  town  had  its  own  method.  Furthermore,  we  counted 
mo:c  than  100  different  types  of  trawls,  which  big  number 
seemed  not  to  be  justified  in  such  a  limited  area  as  the  Baltic 
Sea  is.  By  introducing  a  definite  and  legal  method  of  taking 
measurements  we  found  that  this  number  could  be  reduced 
to  about  23  white  fish  trawls  and  about  18  herring  trawls 
This  standardization  is  in  process  of  development  and  we 
aim  to  arrive  at  a  more  restricted  number  of  trawl  types 
for  each  of  which  there  will  be  5  sizes.  This  difference  in 
size  will  only  affect  the  front  part  of  the  trawls  as  the  belly 
and  codcnd  for  each  type  will  remain  the  same.  The  material 
is  being  standardized  too.  From  about  200  different  types 
of  netting,  we  are  using  about  40  for  the  cutter  trawls  in  the 
Baltic.  At  present  we  specify  the  size  of  a  trawl  by  the  length 
of  the  headline  where  netting  is  attached  and  half  of  the 
stretched  circumference  of  the  front  edge  of  the  belly.  These 
two  numbers  are  sufficient  for  the  experienced  net  maker 
and  net  designer  to  estimate  the  size  of  a  trawl  net.  As  a  third 
number  the  mesh  size  of  the  codcnd  could  be  added  to  give 
an  idea  of  its  selectivity. 

Another  matter  I  would  like  to  mention  is  the  necessity 
of  a  dictionary  on  fishing  gear  to  facilitate  the  interpretation 
of  gear  descriptions  from  different  parts  of  the  world.  Yet 
it  seems,  that  before  one  can  compile  a  dictionary  we  must 
first  come  to  an  internationally  accepted  classification  of 
fishing  gear. 

Mr.  G.  Albrechtson,  Chairman.  The  point  made  by  Mr. 
Zebrowski  on  the  need  for  a  more  accurate  determination  of 
the  size  of  trawl  nets  appears  to  be  quite  acceptable.  In 
Sweden  the  size  of  trawls  is  determined  by  the  length  of  the 
headline  alone,  but  this  does  not  give  full  information  as  it 
does  not  give  any  indication  of  the  size  of  the  belly.  Standard- 
ization of  the  method  of  designation  would  be  a  step  forward. 

Mr.  J.  W.  Phillips  (U.K.):  My  colleagues  and  I  have  made 
and  tested  many  models  of  floats,  some  in  swimming 
pools  using  a  winch  to  tow  the  models,  with  weights  attached 
to  them,  comparing  the  amount  of  lift  that  one  can  obtain 
from  various  shapes  and  designs  of  floating  equipment. 
The  most  promising  types  were  then  very  carefully  tested 
hydrodymechanically  in  the  tanks  of  Messrs.  Saunders- 
Roe  the  aircraft  manufacturers  on  the  Isle  of  Wight,  and 
we  have  learned  how  very  much  more  there  is  to  testing 
than  we  thought.  For  instance,  I  have  always  thought, 
that  if  you  make  a  trawl  plane  float  the  right  shape  and  attach 
it  to  the  headline,  the  faster  you  tow  it  the  higher  the  head- 


line will  come.  Well,  that  is  balderdash,  absolute  nonsense. 
The  moment  the  speed  is  increased  beyond  a  certain  velocity, 
the  drag  overcomes  the  buoyancy,  the  angle  of  attack  in- 
creases and  disaster  to  the  headline  is  imminent.  Perhaps 
the  technicians  have  known  this,  all  along,  but  we  had  to 
use  model  testing  to  find  it  out.  I  do  not  think  there  is 
any  real  difficulty  in  designing  and  making  lifting  gear  to 
raise  the  headline  at  any  speed,  provided  you  harness  static 
buoyancy  with  dynamic  lift.  Furthermore,  the  gear  must 
be  very  simple  to  attach  to  the  headline  and  reliable  under 
trawling  conditions.  The  strength  of  the  netting  is,  to  my 
mind,  vital  because  of  the  diverse  forces  at  work  and  there 
is  need  for  a  good  deal  of  research  work  in  designing  and 
making  a  net  if  these  forces  are  not  to  fight  one  another. 
I  feel  that  the  future  development  of  trawling  gear  depends 
to  a  great  extent  on  model  tests,  both  for  the  net  and  for  the 
auxiliary  gear. 

Mr.  I.  Richardson  (Lowestoft):  I  want  to  say  a  few  words 
on  integration  in  the  technological  approach,  because  I 
think  we  are  a  little  at  cross  purposes.  Obviously,  tank 
testing  can  help  in  the  assessment  of  fishing  gear  as  far  as 
hydrodynamics  are  concerned.  But  this  probably,  is  as  far 
as  we  can  go  with  tank  testing.  Next  comes  the  question: 
what  material  arc  we  going  to  use  in  making  the  gear?  In 
a  trawl  gear,  we  are  mainly  interested  in  breaking  strength, 
so  that  we  want  a  measurement  of  the  strength  of  the  gear 
as  it  is  used.  This  is  the  strength  of  the  mesh,  not  of  the 
individual  yarn  and  not  of  a  knotted  twine  pulled  straight. 
We  want  the  mesh  tested  and  we  want  it  tested  wet.  Further- 
more, we  want  comparhon  tables  of  all  material,  synthetic 
and  natural,  for  proper  choice  or  substitution.  This  selection 
of  the  optimal  material  with  sufficient  breaking  strength 
cannot  be  solved  by  model  tests. 

Another  vital  problem  which  also  can  only  be  solved  by 
observations  under  natural  conditions  is  the  reaction  of 
the  fish  to  the  gear.  1  think  that  this  point  has  not  been 
stressed  enough  today.  We  have  only  heard  about  one  or 
two  instruments  which  can  be  used  in  observing  fish  behaviour 
to  gear.  I  believe  that  this  problem  deserves  far  more  atten- 
tion in  the  future  than  has  been  paid  to  it  in  the  past. 

Dr.  A.  W.  Needier  (Canada):  1  would  like  to  talk  not 
only  as  a  Biologist  but  also  as  a  Senior  Government  Adminis- 
trator. In  general,  fishing  gear  is  extremely  inefficient.  In 
any  test  that  I  know  of,  gear  has  been  shown  to  catch  very 
small  proportion  of  the  fish  which  approach  or  which  it 
approaches.  In  the  case  of  scallop  dredges  in  Canada,  for 
instance,  we  have  had  good  estimates  of  the  density  of  popu- 
lation, checked  with  statistical  methods  and  with  photography 
of  the  bottom,  and  we  have  found  that  a  simple  dredge 
moving  over  fairly  smooth  bottom  only  takes  5  per  cent,  of 
the  scallops.  We  had  to  study  the  reactions  of  the  scallops 
to  get  some  explanation  of  this,  and  found  from  frogmen 
observations  that  scallops  escaped  the  dredge. 

Now,  to  overcome  such  inefficiency,  collaboration  is  needed. 
1  am  inclined  to  think  that  the  least  difficult  person  in  this 
picture  might  turn  out  to  be  the  Government  Administrator, 
because  if  the  others  can  bring  their  heads  together  and 
provide  an  apparently  practical  answer,  government  support 
will  be  forthcoming.  Governments  always  yield  to  economic 
pressure,  although  sometimes  they  fail  to  yield  to  what 
you  might  call  intellectual  pressures.  Sometimes,  there  are 


[272] 


DISCUSSION  — RATIONAL     DESIGN 


difficulties  in  establishing  a  working  team  due  to   lack   of 
mutual  understanding. 

Dr.  Rollefsen,  who  was  one  of  the  world's  great  fishery 
biologists,  once  said  in  an  international  conference:  we  have 
to  ask  the  fish  a  lot  of  questions,  and  we  have  to  learn  how 
to  interpret  his  answers.  This  has  a  great  deal  of  bearing  on 
the  rational  design  of  fishing  gear.  One  of  the  big  things 
to  learn  about  fishing  gear  is  what  impulses  it  pushes  ahead 
of  it.  Does  it  make  a  noise?  what  noise?  what  light? 
what  pressure  waves?  what  does  it  do?  The  biologists  should 
join  with  the  technologists  to  investigate  the  reactions  of 


fish  to  gear,  for  which  purpose  tank  testing  techniques  might 
also  be  useful. 

There  is  another  point  which  also  bears  on  this  need  for 
a  broad  approach:  and  this  is  the  economic  side  of  fishing 
methods  and  gear  development  in  general.  When  for  instance 
harvesting  fish  stocks  which  cannot  yield  any  more  than  they 
are  yielding  or  very  little  more,  the  result  to  be  expected 
will  hardly  be  more  fish  but  only  reduction  in  production 
costs.  This  is  only  one  example  to  show  that  economic 
considerations  are  also  important  for  planning  technical 
development. 


200  ton*  oj  herring  token  in  one  set  \\irh  a  two-boat  purse  seme  off  the  west  coast  oj  Norway.          Photo:  FAG. 

[273] 


Section  9:  Fishing  Gear  and  its  Operation 


CLASSIFICATION  OF  FISHING  GEAR 

by 

A.  VON  BRANDT 

Prof,  und  Dir.,  Institut  fur  Netz-  und  Materialforschung,  Hamburg,  Germany 

Abstract 

The  author  has  divided  fishing  methods  into  thirteen  categories  beginning  with  fishing  without  gear.      Each  category  is  prefaced 
by  a  short  description  and  is  fully  illustrated.  The  names  of  the  several  implements  and  methods  are  given  in  six  languages. 


Rfaime 


Classification  des  engins  de  ptche 


L'auteur  a  divisc  les  m&hodes  de  peehe  en  treize  categories,  en  commencant  par  la  peche  sans  engins.  Chaque  categoric  est 
pitcedee  d*une  courte  description  et  abondamment  illustree.  Les  noms  des  divers  instruments  ct  m&hodes  sont  donnes  dans  plusieurs 
langues. 

Ctasificadon  de  los  artes  de  pesca 
Extracto 

El  autor  ha  dividido  los  m£todos  de  pesca  en  trece  categories,  empezando  por  la  capture  sin  ningun  equipo.  Cada  categoria 
empieza  por  una  breve  description  acompaftada  de  numerosas  ilustraciones.  Los  nombres  de  los  diversos  artes  y  m£todos  de  pesca  se  dan 
en  varios  idiomas. 


THERE  is  at  present  no  uniformity  in  the  terms  used 
to  denote  the  fishing  gear  used  in  commercial 
fisheries  in  different  countries  and  the  name  for 
the  same  gear  may  even  change  from  one  fishing  area 
to  another;  furthermore,  the  name  given  to  gear  in  one 
area   may   denote   a   different   gear  elsewhere.    When 
translating  the  names  of  fishing  gear  from  one  language 
to  another,  these  difficulties  of  nomenclature  increase. 
The  aim  of  this  paper  is  to  present  names  which  allow 
but  one  interpretation  in  different  languages.  But  for 
some  types  of  fishing  gear,  which  are  not  generally  known 
in  other  countries,  new  names  have  to  be  coined  when 
translating  into  other  languages. 

I  am  indebted  to  Mr.  A.  R.  Margetts,  Lowestoft,  for 
his  helpful  recommendations  and  for  the  collection  of 
English  names.  The  following  scientists  have  also  kindly 
given  assistance: 

Messrs.  A.  PERCIER  and  KURC,  Paris,  for  French  fishing 

gear  names. 

Mr.  J.  de  VEEN,  Den  Haag,  for  Dutch  fishing  gear  names. 
Dr.  B.  RASMUSSEN,  Bergen,  for  Norwegian  names. 
Dr.  Aage  J.  C.  JENSEN,  Copenhagen,  for  Danish  names. 

In  attempting  to  compile  a  classification  of  fishing 
gear  and  methods,  founded  on  European  methods,  it  was 
found  necessary  to  establish  13  groups  based  on  different 
principles  of  catching  fish,  and  a  short  definition  of  each 
group  is  given.  To  prevent  misunderstanding,  some 
drawings  illustrating  fishing  gear  of  different  European 
countries  are  included. 

Each  group  is  divided  into  sub-groups,  and  varying 
aspects  of  fishing  are  taken  into  account  for  this  division. 
It  was  not  possible  to  prevent  overlapping  nor  to  separate 


the  interconnections  in  every  case.  The  principle  of  the 
fishing  method  and  its  historical  development  was  first 
considered,  while  other  subjects  that  seemed  of  import- 
ance were  used  for  further  classification.  All  gear  classi- 
fication will  differ  according  to  the  different  interests  of 
the  compilers,  but  it  is  to  be  hoped  that  this  paper  may 
form  a  basis  for  further  discussions. 

1.  FISHING  WITHOUT  GEAR 

This  method  belongs  to  the  simplest  forms  of  obtaining 
food  and  is  practised  on  the  European  coasts  during 
ebb-tide.  Some  of  the  tools  used  are  knives  to  prise 
molluscs  from  rocks,  picks  and  shovels  to  dig  out  shells, 
and  hooks  to  pull  crabs,  cuttle-fish  and  other  fish  from 
their  hiding  places.  Such  tools  should  no  more  be  called 
fishing  gear  than  should  be  the  little  basket  used  in  the 
French  "peche  a  pied",  in  which  the  fisherman  carries 
home  his  harvest. 

Fishing  without  gear  is  also  carried  out  in  freshwater, 
and  in  salt  water  when  fishermen  dive  to  catch  fish  by 
hand.  There  is  also  the  method  of  using  trained  animals 
and  birds,  such  as  the  cormorant,  to  catch  fish. 

2.  WOUNDING  GEAR 

In  his  hunt  for  food,  man  has  lengthened  his  arm  by 
inventing  such  weapons  as  the  lance,  clamps,  rakes  and 
tongs.  He  has  been  able  to  extend  his  range  again  by  the 
use  of  missiles  thrown  by  hand  or  by  equipment.  Lances 
and  harpoons  and  rifles  are  the  wounding  gears  mostly 
used  in  Europe. 


[274] 


CLASSIFICATION     OF     FISHING     GEAR 


3.  STUPEFYING  METHODS 

One  method  to  prevent  fish  from  escaping  is  to  stupefy 
them.  This  can  be  done  by  concussion,  for  example,  by 
hitting  the  ice  under  which  a  fish  is  lurking.  The  same 
effect  can  be  obtained  by  an  underwater  explosion. 
Poisons  can  be  used  to  paralyse  fish,  and  suitable  poisons 
can  be  made  from  both  tropical  and  European  plants. 
Electrical  fishing  is  the  most  recent  development  in 
catching  fish  by  stupefaction. 

4.  LINE  FISHING 

The  principle  used  in  line  fishing  is  to  offer  the  fish  a 
real  or  artificial  bait  to  entice  it  to  bite.  In  principle, 
hooks  are  not  essential;  lines  without  hooks  are  well 
known,  but  as  the  fish  tries  to  spit  out  the  bait  when  it  is 
lifted,  gorges  and  hooks  arc  added  to  prevent  its  escape 
although  the  former  arc  no  longer  used.  Various  forms 
of  hooks  and  gear  are  made  for  angling  different  kinds  of 
fish. 

Lines  are  fished  in  different  ways.  They  can  be  anchored 
or  left  drifting,  or  they  can  be  fixed  in  any  position  from 
the  surface  to  the  bottom.  The  line  was  originally  a  gear 
to  catch  single  fish,  but  in  the  form  of  longlines  it  became 
a  gear  to  catch  large  quantities  of  fish.  Sometimes  a 
fish  becomes  foul-hooked  because  the  hook  has  caught  in 
some  part  of  its  body.  This  has  been  developed  as  a 
catching  method  in  the  form  of  rip-hooks  to  catch 
certain  kinds  of  fish.  Rip-hooks  are  used  either  as  single 
moving  hooks  or  a  close  row  of  sharp  hooks  on  a  longline 
are  placed  across  the  path  of  the  fish  so  that  they  hook 
themselves  by  their  own  movement.  Gaffs  are  used  to 
lift  the  fish  out  of  the  water  and  sometimes  the  gaffs 
themselves  arc  used  as  gear  to  catch  fish,  thus  they  can 
be  classed  as  big  hooks  or  as  a  special  form  of  fishing 
weapon  (see  ''wounding  gear"). 

5.  FISH  TRAPS 

Barriers,  corrals  and  true  traps  are  known  in  hunting 
but  in  modern  fisheries  these  traps  have  lost  their 
importance,  except,  perhaps,  the  basket  trap  fykes,  etc. 
Their  catching  principle  is  based  on  allowing  the  fish 
to  enter  the  trap,  using  valve  nets  to  prevent  escape. 
Except  for  the  entrance,  small  traps  are  completely 
closed  like  cages,  only  large  traps  are  sometimes  open 
above  the  water  surface.  Traps  can  be  made  of  rigid 
material  or  network.  Smaller  fyke  nets  arc  held  open  by 
vertical  hoops  and  braced  horizontally  by  sticks  or  fixed 
between  stakes  driven  in  the  sea-bed.  In  deep  water  or  on 
hard  ground,  the  traps  are  held  in  position  by  anchors. 

6.  TRAPS  FOR  JUMPING  FISH 

Some  fish,  when  in  danger  or  excited,  jump  out  of  the 
water,  and  they  may  act  in  the  same  way  if  they  find 
themselves  confronted  by  an  obstacle.  This  is  so  typical 
of  some  species  that  a  fishing  method  can  be  used  to  take 
advantage  of  such  behaviour.  Artificial  obstacles,  such 
as  a  fish-hedge  or  net  wall,  are  built  to  make  the  fish 
jump.  Sometimes  the  shadow  of  an  object  floating  on 
the  surface  in  the  moonlight  or  sunshine  is  all  that  is 
needed.  A  horizontal  floating  net,  a  raft  trap,  or  even  a 
boat  or  box,  can  be  used  to  catch  the  fish  as  they  fall  back. 


7.  BAGNETS    WITH    FIXED    MOUTHS 

A  bagnct  has  a  totally  or  partially  framed  mouth.  The 
fixed  form  of  bagnet  is  kept  open  by  the  force  of  the 
water  flowing  through  it,  the  mobile  type  by  being  drawn 
through  the  water.  The  fish  are  actually  tillered  from  the 
water. 

Two  groups  of  bagnets  are  known:  the  small  scoop 
nets,  of  which  there  is  a  large  variety,  and  the  big  gape 
nets.  The  gape  nets  are  important  in  fishing  in  rivers  and 
estuaries.  They  are  fixed  by  stakes  or  anchors,  or  are 
used  from  anchored  boats.  The  moit  developed  bagnet 
is  the  otter  board  stow  net,  the  mouth  of  which  is  kept 
open  by  otter  boards. 

8.  DRAGGED  GEAR 

This  group  contains  all  nets  and  gear  which  arc  towed 
through  the  water,  including  dredges  and  all  vertical 
nets,  Whethersingle  or  multi-walled,  when  dragged  through 
the  water.  They  are  usually  made  of  twine  but  some- 
times of  wire.  The  most  important  in  this  group 
is  the  trawl,  the  mouth  of  which  is  kept  open  either  by 
a  frame,  a  beam,  floats,  sinkers,  otter  boards  or  kites. 
Sometimes  it  is  kepi  open  by  being  towed  by  two  vessels. 
Trawls  arc  not  always  dragged  along  Ihe  sea-bed  as 
ihere  are  also  midwatcr  or  pelagic  irawls. 

9.  SEINE   NETS 

In  seining,  one  end  of  Ihe  nel  is  shot  from  a  fixed 
point,  a  certain  area  is  surrounded  and  the  other  end  of 
gear  returned  to  the  starling  point,  after  which  the  gear 
is  then  hauled.  The  starting  point  can  be  on  the  shore  of 
a  lake  or  pond,  the  banks  of  a  river  or  on  the  sea  beach. 
In  certain  circumstances  a  boat  can  be  used.  The  net 
has  a  pocket  or  bag  in  the  middle  to  collect  the  fish.  The 
seine  is  mostly  worked  on  the  sea-bed  but  it  is  also  used 
in  pelagic  fisheries. 

10.  SURROUNDING    NET 

The  task  of  surrounding  nets  is  to  encircle  a  detected 
fish  school,  which  is  sometimes  scooped  out  with  dip 
nets.  The  simplest  example  of  this  method  is  to  close  a 
creek  with  a  barrier.  Some  forms  of  surrounding  nets 
can  be  shot  out  in  a  spiral  form,  catching  the  fish  as  in  a 
labyrinth.  The  best  known  forms  of  surrounding  nets 
are  the  ring-nets  and  the  purse  seines.  If  the  water  is  not 
too  deep,  the  ring  net  sinks  from  the  surface  to  the  bottom. 
The  most  important  use  of  this  method  is  purse  seining 
in  the  deep  sea.  In  this  case  the  lower  end  of  the  purse 
seine  is  closed  and  the  encircled  fish  are  unable  to  escape. 
Then  the  net  is  partially  lifted  and  the  fish  arc  corralled 
and  caught. 

11.  DIP  OR  LIFF  NETS 

This  method  is  to  submerge  a  hanging  net,  then  pull  it 
rapidly  out  of  the  water  so  as  to  capture  any  fish  or 
crustaceans  which  happen  to  be  over  it.  The  smaller 
nets  of  this  type  are  hand  operated,  but  the  bigger  ones 
need  a  mechanism  on  land  or  on  a  boat.  The  netting*  is 
supported  on  a  round  or  rectangular  frame.  The  espec- 


1275] 


MODERN     FISHING     GEAR     OF    THE     WORLD 


ially  big  sized  lift  nets  used  in  the  Norwegian  sea  fishery 
are  held  at  the  four  corners  by  boats. 

The  same  idea  is  used  in  the  French  water-wheels  to 
pull  the  fish  out  of  the  water. 

12.  FALLING  NETS 

This  method  is  to  cover  the  fish  with  a  cover  pot  or  a 
cone-shaped  net  with  a  stiff  opening.  Cast  nets,  skilfully 
thrown  out  on  the  water  surface,  enclose  the  fish  as  they 
sink,  and  the  fish  are  trapped  in  special  pockets  at  the 
lower  end  of  the  net.  Besides  these  hand-thrown  nets, 
there  are  also  much  bigger  cast  nets  that  need  gallows 
and  boats. 

13.  GILLNETS  AND  TANGLE  NETS 

There  are  two  main  types:  single  wall  nets  and  multi- 
walled  or  trammel  nets.  They  arc  operated  either  as 
drift  nets  or  anchored  to  the  bottom.  These  nets  are 
fished  close  to  the  bottom,  in  mid  water  or  at  the  surface. 


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Harpunen 
Handharpur 
Gewehrharp 

Kanonenhai 

Gewehre 
Blasrohre 

7.J7     Fishing  with  cormorants  in  China. 


(2761 


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CLASSIFICATION     OF     FISHING     GEAR 


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MODERN      FISHING     GEAR     OF     THE     WORLD 


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VII. 


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[280] 


CLASSIFICATION     OF     FISHING     GEAR 


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CLASSIFICATION     OF     FISHING     GEAR 


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CLASSIFICATION     OF     FISHING     GEAR 

1 


285  ] 


MODERN     FISHING     GEAR     OF    THE     WORLD 


(Illustrations    *.4  cnntlituftl  "tt  p.    289) 


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[294] 


CLASSIFICATION    OF    FISHING    GEAR 


No. 
2.12 


Origin 
England 


II.  Austria 

III.  Sweden 

IV.  Sweden 

V.  England 


ORIGIN  OF  THE  DRAWINGS 

Sources  No.  Origin 


VI. 
VII. 


2.13 


X 

XI. 

II. 
in. 

IV. 


2.14 


2.15 


2.31 


2.32 
2.33 


3.3 


II. 
111. 


II. 


I. 
II. 

HI. 


IV. 


4.1 
4.21 


4.22 


I. 
II. 

III. 


Italy 
Malta 


VIII.  Norway 

IX.  Malu 


France 

France 

(Cannes) 
Germany 

(Schlutup) 
Denmark 

(Viborg) 
Denmark 

(Esbierg) 
Germany 

(Schlutup) 
Germany 


Malta 

France 

Italy 

France- 


Norway 
Norway 


Germany 
Germany 

Norway 


U.S.S.R. 


Germany 
France 


Spain  (San 

Sebastian) 
Iceland 


4.22112 
4.22113 


4.2221 


4.22221 


Davis,  F.  M. :  An  Account  of  the  4.221 1 1 

Fishing  Gear  of  England  and 
Wales.  London  1937,  p.   123 
Original 
Original 
Original 

Davis,  F.  M.:  An  account  of 
the  Fishing  Gear  of  hngland 
and  Wale?,  p.  123. 
Original 

Burdon.T.  W.:  A  Report  of  the 
Fishing  Industrie  Malta,  Singa- 
pore, 1956,  p.  30. 
Brobak,  L.:  Farl0y  og  Redskap. 
Fabritius  and  Sonners  Verlag, 
Oslo  1952,  p.  108. 
Burdon,  T.  W.:  A  Report  of 
the  Fishing  Industry  of  Malta, 
p.  30. 

Boudarel,  N.:    Les  richcsses  de 
la  mer.  Paris  1948,  p.  409. 
Photographic 

Original. 
Photographic. 
Original. 
Original. 

Bencckc,  B.:  Fische,  Fischere 

und    Fischzucht  in    Ost  und 

Westprcusscn.         Konigsberg 

1881. 

Burden,  T.  W. :  A  Report  of  the 

Fishing  Industrie  of  Malta,  p. 

30. 

Original. 

Naintre,  L.,  Oddenino,  C.  u.M. 

Laurens:    La   peche   en   mer. 

Paris  1948. 

Bertucciolo,  U.:  II  primo  libro 

del   pescatore.    Venedig    1955, 

p.46.  4.22222 

Chenard,    M.,    P.    Desbrosses 

and   J.    Le   Gall:    Revue   dcs 

Travaux  dc  POffice  des  Peches 

XVI,  1951,  p.  105. 

Original. 

Schubert,  K.:  Der  Walfang  der 

Gegenwart.          Handbuch   d. 

Seeflschcrei   Nordeuropas  XI, 

6,  Stuttgart  1955,  p.  96.  4.23 

Original. 

Meyer,  P.  F.:  Elektrizitat  und 

Fischfang.  Orion  3,  1953,  p.  96. 

Schubert,  K.:  Der  Walfang  der 

Gegenwart.         Handbuch   d. 

Seefischerei   Nordcuropas  XI, 

6,  Stuttgart  1955,  p.  98. 

Skosziewicz:  Die  ncucstcn 

Fischfangmethoden   in   der 

UdSSR.  Leipzig.  1954,  p.  57. 

Original. 

v.  Brandt,  A.:  Fischfanggerate 

und   Fangmethoden.   Prot.   z. 

Fischereitechnik.  Heft  9,  1953, 

p.    19. 

Original. 

Original. 

Original. 


4.24 
5.11 


II. 

HI. 

IV. 


V. 
VI. 


VII. 

VIII. 

IX. 

1. 
II. 
III. 
I. 

II 

111. 
IV. 


I. 

II. 

III. 

IV. 

J. 

II. 


IV. 
I. 

1I.-V. 

VI. 


VIII. 

I. 

II. 

111. 

IV. 
V. 


VI. 
VII. 


Ireland 
Ireland 
Germany 
Germany 


Faroe  Isles 
Spain 


England 

( Lowestoft) 
France 

France 


Switzerland 
Germany 

Norway 
France 


Germany 

France 

Germany 

Denmark 

Germany 

Germany 


III.        Germany 


Sweden 
Norway 
Spain 


VII.       Norway 


Portugal 
Sweden 
Iceland 
Germany 
(Kiel) 
Iceland 

(Reykjavik) 
France 


Russia 

Russia 
France 


I.  -II  I.    Rumania 


Sources 

Original. 

Original. 

Original. 

Peters,  N.:  Angeln.  Handbuch 

d.   Seefischerei      Nordeuropas 

IV,  1942. 

Original. 

Rubio,  M.:  Artes  y  hmbarca- 

ciones  dc  Pcsca  en  Cataluna. 

Barcelona  1955,  p.  72. 

Original. 

Boudarel,  N.:  Les  richcsses  dc 
la  mer.  Paris   1948,  p.  410 
Boudarel,  N.:  Les  richcsses  dc 
la  mer.  Paris  1948,  p.  411. 
Original. 
Original. 
Original. 

Hager,  E.-  Der  Fang  der 
Seeforcllc.  Schweizerische 

Fischcrei-7tg.  65,  1957,  p.  7. 
Meschkat,  A.:  Neues  iiber  die 
Schleppangel.  Fischereiwelt  2, 
1950,  p.  153. 

Brobak,  K.:  Partly  og  Red- 
skap. Oslo  1952,  p.  105. 
de  La  Tourassc,  G. :  Revue  des 
Travaux  de  I'Office  des  Peches 
XVII,  1951,  p.  6. 
Original. 
Original. 
Original. 
Original. 
Original. 

Hcnking,  H.  :  Die  Ostsee- 
fischerei.  Handbuch  d.  See- 
fischerei Nordeuropas  V,  1929, 
p.  87. 

v.  Brandt.  A.:  Fischfanggertae 
und   Fangmethoden.   Prot.   z. 
Fischereitechnik.  Heft  9,  1953, 
p.  24. 
Original. 
Original. 

Brobak,   K.:   Fartoy  og  Red- 
skap, Oslo  1952,  p.  102-103. 
Rubio,  M.:  Aries  y  Embarca- 
ciones  de  Pesca  en  Cataluna. 
Barcelona  1955,  p.  79. 
Brobak,  K.:   Fart0y  og  Red- 
skap. Oslo  1952,  p.  10!. 
Original. 
Original. 
Original. 
Original. 

Original. 

Lc  Manuel  des  Peches  Man- 
times.  Memoires  dc  rOffice  des 
Peches  Maritimes  No.  10,  fasc. 
2,  1935,  p.  90. 

Soljan,    T.:    Classification    of 
fishing  boats,  gear  and  methods. 
1956.  Illustrations  p.  19. 
N.N. :  Fishing  gear  of  the  Casp- 
pian  Sea.  Moscow  1951,  p.  197. 
Naintre,  L.,  C.  Oddenino  and 
M.  Laurens:  La  peche  en  mer, 
Paris  1948,  after  p.  112. 
Antipa,  Gr.r  Pescaria  si  Pescui- 
tul.  Bukarcst  1916,  p.  541  ft 


[2951 


No. 


MODERN     FISHING     GEAR    OF    THE    WORLD 

Origin  Sources  No.  Origin 


Sources 


IV.        Yugoslavia 


5.12 


5.31 
5A 


V. 
VI. 


II. 


l.-lll. 


Sweden 
France 

Germany 
Germany 

Sweden 


5.41 


IV.         Germany 


I.-V 


Soljan,  T.:  Classification  of 
fishing  boats,  gear  and  methods. 
1956.  Illustrations  p.  21. 
Seligo,  A.:  Strandgewasser 
Mitteleuropas  V,  1925,  p.  74. 
Boudarel,  N.:  Les  richesses  de 
la  mer,  Paris  1948,  p.  426. 

v.  Brandt,  A.:  Fischfanggerate 
und  Fangmethoden.  Prot.  z. 
Fischereitechnik,  Heft  9,  1953, 
p.  31. 

v.  Brandt,  A.:  Fischfanggerate 
und  Fangmethoden.  Prot.  z. 
Fischereitechnik,  Heft  9,  1953, 
p.  31. 

Original. 

v.  Brandt,  A.:  Fischlanggeratc 

und    Fangmethoden.    Prot.   z. 

Fischereitechnik,  Heft  9,  1953, 

p.  36. 

v.  Brandt,  A.:  Fischfanggerate 

und   Fangmethoden.   Prot.   z. 

Fischereitechnik,  Heft  9.  1953, 

p.    36. 

v.  Brandt,  A.:  Fischfanggerate 
und  Fangmethoden,  Prot.  7. 
Fischereitechnik,  Heft  9,  1953, 
p.  41. 


5.411 

1. 

Germany 

Original. 

(Varel) 

11. 

Germany 

Original. 

(Hamburg) 

III. 

Italy 

Original. 

IV. 

Germany 

Original. 

(Emsmundung) 

V. 

England 

Fisheries  notice  No.  9,  Capture 

of  Eels.  1954,  p.  12. 

VI. 

France 

Photographic 

(Tregunc) 

VII. 

France 

Photographic. 

(Malinbay) 

5.412 

France 

Original. 

5.4131 

Germany 

Original. 

5.4132 

Germany 

Original. 

5.414 

1. 

Germany 

Original. 

(Heligoland) 

II. 

Ireland 

Photographic. 

5.41521 

1. 

Germany 

Breitenstein,  W.  and  K.  Jager: 

Die     derate     der     Seen-     u. 

Flussfischerei     in     Wundsch, 

H.H.:  Fischereikunde.Radebeul 

and  Berlin,  1953,  p.  179. 

II. 

Germany 

Meyer,  P.  F.  and  L.  RUbenbcrg: 

(Rugen) 

Die  Ankerreuse.  Zeitschrift  f. 

Fischerei  40,  1942,  p.  303. 

III. 

Denmark 

Klust,  G.:  Bundgarne.  Fisch- 

erei we  It  5,  1953,  p.  78. 

IV. 

Russia 

N.N.:    Fishing    Gear   of    the 

Asow-  and  Black  Sea.  Moscow 

1952,  p.  41. 

5.41522 

I. 

France 

Bouderal,  N.:  Les  richesses  de 

la  mer.  Paris  1948,  p.  430. 

5.42 

Norway 

Brobak,  K.:  Fart0y  og  Red- 

skap,  Oslo  1952,  p.  92. 

6.1 

I.-II. 

Italy 

v.  Brandt,  A.:  Fischfanggerate 

(Adria) 

und   Fangmethoden.   Prot.   z. 

Fischereitechnik,  Heft  9,  1953, 

p.  70. 

III. 

France 

Boudarel,  N.:  Les  richesses  de 

la  mer,  Paris  1948,  p.  449. 

6.2 


7.11 
7.12 
7.13 
7.21 

7.22 


7.23 
8.1 


Portugal 


8.311 
8.312 

8.313 
8.322 

9.1 
9.2 
9.22 


10.3 
11.1 

11.2 


Germany 
(Norddeich) 
Germany 
(Rhein) 

1. 
11. 

Germany 
(Elbe) 
Germany 
(Husum) 
France 

111. 

Russia 

1. 
II. 

II. 

Germany 
(  Carol  inensiel) 
Germany 
Germany 
Germany 
Denmark 
Sweden 
Germany 

Germany 

Denmark 

Norway 

Germany 

1. 
11. 

Germany 
(Oberrhein) 
Russia 

11.3 

Norway 

11.4 

12.1 
12.21 

Germany 
(Ammersee) 
I.-IV.     Switzerland 

12.22 
13.1 

Germany 
(Hamburg) 
1.           Germany 

13.21 
13.23 

II.                — 
I.-III.    Germany 
I.           Germany 
II.          France 

Baldaque  da  Sil  va,  A.  A. :  Estado 

actual  das  Pescas  em  Portugal. 

Imprensa     Nacional,     Lisbon 

1892. 

Original. 

Original. 

Original. 

Original. 

v.  Brandt,  A.:  Fischfanggerate 
und   Fangmethoden.   Prot.   z. 
Fischereitechnik,  Heft  9,  1953, 
p.  47. 
Original 

Photographic. 

I  c  Manuel  dcs  Peches  Mari- 
times.  Memoires  de  POffice  des 
Peches  Maritimes,  1935. 
N.N.:    Fishing    Gear    of    the 
Asow-  and  Black  Sea,  Moscow 
1952,  p.  145. 
Original. 

Original. 
Original. 
Original. 
Original. 
Original. 

Seligo,  A.:  Die  Fanggerate  der 
dcutschen  Binnenfischerei,  Ber- 
lin 1914,  p.  97. 
Seligo,  A.:  Die  Fanggerate  clcr 
deutschen  Binnenfischerci,  Ber- 
lin 1914,  p.  180. 
Mortcnsen,  F.  V.  u.  A.  C. 
Strubberg:  Die  danische  See- 
fischerei.  Handbuch  d.  Sce- 
fischerei  Nordeuropas  VIII, 
1931,  p.  66-67. 

Brobak,  K.:  Fart0y  og  Red- 
skap,  Oslo  1952,  p.  61. 
v.  Brandt,  A.:  Fischfanggerate 
und    Fangmethoden.    Prot.   z. 
Fischercilechnik,  Heft  9,  1953, 
p.  62. 
Photographic. 

N.N.:    Fishing    Gear    of   the 

Caspian   Sea.    Moscow,    1951, 

p.  189. 

Brobak,  K.:  Fart0y  og  Red- 

skap,  Oslo  1952,  p.  85. 

v.  Brandt,  A.:  Fischfanggerate 

und   Fangmethoden.   Prot.   z. 

Fischereitechnik,  Heft  9,  1953, 

p.  62. 

Photographic. 

Steinmann,  P.:  Schweizerische 
Fischkunde,  Aarau  1948,  p.  1 1 1 
Photographic. 

Breitenstein  W.  and  K.  Jager: 

Die    Cerate    der    Seen-    und 

Flussfischerei  in  H.H.  Wundsch 

Fischereikunde,  Radebeul  und 

Berlin,  1953,  p.  181. 

Original. 

Original. 

Original. 

Original  by  A.  Pcrcier. 


[296] 


TRAWLING   GEAR 

by 

H.  N.  BINNS 
Great  Grimsby  Coal,  Salt  and  Tanning  Co.  Ltd.,  Grimsby,  U.K. 

Abstract 

Otter  trawls  vary  from  country  to  country  and  from  port  to  port  and,  says  the  author,  who  shall  decide  what  is  right  and  what  is 
wrong?  The  general  principle  of  the  assembly  of  the  trawl  is  universally  accepted  and  in  these  days  of  high  towing  speeds  the  question  of  the 
distribution  of  the  strain  on  the  net  is  very  important.  There  is  always  a  weakest  spot  and  that  is  where  the  trawl  tears  and,  the  netmaker's 
problem  is  to  find  these  spots  and  strengthen  them.  A  general  account  of  the  rigging  of  the  common  otter  trawl  is  given  and,  in  addition,  herring 
trawls  and  the  midwater  trawls  are  discussed.  The  value  of  synthetic  fibres  is  otter  trawl  construction  has  yet  to  be  proved  and  the  high 
initial  cost  is  against  their  general  use,  for,  while  a  synthetic  fibre  codend  may  last  for  months,  the  possibility  of  the  total  loss  of  the  trawl  is 
always  present.  The  author  lists  seven  important  factors  which  are  involved  in  spreading  or  setting  the  modern  trawl  gear,  and  he  suggests 
that  the  successful  "skipper"  is  the  man  who  is  able  to  bring  all  these  factors  under  proper  control  so  as  to  produce  the  most  effective  fishing 
instrument  which  in  turn  matches  his  own  skill  and  intelligence. 

Chaluts 
Resume 

Les  chain ts  a  plateaux  different  suivant  les  pays  et  meme  les  ports,  et,  se  demande  Tauteur,  qui  peut  dire  lequel  est  bon  et  lequel  est 
mauvais?  l.c  principe  general  dc  montage  du  chalut  est  universellement  adopt£  et,  a  notre  6poque  ou  les  vitesses  de  chalut  age  sont  elevees,  le 
probleme  de  la  repartition  de  I 'effort  sur  le  filet  est  tres  important.  11  existe  toujours  des  points  faibles  ou  le  chalut  se  dechire,  et  le  probleme 
du  fabricant  de  filets  consiste  d  localiscr  ces  points  et  a  les  rcnforcer.  L'auteur  fait  un  expose  d'ensemble  du  greement  du  chalut  ordinaire  a 
plateaux  et  etudie  en  outre  le  chalut  a  harengs  et  le  chalut  flottant.  La  valeur  des  fibres  synthetiques  pour  la  fabrication  des  chaluts  a  plateaux 
rcste  a  ddmontrer,  et  Icur  cout  initial  £lcv£  s'opposc  A  la  generalisation  dc  leur  cmploi,  cur  si  un  cul-de-chalut  de  fibres  synthetiques  peut  durer 
des  moist  la  possibilite  de  la  perte  totale  du  chalut  est  toujours  presente.  L'auteur  en umere  sept  facteurs  importants  qui  conditionnent  la  diffusion 
ou  ('adoption  des  chaluts  modernes  et  il  suggere  que  le  "patron  de  peche  qui  r£ussit"  est  celui  qui  est  en  mesure  de  controler  tous  ces  facteurs 
de  facon  a  produirc  1'engin  de  peche  le  plus  efficace  qui  a  son  tour  s'harmonisc  avec  son  habile  te  et  son  intelligence. 

La  red  de  arrastre 
Lxtracto 

Las  redes  de  arrastre  varian  de  pais  a  pais,  dc  puerto  a  puerto  y,  segun  el  autor,  nadie  puede  asegurar  cual  es  mejor.  En  general  se 
acepla  el  principio  que  rige  para  la  armadura  de  las  redes  de  arrastre  pero,  actualmente,  con  el  cmplco  de  grandcs  yelocidades  de  arrastre, 
el  problcma  de  la  distribution  de  los  esfuerzos  sobrc  cl  artc  son  muy  importantes.  Como  siempre  existen  puntos  d£biles  por  donde  la  red  se 
rasga,  la  dificultad  del  fabricant e  estriba  en  encontarlos  y  reforzarlos.  En  el  trabajo  tambien  se  describe,  en  general,  la  manera  de  armar  una 
red  de  arrastre,  y  analiza  un  arte  de  este  tipo  para  la  pesca  dc  arenque,  adcmas  de  otro  que  trabaja  a  profundidades  intermedias.  Todavia  no 
se  ha  probado  el  valor  de  las  fibras  sintdticas  en  las  redes  de  arrastre  y  su  alto  costo  inicial  va  contra  de  la  general izaci6n  de  su  uso,  dado  que, 
si  bicn  un  copo  de  este  material  puede  durar  meses,  siempre  existe  la  posibilidad  dc  la  perdida  total  de  una  red  de  arrastre.  El  autor  da  una 
lista  de  varios  factores  de  importancia  en  el  calamento  de  una  red  de  arrastre  modern*  y  sugiere  que  un  "patr6n  capaz"  es  el  hombre  que 
puede  regular  todos  estos  factores  para  obtener  el  instrumento  de  pesca  mas  efectivo,  posible  dc  comparer  con  su  propia  abilidad  e  inteligencia. 


THIS  paper  deals  with  the  otter  trawl  and  its  varia- 
tions, together  with  the  ropes  and  fittings  which 
go  to  make  the  whole  into  a  fishing  instrument. 
No  two  countries  use  this  gear  in  just  the  same  form 
and  even  neighbouring  ports  have  their  own  ideas  as 
to  detail.  Underwater  photography  does  show  something 
of  what  happens  when  the  gear  is  spread,  but  we  still 
lack  certain  knowledge  as  to  what  happens  when  heavy 
gear  is  being  towed  in  deep  water. 

SETTING  OF  OTTER  TRAWL  GEAR 

There  are  several  factors  involved  in  spreading  or  setting 
modern  otter  trawl  gear.  Unless  they  are  all  in  proportion 
to  one  another,  maximum  efficiency  is  not  attained. 
These  factors  are: 

1.  Speed  of  tow. 

2.  Length  of  warp  used  in  relation  to  depth  of  water. 


3.  Angle    of  attack    of  otter   boards,    determined 
largely  by  the  placing  of  the  brackets. 

4.  Length  of  sweepline. 

5.  Weight  of  footrope. 

6.  Floats  on  headline. 

7.  Proportions  and  mounting  of  the  trawl  itself. 

There  are  cases  where  two  similar  trawlers  fish  side 
by  side  and  one  consistently  out-fishes  the  other.  I  suggest 
that  the  man  who  is  successful  is  the  one  who  manages 
to  bring  all  the  features  listed  above  into  such  proportion 
as  to  produce  the  most  successful  fishing  instrument. 

THE  OTTER  TRAWL  NET 

Broadly  speaking,  most  of  the  vessels  carrying  out  bulk 
fishing  in  Arctic  waters  use  trawl  nets  with  headlines 
ranging  from  78  to  105  ft.  In  Britain,  the  most  popular 
type  is  still  the  Granton  Trawl  in  its  two  versions,  i.e., 


[297] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


the  smaller  type  with  78  ft.  headline  and  116  ft.  footrope, 
and  the  larger  one  with  10  ft.  longer  wings  and  hence 
98  ft.  headline  and  136  ft.  footrope.  It  appears  that  some 
French  vessels  have  shortened  their  upper  and  lower 
wings  so  much  that  there  is  only  60  ft.  of  headline  with 
net  on  it.  Legs  extend  beyond  the  headline  and  footrope 
so  that  there  is  a  normal  length  to  the  danlenos.  The 
sideline  is  also  continued  to  the  danleno.  It  is  a  common 
practice  now,  when  fishing  on  rough  ground,  to  detach 
the  lower  wing  from  the  footrope  in  its  forward  parts, 
or  to  eliminate  the  forward  end  of  the  wing  altogether. 

Although  there  are  variations,  the  general  principle 
of  the  assembly  of  an  otter  trawl  net  is  internationally 
accepted.  Measuring  on  the  side-seam  of  the  wings,  the 
lower  wing  is  10  per  cent,  longer  than  the  square  and 
upper  wing.  This  allows  the  sideline  to  lift,  following 
the  upward  pull  of  the  headline,  and  thus  the  lower 
wing  is  more  or  less  vertical  in  the  water.  The  footrope 
from  the  wing  end  to  the  quarter  point  is  again  some 
10  per  cent,  shorter  than  the  sideline,  thus  throwing 
the  main  towing  strain  where  it  should  be,  namely, 
from  the  footrope  legs  down  the  footrope  wing  pieces 
and  the  bobbin  bunts  to  the  quarter  points,  then  down 
the  belly  lines  to  the  sidelines  and  so  down  to  the  codend. 

In  these  days  of  powerful  modern  vessels  towing 
their  gear  at  speeds  up  to  five  knots,  the  question  of 
distribution  of  strains  is  of  great  importance.  However 
carefully  one  adjusts  the  net  on  its  supporting  lines, 
there  are  bound  to  be  weak  points  causing  tears,  with 
loss  of  fish  and  fishing  time.  The  problem  of  the  net- 
maker  is  to  strengthen  these  parts  as  much  as  possible. 
Something  can  be  done  by  increasing  the  area  of  double 
netting  around  the  upper  and  lower  bosoms,  by  using 
double  twine  for  several  meshes  inside  the  leading 
edges  of  the  wings  and  by  running  a  width  of  double 
twine  down  the  bellies  just  inside  the  side-seam.  Some 
skippers  use  nylon  or  similar  synthetic  fibres  in  the  place 
of  manila  around  the  quarter  meshes.  This  is  the  point 
where  most  damage  occurs,  which  seems  inevitable. 
We  know  that  the  headline  and  footrope  will  tend  to 
form  an  arc,  and  to  this  arc  we  attach  sections  of  netting 
which  have  distinct  angles  at  the  quarter  points.  It 
might  be  a  worthwhile  experiment  to  make  a  net  in  such 
a  manner  that  the  upper  and  lower  mouth  is  rounded 
instead  of  having  sharp  corners  as  at  present. 

OPENING  WIDTH 

It  would  seem  that  during  operation,  a  trawl  with  an 
80  ft.  headline  has  a  distance  of  approximately  50  ft. 
between  wing  ends  but  this  is  pure  conjecture  arrived 
at  by  experimenting  with  models.  If  the  trawl  opening 
is  too  wide,  it  would  clearly  be  more  difficult  for  the 
floats  to  lift  the  headline.  I  think  we  have  less  opening 
with  legs  and  sweeplines  than  we  had  years  ago  when  the 
otter  boards  were  on  the  wing  ends.  The  "pony  boards", 
which  some  vessels  use  instead  of  danlenos,  seem  to 
give  a  wider  mouth  opening  with  a  corresponding 
reduction  of  headline  height. 

OPENING  HEIGHT 

The  question  of  headline  lift  will  have  been  discussed 
elsewhere.  Briefly,  the  lateral  pull  of  the  otter  boards 
tries  to  bring  the  headline  down;  some  of  the  resistance 


of  the  codend  falls  on  the  square  and  thus  tries  to  pull 
the  headline  back.  The  thrust  of  the  water  as  the  trawl 
advances  also  tries  to  push  the  headline  and  its  floats 
back  towards  the  codend.  In  spite  of  these  various 
stresses,  there  is  no  doubt  that  modern  floats  and  kites 
do  lift  the  headline  to  a  considerable  height. 

OTHER  GEAR  COMPONENTS 

There  are  many  variations  in  otter  trawl  gear  to  be 
found  in  different  parts  of  the  world.  North  Europeans 
like  the  steel  bobbins;  in  America  and  Canada  these  are 
little  used.  The  British  prefer  the  steel  danleno,  but  in 
Germany  the  pony  boards  are  popular,  while  in  the 
Americas  no  danlenos  are  used  at  all.  Sweeplines  vary 
in  length  from  15  fathoms  to  125  fathoms,  according 
to  individual  opinion  and  circumstances. 

HERRING  TRAWLS 

Before  discussing  the  mid  water  trawls,  it  is  necessar> 
to  review  the  developments  which  have  taken  place  in 
the  conventional  otter  trawl  in  order  to  adapt  it  for 
catching  herring  and  mackerel.  Originally  a  very  large 
net  with  small  mesh  throughout  was  used.  In  the  late 
twenties  it  became  the  practice  to  use  large  meshes 
in  the  front  parts  of  the  trawl  net,  together  with  a  kite 
so  rigged  as  to  give  maximum  lift  to  the  headline. 
Today  the  main  practice  is  to  use  two  kites,  which  can 
be  rigged  in  different  ways,  and  travel  at  a  considerable 
height  from  the  bottom,  i.e.  the  lower  kite  pulls  up  the 
headline,  and  the  higher  one  breaks  up  the  shoal  of 
fish  and  sends  part,  at  least,  downwards  into  the  path 
of  the  net.  Some  countries  use  a  35  m.  headline  trawl, 
others  one  of  20  m.  but  with  very  deep  wings.  Both  fish 
very  successfully. 

MIDWATER  TRAWLS 

For  a  generation  fishermen  have  been  trying  to  tow  a 
trawl  in  midwater.  This  can  now  be  done  both  by  a  net 
towed  from  a  single  ship  and  by  one  towed  between 
two  vessels.  It  seems,  however,  that  the  results  have  not 
been  as  good  as  predicted  after  the  first  experiments. 
The  Larsen  gear  is  used  successfully  every  winter  in  the 
herring  fishing  off  Skagen.  At  that  time  and  at  that 
place  the  water  is  somewhat  cloudy  and  the  herring  very 
plentiful.  This  success  has  not  been  repeated  in  the  clear, 
deeper  waters  of  the  North  Sea.  Herring  are  often  caught 
in  quantity  with  the  two-boat  gear  and  sometimes  with 
one-boat  gear  in  the  English  Channel,  where  again 
the  water  is  cloudy. 

A  similar  net  towed  by  one  vessel  has  been  tested 
very  extensively  by  the  British  Ministry  of  Fisheries 
in  the  North  Sea,  and  by  the  Canadian  Fisheries  Depart- 
ment in  the  Vancouver  district.  The  experiments  were 
carried  out  by  the  drifter  or  small  trawler  type  of  vessel. 
Icelandic  fishermen  have  for  some  years  used  a  box- 
shaped  midwater  net  towed  by  a  large  single  trawler 
on  a  ground  near  the  Westman  Islands,  usually  called 
"the  Stones",  which  is  far  too  rough  to  be  fished  by  the 
usual  bottom  trawl.  In  the  spring,  cod  arrive  in  great 
numbers  and  the  shoals  are  so  large  that  the  trawl,  if 
well  opened,  can  hardly  fail  to  sweep  up  large  quantities 
of  fish. 

One  is  tempted  to  conclude  that  midwater  gear  has 


[298] 


TRAWLING     GEAR 


only  been  used  with  success  where  ideal  conditions  exist. 
This  means  either  shoals  so  extensive  that  a  net  dragged 
through  the  area  is  almost  certain  to  catch  them,  or 
turbid  water  in  which  the  fish  do  not  see  the  net  approach- 
ing. Experiments  continue,  and  it  seems  likely  that 
midwater  trawls  will  come  more  and  more  into  the 
picture.  The  difficulties  are  obvious.  Except  where  fish 
are  abundant,  the  shoal  has  to  be  located  and  the  depth 
of  the  gear  quickly  adjusted.  Sonic  instruments  are 
now  available  which  show  the  skipper  the  depth  at 
which  his  trawl  is  fishing.  It  would  seem  that  the  bulk 
of  our  white  fishing  operations  will  continue  to  be 
carried  out  with  conventional  bottom  trawls  but  that 
with  time  fishing  grounds  will  be  found  where  the  con- 
ditions exist  which  make  midwater  trawling  a  commercial 
possibility. 


SYNTHETIC  NET  MATERIAL 

Without  questioning  the  virtues  of  nylon  and  other 
synthetics,  it  can  be  stated  that,  for  the  trawl  net,  these 
fibres  do  not  have  the  same  advantages  over  vegetable 
fibres  which  they  have  for  the  gillnet.  Size  for  size,  they 
have  approximately  twice  the  breaking  strength  of 
manila  and  are  virtually  impervious  to  rotting.  A  big 
disadvantage  in  trawling  is  their  high  cost,  and  the 
possible  loss  of  all  or  part  of  the  net  is  always  present. 
However  well  designed  a  trawl  may  be  it  must,  in  parts 
and  at  times,  chafe  along  the  bottom.  A  codend  made 
of  synthetic  twine  and  well  protected  with  hides  may 
last  for  six  months  and  pay  for  itself  several  times  over. 
Only  time  can  show  whether  the  material  will  be  widely 
adopted  for  trawling. 


A  deckload  of  cod. 
{299] 


GERMAN  CUTTER  TRAWLING  GEAR 

by 

J.   SCHARFE 

Fishing  Gear  Section,  Fisheries  Division,  FAO 


Abstract 

This  paper  concerns  exclusively  bottom  trawling  gear  omitting  midwater  trawling,  which,  to  a  certain  extent,  has  been  adopted  by 
the  German  cutters,  particularly  in  recent  years.  After  a  short  description  of  vessels,  crew  and  auxiliary  equipment,  characteristic  examples 
of  the  following  types  of  gear  are  described  in  detail:  flatfish  and  round  fish  trawl,  herring  otter  trawl,  and  herring  pair  trawl.  The  main 
emphasis  is  laid  on  design  and  construction  of  the  nets,  including  a  short  paragraph  on  the  use  of  machine  braided  webbing. 

Les  chaluts  du  coutre  allemand 
Rfeume 

Get  article  concerne  exclusivement  les  chaluts  de  fond,  les  chaluts  flottants  etant  exclus,  qui  ont  etc  adoptes  r&ement  jusqu'a  un 
certain  point  par  les  coutres  allemands.  Apres  une  courte  description  des  bateaux,  des  Equipages  et  de  requirement  auxiliaire,  1'auteur 
d6crit  en  d6tail  des  exemples  caract6ristiques  des  types  d'engins  suivants:  chalut  £  poissons  plats  ct  autres  poissons,  chalut  £  plateaux  £  hareng 
et  chalut-boeuf  &  hareng.  L'auteur  insiste  sur  le  dessin  et  la  construction  des  filets,  un  court  paragraphic  est  consacr£  &  Tutilisation  de  filet 
fabriqu6  mdcaniquement. 

Los  artes  de  los  ciiteres  alemanes 
Extracto 

Esta  ponencia  trata  exclusivamente  de  los  artes  dc  arrastre  dc  fondo,  omitiendo  los  artes  flotantes  que  han  sido  adoptados  reciente- 
mente,  hasta  cierto  punto,  por  los  cuteres  alemanes.  Despues  de  reseftar  brevemente  las  embarcaciones,  las  tripulaciones  y  el  equipo  auxiliar, 
el  autor  describe  con  pormenores  ejemplos  caracteristicos  de  las  siguicntes  clases  de  material:  artes  de  arrastre  para  la  pesca  de  peces  pianos 
y  otros,  artes  de  arrastre  de  puertas  para  la  pesca  del  a  ran  que  y  artes  de  arrastre  remolcados  por  parejas  para  la  pesca  del  arenque.  HI  autor 
hace  hincapi£  en  el  proyecto  y  construed  on  de  los  artes  y  dedica  un  pdrrafo  al  empleo  de  redes  fabricadas  mecanicamente. 


GENERAL 

TRAWLING  is  the  most  important  fishing  method 
used  in  the  German  sea  fishery.  Except  for  the 
shrimp  fishery,  which  is  carried  out  in  coastal 
waters  by  small  cutters  (10  to  15  m.  long)  using  the  beam 
trawl,  fishing  is  done  by  otter  trawling  and  pair  trawling. 
There  are  three  classes  of  ships  engaged  in  otter  trawling: 
big  trawlers,  trawling  luggers  (which  are  also  equipped 
for  drift  net  fishing)  and  big  cutters,  the  operating  range 
of  these  vessels  being  in  relation  to  their  size.  The  gear 
of  these  vessels  differs  not  only  in  size,  but  also  in  material, 
construction,  and  rigging. 

The  big  trawlers  fish  exclusively  for  round  fish  and 
herring  and  use  two  basic  types  of  gear  specially  developed 
for  these  fisheries.  The  luggers,  which  fish  exclusively 
for  herring,  use  the  same  type  of  herring  gear  but  of 
smaller  size  when  trawling.  The  big  cutters,  which 
operate  for  flatfish,  roundfish  and  herring,  also  use  two 
types  of  gear,  one  for  roundfish  and  flatfish  and  another 
for  herring.  These  vessels  are  used  for  otter  trawling  as 
well  as  pair  fishing. 

The  big  cutters  have  recently  started  midwater  trawling 
for  herring  and  sprat  in  the  south-eastern  North  Sea 


during  the  winter,  generally  using  the  two-boat  type  of 
gear.  This  paper  deals  exclusively  with  this  class  of 
vessels. 

The  Vessels 

The  size  of  the  big  cutter  trawlers  lies  between  14  and 
26  m.  overall  (fig.  1). 

Propulsion  is  exclusively  by  diesel  engines  of  90  to 
200  h.p.  giving  a  cruising  speed  of  about  9  knots  and  a 
towing  speed  of  2-5  to  3-5  knots.  Fishing  is  done 
exclusively  over  the  side,  preferably  to  starboard.  The 
main  fishing  areas  are  the  south-eastern  North  Sea  and 
the  Baltic  Sea.  Many  of  the  boats  are  fitted  with  radio 
telephone,  radio  direction  finders  and  echo  sounders 
for  navigation  and  fish  location.  The  crew  ranges  from 
3  to  5  men,  depending  on  the  size  of  the  vessel;  all  hands 
are  fully  qualified  fishermen.  Fishing  trips  rarely 
exceed  one  week.  The  winches  of  the  smaller  boats 
are  often  constructed  from  the  rear  axle  of  a  truck  and 
are  driven  from  the  main  engine  by  means  of  a  driving 
belt  (fig.  2). 


[300] 


GERMAN     CUTTER     TRAWLING    GEAR 


Fig.  1.     Modern  German  cutter  (Type  KFK). 


Fig.   3.     Fore  gallows   with   two  blocks  for  both    warps. 

The  two  drums  usually  have  a  capacity  of  about 
400  fm.  of  warp  (wire  cable  of  about  9  to  14  mm.  0) 
but  are  not  big  enough  to  take  up  the  bridles  as  well. 

The  two  gallows  arc  arranged  at  the  side  in  the  same 
manner  as  on  the  bigger  trawlers.  For  pair-fishing, 
however,  the  aft  gallows  are  inconvenient  and  fore 
gallows  have  been  constructed,  carrying  two  blocks  to 
accommodate  both  warps  (fig.  3). 

This  leaves  the  deck  and  rail  free  for  better  handling 
of  net  and  lines,  but,  on  the  other  hand,  requires  addi- 
tional care  when  shooting  and  hauling  to  prevent  the 
gear  being  fouled. 

The  bridles  are  hauled  in  on  the  winch  barrels,  and  to 
facilitate  hauling  and  prevent  chafing  of  the  bridles, 
special  rail-rollers  and  fairleads  are  used,  to  give  the 
rope  the  right  direction  to  the  barrels  (figs.  4  to  6). 


Fig.  2.     Cutter  winch  made  out  of  a  rear  axle  of  a  truck.    C= 

centre-bollard;  L^  leading-bollard  to  give  the  bridle  the  right 

direction  to  the  barrel  of  the  winch. 


Fig.  4.     Example  of  the  arrangement  of  centre-bollards  (C)  and 
leading-bollards  (L). 


[301  ] 


MODERN     FISHING    GEAR     OF    THE     WORLD 


Very  simple  devices  keep  the  warps  together  near  the 
stern  when  towing.  Sometimes  the  warps  are  only 
belayed  round  a  common  bollard  or  put  into  a  lip.  Some 
vessels  use  a  special  type  of  clamp  fixed  to  the  gunwale, 
which  facilitates  the  release  of  the  warps  before  hauling 
(figs.  7  and  8). 

The  Trawling  Gear 

The  warps  are  about  9  to  14  mm.  0. 

The  otter  boards  are  made  of  wood,  with  iron  rein- 
forcement. The  size  is  from  0-85  to  1-0  m.  in  height 
and  1  -8  to  2-0  m.  in  length,  and  the  weight  lies  between 
50  and  100  kg.  each.  They  are  relatively  higher  than  the 


boards  used  by  the  bigger  trawlers.  To  obtain  the 
required  angle  of  attack,  iron  brackets  are  used,  but 
chain  strops  or  a  combined  chain  bracket  arrangement 
can  also  be  found  (fig.  9). 

The  "chain  boards"  are  more  difficult  to  operate 
because  they  foul  more  easily.  The  most  modern  form 
has  rigid  iron  brackets  instead  of  the  folding  pair  type 
(fig.  10).  This  construction,  which  originated  in  Den- 
mark, is  now  well  introduced  in  Germany.  Very  often 
such  boards  have  glass  or  aluminium  floats  attached  near 
the  upper  edge  to  keep  the  board  upright  on  the  ground 
when  towing  is  interrupted  (fig.  11). 

The  boards  are  often  fitted  with  shoe-plates  (8  to  10 
cm.  wide),  made  out  of  iron  sheet  about  3  mm.  thick, 
to  prevent  them  ploughing  too  deep  in  the  muddy  bottom. 

In  pair-fishing,  the  width  of  the  net  opening  is  con- 
trolled by  the  distance  of  the  two  boats  and  no  otter 


Fig.    7.     Special  clamp  serving  as  towing-block. 


Figs.  5  and  6.    Two  different  types  of  rail-rollers  for  leading  the 
bridles    when   heaved  over   the   rail.    5  --=  removable   roller. 
6  -r-  fixed  roller. 


Fig.  8.     Rail-roller  used  to  keep  the  warps  together  (Pair-fishing). 


[302] 


GERMAN     CUTTER     TRAWLING     GEAR 


Ftp.  V.      Otter  hoanl  with  combined  chain-bracket  arrangement . 

boards  are  required.    Instead  heav>  sinkers  are  used  to 
weight  the  gear  down  (fig.   12). 

A  main  sinker  (M)  of  about  60  to  120  kg  replaces  the 
otter  board  and  is  attached  at  the  connection  between 
warp  and  bridle.  A  second  sinker  (D)  of  about  15  to 
20  kg.  is  fixed  at  the  lower  edge  of  the  danleno,  whilst 
a  third  weight  (W)  of  about  8  to  10  kg.  may  be  attached 
at  the  connection  of  the  ground  rope  leg  to  the  wing  end. 
These  weights  are  often  made  out  of  bunched  chains 
instead  of  solid  iron  because  chain  bundles  are  less  likely 
to  cause  damage  to  the  ship's  side  when  hauling  in  bad 
weather. 


Fig.  //.     Otter  boaid  with  glass  floats  to  prevent  the  hoard  from 
falling  flat   when  towing  has   to  be  interrupted. 

THE  FLATFISH  AND  ROUNDFISH  GEAR 

There  are  great  differences  in  the  fishing  gear  of  the 
cutters  due  to  differences  in  fishing  conditions,  size  of 
the  boats,  historical  origin,  etc.  In  the  following,  some 
characteristic  examples  are  given  for  each  of  the  main 
types  used. 

Although  quite  similar  gear  can  be  used  for  flatfish 
and  roundfish,  specific  types  of  gear  are  usually  in  use 
too.  For  the  cod  fishery  in  the  Baltic  Sea,  several  types 
of  nets  with  different  mesh  sizes  and  twine  strengths 


Fig.  10.    Modern  otter  board  with  rigid  iron  bracket  (Danish 
Type).      A  —  Point  of  attachment  for  the  warp. 


Fig.    J2.     Weights  for  bottom  pair-fishing. 


[  303  1 


MODERN     FISHING     GEAR     OF    THE    WORLD 


have  been  specially  developed  for  different  fishing 
conditions.  The  most  common  method  is  otter  trawling, 
although  pair-fishing  is  sometimes  practised  for  cod. 
When  operating  for  flatfish,  the  ground  rope  must  have 
good  contact  with  the  bottom,  whereas  the  opening 
height  is  of  less  importance.  For  catching  roundfish, 
however,  the  headline  must  have  a  certain  opening  height, 
and  the  ground  rope  may  travel  lighter  over  the  bottom. 
These  conditions  can  be  realised  to  a  certain  extent  with 
the  same  gear  by  choosing  an  adequate  rigging  of  the 
legs,  the  right  type  of  ground  rope,  and  by  suitable 
lifting  devices  for  the  headline.  The  ground  rope  has  to 
be  lengthened  in  relation  to  the  headline  to  gain  good 
contact  with  the  bottom. 

The  bridles  are  usually  of  hard-laid  manila  rope  of 
16  to  24  mm.  0.  They  are  often  made  up  of  two  to  three 
parts  of  equal  length,  being  thicker  in  the  section  nearer 
the  net.  Sometimes,  combined  rope  of  16  to  20  mm.  0 
is  used.  The  length  depends  to  a  certain  extent  on  the 
depth  of  the  water  and  lies  between  60  and  80  fm., 
longer  bridles  being  used  in  deeper  water.  The  connec- 
tions of  the  bridle  with  the  otter  board  at  one  end  and 
the  danleno  at  the  other,  arc  made  by  means  of  shackles 
with  swivels;  (V.  D.  Kelly  eyes  and  links  are  not  used). 

The  danlenos  are  made  of  hardwood  with  iron 
fittings  and  the  lower  end  is  weighted.  The  length  varies 
between  0-6  and  0-7  m.  They  are  equipped  with  a 
strop  for  fixing  the  bridle  and,  with  loops  for  the  attach- 
ment of  the  legs  or  the  net  respectively. 


As  a  rule,  the  net  is  fastened  directly  to  the  danleno, 
but  legs  are  occasionally  used.  If  necessary,  short 
pieces  of  wire  are  inserted  for  regulating  the  length  of 
the  ground  rope.  Chain  is  used  for  the  ground  rope  when 
fishing  for  flatfish,  and  a  wire  of  about  10  mm.  0,  served 
with  yarn  and  rope,  with  eyesplices  at  both  ends,  is  used 
for  roundfish.  Additional  chains  may  be  wrapped 
round  to  increase  the  weight.  A  bolshline  is  laid  on  to 
attach  the  net  in  the  usual  way. 

The  headline  is  also  wire  of  about  10  mm.  0  and 
served  with  yarn.  Sometimes  combination  rope  of 
comparable  strength  is  used.  Kites  are  rarely  used,  and 
buoyancy  is  assured  by  a  varying  number  of  8  in.  floats 
(from  10  to  35),  made  of  glass  or  synthetic  material.  The 
connection  of  the  webbing  to  the  headline  is  usually 
direct,  without  a  bolshline. 

Cutters  of  120  to  180  h.p.  fish  with  90  ft.  nets  of  this 
type.  This  value  of  the  ground  rope  length  is  nothing 
but  a  name.  Flatfish  nets  are  handbraided  from 
manila  twine,  while  roundfish  nets  for  the  Baltic  Sea 
usually  are  assembled  from  sections  of  machine-made 
cotton  netting.  The  following  twine  strengths  are  suitable: 


Part  of  the  net 

Upper  wing 
Lower  wing 
Square 
Belly 
Codcnd 

26-30 


Breaking  strength  (wet) 

45  kg.  (99-21bs.) 

45  kg.  (99-21bs.) 

45kg.  (99-2  Ibs.) 

48-50  kg.  (105-8-I10-31bs.) 

90kg.  (198 -5  Ibs.) 


12-16 


meshsize 

mm 


100-110 


90  -100 


90 


wing 


lower- 


belly 


meshpiece  with  pocket 

t 
codend 

I 

Fig.  13.    Construction  plan  of  90  ft.  flatfish  trawl. 
[304] 


GERMAN    CUTTER     TRAWLING     GEAR 


The  nets  consist  of  an  upper  and  a  lower  part,  which 
are  made  up  of  similar  sections.  An  example  of  this 
type  of  net  is  given  in  fig.  13. 

The  first  part  of  the  headline  side  of  the  upper  wing, 
starting  from  the  bosom,  may  be  baited,  every  row  instead 
of  every  other  row.  This  results  in  a  quickly  diminishing 
width.  Further  towards  the  wing  tip,  it  may  be 
continued  with  fly-meshes  until  the  end.  The  lastrich 
side,  consequently,  cannot  be  straight  but  must  have  a 
number  of  creasings  inserted,  to  compensate  this  quick 
loss  of  meshes  on  the  headline  side  in  order  to  obtain  the 
desired  length  of  about  8  to  1 1  m.  The  mesh  size  is 
generally  100  to  1 10  mm. 

The  square  has  the  same  mesh  size  as  the  upper  wings. 
The  width  may  vary  between  about  170  and  214  meshes 
at  the  top  and  about  150  and  180  meshes  at  the  bottom. 
Depending  on  the  baiting  rate,  the  depth  may  differ  and 
the  total  length  can  vary  between  about  1  -7  and  3 -7m. 
Usually,  a  longer  square  is  combined  with  a  shorter  wing 
and  vice  versa. 

To  obtain  the  necessary  slack,  the  lower  wing  is  made 
about  1/6  to  1/5  longer  than  the  upper  wing,  plus 
square.  The  ground  rope  side  is  fly-meshed,  and  to 
obtain  the  required  length,  the  lastrich  side  has  a  com- 
pensating number  of  creasings. 

As  almost  no  reduction  of  the  mesh  size  is  needed,  the 
belly  is  usually  an  undivided  piece  of  equal  mesh  size 
(about  100  to  110  mm.).  It  is  about  150  to  180  meshes 
wide  at  the  start  and  about  80  meshes  wide  at  the  end. 
According  to  the  width  at  the  top  and  the  baiting  rate, 
the  total  length  lies  between  about  6-0  and  10-0  m. 

The  throat  is  usually  about  80  meshes  wide  at  the 
start  and  about  40  meshes  wide  at  the  end.  The  mesh 
size  may  be  slightly  smaller  than  in  the  belly  and  lie 
between  90  and  100  mm.  The  total  length  may  vary 
between  about  3-0  and  4-0  m. 

The  floppa,  or  the  so-called  "pocket"  are  arranged  in 
the  throat.  The  floppa  is  usually  a  trapezium-shaped 
piece  of  netting  attached  along  its  upper  edge  to  the  upper 
net,  and  at  the  sides  to  the  lower  net  along  a  halfcr.  But 
there  are  also  rectangular  floppas  in  use,  the  front  edge 
of  which  is  attached  to  the  upper  net  in  the  usual  way, 
while  the  side  edges  are  laced  in  the  lastrichs  leaving 
about  0-5  m.  of  the  end  free.  To  form  the  "pockets" 
upper  and  lower  net,  starting  from  the  lastrichs,  are  laced 
together  on  the  halfer  leaving  an  opening  of  only  about 
20  meshes  in  width  free.  To  prevent  the  fish  from  being 
pressed  into  the  corners  formed  by  the  seams  and  the 
lastrich,  the  tip  of  the  pocket  is  closed  by  lacing  upper 
and  lower  net  together  on  the  full  mesh.  Such  pockets 
are  only  used  for  the  flatfish  fishery,  and  the  floppa  is 
preferred  for  roundfish. 

Usually,  the  codend  has  no  baitings.  The  width  may 
be  about  38  to  40  meshes  of  about  90  mm.  mesh  size,  so 
that  the  length  is  about  4-5  to  5-5  m. 

Halving  beckct  and  bull  ropes  are  not  customary. 
The  net  is  hauled  in  by  hand,  the  codend  stropped  and 
hove  on  deck  by  means  of  a  gilson. 

THE  HERRING  OTTER  TRAWLING  GEAR 

For  herring  and  sprat  fishing,  the  cutters  use  a  very  light 
gear  adapted  to  their  limited  engine  power.  This  gear 
has  a  high  net  opening,  5  to  8  m.  with  the  ground  rope 
travelling  lightly  over  the  bottom. 


The  same  otter  boards  are  used  as  for  the  flatfish 
fishery.  The  bridles  are  handled  without  pennants  and 
are  made  of  combination  rope  of  about  18  to  20  m.  0. 
The  length  varies  between  about  30  and  60  fm.  and  vanes 
with  the  depth  of  water.  The  bridles  are  fixed  in  the 
same  manner  as  already  described. 

The  danlenos  are  similar  to  those  used  for  flatfish  gear 
except  for  their  size,  and  may  be  between  1  •  2  and  1  •  3  m. 
long. 

The  legs  are  made  of  wire  rope  of  12  to  14  mm.  0. 
There  are  usually  three  legs,  i.e.  one  each  for  the  headline, 
the  lastrich,  and  the  ground  rope.  Their  length  varies 
from  about  12  to  15  m.  Usually  the  headline  and  the 
lastrich  leg  are  fixed  at  the  upper,  and  the  ground  rope 
leg  at  the  lower,  loop  of  the  danleno.  The  usual  length 
relation  of  these  legs  is  maintained.  A  lengthening  of  the 
ground  rope  or  its  legs  results  in  sharper  travelling  over 
the  bottom. 

The  headline  is  usually  made  of  combined  rope  of 
about  12  mm.  0.  It  has  eycsplices  at  both  ends  for 
connection  with  the  legs  by  means  of  shackles.  Bolsh- 
lines  arc  unusual  with  this  type  of  net,  which  is  attached 
directly  to  the  headline.  A  varying  number  of  floats 
(about  15  to  25)  are  attached  to  give  buoyancy  and,  if  a 
kite  should  be  used,  which  is  very  seldom,  it  is  attached 
directly  to  the  headline. 

The  ground  rope  is  also  usually  made  of  combined 
rope  but  of  about  14  mm.  o.  Like  the  headline,  it  also 
has  eyesplices  at  both  ends  for  connection  with  the  legs. 
In  contrast  to  that  used  in  the  flatfish  gear,  this  ground 
rope  is  not  served  with  ropes  or  anything  else.  The  net 
is  attached  directly  without  bolshlines.  To  give  weight, 
iron  rings  of  about  20  cm.  0,  and  weighing  about  1  kg. 
each,  are  fixed  by  means  of  short  strops;  the  distance  of 
the  ground  rope  from  the  bottom  can  be  regulated  by 
the  length  of  these  strops.  The  advantage  of  such  rings 
is  that  they  cannot  hook  into  or  fall  through  the  meshes, 
so  that  fouling  is  avoided.  About  five  rings  may  be 
distributed  over  the  bosom,  while  in  the  wing  sections 
they  are  distributed  at  a  distance  of  about  1  fm. 

The  lastrich  is  strengthened  by  combined  rope  of 
10  to  12  mm.  0  down  to  the  codend;  manila  rope  is 
preferred  on  the  lastrichs  of  the  codend.  The  wingends 
of  these  strengthening  ropes  connect  with  the  legs. 

Differences  between  the  length  of  the  netting  at  the 
headline,  lastrich,  and  ground  rope  in  the  wingends,  are 
compensated  by  the  length  of  headline,  lastrich  rope  and 
ground  rope  respectively.  As  a  rule,  these  ropes  extend 
the  netting  accordingly. 

A  common  size  of  net  used  with  this  type  of  gear  is 
90  to  100  ft.  (fig.  14). 

In  contrast  to  the  flatfish  net  described  above,  these 
nets  or,  at  least,  the  smaller  meshed  parts,  are  always  made 
from  machine-made  netting.  The  material  iscottonorman- 
madc  fibre,  such  as  Perlon  or  nylon.  The  following 
twine  strengths  are  suitable  for  the  different  net  sections. 

Part  of  the  net  Breaking  strength  (wet  twine) 

Upper  wing  14  kg.  (30  9  Ibs.) 

Lower  wing  14  kg.  (30-9  Ibs.) 

Square  14  kg.  (30-9  Ibs.) 

Belly  6-5  to  11  kg.  (14-3  to  24-3  Ibs) 

Tunnel  8-5  kg.  (18-7  Ibs.) 

Codend  11  kg.  (24  3  Ibs.) 


[305] 


MODERN     FISHING    GEAR     OF    THE     WORLD 


90'-  108'  h»rnng-n»C 


meshsize 


150-160 


20-28 


I150-168_ 

Fig.  14.     Construction  Plan  of  VO  to  108  ft.  herring  trawls. 


The  twine  used  in  this  type  of  net  is  of  relatively  low 
breaking  strength  and  such  light  nets  need  careful 
handling,  particularly  in  bad  weather.  On  the  other 
hand,  this  net  construction  shows  that  the  twine  strength 
used  is  often  higher  than  necessary.  Thinner  twines 
mean  lower  towing  resistance  and  a  further  saving  of 
material.  The  latter  is  particularly  important  when  the 
more  expensive  synthetic  materials  are  used. 

The  headline  side  of  the  upper  wing  is  usually  fly- 
meshed  or  cut  on  the  halfer.  The  lastrich  side  is  not 
straight  but  usually  runs  out  at  the  same  baiting  rate  as 
the  side  edges  of  the  square.  Some  nets  have  long  wings 
and  others  short  wings,  so  that  the  length  may  vary 
considerably,  i.e.,  between  about  4  to  10  m.  The  base 
of  the  upper  wings  are  rather  wide  compared  with  the 
width  of  the  bosom.  The  end  of  the  shorter  type  of  wings 
may  be  wide  (about  40  to  65  meshes).  Often  such  short 
wings  end  with  an  additional  triangular-shaped  piece  of 
netting.  This  piece,  which  is  fly-meshed  or  cut  on 
the  halfer  at  both  edges,  is  attached  mesh  to  mesh  at  the 
normal  end  of  the  wing.  While  the  headline  edge  is 


fixed  at  the  headline,  the  opposite  side  is  not  laced  into 
the  lastrich  but  attached  to  the  fishline  (combination 
rope  of  about  8  to  10  mm.  0)  which  leads  from  the 
groundrope  over  the  lastrich  to  the  headline.  In  this 
case,  an  additional  lengthening  of  the  shorter  lastrich  is 
needed  for  compensation  with  headline  and  ground  rope. 

The  length  of  the  square  is  about  4-0  to  4-5  m.  The 
mesh  size  is  usually  the  same  as  in  the  upper  wings, 
i.e.  150  to  160  mm. 

The  lower  wings  have  the  same  mesh  size  as  the  upper 
wings  and  square.  The  total  length  may  exceed  the 
length  of  the  upper  wing  plus  square  by  the  usual  1/6, 
but  more  often  the  lengths  of  upper  and  lower  net 
are  equal.  As  the  ground  rope  side  is  fly-meshed,  the 
lastrich  side  has  a  compensating  number  of  creasings 
to  obtain  the  necessary  length.  If  the  upper  wing  ends 
have  a  triangular  shaped  piece  of  netting,  the  lower  wings 
are  constructed  in  the  same  way. 

The  mesh  size  diminishes  considerably  in  the  bell> 
which  consequently  is  divided  into  several  sections. 
The  example  given  in  fig.  14  shows  that  great  variations 


306  ] 


GERMAN     CUTTER     TRAWLING     GEAR 


Fig.    15.     Heaving   codend. 

in  the  construction  exist.  The  belly  usually  ends  with 
100  to  105  meshes,  including  the  throat.  The  throat  forms 
the  last  section  and  usually  contains  the  floppa.  The 
total  length  of  the  belly  may  vary  between  18  and  21  m. 

The  floppa  is  a  trapezium  or  a  rectangular  piece  of 
netting  about  2  m.  long,  and  is  inserted  in  the  way 
already  described.  No  floppa  is  needed  in  water  depths 
of  less  than  about  20  m.,  because  the  length  of  tunnel 
and  codend  prevents  the  fish  from  escaping. 

The  tunnel  is  rectangular  and  has  a  width  of  120  to 
150  meshes.  Its  length  can  be  from  about  7  to  9  m. 
The  mesh  size  of  this  piece  is  usually  somewhat  smaller 
than  in  the  last  section  of  the  belly. 

The  mesh  size  and  number  of  meshes  in  the  width  of 
the  codend  depend  on  the  species  of  fish  to  be  caught. 
For  herring  a  mesh  size  of  about  28  mm.  is  suitable.  In 
this  case,  the  codend  would  have  the  same  number  of 
meshes  in  width  as  the  tunnel.  For  catching  sprat, 
however,  a  smaller  mesh  size  (about  20  mm.)  is  needed. 


Fig.  76.     Example  of  closing  the  codend  by  means  of  a  cod  line 
with  a  simple  knot. 


/-'iff.  17.     Example  of  closing  the  code  ml  hy  means  of  a  special 
lock  ( Danish  type).      For  c  losing  >  lock  has  to  be  pulled. 

and  the  number  of  meshes  in  the  width  must  be  corres- 
pondingly increased.  The  length  of  the  codend  may 
vary  between  about  3-5  and  9-5  m.,  depending  on  the 
preference  of  the  skipper  and  the  size  of  catches  expected. 

An  additional  codend,  made  of  manila  or  Perlon 
twine  (breaking  strength  about  65  kg.),  with  a  mesh 
size  of  about  120  mm.,  is  used  for  heaving  the  bag 
aboard.  This  "heaving  codend"  covers  the  last  part 
of  the  real  codend  and  is  about  2-5  m.  long  (fig.  15). 

Its  front  part  may  be  made  of  single  twine,  but  the 
heaving  part  is  often  double  braided.  The  codend 
protrudes  through  the  opening  in  the  heaving  codend 
and,  by  closing  this,  the  real  codend  is  also  secured.  This 
can  be  done  by  means  of  a  codline  and  use  of  a  simple 
knot  (fig.  16)  or  by  a  special  design. 

The  apparatus  shown  in  fig.  17,  developed  in  Denmark, 
allows  for  quicker  opening  and  closing  when  big 
catches  have  to  be  divided  into  several  bags.  The 
lastrich  strengthening  ropes  of  the  codend  may  end  in 
cyesplices  which  can  be  used  for  fixing  a  line  to  a  buoy 
or  an  auxiliary  rope.  The  latter  arc  often  attached 
to  prevent  loss  of  the  aft  part  of  the  net  in  case  of  damage. 

A  halving  becket  with  bullropc  is  provided  for  heaving 
the  bag.  In  contrast  to  the  gear  of  big  trawlers,  this 
halving  becket  in  addition  to  the  loops  at  the  lastrichs — 
is  rove  through  a  number  of  iron  rings  which  are  fixed 
at  intervals  around  the  codend. 

To  obtain  a  high  opening,  this  net  is  rigged  so  that  the 
main  pull  lies  on  the  lastriches.  The  netting  stretches 
in  use  and  this  loosening  of  the  rigging  results  in  a  higher 
towing  resistance.  As  a  tight  rigging  is  preferred 
nowadays,  the  net  has  to  be  cut  off  about  every  month 
and  rigged  up  tightly  again. 

Fig.  18  shows  the  way  in  which  the  different  pieces 
of  such  a  net  can  be  cut  out  of  machine-made  netting 
with  as  little  loss  as  possible.  Of  course,  different  pieces 
or  bales  of  machine-made  netting  have  to  be  used  for 
the  parts  with  different  mesh  sizes  and  twine  strengths. 
The  nelmakers  order  such  bales  from  the  factories  in  the 
necessary  width,  according  to  the  length  of  the  respective 
part  of  the  net. 

The  square  and  wings,  which  usually  have  the  same 
mesh  size  and  twine  strength,  arc  often  constructed 


t  307  ] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


;  -•    /•"•/ 

i  /        / 


square 


upper-          ends  of          upper- 
wing          lower-wings      wing 


•  4  » 

'  \'f 


base  of 
lower- wings 


lower- 


upper- 


one  section 


Fig.  18. 


Method  of  cutting  90  to  108  ft.  hen  ing  trawls  out  of 
machine-made    netting. 


in  such  dimensions  that  they  can  be  cut  out  of  one  and 
the  same  bale.  A  certain  difficulty  may  arise  if  the 
lastrich  side  of  the  base  part  of  the  lower  wings  is  baited 
in  another  way  to  those  of  square  and  upper  wings. 
(fig.  14).  This  means  that  all  sections  cannot  be  cut  out 
of  one  piece  of  machine-made  netting  continuously,  and 
two  bales  are  needed  to  avoid  loss.  Fig.  18  shows  how 
this  can  be  done  by  putting  the  triangular  shaped  pieces 
cut  from  the  front  edge  (a,  b)  to  the  back  edge  (a',  b'). 
As  the  attachment  of  these  pieces  means  additional  work, 
it  is  more  economic  to  cut  the  parts  of  more  than  one 
net  out  of  one  machine-made  piece  of  netting,  as  the 
operation  has  then  only  to  be  done  once.  Netmakers, 
therefore,  usually  use  two  bales,  one  for  the  square, 
upper  wings,  and  the  fore  parts  of  the  lower  wings,  and 
one  for  the  base  parts  of  the  lower  wings,  from  which 
the  respective  pieces  are  cut  when  needed.  The  piece 
cut  away  at  the  beginning  then  may  be  added  to  the  back 
edge  when  the  end  of  the  bale  is  reached. 

If  triangular  shaped  wing  tips  are  needed,  a  loss  of 
netting  can  be  avoided  only  if  separate  rectangular 
pieces  of  machine-made  netting  are  available.  As 
already  mentioned,  both  side  edges  of  these  end  pieces 
are  cut  on  the  halfer.  A\S  the  bases  of  those  for  upper 
and  lower  wing  are  different,  the  lengths  are  different 
too.  If  netting  of  the  proper  width  is  available,  loss  can 
be  avoided  by  using  the  triangular  shaped  pieces  cut  off 
at  the  beginning,  in  the  way  described  above. 

The  different  sections  of  the  belly  have  different  mesh 
sizes,  twine  strengths,  and  lengths  and,  therefore,  have 
to  be  made  out  of  separate  bales.  The  upper  and  lower 
part  of  each  section  are  cut  as  shown  in  fig.  18,  i.e.,  the 
two  parts  appear  inverse.  The  triangular  shaped  piece 
cut  off  in  the  beginning  is  handled  as  usual. 

No  explanation  is  necessary  for  cutting  tunnel  and 
codend,  which  are  rectangular. 

All  nets  can  be  made  from  machine-made  netting, 
but  variations  are  possible.  For  instance,  missing 


triangular  edge  pieces  can  be  added  by  handbraiding. 
The  selection  of  the  actual  method  is  determined  mainly 
by  economic  considerations. 

To  repair  destroyed  parts,  fishermen  take  a  collection 
of  different  pieces  of  netting  with  them  so  that  whole 
sections  can  be  replaced. 

THE  HERRING  PAIR-FISHING  GEAR 

The  length  of  the  ground  rope  in  this  type  of  gear, 
which  is  towed  by  two  cutters,  is  similar  to  that  found  in 
the  gear  of  big  trawlers,  but  the  length  of  the  net  is 
greater.  The  large  size,  in  relation  to  the  limited  engine 
power  (about  240  to  360  h.p.  both  boats  together) 
is  made  possible  because  the  nets  are  light,  made  out  of 
cotton  or  synthetic  twine.  In  the  North  Sea,  slightly 
smaller  nets  are  used  than  in  the  Baltic  Sea. 

Weights  of  60  to  120  kg.  are  fixed  at  the  connection 
of  warp  and  bridle. 

The  bridles  are  made  of  combined  rope  of  18  to  20 
mm.  0,  and  may  be  40  to  65  fm.  long.  For  deeper  water 
longer  bridles  are  preferred.  They  are  connected  b> 
shackles  with  swivels  to  the  warps  and  the  strops  of  the 
danlenos  or  the  legs,  respectively. 

If  danlenos  are  used,  they  are  of  the  usual  wood/ iron 
construction  and  about  1-3  m.  long.  The  charging 
weight  at  the  lower  end  is  15  to  20  kg.  A  strop  of  2  to 
3  fm.  length  is  provided  for  the  attachment  of  the  bridle, 
whilst  the  legs  are  fixed  to  two  iron  loops  near  each  end. 
The  danlenos  may  often  be  omitted.  In  this  case,  two 
additional  legs,  5  to  6  fm.  long,  lead  from  headline  and 
lastrich  (or  its  legs)  on  the  one  hand,  and  from  the 
ground  rope  (or  its  leg)  on  the  other  hand,  to  the  point 
of  connection  with  the  bridle.  These  additional  legs 
must  have  good  swivels  to  avoid  fouling. 

The  legs  are  made  of  combined  rope  of  12  to  14  mm.  o. 
The  length  may  vary  considerably,  i.e.,  between  4  and 
16  fm.  Sometimes  the  legs  may  be  omitted.  Headline, 


Fix.   /V.     Direct  attachment  of  the  net  to  the  headline. 
S    Strengthening  rope,  fixed  on  the  halter. 


308  1 


GERMAN     CUTTER     TRAWLING     GEAR 


lastrich,  and  ground  rope  are  then  fixed  to  the  danlenos 
directly.  As  with  the  herring  otter  trawling  gear,  headline 
and  lastrich  are  then  attached  to  the  upper  loop,  and  the 
ground  rope  to  the  lower  loop  of  the  danleno. 

The  headline,  usually  is  made  of  combined  rope  of 
about  12  mm.  0,  and  has  eyesplices  at  both  ends  for 
connection  to  the  legs  or  the  danleno.  As  bolshlines  are 
not  used,  the  net  is  attached  to  the  headline  directly 
(fig.  19).  For  lifting,  a  varying  number  (25  to  35)  of 
floats  (aluminium,  plastic,  or  glass,  8  in.  0)  are  distri- 
buted over  the  whole  length.  Kites  or  false  headlines  are 
not  normally  used. 

The  ground  rope,  also,  of  combined  rope  is  of  14  to 
16  mm.  0.  Like  the  headline,  it  has  eyesplices  at  both  ends. 
It  is  not  served  with  ropes  and  the  net  is  fixed  on  directly. 
For  charging,  iron  rings  are  used  in  practically  the  same 
way  as  described  for  the  otter  trawling  gear.  If  legs  are 


used,  an  additional  weight  of  8  to  10  kg.  is  usually  attached 
to  each  wing  end  at  the  point  of  connection  with  the 
legs. 

The  lastrichs  are  strengthened  down  to  the  codend  by 
combined  rope  of  10  to  12  mm.  0.  For  the  lastrichs  of 
the  codend  itself,  manila  rope  of  about  14  mm.  0  is 
preferred.  If  triangular  shaped  wing  tips  are  used,  the 
wing  ends  of  these  strengthening  ropes  must  extend  the 
netting  sufficiently  to  compensate  the  greater  lengths  of 
the  headline  and  the  ground  rope.  The  ends  have  eye- 
splices  for  connection  with  the  legs  or  the  danleno. 

Usually,  140  to  160  ft.  nets  are  used  with  this  type 
of  gear.  A  scheme  for  the  construction  of  such  nets  is 
given  in  fig.  20.  It  must  be  mentioned  that  the  measure- 
ments as  given  are  only  examples  and  do  not  cover  all 
the  variations  which  are  in  use.  These  nets  are  con- 
structed of  machine-made  netting.  The  material  is 

K0'-160'h*rr-ng-pa.r-nel 


wing 


lower- 


belly 


meshpiece  with  floppa 

t 

tunnel 


+ 

cod-end 
* 

Construction  plan  of  140  to  /60ft  henhiK  pair-na\vl.\ 
[309  1 


MODERN     FISHING    GEAR    OF    THE    WORLD 


cotton  or  synthetic  fibres.  The  following  twine  strengths 
are  suitable  for  the  different  parts: 

Part  of  the  net  Breaking  strength  (wet) 

Upper  wing  14  kg.  (30-9  Ibs.) 

Lower  wing  14  kg.  (30-9  Ibs.) 

Square  14  kg.  (30-9  Ibs.) 

Belly  6 -6  to  11 -0  kg.  (14  to  24-3  Ibs.) 

Tunnel  8-5to  12-5kg.(18-7to27-6ibs) 

Codend  14-Oto  17-5  kg.  (30-9  to  38-61bs.) 

These  values  are  almost  the  same  as  for  the  otter  trawl 
gear  except  that  the  tunnel  and  codend  are  usually  more 
strongly  made  to  cope  with  the  bigger  catches  obtained 
by  pair  fishing  with  such  big  nets.  However,  the  twine 
strength  is  much  lower,  although  the  nets  are  as  large  as 
those  used  in  big  trawlers.  This  results  in  lower  towing 
resistance,  saving  of  material,  and  better  catching 
ability. 

As  is  customary,  the  headline  side  of  the  upper  wings 
is  fly-meshed  or  cut  on  the  halfer.  The  lastrich  side 
is  usually  not  straight,  but  either  runs  in  the  same  slant 
as  the  side  edges  of  the  square  or  at  least  has  some 
creasings.  The  length  may  vary  between  about  11-0 
and  12*5  m.  There  are  types  with  straight  terminal 
edges  of  about  25  meshes  in  width,  and  others  terminating 
in  one  mesh.  The  latter  are  constructed  in  the  same  way 
as  already  described  for  the  triangular  shaped  tips  of  some 
herring  otter  trawl  nets.  The  mesh  size  is  about  160  mm. 

The  square  has  the  usual  trapezium  form.  The  mesh 
size  is  the  same  as  in  the  upper  wings.  The  length  may 
be  about  4  m. 

The  ground  rope  side  of  the  lower  wings  is  fly- 
meshed  or  cut  on  the  halfer.  The  lastrich  side  may  run 
partly  the  same  way  as  the  side  edge  of  the  square.  More 
creasings  have  then  to  be  inserted  in  the  remaining  part 
to  gain  the  necessary  length.  The  lower  wings  usually 
end  with  a  triangular-shaped  part  which  terminates  in 
one  mesh.  The  real  lastrich,  in  which  upper  and  lower 
net  are  connected,  does  not  extend  further  than  the  point 
where  these  triangular  pieces  begin.  The  side  edges 
opposite  the  ground  rope,  which  are  fly-meshed  or 


cut  on  the  halfer,  are  then  attached  to  the  fishline.  This 
fishline,  usually,  is  made  of  combined  manila  wire  rope 
of  10  to  12  mm.  0. 

In  the  belly  the  mesh  size  has  to  be  diminished  con- 
siderably. It  usually  consists  of  4  to  5  sections  not  only 
with  different  mesh  size,  but  with  different  twine  strength 
and  also  a  different  baiting  rate.  The  total  length  of  the 
belly  amounts  to  about  26  to  28  m.  This  means  that  it  is 
longer  and  the  net  is  more  slender  than  the  herring  net  of 
big  trawlers.  The  end  of  the  belly  may  be  105  to  112 
meshes  wide  including  the  mesh  piece.  The  floppa  is 
inserted  in  the  last  section  of  the  belly.  It  may  be 
trapezium  or  rectangular  and  is  inserted  in  the  same 
way  as  already  described  for  the  other  types  of  cutter 
nets. 

The  tunnel,  as  usual,  is  rectangular.  As  its  mesh  size 
usually  is  a  bit  smaller  than  in  the  last  section  of  the  belly, 
the  number  of  meshes  in  the  width  must  be  correspond- 
ingly higher.  The  total  length  may  be  5  to  6  fm.  If  the 
rest  of  the  net  is  made  of  cotton,  for  the  tunnel  and  codend 
synthetic  fibres  are  preferred. 

The  codend  usually  has  the  same  mesh  size,  and  also 
the  same  number  of  meshes  in  width,  as  the  tunnel.  A 
smaller  mesh  size  has  to  be  used  for  catching  smaller  fish 
than  herring,  i.e.  sprat.  In  this  case,  the  number  of 
meshes  in  width  must  be  correspondingly  higher.  The 
total  length  may  be  5  to  6  fm.  The  same  type  of  heaving 
codend  is  used  as  described  for  the  herring  otter  trawling 
gear. 

The  method  of  cutting  such  nets  out  of  machine-made 
netting  is  almost  the  same  as  described  for  the  herring 
otter  trawling  gear.  The  only  difference  arises  with  the 
long-winged  type.  But  the  length  of  such  wings  is  a 
multiple  of  the  square  length  so  that  they  can  be  easily 
cut  with  the  square  and  the  lower  wings  from  the  same 
cale  of  webbing. 

This  last  type  of  cutter  gear  can  be  considered  as  the 
most  progressive  and  effective.  Pair  fishing  seems  to  be 
an  economical  method,  particularly  for  boats  with  lower 
engine  power.  The  catch,  usually,  is  more  than  twice 
that  which  each  cutter  could  obtain  by  otter  trawling. 


[310] 


SHRIMP   TRAWLING   GEAR   AS   USED    IN   THE   GULF   OF  MEXICO 

by 

JOHN  S.  ROBAS 

Commercial  Fisheries  Consultant,  Fernandina  Beach,  Florida,  U.S.A. 


Abstract 

In  place  of  the  usual  80  to  100  ft.  single  trawl,  typical  trawlers  (about  67  ft.)  have  started  to  operate  two  trawls,  each  about  45  ft. 
headline,  from  outriggers  on  each  side.  These  two  otter  trawls  may  be  of  the  flat  or  the  balloon  type.  A  three-drum  winch  is  used,  with  one 
drum  for  each  trawl,  and  one  for  the  try-net.  The  boats  operate  to  a  depth  of  30  fm.  A  typical  Campeche  shrimp  trawler  makes  6  to  7  trips 
per  year  and  obtains  an  average  catch  of  about  90,000  Ib./year  of  21  to  25  Ib.  shrimp,  heads  off  basis.  The  complete  gear  is  described  in  detail. 


Resume 


Le  Dispositif  pour  la  peche  des  cervettes  au  chalut  dans  le  golfe  du  Mexique 


Au  lieu  du  simple  chalut  habituel  de  80  a  100  pi.  (24.38  a  30,48  m.),  les  chalutiers  ty piques  (d'environ  67  pi.,  soil  20,43  m.)  ont 
commence  a  trainer  deux  chaluts  ayant  chacun  une  corde  de  dos  de  45  pi.  (13,72  m.)  environ,  &  1'aide  de  tangons  monies  de  chaque  cdt6. 
Ces  deux  chaluts  &  plateaux  peuvent  elre  du  lype  plat  ou  ballon.  On  utilise  un  Ireuil  &  irois  tambours,  un  tambour  chaque  chalut  et  un  pour 
le  chalut  d'essai.  Les  bateaux  pechent  £  une  profondeur  de  30  br.  (54  m.).  Un  chalutier  £  crevettcs,  typique  de  Campeche,  accomplit  jusqu'a 
6  ou  7  sorties  par  an  el  fait  une  peche  moyenne  d'environ  90,000  Ib./an  (40,820  kg./an),  de  crevettes  de  moule  21  &  25/lb.  (47  A  55/kg.). 
crevettes  etdtdes.  Tout  le  dispositif  est  deer  it  en  detail. 


Redes  de  arrastre  para  la  pesctt  de  cameron  utilizadas  en  el  golfo  de  Mexico 
Extracto 

En  lugar  de  la  red  unica  de  80  a  100  pies  de  longiiud  utilizada  correintemente,  los  arrastreros  lipicos  (alrededor  de  67  pies)  han 
empezado  a  trabajar  con  dos  redes  de  arrastre,  cuya  relinga  superior  tiene  45  pies,  operadas  desde  batangas  situadas  a  los  costados.  Hstas 
redes  de  puertas  pueden  ser  del  tipo  piano  o  de  globo.  Se  utiliza  una  maquinilla  con  tres  tambores,  usando  dos  de  eslos  para  las  redes  de 
arrastre  y  otro  para  la  red  de  sacar  muestras.  Los  barcos  actuan  a  una  profundidad  de  30  brazas.  Un  barco  lipico  de  tipo  Campeche  dedicado 
a  la  pesca  de  camarbn  hace  de  6  a  7  sal  id  as  al  afto  y  obtiene  una  capture  media  de  unas  90.000  libias  de  camarones  por  afto  siendo  el  tamafio 
de  estos  equivalente  a  21  a  25  ejemplares  sin  cabeza  por  libra.  Se  describe  con  detalle  el  arte  complete. 


THERE    have    been  broad  and  significant  changes 
during  1957   in   the  gear  of  the  Gulf  of  Mexico 
shrimp  trawler.   Briefly,  the  typical  67  ft.  trawler, 
working  in  waters  to  a  depth  of  30  fm.  for  pink  and  brown 
shrimp,  has  abandoned  the  usual  single  trawl  towed  off 
the  starboard  outrigger  and  has  switched  to  towing  two 
smaller  trawls,  one  port  and  one  starboard. 

In  place  of  the  usual  80  to  100  ft.  single  trawl  the 
typical  trawler  now  tows  two  40  to  45  ft.  trawls  and  finds 
that: 

(a)  the  gear  is  easier  to  tow  and  handle; 

(b)  the  gear  is  safer  for  the  crew; 

(c)  the  two  trawls  produce  more  shrimp  per  unit  of 
effort  than  a  single  large  trawl; 

(d)  gear  losses  from  wrecks  and  hangs  are  lower  as 
only  one  trawl  is  usually  involved  and  cost  of 
repair  or  replacement  is  lower. 

Handling  two  trawls  simultaneously  is  done  by  using 
longer  out-rigger  booms,  usually  about  24  ft.  long, 
constructed  of  heavy  duty  steel  pipe  with  \  in.  wall 
thickness,  braced  by  external  welded  struts. 

The  typical  shrimp  trawler  has  always  fished  from  out- 


riggers rather  than  from  a  gallows  frame  and  so  the 
change  merely  involves  longer  and  stouter  out-riggers. 
Instead  of  using  two  &  in.  trawl  warps  running  to  the 
single  trawl,  the  vessel  now  fishes  each  pair  of  doors  on 
a  long  bridle,  with  a  single  trawl  cable  spliced  into  each 
bridle. 

The  typical  trawler  is  equipped  with  a  three-drum 
winch.  The  top  drum  handles  the  port  side  warp,  the 
bottom  drum  handles  the  starboard  side  warp,  and  the 
centre  drum  is  reserved  for  the  try-net,  the  8  ft.  trawl  with 
miniature  doors  which  is  used  to  locate  the  shrimp. 
Both  nets  are  set  and  lifted  simultaneously,  generally  on 
a  straight  automatic  pilot  course.  The  port  trawl  is 
fished  about  25  fm.  behind  the  starboard  trawl  to  avoid 
the  possibility  of  fouling.  The  winch  man  engages  both 
winch  drums  to  take  up  his  gear  and  commences  hauling. 
The  starboard  trawl  is  taken  in  first,  using  a  long  boat 
hook  to  reach  out  for  the  lazy  line  attached  to  the  codend. 
The  codend  is  then  lifted  over  the  rail  by  means  of  the 
winch  barrel  and  dumped  on  deck.  By  this  time,  the 
port  trawl  is  ready  for  the  same  procedure. 

The  two  smaller  trawls  actually  catch  more  shrimp 


[311  ] 


MODERN     FISHING     GEAR     OF    THE    WORLD 


Fig.  7.     A  typical  mass-produced 67  ft .  American  shrimp  trawler, 
showing  the  trawling  outriggers  in  the  raised  position.     Note 
external  bracing  of  outrigger  booms.     This  vessel's  main  pro- 
pulsion engine  develops  150  h.p. 

than  an  equal  size  single  trawl.  Since  towing  speed  is 
generally  higher  with  the  smaller  trawls,  it  is  possible 
that  the  increased  production  is  merely  the  result  of 
covering  more  ground.  This  is  not  believed  to  be  the 
whole  story  and  the  answer  may  well  lie  in  the  reaction 
of  the  shrimp  to  the  trawl  itself. 

The  two  trawl  rig  has  not  been  generally  adopted  for 
catching  white  shrimp,  which  have  a  tendency  to  assemble 
in  small  schools,  so  that  much  manoeuvring  is  required 
to  stay  in  the  shrimp.  Many  white  shrimp  vessels  on  the 
Atlantic  coast  are  still  using  a  single  trawl  rig. 


Fig.  3.    Deck  plan  of  a  67ft.  American  shrimp  trawler  showing 
location  of  3-drum  trawl  winch  in  relation  to  deckhouse. 

TRY-NET 

The  feature  which  distinguishes  the  American  shrimp 
trawler  from  almost  any  other  trawler  in  the  world 
is  the  try-net,  the  small  trawl  equipped  with  miniature 
doors  which  is  used  to  sample  the  bottom  for  shrimp 
before  the  main  trawl  or  trawls  are  put  overboard. 

The  try-net  is  fished  from  a  small  davit  on  the  stern 
of  the  vessel. 

As  it  is  light  and  easily  handled,  it  can  be  set,  towed 
for  2  to  5  min.,  and  lifted,  air  by  one  man.  A  competent 
captain  keeps  his  try-net  fishing,  even  while  his  regular 
gear  is  set,  to  ensure  staying  in  the  shrimp.  This  is 
particularly  important  in  the  white  shrimp  fishery.  An 
experienced  captain  soon  learns  how  to  interpret  his 
try-net  catch,  which  may  consist  only  of  two  or  three 
shrimp,  in  terms  of  what  it  will  produce  in  his  large 
trawl  and  is  also  able  to  evaluate  the  presence  of  trash 
fish  on  the  shrimp  grounds. 

As  power  is  required  to  handle  this  small  trawl  with 
ease,  the  try-net  accounts  for  the  fact  that  virtually  all 
shrimp  trawlers  are  equipped  with  three-drum  winches. 
In  the  case  of  a  two-trawl  vessel,  the  centre  drum  on  the 
winch  is  usually  reserved  for  the  try-net. 

DOORS  AND  WARPS 

A  typical  150  h.p.  two-trawl  vessel  is  equipped  with  125 
to  150  fm.  of  A  in.  steel  galvanised  wire  rope  on  two 


Fig.  I.    Rigging  profile  of  a  typical  two-trawl  shrimper.    On 


Fig.  4.    A  new  3-drum  winch  especially  developed  for  two-trawl 
operation,  which  is  more  compact  and  efficient  than  the  common 
units.   The  top  drum  handles  the  try  net  and  the  other  two  drums 
one  trawl  eack\  The  bridles  are  wound  uy  on  the  winch  drum. 

J 


SHRIMP    TRAWLING    GEAR 


"S" 


Fig.  5.     Construction  details  ofstandard84  in.  by  32  in.  trawl  door 
used  on  two-trawl  shrimpers. 


drums  of  the  winch  and  a  shorter  length,  generally  the 
same  type,  on  the  centre  drum  for  the  try-net. 

Two  light  trawl  doors,  approximately  84  in.  long  by 
32  in.  high,  rigged  with  chains  and  15  to  20  fm.  wire 
rope  bridles,  spread  the  42  ft.  trawls,  which  may  be  the 
conventional  flat  net  or  the  balloon  trawl.  The  doors 
used  by  the  United  States  shrimp  fleet  are  much  lighter 
in  construction  than  doors  used  in  other  fisheries. 
Their  relative  lightness  makes  them  easy  to  handle  by 


small  crews  and  they  have  been  successfully  used  for 
many  years. 

Fig.  5  shows  the  construction  of  a  typical  shrimp 
door  84  in.  long  by  32  in.  high,  normally  supplied  to  the 
Gulf  of  Mexico  shrimpers  using  the  two  trawl  system. 
The  door  is  constructed  of  \\  in.  pine  timber,  planed 
on  both  sides  to  a  thickness  of  1  in.  Other  size  trawls 
require  larger  or  smaller  doors;  for  example,  a  100  ft. 
flat  net  will  require  a  door  approximately  144  in.  long 
by  45  in.  high. 

The  American  fisherman  normally  buys  his  doors 
by  the  pair,  completely  rigged  and  ready  to  fish.  However, 
by  studying  the  illustration,  the  construction  on  the 
door  can  be  readily  deduced.  The  formula  generally 
used  for  spacing  the  chain  bridles  is  as  follows: 

L  is  the  total  length  of  the  door,  in  inches. 

L 

1    is  the  distance  in  inches  from  the  front  of  the 

4  door  to  the  centre  of  front  chains. 

L 

2    is  the  distance  in  inches  between  the  front 
4  and  rear  chains. 

L 

-f-  1    is  the  distance  in  inches  from  the  rear  of  the 
4  door  to  the  rear  chains. 


LOOKING     DOWN   TO    OPEN    RIG 


DETAIL- CHAIN   HOOK 
TO    BOARD 


Fig.  6.    CoiatriKtumal  details  of  the  try-net  gear. 
f3I3) 


MODERN    FISHING    GEAR    OF    THE    WORLD 


TRIM  A«0  MW  TO  FOWM  COO -END 


ft,    C.  •  0     ARC    CORNER    PICCCS.    AftC    SfWfD    T06TTHER 
ON   STRAIGHT    COGC   OIVC    A(     IOCNTICAL     TO    B.C.D 


POINT   8  — --.  r  -»_  - 


Fig.  7.     Construction  and  method  of  cutting  the  try-net. 

In  establishing  the  length  of  the  chains,  it  is  con- 
sidered a  good  practice  for  the  front  chains  to  be  about 
45  to  50  per  cent,  the  length  of  the  rear  chains.  Further, 
the  top  front  chain  is  set  1  link  longer  than  the  bottom 
front  chain  and  the  top  rear  chain  is  set  2  links  longer 
than  the  bottom  rear  chain.  Set  in  this  manner,  the  doors 
have  an  outward,  downward  thrust  when  towed  and 
dig  into  the  bottom  to  produce  maximum  catches. 

In  rigging  the  chain  bridles,  4  or  5  links  are  added  to 
each  length  of  chain  so  as  to  provide  for  adjustment  as 
needed.  The  chain  settings  shown  in  fig.  7  are  counted 
from  the  inner  face  of  the  door  to  the  I  in.  shackle 
attached  to  each  length  of  chain.  The  4  end  shackles 
are  all  hooked  into  a  i  in.  swivel  on  each  door.  The  towing 
bridle  is  attached  to  the  swivel. 


The  usual  criterion  for  efficient  performance  is  the 
polishing  of  the  steel  shoe  plates;  if  these  develop  rust) 
or  dirty  spots  it  can  be  assumed  that  the  doors  are  not 
fishing  properly  and  should  be  adjusted.  Adjustment  is 
quickly  made  by  lengthening  or  shortening  the  chains 
by  means  of  slotted  steel  plates  or  locks  on  the  outer 
face  of  the  door.  The  I  in.  proof  coil  welded  chain  used 
today  is  galvanized  and  weighs  168  Ib./lOO  ft.  It  has  an 
outer  length  of  2.V*  in.  per  link  and  averages  9-75 
links/ft. 


BLOCKS  AND  BOOMS 

The  trawl  blocks  for  handling  the  warps  are  of  particular 
importance.  They  must  have  a  broad,  gently  V-shaped 
cross  section  so  that  splices,  bridles,  etc.,  will  pass  over 
them  freely,  permitting  the  leading  edge  of  the  trawl 
door  to  come  up  to  the  block.  The  blocks  should  be 
galvanised  and  the  sheave  flame-hardened  and  mounted 
on  heavy  roller  bearings. 

In  rigging  the  out-rigger  boom  topping  lifts,  care 
must  be  taken  to  insure  that  the  trawl  block,  in  fishing 
position,  is  at  least  12  ft.  off  the  water  under  calm  con- 
ditions, otherwise  the  out-riggers  will  dip  into  the  water 
when  the  vessel  rolls.  As,  in  the  raised  position,  the  heavy 
out-riggers  affect  the  stability  of  the  vessel,  they  are 
generally  lowered  into  fishing  position  when  running 
from  one  fishing  ground  to  another.  Many  vessels  also 
mount  their  out-riggers  on  the  mast  by  means  of  a 
revolving  pin  so  that  the  out-rigger  forestays  can  be 
slacked  in  rough  weather  and  the  booms  lowered  to 
the  after  deck  for  safety. 

The  trawler's  main  topping  boom  must  be  securely 
stayed  to  withstand  the  extra  stresses  involved  and  it 
is  customary  to  install  a  welded  steel  ladder  from  the 
tip  of  the  topping  boom  to  the  centre  of  the  transom 
caprail.  This  ladder  is  also  useful  in  clearing  a  fouled 
block  or  doing  repairs  on  the  boom  itself. 

NETS 

Three  common  shrimp  trawls  are  shown  in  figs.  8,  9 
and  10.  There  are  innumerable  variations  in  size  and 
construction,  depending  upon  the  area,  size  of  vessel 
and  type  of  shrimp.  As  this  discussion  is  concerned  with 
the  typical  two-trawl  Gulf  of  Mexico  trawler,  emphasis 
will  be  placed  on  the  40  ft.  flat  trawl.  Individual  varia- 
tions of  this  size  will  be  found  from  43  to  45  ft.  in  width. 

Fig.  10  shows  a  40  ft.  no-overhang  trawl,  i.e.,  the 
headline  of  the  trawl  rides  directly  over  the  foot- 
rope.  Many  fishermen  prefer  to  make  their  nets  with 
12  to  18  meshes  of  overhang  so  that  the  headline  precedes 
the  footrope,  the  theory  being  that  the  shrimp  will 
leap  upward  when  disturbed  by  the  footrope  and 
strike  against  the  forward-projecting  top  of  the  trawl. 

In  hanging  this  net,  which  is  one  of  the  simplest  to 
construct,  &  in.  manila  rope  is  used  for  both  headline 
and  footrope.  Three  meshes  are  caught  per  hanging, 
with  ties  spaced  about  4J  in.  About  4  in.  between  the 
headline  and  the  webbing  and  about  6  in.  between  the 
footrope  and  the  webbing  are  considered  standard 
measurements.  Three-inch  corks  are  spaced  across  the 
headline,  1  every  10  ties;  4  oz.  leads  are  added  to  the 


[314] 


SHRIMP    TRAWLING    GEAR 


ItOM 

BOTTOM    BODY 


J>***<*.  •"••— 

^t   MIVTft 


CODEND 


.  #.     Construction  diagram  oj  typical  74ft.  Balloon  trawl  Jor  single  trawl  operation.     Wing.\%  body\  dogears  and  jibs  are  18-thread,  2\  in. 
stretched  mesh  cotton  webbing.     The  codend  is  42-thread,  2  in.  stretched  mesh  cotton  webbing. 


Fig.  9.     Construititw  Jitintint  of  typical  100  ft.  flat  shrimp  trawl  with  36  mesh  overhang,  used  for  single-trawl  operation  Wing.^ 

and  throat  an    is-thn-ad.  2\  in.  stretched  mesh  cotton  webbing.    The  codend  is  42-thrcad,  2  in.  stretched  mesh  cotton  webbing. 


315 


MODERN     FISHING     GEAR    OF    THE    WORLD 


ground  rope  as  required,  generally  1  every  5  ties  in  the 
body  and  1  every  3  ties  in  the  jibs. 

PRODUCTION 

The  typical  67  ft.  Campeche  shrimp  trawler  averages 
6  to  7  trips  per  year  from  the  coast  of  the  United  States 
to  the  fishing  grounds  off  the  coast  of  Yucatan  Peninsula, 
Mexico.  An  average  catch  for  a  well-equipped  vessel 
with  a  capable  crew  is  about  90,000  Ib./year  of  21  to 
25  per  Ib.  shrimp,  heads  off  basis,  but  many  vessels  fall 
short  of  this  production.  With  the  advent  of  the  two- 
trawl  system,  many  owners  are  examining  the  possibility 
of  using  larger  vessels  with  greater  horse-power,  capable 
of  handling  larger  nets,  i.e.  two  70  ft.  balloon  or  flat 
trawls  instead  of  the  conventional  45  ft.  size. 


Fig.  JO.  Construction  diagram  of  a  40  ft.  no-overhang  flat  trawl. 
Wings*  body\  jibs  and  throat  are  15-thread.  2\  in.  stretched  mesh 
cotton-webbing.  Codend  is  42-threatt,  2  in.  stretched  mesh  cotton. 


A  small  shrimp  trawler  on  the  U.S.A.  Gulf  Coast. 


316] 


TRENDS    IN    TRAWLING    METHODS   AND    GEAR    ON    THE   WEST 

COAST   OF   THE   UNITED   STATES 

by 

DAYTON  L.  ALVERSON 

Fishery  Biologist,  Department  of  Fisheries,  State  of  Washington,  U.S.A. 

Abstract 

Most  of  the  changes  that  have  taken  place  in  the  otter  trawling  fleet  since  the  introduction  of  the  V.D. -method  have  occurred  since 
the  last  war,  the  most  spectacular  ones  being  the  assimilation  of  electronic  devices  such  as  depth  finders,  I.oran,  radar,  and  fishfinders. 

The  high  cost  of  radar  slowed  down  the  installation  of  this  device  but  about  25  per  cent,  of  the  off-shore  trawlers  are  now  equipped 
with  instruments  using  3  or  10  cm.  wave-lengths.  The  value  of  C.R.T.  fishfinders  has  not  yet  been  properly  assessed  because  there  seems  to  be 
some  difference  of  opinion  as  to  their  ability  to  locate  and  increase  catches  and  it  is  felt  that  too  much  interpretation  is  left  to  the  fisherman. 
One  important  result  of  the  use  of  electronic  devices  has  been  the  expansion  of  the  fishing  grounds,  and  the  average  maximum  fishing  depth 
has  i  ncreased  I'rorn  144  fm.  in  1952  to  280  fm.  in  1956.  In  addition  to  these  post-war  innovations  two  other  mechanical  devices,  the  Trawl 
Cable  Meter  and  the  Drum  Trawl  technique  have  been  developed,  but,  notwithstanding  all  these,  the  skill  and  intelligence  of  the  skipper  and 
crew  are  still  the  key  factors  governing  the  efficiency  of  the  fleet. 

F.volution  des  methodes  et  engins  de  chalutago  sur  la  cote  occidental?  dos  FJats-l'nis 
Resume 

La  plupart  des  changements  intervenus  dans  la  flotte  des  chain  tiers  depuis  ('introduction  de  la  mcthodc  V.I)  ont  eu  lieu  depuis  la 
derniere  guerre;  les  plus  importants  ont  consiste  dans  I'milisation  d'apparcils  electroniqnes  tels  que  les  sondeurs,  'e  Loran.  le  radar,  et  les 
appareils  a  reperer  le  poisson. 

Le  prix  6lcv6  du  Radar  a  freine  Pemploi  de  ce  dispositif  mais  a  I'heurc  actuclle  25  pour  cent  environ  des  chaluticrs  de  hauie  mer  sont 
dquipes  d 'appareils  fonctionnant  sur  des  longueurs  d'onde  de  3  ou  10  cm.  La  valeur  des  detecteurs  de  poisson  equipes  de  lampe  &  rayons 
cathodiques  n'a  pas  encore  el£  etablie  avec  precision  car  les  avis  sont  partages  sur  leur  aptitude  a  localiscr  le  poisson  ct  i  augmenter  le  volume 
des  peches  et  Ton  considere  qu'une  trop  grande  marge  d'interpr6tation  est  laissee  aux  pecheurs.  Un  des  res ul tats  importants  obtenus  a  1'aidc 
des  appareils  dectroniques  a  6te  Pextension  des  licux  de  pec  he,  ct  la  profondeur  maximum  moyennc  dc  pechc  est  passec  de  144  brasses  en  1952 
a  280  brasses  en  1956.  Outre  ccs  innovations  d'apres-guerrc.  on  a  mis  au  point  deux  autres  dispositifs  mecaniques.  I'appareil  de  mesure  du 
cable  de  chalut  et  le  tambour  de  chalut;  mais  en  depit  de  tous  ces  perfect ionnements,  c'est  Phabilete  et  I'intelligence  du  patron  de  peche  et  de 
son  equipage  qui  sont  encore  les  facteurs  essentiels  du  rendemcni  dc  la  pechc. 

Tendenoias  sobre  metodos  y  uso  de  redes  dc  arrastre  ohscrvadas  en  hi  rosta  maiden  la  I  de  los  E.t'.A. 
Extracto 

La  mayoria  de  los  camhios  experimentados  por  la  flota  de  arrastreros  desdc  la  introduction  del  mctodo  V-D,  turvo  Sugar  dcspues 
de  la  gucrra.  siendo  el  mas  espectacular  el  uso  dc  dispositivos  electrcSnicos  como:  ecosondas  o  localizadores  de  peces,  loran  y  radar. 

El  alto  costo  del  radar  demoro  la  instalacion  dc  este  equipo,  pero  un  25  pour  cent  de  los  arrasireros  de  altura  cuenta  ahora  con 
mstrumentos  que  usan  ondas  dc  3  o  10  cm.  de  longitud.  F.I  valor  de  los  locali/adores  de  peces  con  mhos  de  rayos  catodicos  no  ha  podido 
cvaluarse  en  forma  ad  ecu  ad  a  porque,  al  parecer,  existen  ciertas  diferencias  de  opinion  en  cuando  a  sus  posibilidades  para  locali/er  y  aiimentar 
la  pcsea,  creyendose  que  se  deja  demasiada  interpretation  al  pescador.  Rcsultados  importanics  del  uso  de  disposibiivos  elect  romcos  han 
sido  el  aumenlo  dc  la  supcrlicic  de  cxplotacion  de  los  bancos  de  pesca  y  de  la  profundidad  maxima  de  captura  desde  144  brazas  en  1952  a 
280  brazas  en  195ft.  Adcmas  de  estas  innovaciones  de  posguerra  se  han  ideado  otros  dos  dispositi\os  inccantcos:  el  medidor  dc  la  longitud 
del  cable  de  arrastre  y  la  de  tecnica  recoger  la  red  dc  arrastre  nicdiante  un  carrctel.  No  obstante  esto.  la  hahilidad  e  inteligcncia  del  patron  y 
dc  la  tripiilacion  son  todavia  los  fact  ores  principales  que  regulan  la  eficacia  de  la  flota  pesquera. 


OTTER     trailers     from    ports    along    the    Pacific 
Coast  operate  in  the  offshore  waters  from  Santa 
Barbara,     California     (U.S.A.)     northward     to 
Hecate  Strait,  British  Columbia  (Canada),  a  distance  of 
nearly    1,500   miles.   The  vessels  are   mostly  converted 
seiners  and  halibut  schooners  from  45  to  100  ft.  in  length 
(figs.  I  and  2),  the  average  being  about  60  ft.  The  fish 
capacity  of  these  trawlers  is  about  33  short  tons.  Three 
or  four-man  crews  are  used  although  five  and  six-man 
crews  were  common  during  the  war  years.  Almost  all 
West  Coast  trawlers  use  gallows  placed,  one  on  each 
side,  near  the  stern.  Normally  the  net  is  shot  directly 
over  the  stern  and  hauled  in  over  the  starboard  side. 
The  use  of  the  seine  or  schooner-type  trawler  has 
allowed  operators  to  convert  and  rig  quickly  for  salmon, 


halibut  or  albacore  \\hene\er  these  fisheries  may  be 
more  lucrative.  Because  of  the  seasonal  nature  of  the 
fisheries,  trawling  has  been  a  part-time  "off  season" 
operation  for  many  vessels. 

KI-EC TRONIC  AIDS 

The  more  important  changes  in  fishing  gear  and  methods 
influencing  the  efficiency  of  trawling,  following  the 
adoption  uf  the  Vignoron-Dahl  fishing  method,  have 
evolved  during  the  last  10  to  15  years.  The  most  spec- 
tacular trend  one  which  has  occurred  in  major  fisheries 
throughout  the  world  has  been  the  assimilation  of 
various  electronic  devices  developed  and' or  modernized 
during  World  War  II.  An  important  consequence  of 


[3171 


MODERN     FISHING     GEAR     OF    THE     WORLD 


this  trend  has  been  the  increasing  dependence  of  trawl 
fishermen  on  this  equipment.  Depth  recorders,  Loran, 
radar,  and  fishfinders,  which  represent  post  war  refine- 
ments in  ultrasonic  techniques,  are  all  carried  aboard 
the  better  equipped  vessels. 

DEPTH   RECORDERS 

The  use  of  echo  sounders  aboard  West  Coast  trawlers 
preceded  World  War  II  and  Scofield  (1948)  reported 
25  per  cent,  of  the  California  trawl  fleet  was  equipped 
with  them  by  1947.  Practically  all  offshore  trawlers 
were  using  them  by  the  end  of  1948  and  reliance  on  the 
device  had  become  so  complete  that  the  failure  of  the 
echo  sounder  meant  a  return  to  port  for  most  trawlers. 

LORAN 

Interest  in  war  surplus  Loran  sets  increased  in  1949, 
especially  in  the  Pacific  Northwest  (U.S.)  where  adverse 
weather  conditions  made  accurate  offshore  navigation 
difficult.  The  advantages  of  precise  fixes  for  trawl 
fishing  were  quickly  recognized  and  by  1954  Loran  was 
being  used  by  most  trawlers  fishing  between  Eureka, 
California,  and  the  Canadian  border. 


The  adoption  of  Loran  resulted  in  a  new  era  and  meth- 
odology for  the  West  Coast  trawl  fishermen.  Precise 
navigation  increased  the  effectiveness  of  trawling  and 
it  was  especially  important  in  making  deep-water  fishing 
a  profitable  operation.  The  rugged  submarine  topography 
of  the  offshore  grounds  had  previously  made  their 
exploitation  difficult  and  at  times  unprofitable.  Loran, 
coupled  with  echo  sounders,  gave  trawlers  the  accuracy 
needed  to  define  and  maintain  their  position  while 
fishing. 

Since  1954,  Loran  has  so  completely  dominated  the 
navigation  and  fishing  of  trawlers  that  reference  to  trawl 
grounds  is  seldom  related  to  prominent  headlands  or 
banks  but  more  often  as  a  "Loran  microsecond*'  reading. 
Loran  readings  have  in  many  instances  replaced  other 
methods  of  recording  position  in  trawlers'  log  books 
and  the  practice  of  towing  by  time  intervals  has  been 
discarded  by  some  skippers  in  favour  of  towing  actual 
distances  as  measured  by  microsecond  intervals. 

RADAR 

Incorporation  of  radar  progressed  somewhat  slower  and 
the  first  sets  were  not  installed  until  1954. 

The  trawl  fishermen  were  well  aware  of    the  safety 


rig.  I.     Schooner  type  trawler  (Neah  Bay.  Washington). 

[318] 


U.S.     PACIFIC     COAST    TRAWLING 


advantages  of  this  device  and  the  possibilities  of  "around 
the  clock"  fishing.  Apparently,  the  basic  cost  has  been 
the  major  reason  why  radar  has  not  been  installed  aboard 
a  larger  number  of  vessels.  In  the  Pacific  Northwest  area 
about  25  per  cent,  of  the  offshore  trawlers  are  now 
equipped  with  radar,  using  wave-lengths  of  3  or  10  cm. 

F1SHFINDERS 

The  cathode  ray  tube  presentations,  which  allow  the 
operators  to  select  and  expand  the  echo  trace  for  a  par- 
ticular depth  interval,  have  been  adopted  by  only  a 
small  portion  of  the  fleet.  The  ability  of  these  devices 
to  locate  fish  and  increase  catches  is  not  as  yet  definite 
and  mixed  opinions  exist  between  trawl  fishermen  who 
have  used  them.  Interpreting  echo  traces  requires  con- 
siderable experience,  and  increased  yields  which  might 
result  from  the  ability  of  these  devices  to  detect  ground 
fish  is  dependent  on  the  operator's  proficiency,  as  they 
leave  considerably  more  for  the  fishermen  to  evaluate 
than  either  Loran  or  radar.  In  spite  of  this  handicap, 
some  skippers  who  have  used  fishfinders  are  convinced 
of  their  ability  to  locate  schools  of  cod  or  rockfish. 


DEEP  WATER  TRAWLING 

It  is  difficult  to  asses  the  individual  or  cumulative  effect 
that  various  electronic  devices  have  had  on  the  trawl 
fisheries.  It  is  obvious,  however,  that  accurate  offshore 
navigation,  the  ability  to  survey  quickly  the  bottom 
contours,  and  to  maintain  desired  depths  and  position, 
have  increased  the  fishermen's  knowledge  of  fishing 
grounds  and  subsequently  the  ability  to  harvest  ground 
fish.  These  aids  have  undoubtedly  played  an  important 
role  in  the  recent  expansion  of  trawling  to  deeper  waters 
along  the  continental  slope.  Active  deepwater  trawling 
started  off  the  Northern  Californian  coast  shortly  after 
World  War  II  and,  by  1950,  trawlers  were  prospecting 
deeper  banks  off  the  states  of  Oregon  and  Washington. 
Increased  deep-water  fishing  is  demonstrated  by  records 
maintained  by  the  State  of  Washington  Department  of 
Fisheries.  Prior  to  1950,  less  than  5  per  cent,  of  the 
annual  catch  was  estimated  to  have  been  caught  at 
depths  greater  than  100  fm.  By  1954,  close  to  25  per  cent, 
of  the  total  catch  was  being  harvested  at  depths  ranging 
between  100  and  200  fm.,  and  during  1956  trawlers 
often  reported  catches  at  depths  exceeding  250  fm.  Off 


Fig.  2.     Seiner  type  trawler  (Seattle,  Washington). 

f3191 


MODERN    FISHING    GEAR    OF    THE    WORLD 


Eureka,  Californian  vessels  have  prospected  at  depths 
exceeding  300  fm.  This  expansion  to  offshore  grounds 
was  made  feasible  and  practical  by  electronic  aids. 

NETS 

Trawl  nets  in  use  along  the  Pacific  Coast  vary  consider- 
ably in  size  and  design  according  to  the  geographic 
areas  and  racial  background  of  the  fishermen.  Both 
the  eastern  two-piece  net  and  the  western  four-piece 
trawls  are  common  and,  in  Central  California,  fishermen 
of  Italian  descent  use  a  modification  of  the  paranzella 
net  for  otter  trawling.  Nets  vary  in  cut,  twine  size, 
width  and  length  according  to  fisherman  preference 
but  are  essentially  similar  to  nets  used  in  all  the  world's 
major  trawl  fisheries.  Cotton  nets  have  been  in  use  since 
the  inception  of  the  fishery,  but  the  use  of  imported 
manila  and  hemp  nets  has  increased  during  the  past 
five  years.  Between  1950  and  1953,  nylon  codends  were 
tried  on  board  several  northwest  trawlers,  but  the  gravel 
and  hard  bottom,  which  are  typical  of  many  trawl 
grounds  in  this  area,  caused  serious  abrasion  and  wear 
of  the  nylon  and  most  fishermen  returned  to  the  use 
of  cotton. 

The  only  notable  change  in  net  usage  has  been  the 
adoption  of  larger  meshed  nets  or  codends  in  confor- 
mance  with  mesh  regulations  promulgated  by  Pacific 
Coast  fisheries  agencies.  A  minimum  of  4 A  in.  stretched 
mesh  (with  minor  variations  between  States)  is  now 
adopted  as  a  regulation  along  the  Pacific  Coast. 

MECHANICAL  AIDS 

Two  innovations  of  trawl  gear  developed  since  1950, 
and  unique  to  the  Pacific  Coast  States,  are  the  trawl 
cable  meter  and  drum  trawlers.  Both  were  pioneered  and 
developed  in  the  Puget  Sound  area  of  Washington  State. 

CABLE  METER 

The  first  trawl  cable  meter  was  designed  around  the 
specific  needs  of  Puget  Sound  trawlers  by  the  Olympic 
Instrument  Laboratories,  Vashon,  Washington.  The 
meter,  which  has  become  increasingly  popular  among 


local  fishermen,  replaces  the  use  of  cable  markers  to 
determine  the  amount  of  warp  spooled  off  winch  drums. 

DRUM  TRAWLERS 

During  1954  several  Puget  Sound  trawlers  changed  over 
to  this  method.  Drum  trawlers  use  a  power  driven  drum 
similar  to  that  used  with  the  "drum  seine",  but  with  a 
somewhat  smaller  spool.  In  operation,  the  otter  boards 
are  brought  to  the  gallows,  the  wings  of  the  net  shackled 
to  the  reel  core,  and  spooled  on  the  drum.  The  codend 
is  picked  up  in  the  conventional  manner,  with  slight 
variations.  The  drums  are  powered  by  hydraulic  drive 
and  are  designed  with  four  speeds  forward  and  reverse. 
Operators  of  drum  trawl  vessels  report  that  they  can 
fish  in  heavier  weather  and  can  haul  the  net  more  quickly 
than  the  conventional  trawlers.  Drum  trawling  has 
eliminated  restacking  the  net  and  no  lifts  of  the  net 
are  necessary  prior  to  bringing  the  catch  aboard.  The 
drum  trawl  method  of  fishing  has  been  patented  and  its 
use  up  to  the  present  time  has  been  restricted  to  vessels 
owned  by  the  designers. 

SUMMARY 

Progressive  changes  which  have  occurred  in  a  group 
of  Puget  Sound  trawlers  from  1952  to  1956  arc  sum- 
marized below. 

The  addition  of  better  fishing  gear,  new  net  designs, 
electronic  aids,  increased  mechanization,  better  vessel 
design  and  increased  power  have  all  combined  to  increase 
trawl  efficiency.  Theoretically,  boat  efficiency  might  be 
expressed  (mathematically)  as  the  cumulative  total  of 
the  efficiency  contributed  by  each  item  to  the  fishing 
effort.  The  most  important  variable  in  such  an  equation, 
which  influences  the  efficiency  of  any  item  or  the  total 
efficiency  of  the  gear  and  vessel,  is  the  experience  of  the 
crew  and  skipper.  Fishing  knowledge,  experience, 
and  intelligence,  therefore,  remain  the  key  factors. 


TABLE  I 

Gear  changes  for  20  trawlers  fishing  during   1952  to   1956 
(Puget  Sound  area) 

Number  ami  percentage  of  vessels  equipped 

with  indicated  electronic  devices 
Equipped  with  1952  1955  1956 

Per  Per  Per 

Number   cent.    Number    cent.   Number   cent. 


Echo  sounder             20 

100 

20 

100 

20 

100 

Radio  D/F    . 

17 

85 

19 

95 

19 

95 

Loran 

6 

30 

20 

100 

20 

100 

Radar 

0 

00 

5 

25 

5 

25 

Fishfinder 

0 

00 

4 

20 

5 

25 

Cable  meters 

0 

00 

2 

10 

6 

30 

Drum  trawlers 

0 

00 

2 

10 

3 

15 

1952 

7955 

1956 

Average  maximum  Mshable  depth 

144 

240 

280 

Average  maximum  h.p. 

144 

152 

158 

*  Average  number  of  meshes  in 
circumference  of  net  (at  throat) 


406 


370 


370 


Fig.  3. 


"Drum  Trawler"  Sunbeam. 
Washington). 


Stern  view  (Seattle. 


'Smaller  number  of  meshes  resulted  from  increase  in  mesh  size. 


[320] 


STERN   TRAWLING   VERSUS   SIDE   TRAWLING 

by 
C.  BIRKHOFF 

Bremerhaven,  Germany 


Abstract 


This  paper  concerns  the  problems  connected  with  stern  trawling  and  also  gives  account  of  the  reasons  which  have  led  to  tne  develop- 
ment of  this  type  of  operation.  Design  of  the  vessel  and  arrangement  of  the  deck -gear  are  inseparable  from  the  operations  of  shooting  and 
hauling  of  the  gear.  The  author  describes  the  deck  arrangement  of  auxiliary  gear,  together  with  the  mode  of  operation  on  board  different 
types  of  stern  trawlers:  the  Fairtrv  and  the  fair  free,  built  in  Britain,  and  the  Puschkin  class,  the  Heinrich  Meins  and  the  Carl  Kampf  built 
in  Germany.  The  advantages  of  the  transverse  gantry  with  roving  pulleys,  over  the  usual  gallows  are  especially  pointed  out. 


Resume 


Comparaison  du  chalutage  par  Parricrc  et  du  chalutage  sur  le  cdt£ 


L'auteur  examine  les  problemes  quc  pose  le  chalutage  par  Tarriere  et  expose  les  raisons  qui  ont  conduit  a  la  misc  au  point  de  ce 
type  de  pechc.  Les  plans  du  bateau  el  la  disposition  des  engins  de  pom  sont  conditionnds  par  les  operations  de  mise  £  1'eau  et  de  rclevage  du 
chalut.  L'auteur  decrit  la  disposition  des  appareils  auxiliaires  sur  le  pont  ainsi  que  la  manoeuvre  a  bord  de  quatre  types  tlifle rents  de  navires 
equipes  pour  le  chalutage  par  Tamere:  le  Fair  try  et  le  t'airjree,  const  mils  en  Grande  Bretagne,  les  bateaux  de  la  classe  Puschkin,  le  Heinrich 
Meins  el  le  Carl  Kampf  constants  en  Allcmagne.  II  soulignc  lout  particulierement  les  avantages  du  portique  transversal  a  poulies  coulis- 
santcs,  sur  les  potences  con  vent  lonncl  les. 


Extracto 


I1'!  arrastre  de  la  red  por  la  popa  y  por  el  costado  de  la  embarcacion 


Este  trabajo  trata  de  los  problemas  relacionados  con  el  arrastre  de  la  red  por  la  popa  y  describe  las  razones  quc  han  inducido  a 
perfeccionar  este  lipe  de  operacion.  M  proyccto  del  barco  y  la  distribucion  del  cquipo  de  cubierta  son  inseparables  de  las  mamobras  que 
requiere  el  calamcnto  y  recogida  de  Ui  red.  Por  este  motivo,  el  autor  describe  la  distribucion  y  funcionamiento  del  equipo  auxiliar  montado 
en  la  cubierta  de  cuatro  tipos  dc  urrastreros  que  remolcan  la  red  por  la  popa:  el  Fairtryy  el  Fuirjree,  construidos  en  Gran  Brclana.  el  Puschkin, 
el  Heinrich  Meins  y  el  Carl  ka nip/  en  Alcmania,  haciendo  notar  las  vcruajas  de  un  puentc  transversal  con  polcab  dc  vaiven  sobre  los  pcscantes 
de  arrastre  comuncs. 


THE  design  of  trawlers  is  undergoing  a  fundamental 
change  at  present.  Instead  of  the  single-deck 
trawler  arranged  for  handling  the  gear  over  the 
side,  recent  developments  point  towards  the  use  of  shelter- 
deck  vessels  arranged  for  handling  the  trawlgear  over 
a  ramp  at  the  stern.  This  rather  radical  deviation  from  a 
fishing  method  which  had  become  standard  practice 
throughout  most  of  the  world,  was  caused  by  the  desire 
to  convert  trawlers,  designed  exclusively  for  catching 
and  transporting  the  fish,  into  factory-trawlers.  Pro- 
cessing of  the  catch,  the  increased  number  of  men 
required  and  the  placing  of  additional  machinery,  raised 
space  problems,  the  solution  of  which  required  either 
sacrificing  fish  hold  capacity  or  gaining  space  by  adding 
a  shclterdeck,  or  a  "semi-shelterdeck",  i.e.  on  one  side 
only  or  a  trunk  deck. 

All  these  proposals,  however,  when  combined  with 
the  usual  method  of  handling  the  net  over  one  side 
of  the  ship,  meant  difficulties  in  placing  the  trawl-winch 
and  leading  the  warps  and,  moreover,  seriously  curtailed 
the  space  on  the  working  deck  for  handling  the  catch  in 
a  continuous  flow. 

The  "side  trawler"  must  heave  the  gear  on  the  wind- 
ward side  to  avoid  drifting  over  the  net.  In  this  position 


she  is  parallel  to  the  waves,  which,  of  course,  in  bad 
weather  causes  heavy  rolling  of  the  ship.  These  "intended" 
movements  are  of  material  assistance  to  the  crew  when 
pulling  in  the  belly  of  the  net  but  are,  at  the  same  time, 
a  serious  disturbance  when  the  trawler  is  fitted  out  with 
conveyor  belts  and  processing  machinery. 

Such  considerations  caused  the  British  designers  of 
factory  ships  to  search  for  ways  and  means  of  handling 
the  trawl  gear  over  the  stern.  It  took  quite  a  while  to 
overcome  the  scepticism  of  the  conservative  fishermen 
towards  this  new  idea  but,  due  to  the  successful  trials  on 
the  British  factory  trawler  Fair  try  and  the  numerous 
Russian  factory  trawlers  of  the  Pushkin  class,  developed 
by  the  Howaldtswerke,  Kiel,  the  way  is  now  paved  for 
a  more  general  application  of  the  stern  handling  method. 
The  fear  that  the  fish  would  be  squashed  when  pulling 
the  codend  up  over  the  stern-ramp  proved  unfounded. 
The  normal  round-fish  required  neither  roller  conveyors 
(as  proposed  by  the  Stiilckenwerft,  Hamburg),  nor  a 
lifting  platform  at  the  stern  (proposed  by  Dr.  Lehmann) 
nor  an  inclined  conveyor  belt  (proposed  by  the  writer 
for  sardines),  nor  a  large  diameter  roller  at  the  upper 
end  of  the  ramp  as  used  on  the  Fairfree  and  the  Fairtry 
(proposed  by  Sir  Dennis  Burney).  Instead,  a  simple  ramp, 


f  321  ] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


without  any  moving  parts,  proved  absolutely  sufficient 
when  having  a  parabolic  cross  section  at  the  lower  end 
and  a  large-radius  rounding  at  the  deck  end. 

The  method  of  handling  the  intricate  trawl  gear  over 
the  stern  has,  however,  caused  many  difficulties.  All 
yards  and  owners  have  tried  various  ways  and  have, 
when  possible,  put  them  under  patent  protection.  The 
most  difficult  problem  to  solve  was  a  method  of  shifting 
the  pull  of  the  warps  from  the  gallows  to  the  stern  ramp 
and  vice  versa. 

On  the  Fairfree  and  Fairtry,  auxiliary  wires  (sweepline 
gilsons)  were  used  to  haul  the  sweepline  from  the  trawl- 
doors  to  the  ramp  while,  at  the  same  time,  the  warps 
were  slackened  (see  fig.  la). 

The  complete  manoeuvre  of  hauling  is  carried  out  as 
follows:  The  warps,  leading  from  the  gallows  at  the  side 


Fig.  la.    Arrangement  of  using  sweepline-gilsons  on  the  Fairtry 
(patented). 


of  the  stern  and  over  two  fairlead  bollards  to  the  winch 
drums,  pull  the  boards  up  to  their  lifted  position.  The 
boards  are  then  hung  up  and  disconnected  from  the 
warps  and  sweeplines,  which  are  connected  by  the 
pennants.  On  each  side  of  the  slip-deck  and  the  ramp, 
auxiliary  wires  (sweepline-gilsons)  are  carried  around 
the  corresponding  sides  of  the  stern,  connected  to  the 
forward  end  of  the  sweepline  and  heaved  in  by  means  of 
warping  heads  on  the  ends  of  the  trawl  winch.  At  the 
same  time  the  warps  are  slackened  to  the  necessary 
extent.  This  manoeuvre  heaves  the  net  on  to  the  main 
deck  until  the  danlenos  reach  the  winch.  The  free  length  of 
deck  is  about  20  to  25  m.  (70  to  80  ft.)  which  is  not 
sufficient  to  heave  the  entire  net  aboard  in  a  single 
operation  as  the  net  is  about  double  this  length.  The 
sweeplines  are  therefore  clamped  to  the  deck  to  free  the 
warping  drums  for  a  second  pull  again  by  means  of 
gilsons  until  the  codend  has  been  hauled  on  deck. 

On  the  Fairfree  *  which  is  a  converted  corvette  of  the 
British  Navy,  the  deck  runs  continuously,  with  small 
sheer  right  through  from  stem  to  stern.  The  deck  is 
rounded  off  on  the  sides  of  the  ramp  and  underneath 
the  gallows,  which  are  placed  to  both  sides  of  the  stern 
and  inclined  over  the  sides.  An  observation  bridge  for 
directing  the  manoeuvres  is  arranged  right  over  the 
ramp  (fig.  2).  To  reduce  the  friction  of  the  net,  the  top 
end  of  the  ramp  was  fitted  with  a  long  roller  of  large 
diameter.  Vertical  fairlead  rollers  flank  the  sides  of  the 
gap  between  the  low  end  of  the  ramp  and  the  observation 
bridge  to  guide  the  warps  when  fishing  along  "steep 
edges"  where  much  turning  is  required.  These  rollers, 
proposed  by  Sir  Dennis  Burney,  are  useful  also  for 
leading  the  various  wires  between  the  gallows  and  the 
ramp. 

The  experience  of  the  Fairfree  formed  the  basis  for  the 
design  of  the  Fairtry  which  has  since  become  the  prototype 
of  subsequent  factory  trawlers.  Fairtry  has  a  shelterdeck 
extending  almost  her  entire  length,  which  ends  in  a  step 
down  to  the  main  deck  shortly  forward  of  the  gallows. 
The  gallows  arc  placed  at  both  sides  near  the  stern  but 
somewhat  further  forward  than  on  the  Fairfree.  The 
fishing  observation  bridge  is  placed  forward  of  the  upper 
end  of  the  ramp,  and  the  vertical  fairlead  rollers  at  the 
low  end  of  the  ramp  are  omitted  (see  fig.  Ib).  The  24 
factory  trawlers  of  the  Pushkin-class  followed  in  1954/55. 


Fig.+lb.'£.Stern  view  of  the  prototype  factory  trawler  Fairtry. 


Fig.  2    Safety  rollers  for  stern-ramp  on    Fairfree,   ace.    to 
Burney  (patented). 


[322] 


STERN     TRAWLING     VERSUS     SIDE    TRAWLING 


/?#.  J.     Leading  warps  with  roller-trollies  ace.  to  method  of 
"Kieler  Howaldtswerke  AC."  (patented). 

Here  again  we  find  the  shelterdeck  ending  shortly  forward 
of  the  stern  with  a  step  down  to  the  main  deck.  The 
ramp  rises  from  the  water  surface  to  the  height  of  the 
shelterdeck.  Instead  of  leading  the  warps  over  gallows 
rollers,  they  are  supported  by  rollers  attached  to  trollies 
which  run  on  horizontal  rails  fitted  to  either  side  of  the 
ramp  and  extending  to  the  rear  end  of  the  vessel  (fig.  3). 
This  arrangement  keeps  the  deck  and  the  ramp  clear 
when  the  warps  arc  fastened  or  unshackled  from  the 
boards  and  allows  shooting  the  trawl  gear  from  the 
ultimate  rear  end  of  the  vessel,  which  is  considered  a 
desirable  convenience. 

After  the  net  has  dropped  down  the  ramp  by  its 
own  weight,  it  is  drawn  away  from  the  ship  by  the  wake 
of  the  vessel  and  pulls  the  trollies,  to  which  the  danlenos 
are  fastened  by  a  chain  and  slip-hook,  to  the  rear  end  of 
the  rails  where  they  are  blocked.  Then  the  chains  from 
the  danlenos  are  released,  the  sweeplines  are  run  out, 
and  the  warps  are  connected  to  the  boards  which  are 
hung  ready  for  fishing.  By  running  out  the  warps  further 
the  boards  spread  the  net  while  sinking  down  to  the 
bottom. 

The  tension  on  the  warps,  their  downward  and 
forward  pressure  on  the  rollers,  allows  the  trollies  to 
move  automatically  forward  to  their  initial  position 
as  soon  as  they  are  unblocked.  The  whole  manoeuvre 
of  shooting  takes  place  without  the  least  deviation  from 
the  fishing  course  and  much  time  is  saved. 

The  trawl  observation  bridge  on  the  Pushkin  trawlers 
was  combined  with  the  bridge  deck,  forming  its  rear  end. 
The  manoeuvres  of  the  gear  on  the  slip  deck  can  be 
easily  observed  and  the  captain  can  move  quickly  from 
the  navigation  bridge  to  the  trawl  observation  bridge. 

In  reviewing  the  above  arrangement,  the  costs  cannot 
be  overlooked  which  makes  desirable  a  simpler  design 
for  smaller  stern  trawlers.  On  two  such  trawlers  built 
in  1956-57  by  the  Rickmerswerft  in  Bremerhaven,  the 
shelter-deck  was  carried  continuously,  i.e.  without  step 
to  the  vessel's  stern.  On  both  vessels  the  portal-gallows 
rollers  design  was  again  used,  the  ramp  is  the  same  and 
the  trawl  boards  are  hung  up  against  the  flat  sections 
of  the  stern  on  either  side  of  the  ramp.  The  face  of  these 
flat  sections  is  inclined  forward  from  the  water  to  the 
deck  and  fitted  with  guide  bars  for  the  boards. 

The  first  of  these  two  boats,  the  Heinrich  Meins, 


Fig.    4.     Transversely    movable   gallows    rollers   on   "Portal- 
Gattows"  ace.  to  method  patented  by  "GHG*",  Bremerhaven. 

built  for  the  GHG  in  Bremerhaven,  shows  a  gantry- 
shaped  gallows  (fig.  4)  extending  from  one  side  of  the 
ship  to  the  other.  The  gallows  rollers  are  hung  on 
trollies  which  traverse  between  two  horizontal  U-beams. 
The  trollies  can  be  blocked  in  side  position  when  the 
trawl  boards  are  hung  up  and  also  in  a  position  near 
the  centre  line  of  the  ship.  They  are  moved  between 
these  two  positions  by  a  system  of  pulling  chains  and 
sheaves.  When  shooting  the  trawl  gear,  and  during  the 
trawling  operation,  the  gallows  sheaves  are  in  their 
central  position  and  support  the  warps.  This  leaves  the 
deck  and  the  ramp  free  for  the  net.  Before  hauling  the 
net,  the  sweeplines  can  be  thrown  down  from  the  open 
gallows  blocks  to  the  ramp  by  an  eccentric  movement  of 
the  sheaves  and  conical  slip  irons.  Towing-up  gilsons 
are  used  for  lifting  the  warps  into  the  gallows  sheaves. 
The  trollies  for  the  gallows  sheaves  are  moved  into  their 
position  near  the  ship's  side  when  lifting  and  hanging 
up  the  trawl  boards  after  hauling,  and  when  fastening 
the  warps  and  sweeplines  to  the  boards  before 
shooting. 

Two  Voith-Schneider  propellers  drive  the  Heinrich 
Meins  from  under  the  fore-ship.  This  feature  allowed 
the  super  structure,  with  the  navigating  bridge,  to  be 
placed  so  far  forward  that  a  free  upper  deck  of  about 
50  m.  (160  ft.)  length  came  behind  the  superstructure. 
The  net  can  be  hauled  up  on  this  deck  in  a  single  heave, 
thus  saving  the  time  of  a  second  heave  by  means  of  a 
gilson. 

The  transverse  gallows  rollers  were  considered  too 
expensive  and  too  complicated  for  rough  service  on  the 
still  smaller  trawler,  Carl  Kampf.  Though  the  overhead 
gantry  is  used,  the  gallows  sheaves  have  a  fixed  position 
over  the  places  for  the  trawl  doors,  which  permits  a 
relatively  simple  handling  of  the  gear,  as  compared  to 
the  handling  aboard  side  trawlers. 

Hauling  of  the  sweepline  gilsons  could  be  done  as 
on  Fairfree  and  Fairtry  by  using  the  side  barrels,  but — 
in  order  to  save  time — two  special  sweepline  gilson  drums 
have  been  fitted  between  the  warp  drums.  The  danlenos 
can  be  hauled  on  deck  up  to  the  trawl  winch  by  these 
drums.  Then  it  is  possible  to  heave  in  the  rest  of  the  net 
with  gilsons  worked  over  the  barrels  on  the  ends  of  the 
winch  (see  fig.  5a). 

The  latest  improvement  is  a  special  hauling  block, 


[  323  ] 


MODERN     FISHING     GEAR     OF    THE     WORLD 


installed  high  enough  so  that  the  codend  can  be  hauled 
in  line  with  the  slope  of  the  ramp. 

On  the  rather  small  Carl  Kdmpf,  the  superstructure 
for  the  navigating  bridge  is  so  short  that  the  trawl 
observation  bridge  is  arranged  immediately  behind  the 
chart-room  (see  fig.  5b). 


At  present  opinions  differ  concerning  the  various 
alternative  methods  of  handling  the  trawl  gear  over  the 
stern.  It  must  be  left  to  practical  experience  in  the  future 
to  find  a  standard  solution  to  this  problem,  combining 
simple,  rugged  construction,  simple  handling  and  easy 
control. 


.- — x" 


Fig.  5a.     Leading  warps  with  "Portal-Gallows"  ami  sweepline- 
gihons  ace.    to   method  oj  "Rickmers    Werft" ,    Bremerhaven 
(patented). 


Fig.  5h.     The  stern-trawler  Carl  Kampf  011  trials. 


Hauling  the  codend  over  the  stern  chute  of  a  large  Russian  factory  trawler. 

[3241 


POWER   REQUIREMENTS   FOR   DEEP   SEA   TRAWLING 

by 
G.  C.  EDDIE 

Food  Investigation  Organization,  Department  of  Scientific  and  Industrial  Research,  Torry  Research  Station,  Aberdeen, 

U.K. 


Abstract 

With  the  present  methods  of  preservation  of  the  fish  and  the  present  rates  of  capture,  the  point  has  almost  been  reached  beyond 
which  increases  in  speed  of  long  distance  trawlers  will  be  uneconomic,  but  if  improved  preservation  methods  were  available,  the  optimum 
speed  might  change. 

In  order  to  get  the  maximum  economic  advantage  from  up-to-date  freezing  methods,  it  may  be  necessary  to  design  ships  with 
machinery  which  has  a  power  output  only  slightly  in  excess  of  that  needed  for  trawling,  since  the  need  for  very  fast  runs  to  market  would  be 
reduced.  Observations  of  the  h.p.  developed  under  various  fishing  conditions  were  made  on  three  different  trawlers  and  the  tentative  con- 
clusion is  that  about  600  s.h.p.  are  required  for  a  deep  sea  trawler  towing  the  trawl,  and  a  maximum  of  750  or  800  s.h.p.  are  needed  for  run- 
ning free.  This  paper  should  provoke  discussion  on  the  economics  of  deep  sea  trawling. 


Resume 


La  puissance  ncrcssaire  pour  le  chalutagc  hauturier 


En  raison  des  progres  realises  dans  la  preservation  des  poissons  et  I'efficacite  des  engins  de  peche,  on  a  presque  atteint  la  limite 
au-dela  de  laquelle  Paccroissement  dc  la  vitesse  des  chalutiers  hauturiers  n'est  pas  rentable;  mais  si  Ton  pouvait  ameliorcr  ies  methodes  de 
preservation,  la  vitesse  optima  pourrait  s'en  trouver  modifier. 

Pour  tirer  Ic  maximum  d 'a vantages  6conomiques  des  systemes  modernes  de  congelation,  il  peut  etre  n£cessaire  de  dessiner  des 
navires  dont  la  machinerie  n'a  qu'une  puissance  legerement  supenctirc  a  celle  qu'exigent  Ies  operations  de  chalutage,  car  la  necessite  des 
retours  a  tres  grande  vitesse  au  port  de  debarqucment  serait  moias  impcrieuse.  Des  observations  de  la  puissance  en  c.v.  absorbee  dans 
differentes  conditions  de  pdche  pnt  ete  cfifectuees  sur  trois  chalutiers  et  Ton  a  conclu  provisoirement  qu'un  chalutier  hauturier  avait  besom 
d'unc  puissance  dc  600  C.V.  environ  pour  tirer  le  chalut,  et  un  maximum  de  750  &  800  C.V.  pour  naviguer.  Cette  etude  pourrait  servir  de 
document  de  base  a  des  debuts  sur  Peconomie  du  chalutage  hauturier. 


Potencia  rcquerida  para  la  pesca  dc  arrastrc  en  aguas  profundas 
Extracto 

Con  los  metodos  actuates  de  prescrvacion  del  pcscado  y  el  volumen  de  las  capturas  casi  se  ha  alcanzado  un  punto  sobre  del  cua 
seria  anti-economico  aumentar  la  marcha  de  los  arraslreros  dc  altura;  no  obstante  al  disponer  de  metodos  de  conservaci6n  adecuados  podria 
cam  hi  arse  la  vclocidad  6ptima. 

A  tin  de  obtcncr  al  maximo  beneficio  economico  de  los  modernos  sistemas  de  congclacion,  seria  necesario  proyectar  barcos  con 
maquinaria  de  una  potcncia  ligcramcnte  superior  a  la  requerida  para  las  facnas  de  arrastre,  por  rcducir  la  neccsidad  de  viajes  de  rcgreso 
rapidos  a  las  bases  de  operaciones. 

A  bordo  de  tres  arrastreros  de  altura  dife  rentes  se  hicieron  observaciones  para  dc  term  mar  la  potencia  desarrollada  en  las  diversas 
condiciones  de  pesca,  Megan  dose  a  la  conclusi6n  provisional  de  que  un  barco  de  este  tipo  requiere  600  C.V.  al  frcno  en  el  lance  y  un  maximo 
de  750  a  800  C.V.  durante  la  navjgacibn  normal.  Sc  estima  que  este  trabajo  debe  provocar  una  discusion  sobre  los  aspectos  econ6micos  de 
a  pesca  de  amistre. 


THE  efficiency  of  exploitation  of  a  fishery  can  be 
measured  by  such  criteria  as  the  weight  of  fish 
landed  per  man-day  of  effort,  per  ton  of  fuel 
consumed,  per  unit  of  capital  employed  and  per  unit 
cost  of  maintenance  of  ship  and  fishing  gear.  By  con- 
verting each  of  these  into  monetary  terms  it  is  possible 
to  establish  both  a  basis  for  economic  analysis  of  the 
fishing  operations  and  a  criterion  of  overall  efficiency. 
The  design  of  the  fishing  gear  and  its  method  of  use 
affect  the  rate  of  catch;  in  addition,  such  factors  as 
distance  of  the  fishing  grounds  and  method  of  preserva- 
tion of  the  fish  can  affect  the  number  of  crew,  the  fuel 
consumption  and  the  size  of  the  ship. 

The  interaction  of  catching  rate,  distance  and  method 
of  preservation  is  well  known.  The  ships  of  the  British 
distant-water  fleet  return  on  average  with  about  a  half 
load  of  fish  because  of  the  limitations  of  crushed  ice 


as  a  means  of  preserving  the  catch.  The  size  of  the  ships 
is  determined  partly  by  considerations  of  weather; 
the  real  advantage  of  the  larger  ships  is  the  ability  to 
maintain  speed  in  bad  weather.  Speed  can  reduce  the 
time  to  and  from  the  fishing  grounds  during  which  men 
and  capital  are  idle,  but  causes  increased  capital  outlay, 
maintenance  costs  and  increased  fuel  consumption, 
the  latter  being  the  biggest  single  item  in  the  costs  of 
a  distant-water  trawler. 

EFFECT  OF  PRESERVATION  METHODS  ON  THE 
SPEED 

Improvements  to  methods  of  preservation  are  usually 
directed  towards  the  quality  of  the  fish,  but  the  economic 
benefits  of  improved  quality  are  not  easily  forecast 
whereas  additional  costs  of  the  new  methods  are  often 


[325] 


MODERN     FISHING     GEAR     OF    THE    WORLD 


obvious.  Freezing  at  sea,  for  instance,  is  more  costly 
than  the  present  method  of  icing.  However,  freezing 
at  sea  can  give  direct  economic  advantages,  for  instance, 
by  increasing  edible  weight  landed  per  unit  of  fishing 
effort  e.g.  by  avoiding  weight  losses  in  stowage,  condem- 
nations, and  the  necessity  to  send  surplus  fish  to  the 
fish  meal  factory.  Yet  another  possibility  is  that  the  use 
of  a  freezing  plant  would  tend  to  reduce  the  need  for 
high  speed,  and  it  was  the  examination  of  this  point 
which  led  to  the  observations  recorded  below. 

With  the  present  size  and  quality  of  catches,  the  point 
has  almost  been  reached  beyond  which  increases  in 
speed  in  long  distance  trawlers  will  be  uneconomic, 
since  the  extra  length  and  power  required  would  make 
costs  excessive  in  relation  to  the  value  of  the  landings. 
With  an  improved  method  of  preservation  the  optimum 
speed  might  change.  For  instance,  if  the  use  of  antibiotics 
in  association  with  crushed  ice  were  found  to  be  feasible 
it  would  extend  the  keeping  time  or  "shelf  life"  of  the 
fish  by  some  days,  and  the  period  of  fishing  might  be 
extended.  Speeds  could  then  remain  as  high  as  at 
present,  or  they  might  even  increase  very  slightly,  since 
the  extra  catch  might  justify  higher  costs.  If  fishing  were 
extended  by  a  shorter  period  than  the  extension  of 
**shelf  life",  as  seems  more  reasonable,  then  some,  or 
all,  of  the  time  gained  might  be  used  for  only  slightly 
slower  but  much  more  economical  running  from  the 
fishing  grounds  to  the  home  port. 

Freezing  renders  the  fish  virtually  imperishable  and  in 
this  extreme  case  it  would  be  necessary  to  consider 
Arctic  trawler  designs  with  the  whole  range  of  possible 
powers  down  to  a  lower  limit  set  by  the  requirements  of 
trawling.  The  optimum  power  for  a  given  size  of  freezing 
trawler  might  be  quite  near  the  minimum;  freezing  plant 
uses  up  space  and  capital  but  this  might  be  saved  by 
adoption  of  smaller  main  engines  and  fuel  tanks.  It 
may  be  economically  attractive  to  fit  a  freezing  plant 
into  a  hull  of  185  ft.  b.p.  or  very  little  more.  The  adoption 
of  freezing  need  not  then  bring  the  problem  of  physical 
size  of  ship  and  length  of  voyages  associated  with 
factory  trawlers.  Gradual  development  in  size  of  ship 
can  take  place  as  and  when  this  seems  worth  while. 

The  immediate  problem  is  to  determine  what  shaft 
horse  power  and  thrust  are  really  necessary  in  the  trawling 
condition.  Opinions  seem  to  vary;  a  figure  of  800  s.h.p. 
is  often  quoted  as  necessary  for  a  long-distance  trawler 
and  often  figures  as  high  as  1,200  h.p.  are  mentioned. 
Knowledge  based  on  actual  records  seems  to  be  scarce. 

Such  knowledge  would  also  be  useful  to  the  designer 
of  multi-engined  trawlers.  It  is  desirable  to  trawl  on 
fewer  than  the  maximum  number  of  engines  available, 
and  it  would  obviously  be  more  satisfactory  if  the  choice 
of  engines  could  be  such  that  those  actually  in  use 
during  fishing  were  running  either  at  maximum  efficiency 
or  near  maximum  economic  power. 

TRAWLING   POWER 

Some  efforts  were  made  by  the  staff  of  the  Torry  Research 
Station  to  determine  trawling  powers.  There  was  no 
attempt  made  at  a  systematic  study  of  the  effects  of 
different  gear,  or  varying  depths,  weather  and  catch, 
or  of  the  design  of  propellers,  since  these  subjects  are 
not  within  the  terms  of  reference,  but  some  of  the 


observations  may  be  of  interest  to  the  appropriate 
research  organizations. 

Vessel  A  is  the  Sir  William  Hardy,  a  steel  trawler 
of  130  ft.  b.p.  This  is  far  smaller  than  a  typical  distant- 
water  trawler,  but  the  trawling  gear  itself  is  of  the  same 
size,  perhaps  slightly  heavier.  Special  facilities  were 
available  for  making  accurate  measurements  of  power 
and  speed,  so  that  the  power  developed  by  a  larger 
trawler  could  be  deduced  from  powers  measured  on  this 
vessel  when  trawling  and  running  free. 

The  readings  were  taken  during  a  series  of  special 
trials  conducted  jointly  by  the  British  Shipbuilding 
Research  Association,  the  Ship  Division  of  the  National 
Physical  Laboratory  and  the  Torry  Research  Station. 
The  propulsion  machinery  is  diesel-electric  and  power 
readings  were  taken  by  sub-standard  electrical  instruments 
specially  fitted  for  the  trials.  Speeds  were  taken  by  the 
ship's  Pitometer  log,  calibration  curves  for  this  instru- 
ment being  available  from  the  comprehensive  measured 
mile  trials  which  formed  the  greater  part  of  the  whole 
series.  Wind  speeds  were  measured  by  cup  anemometer. 

The  trawling  tests  were  carried  out  in  fine  weather 
in  a  depth  of  30  fathoms  on  a  smooth  and  level  bottom 
of  estuarine  mud.  The  method  was  to  tow  at  constant 
engine  settings  into,  across  and  down  the  wind,  the 
ship  being  kept  on  each  course  for  a  sufficient  time  to 
get  a  series  of  steady  readings.  On  the  fourth  side  of  the 
square,  the  engine  settings  were  altered  to  give  increased 
trawling  speed.  In  this  way  figures  were  obtained  at  three 
different  speeds,  using  a  trawl  with  a  rope  bosom.  The 
highest  speed  runs  were  repeated,  using  the  same  trawl 
with  15  inch  diameter  spherical  steel  bobbins. 

Fig.  1  shows  the  results  which  are  recorded  in  Table  I. 
The  effects  of  tide  have  not  been  eliminated  and  this 
might  explain  why  there  is  little  difference  in  the  per- 
formances across-wind  and  down-wind.  The  placing 
of  the  points  relating  to  the  run  with  bobbins  is  not 
compatible  with  those  for  the  rope  bosom,  the  down- 


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Fig.  /.     Relation  between  power  requirements  and  towing  speed. 


[3261 


POWER     REQUIREMENTS    FOR     DEEP    SEA    TRAWLING 


TABLE  I 
Series  A 

Ship's 

Wind  Speed 

Speed 

Shaft 

r.p.m. 

Depth 

and  Direction            Sea 

Knots 

h.p. 

fm. 

off  Bow 

Trawl 

with  Rope 

Bosom 

4 

•5 

274 

133-9 

29 

17 

4K—  180° 

4 

•5 

266 

131-7 

29 

22 

•4K     80  S 

3 

•9 

264 

131  9 

32 

23 

2K-  KVP 

Moderate  swell 

4 

•9 

352 

146-5 

29 

15 

•5K     1  65  'S  from  about 

4 

•9 

356 

145-3 

30 

23 

IK  -70S 

221°  (true) 

4 

•4 

356 

145-7 

31 

24 

5K-10"P 

wave  height 

5 

•0 

426 

154-9 

29 

11 

OK     160"Sabout4ft. 

5 

•1 

427 

154-2 

30 

22 

3K—  70US 

4 

•6 

427 

154-5 

30 

23 

•2K     10'P 

Trawl  with  Bobbins 

4 
5 

•7 
•3 

422 
419 

153-1 
152-9 

31 
30. 

15 
21 

5K—  I65°S 
8K-   70°S 

Also  choppy 

5 

•0 

418 

154-3 

32 

21 

5K    5  P 

surface 

wind  being  the  slowest  instead  of  the  run  into  the  wind. 
It  was  found  that  the  bobbins  had  been  digging  into  the 
mud  so  possible  explanations  are  that  the  net  gradually 
filled  with  mud  during  the  runs,  or  that  the  bobbins 
dug  in  more  at  some  times  than  at  others. 

The  power  required  to  drive  the  ship  running  free  at 
speeds  from  three  to  five  knots  was  measured  in  the 
speed  trials  and  found  to  be  15  s.h.p.  at  3  knots,  25  at  4 
knots  and  40  at  5  knots  in  a  flat  calm  and  at  the  condition 
of  the  trawling  tests.  Allowing  for  reduced  efficiency, 
slight  seas  and  change  of  trim,  the  power  required  to 
drive  the  ship  alone  (when  trawling  at  4  knots)  would 
not  be  more  than,  say,  40  s.h.p. 

It  is  proposed  that  these  observations  be  supplemented 
by  readings  taken  during  normal  voyages,  this  task 
being  facilitated  by  the  calibrated  electrical  horse  power 
meters,  logs  and  r.p.m.  indicators.  Some  idea  of  the 
effects  of  depth  and  wind  would  then  be  obtained. 
As  far  as  can  be  deduced  from  readings  taken  up  to  the 
time  of  writing,  the  extra  torque  required  to  trawl  into 
a  force  8  wind  would  result  in  an  increase  in  power  of 
less  than  100  s.h.p. 

In  the  meantime  it  has  been  possible  to  take  readings 


TABLE  II 

Series  B 

Ship's  Speed 
Knots 

Shaft 
h.P. 

r.p.m. 

Depth 
fm. 

Wind  Force  and 
Direction  off  Bow. 

Approx.  3  —  4 

475 

100 



2  to  3—    45°S 

525 

100 

90-150 

6  t  o7  —    45°S 

320 

90 

— 

6         —  145°P 

500 

100 

— 

4          —    70°S 

560 

100 

._. 

6  to  7  —      0° 

390 

92 

90 

3         —    70nS 

480 

99 

100 

4         —    70°S 

500 

95 

60-80 

5               100°S 

450 

90 

100 

— 

500 

95 

— 

6         — 

550 

95 

— 

7  to  8  — 

500 

95 

— 

2  to  3  — 

during  normal  voyages  in  two  commercial  trawlers  of 
about  180  ft.  b.p.  One  trip  was  made  in  each  vessel 
primarily  for  the  study  of  fish  spoilage,  and  readings 
of  trawling  powers  and  speeds  were  taken  only  when 
convenient. 

Table  II  gives  the  readings  taken  on  a  trip  to  Iceland 
and  Faroe  in  Vessel  B,  a  diesel-electric  trawler  of  190  ft. 
b.p.  The  figures  for  ship's  speed  arc  very  approximate 
and  no  great  reliance  should  be  placed  upon  them. 
The  propulsion  system  had  a  variable-torque  character- 
istic. 

The  level  of  power  developed  is  not  very  different  from 
that  of  the  small  ship  in  shallow  water.  A  power  of 
560  s.h.p.  was  observed  when  towing  into  a  force  7  wind. 

Vessel  C  was  a  steam  trawler  of  181  ft.  b.p.  with  triple- 
expansion  engines  and  a  Bouer-Wach  exhaust  turbine. 
The  readings  taken  during  a  trip  to  Bear  Island  are 
given  in  Table  III.  Power  was  measured  by  a  Siemens 
torsion  meter  and  ship's  speed  both  by  SAL  log  and  by 
timing  the  passage  of  a  floating  object. 

Again,  the  general  level  of  powers  developed  is  similar 
to  that  in  Ship  A.  Again,  the  torque  probably  varies 
from  one  reading  to  the  next,  since  the  normal  engine 
setting  when  towing  is  well  below  maximum  torque  and 
the  settings  were  varied  to  give  constant  r.p.m.  as  near 
as  possible. 

The  last  two  tests  in  Table  III  were  an  experiment  in 
which  maximum  torque  was  applied.  It  is  interesting 
to  note  that  nearly  normal  maximum  power  (which  is 
1,300  s.h.p.)  could  be  developed  under  these  conditions. 
In  spite  of  the  threefold  increase  in  power,  the  speed  of 
trawling  rose  by  only  20  per  cent,  in  relation  to  the  most 
strictly  comparable  runs  at  normal  settings.  Judging  by 
the  other  observations  from  ships  A,  B  and  C  (fig.  2), 
the  increase  in  power  absorption  is  to  be  attributed  to 
the  propeller  working  at  a  point  of  low  efficiency  rather 
than  to  a  sudden  increase  in  the  drag  of  the  gear  between 
4-2  and  5-0  knots,  but  this  is  not  conclusive;  it  is  possible 
that  a  sharp  increase  in  the  drag  of  the  gear  used  in 
ship  C  occurs  at  a  lower  speed  than  for  the  gear  used 
by  ship  A. 

The  result  would  explain  the  opinions  that  800  to  1200 
horse-power  is  developed  by  a  big  trawler  when  fishing. 


TABLE  III 
Series  C 

Ship's 

Speed 

Knots 

Shaft 
h.p. 

r.p.m. 

Depth 
fm. 

Wind  Force 
and  Direction 
off  Bow 

Sal.  Log 

Floating 
Object 

3-5 



435 

78 



0                0 

3-4 

._ 

406 

76 

170 

0                 0 

3-5 

— 

462 

79 

230 

0                0 

4-2 

.„_ 

417 

78 

80 

0                0 

3-4 

— 

405 

78 

.,- 

3-8 

4-0 

380-425 

78 

160-180 

0                0 

3-2 

3-27 

480 

80 

76 

4     —    45  P 

4-3 

4-2 

430 

78 

76 

4            45°P 

4-0 

— 

412 

77 

76 

4     —     180° 

3-7 

36 

430 

78 

76 

4             180° 

4-8 

1230 

110 

100 

0                0 

5-2 

— 

1270 

110 

100 

0                0 

[327] 


MODERN     FISHING     GEAR     OF    THE    WORLD 


,_  ^^ 

MB.   _      __    —J~,     _„     _  _     _  ___« 


1  * 


1 

1                      1 

1 

4 

s 

/•iff.  2.     Relation  between  power  requirements  and  towing  speed. 

It  had  been  assumed  earlier  by  the  author  that  these 
figures  were  rational  deductions  from  the  propeller 
r.p.m.  known  to  be  employed  when  trawling,  on  the 
assumption  that  maximum  torque  was  used.  It  may  be 
that  in  some  cases  high  torques  are  used. 

If  some  large  trawlers  do  indeed  have  to  develop  800 
to  1,200  s.h.p.  to  trawl  effectively,  it  indicates  either  great 
variation  in  the  quality  of  propeller  design,  which  is 
hardly  likely,  or  in  the  rigging  of  the  trawls.  The  experi- 
ment recorded  above  shows  however  that  a  vessel  with 
a  propeller  of  high  enough  efficiency  in  the  trawling 
condition  to  allow  of  powers  being  kept  as  low  as  400  h.p. 
can  be  operated  so  inefficiently  that  the  power  absorption 
rises  to  1 ,250  h.p. 

Since  there  is  more  than  adequate  torque  for  fishing 
available  on  a  big  trawler,  the  choice  of  engine  settings 
is  to  a  large  extent  at  the  mercy  of  arbitrary  decisions. 
In  British  trawlers  the  skipper  chooses  the  r.p.m.  Many 
years  ago  the  largest  trawlers  were  steam  vessels  of  the 
size  and  power  (600  s.h.p.  maximum)  of  the  vessel  A. 
Such  a  vessel  had  to  develop  maximum  torque  in  order 
to  produce  the  thrust  required  to  trawl  at  acceptable 


speeds;  the  r.p.m.  were  then  about  two  thirds  to  three 
quarters  of  the  maximum  r.p.m.  when  running  free. 
As  the  ships  have  increased  in  size  and  power,  the  skippers 
have  continued  to  trawl  at  the  same  r.p.m.,  and  by  many 
skippers  r.p.m.  is  still  regarded  as  a  direct  measure  of 
power.  It  is  possible  that  a  slight  improvement  in 
efficiency  in  either  the  running  free  condition,  or  in  the 
trawling  condition,  or  in  both,  might  be  achieved  if 
the  designer  of  the  propeller  were  allowed  a  free  choice 
of  r.p.m.  for  trawling.  Ship's  speed  and  the  thrust 
required  to  pull  the  trawl  at  that  speed  would  have  to  be 
known. 

Guidance  on  the  best  speed  for  towing  a  trawl  would 
be  very  welcome.  Given  favourable  conditions,  sufficient 
torque  and  a  suitable  propeller  trawls  could  be  towed 
at  much  higher  speeds  than  usually  achieved  by  large 
British  commercial  distant-water  trawlers.  It  has  been 
suggested  that  by  towing  too  fast  the  configuration  of  the 
gear  is  upset  or  that  the  gear  is  lifted  off  the  seabed.  The 
skipper  of  ship  C  was  unwilling  to  make  the  maximum 
torque  tests  as  he  expected  the  gear  to  be  damaged. 

CONCLUSIONS 

The  tentative  conclusions  to  be  drawn  from  the  observa- 
tions recorded  in  this  paper  are  that  a  deep  sea  trawler 
towing  a  trawl  of  conventional  design  and  with  a  suitably 
designed  propeller  needs  to  be  able  to  develop  500  s.h.p., 
or  600  s.h.p.  at  most.  Given  constant-power  characteris- 
tics a  total  power  of  600  s.h.p.  would  be  ample.  Given 
constant-torque  characteristics  perhaps  750  or  800  s.h.p. 
maximum  when  running  free  would  be  required. 
Any  power  installed  in  excess  of  this  figure  should  be 
economically  justified  by  the  extra  speed  it  produces 
in  service.  The  size  of  the  ship  has  relatively  little  effect 
on  the  power  requirements.  It  is  quite  easy  to  increase 
fuel  consumption  of  a  big  trawler  by  a  large  amount 
with  relatively  little  effect  on  the  speed  of  the  trawl. 
It  is  not  known  if  this  effect  is  due  entirely  to  the  propeller 
working  at  a  point  of  low  efficiency  or  to  a  sudden  increase 
in  the  drag  of  the  trawl,  the  increase  occurring  at  lower 
speeds  for  some  trawls  than  others.  Information  would 
be  welcomed  on  the  optimum  speed  of  trawling  and  on 
the  thrust  required  to  low  the  gear  at  that  speed.  Improve- 
ments in  propeller  design  might  follow  as  well  as  more 
accurate  knowledge  on  the  power  requirements  for  deep 
sea  trawling. 


328] 


FLEET  OPERATION   OF  TRAWLERS   WITH   A   MOTHERSHIP 

by 

C.  BIRKHOFF 

Bremerhaven,  Germany 

Abstract 

Two  future  possibilities  in  distant  water  fishing  are  the  self-producing  factory  trawler  and  the  big  mothership  with  accompanying 
trawlers.  The  main  drawback  of  the  latter  has  been  the  difficulty  of  transferring  the  catch  from  the  trawler  to  the  mothership  especially 
in  rough  seas.  This  paper  shows  how  this  difficulty  can  be  overcome,  by  using  detachable  codcnds  which  can  be  left  floating  in  the  sea 
attached  to  a  buoy  and  a  radar  float,  later  to  be  picked  up  by  the  mothership.  The  handling  of  the  fish  would  be  done  entirely  on  the  parent 
vessel,  which  would  allow  the  trawlers,  equipped  with  numerous  spare  codends,  to  devote  more  time  to  fishing,  and  the  adoption  of  such  a 
method  might  lead  to  changes  in  the  design  of  trawlers,  for  the  large  fish  holds  would  no  longer  be  necessary. 


Les  operations  d'unc  flottille  dc  chalutiers  avec  un  bateau-mere 

II  y  a  deux  possibilites  futures  pour  la  pechc  dans  les  eaux  eloignees:  le  chalutier  navire-usine  pechant  lui-mcmc  et  1?  gros  bateau- 
mere  accompagne  de  chalutiers.  Lc  principal  inconvenient  de  ce  dernier  a  etc  la  difficult^  dc  transf<6rer  la  pechc  du  chalutier  an  bateau-mere, 
particulierement  par  mcr  forte.  Le  present  article  montrc  comment  ccttc  difficulte  pent  etre  surmontec  en  utilisant  dcs  culs-dc-chalut  amovibles 
quc  Ton  peut  laisser  flotter  dans  la  mcr,  attaches  a  une  bouee  et  a  un  flott cur-radar  pour  etre  ra masses  plus  tard  par  le  bateau-mere.  Lc 
traitement  du  poisson  serait  cflectue  entierement  sur  le  bateau-mere,  ce  qui  permettrait  aux  chalutiers  munis  de  nombreux  cuh-de-chalut  de 
rechangc  de  consacrcr  plus  de  temps  a  la  pechc.  L'adoption  d'une  telle  methode  pourrait  conduirc  d  changer  le  dessin  des  chalutiers  etant 
donnd  que  les  grandes  cales  a  poisson  nc  scraient  plus  necessaires. 

Explotacion  de  arrastreros  que  trabajan  con  un  buque  fftbrica 
Extracto 

Las  posibilidades  futuras  de  la  pesca  de  altura  son  cl  arrastrero-fabrica  y  el  gran  buque  madre  dc  varios  arrastreros.  HI  principal 
inconveniente  de  este  ultimo  reside  en  la  dificultad  de  transferir  la  pesca  de  la  embarcacion  quc  la  captura  al  buque  madre,  especialmentc  con 
mar  gruesa.  Fn  el  presentc  trabajo  se  demuestra  la  manera  dc  cvitar  csta  dificultad  usando  copos  separables  quc  pcudcn  dcjarsc  flotando 
en  el  mar  atados  a  una  valiza  o  boya  de  radar  a  fin  dc  scr  posteriormente  recobrudos  por  el  buque  madre.  Al  manipular  la  pesca  en  esta 
cmbarcacibn,  los  arrastreros  provistos  de  numerosos  copos  ticnen  ticmpo  dc  cfcctuar  mas  lances.  La  adopcion  de  dicho  mctodo  puedc 
inducir  a  modificar  cl  proyccto  de  estos  ultimos  barcos,  en  atenci6n  a  quc  no  seria  necesario  disponcr  dc  grandes  bodegas  para  el  almacen- 
amiento  del  pcscado. 


WHEN  comparing  the  possibilities  of  self-pro- 
ducing factory  trawlers  and  big  factory  mother- 
ships  with  accompanying  trawlers,  the  decisive 
argument  against  the  latter  has  been  hitherto  the  unsolved 
problem  of  handing  over  the  catch  in  rough  sea,  especially 
in  the  Atlantic  regions.  Because  of  this  difficulty,  and 
despite  economic  considerations,  the  use  of  the  factory 
trawler  has  been  given  preference.  But  there  are  increasing 
indications  that  experts  realize  that  the  development 
of  fishing  techniques  would  be  considerably  influenced 
if  the  problem  of  handing  over  the  catch  from  the  trawler 
to  the  main  vessel  could  be  solved.  So  far,  little  success 
has  attended  efforts  to  solve  this  problem.  Some  of  the 
methods  which  have  been  used  are  as  follows: 

TRANSFER  BY  MEANS  OF  DERRICKS 

For  this  method  shelter  is  needed  as  offered  by  islands 
or  nearby  coasts,  so  it  is  restricted  to  certain  areas. 
Furthermore,  this  method  presents  a  problem  when 
hauling  up  a  well  filled  codend  as  there  is  a  big  risk  that 


the  net  might  split.  At  present  this  technique  is  used 
by  the  Japanese,  Americans  and  Russians. 

TRANSFER  BY  MEANS  OF  TACKLE  RUNNERS 

Poland  and  the  USSR  some  time  ago  adopted  the  method 
of  using  tackle  runners  with  containers.  The  vessels  lie 
alongside,  protected  by  means  of  several  big  triple-tyre- 
fenders.  This  method  underlies  the  same  restrictions  as 
those  just  mentioned. 

TRANSFER  BY  MEANS  OF  ELEVATORS 

Also  this  method  can  be  practised  in  calm  sea  only. 
The  substantial  loss  in  quality,  moreover,  restricts  the 
use  of  this  method  to  ships  producing  fish  meal,  as,  for 
instance,  the  Norwegian  factory  ship  Clupea. 

TRANSFER  BY  MEANS  OF  PIPELINES 

One  of  the  earliest  experiments  with  flexible  pipes  and 
a  transfer  pump,  the  "Yeoman  Pump",  took  place  in 


[329] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


Gallows 
Fig.  L     Common  Trawler  with  "Roscher  Biinn". 

the  USA,  but  this  method  cannot  be  maintained  in  rough 
seas  because  of  constant  leakages  at  the  pipe  connections. 
It  is  evident  that  all  these  methods  are  useless  for 
our  fishing  regions  because  of  the  generally  unfavourable 
weather  conditions. 

TRANSFER  BY  MEANS  OF  SPECIAL  NET  BAGS 

An  interesting  proposal,  made  by  Roscher,  is  based  on 
the  provision  of  a  stern  chute.  According  to  this  plan, 
the  fish  should  be  gutted  aboard  the  trawler  and  then 
put  into  a  reservoir  net  equipped  with  a  lightbuoy  and 
directional  transmitter  (fig.  1 ).  These  operations  demand 
the  same  number  of  crew  on  the  trawlers  as  before  and, 
as  a  factory  mothership  is  used,  there  is  no  saving  of 
labour. 

Unfortunately,  this  proposal  has  not  been  put  to  a 
practical  test  as  yet  so  no  judgement  can  be  made  with 
regard  to  its  performance.  Nevertheless,  the  proposal 
certainly  presents  a  possible  practical  method  for  dis- 
posing of  additional  catches  of  older  trawlers  which  are 
not  equipped  with  a  fish  meal  plant. 

TRANSFER    BY    MEANS    OF    DETACHABLE 
CODENDS 

If  hauling  up  the  codend  and  gutting  the  fish  were  done 
by  the  factory  ship,  the  crew  of  the  trawlers  could  be 
reduced.  Such  a  development  could  lead  to  specially 
constructed  trawlers  having  no  fish  holds.  This  could 
be  effected  if  it  were  possible  to  transfer  the  ungutted 


catch  to  the  factory  ship  in  the  codend  itself,  which, 
furthermore,  would  be  much  less  bulky  than  the  heavy 
reservoir  proposed  by  Roscher. 

For  this  purpose,  the  net  is  divided  transversely  across 
the  lengthening  piece  (fig.  2)  and  thimbles  are  attached 
to  the  two  sets  of  joining  meshes.  A  rope  passing  through 
these  thimbles  joins  the  two  parts  together  and,  in  a 
matter  of  a  few  seconds,  the  codend  can  be  released. 

The  trawler  hauls  the  net  only  as  far  as  necessary  to 
get  hold  of  the  connection  so  as  to  detach  the  codend 
which,  before  releasing,  is  then  securely  closed  and  to 
which  a  buoy,  equipped  with  radar  reflector  and  a  lamp 
is  attached  (fig.  3).  As  the  first  codend  floats  away, 
waiting  to  be  picked  up,  another  codend  is  laced  to  the 
lengthening  piece  and  fishing  begins  again. 

Practical  experiments  on  the  German  research  vessel, 
Anton  Dohrn,  have  proved  the  possibility  of  trawling  with 
such  a  detachable  codend  as  well  as  the  practical  handling 
of  the  catch  by  this  method.  A  thorough  examination 
of  the  quality  of  the  fish  after  varying  periods  of  drifting 
has  indicated  no  loss  in  quality  if  the  fish  are  processed 
immediately  after  they  are  taken  aboard  the  ship,  which 
is  what  we  are  aiming  at. 

Thus  the  method  seems  to  be  suitable  for  transferring 
the  catch  even  under  the  unfavourable  sea  conditions 
prevailing,  for  instance,  in  the  North  Atlantic,  and  we 
are  now  in  a  position  to  start  fundamental  studies  with 
regard  to  new  types  of  fishing  vessels. 

OPERATION  OF  TRAWLER  FLEETS 

The  Russian  trawler  fleet,  like  the  first  English  fleets, 
uses  the  flotilla  system,  with  a  "buoy  boat"  searching 
for  a  good  fishing  ground.  As  soon  as  a  new  ground  has 
been  found,  the  trawlers  gather  there,  but  one  vessel 
remains  on  the  old  ground  until  the  new  one  is  found 
to  produce  good  catches. 

THE  PACIFIC  EXPLORER 

The  flotillas  of  the  American  and  Japanese  factory 
motherships  work  in  a  similar  manner,  and  there  are 
reports  that  the  Pacific  Explorer  a  freighter  which, 
at  the  instigation  of  the  American  Government  during 


The  "NEKRASOV",  one  of  the  new  refrigerator  trawlers  of  the  Murmansk  trawler  fleet. 

[330] 


TRAWLER     FLEET    OPERATION    WITH     MOTHERSHIP 


BELLY  AND  SQUABC 
CONNECT/ON 


COD  END 


LEHGTHENER 


Fig.  2.     Shows  the  construction  of  the  trawl  with  a  detachable  codend. 


the  last  war,  was  converted  to  a  factory  ship  has  been 
employed  as  mothership  for  10  smaller  vessels  catching 
halibut  and  king-crab  in  Alaska  for  periods  of  90  sea- 
days.  The  fishing  vessels  are  often  chartered  at  ports  near 
to  the  fishing  grounds.  These  search  prospective  fishing 
grounds,  which  were  marked  by  buoys  until  the  flotilla 
could  concentrate  in  one  spot.  During  certain  radio 
service  hours  there  was  a  routine  exchange  of  information 
between  the  units  of  the  flotilla. 

Because  of  the  rather  small  size  of  the  fishing  vessels 
and  the  bad  weather,  the  fishing  had  to  be  interrupted 
for  about  20  per  cent,  of  the  time.  The  catch  was  trans- 
ferred by  means  of  derricks,  the  operation  was  carried 
out  under  the  lee  of  nearby  islands.  The  main  problem 
for  the  factory  vessel  was  the  varying  amount  of  daily 
catches.  The  plant,  therefore,  had  to  work  intermittently 
and  the  crew  had  to  be  given  a  catch  premium  to  com- 
pensate for  this  differing  volume  of  work.  While  the 
watch  on  board  was,  as  usual,  four  hours  twice  a  day, 
the  plant  watch  was  seven  hours  twice  a  day  when  the 
hauls  were  successful.  The  premium  system  provided 
for  an  increase  of  25  per  cent,  of  the  hourly  rates  if 
the  work  lasted  for  at  least  two  hours  uninterruptedly. 
This  led  to  the  situation  that  the  crew  refused  to  take 
over  a  smaller  haul  because  they  would  not  take  two 
hours  to  deal  with  the  catch. 

American  Labour  Unions  have  laid  down  a  clear 
division  of  work  in  these  factory  ships  and  ha ve  forbidden 
the  deck  crew  to  work  in  the  plant,  even  when  the  plant 
was  short-handed. 

The  long  periods  of  employment  of  the  vessels 
subject  to  the  season-  called  for  extended  time  in  port 
when  the  crews  constantly  leave  for  other  jobs.  On  every 
trip  new  men  had  to  be  trained.  The  whole  undertaking 
in  the  USA  was  suffering  because  the  crews  could  not 
be  tied  to  the  ship  and,  furthermore,  the  Labour  Unions 
did  not  grant  special  regulations. 

THE  MORSKA   WOLA 

Another  report  concerns  the  Polish  flotilla  experiment 
in  herring  fishing  with  the  Morska  Wola~-a  former 
German  motorship  of  3350  GRT  and  10  knots  speed— 


which  took  place  after  the  initial  Russian  trials  in  1953. 
The  long  distance  from  the  Polish  ports  to  the  herring 
fishing  grounds  in  the  North  Sea,  for  example,  the 
Fladen  Ground,  led  to  the  idea  of  releasing  the  trawlers 
from  transport  tasks.  The  catches  of  herring  were 
salted  in  barrels  for  supply  to  the  canning  industry. 
Special  mention  is  given  to  the  fact  that  there  was 
absolutely  no  indication  that  the  fishermen  refused 
to  cooperate  with  the  factory  ship. 

The  experience  gained  at  the  Fladen  Ground  suggested 
that  the  cutters  did  not  work  satisfactorily  as  they  were 
constantly  in  need  of  technical  repair.  The  factory 
mothership  was  supplied  from  20  to  30  fishing  vessels, 
and  the  transfer  of  the  catches  by  means  of  tackle 
runners  was  very  much  subject  to  weather  conditions. 
At  wind  force  2  to  3  Beaufort,  an  average  of  one  barrel 
per  minute  could  be  handed  over.  Consequently,  the 
transfer  of  the  full  load  of  a  fishing  vessel  of  trawler 
or  lugger  size  took  about  five  hours. 

Radio-telephone  communication  was  established 
between  the  Morska  Wola  and  the  fishing  vessels.  The 
fishermen  received  weather  reports  by  radio  and,  in 
cases  of  sickness,  medical  instructions  were  given  from 
the  mothership.  Furthermore,  appointments  for  the 
transfer  of  catches  were  made  by  radio,  and  repairs  to 
the  fishing  vessels  were  executed  by  expert  workers  from 
the  mothership  ferried  over  by  launches. 

THE  RUSSIAN  FLOTILLAS 

The  reports  of  the  Russian  flotillas  show  that  a  start 
was  made  in  1951  from  Lithuanian  ports  with  two 
trawlers  operating  in  the  North  Atlantic.  Today  there 
are  100  trawlers  and  six  big  factory  ships  operating  from 
these  ports  and  other  factory  ships  operating  from  the 
port  of  Murmansk.  Factory  vessels  with  landing  space 
for  helicopters  are  now  being  built  at  Memel  and  in 
Poland  while  other  fish  transport  vessels  for  Russia  are 
being  built  in  Sweden  and  Eastern  Germany,  all  of  which 
indicates  the  long-sighted  planning  devoted  to  building 
up  the  flotilla  system  in  the  USSR. 

The  Russian  reports  again  and  again  refer  to  the  fact 
that  the  tremendous  increase  in  their  herring  production 


[331] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


depends  on  the  use  of  these  factory  motherships  as  a 
"base".  The  average  time  these  ships  spend  in  port  for 
landing  the  catches,  repairs,  and  taking  in  provisions 
amounts  to  a  third  of  a  year. 

The  short-term  conversion  of  freighters  for  seasonal 
employment  in  the  fishery,  however,  is  being  violently 
criticized  in  Russia,  while  it  also  seems  that  the  rather 
sudden  and  extensive  employment  of  flotillas  has  shown 
organizational  discrepancies.  The  use  of  several  factory 
motherships  without  having  fishing  vessels  definitely 
allotted  to  them  has  made  it  difficult  to  control  their 
work,  with  the  result  that  the  pressure  of  work  on  the 
factory  ships  was  irregular,  and  at  times,  because  the 
supply  of  barrels  was  insufficient,  fishing  had  to  be 
stopped.  The  great  difficulties  in  transferring  the  catches 
or  barrels  are  repeatedly  mentioned  in  the  reports.  It 
is  also  revealed  that  the  ships,  operating  outside  the 
three-mile  zone,  in  the  lee  of  the  Faroe  Islands,  had 
constantly  to  change  position  with  every  shift  of  the 
wind.  When  this  was  not  done,  there  were  so  many 
accidents,  that  the  ships'  hospitals  became  overcrowded 
and  patients  had  to  be  sent  to  the  Thorshavn  hospital. 
One  cannot  dismiss  the  project,  however,  because  of 
these  "growing  pains"  and,  indeed,  the  new  Russian 
plans  are  expected  to  lead  to  much  better  organization. 

RECENT  AND  FUTURE  DEVELOPMENTS 

This  flotilla  system  has  been  practised  by  the  Japanese 
for  a  long  time  with  success,  because  the  transfer  of  the 
catch  holds  no  difficulties  thanks  to  numerous  islands 
within  the  catching  region.  The  Russians  now  intend 
to  have  the  factory  motherships  enlarged  into  a  "'catching 
base"  equipped  as  a  shore  station  with  accommodation, 
such  as  hotel,  post  office,  theatre,  movie  theatre,  etc. 
Poland,  having  rented  a  pier  of  the  Cuxhaven  fishing 
port,  prefers  the  advantage  of  this  shore  station  for 
transferring  the  herring  catch  to  transport  vessels, 


while  other  countries  endeavour  to  establish  such  shore 
stations  near  to  the  fishing  grounds  for  white  and  red 
fish  as,  for  instance,  in  Greenland  or  Spitzbergen.  There 
is  even  talk  about  constructing  artificial  islands  for  use 
as  ports  and  air  bases. 

For  West  Europe,  where  the  herring  grounds  are 
fairly  near,  flotilla  operations  would  mainly  be  needed 
for  white  and  red  fish.  The  main  hindrance  to  such 
flotilla  operation  has  been  overcome  now  that  the  prob- 
lem of  handing  over  the  catch  has  been  solved  by  using 
detachable  codends.  Each  trawler  in  the  flotilla  group 
should  work  with  hauls  of  equal  duration  but  the  timing 
of  the  hauls  should  be  staggered,  so  that  the  factory 
ship  can  steadily  pick  up  the  floating  codends  and  main- 
tain continuous  processing. 


Fig.  3.    Detachable  codemi  prepared  for  drifting. 


Mothership  loading  up  catches  from  the  fishing  vessels  on  the  high  seas  at  Sakhalin  Island. 

[332] 


MIDWATER   TRAWLS   AND  THEIR   OPERATION 

by 

B.  B.  PARR1SH 

Marine  Laboratory,  Torry,  Aberdeen,  Scotland 

Abstract 

These  trawls  are  a  comparatively  recent  development  and  although  certain  types  are  being  used  successfully  in  many  different  fisheries, 
their  design  can  still  be  said  to  be  in  a  state  of  evolution.  The  general  pattern  and  use  of  the  trawls  arc  dependent,  to  a  much  greater  extent 
than  in  most  other  gears,  on  the  behaviour  and  biology  of  the  species  for  which  they  arc  intended.  This  paper  deals  fully  with  the  main 
biological  factors  which  are  necessary  for  the  successful  use  of  the  mid-water  trawls  and  also  with  the  general  principle  of  their  design.  The 
two  types  of  trawl,  those  operated  by  single  vessels  and  those  towed  between  two  vessels,  are  fully  described  and  the  author  concludes  with 
the  suggestion  that  perhaps  the  most  profitable  future  development  is  towards  the  dual-purpose  bottom  and  midwater  trawl  that  can  be  used 
alternatively  on  or  off  the  seabed.  In  this  way.  fish  like  the  herring,  cod  and  mackerel  which  inhabit  various  water  levels,  can  be  exploited 
to  the  full. 

