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.
FISHING NEWS (BOOKS) LTD
Ludgate House, 1 10 Fleet Street
London, E.C.4
England
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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14
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15
15
15
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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
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84
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
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100
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100
101
101
102
103
104
105
106
107
107
108
109
110
110
110
111
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
139
140
141
141
141
142
143
143
144
147
147
148
148
148
148
148
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
214
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 ....
Page
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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 ......
Page
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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
No.
336
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
Page
No.
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 .....
Page
No.
483
483
484
485
486
486
486
488
488
489
491
491
492
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
Page
No.
509
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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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
*k 1
,..
Cj
D
A
j: '50.
„ D
^T* 40.
r'
tf 3*
B
A C D
A
Q
D
«£ 20
B
...
B
nn
_.
o> 55
B
S| 10.
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
•M A
J* 60.
A
tiO
h"
">-
...
sii extension
sile strengt
CO ^
p (0
B
C
ft.
CD
A
B
C
D
B
A —
C D
A
B
C
D
A
B
C
D
A
B
w*
C
£
B
A CD
A
B
CD
rOn
D
...
...
.-
o g 20-
•-..
—•
...
r
0.
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.
e70
0 -
g
B
Sea
x
ft)
n Jl
A
S5a
•Vi
Is
A
C,
D
A
JB
I
C D
A
D
*
A
A
C D
A
R
CD
A
E
CD
A
D
A
..
^2fx
B
IW!
...
•-
..,
C
D
C
D
B
...
...
q
n
i>
43
wlO-
W
•> 0-
"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
A
A
r
A
...
50.
^40.
8
r
JJ
C
m
,D
•n
CD
A
B
A
:
TX
A
B
C
D
_
C
D
"S3Q.
BCD
Bj
..,
Cj
D
<J
...
C
u
...
C
D
...
f201
B
...
rwi
B
...
...
B
•M
B
m
I10-
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.
A
a50-
...
s
A
A
B
C
A
B
d " A
...
A
A
—
A
3
r1 "^
—
...
i—
CD
rsr!
C30-
— .
^
A-/
C P
Tl ^
_ Ffc
BC D
^20.
.. «
...
£
—
\^
nm
C
D
C
-..
#•.
...
C
"10.
**
Q) f\
5
S
Ql
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
«60_
...
..,
A
MW
r°~
A
•40.
A
mSm
n
CD
D
jf 30_
...
_
CD
ft
D
=5
C
...
_ D
._
...
C
• — •
tt 20
...
B
B
<J
B
§10.
B
rrra
B
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/^^ , ^
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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
Textile Journ.. 74-76. 1953.
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-
burgh, 1948.
18 Klust, G.: Perlonschnure, ihre Herstellung und
Eigenschaften unter Beriicksichtigung ihrer Verwendung in der
Fischerei. Ulm/Donau, 1957.
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
aid in certain forms of commercial fishing and in various problems
of fisheries research. F.F.B. -*(3), 1955, 130.
3 dc Boer, P.A. Trawl gear measurements obtained by under-
water instruments. Interim copy.
4 Fukutomi, T. *A mjthod of approximate estimation for lee
way or oce in current from the inclination of the lead line. J.O.C.
-?<3), 1943, 19.
fi Fujitu. H. and T. Yokota. The drag action on a net in a uniform
current. I. J.A.P. 20(2). 1951. 59.
6 Fujita, H. The drau action on a net in a uniform current. II.
M,A.K. 66, 1953, II.
7 Hamuro, C. *A study of Danish seine by means of recording
the height of entrance and direction of ground rope. B.S.F. 20(5),
1954, 353.
8 Hamuro, C. and K. Ishii. *Study on the automatic net depth
meter, automatic net length meter for salmon gillnet and the results
of measurement at sea. T.R.F. #, 1956, 29.
9 Hamuro, C. *Study of the automatic net depth meter, net
height meter for crab tangle net and its practical application.
T.R.F. 5, 1956, 183.
10 Hayashi, H. *An investigation of a spreading device for
trawling net. J.F.E. .?, 1933, 37.
II Hosino, S. and N. Sato. (An experiment on the forms of the
fishing net piece in the air currents.) F.l.S. #, 1941, 1.
