MODERN FISHING GEAR OF THE WORLD: 2
MODERN FISHING GEAR
OF THE
WORLD 2:
Arranged by
THE TECHNICAL STAFFS
OF
FISHING NEWS INTERNATIONAL
AND
FISHING NEWS
from the papers and discussions at the
Seconcf FAO World Fishing Gear Congress, London, 1963
Published by
FISHING NEWS (BOOKS) LTD.
LUDOATEf HOUSE, 110 FLEET STREET, LONDON, E.C.4., ENGLAND
JUNE 1964
NOTICE TO READERS
TUs book embodk* the text and illustrations of the papers presented at the Second
Worid Fishing Qear Congress, London, 1963, which WM organized by the Food and
AgrkuUnre Orgmniadoo of the United Nations (FAO). However, tins is not an
FAO publication.
The Organization authorized the repnbUcatkHi of the material here presented as
a service to the fishing industry, hot the responsibility for the reports of the discussions
fbOowing the presentation of the various papers rests with the edtorial staff of the Fishing
News Intenutioittlt^ of the pabtohei^ Fishing News (Books) L^ The text of the
reports of the discussions has been reviewed for accuracy.
The views expressed, both hi the papers and the discussions are those of the
htdrridBab concerned and do not necessarily represent the views of FAO.
© COPYRIGHT 1964
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 are not necessarily those of the Food
and Agriculture Organization of The United Nations.
MADE AND PRINTED IN BRITAIN
PRINTED BY MERRTTT & HATCHER LTD., High Wycombe and London
ADVERTISEMENT SECTIONS
For con?miMC6 of prawntation tUs book is dhMod into tteoo
relating to different aspects of the indartry.
At the end of Parts I and H there are advertisement sections.
Hew are mdoded because it to appreciated that practical commercial
•odd be readily available about naUng gear and equip-
procnnubk from Tirious sources for the bettoi'nmt of fluMng
An indra of ttese idvertisers' ^BMy^fffUmiffltfi UDMTS in
CONTENTS
Notice to Readers .................. IV
Copyright ...................... V
List of Contributors .................. XI
PART I. MATERIALS FOR NETS AND ROPES
Section 1— Terminology Page
No.
Standardisation of Terminology and Numbering Systems for Netting Twines Gerhard Klust 3
Discussion on Terminology .......... 8
Section 2— Test Methods
Test Methods for Fishing Gear Materials (Twines and Netting) A von Brandt and P. J. G. Carrothers 9
Discussion on Test Methods .......... 49
Section 3— New Net Materials
Netting Twines of Polypropylene and Polyamide Compared .... Gerhard Klust SO
Polypropylene Twines in Japan ..... Katsuji Honda and SMgeru Osada 55
Use of 'Ulstron* Polypropylene in Fishing . . . C. L. B. Carter and K. West 57
Synthetic Fibre Fishing Nets and Ropes made in Japan . Japan Chemical Fibres Association 64
Production and Characteristics of Synthetic Nets and Ropes in Japan . Yoshinori Shimozaki 66
Japanese Fish Netting of Synthetic Fibres ...... Iwao Tani 71
New Synthetic Herring Driftnets Used in the North Sea ... Janusz Zaucha 73
Discussion on New Net Materials ......... 78
Section 4— Ropes, Knofless Nets and Monofflaments
Ropes of Polyethylene Monofilaments . . . . C. C. Kloppenburg and J. Renter 81
Tests on Knotless Raschel Netting . . . . . . .A. von Brandt 88
Knotless Netting in the Norwegian Fisheries ..... Norvald Mugaas 96
Knotless Fishing Nets on Raschel Equipment in Italy .... Mario Damiani 97
Resistance & la Rupture de Filets sans Noeuds .... Francesco Pianarali 101
Monofilaments in Fishing ...... D.F.C.EdeandW.Henstead 103
Nylon Monofilament in the Viet-Nam Fisheries . . Tran-Van-Tri and Ha-Khac-Chu 108
Monofilament Oillnets in Freshwater Experiment and Practice . . . R. Steinberg 111
Etudes sur to Freinage et lfUsure des FilsdePftche . Maurice Bombeke 115
Discussion on Ropes, Knotbss Nets and Monofilaments . . . . * .118
€• Nets afld HMOS • • • • • « • • 121*144
PART EL BULK FISH CATCHING
Section 5— Stem Trawling Page
No.
The Stern Trawler— a Decade's Development in Trawl Handling . . Conrad Birkhoff 147
Some Small Stern Trawlers E. C. B. Corlett 153
Ross Daring— Experiment ....... Dennis Roberts 158
Discussion on Stern Trawling . * . . . * . . . .160
*••*
Section 6— Bottom Trawling
Some of the General Engineering Principles of Trawl Gear Design . . . P. R. Crewe 165
Some Comparative Fishing Experiments in Trawl Design .... W. Dick son 181
Development of an Improved Otter Trawl Gear .... Chikamasa Hamuro 191
Towing Power, Towing Speed and Size of Bull Trawl . . . Chikamasa Hamuro 199
Suggestions for Improved Heavy Trawl Gear Eldon Nichols 204
Development of Soviet Trawling Techniques . . . . . A. I. Treschev 206
Double-Rig Shrimp Beam Trawling . . . . . /. Verhoest and A. Maton 209
Discussion on Bottom Trawling . . . . . . . . . .218
Section 7— Midwater Trawling
One-Boat Midwater Trawling from Germany ...... S. Schdrfe 221
Universal One-Boat Midwater and Bottom Trawl . . . . • S. Okonski 229
Two-Boat Midwater Trawling for Herring with Bigger Boats ... R. Steinberg 235
Development of the Cobb Pelagic Trawl— A Progress Report . . . Richard L. McNeely "240
Underwater Telemeters for Midwater Trawls and Purse Seines . Chikamasa Hamuro and Kenji Ishii 248
Reaction of Herring to Fishing Gear Revealed by Echo Sounding ...//. Mohr 253
Discussion on Midwater Trawling ......... 257
Section ft— Gillnetting, Longlining and Traps
King Crab Pot Fishing in Alaska R. F. Allen 263
Les Madragues Atlantique et Sicilienne ....... Vito Fodera 271
Eel Traps Made of Plastic H.Mohr 277
Types of Philippine Fish Corrals (Traps) . . Arsenic N. Roldan Jr. and Santos B. Rasalan 279
A New Fish Trap used in Philippine Waters . . . Santos B. Rasalan 282 and 593
Dropline Fishing in Deep Water Ronald Powell 287
Discussion on Gillnetting, Longlining and Traps . . . . . . .291
vra
Section 9 — Purse Seining Page
No.
Recent Developments in Icelandic Herring Purse Seining . . . Jakob Jakob sson 294
Sonar Instruction Courses for Fishermen ..... G. Vestnes 306
Discussion on Purse Seining . . . . . . . . . .310
Section 10— Deck Machinery
The Application of Hydraulic Power to Fishing Gear . . . . D. W. Lerch 314
Advances in Centralised Control and Automation . . . . H. E. H. Pain 338
Driftnet Hauler for Salmon Fishing ...... Chihlro Miyazaki 347
Mechanization of Driftnet Fishing Operations . . . . P. A. Kuraptsev 352
The Complex Mechanization of Beach Seining . . . . . S. S. Torban 355
Discussion on Deck Machinery . . . . . . . . .361
Section 11— Fish Detection
A Comprehensive Echo-Sounder for Distant-Water Trawlers G. H. Ellis, P. R. Hopkin and 363
R. W. G. Hasten
Sector-Scanning Sonar for Fisheries Purposes . . . D. G. Tucker and V. G. Welsby 367
A New Sonar System for Marine Research Purposes . . . T. S. Gerhardsen 371
Detection et Localisation des Banes de Poissons .... Robert Lenier 376
Echo-Detection of Tuna ........ Minoru Nishimura 382
Echo-Sounder Measurement of Tuna Longline Depth Kyotaro Kawaguchi, Masakatsu Hirano and 385
Minoru Nishimura
A 200 Kc/28 Dual Frequency Echo-Sounder for Aimed Midwater Shrimp Trawling . . 388
Masakatsu Hirano and Takashi Noda
Ddtecteur de Poisson "Explorator" ......./. Fontaine 396
Bio-Acoustical Detection of Fish Possibilities and Future Aspects . . G. Freytag 400
Study of Acoustical Characteristics of Fish . . . . E. V. Shishkova 404
Frequency Analysis of Marine Sounds . . Tomiju Hashimoto and Yoshinobu Maniwa 410
Identifying Pacific Coast Fishes from Echo-Sounder Recordings . . . E. A. Best 413
Echo-Sounding through Ice . Tomiju Hashimoto, Yoshinobu Maniwa, Osamu Omoto and
Hidekuni Noda 415
Discussion on Fish Detection . . . . . . . . . .417
Section 12— Fleet Operations
Japanese Mothership and Fleet Operations for Salmon, Crab, Longlining and Tuna
Hiroshi Tominaga, (2) Masatake Neo, Nippon Suisan Kaisha Ltd., and Goro Okabe 423
Las Pesqucrias Espanolas Austro-Atlanticas ..... K. Paz-Andradc 438
Discussion on Fleet Operations .......... 445
Advertisement Section on Gear and Equipment etc. 447-468
DC
PART m. TECHNICAL RESEARCH
Section 13— Gear Reaearck Page
No.
The Theory of Designing and Testing Fishing Nets in Model . . Tasae Kawakami 471
The Development of a Midwater Trawl P. Dale and S. Mailer 482
Fishing Methods and Gear Research Institutes: Their Organization and Scope A. von Brandt 489
Discussion on Gear Research .......... 493
Section 14— Instruments for Testing Gear
Trawl Gear Instrumentation and Full-Scale Testing /. Nicholls 497
Some Japanese Instruments for Measuring Fishing Gear Performance
Chikamasa ffamuro and Kenji Ishii 513
Trawl Studies and Currents . J. N. Carruthers 518
Performance of the Granton Trawl ....... W. Dickson 521
Discussion on Instruments for Testing Gear ........ 525
Section 15-Fish Behaviour
The Importance of Vision in Fish Reaction to Driftnets and Trawls
/. H. S. Blaxter, B. B. Parrish, and W. Dickson 529
Importance of Mechanical Stimuli in Fish Behaviour Especially to Trawls C J. Chapman 537
The Use of Air-Bubble Curtains as an Aid to Fishing . . . Keith A. Smith 540
Utilization of Fish Reactions to Electricity in Sea Fishing . . Conradin O. Kreutzer 545
Problems of Electro-Fishing and Their Solution .... Jurgen Dethloff 551
Notes on the Importance of Biological Factors in Fishing Operations
B. B. Parrish and J. H. S. Blaxter 557
Tuna Behaviour Research Program at Honolulu .... John /. Magnuson 560
Shrimp Behaviour as Related to Gear Research and Development
Charles M. Fuss Jr. andF. Wathne 563
Experimental Dispersion of Chum ..... Kentaro Hamishima 570
Evolution de la Pdche & la Lum&re dans les Lacs Africains . . . A. Collart 573
Pump Fishing with Light and Electric Current . . . . . /. K Nikonorov 577
Discussion on Fish Behaviour .......... 579
Section 16— Science and the Future
Prospective Developments in the Harvest of Marine Fishes Dayton L. Alverson and
Norman /. Willmovsky 583
Automatic Data Processing and Computer Use in Fisheries . . Benjamin F. Leeper 590
Discussion on advance in Science and the Future . . . • • • .591
LIST OF CONTRIBUTORS
Page
No.
ALLEN, ROBERT F. 160, 164, 263, 311, 362
Marine Construction and Design Co., 2300, W, Commodore
Way, Seattle, Washington, U.S.A.
ALVERSON, DAYTON, L. 161, 219, 260, 421, 422, 526,
527, 583, 591, 593
U.S. Bureau of Commercial Fisheries, Exploratory Fishing and
Gear Research Base, 2725, Montlake Boulevard, Seattle, 2,
Washington, U.S.A.
ANDERSON, A. W 582
American Embassy, Copenhagen, Denmark.
BALLS, RONALD 582
3, South Beach Parade, Great Yarmouth, U.K.
BAYAGBONA, EDWARD . . . .582
Federal Fisheries Service, Lagos.
BEST, E. A. 413
Marine Resources Operations Laboratory, 411, Burgess Drive,
Menio Park, California, U.S.A.
BLAXTER, J. H. S 557, 579
Marine Laboratory, Tony, Aberdeen, U.K.
BIRKHOFP, CONRAD . . . 147, 161
Schiffbau-Dipl, Ing. Fischereitechnische Konstruktioncn
Schroderstiftstr, 30, Hamburg, 13, Germany.
BOMBEKE, MAURICE . . . .115
Ets Cousin Freres, Wervicq-Sud (Nord), France.
BOSCH, IR. J. V. D. . . . 495, 528
Mekelweg 2, Delft, Netherlands.
VON BRANDT, PROF, DR. ANDRES 8, 9, 49, 79, 80, 88,
119, 489
Direktor, Institut fur Netz- und Materialforschung, Palmaillc 9,
Hamburg- AJtona 1, Germany.
CARLSON, W. W 161, 362
Blount Marine Corp., Warren, Rhode Island, U.S.A.
CARROTHERS, P. J. G 9
Fisheries Research Board of Canada, St. Andrew's, New Bruns-
wick, Canada.
CARRUTHERS, DR. J. N. , . . 495, 518
National Institute of Oceanography, Wormley, nr. Godalming,
Surrey, U.K.
CARTER, C. L. B 57
Industrial Fibres Division, Imperial Chemical Industries Ltd.,
Harrogate, Yorks, U.K.
CHAPMAN, C. J 537
Marine Laboratory, lorry, Aberdeen, U.K.
CHAPMAN, DR. WILBERT McLEOD 495, 581, 582, 592
Van Camp Foundation, 739, Golden Park Avenue, San Diego 6,
California, U.S.A.
Page
No.
COLE, DR. H. A. . . . 420, 527, 580, 592
Fisheries Laboratory, Lowestoft, Suffolk, U.K.
COLLART, A. 573
FAO/EPTA Economiste des Ptehes, Cotonon, Dahomey.
CORLETT, E. C. B. . . . 153, 162
Burness, Corlett & Partners Ltd., Basingstoke, Hants., U.K.
CRAIG, R. E 417, 421
Marine Laboratory, Aberdeen, Scotland, U.K
CREWE, P. R. . . . 165, 219, 494, 495
Westland Aircraft Ltd., (Saundcrs-Roe Division), East Cowes,
Isle of Wight, U.K.
CREWDSON, B. W. .... 79, 80
The Great Grimsby Coal, Salt and Tanning Co. Ltd., Fish
Dock Road, Grimsby, U.K.
GUSHING, DR. D. H.
Fisheries Laboratory, Lowestoft, U.K.
419
DALE, P 482
Arbeidsutvalget for utvikling av pelagisk enbatstral, C. Sundtsgt,
3, Postboks 37, Bergen, Norway.
DAMIANI, ING. MARIO .... 97
Society Rhodiatoce, Piazza Erculea, 15, Milano, Italy.
DE WIT, J. G 219
Ministry of Agriculture A Fisheries, The Hague, Netherlands.
DETHLOFF, JURGEN . . . 551, 582
International Electronics Laboratories, GmbH, 52, Lottestr,
Hamburg-Lokstedt 1, Germany.
DIAZ DE ESPADA, PEDRO . . . .581
Pesquerias y Secaderos de Bacalac de Espafta, S.A. (PYSBE),
Aguirre Miram6n, 2, San Sebastian, Spain.
DICKSON, W. . 163, 181, 493, 496, 521, 525, 528,
529
Marine Laboratory, Torry, Aberdeen, Scotland, U.K.
DORVILLE, F. 163
MacGregor-Comarain, S. A., 96 bis rue du Ranelagh, Paris,
16 tone, France.
DOUST, DAVID J 163, 591
Ship Hydrodynamics Laboratory, Faggs Road, Feltham, Middx. ,
U.K.
EDE, D. F. C 103
British Resin Products Ltd., Devonshire House, Piccadilly,
London, W.I., U.K.
EDDIE, GORDON, C. . . . 445, 446
White Fish Authority, Industrial Development Unit, St. Andrew's
Dock, Hull, U.K.
XI
Page
No.
ELUS, G. H. 363
S. Smith & Sons (England) Ltd., Kelvin Hughes Division,
St. dare House, Minorics, London, E.C.3., U.K.
FODERA, Vrro .
FAO/EPTA, Fishery Adviser, Tunis, Tunisia.
271
FONTAINE, JEAN-GABRIEL 362, 396, 418, 420, 421
Compagnie g6nerale de t£16graphie Sans Fil (CSF)
79, Boulevard Haussman, Paris 8 erne, France.
FRECHET, JEAN . . . 163, 291, 312
Federal Department of Fisheries,
Sir Charles Tupper Building, Ottawa, Ontario, Canada.
FREYTAG, DR. G. ... 400, 582
Institut fur Nctz- und Materialforschung, Palmaille 9,
Hamburg-Altona 1, Germany.
Fuss, CHARLES M. JNR. . . . .563
U.S. Bureau of Commercial Fisheries,
Gear Research Station, Panama City, Florida, U.S.A.
GERHARDSEN, ENG. TORVALD S. . .371
Simonsen and Mustad A/S, Morten, Norway.
GRONNINGSAETER, CDR. A. ... 220
Flyveien 13, Holmen, Norway.
HARPER-GOW, L. M. . 80, 161, 219, 260, 581, 582
Chr. Salvesen & Co., 29, Bernard Street, Leith, Edinburgh 6, U.K.
GUNDRY, E. F 80
Joseph Gundry & Co. Ltd., Bridport, Dorset, U.K.
HA-KHAC-CHU 108
Fisheries Directorate, Saigon, Viet-Nam.
HALLIDAY, W 421
S. Smith & Sons (England) Ltd., Kelvin Hughes Division,
St. Clare House, Minorics, London, E.C.3.
HAMASHIMA, KENTARO . . . 311, 570
Nagasaki Prcfcctural Fisheries, Matsugae-Cho, Nagasaki City,
Japan.
HAMURO, CHIKAMASA PROF. C. 191, 199, 248, 513
Fishing Boat Laboratory, Fisheries Agency 1,
2-Chome Kasumigaseki, Chiyoda-ku, Tokyo, Japan.
HASHIMOTO, PROF. T. 410, 415
Fishing Boat Laboratory, Fisheries Agency 1,
2-Chome Kasumigaseki, Chiyoda-ku, Tokyo, Japan.
HASLETT, DR. R. W. G. . . . 363, 419
S. Smith & Sons (England) Ltd., Kelvin Hughes Division,
Barkingside, Essex, U.K.
HHNSOHN, DIPL. ING. H. . 160, 261, 362
Richmers Werft, Postfach 2066, Bremerhaven 2, Germany.
HENSTEAD, W 80, 103
British Celanese, 345, Foleshill Road, Coventry, U.K.
HINDS, V. T 527, 581
Fisheries Department, Aden.
HIRANO, MASAKATSU . . . 385, 388
Sanken Electronics Co.,
1997 Sembon gorin, Numazu City. Japan.
HONDA, DR. KATSUJI ... 55, 79
Tokyo University of Fisheries,
6-Chome Shibakaifcan-dori, Minato-ku, Tokyo, Japan.
xn
HOWARD, D. S. .
Bridport Industries Ltd., Bridport, Dorset, U.K.
Page
Mi-
261
HOPKIN, P. R. . . . . .363
Kelvin Hughes, St. Clare House, Minories, London, E.C.3.
ISHII, PROF. K 248, 513
Fishing Boat Laboratory, 2-Chome, Kasumigaseki, Chiyoda-ku
Tokyo, Japan.
JAKOBSSON, JAKOB . . . 259, 294, 311, 313
Fiskideild Skirlag. 21, Reykjavik, Iceland.
JAPAN CHEMICAL FIBRES ASSOCIATION . . 64
3, 3-Chome, Muromachi, Nihonbashi, Chuo-ku, Tokyo, Japan.
KAWAKAMI, DR. T. . . . .471
Department of Fisheries, Kyoto University, Maizuru,
Kyoto Prefecture, Japan.
KAWAGUCHI, KYOTARO .... 385
Kanagawa Prefectural, Fisheries Experimental Station, Misaki,
Miura City, Japan.
KLOPPENBURG, C. C. . . . 81, 119
Kunstzijdespinnerij "Nyma" N.V., Nijmegen, Netherlands.
KLUST, DR. GERHARD . . 3, 8, 50
Institut fur Netz- und Materialforschung, Palmaille 9,
Hamburg- Altona 1, Germany.
KNOX, B. M
W. & J. Knox Ltd., Kilbirnie, Ayrshire, U.K.
8
KOBAYASHI, MASAO . . . .119
Nippon Seimo Co. Ltd., 6, 1-Chome, Ginza, Chuo-ku, Tokyo,
Japan.
KREUTZER, DR. CONRADIN O. . 545, 582
Smith Research & Development Co., Inc., Lewes, Delaware,
U.S.A.
KRISTJONSSON, HILMAR . 80, 119, 262, 292, 313
Fisheries Division, FAO, Rome, Italy.
KURAMOTO, K.
Chairman Japan Chemical Fibres Association.
64
KURAPTSEV, P. A. .... 352
Institute of Marine Fisheries & Oceanography (VNIRO),
Moscow, U.S.S.R.
KWEI, ERIC
Fisheries Inspectorate, P.O. Box 630, Accra.
79
LARSSON, KARL-HUGO . . .258
Consulting Naval Architect, Stadsgarden 10, Stockholm So,
Sweden. .
LEAKEY, R. D 292
R. & B. Leakey, Sutcliffe House, Settle, Yorks, U.K.
LEEPER, BENJAMIN F. 590
Univac Division, Sperr> Rand Corp., Box 1549, Baton Rouge,
Louisiana, U.S.A.
LENIER, CDT. ROBERT . . 218, 311, 376, 419
Commission Marine Marchande et Peche, S.P.E.R.,
11 bis, rue St.-Augustin, Asnicres (Seine), France.
LERCH, D. W.
Marine Construction and Design Company,
Seattle, Washington, U.S.A.
314
Page
No.
LEVY, HAIM 311, 582
Place de I'Jndependance, Safi, Morocco.
LIBERT, Louis ...» 261
Institut des Ptehes Maritimes, Qua! de la Poste,
Boulogne-sur-Mer (P de Q, France.
LUSYNE, PIERRE . . 49, 78, 80, 119, 362, 421
Fishing Gear Section, FAO, Rome, Italy.
MAGNUSON, J. J. . . . .560
Bureau of Commercial Fisheries, Honolulu, Hawaii, U.S.A.
MANIWA, Y. .... 410, 415
Fishing Boat Laboratory, Fisheries Agency, 1, 2-Chome
Kasumigaseki, Chiyoda-ku, Tokyo, Japan.
MARESCHAL, Y. J. A. . . . .311
Anciens Chantiers Dubigeon, 7, rue Caumartin, Pans, France.
MAJIGETTS, A. R 218, 219, 220
Fisheries Laboratory, Lowestoft, Suffolk, U.K.
MAURIN, C. . . . . .162
Institut des Ptehes Maritimes, 6, rue Voltaire, Site, France.
MOCRACKEN, FRANK . . . .260
Fisheries Research Board, St. Andrew's, New Brunswick,
Canada.
McNEELY, R. L 240
Research Bureau of Commercial Fisheries, Seattle Washington,
U.S.A.
MOHR, DR. H. . . . 253, 277
Institut fur Netz- und Materialforschung, Palmaille 9,
Hamburg-Altona 1, Germany.
MATON, DR. ANDRE . . . .209
Rijksstation voor Boerdeijbouwkunde, University of Agiiculture,
Ostend, Belgium.
MIYAZAKI, DR. CHIHIRO 219, 291, 293, 347, 445, 527
Tokai Regional Fisheries Research Laboratory, Tsukishima,
Chuo-ku, Tokyo, Japan.
MOLLER, S. ..... 482
Bergens Mekaniske, Verksteder, P.O. Box 858, Bergen, Norway.
MROSS, SIEGFRIED . . . .418
Atlas-Werke A. G. Bremen, Paschenburgstr. 63, Bremen,
Germany.
MUGAAS, NORVALD .... 96
Engineer, Statens Fiskeredskapsimport, Bergen, Norway.
NALL, A. F. B 49
British Standards Institution, Coronation House, Market Street,
Manchester, 1., U.K.
NEO, MASATAKE . . . 423, 428
Nichiro Gyogyo Kaisha Ltd., Marunouchi Building, Tokyo,
Japan.
NICHOLS, ELDON . . 204, 219, 220, 527
American Telephone & Telegraph Co., Ocean Cables Division,
32 Ave. of the Americas, New York, 13, N.Y., U.S.A.
NICHOLLS, J. .... 497, 527
Westland Aircraft Ltd., (Saunders-Roe Division), East Cowes,
Isle of Wight, U.K.
NILSSEN, E. ALLERS .... 80
Bergens Notforretning, P.O. Box 868-870, Bergen, Norway.
Page
No.
NIKONOROV, I. V. . . . . .577
Caspian Institute of Marine Fisheries & Oceanography,
Astrakhan, U.S.S.R.
NIPPON, SUISAN KAISHA LTD. . . .423
Tokyo Building, Marunouchi, Chiyoda-ku, Tokyo, Japan.
NISHIMURA, M. 382, 385
Fishing Boat Laboratory, 1, 2-Chome, Kasumigaseki, Chiyoda-
ku, Tokyo, Japan.
NODA, HlDEKUNI
Shibaura Technical Institute, Tokyo, Japan.
415
NODA, TAKASHI ..... 388
Sanken Electronics Co., Ltd., No. 1907 Sembon-Gorin, Numazu
Shizuoka Pref., Japan.
NORMAN, J. , . . . . .79
Ministry of Agriculture, Fisheries and Food, 10, Whitehall Place,
London, S.W.I., U.K.
OCRAN, R. K. N 80, 311
Mankoaeze Fisheries Ltd., P.O. Box 103, Tema, Ghana.
OKABE, GORO .... 423, 436
Taiyo Gyogyo Kabushiki Kaisha 4, 1-Chome Marunouchi,
Chiyoda-ku, Tokyo, Japan.
OKONSKI, S. ..... 229
Sea Fisheries Institute, Al Zjednoczenia 1, Gdynia, Poland.
O'MEALLAIN, SEAMUS .... 260
Fisheries Division, Department of Lands, 3, Cathal Brugha
Street, Dublin, Ireland.
OMOTO, OSAMU . . . , .415
Shibauru Technical Institute, Tokyo, Japan.
OSADA, S 55
Nippon Gymo Sengu Kaisha Ltd., Tokyo, Japan.
PAIN, CMDR. H. E. H. . . . 338, 361
S. G. Brown Ltd., Shakespeare Street, Watford, Herts., U.K.
PANZER, HELMUT , . . .119
Plutte, Koccke & Co., Ottostr 1, Wuppertal-Barmen, Germany.
PARAMANANTHARAJAH, T. .
Department of Fisheries, Colombo, Ceylon.
PARRISH, B. B.
Marine Laboratory, Torry, Aberdeen, U.K.
292
557
PARSEY, M. R 261
Fibres Division, Imperial Chemical Industries Ltd., Harrogate,'
Yorks., U.K.
PAZ ANDRADE, VALENTIN . . 438, 445
Revista "Industries Pesqueras", Policarpo Sanz 22, Vigo, Spain.
PETRICH, BORTI . .311, 312, 581, 582
U.S. Net & Twine Co., P.O. Box 9206, Long Beach, California,
U.S.A.
PIANAROU, ING. FRANCESCO . . .101
Retificio Carlo Badinotti, Viale Beatrice d'Este, 18, Milano, Italy.
POWELL, RONALD 287
Government of the Cook Islands, Rarotonga, Cook Islands.
PRAET, MARCEL . . . . .119
U.C.B., Division Fabelta, Application Industrielle, 18, Chausaee
de Charleroi, Bruxelles 6, Belgium.
xra
No.
RANKBN, CMDR. MICHAEL B. F. . .446
J. ft. E. Hall Ltd., Dartford, Kent, U.K.
RASALAN, SANTOS B. . . 279, 282, 593
Philippine Fisheries Commission, Diiiman, Quezon City,
Philippines.
REMOY, SVERRE 312
Government School of Fisheries, P.O. Box 58, Florce, Norway.
REUTER, DR. J.
81, 118, 119
Nederlandsch Visschery Proefstation, Mailiebaan, 103, Utrecht,
Netherlands.
ROBERTS, CAPT. DENNIS A. 79, 158, 162, 220, 494, 581
Ross Trawlers Ltd., Fish Docks, Orimsby, Lines., U.K..
ROBINSON, A.
8
Wm. Kenyon ft Sons (Rope and Twines) Ltd., Dunkinfield
Cheshire, U.K.
ROLDAN, JR. ARSENIC M.
279
Philippine Fisheries Commission, Diiiman, Quezon City,
Philippines.
ROWE, K. C T.
582
Great, Orimsby coal Salt & Tanning Co., Ltd., Battery Oreen,
Lowestoft, Suffolk, U.K.
SCHABFBRS, E. A. . . 80, 310, 312, 313, 361
Branch of Exploratory Fishing, U.S. Bureau of Commercial
Fisheries, Washington 25, D.C., U.S.A.
SCHARFE, DR. J. 220, 221, 257, 260, 261, 262, 312,
361, 418, 495, 526, 527, 528
Institut fur Netz- und Materialforschung, Palmaille 9,
Hamburg-Altona 1, Germany.
SCHMIDT, P. G.
419
Marine Construction ft Design Co., 2300 Commodore Way,
Seattle, 99, Washington, U.S.A.
SfflMOZAKI, Y.
66
Fishing Gear Division, Tokai Regional Fisheries Research
Laboratory, Tsukishima, Chuo-ku, Tokyo, Japan.
SHISHKOVA, E. V. .
404
Institute of Marine Fisheries & Oceanography (VN1RO),
Moscow, U.S.S.R.
.SLEIGHT, R. .
528
Decca Radar Ltd., North Quay Fish Docks, Grimsby, Lines.,
SMITH, K. A.
540
Fish ft Wildlife Service, Gear Research Base, State Fish Pier
Gloucester, Massachusetts, U.S.A.
STEINBERG, DR. ROLF . . . ill, 235
Institut fur Netz- und Materialfonchunf , Palmaille 9,
Hamburg-Altona 1, Germany.
TANI, IWAO
&** 1*^4^ *?* Nct
Chuo-ku, Tokyo, Japan,
71
Association, Bchizenbori,
... 292
THOMAS, DAVIDSON ....
4/0 Ministry of Agriculture, Maiduguri, Northern Nigeria.
]XIV
No.
423, 435
Taiyo Fishery Co., 4, 1-Chome Marunouchi, Chiyoda-ku,
Tokyo, Japan.
TOMINAGA, HlROSHI .
TORBAN, DR. S ...... 355
Research Institute of Marine Fisheries (VNIRO)
17v Krasnoselskaya, Moscow, U.S.S.R.
TRAN-VAN-TRI . . . . .108
Fisheries Directorate, Saigon, Viet-Nam.
TRAUNG, J. 0 ..... 527, 528
Chief, Fishing Boat Section, Fisheries Division, FAO, Rome,
Italy.
TRESCHBV, A. 1 ..... 206, 259
Chief,' Fishing Technique Laboratory, Institute of Marine
Fisheries (VNIRO), Moscow, U.S.S.R.
TROUT, G. C
Fisheries Laboratory, Lowestoft, Suffolk, U.K.
163
369
TUCKER, PROF. D. G.
University of Birmingham, Birmingham]! 5, U.K. %
TVBDT, JOHN A. . . . 219, 260, 527
Research and Fishing^Ltd., 1, Chesham Place, London, S.W.I.,
U.lv.
VALDEZ, V. . 79, 80, 313, 119, 420, 421
Institute de Investigation de los Recursos Marines, Avenida
Bologncsi 22, La Punta, Callao, Peru.
VERHOEST, J. . . . 209, 261
Commissie T.W.O.Z., Zeewezen Rebouw, Natienkaai, Ostende,
Belgium
VESTNES, GUDMUND . . . .306
Institute of Marine Research Nornesparken 2, Bergen, Norway.
WAKEFIELD, LOWELL . . . .292
Wakefield Fisheries, Port Wakefield, Kodiac, Alaska, U.S.A.
WATHNE, FREDERICK
563
U.S. Bureau of Commercial Fisheries, Gear Research Station,
Panama City, Florida, U.S.A.
WATSON, C. E. P.
Fisheries Department, Kenya, East Africa.
293
WELSBY, V. G. . . . 307, 420, 582
University of Birmingham, Birmingham, U.K.
WEST, K
Imperial Chemical Industries Ltd., Fibres Division,
Harrogate, York*., U.K.
WILIMOVSKY, NORMAN, J. ...
Institute of Fisheries, University of British Columbia,
Vancouver, British Columbia, Canada.
WORSFIELD, D. L.
British Nylon
Pontypool, Monmout]
Textile Development Dept.,
ire, U.K.
ZAUCHA, JANUSZ .
Sea Fisheries Institute, Zjcdnoczcnia lt Gdynia, Poland.
57
583
119
73
PREFACE
THE book Modern Fishing Gear of the World, resulting from the first FAO Fishing Gear
Congress held in Hamburg in 1957, sold out and had to be reprinted. This confirmed
the belief that there existed a great need for a comprehensive work on fishing gear and
fishing technology. The book was widely hailed and has undoubtedly stimulated gear
development and the application of modem methods in commercial fishing. Innovations that were
pure experiments when reported at the 1957 Conference have proved themselves and are now widely
used. Other innovations and developments have occurred and many more studies have yielded
further data on existing gears.
These are dealt with in the present book which comes as a result of the Second FAO World
Fishing Gear Congress, which was held in London at the kind invitation of Her Majesty^ Govern-
ment in the United Kingdom in May, 1963. Though this Congress dealt with fewer subjects than
the former one in 1957, the book contains more matter, despite considerable condensation. This
comes about because subjects are dealt with in 'depth', much more detailed work having been done
than heretofore. It is hoped that this book, in its turn, will serve to further stimulate interest and
progress in its field and be useful to the fishing industry in decision making.
While dealing with Materials, Gear and Fishing, and Gear Research in some 88 papers,
certain trends stood out. Two of these were the emphasis which the Congress placed upon (1)
rational gear research and (2) the standardization of testing methods. It was felt that much more
should be done on these lines to supplement the trial and error approach. Another is the work
that is reported on the hydrodynamics of trawls. Much has been learned about the movement of
gear through water and the efficiencies that may be obtained, but it is found that it does not follow
that the hydrodynamically perfect trawl will necessarily be the best fish catcher. This depends
upon the reaction of the fish themselves towards moving bodies. This is an aspect of fish behaviour —
a subject which, in spite of its difficulties, is attracting increased attention, though one might say
that it has scarcely begun.
When one goes over the papers in this book one is impressed with the growing complexity
of gear technology, as well as the growing demands upon the skippers who use it. Though there is
still a great deal of subjective judgment in successful fishing, these judgments are being aided in-
creasingly, if not being gradually supplanted, by the intricacies of instrumentation. Aviators faced
the same situation yesterday. Successful flying used to be done 'by the seat of the pants', but this
is gone today.
This points to the need for better instruction in the training of skippers in a wide range
of techniques and of instrumentation as modern fishing vessels become more complex. It is only
in this way that the increased expenditure of the capital required can be justified. Perhaps this
book may be useful in this respect. Once again on the matter of training, the facilities offered to
XV
gear technologists are meagre. Only a few countries have gear research institutes at the present
time. As has occurred in other kinds of oceanographical research, perhaps the gear scientists will
find it profitable to organize team work between their institutions, so that they may share their
skills and observations, especially in the more expensive and more complex operations.
The papers embodied in this book come as a result of much patient work on the part of
both the authors and the Secretariat of FAO, who devised and organized the Congress. It is grati-
fying to think that the two years spent in preparation have yielded this result. Again, it is fortunate
that the Congress took place in London. The Ministry of Agriculture, Fisheries and Food and
the Foreign Office generously placed their services at the disposal of FAO, and many thanks are
due to the Fisheries Secretary, Mr. Hugh Gardiner, and his staff for the excellent arrangements.
It was fortunate, too, that the Congress coincided with the First World Fishing Exhibition which,
through the courtesy of World Fishing, enabled the participants to view the latest improvements
in fishing materials and equipment.
DR. D. B. FINN,
Director, Fisheries Division,
Food and Agriculture Organization
Rome, January 15, 1964. of The United Nations
XVI
PART 1
MATERIALS FOR NETS AND ROPES
Page
Section 1— Tenninology
Standardisation of Terminology and Numbering Systems for Netting Twines . . Gerhard Klust 3
Discussion on Terminology 8
Section 2— Test Methods
Test Methods for Fishing Gear Materials (Twines and Netting) A. von Brandt 9
and P. J. G. Carrothers
Discussion on Test Methods 49
Section 3— New Net Materials
Netting Twines of Polypropylene and Polyamide Compared Gerhard Klust 50
Polypropylene Twines in Japan Katsuji Honda and Shigeru Osada 55
Use of 'Ulstron' Polypropylene in Fishing C. L. B. Carter and K. West 57
Synthetic Fibre Fishing Nets and Ropes Made in Japan. . . .Japan Chemical Fibres Association 64
Production and Characteristics of Synthetic Nets and Ropes in Japan. . . . Yoshinori Shimozaki 66
Japanese Fish Netting of Synthetic Fibres Jwao Tani 71
New Synthetic Herring Driftnets Used in the North Sea Janusz Zaucha 73
Discussion on New Net Materials 78
Section 4— Ropes, Knotless Nets and Monofilaments
Ropes of Polyethylene Monofilaments C. C. Khppenburg and J. Reuter 81
Tests on Knotless Raschel Netting A. von Brandt 88
Knotless Netting in the Norwegian Fisheries Norvald Mugaas 96
Knotless Fishing Nets on Raschel Equipment in Italy Mario Damiani 97
Resistance & la Rupture de Filets sans Noeuds Francesco Planaroli 101
Monofilaments in Fishing D. F. C. Ede and W. Henstead 103
Nylon Monofilament in the Viet-Nam Fisheries Tran-Van-Tri and Ha-Khac-Chu 108
Monofilament Gillnets in Freshwater Experiment and Practice R. Steinberg 1 1 1
Etudes sur le Freinage et TUsure des Fils de P6che Maurice Bombeke 115
Discussion on Ropes, Knotless Nets and Monofilaments 118
Advertisement Section
Trade announcements relating mainly to nets, netting and ropes, together with equipment . . 121-144
1
A
Part 1 Materials for Nets and Ropes
Section 1 Terminology
Standardisation of Terminology and Numbering
Systems for Netting Twines
Abstract
Some terms and definitions of netting twines recommended by the
International Organisation for Standardisation are reviewed, as
well as presently used twine numbering systems, and the need for an
internationally accepted numbering system is pointed out. The
tex-system. recommended by ISO, expresses the mass in grams for a
unit length of 1,000 m of yarn or twine, and has the advantage,
as an international unit, that it is based on the metric system and
gives a direct estimation of the twine size, as the heavier the twine
is the greater the tex value becomes. For the purpose of defining a
simplified but generally useful numbering system for netting
twines, the author then discusses the relevant characteristics of
netting twines in relation to the information required by the fisher-
men, netmakers and gear technologists. There are two basic ways of
describing twines by the tex-system; the most detailed one gives
the tex value of each yarn and the number of yarns and plies, e.g.,
23 texx5x3. To give the weight per unit length of the twine,
further information must be given as to the degree of twisting and
chemical treatment. The simpler form gives only the resultant
linear density of the twine, called R tex, i.e., the actual weight in
grams of 1,000 m of the twine. This value includes the effect of
twisting operations, and will thus replace the conventional indication
of runnage (m/kg and yds/lb). The author feels that R tex alone does
not provide sufficient information in the case of thin twines but
would be adequate for thicker twines. The complicated and multi-
plex construction of braided twines and of twines made of dissimilar
components only allow designation by R tex values.
Standardisation de la terminologie et des systfcmes de numeration
rlesfllsdefllets
Rfemnt
Quelques termes et ddfinitions de fils de filets recommandees par
TOrganisation Internationale de Standardisation sont recapitulees
ainsi que les systemes de num6rotage utilised internationalement.
Le systeme tex, recommand6 et approuv6 par l'"OIS" donne la
masse en grammes par unite de longueur de mille metres de fil de
caret ou de fil retprdu. Ce systeme a 1'avantage d'etre bas6 comme
toutes les unites international sur le systeme metrique et donne
une estimation directe de la mesure du fil puisque plus lourd sera
le fil, plus grande sera la valeur en tex. Pour definir un systeme de
numerotage simplifi6 mais utilisable dans la fabrication de filets,
1'auteur examine les caracteristiques applicables aux fils de filets en
relation avec les informations demandees par les pgcheurs, les
fabricants de filets et les techniciens de peche. II y a deux manieres
de base pour decrire les fils retordus par le systeme tex; la plus
detaillee donne la valeur en tex de chaque fil et le nombre de fils et
de brins, c'est-a-dire, par example: 23 texx5x3. Pour obtenir le
poids par unitd de longueur d'un fil retordu, on a besoin de donnees
sur le degrg de retordage et le traitement chimiquc. Le systeme plus
simple donne seulement la densit6 lineaire resultant du fil retordu,
appelee R tex, c'est-a-dire le vrai poids en grammes de 1000 m de
ce fil retordu. Cette valeur comprend Teffet du retordage et rem-
placera 1'indication conventionnelle m/kg et yds/lb. D'apres 1'auteur
Vindication en R tex seule, ne donne pas asses d'informations dans
le cas des fils fins mais decrit d'une fason adequate les fils lourds.
La construction compliquee et multiplex des fils tresses et des
fils composes de matieres dissemblables admet seulement la ddsigna-
tion par les valeurs R tex.
Normalizadfa de la terminologia y de los sistemas de nmneracion
dehilos para redes
Extracto
Se reseftan apelaciones y definiciones de hilos para redes recomen-
dadas por la Organizaci6n Internacional para Normalizaci6n,
sistemas de numeraci6n de hilos empleados actualmente y se pone
de relieve la necesidad de disponer de un sistema de numeraci6n
accptado internacionalmcntc. El sistema tex, recomendado por
by
Gerhard Klust
Institut fur Netz und Material-
forschung, Hamburg
la OIN, expresa la masa en gramos por unidad de longitud de mil
metros de hilo; como patr6n internacional tiene la yentaja de que
se basa en el sistema metrico y de que da una estimacidn directa del
tamaflo del hilo ya que cuanto mayor es este mas grande es el
niimero tex. Para dennir un sistema de numeration de hilos para
redes sencillo y de utilidad general, el autor examina las caracteris-
ticas principales de dichos hilos con respecto a la informaci6n que
necesitan Pescadores, fabricantes de redes y tecnicos en material de
pesca. Existen dos maneras principales de describir hilos por el
sistema tex ; el mas dctallado da el numero tex y torsibn en cada hilo,
por ejemplo: 23 tex x 5 x 3. Para poder dar el peso por unidad de
longitud se necesita mas informaci6n respecto al grado de torsidn y
tratamiento quimico. La forma mas sencilla solo da la densidad
lineal resultante del hilo, llamada R tex, es decir: el peso real en
gramos de mil metros de hilo. Este niimero no s61o comprende
el efecto de la torsi6n, con lo que sustituira la indicaci6n normal
de m/kg y y/lb. Considera el autor que el niimcro R tex solo no
facilita suficiente informaci6n en el caso de hilos finos pero basta
para los mas gruesos. La complicada y multiple fabncaci6n de
hilos trenzados y ole materiales distintos s61o permite que sean
designados por numeros R tex.
1. TERMINOLOGY FOR TWINES
AS this paper is written for the fishing and not the
textile industry, only those terms are defined which
should be used for communications in the English
language for international trade with fishing gear, in the
exchange of information between fishery scientists and
in the literature of fishery science. The following terms
and definitions are based on recommendations of the
International Organisation for Standardisation (ISO
Refs. 6 and 7).
1*1 Material for fishing nets
(a) Discontinuous fibres (or staple fibres), which may
be combined by twisting to form spun yarn.
(b) Continuous filaments, which are usually, but not
always, combined by twisting to form filament
yarn, also described as multifilament yarn.
(c) Monofilaments1, continuous filaments which have
greater diameter and stiffness than those used in
1 By ASTM Standards, monofilament is a "single filament of
sufficient size to function as a yarn in normal textile operations**.
(Ref. 1).
multifilament yarn. They may be used singly to
make netting such as for fine gillncts, in which case
the "y&m" consists of only one monofilament.
Two or more monofilaments may be twisted
together to form folded monofilament yarn (or
plied monofilament yarn).
1*2 Single yam is "the simplest continuous strand of
textile material" (Ref. 6). The term "yarn" is a general
term; when meaning the components of the final
product the term single yarn should be used.
M Netting twine. At the first meeting of the sub-commit-
tee "Textile Products for Fishing Nets" of the ISO
Committee "Textiles" on 23 May, 1962, in Hamburg,
it was suggested that the term netting twine should
be used instead of "net twine", "fishnet twine",
"fishing twine" or other terms, as the general term
for any kind of yarn or combination of yarns (inclu-
ding braided) usable for the manufacture of netting
(Ref. 7). Fishing nets will seldom be made of single
yarns. The usual netting twines are:
Folded yarns (or plied yarns)
Cabled yarns
Braided twines.
The terms "strand" (as a component of twisted
twine) and "cord" should not be used.
Netting twine in the form of folded yarn (plied
yarn) consists of two or more single yarns twisted
together by a single twisting operation.
Netting twine in the form of cabled yarn is made
by two or more twisting operations. Firstly, two or
more single yarns are twisted together to form a
folded yarn (plied yarn) and then two or more of
these folded yarns are twisted together to form the
cabled yarn (Fig. 1). For very strong netting twines,
cabled yarns can be combined by a third twisting
operation.
Netting twine in the form of braided twine. A
variety of single yarns or of folded yams may be
combined in the braiding process. The possibilities
of varied constructions are greater than with the
twisted twines (see 6'3).
2. NUMBERING SYSTEMS FOR NETTING TWINES
2-1 Conventional systems
There are many systems in use throughout the fishing
industries of the world. The most important are probably:
The international denier system (Td)
The metric count (Nm)
The English cotton count (Nee)
The international denier system gives the weight in
grams of 9,000 m of single yarn. This system seems to
obtain in all countries. It is used for netting twines made
of continuous synthetic filaments and (Ref. 13) also for
silk twines. Example 1: 210 denx5x3 (abbreviated:
210 den x 15).
The metric count is the number of kilometres of the
single yarn which weigh one kg. It is mainly used in
European fishing industries for cotton netting twines
and for netting twines made of discontinuous (staple)
synthetic fibres and in some countries for hemp netting
twines. Also netting twines made of continuous filaments
are so described. Example 2: Nm 20/5/3 (abbreviated:
Nm 20/15).
The English cotton count of a single yarn is the number
of hanks, each of a length of 840 yds, which weigh one
English pound (Ib). The system is used in Great Britain,
U.S.A., Japan, Canada and other countries for cotton
twines and for twines made of discontinuous (staple)
synthetic fibres. Example 3: Nec 20/5/3 (abbreviated:
Nec 20/1 5).*
2*2 Conventional systems of less importance or restricted
use
The English linen count (lea) of a single yarn is the num-
ber of hanks, each of a length of 300yds, which weigh one
English pound (Ib). It is used for netting twines made of
ramie and flax (Refs. 2, 3 and 13).
The equivalent English cotton count (C.20). In this
system yarn count is related to a yarn of Nec20. It is
used especially in Japan.
"The numbers of yarn for C.20-equivalent twine are
obtained by dividing the total denier of the twines with
the thickness of the C.20-equivalent. It should be remem-
bered, however, that the thickness C.20-equivalent
determined for these twines does not necessarily repre-
sent, in a strictly physical sense, the true thickness of
cotton 20's." (Ref. 1 2). In Canada and the U.S. A."medium-
laid, continuous multifilament synthetic twines are
numbered according to what the manufacturer thinks
is the equivalent Nec cotton twine, although there is
disagreement as to what constitutes equivalence".
(P. J. G. Can-others).
The rope-yarn number m/kg (Nt or NR) is used in
some European countries, for instance in Germany
(Ref. 9) for trawl twines made of manila, sisal and syn-
thetic filaments. Example 4: Nt 3/900 (three single yarns
are connected in the netting twine; each single yarn with
a length of 900 m has the weight of one kg).
The yards per pound number is used for trawl twines
in Great Britain and Canada and, to some extent, for
seine twines in Canada.
2 In the examples 1, 2 and 3, the first figure denotes the s
yarn count, the second figure the number of single yarns constitu
the folded (plied) yarn and the third figure the number of fol
yarns constituting the final product (netting twine). In the inter-
national denier system, which is a direct numbering system, the
figures are connected by the multiplication sign x . If an indirect
system is used (Nm and Nec) they are separated from each other
by the solidus (oblique stroke) /.
Hemp yarn may be identified by the number of pounds
per spindle (14,400 yds); (Ref. 3).
The diameter in millimetre or inch is generally used
for monofilaments.
The runnage in m/kg in yds/lb or in g/100 m (the
fatter e.g., in Italy) of the final product (netting twine)
is of great importance for netmakers and fishermen.
It is generally used in connection with other count sys-
tems.
This survey is not complete. In nearly all countries
further methods of identification of netting twines are
used, though not all of them can be classed as numbering
systems. Manufacturers, for instance, sometimes denote
their products with letters A, B, C ... or with Figs. 1,
2, 3 and so on. On the other hand, fishermen often prefer
their own traditional denominations, taking little heed
of the other systems. (Ref. 13). At the first FAO Fishing
Gear Congress the need was expressed for replacing
the various numbering systems by a single system applic-
able to all kinds of netting twines. The general adoption
of such a system on an international scale would avoid
confusion and facilitate international trade and the
exchange of information between fishery technicians.
Because of its many advantages and its acceptance as
universal yarn count system by the textile industry, the
tex-system established by ISO should be used for netting
twines.
3. THE TEX-SYSTEM
In 1956, Technical Committee Textiles of International
Organisation for Standardisation agreed to recommend
a direct system based on metric units for international
adoption in place of the various traditional methods of
numbering. The basic unit of the new system is the tex.
"The linear density (or number) of a yarn in tex expresses
the mass in grams of yarn having a length of one thou-
sand m. Thus a thread designated one tex has a mass of
one gram per thousand m of its length." (Ref. 5).
TABLE I— Conventional numbers and tex values
1 tex=-
lg
1,000m
The higher the tex value, the heavier the yarn. The
numerical value is followed by the term "tex", e.g.,
23 tex.3
Formula for converting the various conventional
yarn numbers into the tex-system (Ref. 5):
^ 1000 590-5 1000000 496055
tex-0,llllxTd ==— - = - = — ^-r-zr-
Nm Nec m/kg yds/lb
(Td= international denier system; Nm= metric count;
Nec= English cotton count)
In order to facilitate the conversion into the tex-system
the following conversion tables are given with values
rounded off according to ISO (Refs. 4 and 5):
Td^tex
Nm=tex
Nee = tex
m/kg « tex
70= 7-6
200- 5
120= 5
5000= 200
75= 8*4
160= 6-4
100- 6
4500- 220
90= 10
120= 8-4
90= 6-4
4000= 250
100= 11
100- 10
70« 8-4
3600- 280
110= 12
90= 11
60= 10
3000= 340
125= 14
85- 12
50= 12
2500= 400
150- 17
70= 14
40= 15
2250= 440
180= 20
60= 17
30= 20
2000= 500
190= 21
50= 20
26= 23
1800= 560
200= 22
43= 23
20= 30
1600- 640
210- 23
36= 28
12= 50
1500= 680
250= 28
34- 30
10= 60
1400= 720
265- 30
30= 34
8= 72
1200= 840
300- 34
20= 50
6=100
1000=1000
420= 46
18- 56
900-1100
630= 69
14= 72
800=1250
840- 92
10=100
750=1300
1000=110
700=1400
1100-120
600=1700
500=2000
400=2500
350=2800
3 In order to avoid mistakes, the multiple (kg per 1,000 m) and
sub-multiple (milligram per 1,000 m) of the tex unit should not be
used for the fishing industry. It seems to be better for netmakers and
fishermen to have only one unit for designating all sizes of single
yarns and netting twines.
4. DESIGNATION OF TWINES
4-1 Technical description
A complete technical description of a twine in the form
of folded yarn (plied yarn), based on the linear density
of the single yarn, comprises (Ref. 6) :
Linear density (number) of the single yarn in tex
Direction of twist of the single yarn (Z or S)
Amount of twist of the single yarn (expressed as the
number of turns per metre, or of turns per inch, of
the twisted single yarn)
Number of single yarns twisted together
Direction of folding (plying) twist (Z or S)
Amount of folding (plying) twist (t/m or t/i)
Example 5 (Ref. 6) : 34 tex S600 x 2Z400; R 69'3 tex.
For twine in the form of cabled yarns the following
data must be added:
Number of folded (plied) yarns cabled together
Direction of cabling twist (Z or S)
Amount of cabling twist (t/m or t/i)
Example 6 (Ref. 6): 20 tex Z700x2S400x3Z200;
R 123 tex.
In examples 5 and 6 the values R 69*3 tex and R 123 tex
are the resultant linear densities of the twines. They are
given for additional information and separated from the
proceeding parts of the twine counts by a semi-colon.
The symbol R is set before the numerical value (Ref. 6).
4*2 Resultant linear density
The resultant linear density is the linear density (tex
number) of the final product (netting twine) resulting
from twisting, folding or cabling operations. It takes
into account the increase in mass per unit length by the
twisting (or braiding) operations.
Resultant linear density is greatly dependent on the
amount of twist of single yarns, folded yarns and the
final product. Some examples for netting twines made of
polyamide filaments (without chemical treatment) are
given in Table II. In column 2 of this table the product
of single yarn tex x number of yarns is given. The values
in column 3 are the coefficients of twist a of the final
products which are netting twines in the form of cabled
yarn. The degrees of twist of single yarns and folded
yarns have not been considered. The values of a have
been calculated by means of the formula :
turns/metre
Nm is the metric number of the twine. The actual
resultant linear densities in column 4 are rounded arith-
metic means of different numbers of test results.
TABLE II— Coefficients of twist and resultant tex values
1 2
Twine Yarn tex x
construction number of
yarns
3
coefficient
of twine-
twist, a
4
Measured
R tex
5
Increase
in number
%
23texx2x3
(23tcxx6)
138
118
136
168
149
152
172
8-0
10-1
24-6
23tcxx4x3
(23 tex x 12)
276
129
145
166
299
309
332
8-3
12*0
20-3
23tcxx5x3
(23texxl5)
345
127
189
238
368
423
450
6-7
22-6
30*4
23texx6x3
(23 tex x 18)
414
126
179
218
456
489
510
10-1
18-1
23-2
23texx7x3
(23 tex x 21)
483
122
162
216
529
558
618
9-5
15-5
28-0
23 texx8x3
(23 tex x 24)
552
141
198
614
703
11-2
27-3
23 texx9x3
(23texx27)
621
134
247
681
725
9-7
16-7
5. SELECTING A NUMBERING SYSTEM FOR
NETTING TWINES
In selecting and recommending an international number-
ing system, the following points of view should be taken
into consideration:
5*1 The number of a netting twine is only one of the
specifications fishermen give when ordering netting
twine and netting. The fibre material, e.g., cotton
or nylon, must, of course, also be mentioned.
Other additional information concerning proper-
ties of the twine, e.g., its thickness or breaking
strength, may be given too. But these details do
not belong to the number of the twine.
5*2 The new system should be based on the tex values
and on the recommendations of the International
Organisation for Standardisation but, for use in
the fishing industry, simplifications are necessary.
The complete twine count (see examples 5 and 6) is
useful for the manufacturers of twine and netting
and for fishing gear technologists but not very
practical for the fishermen and most of the net
distributors.
5*3 In order to facilitate the introduction of the new
system into the fishing industry, traditional
methods of notation should not be entirely dis-
regarded. They should be respected to a certain
extent if they are not in contrast to the principles
of the new system (see point 7).
5*4 Proposals for simplification (if some details of twine
construction must be indicated): The meaning of
the numerical values of the amount of twist is
scarcely known by fishermen and, therefore, should
not be indicated. A general term, such as soft-laid,
medium-laid, hard-laid, would be preferable.
The twist direction of the final product only is,
to some extent, of importance to the fisherman
and netmaker but not the twist direction of the
components of the twine. With twines in the form
of cabled yarns it is not necessary to give the
numbers of the single yarns and of folded yarns
separately but only the total number of single
yarns. This notation is already spread in Japan,
Italy and Germany and in many other countries.
Examples for this simplified twine indication are
given in Table II, column 1, in brackets (Ref. 10).
5-5 Netmakers and fishermen often require the weight
of the netting twine. Resultant tex values can be
used for calculating the weight of netting twine and
netting and thus also the price. In the new system
they should replace the twine runnage (m/kg or
yds/lb) which should no longer be used.
6. NUMBER NETTING TWINES ONLY BY
RESULTANT TEX (R TEX)
There are recommendations for simplifying the designa-
tion of netting twine to one value only: the resultant tex.
On account of the influence of twisting, R tex cannot
be determined by only multiplying the yarn tex (as in
column 2 in Table II). Proposals have been to approxi-
mate the R tex value by applying a correction as twist
factor of 10 per cent which often holds true for medium-
laid twines. But the resultant linear density should be
indicated with sufficient accuracy as the weight of 1000 m
of the twine to be used as a basis for calculating the
weight of netting. This R tex value basing on the actual
weight of the twine could be used alone for thick trawl
twines (see 6*1), for twines of dissimilar components
(see 6*2) and for braided twines (see 6'3).
6-1 Thick netting twines
The actual R tex values could be used singly, without
additional description for designating the thick netting
twines of big bottom trawls.
Examples for strong twisted trawl twines made of
continuous poly amide:
R 8000 tex (125 m/kg)
R 6250 tex (160 m/kg)
R 5000 tex (200 m/kg)
R 4000 tex (250 m/kg)
R 3333 tex (300 m/kg)
Hfere the differences between the numerical values are
so evident that the kinds of twine may be clearly separ-
ated from each other by the R tex. If necessary, the
direction of twist of the twine may be put at the end,
e.g., R 8000 tex S.
6*2 Netting twines of dissimilar components
These are produced mainly in Japan and are of some
importance for the fishing industry. They may be com-
posed of yarns made of different kinds of fibres (e.g.,
R 3000 tex (333 m/kg)
R 2500 tex (400 m/kg)
R 2000 tex (500 m/kg)
R 1650 tex (600 m/kg)
'Livlon', made of poly vinylidcnc chloride and polyamidc
fibres, or 'Marlon', made of polyvinyl alcohol and poly-
amide fibres) or of spun yarns and filament yarns of the
same kind of fibre. The clear notation of such twines
being very complicated (Ref . 6), they should be numbered
by R tex with additional information concerning the
fibre material.
6-3 Braided netting twines
They are generally round or tubular braided, but braids
of flat structure are also manufactured for fishing nets.
There is an infinity of construction possibilities for these
twines:
The number of strands, braided together, depends on
the number of bobbins of the braiding machine
(e.g., 6, 8, 12, 16, or others).
The strand may consist of one or more (Fig. 2) yams,
twisted together or not.
Each strand may include the same number of yarns or
not.
The braid may contain a core (Fig. 2) or not.
The core may consist of only one yarn, or of several
yarns, twisted together or not.
The core yarns may be of the same count as those of
the strands or of another fineness.
This complicated structure of braids does not allow
a simple notation. Only resultant tex can give a practical
and useful designation.
5/oQ/f yarns
tf*oi number: 6
netting twin*
(cabltd yarn)
23 tex x2 x3'j R~.texZ
Fig. 1. Construction of a netting twine in the form of a cabled yarn.
braidtd ntttina twir*
toc/i i frond with 3 aing*
R.-tex
Fig. 2. Construction of a braided netting twine.
7. IN FAVOUR OF DETAILED DENOTATION
As can be seen from Table II, the R tex values of netting
twines of the same single yarn tex and the same number
of yarns but with different amount of twist can differ
in a high range. It is even possible that a hard-laid
twine with a lower number of single yarns has a higher
R tex than another soft- or medium-laid twine containing
more single yarns. (See the examples 23 tex x 21, 23 tex x
24 and 23 tex x 27 in Table II.)
The resultant tex alone does not supply full indication
in most cases and netting twine, except those mentioned
above (see points 6*1-6*3), should therefore be designated
by the count of the single yarn and the number of single
yarns constituting the twine and, additionally, by the
resultant tex. (See Fig. 1.)
There is another reason for suggesting this notation:
If the types of fishing gear are classified according to the
strain on the net material (Ref. 8) most of them belong
to the medium-strained group, e.g., seines and purse
seines, bottom trawls of small vessels, most midwater
trawls, gillnets for herring, cod and salmon, most river
stownets, dipnets, fykenets, trapnets, castnets and so on.
Studying fishery literature we can find that the netting
twines for these types of gear are nearly always designated
by the single yarn count and number of single yarns.
The systems used may be different (Td, Nm, Ne or
others) and so may be the way of writing the notation
but nearly always we may find the two components
(often supplied by the runnage in m/kg or yds/lb).
Fishing industry, netmakers and distributors are
accustomed to this method of numbering; they are not
textile experts; for them the number of a netting twine is
what they ask for when ordering so as to receive a twine
of a certain construction and breaking strength. Numbers
such as Td 210 x 15, Nm 20/15 or Ne 20/15 are names of
these kinds of twine well known to the fishing industry.
This should be taken into account when introducing the
tex-system. Example: 23 texx5x3; or simplified:
23 tex x 15; R 380 tex Z. Here, "23 tex x 15" is the
unchanging fixed nominator of the twine, similar to the
conventional systems used in most countries and there-
fore easily understandable to the fishermen. The resultant
tex value, rounded to a certain degree, changes according
to the amount of twist and may be used for calculating
the weight of twine and netting.
8. CONCLUSIONS
All numerical values of netting twines should be given in tex.
The resultant tex value (R tex) could be used alone for
those types of netting twine which are unmistakably
identified by this notation or have a complicated con-
struction (as e.g., braided twines or twines of dissimilar
components). It should replace the conventional runnage
in m/kg or yds/lb.
Types of twine which are not unmistakably identified
by R tex alone should be designated by single yarn count
in tex, number of single yarns and, additionally, by the
resultant tex. (As in Fig. 1.)
If necessary, the direction of twist (S or Z) of the netting
twine could be put at the end of R tex, as well as "soft-"
"medium-" or "hard-laid".
The author wishes to thank the following experts for their very
useful advice and comments: Dr. Arzano (Italy), Mr. Can-others
(Canada), Mr. Hentschcl (Germany), Mr. Lonsdalc (Great Britain),
Dr. Reuter (Netherlands) and Prof. Takayama (Japan).
Not all of them concur in all details with his opinion.
1. (1956) American Society for Testing Materials: ASTM Stand-
ards on Textile Materials, Philadelphia.
2. (1958) Canadian Government Specifications Board: Specifica-
tion for nets; fishing. Ottawa.
3. (1959) Carrothers, P. J. G. : The physical properties of netting
and twines suitable for use in commercial fishing gear.
In: Modern Fishing Gear of the World.
4. (1958) Deutscher Normenausschuss: DIN 60 905 and DIN
60 910. Berlin.
5. (1961) ISO, TC 38: Implementation of the tex-system for
designating the size of textile fibres, yarns and similar structures.
Draft ISO recommendation, No. 391.
6. (1962) ISO, TC 38, Sub-committee 4: Proposal for draft ISO
recommendation — Designation of yams.
7. ( 1 962) ISO, TC 38, Sub-committee 9 : Basic terms and definitions
for textile products for fishing nets.
8. (1959) Klust, G.: The efficiency of synthetic fibres in fishing
especially in Germany.
In: Modern Fishing Gear of the World.
9. (1961) Klust, G.: Fischereiliches Tauwerk aus Hart- und
Bastfasern. Teil I: SchnUre. Archiv fUr Fischereiwissenschaft, XII,
138-160.
10. (1963) Klust, G.: Uber die Verwendung der tex-Werte zur
Kennzeichnung der Netzgarnfeinheit. Deutsche Seiler-Zeitung,
82, Nr. 1, 163-164.
1 1 . (1960) Lonsdale, J. E. : Numbering systems for fishing twines
—Report of Working Party.
12. (1959) Shimozaki, Y.: Characteristics of synthetic twines
used for fishing nets and ropes in Japan.
In: Modern Fishing Gear of the World.
13. (1959) Takayama, Sh.: Terminology and numbering systems
used in Japan.
In: Modern Fishing Gear of the World.
DISCUSSION
Dr. G. Klust (Germany) Rapporteur: Following the
expressed desire of the participants at the First Gear Congress
(1957) that the different numbering systems should be replaced
by one single International system usable in all countries and
applicable to all kinds of net materials, a working party was
established to review the position and make a recommenda-
tion. Its Chairman, Mr. Lonsdale, submitted a report in
December 1960 of which the conclusions were that the uni-
versal system might be either a conventional system, preferably
metric, or an entirely new one in which case the tex-system
should be selected.
Today the decision is in favour of the tex-system which
the textile industries of many countries have already accepted.
The netting industry is only a small special branch of textiles.
It is therefore important that the tex-system should be accepted
by the fishing industry without further discussion. The
unit of the tex-system is the "tex" based on metric values.
The linear density, or the number of a single yarn, in tex
expresses the mass in grams of a yarn having a length of
one thousand metres. For instance, 23 tex means a single
yarn, which, with a length of 1,000 metres, weighs 23 grams.
Besides this unit "one tex" the International Organisation
for Standardisation recommends two other units; one is
8
the multiple of tex, kilotex, meaning kilograms per
thousand metres and the other is the sub-division of tex,
millitex, meaning milligrams per thousand metres. But
1 propose that these further terms be not used in fishing in
order to avoid new confusion. It would seem to be better
to have high numerical values in tex for thick twines than to
number by kilotex, and the lighter twine by tex or millitex.
The advantages of the tex-system are that it is direct and
can be applied to twine as well as yarn. Because of the
advantages outlined in detail in the paper the sub-committee
"Textile products for fishing nets" of ISO, has resolved to
recommend the following notations for netting twines.
Notation may give details of the single yarns making
up the product
(e.g., 23 tex X 5 X 3 ; R 380 tex)
or when especially constructed twines are described, may
be limited to the resultant count and final twist direction
(e.g., R 380 tex S)
or when describing braided twines, may give the resultant
count alone
(e.g., R 380 tex).
Netting industries and fishing industries should accept
these notations and FAO should support them. Manu-
facturers of twine and netting should use, in the first place,
the tex value and then the conventional number, maybe in
brackets.
Mr. A. Robinson (UK) questioned the wisdom of the
recommendation that the term "strand" should not be used
to indicate ply in the twine. He thought, to the trade
generally, it would be clearer than the alternative of "folded
yarn". "Strand" was well known because of its use in ropes
where it was the standard term. In relation to the claim that
the description should be "unmistakable" he pointed out
that if the term was given up, it could leave doubt as to whether
twine was two ply or three ply— it could be 6 X 2, 4 x 3,
or 3 X 4 — and manufacturers would often be left to guess.
Dr. von Brandt: These points have been considered and
the measures suggested are deemed to be most suitable.
Mr. B. M. Knox (UK): Since the introduction of
synthetic fibres there has been a great advance and most
manufacturers do now designate twine in either metres per
kilo, or yards per Ib which is an advance on the old method.
This data does assist the fishing industry in understanding
the selling numbers of twine. My company will work with
others to achieve some sort of standardisation.
Dr. Klust summing up, stressed that descriptions must be
as simple as possible. The terms "ply" or "fold" are recom-
mended by ISO for International use. "Strand" is generally
used for the components of ropes only and not of twines.
If the first twine numbering proposal of ISO sub-committee
is accepted (example: 23 tex X 5 X 3), it could leave no doubt
as to the number of plies. The amount of twist of yarn, ply
and twine has a great effect on the properties of netting twine
(as can be seen from tables in the paper). But fishermen do
not understand numerical values of twist given in turns per
metre or per inch and only need to know the direction of the
twist of the final product, the netting twine as well as the
the terms "soft", "medium", or "hard". The problem of
trade names was only one for the manufacturer of fibres.
The main problem was to introduce the tex unit and that
depended on the manufacturers of twine and netting. The
question of how to write the number of twine is of secondary
importance.
Part 1 Materials for Nets and Ropes
Section 2 Test Methods
Test Methods for Fishing Gear Materials
(Twines and Netting)
Abstract
The present paper is the result of the Working Party on Testing
Methods set up during the first FAO World Fishing Gear Congress
and of a recommendation made by the ISO Sub-committee 9 —
Textile Products for Fishing Nets. It represents an effort at unifica-
tion of the various methods used for testing the characteristics of
fishing twines and netting, resulting from the suggestions and
exchange of ideas of many gear technologists working in this field.
For twines the testing methods used in the textile industry can be
employed; however, properties in the wet condition are of main
importance in fishing gear. Special methods have to be developed
for testing fish netting, since no applicable textile methods exist.
The testing methods described cover both physical, chemical and
biological tests, and for each item define a characteristic to be
tested, and the range and accuracy of the instruments to be used.
The report covers testing of netting materials for the various
forms of breaking strength, elasticity, elongation, extensibility,
flexibility, abrasion resistance, weight (under different environ-
mental conditions), length and thickness, effect of heat, weathering,
chemicals and deterioration of a biological nature. The paper
furthermore includes suggestions towards uniform methods for
testing the properties of netting such as mesh size, strength, knot
stability and weight under the various forms applicable to fishing
nets. It had been the intention of the FAO Secretariat to bring this
matter to the Congress ready for discussion, aimed at definite
recommendations towards an internationally agreed standardisa-
tion of testing methods for fishing twines and netting. For the
testing of some properties, a final selection has not been made from
among the two or more alternative testing methods described.
Apart from these few exceptions, the report suggests one set of
definitions, type of equipment and methodology for testing the
characteristics of netting twines and netting, and it is hoped that
these will be accepted as standard in the trade and among fisheries
technologists.
Epreuves sur materiaux textiles utilises dans les engins de peche
Rtami*
La presente communication est le resultat du Groupe de Travail sur
les Methodes d'Epreuves organist pendant le Premier Congres
Mondial des Engins de peche et d'une recommandation faite par
le sous-comite 9 (Produits textiles pour filets de Peche) de 1' Organisa-
tion Internationale de Standardisation. Elle traduit un effort
tendant a unifier les diflferentes methodes utilisees pour eprouver
les caracteristiques des fils et filets de peche et est le resultat d'
^changes d'idees entre de nombreux tcchnicicns des peches tra-
vaillant dans ce domaine. En ce qui concerne le fils, les methodes
utilisees dans 1'industrie textile sont valables cependant il est tres
important de rechercher des methodes pouvant etre appliquees dans
le domaine des ptehes a des fils mouillds. Les methodes d'essais
dtaites couvrent en mftme temps les tests physique, chimique
ct biologique et pour chacun, definissent les caracteristiques
a eprouver et aussi la portec et la precision des instruments a
utihser. Le rapport traite dgalement des epreuves des materiaux
written by
Andres von Brandt
Institut fur Netz und Materialforschung,
Hamburg, Federal Republic of Germany
and revised by
P. J. G. Carrothers
Fisheries Research Board of Canada,
St. Andrews, New Brunswick, Canada
A. von Brandt
P. J. G. CuTothen
de filets, des differcnts aspects de force de rupture, elasticity
allongement, extensibility, flexibility, resistance a rusure, poids
(sous differentes conditions d'environnement), longueur et
epaisseur, efTets thermiques, atmosph6riqucs, chimiques et
deterioration de nature biologique (animaux marins). En outre,
la publication contient des suggestions pour {'unification des met-
hodes d'epreuves, dimensions des mailles, resistance a la rupture,
stability des noeuds et poids sous diflferents aspects, applicables
aux filets de peche. Le Secretariat de la FAO avail Pintention de
porter cette question au Congres, prfcte pour discussion et arriver
ainsi a des recommandations definitives Internationales pour la
standardisations des methodes d'epreuves des fils et filets mais
pour les epreuves de certaines proprietcs, une selection finale n'a
pas encore pu toe etablie. Le rapport cependant suggerc une liste
de definitions, type d'equipement et methodologie pour eprouver
les caracteristiques des fils et filets et on espere qu'elle pourra 6tre
acceptee oomme standard par le commerce et les techniciens des
pdches.
Extracto
Debesc esta ponencia a los rcsultados obtcnidos por el Grupo de
Trabajo sobre Metodos para Ensayos creado durante el Primer
Congreso Mundial de Artes de Pesca de la FAO y a una recomen-
dacion hccha por el Subcomite 9 de la ISO sobre Productos Tex-
tiles para Redes de Pesca. Apira a unificar los dlversos metodos
empleados para ensayar las caracteristkas de los hilos y redes de
pesca, resultado de las sugercncias e intercambio de ideas de muchos
especiaJistas en material de pesca ocupados en este sector. Para los
hilos pueden emplearse los metodos de ensayo de la industria tcxtil,
pero como lo que mas interesa en el material de pesca son sus pro-
piedades cuando esta hiimcdo, se tiencn que formular metodos
especiales para dlo, ya que los de dicha industria textil no son
aplkables. Los metodos descritos, flsicos, qufmicos y bio!6gicos,
definen para cada caso una de las caracteristicas que se tienen que
ensayar y dan el alcancc y exactitud de los instrumentos empleados.
Menciona el autor el ensayo de los materiales para redes en cuanto
a su resistencia a la rotura, elasticidad, estiramiento, flcxibilidad,
resistencia a la tracci6n, peso (en diversas condiciones ambientales),
longitud y diametro, efecto del calor, resistencia a la acci6n
atmosferica y a las sustancias quimicas y deterioraci6n de naturalcza
bioldgica. Se hacen tambien sugerencias para uniformar los metodos
para ensayar propiedades de las redes, como tamafto de la malla,
resistencia, estabuidad del nudo y peso en las diversas condiciones
en que trabajan en la pesca. Intentaba la Secretaria de la FAO
someter esta cuestion al Congreso, lista para discutirla, con objeto
de que se hicieran recomendaciones definitivas para normalizar
iriternacionalmente, tras previo acuerdo, los metodos para ensayar
hilos y redes de pesca. Para la prueba de algunas propiedades
todavia no se ha hccha una selecci6n definitiva entre los diyersos
metodos descritos. Salvo estas excepciones, en la comunicaci6n se
propone un grupo de dcfiniciones, clase de material y metodologia
para ensayar las caracteristicas de los hilos y las redes y se confia
que sea aceptado como patron en la industria y entre los tccndlogos
pesqueros.
AS a result of efforts in all parts of the world to increase
/\thc fishing yield, fisheries science has become more
interested in fishing methods and fishing gear than ever
before. A number of research institutes have been founded
specifically to study problems of fishing techniques and
others have added such studies to their research pro-
grammes. The First International Fishing Gear Congress,
convened by FAO in Hamburg, October 1957, demon-
strated that this field of study can be approached in
many different ways and that great possibilities to im-
prove fishing methods still exist. As a result of these
studies, fishing gear will be designed and constructed on
a more rational basis and the fishing effort will become
more effective.
It is logical not only to study the fishing gear to improve
its construction and efficiency but also to study the
materials from which the gear is made. In particular,
new man-made fibres will continue to be invented and
will continually become available for making nets. It is
important to test these new netting twines and the netting
made from them to assess their usefulness before trying
them in full-scale fishing gear. For this purpose it must
be known what properties of the netting materials are
important for fishing gear, and appropriate methods for
testing these properties quantitatively must be available.
A knowledge of these properties, e.g., of newly developed
netting twines, permits a decision to be made whether or
not the material can be used in fishing gear and how
it can be used to greatest advantage. It is possible to
avoid unnecessary and expensive tests with commercial
fishing gear if the properties of the netting materials are
measured first.
10
In some cases netting materials and netting can be
tested by methods which have already been developed
by the textile industry and which, in many instances,
have already been standardised. On the other hand,
the fishery is often interested in material properties in
both the dry and wet conditions, whereas these proper-
ties are of little or no interest for usual textile applications
and hence have so far not been measured. Thus, the
fishery must develop new test methods of its own or
modify existing methods to its own purposes.
In the Netherlands, a special institute for fisheries
has tested netting materials since 1911. It is interesting
that Het Nederlandsche Visscherij-Proefstation en
Laboratorium voor Materialen-Onderzoek, Utrecht, was
founded by fishermen organisations. In Germany, tests
in connection with gear research work began in 1928, and
a special institute for fishing technique, now located in
Hamburg, was founded in 1936. In Canada, the Fisheries
Research Board has been interested in netting properties
since 1934, and in 1954 the Canadian Government
Specifications Board founded a Committee on Speci-
fications for Fishing Gear at the request of the commer-
cial fishery. In Japan, the Tokai Regional Fisheries
Research Laboratory in Tokyo became the centre of
fishing gear research, and, in India, the Central Institute
of Fisheries Technology, Craft and Gear Wing, in
Cochin, should be mentioned in this respect.
Some of these, and other institutions also, have devel-
oped test methods for netting materials and netting,
often without knowledge of the other's work, and have
collected experiences which should be used as the basis
for future research. Unfortunately, the methods have
either not been published or exist in literature which is
difficult to find. Thus, it seems advisable to summarise
these test methods for general use.
A Working Party on Testing Methods, founded at
the First International Fishing Gear Congress, was
asked by FAO to collect all testing methods used for
fishing gear materials, with the ultimate objective of
establishing international standard test methods for
netting twine1 and netting.
A number of interested countries participated in this
Working Party, and the following submitted contribu-
tions:
Canada (Fisheries Research Board of Canada).
Denmark (Teknologisk Institut, Kopenhagen).
France (Chambre Syndicate des Filets de France,
Paris).
Germany, Federal Republic (Institut tlir Netzfor-
schung, Hamburg).
Germany, Eastern (Institut fiir Hochseefischerei,
Rostock).
Japan (Tokai Regional Fisheries Research Labora-
tory, Tokyo).
1 The International Organisation for Standardisation (ISO)
Sub-committee 9, Textile Products for Fishing Nets, has prepared a
draft dealing with basic terms and definitions for textile products
for fishing nets. In this draft "Netting twine" is denned as a
"general term for any kind of yarn or combination of yarns (includ-
ing braided) usable for the manufacture of netting".
The Netherlands (N.V. Onderzoekingsinstituut Re-
search, Arnheim).
(Het Nederlandsche Visscherij— Proefstation,
Utrecht),
U.S.S.R. (Laboratory of Fishing Technique, Mos-
cow).
In addition to these papers, recent contributions,
particularly those published in Modern Fishing Gear
of the World (1959), have been considered in so far as
they are concerned with test methods.
Different methods have reached different stages in
their development. Test methods for some properties,
particularly those in which the textile industry is also
interested, have been highly developed whereas test
methods for other properties have been only slightly
developed or do not exist at all.
Before adoption, a test method should be proven in
practice; its results should correlate well with experience
and results in the fishery. It is not possible to develop
test methods only in the laboratory. The following
collection of methods includes only those which have
actually been used in different countries. It is quite
possible that, as a result of knowledge gained in the
future, certain methods will have to be modified or
completely replaced by other methods. As new materials
become available, new problems will arise and new test
methods will have to be developed to measure properties
not yet recognised.
In spite of this, the following collection of test methods
for netting twine and netting may be useful to those
starting investigations in this field which is so important
to the fishery.
TESTING FISHING GEAR MATERIAL
For testing netting it will usually not be possible to
base the test method on methods already developed,
for example, for woven and knitted fabrics. In most
cases, these other materials differ from netting, even
from their initial manufacture. The situation is different
with netting twine, in the form of yarns, twines, and
monofilaments, which may often be tested by methods
already standardised as mentioned above (i.e., by national
standard methods of different countries such as the
ASTM Standards on Textile Materials in U.S.A, or
"Deutsche Normen" in Germany, and by the different
recommendations of BISFA). Yet, very often, these
methods were developed for finer materials such as
fibres and yarns and consider only the properties, e.g.,
breaking strength, in the dry condition. Such tests require
a room having standard atmospheric conditions (see
below) in order to give comparative results.
It must be emphasised that the precision which is
often desirable in the textile industry cannot be achieved,
and even may not be required, with the rough materials
often used for making nets. Further, the properties of
netting materials and netting are usually of interest
only in the wet condition. The usual dry strength data,
for example, are normally of no value to the fishery and
can even be dangerous when they lead to an incorrect
decision. Only properties in the wet condition are
important. This results in an advantage in testing
textile materials for fishing gear because rooms having
standard atmospheric conditions are not essential. This
fact is of particular interest to those fishing institutes
which wish to start testing netting materials and netting.
On the other hand, it cannot be denied that the price
of netting twines and netting is usually based on the
air-dry weight, and this can be determined accurately
only in a properly conditioned atmosphere. It is not
good enough to measure moisture content and correct
the weight to a standard moisture content because the
amount of moisture in the material affects its physical
dimensions. For example, a piece of cotton twine is
longer when it has a low moisture content than when it
has high moisture content so that measuring its runnage
(m/kg) when its moisture content is below standard,
then correcting the weight to standard moisture content,
results in the measured runnage being too high. This
error is particularly important if the twine is identified
by its runnage (e.g., m/kg) or by its weight (e.g., resultant
tex). Thus, a room having standard atmospheric condi-
tions is useful even for testing fishing net materials and
netting. However, absence of such a room is not sufficient
reason for not starting tests with wet materials.
DRY AND WET TESTS
The standards already established for textile tests
specify, as already mentioned, that the specimens be
conditioned in a standard atmosphere for at least 24
hours or until the weight becomes constant. The usual
standard atmospheric conditions are:
A temperature of 20 ± 2°C (70 ± 2°F) and
A relative humidity of 65 ±2%.
The test must be performed in the same standard
atmosphere as that in which the test specimens were
conditioned. Unless extreme accuracy is required, air-
dry specimens kept in the normal testing room atmos-
phere will give dry test results adequate for fishery
purposes.
For the much more important wet tests, most investi-
gators immerse the specimens in clear tap water or sea
water at ordinary room temperature for at least 12 hours
before the test. Tests with netting materials made of
cotton, polyvinyl alcohol, polyamide. or polyester show
no differences in wet strength whether soaked in distilled
water, fresh water, brackish water, or sea water. Net
materials and netting samples which have already been
used for fishing become soaked in a relatively short time.
This is also true with new bast fibres and untreated man-
made netting materials. Other natural fibres require
more time. Care must be taken that specimens of new
netting twines made of cotton or of lubricated hard
fibres are really submerged below the surface of the water
and do not float without becoming wetted. Soaking
must be done in containers which are large enough to
permit clear observation that the samples are wholly
immersed.
11
If, under certain circumstances, the specimens must
become soaked more quickly, they may be immersed in
a solution of a wetting agent.2 Tests may be conducted
after the specimen has been immersed for 15 minutes.3
These agents often have the disadvantage that they
decrease the slipping resistance of the knots, hence,
they should be avoided if knots are to be tested.
Tests with netting twines made of polyamide, polyester,
or polyvinyl alcohol fibres or polyethylene monofila-
mcnts have shown that the time of immersion in wetting-
agent solutions has no effect on the results of wet break-
ing strength tests.
A. Testing of netting materials
The term "netting materials" includes all materials
used to make netting. They may be made of natural or
synthetic fibres or even of metal wire. They may be
monofilaments or they may be twisted or braided4 into
one of many various forms. Traditionally, the terms used
by fishermen do not always agree with the definitions
used in the textile industry. There have been some pro-
posals for world-wide standardisation of terms used for
fishing, especially those for nets. 5 The following remarks
may be helpful to an understanding of the meaning of
terms used in the textile industry:
"Yarn" — a generic term for continuous strands of
textile fibres or filaments suitable for laying, braiding,
knitting, weaving, etc., into larger structures. It may
be a number of fibres twisted together, a number of
continuous filaments laid together with or without
twist, or a monofilament. The term "netting yarn" as
used in the fishing industry is very often not in accord
with this definition.
"Twine" — a balanced, plied yarn made by twisting,
laying, or braiding several yarns together. Only multi-ply
yarns are called "twines". The term "netting twine" as
used in the fishing industry is usually in accord with this
definition, but it is not always used by the fishermen.
"Cord" — a balanced structure formed by twisting or
laying two or more plied yams together.
"String" — not generally used in the textile or fishing
industries because it does not have a precise definition.
Most tests applied to netting twines or netting may be
classed as physical, chemical, or biological, but there
are also other tests related to manufacturing, handling,
or fishing effectiveness. Many tests have to be conducted
with the fishing gear in action, but the following descrip-
tions include only those test methods which may be
used in the laboratory.
* The term "vetting ayent" refers to chemical products which
decrease the surface tension of the water so that the water can
easily soak into the specimen, displacing the air. Typical wetting
agents for textiles are 'Aerosol1 and 'Nekal BX' in a 0-1 per cent
solution.
8 Canada specifies that the specimen be soaked for five min
(Canadian Government Specifications Board, Specification for
Nets: Fishing, 55-GP-l, 25 April, 1958).
4 The terms "braid" and "plait" both mean "to intertwine or
interweave yarns", but "braid" is usually used with reference to
twines because "plait" also means "to fold cloth".
6 See footnote *.
12
(a) Physical tests
Most of the netting twine tests under discussion are
physical in nature. Surely breaking strength is of greatest
interest to the fishery and should be reported in every
case. However, there are numerous other physical
properties of differing significance. The order in which
the test methods are presented in the following is arbi-
trary and does not imply any relative importance.
(1) Breaking strength
Breaking strength is defined as that load which causes
the netting material to break. Wet or dry netting yarns
may be tested. As already stated, the dry strength is
usually of no interest to the fishing industry but, for
its measurement, the test methods standardised in many
countries may be used. Otherwise, the methods for
wet netting materials described in the following have
to be modified to accommodate the dry materials. The
test report must state whether the materials were dry or
wet at the time of testing.
Fig./
Both the wet strength (e.g., of unknotted twine) and
the wet knot strength of the netting material are impor-
tant to the fishing industry, and special testing machines
are required for their measurement.
Strength testing machines: Testing machines known
as dynamometers or tensiometers are used to measure
breaking strength. The test specimen of netting twine is
fastened between two clamps and is stretched by an
increasing load until it breaks.
Strength testing machines may be classified as follows
into three basic types, according to the way in which the
load is applied to the test specimen:
Constant rate of load.
Constant rate of extension.
Constant rate of loading-clamp traverse.
The widely used pendulum-type tester (Fig. 1 and 2)
belongs in the last class. The upper clamp is connected
with the weighted pendulum which, in turn, is lifted by
the increasing tension in the test material. This tension
increases irregularly in accord with the elongation
characteristics of the material, for tension is reduced as a
result of elongation, particularly at the beginning
of the test. Similarly, the length of the specimen increases
irregularly because the upper clamp moves irregularly in
accord with the irregular increase of tension in the test
specimen. These pendulum testers have unofficially
been called "constant rate of nothing" because, so far
as the test specimen is concerned, nothing changes at a
constant rate.
The recently designed, electronic, recording dynamo-
meters (Fig. 3) are very precise and accurate because
there is no frictional resistance in die tension-measuring
unit. In these testers, the first clamp is moved from the
second clamp at a constant velocity and the tension is
measured by an electrical element to which the second
clamp is attached. Only testers with these recording
electronic dynamometers give a constant rate of exten-
sion because the clamp on the dynamometer moves a
negligible amount.6
The inclined plane, or constant-rate-of-load, machines
can be used only for lighter materials up to 2 kg breaking
strength and hence are not generally used for fishing
gear materials.
The clamps usually supplied with the testing machines
may be used for testing many netting materials. However,
if smooth or heavy materials, because of their fibre
properties, construction, treatment, or high strength
(e.g., twines made of hard fibres or certain man-made
filaments), slip in the usual clamps before breaking or
break at the clamps, then special clamps (Fig. 4) have to
6 These electronic testing machines are relatively expensive. For
testing only in the field, home-made testers, usually of the spring
balance type, may be used. Such testers have been described, e.g.,
by Czensny and Meseck: Untersuchungen Uber den Netzfrass
nicdcrer Wassertiere, Zeitschrift fur Fischerei XXVI, 237-310, 1938;
by Park, Y.C. and An, U.H.: Tensile Strength of Netting Cords,
Bull, of Pusan Fisheries College 11, 4-6, 1958; Miyamoto, H. and
Shariff, A.T.: Experiments on Fishing Net Preservation, Indian
Journal of Fisheries 6, 145-185, 1959.
Fig. 2. Pendulum tester:— (a) Koch (Schopper) (0-2. 0-5 and 0-10 kg) (b) Frank (0-10, 0-50 kg) (c) Wotpert (0-100, 0-500 kg).
13
Fig 3. Electronic recording dynamometer ' Teslatron" Wolpert.
Fig. 4
be used. These must be constructed in such a way as to
avoid sharp edges. 7
If a pendulum-type testing machine is used, its load
capacity should be chosen so that the angle of the pendu-
lum from vertical at the time of breaking is not greater
7 See Modern Fishing Gear of the World, p. 77, Fig. 3, and sketch
p. 96.
14
than 45° nor smaller than 5°. Further, the average break-
ing load should lie above the first fifth of the measuring
range. The velocity8 at which the loading clamp moves
should be 100 mm/min.9
It is convenient to use testing machines with different
overlapping ranges for testing netting materials, e.g.,
0-2 kg, 0-5 kg, 0-10 kg, 0-50 kg, 0-100 kg and 0-500 kg. To
a certain extent the testing range of the same machine
may be altered by changing the pendulum weight (by
adding or removing weights). To be specific, then, three
machines are needed, viz. 0-10 kg, 0-50 kg, and 0-500 kg,
and the other ranges may be obtained by changing the
pendulum weight.
There is considerable variation in the length of the
specimen specified between the clamps at the beginning
of the test. For normal clamps this should be 200 mm10
so that a netting twine sample 30 cm (ca11 12 in) long
is required for each test. If, as mentioned before, special
clamps have to be used for those netting materials which
slip or break in normal clamps, the samples may have to
be 100 cm (40 in) or more in length depending on the
construction of the clamps. The actual test length
should still be about 200 mm.
Care must be taken that twisted materials do not lose
twist while the test specimens are being cut to the
lengths described above.
Wet netting materials are tested at a room temperature
of about 20°C (68-70°F). It is suggested that higher or
lower temperatures can affect the result, particularly
if the material under test has been manufactured from
man-made fibres (see Section 9: Thermal Reaction).
The testing machines can become damaged as a result
of tests on wet specimens. Therefore, the machines
should be dried and oiled very carefully after use.
Wet breaking strength: The netting materials which are
to be tested for wet breaking strength are cut into test
pieces each 300 mm long and, as described, are immersed
in water for at least 12 hours. The netting materials are
tested as soon as they are removed from the water.
They are straightened and are secured in the clamps of
the testing machine with 200 mm actual test length
between the clamps. If breaking strength alone is to be
tested, it is not necessary to use any particular pre-tension
contrary to official standards.
Breaking strength is reported as the average of all
test results. Thus, the test specimens should be taken from
8 International standards do not specify the rate at which the
clamp travels, but, rather, specify that the testing time (time during
which the specimen is loaded to rupture) shall be 20 ± 2 sec.
9 Canada (CGSB Schedule 4-GP-2, Method 9.B), Japan (Modern
Fishing Gear of the World, p. 66), and U.S.A. (ASTM Designation :
D76-53) specify that the rate of loading clamp traverse shall be 12 ±
0*5 in/mm = 30 cm/min.
1° Canada specifies 10 ± 0-25 in in CGSB Schedule 4-GP-2,
Method 9B; Japan specifies 25 cm in Modern Fishing Gear of the
World, p. 66, 1959; U.S.A. specifies 10 ± 0-05 in in ASTM Desig-
nation: D204-57T. Man-made fibres are usually tested according tu
BISFA rules, viz. 50 cm between grips and 20 ± 2 sec breaking time.
11 All conversions from the metric system are approximate.
the netting material in such a way that they are represen-
tative of the whole lot that is to be tested, e.g., not from
the first portion of a spool or strand. In many cases,
particularly if a new material is being tested for the first
time or if the material is not uniform (e.g., from hard or
bast fibres), at least 20 test breaks are required to get a
sufficiently precise result. As a rule, 10 test breaks are
sufficient for routine tests. Where a series of tests are
being performed to show loss in strength by rotting of
preserved, natural fibre netting materials, only five
test breaks are required for significant results. This is in
contrast to the usual procedure for tests of yarns and
twines where the number of test breaks is determined
by the non-uniformity of the material and by the desired
precision of the average. Twenty, ten or five test breaks
are sufficient for fishery purposes under the above
conditions.
The test result is disregarded and an additional test
break is performed if the test specimen slips in the clamps
of the testing machine, or breaks in or at the edges of
the clamps, or if the result appears to be different from
other results in the test as a consequence of unusual or
irregular faults in the material.
The breaking strength is reported in kg to two signi-
ficant figures : averages greater than 1 kg are rounded off
to the first decimal place and averages less than 1 kg are
rounded off to the second decimal place.
Wet knot breaking strength: In testing wet knot breaking
strength, the type of knot is important. In most labora-
tories, a simple, overhand knot (Fig. 5) is tied in the
centre of a single piece of the netting material, because
only specially trained people can tie typical netting knots
without losing some of the twist in the netting twine.
Further, it has been found that the results of tests with
knots tied by hand in netting twine do not correlate well
with the results of mesh-strength tests applied to netting
made by machine using the same knot. The value of
tests involving the overhand knot is thereby increased.
In order to test a knot as used for making netting, two
pieces of the netting twine under consideration, each
SO cm long, are tied together by the weaver's knot
(English knot, Fig. 6a) or by the double knot (Fig. 6b)
developed from it. Also, the reef knot (Fig. 7) and other
knots may be tested in so far as they are used to make
netting.
However, this test cannot always be conducted satis-
factorily with all netting materials because the knots some-
times slip under the increasing load and can slip out
completely without breaking if only one leg of each side
of the knot is secured by the clamps (Figs. 6 and 7).
Where slippage occurs, the two legs on each side of the
knot must be fixed in each respective clamp (Fig. 37) or,
unless there are other reasons to the contrary, the over-
hand knot (Fig. 5) should be used for knot strength
tests. This latter is particularly true because the usual
hand-tied or machine-made knots are otherwise tested
in connection with wet mesh strength and knot stability
(see Sections 19 and 20).
The force with which the knots are tightened before
the test can affect the result but this effect becomes negli-
gible with the overhand knot. Loosely tied knots are
usually weaker and their strength more variable than are
tightly tied knots. The time between tying the knot and
testing for strength does not affect results significantly
so long as it exceeds 24 hours. However, all knots should
be tied shortly before the test and should be tightened
once more immediately before the test.
For testing, the wet, knotted twine is fastened in the
clamps so that the knot is about midway between the
clamps. The results of breaks which for some reason do
not occur at the knot are disregarded; only results from
breaks which occur at the knot are used.
The wet knot breaking strength is reported as the
arithmetic mean of all allowable test results, as was the
case when reporting wet strength. In addition to this
absolute value, the relative knot strength is also reported
as a per cent of the wet strength of the unknotted material
as follows:
wet knot breaking strength x 100
wet breaking stength (unknotted).
Calculations for comparing results: If materials of different
sizes or weights are to be compared concerning strength
efficiency, this can be done by converting strength data
to a common basis such as linear density or cross-sec-
tional area. Canada proposes to use the term "specific
strength" to identify all such converted data used for
comparative purposes. The three following types of
converted strength data differ in the units used, but all
may be employed directly to compare strengths of differ-
ent-sized materials.
(a) Breaking length: This figure is the length of the
material whose weight at the surface of the earth
equals its breaking strength. It is the maximum
length of the material which can be supported by
one end of itself without that end being broken by
the weight of the material. Contrary to common
practice, it is better to use this parameter for com-
paring only materials of the same specific weight.
15
It is calculated as follows:
breaking length (km) =
breaking load (g) x resultant Nm (km/kg).
_
However, since netting twines are made of several
yams, the resultant Nm has to be estimated from
the yarn Nm, and this is difficult because the various
hard, medium, and soft twists result in different
twist contractions. Thus, the breaking length may
be calculated more conveniently as follows:
^ ,. , , „ x breaking load (kg)
breaking length (km) = —
linear density (g/m).
To be precise, this calculation can be made only
with data, particularly for weight, from tests
performed in the standard atmosphere. In a
temperate climate, however, tests performed under
normal room conditions will be sufficiently
accurate for netting materials. The result is roun-
ded off to the nearest whole number.12
(b) Unit strength: Another way to compare strengths
of different netting twines is to refer the strength to
the cross-sectional area of the fibre by calculating
the unit strength, kg/mm2. Measurement of the
actual fibre cross-sectional area can be affected by
stretching or swelling the fibre. Thus, the area is
estimated from the runnage and the density of
fibre and the unit strength may be estimated as
follows:
unit strength (kg/mm2) =
breaking length (km) x fibre density (g/cm3)-
(c) Tenacity: The tenacity may be calculated as g/den
or g/tex. The calculation in g/den is made as
follows:
breaking strength (g)
tenacity (g/den) ==-
resultant denier.
For this formula, the denier figure is normally given
for the single yarn, as was the case when calculating
the breaking length above. Netting materials, on
the other hand, are usually plied twines, as already
mentioned. Therefore, the resultant denier of the
whole twine must first be determined. This is the
weight in grams of 9,000 m of the netting twine. It
is easier to compute the tenacity from the breaking
length as follows :
•* / /j ^ breaking length (km),
tenacity (g/den) = ^— ^ — - — -
Tenacity in g/tex is numerically equal to the break-
ing length in km.
(2) Extensibility
Extension is the change in dimension of the netting
material in the direction of the load, i.e., along the longi-
tudinal axis. It is measured at the same time and on
the same machine as strength (see above). For indicating
12 Where tests are performed in the standard atmosphere,
results may be reported to the second decimal place.
16
extension the testing machine must be equipped with a
suitable autographic device for recording the elongation
of the test specimen. The resulting graph shows the
amount of extension under the various loads.
Test conditions for extension are the same as those
described above for strength. Here, too, tests performed
on wet materials are more important than those perfor-
med on dry netting materials. For this test, it is essential
that the netting twine be placed in the tester under a
standard pre-tension, usually equal to the dry weight of
a 250-m13 length of the netting material. As a rule, this
weight is sufficient to straighten netting materials which
have become kinked or hardened as by preservatives.
The pre-tension is applied by hanging appropriate weights
from the test specimen. There are even some testing
machines on which the pre-tension can be applied
directly.
One difficulty in measuring the extension of knotted
netting twines is caused by twine coming out of the knot
when loose knots are tightened during the test, thereby
causing a false extension. Thus, extension tests should
be carried out with unknotted materials unless there are
particular reasons to the contrary.
Extension at break: The extension at the moment of
break is called the breaking extension (total extension,
stretch at rupture). At least 10 tests should be carried
out and the average calculated. The result is usually
reported as a per cent of the initial length :
Extension at break (%) =
length at break — initial length
; — ~r~. — r~i 1 ^ 100.
initial length
Usually testing machines plot the increase in length
directly, and some are even calibrated to give immediately
the per cent increase in length for certain initial lengths.
For an initial length of 200 mm half the extension in mm
is the per cent extension.
Load-elongation curve: In most cases, e.g., if netting is
being considered, the extension at break for unknotted
twine is of little interest to the fishing industry. In
netting, the knots (or the junctions in knotless netting)
break at loads lower than do corresponding unknotted
twines. This is also true of twines used in other ways,
e.g., for fishing lines. Thus, for the manufacture and use
of netting materials, it is important to know extensions
at loads lower than the breaking load.
The extension under different loads is plotted in the
so-called load-elongation curve, with the extension on
the abscissa and the load on the ordinate (Fig. 8a).
Where this graph is not drawn by an automatic recorder,
it may be plotted if two persons read off the load and
extension at the same time during the test. It is not
advisable for one person to do this by stopping the tester
periodically to measure the load and extension, because
ls The amount of pre-tension is variously specified. The 250-m
weight agrees with most standards. The international BISFA
specifies the 500-m weight for filament yarns. The measurement of
the meter-weight is described in Section 5.
some stress relaxation at constant extension occurs. If
single values of extension are to be reported, these should
be at 1/20, 1/10, 1/5, 1/4, 1/2, and 3/4 per cent and at
the full breaking load. Usually it is better to read off
extensions under low loads at small, regular intervals,
and extensions under higher loads at greater intervals,
e.g., of load in kg. Five tests should be conducted.
too
In order to compare the extensibilities of netting mat-
erials of different breaking strengths, the extension
values are not plotted against the absolute load in kg but,
usually, against the per cent of the breaking load, as
shown in Fig. 8b for netting materials made of different
man-made fibres. Another procedure is to plot the
extension against a specific load such as g/den, km-
weight, or g/tex. Because strength is more variable
than (and hence cannot be measured so accurately as)
twine weight, the latter procedure is often the more
precise.
Elasticity: Under stress, netting materials are extended
by an amount called "total elongation". When the stress
is removed, this extension may decrease totally or in
part. This ability to recover extension is called "elas-
ticity". If, after the stress is removed, the extension
does not decrease completely, the remaining extension
is called "permanent" or "irreversible elongation". The
extension which decreases when the stress is removed is
called "elastic elongation". With most netting materials,
part of the elastic or reversible extension decreases as
soon as the stress is removed and part decreases only
POLYESTER,
continuous
B POLYACRYLOWTRIE,
continuous fiUmint
POLYAMIDE,
continuous fH»m*nt
IV POLYVINYL ALCOHOL,
stapl* Kbr*.
POLYAMIDE.
2 4 6 • 10 18 14 M M EOttt4MttlOB8»4a«M404B44404t50
after a period of time. That part of the elastic extension
which decreases at once is called "immediate elasticity"
and that which decreases slowly after removal of the stress
is called "delayed elasticity". Thus, "total elongation"
consists of three parts, viz. "immediate elasticity",
"delayed elasticity", and "irreversible elongation", and
all three parts should be considered when studying
elasticity.
Elasticity is important in some fishing gear, particu-
larly that from which fish can escape by slipping through
the meshes or in which the fish are caught by their heads
(e.g., gillnets). Further, elasticity is important to mini-
mise the effect of sudden, jerking stresses (e.g., cyclic
shock loads).
The test procedure for measuring elastic extension in
textiles has not yet been standardised. It is usually done
on strength testers, as described before, by stressing and
relaxing the test specimens on a definite cycle. M
For the fishery, several proposals for testing elasticity
have been advanced, but not yet compared. The tests
are conducted with wet, unknotted, netting material.
Proposal (a):15 Specimens of netting twine (wet or
dry) from the same sample are fastened successively in
the strength tester with 200 mm test length under the
500-m weight pre-tension. Each specimen is stressed in
turn by the machine to either 10, 20, 30, 40, 60, or
14 See DIN proposal 53835, ASTM Designation 1333-54T, or
the Japanese proposal in Modern Fishing Gear of the World, p. 66,
items 5-11.
16 Institut ftir Hochseefischerei, Rostock-Marienehe, according
to DIN 53835.
17
80 per cent of the average breaking load for 10 min. Then
the load is removed for IS min. For quick tests for
comparative purposes, one test to SO per cent of the
breaking load is sufficient.
On the strength tester, the extension is read as soon as
the desired stress has been applied. The stress is then
maintained for the 10 min, by continually adjusting the
stress on the testing machine to the desired value, while
the extension of the twine increases further to a constant
value.
Then, the stress is reduced to the pre-tension value,
and the immediate change in length for the appropriate
degree of stress is recorded. To do this, the initial
200-mm test length is marked on the material and the
lower clamp is loosened so that only the pre-tension
affects the specimen. By moving the lower clamp to the
mark, the remaining extension can easily be read. The
test specimen is then removed from the tester and im-
mersed in water without stress for 15 min after which the
final extension is measured under the above pre-tension.
Each test to each degree of stress has to be carried out
with a new specimen.
According to DIN 53835, the test has to be repeated
for each load until constant values are obtained. At
least five tests have to be conducted at each load.
s
*• foatft* for f *»l/r,
. . jHUMriiatofv ft/far
e. ess* yair
Proposal (b):16 For the test, a 50-cm length is marked
on a 1-m piece of the netting material under 250-m
weight pre-tension.
The material is stressed to 30 per cent of its average
breaking strength for one hour, then the marked length
is measured. From this measurement, the total elonga-
tion at this load is calculated.
Then the load is reduced to the above pre-tension and
the marked part is measured once more. All the while,
the specimen is kept wet by spraying it with distilled
water. After each pair of measurements, at test load and
V-2,5
14 16 1* 20 22 24 2t 26 X) & 3435
f «4A
•f
o
150
i jlU *
1
, ,,..i
520
Flg.9
at pre-tension, the specimen is soaked in water. These
measurements are repeated periodically with the same
specimen until the readings are constant. Some materials
require several days to reach equilibrium. The final
length under pre-tension gives the permanent extension.
Elastic extension may be computed as the difference
between the final total extension underthetest load and the
permanent extension. As mentioned, elasticity may be
computed as the ratio of elastic extension to total extension.
The test load, equal to 30 per cent of the breaking
strength, was chosen arbitrarily. There may be advantage
in carrying out this test to other degrees of stress.
Fig. 8c demonstrates the method of data presentation
16 Institut filr Nctzforschung, Hamburg.
18
and gives results of extension and elasticity tests on
'Perlon* continuous-filament trawl twines at stresses
equal to 10, 20, 30 and 40 per cent of the average breaking
strength.
(3) Flexural stiffness
The term flexural stiffness refers to the resistance of the
netting twine to lateral or bending deformation. More
specifically, it may be defined as the force required to
cause a unit of bending deflection. By this definition,
when a fish touches a net, flexural stiffness is a measure
of the degree to which the netting itself resists the move-
ment of the fish. Thus, any method for measuring
flexural stiffness should assess the force which causes the
deflection. As defined here, flexural stiffness depends
primarily on the elastic modulus of the twine (governed
by fibre stiffness and twine construction), on the distance
between the point of support and the point at which the
force is applied, and on the diameter of the twine. This
last is a very important factor. The stiffness of the
netting material is reported to affect the efficiency of
many types of fishing gear. Usually, low stiffness in-
creases catching efficiency whereas high stiffness makes
the nets easier to handle. Sometimes the antonym
"flexibility" is used as the characteristic of netting
materials, as, for example, when the term "softness"
is used to characterise the desired low resistance to
deformation of soft-laid materials.
Different workers, working with different materials,
have recommended widely differing procedures for
measuring the stiffness of netting twines. Theoretically
this property may be measured according to either of
two basic principles: either the force which causes a
certain deflection (proposals (b) and (c)) or the energy
associated with this force (proposal (d)) is measured,
or deflection of a horizontally mounted specimen under
its own or an added weight (proposal (a)) is measured.
This last procedure is widely used for testing textile
fabrics and paper.17
Proposal (a):18 The flexural stiffness is measured by
fastening the specimen by one end only in the holding
device (Fig. 9).
The wet specimen is slipped horizontally under the
vertically adjustable upper jaw of the clamp in such a
way that it does not touch that jaw. The leading end of
the specimen approaches a horizontal reference line,
3 cm below the face of the clamp, at a point nearer to or
farther from the clamp according to the linear density
and flexibility of the specimen. The specimen is advanced
until it touches the graduated reference line, and the
horizontal distance of the point of contact from the jaw
is read directly as the measure of stiffness.
Five specimens of each material are each tested 10
times. The average gives a figure for stiffness which is
greater for stiffer materials.
17 Further proposals for measuring the flexibility of netting
materials, particularly in meshes, are presented in Roessingh, M.:
Recent experiments on tear and mesh selection in the Netherlands.
ICES-CM-1959, No. 88.
18 Institut flir Hochseefischerei, Rostock-Marienehe.
Care must be taken that the netting material has no
initial deformation which would cause errors. Such
errors are particularly possible with wet netting twines
of medium count.
Fibres of different specific weights cannot be compared
by this procedure because the twine is deflected by its
own weight. The same is true of twines treated with
preservatives of different weight, even though the twines
may be of the same number.
Proposal (b): " The principle of this procedure is to
measure the force required to deform the netting material
(cf. the force required for a fish to pass through the mesh).
Fig. 10 shows the required apparatus.
19 Institut fUr Netzforschung, Hamburg: Ldtzener
Mcthode v. Brandt, A.: Arbeitsmethoden der Netz-
forschung, Stuttgart, 1947, pp. 44-49.
Ffg.lO
19
Twenty cm (8 in) of the specimen are bent in such a
way that they form a loop (Fig, 10, A-B) and the ends
are fixed in the metal clamp (Fig. 10, 3). The stiffer
the material the wider the loop will be. A light 'Cello-
phane' vessel (Fig. 10, 2) of about 3 gm weight and SO cc
capacity, is hung from the loop. Care must be taken that
the vessel is suspended from the bottom of the loop.
Water is fed into the vessel from a burette, preferably
automatically filled, adjusted in such a way that the flow
is by steady drops rather than by a continuous jet (about
40 cc/min). The flow of water gradually draws the loop
together. The opening of the loop at its widest point (A-B)
is constantly observed with the assistance of a gauge and
the flow of water is stopped as soon as the opening has
decreased to 5 mm. The weight of the vessel and the
quantity of water which has dropped into it, as read
from the burette to the nearest cc, is used as the measure
in grams for the stiffness of the material being tested.
Here 1 cc of water is taken to weigh 1 g. The test should
be repeated 10 times for a good average.
It is desirable to use the same vessel for all weights of
material, but it may be necessary to use smaller vessels
for softer materials and larger vessels for harder materials.
For comparing different preservatives and the stiffen-
ing caused by the treatment, the following classification
is used:
up to 2 gm = extremely soft
3-10 gm = very soft
11-1 5 gm = soft
16-25 gm = medium hard
26-50 gm = hard
more than 50 gm = wire-like.
Special variations of the procedure have been proposed
for particularly soft net materials and for small samples.
Because a vessel must be used to contain the water, a
certain initial weight cannot be avoided. Thus, if the
specimens are very soft, measurement may be impossible
because the loop opening is drawn together to less than
5 mm by the weight of the vessel alone. The weight of
the vessel may be counterbalanced by the arrangement
shown in Fig. 11. 20 In this, one pan of the balance is the
vessel into which the water is allowed to flow, and its
weight is off-set by the weight of the other pan. A little
glass hook is hung into the loop of twine and is
attached to the balance by a fine strand ('Perlon' or
nylon). The initial stress on the loop can be decreased to
0*1 gm in this way. Water flow is regulated to about
5 cc in 60-100 sec by a burette with a very small opening,
thus permitting easier regulation for softer materials.
With this modification, results can be reported to the
nearest 0*1 gm. Otherwise, the procedure is the same as
described above.
But even with this modification, it is possible that
extremely soft materials cannot be tested because the
loops formed with 20 cm of the material close under
their own weight to less than 5 mm loop-opening width.
Proposal (c): 21 Twenty turns of the material are
wound close to each other around a rod 4 cm in diameter
and are held together by a narrow strip of adhesive
tape. This coil is removed from the rod and its diameter
is reduced from its initial value of about 4 cm to 2*5 cm
(1 in) by an increasing diametrical load.22
Two flat plates about 10 cm in diameter are fitted to
the clamps of an electronic dynamometer. The moving
clamp is lowered a few centimetres and the cord cylinder
is placed on the lower flat plate (Fig. 12). The lower
clamp is then moved upwards until the cylinder becomes
an ellipse with a short axis of 2-5 cm. The force required
for this is taken as the measure of flexural stiffness.
Proposal (d): 23 For measuring the flexural stiffness
of coarser netting materials (and cordage) the specimen
is suspended with a weight at one end like a pendulum
and is allowed to swing. Stiffer materials resist the swing
more strongly and stop the swinging sooner.
20 According to an unpublished proposal by E. Schoeninger.
21 van Wijngaarden, J.K. : Methods of testing net cords and nets
especially made of synthetic material. Rayon Revue 1 1, pp. 146-160,
1957.
22 This testing technique is similar to those used in the textile
industry for cloth.
28 Taylor, H. F. and Wells, A. W.: Properties and values of
certain fish-net preservatives. Doc. No. 947, U.S. Bureau of Fish-
eries, Washington, D.C., 1923.