Le?  chaluts  flottants  et  leur  manoeuvre 
Resume 

Ccs  chaluts  sont  de  creation  relativement  reccnte  et  bien  que  certains  types  soient  utilises  avec  succes  dans  un  grand  nombre  de 
peches  differentes,  on  pent  dire  qu'ils  sont  encore  en  pie  me  evolution.  La  conception  gdnerale  et  1'emploi  des  chaluts  dependent,  dans  une 
mesure  beaucoup  plus  grande  quc  pour  la  pi  apart  dcs  autres  engins,  du  comportemcnt  et  de  la  biologie  dc  1'espece  de  poisson  a  la  peche  de 
laqucllc  ils  sont  destines.  L'auteur  examine  les  principaux  facteurs  biologiques  qu'il  est  necessaire  de  con  na  it  re  pour  obtenir  de  bons  rcsultats 
avec  les  chaluts  flottants,  ainsi  quc  les  pnncipes  generaux  de  leur  conception.  II  d6crit  en  detail  les  deux  types  de  chaluts  (les  uns  manoeuvres 
par  un  scul  bateau,  et  les  autres  remorques  entre  deux  navires)  et  il  concha  en  suggerant  que  le  chalut  de  1'avenir  qui  donnerait  les  meilleurs 
resultats  sera  it  probablement  celui  qui  pourrait  etre  utilise  a  volont6  au  fond  ou  entre  deux  eaux.  On  pourrait  ainsi  exploiter  completement 
les  especes  de  poissons  t  el  les  que  le  hareng,  la  moruc  et  le  maquereau  qui  vivcnt  a  des  niveaux  differcnts. 

Las  redes  dc  arrastre  para  profundidades  intermedia*;  y  sus  usos 
Kxtracto 

Puede  decirse  que  algunos  tipos  de  estas  redes  de  arrastre.  ideadas  comparativamente  hace  poco  tiempo  y  usados  con  exito  en 
diversas  pesquerias,  estan  todavia  cvolucionando. 

Las  caracteristicas  generales  y  el  uso  de  estos  artes  dcpenden  de  la  reaction  y  biologia  de  las  espccies  a  que  se  destinan.  For 
estos  motivos,  en  el  trabajo  materia  dc  cste  cxtracto  se  estudian,  en  detalle,  los  principals  factores  bio!6gicos  que  son  neccsarios  para  usar 
con  6xito  las  redes  dc  arrastre  destinadas  a  las  pesca  en  profundidades  i n termed i as,  y  el  principio  general  que  rige  la  construcci6n  de  dos 
tipos  dc  redes  de  arrastre,  a  saber:  remolcados  por  una  y  por  dos  embarcaciones.  El  autor  ha  llegado  a  la  conclusidn  de  que  tal  vcz  en  el 
futuro  el  modelo  mas  adccuado  sea  uno  que  permita  tanto  la  pesca  de  fondo  como  a  profundidades  intermcdias,  y  pueda  usarse  junto  al  lech 
mar  o  inmediatamcnte  sobre  este.  Asi  se  podrian  pescar  especies  como  arcnque,  cabal  I  a  y  bacalao  que  viven  a  diverso  nivel. 


INTRODUCTION 

SINCE  the  Second  World  War  midwater  trawls  (also 
known  as  floating  or  pelagic  trawls)  have  been 
introduced  in  the  commercial  fisheries  of  some 
countries  to  exploit  concentrations  of  fish  in  the  water 
layers  away  from  the  seabed.  Midwater  trawling  is 
still  untried  or  unproven  in  many  parts  of  the  world,  and 
is  still  restricted  to  the  exploitation  of  only  a  few  species 
of  fish,  principally  herring,  sprat  and  cod.  Although 
intensive  experimentation  is  being  carried  out,  the 
designs,  detailed  rigs  and  methods  of  operation  of  mid- 
water  trawls  are  still  less  well  defined  than  in  the  ground 
trawls,  which  have  evolved  over  a  much  longer  period  of 
time  to  suit  specific  conditions.  The  design  and  operation 
of  these  two  types  of  trawl  differs  in  important  aspects. 
The  pattern  is  governed  by  the  behaviour  characteristics 
of  the  fish  and  the  milieu  in  which  these  trawls  operate. 

FACTORS    GOVERNING    DESIGN,    OPERATION 
AND   EFFICIENCY  OF  MIDWATER  TRAWLS 

Regulation  of  Fishing  Depth 

A  fundamental  feature  of  midwater  trawling  is  that  the 
zone  of  operation  extends  over  the  whole  water  column; 


the  gear  must  work  at  different  depth  zones  according  to 
the  particular  depth  distribution  of  the  fish.  This  varies 
widely  in  space  and  time.  For  some  species  the  depth 
range  is  very  narrow,  and  constitutes  a  very  small  fraction 
of  the  total  depth  column.  These  features  dictate  the 
fundamental  first  essentials  for  success  in  midwater 
trawling.  These  are: 

(i)    knowledge  of,  or  the  means  for  detecting,  the 

depth  of  the  fish  concentrations; 
(ii)    the  means  for  ensuring  that  the  trawl  operates  at 

the  required  depth. 

Thus,  whereas  most  ground  trawling  is  still  conducted 
without  the  use  of  specific  fish  detection  devices,  their 
use  in  midwater  trawling  is  of  fundamental  importance 
and  midwater  trawling  has  developed  in  close  association 
with  fish  detection  devices,  especially  the  echo  sounder. 
However,  the  provision  of  fish  detection  devices  alone  is 
not  sufficient  to  ensure  success.  The  second  fundamental 
requirement  is  of  equal  importance. 

Sometimes  the  depth  range  within  which  the  fish  are 
concentrated  is  no  greater,  or  very  little  greater,  than  the 
vertical  opening  of  the  trawl  itself,  in  these  instances  very 


[333] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


small  errors  in  depth  regulation  of  the  gear  will  reduce 
seriously  the  size  of  catch. 

Pelagic  fish  concentrations  (e.g.  herring)  also  often 
exhibit  marked  changes  in  depth  distribution  from  time 
to  time  and  from  one  locality  to  another,  and  even  in 
the  course  of  a  single  haul.  Therefore,  not  only  must  the 
operator  be  able  to  set  the  midwater  trawl  at  the  required 
depth,  but  he  may  also  change  its  fishing  depth  during 
the  course  of  a  haul.  Information  on  such  changes  in 
depth  distribution  of  the  fish  can  also  be  gauged  from 
echo-sounding  recordings  taken  during  the  tow.  There- 
fore the  use  of  these  devices  should  be  an  integral  part 
of  each  trawling  operation. 

The  most  direct  way  of  gauging  the  depth  of  the  trawl 
is  to  attach  to  the  trawl  a  suitable  depth  measuring 
device  which  records  continuously  on  board  the  towing 
vessel.  Only  in  this  way  can  the  fisherman  know  at  what 
depth  the  trawl  is  fishing.  Many  such  instruments  have 
been  produced  and  tested  in  recent  years.  They  are  of 
two  main  types:— 

(1)  Ultrasonic  oscillators  or  transducers  as  used  in 
normal    echo-sounding,    which    are    linked    with 
recorders  on  the  towing  vessels  by  an  electric  cable 
and  which  are  fixed  to  a  part  of  the  trawl  and  shot 
with  it  during  trawling  operations.  The  operation 
of  these  instruments  is  essentially  the  same  as  in 
echo-sounding;  records  of  the  depth  of  the  trawl 
from  the  seabed,  the  height  of  opening  of  the  mouth 
of  the  net,  and  the  fish  in  the  vicinity  of  the  mouth 
of  the  net,   are  conveyed  from  the  transducer 
through  the  cable  to  the  recorder  on  board  the 
towing  vessel.  This  type  of  instrument  has  been 
developed  and  marketed  in  the  United  Kingdom 
by  Pye  Marine  Co.  Ltd. 

(2)  Battery-operated  "telemeters"  which  are  fixed  to 
the  trawl  and  transmit  coded  signals.  These  are 
picked  up  on  board  the  towing  vessel  by  a  receiving 
apparatus.  Two  different  transmitting  systems  have 
been  employed  in  instruments  developed,  or  being 
developed,  in  the  United  States,  Germany  and  the 
United  Kingdom.    In  the  American  Depth  Tele- 
meter, developed  at  the  Marine  Laboratory  of  the 
University   of   Miami17,    a   directional,  variable 
frequency  transmitter  produces  beamed  acoustic 
signals  between  21    and  35   kcs.   the  frequency 
changing  with  depth.  These  signals  are  picked  up 
by  the  directional  hydrophone  at  the  ship  and  passed 
to  a  radio  receiver.     The  same  general  system  is 
used  in  the  German  instrument,  but  instead  of 
frequency  modulation  to  indicate  the  depth  of  the 
net,  the  transmitter  works  on  a  fixed  frequency  of 
IS  kc.,  and  emits  groups  of  intermittent  acoustic 
signals,  the  numbers  of  which  vary  according  to 
depth. 

These  instruments  have  proved  efficient  in  experi- 
mental trials,  and  both  types  have  advantages  and  dis- 
advantages. The  first  type  gives  the  most  information 
since  it  records  the  height  of  the  mouth  opening  of  the 
net  and  the  behaviour  of  fish  in  its  vicinity,  as  well  as 
the  depth  of  the  net  in  the  water.  However,  it  has  the 
disadvantage  of  requiring  a  cable  link  with  the  recorder 
on  board  ship. 


Instruments  of  the  second  type  need  no  cable,  but  use 
a  transmitting  system.  For  effective  use,  the  transmitters 
must,  however,  be  directed  towards  the  towing  vessel 
which  places  restrictions  on  the  positions  in  which  they 
can  be  fixed  to  the  gear,  and  the  presence  of  "noise"  from 
other  sources  may  reduce  the  efficiency. 

An  obvious  operational  modification  of  the  first  type 
is  already  receiving  attention,  i.e.  to  tow  the  transducer 
free  from  and  above  the  trawl.  This  reduces  many  of 
the  inherent  handling  difficulties,  since  the  oscillator  can 
be  launched  after  the  trawl  has  been  shot.  It  also  gives 
information  about  the  distribution  of  fish  above  as  well 
as  below  the  headline  of  the  net4 

These  instruments  have  not  yet  become  an  everyday 
part  of  commercial  midwater  trawling  gear,  and  the 
marketing  of  a  relatively  cheap,  reliable,  easily  handled 
and  robust  instrument  for  commercial  use  is  an  urgent 
requirement.  Meanwhile,  the  regulation  of  the  fishing 
depth  of  midwater  trawls  must  be  effected  by  less  direct 
and  often  less  accurate  methods.  The  most  common  is  to 
regulate  the  speed  of  towing  and/or  the  length  of  the 
towing  warp,  based  on  experience. 

Experiments  with  different  trawls  have  been  made, 
using  varying  warp  lengths  and  different  towing  speeds. 
The  depth  of  fishing  has  been  measured  by  recording 
depth  gauges  attached  to  the  net  or  by  echo  sounding 
from  vessels  following  the  towing  vessel  14-  ir>*  1H.  These 
experiments  have  provided  measures  of  the  towing  depth 
and  the  angle  of  the  towing  warps  at  the  ship  for  a  range 
of  warp  lengths  and  towing  speeds.  Tables,  showing  the 
relation  between  these  variables,  and  protractors  for 
measuring  the  angle  of  the  towing  warp,  are  issued  by  the 
manufacturers  of  midwater  trawls  from  which  the 
fishermen  can  gauge  the  length  and  angle  of  warp  to  use 
at  a  given  towing  speed.  The  protractor  can  be  used  at 
intervals  during  each  haul  to  provide  a  rough  check  on 
the  depth  of  the  trawl. 

Experimental  trials  have  shown  that  this  method  is 
only  useful  for  providing  an  approximate  guide  to  the 
depth  of  fishing.  Factors  other  than  towing  speed  and 
warp  length  also  affect  the  depth  of  the  net,  i.e.  the  size 
of  the  warps,  the  detailed  rig  of  the  gear,  the  material 
from  which  the  net  is  constructed,  the  strength  and 
direction  of  the  water  currents,  wind  strength,  and 
possibly  also  the  size  of  the  catch.  It  is  inevitable  that 
sole  adherence  to  the  data  supplied  by  the  trawl  manu- 
facturers may  sometimes  result  in  substantial  errors  in 
fishing  depth.  It  is  advisable  for  each  operator  to  conduct 
his  own  depth  determinations  for  different  warp  lengths 
and  towing  speeds  (most  commonly  measured  in  numbers 
of  engine  r.p.m.),  using  the  complete  rig  where  fishing 
is  to  be  done.  This  can  be  achieved  with  relatively 
simple  and  cheap  depth  recorders,  obtainable  in  most 
countries  today,  or  in  cooperation  with  other  vessels 
equipped  with  echo  sounders.  While  such  trials  will  tend 
to  increase  the  accuracy  of  depth  regulation,  the  need 
for  the  direct  recording  depth  indicators  must  be  stressed, 
because  of  variations  from  haul  to  haul  and  from  one  set 
of  operational  circumstances  to  another. 

IMPORTANCE  OF  BIOLOGICAL  FACTORS 

Factors  related  to  the  behaviour  and  general  biology  of 
pelagic  fish  concentrations,  and  the  differences  between 


[334] 


M1DWATER    TRAWLING 


them  and  bottom  living  fish  are  responsible  for  many  of 
the  departures  in  design  and  operation  of  midwater 
trawls  from  the  basic  ground  trawl  pattern. 

The  biological  features  of  greatest  importance  are 
given  below: 

(i)  Most  pelagic  fish  are  active  and/or  fast  swimmers 
and  probably  react  quickly  and  more  violently  to 
stimuli,  i.e.  the  approaching  trawl,  than  do  species 
inhabiting  the  seabed.  Information  on  the  flight 
reactions  of  pelagic  fish  is  still  very  scanty,  but  echo 
sounder  recordings"*  3  indicate  that  most  of  them 
are  downwards  into  deeper  water,  an  avenue  of 
escape  which  is  available  to  pelagic  but  not  to  the 
bottom  living  species. 

(ii)  Many  of  the  fish  exploited  by  midwatcr  trawls  are 
in  compact  schools,  especially  during  daylight,  and 
respond  as  a  body  to  disturbance  stimuli.  The 
shoaling  habit  is  much  less  marked  amongst 
bottom  living  fish,  which  tend  to  have  a  small 
depth  distribution  and  a  relatively  extensive  area 
distribution,  and  probably  react  more  individually 
to  disturbance  stimuli. 

(iii)  Pelagic  species  have  well  developed  organs  of 
of  sight  and  hearing  (including  those  for  the 
perception  of  low  frequency  vibrations).  These  may 
also  facilitate  their  avoidance  of  the  gear,  parti- 
cularly in  daylight  when  the  warps,  otter  boards 
and  the  net  will  be  seen  more  easily.  In  bottom 
trawling,  the  low  visibility  may  be  made  worse  by 
the  disturbance  of  the  seabed  caused  by  the  otter 
boards,  sweeplines  and  foot  rope.  Accurate  inform- 
ation on  the  part  played  by  vision  and  sound 
perception  in  avoidance  is  very  scanty,  but  they  are 
believed  to  be  of  a  major  importance  and  have  been 
taken  into  serious  consideration  in  the  rigging  of 
midwater  trawls.  They  are  held  responsible  for  the 
failures  encountered  in  midwater  trawling  in  some 
areas  and  with  some  designs  of  trawl.  It  is  signifi- 
cant that  midwater  trawling  has  generally  proved 
more  effective  at  night,  especially  in  regions  of 
clear  water  of  low  phosphorescence  level,  and  has 
been  most  successful  with  "pair"  trawls,  towed  by 
two  vessels,  which  have  no  otter  boards  to  cause  low 
frequency  vibrations.  The  noise  and  propeller 
wakes  from  the  towing  vessels  are  also  removed 
further  from  the  path  of  the  trawl  than  with  the 
one-boat  trawls. 

(iv)  The  temperatures  of  the  upper  water  layers  are 
usually  higher  than  those  near  the  seabed.  This 
tends  to  increase  the  activity  of  pelagic  fish  relative 
to  bottom  fish  inhabiting  the  same  region  and  hence 
stimulate  their  avoidance  ability.  Information  on 
the  effect  of  temperature  on  activity  and  avoidance 
is  also  scanty,  but  it  is  significant  that  most  of  the 
midwater  trawl  fishing  in  Europe  takes  place  in 
winter  when  water  temperatures  are  low. 

Thus,  the  behaviour  and  habits  of  the  fish  probably 
govern  the  effectiveness  of  midwater  trawls  to  a  critical 
degree  and  also  influence  the  design  and  rigging  and 
operation  of  the  gear.  Success  or  failure  can  usually  be 
traced  to  differences  in  behaviour  and  habits  of  the 
species  between  areas  and  seasons.  The  European 
herring  constitutes  a  good  example.  This  species  is 


successfully  caught  by  midwater  trawl  in  a  number  of 
localities,  mostly  during  the  winter  months,  but  results 
are  poor  or  only  moderate  in  the  summer  when  the 
herring  are  probably  more  active  and  are  relatively  widely 
distributed  in  small  compact  schools.  The  most  favour- 
able conditions  for  midwater  trawling  are  probably 
those  in  which  (a)  the  fish  concentrations  (schools  or 
aggregations)  are  fairly  large  and  remain  approximately 
stationary;  (b)  the  fish  are  relatively  "inactive"  either  by 
virtue  of  low  water  temperatures  or  their  physiological 
slate  (in  general,  "spawning"  and  "spent"  fish  are 
probably  less  "active"  than  the  feeders);  (c)  the  fish  do 
not  undergo  rapid  diurnal  depth  migrations,  and  their 
depth  distribution  is  fairly  constant  over  the  fishing 
locality;  (d)  the  water  is  shallow  and  turbid  or,  if  clear, 
contains  low  concentrations  of  phosphorescent  organ- 
isms; (e)  the  light  intensity  is  low. 

While  success  in  midwater  trawling  may,  of  course,  be 
achieved  when  some  of  these  conditions  are  not  satisfied, 
particularly  when  the  fish  concentrations  are  very  large 
and  dense  and  when  large  midwater  trawls  are  towed  at 
high  speeds  by  powerful  vessels,  they  do  define  the 
generally  most  favourable  conditions  and  they  may  serve 
as  a  guide  to  the  potentialities  in  new,  untried  situations, 
and  to  the  trawl  manufacturer  in  the  design  and  construc- 
tion of  the  gear. 

GENERAL    FEATURES    OF    MIDWATER    TRAWL 
DESIGN  AND  OPERATION 

The  general  features  outlined  above  define  the  general 
requirements  in  design  and  operation  of  the  gear.  These 
requirements  can  b~  categorized  as  follows: 

(1)  A  net  with  a  large  vertical  as  well  as  horizontal 
mouth  opening. 

This  feature  is  necessary  in  view  of  the  vertical 
as  well  as  the  horizontal  distribution  of  the  fish 
concentrations,  and  to  increase  stability  of  the  gear 
in  midwater.  In  consequence,  most  midwater  trawls 
have  a  square  or  rectangular  mouth  opening  in 
which  the  depth  is  equal  to  or  little  smaller  than  the 
width.  This  is  achieved  at  the  expense  of  wings, 
which  are  relatively  small  or  absent,  and  by  the 
insertion  of  large  side  panels  between  the  top  and 
bottom  net  surfaces.  The  headline  and  footrope 
are  usually  of  the  same  thickness,  but  to  achieve 
a  large  vertical  opening,  floats  or  shearing  devices 
are  attached  to  the  headline  and  weights  and  or 
depressing  devices  to  the  footropc. 

These  features  mark  the  most  striking  lines  of 
departure  in  general  design  from  the  ground  trawl 
pattern. 

(2)  The  lower  surface  of  the  net  extending  as  far  for- 
ward as  the  upper  surface  (and  possibly  beyond  it). 

This  is  necessary  to  counter  the  suspected  down- 
ward escape  reaction  of  the  fish.  In  most  pelagic 
trawls  in  use  at  present  the  upper  and  lower 
net  surfaces,  and  the  headline  and  footrope,  are 
made  of  equal  length,  but  some  designers  have 
advocated  the  extension  of  the  lower  surfaces  of 
the  trawl  in  front  of  the  upper  surface18. 

This  feature  is  again  in  striking  contrast  to  the 
ground  trawl  design.  The  differences  in  general 


[335] 


MODERN     FISHING     GEAR     OF    THE    WORLD 


shape  between  the  mouth  regions  of  ground  and 
midwater  trawls  are  shown  in  fig.  1 . 
(3)  Smooth  water  flow  characteristics. 

These  are  important  in  both  midwater  and  ground 
trawls,  but  their  importance  is  particularly  great 
in  midwater  trawling,  in  which  turbulence  near  the 
front  of  the  net  may  result  in  violent  flight  responses 
by  the  fish.  To  meet  this  requirement,  and  to 
minimise  the  escape  of  fish  through  the  larger 
meshes  at  the  front  of  the  net,  midwater  trawls  are 
relatively  long,  finely  tapered,  funnel-shaped  bags, 
with  long  extension  pieces  and  codends  which  have 
no  "flappers".  The  shape  and  absence  of  flappers 
reduces  the  likelihood  of  fish  becoming  "meshed" 
in  front  of  the  codend.  The  long  codend  also 


headline  with    floats 
or    elevator    toads. 


foot  rope  r          we  ights 
or  depressor    toads. 

MID- WATER       TRAWL 
headline  with    floats 


upper  t  lower 
wings 


square 


Side  seam 


bosom  of    ground- rope. 


GROUND     TRAWL 
Fig.  L    Mouth  regions  of  midwater  and  bottom  trawls. 


reduces  the  escape  of  fish  from  the  net  during 
hauling  and  facilitates  the  handling  of  large  catches. 

(4)  Fast  towing  speed. 

This  is  especially  important  because  pelagic  fish 
are  fast  swimmers.  Since  midwater  trawling  is  at 
present  carried  out  mainly  by  relatively  small, 
low  powered  vessels,  it  is  important  that  the 
overall  drag  of  the  gear  should  be  reduced  to  a 
minimum  to  permit  the  fastest  possible  towing 
speed.  In  consequence,  midwater  trawls  are 
usually  made  of  the  lightest  materials,  compatible 
with  the  required  strength  and  durability;  the  sizes 
of  ropes  and  warps  are  usually  reduced  to  a  work- 
able minimum,  and  shearing  devices  (otter  boards, 
etc.)  are  reduced  in  size  and  weight  and/or  designed 
to  provide  a  better  lift/drag  ratio.  The  smooth 
water  flow  characteristics  and  absence  of  net 
attachments,  such  as  flappers  and  "cowhides"  or 
"false  bellies",  also  contribute  to  the  overall 
minimizing  of  drag.  Most  midwater  trawls  used 
hitherto  have  been  made  of  different  grades  and 
sizes  of  cotton  twine,  variously  treated  to  reduce 
water  absorption,  increase  the  smoothness  of  water 
flow  and  arrest  deterioration,  but  the  superior 
qualities  of  synthetic  fibres  has  led  to  their  increas- 
ing use. 

(5)  Minimum  visibility,  noise  and  vibration  of  gear 
units. 

These  requirements,  which  are  very  difficult  to 
achieve  with  complete  satisfaction,  minimize  the 
flight  responses  of  the  fish.  Various  rigs  have  been 
adopted,  chiefly  concerned  with  keeping  the  warps 
and/or  the  otter  boards  as  far  from  the  mouth  of  the 
net  as  possible  when  towing,  and  with  improving 
the  overall  stability  and  hydrodynamic  flow  char- 
acteristics of  the  net  and  other  gear  components. 
Measures  taken  have  been: 
(i)  the  use  of  special  shearing-boards,  having  low 

turbulent  flow  characteristics; 
(ii)    mounting  the  otter  boards,  on  side  cables 

attached  to  the  main  towing  warp; 
(iii)  the  adoption  of  the  "pair"  fishing  method 

without  otter  boards. 

TYPES  OF  MIDWATER  TRAWLS 

Present  midwater  trawls  can  be  divided  into  two  opera- 
tional categories:  (a)  trawls  towed  by  two  vessels;  (b) 
trawls  towed  by  a  single  vessel. 

The  former  was  the  first  to  be  introduced  on  a  com- 
mercial scale  with  the  invention  in  1948  of  the  Larsen 
"Atom"  trawl  by  Mr.  Robert  Larsen  of  Skagen,  Den- 
mark. This  trawl,  or  closely  similar  trawls,  has  proved 
successful  commercially  in  Europe  for  catching  clupeoid 
species,  mainly  herring  and  sprat,  and  its  use  has  in- 
creased greatly  in  recent  years  in  some  European 
fisheries. 

The  one-boat  pelagic  trawls  have  appeared  on  the 
scene  since  the  introduction  of  the  "Atom"  trawl.  While 
no  one  type  has  yet  proved  successful  commercially  on  as 
wide  a  scale  as  the  two-boat  trawl,  several  types  have  been 
developed  in  different  parts  of  the  world  and  have 
received  extensive  publicity  following  operational  trials 
and/or  limited  adoption  commercially.  Amongst  the 


[336] 


MIDWATER    TRAWLING 


most  prominent  are:  the  Larsson  "Phantom"  trawl,  the 
Icelandic  "Breidfjord"  trawl,  and  the  British-Columbian 
trawl,  developed  recently  in  western  Canada.  These 
types  also  illustrate  the  more  important  variations 
in  the  design  and  rigging  of  one-boat  midwater  trawls. 
The  feature  of  other  types  of  midwater  trawls,  not 
dealt  with  here,  can  be  found  in  trade  journals  of  most 
countries  and  a  number  of  them  have  been  patented. 

THE  LARSEN  TWO-BOAT  TRAWL 

This  trawl  was  originally  developed  for  use  in  the  herring 
fishery  off  the  coasts  of  Denmark.  The  dimensions, 
design  details  and  method  of  operation  were  planned  for 
the  type  and  size  of  vessels  operating  in  that  fishery, 
and  with  a  view  to  the  minimum  of  change  in  boat  lay-out 
and  equipment.  The  use  of  this  gear  successfully  initiated 
was  then  extended  to  fisheries  in  other  countries,  having 
different  types,  sizes  and  powers  of  vessels  working  under 
different  conditions.  In  consequence,  trawls  of  different 
dimensions  have  been  constructed  and  differences  in  the 
detailed  rig  and  method  of  operation  have  arisen  to 
suit  local  conditions,  vessel  characteristics  and  fisher- 
men's preferences.  One  such  trawl  is  the  "Jet  Fighter", 
designed  for  fishing  for  herring  and  sprats  in  the  Baltic11. 
The  trawls  produced  by  Mr.  Larscn,  or  manufactured 
under  licence,  have  been  designed  for  vessels  ranging 
from  about  40  to  100  ft.  (13  to  30  m.)  in  length  and 
from  40  to  250  h.p.  They  are  available  therefore  for 
almost  all  classes  of  vessels  other  than  the  large  deep-sea 
trawlers.  In  practice,  the  upper  limit  of  vessel  size  for 
this  type  of  gear  is  set  by  economic  considerations  rather 
than  by  inherent  technical  limitations. 

The  Net 

The  Larsen  trawl  net  conforms  with  the  general  form  of 
midwater  trawls  already  outlined.  It  has  a  square  mouth 
opening;  it  is  of  square  cross  section  throughout  its 
length;  it  constitutes  a  long  finely  tapering  bag  termin- 
ating in  a  long  codcnd;  and  it  is  made  throughout  of 
light  material. 

The  nets  at  present  in  use  in  commercial  operations  are 
made  of  varying  grades  of  cotton  or  synthetic  twine  and 
range  in  size  from  about  28  by  28  ft.  to  60  by  60  ft. 
mouth  opening  and  96  to  180  ft.  total  length.  The  smallest 
of  the  sizes  mentioned  above  are  suitable  for  use  on  boats 
of  30  to  50  h.p.,  while  the  largest  are  used  on  vessels  of 
over  200  h.p.  Most  of  the  nets  used  commercially  lie 
between  these  extremes.  The  commonest  sizes  are: 
(a)  nets  with  a  mouth  aperture  of  about  48  by  48  ft.  for 
use  on  vessels  from  30  to  150  tons,  with  engines  from 
100  to  250  h.p.;  (b)  nets  with  a  mouth  aperture  of  about 
36  by  36  ft.  for  use  on  boats  from  20  to  50  tons,  with 
engines  from  50  to  120  h.p. 

The  nets  are  of  simple  design,  consisting  of  four 
sections,  the  top,  bottom  and  two  sides,  of  equal  size  and 
shape,  which  are  attached,  on  assembly,  to  sidelines 
2  to  2\  in.  in  circumference  running  down  the  length 
of  the  net.  The  shape,  with  the  approximate  dimensions 
of  a  single  net  section  for  a  48  by  48  ft.  trawl  is  shown 
in  fig.  2,  and  the  whole  net  is  shown  diagrammatically 
in  fig.  3. 

The  net  sections  (fig.  2),  are  shaped  to  provide  an  even 
taper  throughout  their  length  down  to  the  codend  and 
are  provided  with  short  wing  sections,  so  that  when  in 


action  the  top,  bottom  and  side  leading  edges  of  the  net 
are  swept  back  from  the  four  corner  towing  points.  This 
feature  minimises  differential  strains  on  the  netting 
near  the  mouth,  and  improves  the  water  flow  in  this 
region.  The  leading  edges  of  the  four  sections  are 
attached  to  2  to  2 \  in.  ropes,  that  are  attached  to  the 
top  section  forming  the  headline  and  that  to  the  bottom 
section  the  footrope. 

The  mesh  sizes  vary  according  to  the  size  of  the  net 
and  the  kind  offish  which  is  being  caught,  ranging  from 
5  to  6  in.  (120  to  150  mm.)  stretched  in  the  wing  sections 
to  \  to  |  in.  (12  to  20  mm.)  stretched  in  the  codend.  The 
codend  is  also  usually  covered  by  a  larger  meshed  nylon 
bag  for  protection  and  to  facilitate  handling,  especially 
when  hoisting.  For  this  purpose  also,  the  codend  is  sup- 
plied with  a  ''splitting  strap",  5  to  10  ft.  from  its  end, 
which  consists  of  a  wire  or  rope  strop,  about  12  ft.  long, 
encircling  the  outer  nylon  covering  of  the  codend  and 
passing  through  galvanised  rings  attached  to  the  webbing. 
The  two  ends  of  the  splitting  strap  are  shackled  to  a  rope 


T 


tfimcnilMt  coMpriM 


Ot      «Mct    ll 

footrofM  on4  iMt 


600m 
I  in  6  toper 


2mm 
12/9 


T 


200m 


1 


100 


1 

T 

section 


Fig.  2.     Dimensions  of  single  panel  of  48  ft.  by  48  ft.  Larsen 
"Atom"  trawl.    Lengths  and  widths  of  net  sections  given  in 
number  of  meshes.     Mesh  sizes  in  millimetres. 


(  337  ] 


MODERN     FISHING    GEAR     OF    THE    WORLD 


Fig.  3.     Larsen   net   ami  bridle  attachments    shown    diagrammatical  I y. 


("lazy  line",  "pork  line"),  longer  than  the  total  length 
of  the  net,  which  hangs  free  from  the  net  and  is  attached 
to  one  of  the  lower  towing  corners.  When  the  net  comes 
alongside  the  ship  during  hauling,  this  rope  can  be  used 
to  haul  up  the  codend,  and  the  catch  of  fish  if  large  can 
be  split  into  a  number  of  bags.  A  "pursing  rope"  running 
between  the  bottom  and  top  corners  on  one  side  of  the 
net  is  also  used  by  some  fishermen. 

Floats  and  weights  are  attached  along  the  headline  and 
footrope  respectively  to  produce  the  necessary  vertical 
opening  of  the  mouth  of  the  net.  Some  20  to  30  evenly 
spaced  8  in.  metal  or  plastic  floats  along  the  headline 
and  17  to  18  Ib.  (approximately  8  kg.)  of  weights  dis- 
tributed along  the  footrope,  are  probably  adequate  for 
the  medium  sized  48  ft.  net.  Additional  heavier  weights 
are  sometimes  attached  to  the  lower  towing  corners  and 
along  the  towing  bridles  close  to  the  net,  and  other  larger 
floats  or  "buffs"  are  also  sometimes  attached  to  the  upper 
towing  corners  to  provide  extra  lift. 

The  Bridles 

Each  of  the  four  corners  is  attached  to  a  rope  or  combina- 
tion wire  bridle.  The  bridles  from  the  upper  and  lower 
corners  of  one  side  of  the  trawl  are  attached  to  the  warps 
from  one  of  the  towing  vessels  and  those  from  the  other 
side  of  the  net  to  the  other  vessel  (see  fig.  4). 

The  lengths  of  the  bridles  vary  according  to  the  size 
of  the  trawl,  the  lower  bridles  being  usually  about  2  fm. 
longer,  and  lighter  than  the  upper  ones.  For  the  smaller 
sizes  of  trawl,  the  upper  bridles  are  usually  made  of  2}  in. 
circumference  combination  wire  and  are  16  fm.  long, 
while  the  lower  bridles  are  of  2  in.  combination  wire  and 
18  fm.  long.  For  the  larger  trawls,  the  corresponding 
dimensions  arc: 

Upper  bridles:    26  fm.,  2\  in.  combination  wire;  or 

4  in.  manila  rope; 

Lower  bridles:  28  to  29  fm.,  21  in.  combination  wire, 
or  3.}   in.  manila  rone. 

Eyes  are  sometimes  spliced  at  intervals  along  the  lower 
bridles  for  the  attachment  of  weights. 

One  end  of  each  bridle  is  attached  to  a  corner  of  the 
net  by  a  swivel  which  is  connected  to  a  thimble  at  the 


end  of  the  headline  or  footrope  extensions  with  harp- 
shaped  shackle  (fig.  3).  At  its  other  end,  the  bridle  is 
attached  to  the  towing  warp  with  a  special  sliphook 
(fig.  3).  Between  sliphook  and  swivel  of  the  lower  bridles 
is  a  shackle  for  the  attachment  of  a  further  weight, 
usually  a  cylindrical  block  of  iron  or  a  bundle  of  heavy 
chain.  The  weights  vary  between  130  to  350  Ib.  (60  to  160 
kg.)  according  to  the  si/e  of  the  net,  the  power  of  the 
towing  vessel  and  the  depth  at  which  it  is  required  to  fish. 

Operation 

The  method  of  operating  the  Larsen  gear  has  been 
described  in  detail  by  Glanville  6>  7,  so  that  attention  will 
be  paid  here  only  to  the  more  important  features  of  the 
operation,  which  since  it  is  conducted  by  two  vessels 
working  together,  is  more  complex  than  with  the  one- 
boat  trawls  and  demands  a  high  degree  of  skill  and  co- 
operation by  the  crews  of  the  vessels.  The  operation  also 
makes  specific  demands  on  the  towing  vessels,  both  of 
which  should  be  of  the  same,  or  nearly  the  same  si/e  and 
power,  and  of  high  manoeuvrability.  This  last  feature  is 
one  of  the  most  important  in  setting  the  upper  limit  of 
vessel  size  for  the  effective  operation  of  this  gear.  No 
special  demands  are  made  on  deck  layout,  but  sufficient 
deck  space  aft  is  required  for  the  exchange  of  towing 
bridles  between  the  two  vessels.  The  basic  deck  equip- 
ment required  on  each  ship  for  operating  the  gear  is  the 
same  as  for  normal  bottom  trawling;  this  comprises  a 
winch,  with  a  capacity  of  500  to  600  fm.  of  I  i  to  U  in. 
steel  towing  warp,  gallows,  the  after  one  fitted  with  an 
extra  sheave,  and  the  normal  gilson  or  hauling  derrick 
for  landing  the  catch.  In  addition,  two  bollards,  one  just 
forward  of  the  after  gallows  and  the  other  aft  of  the 
forward  gallows,  are  desirable  accessories  for  use  in 
shooting  the  gear. 

The  vessel  holding  the  net  lies  broadside  on  to  the  wind 
and  the  net  is  paid  out  over  the  side  as  in  normal  trawling 
operations.  Three  to  four  fathoms  of  both  sets  of  bridles 
are  then  paid  out,  and  held  on  the  bollards.  The  second 
vessel  approaches  from  astern,  passes  under  the  lee 
quarter  and  stops.  A  heaving  line  for  taking  the  ends  of 
the  after  bridles  is  thrown  between  the  vessels  and  the 
bridles  are  hauled  aboard  the  second  vessel  and  made 


[338] 


MIDWATER     TRAWLING 


fast  to  the  sliphooks  on  the  warps,  the  aft  warp  being 
already  in  position  over  the  after  gallows  with  the  weight 
attached,  and  the  forward  warp  brought  aft  to  the  gallows 
for  attaching  its  bridle.  At  the  same  time,  the  pair  of 
bridles  remaining  on  the  vessel  shooting  the  net  are 
attached  to  its  warps  in  the  same  way.  The  bridles  are 
then  thrown  off  from  the  rail  bollards  and  the  boats 
move  slowly  ahead  and  away  from  one  another.  At  the 
same  time  the  forward  warps  on  the  shooting  vessel  are 
slackened  out  until  they  are  level  with  the  after  gallows, 
when  they  are  picked  up  and  put  into  the  extra  gallows 
sheave.  Both  vessels  now  steam  on  course  and  increase 
speed  and  continue  to  bear  slightly  away  from  one 
another,  paying  out  warp  as  they  go.  Communication 
is  maintained  during  this  operation  either  by  radio 
telephony  or  by  shouting. 

Warp  is  run  off  until  long  enough  to  set  the  net  at  the 
required  depth.  At  present,  this  is  gauged  indirectly  from 
tabulations  of  the  depth  of  towing  for  different  warp 
lengths,  towing  speeds  and  warp  angles,  the  latter  being 
measured  with  a  simple  pendulum  protractor  supplied 
with  the  gear.  Then  the  two  vessels  steam  on  parallel 
courses  for  the  duration  of  the  tow. 

The  distance  between  the  vessels  is  an  important 
feature  of  the  operation,  the  optimum  distance  being 
largely  a  matter  of  experience  gained  from  repeated 
operations.  A  distance  of  about  one  half  the  warp  length, 
excluding  the  bridles,  is  commonly  adopted.  Station 
keeping  is  difficult  and  requires  practice,  and  in  the  early 
stages  of  operation  a  measured  rope  is  sometimes  used 
between  the  vessels.  Practised  fishermen,  however, 
usually  keep  station  solely  by  eye. 

At  the  end  of  the  tow,  the  two  vessels  head  down  wind 
and  move  in  towards  each  other  until  they  are  about  a 
boat's  length  apart  and  the  gear  is  lying  astern.  They  then 
move  slowly  ahead  and  heave  in  the  warps  until  the 
weights  and  bridles  have  been  brought  up  to  the  after 
gallows,  when  they  stop  and  one  set  of  bridles  is  passed 
back,  with  a  heaving  line,  to  the  original  shooting  vessel. 
These  are  passed  round  the  stern  to  the  windward  side 
of  the  vessel  and  the  lower  one  fixed  to  a  warping  head 
of  the  winch,  slack  being  obtained  by  moving  the  vessel 
astern.  The  four  bridles  are  then  hauled  in  together  and 
are  neatly  coiled  in  preparation  for  shooting,  the  lower 
by  the  winch  and  the  upper  pair  by  hand.  The  net  is  then 
hauled  as  in  normal  trawling  practice  and  the  catch 
brought  aboard  in  one  or  more  "bags". 

This  type  of  fishing  operation  is  beset  with  a  number  of 
formidable  inherent  difficulties  and  drawbacks  which  are 
not  present  with  one-boat  trawling.  Operational  costs  are 


higher  for  vessels  of  the  same  size;  and  more  manpower  as 
well  as  a  higher  level  of  operational  skill  and  seamanship 
are  required.  Weather  is  also  a  more  serious  limiting 
factor.  Such  factors  undoubtedly  prompted  the  develop- 
ment of  the  one-boat  midwater  trawls.  However,  the 
two-boat  method  has  a  number  of  important  advantages 
which  tend  to  offset  these  drawbacks.  In  most  regions 
where  the  one-boat  and  two-boat  trawls  have  been  used 
together,  the  latter  have  so  far  proved  the  most  effective: 
this  is  particularly  evident  in  the  European  herring 
fisheries.  The  factors  which  contribute  to  this  greater 
efficiency  probably  relate  to  a  number  of  technical  features 
of  the  gear  and  method  of  operation  which  affect  the 
behaviour  of  fish  near  the  trawl  mouth.  The  features 
which  are  possibly  of  special  importance  are: 

(i)  the   absence    of  otter   boards.    This   avoids    the 

"noise"  and  other  disturbances  in  the  vicinity  of 

the  mouth  of  the  trawl, 
(ii)  the  absence  of  propeller  noise  in  the  direct  path 

of  the  trawl, 
(iii)  the  generally  marked  divergence  of  the  warps  from 

the  mouth  of  the  net. 

The  specific  design  of  the  trawl  itself  may  also  play 
a  part. 

ONE-BOAT  MIDWATER  TRAWLS 

Developments  in  the  one-boat  trawl  have  taken  place 
independently  in  different  parts  of  the  world  and  these 
have  led  to  differences  mostly  in  the  structure  and  rigging 
of  the  gear  components  (spreading  and  lifting  devices, 
ropes,  bridles,  etc.)  rather  than  in  the  specific  design  of 
the  nets,  which  have,  in  the  main,  conformed  with  the 
general  pattern  and  basic  characteristics  as  exemplified 
by  the  Larsen  trawl.  These  differences  reflect  the  attempts 
by  designers  to  satisfy  one  or  more  of  the  requirements 
of  high  stability,  large  mouth  aperture,  low  turbulence 
and  disturbance  near  the  mouth  of  the  net  and  low 
overall  drag.  The  differences  are  well  illustrated  by  the 
Larsson  "Phantom"  trawl,  the  Icelandic  "Breidfjord" 
trawl  and  the  British  Columbian  trawl.  These  possess 
features  of  design  and  rig  differing  markedly  from  one 
another  and  together  they  mostly  cover  the  range  of  the 
gear  make-up  of  present  day  one-boat  midwater  trawls. 

The  Nets 

The  nets  conform  with  the  main  characteristics  of  mid- 
water  trawls  in  being  approximately  square,  or  rect- 
angular in  shape  down  to  the  codend.  Nets  of  different 


Dttl«nc«   bttvct 
vc licit  appro* 
half  warp  length 


Upper  bndlci 


-X-  -  .---.-is 


Fig.  4.     Diagrammatic  view  of  iMrsen  net  under  low. 

[339] 


MODERN     FISHING    GEAR    OF    THE     WORLD 


dimensions  have  been  designed  for  use  on  different 
classes  and  sizes  of  vessels  and  for  different  species  or 
fish.  The  Larsson  "Phantom"  trawl  was  developed 
principally  for  use  in  the  European  herring  fisheries  by 
vessels  of  the  same  range  of  size  and  power  as  for  the 
Larsen  trawl.  The  "Breidf jord"  trawl  was  designed  for 
use  by  larger  vessels  in  the  Icelandic  cod  fishery.  The 
British  Columbian  trawl  was  intended  for  use  in  the  local 
herring  fishery  by  stern  trawling  vessels  of  150  to  175  h.p. 

The  general  make-up  of  the  nets  is  illustrated  in  fig.  5. 
The  data  for  the  Larsson  trawl  were  obtained  from 
Messrs.  Albrechtson  and  Company  of  Gothenburg, 
Sweden,  and  those  for  the  British  Columbian  trawl  were 
taken  from  information  published  by  Barraclough  and 
Johnson1. 

The  nets  differ  in  some  important  dimensional  details. 

(i)  In  the  Larsson  trawl,  the  side  panels  are  narrower 
than  the  upper  and  lower  panels  whereas  in  the 
British  Columbian  trawl  they  are  of  the  same  size. 
The  Larsson  trawl  is  therefore  of  rectangular  cross 
section  and  the  British  Columbian  trawl  of  square 
section. 

(ii)  The  British  Columbian  trawl  is  much  larger  overall, 


the  result  principally  of  differences  in  the  sizes  and 
power  of  vessels  for  which  the  trawls  were  intended. 

(iii)  The  mesh  sizes  in  the  British  Columbian  trawl  are 
larger  overall.  This  is  undoubtedly  due  in  part  to 
differences  in  the  size  composition  of  the  fish 
schools  in  the  regions  fished,  but  the  provision  of  a 
larger  mesh  in  the  forward  parts  of  the  net  is  held 
by  many  experts  to  improve  water  flow  and  reduce 
drag  and  turbulence  in  the  net. 

in  each  of  these  trawls  the  net  sections  are  laced 
together  and  fixed  to  sidelines  running  down  their  lengths 
to  the  codend  for  strengthening  purposes,  and  the  leading 
edges  arc  fixed  to  ropes  to  provide  headline,  footrope 
and  sidelines  respectively.  Larsson  trawls  have  mainly 
been  made  of  cotton  twine  for  the  webbing  and  manila 
for  the  ropes  while  in  the  British  Columbian  trawl  nylon 
has  generally  been  used  for  the  net,  sidelines  and  footrope, 
and  combination  rope  (manila  and  six-strand  wire)  for 
the  headline.  The  British  Columbian  trawl  has,  in 
addition,  a  "zipper"  device  for  opening  the  codend 
along  its  length,  so  that  the  catch  can  be  brailcd  from  the 
net  without  hauling  the  codend  aboard.  This  probably 
saves  time  with  large  catches  and  also  permits  the  catch 


LARSSON  'PHANTOM*  TRAWL. 


BRITISH     COLUMBIAN   TRAWL. 


IXTINSION  Pircr    »  toot  NO  _ 


Dimensions  of  upper,  lower  and  side  panels  of  iMrxson  and  British  Columbian  trawls.      In  the  British  Columbian  trawl  all  panels 
have  the  same  dimensions,  but  in  the  Larsson  trawl  the  side  panels  have  different  dimensions  from  the  upper  and  lower  panels. 

[340] 


MIDWATER    TRAWLING 


to  be  taken  aboard  in  good  condition.  In  the  Larsson 
trawl,  the  codend  is  tied  in  the  usual  way  with  a  codline 
and  the  catch  is  hauled  aboard  as  in  normal  trawling 
operations.  A  splitting  strap  is  usually  attached  to  the 
codend  for  dividing  the  catch  into  "bags". 

Arrangement  of  ropes,  bridles  and  otter  boards 

The  differences  in  the  construction  and  rigging  of  the 
ropes,  bridles  and  otter  boards  are  the  result  of  attempts 
to  achieve  operational  efficiency  and,  in  particular,  to 
minimise  disturbance  by  bridles  and  otter  boards  near 
the  mouth  of  the  net,  and  to  ensure  a  large  vertical 
mouth  opening.  The  general  arrangement  of  these 
components,  with  the  dimensions  of  each,  for  the  three 
trawl  types  are  shown  in  fig.  6. 

The  simplest  arrangement  is  displayed  by  the  Larsson 
trawl,  in  which  no  special  provision  is  made  to  offset  the 
otter  boards  from  the  path  of  the  net,  and  the  most 
elaborate  by  the  British  Columbian  trawl,  in  which  the 
otter  boards  were  initially  fixed  to  side  cables,  attached 
to  the  main  towing  warp.  In  later  models  this  side  cable 
or  "pennant"  arrangement  has  been  abandoned. 

Otter  Boards 

Special  otter  boards  have  been  constructed  for  use  with 
both  the  Larsson  and  the  British  Columbian  trawls 
(fig.  7). 

The  "wing  door"  developed  by  Mr.  K.  H.  Larsson 
has  greater  spreading  power,  less  drag  and  higher 
stability  than  plane  boards  of  the  same  surface  area.  It  is 


bridle          donleno 


line  leg 


dcpreiior 
toodi 

foot  rope  I* 


LARSSON        TRAWL 


Otter  board 

swivel  J  too  trope  I 

towing  warp  -^    ^*S'{j|€  f   \  (." frn 


ICELANDIC     TRAWL 


trawl   booid          >^v    > 
p.noont     ^   / 
00  fm.)       V 


dual   tin"  otter  board 


V- 


rpre»iorX   | 
. . --"*">.  / 


V 


loW«r  leu 


BRITISH  COLUMBIAN  TRAWL 


Fig.  6.     Arrangements  oj  nets,  bridles,  trawl-boards  and  lowing 
warps  of  Larssoh,   Icelandic  and   British   Columbian   trawls. 


constructed  of  wood,  capped  with  metal;  it  is  elongated 
with  tapered  ends,  the  bottom  one  of  which  is  weighted. 
It  is  of  aerofoil  cross  section  with  a  smooth  leading  edge 
and  a  more  pointed,  V-shaped  trailing  edge.  The  warp  is 
fixed  to  the  board  by  two  lengths  of  chain  attached  to  the 
board  mid-way  along  its  length,  and  one  of  the  points 
of  attachment  is  adjustable  in  order  that  the  angle  of 
attack  of  the  door  can  be  changed.  The  bridle  from  the 
danleno  is  attached  to  the  board  via  a  special  ring  which 
is  attached  to  the  trailing  edge  of  the  board  by  wire  or 
chain  cables  of  equal  length.  The  ring  has  a  series  of 
holes  round  its  circumference  by  which  the  positions  of 
attachment  of  the  bridle  and  door  cables  can  be  changed 
and  so  alter  the  angle  of  incidence  of  the  board  co  the 
vertical  and  hence  its  depth  seeking  properties.  The  ring 
is  dispensed  with  by  some  operators,  who  prefer  to  use 
a  pair  of  simple  chain  backstrops  attached  to  the  two 
ends  of  the  board  and  joined  to  the  bridle  by  a  shackle. 
With  this  arrangement,  of  course,  the  provision  for 
quickly  changing  the  angle  of  incidence  of  the  board  is 
lost.  Holes  are  also  provided  at  the  upper  and  lower  ends 
of  the  doors  for  fitting  floats  and  weights  respectively, 
if  required  during  operations. 


..  (coding  edge 
.'^'     (metal  covered) 


"~^b 

metal    lower  _f  _.y->       /  I 
\^*r''                   * 
2-  4»  _ ^ 

OF      PRESSURE SURFACE 


trailing  edge 


towing    bracket 
CROSS    SECTION     OF    POftRL.AT   CENTRE      sijDE    VIEW    OF    TOWING    BRACKET^ 


Fig.    7.       Diagrammatic   representation   of  British    Columbian 
"dual-Jin"  otter  board  (  after  ),  and  of  Larsson  *V/iv"  board 


341  ] 


MODERN    FISHING     GEAR    OF    THE    WORLD 


The  "dual-fin"  otter  board,  designed  for  the  British 
Columbian  trawl1,  is  constructed  of  curved  f  in.  laminated 
plywood,  measuring  54  ft.,  having  upper  and  lower 
horizontal  stabilizing  fins,  also  made  of  laminated  ply- 
wood (J  in.),  attached  to  the  main  surface  by  angle  iron 
strips  on  the  upper  and  lower  edges.  Two  adjustable 
vertical  fins,  of  J  in.  plywood,  are  also  bolted  to  the  upper 
and  lower  horizontal  fins.  Four  towing  lugs  are  attached 
to  the  concave  surface  of  the  door,  on  two  of  which  a 
series  of  alternative  holes  are  provided  for  attaching  the 
towing  bracket  chains  (g  in.)  the  free  ends  of  which  are 
shackled  to  a  steel  towing  ring.  The  towing  penant  is 
also  attached.  The  upper  chains  have  16  links  in  the 
fore-section  and  26  in  the  after  section  while  the  lower 
chains  have  17  and  27  links  respectively.  (These  dimen- 
sions can,  of  course,  be  changed  according  to  the  opera- 
tor's wishes.)  The  board  is  weighted  with  lead  bars, 
totalling  135  Ib.  to  provide  a  satisfactory  balance  to  the 
boards  during  operation. 

There  appears  to  be  no  specially  designed  otter  board 
for  use  with  the  Icelandic  trawl,  which  has  been  worked 
with  standard  plain,  wooden  otter  boards.  However, 
it  can  be  used  equally  well  with  boards  of  the  Larsson 
or  other  types. 

Danlenos 

In  the  beginning,  danlenos  were  used  with  the  Larsson 
trawl  but  not  with  the  Icelandic  or  British  Columbian 
trawls.  They  were  of  the  "stick"  type,  about  9  ft.  long 
6  in.  wide  and  3  in.  thick,  made  of  wood,  capped  with 
metal,  and  shaped  so  as  to  provide  a  shearing  effect  when 
towed  through  the  water.  The  inner  (pressure)  surface 
was  plain  and  the  outer  surface  curved. 

Fastening  rings  were  attached  at  each  end  of  the  danleno 
to  which  the  bridles  to  the  net  and  the  boards  were 
attached.  The  bottom  end  of  the  danleno  was  weighted. 

Floats,  Elevators  and  Depressors 

A  large  vertical  mouth  opening  is  a  well-known  require- 
ment for  midwater  trawls,  and  for  both  the  Larsson  and 
the  British  Columbian  trawls,  special  devices  have  been 
developed  to  achieve  this. 

The  devices  developed  by  Mr.  Larsson  comprise 
"elevator  toads",  which  take  the  place  of  the  customary 
floats  along  the  headline,  and  of  "depressor  toads", 
which  take  the  place  of  weights  for  the  footrope  (fig.  8). 

The  elevator  and  depressor  toads  are  constructed  alike, 
except  that  while  the  bodies  of  the  elevator  toads  are 
lighter  than  sea  water,  giving  them  a  positive  buoyancy, 
the  heads  of  the  depressor  toads  are  weighted  with  lead 
or  other  metal  to  make  them  heavier  than  water  and  to 
make  the  head  heavier  than  the  tail.  The  body  of  each 
toad  has  a  pressure  surface,  which  is  roughly  V-shaped 
in  cross  section,  and  a  suction  surface,  which  is  convexly 
rounded  in  front  and  channel-shaped  at  the  rear.  The 
thickness  of  the  body  decreases  at  both  ends  from  maxi- 
mum near  the  front  end.  A  metal  stabilizing  fin  is  attached 
to  the  pressure  surface  of  the  body  at  the  rear  end,  and  a 
pin  for  fixing  the  line  for  attaching  the  toad  to  the  head- 
line or  footrope  of  the  net,  is  inserted  a  short  distance  in 
front  of  the  pressure  centre  on  the  pressure  surface. 

The  elevator  and  depressor  toads  provide  stable  lifting 
and  depressing  forces  increasing  with  the  towing  speed. 


The  toads  commonly  used  with  the  Larsson  trawl  have 
a  body  length  of  about  14  in.  (excluding  the  tail  fin)  and 
maximum  width  of  9  in.  and  these  develop  a  lift  of  about 
1 5  Ib.  at  3  knots. 

The  number  of  elevator  and  depressor  toads  required 
for  effective  operation  of  the  trawl  varies  with  the  size 
and  material  of  the  trawl  and  the  angle  of  attack  of  the 
otter  boards.  Furthermore,  this  is  subject  to  wide  differ- 
ences of  opinion  by  operators.  However,  an  arrangement 
recommended  by  the  suppliers  of  the  gear  is  5  elevator 
and  7  depressor  toads  attached  in  the  central  parts  of  the 
headline  and  footrope  respectively,  with  ordinary 
spherical  floats  attached  to  the  wing  sections  of  the 
headline,  on  either  side  of  the  elevator  toads.  An  elevator 
and  depressor  toad  at  each  of  the  upper  and  lower  wing 
tips  respectively  is  also  often  recommended. 

The  only  special  devices  designed  for  use  with  the 
British  Columbian  trawl  are  two  depressors  which  are 
shackled  to  the  lower  bridles  (figs.  6, 8)  just  in  front  of  the 
wing  tips  of  the  net.  They  are  attached  so  that  they  can 
slide  freely  on  the  bridles  and  facilitate  handling. 

In  the  Canadian  experiments  with  the  British  Colum- 
bian trawl1  headline  flotation  was  effected  with  eleven 
8  in.  Phillips  trawl  plane  floats,  attached  about  2  ft. 
apart  along  the  bosom  of  the  headline.  The  type  and 
number  of  floats  to  use  with  this  gear  is  again  subject  to 
the  operator's  personal  choice  and  experience.  Weights 
are  attached  at  intervals  along  the  length  of  the  footrope. 

It  seems  that  no  special  elevator  or  depressor  devices 
have  been  developed  for  use  with  the  Icelandic  trawl. 


fastening  rings 
headline  bridle     S\  door  bridle 


trailing  edge 
(capped  with  metal) 


footrope  bridle- 


leading  edge 
(capped  with  metal) 


9'6" 


door  bridle 


SIDE   VIEW  OF  DAN   LENO 


trailing  edge 


leading 


pressure  _ 
surface 

CROSS  SECTION  OF   DAN  LENO 
Fig.  8.    Diagrammatic  representation  of  Larsson  danleno. 


[342] 


MIDWATER    TRAWLING 


except  that  small  wooden  kites  are  attached  to  the  upper 
corners  of  the  mouth  of  the  net  to  provide  extra  lift. 
Standard  floats  and  weights  have  been  used  for  the 
headline  and  footrope. 

Operational  Features 

Both  the  Larsson  and  Icelandic  trawls  were  designed  for 
use  on  standard  European  trawlers  equipped  for  shooting 
and  hauling  from  the  side,  and  they  can  be  readily 
handled  in  the  manner  customary  for  bottom  trawls.  The 
British  Columbian  trawl,  on  the  other  hand,  was  designed 
for  shooting  and  hauling  over  the  stern  and  the  rigging  of 
the  trawlboards  on  side  pennants  presents  no  operational 
difficulties,  but  it  might  present  difficulties  on  vessels 
equipped  for  side  handling. 

Since  no  comparable  data  on  the  relative  efficiency  of 
these  trawls  are  available,  it  is  difficult  to  itemise  their 
relative  merits.  Observations  indicate  that  each  is  stable 
under  tow  and  each  provides  a  satisfactory  vertical 
mouth  opening  when  towed  at  speeds  between  3  to  5 
knots.  Heights  of  from  4  to  7  fm.  have  been  recorded  for 
the  Larsson  trawl,  of  7  to  8  1m.  for  the  British  Columbian 
trawl1,  and  of  4  to  5  fm.  for  the  Icelandic  trawl  (un- 


hocizontol    stabilising  f»n 


Top  view      (pressure    surface) 


metal  weight 


Side  < 


Cross    section   through   A- A 


Cross    section    through    B  B 


Bottom  view  of    depressor    tood 
(•action   surface J 


IARSSON   ELEVATOR  AND   DEPRESSOR 


V  shackles 


sweep  line 


V  steel  plate 
DEPRESSOR      FOP     BRITISH     COLUMBIAN     TRAWL 

Fig.  9.    Larsson  elevator  and  depressor  toads    and   British 
Columbia  depressor. 


published  information  held  at  the  Scottish  Home  Depart- 
ment Marine  Laboratory,  Aberdeen). 

At  this  stage  in  midwater  trawling  development,  which 
is  still  largely  exploratory  and  experimental,  the  criteria 
by  which  to  gauge  the  potentialities  of  a  particular  design 
or  rig  of  trawl  are  difficult  to  specify,  and  the  factors  which 
govern  the  efficiency  of  the  gear  are  not  accurately  known. 
Extensive  comparative  trials  with  a  variety  of  net  designs 
and  gear  rigs  are  required  to  obtain  the  necessary  know- 
ledge. In  particular,  more  information  is  required  of  the 
reactions  of  pelagic  fish  to  the  gear  in  different  regions, 
seasons  and  depths.  Such  information  is  now  accumu- 
lating in  different  parts  of  the  world  and  new  designs 
and  rigs  of  gear  are  being  developed.  It  is  likely  therefore 
that  new  designs  and  rigs  of  midwater  trawl  will  be 
developed  to  supersede  those  described  here. 

Perhaps  the  most  profitable  line  of  future  development 
is  towards  dual-purpose  bottom  and  midwater  trawls 
which  can  be  worked  alternatively  on  or  off  the  seabed. 
Many  of  the  species  of  fish  inhabiting  the  continental 
shelf  and  which  are  exploitable  in  midwater,  also  spend 
part  of  their  lives  close  to  the  bottom.  Herring,  cod  and 
mackerel  are  notable  examples.  This  alteration  between 
the  demersal  and  the  pelagic  habit  is  often  sporadic  and 
unpredictable,  although  the  herring  exhibits  a  generally 
regular  diurnal  movement.  The  development  of  a  dual- 
purpose-gear,  or  one  in  which  the  demersal  gear  could  be 
quickly  modified  at  sea,  would  permit  greater  flexibility 
in  commercial  operations  and  would  reduce  the  cost  of 
fishing  for  species  which  habitually  move  between  the 
bottom  and  midwater. 

REFERENCES 

1  Barraclough,   W.  t.  and  Johnson,  W.  W.  "A  new  mid-water 
trawl  for  herring".  Bull,  Fish.  Res.  Bd.  Can.,  no.  104,  pp.25.  1956. 

2  Brandt,     A.     von.      "Schwedisches     tinschiff-Schwimmtrawl 
Fantom."  Fischwirtschaft,  5,  25.  1953. 

3  "Frprobung  des  schwedischen    Einschiff-Schwimmtrawls   von 
l^arsson."  Protokolle  Fischereitech.,  .?,  3-13,  1954. 

4  Craig,    R.    E.,    "Echosounding   and   lish    detection/'    Paper 
presented  to  this  Congress,  1957. 

5  Delpierre     H   J-B.     "Le     chulut     fiottant      islandais."     Le 
Mann,  nos.  313-4,  1953. 

6  Cilanville,   A.,    "The    Larsen    midwater    trawl."    Fish    Bull. 
F.A.O.,  9,    113-29,    1956. 

7  "Design  and  operation  of  the  Larsen  trawl."  World  Fishing, 
5(12),  38-42,  and  6  (I)  34-7.  195(>-57. 

8  Larsen,  R.  "Floating  trawl."  British  Patent  Specification,   no. 
695,  361,  1953. 

*  Larsson,  K.  H.  "Improvements  in  trawl  nets."  British  Patent 
Specification,  no.  670,  222,  1952. 

10  "Improvements  in  shearing  boards".  British  Patent 
Specification,  no.  674,  91 1.  1952. 

J1  Mcschkat,  A.  "Der  'Dusenjager',  ein  ncuer  Schleppnetztyp." 
Fischereiwelt,  2,  177-9,  1950. 

J2  Richardson,  I.  D.  "Some  problems  in  midwater  trawling". 
World  Fishing,  6  (2),  28-3 1 ,  1957. 

13  "Midwater  trawling.  Pair  fishing  experiment  on  herring 
1955-56."  Fish.  Lab.  Lead.,  Lowestoft,  no.  12,  pp.  26,  1957. 

14Scharfe,  J.  "Messung  der  Oefnungshoe  von  Schleppnctzen 
mit  Echolot."  Fischwirtschaft,  5,  282-4,  1953. 

16  "Uber  Messungen  an  Schleppnetzcn."  Arch.  Fisch.  Wiss., 
6,  64-83,  1955. 

16  Sigurdsson,  J.  "Improvements  in  or  relating  to  trawl-fishing 
nets."  British  Patent  Specification,  no.  699,  206,  1953. 

17  Stephen.    F.    H.    and    Shea,    F.    J.    "Underwater    telemeter 
for  depth  and  temperature."  Spec.  sci.  Rep.  Fish.  U.S.  Fish  Wildl. 
Scrv.,  no.  18 1,  pp.  23,  1956. 

18  Wood,  H.  and  Parrish,  B.  B.  "Echo-sounding  experiments- 
on  fishing  gear  in  action."  J.  Cons.  Int.  Explor.  Mer.  /7,  25-26, 
1950 


[343] 


SCANDINAVIAN   EXPERIENCE  WITH  MIDWATER  TRAWLING 

by 
KARL-HUGO  LARSSON 

Stockholm,  Sweden 

Abstract 

Although  the  pelagic  trawl  has  only  been  in  use  in  Scandinavia  for  nine  years,  the  idea  of  using  these  trawls  is  an  old  one,  and  patents 
were  granted  for  these  special  nets  at  the  beginning  of  the  century.  In  this  paper,  two-boat  trawls  and  one-boat  trawls  are  described  and  the 
advantages  and  disadvantages  of  each  are  mentioned.  For  example,  the  author  states  that  the  two-boat  trawl  must  be  limited  to  the  use 
of  small  fishing  craft  and  that  one-boat  nets  arc  best  used  by  the  more  powerful  trawlers.  The  technique  of  lifting  the  headline  and  depressing 
the  foot  rope  in  order  to  obtain  the  maximum  amount  of  opening  for  the  mouth  of  the  net  is  fully  described,  and  the  use  of  "trawl-toads" 
with  the  Phantom  Trawl,  designed  by  the  author  is  also  discussed.  The  Sterner  Pcrsson  one-boat  trawl,  with  three  towing  lines  on  each  side 
is  used  for  catching  "stroming"  (Baltic  herring)  in  the  Baltic  and  a  catch  of  10  tons  in  A  hr.  has  been  reported.  The  author  feels  that  there 
is  a  good  future  for  pelagic  trawls. 

Experience  acquise  en  Scandinavic  avec  le  chalut  flottant 

Bicn  que  le  chalut  pclagiquc  ne  soit  utilisd  en  Scandinavie  que  depuis  neuf  ans,  I'idee  en  est  andene  et  des  brevets  rclatifs  &  ces 
types  spcciaux  de  filets  ont  etc  pris  au  debut  de  siecle.  L'auteur  dccrit  les  chaluts  a  deux  bateaux  et  les  chaluts  a  un  bateau,  et  expose  les 
avantages  et  inconvenients  de  ces  deux  types  d*cngin.  C'est  ainsi  que  scion  Fauteur,  Pemploi  du  chalut  a  deux  bateaux  doit  ctrc  Iimit£  aux 
petits  bateaux  dc  peche,  t  and  is  que  le  chalut  a  un  bateau  est  utilis£  dans  les  mcilleurcs  conditions  par  d  >s  chalutiers  a  vapcur  plus  puissants. 
L'auteur  fait  un  expos6  detaille  de  la  technique  employee  pour  soulever  la  ralingue  superieurc  et  abaisscr  la  ralingue  inferieure  afin  d'obtenir 
le  maximum  d'ouverture  dc  la  gueulc  de  chalut;  il  ctudie  egalcment  I'cmploi  des  "crapauds  de  chalut"  avec  le  chalut  "Phantom"  dont  il  est 
I'inventeur.  On  sc  sert  du  chalut  Sterner  Pcrsson  d  un  bateau,  avec  trois  funes  de  chaque  cote  pour  pechcr  les  "stromming"  dans  la  Baltiquc; 
on  a  capturd  de  la  sorte  10  tonnes  dc  poisson  en  unc  demi-hcure.  L'auteur  estimc  que  les  chaluts  pclagiques  ont  un  bcl  avcnir. 

Ensayos  hechos  en  Escandinavia  con  una  red  de  arrastrc  para  pescar  a  profundidades  intermedias 
Extracto 

Aunque  s61o  durante  los  ultimos  anos  ban  comen/ado  a  usar  en  Hscandmavia  la  red  dc  arrastrc  pelagica,  la  idea  dc  utilizar  este  tipo 
de  arte  es  muy  antigua,  habicndose  otorgado  patcntcs  de  invention  a  principles  de  siglo.  En  estc  trabajo  se  describen  las  redes  remolcadas 
por  una  y  dos  embarcaciones,  asi  como  las  ventajas  e  inconvenientes  de  am  has.  Por  ejemplo,  segun  cl  autor,  la  primera  es  utilizada  por 
arrastreros  dc  vapor  potentes,  mientras  que  el  uso  dc  la  segunda  se  limita  a  embarcaciones  pcsqucras  pequcnas. 

Se  describe,  en  dctalle,  la  tecnica  dc  hacer  subir  la  relinga  de  boyas  y  de  bajar  la  de  plomos  para  que  la  boca  sc  abra,al  maximo  y 
cl  uso  de  "trawltoads"  en  la  red  de  arrastre  "Phantom"  proyectada  por  el  autor.  La  red  dc  arrastrc  "Sterner  Persson"  remolcada  por  una 
cmbarcaci6n,  mcdiante  3  cables  unidos  a  cada  costado,  se  lisa  para  pescar  "stromming"  en  el  mar  Baltico,  habicndose  logrudo  lances  de  10 
toncladas  en  media  hora.  El  autor  crce  en  cl  porvcnir  dc  las  rcdcs  dc  arrastrc  pelagicas. 


THE  idea  of  catching  fish  shoals  between  the  surface 
and  bottom  by  means  of  trawls  seems  to  be  of 
about  the  same  age  as  the  ordinary  bottom 
trawling.  Patents  have  been  granted  for  several  pelagic 
trawl  constructions  since  the  beginning  of  this  century, 
but  none  has  become  popular  among  fisherman. 

The  main  reason  might  be  that  methods  for  locating 
fish  in  the  water  were  rather  poor  and  unsatisfactory. 
This  was  changed  by  the  development  of  echo  sounding 
and  asdic  during  World  War  II,  and  its  later  application 
to  fish  location.  Once  depth  and  thickness  of  fish 
shoals  can  be  ascertained,  pelagic  trawling  is  given  a 
raison  d'etre. 

Much  research  has  been  done  in  several  countries  to 
develop  reliable  midwater  trawls.  In  Scandinavia,  this 
work  started  during  the  war  and  pelagic  trawling  has 
been  practised  there  for  about  nine  years. 

Two  different  systems  exist:  the  one-boat  and  the  two- 
boat  method. 

The  first  reaction  of  the  fishermen  was  in  favour  of  the 


two-boat  trawl,  which  is  towed  by  two  vessels  side  by 
side  at  a  certain  distance  from  each  other,  with  the  warps 
spreading  outwards  from  the  net  to  the  boats.  There  is 
nothing  in  front  of  the  net  to  frighten  the  fish.  A 
special  advantage  of  the  two-boat  trawl  is  that  no  otter 
boards  are  needed.  Otter  boards  cause  a  certain  resist- 
ance, so  their  absence  means  saving  fuel  or  increased 
trawling  speed,  which  in  many  cases  can  give  better 
catches. 

Many  objections  have  been  made  against  the  one-boat 
system.  Some  fishermen  considered  it  impossible  to 
catch  any  kind  offish  with  it.  The  noise  of  the  propeller, 
otter  boards  and  warps  was  supposed  to  frighten  the  fish 
away  from  the  net  mouth.  Theoretically,  this  sounds 
reasonable,  but  the  many  good  catches  made  with  one- 
boat  trawls  have  demonstrated  it  must  not  always  be  true. 

The  two-boat  system,  has  disadvantages  too.  It  is 
very  important  that  the  two  boats  move  at  the  same 
speed  and  keep  a  constant  distance  from  each  other.  It 
is  difficult  to  comply  with  these  requirements  in  bad 


[344] 


MIDWATER    TRAWLING    IN    SCANDINAVIA 


weather.  Currents  might  bring  the  net  to  one  side,  and 
this  causes  an  increased  pull  for  one  of  the  boats.  It  is 
often  said  that  one  should  have  two  boats  of  the  same 
size  and  engine  power  to  get  good  results.  It  seems, 
however,  more  important  that  there  should  be  two 
skippers  on  the  boats  who  co-operate  and  who,  in  the 
long  run,  have  the  same  idea  of  fishing  places  and  other 
conditions  which  influence  the  fishing  result.  There  is  a 
saying  that  "brother  skippers  do  not  know  each  other" 
after  having  fished  with  a  two-boat  gear  for  some  time. 

It  seems  impossible  for  two  big  trawlers  to  fish  together, 
and  the  one-boat  floating  trawl  therefore  has  come  more 
and  more  into  use. 

Designing  a  one-boat  midwater  trawl  is  a  more 
difficult  technical  problem  than  making  a  two-boat  gear. 
Otter  boards  are  needed  to  ascertain  a  satisfactory  hori- 
zontal opening.  The  ordinary  flat  otter  board,  which  is 
used  in  bottom  trawling,  does  not  quite  meet  the  require- 
ments of  midwater  trawling.  Its  angle  of  attack  is  30 
to  35  degrees,  and  when  it  loses  contact  with  the  bottom, 
it  often  starts  swinging  in  the  water.  Its  sheering  ability 
is  not  very  good.  Another  problem  is  to  obtain  a 
sufficient  opening  height  without  using  heavy  weights, 
which  are  normal  on  the  two-boat  trawls.  Floats  on 
bottom  trawls  generally  are  balls  made  of  glass,  steel, 
light  metal  alloy,  or  plastic.  The  lifting  capacity  of  a 
ball  is  affected  by  the  towing  resistance  which  increases 
as  the  square  of  the  speed.  This  means  that  with  ball 
floats  the  trawling  speed  is  limited,  and  the  towing  power 
required  is  rather  high. 

For  a  midwater  trawl  it  is  essential  that  the  height  and 
width  of  the  mouth  are  independent  of  the  speed.  There 
should  be,  however,  a  balance  between  vertical  and 
horizontal  powers  affecting  the  mouth  of  the  net.  While 
the  lifting  capacity  of  ball  floats  decreases  with  increased 
speed,  the  sheering  power  of  the  otter  doors  increases  as 
the  square  of  the  speed.  This  problem  can  be  solved 
by  sheering  devices,  working  upwards  and  downwards  on 
headline  and  footropc.  Different  kinds  of  such  devices 
are  on  the  market. 


The  midwater  trawl  nets  in  Scandinavia  generally  are 
constructed  with  identical  upper  and  lower  parts  and 
identical  side  parts.  Sometimes  all  four  parts  have  the 
same  dimensions.  While  cotton  is  normally  used,  in 
recent  years  good  results  have  also  been  obtained  with 
nylon  and  Perlon.  The  higher  breaking  strength  of  these 
new  materials  allows  for  thinner  twines  and  thus  for 
greater  towing  speed. 

It  has  been  found  difficult  to  tow  the  net  in  the  right 
depth  and  many  different  methods  have  been  tried.  One 
way  is  to  observe  the  angle  of  the  warps  to  the  horizontal. 
Knowing  the  length  of  the  warps,  the  depth  of  the  net 
can  be  calculated,  under  the  assumption  that  the  warps 
run  in  a  straight  line.  A  correction  of  15  to  20  per  cent, 
has  to  be  subtracted  from  the  calculated  result  to  com- 
pensate the  actual  slope  of  the  warps.  This  method  is 
simple  but  not  quite  reliable,  because  currents  may  have 
an  influence  on  the  depth  of  the  net  without  changing 
the  warp  angle.  Simple  instruments  have  been  developed 
which  can  be  lowered  along  the  warp  and  pulled  up 
again  during  towing.  After  years  of  pelagic  fishing  many 
skippers  know  from  experience  how  to  bring  the  trawl 
to  the  right  depth.  But  there  is  still  a  demand  for  a 
reliable  instrument  which,  in  a  simple  way,  i.e.  preferably 
without  wire  connections  between  net  and  ship,  informs 
the  skipper  continuously  about  the  depth  of  the  net. 

Midwater  trawls  have  been  used  by  Scandinavian 
fishermen  mainly  for  catching  herring  in  the  Skagerack 
and  Kattegat  during  the  winter  season.  From  about 
the  end  of  November  to  the  beginning  of  March  a  big 
herring  fishery  goes  on  there.  Previously,  purse  seines 
were  used,  but  midwater  trawling  was  started  on  a 
commercial  scale  for  the  first  time  in  the  1948-1949 
season.  Today  most  of  the  herring  is  caught  by  mid- 
water  trawls. 

In  the  beginning,  midwater  trawling  was  carried  out 
during  the  night  when  the  herring  is  found  at  10  to  30  fm. 
depth,  depending  upon  weather  conditions  and  darkness. 
Later,  it  was  also  found  that  the  method  can  be  used  in 
daytime.  Now,  more  and  more  fishing  takes  place  in 


Fig.   I.     Robert  Larsen's  two-boat  midwater  trawl. 

I  345  ] 


MODERN    FISHING    GEAR     OF    THE    WORLD 


f-^ 

\Tnhauier    c        Clip  link 


~1  Weight 
Chain  or  lead 
.  2.     Sterner  Persson  midwater  trawl  or  one  or  two  boats.     When  towing  with  two  boats  weights  are  used  instead  of  otter  boards. 


daylight  when  the  herring  usually  are  found  in  50  to 
80  fm.  depth. 

The  echo  sounder  tells  the  skipper  the  depth  of  the 
shoals  and  their  size  so  that  he  can  regulate  the  length  of 
the  haul,  which  might  vary  between  10  min.  and  2  hours. 

Some  fishermen  believe  that  echo  sounding  disturbs  the 
fish  and  frightens  them  away,  but  the  results  of  experi- 
ments do  not  support  this  theory.  Some  observations 
made  during  night  fishing  with  a  one  boat  trawl  in  the 
Skagerack  are  worth  mentioning.  During  towing,  the 
echo  sounder  recorded  shoals  of  fish  of  regular  character 
but  when  it  was  time  for  hauling  and  the  deck  lights  were 
switched  on,  the  shoals  immediately  disappeared.  After 
dimming  the  lights,  the  shoals  very  soon  appeared  again. 
This  observation  was  repeated  several  times  with  the 
same  result.  As  the  engine  and  propeller  were  running 
the  whole  time,  it  seems  as  if  the  sound  from  the  propeller 
and/or  the  engine  does  not  disturb  the  fish  but  the  deck- 
lights  do. 

Up  to  now,  midwater  trawling  has  been  done  relatively 
near  the  surface,  but  it  is  likely  that  in  the  future  midwater 
trawls  will  also  be  used  near  the  bottom.  When  large 


fish  shoals  are  on  the  bottom,  extending  to  several 
fathoms  in  height,  a  normal  bottom  trawl  can  only 
catch  a  certain  part  of  them  because  its  mouth  is  often 
too  low.  A  midwater  trawl  has  a  higher  opening  and 
consequently  may  catch  more.  For  proper  regulation 
of  the  depth  of  the  net,  it  seems  the  best  to  have  the  trawl 
travel  at  a  certain  distance  above  the  seabed.  This 
could  be  arranged  by  using  trawl-toads. 

MIDWATER  TRAWL  TYPES   IN   USE 

The  best  known  types  used  in  Scandinavian  waters  are 
Robert  Larsen's  two-boat  trawl,  Sterner  Persson's  one- 
boat  trawl  and  the  Phantom  trawl,  a  one-boat  trawl 
designed  by  the  author  of  this  paper. 

The  functioning  of  the  Larsen  two-boat  trawl  is  shown 
in  fig.  1. 

The  net  is  made  of  four  identical  pieces.  No  otter 
boards  are  needed.  Each  boat  has  an  upper  and  a  lower 
warp.  Big  weights  attached  to  the  lower  warps  at  a  certain 
distance  in  front  of  the  net  together  with  weights  on  the 
footrope  and  floats  on  the  headline,  give  the  net  its 


Upper  trawltoad 
Float 


Wingboard 

Depth  regulating  ring 


Lead- 


-  Lower  trawltoad 


rig.  3.    Phantom  trawl  designed  by  the  author.     This  is  a  one-boat  trawl  equipped  with  special  wing-boards  and  trawl-toads  on  headline 

and  footrope. 

[346] 


M1DWATER     TRAWLING     IN     SCANDINAVIA 


Fig.  4.     Wing-board  oj  the  Phaniom  trawl  with  depth  regulating 
ring. 

vertical  opening.  The  depth  of  the  trawl  is  usually 
regulated  by  the  length  of  the  warps  and  can  also  be 
influenced  by  the  distance  between  the  two  towing  boats. 
Normal  trawling  speed  is  said  to  be  3  to  4  knots. 

The  Sterner  Persson  trawl  (fig.  2)  has  a  six-wing  net 
with  three  legs  on  each  side  between  the  otter  boards 
and  net.  Short  chains  in  upper  end  lower  legs  and  close 
behind  the  otter  boards  serve  for  easy  regulation  of  the 
amount  of  pull  on  the  middle  legs.  The  Persson  trawl 
has  ordinary  otter  boards,  with  four-chain  brackets  for 
the  warps  and  the  legs  for  a  better  control  of  the  working 
performance.  The  vertical  opening  is  ascertained  by 
floats  on  the  headline  and  weights  on  the  footrope.  The 
depth  of  the  trawl  is  also  regulated  by  the  length  of 
warps.  A  number  of  these  trawls  are  said  to  be  in  use  in 
the  Baltic,  where  good  catches  of  the  Baltic  herring 
("stromling")  have  been  recorded.  A  catch  of  10  tons 
of  herring  in  a  haul  of  $  hour  and  a  total  catch  of  36,000 
kg.  in  one  week  of  fishing  have  been  reported. 

The  Phantom  trawl  (fig.  3)  has  been  designed  on 
results  of  tank  tests  at  Statens  Skeppsprovningsanstalt  in 
Goteborg.  By  testing  15  different  types  of  sheering 
boards  for  the  Swedish  Navy,  the  author  found  that  the 
wing-board  (figs.  4  and  5)  has  twice  the  sheering  ability 
of  an  ordinary  board  of  the  same  area.  As  it  works  with 
an  angle  of  attack  of  13  to  14  degrees,  it  moves  steadily 
and  smoothly  through  the  water.  By  a  very  simple 
arrangement  the  depth  regulating  ring  an  upward  or 
downward  sheering  component  can  be  created.  Instead  of 
common  floats,  trawl-toads  are  fixed  to  headline  and 
footrope  to  open  the  net  mouth  vertically.  The  balance 
between  the  vertical  and  horizontal  forces  is  secured  as 


fig.  5. 


A  Lars  son  wing-board  hanging  in   the  gallows  of  a 
Swedish  cutter. 


the  sheering  power  of  both  wing-boards  and  toads 
increases  equally  at  the  square  of  the  speed.  Thus  a 
rather  high  towing  speed  of  4  to  5  knots  is  made 
possible.  At  a  speed  of  4  knots  the  lifting  power  of  one 
trawl-toad  of  normal  size  (length  about  40  cm.)  is  about 
16  kg.  compared  with  little  more  than  2  kg.  for  a  spherical 
float  of  8  in.  diam.  At  the  same  speed,  the  resistance  of 
the  spherical  float  is  about  6  kg.  and  of  the  trawl-toad 
about  8  kg.  That  means  that  the  lifting  force  of  a  trawl- 
toad  is  7  to  8  times  that  of  a  spherical  float  at  only  slightly 
higher  resistance.  With  increasing  speed  the  difference 
between  the  trawl-toad  and  the  spherical  float  changes 
more  and  more  in  favour  of  the  trawl-toad.  Measure- 
ments made  under  fishing  conditions  have  demonstrated 
that  by  using  5  to  7  toads  on  the  headline  of  an  ordinary 
bottom  trawl,  the  opening  height  can  be  doubled.  By 
attaching  heavy  trawl-toads  to  the  footrope  of  a  trawl 
it  becomes  possible  to  keep  it  just  off  the  bottom.  The 
author  is  of  the  opinion  that  this  will  help  to  save  small 
fish  and  spawn  and  thus  might  be  a  way  to  prevent  over- 
fishing  which  now  seems  to  affect  the  North  Sea  stocks. 
It  is,  therefore,  suggested  that  it  would  be  worthwhile 
to  arrange  for  comparative  fishing  tests  with  a  trawl 
with  wing-boards  and  trawl-toads  on  headline  and  foot- 
rope  and  an  ordinary  trawl  of  equal  size. 

Thorough  tests  of  the  Phantom  trawl  gear,  including 
towing  speed,  towing  power,  depth  of  the  trawl,  opening 
height,  etc.,  under  different  conditions  have  been  made 
by  experts  from  the  Institut  fur  Netzforschung  in  Ham- 
burg, in  the  presence  of  British,  Danish  and  Swedish 
observers  and  the  results  have  been  reported  in  the  fishing 
Press. 


[347] 


THE  THAMES  FLOATING  SPRAT  TRAWL 

by 

H.  S.  NOEL 

Staff  Writer,  "World  Fishing",  London,  U.K. 

Abstract 

The  author,  a  former  Whitstable  (U.K.)  inshore  fisherman,  reports  that  after  the  last  war  two  Essex  fishermen,  Messrs.  Alf  and  George 
I^ggatt,  decided  to  depart  from  the  traditional  stow  net  method  of  catching  sprats  in  the  Thames  Estuary  because  they  wanted  something 
more  mobile  and  more  positive.  Accordingly,  they  devised  a  two-boat  net  which  they  use  from  their  two  39  ft.  shallow  draught  diesel  trawlers 
of  almost  revolutionary  design.  After  many  trials,  the  net  was  perfected  and  then,  with  the  use  of  the  echo  sounder,  it  proved  to  be  a  first- 
class  producer  of  good  quality  sprats.  This  paper  deals  intimately  with  the  design  and  operation  of  the  net,  the  hauling  and  shooting,  and  the 
author  points  out  that  while  this  method  of  fishing  has  many  advantages  over  the  drift  net  and  stow  net,  it  has  the  one  failing  that  it  is 
unsclcctive  and  may  at  times  produce  quantities  of  small  fish.  This  is  perhaps  unavoidable,  because  to  increase  the  size  of  the  mesh  would 
increase  the  number  offish  meshed  in  the  mouth  and  funnel  and  the  net  could  quite  easily  become  unmanageable. 


Rtaim* 


Le  chalut  flottant  de  la  Tamise  pour  la  p£che  des  sprats 


L'auteur,  ancien  pecheur  cotier  de  Whitstable  (Royaume-Uni)  raconte  comment  deux  pecheurs  de  I'Essex,  MM.  Alf  et  George 
Leggatt,  deciderent  apres  la  guerre  de  rompre  avec  la  meihode  traditionnelle  de  la  neche  des  sprats  dans  Pestuaire  de  la  Tamise  a  bord  de 
batcuax  ancres,  car  ils  voulaient  un  systeme  plus  mobile  et  plus  positif.  Us  construisirent  done  un  filet  manoeuvre  par  leurs  deux  chalutiers 
Diesel  &  faible  tirant  d'eau  et  de  39  pieds  de  long,  d'un  type  presque  revolutionnaire.  Le  filet  fut  perfection^  apres  de  nombrcux  essais. 
puis,  a  1'aide  de  I'echo-sondeur,  il  s'avcra  gtre  un  engin  de  premiere  classe  donnant  des  sprats  de  bonne  qualit£.  L'autcur  expose  en  detail  la 
conception  et  la  manoeuvre  du  filet,  la  facon  de  le  mettre  a  1'eau  et  de  le  relcver,  et  fait  observer  que  tout  en  possedant  de  nombrcux  avantages 
sur  le  filet  d£rivantet  le  filet  fixe,  ce  systeme  a  ['inconvenient  de  ne  pas  etre  select  if  et  d'etre  parfois  susceptible  de  capturer  d'importantes  quantites 
de  petits  poissons.  II  semble  que  ce  deTaut  soit  inevitable,  car  si  Ton  augmenta.it  la  dimension  des  mailles,  les  poissons  captures  dans  la 
gueule  et  le  corps  seraient  plus  nombreux  et  il  deviendrait  rapidement  impossible  de  manier  le  filet. 


Le  red  de  arrastre  flottante  para  espadfin  usada  en  el  rio  Timesis 
Eitracto 

El  autor,  un  ex-pescador  de  bajura  de  Whitstable,  en  el  Reino  Unido,  informa  que  despues'de  la  ultima  guerra  los  Srs.  Alf  y  George 
Leggatt,  dc  Essex,  decidieron  apartarse  del  metodo  tradicional-que  usaba  una  red  de  copo  fija  a  una  embarcacion  al  ancla — empleado  en  el 
estuario  del  Tamesis  para  capturar  espadin,  a  causa  de  necesitar  un  procedimiento  mas  movible  y  positivo.  Para  cstq  idearon  una  red  de 
arrastre  que  remolcaron  con  sus  dos  arrastreros  de  poco  calado,  39  pies  (11,9  m.)  de  eslora  y  construccidn  casi  revolucionaria,  provistos  de 
motores  Diesel.  Despuds  de  muchos  ensayos  lograron  perfeccionar  una  red  que,  mediante  el  uso  de  la  ecosonda.  demos  tro  tener  nuiy  buenas 
condiciones  para  capturar  espadin  de  buena  calidad. 

Este  trabajo  tambien  se  refiere,  en  detalle,  a  su  construcci6n  y  manipulaci6n  -  calamcnto  y  recogida — senalando  el  autor  que  si 
bien  ofrece  muchas  ventajas  sobre  los  artes  de  deriva  y  la  red  de  copo  fija  a  una  embarcaci6n  anclada  ("stow  net1*),  tiene  el  inconveniente 
de  no  seleccionar  la  pesca  y  de  capturar  a  veces  peces  pequenos.  Esto  es  talvcz  inevitable  a  causa  del  aumento  del  tamano  de  la  malla,  que 
incrementaria  el  numero  de  peces  enmallados  en  la  boca  y  el  cngullidor  dificultando  considerablemente  la  mampulacion  de  la  red. 


A  THOUGH  the  practice  of  two-boat  trawling 
for  sprats  is  only  in  its  seventh  year,  a  thriving 
sprat  fishery  existed  in  the  Thames  Estuary 
prior  to  the  Second  World  War.  Fishermen  used  the 
stow  net,  drift  netting  being  precluded  by  heavy  steamer 
traffic  and  shallow  water.  This  net,  held  open  by  baulks 
of  timber,  was  streamed  in  the  tideway  from  an  anchored 
smack.  Catches,  though  often  heavy,  were  unpredictable 
owing  to  the  immobility  of  the  net,  and  landings  often 
far  from  fresh,  several  tides  sometimes  being  needed 
to  produce  a  full  load. 

The  introduction  of  the  Larsen  trawl  in  1948  prompted 
two  Essex  fishermen,  Alf  and  George  Leggatt,  who  had 
for  some  time  believed  that  this  lack  of  mobility  could 
be  overcome  by  the  use  of  two  boats  to  spread  and 
position  the  net  under  power,  to  initiate  experiments. 
They  already  possessed  two  39  ft.  shallow  draught 
diesel  trawlers  of  modern  almost  revolutionary 


design,  equipped  with  small  trawl  winches  made  from 
back  axles  of  cars  and  capable  of  holding  70  fm.  of  wire. 
A  net  was  therefore  ordered,  to  their  own  design,  from 
the  Great  Grimsby  Coal  Salt  and  Tanning  Co.,  and 
trial  hauls  were  made  in  March  1950  as  the  sprat  season 
was  closing.  Results  were  disappointing,  and  it  was 
apparent  that  the  presence  of  gulls  was  no  reliable 
indication  of  the  presence  and  depth  of  fish. 

A  Kelvin  Hughes  MS24  echo  sounder  was  therefore 
installed  and  in  the  following  summer  they  steamed  west 
to  Cornwall  to  assess  the  new  technique's  usefulness 
in  catching  pilchard.  The  experiment  was  not  a  success, 
but  with  the  assistance  of  a  frogman,  the  late  J.  H. 
Hodges,  it  was  found  that  the  net  was  seldom  at  the 
depth  estimated,  and  that  the  vertical  opening  was  far 
less  than  the  intended  24  ft.  A  new  net  was  then  ordered, 
having  a  theoretical  opening  of  36  ft.  by  36  ft.,  which 
proved  to  give  a  vertical  opening  of  24  ft.  in  the  water. 


[348] 


THE    THAMES     FLOATING     SPRAT     TRAWL 


Armed  with  this  knowledge,  an  echo  sounder,  and  a 
good  deal  more  experience,  the  brothers  returned  to 
Whitstable,  and  in  the  autumn  were  able  to  locate 
good  schools  of  sprats  in  the  Estuary  and  to  land 
heavy  catches  regularly. 

This  regularity  of  landings  and  the  fresh,  undamaged 
quality  of  the  fish,  found  a  ready  market  with  the  canners, 
and  a  third  boat  was  engaged  to  carry  fish.  Detachable 
codend  sleeves,  when  full,  were  passed  to  this  boat  so 
that  fishing  could  be  resumed  by  the  main  pair.  This 
codend  sleeve  was  drawn  over  the  end  of  the  main  net, 
and  held  by  four  lashings. 

The  Leggatt's  example  was  soon  followed  by  other 
local  fishermen,  and  although  Mr.  Larsen  himself 
introduced  his  trawl  at  Tollesbury  and  Harwich,  the 
net  used  by  this  now  thriving  fishery  is  of  the  Leggatt 
pattern.  It  is  to  this  gear  and  its  operation  that  the 
following  description  applies. 

Broadly  speaking,  the  net  is  a  tapering  funnel  of 
square  section  with  wings  tapering  to  the  four  towing 
points  formed  by  the  roping.  Mesh  in  the  entry  is  of 
2  in.,  reducing  to  1  in.  in  the  codend  of  the  main  net, 
to  which  are  attached  three  20  ft.  lengthening  pieces  of 
proofed  cotton  net  of  I  in.  mesh,  ISO  meshes  wide, 
giving  an  overall  length  of  186  ft.  For  the  main  net, 
nylon  twine  and  nylon  roping  have  proved  to  be  far 
superior  to  natural  fibre  for  strength,  water  resistance 
(and  fuel  economy),  and  for  its  ability  to  withstand  rot. 
Up  to  the  present,  cotton  has  proved  satisfactory  for  the 
lengthening  pieces  as  these  are  soon  weakened  by  loading 
operations  and  are  renewed  before  rot  begins.  The 
headline  is  supported  by  six  or  eight  spherical  floats 
6  in.  in  diameter,  while  the  lower  wings  are  sunk  by  a 
67  Ib.  weight  on  either  wing  end,  shackled  on  to  a 
foot  of  light  chain. 

The  boats  should  have  engines  of  not  less  than  30 
b.h.p. — more  if  possible.  It  is  a  great  advantage  if  a 
"pair"  arc  matched,  or  have  at  least  the  same  draught, 
dimensions  and  engines,  with  revolution  counters,  so 
that  towing  effort,  drift,  and  leeway  are  identical  on 
each  side  of  the  net.  If  fish  are  to  remain  in  good  con- 
dition, a  spacious  fish  hold  is  essential,  so  that  the  depth 
of  fish  is  never  such  as  to  crush  those  underneath.  The 
boats  in  question  have  a  load  capacity  of  7  tons  without 
crushing,  while  the  wheelhouse  siting  gives  ample  room 
for  net  handling  and  loading,  allowing  great  scope  for 
the  warps  when  manoeuvring. 

The  winch  must  be  of  the  twin  drum  type,  and  should 
carry  sufficient  wire,  of  at  least  &  in.  diameter,  to  get 
the  net  into  whatever  depth  is  to  be  worked.  A  roller 
fairlead  of  the  seine  type  is  fitted  on  each  quarter. 

To  prepare  the  gear  for  fishing,  the  net  is  flaked  down 
aft  on  the  leader  boat  and  the  wings,  marked  to  identify 
upper  and  lower,  laid  out  ready  to  pass  to  the  other  vessel. 
The  codend  is  tied  about  6  ft.  back,  to  provide  slack 
net  for  loading,  using  a  codend  float  rope  of  ample 
length.  On  locating  a  satisfactory  school,  the  leader 
boat  steams  down  tide  to  overrun  it,  then  turns  into  the 
tide,  streaming  the  net  as  quickly  as  possible.  When  the 
net  is  streamed  as  far  as  the  entry,  speed  is  dropped 
to  steerage  way  and  the  net  checked,  while  the  other 
vessel  comes  alongside  to  receive  his  wings  from  the 
third  hand,  with  instructions  as  to  warp  length,  speed 


and  so  on.  To  avoid  delay,  the  second  boat  must  always 
stay  close  behind  the  leader,  and  it  is  advisable  to  shackle 
heavy  clip  hanks  to  the  warps,  so  that  the  wings  can  be 
attached  without  delay.  As  the  boats  part,  the  lower 
warps  are  released,  followed  by  the  upper  warps,  until 
the  required  marks  are  reached  on  the  wire.  Due  to  its 
greater  angle,  extra  length  is  required  on  the  lower  warp, 
according  to  the  depth  fished. 

No  hard  and  fast  rule  can  be  laid  down  for  the  length 
of  the  warp  needed  to  fish  any  given  depth,  as  this 
depends  on  the  varying  factors  of  speed,  type  of  net, 
and  distance  between  boats.  While  accurate  electronic 
aids  are  being  developed,  a  guide  can  be  obtained  by 
the  measurement  of  warp  angle,  and  the  calculation, 
with  the  aid  of  tables,  of  net  depth.  Station  keeping 
and  speed  affect  net  depth  considerably,  and  while  the 
former  comes  with  practice,  the  revolution  counter  is 
the  answer  to  the  latter.  Close  station  will  raise  the  net, 
wide  station  lower  it,  due  to  the  loss  of  way:  the  distance 
between  boats  should  be  such  as  to  extend  the  angle 
formed  by  the  net  itself. 

An  echo  sounder  on  each  boat  is  an  advantage,  to 
ensure  that  both  are  over  the  fish,  while  R/T  communica- 
tion is  a  valuable  adjunct  to  hand  signals  and  enables 
wider  searching.  The  sounders  should  be  compared  for 
depth  and  sensitivity  at  frequent  intervals. 

It  is  often  necessary  to  turn,  in  order  to  pass  through 
a  school  for  a  second  time,  the  outer  boat  turning  fast 
round  the  inner,  which  must  keep  a  pull  on  the  warps 
to  avoid  fouling  the  net  or  allowing  it  to  drop.  Towing 
against  the  tide  need  not  produce  headway,  it  being 
sufficient  to  stem  the  tide,  or  even  make  sternway 
against  it  when  very  fast-running. 

Length  of  haul  is  determined  by  the  density  of  the 
school,  and  it  is  possible,  under  ideal  conditions,  to 
catch  the  net's  capacity  of  200  bushels  (approximately 
5  •  1  tons)  in  as  little  as  15  minutes,  when  an  indication  is 
given  by  the  codend  float  pulling  under  and  by  loss  of  way. 
A  really  full  net  is  a  liability,  as  the  time  taken  to  get 
5  tons  of  tightly  packed  fish  alongside  safely  is  often 
more  than  that  taken  for  another  haul. 

To  haul  the  gear,  the  boats  are  sheered  alongside  and 
made  fast  with  prepared  breast  ropes  of  4  in.  coir,  and, 
in  bad  weather  also  by  springs.  One  cannot  have 
too  many  fenders,  preferably  doubled  tyres  on  chain  or 
wire.  Properly  made  fast,  two  matched  boats,  well 
fendered,  should  lay  together  without  damage  in  a  gale, 
but  if  damage  seems  likely,  the  wings  can  be  passed  to  the 
leader,  and  the  other  vessel,  with  one  man  on  board, 
taken  out  of  danger.  Hauling  is  generally  carried  out 
stern  to  wind,  keeping  the  warps  even,  until  the  wings 
are  reached,  when  the  slack  net  is  hand  hauled  over  the 
transom  or  quarter,  engine  power  being  used  to  keep 
the  boats  square,  and  to  tow  up  the  sleeve  should  it 
begin  to  sink.  Should  the  haul  be  good,  the  sleeve  is 
taken  forward,  overhauled,  and  supported  every  8  ft. 
by  chain  weighted  "girtles"  12  ft.  long,  which  arc  passed 
round  and  down  the  sleeve,  until  it  is  made  fast  in  bights 
to  the  bow.  Should  this  not  be  done,  the  fish  will  sink 
the  sleeve  vertically  when  they  die  and  lose  buoyancy, 
making  the  sleeve  difficult  to  recover.  In  this  event  the 
codend  float  and  line  are  used  to  help  bring  it  to  the 
surface. 


[349] 


MODERN     FISHING    GEAR     OF    THE    WORLD 


A  "cutting-off  ring"  of  A  in.  iron,  3  ft.  in  diameter, 
mounted  on  a  stout  staff,  is  used  to  cut  off  manageable 
quantities  of  fish  which  are  run  down  to  the  slack  net 
gained  after  removing  the  float  rope,  when  they  can 
be  lifted  inboard  manually  or  by  derrick. 

This  form  of  fishing  has  considerable  advantages  over 
drift  netting,  being  more  positive  in  action,  and  pro- 
ducing fish  that  are  unmarked  by  the  gillnet.  On  the 
adverse  side,  it  cannot  be  said  to  be  so  selective,  for  the 
catch  frequently  contains  a  proportion  of  smaller  fish. 
Without  the  use  of  a  mechanical  grader,  this  tends 
to  preclude  the  fresh  market,  leaving  only  the  canning 
industry  and  the  fishmeal  plant,  with  consequent  lower 
prices.  An  increase  in  mesh  size  has  been  suggested, 
but  in  the  writer's  experience  this  is  not  practical, 
as  with  the  existing  mesh  the  gear  becomes  at  times 
almost  unmanageable  owing  to  the  number  of  fish 


gilled  in  the  entry  and  funnel.  Should  the  mesh  of  the 
lengthening  pieces  be  increased,  the  same  would  occur 
over  another  60  ft.  preventing  free  water  flow,  damaging 
fish  and  obstructing  the  escape  of  "whitebait",  a  mixture 
of  immature  herring  and  sprats  which  abounds  in  the 
Estuary. 

It  seems  possible,  with  reservations,  that  this  form 
of  fishing  could  be  used  for  other  types  of  pelagic  fish. 
The  scaring  effect  of  the  net,  negligible  in  the  muddy 
Thames,  may,  however,  count  against  it  in  clear  water. 
The  poor  hauls  experienced  by  single  boats  towing  a 
modified  trawl  of  this  type  on  shallow  settings  would 
seem  to  indicate  that  the  sprat  is  able  to  evade  the  net 
when  startled  by  propeller  noise  above  it,  and  that  the 
sight  of  the  net  in  clear  conditions  might  have  the  same 
effect.  Further  information  on  this  point  may  soon  be 
available  after  trials  on  Cornish  pilchards. 


Underwater  photo  of  a  midwater  trawl  model,  scale  1  :  10,  with  a  hydrofoil  kite.     Photo:   J.  Sch&rfe 

[350) 


THE  DEVELOPMENT  OF   A  NEW  HERRING  TRAWL  FOR   USE 
IN  MIDWATER   OR   ON   THE  BOTTOM 

by 
W.  E.  BARRACLOUGH  and  A.  W.  H.  NEEDLER 

Fisheries  Research  Board  of  Canada,  Nanaimo,  B.C.,  Canada 

Abstract 

With  I  he  assistance  of  experienced  fishermen  and  the  testing  of  models,  the  trawl  described  in  Fisheries  Research  Board  of  Canada 
Bulletin  No.  104  was  developed  and  demonstrated,  it  is  a  one-boat  (175  h.p.)  trawl  of  light  nylon  construction  with  specially  designed 
curved  dual-fin  otter  boards  attached  on  pennants  to  keep  the  boards  and  towing  cables  away  from  the  front  of  the  net.  A  special  feature  is 
a  provision  for  opening  the  codend  while  still  in  the  water  to  remove  herrings  by  brailing.  Harry  in  1955  catches  of  up  to  35  tons  of  winter 
herrings  were  taken  in  20  min.  tows  in  depths  as  great  as  50  fm. 

Later,  improvements  were  made  by  which  the  trawl  was  used  commercially  with  success  both  in  midwater,and  with  the  otter  boards 
on  the  bottom  and  the  net  just  off  it,  but  even  with  a  still  stronger  version  of  the  original  net  and  towing  speeds  up  to  4*  knots,  it  has  failed 
to  caich  commercial  quantities  of  the  faster-moving  summer  herring.  Attempts  are  now  being  made  to  overcome  this. 

Misc  au  point  d'un  nouvcau  chalut  flottant  ou  de  fond  pour  la  peche  au  hareng 
Resume 

I  e  chalut  decril  dans  Ic  Bulletin  No  104  du  Fisheries  Research  Board  of  Canada  a  etc  mis  au  point  et  essaye  avec  Faide  de  pecheurs 
experiments  et  a  la  suite  d'cssais  sur  maqucttcs.  C'cst  un  chalut  en  nylon  fin  destine  a  etre  remorque  par  un  seul  navire  tie  175  CV  et 
equipe  dc  plateaux  a  deux  ailerons  d'une  cotirbe  speciale,  fixes  a  deux  cables  frapp6s  sur  les  funes  destines  a  maintenir  les  plateaux  et  les  funes 
ecartes  de  la  gucule  de  chalut.  C  et  engin  comporte  un  dispositif  special  d'ouveriure  du  cul-de-chalut  lorsque  ce  dernier  cst  encore  dans  1'eau 
de  facon  a  dechargcr  les  harcngs  a  repuisette.  Au  debut  de  1955,  on  a  peche  avec  ce  chalut  jusqifa  35  tonnes  de  harengs  d'hiver  en  20 
minutes  a  des  profondeurs  atteignant  50  brasses. 

Par  la  suite,  des  ncrfcclionncments  apportcs  a  ce  chalut  ont  permis  dc  I'utiliser  avec  de  bons  resultats  aussi  bien  entre  deux  caux 
qifavcc  les  plateaux  sur  Ic  fond  et  Ic  filet  legerement  au  dessus;  mais  on  n'a  pas  reussi  a  pechcr  avec  cet  engm  des  quantites  importantcs  de 
harengs  d'ete.  plus  vifs,  mcme  en  renforcant  encore  Je  chalut  et  en  uugmentant  la  Vitesse  de  chalutuge  jusqifa  4.5  noeuds.  Des  essais  sont  en 
cours  pour  rescind  re  ce  probleme. 

Evolution  de  una  nucva  red  de  arrastre  para  pcscar  arcnque  en  profundidades  intermedias  o  sobre  el  fondo  del  mar 
I-xtractu 

Con  ayuda  de  Pescadores  expenmentados  y  priiebas  de  modelos  se  dcmostro  y  perfecciono  el  funcionamiento  de  la  red  de  arrastre 
dcscrita  en  el  boletin  No.  104  del  Fisheries  Research  Board  of  Canada. 

T.ste  artc  se  proved 6  para  trahaiarcon  un  solo  barco  (175  C.V.)  ;  es  de  nylon  dclgado  y  tiene  puertas  curvas  con  dos  aletas,  u nidus 
mediantc  cabos  a  los  cables  de  arrastre  para  desviarlos  del  f rente  de  la  red.  Entre  sus  caracteristicas  especiales  figura  el  hecho  de  permitir  la 
abertura  del  copo  cuando  esta  aim  en  el  agua  para  sacar  el  arcnque  de  mvicrno  mcdiante  un  salabardo. 

A  pnncipios  de  ll>55,  en  20  minutos  sc  obtuvieron  con  este  arle  hasta  35  toneladas  de  arcnque  a  50  brazas.  Posteriormente  se 
perfecciono  y  uso  con  exito  comercial  en  profundidades  intermedias.  Al  utili/ar  puertas  dc  arrastre  se  pudo  pcscar  con  la  red  muy  cerca  del 
fondo,  pero  aim  con  modelos  mas  pesados  que  el  original  y  velocidades  hasta  de  4  1/2  nudos,  fue  imposible  capturar  gran  cantidad  de  arenquc 
de  vcrano  que  nada  mas  rapidamcnte.  hn  la  actualidad  sc  cstan  haciendo  nuevas  pruebas  para  cvitar  este  inconveniente. 


THE  introduction  of  midwatcr  trawling  for  herring  in 
European    waters,    and,    more    particularly,    the 
success  of  the  two-boat  trawl  developed  by  Robert 
Larsen    in    Denmark,    led   to   experiments    by    British 
Columbia  fishermen.  These   met  with  only   moderate 
success  and  a  demand  arose  for  the  Government  to 
develop   a   trawl    suited   to   conditions   in    the    British 
Columbia   herring  fishery. 

In  1954  work  was  started  by  the  Fisheries  Research 
Board's  biological  station  at  Nanaimo,  B.C.,  using  funds 
provided  by  the  Industrial  Development  Service  of  the 
Department  of  Fisheries,  to  develop  a  suitable  trawl  for 
use  by  moderate-sized  trawlers  or  multi-purpose  vessels. 
Such  a  trawl  has  been  developed  and  demonstrated,  and 
is  now  in  commercial  use  in  the  autumn  and  winter 
herring  fishery. 


The  broader  purpose  of  the  work  was  to  develop  a 
single-boat  mid-water  trawl  effective  in  exploratory  or 
commercial  fishing  for  faster-swimming  fish  than  the 
winter  herring.  In  this,  success  has  been  limited  and 
work  is  still  proceeding. 

DESIGN  AND  TESTING  OF  A  MIDWATER  TRAWL, 
1954-55 

The  first  stage  was  the  design  and  testing  of  the  midwater 
trawl  described  in  Bulletin  No.  104  of  the  Fisheries 
Research  Board  of  Canada.  A  trawl  was  needed  which 
could  be  towed  from  a  single  boat  of  about  150  to  1 75  h.p. 
The  fishermen  felt  that  the  conventional  mounting  of 
otter  boards  near  the  mouth  of  the  net  tended  to  scare- 
fish  away  and  should  be  avoided.  The  trawl  should  be 


[351] 


HANG   DIRECT    TO      t/«€'  BELFLEX    ROPE 
OATMCR    IN     S    MESHES  FRO*  EACH   WNG 

SQUARE     BETWEEN  WINGS  MONO     I  I*" 


BODY 

TAPER     4  BAR     I  POINT 


MODERN     FISHING     GEAR     OF    THE    WORLD 

5/«-    »RAIDED   NTLON 


SEIZING 


ZIPPER    STARTS  HERE 


TAPER  I  BAR    4  POINT 
END  Of  ZIPPER 


Fig.  I.     Sectional  view  of  the  first  net 

large  enough  to  make  paying  catches;  in  particular,  it 
should  have  a  large  vertical  opening  for  fishing  layered 
schools  of  herring,  but  be  light  enough  to  handle  easily 
on  small  vessels.  It  was  considered  important  to  be  able 
to  tow  the  trawl  at  a  higher  speed  than  in  the  past. 

Nylon  was  selected  for  less  weight  and  towing  resist- 
ance. It  was  decided  to  provide  for  opening  the  codend 
for  the  removal  of  herring,  thus  making  it  possible  to 
handle  substantial  catches  with  a  lighter  net  than  is 
required  if  the  whole  catch  is  to  be  lifted  aboard  in  the 
trawl  itself.  A  number  of  means  of  keeping  the  mouth 
of  the  net  open  were  explored  and  finally  a  special  otter 
board  was  designed  and  tested,  first  on  a  small  net  and 
then  on  a  full  scale  size.  The  board  was  suspended  on 
a  pennant  so  that  it  would  be  far  from  the  mouth  of  the 
net,  and  some  use  was  made  of  models  in  testing  pre- 
liminary designs. 

The  net.  The  mouth  of  the  net  is  35  ft.  square  and  the  net 
is  square  in  cross-section  throughout  its  whole  length  of 
180  ft.  Fig.  1  shows  its  construction  diagrammatically, 
indicating  the  size  and  number  of  the  meshes  in  each 
section. 

The  mouth  of  the  net  is  formed  by  lacing  the  forward 


Fig.  2.     Construction  ami  position  of  the  "zipper" 

Fig.  3.     Dual  fin  otter  hoard. 

Fig.  4.     Moore  depressor. 


SEIZING 


GALVANIZED   METAL   RINGS  -  I  !/•' 
SPACED     !•"  APART 


FORWARO 


ZIPPER   IS  LOCATED  AT  THE 
FORWARD  STARBOARD 


V  STCtL    PLATE 


[352] 


BRITISH     COLUMBIA     HERRING    TRAWL 


edges  of  the  triangular  wings  and  the  free  edges  of  the 
first  body  section  ("square")  to  four  lines  of  combination 
manila  wire  rope  ("Belflex")  cable  each  75  ft.  long.  These 
headlines  are  extended  forward  three  feet  to  eye-splices 
for  attachment  of  the  towing  lines,  so  that  the  shackles 
can  be  kept  free  of  the  netting  when  setting  or  stowing  the 
trawl. 

The  four  edges  of  the  net  are  supported  by  sidelines 
of  A  in.  braided  nylon  running  from  the  tips  of  the 
wings  to  the  end  of  the  codend.  An  opening  (the  "zipper") 
is  provided  extending  about  36  ft.  from  the  forward  end 
of  the  codend,  hack  along  the  upper  starboard  scam 


(fig.  2).  The  sidelines  are  doubled  to  form  this  opening, 
which  is  closed  by  lacing  the  nylon  rope  through  rings 
attached  along  each  side  at  18  in.  intervals.  The  codend 
is  closed  by  a  "poke-string"  of  ^  in.  braided  nylon 
which  passes  through  rings  near  its  end.  The  same  rope 
is  passed  forward  and  attached  to  the  foremost  ring  of 
the  "zipper"  to  be  available  for  hauling  the  codend 
forward  along  the  side  of  the  vessel  when  the  trawl  is 
brought  to  the  surface. 

Otter  Boards,  Floats  and  Depressors.  The  curved  otter 
boards  5,1  ft.  long  and  3  ft.  high  are  constructed  of 
laminated  plywood  bolted  to  angle-iron  strips  (fig.  3) 
to  which  upper  and  lower  horizontal  fins  are  attached 
for  stability.  The  correct  angle  of  attack  is  maintained 
by  the  length  and  position  of  the  chain  brackets  and 
by  small  vertical  fins.  Proper  balance  of  the  otter  boards 
is  obtained  by  the  adjustment  of  lead  weights  bolted 
parallel  to  the  lower  horizontal  fin. 

Eleven  Phillips  trawl  planes  are  lashed  about  2  ft.  apart 
along  the  headline  bosom.  Braided  nylon  leadline,  with 
about  25  Ib.  of  small  sectional  leads,  is  attached  at  18  in. 
intervals  to  the  footrope. 

Depressors  (fig.  4)  arc  used  on  the  bridles  just  in  front 


THWIBLE  a 

EYE  SPLICE  > 


WING 


Fig.  5.     Broiling  from  the  "zipper 


12  rings  toshcd  to 
f"»?4  nylon  rope 
tpoced  37  mtsht* 


f'nyfon  ropt  to 
trim  codtnd 


Fig.  6.    Modification  of  the  net  for  strengthening  and  reduction  of  the  towing  resistance. 


[353] 


MODERN     FISHING     GEAR     OF    THE    WORLD 


of  the  lower  wings  to  assist  in  keeping  the  lower  side  of 
the  mouth  opening  down. 

Bridles.  Four  30  fm.  lengths  of  $  in.  diameter  galvanized 
wire  rope  are  coupled  with  swivels  to  the  eyes  in  headline 
at  the  tips  of  the  four  wings,  the  lower  ones  being 
lengthened  with  a  fathom  of  chain  for  adjustment  to 
make  the  trawl  tow  horizontally.  The  two  bridles  from 
each  side  of  the  trawl  run  forward  to  an  eye  or  "tow- 
point"  from  which  the  warp  runs  to  the  vessel.  The  otter 
board  on  each  side  is  attached  to  the  tow-point  by  a 
10  fm.  pennant  of  £  in.  galvanized  wire  rope. 


Operation  of  the  Trawl.  Schools  of  herring  are  first  found 
by  echo-sounder  and  the  trawl  then  is  set  over  the  stern 
in  much  the  same  manner  as  the  conventional  otter  trawl 
in  British  Columbia.  The  depressors  are  slid  down  the 
lower  bridles  as  soon  as  the  net  is  in  the  water.  The  otter 
boards  are  quickly  shackled  to  the  pennants  as  the  latter 
are  unwound  from  the  winches  with  the  bridles.  Thedepth 
of  the  trawl  is  controlled  by  the  towing  speed  and  the  length 
of  the  warps,  and  is  calculated  from  the  angle  of  the  warps 
to  the  horizontal. 

When  the  trawl  is  hauled  the  otter  boards,  depressors 
and  body  of  the  net  are  taken  aboard  and  the  end  of  the 
codend  is  then  passed  forward  to  be  attached  near  the 
bow  of  the  vessel  (fig.  5),  using  the  extension  of  the 
poke-string  noted  above.  The  "zipper"  is  released  and 
the  net  held  open  with  poles  so  that  the  catch  can  be 
removed  by  brailing. 

Fishing  Tests.  Practical  fishing  tests  were  carried  out  in 
January  and  February,  1955,  from  a  62  ft.  1 75  h. p.  trawler 
with  typical  British  Columbia  equipment  and  layout,  i.e., 


free  working  space  aft,  two  trawl  winches  and  two  gal- 
lows on  the  stern  quarters.  Herrings  occur  at  this  time 
of  the  year  in  dense  layered  schools  5  to  10  fm.  thick 
which  tend  to  rise  off  the  bottom  in  the  evening  and 
descend  at  daybreak.  Catches  of  20  to  30  tons  were  made 
in  20  minute  midwater  tows  at  depths  of  15  to  30  fm.  but 
were  smaller  when  the  herring  were  more  scattered  dur- 
ing the  night.  Two  daylight  tows  at  between  45  and 
50  fm.  took  15  and  35  tons. 

Although  these  trials  were  successful  in  catching 
herring  in  midwater  in  commercial  quantities,  two 
difficulties  were  emphasized.  The  plywood  otter  boards 
were  designed  for  working  only  in  midwater  and  were 
readily  damaged  by  striking  bottom,  and  difficulty  was 
experienced  in  judging  and  controlling  the  depth  of  the 
net  accurately.  The  combined  result  was  that  concen- 
trations of  herring  close  to  the  bottom  could  not  be 
fished  effectively. 


ADAPTATION  OF  THE  TRAWL 
TO  THE  BOTTOM 


FOR  USE  CLOSE 


Modification  of  the  Gear.  By  doubling  the  sidelines  in 
the  front  part  of  the  net  and  attaching  them  to  the  quarter 
points  by  rings,  the  trawl  was  made  capable  of  standing 
greater  towing  strains  (fig.  6).  The  resistance  of  the  net 
was  also  reduced  by  using  less  webbing  hung  in  such  a 
way  as  to  give  the  same  mouth  opening  (fig.  7).  The 
combined  result  was  a  stronger  and  lighter  trawl. 

By  using  conventional  otter  boards  and  by  providing 
additional  lift  by  special  hydroplane  floats  on  the  upper 
bridles,  the  trawl  could  be  used  close  to  the  bottom  (fig.  8). 


-  -  NO  TAPER 


BAR      4  POINT   TAPER 


CODE NO 


[ 

M 

.  i 

— *)  lOOm 


__ 400m 


-HI- 


-  1 1/4*  MESH 


••ction  D 


••ction  A 


-75m  H(.^som._H( — ^  H^ — ^^ 

Ji/l-MCSM    — j    |— 4i*'MESM--|    | 5*  MC8M-  -j    |~       5'HCfM  J 


ff 

T 

I 

i« 

If 

T 


f7/^.  7.     Plan  of  the  modified  trawl  net.     Dimensions  in  meshes. 

Material  required: 
Nylon  seine  netting 

Twine  size                 Mesh  size  Depth                   Length 

Wings                .                                      9                            Sin.  50m.                    200m. 

Body  section  A                                         9                            Sin.  50m.                    868m. 

section  B                                         9                          4k  in.  50m.                    684m. 

section  C                                        9                          3k  in.  75m.                   510m. 

Codend             .                                     6                          /i  in.  100  m.                4t440  m. 

[354] 


BRITISH     COLUMBIA     HERRING     TRAWL 


Fig.  8.     Operation  of  the  modified  gear  with  the  otter  boards  on  the  bottom  and  the  net  about  2  fm.  clear  of  it. 


Fig.  9.    Procedure  or  "hook  up"  for  hauling  the  modified  fear. 

Stage  I.  Otter  board  in  trawling  position.  Pennant  is  slack. 
Note  the  5ft.  extension  in  upper  bridle. 

Stage  2.  Otter  board  unlocked  from  warp  link.  As  hauling 
begins  the  pennant  tightens  and  advances  the  upper 
bridle.  The  5  ft.  extension  slackens  as  it  advances. 

Stage  3.  Otter  board  free  from  strain.  The  5ft.  extension  is  now 
switched  to  the  lower  bridle  and  the  stopper  has  left 
the  link,  the  lower  bridle  passing  through  the  ring. 


In  order  to  have  the  net  operate  horizontally  in  this 
position  it  was  necessary  to  make  the  upper  bridles 
longer  than  the  lower.  This  in  turn  required  a  special 
hook-up  (fig.  9)  to  keep  the  headline  evenly  taut  as  the 
trawl  is  being  hauled  and  thus  avoid  the  fouling  of  the 
floats  with  the  netting. 

Fishing  Tests.  Attempts  to  catch  the  moderately 
active  early  autumn  herring  in  1955,  using  the  stronger, 
lighter  trawl  in  midwater  with  the  special  curved  otter 
boards  (fig.  3),  met  with  moderate  success.  Numerous 
catches  of  5  to  15  tons  were  made  in  30  min.  tows  in 
depths  of  30  to  50  fm.  —below  the  normal  fishing  depth 
of  the  conventional  purse  seine.  Catches  of  20  to  75 
tons  were  made  in  20  to  30  min.  tows  in  daylight  in  depths 
from  40  to  55  fm.  when  the  trawl  was  used  with  conven- 
tional otter  boards  on  the  bottom,  the  net  being  about 
2  fm.  off  the  bottom. 

Commercial  use.  A  number  of  commercial  trawlers,  in 
the  late  autumn  and  winter  of  1955-56,  used  the  trawl 
close  to  the  bottom  with  conventional  otter  boards. 
Seven  trawlers  took  2,000  tons  of  herrings  averaging  5  to 
6  tons  per  half-hour  tow.  In  the  winter  of  1956-57,  19 
trawlers  took  part  in  the  fishery. 

Development  of  a  dual-purpose  otter  board.  To  facili- 
tate quick  changes  from  strict  midwater  trawling  to 
trawling  with  otter  boards  on  the  bottom,  or  vice  versa, 
a  dual-purpose  aluminium  otter  board  was  developed  in 
1956.  A  V-shape  gives  stability  at  speeds  from  2  to  6 
knots.  Vertical  stability  is  aided  by  an  air  tube  along  the 
upper  edge  and  horizontal  fins  reduce  oscillations.  The 


[355] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


angle  of  attack  of  the  otter  board  under  tow  is  controlled 
by  conventional  chain  brackets  and  a  vertical  stabilizing 
fin.  These  boards  have  now  replaced  the  curved  plywood 
otter  boards  and  are  manufactured  under  patent  in 
Canada. 

FURTHER  DEVELOPMENTS 

Midwater  trawl  for  small  vessels.  A  small  version  of  the 
modified  stronger  trawl  (fig.  6)  was  built  for  use  on  small 
trawlers  of  about  45  ft.  When  used  with  the  aluminium 
dual-purpose  otter  boards  in  the  autumn  of  1956  it 
caught  herring  in  commercial  quantities  both  in  midwater 
and  at  the  bottom. 

Midwater  trawls  for  faster-swimming  fish.  Attempts 
are  being  continued  to  develop  a  midwater  trawl  for 
catching  in  commercial  quantities  summer  herring  and 
other  fish  more  active  than  winter  herring.  Two 
approaches  are  being  made  using  modifications  of  the 


midwater  trawl  described  above— fast  towing  of  a  speci- 
ally strengthened  net  and  slow  towing  of  a  large-mouthed 
net.  A  trawl  is  also  being  tested  which  embodies  the 
piinciple  of  the  high-speed  plankton  net,  i.e.,  free  passage 
for  water  in  the  centre  of  the  net,  but  it  is  too  early  to 
assess  the  performance  of  these  nets. 

CONCLUSION 

The  success  of  midwater  trawls  in  catching  winter 
herring  is  encouraging.  Efforts  to  improve  midwater 
trawls  as  tools  for  exploratory  or  commercial  fishing  for 
more  active  species  seem  highly  desirable.  The  experi- 
ments described  in  this  paper  have  been  carried  out  with 
a  practical  trial-and-error  approach.  There  is  obviously 
need  also  for  a  fundamental  scientific  approach  if  the 
potential  of  this  kind  of  fishing  is  to  be  fully  explored, 
and  physicists  and  engineers  must  be  enlisted  to  help  the 
fishermen  and  the  biologists. 


Big  hydrofoil  (" Suberkrub^)  oner  board  rigged  for  bottom  trawling.     The  attached  casing  contains  the  recording  unit 
of  a  dynamometer  measuring  the  resistance  of  the  net  during  towing.  Photo:  J.  Scharfe 

[  356  ] 


ON  THE  USE   OF  MIDWATER   TRAWLS  FOR   ANCHOVY   IN  THE 

BLACK  SEA 

by 

ERDOGAN  F.  AKYIJZ 

Fishery  Research  Centre,  Meat  and  Fish  Office,  Istanbul,  Turkey 

Abstract 

This  paper  describes  experiments  made  with  a  Danish  Vinge-trawl  (a  high-opening  ground  trawl)  which  was  rigged  for  fishing  in 
midwater  and  towed  by  a  single  boat.  Very  dense  schools  of  anchovy  were  found  and  hauls  of  over  1  ton  were  taken  in  about  10  to  20  min. 
A  second  series  of  experiments  confirmed  that  the  introduction  of  this  type  of  gear  would  produce  large  quantities  of  anchovies  in  the  winter 
months  when  the  fish  were  readily  accessible  to  the  midwater  trawl. 


Rfeume 


Sur  1'emploi  du  chalut  flottant  pour  1'anchois  dans  la  mer  Noire 


L'autcur  decrit  des  experiences  effectuees  avec  un  chatut  danois  Vinge  (un  chalut  de  fond  a  ouverture  £levde)  qui  etait  gr66  pour  la 
peche  entre  deux  eaux  et  6tait  remorqud  par  un  seul  bateau.  On  a  trouve  des  banes  d'anchois  tres  denses  et  des  traits  dc  chalut  de  plus  d'une 
tonne  ont  effect u£s  en  10  a  20  minutes  environ.  Une  seconde  serie  d'expSriences  a  confirme  que  Tintroduction  dc  cc  type  d'cngin  p rod ui rait 
de  gran  des  quantites  d'anchois  pendant  les  mois  d'hiver  quand  less  poissons  sont  £  la  portee  du  chalut  flottant. 

Uso  de  redes  de  arrastre  pelagicas  para  la  pesca  de  anchoa  en  el  mar  Negro 
Extracto 

En  este  trabajo  so  describen  las  pruebas  hechas  con  una  red  de  arrastre  "Vinge"  (arte  de  fondo  con  boca  dc  gran  altura)  construida 
para  la  pesca  en  medias  aguas  con  ayuda  de  una  sola  embarcacion.  Durante  los  lances  sc  encontraron  cardumenes  muy  densos  que 
permitieron  capturar  mas  de  una  tonelada  dc  pescado  en  10  a  20  minutos.  En  una  segunda  serie  de  pruebas  se  confirm6  que  la  introducci6n 
de  cstc  lipo  de  red  permitiria  obtener  grandes  redadas  de  anchoas  durante  los  meses  de  invierno,  cuando  los  peces  pueden  scr  capturados  con 
facilidad  mediantc  redes  de  arrastre  remolcadas  a  profundidades  intermcdias. 


THE  Turkish  Meat  and  Fish  Office  has  emphasized 
in  development  plans  the  importance  of  increased 
catches  of  anchovy  in  the  Black  Sea,  as  this  fish 
is  to  be  the  principal  raw  material  for  the  fish-meal 
plant  which  is  being  built  in  Trabzon.  Until  now,  the 
anchovy   has   been   fished   by  a  two-boat   purse  seine 
(local  name  girgir)  but,  in  February,  1956,  experiments 


Fig.  1.     Construction  plan  of  Vinge-trawl  net. 


were  made  in  Fatsa  Bay,  on  the  Turkish  Black  Sea 
coast,  using  a  Danish  Vinge-trawl.  This  was  specially 
rigged  for  midwater  fishing  of  the  dense  anchovy  schools 
which  are  frequently  found  in  deep  water  during  the 
day  time.  This  paper  reports  the  results  obtained  and 
makes  some  suggestions  as  to  the  use  of  such  trawls 
in  this  fishery. 

EXPERIMENTAL  HAULS 

Details  of  the  vessel  and  gear  and  their  operation  are 
given  below  (figs.  1  and  2). 

1.  Vessel:  MjV Arar,  starboard  side  trawler;  380  h.p. 
dicsel  propulsion,  28  m.  overall  length, 
173  gross  tons. 


Fig.  2.     Connection  of  wing  tips  to  otter  hoards. 


[3571 


MODERN     FISHING     GEAR    OF    THE    WORLD 


2.    Gear:      Danish  Vinge-trawl,  23  -6  m.  long  headline; 

31-8  m.  long  footrope;  1-8x0-9  m.  long 

otter  boards. 
Mesh  size:     upper  wings  18/1 1  cm.;  lower  wings  18/1 1/9 -5 

cm.;   square  9-5  cm.;  belly  and  batings 

7-3/3/1-6  cm.;  codend  1  cm. 

Eight  hauls  were  made  in  all,  and  the  duration  of 
each  varied  from  10  to  15  minutes. 


Date      No.  of  Duration    Warp     Depth  ofHsh  Time  of  Ancftovy 
haul  paid  out        school  (ley 


February     1 
22,  1956      2 


12  min. 
16  min. 


25  fm.    10  to  40  fm.  Daylight 
75  fm.     8  to  30  fm. 


3 

15  min.  100  fm.  30  to  70  fm.  Daylight 

4 

15 

75 

10  to  40 

Night    -2 

February 

5 

15 

75 

10  to  40 

-2 

24,  1956 

6 

15 

75 

15  to  40 

7 

19 

75 

20  to  40 

•4 

8 

17 

75 

20  to  45 

Notes. —   1.   Duration   is    the    time    in    minutes  from   warps 
blocked-in  to  knock-out. 
2.     Engine  speed  in  all  hauls —230  r.p.m. 

With  the  exception  of  the  first  haul,  the  quantities 
of  fish  caught  did  not  vary  appreciably.  It  is  apparent 
that  not  enough  warp  was  paid  out  during  the  first 
haul  to  enable  the  trawl  to  fish  at  the  proper  depth. 

These  few  hauls  give  no  indication  of  the  relation 
between  catch,  length  of  haul,  time  of  day,  or  the  phos- 
phorescence made  by  the  net. 

The  time  required  for  hauling  and  re-shooting  (in- 
cluding the  time  spent  searching  for  a  new  school  or 
running  back  over  the  same  school  to  shoot  again) 
averaged  45  minutes.  However,  this  time  could  be 
longer  or  shorter,  depending  on  the  amount  of  fish 
caught  and  the  time  taken  in  handling  the  catch. 

The  market  value  of  the  anchovy  at  the  time  of  the 
experiment  was  0-50  Turkish  Lire  per  kilo  and,  with 
an  average  value  of  500  Turkish  Lire  per  haul,  it  seems 
that  a  highly  profitable  trawl  fishery  could  be  established. 

Echo  sounder  observations  showed  that  anchovies 
have  a  tendency  to  congregate  in  the  submarine  valleys, 
which  may  make  trawling  rather  difficult. 

At  the  time  of  these  brief  observations,  the  anchovy 
appeared  to  show  negative  reaction  to  both  natural  and 


artificial  light.  During  the  day  the  schools  stayed  very 
deep,  the  lower  limit  being  160  m.  No  fish  school  was 
detected  below  that  depth,  due,  I  believe,  to  the  lack 
of  oxygen. 

DISCUSSION 

No  midwater  trawl  was  available  so  a  Danish  Vingc-trawl 
was  used.  This  high-opening  bottom  trawl  was  rigged 
for  one-boat  midwater  trawling  (see  fig.  2)  by  FAQ 
Master  Fishermen  (from  Iceland)  who  worked  in 
Turkey,  helping  to  improve  local  gear  and  introduce 
new  fishing  methods.  The  results  show  that,  in  the 
anchovy  fishery  of  the  Black  Sea,  pelagic  trawling  would 
be  profitable.  Such  trawls  have  some  definite  advantages 
over  the  girgir  seines  (the  old,  traditional  two-boat 
purse-seines)  such  as  possibility  of  fishing  in  rough 
weather,  both  day  and  night,  and  at  greater  depths  than 
can  be  done  with  the  seines.  Introduction  of  this  gear 
to  the  fishery  would  be  a  very  useful  step  in  creating 
a  balanced  industry  because  the  fish  are  highly  accessible 
to  the  trawl  during  the  winter  season. 

In  March/April  1957,  a  FAO  fishing  expert  again 
fished  experimentally  for  anchovy  with  a  Vinge-trawl, 
rigged  in  a  similar  manner  and  operated  from  a  180  h.p. 
boat  of  the  Pacific  seiner  type.  He  obtained  catches  of 
10,000  to  over  20,000  Ib.  per  day  (6  to  8  hauls  of  20 
minutes  each)  and  once  he  got  a  full  trawl  in  a  10  minute 
tow  (approximately  10  tons). 

The  fish  were  landed  in  good  condition  and  trucked 
to  Ankara  for  marketing. 

There  is  little  doubt  that  pelagic  trawls  would  be  a 
very  efficient  gear  for  fishing  anchovy.  If  schools  are 
detected  very  deep  during  the  day,  their  lower  limit  could 
be  considered  as  a  "bottom"  and  the  trawls  could  be 
operated  at  that  depth.  This  will  minimise  the  escape 
of  fish  below  the  footrope.  Furthermore,  the  trawl 
could  be  rigged  with  kites  to  lead  the  fish  into  the  net, 
thus  increasing  their  vulnerability  to  the  gear.  It  is 
believed  that  the  fish  will  not  dive  deep  due  to  certain 
hydrographic  conditions  obtaining  in  these  waters. 
LITERATURE 

Aascn,  Olav  and  Akyuz,  Erdogan  (1956).  "Fishery  Investigations 
in  the  Turkish  Black  Sea  Waters  with  Special  Reference  to 
Anchovy.'* 

Reports  from  the  Fishery  Research  Centre,  Meat  and  Fish 
Office,  Vol.  It  No.  7,  Istanbul. 


358 


OTTER   BOARDS   FOR  PELAGIC  TRAWLING 

by 

F.  SUBERKRUB 

Hamburg,  Germany 


Abstract 

After  discussing  the  disadvantages  of  the  common,  plane  otter  boards  for  pelagic  trawling,  a  hydrofoil  type  of  otter  board  constructed 
by  the  author  is  described.  By  using  a  simply  curved  profile,  turbulence  is  avoided  and  towing  resistance  decreased.  An  u asymmetrical 
positioning  of  the  bracket  (for  attachment  of  the  warp)  is  suggested  to  gain  a  slightly  upward  directed  component  of  the  shearing  power  for 
facilitating  the  depth  regulation  of  the  trawl. 


Resume 


plateaux  de  chalut  pour  Ic  chalutage  entre  deux  eaux 


Apres  avoir  examine  les  inconvenients  des  plateaux  de  chalut  ordinaires,  plans,  pour  la  pechc  cm  re  deux  eaux,  Tauteur  decrit  un 
plateau  du  type  a  surface  hydro-dynamiquc  qu'il  a  construit.  Hn  utilisant  un  profil  simplemenl  courbc,  on  evite  la  turbulence  et  la 
resistance  au  remorquage  est  diminucc.  (I  est  suggcre  de  placer  ('attache  de  la  fune  en  position  asymetnquc  pour  tirer  profit  d'une  composante 
tegeremcnt  dirigee  vers  le  haul  de  la  force  d'ecartement  el  faci liter  Ic  reglage  de  la  profondeur  clu  chalut. 


Pucrtas  de  arrastre  para  redes  pel&gicas 
Extracto 

Despucs  de  analizar  los  inconvenientes  dc  las  puertas  de  arrastre  comunes  utilizadas  en  las  redes  pelagicas,  el  autor  describe  un 
dispositivo  hidrodinamico  que  ide6  para  este  tipo  de  artc  dc  pesca.  Al  utilizar  un  perfil  curvo  scncillo  se  evita  la  turbulencia  y  disminuye 
la  resistencia  al  arrastre.  Para  obtener  un  ligero  esfuerzo  separantc  hacia  arnba  que  facilite  la  rcgulacion  de  la  prof undi dad  de  la  red,  se 
sugiere  colocar  asimetricamcnte  el  brazo  de  la  puerta  que  se  conecta  con  el  cable  de  arrastrc. 


OTTER  boards  commonly  used  in  bottom  trawling 
are  rigged  in  such  a  way  that,  when  they  are  not 
in  contact  with  the  bottom,  a  certain  part  of 
their  shearing  power  is  directed  obliquely  downwards. 
This  results  in  difficulties  when  such  boards  are  used 
for  midwater  trawling  and  are  lifted  off  the  bottom. 

The  depth  of  a  pelagic  trawl  can  be  regulated  by 
altering  either  the  towing  speed  or  the  length  of  the 
gear.  At  a  constant  speed  of  the  vessel,  the  shortening 
of  the  warps  produces  an  increased  towing  speed  of 
the  gear  itself  during  the  time  of  heaving,  and  this  in 
turn  increases  the  downward  directed  component  of  the 
shearing  power  of  such  boards.  The  result  is  that  the 
lifting  process  is  retarded.  This  also  happens  when  the 
gear  has  to  be  lifted  during  fishing  by  increasing  the 
towing  speed  of  the  vessel,  especially  when  the  gear 
operates  near  the  surface.  In  this  case,  furthermore,  the 
angle  of  the  warps  to  the  horizontal,  and  consequently 
the  upward  directed  component  of  the  towing  force, 
becomes  small,  whilst  the  downward  directed  component 
of  the  shearing  power  of  the  boards  remains  unaltered. 

Another  disadvantage  of  the  common  boards  lies 
in  the  intermittent  turbulence  which  is  set  up  and 
detaches  itself,  causing  variations  of  the  shearing  and 


resistance  forces.  In  the  same  way  as  toy  kites  weave  and 
stall,  the  boards  travel  unsteadily  and  may  even  turn 
over,  fouling  the  gear  extensively  and  with  astonishing 
speed. 

These  are  the  main  reasons  why  common  otter  boards 
are  not  suitable  for  pelagic  trawling.  Boards  are  needed 
which: 

(1)  Create  little  or  no  turbulence;  and 

(2)  Have  no  downward  shear  and  act  only  in  a 
horizontal,  or  even  in  a  slightly  upward,  direction. 

The  turbulence  can  be  avoided  by  choosing  short 
upper  and  lower  edges  and  by  using  curved  profiles 
instead  of  a  plane.  The  author,  therefore,  constructed 
high  and  narrow  boards  with  a  curved  profile.  Figs.  1 
to  4  illustrate  constructional  details,  and  figs.  5  to  7  show 
the  board  in  different  positions,  according  to  different 
points  of  attachment  of  the  bracket  which  takes  the 
warp.  To  simplify  the  explanation,  only  one  bracket  is 
shown  in  the  drawings. 

With  no  other  forces  acting,  the  centre  of  gravity, 
as  is  well  known,  always  takes  a  position  directly  below 
the  point  of  suspension.  If  the  bracket  is  fixed  in  the 
middle  of  the  board,  the  board  will  therefore  take  up 


[359] 


MODERN     FISHING     GEAR     OF    THE    WORLD 


Bracket  fur  attach- 
nt of  the  warp 


me 
low 
high 


jsh 


caring  power 


\\- 

\\ 
\\ 


xo 


1.  -Board 

2.  -Ballaat  weight 

3.  -Bracket  for  attachment 

of  the  warp 


4   "Point  of  application  of  the  warp 

5.  •  Holes  for  attachment  of  the  warp 

6.  'Shearing  power 


an  oblique  position  under  tow  and  the  outward  tilt 
will  produce  a  shearing  power  which  is  partly  directed 
downwards  (fig.  5). 

If  the  bracket  is  fixed  above  the  middle  of  the  board, 
the  centre  of  shearing  will  lie  below  that  point  so  that 
the  lower  part  of  the  board  will  turn  outwards  and  the 
board  may  take  up  a  somewhat  vertical  position  (fig.  6). 

If  the  bracket  is  fixed  higher  above  the  middle,  the 
shearing  power  of  the  lower  part  of  the  board  will 
increase.  As  the  downward  directed  power  of  the  ballast 
remains  the  same,  the  lower  part  of  the  board  will  go 
beyond  the  vertical  position  and  the  board  will  gain 
a  lifting  component  (fig.  7),  the  strength  depending 
on  the  actual  position  of  the  board. 

This  position  depends  not  only  on  where  the  bracket 
is  fixed  but  also  on  the  towing  speed.  This  may  be  ex- 
plained by  an  example:  with  an  otter  board  of  4m2 
size  the  part  below  the  bracket  may  be  2-3  m2  and  the 
upper  part  1  -7  m-.  If  it  is  assumed  that  with  a  3-5  knot 
towing  speed  the  shearing  power  of  the  whole  board  is 
800  kg.,  the  part  below  the  bracket  would  have  460  kg., 
while  the  upper  part  would  only  have  340  kg.  The 
difference  of  120  kg.  would  push  the  lower  part  of  the 
board  outwards. 

If,  in  order  to  lift  the  gear,  the  towing  speed  is  in- 
creased, for  instance,  to  4-0  knots,  the  shearing  power 
of  the  whole  board  would  be  increased  to  J,040  kg. 
(increasing  as  the  square  of  the  speeds).  The  lower  part 
would  then  have  600  kg.  and  the  upper  part  440  kg., 
i.e.  the  difference  would  increase  from  120  kg.  to  160  kg. 
As  the  ballast  remains  the  same,  the  lower  edge  of  the 
board  would  be  pushed  outwards  even  more  and  the 
lifting  component  of  the  shearing  power  would  become 
stronger.  This,  together  with  the  increased  pull  on  the 
warps,  would  aid  in  lifting  the  gear  towards  the  surface. 

Thus,  by  establishing  the  proper  proportions  for 
the  upper  and  lower  parts  of  the  board  and  the  ballast, 
and  by  selecting  the  suitable  position  of  the  bracket 
for  the  warps,  it  becomes  possible  in  using  such  boards 
to  obtain  speedy  depth-regulation  of  pelagic  trawls. 


Echo  trace  of  a  pelagic  sardine  school  off  the  Brittany  coast. 

[360] 


Photo:  Elac,  Kiel 


CONSIDERATIONS   ON  A  NEW  TYPE  OF  MIDWATER   TRAWL 

by 

Y.    GROUSELLE 

Saint-Malo,  France 

Abstract 

When  considering  the  shape  a  one-boat  midwatcr  trawl  should  have,  we  find  that  the  two  most  important  factors  are  the  total  opening 
and  the  stability  of  the  trawl  during  fishing. 

The  author  claims  that  this  can  best  be  obtained  by  giving  the  net  a  triangular  opening  and  by  having  a  sufficiently  powerful  central 
lifting  apparatus  which  would  facilitate  the  stabili/ation  of  the  net's  position  in  the  water  when  towing.  Such  a  triangular  opening  would 
allow  the  net  to  have  an  apron  instead  of  overhang,  so  that  the  headline  would  come  well  back  of  the  foot  rope.  By  using  suitable  shearing 
apparatus,  such  a  midwater  trawl  would  be  easier  to  keep  at  any  required  depth. 

It  is  claimed  that  the  Rxocct  board  possesses  the  shearing  power  required,  both  for  vertical  lift  of  the  headline  and  for  opening  the 
wings  of  the  trawl.  In  the  latter  function,  by  adjusting  the  shear  to  the  proper  angle,  it  would  mean  an  improvement  on  the  present  use  of 
weights.  The  shearing  power  of  the  Exocet  increases  with  the  speed  of  towing,  whereas  the  downward  action  of  towed  weights  decreases 
as  the  speed  increases. 

The  paper  further  gives  details  of  the  construction  and  operation  of  the  Exocet. 


Resting 


Considerations  sur  un  Nouveau  Type  de  Chalut  Flottant 


Quand  on  considere  la  forme  que  doit  avoir  un  chalut  flottant  traind  par  un  seul  bateau,  on  trouve  que  les  factcurs  les  plus  import  ants 
sont  1'ouvcrture  totale  et  la  stability  pendant  la  peche. 

L'auteur  declare  que  cela  pcut  ctrc  obtenu  de  la  me  i  I  leu  re  facon  en  donnant  au  filet  une  ouverture  triangulaire  ct  en  ayant  un  dispositif 
d'ouverture  vcrticalc  suffisamment  puissant  qui  faciliterait  la  stabilisation  de  la  position  du  filet  dans  1'eau  quand  il  est  remorque.  Une  telle 
ouverture  triangulaire  permet trait  au  filet  d 'avoir  la  partie  inferieure  de  la  gucule  depassant  et  ainsi  la  corde  de  dos  serait  bien  en  retrait  par 
rapport  au  bourrclet.  En  utilisant  un  dispositif  de  plongdc  bien  adaptd,  ce  chalut  flottant  pourrait  plus  facilement  etre  maintenu  £  la  pro- 
fondeur  requise. 

Le  pannea u  clevatcur  Exocet  possede  la  pouss£c  ascension nelle  necessaire  pour  lever  verticalcmcnt  la  corde  de  dos  et  ecartcr  les  ailes 
du  chalut.  Dans  cette  derniere  fonction  en  reglant  le  panneau  selon  le  bon  angle,  il  apporte  une  amelioration  a  Temploi  actuel  de  poids.  La 
puissance  ascensionnelle  de  1'Exocet  augmente  avec  la  vitessc  de  remorquage,  alors  que  Faction  de  plong£e  des  poids  remorques  dimnue 
quand  la  vitcsse  augmente. 

La  communication  donnc  des  details  sur  la  construction  et  le  fonctionnement  de  P Exocet. 

Consideraciones  sobre  un  nuevo  tipo  de  red  de  arrastre  flotante 
Extracto 

Al  considcrar  la  forma  de  una  red  de  arrastre  flotante  remolcada  por  una  sola  embarcaci6n,  encontramos  que  los  dos  factores  mas 
importantcs  son  la  abertura  total  de  la  boca  del  arte  y  su  estabilidad  durante  cl  lance. 

El  autor  afirma  que  esto  puede  lograrse,  en  mcjores  condiciones,  con  una  red  de  boca  triangular  y  mediante  un  poderoso  elevador 
central  que  facilite  la  estabilidad  del  arte  en  cl  agua  al  arrastrarlo.  Una  boca  como  la  descrita  permitira  a  la  red  disponer  de  una  anteeamara 
en  vez  de  visera,  de  manera  que  la  relinga  superior  se  halle  detras  de  la  inferior.  Con  dispositivos  adecuados  serfa  mucho  mas  facil  mantener 
un  arte  de  esta  naturaleza  a  la  profundidad  descada. 

Se  afirma  que  el  elevador  Exocet  posee  las  condiciones  requeridas  tanto  para  elcvar  la  relinga  superior  al  nivel  dcseado  como  para 
scparar  las  bandas  de  la  red.  Esto  ultimo  sc  logra  mediante  la  colocaci6n  del  elevador  en  el  angulo  adecuado,  obtenidndose  mucho  mejor 
rcsultado  que  con  los  plomos  dc  uso  corriente.  La  accion  del  Exocet  aumenta  con  velocidad  de  arrastre,  mientras  que  la  sumersion  de  los 
pesos  durante  el  remolque  es  inversamente  proporcional  a  la  velocidad. 

El  trabajo  tambi£n  contiene  detalles  de  la  construcci6n  y  funcionamiento  del  Exocet. 


IT  is  particularly  important  that  a  one-boat  midwater 
trawl  should  have  a  large  opening  with  good 
stability  when  fishing.  Such  stability  cannot  be 
obtained  from  the  symmetry  of  the  net  alone;  there 
must  be  a  force  acting  in  the  vertical  direction  to 
counteract  the  wobbling  and  weaving  of  the  otter  boards. 

It  therefore  appears  logical  to  give  the  opening  of  the 
net  a  triangular  or  trapezoidal  form  because  that  would 
enable  one  or  several  lifting  devices,  acting  at  the  summit 
of  the  triangle  or  trapezium,  to  produce  a  stabilizing 
effect. 

The  ordinary  bottom  trawl  net  has  an  overhang  and  the 
headline  comes  well  ahead  of  the  footrope.  All  fish 
passing  under  the  headline  are  led  to  the  codend,  and 
the  sea  bottom  forms,  as  it  were,  the  complement  to 
the  overhang. 

The  case  is  different  with  midwater  trawls  as  the  fish, 


which  have  a  tendency  to  dive,  must  be  stopped  and  led 
further  into  the  net  by  the  belly  webbing.  In  the  midwater 
trawl,  therefore,  the  footrope  should  jut  further  forward 
than  the  top  of  the  net,  which  suggests  that  the  tetragonal 
form  (fig.  \d)  or  trapezoidal  (fig.  \b)  would  be  preferable 
to  the  pyramidical  form.  Such  a  form  can  be  roughly 
obtained  by  turning  the  trawl  on  its  back,  so  that  the 
square  is  on  the  lower  lip  of  the  net,  well  forward  of  the 
rest  of  the  body.  It  should  be  of  rather  smaller  mesh 
to  lead  the  fish,  which  tend  to  dive,  further  into  the 
belly.  The  horizontal  opening  of  the  net  could  further  be 
improved  by  the  use  of  shearing  devices,  in  which  a 
suitable  amount  of  depressor  action  could  be  introduced 
to  help  bring  the  trawl  to  the  proper  depth  of  operation. 
This  is  important  as  the  diverging  force  of  such  deflectors 
increases  with  the  towing  speed,  whereas  the  downward 
action  of  sinkers  decreases  with  the  speed. 


[361] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


Exocet  Kibes 


back   line 


Side  Line 


Exocet  Stabiliser -^y 
( 

Fig.  2.    Midwater  trawl  with  4  Exocet  shearing  boards,  two  of  which  act  as  a  kite. 


With  the  help  of  FInstitut  Scientifique  des  Peches 
Maritime*,  experiments  were  carried  out  from  the 
research  vessel,  President  Theodore  Tissier,  in  July  1957, 
from  which  it  appeared  that,  by  using  four  Exocet 
boards  on  a  net  constructed  as  described  above,  good 
opening  and  stability  are  obtained  (fig.  2). 

THE  EXOCET  KITE 

The  apparatus  consists  of  a  board  set  in  an  aluminium 
frame  and  armed  with  two  or  three  floats  to  give  initial 
lift  (fig.  3).  The  aluminium  frame  is  attached  to  the 
middle  of  the  headline  and  allows  the  shearing  board 
freedom  of  movement.  The  board  itself  takes  an  angle 
of  about  30  degrees  to  the  horizontal  as  soon  as  the 
net  is  towed,  thereby  producing  a  lifting  force  which 
increases  with  the  towing  speed. 

EXOCET  STABILIZERS 

In  one-boat  midwater  trawling,  one  of  the  main  prob- 
lems is  the  instability  of  the  otter  boards.  The  Exocet 


Codend 


Headline 


Foot  rope 

Fig.    I  (a).    The  shape  of  the  proposed  midwater  trawl  with 
triangular  opening  and  protruding  lower  lip. 


Codend 


Headline 


Footrope 


Fig.  I  (b).    Proposed  shape  of  midwater  trawl  of  trapezoidal 
opening  and  protruding  lower  lip. 


Fig.   3.     The  Exocet  kite. 


stabilizer  attached  to  the  normal  type  of  otter  board 
makes  it  function  efficiently  in  any  weather  and  trawling 
conditions. 

The  stabilizer  is  a  rectangular  panel  made  of  aluminium 
alloy  to  which  are  fitted  5  or  7  floats  near  the  upper  edge. 
The  lower  edge  is  ballasted  so  that  the  panel  is  com- 
pletely stable  in  itself. 

The  stabilizer  is  attached  to  the  otter  board  by  rope 
brackets  and  a  swivel  joint,  in  such  a  way  that  the  inherent 
tendency  of  the  board  to  capsize  is  counteracted.  As 
the  outward  and  downward  thrust  of  the  stabilizer 
increases  in  proportion  to  the  towing  speed,  the  in- 
creased resistance  of  the  net  at  higher  speeds  is  counter- 
acted by  increased  opening  force.  The  net  opening 
is  therefore  stable  at  all  towing  speeds. 

The  downward  thrust  of  the  otter  boards  equipped 
with  Exocet  stabilizers  is  governed  by  the  towing  speed, 
so  that  the  depth  of  operation  is  fixed  by  the  length 
of  warps  for  a  given  speed. 

When  used  in  conjunction  with  lifting  kites  attached 
to  the  leadline,  the  whole  net  becomes  a  stable  unit. 


[362] 


A  PRACTICAL  DEPTH  TELEMETER   FOR   MIDWATER   TRAWLS 

by 

R.  L.  McNEELY 

Bureau  of  Commercial  Fisheries,  U.S.  Fish  and  Wildlife  Service,  Seattle,  Washington,  U.S.A. 

Abstract 

A  direct-reading  elcctric-il  depth  telemeter  for  midwater  trawls  has  been  developed  and  used  successfully  in  the  north  eastern  Pacific. 
The  svstcm  utilizes  an  electrical  trawl  cable  to  transmit  continuous  depth  information  from  a  pressure-sensing  unit  on  the  gear  to  a  pilot  house 
meter  which  shows  trawl  depth  in  feet  and  fathoms.  Slip  rings  and  brushes  on  the  trawl  winch  complete  the  electrical  circuit,  which  is 
powered  by  a  45V.  battery  located  in  tne  control  box  in  the  chart  room.  Maximum  depth  range  of  the  system  with  the  present  potentiometer 
is  225  fm.,  but  this  can  be  increased  or  decreased  as  may  be  required.  Advantages  of  the  system  are  its  simplicity  and  practicability,  requiring 
no  extra  handling  on  deck  and  no  specially-trained  operator.  It  has  bee.i  tested  and  used  successfully  during  the  spring  and  summer  of  1957 
aboard  the  U.S.  Fish  and  Wildlife  Service's  exploratory  fishing  vessel  John  N.  Cobb  based  at  Seattle. 


Rfenime 


Telemetre  de  conception  pratique  pour  mesurer  la  profondeur  des  chaluts  flottants 


On  a  mis  au  point  un  telemetrc  llcctrique  £  lecture  directe  indiquant  la  profondeur  a  laquelle  operent  les  chaluts  flottants  et  qiii  a  eld 
utilised  avec  succes  dans  le  Nord-1-st  du  Pacifique.  Le  systeme  comportc  une  fune  cable  elect rique  qui  transmet  d'une  facon  continue  a  un 
cadran  montd  dans  la  timonen,  la  profondeur  du  chalut  en  pieds  et  en  brasses  indiquee  par  un  appareil  fixe  sur  le  chalut  et  dont  le  fonctionne- 
ment  est  base  sur  la  variation  de  pression  selon  la  profondcur.  Des  bagucs  et  balais  collect  curs  months  sur  le  treuil  du  chalut  completent  le 
circuit  electrique  qui  est  alimcnte  par  une  ha tt eric  de  45  volts  logce  dans  le  bo i tier  de  commando  installe  dans  la  chambre  des  cartes-radio. 
Le  potentiometrc  actuellement  niontc  sin  1'apparcil  permet  une  portee  maximum  de  225  brasses  en  profondeur,  mais  elle  pent  etre  augmentee 
ou  diminuee  suivant  les  besoins.  Ce  systeme  a  1' a  vantage  d'etre  simple  et  pratique,  de  ne  pas  demander  dc  manoeuvre  suppldmentaire  sur  le 
pont  ni  de  sp£cialistc  pour  le  faire  (onctionner.  II  a  <fte  essayc  et  utilis£  avcc  succes  au  cours  du  printemps  et  de  l'6tl  1957  a  bord  du  navire 
arnericain  John  N.  Cohh,  base  a  Seattle  ct  operant  pour  le  compte  du  Fish  and  Wildlife  Service  des  Hlais-Unis. 

Una  ecosonda  practica  para  redes  de  arrastre  que  pescan  a  profundidades  intermedias 
Extracto 

tin  el  Pacifico  nororiental  se  ha  usado  con  cxito  una  ecosonda  ultrasonora  de  lectura  directs,  ideada  para  determinar  la  profundidad 
de  trabajo  de  las  redes  de  arrastre  que  pescan  a  profundidades  intermedias.  Este  instrumento  se  vale  de  un  cable  de  remolque  que  actiia 
como  conductor  etectrico  para  transmitir  en  forma  continua  la  profundidad  (en  pies  y  biazas)  mediante  un  dispositive  sensible  a  la  presi6n 
montado  en  el  arte,  que  se  conecta  con  el  mcdidor  instalado  en  la  caseta  de  gobierno. 

Anillos  y  escobillas  colectorcs  dispuestos  en  Ja  maquinilla  de  arrastre  completan  el  circuito  alimentado  por  una  boteria  clectrica  de 
45  voltios,  localizada  en  la  caja  dc  control  que  sc  halla  en  el  cuarto  de  derrota  y  radiotclegrafia.  Esle  aparato  permite  determinar  profundi- 
dades hasta  de  225  brazas  con  el  potenci6metro  usado  en  la  actualidad,  pero  dicho  limite  puede  aumentar  o  disminuir  segun  las  circunstancias 
lo  requieran.  Entrc  las  ventajas  del  sistema  descrito  figuran  su  sencillezy  utilidad,  no  requiriendo  ninguna  manipulaci6n  adicional  en  la 
cubierta  ni  un  operador  especialmente  adiestrado.  Este  aparato  se  cnsay6  y  us6  con  £xito  durante  la  primavera  y  verano  de  1957  a  bordo  del 
barco  de  exploraci6n  pesquera  John  N.  Cobb*  del  Servicio  dc  Pesca  y  Vida  Silvestre  dc  los  E.U.A.,  con  base  en  Seattle.  Wash. 


A  DEPTH  telemetering  system,  utilizing  a  low- 
voltage  electrified  trawl  cable  for  determining  the 
depth  of  midwater  trawls,  was  installed  and  used 
successfully  aboard  the  U.S.  Fish  and  Wildlife  Service's 
exploratory  fishing  vessel  John  N.  Cohb  in  the  north- 
eastern Pacific  during  1957. 

Accurate  knowledge  of  the  depth  of  the  net  is  essential 
to  successful  midwater  trawling  as  the  net  must  operate 
at  the  depth  indicated  by  fish  signs  on  the  echo  sounder 
or  other  instrument.  Many  methods  have  been  used  in 
various  parts  of  the  world  to  determine  the  operational 
depth  of  midwater  trawls,  but  there  is  still  need  for  an 
instrument  which  is  accurate,  simple  to  use  and  reasonably 
economical  for  commercial  fishermen.  The  electrical 
depth  telemeter,  which  was  designed,  constructed  and 
installed  at  Seattle  by  Service  personnel,  appears  to  meet 
this  need. 

Although  midwater  trawling  by  commercial  fishing 
vessels  thus  far  has  been  limited  primarily  to  herring  in 


northern  Europe  and  British  Columbia,  there  is  evidence 
that  other  species  of  fish  may  be  available  to  midwater 
gear,  thus  opening  up  vast  new  fishing  areas  of  the  ocean. 
Echo  sounders  and  sonar-type  instruments  have  shown 
that  schools  of  fish  may  be  found  at  any  depth.  Some 
schools  of  fish  occupy  a  relatively  thin  vertical  layer  of 
water  and  can  be  missed  easily  if  the  net  is  a  few  fathoms 
too  high  or  too  low.  During  a  single  tow  separate  schools 
of  fish  may  be  found  at  different  depth  levels,  necessitating 
raising  or  lowering  the  net5.  Also,  when  attempting  to 
catch  fish  very  near  a  hard  or  uneven  bottom,  the 
position  of  the  gear  must  be  accurately  known  to  avoid 
contact  with  the  bottom  which  could  damage  the  gear. 

DESCRIPTION    OF    THE    ELECTRICAL    DEPTH 
TELEMETER 

The  system  transmits  continual  depth  information  from 
the  midwater  trawl  gear  to  the  pilot  house  of  the  vessel. 
A  small  pressure  sensing  unit  (see  fig.  2),  located  on  the 


[363] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


Fig.  I.     Pilot  house  depth  meter,  calibrated  to  show  depth  of  the 
trawl  in  feet  and  fathoms. 


end  of  the  trawl  cable  at  one  trawl  door,  actuates  a 
mill iam meter  in  the  pilot-house  which  is  calibrated  to 
read  depth  in  both  feet  and  fathoms  (see  fig.  1).  Electrical 
continuity  at  the  trawl  winch  is  through  a  slip-ring  and 
brush  assembly  mounted  on  the  outside  of  the  winch 
drum.  Steel  trawl  cable,  having  insulated  conductors  for 
a  core,  provides  a  full  electrical  circuit  for  the  system. 

The  dial  of  the  depth  meter  in  the  pilot  house  is 
calibrated  in  1  fm.  and  5  ft.  intervals  from  0  to  50  fm.  and 
0  to  300  ft.  An  off-on-range  selector  switch  permits  selec- 
tion of  successive  50  fm.  segments  from  0  to  225  fm. 
(the  maximum  depth  of  the  particular  pressure  potentio- 
meter used).  The  captain  refers  to  the  meter  and  adjusts 
the  length  of  towing  cable  or  speed  of  the  vessel  in  order 
to  raise  or  lower  the  trawl  to  any  desired  depth. 

Sensing  unit  and  housing.  The  sensing  unit  consists  of 
a  precision  pressure  potentiometer  encased  in  a  Tobin 
bronze  pressure  vessel  3«  in.  long  and  2&  in.  in  diameter 
(see  fig.  3).  Threaded  cap  and  "O"  ring  seal  provide  a 
watertight  access  port.  Stuffing-tube  type  feed-throughs 
for  the  electrical  conductors  are  located  in  the  housing 
cap.  A  small  hole  in  the  centre  of  the  cap  admits  sea 


Fig.  2.    Pressure-sensing  unit  attached  to  the  end  oj  the  electrical 
trawl  cable  just  in  front  of  one  of  the  trawl  doors. 


Fig.   3.     Bronze  pressure   vessel   with   cap   removed  to  show 
pressure  potentiometer,  feed-throughs 9  and  **O"   ring  seal. 


water  pressure  to  the  castor  oil-filled  bourdon  tube  of  the 
potentiometer  (sec  fig.  4).  The  sensing  unit  is  placed 
inside  a  steel  housing  lined  with  sponge  rubber,  which 
screws  on  to  the  cable  termination  socket.  There  is  a 
shackle  hole  in  the  opposite  end  of  the  housing  for  con- 
nection to  the  bridle  lines  or  chain  of  the  midwater  trawl 
gear.  The  housing  is  7$  in.  long  by  3  in.  in  diameter, 
overall  size. 

The  sensing  unit  potentiometer  has  a  pressure  range 
of  0  to  600  p.s.i.  with  an  electrical  resistance  differential 
of  10,000  ohms,  thus  the  depth  range  of  the  instrument 
is  0  to  225  fm.  Linearity  deviation  is  less  than  one  per 
cent,  with  friction  ofthe  potentiometer  slider  accounting 
for  the  major  part. 

Other  pressure  potentiometers  having  greater  or  lesser 
pressure-resistance  values  are  available  commercially. 
The  225  fm.  depth  range  was  selected  as  the  most  practical 
for  present  use. 

Cable  and  termination.  The  electrical  trawl  cable  is 
•528  in.  outside  diameter,  double-armoured  steel, 
consisting  of  an  electrical  conductor  core  and  two  layers 
of  24-strand  opposed  helical-wound  high  tensile  galvan- 
ized steel  (see  fig.  5).  The  six  rubber-covered  conductors 
are  each  made  up  of  seven  strands  of  -012  in.  diameter 
copper  wire  and  are  wrapped  around  a  solid-rubber 
centre  filler.  Only  three  conductors  are  used,  the 
remaining  three  being  spares.  The  wire  size  of  each 
conductor  is  equal  to  No.  21  a.w.g.,  and  resistance  is 
11-1  ohms  per  1,000  ft.  Nylon  fillers  and  sheath  encase 
the  conductors,  making  a  round  electrical  core  approxi- 
mately fa  in.  in  diameter.  Breaking  strength  of  the  cable 
according  to  the  manufacturer,  is  18,000  Ib. 

The  type  of  termination  developed  for  the  cable  used 
on  the  John  N.  Cobb  is  an  extreme  wide-angle  and 
shallow  poured-babbit  socket  (see  fig.  4).  Glass  tape  is 
wrapped  around  the  conductors  for  protection  during 


[364] 


DEPTH     OF    TELEMETER     FOR     MIDWATER    TRAWLS 


.-  PRESSURE  POKT 

NYLON  SHEATH 


Ftp.  4.     Sen\ing  unit,  housing  ami  cable  termination  of  the  electrical  depth  telemeter. 


babbiting.  This  termination  relieves  external  pressure  on 
the  conductors,  as  opposed  to  the  common  deep  narrow- 
angle  socket  which  tends  to  squeeze  and  cause  shorting. 
The  wide-angle  socket  also  requires  a  minimum  of  length 
making  it  possible  to  contain  the  cable  termination  and 
pressure-vessel  sensing  unit  in  a  single  small  housing 
which  will  pass  through  the  trawling  blocks  and  wind  up 
on  the  winch  (see  fig.  6). 

Slip  rings  and  brushes.  A  set  of  three  bronze  face-type 
slip  rings  are  groove  mounted  in  plexiglass  and  installed 
on  the  outside  of  the  drum  near  the  shaft  (see  figs.  7  and  8). 
In  order  to  utilize  a  minimum  of  space  in  the  winch-drum 
housing  area  and  avoid  disassembly  of  the  winch,  the 
rings  and  mountings  are  split  halves  with  the  ring  joints 


rotated  45  degrees  so  that  on  assembly  around  the  winch 
shaft  they  become  a  solid  unit.  Jumper  wires  on  the  back 
of  the  mounting  provide  electrical  continuity  across  the 


Fig.  5.     Electrical  trawl  cable  ready  for  splicing,  showing  core, 
fillers,  conductors  and  the  two  layers  of  steel  strand*. 


Fig.  6.    Electrical  trawl  cable,  midwater  trawl  bridles  and  the 
telemeter  sensing  unit  on  winch  of  the  John  N.  Cobb. 


365  ] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


Fig.  7.     Access  port  of  the  trawl  winch  showing  location  of  the 

flip  rings  between  the  winch  shaft  bearing  cap  and  the  drum 

flanges. 

ring  joints.  Spring-mounted,  solid  brass,  button-type 
brushes  with  direct  connected  pilot  house  leads  are 
bolted  to  the  winch  shaft  bearing  cap.  The  winch  end  of 
the  electrical  trawl  cable  is  fed  through  the  clamp  hole 
of  the  drum,  and  the  conductors  are  connected  to  the  slip 
ring  terminals  to  complete  the  circuit  from  potentiometer 
to  pilot  house.  A  weather-tight  cover  on  the  winch 
housing  protects  the  slip  ring  assembly. 

Indicator  and  controls.  Electrical  resistance  differen- 
tial in  the  precision  potentiometer  is  measured  by  a 
simple  electrical  bridge  circuit.  This  difference  in  resist- 
ance, when  fed  with  proper  line  voltage,  is  shunted  across 
a  milliammeter  calibrated  to  read  depth  in  fathoms  and 
feet.  The  meter  is  provided  with  a  100  ohm,  one  milli- 
ampere  actuation  coil  for  AV  full-scale  deflection, 


Fig.  9.     Arrangement  of  instruments  in  the  pilot  house  of  the 
John  N.  Cobb  showing  trawl  depth  meter  in  lower  right  corner. 

With  a  bridge  unbalance  of  -095  V.  giving  a  readout  of 
50  fm.,  the  remaining  five  per  cent,  of  the  available 
pointer  travel  is  used  as  a  line  voltage  test.  A  small 
three-pole  triple-throw  rotary  selector  switch  connects 
either  the  pressure  potentiometer  or  a  pre-set  calibrating 
test  potentiometer  to  the  meter  bridge  circuit  (fig.  11). 

Circuitry,  battery,  control  and  test  mechanisms  are 
housed  in  a  small  metal  box  mounted  on  a  bulkhead  in 
the  chart  room  in  a  manner  that  allows  the  depth  meter 
and  the  line  control  and  off-on-range  selector  switch  knobs 
to  be  mounted  on  the  opposite  side  of  the  bulkhead  in  the 
pilot  house  (see  fig.  10).  Holes  drilled  in  the  bulkhead 
connect  the  two  units  and  provide  compactness  of 
installation.  A  45  V.  "B"  battery  located  in  the  control  box 
is  the  voltage  source.  Battery  drain  is  4-2  milliamperes, 
which  should  require  a  minimum  of  battery  replacements. 
Actual  line  voltage  is  28  V.;  thus  the  32  V.  battery  system 
carried  on  most  fishing  vessels  could  be  used  as  a  power 
source  provided  that  voltage  changes  were  checked  and 
compensated  for  during  telemetering  operations.  The 
low  voltage  used  presents  no  hazards  to  personnel. 

Range  selection  is  divided  into  4£  50  fm.  increments. 
To  accomplish  this,  eight  precision  2222  ohm  resistors 


Fig.  8.    Slip-ring  and  brush  assembly  of  the  electrical  depth 
telemeter. 


Fig.  JO.     Control  box  in  radio-chart  room  with  cover  off  to  show 
battery,  controls  and  test  mechanisms. 


[366] 


DEPTH     TELEMETER     FOR     MIDWATER    TRAWLS 


•=T  BATTER' 


-T  OFF-ON-RANGE 
f  SELECTOR 

SWITCH 


.    77.     Schematic   layout  of  electrical  depth   telemeter-ing 
system  installed  on  M.  V.  John  N.  Cohb. 


are  mounted  on  a  two-pole  six-throw  rotary  selector 
switch,  which  is  used  to  return  the  meter  pointer  to 
zero  at  the  end  of  each  50  fm.  deflection. 

SEA    TESTS    AND    TRIALS 

A  series  of  calibration  tests  was  made  aboard  the  John 
N.  Cobb  at  sea  by  lowering  and  raising  the  sensing  unit 
to  measured  depths.  A  ten-minute  warm-up  period 
with  the  sensing  unit  immersed  in  sea-water,  to  neutralize 
capacitance  and  temperature  effect,  preceded  aU  tests. 
Accuracy  of  the  electrical  depth  telemeter  was  found  to 
be  at  least  98  per  cent.  A  slight  lag  of  \  fm.  was  noted 
during  ascending  and  descending  at  normal  winch  speed. 
Depth  readings  of  the  telemeter  agreed  closely  with  two 
types  of  echo  depth  sounders  during  comparison  tests 
when  the  sensing  unit  was  dropped  to  the  bottom  at 
intervals  to  a  maximum  depth  of  187  fm. 

Chief  concern  during  construction,  testing  and  early 
use  of  this  new  telemeter,  was  the  questionable  ability  of 
the  electrical  trawl  cable  to  withstand  the  punishment  of 
regular  fishing  operations.  Full  power  test  runs  towing  a 
70  ft.  square-opening  nylon  midwater  herring  trawl  were 
executed  with  normal  turns  and  excess  cable  played 
from  the  opposite  drum  to  put  the  greater  load  on  the 
electrical  trawl  cable.  A  cable  dynamometer  showed  a 
maximum  cable  strain  of  4,700  Ib.  at  full  throttle  with 


360  fm.  of  cable  out  and  the  net  at  83  fm.  To  date  the 
cable  has  been  used  during  some  50  tows  with  no  sign 
of  damage  or  fatigue.  There  has  been  no  apparent 
damage  to  the  electrical  conductors. 

ADVANTAGES  AND  DISADVANTAGES 

The  greatest  advantage  of  the  electrical  depth  tele- 
metering system  is  its  simplicity  and  practicability.  Since 
it  is  a  direct-reading  instrument  with  a  simple  off-on- 
range  selector  switch  and  line  control  rheostat  to  set, 
no  specially-trained  operator  is  needed.  Likewise,  no 
special  handling  on  deck  is  required  as  the  sensing  unit 
is  attached  as  a  permanent  part  of  the  fishing  gear. 

Being  electrical,  the  system  is  not  affected  by  distance, 
directivity,  water  currents,  wake,  ambient  sea  noises,  etc., 
as  are  acoustic  telemeters. 

The  225  fm.  range  can  be  increased  by  the  installation 
of  a  suitable  pressure  potentiometer,  and  recalibration. 

Use  of  the  system  on  bottom  trawls  is  feasible  due  to 
the  small  size  and  rugged  construction  of  the  sensing 
unit  and  housing. 

Routine  maintenance  can  be  performed  by  relatively 
unskilled  personnel. 

The  accomplishment  of  connecting  an  electrical  circuit 
from  the  pilot  house  of  a  fishing  vessel  to  a  trawl  deep 
beneath  the  ocean  surface  makes  possible  the  transmis- 
sion of  other  types  of  information  to  the  vessel  operator. 
Constant  monitoring  of  water  temperature  at  trawl  depth 
is  possible  with  the  addition  of  a  small  thermistor  inside 
the  pressure  housing  of  the  sensing  unit,  similar  to 
the  S-T-D  used  by  oceanographers6. 

Ink  pen  recordings  of  depth  and  temperature  can  be 
made  if  permanent  records  arc  desired.  Also,  graphic 
presentation  of  telemeter  depth  readings  on  to  the  echo- 
sounder  recording  paper  used  during  fishing  operations 
is  entirely  practical.  Even  some  form  of  automatic  or 
adjustable  controls  on  the  fishing  gear  could  be  installed 
if  found  to  be  desirable  and  practical  in  the  future4. 

Apparent  possible  disadvantages  of  the  electrical  depth 
telemeter  are  few  and  may  prove  to  be  of  minor  import- 
ance with  continued  use  of  the  system. 

Splicing  the  electrical  trawl  cable  is  more  difficult  and 
time-consuming  than  splicing  standard  cable  used  on 
fishing  vessels.  A  50  ft.  long-splice  is  required,  which  was 
found  to  be  not  unduly  difficult  after  some  experience. 
The  3,000  ft.  cable  in  use  on  the  John  N.  Cobb  is  made  up 
to  two  sections  which  were  spliced  together  by  two  staff 
members  in  approximately  two  working  days. 

The  present  cost  of  the  electrical  cable  is  roughly  60 
per  cent,  higher  than  the  cost  of  regular  plow  steel  trawl 
cable,  but  this  cost  differential  cannot  be  properly 
evaluated  until  the  life  expectancy  of  the  new  cable  is 
determined  through  actual  service  over  an  extended 
period  of  time. 

REFERENCES 

1  Barraclough,  W.  E.  and  Johnson,  W.  W.    1956.    A  new  mid- 
water  trawl  for  herring.     Bulletin  No.  104,  Fisheries  Research 
Board  of  Canada,  Ottawa. 

2  Collias,  E.  E.,  with  Barnes,  C  A.   1951.  The  salinity-tempera- 
ture-dcpth  recorder.      University  of  Washington  Oceanographic 
Laboratories,  Seattle,  August. 

3  Dow,  Willard.    1954.    Underwater  telemetry:  A  telemetering 


[367] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


depth  meter.  Woods  Hole  Oceanographic  Institution  Ref.  54-39, 
Woods  Hole,  Mass. 

4  Fryklund,  Robert  A.  1956.  Controllable  depth  maintaining 
devices.  U.S.  Patent  Office  publication,  Patent  No.  2,729,910, 
Washington.  D.C.,  January  10. 

6  Richardson,  I.  D.  1957.  Some  problems  in  midwater  trawling. 
World  Fishing,  vol.  6,  No.  2,  John  Trundell,  Ltd.,  London, 
February. 


6  Smith,  Keith  A.    1957.    An  experimental  air-pressure  depth- 
meter  for  use  with  mid-water  trawls.  Commercial  Fisheries  Review, 
U.S.   Fish  and   Wildlife  Service,  Department   of  the  Interior, 
Washington  25,  D.C.,  vol.  19,  No.  4.  April,  pp.  6-10. 

7  Stephens,  F.  H.  Jr.,  and  Shea,  F.  J.    1956.    Underwater  tele- 
meter for  depth  and  temperature.     Special  Scientific  Report- 
Fisheries  No.  181,  U.S.  Fish  and  Wildlife  Service,  Department  of 
the  Interior,  Washington  25,  D.C.  June,  15  pp. 


Thre  t~drum  winch  on  a  Swedish  cutter  fitted  for  trawling  over  starboard  side.    The  after  warp  is  hauled  on  the  starboard  drum,  forward 
warp  on  the  port-side  drum,  while  the  anchor  line  is  hauled  on  the  centre  drum.  Photo:   FAO. 


368 


OLYMPIC  TRAWL   CABLE  METERS 

by 

CARLYLE  A.  CRECELIUS 

Olympic  Instrument  Laboratories,  Vashon,  Washington,  U.S.A. 

Abstract 

This  is  a  device  for  measuring  the  amount  of  trawl  warp  being  paid  out,  and  the  importance  of  being  able  to  match  the  two  warps, 
especially  in  deep  water,  cannot  be  over-emphasised.  The  meters  arc  fitted,  one  for  each  warp,  close  to  the  winch,  and  irrespective  of  any 
stretching  that  may  have  taken  place  in  the  warps,  the  exact  length  paid  out  is  always  visible  on  the  counters.  The  meters  are  claimed  to 
supersede  the  old  method  of  measuring  warps  by  markers.  They  are  sturdy  in  construction,  unaffected  by  corrosion  and  are  easily  maintained. 


Resume 


Compteur  Olympic  pour  funes  de  chalut 


Get  appareil  permet  dc  mesurcr  la  langueur  exacte  de  chaque  fune  de  chalut  qu'on  laissc  filer  ct  Ton  ne  saurait  attacher  trop  d 'import- 
ance a  la  possibility  de  pouvoir  donner  exactement  la  meme  longueur  aux  deux  funes,  surtout,  en  eaux  profondcs.  Les  appareils  sont  places, 
un  par  fune,  pres  du  trcuil,  et  independamment  de  toute  elongation  subie  par  les  funes,  on  pent  toujours  lire  sur  les  compteurs  la  longueur 
fi!6c.  Ces  appareils  remplacent  la  vieillc  mdthode  de  mesurc  des  funes  an  moycn  de  rcperes.  11s  sont  dc  construction  robuste,  insistent  a  la 
corrosion  et  leur  entretien  est  facile. 

El  medidor  "Olympic"  para  cables  de  arrastre 
Extracto 

Este  dispositive  tiene  por  objeto  medir  la  longitud  exacta  de  los  cables  largada  durantc  un  lance  con  red  dc  arrastre,  dada  la  gran 
importancia  quc  tienc  la  igualacion  del  largo,  csnecialmente  en  aguas  profundas.  Junto  a  la  maquinilla  de  arrestre  cada  cable  se  hace 
pasar  por  uno  de  estos  medidores  quc  permite  leer  su  longitud,  pero  no  asi  el  alargamiento  cxpcrimentado  durantc  el  curso  del  lance.  Este 
m£todo  reemplaza  al  antiguo  procedimiento  de  medir  el  largo  de  los  cables  dc  arrastre  mediante  marcas.  El  instrumento  csde  construcci6n 
fuerte  y  no  sufre  los  cfectos  de  la  corrosi6n. 


THE  importance  of  being  able  to  balance  the  warps, 
especially    in    deep    water    trawling,    cannot  be 
over-emphasized.    A  new    Trawl    Cable    Meter, 
manufactured  by  the  Olympic  Instrument  Laboratories, 
Vashon,  Washington,  U.S.A.,  is  meant  to  replace  the 
markers  generally  used   for  trawl  warps.   It  gives  a 
continuous  indication  of  the  length  of  trawl  warp  as  it 
runs  off  the  winch. 

Marking  of  cables  has  several  obvious  disadvantages: 

(1)  Markers  frequently  need  replacing  which  means 
additional  work; 

(2)  if  either  trawl  warp  stretches,  the  markers  become 
inaccurate  and  cause  improper  alignment  of  the 
otter  boards: 

(3)  a  parted  trawl  warp  requires  a  splice  which  makes 
the  other  markers  inaccurate; 

(4)  trawl  warps  can  be  damaged  by  the  markers, 
increasing  the  possibility  of  a  break  where  markers 
have  been  inserted. 

When  a  pair  of  Trawl  Meters  is  used,  the  true  length  of 
stretched  or  repaired  cable  is  indicated  at  any  stage  of 
the  operation. 

Model  685  Olympic  Trawl  Cable  Meters  are  made  of 


bronze  and  stainless  steel,  are  watertight  and  corrosion 
is  reduced  to  a  minimum  (fig.  1). 

The  melers  will  accommodate  cable  from  |in.  to  fin. 


Fig.  1.     Model  685  Olympic  Trawl  Cable  Meter  for  wire  rope 

to  i  in.  diameter  including  splices  and  markers,  on  to  i  in. 

diameter  without  splices  or  markers. 


[369] 


MODERN     FISHING     GEAR     OF    THE    WORLD 


diameter,  with  or  without  markers,  but  arc  limited  to 
about  £  in.  diameter  cable  when  normal  splices  are  used. 
The  meters  are  easily  fitted  and  operate  close  to  the  winch 
in  plain  view  of  the  operator.  They  are  secured  by  pre- 
venters which  permit  the  necessary  movement  as  the 
warps  wind  off  and  on.  Lengths  are  indicated  in  fathoms 
to  999  and  repeat,  if  necessary.  The  counters  may  be  set 
to  zero  at  any  time.  No  special  tools  are  required  and 
only  average  mechanical  ability  is  needed  to  replace  worn 
parts.  The  meters  are  in  use  commercially  and  in  scientific 
work  in  many  parts  of  the  world,  particularly  in  the 
Pacific  northwest  of  America. 

As  many  big  trawlers  use  warps  as  large  as  lin. 
diameter,  a  Trawl  Meter  with  capacity  from  A  in.  to  1  in. 
cable  plus  any  splices  or  markers  has  been  designed.  This 
new  Meter,  Model  750,  reached  the  semi-production 
stage  in  June  1957  (fig.  2). 

Prototypes  have  received  extensive  tests  ashore  and 
afloat,  and  are  proving  quite  satisfactory  in  mid  water 
trawl  tests  in  Atlantic  coastal  waters. 

A  pair  of  Model  750  Meters  mounted  on  similar  warps 
will  normally  measure  consistently  within  1  fm.  in  500. 
Counters  can  be  furnished  to  indicate  in  fathoms,  feet 
or  meters.  Remote  indicating  electrical  counters  are 
available  on  special  order. 

Careful  selection  of  aluminium  and  stainless  steel 
alloys  achieves  light  weight  (about  10  kg.)  while  assuring 
adequate  strength  and  corrosion  resistance  in  Model  750. 

Fig.  2.     Model  750  Olympic  Trawl  Cable  Meter  for  wire  rope 
i  in.  to  1  in.  in  diameter  including  splices  and  markers. 


The  "  Janet  Helen  " 

[370] 


OCEAN   CABLES   AND  TRAWLERS 
THE  PROBLEM   OF  COMPATIBILITY 

by 

R.  A.  GOODMAN  and  C.  S.  LAWTON 

The  Western  Union  Telegraph  Company 

Abstract 

Both  the  fishing  industry  and  the  ocean  cable  companies  make  use  of  the  seabed  and  this  paper  expresses  the  hope  that  the  two  can 
carry  out  their  respective  duties  without  damaging  each  other.  The  problems  of  the  cable  companies  are  discussed  and  it  is  pointed  out  that, 
between  1946  and  1950,  367  cable-days  were  lost  in  the  Trinity  and  Conception  Bays  area  of  Newfoundland  through  trawler  damage  and  until 
1946  the  cables  had  been  untouched  since  they  were  laid-  some  of  them  75  years  ago.  The  cable  service  is  increasing  and  the  trawlers  arc 
seeking  new  grounds,  so  the  risk  of  damage  is  always  there.  A  British  publication  in  1908,  commenting  on  the  recommendations  of  an 
Interdepartmental  Committee  on  damage  to  submarine  cables,  asked  "Why  should  not  the  onus  of  initiating  fair  play  be  put  upon  the  fisher- 
men?"— and  from  the  cablcmarTs  viewpoint,  this  is  still  a  good  question. 

Lcs  cables  sous-marins  et  les  chalu tiers 
Resume 

L'industric  des  pechcs  et  les  compagnies  de  cables  transoceaniques  utilisent  le  fond  des  mcrs  et  les  auteurs  expriment  1'espoir  q  if  el  les 
pourront  toutes  deux  accomplir  leurs  laches  sans  se  nuirc  reciproqucment.  Us  examment  les  difficultes  des  compagnies  de  cables  et  font 
observer  quc  dc  1946  a  1950,367  jours  de  fonctionnement  dc  cables  ont  etc  perdus  dans  la  region  des  baies  de  la  Trinite  ei  de  la  Conception 
(Terre-Neuve)  par  suite  des  degats  provoques  aux  cables  par  les  chalutiers,  tandis  qu'avant  cette  periode  les  cables  ctaicnt  demeures  in  tacts 
depuis  Icur  pose,  qui  rcmontail  pour  certains  a  75  ans.  D'un  cote,  le  nombrc  de  cables  sous-marins  augmcnte  et  de  I'autre  les  chalutiers 
recherchent  de  nouvcaux  lieux  de  pechc  en  sorte  que  les  risques  d*u  varies  sont  toujours  presents.  Commentant  les  rccommandations  d'un 
Comite  intcr-ministeriel  sur  les  degats  subis  par  les  cables  sous-marins,  une  publication  britanniquc  posait  en  1908  la  question  suivantc: 
"Pourquoi  ne  pas  laisser  aux  pecheurs  Tinitiative  du  fail  play  ?"  ce  qui,  du  point  de  vue  des  compagnies  de  cables,  est  encore  valable  aujourd- 
'hui. 

Los  cables  oceanicos  y  los  arrastreros 
Extracto 

Como  la  industria  pcsqucra  y  his  companias  de  cables  oceamcas  utilizan  el  fondo  del  mar,  en  este  trabajo  sc  expresa  la  esperanza 
de  que  ambas  conti'iiien  sus  funciones  sin  causarse  danos.  Al  anali/,<ir  los  problcmas  de  estas  ultimas  sc  hace  notar  el  hecho  dc  que  entre 
1946  y  1950  se  pcrdicron  367  dias-cables  en  la  zona  de  las  bahias  de  Trinidad  y  Conccpcion,  en  Tcrranova,  a  causa  del  dano  producido  por 
los  arrastreros  a  conductores  que  no  fueron  tocados  dcsde  quc  se  lendieron — en  algunos  casos  hace  75  anos — hasta  1946.  Al  aumento  del 
scrvicio  dc  cables  submarinos  se  suma  la  busquedade  nuevos  bancos de  pesca  dc  arrastre,  lo  cual  const ituyc  un  continuo  peligro.  En  una 
publicacion  britanica  de  1908  comentando  las  recomendaciones  de  un  Comile  Interdepartamcntal  sobre  el  dano  ocasionado  a  los  cables 
submarinos  se  prcguntaba:  i  por  que  no  dejar  en  manos  de  los  Pescadores  la  responsabilidad  de  proceder  cor  reel  arncn  te  ?  Desde  el  punto 
de  vista  de  las  companias  de  cables  oceanicos  esta  es  todaviu  una  huena  pregunta. 


WHEN  commercial  fishing  was  carried  on  largely 
from  dories  or  other  small  craft  with  handlincs, 
there  was  no  appreciable  menace  to  ocean 
cable  telegraphy,  but  as  soon  as  trawling  became  popular 
the  otter  board  became,  and  has  remained,  a  most 
serious  menace.  Today,  with  more  powerful  trawlers  in 
greater  numbers  going  farther  afield  and  using  gear  which 
reaches  to  greater  depths,  the  risk  of  fouling  cables  has 
greatly  increased.  To  satisfy  the  rising  demand  for 
communications  facilities,  new  cables  of  high  message 
capacity  have  been  laid  and  older  cables  adapted  for 
transmission  at  much  higher  speeds.  Many  of  the  older 
transatlantic  telegraph  cables,  normally  still  serviceable 
and  dependable,  arc  not  as  well  equipped  as  a  new  cable 
would  be  to  withstand  damage  from  sharp  blows,  or 
the  fouling  of  otter  boards,  particularly  when  a  skipper, 
in  an  understandable  effort  to  free  his  gear,  causes  the 
board  to  ride  along  the  cable,  straining,  and  often  break- 
ing it. 


COST  OF  CABLE  INTERRUPTIONS 

An  outbreak  of  cable  interruptions  by  trawlers  may  occur 
quite  suddenly  in  an  area  where  it  has  not  happened  before. 
For  instance,  Western  Union  has  transatlantic  cables 
landing  in  Trinity  and  Conception  Bays,  Newfoundland. 
All  of  these  had  remained  undisturbed  by  trawlers  on  the 
western  side  of  the  Atlantic  since  they  were  laid,  some  of 
them  for  almost  75  years;  then  in  1946  the  trouble  began. 
Within  10  weeks  one  of  our  cables  was  broken  three  times 
by  trawlers  working  on  the  northern  edge  of  the  Grand 
Banks  off  St.  John's.  Here  is  the  record  for  this  area  for 
five  years: 


Year 

1946 
1947 
1948 
1949 
1950 


No.  of 

W.V.  Cables 

affected 

\ 

2 

5 

3 

4 


Total 
No.  of 
Failures 
3 
5 
15 
13 
10 


Total 
Cable    Days 
Lost   Time 
32 
32 
116 
57| 
130 


[371   ] 


MODERN     FISHING     GEAR     OF    THE    WORLD 


The  Commercial  Cable  Company  and  Cable  and 
Wireless  Ltd.,  also  suffered  heavily. 

By  1950  costly  large-scale  diversions  of  Western  Union 
cables  were  initiated  which  resulted  in  substantial  relief 
for  several  years,  but  in  1955  the  trawlers  extended  their 
fishing  ground  again  and  the  trouble  recommenced.  This 
time  it  was  just  off  the  mouth  of  Trinity  Bay  where  there 
was  no  previous  record  of  damage  and  where,  due  to  the 
restricted  area  and  the  presence  of  other  cables,  diversions 
to  avoid  the  damage  are  not  practicable.  The  record  to 
22nd  August,  1957,  for  trawler  damage  off  Newfoundland 
is  given  below: 


Afo.  of 

W.U.  Cables 

affected 

1 
1 


Total 
No.  of 
Failures 

11 

18 

17 


Total 

Cable  Days 

Lost  Time 

66 

102 

79 


Year 
1955 
1956 
1957 
(to  22nd  August) 

The  repair  of  each  break  usually  reveals  several  miles 
of  trawler-ridden  cable,  broken  armour  wires,  electrical 
faults  and  a  strained  condition  throughout  which  makes 
it  impossible  to  lay  the  cable  flat  on  the  bottom  again. 

The  value  of  cable  which  has  to  be  renewed  per 
repair  commonly  runs  anywhere  from  $2,000  to  $20,000, 
depending  upon  the  length  and  armour  type.  A  cable 
ship  has  to  carry  a  crew  many  times  larger  than  a  trawler's, 
a  number  of  whom  are  highly  trained  specialists,  in 
addition  to  an  expensive  stock  of  repair  cable.  The  cost 
of  operation  therefore  is  correspondingly  higher.  At 
present  cable  damages  off  Newfoundland  are  costing  the 
Company  $720  an  hour,  and  the  total  loss  for  this  season 
can  be  expected  to  run  over  half-a-million  dollars. 

THEORIES  AS  TO  HOW  FOULING  OCCURS 

All  sorts  of  statistical  studies  have  been  made  in  an 
attempt  to  co-ordinate  the  hooking  of  cables  with  such 
diverse  things  as  the  phases  of  the  moon,  the  design  and 
maintenance  of  the  boards,  the  habits  of  fish,  the  man- 
oeuvring of  the  trawlers,  etc. 

It  seems  reasonable  that  boards  which  are  well 
maintained  have  a  better  chance  of  keeping  clear,  and 
that  the  design  of  the  leading  edge,  the  towing  bracket, 
and  other  details  can  influence  this,  but  it  is  not  easy  to 
establish  scientific  proof,  since  cables  must  be  crossed 
hundreds  of  times  for  every  time  the  boards  foul  them. 
It  also  appears  that  whenever  a  trawler  finishes  its  tow 
and  swings  around  to  heave  in  its  catch,  the  boards  fall 
flat  and  in  being  dragged  home  any  board  may  work  its 
leading  edge  under  a  cable. 

A  flat  even  bottom  offers  more  security  than  an  uneven 
one  because,  particularly  in  the  shallower  depths,  it  is 
difficult  to  lay  cable  without  some  residual  bottom  tension 
and  thus  there  is  always  the  tendency  for  the  cable  to 
remain  suspended  over  indentations  in  an  irregular 
bottom.  If  slack  is  paid  out  it  may  accumulate  and  not 
lie  flat  in  places.  Some  cable  men  have  tried  laying 
cable  bar-light,  with  no  slack,  through  trawling  areas, 
as  the  lesser  of  two  evils. 

Repair  ships  have  to  work  under  emergency  conditions 
to  restore  communications  as  speedily  as  possible,  and 
cannot  pick  up  and  re-lay  the  cable  from  one  end  of  the 
trawling  area  to  the  other.  That  would  be  prohibitively 


costly  and  time-consuming.  Wherever  a  repair  ship 
finishes  her  work,  she  must  leave  some  excess  cable  on 
the  bottom  because  her  last,  or  "final"  splice  has  to  be 
made  on  the  bight.  Efforts  to  lay  out  the  slack  bight 
laterally  have  not  resulted  in  much  improvement  because 
even  if  it  is  weighted  down  with  chain,  the  slack  cable  may 
not  lie  flat  on  the  bottom.  Thus,  the  more  the  cable  is 
repaired,  the  more  vulnerable  it  becomes. 

EFFORTS  TO  MINIMIZE  DAMAGE 

In  1884  an  International  Convention  was  called  which 
made  recommendations  for  legislation,  later  enacted  by 
participating  countries,  making  it  a  misdemeanour  wil- 
fully to  damage  or  break  a  telegraph  cable,  to  approach 
within  a  nautical  mile  of  a  ship  engaged  in  laying  or 
repairing  a  cable  or  within  a  quarter  of  a  mile  of  a  buoy 
marking  the  route  of  a  cable  being  laid  or  undergoing 
repair. 

An  exhaustive  report  made  to  the  British  Parliament 
by  an  Inter-departmental  Committee  in  1908  showed  that 
these  measures  did  not  have  the  desired  effect.  Great 
hopes  were  placed  upon  being  able  to  persuade  trawler 
fishermen  to  use  otter  boards  of  a  design  recommended  as 
being  best  able  to  avoid  fouling  cables  and  also  to  keep 
their  boards  well  maintained.  Many  official  bulletins  have 
been  issued  to  fisherman  containing  instructions, 
warnings,  admonitions,  etc.,  but  they  have  not  solved 
the  problem  of  two  expanding  industries  attempting 
to  occupy  a  common  domain. 

The  cable  companies  have  often  issued  charts  showing 
the  locations  of  their  lines,  to  the  extent  permitted  by 
security  regulations,  but  without  much  effect.  In  general, 
the  fishermen  must  know  pretty  well  where  the  cables  lie 
because  they  have  encountered  them  so  many  times. 
Unfortunately,  fish  do  not  seem  to  mind  the  presence  of 
cables  and  when  they  congregate  in  areas  where  cables  lie 
it  is  only  natural  for  fishermen  to  seek  them  out  unless 
prevented  by  force  majeure. 

The  old  argument  runs:  "The  fish  were  here  before 
the  cables"  and  the  time-worn  counter-argument  is: 
"Not  the  fish  which  you  are  catching!" 

In  the  early  1920's  seven  cable  companies — Western 
Union,  The  American  Telephone  and  Telegraph  Com- 
pany, Imperial  and  International  Communications  Ltd., 
The  Commercial  Cable  Company,  The  Great  Northern 
Telegraph  Company,  the  French  Cable  Company,  and 
the  Deutsche  -  Atlantische  Telegraphengesellschaft  — 
jointly  employed  an  ex-officer  of  the  British  Navy  to  act 
as  a  "Cable  Damage  Inspector".  He  spent  several  years 
visiting  trawler  ports  on  the  European  side,  inspecting 
trawl  gear,  calling  the  fishermen's  attention  to  conditions 
which  he  considered  liable  to  cause  cable  damage, 
and  generally  acting  as  a  liaison  with  the  fishing  industry. 
He  developed  an  otter  board  and  towing  bracket  which 
he  claimed  were  relatively  safe  from  fouling  cables. 
A  trawler  was  chartered  to  prove  his  claim  and  the  gear 
was  towed  over  the  known  line  of  an  abandoned  cable 
without  hooking  it,  but,  in  the  absence  of  any  con- 
clusive data  that  he  was  catching  more  fish  with  it, 
the  fishermen  remained  unimpressed. 

When  trawlers  were  small  and  their  gear  weak, 
heavier  armour  cable  could  be  counted  upon  to  be 
effective.  The  skipper  who  fouled  his  gear  slid  it  along  the 


[372] 


OCEAN    CABLES    AND    TRAWLERS 


cable  until  he  freed  it  or  lost  it.  Today,  heavier  armour 
helps,  chiefly  in  enabling  the  cable  to  withstand  sharp 
blows  from  otter  boards  without  becoming  electrically 
faulty.  But  any  trawler  which  actually  fouls  the  cable 
and  has  power  enough  to  raise  it  to  the  surface  on  the 
bight  can  cut  it  easily  with  an  acetylene  torch. 

Three  factors  limit  the  size  and  weight  of  cable  which 
can  be  used  in  trawler  areas:  the  storage  capacity  of  the 
repair  ship  which  has  to  transport  it,  the  power  of  her 
cable-handling  gear,  and  the  cost.  With  no  positive 
assurance  that  still  more  expensive  cable  will  be  effective 
against  the  ever-growing  size  and  power  of  the  trawling 
vessels,  there  is  a  natural  reluctance  to  embark  on  such 
big  expenditures.  The  most  which  can  reasonably  be 
done  is  to  provide  and  lay  new  cable  of  fairly  high  strength 
and  weight  along  the  best  available  route.  A  renewal, 
to  be  effective,  must  be  done  in  one  continuous  operation, 
avoiding  all  bight  splices  and,  if  possible,  any  and  all 
ship-made  splices  because  of  the  relative  inflexibility  they 
introduce  where  the  armour  is  overlaid  for  five  or  six 
fathoms  on  one  end. 

In  the  early  1930's  trawler  damage  southwest  of  Ireland 
became  so  troublesome  that  Western  Union  developed  a 
tool  for  trenching  a  cable  into  the  ocean  bottom.  This 
was  used  successfully  on  four  transatlantic  cables  across 
an  active  trawler  area  of  some  twenty  miles  wide 
and  between  the  depths  of  100  and  400  fm.  Un- 
fortunately, the  tool  has  a  very  limited  application.  The 
bottom  must  be  free  from  out-cropping  rock  so  that  a 
continuous  trench  can  be  dug  and  surface  currents  must 
be  moderate  enough  to  enable  the  ship  to  manoeuvre 
when  towing  the  plough  at  slow  speed.  The  operation, 
once  begun,  must  be  continuous  and  requires  navigation 
and  seamanship  of  the  highest  order  by  specialists  with 
many  years  of  training.  Had  conditions  off  Newfoundland 
been  favourable,  trenching  would  have  been  attempted. 
However,  before  this  can  be  considered,  a  better  tool 
must  be  developed  to  deal  with  rock  and  to  stand  up  to 
much  longer  continuous  operation.  Surface  currents  are 
a  problem  but  this  part  is  felt  to  be  surmountable  in  time 
through  use  of  modern  navigational  aids  of  greater 
accuracy  and  ships  of  greater  manoeuvrability  at  slow 
speed. 