12 Isouti, N. and T. Kawakami. Mechanical studies on the
trolling gear. I. Estimation of the fishing depth. M.A.K. 76, 1957, 1.
13 Kawakami, T. and H. Tubota. On the configuration and distri-
bution of tension of a rope in a uniform stream. M.A.K. 66,
1953, I.
14 Kawakami, T. Mechanical action of the otter board of the
trawl net. B.S.F. 79(4). 1953, 228.
16 Kawakami, T. Equilibrium configuration of a rectangular
strip of net subjected to a uniform current. M.A.K. 72, 1955, 1.
16 Kawakami, T. Mechanical characters of the drag net. M.A.K.
72, 1955, 5.
17 Kawakami, T. and Y. litaka. On a simple instrument, Siomi-
iro, for detecting underwater current. B.S.F. 20(1 Ij, 1955, 962.
18 Ketchen, K. S. Preliminary experiments to determine the
working gape of trawling gear. P.P.C. 88, 1951, 62.
19 Kobayashi. K. and H. Takahashi. *A study on fishing trawl
perated in any depth of sea-water. B.F.H. 7(3, 4), 1951, 139.
20 Kobayashi, K. and H. Takahashi. *A study on fishing trawl
operated in any deptn of sea-water (Continued). On the relations
between the length of warp and the frontage of otter board. B.F.H.
2(1), 1951, 86.
21 Kobayashi, K. and T. Deguchi. *An experiment of a beam-
type trawl net for fish larvae at the various depths of sea-water.
B.F.G. j(l), 1952, 104.
22 Kobayashi, K., H. Takahashi and M. Ucno. * Fundamental
studies on spherical glass floats for fishing nets. II. On the water
resistance of glass floats. B.F.H. 4(4), 1954, 348.
-3 Kobayashi, K. *An experiment on a mid-water trawl. IV
On its mechanism and a trial practice. B.F.H. 7(1), 1956, 21.
24 Koga, S. *On the extension of the wing of trawl net. B.F.N. 3
1955, 15.
25 Kumakori, T., C. Hamuro and K. Ishii, O. Furuya. (Catch
gauge for two boat trawl.) T.R.F. J, 1952, 139.
26 Kumakori, T. and K. Ishii. (F.xperiments on manufacturing
of remote control towing thermometer for mid-water.) T.R.F. 3
1952, 159.
27 Kumakori, T., C. Hamuro, K. Ishii and O. Furuya. (Catch
gauge for two boat trawler.) T.R.F. 4, 1953, 112.
2ft Kumakori, T. and K. Ishii. * Automatic net-height meter and
automatic ground rope indicator for two-boat trawler and results
of experiment for these apparatuses. T.R.F. 5, 1954, 73.
29 Kumakori. T.. C. Hamuro, and K. Ishii. *Automatic net-
height meter and automatic ground rope indicator for trawler and
results of experiments for these apparatuses. T.R.F. 6, 1955, 41.
30 Miyuke, Y. On the plane net. I. Resistance of plane nets in
water. f.F.L 23(2). 1927, 21.
:n Miyamoto. H. *The excess buoyancy of gillnel and the ten-
sion of them. B.S.h.^3), W4, 147.
32 Miyamoto, H., M. Nomura and Y. Shimozaki. *Rcsistance
of plane net against the flow of water. I. Effect of knot type on
the resistance of net. B.S.F. 77(8, 9), 1952. 249.
33 Miyamoto, H. and M. Nomura. *Resistancc of plane net
against the flow of water. II. Effect of different shapes of the
mesh upon the resistance. B.S.F. M(7), 1953, 327.
34 Nomura, M. and Y. Nozawa. ^Resistance of plane net
against the flow of water. IV. On the inclination of threads in a
current. B.S.F. 20(V). 1955, 762.
3<t) Nomura, M. and K. Mori. * Resistance of plane net against
flow of water. III. Effect of kind of fibres on the resistance of net.
B.S.F. 27(11), 1956, I 1 10.
36 Nomura, M. and K. Mori. * Resistance of plane net against
flow of water V. On the inclination of net in a current. B.S.F.
27(11), 195(>, 1114.