20
A wet specimen 30-cm (1 ft) long is attached to a clamp
and is stressed by a 50-g24 weight (Fig. 13a). The wooden
base is sawn to form a sector of a circle of 30-cm radius
and is placed 30 cm from the clamp. The pendulum is
raised along the sector for a distance of 15 cm (6 in) and
is released without acceleration. The number of swings
for the amplitude to decrease to half its original value
are counted as the measure of stiffness.
The test is repeated 10 times with each of 10 samples.
If the sample becomes unduly softened at the clamps
by the test, the number of swings may be reduced by a
stopping device as shown in Fig. 13b.
If fine, twisted netting materials are being tested,
care must be taken that the twine does not lose twist.
If it does, the pendulum will not swing always in the
same plane and the number of swings to half amplitude
cannot be counted.
Fig. I 3
(4) Abrasion resistance
The term "abrasion resistance" refers to the ability of
the material to withstand abrasion under defined,
conventional conditions. It is said to correspond to the
mechanical wearing out with use. But it is difficult to
simulate in the laboratory the conditions which are
experienced by fishing gear materials during fishing. It
is not possible to obtain absolute values from test pro-
cedures because actual mechanical wear is a complex
process. The original nature of the material and subse-
quent damage by light or micro-organisms can affect
abrasion resistance. Also, the results of abrasion tests
are affected by the type of testing machines.
There are two ways in which abrasion can occur in
fishing gear:
(a) The netting materials may be abraded against
other harder objects such as the sea floor or the hull
of the boat.
(b) The netting materials may be abraded against one
another as, for example, in the knots or in the
seams of the net.
In textile-abrasion tests, the specimens may be rubbed
until they break or otherwise may be rubbed for a certain
period, then the change in other properties such as
strength studied.
Abrasion against hard objects: Abrasion of netting mat-
erials against comparatively hard objects is usually
regarded as the more significant procedure. For this, a
special testing machine is used in which the test specimen
is stressed and is rubbed on or by a hard object. The
number of rubs required for a given amount of wear
decreases as the load is increased. Fig. 14a represents the
Sander test machine,25 while Fig. 14b represents a
design reported by Shimozaki.26 Both testing machines
work according to the same principle and the specimens
are kept constantly wet during the test, but the angle
described by the netting material at the point of contact
with the abradant is different on the two machines. In
the Sander testing machine, the specimen assumes an
angle of about 90° at the abradant as shown in Fig. 14a,
but, in the design reported by Shimozaki, the correspond-
ing angle is about 150°. In the Sander testing machine
the specimen is pulled back and forth, whereas in the
design reported by Shimozaki, the specimen is held fast
while the abradant is moved back and forth.27
24 It would be more suitable to use the 250-m weight.
26 Klust, G.: Untersuchungen liber die Scheuerfestigkeit von
Fischnetzschnurcn. Protokolle ziir Fischereitechnik 3, pp. 64-88,
1954. This machine uas originally developed for testing textile
fabrics, not netting materials.
26 Shimozaki, Y.: Characteristics of synthetic twines used for
fishing nets and ropes in Japan. Modern Fishing Gear of the World,
pp. 19-29, 1959.
27 The Institut fur Hochseefischerei in Rostock-Marienche uses
an abrasion testing machine designed by Bobeth-Hahn (DRP No.
8961). On it, the load on the specimen may be held constant or it
may be changed between maximum and minimum values at every
complete rubbing cycle.
21
The water pouring steadily over the specimen serves
not only to keep the specimen wet but also to keep it
cod and to wash away loose particles as they are rubbed
off the material.
The netting twines are mounted as shown in the draw-
ings.20 Several specimens may be mounted beside one
another. Loads are attached to the free ends of the
netting material, equal to or greater than the 250-m
weight according to the purpose of the test.
On the Sander machine, the test specimen is pulled
back and forth over a corundum rod in which particles
of corundum are bound in a ceramic material. The size
of the corundum particles is standardised, is chosen
according to the material under test, and must always
be reported. The machine described by Shimozaki used
oil stones for (he abradant. The Sander machine imposes
63 rubbing cycles per minute, whereas the machine
described by Shimozaki imposes 80, and these cycles
are counted automatically on both machines. On the
Sander machine the stroke amplitude, E-F, is 6 cm
(2f in) if the machine is set up as shown in Fig. 14a.
The specimen is rubbed until it is completely worn and
breaks. The test is repeated at least 25 times with each
sample, and the arithmetic mean of the results is taken as
the abrasion resistance.
The type of abradant, the magnitude of the load on
the specimen, the speed of rubbing, the angle described
by the specimen at the abradant, and the length of the
rubbing stroke all affect the result and should be included
in the report.
Abrasion against itself: For testing abrasion resistance
between similar or different netting twines an apparatus
was proposed by Taylor and Wells.29 As can be seen
in the diagram (Fig. 15a), the two twines A-B-F-C and
D-F-E are rubbed against one another. The first twine
is fastened eccentrically to disc A, is led over the small
roll B, through the loop in the second twine at F, and is
attached to the table at point C. The second twine is
fastened to the table at point D and is led through the
loop of the first twine at point F. The free end of the
second twine is stressed by the weight E.80
The weight should be between a fifth and a twentieth
of the average breaking strength of the specimen and
should be chosen so that between 100 and 200 complete
rubbing cycles are required before the specimen breaks.
Taylor and Wells recommend that SO tests be averaged
for each result. The load, the speed of rubbing, and the
angles of contact should be reported with each result.
AC ]A
18 A-B= netting material under test, C = abradant, D= weight,
E-F = direction of reciprocating motion, O = clamp for test
specimen, H = cycle counter, 1 = crank, J = motor, K = water
tank supplying water to specimen, L = water tank catching water
from specimen.
" Taylor, A. F. and Wells, A. H.: Properties and values of
certain fish-net preservatives. Document 947, U.S. Bureau of
Fisheries, 1923.
86 It is important to keep the angles of the abrading materials
equal for all comparative tests.
22
T S
T
Old M«thod Ntw Mtthod
(5) Weight
Accurate tests for material weight, as required for the
calculation of breaking length, can be conducted only
in a standard atmosphere which complies with official
international standards for temperature and relative
humidity (see above). In addition to tests for twine
count which are not within the terms of reference for
these discussions, the weight of the netting materials in
these standard atmospheric conditions is of interest for
comparison with the wet weight. Also, the weight of the
netting material in water is important.81
Dry weight: The old Canadian procedure,32 as shown in
Fig. 15b, is to fasten the specimen of netting twine, T,
in a clamp, A-A, by its upper end, allowing it to hang
vertically (not horizontally as shown in Fig. 20 and Fig.
21). The required pre-tensioning weight, W, is hung
from the lower end of the specimen by means of the clamp
B-B. This vertical arrangement avoids the effect of
friction in pulleys, etc. The specimen is cut first, while
under appropriate pre-tension, against the scale S, at a
point one m below the upper clamp as shown at K, and
is then cut a second time at the lower face of the upper
clamp, A-A. Ten such 1-m specimens are weighed to-
gether and the linear density or runnage of the netting
material is computed.
A refinement of this apparatus is shown as the new
method in Fig. 1 Sb. It consists essentially of two clamps,
A-A and B-B, mounted on a vertical support. A steel
meter-scale, S, divided to the nearest 0*5 mm, is mounted
behind the clamps so that the distance between the
clamps may be set accurately. A trip balance for pre-
tensioning the specimen is mounted above the upper
81 See Chapter 21 : Weight of netting.
** Can-others, P. J. O.: Fisheries Research Board of Canada,
St. Andrews, N.B.
clamp, A-A. The upper end of the specimen is fastened
to the hook, H, under the pan of the balance and the
specimen is allowed to hang between the open jaws of
the clamps. The weights, W, on the balance beam are
set to exert a prc-tcnsioning force on the netting material
equal to the 500-m weight and the clamps are set so that
their facing edges are exactly 1 m apart. The lower end
of the specimen is pulled by hand as shown at P until
the balance pointed reads "O" on the balance scale, and
the jaws of the clamp B-B are tightened. The jaws of
clamp A-A are then tightened. The specimen is cut at
the facing edges of both jaws as shown at K-K and the
1-m piece is removed for weighing. As before, 10 such
1-m pieces of the netting material are weighed together
and the runnage (m/g) or the linear density (g/km) can
easily be calculated.
Wet weight: For measuring the wet weight of netting
twines out of water, the test specimens are cut as des-
cribed and soaked. The resulting shrinkage is not
considered because the greatest interest lies in how much
more the specimen weighs wet than dry.
When the netting material has become completely
soaked, it is removed from the water, allowed to drain,
and is weighed.33 The wet weight is reported as a per
cent of the dry weight according to the formula :
Wet weight (%) = -
wet weight (g) x 100
dry weight (g)
or as the per cent increase in weight according to the
formula :
,«^ wet weight — dry weight </wv
Water absorption ( %) =- f r~ - X 1 00.
dry weight
Weight in water: The weight of netting materials as well
as of netting in water depends not only on the specific
gravity of the fibre but also on the speed with which
water soaks into the material and on the tendency for
air bubbles to remain in or on the material. Because
of this air entrainment, quite dense materials can float
at or near the surface of the water unless they are loaded
to cause complete immersion.
The apparatus shown in Fig. 16 may be used to mea-
sure the weight in water.34 A specimen of netting twine
of known weight (e.g., 15 g) is completely soaked as
described above and is fastened to the bottom of the
balance pan as shown in Fig. 16. It is suspended in
water (distilled, fresh, or sea) at about 20°C (68°F). As
distilled water contains no dissolved air, the formation
of bubbles on the specimen may be minimised by its use.
The weight of the specimen under these conditions is
measured and is reported as a per cent of the weight of
the air-dry specimen.
88 In Modern Fishing Gear of the World, p. 21, Shimozaki gives
factors for the estimation of the dry weight (linear density— mg/m)
and the wet weight of several man-made and cotton netting mater-
ials.
84 v. Brandt, A.: Arbeitsmethoden der Netzforschung, Stuttgart,
1947, pp. 107-108.
Fig. I 6
™ • u. • * /o/x weight in water (g) x 100
Weight in water (%) = ;— r-rr- ; :
e air-dry weight (g)
It is also possible to compute the weight in sea water from
the formula:35 36
Weight ( %) in sea water =
specific gravity of sea water\
x 100.
\ specific gravity of material
Similarly, the specific gravity of the fibres may be com-
puted from the following formula:
0-998
Specific gravity = r-r- - — .. ... « —
weight m distilled water
air-dry weight in air.
Floating ability and sinking speed: Of particular interest
with netting twines and still more important with certain
complete nets, is the time for which they remain buoyant,
i.e., whether they float for a longer or shorter time or
whether they sink quickly. Whether the ability to float
for a long time (e.g., netting materials made of fibres
having specific gravity less than one) or the ability to
sink quickly is important depends on the type of fishing
gear. Different procedures for measuring this property
have been developed for different reasons.
Proposal (a), Floating ability:37 The ability of a speci-
men to retain its lightness in water can be measured on
the same apparatus as was used for measuring the weight
in water.
The specimen of netting material is suspended in a
vessel which is filled with fresh water or sea water.
85 Can-others, P. J. G.: The physical properties of netting and
twines suitable for use in commercial fishing gear. Modern Fishing
Gear of the World, pp. 69-74, 1959.
» In Modern Fishing Gear of the World, p. 22, Shimozaki gives
factors for estimating the weight in water from the dry weight for
materials made of several man-made fibres and of cotton.
87 v. Brandt, A.: Arbeitsmethoden der Netzforschung, Stuttgart,
pp. 107-108, 1947.
23
Weights are placed on the specimen to hold it below the
surface of the water. At intervals of time, first of hours
and later of days, the added weights are removed and
the weight of the specimen alone in the water is measured.
This procedure is repeated over a period of time until
the weight in water remains constant.
The weight in water may be reported as a per cent of
the weight of the air-dry specimen as described above.
For better understanding, the changes in floating ability
may be plotted graphically. The time in hours or days is
plotted against the abscissa and the increase of weight in
water, expressed as a per cent of the dry weight, is
plotted against the ordinate. Fig. 17 shows three cotton
twines prepared by different methods. According to the
preparation the floating ability will be lost immediately
(I and II) or after some hours (111).
flg.17
ftg.19
Proposal (b), Sinking speed:38 For determining sinking
speed, a piece of netting twine 2-cm ( it in) long is knotted
in the middle and is soaked in clear water for 12 hours.
A glass vessel is filled to a height of 50 cm with water at
20 ± 5°C (Fig. 18) and the test specimen is allowed to
fall from the surface of the water. The time required for
the sample to fall the distance of SO cm is measured in
seconds and the average sinking speed is calculated in
cm per sec. Three tests should be performed with each
specimen. Results from tests in which the sample sinks
at an angle or close to the wall of the vessel should be
rejected.
(6) Diameter
For measuring diameter, generally speaking, special
thickness gauges of different types are used. With these
a certain diametrical pressure on the netting material
cannot be avoided so that the results of measurements
on soft-laid or loosely braided materials or on those
made of staple fibres are too small.39 Therefore, it has
been proposed to test these materials according to an
ASTM procedure (proposal (b)). The facts that the
diameter of netting twine is decreased as a result of
extension under tensile load and that the cross-section
w Japan Chemical Fibres Association. The manufacture and
testing of synthetic yarns and fibres used in Japanese fishing gear.
Modern Fishing Gear of the World, p. 68.
89 Carrothers, P. J. G. : The physical properties of netting and
twine suitable for use in commercial fishing gear. Modern Fishing
Gear of the World, pp. 69-74, 1959.
24
of the twine may be elliptical or even rectangular rather
than circular must be taken into consideration.
Proposal (a), for measurement of monofilaments and
firm netting twines: A manual thickness gauge40 (Fig.
19a) may be used for this measurement. The test specimen
is placed between the two circular pressure feet (A^A* in
Fig. 19a) which are completely flat and are set parallel
to one another. The feet separate to 10 mm ( ft in) apart
without pressure so that all kinds of netting materials and
even thinner cordage used in the fishing industry can be
measured. By pressing lightly on the key (B in Fig. 19a)
the thickness of the material may be read directly from
the scale (C in Fig. 19a) to the nearest 1/100 mm. The
results of at least 20 measurements should be averaged,
and with very irregular materials even more measure-
ments may have to be made. As mentioned, it must be
noted whether or not the specimens really have a circular
cross-section.
Alternatively, a dead-weight dial gauge41 (Fig. 19b)
may be used for this measurement. In this, four lengths
of the specimen are laid parallel on the plane anvil, A,
and the movable foot, F, which is plane and parallel to
the anvil, is lowered gently onto the specimen by means
of the handle, H. The weight, W, is chosen so that the foot
exerts a standard total force of 6 oz (170 g) on the
specimen. The thickness of the specimen is read to
the nearest 1/1,000 in directly from the dial.
Proposal (b): Particularly for soft, compressible netting
materials. Canada has suggested that ASTM Desig-
nation D578-61 for glass yarns be used for measuring the
diameter of netting materials. This specification reads:
"(a) This method is based upon the use of a microscope
equipped with either a micrometer eye-piece with
a movable scale or a filar micrometer eye-piece.
(b) Apparatus. A microscope having a movable stage
which can be rotated to bring the yarn parallel
40 Klust, G.: Verflnderung des Durchmessers in Netzganen
durch Wasserung. Protokolle zur Fischereitechnik IV, pp. 84-106,
1956.
41 ASTM Designations D204-57T and D885-62T, CGSB
Schedule 4-GP-2, Method 37, and B.S. 2544: 1954.
to the cross hair shall be used. The magnification
shall be of such power that the yarn shall cover
approximately one-quarter of the field of view.
(c) Procedure. Mount the yarn on the movable stage
of the microscope by means of a yarn carrier which
is provided with suitable guides to maintain a
constant tension. Take care that no change in
twist occurs in mounting the yarn. Rotate the
stage until the yarn is parallel to the cross hair.
Determine the diameter of the yarn as the differ-
ence in the micrometer settings when the cross hair
is moved from one edge of the yarn to the other.
(d) Number of measurements. Make 20 measurements
at least 1 ft apart, and take the average as the
diameter of the yarn."
The Fisheries Research Board of Canada has found a
projection microscope to be just as accurate as, and more
convenient to use than, the movable stage micrometer
eye-piece instrument described above. First, the image
of a stage micrometer is projected by the microscope
on the screen on the bench and an appropriate scale and
vernier for the screen, reading to the nearest 0*01 mm at
the stage, is prepared from the image. The netting
material is then mounted on the stage of the projection
microscope, as described above by ASTM, and its
diameter is read directly by adjusting the specially pre-
pared scale and vernier to the image of the material on
the screen. As in the ASTM designation, 20 measure-
ments should be made. An F = 32 mm Macro-Tessar
lens has been found suitable for most materials.
Besides avoiding compression of the material, these
optical procedures permit study of the profile of the
material for irregularities and fuzziness, and may be
adapted to studying diameters of the material in water
and with different tensile loads on the material.
Proposal (c): Coil winding method.42 Twenty turns of
the netting material are wound closely and parallel to
one another around a cylinder of about 5 cm diameter.
The overall width of the 20 turns is measured with callipers
and is divided by 20. The result is reported as the thick-
ness of the material in mm. Five tests should be per-
formed. The difficulty is that uneven tension in the speci-
men and variations in the angle at which the material
is fed on to the rod can affect the result.43
(7) Surface roughness
Even if there is little danger of the netting material
becoming compressed during the measurement of dia-
meter, there is still the possibility that netting materials
made of staple fibres will be much thicker in water as a
result of protruding fibres than is indicated by the meas-
urements described in Section 6.44 This surface roughness
42 Japan Chemical Fibres Association. The manufacture and
testing of synthetic yarns and fibres used in Japanese fishing gear.
Modern Fishing Gear of the World, p. 65.
43 Hamilton, J. B.: Direct method for measuring yarn diameters
and bulk densities under conditions of thread flattening. The Journ-
al of the Textile Institute, Transactions, 50, pp. T655-672, 1959.
44 Can-others, P. J. G.: The physical properties of netting and
twines suitable for use in commercial fishing gear. Modern Fishing
Gear of the World, pp. 69-74, 1959.
also affects several other properties, such as knot
stability (see Section 20), pollution of nets (see Section
22), hydrodynamic drag, and selectivity of catch. So
far, however, a procedure for measuring surface rough-
ness has not been developed and, until now, tests have
been restricted to visual observations, such as by photo-
graphs.45
(8) Shrinking and lengthening
Generally speaking, the length of netting materials
changes when they are soaked in water. Usually a
shrinkage occurs, but sometimes a lengthening takes
place. Of greatest importance is the fact that this change
in length affects the size of the mesh (see Section 18).
These length changes may be determined either on the
unknotted material or by measuring the size of mesh.
However, the two results are generally not in agreement
because of the effect of the knots, quite apart from the
effect of any difference between the measuring procedures.
The main source of the difference in results lies in the
fact that shrinkage in the netting material is usually
measured under a pre-tension which is graduated accord-
ing to the linear density of the material, whereas the
mesh size is measured either without pre-tension or with
a pre-tension which has been agreed upon by international
conventions but which is not related to the runnage of the
material.
Tests for length change may be conducted to deter-
mine the effect of cold tap water or they may be conducted
to determine the effect of higher temperature, e.g., by
boiling during dyeing or hot treatment with a preserva-
tive. The test procedures which have been proposed for
the fishing industry to date do not differ significantly
from one another.
Proposal (a):46 For measuring length, the specimens
are mounted in the apparatus as shown in Fig. 21. To
avoid errors resulting from pulley friction, scale parallax,
and twine sag, it has been recommended that the appa-
ratus of Fig. 21 be mounted vertically and that the test
specimen be hung very near to the scale. The specimen,
more than 2 m long, is attached to point A (Fig. 21)
and is led over the roll B, 200 cm away. A load equal to
the 250-m weight47 is applied to the free end.
45 v. Brandt, A.: Zur Knotenkonstanz von Fischnetzen. Archiv.
fUr Fischerciwissenschaft 9, pp. 244-265, 1958.
46 Klust, G.: Lftngenverftnderung von Netzgarnen, Protokolle
zur Fischercitcchnik VI, pp. 70-83, 1956.
47 BISFA rules for synthetic fibres specify a pre-tension equal to
the 500-m weight.
25
For determining changes in length due to wetting,
two or more points are marked on the dry, pre-tensioned
specimen before it is soaked, and the distance between
these points is measured. The specimen is soaked for
at least 12 hours in fresh or sea water at normal tempera-
tures. Otherwise, the specimen may be boiled or treated
in any other way as by preservatives or by dyeing. Then
the distance between the marked points is measured
once more in the manner described above.
The final length is reported as a per cent of the initial
by length:
Final length^ Final length (cm) x 100
Initial length (cm).
Proposal (b):48 Five knots are tied in a piece of netting
twine at intervals of about 1 m and each part of the
material, as defined by the knots, is mounted in turn
on the measuring board as shown in Fig. 20. As men-
tioned for Proposal (a) above, the apparatus would
preferably be mounted vertically. The specimen is
stressed by a standard pre-tension and the distance
between each pair of successive knots is measured to the
nearest 1 mm. The sample is then placed, unstressed, in
cold or warm water as required, and the distances
between the knots are subsequently measured again. The
changes in length may then be reported as:
_. . , /oyv Initial length — final length
Shrinkage (%) ,.*... — - — x 100
Initial length
or:
Length change to wetting ( %) =
Final length — initial length
x 100.
Initial length
In this latter case, shrinkage is reported as a negative
quantity and elongation is reported as a positive quan-
tity/9
Proposal (c): 50 Tests in cold water. A distance of
1,000 mm is marked on the test specimen under standard
pre-tension and the specimen is tied outside the marks
to form a loop. The specimen is immersed in water at
room temperature for 12 hours or until it has become
thoroughly soaked, and then it is dried. The new length
is then measured in mm, as above, and the shrinkage is
calculated by the first formula in Proposal (b) above.
M van Wijngaarden, J. K. : Methods of testing net cord and nets,
especially those made of synthetic material. Rayon Revue, 11, pp.
146-160, 1957.
49 Can-others, P. J. O. : The physical properties of netting and
twine suitable for use in commercial fishing gear. Modern Fishing
Gear of the World, pp. 69-74, 1959.
60 Japan Chemical Fibres Association: The manufacture
*nd testing of synthetic yarns and fibres used in Japanese fishing
gear. Modern Fishing Gear of the World, pp. 62-68, 1959.
26
Tests in boiling water: A test specimen more than 1 m
long is folded in half and the two free ends are secured
together. A standard, prc-tensioning load is applied to
the loop and both sides of the loop are marked at a
distance 50 cm from the fold. The test specimen is
then boiled, unstressed for 30 min and is dried. The
marked distance is measured again, as above, and the
shrinkage is calculated as follows:
et. • i ,o/x
Shrinkage (%) =
500 — final length (mm)
— ~ — -v - '
500
^
x 100.
At least five tests should be carried out for each average.
(9) Thermal reaction
The actual properties of netting materials, particularly
those made of man-made fibres, can be different from
the measured properties at temperatures which differ
from the specified standard temperature at which the
properties were originally measured. This is true both
for the higher temperatures experienced in the tropical
fishing industry and for the lower temperatures experi-
enced in the winter fisheries of most countries in the
temperate and arctic climates. In particular, changes
can be expected in strength, extensibility, and stiffness.
Special equipment, required for measuring physical
properties of netting materials at extreme temperatures,
has not yet been developed by fishery researchers. To
determine whether or not netting materials can with-
stand temperatures required for preserving or dyeing,
boiling tests are usually carried out, particularly to
establish resulting changes in length. Such tests demon-
strate that the materials can be affected by temperature.
The methods developed by the textile industry for
measuring resistance to cold and heat should be remem-
bered.51
(10) Weather resistance
Some properties of netting twines, especially strength,
are adversely affected by light, particularly by ultra-
violet radiation. Usually, light damage is studied by
exposing the unprotected samples to natural sunlight in
the open air. Since light is not the only factor in such
exposures, the tests are often described as being for
weather resistance or atmospheric resistance. In this
test, quite a number of other factors affect the material : 52
Primary exposure factors:
chemicals in the air — oxygen, carbonic acid,
water (steam, fog, dew, rain, snow, ice)
temperature
air movements (wind)
solar radiation.
Secondary exposure factors :
dust
shifting sand
micro-organisms and insects.
81 E.g., Sommer, N. and Winkler, F.: Prilfung der Tcxtilien.
In: Handbuch der Werkstoffprilfung V, 1312-1335, 1960.
69 Hofmeier, H.: Klimapriifung zur Ermittlung der Gebrauch-
seigenschaften von Kunstatoffen und anderen Werkstoffen. Kunst-
stoffe41,pp. 179-180, 1951.
In every case, changes in strength or abrasion resis-
tance or fading of dyed materials have to be measured.
When different materials are being compared, it should
be recognised that light rays do not penetrate deeply so
that materials of different diameter are differently
affected.
For determining the effect of the sun's rays and weather
in the open air, the samples are stretched in a frame
(Fig. 22). These frames are set in an unshaded place
(e.g., on a roof) free from dust and smoke if possible.
They are faced in a southerly direction and are secured so
they are not damaged by wind. The frames are built of
wooden laths, 3 cm wide by 2 cm thick, and are varnished
for protection against the weather. The size of the frame
is governed by the size and number of the samples.
Where strength is used as the measure of weather damage,
a test length of 200 mm is required (see Section 1) so
that the frame should be 30 cm wide. Several frames
may be exposed together.
The materials are suspended, without tension if possible,
on aluminium nails or screws set 1 cm apart on the frame.
The nails or screws on opposite sides of the frame are
off-set \ cm so that the test material assumes a zig-zag
shape (see Fig. 22).
Every two or three months test specimens are removed
for measuring changes in strength or other properties.
The measured result is referred to the total time of
exposure or to the number of hours of sunlight measured
during the exposure as reported by the local weather
station. "
Resistance to sunlight: For determining the effect of
sunlight completely exclusive of rain and dust, the
materials may be exposed in a box covered with ultra-
violet transparent glass. As for the weathering test, the
box is set at an angle of 45° facing south, and the bottom
of the box is ventilated to avoid overheating.54
51 In connection with weathering tests, the amount of sunlight
and precipitation and the relative humidity should be measured
each day.
M Standard procedures specify that the ventilation be such that
temperatures inside the box are no more than 2°C above the outside
temperatures.
The sample is wound around an opaque board, 30 cm
wide, which is mounted in the box so that its upper face
is 5 cm from the glass cover. The specimens on the
shaded side of the board are used as controls for com*
parison with the specimens on the exposed side of the
board. As mentioned above, the resulting change in
properties is related to the number of hours of sunlight
during exposure.
Resistance in weathering machines: It has also been
proposed that accelerated tests be conducted with
artificial radiation instead of with sunlight which requires
a long exposure time. These weathering machines are
constructed so that the samples may be exposed to
artificial rain as well as to artificial sunlight, e.g., The
American Weather-Ometer.55
(b) Chemical tests
(11) Resistance to preservatives, oik, etc.
Netting twines made of man-made fibres may be damaged
by substances such as creosote oils and tannins which
are used to preserve natural fibres as well as by fish
slime and by other chemicals (e.g., light and heavy oils)
used in the fishing industry which also damage natural
fibres. Their effect, particularly on strength, should be
studied.56
(c) Biological tests
Fishing gear made of natural or synthetic fibres can
become damaged by various organisms both in storage
and in use. However, storage tests which study damage
in net warehouses will not be included here. Only those
tests which assess damage to fishing gear by water-borne
organisms will be discussed.
For netting materials made of plant fibres, cellulose-
digesting microbes and fungi living in the water and in
most storehouses are the major cause of rot. Similar
damage by micro-organisms can also be experienced by
fibres of animal origin.
Netting materials or natural and synthetic fibres can
also be damaged by several higher-order organisms in
the water. Here, our thoughts are restricted to those
organisms which eat dead vegetable matter or gnaw at it
for some other reason and, hence, which attack netting
made of plant fibres, too. Those water animals which
have become trapped in fishing gear and damage it in
an effort to become free will not be considered. Among
these could be members of all types of animals living in
the water, including the fish themselves.
Finally, within the scope of biological tests is a study of
that damage by communities of living animals or plants
known as fouling, whereby the organisms move actively
onto fishing gear which has been in the water for a
while."
65 Japan Chemical Fibres Association: The manufacture and
testing of synthetic yams and fibres used in Japanese fishing gear.
Modern Fishing Gear of the World, pp. 62-68, 1959.
M See Chapter 16: Dyeability and treatability, and Chapter
17: Storability. . ,
57 Concerning organisms which are moved passively onto the
nets, see Chapter 22: Pollution.
27
(12) Rotting rabtance
If treated or untreated netting materials made of natural
fibres are to be tested for rotting resistance, the condi-
tions experienced by the fishing gear during fishing
must be considered. Of great importance is the constant
rinsing action by the water.58 Tests, as conducted in
textile research with pure cultures of certain micro-
organisms such as fungi or with mixed cultures enriched
to a greater or lesser degree with suitable micro-
organisms as in the soil burial test,59 can yield incorrect
results with fishing gear materials, unless storability tests
are being conducted (see Section 17), because there is no
constant rinsing and consequent leaching of preservative
agents.
The two tests described below refer, in the one case, to
a field test in natural water and, in the other case, to a
laboratory test. The objection60 to the field tests in
natural water is that non-biological factors such as
currents, rinsing time, and mechanical effects can affect
the result. The objection to laboratory tests, e.g., in
aquaria, is that the test vessel can become poisoned by
preservative agents leached from the test specimens and
that leaching is slower than in the field so that rotting
may be lower in the experiment than during fishing. So
far, these difficulties have not been overcome. Neverthe-
less, by eithei procedure, comparable results may be
obtained.
Proposal (a): test under natural conditions.61 For
measuring the resistance to rot under natural field con-
ditions, the specimens are exposed in a suitable location
under the surface of the water. Damage by the micro-
organisms is measured by testing the strength of the
specimen both before and periodically during the exposure.
The lower the rot resistance of the material, the quicker
it will decrease in strength, and vice versa. However,
the rate of strength decrease depends also on the rotting
activity of the water. Therefore, this must be measured
at the same time. Thus, the duration of the test is deter-
mined on the one hand by the rotting activity (exogene
factor) of the water and on the other hand by the rotting
resistance (endogene factor) of the netting material.
The exposure should be continued, if possible, until the
specimen has lost at least 50 per cent of its initial strength.
The rotting activity which causes the netting material
to lose 30 per cent of its initial strength is taken as the
measure of rot resistance. A greater required rotting
activity to cause this loss indicates greater rotting
resistance of the material.
M For netting materials made of synthetic fibres, which as a rule
are not destroyed by cellulose- or protein-digesting micro-organisms,
the methods described here may be used to measure their resistance
to damage by the water or other agents in it.
M See proposal DIN 53933 and ASTM Designation D684-54.
60 Zaucha, J.: Evaluation of rot-retarding net preservatives.
Modern Fishing Gear of the World, pp. 128-132, 1959.
61 v. Brandt, A.: Arbeitsmethoden dcr Nctzforschung, Stuttgart,
pp. 64-83, 1947, and Method of testing resistance of net materials to
micro-organisms. Modern Fishing Gear of the World, pp. 133-136,
1959. This method is based on the so-called "physical-activity-
detennination" by Meseck (1929).
28
Thus, the test consists of two parts: first, the rotting
activity of the place where the exposure is to be made is
determined and, second, the strength loss experienced
by the specimen is measured. The rot resistance of the
specimen is computed from both these values.
If the rotting activity of several test places is known,
comparative tests can be conducted in these different
places even during different seasons. Thus, the test
results are independent of differences in exposure site
and exposure time.
The monthly rotting activity of a new exposure site
may be measured as follows :
Special cotton test twine of metric No. 50/1 5 (20 tex x
5 x 3)62 is suspended in the water at the site in question
on the first day of the month. Prior to exposure, the
twine is first extracted by boiling in distilled water and
its initial strength in the wet condition is measured;
then pieces of it, each 30 cm long, are tied together by
means of a rot-resistant material into four bundles of
10 pieces of twine each. After the first, second, and third
week and on the last day of the month, one of the bundles
of test twine is removed and its loss in strength is deter-
mined. The per cent loss in strength after one month is
the rotting activity of that site during that month. For
the above procedure, the loss in strength should not
exceed 70 to 80 per cent of the initial strength.
In areas where rotting activity is high, a loss in strength
equal to 70 to 80 per cent of the initial strength may be
reached before the end of the month, even before the
end of the first week. In this case, it is not possible to
determine the monthly rotting activity by the above
procedure.
The procedure must be modified so that the exposed
specimens are replaced by new ones after they have lost
| of their initial strength. The time at which this loss
occurs may be found easily by testing exposed specimens
for strength loss every week as described above. For sites
which have been used experimentally for several years,
the time required for the strength loss to equal £ of the
initial strength during any particular month is already
known approximately. In such cases, the specimens may
be replaced every week or 10 days as indicated. The
per cent strength losses of all successive specimens,
replaced one or more times during the month, are added
together. 63 The total result is the rotting activity of that
site during the month in question. Similar determination
of the monthly rotting activity of a given site, by exposing
the untreated specimens until they have lost only
62 In tropical areas, where the rotting activity is higher, heavier
twines such as metric No. 20/45 (50 tex x 15 x 3) have been used.
63 The rate of strength loss with time is not constant, being much
slower at the beginning of the microbiological destruction than at
the end. For this reason, strength losses greater than 70 to 80 per
cent of the initial strength are not included in rotting activity
estimates. Even so, some error cannot be avoided, but the simple
summation of per cent strength losses can still be useful.
SO per cent of their initial strength, has already been des-
cribed elsewhere.64 Because the moment of the desired
standard strength loss does not always fall at the end of
the month, or because the exposure is not always started
at the beginning of the month, daily values for per cent
strength loss may be calculated as the basis for estimating
the rotting activity which affects a specimen for periods
less than a month.
In only very few cases are untreated netting materials
tested for rotting resistance. Usually the materials have
been treated with a preservative.
For measuring the efficiency of a preservative in
reducing rot, cotton twines, e.g., metric No. 50/15
(20 tex x 5 x 3), or materials made of other fibres sub-
ject to rot, are treated according to directions for the
preservative. For comparing different preservatives with
one another, the same size twine should be used through-
out.
The twine is cut into 60 cm lengths, the ends are knotted
to conserve twist, and the specimens are treated. Then
they are tested for increase in weight, content of certain
active preservative agents, changes in flexural stiffness
(Section 3), changes in length (Section 8), changes in
strength (Section 1), etc. The pieces of treated netting
material are folded in two and tied into bundles 30 cm
long. Care must be taken that the material used for
tying the bundles does not affect nor is affected by the
treated test specimens, and that it does not rot itself.
Otherwise, the bundles will become loosened before the
end of the test period.
In test places where the current is strong, e.g., offshore
or in rivers, etc., the specimens may have to be tied to-
gether at their lower ends as well as at their upper ends to
prevent mutual damage or entangling. It must be noted
that the specimens are rinsed more in sites having strong
currents than in still locations. This stronger rinsing
can decrease the resistance to rot. Results from places
which differ widely in these non-biological factors should
not be used for comparative tests.65
Each bundle of treated netting twine should consist
of 60 test pieces (30 double twines). This number is
sufficient for six tests of 10 twines each or 12 tests of
five twines each. Larger bundles than this are not satis-
factory because the outer twines protect the inner ones
from the rinsing action of the water, particularly if the
bundles are tied at both ends.
It is not necessary to rinse the bundles of treated twine
before the test because, as with fishing nets, rinsing
occurs automatically during the first few days of exposure.
64 Zaucha, J.f Evaluation of rot-retarding net preservatives,
Modern Fishing Gear of the World, pp. 1 28-1 52, 1 959, has proposed
to use the 50 per cent strength loss rather than the 70 to 80 per cent
strength loss as the limiting value. In Proposal (b), the per cent loss
in strength is not specified at all. Rather, the time for total rotting
of the untreated twine is called a period, and the number of periods
required for the treated twine to lose a specified per cent of its
initial strength is used as the measure of rotting resistance.
66 Results from test sites having low rotting activity will usually
be lower than results from test sites having high rotting activity
because rinsing in the former sites has greater effect through the
longer test period.
Specimens which have been treated with different pre-
servatives should not be exposed close together, other-
wise they may affect one another.
The specimens are exposed in such a way that they do
not touch the bottom, or piles, or any other object. In
offshore regions in the presence of tide, the specimens
should be mounted deep enough so that they do not
become exposed to the air at any time.
For assessing a new preservative, twines treated with
it are exposed together with twines treated with a known
standard preservative for comparison. All samples are
exposed in the water at the same time. Under European
conditions, the first test for strength loss is applied to the
required number of specimens, e.g., 10, each 30 cm long,
removed from the bundle after two or even three months
of exposure.66 As a rule, tests are repeated every two
months in summer and every three months in winter
unless an abnormal strength loss requires more frequent
examination. The rotting activity is measured at the
same time by the procedure described above.
The specimens taken from the exposed bundle are
rinsed, any fouling, etc., is removed, and the breaking
strength is determined immediately. If it is not possible
to conduct the strength test immediately, the specimens
are dried quickly and thoroughly and are stored under
suitable conditions for later test for wet strength.67
The loss in strength of the treated specimens is com-
pared with the rotting activity of the water, measured
at the same time (see above). As already described, the
rotting resistance of the treated specimens is reported
as that rotting activity of the water which causes the
treated specimens to lose 50 per cent of their initial
strength. Usually, the exact rotting activity for this 50
per cent strength loss cannot be determined directly by
experiment but must be computed by linear interpolation
from the test strengths and rotting activities before and
after the 50 per cent strength loss as follows:
« . .
Rotting resistance =*
where:
(ti — tO X
RBO)
+ ^
tj = rotting activity which causes less than 50%
strength loss
t2 = rotting activity which causes more than 50%
strength loss
R! = breaking load after exposure to rotting acti-
vity ti
R2 = breaking load after exposure to rotting activity
ta
RBO =50% of initial breaking load
66 In tropical areas, specimens should be removed for test after
two or three weeks, or, at most, one month.
67 It has also been proposed to preserve specimens in formalin
when they cannot be tested immediately. The German DIN 53932
(draft) has proposed 0-1 per cent Trcventol CMK' for this purpose.
This procedure should be satisfactory particularly for untreated
twines which are being used to measure the rotting activity of the
water.
29
the desired value for rotting activity is between tt and
t*"
The results may be grouped as follows for classifying
the efficiency of various preservatives:
rotting resistance up to 200
200 to 500
500 to 1,000
1,000 to 2,000
no practical effect
minor effect
medium effect
: good effect
more than 2,000 : very good effect.
Proposal (b): tank test in laboratories.69 The tank
consists of a 12-litre bottle with a tightly fitting rubber
stopper carrying a fermenting tube (Fig. 23). The tank is
placed in a constant temperature water bath at 28°C (82°F).
Ten litres of sewage sludge is collected in a bottle from
the local sewage treatment plant as the source of supply
for the rotting medium. This bottle of sludge is placed
alongside the rotting tank in the water bath as soon as
possible. Meanwhile, the following nutrient solution
is prepared and placed in the rotting tank :
8 litres of tap water
10 g secondary potassium phosphate
10 g ammonium sulphate
0*1 g sodium chloride.
To this solution two litres of sludge from the supply
bottle and a few sheets of filter paper torn into small
pieces and shaken in water are added.
The rotting tank is allowed to remain at 28°C until the
bacteria have begun to multiply (usually between one and
two days). Then the pH is measured with an electric
pH meter and is corrected to 7-5 with potassium car-
bonate. The rotting tank is now ready for use.
Samples of cotton twine treated with the preservative
being tested are placed in the rotting tank along with
samples of untreated twine as a control. The tensile
strength of the untreated sample is checked daily. The pH
is measured and adjusted every three days. When the
bacterial growth ceases to cause a surface scum, more
filter paper is added. The tank is stirred daily.
As soon as the tensile strength of the untreated twine
has dropped to 0, all the samples are removed from the
tank and are rinsed. This concludes the first exposure
period. Thus, in this procedure, one period is defined
as the time required for the complete deterioration of a
sample of untreated twine of the same metric number as
the treated samples.
Before a new exposure period is started, the samples of
twine are rinsed in running water for three days. Mean-
while half the contents of the rotting tank are removed,
one litre of sewage sludge is added from the supply bottle,
the rotting tank is topped up with nutrient solution at
28°C, and the pH is adjusted to 7*5. Then the test samples
are returned to the rotting tank along with a fresh sample
of untreated twine as a control.
After each exposure period the tensile strength of the
wet test samples is measured by performing five test
48 For an example of this method see: v. Brandt, A. : Method of
testing resistance of net materials to micro-organisms. Modern
Fishing Gear of the World, pp. 133-136, 1959.
* Teknologisk Imtitut, Copenhagen.
30
breaks on the strength-testing machine using a 10 cm test
length. The results of these tests are tabulated, reporting
first the tensile strength after treatment with the preserva-
tive, then the percentages of this strength which remain
after the first, second, third, etc., exposure periods.
Experimental procedure: Cotton twine, Nm 20/6
(50 tex x 2 x 3) (tensile stength = 6'0 to 6-5 kg), is
used for this experiment. For each test, a total of about
40 pieces of cotton twine, each about 50 cm long, are cut.
These pieces are folded in half, gathered into small
skeins about 25 cm long, and treated with the preserva-
tive as desired.
Nylon lines, 30 cm long, are fastened between two
circular plates made of a dense material. The test skeins
of twine are fastened to these lines, and are identified by
numbers marked on the circular plates.
In addition to the hole drilled for the fermenting tube,
another hole is drilled in the centre of the large rubber
stopper used to seal the rotting tank, and a glass tube
20 mm in diameter is inserted. This glass tube can be
sealed with a smaller rubber stopper.
When the two circular plates with the sample skeins
attached are lowered into the rotting tank, the lower
plate settles to the bottom of the tank while the upper
plate is suspended just below the surface of the fluid by a
nylon line which is secured between the central glass
tube and its smaller rubber stopper. Thus, the sample
skeins are held in a vertical position in the rotting tank
without touching each other. The contents of the tank
are stirred daily by vertically moving the nylon line from
which the upper circular plate is suspended.
The upper end of the sample of untreated cotton twine
is similarly led through the central hole in the large
rubber stopper, while a 250-g weight (less than five per
cent of its initial tensile strength), tied to the bottom end
of the sample, rests on the bottom of the rotting tank.
After this, the tensile strength of the control twine is
checked daily simply by lifting it by hand. When the un-
treated twine breaks, its strength must be between 0 and
5 per cent of its initial tensile strength (see Fig. 23)
This procedure gives results which agree quite well
with those from field experiments, although there are
certain exceptions.
1. When the preservative treatment gives the netting
twine a coating of tar, plastic, etc., the results in
the rotting tank are often better than in practice
because the coating frequently forms an effective
physical barrier against the bacteria in the tank
whereas in practical use flexing and natural rinsing
cause local breaks in the coating, thereby laying the
twine open to bacterial attack at these places.
2. Certain preservatives are simply deposited on the
twine. Since these agents may be washed very slowly
from the twine, it is possible that the three-day
rinse is insufficient and that the loading of preserva-
tive on the twine does not reach equilibrium
until after the sample has been placed in the rotting
tank. In this case, the preservative agent will
migrate from this test sample into the water and
onto the other test samples, thus causing erroneous
results.
tut*
(13) Resistance to macro-organisms
Because damage by other water-borne organisms (some
insect larvae particularly of caddis and various sand-
hoppers which gnaw the material, or wood borers such
as ship-worms and gribbles which also attack cordage)
cannot be tested during practical use, this must be done
in laboratory aquaria70 or in lath boxes which are im-
mersed in natural waters.71 The boxes are constructed in
such a way that the organisms approach the test material,
which is stretched and fastened in the boxes, from the
side. The bottoms of the boxes are solid so that organisms
which fall off the test material during exposure are not
lost.
In laboratory aquaria the test may be conducted in
such a way that the organisms are offered their ordinary
food as well as the specimen of netting material. Other-
wise, according to procedures developed for wood and
textile vermin,72 only the specimens of netting material
are offered, nothing else. In the first circumstance the
animals can select their food, whereas in the second
circumstance they are forced to eat the netting material
or die. The latter arrangement is the more severe test,
and it will indicate whether or not the netting material
will be taken as food in an emergency. Caddis use
netting materials particularly to build their tubes.
(14) Fouling resistance
If netting materials and cordage are allowed to stand in
70 v. Brandt, A.: Arbcitsmethodcn dcr Netzforschung, Stuttgart,
pp. 83-85, 1947.
71 Meseck, G.: Untersuchungen liber den Netzfrass niederer
Wassertierc, etc., Zeitschrift ftir Fischerei XXVI, pp. 237-310, 1938.
72 Becker, G.: Biologische Untersuchungen an Tcxtilien.
Handbuch der Werkstoffjprttfung, 5 : Prttfung der Textilien, pp. 971-
1007, 1960.
water for a long time, plant and animal organisms can
actively settle on them. The settling of these communi-
ties of living organisms on dead objects is known as
fouling. Special test procedures have been developed for
studying fouling on ships. 73 Such a test will be successful
only if it is carried out in the field under natural condi-
tions. The fouling conditions at the test site must be
known. In this matter, the following has to be reported :
(a) The general development of the fouling, its com-
position and intensity during the course of one year
(b) Whether there is only one or several periods of
fouling
(c) When the fouling periods begin and end
(d) The seasonal appearance of the different types of
organism, singly or in groups, and the order of their
arrival
(e) All organisms which settle first prerequisite to
subsequent organisms.
As fouling can consist of many species of different animal
and plant organisms,74 it must be realised that treatments
which can prevent fouling of netting material in one
place can be useless in another where there are completely
different organisms.
It is possible to assess the fouling activity either by the
density or by the weight of the fouling organisms. The
test may be conducted with gear used for fishing or with
netting twines or cordage exposed specially for this
purpose. The procedure for the test depends on the type
of fouling.
Proposal (a):75 Netting materials, netting, or cordage
is exposed under water in a suitable place, if possible before
the beginning of the fouling period. Fouling is observed
continuously, particularly more frequently during the
initial period of immersion.
For measuring fouling, the test sample is removed from
the water for a short time and the fouling intensity is
recorded photographically. For comparative values, the
fouling density is reported as the per cent of the surface
area which has become covered during the exposure
period. Fouling density should be reported more preci-
sely when it is light than when it is heavy. For example,
the following classes of fouling intensity are proposed :
free of fouling =- 0%
partial fouling = 1, 3, 5, 10, (15), 20, 30, 50,
60, 80, 90%
maximum fouling =100%.
Finally, the fouling is removed at the conclusion of the
exposure to determine to what extent the surface fibres
of the netting materials have been affected by the fouling.
Proposal (b):76 The specimens of netting material are
immersed in water for a sufficiently long time to become
78 KUhl, H.: Arbeitsmethoden fUr Untersuchung von Bewuchs-
schutzmitteln. VcrdfTentlichungen des Institute fur KUsten-und
Binnenfischerei, Nr. 16, 1957.
74 Ray, D. L.r Marine boring and fouling organisms. Seattle,
1959.
76 In accord with fouling tests for ships. KUhl, H. : see n above,
76 Takayama, S. and Shimozaki, Y.: Development of fishing
net and rope preservation in Japan. Modern Fishing Gear of the
World, pp. 113-122, 1959.
31
thoroughly soaked, then they are weighed on a spring
balance and exposed at the test site. Periodically, the
specimens are removed from the water, allowed to drain,
and weighed, complete with the organisms which have
settled on them, on the spring balance. Results with
variously treated materials are shown in Fig. 24.
of touting (%)
10
^W A
CMf-ftr
(d) Tests for suitability
The way in which different netting materials are best
used in fishing gear and the way in which they are best
handled during fishing can be determined only during
practical use. Usually the assessment will be a subjective
one. Sometimes quite effective netting material is rejected
by the fishermen because it has other disadvantages, e.g.,
if it is too hard or cuts the hands when handled. Only a
few of these properties, which are important during
practical fishing, can be measured in the laboratory.
Also in this category should be mentioned the ease with
which the netting twine can be made into netting, its
amenability to or need for dyeing or treating with a
preservative or stiffening agent and its storability when
not in use.
"
(15) ProcessabilHy
Netting materials, e.g., twines, can be manufactured into
netting by manual, semi-mechanical, or mechanical
means. This is usually accomplished by knotting the
netting materials. Recently it has also become possible
to manufacture netting mechanically without knots.
Whether or not the netting material can be knitted easily
by manual or mechanical means must be determined by
practical tests. This processability depends upon strength,
particularly knot strength (Section 1), elasticity (Section
2), flexibility (Section 3), and on surface roughness
(Section 7). The last two properties also affect knot
stability (Section 20) and mesh strength (Section 19).
Manufacturing time can be measured, for either man-
ual or mechanical netmaking, to determine whether the
processing speed for a material under test is greater or
less than that for a known material of equal value.78
77 v. Brandt, A. : Arbeitsmcthoden der Netzforschung, Stuttgart,
pp. 54-63, 1947.
78 Different netting twines are of equal value if they may be
interchanged for practical fishing. Between twines made of different
fibres, there are seldom two of identical count. Very often substitu-
tion is made on the basis of equal wet knot strength, but this cannot
always be done, e.g., if one of the twines is so fine that it is difficult
to handle.
32
The major causes of delay during manufacture of netting
are breakage of the netting material, excessive surface
roughness, adhesion as a result of static electricity, and
the formation of unwanted loops and kinks. For asses-
sing netting materials in this way, certain types of fishing
gear or pieces of netting are manufactured by experienced
netmakers. Manufacturing time is measured and manu-
facturing difficulties are recorded as the basis for com-
parison between the new and conventional materials.
This test procedure can be similarly applied to mechanical
netting manufacture. Where the netting manufacture is
fully mechanical, the average number of rows of knots per
unit time and the amount of waste are measured. The
average number of rows of knots tied per unit time will be
decreased every time the machine is stopped because of
difficulties. Also the amount of waste cut from the netting,
expressed as per cent of the amount of good netting
manufactured, will be increased by a greater difficulty
in processing.
(16) Dyeability and treatability
Before new fibres can be used for practical fishing, it
should be determined whether or not they can be dyed or
otherwise treated for stiffening, etc., without damage.
Some synthetic materials are thermoplastic, and soften
and shrink (see Sections 8 and 9) even at temperatures
which are commonly used without damage for dyeing
or treating netting materials made of natural fibres.
It is also possible that other chemicals, which are used
for treating fishing gear or which are used for other
purposes in the fishing industry and come in contact
with the netting twines, can damage these netting mater-
ials (see Section 11, Resistance to preservatives, oils, etc.).
Often the textile industry offers dyes for man-made
fibres which are too expensive for the fishing industry
or which cannot be used by the fishing industry because
they require special dyeing equipment. In such cases,
other suitable processes must be sought.
In this matter, the fastness of the dyes, e.g., against
exposure to light (Section 10), as well as dyeability
must be considered. The fastness of dyes or of other
treatments may be measured during the tests for weather
resistance (Section 10), for rot resistance (Section 12) or
by one of the following proposals.79
Proposal (a), salt-water test: Two samples of the
netting material are placed in a solution containing three
per cent sodium chloride (NaCl) and 0-5 per cent mag-
nesium chloride (MgCla). After 24 hours immersion the
samples are rinsed in fresh water and dried. The changes
in colour and in stiffness (through loss of treating agent)
are noted.
Proposal (b), agitating test: Samples of netting
larger than 10 cm2 are subjected to the action of a suitable
agitator (washing machine) for 10 hours in water at
20 ±5°C. Then they are rinsed in fresh water and dried.
The change in colour is noted and the loss of treating
agent is measured by weighing.
79 Japanese Industrial Standard (draft) J.I.S.L., 1958, Method
(17) Storability
Untreated or treated netting materials can also suffer
damage even by nothing more than protracted storage
under normal climatic conditions. This can be observed
particularly with treated materials which have not yet
been used but which have experienced a loss in strength.
It is possible, too, that netting materials stored under
pressure can be damaged by spontaneous combustion,
particularly if they are damp.80
For testing storability, netting materials should, as a
rule, be held for a long time in a dry, shaded, and well
ventilated place to simulate storage conditions. Strength
of the netting material is measured periodically as des-
cribed in Section 1. Of course, the intervals of measuring
have to be very long.
For testing spontaneous combustion, the netting
materials are stored in large piles which may be moist.
Thermometers may be inserted into the pile of netting,
even by the fishing industry itself, to measure its tempera-
ture. Smaller netting samples, or even the agents which
cause spontaneous combustion, may be tested by the
Mackey test for which the procedure has not been
standardised.81
The effect of warm and moist storage can be deduced
from soil-burial tests. The test described above (Section
12) for rot resistance in water cannot be used for testing
storability. Similarly, the soil-burial test cannot be used
for testing rot resistance under fishing conditions.
Proposal (a), soil-burial test for assessing storability
under damp conditions.82 For this test, soil-burial boxes
(e.g., 45 X 30 X 25 cm), of an air-permeable material
(e.g., wood or asbestos-cement, but not glass), are filled
with soil to at least 1 50 mm deep. The soil should be a
well-seasoned compost, and should have a moisture
content of about 20-35 per cent, based on the dry weight.
Soils which contain loam or clay are not suitable. The
boxes are kept at 29 ± 1UC (84' F) in a room whose
atmosphere is saturated with moisture (100% RH).
For determining soil activity six cotton test twines, of
metric number 50/15 (20 tex x 5 x 3) and each 30 cm
long, are buried vertically in the soil so that only a short
part (ca 5 cm) is not covered, and the soil is pressed
lightly toward the test twines. The samples should be
at least 50 mm apart. The boxes are weighed at the
beginning of the test and the weight is checked daily.
Water lost by evaporation is replaced by spraying warm
(29°C = 84°F), chlorine-free water over the test soil.
After four days in the soil the twine samples are removed
and tested for strength. When the twine strength decrea-
ses by 50% ±10 during the four-day burial period
(80 ±10% during a seven-day period), the mixture in
the box is active enough for use.
80 v. Brandt, A.: Arbcitsmethoden der Netzforschung, Stuttgart,
pp. 87-91, 1947.
81 Schniffher, R.: Untersuchungen von textilien Hilfsstoffen.
Handbuch der WerkstoffprUfung Vol. 5: Prufung der Textilien,
pp. 858-912, 1960.
88 In accordance with DIN 53933. See also ASTM Designation
D684-54, ASA L 14.55-1960, Norme Fran$aise NF X 41-503-1955.
Indian Standard 1623 and 1633, 1960.
Meanwhile, the samples which have already been
treated with the preservative are rinsed in tap water
for 24 hours at 20°C (68°F) a flow rate at such that the
water is renewed five times per hour. They are then ready
for testing.
If the boxes of soil are sufficiently active, up to 20
specimens of treated twine each 30 cm long are buried as
described above along with six similar, untreated twines
for control. The untreated control twines are replaced
every four days. Five of the treated twines under study
are removed every two weeks, rinsed, and tested for
strength. The test is seldom continued for longer than
eight weeks. Usually four weeks is sufficient.
By testing the untreated control twines it is possible to
determine the rotting activity or the number of rotting
periods affecting the treated samples during the test
(see Section 12).
It must be pointed out that the results of soil-burial tests
are inherently affected by experimental conditions, and
since conditions of soil burial differ somewhat from condi-
tions in damp storage, the results from the soil-burial
test should be interpreted cautiously.
A better test for storability may be to inoculate sam-
ples of the netting material with a standard culture of
fungi, to incubate the infected sample in a warm, moist
atmosphere, and to check the strength samples at regular
time intervals. Such tests require special bacteriological
laboratories which are usually not associated with fishery
research institutes.
B. Testing of netting
The term "netting" refers to textiles which consist of
one yarn or one or more systems of yarns which are
crossed or joined so as to form meshes in the final pro-
duct, or to meshed structures which are formed by other
means such as by stamping or cutting sheet materials
or by extrusion.83 Usually the mesh assumes a diamond
shape, seldom square. In knotless netting the meshes are
sometimes more or less honeycomb shaped (hexagonal).
Netting may be classified as follows, according to the
structure of the fabric,84 see Fig. 25:
1. Knotted netting — usually made with the weaver's
knot or reef knot or double-knot modifications of
both types.
2. Knotless netting — either by the twisting technique or
by the Raschel procedure.
3. Marquisette — leno or mock leno weave (cf. gauze).
4. Woven netting — open plain weave (cf. cheese cloth).
The chemical, biological and suitability tests described
above for netting twines may be similarly used for testing
netting. However, it is usually more convenient to
apply such tests to netting twines than to netting.
There are, however, quite a few physical tests which
refer particularly to the mesh or to the knot. These can
be of great importance in the evaluation of netting
fabrics and, hence, also of netting twine.
88 According to draft proposal by ISO/TC38/SC9.
84 According to Japanese Industrial Standard (draft) J.I.S.L.
1948: Testing methods for synthetic fishing nets, Sections 2-8.
33
knotted netting
straight type
(ordinary type)
minnow net
(twined net)
jtigzag type
N • 222
woven net
Fig. 25
tortoise type
(18) Netting Dimensions
Two types of dimension pertinent to netting have to be
considered: the overall proportions of the netting fabric
and the size of the meshes. Particularly for measurement
of mesh size, quite a variety of instructions have been
issued, because most regulations for the protection of
fish populations are based on the specification of mesh
size.
Length and breadth: The length of a piece of netting
may be indicated either by the number of meshes or by
the number of meters. The breadth (depth) is always
34
indicated by the number of meshes which lie in one row
oriented in the direction at right angles to the direction
of the length indication.85
Mesh size: The size of the mesh may be indicated as
the length or as the opening of the mesh. The mesh
length in knotted (or knotless) netting is the distance
between the centres of two opposite knots (or opposite
joints) in the same mesh when the mesh is fully extended
at right angles to (or parallel to) the continuing direction
of the twines (Fig. 26, (a) — (b)), whereas the opening of
the mesh in knotted (or knotless) netting is the inside
distance between two opposite knots (or opposite joints)
in the same mesh when the mesh is fully extended at right
angles to (or parallel to) the continuing direction of the
twines (Fig. 26, (c)— (d)).8«
Fig.26
The mesh size may be measured in many different
ways. In addition to the fact that with hand-made
netting the circumference of the spool is taken as a
measure of mesh size, there are three basic principles
underlying all these different ways for measuring mesh
length. These are:87
" According to DIN 61250 (draft).
86 ISO/TC38/SC9: Basic terms and definitions for textile pro-
ducts for fishing nets. Draft, May 1962.
87 v. Brandt, A.: Zur Technik der Bcstimmung der Maschen-
grttsse. Archiv fUr Fischereiwissenschaft 6, pp. 54-64, 1956.
(a) By direct measurement of knot spacing. The dis-
tance between knots on opposite sides of the same
mesh or beside one another in the same mesh is
measured with a graduated rule. The place on
the mesh where the rule is read depends on whether
the mesh length or the mesh opening is being
measured. It makes a difference whether or not
the knot is included.
(b) By inserting a mesh gauge into the mesh to measure
the space surrounded by the mesh. Recently, the
type of mesh gauge and the required force have
been standardised.88
(c) By counting the number of knots per unit length.
In this procedure, the netting is fully extended and
the number of knots within a known length are
counted.
Since the recommendations for mesh size are designed
to allow smaller fish to escape, only the mesh opening is
significant. Therefore, only those procedures which
have been developed for measuring mesh openings will
be treated here. Knot size and twine diameter of the
netting material are not considered. Particularly in
heavy netting it makes a difference to the result whether
or not they are included in the measurement. In finer
(ratio of twine diameter to mesh length is small) netting
it makes little difference whether or not the knot is
included.
In the netting trade it is customary to measure the
mesh size by gathering the netting lengthwise and by
measuring the distance between opposite or successive
knots, including one knot.
In the following proposals for mesh-length measure-
ment, only wet netting will be considered because gear
selectivity is determined by the size of the wet meshes. 8g
For the netting manufacturer it is important to know the
relationship between the sizes of wet and dry meshes.
Thus, the length of dry meshes may also be measured by
the following procedures to determine changes in mesh
size (shrinking or lengthening):
As indicated above, a mesh gauge is inserted into the
mesh to measure its opening. For this, there must be a
certain force to straighten the mesh. There is no general
agreement as to how great this force should be for diff-
erent materials. As discussed above for unknotted
netting materials, any standard should relate the magni-
tude of the straightening force to the weight of the
material, e.g., equal to the weight of a certain length of
the material, preferably the 250 m weight. (See proposal
(d) in this Section). Because it is difficult to measure the
weight per unit length of knotted netting materials
which have already been used, it becomes necessary
simply to specify a certain force during measurement of
mesh size and to relate the measurements with different
materials to the fishing selectivity of those materials.
88 Prospectus of the ICES Mesh Gauge. Le Bureau du Conscil
Permanent International pour 1'Exploration de la Mer, Charlotten-
lund, Denmark, 1962.
89 However, gear selectivity depends not only on mesh size but
also on the shape of the meshes and on other internal and external
factors.
So far, no method has been developed for measuring
mesh size whereby the relation between the measured
mesh size and the selectivity of the fishing gear is the same
for all types of material.
Proposal (a): mesh measurement in trawls for the
marine fishery.90 The mesh is measured by the so-called
"ICES Mesh Gauge" with locking device,91 which acts
longitudinally to exert a standard force on the mesh.
(Fig. 27) This gauge yields more consistent results with
90 According to a proposal of ICES, Report of the Mesh Selec-
tion Working Groups Meeting in Copenhagen, December 1959 and
1960, Part IV, General Considerations on Trawl and Seine Mesh
Selection and its Measurement.
91 Fig. 27 shows different types of gauges which exert a force on
the mesh. (1) ICNAF type, (2) Scotch longitudinal-force mesh
gauge on which the ICE Smesh gauge was based, (3) Polish longi-
tudinal-force mesh gauge, (4) Lowestoft "scissors" gauge.
AM 4
a * * a « r *
n M m H an**9j f t i 4 t i i t
Fig.27
V
4M.J
35
different people92 than does any other gauge by virtue of
its design and construction. Because measurements of
mesh size must be exactly c nparable for mesh selec-
tivity studies, it is recommended that all other gauges
used for such work be calibrated against this type of
gauge as standard.
In particular, a force of 4 kg is exerted on the mesh
when measuring mesh size in codends of trawls and seine
nets. It is recommended that the gauge be recalibrated
regularly. The codend must be completely wet. It is
recommended that the meshes chosen for measuring
should be in straight lines running fore and aft along the
top side (not near the selvedges) of the after half of the
codend, starting from the third row behind the codline.
Meshes adjacent to strengthening ropes, meshes with a
join in any bar, and meshes in any repaired pait of the
codend should not be measured. The measurements
ra v. Brandt, A. and Bohl, H.r Ma&chenmessung
mit Druckrnessgerftten bei Schlcppnetzen. Protokoile
ztir Fischereitechnik 5, pp. 277-295, 1959.
Q
I
1,00 KO
mit Mttstob
Fig. 2 8
B
should be made immediately after every haul so that
any change in average mesh size with time can be detected
and reported.
The number of meshes to be measured should not be
fixed arbitrarily because the minimum number of mea-
surements is governed by the desired accuracy of the
average and on the standard deviation of the mesh size
which, in turn, varies with the original construction and
use history of the codend. Usually, the average mesh
size must be determined with an error less than two per
cent. For this, it will usually be sufficient to measure
25 meshes if the observed 95 per cent range of mesh sizes
is 20 mm and the average of measurements must be
within 2 mm of the actual mesh size. In scientific reports,
the number of measurements and the standard error of
the average mesh size should be reported along with the
average mesh size and range of mesh sizes.
The mesh size is reported in mm and the average is
reported as the mesh size for that codend.
Proposal (b): mesh measurement in gillnets made of
fine material.93 This measurement is made on a special
gauge as represented in Fig. 28. A panel of the netting,
five meshes long by five meshes broad (deep) is cut from
the gillnet. The rows of meshes along the upper and lower
edges of the panel are respectively attached to hooks A
and B and the gauge is suspended vertically from point C
as shown in Fig. 28. The load D (1 kg) applies a tension
to all five rows of meshes and the total length of five
meshes is read at point F on scale E. This length divided
by 10 gives the average length in mm of each mesh bar04
taken from one knot to the next, including one knot.
Proposal (c):95 The average measurements of 12
individual meshes in gillnets is taken as mesh size for the
net. Three of these meshes are selected from each
lengthwise quarter of the net, one randomly from the
top third of the net (i.e. from the third nearest to but not
including the corkline selvedge), one from the middle
third, and one from the bottom third exclusive of the
leadline selvedge.
The following measuring devices are recognised for
measuring the mesh size (Fig. 29): the Allen net rule, the
Hovey gauge, the Selkirk gauge, the flexible rule, and the
straight rule. These devices apply a tension between
three ounces (85 g) and eight ounces (225 g) to the mesh
at the time of measurement.
The Allen gauge consists of a threaded rod along which
the internally threaded sleeve C (Fig. 29) can travel. The
block D is moved along the rod by turning sleeve C
to which it is attached, but the block itself is restrained
from turning on the rod by flats which are milled on
either side of the rod. The mesh to be measured is placed
98 Florin, J.: Ein Messger&t ztir einheitlichen Bestimmung der
Netzmaschenweiten. Schweizerische Fischerci Zeitung 65, pp. 243-
244, 1957.
94 According to the ISO draft mentioned above, the term "bar"
is defined as the distance between two sequential knots or joints,
measured from centre to centre.
95 According to Canadian Government Specification Board,
Specification for Nets: Fishing, 25 April, 1958, and Fisheries
Research Board of Canada, St. Andrews, N.B.
36
in the notches at A and B, and the sleeve C is turned by
hand, moving the block D and the hook B away from
hook A until the mesh becomes tight. The tension in
the mesh pulls on the lever at B so that it pivots about
point P and compresses the spring until the dog on the
lever drops into one of the notches in sleeve C, stopping it
from turning. The mesh opening is then read directly from
the scale which is engraved on one of the flats on the rod.
By proper selection of the spring, this gauge exerts a
consistent tension of about five ounces (140 g) on the
mesh at the time of measurement and gives more consis-
tent results with different persons than do the other
gauges.
Alltn Gouge
Hovey Gouge
Fig. 2 9
D
•
-n-
s
-
s<
w
ilkirk Goug
u
6
The Hovey gauge (Fig. 29) operates on the same prin-
ciple as the 1CN AF gauge. The weight W is inserted into
the mesh until the edges of the wedge rest against oppo-
site knots in the mesh. The mesh is held horizontal so
that the gauge hangs freely and the weight W (8 oz)
exerts a standard force on the mesh. Care must be taken
not to overstress the mesh while holding it. The mesh
opening is then read directly from Scale S.
The Selkirk gauge (Fig. 29) consists of a cylindrical
tube with a longitudinal slot cut in its wall, with a scale
S engraved against the slot, and with a hook A fastened
near its upper end. A rod W of standard ( 8 oz) weight is
inserted into the lower end of the tube and the hook B
is fastened near its upper end. The mesh to be measured
is placed in the notches at A and B and the gauge is
suspended from its ring R so that the weight W exerts the
standard tension on the mesh. The mesh opening is then
read directly from Scale S.
The flexible rule is a go-no-go gauge used for enforcing
fishery regulations. It is a piece of flexible steel, \ in
(1 cm) wide by 1/100 in (£ mm) thick and the same
length as the minimum allowable mesh opening. The rule
is flexed to a slight curvature and is inserted into the
mesh so that each end rests against one of the diagonally
opposite knots in that mesh. The rule is then released.
If the rule straightens out, the mesh opening is satisfactory
if the rule is held curved by the mesh, the mesh opening
is too small.
The straight rule is a thin, graduated rule, usually made
of steel. The index end of the rule is placed against one
of the knots in the mesh to be measured and the opposite
knot is pulled along the rule until the mesh is straight
but not stretched. The mesh opening or the mesh
length is then read directly from the scale on the rule.
The measurement is usually made by these gauges
inside the mesh, i.e., between the knots (mesh opening),
and with the mesh fully extended in the direction normal
to the selvedge.96 The result is reported in inches to three
significant figures.
Proposal (d):97 Although in this method, as in Pro-
posal (b), the knot is included in the mesh size measure-
ment, this is of little importance when the mesh opening
in fine netting is being measured.
The mesh size is measured by a metric scale as the
distance between the centres of opposite knots (mesh
length) while the mesh is loaded by a pre-tension which
causes no noticeable extension of the sample. For man-
made fibres, this pre-tension in grams equals 1/30 of the
total denier of the netting twine. Neglecting twist
contraction, this is equivalent to the 300 m weight.
At least three measurements are made and the average
is reported in cm to the first decimal place.
(19) Strength and extensibility
The term "strength of netting" is usually used to mean
mesh strength, although, less frequently, it refers to the
bursting strength of a piece of netting or to the tearing
strength. If the usual strength-testing machines are used,
the extension of the piece of netting or of the single mesh
under load can be measured at the same time as the
strength.
Mesh strength: To determine the strength of finer
netting, the fisherman places two fingers in the mesh
and tries to break it. He judges the mesh strength not
only from the resistance to breaking but also from the
sound of the break. The quantitative test is carried out
in a similar way. The strength-testing machine is fitted
with two pins which go into the mesh and pull it by an
increasing force until it breaks.
Proposal (a):98 The test for the strength of dry or wet
netting is applied to single meshes. These test meshes
are cut from the netting in such a way that the cut ends
of twine are at least | in (12 mm) long if the mesh is
large enough, and, in any case, no closer to the test
mesh than the mid-point of the cut bar.
96 The selvedge is considered to run parallel to the general course
of the twine through the netting.
97 Japanese Industrial Standard (draft) J.I.S.L., 1958: Testing
methods for synthetic fishing net (2-9).
98 According to Canadian Government Specifications Board.
Specification for "Nets: Fishing", 55-GP-l, 25 April, 1958.
37
The teat is conducted on the usual strength-testing
(gee Section 1) on which the damps have been
replaced by pins which stand at right angles to the direc-
tion of pull. These should be sufficiently rigid that they
do not bend noticeably when stressed by the test mesh.
On the other hand, they should be fine enough that the
mesh can be placed on them without any of the knots
touching them, because, for the test, the mesh should
be placed over the two pins in such a way that none of the
four knots touches either of the two pins (Fig. 30).
Thus, the test is conducted with the mesh stressed diag-
onally to the usual mesh shape.
The average of ten test breaks is reported as the mesh
strength, and this may also be reported as a per cent of
twice the strength of the netting twine to indicate mesh-
strength efficiency.
Proposal (b):<"> For this test, too, the usual strength-
testing machine on which the usual clamps have been
replaced by hooks five to eight mm in diameter is used.
Either a piece of netting or a single mesh may be tested
(Fig. 31). The mesh is stretched over the hooks in its
usual shape or at right angles to it, i.e., so that one of the
knots touches each hook. Otherwise, the mesh may be
stretched diagonally without any of the knots touching
either of the hooks as described in Proposal (a), Fig. 30.
» Japanese Industrial Standard (draft) J.I.S.L., 1958 (5-10).
The initial distance between the two hooks is 10 cm
and the rate of loading hook traverse is 15-30 cm/min.
The mesh strength is taken as the average of 10 tests and
is reported in kg to three decimal places.
Proposal (c):100 Knotless netting is tested in the same
way as described in Proposal (a), except that loops of
twine of sufficient diameter and strength are used instead
of the pins.
Sometimes a single mesh which has been cut from a
piece of netting cannot be tested because of loosening
and slipping in the junctions. In this case, instead' of
cutting off a single mesh, small pieces of netting are cut
and the single mesh in the centre of each piece is tested
according to Proposal (a).
In contrast to knotted netting, knotless netting which
has been made by the Raschel process has noticeably
a different strength when stressed in its usual shape
(i.e., in the direction of the breadth or depth of the netting
normal to the general course of the yarns) than when
stressed at right angles to this (i.e., in the direction of the
100 Institut fur Netzforschung, Hamburg.
Fig.32
XX
length of the netting). The strength when stressed vertically
(former case) is usually the lower one. For this test, rows
of single meshes are cut from the netting both vertically
and horizontally and each row is stressed in the usual
fabric clamps on the strength-testing machine (Fig. 32).
The initial distance between the clamps is 10 cm. Care
must be taken that no knots or joints are caught between
the faces of the clamps, otherwise the twine may become
cut at that knot or joint before the mesh breaks.
The strength in both directions is measured and the
difference is reported as a per cent of the higher value.
Netting strength: The term "netting strength" is used
in more than one way. Sometimes it means the bursting
strength of uniformly stressed netting and sometimes
it means the diagonal tearing strength of pieces of netting
such as those used for mesh-strength tests in Proposal (b).
The bursting-strength testers used in the textile industry
cannot be used for measuring the bursting strength of
netting under uniform load, particularly if the netting
has large meshes. The knots are squeezed by the frame
and the netting breaks there. m 102
101 Results of bursting-strength tests with fine (Nm «= 70 and
higher) netting on makeshift apparatus have been reported by
v. Brandt, A.: Untersuchungen iiber die Maschengrosse. Proto-
kolle ziir Fischereitechnik, 5, 1951.
102 For woven or other very small mesh netting the usual bursting-
strength tester of the textile industry may be used. The average
of the results of 10 tests is reported in kg/cm2, Japanese Industrial
Standard (draft) J.I.S.L., 1958 (5-13).
Fig.33b
Fig. 33 a
Fig. 33a. Preparation of net sample for the determination of net strength.
Fig. 33b. Way of hooking a net sample onto the dynamometer for
determining the net strength.
Two methods for netting strength have been proposed,
the first applying the load to a single mesh, and the second
applying the load to several meshes at once.
It is not always true that the measurement of mesh
strength by Proposal (a) in Section 19 is preferred to the
following proposal by van Wijngaarden even though the
former can be conducted more easily and requires less
material, because the latter permits a better study of knot
stability (see Section 20) at the same time.
39
Proposal (a): 108 For the test, machine-made netting is
cut in the manner shown in Fig. 33a. The twines from
the creel on the netting machine must be distinguished
from the twines from the shuttle. The twine from the
shuttle makes the knot whereas the twine from the creel
only forms the loops. The mesh bars are cut at a, b, c,
and d in Fig 33a, but not at e, yet again at f. The netting
is stressed in a strength-testing machine, the same as is
used for measuring mesh strength, by the upper hook A
and the lower hook A1, shown in Fig. 33b.
The netting strength is that force in kg which breaks the
netting at the uncut mesh bar shown as e in Fig 33a and
b. The speed of the loading jaw traverse • equals
one per cent of the initial specimen length per second.
For the subsequent test break, the netting is mounted
on the testing machine with the hooks in meshes B and
B1 (Fig. 33a) and the mesh bar at h is cut.
This test cannot be conducted if the knots slip.
w/m
a. 5 knots
4m*sh
9 ttgs
Fig.34
Proposal (b):104 For measuring netting strength,
strips of netting four meshes (five knots) wide are cut in
both the lengthwise and breadthwise (depthwise)
directions from the net, and nine bars of the strip are
fastened between the clamps of a suitable tensile tester
for each test break (see Fig. 34a). The initial distance
between the clamps should be no more than 20 cm and
the speed of the loading-clamp traverse should be 15-30
cm per minute. If necessary, fewer meshes in breadth or
in length may be taken to accommodate the limitations
of the tester, but in such cases the size of the specimen
must be reported (see Fig. 34b).
At least 10 test breaks should be conducted, and the
average is reported as the test strength.
The way in which the specimen is mounted in the
clamps on the tester is of utmost importance. The knots,
109 van Wyngaarden, J. K.: Method for determining the tear
resistance of nets, the so-called net strength. N.V. Research,
Arnhem, Preevenverslag, 1959.
104 Japanese Industrial Standard (draft) J.I.S.L., 1958 (5-9).
40
being thicker than the twine, can be broken by the pres-
sure cutting the twine in the knot if the knots are caught
between the faces of the clamps. Also, it is difficult to
mount the specimen in such a way that the load is equally
distributed among all the mesh bars across the sample.
Therefore, Carrothers 106 threads a rod through the
meshes on the upper edge of the specimen and another
rod through the meshes on the lower edge, and hooks
these rods onto the strength-testing machine so that
the machine pulls these rods one from the other until the
specimen breaks. For such mounting, the test specimen
must be a whole number of meshes wide, not, for example,
\\ meshes as shown in Fig. 34b.
Rending strength:106 The specimen may be cut from
the netting either lengthwise or breadthwise (depthwise)
as shown in Fig. 35. The specimen should be larger than
ten knots = nine meshes square. A cut about 15 cm
long is made into the specimen so that five knots = four
meshes remain uncut. The parts marked A and B in
Fig 35 are pulled apart by the strength-testing machine.
The initial distance between the clamps is 10 cm and the
speed of loading jaw traverse is 15-30 cm per minute.
The rending strength is that force which is required to
tear the sample for five knots or more. At least 10 test
breaks should be made.
Fig. 35
Frictional strength in minnow netting:107 The proce-
dure for testing the strength of woven minnow netting is
different from that for other types of netting. The test
specimen is cut 30 cm long in the direction of the warp
by 10 cm wide in the direction of the woof or filling.
The specimen is prepared as shown in Fig. 36. At the
upper end of the specimen, all but the central warp are
cut short and only the central warp is attached to the
upper clamp of the strength-testing machine. At the
lower end of the specimen, the central warp is cut short
and all the warps but it are attached to the lower clamp.
The number of woof or filling threads is limited to 15. The
specimen is pulled apart at 15-30 cm per min and the
force required to pull the central warp out of the specimen
is measured. At least 10 tests are made. This procedure
can be applied only to minnow netting which has been
treated with a resin or some such similar bonding material.
105 Modern Fishing Gear of the World, p. 72, 1959.
106 Japanese Industrial Standard (draft) J.I.S.L., 1958 (5-11).
107 Japanese Industrial Standard (draft) J.I.S.L., 1958 (5-12).
V/////////////A
I I
cut off
«*c*pf- only c*nt*r warp
~-Jr c*nt9r warp cutoff
0
0
cutoff woof Fig. 36
(20) Knot stability
When netting materials of synthetic fibres came into use,
the question of knot stability or knot constancy arose.
The term "knot strength" is sometimes used wrongly
in this context. Knot stability is the ability of the knot to
retain its original form, both resisting inversion into an-
other form without twine slip and resisting loosening with
resulting twine slip but without inversion. Knot stability
is a prerequisite for constant mesh size in a fishing net.
The following test methods refer to the resistance of the
knot to inversion, of the twine to slipping in the knot,
and of the knot to loosening.
Inversion resistance: When netting is made with the
weaver's knot, it is possible that the loop C-D (Fig. 37)
does not fall correctly when the knot is tightened so
that it forms an insecure knot with the mesh of the pre-
ceding row. This insecure knot can also be formed from
a proper weaver's knot if the mesh legs A and B are
pulled apart strongly while legs C and D remain slack.
A similar insecure knot can also be formed in the same
way, and usually more easily, from a reef or square knot.
It can be seen, as shown for the weaver's knot in Fig. 37,
that, in these insecure knots, the netting material C-D
can readily slip on netting material A-B.
Proposal (a):108 This procedure is based on the fact
that the knot can be inverted into a slip knot as described
above. The legs A and B (Fig. 37) are stretched between
the clamps of the strength-testing machine while the
legs C and D remain free. The force at which the inver-
sion of the knot occurs is measured and may be reported
in kg or as a per cent of the knot strength.
108 van Wungaarden, J. K.: Testing methods for net twines and
nets. Modern Fishing Gear of the World, pp. 75-81, 1957.
D C
When the knots for this test have not been cut from
netting but have been formed by hand, the force by which
the knots are tightened is important. To minimise the
effect of this variable, the knots should be pulled together
by a force equal to about 20 per cent of the knot strength.
Proposal (b):109 For this test, several weaver's knots
C and D are tied in the netting material A and fi as
shown in the first drawing of Fig. 37 and Fig. 38. The
material A and B is loaded by a series of frequent jerks
in an effort to invert the knots. The arrangement is
shown in Fig. 38.
CDCDCDCD
a
Fig. 38
The knotted test specimen is stretched horizontally
so that the resulting deformation of the knots may be
better observed. The loading mass E equals that of the
500-m weight. For the test, the mass is lifted 50 cm and is
allowed to fall every two seconds, thereby jerking the
specimen.
109 Treschev, A. J.: Laboratory of Fishing Technique. Research
Institute of Marine Fisheries and Oceanography, Moscow.
5 10
20
40 50%
Fig. 39. Tightening force of knots In per cent of the cord strength:
(/) treated cotton knots: (2) untreated cotton knots: (3) Capron knots
fixed at temperature 100°C: (4) Untreated cotton flap knots.
41
The number of jerks required to invert the knot is
taken as the measure of its stability or resistance to
inversion. This, in turn, is largely dependent on the
force by which the knots are tightened, usually taken as a
per cent of the twine strength as shown in Fig. 39.
Proposal (c):no By this proposal, the inversion resis-
tance discussed above and the knot-slip resistance men-
tioned below are both measured while testing for mesh
strength by Proposal (a) for mesh strength in Section 19
above. The strength tester is the usual pendulum type
with an automatic load-elongation recorder and reverse
stop-pawls on the pendulum. The test mesh is mounted
on the tester as shown in Fig. 30 with the pins a known
distance apart. Thus, the distance for which the recorded
line follows the elongation axis (load = 0) when added
to the initial spacing of the pins, gives the mesh opening
under zero load. If one of the knots inverts, this shows as
a short distance on the recorded line which is parallel to
the elongation axis (recorded load = constant = inver-
tion resistance). Twine slip in a knot also shows as a
straight portion in the recorded line parallel to the elonga-
sion axis, but this is usually longer than the knot inversion
indication (recorded load=constant ^knot-slip resistance).
In any one mesh, only one knot can invert and only two
can slip. Therefore, it is necessary to watch only three
knots during test to distinguish inversion from slip for the
record. For this test, none of these knots should touch
either of the pulling pins. If the mesh eventually breaks,
the inversion resistance and the knot-slip resistance may
be reported as a per cent of the mesh strength. Thus, by
this one procedure, as many as four different properties
may be measured. This procedure has the further advan-
tage that variations resulting from variously tightened
hand-tied knots are avoided by using netting meshes.
Knot-slip resistance:111 Slipping can also occur
without any deformation of the knot. If the knot is
stressed from one side only, that leg can slip through the
knot without the knot becoming broken.
For measuring slippage in a knot, the legs C and D vs.
A (Fig. 40) are stressed in the strength-testing machine.
The force which causes 5-cm of the free leg B to slip out
of the knot is reported. (The 5-cm length is used only
with knots tied specifically for the test.) In this test, the
angle between legs C and D should be 0°, and the rate
of loading-clamp traverse should be one per cent of the
initial separation per second. Manually tied knots should
be tightened for 10 sec, 5 min before the test, by a force
equal to 20 per cent of the knot strength.
The knot-slip resistance is reported as that force,
expressed in kg or as a per cent of the knot strength,
at which slipping first occurs for more than one second.
Shorter jerks which result from tightening of the stressed
knot are ignored. Knot-slip can be seen particularly
well if the extension of the specimen is recorded auto-
graphically at the time of testing. Then the following may
be observed (Fig. 41):
110 Can-others, P. J. O.: Fisheries Research Board of Canada,
St. Andrews, N.B., Canada.
111 van Wyngaarden, J. K. : Testing methods for net twines and
nets. Modern Fishing Gear of the World, pp. 75-81, 1957. Also,
Methods of testing net cord and nets. Rayon Revue 11, pp. 146-160,
1957.
42
CUD
Fig. 40
Dthnung ilongttion
Fig. 41
(a) The knot does not slip but the specimen is extended
and breaks in the knot (Fig. 41a).
(b) After a certain load has been applied the knot slips
in a jerking manner. However, the material breaks
before the free leg B has slipped completely out
of the knot (Fig. 41 b).
(c) A more or less steady slipping occurs until the free
leg B has slipped completely out of the knot
(Fig. 41c).
At least 10 tests are carried out. Knot-slip resistance
may also be evaluated by reporting the frequencies at
which the above alternatives occur as per cents of the
total number of tests.112
Loosening resistance: For this, the knots are placed in
a rotating box and their opening-up is observed.
Proposal (a) :118 For testing loosening resistance, knots
are tied between pieces of the netting material about
20-cm (8 in) long. The knots are tightened by fastening
a weight equal to 1 g per five resultant twine tex (200 m
weight) to both ends of the same twine and allowing
the weight to fall 20 cm before the knot takes the load.
Five of the knots thus formed are placed in a rotating
wooden box (20x30x20 cm) which turns at 60 revolu-
tions per min (Fig. 42). The box is stopped at regular
time intervals and the number of knots which have
become loosened is counted.
118 v. Brandt, A.: Zttr Knotenkonstanz von Fischnetzen.
Archiv flir Fischereiwissenschaft, 9, pp. 244-265, 1958.
118 von Wyngaarden, J. K.: Testing methods for net twines and
nets, etc. Modern Fishing Gear of the World, pp. 85-91, 1959.
Also, Methods of testing net cords and nets. Rayon Revue 11,
pp. 146-160, 1957.
JO em
>i 20cm
Fig 42
Proposal (b):114 Knots from netting or knots formed
under a known tension from netting twine are cut in
such a way that their legs are 1'5-cm ( tf in) long. For
synthetic netting twines the twine ends are fused and for
natural fibre netting twines the twine ends are cemented
to prevent them from unravelling.
At least 10 of these knots are put into a rubber-lined
box which rotates at 60 turns per min. A small rubber
cylinder is also placed in the box. The box is made of
metal, to permit wet tests, and is 15 x 10 x 10 cm. After
10, 20, and 30 min, the number of knots which have
become loosened during each period is counted, and is
reported as a per cent of the total number of specimens.
(21) Weight of netting
The dry weight of uncut netting is of interest for com-
mercial purposes. However, the wet weight is also of
interest because the netting is usually wet when lifted
through the air during fishing operations. Finally, the
buoyed weight of netting while still in the water is of
interest. The manner used for stating the weight of netting
varies because of the different ways in which the measure-
ments are made, e.g., length may be stated as the number
of conventional length units or as the number of meshes
(see Section 18) and weight may be stated in metric or in
other units.
Dry weight of netting: stated exactly, the weight of
netting for commercial purposes is not its actual, air-dry
weight, but is its bone-dry weight plus a standard mois-
ture content. The corrected weight of the netting can be
computed from the following formula:116
100 + Re
Standard weight == measured weight x R
where R = measured moisture content ( %)
Re = standard moisture content ( %).
Standard moisture contents for various textile materials
may be different in different countries, and this should
be recognised when "official" moisture ratios are quoted.
Standard moisture contents for natural fibres are all
higher than the contents usually found actually to exist.
114 Stutz, H.: Lateral strength and knot-firmness of synthetic
twines for fishing purposes. Modern Fishing Gear of the World,
pp. 87-92, 1959. Also, v. Brandt, A.: Zttr KnotenkonsUuu : von
Fwchnetzen. Aichiv fur Fischereiwisscnschaft 9, pp. 244-265, 1958.
115 Japanese Industrial Standard (draft) J.I.S.L., 1958 (2-12).
The actual moisture content is found by drying the
specimen at 105-1 10°C until its weight is constant. This
determination cannot be made if the thermoplasticity of
synthetic fibres is such that they must be dried below
60°C.
The following is an official moisture content for various
textile fibres:
Synthetic fibres
polyamide 4*5%
polyester 0-4
vinyl chloride 0
polyvinyl alcohol 5*0
Natural fibres
cotton 8-5%
flax 12
hemp 12
ramie 12
sisal 12
manila 12
Proposal (a):116 The proposal is based on the state-
ment of the length of the netting in metres and of its
breadth (depth) in the number of meshes. If the weight
(c) in kg is known for a piece of netting of a given twine
number and mesh size, 100 m long by 100 meshes broad
(deep), the weight of any other piece of the same style of
netting may be estimated from the formula:
«, . , „ x Ien8th x breadth (depth) x c.
We,ght (kg)= 1M— ^
Values for the constant c are given in Tables 1 , 2, 3, and 4.
They are the weights of pieces of netting, 100 m long
when fully extended by 100 meshes broad (deep),117
which have been measured by the netting manufacturers
under prevailing conditions of temperature and humidity,
not in conditioned rooms with the standard atmosphere.
Thus, these values may vary between different factories.
Also, differences in these weights may be caused by the
knots being more or less strongly tightened.
These tables give the values for air-dry netting made
with the single weaver's knot using medium-laid twines
of cotton (Tables 1 and 2), flax (Table 3), or hemp (Table
4). Of course, these tables are valid only in a climate
similar to that of Central Europe, hence they are presented
only as an example. These lists of data may be examined
further and presented graphically. The resulting curves
should not have a discontinuity. If, for a given twine
size, the weights for pieces of netting of only a few mesh
sizes are known, the weights for intermediate mesh sizes
may then be determined by graphical interpolation.
In these tables, the mesh sizes have been reported as
the distance between the centres of neighbouring knots
(mesh size = mesh-side length = \ mesh length).
Proposal (b):118 This proposal is based on the state-
ment of the length of the netting in fathoms (1 fm =
6 ft = 1*83 m) and of its breadth (depth) in the number
of meshes. The weight is estimated in Ib.
116 v. Brandt, A.: Arbeitsmethoden der Nctzforechung, Stutt-
gart, p. 101, 1947.
117 Schmidt, K., and Amwand, K., have also published con-
stants for estimating the weights of netting which is not fully
extended but is hung to various ratios: Tabcllcn zur Gcwichts-
bcrcchnung von Baumwollnctzen untcr BerUcksichtigung cincs
Einstellungsverhaltnisses. Deutsche Fischcrci Zeitung 5, pp. 100-
104, 1958.
118 Fisheries Research Board of Canada, Technological Station,
Vancouver, B.C., Canada.
43
Table 1
" ^
5 6 8 10 12 14
270/6
0,64
250/6
1,00 0,90 0,82 0,76 0,73 0,71
0,70
200/6
1,20 1,10 1,00 0,90 0,88 0,86
0,82
160/6
1,45 1,35 1,20 1,10 1,07 1,04
1,00
140/6
1,95 1,80 1,60 1,35 1,33 1,31
1,29
100/6
2,15 2,00 1,87 1,80 1,76 1,73
1,68
100/9
3,80 3,50 3,10 2,80 2,70 2,60
2,54
85/6
2,95 2,80 2,45 2,20 2,14 2,08
2,03
85/9
3,90 3,80 3,74 3,50 3,40 3,33
3,25
70/6
3,60 3,20 2,90 2,84 2,75
2,65
70/9-
5,20 4,70 4,10 4,00 3,90
3,80
-H» 18 20 25 30 40 50 •»
270/6
0,54 0,52 0,50
250/6
0,69 0,69 0,6? 0,65 0,62
800/6
0,80 0,78 0,74 0,73 0,72
160/6
0,98 0,96 0,94 0,92
140/6
1,27 1,25 1,20 1,15
100/6
1,67 1,65 1,60 1,55 1,50
WO/9
2,52 2,50 2,45 2,40 2,30
85/6
1,98 1,95 1,90 1,88 1,85
85/9
3,17 3,10 2,95 2,90 2,82 2
,80
70/6
2,57 2,50 2,45 2,40 2,30
70/9
3,70 3,65 3,60 3,55 3,50
Table 2
Km
6
8
10
12
14
16
18 20 22 25
30
35
40
4«| B^
30/9
7,0
6,4
5,8
50/12
10,0
8,7
8,2
6,1
6,0
50/15
50/18
50/21
14,0
12,0
14,0
18,0
10,8
13,4
16,3
10,2
12,5
15,5
9,6
12,0
14,7
9,0
11,7
13,9
8,9 8,8 8,6 8,5
11,2 11,0 10,6 10,6
13,5 13,2 13,0 12,6
8,0
10,0
12,2
7,9
9,8
11,8
7,7
9,6
11,5
7,5
9,4
20/6
20/9
u,o
12,0
21,0
10,9
17,6
10,4
17,1
9,8
16,3
9,3
15,3
8,9 8,7 8,5 8,3
14,7 14,0 13,8 13,4
7,8
12,7
7,7
12,5
7,6
12,2
20/12
20/15
20/18
27,1
26,0
34,2
42,0
24,5
31,6
39,6
23,5
30,0
37,9
22,5
28,8
35,9
21,7 20,4 20,1 19,3
27,8 27,0 26,0 25,3
34,8 34,4 33,6 32,3
17,9
24,0
30,7
17,7
22,9
29,3
16,7
21,6
27,5
fa
48
20
25
30
35
*P
50 60 70
80
100 i
n
20/21
20/24
20/27
40
39
47
53
37
44
50
36
42
48
34
40
46
31
38,
45
30 29 28,5
5 36 34 33,5
42 40 39
28
33
38
27
32
37
20/30
20/36
20/42
20/48
60
74
88
102
57
70
82
95
55
66
77
88
53
63
73
83,5
51
59
71
81
46,5
57 54
67 65
76,5 74
44
T«bl« 3
Jta
10
12
17
29
22
25
60
10,6
10,0
9,6
8,3
8,3
8,2
7,9
7,7 7,6 7,5
7,4
18/9
16,2
15,4
13,9
12,8
12,7
12,6
11,8
11,6 11,0 10,8
10,7
18/12
83,0
21,7
20,4
19,0
18,2
17,5
17,2
16,4 16,1 16,0
15,8
16/4
7,9
7,5
7,0
6,3
6,2
6,1
6,0
5,9 5,8 5,7
5,6
16/6
11,9
",5
10,8
9,4
9,3
9,2
9,1
8,9 8,7
16/8
16,5
15,5
14,5
13,2
12,8
12,3
12,0
11,8 11,6 11,4
11,2
16/12
28,0
26,0
23,9
21,2
20,5
20,0
19,2
19,1 18,7 18,4
18,2
16/15
40,0
38,0
33,5
29,5
28,5
27,5
25,5
24,7 24,0 23,7
23,3
23,0
22,6
16/18
52,0
47,5
42,5
37,3
36,0
34,5
32,0
30, * 30, 4 29,9
28,4
27,6
26,2
10/6
19,5
18,0
16,4
15,0
14,7
14,6
14,3
14,1 13,8 13,2
13,0
10/12
58,6
53,0
46,9
41,2
40,0
38,0
37,6
33,3 31,0 29,1
27,6
25,8
TftbU 4
lhil
6
8
10
1
2
jj
16
20
M
_£
*••
70/6
in/to
3,0
4n
2,8
3 a
2,6
2
,4
2,2
2,2
2,1
2,1
2
,1
70/9
75/9
,0
»8
sJi
3,3
3,2
85/6
2,2
2,2
2,1
2
,0
1,9
1,8
1,8
1,8
1
,8
85/9
4 AA /£
3,1
1Q
3,1
1a
3,0
2
,9
2,8
2,8
2,7
2,7
2
,6
100/0
100/9
,»
2,7
,8
2,7
2',6
2
,5
2,4
2,3
2,2
2,2
2
,2
120/6
1,5
1,4
1,3
120/9
2,2
2,0
1,9
140/6
1,5
1,4
1,3
1
,2
1,1
1,1
1,1
1,1
1
,1
160/6
1,5
1,08
1,04
0
,99
0,95
0,92
0,89
0,89
0
,89
200/6
0,8
0,79
0,78
0
,77
0,76
0,73
0,70
0,70
0
,69
240/6
0,82
0,70
0,68
270/6
0,64
0,57
0,54
Na
28
30
32
?
4
?6
38
40
45
5
0 am
^•'•••••••••••OHVW
70/6
2,1
2,1
2,0
2
,0
2,0
2,0
2,0
1,9
i
,9
70/9
3,2
3,1
3,1
3
,1
3,1
3,1
3,1
3,0
3
,0
75/9
2,8
2,8
• »^
85/6
1,7
1,7
1,7
1
,7
1,7
1,7
1,6
1,6
1
,6
85/9
2,6
2,6
2,6
2
,5
2,5
2,4
2,3
2,2
2
,2
100/6
1,5
1,5
1,4
1
,4
1,4
1,4
1,3
1,3
1
,3
100/9
2,2
2,2
2,2
2
,2
2,2
2,2
2,1
2,1
2
,1
120/6
1,2
1,2
120/9
1,8
1,7
140/6
1,1
1,0
1,0
1
,0
1,0
1,0
1,0
1,0
1
,0
160/6
200/6
0,89
0,68
0,88
0,67
0,88
0,66
0
0
,88
,66
0,88
0,66
0,88
0,65
0,88
0,64
0,85
0,64
0
0
,80
,64
240/6
0,65
0,63
270/6
0,52
0,50
45
(23) Visibility
The assessment of the absolute visibility of netting, partic-
ularly of gillnets under different conditions, or the
study of whether or not the netting blends with the
surroundings in the water, can be achieved by direct
observation as by a SCUBA diver or with the help of a
bathyscaphe or television. As a record is usually required
for comparisons, photography is used. Probably these
visibility studies should be made with natural light
rather than with artificial light, and the turbidity of the
water as measured by oceanographers should be reported
with the test results.
The visibility can be varied by lowering the netting
and the camera together to the desired depth and photo-
graphing by remote shutter release. In this instance it is
proposed to study only pieces of netting. These are
stretched on a frame and are mounted with the camera
on a base as shown in Fig. 44.126
Fig. 44
The apparatus consists of a base bar A, about 1-5 m
long, on which the waterproof case with the camera, B,
and the frame carrying the netting under study, C, have
been fixed. The frame takes a piece of netting 50 x 50
cm which may be interchanged with similar pieces of
different types of netting for comparison.
The camera should preferably be a model with auto-
matic winding. In this way, several photographs can
be made in sequence with the help of cable D without
removing the camera from the water. It should be noted
that, for example, at an actual distance of 1*3 m between
the netting and the camera, the camera should be focused
at 1*0 m because of the higher index of refraction in the
water. At an actual distance of 1*2 m an optical distance
of 0-9 m should be used. Preliminary test photographs
should be taken to determine the exposure, particularly
if colour pictures are being taken.127
126 Ulrich, B.: Bin ncues Gebiet ftir den Robot-Fernausldser.
Neue Fotolinic, No. 6, pp. 15-19, 1951.
117 Park, Y. J. : Visibility of webbing dyed with several kinds of
colours under the ocean. Bull, of Fisheries College, Pusan National
University, Vol. VI, pp. 7-10, 1 962. Describes measuring the visibility
of netting by fastening pieces of netting to a frame and by carefully
and slowly lowering the whole into the water until the observer
cannot obviously recognise the netting through a water glass. The
depth in cm is taken as a measure of the visibility of the netting
under test.
48
(24) Hydrodynamic resistance of netting
Since the hydrodynamic resistance of netting cannot
readily be estimated from the towing resistance of towed
gear, the netting can be stretched on a frame for measur-
ing this property. Such measurements can conveniently
be made in a tank in which other objects, such as ship
models, can also be tested. The netting is towed at
different speeds, preferably those at which full-scale
gear is dragged, expressed in nautical miles per hour
(knots) or metres per second (Fig. 45). One square
metre of netting has been used for such tests.128 The
resistance of the frame alone is measured separately,
and this is subtracted from the resistance of the mounted
netting.
Wgr
25000
20000
15000
Manila 3/900 No. 1
! /Ptrlon 400 m/kg braid* No. 2
/,5
m/5»c.
As the use of ship-model tanks is very expensive,
simpler makeshift apparatus has been used.129 Fig. 46
128 Institut fur Netzforschung, Hamburg. Also, Deutsche
Seiler Zeitung 79, pp. 146-197, 1960.
129 Miyamoto, H.: Hydrodynamic resistance of a plane net:
International Shipbuilding Progress 2, pp. 291-295, 1955. Nomura,
M., and Mori, K.: Resistance of a plane net against flow of water
HI— Effect of kind of fibres on the resistance of net. Bull. Japanese
Soc. Sc. Fish., Vol. 21, pp. 1110-1113, 1956.
represents the required arrangement. A piece of netting
is stretched on a metal or wooden frame, loaded by a
weight and balanced by a counter-weight. In this the
netting may be set at right angles to the direction of
motion or at any other desired angle to it. The frame is
allowed to sink through the water under a known force
both with and without the netting to be tested. The
resistance of the netting to motion through the water
may then be computed either from the times required
for the frame to travel a certain distance or from the
distances travelled by the frame in a certain time.
DISCUSSION
Dr. A. von Brandt (Germany) Rapporteur: There is no
doubt that clear definitions are the basis of all our discussions.
Just as we need a numbering system, we also need an agree-
ment on testing methods for the properties of materials. It
was therefore a very important achievement that, at the first
International Fishing Gear Congress, a working group was
set up to advise on and recommend standard methods of
testing net materials. To help that working group scientists
of Canada, Denmark, France, West and East Germany,
Japan, the Netherlands and the USSR gave co-operation in
providing information about the methods actually used in
their country for testing netting twine and netting. The
group had to limit its work to these two items, but there
was no doubt that agreement was also needed on testing
methods for ropes and lines as used in fisheries as well as an
agreement on methods of testing the different types of fishing
gear either in model or full scale.
The version of the work on testing netting twine and netting
now presented by P. J. G. Carrothers (Canada) and myself
should be considered as a draft and not as a final form. It
does, however, review the situation as it is now and we hope
it will help all who want to begin testing materials for fishing
In the meantime, national working groups for definitions
and testing methods have been set up in different countries.
Furthermore, some international groups have discussed
special items, e.g., the International Council for the Explora-
tion of the Sea and the International Council for Northwest
Atlantic Fisheries have discussed methods for testing mesh
size; and a new international working group on biological
deterioration of material, including the rotting of fish nets in
water, was established two weeks ago by the Organisation for
Economic Co-operation and Development in Paris.
Standardisation, however, could not be effected by these
bodies but only by the International Organisation for Stand-
ardisation, ISO, in London. This organisation had set up
last year a special sub-committee for textile products as used
for fishing nets and the second meeting of this sub-committee
was held in London recently. Actually many fishery experts
in this field were members of this ISO sub-committee and
they were in a position to endeavour to make certain that
ISO would decide on standards which were in accordance
with the wishes of fisheries as well as of the textile industry.
In order to rationalise and to avoid duplication of effort,
I suggest the FAO working committee for standardisation
of testing methods set up during the First Gear Congress be
dismissed and its functions transferred to that sub-committee
of the International Organisation for Standardisation.
Mr. A. F. B. Nail (UK) a Member of the British Standards
Institution, outlined the position of ISO. It was the recog-
nised body for international work on standards of quality,
performance, size or terminology. It was set up by the
United Nations during the period 1941-1948 and it worked
through the national standards organisation of member
countries. There were over 100 technical committees
belonging to JSO and among them was Committee No. 38
dealing with textiles. This comprised representatives of 41
countries from all parts of the world. There were specialised
sub-committees dealing with textile products for fishing needs.
Germany was responsible for the Secretariat of that committee
and at the last meeting there were representatives of nine
countries present and some of those representatives have been
members of the FAO working group. There were strong
links between the two bodies in both membership and
purposes.
49
o
Part 1 Materials for Nets and Ropes
Section 3 New Net Materials
Netting Twines of Polypropylene and Poly amide
Compared
Abstract
This paper concerns experiments to date conducted with polypro-
pylene, a synthetic fibre developed in 1954. Properties of polypro-
pylene netting twines which were tested include breaking strength
dry and wet, knot breaking strength, extensibility and influence
of twine construction. A rather complete comparison is made
between polypropylene twines and pplyamide twines. It is pointed
out that at the first International Fishing Gear Congress the authors
generally judged the efficiency of the synthetic net material by
comparing their properties with those of natural fibre twines.
At the present time, however, new net materials have to compete
with high-class synthetic twines, especially polyamide. Comparisons
of new synthetic twines with twines of natural fibres rather than with
established synthetic twines cause little interest among members of
the fishing industry. Comparative tests between polypropylene
twines and polyamide twines reported on in this paper include
breaking strength and R tex, breaking strength and diameter,
abrasion resistance, extensibility and elasticity. A resume is given
of actual field work undertaken by other researchers on polypro-
pylene fishing nets in gillnetting for salmon and cod and in bottom
trawl fishing. All sections of this paper are thoroughly supplemented
with graphs and tables.
Fib et filets de potypropyKne et polyamide comparaison de lews
Proprietes
Rfeumt
La communication traite des experiences conduites jusqu'a ce
jour sur le polypropylene, une fibre synthetique developpec en 1954.
Les proprietes des fils de filets polypropylene eprouvees 6taient les
suiyantes: resistance a la rupture & sec, mouilles et noues, extensi-
bilite et influence de la fabrication. II y est fait une comparaison
assez complete des fils de filets polypropylene et polyamide. Alors
qu'au Premier Congres des Engins de Pechc on jugeait refficacitd des
fibres synthetiques en comparant leurs proprietes avec celles des fils
en fibres naturelles, aujourd'hui par centre, les nouveaux materaux
de filets doivent fctrc compares avec des fibres synthdtiques
superieures et speciakment, le polyamide. La comparaison des
nouveaux fils synthetiques avec des fils en fibres naturelles n'a
plus aucun interdt pour 1'industrie des pcches. Les epreuves com-
paratives cntre fils en polypropylene et fils en polyamide mention-
nees dans cette communication ont trait a la force de rupture et
la grosseur en R tcx, la force a la rupture et le diametre, la resistance
a 1'abrasion, 1'extensibilite et 1'elasticite. Un resume des essais
actuels entrepris par les chercheurs sur des filets en polypropylene
dans la pcche au saumon, au cabillaud et dans le chalutage de
fond est compris dans cette etude. Toutes les parties de cette
documentation sont largement completees par des graphiqucs et
des tableaux.
HOos para redes hechas de propileno y poliamido— Comparackta de
Extracto
Alude esta ponencia a los expcrimentos realizados hasta ahora con
polipropileno, una fibra sintetica descubierta en 1954. Entre las
propiedades de los hilos para redes de polipropileno se ensayaron
la resistencia a la rotura estando humedos y secos, resistencia a la
rotura del nudo, estiramiento y efecto de la manera de fabricar el
hilo. Se hace una comparacibn bastante completa entre los hilos de
polipropileno y los de poliamidos. Se menciona que en el Primer
Congreso Internacional de Artes de Pesca los autores, en general,
juzgaron la eficacia de un material para redes sintetico comparando
sus propiedades con las de hilo de fibras naturales. En la actualidad
los nuevos materiales para redes tienen que competir con hilos
sinteticos de gran calidad, particuiarmente poliamidos. Las com-
paracipnes de los nuevos hilos sinteticos con los de fibras naturales,
mas bien que con los hilos sinteticos cstablecidos, des picrtan poco
interes entre el personal de la industria pesquera. Los ensayos
comparatives entre hilos de polipropileno y de poliamidos de que
50
by
Gerhard Klust
Institut fur Netz und Materialforschung,
Hamburg
se informa en esta comunicacidn comprenden resistencia a la rotura
y R tex, resistencia a la rotura y diametro, resistencia al desgaste,
estiramiento y elasticidad. Se hace un resumen de experiencias
practicas realizadas por otros investigadorcs con redes de poli-
propileno empleadas para pescar al enmallc salmon y bacalao en los
artes de arrastre de fondo. Todas las sccciones de esta ponencia estan
muy bien documentadas con ilustraciones y tablas.
THE introduction of synthetic net materials is certainly
one of the three main technological revolutions in
modern fishing. Nowadays fishing nets made of poly-
amide (nylon) are found all over the world. Polyvinyl
alcohol nets have also gained great importance in some
countries, while the use of polyvinyl chloride, poly-
vinylidene chloride, polyethylene and polyester for
fishing nets has remained more limited. In 1954, a new
synthetic fibre, polypropylene, was developed in Italy
and is now manufactured in several other countries.
The author has tested more than 40 samples of netting
twines made of this fibre, ranging in size from fine gillnet
twines to thick trawl twines, and originating from Great
Britain, Denmark, Italy, Japan and Germany. In this
paper only continuous multifilament twines of high
fibre quality are considered.
Testing methods
R tex values — i.e., the actual weight in grams of 1,000 m
of the netting twine — and runnage in m/kg were calcula-
ted from the weight of 10 m of the samples. The values
of diameter are average data of 20 single measurements
taken on a twine length of 10 m with a dial gauge
(Frank).
Twines which were tested in wet condition were im-
mersed in water for one day or, if not enough time was
available, for one hour with the wetting agent 'Nekal BX'
added.
Breaking strength was tested by a modern electronic
recording dynamometer (Testatron'). The average
data are based on IS to 20 single tests. Two kinds of
knots were used for testing the knot breaking strength,
the common overhand knot and the English knot
(weaver's knot). In the latter case the knot was tied with
two pieces of netting twine and the two ends (bars) of
each piece, were fixed in one of the two clamps of the
testing machine respectively.
For comparing the results of the weaver's knot with
those of the overhand knot the former values must be
halved. The knot strength efficiency is related to the
breaking strength of the unknotted twine in wet condi-
tion. The load-elongation curves were recorded by the
strength-testing machine. They extend only to a load
corresponding to the knot breaking strength.
The elasticity was tested by loading the freely hanging
netting twine samples with a weight of 30 per cent of
their breaking strength.
Amount of twist (the number of turns per metre of
twine), was determined with a twist tester (Frank). With
these values (ts/m) the coefficient of twist was calculated
by the formula:
ts/m
a = — — where Nm is the metric number of the netting
Nm °
twine (not of the single yarn). The abrasion resistance was
tested with the Sander test machine by rubbing the
samples (constantly kept wet) against a rough corundum
rod under a load of 200 g. The measured value is the
number of double friction movements until the twine
breaks. At least 30 tests on each sample.
Properties of polypropylene netting twines
The average data for results of most tests are in Tables 1
and II. The comments are supplementary.
Breaking strength dry and wet — Unlike twines made
of polyamide and polyvinyl alcohol but like twines made
of polyethylene and polyester, polypropylene twines have
practically the same breaking strength wet as dry.
In breaking length, polypropylene twines are of the same
high quality as polyamide twines with respect to strength.
Their tenacity is higher than that of netting twines made
of all other groups of synthetic fibres except polyamides.
Knot breaking strength— With twisted twines, the
English knot gave better results than the overhand knot.
With braided twines, there was no such difference, thus
being similar to polyamide twines. Losses in strength by
knotting somewhat depend on twine fineness.
Losses in strength by :
Finest netting twines (twisted)
Twisted netting twines of medium strength,
approximately from R 130 tex to R 500
tex
Thicker netting twines, twisted
Braided trawl twines
English Overhand
knot knot
24% 30%
39%
50%
49%
45%
53%
49%
Extensibility — Continuous polypropylene twines have
a relatively low extension. The form of the load-elonga-
tion curves is very remarkable and typical for polypro-
pylene material— Figs. 4, 5 and 6 show they are almost
straight.
Influence of twine construction — Physical properties
of twines depend on the quality of the fibre but also
to a high degree on the twine construction. The single
yarns composing the twines described differ in fineness.
Most have the relatively low yarn number of 170 or 190
denier and are combined to twines in the form of cabled
yarns. Trawl twines numbered 35,21 and 22 (Table II)
are made of a few single yarns with the high yarn number
3,000 denier; 36 is made of a single yarn of 2,000 denier.
Samples 23 to 28, single yarns of equal fineness about
800 denier. Although differences in fibre quality, fineness
of single yarns and the number of yarns in the twines
may explain some of the variations shown, the influence
of the amount of twist is definitely more evident. It is
already known from other continuous multifilament
materials, that the breaking strength decreases with
i. NettfnantviiiM
Ho of testing 1 U 12 13 14 2 15
Promotion A B B B B 1 C
3
4
5
6
7
8
9
16
C
19
C
10
A
20
0
No in Denier 190x2 170x3 170x4 170x6 170x9 190x9 190x9
Rtex (1000 »*) 43,5 63,9 63,4 130 189 211 208
Bunnage, «/*«• 22900 15657 11990 7686 5291 4740 4805
190x12
284
3517
190x15
351
2850
190x18
428
2336
190x24
547
1830
190x27
647
1546
190x30
731
1368
190x36
921
1065
570x16
1243
805
1272
786
190x60
1453
680
570x27
1974
507
DiajMter.sm. 0,23 0,33 0,35 0,44 0,52 0,60 0,58
0,73
0,81
0,89
0,99
1,18
1,19
1,38
1,57
1,55
1,72
2,14
Brkg. strength
etraifiht,dry,kg. 2,36 3,67 ,4,50 7,38 11,21 13,1 11,0
•trsight,wet,kg. 2,35 3,63 4,13 7,43 11,31 12,9 10,8
17,4
17,2
.