DISREGARD  OF  REGULATIONS 

It  is  regrettable  that  in  many  instances  cables  heaved  to 
the  surface  when  fouled  by  trawl  gear  are  deliberately 
cut  with  a  saw,  axe,  or  acetylene  torch,  these  being  the 
quickest  and  easiest  methods  of  freeing  the  otter  board. 
It  is  almost  impossible  to  establish  the  responsibility  of 
the  trawler  concerned  because  of  the  unavoidable  lapse 
of  time  between  the  act  of  damaging  the  cable  and  the 
arrival  of  the  repair  ship.  The  prevalence  of  ice  and  fog 
in  some  areas  adds  to  the  difficulty.  It  is  not  uncommon 
to  find  as  many  as  thirty  trawlers  all  working  across  the 
cable  line  in  the  vicinity  of  the  break  or  fault  and  in 
some  cases  the  repair  ship  reports  a  portion  of  the  cable 
missing  entirely.  For  instance,  recently  off  Newfound- 
land 1£  nautical  miles  of  cable  were  removed  in  one  case 
and  3  n.m.  in  another,  with  a  large  group  of  trawlers 
working  unconcernedly  through  the  gap  thus  created. 

On  two  occasions  this  summer  a  Western  Union  cable 
ship,  while  picking  up  cable,  has  had  a  trawler  cross 


directly  ahead  in  spite  of  whistled  warnings.  In  both 
cases  the  cable  ship  has  stopped  and  eased  away  the 
cable  at  the  bow  in  an  attempt  to  avoid  a  foul.  In  both 
cases  the  cable  has  been  fouled  and  the  ship  has  had  to 
cut  it  away  under  heavy  strain  without  being  able  to 
identify  the  trawler.  On  other  occasions  the  moorings 
of  the  buoys  streamed  by  the  cable  ship  to  mark  the  line 
of  the  cable  have  been  fouled  by  trawlers  working  close 
to  them  and  cutting-corners  around  them. 

Anonymity  has  given  these  fishermen  freedom  to 
flaunt  the  regulations  with  impunity  and  some  of  them 
are  not  loath  to  do  so  wherever  and  whenever  the  cables 
or  the  repair  ship  are  obstacles  to  good  fishing. 

Lest  this  be  interpreted  as  a  diatribe  against  the  fishing 
fraternity  in  general,  let  us  say  that  it  is  not  so  intended. 
Several  times  a  year  Western  Union  and  the  other  cable 
companies  receive  claims  filed  by  fishermen  who  have 
cut  away  their  gear  when  foul  of  a  cable  and  reported  to 
us  the  position  and  depth.  In  all  such  cases  where  the 
data  given  warrants  the  conclusion  that  the  cable  was 
ours,  and  that  the  amount  claimed  is  reasonable,  pay- 
ment in  full  is  made  promptly  and  with  gratitude. 

NEED  FOR  CLOSER  CO-OPERATION 

The  pressing  need  for  closer  co-operation  between  the 
fishing  and  communications  industries  must  be  evident 
to  both  parties.  There  can  be  little  doubt  that  fishermen 
lose  too  much  gear  and  valuable  time  because  of 
cables.  The  most  hopeful  sign  on  the  horizon  is  the 
discovery  that  fish  communicate  by  sound  and  that  it 
may  be  possible  to  devise  an  acoustical  method  of 
attracting  fish  into  nets.  Think  of  the  saving  in  wear  and 
tear,  not  to  mention  ship's  fuel!  The  converse  idea  of 
repelling  fish  from  cables  raises  certain  bothersome 
problems  of  power  distribution  and  range  of  emission. 
It  also  would  be  of  no  benefit  to  fishermen.  But,  seriously, 
would  it  not  be  worthwhile  to  put  some  money  and  effort 
into  finding  a  better  method  of  fishing  than  that  involving 
the  otter  board?  The  method  of  using  two  ships,  popular 
with  the  Spaniards  some  years  ago,  was  much  less  des- 
tructive to  cables.  There  must  be  others. 

Despite  wireless  communication,  the  cables  remain 
the  work  horses  of  the  trans -ocean  communication 
system  and  perform  a  vital  role  in  the  world's  business. 
Ocean  cables,  apart  from  trawler-damaged  portions  and 
shore  ends,  have  an  average  life  of  well  over  fifty  years 
and  with  normal  maintenance  can  be  gradually  renewed 
to  last  so  long  that  obsolescence,  rather  than  physical 
depreciation,  tends  to  be  the  limiting  factor  in  their 
useful  economic  life.  Recently  there  have  come  into 
the  picture  two  new  factors  which  have  revolutionized 
cable  design.  These  are  the  submerged  "repeater"  or 
amplifier,  and  the  coaxial  cable,  together  capable  of 
handling  voice  frequencies  across  the  ocean.  Such 
cables  can  be  used  for  telephone  or  telegraph  or  a 
combination  of  both.  It  is  a  well-known  fact  that  any 
voice  channel  can  be  made  to  handle  20  or  more  telegraph 
channels.  The  capacity  of  the  recently-laid  transatlantic 
telephone  cables  is  so  great  that  it  does  not  take  much 
imagination  to  visualize  either  the  replacement  of  the 
older  telegraph  cables  with  relatively  few  telegraph  cables 
of  modern  coaxial  design  using  fewer  submerged  repeaters 
than  are  required  for  telephony,  or  the  gradual  assign- 


[373] 


MODERN     FISHING     GEAR     OF    THE    WORLD 


ment  of  more  frequency  space  in  telephone  cables  to 
telegraphy. 

Whichever  takes  place,  the  result  will  benefit  fishermen 
for  a  time,  since  the  number  of  cables  laid  across  fishing 
areas  probably  will  decrease  rather  than  increase. 
However,  it  would  be  unsafe  to  count  on  this  for  long, 
since  overseas  telecommunications,  as  the  result  of 
recent  strides  in  transmission  techniques  are  getting 
cheaper.  If  this  is  accompanied  by  improved  quality  and 
dependability,  the  result  is  bound  to  be  a  tremendous 
increase  in  demand  and  soon  there  will  be  just  as  many 
cables  as  before. 

At  the  present  time  a  new  all-British  transatlantic 
telephone  cable  is  projected  which  will  extend  from 
Canada  to  Scotland  and  another,  to  be  jointly  owned  by 
the  Americans,  French,  and  Germans,  will  run  from 
Canada  to  France.  A  telephone  cable  has  been  laid 
from  the  United  States  of  America  to  Alaska  and  one 
has  just  been  completed  to  Hawaii. 

The  point  which  the  fishing  industry  should  ponder  is 
that  coincidentally  with  the  rapid  growth  of  trawlers  in 
size,  range  and  power,  there  has  come  a  corresponding 
growth  in  the  importance  of  ocean  cables.  Whereas  a 
further  increase  in  the  world  catch  of  fish  may  deplete 
areas  so  that  new  grounds  must  always  be  under  explora- 
tion, the  communications  industry  thus  far  has  barely 
scratched  the  surface  of  its  world  market  and  there  is  no 
corresponding  scarcity  of  product  to  hamper  its  rapid 
future  expansion. 

Breaking    or    damaging    an    ocean    cable    handling 
communications  valued  at  more  than  $50,000  a  day 
and  such  cables  already  exist     is  going  to  cause  quite  an 
international   furore.     Would   it   not  be  well   for  the 
fishing  industry  to  discipline  itself  before  this  happens? 

In  1908  a  British  publication  commenting  upon  recom- 


mendations made  by  the  Interdepartmental  Committee 
in  its  report  to  Parliament  on  damage  to  Submarine 
Cables,  posed  this  question: 

"To  put  the  matter  on  another  footing,  it  may 
fairly  be  asked,  why  should  it  be  the  cable  companies' 
duty  to  cultivate  the  trawling  community?  Why  should 
not  the  onus  of  initiating  fair  play  and  good  feeling 
be  put  upon  the  fishermen,  who  have  the  whole  sea 
in  which  to  pursue  their  calling?" 

This  still  is  a  good  question  from  the  cableman's 
point  of  view  if  the  present  methods  of  catching  fish 
commercially  are  to  be  perpetuated. 

HOPES  FOR  THE  FUTURE 

Industries  which  have  made  the  greatest  strides  in  this 
modern  world  are  those  which  have  had  the  foresight  to 
set  aside  some  of  their  earnings  regularly  for  development 
and  research.  It  is  the  earnest  hope  of  those  of  us  who 
are  engaged  in  the  business  of  communications  that  the 
fishing  industry  will  find  a  way  to  lean  less  upon  govern- 
ments in  this  respect  and  more  upon  its  own  resourceful- 
ness, perhaps  using  the  facilities  of  oceanographic 
institutions  more  effectively.  It  will  take  organization, 
but  the  job  is  rewarding  enough  to  make  it  well  worth 
doing.  You  will  find  the  communications  people  ready 
and  anxious  to  do  everything  within  reason  to  promote 
collaboration  because  it  is  their  belief  that,  once  aware 
of  tl\e  growing  seriousness  of  the  problem,  the  fishing 
industry  can  bring  about  another  transformation  in 
fishing  methods  no  less  spectacular  than  the  change  from 
linefishing  to  trawling.  If,  in  doing  so,  it  can  eliminate 
the  hazard  to  cables  it  will  have  performed  a  great  service 
to  both  industries. 


An  oval  otter  board  for  bottom  trawling  of  Russian  design.     Besides  its  other  advantages  this  type  of  board  should  be  more 
suitable  to  avoid  fouling  with  underwater  cables  than  the  common  otter  boards.  Photo:  FAO. 

1374] 


THE  USE  OF  THE  DANISH  SEINE  NET 

by 

W.   DICKSON 

Marine  Laboratory,  Aberdeen,  Scotland 


Abstract 

A  comparison  is  made  of  the  relative  advantages  and  disadvantages  of  Danish  seining  versus  trawling.  A  detailed  description  is 
given  of  anchor  seining  and  fly  dragging,  with  particulars  of  the  gear,  boats,  and  fishing  operations. 

Fcche  a  la  Sennc  Danoisc 
Resume 

L'auteur  compare  les  avantages  et  inconvenients  relutifs  de  la  senne  danoise  par  rapport  au  chalut.  11  donne  une  description 
detail  lee  de  la  peche  a  1'ancre  el  de  la  peche  en  route,  avcc  les  particularity  de  I'cngin,  dcs  bateaux  et  des  operations  dc  peche. 

Pesca  con  red  de  arrastre  dancsa 
Extracto 

Sc  comparan  las  ventajas  c  inconvenientes  relativos  de  la  pesca  con  redes  de  arrastre  de  tipo  danes  y  con  las  de  tipo  cornente.  Se 
describcn  minuc iosamente  las  operaciones  de  la  pesca  con  el  arte  anclado  y  las  de  la  pesca  en  marcha,  dando  noticias  de  los  artcs,  embarca- 
ciones,  y  metodos  de  pesca  aplicados. 


DANISH  seining  is  a  method  whereby,  without  the 
use  of  otter  boards,  a  net  can  be  dragged  across 
the  sea  bottom  by  one  boat.  The  gear  in  the  water 
consists  in  essentials  of  nothing  more  than  two  long  ropes 
and  the  net.  The  pattern  in  which  the  gear  is  laid  is 
roughly  pear-shaped,  with  the  net  where  the  eye  would 
be  and  the  boat  where  the  stalk  would  be.  The  boat 
may  be  either  moored  (anchor  seining)  or  moving  (fly 
dragging).  As  the  warp  is  hauled  aboard  the  boat,  the 
net  is  hauled  forward,  closing  gradually  but  held  open 
for  a  time  by  the  friction  of  the  ropes  on  the  bottom  and 
by  their  resistance  to  the  water.  A  general  impression  of 
the  operation  of  the  gear  is  given  in  fig.  1. 

DANISH   SEINING    VERSUS   OTTER   TRAWLING 

Danish  seining,  henceforward  simply  referred  to  as 
seining,  has  certain  points  of  superiority  as  well  as 
certain  disadvantages  compared  with  otter  trawling. 

Advantages 

( 1 )  Seining    is   a   mechanically   efficient    method    of 
fishing,  in  that  a  comparatively  large  net  can  be 
worked  with  only  a  modest  expenditure  of  power. 

(2)  The  encircling  movement  of  the  net  and  warps 
gives  a  high  fishing  efficiency,  and  a  seiner  can 
often   compete   to    advantage    with    larger   otter 
trawlers. 

(3)  It  can  be  worked  from  relatively  small  boats,  30  to 


80  ft.  (9  to  24  m.).  The  smallest  need  no  winches, 
two  powered  warping  drums  being  sufficient. 

(4)  The  method  allows  pinpoint  searching  as  compared 
with  trawling,  where  it  is  not  clear  at  what  point 
during  a  long  tow  fish  have  been  struck. 

(5)  It  is  possible  to  work  small  patches  of  good  ground 
among  the  rough  provided  their  extent  is  known. 

Disadvantages 

(1)  The  seiner  is  more  limited  in  the  areas  it  can  work 
in  that  it  is  not  possible  to  work  on  as  rough  ground 
as  can  be  done  with  the  otter  trawl. 

(2)  The  strength  of  tide  imposes  a  greater  restriction 
in  its  use. 

(3)  Much  time  is  lost  because  of  snag.\\  with  conse- 
quent retracing  and  lifting  some  or  all  of  the  warps 
and  net. 

(4)  Operations  arc  held  up  by  fog  unless  navigational 
aids  are  available  to  bring  the  boat  accurately 
back  to  the  dahn  when  shooting. 

(5)  It  is  usual  to  work  by  day  only. 

(6)  The  gear  needs  constant  attention  during  fishing 
operations,  and  to  this  extent  work  on  deck  tends 
to   be   more   protracted,   if  not    harder,   than   in 
trawling. 

(7)  The   set   cannot    be   much   speeded   up   without 
altering  the  length  of  warp  so  that  the  duration  of 
a  haul  is  not  a  matter  of  choice  as  it  is  in  trawling. 


[375] 


MODERN     FISHING     GEAR    OF    THE    WORLD 

i  Barrel  or  Buoy. 


ItVKrt 
3,  Depth 


s-isCoils  per  side 
och. 


Net   shepherding    stage. 


Of  DOFrni.  C 

2V  Manila  warp. 


Fig.  1.     Anchor  seining  operation. 


ANCHOR  SEINING  VERSUS  FLY  DRAGGING 

Fly  dragging,  being  half  way  between  trawling  and 
anchor  seining,  has  its  own  advantages  and  disadvantages. 
The  relative  merits  of  these  two  seine  net  methods  are 
hotly  argued.  The  following  gives  a  comparison  of  the 
salient  differences  between  them. 

Anchor  Seining 

(1)  Reputed  to  be  better  for  flatfish. 

(2)  More  economic  in  conditions  of  light  fishing. 

(3)  Smaller  crew  required,  usually  four. 

(4)  Not  so  much  power  required,  and  fuel  consumption 
lower.  Greater  mechanical  efficiency. 

(5)  Fewer  gears  required  on  the  winch,  usually  three. 
Winch  speeds  in  the  early  stages  of  hauling  are 
comparatively  fast.  Anchor  windlass  required. 

(6)  Less  stress  on  the  gear  and  thinner  warp  than 
needed  for  fly  dragging. 

(7)  The  ground  can  be  covered  more  systematically, 
and  towing  back  to  the  dahn  puts  the  gear  in  a 
better  starting  position. 

(8)  More  comfortable  for  the  crew  with  the  vessel  at 
anchor. 

(9)  Gear  stays  open  longer. 

Fly  Dragging 

(1)  Can  sometimes  be  better  for  demersal  roundfish. 

(2)  Less  time  to  shift  ground. 

(3)  Crew  size  four  to  seven. 

(4)  More  independent  of  direction  of  tides  and  can 
be  worked  in  stronger  tides,  hence  more  freedom 
to  tackle  a  patch  of  ground  from  any  advantageous 
direction. 

(5)  Winch  usually  has  four,  and  sometimes  even  six, 
gears.  Winch  speeds  in  early  stages  of  hauling  are 
comparatively  slow;  no  windlass  required. 


(6)  Net    moves   further   across    the    bottom    before 
closing,  but  gear  closes  with  less  warp  inboard. 

THE  DEVELOPMENT  OF  DANISH  SEINING 

The  development  of  the  method  has  encountered  a 
series  of  obstacles  but  overcoming  these  has  led  each  time 
to  an  expansion  of  its  use.  To  Jens  Vaevcr  of  Denmark 
goes  the  credit  of  introducing  the  method  just  over  one 
hundred  years  ago.  A  rowing  boat  was  then  used  to 
shoot  the  gear,  ready  for  hauling  by  hand  from  an 
anchored  cutter.  Similar  methods  are  still  in  use  today, 
such  as  with  the  Paithu  vafa  or  boat  seine  of  the  Malabar 
coast  of  India.  At  the  turn  of  the  century,  first  the  cutter, 
and  later  the  auxiliary  boat,  were  powered  with  hot-bulb 
engines.  The  elimination  of  the  small  boat  and  the 
introduction  of  a  powered  winch  followed.  The  scene  was 
then  set  for  a  rapid  development  of  the  method  but 
there  was  a  limit  to  the  amount  of  rope  that  could  be 
coiled  by  hand,  and  it  was  not  until  after  the  introduction 
of  the  mechanical  coiler  in  the  1920s  that  its  greatest 
development  took  place,  spreading  outwards  from 
Denmark  to  Sweden,  England,  Scotland,  Ireland, 
Iceland  and  farther  to  Australia,  New  Zealand  and 
Newfoundland.  During  this  time  the  method  was  adapted 
to  fishing  for  roundfish  as  well  as  flatfish.  The  Swedes 
now  use  the  anchor  seine  in  deep  water  up  to  100  fm. 
(183  m.)  and  the  Scots  work  just  as  deep  by  fly  dragging. 
But  a  difficulty  is  now  being  met  in  that  it  is  not  conveni- 
ent so  far  as  coiling,  handling  and  stowing  the  required 
length  of  rope  is  concerned,  to  work  with  ropes  larger 
than  2J  in.  (6-4  cm.)  circumference.  The  bigger  fly 
dragging  seiners,  of  about  75  ft.  (23  m.)  in  length,  put  a 
high  stress  and  cause  rapid  wear  on  such  ropes. 

Meanwhile,  the  Japanese  have  been  developing  fly 
dragging  along  somewhat  different  lines,  using  much 
heavier  warp.  Their  warp  is  shackled  together  in  lengths 


[376] 


DANISH     SEINING 


ANCHOR  SEINER  SHOOTING. 

SCoils. 

<.  or  B 

tow 


ANCHOR  SEINER  HAUUNG. 


Long 

tow  bock 
\    todohn 


Tide    indicoti" 
_    Hoot 


CotU    upside  down. 


Hcadhne» 


WIND 


TIDE 


Rg.2. 


FLY     DRAGGER   SHOOTING. 

3  Coil*  4 
3. Coils 


TO      MOORINGS. 

Fig.  3 
FLY      DRAGGER    HAULING. 


Coils 


Back    to    dahn 

with    warp   to     spare. 


Rort  bag. 


Fig  4. 


WIND 


TIDE 


AUSTRALIAN.STYLJE    FLY  DRAGGER 

SHOOT!  NG. 


AUSTRALIAN  STYLE    FLY   DRAGGER 

HAULING. 


Starboard    bag 


Coils    upsid«  down 


WIND 


TIDE 


Fig  6. 


TIDE. 


Fig? 


Figs.  2  to  7.      Deck  layout  of  the  boats  and  operation  of  the  main  seining  methods. 

[377  ] 


MODERN     FISHING     GEAR    OF    THE     WORLD 


increasing  from  3  in.  (7-6  cm.)  circumference  at  the 
ship  to  as  much  as  51  in.  (14  cm.)  circumference  at  the 
cross-rope  (that  part  of  the  rope  which,  together  with 
the  net,  forms  the  base  of  the  triangular  pattern  in  which 
the  gear  is  shot).  It  is  usual  to  introduce  a  length  of 
heavy  chain  at  the  end  of  the  cross-rope  and  this, 
together  with  the  large  and  changing  diameters  of  the 
warp,  precludes  the  use  of  a  coiler.  The  length  of  warp 
that  can  be  so  handled  is  limited  to  about  four  coils  per 
side.  With  this  heavier  gear  a  larger  net,  called  Teguriami, 
is  used.  The  gear  is  towed  to  a  close  with  the  tide  and 
then  winched-in  over  the  stern  by  warping  drums  located 
on  either  side  of  the  engine  casing,  aft  of  the  wheelhouse. 
Naturally  they  use  their  own  traditional  style  of  boat. 

BOATS 

The  lengths  of  most  seiners  fall  between  30  ft.  (9  m.)  and 
80  ft.  (24  m.).  An  engine  upwards  of  15  h.p.  is  required 
for  anchor  seining,  and  30  h.p.  rising  to  160  h.p.  is 
required  for  fly  dragging.  Towing  capabilities  are  essential 
for  a  fly  dragger,and  the  usual  practice  in  Scotland  at  least, 
is  to  have  a  medium  speed  engine  (600  to  1,000  r.p.m.), 
with  a  reverse  reduction  gear  (2:1  to  3: 1 ).  Although  not 
in  common  use  in  Scotland  as  in  the  Scandinavian 
countries,  the  variable  pitch  propeller  would  appear 
to  have  certain  advantages  for  fly  dragging.  The  propeller 
must  also  be  deeply  enough  immersed.  It  is  essential  for 
any  seiner  to  have  the  main  engine  controls,  throttle, 
ahead  and  reverse  gear,  or  propeller  pitch  control, 
operable  from  the  wheelhouse.  It  is  also  highly  desirable 
to  have  a  meter  in  the  wheelhouse  to  show  the  engine 
revolutions. 

Some  boats  are  purely  seiners  and  their  usual  deck 
layout  is  shown  in  figs.  2  to  7,  but  often  enough  with 
these  small  boats  seining  is  not  the  only  purpose  for 
which  they  are  used.  Their  layout  and  the  layout  of 
their  deck  machinery  must  then  be  designed  to  meet 
other  demands.  Some  Swedish  and  Danish  anchor 
seiners  can  also  trawl,  two  extra  trawl  barrels  being  fitted 
in  line  with  the  barrel  containing  the  mooring  wire.  Some 
Irish  fly  draggers  have  a  combination  winch  containing 
two  small  trawl  barrels  in  line  with  the  warping  drums, 
together  with  an  extended  coiler.  With  the  Scottish 
drifter/seiners,  all  that  is  required  to  make  the  conver- 


sion is  the  replacement  of  one  of  the  warping  drums  by  a 
larger  drift-net  one,  but  the  boat  must  also  have  the 
large  hatch  required  for  drift  netting. 

An  essential  requirement  for  any  seiner  is  that  the 
deck  and  rail,  from  which  the  warps  have  to  be  shot,  must 
be  clear  of  obstructions.  There  should  also  be  a  clear 
space  aft,  usually  with  a  built-in  platform  on  which  the 
net  is  stacked  ready  for  shooting.  It  is  a  considerable 
encumbrance  to  have  a  small  boat  aft,  although  it  is  still 
possible  to  shoot  the  net  over  the  quarter. 

If  the  sequence  of  shooting  and  hauling  on  a  fly 
dragger  are  thought  out  (figs.  4  to  7),  it  will  be  seen  that 
positioning  the  winch  aft  of  the  casing,  and  having  a  clear 
working  space  aft,  holds  some  advantage  in  the  readiness 
with  which  the  ends  of  the  warp  can  be  led  direct  to  the 
winch.  Such  a  re-arrangement  of  deck  machinery  and 
wheelhouse  must  mean  a  corresponding  re-arrangement 
of  fish-hold,  engincroom  and  crew's  accommodation 
below  decks,  not  to  mention  the  effect  of  these  on  hull 
design.  Whether  in  total  all  these  changes  are  so 
advantageous  is  another  question.  The  Australians,  at 
any  rate,  have  chosen  this  alternative  arrangement.  On 
Japanese  seiners,  too,  with  their  winch  barrels  protruding 
from  either  side  of  the  winch  casing  itself,  the  warps  can 
be  handled  in  a  similar  manner. 

Many  factors  other  than  thoscconcerningthesuccessful 
operation  of  the  fishing  gear  affect  the  choice  or  design  of 
a  good  seiner.  It  is  up  to  the  intending  owner  to  make 
his  own  choice  of  boat  type  and  size  in  the  light  of  local 
conditions  and  the  fishery  to  be  exploited. 

WINCHES 

Figs.  8  to  10  show  an  anchor  seiner's  winch  and  two  fly 
draggers'  winches,  the  last  with  a  rather  novel  design  of 
coiler.  Seine  net  winches  are,  for  the  most  part,  belt- 
driven  off  the  fore  end  of  the  main  engine,  but  some 
hydraulic  winches  have  also  made  their  appearance. 
The  warping  drums  are  often  about  8  in.  (20-3  cm.)  in 
diameter  but  may  be  bigger  to  advantage,  thus  causing 


Some  details  of  the  design  of  a  Scottish  type  drifter/seiner, 
the  Silver  Scout  arc  to  be  found  in  Jan  OJof  Traung's  "Improving 
the  design  of  fishing  boats"  -  F.A.O.  Fisheries  Bulletin  Vol.  4, 
Nos.  1  to  2,  Jan./Feb.-Mar./ April,  1951. 


.  8.     Anchor   seining    winch. 


Fig.  9.    Fly  dragger  winch. 


[378] 


DANISH    SEINING 


C  TENSION  W***i 


Fig.  10.      fly  dragger  winch  with  novel  coiler  design. 

less  wear  on  the  ropes.  Having  made  the  choice  of 
engine  with  a  known  maximum  r.p.m.,  and  of  winch  with 
a  known  number  of  gears  and  their  ratios,  the  one  must 
be  matched  to  the  other  to  give  the  desired  hauling  speed 
on  the  warps.  This  can  be  done  by  choosing  the  ratio  of 
the  down-drive  (if  there  is  one)  at  the  forward  end  of  the 
main  engine  and  also  by  choosing  the  sizes  of  the  pulley 
wheels  on  the  up-drive  between  the  forward  transmission 
shaft  and  the  winch  driving  shaft  (fig.  1 1). 

The  tables   of  winch   revolutions  which   follow  are 
typical   though   not   universal. 


Fig.  12.     Operational  principle  of  the  warp  coiler. 

The  boat  in  question  had  a  95  h.p.  main  engine  at  a 
maximum  speed  of  900  r.p.m.  There  was  a  1 J  :1  reduction 
in  speed  between  the  main  engine  and  the  winch  gear  box. 


Main 

Engine  Winch 

Revs.  Revs, 

r.p.m.  r.p.m. 


Fly   Dragging 


315 
315 
320 
320 


21 

42 

80 

160 


Winch  Gearing 


15:1— 1st  gear 

7J:1     2nd  gear 

4:1 — 3rd  gear 

2:1— 4th  gear 


Hauling  Speed 


42  ft./min. 

84  ft./min. 
J  60  ft./min. 
320  ft./min. 


(  13  m./min.) 
(  26  m./min.) 
(  50  m./min.) 
(100  m./min.) 


Anchor    Seining 


Main 
Engine 
Revs, 
r.p.m. 

Winch 
Revs, 
r.p.m. 

Winch  Gearing 

Hauling  Speed 

305 
350 
660* 

65 
100 
250 

2?f:J  —1st  gear 
2:1  2nd  gear 
lj:l  —  3rd  gear 

130  ft./min. 
200  ft./min. 
500  ft./min. 

(  40  m./min.) 
(  60  m./min.) 
(150  m./min.) 

*The  hauling  speed  shown  in  3rd  gear  was  not  measured  when 
hauling  the  net  but  when  winching  back  the  warp  to  a  fastener. 
It  is  included  to  show  just  how  fast  warp  can  be  brought  in  and 
coiled  down. 


•J°Ssti!?*9 


Fig.  IL    Seiner  winch  drive. 


This  boat  had  a  main  engine  of  132  h.p.  at  a  maximum 
speed  of  750  r.p.m.,  with  the  propeller  driven  through  a 
2:1  reduction  gear.  The  speed  ratio  between  the  main 
engine  and  the  winch  driving  shaft  was  1:1.  For  such  a 
boat  the  warp  tension  has  been  observed  to  rise  to  as 
much  as  J  ion.  The  brake  h.p.  delivered  by  such  a  winch 
then  rises  to  20  h.p.  during  fast  hauling  and  it  may  at 
times  be  greater. 

There  are  many  makes  of  coilers  and  winches,  but  the 
operational  principle  most  common  is  shown  diagram- 
mutically  in  fig.  12.  After  the  warp  leaves  the  warping 
drum  A,  usually  after  four  turns,  it  passes  over  the 
grooved  wheel  B  and  between  it  and  the  idler  wheel  C. 
Wheel  B  is  chain  driven  from  the  winch  itself  and  is  either 
of  the  same  diameter  as  the  warping  drums  with  a  1:1 
gear  ratio,  or  is  geared  to  have  the  same  peripheral  speed. 
The  idler  wheel  C  puts  tension  on  the  warp,  so  that,  on 
releasing  the  tension  spring,  the  warp  slips  on  the  warping 
drum  without  hauling.  This  is  called  surging.  The  warp 
is  led  through  the  gate  into  the  middle  of  pinion  E  which 
is  driven  by  pinion  D  at  a  reduced  speed,  so  that,  after 
emerging  from  the  spout  attached  to  E,  the  warp  is 
coiled  in  a  convenient  diameter. 


[379] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


Fig.  13.     Mooring  gear  for  anchor  seining. 

GEAR 

Moorings 

An  anchor  seiner's  mooring  gear  is  set  out  in  fig.  13. 
The  anchor  itself  is  1|  to  If  cwt.  (63  to  98  kg.).  The 
flukes  must  be  broad  or  have  flat  plates  bolted  to  them. 
The  stock  should  be  long  enough  so  that  if  the  end  of  it 
touches  the  ground  first,  the  anchor  rolls  on  its  crown 
without  the  flukes  touching  the  ground  until  the  anchor 
rolls  right  over.  The  £  in.  (19  cm.)  heavy  chain,  with  a 
swivel  at  each  end,  may  be  15  to  30  fm.  (27  to  54  m.) 
long,  depending  on  the  weight  of  the  ship.  Of  course, 
very  small  boats  would  have  lighter  gear.  The  mooring 
wire  is  about  three  times  the  depth,  with  a  basic  length 
of,  say,  50  fm.  (90  m.),  extra  lengths  with  swivels  between 
them  are  kept  for  shackling  on  as  necessary.  The  arrange- 
ment here  depends  on  the  depth  of  water  in  which  it  is 
customary  to  fish.  A  50  fm.  (90  m.)  and  a  25  fm.  (45  m.) 
length  are  shown  in  fig.  15.  There  is  then  a  large  link  into 
which  a  barrel  or  buoy  on  6  to  9  ft.  (1-8  to  2-7  m.)  of 
chain  is  shackled.  The  barrel  must  have  its  own  swivel. 
It  is  nowadays  customary  to  use  a  large  canvas  buoy 
because  it  is  lighter  to  handle,  but  a  barrel  serves  quite 
well  and  should  have  false  ends  fitted  to  it  to  protect  the 
weakest  part.  Between  ship  and  buoy  there  is  a  further 
15  fm.  (27  m.)  of  mooring  wire.  All  the  mooring  wire  is 
1£  in.  (38  mm.)  or  even  2  in.  (51  mm.)  in  circumference.  A 
slip  hook  over  the  roller  in  the  bows  of  the  vessel  clips 
into  a  large  link  or  bow  shackle  at  the  end  of  the  mooring 
wire.  A  4  fm.  (7  m.)  rope  picking  up  piece  is  clove-hitched 
on  to  the  end  of  the  mooring  wire.  The  dahn  is  clipped 
into  the  long  end  and  the  warp  to  the  shorter.  A  couple 
of  smaller  canvas  buoys  at  the  end  of  the  mooring  wire 
serve  to  keep  the  picking  up  end  afloat.  Sometimes  a 
third  is  put  further  along  the  wire. 

Warp 

Figs.  14  and  15  show  a  clip  spliced  into  the  warp  and  a 
warp  splice.  Note  the  staggering  of  the  ends,  which  would, 
of  course,  be  cut  off.  The  splices  are  made  this  way  so 


Fig.   14.     Clip  spliced  into  the  warp. 


Fig.  15.     Warp  splice. 

that  there  is  less  chance  of  them  sticking  in  the  coiler. 
The  most  common  size  of  warp  is  2]  in.  (5*7  cm.)  but  the 
larger  fly  draggers  use  2g  in.  (6  cm.)  circumference.  The 
rope  must  be  hard  laid  and  is  made  of  manila.  Manu- 
facturers make  seine  net  warps  specially  for  the  purpose. 
When  splicing  on  a  new  coil,  it  is  laid  in  position  on  the 
deck  and  the  inside  end  is  spliced  to  the  end  of  the 
previous  coil.  It  must  be  ensured  that  the  rope  is  coming 
out  of  the  inside  of  the  new  coil  in  the  opposite  direction 
to  that  in  which  it  was  coiled.  Seine  net  rope,  being 
always  right-hand  laid,  is  coiled  down  right-handed  and 
shot  so  that  it  comes  out  of  the  coils  anti-clockwise. 

Nets 

In  anchor  seining,  the  size  of  the  net  is  more  dependent 
upon  the  size  of  the  crew  available  to  handle  it  than  upon 
the  weight  and  power  of  the  vessel.  The  fly  dragger's  net 
must  not  be  beyond  the  lowing  capabilities  of  the  ship. 
All  seine  nets  are  long  in  the  wings  so  that  headline 
length  is  a  poor  determination  of  the  size  of  a  net.  Much 
better,  as  a  rough  determination  of  its  relative  size,  is  the 
number  and  size  of  the  meshes  round  the  mouth  of  the 
bag. 

Seines  are  made  in  quite  a  different  manner  from  trawls. 
Whereas  a  trawl  comprises  an  upper  and  lower  half,  a 
seine  is  made  by  joining  together  identical  right-hand  and 
left-hand  halves.  The  component  parts  of  a  seine  are: 
the  wings,  shoulders,  crowns,  bag  or  funnel,  and  codend. 
As  a  result  of  this  construction,  the  bating  meshes, 
giving  the  taper  to  the  bag,  are  to  be  looked  for  down 
the  middle  of  the  net  and,  again  unlike  a  trawl,  there 
are  no  selvedges.  In  plaice  seines,  the  shoulders  are 
either  small  or  absent;  the  net  is  made  in  large  mesh,  with 
the  bag  and  codend  usually  made  of  sisal  or  manila, 
although  completely  cotton  plaice  seines  are  also  made. 
The  bating  meshes  in  the  wings  are  down  the  middle. 
The  headline  is  little  longer  than  the  footrope.  Some- 
times, if  there  are  a  lot  of  shells  on  the  ground,  a  few 
rows  on  the  under  side  of  the  bag  are  replaced  by  extra 
large  mesh  to  allow  the  shells  to  fall  through. 


[380] 


li 

2 

9           f 
O*O^ 

rj 

UPPER  2"] 
CROWN  £ 

Scale  Vtaift^ 

-- 

DANISH     SEINING     NET 

HEADLINE. 
FOOTROPE. 


CROWN. 


148    (a*V  SHOULDEP  +  a  SWINGS.  ^ 

I*"  TABUED    HEMP. 

ISO    (**4  a'SHOuUfcPS  +  2x70*WlMGs) 
I'/ TARRED     HEMP. 


CROWN. 


WINGS. 


—Jfc 


SHOULDERS 
CROWN. 


BAG 


3'  4" 

-4-    -  • 

16  row  i 

rs"   2'r 

6r     iQrowJ 

If                " 

^i 

J  0-      £ 
|OV°S 

—   ^J  i.) 

^^        rt 

LOWER 
CROWN 

Scaled  ta4 

l! 

CROWN. 


.    /6.     /Vast  of  a  plaice  seine. 


A  plan  of  a  plaice  seine  is  given  in  fig.  16.  Typical 
flotation  for  such  a  net  is  5  5  in.  (12-7  cm.)  diameter 
balls  on  each  wing  and  a  1  8  in.  (20  cm.)  diameter 
ball  to  mark  the  crown  at  the  centre.  Mounted  on  the 
footrope  are  236  \  2  oz.  (57  gm.)  conical  leads  (8  at  the 
crown  and  1 14  on  each  wing).  A  bosom  is  formed  by 
reeving  a  2  in.  (5  cm.)  coil  rope  round  the  footrope, 
closely  wound  at  the  lower  crown  and  more  sparsely 
towards  the  wing  tips.  The  codend,  being  of  such  heavy 
twine,  contains  4  x  5  in.  (12-7  cm.)  balls  to  help  float 
and  keep  it  from  chafing  on  the  bottom. 


Specification  of  a  Plaice  Seine 

Headline      148  ft.     (2x4  ft.  shoulders  J  2 x 70  ft.  wings) 
U  in.  tarred  hemp. 

Footrope     150  ft.     (2x4   ft.    3   in.   shoulders-!  2  •  70   ft. 
wings)  1]  in.  tarred  hemp. 

Wings  71  m./28  m./350  rows  (73  ft.)  long 

12/18s  tarred  cotton  twine  5  in.  mesh 

Baiting  up  the  middle  of  the  net 
every  8th  row  for  43  baitings 


Shoulders  63  m./63  m./32  rows  (6  ft.  8  in.)  long 

125s  tarred  maniki  twine  5  in.  mesh 
shoulders  only  extend  to  the  end  of 
the  crowns. 

Crowns  24  m.,'4  m.  '32  rows  (6  ft.  8  in.)  long  1 25s 

(upper  and  lower)  tarred  manila  twine,  double  5  in.  mesh 


the  same) 


Bag 


9    in. 

71 
28 

43 
8 

344 


87 


Flapper 
Codends 


meshes 


Set  up  at  24  m.  across  in  double  twine 

Continue  for  10  plain  rows 

Then  leaving  10  bosom  meshes 

3  fly  meshes  on  insides  to  bring  each 

side  to  4  m.  across 

Then  plain  to  the  end 

174  m.  round/70  m.  round '164  rows 

(34  ft.  2  in.)  long 
125s  tarred  manila  twine  5  in   mesh 

52  batings  on  top  and  52  underneath 
bating  every  3rd  row  then  plain  to  end 


None 

70  m.  round  '70  rows  (1 3  ft.  1  £  in.)  long, 
75s,  4  strand  tarred  manila  4\  in.  mesh 


87 
35 

52 
3 

156 


The  haddock  seine  is  made  entirely  of  cotton,  occa- 
sionally even  of  nylon.     It  has  large  shoulders  which 


[381  ] 


MODERN     FISHING     GEAR     OF    THE    WORLD 


together  with  the  bag  and  codend  are,  in  the  North  Sea 
at  least,  of  the  70  mm.  (stretched)  regulation  mesh  size. 
The  batings  in  the  wings  and  shoulders  are  in  the  headline 
side  only.  The  net  usually  has  a  considerable  overhang, 
the  headline  being  markedly  shorter  than  the  footrope, 
As  the  stress  of  towing  is  thus  put  predominantly  on  the 
headline,  it  is  often  made  of  combination  rope.  The 
footrope  is  not  made  of  combination  rope  but  hemp,  so 
that,  if  it  does  become  solidly  snagged,  it  will  part  and 
the  net  may  not  be  wholly  lost.  As  the  hemp  shrinks 
much  more  than  the  combination  rope,  it  must  be 
ensured  that  the  footrope  is  wetted  and  stretched  before 
the  net  is  mounted;  otherwise  the  proportional  lengths 
of  headline  and  footrope  will  be  wrong  after  the  net  has 
had  its  first  wetting. 

Specification  of  a  Haddock  Seine    (Stuart  No.  18) 

Headline  190  ft.  (2  x  18  ft.  shoulders  F 2  x  77  ft.  wings) 

li  in.  combination  rope 
Footrope  197  ft.  (2  x 20 ft.  6  in.  shoulders -f  2  x  78  ft.  wings) 

1  ]  in.  tarred  hemp  rope 
Wings  100  m./45  m./440  rows  (91  ft.  8  in.)  long 

12/1 2s  twine  5  in.  mesh  100 

Bating  every  8th  row  on  top  only  45 

55 
8 

440 


Crowns 


Bag 


5  m.+20  m.-f  5  m.  20  rows  (2  ft.  51  in.)  long 
Double  12/24s  twine    70  mm.  mesh 
Set  up  at  30  m.  across  for  10  plain  rows, 
then  leaving  20  bosom  meshes  carry  on  fly 
meshing  on  the  outside  for  10  rows  more. 
The  uppercrown  has  an  additional  14  rows 
(1  ft.  8J  in.)   in  single    12/12s  twine. 
416  m.  round/192  m.  round/84  m,  round/52  ft.  long 
12/1 2s,  12/1 8s,  12/36s  twine  70  mm.  mesh 
1st  sheet  208m./96m./230  rows(28  ft.)long.       208 
12/12s    twine  96 

6  plain  rows  then  bating  every  4th        2/112 

row  for  56  batings  56 

4 

224 

2nd  sheet  96m./42m./l  18  rows(  14  ft.  4  in.)  long    96 
12/1 8s  twine.     Bating  every  4th   row         42 


Flapper 


Shoulders  188  m./175  m./220  rows,  (26  ft.  9  in.)  long 

12/1 2s   twine    70  mm.     mesh 
38  plain  rows  then  bating  every  14th  row 

on  top  only. 

Join  on  to  wings  by  taking  up  3  meshes  in 

7  in  sequence  1:2.  1:2,  1:2,  1:1 
196  rows  set  on  to  headline 
210  rows  set  on  to  footrope 


Scale**  id* 


175 

13 
14 

182 


Codend 


for  27  batings 


3rd   sheet   42  m./42   m./   9   ft.   8   in.   long 

12;36s  twine 
90   m./30   m./60   rows  (7  ft.    3  A    in.)  long 

I2/12s   twine     70  "mm.   mesh 

Bating  every  2nd  row 

Join  on  8  rows  from  end   of 
1st  sheet  of  the  bag 


84  m.   round/ 100  rows  (12  ft.  2  in.)  long 
12/4 2s  twine       70  mm.  mesh 


2/54 

27 
4 

ION 


30 

2 '60 

30 

-> 

60 


The  plan  of  a  large  haddock  seine  is  given  in  fig.  17. 


C2*I8'  SHOULDERS      I        2*77  WINGS.} 
Ca-206"     do.  »      *2*7a'     40.     ) 


FLAPPER 


Scok*toT. 


X2or9ny  imsfc 
im»  pvt  in  H€f«  to 
•troin  on  mt. 


Fig £17.    Plan  of  a  haddock  seine. 

1382] 


DANISH    SEINING 


Fig.  18.    Foot  rope  of  a  haddock  seine,  showing  the  ring 
leads  and  the  attachment  of  the  net. 


The  net  rides  more  lightly  on  the  bottom  than  does  a 
plaice  seine,  by  virtue  of  its  proportionally  shorter  head- 
line, its  greater  flotation  and  its  looser  method  of 
attachment  to  the  3i  in.  (9  cm.)  coir  rope  (fig.  18). 
Mounted  on  this  coif  rope  are  166  «"  4  oz.  (114  gm.) 
ring  leads.  Typical  flotation  is  43  ,\  5  in.  (12-7  cm.) 
balls,  10  on  each  wing,  9  on  each  shoulder  and  5  on 
the  crown.  Fig.  19  shows  the  details  of  the  upper  crown 
of  a  smaller  haddock  seine.  The  double  row  at  the  bottom 
of  the  picture  marks  the  join  of  bag  and  shoulders. 
Note  the  extra  five  rows  of  overhang  on  top  between 
crown  and  bag.  The  harness  lines  run  down  the  net 
diagonally  to  the  point  where  they  meet  similar  ones 
coming  from  the  lower  crown  and  thence  to  the  codend. 
The  detail  of  the  stitch ing-in  of  the  flapper  is  seen  in  fig. 
20.  Note  the  join  of  the  two  halves  of  the  net  above  and 
below.  The  normal  type  of  danleno,  with  its  bridles, 
swivel  and  clip  link,  is  shown  in  fig.  21.  A  length  of  Jin. 
(6-3  mm.)  or  Jin.  (9-5  mm.)  chain,  bet  ween  4  ft.  (1-2  m.) 
and  24  ft.  (7*3  m.)  long,  is  often  connected  between  the 
danleno  and  the  warp.  Fig.  22  shows  the  hoop  type  with 
a  wrapping  of  lead  sheet  at  the  bottom.  This  type  has 
the  advantages  that  it  is  less  apt  to  stick  in  muddy 
bottom  and  also,  if  and  when  a  splice  does  stick  in  the 


rig.  20. 


View  into  a  haddock  .\eine,  showing  the  attachment 
oj  the  flapper. 


coiler  during  fast  hauling  and  when  the  wing  lips  are 
together,  the  danleno  is  less  likely  to  catch  and  tear  the 
opposite  wing. 

The  specifications  of  a  Japanese  seine  net  and  asso- 
ciated gear  are  given  in  fig.  23. 

GENERAL   DESCRIPTION 

It  is  instructive  to  take  a  length  of  light  rope  and  a  piece 
of  light  chain  to  simulate  the  net,  to  lay  them  out  on  a 
sandy  beach  in  the  same  pattern  as  seine  net  gear,  then 
to  haul  them  in  by  hand.  The  sequence  of  events  is  better 
understood  from  such  a  rough  working  model  than  from 
pages  of  explanation.  At  first  the  net  does  not  move  at 


Fig.   21.     Common  danleno. 


Fig.  19.     Upper  crown  of  a  smaller  haddock  seine. 


Fig.  22.     Hoop  type  danleno. 


[383] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


all,  but  the  pattern  of  the  warp  changes  (fig.  24),  tending 
to  shepherd  the  fish  into  the  path  of  the  net,  which  only 
starts  to  move  after  warp  and  wing  tips  have  assumed  a 
position  in  which  the  tension  in  them  has  a  significant 
forward  component.  The  ground  rope  of  the  net  now 
shepherds  the  fish  forward  and  inward,  but  as  yet  few 
go  into  the  net.  As  the  warps  become  straighten  the  speed 
of  the  net  increases  and  fish  pass  down  the  funnel  to 
the  codend  (fig.  25).  The  headline  height  gradually  falls 
towards  a  steady  level  and  the  net  takes  up  its  fishing 
shape  very  much  like  that  of  a  trawl.  The  height  of  the 
headline  depends  on  the  rig  of  the  net  and  the  amount 
of  flotation,  but  a  final  steady  value  of  8  ft.  (2 £  m.)  may 
be  taken  as  represe  ative  of  a  medium  sized  haddock 
seine.  The  net  gradually  closes  (fig.  26)  until  it  has 


A  DIAGRAM 


B  CONSTRUCTION  OF  NET 


»•**" 


Part  of  Net 


NETTING 


Material 


Lower  belly 

Cotton 

45 

Side  panel      .  . 

Cotton 

30 

Upper  belly 

Cotton 

30 

Bottom  piece 

Cotton 

45 

Square 

Cotton 

30 

Flapper 

Cotton 

21 

Wing      .  .      . 

Cotton 

36 

Extension  Wing 

I  lemp  or 

3 

Manila 

No.  of        Size  of  Mesh     Length  (Stretched) 
Strands       (cm.)     ins.        (m.)               (ft.) 

45 

6 

2". 

22-7 

74-5 

30 

6 

2  • 

22-7 

74-5 

30 

6 

2  ' 

22-7 

74-5 

45 

6 

2- 

7-3 

23-8 

30 

6 

2  • 

7-3 

23-8 

21 

6 

2- 

14-6 

47-7 

36 

6 

2  • 

21-8 

71-6 

3 

48 

19-0         27-3 

89-5 

ROPES: 


Manila  diam.  33  mm.  with  glass  floats  (diam.  20  nun. 
Manila  diam.  43  mm. 


Headrope  : 
Footrope : 
Tow  rope  :  6  pieces  of  manilu  rope  shackled  together. 

Diameters  from  45  to  25  mm.  decreasing  from  wing  to  ship  chain 

of  about  37  kg. 

Total  length  of  tow  rope:  about  900m. 


Fig.  23.    Japanese  seine.  (F.W.S.  Fishery  Leaflet  389.) 


I.  Ropes  and  net  as  shot 
2.Net   bellying  out  and 
Starting  to  move. 


3.  Net  moving  si  owl/ 

and  shepherding  ffsh. 
4. Net    fishing. 
5.  Net  closed. 


Fig.   24.     Simplified  diagram  showing  the  stages  of  hauling. 

ceased  fishing,  with  the  codend,  we  hope,  swollen  ou 
like  a  balloon.  After  that  it  is  a  case  of  bringing  the  ne 
back  to  the  boat,  as  fast  as  possible,  depending  on  th< 
stress  on  the  ropes. 


Fig.  25.    Flatfish  in  the  codend. 
(A  Still  from  the  Scottish  Home  Department's  film,  "Fish  and  the  Seine  Net."] 


[384] 


DANISH     SEINING 


Fig.  26.  The  shape  of  the  headline  with  the  wings  almost  closed.  (Still 
from  the  Scottish  Home  Department*  sjilm  "Fish  and  the  Seine  Net") 

Warp  Length 

The  length  of  warp  used  increases  with  the  depth  worked, 
with  the  stowage  space  for  it  on  the  boat  and  with 
confidence  in  the  knowledge  of  a  clear  bottom.  A  small 
boat  of  40  ft.  (12  m.)  may  use  five  to  six  coils  per  side  in 
water  up  to  40  fm.  (73*  10  m.).  A  large  seiner  may  use 
ten  or  a  dozen  coils  per  side  in  water  up  to  100  fm. 
(183  m.),  but  even  in  depths  of  a  few  fathoms,  where  it  is 
known  there  is  clear  ground,  anchor  seiners  fishing  for 
plaice  may  use  up  to  sixteen  coils  per  side.  It  is  common 
practice  to  have  a  set  of  warps  of  the  minimum  length 
used  and  to  clip  on  to  these  a  set  of  extra  coils  as  neces- 
sary. A  rough  guide  to  the  amount  of  rope  used  by  fly 
draggers  is  as  follows: 

For  a  boat  with  an  engine  40  to   50  h.p.  —   5coilsaside 

60  to    70  h.p.  —  7     „     „ 
SO  to    90  h.p.       9     „     „ 
130  to  140  h.p.  -11     „     „ 
Echo  Sounder 

A  reliable  echo  sounder  is  an  important  adjunct  in  seine 
netting,  not  so  much  from  the  point  of  view  offish  finding, 
as  yet,  but  for  determining  the  nature  of  the  bottom.  It 
is  common  practice  on  unfamiliar  ground  to  keep  the 
sounder  running  all  the  time  when  the  warps  arc  running 
out,  and  often  enough  the  pattern  of  the  set  has  to  be 
modified  on  "advice"  from  the  echograph.  It  is  well  to 
have  an  echo  recorder  with  a  paper  speed  on  which  the 
whole  of  the  ground  covered  during  shooting  can  be 
surveyed  at  a  glance,  neither  loo  cramped  not  too 
extended.  Secondly,  it  is  well  to  have  a  paper  recording 
from  which  the  nature  of  the  bottom  can  be  interpreted. 
This  is  one  of  the  arts  of  seining. 

ANCHOR    SEINING    OPERATION 

When  an  anchor  seiner  reaches  the  ground,  it  is  common 
enough  to  have  a  fly  drag  before  deciding  to  lay  her 
moorings.  The  Danes  and  Swedes,  with  variable  pitch 
propellers  on  their  boats,  can  adjust  the  pitch  until  the 
boat  is  just  making  headway.  This  can  be  tested  in 
shallow  water  by  keeping  a  sounding  lead  bouncing 
across  the  bottom. 


The  decision  whether  to  shoot  to  starboard  (clockwise) 
with  a  port  bag,  or  vice  versa,  must  be  made  before  the 
set  commences  because  of  the  way  the  net  must  be 
stacked  down.  With  a  port  bag  the  left-hand  wing  is 
flaked  down  first  so  that  it  runs  off  the  stern  platform  last, 
the  headline  is  to  the  starboard  side  and  on  top,  the 
footrope  to  port  and  underneath.  The  danleno  at  the 
end  of  the  right-hand  wing  is  left  hanging  just  over  the 
stern,  allowing  the  net  to  start  running  away  clearly.  The 
set  is  made  so  that,  if  possible,  the  net  is  shot  before  the 
wind  and  on  returning  to  the  dahn  all  the  gear  is  then  on 
the  weather  side.  Attention  has  also  to  be  paid  to  the 
manner  in  which  the  tide  is  changing.  During  the  tow 
back  to  the  dahn  (fig.  2B)  the  net  moves  sideways,  and 
it  should  finish  in  such  a  position  that  the  tide  flows 
through  it  during  the  haul.  A  tide  float  is  sometimes  hung 
over  the  stern  of  the  vessel  while  she  is  at  her  moorings. 
This  helps  the  skipper  to  judge  the  best  way  to  make  his 
next  set. 

Once  the  moorings  are  laid,  the  dahn  with  its  picking- 
up  piece  is  attached  to  the  end  of  the  mooring  wire.  The 
dahn  must  be  tall  enough  to  be  seen  at  a  considerable 
distance  and  yet  light  enough  to  be  easily  brought  on 
board.  The  end  of  the  seine  net  warp  is  clipped  into  the 
end  of  the  mooring  wire  and  the  boat  commences  its  set, 
shooting  down  with  the  tide.  The  warp  can,  with  practice, 
be  run  out  at  full  speed,  but  here  is  one  of  the  most 
dangerous  aspects  of  seine  netting,  because  a  man  can  be 
snatched  overboard  through  stepping  inside  a  coil  or  bight 
of  rope.  If  he  is,  the  rope  should  not  be  cut  as  he  will  go 
down  with  the  rope  unless  he  frees  himself.  It  is  better  to 
bundle  slack  warp  overboard  and  then  come  back  on  it. 
The  boat  is  slowed  down  just  before  the  net  runs  out  and 
is  speeded  up  afterwards.  As  the  last  few  fathoms  of  warp 
prior  to  the  net  run  out,  a  bight  of  rope  is  lifted  over  the 
shooting  post  (fig.  2),  otherwise  the  net  will  be  dragged 
round  it  instead  of  going  over  the  stern.  As  the  crown  of 
the  net  comes  over  the  rail,  the  codend  is  thrown  by 
hand  clear  and  to  the  outside  of  the  rest  of  the  net. 
Often  enough,  the  last  danleno  to  go  overboard  is  held 
for  a  moment  by  hand,  helping  to  spread  the  net  as  far 
as  possible.  The  boat  carries  on  shooting  her  cross-rope 
before  turning  back  to  her  dahn.  An  anchor  seiner 
shoots  all  her  rope  out,  keeping  to  the  outside  of  her 
dahn  and  then  towing  her  end  of  the  warp  back  to  it. 
By  doing  this,  strain  is  put  on  the  gear,  which  is  brought 
into  a  more  advantageous  position  for  hauling  by  the 
winch.  The  engine  is  slowed  down  before  the  warp  is  all 
run  out,  and  the  low  back  to  the  dahn  is  made  at  1 1  to 
2  knots.  The  nature  of  the  ground  may  preclude  shooting 
as  much  cross-rope  on  one  side  as  the  other,  and  it  is 
not  always  desirable  to  do  so.  The  amount  of  cross-rope 
to  shoot  and  the  length  of  tow  back  to  the  dahn  is  a  matter 
for  the  skipper's  consideration/ 

The  Danes  have  evolved  a  special  method  of  shooting 
in  fishing  for  plaice  (fig.  2B).  Here  one  side  of  the  warp 
acls  as  a  shepherding  arm  and  the  set  covers  a  much 
larger  area  of  ground  than  usual.  In  such  circumstances, 
it  may  take  anything  up  to  1  i  hours  to  tow  back  to  the 
dahn.  In  these  days  of  various  radio  navigational  aids, 
the  moorings  can  be  laid  with  precision.  Ground  can 
then  be  covered  systematically  by  working  right  round 
the  moorings  as  the  tide  turns.  This  is  known  as  star 


[385] 


MODERN     FISHING     GEAR     OF    THE    WORLD 


ringing.  For  roundfish,  and  in  deep  water,  a  tow  back  to 
the  dahn  buoy  of  half  a  coil's  length  is  more  usual. 

If  the  tide  is  very  strong,  the  mooring  barrel  and  the 
dahn  can  be  ridden  down  during  this  towing  stage.  Even 
less  tide  than  this  can  cause  the  gear  to  close  quickly 
after  hauling  commences  and  the  net  to  make  very  little 
advance  before  closure. 

Due  to  straining  the  ropes  on  obstacles,  as  well  as  to 
shrinkage  and  to  splicing,  there  is  always  some  difference 
in  the  lengths  of  the  two  warps.  Yet  it  is  necessary  for  the 
net  to  be  brought  in  evenly.  If  it  is  not,  pockets  form  in 
the  net  out  of  which  fish  can  escape.  The  shorter  warp, 
which  has  the  greater  tension  in  it,  rides  high  and  so 
meets  the  water  further  away  from  the  ship's  side.  By 
surging  at  the  winch  on  the  tight  one,  the  two  warps  can 
again  be  brought  level.  It  is  possible,  using  this  judgment 
by  eye,  to  bring  up  the  danlenos  level  with  each  other  to 
within  a  few  inches.  Another  help  is  to  press  down  on 
the  ropes  as  they  come  over  the  rail  to  judge  by  their 
resistance  the  evenness  of  the  tension  in  them. 

It  must  happen  sooner  or  later  that  the  gear  comes  fast. 
What  is  to  be  done  then  depends  on  the  stage  of  hauling 
reached  when  this  occurs.  If  but  little  warp  has  been 
brought  abroad,  the  free  end  is  made  fast  to  the  moorings, 
which  are  then  slipped,  and  the  boat  winched  down  to  the 
fastener  on  the  other  end.  On  clearing  it,  the  boat 
returns  to  the  moorings,  shooting  out  the  warp  again, 
and  resumes  hauling.  If,  however,  the  haul  is  well 
advanced,  then  the  moorings  have  to  be  slipped  completely 
and  the  boat  winched  back  along  both  warps  until  they 
come  clear.  If  both  warps  are  fast,  a  time  will  come  when 
one  will  be  pointing  one  way  and  one  the  other.  The 
procedure  is  then  to  cut  and  buoy  one,  at  a  splice  if 
possible,  and  winch  along  the  other.  Once  the  warp  is 
cut  it  may  help  to  take  the  free  end  down  tide  of  the 
fastener. 

The  coils  have  to  be  removed  from  below  the  coiler 
to  be  dragged,  one  set  forward  and  one  set  aft.  To  do 
tin's,  a  4ft.  length  of  old  rope  is  placed  below  the  spout 
of  each  coiler,  and,  when  the  warp  is  piled  high  enough, 
the  end  of  this  piece  is  brought  up  through  the  centre  of 
the  coil  and  used  to  drag  the  coil  along  the  deck.  In 
figs.  2  and  3  it  can  be  seen  that,  as  far  as  the  shooting 
sequence  is  concerned,  the  after  coils  are  coiled  upside 
down  as  they  come  from  the  winch.  The  usual  practice 
is  to  turn  the  first  after  coil  upside  down  on  the  deck  as  it 
is  dragged  from  the  winch  and  to  stack  the  remaining 
coils  against  it.  These  can  then  be  quickly  flicked  over 
just  prior  to  the  next  set.  It  would  also  be  possible  to 
change  the  after  coils  end  for  end  without  turning  them 
over,  but  this  is  not  good  practice.  The  rope  nearest  the 
net  wears  most,  and  it  is  better  to  keep  the  wear  in  one 
place,  cutting  off  the  worn  coils  and  adding  new  ones  at 
the  other  end,  so  that  the  warp  is  renewed  continually. 

For  plaice,  the  winch  may  be  kept  in  first  gear  until 
the  warps  close;  for  other  species,  hake,  for  instance,  the 
hauling  speeds  can  be  faster.  The  time  taken  for  a  set 
depends  on  several  factors,  but  an  estimate  of  it  can  be 
made  beforehand,  which,  apart  from  snags,  is  usually 
correct  to  within  a  few  minutes.  It  is  here  that  a  knowledge 
of  the  w\nch  gear  and  pulley  ratios  come  in  useful.  The 
length  of  warp  and  the  speed  of  the  boat  shooting  it  are 
known.  The  length  of  tow  back  to  the  dahn  is  determined 


by  the  pattern  of  the  set,  while  the  speed  of  tow  can  be 
chosen.  There  is  usually  an  engine  revolution  meter  in 
the  wheelhouse,  so  that  the  engine  speed  can  be  set  to 
give  the  desired  hauling  speed  on  the  ropes.  To  all  this 
must  be  added  a  few  minutes  for  clipping  on  to  the 
moorings  and  for  the  lifting  aboard  of  the  net. 

FLY  DRAGGING  OPERATION 

As  the  movement  of  the  net  is  due  partly  to  the  headway 
of  the  ship  and  partly  to  the  hauling  of  the  winch,  the 
winch  speeds  in  the  early  stages  of  the  hauling  are  kept 
slow.  In  general,  a  set  tends  to  take  longer  by  this 
method,  but  there  can  be  no  certainty  about  this  because, 
even  with  the  hauling  period  longer,  due  to  the  time  spent 
in  towing  before  the  winch  is  started  and  to  the  slower 
initial  winch  speeds,  the  shooting  period  is  shorter  since 
no  tow  back  to  the  dahn  is  made.  Some  skippers  like  to 
start  the  winch  as  soon  as  the  gear  is  "all  square",  while 
others  prefer  to  tow  for  anything  up  to  20  minutes  before 
starting  the  winch.  With  a  very  small  boat  and  little 
power,  it  is  quite  a  successful  practice  to  tow  the  gear  to 
a  close  and  then  to  use  the  power  for  winching  in  over 
the  broadside. 

The  speed  at  which  a  fly  draggcr's  engine  is  set  is  not 
dependent  upon  the  requirements  of  the  winch  but  upon 
what  is  necessary  to  make  headway  over  the  ground. 
Against  wind  and  tide,  more  engine  revolutions  are 
required,  and  it  is  such  considerations  that  lead  to  the 
greater  number  of  winch  gears,  though  all  of  them  may 
not  be  used  every  drag.  Similarly,  the  choice  of  the  pulley 
ratios  is  a  matter  to  be  given  some  thought.  An  inch  or 
two  more  on  the  upper  or  lower  pulley  can  make  quite 
a  difference. 

By  steaming  ahead,  the  seiner  makes  its  own  tide,  as  it 
were,  so  reaping  certain  advantages.  It  is  possible  to  work 
against  a  stronger  tide  and  to  tow  before  it  or  even 
partly  across  it.  There  is  more  freedom  to  tackle  a  piece 
of  ground  from  any  advantageous  direction,  remembering 
amongst  other  things  that  there  are  some  types  of 
grounds,  like  the  belly  of  a  sprat,  where  net  and  ropes  will 
come  one  way  but  not  the  other.  In  towing  before  the 
tide,  two  things  to  be  remembered  are: — 

( 1 )  The  net  moves  a  long  way  before  it  closes,  so  good 
ground  is  needed. 

(2)  As  the  net  sinks,  the  codend  trails  above  it  and  may 
be  carried  over  the  lop  of  the  headline  when  the 
net  reaches  the  bottom  and  becomes  stuck  inside  it 
when  the  net  starts  to  move.  A  whole  drag  can  be 
lost  in  this  way  though  the  shooting  has  been 
perfect,  and  it  is  a  precaution  either  to  weight  or 
float  the  codend  on  such  occasions. 

The  gear  takes  some  time  to  sink  to  the  bottom,  and 
under  the  action  of  the  tide  it  will  not  reach  the  bottom 
directly  below  where  it  is  shot.  Allowance  has  to  be  made 
for  this  on  some  confined  grounds.  The  ropes  themselves 
sink  at  about  12  fm.  (22  m.)/min.  They  sink  faster  than 
the  light  haddock  seine  and  carry  the  net  down  with 
them.  When  intending  to  tow  partly  across  the  tide,  the 
uptide  warp  should  be  shot  first  or  the  gear  will  tend  to 
close  up  on  itself. 

Just  how  far  and  fast  the  boat  advances  from  the  point 
at  which  the  dahn  is  lifted,  and  how  far  the  net  advances, 


[3861 


DANISH     SEINING 


o  /  !*'« 


i*    I   K^^ 


POSITION    AT  CLOSURE  POSITION    AT    SHOOTING 

Fig.  27.     Fly  dragging  operation. 

depend  on  many  factors,  tide,  winch  speeds,  length  of 
rope  and  the  pattern  of  shooting,  power  of  the  vessel  and 
times  of  changing  gear.  If  too  much  rope  is  used,  the 
vessel  can  be  as  good  as  anchored,  the  advantages  of  fly 
dragging  being  lost  without  securing  the  advantages  of 
working  at  moorings.  In  general,  a  fly  dragger  would  use 
less  rope  than  an  anchor  seiner  of  the  same  size.  Fig.  27 
gives  some  idea  of  what  can  be  done  with  comparatively 
little  rope.  A  typical  example  for  a  small  40  ft.  (12  m.) 
boat  is  to  tow  for  10  min.,  take  half  a  coil  in  1st  gear,  one 
coil  in  2nd,  one  and  a  half  coils  in  3rd,  by  which  time 
the  net  is  closed,  and  the  remaining  three  coils  in  4th.  A 
small  boat  scores  in  this  respect  because  once  the  net  is 
closed  she  can  take  in  all  remaining  rope  in  top  gear,  thus 
saving  time.  With  a  larger  ship,  and  particularly  with  a 
following  sea,  one  has  to  take  things  more  gently.  The 
speed  of  advance  of  the  boat  across  the  ground  may  be 
taken  roughly  as  falling  from  2  knots  when  towing,  to 
standstill  in  top  gear,  with  the  speed  of  the  net  rising 
from  zero  to  a  maximum  of  2\  knots  at  closure.  The 
amount  of  cross-rope  to  shoot  is  a  matter  of  choice 
dependent  on  the  total  length  of  rope  and  nature  of  the 
ground.  It  is  a  mistake,  however,  to  think  that  the  more 
cross-rope  shot,  the  further  the  net  will  advance  before 
closure.  Too  much  cross-rope  simply  means  that  the 
boat  has  to  move  farther  ahead  and  winch  in  more  rope 
before  the  net  starts  to  move  at  all. 

The  shooting  procedure  is  much  the  same  as  in  anchor 
seining  except  that  the  coils  are  laid  out  on  either  side  of 
the  vessel,  as  shown  in  figs.  4  to  7.  The  shooting  bar  on 
the  rail  is  shown  in  fig.  28.  The  vessel's  head  is,  as 
before,  kept  to  the  outside  of  the  dahn  on  the  return 
journey  to  it,  but  enough  warp  is  left  to  reach  the  dahn 
and  the  slack  warp  is  thrown  overboard  as  the  dahn  is 
approached.  When  hauling,  the  warp  is  led  through  a 
single  roller  (see  fig.  29).  The  roller  is  let  into  any  of 
several  holes  along  the  rail  on  the  weather  quarter.  The 
stronger  the  wind,  the  further  forward  the  roller  is  put 
to  maintain  steerage.  A  mizzen  sail  is  often  a  help,  too. 
The  procedure  on  coming  fast  is  the  same  as  for  anchor 
seining,  except  that  there  are  no  moorings  to  slip. 


Fig.    28.        Shooting  bar. 
Fig.  29.     Rail-roller  for  hauling. 

CONCLUSION 

Compared  with  trawling  on  a  heavy  ship,  seining  is  as 
the  rapier  to  the  sledgehammer,  not  necessarily  always 
more  effective  but  requiring  a  difference  of  outlook  and 
technique.  The  beginner  need  hardly  expect  immediate 
success  with  cither  of  the  two  methods  of  seining,  but, 
with  practice,  these  can  be  most  effective  in  the  right 
places.  Accurate  knowledge  of  the  grounds  and  tides 
must  be  acquired,  and  knowledge  of  echo  sounder 
interpretation  is  a  great  help.  Seining  has  its  physical 
dangers,  too,  particularly  those  of  being  hauled  overboard 
by  the  warp  during  shooting  and  being  carried  into  the 
winch  at  high  speed  when  hauling.  The  shooting  pro- 
cedure is  often  different  from  one  set  to  the  next,  and 
this  is  difficult  for  a  raw  crew  to  master  and  learn  to 
accomplish  with  precision.  In  spite  of  its  difficulties,  the 
reward  of  persistence  at  Danish  seining  has  been  sufficient 
to  bring  it,  within  recent  years  and  in  more  than  one 
country,  to  being  a  principal  method  of  fishing  for 
demersal  species. 


387  ] 


COMPARISON   OF  STARBOARD  AND  PORT  SIDE  OPERATION 

FOR  (DANISH)  SEINING 

by 

ICHIRO  SAITO 

The  Faculty  of  Fisheries,  Hokkaido  University,  Hakodate,  Japan 


Abstract 

In  most  Japanese  boats  the  fishing  gear  has  generally  been  worked  from  the  port  side,  merely  as  a  matter  of  custom,  due  no  doubt 
to  the  fact  that  the  earliest  boats  were  steered  by  an  oar,  mounted  on  the  port  quarter.  The  author,  after  observing  the  operation  of  the 
seines  (similar  to  the  Danish  seine),  deduces  that  better  and  safer  handling  of  the  net  is  obtained  if  the  starboard  side  of  the  vessel  is  used. 


Resum^ 


Comparaison  de  I'utilisalion  a  tribord  et  a  babord  d'un  filet  de  pechc  de  type  scnne  danoisc 


La  pi u part  des  bateaux  japonais  vitiliscnt  generalement  les  cngins  de  peche  ;\  babord,  simplement  par  habitude,  sans  doute  parce 
que  les  premiers  bateaux  ctaicnt  gouvcrnes  au  moyen  d'un  "Ro,"  ou  aviron,  mont£  sur  1'arrierc  a  babord.  Dans  cet  article,  I'autcur,  apres 
avoir  observd  le  fonctionnement  d'un  chalut  moyen  a  un  bateau  (en  fait,  une  variet£  de  scnnc  danoise)  conclut  qu'il  est  preferable  et  moins 
dangereux  de  manoeuvrer  le  filet  a  ti  ibord.  II  montre  comment,  avec  unc  helice  tournant  vcrs  la  droiic,  on  doit  logiquement  opdrer  a  tribord 


Comparacion  del  calamento  de  una  red  de  arrastre  danesa  por  babor  y  estribor 
Extracto 

A  causa  de  la  costumbre,  y  quizes  por  el  hecho  dc  que  las  primeras  embarcaciones  eran  timoneadas  mediante  un  **ro"  o  remo 
montado  en  el  cuarto  de  babor,  la  mayoria  de  los  barcos  japoneses  calan  los  artcs  de  pesca  por  esc  costado.  En  este  articulo  el  aulor  despues 
de  observar  el  calamento  y  rccogida  de  una  red  usada  por  un  "arrastrero  mediano"  (en  realidad  un  tipo  de  harco  para  red  de  arrastre  danesa), 
deduce  que  la  red  se  manipula  en  mejores  condiciones  y  con  menor  peligro  por  el  costado  dc  estribor  de  la  embarcaci6n.  Tambien  dcmuestra 
como  al  usar  una  helice  que  gira  a  la  derecha,  estribor  seria  el  costado  16gico  de  trabajo 


THE  seining  method  discussed  here  has  been  devised 
in  Japan  and  is  most  popular.  The  boats  of  15  to 
75  gross  tons  used  for  this  seining  are  the  most 
numerous  in  the  Japanese  fishing  fleet.  The  operation 
of  the  gear  (which  is  practically  identical  with  Danish 
seining. — The  Editor)  is  as  follows: 

The  two  manila  ropes  may  vary  in  length  from  1,600 
and  3,200  m.  A  buoy  fixed  to  the  free  end  of  one  rope 
is  thrown  overboard,  and  the  warp,  the  net  and  the 
second  warp  steamed  out  according  to  the  scheme  shown 
in  fig.  1 .  After  having  picked  up  the  buoy  with  the  free 
end  of  the  first  rope,  both  ropes  are  hauled  while  keeping 
the  boat  before  wind  and  current. 

In  Japan  the  gear  is  customarily  operated  on  the  port 
side  but  the  reason  for  this  is  not  clear.  In  the  early 
days  the  "Ro"  (Japanese  oar)  for  steering  was  set  at  the 
port  side  of  the  stern,  so  the  fishing  was  also  done  from 
the  port  side.  This  is  still  the  practice  even  though  the 
boats  are  now  equipped  with  propeller  and  rudder. 


According  to  seamanship  theory,  the  starboard  side 
should  be  used.  The  author  (1949)  pointed  out  that,  for 
otter  trawling,  the  starboard  side  is  preferable  because 
of  the  effects  of  propeller  and  rudder,  and  because  of  the 
rules  of  the  International  Regulations  for  Preventing 


Wind  and  wave 


Fig.  I.     Method  of  steaming  out  the  gear  when  fly-dragging. 


[388] 


COMPARING     STARBOARD     AND    PORT     SIDE     OPERATION 

Collisions  at  Sea  (1948).  As  seiners  usually  have  a  right- 
handed  propeller,  seining  should  also  be  done  from  the 
starboard  side.  The  author's  studies  of  commercial 
seining  have  confirmed  this. 

The  relation  between  the  ship  and  the  net  in  port  side 
hauling  is  shown  in  fig.  2.  From  the  positions  b  and  c, 
to  the  situation  shown  in  d,  the  ship  is  propelled  back- 
ward, and  the  net  is  taken  in.  When  using  the  engine 
for  going  astern,  the  righthand  propeller  turns  the  stern 
of  the  ship  to  port  and  the  head  to  starboard  so  that  the 
after-wing  of  the  net  is  in  danger  of  becoming  entangled 
with  the  propeller  (fig.  2,  e).  The  helm  must  be  hard 


wind  and  wave 


(d) 


fig.  2.     Operation  of  the  net  and  movements  of  the  boat  when 
fishing  over  the  port  side. 


Fig.  3.     Operation  of  the  net  and  movements  of  the  boat  when 
fishing  over  the  starboard  side. 


[389] 


MODERN    FISHING    GEAR     OF    THE    WORLD 


to  port  and  the  skipper  must  depend  on  the  action  of  the 
propeller  to  reach  the  situation  in  fig.  2,  d.  The  effect 
of  the  rudder  is  very  slight  at  such  low  speed  so  steering  is 
difficult. 

When  the  wind  exceeds  Beaufort  Scale  4,  it  is  better 
to  have  the  net  with  the  ship  heading  to  the  wind  and  sea. 
But,  as  for  this  purpose  the  engine  must  be  used  ahead 
and  astern,  there  is  still  danger  of  the  net  at  port  side 
being  drawn  in  the  propeller  because  of  the  port  side 
tendency  of  the  boat. 

The  reasons  for  this  effect  of  the  propeller  on  the  boat 
are  the  following:  While  the  engine  is  going  ahead,  the 
discharge  current  of  a  righthand  propeller  strikes  the 
lower  part  of  the  rudder  on  the  starboard  and  the  upper 
part  on  the  port  side.  In  the  usual  type  of  rudder,  which 
is  narrow  in  the  upper  part  and  wide  in  the  lower  part, 


the  pressure  on  the  lower  part  is  stronger,  which  tends 
to  turn  the  stern  to  the  port  and  the  head  to  starboard. 

When  the  propeller  goes  astern,  the  discharge  current 
strikes  the  upper  part  of  the  stern  on  the  starboard  side, 
and  the  lower  part  on  the  port  side.  Most  of  the  pressure 
on  the  lower  part  escapes  under  the  keel,  therefore  the 
pressure  on  the  starboard  side  is  greater,  tending  also 
to  turn  the  stern  to  port  and  the  head  to  starboard. 
The  magnitude  of  this  effect,  of  course,  depends  on  the 
construction  of  the  stern  and  rudder  and  also  on  the 
actual  weather  and  current  conditions. 

Seiners  with  a  righthand  propeller  should,  therefore, 
fish  from  the  starboard  side,  particularly  in  stormy 
weather  because  the  vessel  can  turn  in  a  smaller  circle 
and  the  danger  of  fouling  the  net  in  the  propeller  is 
avoided  (fig.  3). 


Bench  seining  for  Anchoveta  and  other  small  fish  for  use  as  live  bait  in  tuna  fishing—  Haiti. 


Photo:     FAQ. 


[390] 


FISHING   WITH   SOUTH   AFRICAN   PURSED   LAMPARA 

by 

C.  G.  DU  PLESSIS 

Director,  Division  of  Fisheries,  Beach  Road,  Sea  Point,  Cape  Town,  Union  of  South  Africa 

Abstract 

This  short  paper  gives  a  conslruction.il  picture  of  the  pursed  Lampara  net  used  in  South  Africa  for  catching  Pilchard,  Maasbankcr 
and  Mackerel.  The  net  is  made  of  Marlon  (a  synthetic  material  consisting  of  a  mixture  of  nylon  and  vinylon)  and  comprises  K»  panels  of 
netting  arranged  around  a  central  bag  in  the  orthodox  manner.  This  gear  has  been  in  use  in  South  Africa  for  about  15  years,  and  with  the 
exception  of  minor  variations  according  to  the  ideas  of  individual  fishermen,  has  become  a  standard. 


Resume 


Pcchc  a  la  senne  tournante  au  lamparo  en  Afrique  du  Sud 


L'auteur  decril  d.ms  cette  courte  etude  le  filet  utilise*  en  Afrique  du  Sud  pour  la  peche  au  larnpare  du  Pilchard,  du  Chinchard  el  du 
Maquereau.  Le  filet  est  en  Manen  (materiaii  synthetique  constilud  d'un  melange  de  Nylon  et  de  vinylon)  ct  comprend  16  pieces  de  filet 
disposers  scion  la  methode  habituelle  aulour  d'un  sac  central.  Cet  engin  est  utilise  depuis  une  quinzaine  d'annees  en  Afrique  du  Sud,  et, 
si  Tou  excepte  de  legcres  modifications  dictees  par  les  idees  pcrsonnelles  de  certains  pecheurs,  il  est  devcnu  un  modele  standard. 


Tipo  dc  red  "lampara"  con  jarcta  usada  en  la  Union  Sudafricana 
Extracto 

En  este  breve  estudio  se  describe  la  construction  de  una  red  "lampara"  tipica  de  la  Union  Sudafricana  para  pescar  sardina,  jurel 
y  cabal  la.  El  arte  csta  compuesto  por  16  panos  de  *4marl6n"  (producto  sintetico  que  consiste  en  una  mezcla  dc  nylon  y  vinilidon).  distribuidos 
alrcdcdor  de  una  bolsa  central  en  la  forma  acostumbrada.  IZsta  red  sc  ha  usado  en  la  Union  Sudafricana  durante  unos  15  anos  con  solo 
pequenos  cambios,  scgun  las  ideas  de  cada  pescador,  convirtiendose  en  un  mctodo  de  pesca  corriente. 


THIS  method  of  fishing  has  been  in  use  for  about 
15  years  and  has  been  considerably  modernized 
during  this  time.     The  advent  of  synthetic  fibre 
netting,  more  efficient  winching  systems,  and  craft  of 
greater  carrying  capacity  have  been  the  main  influencing 
factors  in  the  evolution  of  the  gear  and  the  method  of 
operation.  Today,  the  construction  of  the  net  and  its 
operation  are  almost  standard  with  only  minor  variations 
due  to  the  idiosyncrasies  of  individual  fishermen. 

CONSTRUCTION    OF    GEAR 

Fig.  1  (not  to  scale)  shows  a  typical  lampara  net  made  of 
Marlon,  a  mixture  of  nylon  and  vinylon  which  is  widely 
used  in  the  pilchard,  maasbanker  and  mackerel  fisheries. 
The  net  is  seamed  in  approximately  1 70  fm.  corkline  and 
approximately  145  fm.  leadline.  The  netting  itself  is  1 J  in. 
mesh  throughout  and  comprises  a  set  of  panels  each 
of  the  same  width  on  either  side  and  the  same  width 
top  and  bottom.  Particulars  of  these  panels  (see  fig.  1), 
are  as  follows:  ~ 

Panel  No.         Meshes  wide      Meshes  deep 


Panel  No. 


Meshes   wide      Meshes  deep 


Twine 
(Mai Ion   Number) 


1 

800 

900 

M4 

2 

800 

950 

M4 

3 

800 

1000 

M4 

4 

800 

1050 

M4 

5 

800 

6 

800 

7 

400 

8 

400 

9 

1000 

10 

900 

11 

800 

12 

700 

1100 
1150 
1200 
1250 
400 
400 
300 
250 


M4 

M6 

M8 

M10 

M12 

M12 

M10 

M10 


To  create  the  curvature  or  bulge,  the  outer  side  of  each 
panel  is  reduced  by  stealing  or  tucking  in  an  appropriate 
number  of  meshes  to  match  the  depth  of  the  inner  side 
of  the  adjoining  panel.  This  procedure  is  followed 
throughout  the  wings  with  the  result  that  the  bottom  of 
the  net  lies  straight  yet  tapering  towards  the  wing  ends. 

The  corks  are  attached  directly  to  a  £  in.  diameter 
synthetic  rope  and  the  leads  to  a  A  in.  diameter  synthetic 
rope.  Approximately  1,000  5  in.  corks  are  used  for 
the  bag  and  shoulders  and  from  750  to  1,000  4  in.  corks 
for  the  wings.  The  weight  of  the  leads  and  the  distance 
between  them  depend  on  the  thickness  of  the  twines  used 
in  the  various  sections  of  the  net. 

The  purse  lines,  each  approximately  80  fm.  long  and 
usually  of  manila  rope  of  about  3  in.  circumference,  are 
shackled  to  the  tongue  (i.e.  the  deepest  portion  of  the  bag,  at 


[391  ] 


MODERN     FISHING     GEAR     OF    THE    WORLD 


Cork  line 


Lead  line 


Putse  line 


Tongue 


Fig.  1.     Construction  diagram  of  a  Lampara  seine  net. 


which  point  about  600  meshes  are  bunched  together  to 
cover  an  area  of  not  more  than  18  in.  wide)  in  the  following 
manner:  a  strop  about  2  ft.  long  with  a  swivel  at  each 
end  is  attached  to  the  leadline  of  the  tongue  and  the 
purse  lines  are  attached  to  these  swivels.  Below  this 
strop  another  rope  is  attached,  about  1£  in.  in  circum- 
ference, carrying  up  to  3  dozen  leads  to  weight  the  bag. 
The  purse  lines  are  passed  through  rings  about  4  in. 
in  diameter  which  are  hung  from  the  leadline  by  the 
ring  lines,  which  are  1£  in.  in  circumference,  1  fm.  in 
length  and  spaced  2  to  3  fm.  apart. 

OPERATION 

The  average  vessel  using  a  net  of  the  above  size  is  55  ft. 
overall,  has  a  beam  of  19  to  20  ft.,  an  engine  of  150  h.p. 
and  a  total  carrying  capacity  of  about  100  tons  of  fish, 
i.e.  80  tons  in  the  hold  and  20  tons  on  deck.  Its  super- 
structure is  well  aft  and  comprises  mainly  a  wheelhouse, 
skipper's  cabin  and  galley.  The  average  number  of  crew 
is  10.  Most  boats  have  R.T.  and  echo  sounding  equip- 
ment. 

The  lampara  net  is  used  on  the  starboard  side  of  the 
boat.  The  front  wing,  which  is  stacked  on  top  of  the 
net,  is  first  released.  The  purse  line  attached  to  this  wing 
is  made  fast  to  a  one-man  rowing  boat  (dinghy)  about 
14  ft.  long,  while  the  purse  line  on  the  other  wing  is 
usually  attached  to  the  mast. 

The  method  of  coiling  is  to  begin  from  the  tongue  at 
the  front  wing,  care  being  taken  to  match  the  lengths  of 
the  leadline,  the  ring  lines  and  the  purse  line.  This  wing 
is  coiled  over  the  starboard  gunwale  in  such  a  manner 
that  the  weight  of  the  net  is  distributed  evenly  in-  and 
outboard.  The  back  wing  and  the  purse  line  on  this 
side  are  coiled  on  the  starboard  foredeck. 


As  the  amount  of  netting  in  the  water  increases  while 
the  boat  moves  in  a  circle  to  encompass  the  school  of 
fish,  the  purse  lines  are  pulled  through  the  rings  and,  on 
the  tightening  of  the  leadline,  the  ring  lines  pass  over  the 
side.  When  the  school  is  encircled  the  man  in  the  dinghy 
throws  up  a  heaving  line  to  which  the  purse  line  is 
attached.  The  two  ends  of  the  purse  lines  are  passed  over 


Fig.  2.     The  net  is  being  hauled  in  after  pursing  with  a  winch 
set  at  an  angle  in  front  of  the  wheelhouse. 


(392) 


THE    SOUTH     AFRICAN     PURSED     LAMPARA 


a  roller  on  the  gunwale  on  to  the  winch  drums  and  hauling 
commences.  The  dinghy  proceeds  to  the  corkline  of  the 
bag  and  the  dinghy-man  makes  fast  to  the  shoulders  of 
the  net  two  sealed  empty  drums  and  then  fastens  his 
dinghy  to  the  centre  of  the  bag  corkline.  This  is  to  prevent 
the  net  from  sinking  through  an  excessive  weight  offish. 
The  wings  are  drawn  in  evenly  or  one  before  the  other, 
depending  upon  the  behaviour  of  the  fish.  The  opening 
between  the  purse  lines  decreases  as  the  tongue  is  brought 
nearer  the  surface  and  as  soon  as  the  purse  lines  lie 
parallel,  the  tongue  is  brought  alongside.  The  winch 
is  stopped  and  the  tongue  made  fast,  thus  enabling  the 
crew  to  retrieve  the  leadlines  which  at  this  stage  hang  in 
bunches  in  the  water  alongside  the  boat.  While  the 
leadlines  are  retrieved  and  the  wings  brought  in,  the 


area  of  the  net  in  the  water  decreases  until  finally  the  fish 
are  contained  in  the  bag  and  inner  portions  of  the  shoul- 
ders. 

It  is  at  this  moment  that  brailing  begins  by  means  of 
a  steel  pole  about  10  ft.  long,  attached  to  a  steel  hoop  and 
a  bag  of  2  in.  stretched  mesh  synthetic  fibre  netting  with  a 
capacity  of  about  500  Ib.  of  fish.  The  bottom  of  the 
brailer  contains  a  number  of  cringles  through  which 
passes  a  zipper  chain.  The  brailer  is  attached  to  a 
derrick  on  the  mast  and  is  manipulated  with  the  aid  of 
the  winch.  Up  to  60  tons  offish  can  be  brailed  in  an  hour. 

Sometimes  up  to  400  tons  of  fish  are  encircled  in  one 
set.  When  this  happens  other  boats  are  called  and 
they  help  to  manage  the  net  while  the  catch  is  brailed 
into  their  holds. 


fitfm^s  •&&* **™  r*'"1: 


•-&&1"     <v^W#^ 

'"••  *^Ss?^^^       -*$tf ''< 


Indian  fishermen  on  the  Coromandel  Coast  hauling  the  bag  of  their  lampara  type  bag  net  between  two  log  rafts. 

(393] 


MENHADEN   PURSE   SEINING 

by 
JOHN  S.  ROBAS 

Commercial  Fisheries  Consultant,  Fernandina  Beach,  Florida,  U.S.A. 

Abstract 

Vessels,  gear  and  fishing  operations  are  described  in  detail.  In  U.S.  menhaden  purse  seining,  fish  concentrations  are  located  visually 
from  the  crow's  nest  of  the  main  vessel  or  by  aircraft  spotting,  with  one  airplane  usually  serving  5  seiners.  About  22  to  30  men  arc  needed 
to  handle  the  big  purse  seine  which  is  about  16,(X)()  to  22,000  meshes  U  in-  har)  long  and  about  800  to  1,000  meshes  deep.  The  purse  seine 
is  operated  by  two  motorized  purse  boats,  each  about  34  ft.  in  length,  which  are  carried  to  the  fishing  grounds  by  the  main  vessel  (UK)  to 
200  ft.  overall).  These  big  seiners  often  have  refrigerated  fish  holds  to  carry  between  150  and  1,200  tons  of  fish.  The  catch  is  generally 
transferred  from  the  net  into  the  main  vessel  by  fish  pumps.  Synthetic  materials  are  replacing  the  conventional  cotton  and  cork  in  nets  and 
floats.  A  top  seiner  will  catch  between  5,000  and  10,000  tons  in  a  5-month  season. 


Resume 


La  Pdche  du  Menhaden  a  la  pcche  Tournante 


L'auteur  decrit  en  detail  les  bateaux,  les  engins  et  les  operations  de  pcche.  Dans  la  peche  du  menhaden  aux  Htats  Unis,  les  con- 
centrations de  poissons  sont  reperes  visuellement  du  n  id -de-pie  du  navire  principal  on  par  rcpdragc  aerien,  avec  avion  opcrant  gendralcment 
pour  5  senneurs.  II  faut  environ  dc  22  a  30  hommes  pour  manipuler  la  grandc  scnne  tournante  qui  mcsurc  environ  16,000  a  22,000  mail  les 
(de  |  de  pouce,  soit  2  cm.  cntre  noeuds)  de  long  et  environ  800  a  1,000  mail  les  de  chute.  La  scnne  tournante  est  manoeuvres  par  deux 
bateaux  a  moteur,  d'cnviron  34  pi.  (10,36  m.)  de  long,  qui  sont  amends  sur  les  lieux  de  peche  par  le  navire  principal  (100  a  200  pi.,  soit  30,48 
a  60,96  m.,  hors-tout).  Ces  grands  senneurs  possedent  souvent  des  calcs  a  poissons  refrigeres  pouvant  porter  entre  1 50  et  1 ,200  tonnes  de 
poissons.  Generalement  on  utilise  des  pompes  a  poisson  pour  transporter  la  peche  du  filet  a  bord  du  navire  principal.  Dans  la  fabrication 
du  filet,  les  materiaux  synthetiques  remplacent  le  colon  du  filet  et  le  liege  des  flotteurs.  Un  excellent  senneur  peche  entre  5,000  et  10,000  tonnes 
de  poisson  en  5  mois  de  campugne. 

La  Pesca  de  lacha  con  red  de  cerco  de  jareta 
Extracto 

Se  describen  minuciosamcnte  las  embarcaciones,  artes  y  procedimientos  pesqueros.  En  la  pssca  de  lacha  con  red  de  cerco  de  jareta 
en  los  Fstados  Unidos,  las  concentraciones  de  pcces  se  localizan  visualmente  ya  sea  desde  la  torre  del  vigia  del  barco  principal  o  mediante  su 
senalamiento  desde  un  avion  que  sirve  por  lo  general  a  5  pesqueros.  Se  precisan  alrededor  de  22  a  30  hornbres  para  manejar  la  grandisima 
red  q.ue  tiene  dc  16.000  a  22.000  mall  as  (j  de  pulgada  el  lado  del  cuadrado)  de  longitud  y  unas  800  a  1.000  mallas  de  profundidad.  El  arte 
se  acciona  desde  dos  embarcaciones  molorizadas  de  34  pics  eslora,  a  las  que  lleva  hast  a  los  bancos  de  pesca  la  cmbarcaci6n  principal  (de  100 
a  200  pies  de  eslora  total).  Estos  barcos  grandes  disponen  a  veces  dc  bodegas  refrigeradas  para  transportar  de  1 50  a  1 .200  toneladas  de 
pescado.  La  pesca  se  traslada  gencralmente  desde  la  red  al  barco  principal  por  medio  de  bom  has  de  pescado.  Los  materiales  sinteticos 
estan  sustituyendo  al  algodon  y  corcho  usados  normalmente  en  las  redes  y  en  los  flotadores.  Uno  de  estos  barcos  grandes  captura  entre 
5,000  y  10,000  toneladas  de  pescado  en  una  campaha  de  5  meses. 


THIS  fishery  is  based  on  the  menhaden,  a  herring-like 
fish  of  the  genus  Brevoortia,  which  migrates  along 
the  Atlantic  and  Gulf  coasts  of  the  United  States, 
usually  in   the  shallow  waters  near  shore.    This  fish 
is   used   exclusively    for   reduction    purposes   and   the 
production  of  meal,  oil,  and  condensed  solubles. 

In  the  Gulf  of  Mexico,  the  catching  season  usually 
starts  in  May  and  extends  through  September;  along  the 
Atlantic  coast,  the  season  starts  in  April  in  Southern 
waters  and  is  followed  by  the  northern  fishery  near 
Virginia  and  New  Jersey  in  May.  Only  one  winter  fishery 
is  worked  in  the  waters  of  North  Carolina,  where 
migrating  spawning  fish  are  caught  near  Cape  Hatteras 
from  November  through  December. 

VESSELS 

In  the  United  States  fishing  fleet,  the  menhaden  purse 
seiners  are  distinguished  by  their  size,  which  reaches 
200  ft.  in  length,  with  a  carrying  capacity  of  approxi- 


mately 1,200  tons;  the  smallest  profitable  vessels  in  use 
are  approximately  100  ft.  overall  and  have  a  carrying 
capacity  of  about  150  tons  of  fish.  Most  of  the  newer 
boats  being  built  are  of  steel,  and  many  of  them  are 
being  equipped  with  refrigeration  facilities  to  keep  the 
fish  fresh.  A  typical  menhaden  seiner,  however,  still 
returns  to  port  each  day  or  every  two  days  to  unload 
the  catch. 

The  actual  fishing  operation  is  done  from  two  steel 
purse  boats,  each  handling  one  half  of  the  large  purse 
seine.  The  industry  is  still  largely  dependent  on  manual 
labour  and  for  this  reason  the  crew  of  a  typical  menhaden 
vessel  numbers  22  to  30  men,  the  majority  of  whom  are 
used  in  the  purse  boats,  pulling  the  net  by  hand.  The 
function  of  the  main  vessel,  therefore,  is  merely  to  carry 
the  fishing  gear  and  crew  to  the  fishing  grounds  and  to 
return  home  with  the  catch  as  rapidly  as  possible. 

It  goes  without  saying  that  speed  is  an  important 
characteristic  of  the  menhaden  vessel:  some  of  the  larger 
vessels  are  powered  with  two  750  h.p.  engines  driving 


[394] 


MENHADEN    PURSE     SEINING 


Fig.  I.     View  from  the  mast -head  of  a  menhaden  purse  seiner 

lowing  her  purse-boats  and  the  striker  boat.     The  steel  davits 

on  each  side  of  the  engine  house  are  used  for  carrying  the  purse 

seine  boats  to  the  fishing  grounds. 


twin  screws  and  attain  a  speed  of  16  knots.  Vessels 
working  in  northern  waters,  where  fog  is  encountered, 
usually  have  radar  and,  of  course,  most  of  them  carry 
depth  finding  and  radio-telephone  equipment,  particularly 
of  the  FM,  private-channel  variety. 

The  menhaden  seiner  can  generally  be  distinguished 
by  her  high  mast,  topped  by  a  crow's  nest,  her  large 
and  roomy  deckhouse  forward,  and  a  relatively  low 
engine  house  aft,  with  her  purse  boats  suspended  from 
davits  on  each  side  of  the  engine  house. 

The  purse  boats  are  generally  powered  with  100  h.p. 
gasoline  engines,  and  are  used  to  set  the  purse  seine 
around  the  school  offish.  The  captain  hoat  is  commanded 
by  the  master  of  the  seiner  and  the  mate  hoat  by  the  mate. 
Pursing  the  net  is  done  by  gypsy  spools  in  the  captain  boat. 

The  striker  boat  or  drive  boat  is  a  12  ft.  dory  rowed  by 
one  man,  and  is  used  to  keep  track  of  a  school  of  fish 
until  the  captain  is  ready  to  set.  After  setting  the  striker 
boat  helps  hold  up  the  net  opposite  the  purse  boats. 

OPERATION 

The  actual  fishing  is  done  from  the  two  34  ft.  by  8  ft. 
gasoline-powered  purse  boats.  When  the  captain  sees  a 
school  of  fish  he  orders  his  purse  boats  lowered,  manned 
by  almost  the  entire  crew;  only  the  engineer,  the  pilot, 
and  the  cook  remain  aboard  the  main  vessel.  The  small 
12  ft.  wooden  striker  boat  is  quickly  picked  up  by  a 


winch  and  is  also  lowered  into  the  water.  The  striker 
boatman  jumps  into  this  little  dory  and  rows  quietly 
standing  up  and  facing  the  fish,  to  the  edge  of  the  school. 
He  then  keeps  track  of  the  movements  of  the  fish  while 
the  captain  is  readying  his  purse  boats.  As  the  purse 
boats  approach  the  school  in  setting  positions,  usually 
with  the  sun  behind  them  so  that  the  tendency  of  the  fish 
to  rush  toward  the  sun  will  carry  them  further  into  the 
net,  the  striker  boatman  indicates  the  extent  and  direction 
of  travel  of  the  fish  by  signalling  with  his  oar.  The  purse 
boats  then  shoot  the  net  and  surround  the  school, 
meeting  on  the  far  side  with  the  fish  trapped  within. 
Once  in  the  net,  the  goal  is  to  purse  the  bottom  of  the 
net  before  the  fish  can  escape.  Pursing  is  done  with  a 
SJ  in.  Italian  hemp  purse  line  running  through  the  brass 
purse  rings  along  the  bottom  edge  of  the  net,  which  is 
hauled  by  means  of  the  winch  located  in  the  captain  boat. 
The  menhaden  generally  strike  horizontally,  rather 
than  vertically,  and  thus  a  small  delay  in  pursing  can  be 
tolerated.  The  menhaden  pursing  technique  is  radically 
different  from  any  other  method  used  in  the  United 
States.  Since  the  fish  strike  horizontally  the  water  usually 
is  shallow  enough  to  permit  the  lower  edge  of  the  seine 
to  reach  bottom,  a  600  to  700  Ib.  lead  weight  is  first 
dropped  over  the  side.  Called  a  torn  weight,  or  torn,  it 
has  t\\o  brass  blocks  attached.  The  purse  line  thus 


Fig.  2.  Menhaden  purse  boats  preparing  for  the  set:  note  the 
thick  webbing  used  in  the  centre  of  the  seine  to  stand  heavy 
stresses  during  brail  ing  offish.  These  purse-boats  are  oj  wood 
construction  covered  with  three  plies  of  fibreglass  armour. 
Natural  corks  shown  on  seine  have  been  largely  replaced  by 
synthetic  foamed  plastic  floats. 


[395] 


MODERN     FISHING    GEAR     OF    THE    WORLD 


runs  through  the  brass  purse  rings  along  the  lower  edge  of 
the  net,  passes  through  the  two  blocks  on  the  torn  weight 
and  then  goes  vertically  to  the  surface  whence  it  is  led  to 
the  barrels  of  the  winch  in  the  captain  boat.  The  torn 
weight  thus  counteracts  the  tendency  of  the  seine  to  rise 
off  the  bottom  and,  if  pursing  is  carried  out  properly,  the 
lower  edge  of  the  seine  moves  inward  to  the  closed 
position  without  rising  in  the  water.  When  pursing  is 
complete,  the  torn  weight  is  taken  back  aboard  and  stowed 
until  the  next  set. 

With  the  net  tightly  closed  at  the  bottom,  the  crew 
commences  to  haul  in  the  wings,  carefully  piling  each 
end  in  the  purse  boats  as  it  comes  in  until  they  have  the 
fish  pocketed  in  the  heavy  centre  section. 

The  captain  then  signals  the  seiner  to  come  carefully 
alongside  the  net,  which  is  then  fastened  to  her,  and  the 
crew  commence  the  operation  of  drying  up  the  fish  for 
brailing.  While  the  crew  harden  up  the  fish,  the  engineer 
aboard  primes  his  10  in.  centrifugal  fish  pump. 

The  lOin.  rubber  suction  hose  is  lowered  into  the  fish 
and  pumping  starts  at  the  rate  of  1  ton  of  fish/min.  It 
does  not  take  long  to  load  the  average  catch  of  perhaps 
20  or  25  tons  offish  which  land  in  the  hold  undamaged 
and  alive.  As  soon  as  pumping  is  finished,  the  purse 
boats  go  to  the  stern  of  the  seiner  and  are  hooked  on  for 
towing  to  the  next  set. 

Individual  sets  offish  may  run  as  high  as  200  tons  but 
anything  in  the  neighbourhood  of  100  tons  is  considered 
exceptionally  good.  When  his  vessel  is  loaded,  the 


Fig.  3.     The  600  to  100  Ib.  torn  weight  insures  that  the  net  stays 
on  the  bottom  during  pursing. 


captain  picks  up  his  purse  boats  in  the  davits  and  heads 
home  to  the  reduction  plant  where  he  is  unloaded  very 
quickly  by  suction  pumps  on  the  dock. 

Some  mention  of  quantity  is  also  indicated:  a  first 
class  vessel  operating  in  the  northern  fishery  will  catch 
10,000  tons  of  fish  in  a  5  months'  fishing  season;  in  the 
Gulf  of  Mexico  the  average  catch  is  probably  half  this 
amount  of  fish  for  a  slightly  longer  season. 

PURSE  BOATS 

The  menhaden  purse  boat  was,  many  years  ago,  built  of 
wood,  usually  thin  cedar  planking  over  oak  frames. 
Caulking  and  other  maintenance  expenses  were  high  so 
the  industry  switched  to  steel  welded  construction  with 
built-in  air  flotation  tanks.  Despite  expensive  galvanising 
these  steel  purse  boats  were  still  costly  to  maintain  and 
there  has  been  a  trend  recently  toward  using  wooden 
boats  again,  this  time  covered  with  a  three-ply  layer  of 
"fibreglass"  armour. 

Built  as  a  double-ender  boat,  the  propellers  are  well 
protected  by  metal  baskets.  Since  the  net  is  set  from  the 
after  quarter  of  the  boat,  the  engines  are  located  forward 
and  a  roomy  cockpit  is  left  aft  for  piling  the  net.  With  a 
100  h.p.  gasoline  engine,  the  boat  is  relatively  speedy 
when  running  light  but  the  drag  of  the  net  during  setting 
cuts  speed  sharply  and  it  takes  1  min.  to  set  a  1,200  ft. 
seine  around  a  school  of  fish. 

SEINE  NET 

The  outstanding  characteristic  of  the  menhaden  purse 
seine  is  its  hanging.  Unlike  Pacific  coast  seines,  which  are 
hung  to  capture  vertically  striking  fish,  the  menhaden 
seine  is  hung  to  capture  a  horizontally  striking  fish. 
This,  simply  stated,  means  that  when  the  fish  rush  toward 
the  sun  (as  they  usually  do)  there  will  be  ample  loose 
webbing  to  contain  their  rush  without  sinking  the  cork 
line. 

Since  there  are  major  differences  in  hanging  a  net, 
depending  upon  area,  type  of  fish,  and  many  personal 
prejudices  by  individual  captains,  no  attempt  will  be 
made  here  to  detail  the  procedure.  However,  a  first-class 
Virginia  captain  reported  his  hanging  technique  as 
follows: 

"With  respect  to  my  seine,  a  tarred  cotton  net  with 
wings  of  20/9  twine,  2-J  in.  stretched  mesh,  13,000 
meshes  long  by  800  meshes  deep,  I  start  at  the  centre 
of  the  net  and  hang  toward  each  end.  I  commence  by 
hanging  72  meshes  to  each  fathom  of  line;  1  hang  this 
amount  of  webbing  per  fathom  for  5  fm.,  then  drop  to 
71  meshes/fm.  for  the  next  5  fm.  I  continue  dropping 
one  mesh  per  fathom  each  5  fm.  until  I  reach  the  end 
of  the  net.  I  hang  the  foot-  and  headlines  alike.  To 
rig  the  purse  ring  bights,  I  start  at  the  centre  of  the 
net  and  work  toward  the  ends,  tying  one  ring-bight 
per  5  fm.;  after  the  5th  ring  I  space  them  4-J  fm. 
apart  until  the  llth  ring.  From  the  llth  ring  out  to 
the  end  of  the  net  I  space  them  4  fm.  apart.  Length 
of  the  ring  bights  is  approximately  2  ft." 

Floats  of  foam  plastic  are  used  today.  Between  1,600 
and  2,000  4 in.  x  2  in.  plastic  floats  are  required  for  one 


1396] 


MENHADEN     PURSE    SEINING 


fiyp. 


TVflw 


is  required  to  dry  up  the  fish  prior  to  pumping.     Note  the  purse  line  coiled  on  spool,  right  foreground.     To  the  left  of 
the  purse  line  spool  is  the  double  barrel  winch  used  for  pursing. 


seine.  Smooth  brass  purse  rings  each  weigh  about  2  Ib. 
and  have  a  2  in.  centre  opening. 

Formerly,  cotton  nets  were  used  exclusively  and  the 
crew  had  the  extra  work  of  brining  the  net  each  day  after 
fishing,  using  strong  salt  solution,  to  slow  down  rotting 
of  the  tarred  seine.  A  typical  cotton  seine,  for  example, 
would  be  16,000  meshes  long  by  1,000  meshes  deep,  Jin. 
bar  (1  Jin.  stretched  mesh)  made  of  20/9  hawser  twine, 
scotch  knot.  The  centre  of  the  net  would  be  heavier 
twine  to  accommodate  the  strain  and  extra  wear  of 
brailing  the  fish. 

Net  size  varies  by  locality  and  while  a  Gulf  of  Mexico 


Fig.  5.  Pumping  the  catch  aboard  the  seiner.  The  captain, 
left  foreground,  signals  the  engineer  that  the  hose  is  lowered 
sufficiently.  The  JO  in.  I.D.  rubber  suction  hose  is  specially 
reinforced  with  stainless  steel  internal  wires.  Fish  discharge  from 
chute,  right,  and  excess  water  passes  overboard  through  another 
chute. 


f397 


MODERN     FISHING    GEAR    OF    THE    WORLD 


—  18.000    MESMFS  

I  1/2"  STRETCHED   MESH 


gO/12      BORDER 


?0/»g       BORDER 


20/12 
HAWSER 


30O 
MESHES 


21 
THREAD 


15  THREAD 
700  MESHES 


*9  MEDIUM 


20/12 
HAWSER 


300 
MESHES 


_20/i? .  BORDER 


|  _7  "   50  MFSHfS 


(NOT  DRAWN  TO  SCALE) 


Fig.   6.     Typical  Gulf  of  Mexico  menhaden  purse  seine. 


fisherman  would  probably  use  a  J  in.  bar  seine,  a  New 
Jersey  or  New  England  fisherman  would  prefer  to  use 
li  in.  bar  or  possibly  1J  in.  bar  webbing.  The  size  of 
the  webbing  naturally  depends  on  the  smallest  fish  likely 
to  be  captured,  for  if  a  large  mesh  seine  is  set  around  a 
school  of  small  fish,  the  fish  will  gill  in  the  webbing  and 
cause  the  crew  much  extra  work  in  their  removal.  Like- 
wise the  depth  of  water  has  much  to  do  with  the  depth 
of  the  seine  and  every  individual  captain  has  his  own 
preferences  in  this  regard.  Length  again  depends  upon 
local  conditions  but  a  length  of  22,000  meshes  of  J  in. 
bar  webbing  might  be  considered  the  practical  maximum. 
With  the  advent  of  synthetic  webbing,  the  industry  is 
trending  toward  the  complete  abandonment  of  cotton. 
At  present,  the  Japanese  synthetic  nettings  seem  the  most 
popular,  along  with  United  States  nylon  nets.  Additional 
interest  is  being  shown  in  knotless  webbing  and  several 
knotless  nets  are  under  test.  Jn  Florida,  trials  with 
inexpensive  tarred  spun  nylon  webbing  from  Scotland 
seems  to  be  working  out  well  and,  as  yet,  no  knot  slippage 
problem  has  been  encountered.  Unless  an  extremely 
efficient  cotton  net  preservative  can  be  offered  the  industry 
quickly,  it  seems  a  foregone  conclusion  that  it  will 
shortly  abandon  cotton  webbing  completely. 

FISH  PUMPS 

Fish  pumps  have  been  almost  generally  adopted  by  the 
industry  over  the  past  five  years.  Built  around  a  10  in. 
diameter  centrifugal  pump,  they  bear  a  strong  resemblance 
to  sewage  pumps.  Pump  speed  reaches  a  maximum 
of  550  to  600  r.p.m.,  when  powered  by  a  90  h.p.  gasoline 
engine,  and  will  deliver  1  ton  of  fish/min.  to  the  fish  hold. 
Fish  are  sucked  up  through  a  10  in.  heavy  duty  rubber 
suction  hose  roughly  36  ft.  long,  pass  through  an  inclined 
de-watering  screen  (whence  excess  water  returns  over- 
board) and  fall  into  the  fish  hold  undamaged  and  alive. 


A  subsidiary  advantage  of  the  system  is  thai  menhaden 
have  a  tendency  to  lay  in  (he  bottom  of  the  seine  and 
resist  efforts  to  raise  them.  The  long  rubber  hose  lowered 
into  the  bottom  of  the  net  helps  reduce  the  labour  re- 
quired to  dry  them  up. 

RADIO-TELEPHONE  EQUIPMENT 

Jn  the  past,  conventional  radio-telephone  equipment 
operating  on  the  2,000-3,000  kilocycle  band  was  used 
but  over-crowding  of  the  airwaves  from  other  vessels 
plus  the  advent  of  the  spotter  airplane  has,  in  the  past 
five  years,  caused  a  shift  to  FM  (frequency  modulated) 


Fig.  7.     Schematic  diagram  of  a  fish  pump  installation. 


[3981 


MENHADEN     PURSE     SEINING 


private-channel  equipment.  Most  fleets  of  menhaden 
vessels  are  now  linked  together  and  to  the  reduction 
plant  and  spotter  planes  by  such  FM  equipment.  In 
addition,  the  captains  carry  portable  FM  transmitters 
into  the  purse  boats  with  them. 

Operating  on  the  154  megacycle  band,  or  near  it,  this 
VHF  system  provides  privacy,  freedom  from  static, 
and  crystal  clear  reception  over  line  of  sight  distances. 
For  spotter  airplanes  it  is  almost  mandatory. 

AIRCRAFT  SPOTTING 

The  importance  of  aircraft  spotting  to  the  menhaden 
industry  can  be  shown  by  the  following  example:  Ten 
years  ago  captains  stubbornly  dismissed  the  spotter  pilot 
as  an  unskilled  nuisance  who  led  them  on  wild  goose 
chases.  Today  it  is  considered  most  efficient  to  have  one 
airplane  serving  each  five  vessels  on  the  fishing  grounds. 
Capable  of  long  range  reconnaissance  flights  along  the 
shore,  the  typical  spotter  plane  is  a  single  engine,  high 
wing  monoplane,  similar  to  the  Piper  Super  Cruiser  or 


the  Cessna  180.  A  plane  on  floats  may  be  used  over 
swampy  areas  which  are  devoid  of  good  landing  sites. 

A  skilled  spotter  pilot  will  not  only  accurately  locate 
the  fish  for  his  vessels  but  also  estimate  the  quantity  of 
fish  in  the  schools,  frequently  to  quite  narrow  limits. 
He  is  in  constant  radio  contact  with  his  vessels  during  the 
daylight  hours  and  also  renders  other  valuable  services: 
supply  of  spare  parts  and  gear,  removal  of  injured  men 
to  hospital,  etc. 

In  the  fall  and  early  winter,  the  spotter  is  invaluable. 
At  this  time  of  year  the  fish  have  a  tendency  to  run 
"sunk"  under  the  surface  where  the  captains  cannot  see 
them.  But  flying  up  to  perhaps  as  much  as  6,000ft. 
instead  of  the  usual  800  ft.  the  spotter  pilots  locate  these 
hidden  schools.  Communicating  to  the  purse  boats  via 
the  portable  FM  transmitters  carried  by  the  captains,  the 
spotter  pilots  guide  the  purse  boats  to  the  fish,  tell  them 
when  to  separate  so  as  to  set  the  net  and  when  to  come 
together  so  as  to  trap  the  fish.  Many  startling  and 
successful  catches  have  been  made  this  way  and  it  is  now 
considered  a  standard,  every-day  procedure. 


Portuguese  purse  seine  net. 
[399] 


THE  PURETIC  POWER  BLOCK  AND  ITS  EFFECT  ON  MODERN 

PURSE  SEINING 

by 

PETER  G.  SCHMIDT,  JR. 

Marine  Construction  and  Design  Co.,  Seattle,  99,  Washington,  U.S.A. 

Abstract 

This  paper  describes  the  latest  development  in  the  power-handling  of  large  nets  of  the  purse  seine  type.  The  self-powered  sheave 
can  accommodate  the  passage  of  the  entire  net  and  the  angle  of  the  sides  of  the  sheave  has  been  so  designed  as  to  allow  the  float  line  and  the 
leadline  to  pull  evenly  and  thus  avoid  tugging  the  net  out  of  square  during  hauling.  There  are  two  methods  of  driving  the  block— the  rope 
drive  and  the  hydraulic  drive — and  these  together  with  the  details  of  installation  in  many  kinds  of  fishing  craft  are  discussed  fully.  The  effect 
of  the  power  block  on  present  fishing  methods,  net  size  and  design,  and  its  influence  on  the  type  and  design  of  fishing  vessels  is  also  mentioned. 


Rfeume 


La  poulie  mecanique  Puretic 


L'auteur  ddcrit  1'apparcil  le  plus  recent  mis  au  point  pour  la  manoeuvre  m&anique  des  grands  filets  du  type  de  la  senne  tournantc. 
Le  r&a  mecanique  permet  le  passage  du  filet  tout  cntier,  el  Tangle  donn£  aux  bords  du  rea  £te  calcule  de  fac.on  a  pcrmcttrc  une  traction  dgale 
de  la  ligne  de  flottes  et  de  la  ligne  de  plombs,  ct  d'evitcr  ainsi  de  tirer  Ic  filet  en  biais  pendant  Ic  relevagc.  II  cxiste  deux  systcmcs  d'entrainc- 
ment  de  la  poulie:  hydrauliquc  et  par  courroie  de  corde,  donl  I'autcur  fait  une  etude  approfondie  en  donnant  egalemcnt  les  details  d'installation 
a  bqrd  d'un  grand  nombrc  de  types  de  bateaux  de  peche.  II  indique  Teffet  de  la  poulie  mecanique  sur  les  methodes  de  peche  actuelles,  sur 
la  dimension  et  la  forme  des  filets,  ainsi  que  sur  le  type  et  la  conception  dcs  bateaux  de  peche. 

La  polea  motriz  "Puretic** 
Extracto 

En  este  trabajo  sc  describe  el  ultimo  fnvento  en  materia  de  manipulaci6n  mccanicu  de  grandcs  redes  de  cerco  de  jareta,  quc  consiste 
en  una  polea  o  mot6n  motriz  el  cual  permite  el  paso  del  arte  por  su  caja.  Los  angulos  de  las  paredes  de  la  garganta  de  la  roldana  de  este 
moton  se  calcularon  de  manera  que  tiren-las  relingas  dc  plomos  y  corchos  en  forma  pareja  evitando,  dc  este  modo,  esfuerzos  diagonales  sobre 
las  mallas  al  izar  Ja  red  a  bordo.  Hxisten  dos  metodos  para  accionar  la  roldana—  mediantc  una  transmisi6n  de  cuerda  c  hidraulicamente— 
que  se  analizan  en  forma  muy  completa  con  los  dctalles  de  la  instalacion  en  divcrsos  tipos  de  embarcaciones  pesqueras.  Tambicri  se  mcnciona 
el  efecto  de  la  polea  motriz  sobre  los  metodos  de  pesca  usados  en  la  actualidad,  tamaho  y  modclo  de  las  redes  c  influcncia  sobre  el  tipo  y 
proyecto  de.  embarcaciones  pesqueras. 


NEARLY  all  fishing  operations  have  been  improved 
with  mechanization  except  the  handling  of  large 
nets  which  are  hauled  from  the  sea  by  brute 
strength. 

The  search  for  new  methods  of  hauling  such  nets  has 
led,  during  the  last  thirty  years,  to  the  development  of 
the  power  roller  on  the  smaller  purse  seine  vessels,  and 
the  strapping  method  on  the  larger  vessels.  The  power 
roller  helps  bring  the  net  aboard,  but  falls  far  short  of 
being  a  mechanical  system.  The  strapping  method  takes 
advantage  of  the  mast  rigging  and  the  long,  sturdy  boom, 
whereby  the  net  is  brought  aboard  in  bights  by  the  use  of 
winch  and  single  fall  from  the  end  of  the  boom.  This 
method  also  falls  far  short  of  providing  a  satisfactory 
means  of  hauling  the  net  out  of  the  water  because  it  is 
slow  and  tedious.  The  next  major  development  was  made 
in  1951,  when  Nicholas  Kelly  of  Nanaimo,  British 
Columbia,  introduced  a  drum  with  level  winding  mechan- 
ism for  handling  an  entire  purse  seine.  Between  1951  and 
1957,  some  fifty  purse  seiners  in  the  Pacific  Northwest 
have  been  equipped  with  some  type  of  drum  for  handling 
purse  seine  nets  and  the  drum  has  also  met  with  some 


success  in  handling  certain  types  of  salmon  purse  seines. 
It  is,  however,  too  complicated,  and  not  nearly  adaptable 
enough,  to  be  the  ultimate  solution  for  handling  large 
fish  nets. 

A  mechanical  method  was  needed  that  would  lift  a 
large  net  out  of  the  water  and  deposit  it  on  the  deck.  The 
net  should  be  hauled  much  faster  than  by  present  means, 
use  less  manpower,  and  cause  no  damage  to  the  net.  The 
mechanism  should  be  relatively  inexpensive,  simply 
constructed,  easily  installed,  allow  fishing  in  rough 
weather,  and  be  adaptable  to  all  types  of  fishing  vessels 
and  as  many  types  of  nets  as  possible. 

THE  CONCEPTION  OF  THE  POWER  BLOCK 

In  1953  Mario  Puretic,*  an  experienced  tuna  and  sardine 
fisherman  of  San  Pedro,  California,  U.S.A.,  conceived 

*  U.S.  Patents  2,733,530  and  2,733,531,  Canadian  Patent  522,263 
have  been  granted  on  the  Puretic  Power  Block  and  method  of 
hauling  nets;  and  patents  have  been  granted  or  are  pending  in  all 
principal  maritime  countries  of  the  world.  Marine  Construction 
and  Design  Co.,  2300  Commodore  \Vay,  Seattle  99,  Washington, 
is  the  exclusive  licensee  under  Puretic's  world-wide  patents. 


[400] 


THE    POWER     BLOCK     AND    PURSE     SEINING 


the  basic  idea  of  passing  the  entire  purse  seine  net  through 
an  elevated  free-swinging,  self-powered  V-sheave,  so 
constructed  that  gravity  would  wedge  the  net  into  the 
sheave,  giving  it  the  necessary  traction  to  pull  the  net 
out  of  the  water.  In  1954  Puretic  built  the  first  power 
block,  which  was  tested  on  December  22  of  that  year  on 
the  tuna  seiner,  "Anthony  M" — the  largest  vessel  purse 
seining  on  the  Pacific  Coast  at  that  time. 

In  April  of  1955  the  first  power  block  was  introduced 
in  the  Pacific  Northwest  area  of  the  United  States  and 
in  British  Columbia,  Canada,  and  its  success  has  been 
phenomenal.  In  the  2i  fishing  seasons  since  the  power 
block  was  introduced,  approximately  1,000  purse  seine 
vessels,  primarily  on  the  Pacific  Coast  from  Alaska  to 
South  America,  have  been  equipped  with  this  device. 
At  present,  the  power  block  is  being  used  in  small  but 
increasing  numbers  on  the  East  Coast  of  the  United 
States. 

Introduction  of  the  power  block  has  started  in  many 
other  countries,  including  Iceland,  Norway,  Portugal, 
South  Africa,  Morocco,  Pakistan,  Korea  and  Mexico. 

ADVANTAGES  OF  THE  POWER  BLOCK  METHOD 

As  soon  as  fishermen  began  using  this  new  method,  many 
additional  advantages  became  apparent. 

Fishing  in  rough  weather  is  possible  with  the  power 
block  because  of  the  greater  force  exerted  on  the  net  and 
the  stabilizing  influence  of  the  net  on  the  vessel. 

Net  wear  has  been  reduced  in  many  of  the  types  of 
fishing. 

The  power  block  is  of  particular  help  to  the  fisherman 
in  loading  and  unloading  the  net  from  the  vessel  and  in 
handling  the  net  while  stacking  and  making  repairs, 
and  has  resulted  in  a  large  increase  in  manpower 
efficiency. 

The  power  block  increases  the  efficiency  of  the  fishing 
operation,  particularly  when  sets  are  made  in  which  there 


are  little  or  no  fish.  In  this  case,  the  net  can  be  retrieved 
in,  say,  10  min.  for  a  300  fm.  seine,  allowing  the  vessel 
to  make  repeated  sets  on  the  school  that  it  missed. 

The  power  block  makes  it  possible  to  save  nets  in 
times  of  emergency,  such  as  when  the  net  and  catch  are 
attacked  by  sharks,  when  the  net  is  snagged  or  when, 
during  strong  tides  and  winds,  it  is  necessary  to  get  the 
net  aboard  in  a  hurry. 

DESCRIPTION    AND    APPLICATION    OF    THE 
POWER    BLOCK 

Principle  and  Construction 

The  power  block  (fig.  1)  is  a  large  self-powered,  free- 
swinging  V-sheave,  supported  by  a  rigid  frame  at  an 
elevated  position  on  the  fishing  vessel,  such  as  from  the 
end  of  a  boom,  crane,  etc. 

The  basic  components  of  the  power  block  are  the 
sheave,  the  frame,  the  chutes  -which  are  the  smooth 
guiding  surfaces  extending  above  the  frame — the 
supporting  yoke  or  eye,  and  the  power  source.  The 
sheave  is  so  designed  that  it  can  accommodate  the  passage 
of  the  entire  fish  net,  and  so  shaped  that  the  net  will  be 
wedged,  due  to  its  weight,  which  provides  the  necessary 
traction  for  pulling.  Most  of  the  sheaves  which  have  been 
used  to  date  are  covered  with  vulcanized  rubber  and 
vulcanized  rubber  cleats,  which  increase  the  traction, 
but  experiments  arc  being  made  with  aluminium  surfaces 
and  aluminium  cleats  on  the  sheave.  The  angle  of  the 
sides  of  the  sheave  has  been  determined  by  experimenting 
with  various  types  and  sizes  of  nets  to  give  maximum 
grip  and  to  allow  the  net  to  come  in  evenly-  that  is,  to 
allow  the  cork  line  and  lead  line  to  pull  evenly  so  that  the 
net  does  not  come  unnecessarily  out  of  square  during  the 
hauling.  The  root  at  the  bottom  of  the  sheave  varies 
from  approximately  ,J  to  4  in.  depending  on  the  type  of 
net  and  its  bulk. 


MOOtL>**A 


sac 


MODEL  tS» 


MODEL   10A 


MOOCL.  ItA 


Fig.  1.    Puretic  power  blocks. 
[401  ] 


BB 


MODERN     FISHING    GEAR     OF    THE    WORLD 


Fig.  2.     Western  style  purse  seiner,  illustrating  installation  of  rope  drive. 


The  power  block  is  constructed  primarily  of  cast 
aluminium,  using  a  zinc  aluminium  alloy  which  has  high 
tensile  strength  and  is  extremely  resistant  to  salt  water 
corrosion.  Aluminium  answers  the  problem  of  provid- 
ing a  strong,  lightweight,  rigid  structure.  The  shafts  and 
all  pins  are  made  of  stainless  steel,  and  aluminium  is  used 
for  the  gearbox  housing  and  all  other  small  fittings. 

Drive 

The  power  block  can  be  driven  in  two  different  ways. 
The  first,  devised  by  Puretic  to  aid  in  the  initial  intro- 
duction of  the  power  block,  is  the  rope  drive  (fig.  2). 

It  consists  of  an  endless  rope  or  cable,  running  through 
small  fairlead  pulleys  to  a  V-sheave  which  is  on  the  side 
of  the  main  net  sheave.  The  fairlead  pulleys  are  so 
arranged  that  they  lead  to  the  driving  sheave,  regardless  of 
how  the  power  block  swivels.  The  endless  rope  or  cable 
is  then  led  to  a  gypsy  head  of  the  purse  winch.  Several 
turns  are  taken  around  the  gypsy  head  and  a  small 
tightener  pulley  is  employed  on  the  slack  side.  The  rope 
drive  made  it  possible  for  nearly  300  boats  to  adopt  the 
power  block  within  two  months  of  its  introduction  on 
the  Pacific  coast.  As  each  purse  seine  vessel  had  a  purse 
winch,  it  only  required  hoisting  the  block  to  the  end  of 
the  boom  and  splicing  a  suitable  drive  rope,  to  convert 


the  purse  seiner  from  hand  hauling  to  the  Puretic  method. 
Mechanical  drives  of  all  types  have  been  considered 
for  the  power  block,  including  air,  electric  and  hydraulic. 
Of  these,  the  hydraulic  drives  provides  the  ideal  solution. 
The  power  requirements  on  the  various  models  of  the 
power  block  are  from  2  to  20  h.p.,  depending  on  the  size 
of  the  boat  and  size  of  net.  The  speed  requirement 
varies  from  about  20  to  30  r.p.m.  The  high  pressure 
hydraulic  drive  is  ideal  for  meeting  these  requirements. 
It  provides  the  necessary  high  starting  torque  together 
with: 

Low  initial  cost  per  h.p. 

Small  size  and  weight 

Low  maintenance  cost 

Flexibility  of  control 

Simplicity  of  installation  and  maintenance 

Dependability 

A  typical  hydraulic  installation  diagram  for  a  Model 
35  power  block  is  shown  in  fig.  3. 

High  pressure  hydraulic  systems  have  been  selected 
rather  than  the  lower  pressure  type  commonly  used  in 
Europe,  to  obtain  maximum  torque  with  the  smallest 
possible  size  and  least  weight.  The  hydraulic  equipment 
selected  is  manufactured  by  Vickers,  Inc.  1400  Oakman 
Boulevard,  Detroit,  Michigan,  U.S.A.,  and  is  of  the  vane 


[4021 


THE    POWER    BLOCK     AND    PURSE     SEINING 


Fig.  3.     Typical  hydraulic  drive  installation  diagram. 

type.  Its  normal  operating  pressure  is  1,000  Ib./sq.  in. 
(68atm.);  however,  a  relief  valve  setting  of  1,200  Ib./sq.  in. 
(81-6  atrn.)  is  used  in  most  cases.  In  a  few  cases,  this 
equipment  is  being  operated  up  to  maximum  pressures  of 
1,500  Ib./sq.  in.  (102  atm.). 

Installation 

A  high  pressure,  vane  type  hydraulic  pump  is  installed 
and  driven  in  one  of  the  following  ways: 

1.  Through  a  power  take-off  from  the  main  engine.  In 
this  case,  a  pump  size  is  selected  to  produce  the 
required  volume  of  oil  while  the  engine  is  operating 
at  its  normal  idling  speed,  but  which  can  run  up  to 
full  engine  speed  without  being  damaged.  Normally, 
however,  when  the  engine  is  running  at  full  operat- 
ing speed,  the  clutch  would  be  disengaged. 

2.  By  an  auxiliary  engine  in  the  engine  room. 

3.  On  boats  with  large  capacity  of  110  or  220  V. 
A.C.  or  D.C.,  an  electro-hydraulic  power  unit  may 
be  installed. 

The  hydraulic  pump  takes  the  oil  from  an  expansion 
tank  through  a  suitable  filtration  system  (fig.  3).  The 
hydraulic  fluid  then  passes  to  a  control  panel  which,  on 
most  of  the  Pacific  Coast-type  vessels,  is  located  on  the 
after  side  of  the  main  deckhouse.  The  control  panel 
consists  of  a  reversing  control  valve  and  a  by-pass  or 
needle  valve  for  regulating  power  block  speed.  A  relief 
valve  is  generally  built  into  the  system  at  this  location, 
and  a  pressure  gauge  installed  convenient  to  the  operator. 


The  control  valve  is  operated  in  such  a  manner  that  in 
the  first  position  the  power  block  rotates  in  one  direction; 
in  the  middle  position  the  power  block  is  locked  hydraul- 
ically;  and  in  the  third  position  the  power  block  rotates  in 
the  opposite  direction.  From  the  control  panel  the 
hydraulic  fluid  goes  to  the  power  block  and  returns 
through  hydraulic  hoses  to  the  expansion  tank.  For  this 
purpose,  wire  braid,  neoprene-covered  hydraulic  hose  is 
used,  which  is  good  for  working  pressures  of  approxi- 
mately 2,000  Ib./sq.  in.  (136  atm.).  Single  wire  braid  is 
used,  for  hose  up  to  .J  in.  diameter,  and  double  wire  braid 
for  hose  from  \  to  1  in.  diameter. 

In  most  cases  to  date,  the  power  block  has  been 
supported  from  the  end  of  the  boom  on  the  fishing  vessel. 
Some  fishermen  support  the  block  from  a  single  fall  at 
the  end  of  the  boom  so  that  it  can  be  raised  and  lowered 
as  necessary  for  inspection,  maintenance  and,  in  some 
cases,  to  allow  the  block  to  be  opened  to  insert  or  remove 
the  net.  In  this  case,  the  hydraulic  hoses  are  run  from  the 
control  panel  to  the  power  block  in  one  or  two  large 
bights,  generally  supported  in  the  middle  by  a  line  carried 
to  a  small  block  on  the  mast,  or  in  the  rigging.  In  other 
cases  the  power  block  is  shackled  to  the  end  of  the  boom, 
and  the  hydraulic  hoses  are  strapped  or  lashed  to  the 
boom.  In  some  installations,  black  iron  pipe  is  used  in 
piping  up  the  hydraulic  circuit  from  the  engine  room 
to  the  control  panel  and  along  the  boom,  with  hydraulic 
hose  being  used  only  where  flexibility  is  desired.  In  general 
the  most  satisfactory  installations  have  been  made 
using  hydraulic  hose  throughout. 

PRESENT  MODELS  OF  POWER  BLOCK 

The  model  number  of  the  power  block  designates  the 
size  of  the  sheave  in  inches.  The  letter  following 
indicates  the  type  of  or  modification  of  the  given  model. 
Each  one  of  the  five  power  blocks  shown  (fig.  1)  was 
originally  designed  for  a  specific  fish  net  and  area  of 
fishing.  The  range  of  blocks,  however,  has  been  found  to 
cover  most  new  applications  to  date.  In  general,  all  power 
blocks  are  designed  on  the  snatch  block  principle,  which 
allows  the  top  to  be  opened  for  removal  of  the  net,  if 
this  should  be  necessary. 

The  first  power  block  to  receive  wide  acceptance  was 
the  Model  28,  which  was  originally  designed  to  handle 
the  300  fm.  long  salmon  purse  seines  used  on  boats  from 
40  to  80  ft.  in  length  on  the  Pacific  Northwest  coast. 
Originally,  most  of  these  boats  were  equipped  with  the 
rope  drive  model  (fig.  2)  but  more  than  half  have  con- 
verted to  hydraulic  drive.  There  are  approximately  500 
seine  boats  using  the  28  in.  power  block  which  is  the  most 
universal  of  all  models,  as  it  has  been  successfully 
applied  to  the  smaller  herring  nets  used  in  the  Pacific 
Northwest,  California  sardine  nets,  Peruvian  bonito  and 
anchovy  seines,  and  the  smaller  Southern  California  tuna 
seines.  This  model  would  also  be  adequate  in  size  to 
handle  the  Icelandic  and  Norwegian  herring  seines, 
Portuguese  sardine  seines,  South  African  lampara  seines, 
and  East  Coast  menhaden  seines.  The  28  in.  power  block 
has  also  been  built  with  two  types  of  hydraulic  drive 
which  arc  capable  of  exerting  line  pulls  up  to  U  tons. 
Its  weight  is  210  to  260  Ib.,  depending  on  drive  arrange- 
ment. 

The  next  model  developed  was  the  35  in.  power  block, 


[403] 


MODERN     FISHING    GEAR     OF    THE    WORLD 


CACH  IMP   -  JOO»TMrt» 
W.ASTIC  FLOAT*  PfH  FTM 


ar 


Fig.  4.     West  Coast  Canadian  herring  seine. 


for  use  on  the  large  Canadian  and  Alaskan  herring  seines, 
and  large  Southern  California  seines.  This  model  has 
ample  power  and  capacity  to  handle  efficiently  the  largest 
purse  seines  known  to  the  author.  Currently,  the  model 
is  being  used  in  Alaska  and  British  Columbia,  Canada, 
on  herring  seines  similar  to  that  shown  in  fig.  4. 

It  is  also  being  used  in  Southern  California  boats  purse 
seining  for  tuna,  and  in  Iceland  ona  Canadian  style  herring 
seine.  This  model  weighs  approximately  425  Ib.  including 
the  hydraulic  drive,  but  is  not  available  for  rope  drive. 

The  next  model  developed  was  the  18  in.  power  block 
weighing  from  95  to  125  Ib.  designed  to  handle  small 
nylon  and  dacron  salmon  purse  seines  used  on  boats  of 
30  to  40  ft.  length  in  the  shallow  fishing  areas  of  Alaska, 
principally  around  Kodiak  Island.  Over  200  of  this 
model  are  in  use,  both  with  rope  and  hydraulic  drive, 
and  a  few  are  being  used  for  handling  gillnets. 

The  Model  25  power  block  weighing  250  Ib.  including 
the  hydraulic  drive  was  designed  to  meet  the  requirements 
of  the  menhaden  fishery  on  the  East  Coast  of  the  United 
States.  This  power  block  is  unique  in  that  it  is  entirely 
supported  from  one  side,  with  the  other  side  hinged  (fig.  1). 
This  feature  was  necessary,  because,  in  menhaden 
fishing,  a  considerable  amount  of  cork  line  is  pulled  while 
the  net  is  being  pursed.  By  hinging  the  side  open  and 


lowering  the  block,  the  cork  line  only  can  be  inserted  into 
the  sheave  to  aid  in  this  operation.  After  the  net  is  pursed, 
the  entire  net  is  inserted,  the  side  closed,  and  the  power 
block  elevated  while  pulling  the  remainder  of  the  webbing. 
The  12  in.  power  block  was  developed  for  pulling  very 
small  nets,  such  as  small  gillnets  and  lead  nets  used  in 
connection  with  the  larger  salmon  purse  seines  and 
certain  types  of  fish  traps  (fig.  9). 

SELECTION  OF  PROPER  SIZE  AND  TYPE 

It  has  been  found  advantageous  in  selecting  the  type  and 
size  of  power  block  to  have  the  information  shown  in 
fig.  5. 

The  factors  that  influence  the  size  of  the  power  block 
are  the  depth  of  the  net,  which  may  be  indicated  both 
in  fathoms  and  by  total  number  of  meshes,  the  size  of 
twine  and  size  of  mesh.  In  general,  there  is  adequate 
reserve  capacity  at  the  top  of  the  power  block  between 
the  chutes  to  handle  any  necessary  size  of  cork  line,  even 
including  inflatable  floats  such  as  the  montara  float,  used 
in  California,  and  the  inflatable  floats  used  in  some  of  the 
larger  herring  seines  in  Canada  and  Alaska.  Likewise, 
there  is  ample  space  at  the  top  of  the  power  block  to  pass 
the  cork  line  should  it  become  bunched  up,  and  a 
nominal  quantity  of  gilled  fish. 


[4041 


THE    POWER    BLOCK     AND    PURSE     SEINING 


MPT 


Fig.  5.     How  to  measure  nets  Jor  the  Puretic  power  block. 


The  Model  35  power  block  is  adequate  to  pass  gilied 
tuna  and  occasional  sharks.  The  best  indication  of  the 
size  of  the  net  as  regards  selection  of  the  power  block  is 
the  circumference  measurement  of  the  net  when  gathered 

TABLF   I 
Capacities  of  the  different  power  block  models  for  different  types  of  net 


owei 
Uock 
lode 

f  Max.  Circum. 
of  Com- 
l        pressed 
webbing, 
inches 

Tuna  Webbing 
4J  w. 
stretched; 
42  thread 

Salmon  Webbing 

stretched; 
15  thread 

Menhaden  or 
Herring 
webbing  1  1 
/ft.  stretched 
9  thread 

35 

48 

900  meshes 

1500  meshes 

3000  meshes 

max.  ; 
300  meshes 

max.; 
450  meshes 

max.; 
800  meshes 

28 

38 

min. 

min. 
750  meshes 

min. 
1600  meshes 

max.; 
200  meshes 

max.  ; 
400  meshes 

25 

34 

min. 

min. 
1200  meshes 

max.; 
600  meshes 

18 

23 

300  meshes 

min. 
800  meshes 

max.; 
50  meshes 

max.; 
100  meshes 

,2 

„ 

min. 
1  50  meshes 

min. 

• 

max.; 
50  meshes 

min. 

together  and  measured  by  a  tape.  This  measurement  is 
taken  in  between  corks  with  the  tape  clinching  the  net 
firmly,  but  not  tightly,  at  the  deepest  part  and  also 
through  the  bunt.  It  is  best  that  the  webbing,  when  in  the 
power  block,  does  not  fill  the  block  much  higher  than  the 
sheave.  This  allows  reserve  space  on  top  for  the  corks, 
miscellaneous  gillcd  fish,  and  the  extra,  heavier  webbing 
which  may  be  in  the  bunt.  Typical  power  block  capacities 
for  different  types  of  nets  arc  indicated  in  the  table  below 
and  include  ample  reserve  for  leadline,  purse  line,  corks 
and  floats.  Capacity  is  increased  by  10  to  30  per  cent, 
when  nylon  nets  are  used.  Particular  attention  is  directed 
to  the  column  showing  maximum  circumference  of 
compacted  webbing. 

BASIC  SYSTEMS  OF  HAULING  WITH  THE  POWER 
BLOCK 

The  first  system  can  be  called  the  Western,  or  American 
style,  in  which  large  purse  seines  are  hung  with  the  bunt 
in  the  end  of  the  net.  Normally,  in  this  system,  the  vessel 
has  the  machinery  and  deck  house  forward,  with  the  net 
stacked  on  a  turntable  at  the  stern  of  the  vessel.  All 
boats  of  this  type  are  equipped  with  substantial  rigging 
and  booms,  which  provide  an  adequate  support  for  the 
power  block  (fig.  6). 

To  convert  to  the  Puretic  system,  these  vessels  require 
onlyHhe  addition  of  the  power  block.  The  nets,  which  are 
250  to  400  fm.  in  length,  are  shot  from  the  stern  of  the 
vessel,  generally  with  a  power  skiff  at  the  end  of  the  net. 


[405] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


Fig.  6.     Power  block  in  operation,  showing  Western  me  thud  of 

hauling  purse  seines.     Top:  Alaska  type  salmon  purse  seiner; 

bottom'.  Canadian  herring  seiner. 

After  completing  the  set,  the  seine  vessel  picks  up  the  end 
of  the  net  from  the  skiff.  Purse  lines  are  led  to  the  purse 
winch,  which  is  generally  located  just  aft  of  the  deck- 
house. When  pursing  is  complete,  the  rings  are  hoisted 
on  deck,  using  a  winch  and  a  sling  from  the  boom.  The 
purse  line  is  then  split  in  the  middle  by  opening  a  con- 
nector or  a  figure  eight  link  which  is  pulled  out  of  the 
rings.  The  wing  of  the  net  is  then  started  through  the 
power  block,  either  by  lowering  the  block  or  pulling  it 
up  with  a  light  line  which  has  been  reaved  through  the 
power  block  sheave  for  this  purpose.  The  entire  net  is 
then  pulled  through  the  power  block,  with  the  rings  and 
leadline  coming  from  the  deck  of  the  vessel,  as  shown  in 
fig.  2,  an  operation  which  takes  from  8  to  15  min.  for 
nets  of  300  fm.  in  length.  The  speed  of  hauling  depends 
on  the  speed  at  which  a  man  can  stack  the  cork  line. 
In  British  Columbia  and  Canada  it  is  common  for  3  men 
to  stack  herring  nets  up  to  400  fm.  long  and  45  fm.  deep 
in  approximately  20  min. 

The  net  is  hauled  until  the  fish  are  hardened  up  suffi- 
ciently for  brailing.  With  large  catches  of  tuna,  sardines, 


or  herring  it  may  be  necessary  to  "cut  the  net",  i.e. 
section  off  the  catch  and  brail  the  haul  in  several  "cuts". 
With  adequate  hydraulic  power,  it  is  possible  to  harden 
up  completely  even  large  sets  of  around  500  tons.  During 
the  hauling  operation,  it  is  common  for  the  power  skiffs 
to  take  a  towing  bridle  on  the  side  of  the  seine  vessel 
opposite  the  net,  towing  the  vessel  sideways  from  the  net 
and  into  the  wind.  When  most  of  the  net  is  aboard  and 
hardening  of  the  fish  commences,  the  skiff  takes  a  position 
along  the  corkline  to  help  support  the  bunt  for  brailing. 

The  second  basic  system  of  fishing  with  the  Puretic 
power  block  can  be  called  the  small  seine  boat  system  or 
purse  boat  system  see  figs.  7  and  8. 

This  system  employs  two  small  open  boats  of  approxi- 
mately 32ft.  in  length.  It  is  used  on  the  East  Coast  of 
the  United  States  for  catching  menhaden,  where  there 
are  approximately  300  pairs  of  such  boats  operating, 
but  the  largest  use  of  this  system  is  in  the  Norwegian 
herring  fishery  and  in  Iceland.  The  power  block  was 
originally  conceived  and  devised  for  use  on  the  larger 
Western  style  boats.  A  successful  adaptation  of  this  idea 
to  small  purse  boats  has  required  a  considerable  amount 
of  experimentation  and  instruction  to  local  fishermen  to 
evolve  a  method  and  arrangement  which  would  obtain 
the  maximum  benefit  from  the  power  block. 

The  lampara  seining  system  would  employ  the  use  of 
two  power  blocks  on  one  vessel  to  haul  in  both  ends  of  a 
lampara-type  seine,  which  has  the  purse  bag  in  the  middle. 
This  system  of  fishing  is  most  widely  used  in  the  Union 
of  South  Africa. 

The  fourth  basic  purse  seining  system  is  that  used  by 
some  of  the  mackerel  seiners  in  New  England  and  some 
herring  seiners  in  Iceland,  where  one  small  net  carrying 
seine  boat  is  towed  alongside  a  larger  vessel.  The  small 
boat,  in  most  cases,  has  no  power.  In  this  system,  the 
power  block  would  be  installed  on  the  larger  vessel, 
which,  after  the  net  is  pursed,  would  lift  and  deposit  it  in 
the  small  boat,  where  it  would  be  stacked. 

There  are  two  other  types  of  purse  seine  fishing 
systems.  In  the  first,  the  vessel  carrying  the  net  also 
carries  the  fish,  and  in  the  second,  the  net-carrying  vessel 
is  used  only  for  hauling  and  setting  the  net,  while  the 
mother  vessel  brails  or  pumps  the  fish  and  carries  them. 

ADAPTATION    OF    THE    POWER    BLOCK    TO 
DIFFERENT  METHODS  OF  PURSE  SEINING 

In  each  area  where  the  power  block  has  been  introduced, 
the  first  attempt  has  been  to  use  the  equipment  as  a 
substitute  for  manpower  in  hauling  the  nets.  In  some  of 
the  fisheries,  the  power  block  has  been  applied  with 
practically  no  change  in  method,  net,  or  basic  system. 
In  others,  it  has  been  necessary  to  devise  additional 
equipment,  make  minor  modifications  to  boat  and 
equipment,  and  develop  modifications  to  the  traditional 
fishing  systems.  In  a  number  of  fisheries,  it  has  been 
apparent,  that  through  the  use  of  the  power  block  and 
its  ability  to  handle  larger,  longer,  and  deeper  nets 
rapidly,  there  would  eventually  be  a  change  in  size  and 
design  of  net  and,  very  possibly  a  complete  change  in  the 
basic  system  of  purse  seining. 

Described  now  are  the  various  types  of  fishing  where 
the  power  block  is  currently  being  used  and  in  many 
cases  adopted  as  the  standard  method  of  fishing. 


[4061 


THE    POWER     BLOCK     AND     PURSE     SEINING 


Menhaden  purse  seining  with  the  Pure  tic  power  block,  using  new  aluminium  purse  boats. 


WEST  COAST  SALMON  FISHING 

Salmon  is  one  of*  the  major  fisheries  in  the  North  Pacific 
where  seine  vessels  range  from  30  ft.  in  length  to  about 
85  ft.  Power  blocks  of  18  in.,  28  in.,  and  35  in.,  are  used 
on  these  vessels  depending  on  the  size  of  the  vessel  and 


Fig.  8.     Improved  power  block  crane  and  its  installation  in  steel  menhaden  purse  s>ine  boat. 

[4071 


MODERN    FISHING    GEAR    OF    THE    WORLD 


size  of  net.  The  12  in.  power  block  is  used  in  the  skiffs 
which  are  carried  by  the  salmon  seiners  for  pulling  the 
lead  nets  to  lead  the  salmon  into  the  larger  purse  seines. 
Previously  the  normal  crew  consisted  of  8  to  9  men, 
including  captain  and  the  man  in  the  skiff.  With  the 
application  of  the  power  block,  most  vessels  have  cut 
at  least  2  men  from  their  crew.  Further  reduction  in  the 
crew  are  possible  with  improvement  in  the  pursing  and 
other  operations. 

After  the  first  season's  operation,  the  International 
Sockeye  Commission,  which  is  the  conservation  authority 
for  the  red  salmon  resource  of  the  great  Frazer  River 
spawning  area,  reported  that  the  power  blocks  increase 
the  efficiency  of  the  seiners  by  more  than  15  per  cent. 
However,  it  appears  that  in  some  types  of  purse  seining 
the  increase  in  productivity  may  be  many  times  higher. 

The  only  basic  change  in  the  salmon  vessels  to  date 
has  been  the  strengthening  of  their  booms,  while  new 
vessels  are  being  built  without  turntables. 

PACIFIC  COAST  HERRING  FISHING 

The  vessels  used  in  this  fishery  are  of  the  western  style, 
with  deckhouse  and  machinery  forward  and  large 
turntable  aft,  on  which  the  net  is  stacked.  These  vessels 
are  very  similar  to  the  California  sardine  and  tuna 
seiners  and  to  the  smaller  salmon  purse  seiners.  In  this 
fishing,  the  power  block  replaced  the  strapping  method 
and  speeded  up  the  operation  by  as  much  as  300  or  400 
per  cent. 

The  35  in.  power  block  is  used  by  the  herring  vessels, 
with  a  few  of  the  smaller  ones  using  the  28  in.  block.  A 
typical  Canadian  herring  seine  is  shown  in  fig.  4.  These 
nets  are  usually  about  300  to  400  fm.  long.  The  depth 
varies,  depending  on  the  time  of  the  year  and  area  fished, 
and  it  is  not  uncommon  for  the  nets  to  be  as  deep  as 
45  fm.  The  vessels  fish  with  a  crew  of  8  men,  including 
the  captain  and  the  man  in  the  skiff.  They  can  handle 
sets  which  frequently  run  to  500  tons  and  have  occasion- 
ally exceeded  1,000  tons.  The  vessels  both  carry  fish 
themselves  and  brail  their  large  sets  into  carrier  vessels 
operating  with  the  fleet.  Everything  is  operated  by  power, 
and  nothing  depends  on  the  physical  strength  of  the 
fishermen.  The  operations  sometimes  take  place  in  very 
rough  winter  weather  in  open  water,  while  at  other 
times  these  80  to  90  ft.  vessels  fish  in  small  fjord-like 
inlets  similar  to  the  herring  fishing  areas  of  Norway. 

WEST  COAST  SARDINE  SEINING 

The  power  block  is  beginning  to  be  used  by  the  West 
Coast  sardine  fleet,  but  as  this  fishery  has  been  unusually 
inactive,  very  few  vessels  have  been  fishing  it  since  the 
advent  of  the  power  block.  The  vessels  and  the  system  of 
fishing  are  very  similar  to  that  used  in  northern  herring, 
and  the  power  block  functions  in  a  similar  manner. 

TUNA  SEINING  OFF  THE  COASTS  OF  SOUTHERN 
CALIFORNIA,  MEXICO  AND  SOUTH  AMERICA 

The  nets  used  in  this  fishery  are  probably  the  largest 
purse  seines  in  the  world,  while  the  vessels  measure  from 
70  to  130  ft.  in  length.  A  normal  crew  is  12  men. 
The  vessels  are  similar  to  the  western  sardine  and  her- 


ring seiners,  but  larger.  They  are  equipped  with  220  V. 
A.C.  throughout,  and  the  prime  source  of  their  hydraulic 
power  is  a  220  V.  electro-hydraulic  power  plant.  Through 
their  fishing  off  South  America,  the  vessels  have  helped 
introduce  the  power  block  to  Peru,  where  it  is  being 
installed  on  a  number  of  bonito  and  anchovy  seiners. 
The  Peruvian  boats  are  installing  28  in.  power  blocks, 
while  the  American  boats  which  fish  in  Peruvian  waters 
primarily  use  35  in.  power  blocks. 

Tuna  in  these  areas  are  caught  both  by  purse  seining 
and  by  large  tuna  clippers.  The  increase  in  efficiency  of 
the  seine  boats  due  to  introduction  of  the  nylon  nets 
handled  by  the  power  block,  has  created  interest  in 
converting  some  of  the  larger  tuna  clippers,  of  130  to 
1 50  ft.  length,  to  purse  seiners,  handling  the  larger  tuna 
net  on  the  stern  after  the  style  of  the  other  western 
seiners.  One  boat  has  already  been  equipped  in  this 
manner. 

MENHADEN  SEINING 

The  menhaden  fishery  on  the  East  Coast  of  the  United 
States,  from  New  England  to  the  Gulf  of  Mexico  is  one 
of  the  largest  reduction  fisheries  in  the  world.  More  than 
1,000,000  tons  of  menhaden  are  caught  each  year.  The 
vessels — which  are  still  called  steamers — are  diesel- 
propelled.  They  range  from  about  90  to  220  ft.  in  length, 
and  operate  up  to  approximately  100  miles  from  the 
reduction  factory.  Each  steamer  carries,  in  davits  at  the 
stern,  two  purse  boats  of  approximately  32  ft.  in  length 
by  8  ft.  beam.  These  boats  are  similar  to  the  Norwegian 
herring  seine  boats,  except  that  the  gasoline  engines  are 
in  the  bow  rather  than  the  stern.  Each  boat  carries  half 
of  the  seine  net,  which  is  approximately  200  fm.  long  by 
1,000  meshes  deep,  of  1.J  in.  mesh  stretched.  Each  boat 
has  a  crew  of  about  12  men,  a  total  of  24  fishermen,  which 
is  an  extremely  large  crew  to  handle  a  relatively  small 
purse  seine. 

Puretic  devised  a  hydraulically-operated  power  block 
crane  which  is  installed  in  the  purse  boat  (fig.  8). 

The  two  purse  boats  set  around  the  fish  and  purse  in 
the  usual  manner.  The  author  and  Puretic  introduced 
the  Norwegian  type  of  snap  purse  rings  to  solve  the  prob- 
lem of  having  to  split  the  purse  line  in  the  middle  in  each 
set.  While  the  net  is  being  pursed,  the  specially  construct- 
ed menhaden  power  block,  model  25B  (fig.  1),  is  hinged 
open  and  the  cork  line  is  hauled.  As  soon  as  pursing 
is  completed,  the  entire  net  is  put  into  the  power  block, 
the  crane  is  elevated  and  swung  to  convenient  position 
and  hauling  commences.  The  crew  has  been  reduced 
by  about  6  by  using  this  system,  constituting  a  saving  of 
25  per  cent,  in  manpower. 

The  power  block  crane  is  so  designed  that  it  can  be 
swung  in  a  90  degree  arc  from  side  to  side  and  raised 
and  lowered  by  hydraulic  power.  Puretic  provided  a 
pantograph  motion  on  the  extended  jib  so  that  the 
power  block  could  be  semi-rigidly  attacked  through 
rubber  mountings  and  yet  would  remain  level  at  all 
heights  of  the  crane.  This  was  an  important  feature,  as  it 
is  necessary  for  the  power  block  to  be  able  to  swivel 
freely;  but  in  small  boats  of  this  type,  the  power  block 
had  too  violent  a  motion  when  hanging  loose  and 
unrestricted.  Another  unique  feature  of  the  design  is  that 
the  oil  reservoir  is  in  the  column  of  the  crane,  with  all 


[408] 


THE    POWER    BLOCK 


s*.- 


Fig.  9.     American  shrimp  boat  rigped  for  purse  seining  menhaden. 


hydraulic  piping  and  controls  mounted  in  the  unit. 
Installation  requires  only  the  mounting  of  the  hydraulic 
pump  on  the  engine  power  take-off,  running  two  hydraulic 
hoses  to  the  crane.  The  3  levers  on  the  crane  actuate  the 
power  block,  swing  the  crane,  and  raise  and  lower  it  as 
required. 

Most  of  the  leaders  of  the  menhaden  industry  realize 
that  this  is  only  an  initial  step  in  increasing  the  efficiency 
of  their  catching  operations  and  considerable  work  is 
being  done  by  several  of  the  leading  firms  in  using  a 
modified  western  technique  for  catching  menhaden. 
Conversion  of  an  American  shrimp  trawler  for  purse 
seining  menhaden  is  shown  in  fig.  9. 


AND    PURSE    SEINING 

ANCHOVY   SEINING 

There  is  a  considerable  amount  of  anchovy  purse  seining 
in  Peru,  done  by  boats  under  50  ft.  in  length,  with  rather 
short,  but  very  deep,  nets  of  approximately  24  fm.  of 
li  in.  mesh,  stretched.  Several  28  in.  power  blocks  are 
being  introduced  into  that  fishery. 

EUROPEAN  HERRING  SEINING 

The  European  system  of  herring  seining,  as  used  in 
Norway  and  Iceland,  is  similar  to  the  menhaden  seining 
described  above.  As  yet,  no  attempt  has  been  made  to 
introduce  the  power  block  in  this  fishery,  but  it  appears 
that  the  block  can  be  used  in  much  the  same  way  as  in 
menhaden.  Applying  the  power  block  to  large  nets  which 
are  handled  from  very  small  boats  presents  a  difficult 
problem,  because  it  is  not  possible  to  get  the  required 
height  and  have  the  necessary  stability  for  ideal  operation. 
A  stable  fishing  platform  is  desirable  for  proper  use  of 
the  power  block. 

LOFOTEN  COD  SEINING 

In  the  northern  part  of  Norway  in  the  Lofoten  Islands, 
codfish  are  caught  by  using  purse  seines.  The  method  of 
fishing  this  seine  is,  in  many  respects,  similar  to  the 
methods  used  in  handling  the  West  Coast  salmon  net. 
Driftnet-type  boats  handle  the  nets  in  the  small  area  aft 
of  the  deckhouse.  One  28  in.  power  block  has  been  sent 
to  one  of  the  leading  Lofoten  fishermen  and,  in  his 
opinion,  it  can  be  successfully  applied  to  handling  the 
cod  purse  seines. 

PORTUGUESE-STYLE  SARDINE  SEINING 

The  Portuguese  and  Angolan  purse  seiners  fish  long, 
deep  nets.  The  system  and  nets  are  very  similar  to  those 
used  for  sardines  on  the  West  Coast  of  the  United 
States.  The  only  basic  difference  is  that  the  Portuguese 


Fig.  10.    Portuguese  type  seiner,  equipped  with  power  block  for  pulling  net  amidships,  similar  to  present  system. 

[409] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


Fig.  II.     Portuguese  tyre  seiner,  showing  power  block  installed  for  pulling  net  on  stern. 


vessels  have  their  machinery  and  deckhouse  amidships, 
and  pull  the  net  over  the  side.  The  traditional  reason  for 
this  seems  to  be  that  their  system  has  been  developed  on 
the  availability  of  plenty  of  manpower  for  pulling  the 
webbing.  The  American  boats  of  this  type  previously 
used  the  strapping  method.  A  crew  of  about  30  pull  the 
Portuguese  net  abroad,  while  on  American  and  Canadian 
boats,  using  large  nets  and  a  crew  of  8,  the  net  is  pulled 
aboard  far  more  rapidly.  Two  methods  of  rigging  the 
power  block  on  Portuguese-type  vessels  to  handle  the 
sardine  purse  seines  are  shown  in  figs.  10  and  11. 

No  basic  changes  in  procedure  or  design  of  the  vessel 
need  to  be  made.  It  is  believed  by  the  author  that  this 
would  produce  a  very  efficient  purse  seining  operation. 
The  first  introduction  of  the  power  block  has  been  made 
in  Portugal,  and  more  activity  is  expected  to  follow  in 
the  near  future. 


Fig.  12.     Typical  South  African  purse  seiner,  equipped  with  one 
power  block  for  experimental  operation. 


SOUTH  WEST  AFRICA  LAMPARA  SEINING  FOR 
PILCHARD  AND  MAASBANKER 

In  South  West  Africa,  vessels  of  55  to  65  ft.  length,  of 
the  type  shown  in  figs.  12  and  13,  are  used  operating  with 
small,  shallow  lampara-type  purse  seines. 

Instead  of  pulling  each  wing  of  the  net  into  separate 
small  boats  as  is  done  in  Norway,  Iceland,  and  on  the 
East  Coast  of  the  United  States,  the  South  Africans  pull 
each  wing  on  to  the  purse  seine  vessel.  The  vessels  are 
fine,  modern  boats  which  carry  a  tremendous  load  for 
their  length. 

In  December  of  last  year,  the  author,  in  co-operation 
with  Wilbur,  Ellis  Company  and  Cooper- Wolmarans  and 


Fig.  13.     South  African  seiner,  showing  proposed  method  of 
hauling  lampara  seine  with  two  power  blocks. 


[410] 


THE    POWER    BLOCK     AND    PURSE     SEINING 


Company,  rigged  one  35  in.  power  block  on  a  typical 
South  African  boat  (fig.  12)  to  see  if  both  wings  of  the 
lampara  seine  could  be  pulled  through  one  power  block. 
The  results  of  the  experiment  showed  definitely  that  this 
could  be  done  but  several  small  difficulties  were  encoun- 
tered which  indicated  that  it  would  be  better  to  use  two 
smaller  blocks  rigged  as  in  fig.  13.  No  operating  results 
have  as  yet  been  obtained  from  this  system,  but  there  is  no 
reason  why  it  should  not  be  very  satisfactory  and 
rapidly  handle  the  lampara  nets. 

The  efficiency  of  the  South  African  system  depends 
entirely  on  speed  and  crowding  the  fish,  it  would  be 
possible  to  use  much  larger  and  deeper  nets  by  intro- 
ducing the  power  block  to  this  type  of  fishing  while 
using  the  same  number  of  crew  or  even  fewer.  By  using 
a  fast,  mechanical  means  of  handling  the  netf  the  need  to 
pull  both  wings  simultaneously  is  eliminated,  which 
suggests  that,  with  the  advent  of  the  power  block,  a 
modified  Western  system  can  be  adapted  to  this  South 
African  fishery. 

BEACH  SEINING 

In  many  areas  of  the  world,  large  nets  are  hauled  on  to 
the  beach.  Generally,  these  nets  have  long  wings  which 
are  simultaneously  hauled.  An  application  of  the  power 
block  may  be  useful  in  this  type  of  fishing,  the  block 
being  supported  on  a  boom  or  A-frame  on  the  back  of  a 
truck  or  other  piece  of  mobile  equipment.  The  hydraulic 
pump  would  be  run  from  the  vehicle's  engine  to  provide 
the  power  source.  The  webbing  could  be  dropped  on  the 
flat  bed  of  the  truck  or  on  the  beach. 

LEAD  NETS 

In  many  types  of  fishing,  including  certain  types  of 
herring  traps,  long  lead  nets  are  used.  These  in  general, 
have  little  bulk,  and  small  skiff  or  launch,  with  a  power 
block  mounted  in  davits,  would  be  of  great  aid  in  picking 


Fix .  14.     18  ft.  Salmon  skiff,  hauling  lead  net  by  means  o)  a  12  in. 

power  block.         Similar  arrangements  are  reported  to   work 

-  satisfactorily  in  handling  gillnets. 


up  the  net.  Salmon  lead  nets,  used  from  the  skiff  of  the 
salmon  purse  seiner,  are  successfully  being  hauled  with 
the  12  in.  power  block  (fig.  14). 

POWER  BLOCK  PRINCIPLE  USED  FOR  GILLNETS 
AND  DRIFT  NETS 

Salmon  Gillnets 

A  large  percentage  of  West  Coast  salmon  is  caught  in 
gillnets,  of  25  to  150  meshes  deep,  of  5i  in.  nylon 
webbing.  In  some  areas,  such  as  Bristol  Bay,  all  of  the 
fish  are  caught  by  gillnets,  and  the  Japanese  ocean 
salmon  fishing  is  done  almost  entirely  with  gillnets.  At 
present,  American  boats  are  equipped  in  one  of  two 
ways.  The  first  uses  a  small  drum  which  winds  up  the 
entire  gillnet,  the  fish  being  picked  out  of  the  net  before 
it  gets  to  the  drum.  This  is  a  one-man  operation,  and  is 
relatively  efficient,  but  is  applicable  only  in  those  areas 
whert  a  high  concentration  of  fish  is  not  frequent.  The 
second,  found  in  the  Bristol  Bay  area  where  500  to  1,000 
gillnettersfish,  uses  a  power  roller  overwhich  thenetsare 
hauled.  Huge  concentrations  of  fish  occur  in  this  area, 
which  make  the  application  of  a  mechanical  hauler  of 
any  type  extremely  difficult.  The  power  roller  now  used 
is  only  an  aid  and  is  of  little  assistance  when  a  large 
quantity  of  fish  is  in  the  net. 

The  power  block  is  being  applied  to  handling  gillnets 
in  a  variety  of  ways  but  as  this  is  a  new  field  the  work 
on  it  can  still  be  considered  experimental.  A  number  of 
fishermen  have  reported  success  in  hauling  gillnets — some 
picking  the  fish  before  reaching  the  block  and  others 
letting  the  fish  go  through.  Purctic  has  devised  a  complete 
salmon  gillnet  method  which  seems  promising  but  is  still 
in  an  experimental  stage. 

Herring  Drift  Nets 

There  are  many  thousands  of  boats  fishing  herring  using 
drift  nets  in  Europe  and  Iceland.  A  proposed  system  for 
using  the  power  block  principle  to  aid  hauling  these  long 
nets  has  been  devised  by  Puretic  and  the  author.  Basically, 
the  net  would  be  hauled  in  the  same  manner  as  at  present, 
using  the  cable  around  the  winch  with  a  modified  power 
block  supported  by  the  boom  across  the  vessel  from  the 
hauling  side.  The  power  block  would  take  the  strain 
on  the  net,  which  would  be  controlled  by  a  foot  pedal  or 
knee-actuated  lever  on  the  gunwale,  operated  by  a  man 
located  in  the  usual  position  for  shaking  out  the  fish. 
The  net  would  come  across  the  gunwale,  where  two  men 
would  pull  it  apart,  shaking  out  a  small  amount  of  the 
catch  at  a  time. 

EFFECT  OF  POWER  BLOCK  ON  PURSE  SEINING 
METHODS 

Existing  purse  seining  methods  are  based  on  conditions 
which  no  longer  exist  and  although  improvements  were 
developed  as  modern  appliances  became  available,  the 
basic  operational  methods  have  never  been  revised. 
With  the  advent  of  a  mechanical  hauling  apparatus 
which  reduces  the  most  time-consuming  part  of  the 
operation,  i.e.  the  hauling,  the  entire  method  of  fishing 
should  be  re-analysed.  As  an  example,  the  use  of  two 
small  seine  boats,  practised  in  Norway,  Iceland,  and 


[411  ] 


MODERN     FISHING    GEAR     OF    THE    WORLD 


on  the  East  Coast  of  the  United  States,  was  developed 
before  boats  had  engines.  It  was  necessary  to  carry  these 
small  boats  to  the  fishing  grounds  on  larger  vessels — 
originally,  sailing  vessels,  and,  later,  steamers— and 
the  boats  had  to  be  kept  small  enough  so  that  they 
could  be  propelled  by  oars  around  the  fish.  This,  then, 
also  controlled  the  size  of  the  net.  Two  boats  were 
used  because,  without  power,  one  boat  could  not  be 
rowed  around  the  fish  fast  enough.  Half  of  the  net  was 
placed  in  each  boat. 

The  Western  style  of  purse  seining  was  tried  on  the 
East  Coast  of  the  United  States,  and  even  in  Norway, 
but  was  not  successful  because  of  inexperienced  crews 
and  the  slow  strapping  method  used.  Though  efficient 
as  regards  manpower,  it  is  not  as  fast  as  the  two-boat 
method. 

In  the  opinion  of  the  author  and  fishing  gear  experts, 
such  as  Icelandic  Captain  Ingvar  Palmason,  the  purse 
seining  system  used  by  the  British  Columbia  herring 
seine  boats  is,  in  general,  the  most  efficient  yet  devised, 
but  it  cannot  be  copied  outright  for  use  in  all  areas  and 
must  be  adapted.  In  most  fisheries  where  the  power 
block  has  been  introduced,  it  is  already  having  an  effect 
on  the  size  and  style  of  the  net  in  that  fishermen  begin 
to  think  in  terms  of  longer  and  deeper  nets. 


EFFECT  OF  THE  POWER  BLOCK  ON  THE  DESIGN 
OF  FISHING  VESSELS 

There  have  already  been  some  changes  made  in  purse 
seine  vessels  because  of  application  of  the  power  block; 
turntables  are  being  removed  because  they  are  no  longer 
essential  and  new  vessels  are  being  designed  with  smooth 
bulwarks.  Wood  booms  are  being  replaced  by  steel 
booms  of  new  design,  the  most  notable  development 
in  this  line  being  the  Puretic  power  boom,  as  shown 
in  fig.  15. 

This  steel  boom  is  provided  with  a  gooseneck  extension 
on  the  end  to  support  the  power  block  clear  of  the  boom. 
It  is  fitted  with  a  hydraulic  topping  lift  and  hydraulically- 
actuated  vangs  (guys).  Hydraulic  cylinders  mounted  on 
the  side  of  the  boom  keep  tension  on  the  vangs  at  all 
times  and  allow  the  boom  to  be  positioned  in  the  most 
convenient  stand.  The  power  slewing,  together  with 
the  hydraulic  topping  lift,  provide  complete  flexibility, 
both  for  use  with  the  power  block  and  in  brailing. 

The  improved  menhaden  purse  boat  shown  in  fig.  8 
was  designed  by  the  author's  company.  In  addition  to 
the  crane  and  power  block;  the  boat  has  other  inno- 
vations, i.e.  the  captain's  controls  are  placed  forward 


Fig.   15.    Hydroulically-operated  power   boom  for  use   with 
Power  Block.    Boom  is  raised  and  lowered  and  slewed  hydraulic- 
ally. 


Fig.  16.        A  steel  74ft.  purse  seiner-trawler  proposed  for  the 
Icelandic  fishery.    Designed  after   West  Coast-style  vessels. 


1412] 


THE     POWER     BLOCK     AND    PURSE     SEINING 


Fig.  17.     39, ft.  by 

where  he  can  steer  and  handle  the  throttle  without 
depending  on  an  engineer.  This  position  allows  better 
visibility  for  setting  the  net  and  handling  the  boat.  The 
double  skeg  arrangement  underneath,  with  the  propeller 
located  in  the  tunnel,  is  designed  to  facilitate  the  use  of 
the  new  system  of  fishing  by  keeping  the  net  out  of  the 
propeller. 

Fig.  16  shows  a  steel  vessel  designed  for  trawling  and 
purse  seining.  It  is  patterned  after  West  Coast-style 
vessels,  but  takes  into  account  the  rougher  weather 
conditions  experienced  around  Iceland.  Fig.  17  is  a 


14  ft.  steel  purse  seiner. 

modified  West  Coast-style  vessel  with  a  raised  fo'c'sle, 
again  designed  for  combination  trawling  and  purse 
seining.  Both  of  these  vessels  would  be  very  efficient 
purse  seiners  with  their  broad,  clear  decks  providing 
excellent  platforms  for  stern  trawling. 

Although  the  new  system  of  handling  nets  is  having  an 
effect  on  methods  of  fishing,  size  and  design  of  fish  nets, 
and  the  design  of  fishing  vessels,  the  principle  is  so 
adaptable  that  it  can  be  applied  to  most  existing  vessels 
whether  the  net  be  hauled  over  the  stern,  amidships,  or 
in  the  fore  part  of  the  vessel. 


Conventional  hand  hauling  of  a  Norwegian  herring  purse  seine. 
[413] 


Photo:  FAO. 


THE  USE  OF  FISHPUMPS  IN  THE  U.S.A. 

by 

D.  W.  BURGOON 

President,  Yeomans  Brothers  Company,  Mclrose  Park,  Illinois,  U.S.A. 

Abstract 

The  United  States  fishing  industry  has  found  through  12  years  use  of  hydraulic  fish  handling  systems  that  loading  and  unloading 
boats  with  pumps  is  equivalent  to  increasing  the  number  of  fishing  vessels  at  a  comparatively  insignificant  cost,  and  to  extending  the  fishing 
operations  by  at  least  13$  per  cent.  Briefly,  ocean-to-boat  and  boat-to-dock  fishpump  systems  have  led  to  more  fresh,  salted  and  preserved 
fish  for  the  table— more  pharmaceutical  and  vitamin  products — more  poultry  and  livestock  feed — more  oils  and  fats  for  food  and  industrial 
purposes— all  without  increase  in  fishing  boats  and  manpower.  The  centrifugal  type  vacuum  suction  pressure  fishpump  is  now  the  accepted 
method  offish  handling  in  the  U.S.A.  when  the  requirements  are  8  or  more  tons  per  day  of  any  type  of  shoaling  fish  such  as  sardine,  herring, 
mackerel,  menhaden,  red  fish,  etc.  Smaller  wharfs  can  be  used  by  the  factories,  for  the  fishpump  system  itself  is  small  and  compact,  and  space 
is  not  required  for  boats  to  wait  for  unloading.  The  economic  advantages  of  this  type  of  mechanization  extend  from  the  owners  and  operators 
down  to  the  crews  whose  work  is  not  only  easier  and  cleaner,  but  whose  income  is  higher. 


R6sumc 


La  peche  industriclle  aux  E.-U.  avec  Ic  systeme  de  pompc  a  poissons 


L'industrie  de  la  p&che  aux  E.-U.  a  trouve,  apres  12  ans  d'cmploi  des  systemes  hydrauliques  de  munutention  du  poisson,  quo  le 
chargement  et  le  dechargcment  des  bateaux  par  des  pompes,  equivaut  a  augmcnter  le  nombre  de  bateaux  de  peche  pour  un  cout  comparative- 
ment  insignifiant  et  a  augmenter  les  operations  dc  peche  d'au  moms  134  pour  cent.  En  bref,  les  systemes  dc  pompes  a  poissons  de  la  mcr 
au  bateau  et  du  bateau  au  quaiont  donn£  plus  de  poisson  frais,sal£  ct  conserve  pour  la  table — plusdc  produits  pharmaceutiques  et  dc  vitamines 
— plus  d'  aliments  pour  lavolaillc  et  le  be  tail — plus  d'huiles  et  de  graisses  pour  les  fins  industricllcs  et  alimentaircs  —  tout  cela  sans  augmentation 
des  bateaux  de  peche  ni  de  la  main-d'oeuvre.  Lctype  de  pompe  &  poissons  centrifuge  &  aspiration  sous  vide  est  maintenant  la  methode  reconnue 
de  manutention  du  poisson  aux  E.-U.  quand  les  exigences  sont  de  8  tonnes  ou  plus  par  jour  pour  n'impprte  quelle  especc  dc  poissons  vivant 
en  banes  comme  les  sardines,  harengs,  maquercaux,  menhadens,  ch&vres,  etc.  Les  usines  peuvent  utiliser  dc  pctits  quais  car  les  pompes  a 
poissons  elles-memes  sont  petites  et  compactes  et  il  n'y  a  pas  bcsoin  de  prevoir  d'espace  pour  les  bateaux  attendant  d'etre  dcchargds.  Les 
avantages  6conomiques  de  ce  type  de  mecanisation  splendent  des  armateurs  et  conserveurs  jusqu*  aux  equipages  dont  Ic  travail  est  non  seule- 
ment  plus  facile  et  plus  proprc  mais  dont  les  revcnus  sont  plus  el  eves. 

Pesca  comercial  en  los  E.U.A.  con  bombas  de  pescado 
Extracto 

La  industria  pesquera  de  los  E.U.A.  ha  encontrado  durante  los  ultimos  12  aflos  que  el  empleo  dc  bom  has  hidrdulicas  en  la  mani- 
pulaci6n  de  pescado  durante  la  carga  y  descarga  de  las  embarcacioncs  equivale  a  aumentar  el  numcro  de  dstas  con  un  costo  insignificante  y 
a  ampliar  las  operacioncs  pesqueras  cerca  de  un  13A  por  cicnto.  En  res u men,  los  sistemas  dc  bombeo  de  pescado  desde  el  oceano  al  barco 
y  de  este  al  muelle  ban  permitidq  obtener  mayor  cantidad  de:  pescado  fresco  salado  y  preservado,  productos  farmaceuticos  y  vitaminados, 
alimentos  para  aves  y  ganado,  aceites  y  grasas  para  la  alimentaci6n  y  fines  industrials,  sin  aumentar  cl  numerp  o  tonclajc  de  las  cmbarcaciones 
pesqueras  ni  la  tripulacion.  En  la  actualidad  en  E.U.A.  tpda  la  industria  acepta  la  bomba  aspirante  dc  tipo  ccntrifugo  como  metodo  para 
manipular  diariamente  8  o  100  mas  toneladas  de  cualquier  tipo  de  pescado  que  vive  en  cardumcn,  a  saber:  sardina,  arcnque,  cabal  la, 
lacha,  cabracho,  etc.  Las  fabricas  pueden  usar  pequeftos  muelles,  en  atcncion  a  que  el  sistcma  de  bomba  cs  pcquefto,  no  requiriendose 
espacio  para  los  barcos  en  espera  de  ser  descargados.  Las  ventajas  econdmicas  de  este  tipo  de  mecanizacion  alcanzan  desde  los  armadores 
a  la  tripulacidn  cuyo  trabajo  es  mas  facil  y  limpio,  ademas  de  permitirle  aumentar  sus  entradas. 


AFTER  12  years'  experience  of  hydraulic  fish  hand- 
ling systems  the  United  States  fishing  industry  has 
found  that  loading  and  unloading  boats  with 
pumps  is  equivalent  to  increasing  the  number  of  fishing 
vessels  at  little  cost,  and   to  extending  fishing  oper- 
ations by  at  least  13£  per  cent.  Briefly,  it  means  an 
increased  turnover  without  increase  in  fishing  boats  and 
manpower. 

Early  attempts  to  pump  fish  had  presented  problems  and 
the  systems  had  several  drawbacks;  only  small  fish 
could  be  handled;  operations  were  limited  to  the  flood 
tide  timetable  to  obtain  ideal  conditions  and  prevent 
damage  to  the  fish;  the  systems  were  complicated  by 
auxiliary  valves  and  control  equipment  with  attendant 
high  labour  and  maintenance  costs.  In  1943  Yeomans 
designed  a  pumping  system  for  unloading  fishing  vessels. 


THE  BOAT-TO-DOCK  SYSTEM 

The  first  fishpump  system  was  installed  commercially 
in  1945  in  Portland,  Maine,  to  unload  herring  or  sardines 
directly  from  the  fishing  vessels  to  storage  bins  in  the 
cannery.  This  installation  was  successful,  800  bushels 
(about  29  cu.  m.)  offish  being  moved  from  the  boat  into 
the  factory  in  17  min.  by  only  two  men,  as  compared 
to  the  six  or  more  hours  and  10  men  previously  required. 

Within  4  seasons,  80  such  installations  had  been  made, 
including  some  in  Norway,  Iceland,  Newfoundland,  and 
South  Africa.  As  would  be  expected,  the  fish  unloading 
system  was  extended  to  include  shrimp,  mackerel,  red 
fish,  pogies,  menhaden,  slid  and  brisling.  Most  varieties 
of  pelagic  fish  up  to  36  in.  long  now  are  handled  easily 
without  damage  to  the  fish,  the  capacities  ranging  from 


[414] 


FJSHPUMPS     IN     THE     U.S.A. 


Fig.  I.     Boat-to-clock.      Water  for  flotation  is  introduced  and 

the  flexible  suction  hose  lowered  into  the  hold. 
Fig.    2.     A     Yeomans    boat-to-dock    installation. 
Fig.  3.     Fishes  being  discharged  on  to  a  dewatering  sluiceway. 

Fig.  4.     Final  transport  of  fishes  hv  a  conveyor  system  into  the 
storage    tank. 


TABIF  I 

Boat-to-dock  System.    Capacity  in  tons/mm     **--  Dimensions      *llcad     distance 
from  water  to  pump  plus  distance  from  pump  to  highest  point.    **1  ton     2.000 Ib. 


Si/e  of 
Pumping 
Unit  in 
inches.       20 


Head  in    leet* 


Approximate  Dimensions 
sec  fig.  5. 


40 


50 


80 


Si/c  of 
Msh  in 
inches 


t-ish   Pump 
\  tt 


Water 

Supply 
Pump 


6  2  ton     I  ton  i  ton 

8  4  ton    2  ton  1  ton 

10  5  ton    4  ton  3  ton     I  ton 

12  12  ton  1 1  (on  l>  ton    6  ton    4  ton      Uf/°    7  ft.  6  in 


12       6  ft    Om.        7  ft.  0  in. 
20       6ft.  0  in       10  It.  0  in. 

20       6ft    A  in.      11  ft.  3  in 

D     1 8  in. 
2  ft.  6  in. 


2  tons/min.  with  the  smallest  system  up  to  12  lons/min. 
with  the  largest  system  available.  Decks  as  well  as  holds 
can  be  unloaded. 

The  Yeomans  boat-to-dock  fishpump  system  operates 
on  the  simple,  efficient  vacuum  suction  pressure  principle. 
The  basic  equipment  consists  of  an  automatic  vacuum 
priming  system  and  a  horizontal  non-clog  centrifugal 
pump  with  a  specially  designed  and  treated  impeller.  The 
system  is  operated  by  two  men — one  at  the  pump  on  the 
dock  and  one  in  the  boat.  There  are  no  complicated 
controls  or  valves  requiring  extra  attendants.  Only  one 
valve  is  used,  this  being  opened  by  the  pump  operator  at 
the  beginning  of  the  pumping  cycle  and  closed  when  the 
hold  and  deck  have  been  unloaded.  A  minimum  of  water 
is  used  as  the  carrying  media  for  the  fish,  the  amount 
depending  upon  the  type  and  the  freshness  of  the  fish 
(the  fresher  the  fish,  the  less  water  required). 

The  operation  is  simple  and  can  be  handled  success- 
fully by  non-skilled  personnel.  A  flexible  hose  attached 
to  the  suction  of  the  pump  is  lowered  into  the  fish  until 
its  end  is  covered.  Then  the  discharge  valve  of  the  main 


VACUUM   PUMP     ;  MOTOR      j          ^-   SUCTION  v  WATER   SUPPLY    PUMP 


r FLUME    TO   STORAGE 
\  AND    SCREEN 


Typical  layout  of  a  boat-to-dock  installation. 
Plan    view. 


415  ] 


MODERN     FISHING     GEAR     OF    THE    WORLD 


Fig.  6.     Ocean-to-boat.     Two  vessels  are  brought  alongside  the 

seine.     The  bigger  boat  is  loaded  by  means  of  ihe  pump  installed 

on  the  smaller  one. 

Fig.  7.     The  flexible  suction  hose  is  lowered  overside  into  the  net. 

Fig.  8.     The  Yeomans  ocean-to-boat  pump  installation. 
Fig.   9.     Fish  being  discharged  from  the  pump  hose  in  to  the  hold. 


pumping  unit  is  closed.  Only  enough  water  is  introduced 
at  the  end  of  the  suction  hose  to  keep  it  covered,  and  to 
prevent  air  entering  it.  The  operator  starts  the  priming 
system  with  a  push  button  control.  This  sytem  is  auto- 
matic and  needs  no  further  attention.  When  the  priming 
system  stops  the  actual  unloading  begins.  The  fishpump 
is  started,  the  discharge  valve  is  opened  manually,  and 
the  unloading  through  the  pump  is  continuous  as  long 
as  the  end  of  the  suction  is  kept  covered  with  fish  and 
water.  The  fishpump  is  under  the  control  of  the  operator 
at  all  times.  While  no  changes  are  required  for  handling 
fish  of  different  sizes,  the  capacity,  or  rate  of  delivery,  can 
be  altered  within  limits  by  an  adjustment  of  the  discharge 
gate  valve.  No  changes  are  required  for  tide  conditions 
as  the  fishpump  is  designed  to  operate  within  a  25  ft. 
suction  lift  range. 

Fish  are  discharged  from  the  pump  through  piping 
to  a  flume  or  sluiceway,  or  on  to  a  conveyor,  or  through 
piping  to  the  factory  or  storage  bins.  The  fish  either 
travel  over  a  stationary  screen  or  through  a  revolving 
screen,  for  dewatering  and  then  the  dry  fish  can  be 
measured  or  weighed  by  a  quartering  box  or  electric 
weighing  device.  The  water,  which  has  been  screened 
out,  can  be  flumed  or  pumped  back  into  the  hold  of  the 
boat  for  re-use  in  the  pumping  operation. 

The  fishpump  is  a  compact  unit,  requiring  only 
approximately  6  by  7  ft.  for  the  smallest  unit — 74  by  12£  ft. 
for  the  largest.  It  can  either  be  permanently  installed 
and  housed  at  the  end  of  the  wharf,  or  mounted  on  a 
truck  and  brought  forth  for  use  as  required. 

The  savings  in  manpower  and  manhours  vary  with  the 
plant.  For  example,  we  can  state  that  one  plant  effected 
a  total  saving  of  $24,900  in  the  first  100  operating  days, 
including  depreciation  on  the  system  which  had  cost 
approximately  86,000. 


VACUUM    PUMP 


GASOLINE   OR 
DIF5.FX    ENGINE    .-- 


DE  WATER  ING     SCREEN 
&    TROUGH 


Fig.  10.     Typical  layout  of  an  ocean-to-boat  installation. 
Plan  view. 


[416] 


FISHPUMPS    IN    THE     U.S.A. 


TABLE  II  Ocean-to-boat  System.  Capacity  in  tons/min.** 
•  -Dimensions.  *  Head -distance  from  water  to  pump  plus 
distance  from  pump  to  highest  point.  **J  ton  2,000  Ib. 


Size  of 

Pumping 

Unit  in 

Inches. 


Head  in  Feet* 


10 


20 


30 


Size  of 
Fi\h  in 
Inches 


Approximate  Dimen- 

sions 
see  rig.   10 


6         I  ton      3/4  ton   J  ton 

8         4  ton      2  ton      1  ton 

12       10  ton      8  ton      6  ton 


Up  to  12  6  ft.  Oin.  7ft.  0  in. 
Up  to  20  6  ft.  0  in.  10ft.  Oin. 
Up  to  28  7ft.  6  in.  15  ft.  Oin. 


THE  OCEAN-TO-BOAT  SYSTEM 

The  first  ocean-to-boat  pumping  ii.stallations  were 
made  in  1949.  Like  the  unloading  fishpump,  the  initial 
installation  was  successful  and  the  following  season 
nearly  a  dozen  boats  belonging  to  the  sardine  and  herring 
fleet  were  equipped  with  the  ocean-to-boat  units. 

Ocean-to-boat  equipment  is  now  considered  essential 
on  U.S.  fishing  vessels  when  the  crew  is  working  on  a 
share  basis,  and  when  the  season  is  limited. 

In  operation,  the  boat  is  brought  alongside  the  net, 
the  suction  hose  is  lowered  into  the  seine,  the  seine  is 
dried  up,  and  the  fish  are  pumped  directly  across  a  screen 
to  eliminate  water,  and  into  the  hold  alive.  A  special 
device  on  the  end  of  the  suction  hose  serves  the  dual 
purpose  of  protecting  the  net  and  keeping  it  from  being 
sucked  itno  the  hose,  as  well  as  preventing  large  fish 
from  being  drawn  into  the  system. 

The  smallest  of  the  three  ocean-to-boat  fishpumps 
available  has  an  average  rated  capacity  of  1  ton/min., 
handling  fish  up  to  12  in.  in  length,  the  largest  unit 
handling  an  average  of  10  tons/min.  and  fish  up  to  28  in. 
long.  Power  for  the  pump  can  be  taken  either  from  an 


auxiliary,  or  from  the  main  engine  of  the  vessel,  if  suffi- 
cient generator  capacity  is  aboard. 

One  branch  of  the  fishing  industry  which  has  found  the 
ocean-to-boat  fishpump  useful  is  the  by-products  group 
interested  in  scale  recovery  for  pearl  essence  and  similar 
uses.  When  used  in  this  operation,  fish  are  pumped  from 
the  seine  through  a  sealer  and  then  into  the  hold  of  the 
carrier  boat.  In  this  manner  the  intermediate  scale  boat 
is  eliminated.  The  fish  are  delivered  alive  into  the  hold 
after  having  passed  through  both  the  pump  and  the 
scaling  equipment. 

While  more  than  200  of  these  two  fishpump  systems 
have  been  installed  in  various  countries,  the  manufacturer 
does  not  produce  the  fishpump  as  a  "stock"  unit.  Each 
is  made  specifically  for  the  user  and  the  service  required. 
The  variables  of  the  requirements  include:  type  and  size 
of  fish  to  be  handled;  capacity  of  the  boats  to  be  loaded 
and/or  unloaded;  power  available  and  current  character- 
istics, in  the  case  of  electric  power;  and,  for  fish  unloading 
systems,  the  capacity  of  the  plant,  the  type  of  equipment 
now  used  for  moving  fish  into  and  within  the  plant, 
vertical  distance  from  the  pump  to  low  and  high  tide 
marks,  vertical  distance  from  wharf  to  point  of  discharge, 
and  horizontal  distance  from  the  wharf  to  the  point  of 
fish  discharge. 

The  centrifugal  type  vacuum  suction  pressure  fishpump 
is  now  the  accepted  method  of  handling  in  the  United 
States  for  8  or  more  tons  a  day  of  any  school  fish,  such 
as  sardine,  herring,  mackerel,  menhaden,  red  fish,  etc. 

The  economic  advantages  of  this  type  of  mechanization 
extend  from  the  owners  and  operators,  who  save  on 
handling  costs  while  being  able  to  handle  more  fish, 
down  to  the  crews,  whose  work  is  made  easier  and 
cleaner  while  receiving  much  higher  wages  because  the 
faster  loading  and  unloading  means  that  more  fishing 
can  be  done,  yielding  a  greater  total  catch  in  the  proceeds 
of  which  they  share. 


South  African  fishing  vessels  unload  their  cat  eh  of  pilchards /masbanker  by  means  of  a  pump.        On  the  left  arc  vessels  of 
different  design  pulling  into  the  jetty  while  view  on  the  right  shows  the  pumping  process  in  action. 

[417) 


cc 


THE  DEVELOPMENT  OF  THE  PHILIPPINE  BAGNET  (BASNIG) 

FOR  INCREASED  EFFICIENCY 

by 

SANTOS  B.  RASALAN 

Bureau  of  Fisheries,  Department  of  Agriculture  and  Natural  Resources,  Manila,  Philippines 


Xtobafnet,ortajfifcisQfk>caloiira     It  has  readied  its  present  stage  of 


contrivance  for  sustenance  fishing  to  one  which  is  operated  on  a  commercial  scale  for  the 
herrings,  sardines  and  mackerel.  The  net  was  formerly  rectangular  or  trapezoidal  in  shape 
with  the  aid  of  a  torch,  hut  it  later  assumed  the  form  of  a  large  inverted  box-like 


tern  which  it  is  operated.  The  boa  is  propelled  by  motor  power  and  equipped  with  . . 

This  gradual  developmentthas  been  attained  through  the  ingenuity  of  the  fishermen  in  order  to  achieve 


net  whose  size  < 
lights  supplied  1 


al  evolution  from  a  simple 
;  species,  such  as  anchovies. 
taa  fibre  doth  and  operated 


i  upon  the  size  of  toe  boat 


are  fully  described  and  illus  trated  in  this  paper. 


wcy,  and  these 


(bass*)  pew  ea 

Le  filet-sac,  ou  basnig,  est  d'origine  locate.    II  est  arrivt  a  sa  forme  actuelle  par  une  Evolution  progressive  qui,  d'un  simple  dispositif 
pour  la  ptebedc  subsistence,  CTI*  fait  uncngm^ 
sardines  ct  tnaQucvsaiix* 

faitiatement,  cc  filet,  de  forme  rectangulaire  ou  trapfeoide,  6tait  fait  d'une  totte  grossfere  en  chanvrc  de  Manille  ct  utilis*  4 1'aide 
d'une  toiche.    II  a  pris  uHArieurement  la  forme  d* une  moustujuaire  invmfe,  comparable  6  une  botte  dont  la  dimension  est  fonction  de  ceUe 

"      *  lues  alimentees  par  un  g6nerateur. 

rattdndie  une  meiUeure  efficatil*  et  ce 


dn  bateau  partir  duquel  on  1'utilise.    Le  bateau,  propuW  par  un  rootcur,  est  i 

Cette  Evolution  progressive  a  6*6  tfeltsfo  grto  A  ringfoiositf  des  p6cheun  qui  s  efforcent  < 


doane  une  description  <ktaill6e  et  illustrte  de  ces  modifications. 


>dd"delo"(bi 


El  "ckloM 


cido  locale 


actual  por  una  evolution  paulatina  de 

a  una  red  perfeccionada  que,  en  hi  actualidad,  se  usa  en  la  captura  de  divenas 
sardina,  arenque,  cabana,  etc.    Originahnente  este  arte  de  forma  rectangular 


o  trapezoidal,  era  tejido  con  gruesas  fibras  de  manila  y  durante  su  calamento  se  empleaba  una  antorcha.  Con  el  tiempo  tom6  la  foima  de 
im  mosquitero  rectangular  cuyas  dimensions 
electricas  ahmentadas  por  un 


En  d  trabajo  tambien  se  describen  e  tlustran,  con  detalle,  las  modificactones  de  este  arte  y  su  pi 
ingeniosidad  desfdegada  por  los  Pescadores  para  aumentar  su  eficacia. 


togr 


tab 


THE  bagnet,  a  fishing  gear  widely  used  in  the 
Philippines  and  domestically  called  basnig,  is  of 
local  origin,  having  evolved  from  a  simple  susten- 
ance fishing  method.  Today,  it  is  commercially  operated 
in  most  fishing  grounds  of  the  Philippines  during  dark 
nights  to  catch  sardines,  herrings,  anchovies,  mackerel, 
and  other  fishes  which  frequent  sheltered  waters.  Of  the 
1  ,238  commercial  fishing  boats  of  more  than  three  tons 
gross,  licensed  by  the  Bureau  of  Fisheries  in  1955,  670 
were  battdg  boats,  producing  30  per  cent  of  the  total 
production  offish  in  that  year. 

This  gear 
varying  accor 

operated,  the  noting  is  made  of  6  strand  twine  of  1  to 
2*5  cm;  stretched  mesh.  Basnig  boats  range  from  53  to 


(fig.  1)  is  a  rectangular  bagoet,  its  size 
rding  to  the  size  of  the  bos^  from  which 


generators  tad  temporary  booms  and  masts.  The 
generator  suppBe*  ttepowcr  for  6  to  14  bulbs  of  1,000 
candle  power  each-  The  booms,  with  the  aid  of  guys, 


ropes  and  pulleys,  serve  to  spread  the  net  under  the  boat. 
Some  12  to  24  fishermen  operate  this  type  of  basnig. 
When  the  boat  reaches  the  fishing  ground  at  dusk,  the 
lamps  arc  lit  to  attract  the  fish,  then  the  net  is  dropped  on 
the  windward  side  and  allowed  to  hang  far  underneath 
the  boat.  After  fish  have  been  attracted,  all  lights  except 
the  two  amidships,  are  doused  and  the  fish  concentrate 
in  the  lighted  area.  The  net  is  raised,  and  the  lights  are 
extinguished  or  covered.  The  windward  side  of  the  net  is 
passed  under  the  boat  to  the  foewatd  side  and  hauled  in 
until  the  fish  are  concentrated  in  a  small  area  of  the  net, 
ready  to  be  braited  and  taken  on  board. 

THE  DEVELOPS 

The  bakmit  (of  northern 
ayan  Island  ai 


doth  of 


[andBo&Otsa 

'Oy  suottgieooo  i 
a  rectangular  or  trapezoidal  net  made  of 


PHILIPPINE    LIFT    NETS 


Fig.  I     Construction  of  the  modern  basnig  net. 

abaca  (Musa  textilis  Nie)  or  maguey  (Agave  tantala  Linn) 
fiber.  It  is  used  to  collect  the  fish  from  the  crib  of  fish 
traps  as  set  up  in  the  5  to  10  fm.  zone  (fig.  2). 

The  net  is  made  of  several  strips  of  abaca  cloth  sewn 
together  to  fit  the  size  and  shape  of  the  crib.  Its  edges  are 
strengthened  with  manila  rope,  about  J  in.  in  diam. 
which  is  25  per  cent,  shorter  than  the  stretched  length 
of  the  cloth  to  form  a  bulge.  Each  corner  and  the  middle 
of  the  sides  are  provided  with  slings  and  ropes  for  hauling. 

The  use  of  light  for  attracting  the  fish  into  the  trap 
increases  the  efficiency.  In  the  early  days,  a  torch  was 
used  for  this  purpose  but  in  1924  kerosene  lamps  with 
mantles  came  into  use  and  the  fishermen  observed  that 


Flf.   3.    The  bintol. 


bigger  catches  were  obtained  with  the  brighter  lights. 

The  bintol,  which  is  a  further  step  in  the  direction  of  the 
lift  net  principle,  is  used  in  Bohol  province.  It  came  into 
popular  use  as  early  as  1920  because  it  was  cheaper 
(fig-  3). 

The  Mff/o/net  was  formerly  made  of  coarse  abaca  cloth, 
rectangular  and  hung  with  a  25  per  cent,  slack.  Later 
fine  handbraided  abaca  webbing,  of  2  in.  stretched  mesh, 
came  into  use,  hung  with  50  per  cent,  slack,  thus  assuring 
greater  bulge.  A  bamboo  frame  has  the  same  size  as  the 
net  and  is  supported  close  above  the  water  surface  by 
means  of  vertical  posts  rammed  into  the  bottom.  The 
lamp  for  attracting  the  fish  is  fixed  over  the  centre  of  the 
frame. 

Four  fishermen,  one  at  each  corner  of  the  frame, 
handle  the  net  by  means  of  ropes  attached  to  the  corners. 

The  so-called  new  look,  which  first  appeared  in  1946, 
is  a  further  development  of  the  bintol.  The  main 
improvement  is  the  introduction  of  additional  posts  to 
make  the  gear  more  resistant  to  high  waves  and  strong 


Fig.  4.    7ft*  new  look,  on 


form  of  the  bintol. 


1419] 


MODERN    FISHING    GEAR    OF    THE   WORLD 


T.  5.    ^  bunig  operated  with  two  small  dugouts. 


current  (fig.  4).  The  net,  which  is  made  of  cotton,  1  to  2 
cm.  stretched  mesh,  is  bigger  and  shaped  like  an  inverted 
mosquito  net.  Small  light-boats  are  sometimes  used  to 
attract  fish  outside  and  lead  them  into  the  enclosure. 
The  fish  are  caught  by  lifting  the  net  after  all  lights 
except  one  over  the  net  have  been  dimmed. 

In  1924  the  operation  of  the  basnig  net  from  boats  of 
about }  ton  gross  was  started  in  northern  Ncgros,  Lcytc 
and  Panay.  Lights  were  used  to  attract  fish  but  the  real 
lift  net  principle  was  not  adopted  until  the  handliners 
began  to  catch  their  bait  by  scooping  small  fish  gathered 
under  the  bright  lamp  of  the  boat.  They  found  that 
brighter  lights  attracted  more  fish  and  eventually  used 
bigger  nets  handbraided  of  fine  abaca  twine  and  operated 
byfour  men.  This  became  the  forerunner  of  the  modern 
basnig. 


The  use  of  these  bigger  nets  was  made  possible  by 
joining  two  boats  together  with  a  common  outrigger 
(fig.  5)  and  operating  the  net  between  them*  This  stage 
of  development  remained  until  1935. 


cial 


THE  MODERN  BASNIG 

The  transformation  from  sustenance  to  com 
fishing  was  brought  about  in  1935  by  the  conversion  of 
several  sapiao  (scoop  seine)  boats  each  operated  by  40 
to  60  fishermen,  into  basnig  boats.  The  reason  was  that 
many  operators  were  experiencing  a  decline  in  their 
catch  as  well  as  difficulties  in  the  hiring  of  fishermen, 
who  had  to  be  recruited  from  places  other  than  the 
operation  headquarters,  taught  fishing  operations  and 
provided  with  food  and  cash  advances.  This  amounted 
to  a  sizeable  investment  for  the  capitalist  and  with  this 
high  cost  and  the  unpredictable  labour  market,  fishing 
became  unprofitable.  Moreover,  only  about  SO  per  cent, 
of  the  fishermen  continued  to  operate  during  the  entire 
fishing  season.  The  operators,  therefore,  had  to  find  other 
fishing  methods  which  required  fewer  fishermen. 