37 Okuno, H. * Model experiment on plates to sink an end
of a trawl rope bclovt the water surface or hydrofoil sinker.
B.S.F. 5(3). 1936, 155.
38 Pode, L. An experimental investigation of the hydrodynumic
forces on stranded cables. R.T.M. 77,?, 1950.
39 Pode, 1.. Tables for computing the equilibrium configuration
of a flexible cable in a uniform stream. RT.M. 687. 1951.
40 Pode. L. Configuration and tension of a light flexible
cable in a uniform stream. (Appendix to the R.T.M. 777), 1956.
41 Sato, N. and 1. Takayama. * Experiments on hydrofoil
sinker for a troll line. J.F.I. J, 1933. 73.
42 Sato, N. and S. Kurita. (An experiment on the resistance
and lift for a float of a seine net.) F.l.S. V, 1943, I.
43 Taniguchi, T. *On the resistance of various codends fixed
in a stream. I. B.S.F. 27(5), 1955. 291.
44 Taniguchi, T. *On the resistance of various codends
fixed in a stream. II. B.S.F. 27(9). 1956. 969.
46 Taniguchi, T. *On the resistance of various codends fixed
in a stream. 111. 27(11), 1956, 1107.
46 Taniguchi, T. *On the resistance of various codends fixed
in a stream. IV. B.S.F. 22(12), 1957, 727.
47 Tauti, M., T. Miura and K. Sugii. Resistance of plane net
in water. J.F.I. 27(2), 1925, 11.
48 Tauti, M. (Holding power of anchors). T.G.K. 77, 1930, 16.
[183]
MODERN FISHING GEAR OF THE WORLD
4tt Tauli, M. The force acting on the plane net in motion
through the water. B.S.F. J(l). 1934, 1.
60 Tauti, M. A relation between experiments on model and on
full scale of fishing net. B.S.F. 3(4), 1934, 1.
61 Terada, T., I. Sekine, and T. Nozaki. (Study on the resist-
ance of fishing net against the flow of water.) J.F.I. 10. 1915, 1.
52 Thews, J. G. and L. Landweber. On the resistance of a heavy
flexible cable for towing a surface float behind a ship. R.E.M.
418. 1936. Appendix I.
** Thews, J. G. and L. Landweber. The tension in a loop of
cable towed through a fluid. R.E.M. 422, 1936. Appendix. Model
Experiment. (Fixed net).
*4 Honda, K. and Y. Matumoto. * Model experiment on the
sea fixed net for fishing amber-fish. B.S.F. 72(3), 1934, 91.
56 Honda, K. and collaborators. Model experiments on fixed
nets for amber-fish, Seriora* used in the coast of Mie Prefecture.
J.T.F. 47(2), 1955, 195.
56 Kanamori, M. *A tentative experiment on the improvement
of yellow-tail setting net (Preliminary report). J.K.C. 7, 1950, 4,
57 Kanamori, M. and S. Enami. ^Studies on the improvement
of yellow-tail setting net on the model experiment of mid-layer
trap-net. I. M.F.K. 2(1), 1952, 2.
58 Miyamoto, H. The form of amber-fish keddle net on the
basis of model experiments. B.S.F. 4 (3), 1935, 153.
69 Miyamoto, H. * Model experiments on sea fixed nets for
fishing amber-fish, Scriola. I. Ettyu keddle net and ordinary keddle
nets. B.S.F. 5(3), 1936, 158.
60 Miyamoto, H. * Model experiments on sea fixed net for
fishing amber-fish, Seriola. II. Trap nets set on the fishing grounds
under strong tidal currents. B.S.F. 5(4), 1936, 227.
61 Miyamoto, H. * Model experiments on sea fixed net for fishing
amber-fish, Seriola. III. Trap nets. B.S.F. 5(6), 1937, 361.
62 Miyamoto, H. * Model experiments on sea fixed net for fishing
amber-fish, Seriola. IV. Keddle nets and trap net. B.S.F. 6(6).
1937, 311.