.
30,6
31,7
-
36,8
36,6
44,5
43,8
68,4
67,0
66,8
66,5
74,3
72,3
93,7
92,3
Brkg. strength, wet
o/nand knot, kg. 1J6 2,29 3,02 4,14 6,34 6,5 6,1
fegliah knot, kg. 3,63 5,07 6,73 9,2 13,1 14,9 14,2
Engl. knot, half kg. 1,62 2,54 3,37 4,6 6,6 7,5 7,1
8,7
18,5
9,3
10,6
21,7
10,9
13,4
28,0
14,0
16,9
29,8
14,9
19,0
43,4
21,7
16,2
40,0
20,0
20,0
44,7
22,4
32,1
76,0
38,0
33,2
33,1
70f6
35,3
41,7
94,6
47,4
*Knot atrength efficiency,
o/nand knot % 74,9 63,1 73,1 55,7 56,1 50,4 56,5
Ingl,knot,half <f> 77,7 70,0 81,6 61,9 58,4 58,1 65,7
50,6
54,1
-
-
53,3
47,0
.
47,2
51,8
45,7
51,2
47,9
56,7
49,9
45,8
48,8
45,2
51,4
Breaking length
atraight,dry,Jrm 54,3 57,4 54,0 56,7 59,3 62,1 52,9
straight,wet km 54,0 56,6 49,5 57,1 59,8 6l,l 51,9
61,3
60,6
-
.
55,9
56,0
-
53,1
52,8
46,3
47,6
55,0
53,9
52,5
52,3
51,1
49,8
47,5
46,8
Bkg.lsngth wet
o/nand knot, to. 40,5 35,8 36,2 31,8 33,5 30,8 29,3
ft.gl.knot,half kn41,6 39,7 40,4 35,4 34,9 35,5 34,1
30,6
32,7
30,8
31,0
31,3
32,7
30,9
27,2
29,4
55,7
24,9
27,4
21,8
24,3
25,6
30,6
26,1
22,8
24,3
21,1
24,0
Bxt. at knot
bkg.strength % - - - 12 12 10 11
10
9
10,5
9
U
11.5
15
12
15
14
15,5
* Knot strength efficiency in the Tables I and II relates to straight breaking strength of wet twines.
51
increasing twist and that extensibility as well as diameter
and weight per-unit-length increase. In Table I the
samples 2, 3, 4, 5, 6 and 7 are soft laid with an average
coefficient of twist of a = 118. The samples 8, 9 and
10 are medium laid (a = 164 in the average) and there-
fore have lower breaking lengths and higher extension
values.
Yabl* II Tr.wJ t»in«* Md« of continuous Polypropyl«n«
to. of toting
Kim (MOO we)
!•• of «laglt jrtn
., «7kg.
1191 1456 3116
569
869 696 321
4320
12
231
3500
33
266
24
A
4709
45
212
25*
A
1706
ia
566
26*
A
27*
A
2131 4795
22 46
469 209
•169'
1,46 1,64 2,31 2J1 2,61 2,66 -
Bkg . •toongth
•trtigfct, dzjr kg.
•tMlgnt, *«t kg.
52.8 59,7 156,7 206,0 164,6 226,5 94,6 114,0 210,0 260.0
52.9 60,4 160,3 210,4 172,1 237,6 94,9 Ho, 4 W,? 282,0
Bkg.
o/hand knot, kff.
fe«. knot, kg.
feg. knot, ike.
Knot
•fficlwojr
o/h»nd knot, kg.
tog. knot **«.
Dkg. Imgth
•t»al«ht, dry ka.
t, w»t lou.
27,8 33,9 77,5 101,2
63.8 76,4 161,0 234,9
31.9 Jb,2 60,5 117,5
73,5 96,7 46,7
160,9 200,2 96,7
6U,5 100,1 46,4
57,5 114,5
113,6 216,3
5b,6 109,*
135,5
275,0
kg. iMgtb ««t
o/hand knot. kn.
tog. knot, ftto.
52,6 56,1 4B.3 48,1 42,7 41,5 51,3 49,7 57,6 47,3
60,3 63,2 50,2 55,6 4b,6 4^,1 51,0 4b,b 54,9 4b,U
45,9 41,0 50.2 48,2 47,0 4«,1 55,5 53,5 4:5,8 47,4
45,9 41,4 51,4 40,7 4J/.2 50,5 5b,& 54,6 41,5 47, «
24,1 23,3 24,9 23,4 21,0 21,0 2tt,5 27,2 23,9 22, b
27,7 26,2 2>,8 27,2 23,0 21,3 2b,J 26,7 2c',b 23,3
breaking itrongth %
* Th« t«iM« 25-26
10 11-12 13-15 12-13 9-10 9-11 13-14 H-12
art br«id«d
Comparing polypropylene and polyamide twines
Here the properties of polypropylene twines for fishing
nets are compared with those of polyamide twines.
The breaking strength of the two types of twines
(see Figs. 1 and 2) is compared wet and knotted with
the weaver's knot, because the tests then come nearest
to the normal strains of fishing. The knot breaking
strengths are compared on the basis of R tex values, Fig. 1
containing those of fine and medium strong twines, Fig. 2
those of heavy twisted and braided trawl twines. The
regression-line for polypropylene in Fig. 1 relates to the
round black points only and not to the crosses which
characterise the breaking strength of twines of lower
quality. Both figures show that with the same R tex
value both types of fibre have nearly the same breaking
strength wet, knotted.
Netting twints mod* of
Potypropyto" •
Polyamid* o
too
300 800
700
Ptar
900 flOO
1300 1500
Figs. 1 and 2. R tex values and knot strength of netting twines made of
continuous polypropylene compared with those of polyamide.
52
HO
130
120
$ no
\ioo.
60
40
Trowt twttm fnodtof
1200 1900 2000 2400 2600 ' 3200 ' 3600' 400o' 4400 4600 3200 5&0 6000
Rt9X
Fig. 2. See caption Fig. 1.
Breaking strength and diameter — Apart from its high
tenacity the most remarkable property of polypropylene
is its low specific gravity of 0*91 as compared with
1-14 for polyamide. Specific gravity and specific
volume being reciprocally proportional, the diameter
increases with decreasing density. Polypropylene twines,
therefore, must be thicker than polyamide twines of
equal weight per-unit-length. Since twines of both fibre
types with equal twine number have nearly the same
breaking strength, the diameters can also be compared
on the basis of the latter property. Fig. 3 shows the result
of this comparison. Very thin twines of both fibres do not
differ in breaking strength and diameter. With stronger
twines those made of polypropylene are on average about
15 to 20 per cent thicker than polyamide twines.
Of-
24
If W
Fig. 3. Knot strength and diameter of fine and medium strong netting
twines made of continuous polypropylene and polyamide.
Abrasion resistance— Results of these tests depend very
much on the method and mainly on the test machine
used and it seems almost impossible to imitate the actual
conditions of friction, which affect nets during fishing.
Table Ill—Abrasion resistance of netting twines, vet (average data)
PnlvnrnnvUnA filament t»~i. • J_ m .\ . e ««iay
Polypropylene filament
R tex Diameter Double
211
284
mm
0*60
0-73
547 0-99
731
H9
frictions
35
56
135
552
Rtex
237
307
489
708
785
917
Polyamide filament
Diameter
mm
0-55
0-64
0-84
1-01
1-05
M6
Double
frictions
89
84
141
364
628
722
Twines of exactly the same R tex or the same diameter
were not available. Comparisons based on R tex values
showed no significant differences between polypropylene
and polyamide. This indicates that polypropylene has a
very high abrasion resistance, for polyamide is known to
be one of the best fibrous materials in this respect.
If twines of equal diameter are compared, the abrasion
resistance of polyamide twines is higher than that of
polypropylene twines.
Fig. 4. Load-elongation curves of polypropylene and polyamide netting
twines (wet, %). Pp= polypropylene. Pa ^polyamide.
Extensibility — The inherent extensibility of synthetic
fibres varies, being relatively large for the polyamide
group and comparatively small for the polypropylene
and polyester groups. The amount of stretching during
manufacture is of great influence. The more the fibres
are stretched, the more does extensibility decrease and
their breaking strength (also per centage losses by knot-
ting), increase. Nearly all polypropylene twines, men-
tioned in this paper, are of highly stretched material,
similar to nylon twines. The form of fibre also influences
twine extensibility; staple fibre twines are more extensible
Fig. 5. Load-elongation curves of trawl twines (wett%). (As in
Fig. 4 the curves only reach up to a load corresponding to the
knot breaking strength.)
than continuous filament twines. The influence of the
amount of twist is shown in Fig. 6 with the nylon twines
2 and 3. It is possible to double extensibility by increasing
the coefficient of twist from, e.g., 150 to about 200.
The influence of increasing loads on twine extension is
demonstrated by the load-elongation curves of Figs. 4
and 5. Here polypropylene and polyamide twines
approximately corresponding in twist and knot breaking
strength, are compared. They show remarkably great
differences of extensibility — polyamide stretch much more
than polypropylene twines — particularly under low
loads. (Figs. 4, 5, and 6.)
Fig. 6. Load-elongation curves of netting twines (wet, %) made of
polypropylene (Pp) and polyamide (Pa). Twines I and 2 medium laid
(a- 750-760), twine 3 hard laid.
„
a
12
n
n
9
6
7
6
5-
4 -
3 -
Polyamide
Fig. 7. Elongation and elasticity of wet netting twines loaded for
one hour with 30 per cent of their (straight) breaking strength.
Elongation: (a)^ immediately after loading. (b)= loaded for one hour,
(^immediately after removal of load. (d)=one hour after removal
of load, (c) **permanent elongation.
Elasticity— Because of the factor of duration of time
of loading, elasticity was tested by special trials, the results
of which are compared in Figs. 7 and 8 and they confirm
once more that polyamide twines are more extensible
than polypropylene twines— but the permanent elongation
of polyamide is lower. A further remarkable difference
53
between the two kinds of twine becomes evident
after longer loading, for instance 24 hours, as in Fig. 8.
Polypropylene twines show a "creep" when subjected to
loads for prolonged times. Polyamide twines do not
change their length during a long period of loading.
They show no creep and have a better elasticity.
1
Fig. 8. Elongation and elasticity of dry netting twines loaded for 24
hours with 30 per cent, of their (straight) breaking strength. Elongation:
(a)« immediately after loading. (b)^loadedfor one hour. (c)=loaded
for three hours. (d)= loaded for 24 hours. (e)= immediately after
removal of load. (/) * one hour after removal of load, (g) = permanent
elongation (measured four days after removal of load).
The first trials with polypropylene fishing nets seem to
have started in 1961. Experiences, therefore, are still
limited.
A report by Carrothers on trials of polypropylene and
nylon salmon gillnets in Canada says polypropylene nets
were easy to handle and their knot stability was superior
to that of nylon nets. In capacity to catch fish, there was
no conformity in the experiences of the fishermen.
In trials with polypropylene cod gillnets during the
Lofoten season carried out by Norwegian fishermen, the
polypropylene nets were about 10 per cent lighter and
lost less strength during fishing than nylon nets, with
about equal catching efficiency.
In a cruise by the USA research vessel Delaware an
otter trawl, "in which the top wings, top belly and square
were constructed of polypropylene fibre twine", was
compared with a trawl, entirely made of manila twine.
After equal numbers of tows, "The polypropylene-
equipped net took 57 per cent fish compared to 43 per
cent with the all-manila net". It also gave easier handling,
lower water resistance and easier towing.
Trials of British trawlers with trawls entirely made of
polypropylene, brought similar results. Especially
braided twines were very suitable. These trawls can be
towed more easily through the water than comparable
manila trawls and give greater headline heights than nylon
using the same number of floats.
54
Mid water trawls
No experiences with polypropylene for this modern gear
type are known to the author. Some general remarks
can, however, be given.
Fishing gears subject to heavy strain, especially
trawls, should on principle be constructed of net material
which combines high breaking strength with a twine
diameter as small as possible. The small diameter gives
less towing resistance. The twines should also have a
relatively high extensibility with a high share of elasticity
in order to absorb kinetic energy so that the trawl nets
can withstand shock loads during fishing and heavy
stress when hauling big catches.
For midwater trawls these demands are of particularly
great importance. The nets have to be larger and be
towed faster than bottom trawls, because pelagic fish
are usually fast swimmers. Light and strong nets are,
therefore, indispensable. Since twine efficiency depends
not only on strength but also on extensibility, midwater
trawl twines are relatively high twisted. As shown in
Fig. 6, the degree of extension can remarkably be increased
in this way. As a high extension obtained by twisting is
connected with a considerable decrease in strength, nylon
twines should not be manufactured with a coefficient of
twist higher than 200. If twine thickness is really of such
great importance for the towing resistance, polyamide
twines are superior to polypropylene twines (see Fig. 3).
Considering extensibility the same fact has to be
stated. Because of the low original extension of polypro-
pylene fibres, twines made of these fibres must be twisted
to a much higher degree than polyamide twines in order
to reach an extensibility suitable for midwater trawls.
But with this hard twisting the breaking strength would
considerably decrease and the diameter would increase
even more.
References
v. Brandt, A. : Das Schwimmschleppnetz. Protokolle z. Fisch-
ereitechnik, V, Nr. 22/23, pp. 201-224, 1958.
v. Brandt, A. and Carrothers, P. J. G. : Test methods for fishing
gear material (Twines and Netting): 2nd World Fishing Gear
Congress, 1963.
Carrothers, P. J. G. : Results of laboratory and field evaluation of
continuous filament isotactic polypropylene fibre for salmon gill-
nets. Fish. Research Board of Canada, Circular No. 66, 1962.
Imperial Chemical Ind. Ltd. : 'Ulstron' polypropylene for fishnets,
1962.
Imperial Chemical Ind. Ltd.. 'Ulstron' trawl trials, 1962.
Imperial Chemical Ind. Ltd.: Trials with 'Ulstron' polypro-
pylene cod gillnets, Lofoten season, 1961, 1962.
Klust, G.: The efficiency of synthetic fibres in fishing, Modern
Fishing Gear of the World, 1959.
Klust, G. : Net materials for trawlnets. Comp. Fishing Committee
No. 87, 1961.
Klust, G. : Polypropylen-Schntire als Fischnetzmaterial. Dtsch.
Seiler-Zeitung, Nr. 4, pp. 203-208, 1963.
Klust, G.: Netzmaterial fur Schwimmschleppnetze der Hoch-
sccfischcrei. Allgcm. Fischwirtschaftszcitung— H20/21, 1963.
Mohr, H. : Reaction of herring to fishing gear studied by echo
sounding. 2. World Fishing Gear Congress, 1963.
N.N.: Delaware report on polypropylene trawl. Bureau of
Commercial Fisheries, Report No. 95, 1961.
Polymer Ind. Chimiche S.p.A.: General description of 'Mera-
klon' Booklet No. 1, Milan, 1960.
Schftrfe, J. and Steinberg, R.: Neue Erfahrungen mit Schwimm-
schteppnetzen (Juni bis Dezember, 1962). Protokolle z. Fischcrei-
technik, VIII, No. 37, pp. 160-230, 1963.
Schftrfe, J.: One-boat midwater trawling in Germany. 2. World
Fishing Gear Congress, 1963.
Polypropylene Twines in Japan
Abstract
Several Japanese firms started production of polypropylene net
twines in 1961. The first products varied in quality, but recently
more stable material has been produced and polypropylene twines
are now used in the construction of nets in Japan. Although
efforts are still in progress to improve the present twines and mono-
filaments, tests were made to ascertain their properties in relation
to other synthetics. Polypropylene has a lower specific gravity
(0-91) and a higher breaking strength (g/den) both dry and wet
straight and knotted, than any other fibre material. Water absorp-
tion is negligible and consequently there is no loss of strength through
wetting. On the basis of equal size (same diameter but lower weight
per unit length) it is equal in strength to 'Amilan' but slightly
inferior to Tetoron* multifilaments. Elasticity and elongation is
small at low loads, but breaking energy is 50 per cent greater than
that of 'Amilan*. Resistance to weathering and abrasion is still
poorer than that of 'Amilan*. No appreciable difference in catch-
ability or ease of handling could be noted during fishing tests
when compared with 'Amilan', except that polypropylene was more
bulky.
Les fils de polypropylene au Japon
Resume
Plusieurs entreprises japonaises ont commence en 1961 a fabriquer
des fils de polypropylene pour les filets de pechc. La qualite des
premiers produits a beaucoup varie et tout r&emment on a mis au
point une matiere plus stable que Ton utilise dej& pour la structure
des filets de peche. Bien que de gros efforts soient en cours pour
ameliorer encore les fils et monofilaments actuels, des essais ont
deja et6 effectues en vue de determiner les qualites specifiques de ces
fils par rapport aux autres fils synthetiques. Le poids specifique du
polypropylene est inferieur (0-91) et sa resistance & la rupture
superieure (g/den) (que la fibre soit seche ou mouillee, nouee ou
non) a ceux de toute autre fibre. Son absorption d'eau est minime et
de ce fait le mouillage n'entraine aucune diminution de sa force.
Sur la base de mfimes dimensions (c'est-a-dire du meme diametre,
mais d'un poids inferieur par unite de longueur), la force du poly-
propylene est egalc a celle de T Am ilan' mais un peu inferieure a
celle des multifilaments de Tetoron1. Son elasticite et son elongation
aux charges faibles sont restreintes, mais sa resistance & la rupture
est de 50 pour cent plus grande que celle de rAmilan'. Sa resistance
a I'usure du temps ou par frottemen reste inferieure a celle de
PAniilan'. Pendant les essais de peche, on n'as pas constate de
difference appreciable entre P Amilan* et le polypropylene quant a
la capacite de capture et la facilite de I'amenagement du poisson,
sauf que le polypropylene se faisait plus volumineux.
Hilos de polipropileno en el Japon
Resumen
Varias empresas Japonesas iniciaron en 196 1 la producci6n dc
hilos de polipropileno para redes. La calidad de los primeros
productos variaba mucho, pero ultimamente se ha obtenido un
material mas estable y estos hilos se emplean actualmente en la
construcci6n de redes. Continuan las investigaciones para mejorar
los hilos empleados actualmente, realizandose, ademas, ensayos para
determinar sus propiedades con respectoaotrosmaterialessintdticos.
El polipropileno tiene una gravedad especifica m£s baja (0-91) y una
mayor resistencia a la rotura (g/den) tanto humedo como seco,
anudado como sin anudar, que cualquier otro material para fibres.
La absorci6n de agua es insignificante y por lo tanto no hay perdida
de resistencia a causa de la mojadura. A igualdad de dimensipnes
(el mismo diametro, pero menos peso por unidad de longitud)
es tan fuerte como * Amilan', pero un poco menos que los multi-
filamentos de Tetoron'. Le elasticidad y el estiramiento son peque
nos a cargas reducidas pero la resistencia a la rotura es un 50 por
ciento mayor que la de 'Amilan'. La resistencia al desgaste y a la
abrasidn es todavia menor que la de 'Amilan'. No se observarpn
diferencias sensibles en cuanto a captures o facilidad de manip-
ulacidn en ensayos de pesca en los que se compar6 con 'Amilan',
aunque el polipropileno result6 ser mas voluminoso.
"EXPERIMENTAL production of polypropylene was
JLJ started by several manufacturers in 1961. Of the
first products quality was unstable and the resultant
Katsqji Honda
Tokyo University
of Fisheries
ShfemOttda
Nippon Gyomo
Sengu Kaisha Ltd
fibres unreliable. Products are now more stable and such
fibres are beginning to be used in fishing gears.
Investigations towards improvement are still being
carried out and the data provided here is not final.
Properties of polypropylene yarns are given in Table I,
compared with others productions. From this it can be
clearly seen that:
(a) Specific gravity of polypropylene is the lowest
among all synthetic fibres. This is a definite advan-
tage for light fishing gear. But it can be a drawback
in some gears where sinking speed is important.
(b) Breaking strength, dry and wet, both straight and
looped, is greater than that of other synthetic
fibres. However, when compared with multifila-
ment yarns of the same size (diameter), polypro-
pylene has the same strength as 'Amilan*, but is
inferior to Tetoron' (polyester). In spun yarns it
equals both Tetoron' and 'Kuralon' of the same
diameter.
(c) As polypropylene absorbs practically no water,
its characteristics undergo no changes in wet
conditions so that wet strength is as high as dry
strength which is an advantage.
(d) The stress-strain curve under loop test is given in
Fig. 1. (Mark P.P. as used throughout Figs. 1 to 7
and Table I means polypropylene.) Compared
with 'Amilan', polypropylene is difficult to stretch
even under low load and its elongation is com-
paratively small. Its breaking energy is SO per cent
greater than that of 'Amilan' ; but if the curves are
made for equal diameter (Fig. 2) then the breaking
energy is 30 per cent lower. This means that it is
weaker than 'Amilan' of the same diameter.
Abrasion tests (Fig. 3) show that polypropylene has a
relatively low abrasion resistance and is therefore not
well suited for trawls, etc.
Tests for weathering on its strength and elongations
(Figs. 4 and 5) made it clear that polypropylene has a
much lower resistance to weathering than either 'Amilan'
or Tetoron'.
55
As a monofilamcnt polypropylene was found to be
superior both in straight and loop tensile strength. For
elastic recovery, the results in Fig. 6 show that the elas-
ticity decreases much faster with increasing load than
for 'Amilan* of the same diameter.
Impact tests and abrasion tests showed that polypro-
pylene was inferior to * Amilan* (Table II and Fig. 7).
Polypropylene is more transparent than 'Amilan' but
rather stiffen
Two hundred and fifty polypropylene gillnets made of
No. 10-5 (2315d) monofilament twine in 121-mm mesh
continued on page 57
p.p.
101
800
Plw)
\ f
' I
^
/
^T
1
1
7
/S
AMM«H 110 0
./
/
T-
1
i
i
If.
/
1
1
1
i
« • tO If 14 Elong*ti*n
Fig. 1. Stress-strain
diagram.
t
PP 100 D
'"'V5
/
/
y
/
A.
lion CIO 0
lo »
/
/
(/
Fig. 2. Stress-strain diagram
for equal diameter.
Fig. 3. Abrasive
resistance.
10 20 50 40 »r
Fig. 4. Loss in strength
from exposure.
E%
100
Totoron 210 D/,5 90
0 24 6 8 10 12 Kg
Fig. 5. Elongation decrease from exposure. Fig. 6. Monofilament elastic behaviour.
/TC
1,0
I
0 500 1000 *•"— ""
Fig. 7. Monofilament abrasive resistance.
TABLE U
XtM
•ultifUOMBt
Spun
Zton tablo Ib
Polypropylono*
Anil*n»
P.P. Milan TotoroB SATOA Toriroa
160 P 210 D 210 D 360 9 300 D
20.24 P15P 24P i P 60 P
p.p. luntloa Spun Bpuft i
nylon Totoroi
20 8 20 8 22 8 20 8
Jbawrod. doniov (a)
2315
2650
•pooiflo fmritj
0.91 1.14 1.38 1.70 1.40
0.91 1.30 1.14 1.38
Crooo oootion
IllipM
Bllipoo
•MOkiMf itTOnfftk
Brookini otronfth (kf)
13.2
13.8
(«/«o») to.
6.5-7.7 6.20 6.50 2.0 2.50
4.3-4*5 4.50 2.70 2.60
" 4.20 2.40 2.65
Broftfcinf olongntlon (%}
20.5
33-5
IrMtiBg rttmgtttlM
Loop otronfftk (kf )
7.8
8.9
"* wit
20.5 18.0 6.0 26.0 20.0
* 19*0 6.5 * 21.0
16.0-16.5 10.0 20.0 23*5
" 13*0 22.0 24.5
Loop olon«Ation (%)
11.0
16.6
Loop *ty+Mffftk
AOOMAM % of otronf th, by loop
36.1
35-5
(«/4-) gy
4.8-5.3 5.10 4.60 1.3 1.90
* 4.53 " " 2.00
3.4-3.6 3.40 2.30 2.25
11 2.80 2.10 2.35
BrooJcinc otroM (kf/M2) itrfti«kt
43
49
(F) D«y
11*5 11.0 7.0 13.5 12.0
12.5-13.5 75 16 5 21 5
(in tho ot»o of MM finonoso) loop
26
32
Wot
• 13.5 7.5 " 13.0
9.5 14.5 23.5
tar loop wot
28.0 13.0 29.0 35*0 22.0
20.0 33.0 13.0 11.0
Item T*bl« II
Poljrpropylww
aonofiluBont
JLnilan
nonofiluitnt
— ^nj| ttrta.
2515D, 121 m
26$00t 121 HP
(»0t) (kf/BII*)
Impact
r^1**
53-46 53 81 31 32
4<MJ 47 53 20 25
35-37 49 14 33
28-30 32 22 29
Br«akia« «n«r«y (k«.n) (T«rtio«l)
0.51
0.67
»^— w
0.1 4*5 0.4 0.1 0.2
5.5
Br*«kin« «n«r«y (kff.n) (horizontal)
0.61
1.39
56
Use of < Ulstron ' Polypropylene in Fishing
Abstract
Production of 'Ulstron' for fishing nets will, by the end of 1963
be greater than the total equivalent consumption of all other U K'
synthetic fibres. An oil derivative, polypropylene was invented in
1954 in Miten. Total world production of polypropylene now
exceeds 300,000 tons per annum but only a small percentage in the
form of fibres, the majority being used as moulding. High-tenacity
•Ulstron' at 8-5 g/den competes with nylon on strength and price
basis. Plaited cords may be produced on conventional machinery
*Ulstron' nets can be constructed on ordinary netting looms such
as Zang, Dandy, Amita and Porlester, and are said to offer advan-
tages in both runnage and mesh strength over equivalent nylon.
Knot firmness is relatively good but loom tensions in netmaking
should aim at 1 -0 g/den followed by knot stretching at 1 -0 g/den at
100/1 10°C for 15 minutes while held to fixed dimensions on a
stretching frame. 'Ulstron' should not be heated over 110°C.
'Ulstron' yarn may be obtained in and dyed into various colours.
Lighter than water, 'Ulstron' is now being used in a variety of
fishing gears such as bottom trawls and midwater trawls, gillnets,
Danish seines and purse seines, etc., as well as in ropes, where it is
in the same strength bracket as nylon and polyester ropes, but has a
considerably greater runnage and is cheaper per unit length of
equal strength.
by
C. L. B. Carter
and
K. West
Fibres Division,
Imperial Chemical Industries Ltd
L"Ulstron' ne doit pas etre chauffe audessus de 110°C U fil de
filet 'Ulstron' peut 6tre obtenu dans diverses tcintes. II flotte sur
1'eau et est maintcnant utilis& pour des engins de pechc varies tels
que: chaluts de fond, chaluts pelagiques, filets maillants, sennes
danoises, sennes coulissantes, etc., aussi bien que pour des cordes ou
1" Ulstron' est l'egal du nylon et du polyester en ce qui concerne la
resistance a la rupture, ma is est beaucoup plus long au kilo et est
beaucoup moins cher par unite de longueur pour la mdme resistance.
L'Utilisation du polypropylene 'Ulstron' dans 1'industrie de la peche El empleo de pollpropileno 'Ulstron' en la industria pesquera
Resume
La production d'4Ulstron' pour les filets de peche sera, a la fin de
1963, superieure a la consommation totalc de toutes les autres
fibres synthetiques pour le Royaume-Uni. Le polypropylene,
deriyS d'une huile minerale, a etc invente a Milan en 1954. La pro-
duction mondialc totale de polypropyleneexcede maintenant 300,000
tonnes par an, mais un faible pourcenlage seulement sert a la
fabrication des filets, la majorite etant utilised pour les plastiques.
L"Ulstron' dc haute tenacite a 8,5 g/den peut rivaliser avec le nylon
au point de vue resistance et prix. Des cordes tressees peuvent
etre produites sur les machines traditionnelles et les filets d" Ulstron'
peuvent etre fabriques sur des m6tiers ordinaires comme Zang,
Dandy, Amita ct Porlester. Us sont sensds offrir des avantages sur
les filets en nylon de denier equivalent, en ce qui concerne le rapport
m/kg et la resistance des mailles. La stabilite des noeuds est relative-
ment bonne mais la tension sur le metier pendant la fabrication devra
6tre dirigee vers 1 g/den, suivie d'un etiragc des noeuds a un g/dcn,
100/1 10' C pendant 15 minutes, en tenant le filet fixe sur un cadre.
Extntcto
La producci6n de 'Ulstron' para redes de pesca sera, para fines de
1963, mayor que la de todas las demas fibras sinteticas en el Reino
Unido. El propileno es un derivado del petrbleo descubierto en
Milan en 1954. La producci6n mundial total de la sustancia excede
de 300.000 tons anuales, pero s61o una pequena proporcibn en la
forma de fibra y la mayor parte para moldes. El 'Ulstron' de gran
tenacidad a 8,5 g/den compite con el nylon en robustez y precio.
Las cuerdas trenzadas pueden fabricarse en las maquinas normales.
Las redes de 'Ulstron' se manufacturan en los telares corrientes
como el Zang, Dandy, Amita y Porlester y sc dice que resultan
ventajosas en cuanto at peso por longitud y resistencia de ia malla
con respecto al nylon equivalente. Elnudo es bastante fir me,
pero las tensiones en el telar, cuando se hacen redes, deberian ser
del orden de 1,0 g/den, seguidas de estiramiento del nudo a 1,0
g/den a 100-1 10 C durante 15 rnin mientras se sujeta a dimensiones
fijas en un armaz6n de estiramiento. El 'Ulstron' no debera calen-
tarse a mas de HOC. Sus hi los pueden prepararsc de cualquier
continued from page 56
size nets were tested in the North Pacific Ocean from May
to July, 1962, for salmon and trout. The catch ratio per
net appeared to average approximately that of 'Amilan'
nets of the same diameter and were as follows:
Polypropylene 1 '30- 1 -45
4 Amilan' nets made in 1 96 1 1 -00
4 Amilan' nets made in 1 962 1 '50- 1 -65.
The polypropylene nets were found to be two to three
times more voluminous than 4Amilan' ones and tended to
blow about easily, making them liable to fouling during
hauling and shooting.
As regards fishing damage it would appear that the
number of tears is twice that of the 4Amilan' nets but the
knot slippage was very much lower.
After the trials, the nets were again tested and it
appeared that the strength had increased during use so
that it was now higher than that of 'Amilan'. No change
in mesh size as a result of the trials could be ascertained.
Polypropylene nets made of multifilament 180d/15
were made in 121 -mm mesh size and operated together
with 'Amilan' and 'Tetoron' nets made of equal diameter
twines.
Catches per net were as good as those of contemporary
'Amilan' nets and normally better than older nets.
In handling they were more bulky and tended to blow
about.
About 600 nets made of polypropylene staple yarn
20s/ 15 (resin treated, singed) were operated from April
to August 1962, for a period of 103 days, together with
nets made of 'Kuralon'.
The experiment showed that catches were higher than
those of the 'Kuralon' nets and, while the handling of the
nets was fairly good, there was more difficulty in retriev-
ing the crabs than with the 'Kuralon* nets.
The nets were tested after use and it was noticeable
that, whereas the monofilamcnt and multifilament nets
slightly increased in breaking strength and decreased in
extensibility, the opposite occurred with these staple
yarn nets.
57
color o tefiirse de Igual manera. Es menos denso que el agua y se
emplcaactualinentccnlafabricad6ndeartcsdean«treyffo
noes de enmalle, artes daneses y redes de oerco de jareta, asi como
en caboi de todas dases en los que tiene la misma robustez que los
de nylon y poiiesteres, pero entran muchos mas metres en kg y es
mis barato por unidad de longitud de la misma resistencia.
4T TLSTRON' polypropylene is rapidly becoming the
vJ leading synthetic fibre for the production of
fish netting in the U.K. It is already used for a wider
range of fish nets than any of the other synthetics, and
production of 'Ulstron' for nets and other fishing appli-
cations will, by the end of 1963, be greater than the total
equivalent consumption of all the other U.K. synthetic
fibres.
Polypropylene is the latest synthetic fibre to be devel-
oped for the fishing industry and this paper outlines the
properties of 'Ulstron', I.C.I.'s brand of polypropylene,
and describes the experience to date in net production and
use.
Discovery and development
Polypropylene has been known for many years as an
oil, although, for various reasons, it had no commercial
importance. In 1954, at the Polytechnic Institute of Milan,
Professor Natta, whose work had been concerned with
the stereospecific polymerisation of defines using organo-
mctallic catalysts of the Zeigler type, discovered how to
make isotactic material with a regular ordered molecular
structure. This raised the melting point for 20 to 40°C to
165°C and altered the whole future of polypropylene as
both a plastic and fibre material. Propylene is derived from
oil, being a gas which is obtained at the same time as
ethylene by cracking petroleum. As a point of interest.
'Terylene' and polythene are also derived from oil while
nylon is derived from coal.
I.C.I, recognised the great potential for this new
material and negotiated a patent licence from the Italian
chemical firm of Montecatini in August, 1960. This
agreement covered the exclusive production and sale in
the U.K. by I.C.I, of polypropylene filament yarn, staple
fibre and textile monofilaments. To the high tenacity
multifilament and monofilament polypropylene yarns,
I.C.I, has given the trade name 'Ulstron'. A large plant,
with an eventual capacity of 5,000,000 Ib per annum, is
now in production and work is proceeding to extend
this capacity still further.
Total world production of polypropylene is now
estimated to exceed 300,000 tons per annum but at
present only a small percentage is in the form of fibres,
the majority being supplied for plastic uses such as
moulding. With most brands of polypropylene fibre
the tenacity (6 g/den) has been considerably below that
of nylon (7^-9 g/den) and this has undoubtedly held
back a more rapid expansion in the fishing industry than
would otherwise have been expected from its outstanding
properties of lightness and low price. 'Ulstron' at 8*5
g/den is the first (and, at present, only) brand of poly-
propylene to be fully competitive with nylon on strength
as well as price. This accounts for its rapid progress at
the expense of nylon and other synthetics which it is
replacing, a pattern which is likely to occur in other
parts of the world during the next few years as other
brands of polypropylene become more fully developed.
Some details of this new material will therefore be of
particular interest.
Properties of 'Ulstron'
'Ulstron' has a low density, high wet strength and extreme-
ly high capacity for withstanding shock loads. Its pro-
perties are compared with those of other yarns used in
fish net manufacture in Table I. Other advantages include
a good resistance to chemical and micro-biological
attack (Table II) and negligible water absorption. It
is currently produced in the following deniers:
190 den (Decitex 21 1), 380 (Decitex 422), 570 (Deci-
tex 633), 760 (Decitex 844), 1,140 (Decitex 1,270), in
both natural and melt coloured green.
Twine making
* Ulstron' twines and nets are constructed in much the
same way as other synthetic materials but, because of
their lower liveliness compared with nylon or Terylene'
Property 'Ulstron'
Tenacity (g/den) . . 8-0-8-5
Extension at Break ( %) 1 8-22
Elastic Recovery (% from 5% 88
Extension)
Working Load Extension 1-11
Initial Modulus (g/den) 90
Toughness x 10*(joiiles/g) 80-100
Wet Tenacity (g/den) . . 8-0-8-5
Wet Extension (%) .. 18-22
Moisture Regain (%).. 0-1
Softening Point .. 160-170
Specific Gravity of Fibre 0-91
TABLE I — Comparative filament yarn properties
Polyethylene Terylene'
4-5-6-0
25-35
88
2-22
45
85-90
4-5-6-0
25-35
0-15
110-140
0-95
6-0-7-0
6-14
90
•91-77
110-130
28-50
6-7
6-14
0-4
260
1-38
Nylon
Polyvinyl
Alcohol
7-0-9-0
3-0-7-0
12-18
15-28
98
60-75
1-5-1-3
at 3% Ext.
0-91
45-55
110
55-89
6-0-7-9
2-6-5-9
19-28
20-30
4-2
5
250
215-225
1-14
1-26-1-3
Cotton*
1-5-2-0
3-10
45
8-4-1-4
12-70
3-4-8-5
1-8-2-4
2-5-8-0
8
1-54
Manila*
2-3-2-9
2-3
Breaks
6-75
148
4-0
2-3-2-9
2-5-3-5
13
148
* The tensile properties quoted for cotton, manila, sisal of necessity refer to spun yarns, which have an overall
tenacity lower than that of the individual single fibres.
Sisal*
2-4
2-2-5
Breaks
6-75
148
3-0
2-2
2-3
13
149
58
Chemical
Acids:
Hydrochloric
Nitric
Sulphuric
Sulphuric
Sulphuric
Formic
Acetic
Alkalis:
Caustic Soda
Caustic Soda
Caustic Potash
Solvents:
Trichlorethylene . .
Trichlorethylene . .
Carbon Tetrachloride
Benzene
Benzene
Mctacresol
Metacresol
Oxidising Agents:
Sodium Hypochlorite
Hydrogen Peroxide
Cone. W/W
34
66
96
96
50
90
100
40
20
40
100
100
100
100
100
100
100
5% active C12
12 volume
TABLE II— Chemical resistance data
Temp.(°C) Time
(hour)
20
20
20
70
70
20
20
20
70
20
30
at the boil
20
70
at the boil
70
100
20
20
100
100
100
20
150
100
10
100
150
100
150
8
150
150
4
150
4
100
100
Residual Strength (%)
Nylon Polyester 'Ulstron'
0 89 100
0 71 100
0 0 100
0 0 95
0 83 90
0 100 100
86 92 100
90 0 90
100 0 100
90 0 90
N.A. N.A. 75
N.A. N.A. 70
100 100 100
100 100 100
85 92 95
0 0 100
0 0 100
N.A. 95 85
N.A. 100 90
N.A.— No data available
twines, twist setting is less important. In twisting * Uls-
tron' twines, it may be found necessary to employ ring
travellers one or two sizes lower than are used for nylon
twines of equivalent numbers of plies, owing to the lower
density of the material.
'Ulstron' twines, matching in twist-hardness nylon
twines of 210-den yarns, will be obtained by using one
190-den yarn for each 210-den yarn and employing twist
levels at all stages which are some five per cent lower than
for the nylon twine. Where high extensibility and high
bending stiffness are required hard twines will be con-
structed; where the net is to be subsequently bonded,
soft twines will be adequate. Twist levels which have been
employed in constructing hard and soft 'Ulstron' twines
based on 190-den yarn are given in Fig. 1.
ILK TWHT.CULiailt
Fig. 1. Ply twist for three-ply twines based on 190-denier ' Ulstron9 yarn
Equivalent twist hardnesses for 'Ulstron' twines,
based on the same number of plies of any other denier,
may be calculated from the formula
T8 = TWWO
d
where d is the yarn denier to be used, T! the twist (tpi)
for twines based on 190-den 'Ulstron' and Tt the twist
to be used in the twine to be constructed. Balanced
twines are produced exactly as for other materials by
making:
cable twist (tpi) = ply twist (tpi)
V
no. of plies
although, because any unbalance in 'Ulstron' twines
tends to decay, there is more latitude for error than in
producing balanced Terylene' or nylon twines.
Plaited or braided cords may be produced on conven-
tional machinery into either hard extensible braids or
soft inextensible braids. Ends/spindle, spindles/braid
and picks per inch can be almost identical for 'Ulstron'
braids based on 190-den yarn to those employed for
210-den nylon braids with equivalent numbers of ends.
Comparative properties
All the advantages which 'Ulstron' yarn offers over com-
petitive yarns are retained when twine properties are
compared. 'Ulstron' yarns are produced in multiples of
190 den and so, when compared with 210-den nylon
twines of equivalent number of plies, they have approxi-
mately 10 per cent runnage advantage. Table III shows
that 'Ulstron' twines have, in addition, a small wet
strength advantage over their nylon equivalents.
TABLE III— Comparative wet knot strength of 'Ulstron' and
nylon twines
Nylon
Wet Knot
Bid (Ib)
42-6
49-1
No. of Plies
190-den ier 'Ulstron'
210-denier Nylon 6,6
36
42
60
62-6
78-4
*Ulstronf
Wet Knot
%Ext
24-0
Bid (Ib)
51-0
%Ext
13-0
22-0
55-0
13-0
26-0
68-7
14-2
21-0
85-6
15-0
N.B. (1) Runnage advantage to 'Ulstron1 - 210—190+9$%
210
(2) Twines constructed at comparable twist hardnesses.
59
Similar comparisons and conclusions can be made
about the strength and weight advantages of 4Ulstron'
in netting form. It can be seen from Table IV below that
'Ulstron' nets offer an advantage in both runnage and
mesh strength over equivalent nylon nets over a wide
range of mesh sizes and constructions. Nylon is used for
a basis of comparison because of its former pre-eminence
in the industry.
TABLE IV— Physical properties of gillnets
Net Construction (1) Runnage (2)
Ply Mesh size 'Ulstron' Nylon
(in)
2 2-5 165-5 154
2 2-6 172 156
2 3-1 184 160
2 4-5 433 411
3 4-5 292 268
9 6 84-4 75-5
15 5-5 44 39
18 4-1 26 23
Wet mesh strength (Ib)
* Ulstron' Nylon
6-1
6-1
6-1
6-2
9-3
27-4
45-8
53-3
5-0
5-2
5-8
5-3
7-9
23-0
42-8
54-4
1 I ) Number of plies of 1 90-denier 'Ulstron' or 2 1 0-denier nylon.
(2) The runnage is expressed as the number of square yards of
net per Ib, the meshes being fully open and squarely aligned.
Net making and finishing
'Ulstron' nets may be constructed on any of the netting
looms in common use such as the Zang, Dandy, Amita
or Porlester. Table V and Fig. 2 show that such nets
show very little knot inversion in fishing and less ten-
dency for knot opening in transportation from loom to
stretching frame than equivalent Terylene' and nylon
nets subjected to identical stretching and stabilising
treatments. Knot slippage may nevertheless occur unless
adequate precautions are taken.
TABLE V — Load in kg to invert double knots tied under a tension
of 625 gms
Nylon 6 x 210 1-73
Early 'Ulstron' 6 x 190 1-85
Current 'Ulstron' 6 x 190 2-30
Current 'Ulstron' 6 x 190, relaxed in boiling \\ater for 15
minutes 2-7
Current 'Ulstron' 6 x 190, stretched under a load of 1
gpd at 1 1 0°C for 5 minutes 3 • 1
I-
i!
Fig. 2. Relation between slippage and knotting tension.
Loom tensions in netmakmg should be as high as is
consistent with smooth running, and maximum levels
of 1*0 g/den should be aimed for. Subsequent stretching
should be at a tension of at least 1*0 g/den if carried out
cold, but preferably the net should be subjected to a
temperature of 100/1 10°C for 15 min whilst held to
fixed dimensions on a stretching frame. * Ulstron' nets
should never be subjected to temperatures exceeding
1 10°C If these precautions are followed, the knot stabi-
lity will be found satisfactory without using twine or
net bonding agents. Good knot stability can also be
obtained by heat relaxing the net at 100/1 10°C for 15 min
after stretching, but only at the expense of some 8 to 10
per cent shrinkage compared with six to eight per cent for
nylon. Satisfactorily stable knots are obtained in 'Uls-
tron' nets made from braided cords even when no heat
treatment is used and at deniers in excess of 20,000 there
is no need to stretch the nets.
Where facilities for heat treatment of 'Ulstron' nets
made from fine twines are not available, it is necessary
either to employ considerably higher net stretching
tensions than usual or to apply a suitable knot bonding
agent. Such agents may also be applied where the fisher-
men require a high degree of stiffness, as for wing trawls.
A number of bonding agents have been examined for
their effect on knot inversion. Not all the agents exam-
ined produced an improvement. In general, bonding
of knots produced an improved stability whilst bonding of
the twine gave a lowered stability. Among those agents
which produce a noticeable increase in stability are
'Bitumen PE4', 'VinamuF N6515, N6530, 'Cuprinol' and
'Marstein'. In most cases, the solution should be diluted
to ensure the correct pick-up which will vary with the
dilution, the size of twine in the netting, and whether
the net is mechanically dried or not but should be as low
as possible compatible with the required improvement
in knot stability and stiffening of the twine. The 'Bitumen
PE4' treatment which is often used for bottom trawls,
besides stiffening the twine, protects it from the abrasive
wear of the sea bed and reduces the uptake of sand and
mud.
'Ulstron' yarn may be obtained in natural colour,
melt dyed green, or in special cases melt dyed black.
If other colours are required, the nets can be dyed with
one of the following range of disperse dyestuffs :
'Supracet' Yellow 2G 'Cibacet' Blue OF
'Resolin' Yellow 5R 'Dispersol' Yellow PP
'Serisol' Fast Pink RGL 'Dispersol' Red PP
'Foron' Brilliant Violet BL 'Duranol' Blue PP.
All these give adequate depth of shade and rub fastness.
These dyestuffs are applied at 85 to 100°C and full
depth of shade may be obtained in some 30 min. Unless
the net can be held to length in dyeing, a net shrinkage of
up to nine per cent must be allowed for.
Mounting requirements
'Ulstron' nets are themselves lighter than water and
therefore need adequate weighting. The mounting
requirements vary according to the type of net used:
60
in mounting gillnets which have already a weighted
sinkerline, it is merely necessary to increase the weighting
to compensate for the lower weight of 'Ulstron'. When
netting which weighs 1 Ib in air is immersed in water, the
actual weight in water is D-— 1 where D is the specific
D
gravity of the fibre. Hence the weight of 1 Ib (air weight)
of various nets immersed in water for long enough to
displace all entrapped air is:
1 Ib (air weight) of cotton netting
1 Ib (air weight) of nylon netting
—0'33 Ib negative
buoyancy
—0*12 Ib negative
buoyancy
1 Ib (air weight) of 'Ulstron' netting — (HO Ib positive
buoyancy.
Thus in changing from cotton to 'Ulstron' netting an
extra 0*43 Ib of weighting should be added (if necessary)
to the sinkerline for every 1 Ib (air weight) of cotton
netting previously used.
In mounting trawlnets the aim is often to secure
maximum headline/footrope separation, which depends
principally on the design of the net. In changing to
4 Ulstron' from the denser fibres, therefore, no great
increase in headline/footrope separation should be
expected but instead it should be possible to reduce the
number of floats without impairing the shape of the net
in fishing. The use of fewer floats will reduce the drag
of the net, which may lead to valuable savings in power.
Fishing performmnce— 4>ottoro trawls
The most important property of a bottom trawl is its
fishing efficiency, which depends upon many factors,
some of which favour the use of 'Ulstron'. Among the
favourable factors is the lower drag which can be obtained
by reducing the number of floats on the headline whilst
keeping the same trawl mouth opening. One unfavour-
able factor is the smooth surface and small knot size of
'Ulstron' multifilament trawl meshes, which may permit
more of the smaller fish to escape, but this applies equally
to other synthetic fibres.
Since catches are highly dependent upon the efficiency
of the skipper, the season, the weather, and the state of
the sea bed, it is difficult to make a meaningful compari-
son of fishing power. All that could be said after a trial,
in which four trawling companies fished 'Ulstron1
trawls for a total of 12 trips, was that there was no signi-
ficant difference between their catching power and of the
manila trawls previously used. Thus although the
'Ulstron' trawls caught an average of 82- 1 kits/net/day
against 69-7 kits/net/day for equivalent-sized boats
fishing the same grounds over the same periods, most of
the difference could be explained by the superior fishing
ability of the boats used in the trial; before the 'Ulstron'
nets were supplied, the boats concerned in the trial
caught on average 12 per cent more fish/net/day than
the mean for all the boats fishing at the same periods.
Although no clear-cut answer emerges on fishing
COARSE GROUNDS
AVERAGE LOSS RATE I TRAWL PER 2 WEEKS
FINE GROUNDS
AVERAGE LOSS
I TRAWL PER 5 WEEKS
BRAIDED
ULSTRON
BRAIDED
ULSTRON
'APPARENT'
LIFE OF TRAWLS
REAL LIFE OF
TRAWLS ALLOWING
FOR LOSS
'8° so ,80
TRAWLS STILL FISHING AFTER — Y WEEKS
Fig. 3. Price / life relationship from trials on fishing efficiency.
loo
61
efficiency from the trials, the price/life relationship is
very much clearer (Fig. 3). In coarse ground fishing the
average life of braided 'Ulstron' trawls (if not lost) is
some five weeks9 fishing. Making allowances for acci-
dental losses by snagging on submerged objects and
similar mishaps, the average life is reduced to some two to
three weeks. The corresponding average life for manila
trawls, again taking accidental loss into account, is some
1 to 1 J weeks, or rather less than one full trip. Since
'Ulstron' Granton trawls in braided multifilament cost
in the region of £160 to £210 compared with some
£130 for manila trawls based on equivalent diameter
cords, the 'Ulstron* trawls have an advantage of about
SO per cent over manila trawls on a price/life basis.
Similar considerations apply in fine ground fishing
where the rates of accidental loss and severe damage are
much lower. Fig. 3 shows that trawls in more costly
materials than * Ulstron' would be uncompetitive in
the market, even if they had a higher resistance to normal
sea-bed abrasion, simply because of the chance of com-
plete loss.
Measurements have shown that 'Ulstron' trawls
constructed from braided cords show no mesh-size
changes greater than three per cent in practical fishing
up to three trips. 'Ulstron' trawls braided from twisted
twines show a small initial mesh-size increase associated
with knot tightening. After fishing for three trips, the
meshes decrease in size owing to penetration of sand
and other particles into the cords; but this shrinkage is
less with 'Ulstron' than it is with other trawl materials
which absorb moisture and so suffer from direct yarn
shrinkage as well as sand penetration. Accordingly, it
is possible to braid 'Ulstron' trawls to their desired
final mesh size more accurately than for other materials.
Closely associated with the relative constancy of mesh
size in an 'Ulstron' trawl, and stemming from its knot
stability and its low moisture uptake, is the fact that the
International Meshsize Regulations governing minimum
mesh size are less likely to be infringed by 'Ulstron'
trawls which are very light. A full bottom trawl weighs
about 250 Ib compared with 450 Ib for the equivalent
net in manila. They are easy to handle and because of
their low moisture uptake behave well in freezing condi-
tions.
Eighty per cent of Norway's large trawlers now use
'Ulstron' nets and they are being increasingly adopted
by nearly all the U.K. trawler companies. The Silver
Cod Trophy, awarded annually by the British Trawlers
Federation to the skipper landing the greatest volume of
fish, was won in 1962 by Somerset Maugham which was
largely using 'Ulstron' netting.
MUwater trawls
Midwater trawls are generally treated with a stiffening
agent to ensure that the nets retain their hydrodynamic
shape. The benefit of using a netting material sufficiently
light to permit a reduction in floats is greater with mid-
water trawls than for bottom trawls and the American
report on the cruise of the U.S.S. Delaware suggests
62
that fuel bills for towing 'Ulstron' midwater trawls may
be somewhat less than for heavier materials.
Results so far available show a wide variation in
reported catching power between boats, with some skip-
pers unenthusiastic whilst their neighbours are reporting
much higher catches. Undoubtedly much depends upon
intelligent mounting of the new material and upon the
use of an adequately rigid bonding agent. Nevertheless,
all the reports welcome the lightness and ease of handling
'Ulstron' midwater trawls.
Gfflnete
As in trawlnets, fish-catching power is of critical impor-
tance. In this case the relative fish-catching power of
nets in two materials is somewhat easier to measure
since the nets to be compared can be fished at the same
time and under the same conditions. The comparisons
have been chiefly with nylon, since the superiority of this
fibre in fish-catching power over other netting materials
such as cotton was already well established before
'Ulstron' was introduced.
Fig. 4 shows measurements of the fish-catching power
of a range of different types of 'Ulstron' and nylon nets.
In some cases the differences can be simply explained.
Thus, the salmon leader nets did not incorporate weights
on the "non-return valves" leading to the net bags so
that on the ebb tide the 'Ulstron' valves, formed of netting
panels, closed too rapidly and prevented the entry of
fish.
Fig. 4. Fish-catching power of " Ulstron* nets.
The other differences are less easy to explain. It was
emphasised in the section on mounting that where an
'Ulstron' net is set on to a shorter support rope it will bag
upwards, whereas a nylon net would bag downwards.
In these circumstances the 'Ulstron' net would catch more
of the fish swimming near the surface and this may explain
the superiority of the 'Ulstron' salmon gillncts. A further
reason for genuine differences in catching power stems
from the difference in extensibility of the two materials.
'Ulstron' meshes are less extensible at low loads than
are nylon meshes so that they will release fewer fish
whose girth is only slightly greater than the mesh size.
Conversely, fewer fish of girth appreciably greater than the
mesh size will be able to penetrate the 'Ulstron' meshes
than could become entrapped in a nylon mesh. To some
extent, therefore, the relative fish-catching power of
'Ulstron'and nylon nets will depend upon the size of the
fish being caught in relation to the mesh size. The differ-
ence in extensibility may be the explanation of two
features reported by a number of fishermen: namely, the
ease of removing the fish by hand from the net and the
reduced damage to fish compared with nylon.
The most extensively documented trial so far carried
out on 'Ulstron' gillnets was a trial with cod gillnets
in the Lofoten Islands. (Fig.4). The result on fish-catching
power showed that in this particular trial, in which six
boats fished a total of 30 nets for 42 days, there was no
statistically significant difference in fish-catching power
between the 'Ulstron' and nylon nets. The trial was
interesting in that it showed that the average loss in
strength of the nets after one year's fishing was 16 per
cent for the 'Ulstron' nets and 24 per cent for the nylon
controls. These results are again so close as to be statis-
tically inseparable but they serve to show that the
strength losses in fishing from all causes are no greatei
than those found with nylon netting.
Some outstanding results were reported from Nyasa-
land where a single 'Ulstron' net caught nearly as much
as 14 nylon nets of similar construction being fished
alongside. Catches during three fishing periods were:
Single 'Ulstron' net 14 Nylon nets
20 dozen 52\ dozen
25 J dozen 50 dozen
104| dozen 82 dozen
150 dozen 184^ dozen— 13 dozen per
single nylon
net
In another trial, supervised by one of the East African
Fisheries Officers, similar 'Ulstron' and nylon nets were
fished for 36 days on small shark. During this period
4Ulstron' caught 416 Ib and nylon 319 Ib. At the end of
this period, however, the nylon nets were no longer
serviceable whereas the 'Ulstron' was still considered to
have half its life remaining.
It is well known that wide variations can occur in fishing
results but it has been reasonable to deduce from these
and other trials that the performance of 'Ulstron' is at
least as good or better than that of nylon and contrary
to some expectations the slightly greater diameter of an
'Ulstron' gillnet twine, compared with equivalent ply
nylon, does not have any detrimental effect on catching
power.
Other nets
'Ulstron' has been found very satisfactory for the produc-
tion of Danish Seine nets and a large number of seiners,
particularly from Grimsby, have now changed to it.
Twisted twine has generally been used in runnages from
460 m/kg to 180 m/kg although there has been some
usage of braided twine, particularly for the codend.
'Ulstron' has made some progress in the wing trawl
market against considerable competition from polythene
monofil, which has been established for some time. The
greater strength, lower price and ease of mending of
'Ulstron' are, however, finding favour with many fisher-
men.
An interesting recent development has been the use of
'Ulstron' in ringnets, a small type of purse seine used
largely in Scotland for herring. In some instances only
part of the net has been made from 'Ulstron9, the balance
being in cotton or nylon. A number of 100 per cent
'Ulstron' ringnets have, however, been produced and
fished successfully. In spite of the buoyancy of 'Ulstron'
no difficulty has been reported in getting the nets to sink
properly and it is therefore likely that, with proper
rigging and weighting, it will be possible to produce
purse seines in 'Ulstron' where the reduction in weight
will give lower prices and easier handling.
Other uses for 'Ulstron' include trap netting, lobster
pots, anglers' keep netting, lines, snoods and dyed
mending twine. Trial quantities of knitted, knotless
netting have also been produced. Further trials are in
progress evaluating 'Ulstron' monofilament yarn and
twines in various types of net.
Ropes
The second largest use for 'Ulstron' is in the production
of ropes, many of which are used for fishing purposes
particularly in Norway.
'Ulstron' rope is in the same strength bracket as nylon
and polyester rope and considerably stronger than poly-
thene rope but has considerably greater runnage than
nylon and polyester as shown below (for 1 in circ) :
Ib per
120fm
4 Ulstron' .. 16-0
Nylon . . 21-5
Terylene' .. 24-75
B/load(lb) Cost per pence/ton/
-1ft*
2,100 Ib
2,240 Ib
2,100 Ib
1001
37s. 6d.
48s. 9d.
58s. 8d.
foot
4-80
5-85
7 «50
* Typical U.K. price
The 'Ulstron' rope has been largely used as mounting
ropes for cod, herring and other types of gillnets; in
some cases the netting itself has been 'Ulstron' but in
many cases 'Ulstron' rope has been used with nylon or
other types of netting. The reports on the ropes have
been very satisfactory.
One particularly interesting development in the rope
field has been the introduction of ropes made partly or
wholly from 40-in length 5,000 diameter polypropylene
staple fibre. This long staple 'Ulstron' has been specially
developed by I.C.I, and a leading U.K. ropemaker. It is
processed on standard hard fibre machinery into either
100 per cent spun 'Ulstron' rope, which is as strong as
'Ulstron' multifil rope, or into 35/65 or 50/50 polypro-
pylene/sisal blended ropes which are cheaper but not
quite so strong and to which the trade name 'Systron'
has been given. These ropes are cheaper than other
synthetic ropes and have performed well in trials as
trawl quarter ropes and for other uses.
Twines can only be produced from this new staple fibre
at runnages less than 900 yds/lb, so its use in the fishing
industry is restricted to ropes and heavy trawl twines.
continued on page 64
63
Synthetic Fibre Fishing Nets and Ropes Made
in Japan
The estimated production of synthetic fibre for fishing nets and
ropes in Japan is estimated at 45,000,000 Ib in 1 962, of which about
32,000,000 Ib was used for making fishing nets, the latter having
increased from 21 ,000,000 Ib in 1 959. On the other hand, the produc-
tion of natural fibre fishing nets (cotton, hemp, etc.) declined from
some 28,000,000 Ib in 1949 (when synthetic fibre net manufacture
started) to only about 3,000,000 Ib in 1962. About 85 per cent of
Japanese fishing nets are now made of synthetic fibres. Initially
nylon, vinylon, polyvinylidene chloride and polyvinyl chloride
fibres were mainly used but after 1959 polyethylene and poly-
ester were added, and polypropylene in 1962. Exports of syn-
thetic fibre fishing nets have increased from 400,000 Ib in 1955 to
about 70,000,000 Ib in 1961, shipped to more than 100 nations.
In 1961 nylon and vinylon each accounted for 41 per cent of syn-
thetic fibre nets, polyethylene six per cent, polyvinylidene five per
cent, polyvinyl chloride two per cent, polyester one per cent and
blended fibres four per cent. In lines and ropes vinylon accounts
for 53 per cent and polyethylene for 23 per cent. Almost the entire
output of ropes of blended fibres is exported.
Filets et cordes fabriques en fibres synthetiqties
R&ume
Au Japon, la production de filets et cordes en fibres synthdtiques
est estimee pour 1962, a 20.000 tonnes, dont 15.000 pour la
fabrication de filets, celle-ci ayant augmente de 9.500 tonnes
riepuis 1959. D'autre part, la production de filets en fibres
naturelles (coton, chanvre, etc.) a diminue de 13.000 tonnes en 1949
(d£but de las fabrication des filets en fibres synthdtiques) a 1.330
tonnes en 1962. Aujourd'hui environ 85 pour cent des filets japonais
sont fails en fibres synthdtiques. Initialement, les fibres de nylon,
vinylon, polyvinylidene chloride et polyvinyl chloride etaient
surtout utilises mais, depuis 1959 on a introduit le polyethylene et
le polyester puis, en 1962, le polypropylene. Les exportations de
filets en fibres synthdtiques vers plus de 100 pays, sont passees de
180 tonnes en 1955 a 32.000 tonnes en 1961. En 1961, 41 pour cent
des filets en fibres synthdtiques etaient fabriquds en nylon et vinylon,
six pour cent en polyethylene, cinq pour cent en polyvinylidene
deux pour cent en polyvinyl chloride, un pour cent en polyester et
quatre pour cent en fibres mixtes. Quant aux filins et cordes, 53
pour cent dtaicnt faits de vinylon et 23 pour cent de polyethylene.
Presque toutes les cordes en fibres mixtes sont exportdes.
by
Japan Chemical
Fibres Association
Redes y cabos de pesca de fibras sinteticas fabricados en Japon
Extracto
La producci6n japonesa de fibras sintdticas para cabos y redes de
pesca se estima en 45 millones de libras en 1962, de las que 32
millones de emplearon para fabricar redes. En 1959 se emplearon
con el mismo objeto 21 millones de libras. For otro lado, la pro-
duccion de redes de pesca de fibras naturales (algoddn, caflamo,
etc.) disminuy6 de unos 28 millones de libras en 1949 (afto en que
comenzo la fabricaci6n de fibras sinteticas), a unos tres millones de
libras en 1962. Actualmente, el 85 por ciento de las redes de pesca
japonesas se hacen de fibras sintdticas. Inicialmente, las que mas
se usaben eran de nylon, vinylon, cloruro de polivinilideno y
cloruro de polinivinilo, pero a partir de 1959 empczaron a usarse
polietilenos y poliesteres y desde 1962 polipropileno. La exportacibn
de redes de pesca de fibras sintdticas ha aumentado desde 400.000
libras en 1955 a unos 70 millones de libras en 1961, que se han
enviado a mas de 100 naciones. En 1961 se fabricaron de nylon y
vinilon el 41 por ciento de las redes de fibra& sintdticas, de polietileno
el seis por ciento, de polivinilideno el cinco por ciento, de cloruro de
polivinilo dos por ciento, de poliesteres el una por ciento y de fibras
mixtas el cuatro por ciento. En cabos y cuerdas el vinilon representa
el 53 por ciento y el polietileno el 23 por ciento. Se exporta casi toda
la produccidn de cabos de fibras mixtas.
THE production of synthetic fibre fishing nets and ropes
increased annually since its commencement in* 1949,
and 10 years later, in 1959, the output totalled some
25,000,000 Ib of which 21,000,000 Ib were used for
manufacturing fishing nets. The production is estimated
to increase further in 1962 to some 45,000,000 Ib of
which 32,000,0001 bare expected to be used for fishing nets.
The 1 949 output of natural fibre fishing nets, lines and
ropes — cotton, hemp, and silk products — totalled some
216,000,000 Ib of which 28,000,000 Ib were used in
continued from page 63
For trawls the material has certain obvious advantages
such as hairiness and relatively large knot size so that
more fish may be retained at a given mesh size than in
equivalent continuous multifilament 'Ulstron' trawls.
Conclusion
Nylon, with its outstanding advantage of high strength,
was introduced to the fishing industry just after World
War II. Polyester with its better resistance to stretching,
was added to the fishermen's resources some five years
later and polythene followed at a similar interval having
the advantage of being lighter than both.
With the introduction of 'Ulstron' polypropylene,
the fishing industry now has a synthetic fibre which to a
large measure combines the outstanding properties of all
these three groups which have formerly been used to meet
the exacting requirements of the fisherman.
64
Appendix: Trademarks
Ulstron', Terylene', 'Disperse!' and 'Duranol' are registered
trademarks, the property of Imperial Chemical Industries Limited.
'Supracet' is a registered trademark, the property of L. B.
Holliday and Co. Ltd., Huddersfield, England.
'Foron' is a registered trademark, the property of Sandoz AG.
Lichtstrasse 35, Basle, Switzerland.
'Rcsolin* is a registered trademark, the property of Farben-
fabriken Bayer Aktiengesellschaft, Leverkusen, Federal Republic
of Germany.
'SerisoF is a registered trademark, the property of Yorkshire
Dyewares and Chemical Co., Kirkstall Road, Leeds, 3, England.
'Cibacet' is a registered trademark, the property of Ciba AG.
Klybeckstrasse 141, Basle, Switzerland.
'Systran' is a registered trademark, the property of Wrights
Ropes Ltd., Universe Works, Birmingham 9, England.
'Vinamul' is a registered trademark, the property of Vinyl Pro-
ducts Ltd., Carshalton, Sumy, England.
'CuprinoF is a registered trademark, the property of Cuprinol
Ltd., 7 Upper Belgrave Street, London, S.W.I., England.
fishing nets. The output dropped to 94,000,000 Ib in
1962, only 3,000,000 Ib were used for fishing net manu-
facture.
At present nearly 85 per cent of the total fisheries need
is met by synthetic fibre nets.
In early days, synthetic fibre nets and ropes were
made principally of nylon, vinylon, polyvinylidene
chloride and polyvinyl chloride fibres. In 1959, however,
polyethylene and polyester fibres were added and, in
1962, polypropylene fibre joined the ranks of materials
for manufacturing fishing nets. The use of combinations
of fibres began in 1955.
While the demand for fishing nets is increasing, prin-
cipally for export, demands for ropes and the replace-
ment of manila ropes with those of synthetic fibres have
also risen steadily.
In 1955, exports of synthetic fibre fishing nets totalled
400,000 Ib but by 1961 the amount jumped to some
70,000,000 Ib which were shipped to more than 100
nations.
Production
Fluctuations in production of synthetic fibre fishing
nets and ropes are shown in Table I.
TABLE I — Production of synthetic fibre nets and ropes
In t,000lh
Nylon
Vinylon
Polyvinylidene
Chloride
Polyvinyl Chloride. .
Polyester
Polyethylene
Twisted blended . .
Total
(Reference) . .
Natural Fibre
Net
Rope
Total
Net
Rope
Total
Net
Rope
Total
Net
Rope
Total
Net
Rope
Total
Net
Rope
Total
Net
Rope
Total
Net
Rope
Total
Net
Rope
Total
1955
1960
3,535
9,293
101
651
3,636
9,944
3,890
11,953
337
3,179
4,227
J5J32
1,148
1,415
128
no
1,276
1,525
—
911
_ .
332
—
1,243
42
42
8,615
566
9,181
24,781
66,935
91,716
1,399
1,399
24,971
4,272
29,243
10,182
87,259
97,441
1961
12,749
1,241
13,990
12,679
5,342
18,021
1,393
194
1,587
574
645
1,219
238
354
592
1,716
2,363
4,079
1,333
7
1,340
30,682
10,146
40,828
4,413
98,987
103,400
A review for 1961 reveals that, for fishing nets, nylon
and vinylon each accounted for 41 per cent; polyethylene
fibre for six per cent; polyvinylidene fibre for five per
cent; polyvinyl chloride fibre for two per cent; polyester
fibre for one per cent; and blended fibres accounted for
four per cent. It is evident that the majority of fishing
nets is made of nylon and vinylon. In ropes and lines
(longlines and those for land use included), vinylon
accounts for 53 per cent; polyethylene fibre for 23 per
cent; nylon for 12 per cent; polyvinyl chloride fibre tor
six per cent; polyester fibre for four per cent; polyvinyli-
dene fibre for two per cent, showing the importance of
vinylon, nylon and polyethylene in this field.
Combinations of fibres are comprised principally
of nylon and vinylon, nylon and polyvinylidene, and
nylon and polyvinyl chloride fibres. Almost the entire
output of ropes of blended fibres is exported.
Production of synthetic fibre fishing nets is expected to
expand steadily in future, as in the past, but even greater
increase of rope production is expected. The output of
natural fibre ropes in 1962 has shown a decrease of some
7,000,000 Ib from the preceding year, but the synthetic
fibre rope output has increased some 1,000,000 Ib, thus
showing a progress in replacing manila ropes with syn-
thetic fibre ropes.
The principal uses of fishing nets made of different
synthetic fibres are shown in Table II.
TABLE II— Principal uses of synthetic fibre
Uses
Nylon: Gillnet, round haul net, longlines, various ropes
Vinylon: Round haul net, setnet, gillnet, longlines, trawlnet and
other small dragnets, various ropes
Polyvinylidene Chloride: Setnet, trawlnet and other small dragnets,
various ropes
Polyvinyl Chloride: Setnet, trawlnet and other small dragnets,
various ropes
Polyester: Longlines, various ropes
Polyethylene: Various ropes, gillnet, trawlnet and other small
dragnets
Polypropylene: Gillnet, various ropes.
Exports
Expansion of export of synthetic fibre products is
notable — see Table 111. Nylon accounted for 60 per
cent of the total, while vinylon accounted for 21 per
cent and blended fibres for 12 per cent. These products
are shipped to more than 100 nations, with South-east
Asia, Africa and North America as the principal markets.
Others importing Japanese fishing nets and ropes in
large quantities include Thailand, Pakistan, Malaya,
the United States, Canada and Iceland.
TABLE 111 — Exports of synthetic fibre nets and ropes
In 1,000 Ib
Nylon
Vinylon
Polyvinylidene Chloride
Polyvinyl Chloride . .
Polyester Fibres
Polyethylene Fibres . .
Twisted blended yarn . .
Total
New synthetic fibres
New synthetic fibres used for fishing nets include
polyethylene and polyester fibres, which were adopted in
1959, and polypropylene fibres first used in 1962. Among
the three types of fibres, polyethylene is used in greatest
quantity, while polyester and polypropylene fibres are
still in an experimental stage. Polyethylene and polypro-
pylene fibres are characterised by high tenacity and low
specific gravity, while polyester fibre is outstanding for its
high resistance against wear and tear.
continued on page 66
65
1955
264
77
42
383
1960
3,901
1,617
29
238
26
2
1,393
7,206
1961
6,191
2,233
141
387
64
266
1,327
10,609
Production and Characteristics of Synthetic Nets
and Ropes in Japan
Afcttrtct
The paper reviews the production and consumption of synthetic
materials in Japan and tries to clarify some of the technological
problems regarding their rational use. The total amount of webbing
and ropes presently used in Japan is about 87,000 tons. Webbings:
27,000 tons of which 90 per cent synthetic. The conversion has been
more complete for netting than for ropes as only about 19 per cent
of the ropes hi use are of synthetic fibres. Summing up, the paper
gives the characteristics of the various synthetic materials, supported
by numerous tables. The main requirements for various fishing gears
are then discussed. For fixed trap nets twines are required which have
a high initial weight in water and smooth surface texture to give
a minimum resistance to the current ; tarred polyvinyl alcohol is
sometimes preferred. For purse seines which also need a high
sinking speed, twines of at least two g/den loop strength are required,
and tarred nylon is mainly used. For the straight gilling types of
gillnets, such as sardine nets, the Japanese fishermen prefer poly-
vinyl alcohol yarn. For other gillnets the polyamide and polyester
groups are generally used. For bottom trawls nylon and vinylon
normally have been used but polyethylene and polypropylene are
gaining ground. However, due to the cost of synthetics, the extensive
wear and occasional loss of such nets, the Japanese fishermen often
use natural fibres when towing on a rough bottom especially for
the underparts of trawls. One of the reasons why man! la rope is
largely preferred by the fishermen is its low elongation (12 to
20 per cent) as compared with 25 to 50 per cent of synthetic fibre
ropes. Recent production of ropes made of mixed materials
and subsequent treatment with resin has improved the handling
of polyethylene ropes but further improvements to rope-making
techniques are required before synthetic material ropes may
be fully exploited in fisheries. Monofilament gillnets are said
to have a 1*2 to 2-3 times higher catch ratio than other types of
twine; nevertheless, these nets present difficulties in bulkiness and
knot-fastness, and further research and development is required.
Production et caracteristkjues des cordes et filets synthftiques au
La communication traite de la production et de la consommation de
materiaiix synthetiques au Japon et essaie d'6claircir certains
problemes technologiques concernant leur utilisation rationclle.
Sur 87,000 tonnes de filets et cordes utilises au Japon, environ 90
pour cent sont de matdriaux synthetiques. La conversion a et£
plus complete pour les filets que pour les cordes dont 19 pour cent
seulement sont en fibres synthetiques. La communication contient
eaalemcnt de nombreux tableaux dormant les caracteristiques des
diffcrcnts materiaux synthetiques utilises ainsi que les principals
conditions requises par divers engins de peche. Pour les trappes
fixes, les fits doivent fitre lourds et avoir une surface lisse afin
d'offrir le minimum de resistance au courant; le polyvinyl alcohol
goudronne* etait pr^ferd jusqu'ici. Pour les sennes coulissantes, une
grande vitesse de coulee et des fils ayant au moins deux gr/d de
resistance en mailles sont n&essaires et le nylon goudronne est le
plus utilised Pour les filets maillants tels que les sardinicres, les
pecheurs Japonais preferent les fils de polyvinyl alcohol; pour les
autres filets, ils emploient gdneralement le polyamide et le polyester.
Pour les chaluts de fond, le nylon a et6 normalement utilise mais
le polyethylene et le polypropylene gagnent du terrain. Cependant,
by
Yoshinori Shimozaki
Tokai Regional Fisheries Research
Laboratory, Tokyo
etant donn6 le prix 6leve des fibres synthetiques, Fusure excessive
et les pertes occasionnelles de ces filets, les pgcheurs Japonais
utilisent souvent des fibres naturelles, particulierement pour le
dessus des chaluts. 11s preferent aussi les cordes de manille dont
reiongation (12 a 20 pour cent) est tres faible compared a celle
des cordes synthetiques (25 a 50 pour cent). Une recente production
de cordes faites de materiaux mixtes puis traitees avec de la restne, a
ameiiore la manipulation des cordes de polyethylene, mais d'autres
ameliorations dans les techniques de fabrication des cordes seront
necessaires avant que la peche puisse exploiter compietement les
cordes faites de fibres synthetiques. Les filets maillants en mono-
filaments sont senses avoir un coefficient de capture superieur de
1-2 a 2-3 a celui des autres fils, mais ils sont trop volumineux,
leurs noeuds manquent de stability et leur developpement requiert
de plus amples recherches.
Produccion y caracteristicas de redes y cables sinttticos fabricados
en el Japdn
Extracto
Resefia el autor la producci6n y consumo de materiales sinteticos
en el Japon y aclara algunos de los problemas tecno!6gicos que
plantea su uso racional. Actualmente en el pais se emplean unas
87,000 tons de paftos y cables, el 90 por ciento de los cuales es de
materiales sinteticos. La transformaci6n ha sido mas completa en
el caso de las redes que en el de los cables pues solamente el 19
por ciento de los que se emplean actualmente es de fibras artificiales.
Menciona el autor las caracteristicas de varios materiales sinteticos
que las da en numerosas tablas. Discute las necesidades principles
de los di versos equipos de pesca. Para las trampas fijas se necesitan
hilos de mucho peso inicial en el agua y de superficies suaves para
reducir la resistencia a la corriente; hasta ahora se ha mostrado
predilecci6n por el alcohol de polivinilo alquitranado. En el caso
de las redes de cerco, que se tienen que hundir rapidarnente, se
necesitan hilos de, por lo menos, dos g/den y se usa principalmente
nylon alquitranado. Para las redes que son principalmente agall-
eras, como los sardinales, los Pescadores Japoneses prefieren hilos de
alcohol de polivinilo; para otras redes de enmalle se usan general-
mente poliamidos y poliesteres. Para los artes de arrastre de fondo
se hap usado normalmente redes de nylon, pero las de polietilcno
y polipropileno ganan terreno. Sin embargo, dcbido al costo de las
redes sinteticas, su gran desgaste y perdida ocasional, los Pescadores
Japoneses emplean con frecuencia fibras naturales, especialmente
en las secciones inferiores de los artes de arrastre. Una de las razones
continued from page 65
Nearly 60 per cent of polyethylene output is used for
manufacturing ropes, including floatline, buoyropes,
seine net ropes, anchor ropes and guyropes. For ship
use, towlines, hawsers, guyropes, lifelines and flaglines
are made of these fibres. Fishing nets made of these
fibres include trawlncts and other small drag-nets, fixed
nets and round haul nets. Polyester fibre is used predomi-
66
nantly for manufacturing trawlnet ropes and longlines.
Polypropylene fibre manufacturing was limited last
year to trial products, and is now being used in crab
gillnets, round haul nets and ropes. Improvements in its
weathering resistance, dyeability and abrasion resistance
are likely to make it popular for manufacturing various
types of fishing nets.
de que muestrcn tanta preddecci6n por el abaca de Manila es su
poco estiramicnto (de 12 a 20 por cicnto) con respccto al de las
fibras sintetteas (de 25 a 50 por ciento). La fabricaci6n reciente de
cables de matenales mixtos y tratamiento posterior con resina
ha mejorado la manipulaci6n de los de polietileno, pero se tcndran
que introducir otras mejoras en las tecnicas de fabricaci6n antes
de que los cables sintdticos puedan aprovecharse completamente
en la pesca. Se afirma que los artes de enmalle de monofilamentos
pescan de 1-2 a 2-3 veces mis que las de otras hilos, a pesar de lo
cual su adopcion no es universal porque son voluminosas y estan
expuestas a que se corran los nudos, defectos que sc tendiin que
corregir mcdiante ulteriores investigaciones y perfeccionamientos.
THE total amount of netting and rope presently
used in Japan is about 87,000 tons, an increase of
15,000 tons since 1940.
Of this tonnage an increasing volume is made from
synthetic fibres as is shown in the range of tables on
various aspects given in this article.
The rapid increase in local consumption is due to the
expansion of high-sea fisheries. The local utilisation of
synthetic fibre for fishing ropes is much less than for
fishing nets. Out of an estimated 57,000 tons of rope in
service in 1962, 10,688 tons (only 19 per cent) consisted
of synthetic rope including the polyvinyl alcohol fibre
used for longline and seine warps. The much slower
conversion is due mainly to the low cost of manila which
is about one-quarter the price of synthetic fibre.
Much care has been taken in assessing the best
material for each particular fishery.
Fixed nets. Knotless nets and netting made of con-
tinuous heavy filament yarn are recommended. Resistance
to sunlight should be high. Netting twines of polyvinyli-
dene chloride and vinyl chloride groups have been
mainly used. Polyvinyl alcohol has poor weather resis-
tance and a low specific gravity but, to compensate,
coal tar is often applied to those parts of the net receiving
the main load or to the whole net when set in an area
where the sea is generally rough. Nylon, though strong
enough, is not very satisfactory as regards specific
gravity and sunlight resistance and is furthermore too
expensive.
The polyethylene group has low specific gravity (0*96)
but is strong and not costly, and in black-coloured
twines is adopted to make the upper parts of set nets.
The black-coloured fibre stands up well to sunlight.
Purse seines. Optimal physical properties of webbing
twines for purse seines : over2*0g/denin wet loopstrength,
20 per cent in elongation, as well as high specific gravity.
Knotless or weaver's knot webbings of nylon, vinylon and
Tetoron' yarns are used in greater amounts for purse
seines than for set nets. For the landing part of purse
seines, however, fishermen prefer nylon or 'Tetoron' to
vinylon even if they use vinylon for most of the other
parts.
Sinking speed is important in purse seines and the
amount of lead used for a nylon net is 30 to 40 per cent
more than for a cotton net. The use of polyester group
for purse seines is developing though these materials
are rather costly. Tarring of the nylon and vinylon
groups has increased the weight and sinking velocity of
large-sized purse seines for skipjack, tunas, and mackerel,
but this old technique cannot increase the specific gravity
of a net.
Gillnets require invisibility in water, fineness and
pliability, elasticity and abrasive resistance. Polyamide
yarns are preferred for gillnets but some fishermen have
turned to polyester yarn for large types of gillnets.
Polyvinyl alcohol yarn, less fine, is mainly used for
sardines.
Trawlnets. Even when nylon, polyethylene or polypro-
pylene is used for other parts of a trawlnet, cottoiTor
Table I Cost ratio of fibre material to other materials used in fishing gear (1957)
tind of foar
flbro iiatorial
Othor Mtorialo
Ooot ratio
(ra * i°°)
00
Xton Aaount Ooot
(Motrio ton) (Ton)
Itoa Aaount Ooot
(luabor or (Ton)
Motrio ton)
rix»d not for
yollcwtail
1957
lottinf, 19*6 5t 660, 000
Ropooi/ 27.8 3t 520, 000
Othor fibro
wtoriolo 10.0 1,580,000
Bamboo 2,163
01*10 float* 441 300,000
Straw bag 17,000
Xgrootuff 480,000
93
ftro-boat trawl
not
(T5.35 ton, 25 hp)
Vottinf 0.191 185,000
Ropoo 2.428 268,000
Olaoo f lotto 100 10,000
Trawl board 2 50,000
Othoro 40,000
89
Two-boat puroo
•oino for tuna
and okipjaok
Wotting 8.894 12,800,000
Bop** 3.04
Othor
Mtorlalo 1.0 1,400,000
(ton)
Loado 1.6 1,490,000
float* 0.5 420,000
Puroo Bin** 0.08 120,000
88
I/ Wire rep* ie inoluAeA in the fibre wrterl»l ae thie Mjr be replaoed by fibre Material in future.
67
vinylon webbings arc used for under parts of the net be-
cause of their low cost. Fishermen have found it practical
through experience not to use costly net materials, which
are often lost when working rough grounds and this
explains why cotton nets are still used side by side with
polyamide, polyvinyl alcohol and polyethylene material.
Table II gives the following percentages of synthetic fibres used in
various fishing gears by 1962:—
fixed nets— large 95, medium 85, small 70.
seine nets— purse seines 100, nuikiri ami (lampara type) 80,
beach seine 70, others 90.
gillnets— for salmon 100, crab 95, sardine 90, herring 90,
others 95.
dipnet — for suary 75, others 80.
trawl— other 100, two-boat 100, medium-sized 50, small-sized
45, others 45.
longline— oceanic tuna 100, coastal tuna 95, others 70.
Longlines. Comparative experiments conducted in
regard to the tensile strength, elongation, and potential
energy left in disused longlines of cotton and synthetic
lines, indicate that vinylon line tested unknotted
showed better characteristics than cotton line, whereas
similar sizes of both lines when tested for loop strength
did not always give comparable results in regard to the
strength and potential energy. This implies that the loop
strength is probably the best criterion for safe operation
of longlines.
In the near future ropes mixed with polyethylene or
polypropylene are likely to be preferred for the mainline
of the longline gear, with improvement of the operation
system, though the vinylon group has until now sur-
passed the other groups of synthetic fibres for the con-
struction of tuna mainlines.
For the branchlines, polyester lines are preferred
because of their high catch efficiency and their favourable
characteristics of specific gravity, tenacity, and fineness.
Ropes. Cost of synthetic fibre ropes, over three times
higher than manila rope, is one of the drawbacks.
Another defect of synthetic fibre ropes in general is
excessive elongation. In contrast to 12 to 20 per cent of
manila ropes, synthetic fibre ropes are susceptible to as
much as 25 to 50 per cent. This can make the ropes
inefficient and sometimes even dangerous.
Webbings
Fibre
Sub Total
Table III — Fishing Gear synthetic fibres produced and exported
Output in metric tons
1955 1957 1961 1962 1955
Export in metric tons
1957 1961
Polyvinyl alcohol
Polyamide
Polyvinylidene chloric]
Polyvinyl chloride
Mixtures*
Polyester
Polyethylene . .
Polypropylene
le
1,768
1,607
572
19
3,368
2,783
631
423
695
5,763
5,792
633
261
606
108
780
5,505
6,592
461
130
797
142
719
23
26
74
7
320
1,286
5
19
652
495 !
2,466 4,1
38
10
315 t
462
9,366
7,900
13,943
14,369
107
2,282
3,786
1962
502
11
31
72
619
45
5,422
Ropesf
Polyvinyl alcohol
Polyamide
Polyvinylidene chloric!
Polyvinyl chloride
Mixtures
Polyester
Polyethylene ..
Polypropylene. .
e
152
46
58
690
103
109
64
5
2,428
564
88
293
3
161
1,074
Sub Total
256
971
4,611
3,320
885
194
365
29
55
1,165
15
6,028
12
4
260
235
5
—
75
105
—
2
4
—
—
—
no
150
5
—
11
~-
—
—
8
"~~ ~
120
99
17
11
569
608
Twines
Polyvinyl alcohol
Polyamide
Polyvinylidene chlorid
Polyvinyl chloride
Mixtures
Polyester
Polyethylene ..
Polypropylene
e
128
84
68
13
201
330
14
58
20
872
1,372
52
90
99
554
963
1,491
66
135
154
620
6
101
53
58
94
187
2
23
19
328
612
5
16
85
140
661
686
2
54
120
4
141
Sub Total .. .. 293
623
3,039
3,275
212
425
1,186
1,668
TOTAL .. .. 4,515
9,474
21,643
23,672
336
2,725
5,541
7,698
* Ropes made of mixed yarns of polyamide and polyvinylidcne chloride or of polyvinyl chloride and polyamide.
t Includes materials for longline fisheries throughout.
68
(Metric tons)
Fixed net
Vinylon
Vinylidcne chloride
Vinyl chloride . .
Polyethylene
Sub-total
Sub-total
Table IV— Local consumption of synthetic fibres for fishing gear
I9*l 1962 (Metric tons)
Trawls and other*
732 651
490 343
89 2
1,311
2,168
996
Purse seine
Nylon
Vinylon
Vinylidene chloride
Vinyl chloride
903
1,709
61
20
538
1,866
34
Combined
4
34
Sub-total
2,697
2,472
Gillnet
Nylon . . .
Vinylon . . .
1,816
342
1,077
304
Vinyl chloride
10
Polyester
77
Polypropylene
—
18
1,476
Nylon . .
Vinylon ..
Vinylidene chlon
Vinyl chloride
Polyester
Polyethylene
Polypropylene
Combined
de
Sub-total
1961
301
2,100
61
79
88
707
3,338
1962
180
1,519
51
2
19
534
5
23
"£333
Ropes
Nylon ... 525
780
Vinylon . .
2,293
3,085
Vinylidene chlorid
e
45
194
Vinyl chloride .
180
215
Polyester
152
47
Polyethylene
K076
1,066
Polypropylene .
—
15
Combined
—
18
Sub-total 4,271
5,420
Total of netting and ropes . . 12,785
12,697
All nets include twinejjsed for constructing or mending.
Table V Tensile strength and elongation of various kinds of twine for comparable diameters
Kind of twin* fibre
Construction
of
twins
Ns
Spsoifio
gravity
Disustsr
dry
(-)
Straight (wst) Trawlar Knot (wst)
Psnsils strsngth
Ilonga-
tion
00
Irsaking Strsngth
•longsj
IB
(Kg) (g/d)
(Kg) (g/d)
Polys thy Isns A Continuous
400d/F 9x3
10,600
0.96
1.84
52.2
4.83
25
65.1
3*01
19
Polys thy Isns B Continuous
400d/F 9x3
10,800
0.96
1.83
49.4
4-57
17
70.5
3.2J
15
Polys thy Isns C Continuous
350d/F 10x3
10,500
0.96
1.80
49.7
4.73
15
40.4
1.92
13
By Ion 6 Continuous
210d/15F 20x3
12,600
1.14
1.80
63.8
5.06
36
75.6
3.00
32
Polys star Continuous
2?0d 20x3
15,000
1.38
1.82
65.6
4.38
15
74.2
2.47
13
Polyvinyl ohlorids
Continuous
300d/60F 20x3
15,300
1-39
1.83
35.2
2.30
26
44.0
1.44
20
Polyvinylidsns ohlorids
Continuous
360d/F 18x3
19,440
1.70
1.80
38.8
2.00
28
45.9
1.18
23
Polyvinyl alcohol
Stapls
20«s 15x3
11,700
1.26
1.79
36.4
3.13
32
42.8
1.73
26
Cotton
20's 16x3
12,480
1.52
1.82
28.8
2.31
33
47.5
1.86
28
Manila
1*45
1.90
39*0
13
52.5
12
Polysthylsns A, B, and C diffsr in basic natarial as wsll as in aanufaoturing proosssss, aooording to
producers.
ThS s amp Is spsoimsns wsrs givsn a medium twist and tsstsd undsr ths following conditions t
room tanpsraturas 20 to 23°C, Isngth of apsoinsm 60 om, pulling vslooityt 0.22 oa psr ssoond
Table VI — Number of major fishing gears by kind and size in Japan,
1960
Kind of gear Small- Medium- Large-
Fixed net. .
Purse seine
Trawlnet
Minor drag nets
Gillnet . .
Longline
Small-type nets are
type
15,789
2,047
3,379
13,186
49,692
30,953
type
1,398
666
1,911
226
2,738
2,287
type
154
255
806
230
1,412
in fixed nets, those operated by under 10
fishermen, and in other gears, those operated with non-powered
boats or less than 10 GT.
Medium-type nets are, in fixed nets, those manned by 10 to 50
men, in other gears, those operated with boats from 10 to 50 GT.
Vinylon ropes are used greatly for warplines of Danish
seine type of trawls seems because of their durability
which is three to four times greater than that of manila
ropes. Recently, a vinylon rope combined with wire
came into existence for the warpline. Fishermen who
employed this combination rope reported that the new
warpline enabled them to operate more efficiently.
Polyethylene ropes are particularly suitable for
the mainline of bottom longlines for cod and cod-
like fish and for floatlines of salmon gillnets. Such
ropes are very often twisted together with spun
69
Table 711 Some properties of tuna longline
VMM ConatruotiQH of linti ftiMMttr
9»n«il« *tr«nfta
frf) (w.t)
•loafttiofl
•(« <*•*:
Main *••
OMMM 80*8 50 x 3 x 3 *»00
265
46
feinUm BnrnofcliM
0mm** 20*8 J5 x 3 * 3 6.10
290
47
*
tfttwft Io.1l(lOOd/> x 10 x 11 + 20*8 11 z t)*J
6*00
315
59
N
Mftn**n JTo.12(lOOd/r x 10 x 12 * 20*8 12 « l)x3
6.40
345
48
M'
foU»*tqrlvM 300d/F x 9 x 33 6.09
319
47
M
Vjlen 210 d/1?F * 36 x 33 4.55
355
65
Brftnohlin*
fetoron 250 d/2411 * 35 * 33 4*50
239
34
w
M*n**n Voe 9
(100 d/7 x 10 x 9 4 20f8 9 x 1) x 3 5*3
250
53
tf
Ihipxp Ho. 10
(100 d/f x 10 x 10 + 20*8 10 x 1) 5.7
285
52
N
im oonstruotvd with iOOd of Tiny Ion •onofilMMnt* whoro •vary 10 yarn* !• ooToxod with Tingrlon
•pan yarn. Tttoron ia a ooMtroial naM of polyaatar produoad in Japan.
Table VIII Result of creep test and abrasion teat of twinee
Material
Relation between time (hour) and elongation (urn) when 2 fi/d of load is Frictions against Frictions
given until creep occurs in twine steel ed^reJJ against oil
stone 2J
Beginning 0.5 1 6 25 50 46 102 152 318 600 (hours.
load 10 (kg)
load 5 (kg)
load 0.5 (kg)
Polyethylene A
10.8 16.6 17.8 22.6 23*6 Creeping
16
22
17
85
101
93
0.71
11 B
9.2 11.4 13.2 16.6 18.2 20.6 Creeping
9
*
*
82
78
81
0.84
11 C
12.0 15.4 16.8 18.2 18.6 19.2 24,0 Creeping
iy
20
22
142
156
174
0.89
Nylon
Italia
Cotton
20.0 21.7 21.7 21.7 21.7 21.7 22.3 22.3 Cree-
ping
3.0 3-3 4-0 4.0 4.3 4-7 Creeping
7.0 7-3 7-3 7-3 7-3 Creeping
24
231
0.70
} broken
within 1,000
tines of
friction Jj
JJ Angle of the steel edge is 90 . _2J fear strength against oil stone is represented by the ratio between the tensile
strength after abrasing twine 5tOOO tines and the original strength. 3 1 No comparable data available for these materials
as they were broken before abrasing 1,000 tines.
yarns of either 'Teviron' or vinylon in order to prevent
slippage that may occur in handling them. The mixing
rate of Teviron' or vinylon with polyethylene twines is
from 10 to 30 per cent, depending upon the producers'
or fishermen's requirements. Laboratory experiments
indicated that the abrasion coefficient of polyethylene
twines against oil stone was 0*18 to 0*20 as compared
with 0*3 to 0*34 for spun yarn twines of Teviron' and
vinylon, the former being more slippery than any other
kind of synthetic twine.
Twisting together with other materials and subsequent
70
treatments with resin has much improved the handling
of polyethylene ropes.
Rope-processing techniques will require further im-
provement before synthetic fibre material ropes can be
adopted for fishing rope to a greater extent.
Table IX gives abrasion coefficients for continuous filament netting
twines by oil stone friction test.
PolyethyteneO-20, 'Amilan* (all nylon), 0«25 'Tevi-Ny' (partly nylon)
0-27, 'Kyokmin' (partly nylon) 0-28, 'Tcviron' 0*28, 'Saran' 0-30,
'Kurehalon' 0-31.
Spun yarn: 'Kremona' 0-24, cotton 0*37 manila 0-41.
Japanese Fish Netting of Synthetic Fibres
Abstract
The total production of fishing nets in Japan for 1961 was approxi-
mately 9,000 tons; half of this was nylon. About 5,500 tons were for
domestic use while about 3,500 tons were exported. Research on
polypropylene fishing twines was recently undertaken and in 1962
several firms started industrial production. Tests on the polypro-
pylene fibres produced in Japan showed that it is stronger than
polyamide 6 for the same denier and has a higher degree of tough-
ness. It is, however, stiffer and more bulky which, at present, seems
a disadvantage in its use as gillnet material. However, development
continues and indications are that this fibre should produce equal
if not superior quality fishing twines. The heat treatment used on
synthetic yarns is aimed at reducing elasticity and increasing
strength; such treatment applied to twines is aimed at stabilising
the twist, while the heat treatment of netting is directed toward
heat-setting the knots. Most heat-treatment methods can be classi-
fied under either the dry or the wet systems, although some methods
use both. Several methods have been evolved using electricity,
chemical baths and even high-frequency. The latest innovation is
based on heat radiation and makes use of ultra-red lamps. The
boiling water treatment of the early days has long since been super-
seded by the use of steam or other heated vapours. Liquid treatment,
however, has the advantage that it avoids all risk of overheating
as the temperature of the bath can be fully controlled. A major
point in all heat treatment is that the shrinkage which accompanies
all such treatments must be within the range of the specific elonga-
tion of the material.
Filets de pfcche Japonais, en fibres synthftiques
Resume
La production totalc de filets au Japon pour 1961 etait approxima-
tivement de 9.000 tonnes dont la moitte gtaient de nylon. Environ
5.500 tonnes gtaient destinies a ('utilisation domestique et 3.500
tonnes, a ('exportation. Des 6tudes surles fils de polypropylene ont
6te entreprises recemrnent, et en 1962, plusieurs firmes en commen-
Caient la production industrielle. Les epreuves des propri6tes du
polypropylene produit au Japon ont montr£ que sa resistance a la
rupture etait plus grande que celle du polyamide 6 pour un mdme
denier et que le polypropylene 6tait aussi plus tenace. Le polypro-
pylene par contre est plus raide et plus encombrant ce qui, a
present, constitue un desavantagc pour son emploi comme matdriau
de filets mai Hants. Cependant le developpement continue et il
semble que cette fibre pourra produire des filsdequalitesuperieure.
Le but du traitement par la chaleur utilise pour les fils synthetiques
est de reduire l'£lasticit£ et accroitre la resistance a la rupture. Un
meme traitement applique aux fils retordus permettra de stabiliser
le retordage tandis que ce traitement applique aux filets fixera
les noeuds. Les diflferentes m&hodes de traitement peuvent
6tre classees dans deux groupes: le systeme "sec" ou le systeme
"mouilte" bien que certaines methodes utilisent les deux
systdmes. Plusieurs m&hodes utilisent I'electricitS, les bains
chimiques et meme du courant a haute frequence. La derniere
innovation est basee sur 1'irradiation de la chaleur a partir de lampes
a rayons ultra-rouges. L'eau bouillante des premiers temps a 6t6
remplacee par la vapeur chauffee.
Redes de pesca Japonesas de fibras sinteticas
Extracto
En Jap6n en 1961 se fabricaron cerca de 9.000 toneladas de redes
de pesca; la mitad eran de nylon. Unas 5.500 tons las absorbi6 el
mercado interne y unas 3.500 tons se exportaron. Recientemente se
inici6 la investigaci6n de los nibs de polipropileno para redes de
pesca y en 1962 varias empresas iniciaron la producci6n industnal.
Los ensayos de las fibras de polipropileno fabricadas en Japon
demuestran que a igualdad de denier son mas robustas y duraderas
que las de poliamido 6, pero tambi&i son ma> rigidas y voluminosas,
lo que actualmente parecc ser un inconveniente para su empleo en
los artes de enmalle. Continuan los estudios y las indications son
que con esta fibra se obtendran hilos iguales o superiores. El trata-
miento tdrmico reduce la elasticidad e incrementa la robustez de las
hilazas sinteticas : cstabiliza la torsidn de los hilos y fija los nudos de
las redes. Casi todos los tratamientos termicos pueden clasificane
como secos o hiimedos, aunque en algunos cases se emptean ambos.
Se recurre tambitn a la electricadad, baftos quimicos e incluso la
by
Iwao Tani
Japan Synthetic Fibre Net and
Rope Association
aha frecuencia. La mas reciente innovacidn se basa en la termo-
radiaci6n de las lamparas de rayos ultra-rojos. Los baftos en agua
hirviendo de los primeros dias han sido sustitufdos por el empleo de
vapores de agua u otros supercalentados. El tratamiento liquido tiene
la ventaja de que evita todo peligro de sobrccalentamiento porqu la
temperature del bafto puede regularse exactamente. Un factor de la
mayor importancia en todos los tratamientos termicos es que el
lincogimiento que los acompafta tiene que quedar dentro do los
emites del estiramiento especifico del material.
Q YNTHETIC fibres used in the manufacture of fishing
>3 nets in Japan are:
Group Type Some brand names
Polyamide Nylon 6 * Amilan* and 'Grilon'
Polyvinyl alcohol Vinylon 'Manryo', 'KuralonVKrcmona'
'Mewlon'
Polyvinylidene Vinylidene 'Saran', 'Kurehalon'
Polyvinyl chloride Teviron*, 'Envilon'
Polyester Tetoron'
Polyethylene *Hi-Zex', 4Pylen-E', 'Ethylon'
Polypropylene 'Pylene'
These fishing twines are each made up of one-fibre
material. Twines made up of a combination of yarns or
fibres of different fibre materials are also produced,
mostly with nylon as the basic material, although twines
are available with vinylon as the basic material with
other fibres twisted in.
Total production of fishing nets in Japan for 1961 was
approximately 9,000 tons; half being nylon. About
3,500 tons were exported, the balance being for domestic
use.
The numbering systems presently used in Japan are
not officially established and are for business dealings
only.
The quality standards of Japanese twines and nets for
use within the country are covered by the Japanese
Industrial Standards (J1S) which has been set up by the
Ministry of International Trade and Industry, These
standards comprise the following:
JIS L 1033-58 Testing method for vinylon spun plied
yarn for fishing net
JIS L 1034-58 Testing method for filament nylon
plied yarn for fishing net
JIS L 1035-58 Testing method for filament vinyli-
dene chloride yarn and filament
vinylchloride plied yarn for fishing
net
71
JIS L 1043-58 Testing method for synthetic fibre for
fishing net
The JIS further comprises inspection standards for the
use of net manufacturers as quality norms for their
product intended for export.
Polypropylene
Polypropylene fibres industrial production only started in
1962. There is therefore very little experience of it as a
netting twine.
It is presently produced by two different techniques—-
that introduced by Montecatini of Italy, and by the
Abisun Corporation of U.S.A. Eight firms have started
manufacture with a total production capacity of 38 tons
per day.
The technique used in spinning, dyeing and general
manufacture of polypropylene filament differs from one
manufacturer to another.
TABLE I — Average characteristics of Japanese polypropylene
Polypropylene fila- Polypropy-
ment I80d (20-24 f) lene spun
No. 20S
Specific gravity
Tenacity (g/den) ..
Extensibility, straight ( °/0). .
Loop strength (g/den)
Decrease of strength caused
by knotting (%)..
Wet breaking strength
(kg/mm2)
Moisture absorption ( %) . .
Dry
Wet
Dry
Wet
Dry
Wet
0-91
6 -5—7 -7
6 -5—7 -7
20-5
20-5
4 -8—5 -3
4 -8-- 5 -3
0-91
4 -3—4 -5
4 -3-^4 -5
16-0—16 -5
16 -0—16 -5
3 -4-3 -6
3 -4-3 -6
Straight
Loop
28-0
53—63
40-43
0-1
20-0
35—37
28-30
The study and research on polypropylene filament and
twines for netmaking is continuing and there are indica-
tions that, with more experience, twines will be produced
of equal if not superior qualities to other synthetic
fibres. The fibre has unique features and characteristics
which have already proved to a certain extent that it can
be used to advantage for the manufacture of twines and
ropes. There are, however, a few problems to be over-
come.
Heat treatment
Heat treatment of synthetic netting twines varies accord-
ing to the kind of fibre, twine count, type of knot and the
end-use intended. Some synthetic fibres do not require any
heat-setting.
Heat treatment is applied for three main reasons:
(a) Twist-setting of twine; (b) Setting of knots in netting;
(c) Stabilising the shape of meshes.
It is primarily aimed at reducing the extensibility and
at increasing the strength of twines although, at the
same time, it sets the twist in the twine. There are many
methods for heat-setting and a variety of devices are in
use. For certain fibres heat treatment is more difficult
72
than for others and only heat treatment of nylon
netting is considered here.
There seems no theoretical or scientific principle for
establishing an optimum heat treatment method. The
techniques in use have been developed and improved by
practical experience, while new methods and new devices
are continually introduced.
Early heat treatment processes were mainly intended to
obtain an "ironing" effect, and usually consisted of
heating and pressing at the same time.
The first 'Amilan' netting twines (type 100) were heat
treated by the "heated ironplate" system. The apparatus
consists of a chamber with a heated ironplate on top.
This heated chamber contains 'Anilene' or ethylene glycol,
and the webbing is run through it. 'Amilan' softens at
180"C, so the heat should be kept below this point.
This treatment is expensive and can only be used for
twines of 210d/6 and finer.
To heat treat coarser twines than 210d/6, a new type of
'Amilan', type 300, was developed. This type did not prove
successful for gillnet twines, being too extensible. The
next ' Amilan' yarn was of type 700 with a very low exten-
sibility and it proved excellent for all types of gillnets.
Heat-treatment methods are of two types— dry and
wet. Sometimes both methods are incorporated in one
type of heat treatment, so that the so-called dry system
also uses water steam. The dry system is mostly used by
net manufacturers while the wet system is mainly applied
to yarns and twines.
The method and the construction of the necessary
equipment vary according to production capacity, type
of nets and nature of heat used. Various methods, based
on heat conduction, use either electric regulators,
'Anilene' baths, melamin-resin, metal baths or high-
frequency.
A different principle is heat radiation by, for example,
the use of ultra-red lamps. Here the net is run over the
radiating elements or alternatively the element is passed
over the netting. In general this method has not proved
very effective.
One heat-treatment method used largely in early days
was to run the nets through boiling water, but today's
wet-treatment systems all make use of water steam or
other vapours.
Treatment with liquid, such as boiling water, has a
big advantage in that it avoids the risk of over-heating.
Nylon, which shrinks in boiling water, lends itself well to
this treatment for setting knots.
Twines usually require heat-setting to reduce their
extensibility and increase the strength, whereas heat
treatment is normally aimed at setting the knots in the
nets; in both cases, however, the heat should be applied
while the material is under tension. For nets, this is
normally obtained by running them between friction
rollers, which furthermore help to reduce irregularities
in the braiding and at the same time pre-set the knots
firmly. The tension to be applied should be chosen
continued on page 73
New Synthetic Herring Driftnets Used in the
North Sea
Abstract
In driftnctting the technique and nets differ from area to area and
even between the nationals operating them, due to the peculiar
traditional habits of the fishermen themselves. Experiments carried
out using a great variety of net constructions made of polyamidc
('Perlon'. 'Steelon', 'Dederon'), polyyinyl alcohol CKuralorT) and
cotton, showed that while polyamide is by far the strongest material,
polyvinyl alcohol lends itself better for herring driftnetting due to
less damage to the fish. Notwithstanding, it was observed that all
synthetic materials must be stiffened to avoid damage to the herring
while shaking out. Two years of subsequent commercial fishing
with *Kuralon' nets have shown that while their catch efficiency is
equivalent to that of cotton nets, they need only one-fifth as much
replacement for loss and damage. By the end of 1962, more than
2,000 'Kuralon' driftnets were in commercial use compared to
8,000 cotton driftnets showing that Polish fishermen are now turning
over to synthetic materials for the driftnet fishery.
Nouveaux materiaux synthetique* utilises dans la peche au hareng en
mer du nord
Resume
La technique de la peche au filet mi i I lam d iff ere solon Ic lieu et
merne la nationality des operateurs, difference due aux habitudes
traditionnelles particulieres des p£cheurs eux-memes. Des essais
effectues avec des filets de constructions tres variees, faits de poly-
amide rPerlon", "Steelon\ *Dederon'), de polyvinyl alcohol
('Kuralon') et de coton, ont montrc que bien quc le polyamide soit
le plus fort des materiaux le polyvinyl alcohol est meilleur pour
fabriqucr des filets maillants pour le hareng parcc qu'il endommage
moins les poissons. Cenendant on a pu observer que tous les
materiaux synthetiqucs doivent etre durcis pour qu'ils n'abiment
pas les harengs lorsqu'on secoue les filets. Deux annces de peche
by
Janusz Zaucha
Sea Fisheries Institute, Poland
commercial avec des filets maillants faits en 'Kuralon* ont montre
que leur efficaeite de capture est equivalente a celle des filets en
coton et ne doivent fitre remplaces pour perte ou d£t6rioration que
dans la proportion cTun cinquieme. A la fin de 1962, plus de 2000
filets maillants en 'Kuralon' etaient employes commercialement
com re 8000 en coton, ce qui demontre que les pecheurs polynesions
commsncent nuintenant a utiliscr les mat6riaux synth6tiques pour
la peche du hareng au filet maillant.
Nuevos materiales sintetlcos para las redes de deriva arenqueras
usadas en el mar del norte
Extracto
En la pesca a la deriva, la t£cnica ya las rejes varian de un pais a
otro incluso dentro del mismo pais dd*!j a las peculiar! dades
tradicionales de los Pescadores. Los expjrim^ntos realizados con
muchas redes de poliamidos ('Perlon, 'Steelon', *Dederon*),
alcohol de polivinilo (' Kuralon') y algoddn, demos traron que
continued from page 72
according to type of fibre and size of material, mesh size
and volume of netting.
Very often, a resin product is applied to set the knots
firmly.
Treatment of * Amilan9 nets
For heat-setting of salmon gillnetting, this procedure is
general :
(a) As the net comes from the braiding machine, it is
passed through a resin solution or polyvinyl
alcohol and is wound on a roller. During winding
the netting is pressed to prevent loosening of the
knots.
(b) The netting is then passed in a stretching machine
to tighten the knots. While under stress, the netting
passes through a vacuum pipe which enhances the
heat conductivity of the material.
(c) After stretching, the webbing is usually inspected
and, if necessary, mended.
(d) A steel bar is inserted in the meshes at both ends
of the section immersed in the boiling water, and
force is applied to the bars in accordance with the
amount of stretch required.
The maximum extensibility of 'Amilan' netting is 1 7 to
20 per cent and netting of 21 Od/ 12 should be stretched
while under treatment 13 to 14 per cent of its original
length.
Steam can be applied as heating agent but the material
should not come into contact with it. The webbing is run
through a steel-plate box while the steam passes through
the double wall of this box and heats the air in the
chamber, where the webbing is run.
Another method is to run the webbing, in firm contact,
over the heated surface of a hotplate. Tensionless heat
treatment in resin emulsion at a temperature of 90 to
100"C is still another method.
Dyeing
Nets are normally dyed after drying, and dyeing should
take place only after the net has been thoroughly washed
and all chemicals used removed. One recent method
applies the dye automatically, by spraying to the net while
it passes through a dye-beck. Such dyes are complex in
composition and are normally applied by a special pro-
cess so that the dye permeates and reaches the filaments
of individual twines.
After dyeing, a resin treatment is sometimes applied ;
either urea-resin or thermoplastic resin may be used;
the latter enhances the effect of dyeing. After such treat-
ment, the net is usually stretched and dried for about
five minutes at a temperature of ISO to 160 C.
73
aunque los poliamidos son, con mucho, los mis robustos, el
alcohol de polivinilo cs m6s a proposito para la pesca a la deriva del
arenquc porque peijudica menos al pescado. Tambien se observ6
que a todos los matcriales sintcticos hay que darles rigidez para
evitar que estropccn al arenquc cuando se sacuden. Dos aflos de
pcsca industrial con redes de 'Kuralon' ban demostrado que su
rendimknto de pcsca es equivatente al de las redes de algod6n, pero
que las averias y perdidas se reducen a la quinta partc. Para fines
de 1962, los Pescadores polacos empleaban 2.000 redes de deriva
de 'Kuralon' y 8.000 de algodon, lo que demuestra que adoptan
los materialcs sinteticos para dichas redes.
T71SHING techniques and gear differ from area to
r area even though they have been developed for
catching the same species offish. This is partly due to the
different types of vessels used by different nations but is
also due to the peculiar traditional habits of the fishermen
themselves. The same applies to the mechanisation and
modernisation of boats and gear for different communi-
ties, in spite of the fact that such communities operate
the same areas for the same species and are in close
contact with one another.
Herring driftnetting, operated for ages by Dutch, Bri-
tish, German and Polish fleets, are all carried out simul-
taneously for the same species and one could expect
that the gear would have developed one standard type.
But this is not so and investigation shows the basic
purpose is different. For most driftnets used it would
seem the main purpose has been to construct strong and
durable nets; with cotton nets numerous and costly
preservation treatments are necessary to prolong working
life and even with conversion to synthetic materials, this
purpose seems to have been maintained. In other words,
Dutch experiments with nylon herring driftnets seem
to be entirely directed towards producing more durable
gear.
The purpose behind the driftnets used by the Soviet
herring fleets is vastly different as the nets are made of
cheap and remarkably thin cotton yarn of only average
quality, with all lines made of hemp. Although the nets
are treated with a preservative they are designed to be
discarded after one fishing season. This, of course, saves
repeated preservative treatment and storage between
seasons. The reason is probably plenty of cheap cotton.
Since countries such as Poland do not have their
own source of cheap natural materials, efforts to moder-
nise the herring gillnets by replacing cotton materials
with synthetics are necessary, so Polish experiments have
aimed at determining the best type of synthetic material
to use.
The need for synthetic gillnets in Polish fisheries is
also based on the practice of trawler-drifters to switch
from gillnets to trawls and vice versa. In these circum-
stances, cotton gillnets, even though well preserved,
were frequently found to deteriorate after only a few
days storage. The first small-scale experiments were
initiated in 1956, and now over 2,000 synthetic gillnets
are used in Polish fisheries.
Experiments to date have resulted in the use of syn-
thetic nets with sisal lines, Their superiority over cotton
gillnets has been proved but it is still felt that gillnets
made entirely of synthetics would be more desirable.
Materials and methods
Even though synthetics such as the polyamide or poly-
vinyl alcohol fibres are well known, they are still not
used in certain types of gear. The Material Research
Section of the Laboratory of the Sea Fisheries Institute,
Gydnia, Poland, selected two basic types of synthetic
products: the polyamide group ('Steelon', 'Perlon', 'Ded-
eron') and the polyvinyl alcohol group ('Kuralon') for
experiments, aimed at developing synthetic driftnets for
the drifter-trawler fleet. The technical data of these
materials, as well as for the cotton (used as a control),
is given in Table I which is linked with Table II.
The coefficient of change in mesh size was calculated
by the formula:
w 100a
Ww - _~
Where Ww is the coefficient of change in mesh size.
a is the elongation of the mesh at break in
mm
b the barlength plus one knot (mesh size).
The coefficient of the mesh stability in connection with
the percentage of untied meshes during the mechanical
analysis, gives an indication of the probable behaviour
of each material during actual fishing operations. The
coefficient of stability Wt of the mesh is evaluated by the
formula :
Wt = 100 — I
where Ws is the mean mesh breaking strength
Wo is the mean strength of the meshes untied
during the analysis
R is the percentage of meshes untied during
the analysis
R is obtained from the dependence
R- i+T.
where I is the number of breakings in the analysis
which served to evaluate the mean
breaking strength
Is is the number of untied meshes during the
experiment.