One  sapiao  operator  of  Punta  Bun,  Tagubanhan 
Island,  converted  his  boat  to  basnig  fishing  in  1935, 
operating  with  pressure  gas  lamps  of  1,000  to  1,500 
candle  power.  Only  8  fishermen  were  employed  but  he 
was  able  to  land  almost  the  same  quantity  of  fish  as  when 
operating  with  saptao  gear. 

Consequently  other  sapiao  operators  lost  no  time  in 
converting  to  the  basnig  method  too.  The  oval  sapiao 
nets  were  made  rectangular  and  given  more  bulge  to 
obtain  a  shape  similar  to  an  inverted,  rectangular 
mosquito  net.  Lights  of  1,500  to  2,000  candle  power  were 
used.  In  1936  also  boats  of  more  than  3  tons  gross  were 
used,  towed  by  motor  launches  to  and  from  the  more 
distant  fishing  grounds. 

Surplus  engines  after  World  War  II  contributed  to 
further  improvement.  Engines  from  25  to  120  h.p.  were 
installed  and  electric  lights  came  into  use. 

Formerly  the  size  of  the  net  depended  on  the  size  of  the 


hoot. 


fig.  7. 


[420] 


PHILIPPINE    LIFT    NETS 


outrigger  of  the  boat.  Now  temporary  booms  (fig.  6) 
have  been  introduced  to  increase  the  net  size  without 
increasing  the  size  of  the  boat  or  its  outriggers.  Sometimes 
2  or  3  additional  light  boats  are  used  to  attract  schools 
of  fish  and  lead  them  to  the  fishing  boat. 

In  1950  also  trawlers  and  fish  carriers,  ranging  from 
70  to  136  ft.  in  length,  16  to  24  ft.  beam  and  5  to  8  ft.  draft, 
with  a  speed  of  7  to  14  knots,  weft  converted  to  basnig 
boats  (fig*  7).  As  they  carry  more  supplies  and  provide 
storage  space  for  more  fish  they  permit  operation  in  more 
distant  fishing  grounds. 

They  are  equipped  with  high  speed  engines  and  a 
generator  of  10  to  30  kw.  with  10  to  20  specially 
constructed  electric  light  bulbs.  Bamboo  booms,  3  to  6  m. 


long,  are  installed  along  the  sides,  held  in  place  by  stays 
and  shrouds  supported  by  an  auxiliary  mast. 

The  temporary  booms  which  spread  the  net  under  the 
boat  as  well  as  the  auxiliary  mast  are  detachable  and  set 
only  on  the  fishing  grounds. 

As  the  gear  is  still  operated  by  hand,  hauling  is  rather 
slow,  with  the  result  that  big  fish  such  as  mackerel,  tuna 
and  bonito,  which  are  also  attracted  by  light,  are  rarely 
caught.  This  leads  operators  to  use  explosives  which  not 
only  kill  all  marine  life  near  the  blast  but  sometimes 
injure  the  fishermen  themselves.  But  with  the  introduction 
of  multiple  winches  the  net  can  be  hauled  in  faster  so 
that  the  bigger  fish  can  be  caught  and,  at  the  same  time, 
fewer  fishermen  need  be  employed  per  boat. 


Photo:  FAO. 


SOME  IMPROVEMENTS  IN  THE  STICK-HELD  DIPNET  FOR 

SAURY  FISHING 

by 

AKIRA   FUKUHARA 

Hokkaido  Fisheries  Experimental  Station,  Japan 

Abstract 

The  usual  method  of  catching  saury  is  by  the  dipnct  supported  on  slicks  which  are  fixed  to  the  ship's  side.  The  fish  are  attracted 
into  the  net  by  lights,  but,  owing  to  the  construction  of  the  bag  of  the  net,  many  fish  escaped  from  the  sides  and  the  operation  had  to  be 
repeated  many  times.  In  addition  the  net  was  inclined  to  fold  flat  when  there  was  little  wind  and  weak  currents,  and  when  the  wind  was 
strong  it  tended  to  rise  to  the  surface,  rendering  it  very  inefficient. 

By  altering  the  shape  of  the  net  and  making  it  more  box-like,  and  by  making  the  sides  more  buoyant,  the  author  has  increased  the 
efficiency  of  the  method,  and  by  using  less  webbing,  he  has  reduced  the  weight  and  made  it  easier  to  handle.  The  author  hopes  that  it  may 
lead  to  a  revival  of  the  saury  fishery  which  has  become  difficult  owing  to  increasing  cost  of  materials. 


Resume 


Amelioration  apportees  aux  epuisettes  soutenues  par  des  piquets,  pour  la  pechc  au  scombrcsoce 


On  capture  habitue! Icment  le  scombrdsoce  au  moyen  d'£puisettes  soutenues  par  des  piquets  fixes  aux  flancs  du  bateau.  Les  poissons 
sont  attirds  dans  le  filet  par  des  lumieres  mais,  en  raison  de  la  construction  du  filet,  un  grand  nombre  de  poissons  s'6chappent  par  les  cotds  et 
J'op^ratipn  devait  etre  rdpetee  un  grand  nombre  de  fois.  De  plus,  lorsqu'il  y  a  pen  de  vent  et  que  les  cou rants  sont  faibles,  le  filet  a  tendance 
a  se  replier  a  plat  ct,  lorsque  le  vent  est  fort,  le  filet  a  tendance  a  rcmonter  vers  la  surface,  ce  qui  lui  fait  pcrdre  la  plus  grandc  partic  dc  son 
efficacite\ 

En  modifiant  la  forme  de  1'epuisette  et  en  la  faisant  ressembler  davantagc  a  unc  boite  el  en  allegeant  les  cot£s,  I'auteur  a  donne  plus 
d'efficacitg  a  la  m£thode.  En  utilisant  moins  de  corde,  il  a  allege  le  filet  et  l'a  rendu  plus  facile  a  manipuler. 

Lc  document  donne  tous  les  details  concernant  le  nouvel  engin  et  contient  de  nombreuses  illustrations.  L'auteur  espdre  quc  cet 
engin  pourra  contribuer  £  redonner  de  la  vogue  £  la  peche  au  scombr£soce,  devenue  difficile  du  fait  du  coiit  croissant  des  materiaux. 

Mejoramiento  del  "cielo"  empleado  en  la  pesca  de  "saury" 
Extracto 

En  la  pesca  de  "saury"  se  utiliza  usualmentc  un  "cielo"  que  cuclga  de  tangones  fijos  al  costado  del  barco.  Los  pcccs  son  atraidos  a  la 
red  mediante  el  empleo  de  luces  pero,  dada  l'i  construcci6n  de  la  bolsa  del  arte,  gran  numcro  de  ellos  escapa  por  los  co&tados,  debiendosc 
rcpetir  esta  operation  muchas  veces.  Ademas,  la  red  tiende  a  plegarse  cuando  hay  poco  viento  o  corrientes  muy  d6  biles  y  a  subir  a  la  super- 
ficic  en  caso  de  soplar  viento,  disminuycndo  considerablemente  su  cficacia. 

Al  modificar  la  forma,  darlc  una  estructura  mas  parecida  a  un  caj6n  y  aumentar  la  flotabilidad  dc  los  costados,  se  Iogr6  una  mayor 
eficacia  con  estc  arte  de  pesca;  por  otra  parte,  el  iiso  de  mcnos  red  icdujo  el  peso  y  facilit6  su  manipulation. 

En  el  trabajo  original  el  autor  incluye  gran  cantidad  de  dctalles  y  numerosas  ilustraciones  dc  la  nueva  red  con  la  esperanza  dc  inducir 
al  cstablecimiento  dc  la  pesca  de  "saury"  quc  se  ha  tornado  dificil  a  causa  del  mayor  costo  dc  los  materiales. 


IT  is  said  that  the  Japanese  saury  fishery  off  the  Pacific 
coast  was  started  about  280  years  ago.  At  that  time 
a  very  ancient  kind  of  blanket  net  was  used,  the 
Yatsude-ami.  This  was  first  replaced   by  a   primitive 
type  of  seine  net  and,  later,  by  drift  nets.  Recently  the 
use  of  the  stick-held  dipnct,  together  with  electric  lamps, 
has  led  to  a  rapid  development  of  the  saury  fishery.  The 
number  of  vessels  employed  exceeds  2,000  and  the  annual 
catch  is  now  375,000  tons. 

Although  the  stick-held  dipnet  is  economical  and 
effective,  and  also  very  efficient  for  taking  other  pelagic 
fish,  it  has  some  defects.  As  a  result  of  his  experience  the 
author  has  invented  a  new  type  of  net. 

PRESENT  STICK-HELD  DIPNET  AND  ITS 
DEFECTS 

The  present  stick-held  dipnet,  as  shown  in  fig.  1  and 
fig.  2,  has  a  flat  form.  It  becomes  baglike  only  under  the 
influence  of  current.  Fishing  with  the  net  is  simple 


because  no  ground  bait  is  needed  and  the  saury  shoals, 
once  attracted  by  light,  do  not  scatter  easily. 

The  operation  is  as  follows: 

The  vessel  arrives  at  the  fishing  ground  at  dusk  and  as 
soon  as  the  sun  sets,  the  search  for  fish  starts.  When  a 
satisfactory  school  of  fish  is  located,  the  vessel  is  stopped 
with  the  fishing  side  to  windward  and  the  lamps  lit  on 
the  opposite  side.  Fish  begin  to  gather  under  the  lamps 
5  to  10  min.  later,  and  when  the  shape  of  the  net,  which 
is  cast  when  the  vessel  is  stopped,  becomes  baglike  (it 
usually  takes  about  5  min.),  lamps  on  the  fishing  side  are 
lit  and  the  others  are  extinguished.  This  induces  the  fish 
to  pass  under  the  bottom  of  the  vessel  into  the  net.  They  are 
then  brailed  into  the  vessel  by  a  scoop  net.  As  all  the  fish 
crowded  around  a  vessel  cannot  be  captured  at  once,  the 
operation  described  above  is  usually  repeated  several  times. 

Although  a  vessel  which  finds  a  large  school  may  be 
fully  loaded  (about  37  •  5  tons)  in  3  to  4  hours,  the  boats 
usually  return  to  port  at  dawn. 


[422] 


JAPANESE     STICK-HELD     DIPNETS 


f/tf.  /.     Present  type  of  stick-held  dipnet. 

The    present    stick-held    dipnet    has    the    following 
defects: 

(1)    The  net  form  is  flat  in  itself  and,  in  spite  of  a  certain 
amount  of  bulging,  it  folds  up  flat  when  wind  and 


Fig.  3.     Newly  devised  type  of  stick-held  dipnet. 

current  are  weak,  and  rises  to  the  surface  when  they 
are  strong. 

(2)  A  large  amount  of  webbing  is  used  to  increase 
bulging  so  that  much  labour  is  needed  to  lift  the  net. 

(3)  Because  of  the  net-form,  the  upper  edges  of  both 
sides  of  the  net  sink  and  70  to  90  per  cent,  of  the 
school  escape  this  way.  Consequently,  the  operation 
must  be  repeated  many  times. 

(4)  As  the  net  sinks  slowly,  it  takes  some  time  before  it 
assumes  the  correct  shape. 


Fix.  2.     Construction  of  present  stick-held  dipnet. 


Fig.  4.     Construction  of  newly  devised  stick-held  dipnet. 


[423] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


Fig.  5.  Newly  devised  type  of  stick-held  dipnet.  I.  Sticks: 
2.  Stretching  line:  3.  WK  floats;  4.  Blocks:  5.  Floats:  6.  Sinker 
lines:  7.  (/)  Fish  fathering  part  of  net,  (2)  Side  parts,  (3)  Bottom. 
(4)  Vessel  side  of  net:  8.  Metal  rings;  9.  Tow  lines  for  setting  the 
net;  10.  Towlines  for  haulinff  the  net:  //•  Towlines  for  hauling 
net  aboard;  12.  Guys. 


PARTICULARS  OF 
NEWLY  DEVISED 
STICK-HELD  DIPNET 

The  newly  devised  stick- 
held  dipnet  was  used  with 
great  success  in  the  Pacific 
and  the  Okhotsk  Sea  in 
October  1952  from  the 
research  ship,  Hokko- 
Maru.  The  greater  part  of 
schools  were  captured  by 
only  one  operation  and 
the  ship  was  fully  loaded 
by  3  to  4  operations.  The 
research  ship  of  the  Hok- 
kaido Regional  Fisheries 
Research  Laboratory, 
Tankai-Maru  No.  3,  cap- 
lured  22-5  tons  in  4 
operations  on  September 
8,  1956,  and  Koyo-Maru 
of  the  Hokkaido  Fisheries 
Experimental  Station, 
48-7  tons  in  4  operations. 
Some  advantages  of 
improved  net  are  as  fol- 
lows: 

(1)  As  the  net  form  is 
cubic,  the  shape  of  the 
net  is  nearly  constant 
irrespective  of  wind 


.  6.   Operation  of  newly  devised 
stick-held  dip  net. 


Fig.  7.     Operation  of  sticks  oj      Fig.  8. 
large  and  medium-sized  vessels. 


Leading  a  school  of  fish 
round  the  how. 


and  current,  thus  increasing  the  fishing  efficiency. 

(2)  The  new  net  required  only  70  to  80  per  cent,  of  the 
webbing  used  in  the  old  type  net  and  only  two  thirds 
of  the  labour  for  lifting  the  net. 

(3)  As  the  net  form  is  cubic  and  the  upper  edge  of  both 
sides  of  the  net  arc  buoyant,  fish  are  induced  easily 
into  the  net  and  cannot  escape. 

(4)  Three  sides  of  the  net  float,  therefore  the  fish  do  not 
escape  even  if  lamps  with  intensive  light  are  used. 

(5)  As  both  edges  of  the  bottom  net  have  lead  sinkers, 
the  net  takes  up  its  proper  shape  more  rapidly. 

(6)  The  fishing  rate  per  operation  is  doubled,  so  the 
time  taken  to  load  the  boat  is  shortened. 

Construction  and  operation 

The  construction  of  the  new  net  is  shown  in  figs.  3  to  5. 
The  net  is  hung  down  in  the  water  from  metal  rings  which 
are  carried  by  two  sticks  (bamboo,  wood  or  metal) 
projecting  from  one  side  of  the  vessel.  Lamps,  reflectors, 
ground  bait  and  so  on  are  used  to  attract  the  fish. 

When  a  school  of  fish  is  located,  the  vessel  is  stopped 
with  the  wind  and  tide  on  the  fishing  side.  Ropes  of 
certain  lengths  keep  the  net  in  position  when  the  tow-line 
(figs.  5  and  6)  is  pulled. 

Just  before  the  operation  begins  two  sticks  (1)  are 
projected  outward  and  are  fixed  by  guy  (12)  ropes,  and 
the  net  is  set  by  pulling  the  tow-lines  (9).  After  the  fish 
have  entered  the  net,  the  tow-lines  (11)  are  pulled,  and 


Fig.  9.     Departure  of  vessels. 


[4241 


JAPANESE    STICK-HELD     DIPNETS 


the  catch  is  safely  confined.  Then,  by  means  of  the  tow 
lines  (10),  the  net  is  hauled  towards  the  vessel  and  fixed 
to  the  ship's  side  (fig.  6,  a-c) 


Fig.  10.     Operation  of  newly  devised  stick-held  dipnei. 


After  the  trapped  fish  have  been  i  railed  by  a  scoop  net, 
operated  from  a  boom,  the  net  is  s?t  again. 

In  the  case  of  medium-sized  and  larger  vessels  the 
following  particulars  are  of  advantage  (fig.  7).  The  sticks 
should  be  made  of  metal.  A  line  (e)  is  stretched  between 
the  two  sticks  to  prevent  any  change  in  the  net  shape,  big 
floats  are  attached  to  the  top  of  the  sticks,  and  the  sticks 
are  connected  with  the  vessel  by  universal  joints  (c). 
Universal  joints  are  particularly  useful  in  case  of  rolling 
and  pitching  in  stormy  weather.  They  also  simplify  the 
shipping  and  unshipping  of  the  sticks. 

In  rough  weather  a  spanker  with  two  sails  is  used  to 
hold  the  vessel  with  the  wind  about  2  points  on  the 
quarter  and  with  the  fishing  side  as  the  weather-side. 
If,  with  larger  vessels,  the  attraction  of  fish  into  the  net 
under  the  bottom  is  impossible  because  of  the  draft,  fish 
are  led  round  the  bow  (fig.  8).  In  this  case,  the  side  of  the 
net  nearest  the  bow  can  be  opened  or  closed  by  means  of 
ropes  and  pulleys,  to  enable  the  fish  to  enter. 

This  improved  gear  is  called  a  moving  dipnet  (Ido- 
Kaku-Ami).  It  is  easily  assembled  and  dismantled.  It 
is  effective  for  fish  such  as  mackerel,  squid  and  sand-eel, 
which  can  be  attracted  by  light,  shadow,  ground  bait 
and  sound. 


Hauling  up  the  bag  of  a  large  Japanese  setnet 

[425) 


Photo:  Japanese  Fisheries  Agency 


THE  METHODS  AND  GEARS  USED  FOR  MACKEREL  FISHING 

IN  JAPAN 

by 

KE1SHUN  MIHARA 

Chiba  Prefectural  Fisheries  Experimental  Station,  Japan 

Abstract 

This  is  a  detailed  account  of  the  hand  line  and  pole  and  line  fisheries  which  are  generally  used  along  the  Japanese  coast.  Tiiere  is 
complete  description  of  the  assembly  and  use  of  the  handlines  and  the  rate  of  fishing  can  be  judged  by  the  fact  that  a  small  boat  of  3  to  5  tons 
can  catch  more  than  2  tons  of  fish  per  day.  At  night,  the  most  efficient  system  is  the  pole  and  line,  which  is  simply  a  bamboo  pole  carrying 
a  line  of  red  or  green  nylon  of  the  same  length  as  the  pole  itself.  The  fish  are  attracted  by  scattering  bait  and  also  by  lights  and  enormous 
rates  of  fishing  are  possible  by  this  method.  A  single  fisherman  can  take  500  to  800  kg.  of  mackerel  per  night,  and  an  80-ton  boat  with  a 
crew  of  45  can  catch  more  than  25  tons  per  night.  The  paper  is  well  illustrated. 


Resume 


Methode  et  engins  utilises  pour  pecher  le  maquereau  au  Japon 


Get  article  d&rit  en  detail  la  peche  aux  palangres  et  aux  lignes  £  canne  courte  utilises  communemcnt  le  long  du  littoral  japonais. 
On  y  trouve  une  description  complete  du  montage  et  de  1'utilisation  des  palangres  et  on  peut  juger  de  I'importance  dc  cette  peche  par  le  fait 
qu'un  petit  bateau  de  3  a  5  tonnes  pent  capturer  plus  de  2  tonnes  de  poisson  par  jour  La  nuit,  le  systcme  le  plus  cfficace  est  la  lignc  a  canne 
courte  qui  est  une  simple  perche  dc  bambou  portant  une  lignc  de  fil  de  nylon  rouge  ou  vert,  de  meme  longueur  que  la  perche  elle-meme.  On 
attire  le  poisson  on  appatant  el  aussi  en  utilisant  des  lumieres;  on  pcut  capturer  ainsi  d'enormes  quantites  de  poisson.  Un  pecheur  operant 
scul  peut  capturer  de  500  a  800  kgs.  de  maquercaux  par  nuit  et  un  bateau  de  80  tonnes  montc  par  45  hommes  pcut  capturer  plus  de  25  tonnes 
par  nuit.  L'article  est  abondamment  illustrt. 

Metodo  y  artes  usados  en  las  pesquerias  japonesas  de  caballa 
Cxtatcto 

Este  trabajo  contiene  una  relaci6n  detallada  y  gran  numcro  de  ilustraciones  sobre  la  pesca  con  Una  y  cafta  usadas  a  lo  largo  de  la 
costa  japonesa.  Se  da  una  descripci6n  completa  de  la  confecci6n  y  uso  del  primero  de  estos  aparejos,  pudicndo  juzgarse  la  proporci6n  de  la 
pesca  por  el  hecho  de  que  una  pequcna  embarcacion  de  3-5  toneladas  pucde  capturar  diariamente  mas  de  2  tons,  de  pescado.  En  la  noche,  el 
metodo  mas  eficaz  es  la  cana  que  consiste  simplemcntc  en  una  vara  de  bambu  con  una  linea  de  nyldn  rojo  o  vcrde  de  la  misma  longitud. 
Los  peces  son  atraidos  esparciendo  carnada  y  tambien  con  luces,  obteniendosc  enormes  cantidades  de  pesca.  Un  solo  pcscador  puede  sacar 
unos  500-800  Kg.  de  caballa  y  una  embarcaci6n  de  80  tons,  con  una  tripulacion  de  45  hombres  mas  de  25  tons,  por  noche. 


MACKEREL  fishing  in  Japan  is  carried  out  by 
surrounding  nets,  longlines,  handlines  and  pole 
and  line,  of  which  handlining  and  pole  and  line 
fishing  are  the  most  common  methods.  Pole  and  line 
fishing  is  generally  used  at  night  and  handlining  during 
the  day. 

These  methods  have  recently  been  improved  and  fishing 
boats  are  now  equipped  with  modern  auxiliary  equip- 
ment which  is  available  at  reasonable  cost.  (Table  I). 


TABLE  I 
Equipment  and  crew  of  mackerel  handlining  boats 


Gross  Tonnage 

1—2 

2-5 

5—10 

10—20 

20—40 

40-80 

Horse  power 
of  Engine 

17—25 

25-45 

30—75 

60—110 

90-200 

200  -250 

Crew 

5—8 

8-10 

8—15 

15—20 

20-30 

30-45 

fish  Finder 

Rare 

Rare 

Common 

Common 

Common 

Common 

Type  of  Radio 

A35-25W. 

A3  10-  35W. 

li  10^35\ 

/ASK: 

HANDLINING 

Handlining,  which  has  developed  most  in  the  Chiba 
Prefecture,  is  practised  in  every  part  of  Japan.  Boats 
from  1  to  40  tons  are  used,  but  the  trend  of  development 
is  toward  bigger  boats.  Artificial  bait  is  particularly 
effective  in  deep  water  where  the  fish  cannot  be  enticed 
up  because  of  low  water  temperature  near  the  sur- 
face. 

Construction  of  the  gear.  Shibuasa,  the  mainline,  is  a 
Japanese  hemp  line  made  of  two  strands  (approximately 
250  g./150  m.).  Cotton  would  not  be  strong  enough,  and 
the  larger  diameter  of  a  cotton  line  of  equal  strength 
would  cause  too  much  resistance  to  water  currents.  Nylon 
is  suitable  but  too  expensive. 

Fishermen  repeatedly  dip  the  hemp  line  in  persimmon 
juice  and  dry  it  in  the  sun.  The  resultant  thin  film  on 
the  line  reduces  resistance  to  currents  and  preserves  the 
rope,  which  retains  its  elasticity  and  strength  and 
becomes  easy  to  handle.  Fishermen  use  300  m.  mainlines 
with  depth  marks  every  30  to  50  m. 


[426] 


JAPANESE    MACKEREL     FISHING     METHODS 


Michi-lto 


ShlbtuM 

< 


Cotton  twine 


(branch) 


Vinylon  fUm 
(protector  of  feather) 


Red  or  pink  vinyl  on  film 
Fig.   3 


cotton  t*  mo 


Michi  -  ito 


Michi-ito 


Y    ^Wooden  boar<T 


Wire  or  bamboo 


2  cm 


Kig.    4 


J:d«-ito 
(branch) 


30  -  40 

I  cm 


Swivel 


Wire 


Cast  -  iron 


Hemp  or  cotton 
thread 


Fig.   6 


Muneyama,  an  intermediate  piece,  is  made  of  three 
strands  of  nylon  monofiJamcnt  weighing  75  g.  per  150  m. 
and  with  0-74  mm.  diam.  The  piece  is  7  to  8  m.  long  and 
is  intended  to  act  as  a  shock  absorber. 

Michi-ito,  the  lower  part  of  the  mainline,  is  usually 
made  of  nylon  monofilament  which  has  a  high  breaking 
strength.  A  draw-back  is  that  the  monofilament  twists 
when  it  is  stretched,  but  this  can  be  avoided  by  boiling 
before  use,  although  such  treatment  slightly  reduces  its 
strength.  The  thickness  of  the  line  varies  according  to 
season,  size  of  fish,  catch,  number  of  hooks  and  current 
strength;  generally  speaking,  a  thinner  line  means  a 
greater  catch.  A  line  of  2  to  2-6fwigara*  (0-74  to  0-84 
mm.  in  diam.)  is  needed  to  carry  50  hooks.  Red  or  pink 
dyed  cotton  twine  (No.  20,  2x3)  is  used  for  fixing  the 
nylon  branches  to  the  line  (fig.  2). 

Nylon  monofilament  between  1  and  1-2  fungara 
(0-52  to  0-57  mm.  in  diam.)  and  10  cm.  Jong  is  used  for 
the  eda-iw.  Snoods  are  attached  to  the  mlchi-iio  (main 
line)  as  shown  in  fig.  2.  The  tying  method  is  important 
for  the  operation  of  the  gear.  The  distance  between  the 
snoods  is  usually  30  to  40  cm.,  depending  on  the  size  of 
fish  to  be  caught. 

The  hook  is  zinc  plated  and  40  to  50  mm.  long  (fig.  3). 
Two  or  three  pieces  of  rump  feather  (5  lo  6  cm.  in  length) 
from  a  white  leghorn  cock,  arc  dyed  red  or  pink  and  are 
attached  to  the  hook.  When  fish  are  plentiful,  fishermen 
also  use  red  or  pink  vinilon  film  (5  70  mm.)  instead  of 
feather.  Each  handline  has  fifty  hooks,  and  2  or  3  lines 
are  put  together  to  make  one  set  of  gear. 

A  gear  coiler,  holding  one  set  of  line  with  hooks,  is 
shown  in  fig.  4.  The  distance  1  is  little  narrower  than  half 
of  the  space  between  the  snoods  while  the  distance  2  is 
equal  to  the  length  of  the  snoods. 

The  sinker  (fig.  5)  is  tied  to  the  end  of  michi-ito  by 
hemp  or  cotton  twine.  It  is  spindle-shaped,  made  of  cast- 
iron  and  weighs  1  to  1  -5  kg.  A  heavier  sinker  is  used 
when  the  current  is  strong  or  more  hooks  are  put  on. 

Yoridome,  the  twist  stopper  (fig.  6),  is  either  a  ring 
(diam.  15  cm.)  made  of  bronze  wire  of  5  to  6  mm.  diam., 
or  a  semicircular  wooden  plate. 

A  spanker  is  used  to  keep  the  boat  "lying-to"  the  wind, 
and  the  engine  helps  to  maintain  that  position  when 
fishing. 

Operation  of  the  gear.  The  fish  are  located  and  the  size  of 
the  school  determined  by  means  of  echo  sounding  (using 
a  fishfinder).  If  the  school  is  big  enough,  the  boat  lics-to 
and  immediately  the  crew  shoot  the  lines  from  the 


I  fungara 


0-3750  gr. 
150  cm. 


Fig.  1.     Construction  of  the  handline  pear. 

Fig.  2.     The  way  of  connecting  the  branchlines  to  the  mainline. 

Fig.  3.     Artificially  baited  hooks. 

Fig.  4.    Line  coiler. 

Fig.  5.     Sinker. 
Fig.   6.     Twist  stopper. 


[427] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


starboard  side.  The  lines  are  veered  to  I  he  required  depth 
while  the  men  hold  the  "gear  coiler"  in  their  hands.  The 
captain,  meanwhile  controls  the  boat  to  prevent  fouling  of 
the  lines.  According  to  the  "feel"  of  the  line  fishermen 
can  assess  where  the  fish  are  biting  and  adjust  the  depth 
accordingly. 

The  line  is  hauled  in  after  passing  through  the  fish 
school  although  the  fishermen  allow  the  line  to  go  down 
again  as  the  hooked  fish  work  against  the  pull.  When 
resistance  is  no  longer  felt,  the  line  is  hauled  up. 

The  lines,  complete  with  hooked  fish,  are  stored  in 
baskets  or  barrels,  and  new  lines  are  shot  as  quickly  as 
possible  because  the  period  in  which  the  fish  bite  is  very 
short.  Later,  when  fishing  is  finished,  the  fish  arc  un- 
hooked and  stored  in  tanks  with  chilled  water. 

Fishing  is  usually  stopped  at  dark  and  the  gear  is 
cleared  while  going  home  or  in  the  port.  For  this  purpose, 
the  michi-ito  part  of  the  mainline  is  unfastened  at  both 
ends  and  the  snoods  removed. 

A  boat  with  a  crew  of  five  fishermen  usually  has  100 
sets  of  micho-ito  with  hooks,  and  a  boat  of  10  to  15  men 
has  250  to  300  sets.  If  lines  get  entangled,  they  are 
pulled  in  together.  A  boat  of  3  to  5  tons  can  often  catch 
over  2  tons  of  fish  a  day.  It  is  estimated  that  380  m.  is  the 
maximum  depth  for  economic  operation  of  this  type  of 
mackerel  handlining. 


POLE  AND  LINE  FISHING 

Pole  and  line  fishing  is  the  most  efficient  mackerel  fishing 
method  at  night.  The  boats  arc  from  1  to  150  tons; 
the  larger  ones  are  equipped  with  D.F.  and  radar. 
The  fishing  grounds  extend  from  2  to  3  miles  off  the 
Japanese  coasts  to  the  East  China  Sea  and  the  northern 
waters  of  Formosa  Island.  The  length  of  a  cruise  may 
range  from  one  night  to  two  weeks.  In  future,  fishing  at 
distant  grounds  may  be  improved  by  the  use  of  a  mother- 
ship. 

The  principle  of  this  method  is  to  attract  the  fish  school 
to  the  surface  by  light  and  by  scattering  bait  (chopped 
sardines).  All  hands  fish  continuously,  using  one  pole 
each.  When  the  school  is  large,  one  man  is  able  to  catch 
from  500  to  800  kg.  a  night.  A  boat  of  80  tons  (250  h.p., 
45  crew)  can  often  catch  more  than  25  tons  of  fish  a 
night  at  a  good  ground,  such  as  in  the  East  China  Sea. 
The  equipment  and  the  method  vary  little  between  the 
boats  of  different  size. 

Construction  of  the  gear.  Bamboo  pole  (fig.  7).  Fishermen 
prefer  bamboo  poles,  produced  in  Japan,  because  they 
are  light  and  pliable.  The  poles  usually  range  from  1  to 
2  m.  in  length,  according  to  the  size  of  boat. 

Fishing  line  and  hook  (fig.  7).  The  mainline,  made  of 
red  or  green  nylon  monofilament  of  1  fungara  (0  •  522  mm. 
in  diam.),  is  exactly  as  long  as  the  pole.  A  hook  is 
joined  to  the  mainline  by  a  10  to  15  cm.  nylon  snood 
(chimoto)  of  0-6  to  0-8  fungara  (0-40  to  0-47  mm.  in 
diam.),  similar  in  colour  to  the  main  line.  In  boats  bigger 
than  20  tons,  the  line  is  30  to  40  cm.  longer  than  the 
pole.  Round  shaped  hooks  of  4-8  cm.  length  (1-6  sun) 
are  generally  used,  but  it  is  better  to  change  the  hook  size 
according  to  the  size  of  fish  available. 


Fig.  7.    Pole  and  line  fishing  gear. 
Fig.  8.     Gaff. 


[428] 


JAPANESE     MACKEREL     FISHING    METHODS 


TABLE  II 
Relation  between  light  equipment  and  tonnage  of  mackerel  pole  and  line  fishing  boat 


Cross  Tonnage 

1 

2-5 

5-10 

10-J5 

15  30 

D.C     1  kW. 

D.C.  2  3  kW. 

D.C.  3-5  kW. 

D.C.  5  7  kW. 

D.C.  7  lOkW. 

Power  Source 

or 

or 

or 

24  V 

24V. 

A.C.  3-5  kVA. 

A.C.  5-7  kVA. 

A.C    7  lOkVA. 

24-tlOV. 

110V. 

1JOV. 

100W. 

100W. 

300  W. 

300  W. 

Candle  Power  and 

or    •    (3-9) 

or    ,    (5-7) 

300  W.    -    (5-9) 

or    -    (8-13) 

or    ,-    (10-19) 

number  of  bulbs 

200  W. 

200  W. 

500  W. 

500  W. 

In  strong  wind,  or  when  the  fish  cannot  be  attracted 
to  the  surface,  a  small  sinker  is  fastened  to  the  snood 
just  above  the  hook  to  sink  it  deeper  and  quicker. 

A  gaff  is  used  to  detach  the  fish  from  the  hook.  It 
consists  of  a  wooden  handle  (20  cm.  in  length,  2  cm.  in 
diam.)  and  a  piece  of  nickel  silver  wire  (2  mm.  in  diam.), 
bent  as  shown  in  fig.  8. 

All  boats  are  equipped  with  D.C.  or  A.C.  generators, 
with  automatic  voltage  regulators,  to  provide  light  for 
attracting  the  fish.  Incandescent  bulbs  arc  normally 
used  but  recently  fluorescent  lighting  has  been  tested.  A 
final  opinion  on  this  lighting  has  not  yet  been  formed. 
The  relation  between  light  equipment  and  tonnage  of 
boats  is  shown  in  Table  II. 

Most  important  for  the  efficiency  of  electric  bulbs  is 
the  angle  of  their  reflectors.  The  best  angle  is  considered 


Lower  -  edge 


Hg.  9.    Spanker. 
Fig.  W.     Method  of  baiting  the  hook. 


to  be  one  which  illuminates  the  water  from  the  boat  to 
the  end  of  the  line.  But  when  many  boats  are  together,  the 
illuminated  area  must  be  enlarged  to  prevent  fish  being 
attracted  by  the  light  from  other  boats,  so  light  reflectors 
are  hung  above  the  heads  of  the  men  sitting  on  the  boat 
side. 

The  spanker  (fig.  9)  consists  of  two  sails,  and  is  used 
very  effectively  for  both  line  and  net  fishing.  It  is  an 
effective  rudder  in  the  wind,  so  it  is  better  to  remove  the 
ship's  rudder  while  fishing.  The  rudder  usually  can  be 
unshipped  in  boats  of  less  than  40  tons. 

The  chopper  is  a  mincing  machine,  worked  from  the 
engine,  to  prepare  the  bait  for  scattering. 

There  are  two  kinds  of  bait,  i.e.  for  the  hooks  and  for 
scattering.  Bait  for  the  hook,  tomoe,  is  mackerel  meat,  off 
the  side  of  the  mackerel,  10  mm.  in  width,  50  to  60  mm. 
long  and  2  to  3  mm.  thick.  Ten  such  pieces  can  be  taken 
from  on  side  of  a  fish  of  500  gr.  The  pieces  are  fixed  on 
the  hooks,  skin  inside,  meat  outside  (fig.  10). 

Chum  bait  is  usually  made  from  frozen  sardines, 
ground  by  the  mincer  and  mixed  with  water.  Fat  sardines 
are  favoured  because  the  meat  does  not  sink  quickly. 
Fishermen  usually  expend  about  350  to  400  kg.  of  bait 
to  catch  8  tons  of  mackerel. 

Operation  of  the  gear.  The  baited  hook  is  thrown  toward 
the  fish  and  the  pole  is  raised  as  soon  as  it  approaches. 
After  a  fish  is  hooked,  the  fishermen  draw  the  line 
through  the  gaffhook  and  force  the  mackerel  to  drop 
off  the  hook  on  to  the  deck,  and  immediately  throw  hook 
and  line  out  again.  Only  about  2  seconds  are  necessary 
to  land  a  fish  and  fishing,  therefore,  is  practically  contin- 
uous. If  the  fish  cease  to  react  well  to  the  bait,  it  is 
renewed  at  once. 

The  men  fish  from  one  side  of  the  boat  only  when  the 
schools  are  scattered.  This  saves  bait,  makes  the  best  use 
of  manpower,  and  simplifies  control.  They  fish  from  both 
sides  of  the  boat  when  schools  are  dense. 

Chopped  bait,  diluted  by  sea  water,  is  thrown  into  the 
water  above  the  fish  school  by  hand  or  by  spoons. 
Usually  one  man  of  a  team  of  3  to  5  scatters  the  bait. 
The  mixture  of  water  and  chopped  meat  is  adjusted  to 
regulate  the  speed  at  which  it  sinks.  The  bait  must  be 
scattered  very  carefully  to  keep  the  fish  near  the  surface, 
and  ensure  good  fishing. 

The  captain  uses  the  spanker  and  the  engine  to  keep  the 
boat  lying-to,  maintaining  constant  contact  with  the 
school.  Fishing  ceases  at  dawn  when  the  boats  return 
to  port. 


[429] 


A  NEW  METHOD   OF   HANDLING   LONGLINE   GEAR 
A   DESCRIPTION   OF  POFI   "TUB"   GEAR 

by 

HERBERT  J.  MANN 

Pacific  Oceanic  Fishery  Investigations,  Honolulu,  Hawaii,  U.S.A. 


Abstract 

Modifications  to  Japanese-type  longlinc  gear,  by  the  introduction  of  a  large,  rotating  tub  from  which  the  line  is  handled,  have 
resulted  in  a  considerable  saving  in  manpower.  Conventional  gear  can  be  operated  in  this  manner  with  few  changes.  Instead  of  using 
individual  baskets  of  line  which  must  be  separated  and  joined  together  for  each  day's  fishing,  a  continuous  mainline  is  set  from,  and  hauled 
into,  a  wooden  storage  drum.  This  innovation  is  the  product  of  continuing  studies  to  increase  the  efficiency  of  longline  gear. 


Resume 


Unc  nouvelle  m£thodc  de  manipulation  des  palangres  Description  de  la  bailie  a  lignes  POFT 


On  a  obtenu  une  importante  economic  de  main-d'oeuvre  grace  a  dcs  modifications  apportees  aux  palangres  japonaises  par  1'intro- 
duction  d'une  grande  bailie  tournante  dans  laqucllc  se  trouvent  les  ligncs.  L'engin  habitue!  peut  etre  manoeuvre  de  cette  fa^on  avec  peu  de 
changements.  Au  lieu  d'utiliser  des  paniers  individuels  de  lignes  qui  doivent  etre  desassemblecs  cl  reassemblees  chaque  jour  pour  la  peche,  on 
met  £  1'eau  et  on  recupere  dans  une  bailie  de  bois  une  ligne  principale  continue.  Cetle  innovation  est  le  resultat  de  recherches  en  cours  pour 
augmenter  le  rendement  des  palangres. 


Extracto 


Nuevo  metodo  para  manipular  palangres.   Description  del  "tambor"  POFI  para  enrrollar  palangres 


Las  modificaciones  introducidas  a  los  palagres  de  tipo  japoncs  mediante  el  uso  de  un  "tambor"  rotatorio  desde  cl  dial  se  manipula 
estc  aparejo  de  pesca  economiza  baslante  mano  de  obra;  adcmas  permite  usar  unidades  de  tipo  convcncional  con  solo  pequenas  modificaciones. 
En  lugar  del  empleo  de  varias  cuerdas  mad  res  que  es  necesario  unir  y  separar  en  cada  salida.  se  procede  a  calar  y  a  Icvantar  una  sola  nlma- 
cenada  en  el  "tambor"  de  madera.  Hsta  innovacion  es  el  producto  de  continuos  estudios  para  aumentar  la  eficacia  dc  los  palangres. 


THE  Pacific  Oceanic  Fishery  Investigations  (POFI) 
of  the  U.S.  Fish  and  Wildlife  Service  has  found 
longlining  to  be  an  effective  means  of  catching 
tunas  in  the  relatively  baitless  waters  of  the  central 
Pacific.  This  report  describes  a  new  method  of  fishing 
longline  gear  recently  developed  at  POFI  following  a 
suggestion  by  Mr.  A.  K.  Akana  Jr.,  POFI  fleet  supervisor. 
The  longline  gear  previously  used  by  POFI  was  copied 
from  conventional  Japanese  designs  and,  although 
adequate  as  a  sampling  tool  for  exploratory  fishing,  it 
required  too  many  men,  by  American  standards,  to  be 
ideal  for  commercial  use.  The  so-called  "tub"  gear 
described  below  is  the  most  successful  effort  to  date  to 
improve  the  efficiency  of  gear  in  this  respect,  in  that  it 
permits  a  material  reduction  in  the  manpower  required 
to  operate  longline  gear. 

The  essence  of  the  tub  method  lies  in  a  novel  means 
of  handling  the  mainline.  Instead  of  the  individual 
baskets  of  conventional  gear,  which  must  be  separated 
and  joined  together  for  each  day's  fishing,  tub  gear 
has  a  mainline  of  one  continuous  length  which  is  shot 
from  and  hauled  into,  a  large  wooden  storage  drum  or 
"tub".  Branchlines  and  floatlines  are  removed  from  the 


mainline  as  they  come  aboard  and  are  reattached  during 
shooting  operations. 

The  tub  method  requires  only  minor  changes  in  the 
design  of  conventional  longline  gear.  Standard  POFI 
gear,  or  any  longline  with  detachable  branchlines  and 
floatlines,  may  be  used  by  adding  "D"  rings  (described 
below)  to  the  mainline. 

DESCRIPTION  OF  THE  GEAR 

The  gear  is  of  a  flexible  design  so  that  it  can  be  altered 
readily  to  meet  the  changing  requirements  of  an  ex- 
ploratory fishing  programme.  Components  are  detach- 
able and  can  be  assembled  so  as  to  fish  varying  numbers 
of  hooks  at  different  depths. 

Shown  in  fig.  1  is  a  schematic  representation  of  a 
single  unit,  or  basket,  of  POFI  gear  of  the  latest  type. 
Sixty  to  a  hundred  such  baskets  are  joined  together  to 
make  a  fishing  set. 

Mainline 

The  mainline  of  each  unit  is  made  up  of  fourteen  identical 
sections  knotted  together  by  double  sheet  bends.  Each 


[430] 


POFI    LONGLINE    TUB    GEAR 


WIRE 


BEAD 


\ 

SWIVEL 

-  SWAP 

BRANCH 
LIMB 


BRANCHLINB 


POLES 


METHOD  OF  ATTACHIJTO 
BRAHCHLUIES  TO  MA1ITLINE 


fir.  /. 


SHOOTING 
FIRS 


Schematic  drawing  showing  the  arrangement  of  a  single  unit,  or  "basket",  of  POH  hngline  gear,  and  the  location  of  the  line 

storage  tub  on  the  stern  of  the  vessel. 


section  consists  of  a  15  fm.  length  of  preserved  261 
thread  hard-laid  cotton  twine  with  an  eye  splice  in 
one  end  and  a  wire  bridle  with  swivels,  "D"  ring,  and 
pigtail  at  the  other.  Baskets  are  joined  by  a  clover-leaf 
knot  which  is  formed  with  an  extra  loop  for  attachment 
of  the  floatline. 

Attachment  of  branchlines 

The  branchlines  are  attached  to  the  mainline  by  the 
"AK"  snap  and  "D"  ring  arrangement  shown  in  fig.l. 
The  "D"  ring  eliminates  a  troublesome  defect  of  old 
gear.  Formerly,  %<AK"  snaps  were  clipped  directly  to 
a  wire  bridle.  This  type  of  attachment  permitted  the 
branchline  to  swivel  around  the  mainline  and  prevented 
tangles  when  gear  was  being  hauled  aboard  ship.  But 
sometimes  it  failed  to  function  properly  when  large 
fish  pulled  the  branchline  parallel  to  the  mainline.  At 
acute  angles  of  pull,  the  snap  ceased  to  have  any  swivell- 
ing action  and  sometimes  was  pulled  out  of  shape  or 
broken.  The  "D"  ring  maintains  proper  swivelling 
action.  Rings  are  fabricated  from  stainless  steel  tubing 
with  a  lower  section  formed  in  a  U  shape  and  welded 
to  the  upper,  straight  tube  section  through  which  the 
bridle  is  rove.  The  wire  bridles  are  made  of  a  6  in. 
length  of  3/32  in.  diameter  7x7  stainless  steel  wire 
rope  with  swivels  connected  to  each  end  by  means 
of  Nicopress  fittings.  These  fittings  consist  of  malleable 


copper  sleeves  which  are  pressed  on  the  wire  by  a  hand 
tool.  Brass  beads  threaded  on  the  wire  on  both  sides 
of  the  "D"  ring  act  as  miniature  thrust  bearings,  and 
aid  the  swivelling  action  of  the  "D"  ring.  The  brass 
swivels  relieve  torque  on  the  mainline  which  occurs 
when  the  line  is  being  brought  in  by  the  hauler. 

Branchlines,  snoods  and  hooks 

The  branchlines  are  made  of  2  fm.  lengths  of  261  thread 
line.  An  "AK"  snap  is  spliced  into  the  upper  end. 
The  lower  end  terminates  in  an  eye  splice  for  securing 
snood  and  hook.  The  snoods  are  fashioned  of  6  ft. 
lengths  of  -066  in.  diameter  7  strand  galvanised  wire. 
The  upper  end  of  the  snood  is  fitted  with  a  section  of 
I  in.  rubber  tubing  which  serves  as  chafing  gear.  The 
hooks,  8/0  or  9/0  tinned,  are  of  a  special  shape  with 
bent  shank  which  allows  them  to  hang  in  line  with 
the  snood. 

Floatlines 

Floatlines  arc  made  from  10  fm.  lengths  of  261  thread 
line.  The  lower  end  of  each  line  has  an  "AK"  snap  for 
attachment  to  the  mainline.  Snaps  are  clipped  to  loops 
in  the  line,  rather  than  to  "D"  rings,  since  it  has  been 
found  that  "D"  rings  and  wire  bridles  do  not  hold  up 
under  the  incessant  jerking  caused  by  the  rise  and  fall 
of  floats  with  sea  and  swell. 


[431  ] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


Storage  tub 

Shown  in  fig.  1  is  a  schematic  drawing  of  the  wooden 
tub  used  for  storage  of  the  mainline.  The  experimental 
model  used  aboard  the  Fish  and  Wildlife  vessel,  "John 
R.  Manning",  is  a  double-walled  plywood  cylinder 
measuring  12  ft.  in  diameter  and  4  ft.  in  depth.  The 
capacity  is  sufficient  for  the  storage  of  100  baskets  of 
gear.  For  greater  ease  in  shooting  the  tub  is  mounted 
near  the  stern  of  the  vessel,  but  adequate  space  between 
tub  and  railing  is  left  all  around.  The  drum  is  supported 
in  the  centre  by  a  heavy  bearing  bolted  to  the  deck; 
the  outer  edge  runs  on  cast  iron  rollers  of  the  type  used 
to  support  the  turntable  of  a  purse  seining  vessel. 
Reinforcing  bands  of  3  in.  strap  metal  are  welded  around 
the  outside  of  the  tub  for  extra  support,  and  33  stainless 
steel  shooting  pins  are  spaced  at  equal  intervals  inside  the 
tub. 

The  gear  is  hauled  by  means  of  a  longline  winch  of 
conventional  Japanese  design.  Electrically  powered  by 
a  3  h.p.  motor,  the  winch  brings  the  line  in  at  a  maximum 
rate  of  about  1,000  ft./min.  and  coils  it  down  automatic- 
ally with  only  slight  assistance  from  the  winch  operator. 
The  hauler  is  mounted  so  that  the  line  is  thrown  just 
over  the  edge  of  the  tub. 

The  shooting  trough,  similar  in  design  to  that  used 
in  the  Northwest  Pacific  halibut  fishery,  is  a  demountable 
sheet  metal  form  used  to  guide  the  outgoing  mainline 
during  shooting. 

OPERATION  OF  THE  GEAR 
Preparation 

Before  sailing,  baskets  of  mainline  are  joined  together 
and  fed  through  the  Japanese  line  hauler  into  the  tub. 
No  attempt  is  made  to  coil  the  line  down  uniformly 
into  regular  piles,  but  it  is  deflected  by  hand  so  that  it 
is  evenly  distributed  in  thickness  over  the  bottom  of 
the  tub.  As  the  44D"  rings  pass  through  the  hauler, 
they  are  caught  and  threaded  upon  the  shooting  pin 
nearest  the  hauler.  All  fourteen  rings  of  one  basket 
are  placed  on  the  same  pin  and  the  knot  marking  the 
end  of  the  basket  is  looped  on  top.  The  tub  is  then 
turned  by  hand  until  the  next  shooting  pin  comes  in 
line  with  the  winch.  To  avoid  piling  up  the  line,  the 
baskets  are  spaced  on  every  other  pin;  when  the  tub 
has  completed  one  revolution,  a  second  and  finally  a 
third  layer  of  line  is  laid  over  the  first  so  that  99  baskets 
are  held  on  33  shooting  pins.  The  tub  is  covered  with 
a  canvas  tarpaulin  to  prevent  the  line  from  being  dis- 
placed in  rough  weather. 

Shooting 

When  shooting  the  tub  is  rotated  by  hand  until  the 
uppermost  basket  comes  under  the  shooting  trough. 
The  vessel  is  steered  on  a  steady  course  at  a  speed  of  up 
to  9  knots.  The  buoy  and  floatline,  marking  the  end  of 
the  set,  are  attached  to  the  mainline,  which  in  turn  is 
led  through  the  trough,  and  the  assembly  is  thrown 
overboard.  Thereafter  the  drag  of  the  gear  pulls  the 
mainline  overboard  as  the  vessel  proceeds. 

Hooks  are  baited  with  sardines,  Sardinops  cacrulea 


(Girard),  or  herring  Clupea  pallasii  (Valenciennes). 
Individual  branchlines  are  attached  to  the  mainline 
by  clipping  the  "AK"  snap  at  the  upper  end  of  the 
branchline  to  the  uppermost  "D"  ring.  They  arc  then 
laid  in  the  shooting  trough  one  by  one  with  the  hook 
and  bait  dangling  overboard.  Floatlines  arc  attached 
in  the  same  manner.  As  the  end  of  each  basket  leaves 
the  shooting  pin,  the  tub  is  turned  to  keep  successive 
baskets  in  line  with  the  shooting  trough. 

Hauling 

The  gear  is  hauled  through  side  rollers  on  the  starboard 
side  of  the  vessel.  The  mainline  is  led  through  the  hauler 
and  coiled  into  the  tub.  As  branchlines  come  aboard, 
the  winch  is  stopped  momentarily  and  the  "AK"  snaps 
are  removed  from  the  "D"  rings.  The  branchlines  are 
coiled  in  specially  built  plywood  boxes  and  are  stowed 
with  hooks  exposed  on  one  side  and  with  the  4kAK" 
snaps  held  by  clips  on  the  other.  Fish  are  gaffed  and 
brought  aboard  through  an  opening  in  the  bulwark. 
The  line  is  stowed  in  the  tub  as  described  before,  with 
three  baskets  of  "D"  rings  being  placed  upon  each  pin. 
Broken  or  tangled  portions  of  mainline  are  removed 
by  unknotting  the  line  section  junctions,  repaired,  and 
joined  to  the  end  of  the  mainline  when  fishing  finishes. 
Because  the  sections  are  identical  they  need  not  be 
restored  to  their  original  positions. 

CONCLUSIONS 

Tub  longlining,  in  particular,  seems  applicable  to  small 
vessels  carrying  a  limited  number  of  men. 

The  saving  in  labour  is  considerable.  Since  a  basket 
of  POFI  gear  weighs  about  40  Ib.  when  wet  and  a 
day's  set  may  consist  of  up  to  l(K)  baskets,  about  4,000  Ib. 
of  wet  line  would  have  to  be  cleared  out  and  rcstowed 
by  hand  for  each  day's  fishing,  when  using  conventional 
methods.  The  POFI  tub  method  eliminates  all  man- 
handling by  flaking  the  line  directly  into  the  tub. 

Five  men  were  found  sufficient  to  operate  the  gear, 
as  against  a  crew  of  11  men  required  for  conventional 
gear.  Both  methods  require,  for  each  basket  of  line, 
about  U  min.  for  shooting  and  3i  min.  for  hauling. 

Some  difficulties  in  operation  have  been  experienced, 
the  most  troublesome  being  excessive  wear  and  tear  on 
the  mainline,  due  to  the  "D"  rings  running  at  high 
speed  through  the  overside  rollers  and  longline  hauler. 
At  high  hauling  speeds,  "D"  rings  strike  the  rollers  or 
hauler  with  enough  force  to  bend  the  rings  or  break 
wire  bridles.  Much  needed  is  a  method  of  joining 
branchlines  to  the  mainline  by  a  system  which  permits 
swivelling  freely  in  any  direction  but  which  offers  little 
resistance  to  passage  through  the  line  hauler. 

One  difficulty  anticipated,  which  failed  to  materialize, 
was  fouling  of  the  mainline  during  shooting  operations. 
Provided  that  the  shooting  trough  is  properly  designed, 
the  outgoing  mainline  runs  smoothly  in  it  and  fouling 
is  less  than  when  shooting  by  conventional  methods. 

All  POFI  tubs  arc  rotated  manually.  Mechanical 
rotation  has  not  yet  been  installed  but,  if  used  com- 
mercially, powered  tubs  should  increase  efficiency. 


[432] 


DRIFTING   LINES  FOR  LARGE  PELAGIC   FISH 


by 

M.    SANCHEZ    ROIG 

Institute  Nacional  de  la  Pesca,  Habana,  Cuba 

Abstract 

This  paper  describes  a  floating  longlme  which  is  left  free  to  drift  independently  of  the  boat  handling  it.  It  is  used  for  the  capture 
of  pelagic  fishes,  such  as  swordfish,  and  different  species  of  marlin,  bill-fishes,  sharks  and  big  tuna,  in  the  open  sea. 

tach  fishing  boat  works  with  several  sections  of  line  but  independently  from  one  another  and  at  a  certain  distance  apart,  usually 
about  1 50  to  200m. 


Resume 


Les  palangrcs  derivantcs  pour  les  grands  poisons  pelagiques 


L'autcur  dccrit  une  palangre  flottantc  qu'cn  laisse  deriver  librement  sans  relation  avec  le  bateau  qui  la  pose.  File  est  utilisee  pour  la 
pcche  des  grands  poissons  pelagiqucs  tels  quc  Tespaden,  le  marlin,  le  voilier,  les  requins  el  Ics  grands  thons  en  haute  mer.  Chaque  bateau  met 
plusieurs  ligncs  a  I  cau,  gcneralement  a  150-120  m.  Tune  de  I'uulrc  et  elles  sont  transportees  sur  une  grande  surface  dc  la  mer  par  les  courants. 
Chaque  ligne  se  compose  d'uns  grande  bouee,  une  ligne  principale  ct  trois  ou  quatre  petites  bouees  portant  chacune  une  ligne  de  peche  avec 
avacon  de  til  d'acicr  ct  hamecon.  Les  lignes  sont  marquees  par  dcs  pavilions  le  jour  et  par  des  lampes  la  nuit. 


Los  palangrcs  de  boyas  para  pesca  en  deriva  de  grandes  especies  pelagicas 
Extracto 

Fn  este  trabajo  sc  describe  un  palanpre  flotantc  de  deriva  para  la  pesca  dc  grandes  especies  pelagicas  como  emperador,  aguja,  tiburon 
y  atun  de  gran  talla  en  alia  mar.  Cada  beta  lienoe  varias  secciones  de  palangre  que  se  situan  a  150  o  200  m.  aparte  unas  de  otras,  yson 
arrastradas  a  gran  distancia  por  las  corrientcs.  La  seccion  esta  compucstapor  el  siguiente  equipo:  una  boya  de  caja  de  aire,  la  cuerda  madre, 
3  o  4  boyarincs,  cordclcs  de  pesca  y  "pclos"  dc  alambre  dc  acero  galvamzado,  banderoles  para  locali/arla  de  diao  y  faroles  que  permilcn 
determinar  su  posicion  duranic  la  noche. 


LARGE  pelagic  fish  such  as  the  several  species  of 
sail-fish,  swordfish,  sharks,  and,  occasionally  out- 
size tunas  have  for  many  years  been  caught  by 
Cuban  fishermen  along  the  north  coast  of  the  province  of 
Havana  and  zones  close  to  Pinar  del  Rio  and  Matanza, 
with  small  boats  measuring  from  16  to  25  ft.  (5  to  7-  5  m.). 
The  boats  carry  a  crew  of  two  men,  hooks  being  usually 
baited  and  the  gear  generally  prepared  before  sailing. 
When  bait  is  not  available  on  the  market,  the  fishermen 
first  go  fishing  for  sandeels  (Ma/acanthus  plumeri, 
Bloch)  which  they  then  use  as  bait.  These  are  found  on 
sandy  bottom  in  between  12  and  30  fathoms  depth. 

Usually  three  sets  of  handlines  were  operated;  one 
called  "el  hondo",  which  was  the  longest,  reaching  a 
greater  depth,  and  taken  care  of  by  the  fisherman  seated 
at  the  stern;  the  other,  called  "de  la  mano",  attended 
by  the  skipper  from  the  central  seat,  and  the  third  on 
the  prow,  fairly  short  and  suspended  from  a  pole. 

THE   FISHING    GEAR 

From  the  above  system  the  presently  used  method  was 
evolved  which  consists  of  fishing  several  independent 
sections  of  "short  longlines"  with  four  hooks  each. 
Each  motorboat  (see  fig.  1)  can  set  from  ten  to  fifteen 
sections  (40  to  60)  hooks.  Each  section  of  line  consists 


of  a  mainline  made  of  hard  twisted  cotton  of  7  mm. 
diameter,  to  which  the  four  branch  lines  made  of  No. 
180  extra  hard-laid  cotton  are  attached. 


/•/#.    /.     Line  boat  with  slacked  gear. 


[433] 


DO 


MODERN     FISHING     GEAR     OF    THE    WORLD 


When  fishing  for  shark  or  swordfish  each  branchline 
has  a  snood  made  of  galvanised  wire  cable  with  a  breaking 
strength  of  920  kg.,  and  No.  2  heavy  type  steel  wire  to 
which  the  No.  16  Mustad  hooks  are  attached  (see  fig.  2). 
When  fishing  for  smaller  fish  species  lighter  snoods  and 
smaller  hooks  are  used. 

Each  section  has  a  marker  buoy  with  flag.  These  buoys 
are  60  cm.  square  boxes  and  about  9  cm.  high;  they  are 
constructed  of  marine  plywood  1-5  cm.  thick  over  a 
hardwood  frame,  copper  nailed.  They  arc  made  water- 
tight by  closing  all  joints  with  marine  glue  and  several 
coats  of  good  quality  paint.  In  one  corner  a  pole  pot  is 
built  in  to  carry  the  marker  flagpole.  The  flagpole  is 
1-60  cm.  in  length  and  carries  a  counterweight  at  the 
lower  end.  Each  buoy  has  2  lights  during  night,  an  oil 
lamp  and  an  electric  light  fitted  to  the  pole  top.  These 
buoys  are  very  strong  and  can  withstand  the  high 
pressure  arising  when  the  buoy  is  pulled  under  by  large 
fish. 

At  each  intersection  of  the  branchlines  with  the 
mainline  a  small  float  is  attached;  these  are  made  of 
solid  wood  of  high  buoyancy  and  have  a  truncated 
pyramidal  shape,  the  base  having  about  13  cm.  side  and 
about  50  cm.  long. 

GENERAL   INFORMATION 

Off  the  Coast  of  Cuba  the  migration  of  While  Marlin 
(Makaira  alhida,  Pocy)  reaches  its  peak  from  April  to 
July.  In  the  case  of  Blue  Marline  (Makaira  ampla,  Poey) 


Fip.  2.     The  hooks  used  in  this  fishery. 


Fig.  3.     Method  oj  attaching  the  bait. 


t-'ig.  4.     Salted  bait  hooked,  ready  Jor  fishing. 


;434] 


DRIFTING     LINES     FOR     LARGE    PELAGIC    FISH 


from  July  to  October  or  November,  but  increase  with 
the  arrival  of  the  first  North  winds.  Swordfish  is  caught 
all  the  year  around,  at  night,  but  mainly  during  the 
Winter  months.  The  days  when  the  catches  are  abundant 
depend  upon  the  intensity  of  marine  currents,  which 
generally  have  a  West  to  East  direction  and  a  rate  of 
about  2i  knots.  Best  catches  are  made  during  strong 
tides  due  to  the  movement  in  the  branch  lines  which 
tend  to  hang  vertical  during  slack  tides. 

On  clear  nights,  with  intense  moonlight,  swordfish 
leave  the  surface  layer  and  longer  branch  lines  are  used 
to  fish  deeper  than  on  dark,  moonless  nights. 

As  bait  any  "white  fish"  of  adequate  size  may  be  used. 
Among  the  appropriate  species  are:  Spanish  Mackerel 
(Scomheromorus  maculatus*  Mitchell),  Bonetish  (Albula 
vulpes,  Linnaeus),  Striped  Mullet  (Mugil  ccphalus)  and 
other  similar  species  such  as  the  Barracuda  (Sphyraena 


guachancho,  Cuvier  et  Valenciennes).  The  bait  should  go 
over  the  hook  so  that  it  covers  the  shank;  only  whole 
fish  are  used  and  usually  placed  on  the  hook  with  the 
head  down  while  the  tail  is  tied  to  the  wire  to  fix  it  in 
position  (see  fig.  3).  Smaller  bait  such  as  sardine  are 
baited  through  the  body  or  through  the  eyes. 

FISHING   OPERATION 

When  a  large  fish  strikes,  it  submerges  pulling  the 
buoy  and  rest  of  the  line  down  with  it.  When  this  is 
observed  from  the  boat  the  fishermen  take  up  the  gear 
and  play  the  line  until  the  fish  can  be  brought  to  surface 
exhausted.  The  fish  is  then  gafled,  stunned  or  killed  and 
finally  hauled  aboard.  The  sections  of  line  are  set  at 
100  to  150  m.  distance,  so  that  a  boat  working  10 
sections  covers  a  distance  of  about  a  mile. 


Shark  longline  being  hauled  mechanically  on  a  22ft.  open  motor  hoat  by  a  h"A  O  expert  off  the  Coromandel  coast  (India}.    Photo:  FAO. 


435  ] 


POWER   HAULER   FOR  LONGLINES 

by 

IZUI  IRON  WORKS  CO.  LTD. 

Tokyo,   Japan 

Abstract 

The  luna  longline  fishery  in  Japan  has  been  made  much  more  effective  since  the  first  line  hauler  was  made  in  1923.  The  old  method 
of  hauling  by  hand  was  extremely  slow  and  as  much  damage  was  done  to  the  fishermen's  hands,  continuous  working  was  impossible,  and  the 
fishery  began  to  decline.  After  many  trials  and  numerous  failures,  the  hauler  was  perfected  and  there  are  now  more  than  8,000  in  use  in  Japan, 
Brazil,  Hawaii  and  the  United  States.  The  hauler  is  operated  from  a  self-contained  motor  or  an  auxiliary  engine  through  a  counter-shaft. 
It  takes  3  h.p.  and  hauls  at  a  speed  of  150  to  180  m.  /  min.,  and  depending  on  the  weather  conditions  and  the  construction  of  the  gear  and  the 
vessel,  it  may  take  up  to  12  hrs.  to  haul  the  300  units  of  lines  which  constitute  the  usual  fleet.  Bach  unit  consists  of  about  200  fm.  of  mainline 
so  the  total  length  of  line  used  is  about  60,000  fm.  The  line  hauler  consists  of  three  parts,  the  lower  one  containing  the  molor  and  the  gears, 
the  middle  one  housing  the  speed  governor  which  adjusts  the  tension  on  the  line  and  prevents  damage,  and  the  upper  part  holding  the  set 
of  three  pulleys  which  wind  in  the  line  automatically. 


Resume 


Treuil  a  paiangres  Izui 


Le  rcndement  de  la  peche  au  tnon  a  la  palangre  a  considerablcment  augmcnte  an  Japon  depuis  I'apparition  du  prcmiei  treuil  & 
palangrc  en  1923.  L'ancienne  methodc  de  halage  a  la  main  etait  cxtremement  Icnte,  et  comme  elle  blessait  Jcs  mains  tics  necheurs  il  etait 
impossible  d'assurer  un  travail  continu  et  cctlc  peche  sf£tait  mise  a  decliner.  Apres  un  grand  nombre  d'essais  infructueux,  le  treuil  a  etc  mis 
au  point  et  il  en  existe  actuellement  plus  de  8.000  en  service  au  Japon,  au  Bresil,  &  Hawai  et  aux  Etats  Unis.  l.e  treuil  est  actionnc  par  un 
motcur  electrique  faisant  corps  avec  Tapparcil,  ou  par  un  moteur  auxiliaire  et  un  arbre  de  transmission.  II  absorbc  une  puissance  de  3  CV 
et  vire  la  palangre  a  raison  dc  150  a  180  metres-minute;  suivant  les  conditions  du  temps  el  Ic  type  d'cngin  et  de  navire,  il  permet  de  rentrer 
a  bord  en  12  heures  au  maximum  les  300  paiangres  qui  constituent  jVquipement  normal  d'un  thonnier.  Chaque  palangre  comporte  environ 
200  brasses  de  ligne  principale  en  sorte  quo  la  longueur  totale  dcs  ligncs  atteint  60,000  brasses  environ.  Le  treuil  a  palangrc  se  compose  de 
trois  elements:  Pelemenl  inferieur  qui  renferme  le  moteur  et  les  engrcnages;  I'element  central  qui  comporte  le  regulateur  de  vitressc  qui  regie 
la  tension  dc  la  ligne  et  empechc  qu'elle  soit  endommagee;  ct  I'clcment  superieur  sur  lequel  sont  monies  les  trois  tambours  qui  recupercnt 
automatiquement  la  ligne. 

Chigre  para   levantar  paiangres 
Extracto 

A  parlir  de  1953  en  que  se  fabric6  el  primer  chigre  para  levantar  paiangres,  la  pesca  de  atiin  con  este  arlc  se  hace  en  forma  mas 
efectiva.  HI  antiguo  metodo  de  i/arlo  a  mano  era  extrcmadamenlc  lento  y  daftaba  considerablemente  la  mano  de  los  Pescadores  imposibilitando 
el  trabajo  en  forma  continua,  lo  cual  influy6  en  la  decadcncia  de  eslas  pesqucrias. 

Dcspues  dc  muchos  ensayos  y  fracases  se  Iogr6  perfeccionar  un  chigre  con  un  motor  acoplado  directamcnte  o  uno  auxiliar  que  se 
conecto  mediante  un  contraeje.  Estc  equipo  permite  recogcr  entrc  150  y  180  metros  de  cuerda  madre  por  minuto  segun  el  estado  del  tiempo, 
la  construction  del  artc  y  del  barco,  yen  12  horas  podria  izar  a  bordo  un  palangre  corricnte  formado  por  300  unidadcs  o  canaslos  de  200  brazas 
que,  una  vez  unidos,  dan  una  longitud  de  60.000  brazas.  El  chigre  consta  dc  3  paries:  la  inferior  con  el  motor  y  engranajes,  la  del  medio 
que  encierra  el  regulador  de  vclocidad  para  ajustar  la  tensi6n  del  arte  e  impedir  que  se  dafte,  y  la  superior  que  sostienc  las  3  poleas 
desttnadas  a  izar  automaticamento  la  cuerda  madre. 

En  la  actualidad  existen  unos  8.000  aparatos  como  el  descrito  en  Brasil,  E.U.A.,  Hawaii  y  Japon. 


GENERAL 

TUNA  fishing  in  Japan  was  quickly  expanded  with  the 
introduction  of  hot  bulb  engines,  but  the  method  of 
operation  remained  old-fashioned  and  primitive.  It 
took  a  very  long  time  to  haul  the  longlines,  and  the  hands 
of  fishermen  were  often  lacerated.    Continuous  working 
was  impossible  and  the  fishing  began  to  decline. 

The  first  Izui  line  hauler  was  completed  in  Shikoku 
Island,  one  of  the  main  bases  for  tuna  fishing,  in  1923. 
But  the  fishermen  hesitated  to  adopt  this  new  method 
before  it  had  been  successfully  demonstrated  by  the 
inventor,  who  owned  a  fishing  boat  himself.  At  present 
some  8,230  Izui  line  haulers  are  used  in  Japan,  Brazil, 
Hawaii,  and  the  United  States. 


CONSTRUCTION 

The  body  of  the  line  hauler  is  made  of  cast  steel  and  the 
pulleys  of  gun  metal  and  rubber  to  prevent  wear  on  the 
lines.  The  middle  and  lower  part  of  the  hauler  contain 
oil  which  is  automatically  fed  to  the  working  parts. 
The  lower  part  contains  the  driving  shaft,  two  gears  to 
change  the  speed,  and  the  stopping  handle.  A  speed 
governor,  installed  in  the  middle  part,  compensates 
for  excessive  strain  which  may  be  caused  by  water 
resistance  or  a  big  catch,  to  prevent  the  wearing  or 
cutting  of  the  lines,  and  at  the  same  time  keeping  the 
fish  on  the  hooks. 

The  upper  part  of  the  hauler  consists  of  three  pulleys 
which   wind  the  line  automatically.    The  line  is  led 


[436] 


JAPANESE    TUNA     LONGL1NE    HAULER 


llotor  room  Side  roller 


Fig.  1.     Construction  drawing  of  Izui  line  hauler  for  longlines. 

1  =  Rope    winding    pulley.     2  -  Rope    drive    pulley.     3 —  Rope 

push  pulley.     4— Stop  handle.     5-  Clutch  handle.     6- Driving 

shaft. 

to   the   hauler   through    fair-leads    in    the    ship's   side. 
The  qualities  of  the  different   models   of  Izui   line 
haulers  available  are  given  below: 


Model 


Height     Height       r.p.m. 
nan.          kg.         of  shaft 


m.  I  min. 


Special  Si/e 

1,504 

402 

220  to  MX) 

184  to  254 

100 

Large  Si/e 

MOo 

282 

2(X)  lo  300 

144  to  216 

30 

(Standard) 

I,arge  Si/e 

1,258 

280 

200  to  300 

144  to  216 

20 

(Lower) 

Medium  Si/e 

1,155 

185 

230  to  2SO 

75  to  91 

10 

Small  Size 

849 

119 

170  to  200 

68  lo  80 

10 

Fig.  2.     Example  of  motor-winch  arrangement  when  driven  by 
the  main  engine. 


Fig.  3.     Example,  of  winch  arrangement  with  auxiliary  engine. 

The  power  is  either  taken  from  a  motor  or  from  a 
main  or  auxiliary  engine  through  an  intermediate  shaft. 
2-5  to  10  h.p.  are  needed  to  drive  the  line  hauler  at 
1 70  to  300  r.p.m.  and  a  hauling  speed  of  68  to  254  m./min. 
It  takes  up  to  12  hours  to  haul  300  units  of  lines,  depend- 
ing on  the  fishing  vessel  and  gear,  and  the  fishing  con- 
ditions. 

OPERATION 

A  Japanese  tuna  longline  consists  of  the  mainline  to 
which  the  floatlines  and  the  branchlines  with  hooks 
arc  attached.  Usually  200  fm.  mainline  make  one  unit 
or  basket.  An  ordinary  vessel  carries  between  300  to  350 
of  such  units,  i.e.  60,000  to  70,000  fm.  of  longline. 

The  length  and  number  of  floatlines  and  branchlines 
per  unit  mainline  depend  on  the  kind  of  tuna  to  be 
caught  and  the  fishing  conditions.  For  blue  Jin  tuna  the 
floatlines  are  between  6  and  20  fm.  long  and  for  alhacore 
even  about  30  fm.  long.  For  blue  fin  tuna  one  branchline 
with  a  big  hook  is  attached  between  two  floatlines 
which  are  100  fm.  apart.  For  albacore  12  to  13  branch 
lines  with  smaller  hooks  belong  to  each  unit  of  mainline. 

When  a  good  fishing  ground  is  found  the  lines  are 
shot  before  sunrise.  Since  the  lines  are  shot  with  con- 
siderable slack  to  bring  the  hooks  into  greater  depth, 
about  100  units  arc  shot  per  hour  and  the  whole  opera- 
tion takes  about  2  to  4  hrs.  The  vessel  then  usually 
returns  and  starts  hauling  the  end  of  the  line  which 
was  paid  out  first.  Sometimes,  however,  depending  on 
circumstances  the  lines  are  hauled  in  again  immediately 
after  shooting  was  finished. 


[437] 


AUTOMATIC  STEERING  SYSTEM  FOR   OCEAN-GOING 

FISHING  BOATS 

by 
SUIYO-KAI 

(A  group  of  manufacturers  of  electrical  equipment) 
Tokyo,  Japan 


Abstract 

Automatic  steering  is  usually  associated  with  the  use  of  the  Gyro  compass,  but  since  1946  some  Japanese  fishing  vessels  have  been 
equipped  with  automatic  steering  which  is  used  in  conjunction  with  the  magnetic  compass.  More  th;in  100  vessels  are  equipped  with  "M.C.P." 
manufactured  under  licence  from  the  Sperry  Gyroscope  Co.,  U.S.A.  and  their  activities  extend  from  the  South  Pacific  to  the  Indian  Ocean. 
The  equipment  operates  by  measuring  the  deflection  from  the  plotted  course  by  a  controller  fitted  to  the  card-bowl  of  the  magnetic  compass, 
and,  by  fitting  a  portable  remote  controller,  the  ship  can  be  steered  by  hand  from  any  position  on  the  ship  outside  the  wheelhouse,  a  fact 
that  will  appeal  to  skippers  supervising  the  shooting  and  hauling  of  their  gear. 


Resume 


Systeme  dc  pilotage  automatique  pour  les  bateaux  de  p£chc  de  haute  mer 


Le  pilotage  automatique  est  g£neralement  associe  a  1'cmploi  du  gyro-compas,  mais  depuis  la  derniere  guerre,  quelques  bateaux 
de  pcchc  japonais  ont  etc  equipes  d'un  systcme  de  pilotage  automatique  utilise  avec  un  compas  magnet  ique.  Cct  appareil,  connu  sous  le 
nom  de  "M.C.P."  a  6te  mis  au  point  par  la  Sperry  Gyroscope  Co.,  Etats-Unis,  et  construit  sous  licence  au  Japon;  it  cst  considere  comme  un 
616ment  indispensable  pour  tons  les  nouveaux  navires  de  pechc  de  haute  mer.  Plus  de  100  unites  dont  1'activite  s'etend  du  Pacifique  Sud  a 
rOc&tn  indien  sont  equipees  de  cet  appareil.  Un  appareil  monle  sur  I'habitaclc  du  compas  magnet  ique  pcrmet  de  mesurer  les  ccarts  de  route. 
Un  dispositif  portatif  de  commande  a  distance  permel  de  gouverner  le  navire  a  la  main  de  n'importe  quel  cndroit  du  navire  silu£  en  dchors 
dc  la  timonerie,  ce  qui  interesse  particulierement  les  patrons  qui  surveillcnt  la  mise  £  I'eau  ct  la  relevage  des  engins  de  peche. 

Sistema  de  gobierno  automatico  para  barcos  pesqueros  de  alta  mar 
Fxtraeto 

El  gobierno  automatico  de  las  naves  esta  generalmcnte  asociado  con  el  uso  del  compas  giroscopico,  pcro  desde  la  ultima  guerra 
algunas  embarcaciones  pesqueras  japonesas  poseen  un  equipo  de  gobierno,  tambien  automatico,  que  se  utili/a  junto  con  el  compas  magn6tico. 
Se  considcra  que  este  equipo,  conocido  con  el  nombrc  de  "M.C.P.1'  y  fabricado  con  licencia  de  la  "Sperry  Gyroscope  Co.",  de  T.U.A.,  es 
indispensable  para  todos  los  barcos  pesqueros  de  alia  mar. 

Mas  de  100  unidades  que  operan  desde  el  Pacih'co  meridional  al  oceano  Indico  cucntan  con  este  equipo,  cuyo  funcionamiento  se 
basa  en  la  mcdida  de  la  desviaci6n  de  la  ruta  tra/ada  mcdiantc  un  regulador  que  se  adapta  a  la  cubeta  del  compas  magnet ico.  Con  un 
dispo&itivo  portatil  de  gobierno  remoto,  el  cual  scguramente  atraera  a  los  palrones  dc  barcos  que  supervisan  las  faenas  del  lance,  es  posible 
maniobrar  el  barco  desde  cualquier  punto  fuera  dc  la  cascta  de  gobierno. 


GENERAL 

A  HUM  AN   helmsman,  can  for  a  short  time  steer 
as  well   as  a  machine,  but  he  will  quickly  tire, 
lose  attention  and  permit  the  vessel  to  wander 
off  course.     How  long  he  can  concentrate  on  his  job 
depends  on  the  manoeuvrability  of  the  vessel  and  the 
state  of  the  sea.     An  automatic  pilot  does  not  tire  and 
will   continue   to   steer   with    utmost   precision,   saving 
navigation  time  and  fuel. 

Automatic  steering  systems  have  been  in  use  on  large 
vessels  for  many  years  and  have  given  excellent  service. 
AH  these  systems,  however,  rely  for  directional  reference 
on  the  gyro  compass  which  because  of  space  and  power 
supply  limitations  is  difficult  to  install  on  small  vessels. 
Since  the  Second  World  War  automatic  steering  has 
become  possible  for  small  and  medium  sized  vessels 
equipped  only  with  a  magnetic  compass.  Over  one 


hundred  fishing  boats  in  Japan  are  today  equipped  with 
the  Magnetic  Compass  Pilot  and  their  activities  extend 
from  the  South  Pacific  to  the  Indian  Ocean. 

The  manufacture  of  the  Sperry  Magnetic  Compass 
Pilot  was  started  in  Japan  in  1953  under  a  licence  agree- 
ment with  the  Sperry  Gyroscope  Co.,  U.S.A.  It  is 
known  as  "M.C.P.",  and  is  now  regarded  as  an  indis- 
pensable instrument  for  ocean-going  fishing  vessels. 

CHARACTERISTICS  OF  THE  M.C.P. 

The  M.C.P.  takes  its  directional  reference  from  a  steady 
dependable  magnetic  compass,  which  is  compensated 
for  deviation  in  the  usual  way.  The  controller  fitted  to 
the  card-bowl  measures  the  deflection  from  the  plotted 
course  and  activates  the  steering  engine  through  a  power 


[438] 


MAGNETIC     COMPASS    PILOT 


Fig.  /.   Magnetic  compass 
and  controller. 


2.     Control  panel  ind  remote  controller. 


unit,  to  position  the  rudder  with  the  exact  amount 
required  to  bring  the  vessel  on  course  again,  as  and  when 
it  diverges  from  the  course  to  which  the  M.C.P.  is  set 
(fig.  1).  While  being  sensitive  to  the  slightest  variation 
in  course,  the  controller  does  not  interfere  with  the 
movement  and  has  no  influence  on  the  magnetic  func- 


Fig,  4.     General  layout  of  installation. 


Fig.  .?.     Power-unit. 

lions  of  the  compass  card.  The  same  compass  can 
therefore  be  used  for  handsteering  in  the  usual  way  and 
the  change-over  from  automatic  pilot  to  hand  steering 
and  vice  versa  is  done  by  an  electric  switch. 

With  the  magnetic  pilot  it  is  not  necessary  to  lay  the 
vessel  on  course  before  setting  to  automatic  steering; 
by  setting  the  required  course  on  the  controller  dial,  the 
vessel  will  veer  to  the  course  set  and  be  held  there.  The 
M.C.P.  is  fitted  with  remote  control  which  makes 
it  possible  to  steer  the  vessel  by  hand  from  any  position 
on  board.  This  allows  the  skipper  to  stand  in  the  most 
suitable  position  to  direct  the  fishing  operations  and  at 
the  same  time  have  full  control  of  his  ship  (fig.  2). 

INSTALLATION 

The  system  requires  220  V  or  1 10  V  D.C.  and  consists  of: 

(a)  Controller    (c)  Control  panel  (e)  Power  unit 

(b)  Amplifier      (d)  Switch  box       (f )  Remote  Controller 
Being  small  and  compact,  it  is  suitable  for  fishing 

vessels.  The  controller  is  fitted  to  the  compass  which 
can  stay  in  its  normal  position  in  the  wheelhouse. 
Amplifier,  control  panel  and  switch  box  are  mounted 
overhead  in  the  chariroom  where  they  are  easily  access- 
ible for  inspection.  The  power  unit  (fig.  3)  is  usually 
installed  in  the  wheelhouse  in  a  suitable  place  for 
connection  to  the  steering  gear.  It  should,  however, 
be  at  least  5  ft.  distant  from  the  magnetic  compass. 

A  plan  showing  the  general  disposition  of  the  units  in 
relation  to  the  steering  mechanism  is  shown  in  fig.  4. 


[439] 


DISCUSSION  OF  CHOICE  OF  FISHING  GEAR  AND 
SOME  NEW  METHODS 


Mr.  W.  Dickson  (U.K.)  Rapporteur:  In  his  gear  classifi- 
cation Dr.  von  Brandt  has  listed  most  gears  that  have 
been  used  at  one  time  or  another  so  that  you  almost  see 
history  marching  past.  We  come  in  at  the  tail  end  of 
this  process,  where  mechanical,  electrical  and  hydraulic 
power  are  firmly  established.  The  tendency  is  to  use  more  and 
more  horse  power  per  member  of  the  crew  with  always  new 
ways  being  introduced  to  eliminate  hand  labour.  This 
process  gives  rise  to  a  distinction  between  active  and  passive 
methods  of  fishing  which  was  not  so  apparent  in  the  past. 
In  Northern  Europe  the  two  really  important  passive  methods 
of  fishing  which  remain  are  gillnetting  and  longlining. 
In  Scotland  and  in  Holland,  too,  I  think,  drift  netting  faces 
somewhat  of  a  crisis,  not  unconnected,  I  believe,  with  the 
increasing  use  of  mechanical  power  in  competitive  methods 
of  catching.  English,  Russian  and  Polish  dual  purpose 
drifter  trawlers  are  to  be  found  drifting  on  the  western  edge 
of  the  Norway  Deeps  in  the  early  part  of  the  year.  It  is 
hardly  likely  that  these  boats  would  be  drifting  if  they  could 
do  better  by  herring  trawling,  in  that  area,  as  herring  trawlers 
are  to  be  seen  in  quite  closely  neighbouring  areas.  There 
is  a  feeling  of  frustration  about  the  passiveness  of  a  drift 
net  which  is  in  the  spirit  of  our  modern  times,  but  if  you  con- 
sider the  water  column  filtered  by  a  bottom  or  pelagic  trawl 
and  compare  it  with  the  probable  water  column  fished  by 
a  drift  net  the  advantages  of  the  active  gear  are  not  quite  so 
obvious.  In  more  than  one  language  there  must  be  an  equiva- 
lent of  our  English  words  "They  also  serve  who  only  stand 
and  wait".  Perhaps  we  could  discuss  a  more  efficient  way 
of  handling  drift  nets  and  give  an  age-old  method  a  new 
lease  of  life. 

The  demersal  longlinc  fishery  extends  to  depths  scarcely 
touched  by  trawls  -down  to  400  fathoms,  and  only  scientists 
have  fished  deeper.  At  such  depths  the  stress  on  the  hemp  line 
caused  by  its  own  weight  and  resistance  through  the  water 
during  hauling  approaches  its  limits.  However,  it  appears 
that  the  Australians  have  developed  a  method  of  longlining 
in  very  deep  water  using  wire  rope.  Lead  pellets  are  clamped 
on  to  the  standing  line  at  intervals,  while  the  snoods  with 
the  baited  hook  at  the  end  arc  clipped  on  to  the  main  line 
as  it  is  shot.  It  is  my  hope  that  something  can  also  be  made 
of  this  in  the  waters  fished  by  our  Scottish  line  men. 

In  my  country  other  passive  methods  have  declined 
in  importance  and  are  only  used  now  for  the  catch  of  lobsters 
and  salmon,  and  this  by  lack  of  choice  in  obtaining  the 
product  in  any  other  way.  Fukuhara,  Rasalan  and  Kanamori 
all  deal  with  types  of  gear  used  in  the  Far  Fast  which  are 
more  or  less  passive,  involving  the  use  of  light  attraction. 
There  is  an  intriguing  comparison  or  choice  in  the  use  of 
power,  whether  to  use  it  to  bring  the  fish  to  your  gear  as 
they  do,  or  use  it  as  we  do  to  bring  the  gear  to  the  fish. 
These  attraction  methods  may  be  of  interest  primarily  to 
the  biologists  to  develop  them  fully  and  will  need  the  coopera- 
tion of  technologists 


This  brings  us  to  the  question  of  cooperation  between 
biologists  and  technologists,  on  which  we  will  hear  more  in 
a  later  section.  There  are  a  number  of  problems  which  the 
biologists  could  help  to  solve  for  the  technologists,  which 
would  help  in  choosing  the  gear  to  use,  or  as  basic  data 
from  which  to  start  designing  new  gear.  Such  as  for  instance: 
at  what  speeds  can  different  species  and  size  of  fish  swim? 
Although  something  has  been  done  in  this  respect,  only 
little  data  of  practical  value  is  available. 

At  what  distance  and  in  which  way  are  fish  effected  by 
vibrations  of  given  strength  and  frequency? 

Which  senses  of  fish  are  affected  by  the  approach  of  gear, 
at  what  distance  and  in  which  way?  Would  it  not  be  possible 
to  work  a  small  trawl  in  a  small  area  walled  off  by  gillncts 
and  by  diving,  observe  the  changed  reactions  of  the  fish. 

The  approach  to  such  problems  of  an  engineer,  left  to 
his  own  devices,  often  appears  quite  inadequate  to  the 
biologist  and  vice-versa.  The  first  step  in  obtaining  good 
answers  is  to  formulate  proper  questions  and  to  this  end 
it  seems  to  me  that  many  of  the  questions  are  best  tackled  by 
a  team  of  two  on  an  equal  footing — one  biologist  and  one 
technologist.  Often  the  physiological  and  technical  problems 
arc  so  intermixed  that  it  is  hard  to  say  who  is  the  better  quali- 
fied. 

How  much  further  will  we  be  when  we  have  answers 
to  questions  such  as  those  posed.  Well,  minimum  towing 
speeds  for  minimum  dimensions  of  mouth  opening  of  pelagic 
trawls  could  be  calculated.  We  would  have  more  idea  of 
how  large  it  might  be  possible  to  make  the  mesh  in  the  fore 
part  of  trawls.  Optimum  theoretical  positions  of  kites  and 
wires  could  be  calculated.  Parts  of  the  gear  could  perhaps 
be  eliminated  altogether  or  altered  to  be  more  effective. 
All  these  things  are  design  parameters  for  the  designer 
which  at  the  moment  he  just  has  to  choose  more  or  less 
by  guess  work,  not  daring  to  deviate  very  far  from  standard 
practice. 

This  leads  us  to  standard  practice  in  bottom  trawl  design. 
Binns  describes  how  the  lower  selvedge  of  a  net  is  made  some 
10  per  cent,  longer  than  the  upper  selvedge.  True  it  is,  but 
does  anybody  know  what  happens  if  it  is  not  10  per  cent, 
longer?  The  Danish  seine  net  and  some  herring  trawls  have 
no  such  provision  for  slack  in  the  lower  half  and  they  work 
all  right.  From  a  study  of  models  on  the  effect  of  the  extra 
length  in  the  lower  wings  and  belly,  if  would  seem  that  when 
taking  a  cross  section  through  the  body  of  the  net,  the  tension 
is  in  the  top  half — the  belly  being  made  longer  but  in  fact, 
constrained  to  the  same  length  as  the  upper  net,  bulges  out 
and  the  selvedges  rise  to  well  above  the  midway  line.  Mark 
that  the  belly  lines  as  they  run  to  the  codcnd  have  to  rise  to 
that  height  and  if  an  attempt  is  made  to  buoy  up  the  headline 
with  floats  to  an  undue  extent,  the  belly  lines  pinch  the  net 
like  a  string  tightened  across  a  cushion. 

A  net  is  like  a  jacket,  you  can  have  a  big  one  or  you  can 
have  a  small  one,  but  having  picked  your  size  you  cannot 


[440] 


DISCUSSION  — FISHING    GEAR    TYPES 


have  your  cloth  in  breadth  and  length  as  well.  You  have  to 
choose  between  spread  and  gape.  Now  to  help  me  under- 
stand what  was  involved  in  this  choice,  I  once  made  two 
separate  nets  that  could  be  mounted  one  above  the  other 
so  that  the  bottom  one  took  fish  between  the  bottom  and 
two  metres  above  it,  and  the  top  one  took  fish  between  two 
and  four  metres  from  the  bottom.  There  were  of  course 
changes  in  the  percentages  of  fish  which  appeared  in  the  top 
codend — changes  for  the  type  of  fish,  by  day  and  by  night 
and  from  one  ground  to  another  where  the  net  was  tried. 
To  cut  a  long  story  short  these  experiments  showed  not 
only  that  sometimes  extra  headline  height  would  be  well 
worthwhile,  but  that  at  others  it  would  be  better  to  have 
your  cloth  in  breadth.  So  think  twice  before  you  load  a 
trawl  up  with  extra  kites  and  floats. 

An  important  point  is  to  have  the  towing  load  on  to  the 
lastrich  of  a  trawl.  As  long  as  the  meshes  are  open  this  can 
only  be  done  as  Ben- Yam i  has  already  described  by  lashing 
along  the  lastrich  a  line  which  is  somewhat  shorter  than  the 
lastrich  itself.  If  a  lastrich  as  braided  is  10  metres  long  it 
will  shorten  to  say  9  metres  as  soon  as  the  meshes  open  up, 
therefore  unless  a  line  of  9  metres  or  less  is  lashed  along 
the  lastrich  it  will  remain  slack  and  no  towing  load  can  be 
put  on  it.  This  however  is  not  generally  accepted  by  net 
designers  and  fishermen. 

There  are  several  papers  covering  midwater  trawling. 
Barraclough  and  Needier  describe  the  whole  process  of 
design,  testing  and  development  of  a  one-boat  trawl.  It 
describes  its  performance  in  midwater  as  well  as  on  the 
bottom,  successful  for  winter  herring  but  somewhat  less  so 
for  the  same  faster  summer  fish.  The  whole  story  is  a  model 
of  what  cooperation  between  fishermen,  technologists  and 
biologists  ought  to  be.  It  includes  a  well  illustrated  and  neat 
way  of  handling  long  double  spreading  wires  from  behind 
the  trawl  board.  Noel  describes  pair  trawling  started  by  the 
Lcggatt  brothers  for  sprats  in  the  Thames.  Akyiiz  describes 
how  even  a  Vingc  trawl,  hardly  the  best  instrument  for 
pelagic  trawling,  caught  plenty  of  anchovy  in  the  Black 
Sea;  Grouselle  describes  a  new  type  of  floating  trawl.  Larsson , 
who  has  spoken  to  us  already  of  his  pioneering  work  on 
this  type  of  fishing  gear,  describes  Scandinavian  experience 
with  three  different  kinds  of  floating  trawls.  Parrish  reviews 
almost  all  the  pelagic  trawls,  the  factors  governing  the 
choice  and  the  successes  and  failures  that  have  been  met. 
There  remains  only  the  use  of  midwater  nets  close  to  the 
bottom  for  cod  which  should  be  given  more  attention. 

Subcrkriib  describes  patent  otterboards  for  use  in  pelagic 
trawling  and  I  suspect  many  of  us  here  would  like  some 
guidance  on  streamlined  otter  boards  from  the  naval  architects 
present.  By  increasing  the  ratio  of  height  to  length  of  the 
board,  by  decreasing  its  angle  of  attack  and  by  shaping, 
the  lift  to  drag  ratio  can  be  very  markedly  improved;  I 
understand,  however,  that  to  keep  the  same  value  of  absolute 
lift,  the  board  would  need  to  be  slightly  increased  in  size. 

The  paper,  by  Baird,  I  have  deliberately  kept  until  last. 
It  deals  with  what  most  people  would  call  a  side  line, 
namely  escallop  dredges.  But  on  careful  reading  it  shows 
how  much  local  fishing  knowledge  goes  into  such  a  seemingly 
simple  decision  as  to  choose  the  length  of  tooth  and  the 
tooth  angle  of  these  dredges.  It  shows  clearly,  I  believe, 
how  detailed  the  knowledge  of  a  gear  technologist  must 
be  and  that  he  should  only  be  expected  to  advise  on  methods 
and  fisheries  which  come  within  his  direct  experience. 


Mr.  L.  Soublin  (France)  Chairman:  I  think  Mr.  Dickson 
has  brought  forward  most  of  the  points  which  should  come 
under  review  so  that  we  are  well  briefed  for  the  discussion 
of  these  subjects.  I  hope  that  we  will  now  hear  the  fishermen 
themselves  tell  us  the  results  of  their  own  experiments.  I 
realize  they  are  a  silent  breed  who  don't  wish  to  talk  of  their 
setbacks  and  are  not  very  anxious  to  boast  of  their  successes 
to  their  colleagues,  because  these  then  become  competitors, 
but  nevertheless  I  feel  it  would  be  very  interesting  to  have 
the  professionals  who  have  worked  in  the  various  fields 
covered  by  the  papers,  to  come  forward  and  tell  us  something 
of  their  problems,  experiences  and  results.  I  have  personally 
made  numerous  tests  with  midwater  trawls  but  1  have 
never  had  a  complete  success  nor  a  complete  failure. 

Captain  D.  Roberts  (U.K.):  Although  as  the  Chairman 
says,  I  do  not  want  to  give  anything  away,  I  agree  that  we 
should  speak  up  here.  In  all  experiments  I  have  made  1 
can  honestly  say  I  have  not  done  myself  any  financial  good. 

I  was  very  interested  in  the  paper  on  inclination  tests  of 
trawl  boards  and  things  like  that.  If  I  could  improve  the 
efficiency  of  my  trawl  doors  5  per  cent,  or  10  per  cent,  every- 
body would  agree  that  that  would  be  very,  very  good  indeed; 
yet,  on  the  other  hand,  I  would  have  to  spend  quite  a  lot  of 
time  fiddling  about  to  do  it  actually  at  sea  and  lose  fishing 
time.  It  is  not  our  job  only  to  catch  fish,  as  1  said  previously 
we  must  catch  fish  that  will  sell. 

The  fishermen  I  have  in  my  crew  work  long  hours  and  want 
to  sec  a  catch  as  the  result  of  their  work.  Fiddling  about 
with  experiments  does  not  appeal  to  them.  I  have  found 
that  my  men  like  best  to  get  into  a  rhythm  of  work  and  they 
work  best  when  they  know  what  they  are  working  for. 
I  know  what  their  thoughts  are  at  the  moment,  knowing 
I  have  come  to  this  Congress;  it  is  a  fact  that  they  will  be 
saying  "What  the  Hell  will  the  old  man  want  us  to  do  when 
he  gets  back".  They  know  I  am  going  to  come  with  ideas 
and  upset  their  rhythm  of  work. 

So  if  after  six  months'  experiments  I  get  10  per  cent, 
more  efficiency  from  my  trawl  boards,  1  have  lost  possibly 
20  per  cent,  of  cooperation.  I  am  pointing  this  out  to  you 
because  I  want  you  to  see  how  difficult  it  is  for  us  to  do 
experimental  work  in  a  commercial  ship. 

Mr.  Phillips  is  a  man  who  has  ideas  all  the  time  and  he 
comes  along  to  me  quite  often  and  says:  "will  you  try  this" 
or  "what  do  you  think  of  that"  and  I  do  make  some  effort 
to  assist  him  in  his  work. 

Two  years  ago  he  was  playing  about  with  a  practice  golf 
ball  and  thought:  "this  is  what  we  want  on  the  bottom 
instead  of  a  bobbin  filled  with  air" — just  an  ordinary  banded 
sphere  with  very  little  resistance,  no  weight  at  all,  and  he 
manufactured  some  of  these  and  thought:  "now,  I  will  test 
them  severely  and  then  hand  them  to  a  fisherman  to  try". 
He  got  two  cars  and  took  his  string  of  bobbins  down  on  the 
beach  at  home  to  sec  if  they  would  sink  into  the  sand  and  he 
towed  them  along  on  the  sand  at  3  to  4  knots.  They  did  not 
dig  into  the  sand — they  looked  to  be  perfect— he  then  thought: 
"well,  are  these  really  strong  enough" — will  they  stand  up 
to  the  hard  bottom  on  which  he  knows  my  trawler  works — 
so  he  took  this  string  of  bobbins  to  the  Ironworks  at  Scun- 
thorpe  near  Grimsby,  where  there  are  slag  heaps.  A  friend 
of  his  there  had  two  railway  lines  running  parallel  in  amongst 
the  works  and  they  got  two  railway  engines,  and  they  attached 
the  arc  of  bobbins  on  the  wires  just  exactly  as  it  would  run 


[441  ] 


MODERN     FISHING     GEAR    OF    THE     WORLD 


on  the  bottom  of  the  sea  and  towed  them  up  and  down 
at  three  knots.  The  bobbins  didn't  break  they  were  hardly 
marked  and  they  appeared  to  jump  as  they  hit  the  different 
pieces  of  hard  slag,  they  jumped  up  and  over  and  gave  us 
the  impression  they  would  do  the  same  on  the  bottom  and 
lift  the  belly  over  just  as  we  wished.  After  watching  that 
test  for  half  an  hour  or  so  the  speed  was  increased  to  1 5  to  20 
miles  an  hour,  purposely  to  break  these  bobbins,  but  nothing 
happened. 

Afterwards  Mr.  Phillips  showed  me  a  film  of  the  test. 
He  said,  "1  do  not  want  you  to  try  anything  that  is  not  good, 
look  at  this  film,  see  what  1  have  done,  and  then  I  want  you 
to  test  my  bobbins."  After  viewing  the  film  I  was  convinced 
that  1  was  on  to  something  really  good  :  I  took  the  bobbins  to 
sea  and  we  tried  them  out  first  in  rather  muddy  ground,  but  fil- 
led the  trawl  absolutely  full  of  mud.  My  bosun  who  goes  under 
the  codend  to  release  the  catch-  he  does  not  think  much  of  Mr. 
Phillips  now !  The  next  day  I  shot  my  gear  on  a  hard  bottom 
where  I  normally  fish  for  flat  fish.  By  engine  pressures  we 
noticed  that  the  gear  gave  there  more  resistance  so  that 
while  we  thought  those  bobbins  would  give  much  less  resis- 
tance, there  was  more.  What  is  more,  after  three  days 
we  had  broken  half  those  bobbins. 

We  do  not  know  what  goes  on  down  on  the  seabed,  as 
one  of  the  biologists  pointed  out  to  me.  He  also  admitted 
that  little  js  known  about  the  pressure  waves  the  trawl  may 
cause  as  it  is  towed  over  the  bottom,  so  there  is  an  awful 
lot  to  learn  as  well.  If.  by  using  something  new,  we  increase 
the  efficiency  of  the  gear  by  10  per  cent.,  but  lose  that  amount 
in  our  crew's  efficiency,  there  is  no  point  in  using  the  new 
gadget.  When  you  consider  the  efficiency  of  gear  you  should 
bear  in  mind  its  effect  on  the  crew;  1  mean  our  fellows  work 
18  hours  a  day  and  they  have  six  hours  of  rest.  By  upsetting 
their  rhythm  of  work  and  making  them  do  work  they  are 
not  used  to— we  are  losing  more  than  10  per  cent,  in  their 
efficiency  and  their  cooperation  and  loyalty  to  the  skipper. 

Dr.  A.  W.  H.  Needier  (Canada) :  I  was  very  much  interested 
in  what  Mr.  Roberts  said  because  he  is  speaking  about  some 
very  real  difficulties.  But  the  other  aspect  of  the  problem 
is  that  in  order  to  test  new  kinds  of  gear  we  have  to  have 
fishermen  and  so  the  only  solution  that  I  know  of  is  that  aflcr 
we  have  done  what  we  can  with  research  vessels  and  in 
laboratories  and  with  models,  we  have  to  persuade  experienced 
professional  fishermen  to  test  it  under  fishing  condition  before 
any  piece  of  gear  may  be  recommended  to  other  fishermen 
to  use  in  making  their  living. 

1  am  being  asked  to  say  something  more  on  the  British 
Columbian  midwatcr  trawl.  Well,  1  don't  have  anything 
particularly  to  add  to  the  paper,  except  to  say  that  although 
we  do  not  claim  that  this  is  the  best  gear  under  all  circum- 
stances, it  does  work  under  certain  conditions  to  catch  slow 
moving  schooled  herring  but  less  on  fast  moving  herring. 

At  the  moment  we  are  unable  to  carry  this  experiment 
further  for  reasons  of  personnel  and  finance. 

The  scientists  in  catching  plankton  now  use  a  high  speed 
plankton  net,  which  allows  water  to  flow  freely  towards  the 
centre  of  the  net,  but  nevertheless  collects  small  animals 
better  than  the  net  which  has  no  hole  through  the  middle. 
We  made  some  preliminary  experiments  with  this  but  the 
net  that  we  constructed  broke  up— not  being  constructed 
strong  enough.  The  idea  may  however  well  be  worthwhile 
following  up.  The  aim  is  to  construct  a  net  that  could  be 


towed  faster  and  which  would  guide  fish  into  the  codend 
while  relieving  the  hydrodynamic  resistance  of  the  net  by 
allowing  a  free  flow  through  the  middle.  The  indications 
are  that  such  a  net  can  catch  fish — we  know  that  much — 
and  I  think  ideas  such  as  these  will,  in  the  long  run,  pay  off. 
1  may  be  pleading  for  a  revolutionary  idea,  but  very  often 
it  is  the  things  that  look  a  little  bit  silly  at  first  that  pay  off 
later.  The  use  of  electricity,  as  we  know  it  today,  all  came 
from  an  imaginative  old  bird  who  pushed  magnets  through 
loops  of  wire  and  wondered  what  would  happen;  he  had 
no  idea  that  this  was  going  to  be  one  of  the  principles  most 
useful  in  industrial  development. 

Commander  M.  Melo  do  Carvalho  (Portugal):  1  think 
that  most  of  us  at  this  session  have  read  with  interest  Dr. 
Miyamoto's  paper  dealing  with  the  relation  between  otter 
trawling  gear  and  towing  power.  I  think  it  would  be  very 
important  to  know  from  the  technologists  present  and  from 
fishermen  what  their  points  of  view  are  in  this  matter. 

The  weight,  the  dimensions,  the  shape  of  the  otter  doors, 
the  position  of  the  brackets,  the  best  working  position  for 
the  towing  point,  the  length  and  the  size  of  the  warps,  are 
arrived  at  by  hit  and  miss  methods  by  the  fishermen  in  my 
country.  I  would  like  to  know  from  the  technologists  and 
the  experienced  fishermen  present  how  they  go  about  deciding 
on  such  measurements. 

Mr,  A.  O'Grady  (Australia):  Those  concerned  with 
fisheries  in  Australia  are  particularly  pleased  that  improve- 
ments in  fishing  gear  and  methods  are  being  sought  on  an 
international  basis.  I  say  we  are  particularly  pleased,  because 
Australia  is  now  at  the  commencement  of  a  programme  in 
fisheries  development  by  way  of  improvements  in  the  gear 
at  present  in  use,  and  change  over  to  new  gear;  also  in  the 
seeking  of  new  fishing  grounds  following  on  the  fisheries 
at  present  being  conducted. 

Our  tuna  fishermen  use  a  lampara  net  for  the  capture  of 
live  bait  at  night,  which  is  very  light  and  comparatively 
cheap  due  mainly  to  the  large  meshes  in  the  wings.  It  is 
very  successful  for  the  taking  of  live  bait  using  light  attraction 
at  night,  but  unfortunately  docs  not  seem  to  give  the  same 
results  during  daylight.  I  have  conducted  experiments  with 
a  model  and  the  results  suggest  that  in  daylight  the  net 
should  be  partly  pursed.  This  will  be  tested  under  actual 
fishing  conditions  at  the  first  available  opportunity. 

Mr.  Dickson,  Captain  Ben-Yami  and  Captain  Hodson 
have  spoken  already  of  their  methods  of  allowing  the  headline 
to  rise  high  by  taking  the  strain  off  the  head  and  placing  it 
on  the  lastrich  line.  This  is  actually  being  done  with  one 
type  of  prawn  trawl  used  in  Australia  for  catching  one 
particular  species  which  apparently  swim  much  higher  than 
the  bulk  of  the  prawns  which  arc  caught  in  Australia.  Some 
of  our  fishermen  have  actually  measured  the  rise  above 
the  bottom  and  in  some  cases  it  has  been  12  ft.,  which  is 
quite  an  improvement  on  the  height  at  which  we  fish  normally. 

I  was  surprised  to  learn  from  Mr.  Dickson  that  in  Great 
Britain  the  Danish  seine  net  is  hung  so  that  the  headline 
and  footrope  are  of  equal  length.  It  has  been  our  practice 
for  many  years  now  to  have  the  headline  shorter  than  the 
footrope,  so  that  the  footrope  is  allowed  to  work  the  bottom. 
Some  years  back  when  1  was  actually  engaged  in  this  fishery, 
we  were  hanging  so  that  each  wing  of  the  footrope  was  one 
foot  longer  than  its  headline  counterpart.  Today  that  has 


[442] 


DISCUSSION"    FISHING     GEAR    TYPES 


been  reduced  to  approximately  five  or  six  inches,  the  foot- 
rope  therefore  being  10  to  12  in.  longer  than  the  headline. 
I  have  been  asked  to  speak  on  advanced  developments 
in  fishing  gear,  and  one  has  been  suggested — the  use  of  wire 
longlines.  Wire  longlines,  made  of  £  in.  circumference  steel 
wire  rope  have  been  used  for  the  capture  of  shark  for  table 
food  in  in  the  Southern  part  of  Australia.  However,  one 
operator — a  very  good  and  a  very  successful  shark  fisherman 
and  owner  of  three  vessels  tested  the  steel  longlinc  gear,  and 
found  that  he  had  to  revert  to  the  use  of  the  gear  in  common 
use,  that  is  natural  fibre  rope — manila  or  sisal — because  of 
the  heavy  losses  sustained  by  fouling  up  at  the  bottom,  it 
was  an  uneconomic  proposition.  However,  another  method 
was  actually  introduced  by  Captain  Ardiani,  the  master  of 
a  fisheries  research  vessel  operated  by  the  Division  of  Fisheries 
and  Oceanography  of  the  CSRIO.  For  this  method  a  flexible 
galvanised  steel  wire  rope  approximately  A  in,  circumference 
is  operated  from  a  hydraulic  reel.  A  sinker,  of  course,  is 
attached  to  the  end  of  the  line  and  several  leads  arc  stopped 
on,  approximately  one  fathom  apart  where  snoods  and 
hooks  are  attached.  This  method  has  been  used  in  depths 
of  up  to  300  fathoms.  As  in  such  depths  it  would  be  impossible 
to  feel  when  the  sinker  and  hooks  had  touched  the  bottom, 
it  has  to  be  used  in  conjunction  with  a  depth  recorder  and  a 
metre  for  measuring  the  length  of  wire  rope  which  has  been 
lowered  into  the  water.  Apparently  this  method  is  already 
successful,  the  particular  Captain  1  have  referred  to  who 
developed  it  is  now  launching  his  own  vessel  to  engage  in 
this  fishery. 

Mr.  W.  Dickson  (U.K.):  I  just  want  to  correct — 1  seem 
to  have  given  the  wrong  impression  to  Mr.  O'Grady,  about 
the  hanging  of  seine  nets.  Actually  in  Scotland,  we  do  have 
the  headline  considerably  shorter  than  the  groundrope, 
for  haddock  and  whiting  fishing  it  is  certainly  as  much  as 
seven  feet  shorter  than  the  groundrope.  For  plaice  fishing, 
well  the  two  have  either  the  same  length  or  the  headline  is 
only  very  little  shorter  than  the  groundrope.  But  that  was 
not  what  I  meant. 

I  meant  that  the  hanging  of  the  webbing  is  not  more  loose 
on  the  footrope  than  it  is  on  the  headline,  whereas  in  trawl 
nets  the  netting  on  the  headline  is  set  more  tightly  to  the 
ropes  than  it  is  along  the  footrope  where  more  slack  is  set 
into  it. 


Mr.  J.  Jakobsson  (Iceland):  I  am  really  talking  here  on 
behalf  of  the  fishermen  of  Iceland,  who  are  attending  this 
Conference.  They  should  themselves  tell  of  their  experiences, 
but  I  think  they  find  the  language  difficulties  rather  too 
great  to  come  up  here. 

As  opposed  to  the  experiences  of  Captain  Roberts,  some 
of  our  captains  have  had  real  success  in  experimenting  with 
new  gear.  I  am  especially  referring  to  Captain  Ingimarsson, 
one  of  Iceland's  crack  skippers — as  far  as  1  know  he  still 
holds  a  sale  record  in  Aberdeen  where  he  sold  the  catch  of  a 
single  trip  for  over  £19,000  in  1948.  Captain  Ingimarsson 
realized  very  soon  after  the  development  of  the  echo  sounders, 
there  were  sometimes,  especially  during  April,  very  heavy 
concentrations  of  fish  over  a  rough  bottom  off  the  south 
coast  of  Iceland.  It  had  been  tried  to  trawl  there  time  and 
again  and  usually  the  nets  got  torn  very  badly.  Not  only 
were  these  heavy  concentrations  of  fish  over  very  rough 
bottom,  but  also  they  were  quite  often  in  midwater.  The 


idea,  therefore,  occurred  to  him  and  many  other  people 
fishing  on  these  grounds  that  midwater  trawling  was  the 
answer  and  in  1952  the  captains  themselves  for  the  part 
with  some  assistance  from  shore  people,  developed  what  is 
known  as  the  Breidfjord  trawl.  It  has  been  successful  for 
cod  fishing  in  late  winter  during  the  spawning  season  when 
there  are  very  heavy  concentrations  of  fish. 

The  special  feature  of  this  trawl  which  I  think  is  rather 
unlike  most  of  the  midwater  trawls  I  have  seen  drawings  of, 
is  that  the  headline  legs  are  not  connected  to  the  otter  doors 
but  to  the  actual  trawl  warps  far  in  front  of  the  otter  doors, 
which  means  that  the  vertical  opening  of  the  trawl  is  secured 
and  no  extra  kites  or  other  lifting  devices  are  needed.  This  is 
actually  drawn  in  Parrish's  review  of  various  midwater 
trawls.  Parrish  does  suggest  that  kites  arc  sometimes  used 
but  for  cod  fishing  I  do  not  know  of  any  skipper  who  has  used 
kites.  The  vertical  opening  is  actually  secured  by  the  nature 
of  the  arrangement  of  wires. 

Now,  after  this  trawl  had  been  used  successfully  for  some 
years,  we  tried  to  fish  herring  as  well  as  cod  and  I  was  lucky 
enough  to  he  offered  to  work  on  this  with  Captain  Ingimarsson 
last  November.  We  had  a  fairly  big  nylon  midwater  trawl 
with  a  square  mouth  of  70  to  62  ft.  and  we  tried  this  rope 
arrangement.  Unfortunately,  we  were  extremely  unlucky 
with  the  weather — the  Atlantic  then  was  at  its  very  worst. 
We  had  gale  after  gale  for  three  weeks  but  we  got  the  trawl 
into  the  sea  several  times  and  we  were  not  successful  using 
the  customary  arrangement  of  wires  which  is  used  for  the  cod 
trawl.  There  were  quite  heavy  concentrations  of  herring, 
but  we  only  caught  very  small  amounts  and  we  suspected, 
without  knowing  of  course,  that  the  wires  going  from  the 
headline  to  the  trawling  wires  were  very  much  above  and 
directly  ahead  of  the  mouth  of  the  trawl;  even  if  they  did  not 
scare  or  frighten  cod,  it  might  affect  herring.  So  we  tried  to 
connect  the  headline  legs  to  the  otter  boards  and  then,  of 
course,  the  old  question  of  vertical  opening  immediately 
became  very  acute.  We  therefore  used  kites  and  heavy  weights 
at  the  ground  rope  and  considering  the  echo  recordings,  1 
think  that  our  catches  were  reasonable.  But  in  actual  fact  this 
was  an  unfinished  experiment  because  of  the  bad  weather. 
We  hope  that  we  will  be  able  to  continue  this  very  soon  and 
experiment  further  with  pelagic  herring  trawling  in  Icelandic 
waters. 

The  British  Columbian  trawl  has  been  tried  out  to  some 
extent  in  Icelandic  waters  by  a  relatively  small  boat,  but  it  has 
not  met  with  success  so  far. 

1  would  also  like  to  point  out  that  Captain  Ingimarsson  and 
I  were  using  a  large  trawler,  a  180  footer  with  approximately 
l,000h.p. 

Mr.  B.  Parrish  (United  Kingdom):  We  have  heard  many 
examples  of  the  kind  of  problem  which  fishermen  and 
technologists  and  those  concerned  with  the  effective  operation 
of  fishing  gear  are  faced  with  in  different  situations,  in  different 
areas  and  at  different  times.  All  the  problems  appear  to  be  a 
combination  of  two  main  factors;  the  technological  efficiency 
of  the  gear  and  the  biological  characteristics  of  the  species  of 
fish  which  they  arc  catching.  The  biological  features  of  the  fish 
one  is  trying  to  catch  are  important  when  making  a  choice  of 
fishing  gear.  I  n  areas  where  fishing  is  being  introduced  the  first 
problem  will  be  to  decide  what  type  of  gear  to  use.  There  are  two 
ways  to  tackle  this  problem,  firstly,  one  can  take  all  the  existing 
types  of  gear  and  try  them  out,  retaining  the  gear  which 
proves  successful.  This  would  obviously  be  completely 


443  ] 


MODERN    FISHING     GEAR    OF    THE    WORLD 


uneconomical.  The  second  way  would  be  to  collect  informa- 
tion on  the  biological  characteristics  of  the  fish  in  the  region, 
and  make  the  choice  on  the  basis  of  such  knowledge. 

An  example  of  this  is  the  problem  we  are  faced  with  in 
Northern  Europe  in  relation  to  herring.  Traditionally,  the 
herring  fisheries  were  drift  net  fisheries  because  of  the  parti- 
cular biological  characteristic  of  the  herring  to  come  to  the 
surface  layer  at  night.  Then  trawling  was  introduced  in 
herring  fishing  in  Northern  Europe  due  to  the  other  biological 
characteristic  of  herring,  that  during  the  daytime  herring 
seeks  the  bottom  layer  near  the  seabed,  and  can  then  be 
caught  in  traditional  trawl  gear.  Trawling  for  herring  which 
was  developed  in  the  regions  where  distribution  of  the  herring 
near  the  seabed  was  particularly  suitable  proved  profitable. 
Since  the  last  war,  there  has  been  a  tremendous  expenditure  of 
effort  in  assessing  whether  herring  trawling  would  be  a  suitable 
alternative  to  drift  netting  in  different  parts  of  the  Northern 
European  region. 

Many  of  these  experiments  have  been  conducted  wastefully 
because  had  the  biological  characteristics  of  the  herring  been 
studied  in  those  regions  before  the  experiments  were  carried 
out,  it  would  have  been  quite  clear  that  the  herring  trawling 
was  not  the  practical  alternative  to  drift  netting,  simply  because 
this  habit  of  herring  seeking  the  lower  water  layers  during  the 
day  time,  is  not  a  general  characteristic  and  in  fact  in  some 
regions,  the  herring  distribute  themselves  in  mid  water. 

Mr.  K.  H.  Larsson  (Sweden):  It  is  a  well  known  fact  that 
the  trawl  takes  almost  everything  that  comes  before  its  mouth 
-  big  fish  and  small  fish  alike  together  with  other  sea  animals  - 
and  therefore,  in  the  last  years  much  concern  has  been  felt 
regarding  over-fishing  for  instance,  in  the  North  Sea.  Many 
people  are  also  worried  about  the  damage  wrought  by  the 
bobbins  and  footropes.  The  North  Sea  Convention  has  been 
established  to  preserve  the  fish  stock  and  it  must  be  kept  in 
mind  that  trawling  gear  should  be  so  arranged  as  to  save  the 
small  fish  if  possible. 

Trawl  doors  of  the  usual  oblong  shape  make  much  damage 
to  the  sea  bottom  and  the  animals  living  on  it,  when  they  are 
ploughing  through  the  mud.  In  designing  the  midwater 
trawl,  I  constructed  trawl  doors  which  can  be  towed  along 
the  ground  a  little,  say  2  to  10  ft.  above  the  seabed.  The  foot- 
rope  of  the  trawl  net  then  does  not  touch  the  bottom,  but 
travels  just  above  it.  1  think  this  would  be  a  good  idea  to 
avoid  damage  to  the  bottom  and,  at  the  same  time,  save  most 
of  the  small  fish.  Another  point  to  be  mentioned  in  connection 
with  such  a  trawl,  is  that  the  resistance  of  the  whole  gear  will 
decrease  considerably. 

Dr.  J.  Scharfe  (FAO):  First,  I  would  like  to  underline 
what  Captain  Roberts  said,  that  it  cannot  be  expected  of 
commercial  fishermen  to  make  experiments.  Of  course,  this 
work  has  to  be  done  by  governmental  institutes  using  research 
ships.  Furthermore,  it  is  not  enough  to  prove  the  technical 
superiority  of  a  certain  design  but  it  also  has  to  be  worked  out 
until  it  is  completely  reliable  for  commercial  use. 

One  example  of  this  arc  hydrofoil  otter  boards.  Such 
boards  have  been  tested  in  Germany  since  about  30  years  and 
most  of  the  experiments  were  carried  out  in  close  collaboration 
with  commercial  fishermen  and  resulting  increased  efficiency 
of  these  boards  was  acknowledged  also  by  those  commercial 
fishermen  involved.  But  nevertheless  such  boards  have  not 
been  accepted  by  the  fishery,  presumably  because  of  the 


unsolved  preliminary  difficulties  in  operation.  We  now  hope 
that  hydrofoil  otter  boards  may  be  introduced  in  midwater 
trawling  for  which  purpose  they  are  very  effective.  I  believe 
that  also  the  commercial  comparative  fishing  test,  which  has 
to  follow  the  technical  experiment  to  prove  the  efficiency  of  a 
new  design,  cannot  be  expected  from  commercial  trawlers 
but  has  to  be  carried  out  by  governmental  research  vessels. 

I  would  like  to  give  another  example  in  answer  to  the 
remarks  Mr.  Parrish  made  to  the  part  the  biologist  has  to  take 
in  the  development  of  gear.  Several  years  ago  experiments  were 
carried  out  to  catch  the  big  herring  schools  approaching  the 
Norwegian  coast  in  the  early  spring  by  midwater  trawling, 
because  the  depth  in  which  they  appear  outside  the  national 
limits  is  too  great  for  other  methods. 

It  was  well  known  that  very  thick  schools  were  to  be 
expected  and  we  really  saw  them  on  the  echo  sounder. 
Furthermore,  there  was  no  doubt  that  the  trawl  worked  in 
the  right  depth,  but  nevertheless  we  could  not  get  them.  The 
reason  may  be  that  the  behaviour- of  this  herring  being  on  a 
migration  to  the  coast  is  quite  different  from  the  behaviour  of 
other  herring  schools,  which  had  very  successfully  been 
caught  by  midwater  trawling  in  other  areas  and  at  a  different 
season.  These  herrings  obviously  were  too  alert  and  avoided 
the  gear.  Similar  experiences  were  made  with  the  well-known 
Larssen  two-boat  midwater  trawl — which  works  very  success- 
fully, for  instance  in  the  Kattegat  but  gave  poor  results  in 
the  northern  North  Sea  and  in  Icelandic  waters,  despite  the 
strong  concentrations  of  herring  there. 

In  the  last  two  years  a  successful  herring  trawl  fishery,  also 
in  midwater,  has  been  developed  in  the  southern  North  Sea 
in  winter  time.  This  fishery  is  interesting  in  regard  to  one  still 
pending  question  in  midwater  trawling — that  is  the  frightening 
effect  of  the  warps.  It  is  commonly  thought  that  with  two 
boat  midwater  trawls,  the  frightening  effect  of  the  warps 
leading  from  the  boats  to  the  trawl  improves  the  catching 
ability  of  the  net.  On  the  other  hand  with  the  one-boat  trawl 
the  warps  lead  from  one  point,  the  sliphook,  to  the  two  wings 
of  the  trawl  and  then  the  frightening  effect  might  chase  the 
herring  away  from  the  net  opening.  Now,  the  German 
cutters  in  the  southern  North  Sea  use  not  only  the  two-boat 
trawl  but  they  tried  also  one-boat  trawls.  As  they  did  not 
have  sufficient  experience  in  bringing  and  keeping  them  in  the 
depth  desired,  they  attached  big  floats  to  the  otter  boards 
with  strops  of  such  length  that  the  gear  fished  at  the  desired 
depth  and  they  also  got  the  herring. 

1  myself  made  some  observations  on  the  behaviour  of 
herring  towards  the  warp  by  means  of  echo-sounding.  The 
echograms  obtained  over  the  warps  compared  with  those 
taken  by  the  towing  vessel  indicate  that  there  is  a  frightening 
effect,  but  not  a  very  effective  one. 

The  herring  avoid  the  warp  and  keeps  a  distance  of  about 
2  to  4  m.,  but  after  the  warp  has  passed  the  school  closes 
again. 

Mr.  S.  Springer  (U.S.A.):  A  very  large  part  of  the  work  that 
I  have  been  associated  with  recently,  has  been  in  the  Gulf  of 
Mexico,  where  there  is  a  well-established  fishery  for  shrimp, 
for  red  snapper  and  for  menhaden.  The  point  that  Mr. 
Parrish  made  about  the  importance  of  the  cooperation  of 
biologists,  fishermen  and  technologists  sef  ms  to  me  to  be  a 
very  important  one. 

In  the  Gulf  of  Mexico  very  little  is  known  either  by  the 
biologists,  the  fishermen  or  the  technologists.  In  this  case  it 


[444] 


DISCUSSION  — FISHING    GEAR    TYPES 


will  have  to  be  the  fisherman  who  attacks  the  problem  first, 
because  until  the  fish  are  caught  ths  biologists  have  no  data 
to  work  on. 

We  have  at  times,  I  think,  used  the  wrong  approach  to  some 
of  our  problems,  as  for  example  when  we  attempted  to  use 
the  midwater  trawl  for  catching  menhaden.  If  we  had  realised 
before  we  started  this  work  that  the  menhaden  are  extremely 
active  and  fast  swimmers,  we  problaby  would  have  saved 
ourselves  quite  a  lot  of  extra  work.  By  aerial  observation  we 
found  that  in  using  a  midwater  trawl  or  almost  any  kind  of 
trawl,  the  fish  could  easily  be  got  in  the  mouth  of  the  trawl, 
but  they  could  not  be  got  into  the  codend.  We  operated  with 
three  fishing  boats  for  nearly  a  month  only  to  find  out  that 
we  could  not  catch  menhaden.  However,  this  trial  and  error 
technique  is  sometimes  necessary  in  fishing  for  species  whose 
behaviour  is  insufficiently  known. 

There  is  now  a  small  longline  fishery  for  yellow  fin  tuna 
in  the  Gulf  of  Mexico.  It  was  not  until  1952,  I  think,  that 
the  first  individual  specimen  of  yellow  fin  tuna  was  taken. 
Fishermen  travelling  over  the  area,  had  not  observed  them, 
probably  because  they  do  not  show  on  the  surface.  We  tried 
to  catch  them  by  trolling  and  were  phenomenally  unsuccessful. 
Live  bait  fishing  and  purse  seining  operations  also  failed 
so  that  we  wasted  a  whole  year  before  trying  longlining, 
which  in  this  area  is  now  modestly  successful. 

Captain  A.  Hudson  (U.K.):  I  would  just  like  to  mention 
that  we  all  know  from  practical  experience  that  between  ships 
of  the  same  firms,  the  same  horse  power,  using  the  same 
fishing  gear  as  designed  and  adapted  by  the  owners,  the  catches, 
can  differ  vastly  and  consistently.  This  can  be  due  to  more 
skilled  fishing  technique,  but  also  to  a  difference  in  rigging-up 
of  the  gear,  which  was  found  by  experimenting.  Such  experi- 
ments carried  out  by  individual  skippers  are  unluckily  mostly 
held  secret  and  should  be  made  public. 

With  regard  to  the  question  of  over-fishing  1  would  like 
to  say  that  I  don't  agree  with  Mr.  Larsson  that  otter  boards 
cause  much  damage  to  the  bottom.  Further  that  over-fishing 
should  not  be  attributed  so  much  to  the  method  of  trawling 
but  to  the  use  of  small  meshed  nets,  such  as  those  used  for 
catching  sandeel.  That  old  saying  4*we  cannot  take  out  more 
than  we  put  in"  holds  true  here.  In  my  opinion  unbridled 
catching  of  small  runner  fish  food  such  as  sandeel  should  be 
controlled  because  they  are  food  for  the  large  fish  and  without 
that,  the  stock  of  some  other  species  will  soon  be  depleted. 

Mr.  M.  Svetina  (Yugoslavia):  I  represent  the  fishermen 
from  Yugoslavia.  Much  has  been  said  about  fishing  gear, 
but  unfortunately  very  little  about  fresh  water  fishing.  1 
would  like  to  say  a  few  words  about  salmon  fishing  in  fresh 
water,  which  is  carried  out  in  my  country  as  well  as  other 
parts  of  the  world.  For  some  years  now  we  have  successfully 
used  drift  nets  in  our  lakes.  Very  poor  results  had  previously 
been  obtained  with  cotton  nets,  but  since  we  use  monofila- 
ment  nets  which  are  hardly  visible,  we  can  produce  rather 
important  quantities  of  trout  and  other  fish.  We  also  use 
with  great  success  monofilament  nets  to  capture  minnows 
for  breeding  purposes.  I  should  like  to  obtain  some  informa- 
tion about  lake  fishing  in  other  parts  of  the  world. 

Another  question  concerns  the  fishing  in  artificial  lakes. 
During  the  construction  of  such  lakes,  usually  for  electricity, 
no  steps  are  taken  to  clear  trees  and  other  obstructions  from 
the  bottom,  which  later  present  very  serious  obstacles  to 


fishing.  We  find  these  obstacles  are  quite  a  problem  and  I 
would  like  to  know  what  is  done  in  other  countries  in  this 
matter. 

Mr.  R.  S.  Rack  (Northern  Rhodesia):  I  would  like  to 
say  a  few  words  on  the  application  of  new  fishing  methods 
to  areas  where  primitive  fishery  exists,  with  special  reference 
to  lake  fishing. 

When  introducing  new  ways  of  fishing  it  is  best  to  first 
study  the  indigenous  methods  of  fishing.  These  methods  may 
not  be  the  best,  but  they  are  the  result  of  a  great  deal  of 
experimenting  and  practical  experience  by  fishermen.  Some- 
times new  methods  fail  when  introduced  in  primitive  areas, 
not  on  technical  grounds,  but  because  of  the  attitude  of  the 
natives  towards  it,  who  for  various  reasons  may  object  to  its 
use.  It  is  best  to  start  by  introducing  better  material  from  which 
to  construct  one  of  the  locally  known  gear. 

In  Northern  Rhodesia  the  amount  of  fishing  has  increased 
8-  or  10-fold  through  the  introduction  of  nylon. 

The  question  of  cooperation  between  the  biologist  and 
the  technologist  seems  to  be  no  problem  in  our  lake  fisheries 
— each  has  his  well  defined  part  to  do.  The  technologist 
implements  the  result  of  the  biologist's  survey  by  testing 
appropriate  methods  and  passes  his  own  results  on  to  the 
fishermen.  In  the  lake  fisheries  we  arc  particularly  interested 
in  gillnet  fishing  because  it  enables  us  to  control  the  size  and 
the  amount  of  fish  which  is  taken. 

Mr.  R.  Ocran  (Ghana):  Most  of  our  fishermen  still  fish 
from  dug-out  canoes,  but  during  the  last  five  years  motor  boats 
are  being  brought  in  and  used  very  successfully,  especially 
during  the  herring  seasons. 

We  also  have  a  good  line  fishing  whereby  dug-out  canoes 
manned  by  6  men  go  out  60  miles  along  the  shore,  sound  the 
rocks  and  fish  for  snapper  over  them.  Each  line  has  five 
hooks  and  these  dug-outs  can  hook  up  to  one  ton  of  snapper 
in  two  hours.  We  have  also  beach  seine  fishing,  and  gillnet 
fishing.  We  have  experienced  the  advantage  of  nylon  and 
this  is  being  used  in  increasing  quantities. 

Our  problem  is,  however,  the  choice  between  the  different 
kinds  of  synthetics  that  are  being  offered,  such  as  perlon, 
nylon,  marlon  and  others. 

Mr.  D.  L.  Alverson  (United  States):  I  would  like  to  make  a 
few  comments  at  this  time  on  some  of  the  statements  we  have 
heard.  With  reference  to  earlier  speakers  on  the  effects  of 
otter  trawl  doors  on  the  bottom,  I  would  like  to  state  that  I 
also  don't  think  that  they  are  detrimental  to  fish  stocks. 

Secondly  on  the  problem  of  selecting  a  gear  for  a  new  area 
I  agree  that  theoretically  and  possibly  academically  the 
biologist  should  first  study  the  behaviour  pattern  to  provide 
the  gear  technologist  with  data  on  which  to  base  his  choice  of 
gear;  but  let  us  get  down  to  reality.  If  we  are  in  a  new  area 
or  a  new  fishery  is  to  be  started  where  we  know  the  fish  exist, 
to  begin  with  there  is  the  difficulty  of  obtaining  funds  to  study 
a  fishery  which  does  not  exist.  In  the  second  place,  the  in- 
dustry cannot  afford  to  waste  the  time  that  is  necessary  to 
carry  out  such  biological  experiments  which  take  considerable 
time.  I  have  observed  several  fisheries  evolve  on  the  Pacific 
Coast  of  the  United  States  and  being  a  scientist  I  do  appre- 
ciate the  importance  of  experimentation.  But  to  advise 
fishermen  or  to  disseminate  information  on  the  selection  of 


[445  J 


MODERN    FISHING    GEAR    OF    THE    WORLD 


fishing  gear  for  a  new  fishery,  1  believe  the  first  step  is  to 
review  the  nature  and  methods  that  are  in  use  and  to  discuss 
their  efficiency  with  the  local  fishermen.  Furthermore,  the 
trial  and  error  method,  may  not  be  the  most  thorough  way 
of  experimenting,  but  in  the  case  of  a  new  fishery,  it  is  often 
the  quickest  way  to  obtain  results.  Where  it  concerns 
government  research  for  the  development  of  gear,  I  think  it  is 
important  that  the  biologist,  the  gear  technologist  and  the 
commercial  fisherman  work  together.  This  is  being  done  by 
the  U.S.  Fish  and  Wild  Life  Service;  the  crew  for  the  research 
vessel  John  N.  Cobb  consists  of  top  fishermen  in  trawl  fishing, 
in  gillnetting  and  seine  fisheries  and  they  are  paid  well. 

The  use  of  top  notch  fishermen  working  close  together  with 
the  gear  technologists  on  the  spot  has  contributed  largely  to 
the  success  of  the  cruises  undertaken. 

Mr.  I.  Richardson  (U.K.):  Captain  Hodsori  raised  the 
matter  of  the  type  of  vessel  used  for  mid-water  experimental 
trawling  being  not  typical  of  the  type  of  vessel  that  is  com- 
mercially used.  I  may  just  very  briefly  outline  the  problems 
which  have  presented  themselves  to  us  during  this  work.  Any 
gear  of  this  square  opening  type  of  trawl  is  fairly  easy  to  con- 
struct and  observations  show  that  this  gear  is  opening  and  has 
roughly  the  right  shape.  The  real  problem  is  still  to  catch 
the  fish  and  the  only  way  to  ascertain  that,  is  by  trying  it  out  in 
your  own  particular  locality.  Because  a  gear  works  somewhere 
does  not  prove  it  will  work  in  your  area.  It  is  the  behaviour 
of  the  fish  in  a  particular  area  that  finally  decides  whether  a 
gear  is  effective — not  the  mechanics  of  the  gear,  so  that  all 
such  tests  must  finally  be  carried  out  for  a  particular  fish  in 
the  particular  area. 


When  we  started  experimenting  on  the  midwater  trawling 
in  the  southern  North  Sea  for  herring,  we  found  that  although 
we  were  sure  of  the  mechanics  of  the  gear,  we  still  didn't 
catch  them.  Not  knowing  how  the  fish  behaved  towards  the 
trawl  we  decided  that  towing  faster  we  could  neutralize 
whatever  evading  action  the  herring  took,  so  a  bigger  vessel 
was  used.  This  raised  a  new  problem  because  immediately, 
the  nets  started  splitting.  The  water  resistance  was  too  great 
for  this  type  of  net,  made  of  the  materials  which  we  were 
using.  This  is  where  the  technologists  and  the  net  makers 
could  help  us,  to  define  the  exact  size  and  type  of  material  to 
use.  A  new  net  of  synthetic  fibre  was  constructed  and  we 
began  to  catch  a  few  fish  but  not  enough.  By  then  an  instru- 
ment became  available  that  told  us  something  of  the  behaviour 
of  the  fish- -the  headline  oscillator  described  by  Woodgate. 
This  instrument  showed  us  that  the  fish  were  not  going  in 
to  the  mouth  of  the  net,  but  passed  just  under  the  footrope 
all  the  time.  Lowering  the  net  didn't  help,  they  were  still 
going  under  the  footrope.  With  the  synthetic  net  made  of 
thinner  twine,  we  were  getting  a  considerably  higher  speed. 
It  was  then  observed  that  the  fish  were  not  escaping  below 
the  footrope  any  more,  although  the  water  was  now  clear 
instead  of  turbid  as  in  the  first  trials.  The  net  and  the  boat 
obviously  had  a  scaring  effect  on  the  fish  so  that  they  took 
avoiding  action.  But  to  find  this  out  some  type  of  instru- 
ment was  needed  which  indicates  the  fish  distribution,  the 
towing  depth  and  shows  how  the  fish  behave.  The  echo 
sounder  with  headline  oscillator  provides  these  data  and  I 
think  we  can  now  go  ahead  with  our  experiments  to  find 
the  optimum  towing  speed. 


'Chinese"  liftnet  in  Cochin,  India. 
1446} 


DISCUSSION   ON  EFFICIENT  HANDLING   OF  FISHING   GEAR 


Mr.  H.  Kristjonsson  (FAG),  Rapporteur:  The  handling  of 
fishing  gear  and  of  the  catch  is  closely  connected  with  man- 
power, and  the  fishing  industry  in  several  countries  is  now 
facing  grave  difficulties  in  attracting  men  to  the  trade  because 
of  better  opportunities  offered  by  land  based  industries. 

The  only  way  of  combatting  this  crisis  is  to  improve  the 
efficiency  of  fishing  operations  and  to  save  manpower.  This 
can  be  achieved  in  some  cases  by  improving  the  method  of 
operating  the  gear,  in  others  by  improving  the  work  on  board 
by  better  deck-layout  and  more  use  of  mechanization.  Line 
fishing  is  one  of  the  fields  where  much  effort  has  been 
made  to  improve  the  efficiency  of  gear  operation.  Yet  it  is 
surprising  that  such  elementary  and  simple  mechanical 
equipment,  as  the  vertical  line  hauling  gurdy,  so  very  com- 
monly used  on  all  longline  fishing  boats  in  the  Scandinavian 
countries  during  several  decades,  is  still  practically  unknown 
in  many  other  parts  of  the  world.  The  Japanese  tuna 
longline  hauler  described  by  the  1ZUI  Iron  Works  is  a  more 
complex  apparatus.  The  floating  tuna  longlines  arc  extremely 
long,  often  60  miles  and  more,  and  are  hauled  at  a  rate  of 
5  miles  per  hour  as  compared  to  cod  lines  in  Northern 
European  waters  which  are  hauled  at  a  speed  of  about  2 
miles  per  hour. 

Japanese  haulers  haul  automatically,  and  by  using  line  of 
appropriate  stiffness,  coil  the  line  down,  so  that  no  handling 
of  the  lines  is  needed,  only  supervision.  Such  line  haulers 
could  perhaps  be  adapted  to  longlimng  for  smaller  fish,  not 
so  much  to  speed  up  hauling,  but  perhaps  to  reduce  the 
number  of  crew  needed  at  present  and  make  work 
easier. 

The  exploratory  fishing  conducted  in  recent  years  has 
shown  that  productive  tuna  grounds  extend  really  all  round  the 
world  in  a  fairly  broad  belt  in  the  tropics.  Tuna  longlining 
has  therefore  a  much  greater  potential  than  ever  before,  but 
so  far  only  Japanese  vessels  have  been  operating  on  a  com- 
mercial scale,  except  for  a  few  commercial  boats  which  I  under- 
stand, are  working  in  the  Gulf  of  Mexico.  The  Americans  have 
tried  out  several  modifications  to  streamline  the  operation 
and  Mann  describes  such  a  new  method  of  handling  longline 
gear.  The  line  is  handled  from  a  large,  rotating  tub,  with  a 
considerable  saving  in  manpower.  Instead  of  individual 
baskets  of  line  which  need  joining  and  separation  at  each 
setting  and  hauling,  a  continuous  mainline  is  set  from  and 
hauled  into  this  tub. 

The  line  is  set  at  a  speed  of  9  knots  running  out  over  a 
setting  trough  or  line  shute.  This  is  a  simple  device  which 
has  been  used  in  Scandinavian  countries  for  decades.  It 
consists  of  a  simple  trough  which  pays  out  the  line  and  allows 
the  branch  lines  to  swing  out  clear  as  the  line  is  streamed  out 
at  full  speed.  The  use  of  such  a  line  shute  is  much  more 
efficient  than  paying  the  lines  out  by  hand  at  slow  speed, 
getting  things  tangled  up  and  the  hands  wounded.  A  step 
in  the  same  direction  is  the  steel  wire  line  described  this 
morning  by  Mr.  O'Grady  in  which  the  line  was  wound  on  a 


hydraulically  powered  reel.  A  similar  method  was  also  tried 
for  lingcod  on  the  American  West  Coast  some  years  ago. 
The  idea  was  to  have  a  system  that  would  disconnect  the 
branchlines  from  the  mainline  as  it  came  aboard,  stack  them, 
bait  the  hooks  mechanically  and  join  them  to  the  mainlines 
again  as  the  line  was  paid  out  during  setting. 

This  brings  me  to  a  question  asked  by  Mr.  Ocran  from 
Ghana  about  synthetic  materials  for  longlines.  In  Japan 
considerable  use  is  being  made  now  of  Vinylon,  under  its 
different  trade  names,  as  a  substitute  for  the  previously  used 
cotton  lines.  When  I  was  in  Tokyo  last  fall,  Vinylon  lines, 
which  had  been  used  for  4  years,  were  still  as  new.  It 
appears  that  cotton  lines  of  about  6  mm.  diameter,  when 
new,  are  thinned  down  to  5  mm.  by  stretching,  having 
made  about  I(X)  sets  during  annual  season.  Although  they 
still  retain  test  strength,  they  have  lost  their  elasticity  and  snap 
easily  due  to  sudden  jerks  and  are  therefore  unsuitable  for 
the  big  tuna.  This  is  pointed  out  as  one  of  the  advantages 
of  synthetic  line. 

We  now  come  to  a  more  recent  step  forward  in  efficient 
handling  of  gear:  the  Puretic  Power  Block.  On  the  Pacific 
Coast  of  America  the  generally  high  standard  of  living,  and 
high  income,  has  forced  the  fishermen  to  be  more  efficiency 
conscious  than  perhaps  any  other  part  of  the  world;  in  the 
handling  of  large  nets,  such  as  purse  seiners,  muscle  power 
proved  to  be  much  too  expensive.  Already  two  or  three 
decades  ago,  power  driven  rollers  and  later  strapping  of  the 
nets  from  a  high  boom  over  the  stern  of  the  vessel,  started 
the  trend  to  mechanise  the  handling  of  these  nets.  Later 
drum  seining  was  introduced,  in  which  the  entire  net  is  wound 
on  a  large  drum.  This  has  found  only  limited  application, 
mainly  in  salmon  seining  where  you  can  use  seines  of  a  certain 
shape  suitable  to  this  application.  The  power  block  is 
an  entirely  new  approach  to  the  handling  of  such  nets.  It  is 
claimed  that  at  least  two  men  can  be  cut  from  the  traditional 
crew  of  herring  and  sardine  boats  on  the  Pacific  coast.  The 
blocks  can  be  driven  cither  mechanically,  hydraulically  or  with 
a  rope  drive  and  are  available  in  five  different  si/es  to  suit 
most  sizes  and  types  of  nets.  Apart  from  their  now  proved 
efficiency  with  purse  seines,  they  can  also  be  used  with  other 
nets. 

Hauling  of  a  purse  seine  with  the  power  block  is  much 
quicker,  particularly  when  there  is  no  catch,  taking  only  «S  to 
10  minutes  to  haul.  This  greatly  increases  the  chances  the 
boats  have  of  getting  a  school  that  was  missed  in  the  first  try. 
It  facilitates  the  work  of  the  crew  as  no  heavy  weights  need 
lifting  by  hand.  We  really  need  more  fishermen  like  Puretic, 
who  have  a  brain  wave  like  this  and  the  perseverance  to  see  it 
through. 

Saito  in  his  paper  points  out  the  advantages  of  using  the 
starboard  side  in  operating  a  fishing  net  of  the  Danish  seine 
type  from  a  vessel  with  a  righthanded  propeller.  Birkhoff 's 
paper  deals  with  operating  the  trawl  over  the  stern  as  is  used 
in  some  parts  of  the  world  and  is  in  fact  common  practice  in 


447  ] 


MODERN    FISHING    GEAR     OF    THE    WORLD 


the  Mediterranean  and  on  the  Pacific  North  West  Coast. 
This  method  is  better  especially  when  using  light  gear  with  no 
heavy  footropes  and  no  bobbins.  It  simplifies  manoeuvring 
and  leads  to  a  more  rational  handling  of  the  gear  on  board. 
This  method  has  lately  been  brought  very  much  to  everybody's 
attention  in  the  Northern  European  countries  because  of  the 
recent  experiences  with  big  stern  trawlers,  such  as  the  Fairtry 
the  Russian  Pushkin  class  and  the  new  German  stern  trawlers. 

Birkhoff  in  his  paper  explains  the  various  systems  of  hand- 
ling the  heavy  trawl  gear  used  in  the  North  Atlantic  over  the 
stern  shutes,  and  particularly  the  arrangement  of  the  warps 
and  the  hanging  of  the  trawl  doors  with  and  without  gallows. 

Although  the  systems  described  work  quite  well,  experi- 
mentation is  still  going  on  to  improve  the  efficiency  and  to 
speed  up  the  operation.  A  particularly  efficient  solution 
appears  to  be  that  used  on  the  German  vessel  Heinrich  Me  ins 
which  has  a  50  m.  long  deck  so  that  the  net  can  be  heaved 
aboard  in  one  operation. 

The  Russians  seem  to  have  no  special  difficulties  with  their 
big  stern  trawlers  except  in  handling  very  big  catches,  when  it 
sometimes  happens  that  the  codend  or  the  lengthening  piece 
burst  and  the  catch  is  lost.  I  understand  they  are  seeking 
a  solution  to  this  problem  in  two  different  ways,  first  by 
strengthening  the  construction  of  the  net  and  secondly  by 
modifying  the  angle  of  hauling  relative  to  the  slope  of  the 
stern  shutc. 

This  brings  up  another  related  problem;  that  of  shelter 
decks  which  can  improve  the  working  conditions  on  board  and 
so  the  efficiency  of  the  whole  operation.  A  shelter  deck  such 
as  built  on  the  Anton  Dohrn  must  make  a  tremendous  differ- 
ence to  the  fishermen  who  don't  have  to  wear  heavy,  cumber- 
some scaclothes  when  gutting  and  otherwise  handling  the 
fish. 

In  another  paper  Birkhoff  describes  fleet  operation  of  trawlers 
with  a  mothership  but  mainly  in  connection  with  the  transfer 
of  the  catch  from  the  catcher  boats  to  the  mothership.  He 
mentions  several  methods  used  in  such  fleet  operations  and 
comes  to  the  conclusion  that  one  very  efficient  way  is  to  detach 
the  codend  from  the  trawl  and  attach  a  buoy  with  radar 
reflector  and  light  to  it.  The  floating  codend  is  then  picked 
up  by  the  mother  ship  and  taken  on  board.  Fleet  operation 
is  developing  rapidly;  the  Russians  now  use  hundreds  of  vessels 
in  such  operations  and  their  factory  ships  are  already  operat- 
ing in  the  North  Atlantic,  while  others  are  being  built.  The 
Japanese  have  since  long  operated  salmon  and  crab  fishing 
with  many  catcher  boats  from  motherships  in  the  North 
West  Pacific.  Using  advanced  techniques  of  fish  detection 
the  "Admiral"  in  charge,  sends  out  pilot  boats  in  various 
directions  to  test  the  grounds.  Their  reports  are  interpreted 
on  the  main  ship  after  which  orders  are  sent  to  the  catcher 
boats  who  are  allocated  certain  fishing  areas  to  operate  in. 
By  such  methods  it  is  possible  to  find  where  the  concentration 
is  heaviest  and  stay  on  the  fish. 

Closely  connected  to  efficient  handling  of  gear  is  the  efficient 
handling  of  the  vessel,  and  Suiyokai,  a  group  of  electronic 
manufacturers  in  Japan,  describe  magnetic  compass  auto- 
matic pilot  which  is  used  by  several  hundred  boats  in  Japan. 
By  using  a  portable  remote  controller,  the  boat  can  be  steered 
from  almost  any  point  on  deck  which  is,  of  course,  a  great 
advantage  to  the  skipper  during  hauling  and  shooting  of  the 
gear. 

In  America  thousands  of  large  and  small  fishing  boats  arc 
using  similar  equipment,  which  is  found  to  save  labour  and  fuel 


It  would  seem  that  the  use  of  automatic  and  remote  controls 
aboard  fishing  vessels  has  made  less  headway  than  could 
have  been  expected,  considering  their  widespread  use  in 
industry  on  shore,  and  considering  that  fishing  boats  are  in 
other  respects  often  generously  equipped  with  complex 
equipment.  A  point  to  keep  in  mind  here,  however,  is  that  of 
watchkceping. 

Navigational  aids  are  described  by  Alverson  and  the  Dccca 
Navigator  Co. 

Alverson  says  the  skippers  of  trawl  boats  in  the  Pacific 
North  West,  which  arc  commonly  equipped  with  Loran,  now 
even  refer  to  trawl  grounds  and  positions  at  sea,  not  by 
relation  to  headlands  or  to  the  marks  on  shore,  but  to  the 
Loran  microsecond  reading.  The  value  of  navigation  aids 
as  a  help  to  stay  on  the  fish  is  quite  obvious—for  instance  in 
covering  a  trawl  ground  systematically  without  using  a  dahn 
buoy  or  to  back-track  to  the  exact  spot  where  fish  has  been 
indicated  on  the  echo-sounder,  or  to  avoid  rough  spots  or 
obstructions  which  have  been  pinpointed  previously  and 
marked  on  the  Dccca  chart.  A  further  advantage  lies  in  the 
use  of  the  'tracker',  which  gives  a  record  of  the  track  taken 
and  facilitates  the  entry  of  harbours  in  poor  visibility.  The 
high  cost  of  this  equipment  and  the  limited  areas  covered  by 
the  stations  so  far  has  limited  the  use  of  this  equipment, 
to  certain  areas  and  to  fairly  large  si/e  trawlers.  It  will  be  to 
the  manufacturer's  advantage  to  bring  this  very  useful  equip- 
ment within  the  economic  means  of  the  smaller  fishing  boats, 
as  there  arc  several  times  more  small  boats  in  the  world  than 
big  ones. 

Crecelius  stresses  the  importance  of  being  able  to  match 
exactly  the  length  of  the  two  warps,  especially  during  trawling 
in  deep  water.  The  meters  described  are  fitted  one  on  each 
warp  near  the  winch  and  measure  the  amount  of  cable  payed 
out.  Although  he  does  not  mention  it,  I  think  such  meters 
could  be  very  useful  in  midwater  trawling  too  where  the 
amount  of  warp  used  is  very  important. 

1  would  also  like  to  draw  your  attention  to  the  paper  by 
Goodman  and  Lawton  on  the  damage  caused  by  fishermen 
to  telegraph  cables.  Some  six  companies  have  shown  very 
keen  interest  in  bringing  before  this  Congress  this  serious 
problem.  Trawlers  are  seeking  ever  deeper  grounds  and 
intensifying  the  operations  in  areas  not  previously  affected. 
The  paper  recounts  the  tragic  story  of  the  tremendous 
financial  Josses  sustained  when  cables  are  cut  and  days  are 
needed  to  splice  together  broken  ends  to  replace  missing 
sections.  We  should  try  to  find  the  means  of  solving  this 
problem  very  soon  and  one  way  is  perhaps  the  use  of 
streamlined  doors,  which  are  still  in  the  experimental 
stage. 

One  example  of  this  are  the  oval  shaped  doors  used  by  the 
Russians  now.  Such  boards  would  ride  more  easily  over  cables 
and  cause  less  damage.  Hydrodynamically  shaped  doors  like 
the  Larsson  wingdoor,  Suberkriib  boards  and  other  similar 
rational  and  radical  designs  may  also  ride  more  easily  over 
cables. 

Eddie  enlarges  on  the  very  interesting  subject  of  power 
saving  on  trawlers.  After  discussing  the  power  expended  by 
trawlers  of  different  sizes  and  basing  on  data  obtained  during 
both  experimental  and  commercial  use,  he  comes  to  the 
conclusion  that  very  often  the  skippers  are  using  much  more 
engine  power  than  necessary  for  the  fishing  operation.  He 
explains  that  on  reaching  a  certain  towing  speed,  doubling 
or  tripling  the  power  expenditure  results  in  only  a  very  small 


[448] 


DISCUSSION  — EFFICIENT     HANDLING    OF    FISHING    GEAR 


increase  in  towing  speed.  It  seems  particularly  important, 
then  to  develop  some  way  of  measuring  the  power  developed 
by  the  engine  at  exactly  measured  trawling  speeds,  so 
that  the  economic  towing  speed  can  be  determined.  He  is 
however  convinced  that  much  more  research  is  really  needed 
on  this  important  subject. 

That  brings  us  to  the  matter  of  gear  research  and  to  the 
need  for  more  rational  methods  of  developing  and  testing  gear 
which  cannot,  as  Captain  Roberts  has  already  explained  to  us, 
be  expected  to  be  made  by  commercial  vessels  working  under 
economic  pressure.  For  several  years  now  biological  and 
processing  laboratories  have  been  in  operation  in  practically 
every  fishing  country  and  in  some  cases  doing  some  quite 
expensive  research,  which  is  accepted  now  by  everybody  and 
people  are  gradually  realising  that  this  money  is  well  spent. 
That  such  research  pays.  The  time  has  now  come  for  us  to 
show  that  research  in  the  Held  of  fishing  gear  technology  also 
pays  and  to  underscore  its  importance  as  it  is  quite  obvious 
that  government  funds  will  have  to  sustain  such  research;  not 
only  must  tests  be  carried  out  on  materials  ashore,  but  also 
with  complete  gears  at  sea.  To  guide  the  fishing  industry, 
auxiliary  equipment  must  be  tested  to  ascertain  that  it  suits 
the  particular  fishing  conditions  in  different  areas;  also  to 
guide  the  manufacturers  into  developing  the  right  kind  of 
equipment  needed  by  the  fishing  industry.  New  ideas  in  gear 
development  must  be  tested  using  both  rational  design  and 
the  time  honoured  *4try,  try  and  try  again"'.  The  following 
message  should  quite  clearly  emerge  from  this  Congress— 
that  now  that  biological  research  is  established  and  gathering 
way,  the  time  has  come  to  start  and  concentrate  on  the 
study  of  the  efficient  design,  construction  and  operation 
of  ihe  gear  that  has  to  catch  the  fish.  This  is  becoming  more 
economically  pressing  every  year. 

Mr.  P.  G.  Schmidt  (U.S.A.):  Until  now  most  of  the  dis- 
cussions were  centred  around  trawling,  which  is  certainly  one 
of  the  most  important  fishing  methods.  But  other  methods 
are  also  very  important  and  personally  I  am  more  interested 
in  purse  seining.  Purse  seining  seems  to  have  been  neglected 
and  the  working  method  has  not  been  improved  in  relation  to 
its  importance,  when  compared  with  trawling.  One  of  the 
reasons  for  this  is,  that  traditionally  a  large  number  of  men 
were  required  to  pull  in  the  net,  and  therefore,  we  find  that 
on  the  purse  seine  vessels  many  other  operations  are  still  done 
manually  because  this  large  man  power  was  always  avail- 
able. 

I  am  basically  an  engineer  and  a  boat  builder,  but  I  have 
become  thoroughly  interested  in  the  efficiency  of  fishing 
operations.  In  the  course  of  my  travels  connected  with  the 
introduction  of  the  Puretic  Power  Block  in  purse  seine  fishing, 
I  have  had  an  opportunity  of  visiting  most  of  the  important 
purse  seine  fisheries  in  the  world.  1  have  not  been  able  to 
find  out  why  the  methods  used  in  each  area  arc  so  widely 
different.  Why  are  they,  for  instance,  in  South  Africa,  using 
the  Lampara  purse  seine  with  two  tapered  wings  hauled  on  to 
one  boat;  why  in  Norway  do  they  use  two  small  seine  boats 
working  with  a  mother  vessel;  why  do  they  use  a  similar  system 
for  catching  menhaden  off  the  U.S.  Coast  .which  is  one 
of  the  world's  largest  fish  producing  areas;  why  in  Iceland 
where  originally  a  system  similar  to  the  Norwegians  was  used, 
are  they  now  working  with  two  or  three  different  systems, 
including  the  use  of  one  small  seine  boat  carrying  the  net, 
with  a  mother  vessel.  Why  is  it  that  in  Portugal  they  use  a 


net  which  they  call  American  style,  with  one  boat  and  a  small 
skiff,  and  why  do  Japanese  have  a  method  where  two  big  tuna 
seiners  carry  each  one  half  of  a  monstrous  net  and  work 
together?  I  am  sure  that  there  are  reasons  for  using  such 
different  methods.  However,  I  believe  that  in  most  cases 
these  reasons  are  historical  and  that  with  present-day  tech- 
niques, simple  machinery,  fittings  and  devices  available,  many 
of  these  methods  have  become  obsolete. 

In  Norway  for  instance,  the  very  best  fishing  captains  will 
tell  you  that  they  have  refined  their  methods,  have  been  working 
the  method  for  years  and  years,  catch  more  herring  than  any 
other  country  in  the  world,  and  that  the  method  is  therefore 
efficient.  I  wonder,  if  it  is  really  the  most  efficient  method. 
I  wonder  whether  the  reasons  for  using  two  small  open  boats 
are  still  valid,  and  if  it  is  still  necessary  to  have  18  to  30  men 
for  handling  nets  which  in  comparison  with  those  used  in 
Japan  and  on  the  West  Coast  of  the  United  States  may  be 
considered  to  be  rather  small  nets. 

The  previous  discussions  were  mainly  concerned  with  the 
design  of  nets,  the  materials  for  making  nets  and  so  forth.  I 
believe  that  in  purse  seine  fishing,  these  things  are  not  nearly 
as  important  as  the  operational  method  in  connection  with 
the  economical  use  of  manpower  and  the  appropriate  and 
effective  use  of  equipment.  Yet  the  power  block  is  but  one  piece 
of  the  mechanical  equipment.  For  instance,  the  Lampara 
seine  fishery  in  South  Africa  is  fairly  new  and  I  think  most 
of  our  South  African  representatives  can  remember  that  about 
10  years  ago,  maybe  a  little  longer,  it  was  a  tremendous  step 
forward  to  get  the  first  purse  winch  introduced.  We  from  U.S. 
could  not  possibly  conceive  of  handling  and  pursing  a  large 
net  without  a  purse  winch,  and  yet  this  was  a  big  step  in  an 
area  which  had  not  been  using  such  nets  before. 

We  have  now  found  a  new  way  to  haul  large  nets  on  to  the 
fishing  vessel  more  efficiently,  faster,  with  less  men,  with  less 
wear  on  the  nets  than  any  historical  hand  hauling  method. 

When  discussing  purse  seining  with  the  local  people,  we 
always  suggest  that  they  should  first  attempt  to  increase  the 
efficiency  of  the  method  used  on  the  present  boats.  After  all, 
they  have  large  fleets  of  boats  and  it  is  impossible  for  them  to 
dispose  of  these  and  build  new  boats,  which  is  furthermore 
unnecessary  as  the  method  can  be  adapted  to  practically 
every  boat.  On  trying  to  get  them  used  to  the  idea  of  hauling 
the  net  by  power,  we  immediately  meet  scepticism;  what  are 
we  going  to  do  with  the  purse  rings:  how  are  we  going  to 
get  them  off  from  the  purse  line,  how  will  the  catch  be  hardened 
in  the  bag,  etc.?  We  know  from  experience  that  this  hardening 
is  not  a  problem,  but  in  the  minds  of  the  local  fishermen  it  is. 
The  fact  is  that  on  the  existing  boats,  booms  and  other 
equipment  does  not  fit  in  with  mechanical  hauling  and  needs 
some  changes. 

Three  years  ago  we  thought  that  introducing  more  efficient 
methods  to  the  menhaden  industry  on  the  East  Coast  of  the 
U.S.  would  be  easy.  They  have  approximately  600  small 
seine  boats  operating  as  pairs  with  about  300  large  vessels 
which  they  call  steamers.  Our  problem  was  not  operating  the 
net,  but  in  finding  support  for  the  power  block  on  such  small 
open  seine  boats  and  to  find  a  system  to  handle  the  purse 
rings,  the  cork  lines  and  some  more  details.  After  working 
with  the  industry  now  for  two  and  a  half  years,  most  of  these 
problems  have  been  solved  with  the  result  that  we  have 
groups  of  boats  which  are  now  fishing  with  six  men  less  per 
net. 

In  Iceland  they  are  starting  to  develop  methods  of  reducing 


[449] 


EE 


MODERN    FISHING     GEAR    OF    THE    WORLD 


crews,  because  in  Iceland  there  is  a  great  shortage  of  manpower 
and,  of  course,  these  are  the  areas  where  it  is  easiest  to 
introduce  mechanization. 

I  might  just  very  briefly  comment  on  some  specific  questions 
which  were  brought  up  by  others  concerning  other  fishing 
methods  in  the  North  West. 

Mr.  Kristjonsson  mentioned  the  automatic  pilot.  On  the 
Pacific  Coast  I  would  guess  we  have  maybe  3,000  fishing 
boats  of  one  type  or  other  and  J  would  say  they  are  virtually 
100  per  cent,  equipped  with  a  very  inexpensive  automatic 
pilot,  costing  somewhere  around  U.S.S  300  to  400. 

Concerning  longline  hauling,  there  arc  two  firms  in  the 
North  West  of  the  United  States  who  have  experimented  with 
automatic  longline  haulers  which  are  a  bit  different  from  what 
has  been  used  elsewhere.  They  consist  of  a  device  which  has 
drums  on  which  the  wire  longline  gear  is  reeled  and  which 
are  driven  mechanically  and  operated  with  clutches  and  brakes 
so  that  the  branchlincs  can  be  attached  as  the  gear  is  shot. 
This  was,  however,  never  generally  adopted  in  the  area, 
although  it  is  felt  in  our  area  that  this  gear  docs  have  merits 
and  it  should  be  further  developed. 

Mr.  E.  R.  Geroult  (France):  1  would  like  to  confirm  what 
Mr.  Eddie  has  stated  in  his  very  interesting  paper.  Each 
time  when  it  has  been  possible  to  take  measurements  of  power 
requirements  of  trawlers  at  sea,  we  have  found  that  less  power 
was  actually  used  than  is  continually  requested  by  the  skippers. 
We  think  therefore  that  500  to  1,000  h.p.  is  about  what  we 
need  for  a  bigger  trawler.  About  500  h.p.  arc  needed  for 
trawling,  and  about  850  or  even  a  little  less  for  free  running. 

As  regards  stern  trawling,  there  are  in  Portugal,  in  Spam 
and  in  Africa  a  certain  number  of  boats  who  use  this  method. 
They  allow  for  interesting  comparisons  with  the  new  big  stern 
trawlers  recently  developed  in  Northern  Europe  which  have  a 
stern  shute.  Most  likely  something  useful  could  be  learned 
from  these  techniques,  particularly  as  regards  the  hauling 
operation.  With  the  big  stern  trawlers  there  are  problems 
concerning  the  handling  of  larger  catches  over  the  stern 
shutc.  There  might  be  less  danger  of  wearing  or  even  bursting 
the  codcnd  and  less  damage  to  the  fish  with  the  Mediterranean 
method  of  hauling  the  catch  on  deck  almost  vertically  and 
without  a  stern  shute.  For  bigger  boats  and  larger  catches  this 
would  require  some  special  crane  arrangement. 

Mr.  A.  I.  Treschev  (U.S.S.R.):  According  to  our  experience 
stern  trawling  is  quite  efficient  in  comparison  with  side 
trawling.  The  slightly  longer  time  needed  for  operation  in 
stern  trawling  is  fully  compensated  by  the  possibility  to 
continue  fishing  in  bad  weather  conditions.  In  our  opinion, 
stern  trawling  is  more  convenient  from  the  point  of  ease  of 
operation,  in  regard  to  the  fishing  operation  itself  and  to 
provide  the  space  needed  for  fish  processing.  There 
are  some  difficulties  in  hauling  big  catches,  but  we  hope 
to  overcome  these  pretty  soon. 

Mr.  J.  G.  de  Wit  (Netherlands):  1  feel  obliged  to  warn 
against  automatic  steering  gear,  particularly  if  it  is  of  the  type 
that  can  be  operated  by  hand  from  any  point  outside  the 
wheelhouse.  According  to  my  experience,  fishermen  are 
inclined  to  neglect  watch  keeping  generally.  I  expect  that 
automatic  steering  will  aggravate  this  trend  and  will  subse- 
quently do  more  damage  to  fishing  gears  and  lead  to  more 
collisions. 


Particularly  in  trawl  fishing  there  is  a  close  relation  between 
the  ship  and  the  gear.  The  ship  has  to  tow  the  trawl  gear  at  a 
certain  speed  for  which  a  certain  amount  of  towing  power  is 
needed  which  is  supplied  by  the  main  engine  through  the 
propeller.  The  lowing  power  requirement  should  be  the  starting 
point  for  the  estimation  of  the  main  engine.  As  the  towing 
resistance  increases  practically  with  the  square  of  the  speed, 
the  fishermen  should  be  careful  in  their  power  requirements 
because  too  big  an  engine  means  excessive  initial  and  running 
costs.  The  risk  of  spoiling  the  economy  of  the  vessel  is  not 
imaginary. 

In  the  Netherlands  also,  many  owners  and  skippers  are 
inclined  to  increase  the  engine  power  of  their  trawlers. 
Recently  some  trawlers  of  275  to  310  tons  gross  were  put  into 
service  and  they  have  main  engine  power  of  1,000  h.p.  at  a 
speed  of  1 1  knots.  The  length  of  these  vessels  is  less  than  140 
feet  between  perpendiculars. 

In  this  connection  1  also  wish  to  underline  the  remarks 
Mr.  Eddie  made  in  his  paper  on  the  power  measurements  he 
carried  out  on  three  180  ft.  trawlers  during  trawling.  From 
these  valuable  remarks  it  can  be  concluded  that  vessels  of 
132  ft.  and  1,000  h.p.  are  over-powered. 

The  pull  which  the  vessel  has  to  exert  on  the  warps  consists 
of  the  net  resistance  and  the  resistance  of  otter  boards  and 
warps.  Speaking  of  the  resistance  of  bottom  trawls,  one  has 
to  take  into  account  the  fact  that  this  consists  of  hydrodynamic 
resistance  and  bottom  friction.  Each  of  these  factors  follows 
its  own  laws.  A  recent  research  into  the  pull  of  a  hydraulic 
trawl  winch  during  the  hauling  operation  showed  that  this 
pull  was,  at  the  beginning,  about  10  to  11  percent,  higher  than 
towards  the  end  because  the  frictional  resistance  had  dis- 
appeared and  the  hydrodynamic  resistance  of  the  warps  had 
decreased. 

Personally,  1  regret  that  so  few  papers  are  dealing  with  the 
resistance  of  the  otter  boards.  The  otter  boards  are  supposed 
to  provide  shear.  The  drag  has  to  be  accepted  as  a  necessary 
evil.  The  conventional  flat  otter  boards  arc  not  the  most 
suitable  ones  from  a  hydrodynamical  point  of  view,  as 
Schiirfe  states  in  his  paper.  But,  perhaps  the  otter  boards 
also  have  a  function  in  ploughing  the  ground  and  stirring 
up  the  mud  in  front  of  the  trawl  net  to  camouflage  it.  It 
would  be  worth  while  to  know  whether  this  action  is  of  real 
significance  or  not. 

Decreasing  the  towing  resistance  means  decreasing  the  fuel 
bill  and  raising  the  economy  of  the  vessel.  I  cannot  under- 
stand why  otter  boards  like  the  Subcrkrub  boards  for  instance, 
did  not  come  into  a  more  general  use.  The  same  applies  to  the 
warps.  Adoption  of  warps  of  a  smaller  diameter  and  a  higher 
tensile  strength  decreases  the  resistance  of  the  warps  in  the 
water. 

There  seems  to  be  a  difference  of  opinion  between  Lewis 
and  de  Boer  concerning  the  changes  of  the  resistance  of  the 
fishing  gear  in  relation  to  the  amount  of  catch.  Lewis  mentions 
the  closing  of  the  trawl  warps  and  the  reduction  in  engine 
revolutions  as  an  indication  of  an  increasing  amount  of 
catch.  Both  factors  point  to  an  increase  in  the  resistance 
of  the  fishing  gear.  De  Boer,  however,  states  that  filling 
the  codend  results  in  a  decrease  of  the  pull. 

The  solution  of  this  puzzle  might  be  that  both  are  right. 
The  dynamometer  in  de  Boer's  case  was  arranged  between 
the  net  and  the  otter  board.  The  decrease  of  the  net  resistance 
meant  a  decrease  of  the  pull  of  the  net  to  the  otter  boards, 
resulting  in  an  increase  of  the  angle  of  attack  of  the  otter 


[450] 


DISCUSSION       EFFICIENT     HANDLING     OF     FISHING     GEAR 


boards  and  consequently  an  increase  of  the  total  drag.  This 
increase  of  the  total  drag  results  in  a  closing  of  the  trawl 
warps  and  a  decrease  in  the  number  of  revolutions.  If  this 
suggestion  is  right,  it  would  underline  the  importance  of  the 
drag  of  the  otter  boards. 

At  a  speed  of  about  5  knots  something  seems  to  happen  with 
the  trawl  gear.  Eddie  refers  to  a  slower  increase  of  the  drag. 
In  one  gear  this  seems  to  occur  at  a  lower  speed  than  in  another 
gear.  1  should  like  to  stress  that  the  trawlers  should  be  designed 
for  optimal  economic  performance. 

This  involves  careful  consideration  of  problems  such  as 
what  is  the  best  towing  speed?  which  pull  must  consequently 
be  expected?  and  is  it  not  possible  to  modify  the  trawl  gear 
in  order  to  reduce  the  drag  without  affecting  the  fishing 
efficiency? 

These  questions  also  apply  to  the  auxiliary  engines  of 
diesel  trawlers,  particularly  the  winch  drive.  There  is  a  trend 
to  increase  the  hauling  speed  of  the  winches,  but  does  the 
saving  of  a  few  minutes  of  the  hauling  time  really  justify  the 
higher  costs  of  the  much  more  expensive  winch  installation? 
Such  questions  are  particularly  important  for  trawlers  which 
arc  not  designed  for  the  high  free  running  speeds  of  1 3  knots 
and  more.  All  such  problems  require  a  close  cooperation 
between  the  fisherman  who  operates  the  boat  and  the  trawl 
gear,  the  designer  of  the  trawl  gear,  the  scientist  studying 
the  behaviour  of  the  trawl  gear  and  the  naval  architect.  I 
wish  to  express  the  hope  and  the  expectation  that  this  Con- 
gress and  the  Fishing  Boat  Congress  may  further  this  co- 
operation. 

Mr.  P.  A.  de  Boer  (Netherlands):  In  my  opinion,  with  a 
certain  amount  of  catch,  the  meshes  of  the  net  arc  stopped  up 
and  the  water  overflows.  This  results  in  a  decrease  of  the 
net  resistance.  In  the  conventional  otter  board  adjustment 
the  pull  of  the  net  has  a  regulating  effect.  Lower  pull  allows 
an  increase  and  higher  pull  forces  a  decrease  of  the  angle  of 
attack.  If,  due  to  low  net  resistance,  the  angle  of  attack  exceeds 
a  certain  limit,  not  only  the  drag  goes  up  considerably  but  also 
the  shear  goes  down.  This  effect  might  be  responsible  for 
both  the  increase  in  total  resistance  at  decreased  net  resistance 
and  the  closing  of  the  warps.  As  the  decrease  of 
the  net  resistance  seems  to  be  more  significant  than  the 
increase  in  the  resistance  of  the  boards  which  is  partly 
camouflaged  by  the  former,  I  suggest  that  for  determining  the 
amount  of  catch  by  changes  in  the  towing  resistance,  the 
pull  measurements  should  be  made  behind  the  otter  boards. 
Suitable  equipment  for  such  underwater  measurements  is 
available  and,  with  a  cable  connection  as  described  in 
McNccly's  paper,  an  indication  or  even  recording  could 
easily  be  arranged  in  the  whcclhouse. 

Mr.  A.  O'Grady  (Australia):  The  trap  fishery  in  Australia 
is  rather  an  important  fishery  for  snapper  and  other  bottom 
fish.  The  type  of  trap  used  is  a  wooden  framed  rectangular 
shaped  trap,  sometimes  5  to  6  ft.  in  length,  2J  to  3  ft.  in  height 
and  a  similar  width.  It  is  completely  covered  with  2A  in. 
diameter  galvanised  wire  netting.  These  traps  arc  baited  and 
lowered  to  the  bottom  and  are  secured  with  a  buoy  rope  of 
1  to  1}  in.  sisal  or  manila  rope,  and  the  buoy  line  is  held  up 
by  6  in.  glass  floats.  Now,  we  have  quite  a  problem  with  this 
fishery.  The  traps  arc  set  in  waters  up  to  about  50  fms.  in 
depth,  where  normally  a  strong  current  is  running,  but  slackens 
from  time  to  time.  We  find  that  the  current  assists  the  fishing, 


for  when  the  current  is  slack  very  few  fish  enter  the  traps.  How- 
ever, when  the  traps  are  lowered  to  the  bottom  during  strong 
current,  it  is  not  long  before  the  glass  floats  submerge  and 
disappear.  These  traps  may  stay  submerged,  including  the 
buoy  line  and  the  floats  for  sometimes  up  to  7  or  8  days.  So 
that  when  the  fishermen  go  out  to  pick  up  their  traps  they 
often  cannot  find  them.  Jt  is  thought  that  the  traps  continue 
to  fish  until  the  bait  is  all  eaten.  Furthermore,  when  the  fish 
are  left  unnecessarily  long  in  the  trap  they  are  frequently 
mutilated  by  colliding  with  the  wire  netting. 

Possibly  the  use  of  a  fine  galvanised  wire,  instead  of  the 
thicker  buoy  lines  may  be  the  answer  to  this  problem.  I 
understand  from  Mr.  Ocran  of  Ghana  that  they  are  engaged 
there  in  a  hand-line  fishery  for  snapper  (the  same  kind  of  fish 
we  trap  for)  and  that  they  propose  to  engage  a  trap  fishery 
similar  to  that  used  in  Australia.  Perhaps  this  current 
problem  in  trap  fishing  has  been  solved  somewhere  else  and 
I  would  be  glad  if  1  could  get  some  advice. 

Mr.  H.  Kristjonsson  (FAO):  I  think  Mr.  O'Grady's  problem 
of  submerging  floats  is  much  more  widespread  than  just  trap 
gear.  It  also  concerns  set  longlines  and  in  fact  any  gear  that 
employs  anchored  marker  floats  or  buoys.  Light  wires  are 
in  fact  used  by  trawlers  for  these  marker  buoys  when  set  in 
deep  water.  Light  wire  has  also  been  used  for  many  years 
in  France  and  other  countries  for  traps  set  in  deep  water  and 
small  hand  reels  are  then  used  to  haul  the  pots.  To  avoid 
the  traps  being  carried  away  by  the  tide  in  such  strong 
currents,  most  fishermen  use  small  grapnels  instead  of  the 
traditional  heavy  stones. 

The  submerging  of  the  floats  can  be  partly  avoided  to  some 
extent  by  the  use  of  several  floats  spaced  along  the  end  of  the 
buoy  rope,  but  is  not  altogether  satisfactory.  Such  floats  as 
designed  by  Larsson  which  combine  static  and  hydrodynamic 
buoyancy  may  be  the  answer  to  this  problem. 

Dr.  Ch.  Hennings  (Germany):  I  am  concerned  with  handling 
and  processing  of  fish  rather  than  catching  it  and,  therefore, 
mainly  interested  in  quality  problems.  It  is  well  known  that 
the  quality  of  fish  depends  very  much  on  how  it  was  caught 
and  how  it  was  treated  on  board  the  fishing  vessel. 

It  is  for  instance,  known  that  fish  caught  with  gill  nets  or 
longlines  is  often  more  palatable  and  more  valuable  food 
than  trawler  fish.  We  also  know  that  fish  which  is  maltreated 
or  is  not  adequately  stored  on  board,  consequently  loses  in 
nutritive  and  market  value  and  may  be  unsuitable  for  any 
further  processing  for  human  consumption.  1  am  aware  that 
such  problems  are  not  exactly  within  the  scope  of  this  Congress 
and  therefore  would  not  go  into  further  details.  But,  J  think 
it  might  be  useful  just  to  mention  that  when  considering  fishing 
gear  design  and  operation  the  problem  of  fish  quality  must 
be  kept  in  mind  too. 

Captain  I.  R.  Finlayson  (U.K.):  I  command  a  cable  ship 
and  1  am  speaking  on  behalf  of  operators  of  submarine  cables, 
who  own  thousands  of  miles  of  telegraph  and  telephone  cable 
lying  on  the  seabed  in  fishing  areas  throughout  the  world.  We 
have  a  problem  which  has  been  mentioned  in  the  paper  by 
Goodman  and  Lawton  which  was  so  ably  amplified  by  the 
Rapporteur.  We  feel  that  this  problem  is  not  generally 
known  in  the  fishing  industry. 

Submarine  cables  are  armoui  ed  with  steel  wires,  they  vary 
in  diameter  from  1  to  3  in.  They  are  sometimes  fouled  by 


[451  ] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


trawling  gear,  particularly  by  otter  boards  when  the'se  have 
defective  shoes  or  protruding  bolts. 

Sometimes  the  otter  boards  skid  along  the  cable  for  hundreds 
of  fathoms  damaging  the  cable  before  it  frees  itself  and  other 
times  the  cable  is  torn  in  two  by  the  fishing  vessel.  Occasion- 
ally the  cable  is  even  hauled  up  to  the  surface  together  with 
the  fouled  gear  and  then  cut  away.  This  can  be  highly  danger- 
ous to  the  fisherman,  as  some  of  these  cables  now  carry  up  to 
3,000  volts. 

This  damage  and  the  consequent  interruption  of  inter- 
national communication  is  a  very  serious  matter  for  everyone 
and  the  cost  of  repairing  is  high.  During  the  last  year,  there 
have  been  more  than  one  hundred  interruptions  throughout 
the  world  due  to  trawlers  and  other  fishing  vessels.  1'he  cost 
of  repairing  these  runs  into  millions  of  pounds,  and  the  loss 
of  revenue  while  the  cables  are  broken,  cannot  be  estimated. 

I  ask  designers  and  manufacturers  of  trawl  gear  and  other 
fishing  equipment,  to  bear  the  cables  in  mind  and  ensure  that 
there  are  no  projections  on  otter  boards  or  fittings.  The 
gear  should  be  designed  to  avoid  fouling  submarine  cables 
and  so  reduce  the  risk  of  damage  to  both  the  fishing  gear  and 
the  cables. 

I  would  also  like  to  remind  owners  and  skippers  of  fishing 
vessels  that  under  Article  7  of  the  International  Convention 
for  the  Protection  of  Submarine  Cables,  1884,  when  they 
sacrifice  gear  to  avoid  damaging  submarine  cables  they  shall 
and  I  repeat  they  shall,  receive  compensation  from  the  cable 
owner. 

Mr.  J.  R.  Lenicr  (France):  I  should  like  to  reply  to  the 
question  raised  by  the  Rapporteur;  on  why  the  Decca  system 
is  not  used  on  small  boats.  The  existing  Decca  chains  would 
allow  for  that,  at  least  in  the  North  Sea  and  considerable 
parts  of  the  Atlantic  and  most  of  the  big  trawlers  use  this 
simple  and  reliable  method  for  spot  plotting  their  position. 
I  think  that  this  navigational  aid  would  be  even  more  valuable 
for  the  smaller  boats,  the  skippers  of  which  usually  do  not  have 
the  same  navigational  training. 

Now,  in  France,  and  I  do  not  know  whether  this  goes  for 
other  countries,  the  small  fishing  boats  of  16  to  20  m.  in  length 
simply  do  not  have  the  space  for  the  installation  of  the  rather 
bulky  equipment.  They  therefore  have  to  use  simpler  equip- 
ment like  radio  direction  finders.  I  think  it  is  mainly  up  to 
the  boat  designers  to  provide  the  space  needed  for  the  equip- 
ment and  the  special  maps  and  then  Decca  could  also  be 
used  by  small  boats. 

Prof.  £.  Halme  (Finland):  In  1948  the  Fisheries  Foundation 
in  Finland  announced  a  prize  competition  for  fishermen's 
accounts  of  their  experiences  and  discoveries  concerning  the 
habits  of  fish  and  special  methods  or  devices  of  fisheries 
techniques.  The  primary  purpose  was  to  collect  such  material 
for  scientific  purposes.  It  was  left  to  the  fishermen  themselves 
to  decide  what  kind  of  things  they  considered  important.  To 
this  prize  competition  answers  were  received  from  365  different 


persons  from  the  whole  coastal  area  of  Finland  and  from  the 
inland  waters.  The  majority  of  the  accounts  covered  the  life 
and  spawning  habits  of  the  different  species  of  fish  but  all 
kinds  of  small  devices  concerning  gear  and  fishing  methods 
were  also  given.  These  replies  formed  a  pile  of  about  half  a 
metre  thick  and  study,  screening  and  indexing  will  take  quite 
a  considerable  time. 

This  is  the  guide  to  fishermen's  lore.  It  is  intended  to  have 
this  material  compiled  and  printed  in  form  of  a  book.  Most 
of  the  devices  described  concern  our  specific  problems  of  how 
to  get  fish  from  our  Baltic  area  and  from  our  7,000  lakes.  But 
1  think  that  in  spite  of  this  and  eventual  language  difficulties, 
from  the  many  pictures  in  this  book  you  would  understand 
how  much  information  you  could  get  in  this  way  from  the 
fishermen  of  your  own  countries. 

Mr.  Z.  Zebrowski  (Poland):  This  time  I  am  talking  to  you 
as  a  manager  of  a  fishing  company.  As  such  I  am  responsible 
not  only  for  the  efficiency  of  gear  and  the  speed  of  work,  but 
also  for  the  safety  on  board.  You  may  remember  that  not 
very  long  ago,  one  new  British  trawlet  capsized  because  his 
gear  was  caught  and  the  crew  lost  their  lives.  In  my  company 
we  fortunately  have  not  had  such  a  serious  accident.  But  I  am 
sorry  to  say  that  we  have  had  accidents,  when  the  gear  was 
caught,  and  either  the  wires  broke  and  then  slashed  across 
the  deck,  or  the  bollards  were  torn  away.  In  both  cases 
the  people  working  on  deck  arc  in  serious  danger  of  being 
hurt  by  the  warps. 

I  want  to  ask  how  such  accidents  could  be  avoided.  Is  it 
not  possible  to  have  a  trawl  winch,  the  brakes  of  which  give 
way  when  the  gear  is  caught  and  the  tension  on  the  warps 
exceeds  a  certain  value  well  below  their  breaking  strength? 
Or  could  not  each  gallows  be  provided  with  a  separate  winch, 
operated,  for  instance,  by  a  synchronic  arrangement  from 
the  bridge?  In  the  latter  case  the  warps  would  not  run  across 
the  deck  where  the  people  are  gutting  fish.  I  think  this  is  a 
problem  of  utmost  importance  because  it  is  concerned  with 
the  safety  of  human  life. 

Mr.  L.  Soublin  (Chairman):  I  am  very  grateful  to  Mr. 
Zebrowski  for  having  closed  the  discussion  with  this  humane 
matter.  I  am  glad  to  say  that  in  France  this  problem  is 
already  solved.  As  far  as  I  know,  on  the  latest  French 
trawlers  there  is  a  system  which  allows  the  warp  to  slip  when 
the  resistance  becomes  too  great. 

In  closing  this  meeting,  I  should  like  to  refer  to  what  was 
said  by  Mr.  De  Wit,  who  pointed  out  that  automatic  piloting 
did  not  just  point  to  the  need  for  careful  watch  by  the  skipper. 
J  would  like  to  go  even  a  little  further— nothing,  of  course, 
can  act  as  a  substitute  for  the  intelligence  of  man.  All  the 
technical  improvements  which  are  made  for  the  boats  are  an 
aid  to  this  intelligence  but  nothing  replaces  intelligence. 
Nothing  can  be  a  substitute  for  the  personal  qualities  of  the 
captain  and  nothing  can  act  as  a  substitute  for  the  capacities 
of  energy,  endurance  and  intelligence  of  the  crew. 


[452] 


Section  10:  Location  of  Fish. 


LOCATING   FISH   CONCENTRATIONS   BY   THERMOMETR1C 

METHODS 

by 
G.  DIETRICH 

Institut  fiir  Meereskunde  der  Universitat  Kiel,  Germany 

D.  SAHRHAGE  and  K.  SCHUBERT 

Institut   fiir  Scefischerei,   Hamburg,   Germany 

Abstract 

A  correlation  exists  between  the  distribution  of  water  temperature  and  the  concentration  of  some  species  of  commercial  fishes, 
sometimes  for  the  whole  year,  but  mainly  only  during  special  seasons.  In  those  areas  where  there  are  temperature  differences  during  the  year 
or  season,  the  correlations  are  useful  for  defining  the  distribution  of  the  fish  concentrations.  Temperature  differences  can  be  caused  by 
convergence  or  divergence  of  water  movements  and  also  by  vertical  turbulence  of  the  tidal  stream  and  examples  are  given  of  cod,  haddock, 
yellowfin  tuna  and  herring  in  relation  to  these  differences.  Temperature  may  not  be  the  only  influencing  factor,  but  it  is  the  easiest  to  observe 
and  the  authors  feel  that  the  results  of  their  work  could  well  be  augmented  by  further  surveys  by  specially  equipped  vessels. 

Localisation  des  concentrations  de  poissons  par  les  mcthodes  thermometriques 
Rfeum* 

II  existe  une  correlation  cntre  la  distribution  de  la  temperature  dcs  eaux  et  la  facon  dont  les  concentrations  de  ccrtaines  espcces 
commerciales  de  poisson  sont  reparties,  parfois  toule  Tannee  mais  principalem^nt  pendant  certaines  saisons.  Dans  les  rdgions  ou  la  temper- 
ature varie  au  cours  de  I'annee  ou  de  la  saison,  la  connaissance  de  cette  correlation  est  premie  use  pour  la  localisation  des  concentrations  de 
poisson.  Les  differences  de  temperature  peuvent  Stre  provoquees  par  des  deplacements  de  masses  d'eau  convergentes  ou  divergcntes  ainsi 
que  par  la  turbulence  verticale  des  courants  de  maree.  Les  auleurs  donnent  des  exemples  de  la  repartition  des  banes  de  morues,  de  haddocks, 
dc  thons  et  de  harengs  en  fonction  de  ces  differences.  La  temperature  pcut  ne  pas  etre  le  seul  facteur  en  cause,  mais  it  est  le  plus  facile  a 
observer,  ct  les  auteurs  estiment  que  les  resultats  de  leurs  travaux  pourraient  etre  enrichis  par  d'autres  campagnes  d'observation  exccutees 
par  des  navires  dotes  d'un  6quipcment  appropric. 

Localizacion  de  concentraciones  de  peccs  mediante  metodos  termometricos 
Extracto 

A  veces  durante  todo  el  aflp  y,  de  prefercncia,  dtirante  ciertas  temporadas  existe  una  corrclacidn  entre  la  temperatura  del  agua 
y  la  concentracion  de  algunas  espccies  de  peces  de  importancia  comercial.  En  aquellas  zonas  donde  hay  diferencias  tcrmicas  anuales  o 
estacionales,  esta  correlaci6n  cs  util  para  precisar  la  distrihucion  de  las  concentraciones  dc  peces.  Las  variacioncs  de  temperatura  pueden 
deberse  a  la  convergencia  o  divergencia  de  los  niovimientos  del  agua  y  tambien  a  la  turbulencia  de  las  corricntes  de  mareas.  Para  ilustrar 
estas  diferencias,  en  cl  trabajo  original  se  dan  ejemplos  relatives  a  las  siguientes  especies:  bacalao.  eglefino,  atiin  de  alcta  amarilla  y  arenque. 
Probablemente  la  temperatura  no  es  el  unico  factor  que  ejerce  infiuencia,  pcro  se  observa  con  mayor  facilidad  que  otros,  y  los 
autores  creen  que  los  resultados  de  su  trabajo  podrian  aumentarse  con  nuevos  estudios  mediante  barcos  cquipados  especialmente  para  tal 
objeto. 


GENERAL  BASIS   FOR   FISH  FINDING  BY 
THERMOMETRIC  METHODS 

INVESTIGATIONS  have  established  beyond  doubt 
some  general  facts  which  can  be  summarized  as 
follows: 

1.  The   stocks   of  commercial   fish   are   intimates   of 
ecological  systems  in  which  interrelated  physical  and 
chemical   conditions   of  great   complexity   operate 
and  fluctuate  from   year  to  year.    These  systems 
exercise  a  decisive  influence  on  reproduction,  growth 
and  mortality  of  the  organisms,  and   no  rational 
study  of  food  fish  can  be  undertaken  without  ex- 
haustive   inquiry    into    the    physico-chemical    con- 
ditions. 

2.  Among  the  different  factors  of  the  ecological  systems 
such  as  temperature  of  the  sea  water,  salinity,  depth, 
pressure,    currents,    content    of   nutrients,    oxygen, 
intensity  of  light,  osmotic  pressure,   hydrogen-ion 


3. 


concentration,  food,  etc.,  temperature  and  food  are 
the  most  outstanding.  Food,  however,  consists 
either  of  other  animals,  which  are  subject  to  similar 
physico-chemical  influences  in  general  and  the 
temperature  in  particular,  or  of  marine  plants 
depending  entirely  upon  certain  elements  dissolved 
in  sea  water,  and  certain  intensity  of  light  and 
temperature.  Therefore  the  temperature  may  be 
used  as  the  most  practicable  indicator  of  ecological 
conditions. 

(a)  In  many  cases  the  hatching  of  fish  eggs  is  to  a 
high  degree  dependent  on  just  the  right  temperature 
of  the  surrounding  water. 

(b)  In  their  growth  and  ripening  from  the  larval  to 
mature  stages  fish  continue  to  be  influenced  directly 
by  temperature. 


[453] 


MODERN     FISHING     GEAR    OF    THE     WORLD 


(c)  At  the  spawning  time  many  species  of  fish  have 
to  find  just  the  right  temperature  of  water  in  which 
to  deposit  their  spawn, 

4*  In  addition  to  these  general  findings,  the  special 
temperature  range,  which  some  species  of  commercial 
fish  prefer,  is  known.  This  is  especially  true  for  the 
spawning  conditions,  and  partly  for  other  periods 
of  their  life. 

This  relation  existing  between  the  concentration  of 
certain  commercial  fish  and  the  water  temperature  can 
be  utilized  in  practical  fishing.  Since  measurement  of 
the  temperature  is  easy,  it  should  facilitate  location  of 
fish.  But  one  cannot  expect  completely  correct  con- 
clusions by  thermometric  methods,  as  temperature  is 
only  one  factor  influencing  fish  concentration. 

Thermometric  methods  for  locating  fish  have  been 
applied  for  a  long  time  with  varying  success.  The 
prediction  of  the  size  of  the  stocks  in  different  seas  based 
on  analogies  in  temperature  data  may  be  called  fishery 
strategy.  Here  we  deal  with  the  task  of  fishery  hydro- 
graphy in  the  form  of  tactical  information  for  fishermen 
with  regard  to  the  special  fishing  ground  based  on  the 
water  temperature.  For  such  tactical  information: 

(1)  The  distribution  of  temperature  must  be  known. 

(2)  The  differences  in  temperature  must  reach  several 
degrees  centigrade. 

(3)  The  correlation  between   temperature  and  con- 
centration of  fish  must  be  known. 

So  far,  information  on  the  two  first  items  can  be 
given  only  with  the  aid  of  direct  observations  of  tempera- 
ture. Data  with  reference  to  item  3  are  available  only 
for  limited  areas  and  for  some  commercial  species  of  fish. 


IT' 


20- 

n— • 


KEY 

OB  OVER  1 20  BASKETS  COD  P.  H. 
^40  -120  "  •  •  • 
Z3  15-40  "  '  -  • 

.TRAWL  HAUL 

•  BOTTOM  TEMPERATURE  OBS. 
+  HYDROGRAPWCAL  SERIES 

ISOBATHS 

200  M.  400  M 


BEAR  IS. 


A  general  application  of  thermometric  methods  for 
locating  fish  cannot  be  expected,  but  its  use  in  selected 
regions  and  for  certain  species  of  fish  demonstrates  the 
line  to  be  followed  to  achieve  success. 


RESULTS  OF  FISH  FINDING  BY  THERMO- 
METRIC   METHODS    IN    SELECTED 
HYDROGRAPHICAL  REGIONS 

Differences  in  temperature  within  narrow  strips  in  the 
sea  result  from  these  three  different  processes,  namely  the 
convergence  of  water  masses  of  different  origin,  the 
divergence  in  thermally  stratified  water  whereby  colder 
water  gets  to  higher  levels,  and  vertical  turbulence  and 
its  local  differences.  This  turbulence,  which  can  be 
generated  either  by  wind  or  tidal  stream,  contributes 
to  the  formation  of  the  thermocline  and  its  local  differ- 
ences. 

Zones  of  Convergence 

The  most  conspicuous  example  of  convergence  is  the 
Polar  Front  in  the  northern  and  southern  hemisphere. 
Here  cold  polar  and  sub-polar  water  masses  meet  the 
warmer  water  of  the  temperate  latitudes.  The  concentra- 
tion of  food  fish  at  this  Front  has  long  been  made  use 
of  by  fishermen.  An  example  of  relationship  between 
temperature  and  fish  distribution  is  that  of  the  Bear 
Island  region.  Westward  and  southward  of  Bear  Island 
there  ib  a  convergence  between  cold  arctic  water  and 
warmer  Atlantic  water. 

The  investigations  made  from  the  English  research 
vessel  Ernest  Holt  since  1949  into  the  relationship 
between  bottom  temperature  and  cod  distribution  have 
yielded  some  remarkable  results.  They  show  that  the 
thermal  structure  seems  to  assist  a  concentration  of 
cod  in  a  narrow  strip  at  certain  times.  The  best  yields 
were  obtained  in  winter  on  grounds  with  bottom  tem- 
peratures between  1  -75  degrees  C.  and  3  degrees  C. 

But  it  is  supposed  that  such  thermoclines  do  not 
provide  a  universal  indicator  of  where  to  fish  successfully, 
but  only  that  there  is  a  greater  probability  of  finding 
remunerative  quantities  of  fish. 

The  temperature  distribution  at  the  Polar  Front 
shown  in  fig.  1  changes  in  the  course  of  time.  There 


Fig.  /.     Distribution  of  bottom  temperature  and  cod  in  the  Bear 
Island  area,  20  to  28  November  1949   (After  6). 


J       J    _  '    _        l- 

»'"""  ir~"         "Hi1""         ~tr~' 


Fig.  2.    Distribution  of  surface  temperature  in  the  Greenlandic- 
Icelandic   Waters  in  August  1956  (After  5). 


[454] 


LOCATING     FISH     BY     THERMOMETRIC     METHODS 


is  no  published  data  to  give  more  information  about 
these  changes.  Observations  have  been  made  in  a  similar 
region,  namely  the  Greenland-Iceland-Ridgc,  where  polar 
water  of  the  East  Greenland  Current  and  Atlantic  water 
of  the  Irminger  Current  meet.  From  the  temperature 
distribution  at  the  surface,  determined  by  the  German 
R.  V.  Gauss  in  August  1956,  it  is  obvious  that  the  Polar 
Front  meanders  along  an  axis  which  follows  the  edge 
of  the  shelf,  approximately  the  500  m.  depth  line  (fig.  2). 
Probably  these  meanderings  are  in  the  direction  of 
the  axis,  which  is  also  the  prevailing  direction  of  the 
East  Greenland  Current.  These  variations  also  influence 
the  temperature  distribution  on  the  bottom,  so  the 
information  given  to  fishermen  should  take  into  account 
not  only  the  observed  distribution  of  temperature  but 
also  the  variations  in  the  course  of  time.  As  the  fishermen 
will  have  neither  the  time  nor  be  equipped  to  gather 
information  about  the  thermal  conditions  existing  in 
the  fishing  ground,  the  locating  of  fish  concentrations 
by  thermometric  methods  would  very  soon  become 
discredited.  The  work  should  therefore  be  carried  out 
by  systematic  fishery  biological  and  hydrographical 
investigations  on  the  main  fishing  grounds  and  the 
results  distributed  to  the  fishermen. 

Zones  of  Divergence 

Zones  of  divergence  are  the  upwelling  /ones  at  the  west 
coasts  of  the  continents  in  the  temperate  latitudes  and 
the  upwelling  at  the  equator,  where  fishermen  find 


£*> 

i 


300 


SCO 
€OO 


DRIFT    VECTOR 


ZOOPLANKTON 


YELLOWFIN 


5*S 


Fig.  3.     Surface  temperature,  vertical  temperature  field,  long- 

line  drift,  zooplankton  abundance  in  the  upper  200  m.,  and  the 

yellow/in  tuna  catch  along  long.  150  degrees  W.  August  22  to 

September  25,  7952  (After  II). 


paying  quantities  of  fish.  These  zones  can  be  found  by 
thermometric  methods.  The  impressive  results  of  the 
Pacific  Oceanic  Fishery  Investigations  (POFI)  of  the 
U.S.  Fish  and  Wildlife  Service  in  investigating  the 
fishery  potential  of  the  tropical  mid-Pacific  waters, 
show  that  yellowfin  tuna,  Neothunnus  macropterus,  and 
skipjack,  Katsuwonus  pelamis,  are  concentrated  in  the 
upwelling  zone  at  the  equator  and  in  the  narrow  strip 
south  of  the  Equatorial  Counter  Current 2t 7- n  andothcrs 

Fig.  3  shows  an  example  of  the  results  along  150  degrees 
W  between  5  degrees  S  and  15  degrees  N,  i.e.  south  of  the 
Hawaiian  Islands.  Other  investigations  by  POFI  prove  that 
the  tuna  larvae  are  also  concentrated  in  great  numbers  in 
the  same  zone. 

Further  observations,  discussed  by  G.  J.  Murphy  and 
R.  S.  Shomura7,  show  that  the  quantities  offish  caught 
and  the  positions  of  the  zones  vary  considerably  in  the 
north-south  direction,  probably  in  response  to  the  value 
of  the  upwelling  and  meandering  of  the  current  system. 

It  may  be  mentioned  that  the  divergence  of  the 
northern  frontier  of  the  Equatorial  Counter  Current, 
recognizable  in  fig.  3  at  about  9  degrees  N,  brings  about 
no  enrichment  cither  in  zooplankton  or  in  yellowfin 
tuna.  The  stability  of  the  surface  layer  is  very  high  and 
therefore  the  upwelling  of  water,  rich  in  nutrients  which 
exist  in  this  zone,  does  not  reach  the  surface  layer. 

When  one  realizes  the  full  potentials  of  this  equatorial 


Fig.  4.     Distribution  of  maximum  velocities  (cm/sec.)  of  tidal 
streams  in  the  North  Sea  at  springtide  (After  .?). 


[455] 


MODERN     FISHING     GEAR     OF    THE     WORLD 


area  the  enormous  resources  available  for  a  high  seas 
fishery  are  evident.  Informing  fishermen  of  possible  fish 
concentrations  by  research  ships  becomes  possible  when 
thermometric  methods  arc  applied  to  determine  the 
variations  in  the  current  system.  This  could  be  applied 
also  in  other  regions  with  divergent  water  movements. 

Zones  of  Local  Differences  in  Vertical  Turbulence 

Local  differences  of  vertical  turbulence,  observed 
frequently  in  shelf  waters,  cause  and  maintain  local 
differences  in  temperature  in  stratified  waters.  The 
differences  of  turbulence  increase  with  the  maximum 
velocity  of  the  tidal  stream  which  stretches  from  the 
surface  to  the  bottom.  The  temperature  distribution  in 
stratified  waters  is  influenced,  especially  during  summer, 
in  those  seas  in  which  great  local  differences  in  tidal 
streams  exist.  An  interesting  example  is  the  North  Sea. 
Here  the  distribution  of  maximum  velocity  of  tidal 
streams  as  well  as  the  temperature  distribution  are 
known,  and  some  relations  between  them  and  the  con- 
centration offish  are  worth  using  for  tactical  information 
for  fishermen. 