63 Miyamoto, H. * Model experiments on sea fixed net for
fishing amber-fish, Seriola. V. The relation between the weight of
sinkers and the deformation of amber-fish nets subjected to various
currents. B.S.F. 7(6), 1939, 319.
w Miyamoto, H. * Model experiments on sea fixed net for fishing
amber-fish, Seriola. VI. Fishing gear in relation to the current
prevailing on the fishing ground. B.S.F. 9(5), 1941, 181.
65 Miyamoto, H. * Model and full scale experiment on set net.
Preliminary report. I. Double trap net for cod. B.S.F. 13(2), 1947,
51.
66 Miyamoto, H. * Model and full scale experiment on set net.
Preliminary report. II. Double trap net for cod. B.S.F. 7J(6), 1948,
263.
e7 Miyamoto, H. * Model and full scale experiments on set net.
Preliminary report. III. Trap net for sardine. B.S.F. 74(1), 1948, 31.
68 Miyamoto, H. 'Study on the set-net. B.T.I.. 2,1951, I.
69 Miyazaki, T. * Model experiments on fixed nets for fishing
salmon in Kamchatka. B.S.F. S(3) 1939, 129.
70 Miyazaki, T. (Model experiment on the leading net). B.S.K
77(6), 1942, 287.
71 Miyazaki, T. (Model experiment on a leading net. II.) B.S.F,
70(2), 1942, 63.
72 Miyazaki, T. (Model experiment on a fixed net). B.S.F. 77(3).
1942,101.
73 Mori, Y. * Model experiment on the sea fixed net for fishing
amber-fish. B.S.F. 70(6), 1942, 278.
74 Okahe, G. * Model experiments of a fishing net, Anko-ami.
B.S.F. 6(6), 1938, 305.
76 Pack, Y. *A model experiment of sardine trap net "Hisaxo-
amr. B.S.F. 22(11), 1957,657.
76 Saito, T. *Model experiments on the two kinds of sea bottom
fixed nets for fishing "7ar<T. B.S.F. 2(5), 1941, 185.
77 Sato, N. *On a tank experiment with seine nets. J.F.E. .?.
1933, 49.
78 Sato, N. (An experiment with models of a surface seine net).
F.I.S. 9, 1943, 35.
79 Simizu, H. '"Model experiments on the sea bottom fixed nets
for fishing "Tara". B.S.F. 9(5), 1941, 191.
H0 Sugano, S. *The model experiments on fixed nets for fishing
salmon in Kamchatka. B.S.F. 4(6), 1936, 397.
(Towed net).
81 Barraclough, W. E. and W. W. Johnson. A new mid-water
trawl for herring. B.F.C 704, 1956.
82 Imamura, Y. (Model experiment of the British herring trawl)
B.S.F. 70(5), 1942, 221.
83 Imamura, Y. (Model experiments on a trawl net). B.S.F. 77(2),
1942, 61.
84 Koike, A. * Model experiments on a Teiso-hikiami (trawl net).
B.S.F. 79(1), 1953,8.
86 Miyamoto, H. The model experiments on drifter drag net
for fishing "Sirauwo" in Lake Kasumigaura. B.S.F. 4(5), 1936,
310.
8fl Miyamoto, H. The model experiments on sailing trawl net
used in Mikawa Bay. B.S.F. 4(6). 1936, 391.
87 Miyamoto, H. * Model experiments on trawl nets. B.S.F. 5(1),
1936, 19.
HR Miyazaki. T. Preliminary report on experiments with small
trawl nets. I. R.U.M. 7(2), 1952, 215.
*9 Miyazaki, T. Preliminary report on experiments with small
trawl nets. I. Change in the height of square part at the mouth of
the net with changes in the tension of the rope of the net. R.U.M.
7(2), 1952, 223.
90 Miyazaki, T. The relation between the mouth height of the
square part of small trawl net and its pulling velocity. B.S.F. 79(4),
1953, 223.
01 Nomura, M. * Model experiments on two boat type trawl net.
B.S.F. 76(8), 1951, 347.
92 Nomura, M. and T. Yasui. * Model experiments on trawl
nets of various types. B.S F 7*(I2), 1953, 727.
93 Ogura, M. *Model experiment on a Tyus(9~hikiami(\rw\ net).
B.S.F. 20(4), 1954, 259.