The yarn diameter was measured with a thickness
gauge with accuracy of 0-01 mm with an initial charge of
50 g.
The investigations were carried out during two voyages,
each lasting for about two months.
Eighty experimental driftnets were made of these
materials for the first voyage and 1 10 for the second.
Experimental nets were constructed to the same design
and specifications as those successfully used by all
western nations fishing herring in the North Sea. The
net sections consisted of a central panel of 780 meshes
long and 340 meshes wide, with 10 mesh wide selvedges
made of thicker yarn. The webbing of the central panel
and the side selvedges were hung so that the horizontal
strain was "with the knots" while the upper and lower
selvedges were hung so that the vertical strain was "with
the knots" (see Fig. la).
In order to investigate if the arrangement of the netting
and the selvedges have an influence on the practicable
use, nets were made with the netting arranged in the
different ways illustrated in Fig. 1.
I
I
1 -
1
4
*
r~
i
J
Fig. 1 Mesh direction arrangement of gillnets.
& normal arrangement
b normal simplified
c unstrengthened
d simplified inverted
e nonnal inverted
The experiments were carried out on a typical com-
mercial herring fishing vessel with a crew partly composed
of scientists for recording the necessary data. Data
recording during and after use of experimental nets
included:
(a) Breaking strength of netting after the fishing period.
(b) Fishing yield for each type of gillnct.
(c) Estimation of the amount and the degree of
damage to the fish.
(d) Estimation of the number of fish gilled.
(e) Degree of deterioration of the gillnets after the
fishing period.
(f ) Estimation of each particular type of net material
for commercial use.
(g) Evaluation of work required of the crews for opera-
ting the different kinds of materials.
Experimental operations
The two experimental voyages lasted approximately
two months each. The first one took place during
October-November 1959, and the second in August-
September 1960. Due to bad weather in 1959, the number
of observations, experiments and measurements carried
out were fewer than in 1960. About 40 operations
during 1960 gave catches totalling 77 tons of herring
compared to about 20 tons in 1959.
After each fishing operation samples of netting from
all driftnets were analysed for mechanical strength and
to define the eventual decrease in strength— sec Table 11
(linked with Table I).
Polyamides have a relatively high strength, and the
nettings of thinner yarns, e.g. 265 den/9 were nearly twice
as strong as cotton netting; the thicker yarns of 265 den/
12 were 2-5 times stronger than standard cotton ones.
The average strength of 'Kuralon' gillnets (omitting
the badly chosen Nm 40/9) corresponds in general
to the strength of high-quality cotton gillnets.
As indicated in Tables 1 and II, polyamide material
has higher breaking strength than 'Kuralon', whose
strength is near to that of the very good cotton.
The mesh size of the experimental nets in 1959 was not
suited to the mean size of the fish caught and the nets
made of different materials had various mesh sizes.
The nets in 1960 were all of more or less equal mesh size.
Ttfchnicol .jL«t_
Breakin
in k,;
=» of !i«.rriii
Table 1
, drift net i u
ieu uurin
. the ttptruiOTltti
?<K3f<I.JC(ll UDfOlfiC tiO
Table II
ntt of oat*}
•i«l.. uned uariru 1959-60
;ns.
Jen
hnixe I'enii change
coefficient
Leah of broken
tenacity mebhes
Breaking Low. in »trtngth
Mtrtngth through braiding
in ke
T e hnite in an
(Ik; prt-ttnuion)
cotffioitnt
MtshMist KnotfuttncN
Mater itl
Dry
7,'tt
«n
l>ry
Wet
Dry
,tt
Dry Wet
l>ry -tt Dry
*tt.
Dry
/tt
Dry
Wtt
wet
Perlon
210 d'9
'•.9
15.9
0.6C
54
57
46.4
46.9
99. H - 11
1S9 12.6 -12.6
-9.4
57
56
51.5
52.5
99.6
1'erlon
** i/«
r-.-1
14.2
M.W
44
46
•#.7
'>5.1
99.9 99. » 6
15.3 15.9 -0.6
-2.1
45
47
56.7
96.2
99.e
l»»d.rcm
trt d/'>
M
12.9
0.81
50
52
40.4
41. t?
9'>9 9^.7 9-
15.1 12.5 0
-5.1
49
52
46.0
50.9
99.7
]Jcd«ron
».'.• d/9
•>.«,
14. .
O.flH
51
S2
46.1
45.4
'/i. 9 yy.9
15.9 14.5 xl.y
xi.4
52
•52
48.0
47.0
99.5
fteelcm
26S d/y
t,.f.
15.9
o.Pi,
54
5B
55.7
• 9.1
99.«. 9*^.6 11
16.5 14.1 «O.6
*1.4
55
59
50.0
50.9
99.7
Gteeloji
265 d/9
%5
14.1
„
51
55
46.5
49-5
99.5 99-5 25
15.4 14.0 -0.6
-0.7
54
56
56.5
45.5
-
Sttelon
265 d/9
17.9
If,. 1
0.91
55
56
54-9
56.9
99.6 99.0 1
17.1 15. n -4.5
-1.9
56
56
52.7
50.9
100
54
56
51.9
47.3
100
F.teelon.
265 d/12
19.2
17.4
0.89
15
52
55.5
51.2
100 100
18.2 16.4 -5.2
-5.7
52
53
52.7
50.9
100
Cttelon
265 d/12
22.4
19.9
1.07
54
56
57.6'
54-2
100 100
22.2 20.7 -0.*9
«4.0
54
55
50.9
54.5
100
Btttlon
265 d/12
20.4
17*2
1.01
52
53
56.9
t>5.0
100 100
20.2 17.fi -1.0
X2.5
50
52
65.4
61.1
100
Steelon
265 d/12
2?. 2
21.6
1.15
50
52
66.9
66.4
ion 100
22.1 21.2 -0.5
-1.6
50
52
58.8
51.0
100
Xurolon
H» 40/9
6.1
4.9
0.66
52
51
/(n.O
49.0
100 100 15
5.1 4.6 -16.4
-6.1
53
51
40.0
43.1
100
XUrnlon
Vm 54/15
7.2
5.9
0.85
54
51
45.0
45.1
9% 5 99.8 5
6.4 5.5 -ll.o
-7.0
52
49
50.0
55.1
100
Xuralon
»« 54/1 5
8.0
•7.1
0.89
51
50
57.9
54.2
KX) 100
8.0 6.6 0
-7.0
54
52
56.0
66.0
100
Kuralon
Km 40/15
6.7
7.5
0.97
57
52
57.7
67.$
99.6 99.7 5
7.8 7.1 -10.0
-5.0
50.
46
76*0
77.5
100
Kuralon
H» 40/15
9.7
8.9
1.04
54
52
80.1
79.2
100 100
9.6 6.5 -1.0
-7.0
52
52
56.0
35*6
200
Kuralon
1.20/9
8.5
6.6
0.80
55
52
57.2
56.8
100 100
7.9 6.2 -4.8
-6.1
53
47
51.0
48.9
100
Kurtlon
fa 20/9
9.9
8.1
0.90
55
46
51.1
49.7
100 100
9.5 7.4 -6.0
-9.0
53
50
25.8
31.6
loa
Cotton
Cotton
Cotton
»» 50/15
Mm 54/15
7.0
7.9
6.0
6.9
6.5
8.8
1.05
0.65
0.89
55
54
55
50
54
50
26.9
36.5
42.5
54.6
44.2
44.0
100 100
100 100
100 100
6.6 6.5 -2.9
6.4 7.0 -19.0
8.0 8.1 0
-5.6
-15.7
-8.0
52
51
50
29.6
51.5
40.0
>8.8
100
100
75
The mesh size of polyamidc nets increased appreciably
when wet, while the increase in the mesh size of 'Kura-
lon' nets was much less. The coefficient of change in
mesh size is characteristic for the particular type of
material and increases very much as a result of the tar
treatment. The meshes of the untreated polyamide
nets became easily distorted, causing considerable
change in their dimension. It should be stressed that all
the synthetic nets tested are commercially adaptable
except those appearing as items 1 and 12 in Tables I and II.
Among examined materials, dyed 'Per Ion' must be
rejected for commercial use owing to its lack of mesh
stability, and 'Kuralon' Mm 40/9 also because of in-
sufficient initial strength.
Fishing efficiency
The most important factor is, of course, fishing efficiency.
It also plays an important part in experimental catches
in the interpretation of the obtained indices. However,
due to the small number of operations carried out, no
final conclusions concerning commercial possibilities
can be drawn at this time.
The fishing results obtained with the different kinds
of nets in both 1959 and 1960 are presented in Tables
111 and IV, and, as can be seen, the fishing results of all
net types were, on average, satisfactory. The relatively
low catches of some of the polyamide nets during the
first experiment were caused by the mesh size of the nets
being too large for the average size of the herring.
Table III Firftt experiment
ng rosul'tu or
of nct& in
Material
* twint
H^fljh^j*
ttaan catofar per Total fish
Treatment net/day in ktf. per net in
tar.
l.Perlon 263 d/9
dyed
15
85
2.Perlon 265 d/9
Ivrown
45
2?0
J.Dederon 265 <*/9
brown
50
210
A.Dederon 265 d/9
5.St*elcm 265 <V9
tarred
oaprolaotm
30
15
150
70
6.Steelon 265 d/9
brown
20
110
T.bteelon 265 d/9
tarred
15
90
e.JCuralon Ha 40/9
brown
45
345
9.Cotton Nm 50/15
oil coated
35
215
lO.Cotton Jfin 54/15
ohro-oopper
30
195
ll.Cotton Nm 54/15
tarred oliro-oop.
25
140
When analysing the catch data per net per day, the
mean catches of the polyamide nets reached 37 kg, of
the 'Kuralon' nets 31 kg, and the cotton nets 31 kg.
While the fish catches of driftnets depend highly on the
proper choice of the mesh size, it is also important to
obtain proper correlation between the fish size, the kind
and thickness of the material and the quality of the
net treatment.
The more soft the material, the higher its fish catching
ability. This explains the high efficiency of untreated
polyamide driftnets and the efficiency of thin 'Kuralon'
driftnets. The thicker and stiffer the netting, the lower
the efficiency. This can be seen from the tables but this
conclusion is also based on additional observations
mentioned later in the paper.
Type and degree of fish damage
The first experiments with polyamide driftnets in com-
mercial lugger fisheries were failures. When Polish
76
fishermen were given 'Steelon' nets of equal strength to
the best cotton driftnets, they handed them back after
only a few trials and reverted to the standard cotton
nets. This emphasised the need for a detailed study of
synthetic materials for driftnets as the future of the
Polish lugger fisheries depend on developing better
gear than the traditional cotton nets. The damage of
the netting to the fish caused by the synthetic material,
was partially responsible for this initial negative reaction
Table IV - seoond experiment
Pithing results of nets in I960. Codei S - Steelon, K • Kurulon, C-
Cotton, D - Dederon.
• Material
& twint
number
Kind of
finiehina
I.oah catch
per net/day in kw%
Total fieh per
not in k(>
l.S 265 d/9
tarred
35
490
2.D 265 d/9
tarred
35
493
3.S 265 d/12
4.3 265 d/12
5.S 265 d/12
6.S 265 d/12
caprolactaa
11 + 2/: tar
eaprolaotam Ify'
oaprol. + 'tar
45
45
25
35
595
655
333
462
7.K Nm 54/15
not finished
30
438
8.K Nm 54/15
tarred
30
385
9. K Nn 40/15
not finished
35
473
10. K Nm 40/15
tarred
25
333
11. K Km 20/9
orown
25
315
12. K NB 20/9
tarred
25
326
13.C Nm 54/15
U.C Nm 54/15
chrocate-copper
])luc tar
35
30
539
389
of the fishermen. In driftnets, the herring should be
gilled strongly enough to prevent its falling out while
the net is still in operation, but not so strongly that
they are prevented from being easily shaken out during
hauling. Too flexible a mesh causes damage to the fish
which diminishes its value, and requires additional work
from the crew. In this regard, cotton net gives a good
performance and the tendency is to make nets of syn-
thetic materials as much as possible similar to cotton
nets. This can only be done by special physico-chemical
treatment of the presently available synthetic material.
During 1959 and 1960, trials were made to define the
influence of particular materials on the quality of the
fish captured. The index was decided by taking the aver-
age percentage of damaged fish taken from each kind
of gillnet. The results obtained are given in Table V and,
as can be seen, there is a certain relation between the
kind of material and the degree of damage to the fish.
Polyamides, owing to their mechanical properties and
especially their elasticity, are among the most gripping
materials and consequently have the greatest percentage
of damaged fish (more than 25 per cent).
The use of thicker and stiffer materials diminishes the
percentage of damaged fish (see items 5 and 8 in Table V).
The same result is obtained when the nets are treated
with a stiffening preparation (see items 3, 4, 6, 7 and 9 in
Table V). The simultaneous increase of the twine dia-
meter and application of physico-chemical treatment
reduces the percentage of damaged fish by the polyamide
nets to that of the cotton nets.
This applies also to 'Kuralon' nets which, however,
give a lower percentage of damaged fish in the untreated
condition than polyamide. It suffices to select a proper
thickness of 'Kuralon' twine to obtain even better
average results than with cotton fabrics.
for different kinds of driftnets. P -
Porlon. D «
De dor on
S . Steelon, K « Kuralon.
C - Cotton
Treatment
'Percent
dtuartged fish
material &
Twine Number ,
diam am
1. P 210 d/9
0.66
dyed
26.7
2. P 265 d/9
0.82
brown
12.9
5. D 265 d/9
4. T) 265 d/9
0.81
0.88
brown
tnrred
27. B
10.1
5. S 265 d/9
0.85
cnprolactuni
21.2
(>. S 265 d/9
-
brown
?3.5
7. r> 26 r. d/9
0.91
tarred
9-4
8. S 265 d/12
0.89
caproluctam
#- 14.7
9. S 265 d/12
1.07
it
+ tar
" 7-6
10. S 265 a/1?
1.01
caprolactam
10/< 5.2
11. 5 265 d/12
1.13
it it
+ tar
6.4
12. K to 40/9
0.66
dyed
9-9
15. K Kin 54/15
0.8J
untreated
9."
14. h ton 54/1?
0.09
t»:rred
7-4
15. K Urn 40/15
0.97
untrented
16. K Nm 40/15
1.04
tnrred
3.*5
It. K Nn 20/9
0,30
brown
10.0
18. K Urn 20/9
0.9f»
tarred
9-0
19. C Nm 50/15
l:03
oil coated
6.6
?0. 0 JJm' 54/15
C:iror.'inte-cop
P*r 6,0
2). C Jfoi 54/15
0.89
• • M
tarred
4.6
When determining the coefficient of damage to fish
by the netting, we should stress that results may vary
from ship to ship. For example, highly qualified crews
may cause less damage by skilful handling of the nets
during shaking, than inexperienced crews. It is most
difficult to assess the damage caused by inefficient
manipulation of the nets during shaking as the effect
is combined with that of the gripping qualities of the
material as well as the mesh size in relation to the girth-
size of the fish. However, some indication can be obtain-
ed by the number of fish remaining gilled in nets of
different materials after identical shaking. In Table VI
the results obtained are reduced to a comparative scale
with the chromate copper treated cotton nets as control.
These indices again stress that the average performance
of the driftnets depends on the accurate choice of the
material and the mesh size.
Wearing rate of gillnets after fishing
After every voyage of comparable length to commercial
voyages, a visual inspection of the gear was carried out
by a group composed of skilful quality control inspectors.
They assessed the necessity of repairs to the nets both
as to quantity and quality, to restore it to full technical
efficiency for the next voyage.
An estimation of the wear was based on the following
conventional numerical scale used in Poland:
VI uantity of fiah rem'iininA gilled
Knterial
after shaking
Treatment
Index
Steelon
iintr^.atea or onJLy
dyed.. 2.5 - 5
n
tarred
1.5 - 2.5
Kurfjlon
untr^otc-ci or only
dyed. 1.2 - 1.8
it
tftrrfcd
0.6-1.2
Cotton
chroma te-coiipcr
prtaerveition
1
S — single small holes in the net.
10 — numerous holes of several mesh sizes, single
tears.
25— multidirectional tears of more than 10 meshes
in length spread over the net, but not to the
extent that replacement is necessary.
75 — tears and holes such that only parts of the
netting can be saved to patch others.
T « I'crlon, 1) - Dederon. b - Steelon* K • Kuralon. C « Cotton
Totorial &
Trine Number
Treatment
Decree Uenarkv
1. T 180 d/9
dyed
25 after two exper-
?. Jj 26 S d/9
brown dyed
10 imental voyngei.
1. D 265 d/9
tarred
5
4. S 265 d/9
oaprolactwn
5 after two
S y 2^5 d/9
brown dyed
10 voyojee*
6. S 265 d/9
tarred
5
7. !> 26^ d/12
cuprolnotom 2>
5
H. S 26 b d/12
" * tar
0
9. S i?63 d/12
eaprolactaa 10
f 5
!•). S 2t5 d/12
" 4 tar
0
11. K NCJ 40/9
brown vlynd
75
12. K NDJ 54/15
\antreatod
75
13. K 2ha 54/15
tarred
10
U. K I-Jm 40/15
untreated
5
15. K Un 40/15
16 K Win 20/9
tarred
brown dyed
0
25
17. K Nm 20/9
tarred
5
18. C No 50/15
oil coated
10 old nets
19. C Nm 54/15
otoron--.te-oop|ie
r 25
20. C Nm 54/15
ditto -t- tar
0
Cotton
oil or tar COP ted
0.5 - 0.7
As indicated in Table VII, the wear rate depends on
the type of material and proper preservative physico-
chemical treatment considerably reduces net wear. So far
as the polyamide untreated nets are concerned, a low
mesh stability resulted in significant mechanical wear
despite their high initial strength. The treated polyamide
nets are highly resistant to abrasion. Untreated 'Kura-
lon' should not be used, since it has many disadvantages
which can be overcome by proper treatment.
The following general conclusions may be drawn from
this part of the experiment:
(a) Driftnets should be made of materials with a rela-
tively high strength and sound mesh stability.
(b) A careful selection of the twine is necessary to
obtain proper relation between twine strength
and the forces acting on it during the fishing opera-
tion.
(c) A proper physico-chemical treatment of the net
materials is of great importance as it reduces
mechanical wear.
Effect of mesh arrangement
The driftnet known in Poland as the Dutch driftnet is a
basic type commonly used in the North Sea. In the
main netting the run of the meshes is with the knots in
the lengthwise direction of the net, but across the knots
for the top and bottom selvedges.
In the opinion of many observer-fishermen, this arran-
gement was best because of the reduced number of fish
falling out during hauling, and also because the main
forces during hauling, act "with the knots". It seemed
77
advisable to try various arrangements and this was done
as indicated in Table VIII.
Tablt VIII ?;tt»h arrnn.*aentH of the driftnets used
K • normal* S « giaBlifiedt St. S tee Ion. K.« Kuralon. C • Cotton
twin* jise Twin* eiee Treat- No.
of main netting of stlveds*»9 Ijaterial nent £ of nets
1.X oinrolifitd
265 d?9
265 d/9
St.
tarred 53.0
5
2.K simplified
265 d/12
265 d/12
' -t.
none
3
3.N Bimplifi.d
265 d/12
265 d/12
fet.
turrod 31.3
5
4.N une lengthened
Nm20/9
None
X
dyed
5
5 •N un«tren^th«ned
Km 20/9
None
K
tarred 20.6
18
6,N unotrencthened
Km 40/15
None
K
none
2
7.H unatrengthened
Nm 40/15
None
K
tarred 34.25
2
6.S inverted
Nut 20/9
Nn 20/12
K
dyed
5
9.3 inverted
Nm 20/9
Nra 20/12
K
tarred 34.3
20
10. S inverted
265 d/12
265 d/12
St.
tarred 31.3
3
11. N inverted
Km 50/15
Km 50/18
C
tarred 27.9
10
These gillneti were uaed simultaneously with normal driftnets,
On the basis of these experiments, a simplified arrange-
ment for thinner materials and a simple unstrengthened
arrangement of the netting of greater mechanical strength
has been chosen for industrial manufacture.
Estimate of crew work
The amount of crew work differed according to the
material used. In general, work increased when polya-
mide and 'Kuralon' driftnets were used, decreased when
oil-coated cotton gillnets were used.
The amount of work was estimated directly and
indirectly by measuring the operation time for each
type of driftnet, the fatigue rate of the crew and the
eventual frequency of shifts during removal of the
herring.
In general, the thin untreated polyamide nets, with
yarn number Nm 50/9 and Td 265/9 or Nm 34/9 are very
tiring, and in fact are rather unsuitable as a commercial
fishing gear. 'Kuralon' is better to handle than polyamide,
especially when coated with a tar substance.
Based on the experiments, 'Kuralon' twines of Nm
34/12 have been selected for use in commercial fisheries.
tl experiments
Commercial experiments were planned and carried out
in 1961 and 1962. They consisted in equipping a com-
mercial vessel with an experimental set composed of
either 'Kuralon' or cotton and 'Kuralon' driftnets. The
crew and the skipper fished this set according to typical
commercial principles and prepared a report after each
voyage.
During the two years mentioned, 20 commercial
voyages were made, during which it was concluded that
the fishing effectiveness of the 'Kuralon9 driftnets does
not differ from that of standard cotton gillnets. The
wear of 'Kuralon' nets was, on average, 10 times smaller
78
compared with cotton. This may not hold true in future
experiments as some of the cotton nets had been used
whereas all the 'Kuralon' nets were new. The skipper, as
well as the crew, indicated that the amount of work
for the operation of the 'Kuralon' nets is similar to that
required for the operation of cotton gillnets.
On these experiments, Polish fishermen have gradually
started to use 'Kuralon' nets made of Nm 34/12, and
treated with petroleum tar (coating of 30 per cent.).
By the end of 1962, more than 2,000 'Kuralon' driftnets
were in use compared to 8,000 cotton driftnets. From the
wear data for the year 1962, it transpires that the replace-
ment for the 'Kuralon' driftnets (losses at sea and normal
wear) averages nine per cent per year compared to approx-
imately 50 per cent for cotton nets.
Our further work will deal with the elaboration of
driftnets made entirely of synthetic fibres.
General conclusions
1 . Of the analysed materials, polyamides as well as
polyalcohol vinyl ('Kuralon') are suitable as material
for herring gillnets.
2. In spite of the many qualities of polyamide (great
strength, fishability, gripping quality) 'Kuralon' has
better overall utility for herring driftnets and costs less.
3. The special treatment required for the polyamides
and the necessity of a greater thickness of the twines for
handling limit to some extent their use as herring gillnets.
4. The simpler treatment of the 'Kuralon' and good
fishing results indicated that this material is better suited
for herring driftnets.
5. The number of fish damaged during shaking
depends on the quality of the material, the thick-
ness of the twine and the degree of stiffening.
6. The mesh arrangement in a gillnet has no direct
influence on the fishing efficiency of the net. Based on
observations, however, the best arrangement seems to be
that normally used in the North Sea herring driftnet
fisheries.
7. The commercial fishing experiments have confirmed
the high qualities of the 'Kuralon' twines for driftnets.
8. On the basis of two years of experiments, 'Kuralon*
nets (with treated sisal lines) need only one-fifth as much
replacement as cotton driftnets. Into consideration there
was taken Polish type of fishing operations, in which
vessels catch with driftnets through a part of the year and
with trawls through another one.
Discussion:
New net materials
Mr. Pierre Lusyne (FAO) Rapporteur: At the first World
Fishing Gear Congress in 1957 the characteristics and use of
several synthetic fibre materials were much discussed because
of their newness. Since then some have practically dis-
appeared while others have firmly established themselves,
such as nylon, polyester, polyethylene, polyvinyl alcohol and
its various combinations. To choose appropriate material
for a specific fishing gear is not simple and among the syn-
thetics now used in fishing, many factors such as wet knot
strength, elasticity, abrasion resistance, etc., must be carefully
considered.
A new material is polypropylene which belongs to the
polyolefin group and has many of the characteristics of
polyethylene which was brand new six years ago, but is now
well established as trawl twine material. The characteristics
of polypropylene are fully described in Dr. Klust's paper
after testing some 40 samples. Polypropylene has the lowest
density of all materials used for twine manufacture and has
the same strength in wet or dry condition. Based on breaking
length it has the same strength as nylon, but will, for the same
strength, be thicker due to its low density. Klust also
showed that in the wet knotted condition polypropylene and
nylon of comparable R tex value have the same breaking
strength.
He found a remarkable difference in extensibility; poly-
propylene stretched much less than nylon and the load
extension increase was almost linear. This is very different
from nylon which has a great extension at low load but with
increasing load gives a constantly decreasing percentage of
stretch. Polypropylene, on the other hand, has a high creep
value when under continued load, so that the permanent
elongation is higher than that of polyamide. This is an
important factor as it may affect its suitability for net parts
where heavy and consistent loads are applied, which then
cause undesirable mesh changes.
The paper by Carter and West on the properties of 'Ulstron'
one of the first polypropylenes used (at least in the fishing
twines), gives much the same figures as Klust in regard to its
properties, but refers to other synthetic twines such as
polyethylene, polyester, nylon, polyvinyl alcohol and some
of the natural fibres and provides a very useful basis for
comparison.
Polypropylene softens at 160°- 170° C and should therefore
not be submitted to high temperatures during stretching and
heat-setting, although 100° C (and therefore boiling water)
is quite safe.
The fact that some papers vary in details of the catching
efficiency of polypropylene compared to other twines is no
doubt due to the need for more experience and further adaption
of gears. Klust quotes Carrothers in giving polypropylene
a slightly higher catch efficiency than nylon both in gillnets
and bottom trawls. Carter and West's results are approxi-
mately the same. The Japanese, however, found poly-
propylene gillnets to be rather bulky for handling and their
catching efficiency slightly lower than that of nylon. Poly-
ethylene, in the meantime, has been fully accepted and is
now extensively used especially as a trawl net material.
According to the Japan Chemical Fibres Association
78,000 tons of synthetic nets and ropes are now in use in
Japan compared to about 9,000 tons of natural fibre material,
which shows their highly modern outlook. It is, however,
remarkable that while the conversion to synthetics is 90 per
cent for netting, about 80 per cent of the ropes used are still
of natural fibres.
Much headway has been made since the last Gear Congress
in the construction of braided twines. On this point
Bombeke reports tests which showed that the drag of braided
nylon twines is lower than that of twisted twines; both showed
one-half less drag than cotton at a speed of three knots.
The actual testing was done in an air tunnel and the results
recalculated to give the drag in water.
Mohr reports on experiments carried out in Germany to
use plastic material for creels and pots. While first efforts
were not quite satisfactory, plastic traps were later made
which caught as many eels as the usual rattan traps. Such
plastic traps, though initially more expensive, had a much
longer useful life.
Dr. Katsuji Honda (Japan): As a result of continued effort in
recent years, the strength and quality of the twine made from
polypropylene had been considerably improved as these
figures show:
Multifilament Spun
yarn yarn
$.5—6.0 g/d 3-7— 3 -8 g/d
6-0—7-0 „ 4-0-4-5 „
7-5—8-0 „ 5-5—6-5 „
Monofilatnent
yarn
5*5— 6-5 g/d
5-5—6-5 „
6-5-7-0 „
1961
1962
1963
In strength it has been made even stronger than nylon
and in weathering there would be no further problem. As
with other fibre materials all kinds of industrial uses can be
expected of this material but experimentation was still
required regarding its use as rope. It had been found that
in strength it was stronger than, or almost as strong as,
polyvinyl alcohol but not yet as strong as nylon. There are
various advantages, however, because of its light specific
gravity, its flexibility and abrasion resistance; and it is expected
to be adequate for use as rope. For further improvement
they were studying prices and economic conditions and also
the question of durability.
Mr. V. Valdez ( Peru) : There are now 1 ,200 boats operating
in the Peruvian anchovetta industry. The great richness of
plankton in that area caused cotton nets to rot quickly but
synthetic nets do not rot and all the nets now used are of
nylon.
Mr. D. Roberts (UK): In trawling some years back, we
were not too sure about synthetics but I am now 100 per cent
convinced and we never want to buy anything but synthetics
for trawling in future.
Mr. J. Norman (UK): As a District Inspector of Fisheries
my duties necessarily include enforcement of regulations.
With the introduction of synthetics it has become difficult to
distinguish between the four main groups and 1 would like to
know if there are any simplified methods of identifying them.
Dr. von Brandt: That problem has been much discussed.
There are many fibre producers here. Can anyone give an
answer for the main types of synthetic fibre especially those
used in trawls ?
Mr. Eric Kwei (Ghana): In using synthetic nets we found
that in our tropical waters fish that were gilled died quickly
and when they were taken out had already begun to putrefy.
What is the possible remedy ?
Mr. B. W. Crewdson (UK): The question about mesh sizes
as interpreted by Government Authorities is a very vexatious
one. Dealing with synthetics is very different from natural
fibres. The material itself might not shrink but when put
into a net form there is very pronounced shrinkage. It is our
trade custom to measure mesh sizes between net knot centres,
whereas government authorities measure the mesh opening
(lumen). Braiding nets is not an exact science. If done by
hand there is variation. If done by machinery it is a matter of
relation. If you are A th of an inch below you are in the red
with the Ministry and if above, with the Skipper. We are
in a most difficult position. There should be some standard-
isation which, at the moment, there is not. In America, nets
are measured before use and they are sealed, but here the mesh
is supposed to be measured after use and when wet. That is
the question of mesh size in relation to synthetics and it is very
serious. Can manufacturers tell us what proportion of
shrinkage there is in the product they produce?
79
Dr. von Brandt: This problem we have also discussed on
the Permanent Commission. It is very difficult not only for
synthetics. Specifications of some nets call for mixtures
which differ from time to time. There is no real answer.
Mr. W. Hewtead (UK): We can all sympathise with Mr.
Crewdson, but yarn manufacturers are entitled to sympathy
too. We know this problem and we are doing all we can to
combat it. One constructive suggestion we made was that
there should be one regulation for monofilaments, one for
staple fibres, and one for multifilament yarn.
Mr. Harper Gow: Can anyone give details of the United
States method and say whether it is satisfactory to the net
maker and the skipper?
Mr. Lusyne thanked Dr. Honda for clarifying the position
in Japan regarding the production and properties of poly-
propylene. For distinguishing different fibres, various means
were known, such, for instance, that burning nylon gave off
white smoke and left a residue of hard black quality; other
fibres reacted differently, but the real tests were chemical and
they could only thus distinguish the different fibres.
There were practical methods which had been published
and re-published. We could suggest that FAO include in
the Fishing Gear Designs Catalogue some simple methods for
testing fibres.
The only practical answer to Mr. Kwei's problem of fish
rotting in synthetic gillnets was to haul the nets and clear
them more frequently.
To Mr. Crewdson's problem he could see no real answer.
Stretch could arise not only from the material but from the
knots themselves tightening or giving. There would always
be movement in the mesh of knotted nets. He thought the
regulations should state clearly whether, and how many
meshes should be taken or sampled in different parts of the
net to decide whether the average mesh size conformed with
the regulations. The nets did change in use and the regula-
tions should recognise that.
Dr. von Brandt: That is also a problem for the Permanent
Commission and he thought most delegates of the different
countries would discuss the right regulations for checking
mesh sizes.
Mr. Kristjonsson (FAO): We should be discussing here
mainly the suitability of the different materials for different
fishing methods and fishing gear. He suggested that poly-
propylene might advantageously be used in some purse seines
because of its lightness and capacity for floating in the water.
This should reduce fouling when making sets in shallow
water provided the necessary weights were concentrated in the
purse rings instead of using a leadline. He would like to
hear other provocative statements about the use of certain
types of materials for different fishing needs.
Mr. VaWez (Peru): Our Fishery Institute is considering
buying polypropylene in order to test it for purse seines.
Mr. R. K. N. Ocran (Ghana): Our traditional methods
were to fish with natural fibres but cotton nets rotted after
two months. After the Hamburg Congress and his own
experience in Scotland, they tested nets of cotton, 'Kuralon'
and nylon. 'Kuralon' was very heavy and sank quickly and
tended to get caught on the rocks. With nylon used in
shallow water, they had some success, but in deeper water,
because the net was light and slow to sink, 75 per cent of the
fish in a nine-months trial of seine nets escaped before they
could catch them. If they used leadlines the net would sink
quickly enough but the net would foul. To solve that
problem they were now trying another net from a different
maker.
Mr. E. F. Gundry (UK): One of the problems of a netmakcr
is the multiplicity of synthetic fibres. This is handicapping
the commercial success of netmakers. Some settlement
should be arrived at as to the ideal materials to be used for
different fisheries so that netmakers would not have to carry
so much stock in different lines. This would contribute to
the economy of the netmaking industry.
Mr. £. Alters Nilssen (Norway) agreed it would be far easier
for manufacturers to have on*y a few synthetic fibres. Syn-
thetics certainly floated more and sank slowly, therefore they
had tried to find out the right kind of dressing to keep purse
seine nets upright in the water. He did not agree that
polypropylene would be of any advantage in purse seining.
On the contrary. On the setting of knots and mesh sizes,
their experience was that if netmakers used synthetic fibre
they could know exactly what the shrinkage was. If a net
was really properly knot set there would be very little variation
in the mesh size.
Mr. £. A. Schaefers (USA): In experiments of synthetic
materials tar has reduced the abrasions on the fish caused by
the material. It has also probably reduced the catchability of
the nets but they still maintain the durability which is one of
the main features of synthetics against cotton fibres.
Mr. Ocran: The difference between fish caught by cotton
and nylon is that when the fish struggle the fine twine cuts and
the fish deteriorates before it reaches the consumer so the
impression in our country is that nylon-caught fish deterior-
ates more than cotton caught fish.
Mr. C. Crewdson (UK), on the point of dressing synthetics
with bitumastics or pitch material, said he had always
understood the fibres were impervious to bacterial action.
They did not need dressing to give durability. His firm gave
bitumastic treatment to synthetic nets merely to give rigidity
because those nets were so limp that they tended to foul in
handling.
Mr. Schaefers: I have been misunderstood. I was merely
trying to point out that the treatment of synthetics with tar
was to reduce abrasion on the fish and not to extend the life
of the material. 1 agree completely with Mr. Crewdson.
Mr. Lusyne, in summing up, said that Mr. Gundry's
problem of limiting the number of materials could not be
easily met. They could not just say that any particular
material would make a good net. They had to qualify that by
saying it is a good net at certain places at certain times. One
fisherman would succeed with it and another would not.
Their individuality had to be met. The gear the fisherman
used was that which best suited his operation. No agreement
would be possible in the way of manufacturers limiting
themselves to three or four materials. Furthermore such
standardisation would mitigate against progress.
Part 1 Materials for Nets and Ropes
Section 4 Ropes, Knotless Nets and Monofilaments
Ropes of Polyethylene Monofilaments
Abstract
Polyethylene monofilaments are manufactured by extrusion,
followed by an orientation process. Two factors have a significant
effect on the properties of these filaments, i.e., type and quality of
the polymer used for production and the degree of stretch applied
at the orientation stage. The properties of monofilaments made from
three different polyethylene types have been compared with the
normally given specifications for these raw materials. The influence
of the stretching percentage on the filament properties, has been
determined quantitatively. Polyethylene monofilaments with the
same diameter, but with different properties, were twisted and stran-
ded into twines and ropes. The mechanical properties of these
twines and ropes were compared with the filament properties. The
conclusions arrived at were as follows : ( 1 ) Melt index and specific
gravity are insufficient specifications for characterising polyethylene
raw material, suitable for monofilament production. (2) Under
comparable circumstances ethylene copolymers give stronger
monofilaments than ethylene homopolymers. (3) The monofilaments
of highest breaking strength do not give the best net yarns in respect
of the knot tenacity in netting. (4) For rope making, highly stretched
monofilaments are desirable.
by
Resume
La production des monofilaments de polyethylene s'execute en deux
phases suivies Tune apres Tautre: rextrusionct K orientation. Deux
facteurs importants infiuencent les proprietes des filaments obtenus:
le type et la qualite du polymer destine a leur production et le degre
de Tetirage appliqu6 lors du proccs de Torientation.
Les proprietes des filaments fabriques a base de trois types
differents de polyethylene ont etc comparees ayec les specifications
donnees reguliercment pour ces matures premieres. L'influence de
Petirage sur les proprietes des filaments a et£ determinee quantita-
tivement. Des monofilaments d'un meme diametre, mais avec des
proprietes differentes ont 6t6 rctordus et assembles en cables ct en
cordes. Les proprietes mecaniques de ces cables et de ces cordes ont
et6 comparees avec les proprietes des filaments.
Les auteurs sont arrives aux conclusions suivantes: ( 1 ) Klndice de
fusion (melt index) ct la densitS sont insuffisants pour caracteriscr la
matiere premiere servant de base a la fabrication de monofilaments.
(2) Dans des circonstances comparable*, les copolymers d'ethyldne
produisent des monofilaments plus resistants quc les homopolymers
d'ethylene. (3) Les monofilaments ayant la plus grande resistance
de rupture ne sont pas les meillcurs pour la confection des filets, par
rapport a la tenacit£ au noeud. (4) Pour la fabrication de cordes,
il est souhaitable quc les monofilaments aient etc soumis a un
etiragc tres eleve.
Extracto
Se fabrican los monofilamentos de polietileno por medio de extru-
si6n, seguido por un proceso de eslirar. Dos factores licnen un
efecto significante sobre las propiedades del producto: la clase y
calidad del polimero y el grade de estiramiento aplicado en la fase
de orientacibn. Las caracteristicas de los filamentos hechos de tres
tipos diferentes de polietileno han sido comparadas con las especih-
caciones usuales para esta materia prima. La influencia del estirar
sobre las propiedades de los monofilamentos ha sido establecida en
cantidades determinadas.
Hilos monofilamento del mismo diametro pero con caracteristica
diferentes han sido retorcidos y ensamblados en cables y cordonos.
Las propiedades mecanicas de estos cables y cordones ban aido com-
paradas con las propiedades del monofilamento. Se Ileg6 a las
conclusiones siguientes: ( 1) El indice de fusi6n y la gravedad especf-
fica son insuficientes para caracterizar el polietileno bruto que reuna
buenas condiciones para la produccibn de monofilamentos. (2) tn
condiciones andlogas dan monofilamentos mas fuertes los copoli-
meros que los homopolimeros de etileno. (3) Los monofilamentos
de mayor resistencia a la rotura no producen los mejores hilos para
redes rcspecto a la tenacidad de los nudos en los paftos. (4) Enjas
fabrication de cuerdas convien
estirados.
' conviene emplear monofilamentos muy
C. C. Kloppenburg
J. Reuter
IT is well known that monofilaments from polyethylene
are manufactured by an extrusion technique, followed
by an orientation stage. In principle one can distinguish
two different processes. The whole operation can be
done in a single "in-line" unit or in two separate units,
the first devoted to extrusion and the second to orienta-
tion. Fig. 1 gives a comparison of these two manufac-
turing methods. At the top is single-stage production.
The polymer is molten in an extruder, forced through a
multihole die and quenched in warm water. Directly
after that, filaments are stretched up to eight to ten times
their original length in boiling water, and then wound
up under constant tension. This process can also be
done in a discontinuous way, as is shown below. Here,
the unstretched filaments are spooled and afterwards
stretched in a second operation.
Both methods have advantages and disadvantages in
production. The in-line method requires fewer pieces of
ORIENTATION BATH QUENCH EXTRUDER
TWO-STAGF MONOFll AMLNT PRODUCTIO
BOBBINS
ORIENTATION BATH
UNSTRE1CHED
F::.AM( NT
SPOOL
Fig. I. Single-stage and two-stage monofilament production.
81
equipment and a simpler attention than the two-unit
method. The speed of stretching, however, is in point of
principle limited by the extrusion speed. This is a draw-
back in regard to output. The final selection of the method
to be used will depend, among other things, on the
number and diameter of the monofilaments. The manu-
facturing process for polyethylene monofilaments may
be relatively simple but needs to be carefully controlled.
Different manufacturing conditions can be used and
they have more or less influence on the properties of the
oriented monofilaments. But this influence is not always
clear and some investigations contradict each other.
Two factors, however, have a significant effect on the
properties of the end product. These are : type and quality
of the ethene polymer used for the production and the
stretching percentage (draw ratio) applied in the orienta-
tion stage. Both factors are discussed.
The raw material
For producing polyethylene monofilaments, mainly three
types are considered:
(a) Homopolymer of ethylene, manufactured by
Ziegler procedure.
(b) Product obtained by copolymerising ethylene and
1-butene by Phillips process.
(c) The newer commercial ethylene copolymer, ob-
tained by Ziegler process.
These materials are only specified by melt index and
specific gravity and sometimes by viscosity data.
For filament production, the types with specific gravity
= 0*95 and melt index 0*1 0*3 are recommended. It
turned out, however, that these two or three polymer
specifications are insufficient for characterising poly-
ethylene material suitable for filament production. In
other words, the suitability of an ethylene homo- or
copolymer for making monofilaments is not guaranteed
when the specific gravity and melt index are within the
above-mentioned limits. This does not only relate to the
economy of the process but also to the mechanical
properties of the obtained filaments (Table I).
From eight different ethylene homopolymer batches,
all produced according to the Ziegler process, were
determined: density, melt index and intrinsic viscosity.
Afterwards, output samples were taken for manufactu-
ring monofilaments in exactly the same way, using a
special investigation equipment, and applying a draw
ratio of 8 : 1 . The final diameter of the filaments amoun-
ted to 0*38 mm (0*0150 in), corresponding with tex
108 (tex=weight in grams of 1000 m of monofilament).
Table I indicates the average values of some typical
properties of the obtained monofilaments, namely the
tenacity determined with and without an "overhand
knot", and the elongation at break. These determinations
were carried out with a Scott IP4 inclined plane tester.
Drawing time: 13 sec. The tenacities are expressed in
grams per tex. Dividing these figures by 9, gives tenacity
expressed in grams per denier.
82
polyathylana Ha
Raw satariala
XonofilaMata
fetch or
Lot
mwbar
Datarainad apaoifioationa
Maoftanloal propartUa
Danaitjr
felt
index
Intrinaio
viaooalty
Tenacity (g/ta*)
Elongation
normal
ovarhand
knot
1
0.947
0.16
2.16
49-3
37.3
19 %
2
0.950
0.31
1.72
46.0
33.2
19 %
3
0.949
0.29
1.64
40.3
32.3
18 %
4
0.947
O.JO
1.73
40.4
31.5
21 %
5
0.947
0.28
1.72
44.3
32.1
17 *
6
0.950
0.20
2.16
46.3
38.0
22 H
7
0.949
0.17
2.16
44.7
34-7
19 *
8
0.947
0.23
1.90
48.0
32.5
18 %
i>iM* mtio
lJiaiurt»r monofil«««»nt«
Tex monofil amenta
- 6 i 1 """"""
* 0.30 M (0.0150 in.) 45.2
- 108 a/km.
34.0
19.1*
From these data the conclusion is well founded that,
within the investigated range, there is no correlation
between density, melt index and intrinsic viscosity on
the one side and tenacity or breaking elongation on the
other. Starting with the same type of raw material, there
are remarkable differences in filament properties depend-
ing on the polymer batch that has been used. It is inter-
esting to compare these results with the figures obtained
by experimenting, in exactly the same way, with ethylene
copolymers produced according to either the Phillips or
the Ziegler process (Table II).
Tabla II. konofilaaanta from athylana oopolvaai-a
l)raw ratio • 8 t 1. Konofilahent diamvUr - 0.38 DM
(0.0150 in.)
lav aatariala
Monofiluanta
Baton or
Datarainad apaoifioationa
Maohanioal propartiaa
Lot
Nalt
Intrinaio
Tanaoity (g/tax)
Elongation
nuabar
indax
viaooaity
normal OTJ|Jj*d
Phillipa oopolyaar
11
0.950
0.28
1.80
51.0
37.0
20 %
12
0.951
0.16
2.12
58.8
36.9
20 %
13
0.949
0.16
2.09
56.1
34.2
. 18 %
14
0.950
0.15
2.10
52.0
38.5
19 %
15
0.951
0.22
1.96
52.0
36.5
22 %
16
0.950
0.30
1.70
47.2
33-3
22 %
17
0.948
0.19
1.91
53.3
35-7
20 %
18
0.948
0.21
1.66
47.0
33-5
21 %
52.2
35-7
20.#
giaglar oopolyaar
21
0.949
0.26
2.10
57-1
40.6
19 %
22
0.949
0.29
1.86
55.3
38.1
20 %
23
0.948
0.35
1.84
51.3
36.0
23 %
24
0.949
0.26
1.85
49-5
38.5
21 %
25
0.947
0.25
2.12
48.5
38.1
22 %
26
,0.948
0.28
1.96
56.8
38.5
17 %
27
0.946
0.34
2.01
57.3
35.5
17 %
26
0.946
0.15
2.13
56.6
38.5
19 %
54.1
38.0
19.6*
SRCAKING TENACITY (0/ttx)
ffff
40
2i
20
Normol
Ovtrhond knot
BREAKING ELONGATION (%)
96
*
<^
20
^
"^
1 •
15
10
s
o
Vs
c^,
^••ll^
•>-
*•»»*
* •
— — 1
Prow Ratio
9:1
12:1
Fig. 2. Mono/Moments made from Ziegler-type polyethylene. Homo
polymer. (Filament diameter— 0-41 mm).
BREAKING TENACITY (0 tex)
70 I
X
^s
Overhand Knot
•MAKING
PLQI JC/VI ION I
Draw Ratio
Fig. 3. Monofilaments made from the Ziegler-type polyethylene.
Copolymer. (Filament diameter=0-41 mm).
Table III. Prop«rti«B of
Ziegltr typ« poly •thy Itnsi
(0.0161 in.}
Draw ratio
Breaking ttnaoity in
o/ttx
Breaking
elongation
Normal
Overhand knot
Homopolyiner
50.1
23.8
23.#
6.5 s 1
7.25t 1
40.4
31.2
21.*
7.8 t 1
44.7
33.6
20.#
8 t 1
45.3
33.8
19.(#
6.25s 1
47.5
32.7
16.#
8.5 i 1
50.3
32.6
16.7*
9 * 1
50.6
26.7
14. #
9.75» 1
51.9
24.0
13.#
10.4 > 1
52.8
21.4
12.#
11.5 t 1
54.6
16.1
ll.Qt
CopoXymer
55.7
'26.9
27.$
6.5 i 1
7-25* 1
47.5
33.6
24.#
7.8 t 1
54.3
38.1
22.#
8 s 1
54.2
37.9
19.7#
8.25s 1
57.0
39.5
19.49&
8.5 s 1
59.5
37.6
18.4#
9 « 1
61.2
35.5
16. #
9.75s 1
61.6
29.3
14.79&
10.4 s 1
62.7
24.5
13.7#
11.5 s 1
65.0
20.6
12. <#
Here the same picture is seen. The filament tenacities
may diverge to 20 per cent depending on the polymer
batch that has been used. It is not possible to predict the
mechanical properties of the monofilaments from such
specifications as: density, melt index and viscosity.
These data are apparently insufficient to describe the
behaviour of an ethylene polymer for filament production.
We can only hope that, in the near future, the suppliers
of the raw material will be able to furnish stricter speci-
fications; such as: molecular weight distribution and
data concerning unsaturation and branching.
The draw ratio
It is common knowledge that the draw ratio applied in
the orientation stage influences the elongation and
tenacity of the resulting monofilaments. As a general
rule one can say: the higher the stretching percentage,
the higher the tenacity and the lower the elongation.
This correlation has been investigated quantitatively
by extruding monofilaments with different diameters
and stretching the said monofilaments in such a way that
the final diameters of the obtained products were the
same. The applied draw ratios in this investigation
ranged from 6*5: 1 to 11-5:1. From the resulting mono-
fils not only the tenacity and elongation were determined,
83
but also the knot tenacity. Table 111 gives the results
obtained with monofilaments produced from a Ziegler-
type homopolymer and a Ziegler-type copolymer, both
batches representing an average quality. From these data
the two graphs, shown in Figs. 2 and 3, have been made.
Fig. 2 relates to monofilaments made from the
Ziegler-type homopolymer. On the x-axis is plotted
the applied draw ratio during the production, and on the
y-axis the determined tenacity (in grams per tex) and
elongation of the obtained filaments. As it turns out,
the tenacity increases to a certain level and the elonga-
tion decreases when we increase the stretching percentage.
This is not the case with the knot tenacity which exhibits
a distinct maximum. It is difficult to explain the occurrence
of this maximum because a simple "overhand knot",
regarded physically, is a rather complex whole in which
among other things, a bending modulus, a torsion mod-
ulus and a sliding modulus play a part.
Fig. 3 relates to monofilaments made from the Ziegler-
type copolymer. The three curves exhibit the same shape
as in Fig. 2 but are all at a higher level. This finds better
expression in Fig. 4, where the curves are taken together.
The drawn lines indicate the results obtained by investi-
gating the Ziegler copolymer and the dotted lines repre-
sent the behaviour of the Ziegler homopolymer. The
knot tenacity curves have different maxima, concerning
both the draw ratio and the tenacity.
Investigation
The foregoing poses the questions as to what raw material
should be used, and what draw ratio applied to get
the best products for rope and twine makers. The answer
may seem rather simple. From the graphs shown, it is
possible to find the circumstances for getting monofils
with the highest normal strength. According to general
opinion, these monofils will automatically yield the best
ropes and net yarns, and are therefore most desirable.
To test this the following investigation was carried out
(Fig. 5).
Pilom«nt Dlam*t«r « o.+lm.m (o.oiei in)
66
60
95
50
45
4O
35
3Q
25
20
Brcokino
Elongotion
30
25
20
16
IO
5
s,
X
Notn«l
Biti! tiggd ^n°*
0:1
to: I
Draw Ratio
ICI I2:
Fig. 4. Monofilaments made from the Ziegler-type polyethylene.
Homopolymer compared with copolymer. The draw ratios applied to
the investigation as performed are shown by the thick down lines.
Zi«ffUv-typ«
Poly»thjrl«M
OTMT latio
MonofiI«M0t
AUMUr 0.41 ••
(0.0161 in.)
•op« jurat 48110 x 3X65
Trwrl twin* t 481^0 z 3Z70
3 « 35 /•«•
lOMBI
2-3/4 ia.
almiMf.
Fig. 5. Programme of Investigation.
84
Starting from both the Ziegler-type homopolymer
and the Ziegler-type copolymer, eight different kinds
of polyethylene monofils were made using four different
draw ratios during the production, viz. : 6-5 : 1 ; 7-8 : 1 ;
9 : 1 and 10'4 : 1. The resulting products all had a
final diameter of 0'41 mm (0-0161 in). To prevent
confusion, the monofilament types were made in different
colours. Each filament type was used for making both
rope yarn, with the construction 4 S 1 10 x 3 Z 65, and
trawl twine with the construction 4 S 1 50 x 3 Z 70. From
3 x 35 rope yarns, two types of rope were manufactured:
rope 1 with a normal lay, and rope II with a hard lay.
The runnage of the resulting ropes was about 4-5 m/kg,
corresponding with a circumference of 70 mm (2| in).
Table IV. Mechanical properties of polyethylene
monofilaments; diameter 0.41 mm (0.0161 in)
(average values of 20 determinations) .
1WM
HroakUn; «tr»n«th
KlMVMtion
knot breaking
kg.
«/t«
brMk
»try
kc
fl"
6,'j i 1
yellow
1?1.9
3.67
30.1
-3.fi
2.91
23.9
70u
7.0 « 1
red
r/i U)T ^
600
126. V
'..<.7
4,. 7
Od.9
4.i?6
JJ.fc
homopol/ i *P
9." i 1
«r*«
•jOO
1/6.7
<..4S
S('.9
11. '<
5.M
-•b.7
10.4 i 1
brmm
4'.X)
128.9
6.60
W.u
'•'•''
2.76
21.4
6.5 • 1
•hit*
800
1?H.5
4.'>e
35.7
27.1
3.45
26.9
7.6 i 1
li«ht
y»lio«
100
H'5.5
b.Bl
S4.J
J2.9
4.78
30.1
^ie«)tr type
copolyntr
9.0 i 1
tr inapar-
•nt.200
12L.9
7.76
bl..'
16.3
4.50
35.5
10.4 i 1
bl»ok
VX)
12SO
7.84
"'7
14.0
3.08
24.9
Filament properties
Table IV surveys the mechanical properties of the fila-
ment types used.
Not only were the elongation and breaking strength
determined, but also the "cold flow'* under a permanent
load of 12 g/tex.
Fig. 6. Cold flow of polyethylene monofilaments. Permanent load:
12 g/tex, monofilament diameter: 0-4 J mm (0-0161). The per cent
elongation is shown in relation to the time in hours.
Fig. 6 shows remarkable differences among the investi-
gated monofilaments. The lower the stretching percentage
during production, the higher the fibre creep afterwards.
One can realise that the two monofilament types 1 and la,
which have a creep behaviour as is shown, having been
stretched to only 550%, cannot be used for practical
purposes. Further, it should be noted that the copoly-
mers la, 2a, 3a and 4a show more fibre creep than the
corresponding homopolymers 1,2,3 and 4.
Trawl twines
To judge net yarns, the main requirement should be the
ability to give a strong net. As the examination of net
strength is not so simple, the different net yarns were
compared on the basis of their dry "knot strength",
using a not too tightly drawn "overhand knot".
The necessary yarns were all manufactured in the
same way and on the same machine. To determine
breaking strength, use was made of a horizontal appa-
ratus of the pendulum type, constructed so that only the
maximum load could be determined. The measuring
accuracy amounted to 1 Ib and the drawing speed to
80 cm (31 '5 in) per min. To test the yarn, it was wound
round smooth rollers with a circumference of 19 cm
(7-5 in). The distance between the rollers amounted to
50 cm (19-5 in). The yarn always broke either between
or below the rollers but never on the rollers. Table
V gives the results found by testing the above-mentioned
eight different trawl twines.
Table V. Mechanical properties of polyethylene
trawl twines (made from 3x4 monofilaments).
Runnage in m/kg.
Colours
Tex
Runnage
Breakin
a strength
Knot breaking
•trexucth
kg
g/teac
kg
g/tex
yellow
1590
626
36.0
22.6
26.0
16.4
700
red
600
1630
606
54.0
32.7
32.6
19.7
green
500
1710
585
61.2
35-7
28.2
16.5
orown
400
1690
593
62.7
37-1
22.3
13.2
white
600
1630
613
39.0
23-9
30.3
18.6
light
yellow
100
1660
594
61.7
36.7
34.1.
20.3
transp-
arent
200
1700
587
66.0
38,7
32.3
18.9
black
500
1660
602
68.7
41.3
26.1
15.7
These values are an average of 20 determinations. The
only reasonable way to correlate these figures with the
monofilament data, is on the basis of the breaking length.
From Table VI it appears that the higher the breaking
length of the monofilaments, the higher the breaking
length of the net yarns, as would be expected. For the
netmaker, however, it is far more important to compare
the breaking length of the monofilaments with the so-
called knot breaking length of the trawl twines. If this
thought is followed, it is found that the strongest monofils
do not give the best net yarns. There exists a maximum
in the knot strength of thejiet yarns, depending on
Ac draw ratio applied during the production of the
monofilamcnts.
Tabl« TI. Broking lamrthi. (Trawl
twin** ooapar*d with BonofilaMnta )
Colour
Draw ratio
during th*
production
of aonof il*
MOBO-
Trawl
twin**
Trawl twin**
with over-
hand knot
/•How
700
6.5 t 1
30.1
22.6
16.4
r*d
600
7.8 i 1
44.7
32.7
19.7
Zi*ffl*r
*p*
ho»opoly»«r
^500
9.0 i 1
50.9
35.7
16.5
brown
400
10.4 > 1
52.8
37.1
13.2
whit*
800
6.5 i 1
35.7
23.9
18.6
light
Ziaglm*
100
7.8 j 1
54.3
36.7
20.3
typ*
oopoljr»er
trans*
parwit
200
9.0 t 1
61.2
38.7
18.9
laok
300
10.4 t 1
62.7
41.3
15.7
Ropes
The samples for the following investigation were made
as follows: from each of the eight monofilament types,
rope yarns were manufactured with the construction:
4 S 110 x 3 Z 65. This work was carried out on one
machine and the resulting rope yarns were tested in
the same way as described for the net yarns. The strands
were all made from 35 rope yarns and manufactured
under the same circumstances. From these strands
two types of 2f-in ropes were produced, one with a
normal lay and one with a somewhat harder lay.
The testing of the ropes was performed by means of a
hydraulic breaking strength machine fitted with grips. The
distance between grips was 50 cm (19*7 in), and the rate
of movement of the straining head 12-5 cm (4-9 in) per
min. From the 32 determinations, only three breakings
occurred in the grips. In these cases, however, no differ-
ent values were obtained. Elongation was also measured
under different loads. This was done by marking a
distance of about 20 cm (7-9 in) on the rope, and meas-
uring the increase of this length under increasing load.
In this manner, load-elongation curves of these poly-
ethylene ropes could be determined.
Fig. 7 shows four characteristic load-elongation curves
obtained by testing ropes made from homopolymer
monofilamcnts. The two lower curves show a normal
behaviour: increasing elongation with increasing load.
The two upper curves, however, exhibit a rather strange
behaviour. Here the load increases till a maximum value
is attained (by which the rope does not break) and then
decreases with increasing elongation. These two curves
are found by testing the yellow and red ropes. From the
preceding graphs it will be remembered that the yellow
and red monofilaments got the lowest stretch during
production and, moreover, showed a high creep when
86
Fig. 7. Ropes made from ethylene homopolymer (normal lay). The
per cent elongation is shown in relation to the load in kg. The draw
ratio of yellow was 6-5:1, red 7-8: 1, green 9:7, and brown 10-4:1
exposed to a permanent load for several hours. The
load-elongation curves of the ethylene copolymer ropes
exhibit an analogous behaviour, as is shown in Fig. 8.
The conclusion of these experiments is obvious. To
avoid undesired creep in the rodes, the monofilament
producer has to take care that the stretching percentage
in the orientation stage is not too low. The cold flow of
monofilaments, when exposed to permanent load, gives
already some indication of the rope creep to be expected.
Table VII gives a survey of the data that have been
determined by testing the four rope types made from
ethylene homopolymer. Each rope has been tested in
duplicate and the data show little or no differences. The
rope figures can be compared with the yarn properties,
indicated at the left side of the table. From the foregoing
it is clear that the maximum load does not always corres-
pond with the breaking load. Further, it can be stated
that the higher the breaking length of the yarn, the higher
the breaking length of the corresponding rope. The hard-
laid ropes show a somewhat lower breaking load than the
normal laid ones.
% ELONGATION
Fig. 8. Ropes made from ethylene copolymer (normal lay). The per cent
elongation is shown in relation to the load in kg. The draw ratio of
white was 6-5:1, light yellow 7-8:1, transparent 9:1, black 10-4:1.
Table Vlll gives the data found by testing ropes made
from ethylene copolymer. The tendency is the same as
in Table VII, but the figures are on a higher level.
Fig. 9 shows that a linear correlation exists between
the breaking length of the rope yarn and the breaking
length of the rope.
continued on page 88
Table VII Ropea made from ethylene Homopolymer. N » normal lay, H - hard lay
Ropa yarns aada from
3 x 4 monofilamanta
Ropes made from 3 x 35 yarns
Colour
«/»
manage
•A*
break inj
Ioa4
In kff
breaking
length
•
lay
C/»
number
•f laye
per m
oiroum-
ferenoa
In mm
breaking
load
in kg
max.
load
in kg
breaking
length
th«OTCtlMl
teMklat
lo»« IB k|
Mf«
jrl«ld
1,51
662
36,2
25,3
H
204
16,7
67
2125
2600
12,6
4010
65*
V
204
16,7
68
2125
2600
12,8
65*
Ttllow
H
212
18,5
68
2025
2550
12,0
64*
H
210
18,5
68
2025
2330
12,1
64*
1,51
663
58,4
38,7
N
210
16,3
69
3325
4225
20,1
61JO
69*
N
214
16,7
70
3325
422J
19,7
69*
Bed
H
220
18,4
69
3325
4125
16,6
67*
H
218
18,4
68
3200
4125
16,9
67*
1,66
604
66,6
41,5
H
224
15,9
70
5023
5030
22f5
7220
70*
N
224
16,3
70
5025
5050
22,5
70*
Qreea
H
234
18,5
70
4850
4925
21,1
6S*
. _
H
234
18,0
71
4925
4950
21,2
69*
1,65
612
68,5
42,3
*
216
15,9
67
5350
5350
24,6
TWO
74*
i
i
i
I
216
15,7
68
5350
5350
24,6 ,
74*
Brown
H
226
17,8
67
5100
5100
22,6
71 *
I
i
j
H
228
17,7
68
5100
5100
22,4
71*
Table VIII Ropes made from ethylene Copolymer. N - normal lay, H - hard lay
Rope yarna made from
3x4 monofilamenta
Ropea made from 3 at 35 yarns
Colour
ff/»
runnage
»Ag
breaklx*
load
in kg
breaking
length
ft
lay
«/«
number
of laya
per m
oiroum-
ferenoe
in mo
breaking
load
in kg
max.
load
in kg
breaking
length
theoretical
breaking
load in kg
rope
yield
1,60
624
40,7
25,4
N
218
17,1
70
1850
2650
12,2
4270
62 *
H
220
17,1
70
1925
2650
12,1
62*
Vhlte
R
224
16,6
70
1850
2600
11,6
61 %
H
224
18,8
70
1650
2600
11,6
61 %
1,64
606
63,3
36,3
V
224
16,9
70
2650
4525
20,2
6630
68*
Light
yellow
N
H
226
234
16,9
16,1
70
70
3125
3400
4525
4400
20,0
18,8
68*
66 *
H
232
17,9
70
2650
4400
19,0
66*
1,61
612
72,3
44,2
If
218
17,0
70
5525
5550
25,4
7590
73*
Transparent
N
H
218
226
16,7
18,0
68
69
5500
5325
5525
5400
25,3
23,9
73*
71 *
H
226
16,0
67
5325
5400
23,9
71*
1,63
614
80,7
49,5
N
216
16,0
68
5900
5900
27,1
8470
70 %
Black
•
N
H
216
226
16,2
17,9
69
66
5900
5800
5900
5600
27,1
25,7
70*
68*
H
226
16,0
66
5750
5750
25,4
66* i
Tests on Knotless Raschel Netting
Abstract
Knotless nets are of increasing interest for fishing. They are made by
two different methods: the Japanese twisting method and the
Raschel knitting method. Knotless nets are said to have certain
advantages in comparison with knotted nets. Some of these pro-
perties (mesh strength, weight, resistance in water, constancy of
meshes, and catching efficiency when used for herring driftnets)
have been tested for fine knotless Raschel netting. As the main
criterion for the exchangeability of knotted and knotless netting,
equal mesh strength in wet condition has been adopted. With
knotless Raschel netting made of the same fibre material, the mesh
strength, the diameter of the bars and the weight per unit area
depend to a great extent on the type of construction of twine
(bars) and connections. The Raschel machines allow a wide range
of various constructions. On the base of equal mesh strength in
wet condition, the types of Raschel netting tested in comparison
with knotted netting, were found to have (a) less weight per unit
area, (b) bigger diameter of bar, and (c) same towing or current
resistance in water. The constancy of the mesh size of Raschel
netting is better than that of knotted netting. For herring drifting
in the North Sea, the catching efficiency of Raschel netting was
found to be equal to that of conventional knotted and treated cot-
ton netting of equal twine (bar) diameter (one mm). (The same was
found earlier for knotless netting of the Japanese twisting method.)
Epreuves de filets sans noeuds, type Raschel
Rfeum*
Pour la pgche, les filets sans noeuds offrent un interet croissant.
Us sont fabriques de deux facons differentes : selon la methode
japonaise de retordage et selon la methode de tricotage Raschel.
Les filets sans noeuds sont supposes avoir certains avantages sur
les filets noues traditionnels. Certaines de lews proprietes (resistance
des mailles a la rupture, poids, resistance a Pavance dans Peau,
longueur des mailles et efficacite de capture lorsqu'ils sont employes
dans les filets maillants pour la peche au hareng) ont etc eprouvees
pour les filets sans noeuds Raschel, en fil fin. Le principal critere
de cpmparaison est la force de rupture des mailles en condition
mouillee. Avec les filets sans noeuds Raschel, constants dans un
mSme mat£riau la force de rupture des mailles, le diametre du fil
entre les connexions et le poids au metre carr£ dependent du type de
fabrication de ce fil et des connexions ellesmemes. Les machines
Raschel pcrmcttent une grande varied de fabrications. Sur la base
d'egales forces de rupture des mailles, en condition mouillee, les
types de filets Raschel eprouves avaient en comparaison avec les
filets noues: (a) un moindre poids au metre carre; (b) un plus grand
by
Dr. A. von Brandt
\
Institut fur Netz und Materialforschtmg,
Hamburg
diametre de fil; (c) une meme resistance a 1'avance dans Peau.
La dimension des ma i lies est plus stable dans le filet sans noeuds de
type Raschel que dans le filet nou6. Pour la peche au hareng dans la
Mer du Nord, avec des filets maillants, les filets sans noeuds Raschel
ont permis des captures egales a eel les effectuees avec des filets
npues conventionnels de meme dimension de mailles, pour un
diametre de fil de un mm. (La meme constatation avait et£ faite
avec les filets sans noeuds de type retordu).
Ensayos con redes Raschel sin nudos
Extracto
Las redes sin nudo despiertan cada vez mas interes en la pesca;
se hacen de dos manera distintas: la japonesa de la torsi6n y la
Raschel del tejido. Las redes sin nudos tienen ciertas ventajas
sobre las anudadas. De las redes Raschel sin nudos se han ensayado
algunas propiedades, entre las que estan la robustez de la ma I la,
peso, resistencia en el agua, firmeza de las mallas y rend im lento de
pesca cuando se emplean en los artes de deriva arenqueros. Como
criterio principal de la posi bill dad de intercambiar redes con y sin
nudos, se ha adoptado la igualdad de robustez de la malla cuando
esta humeda. Con las redes Raschel sin nudos hechas de la misma
fibra, la robustez de la malla, el diametro de los hilos entre nudos y
el peso por unidad de area dependen mucho de la manera de fabri-
carlas y de las conexiones. Las maquinas Raschel son capaces de
fabricar muchas clases de red. Basandose en la igualdad de la robus-
tez de la malla humeda, las redes Raschel ensayadas eran, con
respecto a las anudadas, (a) menos pesada por unidad de area, (b) de
mas diametro entre nudos y (c) de la misma resistencia al remolque
o a corriente de agua. Las mallas de las redes Raschel son mas con-
stantes que las de las anudadas. En el caso de la pesca a la deriva
del arenque en el Mar del Norte, la capacidad de captura de las
redes Raschel era igual que la de redes de algodon tratado y
anudado normales de hilo del mismo diametro entre nudos (una
mm). El mismo resultado se habia pbtenido anteriormcnte para
redes sin nudos hechas con hilos torcidos.
continual from page 86
9rw*9.*i
3O
28
26
24
22
specifications for characterising polyethylene raw material
suitable for rnonofilament production.
2. Under comparable circumstances, ethylene copoly-
mers give stronger monofilaments than ethylene homo-
polymers.
3. The knot tenacity of monofilaments shows a maxi-
mum as function of the draw ratio.
4. The strongest monofilaments do not give the best
net-yarns regarding the knot tenacity.
5. For rope making, highly stretched monofilaments
are desirable.
Fig. 9. Freaking length rope versus breaking length rope yarn.
Conclusions
From the investigations come these conclusions:
1. Melt index and specific gravity are insufficient
88
Acknowledgments
The monofilaments were manufactured and tested by Nyma
Rayon Works (Kunstzijdespinnerij NYMA N.V.), Nijmegen,
Netherlands.
The trawl twines and ropss were made by Esbjerg Tovvaerks-
fabrik A/S, Esbjerg, Denmark, and tested by "Nederlandsche
Visserij-Proefstation", Utrecht, Netherlands.
AT present knotted netting is still the main material
for fishing nets, including modern fishing gear.
Knotted netting is made by hand braiding or by semi-
or fully automatic machines. The weaver's knot and the
reef knot (especially in Asia) are the main knot types
used in netmaking. Double knots developed from these
two basic types are also used (v. Brandt 1957).
Recently so-called knotless netting is becoming
increasingly important. This interest stems from reports
that knotless netting is cheaper than knotted netting and
has certain technical advantages. This development
makes it necessary to establish testing methods for deter-
mining the qualities of knotless netting which are impor-
tant for fishing purposes and to establish rules for substi-
tuting knotless for knotted netting in fishing gear.
Manufacturing techniques
Knotless nets have been well known for a long time.
Loosely woven fabric as used, for example for Japanese
minnow nets, can also be regarded as knotless netting.
Such small meshed material, which cannot be manu-
factured by knotting, will retain its importance in Asia
for catching small fish.
Introduction of chemical fibres for fishing brought the
idea of welding or glueing synthetic fibres together to
form netting. Stamping or moulding finished net sheets
has been tried. Such techniques so far have not produced
satisfactory fishing nets.
In 1922, knotless netting made by a twisting technique,
was introduced in the Japanese fishery. (Nippon Seimo
1959.) This type of knotless netting is made of twines
consisting of only two yarns (Fig. la). The connection
of the twines to form meshes is made by interlacing
the yarns of two twines, once or several times. According
to the construction of the joining points, the twines run
diagonally through the netting or in a zig-zag line. If
the interlacing of the twines at the joining points is done
several times, the shape of the mesh may be changed
from rhombic into hexagonal. This Japanese twisting
technique stimulated efforts elsewhere to develop pro-
duction techniques for new types of knotless netting.
As a result of these efforts, another type of knotless
netting was introduced into fisheries around 1951. The
manufacture of this type is based on the Raschel tech-
nique, well known in curtain-making for at least 100
years (Fig. Ib). This development has been especially
promoted in the U.S.A., Belgium, Italy and Germany.
The Raschel technique of knitting is done by special
machines (Reichel 1960).
The bars of the meshes are built up by one or two knit-
ted strands and one to three additional woofs for
strengthening (Fig. 2). For each strand two guide bars
are needed. Usually knitting machines with six to eight
guide bars are used for fish netting.
The Raschel machine not only makes the connections
to form meshes but also knits the mesh bars. Therefore
the Raschel machine produces per unit time, not a
certain number of connections but a certain area of
netting. The size of this area is not influenced by the
Fig. 1. The two main types of knotless netting used in fishery,
(a) Japanese twisted type. (/>) Raschel type.
mesh size, i.e., with the same type of material the pro-
duction of a certain mesh size needs double the time
needed for producing a mesh of half that size. Thus
Raschel machines operate quite differently from knotting
machines. The knotting machines work with finished
twines and produce only the knots. Per unit time they
produce a certain number of knot-rows on which the
89
mesh strength can be produced, depending on the type
of construction. In all cases tested here, the mesh strength
of Raschel netting is higher than that of the knotted
netting made of the same fibre material. But it must not
be forgotten that the construction of the twine (bar) in
the Raschel netting is completely different (Fig. 4) and
that the twine number given for the Raschel bar is there-
fore not directly comparable with the conventional
twine number.
Weight of knotless Raschel netting
In general the trade with netting is based on the weight
of a net sheet 100 m long and 100 meshes wide, absolutely
dry, plus an official moisture content which depends on
the kind of fibre. With decreasing mesh size, the weight
of a given area of netting increases very quickly because
the number of mesh bars and knots is increasing. In the
heavy codends of bottom trawls made of manila,
double braided, with a mesh size of 70 mm stretched,
the weight of the knots can be more than 60 per cent
of the total weight. The share of the weight of the knots
increases with decreasing mesh size and increasing twine
diameter.
Since for the present tests only small samples were
available, the tables commonly used by netmakers for
calculating the weight of net sheets could not be used,
and the net weight had to be newly determined. This
was done for pieces of netting of different size but of
32 meshes each made of medium-laid continuous multi-
filament nylon twines (Fig. 5).
height of knotted n*tttn£
with 12 m«Bh«« v»r»u»
m**li»tx* («tr«tched) for
nylon continuous nultlflli
- - -- 6 to 210
Fig. 5. Weight of knotted netting per unit area versus mesh size
(stretched) for nylon continuous multifilament twines.
Td 210 x 6 to 210 x 60.
The mesh size (stretched) was determined with a special
pressure gauge (1 kg) constructed for fine netting (Florin,
1957).
For the comparison of weights of netting it must be
taken into account that knotted netting very often and
knotless netting, especially when made of synthetic
fibres, are practically always treated during manufacture
or later with some bonding agent which adds to the
weight. The data in Fig. 5 are for untreated knotted
netting and can only be compared with untreated knotless
netting.
The weight of the Raschel netting samples in Table I,
in comparison with knotted netting of the same mesh
strength according to Table II, is given in Table III.
TABLE 111
Mesh size
Raschel netting Weight
stretched
(mm)
(8)
19-6
210x11 normal 0-5
50-0
210 x 11 normal
•1
80-0
210x11 normal
•7
19-6
210x11 special (
)-5
50-0
210x11 special
•2
80-0
210x11 special
•8
19-6
210x11 super (
)-7
50-0
210x11 super
•4
80-0
210x11 super
•8
Knotted
Weight
netting
210x18
ft
210x18
1-3
210x18
1-9
210x21
0-9
210x21
1 -6
210x21
2-2
210x24
1-0
210x24
1-8
210x24
2-5
According to this small number of samples at equal
mesh size and equal mesh strength, of the three construc-
tions tested, Raschel netting is lighter than knotted
netting. This means that Raschel netting of the tested
types gives more net per unit weight than knotted netting
of the same mesh size and mesh strength.
Towing resistance of Raschel netting
The towing or current resistance of netting in water is
of high interest for all towed or dragged fishing gear,
as well as for gear set in a current, because the resistance
and the water pressure wave caused thereby have a
bearing on construction and power requirements and
may also influence the catching efficiency of the gears.
The current of towing resistance depends on mesh size,
twine diameter (mesh bars) and size and kind of the
joining points of the meshes.
The diameter of normal net twines is relatively easy to
measure. Some values for medium laid, continuous
multifilament nylon twines are given in Table IV (Klust
1960).
To determine, for comparison, the diameter of the
mesh bars in Raschel netting is more difficult. Due to the
construction, the cross-section of the bars in Raschel
netting is not circular, but more or less rectangular.
Since it cannot be predicted which side of the bar will
face the waterflow in a fishing gear, the wider side has to
be considered.
The following comparison has been made with the
same samples of Raschel netting (Table I) versus the
same corresponding samples of knotted netting (Table II).
As can be seen in Table I, the diameter of the bars in
Raschel netting can vary considerably according to the
structure, even if the fibre material is the same. Due to
the specific structure, the same twine number or count
results in a thicker twine or mesh bar in Raschel netting
Td.
Diameter
(mm)
210x2
0-28
3
0-32
6
0-45
9
0-60
12
0-67
15
0-78
TABLE IV
Td. Diameter
(mm)
Td. Diameter
(mm)
210x18
21
24
27
30
33
0-84
0-92
1-00
1-05
1-10
1-20
210x36
48
54
60
72
•30
•40
•50
•57
•72
92
compared with normal twisted net twine in knotted
netting. The reason is that in the knitted bars of Raschel
netting the main strands are reversed in direction by
knitting and appear in the cross-section several times
(See Fig. 2). The comparison of the diameters of the bars
of Raschel netting with those of knotted netting of equal
mesh strength is given in Table V.
For equal mesh strength the bars of Raschel netting
are always thicker. This could be a disadvantage of
knotless Raschel netting in comparison with the knotted
netting but, as mentioned before, the towing or current
resistance of netting in water does not depend on the
diameter of the bars alone, but also on the area and shape
of the connections.
Raschel netting
material structure
210x11 normal
21 Ox 11 special
210 x 11 super
TABLE V
Diameter Equal to knotted
(mm) netting material
1-05 210x18
1
10
14
210x21
210x24
Diameter
(mm)
0-84
0-92
1 -00
The determination of the area of the joining points as
a criterion for towing resistance is even more difficult.
The joining points can be projected on a plane and the
area measured by planimetering (v. Brandt 1958a).
This is, however, not enough for a comparison of knotted
and knotless connections because the differences in the
three-dimensional shape of the joint points is neglected
and this will also influence the resistance in water.
The determination of the projected area of netting is
therefore considered an insufficient guide in comparing
towing or current resistance of netting. For reliable
comparative results it has to be measured with samples
of complete netting (bars and joining points together)
in a towing tank. For the present studies netting samples
of 1 m2 were tested normal to the direction of tow
(N.N. 1960). Since such towing tests are rather expensive,
the number of samples tested so far is limited and com-
prehensive tables for the towing resistance of knotted
netting of different materials and different mesh sizes
are not available yet.3
For the present comparison the same samples (Table f
and Table II) have been tested. To keep the number of
samples small, this comparison was limited to a mesh
size of 50 mm stretched (No. 4, 5 and 6 in Table I and II).
The results for a towing speed of 0*6 to 2*0 m/sec (1 '2 to
3-8 knots) are shown graphically in Figs, 6 to 8.
Fig.
No.
Material
5
2a
2b
Raschel, normal
knotted, 210x18
6
3a
3b
Raschel, special
knotted, 210x21
7
4a
4b
Raschel, super
knotted, 21 Ox 24
Mesh size
Diameter
of bar
(mm)
(mm)
49
1-05
49
0-84
50
I -10
50
0-92
50
1 -14
50
1-00
3 The present tests have been made in the towing tank of
Ingenieurschule, Hamburg, by Oberbaurat BischofT.
100
KP
-tr
nfir
Fig. 6. Towing resistance of samples of I m* of netting of equal mesh
strength normal to the towing direction. Mesh size 49 mm stretched
mesh shape square. 2a. Raschel, normal, 210x11, diameter of bar
1-05 mm. 2b knotted, 2 WxJ89 diameter oj bar 0-84 mm.
In spite of the thicker bars of the Raschel netting, no
difference in the towing resistance could be found in
comparison with knotted netting of the same mesh
strength. Obviously the higher towing resistance of the
thicker mesh bars of the Raschel netting is just compen-
sated by the lower resistance of the joining points. As
regards towing resistance, knotless Raschel netting and
normal knotted netting are practically equal.
Constancy of mesh size in Raschel netting
Mesh-size measurements for comparison have to be
made with mesh measuring gauges applying defined
pressure. For fine netting the official pressure gauge of
the Lake of Constance can be used (Florin 1957). For
heavier netting, e.g., bottom trawls of deep-sea trawlers,
a special pressure gauge has been developed for the area
of ICES and ICNAF which measures the opening
of the meshes only, excluding the knots (Bohl 1961 a).
For testing the constancy of mesh size, a representative
number of meshes in a net sample is measured and the
standard deviation is determined. The test may be done
with new or used netting according to the purpose of
the study.
For the present study, roundfish bottom trawl codends
made of 'Perlon' were compared. One was single braided
93
r
.
F<^. 7. Towing resistance of samples ofl m2 of netting of equal mesh
strength normal to the towing direction. Mesh size 50 mm stretched,
mesh shape square. 3a Raschel, special 2/0x77, bar diameter 1-10
mm. 3b knotted, 210x21, bar diameter 0-92 mm.
Raschel netting, with short joining points. Two others
were of knotted netting. With the knotless codend
some heavy redfish catches had been made. The results
are given in Table VI (Bohl 1961b).
Codend type
knotless
knotted
knotted
TABLE VI
Number of
meshes
measured
360
368
232
Mesh size in average
121 -3 ±0-1 mm
128 -8 ±0-2 „
145 -7 ±0-2 „
Standard
deviation
1-7
4-2
3-5
The standard deviation for the mesh size is much less
in the knotless Raschel netting than in knotted netting.
Of course, the type of the Raschel connections will
influence the constancy of the mesh size, and other con-
nection types may give different results.
Comparative catching efficiency of knotted and knotless
herring driftnets
As mentioned, the mesh strength is not always the only
criterion for the exchangeability of knotted and knotless
netting. In driftnets used for herring fishing in the North
Sea the bars of the meshes must have a certain diameter
(1 mm). A smaller diameter would damage the gilled
94
fish. Conventional cotton gillnets (knotted), preserved
against rotting and stiffened as used in German herring
drifting, have been compared with knotless Raschel
netting, normal construction (Fig. 4a), Terlon' 420 X 3,
also treated to give similar physical properties (Table
VII).
TABLE VH
Mesh size (bar)
Mesh strength:
dry
wet
Bar, diameter
Weight
Cotton Terlon'
knotted knotless
25 -3 mm 24 -9 mm
6 -Okg 15 -5 kg
6 -3 kg 14 -2 kg
1 -0 mm 1 -0 mm
2 -05 g ] -05 g
Table Vll shows the mesh strength of the Raschel
nets was much higher than that of the knotted cotton
nets. This could not be avoided, because it was necessary
to have the diameter of the mesh bars no smaller than
1 mm. The knotless nets were much lighter but this, to a
certain extent, was due to the difference in treatment with
bonding and protecting agents.
For this experiment, 11 knotless driftnets were in-
serted between conventional knotted cotton nets in the
same fleet of driftnets. For comparative tests of the
catching efficiency of certain single driftnets operated
KP
*A
1.2 M io aW M xt »• xn 40
Ff^. 5. Towing resistance of samples of 7m2 of netting of equal mesh
strength normal to the towing direction. Mesh size 50 mm stretched,
mesh shape square. 4a Raschel, super 210 x 11, bar diameter /* 14 mm.
4b knotted, 210 x 24, bar diameter 1*00 mm.
with a great number of conventional driftnets in the
same fleet, the catch in all the nets of the whole fleet has
to be considered. If a small school hits a fleet of driftnets it
will fill the nets in the section corresponding to its size
and leave the rest empty. The resulting uneven distribu-
tion of catch over a long fleet of nets is quite normal in
herring drifting. In order to avoid that the nets under
test do not meet statistically inadequate conditions in a
limited number of observations, a special observation
method has been developed (v. Brandt 1955). The number
of fishes gilled in each single driftnet is counted or
estimated, and represented graphically. In this way it
can be seen how the catches vary from net to net in a
fleet. Where the efficiency of nets under tests differs from
the conventional nets, the more or Jess smooth curve
of the diagram will be interrupted.
This method was used earlier to compare catching
efficiency of knotless *Manryo'-nets (Japanese twisting
technique) with conventional knotted cotton driftnets for
herring in the North Sea. No difference could be found
(v. Brandt, 1958b).
For the present comparison of conventional knotted
cotton nets with knotless Raschel nets (Table VII) the
results of eight sets have been evaluated.4 Different
quantities of herring were caught per set (2'5 to 10*5
tons). The catching efficiency of the Raschel nets tested
did not differ from the conventional knotted cotton
nets.
For two of these sets the graphs showing the number
of gilled fish in each net are given in Fig. 9. The knotless
Raschel nets are marked by underlining. In the other
sets the position of the nets under test in the fleet of
nets was also altered. As can be seen from the graphs
the quantity of the catch in the knotless nets fits well in
the catch distribution of all nets in the whole fleet and
the catching efficiency of both net types can therefore
be considered equal. This is of interest because knotless
Raschel nets, due to their lower weight, are easier tc
handle.
4 These experiments have been made with the help of Vereinigte
GlanzstoffFabriken A.C., Wuppertal-EIberfeld, and Plutte, Koecke
and Company, Wupperta I- Barmen. The Bremen-Vcgesacker
Fischerei-Gesellschaft kindly allowed the experiments to be
conducted on one of their drifters by Dipl. Biol. Koops.
Fig. 9. Example of graphical evaluation of the catches in a fleet of
herring driftnets for the determination of the comparative catching
efficiency of single nets of different types. The tested nets are underlined
References
Bohl, H.: 1961 a. Vergleichende Untersuchung Uber die Eignung
von Druckmessgeraten zur Maschenmessung an Schleppnetzcn.
Arch.f. Fischereiw. XII, pp. 117-137.
Bohl, H. : 1961 b. German mesh selection experiments on redfish.
ICES, Comp.Fish.Committee No. 88.
v. Brandt, A.: 1955. The efficiency of driftnets. Journ. du Cons.
Int. pour 1'Explorat. de la Mer, XXI, pp. 8-16.
v. Brandt, A. : 1958 a. Knotenlose Netze. Protokolle zur Fischerci-
technik 5, pp. 100-112.
v. Brandt, A.: 1958 b. Die Fa'ngigkeit knoten loser Heringstreib-
netzc aus Manryo. Informationen flir die Fischwirtschafl, 5, pp. 192-
196.
Florin, J.: 1957. Em Messgera't zur einheitlichen Bestimmung
von Netzmaschenweiten. Schweiz. Fischerei-Ztg. 65, p. 243.
Klust, G.: I960. Synthetisches Netzmaterial fiir die Kutter- und
Kustenfischerei. Land-u. hauswirtschaftlicher Auswertungs- u.
Informationsdienst No. p. 191
N.N.: I960. Uber Widerstandsmessungen an Schleppnetzcn.
Dtsch. Seiler-Ztg. 79, pp. 196-197.
The Nippon Seimo Co. Ltd.: 1959. The knotless net. Modern
Fishing Gear of the World, pp. 107-109.
Reichel, H.: 1960. Herstellung und Eigenschaften knotenloser
Netze. Fischereiforschung 3, No. 10, pp. 3-22.
van Wijngaarden, J. K. : 1959. Testing methods for net twines and
nets, etc., Modern Fishing Gear of the World, pp. 75-81.
95
Knotless Netting in the Norwegian Fisheries
Abstract
The use of knotlcss netting in the Norwegian fisheries has increased
from 17 tons in 1960 to about 200 tons in 1962. Initially all was
imported but by 1962 19 machines for the production of knotless
netting of Raschel type had been installed by Norwegian net manu-
facturers. Consequently only three per cent of the present consump-
tion of knotless netting is now imported. High tenacity polyamide
continuous multifilament yarn is used. Most of this netting has up
until now been of small mesh size and used in purse seines for
small herring and sprat. Table II gives denier values of equivalent
plied (twisted) twines and Raschel; the latter tend to be of appreci-
ably heavier denier for the same breaking strength, but a much
smaller proportion of the twine length is used in the interlacings
than in the case of knotting. At present prices in Norway, purse
seines made from small-meshed knotless netting are 25 to 30 per
cent cheaper than if made from knotted netting. However, with
increasing mesh size, a point is reached where knotted netting can
be produced more economically on conventional machines.
Filets sans noeuds dans la ptehe Norvegienne
l/emploi de filets sans noeuds dans la peche Norvegienne a aug-
ment* de 17 tonnes en 1960 jusqifa 200 tonnes en 1962. Au d£but,
tous les filets sans noeuds 6taient importes mais en 1962 les pro-
ducteurs de filets norvtgiens ont installe 19 machines pour la
fabrication de ces filets, du type Raschel. De ce fait, trois pour cent
settlement de 1'actuelle consommation de filets sans noeuds, sont
importes. Les fils sont fails de polyamide a haute tenacite, en multi-
filament continu. La majeure partie de ces filets ont des petites
mailles et sont utilises dans les sennes coulissantes pour le hareng
et Tesprot. Le Tableau II donne les valeurs en deniers des fils equiva-
lents, tournes et en tresse Raschel ; le dernier est plus lourd en deniers
pour le meme force de rupture mais dans les entrelacs des mailles, on
utilise une longueur inferieure a celle des filets noues. En Norvege £
present, les sennes coulissantes faites de filets sans noeuds, a
petites mailles sont de 25 a 30 pour cent rnoins chers que les filets
noues. Cependant, avec des mailles agrandies on parvient a obtenir
sur des machines conventionelles, des filets noues plus economi-
ques.
Redes sin nudos en la industria pesquera Noruega
Extracto
El empleo de redes sin nudos en la pesca Noruega ha aumentado
desde 17 toneladas en 1960 hasta unas 200 en 1962. Inicialmente se
importaban todas pero para 1962 los fabricantes noruegos habian
instalado 19 mdquinas para tejer redes sin nudos del tipo Raschel
con lo cual solamente el tres por ciento de las redes sin nudos
empleadas actualmente son de importacidn. Los hilos son de
poliamidos de varios filamentos continups de gran resistencia.
Hasta ahora casi todas estas redes han sido de malla pcquena y
se han dedicado a la construcci6n de artes de cerco de jareta para
la captura de espadin o arenque pequeflo. La Tabla II da los
valores denier de hilos torcidos y Raschel equivalentes; los ultimos
tienden a ser de denier bastante mayor para igual resistencia en la
rotura, pero se emplea una proportion menor de hilos en las ligadas
que en el caso de las redes de nudos. Actualmente en Noruega las
redes de cerco de jareta de malla pcquena sin nudos cuestan de un
25 a un 30 por ciento menos que las anudadas, pero con el aumento
del tamano de la malla se llega a un punto en el que las anudadas se
fabrican mas economicamente en las m&quinas corrientes.
BECAUSE of very limited local production, knotless
netting used in Norwegian fisheries since 1960 has
been mainly imported. Annual consumption of this
type of netting (about 1 7 tons in 1960) gradually increased
to almost 200 tons in 1962, as can be seen from Table I.
By 1962, 19 machines for making knotless netting had
been installed by Norwegian net manufacturers, and
imports were gradually reduced to only 6'7 tons in 1962,
96
by
Norvald Mugaas
Statens Fiskeredsksimport
— about three per cent of the total knotless netting used
that year. Norwegian knotless netting is mainly made of
high tenacity polyamide continuous multifilament yarn,
and most of it has been of the small mesh type used in
purse seines for small herring and sprat.
TABLE I — Import, production, and consumption of knotless
netting in Norway 1960-1962
1960
Imported polyamide knotless
netting, tons . . . . 15-0
Knotless netting locally produced
from imported polyamide
yarn
Total Consumption
1 -6
16-6
tons
1961
82-1
86-3
168-4
tons
1962
6-7
192-5
199-2
tons
When knotless netting was first introduced at the
beginning of 1960, experiments were carried out by
inserting sections of knotless netting in the purse seines
made traditionally of knotted netting. This type of
netting has also been used on a smaller scale in purse
seines for big herring which have a somewhat larger
mesh size, as well as in shrimp trawls and other gear.
The knotless netting used is of the warp knitted type,
produced on Raschel type machines, mainly of six-bar
patterns. This construction method allows a great variety
of mesh shapes, all based on the same yarn size, depend-
ing on the number of interlacings; this in turn influences
apart from shape of mesh, also the strength and weight
of netting. With only one or two interlacings at the
joints, the meshes will be square as with knotted netting;
by increasing the number of interlacings the mesh be-
comes hexagonal in shape. The square-shaped mesh
seems to be the best as it provides equal mesh bars in all
directions.
Experience in making knotless netting indicates that
the best ratio of strength to weight is got by using filament
yarn of the same count throughout in each section, i.e.,
by not mixing different yarn counts in one netting
construction. Basic yarns used for different types of
netting are standard counts available in polyamide
yarn, such as 210d, 300d, 420d, 630d and 840 denier.
Table II gives the Raschel construction denier values
which can be substituted for common plied twines. The
indicated total denier is the approximate value and takes
into account the shrinkage of the yarn due to twisting
in plied twines and to knitting in Raschel construction
respectively.
continued on page 97
Knotless Fishing Nets on Raschel Equipment
in Italy
Abstract
At the end of 1962 about 20 Raschel looms were in operation in
Italy. Each machine has a mean capacity of 30,000 kg per year
The production of knotted nets of polyamide fibre has in the past
three years, averaged 500 tons per year in Italy. The main reason for
the rapid introduction of the Raschel method is lower production
costs due to saving in material and higher operating speed, ranging
from 350 courses per minute (eight guide-bars) to 600 courses (four
guide-bars) ; the coarser the yarn, the fewer are the courses in the unit
length. Depending on the thread counts, the production of 100 kg of
net on a modified Raschel machine, four guide-bars, 700 meshes,
takes from three hours (210/90) to 22 hours (210/4). The smaller
the mesh, the more apparent the advantage of the Raschel method
becomes; 40 mm is considered the break-even limit, except for
nets made of high-denier yarns. In Italy, knotless nets are less
expensive than knotted for almost all mesh dimensions. High
tenacity polyamide multifilament yarns, bright 210 denier (and
multiples) are generally used. The best combination of denier/
interlacing/machine gauge has not yet been determined and is still
the subject of research. Run-proof nets are aimed at but not yet
achieved. Heat-setting is often still used according to the technique
for knotted nets but equipment for continuous dry setting is being
developed.
Filets>aTis>oeuds>roduits>ur]equipement]Raschel en Italic
A la fin de 1 962, environ 20 metiers Raschel fonctionnaient en Italic.
Chaque machine produit, en moyenne, 30.000 kilos par an. La
by
Mario Damiani
Societa Rhodiatoce S.p.A.
Milan
production de filets sans noeuds en fibre polyamide, pendant les
trois dernieres annees a 6te en moyenne de 500 tonnes par an pour
r Italic. La raison principale de 1'introduction rapide de la m6thodc
Raschel est le prix de revient de production plus has. La mdthodc
Raschel en efiet, permet une economic de materiel et une Vitesse
d'operation plus elevee, entre 350 et 600 courses par minute; plus
grossier sera le fil, moins nombreuses seront les courses par unite
de longueur. Selon le nombre des fils, la production dc 100 kilos de
filet sur une machine Raschel de type modifte, quatre barreaux de
guide, 700 mailles, demande de trois heures (210/90) & 22 heures
(210/4). L'avantage de la mtthode Raschel est plus apparent pour
les petites mailles; des 40 mm, il devient plus coftteux que pour les
filets nou6s except&s pour les filets fails de fil de fort denier. En Italic
continued from page 96
TABLE II — Denier values of equivalent plied twines and Raschel
twines
Twine No.
Total Denier
Raschel Con-
Total Denier
Denier
in twine
struction
in Raschel
Denier
210/2x2
900
210/3
1260
210/2x3
1400
300/3
1800
210/3x3
2140
420/3
2520
210/4x3
210/5x3
2800
3600
630/3
3780
210/6x3
4300
840/3
5040
Of the total length of twine used in making a mesh, a
substantial part goes into the knot in knotted netting
and this portion increases as the mesh size becomes
smaller. In Raschel netting a much smaller proportion of
yarn or twine length is used in interlacings. Therefore,
the ratio of total denier for plied twine and Raschel
twine does not represent the correct ratio between the
total material in the same size panels of knotted and
knotless construction. Furthermore, owing to the differ-
ent construction of the joints (knots and interlacings),
the percentage of twine strength lost in the joints cannot
be compared (see Table III).
Production capacity of Raschel machines, compared
with that of machines for producing conventional knotted
netting, presents most favourable conditions for pro-
ducing small mesh knotless netting (as used in purse
seines) and the bulk of knotless netting used in Norway
has been for such nets.
As mesh size increases, a point is reached where knotted
netting can be produced more economically on conven-
tional machines. Where production costs for both types
of netting are the same, the deciding factors must then
be the serviceability of the netting in operation, such as
bulk, resistance, etc. As knotless nets are rather new
in Norwegian fisheries, it is most difficult to secure a
valid opinion. However, the increasing demand for
knotless netting shows the great interest created. At
present prices in Norway, a complete purse seine made
from small-meshed knotless netting will be approximately
25 to 30 per cent cheaper than if made from the conven-
tional type of knotted netting.
TABLE 111— Weight and strength of comparable knotted and
knotless netting
Mesh strengths and weight of panel 600 meshes x 100 metres
Polyamide knotted netting Polyamide, knotless netting
(single knots)
Mesh size: 35-7-5 mm
Twine No.
Weight
Wet mesh
Raschel
Weight
Wetmssh
Denier
(kg)
strength
Denier
(kg)
strength
(kg)
(kg)
210/2x2
16-20
5-5-6-0
210/3
22-3
5-5-6-0
210/2x3
25-31-5
7-8
300/3
32-0
7-8
210/3x3
37-50
11-12
420/3
44-6
12-13
210/4x3
210/5x3
53-72
71-96
14-15
17-18
630/3
66-9
17-18
210/6x3
91-5-123-5
21-22
840/3
89-2
24-25
The knotless netting is given preservative or stiffen-
ing treatment similar to the knotted netting; coal-tar,
bonding, dyeing, etc., are used depending on the end
use. Coal-tar increases the mesh strength of knotted
netting but this effect is less noticeable with knotless
netting.
97
ks filets sans nocuds sont moins chcrs que les filets noufc pour pros-
que toutcs les dimensions de mailies. Les fils de polyanude multifila-
ments de haute tenacite, brillants, de 210 denier (et multiples) sont
generalemcnt utilises pour la fabrication des filets. La meilleure
etc determinee et la recherche se poursuit. On essaie de produire
des filets qui sont indemaillables mais on n'y est pas encore parvenu.
La fixation par la chaleur est encore utilise* tres souvent suivant la
technique des filets noues mais Tequipement pour la fixation & sec
est toujours en cours de developpement.
Redes sin irados producktas por las
Raschel en Italia
Extracto
A fines de 1962 funcionaban en Italia unos 20 telares Raschel. Cada
telar ticnc una cajpacidad media de 30.000 kg al afto. En los ultimos
tres afios se ban fabricado en Italia, por tcrmino medio, 500 tons de
redes anudadas de fibres de poliamidos. La principal raz6n de la
rapida propagaci6n del metodo Raschel es que los costos de produc-
tion son mas bajos debido a un ahorro de material y a una mayor
velocidad de funcionamiento que va de 350 puntos por minuto
(ocho barras-guia) a 600 puntos por minuto (cuarto barras-guia).
Cuanto mas grueso es el hilo menos puntos hay que dar poJ
unidad de longitud. Segun el numero del hilo, la producci6n de
100 kg de red en una maquina Raschel modificada de cuatro barras-
guia, 700 mallas, tarda de tres horas (210/90) a 22 horas (210/4). Las
ventajas del metodo Raschel son mayores cuanto mas pequefla es
la malla, considerandose la de 40 mm como el limite, excepto en el
caso de redes hechas de hilo de mas denier. En Italia las redes sin
nudos son mas baratas que las anudadas en casi todas las dimen-
siones de malla. Generalmente, se emplean poliamidos de varies
filamentos de 210 denier (y multiples). Todavia no se ha detcrminado
la mejor combinaci6n de denier/ligamcnto/y calibre de la maquina,
por lo que continuan los estudios. Tampoco se ha conseguido
fabricar redes en las que no se corran los nudos. Todavia se usa con
frecuencia la fijacion termica segun la tecnica para las redes
anudadas, pero se fabrican maquinas para la fijacidn continua en
seco.
FROM West Germany the Raschel method of manu-
facturing knotless fishing nets spread quickly to
other European countries. In Italy lately, and especially
last year, all the most important net manufacturers have
added Raschel looms to the conventional knotting
machines.
At the end of 1962, about 20 Raschel looms were in
operation.
Most of these looms have six guide-bars (also four
and eight), 124-inch and 139-inch width (also 100-inch),
while gauges range between 18 and 28 needles per two in.
The most popular type is 24 gauge.
For a country like Italy, where the production of
knotted nets (polyamide fibre) has averaged 500,000 kg
per year in the last three years, this number of Raschel
looms is undoubtedly high considering the production
capacity of these machines — on either light or heavy nets,
a machine has a mean capacity of 30,000 kg per year.
The reason for this new trend is economic; production
costs will be lower with a considerable saving of material
by eliminating knots. The weight of knots is often out of
proportion to the weight of net. Elimination of knots
becomes more important as meshes grow smaller and
the threads become coarser. In addition, towing speed
can be noticeably raised.
Actual knitting speed ranges, roughly, from 600
courses (four guide-bars) to 350 courses per minute
(eight guide-bars).
The quantity of net produced in the unit of time (most
important as nets are sold by weight) depends also upon
the number of courses, the width of the machine and
98
the denier of the yarn used. The coarser the yarn, the
fewer are the courses in the unit length; the runnage per
hour for 840 den (8-9 courses per cm) is about twice that
of 210 den (15-16 courses per cm), while the weight of
the net produced is three times as much (Table 1).
Table I— Production of 100 kg of net on a modified Raschel machine,
four guide-bars, 28 gg, 700 meshes (thread counts are expressed in
deniers, as is customary for the twines used in knotted fishing nets)
210den:(210x4)=22hr
(210x6)- 16,,
(210x9)=12,,
840den:(210xl8)-8hr
(210x27) =5,,
(210x48)=4,,
(210x90) -3,,
There are two advantages: in material and in proces-
sing. Firstly, for Raschel machines either flat or low-
twist yarns are used, whereas for knotted net machinery
higher priced twine is required. Average price ratio of
ply yarns (on beams) versus twine is 1/1*2. Secondly,
Raschel looms are equipped with warp beams on which
a great length of yarn is wound ; this means that they can
be operated for a much longer time than conventional
machines, the performance of which is limited because
of low bobbin capacity.
The advantage of the Raschel method over the conven-
tional one becomes more apparent the smaller the mesh.
Actually, in the first case the yield, which does not depend
upon the mesh size, is inversely proportional to the
number of courses per cm; in the second case the yield
increases with the size of the mesh. Admittedly 40 mm is
the limit beyond which the knotting machines become
more economical. It is clear, however, that this limit
increases considerably in the case of nets made of high
denier yarns.
In Italy, knotless nets are less expensive than others
for almost all mesh dimensions, as indicated in Fig. 1
which shows the price in lire per kilo of the two types
of commercial nets on the basis of mesh size and weight
of the net.
6000
2
Mg.l. Frio* llrw'lcg of two tyiws of ooima«roial
—
Of the ri«t.
a? 5000
J5
/A
Q— __
.... . . s /// p
u 4000
a.
^
/^v^4"6"9
3000
*om8to27
•
48
.- ^^21*24
48
froml8to27'
trn
mAAtaQf
i -. Prom48to90
2000
10
20
30 40 ^0
mcsbiizt: mm
In the manufacture of knotless nets high tenacity
polyamide multifilament yarns bright 210 den (and its
multiples 420 and 630 den) and 840 den (and its multiples
up to 8,400 den) are generally used. For very fine nets
also 100 den finds some use.
The most important characteristics of these yarns are
shown in Table II (the figures refer to Rhodiatoce
*Nailon'1 yarns).
TABLE II
Denier Tenacity Extension Breaking 3-pIy yarns Knot
g/den at break length/ breaking strength
% km length/km %
210 7-5 22 67 59 68
840 8-2 16 73 56 55
Singles are generally twistless or low-twist (130-160
turns/metre). As a rule the twist for ply yarns ranges
from 150 to 110 turns/metre. Usually the yarns are
supplied on knitting beams. The choice of the denier is
dependent on the type of net desired and on the gauge of
the loom.
Table 111 shows the relation existing between these
data. (The figures refer to a six guide-bar machine.)
Construction of knotless nets does not differ substan-
tially from that adopted in other countries. Nets are
composed of many threads forming the mesh sides,
which are interlaced at regular intervals at "cross-over
points" at the apex of the mesh. Threads correspond
to twines in the same way as cross-over points corres-
pond to knots in the knotted nets.
1 'Nailon' is the registered trademark of Socict& Rhodiatoce
S.p.A. in Italy.
In the most common version, a thread is formed by
three ends; two laid-in threads (1 and 3) and one (2)
looped, entwined together. The laid-in threads are in
an almost rectilineal position, the looped threads (2)
follow a more complicated path (Fig. 2).
Owing to this construction, the task of bearing the
net load is mostly carried by laid-in threads, the length
of which is about one-fourth that of the looped thread
(2). In general, the looped threads (2) may be lower of
denier than the other two. Yet, as the exact value of the
possible reduction in count has not been established so
far, this technique is not adopted in Italy at present.
At the mesh apex there are different types of joints,
depending upon the way the yarns are interlaced. The
more complex the structure of these joints, the stronger
and more durable they are, according to whether only
the looped threads or also the laid-in threads are en-
twined and depending upon the number of binding
points (generally two to four).
In Fig. 3 the most common stitches used in Italy are
shown.
It has not been established as yet which are the most
suitable constructions for maximum mesh wet-strength.
To avoid damaging the catch, round threads are being
developed, but this is only a minor point.
213
123
6,34^5
Fig. 2.
Max. denier
840x10-8400
840x6=5040
840x4-3360
840x3=2520
840x2=1680
840
210x3=630
210x2=420
Fig. 3.
TABLE III
gauge Needles width 100 in width 124 in
per in needles meshes needles meshes
8 4 400 200 496 248
12 6 600 300 744 372
14 7 700 350 868 434
16 8 800 400 992 496
18 9 900 450 1116 558
24 12 1200 600 1488 744
28 14 1400 700 1736 868
32 16 1600 800 1984 992
Fig. 4.
width 139 in
needles
556
834
937
1112
1251
1668
1946
2224
meshes
278
417
486
556
625
834
973
1112
99
The best combination of dcnicr/interlacing/machinc
gauge has not yet been found and research is being carried
out in this direction. The field is still too new. For exam-
ple, it has already been seen that interlacing A (and almost
all new machines are equipped with the glider chains for
this interlacing) does not give very satisfactory results.
In particular the threads have a tendency to ladder
when some ends break. For this reason several net
manufacturers favour stitch B.
The system, followed by some manufacturers, of letting
yarns (2) pass from one mesh to another (Fig. 4) does
not seem to produce much stronger joints but makes
them more resistant to laddering. Absolutely run-proof
nets have not yet been produced. Actual experience,
though limited, has shown, however, that heat-setting and
impregnation of the nets ensure satisfactory performance
in this respect. Often heat-setting is still carried out
according to the technique used for knotted nets, i.e.,
by wrapping the net around a steel cylinder and setting
it in either boiling water or steam. Equipment for con-
tinuous dry setting by means of hot air is, however,
being developed.
Table IV gives figures of the outstanding characteristics
of several types of knotless nets with type A joint made
from 'Nailon' yarn. For two of them a direct comparison
is established with corresponding knotted nets.
YAEHS
BRAIDS
NETS
(TUNES)
Moat M! denier
dtMtttr
actual
tftnltr
dry k*- „<
I.
couraas/ca.
•esh
slza
•flf
braakfng
atrangth
dryU) wet
tearing
atranoth
dry (3Jtet
price
raiio
(4)
T. C.
M
dan
fcg I
ka
M
g
7 kg
kg
420 420
0,82
3690
8,3 94*
20,2
12
18
8,8
16 1 14
7 6
100
13 -* 12
210x9
0,65
2100
12,4 8tf
S3
«
17.5
7,9
14 t 12
10 8
97
14 -^11
840 840
1,2
7360
16 94*
19,8
8
17
17,8
30 I 29
13 12
100
23 -» 21
210x21
0.98
5400
30 90*
51
-
22
15,8
24 1 22
23 19
158
21 -* 19
2520 3360
2,1
5,5
13
88
125 4 97
48 47
^. Table IV
93 •* 83
Tha email figures rafar to knot tad nata corraapondfng to tha knotlaaa nits considered.
CAPTION: - T • Ufd-in threads
C • looped threads
CR • braakfng load
I . breaking length
(1) 1 • x 10 aaahaa
(2) Msh'sfreclnens: 20 ct x 1 aesh : 4 lengthwise ; -» crosswise
(3) according to drawing
(4) M«nlng 100 it th« prlc* of th* knotUis ntt.
100
Resistance a la Rupture de Filets sans Noeuds
La construction de filets sans noeuds du type Raschel s'est de" velop-
p6e en Europe depuis la guerre. Un des avantages de ces filets sur
les filets nou6s de type traditionnel est le manque de stability des
noeuds de ces derniers. Les filets peuvent etre construits sur des
machines Raschel utilises par Industrie textile depuis de nom-
breuses ann6es, Pour les filets de pdche les barres des mailles sont
composes d'un ou plusieurs composants tricot6s, support6s par des
composants entrelac&s et il existe diff6rentes fa$ons de faire la
connexion des mailles. Jusqu'ici les filets sans noeuds ont une
resistance a la rupture plus faible dans le sens transversal que dans
le sens longitudinal. A la suite d'une etude faite en Italic, un nouveau
type de filet sans noeuds a 6t6 d6velopp6 dont la force de rupture est
a peu pits ggale dans Unites les directions. La communication
decrit la construction de ce filet dont les composants simples se
divisent a chaque intersection de mailles, suivant une sequence
sinueuse pendant laquelle, non seulement le nombre de composants
formant les barres des mailles mais aussi le fil des barres, varient
ce qui fait que les forces s'exerc^uit sur le filet sont ^parties sur
tous les fils. Les r&sultats des dpreuves montrent que pour le type
normal de filet sans noeud perd de 15 a 20 pour cent de resistance
transversale tandis que la nouvelle construction de filet ne perd que
4,5 pour cent en comparaison avec la direction longitudinale.
Breaking strength of knot less webbing
Abstract
The construction of knotless net of the Raschel type has developed
in Europe since the war. One of the advantages of knotless nets over
the traditional knotted types lies in the knot-fastness. The nets can
be made on modern Raschel machines used for many years in the
textile industry. For fishing nets the mesh bars are composed of
one or more knitted components supported by intertwining com-
ponents and there are various ways of making the mesh connections.
In most cases the webbing has a lower strength when pulled across
the connections. Resulting from a study made in Italy, a new type
of knotless net was developed which has almost equal strength in
all directions. The paper describes the construction of such netting
whereby the number of straight components making up the bars
divide at each mesh connection according to a regular sequence.
The knitted components as well as the straight furthermore change
from bar row at regular intervals, resulting in a thorough spread
of the components as knitting progresses. Test results showed that
the usual type of knotless webbing had 1 5 to 21 per cent less breaking
strength across the connections than with the connections. The new
type of webbing loses only 4*5 per cent when tested in the across
the connections direction.
Resistencia a la rotura de redes sin nudos
ExtVBCtO
La fabricaci6n de redes sin nudos de tipo Raschel comenzo en
Europa despufc de la guerra; con ellas se ha vencido el grave
inconveniente del deslizamiento o corrimiento de nudos que ocurre
en las tradicionales. Pueden fabricarse en las m£quinas Raschel que
cmplea la industria textil desde hace varios aflos. Para las redes de
pesca las mallas las forman uno o mas componentes trenzados
sustentados por componentes entrelazados y existen diversas
maneras de hacer las conexiones de las mallas. Hasta ahora los
pafios sin nudos tenian una resistencia a la rotura mas debil en el
sentido transversal que en el longitudinal. Como resultado de un
estudio realizado en Italia se fabrica un nuevo tipo de redes sin
nudos en el que la resistencia es casi igual en todas las direcciones.
Describe la poncncia la fabricaci6n de estas redes en las que los
componentes senciUos se dividen en cada interseccidn de mallas
segun una serie sinuosa en la que el numero de componentes que
forman las mallas, asf como el hilo de cstas, varian de manera aue
las fuerzas a que est* sometida la red se reparten por todos los
hilos. Los resultados de los ensayos son que la clase normal de
redes sin nudos tienen de 15 a 21 por ciento menos resistencia trans-
versal, en tanto que con la manera nueva de fabncar solo se pierde el
cuatro por ciento de resistencia cuando la red se somete a esfuerzos
transversales.
by
Francesco Pianaroli
LA fabrication des filets sans nocuds de type Raschel
s'est dlveloppie en Europe depuis la fin de la guerre.
Ces filets se sont r£vil£s tris utiles par suite du manque
de stability des noeuds des filets traditionnels fabriquis
avec des fils en fibres synth6tiques. Un autre avantage
des filets sans noeuds est la dimension constante des
mailles. Ces filets pouvant etre tricot£s sur des metiers
Raschel normaux, leur emploi dans le domaine de la
pfiche s'est rapidement diveloppi.
Les filets sans noeuds de type Raschel sont fabriqu6s
de la facon suivante: les fils tricotds en chainette, s'entre-
lacent £ des distances d£termin£es pour former les mailles.
Les fils tricotis servant de base peuvent etre renforc6s
par rinsertion d'un ou plusieurs fils simples, suivant
diflterentes techniques. II existe aussi plusieurs mithodes
de connexion des mailles et parmi les plus typiques, on
peut citer celle propos£e aux U.S.A. par M. Henry
Goldsmith et celles utilises par quelques fabricants
europdens et japonais, depuis 1955.