The  southern  and  the  western  areas  of  the  North 
Sea  are  comparatively  rich  in  tidal  streams.  When  the 
thermocline  begins  to  develop  in  spring  it  is  influenced 
by  the  tidal  stream  turbulence.  Observations  have  shown 
that  the  central  and  north-east  is  less  affected  than  the 
southern  and  western  areas.  The  resulting  character 
of  the  summer  thermoclines  can  be  seen  in  fig.  5,  from 
the  hydrographic  sections  through  the  North  Sea  made 
in  August  1955,  while  the  courses  of  these  sections  are 
shown  in  fig.  6. 

1.  The  intensity  of  the  thermocline,  i.e.  the  vertical 
gradient  of  temperature  in  the  discontinuity  layer, 
is  as  small  in  the  northern  North  Sea  as  in  the 
neighbouring  ocean. 


2.  In    the    central    North    Sea   the    thermocline    is 
narrower,  therefore  the  gradient  is  bigger. 

3.  South  of  the  Dogger  Bank  an  extraordinarily  large 
temperature  change  of  about  9  degrees  C.  in  2  or 
3  m.  vertical  distance  (sometimes  in  less  than  1  m.), 
can  be  observed. 

4.  The   thermocline   disappears   totally   as   soon   as 
the  tidal  turbulence  is  strong  enough  to  prevent 
a  permanent  existence  of  temperature  in  vertical 
direction.    That  is  the  case  in  most  parts  of  the 
southern  and  western  North  Sea. 

The  distribution  of  temperature  is  such  that  in  some 
areas  the  thermocline  preserves  the  cool  winter  water, 
but  in  the  southern  and  western  North  Sea  summer  heat- 
ing reaches  the  bottom  because  of  the  turbulence  of  strong 
tidal  streams.  Great  horizontal  differences  in  bottom 
temperature  result  (fig.  6).  The  temperature  is  less  than 
6  degrees  C.  in  areas  with  small  mixing  and  more  than 
1 1  degrees  C.  in  regions  with  high  mixing.  In  the  north- 
east, the  cool  bottom  water  is  delimited  by  warmer 
Atlantic  water  entering  the  Norwegian  Deep  in  the  north. 
Taking  into  account  the  great  counter  clockwise  swirl 
on  the  Flatten  Ground  in  the  northern  North  Sea,  it  is 
clear  that  bottom  water  cast  of  Scotland,  comparatively 
warm  by  great  tidal  mixing,  flows  to  the  cast  and  divides 
the  cool  winter  water  into  two  cool  water  masses.  Separ- 
ated by  the  shallow  Dogger  Bank,  there  is  a  third  core 
of  cold  bottom  water  in  the  south-eastern  North  Sea 
in  summer,  which  has  been  observed  repeatedly  in  the 
summers  of  different  years. 

Fluctuations  from  year  to  year  influence  three  points: 

1 .  The  position  of  the  three  cores  of  cold  winter  water. 

2.  The  temperature  of  these  cores. 

3.  The  structure  of  the  thermocline. 

The  positions  of  the  cores  are  influenced  to  a  certain 
degree  by  the  current  system,  which  changes  with  the 


Fig.  5.     Vertical  distribution  of  temperature  in  a  north-south  and 

in  a  west -east  section  through  the  North  Sea  in  August  7955, 

for  course  of  the  sections,  see  fig.  6. 


Ftg.  6.    Bottom  temperature  of  the  northern  and  central  North 

Sea  in  August  7955,  the  broken  line  shows  the  course  of  the 

sections  represented  in  fig.  5. 


1456} 


LOCATING     FISH     BY    THERMOMETRIC     METHODS 


wind  conditions.  The  temperature  of  the  cores,  for  the 
last  50  years,  investigated  by  G.  Prahm",  seems  to  be 
determined  by  the  severity  of  the  preceding  winter.  The 
structure  of  the  thcrmocline  is  influenced  by  the  wind 
conditions,  and  the  heat  exchange  between  the  sea  and 
the  atmosphere  during  late  spring  and  early  summer. 
This  points  to  the  fact  (hat  temperature  distribution, 
determined  by  the  local  difference  in  vertical  turbulence, 
is  complicated  by  weather  conditions. 

Some  information  on  the  relationship  between  dis- 
tribution of  temperature  and  concentration  of  fish,  i.e. 
haddock  and  herring,  based  on  recent  investigations 
by  the  German  F.R.S.  Anton  Dohrn  in  the  North  Sea 
is  given  below. 

CONCENTRATION  OF  HADDOCK  IN  RELATION 
TO  BOTTOM  TEMPERATURE  IN  THE 
DOGGER  BANK  AREA 

During  two  cruises  in  September  and  October  1956, 
experimental  trawling,  combined  with  hydrographical 
investigations,  was  carried  out  in  the  Dogger  Bank  area 
and  the  southern  North  Sea".  In  figs.  7  and  8,  the  number 
of  haddock  caught  per  haul  of  half  an  hour's  duration 
are  marked  for  all  trawl  stations.  As  shown  in  tig.  7, 
the  haddock  seems  to  avoid  areas  of  bottom  temperature 
below  5  degrees  C.  and  also  water  masses  of  more  than 


13-5  degrees  C.,  the  biggest  yield  of  this  species  of  fish 
being  in  areas  of  5  to  8  degrees  C.  This  can  be  seen  clearly 
from  the  course  of  the  interpolated  isoplath  for  a  catch 
of  5(K)  haddock  per  \  hr.  trawling.  It  delimits  the  areas 
of  higher  haddock  concentration  from  those  of  lower 
concentration  in  the  cold  water  masses  north  of  the 
Dogger  Bank,  and  in  the  warmer  waters  of  the  southern 
and  south-eastern  regions  of  the  North  Sea. 

One  isolated  exceptionally  good  haddock  catch  in 
waters  of  nearly  14  degrees  C.  (2,124  haddock  per  J  hr. 
trawling)  was  made  south  of  the  Dogger  Bank.  Presum- 
ably this  accumulation  of  fish  may  be  connected  with 
the  current  that  flows  around  the  south-western  edge  of 
the  Dogger  Bank  in  an  easterly  direction.  This  current 
is  indicated  by  a  strongly  marked  strip  of  water  with 
comparatively  high  salinity  (34-6  per  mi  lie.),  in  which 
the  station  mentioned  above  is  situated,  and  also  by 
the  course  of  the  isotherms. 

One  month  later,  in  October  1956,  similar  investiga- 
tions also  seemed  to  prove  a  relationship  between 
haddock  distribution  and  temperatures  (fig.  8).  Only  in  the 
area  of  colder  bottom  temperatures  (below  1 1  degrees  C.) 
northwest  of  the  Dogger  Bank  were  sizeable  catches  of 
haddock  obtained,  while  the  amount  caught  in  the 
region  of  warmer  waters  was  very  small  or  absolutely  nil. 

The  correlation  between  the  concentration  of  haddock 
(as  measured  in  catch  per  unit  effort)  and  the  temperature, 


57- 


Fig.  7.     Catches  of  haddock  per  half-hour  trawling  [•"*  in  the  North  Sea  made  hy  F.  R.  S.  Anton  Dohrn  in  September  1956  and 
isotherms  for   bottom  temperatures  stated  simultaneously  (Temperature   observations    [•];    Isoplath  Jor  500  haddocks  per  half-hour 

trawling  drawn  in). 

[457] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


{  ^}^^* 
[  2<%Z  r:'>— -r 

wr  f  ""ft"- 


Fig.  8.  Ca/rAej  <>/  haddock  per  half-hour  trawling  \  • ;  m  r//r 
7V0r/A  5ra  made  by  F.  /?.  5.  Anton  Dohrn  1/1  October  1956  and 
corresponding  isotherms  for  surface  temperatures  (uninterrupted 
lines).  In  the  area  of  thermal  stratification  also  bottom  tem- 
peratures are  given  (dotted  lines).  (Temperature  observations  ( •  J). 

is  not  always  very  close.  It  may  be  influenced  and  veiled 
not  only  by  other  environmental  factors  such  as  hydro- 
graphical  conditions,  food  supply,  depth,  etc.,  but  also 
by  factors  based  on  the  fish  stock  itself,  e.g.  the  stock 
density  and  migrations.  Thus,  influenced  by  the  very 
high  density  of  the  haddock  stock  in  1956  (on  the  basis 
of  numerous  year-class  1955),  the  correlation  becomes 
evident,  whereas  observations  made  in  the  same  area 
the  year  before  showed  no  results,  because  the  haddock 
stock  was  comparatively  poor. 

In  fig.  9  the  number  of  haddock  caught  per  £  hour 
trawling  (fig.  7)  is  plotted  against  the  corresponding 
bottom  temperature  in  order  to  show  the  temperature- 
haddock-density  correlation  in  greater  detail.  On  the 
whole,  there  is  a  wide  range  of  variation.  The  correla- 
tion, however,  becomes  more  evident  by  hatching  the 
whole  area  in  which  all  points  are  situated.  As  shown 
by  the  contour  of  this  hatched  region,  the  optimum 
temperature  for  haddock  lies  between  6  to  8  degrees  C. 
whereas  minimum  conditions  are  found  to  be  below  5 
degrees  C.  and  above  14  degrees  C.  respectively,  the 
latter  with  the  one  exception  mentioned  above. 

Summing  up,  we  may  conclude  that  a  correlation 
between  the  concentration  of  haddock  and  temperature 
exists  in  the  region  of  the  Dogger  Bank  as  soon  as  the 
total  number  of  fish  in  this  area  and  the  differences  in 
temperature  reach  a  sufficient  level. 

Bottom  temperature,  however,  changes  in  space  and 
time.  Profitable  locating  of  haddock  concentrations  by 
thermometric  methods  may  be  possible  in  this  area 
only  by  systematic  hydrographical  observations  con- 


2500 


2000 


1500 


1000 


bOO 


V 


T T"9      10 

temperature  (°C) 


Fig.  9.  Correlation  between  catch  per  unit  effort  of  haddock  in 
the  Dogger  Bank  area,  September  1956,  and  corresponding 
water  temperatures  on  the  bottom  (according  to  data  oj  fig.  7). 

ducted  by  a  research  vessel.  Thermometric  observations 
made  by  fishermen  are  considered  helpful  only  in  special 
cases. 

CONCENTRATION  OF  HERRING  IN  RELATION 
TO  TEMPERATURE  DISTRIBUTION  IN  THE 
NORTHERN  NORTH  SEA 

Four  interrelations  seem  to  exist  between  the  con- 
centration of  herring  and  the  distribution  of  temperature 
in  the  northern  North  Sea: 

1.  In   summer  and  autumn   the  herrings  are  con- 
centrated in  the  core  of  the  cold  bottom  water. 

2.  The  lower  the  temperature  of  this  cold  water,  the 
longer  is  the  duration  of  the  concentration. 

3.  The  geographical  position  of  this  concentration 
fluctuates  with  the  dislocation  of  the  centre  of  the 
cold  water. 

4.  The   daily   vertical    movements   of  the   herring 
schools  are  influenced  by  the  structure  of  the 
thermocline. 

Recent  observations  prove  these  statements10. 

During  a  cruise  from  6th  to  20th  August  1955  the 
F.R.S.  Anton  Dohrn  investigated  the  relation  between 
the  distribution  of  the  herring  stocks,  the  activity  of  the 
trawlers,  and  the  hydrographical  conditions  in  the 
northern  North  Sea  (fig.  10),  The  Atlas  "Fischfinder" 


[458] 


LOCATING    FISH    BY    THERMOMETRIC    METHODS 


f«r 


59' 


58V 


57V 


69* 


W     (T 


Fig.  10.  Distribution  of  herring  in  the  Fladen  Ground  area 
(catch  per  half-hour  trawling*  dotted  line:  Isoplath  for  1,000 
herrings);  Records  by  echo  sounding  of  herring  ( x  x  x  Frequent, 
-f +  -t-  Moderate,  :::.  Rare);  and  hydrographical  conditions 
(temperature  and  salinity)  based  on  the  investigations  with 
F.  R.  S.  Anton  Dohrn  in  August  1955. 

was  running  throughout  the  cruise.  The  greatest  con- 
centration of  herring  was  observed  in  the  small  area 
on  the  Fladen  Ground  and  Bressay  Shoal.  The  isoplath 
for  1,000  herrings  per  4  hour  trawling  corresponds  well 
with  the  cold  centre  of  the  bottom  water,  which  covers 


3* 


Fig.  II.     Activity  of  the  German  trawlers  from  1  to  20  August 
1955  [Number  of  trawlers,  fishing  days  (in  brackets),  total  yields 
and  average  yield  in  baskets]  and  distribution  of  bottom  tempera- 
ture in  August  1955. 

this  area  in  summer-time.  This  area  also  corresponds 
with  the  activity  of  the  fishing  fleet,  and  fig.  1 1  shows  the 
number  of  trawlers,  the  fishing  days,  the  total  yields  and 
the  average  catch  per  vessel  for  the  period  1st  to  20th 
August  1955. 

The  figures  show  that  most  fishing  days  and  most  of 
the  fleet,  which  takes  the  major  part  of  the  catch,  are 


TABLE  I 

The  Water  Temperatures  (Degree  C.)  and  the  Number  of  Fishing  Days 

on  the  Fladen  Ground  in  different  Years 

1935 

1936 

1937 

1938 

1947 

1948 

1949 

1950 

1951 

1952 

1953 

1954 

7955 

1956 

May 

surface 
bottom 

80 
9 

7-5 
6-2 

9 
5:9 

8-1 
7-1 

6-2 
5-9 

8-1 
8-0 

7-8 

7-2 

81 

7-2 

9 

9 

9 
•> 

9-5 

6-7 

80 
6-7 

8-0 
7 

7-5 

9 

June 

surface 
bottom 

7 

9 

11-3 
6-1 

11-1 

5-5 

9 

7-0 

9 

5-8 

11-6 

7-4 

10-5 

7-4 

10-4 

7-3 

9 

6-1 

9 

6-4 

11  2 

6-7 

110 
70 

11-0 
60 

10-0 
5-8 

July 

surface 
bottom 

14-4 
6-8 

7 
7 

14-1 
5-6 

13-5 
6-9 

15-0 
5-5 

12-1 
7-0 

12-0 
6-3 

13-6 

7-2 

•y 
9 

7 

9 

14-5 
6-7 

1  1-5 
7-0 

12-5 

9 

12-0 

o 

August 

surface 
bottom 

.  12-5 
6-8 

14-1 
6-3 

14-1 
5-6 

9 

7 

16-4 
6-0 

13-3 
7-6 

14-3 
7-1 

14-1 

7-3 

9 

6:0 

9 

7-0 

14-4 

7-2 

14  1 
6-9 

14-2 
5-8 

13  0 

5-7 

September 

surface 
bottom 

.  11-8 
6-9 

7 

7 

9 

7 

13-4 

7-5 

16-5 
5-5 

13-0 
7-1 

14-2 
7-6 

9 
«i 

9 

7 

9 
9 

12  0 

9 

12-5 

9 

14-0 
7 

12-5 
7 

Fishing 
Fishing 

start 
termination 
days 

.  28.6. 
6.9. 
71 

28.6. 
17.9. 
82 

27.6. 
25.9. 
91 

24.6. 
5.9. 
74 

18.7. 
28.10. 
70.? 

14.7. 
18.9. 
67 

10.7. 
21.9. 

73 

25.7. 
21.9. 
59 

25.7. 
25.9. 
58 

5.7. 
10.9. 
67 

4.7. 
23.9. 
82 

24.6. 
15.9. 
84 

16.6. 
23.9. 
100 

10.6. 
19.9. 
702 

Air  temperature  in 

February/March 

(4-rel.  warm)  (— rel.  cold) 


[459] 


MODERN     FISHING     GEAR     OF     THE     WORLD 

B 


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.  72.     ^4-/>  Distribution  of  the  fishing  fleet  in  August  1953  to  1956  according  to  weather  reports. 

[460] 


LOCATING     FISH     BY     THERMOMETRIC     METHODS 


TABLE  II 
The  German  Herring  Trawl  Fishery  in  August  1953-1956  on  the  Fladen  Ground 


MIDDLE 


NORTH 


EAST 


SOUTH 


WEST 


rom 

FLADEN  GROUND 

no.  of  aver,  fish-  no.  of  aver,  fish-  no.  of  aver,  fish-  no.  of  aver,  fish-  no.  of  aver,  fish-  no.  of  aver.  fish- 
traw-  no.  of  ing  traw-  no.  of  ing  traw-  no.  of  ing  traw-  no.  of  ing  traw-  no.  of  ing  traw-  no.  of  ing 
lers  basket*  days  lers  baskets  days  lers  baskets  days  lers  baskets  days  lers  baskets  days  lers  baskets  days 


1953 

47 

1507 

18 

218 

1358 

22 

3 

350 

3 

59 

984 

17 

225 

1526 

22 

72 

1408 

14 

1954 

7 

962 

5 

82 

1466 

15 

3 

225 

3 

7 

536 

4 

167 

1590 

21 

236 

1819 

24 

1955 

165 

2262 

23 

205 

1K45 

24 

0 

0 

0 

438 

2332 

31 

3 

150 

1 

922 

2407 

31 

1956 

66 

1838 

20 

246 

1416 

28 

45 

1057 

11 

123 

930 

22 

108 

1567 

22 

1127 

1622 

31 

to  be  found  in  the  area  with  bottom  temperature  below 
6  degrees  C.  Unfortunately,  no  hydrographical  survey  of 
the  whole  area  was  made  in  1953,  1954  and  1956.  We 
know,  however,  by  chance  observations  that  the  greatest 
accumulation  of  trawlers  and  the  highest  yields  were  also 
found  in  the  cooler  part  of  this  area. 

As  shown  in  Table  I,  the  period  for  the  summer 
trawl  fishery  in  several  years  (1935  to  1938  and  1947  to 
1956)  ranges  from  58  to  103  days. 

In  years  of  cooler  bottom  temperatures  (5  to  6  degrees 
C.),  the  fishing  period  is  longer  (91  to  103  days)  than  in 
years  with  higher  bottom  temperature,  which  indicates 
that  the  length  of  the  fishing  period  is  dependent  on  the 
temperature  of  the  bottom  water.  The  bottom  tempera- 
ture itself  obviously  depends  on  the  weather  conditions 
in  spring.  The  relation  between  fishing  time  and  bottom 
temperature  may  be  explained  by  the  fact  that  the 
development  of  the  grounds  depends  on  the  temperature 
of  the  surrounding  water,  and  consequently  the  herrings 
seem  to  stay  longer  in  cold  areas  before  they  are  able 
to  spawn. 

From  Table  J I  it  is  evident  that  the  catches  in  the 
different  areas  of  the  Fladen  Ground  fluctuate  from  year 
to  year.  In  1953,  1955  and  1956,  the  catches  in  the  area 
"middle"  were  good,  whereas  in  1954  they  were  a  failure. 
In  the  area  "east",  the  fishing  activity  was  always  poor. 
The  area  "south"  shows  a  decline  in  1954,  but  good 
fishing  in  1955  and  1956.  In  the  area  "west",  the  fishing 
was  a  failure  in  1953.  It  is  remarkable  that,  since  1953, 
the  more  north-western  and  northern  areas  of  the 
Fladen  Ground  have  shown  an  increase  in  the  fishing 
activity.  Fig.  12  a  to  d  shows  the  distribution  of  the 
fleet  in  August  from  1953-56  according  to  the  daily 
weather  reports,  which  corresponds  well  with  the 
results  from  Table  II. 

It  can  be  concluded  that  the  movement  of  the  fishing 
fleet  reflects  the  accumulation  of  herring,  which  is 
directed  by  the  dislocation  of  cold  water  masses  in 
connection  with  the  variation  of  the  current  system. 

The  daily  vertical  movements  of  herring  to  the  surface 
are  limited  by  the  thermocline  in  summer  time.  The 
herrings  rise  during  the  night  from  the  bottom  up  to  the 
thermocline  and  descend  down  to  the  bottom  again  in 
the  morning.  A  concentration  of  herring  food  is  to  be 
found  in  the  thermocline  as  soon  as  it  develops.  The 
herrings  seem  to  feed  at  night  on  the  plankton  concen- 
tration in  the  thermocline  and  do  not  go  up  to  the  surface 
as  they  do  when  the  thermocline  is  missing  or  poorly 


developed,  in  which  case  the  drifters  generally  have  good 
catches.  The  catches  of  the  drifters  in  the  central  area 
decline  quickly  from  May  to  August  because  the  vertical 
movements  of  the  herring  are  restricted  and  they  no 
longer  come  within  the  range  of  the  drift  nets.  On  the 
other  hand,  successful  trawl  fishing  begins  every  year 
between  the  middle  of  June  and  the  middle  of  July,  as 
soon  as  the  surface  temperature  rises  above  12  degrees  C. 
The  phenomena  1 ,  2  and  4  given  above  may  be  considered 
as  safely  established  by  observations  and  can  therefore 
be  used  as  a  source  of  tactical  information  for  fishermen. 
In  addition,  phenomenon  2  can  be  used  in  making 
strategical  predictions  as  early  as  March  and  April, 
when  the  cold  winter  bottom  water  is  formed,  concerning 
the  probable  duration  of  the  fishing  in  the  following 
summer  and  autumn  in  the  northern  North  Sea.  The 
location  of  herring  concentrations  by  thermometric 
methods,  therefore,  is  possible  but  presumes  that  a 
research  vessel  will  carry  out  systematic  hydrographical 
and  fishery  biological  observations. 

REFERENCES 

1  Carruthers,  J.  N.    Some  inter-relationships  of  oceanography 
and  fisheries.   Arch.    Meteorol.   Geophys.  Bioklimat.,  Ser.  B,  6. 
1955. 

2  Cromwell,  T.  Circulation  in  a  meridional  plane  in  the  central 
equatorial  Pacific.  J.  Mar.  Res.  12.  196-213.    1953. 

3  Dietrich,    G.        Die    natiirlichcn    Regioncn    von    Nord-und 
Ostsce  auf  hydrographischer  Grundlage.  Kicler  Meeresforsch.  7. 
1950. 

4  Dietrich,  G..  Wyrlki,  K.,  Carruthers,  J.  N..  Lawford,  A.L., 
Parmenter,   H.   C.    Windverhaltnisse   iiber  den   Mecrcn  um  die 
britischen    Inseln    itn   zeitraum    1900-1949.     Dtsch.    Hydr.    Inst. 
Hamburg.    1952. 

5  Dietrich    G.      Ozeanographische    Probleme    der    deutschen, 
Forschungsfahrten    im    Internationalen    Gcophysikalischcn    Jahr 
1957/58.  Dtsch.  Hydr.  Z.  JO.    1957. 

6  Lee,  A.  J.  The  influence  of  hydrography  on  the  Bear  Island  cod 
fishery.    Rapp.  Proc.-Verb.  des  Reunions.  Vol.  CXXX1.  Kopen- 
hagen.     1952. 

7  Murphy,  G.  J.  and  Shomura,  R.  S.   Longline  fishing  for  deep- 
swimming  tunas  in  the  central  Pacific,  January  to  June,   1952. 
Spec.  Sci.  Rep.  U.S.  Fish  Wildlife  Serv.,  Fish  No.  108.  1  to  32.  1953. 

8  Prahm,  G.  Summerly  bottom  temperature  in  two  selected  areas 
of  the  Central  North  Sea  in  the  years  1900  to  1956.  Ann.  Biol.  (1956) 
13.  1958. 

9  Sahrhage,  D.  Haddock  of  the  North  Sea.  German  investigations. 
Ann.  Biol.  (1956)  13.     1958. 

10  Schubert,  K.    The  conditions  of  the  Herring  Fishery  in  the 
Northern  North  Sea  during  August  1955.  Ann.  Biol.  (1955)  12. 
1957. 

11  Sette,  O.  E.    Consideration  of  mid-ocean  fish  production  as 
related  to  oceanic  circulatory  systems.  Jour.  Mar.  Res.  14,  398-414. 
1955. 


[461  ] 


RADIO  DIRECTION  FINDERS   AND   RADAR  USED  BY  JAPANESE 

FISHING  VESSELS 

by 
SUIYO-KAI 

Tokyo,  Japan 


Abstract 

In  addition  to  echo  sounders  and  a  comprehensive  range  of  radio,  the  Japanese  fishing  fleet  is  being  equipped  with  many  modern 
devices  which  help  in  both  navigation  and  fishing.  The  modern  radio  direction  finder  uses  a  Cathode  Ray  Tube  which  instantly  gives  the 
correct  bearing  of  the  transmitting  station.  New  radars  have  been  produced  for  use  in  small  fishing  craft.  In  conjunction  with  these, 
radar-reflecting  buoys  are  used  to  enable  the  fisherman  to  keep  in  touch  with  his  nets  at  all  times. 

Equipement  electronique  utilise  a  bord  des  bateaux  de  peche  japonais 

Outre  les  echo  sondeurs  et  une  gamme  tres  complete  d'appareils  de  radio,  la  flotte  de  peche  japonaise  cst  dotee  d'un  grand 
nombre  de  dispositifs  modernes  qui  facilitent  aussi  bien  la  navigation  que  la  p&chc.  Le  radiogoniomctrc  est  equipe  d'une  lampe  a  rayons 
cathodiqucs  qui  donnc  instantanement  la  direction  exacte  de  la  station  emettrice.  II  existe  egalemcnt  un  radiotelemetre  &  longueur  d'onde 
unique  permettant  de  determiner  avec  une  grande  precision  la  distance  d'une  station  fixe  ou  flottante,  et  de  nouveaux  radars  ont  etc  mis  au 
point  a  Pintention  des  petits  bateaux  de  peche,  qui  utilisent  avec  ces  appareils  des  bouees  munies  d'un  r^flccteur  de  radar  pour  rcperer  £  tout 
moment  leurs  filets. 


Extracto 


El  equipo  electronico  usado  en  los  barcos  pesqueros  japoneses 


Ademas  de  las  ecosondas  y  una  gran  colecci6n  de  equipo  de  radio,  la  flota  pesquera  japonesa  cuenta  con  varios  inventos  modernos 
que  sirven  comp  ayudas  para  la  navegaci6n  y  la  pesca. 

El  radiogoni6metro  usa  un  tubo  de  rayos  cat6dicos  que  da  instantdneamente  la  posici6n  correcta  de  la  estaci6n  transmisora.  Tarn- 
bi6n  hay  un  equipo  de  radiotelemctria — que  usa  un  sistema  de  onda  unica  mediantc  el  cual  puede  dctcrminarse  con  gran  cxactitud  la  distancia 
a  que  se  encuentra  una  cstaci6n  fija  o  flotantc — y  nuevos  aparatos  de  radar  para  uso  en  embarcaciones  pesqueras  pequenas.  Con  este  equipo 
se  usan  boyas  provistas  de  radar  para  que  cl  pescador  se  mantenga  en  contacto  continuo  con  sus  redes. 


RADIO  DIRECTION  FINDING 

WITH  the  hand  operated  direction  finder  originally 
used  on  fishing  vessels,  the  bearing  of  the  radio 
station  was  found  by  rotating  the  loop  antenna 
until  a  minimum  of  signal  was  heard. 

This  system  is  now  replaced  by  the  goniometer  system 
using  two  fixed  loop  antennas  adjusted  at  right  angles 
to  each  other  and  mounted  in  a  suitable  position  on  the 
superstructure  of  the  vessel.  The  minimum  of  reception 
is  found  by  turning  a  goniometer  coil  which  can  be 
hand  operated  or  automatically  revolved.  The  indica- 
tion of  the  reception  is  no  longer  acoustical  but  is 
effected  by  means  of  a  Cathode  Ray  Tube  (C.R.T.). 
With  the  hand  operated  equipment,  the  bearing  of  the 
radio  station  is  given  by  the  angle  of  the  goniometer 
coil  at  the  point  of  minimum  reception  which  is  deter- 
mined from  the  size  of  the  amplitude  indicated  on  the 
C.R.T.  With  the  automatic  system,  the  goniometer  is 
revolved  at  a  high  speed  and  the  bearing  of  the  radio 
station  can  be  determined  continuously  from  the  shape 
of  the  amplitude  configuration  on  the  C.R.T.  screen. 


The  automatic  system  is  easy  to  handle  and  gives 
quick  and  accurate  measurements  under  the  stringent 
conditions  as  are  common  in  fishery.  Because  of  this 
superiority,  it  is  gradually  replacing  the  hand  operated 
system. 

The  set  consists  of  a  goniometer,  rectifier,  loop 
antennas,  speaker,  inverter  and  the  direction  indicating 
C.R.T.  which  are  contained  in  one  housing.  After 
switching  on  the  instrument,  the  dial  is  turned  to  the 
frequency  of  the  radio  station  of  which  a  bearing  is 
to  be  taken.  A  propeller  shaped  image  then  appears 
on  the  C.R.T.  When  the  receiver  is  not  properly  tuned 
only  a  round  shaped  image  will  appear  on  the  screen. 

The  angle  at  which  this  propeller  is  tipped  indicates 
the  direction  from  which  the  radio  beam  is  coming. 
When  the  sense  button  is  pressed,  the  image  tilts  to 
the  right  or  left  of  the  direction  line,  thus  indicating 
the  bearing  by  means  of  a  360  degree  scale. 

The  frequencies  used  in  direction  finding  have  been 
between  200  kc.  and  500  kc.,  but  higher  frequencies  of 


[462] 


DIRECTION     FINDERS     AND     RADAR     IN     JAPAN 


f-'iff.  /.     Typical  installation  of  a  double  loop  anttnna  on  fop  of 
a  radar  mast. 

1  -7  me.  to  4  me.  or  more  are  now  being  used  by  fishing 
vessels  too. 

The  main  and  at  present  unavoidable  causes  of  errors 
in  radio  direction  finding  arc: 

1.  Distortion  of  signals  by  the  ship  and  its  super- 
structure (mast,  funnel,  and  antenna) 

2.  Reflection  of  signals  from  the  Heaviside  layer 

3.  Terrestrial   conditions 

Since  the  causes  given  under  2  and  3  do  not  occur  in  the 
open  sea,  they  constitute  no  serious  problem,  but 
the  errors  arising  from  the  ship's  hull  and  superstructure 
increase  with  the  frequency  and  in  extreme  cases,  they 
may  even  cause  resonance  with  the  radio  signals.  There- 
fore when  using  high  frequencies  on  comparatively 
large  vessels,  the  antenna  must  be  installed  as  far  away 
from  the  hull  as  possible.  In  Japan,  good  results  are 
obtained  by  mounting  the  antennas  on  the  top  of  masts 
or  radar  towers  (fig.  1). 

It  is  now  possible  for  300  ton  fishing  vessels  to  use 
frequencies  up  to  3,500  kc.  and  even  10,000  ton  whalers 
can  use  up  to  3,000  kc. 

SMALL  MARINE  RADARS 

In  1951  several  manufacturers  started  or  resumed  their 
study  and  production  of  marine  radars,  and  at  present 
at  least  ten  companies  are  manufacturing  different  kinds 
of  radars  and  more  than  800  Japanese  ships  are  equipped 
with  these  instruments. 

Marine  radars  can  be  classified  into  three  size  groups 
according  to  the  size  of  the  ships  they  are  used  for. 

As  examples  for  the  models  designed  for  fishing 
purposes,  the  following  are  some  of  the  special  features 
of  the  BR-10  and  AR-25  type  marine  radars. 

(a)  Current  consumption  less  than  550  W. 

(b)  Weight  less  than  160  kg. 

(c)  Easy  maintenance 

(d)  Low  price 

(e)  Sturdy    manufacture    to    withstand    the    rough 
working  conditions. 

The  technical  details  of  these  two  models  are  given 
in  Table  I  and  the  indicator  unit  of  model  BR-10  is 
shown  in  fig.  2. 


Fig.   2.     Indicator    unit  of  the  model  BR-W  radar. 

APPLICATION  TO   FISHING   OPERATIONS 

The  small  marine  radar  plays  an  important  role  in  the 
operation  of  fishing  boats,  especially  in  combination  with 
a  corner-reflector.  Extensive  studies  and  experiments  in 
the  use  of  these  aids  are  being  made  in  the  salmon  and 
trout  fisheries  in  the  Northern  Pacific. 


Item 


TABI  f     I 
AR-25 


BR  10 


Frequency  Band 

9345     9405  MC 

9320     9430 

Output 

10  kW. 

10  kW. 

Pulse  Width 

0-27     0-33(1  sec. 

0  35  (jisec. 

Pulse  Repeating  i  re 

1  000  c.;  sec. 

800  c.  sec. 

quency 

Type  of  Antenna 

Double  Cheese  type 

Reflector  and  Horn   type 

System 

Width  of  Scanner 

3  ft. 

4  f|. 

Revolution  of  Antenna 

15  r.p.m. 

15  r.p.m. 

Bearing  Resolution 

3  degrees 

2  degrees 

Range  Resolution 

60  in. 

70  m. 

Minimum  Range 

70  m. 

70m. 

Diameter  of  Scope 

7  in. 

7  in. 

Range 

I,  3,  10.  25  mile 

1,  5,4,  10,  20  mile 

Range  Accuracy 

i  2  per  cent. 

!    2  per  cent. 

Power 

550  W. 

Antenna   D.C.    100  V.  or 

D.C.  24V..  100  V., 

A.C.    100  V.,    150  W. 

220  V. 

Others     D.C.24     V.     or 

A.C.  optional 

100  V..    350    W.  or  A.C. 

100  V.,    50c.  sec.  200  W. 

Weight 

Scanner 

45  kg. 

60kg. 

Mast 

35  kg. 

Indicator 

18  kg. 

35  kg. 

Modulator 

18kg. 

Rotary  Converter 

15  kg. 

15  kg. 

Automatic  Voltage- 

Regulator 

35  kg. 

10kg. 

Position  of  Installa- 

Optional 

Indicator  must  be  directlv 

tion 

beneath  the  antenna  mast. 

[463J 


MODERN     FISHING     GEAR     OF    THE    WORLD 


Fig.  3.     Triple  reflection  at  a  corner  reflector. 

Corner  reflectors  are  frequently  used  in  order  to 
increase  the  range  of  radar  with  respect  to  small  objects 
such  as  fishing  boats  and  buoys.  A  corner  reflector 
consists  of  three  metal  plates  adjusted  normal  to  each 
other.  This  adjustment  results  in  all  incoming  waves 
being  reflected  to  the  point  of  origin  (fig.  3).  A  corner 
reflector,  therefore,  provides  optical  reflection  indepen- 
dent of  its  actual  position  to  the  direction  of  the  radar 
beam.  The  maximum  allowance  for  the  right  angle 
adjustment  is  ±  J  degree.  The  construction  must  be 
rigid  enough  not  to  be  deformed  by  waves  and  wind 
pressures,  which  usually  means  a  rather  heavy  weight. 

When  used  for  marking  the  position  of  fishing  nets, 
the  reflectors  have  to  be  attached  to  rafts  or  other  floats 
for  which  purpose  their  heaviness  makes  them  very  in- 
convenient. The  buoy-type  corner  reflector  was  designed 
to  overcome  these  difficulties  (fig.  4).  The  reflecting 
plates  are  made  of  thin  aluminium  plate,  which  is  filled 
in  with  polystyrene  foam,  through  which  micro  waves 
pass  without  loss.  As  illustrated  in  fig.  5  its  exterior  is 
a  ball  which  maintains  high  mechanical  accuracy  and 
strength.  The  specific  gravity  of  the  foam  polystyrene 
being  approximately  0-03,  the  ball  can  float  on  the  water 
surface. 


TABLE  11 

Diameter 

35  cm. 

70  cm. 

Weight 

Approx.  400  g. 

Approx.    1,200  g. 

Maximum  Range 

A.  approx.  2-2  naut. 

A.  approx.  3-4  naut. 

(by  Radar) 

miles  (when  floating 

miles  (when  floating 

by  itself) 

by  itself) 

B.  approx.  4  -2  naut. 

B.  approx.  5  -8  naut. 

miles  (when 

miles  (when  attached 

attached  at  1  -Om. 

at  1  -Om.  above  the 

above  the  surface) 

surface). 

Two  models  of  different  size  are  available  at  present, 
the  main  features  of  which  are  given  in  Table  II. 

Fishing  in  the  Northern  Pacific  Ocean  is  governed 
by  the  Japan-U.S.S.R.  Fishing  Treaty,  which  strictly 
limits  the  length  of  the  nets  and  the  space  between  the 
nets  of  neighbouring  boats.  Hence  it  is  very  important 
to  keep  the  relative  position  between  the  nets  of  different 
ships  as  specified  which  may  be  difficult  in  rough  seas 
where  visibility  may  be  reduced  to  zero.  The  small 
marine  radar  solves  this  problem.  Corner  reflectors  are 
attached  to  the  nets  at  several  points  to  make  their 
position  visible  on  the  radar  screen.  This  enables  a 
skipper  to  determine  his  own  position  in  relation  to  the 
others  and  their  nets  and  shoot  his  own  nets  at  the 
specified  distance. 

Radar  can  also  be  used  as  a  very  effective  aid  in  crab 
fishing.  The  crab  nets  are  usually  set  parallel  to  the 
coast  and  at  intervals  of  200  to  300  m.  The  total  length 
of  one  series  of  nets  is  approximately  4  nautical  miles.  By 
watching  the  radar  traces  of  the  corner  reflectors  attached 
to  the  net  buoy,  the  skipper  can  shoot  his  nets  even  in 
fog  at  the  proper  distance  from  the  gear  of  other  boats 
making  allowance  for  tide  and  current.  Formerly,  the 
whole  operation  had  to  be  suspended  in  dense  fog. 
In  this  fishery  the  radar  must  be  very  accurate  within  the 
range  of  100  to  300  m. 


Fig.  4.     Shape  and  adjustment  of  the  reflecting  plates  of  a  buoy 
tvt>e  corner  reflector. 


Fig.  5.     Complete  buoy-type  corner  reflector. 


[464] 


RADIO   COMMUNICATION   APPARATUS   FOR   FISHING  BOATS 

IN   JAPAN 

by 

K.  KODAIRA 

Abritsu  Electric  Co.  Ltd.,  Tokyo,  Japan 


Abstract 

The  author  gives  a  general  survey  of  ihe  radio  equipment  used  at  present  in  Japanese  fishing  vessels;  the  frequency  bands  used  and 
the  intercommunication  facilities.  He  deals  further  with  the  Radio  Rotary  Beacons,  whose  signals  can  be  picked  up  with  an  ordinary  receiver 
and  the  use  of  automatic  radio-buoys  to  mark  nets  and  captured  whales,  using  low  powered  transmitting  sets. 

The  author  stresses  the  great  need  for  improved  radio  transmission  and  reception  to  avoid  interference  and  disturbance  during 
communication. 


Resume 


Les  appareils  de  radio  communication  pour  les  bateaux  de  p£che  au  Japon 


L'auteur  donne  une  vue  d'ensemble  de  I'equipement  radio  utilisl  aclucllemcnt  a  bord  dcs  bateaux  de  peche  japonais,  des  bandes  de 
frequence  utilisees  et  des  installations  ^intercommunication.  II  traite  en  outre  des  radiophares  tournants  dont  les  signaux  peuvent  etre  recus 
par  un  recepteur  ordinaire  et  de  1'emploi  de  bouees  radio  automatiqucs  pour  marque r  les  filets  et  les  baleines  capturecs  en  utilisant  des 
emctteurs  a  faible  consommation  d'energie  6lectrique. 

L'auteur  souligne  le  grand  besoin  d'une  meillcure  emission  et  reception  radio  pour  eviter  les  interferences  et  les  brouillages  pendant 
les  communications. 

Los  aparatos  de  radio  comunicacion  dc  los  barcos  de  pesca  del  Japon 
Extracto 

El  autor  hace  unc  cxposicidn  general  del  equipo  de  radio  empleado  actualmente  a  bordo  de  los  barcos  de  pesca  japoneses,  de  las 
bandas  de  frccucncia  usudas  y  de  los  medios  de  intercomunicacion.  Pasa  a  ocuparsc  de  los  radiofaros  giratorios,  cuyas  senates  Jas  puede 
recoger  un  receptor  corriente  y  del  empleo  de  radiobalizas  automations,  para  marcar  artes  de  pesca  y  ballenas  capturadas,  empleando  emisores 
de  poco  consumo  de  energiu  e!6ctrica. 

El  autor  subraya  la  gran  necesidad  que  existe  de  mejorar  la  emision  y  rcccpcion  de  radio  para  evitar  las  interfcrcncias  y  perturba- 
cioncs  durante  la  comunicaci6n. 


RADIO  FACILITIES  ON  BOARD  THE  VESSELS 

ALL  Japanese  fishing  boats  over  100  tons  are 
obliged  to  carry  radio  transceivers,  the  effective 
range  of  which  is  shown  in  the  table  below.  They, 
therefore,  arc  equipped  with  MHF  (medium,  high 
frequency)  transceivers,  with  frequencies  between 
410  kc.  and  4,000  kc.  In  addition,  some  have  HF  (high 
frequency)  transceivers  between  4  Me.  and  23  Me. 
(Megacycles)  for  long  distance  communication  and 
eventually  also  telephony  transceivers  for  the  150  Me. 
or  27  Me.  band,  for  short  distance  communication. 
Most  boats  of  under  100  tons  also  have  MHF  trans, 
ceivers;  boats  which  communicate  only  over  shor- 
distances  use  27  Me.  or  150  Me.  band  radio  transceiverst 


Tonnage  of  boats 


1,600  tons  and  over 
500  to  1 ,600  tons 
300  to  500  tons 
100  to  300  tons 


Effective  distance  of  communication  at  A2. 
500  kc.  davtime 


Main 
Equipment 


Emergency 
Equipment 


More  than  280  km.     More  than  190  km. 
,.       „       1^0  „           „       ,       140   „ 
„       „       100 95   „ 

M  M  100       „  -  ~  -" 


The  number  of  fishing  boats  fitted  with  radio  equip- 
ment  is  given  below:  - 


Equipment 

Telegraphy 

Telephony 

Both  telephony  and  telephony 

VHP  (very  high  frequency)    . 


\umher  of  boats 

1,262 

3.928 

957 

280 


FREQUENCY    BANDS 

Besides  the  MF  and  HF  bands  generally  assigned  to 
marine  mobile  stations,  some  frequency  bands  are  allotted 
exclusively  for  fishery.  The  frequency  bands  commonly 
used  are: 


Frequency  Range  Number 

MHF  band  (410  to  4,000  kc.)  49 

27  Me.  band         ...  9 

I  SO  Me.  band       .           -           .  10 

MHF  band  for  radio  buoys       .  13 


[465] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


COAST   STATIONS 

Communication  for  fishing  boats  is  two-fold,  between 
boats,  and  between  boats  and  special  fishery  coast 
stations.  Communication  with  coast  or  harbour  stations 
for  general  shipping  traffic  is  exceptional.  The  fishery 
coast  stations,  which  are  located  at  all  the  main  fishing 
ports,  are  founded  by  prefectures  or  fishing  guilds  and 
communicate  with  the  guild  members'  boats.  The  num- 
ber of  stations  is  given  below: 

Coast  stations  for  fishing  boats        Number  of  stations 


Communicating  mainly  by  telegraphy 
telephony 
VHP  band 
27  Me.  band 


48  (HF  Stations  13) 
35 
27 
109 


EQUIPMENT 
Radio  Telegraphy 

For  boats  over  100  tons,  MF  transmitters  (410  kc.  to 
535  kc.)  arc  obligatory,  but  MHF  (1605  kc.  to  4000  kc.), 
which  is  the  main  frequency  band  of  fishery  communi- 
cation, may  also  be  used:  since  HF  for  long-distance 
communication  is  also  utilized,  there  are  three  bands  with 
10  to  1 !  channels.  The  antenna  power  of  the  transmitters 
lies  between  50  W.  and  500  W.  When  the  power  is  less 
than  150  W.,  radio  telephony  is  generally  confined  to  the 
MHF  band.  Big  fishing  vessels  have  three  transmitters, 
MF,  MHF  and  HF,  and  an  emergency  set.  The  usual 
power  of  the  emergency  set  is  25  W.  or  50  W. 

The  high  frequency  bands  mainly  used  have  been 
4  Me.,  6  Me.  and  8  Me.,  but  recently  12  Me.,  16  Me.  and 
22  Me.  bands  have  become  popular  because  of  congested 
communication.  Consequently,  increasing  use  of  the 
HF  bands  is  evident  even  in  middle  class  and  smaller 
boats.  All  transmitters  have  the  quartz  crystal  controlled 
power  amplification  system,  the  break-in  communication 
system,  and  the  one  switch  channel  change  system,  except 
in  the  HF  band. 

MHF  Radio  Telephony 

Most  of  the  bigger  fishing  vessels  have  MHF  radio 
equipment  which  can  be  used  for  both  telegraphy  and 
telephony.  The  equipment  of  the  small  boats  is  suitable 
for  MHF  telephony  only,  has  an  antenna  power  of 
about  50  W.  and  mostly  6  channels.  All  these  telephony 
sets  have  also  the  quartz  crystal  controlled  power 
amplification  system,  the  press-talk  system  and  the  one 
switch  channel  change  system. 

HF  Radio  Telephony 

The  27  Me.  band  has  been  adopted  by  the  fishing 
industry  since  1955  to  make  up  for  the  shortage  in  the 
MHF  bands.  It  is  used  for  short  distance  communication 
only.  The  transceivers  which  work  with  A.M.  (amplitude 
modulation),  have  an  antenna  power  of  10  W.  and  2  to 
3  channels. 

Recently  the  150  Me.  band  has  come  increasingly  into 
use  for  short  distance  communication.  Most  of  the 
equipment  works  with  F.M.  (frequency  modulation)  and 
only  some  with  A.M.  The  antenna  power  is  10  W.,  with 
one  to  two  channels. 


Radio  Receivers 

Usually,  large  fishing  boats  have  three  receivers,  one  for 
all-wave,  one  for  HF,  and  one  for  emergency;  middle 
class  boats  have  two  all-wave,  and  smaller  boats  one 
all -wave  receiver. 

Ships  without  radio  direction  finders  can  make  use  of 
the  Radio  Rotary  Beacon  station  if  they  have  beacon 
band  receivers.  Radio  Rotary  Beacon  receivers  which  can 
receive  the  beacon  band  (285  to  325  kc.),  the  MF  broad- 
cast band  (535  to  1605  kc.)  and  the  MHF  band  (1605 
to  7000  kc.),  have  come  increasingly  into  use  for  small 
fishing  boats  which  have  no  other  radio  equipment. 

At  present  there  are  1 5  Radio  Rotary  Beacon  stations, 
but  at  least  30  more  stations  are  to  be  built. 

Radio  Buoys 

Such  buoys  are  fixed  to  captured  whales,  fishing  nets, 
etc.  The  small-sized  transmitter,  which  is  installed  in  the 
buoy,  transmits  signals  automatically  at  regular  intervals, 
which  enable  its  position  to  be  spotted  by  means  of 
radio  direction  finding.  The  most  common  type  uses  a 
MHF'  band  and  has  an  antenna  power  of  2  to  3  W. 
They  are  now  so  common  in  sea  fisheries  that  the  problem 
of  interference  has  arisen.  A  device  to  overcome  the 
difficulty  is  being  studied.  This  will  have  a  receiver  which 
starts  transmission  only  after  selective  receipt  of  a  special 
signal  from  the  respective  fishing  vessel. 

General  tendencies  of  improvement 

The  adaptation  of  electronics  to  fishing  boats  is  increasing 
and  many  new  devices  are  being  developed.  The  future 
trend  will  be  substantially  as  follows:  — 

(1)  More  use  of  radio  equipment  in  small  fishing  boats. 
Many  small  fishing  boats  (up  to  20  tons),  which  at 
present  have  no  radio  equipment  or  only  radio 
receivers,   are    likely   to   adopt    radio    telephony 
transceivers  of  small  power. 

(2)  Improvement  of  the  performance  of  radio  trans- 
ceivers.      Interference  and  disturbance   have  in- 
creased with  the  growing  number  of  users,  and  less 
time  is  permitted  for  communication.  Improvement 
in  the  quality  of  transmission  is  urgently  needed, 
such  as  by  diminution  of  the  frequency  band  width 
of  transmission,  checking  the  radiation  of  spurious 
frequencies,  etc.   Receivers  of  increased  stability 
and  selectivity  are  needed. 

(3)  Speeding-up  of  communication. 

A  sirrtple,  small  high-speed  transceiver  is  being 
developed  to  relieve  the  congestion  and  speed  up 
communication.  This  will  replace  the  conventional 
hand-operated  telegraphy. 

(4)  Adoption   of  SSB  system   in   radio   telephony. 
At  present  DSB  (double  sideband  transmission)  is 
used  exclusively  in  radio  telephony  on  the  HF  band. 
But,  as  the  allotment  of  band  width  is  now  very 
difficult,  the  adoption  of  the  SSB  (single  sideband 
transmission)  system  for  radio  telephony  of  small 
power  is  being  studied.  The  quality  of  communi- 
cation is  thereby  improved  and  because  of  the 
narrower  band  width  needed,  the  channels  assigned 
to  telephony  could   be  doubled.   A   SSB   radio 
telephony  transceiver  recently  manufactured  was 
successfully  tested,  and  appears  to  be  suitable  for 
replacing  the  present  DSB  equipment. 


[466] 


THE   DECCA   NAVIGATOR   SYSTEM   AND   ITS   APPLICATION 

TO   FISHING 

by 

THE  DECCA  NAVIGATOR  CO.  LTD. 

Marine  Sales  Department,  London,  S.W.9 


Abstract 

This  system  consists  of  several  chains  of  land-based  transmitting  stations  and  an  appropriate  set  of  meters  (the  decometcrs)  and 
receiver  installed  on  board  ship.  It  enables  skippers  to  navigate  with  extreme  accuracy  up  to  240  miles  from  a  master  station  and  some 
1,400  fishing  vessels  of  all  classes,  both  in  the  U.K.  and  in  other  countries,  are  now  fitted  with  the  equipment.  Pin-pointing  of  trawling 
grounds  is  accomplished  with  ease  and  a  skipper  can  cover  an  area  without  going  over  the  same  ground  twice  and  without  the  use  of  a  dan 
buoy.  The  use  of  the  Marine  Automatic  Plotter  for  fishing  is  also  described.  This  instrument  removes  the  need  for  manually  transferring 
the  Decca  readings  to  the  chart  by  making  a  continuous  plot  of  the  ship's  track,  and  by  its  aid  a  course  can  be  retraced  with  precision.  Its 
value  in  fishing  is  obvious. 


Resume 


Le  systcnie  Decca  Navigatcur  et  dcs  applications  a  la  peche 


Cc  systeme  comportc  d'unc  part  plusieurs  chairies  de  postes  emettcurs  terrestrcs  et,  dc  I'autrc,  des  appareils  de  mesurc  appropries 
(decometres)  et  un  recepteur  monies  sur  Ic  navire.  II  permet  aux  patrons  de  peche  de  naviguer  avec  une  tres  grande  precision  jusqu'a  240 
mi  lies  d'unc  station  maitresse,  ct  quelquc  1  400  bateaux  dc  peche  dc  tous  types  appartenam  an  Royaume-Uni  et  a  d'autrcs  pays  sont  main- 
tenant  equipes  de  cct  appareil.  I  c  reperage  exact  des  licux  de  chalutage  s'opere  facilement  et  un  patron  de  peche  pcut  chafutcr  une  zone 
sans  repasser  deux  fois  sur  Ic  mcnie  cndroit  m  avoir  rccours  a  des  bouecs  de  reperc,  L'autcur  decrit  egalcmcnt  Tcmploi  du  Marine  Automatic 
Plotter  (traceur  de  route  aulomauquc)  pour  Ic  peche.  t'cl  appareil  qui  donne  un  trace  continu  du  parcours  du  navirc  cvite  ainsi  dc  repo  ter 
sur  la  carte  les  indications  du  Decca  ct  permet  dc  re  tracer  une  route  avec  precision.  l.'interet  qu'il  presume  pour  la  pcche  est  evident. 


Fl  sistcma  4<Dccca  Navigator*1  y  su  aplicacion  en  la  pesca 
Extracto 

Estc  sistcma  csta  formado  por  vanas  cadenas  dc  cstacioncs  trasmisoras  terrcstres  y  aparatos  dc  medida  ("decometros")  adecuados 
que  se  concctan  con  un  receptor  a  bordo  del  harco,  los  cuales  permiicn  a  los  patrones  de  barcos  navegar  con  gran  precision  hasta  240  mil  las 
de  distancia  dc  la  cstacion  macstra.  Unas  1.400  cmbarcaciones  pcsqueras  dc  todas  clascs,  tanto  en  cl  Rcmo  Unido  como  en  otros  paises, 
poseen  en  la  actualidad  dicho  equipo  que  ayuda  a  detcrniinar,  en  forma  precisa  y  con  facilidad.  la  posicion  dc  los  bancos  donde  se  practica 
la  pcsca  de  arrastre,  pudicndo  cl  patron  ctibrir  una  zona  sin  pasar  porella  dos  \cccs  ni  usar  boyas.  hn  el  articulo  tambien  se  describe  el  cmpleo 
del  "Marine  Automatic  Plotter'*  en  la  pcsca,  que  elimina  la  ncccsidad  dc  transporlar  manualmenlc  las  Iccturas  obtcnidas  con  el  "Decca"  a  la 
carta  dc  navegacion,  tra/a  en  forma  continua  cl  rumbo  del  harco  y  mcdiante  su  ayuda  pcrmitc  reproducirlo  con  precision.  Por  las  razones 
mencionadas,  el  valor  dc  cstc  instrumcnto  en  la  pesca  es  cvidentc. 


GENERAL 

THE  most  successful  and  accurate  aids  to  navigation 
which  have  been  developed  over  the  past  50  years 
are  those  based  on  radio  technique.  They  involve 
the  use  of  electromagnetic  waves,  which  usually  enable  a 
fix  to  be  obtained  in  all  types  of  weather  conditions. 

The  first  stage  in  the  use  of  radio  waves  was  radio 
direction-finding,  enabling  the  mariner  to  find  the 
direction  from  which  the  radio  waves  originate,  i.e.  the 
bearing  of  the  transmitting  station.  A  second  stage  was 
the  discovery  of  a  means  for  determining  the  distance,  by 
measuring  the  time  taken  by  a  radio  wave  to  travel  from 
the  transmitting  station.  Radar  and  Shoran  are  examples 
of  this  development. 

The  third  stage  marks  the  development  of  hyperbolic 
systems,  where,  instead  of  the  distance  to  a  fixed  station, 
the  difference  in  the  distances  to  two  fixed  stations  is 
measured,  resulting  in  hyperbolae  as  position  lines.  This 
can  be  done  either  by  measuring  the  difference  in  time 


that  it  takes  the  radio  waves  from  the  two  stations  to 
reach  the  observer,  or  by  measuring  the  difference  in 
phase  between  the  two  radio  waves  at  the  point  of 
observation.  The  Decca  Navigator  System  uses  the  latter 
method. 

DESCRIPTION  OF  OPERATION 

The  system  consists  of  several  chains  of  land-based 
transmitting  stations  and  a  set  of  meters  (the  deco- 
meters)  and  receiver  (the  Decca  Navigator)  installed  on 
board  the  ship.  A  set  of  specially  overprinted  charts  is 
used  to  plot  the  information  from  the  decometers. 

Each  chain  comprises  four  stations  (a  master  and 
three  slave  stations)  which  transmit  continuous  radio 
waves.  The  receiver  which  picks  up  these  transmissions, 
actuates  three  decometers,  designated,  Red,  Green,  and 
Purple.  Each  compares  the  transmission  from  the  master 


[467] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


and  Red  slave,  Green  slave  and  Purple  slave  station 
respectively,  and  is  based  upon  phase-comparison  of  the 
incoming  signals.  As  the  phase  condition  of  a  radio  wave 
in  a  certain  place  at  any  given  moment  depends  upon  the 
wave-length  and  the  actual  distance  from  the  transmitter, 
phase-comparison  is  closely  related  to  a  measurement 
of  the  difference  in  distance  from  the  master  and  respec- 
tive slave  stations.  A  position  line  indicating  a  fixed 
phase  difference  between,  say,  master  and  red  slave 
station  and  therefore  defining  a  fixed  difference  in  distance 
to  the  master  and  the  red  slave,  is  by  definition  a  hyper- 
bola, and  a  lattice  of  hyperbolic  red  position  lines  is 
therefore  set  up  between  these  two  stations.  A  similar 
lattice  of  green  and  purple  position  lines  exist  between 
the  master  and  the  green  and  purple  slave  stations 
respectively.  The  patterns  intersect  in  such  a  way  that 
at  any  place  two  of  them  will  yield  a  pair  of  position  lines 
by  which  the  ship's  position  may  be  fixed. 

The  hyperbolic  position  lines  on  the  chart  are  numbered 
and  the  readings  on  the  three  decometers  correspond  with 
these  numbers.  At  the  start  of  a  voyage  or  on  entering 


the  Decca  coverage,  the  decometers  are  set  up  manually 
to  the  numbers  indicated  on  a  fourth  dial,  the  Lane 
Identification  meter.  Thereafter,  the  decometer  pointers, 
rotating  automatically  as  the  ship  proceeds,  will  give 
readings  corresponding  to  the  ship's  position.  Lane 
Identification  readings  continue  to  be  given  three  times 
every  minute,  thus  providing  a  valuable  independent 
cross-check  in  addition  to  enabling  ocean-going  vessels 
to  set  their  decometers  on  first  entering  the  area  covered 
by  the  System. 

To  fix  his  position,  the  mariner  reads  two  of  the 
decometers  and  finds  on  his  chart  the  intersection  point 
of  the  two  position  lines  bearing  the  numbers  indicated. 
Readings  from  only  two  decometers  are  sufficient;  the 
two  corresponding  to  the  patterns  giving  the  best  angle 
of  cut  in  the  particular  area  (fig.  1). 

The  space  bounded  by  two  adjacent  *in-phase'  hyper- 
bolic position  lines  is  called  a  Decca  lane,  and  the 
pattern  produced  by  one  pair  of  stations  may  contain 
200  or  more  of  these  lanes.  Measured  along  the  inter 
station  base  line,  the  lane  width  is  some  500  yards.  Each 


THE  LAYOUT  OF  THE  ENGLISH  CHAIN  SHOWING 

HOW  A  RX  IS  OBTAINED   FROM   THE    READINGS   OF 
TWO  DCCOMETER  INDICATORS. 


INTERSECTION    OF   TWO 
POSITION      LINES      IS 
FIX    OF     POSITION 


DfCCA 

\   Rf b    I  16-30 


DECCA      COORDINATE 
OUIN    DJS  «0 


PURPLE  GRCFN 

Fig.  1.    Illustration  of  how  the  Decca  Navigator  System  functions. 

[468] 


RFD 


DECCA    NAVIGATOR    SYSTEM 


decometer  is  capable  of  measuring  0-01  of  a  lane, 
representing  about  5  yards. 

The  charts  used  in  conjunction  with  the  System  are 
normal  navigational  charts  over-printed  by  the  Hydro- 
grapher  of  the  Navy  with  the  Decca  hyperbolic  position 
lines  (fig.  2).  In  addition,  special  fishing  charts  are  being 
issued  which  are  similarly  latticed.  As  these  are  normally 
of  a  small  scale,  we  are  now  preparing  special  plotting 
sheets  of  a  much  larger  scale,  say,  1 : 200,000,  which  give 
only  the  Decca  lattice  lines  for  a  particular  area  plus  a 
geographical  reference  in  the  form  of  one  parallel  and 
one  meridian.  The  fishing  grounds  can  be  drawn  in  by 
the  skipper  himself,  with  the  positions  of  various  wrecks 
known  to  him. 

COVERAGE  AND  RANGE 

Due  to  the  frequencies  used,  which  are  in  the  area  of 
100  kcs.,  the  operational  range  is  about  300  to  500  miles. 
As  a  rule,  the  normal  transmissions  of  a  Decca  Chain 
may  be  used  as  a  general  navigational  aid  up  to  240 
miles  from  the  master  stations  in  areas  covered  by 
Decca  charts.  Eight  chains  of  Decca  stations  cover 
North-West  Europe  from  Cape  Finisterre  to  the  Gulf 
of  Bothnia  and  the  Faroe  Islands  (fig.  2).  In  North 
America  three  chains  have  been  recently  established  in 
Newfoundland  and  Nova  Scotia,  which  also  cover  the 
rich  fishing  grounds  in  that  area. 

APPLICATION  TO  FISHING 

The  accuracy  with  which  the  Decca  Navigator  fixes  a 
ship's  position  makes  the  System  particularly  valuable 
to  fishing.  Some  3,200  fishing  vessels  of  all  classes,  such 


•AVKATOR  COVER/ME 
IN  WEST  EUROPE 


Fig.  2.     Decca  Navigator  coverage  in  West  Europe. 


as  trawlers,  seine  netters  and  drifters,  in  the  U.K.  and 
abroad  are  already  fitted  with  this  type  of  equipment. 
The  advantages  may  be  listed  as  follows: 

1.  Pin-point  positions  save  much  wasted  steaming  time 
in  reaching  the  fishing  grounds  and  the  subsequent 
return  with  the  catches.  Exact  courses  can  be  steered 
and  accurate  landfalls  made  by  means  of  a  regular 
check  of  the  decometer  readings.  This  simple  opera- 
tion will  also  enable  the  skipper  to  cover  the  fishing 
grounds    very    thoroughly,    not    leaving   any    spot 
untouched  or  going  over  the  same  place  twice.  Any 
specially  profitable  area  can  be  plotted  with  extreme 
accuracy  and  the  vessel  can  be  taken  back  to  the 
same  locality  again  and  again  with  great  precision, 
without  even  the  use  of  a  dan  buoy  and  regardless 
of  visibility. 

A  common  practice  of  many  fishermen  is  to  trawl 
along  a  certain  Decca  position  line  thus  keeping  one 
of  the  decometers,  say,  the  red  one,  at  a  stable  reading. 
When  the  trawl  is  put  out,  the  reading  of  the  other, 
say,  the  green  decometer,  is  also  noted,  and  this 
procedure  repeated  at  the  end  of  the  track  after 
about  three  or  four  hours.  If  the  catch  is  good  and 
of  the  right  quality  fish,  then  the  skipper  will  en- 
deavour to  retrace  his  course  by  following  the  same 
Decca  lattice  line  on  the  red  decometer  until  he  has 
arrived  at  the  original  point  where  he  obtained  his 
first  reading  on  the  green  decometer.  All  the  time 
his  red  reading  will  tell  him  whether  his  vessel  is 
making  leeway  or  drifting  and  to  what  extent. 

2.  A  noticeable  reduction  in  losses  of  nets  has  been 
experienced    on    board   the   ships   fitted   with    the 
Decca  Navigator.  The  presence  of  wrecks  with  which 
the  trawl  net  and  gear  may  become  entangled  can 
cause  the  complete  loss  of  a  trawl  or  at  least  some 
considerable  damage  plus  loss  of  time  involved  in 
disentangling  the  gear.      Often,  however,  fish  abound 
close  to  wrecks  and  rocks. 

The  Decca  Navigator  enables  every  movement  of  the 
vessel  to  be  immediately  shown  on  the  decometers,  and 
these  dangerous  grounds  can  be  passed  at  very  close 
proximity  once  the  Decca  position  is  known. 

This  is  of  even  more  importance  with  seine  net  fishing. 
Seine  fishermen  use  very  light  fishing  gear  and  a  great  deal 
of  damage  can  be  done  to  a  net  and  the  ropes  by  rocks 
and  wrecks.  They  arc,  therefore,  very  particular  in 
finding  clear  ground  before  shooting  their  net. 

THE  AUTOMATIC  PLOTTER 

This  recently  developed  instrument  has  been  in  use  for 
several  years  for  other  purposes,  such  as  mine-sweeping, 
hydrographic  survey  and  submarine  oil-exploration,  h 
is  a  device  similar  to  the  Flight  Log  used  in  aircraft,  and 
produces  a  continuous  record  of  the  ship's  position  on 
a  roller-mounted  chart  by  means  of  a  plotting  pen.  The 
Decca  information,  which  is  normally  taken  from  the 
decometers  and  plotted  by  hand  on  a  Decca  lattice  chart, 
is  fed  into  a  servo  amplifier  and  translated  into  a  pen 
and  paper  movement,  horizontally  and  vertically, 
respectively.  In  this  way,  a  continuous  plot  is  produced 
of  the  track  made  good.  Various  scales  are  provided,, 
giving  a  movement  of  I  in.  to  4  in.  per  Decca  lane,  whilst 


[469] 


MODERN    FISHING    GEAR     OF    THE   WORLD 


fig.  3.     Decca  Marina  Automatic  Plotter. 

the  Display  Unit  shows  a  chart  area  of  10  in.  by  10  in. 
As  the  plotting  pen  and  the  chart  move  at  right-angles  to 
each  other,  the  hyperbolic  Decca  position  line  patterns 
are  presented  upon  the  chart  in  a  rectilinear  lattice  form 
(fig.  3). 


This  brings  about  a  certain  amount  of  chart  distortion 
which  will  vary  according  to  the  position  of  the  ship  in 
relation  to  the  Decca  Chains  in  use.  However,  compass 
roses  can  easily  be  computed  and  drawn,  informing  the 
user  of  the  amount  of  distortion.  In  practice,  no  dis- 
tortion has  proved  to  be  of  inconvenience  within  150 
miles  from  the  master  stations. 

The  Marine  Automatic  Plotter  has  proved  of  great 
value  to  trawling.  The  Decca  Navigator  enables  the 
fisherman  to  plot  any  specially  profitable  area  and  return 
to  it  again  and  again  with  precision  in  all  visibilities.  To 
accomplish  this,  it  was  hitherto  necessary  to  follow  a 
certain  Decca  lattice  line,  i.e.  to  keep  one  of  the  deco- 
meters  at  a  stationary  reading.  This  restricts  the  user  to 
certain  courses,  depending  also  upon  which  chain  of 
stations  he  has  selected.  For  instance,  many  fishermen 
operating  in  the  German  Bight  make  use  of  the  German 
Chain,  because  its  lattice  lines  run  in  a  more  suitable 
direction  over  the  fishing  grounds,  rather  than  the  Danish 
Chain,  although  the  characteristics  of  the  latter  for  that 
area  provide  better  means  for  accurate  position  fixing. 
With  the  aid  of  the  Marine  Automatic  Plotter,  it  is  no 
longer  necessary  to  follow  a  certain  lattice  line,  as  it 
enables  the  user  to  check  his  positions  at  a  glance  with 
reference  to  his  point  of  origin,  and  steer  the  vessel  back 
to  this  point  along  a  straight  course.  Thus  the  Plotter 
makes  the  Skipper  independent  of  the  chain  pattern. 
Moreover,  by  using  the  largest  scale  in  the  appropriate 
areas,  i.e.  4  in.  to  1  lane  representing  approximately 
U  miles  at  150  miles  from  the  master  stations,  the 
instrument  provides  detailed  and  continuous  information 
of  the  track  made  good,  whilst  at  the  same  time  the 
position  of  wrecks  and  details  of  the  fishing  grounds  can 
be  marked. 

Another  feature  is  the  safe  and  quick  piloting  of  vessels 
over  tracks  previously  made.  When  leaving  harbour 
under  good  weather  conditions,  the  Marine  Automatic 
Plotter  will  draw  a  track  which  can  be  used  when  return- 
ing. Jf,  then,  visibility  should  be  bad,  the  helmsman 
can  steer  the  ship  in  the  opposite  direction  along  the 
track  recorded  on  the  way  out. 

This  method  has  been  adopted  with  success  on  various 
trawlers  in  Great  Britain,  Germany,  Belgium  and 
France. 


470  ] 


DISCUSSION   ON   FISH  LOCATION 


Dr.  A.  W.  H.  Needier  (Canada),  Chairman:  The  general 
strategy  of  fishing  covers  a  broad  subject  concerned  with  how 
to  find  more  fish  as  food  for  our  growing  population,  and 
how  to  better  the  lot  of  the  people  who  produce  the  food. 
This  means  a  much  broader  examination  of  the  problem. 
Three  phases  to  be  considered  are:  location  of  fish;  detection 
of  fish;  and  attraction  of  fish.  These  overlap  to  a  considerable 
degree.  The  first  item  concerns  the  problem  of  where  to  find 
fish. 

Mr.  S.  Holt  (FAO),  Rapporteur:  Detection  is  a  matter 
of  communication  between  fish  and  man,  and  a  communica- 
tion system  consists  of  a  transmitter  which  produces  signals 
to  a  receiver.  The  source  of  the  signal  is  the  fish  and  the  signal 
may  be,  for  instance,  a  sound  or  a  movement  made  by  the 
fish.  The  signal  channel  is  usually  some  characteristic  of  the 
water  itself,  its  conductivity  for  sound,  pressure  waves,  or 
light.  The  receiver  can  be  the  human  eye  or  the  car  or,  more 
likely,  an  instrument  to  amplify  the  signal. 

Attraction  covers  also  repulsion  and  any  other  stimulating 
effect.  While  the  transmitter  in  this  communication  system 
may  be  a  man's  voice,  a  board  he  bangs  on  the  water,  a  light, 
or  an  ultrasonic  oscillator,  the  channel  is  again  provided  by 
one  or  other  properties  of  the  water,  and  the  receiver  is  one 
or  more  of  the  sense  organs  of  the  fish.  F;ish  react  by  becoming 
excited,  turning,  swimming  towards  or  away  f.om  the  source 
of  the  signal,  i.e.  being  directed,  attracted  or  repelled.  For 
attraction  and  detection  the  relevant  properties  of  the  water 
act  as  a  signal  channel:  ability  to  conduct  heat  or  electricity, 
to  flow,  to  propagate  mechanical  or  electro  magnetic  waves, 
the  physical  chemical  property  of  diffusing  dissolved  chemicals. 
Almost  all  these  properties  have  been  used  by  fishermen  and 
scientists  to  direct  or  detect  fish.  The  instruments  used  may 
also  be  the  same  in  both  cases.  In  detection,  the  fish  may 
produce  a  signal  spontaneously,  but  we  can  also  induce  the 
fish  to  give  signals,  as  for  instance,  by  making  a  sound  which 
disturbs  the  fish,  which  can  then  be  seen  by  its  movement. 
An  electric  pulse  may  induce  the  fish  to  make  a  sound, 
which  we  in  turn  hear.  In  detection  methods  using  echoes,  a 
signal  received  back  is  usually  of  the  same  kind  as  that 
originally  sent  out.  But  the  fish  is  not  just  a  passive  reflector. 
The  character  of  the  echo  from  the  fish  depends  as  much  on 
the  behaviour  of  the  fish  as  it  does  on  its  anatomy. 

Usually  a  variety  of  signals  arrives  at  the  receiver.  Not  all  of 
them  will  be  from  the  original  transmitter  and  carry  a  message. 
There  are  others  which  come  from  outside  sources.  They  are 
a  disturbance  for  the  communication  system  and  the  elimina- 
tion or  reduction  of  this  "noise"  is  one  of  the  main  problems 
in  communications  engineering. 

However,  what  is  noise  to  one  man  may  be  a  message  to 
another.  If  we  are  trying  to  detect  fish  by  echo-sounding,  an 
echo  from  the  seabed,  or  from  a  patch  of  plankton,  or  from 
a  water  layer  where  the  temperature  is  changing  rapidly,  may 
be  a  nuisance  in  detecting,  but  any  one  of  them  may  lead  to  a 
way  of  improving  location  methods. 


Location  and  detection  differ  only  in  that  for  location,  we 
may  find  fish  indirectly  by  detecting  not  the  fish  themselves, 
but  something  else,  the  distribution  of  which  is  closely  related 
to  that  of  the  fish,  as,  for  example,  the  type  of  bottom  or  the 
temperature  distribution  in  the  water  which  often  is  associated 
with  plankton  concentrations  which  are  the  food  of  pelagic 
fish.  Furthermore  some  fish  arc  restricted  to  a  certain 
temperature  range.  If,  for  example,  a  certain  kind  of  fish  is 
never  found  in  water  of  less  than  5  degrees  C.  or  more  than 
7  degrees  C.,  then  a  signal  from  the  thermometer  that  the 
temperature  is  3  degrees  C.,  is  a  definite  message:  no  fish 
of  that  certain  kind  can  be  expected.  A  temperature  of 
6  degrees  C.  would  not  mean  that  there  is  fish,  but  only  that 
fish  may  be  present. 

Science  can  help  by  inventing  instruments  to  receive  signals 
man  cannot  himself  receive,  such  as  ultra  sound  or  infra  red 
light.  Science  can  also  improve  methods  of  interpreting  such 
signals,  resulting  in  more  reliable  messages.  Organising  the 
collection  of  data  is  another  means  for  improving  location 
methods.  Here,  the  fishermen,  the  special  research  ships,  and 
science  have  to  collaborate  closely.  Dietrich  ci  aL  conclude 
in  their  paper  on  the  relation  of  fish  to  temperature  that  only 
fully  synoptic  observations,  made  as  a  special  research  project, 
can  lead  to  satisfactory  results.  In  many  cases,  however,  a 
fisherman  himself  can  use  instruments  to  help  him  locate  good 
fishing  grounds. 

If  fish  can  5e  expected  to  stay  where  they  have  been  found 
before,  the  only  location  problem  would  be  to  find  one's  way 
back  to  the  same  place.  If  they  are  spread  about  very  unevenly, 
it  may  be  necessary  to  get  back  rather  precisely  to  the  same 
position,  even  within  a  few  metres.  Kodaira.  the  Suiyokai  and 
the  Decca  Navigator  Company  describe  modern  equipment 
for  such  spot  plotting. 

Other  than  navigational  factors  will  be  important  if  we 
want  to  search  areas  previously  uncxploited;  or  if  the  fish 
move  about  much.  In  the  first  case,  we  can  make  fishing 
trials,  and  cruise  about  with  detecting  instruments.  But  the 
oceans  are  so  vast  that  an  indication  as  to  when  and  where  to 
look  would  be  helpful.  We  now  know  a  little  about  the  parts 
of  the  ocean  where  fish  are  likely  to  be  found,  but  not  enough 
to  advise  confidently  on  the  success  of  a  new  fishing  enterprise. 
We  also  know  a  little  about  the  kinds  of  fish  to  expect  in 
certain  areas. 

A  knowledge  of  the  kind  of  relation  between  the  fish  and 
its  environment  not  only  reduces  the  work  of  searching,  but 
also  helps  in  deciphering  the  detective  signal.  The  instruments 
being  developed  now  for  navigation  and  for  detection,  can 
help  to  improve  location  methods.  They  allow  to  identify 
for  instance  concentration  of  fish  foods,  bottom  types  and 
so  on,  and  find  the  correlation  between  fish  distribution  and 
such  factors  more  quickly  by  measuring  fish  abundance  with- 
out having  to  catch  them. 

Schaefers  and  Powell,  and  Vestnes  show  how  systematic 
detection  of  fish,  and  plotting  migration  rules,  can  help 
predict  the  future  position  of  the  schools,  tcho  sounding 


[471  ] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


can  be  used  for  mapping  the  distribution  of  fish  in  relation 
to  environmental  factors. 

Fish  movement,  local  or  within  the  general  fishing  area, 
may  be  determined  by  some  unchanging  feature  of  the  seabed 
or  the  water,  and  a  simple  fishing  chart  based  on  an  analysis 
of  past  catches  can  be  very  useful.  Fish  may  only  live  in 
certain  places  at  certain  times  of  their  life,  or  at  particular 
seasons  or  times  of  day.  A  useful  chart  must  take  such  changes 
into  account  and  relate  them,  if  possible,  to  other  changes  in 
the  environment,  such  as  the  food  supply.  Atlases  to  help 
the  fishermen  work  fresh  grounds  would  need  to  show  the 
expected  location  of  each  species,  their  abundance,  and 
quality  of  the  fish,  whether  expressed  by  size,  or  age,  fatness 
or  some  other  measure.  They  should  also  show  the  vulner- 
ability of  fish  to  different  gears. 

Other  factors  ruling  fish  migrations  arc  variable  as  for 
instance  light,  temperature,  chemical  composition  of  the  water, 
water  movement,  food  and  enemies.  Some  of  these  affect  the 
fish  food,  others  work  both  directly  and  indirectly.  The 
practical  use  that  could  be  made  of  such  relations  depends  on 
how  close  they  arc  and  how  easily  they  can  be  observed. 
Dietrich  et  al.  point  out  that  temperature  may  only  be 
correlated  with  fish  indirectly,  being  an  indicator  of  "some- 
thing else41  which  is  difficult  to  determine.  But  a  relation  to 
fish  exists  and  temperature  is  easy  to  measure.  The  abundance 
or  lack  of  food  for  example  may  be  indicated  by  the  turbidity 
or  the  chemical  composition  of  the  water.  Plankton  animals 
have  been  used  to  identify  water  masses.  The  presence  of 
sharks  or  birds  may  indicate  the  presence  of  commercial 
fishes.  There  can  be  positive  correlation,  i.e.  a  lot  of  fish 
where  there  is  a  lot  of  food  or  negative  correlation,  fish 
avoiding  areas  that  have  certain  chemical  composition  or 
where  there  are  enemies  or  obnoxious  plankton.  The  correla- 
tion can,  however,  reverse  itself,  starting  as  positive,  ending 
as  negative;  at  one  time  there  may  be  many  fish  where  there 
are  few  food  animals,  most  of  them  having  been  eaten.  Whether 
any  of  these  relations  are  useful  depends  on  whether  they 
are  consistent,  easily  observable  and  can  be  clearly  interpreted. 

1  would  emphasise  the  importance  of  the  exact  way  we 
look  at  each  factor.  We  may  be  concerned  not  only  with  the 
actual  fish  abundance,  or  the  actual  temperature,  but  also 
the  way  these  things  are  changing,  geographically  or  in  time. 
Dietrich  et  til.  show  how  a  temperature  gradient  or  a  rate  of 
change  of  salinity  may  be  much  more  significant  as  an 
indicator  of  fish  distribution  than  a  simple  measure  of  these 
factors. 

Matters  are  complicated  by  the  interference  of  different 
factors.  In  fish  attraction,  the  fish  may  react  to  light  in 
different  ways  at  different  times  and  under  different  conditions. 
In  detection,  the  signal  sent  by  the  fish  may  vary.  In 
detection  and  location,  the  fisherman  must  react  on  the 
same  message  in  different  ways  at  different  times,  depending 
on  other  messages  relating  to  weather  conditions  and  so  on. 

In  the  fishery  strategy  of  nations  and  governments,  the 
broader  aspects  of  location  play  a  very  important  part 
to  know  whether  new  fish  stocks  may  be  found  and  to 
determine  the  size  of  known  stocks. 

To  summarize  the  present,  and,  particularly,  the  future  of 
fish  location  methods:  firstly,  we  know  something  about  the 
general  distribution  of  fish,  but  we  are  still  constantly  being 
surprised  at  the  rapid  development  of  new  and  unsuspected 
fisheries.  Secondly,  we  know  much  less  about  the  factors 
determining  occurrence  of  fish  within  a  particular  area. 


Research  is  needed.  Scattered  information  in  scientific 
literature  needs  to  be  collected.  There  is  a  great  deal  of 
experience  in  the  heads  of  the  fishermen,  which  needs  to  be 
extracted,  and  tested  by  exact  observation  and  experiment. 

We  need  more  statistics  of  catches,  including  location,  time 
and  the  conditions  under  which  the  catches  have  been  taken. 
We  need  more  research  on  the  fundamentals  offish  physiology 
and  behaviour,  especially  in  relation  to  fishing  gear.  Next  to 
putting  promising  methods  into  practice,  we  need  the  technical 
means  of  good  navigation,  communication  and  observation 
at  sea  and  fishermen  to  use  these  instruments.  Synoptic  data 
are  needed  to  which  the  fishermen  can  contribute. 

Mr.  J.  Jakobsson  (Iceland):  During  the  herring  fishery  in 
Iceland,  a  research  vessel  continuously  cruises  on  the  herring 
grounds,  using  Asdic  (echo-ranging)  for  detection.  For 
location,  we  are  chiefly  interested  in  the  distribution  of 
certain  plankton  animals,  i.e.  Calanus  Jinmarchicus,  which 
is  the  main  herring  food  in  these  areas.  A  very  close  correla- 
tion has  been  ascertained  between  the  quantitative  distribution 
of  these  animals.  In  recent  years,  especially  this  summer,  a 
very  interesting  phenomenon  has  been  observed  in  the 
distribution  and  age  composition  of  this  plankton.  When 
the  season  started  there  was  very  little  plankton  on  the  main 
fishing  grounds,  except  at  the  extreme  end  of  the  western 
area.  By  making  a  detailed  analysis  at  the  beginning  of  the 
season,  we  were  able  to  observe  that  the  western  area  plankton 
was  composed  almost  entirely  of  mature  animals.  In  the 
extreme  eastern  area  there  was  a  very  strong  concentration 
of  newly  hatched  plankton.  In  the  middle  area,  little  plankton 
was  found  except  in  a  curved  section  where  the  main  fishing 
takes  place.  During  the  beginning  of  this  season  it  was  quite 
obvious  that  there  would  be  very  little  food  in  the  main  area 
because  of  the  adverse  current.  If  very  thing  pointed  to  the 
fact  that  the  newly  hatched  plankton  would  later  in  the  season 
provide  very  rich  herring  food  off  the  east  coast.  So  1  tenta- 
tively made  the  suggestion  that,  because  of  this  food  distri- 
bution, herring  would  gradually  disappear  from  the  main  area 
and  move  east.  This  prediction  turned  out  to  be  right. 

This  shows  that  the  quantitative  distribution  of  the  plank- 
ton is  not  only  important  for  the  actual  distribution  of  the 
herring,  but  that  a  study  of  plankton  distribution  sometimes 
makes  it  possible  to  indicate  future  movements  of  the  fish. 

Plankton  in  these  waters  practically  rules  the  schooling 
behaviour  on  which,  as  in  most  fisheries,  the  Icelandic 
herring  fishery  depends.  For  purse  seining,  the  schools  must 
rise  very  neap  to  the  surface  and  they  must  be  compact. 
Where  there  is  little  plankton,  the  schools  come  to  the 
surface  and  spread,  consequently  the  fishing  is  poor  even 
though  herring  are  present.  Where  plankton  patches  are 
heavy,  the  schools  actually  break  surface  and  stay  in  catchable 
quantities.  When  using  Asdic,  the  fisherman  can  sometimes 
set  his  purse  seine  according  to  the  Asdic  records,  without 
waiting  for  the  fish  to  break  the  surface. 

Dr.  D.  Sahrhage  (Germany):  Since  1955,  when  the  Re- 
search Vessel  Anton  Dohrn  was  put  in  service,  six  trips  have 
been  made  to  the  North  Sea  to  carry  out  hydrographical 
studies.  The  results  have  strengthened  our  opinion  that  we 
shall  finally  be  able  to  predict  the  distribution  of  fish.  But  to 
establish  this  method  of  fish  location,  a  regular  survey  of  the 
temperature  distribution  in  the  particular  area  has  to  be 
organized,  and  our  knowledge  of  the  relation  between  temper- 
ature and  fish  distribution  has  to  be  improved. 


[472] 


DISCUSSION  — FISH     LOCATION 


Each  area  must  be  dealt  with  separately.  In  particular, 
hydrographical  observations  of  the  herring  grounds  in  the 
North  Sea  would  be  advisable  and  should  be  made  annually, 
i.e.  each  spring.  In  many  cases,  temperature  measurements 
made  by  the  skippers  of  fishing  boats  are  valuable,  but  they 
cannot,  of  course,  replace  surveys  by  research  vessel.  In  1954, 
we  initiated  studies  of  the  area  around  the  Dogger  Bank,  and 
have  repeated  them  every  year  in  spring  and  later.  Restricting 
the  survey  to  a  limited  area  will,  it  is  hoped,  lead  to  quicker 
results.  We  hope,  for  instance,  that  these  regular  surveys  in 
the  Dogger  Bank  area  will  soon  enable  us  to  predict  how  long 
the  herring  season  will  last  and  what  the  catch  will  be. 

Dr.  M.  Ruivo  (Portugal):  I  would  like  to  draw  your 
attention  particularly  to  the  stage  when  data  concerning  fish 


location  become  transferred  to  fishing  maps  and  information 
is  given  to  the  fishermen.  We  need  to  be  very  critical  at  this 
stage  to  ensure  that  the  fishermen  are  not  given  out -dated 
maps. 

While  it  may  be  theoretically  easy  to  deal  with  fish  which 
arc  easy  to  locate,  the  problem  becomes  much  more  compli- 
cated in  dealing  with  pelagic  fish,  which  tend  to  move  over 
vast  areas,  with  variations  in  the  migratory  pattern  from  season 
to  season  and  year  to  year. 

It  would  be  useful  if,  with  these  maps,  practical  information 
could  be  given  to  fishermen  on  fish  location.  We  could,  for 
example,  tell  the  fishermen  about  the  relation  of  sardine 
schools  to  varying  mineral  contents  of  the  water.  This  is  the 
type  of  information  which  they  might  be  able  to  put  to  practi- 
cal use. 


Modern  Japanese  tuna  longline  vessels  like  this  one  which  operate  on  the  high  seas  depend  extensively  on  hydrographic  data  and 
observations  for  locating  fish  concent  tat  ions.       Note  inclined  belt  conveyor  for  taking  the  longline  from  the  foredeck  to  the  stern. 

Fujiyama  in  the  background. 


(473] 


Section  II:  Detection  of  Fish. 


ECHO  SOUNDING  AND  FISH  DETECTION 

by 

R.  E.  CRAIG 

Marine  Laboratory,  Aberdeen,  Scotland 

Abstract 

The  tendency  of  echo  sounder  manufacturers  has  been  to  work  on  a  frequency  of  about  30  kc.  for  fish  detection  but  for  fishing, 
there  is  no  such  thing  as  the  ideal  type.  For  example,  for  locating  fish  a  fairly  narrow  sound  beam,  without  side-lobes,  is  needed,  but  if 
fishermen  want  to  study  the  nature  of  the  seabed  a  wide  beam  and  frequencies  somewhat  lower  than  30  kc.  have  certain  advantages.  A 
narrow  beam  used  from  small  ships  may  be  neutralized  by  aeration  and  the  motion  of  the  ship,  and  some  sort  of  stabilization,  such  as  mounting 
the  oscillator  in  gimbals,  is  called  for.  In  searching  for  fish,  though  not  for  use  during  fishing,  the  oscillator  could  be  mounted  in  a  stream- 
lined housing  (which  the  author  calls  a  shark)  and  towed  at  a  suitable  depth.  This  would  provide  a  stable  sound  beam  and  would  eliminate 
or  reduce  aeration.  For  studying  the  seabed,  frequencies  in  the  lower  range,  10-15  kc.  give  a  clearer  picture  than  the  more  usual  higher 
range.  The  future  trends  in  the  science  of  echo  sounding  are  discussed,  and  it  is  suggested  that  the  recording  of  signals  as  presented  on  a 
Cathode  Ray  Tube  might  be  done  photographically  although  notwithstanding  certain  limitations,  the  recorder  is  likely  to  remain  an  essential 
feature  of  echo  sounders  of  the  future. 


R6sume 


Sondage  a  echo  et  detection  du  poisson 


Les  constructors  de  sondeurs  a  echo  ont  eu  tendance  a  choisir  une  frequence  voisine  de  30  kc.  pour  la  detection  du  poisson,  mais 
il  n'existe  pas,  pour  la  peche,  un  type  d'appareil  ideal.  Par  exemple  il  faut  pour  local iser  le  poisson  un  faisccau  sonore  6troit,  sans  lobes 
Iat6raux,  mais  si  les  pccheurs  veulent  connaitre  la  nature  du  fond,  un  faisceau  large  et  des  frequences  un  peu  inferieures  a  30  kc.  presentent 
certains  avantages.  Sur  les  pctits  bateaux,  un  faisceau  6troit  peut  etre  neutralise  par  Taxation  et  le  mouvement  de  la  coquc,  et  il  faut  avoir 
recours  a  un  systeme  stabilisateur.  tel  quc  celui  qui  consiste  a  monter  1'oscillateur  a  la  cardan.  On  peut  monter  1'oscillateur  dans  une  boite 
profilee  (que  Tauteur  appelle  "requin")  que  Ton  remorque  a  une  profondeur  appropriec  pour  detecter  le  poisson  mais  que  Ton  n'utilisc  pas 
pendant  les  operations  de  p£che  propremenl  dites.  Ce  systeme  permet  trait  d'obtenir  un  faisceau  sonore  stable  et  d'61iminer  ou  de  rdduire 
Pa6ration.  Pour  l*6tude  des  fonds  de  la  mer,  des  frequences  inferieures,  dc  Fordre  de  10  a  15  kc.  donnent  une  image  plus  claire  quc  les 
frequences  habituelles  plus  eievees.  L'auteur  examine  les  perspectives  de  la  science  du  sondage  a  echo  et  suggere  que  Fenregistrement  des 
signaux  reproduits  par  le  tube  a  rayons  cathodiques  pourrait  s'effectuer  photographiquement,  mais  en  depit  dc  certaines  limitations,  J'cnregis- 
treur  demeurera  probablement  un  des  elements  essentiels  des  echo-sondeurs  de  1'avenir. 

El  sondeo  a  eco  y  la  localization  de  peces 
Extracto 

Los  fabricantes  dc  ecosondas  tienden  a  usar  frecuencias  de  aproximadamente  30  Kc.  por  segundo  en  la  local izaci6n  de  peccs,  nero 
esto  no  significa  que  sean  las  mejores.  Por  ejemplo,  para  este  objeto  se  necesita  un  haz  de  ondas  sonoras  bastante  estrecho,  sin  lobulos  laterales, 
pero  cuando  el  pcscador  desea  cstudiar  la  naturaleza  del  fondo  marino,  el  uso  de  un  haz  ancho  y  frecuencias  inferiores  a  30  Kc.  por  segundo 
presenta  ciertas  ventajas.  £1  haz  estrecho  que  utilizan  las  embarcaciones  pesqueras  peg  u  eft  as  puedc  ser  neutrali/ado  por  la  aeraci6n  y  los 
movimicntos  del  barco,  necesit&ndose  cierto  tipo  de  cstabilizacion  como  el  montajc  de  los  osciladores  en  aros  de  suspensi6n  dc  brujulas. 
En  la  busqueda  de  peces — aunque  no  se  usa  durante  las  faenas  de  pesca— el  oscilador  puede  montarse  dentro  de  una  caja  hidrodinamica  que 
el  observador  llama  "shark"  (tibur6n)  y  remolca  a  una  profundidad  adecuada.  Este  equipo  proporcionaria  un  haz  de  ondas  sonoras  estable, 
a  la  vez  que  eliminaria  o  reduciria  la  aeracion.  En  el  estudio  del  fondo  del  mar  se  ha  obtenido  una  imagen  mas  clara  al  reemplazar  frecuencias 
altas  por  otras  mas  bajas  de  10  a  15  Kc.  por  segundo. 

En  el  trabajo  tambien  se  analizan  las  futuras  tendencias  en  la  ciencia  del  sondeo  a  eco,  sugiriendose  que  las  imageries  captadas  por 
un  tubo  de  rayos  cat6dicos  podrian  registrarse  fotograficamente.  No  obstantc  ciertas  limitaciones,  el  mecanismo  regis trader  parecc  que 
continuara  siendo  la  caracteristica  esencial  de  las  ecosondas  del  futuro. 


INTRODUCTION 

SINCE  publication  of  a  wide  summary  of  knowledge 
of  echo  sounding  in  fisheries,  prepared  by  a  Com- 
mittee of  the  International  Council  for  the  Explora- 
tion of  the  Sea1,  the  attention  of  manufacturers  has  been 
directed  to  improving  the  performance  of  echo  sounders. 
The  tendency  has  been  to  settle  on  a  frequency  of  around 
30  kc.  for  fish  detection,  and  to  use  oscillators,  of 
rectangular  form,  in  direct  contact  with  the  sea.  Hori- 
zontal ranging  equipments  (referred  to  hereafter  as 
sonar)  have  been  developed  and  marketed  in  America, 
Norway,  Germany  and  Great  Britain. 

A  DILEMMA  IN  ECHO  SOUNDING 

For  fish  detection,  a  fairly  narrow  sound  beam,  without 
side  lobes,  is  desirable  and  modern  magnet o-striction 


oscillators  are  well  adapted  to  produce  such  a  beam. 
Fishermen  also  use  their  echo  sounders  to  tell  them  the 
nature  of  the  seabed.  For  this  purpose  a  wide  sound 
beam,  and  rather  lower  frequencies  than  30  kc.  offer 
advantages.  Thus  it  seems  that  there  is  no  one  best  type 
of  machine  for  all  purposes. 

FISH  DETECTION— DEMERSAL  FISH 

The  detection  of  fish  either  as  schools  or  individuals  at 
depths  of  up  to  about  ISO  m.  is  possible  with  any  efficient 
modern  machine,  provided  that  the  fish  are  in  midwater 
or  distinctly  above  the  seabed.  Thus,  in  20  m.  depth, 
fish  \  m.  from  the  seabed  can  readily  be  detected,  while 
in  200  m.  of  water,  it  is  not  easy  to  detect  fish  unless  they 
are  3  or  4  m.  from  the  seabed.  This  arises  partly  from 


[474] 


ECHO    SOUNDING    AND    FISH    DETECTION 


Fig.  1.     Schematic  representation  of  the  limits  in  displaying 
bottom  fish  by  means  of  echo  sounding. 


the  difficulty  of  finding  a  distinctive  way  of  presenting 
echo  information  on  paper,  but  more  fundamentally 
from  the  limitations  of  the  wide  acoustic  beam  (see  fig.  1 ). 
Only  a  fraction  of  the  sound  beam  is  usefully  employed 
for  the  detection  of  demersal  fish,  and  this  fraction 
becomes  less  as  the  fish  go  closer  to  the  seabed. 

If,  in  fact,  the  fish  are  at  a  distance  H  above  the  seabed 
and  the  depth  of  water  is  D,  then  the  area  A  in  which 
the  fish  can  be  detected  is  given  by  A  —  2jt  DH  i.e.  it  is 
proportional  to  H.  The  traces  of  fish  outside  this  area 
would  disappear  in  the  bottom  record.  Also  the  maxi- 
mum useful  beam  angle  (0)  is  given  by  Sin0/2  =  V2H/D. 
thus  in  200  m.  depth,  the  maximum  useful  beam  width 
is  achieved  at  a  beam  angle  0=12°  for  fish  1  m.  above 
the  seabed,  and  by  0=6°  for  fish  ]  m.  above  the  seabed. 
These  are  absolute  maxima. 

A  narrow  beam  will  also  give  greater  discrimination  and 
detail  of  the  form  of  fish  schools.  The  use  of  a  sound 
beam  of  greater  width  will  decrease  the  signal-to-noise 
ratio  and  reduce  efficiency. 

FISH  DETECTION— PELAGIC  FISH 

Most  modern  echo  sounders  are  suitable  for  the  detection 
of  pelagic  fish.  The  most  immediate  requirement  is  for 
greater  discrimination  between  types  of  fish  and  types 
of  school.  It  is  possible  that  comparison  of  echoes  on 
two  or  more  frequencies  will  prove  to  be  helpful  in  this 
case.  It  is  likely  that  here,  again,  reduction  in  width  of 


the  sound  beam  may  be  helpful.  A  narrow  beam  gives 
a  greater  chance  of  counting  individual  fish,  and  even 
where  this  is  not  possible,  gives  more  precise  information 
about  the  details  of  school  pattern. 

STABILIZING   THE  BEAM 

A  narrow  beam  can  be  provided  by  increasing  the 
oscillator  dimensions  or  increasing  the  frequency. 
However,  particularly  in  the  case  of  small  vessels, 
considerable  motion  is  normally  to  be  expected.  This 
introduces  problems  of  aeration;  bubbles  of  air  are 
carried  under  the  oscillators  and  performance  is  impaired, 
often  to  a  degree  that  makes  the  echo-sounder  useful 
only  in  moderate  to  fine  weather.  Motion  can  be  expected 
also  to  neutralize  the  benefit  of  a  narrow  sound  beam. 
It  seems  desirable,  therefore,  that  the  oscillator  should 
be  stabilised  to  keep  the  sound  beam  vertical.  There  are 
three  obvious  possibilities: 

(a)  To  hang  the  oscillator  in  gimbals  in  the  same  way 
as  a  compass.  This  would  necessitate  a  fairly  large 
housing,    with    a   covering   diaphragm    of    some 
material  with  good  sound  transmission  qualities. 

(b)  To   maintain    the   oscillator  position    by   servo- 
motors controlled  by  a  damped  pendulum.  While 
this  would   seem   technically   feasible,   it  is   too 
elaborate  for  early  application  in  fishing. 

(c)  To  place  the  oscillator  on  a  streamlined  housing  or 
shark  which  can  be  towed  behind  the  ship  at  a 
suitable   depth.    This    method   can    eliminate   or 
reduce  aeration,  as  well  as  provide  a  stable  beam. 
It  is  unsuitable  for  use  during  some  fishing  opera- 
tions, but  could  be  employed  during  a  search  for 
fish. 

Method  (c)  is  the  only  one  likely  to  be  applied  to  small 
vessels,  while  (a)  or  (b)  might  find  application  in  large 
trawlers  in  which  motion  and  aeration  are  less  serious. 
Other  applications  are  suggested  by  Lochridge3. 

ASSESSMENT  OF  THE  SEABED 

When  a  recording  echo  sounder  with  a  fairly  wide  beam 
is  used,  a  distinction  can  be  made  between  the  recording 
of  a  smooth  seabed,  and  one  of  irregular  character  due 
to  rocks  or  stones.  In  the  case  of  the  first,  the  fringe  of 
the  sound  beam  is  largely  reflected  upwards  and  outwards 
and  is  therefore  not  picked  up  by  the  receiving  oscillator. 
In  the  case  of  the  second,  a  part  of  this  fringe  is  reflected 
upwards  and  inwards  by  irregularities  and  is  picked  up 
and  recorded.  As  a  result,  the  echo  trace  of  a  rough 
seabed  is  very  much  thicker  than  that  of  a  smooth  one. 
Another  feature,  that  is  sometimes  useful  as  a  guide  to 
fishing,  is  the  recording  of  hard  ground  lying  below  a 
layer  of  mud  or  sand.  In  such  cases,  the  limits  of  hard 
and  soft  ground  can  sometimes  be  observed  with  great 
accuracy.  This  information  is  of  value,  for  instance,  in 
seining,  finding  suitable  ground  for  shooting,  and 
sometimes  is  helpful  as  an  indication  of  the  kind  of 
fish  likely  to  be  caught.  The  requirement  for  observing 
such  patterns  is  that  the  pulse,  though  partially  reflected 
at  the  surface  of  the  seabed,  should  be  able  to  penetrate 
the  mud  and  be  reflected  with  sufficient  intensity  from  the 
rock  below.  It  is  known  that  absorption  of  high  frequen- 


[475] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


Fig.  2.  Differences  in  character  of  seabed  echo.  Marconi 
Craphette  echo  sounder,  ship's  speed  8  knots.  The  upper 
echogram  shows  mud  or  sand  overlying  solid  rock  which  outcrops 
on  the  right.  The  lower  record  shows  differences  between 
rough  and  smooth  ground  echoes,  the  extended  echo-length 
being  due  to  reflection  from  a  bigger  area  in  case  of  rough  bottom. 


ties  is  much  greater  than  that  of  low  frequencies.  In  fact 
the  pattern  of  soft  over  hard  bottom  is  more  clearly 
detectable  by  sounders  in  the  10  to  15  kc.  range  than  by 
those  in  the  higher  frequency  30  to  50  kc.  range. 

These  two  effects  produce  the  differences  in  appearance 
of  the  bottom  echo  which  many  fishermen  have  come  to 
recognize  and  use  as  an  aid  in  fishing.  Some  examples  are 
given  in  fig.  2.  It  is  perhaps  worth  noting  that  an  indica- 
tion of  the  surface  shape  of  the  bottom  can  be  obtained 
equally  well,  perhaps  better,  with  a  narrow  beam 
machine,  if  an  inclined  auxiliary  beam  is  provided  that 
can  be  brought  into  use  as  required.  In  fact,  if  this 
auxiliary  beam  is  directed  forwards  at  a  suitable  angle, 
it  should  assist  in  the  prior  detection  of  rough  ground. 

If  knowledge  of  the  composition  of  the  bottom  is 
considered  to  be  essential  for  some  types  of  fishing  then  it 
would  appear  best  to  equip  the  vessel  with  a  low  frequency 
machine,  even  if  a  specialist  machine  is  used  for  fish 
detection.  If,  however,  this  is  not  required,  it  seems 
possible  to  visualise  the  following  equipment  being  used  by 
fishing  vessels  in  the  near  future  (fig.  3):  Echo  sounders, 
in  the  frequency  range  30  to  50  kc.  using : 

(a)  A  fixed  single  oscillator  in  a  limpet  fitting  with  a 
beam  angle  (0)  to  half  power  point  of  about  30° 


Fig.  3.    Possible  arrangement  of  oscillators  for  fishing  purposes. 
Main  beam  for  general  use,  the  auxiliary  beam  for  determining 
the  character  of  the  bottom  surface,  shark  oscillator  for  search- 
ing for  fish. 

in  the  athwartships  direction  and  10°  in  the  fore 
and  aft  direction; 

(b)  an  auxiliary  fixed  oscillator  inclined  forward  by 
about  20°  in  the  fore  end  of  the  limpet,  which  can 
be  switched  in  to  give  an  index  of  seabed  character; 

(c)  very  narrow  beam  oscillator  with  a  symmetrical 
beam  angle  of  8U  or  less,  which  can  be  used  from 
a  shark  when  searching  for  fish. 

THE  PROBLEM  OF  PRESENTATION 

The  simplest  method  of  presenting  echoes  is  to  use  a 
Cathode  Ray  Tube  and  display  the  echoes  on  a  suitable 
time  base.  This  calls  for  continuous  attention  on  the 
part  of  the  operator  and  requires  him  to  act  as  a  kind  of 
integrating  machine,  comparing  the  complex  of  echo 
frequency  and  character  from  area  to  area,  making  due 
allowance  for  changes  in  depth.  Such  comparisons  can 
perhaps  be  better  made  automatically  by  electronic 
means. 

The  traditional  recorder  remains  one  of  the  neatest 
and  most  ingenious  methods  of  presentation,  but  it  has 
two  serious  limitations.  These  are: 

(a)  it  is  a  poor  device  for  the  estimation  of  echo 
amplitude,  as  this  can  be  gauged  only  by  the  intensity 
of  the  markings  on  the  paper; 

(b)  since  «the    mechanical   system   has   considerable 
inertia,  its  timing  cannot  be  conveniently  altered  to 
allow  for  vertical  movements  of  the  ship.  If  these 
are  of  the  same  order  as  the  height  of  fish  above 
the  bottom,  detection  of  fish  on  the  chart  may 
become  very  difficult. 

Two  great  merits  of  the  recorder  are  that  its  indications 
of  fish  have  a  great  deal  of  character— it  is  possible  to 
recognize  large  or  small  schools,  or  individual  fish,  with 
some  confidence — and  its  integrating  properties  provide 
an  impression  of  the  form  of  the  seabed,  which  is  of 
great  value  to  fishing  and  navigation. 

For  this  reason  recording  is  likely  to  remain  an 
essential  feature  of  future  echo-sounders  for  general 
purposes,  and  the  techniques  remain  worthy  of  further 
development.  The  recorder  is,  however,  likely  to  continue 
to  be  supplemented  by  other  more  discriminating 


[476] 


ECHO    SOUNDING     AND    FISH     DETECTION 


equipment  for  the  detection  of  demersal  fish.  The  problem 
of  ship  movement  can  undoubtedly  be  overcome  and  the 
recording  principle  retained,  provided  the  recording  is 
done  photographically.  (A  continuous  recording  camera 
would  be  required,  the  signal  being  applied  to  vary  spot 
brilliance  on  a  Cathode  Ray  Tube.)  However,  this  is  a 
solution  only  for  research  purposes.  If  this  problem  can 
be  overcome,  the  recorder  in  some  specialised  form  may 
continue  to  be  a  basic  equipment  even  for  demersal 
fishing. 

SONAR 

Most  fishing  vessels  are  comparatively  small,  and  so  the 
greatest  problem  in  the  application  of  Sonar  techniques 
is  the  motion  of  the  vessel  and  the  associated  aeration. 


A  useful  discussion  is  given  by  Good-.  To  achieve  a 
satisfactory  range,  which  should  be  at  least  half  a  mile 
for  practical  fishing,  the  searching  beam  has  to  be 
narrow.  Thus  the  method  presents  problems  similar  to 
narrow  beam  echo  sounding,  but  in  more  acute  form. 
The  same  kind  of  solutions  as  have  been  suggested  for 
deep-sea  echo  sounding  may  well  find  adoption  for 
Sonar  search. 

REFERENCES 

1  Hodgson  and  Fridriksson  (cds.)     Report  on  echo-sounding 
and  asdic  for  fishing  purposes.  Rapp.  Cons.  Explor.  Mcr.  139. 
Sept.  1955. 

2  Good,  C.  M.    Asdic  in  the  Fishing  Industry,  World  Fishing, 
March,  April  1956. 

3  Lochridge.        Developments  in  Midwatcr  Trawling.     World 
Fishing,  September  1956. 


Cathode  Ray  Tube  echo  display  of  Herring  in  100  m.  depth  (left)  and  of  Redjish  in  300  m.  dtpth  (right),  both  near  the  bottom. 

Photo:   Flac,   Kiel 


[477] 


STUDIES  ON  THE  BEHAVIOUR  OF  ULTRASOUND  IN    SEA  WATER 

by 
TOMIJU  HASHIMOTO 

Fishing  Boat  Laboratory,  Fisheries  Agency,  Japanese  Government 

and 

YOSHIM1TSU   KIKUCH1 

Research  Institute  of  Electrical  Communication,  Tohoku  University,  Sendai,  Japan 

Abstract 

The  propagation  characteristics  of  ultrasound  in  sea  water  and  its  reflection  loss  on  the  seu-bottorn  or  on  fish-schools  must  he  studied 
for  both  the  design  and  the  practical  operation  of  echo  sounders,  fish  finders  and  Sonars.  This  paper  shows  a  practical  method  for  the 
above-mentioned  study,  together  with  the  results  of  measured  propagation  characteristics  in  both  vertical  and  horizonta  directions,  as  well 
as  the  reflection  loss  on  the  sea-bottom  and  on  fish-schools,  using  28,  50,  200,  300,  and  400  kc. 

The  propagation  attenuation  is  due  mainly  to  spherical  divergence,  but  also  to  absorption.  This  absorption  becomes  larger  at  higher 
frequencies;  in  fact,  the  results  of  experiments  show  that  the  absorption  is  10-20  db./*km.  at  28  kc.,  37-50  db./km.  at  200  kc.,  and  about  120 
db./km.  at  400  kc.  The  reflection  loss  of  the  ultrasound  at  a  fish-school  decreases  as  frequency  increases.  On  the  other  hand,  the  reflection 
loss  of  the  sea-bottom  increases  as  frequency  increases.  At  200  kc.,  therefore,  the  echoes  from  ground-fishes  sometimes  become  stronger 
than  those  from  the  sea-bottom. 

*  db.  -  decibel. 


Resume 


Mesiire  de  la  propagation  dans  Peau  de  mer  des  ultrasons  jusqu'a  400  kilocycles 


II  faut  etudier  les  caract6ristiques  de  la  propagation  des  ultrasons  dans  rcuudemer  et  les  pcrtespar  reflexion  sur  le  fond  ousur  les 
banes  de  poisson  en  vue  de  la  construction  et  de  1'ufilisation  pratique  des  sondeurs  a  echo,  des  detecteurs  dc  poissons  et  des  Sonars.  Cet 
article  decrit  une  methode  pratique  pour  1'etude  en  question  et  indique  les  resultats  des  mesures  des  caracteristiqucs  de  la  propagation  dans 
le  sens  vertical  et  dans  le  sens  horizontal,  ainsi  quc  la  perte  d'ultrasons  par  reflexion  sur  le  fond  dc  la  mcr  et  sur  les  banes  de  poisson,  avec 
des  frequences  dc  28,  50,  200,  300  et  400  kilocycles. 

L'attcnuation  dc  la  propagation  dans  une  direction  perpcndiculaire  &  la  surface  de  la  mer  est  due  surtout  a  une  divergence  sphcrique 
mais  Fattcnuation  en  direction  horizontal  est  due  aussi  en  par  tie  a  un  ph6nomene  d' absorption  qui  devient  plus  prononc£  aux  frequences 
supdrieures.  En  fait,  les  experiences  montrent  que  1'absorption  est  dc  10-20  db*/km.  a  28  kilocycles,  de  37-50  do/km,  a  200  kilocycles  ct 
d'environ  120  db/km.  a  400  kilocycles.  La  peitc  par  reflexion  des  ultrasons  au  niveau  d'un  bane  de  poisson  vane  en  raison  inverse  dc  leur 
frequence.  Au  contraire,  la  perte  par  reflexion  sur  le  fond  de  la  mer  augmcnte  dans  le  meme  sens  que  la  frequence.  II  en  resultc  que,  pour  une 
frequence  de  200  kilocycles,  les  6chos  des  poissons  de  fond  deviennent  parfois  plus  marques  que  les  6chos  donnes  par  le  fond  de  la  mer. 

Medida  de  la  propagacidn  de  las  ondas  ultrasonoras  en  agua  de  mar  usando  frecuencias  hasta  de  400  kc. 
Exiracto 

La  propagacidn  de  las  ondas  ultrasonoras  en  agua  salada  y  su  perdida  por  reflexi6n  en  el  fondo  del  mar  o  cardumencs  deben  ser 
estudiadas  tanto  por  las  personas  que  proyectan  ecosondas,  1  oca lizad ores  de  peccs  y  cquipo  de  "sonar"  como  por  las  practices  en  su  funcio- 
namiento.  En  estetrabajo  se  dan  a  conocer:  un  m£todo  practice  para  efectuar  cl  cstudio  antes  mcncionado,  los  resultados  delas  caractcristicas 
de  propagacidn  medidas  tantp  en  sentido  vertical  como  horizontal,  asi  como  las  perdidas  por  reflexion  ultrasonora  sobre  el  fondo  del  mar  y 
cardumenes  usando  frecuencias  de  28,  40,  200,  300  y  400  Kc. 

La  propagaci6n  en  el  sentido  vertical  es  atcnuada  principalmente  por  las  divergcncias  esfericas,  pero  la  disminuci6n  en  el  sentido 
horizontal  sc  debc,  en  parte,  a  la  absorcidn  que  aumenta  con  frecuencias  mas  altas.  En  efecto,  los  resultados  de  di versos  experiments 
demuestran  que  la  absprci6n  es  igual  a  10—20  db.*  por  Km.  a  28  Kc.  37—50  db.  per  Km.  a  200  Kc.  y  unos  120  db.  por  Km.  a  400  Kc. 
Las  perdidas  por  reflexion  de  las  ondas  ultrasonoras  en  el  card u men  y  en  cl  fondo  del  mar  dismtnuyen  y  aumcntan,  respect ivamcnte,  al  elevarse 
la  frecuencia.  Por  esto,  a  200  Kc.  los  ecos  de  las  ondas  que  se  rcflejan  en  los  pcces  de  fondo  son  a  veces  mas  intensos  que  los  producidos 
contra  el  mismo  fondo  marino. 
*  db.- decibel. 


SINCE  high  frequency  ultrasonic  waves  have  greater 
attenuation  during  their  propagation  in  water, 
lower  frequencies  are  generally  used  for  echo 
sounders  or  fish  finders.  However,  for  sharp  directivity, 
higher  frequencies  are  more  convenient  because  they 
allow  smaller  transducers  to  be  used.  This  is  especially 
true  in  the  case  of  the  Plan -Position -Indication 
for  which  sharp  directivity  is  indispensable.  For 
the  rational  design  of  sounding  and  ranging  equipment, 
comprehensive  knowledge  of  the  propagation  character- 
istics, particularly  of  high-frequency  ultrasound,  is 
needed,  which  has  to  be  collected  by  quantitative  meas- 
urements. 


The  measurements  of  wave-propagations  recorded  in 
this  paper  were  investigated  in  sea  water  with  frequencies 
of  100  kc.,  200  kc.,  and  400  kc.,  and  simultaneously  with 
28  kc.,  allowing  for  a  comparison  between  the  higher  and 
lower  frequency  range. 

CHARACTERISTICS  OF  HORIZONTAL 
PROPAGATION 

Two  vessels  (fig.  1)  with  equipment  as  arranged  in  fig.  2, 
are  needed  for  this  investigation. 

An  ordinary  echo  sounder  is  used  for  reception,  the 
transmission  contact  of  which  is  connected  to  a  radio 


[478] 


BEHAVIOUR    OF     ULTRASOUND    IN     SEA     WATER 


* 


Fig.  /.     The  boats  equipped  for  horizontal  attenuation  measurements. 

transmitter,  so  that  instead  of  an  ultrasonic  impulse  a 
radio  impulse  is  transmitted  when  the  recording  stylus 
passes  the  zero-depth  mark.  When  this  radio  impulse  is 
received  by  the  second  ship,  it  initiates  the  transmission 
of  an  ultrasonic  impulse  in  horizontal  direction  to  the 
first  ship. 

This  ultrasonic  impulse  travels  to  the  receiving  ship 
in  three  different  ways:  ( 1 )  directly,  (2)  with  one  reflection 
to  the  sea-bottom,  or  (3)  with  two  or  more  reflections  to 
the  sea-bottom.  Each  of  these  parts  of  the  impulse,  on 
reaching  the  receiving  transducer,  causes  its  own  mark 
on  the  recording  paper. 

At  a  short  distance  between  the  two  ships  the  recorder 
first  indicates  the  direct  part  only.  With  increasing 
distance,  the  reflected  parts  appear  slightly  below  the 
trace  of  the  direct  part  (fig.  3). 

The  intensity  of  the  received  impulse  can  be  measured 
at  any  time  and  at  any  distance  by  reading  the  attenuator 
which  is  inserted  between  the  receiving  transducer  and 
the  amplifier.  This  attenuator  is  adjusted  so  that  the 
trace  on  the  recorder  disappears  whilst  the  sound- 
pressure  is  measured.  This  method,  started  in  Japan  in 
1943,  is  called  Margin  Text,  the  attenuated  amount  being 


ULTRASON 10 

TKANUIflTTIMC 

SHIP 


ULTRASONIC 
RECEIVING 
SHIP 


RADIO  RECEIVER 


PULSE 
I  GENERATOR 


RADIO 
TRANSM ITTER 


ATTENUATOR 


f 

- 

\ 

DEPTH 
RECORDER 


AMPLIFIER 


ULTRASONIC  - 
UNDER 


.. 

—    —  RECEIVING 
TRANSDUCER 


Fig.  2.     Principle  of  measuring  apparatus  for  horizontal  attenuation. 


s»>  '   ..  ./-,.  -,,C.V ' 

f*j     28  kc 

'.^&*r^jr& 


Fig.  3.     Echograms  oj  horizontal  attenuation. 

the  Margin  of  the  equipment  under  test.  The  depth  scale 
of  the  ordinary  echo  sounder,  of  course,  has  to  be 
doubled  when  measuring  the  propagation  speed  in  the 
way  described. 

In  figs.  4  and  5  the  results  given,  show  the  relation 
between  Margin  and  the  distance  for  the  different 
frequencies  in  question  for  the  directly  travelling  part  of 
the  sound  impulse. 

Since  the  temperature  gradient  near  the  surface  was 
small  during  the  measuring  experiments,  a  bending  of  the 
sound  direction  can  be  practically  excluded. 

By  comparing  the  measured  curves  with  the  theoretical 


10  50  100 

HORIZONTAL  DISTANCE  IN  METER 

Fig.   4.     Curves  of  horizontal  attenuation. 
(In  May  1954) 


500 


[479] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


10 


20   30  40   60  80  100    200  300 
HORIZONTAL  DISTANCE  IN  METER 

Fig.  5.    Curves  of  horizontal  attenuation. 
(In    December    1954) 


curve  for  the  propagation  of  spherical  waves,  the  absorp- 
tion constants  can  be  obtained  for  each  frequency,  as 
shown  in  Table  I. 
The  variations  in  attenuation  are  due  to  different 


water  conditions  at  ihe  different  seasons  at  which  the 
measurements  took  place.  At  present  no  general  con- 
clusions can  be  drawn  in  this  regard  because  our  exper- 
ience is  still  too  limited.  The  absorption  constants 
obtained  are  in  average  for  100  kc.  about  30  db./km.,  for 
200  kc.  about  43  db./km.,  and  for  400  kc.  about  120 
db./km. 

CHARACTERISTICS    OF    VERTICAL 
PROPAGATION 

The  absorption  constant  of  vertical  propagation  was 
measured  in  a  certain  area  with  flat  bottoms  of  equal 
consistency  (mud)  in  different  depths  so  that  the  reflec- 
tion loss  was  kept  equal.  Thus,  by  measuring  the  echo 
intensities  at  various  depths,  the  absorption  constant  of 
vertical  propagation  was  obtained. 

The  results  are  shown  in  figs.  6  and  7  where  the  echo 
intensities  for  the  different  frequencies  are  plotted 
against  the  depth. 

The  absorption  constants  are  shown  in  Table  II.  The 
values  are  much  lower  than  those  for  horizontal  propa- 
gation. 

REFLECTION  LOSS  AT  THE  SEA-BOTTOM 

The  reflection  loss  at  the  sea-bottom  can  be  directly 
measured  by  the  "Sliding  Method"1.  The  principle 
consists  in  finding  the  value  of  the  discontinuity  between 


TABLE  1 
Absorption  Constant  (db./km.)  for  horizontal  propagation 


Frequency 
(kc.) 

April 

May 
In  shallow 
water 
(15  m.) 

In  deep 
water 
(50  m.) 

December 
In  shallow 
water 
(10m,) 

28 
100 
200 
400 

18 
25 
47 

16            20 
33            29 
49            37 

—    20    — 
33     30    — 
50    37     37 

10 

43 
120 

DB 


OU 
70 
60 
50 
40 
30 
20 

10 
0 

1 

BOTTOU-6UB3T4 

i 

JtU 

^Sc 
£°> 

**^, 

S 

"«, 

-^ 

•^ 

~, 

> 
"A 

4» 

V 

X    100 

KC. 

- 

•  200 

KC. 

2        3     4      3  6     8   10             20            40        6 
DEPTH  IN  METER 

0  9 

)  lOO 

Fig.  6.     Curves  of  vertical  attenuation. 
(In  May  1954) 


TABLE  II 
Absorption  Constant  for  vertical  propagation. 


Frequency 
(kc.) 

Absorption  Constant 
(db./km.) 

100 
200 
300 
400 

about     10 
about     10 
25 
50 

10 


20      30  40       60    80100 
DEPTH  IN  METER 


Fig.  7.     Curves  of  vertical  attenuation. 
(In    December    1954) 


[480] 


BEHAVIOUR    OF     ULTRASOUND     IN     SEA     WATER 


TABLE    III 
Reflection  Loss  of  Ultrasound  at  the  Sea-bottom 


Sea-bottom         Frequency  Reflection       Reflection  loss  in 

Substance  (kc.)  loss  (db.)  l^ower  Frequency  Range 

(J0-50kc.)(db.) 


Mud 

200 

24 

11 

Mud 

400 

26 

11 

Sand 

300 

17 

7-8 

the  extrapolated  intensity-distance  curves  of  the  incident 
wave  and  the  reflected  wave. 

The  data  obtained  in  Tokyo  Bay  area  on  mud  or  sand 
bottom  are  shown  in  Table  III.  It  was  found  that  the 
reflection  losses  of  high  frequencies  are  10  to  14  db. 
greater  than  those  previously  obtained  for  lower 
frequencies. 

REFLECTION  LOSS  AT  THE  FISH  BODY 

Our  definition  of  the  reflection  loss  at  the  fish  is  the  ratio 
of  Po  to  P,  where  PO  is  the  sound  pressure  of  the  plane 
wave  inciting  on  the  body  of  a  fish,  and  P  is  that  of  the 
reflected  spherical  wave,  measured  at  a  distance  of  1  m. 
from  the  fish. 


10 
0 
0 
0 
0 

o 
1< 

•> 

fl 

ORSE-MACKE 
v 

;REL 

i 

""" 

*- 

—  • 

— 

M 

M* 

s= 

^ 

:=»• 

*«**« 

ANCHOVY 

:>               2O      3O  4O       6O  801OO          2OO           4O 

FREQUENCY    IN   KC. 
Fig.  8.     Reflection  loss  at  fish  body. 


fa) 
Fig.   9. 


24  kc* 


200  Ice. 


Simultaneous  records  of  bottom  fish   obtained  by 
24  kc.  and  200  kc. 


y 


**{<*'•••/.         t   -,  «  - 
./f't^  ,  ,    -  '•'   .   . '.^v 


Fig.   JO.     Echogram  of  200  kc.  obtained  in  actual  fishing — 
Alaska  Pollack 


A  Sliding  Method  is  used  in  making  measurements 
similar  to  that  used  for  bottom  reflection.  The  data 
indicate  that  the  higher  the  frequency,  the  better  the 
reflection  (fig.  9).  This  suggests  that  the  higher  attenuation 
of  higher  frequencies  might  be  compensated  by  lower 
reflection  loss  at  the  fish.  This  statement  is  almost 
proved  by  several  experiments  with  PPI  type  equipment, 
using  200  kc. 

EXPERIMENTS    ON    FISH   DETECTION   WITH 
HIGH  FREQUENCIES 

High  frequencies  favour  the  resolving  power  due  to  the 
shorter  wave-length,  which  comes  into  the  order  of 
millimeters.  Furthermore,  there  is  the  increase  of  the 
reflection  loss  on  the  sea-bottom,  which  may  help  espec- 
ially in  locating  bottom  fish  by  improving  the  possibility 
for  discriminating  between  the  fish  and  the  seabed. 

Sounding  with  200  kc. 

Simultaneous  experiments  were  carried  out  with  recorder- 
type  fish  finders  using  200  kc.  and  24  kc.  respectively  in 
order  to  compare  the  traces  of  bottom  fish.  The  position 
A  indicated  on  both  echograms  in  fig.  9  correspond  to 
each  other. 

By  inspecting  the  record  of  bottom  fish  traces  it  can 
be  stated  that  the  resolving  power  of  200  kc.  is  better 
than  that  of  24  kc. 

More  examples  of  high  frequency  records  are  shown  in 
figs.  10  and  11,  the  former  from  a  school  of  Alaska 
pollack  (200  kc.)  and  the  latter  from  schools  of  anchovy 
(400  kc.). 


AUCKOVY 
Frequency  400 
b»r  7th 
Tftfcyo  B«y 


Fig.  11.     Echogram  of  400  kc.  obtained  in  actual  fishing- 
Anchovy 


[481  ] 


GG 


MODERN     FISHING     GEAR    OF    THE    WORLD 


/^.   /<2.     Example  of  PPI-Sonar  and  recorder-type  fishfinder 
traces  obtained  simultaneously  from  a  school  of  Anchovy 


Horizontal  ranging  with  PPI  Sonar 

Another  application  of  high  frequency  ultrasound  is  the 
Plan-Position-Indication  (PPI),  using  200  kc.  Fig.  12(a) 
gives  a  photograph  of  the  successive  records  obtained 
from  a  school  of  anchovy  which  was  caught  almost 
entirely  by  the  operation  of  a  pair  of  fishing  boats. 

Fig.  12  (b)  shows  the  cchogram  of  vertical  soundings 
carried  out  simultaneously,  of  the  same  anchovy  school. 


EFFECT  OF  AIR  BUBBLES  ON  ULTRASOUND  OF 
DIFFERENT  FREQUENCIES 

Laboratory  Experiment 

The  data  obtained  from  experimental  studies  and 
theoretical  work  carried  out  in  the  laboratory  on  the 
absorbing  effect  of  dense  bubbles  in  water  are  shown  in 
fig.  13.  Numerous  fine  bubbles  were  generated,  inter- 
secting the  path  of  ultrasound.  The  ordinate  of  the  graph 
corresponds  to  the  increase  of  attenuation  when  the 
bubbles  were  suddenly  generated,  and  the  abcissa 
corresponds  to  the  sound  frequencies.  It  can  be  clearly 
noted  that  the  effect  of  the  air  bubbles  decreases  with 
increasing  sound  frequency. 


ATTENUATZOK  IN  DB4 

M  M  10  K 

o  d  o  oi  o  o 

BUBBLE  DENSITY*-** 
DIA  OF  BUBBLE  1-3™ 

» 

X 

^N 

X 

s* 

s 

«4 

*-*^ 

•—  »^ 

10 


200 


400 


3O       6O  100 

FREQUENCY   IN    kc. 
Fig.  13.     Absorption  due  to  bubbles.  —Laboratory  measurement 


T7/^.  /-/.     Absorption  due  to  wake — simultaneous  records  of  50  kc. 
and   200   kc. 

Propeller  Wake 

The  effect  of  the  propeller  wake  of  a  1-5  ton  fishing  boat 
driven  by  a  4  h.p.  diesel  engine  at  750  r.p.m.  full  speed  is 
shown  in  fig.  14.  The  measuring  boat  sailed  slowly 
across  the  wake  six  times.  The  present  echograms  were 
obtained  simultaneously  and  the  figures  1  to  6  on  both 
records  correspond  to  each  other.  It  can  be  noted  that 
the  bottom  traces  disappear  on  the  50  kc.  records  at 
every  instant  of  crossing,  whilst  the  records  of  200  kc. 
show  no  interruption.  This  observation,  which  may  be 
of  some  practical  value,  is  in  accordance  with  the 
laboratory  experiments  mentioned  above. 

REFERENCE: 

1  T.  Hashimoto,  Report  of  Fishing  Boat  Laboratory,  Fisheries 
Agency,  Ministry  of  Agriculture  and  Forestry,  Japan,  No.  1 , 
p.  13,  Dec.  1953. 


[482J 


RELIABILITY    OF    BOTTOM    TOPOGRAPHY    OBTAINED    BY 
ULTRASONIC    ECHO    SOUNDING* 

by 
T.  HASHIMOTO,  M.  NISHIMURA  and  Y.  MANIWA 

Fishing  Boat  Laboratory,  Fisheries  Agency,  Japanese  Government 


Abstract 

In  this  paper  the  authors  shtm  a  comparison  between  an  echo-survey  of  the  topography  of  an  artificial  lake  with  visual  and  high 
frequency  and  an  actual  survey  by  liiangulation  made  before  the  valley  was  filled  with  water.  Accurate  results  were  obtained  using  high 
frequency  and  a  narrow  beam  angle. 


Resume 


L'cxactitudc  de  la  topographic  du  fond  obtenue  par  les  sondcurs  a  echo  ultrasoniques 


Dans  cette  communication  les  autcurs  momrcnt  la  comparaison  entre  unc  exploration  ulirasoniquc  de  la  topographic  d'un  lac 
artificiel  et  un  relevc  re-jl  par  triangulation  cffcctue  avant  misc  en  eau  dc  la  vallcc.  On  a  obtenu  des  resultats  precis  en  utihsant  line  frequence 
elcvee  et  un  faisceau  de  faible  ouvcriurc. 


Precision  de  la  topognifia  del  fondo  mediantc  sendees  ulfrasonoros 
Kxtraeto 

F:.n  este  trabajo  los  autorcs  comparan  un  Icvantamicnto  topografico  dc  un  lago  artificial  mcdiante  el  empleo  dc  sondas  ultrasonoras 
con  el  obtenido  por  tnangulacion  antes  de  llenar  el  valle  de  agua.  Se  obtuvieron  resullados  precisos  usando  un  estrecho  ha/  de  ondas 
sonoras  de  alta  frecuencia  en  angulo  .igudo. 


GENERAL 

WHEN  investigating  the  boundary  conditions  of 
an  artificial  fish  shelter  by  echo  sounding, 
special  precautions 'have  to  be  taken  to  obtain 
sufficient  accuracy. 

When  using  an  echo  sounder  with  a  wide  sound  beam, 
it  is  very  difficult  to  obtain  the  true  shape  of  a  sloping 
or  undulating  sea  bottom1.  With  the  common  15  to 
3  cm.  sound  waves,  the  necessary  focusing  of  the  sound 
beam  is  possible  only  with  rather  big  and  heavy  trans- 
ducers which  are  not  suitable  for  installation  in  a  small 
craft  used  for  survey  in  shallow  water.  Consequently, 
high  frequency  ultrasound  such  as  200  kc.  is  needed 
to  obtain  the  sharp  directivity  needed  with  small  trans- 
ducers which  can  be  used  in  small  craft. 

While  constructing  an  artificial  lake  along  the  Saikawa 
river  (Nagano  Prefecture),  the  cross  section  was  surveyed 
by  triangulation  before  the  lake  was  filled  with  water 
and  by  echo  sounding  afterwards.  This  paper  contains 
the  results  of  this  experiment. 


*  This  experiment  was  carried  out  in  cooperation  with  the 
Tokyo  Eleciric  Power  Co.  Inc.  in  September  1954  at  a  power  plant 
at  the  Saikawa  River. 


EFFECT  OF  WIDE  BEAM 

A  measurement  with  wide  beam  angle  was  carried  out 
in  November  1953  in  an  artificial  lake  of  the  Yanaizu 
power  plant  (Tadami  river)1.  The  echo  sounder  worked 


:;.. 


Fig.   1.     Echogram  showing  the  accuracy  of  reproduction  of 
a  bottom  profile  with  different  slope.    Frequency  50  Ar.,  half 
power  beam  angle  22  degrees. 


[483] 


MODERN    FISHING     GEAR    OF    THE    WORLD 


a1" 

520 

f30 

&*0 


Echo  sounding 


TriangulAtlon 


Sedimentation  of  sand 


JO  4'fl 


Fig.  2.     Comparison  of  bottom  profiles  obtained  by  echo  sounding 
(see  fig.   1)  ami  triangulation. 


at  a  frequency  of  50  kc.  and  a  half  power  beam  angle 
of  22  degrees.  Fig.  1  shows  the  cross  section  of  the  valley 
obtained  by  echo  sounding  and  fig.  2  gives  a  comparison 
between  the  shapes  obtained  by  triangulation  and 
echo  sounding. 

It  can  be  seen  that  where  the  slope  is  not  too  steep, 
the  shape  obtained  by  echo  sounding  is  comparable  to 
that  obtained  by  triangulation  (figs.  1  and  2,  right). 
But,  as  soon  as  the  slope  becomes  steeper  or  stcplike, 
the  two  shapes  diverge  and  the  conformity  between  echo 
sounding  and  triangulation  is  lost  (figs.  1  and  2,  left). 

Method  and  Apparatus 

For  this  experiment,  first  a  triangulation  was  made  at  a 
cross  section  of  the  under-water  valley  of  Saikawa 
river.  The  slope  of  both  banks  of  the  valley  is  steep. 
The  experiment  was  mainly  made  around  masonry 
revetment  intended  to  separate  the  water  flow  and  situ- 
ated at  the  right  hand  of  the  valley  10  m.  below  the 
water  line.  This  revetment  was  6-2  m.  wide  and  2-8  m. 
high.  It  was  found  by  lead-sounding  that,  after  the  valley 
was  filled  with  water,  the  shape  of  the  bottom  was 
deformed  a  little  by  sand  sedimentation  on  the  right 
side  of  the  revetment. 

In  order  to  keep  exactly  the  same  cross  section,  a 
steel  wire  covered  with  plastic  with  marks  every  metre 
was  extended  between  the  two  marks  which  had  been 
used  for  triangulation.  The  echo  sounding  measurement 


Pig.  4.  Kchogramx  obtained  with  the  experimental  sounder. 
Left:  frequency  100  AT.;  half  power  beam  angle  16  degrees. 
Right:  frequency  200  kc.\  half  power  beam  angle  3-3  degrees. 


Triangulation 


Fig.    5,     Comparison    of  bottom  profiles   obtained  by    echo 

sounding  (see  fig.  4)  and  triangulation  (see  Jig.  3). 
Top:  frequency   100  kc.\  half  power  beam  angle  16  degrees. 
Bottom:  frequency  200  kc.\  half  power  beam  angle  3-3  degrees. 


Original 
(right  shore) 


0        2        4       6        8       10 


nentation  of  «and 

Fig.  3.     Triangulated  shape  and  photo  of  revetment  of  the  profile  used  for  the  experiments 

[484] 


BOTTOM     TOPOGRAPHY    OBTAINED     BY     ECHO    SOUNDING 


was  made  while  travelling  along  this  wire,  marking  1  m. 
intervals  on  the  recording  paper. 

The  echo  sounder  used  in  this  experiment  is  of  experi- 
mental design,  fitted  for  transmitting  and  receiving  100 
or  200  kc.  by  changing  the  transducers,  and  has  a  measur- 
ing range  of  50  m.  The  recording  system  is  of  the  rotating 
type,  with  wet  recording  paper.  The  half  power  beam  angles 
of  the  transducers  are  16  degrees  and  3-3  degrees 
respectively. 

Results 

With  a  half  power  beam  angle  of  16  degrees  ( 100  kc.),  the 
shape  of  the  revetment  is  deformed  considerably  and  it 
is  difficult  to  obtain  the  true  shape  from  the  echogram 
(fig.  4,  left;  fig.  5,  top).  The  narrower  beam  angle  of 
3'3  degrees  gives  much  better  results,  i.e.  the  recorded  width 
of  the  revetment  is  only  20cm.  longer  than  the  true  width 
fig.  4,  right;  fig.  5,  bottom. 


CONCLUSIONS 

From  the  results  of  these  experiments,  the  following 
conclusions  can  be  drawn: 

1.  For  a  sharp  beam  angle,  high  frequency  ultrasonic 
waves  of,  for  instance,  200  kc.  are  favourable  as 
the  transducers  can  be  small  and  compact,  allowing 
installation  in  small  boats  as  needed  for  echo  sound- 
ing in  the  shallow  waters  of  lakes  or  swamps. 

2.  With  a  sharp  beam  angle,  such  as  3-3  degrees,  it  is 
possible  not  only  to   obtain   high   accuracy  in   the 
reproduction  of  the  shape  of  the  bottom  but  also  to 
detect  fish  schools  swimming  quite  near  a  wall,  as, 
for  instance,  an  artificial  fish  shelter. 


REFERENCE 

1     Hashimoto,  T.  and  Nishimura  M.:    Tech.  Rep.  of  Fishing  Boat 
Laboratory  No.  5,  p.   155,  1954. 


Echogram  of  a  stratified  bottom  with  soft  layers  over  harder  ones. 


Photo:  Elac,  Kiel 


485] 


ON  THE  DIFFERENCE  BETWEEN  24  kc.  AND  200  kc.   ULTRASOUND 

FOR  FISH-FINDING 

by 

T.  HASHIMOTO  and  Y.  MANIWA 
Fishing  Boat  Laboratory,  Fisheries  Agency,  Tokyo,  Japan 

Abstract 

The  authors  have  produced  an  experimental  echo  sounder  using  200  kc.  with  a  barium  titanate  transducer,  and  they  consider  that 
this  machine  gives  a  better  picture  quality  than  the  standard  model  using  24  kc.  They  illustrate  this  claim  by  photographs  of  traces  of  fish 
and  of  the  Deep  Scattering  Layer  (DSL),  but  point  out  that  the  200  kc.  instrument  has  a  different  depth  scale  and  a  higher  paper  speed 
than  the  standard  model.  It  is  considered  that  the  higher  frequency  has  the  better  reflection  coefficient  which  compensates  for  some  increase 
in  attenuation. 

Difference  entrc  les  leho-sondeurs  de  200  kc.  et  dc  24  kc. 
Rtsume 

Lcs  auteurs  ont  mis  au  point  un  6cho-sondeur  experimental  fonctionnant  sur  200  kc.  avec  un  emetteur  en  tiumale  de  barium;  ils 
estiment  que  les  images  de  cet  appareil  sont  d'une  qualite  superieure  a  celles  du  modele  standard  fonctionnant  sur  24  kc.  Ils  montrent,  a 
Pappui  de  leur  theone,  des  photographies  de  traces  de  poisson  amsi  que  de  la  couche  dispcrsantc  profonde  (DSL),  mais  font  observer  que 
Teihelle  des  profondeurs  de  leur  appareil  est  diflerente  de  cellc  du  modele  standard  et  que  la  Vitesse  de  deroulement  de  la  bande  est  plus 
£levee.  On  considere  que  la  frequence  plus  clevee  permet  d'obtenir  un  meillcur  coefficient  dc  reflexion  qui  compensc  une  attenuation  un  peu 
plus  import  ante 


Extracto 


Diferencia  entrc  las  ecosondas  que  usan  frecuencias  de  24  y  200  kc. 


Los  autores  han  construido  una  ecosonda  experimental,  provista  de  un  transductor  de  titanato  de  bario.  que  funciona  con  frecuen- 
cias de  200  kc.  y,  scgun  ellos,  produce  una  imagen  mas  clara  que  el  modelo  corriente  dc  24  kc.  llustran  csta  ascrcion  con  fotografias  de 
ecogramas  producidos  por  peces  y  la  capa  profunda  de  dispersion  ("DSL"  en  ingles)  del  sonido  haciendo  notar,  al  mismo  ticmpo,  que  el 
instrumento  de  200  kc.  posee  una  escala  de  profundidad  diferente  y  una  banda  registradora  mas  velo^  que  el  modelo  corremte.  1:1  uso  de 
frecuencias  mas  altas  tiene  un  mejor  coeficiente  de  refleccion  que  compensa  parte  de  las  pcrdidas  causadas  por  la  alenuacion. 


EXPERIMENTAL  ARRANGEMENT 

ATER  having  experimented  with  100  kc.  to  400  kc. 
ultrasound  for  fish  detection1-  z  an  experimental 
200  kc.  echo  sounder  was  designed  to  compare  its 
performance  with  the  24  kc.  model  which  is  in  common 
use. 

The  experimental  model  is  shown  in  fig.  1.  The 
transducer  is  a  circular  disc  of  barium  titanate  of  100  mm. 
in  diameter.  Its  transmitting  surface  is  covered  with  a 
plate  of  synthetic  rubber  5  mm.  thick  and  the  con- 
tainer is  filled  with  castor-oil.  The  beam-angle  used  is 
5  degrees  and  the  peak  power  of  transmission  lies  between 
320  and  480  W. 

The  amplifier  is  a  double  heterodyne  type  (200  kc.  to 
452  kc.  to  12  kc.),  converted  to  direct  current  for  the 
recorder.  The  synthetic  gain  is  155  db.  The  recorder 
is  of  instantaneous  return  type  with  180  mm.  paper 
width  and  50  m.  and  300  m.  measuring  range. 

RESULTS 

We  equalised  the  second  echoes  of  both  the  24  kc.  and 
200  kc.  echo  sounders  to  regulate  the  gains  of  both  instru- 
ments. Their  transmitting  powers,  of  course,  are  not 
strictly  equal  but  this  gives  at  least  a  general  comparison 
The  reproduction  scale  and  the  paper  speed  unfortunately 
are  not  equal  for  the  two  echo  sounders  to  be  compared. 
(The  same  most  probably  is  true  for  the  impulse  period 
and  the  beam  angles — Editor.)  Both  echo  sounders 


were  operated  simultaneously.  Respective  sections  of 
both  records  are  indicated  by  equal  numbers  at  the  lower 
edge. 

Bottom-fish  not  recognizable  in  the  24  kc.  record 
can  be  distinguished  in  the  200  kc.  record  (fig.  2(a)  and 
(b)).  Even  fish  schools  as  near  as  20  cm.  above  the  bottom 
are  recorded  separately  from  the  sea  bottom  by  the  200 
kc.  experimental  sounder  (figs.  2(a);  3(a)  and  (b)). 

Surface   and    midwater    fish    are    represented    more 


Fig.  I.     The  experimental  200  kc.  echo  sounder 


[486] 


ULTRASOUND    FOR    FISH-FINDING 


2.     Comparison  of  fish  records  obtained  with  the  experimental  sounder  (a)  and  a  common  24  kc.  model(b). 
LIU.  TB     At      ,    ,  U*  ;» 


2001w 


Fiff.  J.     Comparison  of  bottom  psh  traces  with  (a)  and  without  (h)  suppressor  circuit. 


Fig.  4.    Comparison  of  fish  and  deep  scattering  layer  traces 

obtained  with  the  experimental  sounder  (a)  and  (b)  and  the 

common  24  kc.  model  (c). 

clearly  by  the  200  kc.  model  (fig.  4(a),  (b)  and  (c)). 
These  are  not  distinguished  from  the  background  noise 
by  the  24  kc.  model,  even  if  the  amplification  is  increased 
as  at  '2'  in  fig.  4(c),  probably  because  the  reflection 
coefficient  for  fish  is  less  for  24  kc.  than  it  is  for  200  kc. 

The  Deep  Scattering  Layer  is  not  clearly  recorded 
by  the  24  kc.  model,  as  it  is  by  the  200  kc.  model  (fig.  4  (c) 
and  (b)). 

The  superior  resolving  power  of  the  200  kc.  experi- 
mental sounder  is  clearly  proved  by  the  records  shown  in 
fig.  5,  (a)  and  (b).  This  200  kc.  echogram  (fig.  5  (a))  is 


f-'ig.  5,     Example  of  the  higher  re  wiving  power  of  the  experi- 
mental echo  sounder  (a)  compared  with  a  50  kc.  model  (b). 


considered  to  be  the  first  to  provide  the  detailed  con- 
struction of  a  fish  school. 

The  use  of  a  suppressor  circuit  for  depressing  the  peak 
of  the  bottom  echo  (white  line)  improves  the  represen- 
tation of  bottom  fish  considerably  also  for  200  kc. 
(fig.  3  (a)). 

The  superiority  of  the  200  kc.  experimental  sounder 
compared  with  the  common  24  kc,  model  in  resolving 
power  and  performance  in  recording  single  fish  and 
the  deep  scattering  layer  are  considered  to  be  due  to  the 
higher  reflection  coefficient  for  200  kc.  of  small  reflectors 
such  as  small  single  fish  and  plankton  concentrations. 


REFERENCES 

1  T.  Hashimoto,  Y.  Maniwa   and    M.  Nishimura;    Propagation 
Characteristics  of  100  kc.  to  400  kc.  Ultrasound  in  Sea  Water, 
Technical  Report  of  Fishing  Boat  No.  8,  March  1956. 

2  T.  Hashimoto   and    Y.  Maniwa;    Study  on  Reflection  Loss  oi 
Ultrasound  of  Millimeter  Wave  on  Fish-body.  Technical  Report 
of  Fishing  Boat  No.  8  and  No.  9,  March  and  September,  1956. 

3  T.  Hashimoto    and    Y.  Maniwa;    Technical  Examination  and 
Experiment  on  Fish-finder  for  Bottom-fish,  Technical  Report  of 
Fishing  Boat  No.  9.  September,  1956. 


[487 


RECENT  TRENDS  AND  DEVELOPMENT  IN  ECHO  FISHING 

by 

R.   W.   WOODGATE 

Pye  Ltd.,  Marine  Division,  Lowestoft,  U.K. 

Abstract 

This  paper  describes  briefly  the  electronic  Pye  Marine  Fish-Finder,  and  then  goes  on  to  discuss  in  detail  the  three  attachments 
which  have  been  designed  to  operate  from  it. 

First  attachment  is  a  horizontal  searching  transducer  designed  to  be  fitted  on  fishing  vessels,  and  suitable  for  shallow  waters  such 
as  are  found  in  the  North  Sea,  as  well  as  deep  water.  This  equipment  is  as  simple  as  is  compatible  with  efficient  operation,  and  can  be 
easily  removed  from  the  vessel  when  required  without  slipping. 

The  second  attachment  consists  of  a  transducer  for  fitting  to  a  midwatcr  trawl,  so  that  soundings  can  be  made  from  the  net  or 
the  ship  at  will.  In  this  way  the  depth  of  the  net,  and  the  opening  of  the  net  mouth  can  be  easily  measured. 

The  third  attachment  is  an  attempt  to  overcome  the  need  to  watch  the  Cathode-Ray-Tubc  of  the  Fish-Finder  when  searching  for 
bottom  fish.  This  "Fish-Counter"  acts  as  an  electronic  observer,  and  using  the  sea  bed  echo  as  a  datum,  looks  back  to  see  if  any  echoes 
appeared  in  the  two  fathoms  immediately  above  the  sea  bed.  If  there  is  an  echo  a  click  is  produced  in  a  loud-speaker.  Alternatively  for 
research  work,  the  instrument  will  operate  a  counter,  and  so  give  a  record  of  the  number  of  fish-echoes  seen. 


Rtaime 


L'utilisation  du  sondeur  a  echo  pour  la  p&he.    Perspectives  et  progres  rfcents 


Ce  document  donne  une  description  rapide  dc  Pappareil  dlectronique  Pye  Marine  Fish  Finder  et  £tudie  en  detail  les  trois  apparcils 
complementaircs  qu'il  doit  faire  fonctionner. 

Le  premier  est  un  emetteur  de  recherche  horizontal,  pouvant  etre  montr&  sur  bateaux  de  peche  ct  convenant  pour  les  eaux  peii 
profondes  comme  eel  les  de  la  mer  du  Nord,  ainsi,  que  pour  les  eaux  profondes.  La  simplicite  de  ce  materiel  est  6gale  &  I'efficacit6  de  son 
fonctionnement  ct  il  peut  dtrc  facilement  demont6  du  bateau  en  cas  de  besoin. 

Le  dcuxieme  se  compose  d'un  &metteur  £  adapter  a  un  chalut  flottant  dc  sorte  que  les  sondages  peuvent  etrc  effectues  a  volont6  & 
partir  du  filet  ou  a  partir  du  bateau.  De  cctte  mamere,  on  peut  facilement  obtenir  la  mesure  de  la  profondeur  du  filet  et  la  mesure  de  son 
ouverture. 

Le  trois  ieme  represente  une  tentative  pour  remedier  £  la  necessitc  de  surveillcr  le  tube  &  rayons  cathodiques  du  detectcur  de  poisson 
quand  on  recherche  les  poissons  de  fond.  Get  "enregistreur  de  poisson'*  se  comportc  comme  un  observateur  electronique  et,  utilisant  comme 
donnee  1'echo  renvoyS  par  le  fond  de  la  mer,  recherche  s'il  ne  s'est  pas  produit  d'echo  dans  les  deux  brasses  situees  immediatement  au-dessus 
du  fond  de  la  mer.  Dans  le  cas  d'un  echo,  un  haut-parleur  fait  entendre  un  signal.  Lorsqu'il  est  employ^  pour  la  recherche,  1'instrument  fait 
fonctionner  un  compteur  et  donnera  ainsi  le  nombre  global  des  6chos  donnes  par  les  poissons. 

Tendencies  recientes  y  perfeccionamiento  de  la  pesca  con  ecosonda 
Extracto 

En  este  trabajo  se  describe  brevementc  el  localizador  electronico  de  peces  "Pye  Marino"  ("Pye  Marino  Fish-Finder*')  y  analizan, 
en  detalle,  los  trcs  accesorios  que  se  proycctaron  para  su  funcionamiento  en  diversas  condiciones. 

El  primer  accesorio  es  un  transductor  para  la  Iocalizaci6n  horizontal  destinado  a  embarcaciones  pesqucras,  que  se  presta  para 
trabajar  en  aguas  somcras  como  las  del  mar  del  Norte  y  tambien  en  zonas  profundas.  Este  equipo  es  tan  sencillo  como  su  funcionamiento 
cficaz  lo  permite  y  puede  retirarse  rapidamente  del  barco  cuando  las  circunstancias  lo  requiercn.  sin  neccsidad  de  proceder  a  la  varadura. 

El  segundo  accesorio  es  un  transductor  que  puede  adaptarse  a  una  red  de  arrastrc  para  la  pesca  cm  re  aguas,  el  cual  permite  efectuar 
sondeos  desdc  el  arte  o  el  barco,  segun  lo  exijan  las  necesidades  de  trabajo.  En  esta  forma  puede  determinarse  facilmente  la  profundidad  a 
que  se  encuentra  la  red  y  la  altura  de  su  boca. 

El  tercer  accesorio  evita  la  neccsidad  de  observar  el  tubo  de  rayos  catodicos  del  localizador  cuando  se  buscan  especies  de  fondo. 
Este  "registrador  de  peces"  actua  como  un  observador  electr6nico  y,  usando  el  fondo  marine  como  piano  de  referenda,  verified  si  aparecen 
nuevos  ccos  en  las  dos  brazas  immediatas  al  fondo.  En  caso  de  producirse  un  ceo,  .el  alto  parlante  emitc  un  ruido  agudo.  Cuando  el 
instrumento  se  usa  para  investigaciones  cientfficas,  puede  hacer  funcionar  un  contador  que  determina  el  numero  de  ecos  producidos  por  los 
peces  que  "vi6"  el  aparato. 


GENERAL  COMPARISON  OF  PAPER-RECORDER 
AND  CATHODE  RAY  TUBE  DISPLAY 

BY  the  early  1940's  the  use  of  the  paper- recorder 
in  searching  for  herring  schools  was  quite  common, 
and  most  of  the  English  East  Coast  Fleet  was  fitted 
with  such  echo  sounders.  The  results,  however,  were  not 
consistent.  Often  herring  echoes  would  appear  suddenly 
and  just  as  mysteriously  disappear,  and  catches  did 
not  always  correspond  to  the  distribution  of  echoes. 

This  effect  became  even  more  noticeable  when  the 
paper-recorder  was  used  by  the  Arctic  trawlers.  Either 
echoes  were  seen  and  no  fish  caught,  or  good  catches 
were  obtained  when  the  paper  was  clear.  This  immediately 


gave  a  clue  to  the  problem  as  the  trawler,  of  course, 
only  catches  fish  very  close  to  the  seabed.  It  seemed 
reasonable  to  suppose  that  the  paper-recorder  was  unable 
to  show  fish  near  the  seabed  as  separate  markings, 
and  that  when  echoes  were  seen  the  fish  were  too  high 
to  be  caught  in  the  bottom  trawl. 

This  theory  was  proved  when  the  Cathode  Ray  Tube 
(C.R.T.)  display  for  fish  searching  was  brought  on  to 
the  market.  The  normal  paper-recorder  does  not  show 
demersal  fish  in  such  a  manner  as  makes  them  easily 
recognizable.  Firstly,  the  scale  length  of  most  recorders 


[4881 


RECENT    DEVELOPMENTS    .IN     ECHO     SOUNDING 


Fig.  1.  The  operation  of  the  Fish-Finder  when  searching  for  bottom  fish.  Left:  C.R.T.  reproducing  the  full  range.  On  top  the  transmitter 
pulse,  in  the  middle  a  secondary  echo  and  below  the  echo  of  the  seabed  with  a  small  fish  echo  on  top  of  it.  Middle:  As  above  with  the  7  fm. 
marker  placed  over  the  fish  echo.  Right:  The  7  fm.  section  expanded  over  the  whole  screen.  The  fish  echoes  are  now  seen  to  extend  3 

fm.  up  from  the  seabed. 


is  in  the  order  of  40  fm.  on  a  paper  6  in.  wide,  and  on 
this  scale  a  fish  school  2  ft.  in  depth  would  make  a 
line  of  about  1/20  in.  if  shown  as  a  separate  mark. 
This  would  be  fairly  easily  seen,  but,  unfortunately, 
owing  to  the  pulse  length  of  the  transmission,  any  echo 
near  enough  to  the  seabed  is  joined  to  the  seabed  echo. 
When  fish  markings  are  seen  as  separate  echoes  clear 
of  the  seabed  they  are  approximately  1  fm.  above 
the  bottom,  and  the  trawl  may  pass  under  the  fish. 

The  Cathode  Ray  Tube  has  no  such  limitation  of  scale, 
and  no  mechanical  moving  parts  to  limit  the  sensitivity 
or  speed  of  display.  It  has  one  more  very  great  advantage 
in  that  it  is  possible  to  compare  the  strength  of  the 
echoes  received.  It  is  chiefly  this  which  makes  it  possible 
to  see  fish  close  to  the  seabed  on  the  C.R.T.  display. 
The  fish  echo  is  always  smaller  than  the  seabed  echo, 
and  is  usually  very  much  smaller.  If  the  fish  are  near 
the  seabed,  their  echo  appears  as  an  extension  of  the 
seabed  echo,  which  is  easily  visible  on  the  C.R.T.  The 
smaller  fish  echo  appears  much  brighter,  which  helps 
to  make  even  the  smallest  easily  identifiable.  There  is 
no  doubt  that  the  C.R.T.  display  is  more  difficult  to 
interpret,  and  requires  considerable  skill  and  experience 
on  the  part  of  the  operator.  If,  however,  the  operator 
approaches  this  problem  with  confidence  he  can  quickly 
attain  the  necessary  skill  to  decide  whether  the  fish  are 


present,  and  to  estimate  his  catch  with  considerable 
accuracy. 

THE  PYE  FISH-FINDER 

The  Pye  Fish-Finder  is  an  electronic  echo  sounder 
solely  designed  for  locating  fish  under  the  conditions 
found  in  deep-water  trawling,  and  seine  net  fishing. 
The  following  facts  were  considered  when  planning 
the  design: 

1 .  Fishing  is  carried  out  at  all  depths  down  to  300  fm., 
but  chiefly  around  80  to  150  fm. 

2.  Using  present  fishing  techniques,  it  is  only  possible 
to  catch  fish  within  3  or  4  ft.  of  the  seabed,  whereas 
the  fish  may  only  be  a  few  inches  from  the  bottom. 

3.  As  the  equipment  is  usually  operated  by  the  skipper, 
who  is  often  alone  on  the  bridge  while  fishing,  the 
information  must  be  presented  simply  and  clearly. 

4.  The    equipment    is    operated    continuously    while 
fishing  and  may  be  required  to  run  for  14  or  15 
days  at  a  time. 

5.  It  is  difficult  to  service  electronic  equipment  at  sea, 
and  there  may  be  no  skilled  personnel  on  board. 

When  using  a  C.R.T.  display,  it  is  inconvenient  to 
watch  a  slow  flashing  trace,  as  is  required  for  a  measuring 
range  of  300  fm.  Since  most  fishing  is  carried  out  at 


Fig.  2.      Different  kinds  offish  echoes  near  the  seabed.       Left  and  middle:  Cod  in  deeper  water  in  the  Arctic  fishery. 

quantity  of  immature  herring  in  35  fm.  in  the  North  Sea. 

[489] 


Right:  Small 


MODERN     FISHING    GEAR     OF    THE    WORLD 


lesser  depths  the  transmission  rate  can  be  speeded  up 
for  sounding  in  this  shallower  water  and  a  steadier 
picture  obtained. 

Therefore  an  impulse  rate  control  is  installed  which 
may  be  set  to  transmit  pulses  at  the  maximum  rate 
according  to  the  actual  depth.  Consequently  the  display 
is  as  steady  as  possible,  and  is  much  easier  to  watch  than 
the  flash  of  a  fixed  pulse  rate  machine.  We  have  also 
fitted  a  long  persistence  C.R.T.  with  a  suitable  colour 
filter,  and  the  set  is  remarkably  easy  to  watch  under  all 
conditions. 

It  is  necessary  to  examine  the  fish  echoes  closely,  and 
determine  their  distance  from  the  seabed.  To  do  this 
we  have  used  the  old  Radar  trick  of  an  expanded  time- 
base  (fig.  1).  A  marker  pulse  7  fm.  in  width  is  placed 
on  the  time-base,  and,  by  means  of  a  variable  delay 
circuit,  moved  up  or  down.  On  pressing  a  key  switch, 
the  7  fm.  enclosed  by  the  marker  can  be  expanded  to 
the  full  tube  width  and  any  portion  of  the  main  trace 
closely  examined.  The  delay  control  or  7  fm.  marker 
control  is  also  calibrated  in  depth,  as  the  delay  circuit 
is  started  at  the  same  time  as  the  transmission.  When 
using  the  expanded  time-base,  it  is  not  necessary  to 
switch  back  to  the  main  time-base  to  read  off  depths; 
also  the  marker  can  be  used  to  measure  the  depths  of 
midwater  schools  while  the  main  time-base  is  used  to 
measure  the  depth  of  the  seabed. 

The  sensitivity  of  the  equipment  is  limited  by  the 
water  noise  picked  up.  Since  the  received  echo  must 
be  of  greater  amplitude  than  this  noise,  the  transmission 
must  be  strong  enough  to  produce  an  echo  of  sufficient 
amplitude  even  from  the  smallest  fish  school  at  the 
greatest  depth  at  which  fishing  takes  place.  The  Pye 
Fish-Finder  produces  a  transmitted  pulse  power  which 
is  capable  of  giving  echoes  from  concentrations  of 
fish  too  small  to  be  worth  trawling  (fig.  2). 

The  controls  are  largely  self  explanatory;  and,  while 
fishing,  it  is  virtually  a  one-knob  instrument.  Once  the 
gain  and  sounded  depth  controls  are  correctly  adjusted, 
the  fish  echoes  can  be  watched  with  only  an  occasional 
touch  of  the  7  fm.  marker  depth  control  to  keep  them  on 
the  tube.  The  gain  control  has  a  calibrated  scale,  and 
both  this  and  the  sounded  depth  and  7  fm.  marker  depth 
scales  are  edge-lit  in  orange  light  and  provided  with  a 
dimmer  to  avoid  upsetting  the  operator's  vision  when 
fishing  at  night. 

The  equipment  is  housed  in  two  cases — the  pedestal 
which  carries  the  transmitter  and  power  unit,  and  the 
display  head  which  houses  the  receiver,  the  C.R.T.  and 
all  the  control  and  pulse  circuits  (fig.  3).  Normally  these 
units  are  mounted  together  as  a  single  console,  but  in 
small  craft  the  pedestal  can  be  sited  in  the  engine  room 
and  the  display  head  in  the  wheel-house. 

The  oscillators  are  of  the  bar  type  and  are  either 
mounted  through  the  hull  or  on  the  keel.  In  wooden 
vessels,  the  transducer  housings  are  small  enough  to  be 
fitted  without  cutting  into  the  frames.  Corrosion  is 
avoided  by  isolating  the  transducer  electrically  from  the 
ship's  earthing  system. 

The  results  obtained  from  the  Fish-Finder  depend 
almost  entirely  on  the  proficiency  of  the  operator;  it 
has  been  found  absolutely  essential  to  send  a  skilled 
man  to  sea  with  each  new  installation. 


Results  have  proved  that  once  the  skipper  is  able  to 
make  full  use  of  the  set,  fishing  efficiency  improves  and 
he  will  never  shoot  his  gear  until  fish  echoes  are  recorded 
on  the  instrument. 


Fig.  3.    Pye  Fish -Finder  in  single  console  installation. 


[490] 


RECENT    DEVELOPMENTS    IN     ECHO     SOUNDING 


Fig.  4.     Depth  gauge  for  midwater  trawling.     Left:  Hand-winch  with  cable.     Right:  Transducer  float  attached  to  the  headline. 


DEPTH  GAUGE  FOR  MIDWATER  TRAWLS 

In  designing  the  Fish-Finder  we  kept  in  mind  that  we 
were  using  a  valve  transmitter  and  able  to  transmit  over 
long  lengths  of  cable,  a  fact  used  to  provide  the  Fish- 
Finder  with  an  attachment  for  measuring  the  depth  of 
any  midwater  trawl.  The  system  comprises  a  float 
attached  to  the  headline  of  this  trawl  and  containing 
a  transducer,  which  combines  the  duties  of  transmitting 
and  receiving  (fig.  4);  a  length  of  high  tensile  cable 
to  connect  the  float  to  the  ship,  and  a  small  relay  panel 
in  the  pedestal,  so  that  the  switch  on  the  display  head 
enables  the  operator  to  use  either  the  ship's  transducer 
or  the  headline  transducer.  For  all  North  Sea  work, 
and  any  depths  up  to  50  fm.,  a  simple  hand-winch  is 
quite  suitable  for  handling  the  cable,  and  no  difficulty  has 
been  found  in  using  the  gear  after  the  first  few  hauls 
(fig.  4.)  Fish  are  usually  found  with  the  normal  ship- 
transducers,  the  trawl  is  shot  and  towed  back  through 
the  school,  adjusted  in  the  depth  to  that  of  the  fish. 

On  the  C.R.T.  the  transmitted  pulse  now  indicates 
the  trawl  headline  instead  of  the  sea-surface  and  below 
it  will  be  seen  the  echo  from  the  footrope,  the  seabed, 
and  any  fish  present.  Therefore  it  is  possible  to  tell 
the  height  of  the  trawl  above  the  seabed,  the  mouth 
opening  of  the  trawl,  and  the  relative  position  of  the 
trawl  in  relation  to  the  fish  school.  It  is  possible  to  know 
immediately  if  the  gear  is  fishing  properly,  and  if  the 
fish  are  entering  the  net. 

This  equipment  has  proved  both  reliable  and  easy 
to  handle,  and  has  given  us  a  great  deal  of  information 
on  the  behaviour  of  both  the  gear  and  the  fish.  Inciden- 
tally, when  using  this  equipment,  we  found  that  the 
angle  of  warp  measuring  method  for  estimating  the 
trawl  depth  is  very  inaccurate,  and  that  paying  out  a 
fixed  amount  of  warp,  and  trawling  at  a  fixed  speed, 
does  not  always  put  the  trawl  at  the  depth  indicated 
by  the  tables. 

We  are  also  hoping  to  make  this  equipment  suitable 
for  deep-sea  work,  using  up  to  500  fm.  of  cable  down  to 
the  transducer.  For  this  purpose  we  have  designed  a 
special  electric  winch,  which  handles  the  cable  auto- 


matically. When  paying  out,  the  winch  holds  a  steady 
100  Ib.  pull  on  the  cable,  and  prevents  it  running  free 
or  tangling  up.  While  fishing,  the  winch  is  held  by  a 
disc  brake  which  slips  at  200  Ib.  and  prevents  sudden 
surges  breaking  the  cable.  On  heaving,  the  winch 
maintains  a  pull  of  up  to  280  Ib.  It  thus  follows  the  trawl 
winch,  and  takes  up  the  slack  as  the  gear  comes  to  the 
ship.  No  extra  manpower  is  required  to  handle  the 
cable.  The  operator  has  a  three  position  switch,  shoot, 
fish,  heave,  and  he  follows  the  skipper's  orders  to  the 
trawl-winch  man. 

HORIZONTAL  RANGER 

Our  second  attachment  is  a  simple  device  for  finding 
midwater  schools,  which  could  well  be  used  in  con- 
junction with  the  trawl  depth  gauge.  We  have  a  particu- 
lar problem  in  the  Fast  Anglian  herring  fishery  in  that 
the  boats  are  used  part  of  the  year  as  drifters,  and  part 
of  the  year  as  trawlers.  The  device  is  therefore  designed 
so  that  it  can  be  quickly  taken  out  of  the  ship  without 
slipping  or  dry  docking. 

In  the  shallow  waters  of  the  North  Sea,  bottom  and 
surface  echoes  can  be  a  problem  with  horizontal  echo 
ranging  and  for  this  reason  we  have  reduced  the  vertical 
beam  angle  to  8  degrees  and  put  up  the  search  speed  to  1  -6 
r.p.m.  Although  interfering  echoes  are  still  picked  up 
when  the  ship  rolls  and  pitches,  the  high  sweep  speed 
does  enable  the  operator  to  distinguish  the  genuine  fish 
echoes  as  they  are  visible  during  the  whole  period  of 
the  roll  while  the  interfering  echoes  appear  and  disappear. 
To  achieve  this  high  sweep  speed,  we  have  made  the 
horizontal  beam  angle  30  degrees  and  the  maximum  range 
just  over  400  fm.  In  spite  of  this  wide  horizontal  beam, 
there  is  no  difficulty  in  obtaining  a  bearing  on  the  schools, 
and  it  does  make  it  easier  to  maintain  contact  with  the 
target  when  the  ship  is  manoeuvring.  The  transducer 
is  raised,  lowered,  and  rotated  electrically,  and  is  con- 
trolled from  a  small  box  mounted  on  the  Fish-Finder 
display  head.  This  box  also  contains  the  indicator  which 
shows  the  heading  of  the  transducer  and  consequently 
the  bearing  at  which  the  echoes  are  picked  up.  The 


[491] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


signals  are  displayed  on  the  C.R.T.  and  they  also 
produce  an  audible  signal,  which  is  used  when  searching. 
Only  when  it  is  necessary  to  home  on  to  the  fish  school, 
or  find  its  range,  is  the  C.R.T.  brought  into  use. 

FISH-COUNTER 

We  have  recently  been  working  on  a  new  device,  the 
Fish-Counter,  which  may  lead  to  the  end  of  the  supremacy 
of  the  C.R.T.  in  this  type  of  echo  sounding. 

We  were  asked  by  the  Ministry  of  Agriculture,  Fisheries 
and  Food  if  we  could  devise  a  system  for  surveying 
bottom  fish  without  having  to  watch  the  C.R.T.  and  time 
the  echoes.  We  eventually  designed  an  equipment 
which  acts  as  an  electronic  observer.  Our  fish  counter 
acts  in  the  same  way  as  the  human  eye.  Using  the  sea- 
bed echo  as  a  reference  level,  it  looks  back,  and  if  there 
is  a  small  echo  within  a  pre-set  distance  of  the  sea  bed, 
it  produces  a  count.  If  there  is  no  bottom  echo,  it 
ceases  to  operate  until  the  echo  returns.  The  results 
are  displayed  on  4  Dekatron  counters,  3  for  counting 
the  fish  echoes  of  differing  amplitudes,  and  the  fourth 
for  counting  the  number  of  transmissions  sent  out. 

During  trials,  the  counts  were  found  to  be  remarkably 
accurate.  It  was  more  accurate  in  good  weather  than  a 
human  observer,  although  the  accuracy  fell  off  slightly 
under  conditions  of  bad  aeration,  so  we  designed  a 
simplified  version  for  the  commercial  fisherman.  This 
is  a  small  box  which  can  be  attached  to  the  Fish  Finder 
and  has  two  pre-set  controls  and  an  external  loud- 
speaker. When  set  up,  the  counter  watches  the  echoes 
on  the  7  fm.  expanded  time-base,  and  produces  a  click 
in  the  loud-speaker  each  time  a  fish  echo  comes  from 
within  2  fm.  of  the  sea  bed.  Three  counters  were  fitted 
on  commercial  ships,  two  trawlers  and  one  seine  netter, 
and  the  skippers  were  enthusiastic  over  their  performance. 
The  counters  are  now  standard  equipment  in  our  range 
of  optional  fittings  to  the  Fish-Finder  (fig.  5). 

OUTLOOK 

The  success  of  this  device  gives  a  clue  to  future  develop- 
ments in  this  field.    Already  the  C.R.T    is  being  used 


Fig.   5.     The  complete  installation  of  Fish-Finder  and  Fish- 
Counter  as  fitted  in  the  (Jrimsby  trawler  Boston  Fury. 

as  an  auxiliary  to  the  counter,  simply  to  check  the  ampli- 
tude and  shape  of  the  echoes.  We  foresee  the  day  when 
we  come  again  to  the  recorder,  with  an  electronic 
device  watching  the  echoes  as  they  are  received  at  the 
ship,  and  after  sorting  them  out,  printing  the  results 
on  a  paper  tape.  When  this  can  be  made  a  reliable  and 
practical  system,  we  shall  see  the  end  of  the  C.R.T.  in 
fish-finding  echo  sounders. 


|492] 


THE  DEVELOPMENT  OF  ECHO  SOUNDING  AND  ECHO  RANGING 

by 
COMMANDER  R.  G.  HAINES 

Messrs.  Kelvin  Hughes  Ltd.,  London 

Abstract 

One  of  the  most  exacting  problems  facing  the  designer  of  echo-sounding  equipment  is  the  detection  of  fish  within  a  few  feet  of  the 
seabed.  The  use  of  the  scale  expander  which  enlarges  the  picture  as  presented  on  the  Cathode  Ray  Tube,  has  helped  to  clarify  the  situation, 
for  by  this  means,  fish  very  close  to  the  bottom  are  presented  as  having  a  brighter  trace  than  the  seabed  itself.  The  C.R.T.,  unfortunately, 
lacks  a  memory  which  the  recorder  has.  In  order  to  get  the  best  of  both  worlds  the  manufacturer  must  produce,  in  a  single  display,  an 
instrument  having  both  qualities.  Owing  to  the  limited  number  of  gradations  or  "tones"  that  can  be  produced  on  the  recording  paper,  fish 
on  the  bottom  may  pass  unnoticed  because  their  echo  merges  with  that  of  the  bottom  and  to  overcome  this  limitation,  an  ingenious  and 
simple  gating  circuit  has  been  devised.  This  operates  only  on  receipt  of  the  bottom  echo  and  has  the  effect  of  cutting  out  the  amplifier  for 
about  1/100  sec.,  thus  producing  a  white  line  which  follows  the  bottom  contour  on  the  paper,  dividing  the  fish  echoes  from  the  bottom  echoes. 
The  author  deals  with  the  problems  and  limitations  of  echo-ranging  and  stresses  the  need  for  the  training  of  fishermen  by  experts  in  the  use 
of  modern  electronic  equipment,  for  only  by  the  correct  use  of  the  instruments  can  complete  liaison  be  maintained  between  the  fishermen 
and  the  manufacturer. 

Perfectionnement  de  I'echo-sondeur  et  du  tel&netrc  a  echo 
Resume 

Un  des  problcmes  les  plus  delicats  qui  se  poscnt  au  technicien  dcs  appareils  de  sondage  a  6cho  est  la  d6tection  des  poissons  situes 
tres  pres  du  fond.  1,'emploi  d'un  "agrandisseur  d'cchelle"  qui  agrandit  F  "image"  donnec  sur  le  tube  a  rayons  cathodiques,  a  permis  de 
realise r  certains  progres  car  il  pcrmet  de  rcproduire  le  poisson  situe  tres  pres  du  fond  sous  forme  d'unc  trace  plus  brilliante  que  eel Jedu  fond. 
Malheureusemcnt,  le  tube  a  rayons  cathodiques  est  d6pourvu  de  la  memoirc  que  possedc  Fenregistreur,  en  sorte  que  pour  obtcnir  les  avantages 
des  deux  systemes  le  fabricant  doit  mettrc  au  point  un  appareil  permcttant  de  combiner  ces  deux  qualites  dans  la  meme  image.  En  raison  du 
nombre  lirnite  de  gradations  ou  de  "nuances"  que  peut  reproduire  la  bande  enregistreuse,  les  poissons  du  fond  peuvent  passer  inapenjus  car 
leur  echo  se  confond  avec  celui  du  fond;  pour  resoudre  cettc  difficulte,  on  a  mis  au  point  un  systeme  simple  et  ingenieux  appele  "circuit  a 
d&rlanchement"  (gating  circuit).  Ce  circuit  ne  fonctionne  qu'a  la  reception  de  Fecho  du  fond  et  a  pour  eflct  de  couper  1'amplificatcur  pendant 
1  /100  de  secondc  environ  en  produisant  une  ligne  blanche  qui  suit  le  contour  du  fond  sur  la  bande  et  s£pare  ainsi  les  6chos  du  poissons  dc  ccux 
du  fond.  L'auteur  examine  les  probtemes  et  limitations  du  telemctre  a  6cho  et  soulignc  la  necessity  de  confier  A  des  experts  la  tache  d'enseigner 
aux  pccheurs  Femploi  des  appareils  electroniques  moderncs,  car  la  collaboration  entre  les  fabricants  et  les  pecheurs  ne  sera  complete  que  si 
ces  derniers  savont  se  servir  correctement  des  appareils. 

Evoluci6n  del  sondeo  y  telemetria  ultrasonoros 
Extracto 

Uno  de  los  problemas  m&s  dificiles  que  debe  resolver  el  proyectista  dc  equipo  para  sondeo  ultrasonoro  es  la  local izaci6n  de  peces  a 
pocos  pies  del  fondo  del  mar.  El  uso  del  "Scale  expander*1,  que  amplia  la  imagen  producida  en  el  tubo  de  rayos  cat6dicos,  ha  ayudado  a 
esclareccr  la  situaci6n,  ya  que  permitc  representar  a  los  peces  que  se  encucntran  muy  cerca  del  fondo  por  un  trazo  mas  brill  ante  que  el  mismo 
lecho  marino.  Desgraciadcmente,  como  el  tubo  de  rayos  cat6dicos  carece  de  la  memoria  que  posee  la  ccosonda  rcgistradora,  para  obtener  lo 
mejor  dc  ambos  instrumcntos  el  fabricantc  debe  producir  un  aparato  con  ambas  cualidades.  A  causa  del  limitado  niimcro  de  gradaciones  o 
"tonos"  que  se  observan  en  el  ecograma,  los  peces  sobre  el  fondo  puedcn  pasar  desapercibidos  cuando  los  ccos  resultantes  de  su  presencia  se 
mezclan  o  fusionan  con  los  del  fondo.  Para  evitar  este  inconvenientc  se  ha  idcado  un  sencillo  circuito  "de  rejilla"  que  funciona  unicamentc  at 
recibir  el  eco  del  fondo  y  tiene  cl  efecto  de  interrumpir  el  funcionamiento  del  amplificador  durante  aproximadamente  1/100  segundo,  pro- 
duciendo  sobre  cl  papel  una  linea  blanca  que  sigue  el  relieve  del  fondo  del  mar. 

El  autor  analiza  los  problemas  y  limitaciones  de  la  telemetria  ultrasonora  y  pone  dc  relieve  la  necesidad  de  que  expcrtos  adiestrcn  a 
los  Pescadores  a  manejar  equipo  electr6nico  moderno,  ya  que  solamenle  cl  uso  correclo  de  estos  instrumentos  puede  coordinar  la  rclacidn 
que  debe  existir  entre  el  pescador  y  el  fabricanle  de  estos  aparatos. 


THE    development    of   echo    sounding  and    echo 
ranging  techniques  as  applied  to  fish  detection 
has  made  rapid  strides  since  the  war.    For  these 
instruments  to   be   used  efficiently  and  for  the   new 
developments  and  refinements  of  technique  that  are 
continually  being  introduced  to  be  mastered  by  the  user, 
the  fisherman,   there   should   be  the  closest  possible 
association  and  interchange  of  ideas  between  manu- 
facturer and  fisherman. 

BOTTOM  FISH 

The  most  exacting  problem  facing  the  designer  of  echo 
sounding  equipment  today  is  the  detection  of  bottom 
fish,  such  as  cod,  when  they  are  on  or  within  1  or  2  fm. 


of  the  bottom.  The  introduction  of  a  Cathode  Ray 
Tube  (C.R.T.),  or  Scale  Expander,  as  a  complementary 
device  to  the  moist  or  dry  paper  recorder,  made  con- 
siderable progress  towards  solving  the  problem.  But 
there  is  a  tendency  to  look  upon  the  C.R.T.  as  an  alterna- 
tive to  the  recorder  chart  instead  of  as  an  additional  aid, 
to  be  used  in  conjunction  with  it. 

When  actually  trawling,  the  skipper  needs  to  assess 
the  density  of  fish  lying  within,  say,  2  fm.  of  the  bottom; 
he  is  only  indirectly  interested  in  anything  above  this. 
Consequently,  it  is  this  layer  of  water  which  must  be 
spread  out  for  display  across  the  chart  or  the  face  of  the 
Scale  Expander  tube.  To  do  this,  a  very  short  time 
base  is  required,  the  echo  time  equivalent  to  2  fm.  being 
just  under  5/1, 000  sec. 


[493] 


MODERN     FISHING     GEAR     OF    THE    WORLD 


Comparison  of  C.R.T.  and  paper  recording 

The  pen  of  a  recorder  would  have  to  travel  at  a  speed 
of  a  100  ft. /sec.  to  cross  a  6  in.  chart  in  this  time— a 
speed  which,  from  the  designer's  point  of  view,  is  un- 
comfortably fast.  On  the  other  hand,  a  spot  can  be  made 
to  move  over  the  screen  of  a  C.R.T.  at  this  speed  with 
no  trouble  at  all.  But  perhaps  the  chief  advantage  of  the 
C.R.T.  presentation  is  that  the  available  dynamic  range 
is  greater  than  with  intensity  modulation. 

The  Scale  Expander  is  also  well  suited  for  the  separation 
of  the  comparatively  weak  fish  echo  just  clear  of  the 


Fig.  1.    Example  of  the  C.R.T.  display  of  the  bottom  echo 
and  fish  echoes. 


bottom  from  the  much  stronger  botto'm  echo  which 
immediately  follows  it,  particularly  because  the  weaker 
fish  echo  shows  up  more  clearly  (fig.  1).  The  deflection 
of  the  spot  on  the  C.R.T.  is  proportional  to  the  strength 
of  the  signal  being  recorded;  consequently  the  spot  moves 
more  slowly  in  response  to  a  weak  echo  than  to  a 
strong  one  and  therefore  leaves  a  brighter  trace. 

However,  the  C.R.T.  has  certain  fundamental  limita- 
tions as  a  fish  indicator.  It  gives  only  the  instantaneous 
picture— it  has,  in  fact,  no  memory.  A  trawler  skipper 
using  a  Scale  Expander  to  assess  the  density  of  fish 
sufficiently  close  to  the  bottom  to  be  below  the  headline 
of  the  trawl,  and  so  help  him  to  decide  how  long  to 
continue  the  tow,  has  to  add  up  the  fish  signals  in  his 
head.  Once  he  has  seen  them,  the  successive  pictures 
are  lost  for  ever— he  cannot  refer  back  to  them.  The 
recorder  chart,  on  the  other  hand,  builds  up  a  trace 
which  can  be  examined  at  leisure  in  one  piece,  and, 
what  is  perhaps  not  so  well  known,  the  fact  of  placing 
successive  echoes  side  by  side,  as  on  the  chart,  does 
actually  make  them  more  visible. 

Here,  then,  is  a  problem  for  the  manufacturer:  to 
produce,  in  a  single  display,  the  scale  expansion  and 
good  echo  discrimination  of  the  C.R.T.  together  with 
the  memory  of  the  recorder  and  the  advantage  it  has 
of  placing  successive  echoes  alongside  each  other.  To 
achieve  this  would  be  to  get  the  best  of  both  worlds. 

The  limitations  of  the  Scale  Expander  appear  to  be 
the  more  fundamental,  so  it  seems  more  fruitful  to 
improve  the  recorder. 

Increasing  the  speed  of  the  pen  is  largely  a  matter  ot 
engineering  and  I  do  not  propose  to  deal  with  that 
aspect  except  to  mention,  in  passing,  that  the  rotating 
pen,  giving  a  curved  recorder  trace,  which  is  still  preferred 
by  my  Company  on  the  grounds  of  simplicity,  may  prove 
to  be  the  only  feasible  way  of  achieving  the  very  high 
pen  speeds  now  required.  The  other  weakness  of  the 
recorder  is  its  relatively  limited  dynamic  range,  that  is  to 
say,  the  comparatively  small  number  of  separately 
discernible  steps  between  the  unmarked  paper  at  one 
extreme  and  the  saturation  echo  at  the  other.  The 
number  of  just  noticeable  differences  between  black  and 
white  has  been  found  to  be  about  50  for  wet  paper,  and 
slightly  less,  about  30,  for  dry.  These  figures  compare 
with  about  100  for  the  Scale  Expander. 

Now,  the  limits  between  which  the  strength  of  bottom 
fish  echoes  can  vary  depending  on  the  depth  of  water 
and  the  size  and  density  of  the  fish  is  greater  than  the 
dynamic  range  of  the  recorder  paper.  To  put  it  another 
way:  with  the  sensitivity  set  to  maximum  so  that  the 
faintest  detectable  fish  echo  will  just  mark  the  chart, 
the  strongest  group-fish  echo  likely  to  be  received  will 
saturate  the  paper;  no  other  echo,  whatever  its  strength, 
can  give  a  darker  echo  mark  than  this. 

It  is  for  this  reason  that  group-fish  traces  can  sometimes 
merge  into  the  bottom  echo  trace,  despite  the  latter 
being  much  stronger,  and  whenever  the  bottom  trace 
is  serrated  due  to  the  vertical  movement  of  the  ship  in  a 
sea-way,  it  may  be  impossible  to  recognise  the  fish 
traces  at  all.  There  is  no  immediate  prospect  of  achieving 
a  fundamental  improvement  in  the  dynamic  range  of  the 
recorder  paper;  after  all,  there  are  only  so  many  grada- 
tions between  black  and  white. 


[494] 


ECHO    SOUNDING     AND     ECHO     RANGING 


White  line  recording 

However,  my  Company  has  just  introduced  a  device  for 
circumventing  the  limitation  in  this  particular  application. 
Like  so  many  clever  ideas,  it  is  extremely  simple  and  will 
prove,  it  is  thought,  to  be  equally  effective  in  practice. 

A  gating  circuit  is  introduced  into  the  amplifier  which 
operates  only  in  response  to  signals  above  a  certain 
amplitude,  this  being  set  at  a  value  in  excess  of  that  of 
the  strongest  fish  echo  that  can  be  received,  but  below 
that  of  the  bottom  echo.  The  operation  of  the  circuit, 
which  only  occurs  on  receipt  of  the  bottom  echo,  has 
the  effect  of  cutting  out  the  amplifier  altogether  for  about 
1/100  sec.,  so  that  the  bottom  echo  is  immediately 
followed  by  a  white  gap,  whereas  the  fish  echo  is  not. 

These  white  gaps  build  up  into  a  white  line  which 
follows  exactly  the  apparent  contours  of  the  bottom. 
The  effect  is  quite  dramatic.  Group-fish  echoes  on  the 
bottom  which  would  otherwise  have  been  undistinguish- 
able  from  it  now  stand  out  clearly  and  can  be  recognised 
at  once  for  what  they  are  (fig.  2). 

This  new  refinement  is  one  of  several  developments 
on  the  way. 

Relation  between  quantity   of  fish  echoes  and  amount 
of  catch 

Now  for  something  much  more  fundamental.  How 
closely  does  the  quantity  offish  echoes  seen  on  the  C.R.T. 
and/or  recorder  compare  with  the  quantity  offish  actually 
caught  in  the  trawl?  And  the  answer  is  —one  may  as 
well  be  candid  about  it  — not  very  well. 

It  can  happen  that  after  seeing  a  good  concentration 
of  fish  within  1  or  2  fm.  of  the  bottom  throughout  the 
period  of  trawling,  the  catch  is  disappointing.  Fifty 
baskets,  perhaps,  instead  of  the  5  or  6  bags  that  might  have 
been  expected.  Less  frequently,  the  opposite  is  experi- 
enced: no  fish  on  the  echo  sounder  and  a  good  haul. 

The  explanation  seems  to  be  an  ambiguity  in  the 
recorded  depth  of  the  fish  echoes.  Only  when  the 
fish  are  directly  below  the  ship  will  their  depth,  as 
recorded  on  the  chart  or  C.R.T.,  be  correct.  However, 
as  the  sound  beam  has  a  certain  width—  it  may  be  likened 
to  a  searchlight  beam — it  will  illuminate  an  area  on  the 
bottom,  rather  than  a  single  point.  Fish  may  therefore 
return  echoes  before  the  ship  gels  up  to  them  and  after 
the  ship  has  passed  over  them;  and  those  which  lie 
slightly  to  one  side  of  the  ship  may  also  be  detected. 
Now,  the  depth  of  any  echo  as  shown  on  the  chart  is 
the  distance  between  the  transducer  and  the  object 
giving  the  echo.  If  a  fish,  for  example,  is  not  vertically 
below  the  ship  at  the  time  then  the  recorded  depth  will 
be  slightly  greater  than  the  true  depth,  consequently 
and  this  is  what  matters  it  will  appear  to  be  nearer 
the  bottom  than  it  really  is.  This  is  why  the  echo  trace 
returned  by  a  single  fish— or  any  other  small  object 
in  the  water —takes  the  form  of  a  crescent  as  the  ship 
passes  over  it.  If  the  ship  passes  exactly  over  it,  then 
the  centre  of  the  trace,  that  is  to  say,  its  highest  point 
above  the  seabed,  will  be  the  measure  of  its  true  position 
in  relation  to  the  bottom,  but  there  is  no  way  of  telling 
whether  the  ship  did,  in  fact,  pass  exactly  over  the  top 
or  slightly  to  one  side.  If  the  latter  is  the  case,  a  fish 
will  be  shown  as  being  nearer  the  bottom  than  it  really  is. 


Moreover,  when  the  fish  are  densely  concentrated, 
the  echoes  received  arc  from  groups  offish;  the  crescent 
shapes  of  each  fish  are  superimposed  on  one  another 
and  the  result  is  a  blob  on  the  bottom.  In  such  conditions, 
the  position  of  the  top  of  the  layer  of  fish  in  relation  to 
the  seabed  can  be  read  off  the  chart  but  the  bottom  of 
the  layer  is  obscured.  So,  in  a  typical  case,  the  chart 
may  show  plenty  of  fish  in  the  layer  from,  say  3  fm. 
off  the  bottom  down  to  the  bottom  itself.  Assuming 
the  headline  of  the  trawl  to  be  about  1  i  fm.  clear  of 
the  bottom,  only  the  lower  half  of  this  layer  will  be 
caught.  But,  as  I  have  explained,  the  trouble  is  that 
most  of  the  fish  may  really  be  in  the  upper  half  of  the 
layer — a  fact  which  will  only  become  apparent  when  the 
trawl  is  hauled. 

This  may  explain  the  reason  for  apparent  over- 
Jogging  of  fish  at  trawl  depth.  Under-logging,  when  it 
occurs,  must  presumably  be  due  to  the  fact  that  fish  on 
the  bottom  are  not  always  recorded. 

A  lot  of  thought  has  been  given  to  developments 
which  may  help  overcome,  or  at  any  rate  reduce,  these 
limitations  in  the  performance  of  echo-sounders.  The 
problem  is  not  an  easy  one,  and,  of  course,  the  designer 
must  bear  in  mind  that  what  is  required  is  not  a  theoretic- 
ally ideal  solution  so  much  as  a  practical  one — practical 
from  the  point  of  view  of  the  maximum  size,  complexity 
and  cost  that  can  be  tolerated  or  accepted  by  the  fishing 
industry. 

Clearly,  what  is  needed  is  even  better  resolution, 
both  in  depth  and  angle.  How  can  this  be  achieved? 
A  more  directional  beam  can  be  obtained  by  using 
a  higher  frequency,  but  this  must  be  at  the  expense  of 
greater  attenuation  and  consequently  reduced  perfor- 
mance at  maximum  depth. 

The  effect  of  the  rolling  and  pitching  of  the  ship 
would  have  to  be  carefully  considered  and  a  means  of 
stabilizing  the  transducer  might  have  to  be  incorporated 
in  the  design.  A  transducer  housed  in  a  streamlined 
body  and  towed  at  a  substantial  depth  below  the  keel 
might  be  the  answer,  but,  here  again,  there  are  obvious 
difficulties  which  would  have  to  be  mastered  before  a 
really  practical  device  of  this  kind  could  be  produced. 

HORIZONTAL  FISH  DETECTION 

By  far  the  most  important  step  forward  in  recent  years 
regarding  midwatcr  fishing  is  the  development  of  the 
technique  which  is  often  referred  to  as  echo  ranging 
as  opposed  to  echo  sounding.  The  principles  involved 
are  the  same  as  those  of  the  Asdic  used  during  the  war 
for  submarine  detection,  but  whereas  the  trend  of 
Asdic  development  for  Naval  warfare  has  been  in  the 
direction  of  greater  complexity,  that  for  fish  detection 
is  towards  greater  simplicity,  without  loss  of  efficiency. 

The  advantage  of  an  echo  ranging  system  for  searching 
fish  is  obvious  enough.  It  is  the  ability  to  look  all  round 
you  instead  of  only  at  your  feet. 

A  vessel  looking  for  herring  schools  can,  with  the  aid 
of  an  Asdic,  search  out  a  lane  of  water  about  2  miles 
in  width — one  mile  on  either  side  of  the  ship's  course. 
The  advantages  of  this  can  be  dramatically  illustrated 
by  comparing  the  coverage  of  an  Asdic-fitted  fishing 
vessel  with  that  of,  say,  a  helicopter  towing  the  transducer 
of  an  echo  sounder  through  the  water  at  50  knots — a 


[495] 


MODERN     FISHING    GEAR     OF    THE    WORLD 


proposal  that  is  made  from  time  to  time.  It  might  be 
supposed  that  the  much  higher  towing  speed  of  the 
helicopter  would  have  the  advantage.  In  fact,  about 
20  helicopters  operated  in  this  way,  would  be  needed 
to  cover  the  same  amount  of  water  in  a  given  space 
of  time  as  a  single  fishing  vessel  steaming  at  10  knots, 
and  using  Asdic. 


Limitations  of  echo  ranging 

However,  the  use  of  the  Asdic  for  fish  detection  is  not 
all  plain  sailing.  It  can  provide,  at  best,  only  a  partial 
aid  to  fishing,  and  is  still  in  an  early  stage  of  development. 
Further  research  may  show  the  way  to  improved  per- 
formance but  the  limitations  of  such  horizontal  detection 


Fig.  2.    White  line  recordings  of  bottom  fish. 
[496] 


ECHO    SOUNDING     AND    ECHO     RANGING 


are  as  much  as  anything  due  to  the  physical  laws  govern- 
ing the  propagation  of  sound  in  water  and,  as  such, 
cannot  be  altered. 

For  example,  as  is  generally  known,  a  sound  beam 
transmitted  horizontally  through  the  water  is  liable  to 
be  bent,  either  up  or  down,  depending  on  the  change 
of  water  temperature  with  depth.  This  can  have  the 
effect  of  limiting  the  maximum  range  of  detection  of 
schools  in  midwater  and  may  sometimes  mean  that, 
whereas  echoes  from  objects  near  the  surface  can  be 
picked  up  at  considerable  ranges,  those  in  deeper  water 
may  only  be  detectable  at  a  few  hundred  yards.  But  a 
more  serious  limitation  to  Asdic  performance  is  due  to 
reflections  from  the  bottom.  In  depths  of  water  less  than 
20  to  30  fm.  these  bottom  echoes  can  be  very  troublesome. 

If  the  seabed  reflected  sound  as  a  mirror  does  light, 
all  would  be  well,  but  the  surface  of  the  seabed  is  too 
rough  for  this,  and  the  sort  of  reflection  you  get  is  more 
akin  to  that  from  a  rough-cast  wall  than  from  a  mirror. 
The  sound  is,  in  fact,  scattered  in  all  directions,  and  so 
some  of  the  energy  of  the  transmitted  beam  finds  its 
way  back  to  the  ship.  These  scattered  echoes  from  the 
bottom  are,  of  course,  amplified  along  with  the  wanted 
echoes  from  fish  schools  and,  unless  the  latter  are  of 
greater  amplitude,  they  will  be  lost  in  the  background. 

A  lot  of  these  background  echoes  are  returned  from 
the  side  lobes  of  the  sound  beam,  or  secondaries ,  rather 
than  from  the  main  beam  itself,  and  so  an  improvement 
in  the  performance  can  be  achieved  by  designing  a 
transducer  in  which  the  energy  transmitted  is,  as  far 
as  possible,  all  concentrated  in  the  main  beam.  The 
performance  of  a  conventional  transducer  can,  in  fact, 
be  substantially  improved  by  the  use  of  a  tapered  array 
—my  Company  has  done  this  in  their  Fisherman's  Asdic 
— but  it  would  be  going  beyond  the  scope  of  this  paper 
to  describe  the  technique. 

There  is  another  aspect  to  be  borne  in  mind:  the  actual 
business  of  operating  the  set  at  a  maximum  level  of 
efficiency.  The  main  problem  is  a  matter  of  under- 
standing the  Asdic  chart,  of  recognizing  the  various 
clues  which  help  to  determine  the  source  of  any  echo 
trace,  and  of  discarding,  by  a  process  of  elimination, 
those  which  emanate  from  objects  other  than  fish, 
such  as  wrecks  on  the  seabed,  wakes  of  other  vessels, 
and  so  on. 

The  operating  problem  differs  from  the  comparable 
one  of  echo  sounding  in  two  ways.  First,  the  direction 
in  which  the  transducer  is  pointed  can  be  varied  at  will 
by  the  operator,  instead  of  remaining  always  fixed  in 
one  direction.  For  searching,  the  transducer  bearing  is 
changed  by  a  small  step  angle  before  each  sound  pulse 
is  transmitted.  This  is  normally  done  automatically  to 
cover  progressively  the  whole  area  of  search,  and  so 
presents  no  problem  to  the  operator.  But  when  an 
echo  is  heard  on  any  particular  bearing  which  has  the 
general  characteristics  of  a  fish  school  echo,  the  operator 
must  himself  take  over  the  directional  control,  and  the 
information  which  is  subsequently  displayed  on  the 
chart  will  depend  upon  his  handling  of  the  transducer. 
What  he  should  do  is  to  train  the  transducer  in  steps 
back  and  forth  across  the  target,  only  reversing  the 
direction  of  training  after  he  has  lost  the  echo  on  one 
side  or  the  other.  He  has  to  remember  that  the  move- 


ment of  the  vessel  will  cause  the  target  bearing  to  draw 
aft;  he  must  decide  at  what  point  to  turn  towards  the 
bearing  of  the  target  in  order  to  have  a  closer  look. 
Obviously,  much  time  would  be  wasted  if  he  were  to  do 
this  every  time  anything  showed  on  the  chart.  All  this 
calls  for  a  certain  amount  of  judgment  and  discrimination 
which  comes  more  easily,  of  course,  with  experience. 

The  second  point  is  this:  fish  echoes  recorded  on  an 
echo  sounder  come  in  the  clear  space  on  the  chart 
before  the  bottom  echo,  but  those  recorded  when  echo 
ranging  are  mixed  up  with  the  returns  from  the  seabed 
and  the  surface  and  have  to  be  sorted  from  them. 
This  means  that  the  fish  school  echo  cannot  be  identified 
as  such  by  its  position  on  the  chart  in  relation  to  the 
bottom  echo  trace,  but  must  be  recognized  by  its  appear- 
ance— width,  regularity,  sharpness  of  edges  and  so  on. 

The  wakes  of  the  vessels  in  the  vicinity  can  undoubtedly 
confuse  the  Asdic  chart,  and  if  there  are  many  of  them 
it  may  become  difficult,  and  eventually  impossible,  to 
pick  out  the  fish  school  trace  from  amongst  them. 

This  is  important  in  cases  which  involve  the  use  of 
ancillary  craft,  as,  for  example,  the  Norwegian  purse 
seine  fishing  in  which  a  small  boat  is  used  to  locate  the 
exact  position  of  the  school  and  direct  the  shooting  of 
the  net.  Unless  this  boat  is  homed  on  to  the  school  by 
the  parent  ship  and  finds  it  almost  at  once,  there  is  a 
danger  that  a  series  of  wakes  will  be  laid  between  ship 
and  school  and  so  considerably  complicate  the  Asdic 
operator's  problem.  No  doubt,  as  Asdics  become  more 
generally  used,  a  method  of  putting  the  boat  over  the 
shoal  will  be  developed  and  perfected  by  the  fishermen 
themselves.  While  the  manufacturer  can  suggest  ways 
of  doing  it,  this  particular  problem  is  really  a  matter 
of  seamanship  and  therefore  one  that  the  seaman  knows 
best  how  to  solve. 

It  is  clear,  then,  that  the  Asdic  can  be  of  relatively 
little  help  when  extensive  herring  schools  close  inshore  are 
being  fished  by  a  large  number  of  boats  at  the  same  time, 
and  it  is  more  a  question  of  finding  clear  water  to  shoot 
the  nets  than  of  finding  the  fish.  On  the  other  hand,  it 
has  tremendous  advantages  in  searching  areas  further 
offshore  where  the  schools  are  more  widely  scattered. 
In  these  circumstances,  too,  the  operating  conditions 
favour  the  use  of  Asdics:  deeper  water,  fewer  vessels  in 
the  vicinity,  etc. 

Training  of  operators 

There  is  a  very  natural  tendency,  when  marketing  this 
new  device,  for  the  manufacturer  to  under-emphasize  the 
operating  difficulties.  He  does  not  want  to  give  the 
impression  that  the  user  must  have  great  skill  and 
experience  if  he  is  to  obtain  useful  results  and,  indeed, 
to  suggest  this  would  be  to  exaggerate  the  problem  very 
considerably. 

Nevertheless,  it  is  no  good  denying  that  there  are 
rather  more  difficulties  involved  in  successful  echo 
ranging  of  fish  than  in  echo  sounding. 

It  is  therefore  incumbent  on  the  manufacturer  to 
supply  not  only  the  equipment,  but  also  guidance  and, 
perhaps,  actual  training  in  its  use.  It  is  also  in  his  best 
interests  to  do  so.  The  skipper  setting  out  to  sea  for  the 
first  time  with  an  Asdic  will  then  know  what  to  expect— 
and  what  not  to  expect — and  how  to  use  it.  He  will. 


[497] 


HH 


MODERN     FISHING     GEAR     OF    THE    WORLD 


Fit;.  3.     Some  examples  of  recordings  obtained  with  the  Fisherman's  Asdic  in  the  Norwegian  purse  seine  fishery. 


of  course,  learn  from  experience  and,  with  other  members 
of  his  crew,  become  more  proficient  as  time  goes  on, 
but  he  should  not  have  to  find  everything  out  for  himself 
the  hard  way.  Written  instructions  are  all  very  well  so 
far  as  they  go  but  they  are  useless  if  they  are  not  read, 
learnt  and  inwardly  digested !  And  perhaps  it  is  rather 
too  much  to  expect  the  fisherman  to  do  this. 

The  subject  is  a  technical  one  and  if  it  is  to  be  treated 
at  all  adequately  some  technical  language  is  unavoidable 
which  is  certain  to  discourage  the  reader  who  has  no 
previous  training  in  acoustics,  electronics  and  so  on. 
An  alternative  is  to  take  people  to  sea  and  give  them 
first-hand  instruction  on  actual  schools,  but  this  is 
always  difficult  to  arrange  in  practice  and  is  both  costly 
and  time-consuming. 