94 Saito, T. and H. Simizu. Model experiments on *47>#//n-ij/w",
a drag-net, for fishing "Tara\ B.S.F. 70(1), 1941, 11.
95 Sato, N. (An experiment on the use of a surface trawl net.)
F.I.S. 7, 1934, 1.
98 Sato, N. (An experiment with models of a surface seine net).
F.I.S. 9, 1943, 35.
97 Sugano, S. * Experiments with models of a surface trawl net
lor fishing salmon. B.S.F. 5(1), 1936, 12.
(Encircling net)
98 litaka, Y. Model experiments on the sardine purse seine
operating in Hiuganada I. B.S.F. 20(7), 1954, 571.
9U litaka, Y. Model experiments on the sardine purse seine
operating in Hiuganada II. B.S.F. 27(1), 1955, 6.
100 litaka, Y. Model experiments on the sardine purse seine
operating in Hiuganada III. B.S.F. 27(7), 1955, 459.
101 litaka. Y. Model experiments on the sardine purse seine
operating in Hiuganada IV. B.S.F. 22(7). 1956, 389.
102 I none, M. Model experiments on a sardine ringnet. B.S.I
79(9), 1954, 942.
103 Uno, M. The form and the tension on pursing line of a
mackerel ring-net used at Kosaito, Tyosen, as known by a model
experiment. B.S.F. 4(3), 1935, 147.
(Other nets)
104 Pack, Y. Model experiments on a saury blanket net (Boukc-
ami). I. B.S.F. 27(9), 1956, 978.
105 Wang, Y. Model experiments of Genziki-anu\ a kind of net
for fishing shrimp. B.S.F. 5(6), 1937, 369.
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
j^P^^i^-iWfc'-:--.^
^;'";u'" .';;;':',' '
^.v^^ftfe'^'-iwlt"
..m
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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[2861
CLASSIFICATION OF FISHING GEAR
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MODERN FISHING GEAR OF THE WORLD
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[290]
CLASSIFICATION OF FISHING GEAR
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MODERN FISHING GEAR OF THE WORLD
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CLASSIFICATION OF FISHING GEAR
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MODERN FISHING GEAR OF THE WORLD
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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-
X WIND A»TCftN-4rt9MCLwWiTMtaAH0
+ WWO JkMAAO - - - -
8 VAKO Asm* •
v
"4O< —
L
lTMtaAH0 1
V
* J
• 4.rr •»*!,> wmt WWo 1
f
- - * - - • J
<»»* 6»*0»
•
SHU** S^cgft - KNOTS
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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or o . /T ,
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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
If
^aW'Tw
'".m $4»tffcfete
v ^IffyHift. ^-H^'**/1
."' "•i^'-riiA-iW^w^*'^;'
' - L1 '• ' 'i ,'4 » b v- •• iiL>< J< .^** ^> ilfv
• ?^|:^;5i^;^
,*!8T' - * s-.t1 •' ^•iii*'?
•. * : . • « « . •»' ' -I
^ • ».'f . • - * • , *.'*"<
CM
ifelHi" ^ -5
^a®^;' .-;'
:-*/tM '.'g'vifl1;. /
« t
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
,„,.--«,. 70
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
* ****** ^^iWf -;
Trawl Depth .r, Vvlu r
H^'u r.i Drjrf*. »r r»th»
T«*i«- -V ,. i K.«Kiiu (|».
*. (.«*«> 9«|.7«t
!*ft-:tt
1 - - , i\ .Jo
----- *|>
4-*nrtur «'<tw»«»r H*r» l
of ripe C»H»H, ». f
• *
Truttl O*|rth ut r«1**v* « I »«d tmrt • SO
Vftttum tWuih MI r»r«.m»< M <
Tm.fc St.rt rt»bjn« (P. s. T > --1*2
ratlin* Tune in Mm»ti»« ... ~)n
'
B
">» <•",'
Truwl D*(»Ui i ft r^tiiutnn (1^*4 I «i«) - 4«J
Tim* ftt*rt K.nh.ni (p! * T * - JOM
Tf««t O^Ui <R K»lhurn» < Ur»d 1 .*»*)- -3TJ
*M*«tlMN*M M^T*
Ka,T) am
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