Aujourd'hui, la qualitl des filets de peche produits dans
le monde est assez satisfaisante bien que I'expdrience ait
d£montr£ qu'il dtait souhaitable d'augmenter encore
les caractSristiques de resistance et de durfe. En Italic,
des essais effectuis en Laboratoire ont confirm6 que par
la recherche, on pouvait encore am£liorer la resistance
des entrelacements.
D'une manure gdn&rale, la resistance des filets sans
noeuds & la rupture par suite de traction transversale est
de beaucoup inftrieure & la resistance & la traction longi-
tudinale. En outre, la tension n'est pas 6galement distri-
bu6e sur les fils composants, dans les barres des mailles
aussi bien que sans les connexions.
Les etudes effectives avaient pour but:
(a) d'augmenter la resistance des filets par une
meilleure distribution des forces;
(b) d'uniformiser la resistance des fils eiementaires
dans les entrelacements afin de lutter aussi bien
contre les tractions longitudinales que transversales ;
(c) de creer une suite de points d'interchangement des
fils uniformement rdpandus dans le filet et avec
la plus grande densite possible, ce qui augmentera la
resistance & la rupture du filet par suite de forces
transversales exercees sur les entrelacements.
De trfcs bons r&ultats ont ete obtenus par la mdthode
que nous decrivons ici dans ses grandes lignes. Les
filets sont constants par des metiers "Raschel" normaux
et les barres des mailles sont consdtu6cs par des fils sim-
ples et des fils tricots en chatnette, les premiers s'entre-
laoant et se croisant dans les seconds de telle fa$on que
101
lorsquc le filet est soumis & ure force, la tension est
rtpartic uniformemcnt sur tous les fils composants.
Pour obtenir ce rfsultat, dcs paires multiples de
"rangs" formant les maillcs dans le sens de la construc-
tion du filet ont M entrelac&s de maniire & ce que les
fils puisscnt s'entrelacer et se Sparer et ce, aussi bien
pour les fils tricotfe en chainettc que pour les fils simples.
Ainsi, dans une maillc les deux barres convergentes de
gauche et de droite ferment une demi-maille; dans
chaquc barrc, les fils composants continuent conjointe-
ment chacun avec sa sequence sinueuse typique, jusqu'4
un noeud du premier rang; & ce moment on proofcde
& un fchange multiple parmi les aiguilles des groupes de
fils voisins, en chainettc, de telle sorte qu'apris s'etre
plusieurs fois crois£s pour constituer 1'entrelacement,
les fils passeiit au rang suivant, dans la "file" voisine
respectivement droite et gauche, pour former la barre
inverse correspondante d'un nouveau rang. En meme
temps, les fils simples de chaque barre, apris s'Stre
crois6s dans Fentrelacement, se divisent en deux groupes
in£gaux, les uns passant dans le rang suivant & la "file"
de droit et les autres dans celle de gauche pour former,
avec les fils tricotfe, les barres d'un nouveau rang.
Successivement, dans les entrelacements du deuxiime
rang, la m£me division des fils simples se reproduit mais
les groupes s'invertissent tous les deux rangs. En meme
temps, au deuxiime rang les groupes de fils tricotls en
chainctte, de deux barres voisines font un double
exchange sur les aiguilles, chacun d'eux restant & la
m£me file, pour former le troisifcme rang. Les entrelace-
ments du troisiime rang seront done une repetition des
mouvements cites pour le premier rang, mais le nombre
de .fils de chaque file sera interverti. Le sch&ma des
mouvements des fils est repr£sent£ dans la figure 1.
Les filets construits selon cette mfthode ont donn6 des
r&ultats tris satisfaisants en ce qui concerne la resistance
& la rupture centre les forces de toutes directions. Le
petit tableau ci-dessous donne les r£sultats obtenus sur
dififerents types de filets examines. Le filet nou£ utilise
£tait fabrique en fibre polyamide 66 210d/9, de 7 g/d de
tenacite et comportait 1000 mailles de largeur sur un m
de hauteur et pesait 0,700 kg. A une t£nacit6 de 7 g/d,
la charge thforique de rupture de chaque fil aurait dfi
Stre 210x9x0,007=13,330 kg mais les valeurs trouvfes
pour les charges de rupture longitudinale et transversale
etaient de 8 kg et 7,200 kg respectivement. Cette dimi-
nution de resistance est due & la perte de 40 pour cent
dans les noeuds.
Des essais ont £t6 effectuds sur quelques types de filets
sans noeuds, fabriqu£s en Italic et dans d'autres pays
d'Europe. Sur des sections de filet de 1000 mailles
sur 1 m, pesant chacune entre 690 et 720 grammes, les
charges de rupture ont £t£ tris inferieures & celles des
filets nouls et accusaient une perte de 15 £ 21 pour cent
dans le sens transversal.
Des essais sur un filet sans noeuds construit d'apres le
syst&me d&rit plus haut out montri que la resistance &
la rupture sous des forces transvcrsales et longitudinales
6tait tris sup6rieure & celle des autres constructions.
Fig. 1. Repartition des fils composants dans la formation de\ mailles.
Type dc filet
Filet nou&, traditionnel poids:
0,7 kg/1 000 mailles haut. 1m
Filet sans noeuds, en g£n£ral
Filet sans noeuds construit d'apres
le systeme 6nonc6
Charge de
rupture
longitudinale
en kg/fil
8
6,100—7,350
Charge de
rupture
transversale
en kg fil
7,200
8,600
Le r^sultat le plus rcmarquable est qu'avec la methode
de fabrication d^crite, la resistance ^ la rupture est
augment^e, bien que la resistance de certaines barres
soit moindre que celle des autres constructions. Ceci
a £t£ obtenu en proportionnant les fils simples et tricot£s
composant les barres des mailles et en faisant participer
plus ftroitement les fils simples dans la formation des
entrelacements.
102
Monofilaments in Fishing
Abstract
Synthetic fibres became widely used in the fishing industry several
years ago, but mainly in the form of twines made of continuous
imiltifilaments (in the range of 2-25 denier/filament). During the
last few years a major breakthrough has been witnessed in the use
of monofilaments, i.e., filaments of 50-1,000 denier. Before mono-
filaments could make any dramatic impact on the traditional
market, polymers had to be produced giving fibres with the follow-
ing characteristics: (a) sufficient flexibility, (b) high tenacity (c)
capability of manufacture at high speed and (d) availability at
commercially attractive prices. The bending moment of a circular
monofil varies in proportion to the fourth power of the diameter.
Shapes of mononl cross-sections are now mainly circular, but
rectangular as well as ribbon-like cross-sections have been used in an
endeavour to increase flexibility and firmness of knot. However,
it seems that to be effective in this way the cross-section must be
extended to a ribbon-like form, but then other factors, notably
abrasion resistance, can become critical. In netting, the wet knotted
strength is of major importance, as well as the load/extension
characteristics. These values are given in tables for monofilaments
of polyvinylidene chloride, nylon, polyethylene and polypropylene,
as well as changes in tenacity at varying temperatures. High tenaci-
ties can be achieved through stretching during spinning, but at the
expense of extensibility, and a favourable balance must be struck
between these properties. From experiments it appears that p. load of
up to two g/den is commonly applied to monofilaments during
normal trawling operations, and the extensibility at such loads is
expressed in graphs in this paper. Test values are also given for
other properties such as creep, elastic recovery and impact strength,
stiffness, knot stability, abrasion resistance, towing resistance, rot-
proof characteristics and resistance to sunlight. The use of mono-
filaments in certain types of fishing gears is discussed, such as in
Danish seines, wing trawls, midwater trawls, deep-sea trawls,
shrimp nets, lobster pots, salmon trapnets and longlines, as well as
the use of monofilaments for ropes.
Monofilaments dans la pgche
Resume
Depuis quelques annees les fibres synthetiques sont bien connues
dans 1 Industrie des peches, principalement sous forme de fils
retordus en filaments continus Centre 2-25 denier/filament). Au
cours de ces dernieres annexes F utilisation des monofilaments c'est-
a-dire des filaments de 50-1.000 deniers s'est d£velopp6e consider-
ablement. Pour que les monofilaments deviennent acceptables
par le march* traditionnel, il fallait des polymers donnant des
fibres ayant les caracteristiques suivantes: (a) flexibility suffisante;
(b) haute resistance a la rupture; (c) possibility de fabrication
acceieree; (d) disponibilite a des prix interessants. Le moment de
flechissement d'un monofil circulaire varie en proportion a la
4eme puissance de son diam&re. Les coupes des monofilaments
sont normalement circulaires mais des formes allant du rectangle
jusqu'au ruban ont et£ utilisees au cours d'essais pour accroitre la
flexibilite et la fermete des noeuds. II semble que pour ces deux
caracteristiques, les fils en forme de ruban sont les plus efficaces
mais alors d'autres facteurs deviennent critiques, notamment la
resistance a 1'abrasion. La force de rupture du filet noue mouille
est de la plus grande importance de meme que les caracteristiques de
force/extension. Ces valeurs sont donnees dans des tableaux pour
des monofilaments de polyvinyliddne chloride, nylon, polyethylene
et polypropylene ainsi que les variations de resistance a la rupture a
des temperatures differentes. De hautes forces de rupture peuvent
6tre obtenues par etirage pendant le filage mais au detriment de
1'extension de sorte qu'un 6quilibre favorable doit fctre trouve entre
ces proprietees. Les r6sultats des experiences ont montre qu'une
force de deux g/den est nonnalement applique aux monofilaments
pendant des operations normalcs de chalutage et 1'extensibilite sous
de telles forces est exprim6e dans un graphique. Sont donnees
egalement les valeurs determiners au cours des experiences pour les
autres proprietes des monofilaments telles que: reprise eiastique,
resistance aux chocs, raideur, stabilite des noeuds, resistance a
Tabrasion, resistance a 1'avance, caracteristiques de poumssement
et resistance aux rayons solaires. L'utilisation des monofilaments
dans certains types d'cngins de pfiche tels que les sennes danoises,
les differcnts types de chalut, les filets a crevettes, ies nasses, les
trappes pour saumon, les palangres aussi bien qu en cordene est
traitec dans cette communication.
by
D. F. C. Ede
British
Resin Products Ltd
W. Henstead
British
Celanese Ltd
Los monofilamentos en la pesca
Extracto
La industria pesquera emplea mucho desde hace varies aftos fibras
sinteticas, principalmente en la forma de hilos hechos de multi-
filamentos continuos (en la gama de 2-25 denier/filamento). En los
ultimos anos se hand hecho prodigiosos adelantos en la fabricaci6n
de monofilamentos, y actualmente se obtienen de 50 a 1 .000 denier.
Antes de que estos monofilamentos pudieran entrar en el mcrcado
traditional, tuvieron que producirse polimeros que dieran fibras con
las siguientes caracteri sticas : (a) suficicnte flcxibilidad, (b) gran
tenacidad, (c) capacidad de manufactura a gran vclocidad y, (d)
disponibilidad a precios interesantes. El momenta de flexibn de un
monofilamento circular varia en proporci6n con la cuarta potencia
del diametro. Las formas de las secciones transversales de los mono-
filamentos son principalmente circulares, pero tambien se emplean
rectangulares y como cintas, para tratar de incrementar la flexibilidad
y firmeza del nudo. Sin embargo, para que sea eficaz, la secci6n
transversal debe ser como una cinta, pero entonces entran en juego
otros factores, principalmente la resistencia a la abrasion, que
pueden llegar a ser criticos. En los paftos para redes la resistencia
del nudo humedo es de la mayor importancia, y tambien lo son las
caracteristicas de carga y estiramiento. Estos indices se dan en
tablas para monofilamentos de cloruro de pplivinilideno, nylon,
polietilcno y polipropileno, asi como los cambios en la tenacidad a
diyersas temperatures. Puede lograrse gran tenacidad estirando
mientras se teje pero a expensas de la extensibilidad, por lo que hay
que encontrar un equilibno favorable entre estas propiedades. De los
experimentos se deduce que una carga hasta de 2 g/den se aplica nor-
malmente a los monofilamentos durante la pesca a) arrastrc; a exten-
sibilidad a tales cargas se expresa graficamente en esta comuni-
cacidn. Tambien se dan valores obtenidos en ensayos de otras
propiedades como deslizamiento, recuperacidn elastica y resis-
tencia a los impactos, rigidez, estabilidad del nudo, resistencia a la
abrasi6n y al remolque, resistencia a la podredumbre y a la luz del
sol. Se examina el cmplco de monofilamentos en artes de pesca
como los de arrastre danses, pernadas de artes de arrastrc, artes
flotantes, artes para la pesca de gran altura, redes camaroneras,
nasas langosteras, trampas y palangres salmoneros, y el empleo de
monofilamentos para cuerdas.
WE deal in this paper with markets and applications
held traditionally by hard fibres, manila, hemp and
sisal, and soft fibres, linen and cotton. Over a period of
years the industry had built itself up to a high degree of
efficiency with stability in price.
Developments in fibre-forming polymers began to
break new ground in the post-war period, nylon being
first as a fibre possessing exceptionally high tenacity.
Other properties were excellent impact and energy
absorbing characteristics, together with resistance to
rotting.
103
These fibres, which were already established in apparel
uses, possessed excellent flexibility due to their fineness
(a range of 5-25 denier/filament). The logical develop-
ment was the use of multifilament yarns in place of soft
fibres, linen and cotton, followed by the introduction of
monofilamcnt yarns. These monofilaments were in the
range 50-1,000 denier/filament.
Having made the distinction between the multifila-
ments and monofilaments, we follow the progress of
monofilament materials in relationship to other syn-
thetic yarns. Before monofilaments could make any
dramatic impact on traditional markets, polymers had to
be produced, giving fibres having these characteristics:
(a) Sufficient flexibility to be capable of yarn, strand
and net production on conventional machinery.
(b) A high tenacity.
(c) The capability of manufacture at high speed.
(d) Availability at a commercially attractive price.
Nylon monofilaments
Earliest monofilaments were made from nylon and were
soon used for very fine gillnets in Northern European
lakes. They had the great advantage of high strength for
small filament diameter and were relatively invisible in
the water. Further development in larger diameter
filaments was limited, firstly, by rapid increase in stiffness
and, secondly by, commercial considerations, which
tended to favour the promotion of multifilament
yarns and staple fibres. More recently, in Germany,
where nylon has reached a high level of acceptance,
nylon monofilaments have been used in large rope
structures alongside nylon multifilaments. The former
confer on the rope a very high degree of abrasion resist-
ance, whilst the multifilaments bear the major load.
It will be seen that the basic stiffness of nylon tends to
limit its use as a fibre except in fine diameter or in large
rope structures where flexibility is not so important.
To some extent these remarks apply also to polypropy-
lene, particularly when aiming for tenacities required by
everyday applications.
Polyolefin monofilaments
It will be appreciated from the foregoing that a more
widespread replacement of traditional fibres by mono-
filaments could occur when a sufficiently flexible filament
in the higher denier range had been developed. Such a
fibre did, in fact, emerge in the earlier days of the poly-
olefins. The earliest fibres produced were spun from
conventional (low density) polyethylene and gave fila-
ments of high flexibility but very low tenacity (in the
range 1-0 to 1-5 gpd). With such a low tenacity there
was little chance of use in cordage applications, and it
was not until linear polyethylene had been spun that
high tenacity filaments were possible. These gave tenaci-
ties of the order of 5-6 gpd but were rather less flexible
than those produced from conventional polyethylene.
It was possible, however, to produce right from the
outset a complete range of twines and rope yarns, with
the exception of very fine twines where the multifilament
yarns still held their own.
104
By 1957/1958, nylon multifilament had secured a very
prominent position in the netting field and was in wide-
spread use in gillnets of all types, for purse seine nets,
principally in Norway, and for herring driftnetting. All
these end uses required excellent twine flexibility for effec-
tive fishing; nylon, and to a lesser extent the polyester
fibres, provided this in ample measure. Although some
synthetic ropes were in existence, their acceptance at
this time in the fishing industry was not high and current
usage was confined more to pure marine fields where
nylon towing ropes and hawsers had proved very effective.
During the last few years, major breakthroughs in
net and rope have been witnessed — much of it associated
with the widespread use of monofilaments.
There has also been a pronounced narrowing of the
price gap between the natural fibres and the man-made
fibres. This review is intended to convey in straight-
forward terms to the twine maker, netmaker and trawler-
owner, the considerations which a yarn producer has in
mind when marketing a yarn intended for the cordage
market. The technical properties of the yarns, allied
to production aspects, are outlined in the following
comparison of monofilament properties and references
made to the economic considerations in some end uses.
Monofilament deniers, shape of cross-section, etc.
These usually fall within the range 100-4,000 den, the
common ones in commercial use in fishing at present
being between 100 and 1,000 den per filament (11-110
tex). The shapes of cross-sections are mainly circular
but, in certain instances, rectangular cross-sections have
been used with a width : depth ratio of approximately
3:1. More recently we have seen experiments with
ribbon-like cross-sections, e.g., having a width : depth
ratio of approximately 50 : 1, and also combinations
of circular and ribbon-type cross-sections, e.g., round
sections spaced at intervals across the cross-section of a
continuous width of ribbon-like monofils.
Much work has been done on flat cross-sections with a
view to increasing both the flexibility of twines and the
firmness of the knots. In the former, the bending moment
is reduced as a result of the thinner axis in one direction
and increased flexibility results (the bending moment of a
circular monofil varies in proportion to the fourth power
of the diameter). In our experience, we have found little
advantage in these respects, unless the cross-section is
extended to a ribbon-like form and, in such cases, other
factors, notably abrasion resistance, can become critical.
Strength
The strength of yams or monofilaments is often quoted
as a selling point and discussed in terms which suggest
that this figure alone determines the strength of net,
twine or rope made from same. In actual use, in a fishing
net for instance, it would seem that the wet knot strength
of the twine is a more proper indication of strength.
Bearing in mind the vast range of twines and twine con-
structions in use, it falls upon the yarn manufacturer to
devise his own standards. These are based on laboratory
and practical tests and aim at an optimum relationship
between tenacity and extensibility for the particular
end uses.
(a) Tenacity
Table I gives a general indication of the tenacity of
monofils used in twines and ropes:
Polyvinylidcne chloride fibres 2 g/den
Nylon 5_7 g/dcn
Polyethylene 5-0-7-5 g/den
Polypropylene 5—8-0 g/den
(In the case of nylon there is a loss in tenacity when wet)
Fig. 1 shows typical load/extension curves for these
fibres (ref. "Textile World", 1962).
UJ
z
UJ
a
to
o
a
2
TYPICAL LOAD/EXTENSION CURVES
CONSTANT RATE OF EXTENSION ZOCMSlMlN
FIG. 2.
20 o/
EXTENSION- /O.
Fig. 2 illustrates the effect on tenacity of various stretch
ratios applied to 'Courlene' high density polyethylene
monofilaments during spinning.
(b) Effect of heat
The tenacity of thermoplastic monofilaments is also
affected by heat but most values quoted relate to tests
carried out under standard conditions at 20°C. Table II
illustrates typical changes in tenacity (g/den) of some
monofilaments at varying temperatures thus:
2°c
20°C
40°C
60°C
•CourleneXr
8-1
7-0
4-9
3-6
Polypropylene
6-0
5-4
5-2
4*6
Nylon
5-4
5-1
4*2
3-7
(Tests carried out in water)
'Saran'
2-1
1-9
1-6
1-4
The above figures apply to monofils of approximately
0-010 in diam which are commercially available. It
will be appreciated that higher tenacities can be readily
achieved. In the case of such monofils, other considera-
tions, e.g., stiffness, susceptibility to fibrillation, give
rise to problems during processing which have not yet
been finally overcome.
(c) Knot tenacities
These are to a large extent dependent upon the tenacity/
extensibility relationship as determined during spinning
of the monofilaments. As the extensibility decreases,
the monofilaments become more brittle to a point
beyond which the increased tenacity obtained by extra
stretching is more than offset by the consequent reduc-
tion in knot strength. (Fig. 3 (Lonsdale 1957).)
MOLECULAR ORIENTATION
FIG.3.
(d) Load-bearing characteristics
These are best considered in relation to the load/extension
curves (Fig. 1). From experiments, it appears that a load
of up to 2 g/den is applied to monofilaments during
normal trawling operations and an idea of the amount
of extensibility at such loads can be obtained from these
curves.
There is also the effect of creep under continuous
loading to be considered. This varies between types of
fibre and also quite substantially between grades of
polymers produced by manufacturers. Some monofila-
ments are criticised on account of creep and Table III
shows test results obtained recently for the various
monofilaments (outlined in Table II) when subjected to
continuous loadings. Fig. 4 shows the time scale which
coincides approximately with a normal trawling cycle of
operation.
(e) Elastic recovery
Extensibility at working loads is largely recoverable in
the case of most synthetic fibres. The monofilaments
considered in Fig. 4 were used in illustration of this
property (Fig. 5).
105
(f) Impact strength
This is the amount of energy necessary to rupture the
yarns. Tests were carried out on a ballistic tester at
20CC and 65 per cent R.H. as shown in Table III thus:
ergs/g
2-7 xJO*
3-1 xlO8
4-2 xlO*
1-2X10*
'Courlene X3'
Polypropylene
Nylon
Polyvinylidcnc chloride
g.cm/cm.den
3-0
3-5
4-7
1-4
Stiffness or flexibility
This is important when considering the winding or
processing of monofilaments. Recovery from creasing
is perhaps the best means of comparison and relative
values obtained on the 0-010 in diam monofils under
consideration are as follows in Table IV:
•Courlene X3'
Polypropylene
Nylon
Polyvinylidene chloride
28
72
102
124
From this table, polyvinylidene chloride might be con-
sidered to be the most difficult monofilament to process
but the bending modulus (resistance to bending) for this
monofilament is low compared with the others.
The crystallinity of fibres is also of importance with
regard to recovery from deformation. Table V (Nichols
1954 and others), gives an indication of values for
normal monofilaments. In general, the high crystallinity
is associated with less recovery from large bending
deformations:
'Courlene X3'
Polypropylene
Nylon
Polyvinylidene chloride
80%
55%
50%
70%
Knot stability
The main properties which determine the ease of
knotting are stiffness, diameter and shape of cross-
TOTAL LOADING - / 0 CRAMS Pffi DCN/FR
CQUBLCNC *1 MXtOBG 4/5 fiCNIEA 0 96GMS/OCN.
section. The crystallinity gives a guide to whether the
monofilaments, once knotted, will tend to become loose.
From these considerations it will be appreciated that
polyethylene has advantages in this respect.
With nylon, the use of double knots, or heat-setting
of knots, is commonly practised to overcome deficiencies
in knot stability.
Laboratory tests have not shown rectangular cross-
sections to be beneficial unless the width : depth axis
ratio is at least 10 : 1.
Monofilaments which are substantially round in cross-
section, but having a serrated outline, have also proved
disappointing from a knot stability point of view.
Abrasion resistance
The results of abrasion resistance testing are known to be
unreliable when relating them to actual usage and
technologists have long been reluctant to be definite
on this subject. A large variety of tests has been devised
over the years and, for comparison, we quote results in
Table VI obtained by a reciprocating motion of the yarn
under 500 g load, across a shot-blasted 'Dural' cylinder
of 3J in diam.
Arbitrary units
54
59
124
24
10
'Courlene X3' (-009 in diam)
Polypropylene (-011 in diam)
Nylon (-010 in diam)
Polyvinylidene chloride ( -01 1 in diam)
830 den nylon multi- (2 tpi)
filament yarn
The same tests carried out in water showed the polyvinylidene
chloride monofils to be almost the equal of the nylon monofils.
These results help to illustrate the effect of denier/
filament, although multifilament yarns can be much
improved by judicious use of twist. Twines made from
monofilaments generally require less twist than compar-
able twines made from multifilament yarns, thereby
avoiding loss of strength due to overtwisting.
The twist used in the make up of the twine has a direct
bearing on abrasion resistance, and a point occurs where
twine strength is adversely affected as twist increases
(Fig. 6 (Shimozaki 1957)).
AFTER I 0 C/W/Pf/V LQAQINQ CYCLE FOR ISO MINUTES
/NC flfC .0 VE, A Y - £>Q GAAMS
~ *
TIME IN MINUTES
lO IO2
TIME IN MINUTES
106
r-
outdoors in temperate climates for many years. A
recent comparison of monofiiaments is given below:
\ V
MVfflONSHtP ifTtfflff Tftf IHlAKlNG STHCHtTH MO
fiti/Upf* Of TWIST3 QIVtN TO Tltf/Nft <«.«.<€. IflOOO. N v^
*_Purst*4MONoriUMiNrs PKQOUCCQ BY oirre*iNr «.
NUMBER OF TWISTS OF TWINE PER3OCMS
Over the past few years, we have carried out many
practical trials on abrasion resistance, incorporating
varying diameters and cross-sections. The results, in-
fluenced by other considerations, have in general borne
out the textile fibre producer's well-known fact, i.e.,
the coarser the fibre, the better the abrasion resistance.
Resistance to drag
There has been evidence in the past (Scharfe 1957) that
nets made from continuous filament yarns offer approxi-
mately six per cent less resistance to towing than com-
parable manila nets, due to the smoother surface of the
twine and also to the fact that a thinner twine could be
used. Monofilament twines could be considered even
better than multifilament twines due to the lower number
of fibres in the twine. Both these types of twine appear
to possess the same advantages over natural twines, and
trawler owners have confirmed a five per cent fuel saving
when changing over from natural fibre nets to 'Courlene'
nets over a trial period of several months.
Rot-proof characteristics
Synthetic monofiiaments have excellent resistance to
sea water. Nylon absorbs some water, with consequent
swelling of the fibre and reduction in breaking load when
wet (Lonsdale 1957).
It is now common practice not to dry synthetic nets.
Preservatives are also regarded as unnecessary unless
used to aid knot stability or abrasion resistance. We
have had experience of polyethylene nets which have
lost none of their properties after immersion in sea water
for one year.
Resistance to sunlight
This can vary between different families of polymers and
similar polymers made by different manufacturers. The
thicker the monofilament the twine or the rope the
better, due to greater resistance in depth to penetration
by ultra-violet rays. Dark colours are helpful in lessening
damage by sunlight. The yarn producer also has access
to a wide range of additives (anti-oxidants), some of
which may be usefully incorporated.
Test sites are located all over the world for comparison
of outdoor tests and laboratory tests. Polyethylene has
been developed to the point where it can safely be used
Polyvinylidene chloride . .
(clear yarn -01 1 in diam)
Polypropylene
(clear yarn -0075 in diam)
Nylon
(clear yarn -0075 in diam)
'Courlene X3'
(clear yarn -0075 in diam)
Use of monofiiaments
% breaking % extensibility
load retained retained
76 89
43
79
90
48
80
86
will
Reference to the physical properties described
facilitate an appreciation of the following end uses.
Polypropylene monofiiaments are not yet in wide-
spread use for nets. The monofiiaments are stiffer and
more difficult to process when making twines or braiding
into nets. Ways of overcoming this problem are:
(a) The use of finer monofiiaments.
(b) Applying less stretch in spinning, which results
in monofiiaments which are more manageable
but lower in strength.
(c) Variation of cross-section.
All these approaches have certain disadvantages in
economy, strength, or abrasion resistance. The develop-
ments over the next few years will be interesting.
Netting
Where first-class durability and abrasion resistance is
required, stout monofiiaments have been proved extremely
suitable. Polyolefin monofiiaments, because of their
low specific gravity, tend to remain clear of the sea bed,
thus avoiding excessive abrasion, whilst the mouth of the
trawl can be maintained in the correct position with
fewer floats.
Danish seine nets.— One of the earliest successes occur-
red in this type of fishing, principally in Scotland.
Within a very short space of time penetration was high,
and today more than 90 per cent of these nets are made
from polyethylene. The material technically was found
to be excellent and the economics decidedly favourable.
A service life of several years is not uncommon.
Wing trawls. — As with Danish seine nets, a very large
number of monofilament wing trawls is now in use and
these are universally accepted as an economic replace-
ment for natural fibres.
Midwater trawls.— In this area of fishing, which is largely
at an experimental stage in the United Kingdom, a
number of nets has been made from both multifilament
and monofilament yarns. All have been successful whilst
the type most likely to be used will be the net providing
minimum drag for a given strength.
Deep-sea trawls. — Deep-sea trawl fishing offers per-
haps the best potential in terms of fibre usage and the
continued on page 108
107
Nylon Monofilament in the Viet-Nam Fisheries
Abstract
Viet-Namcse fishermen have for centuries used different types of
gear on the many fishing grounds surrounding the peninsula.
Different types of gillnets constitute the main equipment used and,
white these have varied little in design, the fishermen have over
recent years adopted the use of synthetic materials. The Viet-Nam
Fisheries Directorate introduced multifilamcnts to the fishermen,
who, of their own accord, changed of er to monofilament for the
construction of their nets. Some of the advantages attributed by
the fishermen to nylon monofilament nets are: they do not retain
spiny shells (limilus sp.) and other impurities drifting around in the
water, and easier to work; spiny fish especially are easier to remove
from the nets; less damage is caused when the nets become snagged
on reefs; the meshes are largely open for true gilling nets; for day-
light fishing the nylon monofilament is nearly invisible, while
multifilaments give a bright reflection, Monofilaments are further-
more cheaper by half. Disadvantages as compared with multi-
filament are: that the netting is more bulky and tends to blow
around during operations; monofilament is more difficult to braid
and tends to become stiff at low temperatures: Of an estimated
11,000 gillnets in operation in the Viet-Nam fisheries, about 8,000
are of monofilament as compared with only 160 multifilament nets.
L'Utffltatkw de monofflaments de nylon dans fes pecfaes Vietnamiennes
by
Tran-Van-Tri
and
Ha-Khac-Chu
Fisheries Directorate, Viet-Nam
Tran-Van-Tri
Les pteheurs Victnamiens ont depuis des siecles utilise" divers
types d'engins pour opercr sur les differents lieux de pfche situes
tout autour de la peninsule. Les differents types de filets maillants
constituant F6quipemcnt principal ont peu vari6 dans leur con-
struction et, au cours des dernieres annees les pecheurs ont utilise
des mat&iaux synthetiques. L'Office des Pfiches Vietnamien a
introduit les fils synth6tiques multifilaments mais les pteheurs, de
leur propre initiative les ont remplac6s par des monofilaments pour
la constructive leurs filets. Quelques uns des avantages attribue*s
par les pteheurs aux filets en nylon monofilament sont les suivants:
ils ne retiennent pas les impuretes derivant dans 1'eau et sont plus
faciles a travail ler; les poissons dpineux en particulier, sont plus
faciles a ctegager des filets; les filets sont moins endommag&s
lorsqu'ils sont accroches dans les coraux; Touverture des mailles
continued from page 107
United Kingdom industry provides good facilities for
experimentation. Penetration, however, has been slow,
not because of technical inferiority or unsuitability, but
due to the unfavourable economic parity ratio arising
from the high incidence of gear loss. Modern detection
methods are already helping to minimise this.
The rate of usage will rapidly increase as the effects of
the improved economic life are realised and particularly
now that a replacement net in synthetics can be purchased
at no more than twice the traditional net.
Shrimp nets and lobster pots. — The ease of cleaning
and long life are particular features of monofilament
nets for this type of fishing.
Salmon trapnets. — Cleanliness, rot-proof characteristics
and long life are important. Protection against scaven-
gers, such as seals, has been afforded by monofilament
nets compared with cotton nets.
Longline fishing. — Monofilament lines and snoods are
renowned for their relative freedom from tangling and
cleanliness in use. The floating characteristics of poly-
olefin monofilaments ensure that snoods tend to rise
in the water and be clear of the sea bed.
Ropes
The last five years have seen a progressive attack on the
whole field of activity where hard fibres have held the
market so completely for so long. Whilst the mono-
filaments had much to commend them at the start of the
development phase, knowledge of the possible polymer
108
deficiencies manifest in fibres compelled caution. If a
filament were subject to high stress at an elevated
temperature for a long period of time, it was known
that an irrecoverable yield would result. The question
was, therefore, do such combinations occur in practice?
A comprehensive testing programme was necessary to
test this and other possible deficiencies in the material,
covering end uses from large marine mooring hawsers
to longlines in Norway. These field trawls have shown
that such a combination of conditions rarely happens in
practice.
For example, 65 in circ-three strand polyethylene ropes
have stood the test of two years continued use as first
ropes ashore on a 20,000-ton tanker, and are still in use.
By comparison, a hard fibre rope is normally replaced
every nine to twelve months.
It is evident that the fishing industry provides an oppor-
tunity for the widespread usage of polyolefin mono-
filaments, since the question of temperature sensitivity
does not arise. A wide range of end use applications has
confirmed the basic suitability of these materials.
Quarter ropes and mooring ropes for all types of
trawlers, together with headropes for seine nets, are all
in regular use. More recently, successful trials have been
carried out with weighted polyethylene ropes for use as
seine warps.
Reference!
Textile World. Man-made Fibre Chart. 1962.
Lonsdale, J. E. Nylon in Fishing Nets. 1957.
Nichols, J. B. J. Appl. Phys. 25. 1954.
Shimozaki, Yoshinori. 1957. Characteristics of synthetic twines
used for fishing nets and ropes in Japan.
Sch&rfe, J. Experiments to decrease towing resistance of trawl
gear.
eat plus constante; pour la p6che pendant la jounce, le nylon
monofilament cat piesque invisible tandis quc Ics multifilaments
donnent ties reflets brillants. DC plus les monofilaments sent de
moiti6 moins chcrs. Par centre, compares a oeux en multifilaments,
les filets monofilaments sont plus volumineux et susceptibles d'etre
gonfles et chasses par le vent pendant les operations; le monofila-
ment est phis difficile a travailkr et devient raide a basse tempera-
utre. DCS 11.000 filets maillants en operation dans les pecheries
vietnamiennes, environ 8.000 sont en monofilaments et seulement
160 en multifilaments.
El empfco de monofllainentos de nykm en las pesquerias de Vlet-Nam
Extacto
Los Pescadores vietnameses emplean desde hace siglos diversas
clases de artes en Jos muchos caladeros que rodean la peninsula. £1
equipo principal lo constituyen diversas clases de redes de enmalle y
aunque sus formas ban cambiado muy poco, en los ultimo* aftos
se emplean materiales sintdticos. La Direcci6n de Pesca de Vietnam
les ofreci6 multifilamentos, pero los Pescadores prefiercn emplear
monofilamentos para construir sus redes, porque entre las ventajas
que les atribuycn estan : menos peligro de enredarse y manipulacibn
mas facil; es mas sencillo descnmallar los peces, particularmentc
los de aletas duras; menos averias cuando las redes se enganchan en
las rocas; la malla es mas constante para los artes verdaderamente
agallcros; los monofilamentos de nylon son casi invisibles de dia,
en tanto que los multifilamentos dan reflejos brillantes; los mono-
filamentos cuestan la mitad, aproximadamente. Los inconvenientes
son: las redes son mas voluminosas; los monofilamentos son mas
dificiles de trenzar y se endurecen cuando la temperatura es baja.
De las 11 .000 redes de enmalle usadas en Viet-Nam, unas 8.000 son
de monofilamentos y s61o unas 160 de multifilamentos.
RICE and fish are basic foods in Southern Viet-Nam.
Along its 1,800 km of coastline a great variety of
gears are used because of the many different fishing
conditions and fish species.
Beach seines, fish traps, pair trawls, gillnets of different
operational types, and the usual variety of hookline gear
have developed. Of these gears, gillnets, and especially
driftnets, are perhaps the most important. Whilst
traditionally made of ramie and cotton, synthetic
materials have been in use for some time.
In 1958 the Fisheries Directorate imported some multi-
filament nylon gillnets of three inches (four fingers) stret-
ched mesh size for use on the east coast where similar
cotton and ramie nets were used. The fishermen at first
were very reluctant to use them. However, a few fisher-
men did insert a few such nets in their own string of nets,
to compare the catch efficiency. The excellent results
completely broke down the fishermen's prejudice-
catches were up to 300 per cent higher than in traditional
nets. Demands for synthetic twines quickly surpassed
the supply available.
Nylon monofilament had for some time been in use
for the construction of snoods in different hook gears
as well as for handlines. Some fishermen, unable to
obtain multifilament nylon for gillnets, turned to the
light monofilament, being convinced that even mono-
filament nylon was better than the natural fibre previously
used.
From the beginning, these monofilament nets obtained
at least as good results as the multifilament nets and, by
introducing small adjustments to mesh size and hanging,
the fishermen very soon obtained even better results
with monofilament nets than with multifilament nets.
Thereafter the demand for monofilament of various
diameters greatly increased.' The nets are used with
local-made plastic floats and tight-fitting leads; both
are available in several sizes to fit the mesh size and type
of operation (Fig. 1).
Fig. 1. Monofilament net with plastic floats and lead sinkers.
The Directorate of Fisheries has to date not yet
conducted suitable comparative fishing experiments to
ascertain whether the monofilament nets do in fact catch
more. The fishermen on the east coast are convinced
that these nets have a higher catch efficiency because of
their transparency. This, on the other hand, does not
justify the preference for monofilament of fishermen
operating on the west coast where the water is very
turbid.
Visibility of monofilament nylon nets is not the only
feature governing the preference of fishermen, and it
may be useful to record some of the advantages and
disadvantages attributed by fishermen to these materials.
Advantages of monofilament nets
(a) The twine is relatively stiff and very smooth and
does not retain spiny shells (limilus sp.) and other
impurities drifting around in the water. Multi-
filament, owing to its high flexibility and very fine
denier, picks up such trash very easily so that
cleaning nets is much more difficult.
(b) Gilled and entangled fish, especially spiny fish,
appear to be much more quickly released from
monofilament than from multifilament nets.
(c) Some of the fishing is done near coral reefs and,
when the nets become snagged on coral, mono-
filament nets are released much more quickly and
with less damage.
109
(d) The fishermen believe that, owing to the stiffness
of the yarn, the meshes remain largely open.
(e) The transparency of monofilament nets reduces
their visibility in the water, particularly in daytime,
whereas multifilament nets give a brilliant reflec-
tion which may scare the fish away. This would
seem to be especially the case round the edges of
tears, so that the hole in the net is very visible
to Hie fish. During daytime monofilament gillnet
fishing is more efficient than multifilament nylon
nets. However, the fishermen remark that multi-
filament nets are particularly good in catching the
Stromateus sp. as the nets entangle the fishes.
(f) The monofilament nets have been found to catch
at least as much fish as the multifilament ones and
are cheaper by half. The reason for the lower cost
of monofilament twines is that multifilament
must still be imported whereas nylon monofilaments
are manufactured in Saigon by six local firms.
Disadvantages of monofilament twines
(a) One disadvantage of monofilament nets is bulkiness.
Fishermen combat this by forcing the netting in the
forward fish-hold and weighting it down (Fig. 2).
Fig. 2. Storage of nets on board.
(b) The netting during shooting and hauling tends to
blow around more than the multifilament nets
and this can slow down the operation. This has
largely been overcome by adapting the shooting
and hauling techniques to this condition.
(c) Monofilament does not braid as easily as multi-
filament because of its stiffness; on the other hand,
no serious difficulty has been experienced as regards
continued on page III
Fig. 3. Joining of twine between knots when mending.
Fig. 4. Mending the nets.
110
Monofilament Gillnets in Freshwater Experiment
and Practice
Abstract
Experimental studies on the comparative importance of the
properties of gillnet materials have shown that in clear water the
visibility is the most important factor. Net materials should have
low visibility, giving little or no contrast to the background and
be transparent; synthetic monofilaments yield the best catches
The softness of the netting was found to be of only secondary
importance. The diameter of the netting material influences the
catch efficiency only inasmuch as thicker twines are more visible
than thinner ones. The elasticity appears to have hardly any in-
fluence on the size of the fish caught while the visibility does appear
to have a selective effect. The breaking strength is only of impor-
tance for large fish and operational considerations, because small
fish such as perch and roach cannot produce the force required to
damage commercial gillnets. The hanging determines the shape of
the meshes, the distribution of the forces in the net and the looseness
in the netting. The framing lines may influence the catch efficiency
by their visibility. The amount of floats and sinkers depends on
the operational conditions, the current and the amount of tension
required in the netting.
Filets maillants de monofllament dans la p§che continental
Resum^
Des etudes experimentales sur I'importance comparative des
proprietes des materiaux pour filets maillants, ont montre que la
visibilite est le facteur le plus important pour la peche en eaux
claires. Les mat&riaux de filets doivent avoir unc faible visibilite
contrastant peu ou pas tout avec le milieu et doivent £tre trans-
parents; dans ces conditions, les monofilaments synthetiques pro-
duisent les meilleures captures. La mollesse des filets est d' importance
secondaire. Le diametre du materiel des filets a une influence sur
refficacite de capture seulement dans la mesure ou les fils epais
sont plus visibles que les fils fins. L'elesticite semble avoir peu
d' influence sur la grandeur des poissons captures tandis que la
visibility a un remarquable cffet selectif. La force de rupture n'est
importante que pour les grands poissons et pour des considerations
cToperation puisque les petils poissons comme la perche et le
garden n'ont pas assez de force pour en ommager les filets com-
merciaux. Le taux d 'assemblage determine la forme des mailles,
la distribution des forces dans le filet et le relachement du filet.
La visibility des ligncs encadrantes peut influer sur Pefficacite de
la capture. Le nombre de flotteurs et le poids dependent des con-
ditions d'operations, du courant et de la tension ndcessaire dans le
filet.
by
R. Steinberg
Institut fur Netz und Material-
forschung, Hamburg
Redes de monofilamentos para la pesca de enmalle en agua dulce
Extracto
Los estudios experimentales de la importancia relativa de las pro-
piedades de los materiales para la fabricacidn de redes de enmalle
ban demostrado que en agua clara la visibilidad es el factor mas
importante. Los materiales para redes deben ser casi invisibles,
crear muy poco o ningun contraste con el fondo y ser transparentes.
Los monofilamentos sinteticos dan las mayores captures. Se demos-
tr6 que s61o tenia importancia secundria que el material fuera
blando. El diametro de los hilos s61o influye en el rendimiento de
la pesca en que los de mayor diametro son mas visibles que los de
menor diametro. Parece ser que la elasticidad apenas tiene efecto
en la talla de los peces capturados y que la visibilidad tiene un
efecto selective. La resistencia a la rotura s61o tiene importancia
para los peces peces grandes y en lo que a la manipulacibn se refiere,
ya que peces pequeftos como la carpa no tienen la fuerza necesaria
para ayeriar las redes de enmalle comerciales. La mancra de armar,
es decir, los metros de pafio por mctros de rclinga, determina la
forma de las mallas, la distribuci6n de las fuerzas en la red y si los
pafios quedan flojos. Las relingas de las cuarteladas pueden influir
en el rendimiento de pesca a causa de su visibilidad. La cantidad
de flotadores y plomos depende de las condiciones de trabajo, la
corriente y la intensidad de la tensibn necesaria en los paftos.
IN spite of relatively high cost, gillnets are of great
importance. By adopting proper mesh size only fish
of the desired range are caught, while undersized fish
continued from page 110
knot slippage as the fishermen make the nets with
double knots and also mend between knots as
shown in Figs. 3 and 4. The great majority of
nets in Viet-Nam are braided by hand by the fisher-
men and in practice it has been found that this
cannot be done well with monofilaments thicker
than 0.90mm diam.
Monofilament seems to be more sensitive to the lower
temperatures and nets made from this material used in the
cooler regions and during the winter season tend to
become stiff thereby becoming even more bulky out of
the water and less flexible during operations. Water
temperatures vary from 22°C to 29°C the year round.
Future of monofilament gillnets
Use of nylon monofilament for gillnets is progressing
very rapidly in Viet-Nam. No accurate figures of the
total nets in use are available but a general estimate
shows that some 1 1 ,000 gillnets are in use. The number
of monofilament and multifilament synthetic fibre nets
in use over the period 1958/62 is given in the table:
1958 1959 1960 1961 1962
Multifilament gillnets 5 50 80 140 160
Monofilament gillnets 2 30 1000 3000 8000
Gillnet fishermen in Viet-Nam normally work on a
share-on-catch basis and the popularity of monofilament
nets is such that the best fishermen engage themselves
only with owners of motorised boats using monofilament
gillnets. The sail boats, many of which still use natural
fibres, are experiencing difficulties in obtaining good
crews and, furthermore, operate at economic disadvan-
tage to motorised craft.
Ill
fort*
f
g. 5 Tht twinning fore* of roaoh
in relation to •!«••
190 tOO ISO MO iSO 400 430 §
A greater number of older and larger fish seems to be
caught with low visibility nets, indicating a more careful
approach towards gillnets than younger specimens. This
influence of the visibility on size of fish, however, varies
according to the fish species; it appears stronger for
perch than roach. The elasticity of the net materials is of
importance for the gilling process and for retaining
gilled fish in the meshes, but this has hardly been studied
so far.
19
2.8
26
2.4
Z2
20
U
16
M
12
10
ft*
0.6
04
0.2
0
• sS I.Pd/tlWtn*
/ ^y 2J1yl»n 210 denx2
3. Poly a mid monofilomtrt 0.25
015
4 Pdytthylen* mtnolllQmtnt 0.25
5. Cotton Nm 140/6
mo/»o///omrnf 0.25
10 12 14 16 18 20 22 24 26 28 30%
Fig. 6 Lo«4-«longation ourres of n«t twin«» of 0.25 » disaster.
Breaking strength — Because of the fineness of the net
materials, breaking strength is of high importance.
Nowadays most gillnets are made of machine-braided
netting, which process already requires a certain minimum
breaking strength and the strength required for handling
and operation must also be considered. Selection of net
material is, therefore, a compromise between the higher
breaking strength needed for lengthy trouble-free opera-
tion and the most desirable diameter for minimum
visibility. The strength of the fish only needs considera-
tion where it concerns large-sized species. High tenacity
materials, therefore, are essential for gillnets.
Constructional properties of gillnets which influence
the catch efficiency are: mesh size, hanging, framing
lines, floats and sinkers and height of the net.
114
Merit size — With true gilling nets the mesh size delimits
the size of the fish caught so that such gillnets are very
selective, catching only fish of a distinct size. Smaller and
larger fish may, however, be entangled if the net is loosely
hung.1 30 This selectivity of gillnets enables the taking
of only those specimens from fish stock which, by their
size, represent a good market value and are mature so
that their loss is of no serious consequence for the preser-
vation of the stock.
The selection of the optimum mesh size requires a
sound knowledge of the composition of the fish stock
and must be determined for each water body in accor-
dance with that knowledge and their needs.
Hanging — While hanging determines the shape of the
meshes and the distribution of the forces in the net, it
may also influence the selectivity.1 It would seem that
for fish with a narrow and high cross-section the mesh
shape should be stretched in vertical direction while for
flatfish the meshes should be stretched horizontally;
the latter, however, gives very tight hanging. The hang-
ing defines thf looseness of the netting and very often
it is desirable to have loose netting so that fish can be
entangled as well as gilled. This looseness can also be
obtained by attaching connecting twines between
floatline and leadline which prevent the netting from
stretching fully.
Framing lines — Gillnets used in freshwater fishing are
normally provided with a complete frame of lines,
bulky enough to handle and strong enough to withstand
shooting and hauling.
The visibility of the framing lines may have an effect
on the catch efficiency.8 26 30 Experiments with nets
framed with differently coloured lines showed that those
with green lines caught 61 perch and 153 roach while
the white-lined nets caught under the same conditions
22 perch and 37 roach respectively.
Floats and sinkers — Floats and sinkers, to achieve the
desired vertical position of the net in the water, must be
distributed uniformly. A large number of small floats
is usually preferable to a small number of large floats
and for submerged nets the total buoyancy must not
produce undue tension1 in the netting.
The shape and colour of the floats should not contrast
unduly with the water or background, otherwise they
may affect the catching efficiency in the same way as
the framing lines.
Net height — Bottom-set gillnets in inland fishing are
usually of small height and experiments showed that an
increase beyond 1*0 to 1*5 m does not normally result
in better catches.
Duration of fishing — Catch efficiency of gillnets decrea-
ses when the fishing time is increased and also with an
increase in the number of fish gilled. In experiments in
the Great Slave Lake17 it was found that, by extending
the duration of a set from 24 to 48 hours, the catch
continued on page 115
Etudes sur le Freinage et L'Usure des Fils de
Peche
La fabrique de fils et cordcs "Cousin Freres", France, precede a des
etudes constantes jx>ur amdliorer ses produits. Lorsque les rtsultats
obtenus sont d'une importance technologique generate, elle consulte
des Instituts in dependants— en ce cas Tlnstitut de Mecanique des
Fluides de Lille et le Laboratoire Federal Suisse de Saint Gall
arm que ceux-ci cffectuent des epreuves de controle, le resultat est
alors porte a la connaissance des pfccheurs, des constructeurs de
filets et des clients en general. La presente communication traite de
trois de ces essais. Les deux premiers conccrnent la trainee compara-
tive de fils de filets; au cours de ces essais il a ete constate qu'a
une vitesse dc trois noeuds, la trainee du coton etait sup6rieure de
50 pour cent a celle du nylon en monofilament continu d'un dia-
metre equivalent et avait trois fois plus de resistance a la traine,
pour d'egales forces de rupture. Les fils en nylon tresse de meme
diametre que les fils en nylon cable avaient une resistance a la
trainee legerement inferieure. Le troisicme essai destine a eprouver
la resistance a 1'abrasion des fils a montrd que le nylon cable avait
sept fois plus de resistance que le manille et le sisal a sec mais que
mouilie, il n 'etait que tres peu superieur au sisal. D'un autre
cdte la resistance a 1'abrasion du nylon tresse est cinq fois plus
grande que celle du nylon cable et sa resistance a 1'abrasion est la
m&me sous les deux conditions. Les fils de nylon tresse ont done un
gros avantage pour la construction des chaluts par suite de leur
haute resistance a 1'abrasion et leur tres basse resistance al a trainee.
Study on the drag and abrasion of fishing twines
Abstract
The net and rope factory of Cousin Freres, France, carries out
by
Maurice Bombeke
Establissements Cousin Fr&res
continuous studies aimed at improving their products. From time
to time when results are obtained which are of general technological
importance, independent institutes, in this case, Institute de Mecan-
ique des Fluides de Lille and Laboratoire Federal Suisse de Saint
Gall, are requested to carry out conclusive tests, the results of which
are then brought to the notice of fishermen, net-makers and clients
in general. The present paper concerns three tests : the first regarding
the comparative drag of twines when made into nets, during which
it was found that at three knots the drag of cotton twines was SO per
cent higher than that of nylon continuous multifilament of equiva-
lent diameter, and about three times higher resistance when com-
pared on equal strength basis. Braided nylon twines of comparative
diameter offer slightly less resistance than cabled nylon twines.
The second experiment on the abrasive resistance of twines showed
that cabled nylon had up to seven times higher abrasion resistance
than manila and sisal in the dry condition but only slightly more
resistance than sisal in the wet condition. On the other hand, the
abrasive resistance of braided nylon was five times higher than that
of cabled nylon, and its resistance is almost the same in dry and
wet condition. Braided nylon twines have, therefore, great advantages
for use in trawls because of high abrasive resistance and low drag.
continued from page 1 14
increased only very little. Similar results were obtained
in Northern German Lakes. Gillnets should be lifted
and cleared at least once a day and even at much shorter
intervals under certain conditions, such as in the tropics
or where predators are abundant.
Bibliography
1 Baranow, J. 1. : Theorie und Berechnung dcr Fischfanggerate.
Moskau/Leningrad. 1939.
2 v. Brandt, A. : Bekommen wir unfaulbare Netzgarne. Fisch.
Ztg., 42, 301-302. 1939.
3 v. Brandt, A.: Arbeitsmethoden der Netzforschung. Stuttgart
1947.
4 v. Brandt, A.: Netzweichheit und Netzharte. Arch.f.Fischerei-
wissenschaft, 1, 173-181. 1949.
6 v. Brandt, A. : Unsichtbare Netze zum Forellenfang. Allg.
Fisch.Ztg., 76, 300-301. 1951.
6 v. Brandt, A.: Erfahrungen mit Platil-Netzen beim Fang von
Edelfischen. Fischwirt, 2, 210-213. 1952.
7 v. Brandt, A.: Kiemennetze aus Perlondraht. Schweiz.Fi.-Ztg.
62, 98. 1954.
8 v. Brandt, A. : The Efficiency of Drift-Nets. Joum.du Cons.,
21,8-16.1955.
y v. Brandt, A. and G. Klust: Neue Faserstoffe fur die Fischerei.
Prot.z.Fischereitechnik, 3, 249-300, 1955.
10 v. Brandt, A. and R. Liepoldt: Ergebnisse von Fangversuchen
im Wolfgangsee mit Stellnetzen aus Baumwolle und Perlon. Oest.
Fischerei, 8, 93-97. 1955.
IJv. Brandt, A.: Net Materials of Synthetic Fibres. FAO
Fisheries Bulletin X, 182-210. 1957.
12 v. Brandt, A. and P. J. G. Carrothers: Test Methods for
Fishing Gear Materials, (submitted to II. Fishing Gear Congress).
1963.
1 3 Einsele, W. : Uber den vom 7. bis 12. Oktober 1 957 in Hamburg
abgehaltenen Internationalcn Fanggerate-Kongress. Oest. Fischerei,
10. 121-126. 1957.
1 4 Geisel B P : 3 Jahre Erfahrungen mit Platilnetzen im Laacher
Sec. Fischwirt, 3, 310-312. 1953. . u . ,
ISHerter, K.: Die Fischdressuren und ihre smnesphysiolo-
gischen Grundlagen. Berlin 1953.
16 Kajewski, G.: Untersuchungen zur Auswirkung der Sicht-
barkeit monofiler synthetischer Fischereigcspinste auf die Reaktion
von Fischen. Fischereiforschung, 1. 1958.
17 Kennedy, W. A.: The Relationship of Fishing Effect by
Gillnets to the Interval between lifts. Jo urn. of the Fish. Res.
Board of Canada, 8, 264-274. 1951.
18 Klust, G. and Kohnke, E.: Stellnetze. Fischwirt, 3, 237-241,
268-273. 1953.
1 ° Mohr, H. : Note on the Behaviour of Herring in a Round
Tank. Comp. Fish. Com., No. 87, Int. Counc. Explor. Sea, CM.
Kopenhagen. 1961.
20Molin, G.: Natmontering och fisklighet. Svensk Fiskeri
Tidskrift, 66, 30-31. 1957.
21 N.N.: Perlon-Drahtnetze haben sich be with rt. Fisch win, 4,
44-45. 1954.
22 N.N.: Nachrichten aus Oberbayern. Fischwirt, 5, 185. 1955.
2'N.N,: Erfolgreiche monofile Stellnetze. Fischwirt, 5, 351.
1955.
2« N.N.: Nochmals Maranennetze aus Kunststoffdrfthten. Fisch-
wirt, 6, 102. 1956.
2* N.N.: Summary Report of the Joint Scientific Meeting of the
International Commission for Northwest Atlantic Fisheries.
Lisbon. 1957.
26 Nomura, M., Mitsugi, S. and R. Nahano: On the Horizontal
distribution of Catch Influenced by the Seam of Net in Sardine
Drift Nets. Tokai Reg. Fish. Res. Lab., Collected Reprints, 245-248.
1957.
27 Niimann, W.: Untersuchungen Uber die Berechtigung einer
verstftrkten Schwebnetzfischerei im Bodenscc. Allg. Fish.-Ztg.,
86, 403-404, 427-428. 1957.
28 Riedel, O.: Contribution to the Experimental Determination
of the Selection Parameter of Gillnets. (unpublished).
2 g Schrader, K.: Kiemennetze und Spiegelnetze aus Platil.
Fischwirt, 4, 104-106. 1954.
'° Steinberg, R.: Die F&ngigkeit von Kiemennetzen fUr Barsch
und Pldtze in Abhftngigkeit von den Eigenschaften des Netz-
materials, der Netzkonstruktion und der Reaktion der Fische.
Arch.f.Fischereiwisscnschaft, XII, 173-230. 1962.
"Wagler, C.: Die Bewirtschaftung der Coregonenseen. Int.
Rev.ges.Hydrobiologie, 37. 1938.
22 Wigutoff, N. B.: Pacific Salmon Drift GUI Netting. Fish and
Wildlife Service,Fisch.Leaflet 386, Washington. 1951.
115
CM to rcsMtmcto y Q6fl0ute cte UkM para
Extncto
La fftbrica de redes y cordelerfa Cousin Frires, Francia, realiza
continuamente estudios para mejorar sus productos y de vez en
cuando, los resultados obtenidos son de importancia tecno!6gica
general, solicita de institutes independientes, en este caso el Institut
de Mecanique des Fhiides de Lila y el Laboratoire Federate Suisse
de Saint Galle que realiccn ensayos definitives cuyos resultados se
comunican a Pescadores, fabricantes de redes y clientela general.
Esta ponencia alude a tres ensayos: el primero sobre la resistencia
oomparativa de hilos cuando se hacen las redes. Durante este
se observd que a tres nudos de velocidad, la resistencia de los
hilos de algod6n es un 50 por ciento mayor que la de los filamentos
continues de nylon del mismp diametro y unas tres veccs mayor a
igualdad de robustez. Los hilos de nylon trenzados ofrecen algo
menos resistencia que los torcidos de diametro analogo. El segundo
experimento, sobre la resistencia al desgastc, demostr6 que el
nylon torcido ofrecfa seite veces mas resistencia al desgastc que el
abaci de Manila y el sisal estando secos, pero s61o un poco mas
que el sisal estando humedo. Por otro lado, la resistancia al des-
gaste del nylon trenzado es cinco veces mayor que la del torcido y la
resistencia a la traccibn casi igual estando seco que humedo. Se
deduce de ello que los hilos de nylon trenzado ofrecen grandes
ventajas en los artes de arrastre por su gran resistencia al desgaste y
pequena a la tracci6n.
DANS le but dc perfectionner leurs produits, les
ftablissements Cousin Frires de Wervicq-Sud,
France, font constamrhent des 6tudes sur les matiriaux
utilises ct les mithodes de fabrication. De temps en
temps lorsque des risultats intiressants sont obtenus,
des 6preuves concluantes sont ex£cut£es dans des con-
ditions contrdl&s par les Instituts reconnus; les risultats
de telles ipreuves sont alors port6s & la connaissance des
pccheurs, des fabricants de filets et des clients en general
La pr£sente communication traite de trois de ces£preuves.
Mesure de la trainee de fib de pfiche
Le but de cette Stude faite par I'lnstitut M6canique de
Lille, dtait de connaitre la force £ employer pour tracter
deux fils cables de meme force £ la rupture : Tun en coton
et 1'autre en nylon.
Les fils & essayer etaient de cinq types:
(a) fil de coton, diamitre approximatif 1 mm
(b) fil de coton, diamitre approximatif 1*8 mm
(c) fil de nylon, diamitre approximatif 0'9 mm, cab!6
(d) fil de nylon, diamitre approximatif 1 mm, tresse*
(e) fil de nylon, diamitre approximatif 1 mm, tressS
Wercord.
Pour les essais, chaque type de fil est monte" sur un
cadre en bois profil£ de une mitre de cote", fix£ rigide-
mcnt aux mats verticaux d'une balance airodynamique.
Les fils sont months verticalement et horizontalement,
de fa<?on i obtenir des mailles carries de 20 mm de cot6
environ. II y a ainsi, pour chaque type de fil 98 mitres de
fil essay*.
Les essais ont 6t6 effectuds dans la Soufflcrie Horizon-
tale de Tlnstitut de Mecanique des Fluides de Lille, de
diam&tre 2*40 mitres en veine guide*e.
Trainee des fib dam I9air
Les resultats des essais effectuds en soufflerie sont con-
si gnds dans les tableaux suivants ; les colonnes A donnent
la pression dynamique en mm eau; les colonnes B
donnent Tefifort de traindc en kg (Tableau I).
116
Courbet £ -f(q)
q mm d'eou
50 100 ISO
Planche 1. Trainee des fils depeche dans Vair.
Trainee des fib dans 1'eau de mer
A partir des essais effectu6s dans 1'air, nous calculons
reffort de trainee s'exergant dans 1'eau de mer sur 100
mitres de fil de chaque type, pour des vitesses d'avance-
ment evaluees en noeuds.
V.D.,
Pour un meme nombre de Reynolds: R on
v
determine d'abord la vitesse du courant d'air corres-
pondant 4 une vitesse d'avancement dans 1'eau. Comme
le diamitre D est constant, on doit avoir:
d'ou
V V
— air= — eau
v v
.. . ,7 v air
V air= V eau x •
v eau
Les valeurs de la viscosite" cinematique v sont:
Air 4 15°: t;=0'15 stokes
Eau de mer & 4° : v=0'01 5 stokes
Ce qui donne: V (air) =10 V (eau)
A une vitesse d'avancement dans Feau de quatre
noeuds par exemple, correspond une vitesse du courant
d'air de 40 noeuds, soit 20*55 m/sec, qui dquivaut & une
d'eau.
Pour une vitesse donnfe, on icrit ensuite, que les
coefficients de trainee dans Fair et dans Teau sont
6gaux:
Fx Fx
Pa Vz air "~pa V2 eau
p eau V eau 2
Ce qui donne: Fx (eau)=Fx (air)x — r— x.. . •
P air V air
Les valeurs de la masse sp&ifique p sont :
Air i 15°: P=l-226 kg/m^
Eau de mer 4 4°: P= 1-026 kg/m3
d'otl: Fx (cau)= Fx (air)=
=8-37 Fx (air)
1-026 1 «
1-226* llf
Dans 1'exemple choisi, on lit sur la P1.2 la valeur Fx
(air) correspondant & q=26'4 mm d'eau, soil Fx (air)=
2-350 kg pour le fil tress6 nylon traitement Wercord.
Pour une vitesse d'avancement dans Feau de duatre
noeuds, on a done, un effort de trainee:
Fx (eau)=8-37x 2-35= 19-620 kg
Les trainees ainsi obtenues dans Feau de mer sont
donnies dans le tableau cidessous, et portees en courbes
P1.2.
Tableau I
Nylon tr«M* traiUment WERGORD
d« 1 mm
Nylon tr«M4 d« 1 mm
A
B A
B
7,60
0,695 M«r>
0,830
19,30
1,790 1*75
1,915
30.10
2,720 ?*M
2.920
40,10
3,650 39.2°
3,980
50,30
4.690 «
4,960
59,20
5,580 57.40
5,800
69,25
6,700 67,60
7,020
80,90
7,880 76
7,860
90
8.9SO M*20
9,130
99.70
9,980 97»2°
10,320
110
11,220% 108»10
11,600
118,20
12,100 11WK>
12,530
134,20
13,820 132^0
14,400
145
15,050 145
15,850
Coton dft 1 yyini
Nylon de 0,9 mm
Colon de 1 ,8 mm *
A
B
~ A
B
A B
11,27
24,80
40.70
46,00
54.75
67,70
77,80
83.50
101,90
106,00
117,70
126,40
132,30
141,50
1,730
3,580
5,745
6,110
7,445
9,090
10,035
11,035
13,215
13,415
15,095
16,155
16,680
17,975
11,45
24,60
40,30
54,30
67,50
. 73,80
78,50
98,09
108,00
125,40
132,00
134,70
151,40
164,20
1,010
2,230
3,660
4,910
6,105
6,570
7,050
8,910
9,750
11,360
11,960
12,365
13,895
15,110
12,75 2.980
28,10 6,595
38,30 8,800
55,30 12,490
66,40 14,825
82,20 18,225
98,00 21,625
109,30 23,940
122,30 26,660
154,00
19,375
165.50
20,775
TABLEAU II— Effort de trainee en kg sur 100 m de fil
Nylon
Vitesse de 0«9 mm
1 noeud 1,275
2 noeuds 5,150
3 noeuds 11,675
noeuds 20,700
Coton
de 1 mm
2,00
7,80
18,00
32,00
Tresse
Nylon
traitement
Coton Wercord
de 1*8 mm de 1 mm
3,270 1,255
13,00 5,010
29,40 11,370
52,00 19,650
Tresse
nylon
de 1 mm
1,420
5,520
12,530
22,200
30-
20-
10-
Courbes Fx «• f(VM notudi)
x
tn kg
1234
Planche 2. Trainee des fils de pechc dans reau.
Remarque
Les caracteristiques des torons qui constituent le
fil de nylon "cabte" de diamitre 0*9 mm sont identiques
£ celles des torons du "tress£ machine'*. La force de
trainee 6tant directement proportionnelle & la surface
frontale du fil, on peut, & partir des risultats obtenus
pour le 0-9 mm, calculer les efforts de train6e s'exer$ant
sur le fil de nylon "c£bl6" de diamitre un mm. Ces
r6sultats portSs en courbes & la planche deux sont &
1'avantage du nylon "tress£" qui, dans 1'eau subit un
effort de trainee tegferement interieur 4 I'effort que
subirait un fil de nylon "cabte" de m£me diamitre.
L'essentiel de cette £tude 6tait de trouver quelle itait
la difference de trainee existant entre les cables et les
tresses: la conclusion est que la tresse nylon offrant
moins de resistance & la traction que le cftbli nylon,
son emploi est vivement recommand£ pour les chaluts
pe"lagiques.
Tenue a 1'abrasion des tresses et cAbKs nylon pour filets de
pfehe
(Etude faite par le Laboratoire F6d6ral Suisse de Saint
Gall)
Articles essay6s:l bobine de tresse en Nylon 4840/3
1 bobine de cdbli-Nylon carte Rouge
502-rtf. 270
1 pelote de manille trois fils 8/10
1 pelote de sisal trois fils 8/10
1 pelote de ficelle CH-D trois fils 8/10
Conditions d'essai:
Le materiel est climatisd & 65 pour cent d'humidit£
relative et 20°C.
Mode d'examen:
Des £prouvettes pr£lev£es de chacun des 6chantillons
sont tressdes dans une plaque pcrfordc. Les boucles
117
sortaot du c6t6 supe'rieur sont frott&s par I'tiiment
frottant plat de 1'appareil Schiefer jusqu'au moment de
la destruction de la premiere boucle. La pcrtc de poids
moycnnc, calculi par 1,000 tours, est determined en
m£me temps. Les deux dchantillons de nylon sont pes6s
apits les premiers 2,000 tours, ceux de manille et de
ficelle CH-D apris les premiers 500 tours et I'&hantillon
de sisal apris les premiers 250 tours.
Afin de determiner les pertes de poids des eprouvettcs
mouillces, tress&s dans les trous du disque perfore et
puis abrasccs pour atteindre le nombre de tours indiqud.
Ensuite elles sont sichies, climatisies et pesies.
R&ultats:
Les essais ont donne" les resultats suivants:
Mb S flta K/10
U » flb »/>U . .
ilk CH-D 3 U» IJIO
3,93
O.M
RgStSTANCK A l.'ABRASlON
0.037
H.VW
0.376
U.3M
7300
5100 I
•J.OM
0.3D2
0.41.'
Cette itude ddmontre la grande resistance a 1'usure de
la tresse nylon, elle a determine Femploi de la tresse
nylon dans la construction, notamment, des chaluts et
des grandes seines tournantes.
En France, Femploi de la tresse nylon pour les chaluts
est maintenant g6n£ral; les chaluts de grande p6che
portugais, espagnols et italiens sont igalement fabrique*s
avec la tresse nylon.
Discussion: Materials for
Lines, Ropes and
Monofilaments
Dr. J. Reuter (Netherlands) Rapporteur: Most synthetic
fibres are produced by extrusion with orientation or stretching
afterwards. As a result of this, textiles, net twines and lines
and ropes are often made of the same stretched fibre. It
seemed strange that different articles used for different
ourposes can te made from the same type fibre and still
have for different purposes, optimal and good properties.
By experiments, he and C. C. Kloppenburg had tried to
find out what fibre type of polyethylene was the best for
special purposes — specifically for (a) a strong trawl twine
which remained strong when knotted; and (b) a strong rope
in the sense of having the highest breaking strength.
E;ght different types of monofilaments — based on poly-
ethylene— made respectively out of homopolymer and
copolymer (with different orientation) were produced. From
all these monofilaments trawl twine was made and tested
on breaking strength and knot breaking strength. For both
types of polyethylene, the filament that gave the highest
breaking strength as twine did not give the highest knot
breaking strength.
Ropes were also made from those eight types of filaments
in normal and hard lay. As an outcome of these tests it
118
could be said that the type of filament that gave the highest
knot breaking strength of trawl twine was the same that
gave the highest rope breaking strength; but the highest knot
breaking strength of the twine must be made from a different
type of filament. This meant that special filaments of
polyethylene had to be made for either ropes or nets. For
ropes they must be made to give breaking strength and for
nets, knot breaking strength.
On knotless nets: It is necessary to point out strongly
that there are two types of knotless nets: (a) the Japanese
type made by a special way of twisting and (b) the Raschel
type made by knitting.
The importance of this last type is pointed out by Mugaas
and by Damiani. Mugaas shows that in the last two years
in Norway the use of this type of netting has increased
from 17 to 200 tons because in the small mesh sizes the
knotless nets are 20 to 30 per cent cheaper than the knotted
ones. Damiani also shows that in Italy the Raschel machine-
made netting is cheaper for smaller mesh sizes but above
a certain size the old knotted ones are less expensive.
Dr. von Brandt's paper on testing Raschel knotless nets
correlates the properties of this netting with fishing experi-
ments made with the nets, whereas Hamuro deals only with
experiments to improve a fishing net by using knotless
netting in parts of the trawl. Their work shows that people
are seeking to discover for what type of fisheries these knotless
nets are better or cheaper than the old ones. It is quite
certain that in one or two years' time certain fisheries will
change over nearly totally to the knotless type of netting.
Dr. von Brandt showed that the construction of the "twine"
and of the joins is rather complicated. But some uniformity,
will be the result of increased testing by discovering the best
methods for these joins. A year ago this mesh strength in
the depth direction of the netting was different to that of the
length direction. It is in fact already claimed now that the
knot strength of this type of netting can be equalled in both
directions. Pianaroli states this, too. These differences in
mesh strength are admitted by von Brandt and I wonder
how much, in consequence of this, new joins with equal
strength in both directions, the comparison of knotted to
knotless nets based on mesh strength will change. This new
basis will also change the weight comparison based on the
respective mesh breaking strength. Although these two
methods of comparison are somewha,t doubtful they do
indicate that the knotless nets have a ittle advantage over
knotted ones.
The total resistance of this type of netting shows no
difference compared with knotted nets of equal mesh-
breaking strength. Used in herring gillnets in the North Sear
the catching capacity of knotless nets was neither higher no,
lower than that of customary knotted gillnets.
It would seem that below a certain mesh size knotless
Raschel nets are absolutely cheaper. Due to a new technique
of making the joins, the mesh breaking strength in both
directions can be made equal and, for that reason, comparison
with knotted nets has to be put on a totally new basis. In
other words a lot of experimenting and testing has to be done
all over again. It is a pity that not much is published in the
same way for comparing Japanese twisted knotless nets,
especially in relation to physical properties and catching
capacities.
On monofilaments: Synthetic fibres for making twines,
lines, ropes and nets can be made in different thicknesses with
few exceptions. The thicker filaments are often preferred
because one thinks that they are more useful and better
adapted for certain tasks. The difference of thickness is the
only reason why one fibre is called multifilament and the other
mononlamcnt.
Multiftlaments are mostly produced in thicknesses of 2 to
25 denier per filament and monofilaments in thicknesses of
50 up to 1,000 denier and even thicker. Even ropes made of
filaments 4 mm thick are available.
This difference of filament thickness creates certain
properties which make the fibres better adapted for certain
work but there are technical reasons which make it sometimes
difficult to use monofilaments. Their very thickness creates
properties not liked by fishermen.
In their paper Henstcad and Ede review these different
properties of monofilaments as made of different materials—
polyamide and polyolefin. The properties considered, to
mention only a few are: (a) tenacity, (b) effect of heat, (c)
non tenacity, (d) degree of flexibility, (e) abrasion resistance.
In practice these monofilaments are made and used as a
single monofil, as twine or as rope, and are used in different
kinds of fisheries. Experience was required to discover in
which fisheries a special part of the netting could be made
best of monofilaments or multifilament products. There are
many fisheries in which monofilaments have played an
enormous role, for instance in Viet-Nam. Filaments made
there are cheaper than imported multifilament and twine but
technical advantages also accounted for their fishermen
preferring monofiiament nets. These advantages are less
fouling, easier to work with, less entangling and catching
not less but sometimes more fish than knotted nets. These
properties compensated for the bulkiness and other dis-
advantages of such monofiiament nets.
Steinberg lists the properties of monofiiament in comparison
with other types of nets as (1) visibility, (2) softness, (3)
diameter, (4) elasticity and (5) breaking strength.
In this complex matter experiments are needed to find out
what is worth while. Monofilaments are used not only as
netting material but to produce twine and rope. Mostly such
products are made out of olefin monofilaments because of
their low specific gravity— I know of only one type of rope
made of monofile polyamide.
We stand at the beginning of a period in which fibres and
filaments will be produced for special purposes. To adopt
monofilaments to the special needs of fishing involves a
tremendous amount of practical experiments.
Dr. C. C. Kloppenburg (Netherlands) explained that the
colouring pigments had been added to the material tested
only for identification purposes and exercised no influence
on strength.
Mr. Masao Kobayashi (Japan) expressed pleasure that
knotless nets were featured in the Congress: his company
had developed twisted knotless nets for many years. They
were used for practically all types of fishing in Japan and the
annual consumption was some 1,600 tons a year. There had
not been sufficient time to study the papers on Raschel nets
in comparison with the Japanese twisted nets, but they would
do so and submit results. Both the twisted net and the Raschel
net were different from the conventional knotted net. Accord-
ingly when such knotless nets were used for purse seines, set
nets and trawl nets the way of assembling the knotless netting
should differ a little from that of knotted nets.
A suitable method of assembling knotless nets was fully
studied in Japan as was shown in Hamuro's paper and it was
used in every type of fishing net except a gillnet. His firm
belief was that the knotless fishing net was most advantageous.
Dr. von Brandt: I would be very interested to compare
types of Japanese and Raschel nets. There are big variations
in the different types: both in making and in the joins.
Mr. V. Valder (Peru), on behalf of Julie Castille, gave
details'of Peru's purse seine fishery in methods and equipment.
Nylon' netting, introduced 10 years ago, had undoubtedly
contributed greatly to the meteoric growth and development
of the industry. Knotless webbing had only recently been
effectively introduced through Raschel knotless webbing
becoming available at the end of 1961. Some earlier supplies
had been too heavily tarred and been awkward to handle and
easily breakable. Latest figures showed that 75 purse seines
of knotless webbing were in use in a total fleet of 1,200 craft.
A survey made in April 1963 by the Marine Resources
Research Institute listed these advantages: (1) lower price
because of lesser weight, (2) non-tangling of baby fish making
operations easier, (3) less friction on the boat and bottom
during hauling, thus promoting durability. A company had
been formed and was beginning large-scale manufacture of
knotless anchovetta nets in a factory containing ten Raschel
machines of 960 mesh width. It was believed development
in use would exceed Norway's growth and sales in excess
of 400 tons were anticipated for 1964. This local manu-
facture was likely to preclude overseas importation.
Mr. D. L. WortfleM (UK): We have a method which we
find very useful for comparison purposes between knotted
and knotless nets. If you take the ratio of the wet mesh-
strength to the weight of a given area of netting, say 1,000,000
meshes (1,000 by 1,000 square), this can be used for both
knotted and knotless nets and this allows for the amount of
twine used in the knot or in the junction of the net. If this—
parameter K, we call it— 1 over K, is plotted against the mesh
size you can find the break-even point as far as the strength is
concerned, for knotted as compared with knotless netting.
Then, by allowing for production costs, this should be very
useful for finding when knotless nets can replace knotted
netting.
Mr. P. Lusyne pointed out that there were at least four
different types of Raschel netting and they should know more
about the joins and also more about the strains of the legs of
the meshes themselves. Whenever they had mesh breaks it
would be useful to know whether they were in the knot or in
the legs of the mesh. It would be helpful also to have some
project for finding which types of knot or netting gave the
greater strength and the greater elasticity. Another point of
interest was the breaking strength according to the direction
of the pull. Much depended here on the type of connection
between each mesh and they needed to know the different
effects of the inner lines crossing or weaving.
Arising from a query from Mr. M. Praet as to the meaning
of the terms "normal, special and super1' in Dr. von Brandt's
paper on testing Raschel nets, Dr. von Brandt explained that
they had been adopted instead of (a), (b), (c) simply to dis-
tinguish three different constructions of material so that they
could see what happened.
Mr. Helmut Panzer (Germany) supplier of the material
gave a technical explanation of their choice of three types of
typical Raschel nets which were already on the market.
The most interesting point was the join (see Fig. 4 in Dr.
von Brandt's paper this section).
Mr. Kristjonsson: Should the term "monofiiament" not be
reserved for one filament used alone and not for two or more
filaments twisted together? They should talk about heavy
multifilaments when using heavy denier for making ropes
as is now being done.
Dr. Renter, summing up, said he considered the most
important subject was testing; the testing of knotted net in
relation to knotless netting. He had tried to get from a fac-
tory making Raschel nets a certain twine but as yet had not
received it. A new system had been devised and people
wished to keep it for themselves. It was impossible to repair
knotless nets without using knots. Knots had to be used.
As to terminology, any change would be difficult because
custom was involved.
119
INDEX TO ADVERTISEMENTS
SECTION 1
Page
AMITA Co. LTD. 122
APELDOORNSE NETTENFABRIEK VON ZEPPELIN & Co. N.V 139
ASAHI Dow LTD 134
BAYER, FARBENFABRIKEN, A.G. 141
BELFAST ROPEWORK Co. LTD., THE 130
BRIDPORT-GUNDRY LTD 141
BRITISH CELANESE LTD. 135
BRITISH NYLON SPINNERS LTD. 125
DAI NIPPON SPINNING Co. LTD 142
DAVIS MILLS INC 138
FISHING NEWS (BOOKS) LTD. 130 & 140
FUKUI FISHING NET Co. LTD 128
HAKODATE SEIMO SENGU Co. LTD 124
HIRATA SPINNING Co. LTD 136
I.C.I. LTD. (FIBRES DIVISION) 139
KENYON, WM., & SONS (ROPES & TWINES) LTD 136
KNOX, W. & J. LTD 132
KURASHIKI RAYON Co. LTD 131
MARCONI INTERNATIONAL MARINE Co. LTD. . . . . . . . . . . . . . . 144
MARINOVICH TRAWL Co 1 32
MARUBENI IIDA Co. LTD 132
MEWES, J. H., & v. EITZEN 138
MOMOI FISHING NET MFG. Co. LTD. 121
MORISHITA FISHING NET MFG. Co. LTD 128
NIPPON GYOMO SENGU KAISHA LTD . . 134
NIPPON RAYON Co. LTD 129
NITTO SEIMO Co. LTD. . .. 137
NORSKOV LAURSEN, CM., MASKINFABRIK I/S . . . . . . . . 138
PERLON-WARENZEICHENVERBAND E.V. 127
R.F.D. Co. LTD 137
RHEINSTAHL HUTTENWERKE A.G .. .. .. .. .. .. .. 143
SAMSON CORDAGE WORKS 123
TEIJIN LTD 126
TOYAMA FISHING NET MFG. Co. LTD. . . . . . . . . 142
UNIVERSAL TRADING Co. LTD. 142
120
Top quality, precision-made MOMOI NETTING is made to
hang right, resist wear, handle easily and fish better
MOMOI FISHING NET MFG. CO., LTD.
Ako, Hyogo-ken, Japan
U.S.A. :
MOMOI CO., INC. .„-..,
1596 Judson Ave., Long Beach 13, Calif.
GERMANY: MOMOI
TAIWAN : Taiwan
. . *T«. muo rriT AT IfnifAf Pfl . T/TD.
a newly invented mechanism
2 'n I functionally
machines by | operator
II *
/Q quicker in action
fishing net machine | model PKA
AIUIITA company limited
Toyohashl Japan
cable address:
'AmlcoToyohashi'
122
10 reasons why Samson Braided Ropes'
help you catch more fish faster
25%
I STRONGER \
The big, value-conscious fishing industries of the West
Coast of the United States and Canada have proven Samson
Braided Ropes are today's most advanced lines for every fish-
ing purpose, under extremes of usage and weather. No other
rope offers such basic advantages for fishing efficiency, dura-
bility, profits. As a matter of business, you should investigate
thoroughly. We will be happy to send literature. Write today,
tell us your area of interest.
Samson Ropes are patented and engineered; backed by most
extensive braided rope laboratory in America, produced by big-
gest, oldest manufacturer in the industry. Advantages are basic;
U.S. Navy, National Aeronautics and Space Administration,
other oceanographic interests have adopted Samson Braided
Ropes for many purposes: retrieval of Space Capsules, suspen-
sion of vital electronic equipment 5 miles deep, etc. Strengths
available (in ropes only 21/8" diameter) up to 120,000 pounds!
1. STRONGER, SIZE FOR SIZE,
Up to 25% stronger than any other
rope of same size, material. Example:
W Samson Braided Nylon Rope with
splice, 8000 Ibs. breaking strength.
V4" Twisted Nylon rope without
splice, 6500 Ibs. Samson rope is made
with latest synthetic fibers.
*Note: Identical ropes are produced
in Canada by our associated corn-
pany, Canada Ropes Ltd., under trade
name ''Buccaneer Brand."
2. CONTROLLED STRETCH
Engineered to correct degree of stretch
required by specific jobs. Some shock-
absorbent. Some, up to 60% less
stretch than other ropes of same size
and material!
3. SPLICES EASILY
You can make faster splices, stronger
splices, when compared with splicing
twisted rope of same material, Trial
splicing kit with line and tools avail-
able—$1.00
4. KINK-FREE OPERATION
. . . And, even after long soaking in
water, Samson Braided Ropes stay
supple, flexible. Can not hockle.
Sold under registered trademarks: Samson 2-in-1 and Buccaneer 2-in-1 Braided Ropes. For further information write to:
EAST COAST OF U S WEST COAST OF U.S. IN CANADA
SAMSON CORDAGE WORKS SAMSON CORDAGE CORP. CANADA ROPES LTD.
Marine and Industrial Division 1215 Commercial St. 1320 Vulcan Way
Boston 10, Mass., U.S.A. Bellingham, Washington, U.S.A. Vancouver, British Columbia
5. ENGINEERED ABRASION RESISTANCE
Wear is spread over 40 to 50% of
surface, not concentrated on a few
high spots.
6. EASIEST HANDLING ROPE
Samson Braided Ropes run smooth in
blocks. Easy on hands.
7. NO BUILT-IN TWIST
Holds freely-suspended loads without
spinning. Braided ropes, unlike twisted
ropes, are naturally twist-free for
easier coiling, easier working,
8. BETTER GRIP ON WINCHES
Contact with winches and blocks is
far better than with twisted rope. 40
to 50% of surface grips winch or
block — not just high spots as with
twisted rope.
9. PERMANENTLY FLEXIBLE
Samson Braided Ropes are the most
flexible ropes ever made. And they
stay flexible.
10. ROT-PROOF
Of course, Samson Braided Ropes
have all inherent advantages of syn-
thetic fibers.
123
A good catch
much depends on using
UROKO'S
FISHING NETS
UROKO S
Thanks to the highest skill and best workmanship as applied by Japan* s
leading manufacturer of fishing net, twines and ropes the "UROKO" label
has built up a tremendous goodwill and popularity with fishermen all over
the world.
HAKODATE SEIMO SENGU CO., LTD.
(HAKODATE FISHING NET ft CHANDLERY MFG.,)
HEAD OFFICE
EXPORT DEPT
TOKTY OFFICE:
OSAKA OFFICE:
HAKODATE, JAPAN
NITTA BLDG., 8-8, GINZANISHI, CHUO-KU, TOKYO, JAPAN
CABLE ADDRESS: SCALENET TOKYO
MITSUI BLDG., 21, JOAN-CHO, KITA-KU, OSAKA, JAPAN
CABLE ADDRESS: SCALENET OSAKA
124
first and
foremost for
fishing
fry I on)
125
TAKE YOUR CHOICE
OF
Polyvinyl chloride fiber
— Tevinye, Tevix —
FISHING NETS & ROPES
Whichever you choose, you get:
* Easy handling
* Non-absorbency and -shrinkage
* Ideal undersea shape-forming
* Abrasion resistance
* Corrosion- and
chemicals-proof properties
Otok.' Tokyo, JM«I
C«to Addrw* "TWIN OSAKA
IN LIMITED
TOKYOM
126
If you had to choose between 'good' and
'excellent' - which would it be?
PERLOlSTof course! Ae an expert
you will quickly appreciate the reasons:
PERLON le reeietantto abraeion.
It doee not rot A PERLON net
ie lighter - therefore eaeier
to handle. Thie means leee work and a
higher towing epeed at the eame
engine power. It meane money eavedi
In recent yeare there's been
an increaeing demand
for knotleee PERLON nete. And no wonder.
Nonelipping and runproof
• they're even more profitable.
PERLON-Warenzeichenverband e.V.
Frankfurt/Main (Germany)
Reuterweg 47
12
MORISHITA FISHING NET MFG. CO. LTD.
Osaka Office:51, Higashishimizumachi, Minami-ku, Osaka, Japan, >X Cable Address: * MORISHITANET* OSAKA
SAVES MONEY
TIME AND
LABOUR
Completely without Knots. Available
m (Sins, to I I6ins. Mesh Sizes. Up to
1,000 meshes depth in one piece. Gives
greater durability and resistance to
abrasion. Also manufacturers of ail
convttntmna! Nets, Ropes mnd Twines
in Natural and Synthetic Fibres,
128
Put fish in your nets . . . be sure, with
Fishing is a man's world and demands the best of equipment.
Fishing Nets and Ropes in N-R-C Nylon are known for
providing toughness, lightness, and resistance to wear and rot,
so the fisherman of experience puts his full confidence in his
gear, when he uses N-R«C Nylon goods. N*R-C Nylon Fishing
Nets and Ropes ensure your haul.
NIPPON RAYON CO., LTD.
NRC
SOI
NYLON.
OSAKA, JAPAN
CABLES: NIPPONRAYON OSAKA
12!
BELFAST
net
made from Manila, Sisal, Nylon
Terylene. Polythene and Ulstron
Manufactured by
fMOf
THE BELFAST
ROPEWORK
COMPANY
LIMITED
FISH CATCHING METHODS
OFgTHE WORLD
by A. von Brandt
This is a fascinating book of value to the
commercial and sporting fisherman as well as
to the scientist, administrator, teacher, ethno-
logist and netmaker.
In nearly 100,000 words and 250 illustrations
it analyses in simple, clear language the basic
methods evolved by mankind for catching fish.
The author shows that these methods are limited
in number but fundamental in character,
although in their application to gear distinct
techniques are employed.
The author heads the Institute for Fishing
Technique and Gear Research at Hamburg and
is a world authority on his subject. Through a
lifetime of research, travel and practical experience
he has built up the most complete and wide
ranging library of literature and pictorial record
on fishing ever assembled by any one individual.
In this book, the first and only one ever
published on so comprehensive a basis, he gives
the reader the essence of his knowledge. The
work is fully indexed and has an appendix
linking the illustrations into the authoratitive
FAO classification of gear.
Price £2 17s. 6d., plus postage 2s. 6d.
($8-75 inclusive)
JAPAN'S WORLD SUCCESS
IN FISHING
by Georg Borgstrom
This book is the first authoritative and de-
tailed analysis of Japan's amazing development
in fishing. Wholly factual and objective in style
and presentation, it outlines in some thirty
chapters with accompanying tables, maps and
graphs the story of how Japan has met her prob-
lem of supporting an expanding population by
turning to the sea.
The work results from the steady collection
of data over many years and a personal visit
to Japan. Fishing operators and administrators
the world over can learn much from it.
Price £2 15s. Od., postage 2s. 6d.
($8-25 inclusive)
Write for our book list.
FISHING NEWS (BOOKS) LTD.
110 FLEET STREET, LONDON, E.C.4
130
New, resistant
KURALON fishing nets and ropes
In use fishing nets and ropes made of Kuralon, an exclusive brand name of
polyvinyl alcohol fibre, exhibit the same remarkable qualities- greater strength,
durability and lightness of weight plus complete resistance to rot, seawater, sun,
fish oil, acids and alkalis.
Remember, if it's made of Kuralon, it will do the job.
For detailed information, please refer to:
KURASHIKI RAYON CO,. LTD.
8, Umeda, Kita-ku, Osaka, Japan
Cable Address: KURARAY OSAKA
Kuralon or Manryo nettings
ft ropes always bear this label.
*/» America and Germany only the trade name "Manryo'' is used.
131
Tr/iu//c Are
I ruVr fa W
WORLD-WIDE
sinj
Tho Linon Throad Co. Product*
• Gold Modal Cotton and Nyak Nottinft
• Barbours* Nylon lolno Notting
• Plymouth Ropos, McKay Chains, Amorican 'Truo Lav" Wiro Rooo
Dtetrlbutor "PIRMA ROPI" TRAWL CABLE
All Sikos Stoekod In Ouantity
Send Your Specifications for Any Type Net or Boards
TRAMMEL NETS Made to Your Specifications
—SHIPMENTS ANYWHERE—
Wherever Good Fishermen Shrimp or Fish World-
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paved the way for more profitable operations.
INSURE LARGER SHRIMP CATCHES WITH
MARINOVICH
Ballerina Balloon Nets
Four-Seam Semi-Balloon Nets
MARINOVICH TRAWL CO. QILOXI, MISSISSIPPI
PhonM* IDIewaod 6-6429 and 6-6524 Iv P.O. Box 194 United Status of America
KNOX
ofKilbimie
The Modern Fisherman is using
KNOX NETS
MANUFACTURED IN
'ULSTRON' ' NYLON * COURLENE
AND OTHER FIBRES
write for samples to:
W. & J. KNOX LIMITED
KILBIRNIE - AYRSHIRE - SCOTLAND
A MEMBER OF THE (.INDUSTRIES GROUP
ET&TWNE
NAMYANG FISHINGNET IND.C0.1TD
PISiV ROMA.
I. P. 0. BOX 1961 CABLE ADDRESS:"NAMYANGNET SEOUL'
EXPORTER: Marubeni-lida Co., Ltd
SARAN
Toujjh and longer Kfe,
strong resistance to
Abrasion, Quick exhalation
of Moisture, Desirable
specific gravity keeping *
good shape. Easiness
removing stickings, and
Saving on expenses
'
FISHING
ASAHI-DOW
V
(1) FISHING NETS, ROPES and ALL OTHER
FISHING EQUIPMENT
« .- v.— W) MARINE EQUIPMENT and HARDWARE
BUSINESS (3) MACHINERY, APPARATUS, ELECTRICAL
LINES APPARATUS, GAUGE SCALE, RUBBER
and PLASTIC PRODUCTS, ETC.
(4) ALL KINDS of METALS
(5) PETROLEUM PRODUCTS
SSI
TNE NIPPON CYOMO SIH6U KAISNA, LT».
Cmttral P.O. ••« M*. 143
Marunoucfel ml**., Chlyodo-ku, Tokyo
TfUi 201*1841 (IS linos)
CAoLI ADMIISCt OYOMO TOKYO
FIRST
AND
FOREMOST
Today, leading fishing fleets use Courlene twines, trawls
and nets because they can depend on them. British
Celanese Limited backed by the resources of the world's
largest producers of man-made fibres, pioneered them from
monofilaments. British Resin Products Limited gave
extra strength by supplying 'Rigidex', the basic raw material of
Courlene. Depend on Courlene twines and ropes in all sizes
up to 10" circ. and in many qualities-from your usual supplier.
Courlene
THE WNU'S IEADIH6 P91YTIHW FUIIN6 TUN
BRITISH CELANESE LIMITED • FOLESHILL RGAL. • COt/ENTPj' • BRITISH RESIN PRODUCTS LIMITED • DEVONSHIRE HOUSC . MAYFAIR PLACE • LONDON W1
135
As a result of a considerable
research programme, Kenyons
are now marketing "Kenlon"
brand bonded 'ULSTRON'
Polypropylene fishnet
twines, which have the unique
property of a high coefficient
of friction of the twine against
itself, with the result that
the knots in the net are secure
and will keep the net in shape
without any danger of mesh
distortion.
FOR
Impregnation is permanent.
Resists sea water.
Increases resistance to
abrasion.
Knots hold.
WILLIAM KENYON &SONS
(ROPES & TWINES) LIMITED
Duklnfkld. Cheshire. England
Tel.: ASHton-under-Lyne I614//37 and
S.T.D. prefix 061
Hirata Spinning Co., Ltd.
FISHING NET DEPT.
President: Sukenobu Munemura
WORLD FAMOUS
HIRATA NET
Spinning-Twisting-Netting
Registered Trade Mark
Foundotlon of the Factory: 1868
Establishment of the Companyt 1918
Capital:
Specialities:
Head Office:
Tokyo Bronchi
Osaka Branch*
Nogoyo Bronchi
¥504,000,000
/All sorts of Fishing nets and
Nets uwines . . Nylon, Kuralon, Saran,
& J Saran-N, Polypropylene, Cotton,
Twines Teviron, Tevi-nylon and oil
other synthetic net.
Ropes . . . Nylon, Kuralon, Polyethylene
(Hopelite) & etc.
Floats . . . Synthetic Float.
32, Amagasuka, Yokkaichi,
Mie-pref., Japan. P.O. Box No. 2
Cable: "HIRATA" YOKKAICHI MIE
10, 4-chome, Nisht-Hacchoborf,
Chvo-ku, Tokyo, Japan. (Senga Bldg.)
Cable: "HIRATABO" TOKYO
3-chome, Blngomachi, HigashMcu,
Osaka, Japan. (Mengyo Kaikan)
Cable: "HIRATANCT" OSAKA
1, 6-chome, Tenma-cho, Noka-ku,
Nagoyo, Japan.
(C Itoh I Co., Bldg. 5 Floor)
136
First choice
of the world's
Fishing Fleets
Up-to-the-minute SAFETY ! It's t well-equipped vessel
that sails with R.F.D. aboard. The life rafts with a
history and success . . . proved time and again In the
roughest, toughest, coldest seas. Ask for full details of
the very comprehensive range of rafts and stowages—-
all backed by an unparalleled Servicing network all
over the world.
R.F.D. COMPANY LIMITED
GODALMING . SURREY . ENGLAND
Tel. : Godalmlng 1441. Telex : 8533
HO
Developed specifically for
fishing fleets.
Fully M.O.T. Approve* 8-,
/O- and 12-man.
Lightweight 3/4 men raft,
for dote to shore
local approval. A/to
6/8 man TEAL' Uforaft
2Q%- 30% stronger, 20%- 40% less weight and
bulk, lower cost, less current resistance, less abra-
sion, no damage to catch, and ever equal meshes.
They're also performance- proved in such uses as
for Trawl, Purse Seine, and Trap Net.
Further details are promptly yours from the Origi-
nators and World's Largest Manufacturers of
Knotless Nets:
NITTO SEIMO CO., LTD.
NO. 6. 1-CHOME, GINZA, CHUO-KU, TOKYO. JAPAN
137
SPECIALISTS
r
141ft
In mechanical
trawl winches
with or without wire steering
seine winches
net rollers on ball bearing
comb/noted winches
' shrimp trawl winches
NORSKOV LAURSEN
ENGINEERING COMPANY
ESBJERG : DENMARK
J. H. Mewes & v. Eitzen
SYNTHETIC - TRAWL NETS - MANILA
For side and stern trawlers
FISHING GEAR ; ROPES ; WIRE ROPES
2 HAMBURG - WEST GERMANY
Neuer Fischereihafen
Phones : 387945-46 Grams : Taunetz
ARE YOU using the NET that offers the
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KNOT-LESS
NYLON NETS
DAVIS
con increase your catch
up to 100%
Now available In polypropylene, Davis Knot-less Nets are not only increasing catches by as
much as 100%, but are also priced, mesh for mesh, as low as ordinary cotton. BECAUSE
THERE ARE NO KNOTS WHATSOEVER, Davis nets are absolutely slip-proof, there are no
concentrated points of abrasion and the mesh size remains constant, with less damage to the
catch, and less repairs for you.
THERE ARE MANY OTHER ADVANTAGES THAT SAVE YOU TINE AND MONEY:
lightweight, easier to handle • stands up better in power blocks
less water resistance and pick-up • non-rotting, resists mold, fungi • immune to fish oils, sea water • great
strength, high tenacity • easier to haul by hand • greater flexibility • can be stored wet * greater use
(Out-time for mending 50% less)
Main Office:
63 W. PALISADE AVE., ENCLEWOOD N.J. U.S.A.
LOwell 9-5055, 5071
DAVIS MILLS, INC.
World's Largest Manufacturer of Knot/ess Nets
The newest man-made fibre of them all
makes the lightest, strongest, quickest-drying fish-netting!
When there's a startling new invention in
synthetics, its various properties indicate
at once how it can be most usefully applied.
'Ulstron' polypropylene yarn will even-
tually be used in all kinds of ways, but its
unique combination of properties is parti-
cularly suitable for fish-netting, because:
<UL8TRON9 is the lightest of all fibres
* ULSTRON' is the strongest fibre, and equally
strong when wet • 'ULSTRON9 is among the
least absorbent fibres, and therefore extremely
quick to dry • 'ULSTRON9 has high abrasion
resistance • 4 ULSTRON9 will not rot • * UL8TRON9
nets compare favourably in price with nets
For farther information about 'Ulstron9
please write to I.C.I. Fibres Division,
Harrogate, York*, England.
• Ulstron' ia the register**
trademark for the poly-
propylene fibre made bv
IMPERIAL CHEMICAL
INDUSTRIES LIMITED
For all your
ANZALON 1100°° super nylon)
Fishing nets Iknotless too)
Lines
Twines
Ropes and
Braided cord
80 years of experience at YOUR SERVICE
APELDOORNSE NETTENFABRIEK
YON ZEPPELIN & CO N.Y.
Apeldoorn-Holland
139
INDISPENSABLE
books for all engaged in every branch of fishing
*t-
FISHING BOATS OF THE WORLD: 2
The biggest and most outstanding book in its
field ever published. Edited by Jan-Olof
Traung, it contains the 55 major papers and
discussions thereon presented at the Rome
Congress on Fishing Boats.
Price: £7 12s. Od. ($22.50) posted.
MODERN FISHING GEAR
OF THE WORLD
Compiled from the 110 papers and discussions
thereon presented at the Fishing Gear Congress,
Hamburg. Edited by Hilmar Kristjonsson,
this valuable book comprises thirteen sections
covering all aspects of gear and fish detection.
Price: £5 8s. 6d. ($15.50) posted.
and many other vital
FISH IN NUTRITION
The handsome volume resulting from the Con-
gress on Fish in Nutrition held in Washington,
1961. Compilation and editorial oversight was
undertaken by experts in Fisheries Division,
F.A.O. with assistance from Commercial Bureau
of Fisheries, Fish and Wild Life Service, U.S.A.
Price: £6 Us. Od. ($19) posted.
ATLANTIC OCEAN FISHERIES
"... describes in great detail the fisheries of
every nation (around the Atlantic) discussing
such aspects as catching data, methods, handling
at sea and in plant, canning, freezing, salting,
drying, antibiotic treatment, by-products, re-
tailing, the economic picture and future possi-
bilities." Authors: Professor Georg Borgstrom
and Arthur J. Heigh way.
Price: £3 I Os. Od. ($10) posted.
FISHERIES HYDROGRAPHY
By Ilmo Hela and Taivo Laevastu. Of this book
Dr. D. B. Finn, Director of Fisheries Division,
F.A.O. says: "It will help fishermen to know
how to apply scientific knowledge in order to
catch more fish more easily ..."
Price: £2 17s. 6d. ($8.25) posted.
fishing books including
HOW TO MAKE AND SET NETS
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PART 2
BULK FISH CATCHING
Section 5 — Stem Trawling Page
The Stern Trawler—a Decade's Development in Trawl Handling . . . . Conrad Birkhoff 147
Some Small Stern Trawlers E. C. B. Corlett 153
Ross Daring— Experiment Dennis Roberts 158
Discussion on Stern Trawling 160
Section 6— Bottom Trawling
Some of the General Engineering Principles of Trawl Gear Design P. R. Crewe 165
Some Comparative Fishing Experiments in Trawl Design W. Dickson 181
Development of an Improved Otter Trawl Gear Chikamasa Hamuro 191
Towing Power, Towing Speed and Size of Bull Trawl Chikamasa Hamuro 199
Suggestions for Improved Heavy Trawl Gear Eldon Nichols 204
Development of Soviet Trawling Techniques . . . . . . . . . . A. J. Treschev 206
Double-Rig Shrimp Beam Trawling . . . . . . . . . . J. Verhoest and A. Maton 209
Discussion on Bottom Trawling . . . . . . . . . . . . . . . . . . ..218
Section 7 — Midwater Trawling
One-Boat Midwater Trawling from Germany /. Scharfe 221
Universal One-Boat Midwater and Bottom Trawl . . . . S, Okonski 229
Two-Boat Midwater Trawling for Herring with Bigger Boats R. Steinberg 235
Development of the Cobb Pelagic Trawl—A Progress Report Richard L. McNeely 240
Underwater Telemeters for Midwater Trawls and Purse Seines Chikamasa Hamuro and Kenji Ishii 248
Reaction of Herring to Fishing Gear Revealed by Echo Sounding H. Mohr 253
Discussion on Midwater Trawling 257
Section 8— Gillnetting, Longlining and Traps
King Crab Pot Fishing in Alaska R. F. Allen 263
Les Madragues Atlantique et Sicilienne Vlto Foder 271
Eel Traps made of Plastic H. Mohr 211
Types of Philippine Fish Corrals (Traps) . . Arsenio N. Roldan Jr. and Santos B. Rasalan 279
A New Fish Trap used in Philippine Waters Santos B. Rasalan 282
Dropline Fishing in Deep Water Ronald Powell 287
Discussion on Gillnetting, Longlining and Traps 291
145
Section 9— Parse Seining
Recent Developments in Icelandic Herring Purse Seining Jakob Jakobsson 294
Sonar Instruction Courses for Fishermen G. Vestnes 306
Discussion on Purse Seining 310
Section 10— Deck Machinery
The Application of Hydraulic Power to Fishing Gear D.W.Lerch 314
Advances in Centralised Control and Automation H.E.H. Pain 338
Driftnet Hauler for Salmon Fishing Chihiro Miyazaki 347
Mechanization of Driftnet Fishing Operations P. A. Kuraptsev 352
The Complex Mechanization of Beach Seining 5. 5. Torban 355
Discussion on Deck Machinery 361
Section 11— Fish Detection
A Comprehensive Echo-Sounder for Distant-Water Trawlers G. H. Ellis, P. R. Hopkin and
R. W. G. Haslett 363
Sector-Scanning Sonar for Fisheries Purposes D. G. Tucker and V. G. Welsby 367
A New Sonar System for Marine Research Purposes T. S. Gerhardsen 371
Detection et Localisation des Banes de Poissons Robert Lenier 376
Echo-Detection of Tuna M inoru Nishimura 382
Echo-Sounder Measurement of Tuna Longline Depth Kyotaro Kawaguchi, Masakatsu Hirano and
Minoru Nishimura 385
A 200 Kc/80 Dual Frequency Echo-Sounder for Aimed Midwater Shrimp Trawling Masakatsu
Hirano and Takashi Noda 388
Ddtecteur de Poisson "Explorator" J.Fontaine 396
Bio-Acoustical Detection of Fish Possibilities and Future Aspects . . . . G. Freytag 400
Study of Acoustical Characteristics of Fish E. V. Shishkova 404
Frequency Analysts of Marine Sounds . . Tomiju Hashimoto and Yoshinobu Maniwa 410
Identifying Pacific Coast Fishes from Echo-Sounder Recordings E. A. Best 41 3
Echo-Sounding through Ice . . Tomiju Hashimoto, Yoshinobu Maniwa, Osamu Omoto and
Hidekuni Noda 415
Discussion on Fish Detection 417
Section 12— fleet Operations
Japanese Motbership and Fleet Operations for Salmon, Crab, Longlining and Tuna Hiroshi
Tominaga (2), Masatake Neo, Nippon Suisan Kaisha Ltd., and Goro Okabe 423
Las Pesquerias Espanolas Austro-Atlanticas V. Paz-Andrade 438
Discussion on Fleet Operations 445
Advertisement Section
Trade announcements relating mainly to fishing vessel gear and equipment. 447-468
146
Part 2 Balk Fish Catching
Section 5 Stern Trawling
The Sterntrawler— A Decade's Development
in Trawl Handling
Abstract
The development of big sterntrawlers started by investigating the
possibilities of processing the catch at sea. It was found that produc-
tion lines inevitably crossed on a large sidetrawler and that the
freeboard with shelter decks became too high for handling fishing
gear. These difficulties disappeared when operating the gear from
the stern. The problem of lifting the heavy gear on board at the
stern was overcome by building a ramp over which the gear could
be hauled without the time-consuming splitting of the codend to
bring the catch on board. The first big sterntrawlers were factory-
ships, such as the converted Fairfree, which led to improved types
such as the Fairtry, Puschkin and Majakowsky class. Fishing
conditions and the size of the vessel must be given due consideration
in the design and construction of sterntrawlers. Matters which are of
minor importance on large vessels may be of critical importance on
smaller vessels. On the other hand, as smaller vessels normally
work under better sea conditions, some problems can be dispensed
with. Apart from more space for fish processing, sterntrawlers
now have advantages over the sidetrawler in faster and almost fully
mechanised handling of the trawl. The paper then describes the
various types which have been realised to date and discusses ways
and means for arranging the deck and winches for operation of the
trawls over the stern. The future of fully mechanised sterntrawling
is brought forward and the author points to the possibility of pro-
viding vessels, even as small as 26 m in length, with a stern ramp and
tilting or fixed bipod posts to haul the gear. Summing up, the im-
portance of close co-operation between the shipbuilder and gear
technologist towards further development of sterntrawling is
emphasised.
Le chalutier & p£che arrtere— Comment il a revolutionne la manipu-
lation de chalut au cours de ces dix dernieres annees
Rfcum*
La construction des grands chalutiers & peche arriere resulte des
recherches sur la possibility de traiter les prises en pleine mer,
a bord du bateau. Sur les grands chalutiers pechant par le cote les
operations de traitement amenent inSvitablement un croisement des
chaines de production et la hauteur de franc-bord de ces bateaux
devient excessive et rend difficile la manipulation de 1 'en gin d*
p&che. Cette difficult6 disparait lorsque Tengin est manipute par
rarriere. Le probleme que posait le relevage par Farriere d'un
lourd engin de p6che a etc resolu par la construction d'une rampe
sur laquelle Tengin peut fctre releve, Svitant ainsi la perte de temps
provoquee par les remplissages et v id ages successifs du cul de
chalut. Les premiers chalutiers a peche arriere furent des navires-
usines tels que le navire transforme Fairfree sur lequel furent bases
les projets relatifs a des types perfectionn6s tels que les bateaux
de la classe de Fairtry, Puschkin, Majakowsky. Les conditions de
p&che, les dimensions du bateau ainsi que sa distribution interieure
doivcnt faire 1'objet d^tudes attentives avant d'etablir les plans du
bateau et de proceder a sa construction car des details d'une
importance relative dans un grand bateau peuyent avoir une
importance capitate dans d'autres bateaux de dimensions plus
reduites; de plus, ces derniers travaillant normalement dans des
conditions de mer plus favorables, certains problemes ne se posent
pas. L'installation de la rampe a rarriere offre non seulement plus
d'espace pour le traitement de la prise, mais a egalement Tavantage,
par rapport aux bateaux pechant par le cdt£, de permettre une
mecamsation presque complete de la manipulation de rengin.
L'auteur decrit plusieurs modeles construits jusqu'a present et traite
des criteres et des systemes d'amdnagemcnt du pont etd 'installation
des treuils qui manipulent les chaluts par rarriere. 11 examine les
perspectives futures et decrit egalement un chalutier a rampe
arriere completement mdcanise, avec une rampe arriere et un mat
bipode basculant ou fixe pour hisscr ie filet (cette mecanisation
pouvant equiper egalement des bateaux n'ayant que 26 m de long)
L'auteur termine en mettant en relief 1'importance de la collabora-
tion entre le constructeur de bateaux et le spccialiste d'engms de
by
Conrad Birkhoff
Naval Architect, Hamburg.
peche en vue du perfectionnement ult&rieur du chalutage par
Tamere.
El arrastrero con rampa a pope— como ha evoludonado la
lacion del arte en una decada
Extracto
La construccibn de grandes arrastreros con rampa a popa se debe
a la investigaci6n de las posibilidades de elaborar la capture a
bordo. Se obscrv6 que al instalar maquinaria elaboradora en
un gran arrastrero que pesca por el costado era inevitable
que las cadenas de produccibn se cruzasen y que el francobprdo
adquiriese demasiada altura para manipular el arte con facilidad.
Esta ultima dificultad desaparecia cuando el arte se manipulaba por
la popa. El problema de meter a bordo por la popa el pesado arte
se solucion6 construyendp una rampa por la que la captura se
podia izar de una vez a cubierta, evitando asi la lenta tarea de llenar y
vaciar el saco repetidas veces. Los primcros arrastreros grandes
con rampa a popa fueron buques fabrica como el transformado
Fairfree, en el que se basaron modelos mejorados como los de la
clase Fairtry, Puschkin, y Majakowsky.
Las condiciones de pesca, las dimensiones del barco y su dis-
tribucton interna tienen que ser objeto de cuidadoso estudio antes de
preparar los pianos de formas y proceder a la construccidn, ya que
detalles de poca importancia en un barco grandc pueden ser vitales en
otro pequeflo; ademas, los barcos pequefios, por trabajar en con-
diciones de mar mas benignas, no plantean ciertos problemas.
El arrastre por la popa no s61o deja m£s cspacio para la elabora-
ci6n de la captura, sino que tiene la gran ventaja con respecto al
arrastre por el costado de que permite mecanizar casi por completo
la manipulaci6n del arte. El autor describe varios modelos construi-
dos hasta la fecha y examina los modos y recursos para la dis-
tribuci6n de la cubierta y de las maquinillas que manipulan las
redes por la popa.
Al examinar las posibilidades futures el autor describia un
arrastrero con rampa a popa totalmentc mecanizado y un barco de
solamente 26 metres de eslora con rampa a popa y pbrtico bascul-
ante a fijo para izar el arte. Termina poniendo de relieve la impor-
tancia que tiene que el constructor de barcos y el especialista en
materiales de pesca cooperen en el pcrfeccionamiento ulterior de
la pesca al arrastre por la popa.
original ideas which led to the development of
A stern trawling were combined with the need for a traw-
ler with a large processing deck. A conveyor belt system
could not very satisfactorily be installed on a sidetrawler:
production lines inevitably crossed. Furthermore, the
high deck of a sidetrawler with an additional proces-
sing deck rendered the handling of gear more difficult.
147
Even such alternatives as the semi-shelter or trunkdeck
ship could not fully solve the problem.
All these disadvantages are avoided if the catch is
hauled in at the stern. Moreover, by this method various
simplifications in trawling can be achieved by mechani-
sation. So the hauling-up ramp was thought of; as used
on whalers. Such a ramp also offered the possibility of
pulling in the entire codend in one operation. The
laborious splitting of bags with larger catches could
therefore be avoided and much valuable time saved.
There were, however, numerous sceptics who at once
saw a danger of damaging the fish. Furthermore, it
was also feared that difficulties could arise in hauling up
large catches, as in fact was at first the case.
Now, after 10 years of progress, the actual possibilities
can be considered in a clearer light. At the same time
we realise a certain change in the problems. It is therefore
interesting to recall some of the stages in this development.
The big factarytrawler
As early as 1945 a patent was applied for on the Ger-
man side for a trawler with a stern-ramp. Similar
efforts were subsequently evident in Great Britain and
it was Chr. Salvesen & Coy., Ltd., there, which mostly
specialised in whaling that had the courage to build the
first experimental vessel in this line. The experiments
with the converted vessels, Oriana and Fairfree, led to the
construction of the new factorytrawler, Fairtry /, and
later on to Fairtry II and ///. Alongside this, in Germany,
a fleet % of 20 ships of the Puschkin-type factorytrawler
were developed for the U.S.S.R. The experience gained
with these vessels led to various new series being con-
structed in the U.S.S.R., Poland and in Eastern Germany
— among others, the well-known Majakowsky Class and,
on account of the special conditions prevailing in the
tropics, the Tropic Type. The size of the vessel justified
equipping the upper deck with a large number of technical
aids for gear handling. Mobile warp trolley-rollers were,
for example, developed on the Puschkin type to simplify
handling the gear. The Majakowsky Class was fitted
with swinging console gallows to meet requirements
during trawling as well as hoisting the trawl doors.