To  some  extent,  of  course,  the  real  thing  can  be  simu- 
lated ashore  by  means  of  specially  designed  training 
apparatus.  An  example  of  this  is  the  Echo  Whale  Finder 
Simulator  designed  by  my  Company  for  the  sole  purpose 
of  training  operators  in  the  use  of  the  Echo  Whale 
Finder,  a  type  of  Asdic  designed  specially  to  assist  the 
gunner  in  quickly  putting  the  ship  within  harpoon- 
firing  range  of  the  whale  he  is  chasing.  This  simulator 


has  been  successfully  used  for  several  years  and  there  is 
no  doubt  that  the  training  it  has  made  possible  has  done 
much  to  ensure  that  the  Whaling  Asdic  is  put  to  effective 
use  at  sea. 

The  operating  skill  involved  in  this  case  differs  con- 
siderably from  the  skills  I  have  been  discussing  and  it 
does  not  follow  that  a  simulator  for  shore  training  would 
necessarily  be  the  right  answer  for  horizontal  fish 
detection. 

The  way  in  which  instruction  and  advice  are  given 
must  depend  on  the  particular  problem  but  the  important 
thing  is  that  the  customers  do  get  it.  If  not,  both  pro- 
ducer and  customer  will  suffer  in  the  long  run. 

Until  quite  recently  the  evolution  of  fishing  has  been 
essentially  local,  in  response  to  local  conditions.  As  a 
result  of  the  introduction  of  modern  aids,  such  as  echo 
detection,  industries  far  removed  from  the  background 
and  traditions  of  the  fishing  ports  have  been  brought 
in  to  play  a  part  in  this  evolution.  If  they  are  to  play  an 
effective  part  there  must  be  a  real  understanding  by  the 
engineer  of  the  problems  of  the  fisherman  and  vice- 
versa,  and  so,  as  I  said  at  the  beginning,  good  lines 
of  communication  are  essential  in  both  directions. 


[498] 


FISH-FINDER  FOR  BOTTOM  FISH 

by 

TOMIJU  HASHIMOTO  and  YOSHINOBU  MANIWA 

Fishing  Boat  Laboratory,  Fisheries  Agency,  Japanese  Government 


Abstract 

Experiments  have  been  carried  out  to  assess  the  resolving  power  of  different  echo  sounders  when  used  for  fish-finding  close  to  the 
seabed.  Using  a  recording  type  with  a  frequency  of  10  to  50  kc.  and  a  depth  scale  of  50  m.  it  was  difficult  to  distinguish  fish  less  than  50  cm. 
above  the  bottom,  but  with  200  kc.  it  was  possible  to  resolve  fish  at  a  distance  of  30  cm.  from  the  bottom. 

With  C.R.T.  presentation  and  an  expanded  scale  the  resolving  power  could  be  10  cm. 

Interesting  results  were  obtained  when  using  a  suppressor  circuit  by  which  the  bottom  echo  on  the  trace  could  be  reduced  without 
affecting  the  fish  trace.  Tank  tests  using  200  kc.  showed  that  the  echo  from  a  brass  disc  40  mm.  in  diameter  could  be  distinguished  from  that 
of  an  iron  plate  when  the  two  were  20  cm.  apart. 


Resume 


Detecteur  de  poisson  pour  esp&ces  de  fond 


On  a  cherchc  a  determiner  experimentalement  le  pouvoir  dc  resolution  de  differents  sondcurs  a  echo  lorsqu'on  les  utilise  a  Ja 
detection  du  poisson  au  voisinagc  du  fond  de  la  mer.  Avec  un  appareil  enregistreur  d'une  frequence  de  10  a  50  kilo-cycles  et  une  gamme  de 
profondeur  50  m.  il  etait  difficile  de  distingucr  des  poissons  sc  trouvanl  a  moins  dc  50  cm  du  fond  mais,  avec  une  frequence  de  200  kilo-cycles, 
il  a  etc  possible  de  dislinguer  des  poissons  se  trouvant  a  30  cm  du  fond. 

Avec  un  tube  a  rayons  cathodiques  et  une  echelle  plus  grande.  le  pouvoir  de  resolution  a  pu  atteindre  10  cm. 

On  a  obtenu  des  resulUits  interessants  au  moyen  d'un  circuit  d'arrel  permettam  d'affailir  du  trace  Fecho  donnc  par  le  fond  sans 
modifier  I'indication  relative  au  poisson,  et  des  essais  en  bassin  avec  des  frequences  de  200  kilo-cycles  ont  montrc  que  Ton  pouvait  distinguer 
Techo  donne  pur  un  dtsquc  de  laiton  de  40  mm  dc  diametrc  de  celui  donnd  par  une  plaque  dc  fer,  lorsque  ces  deux  objets  sc  trouvaient  a 
20  cms  Tun  de  Taut  re. 

tcosonda  para  peces  de  fondo 
Extracto 

Se  han  hccho  experimentos  para  evaluar  cl  poder  de  resolucibn  de  los  difercnies  tipos  de  ecosondas  al  determinar  la  prescncia  de 
pcccs  cerca  del  fondo  del  mar.  Cuando  se  utilize  una  ecosonda  registradora  con  una  frecuencia  de  10 — 15  kc.  y  una  escala  para  profundidadcs 
hasta  de  50  m.  fue  dificil  distinguir  los  peces  que  se  hallaban  a  menos  de  50  cm.  del  fondo,  pero  con  200  kc.,  ha  sido  posible  su  Iocalizaci6n 
a  30  cm.  del  lecho  occanico.  Al  usar  un  tubo  de  rayos  cal6dicos  y  una  escala  mayor,  el  poder  de  rcsolucion  llcga  a  10  cm. 

Tambicn  se  obtuvicron  rcsuhados  interesantes  utili/ando  un  circuito  supresor  que  permitio  reducir  el  tra/o  del  eco  producido  por 
a  onda  cuando  se  reflcja  en  el  fondo  sin  afcctar  el  tra/.o  correspondiente  al  pez.  las  pruebas  en  estanques  con  frecuencias  de200kc., 
demostraron  que  el  ceo  obtenido  al  utili/ar  un  disco  de  laton  dc  40  mm.  de  diamelro  puede  distmguirsc  del  logrado  con  una  placa  de  hierro 
cuando  estos  dos  objetos  se  hallan  scparados  por  una  distancia  dc  20  cm. 


FISH  schools  in  the  range  of  2  m.  above  the  sea 
bottom  play  an  important  part  both  in  trawling 
operations  and  in  the  crab  fisheries.  Four  echo 
recordings  are  needed  to  confirm  the  presence  of  a  fish 
school.  With  normal  equipment  when  the  speed  of  the 
ship  is  10  knots  (300  m./min.),  the  sounding  impulses 
are  transmitted  at  intervals  of  about  3  to  15  m.  For 
accurate  identification  on  the  echogram  the  length  of  the 
school  must  therefore  be  at  least  12  to  60  m.2.  In  order 
to  spot  small  schools,  sounding  impulses  should  be 
transmitted  at  intervals  of  about  30  cm.,  and  conse- 
quently the  speed  of  the  recording  paper  should  be 
high. 

RESOLVING  POWER  OF  COMMON  ECHO- 
RECORDING  AND  CATHODE  RAY  TUBE 
(C.R.T)  DISPLAY  FOR  BOTTOM  FISH 

A  fish  school,  less  than  50  cm.  above  the  sea  bottom, 
is  already  difficult  to  distinguish  by  means  of  a  common 
10  to  50  kc.  recording  type  echo  sounder  with  a  50  m. 


depth  scale.  Tank  and  field  experiments  show  that  the 
resolving  power  can  be  increased  to  30  cm.  when  a 
200  kc.  set  is  used. 

The  C.R.T.  type  fish-finder  with  15  m.  depth  range  has 
a  resolving  power  of  10  cm.  and  bottom  fish  are  distin- 
guished by  single  transmissions.  Fig.  1  shows  the  C.R.T. 
display  of  50  flat  fish  on  a  net  of  4  sq.  m.  (fig.  2)  placed 
on  the  sea  bottom  at  a  depth  of  10  m.,  with  the  boat 
passing  over  at  a  speed  of  about  2  knots. 

A  glass  ball  of  15  cm.  diameter  was  indicated  by  the 
C.R.T.  type  fish-finder  even  when  it  just  touched  the 
sea  bottom  (fig.  3). 

This  superiority  of  the  C.R.T.  results  from  the  con- 
siderable magnification  of  the  image  as  compared  with 
common  recorder  type  echo  sounders. 

When  using  both  systems  simultaneously,  a  bottom 
fish  echo  is  normally  seen  on  the  C.R.T.  screen,  but  can 
also  be  identified  on  the  echogram.  If  a  differential 
suppressor  Circuit  is  used  for  depressing  the  peak  of 
the  bottom  echo,  the  bottom  echo  trace  may  take  the 
form  of  a  bottom  fish  trace  when  the  sea  bottom  is 


[499  ] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


Fig.  1.  C.R.T.  display  of  an  artificial  flat  fish  school  in  JO  m.  depth. 


Fig.  2.     The  artificial  flat  fish  school  being  submersed. 


sloping  or  uneven,  particularly  when  the  ship  is  rolling. 

COMPARISON  OF  200  kc.  RECORDER  AND  50  kc. 
RECORDER  WITH  SUPPRESSOR  CIRCUIT 

Fig.  4  shows  that,  by  suppressing  the  peak  of  the  bottom 
echo,  identification  of  bottom  fish  can  also  be  greatly 
improved  with  the  use  of  50  kc.  The  simultaneous 
record  obtained  with  200  kc.  proves  the  higher  resolving 
power. 

REFERENCES 

1  T.  Hashimoto  and  M.  Nishimura:     Study  on  Application  of 
Echo-sounder  for  Fishing-ground  of  Crab  at  the  Okhotsk  Sea, 
Waters  of  Kamtchatka  Peninsula,  Technical  Report  of  Fishing 
Boat  No.  8,  1956. 

2  T.  Hashimoto:     Technical  Examination  on  Fish-finder.    Fish- 
eries Agency,  Nov.  1955. 

3  T.  Hashimoto:     Study   on    Ultrasonic   Wave   Soundings   and 
Fish-finding,  Society  of  Fisheries  Research,  February,  1951. 


Pig.  3.     C.R.T.  display  of  a  glass  ball  of  15  cm.  diameter  lying  on 
the  bottom  in  10  m.  depth. 


Fig.  4.    Simultaneous  records  of  bottom  fish  with  200  fee. 
conventional  recording  (top)  and  50  kc.  with  suppressor  circuit. 


[500] 


IMPROVED    ECHO  SOUNDING   EQUIPMENT   FOR  THE   DETECTION 

OF  SHOALS  OF  BOTTOM  FISH 

by 
HANS  KIETZ 

Electro-Acoustic  Laboratory,  Atlas  Works,  A.G.,  Bremen,  Germany 

Abstract 

In  the  trawl  fishery,  the  fish  are  usually  caught  within  a  few  feet  of  the  seabed,  so  that  it  is  most  important  that  recording  echo- 
sounders  should  be  able  to  discriminate  between  these  bottom  fish  and  the  sea  bottom  itself.  The  resolving  power  (picture  quality)  of 
a  conventional  echo  sounder  allows  a  fish  to  be  recorded  just  clear  of  the  seabed,  if  it  is  at  least  75  cm.  above  the  bottom.  As  it  is  not 
practical  to  increase  this  resolving  power,  a  special  receiving  amplifier  has  been  made  which  records  the  strong  bottom  echoes  in  a  black  tone, 
and  the  weaker  fish  echoes  in  a  grey  tone,  thus  making  the  discrimination  between  fish  and  seabed  possible.  This  "Grey-Black  Amplifier" 
should  be  of  great  help  in  the  trawl  fisheries. 


Rtsurnc 


Material  perfcctionne  d'echo-sondage  pour  la  reperagc  des  banes  dc  poissons  situes  prts  du  fond 


Dans  la  peche  au  chalut,  le  poisson  est  capture  generalement  a  quelques  pieds  du  fond  en  sorte  qu'il  est  extremement  important 
d'avoir  des  echo-sondeurs  cnregistreurs  qui  permettent  de  donner  des  echos  de  ccs  poissons  que  soient  distincts  de  ccux  du  fond.  Le  pouvoir 
s6parateur  (c'est-a-dire  la  qualite  de  ('image)  d'un  echo  sondeur  convcntionncl  permct  de  rcperer  distinctement  un  poisson  £  plus  de  75  cm. 
du  fond.  Comme  on  ne  peut,  dans  la  pratique,  augmenter  ce  pouvoir  separateur,  on  a  construct  un  amplificatcur  special  de  reception  qui 
enrcgistrc  noir  Ics  6chos  puissants  du  fond,  et  en  gris  ceux,  plus  faibles,  des  poissons,  ce  qui  permet  de  distinguer  les  echos  du  fond  de 
ccux  des  poissons.  Cet  "amplificateur  gris-noir"  pourrait  rendre  de  grands  services  dans  la  pechc  au  chalut. 

Equipo  dc  sondeo  ultrasonoro  mejorado  para  la  localization  de  cardumenes  que  nadan  cerca  del  fondo 
Extracto 

Como  en  la  pcsca  con  red  dc  arras t  re  los  peccs  se  capturan  generalmente  a  pocos  pies  del  fondo  del  mar,  es  de  suma  importancia 
que  las  ecosondas  registradoras  estable/.can  una  diferencia  entre  el  los  y  el  lecho  marino.  Fl  poder  de  resoluci6n  (i.e.  calidad  de  la  imagcn) 
de  una  ecosonda  convcncional  permite  registrar  con  claridad  peces  que  se  ha  Han  a  75  cm.  de  distancia  del  fondo.  Como  no  es  practice 
aumentar  el  podcr  de  resolucion  del  instrumento,  ha  sido  necesario  construir  un  amplificador  dc  rccepcion  especial  que  registra,  en  colores 
negro  y  gris,  los  ecos  fuertes  y  d£biles  producidos  por  las  ondas  sonoras  al  reflejarse  sobre  el  fondo  y  los  pcces,  respect ivamcnte,  hacicndo 
de  esta  manera  posiblc  la  difcrenciacion  entre  ambos.  A  causa  de  esta  caracteristica,  el  amplificador  "gris-negro"  podria  ser  una  gran  ayuda 
para  las  pcsquerias  de  arrastre. 


GENERAL 

WITH  bottom  trawling  only  fish  close  to  the  seabed 
can  be  caught;  with  certain  trawls,  fish  may  be 
caught  in  a  range  up  to  10  m.  above  the  bottom, 
but  the  average  height  is  under  5  m.  Although  modern 
echo  sounders  make  it  possible  to  record  fish  echoes  from 
any  fishable  depth  of  water,  there  is  some  difficulty  in 
clearly  recording  fish  which  are  slightly  above  the  sea- 
bed. Although  such  fish  are  most  important  for  bottom 
trawling,  it  would  be  wrong  to  confine  the  recording 
only  to  this  small  zone.  The  skipper  of  a  fishing  craft 
should  always  be  able  to  get  evidence  about  the  con- 
ditions in  the  whole  area  between  the  surface  and  the 
sea-bed,  for  this  can  often  give  valuable  information. 
For  optimal  detection  of  bottom  fish  a  favourable 
solution  is,  no  doubt,  the  combination  of  a  recording 
type  echo  sounder  with  the  visual  indication  of  the 
echoes  by  means  of  Cathode  Ray  Tube  (C.R.T.)  (fig.  1). 
While  for  instance  the  whole  range  between  0  and  200  m. 


Fig.  1.     A  combination  echo  sounder  consisting  oj  a  recorder 
and  C.R.T.  presentation. 


[5011 


-JU' — 


lima 


MODERN     FISHING     GEAR     OF    THE    WORLD 


Tht  tltctric  impulse  obtatntd 
by  a  condtnstr  discharge 


».         Tht  sound  impulse  in  tha  water 


The  echo  impulse  at  the  output 
of  the  amplifier 


Fig.  2.  The  result  of  a  condenser  discharge  into  a  transmitter 
oscillator  is  an  electric  and  strongly  damped  wave  train  (top). 
The  transmitter  is  shock-excited  and  dies  out  (centre).  The 
receiving  system  transforms  the  impulse  as  shown  in  the  lower 
diagram. 


depth  is  recorded,  a  magnified  image  of  a  small  area  close 
to  the  seabed  is  displayed  on  the  screen  of  the  CRT. 
giving  all  details  of  bottom  fish  echoes. 

However  it  is  still  desirable  to  have  an  echo  recorder 
in  which  the  feeble  echoes  of  bottom  fish  can  be  clearly 
distinguished  from  the  much  stronger  echo  of  the  sea- 
bed. The  echoes  of  the  fish  schools  arrive  earlier  than 
the  bottom  echo,  so  they  are  alone  for  a  short  time, 
before  being  masked  by  the  stronger  echo  of  the  seabed. 

SIGNIFICANCE    OF    LENGTH,    SHAPE    AND 
FREQUENCY  OF  THE  SOUND  PULSE 

Generally  the  shorter  the  echoes  the  greater  the  possibility 
of  distinguishing  between  fish  and  bottom  echo.  A  shorter 
sound  pulse  results  in  a  shorter  echo.  The  length  and 
the  shape  of  the  echo  depend  on  the  electric  pulse 
exciting  the  sound  transducer,  on  the  sound  frequency 
used,  on  the  sharpness  of  resonance  of  the  sound  trans- 
ducer, and  on  the  selectivity  of  the  receiving  system. 
The  shortest  possible  electric  exciting  pulse  is  obtained 
by  the  condenser  discharge  method.  Fig.  2  shows 
what  pulse  shape  can  be  expected  at  the  output  of  the 
receiving  amplifier. 

As  a  general  rule,  when  using  shock  excitation  of 
the  sound  transducer  by  a  condenser  discharge,  the 
amplitude  of  the  echo  pulse  at  the  receiving  amplifier 
builds  up  for  10  cycles  and  dies  out  in  20  cycles.  To 
visualise  the  appearance  of  the  pulse,  as  recorded  or  as 
shown  on  the  screen  of  a  C.R.T.,  the  following  assump- 
tions are  made: 

Sound  frequency,  30  kc.;  recording  range  of  0  to 
200  m.;  paper  width  of  18  cm.;  partial  observation 
range  of  25  m.  above  the  seabed  is  presented  on  the 
screen  of  a  C.R.T.  for  a  length  of  13  cm. 

The  pulse  length  of  the  echo  of  an  individual  fish  at 
the  output  of  the  amplifier  (see  fig.  2,  bottom)  is  about 
30  cycles  i.e.  ^Q  j 

30,000    =     1,000   ^     Considerin8   the 

velocity  of  sound  in  water,  this  time  corresponds  to  a 
depth  of  75  cm.  so  that  on  the  recording  paper  of  the 
echo  sounder  the  echo  of  an  individual  fish  would 
produce  a  black  mark  of  0-7  mm.  length  and,  on  the 


Fish  about  1.5m 
abaft  thtstabtd 


Fish  about  03m 
abort  tht  stated 


fig.  3.      The  echo  of  the  seabed  is  generally  so  strong  that  it 

passes  beyond  the  screen  of  the  C.  R.  T.    The  fish  echo  is  much 

smaller. 


screen  of  the  assumed  C.R.T.,  an  echo  image  of  about 
4  mm.  length. 

If  the  echo  of  a  single  fish  has  a  recorded  length 
equivalent  to  a  depth  measure  of  75  cm.,  it  is  possible, 
with  conventional  recording  echo  sounders,  to  recognise 
that  fish  echo  clearly  separated  from  the  echo  of  the  sea- 
bed only  if  they  are  at  least  75  cm.  above  the  seabed. 

If  fish  are  closer  to  the  seabed,  their  echoes  will 
merge  into  the  strong  echo  of  the  bottom  and  thus 
become  difficult  to  distinguish  from  an  echogram.  This 
can  subsequently  be  observed  on  the  screen  of  the  C.R.T. 
(fig-  3). 

The  pulse  length  of  about  75  cm.  limits  the  resolving 
power  of  most  of  the  currently  used  recording  type 
echo  sounder  and,  generally,  this  resolving  power  is 
sufficient  for  bottom  trawling.  When  using  a  keyed 
lube  oscillator,  instead  of  a  condenser  discharge,  for 
exciting  the  transducer,  the  pulse  length  will  become 
longer  and  the  resolving  power  inferior.  The  resolving 
power  can  be  improved  in  two  ways,  either  by  increasing 
the  sound  frequency,  for  instance,  from  30  to  80  kc., 
or  by  keeping  the  building-up  and  dying-out  time  of 
the  pulse  as  short  as  possible.  This  can  be  effected 
by  shock-exciting  a  sound  transducer  mechanically 
tuned  to  a  frequency  of  25  kc.  with  a  condenser  discharge 

and  by  receiving  the  echoes  through  a  frequency- 
selective  amplifier  tuned  to  30  kc.  By  such  measures,  the 
resolving  power  can  easily  be  improved  from  75  cm. 
to  about  10  to  20  cm.,  so  that  echoes  of  fish  only  this 
distance  above  the  seabed  can  be  clearly  distinguished 
from  the  bottom  echo.  These  measures  have,  however, 
the  disadvantage  of  increasing  susceptibility  to  inter- 
ference and  for  this  reason  a  resolving  power  of  75  cm. 
has  to  be  accepted  for  practical  fishing. 


INFLUENCE  OF  SHIP  MOVEMENTS 

A  high  resolving  power  can  only  be  fully  used  if  the 
sea  is  calm  and  the  seabed  level.  Irregularities  in  the 
contours  of  the  bottom,  small  rocks,  and  movements 
of  the  vessel  in  a  rough  sea,  make  it  more  difficult  to 
recognise  bottom  fish  echoes  in  the  echogram. 

It  is  assumed  that  the  transmitter-transducer  is  dis- 
placed periodically  by  2  m.  in  a  vertical  direction  for  a 


[502] 


ECHO    DETECTION    OF    BOTTOM     FISH 


Fig.  4.    Diagrams  showing  the  effect  of  the  Grey-black  amplifier. 

period  of  6  sec.  owing  to  the  motion  of  the  vessel,  and 
the  paper  speed  of  the  recording  echo  sounders,  having 
the  same  reproduction  scale  as  that  given  above,  is 
60  cm./hr.  corresponding  to  I  mm.  in  6  sec.  Even  if  the 
seabed  is  level,  the  echo  trace  of  the  bottom  would 
appear  serrated  with  an  amplitude  of  about  1-5  mm. 
in  height  and  a  distance  of  1  mm.  between  the  individual 
wave  crests.  As  the  bottom  echo  is  generally  very  strong, 
and  the  echo  recording  consequently  rather  broad, 
echo  traces  originating  from  bottom  fish  cannot  be 
recognised  in  the  troughs  between  the  wave  crests. 

With  the  C.R.T.,  the  echoes  of  each  individual  sound- 
ing are  displayed  and  completely  disappear  afterwards. 
For  this  reason,  the  echoes  from  consecutive  soundings 
do  not  overlap  as  described  above  for  the  recording-type 
echo  sounder.  Thus,  the  echoes  of  bottom  fish  can  also 
clearly  be  seen  despite  the  ship's  movements  in  a  rough 
sea. 

The  echoes  offish  schools  which  form  a  flat  layer  close 
to  the  bottom  are  of  particular  interest  to  fishermen. 
In  calm  weather,  the  presence  of  such  schools  may  be 
recognizable  in  the  echogram  by  a  certain  roughness 
of  the  bottom  contour. 


Fig.  5.   Large  schools  offish 

recorded  by   a    Grey-black 

amplifier. 


Fig.  6.  An  extended  school  of 

fish   just    above   the  seabed 

recorded    by    a   Grey-black 

amplifier. 


fc^^^i 


Fig.  7.  Fish  just  above  the  seabed 
recorded  by  a  conventional  ampli- 
fier. It  is  not  clear  if  schools  of 
fish  or  the  roughness  of  the  sea- 
bed are  indicated. 


Fig.  8.  Comparison  between 
echogram  and  C.R.T.  dis- 
play. 


However,  even  under  such  favourable  conditions,  it 
is  not  easy  to  distinguish  in  cchograms  of  conventional 
machines,  between  a  rough  seabed  and  a  school  of 
bottom  fish. 

GREY-BLACK  RECORDING 

The  difference  in  the  amplitude  offish  and  bottom  echoes 
can  be  used  for  an  improved  recording.  The  echoes  from 
the  bottom  large  stones,  etc.,  are  almost  always  much 
stronger  than  fish  echoes.  By  using  a  special  recording 
amplifier  to  record  the  weaker  fish  echoes  in  a  grey 
tone  and  the  strong  bottom  echoes  in  a  deep  black  tone, 
it  is  possible  to  differentiate  clearly  between  them. 

Examples  (drawn  by  hand  for  the  sake  of  clearness) 
of  the  comparative  performance  of  a  conventional  and 
of  an  amplitude-discriminatory  amplifier,  are  shown 
in  fig.  4.  The  recordings  shown  in  the  top  row  arc  made 
by  a  conventional  amplifier  and  those  at  the  bottom  by  a 
special  amplifier,  the  grey-Mack  recording  amplifier.  The 
recordings  on  the  left  correspond  to  a  rocky  seabed. 
Those  on  the  right  are  from  fish  dispersed  in  several 
schools  close  to  the  seabed. 

In  actual  recordings  obtained  by  a  grey-Mack  amplifier 
aboard  a  trawler  fishing  on  the  Fladen  Ground  in  the 
North  Sea  (figs.  5  and  6),  the  fish  can  be  clearly  recog- 
nized as  grey  shadows  above  the  contour  line  of  the 
seabed.  In  an  echogram  from  a  conventional  type 
amplifier,  however,  it  cannot  be  said  whether  the  irregu- 
larities of  the  depth  line  have  been  caused  by  fish  close 
to  the  seabed  or  by  the  roughness  of  the  seabed  (fig.  7). 

In  fig.  8  an  echogram  from  a  grey-black  amplifier 
installed  in  an  Atlas  Fischfinder  is  compared  with 
photos  of  C.R.T.  displays  obtained  simultaneously.  In 
the  magnified,  C.R.T.  display  the  particular  shape  and 
quality  of  the  fish  echoes  can  be  more  clearly  recog- 
nized than  is  possible  in  the  echogram. 


[503 


SOME  ELECTRO-TECHNICAL  IMPROVEMENTS  IN  ECHO-SOUNDERS 

by 

DR.  FAHRENTHOLZ 

Kiel,  Germany 

Abstract 

Echo-sounders  for  locating  fish  close  to  the  seabed  have  been  improved  by  a  "distinctive  recorder"  on  which  the  fish  cchos  are  made 
to  stand  out  clearly  from  the  seabed,  being  marked  by  a  sharp  black  line.  Thus  it  is  possible  to  discriminate  between  the  bottom  fish  and 
the  sea  bottom  itself. 

A  so-called  "tele-indicator*',  using  two  simultaneously  working  cathode  ray  tubes,  is  described.  The  first  tube  gives  the  whole 
measuring  range  (i.e.  240  m.)  in  four  vertical  parallel  lines,  each  representing  |  of  the  whole  range.  A  particular  feature  of  this  set  is  that 
the  sounding  rate  can  te  adapted  to  the  specific  measuring  range  needed.  By  this  means,  the  highest  possible  sounding  rate  can  always  be 
used  in  favour  of  an  optimal  indication.  The  second  cathode  ray  tube  gives  an  enlarged  representation  of  1 5  m.  range  which  can  be  adjusted 
to  any  depth  wanted  within  the  main  measuring  range  (i.e.  240  m.). 

A  simple,  inexpensive  oscillator  installation  is  available  for  horizontal  location  of  fish  at  a  short  range.  This  makes  it  possible  to 
sound  in  horizontal  direction  with  vertical  echo-sounders.  With  additional  equipment,  four  principal  directions  may  be  chosen,  so  that  the 
fisherman  can,  with  some  practice,  recognize  in  which  direction  he  must  steer  his  craft  in  order  to  obtain  good  catches. 


Quelques  perfectionnements  electro-techniques  dcs  sondeurs  a  echo 

Les  sondeurs  a  echo  pour  le  reperage  des  poissons  pres  du  fond  dc  la  mer  ont  ete  perfectionnes  par  un  "enregistreur  separateur"  dans 
lequel  les  poissons  se  dctachent  netlement  du  fond,  lequel  est  figure  par  une  iigne  noire  tres  precise.  Ainsi,  il  est  possible  de  fairc  la  distinction 
entre  les  poissons  de  fond  et  le  fond  de  la  mer  lui-meme. 

L'auteur  decrit  un  appareil  appele  "tele-indicateur",  qui  utilise  simultanement  deux  tubes  A  rayons  oathodiques.  Le  premier  tube 
donnc  toute  la  portee  du  sondage  (soil  240  m.)  en  4  lignes  verticales  paralleles,  chacunc  reprdsentant  j  de  la  portee  totalc.  Une  caracteristique 
particuliere  de  cet  appareil  est  que  la  frequence  des  sondagcs  peut  ctre  adaptee  a  la  portee  spccifiquc  dc  sondage  dcsirce.  Dc  cette  facon, 
on  peut  toujours  utiliscr  la  frequence  dc  sondages  la  plus  eleves  ce  qui  favorise  une  indication  optimum.  Le  second  tube  cathodique  donnc 
1'agrandissement  d'une  tranche  de  15  m.  que  Ton  peut  deplacer  a  n'importe  quclle  profondeur  desiree  dans  la  portee  totalc  de  sondage  (soit 
240  m.). 

L'installation  simple  ct  bon  march^  d'un  oscillatcur  est  possible  pour  le  reperage  horizontal  du  poisson  a  une  courte  distance.  Cela 
permet  la  telcmctrie  hori/ontale  avec  des  sondeurs  a  echo.  Avec  requipemenl  supplementaire  on  peut  choisir  quatre  directions  principalcs 
et,  avec  de  la  pratique,  le  pecheur  peut  reconnaitrc  dans  quclle  direction  il  doit  diriger  son  bateau  aim  dc  fairc  une  bonne  peche. 


Mejoras  electrotecnicas  en  las  ccosondas 
Extracto 

El  "registrador  difercnciar  ha  permitido  mejorar  la  eficacia  dc  las  ccosondas  para  localizar  peces  cerca  del  fondo  dcstacandolas 
claramente  por  una  linea  bien  marcada.  Dc  cste  modo  es  imposible  confundir  los  peces  que  so  hallun  ccrca  del  lecho  del  mar  con  este. 

En  el  trabajo  tambidn  se  describe  el  "teleindicador"  provisto  dc  lubos  catodicos  que  funcionan  simultaneamente.  El  primcro  da  cl 
alcancc  total  del  aparato  (240  m.)  en  cuatro  lineas  verticales  y  paralelas,  cada  una  de  las  qualo  represent  a  J  dc  toda  la  gatna.  Una 
caracteristica  especial  del  cquipo  lo  const ituyc  la  adaptacion  de  la  cadencia  dc  sondco  a  la  gama  dc  medicion  mas  convenientc.  Por  este 
medio  siempre  es  posible  utilizar  la  mayor  cadencia  posible  en  favor  de  sondeos  mas  precisos.  El  scgundo  tubo  de  rayos  catodicos  pcrmi 
obtcner  una  ampliation  dc  la  gama  dc  15  m.  que  puedc  ajustarse  a  cualquier  profundidad  dcscada  dentro  del  alcance  (240  m.)  del  aparato.tc 

Para  la  localizacion  horizontal  dc  los  peces  a  corta  distancia,  sc  usa  un  oscilador  sencillo  y  barato  que  deja  a  las  ccosondas  verticales 
en  condiciones  de  hacer  sondeos  en  sentido  horizontal.  Con  un  cquipo  adicional  es  posible  clcgir  cuatro  dircccioncs  principals,  dc  manera 
que  el  pcscador  pueda,  con  algo  de  pr&tica,  reconoccr  en  qu6  sentido  debe  dirigir  su  cmbarcacion  para  obtener  buenas  redadas. 


IMPROVEMENT  OF  BOTTOM  FISH 
REPRESENTATION 

TO  be  suitable  for  bottom  trawling  fishfinder  echo 
sounders  should   have  a   representation   system 
which  allows  for  easy  discrimination  between  the 
traces  of  bottom  fish  and  the  bottom  trace  itself.    As 
a  result  of  extensive  work  carried  out  for  many  years 
in  the  author's  laboratory,  a  design  has  been  developed 
which  meets  this  request  in  the  form  of  an  additional 
installation  which  can  be  combined  with  any  existing 
model  of  "Fahrentholz  Echographs". 


For  this  purpose  output  of  the  last  valve  of  the  ampli- 
fier is  throttled  so  that,  also  with  highest  amplification, 
all  echoes  are  recorded  in  dark  grey  only.  There  is 
also  an  additional  circuit  to  the  amplifier  which  operates 
only  in  response  to  signals  above  a  certain  amplitude 
which  is  beyond  the  strongest  fish  echo  and  can  only  be 
reached  by  the  very  strong  echo  of  the  seabed.  This  is 
effected  by  an  additional  thyraton  or  another  valve, 
with  a  high  negative  voltage  on  its  grid,  and  which 
consequently  responds  only  to  an  echo  which  causes  a 
higher  voltage  than  this  negative  potential.  This  high 
echo  strength  is  only  produced  by  the  peak  of  the 


[5041 


SOME    IMPROVEMENTS    IN    ECHO    SOUNDERS 


Fig.  1.     Echogram  showing    "black  tow/"  recording  for  better 
discrimination  of  bottom  fish  traces. 

oscillations  of  the  bottom  echo.  Consequently  the  valve 
responds  only  for  a  very  short  part  near  the  peak  of  the 
bottom  echo  which  hereby  is  recorded  in  deep  black. 
This  results  in  a  deep  black  band  indicating  the  shape  of 
the  bottom  profile,  which  in  a  measuring  range  up  to 
200  m.  is  about  I  mm.  wide.  The  traces  of  bottom  fish 
recorded  in  grey  now  appear  easily  recognizable  on  top 
of  the  deep  black  bottom  profile  band  (fig.  1). 

This  valuable  improvement  is  particularly  prominent 
in  the  echo  sounder  models  which  record  small  sounding 
ranges  on  the  whole  width  of  the  recording  paper. 

CATHODE  RAY  TUBE  REPRESENTATION 

Recently  a  cathode  ray  tube  set  has  been  successfully 
used  in  addition  to  an  ordinary  recording  echo  sounder 
to  magnify  a  small  part  of  the  sounding  range  and  at 
the  same  time  represent  the  strength  configuration  of 
the  echoes,  making  it  possible  to  distinguish  easily 
between  fish  and  bottom  traces.  In  combination  with 
ordinary  echo  sounders  the  pulse  rate  of  the  cathode 
ray  tube  is  necessarily  the  same  as  that  of  the  main  unit, 
which  usually  means  that  for  the  cathode  ray  tube  it  is 
lower  than  desirable  for  a  quick  decision  about  the 
quality  of  fish  shoals. 

The  Fahrcntholz  fish-tclc-indicator  has  been  developed 
to  overcome  this  drawback  (fig.  2).  It  can  be  used  in 
combination  with  recording  or  other  echo  sounders, 
but  is  a  complete  fish-finding  equipment  itself.  The 
"tele-indicator"  has  two  cathode  ray  tubes  of  equal 
size  side  by  side,  one  to  represent  the  entire  sounding 
range  and  the  other  for  magnifying  a  small  part  of  it 


Fig.  3.     Sketch  showing  the  echo  representation  on  the  whole 
\cale  (left)  ami  magnifying  tube  (right), 

which  can  be  chosen  at  will.  The  whole  range  on  the 
left  screen  is  usually  divided  into  four  vertical  parallel 
lines,  e.g.  a  sounding  range  of  240  m.,  consisting  of 
four  parts  of  60  in.  each  using  the  whole  diameter  of 
the  screen.  Formerly  these  four  lines  were  created  by 
vertical  and  horizontal  multivibrators  and  were  con- 
sequently inclined.  The  new  "tele-indicator"  uses 
electronic  switches,  which  bring  the  lines  to  exact 
vertical  position.  The  echoes  appear  as  horizontal 
light  flashes  normal  to  these  "time  lines".  The  depth 
reading  is  done  by  means  of  a  scale  (fig.  3,  left). 


Fig.    2.    Fish-tele-indicaror   set. 


Fig.    4.     Transducer   arrangement  for   alternative    horizontal 
ranging   and   vertical  sounding. 


[505] 


MODERN    FISHING     GEAR    OF    THE    WORLD 


The  moveable,  magnified  range  covers  1 5  m.  (fig.  3,  left). 
A  mark  on  the  whole  range  tube,  which  is  operated  by 
turning  a  knob  marks  the  place  on  the  scale  to  which 
the  magnifying  tube  is  adjusted.  Both  tubes  work 
simultaneously. 

Another  knob  is  provided  to  set  the  impulse  rate  of 
the  whole  set.  This  is  effected  by  an  electronic  switching- 
device  which  assigns  the  length  of  the  scale  of  the  whole 
range  tube.  For  example,  if  the  equipment  has  a  total 
measuring  range  of  240  m.  with  four  time  lines  it  is  possible 
to  cut  the  range  gradually  down  to  60  m.  leaving  only 
one  time  line.  Thus  the  measuring  range  can  be  adjusted 
to  the  actual  depth  and  immediately  after  the  bottom 
echo  has  been  received  a  new  sounding  impulse  can  be 
sent  resulting  in  the  highest  possible  impulse  rate, 
favouring  the  visual  image  of  the  echo  indications. 
The  magnification  on  the  right  screen  is  achieved  in  a 
different  way.  A  continually  working  electronic  circuit 
creates  alternating  current  of  50  cycles,  which,  fed  to  the 
vertical  plates  of  the  cathode  ray  tube,  synchronizes 
another  frequency  of  12-5  cycles  by  means  of  a  counter, 
thus  activating  the  top  to  bottom  movement  of  the 
electronic  ray.  The  four  vertical  time  lines  on  the  whole 
range  screen  are  produced  by  another  counter  which 


creates  a  frequency  of  3-125  cycles  and  actuates  an 
electronic  switch. 

Summarized,  it  may  be  said  that  the  new  set  which  is 
called  "fish-tele-indicator*9  has  two  essential  advantages: 

1.  Simultaneous  indication  of  the  echo  on  the  whole 
range  screen  and  on  the  magnified  screen. 

2.  The  highest  possible  impulse  rate  for  locating  fish. 

SIMPLE  DEVICE  FOR   HORIZONTAL  RANGING 

In  order  to  increase  the  range  at  which  fish  can  be  located 
a  simple  arrangement  of  oscillators  has  been  designed 
for  alternative  vertical  sounding  or  horizontal  ranging 
(fig.  4).  Four  ultrasonic  transducers  with  about  10  degrees 
beam  angle  and  tilted  10  degrees  down  are  fitted  to  the 
keel,  in  forward  and  sideways  direction,  two  to  starboard 
and  two  to  port.  A  switch  box  is  used  to  connect  the 
echograph  to  each  of  these  transducers  and  thus  to  the 
ranging  direction  required  or  to  an  additional  pair  of 
vertical  transducers.  Under  favourable  conditions  and 
with  some  practice,  this  equipment  can  be  used  to 
recognize  the  position  of  fish  schools  in  a  radius  of 
several  100  m. 


Echogram  showing  fish  concentrations  in  a  bottom  depression. 
[506] 


Photo:  Fahrentholz 


FISH  DETECTION  BY  ASDIC  AND  ECHO-SOUNDER 

by 

G.  VESTNES 

Institute  of  Marine  Research,  Fisheries  Directorate,  Bergen,  Norway 


Abstract 

The  success  of  initial  trials  with  Asdic  for  fish  detection  immediately  after  the  war  led  to  the  installation  of  the  apparatus  in  the 
research  vessel  (/.  0.  Sars  in  1950.  Since  then,  regular  surveys  have  been  made  in  winter  and  summer,  the  former  to  locate  the  herring 
shoals  when  they  were  about  120  miles  from  the  coastal  spawning  grounds,  and  the  latter  to  help  in  establishing  a  pelagic  herring  fishery  in 
the  open  Norwegian  Sea.  The  progress  made  in  echo  sounding  is  also  described,  one  of  the  most  important  advances  being  the  introduction 
of  the  bottom  blocking  device  by  which  fish  echoes  can  be  separated  from  bottom  echoes.  Echo-surveys  for  skrei  in  northern  Norway  and 
in  the  Barents  Sea  arc  also  mentioned. 


Resume 


Lc  rcpcrage  du  poisson  par  I'asdic  et  le  sondeur  a  echo 


Le  succes  des  premieres  sorties  d'un  bateau  muni  de  1'asdic  imm6diatement  apres  la  guerre,  a  conduit,  a  1'installation  de  1'appareil  a 
bord  du  navire  de  recherches  G*.  O.  Sars  en  1950.  Depuis  lors,  on  cffectue  des  explorations  regulieres  en  hiver  et  en  6t6,  les  premieres  pour 
localiser  les  banes  de  harengs  quand  ils  sc  trouvcnt  &  120  millcs  environ  des  lieux  de  ponte  cotiers,  et  les  secondes  pour  aider  £  1'etablissemcnt 
d'une  peche  pclagique  de  harengs  dans  la  haute  mer  de  Norvege.  L'auteur  d6crit  aussi  les  progres  realises  dans  Temploi  du  sondeur  a  6cho, 
un  des  plus  importants  progres  etant  1'introduction  du  blocage  dufond,  dispositif  au  moyen  duquel  les  echos  de  poissons  peuvcnt  etre  s£par£s 
des  £chos  du  fond.  II  cst  aussi  fait  mention  des  recherches  des  skrei  (morues  a  maturild  scxuelle)  avcc  le  sondeur  £  echo  dans  la  Norvege 
scptentrionale  ct  dans  la  mer  de  Barentz. 

I/ocalizacion  de  peccs  con  "asdic'*  y  ecosondas 
Extracto 

F.I  exito  que  se  obtuvo  desputa  de  la  guerra  con  una  embarcacion  provista  de  "asdic"  indujo,  en  1950,  a  instalar  estc  aparato  en 
el  barco  de  invest igaciones  pcsqueras  G.  O.  Sars.  Desdc  entonces  regularmente  se  han  hecho  reconocimientos  en  invierno  para  localizar 
cardumenes  de  arenque  que  se  hallaban  a  unas  120  millas  de  los  bancos  de  desove  costeros,  y.  en  verano,  para  ayudar  el  cominezo  de  las 
pesqucrias  pelagicas  en  el  mar  abierto  de  Noruega.  Tambien  se  dcscribcn  los  progresos  alcanzados  con  las  ecosondas,  uno  de  cuyos  adelantos 
mas  importantcs  es  el  dispositivo  bottom  blocking  para  separar  los  ecos  producidos  al  reflejarse  las  ondas  en  los  peces  y  en  el  fondo  del  mar. 
Tambien  se  mencionan  los  reconocimientos  hechos  por  skrei  en  Noruega  septentrional  y  en  el  mar  de  Barents  mediante  el  empleo  de 
ccosondas. 


GENERAL 

DURING  the  winter  of  1945/46  the  Norwegian 
corvette  Eglantine  was  put  at  the  disposal  of 
the  Institute  of  Marine  Research,  Bergen,  to 
investigate  the  possibilities  for  locating  herring  shoals 
by  means  of  Asdic.  After  the  promising  results  of  these 
trials2' 3  it  was  decided  that  the  research  vessel, 
G.O.  Sars,  should  be  equipped  with  an  Asdic  set. 

Since  the  vessel  was  commissioned  in  1950  the  Asdic 
equipment  has  been  in  use  for  more  than  25,000  hrs. 
on  various  cruises  in  the  Norwegian  Sea.  In  winter 
seasons,  from  November  to  February,  the  task  has  mainly 
been  to  trace  and  follow  the  herring  shoals  on  their 
spawning  migration  to  the  Norwegian  coast.  Part  of 
the  work  during  the  summer,  from  June  to  September, 
has  been  to  locate  the  herring  in  the  Norwegian  Sea. 

The  Asdic  has,  however,  given  satisfactory  results 
only  in  locating  concentrated  herring  shoals.  In  recent 
years,  therefore,  we  have  tried  to  improve  the  echo 


sounding  equipment  especially  with  the  object  of  obtain- 
ing reliable  registration  of  cod  and  haddock. 

DETECTION  OF  HERRING  BY  ASDIC 
Winter  Season 

In  November/December,  herring  normally  concentrate 
in  large  shoals  in  the  open  sea,  when  the  temperature 
gradients  are  small  and  no  considerable  bending  of  the 
horizontal  sound  beam  occurs.  Under  these  favourable 
circumstances  Asdic  ranges  of  2,000  to  2,500  m.  can 
normally  be  obtained  in  reasonable  weather  conditions. 

Twenty-four  hours'  Asdic  watches  are  kept  on  the 
cruises.  The  normal  operating  procedure  is  to  train  the 
oscillator  stepwise  from  port  to  starboard  at  a  rate  of  1 
transmission  every  4  sec. 

When  an  echo  is  heard  or  recorded,  the  target  has 
to  be  identified.  Since  echoes  may  be  obtained  from 


[507] 


MODERN     FISHING    GEAR     OF    THE    WORLD 


Fig.    /.     Echogram   of  typical  winter  herring  shoals  in   the 

Norwegian  Sea  (Migratory  Shoals).    Distance  between  vertical 

lines  is  I  nautical  mile.   Depth  scale  0-200  m.  R.V.  G.O.  Sars, 

January  1955. 

aerated  water,  crest  of  waves,  shoals  of  squids,  whales, 
etc.,  the  question  of  identification  is  an  important  one 
because  much  valuable  time  may  be  lost  by  investigating 
false  echoes.  Experience  has  shown  that  a  well  trained 
operator  can,  to  a  certain  extent,  distinguish  between 
echoes  from  herring  shoals  and  echoes  from  other 
sources  by  analyzing  the  sound  quality  of  the  echoes. 
This  procedure,  however,  is  not  infallible  and  it  has  been 
found  advantageous  to  identify  the  Asdic  target  by  the 
vertical  echo  sounder.  To  do  this,  the  ship's  course  is 
altered  to  the  bearing  of  the  echo.  By  training  the 
transmitter  and  noting  the  range  and  the  angle  of  recep- 
tion an  estimate  of  the  width  of  the  shoal  at  90  deg. 
to  the  ship's  course  can  be  obtained.  When  passing 
over  the  shoal,  the  second  horizontal  dimenions  can  be 


measured  by  the  vertical  echo  sounder.  The  vertical 
extension  of  the  shoal  is  rather  difficult  to  measure, 
since  the  length  and  intensity  of  the  marking  on  the 
recording  paper  also  depend  on  the  density  of  the  shoal 
and  several  technical  features  of  the  echo  sounder  being 
used.  It  has  been  our  experience  that  the  echo  markings 
obtained  give  only  a  rough  estimate  of  the  vertical 
dimension  of  the  shoal. 

The  winter  herring  shoals  on  migration  to  the  spawn- 
ing grounds  usually  appear  as  vertical  comets  with  a 
width  of  50  m.  to  about  2  km.  and  with  an  apparent 
vertical  extension  of  10  to  200  m.  (fig.  1). 

When  the  herring  appear  at  the  coastal  banks  (depth 
less  than  200  m.),  the  efficiency  of  the  Asdic  is  reduced 
because  of  less  favourable  water  condition  and  frequent 
occurrence  of  echoes  from  the  rough  bottom.  These 
are  difficult  to  distinguish  from  the  echoes  from  a  herring 
shoal.  In  the  coastal  waters,  all  Asdic  targets  must, 
therefore,  be  classified  by  means  of  a  vertical  echo  sounder. 

To  show  the  migration  route  of  the  herring,  all 
Asdic  records  arc  plotted  on  a  chart.  Our  fishing 
authority  is  daily  informed  of  the  position  of  the  herring 
shoals  and,  when  the  shoals  are  about  120  nautical 
miles  from  the  coast,  the  fishermen  receive  information 
several  times  a  day  by  radio  from  the  research  vessel. 
In  some  years  the  first  catch  has  been  taken  more  than 
100  nautical  miles  from  the  coast1. 

Summer  Season 

It  is  well  known  that  the  range  of  the  Asdic  shows  a 
large  variation  due  to  changes  in  water  condition 


Fig.  2.    Echogram  of  a  pelagic  shoal  of  codlhaadock.     Barents  Sea.  March  1956.    Distance  between  vertical  lines  is  1  nautical  mile. 

Depth  scale   100-300  m. 

[508] 


ASDIC    AND    ECHO    SOUNDERS 


Fig.  3.     Echogram  taken  while  releasing  marked  fish  from  the  R.V.  G.O.  Sars  and  shooting  the  trawl.     Barents  Sea.  March  1956, 


The  distribution  of  salinity  and  temperature  in  the 
summer  season,  is  such  that  the  effective  working 
range  is  reduced  and  ranges  of  more  than  1,000  m. 
should  not  be  expected. 

The  much  greater  horizontal  range  of  the  Asdic  as 
compared  with  an  echo  sounder  has  opened  new  possibil- 
ities of  catching  herring  in  the  open  sea  and  consequently 
a  pelagic  herring  fishery  in  the  Norwegian  Sea  is  now  a 
reality.  Asdic  sets  are  already  used  by  several  Norwegian 
fishing  vessels. 

EXPERIMENTS  WITH  ECHO  SOUNDERS 

Trials  in  1954  to  locate  shoals  of  cod  and  haddock  in 
the  deeper  waters  of  the  Barents  Sea  by  means  of  echo 
sounding  proved  that  ordinary  types  of  commercial 
echo  sounders  were  not  well  suited  for  this  purpose. 
In  1955,  therefore,  a  working  programme  was  established 
in  cooperation  with  Simonsen  Radio  A/S,  Oslo,  to  develop 
an  improved  model  suitable  for  recording  fish  in  deeper 
water  (200  to  400  m.)  and  particularly  close  to  the 
bottom. 

Improvement  of  Sensitivity 

An  increase  of  the  amplification  of  the  received  signal 
above  the  maximum  of  a  normal  Simrad  echo  sounder 
was  first  tried.  The  result  was  a  considerable  improve- 
ment of  the  sensitivity:  echos  of  single  cod/haddock 
could  now  be  clearly  obtained  down  to  200  m.  depth. 
Secondly,  various  types  of  oscillators  were  tried. 
Normally  this  make  of  echo  sounder  is  equipped  with 
an  oscillator  which  gives  a  half  value  angle  of  13  deg. 
alongship  and  22  deg.  abeam.  Great  improvements  in 
results  were  obtained  with  an  oscillator  which  pro- 


Fig.  4.  Echograms  from  normal  echo  sounder  (top)  and  from 
echo  sounder  equipped  with  bottom  blocking  device  (lower), 
showing  concentration  of  cod/haddock  close  to  the  bottom. 
Ship's  speed  4  knots.  Barents  Sea,  April  1957.  (from  Fiskets 
Gang.  No.  2230.  May,  1957). 


509] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


•  > 


Fig.  5.   Echogram  showing  mainly  pelagic  occurrence  of  cod  with  local  bottom  concentration.   Bottom  depth  210-200  m.  Ship's  speed 

10  knots.     East  Skolpcn  Bank,  Barents  Sea,  November,  1956. 


duced  a  beam  of  6-5  deg.  alongship  and  22  deg.  abeam. 
With  this,  the  number  of  echoes  from  deeper  water 
(200  to  400  m.)  increased  markedly. 

Identification  of  bottom  fish 

This  equipment  has  now  been  used  on  a  number 
of  cruises  of  the  R.V.  G.O.  Sars  to  the  coastal  waters 
of  North  Norway  and  the  Barents  Sea,  and  some 
experience  has  been  gained  in  its  application.  Several 
characteristic  types  of  midwater  echoes  are  regularly 
obtained  in  these  areas.  Although  some  of  them  have 
not  been  identified  yet,  we  believe  we  are  able  to  recog- 
nize the  traces  given  by  individual  cod  or  haddock  of 
marketable  size.  Apparently  these  fish  often  live  in 
pelagic  shoals,  usually  of  low  density,  which  makes  it 
possible  to  discern  the  echoes  of  single  fish  (fig.  2). 

Echograms  taken  while  releasing  marked  fish  over 
the  side  of  the  vessel  have  been  of  great  help  in  the 
identification  of  the  traces  of  single  fish  (fig.  3).  Experi- 
ments have  also  been  made  in  comparing  echoes  fish  of 
of  different  sizes,  and  the  echoes  of  live  and  dead  fish 
by  lowering  the  targets  attached  to  a  line.  On  this 
occasion  the  vessel  lay  to  during  the  fish  marking.  The 
nearly  vertical  lines  (fig.  3,  top  left)  are  echoes  of  single 
fish  going  down.  At  a  depth  of  approximately  100  m. 
the  fish  seem  to  stop,  perhaps  in  order  to  adapt  themselves 
to  the  increasing  hydrostatic  pressure.  The  echo  of  the 
trawl  gear  whilst  shooting  is  shown  further  to  the  right 
of  the  echogram.  As  soon  as  the  vessel  takes  up  speed  to 
shoot  the  trawl,  the  long  horizontal  traces  of  single 
fish  are  changed  into  the  dotted  form  characteristic  of 
records  taken  during  steaming  (fig.  3,  right  and  fig.  2). 

Bottom  blocking 

Greater  difficulties  were  encountered  in  our  attempts  to 
obtain  a  quantitative  measure  of  bottom  fish  concen- 
trations. One  step  forward  was  achieved  in  1956  when 
a  technical  improvement  was  made  which  enabled  us 
to  distinguish  between  the  traces  of  bottom  fish  and  the 
bottom  trace  itself.  Fig.  4  shows  echograms  taken 
simultaneously  with  a  normal  echo  sounder  (top)  and 
with  an  echo  sounder  equipped  with  this  bottom  blocking 
device  (bottom)  during  a  haul  in  the  Barents  Sea  in 
April  1957.  By  this  new  method  the  upper  part  of  the 


bottom  trace  is  presented  as  a  thin  line  only  while  the 
echoes  fron  fish  shoals  close  to  the  bottom  are  shown 
as  usual. 

It  seems  probable  that  bottom  blocking  will  prove  of 
great  practical  value  to  fishermen.  Its  application  may 
also  include  means  to  a  rough  estimate  by  the  density 
of  pelagic  shoals  of,  for  example,  herring. 

Fish  Surveys 

This  echo  sounder  equipment  is  being  used  in  the 
Barents  Sea  for  surveying  large  areas  to  study  the 
relation  between  the  distribution  of  the  fish  and  the 
environmental  factors.  We  are  not  yet,  however, 
entirely  satisfied  with  our  instruments  and  methods. 

A  simpler  and  more  practical  application  of  the  high 
sensitive  echo  sounder  is  as  follows.  In  the  Barents  Sea 
shoals  of  cod  and  haddock  have  often  a  largely  pelagic 
character,  but  with  a  local  concentration  closer  to  the 
bottom.  After  detailed  echo  surveys  of  such  shoals  we 
have  repeatedly  been  able  to  find  the  best  bottom  con- 
centrations, so  increasing  our  trawl  catches  from  in- 
significant to  paying  amounts.  Fig.  5  is  a  part  of  an  echo- 


Fig.  6.    Distribution  of  skrei  in  Lofoten  at  the  beginning  of 
March  1957  according  to  echo  survey  by  the  R.V.  G.O.  Sars. 


[5101 


ASDIC     AND     ECHO     SOUNDERS 


gram  of  such  a  search  showing  a  local  bottom  concentra- 
tion in  a  largely  pelagic  shoal  of  cod. 

Local  echo  surveys  were  also  carried  out  in  the  skrei 
district  of  North  Norway  during  the  season  1957  with 
this  equipment  (skrei  is  large  mature  cod). 

Fig.  6  shows  the  distribution  of  skrei  in  the  Lofoten 
area  at  the  beginning  of  March,  according  to  an  echo 
survey  by  G.  O.  Sars.  The  straight  lines  are  the  course 
of  the  ship.  The  figures  along  these  lines  denote  the  tens 
of  the  average  number  of  single  fish  traces  per  nautical 
mile.  The  yields  of  the  skrei  fishery  in  the  various  localities 
in  the  first  part  of  March  showed  a  convincing  corres- 


pondence with  the  quantitative  distribution  indicated  on 
this  chart.  Thus,  in  the  skrei  district,  it  seems  possible 
to  obtain  a  fairly  reliable  picture  of  the  distribution  of  the 
fish  by  means  of  echo  sounding. 


REFERENCES 

Devoid,  F.          Pa  jakt  etter  storsildcn  i  Norskehavet.   Fiskets 
dang,  20  :  21 7-222.    1951. 

2  Gcrhardsen,  T.    Sildeleting  ved  hjelp  av  Asdic  og  ekkolodd. 
Teknisk  Ukehlad,  51  :  3  7.     1946. 

3  Lea,  H.        Asdic  i  fiskeritjenesten.    Fiskets  Gang,  4  :  41.    1946. 


Broiling  herring  from  a  purse  seine  during  the  Norwegian  winter  herring  fishing  which  relies  heavily  on  echo  .wunding  ami  echo  ranging  for 

detecting  the  schools  which  normally  are  submerged. 

[511] 


HORIZONTAL  ECHO  RANGING 

by 

KARL  FEHER 

Electroacoustic  G.m.b.H.,  Kiel,  Germany 


Abstract 

Two  dual  purpose  echo  sounding/ranging  units  have  been  produced  for  fishing;  one,  the  Miniature  Lodar,  for  installation  in  small 
fishing  vessels  and  the  other,  the  Lodar.  for  use  in  medium  and  large  ships.  In  the  former,  the  aim  has  been  to  produce  a  simple  unit  that 
can  be  used  by  fishermen.  It  consists  of  an  indicating  unit  and  a  hoist/sweep  gear  with  a  transducer,  and  has  8  ranges,  covering  0-1,000  m. 
It  uses  1  50  W.  and  operates  on  a  frequency  of  30  kc.  The  transducer  can  be  trained  from  1  35  degrees  to  port  to  1  35  degrees  to  starboard.  This 
combined  horizontal  and  vertical  sounder  can  be  used  at  a  speed  of  10  knots  and  has  proved  in  practice  to  be  an  essential  aid  for  fishing.  The 
Lodar,  i.e.  the  larger  equipment,  is  a  more  highly  developed  instrument  and  operates  with  two  separate  transducers,  one  for  sounding  and 
the  other,  fitted  in  the  hoist/sweep  gear,  for  horizontal  ranging.  This  unit  is  also  equipped  with  loudspeaker  reproduction  of  the  echo  which 
enables  the  operator,  with  experience,  to  discriminate  between  echoes  from  different  sources. 


TCI&netrie  horizontal  *  6cho 

Deux  sondeurs-t61emetres  £  double  eflet  ont  etd  mis  au  point  pour  la  pdche;  Pun,  appele  Miniatur  Lodar  pour  les  petits  bateaux 
de  peche,  et  Fautre,  appete  Lodar  pour  les  na  vires  de  moyen  et  gros  tonnage.  En  realisant  le  premier,  les  constructeurs  ont  cherchi  d  produire 
un  appareil  simple  susceptible  d'etre  utilise  par  les  pccheurs.  II  comporte  un  ecran  et  un  dispositif  de  belayage  vertical  et  horizontal,  avec 
ftnetteur;  11  a  8  portecs  couvrant  de  0  a  1,000  metres.  Sa  consommation  est  de  150  W.  ct  il  fonctionne  sur  30  kc.;  Temetteur  pent  pi  voter 
de  135  degrees  tribord  a  135  degrees  babord.  Get  appareil  a  efict  combing  vertical  et  horizontal  peut  £tre  utilise  d  unc  vitesse  dc  10  noeuds  et 
s'est  aver6  d'une  grande  importance  pratique  pour  la  peche.  Le  Lodar,  c'est-A-dire  1'appareil  de  plus  grandes  dimensions,  est  plus  perfectionne 
et  fonctionne  avec  deux  6metteurs  distincts.  Tun  pour  le  sondage  et  1'autre,  mont&  dans  le  dispositif  de  balayage,  pour  la  teldmetric  horizontalc. 
L'  appareil  est  aussi  muni  d'un  systeme  de  reproduction  de  1'echo  par  haut-parleur  qui  permet  a  Poperateur  experiment^  dc  distingucr  les 
ecbos  provcnant  de  sources  differentes. 

Telemetria  mediante  ondas  ultrasonoras 
Extracto 

Se  han  const  ruido  dos  ecotelemctros  para  pesca:  el  Miniature  Lodar  destinado  a  embarcaciones  nequenas  y  el  Lodar  quc  se  usa  en 
barcos  medianos  y  grandes.  En  el  primer  caso  se  tuvo  por  finalidad  producir  un  equipo  scncillo  para  uso  dc  los  Pescadores,  que  consiste 
en  una  unidad  indicadora  con  un  transductor  de  150  vatios,  8  gamas  de  sondeo  que  abarcan  profundidades  de  0-1,000  m.,  una  frccuencia  de 
onda  de  30  Kc.  por  segundo  y  un  mecanismo  de  clcvacidn  y  exploraci6n  o  "barrido"  que  pcrmite  dirigir  el  transductor  desde  1  35  degrees  a 
babor  hasta  135  degrees  a  estribor.  Este  ecotetemetro  mixto  para  mediciones  verticales  y  horizontals  puede  usarse  a  velocidades  de  10 
nudos,  demostrandp  en  la  practica  ser  una  ayuda  esencial  para  la  pesca.  El  equipo  de  Lodar  es  un  instrumento  mucho  mas  pcrfeccionado 
para  medir  distancias,  que  funciona  con  dos  transductores  independientes:  uno  vertical  y  otro  horizontal,  instalados  en  el  mecanismo  de 
elevaci6n  y  exploracidn.  Este  instrumento  tambien  cuenta  con  un  altavoz  para  reproducir  los  ecos  pcrmitiendo  a  una  persona  con  cxpcriencia 
diferenciarlos  segun  las  diversas  fuentes  de  donde  provienen. 


GENERAL 

RECENT    publications  have  dealt  in   detail  with 
horizontal  ranging  for  fishing,  and  have  considered 
both  the  theoretical  principles  and  the  practical 
viewpoints1'2-3. 

From  a  technical  point  of  view,  horizontal  ranging 
is  a  more  pretentious  procedure  than  conventional 
vertical  sounding;  the  operators  must  therefore  be  better 
informed  and  more  experienced  regarding  the  character- 
istics of  the  apparatus  and  the  properties  of  the  sea  water 
as  a  medium  for  the  propagation  of  sound.  Horizontal 
ranging  offers  a  considerable  extension  of  the  searching 
range  and  direct  hunting  for  distant  fish. 


The  horizontal  ranging  equipments,  of  course,  must 
meet  the  requirements  of  the  customers  regarding 
technique  and  cost.  Small  craft,  for  instance,  which 
hunt  for  fish  in  the  immediate  neighbourhood  can  only 
use  small  equipment  because  of  economic  and  space- 
saving  reasons.  It  must  be  designed  so  that  the  weight 
is  low  and  the  ship's  electrical  supply  is  not  overloaded. 

Large  and  medium  fishing  craft,  however,  can  use 
more  powerful  equipment  by  means  of  which  normal 
fish  targets  can  be  located  even  under  adverse  propaga- 
tion conditions. 

The  Electroacoustic  G.m.b.H.,  Kiel,  have  tried  to 


[512] 


ECHO    RANGING 


h'iK-    1-      Recording    unit    of    the    Minmtute    1  odar. 
Fig.  2.      Hoist  i  \wccp  gear  oj  the  Miniature  Lodar. 

solve  the  technical  and  cost  problems  by  designing  two 
types  of  horizontal  rangers,  which  arc  discussed  below. 

MINIATURE  LODAR 

This  equipment  is  a  small  combined  horizontal  and 
vertical  ranger/sounder,  with  a  minimum  of  technical 
complexity  and  size.  The  equipment  consists  of  two 
parts: 

1.  Recording  unit,  with  electronic  groups  and  power 
supply  (fig.  1). 

2.  Hoist/sweep  gear  with  transducer   (fig.  2). 

The   recording   unit,   which    is   housed    in   a   strong 
light-metal  casing,  consists  of  the  following  components: 

(a)  The  recording  mechanism,  with  8  sounding  ranges, 
the  smallest  coverage  ranging  between  0  and  75  m., 
the  largest  between  400  and  1,000  m.  The  pulse 
rate  is  arranged  to  match  the  respective  working 
range.   The  recording  paper  is  180  mm.  wide  and 
can  be  adjusted  to  advance  at  a  speed  between  0 
and  a  maximum  of  160  mm./min.,  according  to 
the  engaged  range. 

(b)  The  electronic  unit  consists  of  a  generator  of  30  kc. 
and  of  a  highly  sensitive,  selective  echo  amplifier, 
which  is  tuned  for  30  kc.   Besides  the  continuous 
gain  control,  the  amplifier  contains  an  automatic 
device  for  suppressing  the  near  echoes. 

(c)  The  power  supply  unit,  which  delivers  the  95  W. 
(approximately)  consumed  by  the  equipment. 

The  hoist  sweep  gear  of  the  Miniature  Lodar  (fig.  2) 
is  arranged  for  manual  operation  or,  in  a  second  type, 


may  be  operated  by  a  motor.   It  permits  the  transducer 
to  be  moved  in  three  directions. 

(a)  The  transducer  is  housed  in  a  ball  and  is  located 
at  the  lower  end  of  the  hoisting  shaft,  which  has 
an  elliptical  cross-section  and   is  made  of  non- 
corrosive  chrome-nickel  steel.  The  hoisting  length 
of  the  shaft  is  approximately  80  cm. 

(b)  The  transducer  is  arranged  so  that  it  can  be  tilted 
continuously  from  the  horizontal  to  the  vertical 
position.    It  is  thus  able  to  carry  out  horizontal 
ranging  as  well  as  vertical  soundings  with  only 
one  transducer,  and  to  sound  in  a  slant  downward 
direction  from  0  degrees  to  90  degrees. 

(c)  The  transducer  can  also  be  trained  in  the  horizon- 
tal plane  from  135  degrees  port  to   135  degrees 
starboard. 

With  medium  and  large  ships,  the  hoist/sweep  gear 
is  installed  in  the  hull.  This  has  the  advantage  that 
the  ball  is  in  a  completely  protected  position  when 
the  unit  is  hoisted.  For  small  craft,  the  hoist/sweep 
gear  can  also  be  used  as  an  outboard  installation.  The 
three  possible  movements  of  the  hoist/sweep  gear,  i.e. 
hoisting  and  lowering,  and  setting  tne  vertical  or  horizon- 
tal angle,  can  be  controlled  directly  from  the  unit  by 
means  of  handwheels  or  by  actuating  a  remote  control 
from  the  operator's  stand  or  the  wheelhouse. 

The  transducer  is  of  the  magnetostrictive  type  and  is 
used  for  transmission  as  well  as  reception  of  sound  pulses. 
The  beam  angle  of  the  transducer  for  half  power  is 
approximately  _•  6  degrees  in  the  horizontal  and  i  10 
degrees  in  the  vertical  plane.  The  ball-shaped  housing, 
made  of  sound  transmissive  material,  is  meant  to  avoid 


Fig.  3.     Horizontal  ranging  with  Miniature  Lodar     Recording 
range  0  to  600  m. 


[513] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


Fig.  4,     Horizontal  ranging  with  Miniature  Lodar.   showing 
bottom  and  fish  trace.\.     Recording  range  0  to   150  m. 


interfering  noises  from  eddying  when  the  ship  travels  at 
a  high  speed.  The  ball  is  filled  with  water  when  sub- 
merged. The  speed  at  which  the  transducer  is  trained 
during  horizontal  ranging  should  be  adjusted  to  the 
range  for  which  the  Heliograph  has  been  set.  If,  for 
instance,  the  0  to  75  m.  range  is  engaged,  the  pulse  rate 
is  still  J58/min.,  since  the  echoes  from  the  reflecting 
objects  will  return  to  the  transducer  shortly  after  the 
pulses  have  been  emitted.  In  this  case,  the  transducer 
can  be  trained  at  a  relatively  high  speed.  On  the  larger 
ranges  (0  to  600  m.  and  400  to  1,000  m.),  however, 
the  pulse  rate  is  reduced  (19-75/min.),  since  the  echoes 


fig.  5.    Hoist/sweep  gear  and  valve  generator  of  the  big 
Lodar  equipment. 


h'ig.  6.     Recorder  and  operating  unit  of  the  big  Lodar. 

from  a  greater  distance  take  longer  to  reach  the  receiver. 
The  transducer  must,  therefore,  be  trained  at  a  lower 
speed  when  working  large  ranges.  If  this  is  not  observed, 
targets  may  easily  be  lost  from  the  sound  beam. 

Miniature  Lodar  offers  the  possibility  of  tracking  a 
located  fish  target  by  means  of  horizontal  ranging  and 
then  to  approach  the  target  and  measure  its  depth  by 
means  of  slowly  tilting  the  transducer  in  the  vertical 
plane  from  0  degree  to  90  degrees. 

The  horizontal  effective  range  of  the  unit  naturally 
depends  on  the  propagation  conditions,  and  on  the 
kind  and  size  of  the  target.  In  fig.  3,  which  shows  a 
recording  in  deep  water,  horizontally  located  fish  are 
indicated  as  slanted  lines.  The  distance  corresponds  to 
the  vertical  distance  of  the  echo-trace  from  the  zero  line 
at  the  upper  margin  of  the  illustration.  The  paper  move 
was  set  at  low  speed.  Range  0  to  600  m.  Fig.  4  shows 
horizontal  echo  indications  in  relatively  shallow  water. 
The  transducer  was  inclined  to  about  40  degrees  below 
the  horizontal  line.  Measuring  range  was  150  m.  Fish 
being  located  horizontally  appear  as  almost  vertical 
stripes  because  of  the  low  paper  speed.  Also  one  can 
see  a  weak  contour  of  the  vertical  bottom  echo  which  is 
caused  by  a  minor  side  lobe  of  the  transducer  beam  and 
thus  permits  a  simultaneous  observation  of  the  bottom. 
Below  the  bottom  echo  appears  a  wide  bottom  echo 
caused  by  the  main  beam  hitting  the  ground. 

LODAR   EQUIPMENT 

This  very  efficient  equipment  is  designed  for  use  on 
medium  and  large  ships  and  is  also  a  combination 
ranger/sounder.  Unlike  the  Miniature  Lodar,  the  Lodar 
equipment  operates  with  two  identical  transducers.  One 
is  permanently  installed  in  the  ship's  hull  and  is  exclu- 
sively used  for  vertical  soundings,  while  the  other  is 
installed  in  a  hoist/sweep  gear  and  is  only  used  for  hori- 
zontal ranging.  The  transition  from  horizontal  to  vertical 
is  effected  by  means  of  electrically  switching  from  one 
transducer  to  the  other. 
The  equipment  consists  of  the  following  units. 

1.    The  high  frequency  valve  generator  (fig.  5),  which 
has  a  pulse  output  of  T  5  kw.,  and  a  transmitting 


(514] 


ECHO     RANGING 


frequency  of  20  kc.  The  length  of  the  transmission 
pulse  is  automatically  increased  in  the  generator 
when  switching  from  vertical  to  horizontal. 

2.  The  operating  unit  (fig.   6),   which  contains  all 
operating  components  for  switching  on  the  equip- 
ment, training  the  transducer,  and  hoisting  and 
lowering  the  transducer.     It  is,  therefore,  arranged 
in  the  wheelhouse,  near  the  recording  unit.  The 
hoisted  and  lowered  position  of  the  transducer  as 
well  as  the  training  angle  are  indicated  on  the 
operating  unit  by  means  of  pilot  lamps  or  synchro- 
repeaters. 

3.  The  recording  unit  (fig.  6),  which  is  housed  in  a 
cast  light-metal  casing,  exactly  like  the  operating 
unit,  consists  of  the  following  components: 

(a)  The  recording  mechanism,  with  8  ranges,  the 
smallest  extending  from  0  to  300  m.,  and  the 
largest  from   1,600  to  4,000  m.     The  echoes 
arc    registered    on    dry    paper   of  204    mm. 
width,  the  speed  of  which  can  be  manually 
adjusted    to    a    maximum    of  40    mm./min., 
according  to  the  working  range.  The  respec- 
tive paper  speed  is  marked  on  the  recording 
paper  by  means  of  a  time  marker. 

(b)  The     highly     sensitive,     selective     amplifier, 
which  contains  an  automatic  device  for  the 
suppression  of  near  echoes.  A  supplementary 
unit   provides  the  facility  of  simultaneously 
observing  the  targets  acoustically,  which  is  an 
important  aid  in  identifying  the  targets. 

4.  The  hoisi/sweep  gear,  with  the  horizontal  trans- 
ducer (fig.  5)  is  permanently  installed  in  the  ship's 
bottom.     The  transducer  can  be  trained  horizon- 
tally, at  a  maximum  from  150  degrees  starboard 
to  150  degrees  port,  by  means  of  an  electro-motor 
remotely  controlled   from   the  operator's   stand. 
The   automatic   control   can    be   carried   out   at 
0-7  degrees/sec,  in  a  search  sector  of  maximal  90 
degrees,  which  can  be  selected  as  required.  The 
sea  area  is  usually  searched  automatically.   On 
locating  a  school  of  fish,  the  manual  control  is 
switched  on  to  track  the  target.     The  maximum 
training  speed  for  manual  control  is  20  degrees/sec. 
Hoisting   and    lowering   the    transducer    is   also 
remotely  controlled  and  the  limit  positions  are 
indicated  by  means  of  pilot  lamps. 

5.  The  transducers  are  of  magnetostrictive  type  and 
are  suitable  for  transmission  as  well  as  for  recep- 
tion.    The  beam  angle  of  the  transducers  is  d  9 
degrees  in  the  horizontal  plane,  and    L  6  degrees 
in  the  vertical  plane.  The  resonance  frequency  of 
the  transducers  is  tuned  for  20  kc.  transmission. 
The  output  is  1-5  kw. 

The  main  reason  for  supplying  the  Lodar  with 
a  transmission  output  of  1  -5  kw.  was  to  achieve 
a  working  range  for  fish  of  2,000  m.  under  normal 
sounding  conditions,  and  to  have  a  certain  power 
reserve  for  adverse  conditions.  Trials  have  shown 
that  under  good  conditions  ships  and  fish  were 


clearly  indicated  at  great  distances,  e.g.  up  to 
4,000  m.  In  the  Bay  of  Biscay,  vertical  soundings  with 
Lodar  could  be  taken  down  to  depths  of  more 
than  4,000  m.  The  power  reserve  of  the  Lodar 
guarantees  that  a  range  of  1 ,000  m.  is  obtained  even 
under  adverse  conditions. 

The  vertical  beam  angle  of  the  transducer  must  not 
be  too  narrow  for  the  detection  of  fish  schools  at  a  slant 
distance  in  depths.  Narrow  beam  angles  are  of  advantage 
only  in  calm  sea.  In  rough  sea,  however,  the  over- 
emphasised directional  characteristics  would  render  it 
difficult  to  focus  the  objects  while  the  ship  is  rolling  and 
pitching.  Therefore,  beam  angles  of  i  6  degrees  in  the 
vertical  plane  and  !  9  degrees  in  the  horizontal  plane  are 
considered  to  be  optimal. 

If  a  fish  school  is  spotted  near  the  surface,  it  can  be 
closely  approached  by  the  ranging  ship,  which  will 
receive  the  echoes  even  at  a  very  short  distance.  The 


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:-*/tM  '.'g'vifl1;.   / 


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Fig.  7.      Fish  recordings  obtained  with  Lodar  in  the  Kattegat. 

Horizontal  range  7, 000  to  2,000  m.  and  0  to  1,000  m.    Vertical 

range  0  to  200  m. 


[515] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


F/ff .  5.     Traces  c///*/r  schools  ami  a  buoy  obtained  with  Lodar. 
Horizontal  range  0  to  1,000  ///.,  vertical  range  0  to  200  m. 


greater  the  depth  of  a  school,  however,  the  earlier  it  will 
move  out  of  the  sound  beam.  When  nearing  a  target, 
the  echo  recordings  will  steadily  approach  the  zero  line 
until  a  certain  distance  has  been  reached  when  they  will 
be  completely  skipped.  This  is  the  moment  to  switch  the 
equipment  from  horizontal  ranging  to  vertical  sounding, 
in  order  to  determine  the  depth  of  the  school. 

The  supplementary  acoustic  unit  makes  the  transmitted 
pulses  and  the  echo  signals  audible  by  means  of  a  loud- 
speaker, which  is  arranged  in  the  wheelhouse.  These 
audible  signals  give  the  experienced  operator  valuable 
indications  on  the  nature  and  the  character  of  the  targets. 
The  echo  from  a  vertical  wall  of  rocks,  for  instance,  is 
usually  clear-cut,  distinct  tone,  while  the  echo  of  a 
scattered  fish  school  consists  of  a  multitude  of  short 
tones,  as  each  fish  produces  an  individual  echo.  The 
echo  from  the  sandy  seabed,  however,  resembles  an 
elongated  background  noise  because  each  unevenness  of 
the  seabed  reflects  a  portion  of  the  transmitted  energy 
back  to  the  receiver.  A  trace  of  a  fish  school  is  shown  in 
fig.  7.  This  was  picked  up  acoustically  beyond  the  2,000 
rn.  range  of  the  recorder.  As  the  ship  was  headed  for  this 
target  and  approached  it  at  a  constant  speed,  the  trace 
is  a  straight  band  ascending  from  the  left  hand  bottom 
to  the  right  hand  top.  The  upper  edge  of  the  paper  shows 
where  the  working  range  was  switched  to  0  to  1 ,000  range. 
At  the  distance  of  approximately  100  m.,  the  school 
leaves  the  sound  beam.  After  having  switched  to  vertical 
sounding  (0  to  200  m.  range)  and  still  following  the 
same  course,  the  depth  of  the  fish  school  was  measured 
to  be  between  8  and  15  m.  approximately. 

Fig.  8  shows  the  records  of  a  fish  school  and  of  a  buoy. 
The  buoy  was  registered  and  tracked  from  a  distance  of 
800  m.  The  fish  school  was  simultaneously  focused  at 
a  distance  of  1 ,000  m.  The  fish  trace  is  wider  than  that  of 
the  buoy.  It  was  horizontally  tracked  up  to  a  distance  of 
approximately  30  m.  and  then  identified  by  vertical 
sounding.  A  second  fish  school  was  simultaneously 
focused  and  recorded  during  horizontal  operation  (sec 
left-hand  top  of  the  echogram).  This  school  was  verti- 
cally sounded  at  a  depth  between  10  and  20  m. 


REFERENCES 

1  Ahrens,    fc.     Die   Horizontallotung   in   der   Hochseefischerei. 
Bucherci  dcr  Funkortung,  IV,  1954,  Teil  IV,  p.35.     1954. 

2  Ahrcns,  E.    HorizontallolungimFishfang.    Archivf.  Fischerciw. 
6,  1955,  Heft  3/4.  1955. 

3  Ahrens.    E,     Anwendung  dcs  Horizontallotes  bci  der  Wrackor- 
tung,  Bucherei  der  Funkortung,  1957. 


516 


CORRELATION  OF  MIDWATER  TRAWL  CATCHES  WITH  ECHO 
RECORDINGS  FROM  THE  NORTHEASTERN  PACIFIC 

by 
E.  A.  SCHAEFERS  and  D.  E.  POWELL 

Bureau  of  Commercial  Fisheries,  U.S.  Fish  and  Wildlife  Service,  Seattle,  Washington,  U.S.A. 

Abstract 

Midwater  trawling  experiments,  utilizing  a  Sea  Scanar  equipped  with  a  prototype  recorder,  were  conducted  during  the  spring  of 
1956  off  the  coasts  of  Washington  and  British  Columbia.  Catches  of  fish  and  other  marine  organisms  were  identified  from  66  midwater  tows 
made  from  the  U.S.  Fish  and  Wildlife  Service's  exploratory  fishing  vessel  John  N.  Cobb.  Trawl  fishing  depth  ranged  from  10  to  213  fin.  over 
bottom  depths  of  15  to  950  fm.  Catches  varied  widely,  from  no  fish  for  a  60-min.  tow  to  5,500  lb.,  mostly  hake,  in  a  20-min.  tow.  Examples 
of  Sea  Scanar  recordings  made  during  the  tows  show  different  types  of  traces  for  the  various  species  caught,  but  with  no  fully  consistent 
pattern.  Traces  of  dense  schools  of  hake  arc  compared  with  others  showing  scattered  patches  of  rockfish.  Plankton  forms,  particularly 
cuphausiids,  were  abundant  and  caused  traces  on  the  recorder  which  at  first  were  mistaken  for  fish.  With  experience  a  fair  degree  of  success 
was  attained  in  identifying  echoes  and  predicting  species  in  the  catch. 

Correlation  entre  les  quantitcs  pechees  au  chalut  flottant  dans  le  Nord-est  Pacifiquc,  et  les  enregistrements  du  sondeur  a  £cho 
Resume 

DCS  essais  de  chalut  flottant  ont  en  lieu  au  printemps  de  1956  au  large  des  cotes  de  FEtat  de  Washington  et  do  la  Colombie  firitan- 
nique,  en  utilisunt  un  Sea  Scanar  equine  d'un  prototype  d'appareil  enregistrcur.  Ccs  essais  ont  etc  effect  u?s  a  bord  du  navire  de  recherches  sur 
les  peches  John  N.  Cobb.  du  Fish  and  Wildlife  Service  des  Etats-Unis.  Les  poissons  et  autrcs  organismes  marins  captures  en  66  traits  dc 
chalut  ont  etd  identifies,  1-es  profondeurs  de  chalutage  ont  vari6  de  10  a  213  brasses,  sur  des  fonds  de  15  &  950  brasses.  Les  quantites  pechdes 
ont  etc  tres  variables:  dc  z6ro  aprcs  60  min.  de  chalutage,  a  5,500  livres,  en  grande  partie  du  merlu,  en  20  min.  Les  specimens  d'enregistre- 
ments  obtenus  par  le  Sea  Scanar  au  cours  de  la  peche  montrenJ.  differents  types  de  traces  selon  les  especes  de  poisson  caplurees,  sans  que 
chaque  type  soit  absolumcnl  uniformc.  Les  autcurs  com  parent  des  traces  de  banes  scrrcs  de  merlus  avec  d'autres  correspondant  a  des  groupcs 
disperses  de  chevrcs.  Le  plancton,  et  notamment  les  euphausiides,  6tait  abondant  et  a  etc  reproduit  sur  la  bande  enregistreuse  sous  forme  de 
traces  que  Ton  a  d'abord  pris  par  erreur  pour  des  trace*  de  poissons.  L'exp6rience  aidant,  on  est  parvenu  a  identifier  les  £chos  avec  une 
precision  satisfaisante  et  a  prod  ire  ainsi  les  especes  de  poissons  composant  la  peche. 

Correlacion  entre  la  pesca  obtenida  mediante  una  red  de  arrasire  peUgica  y  los  ecogramas  de  sondeos  hechos  en  el  Pacifico  nororiental 

Extracto 

Durante  la  primavera  de  1956  se  efectuaron,  frente  a  las  costas  del  estado  de  Washington,  E.U.A.,  y  de  Colombia  Britanica,  Canada, 
experimentos  con  redes  de  arrastre  pelagicas  rcmolcadas  a  profundidades  intermedia*,  usando  un  Sea  Scanar  con  un  prototipo  de  registrador. 
En  66  lances  hechos  por  el  barco  de  exploration  pesquera  John  N.  Cobb  del  Servicio  de  Pesca  y  Vida  Silvestre  de  los  E.U.A.,  a  profundidades 
de  10  a  213  brazas  (18-3  a  390  m.)  en  zonas  cuyo  fondo  se  hallaba  entre  15  y  950  brazas  (27,4  y  1  -645  m.)  se  identificaron  los  peces  y  otros 
organismos  marinos.  Las  rcdadas  variaron  considerablemente  desde  nada  en  un  lance  dc  60  min.  a  5-500  lb.  (2-041  Kg.)  dc  pcscado,  especial- 
mentc  merluza,  en  20  min.  Los  ejemplos  de  ecos  registrados  por  el  Sea  Scanar  durante  dichos  lances  demuestran  que  los  trazos  correspondicntes 
a  las  diversas  especies  capturadas  no  siempre  ticnen  las  mismas  caracteristicas.  Los  producidos  por  cardumenes  de  arenque  densos  se 
compararon  con  otros  que  indicaban  manchas  de  gallineta  disperses.  Las  formas  plant6nicas,  especialmente  euphausiidos,  prcscntes  en 
gran  cantidad  produjeron  trazos  que  al  principio  se  confundieron  con  los  originados  por  peces.  Con  experiencia  pueden  identificarse  los 
ecos  y  predecirse  las  especies  que  sc  obtendran  en  el  lance. 


GENERAL 

MIDWATER   trawls,   capable   of  fishing   at   any 
depth  from  the  surface  to  the  bottom,  have 
mostly  been  developed  within  the  last  decade. 
They  have  been  used  successfully  to  capture  herring  and 
sprats  in  Scandinavian  and  other  European  countries, 
and  in  the  inside  waters  of  British  Columbia3.    The 
Larsen  two-boat  niidwater  trawl  is  probably  the  best 
known  of  those  now  in  use. 

Two  of  the  major  problems  faced  by  commercial 
fishermen  and  research  workers  using  midwater  trawls 
are:.  (I)  locating  and  identifying  schools  offish  in  mid- 
water,  and  (2)  positioning  the  net  at  the  proper  depth 
to  catch  the  fish.  Experience  has  shown  that  few,  if 


any,  fish  are  caught  by  "blind"  towing,  and  for  good 
hauls  it  is  necessary  to  locate  concentrations  of  known 
species  of  fish,  determine  their  depth  and  devise  some 
accurate  means  of  net  positioning7. 

While  midwater  trawling  thus  far  has  been  concerned 
primarily  with  the  capture  of  herring,  it  has  been  pro- 
posed by  some  fishermen  and  researchers  that  other 
types  of  fish,  which  spend  part  of  their  time  off  the 
bottom,  such  as  rockfish  and  cod,  might  be  available  to 
midwater  trawls  when  they  are  not  available  to  bottom 
gear1.  To  investigate  this  possibility,  the  Fish  and  Wild- 
life Service's  section  of  Exploratory  Fishing  and  Gear 
Research  began  developing  gear  and  equipment  several 


[517] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


F/jp.    /.     7/if    I/.  A".    HA//    and    Wildlije   Service's   exploratory 

fishing  \essel  John  N.  Cobb  midwater  tra\\linp  off  the  coa\t 

of   Washington. 


years  ago.  The  objectives  were  lo  develop  suitable 
one-boat  midwater  trawls  and  accessory  equipment 
which  could  be  used  on  the  Service's  exploratory  fishing 
vessels  in  the  north-western  Atlantic,  north-eastern 
Pacific  and  Gulf  of  Mexico. 

Development  and  testing  of  the  gear  and  equipment 
were  undertaken  at  the  Gear  Research  Station  at  Coral 
Gables,  Florida.  Underwater  television  was  used  for 
direct  observation  of  midwater  trawls  in  action"1 10,  which 
were  also  inspected  and  photographed  from  a  con- 
trollable two-man  diving  sled'*.  An  acoustic  depth 
telemeter  for  midwater  trawl  depth  determination 
was  constructed  for  the  Service  by  the  University  of 
Miami. 

In  the  spring  of  1956  two  mid  water  trawls  and  the 
telemetering  equipment  were  shipped  to  Seattle  for 
fishing  trials.  Exploratory  midwater  trawling  was  carried 
out  aboard  the  M.V.  John  N.  Cohb  (fig.  1)  during  May 
and  June  off  the  coasts  of  Washington  and  British 
Columbia.  Researchers  from  the  Nanaimo  Station  of 
the  Fisheries  Research  Board  of  Canada,  with  their  gear, 
participated  in  part  of  the  cruise. 

GEAR  AND  EQUIPMENT  USED 

A  Sea  Scanar,  equipped  with  a  prototype  recorder, 
was  the  principal  instrument  used  on  the  John  N.  Cohh 
for  locating  fish  in  midwater.  A  standard  type  recording 
echo  sounder  was  employed  primarily  for  sounding 
the  deeper  bottom  contours,  but  it  was  also  useful  in 
confirming  the  location  of  the  more  dense  fish  schools 
detected  with  the  Sea  Scanar. 

The  acoustic  telemeter,  for  determining  constant  depth 
of  the  trawl  gear,  consisted  of  (a)  a  sensing  and  trans- 
mitting unit  which  was  attached  to  the  port  warp 
immediately  ahead  of  the  trawl  door;  (b)  a  hydrophone, 
trailed  on  a  boom  just  beneath  the  surface  amidships, 
for  picking  up  the  sonic  depth  signal:  and  (c)  a  receiving 
set  on  which  the  operator  listened  for  and  determined 
the  signal  frequency.  Sound  frequencies  were  then 


Hf?.  2.     The  cumhersome  acoustic  depth  lelemetei  nnii,  centre, 
has  heen  replaced  by  the  small  electrical  device,   upper  left, 
attached  directly  to  the  end  of  the  it  awl  cahle. 


converted  to  corresponding  depth  by  using  a  prepared 
conversion  table11.  Although  accurate  and  quite 
dependable,  the  instrument  was  too  large  and  com- 
plicated for  practical  use  in  fishing  vessels,  and  it  has 
since  been  replaced  by  a  simplified  electrical  depth 
telemeter  (fig.  2). 

Three  midwater  trawls  were  used,  all  of  nylon: 
the  Canadian  midwater  trawl,  and  40  ft.  and  50  ft. 
square  opening  trawls  made  at  the  Service's  Coral 
Gables  Gear  Research  Station.  The  Canadian  trawl 
had  mesh  sizes  ranging  from  5  in.  in  the  wings  to  IJ  in. 
in  the  codend J. 

The  two  trawls  furnished  by  the  Service  were  similar 
in  design  to  the  Canadian  trawl,  being  made  up  of  four 
equal  side  pieces  and  small  wings  on  each  corner.  Mesh 
sizes  were  4J,  in.  in  the  wings  and  body,  and  3}  in.  in 
the  codend.  The  otter  boards  were  of  plywood,  hydro- 
foil design,  4  -  6  ft.,  rigged  with  a  conventional  bridle 
arrangement,  and  not  attached  at  the  ends  of  separate 
pennants  as  with  the  Canadian  gear.  The  last  9  ft.  of 
each  codend  was  lined  with  IJ  in.  cotton  mesh  to  retain 
some  of  the  small  organisms  which  would  normally 
pass  through  the  larger  mesh. 

The  midwater  gear  was  first  tested  in  inside  waters 
and  it  performed  satisfactorily  after  certain  modifications. 
The  hydrofoil  boards  were  found  to  be  extremely  sensi- 
tive, with  a  tendency  to  collapse  when  set  in  Choppy 
seas,  but  this  fault  was  partly  remedied  by  adjustment 
of  the  chains.  Sounding  the  net  from  a  motor  launch 
revealed  that  the  ground  rope  rode  approximately 
60  ft.  deeper  than  the  depth  telemeter,  which  was  attached 
just  ahead  of  the  port  otter  board,  with  40  fm.  bridles 
between  the  boards  and  the  net.  Consequently,  correc- 
tion factor  of  60  ft.  was  added  to  telemeter  readings  to 
determine  ground  rope  depth  during  towing. 


[518] 


MIDWATER     TRAWL     CATCHES     AND     ECHO     RECORDINGS 


Fig.  3.     Midwater    trawl   catch,  mostly  hake,  aluiizut/c  the 
John  N.  Cobb. 

FISHING  RESULTS 

Catches  of  fish  and  other  marine  organisms  were  identi- 
fied from  66  midwater  tows  made  in  offshore  waters 
between  Grays  Harbour,  Washington,  and  Queen  Char- 
lotte Sound,  British  Columbia,  from  May  19  to  June  21, 


1956.  The  trawls  were  fished  at  depths  ranging  from 
10  to  213  fm.  over  bottom  depths  varying  from  15  to 
950  fm.2.  Sizeable  concentrations  of  fish  at  mid-depths 
were  difficult  to  find  during  most  of  the  cruise. 

Fishing  results  fluctuated  widely,  from  no  fish  in  a 
60-min.  tow  to  5,500  lb.,  nearly  all  hake,  in  a  20-min. 
tow  (fig.  3).  The  wide  variation  in  catches  was  not 
unexpected  on  this  initial  effort.  Although  numerous 
echo  traces  offish  in  the  North  Sea  have  been  identified*, 
it  has  been  noted  that  the  results  do  not  have  world- 
wide application  and  that  intelligent  interpretation  of 
echo  traces  depends  upon  knowledge  assembled  locally6. 
Except  for  inshore  schooling  herring,  characteristic  fish 
traces  and  reactions  of  fish  to  midwater  trawls  were 
unknown  for  the  area  in  which  the  John  N.  Cohh  operated. 

In  addition  to  the  lack  of  identifiable  echo  traces  from 
the  area,  this  was  the  first  attempt  to  use  the  Sea  Scanar 
with  a  recorder  for  midwaler  trawling.  Consequently, 
there  was  no  basis  for  interpretation  of  echoes  received, 
and  many  of  the  early  tows,  made  on  likely-looking 
traces,  caught  only  plankton,  jellyfish,  small  feed,  or 
a  few  larger  fish. 

It  was  only  after  considerable  sounding  with  the  Sea 
Scanar  and  numerous  tows  with  the  midwater  trawls 
that  reasonably  sound  opinion  could  be  formed  as 


V"** 


»T4  MUM  Wwt  rt  Split  ft**. 
C*u-h---40$0  WMM*  Mk«.  t 


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Trm«~*»r7ri^to  <P~.TT. » 14»t 

Kiftiun*  Tfm*  Ift  MtiwiM" 4« 

>tarSplt*r||0rk. 


21 


fcfl* 


JfsS^SJWlJhHi**  Dtpth  i«  r«ihom«  -  --   •«*•-! 


— NvUm  M««l«*t«r  Tr»»J       ' 

L*aUu«*          34  MiJrn  Wmt  of  <  «»•«  J.il«»»tm    W»«J>, 
,-  r»^.C*u >.»-•«•  pound.  «idi.w  r<Mkf**h.  1  wlf        ' 

t  C    'vV-  •  '  •  T 

'^f --'"-•    : 


Fig.  4.    Sea  Scanar  recorder  traces  mailc  during  productive  midwater  tows,  mostly  hake  and  rockfish. 

[  519  ] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


to  whether  traces  were  caused  by  commercial-size  fish, 
small  feed  fish,  or  by  Euphausiids  or  other  plankton 
forms.  Even  then,  sets  were  made  on  doubtful  traces 
to  gain  additional  knowledge  on  the  organisms  present 
in  midwater  in  this  area,  and  these  usually  produced  no 
significant  fish  catches.  The  Sea  Scanar  proved  to  be 
extremely  sensitive  to  plankton  (Euphausiids  were 
abundant  in  most  localities  over  the  continental  shelf), 
and  it  was  often  necessary  to  reduce  the  sensitivity  of 
the  instrument  to  eliminate  much  of  the  plankton  trace 
so  that  fish  echoes  could  be  distinguished. 

There  was  an  indication  from  the  composition  of 
some  midwater  catches  that  hake  and  rockfish  may 
school  together  or  in  close  proximity.  Mixed  catches 
sometimes  occur  in  the  North  Sea  midwater  trawl 
fishery  where  catches  of  brisling  also  contain  herring 
and  mackerel4.  This  degree  of  non-selectivity  of  mid- 
water  trawls  probably  will  not  create  any  greater  prob- 
lems in  sorting  the  catch  than  is  normally  encountered 
on  bottom  trawlers. 

INTERPRETATION  OF  SEA  SCANAR  TRACES 

Examples  of  Sea  Scanar  records  obtained  during  mid- 
water  tows  are  presented  in  figs.  4,  5  and  6.  In  all 
cases  the  Sea  Scanar  was  operated  with  the  transducer 


in  depth-sounding  position  (vertical  beam)  to  show  what 
was  directly  under  the  vessel.  Figs.  4  and  5  show  traces 
obtained  during  tows  which  caught  significant  quantities 
of  fish,  while  those  in  fig.  6  were  made  on  tows  which 
caught  none  or  only  a  few  fish.  The  entire  recordings 
are  presented  for  the  shorter  tows,  while  only  representa- 
tive sections  of  the  longer  tows  are  shown. 

Traces  in  fig.  4A,  B,  and  C  are  examples  of  dense 
schools  of  hake.  4A  resulted  in  the  best  catch  per  unit 
of  fishing  time  5,430  Ib.  of  hake  and  70  Ib.  of  other 
species  in  20  min.  The  catch  rate  differed  because,  as 
shown  by  the  depth  telemeter,  the  net  in  4 A  was  in  the 
most  dense  part  of  the  hake  school  at  the  start  and 
through  most  of  the  tow;  while  in  4B  the  net  was  too 
shallow  for  the  main  body  of  the  school  near  the  start  and 
too  deep  during  the  middle  of  the  tow.  As  for  4C,  the 
net  was  too  shallow  during  most  of  the  tow. 

As  the  net  was  known  to  be  in  the  dense  portion  of 
the  school  in  fig.  4A,  it  was  hauled  soon  after  it  appeared 
that  the  main  body  of  fish  had  been  passed,  which  was 
not  the  case  in  4B  and  C.  Even  though  the  telemeter 
provided  continuous  accurate  information  on  the  depth 
of  the  trawl,  it  was  not  always  possible  to  keep  the 
trawl  in  the  most  dense  portion  of  the  schools,  which 
varied  in  depth  considerably.  The  trawl  depth  was  regu- 
lated by  varying  the  length  of  towing  warp  or  the  speed 


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Truwl  D*(»Ui  i  ft  r^tiiutnn  (1^*4  I  «i«)   -   4«J 
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5.     Sea    Scanar  recorder  traces  made  during  productive  midwater  tows,  mostly  hake  and  rockfish. 

[520] 


MIDWATER    TRAWL    CATCHES     AND    ECHO     RECORDINGS 


of  the  vessel.  After  each  adjustment,  a  short  period  of 
time  was  required  for  the  trawl  to  stabilize  at  the  desired 
depth;  but  by  then  the  position  of  the  school  may  have 
changed,  requiring  further  adjustment  to  raise  or  lower 
the  trawl  to  the  indicated  depth  for  best  results. 

Calculated  towing  speed  in  fig.  4A  and  C  was  from 
3J  to  4  knots.  Speed  in  48  was  slightly  slower  because 
of  wind  conditions.  Normal  towing  speed  during  the 
cruise  was  approximately  3£  knots,  although  the  speed 
was  varied  frequently  as  a  quick  method  of  raising  or 
lowering  the  net  to  the  desired  depth  during  a  tow. 

In  sharp  contrast  to  the  dense  schools  of  hake  shown 
in  fig.  4A,  B  and  C,  are  the  scattered  patches  of  widow 
rockfish  in  4D.  A  95  min.  tow  through  these  traces 
resulted  in  a  catch  of  850  Ib.  of  widow  rockfish,  2 
yellow-tailed  rockfish  and  1  hake.  It  will  be  noted  that 
the  majority  of  the  patches  are  at  approximately  the 
same  depth  although  the  bottom  depth  varies  con- 
siderably. Jt  may  be  a  characteristic  of  this  species  of 
rockfish,  as  well  as  others,  to  gather  near  a  steep  edge 
and  extend  out  over  the  edge  at  about  the  same  distance 
from  the  surface  rather  than  from  the  bottom.  During 
this  tow  (only  a  portion  of  which  is  shown),  the  recording 
indicated  that  fish  were  present  under  the  vessel  only 
sporadically. 


Traces  made  by  hake  and  widow  rockfish  in  fig.  5D 
are  different  from  traces  of  these  fish  in  fig.  4.  Whether 
this  difference  may  be  related  to  time  of  day  (tows  in 
fig.  4  were  made  in  daytime,  fig.  5D  at  night)  has  not  yet 
been  determined.  Also,  no  attempt  is  being  made  at 
this  time  to  draw  any  conclusions  as  to  which  species 
are  represented  by  which  traces  in  5D.  Some  similarity 
can  be  noted  between  the  traces  in  5D  and  6C.  Yellow- 
tailed  rockfish,  shown  in  5 A,  produced  distinctive 
elongated  traces  quite  different  from  any  other  species 
shown. 

The  catch  of  pink  shrimps  in  fig.  6C  is  noteworthy 
since  shrimps  are  common  bottom  dwellers,  but  in  this 
case  the  trawl  was  at  least  20  fm.  above  the  bottom  at 
all  times.  Pink  shrimps  were  also  taken  in  other  night 
tows,  indicating  that  the  species  leaves  the  bottom  to 
swim  around  at  mid-depths  after  dark.  Further  evidence 
to  support  this  conclusion  was  obtained  during  shrimp 
trawling  explorations  by  the  John  N.  Cobb  in  1955  and 
1956  when  night-time  drags  produced  only  a  few  pounds 
of  shrimps  on  the  same  grounds  where  catches  averaged 
1,000  Ib./hr.  during  the  day. 

Midwater  tows  on  traces  in  figs.  5B,  C  and  D 
were  made  in  approximately  the  same  location  on  the 
same  day,  although  the  trawl  was  towed  four  times 


Fig.  6.     Sea  Scanar  recorder  traces,   made  during  relatively  unproductive  midwater  tows,   believed  to  have  been  caused  at  least  in 

part  by  plankton. 

[521  ] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


as  long  on  the  last-mentioned  recording.  It  is  interesting 
to  note  the  changes  in  the  trace  patterns  and  catch 
composition  for  these  three  tows,  with  the  earliest  starting 
before  sunset  and  the  latest  after  dark.  There  was  a 
definite  movement  of  the  fish  toward  the  surface  as 
darkness  approached. 

The  recordings  in  figs.  6A,  B  and  D  are  examples  of 
traces  which  could  have  been  interpreted  as  good 
indications  of  fish,  but  the  total  catch  for  the  three  tows 
was  only  two  fish.  The  net  was  towed  through  what  now 
appears  to  be  a  Scattering  Layer  in  6A  and  B  rather  than 
through  the  large  patches  above,  which  probably  more 
closely  resemble  fish  traces.  However,  in  6D  the  tow 
was  made  through  traces  resembling  the  large  patches 
in  6A  and  B,  with  a  catch  of  only  one  hake  resulting. 

Plankton  forms,  principally  Euphausiids  and  jellyfish, 
were  present  in  many  catches,  even  those  in  which  no 
fish  were  taken.  After  some  experience,  it  was  concluded 
that  some  of  the  Sea  Scanar  recorder  traces,  such  as 
the  layers  in  fig.  6A  and  B,  were  caused  by  dense  con- 
centrations of  Euphausiids,  with  jellyfish  sometimes 
mixed  in.  From  then  on,  a  fair  degree  of  success  in 
predicting  catches  was  possible. 

Exploration  of  these  waters  at  different  seasons  of  the 
year  may  show  that  other  species  of  fish  are  available 
to  midwater  trawls.  It  is  apparent  that  a  great  deal  of 
experience  on  the  local  fishing  grounds  is  necessary  to 
identify  species  of  fish  from  traces  on  the  echo  recorders, 
under  different  conditions  and  times  of  day.  Suit- 
able electronic  fish-finding  equipment,  competently 
operated,  and  an  accurate  trawl-depth  indicating 
method,  are  essential  items  for  successful  offshore 
midwater  trawling  exploration. 


LIST  OF  FISHES 

Yellow-tailed  rockfish     .... 

Orange  rockfish 

Widow  rockfish 


Pacific  ocean  perch 

Hake 

Sabletish 

Arrow-toothed  flounder 

Dogfish 

Pink  shrimp 


Sehastotles  a /it /us 
Merluccius  productus 
Anoplopoma  fimbria 
Atheresthes  siomias 
Squalus  suckleyi 
Panda/us  Jordan! 


REFERENCES 


Sebastodes  flavidus 
Sebastodes  pinniger 
Sebastodes  entomelas 


1  Alvcrson,  Dayton,  L.  and  Powell,  Donald  E.   The  open  ocean 
challenges  the  scientist  and  dares  the  fisherman.  Pacific  Fisherman, 
Portland,  Oregon,  vol.  53,  No.  11.  October  pp.  25,  26  and  29; 
and  vol.  53,  No.  12,  November,  pp.  26  and  27.     1955. 

2  Anonymous.    Promising  results  with  midwater  trawls  by  John 
N.  Cobb  (Cruise  27),  Commercial  Fisheries  Review,  U.S.  Fish  and 
Wildlife  Service,  Department  of  the  Interior,  Washington  25,  D.C. 
vol.  18,  No.  8,  August,  pp.  39-40.     1956. 

3  Barraclough,  W.  E.,  and  Johnson,  W.W.      A  new  midwater 
trawl  for  herring.    Fisheries  Research  Board  of  Canada,  Ottawa, 
Bulletin  No.  104,  25  pp.     1956. 

4  Glanville,  Alan.  The  Larsen  midwater  trawl.   F.A.O.  Fisheries 
Bulletin,  Rome,  vol.  9,  No.  3,  July-September,  pp.  113-129.    1956. 

5  Hodgson,  William  C.  Echo  sounding  and  the  pelagic  fisheries. 
Ministry  of  Agriculture  and  Fisheries,  London,  Fishery  Investiga- 
tion, Series  2,  vol.  17,  No.  4,  25  pp.    1950. 

6  Hodgson,  William  C.,  and  Fridiksson,  A.  (Editors).  Report  on 
echo  sounding  and  asdic  for  fishing  purposes.   Rapports  et  Proces 
— Varbaux  des  Reunions,  Copenhagen,  vol.  139,  September,  49  pp. 
1955. 

7  Richardson,  1.   D.     Some  problems  in  midwater  trawling. 
World  Fishing,  London,  vol.  6,  No.  2,  February,  pp.  28-31.  1957. 

8  Sand,  Reidar  F.    Use  of  underwater  television  in  fishing  gear 
research  (Preliminary  Report).      Commercial  Fisheries  Review, 
U.S     Fish   and   Wildlife  Service,   Department   of  the   Interior, 
Washington  25,  D.C.,  vol.  17,  No.  4,  April,  pp.  1-5.    1955. 

9  Sand,  Reidar  F.     New  diving  sled.     Commercial  Fisheries 
Review,  U.S.  Fish  and  Wildlife  Service,  Department  of  the  Interior, 
Washington  25,  D.C.,  vol.  18,  No.  10,  October,  pp.  6  and  7.    1956. 

10  Sand,  Reidar  FM  and  McNeely,  R.  L.     Underwater  television 
vehicle  for  use  in  fisheries  research.    Special  Scientific  Report- 
Fisheries  No.  193,  U.S.  Fish  and  Wildlife  Service,  Department  of 
the  Interior,  Washington  25,  D.C.,  December,  15  pp.    1956. 

11  Stephens,  F.  H.  Jr.,  and  Shea,  F.  J.   Underwater  telemeter  for 
depth  and  temperature.    Special  Scientific  Report-   Fisheries  No. 
181,  U.S.  Fish  and  Wildlife  Service,  Department  of  the  Interior 
Washington  25,  D  C  ,  June,  22  pp.    1956. 


Echo  traces  of  Tuna  in  the  North  Sea,  swimming  along  under  the  boat. 
[522] 


Photo:  J.  Scharfc 


FISH-FINDING  ON  THE  SALMON  FISHING  GROUNDS  IN  THE 

NORTH  PACIFIC  OCEAN 

by 
T.    HASHIMOTO  and   Y.    MANIWA 

Fishing  Boat  Laboratory,  Fisheries  Agency,  Japanese  Government 

Abstract 

fxpcrimcnts  with  echo  sounders  carried  out  from  a  research  vessel  in  Aleutian  waters  showed  that  salmon  could  be  easily  located 
using  frequencies  of  28  and  2(K)  kc.  Certain  points  in  the  behaviour  of  the  fish  were  noticed.  For  instance,  the  fish  generally  tended  to 
concentrate  at  the  DSL  (Deep  Scattering  Layer)  and  would  rise  and  fall  with  it,  but  on  some  occasions  a  purl  of  the  shoal  would  remain 
at  30  to  50  m.  all  day  long,  and  the  authors  suggest  that  means  should  be  dc\ised  for  catching  these  fish. 

Tests  were  made  with  Gain  Control  and  it  was  found  that  salmon  were  recorded  when  the  gam  of  the  amplifier  of  the  28  kc.  instru- 
ment was  about  120  db,  but  not  when  it  was  95  db.  To  avoid  the  effect  of  aeration  and  noise,  the  transducer  was  used  from  an  iron  arm 
suspended  from  the  side  of  the  boat,  rathei  than  ha\e  it  assembled  m  the  ship's  hull. 


Resume 


Detection  des  poissons  sur  les  funds  de  peche  a  saumon  dans  le  Pacifique  nord 


DCS  experiences  au  moyen  de  sondeurs  a  echo  efTectuccs  a  bord  d'un  na\ire  dc  recherche  dans  les  eaux  voisincs  des  lies  Aleouticnnes 
ont  montre  qif  il  clan  facile  de  detecter  les  saumons  en  ulilisant  des  frequences  de  28  el  de  20  kilocycles.  II  a  etc  possible  de  noter  ccrtaines 
particularites  du  comportement  du  poisson.  C'esi  ainsi  que  les  poissons  ont  en  gcncralcment  tendance  a  se  concentrer  vers  la  D  S  L  (deep 
scattering  layer)  qtfils  suivaicni  dans  ses  rcmontces  et  ses  descentes,  mais  a  plusieurs  reprises  une  panic  du  bane  est  res  tec  toute  la  journee 
entre  30  et  50  metres  el  les  uiileurs  proposent  de  metlrc  an  point  des  met  nodes  pour  capturer  ces  poissons. 

DCS  cssais  ont  etc  faits  en  reglant  I'amplificateur  et  Ton  a  pu  constater  la  presence  de  saumons  lorsque  ramplificatcur  de  1'appareil 
fonctionnant  sur  28  kilocycles  pcrmcttait  d'attcindrc  120  db..  mais  pas  lorsqu'il  n'atteignait  que  95  db.  Pour  compenscr  ('influence  dc  Iteration 
et  du  bruit,  on  a  utilise  le  sondeur  a  echo  non  pas  en  le  montant  dans  la  coque  mais  en  le  fixant  a  une  tigc  dc  fer  suspcndue  sur  cote 
du  bateau. 

Lonili/acion  de  peces  en  los  bancos  de  salmon  del  Oceano  Pucifico  septentrional 
Extracto 

Los  experiments  con  ccosondas  hechos  a  bordo  de  un  barco  de  investigaciones  en  aguas  dc  las  islas  Aleucianas.  demostraron  que 
es  posiblc  locali/ar  salmon  con  facihdad  usando  frecuencias  de  28  y  200  kc.  Durante  el  curso  de  estos  cstudios  se  obscrvaron  cicrtos  puntos 
rclativos  a  la  manera  en  que  reaccionan  los  peces;  por  cjcmpio,  gcncralmentc  tiendcn  a  conccntrarsc  en  la  capa  nrofunda  de  dispersi6n  del 
sonido  ("deep  scattering  layer*')  para  ascender  y  dcscendci  con  ella,  pero  en  algunas  ocasioncs  parte  del  cardumen  pcrmanece  entre  30  y  40. 
durante  todo  el  dia,  sugiriendo  los  autorcs  que  dcbcrian  idearse  mcdios  para  capturar  cstos  peces. 

Sc  hicieron  prucbas  de  control  de  volumcn,  registrandose  la  presencia  de  salmon  cuando  el  aumcnto  de  la  amplification  del  aparato 
que  trabajaba  con  frecuencias  de  28  kc.  fluctuo  alrededor  de  120  db..*  pero  no  se  ontuvieron  resultados  con  95  db.   Para  evitar  cl  efecto  dc 
la  aeration  y  del  ruido.  la  ccosonda  sc  mstalo  en  un  bra/o  dc  hicrro  suspendido  del  costado  del  barco  en  vcv  dc  montarla  en  la  quilla. 
*  db       decibel 


IN  June  and  August   1955  experiments  were  carried 
out  on  the  Aleutian  salmon  fishing  grounds  (fig.  1) 
by  the  Research  Vessel  Katori-maru.  With  a  wind 
velocity  of  generally  3  to  4  m./scc.,  the  height  of  the 
waves  was  almost  always  more  than  3  m.  with  fog  and 
rain. 

Ultrasound  of  28  kc.  and  200  kc.  was  tested.  The 
depth  scale  of  the  fishfinder  was  50  m.,  and  the  pulse 
rate  was  225/min.  The  transducer  was  not  installed  in 
the  ship's  bottom  but  fixed  to  a  vertical  pipe  which  was 
attached  at  70  cm.  distance  to  the  ship's  side.  The  depth 
of  the  transducer  was  1-5  m.  below  the  surface. 

RESULTS 

The  installation  of  the  transducer  at  the  ship's  side  was 
very  successful  in  avoiding  disturbing  aeration,  and  fish 
searching  could  be  continued  even  at  full  speed  (10 
knots),  which  was  impossible  in  the  prevailing  bad 


130*     140°     150°       160°     170°     180"     170°       160° 
Fig.  1.     The  Aleutian  salmon  fishing  grounds  J955. 


[523] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


Fig.  2.     Echogram  of  salmon  in  IS  to  40  m.  depth  obtained  with  28  kc.  on  17  July  1955.    Catch  19-8  fish} 40  m.  net. 


Fig.  3.     Echogram  showing  salmon  concentrated  in  the  Deep  Scattering  leaver  between  30  and  40  m.  depth. 


weather  with  the  conventional  installation  at  the  ship's 
bottom. 

With  equal  paper  width  a  measuring  range  of  50  m. 
is  superior  to  100  m.  for  detailed  observation  of  fish. 

The  higher  pulse  rate  of  225  pulses/min.  proved  to 
be  superior  to  the  144  pulses/min.  rate  for  recording 
fish.  A  short  pulse  length  is  preferable  for  recording 
fish  near  the  surface. 

It  was  found  that  the  distribution  of  the  salmon 
depended  on  the  time  of  day  (illumination  of  the  water). 
Much  attention  was  paid  to  this  point  and  the  echo- 
grams  have  accordingly  been  provided  with  time  indi- 
cators. 

The  density  of  the  present  salmon  schools  is  rather 
low.  Six  fish  per  40  m.  of  net  is  a  usual  catch  and  30  fish 
per  40  m.  of  net  is  considered  exceptionally  high.  The 
example  given  in  fig.  2  shows  a  28  kc.  record  of  a  rather 
good  school.  The  catch  was  in  this  case  19 -8  fish  per 
40  m.  net. 

Some  of  the  salmon  observed  in  30  to  40  m.  depth 
tended  to  ascend  after  sunset  and  descend  again  in  the 
morning.  Some  of  them  even  reached  the  surface. 


The  rest  of  the  salmon,  however,  remained  in  30  to 
50  m.  depth  all  day.  A  new  and  effective  method  should 
be  found  to  catch  the  latter  fish  as  well. 

The  salmon  obviously  concentrate  preferably  in  the 
Deep  Scattering  Layer  (DSL)  and  move  with  it.  Fig.  3 
shows  a  record  of  the  DSL  between  20  and  30  m.  depth 
on  14  July.  Plenty  of  Euphausiids,  probably  from  the 
DSL,  were  found  in  the  stomachs  of  salmon  caught 
here.  The  catch  was  11-2  fish  per  40  m.  of  net. 

The  DSL  appeared  at  the  clear  thermocline  where 
the  temperature  gradient  was  0-8  degree  C./cm.  on  21st 
July.  The  depth  of  the  DSL  and  the  fish  schools  change 
in  accordance  with  the  depth  of  the  thermocline.  Using 
ultrasound  of  200  kc.,  a  study  is  being  made  of  a  DSL 
which  appears  at  200  m.  depth  and  in  which  fish  schools 
were  found.  The  average  length  of  the  salmon  caught 
during  this -experiment  was  about  50  cm. 

REFERENCE 

Hashimoto  T.  and  Maniwa  Y. :  Fish-finding  experiments  on  the 
Salmon  fishing  grounds  in  the  North  Pacific  Ocean,  Technical 
Report  of  Fishing  Boat  Agency,  Nov.  8,  1956. 


[524] 


THE   QUANTITATIVE   USE   OF  THE   ECHO   SOUNDER   FOR 

FISH   SURVEYS 

by 
D.   H.   GUSHING 

Fisheries  Laboratory,  Lowestoft,  U.K. 

Abstract 

The  use  of  echo  sounders  for  fish  finding  is  now  almost  universal,  and  in  this  paper  me  author  shows  how,  by  the  use  of  Cathode 
Ray  Tube  presentation,  an  approximate  correlation  between  "catch"  and  "amplitude"  of  the  returned  signal  can  be  obtained. 

The  total  signal  amplitude,  counted  during  a  trawl  haul,  was  multiplied  by  the  square  of  the  depth  to  correct  for  dissipation  of 
energy  and  this  was  compared  with  the  square  root  of  the  number  of  baskets  of  fish  taken  during  the  haul.  In  fig,  1  the  catch/signal  relation- 
ship in  1955  and  1956  is  shown,  but  in  three  cases  out  of  40  observations  the  relationship  broke  down.  These  cases  were  of  high  signal 
strength  and  no  catch,  and  it  is  considered  that  the  error  was  due  to  fish  swimming  above  the  headline  of  the  trawl,  although  the  returned  signal 
apparently  suggested  that  they  were  actually  below  this  level,  which,  the  author  points  out,  is  quite  possible. 

The  value  of  C.R.T.  observations  as  a  means  of  surveying  a  fishing  area  is  shown  by  three  successive  runs  over  the  grounds  between 
Bear  Island  and  Spitzbergen,  and  during  a  ten  day  period  the  movement  of  the  cod  on  to  the  Bear  Island  Banks  was  clearly  shown. 


Resum* 


Utilisation  quantitative  de  sondeur  a  echo 


Les  sondeurs  a  echo  sont  maintenant  d'une  utilisation  presque  universellc  pour  la  detection  du  poisson.  Dans  cet  article,  1'auteur 
a  montr6  comment,  par  I'emploi  d'un  tube  £  rayons  cathodiques,  on  pouvait  obtenir  un  rapport  approximatif  cntre  la  "capture"  et  P  "ampli- 
tude" du  signal  renvoyt. 

L'amplitudc  totale  dcs  signaux  comptes  au  cours  d'un  trait  dc  chalut  a  £ te~  multiplier  par  le  carr£  de  la  profondeur  pour  compcnser  la 
perte  d'energie  et  cc  chiffre  a  6t£  compart  a  la  racine  carree  du  nombre  des  paniers  de  poisson  captures  pendant  le  trait.  La  figure  I  repr&cnte 
les  rapports  entre  la  capture  et  les  signaux  en  1955  et  1956  mais,  dans  trois  observations  sur  40,  ces  rapports  ne  se  sont  pas  verifies.  Dans 
ces  cas,  les  signaux,  avaient  unc  intensity  elevee  et  les  prises  furent  nulles.  On  s'explique  cette  erreur  par  le  fait  que  les  poissons  se  dgplacaient 
au-dessus  de  la  corde  de  dos  du  chalut,  bien  que  le  signal  rcnvoy&  ait  fait  pcnser  qu'ils  etaicnt  en  realite  au-dessous  de  ce  niveau  ce  qui,  comme 
le  fait  remarquer  Tauteur,  est  tout  a  fait  possible. 

L'interet  dc  ces  observations  r£alis£es  grace  au  tube  a  rayons  cathodiques  en  tant  que  moyen  de  prospecter  unc  region  de  peche,  a 
etc  ddmontrg  en  faisant  trois  passages  successifs  sur  les  fonds  situ6s  entrc  File  aux  Ours  ct  le  Spitzberg  ct,  ainsi  de  suivre  pendant  10  jours 
les  migrations  des  morues  vers  les  banes  de  Tile  aux  Ours. 

El  uso  cuantitativo  de  la  ecosonda 
Extracto 

El  uso  de  la  ecosonda  para  la  locali/acion  de  peces  es  actual  men  te  universal,  pero  el  autor  ha  demostrado  en  cste  trabajo  que  el 
empleo  de  un  tubo  de  rayos.cat6dicos  permite  obtener  una  correlaci6n  aproximada  entrc  la  "pesca"  y  la  "amplitud"  del  ceo. 

La  amplitud  total  de  las  senales  obtenidas  durante  un  lance  sc  multiplied  por  el  cuadrado  de  la  profundidad  para  corrcgir  la  disipacibn 
de  la  encrgia  y  el  resultado  se  compare)  con  la  raiz  cuadrada  del  numero  de  canastas  de  pcscado  que  const ituian  la  redada.  En  la  fig.  1  se 
muestra  la  rclaci6n  entre  la  pesca  y  las  senales  registradas  durante  los  anos  1955  y  1956,  la  cual  no  se  mantuvo  en  solo  3  de  las  40  obser- 
vaciones  que  se  efectuaron.  En  estos  casos  se  captaron  senales  de  gran  intensidad  sin  obtener  nada  de  pesca,  considerandose  como  causa 
de  error  el  hecho  de  que  los  peces  nadaban  sobre  la  relinga  de  boyas  de  la  red  de  arrastre,  si  bien  la  senal  de  vuelta  sugeria  que,  en  real i dad, 
estaban  bajo  estc  nivel,  lo  cual  segun  el  autor  seria  muy  posible. 

El  valor  de  las  observaciones  con  tubos  de  rayos  catodicos  como  medio  para  reconocer  o  evaluar  una  zona  de  pesca,  quedo  demos- 
trado por  medio  dc  tres  pasadas  succsivas  sobre  un  banco  entre  Bear  Island  y  Spitzbergen;  ademas,  durante  un  periodo  de  10  dias  se  establecio 
claramente  el  movimiento  de  bacalao  en  los  bancos  de  dicha  isla. 


GENERAL 

THE  recording  type  of  echo  sounder  has  been  in 
use  since  1948  for  echo  survey,  the  amplification 
being  adjusted  so  that  the  recording  paper  was 
kept  clean  (without  noise  showing);  the  fish  echoes  were 
recorded  necessarily  at  a  high  signal-to-noise  ratio. 
The  quantity  of  fish  was  estimated  by  measuring  the 
width  of  the  fish  traces  per  unit  time.  The  rough  correla- 
tion obtained  between  signal  distribution  and  fish 
distribution  showed  that  the  signal-to-noise  ratio  was 
sufficient,  the  power  output  was  steady  enough,  and  that 
the  variations  in  depth  were  not  too  great1. 

Klust2  has  shown  that  heavier  catches  of  fish  were 
associated  with  signals  of  greater  amplitude  on  the 


cathode  ray  tube  presentation  of  the  echo  sounder. 
In  an  echo  survey  in  the  Bear  Island  area,  we  measured 
cathode  ray  tube  signals  from  fish  at  three  levels  of 
amplitude  in  the  first  fathom  layer  above  the  bottom3. 
Thus  the  minimum  signal-to-noise  ratio  was  reduced 
from  perhaps  10  to  only  3  times,  and  the  method, 
therefore,  could  only  be  successful  if  the  power  output 
was  constant  over  the  period  of  examination. 

METHOD 

A  preliminary  examination  showed  that  a  particular 
signal  close  to  the  bottom  but  separated  from  it  should 
be  disregarded.  We  learned  to  distinguish  this  **X" 


[525] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


echo  from  a  fish  echo  by  its  dull  tone,  sharp  rise  time 
and  serrated  edge;  it  was  probably  a  weak  bottom  signal 
received  from  an  angular  range.  A  fish  echo  was  smoother 
with  slower  rise  time,  and  brighter,  the  amplitude  being 
generally  less. 

In  order  to  measure  the  amplitudes  the  time  base  was 
covered  with  a  dark  strip  of  paper,  which  excluded  the 
noise  traces.  Three  lengths  of  cotton  were  placed  on 
the  screen  parallel  to  the  time  base,  so  signals  could  be 
classified  in  three  levels  of  amplitude;  the  magnitude 
of  the  levels  was  maintained  with  the  use  of  a  signal 
injector  in  series  with  the  receiver.  Signals  were  counted 
by  noting  the  duration  in  seconds  of  signals  at  each 
level  of  amplitude. 

The  levels  of  amplitude  were  frequently  checked  and 
noise  measurements  were  frequently  made.  The  trans- 
ducer was  mounted  on  a  stalk  extending  2  ft.  from  the 
ship's  hull  to  reduce  the  number  of  missing  signals. 
At  the  expense  of  5  //V.  of  noise,  the  number  of  missing 
signals  was  reduced  from  an  average  of  30  to  50  per  cent, 
down  to  about  5  to  10  per  cent.  So  the  counting  method 
could  be  employed  up  to  a  state  of  sea  and  swell  5  and  6. 
If  noise  increased  too  much,  the  counting  levels  were 
increased  to  maintain  the  signal-to-noise  ratio,  so  some 
sensitivity  was  lost. 

THE  RELATION  BETWEEN  CATCH  AND  SIGNAL 

The  signals  counted  were  received  from  a  spherical 
shell  1  fm.  thick,  of  radius  equal  to  the  depth.  Its  extent 
in  angle  from  the  axis  depends  upon  the  directivity  pattern 
of  the  transducers  and  the  qualities  of  the  object  from 
which  signals  are  received.  As  it  was  impossible  whilst 
counting  to  distinguish  between  a  single  fish  and  a  shoal 
of  fish,  the  effect  of  directivity  was  not  dealt  with  ex- 
plicitly. The  total  signal  counted  during  a  trawl  haul 
was  multiplied  by  the  square  of  the  depth  to  correct 
for  the  dissipation  of  energy. 

The  catch,  using  the  "basket"  as  a  unit,  was  trans- 
formed by  the  square  root.  It  is  assumed  that  the  signals 
counted  were  usually  received  from  a  shoal  offish.  When 
catches  were  large,  multiple  signals  were  sometimes 
observed. 

There  are  three  sources  of  error  within  the  method 
of  counting,  that  due  to  a  varying  directivity  pattern, 
that  due  to  differences  in  the  sizes  of  fish  shoals,  and  that 
due  to  the  inability  to  count  multiple  signals  properly. 
There  is  a  further  source  of  error  which  is  an  effect  of 
the  beam  angle  of  the  transducers  used  (size  9x  14  cm.; 
wavelength =5  cm.).  A  signal  presented  in  the  last 
fathom  of  the  cathode  ray  tube  may  be  received  from  any 
part  of  the  shell  of  the  spherical  wave  tangent  to  the 
seabed  which  is  limited  by  the  respective  angle  of 
directivity.  Thus  a  signal  recorded  in  the  last  fathom  of  the 
cathode  ray  tube  may  be  received  from  a  height  of  more 
than  1  fm.  from  the  bottom.  As  it  is  probable  that  a 
fish  swimming  more  than  1  fm.  off  the  bottom  is  swimming 
above  the  headline  of  the  trawl,  the  catch-signal  relation- 
ship in  such  cases  can  break  down.  For  survey  purposes 
this  does  not  matter,  but  for  fishing  purposes  it  does. 

On  fig.  1,  which  shows  the  catch-signal  relationship 
established  in  the  Barents  Sea  in  1955  and  1956,  there 
are  three  observations  (indicated  as  squares)  out  of 
forty  of  high  signal  and  no  catch  where  the  relationship 


is 


fc 


Cruis«  V   1955   RV 
Crui»t  IV  1956     -• 


0*7) 


IO 


IS  D2«0"7 


Fig.  I.     The  relation  between  the  amount  of  catch  (cot/)  and 
received  fish  echo  signals.    Barents  Sea,  June/ July,  7055/56 
5-  signals  *  time;    D^- depth.     Squares    indicate    observations 
when  fish  presumably  above  the  headline. 

broke  down  probably  because  the  fish  actually  were 
above  the  headline.  There  is  a  possibility  that  this  may 
happen  more  often  when  the  fish  are  abundant;  con- 
sequently, it  may  be  a  greater  source  of  error  for  com- 
mercial vessels. 

Different  species  have  not  been  distinguished;  in 
fact,  it  is  quite  reasonable  to  treat  haddock  as  if  they 
were  small  cod.  On  one  occasion  signals  received  from 
catfish  were  less  than  expected  in  comparison  with  cod; 
on  another,  when  they  were  more  abundant,  a  signal 
was  observed  which  looked  on  the  cathode  ray  tube 
like  a  thick  flat  plate  on  the  bottom.  Catfish  have  no 
air  bladder,  so  the  signal  received  from  them  is  probably 
less  than  that  received  from  cod. 

The  most  remarkable  result  from  the  observations 
given  in  fig.  1  is  that  those  of  1955  are  indistinguishable 
from  those  of  1956.  Hence  the  power  output  must  have 
remained  sufficiently  constant  to  allow  us  to  work  at 
a  fairly  low  signal-to-noise  ratio. 

Two  results  of  importance  emerge: 

1 .  the  steady  power  output  of  the  echo  sounder  used 
and  the  steady  level  of  water  noise  permitted  a 
quantitative  use  of  the  instrument. 

2.  the  catch-signal  relation  is  necessarily  rough,  but  it 
would  allow  contour  levels  of  catch  to  be  drawn 
on  survey  at  1,5,  25  baskets/hr.4. 

THE  USE  OF  THE  METHOD  ON  SURVEY 

If  it  is  possible  to  predict  roughly  the  catch  in  a  given 
trawl  haul,  when  steaming  at  3J  knots,  it  is  possible  to 
search  for  the  area  of  highest  abundance.  To  do  this, 
the  ship  has  to  steam  faster  and  the  noise  level  may  be 
high.  At  full  speed  the  noise  is  a  little  greater  than  at 
trawling  speed  with  the  trawl  down;  at  a  little  less  than 
full  speed,  the  noise  is  reduced.  The  mam  component 
of  noise  comes  from  the  propeller. 

An  example  of  such  a  survey  is  shown  in  fig.  2.  The 
first  survey  was  carried  out  by  steaming  on  a  gridded 
course  across  the  edge  of  the  Bear  Island  Bank  and  back 
again  from  Bear  Island  to  Spitzbergen.  Trawl  hauls 


[526] 


QUANTITATIVE     USE    OF    ECHO    SOUNDER 


1-5 
6-10       6-25 
11-39      26  «• 
40* 


Fig.  2      Successive  echo  surveys  made  in  the  Barents  Sea   in  June  1956.    Note  the  movement  of  the  fish  on  the  bank. 


were  made  at  frequent  intervals  to  check  that  the  signals 
were  still  being  received  from  cod,  the  species  in  which 
we  were  interested.  A  second  survey  was  made  on  the 
return  journey  as  an  immediate  check  on  the  first.  It 
will  be  seen  that  the  main  features  are  repeated  with  some 
minor  variations.  The  third  survey  was  carried  out  in 
the  same  way  about  ten  days  later.  In  this  short  time 
fish  have  moved  to  the  shelf  from  the  Storfjordrenna  and 
from  the  N.W.  Gulley;  in  depth  they  have  moved  from 
about  1 50  fm.,  to  70  to  100  fm. 

The  value  of  this  technique  as  a  research  tool  is 
obvious,  but  there  is  no  reason  why  a  fisherman  should 
not  make  some  sort  of  survey  of  this  type  before  starting 
to  fish.  As  an  aid  to  fishing  on  a  particular  ground,  it  is 
possible  that  the  catch-signal  relationship  is  not  suffi- 


ciently accurate  to  show  when  to  haul,  but  it  can  be 
used  to  show  where  the  fish  were  during  a  haul. 

REFERENCES 

1  Gushing,    D.    H.    tcho-survcys   of  fish.    Journ.    du   Cons. 

XVIII,  1,  p.  45.  1952. 

2  Klust,  G.  Ober  die  Grtissenschatzung  von  Schlcppnetzfangen 
auf  Grund  der  Echoan/cigen  der  ELAC-Fischlupe.     Archiv  fur 
Fischereiwissenschaft  III  3/4.  p.  146.  1951. 

3  Cashing,  D.  H.  and  Richardson,  I.  D.  New  Echo  Sounding 
Methods  at  Bear  Island.   World  Fishing,  Vol.  4,  10,  p.  18.  1955. 

4  Cushing,  D.  H.  Studies  on  plankton  populations.    J.  du  Cons., 

XIX,  1,  p.  3.  1953. 

6  Cushing,  D.  H.  and  Richardson,  I.  D.  Echo  Sounding 
Experiments  on  Fish.  Fish.  Invest.  Ser.  II,  XVIII,  4.  1955. 

•  Richardson,  I.  D.,  Cushing,  D.  H.,  Harden  Jones,  F.  R., 
Beverton,  R.  J.  H.  and  Blacker,  R.  W.  Research  on  Echo  Sounding 
in  the  Barents  Sea.  (in  press) 


[527] 


LOCATING  HERRING  SCHOOLS  ON  THE  ICELANDIC  NORTH 

COAST  FISHING  GROUNDS 

by 

JAKOB  JAKOBSSON 

The  University  Research  Institute,  Department  of  Fisheries,  Reykjavik,  Iceland 

Abstract 

The  Icelandic  north  coast  herring  fishery  has  until  1944  always  been  essentially  an  inshore  fishery.  Now,  however,  it  has  become 
almost  an  oceanic  one,  exploited  by  Norwegian,  Russian  and  Swedish  vessels,  in  addition  to  the  large  Icelandic  fleet.  This  move  to  off-shore 
waters  has  made  the  location  of  shoals  more  difficult,  but  the  shoaling  behaviour  of  the  herrings  is  such  that  the  use  of  both  Asdic  and  airplanes 
for  this  purpose  is  possible. 

The  shoals  often  break  the  surface  in  the  evening  and  early  morning  and  when  the  scouting  planes  locate  these,  they  direct  the  fleet 
towards  them.  A  ship  using  Asdic  can  locate  the  shoals  before  they  break  surface  and  before  they  can  be  seen  from  the  planes,  so  by  close 
co-operation,  the  Asdic  and  spotting-plane  become  formidable  weapons  of  attack. 

In  1956,  in  spite  of  many  cancellations  due  to  bad  weather,  some  750,000  square  miles  of  sea  were  surveyed,  and  the  two  planes 
located  herring  shoals  on  60  occasions,  but  it  is  impossible  to  gauge  accurately  the  increase  in  total  catch  produced  by  these  services. 


Resume 


Rep£rage  des  banes  de  harengs  sur  les  lieux  de  pdche  de  la  cdte  septentrionale  de  1'Islande 


Sur  la  cote  septentrionale  dc  1'Islande,  la  pechc  au  hareng  avait  toujours  £te*,  jusqu'en  1944.  une  peche  essentiellcment  cdtiere. 
Mais  maintenant  cllc  est  devenue  presque  oceanique  et  est  pratiquee  par  des  navires  norvlgians,  russcs  et  suedois,  sans  comptcr  I'importantc 
flotte  de  pdche  islandaise.  Cc  emplacement  dc  la  peche  vers  le  large  a  rendu  plus  difficile  le  reperagc  des  banks  de  harengs,  mais  le  comporte- 
ment  de  ces  banes  est  tel  qiTil  permet  1'emploi  d'apparcils  Asdic  et  d'avions. 

Les  banes  remontent  souvent  &  la  surface  dans  la  soiree  et  au  debut  dc  la  matinee,  et  lorsque  les  avions-eclaircurs  les  ont  repe're's  ils 
dirigent  la  flotte  de  peche  sur  leur  emplacement.  Un  navire  equine  de  r Asdic  peut  reperer  les  banes  avant  qu'ils  ne  remontent  en  surface  et 
qu'ils  puissent  etrc  vus  par  les  avions,  en  sorte  qif  utilises  en  etroite  cooperation,  1' Asdic  ct  1'avion-e'claircur  constituent  des  armes  d'attaque 
d'une  efficacite  considerable. 

En  1956,  malgre  le  mauvais  temps  qui  a  limitd  le  nombre  des  vols,  750,000  milles  carrcs  de  mer  ont  ete  explores,  et  les  deux  avions 
ont  repere"  &  60  reprises  des  banes  de  harengs,  mais  il  est  impossible  de  determiner  avec  precision  la  quantite  supplcmentaire  de  poissons 
captures  grace  £  la  mise  en  oeuvre  de  cet  cquipement. 

La  localization  de  cardumencs  de  arenque  en  los  bancos  pesqueros  de  la  costa  septentrional  de  Islandia 
Extracto 

Hasta  1 944,  la  pesca  de  arenque  en  la  costa  septentrional  dc  Islandia  era  en  su  mayor  parte  de  bajura.  Sin  embargo,  en  la  actualidad, 
noruegos,  rusos,  suecos  y  la  gran  flota  islandesa  la  han  tornado  de  altura.  Este  alejamiento  de  las  aguas  costeras  dificulta  la  Iocalizaci6n 
de  los  cardumencs,  nero  la  tendencia  del  arenque  a  congregarse  en  bancos  permitc  el  uso  de  "asdic"  y  acroplanos. 

Los  cardumenes  a  menudo  afloran  durante  el  crepusculo  y  en  las  primcras  horas  de  la  maflana,  permit iendo  a  los  aeroplanos  loca- 
lizarlos  y  dirigir  la  flota  pesquera  hacia  ellos.  Un  barco  provisto  de  "asdic"  puede  locali/ar  los  bancos  dc  peces  antes  de  que  afloren  a  la 
superficic  y  scan  oteados  por  los  aeroplanos.  For  este  motivo,  la  estrecha  cooperaci6n  entre  el  "asdic"  y  el  aeroplano  se  ha  transformado 
en  una  formidable  arma  dc  ataque. 

En  1956,  a  pesar  de  las  numerosas  salidas  canceladas  a  causa  del  mal  tiempo,  dos  aviones  pudieron  inspeccionar  750,000  millas 
cuadradas  local izando  cardumenes  en  60  ocasiones;  no  obstante,  ha  sido  imposible  determinar  con  precision  cl  aumento  de  la  pesca  total 
que  se  obtuvo  con  ayuda  de  estos  medios. 


GENERAL 

AS  an  industry  of  importance  the  Icelandic  North 
Coast  herring  fishery  dates  back  to  the  'eighties. 
Purse  seining  is  the  main  fishing  method  although 
driftnets  have  been  used  as  well.    The  season  usually 
begins  in  late  June  or  early  July  and  ends  after  the  middle 
of  August  or  in  September.   Before  1944,  fishing  took 
place  mainly  in  inshore  waters,  but  in  recent  years  it  has 
largely  developed  into  an  off  shore  fishery  in  which  a  great 
number  of  Norwegian,  Russian  and  Swedish  as  well  as 
Icelandic  vessels  take  part. 

The  Icelandic  purse  seine  fishery  has  proved  successful 
because  of  the  compactness  of  the  schools  and  the 
frequent  surfacing  of  the  fish,  so  that  they  can  be  located 


visually.  As  a  rule  the  schools  break  the  surface  soon 
after  sunset  and  just  before  sunrise,  so  that  in  the 
beginning  of  July  the  schools  can  be  spotted  just  before 
and  after  midnight.  Later  on,  as  the  time  interval 
between  sunset  and  sunrise  increases,  the  times  of  sur- 
facing change  accordingly. 

The  schools  can  be  spotted  from  the  top  of  the  wheel- 
house  of  a  fishing  boat  at  a  distance  of  anything  up 
to  5  nautical  miles.  From  far  away  they  are  observed 
as  small  dark  spots,  almost  like  a  patch  of  soot  floating 
on  the  surface  of  the  sea  (fig.  1). 

In  fact,  when  coal  was  still  the  main  source  of  fuel  in 
the  fishing  fleet,  many  a  soot  patch  was  mistaken  for  a 


[528] 


LOCATING     HERRING     SCHOOLS 


Fig.   I.     Herring  school  in  I  he  Mtrjace. 


herring  school  at  a  distance.  A  closer  view  shows 
thousands  of  dark  blue  herring  backs  breaking  surface. 

The  horizontal  spread  of  a  typical  school  does  not 
exceed  100  sq.  m.  while  its  vertical  spread  may  reach 
30  to  40  m.  Great  variations  arc  observed,  so  that  the 
schools  are  sometimes  very  shallow  with  enormous 
horizontal  spread;  sometimes  they  do  not  break  the 
surface  at  all  or,  in  extreme  cases,  they  may  surface  in 
the  middle  of  the  day  in  bla/ing  sunshine.  Nothing  can 
be  taken  for  granted  and  "typical*'  really  means  the 
behaviour  most  frequently  observed. 

The  purse  seine  nets  generally  used  by  Icelandic 
fishermen  nowadays  measure  approximately  400  *  80  m. 
Each  shot  usually  takes  from  1  to  3  hours,  depending  on 
the  catch  per  shot  which  may  range  from  nil — if  the 
school  dives  before  the  net  can  be  closed — to  some 
200  metric  tons.  This  shows  that  the  schools  may  be 
extremely  compact. 

Owing  to  the  relatively  small  horizontal  spread  and  the 
short  time  that  the  schools  can  be  seen  each  day,  the 
fishing  boats  may  easily  miss  the  herring  concentrations. 
In  a  fishery  of  this  kind  aerial  scouting  would  clearly  be 
advantageous  because  the  schools  can  be  seen  very  dis- 
tinctly from  an  airplane  and  a  large  area  can  be  surveyed 
in  a  short  time. 

Due  to  the  fact  that  the  Icelandic  north  coast  herring 
is  usually  found  in  compact  schools,  vertical  echo- 


surveying  is  of  limited  value  and  may  even  be  misleading 
because  the  chances  of  missing  many  of  the  schools 
are  extremely  great.  On  the  other  hand,  the  compactness 
of  the  schools  makes  it  possible  to  detect  them  effectively 
by  using  horizontal  ranging  equipment  such  as  Asdic 
or  whale  finders.  Both  aerial  and  Asdic  surveys  are  made 
during  the  Icelandic  north  coast  herring  season. 

AERIAL  SCOUTING 

Aerial  scouting  along  the  north  coast  fishing  grounds 
started  as  early  as  1928  when  single  engined  Junkers 
seaplanes  were  used.  From  1931  to  1938  the  scouting 
was  irregular  but  since  1938  it  has  been  of  an  ever 
increasing  importance  to  the  fishery. 

Before  1938  the  immediate  value  of  aerial  scouting 
was  greatly  diminished  because  the  radio  telephone  had 
not  yet  become  a  standard  part  of  the  vessels'  "fishing" 
equipment.  There  were  no  means  by  which  the  results 
of  aerial  scouting  could  be  effectively  communicated. 

In  the  last  few  years  two  airplanes  have  been  chartered 
for  aerial  scouting  and  have  been  operated  from  the 
shore  base  at  Akureyri,  the  position  of  which  is  shown 
on  fig.  2. 

Each  crew  consists  of  two  pilots  and  an  experienced 
herring  skipper  while  two  engineers  at  the  shore  base 
prepare  the  planes  for  each  flight.  One  plane  is  a  twin- 


[529] 


KK 


MODERN    FISHING    GEAR    OF    THE    WORLD 


fig.  2.     Chart  of  the  Icelandic  North  Coast  showing  shore  base  for  aerial  scouting  and  the  main  herring  landing  places. 


engined  De  Havilland  Rapide,  the  other  is  a  twin- 
engined  Beachcraft  C.I 8.  Both  airplanes  are  capable 
of  flying  considerable  distances  on  one  engine  only, 
so  the  added  security  of  using  flying  boats  is  not  con- 
sidered sufficient  to  warrant  their  use,  in  view  of  their 
higher  maintenance  costs. 

In  1956  the  two  planes  were  operated  for  50  days, 
from  the  3rd  July  to  23rd  August,  the  total  flying  time 
amounting  to  286  hours  during  which  37,800  miles 
(statute)  were  covered.  After  many  years  of  experience 
it  has  been  found  that  the  optimum  flying  height  for 
herring  scouting  is  about  800  ft.,  from  which  herring 
schools  can  be  spotted  up  to  a  distance  of  10  miles  so 
that  in  clear  weather  the  plane  covers  an  observational 
track  20  miles  in  width.  In  1956  an  area  of  approx- 
imately 750,000  square  miles  was  surveyed,  in  spite  of 
many  cancelled  flights  because  of  fog  or  gales  or  because 
the  base  of  clouds  was  well  below  800  ft.  If  the  plane 
is  forced  to  fly  much  lower  than  the  optimum  height 
the  observational  field  changes  too  quickly  for  proper 
inspection.  When  herring  schools  are  located  their 
position  and  approximate  size  are  broadcast  to  the  fishing 
fleet,  but  very  often  the  planes  fly  to  the  nearest  group 
of  vessels  and  actually  guide  them  to  the  schools.  This 
practice  has  proved  necessary  in  many  cases  when, 
far  from  the  coast,  the  fishermen  have  some  difficulty 
in  fixing  their  position. 


During  the  above  period  the  two  planes  located 
herring  schools  on  60  occasions  but  it  is  difficult  to 
estimate  the  increase  in  the  total  catch  resulting  from 
the  aerial  scouting.  In  most  of  the  instances,  however, 
the  herring  boats  were  guided  from  areas  where  herrings 
were  very  scarce  to  others  containing  workable  schools 
in  much  larger  quantities,  so  that  this  service  is  greatly 
appreciated  by  the  fishermen. 

ASDIC  SURVEYING 

In  the  autumn  of  1953,  a  Kelvin-Hughes  Asdic  Whale 
Finder  was  installed  in  the  Icelandic  research  ship, 
Aegir,  and  since  then  Asdic  surveys  have  been  carried 
out  on  the  North  Coast  herring  grounds.  During  these 
surveys,  the  best  results  have  been  obtained  by  aiming 
the  centre  of  the  Asdic  beam  horizontally  with  as  small 
vertical  or  lateral  spread  as  possible.  The  maximum 
range  of  the  Finder  is  2,000  yards  and,  during  its  four 
years  of  service  in  the  Aegir  herring  schools  have  fre- 
quently been  recorded  at  that  distance. 

During  surveys  the  Asdic  Whale  Finder  and  a  Simrad 
echo  sounder  are  kept  running  continuously.  The  Asdic 
beam  is  then  kept  stationary  and  directed  60  degrees  to 
90  degrees  to  one  side  or  used  like  a  searchlight,  slowly 
sweeping  through  180  degrees. 

Fig.  3  shows  sample  echograms  of  the  Asdic  (top) 


[530] 


LOCATING     HERRING     SCHOOLS 


V      <  "'«•  ,*  "'(•*'      '   "»  yj*!  » 


Fig.  3.     EchograntA  of  Asdic  (lop)  and  vertical  echo  sounding 
(helow)  showing  traces  of  herring  schools. 

and  of  the  Simrad  echo  sounder  (bottom).  On  the  Asdic 
record  the  diagonal  dark  hands  represent  the  fish  schools. 
Their  distance  from  the  ship  can  be  found  by  calibrating 
the  width  of  the  paper  from  0  to  2,000  yards,  the  zero 
line  being  at  the  top.  On  this  occasion  the  beam  was  at 
first  aimed  70  degrees  to  one  side  so  that  the  trace  of  the 
schools  disappeared  soon  after  they  had  been  observed 
at  a  considerable  distance  from  the  ship.  Thus,  the 
school  marked  "I"  came  into  view  at  the  maximum 
range  of  about  2.000  yards  and  disappeared  at  a  distance 
of  1,700  yards.  On  the  other  hand  the  school  marked 
"2"  came  into  view  only  some  500  yards  away  and  by 
heading  the  ship  to  this  target  it  was  kept  in  "sight" 
all  the  time  until  the  ship  passed  over  it  so  that  it  now 
could  be  "picked  up"  by  the  echo  sounder  (trace  marked 

X>- 

Now  and  again  it  is  found  desirable  to  check  by  echo 

sounding  the  depth  and  size  of  a  school  selected  at 
random.  Some  indication  of  si/e  can  also  be  obtained 
by  considering  the  average  maximum  distance  at  which 
traces  of  the  schools  are  recorded.  The  greater  the 
distance  the  more  compact  must  the  schools  be  assuming 
constant  acoustic  conditions.  At  half  hourly  intervals 


the  number  of  contacts  observed  on  either  instrument 
is  recorded  in  a  special  Asdic/Echo  sounder  logbook, 
also  noting  the  average  maximum  distance  at  which  the 
schools  come  into  view,  their  size  and  depth  according  to 
vertical  sounding,  bottom  depth,  weather  conditions,  the 
ship's  course  and  speed  as  well  as  observations  taken  at 
any  scientific  stations  that  may  have  been  worked  during 
the  period  in  question.  By  plotting  on  a  chart  the  number 
of  schools  along  the  ship's  route,  a  fairly  comprehensive 
picture  of  the  herring  distribution  is  obtained.  In  1956 
the  Acgir  broadcast  information  regarding  the  herring 
distribution  on  45  occasions.  On  some  of  these  the  fishing 
fleet  was  informed  of  new  herring  concentrations  and, 
in  other  cases,  a  general  summary  of  plankton  and 
hydrographical  information  with  information  on  the 
herring  distribution. 

Many  valuable  records  regarding  the  relation  between 
hydrographical  information  was  given  with  information 
on  the  herring  distribution. 

THE    TWO    METHODS    COMPLEMENT    EACH 
OTHER 

When  the  Icelandic  north  coast  inshore  herring  fishery 
changed  to  an  off-shore  or  even  an  oceanic  one,  the 
fishing  grounds  increased  enormously  in  size.  As  a  result, 
the  problem  of  locating  the  herring  concentrations 
became  increasingly  difficult  but  fortunately  it  has  been 
possible  to  find  a  partial  solution  by  employing  modern 
techniques.  Large  areas  are  now  surveyed  quickly  by 
airplanes  but  the  results  of  these  surveys  are  dependent 
on  the  diurnal  schooling  behaviour  of  the  herring. 
If,  for  some  reason  or  another,  they  do  riot  come  to 
the  surface,  aerial  scouting  is  useless.  On  the  other 
hand,  an  Asdic  survey  by  a  research  ship  can  find  herring 
concentrations  regardless  of  the  depth  of  the  schools 
(within  limits).  The  result  of  this  kind  of  survey  is, 
however,  dependent  on  the  compactness  of  the  schools 
and  the  searchers  are  handicapped  by  the  relatively 
small  area  covered  by  ships  as  compared  with  that 
covered  by  the  'planes.  The  two  methods  must  be  con- 
sidered as  complementary.  By  an  Asdic  survey  it  is 
often  possible  to  get  a  general  picture  of  the  herring 
distribution  before  the  schools  come  to  the  surface, 
and  areas  of  herring  concentrations  will  be  watched 
very  closely  by  the  'planes.  The  closest  possible  co- 
operation between  the  two  kinds  of  herring  surveying 
methods  is  thus  of  the  greatest  importance. 


531] 


DISCUSSION   ON  FISH   DETECTION 


Dr.  J.  Scharfe  (FAO),  Rapporteur:  As  the  scope  of  this 
Congress  demands,  the  papers  submitted  on  this  subject  deal 
exclusively  with  the  most  progressive  developments  in  fish 
detection  —of  which  echo  sounding  and  ranging  are  obviously 
the  most  important  ones.  Only  one,  namely  Jakobsson's 
paper,  describes  another  method,  i.e.  aerial  scouting. 

The  main  value  of  this  latter  method  consists  in  the  possi- 
bility of  surveying  large  areas  in  a  short  time.  On  the  other 
hand  it  is  restricted  to  fish  shoals  swimming  at  or  at  least 
very  close  to  the  surface  and  to  favourable  weather  conditions. 
But,  in  spite  of  these  limitations,  certain  fisheries  can  gain 
great  advantage  by  using  it.  In  the  present  paper  it  is  shown 
that  with  not  more  than  two  planes,  a  whole  fishing  fleet  can 
successfully  be  assisted  by  surveying  big  areas,  broadcasting 
the  observations  to  the  fishing  boats  and  thereby  often  leading 
them  from  poor  fishing  areas  to  more  profitable  ones. 

The  great  value  of  echo  sounding  for  certain  fisheries, 
valuable  to  find  suitable  ground  for  bottom-touching  fishing 
particularly  bottom  trawling,  must  be  shortly  mentioned. 
Its  importance  for  navigation  in  shallow  waters,  checking 
D.R.  (Dead- Reckon  ing)  position  in  finding  well  known 
fishing  places,  as  well  as  keeping  to  a  desired  depth  when 
towing  is  commonly  acknowledged.  Alvcrson  gives,  from  the 
offshore  trawl  fishery  of  the  U.S.  West  coast,  an  example  of 
how  echo  sounding  can  increase  the  fishable  depth  and 
thereby  the  operational  area.  Apart  from  the  depth  also 
shape  and  composition  of  the  bottom  is  often  important. 
Both  Craig  and  Hashimoto  and  collaborators  show  how  the 
representation  of  such  bottom  characteristics  can  be  improved 
by  choosing  higher  or  different  frequencies  and  a  sharp  sound 
beam.  The  knowledge  of  the  bottom  conditions  is  not  only 
valuable  to  find  suitable  ground  for  bottom-touching  fishing 
gear  as  trawls  or  Danish  seines,  and  protect  them  from 
damage,  but  may  also  be  helpful  as  an  indication  of  the  kind 
of  fish  likely  to  be  caught.  This  remark  leads  back  to  fish 
detection  as  the  main  subject  of  the  present  report. 

The  possibility  to  locate  fish  acoustically  was  proved  by 
means  of  normal  echo  sounders  designed  for  depth  measure- 
ment only.  Of  course,  such  apparatuses  are  not  optimal  for 
this  new  purpose  and,  due  to  the  efforts  of  physicists  from 
many  parts  of  the  world,  remarkable  technical  progress  has 
been  achieved  since. 

Fish  are  less  effective  reflectors  than  the  sea  bottom,  in 
order  to  get  a  sufficient  working  range  for  fish,  the  sensitivity 
of  the  equipment  has  to  be  increased.  As  a  first  step  in  this 
direction  the  receiving  amplification  was  increased  up  to  the 
level  of  the  disturbing  noise.  Furthermore,  good  success 
was  gained  in  decreasing  this  disturbing  noise  itself  by  optimal 
placing  of  the  transducers  in  the  ship's  bottom,  at  a  short 
distance  below  the  ship's  bottom  or  even  at  the  ship's  side 
(see  Hashimoto  and  Maniwa).  Thereby  the  working  range 
for  worthwhile  fish  schools  could  be  extended  to  about  250  m. 
But  for  deep  sea  trawling,  which  is  carried  out  in  depths  down 


to  about  600  m.,  and  then  particularly  for  the  detection  of 
more  scattered  white  fish,  this  performance  proved  to  be 
insufficient.  For  further  improvement  the  "effective"  trans- 
mitting power  had  to  be  intensified.  This  has  recently  been 
done  by  increasing  the  sound  output.  Another  possibility, 
the  narrowing  of  the  sound  beam,  which  is  proposed  in 
Vestnes'  contribution,  would  also  have  other  advantages  but 
at  the  same  time  leads  to  certain  difficulties  due  to  ship's 
movements  in  rough  sea.  Jt  would  therefore  require  measures 
for  stabilizing  the  transducers  and  some  methods  on  how  this 
could  be  done,  are  described  in  the  paper  of  Craig. 

Besides  the  mere  knowledge  of  the  presence  and  the  depth 
of  fish,  more  detailed  information  was  soon  wanted  such  as 
on  the  size  of  fish  schools  and  the  species  and  therefore  more 
suitable  methods  of  representation  were  needed.  As  is  well 
known,  two  methods  of  representing  the  echo  traces  are 
generally  used  now:  firstly,  the  recording  on  wet  or  dry  paper 
and  secondly,  the  reproduction  by  means  of  a  cathode  ray 
tube  (CRT).  The  characteristics  of  both  methods  arc 
described  in  detail  in  the  papers  of  Cushing,  Woodgate, 
Haines,  Hashimoto  and  Maniwa,  and  Craig.  Each  of  these 
two  methods  has  certain  advantages  and  a  combination  of 
both  in  one  and  the  same  unit  is,  therefore,  the  optimal 
solution  for  acoustical  fish  location. 

Such  combined  fish  sounders  came  on  the  market  some 
years  ago.  Besides  such  combined  units,  now  also  an  existing 
recorder  can  be  combined  with  an  existing  CRT  set  or  a  special 
C  RT  apparatus  can  later  on  be  added  to  an  existing  recorder. 
Such  dual  apparatuses,  particularly  in  combination  with 
high  sound  output,  are  doubtless  the  most  progressive  but, 
of  course,  also  the  most  expensive  ones.  Therefore,  and  also 
because  of  their  great  size,  they  are  mainly  used  by  the  bigger 
craft  as  deep  sea  trawlers. 

The  main  advantage  of  the  CRT  is  its  true  quantitative 
representation  of  the  echo  amplitude  and  the  possibility  to 
easily  expand  the  reproduction  scale  as  desired.  Both  these 
qualities  favour  the  resolving  power  which,  as  is  well  known, 
is  of  the  greatest  importance  for  detailed  observation  of  fish, 
particularly  bottom  fish.  On  the  CRT  the  representation 
of  a  single  sounding  is  complete  in  itself  so  that,  when  in 
rough  weather  some  or  several  soundings  are  lost  because  of 
aeration,  those  which  do  come  through  are  complete  and  give 
all  details.  But  unfortunately  the  CRT  has  no  memory  and 
therefore  requires  continuous  attention  on  the  part  of  the 
operator  who  must  act  as  a  kind  of  integrating  machine 
comparing  the  complex  of  echo  frequency  and  character  from 
area  to  area,  making  due  allowance  for  changes  in  depth. 
This  is  a  severe  disadvantage  which  doubtless  acts  as  a  strong 
handicap  against  the  use  of  CRT  equipment  in  fishery. 

A  highly  interesting  improvement  is  described  by  Woodgate. 
It  consists  of  an  additional  electronic  apparatus  which  gives 
an  acoustical  signal  for  every  fish  echo,  hereby  releasing  the 


[532] 


DISCUSSION  — FISH     DETECTION 


operator  from  continuous  visual  observation.  An  automatic 
counter  of  these  fish  echoes  can  also  be  attached  to  replace 
the  "integrating  duties"  of  the  operator.  Hereby  the  fish 
echoes  arc  at  least  counted,  although,  of  course,  the  quality 
of  the  echoes  is  not  preserved. 

Contrary  to  the  CRT,  the  paper  recorder  has  the  immense 
advantage  that  it  automatically  integrates  the  echoes  in 
completing  a  real  echo  chart  of  a  continuous  sequence  of 
soundings.  This  makes  continuous  observation  unnecessary. 
Furthermore,  not  only  the  number  but  also  the  duration  and, 
to  a  certain  degree,  the  strength  of  the  echoes  is  preserved  in 
a  way  which  results  in  a  very  expressive  picture  of  bottom 
conditions  and  fish  distribution  along  the  course  of  the  ship. 
Unfortunately,  the  reproduction  of  the  echo  strength  is 
unsatisfactory,  due  to  the  limited  sensitivity  of  the  recording 
paper.  This  does  not  matter  so  much  with  pelagic  fish;  but 
with  bottom  fish  the  distinguishing  between  fish  trace  and 
bottom  trace  may  become  difficult  or  even  impossible.  This 
happens  as  soon  as  the  fish  echo  is  strong  enough  to  cause 
recording  with  maximal  intensity  (blackness).  Then  traces 
of  such  schools  which,  because  of  their  position  and  density 
are  of  the  highest  interest  for  bottom  trawling,  may  escape 
the  perception  or,  furthermore,  may  resemble  or  hide  obstacles 
on  the  bottom  and  thereby  cause  wrong  action  resulting  in 
damage  to  the  gear  and  loss  of  time.  Also  estimating  the 
probable  amount  of  catch  which  may  be  important  for 
determining  the  proper  time  for  hauling  the  gear,  often  becomes 
difficult  or  even  impossible. 

The  second  or  double  echo  trace  records  only  the  strong 
bottom  echo  and  not  the  weaker  fish  echoes.  Hereby  a 
discrimination  between  the  two  types  of  reflectors  is  possible. 
But  having  to  record  the  second  echo  decreases  the  reproduc- 
tion scale  and  is  therefore  an  unsatisfying  alternative. 

Three  far  better  solutions  of  this  problem  are  described  in 
five  of  the  present  papers.  One  of  these  solutions,  which 
seems  to  have  been  found  almost  simultaneously  in  three 
different  countries,  i.e.  in  England,  Norway  and  Japan,  is  the 
so-called  "white-line"  method  described  by  Haines.  Tech- 
nically it  consists  of  a  gating  circuit  which  operates  only  in 
response  to  signals  above  a  certain  amplitude.  If  this  gating 
action  is  set  at  a  value  in  excess  of  that  of  the  strongest  fish 
echo  but  below  that  of  the  bottom  echo,  the  amplifier  is  cut 
out  for  about  I/ 100th  of  a  second  on  receipt  of  the  bottom 
echo  resulting  in  a  white  gap  immediately  following  the 
strike  of  the  bottom  echo.  In  a  continuous  sequence  of 
soundings  these  white  gaps  build  up  a  white  line  which  follows 
exactly  the  contours  of  the  bottom  with  the  fish  traces 
appearing  easily  distinguishable  on  top  of  it. 

The  "bottom  blocking  device"  described  by  Vestnes  and 
the  "bottom  suppressor"  described  by  Hashimoto  and 
Maniwa  obviously  have  the  same  effect. 

Kietz  describes  another  method  which  by  courtesy  of  the 
manufacturers  I  myself  have  already  used  for  several  years  for 
the  observation  of  bottom  trawls.  For  this  method,  which  is 
now  available  for  commercial  fishery,  a  special  receiving 
amplifier  was  developed  which  records  the  strong  bottom 
echoes  in  a  black  tone,  and  the  weaker  fish  (or  fishing  gear) 
echoes  in  a  gray  tone.  Hereby  the  operator  can  always  dis- 
tinguish between  the  traces  of  bottom  and  bottom  fish. 
(This  has  later  been  reversed  so  that  the  bottom  echo  is  gray 
and  the  fish  trace  black.— Editor). 

Finally,  Fahrentholz  describes  a  third  method  where  the 
gray  traces  of  bottom  and  fish  are  separated  by  a  narrow  band 


in  deep  black  tone  which  exactly  represents  the  strike  of  the 
bottom  echo,  i.e.  the  surface  of  the  bottom. 

All  three  methods  use  the  great  difference  in  strength 
between  bottom  and  fish  echoes  for  a  clearer  representation 
of  bottom  fish.  Thereby  the  recorder  has  doubtless  gained 
one  of  the  most  important  advantages  of  the  CRT.  Other 
advantages  of  the  CRT  remain,  such  as  the  better  representa- 
tion of  the  single  sounding  and  the  enlarged  scale. 

Another  general  problem  of  echo  sounding  is  the  resolving 
power  which  is  not  satisfactory  with  some  of  the  equipment 
on  the  market.  The  resolving  power  depends  on  the  duration 
of  the  sound  impulse  and  the  width  of  the  sound  beam  and 
can  therefore  be  improved  by  shortening  the  sound  impulse 
and/or  decreasing  the  angle  of  sound  transmission.  A  higher 
resolving  power  is  desired  for  a  better  representation  of 
particularly  the  vertical  dimension  and  also  the  density  of 
fish  shoals. 

High  frequencies  are  favourable  for  short  impulse  length. 
As  the  width  of  the  sound  beam  is  decreased  by  enlarging 
the  value  of  the  quotient  of  transducer  diameter  over  the  length 
of  the  sound  waves,  higher  frequencies  are  favourable  in 
this  regard  too.  Unfortunately  higher  frequencies  are  subject 
to  higher  attenuation  by  which  the  working  range  is  decreased. 
On  the  other  hand,  as  stated  by  Hashimoto  and  Maniwa, 
higher  frequencies  have  a  better  reflection  coefficient  with  fish 
which  may  compensate  the  higher  attenuation  in  regard 
to  fish  to  a  certain  degree. 

A  narrow  sound  beam  is  only  useful  when  it  is  steadily 
adjusted  straight  in  the  direction  desired,  therefore  the  effect 
of  the  rolling  and  pitching  of  the  ship  in  rough  sea  has  to  be 
considered  carefully  and  stabilizing  of  the  transducers  then 
becomes  necessary.  Craig  mentions  several  methods  of  how 
this  could  be  done.  One  possibility,  i.e.  the  installation  of  the 
transducers  in  a  streamlined  body  which  is  towed  at  a  sub- 
stantial depth  below  the  surface  should  have  the  further 
advantages  of  avoiding  the  trouble  connected  with  aeration 
and  the  depth  variations  ca  iscd  by  the  vertical  movements 
of  the  ship  in  rough  sea.  But  all  these  measures  would 
increase  the  price  and  in  the  latter  case  in  addition  cause 
difficulties  in  the  handling  of  the  equipment  and  are  therefore 
not  yet  in  practical  use.  Nevertheless  the  improvement  of  the 
resolving  power  is  an  important  necessity  which  will  have  to  be 
solved  in  the  course  of  future  development. 

Until  now  only  echo  sounding  in  vertical  direction  was 
discussed  by  which  only  a  limited  area  under  the  ship  is 
covered.  The  experience  made  with  Asdic,  i.e.  horizontal 
ranging,  for  locating  submarines  made  the  application  of  this 
method  to  fisheries  purposes  rather  obvious.  Vestnes  des- 
cribes probably  the  first  attempt  in  this  direction.  It  is  very 
impressive  how  successfully  powerful  equipment  of  this  type 
can  be  used  to  survey  big  areas  to  find  out  the  distribution  of 
pelagic  fish  schools  and  observe  their  movements.  Similarly, 
as  described  by  Jakobsson,  the  results  gained  by  one  echo- 
ranging  scouting  vessel  are  utilized  to  guide  the  fishing  fleet 
to  profitable  catching  places.  Unfortunately,  horizontal 
ranging  which,  at  first,  seems  to  accomplish  all  wishes, 
underlies  certain  restrictions  which  are  due  to  physical  laws 
governing  the  propagation  and  the  reflection  of  sound  in 
water.  These  are  closely  discussed  in  the  papers  of  Haines, 
Craig,  Feher  and  Vestnes.  Nevertheless  the  fisherman 
obviously  wants  the  possibility  to  scan  in  horizontal  direction 
so  strongly  that  at  present  almost  all  echo  sounder  manu- 
facturers have  developed  a  horizontal  ranging  device.  To  be 


[5331 


MODERN     FISHING     GEAR    OF    THE    WORLD 


economic  these  "fishery  Asdics"  have  to  be  cheaper  and 
consequently  of  much  less  complexity  than  those  used  for 
Naval  warfare  or  whaling.  Two  fishery  types  may  be  dis- 
tinguished according  to  their  si/e.  The  Lodar  described  in 
Fcher's  paper  is  a  good  example  of  the  big  type.  Besides  such 
big  ones,  smaller  equipment  is  available  which  is  usually 
built  up  by  simply  combining  a  normal  echo  sounder  with  a 
transmitting  device  suitable  for  horizontal  ranging.  Examples 
of  such  smaller  equipment  are  described  in  the  papers  by 
Woodgate,  Feher  and  Fahrcnthol/.  At  present  the  commer- 
cial use  of  echo  ranging  is  almost  completely  restricted  to 
pelagic  fish.  Here  it  proves  very  useful  for  detecting  fish  and 
also  measuring  the  horizontal  extension  of  fish  schools.  The 
ranging  of  bottom  fish  is  much  more  difficult  except  under 
extremely  favourable  conditions.  'The  actual  depth  of  fish 
schools  located  by  ranging  has  to  be  checked  by  sounding. 
Therefore,  ranging  equipment  is  always  combined  with 
a  sounder. 

Finally,  it  must  be  mentioned  that  successful  echo  ranging 
of  fish  is  rather  more  difficult  than  echo  sounding  and 
requires  well-trained  operators  to  obtain  good  results.  In 
spite  of  these  difficulties  the  value  of  echo-ranging  for  solving 
certain  fishery  problems  is  evident.  As  it  is  a  rather  new 
technique  in  fisheries,  more  experience  and  future  technical 
development  will  certainly  extend  the  possibilities  of  its 
application. 

Doubtless  the  technical  improvement  of  the  equipment  will 
further  improve  the  value  of  these  acoustical  methods  for  the 
fishery. 

Cushing's  most  valuable  quantitative  survey,  together  with 
the  reports  of  Jakobsson  and  Vestnes,  give  a  very  good 
example  how,  besides  their  application  to  commercial  fishery, 
these  acoustical  methods  can  be  used  most  successfully 
as  a  tool  for  scientific  work  and  thereby  help  to  improve  the 
knowledge  of  the  distribution  and  behaviour  of  fish 
stocks. 

Mr.  P.  A.  de  Boer  (Netherlands):  Echo  ranging  in  deep 
water  has  been  proved  rather  satisfactory  in,  for  instance, 
the  Norwegian  and  Icelandic  waters,  but  what  about  echo 
ranging  in  shallow  water?  I  have  had  some  experience  of  it 
in  the  shallow  waters  of  the  southern  part  of  the  North  Sea 
and  found  it  quite  possible  although  there  are  some  restrictions 
to  its  use.  In  this  connection  I  object  strongly  to  the  fixed 
position  of  the  oscillator  and  I  will  tell  you  why. 

It  is  generally  thought  that  the  unwanted  bottom  echo  in 
shallow  water  is  a  source  of  much  trouble.  But  the  trouble  is, 
1  think,  theoretical,  not  practical,  because  I  found  that,  for 
example,  a  pier  about  4  m.  under  water  can  be  picked  up  by 
echo  ranging  at  3,000  m.  This  long  range  is,  however,  only 
obtained  if  the  maximum  of  the  sound  beam  is  turned  to 
the  proper  direction.  The  effect  of  the  side  lobes  docs  not 
interfere  seriously.  What  counts  is  the  central  maximum  of 
the  main  beam  which  is  about  only  \  or  1  degree.  I  made  the 
following  experiment  with  the  oscilfator  on  our  research  ship 
which  is  pointed  6  degrees  downwards.  At  a  depth  of  about 
25  fm.  1  found  a  wreck  and  that  wreck  was  shown  on  the 
recorder  at  a  distance  of  1,100  m.  At  a  certain  distance  the 
target  disappeared  from  the  echogram.  Considering  this 
distance  and  the  inclination  of  the  oscillator,  the  water  depth 
at  the  position  of  the  wreck  could  be  calculated.  Furthermore, 
a  calculation  can  be  made  to  determine  how  much  the  tilt 
of  the  oscillator  must  be  decreased  to  bring  the  central 


maximum  of  the  main  beam  clear  of  the  shallow  bottom.    In 
this  particular  case,  it  was  about  2i  degrees. 

With  the  oscillator  in  the  original  position  (6  degrees 
downward  tilt),  echoes  from  the  pier  already  mentioned 
could  be  obtained  only  at  600  m.  and  less.  But  as  soon  as  I 
altered  the  position  of  the  oscillator,  putting  it  up  2,\  degrees. 
1  could  hear  the  pier  at  a  distance  of  3,000  m.  and,  as  1  said, 
it  was  recorded  immediately  at  the  maximum  recording  range 
of  2,000  m.  With  the  oscillator  at  6  degrees  tilt,  the  wreck 
disappeared  at  250  m.  With  2\  degrees  tilt,  it  disappeared  at  a 
distance  of  about  600  to  700  m.  Now,  the  Asdics  exhibited 
at  this  Congress  all  have  the  oscillators  in  a  fixed  position, 
between  0  and  about  2  or  5  degrees  downwards.  With  such  a 
fixed  oscillator,  the  varying  trim  of  the  ship  must  lead  to 
variations  in  the  direction  of  the  sound  beam.  Such  variations 
are  particularly  to  be  expected  with  fishing  craft  where  the 
trim,  for  instance,  depends  on  the  catch  obtained  and  on  the 
varying  load  of  fuel,  water,  ice  and  so  on.  It  should,  therefore, 
be  possible  to  modify  the  position  of  the  oscillator  according 
to  the  trim  of  the  ship,  and  enable  the  instrument  to  be  turned 
from  0  to  8  degrees  downwards.  This  flexibility,  with  the 
compensation  for  trim,  would  enable  the  oscillator  to  be  used 
to  search  for  fish  not  only  near  the  surface  but  also  in  greater 
depth. 

Mr.  Max  Schulte  (Germany):  Dr.  Schiirfe  has  mentioned 
in  his  report  that  horizontal  echo  ranging  for  bottom  fishing 
is  of  little  value,  and  that  this  fishery  must  depend  on  echo- 
sounding  only.  I  should  like  to  say  that,  in  this  statement,  an 
essential  factor  which  plays  an  important  part  in  the  German 
fishery,  has  not  been  taken  into  consideration.  In  our  trawl 
fishery  the  skipper  not  only  wants  to  see  the  fish  but  also  to 
obtain  some  knowledge  of  the  bottom  conditions  of  the 
fishing  ground.  He  is  particularly  interested  in  wrecks  and  in 
rock  formations.  Here  we  can  say  that  Lvdar  has  given  good 
results  by  showing  the  fisherman  that  he  is  going  to  meet  an 
obstacle  which  may  interfere  with  his  trawl. 

Mr.  de  Boer  mentioned  that  to  date  there  was  no  hori/ontal 
echo  ranging  equipment  with  variable  oscillator  tilt,  but  oui 
Mini- Lodar  allows  the  oscillator  to  tilt  anywhere  between 
0  degrees  and  90  degrees. 

One  of  the  Congress  papers  mentions  that  the  side  lobes 
may  cause  trouble.  We  do  not  share  this  opinion,  because  in 
hori/ontal  ranging,  the  side  lobes  can  give  additional  informa- 
tion on  the  depth.  The  echogram  then  shows  not  only  the 
hori/ontal  distance  to  the  target  but  also  the  water  depth  at 
the  ship's  position. 

Mr.  R.  G.  Haines  (U.K.):  I  would  like  to  refer  very  briefly 
to  the  problem  of  the  correct  angle  of  tilt  for  the  transducer 
in  an  echo  ranging  set.  1  think  Mr.  de  Boer  has  slightly  over- 
simplified the  problem.  After  all,  the  trim  of  the  ship  is 
concerned  only  when  the  oscillator  is  trained  ahead;  when  the 
oscillator  is  on  the  beam  it  does  not  matter  what  the  trim  is. 
Also,  of  course,  a  fishing  vessel  at  sea  is  pitching  and  yawing 
all  the  time,  which  means  that  any  very  accurate  setting  of 
the  tilt  will  not  be  so  valuable.  Furthermore,  as  is  probably 
well  known  to  you,  the  horizontal  beam  in  water  is  liable 
to  be  bent  because  of  the  temperature,  salinity,  and  pressure 
of  the  water.  This  varies  in  different  areas  and  different 
places,  so  the  problem  has  to  be  considered  from  a  technical 
point  of  view  in  rather  more  detail. 


Mr.  P.  A.  de  Boer  (Netherlands):    1  should  like  to  answer 


[534] 


DISCUSSION  — FISH    DETECTION 


the  remarks  made  by  Mr.  Schulte  from  £LAC  and  Mr. 
Haines  of  Kelvin-Hughes  about  what  1  said  on  echo  ranging. 
There  is  a  small  Lodar  exhibited  here  in  which  the  oscillator 
can  be  tilted  in  every  direction.  This  is  a  very  useful  design, 
but  1  had  the  big  Lodar  in  mind  in  which  the  oscillator  cannot 
be  so  tilted. 

Mr.  Haines  of  Kelvin  Hughes  mentioned  the  problem  of 
pitching  and  rolling,  which  would  make  useless  any  attempt 
at  careful  directioning  of  the  oscillator  and  compensating 
for  the  ship's  trim.  My  answer  is  that,  as  soon  as  a  ship  starts 
pitching  and  rolling,  spot  aimed  echo  ranging  becomes 
impossible.  It  is  restricted  to  smooth  water.  But  this  docs 
not  make  the  method  useless.  For  instance,  a  ship,  without 
Radar,  which  is  approaching  a  coast  in  a  thick  fog  can  use 
Asdic  to  find  the  entrance  of  the  harbour. 

1  am  fully  aware  of  the  disturbances  caused  by  unwanted 
bottom  echoes,  particularly  in  shallow  water.  A  sandy 
bottom  is  often  irregularly  formed  with  waves  or  dunes,  and, 
in  my  opinion,  it  is  possible  with  Lodar  to  distinguish  between 
such  sand  dunes  and,  for  instance,  herring  schools  near  the 
bottom,  because  most  of  the  sound  reflected  at  the  bottom 
goes  away  in  another  direction  while  the  herring  echoes  are 
mainly  reflected  to  the  sound  source.  Thus,  a  stronger  echo 
is  obtained  from  the  herring. 

Dr.  S.  Fahrentholz  (Germany):  It  was  stated  that  horizontal 
echo  ranging  is  restricted  to  calm  weather.  We  have  developed 
a  special  oscillator  arrangement  fixed  to  the  ship's  bottom, 
and  working  at  a  tilt  of  4  degrees.  1  agree  that  echo  ranging 
in  a  rough  sea  and  in  different  depths  needs  a  turnable 
oscillator,  but  such  equipment,  particularly  for  bigger  working 
ranges,  is  rather  expensive  and  the  ordinary  fisherman  would 
not  be  able  to  afford  it.  We  think,  therefore,  that  a  small  and 
simple  set  ranging  within  a  few  hundred  metres  of  the  vessel 
would  be  useful  too.  The  set  with  fixed  oscillators  we  designed, 
is  fit  for  alternative  horizontal  or  vertical  sounding  and  gives 
rather  good  results.  This  simple  equipment  is  considered  as  a 
temporary  solution  and  we  hope  to  find  a  more  suitable  but 
still  cheap  solution  in  the  future. 

In  order  to  improve  the  representation  of  bottom  fish 
recording,  1  decided  to  use  the  black  line  method  described 
in  my  paper.  This  can  be  effected  by  a  specially  developed 
apparatus  which  can  be  attached  to  any  existing  echo  sounder 
model  of  my  design.  The  sensitivity  of  the  black  line  recording 
can  be  modified  according  to  the  echo  strength.  Bottom  and 
fish  echoes,  which  are  divided  by  the  narrow  black  line,  arc 
both  recorded  in  grey. 

With  regard  to  CRT  representation,  a  new  device  with  two 
tubes  is  also  described  in  my  paper. 

Mr.  Kiyo  Ka/u  Tsuda  (Japan):  Echo  sounders  are  widely 
used  in  the  Japanese  fishery.  Big  fishing  boats  of  more  than 
500  tons  are  usually  fitted  with  two  or  three  echo  sounders. 
The  frequency  usually  lies  between  15-50  kc.  The  beam  angle 
is  around  30  degrees.  Both  paper  recording  and  CRT  display 
arc  used.  Besides  the  normal  frequencies,  200  kc.  and  more 
arc  also  used  and  attempts  are  made  to  improve  determination 
of  the  fish  species. 

Mr.  H.  S.  Noel  (U.K.):  The  tendency  so  far  has  been  to 
regard  the  echo  sounder  as  only  an  agent  to  a  more  general 
form  of  fish  detection,  that  is  to  hydrographical  and  research 
generally,  but  when  midwater  trawling  for  sprat  it  is  virtually 


impossible  to  catch  any  quantity  of  fish  without  an  echo 
sounder. 

You  are  working  in  waters  where  there  is  a  tide  running  of 
about  3  knots  and  the  fish,  which  are  in  fairly  small  shoals, 
cover  quite  a  distance  in  the  course  of  one  tide.  Therefore, 
it  is  essential  to  use  an  echo  sounder,  firstly  to  search  for  the 
shoal,  secondly,  to  assess  its  extent,  and  thirdly  to  assess  its 
depth,  so  as  to  be  able  to  set  the  net  at  the  requisite  depth. 
This  also  applies  to  some  degree,  of  course,  to  the  herring 
fishery. 

Another  aspect  occurs  where  one  is  catching  fish  for  canning 
and  the  fish  have  to  be  returned  to  shore  in  a  very  fresh 
condition.  Here  a  number  of  boats  are  virtually  all  working 
for  the  cannery,  and  they  are  more  or  less  honour-bound  to 
deliver  a  set  quantity  of  fish  day  by  day  in  good  condition. 
Here  an  echo  sounder  is  essential.  Old  fashioned  methods  of 
fishing  for  this  type  of  fish  in  midwater  depended  entirely  on 
indications,  often  wrong,  such  as  were  given  by  the  presence 
of  scabirds  and  boats  were  sometimes  anchored  for  three 
days  before  fish  were  caught  in  sufficient  quantity  to  land. 
It  can  be  said  that  the  canning  industry,  which  uses  most 
pelagic  fish,  is  dependent  on  the  echo  sounder. 

Mr.  H.  Kristjonsson  (FAO):  At  present  not  enough  use 
is  being  made  of  echo  sounding  by  small  craft,  such  as 
open  boats,  of  the  inshore  fishery. 

It  does  not  appear  to  be  generally  known  that  sounders  are 
available  on  the  market,  which  could  be  of  great  help  to  this 
type  of  fishing.  American.  German,  Japanese  and  Norwegian 
firms  now  produce  small  echo  sounders  of  which  the  average 
dimensions  range  around  12  in.  x  10  in.  x  8  in.  while  the 
weight,  excluding  the  transducer,  is  as  low  as  about  20  Ibs. 
The  recording  unit  is  usually  designed  for  easy  removal  after 
use. 

The  sets  are  usually  fitted  for  DC  (direct  current)  power 
supplies  of  6,  12,  24  volt,  and  the  power  consumption  is 
around  50  watts. 

The  width  of  the  (usually  dry)  recording  paper  ranges  from 
about  2  to  6  inches.  Measuring  ranges  differ  according  to 
manufacture  and  type.  A  typical  example  would  be:  0  to  25 
fm.,  25  to  50  fm.,  50  to  75  fm.  However,  lower  and  higher 
ranges  are  available  to  suit  most  needs.  The  transducer  for 
these  small  sets  can  be  mounted  in  different  ways,  either 
permanent  or  movable. 

Permanent.  The  transducer  is  mounted  on  the  hull  inside  a 
cofferdam  in  the  usual  manner  or  at  the  side  of  the  keel  with 
wooden  fairings  fore  and  aft  to  prevent  the  formation  of 
bubbles.  For  removable  recording  units  the  connection  with 
the  transducer  is  made  with  a  good  quality  watertight  plug 
and  socket. 

Movable.  When  boats  are  frequently  beached  or  in  other 
cases  where  it  is  advisable  to  remove  the  transducer  every 
trip,  the  transducer  can  be  mounted  on  the  end  of  a  U  in. 
steel  pipe  which  is  then  clamped  to  the  side  of  the  boat.  The 
length  of  the  pipe  should  be  adjusted  so  that  the  transducer 
is  always  at  least  3  ft.  under  the  surface.  The  way  of  attach- 
ment depends  mainly  on  the  hull  of  the  boat,  but  it  usually 
can  be  done  either  with  2  clamps  fitted  on  the  vessel's  hull  in 
which  the  pipe  is  secured  by  fly  nuts  or,  for  lightly  planked 
boats,  with  an  arm  near  the  top  of  the  pipe  which  is  clamped 
over  the  gunwale.  With  this  last  method  the  pipe  should  be 
secured  in  position  with  fore  and  aft  stays. 

In  order  to  avoid  aerated  water  at  higher  speed  which 


1535] 


MODERN    FISHING    GEAR     OF    THE    WORLD 


would  interfere  with  the  sounding,  the  transducer  is  usually 
fitted  in  a  streamlined  container.  When  properly  formed  and 
mounted  this  side  installing  of  the  transducer  can  give 
better  results  in  avoiding  disturbance  by  aerated  water  than 
the  common  installation  in  the  ship's  bottom. 

Captain  D.  Roberts  (U.K.):  Echo  ranging  for  fishing,  in 
my  opinion,  could  only  be  useful  for  fleet  fishing  and  not  for 
individual  vessels,  except  perhaps  one  particular  case.  In  the 
North  Sea  particularly  (1  do  not  fish  there  but  my  colleagues 
do)  they  are  finding  fish  close  to  wrecks  and  the  fishing  there 
has  been  very  good  at  certain  seasons  during  these  last  two 
years.  But  they  must  fish  close  to  the  wrecks.  Can  the  biolo- 
gists tell  me  why  fish  stay  around  wrecks?  If  there  is  a  good 
reason  for  it,  then  echo  ranging  could  be  very  valuable  for 
pin-pointing  wrecks.  Skippers  have  pin-pointed  the  wrecks 
and  incorporated  them  on  their  Decca  charts,  but  perhaps 
this  information  could  be  valuable  to  more  people. 

Captain  A.  Hodson  (U.K.):  In  my  experience  as  a  skipper 
in  the  North  Sea  I  have  found  that  grounds  littered  with 
wrecks  often  provide  reasonable  catches.  As  you  will  know, 
ships  in  convoy  passed  up  the  east  coast  of  the  British  Isles 
and  during  the  war,  of  course,  many  were  sunk.  We  know 
only  too  well,  because  we  have  caught  them  with  our  nets, 
much  to  our  cost  in  gear  and  fishing  time.  With  the  intro- 
duction of  the  Decca  Navigator  system  individual  skippers 
have  made  their  own  charts  and,  working  in  collaboration 
with  each  other,  have  covered  quite  a  large  part  of  the  area. 
As  regards  echo-sounding  for  wrecks,  1  can  only  say  it  would 
be  a  very  fine  achievement  to  determine  the  presence,  range, 
and  identification  of  the  wrecks  as  distinct  from  other  features 
of  the  fishing  ground.  So  far,  we  cannot  be  certain  whether 
our  gear  has  fouled  a  wreck  or  not.  As  in  other  areas,  we  have 
found  that  it  is  coral  formations  on  the  seabed  that  causes 
considerable  damage,  expense  and  loss  of  time.  If  the 
scientists  eventually  produce  an  instrument  which  enables  us 
to  determine  wrecks  and  other  obstacles,  it  will  be  a  god-send 
from  the  point  of  view  of  British  trawlers  working  in  the 
North  Sea. 

Mr.  B.  B.  Parrish  (U.K.):  1  am  very  sorry  to  say  that  1  am 
in  no  position  to  explain  why  fish  accumulate  around  wrecks, 
but  I  do  think  that  it  is  most  important  that  the  observation 
has  been  made.  There  is  much  confirmation,  particularly 
in  underwater  films  taken  in  the  vicinity  of  wrecks,  of  this 
observation.  Fish  do  appear  to  accumulate  in  the  vicinity  of 
wrecks,  a  fact  which  can  be  made  use  of  in  fishing  operations, 
provided  the  locality  can  be  accurately  determined  and  is 
not  too  littered  with  wrecks  for  effective  use  of  the  gear. 
Yesterday  we  heard  two  excellent  examples  of  fish  location 
devices  used  by  research  vessels,  one  being  thermometers,  and 
the  other  plankton  nets.  These  devices  enabled  the  scientists 
to  deduct  the  whereabouts  of  fish  from  observations  of 
temperature  and  plankton.  The  value  to  a  fisherman  of  such 
devices  relies  very  much  on  an  organized  survey  scheme  by 
research  vessels,  pooling  of  information  and  transmitting  it 
to  the  fishing  fleet.  There  are  devices  which  can  be  used  by 
individual  fishermen  but  these,  to  be  workable,  must  be 
based  on  the  simplest  possible  location  factor.  For  instance, 
it  is  of  no  practical  value  to  a  fisherman  to  know  that  some 
chemical  component  of  the  water  denotes  the  presence  of 
fish  if  he  is  unable  to  estimate  the  quantity  of  this  chemical 


in  the  water,  or  the  process  takes  too  long  or  it  is  too  expensive. 
Captain  Roberts  has  told  me  that  he  had  detected  fish  in 
the  Iceland  region  by  the  colour  of  the  water  and  asked  me 
if  there  was  any  biological  explanation  of  this.  Undoubtedly 
there  is,  but  I  do  not  know  it  and,  I  imagine,  biologists  would 
have  to  conduct  very  intensive  research  over  a  long  time  to 
find  the  actual  relationship  between  water  colour  and  the 
presence  of  fish.  Biologists  naturally  would  like  to  know  the 
answer  but,  so  far  as  practical  fishing  is  concerned,  the  im- 
portant fact  is  the  observation  that  water  colour  and  the 
presence  of  fish  are  related.  There  is  a  danger,  of  course, 
that  the  apparent  relationship  is  not  valid,  but  the  fishermen 
are  probably  much  better  than  scientists  in  detecting  and 
proving  empirically  such  location  factors.  I  think  the  biolo- 
gists and  the  fishermen  would  both  derive  much  benefit  if 
there  were  more  contacts  and  greater  exchange  of  information 
between  them. 

Mr.  H.  Kristjonsson  (FAO):  I  expect  there  is  a  question  in 
the  minds  of  many  people  here,  in  connection  with  the  very 
significant  advances  made  in  detecting  fish  close  to  the  bottom 
by  bottom  suppression,  gating  circuits,  etc.  Now,  will  it  be 
possible  to  build  these  new  features  into  the  echo  sounders 
now  in  use  or  do  the  fishermen  have  to  throw  out  their  existing 
units  and  buy  new  ones? 

Mr.  R.  C.  Haines  (Kelvin  Hughes,  U.K.):  The  white-line 
modification  is  available  for  fitting  as  an  optional  extra  to 
our  full  range  of  fishing  echo  sounders,  including  the  model 
MS:24  which  has  now  been  superseded.  It  is  also  possible  to 
add  the  white-line  modification  to  earlier  models.  However, 
the  cost  of  doing  so  would  hardly  be  attractive  and  naturally 
in  such  cases,  where  the  sets  will  have  been  in  use  for  a  number 
of  years,  we  should  recommend  replacement  with  a  modern 
equipment  incorporating  a  number  of  other  improvements 
in  addition  to  the  white-line  amplifier. 

Dr.  H.  Kietz  (Atlas-Wcrke,  Germany):  The  answer  to  the 
question  "Can  the  grey-black  amplifier  be  built  into  existing 
Atlas  echo  sounders  of  simpler  design?"  is  yes. 

Dr.  S.  Fahrentholz  (for  Bchm-Echolotfabrik,  Germany): 
hvery  existing  Behm-Echolotfabrik  echo  sounder  can  be 
equipped  with  the  black-line  amplifier  unit. 

Mr.  Th.  Gerhardsen  (Simrad,  Norway):  Yes,  we  can  supply 
this  extra  feature,  at  a  moderate  cost,  to  all  our  equipment. 

Mr.  E.  C.  Bindloss  (Marconi,  U.K.):  The  answer  is  yes, 
but  1  would  like  to  bring  out  one  point:  in  order  to  show  up 
bottom  fish  it  has  become  apparent  that  very  much  greater 
transmitting  power  is  required  than  that  used  by  echo 
sounders  up  to  two  or  three  years  ago.  I  think  all  makers 
agree  that  this  is  the  key  to  the  question.  More  powerful 
transmitters  can  be  provided,  of  course,  and  are  being  provided 
with  new  gear,  so,  with  that  provision,  my  answer  is  yes. 

Mr.  J.  Jakobsson  (Iceland):  The  reason  why  the  location 
and  detection  of  fish  has  become  so  increasingly  important  is, 
1  think,  because  the  fishing  fleets  are  growing  bigger  and  the 
ships  arc  costing  more  to  build  and  operate.  It  has  become 
very  important  that  they  should  work  in  the  most  profitable 
areas.  No  matter  how  effective  fish  detection  devices  may 
be,  1  am  quite  convinced  that  no  fishing  skipper  would  have 


[536] 


DISCUSSION  — FISH     DETECTION 


the  time  to  make  a  complete  survey  over  the  whole  fishable 
area  to  make  sure  of  finding  the  best  and  most  profitable 
sector.  My  experience  in  leading  200  herring  boats  convinces 
me  that  a  man  who  continuously  cruises  the  whole  area  and 
surveys  it  thoroughly,  using  the  optimal  devices  provided 
on  a  research  ship,  will  be  better  able  to  direct  a  fleet  to  more 
profitable  grounds  than  the  skipper  who  is  occupied  with  the 
actual  fishing.  I  am  quite  certain  that  it  is  very  effective  and 
very  profitable  to  organise  fish  searching  in  this  way. 

Mr.  D.  L.  Alverson  (U.S.A.):  I  thought  perhaps  you  might 
be  interested  in  some  of  the  experiments  and  statistical 
evaluations  that  we  have  carried  out  on  various  electronic 
gear,  including  fish  finding  devices.  The  population  dynamics 
of  fish  are  important  to  us  and  we  are  interested  in  any 
changes  that  various  gears  m;iy  cause  in  efficiency  of  fishing, 
fishing  power,  or  measures  of  availability.  For  the  past  5 
years  we  have  watched  closely  the  introduction  of  devices 
into  our  trawl  fisheries  and  recently  we  made  a  statistical 
evaluation  of  how  these  devices  were  affecting  our  fishery.  1 
will  not  go  into  the  methods  and  techniques  used  because  they 
are  very  complicated.  The  results  are  to  be  regarded  as  of 
relative  rather  than  absolute  quantitative  character.  The 
devices  we  covered  included  Radar,  Loran  and  the  various 
fish  finders,  particularly  the  Cathode  Ray  Tube  display  which 
expands  the  trace  above  the  bottom.  It  appeared  that,  as 
regards  the  fish-finder,  we  could  nol  demonstrate  statis- 
tically any  change  in  efficiency  of  catching  flat  fish  on  the 
Pacific  coast  in  the  Washington  trawl  fishery.  There  was  only 
some  indication  that  we  were  approaching  statistical  signi- 
ficance with  some  of  the  round  fish  as,  for  instance,  ocean 
perch  and  some  of  the  codfish.  It  may  be  of  some  interest 
that  the  Loran,  in  almost  every  instance,  did  show  a  signi- 
ficant increase  in  efficiency  aboard  the  vessel,  which  suggests 
that,  for  the  trawler  in  this  locality,  maintaining  precise 
position  is  the  most  important  factor. 

The  same  proved  to  be  true  for  Radar  but  only  for  boats 
working  inshore,  inside  of  the  Hekart  Strait  in  the  British 
Colombia  waters,  where  they  use  Radar  for  positioning 
themselves.  But  no  significant  difference  could  be  found  for 
the  same  boats  in  their  offshore  operations.  So  we  had  two 
checks  and  both  seemed  to  indicate  that  positioning  was  of 
utmost  importance  for  trawling.  This  conclusion,  of  course, 
must  not  necessarily  be  true  for  other  regions  where  the  fish 
may  behave  differently  and  where  different  conditions  exist. 

Prof.  Miroslav  Zei  (Yugoslavia):  As  you  probably  know, 
fishing  off  the  Yugoslavian  coast  of  the  Adriatic  Sea  is  mainly 
concerned  with  small  pelagic  species,  such  as  pilchards, 
anchovies,  mackerel.  It  is  mainly  a  seine  fishery  combined 


with  fish  attraction  by  artificial  light.  Usually  two  or  three 
light  boats,  belonging  to  one  owner,  operate  near  each  other. 
As  soon  as  enough  fish  have  been  attracted  under  each  boat, 
they  come  together  and  collect  the  fish  under  one  boat  only. 
Before  the  introduction  of  echo  sounders,  this  method  pre- 
sented no  problems.  Then  a  Simrad  echo  sounder  was 
introduced  and  led  to  the  discovery  that  most  of  the  fish  dis- 
appeared when  the  three  boats  came  together.  Thus  it  seems 
more  efficient  for  the  boats  to  catch  the  fish  separately, 
although  this,  of  course,  takes  much  more  time.  The  question 
now  to  be  answered  is:  why  is  it  that  the  fish  do  not  stay 
together  under  the  one  boat?  This  lack  of  tolerance 
between  the  different  schools  might  be  caused  by  inequality 
in  behaviour. 

But  this  is  only  a  vague  suggestion.  A  satisfactory  explana- 
tion has  not  yet  been  found.  I  think  that  a  combined  hori- 
zontal and  vertical  echo  sounder  could  be  of  great  help  in 
solving  this  and  many  other  biological  problems  about  the 
behaviour  of  fish,  and  it  is  important  for  the  future  develop- 
ment of  fishing  that  we  should  know  the  answers. 

Dr.  J.  Scharfc  (FAO):  Echo  sounders  could  probably 
be  used  effectively  in  connection  with  light  fishing,  for  instance 
in  the  Mediterranean,  both  to  find  fish  concentrations  for 
optimal  placing  of  the  light  boats  and  for  observing  the 
gathering  of  fish  under  the  light  to  determine  the  proper 
timing  of  catching  operations. 

Dr.  D.  H.  Cushing  (U.K.):  As  a  fisheries  biologist  1  am 
interested  only  in  certain  biological  problems  concerned  with 
fish  populations  and  to  understand  how  they  are  distributed. 
Therefore,  I  am  interested  in  echo  sounding  as  a  means  to 
describing  their  distribution  in  the  sea,  to  count  the  fish  and, 
possibly,  identify  them.  By  identifying  them  I  do  not  mean 
establishing  the  species,  but  getting  an  idea  of  the  size  ranges. 
We  already  know  that,  when  single  fish  are  being  recorded,  a 
big  fish  gives  a  stronger  and  more  extensive  signal  so,  in 
certain  circumstances,  you  can  say  there  are  a  lot  of  big  fish 
or  a  lot  of  little  fish  present.  Similarly,  you  could  determine 
from  a  paper  record  that  this  school  is  a  large  one  or  a  small 
one  and,  I  think,  this  line  of  thought  and  approach  may 
eventually  lead  us  towards  getting  some  idea  of  the  sizes  of 
fish  in  the  sea.  We  are  already  progressing  towards  counting 
the  fish  in  the  sea,  which  has  obvious  uses  for  fishery  biologists 
and,  I  think,  for  the  fishermen  too,  because  they  arc  interested 
in  knowing  how  many  fish  there  are  present  before  they  start 
catching  operations.  They  are  also  interested  in  finding  out 
how  the  fish  arc  distributed.  This  is  how,  in  several  ways, 
the  interests  of  the  fishery  biologist  coincide  with  those  of 
the  fishermen. 


537] 


Section  12 :    Attraction  of  Fish. 


SUMMARY  OF  EXPERIMENTS  ON  THE  RESPONSE  OF  TUNA 

TO  STIMULI 

by 

ALBERT  L.  TESTER* 

Director,  Pacific  Oceanic  Fishery  Investigations,  Honolulu,  Hawaii,  U-S.A. 

Abstract 

The  aim  of  the  experiments  described  in  this  paper  was  to  obtain  information  which  might  lead  to  improved  methods  of  tuna  fishing, 
and  especially  in  finding  a  substitute  for  bait  fish.  In  a  series  of  tank  experiments,  it  was  found  that  yellowfin  and  little  tunny  responded  to 
white  light  of  moderate  intensity  but  not  to  a  weak  light.  Their  reactions  to  chemical  stimuli  were  also  observed  and  a  positive  response  was 
obtained  by  using  clear  aqueous  or  alcohol  extracts  of  tuna  flesh.  They  were  repelled,  however,  by  copper  acetate — the  shark  repellent. 
The  possibility  of  electrofishing  for  surface  tuna  was  investigated  and  since  it  was  demonstrated  that  electrotaxis  could  be  induced  in  tuna 
with  an  electric  field  of  known  characteristics,  the  problem  becomes  one  of  electrical  engineering  rather  than  of  biology. 


Resume 


Resume  des  experiences  relatives  a  la  reaction  du  thon  aux  stimuli 


Les  experiences  decrites  dans  cet  article  avaient  pour  but  de  recueillir  des  renseignements  susceplibles  de  conduire  a  Fumdioration 
des  methodes  de  peche  au  thon,  et  plus  particulierement  de  trouver  le  moycn  de  remplacer  I'appat  vivant.  An  cours  d'une  scrie  d'essais  en 
bassin,  on  a  observed  que  les  thons  a  nageoires  jaunes  et  les  "little  tunny*'  (Euthynnus  yaito)  rcagissaient  a  une  lumicrc  blanche  d'intensitc 
moderee  mais  non  a  une  lumiere  faible.  On  a  egalement  observe"  leur  comportement  aux  stimuli  chimiques  ct  Ton  a  obtcnu  une  reaction 
positive  par  I'emploid'extraitsaqueuxoualcooliquesdechairde  thon.  Toutefois,  Pacltate  de  cuivrc,  utilise  pour  ^carter  les  requins,  les  a  eloignes. 
La  possibility  de  pecher  les  thons  de  surface  a  Telectricite  a  etc  etudiec.  et  comme  il  a  6te  d£montr6  que  Ton  pouvait  provoquer  1'electrotaxie 
chez  le  thon  au  moyen  d'un  champ  elect rique  de  caracteristiqucs  connues,  le  problcme  relevc  plus  de  la  technique  de  Tdlectricite  que  de  la 
biologic. 

Resumen  de  los  experiments  sobre  la  rcaccton  del  atun  a  los  estimulos 
Fxtracto 

Los  experimentos  dcscritos  en  cste  trabajotuvieronporobjetoreunirinformacionparacncontrarmetodosmcjoradosdcpcscadeatun  y, 
especialmente,  un  suslitutiv6  del  echo  vivo.  En  una  scrie  de  expcrimentos  en  estanqucs  se  encontr6  que  el  atun  de  aleta  amarilla  y  cl  atunito 
reaccionaban  bien  a  la  luz  blanca  de  intensidad  moderada  pcro  no  asi  a  una  debil.  Ademas  se  observo  cl  cfecto  dc  las  substancias  quimicas 
lograndosc  una  reaecion  al  usar  extractos  acuosos  o  alcohol icos  transparentcs  de  carnc  de  atun.  Sin  embargo  los  peces  se  alejaron  al  utilizar 
acetato  de  cobre  que  se  utiliza  para  rcpeler  a  los  tiburones.  Tambien  se  invesligcS  la  posibilidad  de  ernplear  la  clectricidad  para  la  captura 
de  los  peces  que  nadan  en  la  superficie.  demostrandose  que  es  posible  producir  la  clectrotaxia  en  cl  atun  mediante  un  campo  electrico  de 
caracteristicas  conocidas,  pero  este  problema  cae  mas  bien  dentro  del  terreno  dc  la  ingenicria  electrica  que  de  la  biologia. 


DURING  the  period  1951  and  1956,  several  staff 
members  of  the  University  of  Hawaii  conducted 
experiments,    under  contract    with    the    Pacific 
Oceanic  Fishery  Investigations  (POF1)  of  the  U.S.  Fish 
and  Wildlife  Service,  on  the  response  of  tuna  to  various 
types  of  stimuli.    The  writer  was  leader  of  several  of 
these   projects   prior   to  joining  the   Service  in    1955. 
This  is  a  brief  summary  of  the  work  to  date. 

The  primary  objective  of  the  study  was  to  obtain 
information  which  might  suggest  new  or  improved 
methods  of  tuna  fishing.  Efforts  were  directed  particu- 
larly at  devising  a  substitute  for  baitfish,  the  scarcity 
and  high  mortality  of  which  limits  the  catch  of  surface 
tuna  in  Hawaii  and  prevents  the  expansion  of  the  live- 
bait  fishery  to  potentially  productive  offshore  areas. 

ESTABLISHING  TUNA  IN  CAPTIVITY 

Methods  of  fishing,  transporting  and  establishing  tuna 
in  captivity  are  described  by  Tester*.  Several  species  of 
tuna  and  tuna-like  fish  were  caught  by  surface  trolling 


off  the  windward  shore  of  Oahu,  including  the  skipjack 
(Katsuwonux  pelamis\  yellowfin  (Neothunnusmacropterus), 
and  little  tunny  (Euthynnus  yaito).  Immediately  after 
capture,  fish  were  placed  in  the  vessel's  livewell  (67 Jx 
43  -  32  in.),  which  was  supplied  with  running  sea  water 
(20  to  40  gallons/min.)  but  which,  in  addition,  was 
aerated  with  finely-divided  bubbles  of  oxygen  from  a 
pressure  tank.  They  were  then  transported  to  ponds  and 
tanks  at  the  Hawaii  Marine  Laboratory(l  to  2  hours  run), 
to  which  they  were  transferred  by  dipnet.  Yellowfin 
(about  2  to  8  Ib.  in  weight)  and  little  tunny  (about 
1  to  6  Ib.  in  weight)  were  successfully  established  in  one 
of  the  large  ponds  (350  ft.  long,  65  ft.  wide  and  averaging 
about  6  ft.  deep)  which  was  flushed  by  tidal  currents. 
Yellowfin  and  little  tunny  were  also  maintained  in  a 
concrete  tank  (35  ft.  long,  II  ft.  wide  and  4  ft.  deep) 
which  was  supplied  with  running  salt  water  (about  25 
gallons/min.).  Skipjack  were  not  successfully  established; 


*Now  Senior  Professor  of  Zoology,   University  of  Hawaii, 
Honolulu,  Hawaii,  U.S.A. 


[538] 


RESPONSE    OF    TUNA     TO     STIMULI 


of  numerous  trials  only  one  fish  was  transported  alive 
from  the  fishing  grounds  to  the  laboratory  and  it  died 
shortly  after  being  released  into  the  tank. 

Except  when  on  an  experimental  diet,  the  little  tunny 
and  yellowfin  were  fed  cut-up  fish,  usually  small  tuna 
which  died  during  our  fishing  operations  or  large  tuna 
from  POFI  longline  catches.  The  tunas  generally  started 
to  feed  within  a  week  of  introduction;  otherwise  they 
would  die.  When  feeding  fish  were  already  present  in 
the  pond  or  tank,  newly  introduced  fish  would  usually 
join  the  school  and  start  to  feed  within  one  or  two  days. 

The  established  (feeding)  tuna  survived  for  various 
lengths  of  time,  ranging  from  but  a  few  days  to  over 
2  years.  Some  loss  occurred  from  extraneous  factors 
such  as  poaching  or  accidental  stranding  of  the  fish  on 
dry  land.  Several  of  the  fish  developed  partial  or  com- 
plete blindness  and  died  either  from  starvation  or  injury. 
Others  eventually  became  "sick",  they  moved  slowly, 
listlessly  and  individually,  ceasing  to  feed  and  eventually 
dying.  Mortality  occurred  mostly  during  the  fall  and 
winter,  and  perhaps  was  associated  with  lowered  tempera- 
ture and  salinity  in  the  inshore  environment. 

Yellowfin,  both  in  the  live  well  of  the  boat  and  in 
the  pond  and  tank,  were  relatively  slow,  leisurely  swim- 
mers. When  two  fish  were  present,  they  usually  swam 
together.  When  several  were  present,  they  formed  a 
loose  school  but  often  swam  individually  or  in  two's 
or  three's.  In  contrast,  the  little  tunny  were  rapid  swim- 
mers and  usually  formed  a  single,  compact  school. 
On  occasions,  when  a  large  population  (15  to  30  fish) 
was  present  in  the  pond,  they  formed  two  schools,  but 
rarely  more. 

More  detailed  observations  on  the  captive  fish  are 
included  in  the  papers  summarized  below. 

RESPONSE  TO  VISUAL  STIMULI 

During  the  summer  of  1951,  Hsiao1  studied  the 
response  of  two  yellowfin  and  five  little  tunny  to  arti- 
ficial light.  The  fish  were  confined  in  the  concrete  lank. 
The  experiments  were  performed  after  dark,  with  the 
tank  illuminated  constantly  with  two  60  W.  bulbs. 
He  found  that  both  yellowfin  and  little  tunny  were 
attracted  to  continuous  white  light  over  a  range  of 
moderate  intensity  (about  70  to  450  ft.  candles).  They 
uere  not  attracted  by  a  light  of  weaker  intensity,  and 
they  were  repelled  by  a  light  of  stronger  intensity.  Both 
species  were  attracted  to  coloured  lights  of  moderate 
intensity,  but  to  no  greater  extent  than  to  while  light. 
Similar  results  were  obtained  with  interrupted  white 
light.  There  appeared  to  be  no  relationship  between 
the  strength  of  the  response  and  the  frequency  of  inter- 
ruption of  the  light.  Although  the  tuna  approached  an 
interrupted  light  of  moderate  intensity,  they  were 
repelled  from  Ihe  near  vicinily  at  the  instant  the  light 
flashed  either  on  or  oiV. 

The  above  experiment  suggesl  that  tuna  at  sea  might 
respond  to  continuous  light  of  moderate  intensity.  To 
my  knowledge,  however,  only  tuna  larvae  and  young 
have  been  caught  by  this  method.  If  adults  in  their 
natural  habitat  were  attracted  by  light,  it  seems  likely 
that  this  already  would  have  been  discovered  and  utilized 
by  tuna  fishermen. 

The  response  of  a  school  of  12  captive  little  tunny 


to  moving  objects  of  various  colours  in  the  large  pond 
was  studied  by  Hsiao  and  Tester2  during  the  summer 
of  1952.  During  daylight  hours,  a  pair  of  lures  was 
momentarily  dipped  into  an  "attraction"  area  once  every 
2  sec.  by  a  remote  control  from  a  high  tower.  The  lures 
consisted  of  2  in.  seclions  of  rubber  tubing  of  various 
colours.  The  number  of  "fish-seconds"  in  the  area, 
Ihe  number  of  "passes"  at  the  lures,  and  Ihe  time  were 
recorded  on  a  kymograph.  Observations  were  made 
under  conlrol  conditions,  when  lures  were  introduced, 
and  when  lures  were  used  along  with  an  extract  of 
luna  flesh  (see  later)  which  was  introduced  into  the  pond 
from  one  end.  Activity  of  the  fish  (number  of  school 
entrances,  time  spent  in  the  area,  and  swimming  speed) 
was  greater  when  lures  were  used,  as  compared  with 
control  conditions,  and  was  still  greater  when  lures 
and  extract  were  used  together.  Although,  in  general, 
the  fish  made  more  passes  at  white  than  at  coloured 
lures  (red,  black,  silver),  the  superiority  of  the  white 
lures  was  slight.  It  may  have  been  associated  with  greater 
visibility  rather  than  colour  preference.  There  is  no 
assurance  that  white  lures  would  be  superior  to  coloured 
lures  in  the  open  ocean.  Trolling  data  collected  during 
1951  and  1953  (Tester  and  Nakamura0)  show  no 
preference  by  the  tuna  with  respect  to  either  form  or 
colour  of  the  lure. 

The  above  experiments  deal  with  only  one  aspect  of 
visual  response,  namely  lure  preference.  Specific  pond 
experiments  on  visual  acuity  were  not  undertaken. 
Casual  observations  indicated,  however,  that  both 
yellowfin  and  little  tunny  could  perceive  objects  such  as 
stones  or  pieces  of  food  which  were  thrown  to  them 
while  the  objects  were  still  in  the  air.  The  fish  would 
speed  rapidly  toward  the  point  where  such  objects 
would  hit  the  waler.  If  the  objects  were  not  thrown  so 
that  Ihe  fish  could  sec  Ihem  in  iransit,  the  tuna  could 
be  attracted  either  by  the  splash  made  when  the  object 
hit  the  water,  or  perhaps  by  the  object  itself  over  a  dis- 
tance of  about  50  ft. 

In  view  of  the  conclusion  (see  later)  that  visual  per- 
ception was  of  prime  importance  in  attracting  luna  lo  Ihe 
stern  of  a  fishing  boat  by  the  use  of  livebait,  Matthews3" 
studied  the  comparative  morphology  of  tuna  eyes, 
during  1955-56,  using  species  of  diverse  habits  and 
habitats  (skipjack,  yellowfin,  and  bigeye).  No  significant 
differences  were  observed.  In  addition,  he  conducted 
experiments  to  determine  the  focal  length  of  the  lens 
and  other  optical  properties  of  the  excised  eyes  of 
freshly-killed  tuna  immersed  in  a  long  salt  trough, 
viewing  a  light  source  through  a  "window"  in  the 
chorion  of  the  vitreous  chamber,  but  with  confusing 
results  (unpublished).  Contract  work  in  1956-57  was 
devoted  to  a  study  of  the  comparative  histology  of  the 
retina  in  skipjack,  yellowfin,  bigeye,  and  albacore, 
including  observations  on  the  frequency  and  arrange- 
ment of  Ihe  rod-cone  mosaics.  Preliminary  results'** 
indicate  a  relatively  large  number  of  rods  in  the  retina 
of  the  albacore,  suggesting  relatively  greater  adaptation 
to  dim  light  than  with  the  other  three  species. 

RESPONSE  TO  AUDITORY  STIMULI 


Miyake1  conducted  experiments  with  captive  yellowfin 
and  little  tunny  during  1951-52  to  discover  (1)  if  tuna 


[539] 


MODERN     FISHING    GEAR     OF    THE    WORLD 


produced  any  sound  and  (2)  if  they  could  be  attracted 
or  repelled  by  sounds  of  various  frequencies.  Using  a 
listening  frequency  ranging  from  about  0-1  to  70  kc.,  he 
was  able  to  identify  low  frequency  sounds  produced  by 
the  sudden  movement  of  the  tail  of  the  yellowfin  in 
the  tank.  This  might  have  some  significance  with  respect 
to  the  mechanism  of  school  formation.  No  sounds 
produced  by  tuna  were  detected  at  listening  frequencies 
in  the  moderate,  high,  or  supersonic  range.  In  attempting 
t  o  attract  or  repel  tuna  by  continuous  sound  stimulation 
*n  the  pond,  sounds  were  emitted  at  many  frequencies 
from  0-1  to  70  kc.  Although  the  results  were  incon- 
clusive, there  were  indications  that  yellowfin  might  be 
attracted  by  complex  sounds  of  low  frequency. 

RESPONSE  TO  CHEMICAL  STIMULI 

Van  Weel12  studied  chemoreception  in  both  yellowfin 
and  little  tunny  in  the  concrete  tank  during  1951.  He 
found  that  both  fish  had  a  well-developed  sense  of  smell 
or  taste  whereby  they  were  attracted  to  certain  food 
substances.  They  were  strongly  attracted  to  clear, 
colourless  extracts  of  tuna  flesh.  Moreover,  it  was  found 
that  the  attractant  was  contained  in  the  protein  rather  than 
in  the  fat  fraction  of  the  clear  extract.  In  general,  the 
response  of  the  little  tunny  was  more  pronounced  than 
that  of  the  yellowfin.  On  the  other  hand,  there  was  no 
positive  response  of  either  species  to  conditioned  water 
in  which  baitfish  had  been  living,  nor  to  extracts  of 
either  baitfish  or  squid.  Copper  acetate,  a  shark  repellent, 
was  also  repellent  to  tuna,  although  its  effect  was  not 
as  pronounced  as  on  fish  of  other  species  which  were 
also  present  in  the  tank. 

The  above  study  was  continued  in  1952-53  by  Tester, 
Van  Weel,  and  Naughton10.  In  the  tank,  improvements 
were  made  in  the  techniques  of  observation,  introduction 
of  test  material,  and  measurement  of  the  response. 
Testing  was  hampered  by  accumulation  of  test  materials 
because  of  the  small  volume  of  water  in  the  tank  and  its 
slow  replacement.  This  problem  was  reduced  by  con- 
ducting tests  on  tuna  established  in  the  large  pond, 
introducing  test  material  through  a  continuous  stream 
of  water  supplied  at  one  end  of  the  pond  by  a  pump, 
and  observing  the  response  of  the  fish  from  a  20  ft.  tower, 
which  overlooked  an  "attraction"  area  extending  from 
about  40  to  about  80  ft.  below  the  outlet.  Pond  testing 
was  hampered  by  such  factors  as  poor  visibility  due  to 
weather,  power  failure,  and  erratic  behaviour  of  the  fish. 

The  results  confirmed  Van  Weel's12  observations  that 
a  positive  response  was  obtained  from  clear,  aqueous 
or  alcohol  extracts  of  tuna  flesh.  This  took  the  form  of 
a  feeding  reaction  and  included  one  or  more  of  the 
following  components:  speeding  or  acceleration  of  the 
swimming  rate,  a  return  to  the  area  of  stimulation, 
surfacing,  fanning-out  and  eventually  a  breakdown  of 
school  formation,  circling,  splashing,  and  biting  at 
incidental  objects  on  the  surface  of  the  water.  It  was 
postulated  that  an  attractant  was  present  in  the  flesh, 
viscera  and  blood  of  several  species  of  tuna  and  certain 
other  fish.  Successful  methods  of  concentration  and 
preservation  of  this  substance  were  achieved  in  prepar- 
ation for  large-scale  sea-testing.  Considerable  time  was 
devoted  to  its  fractionation,  purification,  and  identifi- 
cation. 


In  addition  some  40  chemical  compounds  were  tested, 
including  amino  acids,  vitamins,  aromatics,  proteins, 
etc.  With  some  of  these  there  seemed  to  be  a  sensing 
of  the  dissolved  or  suspended  materials  but  in  no  case 
did  the  response  include  all  the  typical  components  of 
a  feeding  reaction  induced  by  fish  extract. 

The  conditioning  of  the  tuna  was  discussed  with 
respect  to  their  response  to  fish  extract.  Obviously  they 
had  become  conditioned  to  feeding  on  inert,  dead  rather 
than  motile,  living  food  for  they  had  little  interest  in 
schools  of  small  baitfish  which  at  times  were  present  in 
the  pond  in  fair  abundance  and  which  would  leisurely 
withdraw  as  the  tuna  approached.  Obviously,  also,  they 
had  become  conditioned  to  being  fed  for  they  would 
mill  close  to  an  observer  on  his  approach  to  the  pond. 
It  was  concluded  that  the  response  of  the  tuna  was  not 
directly  conditioned  by  the  kind  of  food,  i.e.  the  species 
of  fish  used  as  food.  However,  there  was  the  possibility 
that  the  tuna  formed  an  association  between  feeding  and 
the  smell  or  taste  of  the  dead  food,  which  was  cut  up 
or  otherwise  macerated  and  which  might  exude  juices  of 
similar  composition.  If  so,  the  response  would  not 
necessarily  be  obtained  from  wild  fish  at  sea. 

Tester,  Yuen  and  Takata11  reported  on  further  experi- 
ments with  little  tunny  in  the  pond  and  with  skipjack 
schools  at  sea.  Pond  experiments  were  improved  by 
introducing  test  substances  into  a  continuous  stream 
of  water  which  flowed  directly  into  the  attraction  area 
at  the  foot  of  the  observation  tower.  Continued  screen- 
ing of  some  75  known  compounds  indicated  a  mere 
"sensing"  of  strong  smelling  materials.  A  carefully 
designed  experi  ment  involving  feeding  the  fish  at  successive 
weekly  intervals  with  cut-up  squid,  skipjack,  and  shrimp 
and  testing  their  response  to  extracts  of  these  same 
substances  was  conducted.  A  response  to  ail  three 
extracts  was  obtained  regardless  of  the  food  being 
fed,  but  the  response  to  a  particular  extract  was  slightly 
greater  when  the  substance  from  which  it  was  prepared 
was  being  used  as  food.  This  experiment  tended  to  con- 
firm the  suspicion  voiced  earlier  that  the  fish  were  con- 
ditioned to  the  smell  of  juices  exuded  from  the  food 
which  presumably  contained  common  or  similar  sub- 
stances which  stimulated  the  feeding  response.  It  was 
hoped  that,  even  so,  tuna  in  their  natural  environment 
would  respond  to  the  extract,  which  might  contain  a 
substance  exuded  by  living,  injured,  or  uninjured  prey 
to  which  "wild1'  fish  were  naturally  conditioned.  This 
hope  was  riot  realized. 

Large  quantities  of  extracts  of  skipjack,  yellowfin, 
and  anchovy  were  prepared  during  the  winter,  spring,  and 
summer  of  1953,  tested  on  the  captive  little  tunny  and 
presented  to  schools  of  skipjack  at  sea.  In  a  few  instances 
the  schools  consisted  mostly  of  skipjack  but  included 
yellowfin,  little  tunny,  or  frigate  mackerel.  In  some 
tests  the  concentrated  liquid  extract  was  sprayed  on  the 
surface  or  pumped  in  a  stream  in  the  path  of  the  schools. 
In  others,  it  was  presented  to  schools  which  had  been 
chummed*  to  the  stern  of  the  vessel  by  livebait  (Stole- 
phorus  purpureus).  The  results  were  either  negative  or 
inconclusive.  Unchummed  schools  could  not  be  stopped 
by  an  arc  or  circle  of  extract.  Chummed  schools  could 
not  be  held  at  the  stern  even  when  the  extract  was 


+Chum — live  or  chopped  fish  used  as  a  lure. 


[540] 


RESPONSE    OF    TUNA    TO    STIMULI 


bucketed  overboard.  In  a  few  tests,  on  releasing  a  large 
quantity  of  the  extract  after  the  school  had  been  chummed 
to  the  stern  and  chumming  was  stopped,  a  few  fish  were 
seen  jumping  in  or  passing  through  the  material.  It 
was  uncertain  whether  they  were  responding  to  the 
extract  or  chasing  stray  baitfish.  The  apparently  negative 
results  discouraged  any  further  testing  of  extracts  except 
in  conjunction  with  lures  as  reported  below. 

RESPONSE  TO  EDIBLE  AND  INEDIBLE  LURES 

Tester,  Yuen,  and  Takata11  describe  pond  tests  with 
edible  lures,  and  sea  trials  with  both  edible  and  inedible 
lures.  The  pond  tests  showed  that  little  tunny  would  eat 
edible  preparations  such  as  gelatin  capsules,  pieces  of 
macaroni,  and  strips  of  agar  gelatin,  and  that  these 
were  more  avidly  consumed  it  they  were  made  chemically 
attractive,  or  palatable,  with  concentrated  extracts  of 
skipjack  or  anchovy.  In  some  cases,  also,  the  feeding 
response  seemed  to  be  greater  if  the  lures  were  made 
more  visually  attractive,  i.e.,  silvery  in  colour.  The 
fish  showed  a  distinct  preference  for  the  agar  preparations. 

Again,  the  results  of  sea  tests  of  edible  lures  were 
negative.  However,  they  were  conducted  during  the 
autumn  when  skipjack  schools  were  scarce  and  erratic  in 
their  behaviour.  Several  of  the  schools  which  failed  to 
respond  to  our  most  promising  preparation  (agar  strips 
impregnated  with  concentrated  extract  and  with  alu- 
minum powder)  also  failed  to  respond  to  livebait. 

Several  types  of  inedible  lures  were  tested  at  sea. 
These  were  used  as  chum  both  alone  and  along  with 
liquid  extract  after  the  fish  had  been  initially  chummed 
to  the  stern  of  the  ship  with  baitfish.  There  was  a  momen- 
tary response  to  shiny  objects  such  as  strips  of  tin 
(several  of  which  were  eaten),  to  silvery  objects  such  as 
squares  of  aluminum  foil,  and  to  effervescing  objects 
such  as  calcium  carbide  pellets.  Similarly  there  was  a 
momentary  response  to  dead  baitfish.  The  addition  of 
extract  had  no  apparent  effect.  The  results  indicated 
that  the  sense  of  vision  plays  a  much  greater  role  in 
feeding  than  the  sense  of  smell.  However,  neither  the 
visual  lures  nor  a  "drag-lure  array'*  (designed  to  simulate 
a  school  of  baitfish)  towed  behind  the  vessel,  was 
successful  in  holding  the  fish  at  the  stern. 

On  the  hypothesis  that  motion  was  an  essential 
component  in  visual  attraction,  attempts  were  made 
(unpublished)  to  devise  an  expendable,  self-propelled 
motile  lure  which  would  simulate  baitfish  movement,  and 
which  would  be  made  both  visually  and  chemically 
attractive.  Many  types  of  "motors"  (mostly  generating 
air  or  gas)  were  devised,  but  none  was  satisfactory. 
A  compressed-air  "machine  gun"  was  designed  to 
project  and  at  the  same  time  activate  compressed  air 
cartridges.  Following  an  unsuccessful  sea  trial,  hampered 
by  imperfections  in  the  machine  and  poor  behaviour 
of  the  released  cartridges,  this  idea  was  abandoned. 

RESPONSE  TO  ELECTRICAL  STIMULI 

Before  conducting  experiments  on  the  response  of  tuna 
to  electrical  stimuli,  it  was  considered  best  to  obtain 
pertinent  basic  data  on  a  more  readily  available  marine 
species  which  could  be  easily  kept  in  captivity.  During 
1951,  Tester7  studied  the  response  of  aholehole  (Kuhlia 


sandvicensis)  to  interrupted  direct  current  using  a  D.C. 
generator  and  various  types  of  mechanical  interrupters. 
In  a  small  salt  water  tank  (12  <2xl  ft.)  the  fish  were 
forced  to  swim  to  the  positive  electrode  when  subjected 
to  suitable  combinations  of  current  and  frequency  of 
interruption.  The  results  indicated  that,  at  a  frequency  of 
1 5  cycles  a  progressive  saving  of  power  could  be  obtained 
by  reducing  the  fcicurrent-on"  fraction  of  a  cycle  from 
about  0-7  to  0-1.  Power  conservation,  of  course,  is  an 
important  consideration  in  the  practicality  of  marine 
ehctrofishing. 

The  above  results  were  confirmed  and  extended  by 
Miyake  and  Steiger'  in  further  experiments  with  aholehole 
during  1954  and  1955,  using  storage  batteries  as  the 
source  of  power.  They  concluded  that  the  optimum 
frequency  for  electrotaxis  was  10  cycles  and  that  the 
minimum  peak  current  for  satisfactory  response  (12  amp) 
was  associated  with  an  "on-fraction"  of  0-06  to  0-08. 
Total  peak  current  requirements  decreased  with  increase 
in  length  of  the  fish  extrapolating  the  above  results, 
they  concluded  that  a  current  of  130  amp.  at  a  potential 
of  60  V.  would  be  required  to  induce  electrotaxis  in  a 
fish  the  size  of  a  small  tuna  (30  cm.)  using  plane  1 1  x4  ft. 
electrodes  spaced  a  distance  of  33  ft.  This  was  best 
achieved  by  condenser  discharge.  Accordingly  an  appara- 
tus was  constructed  for  experiments  with  tuna  and  other 
large  fish  in  the  concrete  tank.  It  consisted  of  a  bank 
of  capacitors  (55,000  mfd.)  charged  by  any  number  up 
to  10  pairs  of  automobile  storage  batteries  arranged  in 
series  parallel.  The  charging  and  discharging  of  the 
capacitors  was  controlled  by  a  variable  speed  mechanical 
contactor. 

With  electrodes  spaced  at  33  ft.  perfect  electrotaxic 
response  was  obtained  with  a  jack  (Carynx  sp.\  using 
60  V.  (2  banks  of  10  batteries)  at  10  cycles.  However, 
with  little  tunny  and  ycllowfin  the  results  were  either 
inconclusive  or  negative.  By  spacing  the  electrodes  a 
distance  of  16  ft.,  thus  increasing  the  electric  field,  an 
excellent  electrotaxic  response  was  repeatedly  obtained 
with  yellowfin  tuna  (about  50  cm.  in  length).  The  results 
showed  that  power  requirements  decreased  (10  to  6 
pairs  of  batteries)  as  the  frequency  increased  to  20 
cycles,  the  limit  of  the  apparatus.  Theoretical  calcula- 
tions indicated  that  23  times  as  much  power  would  be 
dissipated  in  the  open  sea  as  in  the  closed  tank  and  that 
a  correspondingly  higher  power  output  would  be  required 
to  effect  electrotaxis  with  the  same  electrode  spacing  and 
frequencies 

In  the  same  report,  Miyake  and  Steiger5  presented 
theoretical  studies  of  the  potential,  electric  field,  and 
current  density  for  spherical  electrodes  submerged  in  a 
large  body  of  water.  They  also  investigated  the  theoretical 
relation  ship  between  the  head-to-tail  potential  and 
current  density  in  a  fish  as  determined  by  the  relative 
conductivities  of  the  fish  and  water. 

Following  the  1954-55  results,  reported  above,  the 
possibilities  of  utilizing  an  amplidyne  to  generate  a 
pulsed  direct  current  of  satisfactory  strength,  wave- 
form, and  frequency  for  inducing  electrotaxis  were 
investigated  in  the  laboratory  with  promising  results 
(unpublished).  Unfortunately  continuation  of  the  con- 
tract work  has  not  been  possible  because  the  contractors 
have  assumed  other  responsibilities. 


[541] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


DISCUSSION 

Despite  the  large  amount  of  information  which  has  been 
obtained  on  the  response  of  both  captive  and  wild  tuna 
to  stimuli  of  various  kinds,  little  progress  has  been  made 
toward  attaining  the  main  objective.  This  was  to  devise 
a  method  of  catching  surface  tuna,  which  was  indepen- 
dent of  the  use  of  livebait. 

It  was  realised  at  the  outset  that  our  approach  was 
largely  "trial  and  error"  and  that  the  probability  of 
success  was  small.  It  was  also  realized  that  results 
obtained  on  captive  fish,  conditioned  to  pond  life,  might 
not  be  capable  of  extrapolation  to  wild  fish  of  thfe  open 
sea.  Nevertheless,  the  approach  seemed  worthwhile  in 
providing  leads  which  might  be  tested  by  sea  trials. 

The  pond  tests  demonstrated  that  the  active  little 
tunny  and  the  less  active  yellowfin  both  have  a  keen 
sense  of  smell  (or  taste),  together  with  excellent  vision 
both  through  water  and,  with  a  calm  surface,  through 
air.  The  sea  tests  indicated  that  the  sense  of  smell 
plays  little,  if  any,  part  in  natural  feeding,  although 
this  is  still  an  open  question  (a)  because  of  the  difficulty 
in  making  observations  on  the  response  of  fish  at  sea, 
and  (b)  because  the  materials  tested  may  not  have 
included  those  which  might  be  associated  with  living 
prey.  These  studies  might  well  be  repeated  or  perhaps 
extended  in  view  of  our  recent  discovery  of  a  natural 
laboratory,  a  spot  or  "concourse"  in  the  lee  of  an  island 
where  skipjack  are  always  present  and  where  their 
behaviour  and  response  may  be  readily  observed  by 
underwater  viewing  devices  which  are  presently  being 
perfected. 

Sea  tests  further  indicated  that  vision  played  an 
important,  and  perhaps  a  dominating  part  in  natural 
feeding  and  that  motiiity  of  a  lure,  whether  living  or 
dead,  whether  edible  or  inedible,  was  an  important 
attribute.  These  observations,  also,  can  be  repeated 
and  extended  at  the  skipjack  concourse. 

Although  most  of  our  pond  observations  were  made 
on  little  tunny  and,  to  a  lesser  extent,  on  yellowfin,  our 
sea  tests  were  conducted  mostly  on  schools  of  skipjack. 
This  again  leaves  an  element  of  uncertainty  in  drawing 
conclusions  regarding  the  negative  results  of  sea  tests. 

The  response  of  tuna  to  auditory  stimuli  was  not 
adequately  investigated;  this  work  was  abandoned  in 
favour  of  the  more  promising  fields  of  chemical  and 
electrical  stimulation. 

The  possibility  of  electrofishing  for  surface  tuna, 
either  with  or  without  the  use  of  bait,  remains  a  promising 


area  of  study.  Now  that  it  has  been  demonstrated  that 
electrotaxis  may  be  induced  in  tuna  with  an  electric 
field  of  known  characteristics,  the  problem  is  one  of 
engineering  rather  than  biology. 

Future  efforts  to  devise  new  or  improved  methods 
of  tuna  fishing  will  be  enhanced  by  further  studies  of  the 
behaviour  of  tuna  and  their  response  to  factors  of  the 
environment.  Behaviour  studies  are  rapidly  becoming 
one  of  the  major  areas  of  research  of  the  Pacific  Oceanic 
Fishery  Investigations  and  will  occupy  more  and  more 
of  our  attention  in  the  years  to  come. 


REFERENCES 

1  Hsiao,  Sidney  C.  Reaction  of  tuna  to  stimuli — 1951.  Part  III. 
Observations  on  the  reaction  of  tuna  to  artificial  light.  U.S.  Fish 
and  Wildlife  Service,  Spec.  Sci.  Rept.— Fish.  No.  91,  pp.  36-58. 
1952. 

2Hsiao,  Sidney  C.  and  Tester,  Albert  L.  Reaction  of  tuna  to 
stimuli —1952-53.  Part  2.  Response  of  tuna  to  visual-chemical 
stimuli.  U.S.  Fish  and  Wildlife  Service,  Spec.  Sci.  Rept.— Fish. 
No.  130,  pp.  63-76.  1955. 

3  Matthews,  Donald  C\  MS.  (a)  A  comparative  morphological 
study  of  tuna  eyes.        MS.  (b)  A  comparative  histological  study  of 
tuna    retinae. 

4  Miyake,  Iwao.  Reaction  of  tuna  to  stimuli — 1951.    Part  IV. 
Observations  on  sound  production  and  response  in  tuna.  U.S.  Fish 
and  Wildlife  Service,  Spec.  Sci.  Rcpt.   -Fish.  No.  91,  pp.  59-68. 
1952. 

f>  Miyake,  Iwao  and  Sleiger,  Walter  R.  The  response  of  tuna 
and  other  fish  to  electrical  stimuli.  U.S.  Fish  and  Wildlife  Service. 
Spec.  Sci.  Rept.— Fish.  No.  223  (in  print).  1957. 

6  Tester,  Albert  L.  Establishing  tuna  and  other  pelagic  fishes 
in  ponds  and  tanks,  U.S.  Fish  and  Wildlife  Service,  Spec.  Sci. 
Rept.  -Fish.  No.  71,  33  p.  1952a. 

I  Tester,  Albert  L.    Reaction  of  tuna  to  stimuli— 1 95 1.    Part  I. 
Background  and   summary   of  results.    U.S.   Fish   and   Wildlife 
Service,  Spec.  Sci.  Rept.— Fish.  No.  91,  pp.  1-7.      1952b. 

8  Tester,  Albert  L.    Reaction  of  tuna  to  stimuli  —195 1.  Part  V. 
Notes  on  the  response  of  a  tropical  fish  (Kuhlia  sandvicensis)  to 
interrupted  direct  current.    U.S.  Fish  and  Wildlife  Service,  Spec. 
Sci.  Kept.— Fish.  No.  91,  pp.  69-83.    1952c. 

9  Tester,  Albert  L.  and  Nakamura,  Eugene  L.   Catch  rate,  size, 
sex,  and  food  of  tunas  and  other  pelagic  fish  taken  by  trolling  off 
Oahu,  Hawaii,  195M955.     U.S.  Fish  and  Wildlife  Service,  Sp. 
Sci.  Rept.— Fish.  No.  250,  1957. 

10  Tester,  Albert  L.,  Van  Weel,  P.  B.  and  Naughton,  John  J. 
Reaction  of  tuna  to  stimuli  -1952-53.  Part  1.  Response  of  tuna  to 
chemical   stimuli.      U.S.   Fish  and   Wildlife  Service,   Spec.   Sci. 
Rept.   -Fish.   No.   130,  pp.   1-62.     1955. 

II  Tester,  Albert  L.,  Yuen,  Hceny  and  Takata,  Michio.  Reaction 
of  tuna  to  stimuli,  1953,  U.S.  Fish  and  Wildlife  Service,  Spec.  Sci. 
Rept.— Fish.  No.  134,  pp.  33.     1954. 

12  Weel,  Van  P.  B.  Reaction  of  tuna  to  stimuli— 1951.  Part  II. 
Observations-  on  the  chemoreeption  of  tuna.  U.S.  Fish  and 
Wildlife  Service,  Spec.  Sci.  Rept.— Fish.  No.  91,  pp.  8-35.  1952. 


542] 


FUNDAMENTAL  STUDIES  ON  THE  VISUAL  SENSE  IN  FISH 

by 
TAMOTSU  TAMURA 

Fisheries  Institute,  Faculty  of  Agriculture,  Nagoya  University,  Anzyo,  Aiti-Prefecture,  Japan 


Abstract 

This  paper  deals  with  the  efficiency  of  the  fish's  visual  sense.  The  direction  of  its  optical  axis  and  its  angle  of  vision  and  certain 
facts  concerning  this  subject  are  given. 

The  author  describes  the  anatomy  of  the  various  optical  systems  of  different  fish  and  shows  how  different  visual  axes  are  associated 
with  different  modes  of  living.  Interesting  experiments  on  the  ability  to  see  cotton  and  nylon  threads  were  carried  out,  and  the  fish's  reaction 
to  bait  was  studied.  For  instance,  there  must  be  movement  before  the  fish  is  interested,  and  while  a  fish  can  probably  see  its  prey  at  some 
distance,  it  is  the  motion  of  the  water  produced  by  the  bait  which  makes  the  fish  snap  at  it. 


Resume 


Etudes  fondamentalcs  sur  le  sens  visuel  des  poissons 


Cette  6tude  traite  de  I'cfficacitc  du  sens  visuel  des  poissons.  L'auteur  indique,  1'uxe  optique  et  Tangle  dc  vision  des  poissons  ainsi 
que  certains  faits  relatifs  a  cette  question. 

L'auteur  decrit  I'anatomie  des  systemes  optiques  de  differents  poissons  ct  monlre  comment  les  axes  visuels  diffeients  se  trouvcnt 
associes  a  des  modes  d'existence  egalemcnt  differcnts.  Des  experiences  interessantes  ont  etc  effectuees  sur  I'aptitude  du  poisson  a  vpir  les 
fils  dc  colon  et  de  nylon  ct  la  reaction  du  poisson  a  Pappat  a  etc  etudiee.  11  faut  par  exemple  qu'il  y  ait  mouvement  pour  intercsser  le  poisson, 
et  si  celui-ci  peut  probablementt  apcrcevoir  sa  proie  a  une  certa«ne  distance  c'est  le  mouvement  dc  Peau  engendre  par  1'appat  qui  fait  que  le 
poisson  se  precipite  pour  1'avaler. 

Estudios  fundamentales  sobre  el  sentido  de  la  vista  en  ios  peces 
Extracto 

Este  trabajo  trata  de  la  cficiencia  dc  la  vista  de  Ios  peces  y  da  la  dircccion  del  eje  optico  y  el  angulo  visual;  ademas  trata  de  diversos 
asuntos  rclacionados  con  la  materia. 

F.I  autor  describe  la  anatomia  dc  Ios  organos  tie  la  vista  dc  varias  cspecies  y  da  a  conocer  las  diferencias  dc  sus  ejes  opticos  con  sus 
diferentes  maneras  de  vida.  Se  han  hecho  interesantcs  expcrimentos  para  detcrminar  si  Ios  peces  ven  Ios  hilos  de  algodon  o  ny!6n  y  la 
manera  en  que  rcaccionan  con  Ios  diversos  tipos  dc  cebo.  For  ejemplo,  si  bien  pucdcn  probablemenlc  ver  su  prcsa  a  cierta  distancia,  el  cebo 
debe  mover  el  agua  delante  de  ellos  para  que  dcspierte  su  intercs  y  lo  atrapen. 


FORM  PERCEPTION 

ABILITY  lo  perceive  form  depends  on  two  factors: 
the  resolving  power  of  the  dioptric  system,  and 
the  resolving  power  of  the  retina.    The  first  is  a 
function  of  both  the  resolving  power  of  the  lens  and 
accommodation.    The  second  presumably  depends  on 
the  spacing  of  cones  in  the  retina.     The  direction  along 
which  the  visual  acuity  is  highest  is  considered  to  be 
the  direction  of  the  visual  axis  of  the  fish. 

Resolving  power  of  lens  and  retina 

In  fish,  the  cornea  and  the  vitreous  humour  are  not 
responsible  for  image  formation.  The  crystalline  lens 
is  the  only  dioptric  element  of  the  eye  involved  in  image 
formation.  The  resolving  power  of  crystalline  lenses 
over  approximately  5  mm.  in  diameter  was  found  to 
range  from  54  to  90  seconds  of  arc. 
The  cone  density  varies  with  different  species  and 


also  with  regions  of  the  retina.  The  retinae  of  all  the 
fish  examined  were  therefore  studied  topographically. 
Each  retina  was  divided  into  seven  regions:  temporal, 
dorso-temporal,  ventro-temporal,  dorsal,  nasal,  ventral 
and  bottom.  The  cone  density  of  each  region  was 
measured  from  photomicrographs. 

The  fish  were  divided  into  three  groups  according  to 
the  retinal  region  where  the  cone  density  is  highest: 
i.e.,  dorso-temporal  (Sparus  hasta,  etc.);  temporal 
(Epinenphelus  septemfasciatus,  etc.);  ventro-temporal 
(Trachurus  japonicus>  etc.). 

The  minimum  separable  angle  was  calculated  on  the 
assumption  that  image  lines  can  only  be  resolved  when 
they  fall  on  cones  separated  by  at  least  one  unstimulated 
cone.  The  calculated  angle,  which  varies  from  4-2  min. 
of  arc  in  Epinephelus  septemfasciatus  to  15-4  min.  in 
Chlorophthalmus  albatrossis,  is  obviously  more  than  the 
resolving  power  of  the  lens.  It  may  be  concluded, 


[543] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


therefore,  that  the  resolving  power  is  a  function  of  the 
retina  rather  than  of  the  lens. 

Accommodation  and  visual  axis 

Beer1,  in  his  thorough  investigation  of  accommodation 
in  the  fish's  eye,  showed  that  fish  eyes  are  myopic  in 
the  resting  state,  and  that  accommodation  to  distant 
objects  is  accomplished  by  retraction  of  the  lens  towards 
the  retina.  Retraction  is  effected  by  the  musculus 
retractor  lentis  (Campanula  Halleri). 

The  method  of  measuring  accommodation  used  by 
Tamura7  differed  somewhat  from  Beer's,  and  produced 
additional  information. 

All  the  eyes  examined  in  fish  immovably  fixed  in  sea 
water  between  two  pieces  of  rubber  sponge  (called 
"pre-treatment"),  were  either  emmetropic  or  hyper- 
metropic,  but  became  myopic  in  certain  directions  when 
atropine  or  curare  was  injected  or  when  the  optic  nerve 
was  sectioned  ("post-treatment").  The  results  of  these 
refraction  measurements  are  not  directly  comparable  with 
Beer's.  In  Tamura's  report  the  pre-treatment  state  is 
assumed  to  be  the  resting  state  of  the  eye,  whereas  Beer 
assumed  the  resting  state  to  be  that  produced  when 
curare  or  atropine  was  injected.  Whether  fish  eyes  are 
myopic,  emmetropic  or  hypermetropic  in  the  true 
resting  state  in  nature  is  an  important  problem  for 
future  research. 

Further,  the  degree  of  myopia  obtained  by  Beer  does 
not  necessarily  reflect  the  highest  myopia  of  the  fish, 
because  he  consistently  measured  refraction  along  one 
line  only,  i.e.,  a  line  passing  through  the  centre  of  the 
lens  to  a  point  near  the  scotema.  In  Tamura's  experiments, 
refraction  was  measured  from  six  points  in  the  visual 
field:  fore,  upper-fore,  lower-fore,  upper,  lower,  and 
lateral.  In  most  cases,  the  fore  and  upper-fore  or  lower- 
fore  fell  within  the  binocular  field. 

Beer's  values  of  myopia  ranged  from  —3  to  —10 
diopters,  and  —12  in  one  extreme  case  (mean  —  6-1), 
while  Tamura's  values  ranged  from  —10  to  —25  diopters 
(mean  —14). 

For  experiments  on  the  direction  of  accommodation, 
Tamura  divided  the  fish  into  three  groups:  accommoda- 
tion performed  along  the  lower-fore,  the  fore,  and  the 
upper-fore  directions.  These  directions  are  usually 
served  by  the  retinal  region  of  highest  cone  density  and 
coincide  with  the  visual  axes. 

The  relative  positions  of  the  ligamentum  suspensorium 
and  retractor  lentis  in  relation  to  the  lens  offer  another 
means  of  estimating  the  direction  of  accommodation, 
namely  the  visual  axis.  Fish  having  lower-fore  visual 
axes  have  the  ligament  attachment  on  the  naso-dorsal 
surface  of  the  lens,  and  the  muscle  attachment  on  the 
ventro-temporal  surface  of  the  lens.  Contraction  of  the 
muscle  would  move  the  lens  up  and  back  towards  the 
dorso-temporal  region  of  the  retina.  Similarly,  fish 
having  the  fore  visual  axes  have  the  ligament  attached 
dorso-temporal ly  and  the  muscle  attached  ventro- 
nasally,  indicating  less  movement  along  the  upper-fore 
axis. 

The  ecological  significance  of  different  visual  axes  is 
evident  when  one  considers  the  feeding  behaviour  of 
the  various  fish  examined. 

Pagrosomus,  Sparus,  Evynnis,  Leiognathus  and  Xerusus, 


which  have  lower-fore  visual  axes,  are  bottom  feeders. 

Epinephelus,  Sebastiscus,  Helicolenus  and  Pseudoblen- 
nius,  which  have  somewhat  conspicuous  areae  laterales 
and  fore  visual  axes,  tended  to  take  food  in  front  of 
them.  These  are  fish  which  live  amongst  rocks  and 
seaweed,  where  they  attack  the  small  animals  as  they 
swim  by.  The  eyes  have  considerable  scope  of  movement 
and  are  often  focused  simultaneously  on  an  object 
directly  ahead,  indicating  the  importance  of  binocular 
vision. 

Lateolabrax,  Trachurus  and  Priacanthus,  judged  to 
have  upper-fore  axes,  tended  to  take  food  ahead  and 
above  them.  This  was  particularly  true  for  Priacanthus, 
which  ordinarily  ignored  static  food,  unless  it  was  in 
the  upper-fore  or  upper  portion  of  the  visual  field. 

The  entire  monocular  field,  usually  about  180  deg., 
is,  however,  quite  important  to  all  fish.  Peripheral 
perception  of  movement,  for  which  high  acuity  is  not 
essential,  facilitates  their  detection  of  both  prey  and 
predator  over  the  entire  visual  field.  An  instance  of 
this  movement  perception  in  Lateolabrax  japonicus  is 
given  below.  So  as  to  face  an  interesting  object  which  is 
first  perceived  by  its  movement,  the  fish  bends  or  turns 
its  head  and  body  and  keeps  it  in  the  visual  axis  which 
is,  in  general,  included  in  the  binocular  field.  It  is  then 
that  the  clear  form  perception,  for  which  the  accommo- 
dation and  the  acute  image  are  indispensable,  begins  to 
play  an  important  part  in  discerning  the  object. 

The  binocular  field  tends  to  be  broadest  in  the  direction 
of  the  visual  axis. 

SOME  ASPECTS  OF  THE  VISION  OF  FISH 

Diameter  of  nylon  twine  that  fish  can  recognize 

The  recognition  of  nylon  twines  by  fish  was  investigated 
by  means  of  fish  training  experiments. 

Young  Sparus  aries  and  S.  swinhonis  body  length 
about  3  cm.,  and  fed  previously  on  fresh  but  dead  Ami 
(Neomysis  japonicus),  were  used  as  test  fish.  Two  kinds 
of  nylon  monofilament,  respectively  0-42  mm.  and  0-14 
mm.  in  diameter,  were  tested. 

A  round  tank  (45  cm.  dia.),  having  a  white  inside 
surface,  was  filled  with  sea  water.  Two  white  dishes  3  cm. 
high  were  placed  in  the  tank.  A  30  cm.  wire  pole  was 
fastened  to  the  side  of  each  white  dish  and  a  white 
cotton  twine  was  hung  from  one  of  the  poles  into  the 
dish  (Dt).  The  bait,  Ami,  was  put  in  each  dish.  Five 
fish  were  then  placed  in  the  tank.  The  fish  selecting 
the  dish  (Dt)  with  the  cotton  twine  were  driven  away  by 
a  small  bamboo  stick.  The  fish  entering  the  dish  (Do) 
without  the  twine  were  allowed  to  eat  the  bait. 

Training  was  repeated  many  times  a  day,  and  the 
positions  of  Dt  and  Do  were  constantly  changed.  After 
about  two  days'  training,  most  of  the  fish  learned  to 
avoid  the  dish  with  the  twine. 

Of  the  five  fish,  the  one  which  seemed  to  learn  the 
most  was  subject  to  still  further  training.  When  the 
fish  decided  to  select  the  dish  Dt,  punishment  was 
inflicted  by  a  series  of  electric  shocks  produced  by 
electrodes  placed  in  the  tank.  This  training  was  per- 
formed under  the  dispersed  light  in  the  room. 

When  the  training  was  considered  to  be  complete, 
nylon  monofilament  was  substituted  for  the  cotton 
twine,  and  the  results  were  recorded.  During  this 


[544] 


THE    VISUAL    SENSE    IN     FISH 


period  no  punishment  was  inflicted  even  though  the 
fish  selected  the  dish  with  the  nylon. 

The  results  are  clearly  shown  in  Tables  I  and  II. 

The  thick  nylon  monofilament  (0-42  mm.  dia.)  was 
almost  always  avoided  but  the  thin  one  (0-14  mm.  dia.) 
was  not.  It  could  not,  however,  be  concluded  that  the 
thin  filament  was  not  recognized  by  the  fish. 

In  order  to  see  whether  fish  could  recognize  the  thinner 
twine  S.  swinhonis  were  trained  to  avoid  the  thinner 
nylon  instead  of  the  cotton  twine.  The  results  are  given 
in  Table  III,  which  shows  that  fish  have  the  ability  to 
perceive  a  thin  nylon  monofilament. 

Fish  which  were  trained  to  avoid  the  cotton  twine 
also  avoided  the  thicker  nylon  (0-42  mm.  dia.)  but 
tended  not  to  avoid  the  thinner  one  (0-14  mm.);  they 
did,  however,  have  the  ability  to  perceive  the  thinner 
nylon. 

Food  searching  of  Lateolabra.x  japonicus 

iMteolabrax  japonicus,  popularly  called  Suzuki  in  Japan, 
is  an  important  source  of  food  for  the  Japanese.  It  is 
generally  believed  that  this  fish  lives  on  live  fish  and 
shrimps;  live  bait  is  therefore  used  in  fishing. 

The  experiments  were  carried  out  with  young  Suzuki 
(body  length  from  4  to  6  cm.),  using  for  food  Medaka 
(Japanese  killifish*  Oryzias  latipcs)  (body  length  from 
1  to  1  -7  cm.).  A  hungry  Suzuki  of  such  size  can  usually 
eat  about  ten  Medakas  within  a  few  minutes. 


A  preliminary  experiment  showed  that  Suzuki  placed 
in  complete  darkness  or  blinded  by  removing  both  its 
eyes,  can  barely  find  and  eat  any  bait  unless  it  is  very 
plentiful. 

A  wooden  model  of  a  Medaka  and  a  dead  Medaka 
were  hung  by  a  thin  wire  in  a  tank  which  contained  a 
blind  Suzuki.  Even  when  the  blind  Suzuki  swam  near 
these  baits,  it  showed  no  reaction;  however,  if  the 
bait  was  slightly  agitated  before  the  mouth  of  the  blind 
Suzuki,  it  was  always  snapped  at.  When  the  bait  was 
a  dead  Medaka,  it  was  quickly  devoured;  when  it  was  the 
model,  it  was  vomited  immediately  upon  being  recog- 
nized. 

It  may,  therefore,  be  concluded  that  the  visual  sense 
allows  the  Suzuki  to  detect  its  prey  at  some  distance. 
The  motion  of  the  water  produced  by  the  bait,  either 
by  swimming  or  by  being  agitated,  may  play  an  important 
role  in  making  the  Suzuki  snap  at  the  bait. 

For  the  second  trial,  five  Suzukis  were  put  in  a  large 
wooden  tank.  Two  dead  Medaka  were  used  as  bait.  One 
fixed  bait  was  suspended  in  the  water  by  a  thin  nylon 
monofilament  attached  to  the  tip  of  a  rod  extending 
out  over  one  side  of  the  tank,  the  other  (moving  bait) 
was  hung  on  the  other  side  of  the  tank  so  that  it  could 
be  moved  back  and  forth. 

A  moving  bait,  expecially  that  going  back  and  forth 
rather  rapidly  (i.e.  'irregular'  motion),  at  less  than 
30  sec./cycle,  one  stroke  being  about  20  cm.,  is  more 


TABLI   I 
The   training   results   in   Spams  arics 


Date/  June 

14     15         16     17     18     19     20         21     22     23     24     25 

26         Total 

Cotton 

Correct                 7      9 

7             23 

twine 

Error                    0      0 

0               0 

Thick 

Correct                                      8       5       7     11       4 

35 

Nylon 

F.rror                                          0       1001 

2 

monofilament     Thin 

Correct                                                                                 64455 

24 

Error                                                                                    13123 

10 

TARIF  II 
The  training  results  in  Sparus  swinhonis 


Date/July/August 

20    21     22     23     24    25     26 

27 

28     29     30 

31        1       2 

3 

4     5 

Total 

Cotton 

C. 

7       8       9      4       7     10       7 

5     6 

63 

twine 

P.. 

1000000 

0     0 

1 

Thick 

C. 

2 

665 

19 

Nylon 

E. 

0 

1       0       1 

2 

monoftlament 

Thin 

C. 

2       3       1 

2 

8 

E. 

1       1       3 

8 

13 

Date  /  July  /  August 


TABLE  111 
The  training  results  in  Spams  swinhonis 

20    21     22    23     24    25     26    27    28     29     30    31       1 


Thin  Nylon 
monofilament 


Correct 
Error 


2      4 
4      2 


2      3     12 
243 

[545] 


7     11       2      5     15     15     12     10 
72253000 


LL 


MODERN     FISHING    GEAR     OF    THE    WORLD 


easily  detected  by  Suzukis  than  a  fixed  bait.  Further, 
the  sense  used  in  this  case  was  found  to  be  visual. 

Other  experiments  showed  that  bait  moving  lineally 
and  uniformly  was  by  no  means  more  detectable  than 
fixed  bait. 

It  is,  therefore,  concluded  that  Suzukis  find  their  prey 
at  some  distance  mainly  by  visual  movement-perception; 
a  bait  at  rest  or  moving  in  a  linear  uniform  motion  has 
scarcely  any  attraction.  Suzukis'  sense  (perhaps  lateral 
line  sense)  of  the  motion  of  the  water  produced  by  a 
bait  either  by  swimming  or  being  agitated,  is  indispens- 
able in  making  them  snap  at  it;  the  senses  of  smell  and 
taste  are  not  essential  for  their  food-searching,  although 
these  senses  may  be  indispensable,  along  with  the  tactile 
sense  in  the  mouth,  for  Suzuki  to  ascertain  whether  the 
bait  once  snapped  is  worth  swallowing. 

Light  fishing 

It  has  already  been  reported  by  von  Frisch2  and  others 
that  light  causes  a  shortening,  and  darkness  a  lengthening 
of  the  cones  in  the  fish's  retina.  Welsh  and  Osborn10 
and  Wigger11  and  others  have  shown  that,  when  fish  are 
kept  constantly  in  the  dark,  the  cones  show  greater 
elongation  (extreme  dark  adaptation)  at  midnight  than  at 
noon.  This  may  be  called  diurnal  rhythm  of  the  cone 
shifting. 

This  study  was  carried  out  to  find  the  illumination 
intensity  at  which  the  cones  change  from  dark  to  bright 
adaptation.  This  intensity  varies  by  species  and  times. 

According  to  the  several  sets  of  experiments,  the 
change  in  the  cones  of  Lateolahrax  japonicus  was  seen 
in  the  illumination  intensity  of  0-04  lux  before  midnight 
and  that  of  0-01  lux  after  that.  In  Cvprinus  carpio,  it 
was  0-0005  lux  before  midnight  and  less  than  0-00006 
lux  after.  Generally  speaking,  when  fish  are  kept  in  a 
very  low  intensity  of  illumination,  the  retinae  show 
more  marked  dark  adaptation  before  than  after  mid- 
night. 

Kawamoto  and  Konisr*  have  shown  that  when  fish 
(Girella  punctata)  were  kept  in  a  dark  tank  and  a  small 
part  of  the  water  surface  was  illuminated,  the  fish 
gathered  in  the  bright  region  (220  lux)  during  the  day- 
time and  in  the  less  bright  region  for  which  the  luxmeter 
was  not  available,  during  the  night.  Although  the 
position  of  the  cones  in  the  fish's  retina  was  not  examined, 
it  is  unquestionable  that  the  dark-adapted  retina  in  the 
daytime  showed  less  marked  dark  adaptation  than  in 
the  night  time.  It  may,  therefore,  be  concluded  that  fish 
gather  to  the  dim  region  when  their  cones  are  in  the 
maximum  dark-adapted  condition. 

Considering  this  conclusion,  together  with  the  author's 
findings,  the  phototaxis  of  fish  may  occur  in  lower 
illumination  before  midnight.  This  may  be  one  of  the 
fundamental  reasons  why  fishing  with  use  of  light  is 
usually  more  effective  before  than  after  midnight. 

The  fact  that  the  transitional  situation  of  the  cones 
in  C.  carpio  can  be  seen  in  much  lower  illumination 
than  in  L.  japonicus  may  show  that  the  retina  of  the 
former  fish  is  the  more  sensitive  to  low  illumination. 

Optimal  intensity  of  illumination 

When  a  cone  of  the  fish  retina  is  illuminated,  the  inside 
potential  of  the  cone  changes.  The  changed  potential 


can  easily  be  measured  by  the  ultramicro-capillary- 
electrode  method,  as  shown  by  Svaetichin6,  and  Mitarai 
and  Yagasaki4.  The  amplitudes  of  the  produced  poten- 
tials can  be  changed  by  the  various  intensities  of  the 
light  stimuli,  i.e.,  the  cone  response  is  a  graded  one  and 
does  not  follow  the  all  or  nothing  law.  The  higher  the 
intensity  of  the  light  stimulation,  the  larger  the  amplitude 
of  the  cone  potential.  The  amplitude,  however,  becomes 
constant  at  a  certain  intensity  of  illumination,  called 
the  lowest  intensity  of  illumination,  to  produce  the 
maximum  amplitude. 

From  the  lowest  intensity,  which  is  peculiar  to  the 
particular  fish  species,  the  author  was  able  to  deduce 
the  upper  limit  of  the  most  suitable  illumination  for 
the  daily  life  of  the  fish. 

According  to  the  several  sets  of  experiments,  the  lowest 
intensity  to  produce  the  maximum  cone  response  is 
between  64  and  175  lux  for  Sparus  aries  (Sparidae), 
about  175  lux  for  Cvprinus  carpio  (Cyprinidae)  and  far 
more  than  800  lux  for  Lateolabrex  japonicus  (Serranidae). 

It  may  be  assumed  that  the  cone  can  discriminate 
between  the  intensities  of  illumination  only  when  less 
than  the  lowest  intensity.  In  other  words,  the  cone  may 
be  excited  fully  when  the  illumination  of  the  cone 
reaches  the  lowest  intensity,  because  there  is  no  additional 
increase  in  the  amplitide  even  though  the  intensity  of 
the  light  is  increased  further.  When  the  fish  retina  is 
illuminated  by  higher  intensities  of  light  than  the  lowest 
one,  the  fish  can  hardly  separate  objects  within  the  visual 
field. 

Therefore,  the  lowest  intensity  of  illumination  to 
produce  the  maximum  cone  response  may  be  useful  as 
a  measure  of  the  environmental  illumination  which 
is  suitable  for  the  daily  life  of  the  fish.  Since  this  intensity 
was  measured  as  between  64  and  175  lux  for  S.  aries, 
about  175  lux  for  C.  carpio  and  more  than  800  lux  for 
L.  japonicus ,  it  may  be  concluded  that  these  fish  adapt 
themselves  to  darker  environment  in  this  order.  This 
conclusion  may  be  supported  by  the  fact  that  S.  aries 
lives  in  rather  deep  water  and  eats  its  food  chiefly  at 
night,  C.  carpio  usually  lives  in  turbid  water  and  searches 
for  food  mainly  by  chemoreceptors,  while  L.  japonicus 
usually  inhabits  littoral  clear  water  and  forages  for  food 
in  the  daytime. 

It  was  assumed  above  that  the  cone  sensitivity  for 
C.  carpio  might  be  superior  to  that  of  L.  japonicus. 
This  assumption  also  harmonizes  with  the  present 
findings. 

From  careful  observation  of  records  of  the  cone 
potential,  it  is  supposed  that  the  flicker  fusion  frequency, 
which  is  a  measure  of  the  ability  to  perceive  a  moving 
object,  is  highest  for  L.  japonicus,  lowest  for  S.  aries 
and  intermediate  for  C.  carpio.  Further  studies,  however, 
are  necessary. 

REFERENCES 

1  Beer,  T.  Die  Akkommodation  des  Fischauges,  Pfiiig.  Arch., 
58:  523-650.  1894. 

2  Frisch,  K.  von.  Farbensinn  der  Fische  and  Duplizitatsthcoric. 
Z.  vergl.  Physiol..  2:  393-452.  1925. 

3  Kawamoto,  N.  Y.  and  Konishi,  J.  Diurnal  rhythm  in  phototaxis 
offish.  Rep.  Faculty  of  Fisheries,  Pref.  Univ.  Mic.,  2:  7-17.  1955. 

4  Mitarai,  G.  and  Yagasaki,  Y.  Resting  and  action  potentials  of 
single  cone.   Ann.   Rep.    Research   Institute  of  Environmental 
Medicine,  Nagoya  Univ.  1955,  54-64.  1956. 

6  Svaetichin,  G.  The  cone  action  potential.  Acta  Physiol.  scand., 
29,  Suppl.  106:  565-600.  1953. 


[546] 


THE    VISUAL     SENSE     IN     FISH 


6  Tamura,  T.  On  the  senses  of  food-searching  in  Lateolabrax 
japonicus.  (in  Japanese  with  English  summary).  Bull.  Jap.  Soc.  Sci., 
Fish.,  17:  296-300.     1952. 

7  Tamura,  T.  A  study  of  visual  perception  in  fish,  especially  on 
resolving  power  and  accommodation.  Ibid.  22:  536-557.  1957  a. 

8  Tamura,  T.  On  the  relation  between  the  intensity  of  illumination 
and  the  shifting  of  cones  in  the  fish  retina.  (In  Japanese  with 
English  summary).  Ibid.  22:  742-746.       1957  b. 


9  Tamura,  T.,  Mitarai,  G.  and  Sugita,  Y.  The  lowert  intensity  of 
illumination  to  produce  the  maximum  cone  potential  in  the  fish 
retina  and  its  ecological  meaning.  Ibid.,  23:86-91.    1957. 

10  Welsh,  J.  H.  and  Osborn,  C.  M.  Diurnal  changes  in  the 
retina  of  the  catfish,  Amciurus  nebuhsus.  J.  comp.  Ncurol.  66: 
349-359.     1937. 

11  Wigger,  H.  Diskontinuitat  und  Tagesrhythmik  in  dcr  Dunkel- 
wanderungrctinalcrniemente.  Z.f.vergl.  Physiol.28:421-427.  1941. 


Conical  lift  nets  being  used  for  light  fishing  of  K  ilka  in  the  Caspian  Sea     (U.S.S.R.) 


f  547 


ATTRACTION  OF  FISH  BY  THE  USE  OF  LIGHT 

by 

F.  J.  VERHEYEN 

Laboratorium  voor  Vergelijkende  Physiologic,  Utrecht,  Netherlands 


Abstract 

The  concentration  of  fishes  around  a  lamp  is  usually  attributed  to  "positive  phototaxis"  and  one  view  is  thai  the  concentration  is 
due  to  the  search  for  a  preferred  light  intensity.  The  author  considers,  however,  that  when  the  behaviour  of  light-trapped  fish  is  taken  into 
account,  there  is  reason  to  doubt  this  view. 

He  puts  forward  the  theory  that  the  abnormal  behaviour  is  due  to  the  abnormal  conditions  of  illumination  around  the  artificial  light 
source  and  that,  if  the  central  nervous  system  is  controlled  by  stimuli  from  the  optical  sense  organs,  any  deviation  from  the  fish's  normal 
environmental  illumination  will  ultimately  affect  the  movement  of  the  fish  and  its  behaviour. 


Resume 


Attraction  du  poisson  par  la  lumidre 


La  concentration  des  poissons  aulour  d'une  lampc  est  gdneralement  attribute  a  la  "phototaxie  positive"  et  certains  experts  sont 
d'avis  que  cette  concentration  est  provoquee  par  la  recherche  d'une  intensite  de  lumicre  prefcree  par  les  poissons.  Toutcfois.  1'auteur  con- 
side*  rant  le  comportement  du  poisson  attire*  par  la  lumiere,  estime  qu'il  y  a  des  raisons  dc  doutcr  du  bicn  fond£  de  cctte  opinion. 

II  avance  la  thcorie  d'apres  laquelle  le  comportement  anormal  du  poisson  est  imputable  &  des  conditions  anormales  d'illumination 
autour  de  la  source  artificielle  de  lumiere,  et  que  si  le  systeme  nerveux  central  est  commande  par  les  stimuli  provcnant  des  organes  opliques, 
toute  alteration  des  conditions  normales  d'illumination  du  milieu  ambiant  affectc  en  dernier  ressort  les  mouvements  du  poisson  ct  son  com- 
portement. 

Atraccion  de  peces  mcdiante  la  luz 
Extracto 

La  concentracion  de  peces  alrededor  de  una  him  para  generalmente  sc  atribuye  a  la  "fototaxia  positiva"  y  algunos  invcstigadores 
piensan  que  puede  deberse  a  la  busqueda  dc  la  intensidad  luminosa  preferida.  Sin  embargo,  el  autor  considcra  que  al  tener  en  cuenta  la 
reacci6n  de  los  peces  atraidos  por  la  luz  puede  ponerse  en  duda  este  parecer. 

Ademas,  expone  la  lei  ria  de  que  la  conduct  a  anormal  se  deberia  a  condiciones  anormales  alrededor  de  la  fuente  de  luz  artificial  y  si 
el  sistema  nervioso  central  cs  regulado  por  los  esltmulos  del  organo  visual,  cualquier  cambio  de  la  iluminacion  normal  del  medio  ambicnle 
del  pez  influiria  sobre  sus  movimientos  y  reaccion. 


THE  concentration  offish  around  a  lamp  is  generally 
attributed  to  positive  phototaxis  but  the  mechan- 
isms involved  are  not  yet  clearly  understood. 
The  theory  that  the  concentration  is  due  to  a  search 
for  a  preferred  light  intensity  is  disproved  by  the 
behaviour  of  the  fish  themselves.  The  behaviour  of  the 
Clupeids,  caught  in  many  parts  of  the  world  by  the 
use  of  light,  is  particularly  instructive.  The  fact  that 
some  Clupeids  are  captured  in  the  daytime  with  bottom- 
nets  and  during  night  with  drift-nets  has  already  indicated 
a  diurnal  vertical  migration.  Echo  sounding  has  revealed 
that  this  migration  is  photophobically  produced.  This 
points  towards  a  preference  for  a  low  light  intensity. 
When  considering  the  abnormal  behaviour  of  these 
and  other  fish  in  the  vicinity  of  a  lamp  and  the  negative 
influence  of  the  moon  on  this  fishing  technique,  the 
capture  of  Clupeids  with  lamps  bears  a  striking  resem- 
blance to  the  collection  of  nocturnal  insects  with  lamps. 
This  behaviour  may  be  due  to  the  abnormal  illumina- 
tion conditions  around  the  light  source.  It  is  suggested 
that  the  normal  photo  orientation  of  animals  depends 


on  the  functioning  of  higher  and  lower  levels  of  the 
central  nervous  system,  controlled  by  the  feed-back 
from  the  optical  sense  organs.  The  system's  purposeful 
functioning  can  only  be  maintained  in  conditions  of 
normal  environmental  illumination,  which  are  not 
fulfilled  by  an  artificial  light  source. 

This  theory  is  based  partly  on  the  observation  of 
animals  in  the  vicinity  of  light  traps,  partly  on  similar 
observations  under  experimental  illumination  conditions 
in  the  laboratory,  and  partly  on  an  analysis  of  the  mech- 
anisms of  normal  photo  orientation. 

The  normal  movements  in  search  of  a  preferred  light 
intensity  are  caused  by  higher  or  lower  intensities. 
These  movements  are  guided  by  the  normal  differences 
between  the  illumination  intensities  of  the  photo- 
sensitive surfaces  of  the  two  eyes  and  of  different  parts 
of  the  photo-sensitive  surface  of  each  eye.  The  fixation 
mechanisms  of  the  eyes  are  used  during  more  detailed 
orientation  and  they  are  operated  and  controlled  by 
sign  stimuli  (congener,  prey)  that  are  qualitatively 
different  from  the  many  stimuli  all  around. 


[548  i 


LIGHT     ATTRACTION 


The  normal  values  of  all  these  stimuli  are  controlled 
by  the  normal  light  distribution  in  the  environment. 
This  distribution  is  determined  by: 

1.  the  nature  of  the  light  sources  (the  sun  or  the 
moon); 

2.  the  scattering  capacity  of  the  media  (the  atmos- 
phere and  the  water);  and 

3.  the  reflecting  capacity  of  the  background. 

In  the  vicinity  of  an  isolated  artificial  light  source  in 
an  optimal  "attracting'*  arrangement,  the  influences  of 
factors  (2)  and  (3)  upon  the  illumination  are  modified 
considerably,  resulting  in  abnormal  values  of  the  respec- 
tive stimuli. 

The  abnormal  feed-back  resulting  from  the  abnormal 
differences  between  the  illumination  intensities  of  the 
eyes  and  of  different  parts  of  each  eye  causes  the  animal 
to  deviate  from  its  course.  Moreover,  the  servo  mechan- 
isms of  the  lower  coordination  centres,  controlling  the 
fixation  movements,  become  a  plaything  of  the  stimuli 
from  the  artificial  light  source  that  are  quantitatively 
super-normal  as  compared  with  the  other  available 
light  stimuli.  The  sign  stimuli  that  normally  activate  the 
higher  coordination  centres  of  the  fixation  mechanisms 
lose  their  releasing  power,  thus  the  higher  centres  are 
eliminated  from  the  orientation  process. 

Under  extreme  laboratory  illumination  conditions— a 
lamp  in  a  dark  room  many  creatures,  such  as  insects, 


fish  and  birds,  arc  forced  to  move  in  a  straight  line 
towards  the  light  source,  irrespective  of  factors  that  are 
incompatible  with  survival  (injuriously  high  light 
intensities,  temperature).  This  classical  tclotactic  move- 
ment is  thus  the  result  of  optical  disorientation. 

When  a  lamp  is  introduced  into  the  natural  habitat 
of  animals,  a  drift  towards  this  light  source  is  superim- 
posed upon  their  random  movements.  The  observed 
concentration  of  the  animals  in  the  vicinity  of  the  lamp 
is  the  statistical  result  of  this  drift. 

The  application  of  the  light  trap  technique  will  yield 
optimal  results  only  when  several  conditions  are  realized. 
For  instance,  the  animals  to  be  captured  must  be  active 
at  night  when  the  natural  light  intensities  are  low  enough 
to  permit  the  required  illumination  conditions  around 
the  lamp  (no  moon).  As  for  fish,  the  water  must  be 
sufficiently  clear  to  reduce  the  absorption  of  the  light 
rays  and  to  reduce  the  scattering  that  would  counteract 
the  production  of  the  required  light  conditions.  More- 
over, the  depth  of  the  water  must  be  sufficient  to  eliminate 
reflection  from  the  bottom. 

These  are  the  very  conditions  under  which  fish,  such 
as  sardines  and  anchovies,  are  caught  with  the  help 
of  lamps  in  many  parts  of  the  world. 

REFERENCE 

Vcrheyen,  F.  J.  The  mechanisms  of  th",  trapping  effect  of  artificial 
light  sources  upon  animaJs  -Arch.  Neerl.  Zool.  13,  pp.  1-107.  1958. 


Echogram  showing  the  reaction  of  freshwater  fish  to  artificial  light.     Electric  lamp  of  100  M-.  in  5  m.  depth.        Pholo:  J.  Scharfe 


549 


ON   THE   BEHAVIOUR   OF   FISH   SCHOOLS  IN   RELATION  TO 

GILLNETS 

by 
MASATSUME  NOMURA 

Tokai  Regional  Fisheries  Research  Laboratory,  Tokyo,  Japan 

Abstract 

Reaction  of  fish  to  a  fishing  net  is  an  important  aspect  for  studying  the  performance  of  the  gear  and  thereby  improving  its  efficiency. 
In  this  paper  the  relationship  between  the  reaction  of  the  fish  and  the  performance  of  the  gear  is  investigated  on  the  basis  of  the  data  made 
available  through  gillnet  operations. 

Some  of  the  environmental  factors  that  control  the  activity  of  fish  are  supposed  to  be  the  light  condition  of  the  surrounding  water 
as  well  as  underwater  visibility  of  nets,  which  have  been  studied  in  relation  to  the  behaviour  of  fish.  In  addition,  a  few  different  ways  of 
paying  out  drift  nets  with  the  help  of  fishfinders  in  the  daytime  have  been  compared  to  find  the  best  way  of  shooting  the  nets  under  a  given 
circumstance. 

Les  rapports  entre  faction  du  filet  maillant  et  le  comportement  d'on  bane  de  poisson 
Resume 

La  reaction  du  poisson  a  un  filet  de  peche  cst  un  dement  important  lorsqu'il  s'agit  d'ciudier  les  resultats  fournis  par  un  engin 
ct  par  suite  d'en  accroitre  Pefticacit6.  Dans  cette  etude,  on  a  recherchd  les  rapports  entre  la  reaction  du  poisson  et  les  resultats  fournis  par 
/* engin  en  se  fondant  sur  les  donnees  recueillies  au  cours  dc  1'utilisation  de  filets  maillants. 

Parmi  les  facteurs  du  milieu  qui  influcncent  Pactivit6  des  poissons,  on  pense  qu'il  faut  ranger  la  luminosite  de  Teau  ambiante  ainsi  que 
la  visibilite  sous-marine  des  filets  qui  ont  6te  etudices  d'apres  Ic  comportement  des  poissons.  tin  outre,  on  a  compard,  a  Paide  de  delecteurs 
de  poissons,  un  petit  nombrc  de  manieres  differentes  de  mettrc  £  Peau  les  filers  d&rivants  en  plein  jour,  pour  decouvrir  la  meillcur  maniere 
d'immcrger  les  filets  dans  des  conditions  donnees. 

Kelaci6n  entre  la  manera  como  actua  un  cardumen  y  la  acci6n  dc  una  red  de  enroalle 
Extracto 

La  manera  en  quc  reaccioria  un  pez  ante  una  red  es  de  importancia  para  estudiar  cl  efecto  del  arte  y,  por  consiguiente,  para  mejorar 
su  eficacia.  En  este  trabajo  se  ha  invest igado  la  relaci6n  entre  el  pe/  y  la  forma  como  actua  una  red  sobre  la  base  de  los  datos  disponibles 
durante  las  faenas  con  artcs  de  enmaile. 

Entre  los  factores  ambicntales  que  regulan  la  actividad  de  los  peces  se  han  cstudiado  las  condiciones  de  luz  en  el  agua  que  los  rodea 
y  lavisibilidad  de  la  redes  sumergidas.  Adcmas  se  compararon,  con  ayuda  de  ecosondas,  las  diversas  mancras  de  tender  las  redes  de  dcriva 
durante  el  dia  para  detcrminar  el  calamcnto  mas  conveniente  en  dcterminadas  condiciones. 


T 


HE  fishery   operating  for  sardines  in  the  south-          (fig.  2)  —here  D  is  the  mean  distance  from  the  surface 
western  part  of  the  Japan  Sea   has  advanced  in 
recent  years  partly  because  of  improvement  in  K*n 


fishing  practice.  Since  about  1953  the  majority  of  fisher- 
men here  have  operated  during  the  day  instead  of  at 
night  as  they  used  to  do.  From  1955  until  now,  nearly 
70  per  cent,  of  the  fishing  craft  have  been  equipped  with  ~  2000  •  • 

fish  finders  which  have  increased  their  efficiency.   This        L 

study  is  based  on  data  obtained  from  a  drift-net ter.  Jo         * 

•  *  go         o 

DEPTH  OF  NET  AND  AMOUNT  OF  CATCH  g  • 


Studying  the  relation   between  the  amount  of  catch  20       25        30       35       ^0       45 

C  and  the  length  of  the  buoy  line*  L,  one  may  find  that  , 

under  the  present  conditions  a  good  catch  can  be  expected  L> 

when  length  of  the  buoy  line  is  more  than  30  kens  (45  fig.  I.     The  relation  between  the  amount  of  catch  C  and  length 

metres)  (fig.  1).   In  the  relation  between  C  and  L  to  D  of  the  buoy  lines  L.  (/  ken-  1-5  m.;  J  kan  *-  J-75  kg.) 

•  During  Dec.  29th  1955  to  Jan.  10th,  1956. 

*  The  net  is  suspended  by  buoy  lines  and  L  denotes  the  distance  x        „      Jan.  13th  to  Jan.  21st,  1956. 

from  the  surface  to  the  upper  edge  of  the  net.  o        „      Jan.  22nd  to  Feb.  8th,  1956. 

[  550  ] 


REACTION     OF     FISH     TO    GILLNETS 


3.000 

2.500 
2.000! 

1.500 

l.OOO 

SOO 

0 


.0 


-10        0      +10     +20     +30   K«« 
L  -  D 

/  itf.  2.     The  relation  between  L       D  ami  the  amount  of  catch  C 
(/  ken       1  -5  m.\  J  kan       3  75  A.?.) 

to  the  upper  edge  of  the  fish  traces  il  can  he  seen  that 
a  good  catch  is  made  when  the  value  of  L  to  D  ranges 
from  1  5  to  i  15  kens  (7-5  to  22-5  m.). 

REACTION   OF   FISH  APPROACHING   A   NET 

Data  on  the  depth  distribution  of  the  fish  schools 
have  been  collected  from  fish  finder  records  of  com- 
mercial boats  operated  off  Yamaguchi  Prefecture  in 
1956  and  1957.  To  clarify  the  reaction  of  fish  schools 
approaching  the  net,  the  number  of  fish  schools  traced 
on  a  recording  paper  before  and  after  setting  were 
counted.  Regarding  A  as  the  number  of  the  schools 
found  already  above  the  depth  of  the  floatlinc  expected 
before  setting  and  A'  as  the  one  still  remaining  there 
after  setting,  A — A'  is  assumed  to  be  the  number  of 
schools  that  has  sunk  below  the  floatline  after  setting, 
neglecting  the  schools  reacting  otherwise.  In  the  same 
manner,  B  B'  is  assumingly  the  number  of  schools 
that  has  risen  above  the  leadline  after  setting. 

Fig.  3  indicates  the  relation  between  the  numbers  of 
schools  probably  sinking  and  those  rising.  From  figs. 
2  and  3,  it  appears  that  the  schools  of  sardine  tend  to 
dive  when  they  approach  the  net.  Perhaps  the  fish 
mostly  move  downwards  to  seek  darker  surroundings 
when  they  are  exposed  to  a  stimulus.  This  movement, 
of  course,  will  differ  according  to  the  surrounding 
conditions,  kind  offish,  size  of  school,  degree  of  stimulus, 
and  so  forth. 

INFLUENCE  OF  LIGHT 

In  the  drift  net  fishing  for  bluefin  tuna  off  Ibaraki 
Prefecture,  the  nets  were  lifted  twice  a  day:  about  ten 
at  night  and  four  in  the  morning1.  The  spiny  lobster 


o 


20 
15 


°_  10 


-5     -it      -3      -Z 

Nu-mber  of  Schools  sniKmj  down 

Fig.  3.     The  relation  between  the  number  of  schools  probably 
sinking  down  and  the  number  of  schools  rising  up. 

P.  japonicus,  is  active  mainly  during  the  hours  from  six 
to  eight  in  the  evening  and  two  to  four  in  the  morning2. 
The  activity  of  fish  seems  to  be  closely  related  to  the 
degree  of  light  under  water,  and  as  the  net  is  less  visible 
in  dark  water,  fishing  is  reckoned  to  be  more  favourable 
at  night  than  in  the  day,  in  turbid  rather  than  limpid 
water,  and  with  a  net  of  subdued  rather  than  bright 
colour. 

There  must  be  some  relation  between  the  moon  and 
the  catch  in  gillnetting.  It  is  said  that  during  the  spring 
tide  no  good  yield  of  spiny  lobster  can  be  expected. 
In  drift  netting  for  bluefin  tuna  off  Hokkaido  some  years 
ago,  the  average  catch  was  reported  to  be  abundant 
when  the  moon  was  three  to  eleven  days  old:*.  On  the 
east  coast  of  England,  the  herring  drift  net  catch  is 
said  to  be  greatly  influenced  by  the  phase  of  the  moon4. 
Herring  and  small  pilchard  in  England5  and  herring  off 
Sakhalin,  Russia6,  stay  deep  during  the  day  but  near 
to  the  surface  at  night.  Sardines  in  the  Japan  Sea  are 
said  to  sink  after  sunset  but  rise  again  from  eight  to 
ten  at  night  for  spawning7. 

Fig.  4  shows  the  distribution  in  depth  of  fish  schools 
as  recorded  and  the  underwater  luminosity  measured 
at  the  time.  The  data  were  made  available  from  the 
same  source  referred  to  in  the  preceding  section.  It 
seems  that  the  fish,  having  stayed  at  forty  metres  or 
deeper  till  sunrise,  surface  just  before  sunrise  when  the 
underwater  luminosity  in  the  area  becomes  IO"1  to  ICT" 
lux.  They  submerge  again  deeper  than  thirty  metres  at 
ihe  luminosity  of  10  to  IO3  lux.  The  movement  of 
plankton,  as  influenced  by  light,  must  also  be  taken 
into  account  in  this  activity  of  fish. 

COLOUR  OF  THE  NET 

The  colour  of  the  net  is  another  factor  which  changes 
in  accordance  with  the  depth.  A  test  was  made  with 
nine  coloured  nets  -red,  orange,  yellow,  blue,  green, 
purple,  white,  grey  and  black—to  determine  their  light 
reflection  properties.  At  50  metres  or  deeper,  the 
reflective  light  energy  of  the  different  nets  differs  con- 
siderably, although  the  colours  themselves  are  almost 
lost.  A  comparison  was  made  between  every  two 
adjacent  nets.  The  results  showed  that  a  better  catch 


551  ] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


of    U/atcf 
10       20       30      UP       SO       60      70      80      90      '00  •» 


Ok*. 


Fig.  4.     The  distribution  of  fish  schools  and  the  underwater 

luminosity  according  to  the  time. 
jjne  weaiher%      ....  cloudy  weather. 

can  be  expected  in  daylight  with  the  darker  net,  but  no 
difference  was  found  at  night  (fig.  5). 

VALUE  OF  ECHO-SOUNDING 

Of  various  improvements  made  in   fishing  technique 
since  the  introduction  of  synthetic  fibre  nets*  and  fish 


i.  SOO 

a. 

c 

c 

JT300 

•« 

r?oo 

A 
^  100 

0 


Fig.  5. 


The  relation  between  the  brightness  of  the  coloured  nets 
and  the  amount  of  catch  in  daylight. 


*  About  40  per  cent,  of  sardine  gillnets  of  Yamaguchi  Prefecture 
are  made  of  synthetic  fibre  (Cremona). 


finders,  a  remarkable  feature  is  the  change  from  night 
to  daylight  fishing. 

The  application  of  fish  finders  for  choosing  the 
optimum  fishing  location  and  gear  adjustment  can 
still  be  improved.  A  decision  must  first  be  made  as  to 
whether  the  fish  school  detected  is  likely  to  be  worth 
fishing.  This  is  determined  by  the  number,  size,  type 
and  density  of  the  echo  traces.  Secondly,  the  fishermen 
must  find  out  if  the  school  is  of  the  type  that  tends  to 
sink  when  approaching  the  net.  Present  findings  indicate 
that  a  good  catch  can  be  expected  when  the  nets  reach 
down  5  to  15  kens  (7-5  to  22-5  m.)  deeper  than  the 
fish  school,  and  when  the  school  seems  to  be  swimming 
against  the  current.  In  these  circumstances,  it  is  advisable 
to  pay  out  the  net  in  such  a  manner  that  it  is  drifted  by 
the  current  just  in  front  of  the  fish. 

REFERENCES 

1Isomac,  K.  The  bluefin  tuna  drift  net  off  Ibaraki  Prefecture 
(Japanese).  J.  Fish.  Soc.  Japan,  146.  1894. 

2  Kubo,  I.  The  best  time  for  fishing  the  Japanese  spiny  lobster 
(Japanese).  The  Agriculture,  1  (1).  1953. 

3  Kawana,  T.  Occanographic  conditions  and  bluefin  tuna  fishery 
(Japanese).  Rep.  Fish.  Invet.  of  Hokkaido  Fish.  Exp.  St.,  31.  1934. 

4  Savage,  R.  E.  and  Hodgson,  W.  C.  Lunar  influence  on  the  East 
Anglian  herring  fishery.  J.  du  Conseil,  9  (2).  1933. 

5  Richardson,  I.  D.  Some  reaction  of  pelagic  fish  to  light  as 
recorded  by  echo-sounding.  Fish.  Invest.,  18  (1).  1952. 

6  Yu,  B.  Yudovich.  The  detection  and  fishing  of  herring  at 
Sakhalin  (Russian),  Rybnoe  Khoziastvo,  10.    1954. 

7  Ito,  Y.  and  others.  The  behaviour  of  sardine  at  night  in  Note 
area  of  (he  Japan  Sea  (Japanese).    An.  Rep.  Jap.  Reg.  Fish.  Res. 
Lab.  1.    1957. 


5521 


THE   SIGNIFICANCE   OF  THE   QUALITY   OF  LIGHT   FOR   THE 

ATTRACTION   OF  FISH 

by 

NOBU  YUKI  KAWAMOTO,  D.SC. 

Professor,  Faculty  of  Fisheries,  Prcfectural  University  of  Mie.     Lecturer,  Department  of  Fisheries,  Kyoto  University, 

Japan 

Abstract 

Many  aspects  of  the  influence  of  light  on  the  behaviour  of  fish  have  been  studied  and  in  this  paper  the  author  describes  his  experi- 
ments on  the  effect  of  light  of  different  wave-lengths,  the  correlation  between  wave-length  and  radiant  energy,  the  daily  rhythm  in  phototaxis 
and  the  influence  of  the  moonlight  on  fish. 

Many  types  of  young  marine  fishes  and  Oryzias  talipes  (a  fresh-water  fish)  were  used  in  the  experiments,  and  one,  A  ngui  I  la  japonica, 
the  Japanese  eel.  showed  no  definite  light-seeking  tendency.  Many  fishes  were  especially  attracted  by  blue  and  green  lights  and  it  was  fcund 
that  "spectral  luminosity"  played  an  important  part  in  deciding  which  one  of  two  light  sources  was  the  best  "fish-gatherer".  The  ratio  of  the 
gathering  rates  in  two  lights  is  equal  to  the  ratio  of  their  spectral  luminosities.  It  was  also  found  that  lamps  could  be  used  on  moonlight 
nights  to  attract  fish,  provided  that  the  intensity  of  the  lamp  was  adjusted  to  a  sufficiently  high  level  in  comparison  to  the  moonlight. 


Ittsumt 


Considerations  sur  I'cfficacitc  des  lampcs  pour  attirer  le  poisson 


I  /influence  de  la  lumiere  sur  le  comportement  des  poissons  a  deja  etc  etudiee  a  bien  des  points  de  vue  et  Pautcur  decrit  dans  cet 
article  ses  experiences  pcrsonnelles  sur  I 'influence  dc  I  urn  i  6  res  de  diflfercntes  longueurs  d'onde,  Ics  rapports  cntre  la  longueur  d'onde  ct  l'6nergic 
rayonnanto,  le  rythmc  journal ier  de  la  phototaxie  et  Tinfluence  excrcee  sur  les  poissons  par  la  lumiere  de  la  lune. 

Les  experiences  onl  nort6  sur  de  nombreux  types  dc  jeunes  poissons  mar  ins  ainsi  quc  sur  V  Oryzias  latipes  (poisson  d'eau  douce). 
L'un  de  ces  poissons,  Panguille  japonaise  (Anguilla  japonica)  n'a  manifest^  aucun  phototropisme  net.  Dc  nombreux  poissons  onl  etc  attires 
tout  particulieremcnt  par  le  lumiere  bleuc  ct  la  lumiere  verte  et  on  a  constate  que  la  luminosite  s pec t rale  jouait  un  role  important  pour  choisir 
entre  deux  sources  lumincuses  cellc  qui  incitcrait  le  mieux  les  poissons  &  se  rassembler.  Le  rapport  des  taux  de  rassemblement  dans  ces  deux 
lumieres  cst  6gal  an  rapport  de  leurs  Iuminosit6s  spectrales.  II  a  egaiemcnt  ete  constate  que  Ton  pouvait  utiliser  des  lampcs  pour  attirer 
lepoisscn  pendant  iesnuits  delunc,  a  condition  de  r6gler  I'intcnsite  de  la  lampe  a  un  niveau  assezelcve  par  com  para  ison  £  la  lumiere  de  la  lune. 

Considcraci6n  sobrc  la  eficacia  de  las  lamparas  para  atraer  a  los  peces 
Extracto 

Los  estudios  relativos  a  los  numerosos  aspectos  de  la  influencia  que  tiene  la  luz  sobre  la  manera  como  reaccionan  los  peces  indujeron 
al  autor  de  este  articulo  a  describir  sus  experimented  sobrc  el  efecto  de  la  luz  dc  diversa  longitud  de  onda,  la  correlaci6n  entre  la  longitud  de 
onda  y  la  cnergia  radian te,  el  ritmo  diario  de  la  fototaxia  y  la  influencia  dc  la  luz  dc  la  luna  sobrc  los  peces. 

En  estos  experimented  se  utilizaron  divcrsas  especics  marinas  j6venes,  Oryzias  latipes  (especie  de  agua  dulcc)  y  tambien  una  Anguilla 
iaponica.  Este  ultimo  pez  no  dcmostro  ninguna  tcndencia  a  buscar  la  luz,  pero  muchos  otros  fueron  especialmcnte  atraidos  por  las  luces 
azul  y  vcrde,  encontrandosc  que  la  "luminosidad  espectrar  jucga  un  papel  importante  en  decidir  cual  de  cstas  dos  fuentcs  luminosas  es  mas 
apropiada  para  reunir  peces.  La  rclaci6n  de  la  proporci6n  en  que  estos  animalcs  se  congregan  junto  a  dos  luces  es  igual  a  la  rclacion  de  sus 
luminosidades  espect rales.  Tambien  se  ha  cncontrado  que  en  las  noches  con  luna  pucdcn  utilizarsc  lamparas  para  atraer  peces,  siempre  que 
la  intcnsidad  dc  ellas  sea  lo  suficientcmente  aha  como  para  compararla  con  la  luz  de  la  luna. 


ABORATORY  experiments  were  carried  out  with 
several  species  of  marine  fishes  such  as: 

Oplegnathus  fasciatus  (T.  et  S.) 
Stephanolepis  cirrhifer  (T.  et  S.) 
Scomberomorus  niphonius  (C.  et  V.) 
Fugu  niphobles  (J.  et  S.) 
Sphyraena  japonica  (C.  et  V.) 
Angullla  japonica  (T.  et  S.) 
Afugil  cephalus  (L.) 
Girella  punctata  (G.) 
Fugu  rubripes  (T.  et  S.) 
Oryzias  latipes  (T.  et  S.) 
Pempheris  japonicus  (D.) 
Trachurus  japonicus  (T.  et  S.) 
Plotosus  anguillaris  (L.) 

Experiments   had,   unfortunately,   to   be   restricted   to 
young  fish  about  2  to  15  cm.  in  length. 


THE  SIGNIFICANCE  OF  THE  WAVE  LENGTH 
Experimental  set-up 

A  lustreless,  black,  round  wooden  tank,  100  cm.  in 
diameter  and  25  cm.  in  height,  was  divided  radially  into 
eight  compartments,  open  to  each  other  at  the  centre. 
A  window  in  each  compartment,  covered  with  a  colour 
filter  was  lighted  by  a  60  W.  electric  bulb1. 

The  colour  filters  were  prepared  by  dissolving  the 
respective  colour  in  a  6  per  cent,  gelatine  solution. 
Their  transparencies  were  measured  by  a  recording 
spectrophotometer. 

The  experiments  were  carried  out  during  the  daytime 
in  a  dark  room.  The  fish,  kept  in  an  aquarium  for  about 
one  week  after  being  caught,  were  allowed  to  become 
accustomed  to  the  darkness  in  the  tank  for  at  least  30  min. 
before  each  experiment.  After  lighting  the  8  lamps 
simultaneously,  the  number  of  fish  which  entered  each 


[553] 


MODERN    FISHING    GEAR    OF    THE    WORLD 


compartment  during  a  total  time  of  10  min.  was  recorded. 
Those  leaving  a  compartment  were  neglected  and  those 
present  at  the  beginning  were  counted  as  newcomers. 
During  this  experiment  all  species  were  very  active  and 
changed  their  place  frequently.  The  experiment  was 
repeated  five  times  for  two  different  filter  arrangements. 
The  average  distribution,  the  gathering  rate  for  the  res- 
pective source  of  light,  was  expressed  in  percentages. 

Results 

The  filter  arrangement,  i.e.  in  order  of  the  wave  length  or 
at  random,  had  no  significant  influence  on  the  fish 
behaviour.  All  species  tested,  except  Anguilla,  remained 
mostly  in  the  green  and  blue  compartments.  Anguilla, 
however,  was  indifferent  to  blue,  green,  indigo  and  yellow, 
but  was  attracted  by  violet  and  red1. 

THE   SIGNIFICANCE   OF  THE   RELATION 
BETWEEN    WAVE    LENGTH    AND 
RADIANT  ENERGY 

Experimental  set-up 

Only  two  species,  i.e.  Fugu  rubripes  (T.  and  S.)  (marine 
fish),  and  Oryzias  latipes  (T.  and  S.)  (fresh  water  fish) 
were  tested.  Two  40  W.  light  bulbs  were  placed  10  cm. 
above  slits  on  two  sides  of  the  square  fish  tank.  Mazda 
colour  fitters  (types  V-B2,  V-G1,  V-Y1,  V-R2)  were 
placed  over  the  slits,  and  the  radiant  energy  was  adjusted 
by  inserting  sheets  of  screening  paper  with  a  transparency 
of  75  per  cent. 

For  each  test,  10  fish  were  brought  into  the  tank 
while  the  laboratory  was  dimly  lit.  After  30  min.  for 
accustoming,  both  bulbs  were  lighted  simultaneously, 
and  the  numbers  of  fish  gathered  in  each  illuminated 
field  (13x45  cm.)  were  recorded  in  30  sec.  intervals 
for  a  total  test  time  of  10  min.  To  avoid  accustoming 
due  to  repetition,  different  fish  were  used  for  each  experi- 
ment. 

The  gathering  rate  was  again  expressed  in  percentages. 
The  influence  of  the  different  colour  filters  on  the  radiant 
energy  was  carefully  eliminated  until  equal  gathering 
rates  for  both  lights  were  obtained.  The  following 
combinations  were  tested:  blue  to  white,  yellow  to 
white,  red  to  white,  and  red  to  green. 

Results 

The  total  energy  of  transmitted  coloured  light  can  be 
calculated  from  the  spectral  radiant  energy  of  the  light 
bulb  and  the  transparency  of  the  colour  filter.  The 
values  for  the  coloured  lights  tested  are  shown  in  the 
uppermost  column  in  Table  I. 


TABLE  I 
Visual  Efficiency 


White 

Blue 

Green 

Yellow     Red 

Relative  energy: 

4892 

186 

100 

3464      2179 

Fugu  rubripes 

Visual  efficiency: 

0-16 

0-46 

0-62 

0-15      001 

Oryzias  latipes 

Visual  efficiency: 

0-13 

0-54 

0-53 

0-12    0-004 

Wave  lengths  of  more  than  750  m//.  are  beyond 
the  limit  of  susceptibility  of  fish.  It  was  suggested  that 
the  gathering  rate  may  be  influenced  by  the  energy  of 
the  coloured  light  and  the  perceptibility  of  sensory  cells  of 
the  fish  retina,  and  that  the  quantitative  relation  between 
radiant  energy,  wave  length  and  visual  perceptibility 
affecting  phototaxis  may  be  easily  explainable.  C.  Hess 
(1912)  observed  that  the  visual  curve  of  fish  shows 
fairly  good  accord  with  the  human  rod  vision  curve. 
Heht  (1930)  confirmed  this,  and  H.  Grundfest  (1932) 
has  also  studied  this  problem  of  visual  perceptibility  in 
fish.  For  the  present  considerations,  therefore,  the 
human  visual  curve  was  accepted  with  a  certain  dis- 
placement of  the  maximum. 

The  product  of  the  relative  radiant  energy  (<px)  of 
light  of  a  certain  wave  length  (A)  and  the  relative  visual 
perceptibility  (Vx)  for  light  of  the  same  wave  length  is 
generally  called  the  spectral  luminosity  of  light  of  the 
wave  length  L  With  light  of  a  spectral  distribution 
between  A,  to  Aa  the  total  spectral  luminosity  (H)  may 
be  figured  by  the  expression 


r*2 

H  -Jn 


dx 


The  relative  spectral  luminosity  values  for  each 
experimental  light  source  were  calculated  by  several 
visual  curves  of  500  m//.  to  540  m//.  In  the  experiment, 
white  light  was  combined  in  turn  with  each  coloured 
light. 

It  was  found  that  the  gradual  decrease  in  the  intensity 
of  a  light  in  the  area  where  the  fish  showed  greatest 
aggregation,  resulted  in  an  inversion  of  phototaxis 
between  the  two  lights.  Fish  gathered  round  one  light 
will  migrate  to  the  other.  The  fish  gathered  equally 
under  two  lights  of  equivalent  spectral  luminosity. 
Consequently  the  relationship  between  the  ratios  of 
gathering  rates  of  light  sources,  coloured  (Gc)  or  white 
(Gw),  is  equal  to  the  ratios  of  their  spectral  luminosities 
(Hc/Hw)  : 

GjGw  -  HC/HW 

According  to  the  present  experiments  this  relationship 
holds  also  with  lights  of  different  wave  lengths  and  fish 
of  different  visual  perceptibility.  When  testing  lights 
of  different  wave  lengths  but  equal  radiant  energy  the 
fish  gathered  in  the  light  with  the  spectrum  of  higher 
subjective  visibility.  On  the  other  hand,  the  influence 
of  the  subjective  visual  perceptibility  of  a  certain  wave 
length  can  be  outruled  by  accordingly  higher  radiant 
energy.  It  is  suggested,  however,  that  this  holds  only 
within  a  certain  range  of  radiant  energy. 

The  ratio  of  relative  spectral  luminosity  to  relative 
radiant  energy: 

'?x    vx    dx 


was  called  visual  efficiency. 

In  the  case  of  Fugu  rubripes,  the  green  light  shows 
the  largest  visual  efficiency,  blue  next  and  red  the 
smallest.  Though  radiant  energy  of  red  light  is  22  times 
as  much  as  that  of  green  light,  its  visual  efficiency  is 
1/62  of  green.  In  the  case  of  Oryzias  latipes v  blue  and 


[554] 


QUALITY     OF    LIGHT     AND     ATTRACTION     OF     FISH 

Results 


green  lights  are  equivalent  and  red  extremely  small, 
Differences  with  various  kinds  of  fish  may  be  easily 
understood  as  the  visual  perceptibility  curve  of  Oryzias 
latipes  is  of  shorter  wave  length  than  that  of  Ftigu 
rubripes*. 

DIURNAL   RHYTHM    IN   PHOTOTAXIS 
Experimental  set-up 

A  tank  95  cm.  in  length,  45  cm.  in  width  and  30  cm. 
in  depth  was  used,  with  the  inside  painted  lustreless 
black.  Two  lights  were  arranged  at  each  side  of  the 
tank.  The  required  wave  length  was  ascertained  by 
Mazda  colour  filters  (V-G1,  V-R2).  Ten  fish  were  used 
for  each  test,  allowing  30  min.  for  accustoming.  The 
gathering  rate  was  determined  in  the  usual  way. 

Results 

In  general,  many  kinds  of  fish  show  no  definite  diurnal 
rhythm  in  phototaxis.  Mugil  eephalus,  for  example, 
shows  nearly  constant  gathering  rate  over  a  twenty-four 
hours  period. 

The  behaviour  of  certain  fish,  however,  is  quite 
different.  Gire Ha  punctata,  for  example,  shows  a  notable 
diurnal  rhythm,  an  extremely  strong  light-seeking 
tendency  in  day-time  and  a  less  activity  at  night. 

With  Rudarius  ercodcx  (J.  and  F.)  the  strong  tendency 
to  seek  green  light  during  day-time  is  similar  to  that  of 
Girella  punctata.  Unlike  the  latter,  however,  the  activity 
of  R.  ercodes  decreases  to  a  remarkable  degree  at  night 
and  they  no  longer  respond  to  light  but  fall  asleep, 
resting  near  the  sides  of  the  tub.  In  general,  the  so-called 
"nocturnal"  fish,  i.e.,  Anguilla  japonica,  Plotosus  anguill- 
aris,  showed  no  definite  light-seeking  tendency  and  they 
gathered  in  larger  numbers  in  the  red  light7. 

THE  SIGNIFICANCE  OF  LIGHT  GRADIENT  FOR 
ATTRACTION 

Experimental  set-up 

A  lustreless,  black,  wooden  tank,  3  m.  in  length,  and 
24  cm.  in  height  and  width,  was  placed  in  a  dark  room. 
The  only  light  source  was  installed  at  one  end  of  the 
tank,  separated  from  the  water  by  a  pane  of  glass. 
The  gathering  rate  was  determined  in  regard  to  the 
distance  from  this  light  source. 


The  fish  tended  to  gather  farther  away  from  the  light 
when  the  light  intensity  was  either  stronger  or  weaker 
than  a  certain  optimum  value.  The  differences  of  the 
gathering  rates  obtained  by  day  and  night  coincided 
with  those  found  in  the  experiment  on  diurnal  rhythm2. 

INFLUENCE  OF  MOONLIGHT 
Experimental  set-up 

A  rectangular  net,  3  m.  deep,  20  m.  long,  and  6  m. 
wide  and  made  of  1  -8  cm.  mesh,  12  thread  cotton  yarn, 
was  suspended  from  a  moored  bamboo  float  200  m. 
off  shore.  An  electric  light  was  fbced  less  than  1  m. 
under  the  water  at  one  end  of  the  net:  20  W.,  60  W. 
and  100  W.  bulbs  were  used  with  a  green  glass  filter 
18  cm.  in  diameter.  Adult  horse-mackerel,  Trachurus 
trachurus  (L)  were  tested  and  the  gathering  rates  of 
the  fish  in  the  rectangular  net  were  determined  in  the 
dark  and  in  the  moonlight. 

Results 

The  gathering  rate  decreased  when  the  ratio  of  luminosity 
of  the  lamp  and  luminosity  of  the  moon  decreased  to 
a  certain  value.  It  was  found  that  some  horse-mackerel 
will  gather  at  the  lamp  even  on  a  light  night  if  sufficient 
strength  of  light  is  used6. 

REFERENCES 

1  Kawamoto,  N.  Y.  and  Takeda,  M.  The  influence  of  wave 
lengths  of  light  on  the  behaviour  of  young  marine  fish.  Rep.  Fac. 
of  Fish.,  Pref.  Univ.  of  Mie,  Vol.  I,  No.  1,  pp.  41-53,  Sept.  1951. 

2  Kawamoto  N.  Y.  and  Nagata,  S.  On  the  relation  between  light 
gradient  and  fish  behaviour.  Rep.  of  Fac.  of  Fish.,  Prcf.  Univ.  of 
Mie,  Vol.  lt  No.  2,  pp.  151-173.   1952. 

3  Kawamoto,  N.  Y.  and  Kobayashi,  H.  Influence  of  various  light 
conditions  on  the  gathering  rates  of  fish.  Rep.  of  Fac.  of  Fish., 
Pref.  Univ.  of  Mie,  Vol.  1.  No.  2,  pp.  139-150.     1952. 

4  Kawamoto,  N.  Y.  and  Niki,  T.   An  experimental  study  on  the 
effect  of  leading  fish  by  fish  attraction  lamps.  Rep.  of  Fac.  of  Fish., 
Pref.  Univ.  of  Mie,  Vol.  1,  No.  2,  pp.  175-196.    1952. 

6  Kawamoto,  N.  Y.  and  Konishi,  J.  The  correlation  between 
wave  length  and  radiant  energy  affecting  phototaxis.  Rep.  of  Fac. 
of  Fish.,  Pref.  Univ.  of  Mie,  Vol.  1.  No.  2,  pp.  197-208.  1952. 

6  Kawamoto,  N.  Y.  and  Uno,  H.  Studies  on  the  influence  of  the 
moonlight  upon  efficiency  of  the  fish  lamp.  Rep.  of  the  Fac.  of 
Fish.,  Prcf.  Univ.  of  Mie.  Vol.  1,  No.  3,  pp.  355-364.    1954. 

7  Kawamoto,  N.  Y.  and  Konishi,  J.  Diurnal  rhythm  in  phototaxis 
of  fish.  Rep.  of  the  Fac.  of  Fish.,  Pref.  Univ.  of  Mie,  Vol.  2,  No.  1 , 
pp.  7-17.     1955. 


[555] 


THE   USE   OF  LIGHT  ATTRACTION   FOR   TRAPS   AND   SETNETS 

by 

TADAYOSHI  SASAKI 

Professor  of  Tokyo  University  of  Fisheries  and  Chief  Research  Fellow  of  the  Scientific  Research  Institute,  Japan 

Abstract 

This  paper  describes  tests  which  were  made  to  find  out  if  a  directed  beam  of  light  was  better  for  attracting  fish  than  the  ordinary 
scattered  light.  Experiments  with  bag  nets  and  set  nets  showed  that  greater  catches  were  taken  when  the  directional  beam  was  used. 

A  new  method  of  fishing  with  setnets  has  been  introduced.  Here,  the  fish  arc  guided  into  the  bag  by  switching  on  a  succession  of 
lamps  coupled  together  so  as  to  form  a  lead.  The  lamps  can  be  controlled  from  the  shore  and  several  "fishings"  can  be  made  during  the 
course  of  a  night. 

The  catch  by  this  new  method  was  at  least  twice  as  big  as  by  the  old  method. 


Engins  de  peche  munis  d'un  systeme  de  lampes  pour  attirerles  poissons 

Cc  document  dccrit  des  essats  ayant  pour  objet  de  determiner  si  un  faisceau  lumincux  dirigg  attire  mieux  le  poisson  quc  la  lumiere 
diffusde  ordinaire.  Des  experiences  avec  des  filets-sac  et  des  filets  fixes  ont  montre  quc  les  captures  les  plus  importances  correspondent 
a  1'utilisation  d'un  faisceau  dirige. 

Une  nouvelle  methode  de  peche  au  moyen  de  filets  fixes  a  ete  adoptee.  Dans  cette  m&hode,  les  poissons  sont  altir&  a  1'interieur 
du  filet  par  1'allumagc  en  succession  d'une  serie  de  lampes  associees.  Ces  lampes  peuvent  etre  allumees  dcpuis  le  rivage  et  on  peut  fa  ire  plusicurs 
peches  pendant  une  nuit. 

Les  captures  obtenues  par  cette  m£thode  nouvelle  ont  ete  au  moins  deux  fois  plus  importances  quc  les  captures  obtenues  par  la 
methode  traditionnelle. 

Equipo  de  pesca  con  un  sistema  de  lamparas  para  atraer  los  peces 
Extracto 

En  este  trabajo  se  describen  las  pruebas  efectuadas  para  determinar  si  un  rayo  de  luz  dirigida  da  mejor  resultado  para  atraer  los 
peces  que  la  luz  dispersa.  Los  experimentos  con  "cielos"  y  redes  fijas  han  demostrado  que  se  obtuvieron  redadas  mas  abundantes  al  usar 
haz  luminoso  dirigido. 

Se  ha  puesto  en  practica  un  nuevo  metodo  de  pesca  con  redes  fijas,  a  las  cuales  se  guian  los  peces  cnccndicndo  y  apagando  una 
serie  de  lamparas  conectadas  de  manera  que  formen  una  especie  de  guia  o  rabera.  Estas  luces  pueden  operarse  desde  la  orilla  permitiendo 
hacer  varias  "redadas"  durante  el  curso  de  la  nochc. 

La  pesca  mediante  cste  nuevo  metodo  fuc,  por  lo  menos,  igual  al  doble  de  la  obtenida  con  el  equipo  antiguo. 


GENERAL 

THE  catching  system  of  setnets  consists  in  leading 
the  fish  into  the  bag  of  the  gear  by  means  of 
leader  nets  stretched  across  the  path  of  their 
migration.    It  is  believed  that  the  final  catch  in  the 
bag  net  represents  only  about  20  per  cent,  of  the  total 
fish  coming  into  contact  with  the  leader  net. 

In  order  to  increase  the  catch,  the  author  has  made  a 
series  of  investigations  with  underwater  fish  attraction 
lamps. 

EFFECT  OF  A  DIRECTIONAL  FISH  ATTRACTION 
LAMP 

Experiment  1 

A  directional  light  source  (6  V.,  50  c.p.)  was  submerged 
at  depths  of  1  to  3  m.,  with  the  beam  parallel  to  the 
water  surface.  Immediately,  plankton  and  fry  swarmed 
around  the  light,  and  after  about  1  min.  the  fish  became 
stationary.  The  fish  scattered  in  about  30  sec.  when  the 


light  was  put  out,  but  rcgathercd  within  1  min.  after 
switching  the  light  on  again. 

This  observation  was  then  applied  to  trap  fishing, 
i.e.  three  traps  were  set  as  shown  in  fig.  1 ,  and  a  directional 
light  source  was  placed  at  the  end  of  the  middle  one 
so  that  the  direction  of  the  beam  coincided  with  the 
axis  of  the  trap.  The  result  of  a  comparison  of  the 
number  of  fish  caught  during  one  night  in  each  trap 
(Table  I)  shows  that  this  type  of  light  source  has  an 
attracting  effect  on  some  kinds  of  fish  including,  for 


TABLE 


Trap  No. 


Crab      

3 

4 

0 

Lobster            .... 

0 

22 

0 

Lateolabrax  japonicus  (Cuvier) 

0 

3 

0 

Harengulazunasi  (Sleeker) 

0 

4 

0 

[556] 


USE    OF    LIGHT    WITH    TRAPS 


directional 
light  source 


Fig.  I.     Arrangement  of  three  traps  for  testing  the  effect  of  two 
directional  fish   attraction   lamps  (Experiment    /). 


Fig.   3. 


Experimental  arrangement   of  fish  attraction  lamps 
(/  to  5)  and  a  Y-shaped  trap  (Experiment  .?). 


example,  crab  and  lobster.  The  experiment  indicated 
that  there  were  good  prospects  for  the  use  of  such 
directional  light  sources  for  fishing  purposes. 

Experiment  2 

For  further  investigations  the  same  lamp  was  tested  in 
connection  with  some  of  the  traps  generally  used  in 
Lake  Hamana,  a  salt  lake,  in  the  Shizuoka  prefecture. 
Three  sets  of  traps  of  the  type  shown  in  fig.  2  were 
used,  each  one  consisting  of  two  groups  of  three  bag  nets 
connected  by  a  leader  net:  a  directional  lamp  was  put 
into  one  of  the  bag  nets  of  each  of  the  three  sets  but 
used  only  on  alternative  days.  The  catch  consisted 
of  Snipefish,  Lobster  and  Flatheads  and  the  results 
showed  quite  definitely  that  the  biggest  catches  were 
obtained  from  the  bag  nets  with  the  light. 

Experiment  3 

The  combined  effect  of  directional  light  and  scattered 
light  on  leading  fish  into  a  V-shaped  trap,  was  studied 
in  Sumoto  Bay  in  the  Hyogo  prefecture. 

The  general  arrangement  of  this  experiment  is  shown 
in  fig.  3.  The  water  depth  was  6  m.,  and  the  lamps 
(1,2,  3,  4%  5)  which  were  set  20  m.  apart,  were  1-5  m. 
under  water. 

The  time  required  for  gathering  the  fish  after  lighting 
the  lamp  is,  of  course,  dependent  on  the  intensity  of 


Fig.  2.     Arrangement  of  freshwater  traps  for  testing  the  effect 
of  a  directional  fish  attraction  lamp  (Experiment  2). 


illumination  and  the  transparency  of  the  water.  Lamp 
(5)  was  switched  off  after  5  hrs.  and  the  fish  which  had 
gathered  moved  to  the  next  lamp  (4).  Five  minutes 
later,  lamp  (4)  was  turned  off,  and  so  on  until  all  the 
lamps  except  (I)  were  extinguished.  Finally,  the  fish  were 
led  into  the  bag  and  the  entrance  of  the  net  was  closed. 
The  time  needed  for  this  operation  depends  on  the  kind 
of  fish  and  on  other  conditions. 

Experiment  4 

A  normal  fish  attraction  lamp  was  suspended  from  a 
boat  on  a  selected  fishing  ground  and  the  fish  which 
gathered  around  the  lamp  were  led  to  the  trap  by 
moving  the  boat.  A  directional  lamp  was  placed  before- 
hand in  the  net.  After  the  fish  had  been  led  into  the 
entrance  of  the  net  it  was  closed  as  in  Experiment  3. 
The  results  are  shown  in  Table  II. 

These  experiments  have  furthermore  shown  that  it 
is  not  necessary  to  have  the  string  of  lights  in  a  straight 
line.  The  lights  may  be  placed  in  any  curved  line  accord- 
ing to  the  condition  of  the  sea  and  the  fishing  ground 
where  the  net  is  operated. 

APPLICATION   OF  A   STRING   OF   FISH 
ATTRACTION  LAMPS  TO  A  SETNET 

The  experimental  results  obtained  in  Lake  Hamana  and 
Sumoto  Bay  were  applied  to  setnets  used  in  Atami  Bay 
in  the  Shizuoka  prefecture. 

The  arrangement  of  the  string  of  fish  attraction  lamps 
for  use  with  a  set  net  varies  according  to  the  type  of 
net  and  the  character  of  the  fishing  ground.  In  one  of 
the  experiments,  a  string  of  20  underwater  fish  attraction 
lamps  of  100  to  150  W.,  was  applied  to  a  setnet  (55  \ 
38  m.)  at  a  certain  angle  to  the  leader  net  (230  m.) 
(fig.  4).  The  lamps  were  placed  at  a  depth  of  1-5  m. 
The  distances  between  the  first  to  the  nineteenth  lamp 
were  equal,  but  the  distance  between  the  19th  and  20th 
lamp  was  bigger  (40  m.).  The  reason  for  this  was  that 
the  20th  lamp  fixed  at  the  end  of  the  bag  net  was  a 


TABLE  II 

Leading      Velocity  of  Velocity  of  the 
distance       the  lamp    fish  (swimming 
velocity) 


First 
experiment    1 50  m. 

Second 
experiment    300  m. 


lOOcm./sec.    107-5  cm./sec. 


50  cm./sec.     53 -3  cm. /sec. 


Catch 


horse 
mackerel  210 

horse 
mackerel  175 


[557] 


MODERN     FISHING     GEAR     OF     THE    WORLD 


•ubn*rin«  cabl* 
(310m.) 


__, —  ---  -    ~~   electric  power  source 

(A.C.  100  V.)  on  •hor« 

Fig.  4.     Combination  of  a  fish  attraction  lamp  system  with  a 
big  setnet. 

directional  one  and  the  beam  was  directed  to  the  entrance 
of  the  bag. 

After  sunset,  the  lamps  were  lit  simultaneously  by 
switching  on  the  current  from  the  shore.  After  waiting 
till  enough  fish  had  been  attracted  around  all  the  lamps, 
the  first  lamp  was  turned  off.  Consequently  the  fish 
moved  to  the  2nd  lamp.  The  time  required  to  complete 
such  a  transfer  from  one  lamp  to  another  depends  on 
the  intensity  of  illumination  and  the  kind  of  fish,  but 
generally  1-5  min.  are  sufficient.  After  17  lamps  have 
been  turned  off  successively,  a  fairly  large  amount  of 
fish  is  concentrated  around  the  18th  lamp.  When  the 
18th  lamp  is  turned  off,  the  fish  swiftly  move  through  the 
narrow  entrance  of  the  setnet  (8  m.  in  width)  towards 
either  the  19th  or  the  20th  lamp,  both  of  which  are 
kept  alight  until  the  next  morning. 

With  gear  of  this  size,  it  takes  approximately  I J  hrs. 
to  lead  the  fish  into  the  bag  of  the  setnet,  starting  from 
the  1st  one  and  successively  turning  off  one  of  the  lamps 
every  5  min.  In  order  to  achieve  an  almost  continuous 
catching  operation,  the  1st  and  following  lamps  are 
relit  at  the  same  time  as  the  9th  and  following  lamps 
are  turned  off  in  the  previous  leading  operation.  This 
operation  is  repeated  2  or  3  times  during  the  night, 
with  the  number  of  repetitions  depending  mainly  on 
the  time  required  to  attract  enough  fish.  The  net  is 
hauled  in  the  morning,  usually  just  before  sunrise. 

The  switching  operations  of  the  lamps  can  be  done 
automatically  and,  depending  on  the  facilities  available, 
the  electric  power  may  be  taken  from  shore  or,  for 
instance,  from  a  mother  ship  by  means  of  a  cab-tyre 
cable. 


string  of 

lamps 
(345m.) 


submarine       \ 
cable  (410m.)' 


\ 


\ 


\ 


power 

(  souroe  (A.C*   100  V.) 
A  on  shore 

Fig.  5.     Combination  of  a  fish  attraction  lamp  system  with  a  big 

setnet.       The  string  of  lamps  is  arranged  in  a  certain  way  to 

lead  off-shore  fish. 

During  the  experiment  carried  out  in  Atami  Bay,  the 
string  of  lamps  was  arranged  at  an  angle  of  about  180 
degrees  to  the  leader  net,  in  order  to  attract  off-shore  fish, 
(fig.  5). 

The  catches  made  with  this  new  method  and  with  the 
old  one  were  compared  over  several  months  by  alternate 
testing,  and  it  is  considered  that,  after  many  observa- 
tions, the  new  method  is  at  least  twice  as  efficient  as 
the  old  one. 


[558 


THE  BASIC  PRINCIPLES  OF  FISHING  FOR  THE  CASPIAN  KILKA 

BY  UNDERWATER  LIGHT 

by 

I.  V.  NIKONOROV 

Caspian  Institute  of  Marine  Fisheries  and  Oceanography,  Kaspniro 

Abstract 

Fishing  for  kilka  (a  small  clupjid  fish)  by  underwater  light  has  been  practised  in  the  Caspian  Sea  since  1951.  Before  1954  cone- 
shaped  nets  of  a  very  simple  construction  were  only  used  but  then  an  entirely  new  type  of  gear — the  fish  pump— was  introduced,  following 
extensive  research  on  the  behaviour  of  the  kilka,  including  underwater  observation  and  filming. 

By  1956  the  light  fishing  fleet  had  grown  to  450  vessels,  catching  a  total  of  1,500,000  cwts.;  eleven  of  these  ships  were  equipped 
with  pumps,  but  now  there  are  over  thirty. 

A  suction  hose,  with  electric  lights  attached  at  the  opening,  is  lowered  down  to  the  proper  depth.  Fish  attracted  by  the  light  to  within 
the  critical  range  of  the  hose-opening  are  sucked  in  and  pumped  to  the  fish  hold.  Various  factors  affecting  the  efficiency  of  this  fishing  method 
are  discussed  from  both  a  theoretical  and  empirical  point  of  view. 


Resume 


1/es  principes  fondamentaux  de  la  peehe  a  la  Itimicre  du  kilka  dans  la  (  aspienne 


Depuis  1951  on  pratique  la  peche  du  kilka  (un  petit  clup£id£)  dans  la  mer  Caspienne  a  Taidc  de  lumieres  sous-irurines.  Avant  1954 
on  utilisait  seulement  des  filets  coniques  d'une  construction  tres  simple,  mais  dcpuis  on  a  introduit  un  type  d'cngin  entiercment  nouveau — la 
pompe  a  poissons  apres  des  recherches  poussees  sur  le  comportement  du  kilka  comprenant  des  observations  et  des  prises  de  vues  sous-marines. 

Rn  1956  la  flotte  de  peche  a  la  lumierc  etait  passee  a  450  bateaux,  pcchant  un  total  de  1.500.000  cwts.;  on/e  de  ces  bateaux  etaient 
munis  de  pompes,  mais  maintenant  ils  sont  plus  de  30. 

On  immcrgc  A  la  nrofondcur  voulue  une  munche  d'aspiration  munie  de  lampcs  electriques  a  fouverture.  I.es  poissons  attirds  par 
la  lumiere  dans  le  rayon  d'action  de  la  manche  sont  aspires  et  pompes  dans  la  calc  a  poissons.  Divers  facteurs  afTcctant  1'efticacite  de  cctte  methode 
dc  peche  sont  examines  des  points  dc  vuc  theorique  et  empirique. 


Principals  basicos  de  la  pesca  de  "kilka1'  con  la  ayuda  de  luces  submarines  en  e  mar  B6ltico 
Extracto 

Desde  1951  los  Pescadores  del  m.ir  Caspio  caputran  "kilka"  (clupcido  de  talla  pequena)  atray6ndolo  mediante  luces  submarinas. 
Antes  de  1954  usaban  redes  c6nicas  de  construccion  muy  scncilla,  pero  a  partir  de  esa  fecha  se  introdujo  un  nucvo  tipo  de  arte  (la  bomba 
para  peccs),  despues  de  invest igar  cuidadosamcnte  los  habitos  dc  este  especie,  incluso  valiendose  de  la  observation  y  fotografia  submarinas. 

En  1956  la  flota  dc  pesca  con  hi/  habia  aumentado  a  450  embarcaciones  que  pescaron  un  total  de  150.000.000  libras.  Once  de  estas 
unidadcs  poseian  bombas  pero,  en  la  actual! dad,  mas  de  treinta  cuentan  con  dicho  equipo. 

Para  pescar,  sc  baja  a  la  profundidad  dcseada  la  mangucra  de  succi6n  que  lleva  luces  en  su  extrcmo.  Los  peces  atraidos  por  la  luz 
a  la  zona  critica  de  la  boca  de  la  manguera  son  aspirados  y  bombeados  a  la  bodega,  tn  el  trabajo  sc  analizan  desdc  los  puntos  de  vista 
teorico  y  empirico  los  factorcs  que  influycn  sobre  la  cficacia  de  este  metodo  de  pesca. 


THE  DEVELOPMENT  OF  THE  FISHERY 

FISHING  for  kilka  by  underwater  light  has    been 
successfully  practised  in  the  Caspian   Sea   since 
1951.    Before    1954   the  fish   attracted    by    light 
were  caught  with  a  cone-shaped  net  of  a  very  simple 
construction.  In  1954  an  entirely  new  type  of  gear    the 
fishing  pump — was  introduced. 

The  tjulka  or  kilka  is  a  small  clupeid  fish  of  the  genus 
Clupeonella,  native  to  the  Azov,  Caspian  and  Black 
Seas.  The  Caspian  kilka  is  a  small  pelagic  fish  of 
schooling  habits,  with  a  mean  body  length  (after  Smith) 
of  7  to  11  cm.  and  a  weight  of  3  to  10  g.  There  are  three 
species  which  differ  in  respect  to  size,  colour  and  area 
of  distribution.  The  common  kilka  is  essentially  a  coastal 
fish;  the  anchovy  form  is  found  relatively  far  offshore, 


whereas  the  big-eyed  prefers  deeper  waters  and  very 
rarely  enters  the  coastal  zone. 

The  stocks  of  Caspian  kilka,  especially  those  of  the 
anchovy  form,  arc  very  rich.  In  total  it  ranks  next  to 
three  clupeids:  the  herring  (Clupea  harengus),  sardine 
and  menhaden  (Brevoortia  tyrannus  (menhaden)). 

Before  1951,  the  Caspian  fishery  mainly  exploited  the 
stock  of  common  kilka;  since  1957  following  the  investi- 
gations of  Prof.  P.  G.  Borissov  (1945-47),  a  fishery  for 
the  anchovy  form  of  kilka  was  developed,  using  a  cone 
shaped  net  to  which  the  fish  was  attracted  by  submerged 
electrical  light.  In  1954  an  entirely  new  type  of  gear,  the 
fish  pump,  was  introduced,  based  on  the  results  of 
research  work  carried  out  at  the  Caspian  Institute  of 


[559] 


MODERN     FISHING    GEAR     OF    THE    WORLD 


f 


Fig.  1.  Caspian  kilka:   I —  common  form;  2 --anchovy  form; 
3^  big  eyed  form. 


Marine  Fisheries  under  the  direction  of  the  author. 
The  development  of  fishing  by  light  has  been  very  rapid. 
In  1951,  there  were  170  commercial  fishing  vessels 
engaged  in  this  fishery  and  their  total  catch  amounted  to 
about  7,700  tons.  By  1956,  the  fleet  had  grown  to  450 
vessels,  and  the  total  catch  was  about  68,000  tons. 
Eleven  of  these  ships  were  equipped  with  pumps  and 
their  catch  totalled  4,500  tons. 

THEORETICAL   PREMISES    OF   SUBMERGED 
LIGHT  ATTRACTION 

The  cause  of  the  attraction  exerted  by  underwater  light 
on  kilka  and  on  many  other  fishes,  is  not  yet  established. 
There  is  some  controversy  on  this  question.  The  author 
agrees  with  the  viewpoint  of  S.  G.  Zusser2  and  Borissov1 
that  the  attraction  offish  by  light  is  essentially  a  feeding 
reflex. 

We  know  that  kilka  feed  in  daytime.  Hence  it  may  be 
inferred  that  daylight  acts  as  a  stimulus  for  an  uncon- 
ditional feeding  reflex.  In  the  dark,  an  artificial  light  will 
produce  the  same  effect  and  stimulates  a  feeding  reaction 
inducing  the  fish  to  swim  toward  the  source  of  light. 
This  is  confirmed  by  a  marked  increase  of  the  catch 
before  dawn  when  the  contents  of  the  stomach  sharply 
decreases,  and,  consequently,  the  feeding  reaction 
becomes  stronger  and  the  approach  of  the  fish  to  the 
source  of  light  is  more  intense. 

The  Caspian  kilka  rarely  rises  to  the  surface  and  is 
usually  found  in  deeper  water  layers  so  that  it  does  not 
concentrate  around  a  surface  light  but  is  strongly 
attracted  by  underwater  light  located  at  its  own  depth. 


Fig.  2.   Distribution  of  anchovy  form  kilka  and  commercial 
Jishing  areas. 


The  approach  of  kilka  to  a  source  of  light  (and,  partly, 
its  concentration  within  the  lighted  zone)  depends  to  a 
considerable  degree  on  the  temperature  of  the  water. 

Kilka  will  not  approach  a  source  of  light  placed  above 
or  beneath  the  level  of  optimum  temperature.  If  the 
lamp  is  slowly  raised  or  lowered,  the  kilka  will  follow  it 
for  some  time,  until  it  reaches  unfavourable  temperature 
conditions,  then  it  will  retreat.  This  is  consistent  with  the 
theory  of  I.  P.  Pavlov,  that  animals  react  only  to  those 
external  factors  that  exert  the  greatest  stimulating  in- 
fluence on  the  organism.  In  our  case,  the  conditional 
signal  of  feeding,  determined  by  light,  is  superseded  by 
the  stronger  stimulus  of  the  temperature  of  the  water. 
Consequently  all  attempts  to  induce  kilka  to  rise  from 
the  level  of  optimum  temperature  to  the  warmer  upper 
layers,  or,  inversely,  to  descend  into  colder  waters,  were 
unsuccessful. 

It  has  also  been  observed  that  in  the  presence  of 
predators  a  school  of  kilka  will  assume  a  flattened  shape 
and  swim  in  circles,  rapidly  withdrawing  at  a  considerable 
distance  from  the  source  of  light,  resulting  in  an  abrupt 
decrease  of  catch  both  of  the  cone-net  and  the  fish  pump. 

The  submerged  lamp  may  be  considered  as  a  point 
source,  emitting  a  flow  of  light  equally  in  all  directions. 


[560] 


RUSSIAN     PUMP-FISHING     WITH     UNDERWATER    LIGHT 


,00-    *""* 


low    concentrations 


J    medium 


hiRh 

big.  3.       Depth  distribution  of  kilka  in  regard  to  the  season. 

The  distance  from  which  kilka  are  attracted  by  light 
(radius  of  attraction  R,)  depends  on  the  illumination 

F 

E  (1) 

S 

where  E  --  illumination 
F  ==  flow  of  light 
S  lighted  area 

The  attracting  action  of  rays  of  light  will  depend  on 
the  distance  from  the  light  source.     The  more  distant 


Fig.  4.     The  general  conditions  when  attracting  fish  to  a  lamp. 
R— radius  of  attraction;  7-=  suction  nozzle\  2^ cone-net. 


the  lighted  area  is  from  the  source  of  light,  the  lesser 
will  be  the  flow  of  light  received  per  unit  of  area. 

The  surface  area  S  of  a  sphere,  drawn  from  the  centre 
of  a  source  of  light  of  a  force  J,  with  a  radius  R,  is: 

S  ^  4-Rj- 
The  illumination  of  this  area  will  be: 

F 

F. (2) 

4TTR,2 

Let  us  assume  that  the  boundaries  of  the  sphere  are 
the  limit  at  which  kilka  begin  to  be  attracted  by  light 
(fig.  4);  if  the  flow  of  light  is: 

F  -  4*J. 
Then  the  illumination  is: 

J 
h  (3) 

V 

The  illumination  is  proportional  to  the  force  of  the 
source  of  light  J  and  inversely  proportional  to  the  square 
of  distance  R,2. 

From  two  sources  of  light  with  different  forces  of  light 
J,  and  J2  and  equal  illumination  can  be  obtained  at 
proportional  distances. 

Ji  J,, 

If  E!        and  E2       ---, 


and  it  is  assumed  that  i',     -  E:,,  then: 


or  R., 
R,2 


(4) 


Knowing  the  values  of  S  and  R,  the  radius  of  attraction 
R2  from  a  source  of  light  with  a  force  J2  can  be  deter- 
mined. 

As  kilka  approach  the  source  of  light,  their  concentra- 
tion per  J  cu.  m.  greatly  increases  and  becomes  most 
dense  near  the  lamp.  The  process  of  entering  the  lighted 
zones  being  continuous,  favourable  conditions  are 
created  for  uninterrupted  fishing  at  one  and  the  same 
place.  The  stronger  the  source  of  light,  and  the  wider  the 
radius  of  attraction,  the  denser  will  be  the  concentration 
of  kilka  and  hence  the  greater  the  catch. 

Further  experimental  work  is  necessary  to  define  the 
distance  at  which  the  dense  concentrations  of  kilka 
spread  from  the  source  of  light  in  relation  to  its  intensity, 
and  the  magnitude  of  the  radius  of  attraction.  There  is 
some  evidence  indicating  that  bright  illumination  may 
have  a  reverse  effect  so  that  the  density  of  kilka  will  tend 
to  decrease  near  a  lamp  of  too  great  a  brilliancy. 

OBSERVATIONS  ON  THE  BEHAVIOUR  OF  KILKA 
IN  AN  ILLUMINATED  ZONE 

During  experimental  work  by  the  Caspian  Institute  on 
board  of  a  specially  equipped  vessel  in  1952  to  1954  to 
improve  the  existing  methods  of  fishing  and  to  prove  the 
possibility  of  fishing  with  a  pump,  extensive  observations 
were  carried  out  on  the  behaviour  of  kilka  in  an  illu- 
minated zone  within  the  range  of  action  of  a  fishing  pump. 
Various  methods  of  investigation  were  employed, 
including  underwater  observations  and  underwater 
filming. 

The  investigations  disclosed  that  kilka  begin  to  enter 
the  field  of  light  almost  immediately  (i.e.  in  some 


[561] 


MM 


MODERN     FISHING     GEAR    OF    THE    WORLD 


Fig.  5.  Diagram  of  the  experimental  assembly  used  for  kitka 
fishing  with  a  fish  pump  oj  100  h.p.  I  -suction  nozzle,  2  hose, 
3— return  valve;  4 -fish  pump;  5  -forcing  hose;  6  hauling 
line;  7-  separator;  8— cable  for  lamps;  V  speed  regulator; 
10  electromotor. 


seconds)  after  the  underwater  light  is  turned  on  and  that 
the  fish  approach  the  light  very  closely,  even  brushing 
against  the  lamp.  Commercial  aggregations  are  formed 
in  0-5  to  2-5  min.  depending  on  the  abundance  of 
fish  in  the  region  and  on  the  intensity  of  light.  It  was 
also  observed  that  kilka  approaching  the  source  of  light, 
avoid  narrow,  restricted  spaces.  Therefore  the  construc- 
tion of  catching  devices  for  a  fishing  pump  must  permit 
a  free,  unhindered  approach  of  fish  to  the  source  of 
light;  the  latter  must  be  placed  near  the  intake  nozzle, 
or,  if  fishing  with  a  net,  in  the  centre  of  the  net  mouth. 
The  critical  velocity  of  suction  at  the  intake  aperture  of 
the  nozzle,  where  kilka  are  unable  to  resist  the  sucking 
action  of  the  water  currents,  was  determined  under 
laboratory  conditions  and  found  to  be  0-35  m./sec. 
kilka  approaching  the  critical  zone  try  to  get  away;  if 
the  schools  are  sparse  some  fish  do  escape,  but  when  the 
concentrations  are  dense  the  foremost  fish  are  prevented 
from  swimming  away  by  new  arrivals,  who  push  them 
into  the  critical  zone. 

Some  other  peculiarities  were  observed  in  the  behav- 
iour of  the  fish.  In  summer,  on  moonlight  nights,  when 
kilka  are  fished  at  small  depths,  the  schools  are  small  and 
dispersed  and  much  less  attracted  by  light,  resulting  in 
an  abrupt  decrease  of  catch. 

In  an  electrical  field  of  direct  current,  kilka  swim 
toward  the  anode  only  when  the  lines  of  force  of  the 
electrical  field  are  uniformly  and  horizontally  directed. 


Underwater  sound  of  bells,  or  a  barrier  of  air-bubbles, 
have  no  effect  on  the  behaviour  of  kilka. 


INVESTIGATIONS     OF    PUMP-FISHING      WITH 
LIGHT  ATTRACTION 

The  results  of  investigations  bearing  on  the  behaviour 
of  kilka,  as  well  as  practical  experience  gained  in  working 
with  fish-pumping  gear,  show  that  the  catch  of  kilka 


¥=360° 


Fig.  6. 


Suction  spec  tram  of  a  cylindrical  nozzle,    a-**  with, 
b^  without  shield. 


[562] 


RUSSIAN     PUMP-FISHING    WITH     UNDERWATER    LIGHT 


.•4* 


<*) 


Fig.  7.     Catching  device  for  experimental  nullification  of  the 
field  of  light  at  constant   value  of  suction. 


Fig.  8.     Different  suction  nozzles  for  experiments  with  varying 
suction  characteristics  at  constant  jielit  of  light. 


depends   (all    other   conditions    being   equal)   on    two 
essential  factors: 

(1)  the  value  of  the  critical  sphere  of  suction  at  the 
intake  nozzle  Rkp,  determined  by  the  capacity  of 
the  pump,  and 

(2)  the  value  of  the  field  of  light  Sn  at  the  intake 
nozzle. 

Mathematically  this  relation  can  be  expressed  as: 
Up  -  /(Rkp  ;S,,),  where  Up  -  catch 

The  catch  of  the  cone-net  also  depends  on  these  factors, 
but  then  the  value  of  the  critical  sphere  of  suction  Rkp 
must  be  replaced  by  the  volume  of  water  filtered  by  the 
net  during  hauling.  This  volume  is  determined  by  the 
area  of  the  intake  aperture  and  the  depth  of  the  fish,  or, 
in  other  words,  the  catch  of  the  cone-net  depends  on  the 
area  fished  and  the  speed  of  hauling,  as  well  as  on  the 
value  of  the  field  of  light,  i.e.: 

TtD2 

/( ;Vn;Sn)  where 


Ukc 

Ukc 
D 
Vn 
Sn 


4 

catch 

diameter  of  the  opening  of  the  cone-net 
velocity  of  hauling 
value  of  the  field  of  light 


The  value  of  the  critical  sphere  of    suction  Rkp  at 
the  nozzle  (fig.  6)  is  determined  from  the  formula: 


/    vhc 

/ 

V   Pvn  Vk 


(4) 


which  is  derived  from  the  theory  of  sources  and  dis- 
charges; it  connects  by  a  simple  relation  the  two  relevant 
values  —radius  of  the  sphere  Rkp  and  radial  velocity 
Vkp  by  the  essential  parameters  of  the  flow  Vhc  and 
the  radius  of  the  pipe  r€J,  where  /?,„  coefficient  of 
restriction  of  the  field  of  velocity  (constructive  coefficient). 

At  />'vn-   4  the  fluid  will  be  drawn   from   sphere  y 
360  degrees,  and  at  />vn     2  the  fluid  will  be  drawn  from 
hemisphere  y     180  degrees. 

At  the  cylindrical  nozzle,  limited  by  a  plane  bearing 
the  underwater  lamps,  the  field  of  light  Su  is  restricted 
to  a  hemisphere,  i.e.  Sn  27rR,2  (figs.  4  and  6)  provided 
that  the  circular  shield  on  the  nozzle  fully  reflects  all  the 
rays  of  light  received.  When,  on  the  other  hand,  the 
lamps  are  disposed  around  the  cylindrical  intake  of  the 
nozzle  or  in  the  centre  of  the  cone-net's  opening  without 
a  shield,  the  field  of  light  can  be  assumed  to  form  a 
sphere,  Sn  4,-rRj2.  Generally  speaking,  the  value  of 


1563J 


UP 

MO 
90 

do 
ro 

60 

so 
40 

d 

to 

0 


MODERN    FISHING    GEAR    OF    THE    WORLD 

Up  8"/c 


Oft 


1,0 


2,0 


Fig.  9.     Changes  in  amount  of  catch  in  relation  to  the  value  of 

the  coefficient  of  restriction  of  the  light  field,  obtained  with 

the  equipment  shown  in  jig.  7. 


the  field  of  light  depends  on  the  constructive  form  of 
the  nozzle,  and  is: 

Sn  -      TrR/  .  fisn,  where 

R,  -       radius  of  the  field  of  light  and 

psn    -      coefficient  of  restriction  of  the  field  of  light. 

Consequently  the  theoretical  value  of  the  catch  taken  by 


too 

90 
60 
?0 
60 
-SO 
40 
A) 
20 


12  1,5  2     83  'Pw 

rig.  10.     Changes  in  amount  of  catch  in  relation  to  the  value  of 

the  coefficient  of  restriction  of  the  light  field,  obtained  with 

the  equipment  shown  in  fig.  8. 


100 
30 
80 
70 
60 
SO 


\W 


17,3 


19,3 


22,3 


R  Kp  tCMl 


Fig.  II.     Changes  in  amount  of  catch  in  relation  to  the  value  of 
the  critical  sphere  of  suction  as  obtained  with  the  nozzle  shown 
in  fig.  8  d. 

a  fish  pump  (Up)  or  a  cone-shaped  net  (Ukc)  can  be 
expressed  by 


/     Vbc 
Up       f(rt>    /    :    TrRj2  . 

V   Pvn    Ukp 


3sn) 


-D- 

Uu        f  (      -  -  ;  Vn  ;  -R,2  .  psn) 
4 

To  establish  the  fishing  efficiency  of  the  fish  pump  as 
related  to  changes  in  values  of  the  fields  of  light  and 
velocity,  experiments  were  carried  out  with  a  catching 
device  (fig.  7)  in  which  the  value  of  the  field  of  light 
changed  according  to  changes  in  height  of  the  cylinder, 
whereas  the  field  of  velocity  (suction)  remained  constant, 
i.e.  the  fish  pump  worked  at  a  constant  capacity.  The 
relation  Up  /(//sn)  for  the  different  values  of  the  field 
of  light  was  determined. 

Analogous  experiments  were  carried  out  with  different 
types  of  nozzles  and  different  fields  of  light  between 
ff  110  degrees  to  <p^230  degrees  (fig.  8).  The  value 
of  the  critical  sphere  of  suction  in  the  field  of  velocity 

Up  8% 


7,2     7,8 


8,9 


RKptCM) 


Fig.  12.     Changes  in  amount  of  catch  in  relation  to  the  value  of 

the  critical  sphere  of  suction  as  obtained  with  the  nozzle  shown 

in  fig.  8  a. 


[564] 


RUSSIAN    PUMP-FISHING     WITH     UNDERWATER     LIGHT 


Up  8% 

100] 


a) 


i     ,  r  u   -, 

•*-£/•-  >,  1 

Fig.    13.     Different  nozzle  designs,    a  -cylindrical;  b  — cylin- 
drical  with   a  shear;    c~  conical. 

remained  constant,  except  in  that  of  the  nozzle  tested. 
All  nozzles  were  tested  under  similar  fishing  conditions 
and  the  catch  of  the  pump  was  checked  every  five  min- 
utes. The  variation  of  catches  Up  from  12  observations, 
as  related  to  &„,  could  be  expressed  by  a  straight  line 
to  which  empirical  formulas  were  fitted  (figs.  9  to  10). 

Some  experiments  were  also  carried  out  with  constant 
field  of  light  and  modification  of  the  value  Rkp  of  the 
critical  sphere  of  suction  on  the  field  of  velocity  (fig.  8). 


10 


tlMUHl 


Fig.  14.     Changes  in  amount  of  catch  in  relation  to  the  operation 

of  the  sucking  device.     1 '- nozzle  unmoved;    11 —nozzle  raised 

during   pumping  \    111— nozzle    lowered  during  pumping. 

The  relation  Up=/(Rkp)  was  determined  and  the  result 
is  shown  on  figs.  11  and  12. 

Thus  the  catching  efficiency  of  this  new  fishing  gear  is 


.  /5.     Diesel-electric  motor  vessel  equipped  with  two  fishing  pumps. 

[565] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


determined  by  two  factors,  i.e.  the  values  of  the  fields 
of  light  and  velocity. 

This  fact  explains  the  different  efficiency  of  different 
catching  devices  as  related  to  constructive  parameters, 
and  permit  a  theoretical  approach  to  the  construction  of 
nozzles  with  optimum  fishing  efficiency  by  means  of 
formula  (4): 


/     Vbc 
=  r«  .  / 

V  Pvn  Vkp 


Given  an  unchanged  pump  output  and  r0,  Vbc  and  Vkp 
const,  the  value  of  Rkp  increases  with  decreasing 
coefficient  /Jvn,  characterizing  the  construction  of  the 
nozzle.  A  lower  coefficient  /Jvn,  however,  can  only  be 
obtained  by  an  additional  cone  a  confuser — at  the  end 
of  the  cylindrical  nozzle  (fig.  13c).  This  cone  has  the 
disadvantage  of  increasing  the  distance  between  the 
lamps  as  compared  with  a  cylindrical  nozzle.  It  was 
therefore  necessary  to  devise  a  nozzle  combining  a 
maximum  value  of  the  field  of  velocity  with  a  satis- 
factory arrangement  of  the  electrical  lamps.  The  sloping 
shear  instead  of  a  cone  nipple,  increases  ihe  area  of  the 
intake  aperture  without  increasing  the  diameter  of  the 
nozzle.  Such  a  nozzle  was  designed  and  tested  by  the 
author  during  the  summer  of  1957.  Long  term  investi- 
gations proved  it  to  be  more  efficient  than  conical  and 
cylindrical  nozzles,  and  it  is  now  successfully  used  on 
fishing  vessels  (fig.  13b). 

Similar  relations  were  obtained  for  the  cone-net.  An 
increase  of  the  diameter  of  its  aperture  and  the  speed  of 
hauling,  results  in  increased  catches.  If  the  catch  of  a 
cone  net  with  an  opening  diameter  of  2-5  m.  is  taken  as 
100  per  cent,  then  with  3-0  m.  diameter  the  catch  will 
increase  to  150  per  cent,  and  with  1  -5  m.  diameter  will 
be  reduced  to  52  per  cent. 

Some  other  factors  affecting  the  fishing  efficiency  of 
fish  pumps  were  experimentally  established  and  theoretic- 
ally grounded.  Most  important  are  the  brief  periodical 
changes  of  the  depth  of  fishing,  i.e.  raising  or  lowering 
of  the  nozzle  within  the  layer  of  greatest  abundance  of 
kilka.  This  leads  to  a  denser  concentration  of  the  fish 


around  the  lamp  in  the  sphere  of  suction  and  to  heavier 
catches.  Lowering  the  nozzle  proved  to  be  more  effective 
than  raising,  or  working  at  a  constant  depth  (fig.  14). 
An  analogous  effect  is  achieved  by  a  brief  dimming  of  the 
light,  and  some  effect  by  periodical  extinguishing. 

The  catch  of  a  cone-net  can  be  increased  in  a  similar 
way  by  lowering  and  raising  the  net.  The  fishermen, 
being  aware  of  this  peculiarity,  used  to  sink  the  net 
briskly  before  hauling  up. 

More  than  thirty  ships  are  now  fishing  kilka  with  fish 
pumps  in  the  Caspian  Sea,  and  the  pumping  method  has 
proved  to  be  more  effective  than  the  use  of  cone-nets. 
Pump  fishing  is,  furthermore,  less  affected  by  moonlight. 

The  advantages  of  pump-fishing  are  manifold:  labour 
saving;  lower  operation  costs;  increased  production  and 
better  exploitation  of  the  commercial  fishing  fleet. 
Particularly  good  economic  results  have  been  achieved 
with  the  introduction  of  diesel-electric  motorships  of 
850  tons,  fishing  simultaneously  with  two  150  h.p. 
pumps  (fig.  15). 

With  a  wider  commercial  use  of  this  new  and  progres- 
sive system  of  fishing,  improvements  along  the  following 
lines  are  indicated: 

1 .  More  precision  in  the  design  of  the  catching  device^ 
use  of  a  more  suitable  source  of  light,  and  deter- 
mination of  a  working  scheme  that  would  ensure 
the  formation  of  dense  concentrations  of  kilka 
within  the  critical  (active)  sphere  of  velocity  of  the 
water  suction. 

2.  Possible  additional  attraction  of  kilka  by  means 
of  a  "path  of  light"  and  determination  of  its 
operation   as   related   to   varying  environmental 
factors  as  well  as  to  the  biological  condition  offish. 

3.  Increasing  the  active  sphere  of  suction  at  the 
catching   device    by     using   highly    economical, 
small-si/e  pumps  of  great  capacity. 

REFERENCES 

1  Borissov,  P.  G.  Use  of  artificial  light  in  the  world  fisheries. 
Moscow,  1956  (in  Russian).  1956. 

*  Zusser,  E.  A.  J.  Gen.  Biol.  XLV  (2)  1953  Acad.  Sci.  U.S.S.R. 
(in  Russian).  1953 


Mediterranean  light  boat  with  three  big  gas  lamps  for  attracting  fish. 

I  566] 


Photo:  FAO. 


FISHING   JIGS   IN  JAPAN  WITH   SPECIAL  REFERENCE  TO  AN 
ARTIFICIAL  BAIT  MADE  OF  LATEX  SPONGE   RUBBER 

by 

TAKEO  KOYAMA 

Fishing  Gear  Technologist,  Tokai  Regional  Fisheries  Research  Laboratory,  Tokyo,  Japan 

Abstract 

Various  types  of  lures  are  used  in  the  Japanese  line  fisheries,  all  differing  in  colour  arrangement  and  shape.  The  shaft  of  the  jigs  is 
made  of  horn,  hoof,  bone,  zinc  or  wood  painted  with  powder  of  mother-of-pearl,  while  feathers  or  fish  skin  is  used  for  the  jig-tails. 

In  the  longline  fisheries,  however,  fish  bait  is  preferred  to  artificial  lures.  With  a  view  to  reducing  cost  and  labour  needed  for 
preservation  and  transportation  offish  bait  used  by  the  longline  vessels  when  operating  in  the  tropical  zone,  tests  were  carried  out  with  a  sponge 
rubber  lure  which  has  a  dull  lustre  and  a  squid-like  shape  and  smell. 

Although  the  field  experiments  with  this  artificial  bait  gave  inferior  results  as  compared  with  those  of  fish  bait,  a  close  scrutiny 
of  the  data  shows  that  this  may  be  due  to  the  relatively  unfavourable  position  the  lures  had  on  the  line. 


Resume 


Les  Leurres  Artificiels  au  Japon,  en  particulier  un  appat  Artificiel  de  Caoutchouc-Mousse 


Dans  la  peche  aux  lignes  japonaise  on  utilise  divers  types  de  leurres  qui  different  tous  par  la  disposition  des  coulcurs  et  la  forme 
Le  c&rps  des  leurres  est  en  corne,  en  sabot  d'animal,  en  os,  ou  zinc  ou  en  bois  neint  avec  dc  la  poudre  de  nacre,  alors  que  pour  les  parties 
arrieres  on  emploic  des  plumes  ou  de  la  peau  de  poisson. 

Ccpcndant,  pour  la  p&che  aux  palangrcs  on  pr&fere  les  poissons  comrrtc  appat  plutot  que  les  leurres  artifkiels.  Rn  vue  de  diminuer 
les  coOts  et  la  travail  necessaires  pour  la  conservation  et  le  transport  des  poissons  servant  d 'appat  utilises  par  les  palangricrs  quand  ils  opcrent 
dans  les  regions  tropicales.  on  a  cflectue  des  essais  avec  un  leurre  de  caoutchouc-mousse  mat  qui  possede  une  forme  et  unc  odeur  voisines 
de  eel  les  du  calmar. 

Bien  que  les  experiences  sur  les  lieux  de  peche  avec  cet  appat  artificiel  aient  donng  des  resultats  inferieurs  a  ceux  obtcnus  avec  des 
poissons  servant  d'appat,  une  analyse  serree  des  donnees  montre  que  cela  pent  etre  du  a  la  position  relativement  defavorablc  des  leurres  sur 
la  ligne. 

Anagazas  para  pescar  empleadas  en  el  Japon,  con  especial  mention  de  un  cebo  artificial  hecho  de  caucho  esponjoso  ettstico 
Extracto 

Los  Pescadores  japoneses  que  sc  dedican  a  la  pesca  con  lineas  emplean  diversas  clases  dc  aftagazas,  todas  el  las  de  colores  y  formas 
distintos.  FJ  cuerpo  de  la  artagaza  se  hace  de  cuerno,  pczuna,  hueso,  zinc  o  madera,  pintado  con  polvo  de  mad  re  per  la  y  para  las  colas  se 
emplean  plumas  o  pic!  de  pescado. 

Sin  embargo  los  que  se  dedican  a  la  pesca  con  palangr-es  prefieren  el  pescado  a  las  afiaga/as  artificiales  como  carnada.  Con  objeto 
de  reducir  los  gastos  y  la  mano  de  obra  necesaria  para  la  conservaci6n  y  transporte  del  pescado  empleado  como  cebo  en  los  barcos  palangreros 
que  pescan  en  la  zona  tropical,  se  nan  realizado  ensayos  con  una  aftagaza  de  caucho  esponjoso  que  tiene  la  forma  y  cl  olor  ana  logos  a  los 
del  calamai  y  color  mate. 

Aunquc  los  resultados  obtenidos  en  la  practica  con  este  cebo  artificial  han  sido  inferiores  a  ios  logrados  con  pescado,  el  analisis 
detallado  de  los  datos  indica  que  puede  deberse  a  la  posicion  relativamentc  desfavorahle  que  ocupan  las  anaga/.as  en  el  palangre. 


INTRODUCTION 

JIGS  are  meant  to  imitate  some  kind  of  small  fish  and 
at  the  same  time  retain  a  fishing  efficiency  as  high 
as  that  of  live-bait.  The  jigs  must  therefore  be 
adapted  to  the  particular  type  of  fishing  operation  and 
well  suited  on  the  basis  of  a  physiological  study  of  the 
sensory  functions  of  fish  such  as  vision,  smell,  taste, 
and  touch.  However,  among  dozens  of  different  types 
that  have  been  in  service  for  trolling  and  pole-and-line 
fisheries  in  Japan,  the  majority  have  been  devised,  not 
on  a  biological  ground,  but  on  ideas  coming  from  years 
of  experience  on  the  part  of  skilled  fishermen.  Some  of 
these  jigs  appear  to  be  as  effective  as  live  bait  in  luring 
fish.  Here  a  brief  description  will  be  made  of  some  of 
the  representative  types  of  jigs  employed  by  Japanese 
fishermen  with  special  reference  to  the  result  of  experi- 


mental long  line  fishing  in  which  artificial  bait  made  of 
latex  sponge  rubber  was  used. 

JIGS  FOR  TROLLING  AND  ANGLING 

In  trolling  and  pole-and-line  fishing,  the  jigs  are  designed 
with  special  attention  to  their  shape  and  colour  as  they 
are  intended  to  appeal  mainly  to  vision  of  fish.  Neverthe- 
less, it  is  not  always  necessary  for  the  jigs  to  have  an 
exact  similarity  in  appearance  with  live  fish.  Instead, 
a  substance  roughly  looking  like  a  fish  is  enough  for 
the  purpose,  as  far  as  the  shape  is  concerned.  On  the 
other  hand,  a  subtle  arrangement  of  colours,  much 
brighter  than  actual  ones,  is  needed,  because  the  colours 
appear  to  be  more  important  than  the  shape  in  enticing 


[567J 


MODERN     FISHING    GEAR     OF    THE    WORLD 


Fig.  I.     Skipjack  jigs. 

a — A  riffling  line;  b — Zinc  bar; 

c — fish  skin;  d — Hook; 

e — Horn. 


Fig.   2.     Troll  line  with  floor. 

a-  Angling  line;        b-  Float; 

c-~Jig 


the  fish  to  strike.  In  addition,  the  effect  of  a  jig  will 
be  enhanced  by  quick  and  continuous  movement  in 
and  out  of  the  water,  as  is  the  practice  in  pole-and-line 
fisheries.  Jigs  used  in  this  type  of  angling  are  illustrated 
in  fig.  I.  The  above  facts  explain  the  reason  for  adding 
a  float  to  a  troll  line  as  in  fig.  2  which  gives  the  jig 
a  jerking  movement  in  the  water.  It  also  explains  why 
fish  appear  to  strike  better  in  a  rippling  sea  than  when 
it  is  calm. 

In  some  types  of  angling  for  squid,  mackerel  or  perch, 
jigs  have  been  found  as  good  as,  and  sometime  better 
than  live  bait  (fig.  3). 

When  fish  arc  so  excited  as  in  the  case  of  pole-and- 
lining  for  skipjack  and  frigate  mackerel,  jigs  are  preferred 
to  live-bait  to  save  time  and  labour  (fig.  4).  Even  where 
jigs  are  not  as  effective  as  real  bait,  fishermen  keep  them 
handy  for  use  when  the  stock  of  live  bait  on  board  is 


Fig.  3.  Handline  for  mackerel.       Fig.  4.    Tuna  and  frigate 

mackerel  jigs. 

a— Sisal;    b-  Silk;    c—Gut;    a—Angling  line;     b—Eye  for 

d— Swivel;      e— Branch  line;    tying  the  line;          c—Lead; 

t—Lead'  g — Main  tine;    d— Shell  of  abalone;  c — Inlaid 

h— //£  with  feather.  lead;  f—Fish  skin;  g—  White 

feather;    h — Hook. 


exhausted,  as  in  trolling  for  tuna  or  yellowtail  (fig.  5). 

In  construction,  most  jigs  are  provided  with  a  shaft 
and  one  or  two  hooks.  Those  which  have  only  a  polished 
hook  or  a  hook  furnished  with  fish  skin  or  a  glass  bead, 
form  an  exception. 

The  shaft  is  usually  made  of  horn,  hoof,  bone,  zinc 
or  wood,  and  painted  with  powder  of  mother-of-pearl. 
Bird  feathers,  fish  skin,  or  seaweed  are  used  for  the  tail 
of  the  jigs. 

For  freshwater  angling,  jigs  are  made  to  imitate 
insects  or  flies,  while  for  marine  fisheries  they  are  made 
to  resemble  squid,  sardine,  octopus,  shrimp  or  crab, 
each  being  near  to  life  size.  Some  hooks  have  no  barb, 
this  to  facilitate  dehooking,  others  have  double  hooks 
depending  on  types  of  operation.  Fig.  5  shows  some 
of  the  lures  in  general  use  in  Japan. 

A  rather  new  type  of  jig-tail,  is  thrust  over  a  hook 
in  the  same  manner  as  is  done  with  real  bait.  With  the 
recent  application  of  plastics  and  sponge  rubber  to 
fishing  industry,  this  type  of  jig  is  being  produced  to 
look  like  squid,  crab,  saury  or  fish  eggs.  Sometimes 
they  are  used  together  with,  and  sometimes  independently 
of,  true  baits1. 

ARTIFICIAL  BAIT  FOR  LONGLINE  FISHERY2 

Up  to  the  present  time,  artificial  bait  has  been  rarely 
used  for  longline  fisheries,  probably  because  of  the  lack 
of  movement  in  the  hooks  compared  with  trolling  and 
angling.  An  efficient  artificial  bait  has  long  been  sought 
to  relieve  cost  and  manpower  needed  for  preservation, 
transportation,  and  storage  of  the  bait  fish  used  by  the 
tuna  longline  fleets  operating  far  into  the  equatorial 


Fig.   5.     Trolling  lures. 

a.—Anglinz   line;    b—Zinc;    c—Fish    skin;    <\~-~Hook; 
e—Cattlehorn;    f— Feather:   g— Shell  of  abalone. 


[568] 


ARTIFICIAL    BAIT 


TABLE  I 
Details  of  longline  operations 


Test 

Date 

Position 

Time 

Surface 

number 

(1953) 

Lot.  N. 

Long.  E. 

Setting 

Hauling 

temp.  0°  C 

Baskets 

Bait 

I 

Mar.  15 

5"  35' 

163°  2' 

0335-0755 

1230-2230 

28-6 

330 

Frozen  squid  on  1st 

hook,  lure    on    5th 

II 

Mar.  16 

5°    4' 

163    11' 

0300-0725 

1230-2320 

28-6 

350 

hook.  Frozen  saury 

HI 

Mar.  17 

5°    0' 

163°  2' 

0315-0740 

1230-0020 

28-6 

350 

on  others. 

IV 

Mar.  19 

4°  51' 

163"  1' 

0335-0740 

1230-0020 

28-4 

350 

V 

Mar.  20 

4"  53' 

163r  T 

0345-0740 

1215-2145 

28-1 

320 

VI 

Mar.  21 

4y  58' 

163°  (T 

0345-0725 

1155-2245 

28-4 

350 

Frozen  saury  except 

VII 

Mar.  23 

4"  39' 

163°  39' 

0305-0715 

1140-2300 

28-4 

356 

on  last   5   baskets 

VIII 

Mar.  24 

4°  39' 

162"  30' 

0330-0740 

1205-0100 

28-4 

350 

which   had    lures. 

IX 

Mar.  25 

4fl  48' 

162°  30' 

0330-0750 

1205-2250 

28-4 

335 

X 

Mar.  26 

4°  46' 

162"  30' 

0314-0700 

1145-2045 

28-3 

310 

region.  With  this  in  mind,  a  type  of  artificial  bait 
shaped  like  a  squid  has  been  prepared  from  latex  sponge 
according  to  a  design  of  the  author  (fig.  6). 

A  series  of  field  experiments  with  this  bait  was 
entrusted  to  the  crew  of  the  R.V.  Sagami  Maru  (200 
gross  tons)  of  the  Kanagawa  Prefectural  Fisheries 
Experimental  Station  when  they  operated  for  tuna  in 
the  equatorial  region  in  March  1954.  Because  of  a 
limited  number  of  experiments  and  other  difficulties 
encountered  in  the  field,  the  results  were  inconclusive. 

The  longline  used  for  the  experiments  had  5  hooks 
per  basket  of  225  m.  length  and  is  outlined  in  fig.  7. 
The  design  is  essentially  the  same  as  that  of  the  commer- 
cial longlines  used  in  Japan.  For  the  comparative 
experiments  conducted  ten  times  during  March  15  to 
26,  1953,  the  lure  was  fixed  on  the  5th  hook  of  every 


basket  of  test  1,  while  in  tests  V  to  X,  all  hooks  of  the 
last  five  baskets  of  the  line  had  an  artificial  bait.  The 
other  hooks,  as  a  control,  had  either  frozen  squid  or 
frozen  saury  bait. 

The  artificial  bait,  made  of  latex  sponge  rubber,  was 
painted  with  micaceous  powder  to  give  it  an  opaque 
glimmering  lustre  (fig.  6).  It  measures  35  centimetres 
in  total  length,  and  weighs  40  grams.  About  60  grams 
of  sand  was  stuffed  inside  the  trunk  and  after  soaking 
for  about  5  minutes,  the  weight  was  about  130  grams 
in  air;  the  weight  under  water  being  approximately  that 
of  a  real  squid  of  similar  size.  To  give  it  a  squid-like 
smell  and  taste,  it  was  soaked,  before  use,  in  a  saponified 
solution  of  squid  oil,  and  hooked  through  the  trunk 
as  is  done  with  true  squid  bait. 

Table  II  shows  that  the  fishing  efficiency  of  the  arti- 


Fig.  6.     iMtex  sponge  artificial  squid. 


Fig.   7.     Details  of  one  basket  of  longline. 


Test  I 


Hook  Number 

1 

2 

3 

4 

5 

Bait  used 

SQ 

S 

S 

S 

L 

Yellowfin 

1 

2 

9 

5 

1 

Bigeyc  

1 

~ 

2 

2 

1 

Ground  shark 

- 

2 

1 

1 

1 

Black  marl  in  ... 

2 

1 

4 

2 

— 

Sailfish    

., 

- 

- 

— 

— 

Other  sharks  ... 

- 

- 

2 

_ 

- 

Skipjack 

— 

] 

1 

~ 

- 

Spanish  mackerel 

- 

- 

- 

- 

_ 

Dolphin 

- 

- 

- 

- 

— 

Barracuda 

1 

- 

- 

2 

- 

(Total  catch)  ... 

5 

6 

19 

12 

3 

Total 

18 

6 

5 

9 

0 

2 

2 

0 

0 

3 
45 


TABLE  JI 
Catch  in  Tests  I  to  IV 


7  est  If 

1 

2 

3 

4 

5 

Total 

S 

S 

S 

S 

S 

8 

9 

10 

8 

3 

38 

3 

1 

1 

1 

— 

6 

1 

6 

3 

2 

I 

13 

7 

1 

5 

5 

5 

23 

2 

_ 

1 

1 

_ 

3 

1 

2 

4 

4 

1 

12 

1 

1 

2 

2 

_ 

6 

_ 

1 

_ 

1 

_ 

2 

_ 

_ 

- 

~ 

- 

0 

1 

- 

- 

- 

2 

3 

24  21  25*24  12     106 


Test  111 

1 

2 

3 

4 

5 

Total 

S 

S 

S 

S 

S 

11 

12 

9 

16 

5 

53 

_ 

1 

1 

- 

- 

2 

I 

2 

2 

4 

2 

11 

6 

2 

1 

2 

3 

14 

3 

_ 

. 

1 

2 

6 

4 

2 

2 

2 

4 

14 

_ 

2 

2 

1 

- 

5 

2 

,- 

1 

1 

_ 

4 

- 

1 

_ 

- 

1 

2 

_ 

_ 

_ 

- 

_ 

0 

27 

22 

18 

27 

17 

111 

Test  IV 

} 

2 

3 

4 

5 

Total 

S 

S 

S 

S 

S 

20 

28 

28 

10 

14 

100 

1 

- 

1 

2 

I 

2 

4 

2 

j 

10 

2 

2 

5 

_ 

- 

9 

2 

_ 

1 

_ 

3 

_ 

- 

1 

1 

2 

4 

1 

_ 

2 

1 

_ 

4 

1 

1 

1 

1 

- 

4 

1 

1 

_ 

- 

_ 

2 

_ 

_ 

_ 

1 

3 

4 

27 

36 

41 

17 

21 

142 

[569] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


TABU   III 
Hooking  rates  according  to  position  on  line 


TABLE  IV 
Catch  in  Tests  V  to  X 


Test 
num- 
ber 

Hook 

I 

11 

HI 

IV 

Mean 
for 
Tests 
II  to  IV 

Test 
Number 

Total 
Number  Number 
of           of 
basket       catch 

num- 

V 

320 

142 

ber 

Cn 

/  n 

Cn       X 

n 

Cn       > 

n      A  Cn         n       X  n 

VI 

350 

147 

— 

— 

— 

- 

—  



— 

VII 

356 

150 

1 

5 

0-111 

24      0- 

226 

27      0 

•243     27 

0-190 

0-220 

VIII 

350 

183 

2 

6 

0-133 

21       0- 

198 

22      0 

•  198     36 

0  253 

0-219 

IX 

355 

196 

3 

19 

0-422 

25      0- 

236 

18      0 

•163    41 

0-288 

0-229 

X 

310 

85 

4 

12 

0-267 

24      0- 

226 

27      0 

•243     17 

0-120 

0-196 

Total 

2,021 

903 

5 
C 

3 
45 

0-067 

12      0- 
106 

113 

17      0 
111 

•153    21 

0-148 
142 

0-138 

Mean  rate 

0 

447 

Artificial  bait  Saury-bait 

Number  Number  Number  Number 

of     of  of     of 

basket   catch  basket   catch 


4 
0 
3 
0 
0 
1 


30     8 
0-267 


315 
345 
351 
345 
330 
305 


138 
147 
147 
183 
196 
84 


1,991     895 
0-449 


ficial  bait  on  the  5th  hook  of  every  basket  in  Test  I  is 
inferior  to  the  bait  used  on  the  other  hooks.  However, 
this  may  have  been  caused  by  the  relative  position  of 
the  hook  on  the  line,  a  factor  that  has  been  known  to 
affect  the  efficiency  of  longline  operation.  In  Tests  II 
to  IV  in  which  all  the  hooks  were  provided  with  true 
baits,  the  5th  hook  also  shows  a  lower  efficiency. 

Disregarding  species  of  the  catches,  one  may  indicate 
the  rate  of  fishing  efficiency,  X  n,  of  nth  hook  by 


AW  -        Ol/C 


where 


C 
Cn 


total  catch  of  one  longline  operation 
the  number  of  catch  by  nth  hook. 


Table  III,  showing  A  n  compared  for  each  hook  is 
suggestive  of  this.  In  Tests  II  to  IV  where  only  frozen 
saury  were  baited,  the  3rd  hook  was  the  best,  while 
the  5th  hook  had  the  lowest  average  efficiency.  Because 
of  the  limited  number  of  the  experiments,  and  the 
difference  in  the  number  of  baskets  the  experiment  is 
of  course  inconclusive. 

The  efficiency  of  the  artificial  bait  on  the  5th  hook 
can  be  compared  to  that  of  the  true  squid-bait  on  the 
1  st  hook,  by  comparing  the  ratio  of  efficiency  between 
these  hooks  in  tests  I  to  IV. 

The  calculated  efficiency  of  the  artificial  bait  on  the 
5th  hook  corresponding  to  its  efficiency  if  used  on  the 
1st  hook  may  be  estimated  where: 


A  la 


>5a 


A/5 


x  5  s 


where:   *A  la:  calculated  efficiency  of  artificial  bait  if  on  1st  hook 
X  5a:  catching  rate  of  artificial  bait  on  the  5th  hook 
X  ls\  catching  rate  of  saury  bait  on  the  1st  hook 
x  5s:  catching  rate  of  saury  bait  on  the  5th  hook 

Inserting  the   numerical  values  of  Table  III  into  the 
above  equation  gives: 

0  220 

X  la  -  0-067  x 0-107 

0-138 

This  brings  the  efficiency  of  the  lure  near  to  that  of 
the  true  squid-bait  which  has  0  - 1 1 1 . 

Although  unreliable,  as  the  result  of  only  one  test 
it  may  indicate  that  the  efficiency  of  the  artificial  bait 
is  better  than  its  catch  rate  shows. 

When  the  artificial  bait  was  used  in  the  last  five  baskets 
in  tests  V  to  X,  the  results  as  shown  in  Table  IV  indicate 
that  the  fishing  rate  of  the  artificial  bait  per  basket  is 


again  inferior  to,  or  about  60  per  cent,  of  the  mean  rate. 
In  test  I,  the  artificial  bait  had  a  rate  per  basket  ranging 
45  per  cent,  to  60  per  cent,  of  the  saury  bait.  (Fishing 
rate  per  basket  is  proportional  to  the  catch  per  hook 
in  the  same  haul.) 

Although  the  efficiency  of  the  artificial  bait  appears 
to  be  merely  about  half  as  good  as  the  saury-bait,  one 
should  not  overlook  the  influence  which  the  position 
of  the  hook  or  basket  exerts  on  the  catch,  as  pointed  out 
before. 

If  the  catches  of  Tests  V  to  X  are  arranged,  in  accor- 
dance with  the  position  of  the  baskets,  into  three  sections 
comprising  the  first  five  baskets,  the  intermediate  baskets, 
both  being  baited  with  saury;  and  the  last  five  baskets 
with  the  artificial  bait  (Table  V),  then  the  catch  per 
basket  in  the  first  section  is  no  more  than  50  per  cent, 
of  the  last  section  and  still  lower  than  the  intermediate 
section.  Thus,  so  far  as  the  results  of  the  present  experi- 
ments are  concerned,  it  may  be  reasonable  to  conclude 
that  the  ratio  of  catch  per  basket  between  the  artificial 
bait  and  the  saury  bait  may  be  higher  than  60  per  cent, 
as  has  been  computed  from  the  values  in  Table  IV. 

REFERENCES 

1  Suzuki,    S.     One    hundred    types    of  commercial    anglings. 
Ohashi  and  Co.,  Tokyo,  1931. 

2  Koyama,  T.   Study  on  bait  for  tuna  longline.     I—  An  artificial 
bait  of  latex  sponge  shaped  like  a  squid.    Bull.    Tokat  Reg.  Fish. 
Lab.   No.    15.     1957. 

TABLE  V 
Catch  rates  according  to  line  section 


First  25  hooks 

Middle  section 

Last  25  hooks 

with  saury-bait 

with  saury-bait 

with  lure  bait 

Test 

Number 

Number 

Number 

Number 

Number 

Number 

Number 

of 

of 

of 

of 

of 

of 

basket 

catch 

basket 

catch 

basket 

catch 

V 

5 

1 

310 

137 

5 

4 

VI 

5 

1 

340 

146 

5 

0 

VII 

5 

0 

346 

147 

5 

3 

V1I1 

5 

1 

340 

182 

5 

0 

IX 

5 

0 

325 

196 

5 

0 

X 

5 

1 

300 

83 

5 

1 

Total 

30 

4 

1,961 

891 

30 

8 

Mean  rate 

0-133 

0 

454 

0-267 

[570 


DISCUSSION  ON  FISH  ATTRACTION 


Dr.  M.  Ruivo  (Portugal)  Rapporteur:  The  ultimate  aim 
of  location,  detection  and  attraction  of  fish  is  the  establish- 
ment  of  a  connection  between  man  and  the  fish  through  a 
signal  or  stimulus.  So  far  as  fish  attraction  is  concerned,  man 
would  set  off  a  particular  signal  or  stimulus  with  the  intention 
to  produce  a  directed  response  of  the  fish. 

The  extent  and  the  characteristics  of  the  response  would  be 
a  function  of  the  quantitative  and  qualitative  properties  of 
the  stimulus  used,  of  the  ambient  conditions,  and  of  the 
specific  conditions  of  the  fish. 

The  stimuli  may  be  luminous  (visual),  sonorous  (audial), 
mechanical  or  chemical.  The  response  of  the  fish  to  electrical 
stimuli  will  be  discussed  at  another  session  of  this  Congress. 
These  stimuli  act  separately  or  combine  in  a  more  or  less 
complex  manner.  External  factors  of  the  surrounding 
medium  may  cause  qualitative  and/or  quantitative  changes 
of  a  stimulus  during  its  course  from  origin  to  the  sensory 
organs  of  the  fish.  Factors  specific  to  the  individuals  must 
also  be  considered;  the  anatomy  and  physiology  of  the 
sensory  organs,  the  level  of  reaction,  the  type  of  reaction 
and  its  degree  of  intensity,  vary  according  to  the  physiological 
state  of  the  fish  and  its  adaptation  to  the  ecological  conditions 
of  the  medium  and/or  the  stimulus  itself.  Furthermore,  the 
effect  of  group  reaction  (herd  instinct)  must  be  studied. 

In  applying  methods  previously  used  only  in  laboratory 
experiments  to  the  natural  field,  practicability  and  cost  must 
be  in  reasonable  relation  to  expected  yield. 

The  adaptation  of  the  various  attraction  methods  and 
equipment  to  the  various  types  of  fisheries  is  done  according 
to  the  type  of  fishing  gear  (fixed  or  mobile,  nets,  hooks, 
suction  pumps,  etc.)  and  its  particular  characteristics  (mat- 
erial, colour,  shape,  chemicals  used  for  the  preservation  of 
the  nets)  which  may  influence  the  behaviour  of  the  fish. 

In  commercial  practice  the  attraction  of  fish  may  be  based 
on  either  empirical  factors  or  by  applying  a  strictly  scientific 
method.  When  measured  in  terms  of  time  and  yield,  the 
results  obtained  by  these  two  approaches  arc  very  different. 
Physical  and  chemical  stimuli  (light,  bait,  lures,  sound)  to 
attract  or  direct  the  fish  have  been  used  in  a  more  or  less 
empirical  manner  throughout  the  ages. 

The  evolution  of  light-fishing  over  the  centuries  is  parti- 
cularly demonstrative.  The  light  source  was  at  first  obtained 
by  burning  coal  or  torches.  Then  oil  and  acetylene,  and 
more  recently  gas  and  electric  lamps  came  into  use.  The  fact 
that  electric  lamps  can  be  immersed  has  rendered  "light" 
fishing  far  more  efficient  and  has  permitted  its  use  in  regions 
where  the  state  of  the  sea  and  the  turbidity  of  the  waters 
had  hitherto  been  limiting  factors. 

This  evolution  has,  however,  been  very  slow  and  is  far  from 
having  reached  its  end.  That  these  various  lighting  systems 
still  coexist  in  various  parts  of  the  world,  may  be  explained 
by  the  lack  of  technical  and  scientific  bases,  scarcity  of 
information,  and  the  geographical  isolation  of  certain 
communities  of  fishermen. 


These  factors  arc  stressed  by  Fukuhara,  Kawakami  and 
Mihara,  who  discuss  light-fishing,  its  growing  importance  in 
certain  regions  (i.e.  Japan  and  the  Philippines),  and  suggest  its 
possibilities  in  others. 

Experiments  made  with  a  directed  light  beam  have  led  to 
the  development  of  a  new  method  by  which  the  fish  is  attracted 
and  guided  into  a  fixed  net  by  the  successive  lighting  of  a 
scries  of  associated  lamps  (Sasaki).  This  doubles  the  catching 
capacity  of  the  fixed  net. 

Kawamoto  and  Tamura  demonstrate  how  basic  research 
may  furnish  scientific  information  of  fundamental  importance 
for  the  development  of  new  fishing  methods  and  the  inter- 
pretation of  results  obtained  by  old  ones.  Experiments  with 
various  species  of  fish  on  the  significance  of  the  wave-lengths 
of  light  have  shown  that  blue  and  green  lights  have  the 
strongest  effect.  The  daily  rhythm  in  phototaxis  and  the 
influence  of  moonlight  were  also  investigated.  The  reactions 
of  fish  were  bound  to  be  far  more  complex,  under  natural 
conditions  due  to  the  influence  of  external  factors  (tempera- 
ture, turbidity,  currents),  making  the  interpretation  of  test 
results  difficult. 

Further  studies  on  the  significance  of  the  visual  sense  for 
the  behaviour  and  ecology  of  fish  will  greatly  assist  gear 
technology,  particularly  in  regard  to  the  reaction  of  fish  to 
nets,  hooks,  bait,  etc.  and  subsequently  in  the  problem  of 
gear  selectivity. 

When  catching  fish  by  trolling,  the  lure  must  be  seen. 
Nets,  however,  should  preferably  not  be  clearly  visible. 
Tamura  has  carried  out  experiments  to  determine  the  visual 
reaction  of  fish  to  cotton  and  nylon  lines.  He,  furthermore, 
found  that  the  reaction  to  bait  is  the  result  of  a  complex 
process  in  which  both  the  visual  stimuli  (localization)  and 
the  water  motion  produced  by  the  bait  or  prey  (capture) 
arc  important. 

Useful  data  for  interpreting  the  bathymetrical  distribution 
of  fish,  and  consequently  for  the  design  of  gear  and  the 
planning  of  fishing  operations,  may  be  obtained  by  investi- 
gating the  anatomy  of  the  optical  system  (optical  axis  and 
angle  of  vision),  the  ideal  lighting  conditions  and  the  photo- 
taxis  rhythm,  which  are  closely  connected  with  the  ecology  of 
the  species  under  observation. 

Chemical  stimuli  play  an  important  part  in  the  use  of  bait 
and  lures  (complex  forms  of  chemical,  visual  and  mechanical 
stimuli). 

Certain  fisheries  (for  instance,  tuna  and  cod)  are  to  a 
certain  extent  dependent  on  the  possibility  of  obtaining  live 
or  dead  bait  in  certain  quantities  and  quality  in  a  determined 
period  of  time.  Such  bait  is  often  available  only  in  waters 
far  from  the  fishing  area  and  a  seasonal  species  sometimes 
may  have  to  be  preserved  (frozen,  salted,  etc.  or,  in  the  case 
of  live  bait,  kept  alive  in  tanks).  Capture  may  affect  the 
problem  of  the  conservation  of  natural  resources.  The  eventual 
value  of  the  bait  fish  for  human  consumption  must  also  be 
considered. 


[571  ) 


MODERN    FISHING    GEAR    OF    THE    WORLD 


These  factors  imply  a  number  of  limitations  and  contin- 
gencies to  the  main  fisheries.  The  development  of  artificial 
lures  or  other  stimuli  capable  of  attracting  fish  could  radically 
change  the  economy  of  these  fisheries. 

The  problem  is,  however,  extremely  complex,  and  the 
research  necessary  covers  a  very  wide  field. 

Tester  describes  a  very  interesting  series  of  experiments 
made  with  a  view  to  improving  the  fishing  techniques  for 
tuna,  and  particularly  with  the  aim  of  discovering  a  substitute 
for  bait  fish.  Experiments  were  carried  out  in  fish  ponds  and 
at  sea,  to  determine  the  reactions  of  different  species  of  tuna 
to  chemical,  visual  (artificial  light,  bait  or  moving  lures), 
and  audial  stimuli.  It  was  found  that  in  tuna  the  sense  of 
smell  is  less  important  than  vision,  although  a  positive 
response  to  aqueous  or  alcohol  extracts  of  tuna  meat  was 
observed.  The  response  to  audial  stimuli  was  weak. 

Koyama  refers  to  tests  with  artificial  bait  made  of  latex 
sponge  rubber,  of  squid-like  shape  and  smell,  which  could 
eventually  reduce  the  cost  and  work  necessary  for  the  preserva- 
tion and  transportation  of  bait  fish  needed  for  longlining  in 
the  tropical  zone.  These  lures,  although  found  to  be  less 
attractive  than  natural  bait,  could  very  probably  be  improved. 

Repellent  substances  can  also  be  profitably  utilized  in 
fishing.  Recent  studies  in  Canada  have  revealed  the  repellent 
action  of  mammal  skin  extracts  on  salmon.  These  extracts 
are  extremely  active  even  when  much  diluted,  and  might  be 
used  to  deviate  the  salmon  from  their  course  and  orienting 
them  in  a  determined  direction. 

Another  barely  investigated  sector  is  the  use  of  auditory 
stimuli  to  attract  fish.  In  recent  years,  new  equipment 
(hydrophones,  tape-recorders,  etc.)  has  enabled  some  progress 
to  be  made  and  has  brought  many  new  facts  to  light.  It  is 
possible  that  one  day  cither  artificial  noise  or  the  tape- 
recorded  sounds  of  sea  animals  may  be  used  as  attraction 
signals  along  with  a  particular  fishing  method. 

A  better  knowledge  of  the  behaviour  and  the  reaction  of 
fish  to  the  various  types  of  stimuli,  as  well  as  of  the  processes 
which  are  at  the  basis  of  directed  movement  (be  it  of  attraction 
or  repulsion)  could  increase  the  efficiency  of  fishing  operations 
— either  by  improving  the  catching  possibilities  of  existing 
gear,  or  developing  new  methods. 

Dr.  A.  W.  H.  Needier  (Chairman):  Developing  the  means  of 
locating  fish  by  understanding  their  association  with  con- 
ditions and  understanding  more  of  their  reactions  so  as  to 
attract  them,  is  a  very  interesting  study.  On  the  East  Coast 
of  Canada  there  is  a  small  fishery  for  herring  with  lights. 
The  fishermen  have  found  that  the  yellow  flickering  light 
produced  by  kerosene  soaked  waste,  burning  in  a  basket  on 
the  bow  of  the  boat,  attracted  the  herring  much  better  than 
any  of  the  much  stronger  artificial  lights  which  they  tried  to 
use.  This  sort  of  thing  shows  how  little  we  know  about  the 
behaviour  of  fish. 

Dr.  F.  J.  Verheyen  (Netherlands):  I  came  in  contact  with 
the  light  fishing  technique  in  the  Mediterranean  in  1954  in 
night  fishing  for  sardines,  anchovies,  etc.  I  soon  became 
convinced  that  it  was  erroneous  to  believe  that  the  concentra- 
tion of  fish  around  the  lamp  depended  on  light  intensity.  I 
have  approached  the  problem  from  the  point  of  view  of  a 
comparative  biologist  and  I  have  made  an  extensive  review  of 
data  available  on  fishing  with  lights,  on  attracting  insects  with 
light  and  on  birds  flying  towards  lighthouses.  This  review 


suggests  that  widely  separated  species  of  animals  are  attracted 
to  artificial  light  under  essentially  the  same  conditions,  and  I 
have  found  an  explanation  of  the  mechanisms  involved  in 
this  photic  disorientation.  Certain  features  of  man-made 
illumination  are  abnormal  as  compared  with  natural  illumina- 
tion and  provoke  the  disorientated  concentration  around  a 
lamp.  The  mechanisms  involved  are  too  complicated  to  be 
dealt  with  in  detail  here  but  I  have  given  a  short  outline  of 
them  in  my  paper.  In  this  connection  I  think  it  very  important 
to  point  to  some  misleading  experiments  in  this  field. 

A  number  of  Japanese  workers  have  tried  to  analyse  the 
attraction  mechanisms  by  experiments  in  an  aquarium,  but 
it  can  easily  be  demonstrated  that  the  gathering  of  fish  by  a 
lamp  in  an  aquarium  is  not  comparable  with  the  concentration 
of  fish  around  a  lamp  at  sea.  First,  very  young  fish  were  used 
in  the  experiments,  and  it  is  generally  agreed  that  young  fish 
have  a  higher  light  intensity  preference  than  have  adult  fish 
of  the  same  species.  Secondly,  there  are  variations  in  the 
photo  preference  of  fish  and  many  other  animals.  Some  fish 
that  are  active  in  the  day  time  then  have  a  relatively  high 
intensity  preference  but  much  less  at  night.  Kawamoto  and 
Konishi  in  1955  observed  this  phenomenon  in  some  of  their 
young  experimental  fish.  At  night  some  of  the  fish  even  fell 
asleep  in  the  darkest  area  of  the  tank.  But  it  is  just  during 
this  period,  the  night,  that  fish  must  be  gathered  with  the  aid 
of  light  and,  obviously,  the  incompatibility  of  these  facts 
has  escaped  unnoticed.  A  number  of  workers  have  carried 
out  experiments  with  various  lamps,  such  as  searchlights  and 
water  lamps  to  get  some  insight  into  the  vertical  migration  of 
such  fish  as  the  herring,  the  pilchard  and  other  species.  This 
is  the  equivalent  of  looking  at  nocturnal  insects  flying  towards 
a  street  lamp  or  observing  birds  dashing  themselves  against 
the  lantern  of  the  lighthouse.  When  a  searchlight  is  shone 
into  the  sea  the  behaviour  of  herring  and  pilchards,  as 
indicated  by  the  echo  sounder,  is  a  result  of  an  unknown  and 
very  complicated  interference  between  their  normal  preference 
for  low  light  intensities  and  the  disorientated  attraction  towards 
the  light  source.  The  movements  resulting  from  these  two 
behaviour  patterns  are  dependent  upon  a  number  of  factors, 
the  fluctuating  character  of  which  accounts  for  the  contra- 
dictory results  of  this  kind  of  experiment.  I  should  like  to 
emphasize  that  such  rather  primitive  experiments  are  unlikely 
to  yield  any  consistent  results.  I  should  also  like  to  make  a 
comment  upon  the  distinction  between  detection  and  attrac- 
tion in  this  respect.  Detection  is  perceiving  the  fish  at  a  certain 
location,  while  attraction  is  directing  the  fish  towards  a 
preferred  fishing  place  by  means  of  certain  stimuli.  Now  the 
question  arises,  what  is  the  action  radius  of  a  lamp?  In 
attracting  insects  by  light,  it  is  known  that  the  action  radius 
of  very  strong  lamps  is  small,  perhaps  some  hundred  metres. 
As  the  absorption  of  light  in  water  is  much  greater  than  in 
the  atmosphere,  the  action  radius  of  a  lamp  in  water  will  be 
less.  Fish  only  gather  around  the  lamp  when  they  happen  to 
enter  its  immediate  environment,  thus  a  lamp  might  be 
regarded  as  a  simple  instrument  to  detect  a  school  of  fish 
which  passes  by  accident.  As  such,  it  is  much  inferior  to  an 
echo  sounder.  The  lamp  has  the  advantage  that,  under  certain 
conditions,  the  fish  remain  near  it.  This  light  fishing  technique 
is,  then,  passive  and  based  on  the  random  movements  of  the 
fish,  but  the  technique  can  be  activated  as  described  by  Sasaki. 
Fish  concentrated  around  the  lamp  follow  the  lamp  when  it 
is  moved.  It  would  be  useful  to  know  the  maximum  rate  at 
which  the  fish  can  be  moved  in  this  way  towards  a  favourable 


[572] 


DISCUSSION  — FISH    ATTRACTION 


catching  place.  Perhaps  use  can  be  made  of  floating  lamps 
moved  along  the  water  surface,  thus  increasing  the  chance 
of  meeting  schools  of  fish,  especially  if  a  number  of  lamps 
could  be  moved  from  several  directions  towards  a  catching 
place. 

Mr.  Kristjonsson  (FAO):  Some  Italian  purse  seine 
fishermen  have  in  recent  years  replaced  their  traditional 
kerosene  or  gas  pressure  lamps  with  underwater  electric 
lamps  such  as  the  one  shown  to  the  left  in  fig.  1 .  Normally 
two  500  W  lamps  are  submerged  approximately  3  to  4  ft.  below 
the  surface  under  a  small  boat  which  carries  a  gasoline  or 
dicsel  powered  electric  generator  set  of  2  to  3  kw.  Occasion- 
ally an  additional  light,  as  the  lamp  held  by  the  man  in  the 
figure,  is  also  used  above  the  water.  Such  above-water  lamps 
give  a  wide  horizontal  spread  of  light,  but  naturally  they  arc 
mainly  useful  when  the  sea  surface  is  unruffled.  Generators 
of  32  V  are  normally  used  with  24  V  lamps.  Thus  the 
light  intensity  can  be  varied  from  yellowish  light  over  to 
bright  white  despite  the  voltage  drop  in  the  rubber  cables. 

Commonly  the  intensity  of  the  light  is  kept  constant 
although  theoretically  it  would  seem  advantageous  to  use  a 
light  of  maximum  intensity  and  brightness  during  the  early 
stages,  in  order  to  obtain  maximum  range  for  attracting 
fish  into  the  area  under  the  lamps,  and  then  gradually  dim 
the  light  somewhat  to  compact  the  school  before  setting  the 
net  around  it.  Of  course,  any  change  in  light  intensity  will 
have  to  be  extremely  gradual  so  as  not  to  scare  the  fish  which 
often  seem  to  be  extremely  sensitive. 

Before  sunset  each  purse  seiner  usually  tows  out  to  the 
fishing  grounds  two  light  boats  which  are  anchored  at  a 
considerable  distance  from  each  other.  One  man  is  left  in 
each  boat  to  tend  the  lights  and  to  look  for  signs  of  fish 
gathering  below,  such  as  air  bubbles  rising  to  the  surface 
when  the  fish  jettison  air  from  their  swimming  bladders  in 
order  to  maintain  neutral  buoyancy  when  swimming  up  to 
shallower  depths.  If  no  signs  of  fish  are  noticed  after  two  or 


RUBBER   CABLE 


Fig. 


Fig.  /.   Electric  underwater  lamp  (left)  and  above-water  lamp 
(centre)  used  for  attracting  sardine  and  anchovy. 


three  hours,  the  light  boats 
may  be  moved  to  another 
place. 

From  lime  to  time  the 
purse  seine  boat  communi- 
cates with  the  light  boats  and 
when  it  is  decided  to  make  a 
set,  the  man  in  the  light  boat 
ties  its  anchor  to  a  float  with  a 
small  lantern  and  drifts  or 
rows  very  slowly  a  short 
distance  away,  in  order  to 
enable  the  purse  seiner  to  set 
the  net  around  the  school 
without  interference  from 
the  anchor  rope.  As  soon  as 
the  net  has  been  shot  and 
pursing  begins  the  light  boat 
is  rowed  slowly  towards  or 
across  the  centrepiece  of  the 
cork  line  in  order  to  induce 
the  fish  to  swim  away  from 
the  opening  of  the  net  under 
the  purse  seiner. 

The  light  boat  remains  just 
outside  the  net  until  pursing 
is  completed,  when  it  cither 
returns  to  the  anchor  buoy  or 
else  transfers  to  another  likelier  fishing  location. 

Although  some  underwater  lamps  are  available  as  a  stand- 
ard commercial  commodity,  the  Italian  fishermen  in  Fiumicino 
(a  small  fishing  place  at  the  mouth  of  the  Tiber)  have  their 
lamps  made  in  a  local  workshop.  Such  lamps  can  really  be 
made  by  the  fishermen  themselves  (see  fig.  2). 

The  first  step  is  to  get  hold  of  the  electric  bulb;  then  to 
find  a  strong  rubber  hose,  such  as  an  automobile  radiator  hose, 
which  fits  closely  over  the  neck  of  the  bulb.  An  ordinary 
socket  can  be  used  with  the  bulb  as  it  is  dry  and  well  pro- 
tected by  the  rubber  hose  which  fits  at  the  upper  end  closely 
over  a  brass  cylinder  through  which  is  drilled  a  central  hole 
for  the  rubber  covered  cable  of  not  less  than  3  mnr'  conductor 
area. 

To  prevent  water  from  entering  along  the  cable  a  conical 
rubber  packing  can  be  used,  pressed  down  by  a  screw  plug. 
Finally,  it  is  good  practice  to  apply  several  coats  of  shellac 
where  the  rubber  hose  meets  the  bulb  and  the  brass  cylinder. 

A  reflector  above  the  bulb  will  improve  the  illuminating 
efficiency  but  should  be  small  and  a  heavy  weight  must  be 
attached  below  the  bulb— like  the  thick  iron  ring  shown  in 
fig.  1 — in  order  to  keep  the  lamp  vertical  despite  the  up  and 
down  movement  of  the  boat. 

As  compared  with  the  surface  lights,  the  main  advantages 
of  underwater  lamps  are:  better  utilization  of  the  light  as 
there  is  no  loss  due  to  reflection  from  the  surface  of  the  sea 
and  a  less  powerful  light  is  needed  (caution  :  in  shallow 
water  a  too  strong  light  reflected  from  a  light-coloured 
bottom  may  scare  the  fish);  the  underwater  light  is  steady 
while  the  above-the-surface  lamp  will  give  a  flickering  and 
uneven  light,  especially  when  the  surface  waters  arc  ruffled. 
With  underwater  lamps  the  fishermen  are  able  to  attract 
fish  effectively  in  rougher  weather,  and  less  time  is  lost 
during  the  full  moon  period.  Better  penetration  is  achieved 


[573  1 


MODERN     FISHING    GEAR    OF    THE    WORLD 


in  turbid  water  and  fishing  can  be  resumed  earlier  after  rains 
and  storms. 

I.  V.  Nikonorov*:  Pump  fishing,  which  is  one  of  the 
new  modern  methods  of  fishing,  has  been  continuously  used 
since  1954  in  the  Caspian  Sea  in  the  Soviet  Union.  This 
method  excludes  the  application  of  traditional  conventional 
net  materials  and  it  has  proved  to  be  very  efficient  and  labour 
saving.  The  film  which  was  shown  here  was  shot  in  1954 
and  illustrates  the  contents  of  the  paper  on  this  fishing 
method  better  than  I  could  with  further  words. 

Some  developments  have  been  made  since  1954  and  the 
technique  has  been  improved.  Forty  ships  are  now  (Oct.  *57) 

*  Spokesman  for  U.S.S.R.  participants. 


using  the  light-pumping  method  in  the  Caspian  Sea.  The 
maximum  rate  of  catch  is  about  30  tons  per  24  hours,  and 
this  method  is  gradually  replacing  the  conical  lift  nets  with 
lights,  being  about  50  per  cent,  more  productive.  When  the 
fish  comes  out  of  the  pump  on  to  the  deck  it  is  alive,  and  in  no 
way  damaged. 

Mr.  Traung  (FAO):  I  would  like  to  know  if  these  pumps 
are  in  the  market  for  commercial  use  and  if  they  are  sold 
commercially,  can  Mr.  Juvoy  let  us  know  the  price  and  where 
we  can  buy  them? 

Mr.  Juvoy  (U.S.S.R.):  The  whole  installation  costs  about 
10,000  roubles  and  could  be  bought  in  Astrakhan,  the  main 
port  on  the  Caspian  Sea  coast. 


ump 


+  3  o  o  y/) 


i 


Fig.  1.     Echograms  of  sardine  attracted  by  underwater  electric  lamps  off'  Tunis.     When  the  lights  were  put  on  at  00.45 

hrs.  hardly  any  fish  were  indicated  on  the  echo  sounder  (Japan  Radio  Co.,  "Midgef\  portable  unit).     Towards  01 .00 

his.  some  .scattered  young  fish  have  gathered  near  the  surface  and  a  school  is  formin/t  at  ab^uf  35  m.  depth. 


Fig.  2. 
depth. 


$ 
<> 


A  i  03.00  hrs.  the  echo  sounder  (at  amplitude  U;~  weakest  strength)  showed  more  fish  had  collected  at  30-35  m. 
Without  an  echo  sounder  this  station  might  have  been  abandoned  at  this  stage  as  the  fish  were  so  slow  to 
rise,  probably  due  to  different  temperature  of  the  surface  waters. 


Fig.  3.     After  about  4  hrs.  illumination  the  fish  had  finally  risen  and  formed  a  dense  school  (about  6  tons)  around 
the  light.     As  soon  as  the  light  was  extinguished  (at  05.00  hrs.)  the  fish  dispersed. 

574 


Section  13'     Electrical  Fishing. 


THE  EFFECT  OF  PULSATING  ELECTRIC  CURRENT  ON  FISH 

by 
E.   HALSBAND 

Institut  fiir  Kiisten-und   Binnenfischerei,   Hamburg,  Germany 


Abstract 

In  this  paper  the  author  discusses  the  different  forms  of  electrical  impulses  and  their  effect  on  the  behaviour  of  certain  fishes,  and 
the  results  of  experiments  show  that  the  type  of  impulse  that  has  a  steep  initial  rise  followed  by  a  gradual  decline  of  the  current  is  the  most 
suitable  one  for  fishing  purposes. 

Furthermore,  the  number  of  impulses  per  unit  of  time  is  important  and  it  was  found  that  each  species  offish  had  a  specific  narcotizing 
impulse  limit  at  which,  with  a  minimum  of  electrical  energy,  narcosis  begins.  The  connection  between  the  metabolism  and  current  density 
was  also  investigated. 