The crew required for the complete processing of the
Fig. L Factorytrawler type (Fairtry, Puschkin, Majakowsky) with
normal diesel engine plant, voluminous superstructures^ sufficient deck
space for the trawl but separated navigation bridge B and trawl-
bridge A.
fish made large Uving quarters a necessity, which in
turn required considerable superstructure. Consequently
the navigation and the trawl bridge had to be separated
148
so as to give an adequate view over the stern. In spite
of the large superstructure, sufficient deck space could
be kept free on this big vessel for unimpeded handling
of nets (Fig. 1).
On newer German factorytrawlcrs, powered with
diesel electric drive, the superstructure was eased
forward on top of the forecastle so as to provide the
argest possible trawldeck. If the normal diesel motor
Fig. 2. Factorytrawler type (Heinrich, Meins, Heinrich Kern, Hans
Bockler) with diesel electric plant. Superstructures at fore-ship, long
deck space and combined bridge A and accommodation in forecastle
B.
unit required a division of the hold fore and aft of the
engine-room, by diesel electric drive, the hold could then
be installed in the most convenient place, as far as trim
was concerned, at midships (Fig. 2). Disadvantages soon
arose, however, because of the position of the bridge
and trawl winch, which were too far forward, disadvan,
tages which were apparent from the high acceleration
when pitching and from the poorer view to aft, to the
ramp and gallows. Moreover, the living quarters of the
crew were pushed still further forward, further in fact
than a sterntrawler, according to its very structure-
requires.
With a normal engine plant, the optimum design of
the vessel made it advisable for the engineroom to be
sited aft and the bridge at midships with an aft connected
enginecasing. Consequently, a trawler with superstruc-
ture on one side was proposed, following the model of
Fig. 3. Factorytrawler type (Colonel Pleven II) with semi-side super'
structure amidships and combined console bridge A.
an aircraft carrier, with a combined bridge for navigation
and trawling purposes hanging over the trawldeck
(Fig. 3). Another solution with the same purpose in
mind is the trawler with superstructure on both sides
and a combined ferryboat type of bridge. This had
already been suggested as a possible alternative several
years ago but had not been realised before 1962 (Fig. 4).
IA
Fig. 4. Factorytrawier type (Troms0y I, Lofottral I, II) with two Re-
ward superstructures and combined ferryboat-like bridge A.
The partial lengthening of the actual trawldeck aft
was meant to serve for laying on deck the long trawl
wings but on a rolling ship the floats, shackles and wing-
legs became hooked up and entangled during the process
of handling the gear if both wings were very close to-
gether. Since the trawl is V-shaped, the deck lent itself
to a corresponding division. With side passages for
operating the netwings and laying them out on the deck,
the customary midship superstructure, desirable also
from the point of view of the construction, was made
possible. Behind this deckhouse the trawlwinch is
installed, which now comes as near as possible to the
gallows. In this way the trawling gear is "split" as early
as possible during the hauling-up operation with the
long wings on both sides (Fig. 5).
Fig. 5. Factorytrawler type (Narwal) with diesel engine plant at aft-
V-shaped trawldeck and combined bridge amidships A.
In this case, with ships of over 40 m in length, it is
possible to install the engineroom aft behind the fish
hold which is preferable to all other possibilities. Due
to that arrangement, even for the diesel electric trawler,
a more practical distribution is achieved because cable
lengths are shorter, control is better and one bulkhead
is saved ; more practical, that is, if on the other hand good
trim conditions can be effected in the ship through skilful
arrangement of the tanks.
The deep-sea trawler
Medium-sized sterntrawlers, i.e., midwater and deep-sea
trawlers, require a relatively simple and particularly
stout deck equipment to be able to compare the new
system of handling the gear favourably with the previous
type of sidetrawler. Since the deck area of these vessels is
limited, an especially well thought-out balance between
naval architects' interests and those concerning the pure
fishing technicalities had to be struck.
The desired compromise solution provided a deck
area aft for hauling up the cod-end and some fairly
wide gangways leading forward to the foreship. At
first the trawl winch was built round on three sides, set
into the superstructure, and the bobbin groundrope was
hauled over the tiawl wings, which were dragged up
behind the winch. Disadvantages due to the accumula-
Fig. 6. Deep-sea trawler (Stanislava, Mare de Labrador, Juan de
Urbieta) engine at aft, combined bridge amidships A — and sideward
tracks for the trawl wings B.
tion of the fishing gear, and particularly the trawl wings,
behind the winch, could be avoided by extending the
gangways at the side of the winch as far as possible
towards the fore part of the ship.
Efforts to handle the Pareja gear, of about twice the
bottom gear length in comparison, originally suggested
this course (Fig. 6).
By using additional warping heads or similar drums on
the trawl winch the job of releasing the warps from the
trawlboards was saved, with a gain in time of about
five minutes. At the time, the trawlboards were pressed
flat against the gallows by this action so that work could
be carried on without any danger.
Such warping heads or gilson drums offer the most
varied combination of winch arrangement :
(a) On the same shaft.
(b) Gilson drums on the counter shaft.
(c) Gilson drums on a separate winch or winches,
serving perhaps a dual purpose as ice or unloading
winches.
(d) Combined trawl warp, gilson and hauling drums
separated on each side of the ship.
Solution (b) is contemplated here especially for
smaller vessels.
With medium-sized vessels good possibilities were
presented for comparison with the previous type of ship,
the sidetrawler. The sum total of all the direct and in-
direct advantages, which clearly outnumber the disadvan-
tages (including, among other things, higher building
costs), only now become fully obvious. The most vital
advantage was the shorter time required for handling
the nets; this lengthened the active fishing time, which
in turn produced on an average a 20 per cent profit in
terms of the valuable "trawl on the bottom" time and a
correspondingly better result in the catch.
149
This revolution and change to the new type of vessel
brought with it also a change of the crews from the old
ships to the new ones, which enabled the shipowners of
the latter to have a good selection for crewing the vessels,
so the technical advantage was further strengthened by
these personal influences. Crew problems on the older
type of vessel compelled shipowners in several countries
who were still holding back, to invest in this new solution.
Different bask conditions for near-water and coastal
trawtere
All the disadvantages which are of little consequence
with distant-water trawlers, for example factorytrawlers
and deep-sea trawlers, are of greater significance with
smaller vessels. For this near-water type of vessel the
construction methods must be adapted to the general
weather conditions prevailing on the particular fishing
grounds. Often these small fishing boats work in rela-
tively calm waters and this therefore permits solutions
which would be impossible for rough seas.
In recent years two different ideas have been offered
for this type of craft. One was the type without a ramp,
built with a bipod post which in some cases was tiltable.
The shipowner's fear of possible damage to the fish on
the ramp gave place to the idea of splitting the bag on
sterntrawlers too. Having relatively calm sea, only the
wings, with groundrope and headline, are hoisted over
the bulwark aft. Then the bipod post hoists the bag part
of the codend up out of the water and swings it out over
the deck-ponds. On the backward swing of a tiltable
bipod post, or by means of an additional codend derrick,
the codend is again shot into the water so that the fish
Fig. 7. Near-water sterntrawler without ramp but bipod post with
fallow consoles and codend derrick.
Fig. 8. Near-water sterntrawler with turnabte gantry.
can be swilled quickly back again. This can be accom-
plished in rythmical succession in a really short time
(Figs. 7 and 8). The larger the catch, however, the greater
the time-saving, which the use of a ramp has over this
method.
The sterntrawler with ramp, which is basically intended
to save hauling time, goes a completely different way.
Even if the average catch is relatively small, this vessel,
without any further disadvantages, is better prepared to
handle the larger catches in seasonal periods, too. The
installation of a short ramp is possible even on ships of
up to 28 to 30 m. The codend is guided by the gentle
curving of the ramp in this case and is brought up without
being bounced along against the stern. Damage to the
fish by friction is impossible due to the special construc-
tion of the codend.
The codend is hauled up on deck in the main direction
of the ramp by means of a hoisting block and emptied
out by the tilting tackle on the fixed bipod post. A codend
derrick, of some 4 m in length and pointing aft, takes
care that the gear is lowered into the water behind the
swirl of the ship's wake.
A ramp for hauling up the voluminous fore part of
the gear and also occasionally used for hauling the whole
codend, was considered as a very promising alternative
solution. Indeed in this case, through the combined work
of the tilting tackle on the bipod posts and the shooting
tackle on the codend derrick, an operations area compar-
able with that employing the swinging posts, and possibly
even more favourably sited, could be created ; this would
also permit a similar division of the contents of the codend
(Fig. 9). The strong installation of the solidly built-in
,toO«KIN6 AREA
Fig. 9. Near-water sterntrawler with bipod gallon post, fixed codend
derrick working with both blocks.
bipod posts and codend derrick guarantees safe working
in all weather conditions. The longer trawl tunnel
or lengthening piece, which might perhaps be necessary,
could be compensated many times by the advantage of
having a ramp available and consequently a better control
of the gear. If, in addition, it is possible to tip the con-
tents of the codend out under deck, the gear is again
ready for shooting only a few seconds later, even in
cases of very large catches. The significance of this in
shoal fishing should be considered. The extra cost of
protecting the working deck could, in the majority of
cases, be more than compensated. This solution, after
all, permits even the most sceptical shipowner to re-
consider his prejudice without any disadvantage com-
pared with his more progressive colleagues.
This system of tipping out the entire codend can finally
be put into effect on ships of up to 26 m in length,
taking into account the length of the ramp as a usable
extension to the deck. It is possible to combine the cover
of the filling hatch with the ramp as a swinging-table
type of ramp. To achieve the required security, this
flap can even act as a bulwark when lifted up aft. The
codend is hauled up only on this flat type of ramp and,
after the codline is opened, is emptied out by the tilting
of the flap. With a suitable device for opening the codend
a little more forward and a holding device at the extremity,
even the expensive bipod posts can be spared on such a
type of craft (Fig. 10).
Fig. 10. Sterntrawler with turnable ramp without bipod post.
Where the side spaces are long enough this deck equip-
ment, developed for the single towing trawler, offers the
possibility also of handling a Pareja trawl, as already
mentioned in connection with midwater trawlers. If
one compares the usual practice in Pareja fishing as we
know it at present with these new possibilities, there are
even greater advantages than with the handling of one-
boat trawls. The tremendous simplification of the
manual work by extensive mechanisation produces a
saving in time which is about 30 per cent higher than
with hand-laboured vessels. Consequently, with this
30 per cent longer active fishing time, a corresponding
increase in the results can be expected (Fig. 1 1).
Towards fully automatic trawling
Although school fishing is now an almost fully automated
business, using light and electricity effects as well as
pumps, is relatively complicated to introduce into net
fishing methods. The purse seine can be handled auto-
matically, particularly when one thinks of pumping
out the purse in school fishing. These possibilities cannot
be directly transferred to trawl fishing.
The trawlnet is always working in a water current,
i.e., pulled in a constant direction. Also the instances
are too few when one can really count on constant size
of fish in the catch. Finally, any possible automation
must also be capable of working in different weather
conditions. With these basic conditions and require-
ments in mind one can develop a similar thought in
terms of trawling.
A development such as this goes forward step by step.
When the movement of the trawlboards at the stern
were found to be too violent when the ship was pitching,
guideways and fixed attachments were designed. When
the handling of the wings on the aft deck was found to be
too awkward on a heavily rolling ship, long side alleys for
splitting and laying the wings on deck in one pull were
devised. (Fig. 12a). If the de-shackling of the trawl-
boards and going round them with pennants proves too
troublesome and awkward, we can install a pair of swing-
ing arms, perhaps in the form of guide-rails, to allow
pulling the sweeplines or legs up to the foreship by
using trolley rollers. In this way, the laying of the trawl
gear on deck is fully automatic (Fig. 12b). The last bit
of hard work in handling the nets, which requires six men
on board, will then be spared. With a saving in manpower
there is likewise a saving in accommodation on board
and the extra cost of installing the swinging arms and
guide-rails on deck will be more than offset by this
advantage.
The flap-type ramp makes it possible to empty the
codend with the tilting tackle and this leads to the final
problem, which is the connection between the hoisting
tackle and the splitting strop on the codend. The length
of this line has to be adjusted according to the length
of the available gear deck. If this last piece of manual
work were not so simple, some automatic facility would
have to be found here too.
These suggestions are not final solutions but examples
which will surely, in the future, lead to still other related
ideas.
Trawl gear and fishery ships
Net and gear constructors will no doubt agree that the
sterntrawler is a new development in naval architects9
construction in which the shipbuilder has taken a lot
of trouble to adapt his product to suit the gear construc-
tors9 work. What could and can now be done by the
netmakers to meet the efforts of the naval architect?
Ten years' practical experience with the new type of
vessel has brought with it some alterations in the trawl
gear for sterntrawlers, which should not be forgotten.
First of all, the codend was adapted to the requirements
Continued at foot of page 153
Fig. 11. Pareja sterntrawler with sidewards tracks for the trawl wings,
151
flu 12aondb 12a (bottom) Today's version of a partial aaomat^perated tterntrawkr. 12b (top) Future version of a complete automat-
rigs, tai mu*. v operated sterntrawter.
152
Some Small Stern Trawlers
Abstract
The paper describes some problems arising from stem trawling
operations with small vessels. Bringing the gear on board requires
a long deck space so that the smaller the vessel is, the more difficult
the problem. A number of solutions have been proposed, such as
cantilever deck houses with a winch installed far forward, or a
deck arrangement as on an aircraft carrier with casings all on one
side. On small ships of, say, less than 100 ft, such constructional
changes cannot help as the available areas on deck are too small
at the outset. The mouth of the net must then be brought to the
stern and held there while the codend is lifted over. The use of a
tillable gantry is only one of several ways of doing this, but has
proved to be very effective in use. The Unigan arrangement, as
used on the Universal Star, has incorporated this method. Such
gantries have been fitted in vessels up to 210 ft in length and have
been developed for vessels as small as 50 ft. The functions of the
tiltable gantry during shooting and hauling operations are described
in this paper, and some of the fundamental construction problems
concerning stern trawler construction are discussed and suggestions
given towards their solution.
Quelques petits chalutiers a peche arriere
La communication decrit quelques probtemes d&coulant du chalu-
tage par Tamere avec les petits bateaux. Pour rcmonter le chalut £
bord, le pont doit avoir un espace assez long, c'est pourquoi plus
by
E. C. B. Corlett
Burness, Corlett & Partners Ltd.
le bateau sera petit, plus le probleme sera difficile a resoudre. DC
nombreuses solutions ont 6t6 proposees tclles que: superstructures
en cantilever de chaque c6te du bateau avec treuil install^ tout a
fait £ 1'avant, ou bien arrangement de pont comme sur les porte-
avions avec des revfttements d'un c6te seulement. Sur de petits
bateaux (jusqu'& 32 m) des constructions de ce type nc sont pas
tres efficaces car 1'aire du pont est trop petite. L'ouverture du met
doit dtre halee justqu'& Farriere du bateau et maintenue la pendant
que le sac est montt sur le pont. L'utilisation d'un portique
basculant n'est qu'une des nombreuses maineres de lever le sac et
a prouv& son efficacite dans la pratique. Le syst&ne Unigan utilis*
Continued fro/n page 151
of a ramp. It was fitted with longitudinal or diagonal
ropes as reinforcement and these absorb the whole pull,
and it was divided into two or three longitudinal combs
by vertical walls or seams. The latter improvement made
the codend flat and itiore flexible for the curve in the
ramp. The flat-lying position was often further improved
by floats fastened at the seams, which made rolling
impossible. The underside was fitted with hides and
chafing ropes running lengthwise and close together.
For emptying a heavily filled codend, a second opening
was made at an appropriate distance from the original
codline; for instance, by a ring-splitting connection.
Indeed, this latter idea was subsequently used for
separating and exchanging complete codends as necessary
for transferring the catch or replacing damaged codends,
etc.
Besides the alterations to the codend, the saving of
one "false headline" together with one kite on the herring
gears should be especially mentioned as this easily
becomes entangled in the wake behind the ship when
shooting. In order to maintain the same uplift a special
float (an otter with wing-planes) was used, which is
mounted just before the remaining kite.
On rocky bottoms the advantage of shorter lower
wings had already become evident due to the fact that
fewer stones are collected and so it turned out to be
practical, when hoisting the trawl wings along the deck,
to shorten the upper wings slightly too. The scaring
effect of the longer legs compensated for the loss in
wing length. Depending upon the conditions of the deck,
a slight shortening of the legs is possible without catch
reduction if, in this way, a more practical and satisfactory
constructional distribution can be achieved.
If occasionally, in spite of the ramp which is there,
one wants to bring the contents of the codend on deck
by splitting bags, a longer tunnel or lengthencr is required
in order to have sufficient room in the net for swilling
the fish back.
With still further mechanisation or full automation
in the handling of the trawl, a certain adaption of the
sweeplines and trawlboards may become necessary. In
the case quoted, the length of the sweeplines would have
to be restricted to the length that could be pulled in on
board : some 80 per cent of the ship's length.
The laying out on deck of the entire trawl gear may
render it advisable to divide the net with ringstrop
connections to allow for replacing individual sections
of netting in the event of damage.
These requirements and possibilities are mentioned
only as examples of how the nctmaker can adapt his
work to the arrangement of the new vessel types. The
more the shipbuilder is prepared to mechanise or
automate the handling of the trawl, the more the net-
maker has a duty to develop a trawl gear suitable for
such automation. The advantage in design which is
then achieved may well compensate for certain losses in
catch capacity. Nevertheless, it is the task of the gear
constructor to find such an adaption without vital loss
to the catch results.
The development of the simplest automation and the
production of trawl gear which has the maximum effect
will, in future, call for a closer co-ordination between the
constructors of fishing gear and those of fishing vessels.
On behalf of my colleagues in the shipbuilding industry,
I believe I can especially stress that we are prepared to
do our very best to achieve this end.
153
sur T Universal Star cst base sur ccttc roethode. Cos portiques ont
etc months sur des bateaux dc 70 m de long et developpes pour des
petite bateaux de 16 m. Lcurs fonctions pendant la p6che sont
dtaites dans la prtscnte communication. Quelques uns des probl-
emes fondamentaux relatifs a la construction des chalutiers pechant
par Fan-fere sont posts et des solutions sont suggerees.
Extrftcto
Describe el autor los problemas que plantea la pescaal arrastre por la
popa con foarcos pequenos. Para meter el arte a bordo se necesita una
cubierta larga, de manera que cuanto mas pequeflo es el barco mas
difidleselproblema. Se ban propuesto varias soluciones entre ellas
al puente de mando cantilever con la maquinilla instalada muy a
proa o una cubierta como la de los porta-aviones con las supere-
stmcturas en un lado. En barcos pequenos de, por ejemplo,
menos de 35 m, no se pueden adoptar estos cambios en las estruc-
turas porque, ya para comenzar, las superficies disponibles en
cubierta son demasiado pequeftas. Debido a ello, la boca del arte
se tiene que Uevar a la popa mientras se iza el copo. El portico
basculante es una de las diversas maneras de lograr esta finalidad
y ha resultado excelente en la practica. El sistema unigan empleado
en el Universal Star empka este procedimiento. Estos porticos se
ban montado en barcos hasta de 64 m de eslora y se ban construido
para otros de 15 m nada mas. El funcionamiento del portico
basculante mientras se cala o se recoge el arte se describe en esta
comunicacidn, en la que tambien se examinan algunos problemas
ftindamentales relativos al proyecto y construction de arrastreros
con rampa a popa y se hacen sugerencias para solusionarlos.
THIS paper describes some stern trawling develop-
ments in small trawlers which present quite different
problems from those in larger ships. In all fishing
vessels rising wages and operating costs throw emphasis
upon crew economies and obtaining high utilisation.
In larger ships, crew savings possible with stern trawling
are not as important as with small ships and the pre-
dominant reasons for adopting stern trawling may be
quite separate; for example, increased space in the ship
for processing plant, increased trawling capacity, the
difficulty of side fishing in large ships with considerable
freeboard, etc. However, in small ships where perhaps
three to fifteen men are carried, a saving of three or four
men in say 130 ft. (40 m) ships and one man in the smal-
lest ships is of real economic importance.
Stern fishing allows a ship to fish in worse conditions,
the ship being aligned better with the weather when
shooting and hauling and the crew having much greater
protection at all times. Furthermore, experience is that
the hull design required for practical reasons in stern
trawlers seems to produce ships with greater sea-keeping
capacity than normal side trawlers with their low free-
board amidships. Moreover, the stern trawler with
mechanized systems can handle gear more quickly and it
is difficult to install comparable systems in a side trawler.
As a result, because of better weather ability and quicker
gear handling, the ship can actually fish for a longer
time on a given voyage than can a side trawler. These
arguments are particularly applicable to small ships
ranging from say 40 down to 1 5 m. But there are difficul-
ties in adopting stern fishing with these smaller ships
which are not met with in bigger ones, due mainly to
the inherently limited fore and aft length of the fishing
deck and to sheer lack of space elsewhere.
154
Basic factors affecting stern fbUog
However the gear is brought aboard over the stern,
warps must be led forward. Big ships can use the net
as an extension of the warps and haul the fish bag over
the stern via a ramp, the whole net coming on board
before the fish bag. Clearly this requires space, length
in particular, and is not really possible with small ships,
say under 130 ft (40 m) in length, unless the arrangement
of the whole ship is subordinated to the requirement.
A number of layouts have been proposed and indeed
some have been built using cantilever deck houses with
a tunnel leading to a forward winch installed at the
break of the forecastle. Others have used compact
high-speed machinery installed aft with side funnels and
casings and the net hauled over the top of the engine
room. Alternatively, it is possible to arrange the ship
in aircraft carrier fashion with casings to one side and
still more variations are possible, but generally all these
arrangements lead to an awkward overall arrangement
of a small ship.
If the engine room is forward, Fig. 1 shows a typical
Fig. I. See Text.
layout with cantilever deckhouses, while, if the engine
room is arranged aft, Fig. 2 shows another typical
arrangement. Again there must be cantilever deckhouses
and the net and warps led between twin casings. An
advantage is that the centre of pressure of lateral
Fig. 2. See Text.
windage of the above-water profile is aft and this helps
with handling problems but the arrangement is only
possible if the machinery is of small height and hence of
high revolutions and small volume.
A good example of this type of ship is shown in
Fig. 3 which shows a recently constructed trawler. Here
the main machinery consists of twin engines rated at
788 total continuous bhp at 1,400 engine rpm
returned to the water. The vessel is then given a kick
ahead driving the remaining fish back into the codend
which is then lifted aboard the vessel as before. Fig. 4
shows a typical outline arrangement of a Unigan trawler
Fig. 3. Detail of newly built trawler.
geared together on to a constant speed controllable
pitch screw with a constant speed trawl winch generator
driven off the forward end of the gearbox. The arrange-
ment is very convenient but, as mentioned, is dependent
upon the use of rather specialised machinery.
Up to a point, the smaller the ship the more difficult
is this problem but in very small ships which, anyway
usually fit high-speed engines, the problem eases again
as such engines go under the deck and it is not difficult
to arrange exhausts and casings to suit.
The Unigan tillable gantry solution was developed to
avoid these difficulties, the concept being that during
hauling only the mouth of the net should be brought over
the stern and held there while the codend would be lifted
on board over the stern. This has many advantages,
being simple, quick and convenient, requiring a minimum
of space and manpower and avoiding the length taken
up by a ramp, although the inherent lack of space still
poses some problems. For example, arrangements must
be made for bringing the whole net on board for repairs
but generally this can be handled by bringing the codend
on board and leading it forward on strops from the trawl
winch. Access to an underdeck net store, if fitted, can
be a difficult minor problem.
The Unigan arrangement was first developed on the
well-known Universal Star, has since been fitted to some
twenty vessels and is being fitted to several others either
ordered or under construction. Generally speaking,
the system uses a strongly constructed hinging portal
frame gallows from which the trawl warps are led and
the gear towed. For example, in a 100-ft trawler where
the warp breaking strain is 14 tons, the gantry will weigh
approximately 1-1 J tons and will be designed to move a
gilson block load of 1-5 tons, although much heavier
loads can be handled by hauling in the codend with the
gantry in an upright position. In the case of an extremely
heavy catch it is sometimes desirable to bring the fish
aboard in two or more operations. This double-bagging
procedure is accompanied by tightening the half-bag
becket by means of the gilson wire so splitting the catch.
The codend is then brought aboard in the usual manner.
After the fish have been deposited on the deck the codline
is re-tied, the half-bag becket loosened and the codend
Fig. 4. Outline arrangement of Unigan trawler.
with the engine room forward and showing the lead of
gear from the trawl winch.
The gear has been fitted in trawlers up to 210 ft (64 m)
in length and has been developed for ships as small as
50 ft (15 m). However, the advantages of using the
tillable gantry vary with the size of the vessel. A net of
considerable size can be handled with the gantry on a
small but relatively powerful boat without difficulty,
whereas such a net cannot be manhandled by a small
crew. An example of such a net would be:
ft in
m
Headline 63 0 19*0
Square 13 0 4-0
Baitings 25 0 7-5
Top wing 24 6 7-4
Lower wing 45 0 13-6
with a belly of 300 9-1
Indeed, in day fishing where gutting and heading is
not carried out, such a boat of 50-55 ft (15-17 m)
length and say 250 hp arranged with grouped propulsion
steering, gantry and winch controls, etc., might be opera-
ted by two men in active trawling. The saving is consider-
able as a heavily designed vessel of this size with adequate
power can be made equivalent in fishing ability to a nor-
mal side trawler of some 15-20 ft (4-5-6 m) greater
length which may well have 6-7 men in its crew. One
might say with some confidence that for approximately
equal fishing capacity such ships can halve the number
of crew often required by larger conventional vessels
while, if registered under Part IV of the Merchant
Shipping Act, they can be inside the 25 tons registered
limit.
At the other end of the size range the main attraction
is that the catch is lifted rather than dragged aboard.
With a heavy bag the damage that may be inflicted on
the catch by ramp trawlers when the codend is dragged
155
up the ramp is eliminated and this may be a factor in the
value of the catch when destined for the normal white
fish market. It would seem likely that a gantry which is
really simply added to the normal ramp system will be-
come more and more popular in such ships as, although
derricks and cranes can carry out the same function, they
cannot do it with the same efficiency and economy. The
operation being primarily a transfer of weight fore and
aft, it is best done with a system with only the one rele-
vant degree of freedom. The gantry can be positioned to
facilitate shooting the gear and also handling heavy
gear such as trawl boards, etc. It must be remembered
however that sometimes in ramp stern trawlers the
quality of the catch is not a consideration, as it is under-
stood that a number of large stern trawlers are fishing
mainly for fishmeal production.
Factors in stem f idling vessel design
The design of small stern fishing trawlers offers many
difficulties. Problems of accommodating the gear in
the ship, especially are not simple and the hull design in
itself needs to be sophisticated. Fundamentally the ship
shoots and hauls her gear up-wind or down-wind.
There are advantages, providing the weather is not too
severe, in hauling down-wind as the gear streams naturally
behind the vessel and little or no steerage way is required
for the ship. This basic difference from the methods used
in side trawlers is one of the factors responsible, of course,
for the great improvement in the comfort and safety of
the crew, commonly experienced with stern trawlers.
However, if hauling headed into the wind, if the sea and
wind are not in the same direction or there is a confused
sea running, it may be difficult to prevent the ship's
head blowing off. Furthermore, if a lone stern trawler
is fishing in company with a number of side trawlers it
may not be possible to shoot and haul before the wind
and the ship may have to conform with the general trend,
i.e., with the wind abeam. However, this is a temporary
factor and the time will come when the odd side trawler
will have to conform when fishing with stern trawlers.
Ability to control the ship's head being critical, therefore,
the ship must be designed with deep sections and good
immersion aft.
The pitching centre must be as far aft as possible as
otherwise, although the ship as a whole may be sea
kindly and although the gear may fish very well, it may
be impossible to handle gear on the aft deck or fantail
due to its amplitude of motion. At the same time the
bow must have good immersion and a hard forefoot in
order to assist in preventing the ship's head blowing off
and lowering the steering effort required to keep the ship
on the intended course when hauling. As a result a large
rake of keel is an anomaly, unlike side trawlers where it
may be an advantage and necessary for steering when
towing.
Windage forward, both in terms of area and drag co-
efficient, must be kept to a minimum for the same reasons
generally speaking, therefore, the stern trawler will tend
towards vessels of deep draft, low rake of keel, low block
156
coefficient with a deeply immersed and hard forefoot
profile with flat sections aft, carefully blended, however,
into deep Vees to prevent slamming. If the pitching
centre is far enough aft slamming does not occur with
such sections, providing they are well immersed. Deck-
house and rigging detail, etc. must be as clean as possible
and funnels avoided if the owners will permit this.
The stern must be so designed as to avoid entanglement
of fishing with sterngear especially in the difficult stage
of getting the codend on board. As a result it is desirable
to fit the propeller and rudder well forward which is,
of course, assisted by the low block coefficient inherent
in stem trawlers.
Added protection for the propeller may be given by
the fitting of a steering nozzle as has been done on the
majority of the larger stern trawlers. Other advantages
of a steering nozzle are an improvement in trawling
pull, good steering capabilities at low speeds and the
ability to use relatively high powers in conjunction with
high rpm, without undue loss of efficiency. At the
same time the introduction of a nozzle would appear to
improve the pitching characteristics of the vessel. It is
questionable whether the ideal layout of a stern trawler
may not be obtained with twin screws as in the Universal
Star but with engine rooms aft.
The twin screws naturally lend themselves to this type
of layout as relatively small prime movers are inherent
which may be installed aft and placed so that their
casings are brought up the sides of the ship, leaving the
fishing deck clear in between. There is much to be said
for this layout which gives unequalled controllability
in all phases of the shooting, trawling and hauling
operation. It is not necessarily more expensive as, with
half the power on each propeller, high-speed machinery
in an altogether lighter range of engines may be used
which, although they can be robust and conservatively
stressed, are often cheap per horsepower and can offset
the greater cost of steel and sterngear. Twin rudders are,
however, a sine qua non, and a single rudder would be
far from satisfactory. Prejudice against twin propeller
propulsion does not seem reasonable in the case of stern
trawlers though it may be for side trawlers.
Adequate ballast capacity is essential in a stern trawler,
perhaps more so than with any side trawler. It is not
enough to bring the stern down in order to give adequate
immersion aft, because in such a case the draft at the
bow may still be so light that it will blow off. Neither
is it enough to bring the bow down far enough to give
grip without ensuring at the same time that the stern is
deep enough to bring the pitching centre suitably far
aft. Fundamentally, a smallish stern trawler should be
designed to be able to maintain substantially constant
draft at all phases of her operations. This is not difficult
to achieve.
Recent designs have shown that in most cases ballast-
ing can keep the draft and trim within a few inches of the
design target throughout all phases of the voyage. In
this respect, an owner should not be allowed to dictate
the quantity of ballast in such a ship. This is a delicate
matter falling inside the jurisdiction of the designer who
should not allow himself to be deprived of the necessary
space.
How Unigan operates
For shooting, the gantry is set up with the trawl laid out
on the after deck, codend and body arranged over the
stern ramp. Trawl boards are in a stowed position port
and starboard, between the gantry legs and outer bul-
warks. The boards are stowed in a fore and aft position
just forward of the gantry pivot position and are attached
to the outer bulwarks by short wire strops clipped to the
forward edges of the boards.
The sweep wires are led from the winch under foot
rollers at the base of the gantry and over the hanging
blocks attached to the outboard sides of the gantry legs.
The danleno bobbins and butterflies are attached to
the sweep wires in the usual fashion and are lifted to the
hanging blocks.
When ready to shoot the gantry is swung aft and the
headline legs and towlegs are attached to the butterflies
The codend is pushed over the stern and the drag from
the water streams the net bellies after it. The quarter
ropes are rove through special sheaves on the inside of
the gantry legs and these, with the aid of the winch
warping heads, are used to lift the footrope and heavy
bobbins over the stern. In the case of light gear the drag
from the net in the water is sufficient to stream the entire
wings, footrope and headline over the specially shaped
stern hump without manual effort.
This stern hump, or dwarf vee-shaped bulwark, is a
recent innovation and supersedes the relatively more
expensive hydraulically operated stern flap as fitted to
the Universal Star and some subsequent vessels.
The stern hump gives added protection to the crew
and facilitates the shooting and hauling of the gear.
With the entire net in the water, the warps are slacked
until the strop links engage the Kelly eyes attached to
the trawl board backstrops. At this point the gantry is
swung forward and the warp slackened off. The G-link
assembly can then be attached to the trawl boards, the
warp taken in and the boards lifted to the hanging
blocks. The strops holding the boards to the bulwarks
can now be released and the gantry swung aft to allow
them to drop clear of the stern when the winch brakes are
released. The requisite amount of warp is let out in the
usual manner.
The operation of hauling is in reverse sequence to
that of shooting until the danleno bobbins are brought
up to the hanging blocks. At this point the quarter
ropes are detached from the butterflies and, by means of
ropes rove through the gantry quarter sheaves, are used
to bring the mouth of the net over the stern. A messenger
rope, leading from the winch warping end and passing
through a gilson block at the top of the gantry, is then
attached to the pork line and the codend heaved up to
the gilson block. The gantry can then be swung forward
to deposit the fish in the deck ponds. The entire operation
from the time the trawl boards break the water to deposi-
ting the fish on deck usually takes approximately 10
minutes.
Nothing, however, is static and in some cases it has
been felt that towing from the gantry leads to an unduly
heavy structure which in turn, of course, is reflected in
the size and general cost of the hydraulic gear. An alterna-
tive arrangement has been produced. In this the gear is
towed from gallow posts and the gantry is used to take
the otterboards aft from the stowed position. Hanging
blocks from the top of the gantry are used to lift the gear
over the stern. As no towing and, in particular, no
quartering loads come on the gantry it may be of much
lighter construction, its design being governed only by
the gilson block load. When the boards are brought up
to the block on the gallows posts they are secured by dog
chains, the G-link assembly undipped from the board
and the sweeps hauled in until the danlenos come up to
the blocks (Fig. 5). This variation on the basic gear has
Fig. 5. A board brought up to the block on the gallows post.
worked well and is now afloat in two or three ships. It
is not quite as flexible as the original Universal Star
type gear but, on the other hand, performs well at a
lower initial cost. The tiltable gantry gear may be modi-
fied without much difficulty to purse seining and a com-
bination gear, which can stern trawl and purse seine
without any fundamental alteration to the ship, is under
Continued at foot of page 15*
157
Ross Daring — Experiment
ROM Tftwkrs Limited are presently completing a 99-ft sterntrawter
with a high degree of automation and extensive use of centralized
controls. The vessel will have a 450 hp, 1,800 rpm engine situated
alt and a flsh-hokl foreward of 4,800 ft3 capacity. Vessel, gear and
cww are planned to cope with an average catch of one ton per day.
The crew consists of skipper and four deckhands. All engine room
functions, including servicing, bilge pumps, water supply, heating,
etc, will be fully controlled from the bridge in the same manner as
airplane engines are controlled and serviced from the cockpit.
The vessel will operate a normal wing trawl which will be taken on
deck over a stern roller. Hauling and shooting operations are fully
mechanized and controlled by the skipper from the bridge with the
exception of lifting the codend over the stern. A conventional
sidetrawler of comparable size would need more than ten men on
deck alone.
by
Dennis Roberts
Ross Trawlers Ltd.
L'Experkoce— ROM Daring
Rtamt
La "Ross Trawlers Limited" achieve actuellement un chalutier
pftchant par I'arrttre de 32 m qui aura un haut degre d'automation
et sur lequel les contrdles seront centralists. Le bateau equipe
de machines de 450 cv, 1.800 tpm situees a 1'arriere, a une cale
d'une capacit^ de 135 m3. Lc bateau, les engins et l'£quipage sont
privus pour une capture moyenne de un tonne par jour. L'equipage
est compose d'un patron de pcche et de quatre matclots. Toutes
les fonctions des machines tellcs quc: service, pompe de cale,
distribution d'eau, chauffage, etc. seront entierement contrdlees de
la timonerie, de la meme maniere que les moteurs d'avion, contrdles
et alimentes de la cabine de pilotage. Les operations de peche s'
effectucront avec un chalut & grande ouverture qui sera remonte
par I'arriere, par-dessus un rouleau. Les operations de chalutage,
entierement mecanisees, seront contrdlees par le patron depuis la
timonerie a 1'exception du relevage du sac. Pour un meme travail,
un chalutier traditionnel pechant par le c6t6 aura besoin de 10
homines sur le pont.
Ross Daring— Un experimento
Extracto
Se completa para la empresa Ross Trawlers Ltd. un arrastrero para
pescar por la popa de 99 pies de eslora muy automatizado y en el
que se emplean mucho los mandos centralizados. Tendra a popa
un motor de 450 hp a 1 .800 rpm situado y a proa una bodega de
pescado de 4.800 pies cubicos de capacidad. Esta proyectado para
pescar una tonelada diaria con una tripulacidn formada por el
patr6n y cuadro marineros. Todas las actividades de la sala de
maquinas, incluidos engrase, accionamiento de las bombas de
sentina, suministro de agua, calefacci6n etc., se controlaran desde
el puente de la misma manera que los motores de un aeroplano se
regulan y atienden desde la cabina. Tendra el barcp un arte de
arrastre normal que se metera a bordo sobre un rodillo colocado
en la popa. Las maniobras de largar e izar son totalmente mecanicas
y las hace el patr6n desde el puente, excepcibn hecha del izado
del saco sobre la popa. Un arrastrero que pescara por el costado de
dimensiones analogas necesitaria mas de 10 hombres en la cubierta.
Continued from page 157.
development. This must, however, use the normal
Unigan as a basis.
The system described is one of several; nevertheless it
is relatively widely adopted for small and medium-size
trawlers. Initial teething troubles were not connected
with the gear itself but arose from the differences in
technique required in adapting from side to stern trawling
and also from the twin screw arrangement of the Universal
Star. In particular, headline breakages were traced to
turning too sharply at the end of a steering leg and inad-
vertently collapsing the trawl. The extremely small
turning circle of the Universal Star was largely respon-
sible.
The ultimate, of course, in small stern trawler design
is complete mechanization of fishing operation procedure
allied to centralized engine control in the wheelhouse.
This has, to a fair extent, been accomplished by Unigan
and further developments include an operating bay at the
after end of the wheelhouse with grouped remote con-
trols for gantry and winch. The reduction in crew due
to the increased mechanization improves the economics
of the operation and, at the same time, the better working
158
conditions attract good grade fishermen, with a conse-
quent increase in efficiency.
For Northern waters all gutting should be done under
cover away from the elements and on even relatively
small stern trawlers of say 100 ft (30 m) a shelter deck
can be arranged to give this protection. This means that
only one or two men are needed for a short time on the
weather working deck during shooting and hauling.
Stern trawling has come to stay, and possibly the sole
remaining barrier it has to break through is the reluctance
of many owners to change over from their relatively
new but technically obsolescent side trawlers to the more
comfortable and safe stern trawlers that are now avail-
able. This is understandable and reasonable but the time
is coming when the move has to be made. A develop-
ment which may help is an arrangement for converting
side trawlers into stern trawlers with a high degree of
automation and crew protection at reasonable expense
and without major structural alterations. This has been
done on a trial ship which has proved successful and
perhaps this development may form the transition phase
generally needed. Certainly no one who has seen gear
handled aboard a small stem trawler under difficult
conditions can doubt that in the end the method will
replace side trawling.
BRITAIN has already launched her first
vJT stem-trawler, employing automation, aimed to cut
the cost of fishing. From Cochrane of Selby the vessel
of 99 ft overall will be operating in the North Sea by
October 1963. Ross Daring will be crewed by a skipper
and four men.
The intention is to carry only enough men to handle
the catch. If a vessel requires more men to work the
gear than to handle the catch, then the design is wrong
and it should not be followed.
Man or machine is the basic problem. Factories
ashore have installed machines, whilst fishing vessels up
to now have preserved the man. Each fishing community
has a different problem, a different point of view, and it
is impossible for all of us to move forward at the same
pace and along the same lines of thought. One thing is
certain . . . the more successful a community, the more
conservative will be its thought, and the slower its
development. We must be careful to avoid this lethargy,
that is why automation in fishing has been so long
delayed.
In recent years, fishermen have attained a higher
standard of living and are enjoying more security, so
much so that the younger men are not bothering to
learn their job and hence are very expensive to carry.
When men price themselves out of the market, the oppor-
tunity comes for the machine to replace the man.
This situation has already developed in the North
Sea. Too many expensive men are employed working
the ship, many more than are required, to handle the
catch. The fault lies in the design of the sidetrawler.
Whatever manning scale is attempted, it is a fact that
the conventional trawler requires one man for every
10 ft of length. If we can re-design to make this ratio
1 to 20, then something useful will have been achieved.
Catching capacity
A trawler's capability should not be judged by the number
of men aboard but by overall length. The minimum
length required to operate in the North Sea all the year
round is thought to be 99 ft. This length of vessel is
capable of catching an average of approximately one
ton offish per day; therefore the crew should be no more
than the number required to handle the catch comfortably
— and that is the skipper with four good fishermen.
It is acknowledged that there are sidetrawlers employ-
ing only this number of men, but they are very small
vessels pulling a very light gear. The size of gear suitable
for a 99 ft vessel requires ten men to handle if operated
from the side on a conventional trawler.
As half this number only is being employed, then the
sterntrawler must have a high degree of automation to
simplify the work for these men.
The point can be made here that it is not necessary to
go to sterntrawling in the North Sea if the crew numbers
are not being reduced accordingly.
Are engineers necessary?
The managing director, travelling in his luxury car,
does not have a mechanic on the back seat. Then why
should a fairly elementary diesel engine of this vessel
have two unproductive men to watch it hour by hour?
It is nonsense to say it must be so, in a year when men
are being rocketted into space! It has not happened
before because there has been no demand for such an
engine. It will be commonplace in five years* time.
Control of engines, services, bilge pumps, water supply,
heating, etc., all can be exercised from the bridge.
Integrated control
Control of winches must be integrated with all relevant
information obtained from bridge instruments. At
some point, whether bottom or midwater trawling, the
speed of tow must be adjusted, the length of warp altered,
perhaps the angle of inclination of trawl doors. It is
quite wrong that the control of the winch is on the deck
and that a man must don oilskins and get wet every time
the skipper requires a warp adjustment. This is the
Fig. 1. Single-screw steel motor sterntrawler for operation in the North Sea area. Dimensions: 85ft LBP x 23ft Bmld x 12 ft 6 in Bmld.
KEY: 1. Stern roller. 2. Guide rollers, P. A S. 3. Bobbin trays. 4. Steps to bobbin trays. 5. Gallows, P. A S. 6. Hatch to bobbin store. 7. Mast
house. 8. Derrick. 9. Fish pounds. 10. Warping winch. 11. Trawl winch, P. A S. 12. Gutting room. 13. Tray. 14. Gutting trough. 15. Washing
trough. 16. Chute tofishroom. 17. Trawl lead. 18. Bobbin lead.
159
point where automation does the fisherman a real service.
It is now a fact that the skipper can bring the trawl doors
up into the gallows by the touch of a button.
Will fishermen accept these exciting new vessels?
Yes, certainly. Very few labour-saving devices have been
introduced into the working of the trawl; those that
have, have indeed been welcomed. It is the aim to
improve the standard of living of fishermen still further.
Five good men sharing £7,000 are better off than ten
sharing £10,000 (these figures are an example and not a
proposal).
Fishing gear
The economics of this vessel have been worked out on
using traditional trawls. If there is an improved method
of catching fish, then it will be welcomed. Research
into improving trawls is going on and, until something
more useful is developed, a wing trawl will be used.
Unlike many stern trawlers, the Ross Daring has no
stern chute because of the risk of swamping the fishing
deck in winter conditions. The gear is taken over a wide
roller across the transom and the bobbins are contained
on a sloping tray or trough. Codends are taken over the
quarter, as with Canadian stern draggers.
Both port and starboard winches are hydraulic and
controlled from the bridge, so the skipper is able to
bring the wings up to the winches and the bobbins on to
the tray. Control is then taken over locally by the deck
crew who have a small hydraulic capstan employing two
whipping drums, mounted centrally, to swing-in the
codends, etc.
The fish is sorted and shot into the gutting and washing
house. From there, after processing, it is transported
forward down a chute into the fish-room where it is
packed away in ice in the traditional manner. The
fish-room is forward and has a capacity of 4,800 ft8.
Hatches are only removed for unloading.
The engine room is conveniently situated beneath the
fishing deck and astern of the fish-hold. The engine is a
Ruston Paxman 8 R.P.H.C.M. 450 shp at 1,200 rpm.
The experience obtained from Ross Daring compared
with that from the new American sterntrawler Narra-
gansett will be awaited with great interest. From these
enterprises the development of commercial fisheries
moves forward into the enlightened age of automation.
The sooner we see the end of fishermen slogging away
on an open deck, the better.
Discussion on
Stern Trawling
Mir. Robert F. Allen (USA) rapporteur: Since the last
FAO Fishing Gear Congress, the principle of stern trawling
has been applied on vessels large and small throughout the
world. The basic principle has now been accepted and the
papers this year deal with the refinements and mechanization
of the system. Mr. Birkhoff has summarized the various
types of sterntrawler designs in use today, classifying them as
factory ships, deep sea trawlers or smaller vessels. Further
distinction is made in the deck arrangement and the location
of the navigational bridge. Of particular interest in the
three papers presented this year on trawling are the deck
arrangements and the handling gear for small coastal vessels.
Both Mr. Corlett and Mr. Birkhoff describe the use of the
tillable or turntable gantry to extend the effective working
deck area of the vessel. Mr. Dennis Roberts9 new vessel,
The Ross Daring, will however use a system of lifting the cod
end with a boom similar to that used on the Canadian and
American Pacific Coast system. A similar attempt at design-
ing and building a small mechanized trawler has been
accomplished in the United States. This vessel, the
Narragansett, has a stern ramp and employs a large drum
for winding on the wings so as to minimize the requirement
for deck space.
A designer who is contemplating a new sterntrawler could
do well to review the considerations of sterntrawler design
that Mr. Corlett has proposed. Since the vessel is only part
of the fishing system, considerations should also be given to
the design of the gear to be hauled. Mr. Birkhoff, in his
paper, suggests some modifications which have been made to
the trawl nets which have been found of advantage in stern
trawling. This is a subject in itself and should be discussed
at greater length.
The subject of automation and centralized control, although
applied to all vessels, is extremely important in stern trawling.
Mr. Roberts hopes to reduce his complement on the Ross
Daring to five. A great effort has also been made on the
Narragansett to automate the deck machinery and to centralize
the control in the pilot house. This should also reduce the
complement to a bare minimum. Commander Pain's
paper suggests that not only the operational functions, but
the maintenance functions of caring for the vessel's machinery
can be accomplished from the bridge, thus wholly eliminating
the necessity for engineers on the vessel. Tt would be
appropriate to hear from the participants about their exper-
iences on automation, as we all know that i t is the emergencies
rather than the normal operations which will ultimately
determine the extent to which men can be eliminated from
the vessel.
Dr. I. H. Heinsohn (Germany): My firm has built some
fifteen stern trawlers of medium size operating in Northern
waters for Germany, Norwegian and British owners. They
are mainly from 180 ft bp up to 220 ft and they work mainly
on wet fish. The latest are small service factory vessels.
Referring to Corlett's paper on the Unigan System he endorsed
most of the points, but he would point out in relation to the
gantry that the old fairy tale was repeated that fish hauled
over the ramp suffered damage. This was sheer nonsense.
Tn contrast it would be impossible, in the case of heavy
fishing, to lift thirty tons of fish by the gantry and in that
case what would happen to the fish at the bottom, with 30
tons of fish on top of it ? Every practical consideration was
against the allegation of damage and every trawler operator
would agree to that.
He also could not agree that the Unigan gantry was better
and quicker than the ramp. This was true only in the case of
very small trawlers. In most fisheries, trawling caused dam-
age to the nets and when they were damaged they required
space for laying out and repair so that it was always better
to stretch the net out; therefore, even on smaller vessels,
emphasis should be laid on the need for giving as much space
as possible. Still, he agreed that for very small vessels the
160
gantry was worth its money but on bigger vessels it was not.
He congratulated the Ross Daring experiment as it was
really promising for near-water fishing. They had to save
man power and every mechanical device should be tried to
work with fewer men.
On the bigger trawlers much could be said for twin screws,
if they were properly designed— that is with big propellers.
They gave better efficiency when towing and also gave more
reserve power which should be of advantage when shooting
complicated gear. If, however, twin screws were installed
there should be also twin rudders. A single rudder was no
good with twin screws and slowed trawling speeds.
Birkhoff's paper raised the question of mechanization as
did other papers. On this point he had to pour some water
into the wine. On paper, everything worked very well but
as soon as the skipper had fouled gear, you had to start
working by hand or, if the warp had parted you had to make
the gear ready again and that meant hard manual work.
Mechanical devices were normally installed for performing
any one purpose. They could not be very flexible so that if
anything happened out of the scope of the design you are
in trouble. On this point he had worked out just what an
ordinary trawler would require in the way of full mechaniza-
tion. To adequately handle all the needs of shooting and
hauling a net on a big sterntrawler at least seven winches
would be needed besides the trawl winch. Each winch
should have a hauling power of between one and three tons
depending on the size of the net. Such winches cost anything
between £300 and £1,000 so you could easily run up to
anything between £5,000 or £6,000 for winches. The question
is whether it is worthwhile? Even if you have mechanized
your system, in the case of fouling gear you have still to work
something on the warp drums and you must, in any case have
a few men.
Further, a few men must be on board to operate the
mechanized equipment. He suggested that you could not
work with less than two men on each gallows which makes
four, one man supervising them, and at least one man in
control of the winches and supervising all the gadgets. If
they had so many winches and gadgets to operate it could
easily lead to a mishap and involve accidents. So, at least
under German law, he had some doubt whether full mechan-
ization would be allowed, because control could easily get
out of hand: in any case somebody had to watch and if any-
body got caught in the wire, he could be drawn into the drum
and a severe accident happen.
On the other hand the normal practice was for the men
handling the gear to do gutting as well; therefore he could not
see that, with big gear, they could get down to only two or
three men by mechanization. To haul the gear only by
mechanical means on a big vessel they would need a slipdeck
of from 30 to 40 metres in length and with the hauling winch
placed as far forward as possible. To prevent the danlenos
and bobbins rolling and banging about under the weather
conditions of the Northern waters, there would have to be
preventive structures together with a trolley way running
along the coaming— little as he liked that idea. With the
headline being shorter than the footrope, there would have
to be a U-shaped guard to control the bobbins. All these
points needed consideration if full mechanization was being
contemplated.
If the gear became fast on the ground, a side trawler was
better able to manoeuvre free than a stern trawler as it could
swing and go over the gear more quickly. Quick changes,
of course, were sometimes necessary for various reasons and,
therefore, emphasis should be laid on the need for the aft
trawl blocks being able to allow the warps up to 90 per
cent horizontal clearance on each side without fouling else-
where. The shape of the stem and the position of the stern
blocks demanded care in guarding the trawl boards from
swinging and banging. These, again, would require moving
devices and one strong recommendation to all designers was
to eliminate as much as possible everything that moved
because they involved attention tmd maintenance. All of
these things gave food for further thought. We should not
keep our heads in the clouds too high regarding mechanization,
automation and so on. It was necessary to keep your feet
on the ground.
Mr. W. W. Carlson (USA): My company are the builders
and operators of the Narragansett and other vessels. If we
ever heard of all the problems referred to by the last speaker
we would probably have been afraid to build that vessel.
However, in our ignorance, we went ahead and built it and
have operated it very successfully. We do not have seven
winches and we operate everything from the pilot house;
two men on deck with reasonable intelligence had no trouble
in getting the net over and getting it back. I am no fisherman
and I have done it myself. We operate quite well, do not
tear our nets and have reasonably good catches. The boat
has been really successful.
Mr. Harper Gow: Your point is that automation on your
vessel can be achieved with much less equipment than would
be required on larger vessels.
Mr. D. L. Alverson (USA): There is a difference between
stern trawling and stern ramp trawling. Simple stern trawling
came from the Mediterranean and has been practised in the
U.S.A. since the inception of trawling there. But stern ramp
trawling has only recently developed. On the Pacific Coast
80 ft to 130 ft vessels have worked and continue to work
successfully with four men and sometimes only three men
aboard. Some of their trips range up to 20 days in length.
One vessel of 140 ft has only four men. This is a matter in
which historic development has to be recognised and the psy-
chology of the fishermen themselves; you must also consider
the economics involved and how the catch is shared among
the fishermen. Our West Coast trawlers have changed
somewhat in the last four years. Most of the vessels now use
a drum and roll on the net; this is very close to the stern and
when the codend is reached it is swung round on the small
gantry and lifted over on the side of the vessel. The
Narragansett has a small ramp to bring the codend right up
into the vessel.
Mr. Conrad Blrkhoff (Germany): As a naval architect I
agree with the views of Corlett and Roberts that the use of
the ramp in small ships has limits. Under certain conditions
this ramp is not absolutely necessary and sometimes not even
possible. But the conditions of today might alter and open
possibilities for later use. It has been proved that well
constructed ramps do not cause any damage to fish and that
hauling big catches up the ramp saved much time. He
thought, therefore, they should always, if possible, consider
the construction of a ramp which permitted hauling the gear
as far as the gallows and, when necessary, made it possible
to haul the entire trawl gear onto the deck for mending or
servicing.
The codend suffered less by sliding over the ramp than by
almost vertical hoisting which might involve a swinging of
the bag in rough seas. This idea was involved in Coriett's
161
new bulwark form. He thought, however, that the hauling
during the different phases of the gantry movements could
easily be accomplished by a combination of two blocks on a
fixed framework formed by a bipod post with gallow consoles
at the posts and an outrigger codend derrick aft. This
construction would be cheaper and more robust for being
fixed. The recently developed gantry nose had the same
function as the codend derrick before but involved some
additional costs.
He was not, however, against the idea of a tillable gantry
but it should be contemplated in relation to total building
costs. He recommended, at any rate, a fixed construction of
gallows which was alluded to as an innovation in Corlett's
paper. It would be a further step ahead if the gallows were
built so that the trawl doors could be located close to the
gallow posts. It might then also be possible to exclude from
the tasks of the gantry the hauling of the codend by building
a normal fixed bipod post. For the remaining tasks during
the shooting operations such as moving the codend (and
possibly the quarter ropes) and facilitating the shooting in the
case of the bobbin gear hoisted on deck, a bipod outrigger
derrick leading from the gallow consoles would then be the
lightest, cleanest and most adequate solution. Such a
gantry would be useful for accomplishing these various tasks
and last but not least for the alternative employment of the
ship in different fishing methods. For the normal trawl
fishery he could, however, see no reason why the fixed
gallows should be abandoned.
Corlett's arguments on the necessity of the lateral under-
water and above-water area of the ship should be underlined.
Those ideas were conclusive in relation to the bulbous bows
for this type of vessel and they reduced pitching.
Automation of the remaining phases of handling the net
was more possible on bigger ships than little ones. But the
relative economy of a reduced crew was of greater importance
cm smaller vessels so that due regard should be given to the
idea especially for such vessels. Mr. Roberts* paper showed
that the technical ideas had been well adapted to small
vessels in rough seas. The possibilities of a bobbin track for
various bobbin gears without imposing restrictions on further
gear should be contemplated in detail. The swinging of the
codend over the side near the propeller might be dangerous
in rough seas because of movement in that area.
His view was that the basic character of trawl gear had
remained unchanged for years and had not even been affected
considerably by pelagic nets. But we should be able to cope
with all these assumptions in coming years. He therefore
suggested the automatic positioning of the trawl doors which
did not need pennants any more, if the further hauling of
the net could be done by counter pulling the sweeplines by
means of gillson blocks. Apart from the question of the
crew required for gutting, the most dangerous part of the
work would then be done automatically— even when the
gear became bigger and heavier in the future. If improved
gutting machines did become available in the future then a big
saving of crew would be achieved.
Mr. Dennis Roberts (UK): I had hoped some speakers
would criticise the Ross Daring plan but most speakers had
welcomed it. Dr. Heinsohn had advised them to keep their
feet on die ground. He would say that he was a practical
fisherman with 25 years experience at sea. It had always been
his dream to build a trawler so simply handled that he could
take his wife and children with him and go out and fish. He
was very pleased to learn about the Narragansett. The big
difference between that vessel and the Ross Daring was that
162
the Ross Daring took their gear over a transom stern. When
he had visited the Canadian Pacific Coast and saw their vessels,
the fishermen told him they did not think their type of vessel
was suitable for fishing in the North Sea. He thought a
100 ft vessel with a ramp working in winds of force 7 or 8
would be swamped. A transom stern would be much safer.
A lot of his ideas were based on experience with 60 ft Canadian
vessels.
Mr. E. C. B. Corlett (UK): The gantry system is essentially
for smaller trawlers and he had been surprised that it had been
used in Germany on some of their large trawlers. He was
very interested in designing small vessels and the plan of the
Ross Daring impressed him very much and he wished Mr.
Roberts good luck. If, however, he caught more than one
ton of fish per day he might find difficulty in handling it.
If he caught ten tons a day he would be in difficulties. His
firm had recently designed and completed a trawler almost
identical in size— length 91 ft, beam 23 ft, depth 12 ft 3 in.,
and draught aft under 10ft — almost identical — it carried 12
crew and was also highly automated. It was fishing in the
rough waters north of Norway and was catching ten tons of
fish per day, every day. Three or four men simply could not
cope in those conditions with catches of that size.
On the question of the gantry it had to be built and stressed
so that it would take a full drawing load up to 90 degrees off
the towing line. It was expensive to provide that, but, in
fact, by far the biggest load on the gantry came under those
conditions. That was the reason for adopting the simpler
arrangement for towing from the gallows post. This system
worked very well, although it did have a slight disadvantage
for handling the boards. It was also a good deal cheaper
and the gantry could be built lighter and the hydraulics
involved were less expensive. Mr. Roberts had made a
good point regarding the ramp on smaller trawlers. It was
most important they should have sufficient buoyancy and
protection for the men. The smaller trawler he had mentioned
had very powerful split winches — seven tons pull each.
Fixed gantries were more or less standard practice on larger
trawlers but on smaller trawlers they provided a good deal
of windage and iced up in the Arctic. Remotely controlled
winches were interesting possibilities. They had provided
one which was worked by an operator who had a whole
deckhouse between himself and the winch. He had been
very sceptical about this at first but it did work very well.
Remote control of winches did help the design of stern
trawlers a great deal and he thought the practice would
spread even if full automation was not possible.
Mr. C. Maurin (France): We have had experience of a
gantry based on a German patent. The system originally
used turned out to be fairly difficult particularly so far as
handling the trawl boards was concerned. The hauling was
fairly difficult and use of the gantry in heavy seas was danger-
ous. The catwalk for supervising the hauling was too close
to the sea and in heavy seas created some risk. An alteration
was made whereby the catwalk was raised. This made it
possible to bring the trawl boards in quite easily. Finally the
system was improved by bringing the wings in up to the level
of the winch which meant that the central part of the trawl
could be brought up to the deck directly. At times the trawl
came on to the deck in heavy seas and there was risk from
movement. In later French ships the net has been wedged
alongside the deckhouse. The disadvantages arose from
heavy seas. On the Tallinn an active rudder was used which
meant that they could manoeuvre better in a current This
active steering required a strong motor for working it. In
the case of fouling it was easier to get clear on a side trawler
but in a stern trawler there was available far greater speed in
manoeuvring and speed in handling and with the modification
made, the size of the crew could be cut down compared with a
side trawler. On a stern trawler men had greater protection.
So far as damage to the fish being concerned because
of the slope of the ramp, he had never observed any damage
of this kind. On the contrary, he found that the fish came in
very good condition. Another advantage was that where
fishing was done in calm waters and without wind the trawl
when shot from a stern trawler, went deep into the water. In
a side trawler the trawl sometimes went down incorrectly and
there was a risk of trawl boards crossing, but working from a
stern trawler that difficulty was overcome. The Tallinn, with
the alterations now made, was proving very satisfactory.
They now had working from Lorient two medium tonnage
ships working with a gantry system and the owners of those
ships could give a very satisfactory account of their results.
Mr. F. Dorville (France): The two vessels referred to have a
sheltered control deck and the Unigan System does give the
advantages described. By its use you can place the whole of
the trawl in a very good position for strapping, backing and
also bringing in the trawl boards without danger. The
crews work under better conditions. We did have some small
difficulties in the first place when the boat was rolling and
this resulted in the bag swinging about but this was overcome
by a fairly simple operation in placing two legs between the
gantry which cut down the swinging as the bag came aboard.
I can confirm that these two ships are operating very success-
fully. We can thus say that the Unigan system, because of
its simplicity and automation and concentration on the stern
of the ship, can be applied on various types of ships. We had
two trawlers with covered sterns. Another has a working
area in the bow and there is a conveyor which brings the fish
forward to the working area. Both on the 15 to 20 or 45 to
50 metre length of craft that we have, the system is very
flexible and at the same time gives very useful mechanization
which takes a lot of work off the crew.
Mr. Jean Frechet (Canada): Over the last three years
Canada has been very active in investigating the best trawler
design for small craft and their mechanization. They con-
sidered stern trawling was still in its infancy and many prob-
lems had still to be overcome. One related to the multiplicity
of designs for operating the trawl over the stern. They felt
that stern trawling should be fully operative over the stern
and that the deck layout and equipment should relate to that.
These problems would be solved over the next decade. With
a view to that end, some small trawlers had been built and
would be followed by others so that they could learn by
experience and their mistakes should not be too big. They
now felt safe in going further forward.
Dr. G. C. Trout (UK): For investigations at Lowestoft
we are considering a small trawler of some 35 metres in length.
After his trip on the Fairtry he had been so impressed by the
quality of the fish that they considered going in for a stern
ramp although the vessel was of such small length. He
would like more information about the freeboard. He
understood the Universal Star had a very low freeboard and
it might be that that resulted in her being a wet ship and
therefore on consideration he would not advocate a ramp.
Mr. Birkhoff's design, although not yet built, had a freeboard
of about three metres or more and that might be why there
was no unanimity of opinion about a stern ramp in a small
ship. He was extremely heartened to see the design of the
Narragansett which looked a delightful ship and he was glad
to know it worked well.
Mr. D. J. Doust (UK): The papers presented by Birkhoff
Corlett and Roberts are of particular interest, since they
show the interdependence of the fishing gear and handling
problems with the requirements of good design. For economic
success, the design of vessels for a particular type of fishing
must properly be those in which all the essential technical
factors are fully integrated. What sometimes appears to be
an advantageous innovation from one point of view, may,
however, incur another more serious disadvantage resulting
in an overall loss of earning capacity, or even complete failure.
This factor is clearly brought out in Corlett's discussion of the
design requirements of smaller stern-fishing vessels which
require a pitching axis well aft of amidships to avoid excessive
stern motions and accelerations. Failure to provide for this
feature of the design nullifies the advantages of stern-fishing
and improved fishing gear, purely on account of the physical
inability of the personnel to use the new techniques in adverse
weather conditions.
With the transition from side to stern trawling and the
attendant multiplicity of internal and deck arrangements,
more attention has to be given to this requirement of minimum
motion and acceleration at the stern. Machinery and gear-
handling layouts which favour such a stern trawler design
are more likely to become economically successful, mainly
due to their ability to make more use of the time spent on
the fishing grounds. From the point of view of the vessel's
hull shape, the type of transverse sections which can be
incorporated in such designs is largely governed by the
relative positions of engine and fish rooms. Diesel-electric
machinery for the larger deep-sea trawlers would seem to
offer the maximum flexibility in this respect, since the full
amidships portion of the hull can be utilised to store the catch,
thus minimizing the problem of change of trim with increasing
fish load as the voyage progresses.
The requirement of minimum motion of the stern generally
favours an asymmetric hull shape, with an LCB position
about 4 per cent aft of amidships BP, although there are
possibly further advantages if the LCB position could be
moved even further aft. Vessels with machinery aft, as shown
in Birkhoff's figures 3, 4, 5 and 6, and Corlett's figures 2 and 3,
rather lend themselves to the more favourable hull shapes
on this account, although somewhat increasing the fish-
handling problems on board with the forward fish rooms.
There would, however, on balance, appear to be more
favourable overall design characteristics for the smaller
stern trawlers, by adopting an aft position of engine room,
with the fish rooms below the working deck amidships.
Mr. W. Dickson (UK): The development of stern trawling
raises the question of the types of nets required by them.
The separate winches used for the sweeplines make it much
easier to haul double or triple lines and he did not see why
they should necessarily have danlenos or bobbins. Provided
the sweeps were fairly long, once the doors were disconnected,
the double wire could be hauled to the sweep wire winch.
Regarding Mr. Alverson's comment about winding the net
on the drum, they could not get the whole trawl onto the
drum because of the heavy ground ropes but it might be
possible to get up to say 20 feet of the wings and that would
save 20 feet of working deck. It was not really necessary to
use kites and all those complicated things that could go
wrong when shooting from the stern.
Did anyone tealiy have evidence that those kites did scare
the fiih down in to the net? In the absence of that, they might
juft as well get the headline height by re-designing the net.
There was also the possibility of easier handling of the trawl
boards because the shooting was straightforward. One
could then go in for more refinement in the doors and be
sorer that they would go away all square than with side
trawlers. They thus had the option of securing greater
headline height and greater spread and even a bit of both.
One usually had to pay for those advantages by either increased
damage or increased drag. For practical purposes they had
to choose. They could go to sea with two or three different
nets suitable for fishing but not more. At the moment they
did not really know how much headline height was best.
For design purposes they had to have a figure: 12 or 15 feet
and that height would be determined by what they could find
out about fish behaviour. There were two approaches to
this: (1) either direct observation by camera, TV, netzsonde
equipment or observation by submarine and that was a task
for research workers; (2) the other way was just to try it and
see: make a net and compare it with a standard Granton
trawl To do this one must have accurate measurements on
the fish caught, haul by haul, and measure at least a decent
sample. Some of this could be done by research vessels but
it would be a great mistake to think it could all be done by
them. There should be more co-operation between com**
mercial craft in the industry and the research workers in
doing that work.
Asked by Mr. Harper Gow for a clearer view because he,
(Mr. Gow), had the impression that the higher the net the
better, Mr. Dickson said: I do not know the real answer.
There is not much doubt that if you make a net bigger you are
likely to catch more fish but do you make it by bigger height
or by shaping and spreading the net? The bigger you make
the headline height the more you cut down the spread. You
have to look at the height of the fish off the bottom and
ascertain their concentration and do this at different times of
the year and design a net to suit average conditions the year
round— what proportions go into the height, what into the
spread and what the taper should be. If you have a high
headline you have to have a big taper. You must work out
the average for a reasonable headline height for the whole year.
Mr. Robert F. Alton (rapporteur) summing up said: There
still seems to be many differences in the net handling methods
on large European trawlers and the discussion has reflected
these differences. Strong opinions were expressed by three
of the delegates that experience shows a ramp does not
damage the fish if properly designed. This seems to be a
rather conclusive result. There is still discussion, however,
and differences of opinion on the method of hoisting the codend
and positioning the trawl bag. The minimum deck space
available on smaller vessels is a determining factor in designing
the gear for hoisting the codend aboard. Ramps for this
type of vessel are not generally considered practical because
of the loss in deck space and the hazard of taking sea on board.
Freeboard is a big consideration in the seaworthiness of a
ramp design, but this particular problem is more acute on
the smaller vessels where freeboard is limited.
Although the aim of designers is full automation of the
fishing operation, a major deterrent in achieving this end is
the extenuating circumstances which are encountered when
the gear is stuck or ripped, or when unusually large catches are
encountered. Mr. Corlett has pointed out that there is a
great difference in the number of men required on an auto-
mated vessel catching one ton per day compared with vessels
catching ten tons per day. This would indicate that great
strides could be made in reducing crew if automatic machines
were available to process the larger catches.
In consideration of the comfort of the crew and the safety
to the fishermen, Mr. Doust's comments on hull form with
regard to pitching should be well noted by naval architects.
Lest we forget that the fish are still caught in the nets, Mr.
Dickson reminds us that more experiments are indeed
necessary to provide the most effective gear and to this end
let us strive for more co-operation between commercial craft
in the industry and the research workers
164
Put 2 Bulk Fish Catching
Section 6 Bottom Trawling
Some of the General Engineering Principles
of Trawl Gear Design
Abstract
In 1959 the White Fish Authority, with financial support from the
British Trawlers' Federation and H.M. Government, through
contract, began an extended investigation to develop a distant water
trawl having as its specific features: (a) increased headline height;
(b) increased netmouth width; (c) sound structural design and (d)
improvement in handling. This paper reports on the hydrodynamic
studies which formed a part of this project and is well illustrated with
some 39 figures. Details of experiments, special equipment used,
observations and results concerning the behaviour of the warps,
otter board forces and moments, otter board stability, width and
height of the mouth of the net, and drag of the net are given. A set
of approximate equations from which warp curvature can be esti-
mated with sufficient accuracy was developed. It was found that
when measurements of ship speed and engine r.p.m. are available, a
value of towing pull can readily be deduced. Experiments indicated
that aerofoil type forces can be determined from measurements of
a model in a wind tunnel, a ground being placed adjacent to the
otter board to represent the seabed. The quantities measured are
side-force, drag, lift, pitching moment and yawing moment. A
method was developed for calculating headline height from a
knowledge of the characteristics of the headline lifters and other
factors. The author believes the objectives set forth were accom-
plished and that a basic theory has been provided from which
gears can be designed to meet certain specifications. He also states
that close co-ordination between gear engineering design and fish
behaviour projects is necessary.
Quelques princlpes du genie i
ritin
nt le dessin de chalut
En 1959, la "White Fish Authority" travaillant sous contrat, avec
1'aide financiere de la "British Trawlers' Federation*' et du Gouverne-
ment, a commence une investigation poussee, pour le ddveloppc-
ment des chaluts hauturiers. Les buts de cette recherche etaient:
(a) acroitre la hauteur de Fouverture verticale; (b) accroltre 1'ouver-
ture du filet; (c) 6tablir des principes de base de construction et (d)
ameliorer Foperation de 1'engin. La communication decrit les
etudes hydrodynamiques qui ont constitue une partie de cc projet
et contient 39 illustrations. Sont donnes ggalement, des details sur
les epreuves, 1'equipement special utilise, les observations et resulta
obtenus concernant la configuration des funes, les mpuvcments
et forces des panneaux, leur stabilite, 1'ouverture horizontal^ et
verticale du filet et la resistance & la trainc de 1'engin. Des equations
approximatives ont M developpees & partir desquelles on peut
estimer avec assez de precision la courbure des funes pendant
1'operation. II a etc trouvd que, lorsque la vitesse dans 1'eau et les
tpm de la machine sont connus, la valeur de la force de touage
peut £tre facilement calculfe. Les experiences ont indiqut que les
forces de type aerodynamique, peuvent dtre ddterminees par des
mesures effectuees sur un modele dans un tunnel aerodynamique,
lorsqu'on a pris soin de placer un fond pres du panneau de chalut
pour reprfeenter le fond de la mer. Les valeurs mesurees pendant
les experiences etaient: force laterale, resistance a la trainc, force
d'elevation, importance du tangage et importance d'embardage.
Une m£thode fut developpdc pour calculer Touverture verticale &
partir de la connaissance des caracteristiques des elevateurs et
autres facteurs. L'auteur croit que les objectifs ont etc atteints,
qu'une theorie de base a etc deveioppee et permettra de dessiner
des engins pouvant satisfaire certaines conditions spfcifiques.
Une coordination etroitc entre le dessin technique des engins et
les projets de comportement du poisson est necessaire.
Prlndplos de mecanica general de las fornas del arte de amstre
Extracto
En 1959 la White Fish Authority, con la ayuda financiera de
gobierno y de la Federaci6n de Armadores de Arrastreros de la
Gran Bretafia, inici6 una amplia investigation para encontrar un
arte de arrastre para la petca de gran altura que tuviera como
by
P. R. Crcwe
Westland Aircraft Ltd.
(Saunders-Roe Division)
caracteristicas especfficas : (a) mayor altura de la relinga de corchos ;
(b) mayor abertura horizontal; (c) buenas formas estructuratot, y
(d) mas facilidad de manipulacidn. Esta comunicacion da cuenta de
los estudios que formaron parte de este proyecto. Esta muy bien
ilustrada con 39 figs. Se dan detalles de los experimentos, el
material especial empleado, observaciones y resultados relativos
al comportamiento de los cables, fuerzas, movimientos y estabilidad
de las puertas, altura y anchura de la boca y resistencia de la red.
Se encontraron ecuaciones aproximadas con las cualcs la curvatura
del cable puede estimarse con exactitud suficiente. Se observe que
cuando se conocen la velocidad del barco y las r.p.m. del motor se
puede deducir fAcilmente el valor del tiro de remolque. Los experi-
mentos indican que las fuerzas de tipo aerodinamico pueden deter-
minarse a partir de medidas de un modelo en un ttinel de viento,
colocandosc un suelo junto a la puerta para representar el fondo del
mar. Los valores medidos son: fuerza lateral, arrastre, fuerza
ascensional, momento de cabeceo y memento de guiiiada. Se
encontr6 un procedimiento para calcular la altura de la relinga
de corchos bas&ndose en el conocimiento de las caracteristicas de
sus elevadores y do otros factores. El autor cree que se alcanzaron
las finalidades propuestas y que se ha formulado una teoria fun-
damental con la cual se pueden proyectar artes que satisfagan
determinadas especificaciones. Afirma que es necesario coordinar
los aspectos mccanicos de las formas de los artes y los proyectos
de comportamiento de los peces.
1. FIRMS that engage in aircraft design almost
always have aerodynamics departments, and wind
tunnels for testing models. These departments are
concerned with the airflow round the craft and the way it
affects powering requirements, stability and other such
behaviour. One difference between the Westland Aircraft
Company and other British aircraft contractors is,
however, that its Saunders-Roe Division also has a
hydrodynamics department, and towing tanks. These
undertake corresponding work with regard to craft and
equipment that move on or through the water. In recent
years they have made a number of special investigations
in connection, for example, with a large cargo submarine,
yachts and, currently, trawl gear.
Trawl gear work commenced at Saunders-Roe
with limited investigations for one or two firms, and in
particular for the Lord Line Company, Hull. In 1959
the White Fish Authority, with financial support from
the British Trawlers' Federation and H.M. Government,
165
gave a contract for an extended investigation leading to
the development of a distant-water trawl having as its
specified features:
(a) Increased headline height.
(b) Increased net mouth width.
(c) Sound structural design.
(d) Improvement in handling.
These are purely engineering objectives, which could
be achieved without reference to problems of fish beha-
viour, and consideration of the latter was not required.
However the available information on fish reactions was
borne in mind. This in the main comprised two observa-
tions:
(i) The practical experience of the industry that catch
is increased as the otter boards are moved further
and further ahead of the net, using long sweeplines
or legs.
(ii) Indications of fisheries laboratory experiments
that fish do not appear to react to gear until it is
extremely close to them, unless they can see it.
The present paper seeks to demonstrate that the Com-
pany's know-how and facilities in hydrodynamics,
model testing, structural engineering, instrumentation
design and electronic equipment have proved most
suitable for establishing or confirming the general engin-
eering principles on which good trawl gear design depends
and for designing the trawl gear specified.
2. GENERAL PROBLEMS
Consider the schematic trawl gear shown in Fig. 1.
Logically one should perhaps start with the behaviour
of the net, and work forward to the trawler, but in
practice the performance of the gear depends upon the
Fig. L Typical bottom trawl layout reproduced by permission of the
Gourock Rope Co., Ltd.
amount of warp out, the speed of tow, the depth of
water and the type of sea bottom, so that in analysing
performance it is convenient to start at the ship and
work aft.
166
The instruments used for obtaining the data reported
on in this paper are described in Nicholfs paper in
Section XIV in which also appears Dickson's paper
"Performance of the Granton Trawl".
2-1 The behaviour of warps
The warps take up shapes under tow that are in general
curved both in planform and elevation as shown in
Fig. 2, These curves depend upon warplength, water
TOWING 1LOCK
Fig. 2. Warp curvature.
depth, warp diameter, warp weight in water per fathom
of length, towing speed, tension in the warp, and lateral
distance or spread between the lower ends of the warps.
The warp shape varies with towing speed because of the
hydrodynamic water force that is generated by moving the
warp through the water, and the magnitude of this water
force depends upon the extent to which the warp vibrates.
Special electronic acoustic instruments shown in Fig. 3
have been employed for measuring the spread between
the lower ends of the warps directly. The bodies are
towed by wires which are attached to the warps,
about 25 fm ahead of the otter boards. The conical
tails, which stabilise the bodies directionally, are based
on Kelvin Hughes direct reading current meter practice.
However it is undesirable to need to attach this type of
equipment to the trawl for every instrumented haul,
and in any case other measurements are necessary to
determine the effect of warp vibration. It has, therefore,
been found best to employ an instrument which measures
the "divergence" angle between the warps, just aft of
the block, and the "declination" angle of the warps to
the horizontal, as shown in Fig. 4. The instrument is
simple to install, once the warps are in the block, and
systematic readings with it have been obtained for nearly
all the test hauls made as part of the investigation.
A set of approximate equations has been developed
from which warp curvature can be estimated with
Fig. 3. Electronic Instrument for measuring spread near the otter boards, (a) (on left) Mark II Spreadmeters, the instrument on the right it
ready for attaching to the trawl, (b) (centre) Mark II recorder shown withdrawn from the pressure base, (c) (on right) Mark II Electronic Unit
DECLINATION ANGLE
METER
Fig. 4. Divergence and Declination Meter on warps immediately
aft of towing block.
sufficient accuracy. Four of these equations, taken alone,
provide an adequate account of curvature in a vertical
plane. Theoretical estimates for the declination angle,
appropriate to different amplitudes of warp vibration,
are compared with experimental values given by the
instrument, and the effective amplitude of the vibration
is deduced, as illustrated in Fig. 5. In the case shown,
the "vibration factor" of id is appropriate to a non-
vibrating condition, and thus gives an upper limit for
achievable values of warp declination angle. The vibra-
tion is generally of such a magnitude as to increase the
hydrodynamic forces by about 50 per cent compared
with a non-vibrating wire of the same dimensions
(e.g., a vibration factor of id as compared with id),
but a rather greater effect can occur in deep water or in
conditions that encourage vibration. The vibration is
believed to be due mainly to the wire causing large eddies
which are shed alternately from either side, as illustrated
in Fig. 6. However, ship vibration and disturbances
LENGTH t«27I FATHOM!
ff +ilN-'
DEPTH,
PULL SCALE
MCASUfttMCMTS
(WARP DIAMETER - <) __. * ,. *
—.— I7T 1 THEORY AT MEAN /S OP U'/a.WITM
1 DRAG OP GEAR EXCLUDING WARM
x
«
MERE *r - WEtOMT OP WARP,
Li /FATHOM
\* TOWING SPEED, KNOTS
^\
^
w y^
r;
* ^TK
^^^*^W»
IORITIC4
PER LIMI
L
T
THEORETICAL LIMITS
POR 0 VARIATION
FROM, !§• TO IS»
4
\
^7
^b
s4
J
x.x-:^
•^^
^-^,
v*-^
"--^.
v
"""^
^B
WARP T
DNV.NG
SPEED -
KNOTS
Fig. 5. Variation of warp declination with speed.
VELOCITY THROUGH
WATER
FLUCTUATING SIOC PRESSURES DUE TO PRODUCTION
OF VORTICES
VORTEX LEWES it HMD CIRCULATION OF OPPOSITE
SENSE WHICH CAUSES A SIDCFORCE.
Fig. 6. Vortex street behind a wire rope.
The estimates of warp behaviour require a knowledge
of the tension in the warps* The tension just aft of the
block can be calculated from a knowledge of the trawler
propeHer characteristics and hull form. This has been
confirmed by attaching load cells to the warps a little
ahead of the btock, as shown in Fig. 7. Fig. 8 shows
Fig. 7. Load cells on warps ahead of the block.
variation, with ship speed, of towing pull available, and
compares points deduced from load cell measurements
on a gear of Granton type with curves estimated from
propeller thrust and ship resistance calculations. The
agreement, which is good, is further supported by direct
AM MOUCID 'MOM TMMMTICAL ESTIMATE!
THRUST AND HULL WATCH MMTANCC
ft.V.tftNMT HOLT"
Fig. 8. Comparison of predicted towing pulls with measurements of
warp tension. The figures across the graph indicate the ship's speed
in knots.
measurements of propeller thrust, obtained from a
Michcll thrust meter mounted on the propeller shaft
(see Fig. 9). All results are converted to still air conditions
since the atmospheric wind exerts an appreciable force
on the ship superstructure, which may help or hinder
its motion through the water.
168
It may be seen from Figs. 8 or 9 that when measure-
ments of ship speed and engine r.p.m. are available, a
value of towing pull can readily be deduced. If, for a
given trawl gear r.p.m. is reduced, ship speed and towing
pull required tend to decrease almost along a straight
line, which cuts the towing pull axis at a small positive
value, as shown in Fig. 10. This is because headline
POINTS AM DCOUCID FROM MICCT MCAtUftCMCNTt
Or PftOPCLLC* THRUST
CURVCS AM DEDUCED MOM THEORETICAL CSTIMATCS <
0ROKLLCR THRUST AND HULL WATCH RCSISTAMCC.
»LT."
I I
Fig. 9. Comparison of predicted towing pulls with measurements of
propeller thrust. Figures across the graph show ship's speed in knots.
height generally increases as speed reduces, for otherwise
towing pull required would vary more nearly as the
square of speed. (Further consideration of this point is
given near the end of the paper.) It follows that if checks
of towing pull required are satisfactorily established by
direct measurement, at one speed and r.p.m., the speeds
appropriate to other r.p.m.'s can be estimated from a
knowledge of the ship's propeller characteristics. The
tension in one warp is, of course, nearly enough given by
(total towing pull)/2 (cosine of warp declination angle),
which is approximately equal to (towing pull)/ 1 -85, in
most practical cases.
v I • -WATCH DIHTM
III
Fig. 10. Towing pull requirements. Figures across show ship's speed
in knots.
The tension does not reduce to zero as speed is reduced
to zero since it has to continue holding the warps taut,
which can theoretically be shown to require a value
equal to (depth of water) x (weight of warp in water
per unit length); e.g., a 3-25 in circumference warp
weighing 8*5 Ib per fm, requires a tension of 850 Ib in
100 fm of water. When the gear is under way, the tension
at the bottom of the warp is less than that at the top by
the above quantity together with the component of
force parallel to the warp that is produced by the water
flowing past it. This has been confirmed by measuring
tension both ahead of the block and just ahead of the
otter boards. The load cell designed and built for the
latter purpose is shown in Fig. 11, and the differences
Fig. 11. Load cells for measuring forces in wires. (Top) Load cell
dismantled. (Bottom) Load cells in place as seen from gallows before
shooting.
in experimental tension between the top and bottom of
the warp are compared with theoretical estimates in
Fig. 12. The component of water force parallel to the
warp is given nearly enough by:
ip(*ds)(l-689Vk)*CT,
where p is water density in slugs per cubic foot, d and s
are warp diameter and length in feet, and Vk is speed in
knots. The coefficient CT is due partly to skin friction,
and also to water pressure on the strands of the cable.
Let us now return to the general performance charac-
teristics of the warp. Once the tension (or towing pull)
and the effective magnitude of vibration have been
established, as illustrated in Figs. 8 and 5 respectively,
curves of warp length required to give any desired angle
of the warp to the vertical, at its attachment to the otter
board, can be calculated. A digital electronic computer
was employed for this purpose, a sample set of results
being shown in Fig. 13. These refer to fishing with 275
fm of warp aft, in various depths of water, and at various
ship speeds. The weight of warp in water in Ib per fm,
w, can be given any suitable value when using this dia-
T, - TENSION AT TOP OP WARP , Lt.
Tj - Tf Nf ION AT EOTTOM OF WARP , Lt.
- vr - WEIGHT Of WARP IN WATER , Lt / FATHOM
0 - WATER DEPTH i FATHOMS
TANGENTIAL FORCE COEFFCKNT
CT * '01*
•ABED ON SURFACE AMCA
(THEORY, NEGLECTING
SKIN FRICTION)
o s 10 is ao as
SPEED* (KNOTS')
Fig. 12. Warp tension differences.
gram. It should however be noted that the results only
apply if the drag of the gear is proportional to w. To
obtain specific results, for a case with a drag that is
characteristic of distant-water trawls, a value for w for
WARP LENGTH - 275 FATHOMS /C/a»O
THEORETICAL CURVES AM FOR DRAG OF .*>/
r- GEAR EXCLUDING WARPS - 471 wVfc , Lt. -/--.
C-SOU,
W.S-5 LS./ FATHOM,
C • VERTICAL UP - PULL OF
WARP AT OTTER SOARO. Lt.
(WHERE <* - WEIGHT OF WARP, LI/FATHOM/
Vk • TOWING SPEED, KNOTS ) /
Fig. 13. Typical warp length] water depth curves. The vertical scale
gives the ratio of warp length to water depth and the figures across,
the towing speed in knots.
169
8»$f appropriate to a 3-25 in circumference warp, may
be considered.
1 Each line corresponds to a different constant value of
vertical upward pull of the warp on the otter board
bracket, the appropriate values being given adjacent to
the lines. Experimental fishing conditions for a 3-25 in
warp are shown by points. In these cases the warplength
to water depth ratio used was about 3-5 irrespective of
speed. Some variation with speed should give a rather
better performance, as will now be discussed. To keep a
given up-pull on the board, the warplength to depth
ratio should be varied with speed along one of the
constant up-pull lines, as for example by the variation
marked A- A. If so much warp is paid out that values of
length to water depth lying above line B-B occur, then
there is no up-pull, the lower end of the warp lies on the
seabed and the otter board comes down on its bracket.
For a normal otter board weight in water of about
0'75 ton, the board will be pulled off the bottom when
the vertical up-pull significantly exceeds 0-75 ton. This
occurs for warplength to depth ratios lying below the
line C-C, say. If otter board behaviour depended only
on water forces arising from its motion through the water,
the up-pull would be required to vary as speed squared
to keep the board upright as speed varied. This would
demand an almost constant warplength to depth ratio,
irrespective of speed, as shown by the line D-D. Due to
the presence of weight and buoyancy forces, that do not
vary with speed, the variation required in practice will
be between that of line D-D and the values indicated by
an appropriate line of constant up-pull.
It is of interest to compare the theoretical estimates of
required variation of warplength to depth, with trawler
skippers' "rule of thumb" practice. This is done in
Fig. 14 for varying depths and characteristic trawling
THEORY US«NO f ORAO OP, OEAR EXCLUDING v
\ «AAPS»|4,000 X SPEEP, KNOTS) L»,
I ANP VlfclUTON FACTOR • »/4 .4 .
THEORY , lV4CIRC.»AR»t .l'/3 KMQT*
ft, » 1 71 GIVES A VERTICAL
UP-PULL AT EACH OTTER MARO
OF W4QO Lt.APPROX).
• PRACTICAL RULE USED SY A
i KMOTj
oo aoo
WtTIR DEPTH , FATHOMS
THEORY, I7* CIRC. MM. RP| . 1 KMC
-ifc - aio •ivin vWdSAL^
I UP-FULL AT EACH OTTER tOARD
DEPTH, FATHOM*
— THEORY, rente. «u»»- »'/a KNOT*
___frr-IOOOlV«* A VERTICAL
| OP-PULL AT EACH OTTER IDAHO
| Of fclflft Li. APPHOd).
+ SKIPPEA A\ SOME VALUES UtCO I
PRACTICE •» TWO
KAN Til,
Vfrtrtft DEPTH , FATHOM*
fig. 14. Comparison between theoretical variations of ratio of warp
length to water depth against water depth with values used by practical
fishermen.
170
speeds. It will be seen that the agreement is good, as
would be expected from the very extensive practical
experience upon which such rules of thumb are based.
A characteristic value for the weight in water of a distant-
water otter board is 1,700 Ib so that the lengths of warp
used in practice leave about 300 Ib of the board weight
to press on the seabed. This reserve permits a reasonable
variation of speed and water depth to occur, at a given
length of warp aft, without the board being pulled off
the seabed.
The behaviour of the warp in vertical projection has
been satisfactorily explained, assuming that, both with
and without vibration, and apart from buoyancy, the
resultant water force nearly enough lies in a plane con-
taining the direction of motion and the straight line
joining the ends of the warp, and is perpendicular to the
latter straight line, i.e., it is as Nj in Fig. 15a.
DEFINITION OF
DIRECTION* OF
WATER FORCE
COMPONENTS N, t Na
PQ IS LINE JOINING
THE ENDS OF THE
WIRE)
"4
** / \
^W^ / N, IS IN PLANE CONTAINING. \ S
^^/ PO AND DIRECTION OF MOTION. \£
/^^^ Na IS PERPENDICULAR TO PLANE OF >x/\.
V»lnl/ ^^^^ PQ AND TNE DIRECTION OF jS \
SEPARATION
ASYMMETRICAL
iOUNDARY LAYER
SEPARATION AS A
POUIILE CAUSE
OF Na.
SEPARATION
Fig. 15a. Components of water force acting perpendicular to a wire.
Returning to curvature in plan, it would appear, from
the experimental data so far available, that a "normal"
water force, N2 say, also defined as in Fig. 15a, may act
in addition. For example Fig. 15b gives ratios of theore-
tical spread at the otter boards, assuming N2 to be zero,
to the value which would be obtained from the divergence
meter if the warps were assumed to be straight in plan-
form. In the cases given values of about 1-15 are appro-
priate to representative towing speeds. The corresponding
electronic spreadmeter readings agree rather well with
the estimates, as shown by the test points obtained with
that instrument. In other cases not shown here, experi-
mental factors as high as 1*4 have been obtained.
Investigation of this matter continues and if it is found
that N2 zero theoretical values do not lie sufficiently
close to experiment the theory will be elaborated accord-
ingly. Nt would be non-zero if the "boundary layer"
1 1
WATER DEPTH , 0 RANGE*
I f
SO-ISOPATNOMS
' I
|
j
j i
Dive*
At M
tOINCt At
OASiWID
ttLE
OTTER
•OAROS
1 * " • •
1
NCAM
•LOCK
^^^X— —
T !
^>
I '
~*^
f
I i
"\
•LOCK
r — -
*"«Ste~— .
r
1
f
— «ss^
^v—
-1 i .. .
'1 4
2
^.Jj^"*^"'"
— — ^ 1 is
Q
— -^ — -
* 1
R
ELECTRONIC SPREADMETE
R READMGS • I
s
CORRESPONDING THEORETI
1
ZM. ESTIMATES-)-
'
i
I
I
j j
90 30
TOWING SPEED, KNOTS
Fig. 15b. Warp planform spread factors.
of the flow past the warp separated from the surface
of the wire at different longitudinal stations on the top
and bottom surfaces respectively, as illustrated in Fig.
1 5a. This might occur due to the variation in hydrostatic
pressure from the top to bottom of a transverse cross-
section of the wire, which arises from the density of
water, since this pressure change is of the same order as
the hydrodynamic pressures caused by the motion of
the wire through the water. Again the recently published
reference 7, of 15th February, 1963, describes how
stranded electrical cables, in air, can have this type of
asymmetrical separation, due to the lay of the strands
presenting different profiles to the fluid, at opposite
ends of a diameter of the cable.
In addition to the above possibilities, the lay of a
stranded cable whose axis is inclined to the direction of
motion is known to generate a hydrodynamic force, in
the N2 direction, by causing fluid to swirl round the
wire (see Fig. 16). The magnitude of this component
RADIAL FORCE
IIHCULATtONp
DIRECTION OF
CIRCULATION FORCE
POSITIVE
OF X*' CIRCULATION
react ON FORMftftD
«*M, LIFTING
•LEADING EOOt
ROPE AS SEEN FROM
FORWARD
(CONDITIONS AT
FORWARD WARP)
JO- ft AFT WAR*
APPROXIMATE DECLINATION ANGLE
CHANGCI DUE TO 'LAY CIRCULATION
PLOW' CPPKT
ROPE AS SEEN FROM
AtOVE
(RIGHT HAND
ORDINARY LAY)
of Nt is not likely to be very large, however, and would
be expected to make the planform curvatures of the two
warps different from one another, without appreciably
affecting the spread at the lower end relative to the
divergence angle. Asymmetry in the planform shape of
the two warps, arising from the swirl component of Nt
would be eliminated by using handed warps. Asymmetrical
boundary layer separation might be prevented by encasing
the warps in a smooth plastic sheathing. It would be
of academic interest to make experiments under these
conditions. The relatively small spread factor of about
1-15, that has been measured with traditional laid warps
(see Fig. 15b), is an advantage in practice since only a
small correction has to be made to the readings obtained
directly from the divergence meter, in order to obtain
the spread of the otter boards.
When a gear is turning, the spread at the lower end
can greatly exceed 1 -4 times the value given by the angle
between the warps at the block. This has been confirmed
by the electronic spreadmeter, and an extreme example
is provided by the technique of turning the ship at such
a high speed that the warps cross, and yet the boards
remain well spread.
2-2 Otter board forces and moments
The mathematical evaluation of otter board performance
is a complicated matter. In essence they behave like small
aspect ratio aerofoils but they operate at considerably
greater attitudes and over a considerably greater range
of angles of heel than is usual for the latter (see Figs. 1 7
and 18). Also the otter board may have appreciable
tilt so that it runs on its heel rather than with the sole
plate equally in contact with the seabed all along its
length (Fig. 19).
~T ZERO HEEL > t*EI
KFFICTIVE ATTITUDE
WHEN SOARO IS
°4 DISTURBED INWARDS
Fig. 16. Circulation forces due to warp lay.
Fig. 17. Typical hydrodynamic side force and drag coefficients for
flat rectangular otter boards of aspect ratio^I/2.
171
IFFICT OP VMtYIN* WA*P UIN4TH
(ntMn.Tt PO* A CAMMftf* ftOAftO)
THAWL tMECD
(KNOT!)
«ft»0 HMLtB
LIB ON .MACK
WATCH MFTH • M- «O FATHOM*
TftMML miO
(KHOTt)
Fig. 18. Variations of otter board heel with speed. Full-scale tests.
01
8
O
Ul
o
-2
-4
TRAWL. SPEED (KNOTS)
POSITIVE TILT
NEGATIVE TILT
Fig. 19. Some measured variations of tilt with speed, standard otter
board. Full-scale tests.
The large change in heel with towing speed, at a given
warplength and depth of water, is perhaps the most
immediately striking characteristic of the results shown
in Figs. 17 to 19. From the left-hand side of Fig. 18 it
will be observed that, when using a traditional warp-
length aft, a "standard" flat distant water otter board
was almost upright at the towing speed of about 3*5
knots, at which it was customarily used. From the right-
hand side of Fig. 18, a change of towing speed of one
knot requires a change in warplength of about IS per
cent in order to preserve a given angle of heel. This is,
of course, consistent with rule of thumb trawling prac-
tice.
ment but the change in warplength necessary to maintain
a given heel can also be deduced from theoretical curves
such as those of Fig. 13.
Referring to Fig. 17, the proximity of the seabed alters
the effective aspect ratio, defined as (heights/surface
area, as is the case also with an aerofoil, but, in addition,
so-called "ground shear" forces arise due to seabed
material piling up against the lower face of the board
and sliding along it, or due to the board catching on the
bottom and proceeding in a jerky fashion.
The "aerofoil type*' forces can be determined from
measurements of a model in a wind tunnel, a "ground"
being placed adjacent to the otter board to represent the
seabed (see Fig. 20). The quantities measured are side-
force, drag, lift, "pitching moment" and "yawing
moment", defined as in this figure.
(tAUNDEM HOt WIND TUNNEL TEST* WITH GKOUNO l»«$ENCt)
MAO COEFFICIENT, Cp
80* 10* -10' -10*
ANGLE OP HEEL,«*
SIDE
FOftCE
Fig. 21. The effect of heel on the hydrodynamic forces acting on a
flat rectangular otter board of aspect ratio==]/2.
Typical model test results for an unheeled otter board
are shown in Fig. 17, while Figs. 21 and 22 refer to the
effects of heel and tilt.
When presenting these figures, the customary fluid
dynamic coefficients have been used, instead of giving
forces in Ib or kg. Thus for example sideforce coefficient
^ sideforce, Y Ib, , . A« , .A r
cy = T — g (1.539 y )« e p 1S dcnsity °f watcr
in slugs per cubic foot (£ P is nearly enough 1-0 for salt
water), SB is the area of one side of the board, projected
on a plane through its leading and trailing edges, and
Vk is towing speed in knots. Similar definitions give the
drag coefficient CD and the coefficient of upward vertical
force, CL, which is near enough zero at zero heel (see
Fig. 21). By employing this type of presentation, and
making the usual assumption that the coefficients are
near enough independent of speed and of the size (but
not the proportions) of the board, the forces appropriate
172
USED TO DETERMINE LIFT AND PITCHING MOMENT
j TO MOMENT ARM
RIG EMPLOYED TO FIND DM* SlDEFORCE AND YAK** MOMEHT
TO MOMENT ARM
DEFINITION OF
FORCES- AND MOMENTS
DRAG
YAWING I SIDE FORCE
MOMENT
fig. 20.
(MOMENTS
VALUES A*OUT »/4
wind tunnel tests on model otter board*.
173
SAUNOERS-ROC WIND TUNNEL TESTS WITH GROUND PRESENCE
IO 2O 3O 4O O IO 2O 3O 4O
ATTITUDE,oC (DEGREES) ATTITUDE<*(DEGREES)
Fig. 22. The effect of tilt on the hydrodynamic forces acting on aflat rectangular otter board of aspect ratio === 1/2.
to any chosen speed and size can rapidly be deduced.
In practice the drag coefficient CD shows more tendency
to vary with size and speed than do CY and CL, but to
first order, constancy can still be assumed.
From Fig. 21 it will be seen that maximum sideforce
occurs when the board is heeled inwards (i.e., -ve heel
with bracket towards the seabed). A heel of about —10°
is appropriate to a practical attitude in the 30° to 40°
range. This maximum sideforce is associated with a
rather larger drag than occurs at the positive angle of
heel of the same absolute magnitude but for overall
force efficiency the warp should be long enough to allow
the board to be heeled a little inwards. This does not
necessarily increase the pressure of the board on the
seabed. The water force acts nearly perpendicular to
the board surface so that while lengthening the warp redu-
ces the warp up-pull, heeling-in gives a compensating
increase in vertical hydrodynamic force.
From Fig. 22, a small amount of tilt does not greatly
affect the force efficiency but the sole plate should be
kept at 5°, or less, to the horizontal. As might be
expected from theoretical considerations concerning the
effective aspect ratio of an aerofoil adjacent to a ground,
nose-up tilt reduces sideforce more than nose-down
tilt. The latter can lead to digging in however.
The centre of pressure at which the resultant hydro-
dynamic force acts can be determined from the measured
quantities, including yawing moment, and is as illustrated
in Fig. 23.
174
YAKOVLEV REP. I
FREY ft iOHLE REF. 3
MATROSOV R£F. 4
0-6
0-5
O 4
O 3
0-2
RESULTANT HYDRODYNAMIC
FORCE-
IO 2O IO 4O
ATTITUDE, « DEGREES
Fig. 23. Variation of centre of pressure with attitude: rectangular
flat otter boards of aspect ratio 4=7/2 with ground effect (from model
otter board tests at zero heel).
Influence of camber
So far the performance characteristics of flat otter boards
have mainly been discussed. The considerable increase
in sideforce that can be obtained by giving a board
curvature (i.e., camber) in longitudinal section is shown
by Fig. 24. It will be seen that if a moderate camber is
SAUNDERt -ROE WIND TUNNEL TESTS,
ZERO HEEL i ZERO TILT, WITH GROUND
AND WITHOUT APPENDAGES
(•6
12% GAMIER
SIDE FORCE
O IO 20 3O 40 50
ATTITUDE (DEGREES)
Fig. 24. The effect of simple camber on the hydrodynamic sideforce
and drag of aspect ratio^l/2 otter boards.
used, the drag penalty is small and the sideforce to
drag ratio is thus appreciably improved. These wind
tunnel results have been supported by full-scale measure-
ments. By contrast, neither our tests of oval boards nor
other tests which have been published show such a large
advantage. Furthermore, an oval board occupies a
space behind the gallows which can accommodate a
rectangular board of about 25 per cent more area, and this
alone appears to be enough to counterbalance the rather
greater sideforce coefficient provided by the oval board
as compared with a flat board. Referring again to Fig. 24,
it will, however, be seen that in order to achieve maximum
Values of sideforce coefficient it is necessary to tow the
board at an attitude of about 30°, as compared with the
40° or more that is usual with flat boards. The implica-
tions of this will be discussed in connection with the
stability of otter boards.
Figs. 17, 21, 22, 23 and 24 have been concerned with
hydrodynamic forces arising from flow of water past
the boards. Further attention to "ground shear" forces
will now be given. It is impossible to determine
whether a model simulation of the latter is reliable
without first measuring them full scale. Therefore, this
has been done, employing the specially instrumented
otter boards shown in Fig. 25. Five load cells, of the
Fig. 25. Instrumented otter board with covers removed to show load
cells for measuring ground forces in three directions and cut-outs
for tilt, heel and attitude recorders.
type used in the warps at the otter boards but of smaller
dimensions and capacity, are employed to measure the
ground forces that act on the sole plate in directions
parallel to the plate, and sideways and upwards perpen-
dicular to the plate (see Fig. 26). Some results are shown
FRONT ELEVATION OF MAR OF IQARD «IDE ELEVATION OF REAR OF JOARO
(NOT TO SCALE)
NORMAL FORCE f
LOAD CELL
FRONT ELEWION or REAR OF GOARD
•TANGENTIAL' FORCE
LOAD CELL ^
(TANGENTIAL* LOAD]
r
•TANGENTIAL*
'LOAD CELL
(NJ. FORCE DIRECTION! AM RELATIVE TO tOARD WHICH CAN
•E HEELED AND HAVE TILT.)
Fig. 26. Full-scale otter board ground force measurement instruments
(location and action).
in Fig. 27. It will be seen that the vertical component
can be about 30 per cent of the otter board weight in
water, while the "sideways" component can be about
50 per cent of the weight, and as such can provide a
considerable part of the overall. spreading action of the
board. As might be expected, the ground shear appears
to be more significant on muddy bottoms than on hard
bottoms. However if the board is made rather heavy
in water, so as to get a purchase on the mud, it can sink
in to about one-quarter of its overall height, thus tending
to offset the gain in ground shear by a loss in hydro-
dynamic spreading force. It is tentatively concluded that
when designing boards to achieve ground shear spreading
effects, careful attention must be paid to the type of
grounds on which it is intended to use them.
175
s 4 / \ f
TRAWL SPEED (KNOTS)
TANGENTIAL
•V
SOAftO HCCICD IN
TANGENTIAL
•OARO NEELED OUT
I
^ -~ ^^^NORMAL FORCE
GROUND REACTION
Fig. 27. Full-scale otter board ground force measurements.
2-3 Otter board stability
To design an otter board properly requires considera-
tion, not only of the spreading force and drag that will
act on it, but also of its stability. Static stability is
concerned with its ability to tow steadily at constant
values of spread and constant angles of attitude, heel
and tilt. Dynamic stability is concerned in particular
with three conditions of unsteady motion.
(a) When a board is shot, it must take up a series of
positions, as warp is paid out, that will bring it to a
reasonable spread and nearly unheeled condition
on the seabed. When the warps are blocked up
they will then be level.
(b) When a warp is shortened, the board should pull
off the seabed without heeling or tilting excessively,
so that it will continue to provide spread and can
go down on the seabed again without falling over
on its back.
(c) If the board is disturbed during steady towing, as
for example by being deflected or heeled over due
to an obstacle being encountered, it must auto-
matically return to its condition of steady motion.
The six equations of force and moment equilibrium of
an otter board have been formulated, from which the
bracket and backstrop locations necessary to provide a
desired attitude and angle of heel, at arbitrarily chosen
steady towing speeds, spreads and sweep tensions and
directions, can be determined. The calculated positions
are confirmed by tests on small models in our towing
176
tank. The same equations have been used to calculate
behaviour when pulling a board off the seabed, and in
shooting. The latter can demand wire attachment
positions that differ from those that would give maximum
force efficiency in towing. Some compromise is then
necessary. Model tests of shooting behaviour have been
made and help to clarify full-scale observations. It
must be emphasised that the increased force efficiency
of a cambered board arises from its forward surface being
at a smaller angle to the direction of motion than in the
case of a flat board, thus allowing the water to flow round
it more smoothly. The effect is reinforced by the smaller
attitude, 30° as compared with 40°, at which the cambered
board behaves best when towing. This reduction in
required towing angle, which, at least to some extent,
will be a feature of any board having high force efficiency,
tends to give too small an angle when shooting or hauling.
This can however be overcome by keeping more way
on the board. Prototype devices have also been used
with some success for reducing the attitude of the board,
relative to the warp and backstrop, as soon as it contacts
the seabed.
Stability in steady tow can be studied in terms of force
data such as those of Fig. 17. It will be observed for
example that if a standard otter board, fishing at an
attitude of about 40° is disturbed so as to decrease spread,
then due to its inward motion, effective attitude is
increased, spreading force reduces and the tendency to
decrease spread is encouraged. Such a disturbance
applied to a board which is fishing at attitudes below the
stall should, on the contrary, rapidly be damped out,
due to the positive slope of sideforce against attitude
in that region.
The warp and otter board force and moment equations
that have been elaborated are equally useful for designing
bottom gear and midwater trawl gear. The most suitable
aspect ratios (height2/area) for the different applications
are being investigated.
2-4 Width of mouth of net
Next attention should be turned to the relationships
between bridle tension and angle, board spread, spread
of the net mouth, and drag of the gear. Here, the term
"bridle" will be taken to mean the total wire system
between the wing end of the net, and the otter board,
including legs, cables or sweep wires, and otter board
backstrops.
The headline and ground rope of the net take up
planform shapes that are near enough catenaries with
linear extensions forward. This is reasonable bearing in
mind that the aft pull on these wires, due to the drag of
the net and its appendages, such as floats and ground
rope bobbins, will to a first approximation, be constant
per unit length of wire except in so far as it is concentrated
due to local grouping of the appendages. Confirmatory
experimental evidence has been provided by scratch
marks on the bobbins, full-scale, and by photographs
of tank models, such as illustrated in Fig. 31.
Formulae have therefore been developed on the above
basis, which relate:
Fig. 31. Model net in towing tank.
ground rope planform angle at wing end, 8g.
headline planform angle at wing end, 8h.
width between ends of headline, 2yh>
width between ends of ground rope, 2yg.
and ratio of ground rope width to headline width,
These quantities are defined in Fig. 28. Various headline
ground rope and leg lengths, and various proportions
of straight to catenary length for the ground rope,
defined by a quantity //, have been assumed, the headline
being taken as a pure catenary.
ATE NARY
. HEADLINE
(PULL CATENARY
ASSUMED
GROUND ROPE
SHAPES
fl — TOTAL STRAIGHT LENGTH
TOTAL LENGTH
— O IS A FULL CATENARY )
,G ROUND ROPE
Fig. 28. Definition of plan shape parameters used in Figs. 29-34.
(PDA EXPLANATION OF tVMMU Mt FW »•)
to 20 10
40 §0 iO
Sh(DEGREES)
Fig. 29. Headline and ground rope planform calculation results.
Some results are given in Figs. 29 and 30 together
with confirmatory test results obtained with small
towing tank models, such as that of Fig. 31. The model
test results given in the upper part of Fig. 29 refer to a
(DEGREES)
(DECREE*)
Jh (DIOftElt)
Fig. 30. Headline and ground rope planform calculation results.
177
Oranton type of gear, and indicate that the ground rope
ahead of the bobbins is nearly straight. Full-scale
scratch marks have shown this also. When a gear is
spread wider, // tends to zero however. The lower pan
of the figure indicates that when long legs are used the
wing end spreads at the headline and ground rope can
differ significantly, especially at the higher spread angles.
The nature of the dependence of yg/yh on leg length is
illustrated by Fig. 30.
A development of the theory has been used to calcu-
late the corresponding spread at the forward end of the
leg and cable system, and the planform angle of the cable
at the backstrop, taking account of the effect of water
forces in curving the leg and cable wires in planform.
To perform this calculation it is, however, necessary to
know the ratio of tensions in the head and toe legs. Load
cells mounted in the legs of full-scale gears have given
suitable values for this ratio. Some calculated results
for spread at the backstrops of the otter boards, in
relation to the spread of the groundrope and headline
at the wing end, are given in Fig. 32. The effect of
TRAWL iPUD • 1 KNOT*
ftFMCAD,
*yt
HIAD
LINK
•WtAO,
BOARD SPREAD, 2y( , (FEET)
Fig. 32. Calculated headline and ground rope spreads for a particular
trawl.
changing // will be seen. In practice the headline also
ceases to be a pure catenary, at large spreading angles, due
to constraints applied by the wings, or selvedge ropes.
Fig. 33 compares ground ropes spread against wing end
angle with towing tank model results. In this case a /*
of zero was found to be adequate. Fig. 34 gives estimated
otter board backstrop planform angle against otter board
spread for a given net and leg length.
It will be seen from Fig. 35 that the otter board
spreading force required to provide a given spread of a
gear of known drag can now be determined from the
warp and bridle theories and otter board data that have
been mentioned in this paper. In general allowance must
be made for board heel.
2-5 Drag of the net
It remains to relate net drag to its specification and mouth
height and spread. Both model and full-scale data have
178
OARD SPREAD/FEET)
30
to 20
£9 (DEGREES)
Fig. 33. Calculated ground rope spread with ground rope plan angle
for a particular trawl.
TRAWL SPEED - 3 KNOTS
BOARD SPREAD, 2y , (FEET)
Fig. 34. Calculated otter board backstrop plan angles with board
spread for a particular trawl.
been obtained on this, and theoretical estimates have
also been made, based upon tank model measurements, of
the forces on simple plane specimens of net webbing.
Some results are illustrated by Figs. 36 to 39.
Fig. 36 shows the variation of the resistance of a plane
net specimen having a constant characteristic setting
angle y of 30°, when fl, the angle of the plane of the
webbing to the direction of motion, varies between 0°
and 90°. Also shown are curves of theoretical and partly
empirical formulae for estimating the resistance.
As in the case of otter board forces, it is convenient
to use webbing force coefficients, rather than forces in Ib
of kg. "Twine coefficients*' are based on a "nominal
frontal area49 of solid material in a mesh, 2ad, where a is
MQUMIO MMtAOMQ POftCC
MUfT IOUA4.
jtMtjC* ^.{iOARD DRAft INTRODUCED
Attfi 2 ^ «•• Ja -AMUMED
OR MEASURED DRAG
OF NET I NET APPENDAGES
(E2 » ANGLE OP WARP TO HORIZONTAL
AT IOARD, AS DERIVED PROM WARP
LENGTH THEORY ( USUALLY OP ORDER IO*)
T, TAKEN TO ACT HORIZONTALLY,
I tg. 35. Simple illustration of otter board spreading force require-
ments.
MYAKC far. s )RCWLTJ
WTEMOLATtD TO 4-
ZERO ATTITUDE GlNlRALIZATION Of G.I TAYLOR (lUiF.
THEORY FOR HIGH SOLIDITY .
90 4O SO 60
ATTITUDE, d DEGREES
Fig. 36. Resistance of plane net specimens.
bar length between knot centres, and d is the twine dia-
meter. To obtain the drag per square foot of webbing,
it is necessary to multiply by d/a sin q> cos (p. If the area
of webbing is calculated at a standard setting angle of
45° say, then </> would be taken at this value in the multi-
plier, but not elsewhere.
It will be seen that at attitudes, 0, above about 30°,
a simple theory applies quite well. This assumes that the
water speeds up so that the same volume passes through
the area of the netting as would pass in the absence of the
netting, the bar and knot components of the webbing
having the same drag as would occur if they were each
tested separately in a stream of such increased speed.