Rtsumc 


I/effet  du  courant  elcctrique  pulse  chez  le  poisson 


Dans  cct  article  I'auteur  examine  les  differentes  formes  d'impulsions  electriques  ct  leur  efTet  sur  le  comporlcmeiU  de  certains  poissons. 
I.es  res ul tats  ties  experiences  montrent  que  le  type  d'impulsion  a  elevation  initiate  abrupte  du  courant  suivie  d'une  diminution  graduelle  est 
celle  qui  convient  le  mieux  pour  la  pechc. 

En  outre,  le  nombre  d'impulsions  par  unit6  de  temps  est  important  et  on  a  trouve  que  pour  chaquc  esnece  dc  poisson  il  existe  une 
'Mimitc  d'impulsion  narcosante"  a  1  ague  lie  la  narcosc  commence  pour  une  encrgie  electrique  minimum.  On  a  aussi  fait  des  recherches  sur  la 
relation  entre  Ic  metabolisme  et  la  densite  du  courant. 


El  efccto  de  la  corriente  electrica  pulsatoria  sobre  los  peces 
Extracto 

En  cslc  trabajo  cl  autor  analiza  las  di versus  formas  de  impulsos  elcctricos  y  su  efecto  sobre  la  mancra  en  que  rcaccionan  algunos 
peccs.  Los  rcsultados  de  los  experiments  hechos  demuestran  que  el  tipo  de  impulso  que  aumcnta  en  forma  pronunciada  seguido  por  una 
disminuci6n  gradual  de  la  correinte,  cs  mas  adecuado  para  la  pesca. 

Ademas  juega  un  panel  dc  importancia  cl  numero  dc  impulsos  utilizados  por  unidad  dc  tiempo,  enconirandose  que  cada  especie 
requiere  cicrto  "mdximo  y  minimo  de  impulsos  narcotizanles"  para  la  iniciacidn  de  la  narcosis  con  un  minimo  de  energia  electrica.  Tambicn 
se  investig6  la  rclaci6n  que  existe  entre  el  metabolismo  y  la  intensidad  de  ia  corriente  eleclrica. 


GENERAL 

FISH   swim  towards  the  anode  when   under  direct 
or  pulsating  current  of  a  certain  type,  whereas 
under  alternating  current   they  show  a  so-called 
oscillotaxis  and  take  up  a  transversal  position  to  the 
direction  of  the  current,  in  order  to  tap  off  as  little 
voltage  as  possible.  As  the  cause  of  these  phenomena  had 
up  to  now  not  been  completely  explained,  the  reactions 
of  the  isolated  nerve  were  investigated  to  understand  the 
importance  of  the  principle  of  polar  stimulation  in  the 
behaviour  of  fish  in  electric  fields. 

Stimulation  of  the  nerves  occurs  only  in  response  to 
changes  in  current  and  the  threshold  value  (the  minimum 
voltage  which  produces  a  just  visible  reaction  of  the 
nerve)  is  smaller  when  closing  the  circuit  than  when 
opening  it.  Moreover,  stimulation  occurs  within  the 
range  of  the  cathode  when  closing,  and  within  the  range 
of  the  anode  when  opening  the  circuit.  This  principle 
of  polar  stimulation  causes  the  so-called  Pfluger-effect  of 
vibration.  If  the  nerve  in  a  nerve  muscle  preparation  is 
stimulated  by  placing  the  anode  on  the  nerve  near  the 
muscle  and  the  cathode  at  the  end  of  the  nerve,  there  will 


be  no  muscle  reaction  because  the  anode  ha*  a  paralysing 
effect  (anelectrotonus).  If  the  position  of  the  electrodes  is 
exchanged,  a  vibration  of  the  muscle  will  be  stimulated. 
These  phenomena  observed  in  the  isolated  nerve  and 
muscle  are  parallel  to  the  reactions  of  the  whole  animal. 
If  a  fish  is  brought  into  an  electric  field,  it  will  show  a 
clear  reaction  to  the  anode  when  direct  current  is  applied, 
the  circuit  closed  and  a  certain  threshold  value  reached. 
This  reaction  can  be  explained  by  means  of  the  principle 
of  the  polar  stimulation:  When  the  circuit  is  closed,  a 
stimulation  occurs  under  the  cathode,  which  the  fish 
tries  to  escape  by  swimming  towards  the  anode.  If,  at 
the  moment  of  this  stimulation,  the  fish  is  headed  to- 
wards the  anode,  the  tip  of  its  tail  will  be  strongly 
excited  and  the  jerk  of  the  tail  will  assist  this  movement 
towards  the  anode;  if  the  fish  is  headed  towards  the 
cathode,  the  current  will  produce  only  a  slight  vibration 
of  the  tail,  the  head  will  turn  towards  the  anode,  and 
the  stimulative  effect  of  the  cathode,  gradually  expanding 
over  the  whole  muscular  system  will  cause  a  turning 
towards  the  anode  (fig.  1). 


[575 


MODERN     FISHING    GEAR     OF    THE    WORLD 


nerve 


muscle 


F\K.  I.     The  fish  in  the  Pfliiger  Test. 

THE    EFFECT   OF   VARIATION    IN   THE   PULSE 
TYPE 

The  author  has  investigated  the  reactions  of  fish  in 
relation  to  variations  in  the  pulse  type.  He  found  that 
when  a  current  of  a  very  slow  increase  passes  through  the 
body  of  a  fish,  there  may  be  no  reaction  at  all.  Only  a 
fairly  strong  current  flow  causes  a  reaction.  This  corres- 
ponds to  the  observations  made  on  the  isolated  nerve 
where  a  slow  increase  or  decrease  of  current  has  no 
stimulative  effect,  as  the  nerve  becomes  accustomed  to 
the  current  passing  through  (accommodation  of  the 
nerve).  Not  only  the  change  of  current  but  also  the  speed 
of  this  change  is  of  importance  for  an  effective  stimulation. 
The  more  gradual  the  increase  the  higher  the  threshold 
value  for  producing  an  irritation,  until,  from  a  certain 
point  onwards,  there  will  be  no  more  stimulation. 

When  such  a  slow  rise  of  current  is  combined  with  a 
steep  decline,  a  distinct  opening  vibration  occurs  towards 
the  cathode.  This  is  parallel  to  the  stimulative  effect  ob- 
served in  the  nerve  under  the  anode  when  the  circuit  is 
opened,  provided  the  difference  of  excitation  is  great 
enough.  Whenever  closing  and  opening  reaction  (stim- 
ulation from  both  cathode  and  anode)  come  into  effect 
at  the  same  time,  as  is  the  case  of  a  quick  sequence  of 
impulses  of  this  type,  their  effects  might  counterbalance 
each  other  and  disturb  the  reaction  towards  the  anode. 

When  the  circuit  is  suddenly  closed,  i.e.  when  the  current 
rises  steeply,  the  fish  reacts  anodically.  When  immediate 
opening  follows  the  closing,  the  cathodic  opening 
vibration  does  not  occur,  because  the  difference  of 
excitation  is  too  small  to  effect  stimulation  from  the 
anode. 

When  the  circuit  is  closed  and  opened  only  after  some 
time  (maximum  2  minutes),  the  fish  shows  a  cathodic 
opening  vibration  from  a  certain  current-density  onward, 
which  corresponds  to  the  threshold  value  for  galvano- 
taxis.  This  may  be  called  accommodating  vibration.  In 
this  case  when  the  change  in  excitation  is  great  enough 
the  opening  of  the  circuit  has  a  specific  stimulative  effect 
on  the  fish. 

These  investigations  show  which  type  of  pulsating 
current  is  the  most  suitable  one  for  electrical  fishery. 

A  current  with  slow  increase  and  slow  or  steep  decline 
characteristics  is  unsuited  for  electrical  fishing,  as  it 
excludes  the  desired  movement  towards  the  anode.  With 
steep  increase  and  gradual  decline  of  the  current  the 
fish  reacts  anodically.  The  steeper  the  increase,  the  lower 


Fig.   2.     Shape  of  impulse  discharged  by  an  electrical  fish 
(Hlcctrophorus  elect  ricus). 

is  the  threshold  value  necessary  to  obtain  the  desired 
reaction.  Steeply  increasing  and  slowly  declining  currents, 
which  are  the  most  suitable  for  electrical  fishing,  are 
technically  produced  by  condenser  discharges.  Nature 
itself  produces  this  ideal  type  of  current  in  the  discharges 
of  the  electrical  fish.  This  type  of  current  (fig.  2)  has  a 
maximum  effect  under  most  favourable  conditions  as  far 
as  energy  is  concerned. 

The  impulse  rate  and  the  length  of  impulses  (impulse 
period)  are  also  of  considerable  importance  for  the 
effect  of  pulsating  current  on  the  fish3* 4. 

THE  EFFECT  OF  VARIATIONS  IN  THE  PULSE 
RATE 

Kreutzcr"  pointed  out  that  small  fish  require  higher 
impulse  rates  than  large  fish.  A  large  tuna,  for  instance, 
only  needs  7  to  10  impulses/sec.  The  explanation  is  as 
follows:  Each  individual  impulse  causes  a  muscular 
vibration  in  the  fish.  When  the  next  impulse  follows 
before  the  mechanical  movement  caused  by  the  previous 
impulse  has  been  finished,  the  muscle  is  constantly 
irritated,  which  causes  a  cramp.  The  movement  in  larger 
fish  is  slower  than  in  smaller  ones  as  larger  masses  have 
to  be  moved.  Therefore,  with  bigger  fish  already  a  lower 
impulse  rate  is  sufficient  to  cause  muscular  cramp. 
According  to  Kreutzer,  the  narcotizing  impulse  limit,  i.e. 
the  impulse  rate  which,  with  a  minimum  of  electric 


Narcotizing  impulse  limits  for  s 


TABLE   I 
»me  freshwater  fish  (according  to  Halsband) 


Size  of  the  experimental  basin: 
Size  of  the  electrodes: 
Temperature  of  the  water: 


Species  offish  and 
average  length  in  cm. 


35  x  22  X  22  cm. 
22  x  22  cm. 
15  degrees  C. 

Voltage  of  the 
individual  impulse    Narcotizing 

required  for  impulse 

galvanonarcoxis  limit 

(in  volts) 


Trout  (Trutta  iridea)  15-17 

6-5 

80 

Carp  (  Cyprinus  carpio)  12-15 

7-5 

50 

Catfish  (Silurus  glanix)  14-16 

11-5 

40 

Eel  (Anguilla  vulgaris)  20-22 

13  5 

50 

Goldorfe  (  Idus  melanotus)  13-15 

7-5 

30 

Stone  Perch  (Accrina  cernua)  14-16 

7-5 

50 

White  Bream  (Blicca  bjorkna)  12-15 

11-5 

40 

Minnow  (Phoxinus  laevis)  7-9.  . 

8-5 

90 

Stickleback  (Gasterosteus  aculeatus)  6-7 

13-5 

100 

Perch  (Percafluv.)  12-14 

95 

70 

Loach  (  Misgurnus  fossilis)  24-27 

9-5 

40 

Tench  (Tinea  tinea)  16-18 

7-5 

40 

[576] 


EFFECT    OF     PULSATING     CURRENT     ON     FISH 


TABLE   II 

Narcotizing  impulse  limits  for  some  seafish 
Temperature  of  the  water:  15  degrees  C. 


Species  offish  ami 
average  length  in  cm. 


Voltage  of  the 
individual  impuhe    Narcotizing 

required  J  or  impulse 

galvano-narco  V/'A  limn 

(in  volts) 


A.  ace.  to  Haltband 

Fat herlashcr  ( Cot  to  s  scorpiu.\ )  1 5 - 1 9  6-2 

Lelpout  (Zoarce\  viviparus)  17-21        .  .  1 0-0 

Plaice  (rieuronectes  plafe\\ti)  23-26     .  .  10-0 


I- 1 ve- bearded  Rocklmi;  (Om>\  muMela)  16-17    II    5 

Smelt  (Osmrrus  epcrlanu\)  1822 

I  lounder  (Plcuronette\  fle\\u\)  16-20 


12  0 
II  -5 


B.  ace.  to  Kreutzer 

Herring  of  medium  si/c 
Cod  of  medium  si/c 
Red  Tuna  (200-300  kg.) 


40 
60 
30 
60 
50 
40 


45 

25 

7-10 


energy,  just  narcotizes  the  fish,  is  best  for  electrical 
fishing. 

The  author  made  investigations  on  the  narcotizing 
impulse  limits  of  various  fresh -water  and  marine  fish 
and  found  out  that  each  of  the  tested  species  had  a 
specific  value.  Tables  1  and  ll  give  summaries  of  the 
results  of  the  experiments. 

The  fish  can  be  induced  to  swim  towards  the  anode,  if 
the  strength  of  impulses  required  for  narcosis  and  a 
certain  pulse  rate,  usually  ten  units  below  the  narcotizing 
impulse  limit,  arc  applied.  The  impulse  rate  must  never 
be  so  high  as  to  cause  constant  muscular  cramp.  It  has 
to  be  adjusted  so  that  a  slight  flight  movement  is  still 
possible  between  two  stimulations,  and  the  fish  is 
gradually  directed  towards  the  anode.  Narcosis  should 
not  be  obtained  until  the  anode  is  almost  reached. 

When  the  voltage  of  the  individual  impulse  is  increased 
beyond  the  minimum  required  for  narcoti/ing  effect,  the 
narcotizing  impulse  limit  will  decrease  accordingly.  In 
this  case,  more  electrical  energy  would  be  needed  than 
the  minimum  required  at  a  proper  impulse  rate. 

When  there  is  enough  voltage  to  produce  the  narcot- 
izing effect  and  the  number  of  impulses/sec,  is  increased 
beyond  the  narcotizing  impulse  limit,  the  reaction  of 
the  fish  will  be  decreased  until,  at  a  certain  value 
corresponding  approximately  to  the  doubled  narcotizing 
impulse  limit,  the  same  narcotizing  effect  sets  in  again. 

The  tests  made  with  carp  provide  a  suitable  example. 
A  voltage  of  7-5  V.  at  a  rate  of  50  impulses/sec,  caused 
galvanonarcosis  of  the  fish,  60  pulses/sec,  decreased 
the  reaction,  70  impulses/sec,  caused  a  temporary  loss 
of  equilibrium,  80  impulses/sec,  left  the  fish  in  upright 
position,  and  100  impulses/sec,  caused  loss  of  equilibrium 
again.  The  different  reaction  of  the  individual  species 
of  fish  to  impulse  rates  may  well  be  brought  into  relation 
to  the  different  size.  Yet  it  is  assumed  that  other  factors, 
such  as  different  values  of  chronaxy,  of  speed  of  con- 
ductivity and  of  refractive  time,  also  have  a  decisive 
influence  on  the  specific  behaviour  of  the  fish.  The 
varying  reactions  of  the  species  of  fish  to  different  im- 
pulse rates  make  it  possible,  to  a  certain  extent,  to  select 
the  kind  and  size  offish  in  electrical  fishing. 


THE  EFFECT  OF  VARIATIONS  IN  THE  IMPULSE 
PERIOD 

Scheminzky11  found  out  that  the  physiological  effect  of 
pulsating  current  did  not  change,  when  the  impulse 
period  was  reduced  to  1/10.  In  fact,  this  reduction  means 
a  decrease  in  energy  of,  for  instance,  2  kw.  to  0-2  kw. 
This  saving  of  energy,  however,  is  of  great  importance 
for  the  economy  of  electrical  fishing,  especially  in  salt 
water10.  At  first  it  was  supposed  that  the  impulse  period 
must  not  be  below  the  half  value  time  of  1  msec.  Recent 
investigations  by  American,  Japanese  and  German 
authors1-  K%  {),  however,  prove  that  the  impulse  period 
required  for  effective  fishing  may  be  much  smaller  than 
that.  For  instance,  as  average  value  for  trout,  a  half 
value  time  of  0-3  msec,  may  be  assumed.  These  values, 
however,  vary  according  to  the  species,  size  and  physio- 
logical condition  of  the  fish. 

The  author  made  investigations  into  the  relation  be- 
tween intensity  and  period  of  impulses8. 

It  is  known  from  nerve  physiology  that  each  electrical 
stimulation  must  have  a  certain  minimum  strength 
(rheobasis)  and  a  minimum  flowing  period  (effective 
period)  in  order  to  produce  a  reaction  in  the  nerve.  As 
this  minimum  period  has  the  desired  effect,  any  additional 
time  is  wasted.  Current  intensity  and  time  of  application 
are  in  a  certain  relation  to  each  other  allowing  for  a 
great  number  of  different  effective  periods.  The  greater 
the  intensity  of  stimulation,  the  shorter  the  effective 
period.  When  the  effective  period  is  calculated  for  each 
potential  and  the  values  obtained  are  classified  into  a 
system  of  coordinates,  a  curve  resembling  a  hyperbola 
will  develop.  As  mentioned  before,  the  required  potential 
decreases  with  increasing  period  of  stimulation  until 
a  certain  threshold  value  is  reached,  after  which  the  effect 
remains  constant  throughout  any  extension  of  time,  even 
if  the  pulses  are  infinitely  long.  This  curve,  indicating 
the  potential  needed  for  a  certain  period  of  stimulation, 
thus  runs  in  linear  shape  from  a  certain  point  onward. 
The  turning-point  of  the  hyperbola  is  where  stimulation 
can  be  obtained  with  the  smallest  consumption  of  energy. 
The  author  studied  the  reactions  of  the  fish  under  the 
influence  of  pulsating  current  of  different  potentials  and 
impulse  periods  to  find  out  to  what  extent  the  principles 
prevailing  in  the  isolated  nerve  could  be  applied  to  the 
behaviour  of  the  whole  fish.  Curves  relating  to  the 
potential  and  stimulative  period  were  developed  for  the 
behaviour  of  the  whole  fish  in  order  to  determine  the 
most  favourable  relation  of  energy  and  impulse  period 
needed.  The  energy  was  worked  out  according  to: 
T 

E  -   -  -  V2F  ; 

2R 
where      T        impulse  period  in  sec. 

R  —  resistance  in  Ohm. 

V  —  the  peak  voltage  of  the  pulse. 

F  — -  the  impulse  rate  per  sec. 

Two  kinds  of  freshwater  fish  (trout  and  carp)  of  8  to 
12  cm.  length  were  tested  at  a  water  temperature  of 
15  degrees  C.  As  the  fish  were  of  similar  size,  the  results 
obtained  are  considered  comparable. 

First,  applying  the  maximum  half  value  lime  of  the 
impulses  of  2  msec.,  the  threshold  value  which  just 
produced  a  reaction  in  the  whole  fish  was  determined. 
This  threshold  value  corresponded  to  the  basic  voltage 


[577] 


NN 


MODERN    FISHING    GEAR    OF    THE    WORLD 


energy 


En 

Et 


0.2 


1.0 


2.6 


3.4 


4.2     m««c. 


Fig.  3.     The  significance  of  voltage  and  duration  of  stimulation 

far  the  reaction  of  fish.     Test  fish   =   carp,  9-5  cm    long. 

C— basic  voltage;  Lm^  minimum  of  efficiency;  Et— galvano- 

taxis;    En  =  galvanonarcosis. 


(rheobasis  when  measured  in  the  nerve).  After  that, 
potential  and  impulse  period  were  gradually  increased 
and  the  various  stages  of  reaction  i.e.  first  reaction, 
electrotaxis,  and  electronarcosis,  were  registered  con- 
tinuously. The  values  obtained  were  classified  into  a 
system  of  coordinates  giving  the  half  time  value  of  the 
impulses  in  relation  to  the  voltage  per  cm.  By  this 
procedure  typical  curves  resembling  hyperbolas,  were 
obtained  for  each  stage  of  reaction. 

The  results  of  the  experiments  show  that  the  effects  on 
the  fish  are  similar  to  those  which  are  decisive  in  the  case 
of  the  isolated  nerve.  All  the  fish  used  in  the  experiments 
showed  that  the  pulse  periods  necessary  for  producing 
a  certain  reaction  decrease  continuously  with  the  increase 
of  the  potential  of  the  pulse.  From  a  certain  threshold 
value  onward,  the  curve  presenting  the  reactions  of  a  fish 
runs  linear,  indicating  that  a  further  increase  in  time  has 
no  additional  effect.  Figs.  3  and  4  show  the  results 
of  two  typical  series  of  tests.  These  curves  help  in 
calculating  the  minimum  values  of  efficiency  for  produc- 
ing a  certain  reaction  by  means  of  the  formula: 

T 

E  «  —  V2F. 
2R 

The  results  given  in  figs.  5  and  6  prove  that  the  con- 
sumption of  energy  is  great  when  the  impulse  period  is 
long  or  the  impulse  period  is  short. 
In  between  these  two  extremes  there  is  a  point  of 


Et 


0.2 


1.0 


1.8 


2.6 


3.4 


4*2     msec. 


Fig.  4.  The  significance  of  voltage  and  duration  of  stimulation 

for  the  reaction  offish.     Test  fish  ^  trout,  9-0  cm.  long.      G« 

basic  voltage;  L/n-- minimum  of  efficiency;  £>=- galvanotaxis; 

En — galvanonarcosis. 


1  « 


042 


Thseo. 


Fig.  5.  Energy  curve  for  carp,  9-5  cm.  long.  Lm= minimum  of 
efficiency;  Et— galvanotaxis;  £/i= galvanonarcosis. 

lowest  consumption  of  energy,  the  minimum  of  efficiency, 
marked  by  Lm  in  the  curves.  This  minimum  of  efficiency 
does  not  coincide  with  the  turning-point  of  the  hyperbola, 
but  is  slightly  above  it,  on  the  steeply  rising  branch  of 
the  curve. 

The  results  of  the  experiments  prove  that  the  most 
favourable  impulse  periods  required  for  the  stages  of 
reaction  are  below  the  half  time  value  of  1  msec,  for  either 
species  of  fish.  As  far  as  the  minima  of  efficiency  are 
concerned,  there  exist  slight  differences  between  the 
species.  The  trout  needs  a  smaller  minimum  of  efficiency 
than  the  carp  for  the  stage  of  electrotaxis.  For  the  stage 
of  electronarcosis,  however,  the  trout  requires  a  greater 
minimum  of  efficiency. 

Knowing  the  minima  of  efficiency  for  the  various 
species  of  fish  is  of  practical  importance  as  this  makes  it 
possible,  when  pulsating  current  is  applied,  to  operate 
with  optimum  potentials  and  impulse  periods  under  the 
most  favourable  conditions.  For  instance,  the  curve 
indicating  the  voltage  in  relation  to  the  stimulative 
impulse  period  for  trout  shows  that  the  optimum  period 


0.2 


1.0 


2.2 


3.4 


maeo. 


Fig.  6.    Energy  curve  for  trout,  9-0  cm.  long.    Lm^ minimum 
of  efficiency;   £r«=  galvanotaxis;   En —galvanonarcosis. 


[578] 


EFFECT    OF     PULSATING     CURRENT     ON     FISH 


is  reached  at  the  half  value  time  of  0-3  msec.  For 
example,  if  longer  impulse  periods  of  2  msecs.  are  used, 
the  stimulative  effect  will  not  increase,  but  double  the 
energy  will  be  required,  as  can  be  seen  from  the  energy 
curve.  If  the  impulse  period  of  4  msec,  is  applied,  treble 
the  energy  will  be  required. 

If  the  impulse  period  is  shorter  than  0-3  msec,  the 
desired  effect  can  still  be  produced,  if  the  voltage  is 
increased;  the  energy  required,  however,  is  also  treble 
to  that  of  an  impulse  period  of  0-1  msec. 

THE  INFLUENCE  OF  THE  INTENSITY  OF 
METABOLISM 

We  see  from  the  above  investigations  that  different 
species  of  fish  show  specific  reactions  to  the  electric 
current,  especially  to  pulsating  current.  Sa/monidae,  for 
instance,  require  a  stronger  electric  field  for  narcosis  than 
cyprinidae,  but  they  are  more  easily  excited  by  the  electric 
current  and  induced  to  certain  movements  (galvano-  or 
oscillotaxis).  Furthermore,  the  intensity  of  the  reactions 
in  the  electric  field  may  vary  at  different  times  of  observa- 
tions, even  among  fish  of  the  same  kind.  It  is  known  that, 
for  instance,  trout  are  more  easily  stimulated  in  summer, 
when  the  temperature  of  the  water  is  higher,  than  in 
winter.  Finally,  it  is  supposed  that  the  intensity  of 
metabolism  and  activity  of  the  fish  are  of  importance  in 
this  connection. 

One  of  the  main  factors  influencing  the  physiological 
state  of  fish  is  the  ion  composition  of  the  water.  It  is 
proved  by  earlier  investigations-  that  an  increase  of  the 
concentration  of  the  potassium-ions  in  the  water  results 
in  an  increase  of  metabolism,  whereas  an  increase  of  the 
concentration  of  the  calcium-ions  causes  decreasing 
metabolism.  Potassium  and  calcium  are  also  responsible 
for  the  physiological  equilibrium.  Potassium  increases  the 
excitability  of  the  nerve,  whereas  calcium  reduces  it.  Any 
change  in  the  physiological  state,  however,  will  more  or 
less  influence  the  behaviour  of  the  fish  in  the  electric  field. 

The  influence  of  the  intensity  of  metabolism  on  the 
excitability  of  fish  in  the  electric  field  was  investigated. 
As  before,  two  different  species  of  fish  (trout  and  carp) 
were  compared,  and  different  intensities  of  metabolism 
among  fish  of  the  same  species  were  tested.  The  first 
reaction,  galvanotaxis,  and  galvanonarcosis,  were  ob- 
served. 

The  results  of  the  experiments  proved  that  the  trout,  a 
fish  with  highly  intensive  metabolism,  needed  smaller 
current-densities  than  the  carp  to  develop  the  first  two 
stages  of  reaction.  The  carp,  however,  was  sooner 
narcotized  than  the  trout.  In  this  case,  a  low  intensity 
of  metabolism  is  combined  with  the  paralysing  effect  of 
the  current. 

When  the  fish  became  acclimatized  to  solutions  of 
substances  which  effected  a  decrease  of  metabolism 
(MgCI2 — salt  mixture,  etc.),  the  threshold  values  for 
the  first  reaction  and  galvanotaxis  were  distinctly  higher. 
The  current-density  for  the  stage  of  galvanonarcosis, 
however,  was  lower.  Where  the  level  of  the  metabolism 
of  the  trout  was  increased  (for  instance  by  application 
of  KC1)  the  current-densities  required  for  producing 
the  first  two  stages  of  reaction  were  lower.  Greater 
threshold  values,  however,  were  required  for  narcosis 
(Tables  III  and  IV). 


IABLF   HI 
Test  fish  :  Trutta  iridea 

Temperature  of  the  water:  1  .s  degrees  C. 

Substance    decreasing    metabolism:     salt-mixture    of     the     following 
combination  : 


NaCl  =  0-230  g./l. 
KCI  -  0-005  g./l. 
MgS04.7H20  -  2  - 


MgCL  .  6  H,O        2  386  g./l. 
C  aCL  .  6  H.O    -  0  537  c  /I 


1.  Reaction          Galvanotaxis          Galvanonarcosis 
Length  current-    current-    current-    current-   current-    current- 

offish    Medium   <**"**&     Density     density     density     density     density 
in  cm  in  per  cent.  in  per  cent.  in  per  cent. 

8  of  the          8  of  the          8  of  the 

normal  normal  •  normal 


Fresh- 

9-0 

water 

0-374 

100 

1   430 

100 

4-004 

100 

Salt- 
mixture 

0-660 

177 

1    826 

127 

3-146 

79 

8-5 

Freshw. 

0  396 

100 

-496 

100 

4  202 

100 

Salt-m. 

0  682 

172 

•936 

129 

3-278 

78 

8-0 

Frcshw. 

0  396 

100 

•474 

100 

4-070 

100 

Salt-m. 

0  682 

172 

914 

130 

3  036 

75 

9-0 

Freshw. 

0-396 

100 

430 

100 

4-752 

100 

Salt-m. 

0  704 

178 

804 

126 

3-542 

74 

8-5 

Freshw. 

0  330 

100 

430 

100 

4-070 

100 

Salt-m. 

0  598 

180 

826 

127 

2  970 

73 

80 

Freshw. 

0  352 

100 

408 

100 

4-488 

100 

Salt-m. 

0-660 

188 

804 

128 

3  300 

74 

90 

Freshw. 

0-330 

100 

430 

100 

4   180 

100 

Salt.m. 

0  528 

160 

870 

129 

2  926 

70 

9-0 

Freshw. 

0-308 

100 

•469 

100 

4   185 

100 

Salt-m. 

0  506 

167 

900 

129 

2-706 

65 

TABLF    IV 

Test  fish  :  Trutta  indea. 
Temperature  of  the  water:  15  degrees  C. 
Substance    increasing    metabolism:    KCI.    concentration:    400    mg./l. 


Length 
offish 

Medium 

current  - 
density 

current- 
density 

current- 
density 

current- 
density 

current- 
density 

<  urrent- 
denstty 

in  cm. 

ii 

'i  per  cent. 

in  per  cent. 

in  per  cent. 

K 

of  the 

8 

of  the 

$ 

of  the 

normal 

normal 

normal 

Fresh- 

9-0 

water 

0  315 

100 

I   418 

100 

4   180 

100 

KCI 

0  205 

65 

0  821 

58 

5-348 

128 

Fresh- 

90 

water 

0-368 

100 

1   366 

100 

4  501 

100 

KCI 

0-228 

62 

0-835 

61 

6-098 

134 

Fresh- 

8-5 

water 

0  335 

100 

1-382 

100 

4  300 

100 

KCI 

0  229 

68 

0  829 

60 

5-598 

no 

Fresh- 

80 

water 

0  305 

100 

1-372 

100 

4-449 

100 

KCI 

0  203 

66 

0  851 

62 

5  601 

126 

Fresh- 

8-5 

water 

0  350 

100 

1-392 

100 

4-744 

100 

KCI 

0  220 

63 

0  871 

63 

6-302 

133 

Fresh- 

9-0 

water 

0  342 

100 

1-436 

100 

4-210 

100 

KCI 

0  230 

67 

0  874 

61 

5-426 

129 

Fresh- 

9-5 

water 

0-345 

100 

1-451 

100 

4  499 

100 

KCI 

0-203 

59 

0-884 

61 

5-974 

133 

8-5 

Fresh- 
water 

0-330 

100 

1-319 

100 

4  101 

100 

KCI 

0-202 

61 

0-779 

59 

5  288 

129 

579  ] 


MODERN     FISHING     GEAR    OF    THE    WORLD 


carp 


trout 


1.0  2.0  3.0  4.0 

Fig.  7.     Width  of  narcosis  for  carp  ami  trout.  8  -  curren.  density. 

Active  fish  with  high  intensity  of  metabolism  are  more 
easily  stimulated  by  electric  current,  but  their  resistance 
against  narcosis  is  stronger.  This  phenomenon  may  be 
called  width  of  narcosis.  Such  fish  possess  a  greater 
width  of  narcosis  than  fish  with  smaller  intensity  of 
metabolism  (figs.  7  and  8).  The  physiological  state 
of  a  fish  is  thus  of  great  importance  in  its  reaction  to 
electric  current. 

REFERENCES 

1  Bary,  Mck.  The  effect  of  electric  field  on  marine  fishes.  Mar. 
Res.,  No.  l,pp.  1-32,  1956. 

2  Halsband,  F.  Untcrsuchungen  iiher  das  Verhalten  von  Forelle 
(Trutta  iridca,  W.  Gibb)  und  Dobel  (Squalius  cephalus.  Heck)  bei 
Einwirkung  verschiedener  Ausscnfaktoren.    Zeitschr.  f.  Fischerci, 
2.,  Heft  3/4,  p.  228,  1953. 

s  Halsband,  E.  Untcrsuchungen  iiber  die  Bedcutung  des  polarcn 


in  nalt  mixture, 
acclimatized. 


normal 


in  400  ing. /I.  K  Cl, 
acclimatized 


2.0 


3'.0 


5:0 


6.'0 


Fig   S.     Width  of  narcosis  for  trout  at  different  intensity  of 
metabolism.     8  —  current  density. 

hrregungsgescues  fiir  die  Reaktion  der  Fische  im  clcktrischen  Fcld. 
Arch.  f.  Fischereiw.,  5..  Heft  3/4,  1954. 

4  Halsband,  E.  Untersuchungen  iibcr  die  Bctaubungsgrenz- 
impulszuhlen  verschiedener  Siisswasserfische.  Arch.  f.  Fischereiw., 
6.,  Heft  1/2,  1955. 

6  Halsband.  F.  Die  Beiaubungsgrenzimpulszahlcn  verschiedener 
Salzwasscrfische.  Arch.  f.  Fischereiw.,  6.,  Heft  3/4,  1955. 

8  Halsband,  F.  Die  ariodischc  Reaktion  der  Fische  im  elektrischen 
Feld.  Arch.  f.  Fischereiw.  6.,  Heft  3/4.  1955. 

7  Halsband,  F.  Untersuchungen  uber  den  Einfluss  verschiedener 
Stromarten  auf  den  Stoffwechsel  der  Fische.  Arch.  f.  Fischereiw., 
6.,  Heft  5/6,  1955. 

8  Halsband,  F.  Die  Be/iehungen  zwischen  Intensitat  und  Zeitdauer 
ties    Rci/es   bei   der   elektrischen    Durchstrdmimg   von   Fischen. 
Arch.  f.  Fischereiw.,  7.,  Heft,  1,  1956. 

9  Kreut/cr,  C.  O.   hleklrofischcrci.  Elektrotechn.  Zeitschr.  B, 
Heft  5,  1954. 

10  Ktiroki,  T.  On  the  selection  of  effective  frequencies.  Kagoshima 
Univ.,  Japan. 

11  Meyer- Waardcn,   P.   F.   Electrical  fishing.   F.A.O.   Fisheries 
Study  No.  7,  1957. 

12  Scheminzky,  F.  Versuche  uber  Elektrotaxis  und  Elektronarkose. 
Arch.  f.  d.  gcs.  Physiol.  (Pfliiger),  202.,  p.  200,  1924. 


Electro-fishing  of  small  Bream  with  a  battery  driven  impulse  gear  in  fresh  water.   The  attracted  fish  are  stunned  and 
then  collected  by  means  of  scoop  nets.  Photo:  Inst.  f.Kiisten  u.  Binnenfischerei,  Germany. 

I  580  | 


ELECTRICAL  FISHING  IN  JAPAN 

by 
T.  KUROKI 

Faculty  of  Fisheries,   Hokkaido   University,  Japan 

Abstract 

Patent  rights  for  an  electrical  fishing  apparatus  were  obtained  in  Japan  in  1895,  but  fundamental  studies  were  not  begun  until  1924. 
Before  1940,  the  electrical  fish-screens,  energized  by  commercial  alternating  electric  current,  were  mainly  studied  with  the  object  of  attracting 
fish  schools. 

Remarkable  progress  has  been  made  with  electrical  fishing  in  several  countries  of  the  world  during  the  last  10  years.  Fishing  by  means 
of  low  frequency  electrical  impulse  have  been  studied  in  Japan. 

In  this  paper,  the  author  outlines  the  progress  of  these  investigations,  the  operation  of  electric  harpoons  and  hooks,  and  the  future 
plan  to  use  electric-fences,  etc.  in  Japan.  It  is  emphasized,  however,  that  he  is  not  too  optimistic  about  using  the  apparatus  in  open  sea. 


Resume 


La  peche  electriquc  au  Japon 


Au  Japon,  un  appareil  pour  la  peche  electrique  avait  deja  ete  brevete  en  1895  mais  ('etude  fondamentale  de  ce  systeme  n'a  etc  entre- 
prisc  qu'en  1924.  Avant  1940, 1'etude  des  ecrans  electriques  alimentes  par  du  courant  alternatif  a  la  tension  industrielle  normale  a  ete  princi- 
palcment  entreprise  dans  le  but  de  cherchcr  a  attirer  les  banes  de  poisson. 

Depuis  10  ans  la  peche  clectrique  a  fait  des  progres  rcmarquables  dans  plusieurs  pays,  et  au  Japon  on  a  etudie  la  peche  au  moyen  de 
d&rharges  electriques  a  basse  frequence. 

I/auteur  expose  dans  cette  6tude  Petal  d'avancement  de  ces  recherches,  le  fonctionnement  d'harpons  et  d'hamcc.ons  eleciriques  et  les 
plans  elabores  au  Japon  pour  I'emploi  futur  de  barrieres  electriques.  etc  ....  II  convient  de  souligner  toutefois  qu'il  n'est  pas  tres  optimiste  sur 
Tutilisation  de  la  peche  electrique  en  mer. 

La  posca  con  electricidad  en  el  Japon 
Extracto 

En  1895  el  Jap6n  otorgo  la  primera  palcnte  dc  invcncion  para  un  equipo  de  pesca  con  electricidad,  pero  solo  en  1924  comenzaron 
a  estudiarse  los  principios  fundamcntales  quo  gobiernan  a  estos  aparatos.  Antes  de  1940  ya  se  habian  estudiado  las  rejillas  electrizadas 
con  una  corriente  alterna  industrial,  especialmentc,  como  medio  para  atraer  a  los  peccs. 

bn  varies  paises  se  han  logrado  notables  progresos  en  la  pesca  con  electricidad  mediante  impulses  de  baja  frccuencia. 

El  autor  describe,  en  general,  los  adelantos  logrados  en  este  campo,  el  funcionamiento  de  arpones  y  anzuelos  electrizados  y  los  planes 
para  el  uso  de  barreras  electncas  en  el  Japon.  Tambien  hace  notar  su  pesimismo  en  cuanto  se  refiere  al  cmpleo  de  este  equipo  en  el  mar. 


ELECTRIC  SCREEN,  STUDIED  AND   USED 
BEFORE  1940 

IN  Japan,  Takuhashi  was  granted  a  patent  for  using 
electrical  apparatus  as  fishing  gear  in  1895.  Funda- 
mental studies  on  the  electric  fish  screen  were 
started  by  Tauchi2  in  1924  and  later  continued  by 
Okada3.  They  concerned  methods  of  guiding  fish 
schools  from  the  main  to  a  side  stream,  by  arranging 
several  rows  of  electrodes  to  produce  a  gradient  of 
alternating  current  (A.C.)  voltage. 

Tauchi  and  Yasuda4  then  used  intermittent  current 
in  order  to  save  power,  but  the  results  were  inconclusive 
owing  to  lack  of  electro-physiological  information.  The 
reactions  of  fish  to  intermittent  A.C.  stimuli  were  very 
irregular. 

Miyahara5  studied  the  stimulating  effect  of  high- 
frequency  current  (alternating,  low-frequency  modulated, 
semi-rectified,  etc.)  and  Senuma6  investigated  the  polar- 
ity of  stimulation  by  direct  current  (D.C.) 

As  a  result  of  these  studies,  the  electric  fish  screen  was 
brought  into  use.  Screens  to  guide  migrating  fish  schools 


were  set  up  at  the  electric  power  plants  on  the  Shinano 
River,  Nagano  prefecture,  and  other  places  (1937-1940). 
Estimates  of  the  practical  effectiveness  of  these  screens 
were  conflicting. 

FUNDAMENTAL  INVESTIGATIONS  DURING  THE 
LAST  TEN  YEARS 

The  recent  progress  in  the  study  of  electro-physiology 
has  made  the  influences  of  electric  stimuli  on  fish  some- 
what clearer.  The  optimum  electric  current  (or  voltage) 
and  the  most  effective  pulse-shapes  to  electrocute  fish 
were  determined  by  Kuroki's  investigations  on  the 
relationship  between  chronaxic  and  rhcohase  of  the 
muscles  and  nerves  of  fish7.  Through  the  use  of  electric 
gear  in  longline  fishing,  he  confirmed  that  only  about 
3-5  W.  min.  was  needed  to  electrocute  even  a  large 
and  active  shark8,9. 

Before  these  experiments,  Kuroki  found  that  a  particu- 


[581  ] 


MODERN     FISHING    GEAR    OF    THE    WORLD 


lar,  sensitive  electro-physiological  point,  the  so-called 
**£"  point,  exists  in  the  body  of  some  species  of  fish, 
and  that  the  motion  (forward  or  backward)  of  the  fish 
could  be  controlled  by  shifting  the  point  of  stimulation 
accordingly10.  By  calculating  the  relations  between  the 
swimming  velocities  of  fish  and  the  existence  of  this 
point,  he  determined  the  most  effective  rhythms  of  elec- 
tric stimuli,  i.e.  10  to  20  pulses/sec,  for  electrifying,  and 
I  to  2  pulses/sec,  for  electrocuting11.  Furthermore,  he 
concluded  that  it  was  necessary  to  use  the  "double 
intermittent  method"  in  electrical  trawl  fishing  for 
maximum  catching  efficiency.  If,  for  example,,  the 
electric  energy  for  a  trawl  net  is  supplied  in  the  form  of 
low-frequency  electric  pulses  for  30  sec.,  the  current  must 
then  be  interrupted  for  90  sec.12. 

Lately,  Suzuki  and  Fukuyama13  showed  by  physio- 
logical experiments  that  this  £  point  is  within  the  region 
of  the  vagus  nerve.  They  demonstrated  that  fish  can 
be  made  to  move  clockwise  or  counter-clockwise  by 
a  negative  or  positive  needle-electrode  stabbed  at  this 
point. 

Electrical  fishing  at  sea  has  now  been  introduced, 
using  low-frequency  pulse  current. 

TRIALS  WITH  ELECTRIC  HARPOONS,  HOOKS, 
SCREENS,  AND  TRAWLS 

An  A.C.  electric  harpoon  for  whaling  was  tested  in 
1950  by  the  Japanese  whale  catcher,  Fuml-maru  No.  7.1 
Low-frequency  pulses  were  later  applied  to  harpoons  and 
also  to  hooks  used  in  longlining14.  In  this  apparatus, 
the  low  voltage  of  A.C.  or  D.C.  is  transformed  into 
300-400  V.,  and  low  frequency  shock  waves  (2— 3x  10~4 
sec.)  are  sent  to  the  harpoon  or  hook  electrodes  through 
the  mechanical  contact  points.  Only  a  few  seconds 
are  required  to  bring  large  tunnies  on  board.  Even 
the  fiercest  sharks  can  be  killed  by  electrocution  in 
about  10  sec. 

Electric  hooks  of  this  design  were  operated  success- 
fully about  360  times  by  the  Kuroshio-maru  No.  15  for 
longlining,  decreasing  fishing  time  considerably. 

Trot  line  fishing  with  the  low  frequency  electric  hooks 
is  now  coming  into  use. 

A  low-frequency  screen  was  set  and  operated  by 
Hokkaido  Electric  Power  Co.,  in  Toya  Lake,  at  the  inlet 
water  gate  of  the  Abuta  Power  Plant15,  and  small  electric 
trawls  of  bottom  and  midwater  types  were  operated  on 


Ikeda  Lake  in  the  Kagoshima  Prefecture  by  Kuroki1'. 
It  was  difficult  to  estimate  the  results  of  the  first  ex- 
periment: the  latter  experiment  proved  unsatisfactory. 

FUTURE  POSSIBILITIES 

If  the  towing  speed  in  midwater  trawling  were  to  be 
increased  and  the  depth  of  the  net  controlled  more 
exactly,  the  efficiency  could  be  improved  by  applying 
electricity. 

The  operation  of  a  3-phase  electric  fish  screen,  as 
Kuroki  has  shown16,  could  be  improved  by  using  low 
frequency  pulses. 

While  not  yet  in  actual  commercial  use,  there  are 
various  types  of  defensive  electric  fences  for  surrounding 
shellfish  in  shallow  water,  and  for  defending  the  larvae 
or  eggs  of  fish  from  natural  enemies  on  rocky  seabeds. 

The  economy  of  electric  fishing  in  sea  water,  other  than 
with  harpoons  or  hooks,  will  have  to  be  considered 
carefully  because  of  the  high  initial  and  operational  costs. 

Electrical  devices  can  be  used  for  protecting  the 
larvae  and  eggs  of  some  fish  and  also  for  increasing  the 
efficiency  of  some  types  of  fisheries,  but  they  should  not 
be  applied  to  fisheries  which  may  be  in  danger  of  over- 
fishing.  Even  cheapness  of  operation  would  not  justify 
the  universal  application  of  electrical  fishing  until  the 
problem  of  conservation  of  the  fish  stocks  is  solved. 

REFERENCES 
(In  Japanese;  *  with  summary  in  English) 

1  Kuroki.  "Electrical  Fishing",  Giho-do.  190  pp.  Book.  1955. 

-*  Tauchi  and  Sugii.  J.  Fishery  Sci.,  19,  12.    1924. 

3*  Okada.  J.  Imp.  Fish.  Inst..  24.  2  and  5:  25.  1.     1929. 

4*  Tauchi  and  Yasuda.    Ibid.,  27,  2.    1932. 

6*  Miyahara.  J.  Hiroshima  Tech.  Coll.,  A  4.    1929. 

6*  Senuma.    Suisangaku-kaiho,  5,  2.    1925. 

7*  Kuroki.    Bull.  Jap.  Soc.  Sci.  Fish.,  18,  I.    1952. 

Ibid.,   18,   12.     1953. 

Ibid.  18,  8.     1953. 

Suisan-kagaku-kenkyukai,   Kagoshima    Univ.,   Prints. 


8* 

»* 
10* 

1949. 
11* 

12* 


Bull.  Jap.  Soc.  Sci.  Fish.,  16,  4.    1950. 
Ibid.,  18,  9.     1953. 
13  Suzuki  and  Fukuyama.  Jour.  Physiol.  Soc.  Jap.  75,  4.  Kagaku, 
23,  1.     1953. 

14  *  Kuroki  and  Morita.  Jour.  Kagoshima  Fish.  Coll.,  /.,  1950. 
Report  of  Tateyama  Branch,  Chiba  Pref.  Fish.  Lab.    1952. 

16  Tamori.   Rep.  Lab.  Hokkaido  Elect.  Power  Co.,  No.  1.  1949. 
Shimura.  Ibid.,  No.  2.  1952. 

ia*  Kuroki.  Mem.  Fac.  Fish.  Kagoshima  Univ.,  3,  I.  1953. 
17*  Kuroki  and  others.  Bull.  Jap.  Soc.  Sci.  Fish.,  18.  1.   1952. 


[582] 


ELECTRO-FISHING 


by 

JUERGEN  DETHLOFF 

Dethloff-Electronic,  Hamburg-Lokstedt 

Abstract 

This  paper  shows  the  different  methods  of  electro-fishing  distinguishing  between  the  applications  of  electrotaxis  and  clectronarcosis 
and  "the  first  reaction." 

Some  successful  trials  are  described. 

Proved  methods  are  enumerated  such  as  the  electro  tuna  hook,  a  procedure  for  supporting  the  power  block  operation,  the  gun-pump 
method,  narcotizing  of  sardines,  and  electric  fences.  Possible  developments  considered  are  the  electric  trawl  (for  improving  the  catch  and 
the  quality),  the  "fish  magnet"  and  electric  whaling. 

A  description  is  given  of  the  technique  and  construction  of  pulse  devices.  A  direct  connection  of  pulse  devices  to  3-phase  alternating 
current  generators  instead  of  high  tension  direct  current  generators,  has  now  become  possible.  After  a  short  description  of  the  pulse  devices 
themselves,  there  is  a  paragraph  concerning  security  circuits. 


Resume 


La  p£che  Itoctrique 


Cettc  feuille  11  lustre  les  diflterentes  m&hodes  de  la  p£che  e'lectrique  et  distingue  entre  Tapplication  de  la  "taxis  61ectrique",  la  narcose 
glectrique  et  "la  premiere  reaction". 

Quelques  essais  couronnes  des  succds  font  1'objct  d'une  description. 

Quelques  mdthodes,  ayant  fait  leurs  preuves,  sont,  6num£rees  telles  que  le  hamecon  61ectrique  pour  le  thon,  un  proced6  pour  supporter 
I'operation  du  "power  block",  la  m&hode  de  peche  avec  Electrode  Ianc6e  et  pompe  £  poisson  la  narcotisation  des  sardines  et  les  barrieres 
61ectrique.  Les  cteveloppements  6ventuels,  entrant  en  ligne  de  compte,  sont  les  chaluts  61ectriqucs  (pour  ram61ioration  de  la  prise  et  de  la 
qualit£),  "I'aimant  a  poissons"  et  la  peche  &lectrique  a  la  baleine. 

L'on  donne  une  description  de  la  technique  et  de  la  construction  de  compulsion.  Une  connexion  directe  des  dispositifs  d'impulsipn 
aux  gen^rateurs  de  courant  triphas£  alternatif,  au  lieu  de  cclle  aux  g£n£rateurs  de  courant  direct  de  haute  tension,  est  devenuc  possible  main- 
tenant.  Apres  une  breve  description  des  dispositifs  compulsion  il  existe  un  paragraphe  ayant  trait  aux  circuits  de  securit6.  Apres  une 
presentation  des  grandes  possibilites  de  la  peche  electrique  il  est  egalement  fait  allusion  a  ses  limitations. 


Pesca  por  electricidad 
Extracto 

Esta  hoja  hace  ver  los  distintos  me~todos  de  la  pesca  por  electricidad,  diferenciando  entre  la  aplicaci6n  de  la  "electroitaxis"  y  la 
narcosis  por  elcctrieidad,  asi  como  la  "primera  reacci6n".  Ya  han  sido  descritos  algunos  ensayos  de  buen  resultado.  Se  establecen  algunos 
metodos  acreditados,  tales  como  el  anzuelo  elect rico  para  pescar  atun,  un  proccdimiento  para  apoyar  la  "power-operation",  el  metodo  de  la 
bomba  de  can6n,  y  la  narcotizacidn  de  las  sardinas  por  barreras  de  rejillas  electricas.  Desenvolvimientos  posibles,  tornados  en  consideraci6n, 
lo  constituyen  la  red  de  arrastre,  electrica,  (para  mejorar  la  pesca  y  la  calidad),  el  electroiman  dc  pesca  y  la  pesca  de  la  ballena  por  electricidad. 
Se  describen  la  t&nica  y  la  construcci6n  dc  los  aparatos  de  impulsos.  Ha  sido  ahora  ppsible  una  conexi6n  dirccta  entre  los  aparatos  de 
impulses  y  generadores  de  corriente  trifdsica,  en  lugar  de  con  generadores  de  corriente  continua,  de  alta  tensi6n.  Tras  breve  descripci6n  de 
los  aparatos  de  impulsos  sigueunarticulo  sobre  las  conexioncs  de  seguridad.  Despues  de  las  grandes  posibilidades  de  la  pesca  por  electricidad 
se  describen  asimismo  sus  limites. 


POSSIBILITIES  OF  APPLICATION 

rHE  First  Reaction,  i.e.  the  frightening  effect  at  the 
border  of  the  stimulating  area,  can  be  used  for 
electric  fences,  either  movable  or  fixed,  made  of 
metal  electrodes  or  cables. 

Electrotaxis,  i.e.  the  anodic  attraction,  can  improve 
or  even  replace  (in  certain  cases),  conventional  fishing 
methods,  particularly  in  purse  seining,  hook-fishing 
and  trawling. 

Electronarcosis  and  Electrocution  are  most  useful  in 
angling  and  harpooning  big  fish,  and  in  whaling. 

The  Selectivity  of  electric  stimulation  in  regard  to  the 
size  of  fish  allows  for  a  certain  choice  of  marketable 
species  and/or  size.  Since  small,  immature  fish  react 
little  or  not  at  all,  they  are  not  affected  or  caught. 
Electro-fishing,  therefore,  bears  no  danger  of  over- 
fishing  but  may,  indeed,  be  a  means  of  avoiding  it. 


ESTABLISHED  OR  SUCCESSFULLY  TESTED 
METHODS 

Electric  tuna  hook 

Immediately  the  fish  takes  the  hook  it  is  electro-stunned. 
Any  struggle  is  therefore  avoided,  and  the  fish  cannot 
break  away  after  biting.  Fishing  time  is  saved,  and 
there  is  no  danger  of  loss  of  catch. 

Application  to  purse  seining 

Successful  trials  with  herring-sized  fish  have  shown  the 
advantage  of  an  electrified  suction  hose  for  brailing 
fish  by  means  of  a  pump.  The  fish,  even  those  from  the 
bottom  of  the  purse  seine,  are  attracted  by  electrotaxis 
to  the  mouth  of  the  suction  hose.  Another  advantage 
is  that,  when  using  power  blocks,  the  net  is  released 
from  the  weight  of  heavy  catches  and  hauling  can  be 


[583] 


MODERN     FISHING     GEAR     OF    THE     WORLD 


Fig.  1.     The  gun- pump  method. 

A.  The  electrode  being  shot  into  the  school. 

B.  The  fish  gather  around  the  electrode. 

C.  Pumping  with  the  fish  concentrated  around  the  electrified 
suction  hose. 

continued  smoothly  during  brailing.  If  no  fish-pump  is 
used,  a  simple  electrode  can  support  the  hauling  and 
brailing  operation  by  attracting  the  fish  to  the  surface, 
so  reducing  the  weight  in  the  net. 

Narcotising  sardines 

A  small  hand  electrode,  mounted  on  a  wooden  stick, 
has  been  tested  for  stunning  sardines,  to  prevent  struggl- 
ing after  the  net  is  pursed.  This  reduces  the  risk  of 
damage  and  loss  of  scales,  and  so  increases  the  quality 
and  marketable  value  of  the  catch. 

The  gun-pump  method 

After  locating  a  school  of  fish,  an  electrode  missile  is 
shot,  by  the  Dethloff-Electronic  airblast  gun,  into  the 
centre  of  a  fish  school  (fig.  1,  A).  This  electrode  floats 
like  a  buoy  and  is  connected  by  a  floating  cable  to  a 
pulse  generator  on  board.  The  pulse  current  starts 
automatically  when  the  electrode  touches  the  water. 
The  fish  immediately  concentrate  around  the  electrode 
(fig.  1 ,  B)  and  can  be  pumped  into  the  boat  (fig.  1 ,  C). 
This  combination  of  shooting  and  pumping  has  been 
successfully  tested.  The  power  used  for  this  test  was 
about  80  kw.,  but  can  now  be  decreased  considerably. 
The  gun-pump  method  is  applicable  to  pilchards,  sar- 
dines, and  other  fish  up  to  herring  size.  It  can  also  be 
used  in  combination  with  a  stick-held  dip  net  for  bigger 
fish,  e.g.,  bonitos  and  other  tunas. 
The  diameter  of  the  effective  field  around  the  electrode, 


which  depends  on  the  length  of  the  fish,  is  about  25  to 
35  m.,  when  an  economical  amount  of  power  is  used. 

Fencing 

For  electric  fences,  cables  arc  more  suitable  than  metal 
electrodes,  as  they  make  it  possible  to  obtain  any  form 
of  electric  field.  The  reason  for  this  is  that  pulse  current 
returns  through  the  water  along  the  cable,  which  is 
installed  in  a  "one-way"  system,  i.e.  its  end  is  connected 
to  an  electrode. 

A  smaller  installation  of  this  kind  has  been  success- 
fully tested  by  Dcthloff-Electronic  for  fishing  big  tuna. 
This  trial  provided  the  basic  data  for  the  calculation 
of  larger  installations 

FUTURE   POSSIBILITIES 
The  electro-trawl 

With  trawls,  electricity  can  be  used  in  two  different  ways: 
firstly,  as  a  means  of  improving  the  quality  of  the  catch 
by  reducing  the  period  of  death  struggle;  secondly,  in 
combination  with  the  above,  by  using  electrodes  to 
attract  fish  to  the  mouth  of  the  trawl.  Fish  caught  in 
trawls  arc  gathered  in  the  congested  space  of  the  codcnd, 
where  struggling  causes  the  muscles  to  produce  a  con- 
siderably higher  degree  of  fatigue,  thus  affecting  the 
quality  of  the  flesh.  Electrocuting  reduces  the  period  of 
death  struggle,  resulting  in  a  progress  of  rigor  mortis, 
which  is  more  favourable  in  the  processing  of  the  fish. 
Flexible  non-insulated  bronze  or  copper  cables  are 
interlaced  in  the  webbing  of  the  codend,  and  a  special 
cable  is  connected  with  a  pulse  generator  on  board. 
The  power  needed  for  this  electrocuting  apparatus  is 
less  than  50  kw. 

Electrodes  can  be  placed  in  front  of  the  trawl  mouth 
to  improve  the  catching  ability  of  a  conventional  trawl. 
The  electrodes  attract  and  narcoti/e  the  fish  before  they 
can  escape.  One  of  the  great  advantages  would  be  to 
attract  fish  which  stay  some  meters  above  the  sea  bottom, 
beyond  the  range  of  the  trawl.  Stunned  by  the  electric 
pulses,  the  fish  will  sink  and  be  collected  by  the  net,  as 
shown  in  fig.  2  A  and  B.  This  electric  catching  device 


Fig.  2.    Electro-trawling. 

A.  Bottom  fish. 

B.  Fish  above  the  trawl. 


[5841 


ELECTRO-FISHING 


can  be  combined  with  the  electrocuting  apparatus  with- 
out increasing  the  electric  power. 

Fish  magnet 

The  use  of  electrotaxis  for  leading  fish  by  means  of 
a  movable  electrode  has  been  successfully  tested.  This 
led  to  the  idea  of  constructing  a  "fish  magnet".  This 
could  be  very  useful  for  lifting  a  school  which  stays  too 
deep  for  the  working  range  of  a  purse  seine.  The 
electrode  can  easily  be  watched  by  echo  sounding  and 
switched  on  when  about  3  to  5  m.  above  the  school. 
After  the  fish  have  gathered  around  the  electrode,  it 
will  be  lifted  together  with  the  fish.  The  fish  are  likely 
to  stay  in  electrotaxis  for  1  to  3  min.  before  they  are 
stunned  and  commence  to  sink,  and  the  pursing  opera- 
tion must  be  finished  in  this  time.  The  method  could  also 
be  employed  for  larger  fish,  e.g.  bonitos,  alistados,  etc. 
It  would  be  more  advisable  for  herring-sized  fish  to  be 
pumped  into  the  vessel,  as  described  above.  A  method 
of  collecting  bigger  fish,  other  than  with  a  purse  seine, 
still  remains  to  be  designed. 

Fencing 

Fishing  trials  have  yielded  a  calculation  basis  for  big 
cable  fences  to  lead  fish  in  desired  directions.  Such 
fences  could  be  installed  as  deep  as  IOC)  to  150  m.  While 
technically  the  length  of  such  fences  may  be  unlimited, 
the  actual  fishing  conditions  would  decide  the  economic 
size.  The  main  use  for  such  fences  would  be  to  replace 
the  conventional  leader  nets,  particularly  in  depths 
beyond  about  50  m.,  which  are  now  used  for  the  tonnaras 
or  abnadrabas.  As  50  m.  is  about  the  maximum  depth 
for  real  leader  nets,  the  leading  range  of  such  traps  could 
be  considerably  extended  by  using  electric  fences. 

Whaling 

Electrocution  of  whales  by  pulse  current  has  an  immense 
advantage  over  the  continuous  alternating  current 
method.  In  the  latter  case,  a  short  circuit  ciiused  by  a 
contact  of  the  metal  harpoon  or  its  conductive  head 
with  the  water  will  result  in  a  breakdown.  With  the 
pulse  method  only  the  pulse  length  will  change  but 
not  the  voltage,  i.e.  there  is  no  breakdown.  Further- 
more, the  pulse  method  requires  no  change  m  conven- 
tional guns  and  harpoons. 

TECHNIQUE  AND   CONSTRUCTION   OF  PULSE 
DEVICES 

Latest  developments  have  made  it  possible  to  use  the 
common  three-phase  alternating  current  board  supply, 


with  cheaper  generators  than  the  high  tension  direct 
current  types  previously  needed. 

The  pulse  generators  are  of  simple  and  robust  con- 
struction. The  pulses  are  produced  by  condenser  dis 
charges,  using  ignitrons  as  switches.  The  ignitron — a 
valve  containing  liquid  mercury- is  suspended  in 
gimbals.  The  pulse  generator  is  mounted  inside  the 
vessel.  Remote  control  allows  free  choice  of  location 
of  the  switch  panel. 

As  electric  fishing  methods  need  a  higher  voltage  than 
the  usual  shipboard  voltage,  special  arrangements  for 
prevention  of  accidents  must  be  made.  Existing  regula- 
tions for  the  construction  and  installation  of  the  appli- 
ances give  sufficient  protection,  and  special  care  is  needed 
only  in  the  handling  of  the  electrode  and  cables.  Some 
very  efficient  and  absolutely  safe  security  circuits  have 
been  developed.  They  prevent  a  connection  to  the 
pulse  generator,  if  there  is  any  defect,  before  the  electrode 
touches  the  water.  This  even  includes  the  security  circuit 
itself. 

LIMITATIONS 

The  limitations  for  electrotaxis  and  elect  ronarcosis  are 
set  by  one  physical  law,  expressed  by  the  following 
formula: 


w 
w 

r    - 
Vb 

I, 

if  r 


/     r2.Vb.4:r    \-       Z 

.R         . 

\      L  .  R  /          2 

Wattage 

range  in  m. 

body  voltage  of  each 

species  of  fish  in  V. 

length  of  fish  in  m. 

10  m. 


C    .  10" 


R 
7 
C 


conductivity  of  *ater  in 

«.-m. 

resistance  of  water  in  ft 

pulses  per  sec. 

capacitor  in  \L\: 


10.000 :  .  Vb-.  16-2,  R-      7 

W  -        ""..  .          .  c  .    10"G 

L2.K2  2 

if  r        20  m. 

160,000    .  Vb2.  If*::2.  R2    7 

W         —    .__.(_'.    10~6 

L2  .  K2  2 

This  means  that  doubling  the  range  demands  16-fold 
power.  Experience  has  shown  that  powers  beyond  200 
kw.  are  not  economic.  In  the  author's  opinion,  appliances 
will  be  used  with  a  power  of  less  than  100  kw.,  even  less 
than  50  kw.,  depending  on  the  purpose.  According 
to  a  rough  estimate,  working  range  diameters  of  more 
than  40  m.  for  smaller  fish  (herring  size)  and  50  m.  for 
bigger  fish  (e.g.  1  to  2  m.  tunas)  cannot  be  obtained 
economically  by  present  technical  means. 

Electric  fences,  some  miles  long,  may  be  constructed 
if  so  required  and  their  effective  range  may  reach  from 
100  to  150  m.  Their  limitations  depend  also  on  economic 
considerations  as  well  as  on  the  actual  fishing  conditions. 


[585] 


ELECTRO-FISHING  IN  LAKE  HULEH 

by 

O.  H.  OREN  and  Z.  FRIED 

Sea  Fisheries  Research  Station,  Haifa,  Israel 


Abstract 

Under  the  particular  fishing  conditions  of  Lake  Huleh,  i.e.  very  shallow  water,  narrow  channels,  and  dense  reed  vegetation,  electro- 
fishing  has  proved  to  be  the  only  successful  method  for  catching  the  catfish,  Clarias  lazcra.  The  amount  of  catch  could  be  increased  about 
six  times,  raising  the  percentage  of  this  species  in  the  total  yield  of  Lake  Huleh  from  approximately  8  to  36  per  cent.  The  electric  equipment, 
supplying  7  -  5  kw.,  500  V.  direct  current,  is  installed  in  a  specially  designed  boat.  The  catching  electrode  (anode)  is  combined  with  the  catcher 
net  and  operated  in  the  usual  way  by  a  wooden  stick  of  1  -5  m.  length. 


Resume 


La  peche  a  1'electricite  dans  ie  lac  Huleh 


Dans  les  conditions  particulieres  de  la  peche  dans  Ie  lac  Huleh,  c'est-a-dire,  des  eaux  un  neii  profondcs,  des  chenaux  etroits  et  une 
dense  vegetation  de  roseaux,  la  pdche  a  I'electricitd  s'est  montree  la  seule  mdthode  eflicacc  pour  pecher  Ie  poisson-chat  Clarias  lazera.  Le 
volume  des  captures  pourrait  £tre  augmente  d'environ  six  fois,  en  augmentant  Ie  pourccntage  de  cette  espece  dans  Ie  rendemcnt  total  du  lac 
Huleh  d'environ  8  a  36  pour  cent.  L'6quipement  glectrique,  ctebitant  7,5  kw  sous  500  V  en  courant  continu,  est  install*  dans  un  bateau 
special.  L 'electrode  de  capture  (anode)  est  combines  avec  le  filet  de  capture  et  est  manoeuvres  de  la  facon  habitue  lie  par  unc  pcrche  de  bois 
de  1,5  m  de  long. 


Extracto 


La  electropcsca  en  el  lago  Huleh 


Dados  las  condiciones  especiales  de  la  pesca  en  el  Lago  Huleh,  es  decir,  aguas  muy  poco  profundas,  canales  estrechos  y  densa  vegcta- 
ci6n  acuatica,  la  pesca  con  electricidad  ha  demostrado  scr  el  unico  m£todo  eficaz  para  la  captura  del  bagre  Clarias  lazera.  El  volumen  de 
las  capturas  podria  incrementarse  alrededor  de  seis  veccs,  si  el  porcentage  de  esta  especie  en  el  rendimiento  total  del  lago  Huleh  se  aumentase 
de  8  a  36  por  ciento.  El  equipo  clcctrico,  quc  suministra  7,5  kw  a  500  V.,  corriente  continua,  esta  instalado  en  una  embarcaci6n  especial. 
El  electrode  de  captura  (anodo)  esta  comb  in  ado  con  la  red  de  captura  y  se  maneja  de  la  manera  habitual,  por  medio  de  una  vara  de  madera 
de  1,5  m  de  longitud. 


GENERAL 

LAKE  HULEH  is  the  smaller  of  the  two  natural  fresh 
water  lakes  in  Israel.  Before  the  big  drainage 
project  was  started,  its  main  surface  area  was  about 
13  sq.  km.,  with  an  additional  area  of  water  covered  by 
swamps  and  marshes  of  considerable  size.  Its  maximum 
depth  was  about  3  m.  but  a  large  area  along  the  shores 
was  much  shallower  and  covered  with  a  dense  vegetation 
of  papyrus  (Cyperus  papyrus),  cane(Phragmites  communis), 
water  lilies  (Nymphaea  alba)  and  Nuphar  luteum, 
Polygonum,  etc.,  being  a  serious  obstacle  for  fishery2. 
The  fish  species  of  conventional  commercial  importance 
were  mainly  Tristramella  simonis  a  cichlid,  carp 
(Cyprinus  carpio).  Among  the  many  other  species, 
Tilapia  galilea,  T.  zilli,  Barbus  longiceps,  B.  cams, 
Varicorhinus  damascinus  and  Acanthobrama  lissneri, 
should  be  mentioned  as  of  limited  commercial  import- 
ance. All  these  species  could  be  caught  by  conventional 
methods  and  gear,  such  as  trammel  nets,  gillnets,  beach 
seines,  and  traps4. 

Only  another  species,  the  catfish,  Clarias  lazera,  which 
was  well  known  to  be  abundant,  easily  evaded  these  types 


of  gear  by  keeping  in  the  vegetation,  digging  themselves 
into  the  mud,  or  by  simply  avoiding  the  gear.  Since  no 
suitable  method  could  be  found  locally,  one  of  the  leading 
fishermen  of  the  area  was  sent  on  a  study  tour  to  Europe, 
where  he %  found  that  the  electro-fishing  methods  used 
there  would  probably  be  the  solution  for  his  particular 
problem. 

The  introduction  of  electro-fishing  in  Lake  Huleh 
immediately  had  the  expected  result.  The  catches  of 
Clarias  lazera  increased  considerably  from  about  8 
tons/year  (1949-1950)  over  about  29  tons/year  (1950- 
1951),  to  a  preliminary  maximum  of  52  tons/year 
(1954-1955),  with  the  respective  percentages  of  the  total 
catch  from  Lake  Huleh  of  8  per  cent.,  21  per  cent,  and 
36  per  cent.  With  the  progress  of  the  drainage  project, 
the  area  of  Lake  Huleh  will  be  considerably  reduced, 
only  a  small  part  of  it  being  preserved  as  a  Nature 
Reserve,  so  that  future  data  will  no  longer  be  comparable 
for  the  present  purpose1. 

The  electric  fishing  gear  which  led  to  this  remarkable 
success  was  mainly  developed  locally,  on  information 


[586] 


ELECTRO-FISHING    IN     LAKE     HULEH 


ff.  /.     The  electro-fishing  unit  as  developed  Jor  Lake  Huleh. 


obtained  from  various  European  countries.  Since  it 
represents  a  combination  of  boat  and  electric  equipment 
particularly  designed  for  shallow  canals  and  reed- 
covered  areas,  the  following  description  might  be  of  some 
general  interest1'1. 

BOAT   AND   ELECTRIC   EQUIPMENT 

The  boat  is  of  a  special  design,  constructed  to  suit  the 
unusual  requirements  of  the  lake,  and  the  various 
bottom  conditions.  It  is  built  of  a  light  material  (mainly 
of  laminated  wood)  and  has  a  flat  bottom  and  square 
bow.  Its  dimensions  are:  5  m.  overall  length,  1-5  m. 
wide,  with  a  draft  of  0-2  m.  in  the  stern.  The  boat  lines 
are  such  that  the  bow  is  far  above  the  waterline,  which 
gives  it  an  unusual  quality  of  being  able  to  slide  on  the 
mud  far  into  the  drainage  channels,  the  tight  corners  of 
the  approaches,  and  in  between  the  vegetation.  This 
does  not  interfere  with  the  easy  manoeuvrability  and 
handling  of  the  boat.  Even  so,  it  has  a  capacity  of 
carrying  500  kg.  of  fish  without  losing  these  qualities. 
The  boat  is  divided  by  a  bulkhead  into  two  main  com- 
partments for  the  catch,  and  the  electric  generator  unit 
stands  on  a  wooden  foundation  in  the  stern  (fig.  1 ). 

The  boat  is  driven  by  a  5-5  h.p.  outboard  motor, 
fixed  at  her  transome  stern.  There  is  an  elevated  seat 
providing  the  fishermen  with  maximum  visibility  and 
easy  steering.  Since  a  trip  sometimes  takes  a  full  day, 


from  very  early  morning  until  evening,  the  boat  is 
equipped  with  a  small  locker,  containing  kitchen  utensils, 
for  cooking  and  preparing  light  meals.  A  quantity  of 
spare  fuel  and  lubricants  for  both  engines  is  provided. 
The  forward  part  of  the  boat  serves  as  storage  space 
for  the  catch,  leaving  an  elevated  space  for  the  fisher- 
man who  operates  the  cathode  from  the  tip  of  the  bow. 

The  generator  is  a  sturdy  make,  supplying  7-5  kw., 
5(X)  V.,  direct  current,  at  1500  r.p.m.  It  is  driven  by  a 
16  h.p.  air-cooled  petrol  engine.  The  unit  is  arranged 
in  such  a  way  that  the  stability  of  the  boat  is  not  affected. 
The  electric  unit  is  spray-water  tight,  and  is  coupled  to 
the  engine  with  a  flexible  type  flange-coupling.  To 
reduce  the  vibration  of  the  boat,  the  unit  is  bedded 
on  rubber  mats. 

The  negative  pole  (the  cathode)  is  attached  to  the 
bottom  of  the  boat  to  an  aluminium  plate,  while  the 
positive  pole  (the  anode)  forms  the  metal  ring  (0*5  m.  0) 
of  the  catcher  net  which  is  attached  to  a  1  -5  m.  long 
wooden  stick.  The  anode  is  connected  to  the  generator 
by  a  5  m.  long  rubber  insulated  cable. 

OPERATION 

The  unit  is  operated  by  two  fishermen,  one  of  whom  acts 
as  the  mechanic  and  the  helmsman  of  the  boat,  while 
the  other  is  in  the  bow  and  handles  the  catcher.  While 
the  boat  moves  along  the  bushes  and  the  growth, 


1587] 


MODERN     FISHING     GEAR     OF    THE    WORLD 


Fig.  2.         Small  fishes  being  attracted  experimentally  by  electrotaxis  from  out  of  their  hiding  places  between  stones. 

already  stunned. 


Some  are 


the  fisherman  at  the  bow  has  to  follow  any  movement 
appearing  on  the  surface  and  try  to  detect  the  direction 
of  the  fish.  Since  the  water  is  shallow  and  muddy,  this 
is  sometimes  not  very  easy,  and  only  a  practised  eye  can 
recognize  the  underwater  life.  Some  fishermen  can  even 
tell  the  type  and  size  of  fish.  When  the  catcher  is  sub- 
merged and  the  electric  circuit  is  closed  the  fish  show 
electrotactic  reaction  and  really  jump  into  the  field  of 
the  anode  (the  catcher)  (fig.  2).  While  Clarias  lazera 
reacts  very  strongly,  other  species  are  less  affected,  and 
experience  and  quick  action  is  needed  then  to  manoeuvre 
the  fish  into  the  catcher.  Since  the  fish  are  aware  of  the 
approaching  boat  they  try  to  hide  in  the  undergrowth.  It 
takes  all  the  skill  and  good  collaboration  between  both 
fishermen  to  find  the  fish  and  obtain  good  catches.  Some- 
times the  fish  are  found  in  such  quantity  in  one  spot  that 
the  catcher  had  to  be  used  several  times.  In  this  case  par- 
ticularly quick  action  is  needed  because  the  fish  may  have 
recovered  and  try  to  escape,  before  the  withdrawn 
catcher  is  submerged  again.  This  is  especially  true  for 


smaller  fish,  because  they  are  less  affected  by  the  electric 
current.  Fish  which  for  some  reason  are  not  collected 
are  stunned  only  for  a  few  minutes,  recover  quickly  and 
the  shock  does  them  no  harm.  The  effective  radius 
of  this  gear  is  about  2  to  2  -5  m.  from  the  catcher.  Within 
this  radius  the  fisherman  can  sec  that  the  hiding  fish 
react  a  little  to  the  electric  shock.  After  having  detected 
them  by  this  minor  reaction,  the  fisherman  moves  closer 
to  the  spot,  re-throws  the  catcher  into  the  water,  and 
catches  the  fish  which,  this  time,  have  received  the  full 
power  of  the  electric  current. 

REFERENCES 

1  Division  of  Fisheries.   Fisheries  of  Israel,  Statistical  Bulletin 
Hakiryah,  (in  Hebrew).    1948-1957. 

2  Karmon,  Y.  The  northern  Huleh  Valley,  its  natural  and  cultura 
landscape.  The  Magness  Press,  The  Hebrew  University,  Jerusalem, 
pp.  108,  maps  and  figures,  Jerusalem  (in  Hebrew).     1956. 

3  Lcwin,  S.  Electric  fishing  in  Lake  Huleh,  fishermen  s  Bulletin, 
2  (13):  12-13.  (in  Hebrew).    1957. 

4  Steinitz,  H.   The  freshwater  fishes  of  Palestine.    An  annotated 
list.    Bull.  Res.  Council  oj  Israel,  3  (3)  :  207-227.    1953. 


[588] 


DANGERS  AND  PRECAUTIONS  IN  THE  ELECTRICAL  FISHERY 

by 

A.  HOSL 

Electro-Beratung  Bayern,  Munich,  Germany 

Abstract 

In  spite  of  opinions  to  the  contrary,  the  current  used  in  electrical  fishing  can  be  a  danger  to  human  life,  depending  on  the  amperage, 
type  of  current,  etc.  The  maximum  contact  voltage  permissible  is  about  24  V.  and  direct  current  is  less  dangerous  than  alternating  or  pulsating 
current.  Also,  high  frequency  currents  are  not  dangerous.  The  paper  deals  with  the  changes  in  human  physiology  associated  with  increasing 
amperages  and  the  author  suggests  that  danger  can  be  reduced  by  taking  suitable  precautions  and  by  employing  properly  trained  fishermen. 


Resume 


Les  dangers  presentcs  par  la  peche  clectrique  et  les  precautions  a  prcndre 


En  depit  des  opinions  contraircs,  le  ecu  rant  utilise  dans  la  peche  electrique  peut  presenter  un  danger  pour  la  vie  humainc  scion 
Tintensitc,  la  sorfe  de  courant,  etc.  Le  voltage  maximum  avec  lequel  on  peut  entrer  en  contact  csl  d'cnviron  24  volts  ct  le  courant  continu 
est  moins  dangcreux  que  le  courant  altcrnatif  ou  pulse.  Les  cou rants  a  haute  frequence  aussi  ne  sont  pas  dangereux.  L'auteur  traite  des 
changements  dans  la  physiologic  humaine  associes  a  des  intensites  croissantes  et  il  pense  que  le  danger  peut  etre  diminud  en  prcnant  des  pre- 
cautions convenablcs  et  en  employant  des  pccheurs  correctcmcnt  en  trainees. 


Peligros  y  precauciones  en  la  pesca  con  electricidad 
Rxtracto 

A  pesar  de  diversas  opiniones  contrarias,  la  corriente  usada  en  la  pesca  con  electricidad  pucdc  ser  pcligrosa  para  la  vida  humana 
segun  el  numero  de  amperios,  tipo  dc  cncrgia,  etc.,  utili/ados.  La  intensidad  electrica  maxima  permitida  no  debe  ser  superior  a  24  voltios; 
la  corriente  conlimia  es  menos  peligrosa  que  la  alterna  o  pulsatoria  y  las  de  alta  frecuencia  no  ofrecen  tantos  riesgos.  El  trabajo  tambien  trata 
de  los  cam  bios  tisio!6gicos  en  el  cuerpo  humano  asociados  con  el  aumento  del  numero  dc  amperios,  sugiricndo  el  autor  que  el  peligro  puede 
reducirse  tomando  las  precauciones  adccuadas  o  utili/ando  personal  propiamente  adiestrado. 


SIGNIFICANCE  OF  ELECTRICAL  ACCIDENTS 

THE  human   body  is  adversely  affected  by  electric 
current   which  passes  through    it.     The   conse- 
quences of  this  effect  depend  on  the  amperage, 
the  kind  of  current  (direct  current,  alternating  current, 
high  frequency  current,  pulsating  current),  the  time  of 
exposure  and  the  way  the  current  passes  through  the 
body,  as  well  as  the  individual  resistance. 

According   to    Koeppen1   the   following  degrees   of 
amperage  are  to  be  distinguished: 


Degree  1         0- 1  to  1  mA.:  slight  contrac- 
up  to  25  m  A.  tions  of  the  muscles  in  the 
fingers. 

0-8  to  2 -4m A.:  concussion  of 
the  nerves  in  the  fingers  up  to 
the  underarm. 

9  to  15  mA.:  releasing  the 
contact  still  possible 


increase  of  blood 
pressure  depending 
on  the  amperage,  no 
influence  on  the 
beating  of  the  heart 
and  the  central 
nervous  system. 


19  to  22  mA.:  releasing  the 
contact  impossible  without  help 

Degree  If        28  to  50mA.:  still  bearable         irregular  beating  of 
25  to  80  mA.  amperage,  without  uncon-         the  heart,  increase  of 
sciousness  setting  in  blood   pressure,   re- 

versible standstill  of 
the  heart. 


Degree  III  fluttering  of  the 

more  than  ventricles  of  the 

80  mA.  heart. 

Degree  IV       Pulmonic  palsy  increase  of  blood 

3  to  8  A.  pressure,  standstill  of 

the  heart,  irregular 
rhythms. 

The  intensity  (i)  of  current  passing  through  the  human 
body  depends  on  the  voltage  existing  at  the  moment  of 
contact  (Ue)  hereafter  referred  to  as  contact-voltage 
and  the  electric  resistance  (Ric)  of  the  body.  According 
to  Ohm's  law,  the  formula  is: 
UB 


The  contact-voltage  is  the  amount  of  voltage  which  can 
be  endured  by  the  human  body.  It  can  be  measured, 
for  instance,  between  hand  and  foot  or  between  left 
and  right  hand  respectively,  according  to  the  parts 
of  the  body  where  the  current  enters  and  leaves.  Water 
or  earth  as  well  can  be  current  conductors.  The  electric 
resistance  of  the  body  fluctuates  considerably2.  It 
consists  of  the  resistance  of  the  skin  and  the  internal 
resistance  of  the  body  which  depend  greatly  on  tempera- 
ture, voltage,  period  of  influence,  the  points  of  contact 


[5891 


MODERN     FISHING    GEAR    OF    THE    WORLD 


on  the  body,  pressure  of  the  contacts  on  the  body, 
condition  of  the  skin  and  various  physiological  and 
psychological  factors.  Wet  skin,  which  is  often  a 
condition  with  fishermen,  represents  the  most  unfavour- 
able case,  as  the  total  resistance  then  amounts  to  the 
mere  internal  resistance  of  the  body,  which  has  been 
proved  to  be  about  800  12.  Fifty  mA.  is  the  dangerous 
maximum  amperage  which  means  that  the  dangerous 
contact-voltage  is  800  12  x  0*05  A.  -40  V.  If  this 
voltage  is  exceeded,  the  lives  of  fishermen  will  be  in 
danger.  To  be  on  the  safe  side,  the  maximum  contact- 
voltage  should  not  exceed  24  V.  Lobl3  and  the  Verband 
Deutscher  Elektrotechniker4  (Association  of  German 
Electro-Engineers)  both  recommended  this. 

Experience  shows  that  direct  current  is  not  so  danger- 
ous as  alternating  or  impulse  current  of  40  to  600  cycles 
and  that  high  frequency  currents  of  millions  of  cycles 
are  not  perilous  (e.g.  diathermic  frequency).  The  effect 
of  Faradic  stimulation  on  the  body  can  still  be  felt  up 
to  500  kc. 

Currents  of  more  than  50  mA.  are  dangerous  to  life, 
if  the  influence  exceeds  0-2  sec.,  whereas  brief  shocks 
apparently  have  no  damaging  effects  on  health.  Finally 
the  manner  in  which  the  current  runs  through  the  body 
is  of  importance.  If  the  current  entered  the  index  finger 
and  left  the  thumb  of  the  same  hand,  only  burns  might 
occur,  but  deadly  fluttering  of  the  ventricles  of  the  heart 
and  paralysis  of  the  breathing  organs  must  be  expected 
if  the  heart  and  the  central  nervous  system  are  in  the 
circuit  and  amperage  and  period  of  influence  are  suffi- 
cient. This  is  the  case  when  the  current,  for  instance, 
runs  from  one  hand  to  the  other,  or  from  hand  to  foot, 
or  from  one  foot  to  the  other.  In  the  latter  case,  the 
amperage  has  to  be  double  the  amount  to  show  the 
same  effects. 

PARTICULAR  DANGERS  OF  ELECTRICAL 
FISHING 

Accidents  are  always  possible  because  of  the  kind  of 
current  and  the  voltage  used  for  fishing.  This  applies 
to  direct  current  as  well  as  to  impulse  current.  Because 
of  the  wet  hands  and  feet,  the  resistance  of  the  skin 
is  especially  small. 

The  following  accident  possibilities  exist: 
Freshwater  fishery  with  two  electrodes  (cathode  and 
anode):  The  worst  case  would  be  if  each  hand  touched 
one  of  the  electrodes  at  the  same  time.  The  human  body 
would  feel  the  total  voltage,  the  heart  would  be  in  the 
circuit  and  a  deadly  effect  would  be  unavoidable  if  the 
current  continued  to  pass  through  the  body  for  some 
time.  Similar  conditions  occur  if,  for  instance,  somebody 
is  standing  barefoot  in  a  metal  boat,  used  as  a  cathode, 
and  touches  the  bare  anode  with  his  hand.  In  this 
case,  the  voltage  would  be  between  hand  and  foot. 

Supposing  a  generator  is  installed  in  a  wooden  boat 
and  one  of  the  electrodes  is  connected  with  the  metal 
casing  of  the  generator.  The  fisherman  touches  this 
casing  with  one  hand  and  the  catching  electrode  with 
the  other  hand  at  the  same  time.  He  would  be  exposed 
to  the  total  voltage.  The  danger  would  be  considerably 
smaller  if  only  one  electrode  is  directly  touched.  The 
main  drop  of  voltage  occurs  in  the  closest  neighbourhood 
of  an  electrode,  underwater  or  in  the  earth,  so  that  a 


distance  of  only  0-5  m.  from  the  electrode  the  potential 
decreases  to  about  £  of  that  existing  at  the  electrode 
itself.  Therefore,  if  someone  fell  into  the  water  from  a 
metal  boat  used  as  cathode,  with  his  feet  0-5  m.  away 
from  the  live  anode  submerged  under  water,  and  clinging 
with  his  hands  to  the  metal  boat,  he  would  not  suffer 
the  total  voltage  of,  for  instance,  220  V.  but 
110V. 

110  V.   h 128  V.    His  life  would  still  be  in 

6 

danger,  but  less  so  than  if  he  made  direct  contact  with 
both  electrodes. 

Supposing  that  both  electrodes  are  under  water,  with 
the  voltage  between  them  amounting  to  300  V.,  if 
somebody  in  the  boat  dipped  one  of  his  hands  into  the 
water  at  a  distance  of  0-5  m.  from  one  of  the  electrodes 
and  his  second  hand  at  a  distance  of  1  m.  from  the  same 
electrode,  the  potential  difference  between  both  hands 
in  the  water  would  amount  to  about  15  V.  which  would 
be  absolutely  safe. 

This  distribution  of  voltage  is  independent  of  the 
conductivity  of  the  water  and,  to  a  large  extent,  of  the 
distance  between  the  two  electrodes. 


PRECAUTIONS 

It  is  obvious  that  the  greatest  dangers  occur  when  one 
or  both  electrodes  are  unprotected  so  that  they  can  be 
touched  with  the  hand  or  the  bare  foot.  In  order  to 
prevent  accidents,  only  skilled  electro-fishermen  should 
be  employed.  Fishing  should  be  directed  by  a  respon- 
sible expert,  trained  in  a  special  course,  who  possesses 
authenticated  proof  of  his  qualification  and  permission 
from  the  responsible  authorities  to  operate  the  gear. 
He  should  be  assisted  by  at  least  one  instructed  person. 
People  not  concerned  with  the  fishing  should  be  kept 
away  from  the  gear.  In  Germany,  the  electro-fisherman 
and  his  assistant  have  to  know  the  VDE  0134  "Anleitung 
zur  ersten  Hilfe  bei  Unfallen"5  (Instructions  for  First  Aid 
in  case  of  accidents,  edited  by  the  Association  of  German 
Electro-Engineers)  and  must  be  familiar  with  at  least 
one  life-restoring  method.  The  fishing  gear  must  be 
prepared  carefully.  The  main  electric  cables  especially 
have  to  be  thoroughly  examined  with  regard  to  external 
damage.  Above  all,  the  insulation  must  be  without 
defects.  Special  instructions  for  constructing  electrical 
fishing  gear  and  for  its  operation,  are  being  prepared  in 
Germany,  giving  all  necessary  details. 

Even  the  best  preparations  cannot  prevent  accidents 
which  are  caused  by  defectively  constructed  gear. 
Therefore,  the  use  of  self-made  equipment  must  be 
forbidden,  and,  in  future,  the  construction  of  electrical 
fishing  gear  will  have  to  comply  with  special  instructions 
of  the  VDE,  which  are  being  prepared.  The  operating 
handles  for  instance,  will  have  to  be  made  of  insulating 
material.  All  insulation  will  have  to  comply  with 
VDE  0100,  34.  The  coating  of  the  movable  cables 
leading  to  the  electrodes  will  have  to  be  made  of  insulated 
material  of  especially  high  resistance  against  abrasion 
and  buckling.  An  all-pole  safety-switch,  interrupting 
the  circuit  in  case  of  defective  voltage  or  body  contact, 
will  be  prescribed.  It  will  have  to  be  installed  as  near  as 
possible  to  the  primary  source  of  power,  to  increase 


[590] 


DANGERS    AND    PRECAUTIONS 

working  reliability  as  well  as  safety  and  protection 
against  incidental  contacts. 

Fuses  will  be  provided  in  the  line  to  cut  out  excessive 
current.  The  safety-line-system,  according  to  VDE 
0100, 3.4  is  applicable  to  fishing  equipment  on  metal  boats. 

In  many  cases,  a  Totmann-switch  at  the  catching- 
electrode,  which  is  handled  by  the  electro-fisherman, 
is  recommendable.  This  guarantees  that  the  electrode 
cannot  be  switched  on  before  all  people  concerned  are 
ready  for  the  operation.  In  case  of  failures,  the  circuit 
can  be  interrupted  immediately. 


IN    THE    ELECTRICAL    FISHERY 
REFERENCES 

1  Koeppcn,  S.   Erkrankungcn  der  inneren  Organe  und    des 
Ncrvcnsystems    nach    clcktrischen     Unfallen,     Springcr-Verlag, 
Berlin-Gottingcn-Heidelberg.    1953. 

2  Freiberger,  H.    Der  eleklrischc  Widcrstand  dcs  menschlichen 
Kbrpers  gegen  technischcn  Gleich-  und  Wechsclstrom. 

3  Lobl. O.  Erdung, Nullung und Schutzschaltung, S.ll ;  Springer- 
Verlag,  Berlin.     1933. 

4  VDE  0100.   Vorschriften  nebst   Aiisfuhrungsregeln  fiir  die 
Errichtung  von  Stromanlagen  mil  Betriebsspannungen  unter  1000 
V.,  §  15,  e.;  VDE-Verlag,  Berlin-Charlottenburg.    1957. 

5  VDE  0134.  Anleitung  zur  ersten  Hilfe  bei  Unfallen;  VDE- 
Verlag,   Berlin-Charlottenburg.      1955. 


Chain  of  electrodes  of  an  electrical  fish  barrier  across  a  120  m.  wide  canal  near  the  river  Maas  (Netherlands). 

^nuin  vj  tiv»,  j  ,  DU^«^.    Trtcfitut  fiir  k  iicl«-n  iinH   Rinm»nfl«('herei 


Photo:  Institut  fiir  Kustcn  und  Binncnfischerei.  Hamburg. 


[591] 


DISCUSSION   ON   ELECTRICAL   FISHING 


Prof.  P.  F.  Meyer-Waarden  (Germany)  Rapporteur:  The 
main  points  regarding  electrical  fishing  which  should  be 
considered  arc:  what  is  the  present  situation  of  electrical 
fishing?  and  to  what  extent  can  the  presently  used  electrical 
fishing  method  be  applied  for  catching  fish  and  in  other  fields 
of  fishery?  Very  special  problems  of  basic  research,  such  as 
electro-physiology,  are  not  for  discussion  here. 

The  technical  development  of  electrical  fishing  gear  in  the 
German  Federal  Republic  is  illustrated  by  the  products  of 
various  German  firms.  Sabo,  Dicringhausen,  making  battery 
and  gasoline  engine  generators  (0-4  to  4  kw.),  of  various 
sizes,  for  fishing  in  brooks  and  rivers.  These  have  already 
been  in  use  for  some  years.  Franz  Ploger,  Hamburg,  make  a 
combined  gasoline  generator  and  impulse  gear  for  use  under 
unfavourable  conditions  of  conductivity  in  freshwater 
(Model  Hamburg  II),  and  frightening  gear  for  fencing  rivers, 
for  guiding  fish,  and  for  frightening  fish  away  from  larger 
freshwater  areas.  The  Atlas- Wcrke,  Bremen,  have  an  electrical 
tuna  line,  while  DcthloflF-tilectronic,  Hamburg,  make  impulse 
gear  for  stunning  and  catching  fish  schools,  as  well  as  electrical 
fencing  and  guiding  gear  for  use  in  seawater,  and  electrical 
whaling  gear. 

It  was  only  after  the  war  that  electrical  fishing  gained 
interest,  principally  because  of  the  introduction  of  impulse- 
current.  Until  this  event,  electrical  fishing  gear  only  operated 
with  direct  or  alternating  current,  suitable  only  for  freshwater 
and  with  rather  limited  working  ranges  even  under  parti- 
cularly favourable  conditions  of  conductivity.  The  savings 
in  power  obtained  by  impulse-current  made  it  possible  to 
extend  the  working  range  of  the  gear.  Waters  with  unfavour- 
able conditions  of  conductivity,  such  as  polluted  freshwatcrs, 
and  even  slightly  salty  inland  waters,  became  workable. 
Finally,  the  technological  basis  was  provided  for  stimulating 
marine  animals,  which  had  been  immune  because  of  the 
much  greater  conductivity  of  seawater  as  compared  with 
freshwater. 

Denzer  first  drew  attention  to  this  type  of  current  long  used 
in  electrotherapy,  and  Kreutzer  designed  the  first  impulse 
gear  for  electrical  fishing.  Within  two  years  (1948  to  1950)  the 
working  group  Kreutzer/Pcglow,  in  co-operation  with  the 
Siemens-Schuckert  Werke,  the  German  Federal  Research 
Institute  of  Fisheries,  assisted  by  the  German  fishing  industry 
and  the  A. E.G.,  succeeded  in  solving  the  basic  problems  of 
electro-stimulating  fish  in  the  sea. 

An  experimental  generator  was  developed  with  a  con- 
verted average  output  of  180  to  200  kw.  which  was  able 
to  produce  current-pulses  of  more  than  10,000  amp.  By 
means  of  this  gear,  herring  of  20  cm.  in  length  could  be 
affected  up  to  a  radius  of  10  m.  around  the  anode,  cod  of 
40  cm.  in  length  up  to  a  radius  of  15  m.,  and  cod  90  cm.  in 
length  up  to  a  radius  of  25  m.  Moreover,  Kreutzer  had 
succeeded  in  computing  the  range  of  efficiency  of  the  electrical 
current  depending  on  the  size  of  the  fish.  Furthermore,  a 
trawl  net  for  electrical  fishing  was  designed,  and  Suberkrub 


developed  a  hydrofoil  electrode  which  could  be  towed  in 
front  of  the  net-opening.  These  experiments  were  the  basis 
of  all  future  developments  and  experiments  in  the  electrical 
sea  fishery.  About  the  same  time,  but  independent  of  the 
German  investigations,  the  Japanese,  and  later  the  Americans, 
the  British  and  the  New  Zealanders,  began  to  use  impulse- 
current  for  electrical  fishing. 

The  use  of  impulse-current  opened  many  new  aspects  for 
the  technical  development  of  electrical  fishing,  but  some 
extremely  complicated  and  intricate  conditions  exist  which 
still  require  investigation.  The  most  important  results  of 
recent  research  work  have  shown  that: 

1.  Compared  with  direct  and  alternating  current,  impulse 
current  has  the  greatest  neuro-physical  effect,  but  the 
least  damaging  after-effect  on  fish. 

2.  The  fishing  effect  produced   by   the  impulse-current 
depends   on: 

(i)     the  shape  of  the  current  impulse; 
(ii)    the  average  electrical  flow  of  current; 
(iii)  the  impulse  rate. 

3.  The  optimal  impulse  shape  for  fishing  has  a  steep 
increase  and  a  gradual  decrease.    This  impulse  form 
causes  a  definite  anodic  reaction  in  the  fish,  and  can 
easily   be  produced   by   condenser   discharges.      For 
frightening  fish,    the  impulse  should  have  the  shape 
of  a  quarter  sinus  curve. 

4.  The  duration  of  the  single  impulse  should  not  be  below 
a  certain  minimum.    For  commercial  fishing  it  should, 
furthermore,   correspond   to  the  so-called  period  of 
efficiency,  e.g.  to  that  period  which  guarantees  the 
stimulative  effect  needed   at   the  lowest  expense  of 
energy.  The  optimum  length  of  the  impulse  depends  on 
the  species  and  on  the  size  of  fish.   It  is  usually  below 
the  half-time  value  of  1  m  sec. 

5.  There  is  an  optimal  impulse  rate  for  each  species  of 
fish,  at  which  the  desired  effect  is  attained  with  a 
minimum  effort.      The  larger  fish  require  a  lower 
impulse   rate  for   reaching  elcctrotaxis  and  electro- 
narcosis. 

6.  The  optimum  impulse  rate  ranges  from  7  to  20  for 
tuna,  20  to  25  for  medium-sized  cod,  45  to  50  for  carp, 
and  60  to  65  for  trout.    The  impulse  rate  is  chosen 
according  to  the  effect  wanted,  either  anodic  attraction 
or  killing.     It  has  been  found,  for  instance,  that  an 
active  fish  with  highly  intensive  metabolism  (trout) 
needs  a  lower  impulse  rate  for  electrotaxis  than  a  less 
active  fish  with  less  intensive  metabolism  (carp),  but 
electronarcosis  occurs  more  quickly  with  carp  than 
with  trout. 

7.  Biological  factors  such  as:  (i)  the  biological  character 
of  the  fish;  (ii)  the  particular  physical  condition  of  the 
fish;  (iii)  the  surrounding  conditions  are  also  decisive 
for  the  electrical  effect. 


[592] 


DISCUSSION  — ELECTRICAL    FISHING 


For  electro-stimulation,  the  stage  of  maturity  of  fish  is  also 
important.  The  eel,  for  instance,  is  rather  lethargic  in  maturity, 
and  is  then  considerably  more  resistant  to  electric  current  than 
it  is  when  younger.  Moreover,  exhausted  or  sick  fish,  or  those 
which  have  often  been  subjected  to  electric  current,  may 
be  more  resistant  than  healthy  fish. 

The  degree  to  which  electric  current  affects  fish  depends 
also  on  physical  and  chemical  environmental  factors,  as,  for 
instance,  the  conductivity  of  the  water  in  which  the  fish  live. 
Such  conductivity  can  vary  from  distilled  rainwater  with  1 
million  ohm/m.  to  sea  water  with  0-01  ohm/m.  Average 
conductivity  for  European  freshwater  lies  between  10  and 
10,000  ohm/m. 

Waters  with  medium  conductivity  are  best  suited  to  electrical 
fishing  as  they  possess  the  most  favourable  correlation  be- 
tween the  energy  produced  by  the  gear  and  the  voltage.  The 
conductivity  of  seawater  being  500  times  greater  than  that  of 
freshwater,  electrical  influence  on  the  fish  has  only  become 
possible  by  means  of  pulsating  current. 

Tester  found  that  tuna  react  definitely  to  pulsating  current, 
which  was  produced  by  means  of  condenser  discharges,  by 
moving  towards  the  anode.  An  impulse  rate  of  20/sec.  was 
found  effective  for  yellow-fin  tuna  50  cm.  in  length.  Dethloff 
also  discusses  electro  stimulation  of  tuna  by  means  of  pulsating 
current,  but  without  mentioning  impulse  rates. 

Thus  the  effect  of  pulsating  currents  on  fish  depends  on 
many  factors,  the  correlation  of  which  is  frequently  very 
complicated  and  sometimes  not  quite  recognisable.  These 
complicated  conditions  arc  responsible  for  many  failures 
experienced  not  only  in  electrical  fishing  research  but  also  in 
practical  commercial  application. 

Anodic  (attraction)  effects,  and,  more  recently,  frightening 
effects,  and  killing  effects,  arc  used  in  electrical  fishing. 

The  catching  effect  was  formerly  the  main  one  in  use,  but 
it  was  proved  that  in  the  marginal  zones  of  the  electric  field  a 
strong  frightening  effect  occurs,  which  originally  was  un- 
desirable. This  frightening  effect,  however,  is  now  used, 
for  example,  in  fencing  hydraulic  structures. 

The  killing  effect  was  also  formerly  avoided,  but  is  now  used 
for  special  purposes  as,  for  instance,  in  electrifying  the  tuna 
lines  used  in  the  North  Sea.  This  reduces  considerably 
the  losses  of  hooked  tuna.  The  killing  effect  can  also  be 
used  for  controlling  vermin,  such  as  Chinese  crabs.  Recently 
it  has  been  suggested  that  the  killing  effect  could  be  used  to 
maintain  and  even  improve  the  quality  of  fish.  It  is  already 
possible,  to  kill  sardines  caught  with  the  ringnet  within  2  to 
3  sec.,  and  avoiding  the  loss  of  scales  caused  by  struggling 
in  the  net,  with  a  resultant  decrease  in  the  quality  and  price 
of  the  sardines  landed.  The  Japanese  and  German  investi- 
gations also  indicate  that  the  killing  effect  appears  to  improve 
the  quality  of  the  fish  flesh. 

Many  experiments  have  already  been  made  in  this  field  in 
Japan  and  Kuroki  gives  a  detailed  report.  When  testing  the 
degree  of  freshness  of  electrically  and  normally  killed  fish, 
the  contents  of  lactic  acid,  glycogen,  organic  acid  and  basic 
nitrogen,  as  well  as  the  firmness  and  the  pH  value,  were 
investigated.  The  result  showed  that  electrically  killed  fish 
possessed  more  favourable  values  and  thus  have  a  better 
keeping  quality.  According  to  the  Japanese,  however,  when 
rigor  mortis  is  over,  the  decomposition  of  the  flesh  of  electric- 
ally killed  fish  proceeds  more  quickly  than  that  of  normally 
killed  fish.  Electrically  killed  fish,  therefore,  must  immediately 
be  packed  in  ice. 


In  Germany,  only  organoleptic  tests  have  been  carried  out. 
The  German  Federal  Research  Institute  of  Fisheries  also 
found  that  electrically  killed  fish  usually  have  a  better  storing 
quality  than  other  fish.  Ludorff  and  Kreuzer  in  "Der  Fisch 
vom  Fang  zum  Verbrauch"  (The  Fish  from  Catch  to  Consumer) 
give  the  following  description:  *An  exhausted  fish,  chased 
for  some  time  or  having  struggled  at  the  line  or  in  the  net, 
would  very  quickly  use  up  its  glycogen  content.  That  would 
block  the  contractive  mechanism  in  the  muscle  fibres  and 
consequently  shorten  the  period  of  rigor  mortis  which  is 
very  essential  for  maintaining  the  quality.  Moreover,  there 
occurs  a  shortage  of  basic  substances  for  the  development  of 
lactic  acid  and  for  the  simultaneously  required  decrease  in  the 
pH  value.  The  less  distinct  and  the  shorter  the  rigor  mortis, 
and  the  less  the  decrease  in  the  pH  values,  the  more  the  keep- 
ing quality  of  the  fish  flesh  decreases  and  the  earlier  the  rotting 
bacteria  begin  their  destruction.' 

Certainly,  very  thorough  chemical-physiological  investiga- 
tions have  yet  to  be  carried  out  to  ascertain  the  changes  which 
develop  in  the  electrically  killed  fish. 

I  shall  now  return  to  the  electrical  fishing  gear. 

Easily  portable  gear  already  exists  and  has  proved  efficient 
in  freshwater  fishing.  In  the  Federal  Republic  of  Germany 
the  various  types  of  gear  are  used  for  many  purposes. 

Generally  speaking,  there  are  portable  battery  and  gasoline 
generators  of  various  sizes  of  0*4  to  4  kw,  which  can  be 
used  in  small  or  fairly  large  bodies  of  water  of  medium 
conductivity.  Franz  Ploger,  Hamburg,  with  our  assistance, 
designed  a  combined  gear.  It  transforms  the  direct  current  of 
a  gasoline  generator  into  pulsating  current  in  order  to  increase 
the  working  range. 

In  recent  years  types  of  electric  gear  for  frightening  fish 
have  been  developed  in  America,  Japan  and  in  the  Federal 
Republic  of  Germany. 

Fish  barriers  were  usually  operated  with  alternating  current, 
but  since  the  war  pulsating  current  has  been  introduced  in 
Japan  as  well  as  in  America  and  in  Germany.  The  Japanese 
types  of  frightening  gear,  as  stated  in  Kuroki's  paper,  are 
characterized  by  their  high  pulse  rates  of  1  to  20  per  sec.  In 
Germany  the  pulse  rates  used  range  between  20  and  100  per 
sec.  for  catching,  or  between  10  and  70  per  min.  for 
frightening,  depending  on  the  water  conditions  and  the  species 
of  fish. 

The  frightening  effect  can  be  used  for  many  purposes: 

1.  for  fencing  certain  water  areas  to  prevent  the  fish  from 
migrating  or  escaping; 

2.  for  blocking  the  entrances  of  turbines  and  pumps  to 
prevent  the  fish  from  being  damaged  or  killed; 

3.  for  guiding  the  fish  to  ladders,  ways  and  traps. 
Dethloff  also  describes  the  use  of  such  electrical  barriers  for 

guiding  the  fish  into  large  tuna  traps,  the  so-called  tonnaras 
and  almadravas.  Whereas  hitherto  chains  of  electrodes 
have  been  used.  Dethloff  uses  electrically  loaded  cables.  The 
electric  barriers  used  in  freshwater  have  in  many  cases 
proved  their  efficiency. 

There  already  exists  gear  for  killing  fish,  such  as  the 
electrical  tuna  line  already  mentioned. 

A  gear  for  killing  fish  (for  instance,  sardines)  within  the 
direct  radius  of  the  anode  is  manufactured  by  Dethloff- 
Elcctronic,  Hamburg  and  by  Franz  Ploger,  Hamburg. 
Dethloff  has  also  designed  impulse  gear  for  electrical  whaling. 

The  effect  of  electrical  fishing  gear  in  both  freshwater  and 
seawater  is  limited  to  a  relatively  small  area.  The  gear, 


[593] 


oo 


MODERN    FISHING    GEAR    OF    THE    WORLD 


therefore,  can  scarcely  endanger  the  fish  population.  The 
main  reason  for  this  relatively  small  range  is  the  great  loss  of 
energy,  which  occurs  in  freshwater  as  well  as  in  seawater.  It 
it  particularly  great  in  seawater  so  that  satisfactory  results 
may  only  be  achieved  by  means  of  very  great  energy  (100  kw.). 
In  saltwater  fisheries,  with  medium  conductivity  of  the  water 
and  with  fishing  gear  up  to  4  kw.,  the  limit  of  the  electrical 
effect  is  approximately  4  to  5  m.  According  to  Dethloff,  the 
extension  of  a  fishing  range  in  seawater  from  20  m.  to  40  m. 
would  need  approximately  16  times  the  100  kw.  mentioned 
above. 

Gear  with  extended  range  could  also  be  designed  for 
freshwater,  but  one  should  ask  whether  the  catch  would  be  in 
reasonable  relation  to  the  expended  energy.  This  would  no 
doubt  be  the  case  if  especially  valuable  fish  (tuna,  salmon) 
are  concerned,  or  if  fish  schools  are  sufficiently  concentrated. 
But  whether  it  would  be  profitable  in  the  case  of  bulk  fish, 
such  as  menhaden  or  sardines — particularly  in  view  of  the 
difficult  marketing  and  economic  conditions  of  the  fishing 
industry — must  be  left  to  the  future. 

Efforts  are  being  made  in  Germany  to  fit  electrical  fishing 
gear  with  adequate  safety  equipment.  As  Host's  article 
shows,  it  is  essential  to  take  protective  measures,  as  electrical 
fishing  gear  is  dangerous  to  man.  A  Commission  established 
by  the  Vercin  Deutscher  Elektrotechniker  (Association  of 
German  Electro-Engineers)  is  preparing  regulations  for  the 
use  of  electrical  fishing  gear  in  freshwater  and  in  seawater. 

The  manufacturers  have  spared  neither  money  nor  time 
to  design  efficient  gear.  But  various  types  of  electrical  gear 
must  be  thoroughly  tested  in  practice  and,  generally  speaking, 
this  is  outside  the  scope  of  the  manufacturers.  A  way  should 
therefore  be  found  to  test  these  types  of  gear  on  research 
vessels  in  sea  areas  ideally  suited  to  such  operations  (for 
instance,  sufficiently  large,  with  dense  fish  schools).  Only 
then  will  it  be  possible  to  find  out  whether  the  electrical  fishing 
method  can  become  of  essential  importance  to  the  fishing 
industry. 

Mr.  K.  Schefold  (Austria):  Deep  sea  fishing  is,  of  course, 
more  important  than  inland  or  freshwater  fishing,  but 
freshwater  fishing  has  hardly  been  mentioned  although  it  has 
for  some  countries  considerable  importance,  in  Austria  there 
exist  a  number  of  big  lakes  and  many  smaller  ones,  and  here 
the  interest  lies  in  electro-fishing  in  freshwater  and  the 
possibilities  of  applying  electric  fishing  to  small  lakes  and 
ponds.  The  bottom  of  such  ponds  and  lakes  usually  has 
obstructions  which  make  it  nearly  impossible  to  use  trawl  or 
seine  nets,  particularly  for  deep  lake  fish,  such  as  carp.  The 
fish  also  are  apt  to  escape  into  small  depressions  in  the  lake 
bottom.  In  waters  where  it  was  known  that  several  hundreds 
of  carp  existed,  not  one  could  be  caught  with  the  seine  net. 
Seining  in  these  particular  inland  waters  is  quite  ineffective 
and  here  electric  fishing  might  help  to  produce  better  results. 
Repellent  gear  might  be  particularly  suitable,  but  the  lakes 
and  ponds  are  sometimes  as  much  as  70  m.  deep  and  such 
gear  might  not  be  effective.  Much  depends  on  the  behaviour 
of  the  fish  when  coming  under  the  repelling  influence  of  the 
apparatus;  if  they  tend  to  dive  to  deeper  water  then  such  gear 
would  be  useless.  The  total  amount  of  fish  caught  in  these 
inland  waters  is  quite  important.  Freshwater  fishing  is  subject 
to  very  rigid  laws,  the  fish  can  be  caught  only  at  certain  seasons 
and  the  size  is  restricted;  the  use  of  certain  types  of  gear  is 
prohibited  and  subject  to  very  rigid  legislation  for  conservation 
of  stock. 


Mr.  D.  R.  Lenier  (France):  Mr.  Schefold  may  be  assured 
that  electrical  fishing  in  freshwater  is  more  highly  developed 
on  an  international  level  than  in  sea  fishing,  where  it  is  only 
done  sporadically.  There  is  considerable  literature  on 
freshwater  fishing,  and  this  method  is  constantly  used  with 
success  in  lakes  and  rivers.  It  allows  one  to  catch  only  the 
big  fish  leaving  aside  the  smaller  ones.  At  Prince  Edward 
Island  60  to  70  per  cent,  of  the  fish  are  caught  electrically  and 
unwanted  fish  are  not  touched.  The  structure  of  the  nets  used 
is  well  known  while  the  electrical  apparatus  used  generates 
direct  current  of  1,200  W.  at  115  V.  In  West  Virginia  they 
use  direct  current  of  2,500  W.  at  120  or  230  V.  The  latter 
voltage  seems  to  be  more  effective. 

Dr.  H.  Halsband  (Germany):  Research  work  at  the  Institut 
fur  Kusten  und  Binnenfischerei  has  determined  that  impulse 
current  has  a  great  effect  on  the  fish  and  that  its  influence  on 
the  metabolism  is  less  harmful  than  either  continuous  or 
alternating  current.  With  alternating  current  the  normal 
metabolism  of  the  fish  was  reached  only  after  120  min.,  with 
direct  current  after  70  min.,  but  with  impulse  current  only 
20  min.  were  needed. 

In  our  type  of  fishing  gear  the  anodic  effect  is  used  to 
concentrate  the  fish  around  the  anode.  Besides  this  attracting 
effect  the  frightening  effect  of  the  same  anode,  which  is  caused 
outside  the  range  of  attraction,  can  be  used  for  fishing  gears 
suitable  for  rivers  and  larger  bodies  of  water.  Experiments 
carried  out  by  the  Institute  have  shown  that  it  is  possible  to 
drive  the  fish  into  a  net  even  in  water  32  m.  deep.  Another 
application  of  the  frightening  effect  of  impulse  current 
consists  in  towing  an  electrified  cable  of  up  to  300  m.  length 
between  two  boats  and  driving  the  fish  into  a  trap  or  to  a 
certain  area  in  the  lake  where  they  can  be  caught  by  means 
of  a  seine  or  other  type  of  net.  This  method  is  also  applicable 
in  rivers  where  we  succeeded  in  driving  the  fish  for  kilometres 
towards  nets  to  finally  catch  them  there  electrically.  Such 
gear  could  also  be  switched  from  a  frightening  to  an  attracting 
effect;  for  this  purpose  the  frequency  has  to  be  raised. 

The  significance  of  the  impulse  rate  for  the  reaction  of 
different  species  can  be  used  to  fish  selectively. 

Prof.  S.  Takayama  (Japan):  Experiments  carried  out  by 
the  Japanese  Fisheries  Agency  on  electric  fishing,  divide  the 
subject  into  three  aspects:  physiological  studies,  commercial 
application  and  technical  problems. 

With  regard  to  physiological  considerations,  there  is  a 
point  in.the  body  of  fish  which  is  very  sensitive  to  electricity 
and  which  our  research  workers  call  C  point.  This  point  is 
about  the  middle  of  the  body,  but  the  position  varies  in 
different  species  of  fish. 

If  a  fish  feels  electricity  in  a  part  of  the  body  anterior  to  this 
point,  it  swims  back  out  of  the  electric  field.  On  the  other 
hand,  if  the  fish  feels  electricity  in  a  part  posterior  to  the  C 
point,  it  tends  to  swim  forward. 

By  installing  series  of  electrodes  and  switching  electricity, 
in  accordance  with  the  progress  of  fish  from  one  pair  of 
electrodes  to  the  next  one  in  front  of  them,  it  may  be  possible 
to  lead  fish  in  a  desired  direction.  So  far  these  studies  are  at 
an  experimental  stage.  Efforts  are  being  made  to  determine 
the  position  of  the  £  point  for  various  species  of  fish 
and  to  determine  the  technical  conditions  essential  for  this 
type  of  electric  fishing  such  as  distance  between  electrodes. 

In  Japan  three  types  of  commercial  technique  have  been 


[594] 


DISCUSSION  -ELECTRICAL    FISHING 


attempted  so  far:  (i)  electrified  hook  for  sword  fish,  tuna  and 
sharks,  in  which  case  electricity  is  used  only  for  killing  the 
hooked  fish,  (ii)  electric  shocks  to  whales  through  the 
harpoon  so  that  the  whale  is  killed  instantly.  One  of  the 
difficulties  in  electric  whaling  appeared  to  be  discoloration 
of  the  meat  which  sometimes  changed  to  a  greenish  tint 
specially  when  refrigerated,  (iii)  electricity  for  trolling  lines, 
mainly  for  Spanish  mackerel  and  small  tuna. 

The  amount  of  electric  energy  the  fish  receives  differs 
according  to  the  kind  of  electricity  applied.  When  continuous 


current  is  used,  the  energy  supplied  to  the  fish  is  determined 
by  the  voltage  and  the  amperage  of  the  current. 

In  Japan  the  current  applied  to  whaling  had  in  most  cases 
about  220  V.  and  60  A.  When  low  frequency  shock  current 
is  to  be  employed,  one  has  to  consider  the  impulse  duration 
and  its  rate.  In  experiments  of  electric  harpooning  and  trolling, 
the  duration  was  3/10,000  sec.  at  an  impulse  rate  of  10  per 
second  with  280  V.  However,  these  values  have  to  be 
adjusted  according  to  the  species  of  fish  the  gear  is  intended 
to  catch. 


Electroia.vis  concentration  of  anchovy  around  pump-hose  opening  with  electrode,  during  pump-fixhinp  ten  in  Morocco  (Oct.!  Nov..  1958). 

Photo:  Int.  FJcctronics  Lab.,  Hamburg. 

[595] 


Section  14:  Closing  Remarks. 


DISCUSSION   ON   FUTURE  DEVELOPMENTS 


Mr.  A.  W.  Anderson,  General  Chairman:  We  have  reviewed 
much  of  the  gear  that  is  currently  being  used  and  also  the 
latest  innovations.  Many  of  the  future  developments  therefore 
arc  obvious  to  us  all;  yet  there  are  future  trends,  problems  and 
less  obvious  developments,  especially  those  caused  by  factors 
outside  the  fishing  industry.  For  example,  by  the  end  of  the 
next  20  years  the  United  States  will  need  30  per  cent,  more  tish 
just  to  supply  the  normal  increase  in  our  population.  For  the 
U.S.  Fisheries  this  is  quite  a  problem;  either  our  domestic 
production  or  our  imports  must  increase.  In  either  case 
fishing  gear  must  become  more  productive. 

I  do  not  intend  to  discuss  every  item  on  the  Agenda  in 
detail,  but  some  statements  have  to  be  brought  forward.  It 
has  been  stated  that  the  ideal  fibre  for  nets  is  not  yet  developed. 
This,  then,  certainly  must  be  a  development  of  the  future  and 
if  the  ideal  fibre  can  be  made  into  a  knotlcss  net  we  may  find 
that  the  knotted  net  really  will  become  a  museum  piece,  as 
our  Japanese  colleague  said. 

Then  we  know  too  little  of  fish  behaviour  and  net  action. 
These  arc  both  present  and  future  problems.  I  believe  the 
television  camera  will  unlock  many  of  the  secrets  in  these 
fields.  It  would  not  be  surprising  if  the  next  Fishing  Gear 
Congress  were  overwhelmed  with  reports  of  research  based 
on  the  use  of  television  cameras,  and  I  do  not  think  it  is 
looking  too  far  into  the  future  to  say  that  even  fishing  skippers 
will  be  able  to  look  at  a  television  screen  in  the  pilot  house, 
watch  the  net  and  how  it  operates,  and  observe  the  fish  and 
the  evasive  action  they  may  or  may  not  take. 

Fish  location  is  another  problem.  Finding  solutions  to  this 
problem  through  devices  which  accurately  determine  the 
vessel's  position  or  detect  fish  by  vertical  sounding  or  hori- 
zontal ranging  are  in  good  competent  hands,  as  you  may 
have  noted  during  the  discussion  earlier  this  week.  Surely 
the  competition  in  this  field  will  find  us  advancing  steadily 
and  perhaps  spectacularly,  in  the  next  few  years.  Fish 
Location  by  means  of  studies  of  the  food  and  feeding  habits 
of  fishes,  together  with  allied  factors  such  as  studies  of  the 
currents,  temperatures,  water  colour  and  so  forth,  offer  both 
logical  and  fascinating  possibilities.  More  studies  in  this 
field  can  be  expected  to  be  initiated  when  more  knowledge 
becomes  available. 

Handling  of  the  gear  and  handling  of  the  catch  must  be 
improved.  The  production  of  fish  and  the  fish  themselves 
are  in  keen  competition  with  other  food  products.  As  com- 
petitors lower  costs  with  more  modern  machines  by  increasing 
volume  with  less  labour,  the  fishing  industry  must  keep  pace. 
More  and  more  mechanical  aids  seem  to  be  the  answer.  Not 
only  will  they  cut  costs  and  increase  volume  of  production, 
but  often  they  make  the  handling  of  gear  more  agreeable  to 
the  fishermen,  and  difficulties  in  obtaining  manpower  are  a 
definite  future  problem.  In  the  United  States  we  find  more 
and  more  of  our  fishermen  taking  jobs  ashore  because  of  a 
40  hour  week  for  a  comparable  pay,  easier  work  and  the 
comforts  of  being  at  home  every  night.  The  greatest  possible 


mechanisation  of  the  handling  of  gear  can  help  to  stop  this 
trend  by  making  the  work  less  difficult  and  the  pay  greater. 
This  said,  without  expecting  that  trawler  Captains  will  soon 
be  able  to  sit  before  a  row  of  buttons  and  push  one  to  set  the 
trawl,  another  to  haul  it  back  and  perhaps  a  third  to  open  the 
codcnd,  although  that  certainly  would  be  something  to  look 
forward  to. 

Mechanization  as  well  as  the  use  of  the  most  efficient  gear 
has  sometimes  been  hindered  by  regulations  preventing  their 
use.  1  know  of  no  comparable  industry  that  is  in  some  cases 
bound  to  out-modcd  gear  or  forbidden  to  use  more  modern 
mechanical  devices.  This  occurs  in  the  fishing  industry  when 
measures  are  taken  to  maintain  a  less  perfected  method  of 
production  by  excluding  others,  particularly  more  advanced 
types,  to  maintain  the  maximum  sustained  yield,  considering 
only  the  biological  and  not  the  economical  side  of  the  problem 
hrequently  it  seems  ecisicr  to  regulate  the  less  efficient  gear 
and  methods  so  the  more  efficient  methods  are  thereby  ruled 
out.  Biologists  and  administrators  should  recognize  that 
this  is  only  a  partial  answer  to  the  problem. 

Quality  of  the  catch  is  another  problem.  Fish  from  a 
2 1  day  trawling  trip  is  edible,  perhaps,  but  not  fresh.  The  time 
will  come  when  fish,  which  really  is  a  very  perishable  product, 
will  be  given  the  same  care  as  milk,  butter  and  other  perish- 
able agricultural  products.  Trawlers  freezing  fish  at  sea 
represent  a  beginning  towards  this  objective.  As  they  increase 
in  number,  size  and  complexity  they  will  stimulate  new 
thinking  in  regard  to  gear  and  in  regard  to  handling  methods. 
They  might  well  be  responsible  for  changes  in  the  near  future 
which  otherwise  might  have  been  long  delayed.  There  can 
be  no  doubt  that  their  advent  means  change  or  new  types  of 
gear,  or  new  techniques  for  handling  the  catch. 

Perhaps  we  can  look  for  the  greatest  developments  and  the 
brightest  future  in  mid-water  trawling.  Since  we  now  fish 
the  surface  waters  and  the  shallow  bottoms,  only  the  great 
depth  and  the  midwater  areas  remain.  At  the  moment  the 
researches  in  midwater  look  most  promising.  Much  effort 
has  been  expended  on  midwater  gear  without  appreciable 
result  except  under  special  conditions.  However,  so  much 
work  has  been  done,  and  is  being  done,  on  gear  for  the 
purpose  that  someone  is  bound  to  find  the  answers  we  seek. 

The  final  thing  I  look  for  is  a  much  greater  interest  by 
Governments  in  research  on  fishing  gear.  I  believe  this  will 
occur  for  two  reasons.  First,  it  is  difficult  for  the  fishing 
industry  to  conduct  and  finance  basic  study  needed  to  solve 
the  problems,  and  second  this  Congress  and  the  reports  which 
have  been  made  to  it  should  reveal  to  Governments  the  very 
great  interest  in  the  subject  and  the  need  for  a  solution  to 
fishing  gear  problems. 

Mr.  C.  P.  Halain  (Belgian  Congo):  I  should  like  to  draw 
your  attention  to  a  point  which  is  not  generally  realized  and 
which  I  have  stressed  during  other  Congresses.  A  ton  of 
fish  delivered  in  the  market  for  consumption  represents  the 


[596] 


CLOSING     REMARKS 


value  of  4  to  5  head  of  cattle  at  the  slaughterhouse.  But  in 
order  to  send  4  to  5  head  of  cattle  to  a  slaughtci  house  it  is 
necessary  to  have  35  to  40  animals  in  pasture,  so  that  every 
ton  of  fish  caught  is  equal  to  the  meat  production  of  35  to  40 
head  of  cattle  on  the  range.  A  country  that  has  the  opportunity 
of  catching  100,000  tons  of  fish  per  year  as  the  Belgian  Congo 
presently  does  has  therefore  practically  the  same  wealth  as  a 
country  having  3  J  to  4  million  head  of  cattle  on  the  range. 

A  country  which  possessed  such  a  stock  of  cattle,  would 
use  all  means  to  maintain  this  stock  in  good  health  and 
condition,  as  it  represents  a  considerable  protein  capital.  1 
am  sure  a  great  number  of  laboratories  would  be  set  up  and 
many  veterinarians  employed  to  look  after  such  stock  of 
cattle.  When  producing  100,000  tons  of  fish  each  year,  we 
have  the  same  obligation,  of  course,  to  maintain  research,  to 
maintain  laboratories  and  to  undertake  the  necessary  measures 
to  exploit  this  capital  as  rationally  as  possible.  The  population 
of  the  world  is  increasing  much  faster  than  the  production  of 
food.  The  limit  to  what  can  be  achieved  by  agriculture  is 
perhaps  near.  Every  fish  producing  country  should  value 
fish  production  on  at  least  an  equal  basis  with  other  food 
production  methods  and  provide  in  its  budget  the  means  for 
financing  an  efficient  scientific  and  practical  research,  aimed 
at  conservation  and  rational  development  of  their  fish 
production. 

We  must  endeavour  to  exploit  the  fish  capital  in  this  world 
as  rationally  as  possible.  The  FAG  has  now  shown  us  a  means 
to  achieve  this  by  interchange  of  knowledge  on  an  inter- 
national basis.  Hunger  is  one  of  the  biggest  factors  against 
peace  in  the  world,  if  everybody  in  the  world  had  sufficient 
to  feed  themselves  satisfactorily,  both  quantitatively  and 
qualitatively,  I  think  a  great  many  problems  would  be  solved. 

Mr.  M.  Kawakami  (Japan):  The  development  of  fishing 
gear  and  methods  was  in  the  past  a  very  slow  process,  but 
has,  during  the  past  few  decades,  advanced  very  rapidly.  Our 
future  development  work  must  not  only  take  into  account  the 
technical  progress  but  also  the  conservation  of  the  fish 
resources  for  the  future.  It  appears  we  have  now  reached  the 
point  where  each  new  technical  development  not  only  has  to 
be  assessed  in  relation  to  its  practical  or  commercial  value, 
but  also  in  relation  to  its  biological  effect  on  the  conservation 
of  fish  stocks. 

Due  to  the  increasing  demand  for  fish  food,  the  total 
catches  all  over  the  world  arc  continuously  increasing,  but 
very  little  is  being  done  to  see  that  the  stock  of  young  fish 
is  increased  at  the  same  rate  by  hatchery  or  conservation 
methods.  In  Japan  fishermen  and  scientists  work  together 
also  on  this  problem.  My  company  has  three  Fisheries 
Institutes  where  research  work  is  carried  out  not  only  on 
fisheries  technique,  but  also  on  conservation  methods  with  a 
view  to  preserving  the  fish  stock.  At  one  of  these  Institutes, 
research  is  also  carried  out  on  the  problem  of  fully  utilizing 
the  catch.  For  example,  years  ago  in  whaling  only  the  oil 
was  utilized,  whereas  today  the  whale  meat,  intestines  and 
even  the  blood  is  used,  either  for  human  consumption  or  for 
other  purposes. 

Another  point  which  I  would  like  to  bring  up  is  that  due 
to  language  difficulties  we  Japanese  research  workers  arc 
virtually  cut  off  from  the  other  countries.  Yet  we  would  like 
to  exchange  our  papers  on  research  work  and  if  translators 
could  be  found  we  would  be  able  to  compare  experiments. 
Maybe  FAO  can  also  be  of  assistance  here. 


Mr.  P.  A.  de  Boer  (Netherlands):  In  doing  technical 
research  myself  I  feel  the  need  of  all  possible  assistance  from 
other  people  all  over  the  world.  There  exists  already  one 
valuable  means  for  making  contacts  between  all  who  work  on 
fishery  research  and  that  is  the  FAO  World  Fisheries  Abstracts. 
I  have  found  that  these  Abstracts  often  gave  me  a  lot  of  help 
and  ideas  for  designing  instruments  and  for  improving  my 
research  programme.  As  these  abstracts  are  published 
rather  late  due  to  the  work  involved,  1  would  propose  that 
every  research  worker  should  send  his  own  abstract  immed- 
iately to  FAO  to  avoid  loss  of  time. 

Mr.  A.  Kutsch  (Germany):  I  have  spent  40  years  of  my 
life  making  nets  for  the  fishing  industry  and  speaking  as  a 
net  maker  I  would  like  to  bring  forward  two  of  our  main 
problems. 

1 .  The  netmaker  has  to  keep  pace  with  the  progress  made 
in  the  other  branches  of  the  trade,  principally  when  it  concerns 
the  size  and  type  of  vessels  and  the  improvements  in  deck 
gear  and  operation  techniques,  so  that  the  construction  of  the 
nets  has  to  undergo  constant  changes  and  large  scale  produc- 
tion is  dangerous. 

2.  The  users  of  our  product,  the  fishermen,  are  not  very 
communicative  and  reluctant  to  give  information  on  catches 
and  performance  of  gear.  Whenever  they  discover  something 
they  try  to  keep  it  to  themselves  to  continue  bringing  the 
biggest  catches. 

Many  errors  could  be  avoided  and  much  faster  progress 
made  in  the  matter  of  net  construction  if  the  fishermen  who 
use  the  nets  would  discuss  their  problems  and  ideas  more 
openly  with  the  netmakcr.  We  are  always  willing  to  co- 
operate. 

Mr.  Rack  (Rhodesia):  In  considering  the  future  of  fishing 
gear,  I  may  perhaps  refer  to  the  future  of  the  inexperienced 
people  with  whom  1  work  and  for  the  many  millions  of 
inexperienced  people  not  directly  represented  but  who  are 
served  by  so  many  people  all  over  the  world.  I  refer  parti- 
cularly to  those  people  who  are  emerging  from  subsistence 
fishing  over  the  economic  barrier  to  commercial  fishing.  The 
primary  development  must  always  of  course  be  education. 
Then  they  must  be  shown  how  to  apply  that  education  and  it 
is  in  the  application  of  the  education  where  you  can  help  us, 
you,  the  manufacturers,  scientists,  FAO  and  all  those  who 
represent  the  greater  fishing  industries.  The  proposed 
standardisation  of  materials  and  gear  will  protect  such  people 
from  being  imposed  upon  and  will  strengthen  their  position. 

We  need  equipment  and  our  equipment  may  for  a  long  time 
not  be  on  such  a  scale  as  your  equipment,  but  nevertheless 
it  must  be  good.  Let  us  take  the  case  of  light  fishing;  on 
the  lakes  of  Africa  kerosene  pressure  lamps  are  used  and  it 
may  take  many  years  before  the  natives  can  turn  to  better 
lamps  or  fish  pumping.  But  already  these  lamps  are  bringing 
a  higher  standard  of  living  to  hundreds  of  thousands  of  people 
in  their  small  villages  with  their  small  canoes.  Now  we  are 
still  a  long  way  behind,  but  these  Conferences  nevertheless 
are  helpful  to  us  because  they  enable  us  to  make  contacts 
and  catch  up  on  what  is  being  done. 

Mr.  H.  S.  Drost  (Netherlands):  The  fact  that  so  many 
participants  are  present  indicates  to  me  that  the  interest  in- 
fishing  is  indeed  great.  I  think  we  are  all  aware  that  the 
fishing  industry  is  on  the  way  to  becoming  an  important  and 


597  ] 


MODERN    FISHING    GEAR     OF    THE    WORLD 


modern  industry,  much  more  than  it  has  been  in  the  past. 
The  fishermen  and  the  shipowner  do  not  stand  alone  any 
more;  they  are  working  and  co-operating  with  the  scientists 
and  from  the  very  minute  this  started  we  see  an  amazing 
modernisation  of  the  fishing  industry,  and  it  is  time  this 
happened  for  the  world  food  supply. 

We  see  now  an  accelerating  speed  in  development.  Since 
the  last  world  war  nearly  every  year  some  amazing  technical 
development  in  the  fishing  industry  has  taken  place.  This 
development  was  and  is  furthered  by  international  co-opera- 
tion. We  have  had  a  Fishing  Boat  Congress  in  1953,  a  Fish 
Processing  Congress  in  1955  and  now  in  1957  we  have  this 
Fishing  Gear  Congress.  I  would  not  be  surprised  if  we  had 
in  the  future  a  second  Fishing  Boat  Congress  in  1959,  a 
second  Fish  Processing  Congress  in  1961  and  in  1963  a 
second  Fishing  Gear  Congress;  and  so  on,  every  two  years  an 
International  Congress  concerning  the  fishing  industry.  We 
owe  these  International  Congresses  to  the  FAO.  Although 
future  Congresses  may  be  wishful  thinking,  T  mean  to  speak 
on  behalf  of  all  the  participants  when  I  express  our  thankful- 
ness for  the  work  FAO  has  already  done  so  far  for  the  fishing 
industry.  This  Fishing  Gear  Congress,  I  am  sure,  has  been 
very  useful,  it  has  given  us  all  opportunity  for  international 
contact.  From  now  on  we  will  be  able  to  keep  contact  one 
with  another  far  more  than  before,  Ajiother  very  important 
result  of  this  Congress  is  that  we  have  now,  for  the  first  time, 
a  nearly  complete  inventory  of  the  many  problems  concerning 
fishing  gear. 

1  think  we  all  hope  that  FAO  will  be  able  to  arrange  such 
international  Congresses  in  the  future  for  the  benefit  of  the 
fishing  industry. 

But,  furthermore,  1  suggest  that  we  cannot  just  wait  and 
see  until  1963. 1,  therefore,  propose  that  the  Congress  requests 
from  FAO  Fisheries  Division  to  create  an  official  FAO  Com- 
mittee on  Fishing  Gear  immediately  after  this  Congress.  We 
have  discussed  the  many  problems  on  fishing  gear  and  as  I 
see  it,  they  form  a  wider  and  more  distributed  field  of  work 
than  those  of  the  Fishing  Boat  Congress  and  Fish  Processing 
Congress.  Therefore  I  would  suggest  that  FAO,  furthermore, 
approaches  the  different  governments,  requesting  the  forma- 
tion of  National  Committees  on  Fishing  Gear,  from  which 
representatives  could  be  sent  to  the  meetings  of  the  afore- 
mentioned FAO  Committee  on  Fishing  Gear.  These  National 
Committees  should  include  working  groups  such  as  have 
been  established  at  the  beginning  of  this  Congress  for  stand- 


ardization. The  National  Committees  should  not  consist 
of  government  officials  only  but  also  of  private  experts.  I 
think  this  is  essential  to  keep  the  action  virile.  At  least  50 
per  cent,  of  the  members  of  the  National  Committees  should 
come  from  commercial  firms  who  are  in  business  and  who 
feel  daily  the  financial  responsibility  of  the  industry. 

Stated  briefly,  1  think  the  task  of  the  National  Committees 
would  be  to  go  on  with  research  on  present  and  future  prob- 
lems, and  in  doing  so  to  prepare  for  the  next  Congress. 

The  creation  of  such  National  Committees  will  mean  that 
research  on  fishing  gear  will  have  to  be  established  in  some 
countries  and  improved  in  others.  This  requires  appropriate 
budget  allocations.  I  suggest  that  FAO  should  urge  the 
governments  to  support  research  on  fishing  gear  morally 
and  financially  and  apart  from  the  biological  research  which 
usually  is  already  well  established.  There  is  no  doubt  that 
applied  research,  such  as  on  fishing  gear,  will  pay  itself, 
as  Mr.  Parkcs  stated  already  for  fishing  vessels  during  the 
Fishing  Boat  Congress  in  1953. 

There  is  another  and  most  important  aspect  of  this  Inter- 
national Congress:  it  contributes  to  world  peace.  We  must 
continue  what  we  have  been  doing  this  week:  collaborate  on 
an  international  scale  for  the  benefit  of  the  fishing  industry 
and  for  the  benefit  of  world  peace. 

Prof.  Dr.  A.  von  Brandt  (Germany):  Speaking  for  the 
scientists  working  on  fisheries  problems,  I  would  like  to 
emphasise  that  we  fully  support  the  suggestion  of  Mr.  Drost. 
Some  of  my  colleagues  meet  from  time  to  time  but  we  need 
still  better  international  contacts  with  people  of  our  pro- 
fession, i.e.  not  only  scientists  but  also  net  makers  and  fisher- 
men. We,  therefore,  do  hope  that  Mr.  Drost's  proposal  will 
be  accepted  and  FAO  Fishing  Gear  Committees  be  formed. 

Mr.  A.  W.  Anderson,  General  Chairman:  I  am  sure  that 
FAO  will  give  the  most  serious  consideration  to  what  Mr. 
Drost  has  said.  Fortunately  the  FAO  Conference  meets 
next  month  in  Rome,  and  I  am  sure  that  the  requests  made  at 
this  Conference  for  consideration  of  these  various  matters, 
as  the  establishment  of  Committees,  will  come  up  in  a  very 
short  time  and  will  be  acted  upon  in  the  Fisheries  Panel  at 
that  Conference.  Since  any  Committee  formed  here  would 
be  an  unofficial  one,  the  actual  establishment  of  Committees 
I  think,  should  better  be  left  to  FAO. 


[598] 


DETAILED     INDEX 


INDEX 


Abrasion  resistance  25-28,  39,  40,  44,  46, 

55,  94,  137,  140,  143,  145,  148,  320 
Absorption,  of  sound  waves,  480,  482 
Acetate,  47,  51-53 

—  combined  with  nylon,  148 
Aery  Ian,  copolymer,  141 

Aeration,  effect  on  sound  waves,  474,  477, 

482,  492,  523,  535 
Aerial  scouting.  399,  445,  529,  532 

—  aircraft,  530 

—  flying  height,  530 

Amilan,  properties  and  uses,  2,  20,  24-29, 

44.  150,  151 
Angle  of  attack,  171,  173,  180,  228,  230, 

246,  247 

—  meter,  230-233,  244 
Anzalon.  polyamide,  141 

"Aquarial"  method,  of  testing  net  preserva- 
tion, 128,  129 

Artificial  bait — see  Lures 

Asdic — see  also  Echo  ranging,  472. 477, 491 , 
495,  507,  508,  530,  531,  533,  535 

—  range,  495,  530,  531,  534 

—  training  of  operators,  497.  498,  534 

—  References,  477,  511,  522 
Attenuation,  of  sound  waves,  478,  479,  533 
Attraction  of  fish  by  light,  418-424.  428, 

429,  440,  442,  537,  539,  546,  553-566, 

571,  573,  574 

—  directed  light,  556-558,  566 

—  diurnal  effect,  548,  551,  555,  560,  572 
effect  of  wave  length,  553,  561,  571,  573 

—  electric  lights  above  water,  420,  421, 
428.  429,  555 

—  favourable  conditions.  549.  560,  572-574 
-—  fluorescent  light,  429 

—  indirect  signs  of  fish,  573 

—  intensity  of  light,  419,  561.  572,  573 

—  moon,  551,  555,  573 

—  lamps, surface, 41 8-424, 428,429, 566,  573 

—  lamps,    underwater,    see    Underwater 
lamps 

—  light  gradient.  555,  573 

light  source,  418,  422,  429,  556,  557, 

572,  573 

-  phototaxis,  546,  548,  550,  555,  556,  571 
pump  fishing,  559-566 

—  time  clement,  562,  573,  574 

—  with  liftnets,  418-421,  559,  563,  566.  574 

—  with  pole  and  line,  428,  429 

-    with  setnets  and  traps,  556-558 

—  with  stickheld  dipnet,  422-425 

—  References,   542,   546,   547.   549,   552, 
555,  566,  570 

Background  noise,  497,  508 

—  signal  to  noise  ratio,  490,  525,  526 
Bagnets,  275,  288,  393 

Bait,  164,  390,  434,  435,  571 

—  artificial,  see  Lures 

—  chopped,  428.  429 

Baitings  (balings),  105.  106,  172,  261,  305, 

308,  310 

Balanced  twine,  49,  52 
Basnig.  Philippine  lift  net,  418-421 
Beach  seining,  291,  390,  411,  445 
Beam  angle  (in  echo  sounding  and  ranging), 
475.477,  483, 485, 491. 495, 497,  506,  509, 
513,  515,  516,  526,  530,  534,  535 


Bcdesol  (bonding  agent).  46,  76 
Behaviour  of  fish,  272,  395,  399,  435,  440, 

444,  445,  521,  529,  536,  537,  550,  551, 

555,  556,  557,  560,  561,  571 
--  approaching  a  gillnet,  551 

—  influence  of  light,  448,  551,  556,  557,  561 
influence  of  moonlight,  435,  551,  555 

—  influence  of  temperature,  560 
schooling,  524.  529 

—  shrimp,  521 

—  study  depends  on  fishing,  444   445 
swimming  speed,  440.  445 

Bibliography,  see  References 

Billowing  of  trawl  nets,  210,  215,  216,  221 

Black  line,  505,  535.  536 

Black  varnish  treatment  of  nets,  141,  145, 

158 

Bobbins,  442,  448 
Bonding,  21,  27,  28,  41,  46,  47,  80,  85,  92, 

100,  101,  156,  160 
Bosom,   172,   186-188,   193,   194,   196-199, 

218,  264,  306 
Bottom  echo,  475,  476,  483-485,  503-505 

—  suppression  of,  487,  495,  503-505,  510, 
533,  536 

Braided  twines,  41,  42,  92,  143 

Brailing,   354,  393 

Breaking  length,  1,  11,  14,  16-18,  31,  32, 

37,  59-61,  71,  73,  83-86,  126,  141 
Breaking  strength,  1.  14,  22-24,  28,  31-33, 

37,  38,  44,  50-52,  59-61,  67,  71,  73,  83-86, 

140-143,  153,  157 

-  testing,  60,  72,  76,  77,  138 
Bridles — see  Swecplines 
Bright  yarns,  45,  94,  144 

Cable  meter,  320,  369,  370,  448 
Castnets,   293 

Cathode  ray  tube,  319,  320,  476,  477, 
488-495,  499,  501-503,  505,  525 

—  versus  paper  recorder,  488-490,  492-494, 
532 

—  expanded  time  base,  490,  493 
"Catoba".  acetylated  cotton,  137 
Chernikoff  log,  267 
Chumming  chopped  bait   429 
Classification  of  gear,  165,  274-296,  440 

-  references,  295,  296 
Clinometer,  227 

Cod  distribution,  relation  to  temperature, 

454 
Codend,  164,  165,  180,  210,  214,  215,  305, 

307,  310,  324,  336,  341,  447 

—  cover,  307,  310 

—  detachable,  330,  332,  447 

—  use  of  synthetics,    145,    151,   210-212, 
299,  307,  320 

Colophony  resin,  bonding,  46 

Colour  of  light,  539,  551,  553,  571,  573 

Combination  twines  of  mixed  fibres,  2,  3, 

62.  64,  81,  95,  148,  152-155 
Comparative  fishing,  157-165,  219,  220,  270 
Conductivity   in  electro-fishing,   592-594 
Cone-shaped  liftnets,  547,  559,  566 
--  operation,  566 

Construction  of  twines  and  ropes,  1,  4,  5,  7, 
10,  11,  19,  20,  34-37,  40,  44,  45,  49-52, 
59-61,  70,  72,  76,  90,  109,  149 

[601  ] 


Continuous  mulufi lament  nylon,  148,  149, 

156-158 
Conversion    formulae    and    tables,    yarn 

counts,  3,  7,  9,  11 
Copolymers    (Saran,    Vinylon,    Dynel, 

Acrylan).  2,  141 
Cord,  definition,  1 
Cotton 

—  acetylated,  121,  122,  124-127 

—  export  from  Japan,  64 

—  properties  and  uses  of,  2,  3,  20-28,  30, 
36-40,  44,  63,  84,  96,  139,  141-144,  152, 
160-165 

Courlene,  Courlene  X3,  2.  18,  97,  271 
Crew  size, 

—  basnig,  Philippine  liftnet,  418,  420 

—  Danish  seining,  376 

—  handlming,   Japan,   426 
-     longlining,  432,  447 

—  pole  and  line,  428 

—  purse  seining,  392,  394,  408,  410,  449 

—  trawling,  300,  317,  310 
Cronaxy,  577,  581 

Current,  effect  on  gear,  153,  155,  175-184. 

189-192,  200-204,  209-212,  215,  216,  218, 

221-224,  245-249,  251-259,  266,  271.  336, 

347,  442,  446,  450,  451 
Current  meters,  177,  247,  254 
Cutch—jw  Tanning 
Cutting  machine  made  webbing,  105,  106l 

221,  26K  263 


Dacron,  polyester,  2,  140,   143,  147-149 

—  "Dacron    51",    148 

Danish  seining,  275,  291,  375-390,  441 

—  anchor  seining,  376,  385,  386 
comparing    starboard    and    port    side 

operation,  388-390 

—  fly  dragging,  386,  387 

—  haddock  seine,  382,  383,  443 

—  Japanese  fly  dragging,  376,  378,  388-390 
•--  Japanese  seine,  384 

—  operation,  383-390 

—  moorings.  380 

-  plaice  nets,  380,  381,  443 

—  use  of  echo  sounding,  385 

—  use  of  synthetics.  145 
warps,  380,  385 

—  winches,  378.  379 

Danlenos.  209,  210,  216,  220,  221,  298 

303-305.  308,  342    383 
Decca  navigator,  267,  448,  452,  467-471. 

536 

—  automatic  plotter,  469,  470 
Deck -layout, 

—  Danish  seining,  377,  378,  387 

—  purse  seining,  392,  402,  405,  412,  413, 
449 

—  trawling, 301,  302, 31 1,312, 314, 320-324, 
338,  368,  370.  412,  413 

Deep  scattering  layer,  487,  522,  524 

Delustred  yarns,  45,  94,  144 

Denier,  3,  5,  8,  9,  10,  11,  50-52,  64,  76  84 

110.  148 
Density  of  fibres,  definition,  14 

—  see  also  Specific  weight 

Depth    regulation    of    midwater    trawls, 
242-244,  491 


MODERN     FISHING    GEAR    OF    THE    WORLD 


Depth — continued. 

—  acoustic  telemeter,  334,  446,  491,  518 

—  electric  telemeter,  518 
Detection  of  fish— see  Fish  detection 
Diameter,  of  twines  and  ropes,  21,  35-37, 

40,  41,  59-61,  71,  74,  145,  148,  153,  157, 
158,   160,  210,  218,  245,  447 

—  testing,  65,  70,  72,  74,  93 
Diolen,  polyester,  2,  140,  143 
Dipnets,  275,  292 

—  electrical,   587 
Directed  light,  557 

Direction  finders.  320,  428,  462,  463 

—  frequency  used,  462,  463 
"Doubling"  efficiency,  31,  90 
Drawing  nets,  105,  106,  170-173 
"Drawing"  or  stretching  ace  t>  la  ted  fibres, 

126 
"Drawing"  of  synthetics,  32,  33,  37,  38,  45, 

94,  141 
Dredges,  222-224,  272.  290 

—  absolute  efficiency,  224 
Drift  nets,  276,  294,  411,  440 

—  use  of  synthetics,  142-144,  147-150,  163, 
262 

Droplines,  433-435 

—  of  wire,  443 
Drum  trawlers,  320 
Duchemin's  equation,  179 

Dyeing,  26,  40,  47,  94,  109-111,  113-122, 

137.  144 
Dynamometers,   168,  228,  229,  244,  247, 

267,  270,  356,  450 

Echo  amplitude,  476 

Echographs,  190,  191,  360.  477,  481,  482, 
487,  489,  494,  496,  498,  500,  503,  506, 
508,  509,  510,  514,  516,  519,  520,  521, 
522.  524,  531,  574 

Echo  ranging,  477-537 

—  beam  angle,  477,  491,  497,  509,  513, 
530.  531,  534 

—  effect  of  ship's  motion,  491,  503,  535 

—  horizontal  propagation,  480 

—  limitations,  496,  497,  534 

—  operation,  497,  515,  536 

—  power  requirement,  512 

—  range,  477,  495,  507,  508,  513,  515,  516. 
530,  534 

—  side  lobes,  534 

—  survey,  530,  531,  533 

—  training  of  operators,  497,  498,  534 

—  References,  477,  511,  516,  522 
Echo  sounding,  474-537 

—  beam  angle,  475,  476,  495 

—  effect  of  ship's  motion,  475-477 

—  for  controlling  midwater  trawl,  334,  491 

—  nature  of  bottom,  385, 474-476, 483, 485 

—  range,   535 

—  recording  versus  C.R.T.  533 

—  survey,  525-527,  536,  537 

—  transducer  on   midwater   trawls,    334, 
446,  491 

—  transducer,  474,  475,  478,  515 

—  use  with  lights,  537 

—  References,   527 
Economic  value  of  fish,  597 

Elasticity,  14-18,  25,  31,  38,  39,  55,  67,  95, 
100,  140-143,  148,  149,  157,  447 

—  effect  on  catchability,  148 
Elongation,    1,    14,    15,   38,   39,   67,   73, 

141-143,  149 
English  knot,  23,  24,  46,  63 

—  testing,  66 
Electrical  fishing.  581-595 

—  conductivity,  592.  593,  594 

—  construction  of  gear,  569 

—  current,  A.C.,  581 

—  current,  D.C..  575-580 

—  current  intensity,  587 


Electrical  fishing — continued. 

—  danger  and  precautions,  585,  589-591 

—  dip-net,  587,  588 

—  electric  screen,  581 

—  electric  tuna  hook,  583 

—  electrode,  584,  587 

—  electro-trawl,  583,  584,  592 

—  fish  leaf1  .g,  594 

—  fish  screen.  581,  582,  584,  585,  593,  594 

—  fluorescent  light,  429 

—  gun-pump,  584, 

—  harpoon,  582,  592,  594 

—  impulse  rate,  578,  579,  592 

—  limitations  in  salt  water,  585 

—  longline,  582 

—  operation  range,  585,  588,  592 

—  power  supply,  582,  584,  585,  587,  592, 
593,  594 

—  purse  seining,   583,  585 

—  References,  580,  582,  588,  591 
Electric  field,   584,   588,   592 

Electric  screen,  581,  582,  584,  585,  592,  593, 
594 

—  3-phase,  582 

—  as  leader  net,  585,  594 
Electric  tuna  hook,  583 
Electrocution 

—  voltage,  581,  582,  583,  584.  585 
Electrode,  584,  587 

Elect ronarcosis,  578 

—  potential,  579,  583,  585,  592 
Electrotaxis,  540,  541,  542,  578.  583,  585, 

592 

—  potential  requirement,  540,  541,  578 
Electro-trawl,  584-592 

—  improvement  in  fish  quality,  584 
-     placing  of  electrodes,  584 

—  pulse  rate,  582 

Enkalon,  polyamide,  2,  79,  80,  141 
Envilon,  properties,  20-29,  67 

—  use  of,  62-64 

Escape  offish  from  nets,  164,  165,  173,  174, 

256,  257,  272,  336,  350,  445 
Exocet  kite,  362 

Expanded  time  base  of  C.R.T.,  490,  493 
Extensibility,  1,  14-18,  24,  25,  31.  33,  37-39, 

44,  51,  52,  55,  61,  67,  71,  73,  74,  88-91, 

100,  140-143,  148,  164 

—  testing,  66,  72,  73,  76,  138 

Factors  ruling  fish  migrations,  453, 454, 472 
Factoryships,  321-324,  329,  330 
Fairleads,  301,  302,  322-324,  349 
False  headlines,  172,  173,  186-194,  247 
Filtering  of  trawls,  173,  174,  210,  212,  215, 

216,  218,  221,  310,  336,  350,  356,  442 
First  reaction,  electro-fishing,  579,  583 
Fish  counter,  492 
Fish  detection,  471,  474-537 

—  catch  to  signal  relation,  510,  520,  521, 
526 

—  colour  of  water,  536 

—  near  the  bottom,  319,  474-477,  481,  487. 
485-490,  492-496,  503,  510,  533,  536 

—  pelagic,  334,  336,  348.  349,  358,  475, 
481,  482,  487,  491,  521 

—  range,  474,  477,  535 

—  References,  524 

Fish  echo  traces,  494,  495,  503 

—  illustrations,  190,  191,  477,  481.  482, 
487,  489,  494,  496,  498,  500,  503,  506, 
508,  509,  510,  514,  515,  516,  519,  520, 
521,  522,  524,  531 

—  interpretation,  491,  495,  497,  499,  502, 
503,  507,  508,  510,  516,  519,  520,  521, 
526,  530 

—  near  sea  bottom,  499.  501,  522,  533,  537 

—  relation  to  catch,  488, 495 
Fishermen's  clothing  of  man-made  fibres, 

18,97 

[602] 


Fish  hooks,  279.  427,  428,  431,  434 

Fishing  charts,  472,  473 

Fishing  lamps,  surface,  418-424,  428,  429 
„  underwater,  556-560,  571,  573, 
574 

"Fishing"  method,  of  testing  net  preserva- 
tion, 128-130,  132 

—  testing  twines,  83,  84,  93,  96,  141 
Fishing  power  of  gillnets,  161-165 
Fishing  without  gear,  274 

Fish  magnet,  585 

Fish  pumping,  329,  330,  396,  397,  398, 
414-417 

—  boat  to  dock,  414-417 

—  ocean  to  boat,  416,  417 

—  pump-fishing,  559-566 

Fixed  loop  direction  finders,  462,  463 
Flapper  (in  trawls),  218,  305,  307,  336,  383 
Flax,  properties  and  use,  3,  30,  44,  64,  84, 

139 
Fleet  operation,  329-332,  448 

—  searching,  330,  331,448 

Floats,    157,    169,    179,   200-204,   209-211, 
215,  216,  253,  262,  304,  305,  309,  314, 
345,  347,  349,  391,  396 
-  table  of  buoyancies,  weights,  etc.,  262 

Footrope,  curvature  during  fishing,  215, 
217,  218,  234-240 

—  indicator,  234 

—  shape,  weight,  material,  170-172,  193, 
197-199,  215,  304,  305 

Frequency  of  echo  sounders  and  rangers, 
474-476,  478 

—  attentuation,  478,  481,  482 

—  beam  angle,  475,  476 

—  bottom  contours  and  nature,  474,  476, 
483-485 

—  effect  on  directivity   478,  483 

—  fish  detection,  474,  475,  481,  486 

—  high  versus  low,  474.  476,  478-487,  499, 
500,  533 

—  range,  475,  476 

—  size  and  shape  of  transducer,  474,  475, 
478,  486,  490,  509,  513, 

Future  trends  in  fishing,  598 


Galvanonarcosis,  576,  577,  579 
Galvanotaxis,  575-579 

—  threshold  value,  575,  576,  579 

—  current  density,  579 
Gating  circuit,  495,  536 

Gear  classification,  165,  274-296,  440 

—  References,  295,  296 

Gear  design,  tell-tale  signs,  102,  104,  217, 

267 

Gear  echo  traces,  510,  516 
Gear  materials,  References,  18,  29,  33,  145, 

146,  163 
Gear    research,  collaboration,      440-442, 

444-446 

—  scope  for,  268,  273,  440,  442,  444-446, 
449,  596,  597 

Gillnelting,  276,  294,  440,  445 

—  effect  of  current,  254 

—  fish  behaviour,  551 

—  fishing  depth,  550 

—  in  fresh  water,  445 

—  mechanical  handling,  149,  263,  411 

—  monofilament,  44,  45 

—  synthetics,  42,  52-54,  56,  62,  67,  70, 
95,    96,    103,    104,    112,    137,    142-144, 
147-150,  152-165 

superior  efficiency,  147-165,  445 

Grey-black  line  recording,  503-505,  510. 
533,  536 

Grilon,  polyamide,  2,  67 

Ground-hugging  of  trawls,  seines  and 
dredges,  193,  199,  222-224,  242,  272,  304, 
30S,  314,  327,  383,  442,  444,  445 


Ground  rope,  173,  303-305,  309,  314.  442 

—  bobbins,  442 

Gun-pump,  electro-fishing,  584,  592 
Gussets  at  quarterpoints  in  trawls,  186-188, 
193,  197 

Haddock,  relation  to  bottom  temperature, 

457,  458 
Handlining,  279,  280,  426-428,  445 

—  artificial  bait,  426,  427 
Hanging  coefficient,  104 

—  in  relation  to  area,  104 

Hanging  of  nets  (framing),  101,  103,  104, 
149,  172,  173,  196-199,  210,  211,  214, 
216-221,  262,  305-310,  314,  396,  441,  443 

Heat  resistance,  16-18,  23,  25,  27,  31,  41, 
45,  58,67,  100,  137,  140 

Heat-setting,  15,  27,  29,  31,  41,  45,  47,  97, 
100,  109,  148,  160 

Hemp,  properties  and  uses,  2,  3,  30, 
37-40,  44,  83,  139,  141,  143,  160-165 

Herring,  relation  to  temperature,  458-461 

Herring  trawls,  173,  174,  196-199,  245-247, 
265,  298,  305-310 

High-opening  trawls,  307 

Hydraulic  drive  systems,  402,  407,  412 

—  power  boom,  412 
Hydrodynamic  floats  and  depressors,  179, 

200-204,  208-210,  220,  233,  240,  253, 
270-272,  342,  343,  345,  347,  352,  353, 
362,  451 

Hydrodynamic  forces  acting  on  gear,  153, 
155,  175-184,  189-192,  200-204,  209-212, 
215,  216.  218,  221-224,  245-249,  251-253, 
256-259,  266,  271,  336,  347,  442,  446. 
450,  451 

Hydrofoil  trawl  boards 

—  see  Trawl  boards,  hydrodynamic 

Impulse  current  in  electro-fishing,  576,  577 

—  advantages,  592 

—  effect  of  fish  length,  576 

—  effect  of  variation,  576 
— -  generator,  479    513 

—  impulse  length,  576,  592 

—  impulse  limits,  575 

—  intensity,  577,  579,  592,  594 

—  rate,   576-579,   582,    592-594 

effect  on  carp,  577 

fish  length,  577,  582 

troutf  592 

__  tuna.  592 

—  shape,  576 

—  stimulation,   577,  578 
Instrumentation  for  small  boats,  448,  452 
Intensity  of  light,  539,  546.  554 
Interpretation  of  echo  traces,  319,  491,  497, 

499,  502,  503,  507,  508,  510,  516,  519-521, 
526,  530 

Introducing  new  methods  in  primitive 
fisheries,  445 

Jelly  bottles  for  studying  gear  underwater, 

254,  255,  267 
Joining  of  webbing,  105,  214,  217,  261,  262 

Kanebian,  vinylon,  2,  67 

Kapron,  Russian  polyamide,  2,  141 

Kelly  eye,  304 

Kempff-iog,  247,  267 

Kenlon,  polyamide,  141 

Kites,  172,  173,  179,  185-195,  201,  202,  210, 

233,  240,  247,  253,  271,  298,  304,  309, 

343,  350,  358,  362,  443 

—  angle  of  attack,  187,  189,  247,  362 

—  Exocet,    362 

—  shape.   187,  253 

Knot  firmness,  28,  29,  41,  46,  47,  51,  52, 
79-81,  87-91,  92.  93, '96,  100.  101,  112, 
137,  148,  149,  155,  156,  158.  160 


INDEX 

Knotless   nets,   58,   62,   63,   94,    107-112, 
148,  178,  398, 

—  mending,    108,    112 

—  selvedges,   110,   112 

Knots,  24,  46,  62,  63,  78,  87,   100,   104, 

105,  148,  156,  158.  160,  178,  261 
Knot  strength,  definition,  14,  32 

—  of  twines,  16-18,  23-25,  32,  37,  38,  44, 
45,  51,  52,  60,  61,  67,  70,  71,  73,  74, 
83-86,  89-91,  94,  141.  142,  145,  153 

—  testing,  60,  66,  70,  72,  78-83,  87-92,  112 

—  type  of  knot,  24,  25,  27,  32,  45,  60,  61, 
71,  83-87,  89-91,  94,  110,  112 

Knotting  efficiency,  32,  55,  91,  92,  1 12,  140, 

141 

Knoxlock,  polyamide  141 
Krehalon,  properties  and  uses,  2,  20-29, 

57,  58,  67,  94 
Kuralon,   properties   and    uses,   2,   20-29, 

67,  95,  144,  145,  160 
Kuralon  No.  5,  19,  20,  24,  25,  28,  67 
Kyokurin,  properties  and  use,  20-29,  94 

Lampara  net,  442 
Lampara,    pursed 

—  see  Pursed  lampara 
Lastrich 

-  see  Trawls,  sidelines 
Lateral  strengih  of  twines,  87 
Lay,  influence  of,  59-61 
Legs    (towing    legs),  209,    210,    232-233, 

242-243.  305,  308,  346,  347,  357,  441.  443 
Liftnets,  67,  145,  275,  292 
Light  fishing,  see  Attraction  of  fish  by  light 
Light  gradient,  555,  573 
Light  movement,  539,  573 
Light  source,  429,  556,  557,  572,  573 
Limpet,  mounting  of  acoustical  oscillator, 

476 
Linen,  properties  and  uses,  32,  48,  70,  73, 

153 
Line  fishing,  275,  279-283,  426-437 

—  bait.  — see  Bait 

—  droplining,  433-435,  443 

—  handlintng,  — see  Handlining 

—  longlining,  — see  Longlining 

—  pole  and  line,  281.  428,  429,  445,  571 

—  trolling,   —  see  Trolling 

Lines,  use  of  synthetics  54,  67,  97,  147,  149, 

427,  428.  445,  447 
Live  bait  fishing,  390,  445,  571 

—  tank,  size,  538 

—  tank,  aeration,  538 

Livlon,  mixed  nylon  and  vinylidene,  1 53, 1 55 
Load  extension  of  fibres  (stress/strain),  142, 

143,  147 
Locating  fish,  453-473 

—  by  thermometric  methods,  453-461 

—  References,  461 
Locating  fishing  grounds 

•-  by  Dccca/Loran.  318-320,  467-471,  537 

—  by  Radar,  319,  463,  464 

—  by  R.D.F.  462,  463 
Lock  knot,  24 

Lodar  (echo  ranger),  513-516 

Longline  lures,  568 

Longline  shute,  431,  432,  447 

Longlines  of  wire,  440,  443,  447,  450 

Longlining 

—  artificial  bait,  567-570 

—  demersal,  440,  447 

—  effect  of  current,  255 

—  mechanical  handling.  432, 435-437,  443, 
447,  450,  473 

—  POFI    tub   gear,   430-432 
operation,  432,  447 

—  speed  of  hauling,  437,  447 

—  tuna,  430-432,  436,  437,  445,  447,  455, 
473,  569 

—  use  of  electricity,  581 

[6031 


Long!  ini  ng — continued. 

—  use  of  synthetics,  54,  67,  97,  147,  149 
Loop  strength,  14,  16-18,  32,  60,  61,  87,  148 
Loran,  270,  318,  320,  448.  537 

Lures,  427,  539,  541,  567-570,  572 

—  artificial  taste  and  smell,  569 

—  construction,  568 

—  fishing  efficiency,    567-570 

—  movement    568 

—  sponge  rubber  lure,  567,  568 

—  use  on  longlincs,  568 

—  versus  live  bait,  567,  568 
— -  References,  570 

Magnetic   compass   pilot,   311,   338,    339. 

448-450 

Magneto-striction  oscillators,  474,  475 
Manila,  properties  of,  25,  28,  36-39,  44,  86, 

139,  142,  143,  145 

—  production  and  use  of,  63,  64 

—  rope,  59-61 

Manometer,  differential,  for  measuring  net 

height,  167,  227,  234-235 
Manryo,  — see  Kuralon 
Marlon,  mixture  of  vinylon  and  nylon,  2, 

3,  81,  95,  152,  155,  391 
Materials,  references,  18,  29,  33,  145,  146, 

163 
Measuring  instruments,  167,  213,  225-233, 

234-240,  363-368,  451,  518 
Mechanics  of  fishing  gear,  153, 155,  166-183, 

214-215,  220,  251 

—  References,  174,  183,  184,  195,  221 
Mediterranean  trawl,  213-221,  270 

—  hybrid  net,  220,^270 
Menhaden  purse  seines,  147,  148 

Mesh  shape  of  trawls,  210,  216-218,  221. 

265,  441 
Mesh  size,  selectivity,  72,   101,   103,   142. 

143,  158,  164,  165,  174,  218,  219,  306. 

307,  310,  320,  350,  398 

—  measuring,  261 

Mesh  strength,  72,  78-81,  84-86 

—  testing,  72-74,  78-81 
Mewlon,  vinylon,  2,  67 

Midwater  trawling,  general,  298,  299,  333, 
368 

—  depth  regulation,  177, 180, 242, 268, 271  , 
299,  333-335,  339,  345,   348,  359,  360. 
362-368,  444,  446,  491,  518-521 

by  headline  transducer,  334,  446,  491 

—  design  and  construction,  209-212,  335- 
342,  361-362,  352-354,  518 

—  drawings,  337,  340,  352,  354 

—  echo    sounder    observation,    242-244, 
334,  335,  444,  446,  491,  518 

—  opening  height,  242.  271,  334.  335,  348. 
361,  518 

—  operation,  338,  339,  349,  353,  518,  520 

—  reaction  offish,  299,  334,  335,  346,  350, 
356,  358,  361,  444-446 

—  speed,  334,  446 

—  speed  warp/depth  ratio,  242,  334,  345 

—  used  near  or  on  bottom,  354-356,  441 

—  warp  angle/depth,  334 

—  with  Vinge  trawl,  352,  358 

—  References,  343,  358,  367,  368,  522 
Midwater  trawls 

—  one  boat,  209,  242,  298,  339,  346 
British  Colombia,  212,  298,  340-342. 

351-356 

Icelandic,  298.  340-343,  443 

Larson,  209,  340-343,  346,  347 

Sterner  Persson,  346,  347 

U.S.A.  209-212 

—  two-boat 

Larsen,  298,  336-339.  345,  348,  349. 

^yf>| 

Thames  sprat  trawl,  348-350 


MODERN     FISHING     GEAR    OF    THE    WORLD 


Model  testing,  166-174,  180-189,  252,  253, 
266,  267,  269,  271,  272,  441 

—  in  the  sea,  lakes  or  rivers,  167,  171-174 
-  scale,  166-169,  174,  181,  186,  187,  267, 

269 

—  tanks,  180,  205-208.  266 

-  References,  174,  183,  184,  195 
Modulus  of  elasticity,   14,  31.  39,  52,  55 
Moisture  absorption  (of  fibres),  15-18,  21, 
22,  40,  41,  46,  61,  67,  68,  101,  126,  157 

—  testing,  65 

Moisture  content  of  fibres,  15,  21,  46 
Molecular  orientation,  32,  33,  100 
Monoti  laments 

—  knot-firmness,  92,  156,  158 
-    use  in  gillnets,  96,  156-158 
Moorings,  380 

Mothership  operations,  150,  329-332 

—  transfer  of  catch,  329-332 
Mudropes,  213,  214 


Names  of  fishing  fibres  2,  95,  140 
Narcotizing  impulse  576 

—  limit,  576,  577 

—  elect ronarcosis,  578 
Net  design,   172,   173 

-  tell-tale  signs,  217 
Net  making 

—  cutting  machine  made  webbing,  105, 106, 
221,  261,  304,  307,  308,  310 

—  mechanical,  46,  47,  52,  63,    110,   148, 
154,   159,   160 

hand-operated,   63,    138 

--  rationalization,   110,  111 

tailoring,  102-106,   172,  210,  211,  214, 
217,  261,  305,  308,  310 
Net  preservation,  113-138 

acrylonitrile  agent,   121,   122 

—  "Arigal"  process,  123,  124,  138 

—  bichromate  treatment,    114,    120-122 

—  copper  naphthcnate  bathing,  113-    1 22, 
129,  130,  137 

—  copper  sulphate,  113-117,  122,  137 

-•  cyanoethylation,  also  acel>lation,  121, 
122,  124,  127,  137,  138 

—  dyeing,  -  -see  Dyeing 

resin  coating,  114-119,  122-124,  156 

—  sterilization,  sunlight-drying,   113,   114, 
117,  120-122,  132,  138,  139 

-  synthetic  sterilizers,    114-119 

—  tanning,  -   see  Tanning 
tarring,  — see  Tarring 

testing,  115-119,  121,  122,  128-138 

—  References,   122,   145,   146 
Newton's  law  of  hydrodynamic  force,  180 
Numbering  systems,  1,  3,  5,  7,  8,  9,  10-12, 

64,  70,  76,  98 

—  used  in  Japan,  6,  7,  10,  19,  20 
Nylock,  polyamide,   141 

Nylon  66 

discovery,  13 

—  continuous     multifilament     148,     149, 
156-158 

properties  of,  16,  30-33,  44-49,  56,  67. 
70,  73,  100,  140-145,  148,  152-155 

-  use  of,  2,  33,  62-64,  67,  70,  140-165,  305 
Nylon   -330,  Du  Pont  149 

—  combination  twines,  2,  3,  81,  95,  148. 
152,  155 

Nylon,  spun,     see  Spun  nylon 
Nylon,  staple,  — see  Staple  nylon 

Observing  gear  underwater,  -sec  Under- 
water observation 

Ocean  cables,  damage  by  trawlers,  371-374, 
448,  451,  452 

Opening  height  of  trawls,  —see  Trawls, 
opening  height 


Open-up  resistance  of  knots,  80,  87,  88,  91, 

92 

Organised  fish  searching,  471-473,  530,  531 
Oscillators,  —  see  Transducer 
Oscillotaxis,  575,  579 
Otter  boards,  — see  Trawl  boards 
Overhand  knot,  60,  87 

Pair-trawling,  301-304,  308-310 

PC  U,  polyvinyl  chloride,  2,  140,  141.  143- 

145 

PeCe,  polyvinyl  chloride,  2,    140,    141 
Perl i ton  dyes,  40 
Perlon,  polyamide  6 

—  properties  of  16,  30,  34-41,  85,  88-92, 
140-145 

—  uses  of,  2,  33,  36,  41,  42,  62-64,  67,  70. 
140-145,  159-163,  245,  305,  307 

Phototaxis,  546,  54<*,  550.  555.  571 
Physiological  effect  of  pulsating  current, 
541,  542,  575,  576,  577,  579,  593 

-  current  density,  579,  581,  592 

—  effective  rate,  577,  592 

-  electrocution,    581,    582 
elect  ronarcosis,  578 

-  electrotaxis,  541,  542,  578 

—  galvanolaxis,  577 
ion-composition  of  water,  579 
narcotising  effect,  579 

—  paralysing  effect,  579 
pulse  shape,  581,  592 

—  rheobasis,  577,  578 

—  threshold    value,    577 
References,  580 

Pitometer  log,  326 

Plaited  twines,  — see  Braided  twines 

Plankton  sampling,  472 

Plan-position  indicator,  478,  482 

Polar  front,  convergence  of  water  masses, 

454 
Polar  stimulation,  575 

—  electrotaxis,   540,   541,   542,   578.   583, 
585,  592 

—  oscillotaxis,  575 

—  Pfliiger-effect,  575 

Pole  and  line  fishing,  281,  428,  429 
Pole  and  line  fishing  jigs,  567,  568 
Polyacrylonitrile.  2,  94,  140-144 
Polyamide  6,  Nylon  6,  — see  Perlon 
Polyamide  66,  — see  Nylon  66 
Polyamide  11,  —see  Rilsan 

—  330,   Nylon   330,    149 

—  700,  Nylon  700,   148 
Polyamides,  general,  2,  3,  15,  94 
Polyester   fibres 

—  discovery  of,  13,  17,  43 

—  properties  of,  3,   17,  30,  43-52,  88-92, 
140-145,  148 

—  production  and  uses,  2,  43-45,  52-54, 
140-145,  147 

Polyethylene,  2,  3,  18,  94,  141,  271 

Polyurethane,  2,  141 

Polyvinyl    alcohol   fibres,    properties    and 

uses,   2,   3,    18,   21-29,   94,   95,    140-145 
Polyvinyl  chloride  fibres,  properties,  3,  17, 

55,  56,67,  140,  141,  143-145 

—  production  and  use,  55,  56,  62,  64,  140, 
141,   143-145,   152,   153 

Poly  vinyl  idcne  chloride,  properties,  30,  67, 

94 

•  -  production  and  use,  64 
Pony  boards,  298,  323 
Portable  echo  sounders,  535 
Portable  electrical  fishing  gear,  593 
Power   block,  400-413,  447 
Power  consumption  in  echo  sounders,  535 
Power  supply 

—  electrical  fishing,  582,    584,   585,    587, 
592-594 

[6041 


Power  supply — continued. 

—  light  fishing,  556,  557,  572.  573 

—  pulse  generator,  585 

Preservation  of  nets,  —see  Net  preservation 
Propagation  of  soundwaves  in  water,  478- 
482,    512 

—  References,  487,  500 
Pulsating  current,  577-579,  585 
Pump-fishing  with  light,  559-566 

—  area  of  attraction,  561 

—  cost  of  equipment,  574 

-  critical  range,  563 

—  development,   560,   566,   574 

—  efficiency,  564 

-  field  of  light,  563,  564 

—  fishing  depth,  566 

—  no/./les,  563,   564.   565,   566 

—  suction,  562 

—  References,  566 

Pursed  lampara,  391-393,  442 

—  the  net,  391,  392 

—  operation,  392,  393 
—  use  of  synthetics,  391 
Purse  seines,  292 

-  design  and  construction,  102,  153,  155, 
396-398,  404 

dories,  "395-397,  407,  408 

—  herring,  273,  399    404,  408,  413 

--  menhaden,  147,  148,  394-399,  407.  408, 
449 

one-boat,  404 
tuna,   148,   151,  445 

—  two-bo;M,  273,  357,  394-399,  407,  449 

-  uses  of  synthetics,  21 .  42,  54,  56,  62,  67, 
97.  145,  147,  148,   151-155,  398,  404 

Purse  seining,  general,  155,  391-413,  449 
deck  lavout,  392,  402,  405 

—  drum  seining,  400,  447 

—  one  bo  it,  405,  406 

-  operation.  392,  393,  395,  396,  405-413 
Pacific   Coast   type.  404-410 

power  block,  400-413,  447,  449 

—  power  roller,  400,  447 
strapping,  400,  447 

Pursing  speed,   155,  395 

Pyc  Fish-finder  echo  ranger,  489-492 


Quality  of  Msh,  effect  of  gear  and  method, 
142,  143,  163,  321.  330,  341,  451,  584, 
593,  595,  596 

Quantitative  use  of  echo  sounders,  489,  492. 
495,  507,  508,  510,  520,  524,  525-527 

Quarterpoints,   105,   106 


Radar,  318-320.  428 

—  reflectors,  430,  448,  464 
-  small,  463 

Radio   direction    finders,    -   see   Direction 

finders 

Radio  telephony,  398,  399,  465.  466 
-—  telegraphy,  466 

—  receivers,  466 

—  buoys,  466 

Rail  rollers,  301,  302,  349,  387,  392,  393 
Ramie,  properties  of,  3,  37,  150 

production  and  use,  2,  64 
Range,  echo  ranging,  477,  495,  507,  508, 

513,  515,  516,  530,  534 
Range  limitation  in  electro-fishing,  575,  585 

—  calculation,  585 

Reaction  of  fish  to  gear,  164,  165,  174,  272, 
273,  395,  491,  596 

—  study  of,  440 

—  to  midwater  trawls,  491 

—  to  purse  seines,  395 

—  to  trawls,  298,  312,  314,  335,  443 

—  to  vibrations,  440 


Recorder  paper,  dry  versus  wet,  494 
Recording  echosounders,   475,   476,   481, 
482,  486,  488 

—  versus  C.R.T.,  488-490,  492 
Recording  instruments  for  underwater  use, 

167,    168,  225-240,  244,  267,  268    334, 
356,  441 
Reef  knot,  24,  63 

—  testing,  66 
References 

—  Attraction  offish,  542,  546,  547,  549,  552 
555,  566,  570 

-  -  detection  of  fish,  477,  482,  485,  487,  500 
511,  516,  522,  524    527 

—  efficiency  of  synthetics,   145,   163 

—  electrical  fishing,  580,  582,  588,  591 

—  gear  classification,   295,  296 

—  location  of  fish,  461 

—  midwater  trawling,  343,  358,  367,  368, 
522 

—  net  materials,  18,  29,  33,  145,  146,  163 

—  preservation,  122,  145,  146 

-  -  rational  design  and  model  testing,  174, 

183,  195,  212,  221,  224.  259,  265 
Reflection    loss 

—  at  fish  body,  481,487 

-  at  sea  bottom,  480,  481 
Repellant  substances    572 

Resin  treatment,  21,  27,  28,  46,  47,  148 

Resistance  of  gear  to  water  flow,  153,  155, 
175-184,  189-192,  200-204,  209-212,  215, 
216,  218,  221-224,  245-249,  251-253, 
256-259,  266,  271,  336,  347,  442,  446, 
450,  451 

Resistance  to  chemicals,  16-18,  26,  41,  46 
55,  67 

Resistance  to  sunlight.  16-18,  26,  40,  41, 
47-49,  55,  94,  110,  111,  114,  115,  117, 
120-122,  124,  137.  140,  143-145,  149,  153, 
156-158 

—  testing,  67,  68 

Resolving  power  of  echo  sounder,  481,  487, 
499,  502,  533 

—  influence  of  ship's  movement,  502 
Response  to  auditory  stimuli,  346,  539,  350, 

540,  542,  571 

—  frequency,  539 

Response  to  chemical  stimuli,  540,  567.  571 

—  chemical  compounds,   540 
--  fish  extract,  540 

shark  repcllant,  540 

—  smell,  540 

Response  to  electrical  stimuli,  575  576,  577, 
579,  593 
A.C.  581 

-  -  current  intensity.  541 

—  D.C.  541,  575-580 
electrode  distance,  541 

-  frequency,  541 

Response  to  visual  stimuli,  335,  350,  443, 
444,  539,  541,  543,  546,  551  553,  555, 
567.  571 

-  bait,  541 

—  colour,  539,  551 

—  effect  of  wave  length,  539,  551,  553,  571, 
573 

—  intensity  of  light,  539,  546,  554 

—  light  gradient,  555 
--  lures,  539,  541,  567 

—  movement  of  light,  539 

-  phototaxis,  555 

—  physiological  factors,  548,  549,  590 

—  scaring  effect  of  light,  346,  358,  573 
•-  white  light,  539 

Response  to  stimuli 

—  clupeids,  548 

-  crab,  556 

—  kilka,  559,  561 

—  lobster,  539,  556 


INDEX 

Response  to  stimuli — continued. 

—  tuna,  538,  539,  540 

—  References  542,  546,  547,  549,  552,  555, 
566 

Reynold's  Number,  167,  169,  180,  189,  252 
Rhovyl,   polyvinyl  chloride,  2,    140,    141, 

143-145 
Rilsan,  polyamide  11,  properties  and  uses, 

2,   16,   141 
Ringnets,  292 
Ropes  and  lines,  of  manila,  59-61 

—  use  of  synthetics,  147-149,  155 

Rot  resistance,  16-18,  26,  41,  48,  93,  102, 

113-140.  148,  153,  157 
Rotting  value,  rotting  activity,  131,  133-137 

—  kind  of  water,  140 

Runnagc,  length  per  unit  weight,  35-37, 
44,  49,  59-61,  72,  74,  76,  83-85,  100,  110, 
111,  143,  145,  160 


Saran,  properties  and  use,  2,  20-29,  141,  157 
Scale,  of  echo  sounders,  489,  490 

—  expanded,  489,  490,  493,  494,  499,  505 
Sea  Scanar,  echo  ranger,  518-522 

interpretation  of  traces.  520 
Selectivity  and  conservation,  163-165,  218, 

219,  223,  224,  320,  347,   350,  444,  445 
Sense  of  vision  versus  sense  of  smell,  541, 

542,  572 

Sensitivity  echo-sounding,  509 
Sctncts,  57,  67,  1 12,  256-259,  285,  425,  556, 

557 

"Shark"  oscillator,  475,  476 
Shark  repellent,  540 
Shcltcrdeck,  321-324,  448 
Shooting  trawls,  234-239,  246,  311.  323,  339 
Shrimp  trawling,  311-316 

dual  rig,  311-312 
Shrimp  trawls,  165   311-316,  443 

—  try-nets,  311-314 

Shrinkage,  27,  40,  41,  45,  47,  48,  55,  71, 

74,   UK),   101,   142,   149 
testing  of,  66,  68,  72,  77 
Side  lobes  (of  echo  sound  beam),  474,  534 
Side  trawling,  300-303,  321 
Signal-to-noisc  ratio  490,  525,  526 
Silk  yarns,  properties  and  use,  2,  3,  84,  85, 

187 

Sinkers,  — see  Weights 
Sinking  speed  of  twines,  nets  and  ropes,  21, 

22,  70,  148,  153,  155,  271,  368 

—  testing,   68,   93 

Sisal,  properties  of,  37,  139,  143 

—  production  and  use,  64,  96,  143 
Sonar,     see  Echo  ranging,  474,  477,  482 
Sound  beam 

—  shape,  474,  475 

-  stability,  475,  533 

—  angle,  513,  515,  526,  533 

Sounding  rate,  transmission  rate  or  impulse 

rate,  4^0,  499,  524 
Sound  pulse 

—  absorption,   480 

—  amplitude,  496 

—  attenuation,  480 

-  length,  502.  514,  524 

—  quality,   502 

—  rate,  490.  499,  524 

—  reflection,  480 

—  shape,  502 

Spanker  on  vessels  hove  to,  429 

Specific  breaking  load  (of  fibres),   1,  32, 

72-74 

Specific  tenacity 
-  definition,  3,  72,  73 

—  in  relation  to  diameter.  37 

Specific  weight  (of  fibres),   3,    16-18,   30, 
37,  55    57,  67,  70,  71,  74,  94,  126    141 
153.  155,  168,  271 

f  605  1 


Specifying  gear,    105,    106,    170-172,    186, 

188,  194,  196-199,  211,  214,  220,  260-265, 

272 

Spread  meter,  226 
Spread  versus  gape,  441 
Spun  nylon,  148,  156-158,  398 
Square  of  trawls,  170-172,  197,  214 
Standardisation  of  numbering  systems,   1 

3.  5,  6,  8-12 

Standard i /at ion  of  terminology,  111 
Staple  n>lon,  — see  Spun  nylon 
"Static"  method,  of  testing  net  preservation, 

128-130 
Stern  trawling,  213,  250,  311-317.  321-324, 

354.  413,  448,  450 
hauling  of  big  catches,  448,  450 
Stick-held  dip  nets,  58,  422-425 
Stiffness  of  fibres,  15-18,  26,  39,  58,  141,  144, 

145,  148,  157,  158,  160,  164 

—  testing,  77,  78,  93 
Stownets,  275,  288,  393 

—  use  of  synthetics.  143V  145 
Strain  gauges,  electric,  267 

Stranding  of  twines,  1,  4,  5,  7,  19,  49-52, 

59-61,  76,  109,  111 
testing,  64-66 

Strategy  and  tactics  of  fishing,  453-595 
Strength  efficiency  of  fibres,  74 
Stress  and  strain  in  nets,   103,   104,   140, 

143-145,   176,   186,    198,  209,  210,  212, 

215-221 

—  tell-tale  signs,  217 
Stupefying  methods  of  fishing,  275 

—  concussion,    275 

—  explosives,   275,  421 

-  -  electrical,  — see  blcctrical  fishing 

—  poisoning,  275 

Suberkriib  trawl  boards,  245-247,  356,  359, 

360,  450 
Suppression  of  bottom  echo,  487,  495,  503, 

505,  510,  533,  536 

Surface  fishing  lights,  420,  421,  566,  573 
Surrounding  nets,  275,  292 
Sweeplines  (bridles),    172,    192,   209,   210, 

213-215,  301,  304,  305,  308,  313.  322-324, 

338,  341,  353,  355,  357,  441,  443 
Synthetic  fibres,  production  and  uses  of, 

32,  43,   55,  62-64,  67,  94,  95,   140-160 

—  substituting  for  natural,  69,  70, 96,  1 10 
111,  144,  152-165,  210,  245 

trade  names,  2,  95,  140 

Tailoring  of  webbing.   102-106,  210,  211. 

214,  217,  261,  305,  308,  310 
Tanglenets    276,  294 
Tank  testing,  180,  202,  203,  205-208,  252, 

266,  272 
Tanning,  40,    113-117,   120-122,    124,    137, 

138,  156 
Tapering,  in  netmaking,  105,  172,  211,  214, 

217,  261,  305.  308,  310 
Tarring,   22,   26-28,  41,  42.  48,   58,    109, 

113-117,  120-122,  137,  138,  141,  148,  158, 

396,  398 

Taslan.  textured  nylon,  149 
Tautfs  law  of  similarity  for  model  testing, 

182,   186 
Telemeter  for  midwater  trawls,  268,  334, 

491,  505,  506 
acoustic,  518 

—  electric  363-368,  451,  518 
operation,  520 

-   References,    522 

Television,    underwater,     158,     164,     165, 
209-212,  518,  596, 

—  references.   522 
Telon  dyes,  40 

Tenacity,  definition,  1,  3,  14,  72.  73 
Tenacity  versus  extensibility,  31,  32,  37-39, 
44.  51,  52,  73,  88-91,  94 


MODERN    FISHING    GEAR    OF    THE    WORLD 


Tensile  strength 

—  definition,  14,  73 

—  testing,  60,  61,  66,  70,  73,  76,  77,  82-86, 
96 

—  of  twines  and  ropes.  16-18,  23,  31-33,  37, 
38,  44,  55,  59-61.  67,  7J,  83-85,  88-91, 
126,  140-142,  148,  447 

—  in  relation  to  molecular  orientation,  32, 
33 

—  in  relation  to  temperature,  23 

—  in  relation  to  twist,  22,  23,  59-61 

—  wet/dry,  16-18,  23,  33,  37,  44,  45,  51.  60, 
61,  70,  73,  74,  82-86,  90.  95,  141,  148, 
153,  159,  161 

Terminology 

—  construction  of  twines,  1,  7 

—  names  of  fishing  fibres,  2,  15-18 

—  names  of  gear,  165,  260,  274-296 

—  properties  of  fibres,  1,  14,  15 
Terylene 

—  discovery  of,  2,  13,  43 

--  properties,  17,  30,  31,  43-52,  88-92,  94 

—  uses,  52-54,  94,  157,  160 

Testing  gear  during  commercial  fishing,  269, 

270,  441-444 

Testing  of  twines,  63-68,  70,  72-99 

—  standardization  of  methods,  75,  82,  93, 
95,  97-99 

Teviron 

—  properties.  20-29,  55,  56,  67,  93 

—  uses,  2,  55,  56,  62-64,  93 
Tex,  3,  8-12,  76,  84 
Textured  nylon,   149 
Thermocline,  456 
Tip-over  resistance 

—  of  knots,  78,  79,  93 

—  of  mesh,  81 
Titre,  1,  3 
Toughness 

—  of  fibres,  15-18,  26,  33,  73,  94,  141-143, 
149,   447 

—  index,  71.  73,  74 
Towing  block,  302 

Towing  resistance,  112,  145,  167-170,  177, 
193,  218,  245-249,  251,  253,  266-268,  270, 

271,  306-308,  310,  336,  367,  442,  450 

-  indicating  catch,  2,  14,  215,  221,  233, 

268,450,451 

Training  of  asdic  operators,  497,  498 
Trammel  nets,  151,  294 

—  use  of  synthetics,  151 
Transducer  Oscillator,  474,  475,  478,  515 

—  clamped  on  side  of  boat,  535 

—  fixed  versus  movable,  475,  534 

—  hoist  sweep  gear,  513,  515 

—  limpet  mounting  476 

—  mounting,  475,  476,  486,  490,  491.  495. 
506,  513,  523,  526,  533,  535 

—  on  mid  water  trawls,  334,  491 

—  size  and  shape,  474.  475,  478,  486,  490, 
509.  513 

—  removable,  535 
Transport  of  live  fish,  538 

Traps  and  setnets,  275,  284-287,  421,  425. 
451,  556,  557 

—  use  of  synthetics,  42,  54-58, 62, 140, 144, 
145,  147-149,  157,  158,  160 

Trawling,  297-374 

—  deck-arrangement,  — see  Deck  layout 

—  drum,  320 

—  in  deep  water,  319,  320, 440 

—  economical  speed,  326-328.  448-450 

—  power  requirements,  245-247,  249,  268, 
308,  325-328,  448,  450 

-~  speed,  151,  165,  168,  169.  187,  195, 
200-204,  209,  210,  237-240, 247, 266,  267, 
322,  334,  336,  347,  356,  44W51,  521 

—  measuring,   247,  267,   268,  270,   326, 
327,  334,  448 


Trawling — continued. 

—  midwater,  — see  Mid  water  trawling 

—  stern.  — see  Stern  trawling 

—  two-boat,  301-304,  308-310 
Trawl   boards 

—  angle  of  attack,  171,  179,  180,  228,  230, 
246,  251,  252,  345 

—  brackets,  252,  253,  302,  303,  313,  314. 
342,  352,  360 

—  connecting  links,  304,  314,  355 

~  distance  between,  167,  214,  226-233, 
246,  268 

—  dual   purpose,  midwater/bottom,  355 

—  fouling  of  ocean  cables,  371-374,  448, 
451,452 

—  hydrodynamic,  208,  210,  242,  244-247, 
252,  271,  336,  341,  342,  347,  352,  353, 
356,  359,  360,  444 

—  oval,  244,  374,  448,  450 
righting  moment,  252,  253 

—  sheering  effect  and  drag,  169,  170,  179, 
180,  246,  252,  271,  345,  360,  441,  450,  451 

—  shoe-plates,  302,  314 

-—  size,  weight  and  shape,  210,  213,  246, 
248-250.  252,  302,  303.  312,  360 

—  tilt,  227-232,  252,  314,  359,  360 

—  use  of  floats,  303,  355 

Trawl  cable  meter,  320,  369,  370,  448 
Trawls  classification,  289,  290 

—  design    and    construction,     102      165, 
186-189,     196-199,    209-221,    297,    298, 
304-310  314-316 

—  effect  of  length,  165,  180,  214,  218,  307, 
336 

—  floats,  169,  179,  200-204,  232-233,  236, 
239,  240,  253,  304,  305,  309,  314,  345, 
347,  349,  353 

—  gallows,  301,  311,  322-324,  442 
-  gear,  297-324,  333-370 

—  ground-hugging,  193,  199,  215,  222-224, 
242,  272,  304,  305,  314, 

— headline/footrope  length  ratio,  196-199, 
215,  235,  236,  271.  304,  314,  336,  361. 
362.  443 

—  headline  shape,  size  hanging,  105,  172, 
173,  188,  194.202,204  210,211.215,217, 
218,  264,  265,  304,  305,  309,  314 

—  horizontal  opening,  106,  167,  172,  175, 
187,  194,  197-199,  209,  214,  298 

measuring.  192,209,210,214,226-233 

sideline,  lastrich,  168,  198,  200,  210, 

218,  220,  221,  242,  270,  305-307,  309, 

353,  441,  443 

—  legs,  209,  210,  232,  233,  242,  243,  305, 
308.  346,  347,  357,  441,  443 

-  net  diagrams,  170,  171,  188,  194,  197, 
198,  211,  214,  220,  265,  304,  306,  309, 
314-316,  337,  340,  353,  354,  357 

--  powerhandling,   320-324 

—  opening   height,    165,    167,    172,    173, 
185-202,  209,  210,  215,  218-220,  227-240, 
242,  247,  270,  298,  304,  307,  441,  442 

measuring,  165,  167,  172,  173,  192, 

193,  209,  210,  215,  216,  227-240,  242,  247 
Trawl  types 

—  cod,  265 

—  flatfish,  303-305 

—  German  cutter,  300-310 

—  herring,  305-310, 444 
— -  high-opening,  305-310 

—  roundfish.  303-305 

—  shrimp,  311-316 

—  try-net,  311-314 

—  use  of  synthetics,  42,  53,  54,  67,  94,  96, 
142, 143, 145, 148, 151-155, 196,210,211, 
299,  305,  307,  308,  336, 341,  345,  349, 443 

[606] 


Trevira,  polyester,  properties  and  use,  2, 

88-92,  140,  143 
Trolling 

—  deep,  177,266 

—  jigs,  567 

Try  nets,  shrimp  trawling,  165 

Tuna  purse  seines,   148 

Twine,  definition,  construction,  1,  4,  5,  7, 
10,  19,  20,  76 

Twines  of  mixed  fibres,  2.  3,  62.  64.  81. 
95,  148,  152,  155 

Twist,  5,  7,  20,  22.  23,  26,  27.  31,  37.  41, 44. 
45,  49-52,  59-61,  64,  70,  84,  90,  92,  109, 
112,  141,  143,  155 

Twist  factor,  49-52 

Twist,  testing,  63-66,  76 

Two-boat  trawling  versus  otter  trawling, 
240,  302 

Two-boat  trawls,  underwater  measure- 
ments, 234-23S 

Tynex  polyamide,  141 

Underwater  lamps 

—  construction,  573 

—  depth  submerged,  556,  573 

-  field  of  light,  563,  573 

—  generator,  573 

—  light  intensity,  561,  573 

-  operation,  557,  558,  562,  566,  573,  574 

-  -  range  of  attraction,  560,  561 ,  573 

—  versus  surface  lamp,  560,  571,  573 
Underwater  lights,  — see  Underwater  lamps 
Underwater   measuring   instruments,    167, 

168,  213.  225-233,  234-240,  247,  254,  255 
Underwater  observation  of  gear,  158,  164- 
168,  170-174,  202,  209-212,  213-222,  267, 
348 

—  diving  sled,  210,  213 

—  References,  212,  221,  224 
Underwater  photographing  158,  167,  168, 

171-173,  202,  204,  209,  221,  350,  384,  385 

Vinge  trawls,  173,  357.  358 
Vinyl  fibres,  17 

Vinylidene  chloride  fibres,  properties  of,  1 7, 
56.  67 

-  production  and  use,  2,  62,  64,  152 
Vinylon,  polyvinyl,  alcohol,  properties,  18, 

5fc.  67,  95,  141,  153 

—  production  and  use,  2,   62,   64,    140, 
152-155 

—  use  in  longlines,  447 
Vim/on,  copolymers,  2,  141 

Visibility  of  twines  and  nets,  41.  46,  140. 
IM,  145,  157,  164,  165.  214.  335,  336 
Visual  sense  of  fish,  543,  571 

—  lens  and  retina  of  fish  eye,  543 

—  recognition  of  food,  545 

—  recognition  of  twine,  545,  571 

Warp  leads,   bollards,   blocks,   301,   302, 

314,  322-324 
Warp 

—  length/depth  ratio,  209,  213,  222,  250 

—  angle  of  attack,  232 

—  effect  on  opening  height.  230,  231 

—  effect  on  spread,  230,  232 

—  effect  on  tilt,  230,  231,  232 

—  meter,  320,  369,  370,  448 
Warps 

—  angle  between,  190-194,  214,  246,  450 

—  angle  with  horizontal,  268,  334,  339,  345 

—  curvature,  177,  222,  334,  345 

—  diameter,  213,  301,  311,  334,  336,  338, 
349,  369,  370, 

—  electrical  conductor,  364,  365 

—  importance  of  equal  length,  188,  242, 
369.448 

Water  flow  through  trawls,  — see  Filtering 


Water  speed,  effect  on  gear,  153,  155,  175- 
184,  189-192,  200-204,  209-212,  215,  216, 
218,  221-224,  245-249,  251-253,  256-259, 
266,  271,  336,  337,  442,  446,  450,  451 

Weight  of  twine,  16-18,  21-22.  64-65,  70-74, 
83,  141,  145.153,155,271 

Weights  sinkers,  157,  210,  211,  262,  303, 
305,  308,  349,  353 

Well  for  live  bait 

—  size,  538 

—  aeration  538, 

Wet  strength/dry  strength,  16-18,  23,  33, 
37,  44,  45,  51,  60,  61,  70,  73,  74,  82-86, 
90,  95,  141,  148,  153,  159,  160 


INDEX 

White  line  recording,  487,  495,  533,  536 
Wings,  of  trawls.   170-172,  197-199,  210, 
214,  215,  264,  265,  305,  306,  308 

—  triangular,  306,  308,  309 
Winches 

-  Danish  seine,  378,  379 

—  gillnet,  435 

—  longline,  432,  435-437,  447,  450 

—  purse,  392,  395,  397,  449 

-  trawl,  300,  301,  311,  312,  338,  368,  451 
overload  release,  452 

Working  groups  on  numbering  systems  and 

testing  methods,  12,  97-99.  110 
World  Fisheries  Abstracts,  1 1 1,  597 


Wounding  gear,  274,  276,  277 

Yarn  count,  denier,  1,  3,  5,  7-12,  20,  64,  66 
Yarn  count,  English,  1,  3,  7-12,  20,  64,  66 
Yarn  count,  metric,  1,  3,  8-12 
Yarn,  definition,  1,  4,  7,  10,  75,  76 
Yellow  fin  tuna,  relation  to  hydrographic 

conditions,  455 
Young's  Modulus,  14,  31,  39,  52,  55,  76,  77 

Zones  of  convergence,  454,  455 

—  divergence,  455 

—  local  difference,  456 


[607] 


separated  parts  of  the  world,  considerable 
was  inevitably  occupied  in  the  final  preparat 
that  mass  of  material  for  publication.  The 
however,  is  a  work  of  authority  with,  it  is  be 
as  high  a  standard  of  accuracy  and  compreh* 
ness  as  has  ever  been  achieved. 

The  volume,  designed  as  a  companion  to  F 
Boats  of  the  World  issued  in  1955,  is,  ii 
slightly  larger  in  number  of  pages  and 
in  a  total  of  some  608  pages  there  lie  nearly  f 
words,  with  over  800  diagrams  and  illustr 
and  nearly  200  tables  all  embodying  vital 
mation. 

A  preface  by  Dr.  D.  B.  Finn,  head  of  th 
eries  Division,  and  a  special  introductory 
by  the  editor,  HiJmar  Kristjonsson,  bring  < 
highlights  of  the  work  being  done  to  help 
the  food  production  of  the  world.  Both  ad 
countries  and  under-developed  areas  will  fi 
volume  of  the  highest  value. 

To  make  the  book  of  the  fullest  use  to  ; 
an  audience  as  possible,  abstracts  of  each 
have  been  given  in  French  and  Spanish 
as  in  English.  Thus,  while  the  body  of  th 
is  in  English,  it  will  be  possible  for  those  in 
and  Spanish  speaking  countries  to  apprec 
nature  of  special  chapters  and  seek  fuller  trai 
where  desired. 

The  work  is  essentially  practical.  It  is 
any  sense  a  text  book  although  it  undo 
will  be  a  steady  source  of  reference  for  mar 
to  come  containing  as  it  does,  the  concc 
knowledge  of  the  leading  technicians  and  p 
operators  of  all  the  important  fishing  coum 

A  specially  well   prepared  index   is,   of 
provided  while  the  full  list  of  contribute 
its    own    assurance    as    to    wealth    of   kn« 
drawn   on    in   its   compilation.    Like   the 
Boats  of  the  World  it  does  not  claim  to 
last   word"   on   gear,   for   improvement   M 
be  sought,  but  it  is  "the  last  word"  at  th 
of  time  when  published. 


reprinted  1968 

FISHING  NEWS  (BOOKS) 

Ludgate  House,  110  Fleet  Street 

London,  E.C.4 

England