At smaller attitudes the effective velocity of the stream
is reduced due to water being deflected round the sides
of the specimen. Sir Geoffrey Taylor has produced a
theory for wire meshes of high solidity (i.e., small ratio
of frontal area of holes to total frontal area) at 9=90°.
An attempt has been made to generalise this theory to
small 0 conditions, at which the effective solidity of net
webbing should be high. As will be seen, the peak
resistance given by the generalised theory lies close to
the dashed line empirical curve drawn through the experi-
mental points, but at smaller and larger 0's, it underesti-
mates drag. Finally a simple expression, similar to that
appropriate to large 0 conditions, can be derived for drag
at zero 0, assuming that the speed of flow of water past
the webbing is unaffected by the presence of the webbing,
and that the knots have a negligible effect. This agrees
well with experiment, for setting angles less than about
40°, which should generally bracket the practical range.
At large setting angles interference between longitu-
dinally adjacent webbing components is large, and redu-
ced forces are obtained.
It has been concluded that it is best to employ the
simple theories at high 6 and zero 0, and develop an
empirical formula based on experiment for the "transi-
tion" region lying between the two, such as that shown
by a chain dashed line in Fig. 36.
The sideforce and lift provided by plane net specimens
that have the axes of the meshes located asymmetrically
relative to the stream have also been studied in detail.
Analogous methods have been applied to complete
nets, and some full-scale drag measurements are com-
pared with a partly empirical formula in Fig. 37. This
case refers to a net of varying mouth area, due to varying
headline height. It will be seen that there is a considerable
component of drag even at very small mouth area,
together with a component that increases fairly slowly,
a>u
4-t
4-6
4-4
4*2
4*0
).§
FULL
MEA
SCALE
IURCMCNT
r© ci
s 1^ u
CANTON
kRCE MOt
TH AMA
NET
A
TOW
NG SPCE
D - 3'/2
KNOTS
A
/
/
'ION
/
V
sPRCDlC
/
'"
3*6
/\
3-4
/
4
O 100 200 3OO 40O SOO 4OO
FRONTAL AREA, ATf SQ.FT.
Fig.
37. Variation of drag of tut and net appendages with mouth area.
179
atfd almost linearly, with mouth area. It is believed
that when a net mouth is opened so wide that setting
angles of 45° or more occur, the drag may cease to
increase any further but such large setting angles are
unlikely to be acceptable for structural reasons. The
(iAUNDIM KOt TANK TISTt)
MtSHU Of •** UCMOTN " I • »
TWINE DIA. .-•<>»•'
DRAG, LB.
i^ ^^^
1 • ". J
/
V* VARIATION — *
(POM EQUAL MACS \
AT a FT./ SEC.) \/
/
/
J
f
^
X
o i a i 4 s
TOWING SPEED, FT. /SEC.
Fig. 38. The resistance of a simple conical net of fixed frontal area
(Sounders Roe Tank Tests).
data in Fig. 37 all refer to a single towing speed. If a
simple conical net of fixed mouth area is towed at varying
speeds, the resistance increases almost as the square of
speed, as shown in Fig. 38. A practical net tends to
reduce headline height as speed increases, however,
•OTTOM TftAWLWITH FIXED
HEADLINE MJOVANCY
^S
\
^r
>
IO
|
N:
HCAOLIN
HEI6H1
C
r
HEADLINE
HEIGHT
s AT
-^
CENTRE,FT.
DRAG 4
*/"
ALC
EMINTS
TONS
j
/
/
2
/
/
/
*
FULL SC
MEAiUM
o
o J a » 4 s
TRAWL SPEED, KNOTS
Fig. 39, The resistance of a practical bottom trawl net with frontal
' " * area reducing as speed increases.
180
and the effect is to give an increase in drag that is much
more nearly linear with speed (see Fig. 39). This has
already been mentioned in connection with Fig. 10.
Headline height can, of course, be increased by increas-
ing the number of headline floats or by using dynamic
lifters. A method has been developed for calculating
headline height from a knowledge of the characteristics
of the headline lifters, the details of the leg system, and
the design of the net wings. This makes it possible to
provide a bottom trawl of fixed mouth spread but with
any required height, up to say, 20 ft., without greatly
increasing towing power requirements.
3. CONCLUDING REMARKS
In this paper it has only been possible to give a limited
descriptive and selective account of the extensive
theoretical and experimental investigations that have been
made into the fluid dynamic and engineering principles
on which the scientific design of trawl gear depends.
The author claims that the requirements for a distant-
water trawl, given at the beginning of the paper, have been
met, and at the same time that a basic theory has been
provided from which gears having other requirements
can be designed. Furthermore the theory can be used to
analyse and predict the mechanical and structural pro-
perties of other proposed or existing gears. However, it
would be disingenuous to suggest that the achievement
of given engineering objectives will necessarily catch
more fish.
It would appear that a close co-ordination between
gear engineering design and fish behaviour projects is
necessary to achieve the type of gear that is efficient from
an engineering point of view, and also takes adequate
account of fish behaviour. For example, determination
of an optimum relationship between size of otter board
and size of net may not depend merely upon finding
which arrangements require relatively low towing powers,
and suit the nature of the seabed at the fishing grounds
under consideration.
This paper is published with the permission of the
Westland Aircraft Company, the White Fish Authority
and the British Trawlers Federation, in whose employ-
ment and for whom the work was undertaken. The
responsibility for any statement of fact or opinion is,
however, solely the author's.
References
Yakovlev, A.I. : Fundamentals of the calculation of the hydro-
dynamics of otter boards. Translated from Trans. Inst. Mar. Fish.
U.S.S.R., Vol. 30, pp. 61-76, 1955.
Walderhaug, H. Aa. : and Akre, A. : First IF report on otter board
tests. The Norwegian Ship Model Experiment Tank. Presented at
the IF meeting in The Hague, November, 1962.
Frey, K. and Sohle, H.: Model experiments with various forms
of otter boards, Schiffbau, Schiffahrt and Hafenvau 35, No. 4
(1934), A.C.S.l.L. Translation No. 991.
Matrosov, I. R.: Increasing the stability and spreading effect of
trawl otter boards by the use of a multiple-slotted shape with efficient
moments effects. From "Rybnoe Khozyaystvo", No. 9, September
1958, PRC/3667.
Miyake, Y.r On the plane nets I. 1. Resistance of plane nets in
water. Journal of the Imperial Fisheries Institute, Vol. XXIII,
No. 2. 1927.
continued on page 181
Some Comparative Fishing Experiments in Trawl Design
Abstract
A series of fishing tests was made to compare the relative merits of
a large lightweight wing trawl and a smaller and stronger otter
trawl, both nets suitable for a 180-hp ship. At first the comparisons
were made with the wing trawl skimming just clear of the ground
and the otter trawl in close contact with it. In presenting the results,
the author describes fully the methods he considers necessary for
the treatment of comparative fishing data. Though in some areas
and at some times the big wing trawl skimming the bottom did
very well, it was at other places and times outfished by the small
trawl in close contact with the bottom, even for species such as
whiting. A second series of tests was done with the wing trawl also
in close contact with the bottom. It now outfished the small trawl
steadily but suffered repeated damage when the small trawl did not.
The general conclusion is drawn that the wing trawl is about the
biggest (70-ft headline, 85-ft footrope) and the thinnest twine
(210/30 nylon) that can reasonably be used by that power of ship.
A case is made for carrying on board both a large wing trawl and a
small strong otter trawl for use on hard ground. Another series of
experiments was done to find the best mesh size for the square,
wings and belly of the net. The conclusion was drawn that, in the
northern North Sea, fishing for haddock and whiting, more of the
net should be in 31 inch mesh (89 mm), i.e., smaller mesh than is
present commercial practice, particularly along the sides.
Quelques essais de p6che comparative de chalut
Rfeunit
Une seiie d'6preuves ont 6t6 faitcs pour comparer la valeur relative
d'un grand chalut 16ger a haute ouverture et d'un petit chalut
de plus forte construction, tous deux de dimensions convcnant a
un bateau de 180 c.v. Les premieres pcches comparatives furent
exScutdes avec le chalut a grande ouverture effleurant le fond et le
petit chalut en plein contact avec le fond. En prdsentant les r6sultats,
1'auteur d&rrit les m£thodes qu'il consider^ comme n6cessaires
pour 6 valuer 1'information obtenue par des pSches comparatives.
Bien que dans certains lieux et en certaines occasions le chalut a
grande ouverture effleurant le fond pdchait tr6s bien, le petit chalut
qui pechait en contact avec le fond capturait plus de poisson en
d'autres lieux et occasions, meme pour des especes comme le merlan.
Dans une seconde sdrie d'dpreuves, le chalut a grande ouverture
op£rait aussi en contact avec le fond. II capturait alors constam-
ment plus de poisson que le petit chalut mais 6tait endommagd
tres souvent tandis que le petit chalut ne Tdtait pas. On peut con-
clure qu'un filet de 21 m de ralingue superieure et 26 m de ralingue
infdrieure, construit de fil le plus fin (210/30 nylon), est le plus
grand chalut qu'on peut utiliser raisonnablement pour ce type de
bateau. II est conseille d'avoir a bord d'un chalutier de ce type, up
chalut leger a grande ouverture pour les bons fonds et un petit
chalut robuste pour les fonds durs. Une autre s6rie d'exp6riences
6tait effectude pour determiner la meilleure grandeur de mailles
pour les diffferentes parties du chalut. La conclusion fut que, dans
le nord de la Mer du Nord, pour la peche de merlans et d'&glefins,
plus de filet en mailles de 3,5 pouces (89 mm) devra dtre construit,
c'est-i-dire avec des mailles plus petites que celles utilisees actuelle-
ment dans la peche commerciale.
Ensayos de pesca para comparer foraws de artes de arrastre
Extracto
S realiz6 una serie de ensayos para comparar las ventajas e incon-
venientes de un arte grande de pernadas muy altas y otro mas
by
W. Dickson
Marine Laboratory, Aberdeen
pequefto y robusto de puertas, ambos a prop6sito para un barco
con motor de 180 hp. Al principle, las comparaciones se hicieron
con el arte de pernadas altas pasando por encima del fondo y el
de puertas en contacto con 61. Al presentar los resultados el autor
describe con toda clase de pormenores los m&odos que considera
necesarios para el tratamiento de los datos de pesca comparativa.
Aunque en algunos caladeros en diversas ocasiones el arte de perna-
das muy altas que pasaba por encima del fondo dio bucnos resultados,
en otro lugares y mementos el arte pequeAo en contacto con el
fondo pescaba mas, incluso especies como la merluza americana.
Se realizd una segunda serie de ensayos con el arte de pernadas muy
altas en contacto con el fondo. £1 resultado fue que pesco mas
el arte de puertas pequefto pero sufri6 repetidamente averias que
el otro no tuvo. La conclusi6n general es que el arte de pernadas
muy altas es el mayor (relinga superior de 70 pies e inferior de
85 pies) y el de hilos mas fines (nylon de 210/30) que puede emplear
un barco de esa fuerza. Se dan razones convincentes para llevar
a bordo un arte de pernadas muy altas y otro mas robusto y pequefto
de puertas para emplearlo en fondos sucios. En otra serie de experi-
mentos se determind el tamaflo de mall as mas conveniente para
la visera, pernadas y vientre del arte, llegandose a la conclusibn de
que, en el septentridn del mar del Norte cuando se pesca eglefino y
merluza americana, una mayor proporc|6n de la red deberfa ser
de malla de 3,5 pulgadas (89 mm), es decir, mas pequefia que en la
actualidad, particularmente en las secciones laterales.
SIMPLY to make trawling gear bigger ought to increase
its catching rate, but there are attendant disadvan-
tages. The towing drag is increased, and there are extra
problems in handling gear and increased liability to
damage. The first is a hydrodynamic problem, the
second a practical fisherman's problem and the third
concerns strength of materials but also requires a know-
ledge of the seabed. Damage to netting does not
only arise from its interaction with the seabed, it also
arises from internal localized stresses resulting from
inadequate design in the shaping of the net. A gain in
fishing dimensions is often sought by the use of thinner
twine, but unless stronger material is used a mere 10 per
continued from page 180
Taylor, G. 1. : Air resistance of a fiat plate of very porous material.
Aeronautical Research Council reports and memoranda, R. and M.
No. 2236. 1948.
Curing long-span conductor oscillation. Engineering, 15 February,
pp. 250, 251. 1963.
Acknowledgments
The author would like to give the fullest acknowledgment
to the representatives of the White Fish Authority,
the scientific personnel of the Fisheries Laboratories,
and the many people in the fishing industry who have
provided information and assistance. The full-scale
development tests of gear, forming an important part of
the investigation, have been made on the Fisheries Re-
search Vessels, F.R.V. Explorer and R.V. Ernest Holt.
Finally, the extensive published investigations of earlier
workers in the field, especially the Japanese, Russians
and Germans, have been used as starting points for some
of the work described here.
1R1
cent advantage in twine diameter incurs a 20 per cent
loss of strength. The stronger synthetics allow a useful
strength gain with thinner twine and as important a
mitigating factor as their increased wet knotted strength
is the elasticity and ability to withstand shock loads,
possessed by some of them. Increasing the mesh size
is another way of keeping the drag down, but this may
lead to poorer shepherding in the forward parts of the
trawl and more escapes in the after parts.
This paper describes some experimental attempts to
determine optimum net size, twine size and mesh size by
comparing the catches of different trawls. Optimum size
had of course to take into account the power of the
trawler, in this case 180 hp.
(1) The purpose of comparing a small strong trawl
against a much larger lightweight one was to
assess the catch gain using a big net and to assess
the factors limiting its use.
(2) The purpose of comparing nets, identical except
for the different mesh sizes in their forward parts,
was to narrow the range of choice in this most
important feature of net design, even if only for
one or two important species in localized areas.
Comparisons between two nets are like a series of
football matches. Replays at different times and at
different places lead to different results, and past form
is no guarantee of the results of the next match. When
doing comparative fishing tests on a research ship,
where the attempt is made to measure every fish or else to
sub-sample accurately, a great deal of data is accumu-
lated. The condensing and presentation of this in meaning-
ful form is not easy and, oddly enough, considering the
large amount of data to start with, there rarely seems to
be enough processed material at the finish to be certain
of the conclusions that can be drawn from it. When
comparative fishing with one boat only, the variability
in catch between hauls demands a large number of
replicates, which are seldom achieved! In short, most
answers provided by comparative fishing are provisional.
Any net catches a biased sample from the sea. It will
be better at the capture of some species than of others
and quite apart from codend mesh size, it may tend to
take bigger rather than small fish or vice-versa. In tests
like this the codend mesh size is made small enough to
take everything of interest. Alternatively, the codend is
fitted with a small meshed cover. The objective of this
type of comparative fishing experiment is to discover
the nature and degree of the main bias and to look for
explanations of it.
Wing trawl and otter trawl
The Vinge or wing trawl with its forked wing ends is
used primarily by the continental cutters as a herring
trawl and it was as such that it was first used on our
research vessels, Clupea and Mara, though it soon be-
came obvious to ourselves and others that it had poten-
tialities for demersal species as well. It is a big and
rather complicated net of lightweight material. When
rigged for herring trawling, the net floats (i.e., flotation
182
outweighs the sinkers and altogether the net has positive
buoyancy). The long danleno poles are weighted to
hold the bottom, while the footrope rides clear of it,
save for the loosely hung bights of chain, which must be
just polished at the bottom of the bight. When fishing
for whiting and haddock, the danleno poles are seldom
used.
It soon becomes obvious that some fish are escaping
under the footrope; the temptation is to bring it close to
the ground. The dilemma is spotlighted in Fig. 1, taken
Fig. L Haddock on stony bottom, unfishable by light trawls.
from a flash camera mounted on the trawl headline.
Haddock arc very often as close to the bottom as this
and their chance of escaping under a footrope skimming
one foot clear of the bottom is good. Put the light foot-
rope of a lightweight wing trawl on to such a bottom,
and the net will be severely damaged.
In the first series of tests the wing trawl (Fig. 2) was
kept buoyant, while the otter trawl (on the left of Fig. 3)
was in close contact with the bottom. The tests were
confined to daylight and to good ground. The nets were
tested at the same engine power so that the big net was
towed slightly slower in the ratio 3-4 to 3-6 knots.
Headline heights and spreads are given later.
Method of presentation
(a) Circles are used to illustrate species composition.
The overall area of the circle represents the total number
of fish caught on an hourly basis and it is segmented
by proportion into species. Note that this does not give
the relative commercial value of the catches, because a
small fish counts as a unit here the same as a big one,
and some species have a greater market value than
others. Some of the more interesting examples are shown
in Fig. 4. The circles give the average catching rate over
a period of several days fishing in each area.
(b) Histograms are used to show the number of fish
of each species caught in each one-hour haul. Looking
at these gives some visual idea of the quality of an experi-
ment. It shows how many hauls were made, whether
the gear was changed over reasonably often, the varia-
bility from haul to haul and whether the average catching
rate and species composition as shown by the circles is
likely to have been biased by one or more hauls greatly
HEADLINE- loft (30fl>loft.+3oFt,)
WIN<JIM>f>ES
SCALE.
ft.
f r
Skimming tht (jround
Ntt Buoijont'
2«t ih Plojtic Floor*
m. Plosh'c
orrtr Btord
f tf.t/h « 5Ff.3in.
onfh«
Rig ustd oh Cofchij ground
Fig. 2. Wing trawl.
183
lom
All aooyi/lb. Manilla
txctpf" inn. Mtsh loud./lb.
r.f
JWlS
fc»*»
offcrbtoro!
Ofr. 3in,
. Cokk
5ft DM Ltno
srick
tin.
Fig. 3. Small otter trawls, with different mesh sizes in their forward parts.
larger than the others. Histograms are shown in Figs.
5, 6 and 7. For convenience in presentation, their heights
are shown logarithmically, which means that the net
which is doing better is doing even better than appears at
a glance.
(c) Length composition curves show how the catch of
each species is divided up into the numbers appearing
at each centimetre length. This can be done for every
haul, but results are usually summed for groups of hauls
CLYDE - January 1959.
Whiting
Haddock
Fig. 5. Histograms, haddock and whiting, Clyde, January 7959. Plain
column indicates wing trawl. Black column otter trawl.
184
Cruden Bay
Sept. 1959.
Cruden Bay
Oct. 1959.
HIITINC
O Wing Trawl
• Otter Trawl
Fig. 7. Histograms, whiting only Cruden Bay, September and October,
7959.
in one area at one time. There are two ways of presenting
length composition curves each having advantages. The
length composition can be shown proportionally for
every 10,000 fish caught with each trawl and this is the
best way of showing the bias in sampling between the
two trawls irrespective of quantity caught, or it can be
BUCHAN DEEPS - Early Sept. 1959.
Whiting - left. Haddock - right.
BUCHAN
Mid-Sept. 1959.
Whiting - left
Haddock - right
tor Trawl
Fig. 6. Histograms, haddock and whiting, Buchan Deeps, early and mid-September, 1959.
185
1151
U flit Wkariour
MM M* and tton th« otter
ia Mwe«Mi
f i.hing grounds
CRUDEN BAY
ecr.
Offer TKOW!
32U fish
F#. 4. Oi/c/r composition. Wing trawl buoyant. Otter trawl on
ground.
shown on the basis of the hourly catching rate, which
shows not whether one trawl caught proportionally
more big ones, but whether it actually caught more
big ones. The number of graphs is too large to present
them all here. All showed bias in the same general
direction and a selected few are shown in Figs. 8 and 9.
EHscusskHi of the test results
The wing trawl, skimming clear of the ground, has a
more restricted range of species appearing amongst
its catch. Although it can at times outfish a small otter
trawl, particularly for whiting but to a lesser extent for
haddock also, it does not always do so and is sometimes
quite heavily outfished even for these species, presumably
when they are in close proximity to the bottom. This is
the essential point to be taken from Fig. 4.
The length composition curves (Fig. 8) show that the
wing trawl tends to take proportionally more of the
bigger whiting and the same was true for haddock to a
lesser extent. The curves in Fig. 9 show that even when
outfished in the Clyde the wing trawl took nearly as
many large whiting and when it outfished the otter
trawl in the Buchan Deeps, it took no more small ones.
The bias is noticeable even in the sampling of the baby
186
I
L
Fig. 8. Length composition per 10,000 fish whiting, Clyde, Buchan
Deeps and Cruden Bay, 1959.
J
k. *«
i
.1 Firth o.' Clyde J. in. 1059
r« J !„ ' », l.
fish Ltnqth - cm
Fig. 9. Length composition per one hour fishing, whiting, Clyde and
Buchan Deeps, 1959
brood. There were at this stage of the experiment a
number of possible explanations, some of which are
ruled out later. They may be sought in differences in
fish availability to the nets, differences in avoidance or
differences in mesh selection.
(i) The smaller fish may keep closer to the ground
thus being more likely to escape under the ground-
rope of the wing trawl.
(ii) If the bigger ones swim higher up, they may be
more liable to capture by the wing trawl with its
considerably greater headline height, in the ratio
of about 8 ft as against 5 ft.
(iii) The wing trawl has a greater headline spread than
the otter trawl in the ratio 35 to 25 ft, so it is
possible that the bigger and faster-swimming
fish have a better chance of being shepherded
from the region of the wings into the path of
the bag, while the smaller ones are more likely to
pass through the wings. The thinner twine used
Urn
|il\ **J<
rfr Slock
r **** . /
r60 3Vfn. /I-.3
60m
itfr Slock
(n *k ft. * cafr * nil ft.)
Seek
ao jo
t»
i». ?Ustk Float*.
7fhv Calk
. 70. 5wa// hard ground otter trawl.
in the wing trawl might enhance such an effect,
(iv) The bigger faster-swimming ones may find it
easier to avoid the smaller net.
(v) Jt may be that more small ones succeed in escaping
from the lengthy bag of the wing trawl before
reaching the codend.
A new test
It seemed that something more might be learned if the
two nets could be fished, both with negative buoyancy
and with the same groundrope firmly on the bottom.
Since the wing trawl is quite vulnerable and ought to be
kept clear of the ground, it was decided that both nets
should be rigged with a groundrope comprising 14 in
rubber bobbins in the bosom and 12 in rubber bobbins in
the bunts. (See Fig. 2 for the details of the large trawl
and Fig. 10 for the details of the small trawl.) Note that,
in this series of tests on rougher ground, the forked wing
ends of the big trawl were sewn up to make the net less
liable to snag on the bottom, while the wing ends of the
small net were opened out.
The ground chosen for this comparison was catchy
in places without being really rough. In all 28 hauls were
made on this ground with each net, and on five the large
trawl was seriously damaged. Working on a commercial
basis, it would not have been possible to carry on using
it on that ground, and it was only possible in this instance
by having net riggers ashore mending steadily. The small
trawl was scarcely damaged. It is now suspected that,
even without the net snagging on the bottom, sudden
change of shape of the groundrope as it meets an obstruc-
tion may burst the belly of a lightweight net, even though
the bobbins later surmount the obstacle; and indeed,
when they do surmount it and rush to re-assume an
Fig. 11. Elongation of the meshes where the lower wing Joins the belly
bosom.
187
Btnachit Jlrounu ^tpt.l
J. • All Plat Fith
'A "^
Groundrope Approaching JJnag
Qrouhdrope Utrikea
Snag and Distorts*
Fig. 12. Groundrope meeting obstacle.
even towing position, the resulting localized stresses may
cause further tearing. Fig. 11 although photographed
on a heavy Granton trawl, shows the elongation under
localized tension of the meshes behind the groundrope
quarter (position A in Fig. 12) such as occurs in most
nets. Suppose now the groundrope encounters an obstacle
at X; the groundrope rapidly changes shape and A moves
rapidly forward increasing the localized stress. It is clear
that a lightweight net is more liable to rupture than a
stronger one, that nylon with its ability to withstand
shock loads is a useful material in such situations and that
prestressing, due to inadequate design in such an area
of net liable to be shock loaded, is a source of weakness.
Means of relaxing these localized tensions have been
discussed elsewhere. (Dickson 1961, Garner 1962.)
Further discussion of test results
The species composition, the catch histograms and the
length distribution curves for the new tests with the
groundropes of both trawls firmly on the bottom are
shown in Figs. 13, 14 and IS.
The species composition for the large wing trawl is
now quite different from what it was before. Flatfish
are now making a significant contribution to the catch.
The catch histograms show a favourable result for the
large trawl on this occasion, but remember it was at a
cost in net damage that would have been prohibitive
commercially.
The bias in the length distribution curves is in the
188
Whltiiig
Fig. 13. Histograms, whiting, haddock and flatfish, Benachie Ground,
September, 1961, plain white column is wing trawl and black is otter
trawl.
I*fti Otter Trawl
240 fi eh/hour.
Rights Wing Trawl
735 fi«h/hr.
Fig. 14. Catch composition: both trawls rigged with the same bobbin
groundrope, Benachie Ground, September, 1961.
same direction as when the wing trawl was buoyant.
The possible reasons previously given for this can now be
reviewed. No. i is ruled out because there is now no
reason to suppose that small fish escape more readily
under the groundrope of the wing trawl as the ground-
rope was the same for both nets. No. ii seems unlikely
as a general reason since the same bias appears to be
present in the case of flatfish, which are less likely to be
stacked so that big ones are more susceptible to capture
by a high headline net.
g «-.
L
I -
i
FuK Ufttth.cm.
Fig. 15. Length composition per 10,000 fish, whiting and flatfish,
Benachie Ground, September, 1961.
The general conclusion drawn from the whole series of
tests is that, while a large lightweight wing trawl can be
very effective at times, its use even when rigged with
bobbins is limited to fairly good ground. It seems a
sensible practice to equip ships with both kinds of bottom
trawl or at least to hold them ashore in readiness against
the seasons and the fisheries where each type is required.
There is no such thing as a wonder trawl which is a
winner in all circumstances.
If two such nets are used it is necessary to make the
nets in synthetics so that the one does not rot while the
other is in prolonged service. This at least is now the
practice adopted for our research trawlers. The tests
also set limits to the sizes of twine that it was found
reasonable to use in the large and small nets towed by
a ship of 180 hp. The small net, made in 210/60 nylon
twine and with 210/108 nylon in the front part of the
belly, is a very strong net suitable for using on unknown
rough ground. The trawl is then rigged with bobbins of
course. The big wing trawl was found to be strong enough
for herring trawling with the top and lower wings, the
square, batings and belly all made in 210/30 nylon,
but when working for demersal species it was found better
practice to make the lower wings and belly in 210/45
nylon.
The otter boards, suitable for that size of ship, only
spread the headline of the wing trawl to half the headline
length; it follows that there is not much point in making
the net any bigger.
Having thus reached acceptable upper and lower sizes
for the nets and the twine of which they should be made
(Figs. 1, 2 and 3), the next thing to look at was how the
mesh size should be distributed in the plan of a net.
Mesh size distribution
The usual practice in North Sea nets is to grade the
mesh size from 5i or 5i in in the wings down to the
regulation size for the codend. The grading is usually
done in the belly. With hand braiding it is still a common
practice to change the mesh size in several steps Without
putting in any gaining meshes across the join. As the
tendency towards machine made netting grows, so does
the tendency to have as few mesh sizes in the net as
possible, and this necessitates gaining meshes in the
joining rows. In the ports where haddock and whiting
are the most important species sought, it is a common
practice to have the square and upper body in 3i in
mesh and to grade the mesh size in the lower belly only.
Thus apart from the top wings, the roof of the net is in
small mesh while much of the side formed by the lower
wings and belly is still in large mesh.
With haddock and whiting as the species sought, the
tests were to try and find out whether it was advantageous
to carry the small mesh further forward and to include
the sides of the net. On the other hand it might be possible
to have the square, top and lower wings in much larger
mesh. To try this a net was made with these parts in
11 in mesh; but, so that it would fish at about the same
dimensions as the 5i in mesh net, the twine size of these
parts was also doubled in diameter. This makes a very
strong net. The specifications of the 3£ in mesh and the
firth pf ci/4« toroh 1959.
«l Ortany April 1959.
Fit
. 17. Length composition per one hour fishing, haddock and whiting,
, East Orkney and North Minch, 1*59, Ji to., 51 in, and 11 in
i , mesh nets. '
489
Firth of Clyde - March 1959.
HADDOCK
UK A" MoK Nit.
East Orkney- April 1959
l*SL
5 imj
North Minch - July 1959.
Fig. 16. Histograms, haddock and whiting, Clyde, East Orkney and North Minch, 1959. In the lower histogram the bottom left and right refer
to whiting and the centre to haddock.
1 1 in mesh net are given in Fig. 3 along with that of the
5 1 in mesh net. As compared with the 5^ in and the
1 1 in, the 3| in mesh net loses a little in headline spread,
when being towed. It probably loses a little in speed
too because of the increased net drag.
Discussion of mesh size results
The catch histograms are shown in Fig. 16 and the length
composition curves in Fig. 17. These curves are lumped
together from others, all of which showed the same
general bias though taken in different areas. The sur-
prising thing is how little difference there seems to be
between the catching rates of the three nets for small
fish and how the gain in carrying the sides and roof of the
net forward in 3| in mesh is for the larger faster-swim-
ming fish. Even some marketable fish can, of course,
pass through a 3| in mesh net. The selection factor
(defined as the ratio of fish length to mesh lumen length
190
at which 50 per cent of the fish escape through the meshes
of a codend and 50 per cent do not) may be taken as 3-3
for haddock and 3-7 for whiting. Allowing for the size
of the knot, the lumen size of 3| in mesh is 82 mm,
so that haddock as big as 27 cm and whiting as big as
30 cm could be expected to escape; similarly for S{ in
mesh the haddock and whiting sizes are 44 cm and 49 cm.
Again an explanation of the length composition curves
is uncertain but the bias is in the wrong direction for it
to have been due to mesh selection in the forward parts
of the trawl. Nor does difference in availability seem
likely with the fishing dimensions of the three trawls so
similar. This leaves difference in avoidance as the most
likely explanation, possibly the bigger faster-swimming
fish putting on a spurt to carry them out through the
square and bunts of the large mesh nets.
continued on page 191
Development of an Improved Otter Trawl Gear
AMract
Experiments have shown that the traditional trawl gear has several
shortcomings. The trawlboards tend to dig unevenly into the
bottom, following a zigzag course, thus causing constant changes
in the net opening; the net shows bulges during operation which
causes gilling of the fish; the waterflow inside the net is not evenly
distributed, and is much slower than the towing speed, which causes
turbulence. Studies of the net's behaviour at relatively high towing
speeds led to an improvement in the design of the gear. In this
paper the author gives details of the construction of a four-seam
high-opening trawl and of the concave trawlboards employed.
Experiments conducted with this gear showed that the difference
between the waterflow inside and outside the net was reduced to
0.3 kn. at a towing speed of 4.5 kn. This design has now been
adopted by trawlers in the East China Sea.
Etude du nouveau dessin d'un engin de chalut a plateaux
Resinn*
Les essais ont montr6 qu'il existe plusieurs defauts aux engins
traditionnels du chalut. Trop souvent les plateaux s'enterrent au
fond provoquant une course instable et des changements frequents
dans 1'ouverture du chalut ; le filet se bombe et les poissons s'emmail-
lent. Le cours de 1'eau & 1'interieur du chalut n'est pas uniforme
et beaucoup moins rapide que la v61ocit& du remorquage ce qui
entraine la turbulence. Apres 6tude du comportement du chalut
6 des vitesses relativement elevees, on a dessin£ de meilleurs engins.
L'auteur donne des d6tails sur la construction d'un chalut & quatie
coutures et a grande ouverture, ainsi que sur les plateaux de forme
creuse employes. Les essais efiectuds avec cet engin ont ddmontre,
que la difference entre le cours de Peau £ ttnterieur et & Fexterieur
du chalut a 6t6 reduite & 0.3 noeuds, & une v£locit& de remorquage
de 2.5 noeuds. Ce nouveau dessin a 6t6 adopt£ des maintenant,
pour les chalutiers de la mer orientale de la Chine.
Fabricacion de mejores artes de arrastre de puertas
Extracto
Los experimentos han demostrado que los artes de arrastre tradi-
cionales tienen varios inconveniences; las puertas tienden a
enterrarse en el fondo y causan cambios constantes en la abertura
de la red; los paftos se comban durante la pesca y los peces se
enmallan; el flujo de agua dentro del arte no es uniforme y es
mucho mas lento que la velocidad de remolque, causando turbulen-
cia. Como resultado de un estudio del comportamiento del arte a
velocidades de remolque relativamente elevadas, se han proyectado
artes mejores. El autor da detalles de la construcci6n de un arte de
cuatro relingas de contorno, y gran abertura en altura, asi como
de las puertas c6ncavas empleadas. Los experimentos realizados
con este material demostraron que la diferencia entre el flujo de
agua dentro y fuera de la red se redujo a 0.3 nudos a una velocidad
by
Chikamasa Hamuro
Fishing Boat Laboratory, Tokyo
dc remolque de 4.5 nudos. Este arte lo emplean ya los arrastreros
que pescan en el mar del este de la China.
OTTER trawling and two-boat trawling are widely
used in Japan and bottom trawling accounts for a
major share of the total catch of commercial fishing.
The design and operation of the otter trawl introduced
in 1904 has undergone many changes and improvements.
The nets are made of synthetic twines, and V-D gear has
replaced the traditional leg connection between net and
board but, until recently, very little has been done to
improve the general design and structure of the net
itself.
The Fishing Boat Laboratory undertook a rational
study of otter trawls and by February 1961 had completed
the design and construction of a new modified trawler
gear. This gear was tested from the newly-constructed
No. 50 Akebono-Maru, 1,500 tons, 2,000 hp (Fig 1), of
the Nichiro Fisheries Company and has since been
fishing better than the old gear both on the large stern
trawlers and medium-size side trawlers operating in the
Atlantic and the Bering and East China Seas.
Defects of conventional otter trawls
The general shape of the trawl in action is such that it
continued from page 190
Whatever the reason, it does seem to be profitable, at
least over this size range of fish, to carry the 3£ in mesh
fairly well forward in the top and sides of the trawl. The
limitations on how far forward to bring it may be set
by the amount of mending that has to be done and it
seems likely that part of the belly at least should be in
larger mesh to allow the bottom rubbish to fall through.
The design currently favoured, and used by us for a
North Sea hard ground otter trawl, is of the style shown
in Fig. 10, though it need not be so small. As experiments
along these lines are continuing, the design is not a final
one but it synthesizes what has been learned to date. It is
necessary to remember the limitations of these tests;
they were done in daylight only; they cover only two
species in a limited range of areas and not exhaustively
even there.
A knowledge of fish behaviour, however acquired,
helps in the design of trawls, but so does the fishing of
trawls whose operational dimensions are known help
in the understanding of fish behaviour, always provided
that the trouble is taken to analyse the catch. The design-
ing of nets with greater headline height, greater spread,
etc., is a barren occupation unless the designs are put to
the test of fish catching and put to the test in such a way
that conclusions can be drawn.
References
Dickson, W.: Trawl performance— a study relating models to
commercial trawls. Department of Agriculture and Fisheries for
Scotland, Marine Research Series, No. 1, pp. 30-31. 1961.
Garner, J. : The problem of excess load at the quarters. World
Fishing, Vol II, June, pp. 67-72. 1962.
191
Ffr. /. No. 50 Akebono-Mam.
cannot readily be predicted by mathematical formulae.
Nor can all the various factors which together compose
its catching efficiency as yet be identified and defined or
evaluated mathematically. No part of the gear, for
instance, should interfere with the entrance of fish till
caught in the codend; yet little is known of the precise
reaction of the fish when it detects the gear either by
sight, noise, smell or feeling. From long observations
and experimenting with the gear, the author defined the
trawl boards, bridles and the shape of the net as detri-
mental to efficient operation of the gear.
Trawl boards — These are dragged at a certain angle of
attack over the bottom and should follow a straight
course. This is, however, not always the case. Referring
to Fig 2, every time the board cuts deeper into the sea-
bottom the angle of attack forces the board outward in
the direction of D. The warp tension eventually
overcomes this ploughing effect, and the board moves
back and beyond its original track so that the board
follows a zigzag course. The zigzag movement is increased
if the point of attachment of the warp is below the centre
of the board since this causes the board to tilt out and
plough deeper. Other important factors are the weight
of the board and the width of the shoe.
Fig*
Flg.3
Fig. 2. Course of trawl boards.
Fig. 3. Headline-height changes with conventional-type trawl board
during a four-hour haul.
With the two trawl boards so operating, the distance
between the wing tips of the net is continuously changing.
A narrowing of the net opening results in a relative
increase of the headline height and these fluctuations can
be clearly seen in Fig 3 which represent the net height
192
changes of a conventional type trawl during a four-hour
haul. The conventional height/length relation of 1 :2 for
trawl boards is not very efficient and, furthermore, flat
plates do not give the best shearing power; all of whicljt
results in oversized boards.
Bridles— With the single bridle method, the legs from
the wings are joined at the bridle or to a relatively small
danleno. It is better to take full advantage of the board
height to allow the wings to rise as high as possible,
although this means using two bridles instead of one.
The connection of board to net with two lines, further-
more, helps to steady vertically the board in action. For
this purpose it is better to have a large value for the
ratio H/L of the boards.
Shape of the net — Measurements have shown that
conventional nets, during operation, take a bulky shape
in the forebody with a pronounced narrowing in the
after body (Fig 4). This billowing of the front part
Fig. 4. Shape of conventional net in operation.
Fig. 5. Two-boat trawl being towed at high speed with too great
distance between boats.
of the net is caused by choking up of water which can-
not flow freely through the net. Under such circumstances
the waterflow to the codend is disturbed and causes gilling
in the tapered body of the net. To avoid such gilling the
mesh size is usually decreased which in turn causes even
more resistance and turmoil which impedes capture.
Traditionally, bottom trawls are made of an upper and
lower part laced together at the sides. With such structure
it is difficult to distribute the waterflow evenly through
all parts of the net and the rear part is therefore flattened
out.
When a net is dragged at low speed, the net releases
through its meshes an amount of water which is propor-
tional to the area of the net opening and the towing
speed; the waterflow inside the net is then the same as the
towing speed. However, this is not the case at higher
towing speeds. Measurements with flowmeters showed
that in a 36-8m headline trawl towed at 3-5 kn speed,
the waterflow had decreased to 2*7 kn already at a
distance of only 2 m behind the footrope.
When the volume of water entering the net is greater
than that which can be released by its webbing, a turmoil
is started which at high speeds spills some water back
through the net mouth, possibly taking with it some of
the already caught fish. Pressure builds up inside the net
which can prevent fish from entering and tends to
frighten strong swimmers away from the net entrance.
The above is similar to what happens when a plankton
net is towed too fast. To conclude, differences between
waterflow in the net and the towing speed must be a
minimum in a well-balanced trawl; the measurements
performed on the conventional otter trawl showed a
difference of 0.8 kn which points to poor water release.
Comparison with two-boat trawl
Since the net is towed by two vessels, the net opening is
practically constant (both in horizontal and vertical
direction) and can be regulated to the shape for which
it was designed.
With this type of net the length to breadth relation is
much bigger so that there is better water release. How-
ever, when the distance between the boats is exaggerated
at high towing speeds, the net shows the same defects as
those of the otter trawl (Fig 5). The vertical con-
figuration of the webbing is much better than with the
otter trawl.
Measurements of the waterflow inside a two-boat net
gave 1.9 kn when towed at 2.3 kn. When large-mesh
side panels were inserted at the waterflow inside increased
Leg rope
Pendant wire
Ground rope
Current -^
Fig. 6
Fig. 6. Pattern of water-flow in conventional otter board.
to 1.94 kn at a towing speed of 2 kn, practically
equalizing the waterflow inside and outside the net.
Furthermore, the large-mesh side panels gave good
height.
This indicates that a four-seam construction method
for two-boat trawls has advantages over the conventional
two-seam belly and back structure.
Improved otter trawl
Based on the experiments, a new four-seam net was de-
signed and constructed and operated with concave
boards of a special design.
1.1-
1.6-
14
1.2
VWtr flow
(itsidt tfctntr
Fig. 7. Water-flow characteristics of old-type trawl, two-boat trawl
and new-type net.
Otter boards— The boards were constructed with H to
L ratio of 1/1.12 which had been found to give a good
stabilizing effect during operation. They were equipped
with flat shoes to avoid ploughing and to minimise the
friction with the seabottom so that the boards would
keep a more stable course in relation to the towing
direction. Construction details of the new trawl boards
are given in Figs 8, 9 and 10. The boards were made of
steel and weighed 1200 kg in air.
Net — The net body was designed to be about six times
as long as the fishing width of the net at the lower bosom,
to achieve the required water release. The present net
has a total length of 75.6 m (252 ft) from wing tip to
Fig. 8. New-type otter board
end of codend; the bottom net from footrope bosom
to end of codend is 40 m (133 ft) and the headline is
68.33 m (228 ft). It was estimated that with 250 kg
buoyancy the headline height at the wing tips would be
5 m and at the bosom 8 m. An elevator (Fig 1 1) was
mounted on each wing tip during some of the experiments
to evaluate its effect.
To achieve a smooth flow of water inside the net and
193
Fi£. P. Construction details of new trawl boards.
to avoid bulging, the mesh-size of the side panels was
larger than that of the adjacent webbing of the top and
bottom webbings, and differed by 36 mm at the opening,
scaling down to 6 mm at the codend.
The whole net was made of polyethylene twine of 200
denier filaments of suitable sizes (Fig 12), to obtain
lightness and lift. Except for the wing tips and selvages,
all the webbing was knotless.
Assembly — The net was connected to the boards by
legs of 120 m (400 ft) length, giving an estimated net
opening at the wings of 26 m (87 ft), with a distance
Fig. 10
Fig. 10. Pictorial view of trawl board.
of about 70 m (233 ft) between the boards. Each leg
passes through the stopper rings of strops attached to
the board and during shooting it is stopped by the
shackle joining the leg to the pennant (Fig 13). The
upper strop is 5 m and the lower is 4 m long, giving a
To otttr board
upptr leg
To otter board
lower leg
Fig. 11. Hydro-dynamic elevator.
Fig. 13. Connection of net to boards.
194
Fig. 12 Design of new
four- seam otter trawl.
— Headline: Wirt rope 14mm
— footrope :
Wing
Bosom
18mm fll
11 20mm f $£
— Seamllne • Polyethylene rope 29 mm
— Codlint = Polyethylene rope 16 mm
• — Webbing : Polyethylene 200 denier
V0
m
E
oB
E
(M
o
CM
CE
195
Figs. 14)15. Shooting and hauling sequences.
difference of 1 m between the upper and lower connec-
tions of board and net. The shooting and hauling
sequence is illustrated in Figs 14 and 15.
Experimental results — To ascertain the operational
behaviour of the gear the following automatic measuring
instruments were used:
recording net-height meter
„ depth meter
„ water flow meter
„ dynomometers
Both V-D and two-leg connections were used to
investigate the effect on net height. During a third
series, hydrodynamic elevators were attached to the
with one leg
with two legs
with two legs
Cm 20 <<& CD
Fig. 16. Headline-height measured by recording net-height meter.
106
wing tips. Numerical values obtained during the experi-
ments are given in Tables I, II and III.
The horizontal net opening at the wings was calculated
from the observed warp angles obtained, and was found
to be slightly above 26 m, though decreasing somewhat
at increased towing speeds. At this opening the side
contourlines of the four-seam net make an angle of only
9° to the direction of movement, as compared to 20°
for the conventional two-seam type trawl (Fig 18).
The double-leg connection gave about 2 m higher
net height at the wing tips and at the bosom than the
V-D connection. With the elevators, which give a lift
5 r
*. I0
3.0
4.0
4.6
6.0
Knot
Fig. 17. Height of components of trawl nets at various towing speeds'
1. Height of headline bosom (with two legs and elevator)
2. Height of headline bosom (with two legs)
3. Height of wing headline bosom (with legs)
4. Height of upper net in codend.
5. Height of wing end (with two legs and elevator)
6. Height of wing end (with single leg)
Table I. Data obtained with self-recording net-height meter, depth meter, net - against - water - speed meter, tension meter etc.
Items
Number
Items
V-D rigged
Ho. 1
double legs
Ho. 2
double legs with elevators
Ho. 3
i i
2 1 1
4
1
2
3
1
2
3
4
Humber of
left
1 leg
2 legs
2 legs
Buoyancy of
float
300 kgs
300 kgs
300 kgs
Elevator for
iring net
*
*
*
*
*
*
*
0
0
0
0
weignt or
groundrope
386+90-476 kgs
386+90-476 kgs
386+90-476 kgs
Sea depth
102 n
99
95
100
110
113
120
126
125-7
125
124
Length of warp
300 m
310 m
360 m
Revolution
per minute
150
170
190
210
190
170
200
170 o
190
200
190
Towing speed
3-1 *t
3.6
(1.8)
4.3
4.85
3.9
3.Z7
4.1
3-0
4.4
4O4
4«33
Warp angle
Included angle
left
.
+ 6
+ 7
+4,+5
+10
+10.5
+ 8
+13
+8
+7
46
right
+llw
+12
+10
+8.+11
+1
+ 5
1-3.5
+2
+4
mean
8°
9
8.5
6, 8
8.5
7.7
8
5.7
4.5
5
Depression angle
left
23
22
20
20.20
21
22
20
22
21
21
21
. right
24
22
20
20.22
20
23
22
22
23
21
22
warp tension
2950kgs
3900
4300
5200
6280
—
7060
4100-
4600
5900
b40O-
6900
5200
Height or net
vin*r end
3 m
1.5
—
1.5
—
—
' —
(6-4)
—
(5)
_-
Centre of the
net mouth
10 m
9
8
7.5
11.5
13
10
12
11.7
11
12
Codand
7 A
(8—7-5]
6
11
__
—
Water flow speed
ineid« nat
See Table II
of about 35-40 kgs at 4.5 kn, when wing tips were raised
to 5 m and the bosom to 1 1 m fishing height (Table II).
Table II Towing apa»d at 4.5 knots
\
Itama
luab.r .
Of lagB ^ ^
Haight of
forward
and of wing
Baight of
oantar
of boaon
Haight of
oantral
oodand
Raduotion of
flow apaad
inaida nat
1 lag
2 •
8 m
-
-
2 laga
3-5 »
10 m
-
-
2 logs
with a la vat or
for tha vine
5 •
11 n
6.5 •
0.3 knots
Total tuoyano* waa up to 300 kga. Tha floats (Polyathylana)
woro 24 o» in diaattar, alla^ad to withstand vatar praaaura
of 40 k«/o«.
By observation of chafing on the belly, it was found that
at a distance of about 6-7 m beyond the footrope the
net operated off the bottom, so that the top part of the
codend operated more or less on the same height as the
headline bosom. This is an important feature, especially
for trawls operating on rough bottom.
Waterflow — Measurements of the waterflow inside the
net, obtained during the experiments, are given in Table
III where the values obtained in a conventional two-seam
net are given for comparison. From the table it is clear
that the difference of waterflow inside the net to the
towing speed, is only 0.3 kn at a speed of 4.5 kn. Figs 19
and 20 show the recording of the waterflow in both cases.
Tabla III Bolativo watar flow inaida and out • id* not*
^^^•^a^nd of aat
ZtaM ^**^HV^^
Tha naw typa aat
(4-saaa aat)
ConTaatioaal typo not
(2-aaaa aat)
Outaida tha aat
(4.33kt)(4.4kt) (4.54*t)
2.22m/a 2.26«/a 2.34»/a
(2.64kt)(3.26kt)(3.62kt)
1.35V* 1-6TV. 1.86V.
Inaida tha aat
2.07«/a 2.1 •/• 2.17«/a
1.19»/s 1.16V* l-21"/»
Diffaranoa
0.15«/a 0.16a/a 0.17«/a
Oa6a/, 0.49V* °«65«/«
"\ Kind of iwl
low typo Mt Coavantioaal trawl nat
(4-MM aat) (2-ssaa aat)
It*as v^-
^\
Vatcr flow
(1) -P..*
4.33
4.40
4. 54
2.64
3.26
3.62 k»ot
•4*5 knot
outsida not
2.22
2.26
2.34
1.35
1.67
1.86 a/a
Vatsr flow
(2) apaad
2.07
2.10
2.17
1.19
1.16
1.21 a/.
3.2 knot
inaida Mi
(1) - (2)
•aaad vaftttotioa
0.15
0.16
0.17
0.16
0.49
0.65 •/•
1.3 taaot
of tbo flow
iaaida Us aat
1Q7
Old type net
Secton view of the net
New type net
Plan view
Section view
Plan view
Fig. IS Comparison of shapes of old
Operation— Although the net was much longer than
any net previously used from the No. 50 Akebone-Maru
no difficulties were experienced in operating the gear'
either in shooting or hauling. However, measurements
performed and observation of the net after hauling
indicated the importance of adjusting the warp length
for the angular difference between the ship's course and
the towing direction. This is especially important on
stern trawlers where the distance between the stern
rollers is large.
Om « JO
and new-type trawl nets in operation.
Kg 21 shows the angle made by the warps when towing
with the tide on starboard, ft is clear that the warp
distance from the roller to the otter board is longer for
the portside and that this warp should therefore be
wl!£ I?" £ *?• additl°nal length' equal to D sin «:
where D is the distance between the stern rollers, and
e is the angle of the ship's course to the towing direction
For the No. 50 Akebone-Maru, D was 11.10 m and
during the experiments values as high as 1 m were found
I Of D sin 9.
KNOT
198
Tfcwinj Speed
Figs. 19)20. Difference in flow speed inside and outside new and old-type nets.
Towing Power, Towing Speed and Size of Bull Trawl
Abstract
The engine power of Japanese two-boat trawlers has increased
from 160 to 340 hp in the last 15 years but, due to the use of fixed
blade propellers, the available engine power was not fully utilised
in towing. Experiments were carried put with controllable pitch
propellers and it was found that the towing power could be increased
from 35 to 75 hp. In order to utilise the increased towing power,
a high-opening trawl was designed and constructed and during
comparable fishing operations it was found that, although the
weight of the catch of the new gear was of the same order as that
of the traditional gear, the fish were generally of a larger size and
better value.
by
Chikamasa Hamuro
Fishing Boat Laboratory, Tokyo
Puissance, vitesse de touage et dimensions do chalut a deux bateaux
Resume
La puissance des machines des chalutiers a deux bateaux Japonais a
augment^ au cours des 15 dernteres annexes et est passee de 160
a 340 c.v. mais 6tant donne qu'ils employaient des helices a pas
fixe, cette puissance n'&tait pas utilisee pleinement dans le chalutage.
Des experiences ont 6te conduites avec des helices & pas variable
et on a trouv£ que la puissance de touage pouvait ainsi passer de
35 a 75 c.v. Pour profiler pleinement de cette force de touage
supplemental, un chalut a grande ouverture a 6t6 dessine et
construit et pendant des operations de pdche comparatives, on a
constate que les captures effectives avec le nouveau chalut etaient
egales poids a celles effectives avec le chalut traditionnel, mais que
les poissons captures etaient generalement plus grands et d'une
valeur superieure.
Fuerza y velocidad de remolque y dimensioned del arte
Extracto
La potencia de los motores de las parejas Japonesas ha aumentado
de 1 60 a 340 hp en los ultimos 1 5 aftos, pero por causa del empleo de
helice de palas fijas la fuerza disponible no se ha aprovechado por
completo en el remolque. Se realizaron experimentos con pro-
pulsores de paso controlablc y se demostr6 que la potencia de
remolque podia aumentarse de 35 a 75 h.p. Para aprovechar plena-
mente la mayor fuerza de remolque se proyect6 y construy6 un
arte de mucha abertura en altura y durante actividades de pesca
comparativa se demostrb que aunque la captura total del material
modificado era del mismo peso que la del tradicional, el pescado era
en general de mayor tamaflo y mas valioso.
THHE engine power of the two-boat trawlers in Japan has
JL increased from 160 to 340 hp in the last 15 years.
The main object was to obtain higher speed to minimise
the time spent in steaming from port to the fishing
grounds and back, as well as to speed up the move from
one fishing ground to another.
Due to the use of fixed-blade propellers, the available
towing power did not increase proportionally to the
increase in engine power so that the fishing gear has
remained almost unchanged.
The Fishing Boat Laboratory tested a pitch pro-
peller which increased the effective controllable propeller
power while trawling from 35 hp to 75 hp.
Vessel and propeller
The vessels used during the experiments were the No. 23
and 25 Yamada Maru, both having 98 tons displacement
with diesels of 340 hp at 385 r.p.m. Manganese bronze
controllable-pitch propellers were used, three-blade.
1,600 mm diameter, 7,500 cm2 developed area.
Continued from page 198
Fig. 21. Difference (A-C) in warp length from roller to otterboards
when towing with tide on starboard.
Fig. 22. Net distorted when towing with tidejon starboard.
When towed with equal lengths of warp in the above
case, the weatherside wing was found to contain seaweed
stuck in the meshes while practically none was apparent
in the leeside wing. This pointed to a distorted net
during towing, as illustrated in Fig 22. During subse-
quent hauls, the lee warp was lengthened by the amount
D sin o according to the drift angle, and seaweed then
got caught equally in both wings. The angle 6 changes
as the drift changes and is not constant throughout the
haul. One way of avoiding constant changes in warp
length on stern trawlers would be to centre the top rollers
(towing blocks) at a point from where the warps are
paid out. However, this would adversely affect the
opening width of the net.
The above trawl gear is now being used in commercial
fishing by big stern trawlers in the Bering Sea and the
Atlantic, and also by medium-size trawlers in the East
China Sea, and is giving ISO to 200 per cent better results
than the conventionally used trawls.
199
Such propellers are normally for vessels of the Yamada
Mam class.
Summary of the experiments
Th'e vessels were connected stern to stern with a 209 m
long manila hawser. Towing was done of varying
degrees of pitch and r.p.m. of the propeller (Fig. 1).
Fig. L Operational method during towing tests.
The towed vessel kept a constant pitch on the propeller
but changed the propeller revolutions to allow the towing
vessel to attain the various speeds required for obtaining
the necessary data. The instruments included specially
constructed dynamometers of 2*5 to 5 tons, special logs
designed and constructed at the Fishing Boat Laboratory,
and strain gauges for accurate measuring of the propeller
shaft torque.
The measurements taken included:
(a) Warp tension (pull).
(b) The vessel's speed.
(c) The torque of the propeller's shaft.
(d) The exhaust gas temperature of the main engine.
(e) The mean pressure for each cylinder.
(f) The fuel consumption.
All instruments were installed on the towing vessel
which issued instructions as to speed, etc., to the towed
vessel. The relations were defined between propeller
pitch, propeller speed, fuel consumption, torque and
ship's speed.
Result of the towing tests
The results are summarised in Fig. 2. The results calcu-
lated for towing speeds of 2| to 3£ knots are also shown
in Fig. 3 in another way. The maximum towing pull
normally utilised was extracted from the data provided
in the graph and was taken for the following conditions :
(a) For a maximum exhaust temperature of the main
cylinder during operations of 350°C.
(b) For a maximum mean pressure of the main
cylinder of 5*2 kg/cm2.
(c) For a minimum fuel consumption.
The towing power under the above conditions was
measured and the results are given in Table I.
200
Speed
2-5 knots
3-0 knots
3-5 knots
Tension
(pull)
3-5 tons
3-4 tons
3-3 tons
TABLE I.
Pitch Revolutions
S.H.P.
13-5°
13*8°
14-2°
353 r.p.m. 262 P.S.
Design of the trawl gear
The normal power on two-boat trawlers is 320 to 340 hp
and most vessels have fixed pitch propellers. As the
pitch has normally been selected for free running, power
available for towing is therefore relatively small. Tests
have shown that at 2-3 knots the pull is about two tons;
this means that towing power is around 35 hp. By
Fig. 4. Traditional manila two-boat trawl.
SCO •
100
250
301
300
f
f
t
x>
1
160
trl
Test results for 2.5, 5» 5.5 knots
r.p.m. of Main Engine
J50
300
202
using a controllable-pitch propeller, the towing power
was increased to 75 hp.
Fig. 4 gives the design of the traditional type of two-
boat trawl. A new type of trawl was designed to have a
minimum of resistance during towing at high speeds.
The headline height was found to be about 5 m at 3£ knots.
Construction details are given in Fig. 5. An elevator,
Fig. 6, is attached at each wing for assisting the wing to
reach its maximum height.
Measurements were taken of warp tension and headline
height; these, together with the data previously given,
are shown in Table II.
Towing Speed
2-8 knots 3*5 knots
4-
13°
300
245°C
3-3 kg/cm8
27-5 kg/hr
245m
2-2 tons
4-9 m
14°
340
320°C
4-7 kg/cm8
49-0 kg/hr
230m
2-8 tons
4-5 m
Table II.
Propeller pitch
Propeller r.p.m.
Temperature of the exhaust
Mean pressure
Consumption of fuel
Distance between the boats
Warp tension
Net height
Result of fishing operations
Experimental fishing was carried out on the normal
fishing grounds in the Yellow Sea during the beginning
of September, 1962.
The new gear and the traditional gear were towed
alternately by a pair of trawlers. The catches of large-
and middle-sized yellow croaker were 150 per cent
higher than with the traditional trawl; the catches of
small fish and bottom species were, however, less. The
total weight of the catch was approximately the same but,
the market price for the bigger fish was higher.
This new net has been designed for high opening and
high-speed towing. If the fish population is composed
mainly of bottom fish, the design of the net should be
altered to obtain a wider opening at the cost of a lower
height, conserving the high-speed feature of the trawl.
In any case, any new design must incorporate total use
of the full thrust available from the engine — having a
controllable-pitch blade propeller.
Fig. 5. Design of improved two-boaf trawL
Fig. 6. Elevator used during experiments.
203
Suggestions for Improved Heavy Trawl Gear
Abstract
The author who is an outsider to the fishing industry, presents his
general suggestions for the improvement of heavy bottom trawl
gear as a stimulus for fisheries experts to work out practical solu-
tions to the advantage of both the trawling industry and the sub-
marine cable companies. His proposed modifications aim at
decreasing the towing resistance and the risk of fouling the gear on
obstacles on the bottom. The main suggestions are: To modify
the bracket or bridle arrangement of the otter boards to fend off
objects which otherwise would be hooked or, even better, to operate
the otter boards off the bottom, attaching them directly to the
leadline, and by using a different rig and different designs and/or
other materials (light plastic) for the boards; also to eliminate
bottom-touching sweeplines. To enhance catching efficiency,
sound or electricity should be utilised to guide the fish towards the
net opening. To carry out such gear development projects efficiently
and quickly, the establishment of integrated "systems development"
teams on national, or even international, scale is suggested rather
than widely scattered individuals or small groups working without
co-ordination. If the design objectives include the avoidance of
damage to submarine cables, the international communications
firms might be willing to assist.
Suggestions pour modifier le chalut lourd
L'autcur qui n'est pas un spicialiste dans I'industrie des peches,
soumet dans cette etude certaines idees pour Amelioration des
chaluts lourds de fond, pour lesquels les experts en matiere de
ptehe pourront peut-fctre trouvcr des solutions pratiques interessantes
pour 1 Industrie des peches et les compagnies de cables telephoni-
ques sous-marins. Les modifications proposees tendent a reduire la
resistance de 1'engin et les risques d accrochage aux obstacles du
'fond. Ces suggestions comprennent la modification des brides ou
arrangement d'une chatne sur les panneaux de tellc sorte qu'il ne
soit pas possible d'attraper des objets avec le panneau ou mieux,
d'operer de telle facon que les panneaux soient au-dessus du fond
en les attachant directement a la ligne de fond ou encore par la
modification du dessin de 1'engin et en utilisant pour les panneaux,
les nouvelles matieres comme le plastique, surtout pour empecher
que les bras ne tratnent sur le fond. Pour ameliprer la capture,
pleine utilisation devra fctre faite du son et de I'dlectricitd pour guider
les poissons vers Pouverture de 1'engin. Pour 1'execution rapide et
efficace de ces projets, des "systemes de devcloppcment" etablis
par des £quipes nationales ou Internationales seront certainement
plus efficients que des efforts individuels ou de pet its groups travail-
lant sans coordination. Les compagnies Internationales de com-
munications sont prttes a aider a la realisation de ces projets si les
travaux permettent en m&me temps d'eviter les dommages causes
aux cftbtes sous-marins.
ejorar artes de arrastre pesados
Extmeto
El autor, que es ajeno a la industria pesquera, hace sugerencias
generates para mejorar los pesados artes de arrastre de fondo con
obfeto de estimular a los especialistas pesqueros a encontrar
soiuciones practicas que redunden en beneficio de la industria de
la pesca al arrastre y de las compafiias explotadoras de cables tele-
grancos submarinos. Propone, en sus modificationes, reducir la
resistencia al remolque y d resfo de que el arte se enganche en
obstaculos del fondo. Las princtpales sugerencias son: modificar
el brazo o el enganche del pie de gallo de las puertas para evitar
obstaculos en los que podrian engancharse o, lo que seria todavia
meior, separar las puertas del fondo sujetandolas directamente a la
relinga de ptomos, emptear diversos herrajes, formas o materiales
(plasticos Ugeros) y eliminar las malletas que tocan el fondo. Para
incremcntar el rendimiento de pesca deberian emplearse sonidos o
corrientes ettctricas que guien a los peces hacia la abertura del
arte. Con objeto de que estos proyectos puedan llevarse a la prac-
tica rapida y eficazmente, convendria crear grupos de "estudios de
sistemas" en escala national o internacional, en vez de tener indivi-
duos o grupos pequenos aislados que trabajan sin coordinaci6n.
Si una de las finalidades del proyecto es evitar causar averias a los
cables submarinos, las empresas internaa'onales que los explotan
es posible que esten dispuestas a ayudar.
by
Eldon Nichols
Cables Division, American Telephone &
Telegraph Co.
WHY has heavy trawling gear been improved so little
in recent years? Important advances have been
made in the design of large trawlers and in techniques
for handling and processing the catch but trawling gear
has undergone only minor refinements. The present
trawl works quite well on relatively smooth bottom at
moderate depths (the conditions for which it was designed)
but today much trawling is done on uneven and rough
bottom at depths ranging to more than 300 fm. Under
these conditions, hang-ups and fouling are common
and the width and height of the mouth vary erratically
as the otter boards lurch and heave. Drag is excessive,
limiting the size of the net and the trawling speed and
increasing fuel consumption.
This situation came to attention during studies of the
causes of damage to submarine cables and some consid-
eration was given to it. The results are presented here
with the hope that a look at the subject from an out-
sider's point of view may provoke interest among fisheries
people and lead to effective action.
Need for basic changes
Till now most changes have involved only replacements
and adjustments in one or two specific components,
e.g., the substitution of heavy bobbins for rollers, and
not all have been very beneficial. In fact, matters are often
made worse by attaching weights to doors and reducing
scope to shorten the hauling-in time. This increases the
heaving of doors by sea and swell.
It seems clear that the design of a fundamentally new
trawl is indicated, rather than the mere substitution of
new otter boards, floats, or other items. Although
substantial development work will be required to deter-
mine details of a new trawl, it may be useful to suggest a
general line of approach.
Preview of a new trawl
First, the basic geometry of the trawl needs investigation.
In the present structure the vertical component of the
towing force acting on the warps is opposed and wasted
by the weight and tilt of the otter boards. If the warps were
attached to the headline, they would lift it, eliminating,
or at least reducing, the need for floats. With fewer or
smaller floats there would be less drag on the headline.
This would permit the use of smaller otter boards which
would give further reduction in drag.
These changes would eliminate the groundrope legs
and this should reduce hang-ups to less than a third of
the present number. It may be objected that the otter
boards must be separated from the wings by legs or
sweeplines in order to scare fish into the net. This theory
seems to be questionable when trawling is done in deep
and turbid water, at high latitudes, or at night. Under
these conditions the light is undoubtedly too dim for
the otter boards and lines to be visible for more than a
few feet. And if sound or pressure waves, rather than
light, is thought to be the agent that scares the fish, a
more efficient frightening or guiding device than the
present otter boards and sweeplines could surely be
designed. In recent experiments in the U.S.A., increases
in catches were obtained by passing electric currents
through the water near the mouth of the net. This
appears to be a much more promising method than the
use of sound.
Improvement in the method of attachment is particu-
larly important. The forward end of the present type
of otter board, together with the bracket and the warp,
form a hook that will foul almost any object in its path.
This hook can be eliminated by the use of a bridle
or bracket attached to the top and bottom corners of the
leading edge of the door in such a way as to fend off
objects encountered. This, of course, is only a refinement
of the chain bridle, long in use in small trawls. This
change would interfere with the use of the "G" link,
Kelly's eye and stopper for the quick disconnection of
the otter boards when hauling in. However, with the
otter boards attached directly to the ends of the headline,
as will be the case with the groundrope sweeplines
eliminated, disconnection would not be necessary.
It is also proposed that the otter boards be removed
from the bottom and made "free swimming". This would
do away with the extreme variations in spreading force
which now occur due to the shoe intermittently scraping
up ridges of sediment and jumping over them. Also
eliminated would be the variations in spreading force,
due to the sudden changes in effective aspect ratio
whenever the otter board leaves the bottom or returns to it.
The new otter board should have a little positive
buoyancy which in the past has been unattainable due
to the quick waterlogging of the wooden planks under
pressure. Now it is feasible to make strong, light hydro-
foils of a rigid plastic foam, such as polyurethane,
covered with fibreglass. The very smooth surface,
free from iron braces and bolts, should give a substantial
increase in lift-drag ratio. The foamed plastic core should
be either tapered in density from top to bottom, or
slightly weighted at the bottom, to make it seek a vertical
position in the water. This will correct the tendency of
the present otter boards to fall flat when launched, when
veering or when the warp goes slack for any reason,
such as the heaving of the trawler in the sea.
Second, ways should be found to make better use of
synthetic fibres in nets. Fishermen have been under-
standably reluctant to make full use of them due to
their higher cost and to practical difficulties in tying
them. But if these faults can be overcome, their low
friction, light weight and high strength should pay
dividends in reduced drag and maintenance.
Third, new inventions and ideas should be reviewed
to see if they fit into the general framework of trawl gear
improvement, or if any of them are important enough to
warrant a change in this framework; e.g., a new headline
float intended to give more lift with less drag would be
examined as a possible replacement for the current
types. On the other hand, an invention such as the recent
Canadian one, in which a hydrofoil equipped with a
hydrostatic control always seeks a preset depth, would
be considered as a possible way to eliminate headline
floats.
Further study may show that some of the design features
just proposed are not practical but the point is that
trawls of novel design are possible, that there are new
ideas and materials waiting to be put to work and that
little is being done about this situation, although even
a minor improvement involving so many trawlers would
have substantial value.
Organising for development
If it is agreed that the development of a better deep-water
trawl is possible and desirable, the next question is:
How can it be accomplished? Much of the rapid techno-
logical progress in modern industry is due to the employ-
ment of integrated "systems development" teams capable
of carrying projects all the way through to commercial
operation, rather than widely scattered individuals or
small groups working without co-ordination. Fisheries
people must find ways to employ this powerful method.
A development team capable of handling the project
under discussion should include high-grade fishing
gear technologists, expert designers and makers of nets
and gear, outstanding trawler officers and an able and
experienced leader. Also, there should be arrangements
for occasional consultation with specialists in such fields
as hydrodynamics, marine biology, plastics and elec-
tronics.
In any of several countries, such a team could be
assembled from the competent people now engaged in
fisheries work. A board with members drawn from the
government fisheries department, universities and manu-
facturing and fisheries companies could establish policies,
select the key members of the team and provide general
co-ordination. The funds required to finance the under-
taking might W contributed jointly by the companies
and the government. They should not be much larger
than those now being spent by the several organisations
working separately but the results could be much more
valuable.
Another attractive possibility is the formation of a joint
enterprise in which several countries would co-operate,
perhaps under FAO leadership. In this case, the financial
contribution from each party would surely be within reach.
205
Development of Soviet Trawling Techniques
Abstract
Soviet development is toward active high-sea fisheries. The principal
gear is the bottom trawl. Five stages of its development since 1 878 are
shown in sketches. Most modern trawling is done at 1 50 to 250 m,
at speeds up to 3*5-5-0 knots for periods of 1£ to 3 hours,
with bottom gear of these general specifications: 60 m long; 25 to
40 m horizontal opening; up to 3 m vertical opening. The Novator in
1948, followed by the Pushkin-class vessels, led to the modern
1,900 hp sterntrawler with completely mechanised processing
facilities. Continuing experience dictates changes such as smaller
vertical opening of the net for plaice; larger vertical opening and
greater speed for herring; and variations in vertical and horizontal
opening for cod and haddock. Soviet trawlers use oval, slotted
otter boards which are more durable, lighter and have 19 per cent
more spreading force than ordinary otter boards of comparable
size. Midwater trawling was first tried in the 1920's but is still in
the developmental stage. North Atlantic midwater trawling has
occasionally produced more than 20 tons in 30 minutes' trawling.
A scheme of controlling the net-filling rate is diagrammed. A
transmitter enclosed in a ball attached to the headline is connected
with a transducer installed in the codend; the transmitter sends
signals to a receiver in the wheelhouse. Only some gear parameters
can be studied with present methods and instruments. The author
diagrams what he believes is the most advanced installation for
gear trials. In this, two endless steel ropes are attached to a pon-
toon mounted on two floats. The gear to be tested is secured to the
ropes; the pontoon serves as a platform for cameras, lights and
other instruments to register changes in parameters as the gear is
towed at various speeds across a natural basin which has no current
and is sheltered from the wind. The method permits testing full-
scale gear and one-half and quarter-scale models. The author des-
cribes another special instrument, an under-water dynampgraph
which can continuously record, for 90 minutes, the stresses in rope
and wire to 350 m depth. Another instrument is sketched which
records the net's vertical opening by means of an expandable
tank transmitting pressure variations through a liquid-filled hose
to a recorder. Fish behaviour and gear catchability studies use the
usual aqualungs, bathyscopes and underwater equipment but, in
addition, the special submarine Severyanka permits long-term
observations and photography.
Dtotoppement des techniques de chalutage sovtetique
Return*
Le dcveloppement de la peche sovietique est ax£ sur la peche
hauturiere. L'engin principal est le chalut de fond. Les cinq stades
de dcveloppement depuis 1878 sont racontes par des illustrations.
Le chalutage moderne est gen6ralement pratique1 de 150 a 250 m
de profondeur, & des vitesses de touage allant jusqu'a 4-5 noeuds, par
periodcs de 1£ a 2 heures, avec des chaluts de fond ayant les
caractcristiques suivantes: 60 m de long, 25 a 40 m d'ouverture
horizontal et jusqu'fc 3 m d'ouverture verticale. Le Novateur en
1948, puis les chalutiers de la classe Pouchkine ont conduit aux
chalutiers a peche arriere moderaes de 1900 c.v., entierement
mecanises avec possibilit6 de trailer le poisson. En prolongeant
rexp&ience, il est apparu que certaines transformations devaient
fttre introduites, a savoir: ouverture verticale moins grande pour
les chaluts a plies, ouverture verticale plus grande et vitesse acceteree
pour les harengs, variations dans Pouverture verticale et hori-
zontale pour le cabillaud et 1'eglefin. Les chalutiers sovietiques
utilisent des panneaux ovales, munis de fentcs, qui sont plus
rdsistants, plus legers et ont une force laterale supdrieure de 19 pour
cent a celle des panneaux ordtnaires de dimensions comparables.
Le chalutage, pelagique fut commence en 1920 mais est encore
by
A. I. Treschev
Fishing Technique Laboratory,
Moscow
a un stade de dcveloppement. Dans TAtlantique du Nord il a
parfois produit des captures de plus de 20 tonnes pour une demi-
heure de chalutage. Un diagramme represente un systeme de con-
trdle de la vitesse de remplissage du chalut. Un 6metteur place dans
un globe est attache aux ralingues superieures et relie a un radio-
imcttcur installe sur le sac; les 6chos recus du sac sont transmis
a un r&epteur situe dans la timonerie. Les param&tres des engins
de peche pouvant Stre etudiSs par la pr6sente methode sont limites.
Des diagrammes de 1'installation que 1'auteur suppose Stre la
plus avancee pour faire des 6preuves d'engins de peche, sont
donnds dans la communication. Cette installation comprend
deux cordes d'acier continues, attachees a un ponton montc sur
deux flotteurs; les cordes passent par 1'engin a £prouver; le ponton
porte les cameras, lampes et autres instruments pour enregistrer
les changements des param&tres de 1'engin qui est toue a des vitesses
differentes, dans un bassin nature! n'ayant pas de courant et a
1'abri du vent. La methode a permis de faire des essais a 1'echelle
du modele, et a des echelles reduites de moitie et du quart. La
communication ctecrit un autre instrument special, un dynamo-
graphe sous-marin, qui peut enregistrer continuellement pendant
90 minutes les tensions exercees sur les cordes jusqu'a une profon-
deur de 350 m. Un autre instrument enregistre I'ouverture verticale
du filet au moyen d'un reservoir sensible a la pression qui transmet
a Tenregistreur les variations de pression par rintermidiaire d'un
tube plein d'un liquide. Pour les Etudes du comportement du
poisson et de I'efficacit6 de capture des engins, des bouteilles d'
oxygene, des bathyscaphes et tout un tquipement sous-marin ont
et6 utilises normalement, mais en plus, le sous-marin special
Severyanka a permis de faire des observations de longue duree et
des photographies.
Evoluckta de las tecnicas sovitticas de pesca al arrastre
Extracto
Las actividades sovieticas se encaminan hacia la pesca de gran
altura empleando principalmente el arte de arrastre de fondo.
£1 autor da ilustraciones de cinco fases de su evoluci6n desde
1878. Actualmente la pesca al arrastre se practica en fondos de
150 a 250 m, a velocidades hasta de 4-5 nudos por periodos de
1*5 a 3 hrs, con artes de las siguientes caracteristicas generates:
longitud, 60 m; abertura horizontal de 25 a 40 m y vertical hasta de
3 m. £1 Novator en 1948 y posteriormente los barcos de la clase
Pushkin han culminado en el modernisimo arrastrcro con rampa
a popa con motor de 1,900 hp y rnanipulaci6n y elaboracion
completamente mecanizadas. La experiencia aconseja el empleo de
redes de abertura en altura pequefta para la pesca de peces pianos;
de mayor abertura en altura y velocidad para el arenque; y varia-
ciones en las aberturas horizontal y en altura para bacalao y
Continued from page 205
If the design objectives include the avoidance of damage
to submarine cables, there is good reason to expect that
the international communications interests will be willing
to assist.
Only a single, specific development programme has
been discussed here. If it is carried through successfully
206
there should be strong incentive to retain the organisa-
tional framework and to take up other problems. In
many industries the most healthy and prosperous
companies are those which allocate a significant part
of their budgets to research and development. Can
trawler owners afford not to follow this example.
eglcfino. Los arrastreros sovteticos emplean pucrtas ovaladas,
con ranuras, que duran mas, son mas ligeras y tienen un 1 9 por ciento
mas de fuerza de abertura que las normales de dimensiones ana-
logas. La pesca entre dos aguas se practic6 por primera vez en la
tercera d6cada de este siglo, pero esta todavia en la fase experimental.
Esta pesca, practicada en el Atl&ntico septentrional, ha producido
en ocasiones mas de 20 tons de pescado en lances de 30 min de
duraci6n. Se da un esquema del proyecto para regular la velocidad
a que se llena la red. Un transmisor metido en una esfera ligada
a la relinga de corchos csta conectado con un transductor instalado
en el saco; el transmisor envia senales a un receptor instalado en la
caseta del timbn. Con los m6todos e instrumentos actuales s61o se
pueden estudiar algunos parametros de los artes. El autor da
esquemas de lo que es a su juicio la instalacion mas avanzada para
ensayar artes. En esta, dos cables de acero sinfin se sujetan a un
pontdn montado en dos flotadores; el arte que se va a ensayar se
ata a los cables; el pont6n sirve cpmo plataforma para camaras,
luces y otros instrumentos que registrar) los cambios en los para-
metros cuando el arte se remolca a diyersas velocidades por un
estanque natural en el que no hay corrientes y est£ protegido del
viento. Este mdtodo permite ensayar modelos a la mitad y un
cuarto de la escala total. El autor describe otro instrumento
especial, un dinam6grafo submarino que registra continuamente
durante 90 min los esfuerzos a que estan sometidos Jos cables de
fibra y de acero a profundidades de 350 m. Da un esquema de
otro instrumento que registra la abertura en altura de la red por
medio de un dispositivio de expansidn que, a traves de una man-
guera llena de liquido, transmite las variaciones de la presi6n a un
registrador. El comportamiento de los peces y la capacidad de
pesca de los artes se estudian por buzos aut6nomos, batiscafos y
equipos submarines, asi como con el submarino especial Severyanka
que permite hacer observaciones y tomar fotografias durante
mucho tiempo.
THE Soviet fishing industry is directed mainly toward
active high sea fisheries practised from extremely
sea-worthy vessels.
The main fishing gear is the bottom trawl (Fig. 1).
The most widely used type is about 60 m long with a
horizontal opening from 25 to 40 m; mouth to 3 m,
towing at up to 3*5-5*0 knots.
Trawling is mostly at 1 50 to 250 m but sometimes, in
the redfish (Sebastes) fishery the trawl is down to 500 m
and deeper. Usually trawlers up to 800 to 1,000 hp
bring in salted fish. There are now new freezer trawlers
of 1,900 hp. The first Soviet sterntrawler, Novator, was
built in Murmansk in 1948.
At present trawlers catch demersal species (such as
cod, haddock and redfish) and pelagic species (such as
herring and sardine). Trawling practice indicates that
operations and trawl design for various species should
be changed. For plaice, for instance, the large vertical
Ma inn
Fig. 1. Principal development stage of bottom trawl.
opening of a trawlnet and high trawling speed are not
required, while for herring the vertical opening and
trawling speed are of primary importance. To a certain
extent, a simultaneous increase in vertical and horizontal
openings can be obtained by higher trawling speeds.
In addition to adjustments by rigging the headline and
footrope, the trawl parameters can be considerably
changed by simply employing a more rational design of
otter boards. The Soviet trawling fleet uses oval-shaped
slotted otter boards (Fig. 2). These do not cut so deeply
into the bottom, are more durable, relatively light in
weight and have 19 per cent greater spreading force than
comparable rectangular boards.
Midwater trawls
First attempts to use trawls to catch fish in midwater
were made by Murmansk skippers as early as the 1920's,
but results were poor. After many trials a new method
of midwater .trawling has been worked out which is
Fig. 2. Oval-shaped slotted otter board.
207
used by big trawlers on dense concentrations of Atlantic
herring. In the North Atlantic catches after 30 minutes9
trawling sometimes exceed 20 tons.
Apart from fish location aids, successful "aimed"
midwater trawling requires special apparatus. A catch
indicator which works on a hydro-acoustic principle is
presently used in research work but it is not yet perfected
for commercial use.
The transmitter is enclosed in a ball about the same
size as the ordinary metallic trawl floats. This instru-
ment is attached to the headline and connected by cable
with the transducer installed in the codend. The receiver
is on the pilot bridge. A more detailed description of the
instrument may be found in the journal "Rybnoye
Khozyaistvo" No. 7, 1959. Scientific research in trawl
design development must be closely connected with
studies of fish behaviour near trawl gear.
Gear testing
The determination of technical parameters — the openings
of fishing gears, stresses and strains imposed on warps
and rigs, shapes assumed by gears and so on — is a rather
complicated task and requires special measuring devices,
especially when operating at great depth.
Present methods and instruments so far allow study
of only some of the factors involved, such as resistance
to movement, depth of submersion and vertical and
horizontal openings. The shape assumed by a trawlnet
under different conditions and corresponding internal
stresses in the gear are difficult to study during commer-
cial fishing.
Experimental methods for laboratory investigations
of trawl gear have been mainly oriented toward experi-
ments in natural waters because model experiments in
hydrocanals and wind tunnels with relatively small-sized
models characterise only the qualitative aspect of pheno-
mena, whereas the quantitative results obtained in this
way are not sufficiently reliable.
There are several methods of conducting laboratory
investigations of fishing gear in natural waters. At
present the most advanced method, in the author's
opinion, is that based on the use of a hydrodynamical
installation on steel ropes. In this case a ropeway formed
by two endless steel ropes is used (Fig. 3) running through
snatch blocks in a natural basin with motionless water
sheltered from wind. The endless ropes (1) are driven at
a desired speed by an onshore electric winch (2). To
these ropes is attached a pontoon (3) mounted on two
floats. The tested gear or gear component (4) (trawl, etc.)
is secured to the endless ropes under the pontoon by
special hangings. During the process of towing the
pontoon may shift its position in relation to the gear
being tested. On the pontoon are measuring and con-
trolling instruments (5) which register changes in para-
meters of the gear tested ; movie cameras, searchlights
and other aids.
The return movement of the pontoon is assured by a
reverse operation of the winch. Speed of towing is
regulated by a gradual change of winch r.p.m. through
a frictional speed variator. This method permits testing
both full-sized trawling gear and models scaled down to
2 and £. The tests are conducted under conditions as
near as possible to commercial fishing.
A special underwater dynamograph has been devised
for measuring loading fishing gear operating at great
depths. The working principle is based on the measure-
ment of tractive effort reduced by a lever mechanism
by means of a calibrated spring, and registered by a
self-recorder supplied with multiplying device. The
instrument consists of three essential parts : body, receiv-
ing mechanism and multiplying and recording mechan-
ism. Its purpose is to measure stresses in ropes and
Fig. 3. A hydrodynamical installation on steel ropes.
208
Double-Rig Shrimp Beam Trawling
Abstract
Since 1959 the Belgian shrimp trawlers have steadily been convert-
ing from otter trawling to the double-rig beam trawling. All new
shrimp trawlers have been especially adapted for this fishing method
since 1962. The trawls used have, on average, a beam length of
8 m using a net of 550 meshes width of 26 mm mesh size in the
upper part and 18 mm. in the codend. The whole net is made of
nylon 420 R Tex. The paper describes the operational method and
compares the catch efficiency of otter trawls versus double-rig
beam trawls. The calculation of the resistance and effective mouth-
opening for the two fishing methods shows that the theoretical
advantage of double-rig beam trawls over otter trawls for shrimp
fishing is of the order of 30 per cent. In double-rig operations in
areas having strong tides, a hook-up with one gear is dangerous
and the levering force acting at the boomtip can cause the vessel
to capsize. Full details are given of a safety-rig incorporating a
special safety hook. A survey of the catch results obtained with
both methods in 1962 shows that, for the same horse-power, the
double-rig method resulted in, on average, a 30 per cent higher
catch, which compares well with the calculated 40 per cent greater
horizontal opening.
Chalutage de crevettes avec double-chalut a perche
Resurn*
Depuis 1959, les chalutiers a crevettes beiges ont converti progressive
ment la peche au chalut a panneau en peche avec des chaluts &
perches a double agreement. Depuis 1962, toutes les nouvelles
unites de pfcche aux crevettes sont adapters specialement pour
cette methode de peche. Les chaluts ont une perche d'environ 8
mitres de long et utilisent un filet de 550 mailles d'ouverture dont
les mailles ont 26 mm. dans la partie superieure et 18 mm dans le
sac. Le filet entier est fabriquS en nylon 420 R Tex. L'etude decrit
la methode d'operation et compare Pefficacit£ de capture des chaluts
a panneaux avec celle des chaluts & double greement. Le calcul de la
resistance et de Pouverture effective du filet pour les deux m6thodes
d6montre qu'en theorie les chaluts a perches a double greement
sont surerieurs auc chaluts a panneaux de 30 pour cent. Cependant,
au cours d'une operation a double greement, dans les eaux a fort
courant, un accroshement avec un des engins pounait etre dangereux
by
J. Verhoest
and
A. Maton
University of Agriculture, Ostend
car la force s'exeroant alors sur le bout de la bigue risque de faire
chavirer le bateau. Des details concernant un greement dont la
securitd est basee sur un crochet de stirctd sont tournis dans cette
communication. Une 6tude des resultats de capture obtenus par les
deux methodes en 1962 montre que pour une m£me puissance, le
systeme & double greement produit des captures d'environ 30 pour
cent plus importantes ce qui confirme assez bicn Taugmentation
thgorique calculee pour une ouverture horizontal 40 pour cent
plus grande.
Arte de vara doble para la pesca del camaron
Extracto
Desde 1959 aumenta el ntimero de arrastreros belgas que pescan
camar6n que sustituyen el arte de puertas por otro de varas dobles
Desde 1962 todos los arrastreros para la pesca del camarbn se
adaptan especialrncnte para este metodo de pesca. Por tdrmino
medio, los artes tienen varas de 8 m y emplean paftos de 555 mallas
de 26 mm en la parte superior y de 18 mm en el saco, todos de
nylon de 420 R Tex. £1 autor describe la maniobra y compara su
rendimiento de pesca con el de los artes de puerta. El calculo de la
resistencia y abertura efectiva de la boca en ambos metodos
demuestra que la ventaja te6rica del de vara doble sobre el de
puertas, en la pesca del camar6n, es del orden del 30 por cicnto.
Continued from page 208
cables at up to 350 m depth. Depending on the elasticity
of the spring, the measured effort may vary between
500 and 5,000 kg. The measurements may be recorded
continuously during 90 minutes. The instrument has a
positive buoyancy so that its use does not greatly inter-
fere with gear operation.
The vertical opening can be determined with the help
of an instrument consisting of an expanding tank fas-
tened to the upper point and a recording mechanism at
the lowest point; the tank and the recorder are connected
by an incompressible hose. The principle of this device
being to measure the pressure created by a column of
liquid contained in a closed system.
The manufacture of these devices present certain
difficulties, since they have to be hermetically sealed.
But they are much simpler and easier to handle than the
equipment used in studies of fish behaviour and catching
capacity of fishing nets, which require far more complex,
long-term observations. Recently, such observations have
been conducted with the help of aqualungs, bathyscapes
and underwater television. In the U.S.S.R., a special
submarine, Severyanka, is also used. The employment
of the submarine permits long-term direct observations
of fish near the gear and gives the opportunity to photo-
graph the most interesting phenomena. Fig. 4 shows a
photo of a trawl taken from the submarine.
Fig. 4. Photo of a trawl taken from a Russian submarine.
209
Cuando sc cmptean artcs de vara doble en fondos de corricntcs
fuertea, los enganches y embanes ion pcligrosos porquc la fuerza
que actiia sobre el extremo de la botavara pucde hacer que vuclquc
el barco. Se dan detalles completes de un aparejo con un gancho de
.Elexamendelosresultadosobtenidosconambos
i en 1962 demuestra que, a igualidad de potencia, el arte
de vara doble dio, por termino mcdio, capturas superiores en un
30 por ciento, lo que resulta muy favorable en comparaci6n con la
abertura horizontal calculada en un 40 por ciento mayor.
IN 1959 two traditionally equipped Belgian shrimp
otter trawlers were converted to a double rig beam-
trawling system. The operations were completely
successful from the start, resulting in other vessels follow-
ing suit, so that, by 1962, 28 of the 66 shrimp trawlers
operating on the Belgian coast had changed over to
double-rig operation. This conversion was the result
of a scientific research, undertaken in charge of the
Commission for Applied Scientific Research in Marine
Fishery, presided over by gen. dir. F. Lievens.
Since 1962 new vessels are being built especially
equipped for this type of operation. The fishery has a
definite future and bigger vessels are under construction,
such as those already operating from the Netherlands.
Vessels
The vessels converted up to the present have been mainly
wooden with an overall length of 12 to 17 m, powered
by 40 to 120 hp diesel engines (Fig. 1).
The conversion work consists mainly in erecting a
steel mast based down to the keel which carries the two
booms and the necessary rigging as shown in Fig. 2.
Practically no changes are required to deck arrangements.
The winch used is the traditional double-drum otter
trawl winch; the warps are now led over the booms to
carry one complete fishing gear on each side. The booms
are set to the required stance by tackles, which can be
done by hand as long as the fishing gear is inboard;
once fishing operations have started, the load is too
high. For fishing, the booms are set to an angle of about
30° to the horizontal so that the footrope hangs just
clear of the water (Fig. 3). During bad weather the boom
is lowered to near the horizontal to improve stability.
Fig. L Common Belgian shrimp trawler.
Fig. 3, Diagram showing the length of the derrick (L) as a function of
the beamlength (/) and the vessel's beam (b).
warp
•topper
•topper "block
boon
hoisting taoklt
Fig. 2. Details of boom rigging on converted side trawler.
210
Adjustment of the position of the booms between hauls,
according to fishing conditions, is done by heaving on
the warping heads.
Several new double-rig vessels have lately been built
for this work and the main difference between these
new vessels and the older converted type seems to lie in
the position and type of winch. The new winches are of
the friction clutch type with four drums, two for the
warps and two for topping the booms. The winch,
furthermore, is now housed under the bridge within
reach of the skipper so that the manipulation of the gear,
regulation of the booms' position and steering of the
vessel can now all be done from one position by one
man. Only three men crew these vessels (Fig. 4).
By doing away with the skylight over the engine room
and by leading the warps high above deck, these vessels
now have a much larger working space on deck.
if- >
1M X
Fig. 5. Construction details of a beamnet.
a. warp
to , stopper
c. stopper block
d« boon
e. hoisting tackle
Fig. 4. Details of boom rigging on newly constructed double-rig trawler.
Gear
The net is the usual type consisting of an upper and lower
piece seamed at the sides. Two wedges are inserted
between the square and the lower wings to allow full
lift (Fig. 5). The size depends on the length of beam.
The more generally used type at present has a beam
length of 8 m, a net width on top of 550 meshes of
26 mm mesh size in the upper part and 18 mm in the
codend. The whole net is made of nylon 420 R Tex,
except for the codend where 480 R Tex is used. The
footrcpa, 9-80 m. long, is made of mixed wire rope of
14 mm diam. Full indications for assembling and
mounting the net are given in Figs. 6 to 8. The above
particulars cover both big and small nets as only the
main dimensions differ, the general assembly of the nets
being the same.
The length of beams differs according to the size of
vessel and there is some slight constructional difference
between ports (Fig. 9). The width of the beam shoe at
the sole depends on the type of bottom fished, the softer
the bottom, the broader the sole. Each beam shoe
carries several eyes; one each for the headline, the foot-
Fig. 6. Side panel and square with headline. Fig. 7. Side panel and belly with bolchllne.
Fix, B. Junction of wing and bosom.
211
Cftntrtl fete* ttotl**
bridlt tytt
feotropt and bolehllitt
£. 9. De/fli/s of beam and beam shoe.
Fig. 10. Details of rig-up of beam and foot rope. Inset A: normal
bolchline length. Inset B: shortened bolchline.
212
rope and the bridle. The beam itself is normally construc-
ted in three sections; the outer parts, which are secured
by a chain, slide over the middle. The centre part is
often of a lighter diameter than the outer parts so that
when a hook-up occurs this part will bend and is then
easily replaced without interference with the beam shoes.
The footrope is made up of 13 cm diam wooden rollers
or rubber discs, which are all joined by links at a distance
of 30 cm. centre to centre. At each link there is a connec-
ting chain to the bolchline. For the type of net under
description 32 such rollers are used which together make
up a length of 9*60 m. Under normal conditions the
bolchline operates behind the rollers but in areas where
much bottom trash, starfish, crabs, etc. are picked up,
the bolchline is shortened so that it operates approxi-
mately parallel and above the rollers, allowing the bottom
trash to pass between the rollers and under the net (Fig.
10). The relative tension on headline and footrope can
be adjusted by attaching the footrope ends more or less
forward at the beam shoes.
Operational method
At the fishing grounds both beam nets are hoisted ready
for operation on the booms, which are set at an angle of
approximately 30° and pointing a little more aft than
abeam. The vessel steams ahead on a straight course,
while wearing out both gears, until sufficient warp has
been paid out in relation to the depth. During poor
weather the beams are normally lowered to an almost
horizontal position to improve stability. After hauling,
the process described above is reversed and, when both
nets are heaved to the boomtips, the vessel steams ahead
for a short while to rinse the catch. Only the bat is then
hauled on board with the gilson, the codend emptied
and the gear prepared for the next haul.
Advantages of double-rig fishing
It can be shown that for the same engine power a larger
area is swept with double-rig beam trawls than with one
otter trawl.
The relation between the resistance of nets of identical
construction can be expressed by:
R!
R2
Where: R= resistance; M= stretched mesh size:
N~ number of meshes in rounding;
e= density of twines ; R=runnage of twines ;
v=:towing speed.
Indices 1 and 2 refer to each net (otter trawl; double-
rig beam).
When both nets are constructed of the same material
and mesh size, and are towed at the same speed, the
relation can be simplified to:
Nt
In the above equation N* can be substituted by / x H ;
li /M^y /MA /ClR^
L2 \M2N2/ \M2/ \e2R2/
when the relation of the length (/) of the headlines and
the vertical height (H) of the nets is :
H2
scale
in which case
(«••
/iXH1
.
written
/, x H B
The combined resistance of two small congruent
nets whose total resistance is that of one big net can be
given by: 2Ra = RL
, 4* "I .fclVo M _
therefore -i = — -• = 2 = s*
Kz K2
and s -
Ml; '/ =
H*
= 1-41
The length and height of the opening of one small net
would then be 0-7 of that of the larger net having the
resistance of two such small nets. The shrimp otter trawls
used on the Flemish coast have very short wings and are
therefore of practically the same construction as the
beam trawls.
The length of the headline is on average about 10 m.
and the net is attached to the boards generally without
legs. The trawl boards used are 1-8 m xO*9 m. The open-
ing of such nets differs very little from the height of the
trawl board and, as only few floats are used, the headline
will hardly rise much above the board height. Using the
opening of such a net as a basis, one could calculate the
opening of two small beam trawls having the same resis-
tance:
la = 0-7 x 10 = 7m.
H2 - 0-7 x 0-9 = 0-63 m.
The height of the beam shoes, which represent the
opening height of the beam net, is normally found to be
60 cm, which is very near to the height found for H2.
The horizontal spread of two beam nets would then be
2x7 = 14 m, and is therefore 40 per cent higher than
that of one otter trawl having the same resistance.
Resistance of otter boards and trawl beams
Otter boards— Taking for granted that the otter boards
have an angle of attack giving them an optimum sheering
force for a certain speed, the resistance of the boards of
0-9 x 1-8 m having a towing speed of two knots can
be approximated as below:
p C g- A- v2
R= 2
C — specific resistance coefficient -0*65
sec2
Q = density of seawater - 104 kg — -
A = area of boards in m 2.
v = speed in m/sec.; v = 2 knots = 1*028 m/sec
R = 58kg.
Fhe resistance for two boards works out to 116 kg.
Trawl beams— The resistance R of the cylindric part
of the beam can be read:
P _C. Q. A. v*
K _ 2 .„
C = 1.
A. =0-09 x 8 = 0-72 m2
v = 1-028 m/sec.
R =39-5 kg.
In the same manner we can define resistance of the
beam shoes with C = 1-3 as: R = 8-5 kg.
Total resistance of two beams with beamshoes =
2(39-5 — 8-5) = 96 kg.
Bottom friction resistance
The weight of one trawl beam with beam shoes is more
or less the same as a pair of otter boards and the frictional
resistance of an otter trawl should therefore be the same
as that of the beam trawl. However, trawl boards plough
through the bottom at an angle whereas, owing to the
broad soles on the beam shoes, which act as sleds, the
trawl beam glides over the bottom. It would therefore
seem fair to assess that frictional resistance of two otter
boards amount to about the same as that of two trawl
beams. Summing up, it may be said that for a same
condition of resistance, towing power and speed, the two
beam trawls sweep 40 per cent more bottom, so that the
increase in catch of shrimp and flatfish should also be
about that amount; this is in fact corroborated by sta-
tistical analysis of the catches during one year (see later).
Advantages of the beam trawl
(a) The length of warp has much less influence on
the beam trawl than on the otter trawl ;
(b) The opening does not change during course altera-
tions;
(c) The influence of tides is much less on a beam trawl
than on an otter trawl as the opening is fixed
whereas with otter trawls the resistance and shear
of the otter boards, is highly affected when fishing
in tidal areas;
(d) Shrimp trawling is usually carried out in muddy
and soft bottoms. This does not affect the opening
of the beam trawl as the opening is constant and
the footrope catanary adjusted to an appropriate
and constant "raking" light in the bosom. Such
bottoms do affect the shearing effect of the boards
so that the opening of the trawl varies, with the
result that the footrope tends to dig in soft bottoms
when the boards close;
(e) Double-rig operations allow easier adjustment of
the gear to the fishing conditions as it is possible to
conduct comparative fishing at all times because
two nets are constantly in operation at the same
time.
The direction of wind and tide has no influence on
the shooting of the gear. As only the codend is hauled
aboard, the whole can be mechanized, leading to faster
operation with less crew.
213
Safety
One disadvantage of the method is that the stability of the
vessel can be largely impaired when one of the trawls
becomes hung-up by a bottom obstruction. The forces
acting on the boomtop become so great that the vessel
may capsize. This has in fact occurred on two occasions.
The forces arising from a hang-up of one net may be
taken to culminate at a point at the top of the beam
which is high above and far outside of the centre of
gravity and tends to heel the vessel over. Investigations
were carried out to develop a security release system
which would guarantee stability under all conditions.
Such a release must:
(a) Allow the forces acting at the top of the boom to
be brought to a point lower down on the vessel,
even when there arc winch or other defects which
prevent veering out of the warps;
(b) Transfer the forces to a point which allows the
vessel to manoeuvre the gear clear of the obstacle.
(c) Allow the net to be hauled quickly and in a safe
manner.
Fig. 1 1 shows such a security system which can be used
on any type of double-rig vessel. In this system the warp
coming from the winch is led through a block "a"
attached far forward to the stern and runs over a boom-
block "b" to the gear (Figs. 12 to 16). The boomblock
is itself attached to a second block "c" by a runner and
this runner is secured by a sliphook "d".
When the gear meets an obstruction the sliphook
is released, which in turn releases boomblock "b", so that
the towing forces now act on block "a", which ensures
full stability of the vessel The free net is then hauled over
its own boom, after which the vessel is manoeuvred in
the usual manner to free the hung-up gear from this
obstruction. The gear is hauled hard on to block "a"
where it is stoppered. Boom-block "b" is then hauled
back to its position at the boomtip and the gear is further
hauled in the usual manner.
Sliphook
The construction of the sliphook is given in Fig. 17. As
can be seen, the gear consists of a holder B in which the
hook H can turn freely round its axis E, but blocked by
the lock F. Lock F itself has an axle which has two flat
areas (as shown in detail, cross-section AB) and the
axis carries handle h. The construction of hook H is
such that the forces acting on it tend to turn it round its
axis E, which is however prevented by F when the cross-
section AB locks the hole G at the back end of hook H.
By turning handle h, axis F releases the hook.
The axes E and F are constructed of stainless steel
while the whole mechanization can be released by remov-
ing two slip pins so that maintenance and greasing are
easy.
Economic results
During 1962 the Belgian Fisheries Services made a survey
of the catches of shrimp trawlers. Data were collected
on the engine power of the vessels, the number of hours
Fig. 11. Safety release rig-up, (a) forward block, (b) fishing block, (c) safety block,(d) safety hook, (e) safety runner,(f) warp.
Fig. 12. Warps(f) coming from the winch, running over the forward blocks (a) to the fishing blocks.
Fig. 13. Arrangement at the top of the boom, (b) fishing block, (c) safety block, (e) safety runner, (/) warp.
Fig. 14. The release arrangement with old type of sliphook (d) and runner (e). One can easily see the bowblocks (a) and the warps ( /).
Fig. 15. The safety arrangement. The sliphook (d) was opened and the fishing block (b) came down. The warp (f)is pulling directly on the ship s
bow (e) safety runner.
Fig. 16. The safety arrangement. When the net is hauled and stopped, the warps are loosening and the boomblock is hung in the top of the boom
(h) bridle of the beamnet, ( n wan.
214
Fig. 17. Construction details of sliphook.
at sea for each trip, the number of hours spent in actual
fishing operations and the total catch per trip. These
data were analysed and statistically investigated for the
purpose of ascertaining whether the double-rig fishing
method was superior to the traditional otter trawl fishing
method. The data pertaining to the fishing vessels belong-
ing to the harbours of Ostend and Zeebrugge respectively
were processed separately, partly because the fishing
grounds for these vessels differed slightly but mainly
because the relation of the number of beam and otter
trawlers used was different.
Shrimp catch of Ostend vessels
The shrimp catches were analysed biometrically by two
approach methods. In the first case, the average catch
per hour for every week was calculated from the catch
data for all vessels. In this calculation the time required
to go to and from the fishing grounds was subtracted
so that the catch per hour is in relation to actual fishing
time. The data collected is given in Tables la and Jb.
The results of the biometrical analysis of these figures
in respect of both double-rig and otter trawlers is given
in Table II. As can be seen from this table, the double-
rig trawlers during 1962 caught, on average, 6-21 kg
of shrimp per hr, whereas the otter trawlers caught, on
average, 4-49 kg per hr. The biometrical analysis shows
that there is a significant difference ( 99 per cent) between
the average catches calculated, in favour of the double-
rig vessels, amounting to 1-72 kg per hr which works out
to 38 per cent more catch.
Table II. Reimlt of tht variance analyaoe of the
weekly avera/rea in re]
both fiehin." methods 1
ation to oat oh per hour for
Oitend).
§
•H
f. Theoretical
*i
*g
is
i
*
•i
H
1*
*3
s •**
1*
8
5
1
h
*
t
£
Method
1
70,7954
70,7954
59,28
4,04
7,19
Weeks
47
197,3026
4,1979
3,51
1,61
1,96
Correction
47
56,1320
1,1942
-
-
-
Total 95 324,2300
1. Correction term C •
2756,3266
2. 9*tel » 3080,5566 - C - 324,2300
3. Method* . (298,42)2 + (215,9&)2 - 0 . 70,7954
Week* «
- 0
197,3026
Average ihriap patch/hour for 1962
1. Average *g/)r (double-riff )i
2. Average k$/hr(otter trewl)i
-6,21kiAr
. 4,49
Difference! 6,21 - 4,49- - 1,72
This difference ie iifnifioanti r. oal. (59*28 )> ?.th.(7«19>
215
Details of shrimp catches by
Oat end otter trawlers for 19
'Table Ib - Details of shrimp cutchoe by OB tend
2
i
**
i*
*?
&i
g?
3*
Total trawling
tine per week.
^~<^
t*
ji
0 fc
*&
Average
oatoh/hour
per week
1
2
80
16,49
31
1,87
2
2
75
22,42
42
1,83
3
3
60
11,66
123
10,54
4
11
60
109,24
472
4,32
5
4
55
33,83
209
6,17
6
6
70
75,83
.396
5,22
7
34
82
370,04
1,346
3,63
8
1
96
4,58
9
1,96
9
6
93
55,25
183
3,31
10
10
90
§9,24
270
3,02
11
23
77
230,87
573
2,48
12
33
76
339,24
875
2,57
13
31
71
280,89
1,047
3,72
14
94
67
843,21
3,357
3,98
15
95
70
965,73
3,764
3,89
16
55
69
457,42
1,760
3,86
17
70
69
650,96
2,649
4,06
18
61
69
574,—
2,135
3,71
19
53
67
433,10
2,023
4,67
20
80
70
698,40
3,219
4,60
21
103
70
862,87
4,329
5,01
22
114
71 1
.027,28
4,138
4,02
23
70
73
601,28
2,038
3'*2
24
69
73
555,95
2,773
4,98
25
79
71
682,14
3,289
4,82
26
84
69
741,57
3,054
4,11
27
85
72
768,15
2,948
3,83
28
61
73
538,64
2,080
3,86
29
76
69
709,89
2,646
5,13
30
51
72
407,56
1,615
3,96
31
53
70
445,56
1,664
3,73
32
40
72
310,66
1,395
4,49
33
48
66
433,40
1,782
4,11
34
50
66
412,88
1,980
4,79
35
38
69
315,02
1,389
4,40
36
49
68
427,57
2,277
5,32
37
73
67
621,78
3,706
5,96
38
73
66
639,56
3,918
6,12
39
61
68
522,16
3,298
6,31
40
35
67
318,37
1,871
5,87
41
42
43
33
52
57
381,16
285,70
2,132
1,675
5,59
5,86
43
36
57
294,83
1,971
6,68
44
45
21
47
55
56
163,31
438,85
1,236
2,189
7,56
4,98
46
34
57
288,82
1,307
4,52
47
10
61
74,25
338
4,55
48
2
65
10,25
27
2,63
The second method consisted of using the data con-
cerning the daily catch of all vessels in the biometrical
analysis involving 2,868 figures. The reason is that the
first biometrical analysis depends on the average of
weekly catches, which reduces the spread of the data
considerably, because of the nature of these averages, and
gives rise to the theoretical possibility that a significant
difference could be found, where in fact there is none.
This second analysis, carried out over individual catch
216
I
I
Average hp
of vessels
Total trawling
tine per week
P
3*
Average
oatoh/hour
per week
1
3
93
20,83
111
5,32
2
3
66
45,17
294
6,50
3
4
80
50,25
281
5,59
' 4
3
79
15,50
109
7,03
5
9
75
79,17
526
6,64
6
4
77
46,75
401
8,57
7
9
67
107,90
613
5,68
8
2 -
99
14,50
73
5,03
9
4
77
43,50
257
5,29
10
3
76
38,51
147
3,79
11
5
83
75,83
295
3,89
12
8
77
102,16
476
4,65
13
4
77
26,58
150
5,64
14
18 .
72
162,31
981
6,04
15
14
76
167,47
888
5,30
16
10
62
87,50
435
4,97
17
16
74
163,15
961
5,89
18
15
73
149,89
733
4,89
19
9
78
81,67
565
6,91
20
15
77
132,66
897
6,76
21
18
78
144,72
966
6,67
22
21
78
194,74
986
5,06
23
14
79
116,14
432
3,71
24
10
76
82,57
461
5,58
25
18
77
159,49
962
6,03
26
22
75
191,63
842
4,39
27
20
77
187,49
942
5,02
28
19
78
172,85
896
5,18
29
21
76
187,60
1.001
5,33
30
15
77
103,42
514
4,97
31
14
76
121,91
583
4,78
32
14
81
119,01
662
5,56
33
12
83
115,99
580
5,—
34
14
76
112,67
68?
6,09
s
21
15
76
77
176,24
131 ,,92
'• 906
- 893
5,14
6,76
37
16
65
149, 75' 1*066
7,11
38
14
79
135,91
1.037
7,63
39
17
79
149,83
1.205
8,04
40
14
74
126,91
1.053
8,29
41
14
58
126,83
1.381
10,88
42
21
77
188,15
1.833
9,74
43
44
12
9
76
71
107,43
58,30
924
596
8,60
10,22
45
14
77
127,18
1.313
10,32
46
17
79
167,09
1.241
7,42
47
7
77
73,56
500
6,79
48
4
69
56
209
3,73
data, confirmed that there is a significant difference
(99 per cent) between the catches of the double-rig traw-
lers and the otter trawlers (Table III).
In assessing the much larger catch efficiency of the
double-rig system, trawlers with an average propulsion
power of 76*2 hp were equipped with about 10 per cent
more horsepower than the otter trawlers (69-1). On the
other hand, the catching capacity of a vessel does not
increase linear with the potential engine power. This
fJ22TLl^g^^igJ? ttWtt£ggjSi
Method*
Interaction
m*V
49
1
49
2768
1677,7
2107,Z
1?753,6
1677,7
43,00
4,601
2867 18051,9
1. Oomotlon t«rm C « (14270,14)2 . 203.
.6196 . 71-003
2. Total . 89054,9 - C . 89054,9 - 71. 003 - 18051,9
3. SubolMMd . 76301,34 - C - 5298,3
4. With th« aubolamd! 18051, 9 - 5298,3 - 12753,6
5. *••*• 72516,5-C . 1513,5
6. Mrthodt
3,223 for the double-rig system. The opposite occurred
in Ostend where 603 double-rig catches could be compared
with 2,265 otter trawl catches.
Notwithstanding, the collected data was analysed and
it was found that the double-rig trawlers caught, on
average, 8-88 kg of shrimp, compared with 5*63 kg per
hr for the otter trawlers, resulting in a difference of 3*25
kg per hr, or 57 per cent in favour of the double-rig
trawlers. These figures are, however, not directly com-
parable as there exists an appreciable difference in aver-
age horsepower between the vessels in Zeebrugge. The
average horsepower of the double-rig vessels was found
to be 87-7 whereas that of the otter trawlers was 76-2,
so that the average horsepower of the latter is about
25 per cent lower than the double-rig trawlers. This
must, of course, be taken into account and when the
catch per hour of the double-rig trawlers is reduced for
this difference in horsepower the comparable catch
becomes:
8-88 x 67-2
14*757.583.2
108.755.280.8 - C
From the above it is clear that the catch of the double-
trawlers is still superior to the otter trawlers by 1*12 kg
per hr. It must be noted that the comparison is based,
however, on a linear increase of catch with the horse-
power of the vessel, which is by no means sure.
24.473,6 + 48207,1 - C . 1677,7
7. Interaction 5298,3 - 1677,7 * 1513,5 • 2107,1
Av»rag> •hriaip oatohAour for 1962
1. Av»r*c« kg/hr (double rig)i 6,21 kg/hr
2. Average kg/hr (ott«r-trnrl)t 4,49 kg/17
Difference* 6,21 - 4,49 • 1,72 kg/hr
ThU difference !• significants F oal. (6,69)> F.th.(l,52)
was clearly shown in the study in the matter by De
Poorter (1962). Yet even if we were to accept that the
catch capacity of a vessel is in direct relation to its horse-
power, the catch per hour of the double-rig trawlers
would only be reduced to:
6-21 x 69-1
" 76-2
= 5-63 kg/hr.
which would still mean a difference in catch per hour
of 5-63 - 4-49 =1-14 kg/hr, or 25 per cent, in favour of
the double-rig shrimp beam trawling.
Vessels working from Zeebrugge
Here again a survey was conducted for all vessels;
however, the number of vessels which had converted
to the double-rig system was much greater and did not
provide enough figures for otter trawlers to lead to a
. valied mathematical comparison between the two
methods. During the whole of 1962, only 291 otter
trawl catch figures could be collected, as compared with
Conclusions
The survey conducted has shown that the double-rig
shrimp trawlers operating from Ostend have a catch
efficiency that is about 38 per cent higher than that of the
traditional otter trawlers. Even when reducing the actual
catches in respect of the difference in horsepower, the
net higher catches of the double-rig trawlers can be
expected to be 30 per cent. This 30 per cent increase in
catch shows a good relation to the theoretical 40 per
cent increase in horizontal opening of two beam trawls
as compared with one otter trawl.
The double-rig method has recently been introduced
on fish trawlers and is being applied more and more
to the catch of flatfish. There is not enough data available
at present to assess whether the increase in catch of
fish is of the same order as for shrimps.
References
Bom, J. on de Visser, T.: Rapport betreffende de visserij met
boomkorren in Nederland. -Directic van de Visserjen van het
Ministerie van Landbouw en Visserij.
de Wit, Ing. J. : Het Kromtrekken van de bomen bij de boom-
It or re visserij. Vicuws No. 4. 1962.
de Wit, Ing. J.: Opmerkingen over de veiligheid bij de boom-
korren visserij.
De Poorter, H. : Verder beschouwingen over de invlocd van het
motorvermogen van een vissersvaartuig op zgn rendement. Land-
bouwtydschrift No. 6—7 juni-juli 1962.
Dickson, W.: Trawl Performance — A study relating models to
commercial trawls. Department of Agriculture and Fisheries for
Scotland. Marine Research Service, No. 1. 1961.
Verhoest, J.: Een veilghcidsinrichting vopr de bokkenvisaerij.
Commissie voor toegepast wetenschappeUjk onderzoek in de
zecvisscrij Publicatic No. 9. 1962.
217
Discussion on
Bottom Trawling
A.R. Margetts
Mr. A. R. Margetts (UK) Rapporteur : The bottom trawl
itself has changed remarkably little over the years and those
changes that have been made are certainly not commensurate
with the changes which, in the same period, have occurred in
trawling vessels. The big development in recent years had
been in midwater trawling, while more attention has been
paid to developing from something like a standard bottom
trawl special trawls for particular types of fish and fisheries.
Both experience and experiments have shown that two rather
similar trawlers with not very different trawls could take
catches of quite different composition as regards both quan-
tities and species of fish. Since the last Congress there has
been progress in fitting instruments to gear to discover the
mechanics of how it works. A common assumption in
efforts to improve bottom trawls has been that to increase the
mouth area of the trawl would increase the catch. The
validity of that assumption is perhaps questionable.
The approaches are broadly divisible into two groups.
The first start from the bottom or bottom trawls and work up
by increasing the headline heights, as instanced by Crewe,
Dale and Hamuro. The second group start from the truly-
midwater trawls and work down by adapting them for fishing
just off the bottom, as described by Sch&rfe and Okonski.
Dickson outlined the steps taken to find out by instruments
the dimensions and operating forces in respect of a standard
bottom trawl. The measurements of the standard Granton
trawl given, and generally accepted as typical, are that the
spread of the net (with a 24 m headline) is of the order of 16 m
with headline height or vertical opening of about 2 m, the
whole towed at a load of about 7 tons. Hamuro also meas-
ured existing trawls in his investigations, and to achieve
similar knowledge the Russians, as described by Treschev,
had photographed a trawl from a submarine. It would be
interesting to know more of their technique.
Hamuro was dissatisfied with the behaviour of otter
boards and designed improvements. Crewe and Dale and
Moller also detailed their investigations into the hydro-
dynamics of otter boards. Hamuro measured the flow of
water in the trawl and showed how much water undesirably
spilled out of the front of the net. He then redesigned the
net to successfully reduce this and improve flow characteristics.
Crewe outlined how otter boards and warps behaved and
showed the effect of camber in improving efficiency of the
boards while requiring a different fishing technique. Instru-
ments showed even that warps could appear crossed at the
trawler yet the otter boards be still holding the net somewhat
open. Crewe studied the netting component of the trawl and
tackled the problem of netting resistance at the low angles of
incidence occurring in the trawl; this was relatively very
different from that of sheets of plane netting at big angles of
incidence such as had been investigated in most of the tank
218
experiments hitherto. There was a striking similarity in the
development of trawls by all authors in this section; each had
improved otter boards and two or three used a long leg or
bridle system allowing floats to give a vertical opening to a big
net which had side panels of netting to allow it to be opened*
In the making of these bigger nets, new synthetic materials
had helped by maintaining the necessary strength with finer
twines which gave less drag. The opening of Hamuro's new
net measured 26 m wide by 1 1 m high, an enormous increase
on the 16 m x 2 m mouth of the Granton trawl.
On the mechanical side, big advances had been made and
the engineers now knew enough to develop any particular
trawl required. On catching, Hamuro, with his 26 m X 1 1 m
trawl compared with the Granton, claims that his trawl
(which had been adopted for commercial fishing) has given a
catch increase of 150-200 per cent but he does not mention
what fish are caught. If they are mainly bottom species
it may be that it is only the spread which is mostly responsible.
Sch&rfe, in his midwater trawling, at first got poor catches,
but he persisted and was rewarded by success in catching
herring in a couple of hauls at Iceland. It would be wrong
to expect any newly developed gear to start catching at
maximum efficiency straight away. Time must be allowed
for a new gear to be properly tried and worked with the
co-operation of skilled fishermen before final judgment is
passed on it.
It is abundantly clear from many papers that the link
between gear development and behaviour of fish in relation to
that gear is a very tenuous one and one which must be
strengthened. We still do not properly know how or why a
trawl catches fish. We know that herding of fish in front of
the net does occur but we are not sure how it occurs and this
we must know in order to make more efficient gear. Bottom
trawling has the advantage over midwater in that there is the
bottom against which to trap the fish. But there is always the
problem of how to trawl rough ground. Overfishing and
experience of fish distribution, drives or draws fishermen
to rough ground, and there is need for both rough and smooth
ground trawls.
Contributions by Kreutzer and Wathne might well be
important for future development. Earlier it was thought that
electric fishing in the sea would not be practical. Now,
however, Kreutzer has shown that pulsed DC can overcome
a lot of difficulties. He has already, perhaps, eliminated fish
behaviour as a factor in trawling by electrocuting the fish in
front of the trawl which then swept them up. His first
electrified trawling results indicated improvements in catch
but more thorough testing is needed. Wathne used pulsed
DC (a different technical method of pulsing) to disturb
shrimps from the sea bed and make them jump into the path
of the trawl.
In his paper Nicholls urged more co-operation in gear
development by the inclusion of biologists as well as techno-
logists, engineers and fishermen. A start had been made in
that direction in Britain by a trawl development programme
involving the Saunders Roe Division of Westland Aircraft
Ltd., and in Europe there is an unofficial international group
working for the development of pelagic fishing methods,
which consisted of actively interested workers exchanging
ideas and experiences.
Mr. Robert Lenier (France): It is obvious that the more
bottom trawls work the fishing grounds the more will they
debilitate the bottom of the sea by destroying the natural feed
and larvae. To avoid that damage by heavy trawls, they
should lighten the trawling gear and preserve within the
territorial waters a three-mile limit within which trawling
should not be permitted and so protect the spawning grounds.
Mr, Margetts: I must flatly contradict Mr. Lenier that
trawling badly damages the sea bed and its food value.
Experiments in the United States, Britain and Denmark were
convincing that even heavy trawling did not appreciably
damage the grounds or most of the animals contained in the
surface of the sea bed. The sea bed was not denuded of food.
Grounds had been worked for centuries and were still pro-
ducing fish. Further, most spawning grounds were not inside
the three-mile limit, but there were nursery grounds in inshore
waters and some fish could be protected by keeping trawlers
away from those nursery grounds.
Mr. J. G. de Wit (Netherlands): Our fishermen are using
the double-rig beam trawl the same as the Belgians for shrimps
(described by Verhoest) but they also use it for flat fish.
This was a very old rig which had come into use in an extensive
way. Our fishermen used a number of tickler chains in
front of the footrope— up to five— and this exerted a tremen-
dous force first on the shoes and afterwards on the beam, and
bending of the beam was causing a lot of trouble. The beams
were up to 1 1 metres in length and it was therefore essential to
rig the beams so as to minimise the bending movements as
much as possible. There was a strong tendency to use the
double rig beam trawl for stern trawling. The small trawlers
using double-rig beam trawls had only one mast. If this was
placed aft, the vessel was easily handled, but when working
in strong currents the mast should be put forward as much as
possible in order to give greater manoeuvrability, so that in
the event of the gear becoming fast and the boat swinging
in cross-currents it would be less in danger of capsizing.
Mr. J. A. Tvedt (UK): Many skippers use different types
of gear on the same grounds for the same purpose and each
swears by his particular choice. Obviously they cannot all
be right. It was therefore important to try to find an im-
proved standard gear. As regards the desirable headline
height and net size, his company had worked out a design for
a trawl to work a little way off the bottom. They had reduced
the heavy ground gear so that it will partly roll and partly skim
the surface without scooping up the bottom. To further
reduce the churning up of the bottom their trawl boards were
made more buoyant and operated lightly over the ground
and were given increased lift. They did not subscribe to the
theory that sheer net size was the way to get a better catch.
They had gone the other way and were trying to produce a
reasonable standard mouth area but they had changed the
proportions of the mouth and had settled on something like
twice the Granton headline height and had also reduced the
twine in the trawl to the minimum size they thought necessary
to withstand rough usage. This design had been tried in
laboratory tests and they expected to have practical results
shortly. They expected to get greater spread with it.
Mr. D. L. AKerson (USA): Fishermen have to choose
whether they are going to catch fish on the bottom or above it
and fishermen in some areas carry two nets: one for flounders
on the bottom and one for redfish higher above the ground.
As to raising the trawl a little off the bottom to preclude
damage, his department had carried out some experiments.
For their specific purpose they towed three ways: (1) on the
bottom, (2) half a metre above the bottom, and (3) with a
box form of trawl just above the bottom, for towing half a
metre above the bottom they caught only one-eighth of the
quantity of the fish they caught on the bottom.
When fishing alongside some Russian vessels in the northern
area they found that the Russians rigged their gear so that they
had no weight on the groundrope itself but used a number of
droppers like sash weights each fastened on a fathom of rope
so as to just touch the bottom. They then waited till their
netzsonde showed the redfish had moved off the bottom and
then began towing and got good catches. When the Ameri-
cans in that same area put their gear on the bottom they
got very little fish. By timing their operation the Russians
did very well.
Dr. Miyazaki (Japan): A company in our area which
specialises in shrimp trawling uses two methods. In the first
method a bottom trawl is raised to midwater and in the second
the trawl is in midwater from the beginning. In the first
case, if the presence of shrimp is known, the trawl is raised
from the bottom while carefully watching the net conditions
through the netzsonde. The ratio between the depth and the
warp length should be between 1 to 2-5 and 1 to 2-3 and
the difference between the upper and lower otter board
back-strops about 2-50 m. In the second method, with the
trawl in midwater from the beginning — it is generally about
10 metres above the seabed — the ship's speed must be main-
tained after shooting the net. The netzsonde is again carefully
used. If the school of shrimp happens to be above the net
an adjustment should be made by the same method just
described. When the school is at a lower level, the adjust-
ment has to be made by slowing down the engines. The
maximum speed of lifting the net is 12 metres per minute
and the average about 9 metres per minute. Midwater
trawling can be carried out within a range of about 40 metres
by continuous adjustment in the vertical distance. No precise
formula exists for these operations but the greatest efficiency
depends on the captain's experience and skill.
Mr. Eldon Nichols (USA): Breaks in international cables
due to trawling are 40 to 50 a year in the North Atlantic alone.
Any one break might interrupt 100 telephone conversations
or cablegrams and it would take a boat sometimes a week to
get to the cable to mend it. A new cable had been laid across
the Atlantic between Britain, Iceland, Greenland and Canada
for International Air Control. Since last October up till
May there had been six breaks in that cable because of trawling
and it was still out of action. (Nevertheless he hoped dele-
gates at the congress who were flying the Atlantic would have
happy landings!) In his paper he stressed the desirability
of improving trawl board design if at all possible so that they
would not hook and catch the cables. If the otter boards
could be kept off the bottom it would help, or alternatively
the points of attachment might be made so that they would
fend off rather than hook on the cables.
Mr. Harper Gow: Your company and mine have been in
contact over the years on this problem. These cables spread
completely haphazardly over the best fishing grounds in the
world and the surprising thing really is that so few get caught
by trawlers as they carry out their operations.
Mr. Nichols: The number is being reduced by improved
methods and some cables are being lifted.
Mr. P. R. Crcwc (UK): The present method of attaching
the warps to the otter boards is designed to give them some
219
sort of stability. There was probably more hope of avoiding
damage to the cables by taking the oner boards completely
off the bottom but this might have disadvantages in regard to
fish behaviour, but it might also have advantages. He had
never done any work on fish until required by the research
work for the White Fish Authority, the British Trawlers
Federation and the British Government to take part in the
recent investigation. They were not charged with investigat-
ing fish behaviour but only the engineering aspects of trawl
boards and nets. The nature of their project nevertheless
made it necessary to do some general investigation regarding
the trawls and their mouth area and so on, and he had
attempted to develop general mathematical theories as to how
they behaved. These were set out in his paper which gave a
range of values for mouth areas and the warp lengths.
Dr. Schiife (Germany): In the Mediterranean the sweep:
lines attached to the otter boards are 120 fathoms in length
and are thus so long that they make their trawl something
between a trawl and a Danish seine. Without the long
sweeplines their catch would decrease very much.
Mr. A. Gronningsaeter (Norway): The cable routes should
be marked by electronic devices in the form of a grid so that
they can be avoided.
Mr. Nichols: The United States authorities mark the cables
on all the official charts that are, now issued. These charts
will also show the Loran grid and he understood that the
British Admiralty's charts in preparation would show the
cables as well. The cable companies had distributed tons of
Shifting the point of attachment might not provide the right
charts throughout the world to help trawlers know where
cables were and avoid them if possible. Some countries
had given excellent co-operation but in others progress was
very slow. They could not expect that it would ever be
100 per cent and that was why they looked, if possible, for
the fishing industry to do something about fishing gear.
Mr. Dennis Roberts (UK) agreed with Dr. Scharfe that
otter boards and sweeplines were essential. It was their
experience on the Humber that skippers who used longer
bridles caught more fish. Certainly expenses went up. For
bottom trawling to be successful they would have to be on the
bottom and use maximum length of bridles.
Mr. Margetts summing up said that many of the points
made related to fish behaviour and they were questions to
which biologists and technologists believed the answers to be
important for ensuring future development. On existing
engineering information, it was possible to design nets to
definite specifications, but what they did not know were the
particular values for any parameter as being most suitable for
catching fish. They required to know much more about fish
behaviour and he hoped that that theme would be developed
in other sessions of the Congress. Trawl designers should
heed the comments about submarine cables, but the most
helpful immediate measure will probably be the accurate
charting of cable positions, particularly with fixes by such
navigational aids as Loran, so that fishermen might keep
away from them. Mr. Alversorv s point showed that, in some
fisheries, if the net was raised off the bottom, the fishing would
become non-paying, and that problem related to fish behaviour
as well as to fish distribution.
220
Part 2 Bulk Fish Catching
Section 7 Midwater Trawling
One-Boat Midwater Trawling from Germany
Abstract
From 1958/59 the German efforts towards the development of
midwater trawling for herring were intensified. The present paper
deals only with the one-boat technique for which special nets,
otter boards and a net-depth telemeter ("netzsonde") were improved
step by step. The trials were started with small (24-m length and
150 to 180 hp) trawlers and gradually extended to the big long-
distance sterntrawlers with a stern ramp (70 m length, 2,100 hp).
The objective was to develop a trawl gear suitable for on-the-bottom
trawling, near-the-bottom trawling as well as real midwater trawling.
The gear was designed for operation with normal side- or stern-
trawlers without requiring additional auxiliary gear or special
training of crews. The trawlnets were made of nylon continuous
multifilament twine. The first model was a two-seam, high-opening
design requiring three bridles at each side and with 500 to 800
meshes (200 mm stretched) circumference. Recently a new four-
seam model with 1,200 to 1,400 meshes circumference and rec-
tangular cross-section (side panels narrower and shorter than top
and bottom panels) gave very satisfactory results. Curved all-steel
trawl boards (Suberkrlib) proved to be better than combined steel-
wood construction. Sizes of up to 6 m2 each have been used,
depending on the size of the vessel and the gear. The "netzsonde"
telemeter is an echo sounder connected by cable with a transducer
mounted on the headline, and gives information regarding the net-
depth, the opening-height and fish distribution in and below the
netmouth. Such information is of value for gear studies as well as
for commercial fishing. The catching efficiency of the one-boat
midwater trawl was satisfactory for spawning herring, but at first
rather poor for non-spawning herring in comparison with the two-
boat trawlers. Improvements to the design and size of the net and
boards increased the distance between the boards from about 30
to 50 m up to 65 to 90 m, and resulted in remarkable increase of
catching efficiency for non-spawning herring. Comparative fishing
trials indicate that this improved one-boat trawl will catch practi-
cally the same as two-boat trawls of comparable size. Herring has
been fished experimentally in the North Sea, the English Channel
and the Irish Sea, with main emphasis on the Norwegian coast
(Egersund and north up to about 60"N). In recent trials (February
1963) promising catches of non-spawning herring were also obtained
off South Iceland. It is expected that the new one-boat trawling
technique will become valuable when the conditions for bottom
trawling are less favourable. Apart from herring, also cod, saithe,
mackerel, sardines and other clupeids have been caught in quantities
and it seems likely that other fish stocks could also be fished with
this gear.
Chalutage ptlagique a un bateau, en allemagne
Rfeum*
Depuis 1958 les Allemands ont intensifie leurs efforts en vue de
developper le chalutage pelagique pour les harengs. La presente
etude ne traite que du chalutage pelagique a 1 bateau, pour lequel
des filets speciaux, des panneaux et des tglemetres de profondeur
des filets (Netsonde) ont 6te amdliores progressivement. Les
essais ont commence avec des petits chalutiers (24 m de long et
150 a 180 cv) puis ont etc etendus aux grands chalutiers hauturiers
pechant par Farricre (70 m de long et 2,100 cv) ayant une rampe a
rarriere. L'objectif etait de deyelopper un engin pouvant opercr
sur le fond, pres du fond, aussi bien qu'entre deux eaux. Les engins
ont etc construits pour des chalutiers normaux operant par le cdte
ou par Tarriere, sans necessity d'installation supplementaire d'acces-
soires auxiliaires ni entrainement special de l'6quipage. Les filets
ont etd fabriqucs en fil de nylon continu, multifilaments. Le premier
modele etait un chalut a grande ouverture, a deux coutures avec
trois entremises de chaque cote et avait de 500 a 800 mailles de
circonference (200 mm mailie etirte). Recemment un modele a
quatre coutures, de 1200 a 1400 mailles de circonfference, de coupe
rectangulaire (panneaux latc"raux plus etroits que les panneaux
superieur et inf6rieur) a donne des resultats tres satisfaisants. Des
panneaux de chalut, courbes (SUberkrUb) construits tout en acier
se sont reveles superieurs & ceux d'acier et de bois combines.
Suivant la longueur du bateau et la tailte du filet, des surfaces
by.
J. Scharfe
Institut ftir Netz-und Materiai-
forschung, Hamburg.
ayant jusqu'a 6 m2 chacune, ont etc utilisees. Le telemetrc
Netsonde est un echo-sondeur reli£ par cable electrique & un
dmetteur mont6 sur la ralingue superieure. 11 donne des informations
concernant la profondeur du filet, son ouverture verticale et la
repartition du poisson a 1'interieur et au-dessus de P ouverture du
filet. Ces informations sont de grande importance pour Petude des
engins de peche et pour la peche commerciale. En comparaison avec
celle des chaluts a deux bateaux, 1'efficacite de capture de ce chalut
pelagique a un bateau 6tait satisfaisante pour le hareng en frai
mais beaucoup moins pour le hareng ne frayant pas. Des modifica-
tions apportees au dessin et aux dimensions du filet et des panneaux
ont augmentd de 30-50 m jusqu'a 65-90 m la distance entre les
panneaux et ont permis un accroissement remarquable de Peffica-
cit6 de capture des harengs ne frayant pas. Des essais de peche
comparative indiqnent que ce chalut a un bateau amelior£ peut,
pratiquement, pecher autant que les chaluts a deux bateaux de
dimensions comparables. La peche au hareng a M conduitc
experimentalement dans la Mer du Nord, la Manche et la Mer
d'lrlande mais surtout sur la cdte Norvegicnne (Epersund) et au
Nord, jusqu'a 60°. Au cours d 'essais rccents (fevner 1963) on a
obtenu, au Sud de PIslande, de bonnes captures de harengs ne
frayant pas. On suppose que la nouvelle technique de chalutage
a un bateau deviendra tres int6ressante surtout lorque les conditions
de chalutage sur le fond seront mouns favorables. Outre les harengs,
des cabillauds, des monies charbonnieres, des maquereaux, des
sardines et autres clupeides ont 6t6 pris en quantit6s et il semble que
d'autres especes de poissons pourraient aussi &tre pechtes par cet
engin.
La pesca al arrastre entre dos aguas con una embarcac!6n en aienunia
Extracto
Desde 1958/59 se han intensificado las experiencias alemanas para
perfeccionar la pesca del arenque con artes de arrastre flotantes.
Esta ponencia se ocupa solamente de la tecnica en la que se emplea
una sola embarcaci6n, y para la cual se mejoraron poco a poco
redes y puertas especiales y un tetemetro que indica la profundidad
a que esta el arte (netzsonde). Los ensayos se iniciaron con arras-
treros pequsftos de 24 m de eslora con motores de 150 a 180 hp y
gradualmcnte se emplearon grandes arrastreros de altura con rampa
a popa (70 m de eslora y motores de 2,100 hp). La finalidad era
encontrar un arte que sirviera para pescar en el fondo, cerca de
6ste y entre dos aguas, y que lo pudieran emplear los arrastreros
normales que pescan por el costado o por la popa sin tener que
instalar material auxihar adicional o capacitar especialmente a las
tripulaciones. Las redes eran de nylon de ftlamentos continues.
El primer modelo construido consistia en un arte de deux reiingas
laterales, gran abertura en altura, tres bridas en cada lado y de 500
a 800 mallas de 200 mm de circunferencia. Recientemente un nuevo
modelo de cuatro reiingas laterales de 1,200 a 1,400 mallas de
circunferencia y secci6n rectangular (los paftos laterales son mas
estrechos y cortos que los superiores e inferiores) dio muy buenos
resultados. Las puertas de acero cuvado (SUberkrttb) rcsultaron
ser mejores que las mixtas de acero y madera. Se han empteado
hasta de 6 m1, segun las dimensiones del barco y del arte. El
tel^metro "netzsonde" es una ecosonda conectada mediante un
221
cable a un transductor roontado en la relinga de corchos que da
information respecto a la profundidad a que estd la red, su abertura
en altura y distribucidn de los peces ante la boca y debajo de ella.
Estos datos son de importancia para los estudios de los artes y en
la pesca industrial. El rendimiento de pesca de un artc flotantc
arraitrado por una embarcaci6n fue satisfactorio en el caso del
arenquc en desove, pero al principio, y con respecto a las parejas,
no lo fue para el arenque que no desova. La mejora de la forma y de
las dimensiones del artc y de las puertas incremento la distancia
cntre estas de entre 30 y 50 m hasta 65 y 90 m y produjo un notable
incremento del rendimiento de pesca de arenque que no desova.
Los ensayos de pesca comparativa indican que este arte mejorado
para una sola embarcaci6n pesca practicamente lo mismo que una
pareja de dimensiones analogas. El arenque se ha pescado experi-
mentalmente en el mar del None, Canal de la Mancha y mar de
Irlanda y, sobre todo, en la costa noruega (Egcrsund y hacia el
none hasta cerca de los 60°N). En ensayos recientes (febrero de
1963) al sur de Islandia tamb&n se obtuvieron captures alentadoras
de arenque que no desova. Se anticipa que la nueva tecnica de
arrastre con una cmbarcaci6n adquirirf gran importancia cuando
las condiciones para la pesca en el fondo scan menos favorables.
Adcmas de arenque, tambicn se nan pescado importantes cantidadcs
de bacalao, abadejp, caballa, sardinas y otros clupeidos y es probable
que tambi6n pudieran capturarse en estos artes otras poblaciones
icticas.
A schematic view of the prototype trawl gear developed
/\ during 1958/59 and thoroughly tested later is
given in Fig. 1. Details of the otter board and of the
one-boat midwater trawl net (two panels) are given in
Figs. 2 and 3.
This gear, tested in different sizes and with different
types of trawlers, performed apparently satisfactorily
from a technical point of view. On several occasions
during trials, and also on commercial fishing trips it gave
satisfactory catches of spawning herring in the North
Sea, the Irish Sea and the English Channel, and of cod
off the west coast of Greenland. However, extensive
Me«h- Twine
nix* ntmbcr
Td
too no /M
Cutting M«»h
rate
i«o tie/ 4i
110 HO/ IB
•0 SIO/31
•0 HO/SB
40 IIO/BB
>• IIO/BB doublo
Fig. 3. One-boat midwater trawlnet, two-sideseam type, consisting
of two equal panels.
otter boards warps
Fig. 1. Schematic view of a midwater trawlgear with two-seam net and Netzsonde.
222
fishing trials until mid-1962 showed that the catching
efficiency of this gear for non-spawning herring was not
adequate for commercial operation and very poor indeed
in comparison with that of the two-boat midwater trawls
which at this time were already to some extent in com-
mercial operation.
Fig. 2. Dimensional drawing of a Suberkrub type otter board of
3-2 mz as used in the trials with larger trawlers of up to 1,200 hp.
Observations and experiences collected during these
fishing trials indicated that two shortcomings were
probably mainly responsible for this inferiority of the
one-boat gear, namely (a) insufficient width of the area
swept and (b) less suitable design, particularly mesh
shape, of the two-seam net. To overcome these short-
comings, the distance between the otter boards had to
be increased considerably and the trawlnet design had to
be modified or even a new design had to be developed.
Interesting details covering these preliminary trials and
earlier efforts are available to all interested in the original
paper submitted to the Congress and in the reference
papers listed at the end of this article.
Effective modifications
After a last comparative fishing test in October 1962
(Table I), drastic modifications were made.
Table /.- Comparison of herring catches (in baskets of 50 kg each)
by experimental one-boat midwater trawler Sigofiich with pair-
trawlers Everting and Dahrendorf : Skate Hole, in October, 1962.
S&gefisch Everting, Dahrendorf Catch
relation
baskets per hour baskets per hour fishing day
Oct. per day towing per day towing Slgefisch
7
8
9
10
21
22
23
24
100
43
170
95
100
20
88
180
17
4
19
10
23
2
9
21
Total: 796
425
210
810
550
760
280
390
450
3^875
48
24
105
69
113
25
39
43
4-3
4-9
4-8
5-8
7-6
14-0
4-4
2-5
Modification of the trawl gear
To increase the distance between the otter boards their
size was almost doubled to 6 m2 each and the bridles
were lengthened to 100 to 150 m. For these larger otter
boards the proportions of the Suberkrub design were
retained but the construction was strengthened (Fig. 4).
Fig. 4. Suberkriib otter board of six m2 size constructed for the
modified one-boat trawlgear, during shooting from a side trawler.
The bridles are operated with the trawl winch by means of one mutual
pennant.
Since this larger size corresponds closely to that of the
ordinary flat otter boards of large trawlers, while the
weight was even less, no operation difficulties were
encountered. The mode of operation of the boards and
bridles remained unchanged. For the bridles steel wire
of 16 to 18 mm diameter was chosen.
Depending on the type and size of net, the length of
the bridles, the warplength and the towing speed, these
modifications almost doubled the distance between the
otter boards to about 65 to 90 m without unduly stretch-
ing the net itself sideways.
In order to improve the catching efficiency of the
trawlnets not only a modified two-seam net was designed
but also a new four-seam type, adapted for one-boat
trawling. (Most of the development work regarding
nets was done in close co-operation with the net-making
firm H. Engel, Kiel.) For the practical fishing tests a
223
sidctrawler of 1,250 hp was used, which represents in
type and power a good number of trawlers of the German
deep-sea trawler fleet (Fig. 5).
Fig, 5. The German deep-sea trawler Johannes Kriiss on which the
modified trawlgear was tested. This vessel represents, in regard to type
and power, a good number of sidetrawlers in the German trawler fleet.
A good deal of the midwater trawling for herring in
the North Sea and the North-East Atlantic is conducted
very near to or even on the bottom, with extremely high-
opening nets. In order to reduce the risk of tearing the
net when occasionally fishing on the bottom, the front
edge of the netting along the footrope was reinforced,
the footrope itself served with rope and the front part
of the lower net in some of the nets was made of stronger
twine.
So far the new two-seam type with 1,000 meshes
circumference, shortened wings and modified hanging
for more closed mesh shape has not been thoroughly
tested. It is therefore still in an experimental stage and a
detailed description here appears premature.
Of the four-seam type two sizes were made, one with
1,200 and one with 1,400 meshes circumference (Fig. 6).
The two main differences of this type, as compared with
the ordinary four-seam nets of the pairtrawlers, are the
rectangular (or rather oval) shape of its opening and the
slightly protruding lower panel. The rectangular shape
with the side panels narrower and shorter than the upper
and lower panels was chosen because earlier experience
had shown that it is difficult with the one-boat rig to open
sufficiently in vertical direction a net with equal panels.
This general trawl form, which is well known, e.g., in
Japan and the U.S.A., is so far rarely found in Northern
Europe. The forward extension of the lower panel is
meant to give the net a downward sheering tendency
to secure good bottom contact if desired. This feature,
together with the in-built additional extension of the
lower net by means of special hanging, also makes up
for the sag of the lower bridles caused by the heavy ground
weights (370 to 430 kg each). It furthermore allows the
otter boards to travel above the centre line of the trawl-
gear and this removes the warps, with their supposedly
disadvantageous scaring effect, even more out of the
path of the trawl.
The modified trawlgears technically performed very
satisfactorily. The main technical characteristics, as
were found during the limited number of fishing trials,
are compiled in Table II.
Propeller revolutions and speed were taken from the
instruments installed on board. Net-depth and opening-
height were measured with the Netzsonde and the
trawler's echo sounder. The distance between the otter
boards was deduced from the spread of the warps in the
sliphook. The distance between the wingtips was
calculated from the distance between the otter boards
considering the length of the bridles and of the net bag up
to the front edge of the tunnel.
Since so far only a limited number of measurements
could be taken during two trial trips in December 1962
and February/March 1963 to the Norwegian coast
(Egersund-Stavanger) and the Icelandic south coast
(Skeidardr Deep), the values reported here should not
be taken as final. They give, however, a clear enough
Table II
Gear type
1 ,400 meshes, 4-seam, bridles 1 50 m, ground-
weights 470 kg each
Dittc — groundwcights 370 kg each
1,200 meshes, 4-seam, bridles 1 50 m, ground-
weights 370 kg each
Ditto— bridles 100m..
1,000 meshes, 2-seam, bridles 90 m, ground-
weights 289 kg each
Ditto— bridles 100m, groundweighto 370
kg each
224
Propeller
r.p.m.
90
to
106
(full)
90
to
106
(full)
90
to
106
(full)
86
to
106
(full)
90
to
106
to
98
Speed
knots
3-8
to
4-2
3-3
to
3-9
3-2
to
4-2
3-1
to
4-5
3-4
to
4-1
3-4
to
4-0
Distance
between
Distance
Net depth
Opening
otter
between
Warp.
length fm
(headline)
m
height
m
boards
approx. m
wingtips
approx. m
200
110
20
70
24
to
to
to
to
to
300
170
22
80
27
3CO
110
approx.
approx.
approx.
to
21
75
25
150
150
90
17
85
25
to
to
to
to
to
450
247
22
90
27
350
105
15
60
23
to
to
to
to
to
450
205
19
75
29
300
106
17
60
27
to
to
to
to
450
196
65
29
325
130
17
..
_
to
to
to
350
135
18
I
1
J
i
1
fc
s s ?
I
s
c
ve>
225
Table III
Bail/ midwater trawling oatohee in baakete (50 kg eaoh) of
J« Kiflee (1,200 hp) rereue the oatohea of two-boat trawler* of
the one-boat trawler
different eiie and power
Bat*
J.KrflM
Bv*rlin*
Dan eke r
Thiele
and
Minden
and
Bielefeld
Bet mold
Wind
and «*a
and
and
Sohulte
atadthagen
6 D*o.
390/301-/
250/5
280/25
300/-
200/-
...I/
2-3*
7 H
170/25
160/-
homeward
bound
400/-
280/-
Stf 2
8 «
xxxJ/
XXX
XXX
XXX
a 7-9
9 "
30/3
30/-
XXX
XXX
WN 5 hUh
•¥•11
10 "
100/10
XXX
XXX
XXX
•mr 7
11 «
170/5
ieo/-
XXX
XXX
ohan^ne
12 "
XXX
X X JC
XXX
XXX
"7
HW 7-10
13 H
30/-
40/-
o o o^/
JL X X
2/-
000
mr 4-5
14 »
LJ&) .
450/-
575/-
XXX
XXX
K/
120/-
S-V 5-7
15 "
620/-
honewarC
bound
XXX
000
{L80/BCH
poo)
( 30/-)
(o o o)
I¥ 6-7
16 "
l*ft out b*oau** J.Kra** was in harbour to tak* salt
17 "
XXX
XXX
XXX
XXX
XXX
ao 7-8
18 »
XXX
* X X X
XXX
XXX
XXX
80 8
19 "
120/-
XXX
homewud
bound
40/-
50/-
SO 2-6
Totalt
1630/98
1110/5
855/25
700/-
702/80
201/-
Catoh
p*r day
on th*
ground*
125/*
123/1
122/4
3V-
54/6
34/-
Brarling/Dahrtndorf/ - modazn sidatravlaxv of aiailar •!•• and povar aa
Thiala/tf ••••!• • anall combination drifter/trawlers of 300 hp aaoh
Bmakar/Sohul ta , Mindan/Bialafald and D»tmold/3tadthagan - larga oombination drif tar/
trawl •rn of 600 hp •aoh
tha oatoh
1'ht figUMa in front of tha daah gir* tha herring oat oh. th« f iguraa bahind
of other »ark« table 0peoie*9 mainly aaitbe
* - - - « not proaent on the grounde
xxx- not fishing beoauee of bad weather
000* net damage
5/ • oatoh of one tow with partly damaged net not included
226
impression of the improvements achieved. As regards
the size of the opening, the experimental modified one-
boat trawls are now almost equal even to large two-boat
trawls (up to 1,600 meshes circumference). The variations
of shape and size of the net opening due to speed varia-
tions are within acceptable limits. Also in regard to
sturdiness the modified gears seem to be satisfactory for
commercial operation under reasonably favourable
conditions.
Catching efficiency
After some technical trials, which were required for
determining the proper rig of the modified one-boat gear,
comparative fishing trials were made versus experimental
and commercial two-boat trawlers fishing on the same
grounds. A first series of such tests was conducted in
December 1962 off the Norwegian coast near Egersund
where good schools of non-spawning herring were being
fished by midwater pairtrawlers of several nationalities.
The results are compiled in Table III.
When comparing this table with Table T, a very distinct
change in the relation of the one-boat catches to the two-
boat catches is quite obvious. While in comparison with
the former one-boat trawlgear the pairtrawlers caught in
average about five times more, with the new trawl gear
the one-boat trawler now caught as much and even more
than any of the German pairtrawlers fishing at the same
time on the same grounds. These satisfactory results
show that the one-boat midwater trawl must not neces-
sarily be inferior to the two-boat gear even for non-
spawning herring, and that the modifications made did
actually aim in the right direction.
The table shows further that during the trial period
fishing suffered a good deal from rough weather and
that, according to expectations, the pairtrawlers were
more affected by this than the single trawler. This fact
is of significance for winter fishing in the North Atlantic.
Pairtrawlers usually stop fishing at wind force 5 to 6
while a single trawler can carry on until about wind force
7 to 8. The superiority of single trawlers in this respect
is shown in the last line of the table, i.e., "Catch per day
on the grounds" where all days were considered during
which the vessels fished or could have fished except for
the weather.
The results of this first trial period of only 13 days can,
naturally, not be taken as final. The total catches also
of the pairtrawlers were not too impressive which must
be attributed to the poor weather. There are, however,
no obvious reasons so far why also under more favourable
conditions the one-boat trawler with the new trawl gear
should not be able to keep pace with the pairtrawlers as
well. The largest catch per tow of the one-boat trawler
was 21 tons, and the best daily catch was 31 tons of
herring. The saithe caught was of large size and prime
quality.
These promising results encouraged the testing of the
modified one-boat gear on another non-spawning herring
stock, i.e., the winter herring off the Icelandic south coast.
The programme of the next trial trip, again with the
sidetrawler J. KrUss, included therefore a short trial
period in this area. Thanks to the generous assistance
of the Icelandic colleagues and authorities, good herring
concentrations could be spotted without delay outside
the national fisheries limits at Skeidar£r Deep-Sidugrunn.
At this time of the year (February) this herring is fished by
Icelandic purse seiners during night when it is near the
surface. During the day it migrates downwards and
stays even near the bottom.
Because of the very poor weather (the purse seiners
could not fish) and some technical trouble, /. Kriiss
could actually fish only for one day, on 24th February,
1963. On this day the modified one-boat gear, with the
new large (1,400 mesh circumference), rectangular four-
seam trawlnet, was used. The first tow in the morning
yielded nine tons and the second tow at noon and early
afternoon 23 tons of herring. Due to a misinterpretation
of the Netzsonde echogram, the third tow was extended
too long in order to clear first the deck from the earlier
catch (Fig. 7). Consequently the bulk of the fish, which
Fig. 7. Large catches are taken on board by splitting into bags of about
two tons each as usual on sidetrawlers. The photo shows a catch of
about 23 tons of non-spawning herring taken with the 1,400 mesh,
four-seam net off the Icelandic coast on 24th February, 1963, in a tow
of three hours.
was caught in the very beginning of the tow, had died
and become very heavy. When hauling this catch, the
auxiliary lines (heavy nylon rope) parted and it was only
with the greatest difficulty that the net could be hauled
until the beginning of the completely filled-up tunnel
227
came near the surface. Thus at least a good guess could
be made, according to which the catch amounted to
somewhat more than 20 tons of herring. Unfortunately
the net could not withstand the stress of this heavy catch
which was held from the mast of the vessel rolling in
rather high seas. It parted before splitting could be
started and tunnel and codend with the catch were lost.
A catch of about 30 tons in two tows, or even per day,
is considered good herring trawling anywhere. Regarding
the total catch of this day, together with the 20 tons lost,
the catching efficiency of the modified one-boat trawl for
this Icelandic herring must be considered as very satis-
factory indeed. Although of a preliminary nature, this
positive result could become of significance for the
German sea fishery and it is hoped that the fishing indus-
try will soon start to take advantage of the new possibi-
lities offered.
Together with herring some cod and redfish were also
caught, indicating that the gear may prove to be efficient
also for these species.
Conclusions
The preliminary results of the comparative fishing
trials off the Norwegian coast and of the fishing trial off
the Icelandic south coast appear to indicate that, with
the modifications applied, a significant step forward has
been achieved towards one-boat midwater trawling as
well as for active non-spawning herring in the North
Atlantic. Naturally a great deal of further experience will
be needed for the adaptation of this fishing technique
to other fish stocks and fishing conditions even in this
general area.
For spawning herring, which in the North Sea often
occurs in very dense schools yielding large and extremely
heavy catches, a smaller and much stronger trawlnet will
be required. For cod, saithe and eventually redfish, on
the other hand, the same trawlnet as for non-spawning
herring, only with larger meshed aft belly, tunnel and
codend, will probably be suitable. The operation from
sterntrawlcrs with sternramp will require the usual
strengthening of the aft net, tunnel and codend. Parti-
cularly for large trawlnets, of 1,000 meshes circumference
and more, the four-seam type appears to be better
suited than the two-seam type.
Not only can four-seam nets be made shorter, but they
are also easier to handle because they need only two
instead of three bridles at each side. In order to fully
utilise the large towing power (2,000 hp and more)
of the big, modern sterntrawlcrs, even bigger nets will
be required. In view of the limitation of the length and
also the hauling of heavy catches over the sternramp,
rather new and unorthodox designs will probably
have to be developed for the construction of such large
nets of more than 1,600 meshes circumference.
Further modifications of the rigging will for instance
be concerned with trawling on, or rather close over,
rough bottom. For trawling near the surface, operational
means like towing in circles will probably have to be
developed in order to tow the net outside the propeller
wake which may scatter or disturb the fish schools.
For rational midwater trawling, navigational aids
for spot plotting and optimal means for fish detection
are of particularly great importance. Echo sounding
and echo ranging are actually indispensable for detecting
fish schools in advance and also for observing their
movements until the net arrives.
With the introduction of other modern techniques
new means will, of course, be found for further increasing
the catching efficiency of trawling. This includes the
possibilities of applying electric fields, optimum relation
of towing speed to size and design of the net, improved
release of water through the net to decrease towing
resistance, attraction and guiding of fish, which are
being considered already. Completely new ones will
certainly develop in the future.
This paper deals only with some phases of a continuing
development during which, however, results of some
practical importance have already been achieved. The
one-boat midwater trawl design reached so far is already
considered suitable for careful application in commercial
fishing. Work will be continued to develop this gear
into a still better combined midwater and bottom
trawlgear suitable for a wide range of fishing conditions,
including part of the present conventional bottom trawl-
ing. This gear will, however, not attempt to substitute
for the conventional bottom trawl, in particular the
models for trawling on rough bottom, but rather to make
trawling more versatile and thus help to improve the
economy by extending the scope of operations.
References
v. Brandt, A. and Steinberg, R.: Schwimmschleppnetzversuche
im Gebiet von Egersund (1961). Protokolle zur Fischereitechnik,
Bd. VII, pp. 256-328. 1962.
Sch&rfe, J. : Fernmeldender Tiefenmesser fUr Schwimmschlepp-
netze. Fischereiwelt, Bd. VIII, pp. 24-26. 1956.
Sch&rfe, J.: Das Messen der Tiefenlage von Schwimmschlepp-
nctzen. Rcferat auf der Diskussionstagung des Fachausschusses
Akustik im Fachausshuss flir Funk-und Schallortung, Hamburg.
1958.
Sch&rfe, J. : Report on one-boat midwater trawling experiments
in the North Sea in December 1958 and February/March 1959
FAO Report 59/11/9452, FAO Fishing Gear Section, Rome. 1959.
Scharfe, J.: A New Method for "Aimed" One-Boat Trawling in
Midwater and on the Bottom. Studies and Reviews No. 13, General
Fisheries Council for the Mediterranean. 1960.
Scharfe, J. and Steinberg, R.: Neue Erfahrungen mit Schwimm-
schleppnetzen. Protokolle zur Fischereitechnik, Bd. VIII, Heft 37.
1963.
Steinberg, R.: Einschiff-Schwimmschleppnetz Versuche mit
eincm Logger vom 17.11.— 3.12.1959 in der Irischen See und in
Kanal. Protokolle zur Fischereitechnik, Bd. VI, pp. 235-253. 1960.
SUberkrUb, F.: Otter Boards for Pelagic Trawling. Int. Fishing
Gear Congress, 1957 Paper No. 71 and "Modern Fishing Gear of
the WorlcT, 1959, pp. 359-360. 1957.
228
Universal One-Boat Midwater and Bottom Trawl
Abstract
During recent years experiments were carried out in Poland to
develop a universal one-boat midwatcr and bottom trawl for the
sprat and herring fisheries. The experiments were carried out with
the 1,350 hp deep-sea trawler Miedwie, using a four-seam trawl
of 680 mesh circumference, 220 mm mesh size in the rounding,
with 3,050x1,340 mm flat trawlboards. The paper gives full
details of the gear and the instruments used, as well as the specific
results of the experiments which cover the fishing depth of the
trawl in relation to warplength and towing speed, its resistance and
horizontal and vertical openings. The graphs allow clear inter-
pretation of the results. During the experiments it was found that
by increasing the engine revolutions, the trawl could be lifted safely
to pass over bottom obstructions seen on the depth recorder.
Chalut a un bateau pour la pfehe p^lagique et sur le fond]
Rfeumt
Depuis quelques annees des expeiimences ont 6t£ conduites en
Pologne, dans le but de d6vclopper un chalut & un bateau pour la
ptehe pdlagiquc et sur le fond, destin6 a la pfcche du hareng et de
resprot. Ces experiences ont ite effectuees avec le chalutier hautu-
rier Miedwie de 1,350 cv, utilisant un chalut & quatre coutures,
de 680 mailles de circonference, en mailles de 220 mm, avec des
panneaux ordinaires mesurant 3, 050 x 1,340 mm. La communica-
tion donne les details de la construction des engins et instruments
de mesure employes aussi bien que les resultats obtenus et traite de
la relation entre la longueur des funes et la vitesse de touage d'une
part, la profondeur d'op&ation, la resistance et Touverture hori-
zontale et yerticale d'autre part. Les graphiques permettent
rinterpr&ation claire des rtsultats obtenus. Au cours de ces expSri-
ences on a remarque qu'en accelerant la vitesse des machines, on
pouvait facilement lever le chalut au-dessus des obstacles observ6s
sur Fecho sondeur.
Embarcacifa universal para la pesca con artes pelagicos y de fondo
Extracto
En los ultimos afios se ban realizado experimentos en Polonia para
encontrar una embarcaci6n que pueda emplearse indistintamente
para pescar espadin y arenque en el fondo y entre dos aguas. Los
experimentos se realizaron con el arrastrero de altura Miedwie, de
1,350 hp, empleando un arte de cuatro costuras, de 680 mallas de
circunferencia, mallas de 220 mm, y pucrtas planas de 3,050 x
3,340 mm. El autor da detalles completos del material y de los
instrumentos empleados, asi como de los resultados especfficos
de los experimentos, que comprenden la profundidad de pesca del
arte con relaci6n a la longitud del cable y velocidad de remolque,
su resistencia y las aberturas horizontal y vertical. Las graficas
permiten interpretar claramente los resultados. Se observb durante
los experimentos que aumentando las revoluciones del motor el
arte se levantaba y pasaba por encima de obst&culos del fondo
indkados en el registrador de la profundidad.
POLAND'S first experiments of midwatcr trawls were
conducted on the sprat schools in the Baltic Sea in
1959, during which two-boat mid water trawls were used.
It was then observed that the swimming speed of sprat
allowed them to be caught by one-boat midwater trawls
operated by cutters. So trials were made with one-boat
trawls of synthetic fibres equipped with the ordinary flat
otter boards to operate in midwater.
These proving successful the experiments were extended
to the bigger vessels so that by 1960 the research vessel
Birkut (33 m in length) caught 250 barrels (bbl) of
herring during midwater trawling on Faren Deep grounds
in the North Sea. Subsequently, plans were made for
conducting such a fishery with a supertrawler. The traw-
ler used was the Miedwie, 1,350 hp, and all measuring
equipment was prepared by the technologists of the
by
S. Okonski
Sea Fisheries Institute, Gdynia.
Sea Fisheries Institute and commercial fisheries com-
panies. The objectives were to investigate the behaviour
of the nets and the possibilities of applying ordinary
flat trawlboards to midwater operations, as well .as
to bottom fishing, with this type of vessel.
Although other types of boards have better hydro-
dynamical characteristics, the flat rectangular trawlboards
were used on purpose to avoid difficulties in introducing
the new method in commercial fisheries. It is well known
that fishermen are often unwilling to accept radical
changes, whereas a not too different rig can be readily
accepted. The gear was fully designed, constructed and
assembled ashore to avoid any major work or changes
on board ship. However, some small problems were
encountered during the experimental trip, but they
were solved by the technologists.
The gear
Full specifications of the net are provided in Fig. 1.
All mesh sizes are given for stretched mesh and lengths
in metres. The wings and front part of the body are made
of polyamide (nylon type fibre) double twine of 1*8 mm
diam, while the rest of the body is constructed of single
twine, same diameter. The whole net is made up of four
identical panels, one of which is seen in Fig. 1. Bosom
corners of each panel have a wedge piece to even out
strain.
The headline has 55 spherical aluminium floats of
200 mm diam attached at every 40 cm in the bosom,
every 50 cm at the quarters and every 60 cm in the
wings. The footrope construction was very light, with
a total of 30 kg chain. The chain was divided into short
lengths; six pieces were attached to the bosom of the
footrope, two of them at the extremities of the quarter
wedges and one piece in the middle of the wings. At
the tip of each bottom wing, a weight of 50 to 60 kg
was attached by 60 cm long strops (Fig. 2).
For pulling up the codend, the net was equipped with
two porklincs, one attached to the codend bccket while
the second was attached to the middle of the body.
Both lines were of synthetic fibre of 14 mm diam.
The net had two quarter ropes of 55 m length attached
at the belly quarters and led through the upper quarters.
The net was operated with upper and lower legs which
229
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F/£. 7. De/a// of trawl constructiorTjipnc panel).
differed in length by 2-5 m, the upper legs being 82-5 m
each and the lower legs 80 m. All legs were made of
12 mm diam steel wire, although in some cases the
lower legs were replaced by thicker combination rope
of the type used for the bridles.
The otter boards
The boards were of the normal flat oblong type of 3,050
mm X 1,340 mm, weighing approximately 900 kg each, and
were specially fitted for midwater trawling by means
of two sets of bridles instead of the usual single bridles
at the back of the board (Fig. 3). The angle of attack of
the boards had been calculated for bottom trawling and
the length of the strops differed to ensure that the boards
took up a stable position during operations in midwater,
so that the upper strops were 2-1 m and 3*6 m, while the
lower strops were 2*25 m and 3*75 m, attached as shown
in Fig. 3.
The experiments
The measuring instruments used during the experiments
71*. 4* Baadlim d*pth at rarlou*
- " i (190 r.p.«.)
fi«. 4b B»mdlin* tepth «t rariou* proj»ll«r •p**d»
(at 1^0 » Unffta of vacpa)
Fig. 2. General arrangement of" Universal" trawl. Symbolic haulinfjrawler indicated on top.
230
6100M
lower leg
t* legs
13401*
762mm-
•763ML-
Fig. 3. Details of trawlboard rig.
comprised :
(a) A sal log.
(b) Two dynamometers with a range of 0-15 tons for
measuring warp tension.
(c) A bathygraph with a range of 0-120 m for measur-
ing operational depth of the trawl.
(d) Differential bathygraph with a range of 0-20 m
for measuring the vertical opening of the net.
(e) Wind gauge.
(f) A tachometer for measuring the propeller speed.
(g) An anglemeter for measuring the angle of the
warps at stern.
During the experimental hauls, measurements were
made to ascertain the shape of the trawl in action whilst
towing at different speeds and with different lengths of
warp. The data obtained during these trials were worked
out to averages, which are presented in the graphs and
illustrations given in Figs. 4 to 11.
Fig. 4a shows the graph of the headline depth obtained
during the trials in which different warp lengths were used
at constant engine revolutions. In Fig. 5a this information
is plotted as a curve and, as can be seen, the relation
between the warplcngth and the trawl depth is not linear
at constant speed.
Fig. 4b shows the graph of the headline depth obtained
when changing the towing speed while keeping the warp-
length constant. The values are plotted in Fig. 5b and
the curve shows that the change in headline depth, due
to change of speed, is not constant and is greater at
low towing speed than at high towing speed.
Figs. 5a and b (on tow) (a) Relation of headline depth to warplength at con-
stant speed; (b) Relation of headline depth to towing speed (150 m warplength).
Figs. 6a and b (a) Relation of warp tension to towing speed for 150 m
length; (b) Relation of warp tension to warptength at constant speed.
Drawl resistance ID relation to towing speed and warp
length
The warp tensions obtained during several hauls are
shown in Fig. 6. The total tension at five knots was
about 10 tons. Whereas the load to speed ratio increases
rapidly at higher towing speeds, the increase in load as
more warp is veered out at constant speed is almost
linear. The amount of warp to be veered out to allow
the gear to operate one metre deeper differs according to
the length of warp already veered or, respectively,
the depth at which the gear is operating. It was found
that, at constant engine r.p.m., to allow the net to sink
one metre deeper it required:
7*8 m more warp when fishing with 100 m warp out
6-5 m „ „ 150m
5-9 m „ „ 200m
5-0 m „ „ 250m
4-3 m „ „ 300 m „
The above values are, of course, only valid for the
gear used, so that even a change in the rig-up of the
trawl will result in a different ratio.
As shown in Figs. 7 and 8, the distance between the
otter boards depends on the speed of towing and the
length of warps. Legs of 52 m, 80 m, and 95 m were
tested and, within this range, the length of legs did not
affect much the distance between the boards. On the
Ftgi. 7a and b (top) (a) Relation of horizontal opening to towing speed (150 m
warplength); (b) Relation of horizontal opening to warplength (constant towing
speed).
tf !•«•
other hand, the leg length has great influence on the
horizontal opening of the net. Longer legs give smaller
horizontal net opening. From the purely technological
point of view (leaving out biological considerations),
60 m was found to be the optimum length of legs for this
type of midwater trawl.
Headline height
The headline height of the trawls was measured by differ-
ential bathygraphs constructed by S. Okonski and J.
Zaucha. A typical graph from this instrument is shown
in Fig. 9, one of which gives the headline height variation
Ftgi. it and b Co) Effect of leg length on horizontal opening In relation to warp-
length; (b) Average horizontal opening in relation to warplength.
232
Fit . ?b BMdliM toifht in r«Utioa to
varpl«ofth at ooMtut r.p.B.
in relation to towing speed while the other shows the
headline variation in relation to warplength. The infor-
mation contained in the graphs has been plotted as a
curve in Fig. 10, which clearly shows that the towing
speed has much greater effect on the headline height
than the warplength. At 3 knots the opening-height
was 14-4 m but at 5 knots it was 1 1 -3 m.
Taking into account the very light rigging and the
overall dimensions of the trawl used, the average head-
line height of 12 m at a speed of four knots would appear
to be very satisfactory. This headline height could,
however, be increased by a heavier rigging and the use of
hydrodynamical lifting power. Furthermore, a bigger
19
U
Figs. lOa and b (top) (a) Headline height in relation to warplength at constant
r.p.m.; (b) Headline height in relation to towing speed at 150 m warplength.
Fig. 11 The shape of the trawl during towing.
net could be used as the gear was small for the engine
power of the vessel.
Fig. 12a. Shooting. Arrangement of leg strops on otter board.
Area of the trawlmouth
Using the above data on headline height and horizontal
opening, the catanary of the headline and the footrope
can be calculated and the area of the netmouth is then
obtained for different conditions of towing speed and
warplength.
Keeping the engine at constant revolutions, the area
of the netmouth was found to change as follows for
the different warplengths:
Warplength Area at wingtips Area at bosom
m m2 m2
100 140 117
150 150 132
200 169 141
250 176 145
300 178 147
When the warplength is kept constant with an increas-
ing towing speed, the area of the trawlmouth was found
to be maximum at 185 r.p.m.
at
r.p.m.
150
185
195
205
215
Area at wingtips Area at bosom
m38
158
183
181
176
170
m58
131
151
149
146
140
Fig. 1 1 shows the general configuration of the trawl
in action. The experiments generally conform that the
pelagic trawl is a delicately balanced fishing gear which
required close co-ordination of the many factors in-
fluencing its shape and operation.
Fig. lib. Releasing the quarter rope during hauling (note sinkers on
lower leg).
233
Operetta techniques
The operation of the "Universal" trawl is similar to
that of the ordinary bottom trawls. In hauling and shoot-
ing the main difference lies in the absence of bridles
and danlenos and, on the other hand, the use of long
legs which, being connected through Kelly eyes and
pennants, are led straight onto the winch drums.
The gear can be shot while the vessel is stopped or
steaming round, in the same manner as the ordinary
bottom gear. The wire of the upper and lower legs should
preferably be of right and left twist respectively, to avoid
their twisting together during the hauling. In the rig
used, the lower leg was stoppered through the Kelly
eye, whereas the upper leg was connected to the same
stopper outside the Kelly eye (Figs. 3 and 12 a). The
addition of two leg-strops at the back of the board
improved the stability of their performance, especially
during poor weather. For operation on the shallower
Sandettie grounds in the English Channel, the legs were
shortened to 52 m.
Fishing results
The 1962 herring season on English Channel grounds was
very poor and this gave an opportunity to investigate
the comportment of the gear to other than midwater
towing conditions. The purpose was to assess the possi-
bility of operating on the bottom as well as near the
surface. Three days were devoted to this work and during
some of the tows, though there had been no trace on the
echo sounder, up to 10 bbl of herring were caught.
The distance of the footrope from the bottom could be
regulated very accurately. With the footrope chain
arranged as previously described, the footrope functioned
at 80 cm above the bottom; when 10 kg of chain was
added to the wings this distance was reduced to 30 cm;
with 10 kg more chain in the bosom the net touched
bottom.
During these experiments the possibilities of lifting
the gear over obstacles was also investigated and it was
found that this could be done easily by appropriate
increase of the engine revolutions. Fig. 13 gives the echo-
recording of one occasion when the echo sounder showed
10 to 12 m rocks straight ahead. The dotted lines show
the track followed by the gear when the engine speed was
inciyased. About 1*8 min after increasing the speed, the
net had risen four m; 3-2 min later it had risen llm
and after a further 3-7 min it was 18 m above the bottom,
passing clear of the rocks.
The experiments have shown that this gear can be
operated easily at any depth. There remain, however,
some small problems, one of which is the provision of
cheap and efficient net telemeters for commercial fishing
operators.
Fig. 13. Echogram of the gear being lifted over obstacles.
234
Two-Boat Midwater Trawling for Herring
with Bigger Boats
Abstract
In 1961 and 1962, the German trials with two-boat midwater trawl-
ing for herring in the North Sea were extended to larger vessels
of the lugger type (300 tons, 40 m long, 450 to 600 hp) and big
trawlers (400 to 650 tons, 43 m to 53 m long, 600 to 1,250 hp).
The experiments have shown that the German luggers can adopt
two-boat midwater trawling for herring on a commercial scale
without serious technical difficulties. Since good manoeuvrability
is essential when the boats have to come closer together during
shooting and hauling, controllable pitch propeller, diesel-elcctric
drive and/or active rudder are the more desirable the larger the
vessels are. Even big long-distance side-trawlers can operate a
two-boat midwater trawl if good manoeuverability is secured by
one of the above means. The operation costs of such large vessels
are, however, so high that two-boat trawling would only be profit-
able under exceedingly good fishing conditions. The German
combined luggers can operate at moderate costs. Consequently,
they are adopting two-boat midwater trawling for herring at an
increasing rate and the economic results have so far been highly
satisfactory. In the beginning the larger pair of trawlers used four
seam trawlnets of a similar type as are common for smaller vessels
in northern Europe, but of appropriate size, i.e., 1,400 to 1,600
meshes (mesh size 200 mm stretched) circumference in the mouth
giving an opening height of 14 to 20 m. The trend now is, however,
to adopt a trawl-type with "rectangular" opening and modified
rigging.
Chalutage pelagique a deux bateaux pour grands chalutiers
Resum*
En 1961 et 1962 les essais allemands pour developper 1e chalutage
pelagique a deux bateaux pour le hareng, dans la Mer du Nord, ont
et& etendus a des bateaux plus grands du type lougre, de 300 tonnes,
40 metres de long et de 450 a 600 cv et aussi a des chalutiers
hauturiers (de 400 & 650 tonnes, 43 a 53 metres de long, 600 a
1,250 cv). Les experiences ont montre que les lougres allemands
s'adaptaient au chalutage pelagique a deux bateaux, a une &chelle
commerciale, sans difficultes techniques serieuses. Une helice a
pas variable, un entralnement electrique Diesel, un gouvernail actif
sont des facteurs desirables pour les grands chalutiers car une
grande manoeuvrability est essentielle surtout quand les bateaux
se rapprochent pour echanger les lignes. M6me les bateaux de
peche lointaine operant par le cote peuvent utiliser les chaluts
pelagiques a deux bateaux s'ils obtiennent une bonne manoeuvrabi-
lite par les moyens decrits ci-dessus. Cependant le cout d'operation
de ces grands bateaux est si &leve que le chalutage a deux bateaux ne
peut dtre profitable que sous des conditions extremement favorables,
tandis que les frais des operations des lougres allemands sont plus
moderes. C'est pourquoi les lougres allemands ont adopte rapide-
ment le chalutage a deux bateaux et les resultats ont 6t£ tres
satisfaisants. Au premier, les grands chalutiers "pareja" utilisaient
des chaluts a quatre coutures du meme type que ceux utilises par
les petits chalutiers dans le Nord de PEurope, mais d'une grandeur
appropriee, c'est & dire de 1,400 a 1,600 mailles (de 200 mm, ma i lie
ttiree) de circonference & 1'ouverture ce qui donne une puverture
verticale de 14 a 20m. Neanmoins la tendance aujourd'hui est pour
un chalut avec une ouverture rectangulaire et de montage modified
Empieo de parejas mayores para la pesca al arrastre pdagica del
En 1961 y 1962 los ensayos practicados por los alemanes con artes
de arrastre pelagicos remolcados por las embarcaciones que pescan
arenque en el Mar del Norte, se ampliaron de manera que compren-
diesen otras mayores del tipo llamado "lugger" (300 ton., 40 m de
eslora, motores de 450 a 600 hp) y arrastreros grandes (de 400
a 650 ton, 43 a 53 m de eslora y motores de 600 a 1,250 hp). Los
experimentos ban demostrado que, sin graves dificultades tecnicas,
las parejas de "lugger" alemanes pueden pescar arenque con artes
pelagicos en escala industrial. Como la maniobrabilidad es esencial
cuando los barcos tienen que acercarse para largar y virar la red,
cuanto mas grandes son mas conviene el empleo de propulsores de
paso variable, transmisiones diesel elcctricas y timones activos, o
by
Rolf Steinberg
Institut fur Netz- und Materialforschung,
Hamburg
uno de los dos ultimos. Incluso los arrastreros de altura que pcscan
por el costado pueden emplear el arte pelagico remolcadp en pareja
si se les da buena maniobrabilidad por uno de los medios citados.
Sin embargo, la explotacidn de barcos tan grandes es tan costosa
que la pesca en pareja s61o es provechosa en condiciones excepcional-
mente buenas. La explotaci6n de los "lugger" combinados atemanes
es rclativamentc modesta, por lo que cada vez hay mas parejas de
ellos dedicados a la pesca del arenque con artes pelagicos, habiendo
obtenido hasta ahora resultados muy satisfactorios. Originalmente
las parejas mayores empleaban artes de cuatro relingas de un tipo
parecido a acquellos utilizados para las mas pequenas buques
del norte de Europa que pescan con artes, pero de una dimension
conveniente, as decir de 1,400 a 1,600 mallas (de 200 mm cuando
estan estiradas) de circunferencia en la abertura, lo que de una
abertura vertical de 14 a 20 metros. Hoy dia, pero, la tendencia es
de emplear las artes con una abertura rectangular y aparejo modi-
ficado.
AN extensive and very successful herring fishery with
two-boat midwater trawls has developed in the
North Sea. Until a few years ago only smaller boats —
cutters of 60 to 100 tons— were engaged in this fishery.
These cutters are easily manageable and have relatively
powerful engines (up to 600 hp).
The good commercial results of the cutters suggested
that bigger vessels would also be suitable, so two-boat
midwater trawling trials were carried out by the Institut
fur Netz- und Materialforschung of the Federal Republic
of Germany. The main problem in two-boat trawling
is the passing of lines from one boat to the other during
shooting and hauling. Tt was found that the suitability
of vessels for two-boat trawling depends more on manoeu-
vrability than on size. Accordingly boats fitted with
controllable pitch propeller or diesel-electric drive
proved best suited because propeller performance,
including reversing, can be controlled quickly and
efficiently from the wheelhouse. Boats wijh an active
rudder are also relatively easy to manoeuvre and are
consequently suitable too. Big trawlers without such
equipment are less suitable for two-boat trawling.
Four-seam trawlnets are mostly used in two-boat mid-
water trawling. The German luggers (600 hp) now use nets
of 1,400 meshes circumference at the mouth (mesh size
200 mm stretched— Figs. 1 and 3). For the first trials with
large German trawlers (of about 1,000 hp) a trawlnet
of 1,600 meshes in circumference (mesh size also 200 mm
stretched) was designed and gave very good results
(Fig. 2). The opening height of the lugger nets ranges
between 14 and 18 m, and that of the trawler net between
16 and 20 m.
Because of the very big catches taken occasionally,
235
mx 99 m
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1 P O
1 P 4 * "
1 P 2 b 00
1 P 2 b *0
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2 P 1 b 1 0
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atO » 4« 4*
Fig. L Structure of one panel of a four-seam two-boat midwater
trawlnet for luggers (about 600 hp each). The net consists of four
such panels.
•11 4*
Miati
** / 4M
Fig. 2. Structure of one panel of a four-seam two-boat midwater
trawlnet for larger side trawlers (about 1,000 hp each). The net
consists of four such panels.
50-90(00-90) float* «L)
O ($5) m combined ropt 15(30) kg
4« The rising of tho German two-ton t nidwater tr^iwl gear Tor
luggero and trawler 0 (numbers in brockets).
•f-Cf m
Schematic view of the Oermnn two-boat midwater trawl
gear for lugger a, net with "rectangular" opening.
236
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237
net material of high breaking strength must be used,
particularly since the netting twine should be as thin as
possible in order to provide optimum water flow through
the netting and minimum frightening wake in front of
and in the mouth of the trawl Nylon or 'Perlon*
(polyamide 6) continuous multifilament twines were
mainly used. During the extensive herring fishing trials
in the North Sea the twine strengths given in Figs. 1, 2
and 3 were found adequate. Finer netting is not recom-
mended because it does not withstand the stress of bad
weather and/or large catches. Sufficient elasticity of the
net material is of great importance for absorbing shock
loads and unavoidable distortions of the net.
Arrangement of the legs and weights used in the first
trials is shown in Fig. 4. Of the figures given, the first
one refers to the lugger gear and the second one, in
brackets, to the trawler gear. According to their size
the trawlers use longer legs than luggers do. In this way
the handing over of the legs from one boat to the other
during shooting and hauling is simplified. The 25 mm
wire between the upper wingtips is very important be-
cause it prevents damage to the trawlnet in case the dis-
tance between the two boats becomes too wide during
shooting and hauling. For modified rigging of the two-
boat midwater trawl with rectangular opening see Fig. 5.
Here the big weights are fastened near the lower wingtips
rather than at the juncture of the lower wings and legs.
The warps normally used by these vessels in bottom
trawling are considered too heavy for midwater trawling,
causing a small opening-height of the net mouth. There-
fore the German luggers and trawlers are using warps
of 16 mm diameter for midwater trawling instead of the
normal 19 to 24 mm warps.
As a net depth telemeter, all German luggers and traw-
Fig. 6. Method of shooting the net from the "netzsonde" boat.
238
Fig. 7. Method of hauling the net on the "netzsonde" boar.
lers use the so-called "netzsonde" so that the depth of
the net is recorded continuously during fishing, as well
as the opening-height of the net and the fish passing
through the net mouth, etc. Moreover, it indicates the
behaviour and distribution of those fish so that valuable
conclusions can be drawn for the most efficient tactics
of operation. (Mohr this Congress.) With the "netzsonde"
much bigger catches are obtained on average.
In the two-boat trawling normally only one boat is
equipped with a complete "netzsonde" unit. The other
has only a separate headline transducer with about
120 m of cable attached to her trawlnet. If the net of
this boat is shot, this cable is connected with the main
cable and the electronic unit on board of the "netz-
sonde" boat during shooting (Fig. 6). In this way the
one "netzsonde" echo recorder unit can be used with
the nets of either partner at will.
In two*boat midwater trawling two methods are used
for shooting and hauling (Figs. 6-9). When the
"netzsonde" boat is shooting (and hauling), the method
1 shown in Figs. 6-7 is applied. When the partner boat
is shooting her net, method 2 (Figs. 8-9) is used. In
Fig. 8. Method of shooting the net from the partner boat. The cable
of the headline transducer is connected with the main cable on the
"netzsonde" boat by means of a special plug.
this way the "netzsonde" boat is always on the same
(port) side of the partner boat when towing. This arrange-
ment is necessary because roller and winch equipment
for handling the cable is directed to the starboard side
of the "netzsonde" boat.
The depth of the trawl is regulated by variations of
towing speed and/or the length of warps. This is the
same for one-boat and two-boat trawling. To change
the net depth within about 10 to 15 m, it is normally
sufficient to alter only the speed. If greater changes of
net depth are needed then the warp length has to be
altered.
In two-boat trawling it is difficult to change course
quickly. When fishing deep with long warps, about 45
min would be needed to turn to the opposite course.
Such turns can be achieved much faster by hauling in
the warps up to 50 fm, turning the boats round in a
small circle and veering the warps again. In this way only
about 15 min are needed.
German luggers are taking up commercial two-boat
midwater trawling for herring in the North Sea at an
increasing rate. The economic results have so far been
highly satisfactory. However, it is still an open question
whether a similar development will materialise in the
case of the larger side trawlers. For economic reasons,
and because only a few of these vessels have the required
manoeuvrability it seems likely that if they should take
up commercial midwater trawling at all, they may use
the one-boat technique.
E>
Fig. ?. Method of hauling the net on the partner boat.
239
Development of the Cobb Pelagic Trawl
A Progress Report
Abttract
A large single boat pelagic trawl utilizing hydrofoil otter boards is
under development by the Bureau of Commercial Fisheries
Exploratory Fishing and Gear Research Base in Seattle, Washington.
The general configuration of the huge gear is based on the theory
that a large net travelling at relatively low speed through midwater
or on the surface (Fig. 1) would be more effective in capturing large,
active fish than a small net towed at high speed. Construction
details have primarily resulted from direct observation of experi-
mental nets in action by SCUBA-equipped staff members. These
observations have allowed a series of modifications to be performed,
resulting in attainment of a favourable ratio of net size to horse-
power requirements. A mouth opening of approximately 7,200
sq. ft. and a towing speed of 2.5 kn. using 350 hp. has been accom-
plished.
Actuellcmcnt, au Centre de la Ptehe Explorative et de la Recherche
des Engins de Ptehe est d£vellopp6, par le Bureau de P&che Com-
mcrciale des Etats-Unis, & Seattle (Washington), un grand chalut
ptlagique & un seul navire, 6quip6 de plateaux hydrodynamiques.
La configuration g£n6rale de cct engin immense est fonctee sur
Thypothese qu'un grand chalut remorqud, soil entre deux eaux,
soit pres de la surface (Fig. 1), & une vitesse relativement faible,
serait plus apte & capturer les grands poissons tres actifs qu*un petit
chalut remorqu6 & une grande vitesse. Les details de sa construction
r&ultent principalement d'observations sous-marines de 1'engin en
fonction effectives par des employed du Centre, 6quip6s de scaphan-
dres autonomes.
Ces observations ont pcrmis d'apporter toute une s6rie de
modifications qui ont abouti & une relation plus favourable entre
les dimensions du chalut et la puissance du moteur. C'est ainsi
que Ton a augment^ I'ouverture du chalut jusqu'& pres de 700
metres carres pour une vitesse de touage de 2.5 noeuds et une
force motricc de 350 c.v.
fixtncto
Un arte de grandes dimensiones para la pesca al arrastre pel&gica,
dotado de puertas hidrodinimicas y para ser rcmolcado por una
sola embarcaci6n se construye en la Base de Investigations de
Material de Pesca y de Pesca Exploratoria de la Oficina de Pesca
Industrial, Seattle, Washington. La forma general del cnormc arte
se basa en la teoria de que una red grande que se desplace a poca
velocided relativa entre dos aguas o en la superficie (Fig. 1) pesca
mas peces grandes, activos, que una pequefta remolcada a mucha
velocidad. Los detalles de la construcci6n se deben principalmentc
a observacioncs directas de artes experimentales por invcstigadorcs
equipados con escafandras aut6nomas. Las observaciones han
permitido realizar una serie de modificaciones que han dado por
resultado la obteni6n de una relacidn favorable de dimensiones
de la red a necesidades de fuerza motriz. Se ha logrado una
abertura de red de cerca de 7.200 pies cuadrados a la velocidad de
remolque de 2.5 nudos con un motor de 350 hp.
by
Richard L. McNeely
Research Bureau of Commercial
Fisheries, Seattle
/COMMERCIAL fishermen and marine scientists have
\*/ in the past often considered improvements in the
techniques of harvesting stocks of fish known to inhabit
midwater. Their interest in mid-depth fishing was
accelerated with the introduction of echo sounding
machines which, in addition to registering the depth of
water under the vessel, indicated marine life at inter-
mediate depths (Alverson and Powell, 1955). In recent
years a variety of mid-depth fishing trawls have been
used experimentally in attempts to efficiently harvest
pelagic fishes (Parrish, 1959).
During early 1961 a programme of one-boat midwater
trawl development was undertaken by the Bureau of
Commercial Fisheries, Exploratory Fishing and Gear
Research Base, in co-operation with the Bureau's
Biological Laboratory at Seattle, Washington. A
multi-purpose gear was envisioned which would be both
commercially acceptable by the fishing industry and
useful as a pelagic fish sampling device.
Design concepts
The initial concept of a gear capable of this dual operation
was that it should be of sufficient size to utilize the maxi-
mum horsepower available (350 hp) in the Service's
research vessel John N. Cobb. Prior experiments aboard
the John N. Cobb with a British Columbia herring trawl
(Barraclough and Johnson, 1955) the Larsson Phantom
Fig. L Float line of giant Cobb Pelagic Trawl skims surface 185 fm. aft of the research vessel John N. Cobb.
240
COBB PELAGIC HYDROFOILS
10 ALUMINUM FLOATS
(41 REQUIRED)
-^ELECTRICAL TRAWL CAiLE
DEPTH- TEMPERATURE
SENtmt UNIT
" No. 15 NYLON (oil body mtihtt)
,^'cHAIN LEADLINE
LARSON "PHANTOM" TRAWL HYDROFOILS
COBB PELAGIC TRAWL (196!)
Fig. 2. Otter board and bridle arrangement used to open the Cobb Pelagic Trawl
trawl (Larsson, 1952), and other conventional midwater
trawls designed and constructed by the Bureau of
Commercial Fisheries indicated that fish could be
captured at towing speeds less than 2 kn. Moreover,
directing observations (using SCUBA) and televised
observations (Sand, 1959) of the reaction of fish to the
approach of trawls show that most large pelagic fishes
are capable of swimming speeds in excess of that necessary
to avoid capture. Thus the capturing gear should be of
sufficient size to entrap the fish before it becomes aware
of the net. It was reasoned that a four-door otter board
and bridle system (Fig 2) would be more effective in
opening the mouth of the net than a conventional dual
otter board system requiring unusually long bridles and
unusually large doors.
Gear design experiments
In 1960 a partially constructed experimental two-boat
surface trawl was purchased from a California tuna
vessel owner-operator who had abandoned pelagic
Fig. 3. SCUBA-equipped divers mounted on manoeuvrable sea sled prepare to examine experimental net In action.
241
T~
NOTE: ALL FOUR SIDES IDENTICAL
MATERIALS
WINGS- 4^" No. 15 Contlnous flkjmtnt nylon
BODY— 4^" Na IS Contlnoui fllomtnt nylon
INTERMEDI ATE- 4±" No. 15 Contlnout fllomtnt nylon
COD END-5i"No.9«Centlnout fllomtnt nylon
HEAD LINE-168'xf Brold.d nylon
LEAD LINE- 1*6* *il"G«lvonlitd choln
BREAST LINES-168'xf*Brold«tf nylon
RIBLINES- Duol f nylon rop«.(249* from wing tip
through cod tnd-oll four corntrt)
DANDYLINES-Two 10 fathom Mctlont|"wlrt ropo
Two 20 fathom toot tons fwirt ropt
Two 30 fathom •actlont^'wiro ropt
Two 40 fathom toctlont^wiro ropt
OTTER BOARDS-T*o5'X8' "COBS" hydrofoil
Two LarMnVhofiW trawl doon
FLO ATS- 41 Vwillpt" aluminum trawl floats
2 NaTS^orly^ntumttte floats
COBB PELAGIC TRAWL
Fig. 4. CM Pelagic Trawl
242
trawling experiments. In early 1961 assembly of the huge
net to its original specifications was completed. During
this same period a four-door otter board system was
designed and constructed to provide maximum opening
and single-boat operation of the gear. A desired perform-
ance characteristic of high lift at low speeds of midwater-
otter boards appeared to be similar to aircraft wing
sections used in the 1930's. Therefore, two experimental
hydrofoil otter boards constructed of plywood and tim-
bers were fitted with an appropriate bridle system to
operate in conjunction with two patented "Phantom"
trawl otter boards. The gear was then observed in action
by SCUBA-equipped divers using a sea sled (Fig 3).
The net was found to have several defects which
required extensive modifications to correct. The princi-
pal observed defect was excessive slack or "bagging" of
webbing around the mouth of the net, apparently caused
by the box-shaped wingless design of the net. Breastlines
and footrope were found describing near semi-circular
parabolic curves when viewed perpendicular to the
longitudinal axis of the net. No provision for displace-
ment of these sections from a straight line had been made.
Subsequent to these findings, the net was disassembled,
redesigned and rebuilt with long tapering wings, hung in
along the corner riblines 13.4 per cent. Underwater
observation of the redesigned net, named the Cobb
Pelagic Trawl (Fig 4), revealed a considerable improve-
ment in performance. Most of the slack webbing had
been eliminated, resulting in a more satisfactory ratio
of net opening to number of meshes across the mouth.
The immediate visible effect was a greater mouth opening
and improvement of individual mesh openings which
ranged in configuration from 60° diamonds to 90°
squares. A more equalized distribution of load through-
out the net was observed. Correction of an insufficient
vertical opening was achieved by installation of ft in.
chain for the lead line and increasing the number of
floats on the headrope.
Otter boards
The attempt to provide horizontal spread of the net
through use of large hydrofoil otter doors in conjunction
with "Phantom" travel otter boards has been successful.
A measured opening of 80 ft has been achieved during
trials. The plywood doors which measured 8 ft 2 in in
height and 5 ft in length, have a blunt nose and pro-
portionately thick chord section (Fig 5). It was necessary
to provide sufficient holes on the back (curved) side of
the doors to allow rapid flooding and spilling of water
from each of the hollow compartments during setting
and retrieving operations (Fig 6). A metal shoe was
placed on the lower side of each door to provide weight
for stabilization and to allow use of the doors in future
bottom trawling or near bottom pelagic trawling
experiments. Observed performance characteristics of
the doors showed them to be very stable, even when sets
were made in cross currents. No tendency for the doors
to cross was observed. Since both sides of the doors were
covered with a plywood skin, no deflection vanes were
Fig. 5.
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Prototype Cobb Pelagic Otter Board (1961)
needed to ensure proper setting. The doors responded
effectually to changes in bridle chain lengths which
allowed manipulation of angle of attack for maximum
spread. Changes in differential lengths of the trace chains
at the rear of the doors provided control of rising or
diving in the water. This function was useful during
tests of the gear rigged to fish on the surface.
Four-door hookup
Use of Larsson "Phantom" trawl doors in conjunction
with the larger plywood doors presented initial problems
in hookup. The Larsson doors were later found easier
to set and retrieve when placed ahead of the plywood
Fig. 6. Numerous holes (drilled on curved side only) allow rapid
flooding and drainage of inner compartments.
243
doors. Positioning of the larger doors on the upper
bridle section and the Larsson doors on the lower bridle
section was found desirable since a part of the function
of the upper doors in a four-door arrangement is to
counteract their own weight and diving effect of the lower
doors, thereby stabilizing the depth of the net.
Use of the gear on the surface after use at depths of
ISO to 200 fathoms resulted in a change in the attitude of
the plywood doors due to their becoming water logged.
Prior to mid-depth tows surface tows could be made
using 150 fathoms of cable. Following the mid-depth
tows, attempted surface tows using more than 25 fathoms
of cable resulted in the net sinking beneath the surface.
This observation indicated the advisability of using all
metal doors which would maintain a constant attitude
at all depths.
Modifications in 1962
In early 1962 the Cobb pelagic trawl underwent further
modifications in design to improve operational character-
istics. The redesigned net, "Cobb Pelagic Trawl— Mark
II" (Fig 7) incorporated the following changes:
1 1
Ji
n
A
1
(a) Reduction of mesh size to 3 in (stretched measure)
in the body of the net to eliminate at least part of a
serious gilling problem (Fig 8) encountered with the
original model.
(b) Reduction of length and horizontal size of the net
to compensate for increased drag of smaller meshes.
(c) Elimination of multiple tapers along the corners by
using a straight line taper from wing tip to bag.
(d) Installation of "criss-cross" riblines for better
equalization of strain during instances involving unequal
lengths of towing warp.
COBB PELAGIC TRAWL- MARK TWO 0962)
Fig. 7. Cobb Pelagic Trawl—Mark U
244
Fig. 8. Mackerel and dogfish shark, gllled in intermediate section of
trawl, suggest reduction in size of meshes.
(e) Use of 6 in webbing in the wing sections to reduce
<lrag. ''•'.: .-
' * ' -•» *
Aluminium hydrofoil otter boards ;
Redesign of the large plywood hydrofoils to all alumin-
ium construction was also completed early in 1962. The
new doors (Fig 9) utilized the same basic characteristics of
air foil cross section with vertical size (8 ft) larger than
the fore-aft dimension (5 ft). To develop vertical
stability, 72 3J in glass ball floats were placed in the
ONLY
TWO <f AIR VtNT MOLft TO K ONH.LCO AT TO*
OP CACN COMPARTMENT — ON CUNVCO HOE
(If ACQUIRED)
QUILL
rt wet covcnco t
•LAM FLOATS TOK
SECUMCO IN TO* TWO
CNAMWHS KFONC
INSTALLATION OPSMN
ON PLAT SIDE
MOLES H
(7 REQUIREO)
HK*'
•**••
-•r—
COVCMO WITH .100" ALUMINUM
sc 'HILI-AHC' wt LOCO
CXPOSCO COOCS
ALL ALUMINUM
NCSISTANT
TO SC NISN TCNStLt ANO
TO SALT WATtR CO**OSION
Fig. 9. Cobb Pelagic Otter Board (1962)
245
Fig. JO New aluminium otter board as seen from afterdeck of the John N. Cobb.
upper two rib chambers of each door. Each glass ball
was covered with heavy webbing and packed in fibreglass
to prevent damage during use of the doors.
Underwater observations 1962
Following construction of a new Mark II — Cobb Pelagic
Trawl and modification of existing gear to Mark II
design in early 1962, the trawl, equipped with aluminium
hydrofoils and Larsson "Phantom" trawl doors, was
again observed in action by SCUBA-equipped divers.
With the exception of moderately excess strain noted in
the wing tips, the net appeared to be fully expanded and
exhibited the characteristics of a semi-elastic body being
inflated by an internal force similar to the manner in
which air inflates an airport wind sock. This condition
extended throughout the net from the wings to the
"bitter end" of the codend. A near circular cross section
shape was noted. Meshes from the wings through the
codend were examined by sight and feel to gain know-
ledge of strain distribution. The amount of strain on
individual meshes in all but the forward sections of the
wings was found to be small, even in areas adjacent to
the riblines. The slight increase in strain in wing meshes
was attributed to a greater amount of stretch in ribline
in this area, due to the concentration of total load at the
four wing tips. A further improvement in net per-
formance is expected when corner riblines are rehung
using an incremental hang-in ranging from approxi-
mately IS per cent at the wing tips to approximately
10% at the codend junction. (See note 1 - p. 248)
Observations of the new aluminium hydrofoils from
shipboard (Fig 10) and below water showed them to be
very stable and responsive to adjustments in bridle and
trace chains. Through manipulation of chain linkage
it was possible to tow the net on the surface 185 fathoms
behind the vessel.
Measurements
Direct measurement of horizontal opening was secured
through two auxiliary launches between which a tight
line was suspended in the air over wing-tip mounted buoys
— shown in Fig 1.
Vertical opening measurements were secured by sea-
sled mounted SCUBA divers descending alongside the
net and observing differences in depth gauge readings.
Average towing speed of 2.5 kn was arrived at by
use of radar and Loran to establish start and end positions
of a series of a trial drags, usually of one hour duration.
Depth assessments of the net during experiments at sea
trials were secured through an electrical Depth Telemeter
(McNeely 1959) and conventional wire angle-scope ratio
calculations. Changes in net depth were normally made
by regulating the length of towing cable at the winch.
Preliminary sea trials (1961)
During July and August 1961 a Cobb Pelagic Trawl,
made of 4| inch webbing utilizing plywood hydrofoils in
conjunction with Larsson "Phantom" trawl otter boards,
was fished 25 times in offshore waters to determine
operational characteristics. Most of the 25 tows were set
"blind" (no indication of fish). Ten of the tows were
made in the deep scattering layers (150 to 200 fathoms)
110 miles off the Washington coast to test the ability of
the gear to descend to these depths. These tows produced
small amounts of jellyfish, squid, lantern fish, and
fanged viper fish. Other blind tows made in shallower
depths (less than 50 fathoms) off the mouth of the
Columbia River produced occasional silver salmon and
up to 160 Ib each of hake and jack mackerel. Other
species taken in small numbers include blue shark,
herring, anchovy. English sole, turbot, and black
rockfish.
246
•;<••";
t'^&p3 "".1"V'l.^-"t;A'; ; '*• " •'
^^.••••-^af^^rr" •-••/'
«,#jJ^:-v,^%.,,-,-.,Ty'-';- ',;.-•'.-/ "•
2£ ¥a&Jt^^tt^'^-X< :.' '*J*^-*' .,'
Mfi^^ryT"
Fig. /7. T/ie ma/i stands inside the after-body of the net which is stretched out three-dimsnsionally.
Testing of the gear as a salmon sampling surface trawl
yielded generally poor results. These drags, made on
the Swiftsure Bank, and in Prince William Sound,
Alaska, produced salmon in small numbers ranging
from a single individual to 29 per drag. Large catches of
dogfish shark taken on the Swiftsure Bank repeatedly
damaged the net. Catches of dogfish to 10,000 Ib were
made. During one drag the entire codend was lost
and the intermediate section was severely chaffed.
Jack mackerel and hake were also taken in amounts up to
200 Ib in the Swiftsure area. Although salmon did not
appear to gill easily, dogfish, hake and jack mackerel
catches usually resulted in severe gilling. A reduction
in mesh size from 4J in (stretch measure) to 3 in was
therefore indicated for fish in this size group.
Fishing trials in 1962
In August 1962 the Cobb Pelagic Trawl— Mark II was
used during an extensive survey to determine the relative
abundance of all pelagic species offish at pre-determined
stations off the coasts of California and Mexico. Follow-
ing this work, tests of the gear were conducted in nearby
waters off Mexico, California, Oregon and Washington
to determine its relative efficiency as a biological sampling
tool and possible utility as commercial fishing equipment.
(a) Results of offshore pelagic survey
Forty-four predetermined stations were occupied during
this phase of testing. Oblique tows from 220 fathoms
to the surface were made at each station during daylight
hours. At least one of each series of night tows was
made on the surface. With the exception of one night
surface tow in which 24 mackerel were taken, catch
rates seldom exceeded five Ib per two hour tow. How-
ever, echo-soundings taken at all stations along the track
line, which extended over 600 miles offshore, indicated
no fish concentrations were available. Scatter recordings
were typical of those associated with the deep scattering
layers. During part of the offshore survey simultaneous
sampling was conducted by the Bureau's research vessel
Black Douglass using plankton nets and stramin nets.
A subsequent correlation (by co-operating scientists at
the Bureau's La Jolla Biological Laboratory) of catch
rates made during these trials and catch rates made at
other times by small, high-speed midwater trawls shows
that small, fine mesh nets are as efficient as the Cobb
Pelagic Trawl in taking zooplankton and small fishes
such as stomatoids and myctophids. Catch rates and sizes
of the larger fishes such as anchovy, hake, rockfish,
bonito, sardine, mackerel, barracuda, and ribbon fishes
(up to 6 ft in length) indicate that the large net was
247
Underwater Telemeters for Midwater Trawls and Purse Seines
AMract
Since 1957 the authors have worked on the development of a wire-
less net depth telemeter for commercial midwater trawling. The
general principle and some technical details are given of the five
types of instruments built and tried so far in the course of this work.
Toe net depth is measured by determining the hydro static pressure
and the measured data are coded into ultrasound signals transmitted
to a sound receiver towed by the trawler. They are converted into
electrical signals and led by a 25 m long cable to the depth indica-
tor unit in the wheelhouse. The depth indication was first only by
pointer on a scale, as well as acoustical. Later a paper-recording
system was developed to write the net depth in correct scale on
the echogram of the trawler's echo sounder. For the first four
instrument types the depth measurements were converted into
signals consisting of ultrasound impulses (impulse rate 40 per sec)
the duration of which was in linear relation to the depth. Instru-
ments of this kind could not be made light, cheap and rugged
enough and continuous depth indication could not be achieved.
Then a system of frequency modulation was adopted, i.e., the
freouency of a continuous ultrasound signal was changed according
to the measured depth. The frequency of the transmitted impulses
or signals for all types is approximately 200 kc. The depth-measur-
ing range is 0 to about 250 m with an accuracy of about ± 1-0 m
at about 100 m depth. The transmission range from net to trawler
is about 1,000 m for the first four models and about 2,000 m for
the last model. The last model, which is completely transistorised,
was found to be satisfactory and is available on the market. Already
in 1961 about 70 units were in commercial use, mainly in conjunc-
tion with midwater trawling for shrimp in the East China and Yel-
low Seas. Such depth telemeters are also suitable for measuring
tlie behaviour (sinking speed) and fishing depth of purse seines.
Tele
} pelagiques et i
Rfaumt
La presente ttude d6crit le principe g£n£ral et quelques details
techniques des cinq types d'instruments utilises par les auteurs qui,
depuis 1957, ont travai!16 au d6veloppement d'un sondeur de filet,
actionn£ par radio, pour chaluts p61agiques. La profondeur du
filet est mesuree par la determination de la pression hydrostatique
by
Chikamasa Hamuro
and
Kenji Ishii
Fishing Boat Laboratory, Tokyo
et ks donnees sont transmiscs en signaux ultra-sonores codes et
recus a bord du bateau par un recepteur courant. Ces signaux
sonores sont convertis en signaux dlectriques et transmis a 1'indica-
teur installe dans la timonerie. Au debut, la profondeur 6tait
indiquee sur un cadran ou par signaux audibles; plus tard, grace a
un systeme enregistreur plus perfection^, les profondeurs obtcnues
s'inscrivaient sur I'fchogramme de rinstrument de bord. Les quatre
premiers types d'instruments convertissaient les donnees sonores
en signaux 61ectriques ultra-sonores (frequence de 40/sec) dont la
duree etait en relation lineaire avec la profondeur. Avec octte
m&thode il 6tait impossible de construire des instruments tegers et
assez robustes pour un emploi continu. C'est alors que le systeme
de modulation de frequence fut adopts, c'est-a-dire que la frequence
de 1'onde porteuse fut modulee selon la profondeur mesuree. La
frequence de 1'onde porteuse, pour tous les types, est d'environ
200 kc et Ton peut mesurer des profondeurs de 0 a 250 metres avec
une precision de ± 1 pour cent a environ 100 m. La portee de la
transmission du filet au bateau est d'a peu prcs 1,000 m pour les
quatre premiers modeles et de 2,000 m pour le dernier modele.
Ce dernier modele, a transistors, est tres efficace et est maintenant
disponsible sur le march6. Deja en 1961, 70 unites etaient utilisees
par des bateaux de commerce, notamment pour le chalutage des
crevettes. Ces instruments sont aussi ties utiles pour mesurer It
vitesse de descente du filet et la profondeur de pdche des sennea
coulissantes.
Continued from page 247
capable of sampling a wide spectrum of the large pelagic
vertebrates. In most cases the larger fishes are rarely
if ever taken in small, high-speed midwater nets.
(b) Gear efficiency tests
Following the offshore pelagic survey, 16 surface
towsand 1 1 mid-depth tows were made off the coasts of
Mexico, California, Oregon and Washington. Catch
rates during the near shore tests greatly exceeded catch
rates offshore. Although a wide variety of species were
taken, the largest catches consisted of 1,850 Ib of hake,
1,900 Ib of mixed sablefish and hake, 1,000 Ib of
anchovy, and 600 Ib of ocean sunfish. Most of the
larger catches were made at mid-depth.
Conclusion
Utility of the Cobb Pelagic Trawl for gross biological
sampling8 was demonstrated during sea trials of the gear
by the wide variety of fishes taken— seventy, in fact.
Those sets made on schools of fish located by echo-
sounding in depths greater than 30 fathoms usually
produced fair amounts of fish. Attempts to capture
surface swimming schools usually resulted in poor catches.
248
It should be noted that the total number of drags made
to date is small and has entailed many variables inherent
in development of new gear.
Notes
1. In other gear experiments conducted during 1961 and 1962
involving use of a giant, small-mesh fyke net to strain the total
discharge of water from hydro-electric turbines, the author has
found incremental hang-in to be a distinct advantage to counter
distortion of meshes near the mouth of the net.
2. Towing at predetermined stations without prior echo-sounding
or other observations to determine the presence of fish.
References
Alverson, D. L. and D. E. Powell. The open ocean challenges
the scientist and dares the fisherman. Pacific Fisherman, Vol. 53,
No. 11, (Oct.) and Vol. 53, No. 12, Nov. 1955.
Barraclough, W. E. and W. W. Johnson. Canadian midwater
herring trawl. World Fishing, Vol. 4, No. 8, 1955.
Larsson, K. H. The "Phantom*' pelagic trawl. Fishing News*
No. 2067, 1952,
Parrish, B. B. Midwater trawls and their operation. Modern
Fishing Gear of the World, Fishing News (Books, Ltd.), London,
Sand, R. F. Midwater trawl design by underwater observation .
Modern Fishing Gear of the World/ Fishing News (Books, Ltd.),
London.
Tcto
Ntes de arrastre flotantes y dc cerco de
Extracto
Trabajan los autores desde 1957 en la fabricaci6n de un tetemetro
inalambrico registrador de la profundidad a que esta la red, para
los artes flotantcs empleados en la pesca industrial. Se dan los
principles generates y algunos detalles tecnicos de los cinco modelos
de aparatos construidos y ensayados hasta ahora. La profundidad
a que se encuentra el arte se mide determinando la presidn hidrosta-
tica y los datos se transforman en seriates ultrasonoras y se transmitcn
a un receptor que remolca el arrastrero en el que se convierten en
seftales electricas y a trav6s de un cable de 25 metres pasan al grupo
indicator de la profundidad situado en el puente de mando. En
un principio la profundidad se indicaba solamente mediante un
puntero en una escala y acusticamente ; mas tarde se ideo un sistema
de papel registrador en el que aparecia la profundidad del arte en
la escala del ecograma de la ecosonda del barco. En el caso de los
cuatro primeros modelos la medida de la profundidad se convertia
en seftales que consistian en impulsos ultrasonoros (40 por segundo)
cuya duraci6n estaba en relaci6n aritm6tica con aquella. Los apa-
ratos de esta clase no lograban hacerse lo bastante ligeros, robustos
y baratos y por ello no se obtenia una indicacibn continua de
la profundidad. Posteriormente se adoptd el sistema de modulaci6n
de la frecuencia, es decir: la frecuencia de una seftal ultrasonora
continua se transformaba de acuerdo con la profundidad medida.
La frecuencia de los impulsos o seftales transmitidos por todos los
aparatos es de cerca de 200 kc. La escala para medir la profundidad
va de 0 a cerca de 250 m, con una exactitud pr6xima al ± 1 por
ciento a cerca 100 m. El alcance de la seftal transmitida desde la
red es de unos 100 m en los primeros cuatro modelos y de unos
2,000 m en el ultimo. Este esta completamente transistorizado, da
resultados muy satisfactorios y se encuentra ya en el comercio. En
1961 se empleaban industrialmente unos 70 aparatos, principal-
mente en la pesca del camar6n con artes pelagicas en el mar del
este de la China y en el Amarillo. Estos telemetros tambien se
pueden emplear para medir la velocidad a que se hunden y la
profundidad a que pescan los artes de cerco de jareta.
Q1 UCCESSFUL mid water trawling requires knowing the
>3 net depth under various conditions. Since 1957, the
author and his colleagues have been doing research on
midwater fishing gear and methods and on underwater
telemetry for midwater trawling. Five kinds of tele-
meters were designed and manufactured in four years.
The latest design, Type V, has been installed in fishing
boats and is performing satisfactorily. About 70 sets
are in use in one-boat and two-boat midwater trawling
in the Yellow Sea and other fishing grounds. This tele-
meter has also been adapted to purse seining.
As is known, the depth of the net, even if warp length
and towing speed are constant, can vary due to the
influences of currents and wind.
In order to keep the trawl at a desired depth, it is
therefore necessary to have continuous net depth
measurements by means of a suitable depth meter. The
actual problem for such depth telemeters is not so much
the measurement itself but the transmission of the meas-
ured data from the net to the boat. With the telemeters
described here, this is done by wireless ultrasound
signals.
The first instrument (Type I) was completed in Novem-
ber 1958. This was gradually improved into Type II.
In January 1960, Type III and in February 1961 Type IV
were developed. Type V, which contains further innova-
tions, was completed in June 1961. It is now used in
commercial operation in East China and Yellow Seas.
The complete net depth telemeter consists of the depth
meter and signal transmitter, the signal receiver and the
Otter board
The depth signal transmitter
Sea bottom
Fig. 1. Arrangement of wireless depth telemeters with midwater
trawls.
depth indicator. As shown in Fig. 1, the depth meter and
signal transmitter unit, which transmits signals corres-
ponding to the depth of the meter, is attached to one
warp close in front of the otter board (or the wing tip).
In order to evade the interference of the propeller wake
of the trawler, the receiver is also attached to the warp in
sufficient depth about 25 m astern of the trawler. The
ultrasound signals are converted by the receiver into
electrical signals which are transmitted by cable to the
depth indicator on board the trawler.
Fig. 2. General construction of depth meter and signal transmitter
Fig. 2 shows schematically the structure of the depth
meter and signal transmitter (Types I to IV). For the
net depth the hydrostatic pressure is measured by a set
of bellows. The displacement of the diaphragm in
proportion to the depth actuates a pointer which has
contact with a wire spiral on a cylinder which is driven at
constant speed by a micromotor. Since the pointer moves
in the direction of the axis of the rotating cylinder in
accordance with the depth, the period during which it is
in contact with the spiral is proportional to the depth.
For this period the signal-transmitting circuit is closed
and ultrasound impulses of 203 kc are transmitted at a
frequency of 40 per sec. This impulse frequency is
effected by a commutator attached to the axis of the
micromotor. The duration of the signal of 40 impulses
per sec. thus indicates the depth measured by the bellows.
249
Variations of the voltage of the electrical source are
irrelevant to the accuracy of the measurements.
There is one main shortcoming of this system, i.e.,
measurements are not continuous but can be transmitted
only at the rate of one per rotation of the cylinder. In
the Type I to IV meters the cylinder was designed to
rotate once every 10 sec so that measurements could be
transmitted once every 10 sec which was considered
sufficient for practical fishing.
A pressure switch was provided to cut off the trans-
mitter in less than 8 m water depth to avoid waste of the
battery. The ultrasonic transmitter and receiver were of
the same type, i.e., barium titanate, 30 mm diameter,
beam angle approximately 20°. The unit is shown in
Fig. 3, A. The ultrasound receiver (Fig. 3, B) converts
the received sound signals into electrical signals which,
by means of a 25 m long cable, are conducted to the
indicator unit in the wheelhouse of the trawler.
The indicator unit (Fig. 3, C) consists of the amplifier,
Fig. 3. The wireless net depth telemeter Type /. A— depth meter and
signal transmitter. B— receiver. C— - indicator.
the depth-evaluating part, the depth indicator (with
loudspeaker) and an electrical power source. The depth
indicator is located near the centre of the instrument
panel, and on both sides of this are a voltmeter for the
electrical source and the loudspeaker. The depth signal
S*Q surface
Cweep zone of me md- water trowl net
'
O
10
.•"•••'..' '•' • • V-M956I26 •- • ' •' „
. — * — - — : ' ?Q-
. ;• .•• .../..•• . ..*• »••,/•.•• • •• • 5if
•:: X;:vA;v:;> •'•;<:::: !^ •.:.?:r.::v.;.*-S.
*;:• ••,•..:•. ..
.*:.*/''*•* '**•
*'•.*'• i* •'•••
••/•' . -'^V: .•'•
• « *. ... «. •• •
. . ' . ;.vi. *t.v,jQ
<%: ,'t*» i*r*
•.':••' J*-"*"* ^y• .:• . ti i.i- . .»
^v.';^^;.s.. :V.v..v'-'-.''Mo.
•;••.';•.";'.';"'••'" •".'!.-':. "':"' ., -, . •- v
*•'.':' -'• .' ..-".'•'. ^V- •••':. •;' '*'' v>'''.\?x''^ V-'-;v"'.l.-;i-^-« '*•"•*'.• "*v.'
is amplified and led to the depth evaluation part where
it is converted into a voltage proportional to its duration.
Thus the depth is indicated directly by a voltmeter with
depth calibration. A filtering circuit at the input side of
the depth indicator eliminates the disturbances caused by
external noise. An additional output circuit is provided
to make the depth signals also audible through a loud-
speaker. When the distance to the net becomes too
great (and the signal-input too weak) the depth indicator
may not indicate even at maximum amplification. In
such cases the depth can be estimated from the duration
of the audible signals. The audible signals also help to
determine whether faults of the instrument lie in the
depth meter and signal transmitter or in the receiving
parts. Finally the audible display helps to find the best
position for the receiver.
The Type I depth meter was designed for: range of
depth measurement 0 to 250 m, range of transmission
over 1,000 m, accuracy of depth measurement within
±1 per cent, maximum depth for the depth meter and
transmitting unit 500 m.
Since the directional beam angle of the transmitter
and the receiver is rather narrow, it is best to attach the
depth meter and transmitter unit at the end of the warp;
that is, right in front of the otter board in the case of one-
boat trawling or right in front of the wing tip for two-
boat trawling. The receiver is also attached to the same
warp but short astern of the trawler and just below the
propelling wake. The attachment of transmitter and
receiver to the warp has to be made so that they face
each other over the distance as directly as possible.
From December 1958 to September 1959 various
theoretical and practical experiments were conducted
with satisfactory results on Type I.
The Type II meter developed on the basis of these
experiences incorporated the following improvements : —
(1) The outside diameter of the depth meter and trans-
mitting unit was decreased by 20 mm to 120 mm.
(2) To cover the interval of 10 sec between two
.DSL 9 ;
:\'«W«WIW»«P-- m^^rto^^UBMitun^^^^.iM',
?'*. ^ vT?":--.|l%s
"•:.'• **>V' > * * '" J
N*?* i.'-.' • ••"• :T:I:' 1 '
?2 23 24 *
86 " 70Z,8 29 <3
Worp length ^1 fc Warp length
M7m 153m
32 33
57 58
C3°°
Fig. 4. Schematic drawing ofa/uhfinder echogram with superimposed measurements of the net depth at different warp lengths.
250
depth readings, the indicator was made to main-
tain a constant reading till the next signal.
(3) The frequency filter (40 cycles) was made variable,
so that it could follow the variations in the revolu-
tions of the micromotor of the depth-signal
transmitting unit. This was done by lowering
the voltage of the electrical source. (In the Type I
meter such variations showed up as an error.)
(4) The noise-eliminating circuit was improved, and
the false movement of the indicator was decreased.
(5) The correction of the depth indicator during
operation was made possible. Furthermore, in
Type 1 the time-measurement was done with a
stopwatch, whereas in the new type it is done
electrically.
(6) The indicator was made into a separate unit, so
that it could be placed on the recorder of the fish-
school detector, and the rectifying circuit was
improved to eliminate the chattering of the relay,
which was liable to occur in Type I.
(7) The sound receiver was streamlined, so that the
eddies which it forms would be minimised.
Furthermore, the beam angle was changed to 8°
vertically and 20° horizontally, to simplify the
directional adjustments.
In the Type III meter the depth indication was develo-
ped for recording, superimposed on the echogram of the
trawler's fish-finding echo sounder. The recording of
the depth meter is delayed on the echogram according
to warp length, trawling speed and paper transport.
With this arrangement the adjustment of the net to the
fish schools detected by echo sounding is simplified
considerably. By recording also the variations of the
tension on the warps on the same paper the performance
of the trawl can be judged to a certain extent and the
amount of catch obtained can be estimated (Fig. 5).
The Type III meter has already shown satisfactory
results in catching croaker and Taisho prawns. The
depth meter and transmitting unit is in general the same
as that of Types I and II, but further reduced in
diameter to 102 mm. Since the tension is also recorded,
a depth-tension signal converter was newly installed.
The Type IV meter which was finished in February
1961 is a further improvement. It is still smaller and
more practical than the former types. It is more rugged
and resistant against mechanical impacts and achieved a
lightweight miniaturisation of the transmitter. However,
the modulation method of the depth signal and the
measuring principle remained exactly the same. There-
fore, the transmission of the depth-signal is still made at
intervals of about 10 sec, and the complexity and the
problems involved with the handling of the instrument
were much the same as with the former Types.
Type V (Fig. 6) was first produced and tested by the
I \ N±=^
\Y\\\v:^p:^
\ *
\ OB.
Tension
J {-School of fish
'Net depth
Fish detector
recording pen
i Net depth
'^recording pert
Sea bottom
Sea bottom
S Distance between
fish detector pen
and net depth pen
m ...... Speed of recording
sheet
Fig. 5. Schematic sketch ofmidwater trawling with recording echo sounder, wireless net depth telemeter (Type III) and tension meter for the warps.
251
Fig. 6. The wireless net depth telemeter Type V. A — depth meter
and signal transmitter. B— receiver. C — indicator.
authors in July 1960. It was meant to eliminate the
various shortcomings of the earlier makes, such as: —
(a) Ten-second intervals between depth readings.
(b) Complicated construction.
(c) Too many mechanical parts, making the instru-
ments too delicate to be protected efficiently.
(d) High weight of the depth meter transmitter unit,
making it difficult to handle and operate.
(e) High price.
To overcome these shortcomings a completely different
method of coding and transmission was adopted, i.e.,
conversion of the variations in depth into frequency
variations. As with the former instruments the net
depth is indicated on the dial directly, and it can also be
recorded on the echogram of the fish-finding echo soun-
der of the trawler. In this way the depth of the net is
indicated continuously and without interruption. The
instrument is entirely transistorized to reduce electric
power consumption (electric power consumption of
transmitter: 300 mW; electric power consumption of
indicator 1*2 W) and to make small and lightweight
construction possible. The transmitting range is increased
to more than 2,000 m. Handling and operation is
simplified considerably, and the price is reduced to
almost half of that for Types III and IV. This instru-
ment (Fig. 6) was first completed and tested in June 1961
with a midwater trawl and both the sensitivity and accu-
racy were found quite satisfactory.
The specifications of Type V are: —
(1) Depth measuring ranges: 10 to 130 m. Accuracy
±0*5 m. ; 20 to 240 m. Accuracy ± 1-0 m.
(2) Range of transmission from transmitter to receiver :
about 2,000 m.
Beam angle of acoustic signals: 40°.
(4) Maximum depth for depth meter and transmitter
unit: 500 m.
(5) Depth meter indication: For direct reading and
recording of the net depth from the water surface.
(6) Power source: For indicator: A.C. 100 V, 60
cycles;
For transmitter: 6 dry cells of 1*5 V each in series
giving continuous operation for 40 hours.
A combined recording of this telemeter on the echo-
gram of the trawler's echo sounder is shown in Fig. 7.
Continued at foot of next page
(3)
Depth meter and
signal transmitter
Fig. 8. Mode of application of a depth telemeter to the leadline of a
purse seine.
Fig. 7. Echogram of the trawler's fish finding echo sounder with superimposed net depth recording of the telemeter Type V.
.252
Reaction of Herring to Fishing Gear Revealed by Echo Sounding
Abftract
For successful midwater trawling, the behaviour of the fish
towards the gear is of particularly great importance, because
fish have a better chance to evade midwater trawls than, for
instance, bottom trawl gear. Fish behaviour studies are therefore
of great value for the improvement of midwater trawling. All the
different means of observing fish behaviour, such as diving, tele-
vision, filming of actual conditions, or aquarium experiments,
have their advantages and disadvantages. For the present studies
which were made, along with German midwater trawling trials
in recent years, the echo soundings taken by the trawlers echo
sounder and the "netzsonde" were utilised. The "netzsonde" is a
recording echo sounder on board the trawler, connected by cable to
a transducer mounted on the headline bosom of the trawlnet. By
sounding vertically downwards, the "netzsonde" records the dis-
tance of the net from the sea bottom, the opening height of the
net mouth and fish in and below the net opening. It was found that
herring (Clupea harengus) is easier to catch when spawning because
it then occurs in dense schools and it does not react strongly to the
gear. Otherwise, the herring is much more active and may evade
midwater trawls, particularly when dispersed. Relatively small one-
boat midwater trawls have therefore been reasonably successful in
catching spawning herring on some occasions, although much
inferior to the larger two-boat midwater trawls for catching non-
spawning herring. Since such active herring usually keeps at several
metres distance from lines and netting in the netmouth, the difference
in the size of the "effective net opening" is even more pronounced,
and this is to the disadvantage of smaller nets. Sprat (Clupea
sprattus), which otherwise showed similar schooling and migration
patterns, proved to be much easier to catch than herring. Mackerel
(Scomber scombrus) reacts to midwater trawls in a way similar to
non-spawning herring. Dogfish (Acanthias vulgaris) does not seem
to react to midwater trawls at all.
Etudes aii moyen d'echo-sondeur, des reactions des harengs aux
engins de ptefae
Les chances d'dvasion du poisson 6tant plus grandes dans le chalu-
tage pelagique que dans le chalutage de fond, l'6tude du comporte-
ment du poisson envers les engins de pdche est tres importante.
Tous les moyens d'observation utilises comme: plong6es, television
sous-marine ou experiences en aquarium ont lews avantages et
leurs inconv6nients. Ces Etudes ont 6t6 mendes au cours d'essais
effectugs par des chaluts p61agiques utilisant les enregistrements
des 6chos-sondeurs de bord et du "Netzsonde**. Le "Netzsonde**
est un 6cho-sondeur dont le systeme enregistreur est install a bord
du chalutier et rek'6 par cable a l*6metteur qui est fixe" sur la corde
de dos du chalut. En sondant verticalement le "Netzsonde**
enregistre la distance entre le filet et le fond, I'ouverture du filet et le
passage du poisson dans le filet. Le experiences ont prouvd que le
hareng (Clupea harengus) est plus facile a prendre pendant le frai
parce qu*on le recontre alors dans des banes epais qui ne reagissent
pas vivement & 1'approche de 1'engin de pdche. En dehors de cette
periode, le poisson est beaucoup plus actif et peut s'6vader du chalut
pdlagique. C'est ainsi que les petits chaluts pelagiques ont eu certains
9UCCC6 sur le hareng en periode de frai et que les chaluts pelagiques
tiris par deux bateaux en ont eu beaucoup plus sur les harengs qui
ne fraient pas. En dehors de la p6riode de frai les harengs se tien-
nent & une distance de quelques metres des lignes du filet, dans
Touverture et le resultat est de ce fait, beaucoup plus desavantageux
pour les petits filets que pour les grands. Les espeots (Clupea
sprattus) dans les banes se component comme les harengs et sont
plus facites a capturer; les maquereaux (Scomber scombrus) reagis-
sent de la mfeme facon que les harengs actifs, c'est-a-dire en dehors
by
H. Mohr
Institut fur Netz-und Material-
forschung, Hamburg
de la periode de frai. Les chiens de mer (Acanthias vulgaris) n'ont
aucune reaction a 1'approche du chalut pelagique.
Reacdon del arenque ante los artes de pesca estudfos por medfo de
la
Extracto
Para que la pesca al arrastre entre dos aguas de buenos resultados,
reviste especial importancia conocer el comportamiento de los
peces ante los artes por ser mas probable que eludan los pelagicos
que los de fondo. El estudio de tal comportamiento es necesario
para mejorar el arrastre pelagico. Todos los procedimientos de
observarlo, entre ellos el buceo, la tclevisibn, las peliculas de las
conditioner reales o los experimentos en los acuarios tienen ventajas
e inconvenientes. En los estudios de que trata el autor, realizados
simultaneamente con ensayos de artes de arrastre pelagicos ale-
manes, se emplearon las indicaciones dadas por la ecosonda y la
"netzsonde** del arrastrero. La "netzsonde** es una ecosonda
registradora instalada en el arrastrero y conectada mediante un
cable a un transductor montado en la visera del arte. Sondeando
verticalmente hacia abajo la "netzsonde** registra la distancia
desde la red hasta el fondo, la abertura en altura del arte y los
peces ante la boca y debajo. Se ha observado que el arenque (Clupea
harengus) se pesca mas facilmente cuando desova porque se reune
en espesos cardumenes y no reacciona vigorosamente ante la red.
En otros momentos despliega mucha mas actividad y puede eludir
los artes, articularmente si esta muy desperdigado. Debido a ello,
los artes de arrastre pelagicos remolcados por una embarcatifa
pequefia ban obtenido resultados satisfactorios, en algunas ocasio-
nes, en la pesca del arenque en desove, aunque estos resultados
han sido muy inferiores a los logrados con artes pel&gicos remol-
cados por dos embarcaciones dedicadas a la pesca del arenque
que no desova. Como los arenques activos se mantienen a varios
metros de distancia de los cables y de la boca del arte, la diferencia
en las dimensiones de la abertura eficaz del arte es todavia mayor y
va en perjuicio de los artes mas pequefios. La sardinita (Clupea
sprattus), que mostraba las mismas modalidades de formation de
cardtimenes y movimientos migratorios, result6 ser mucho mas
facil de pescar que el arenque. La caballa (Scomber scombrus)
reacciona ante los artes peldgicos de manera analoga al arenque
que no desova, en tanto que la mielga (Acanthias vulgaris) no
parece reaccionar en absolute.
IN recent years many investigations on midwater
trawling for herring have been carried out by the
Institut ftir Netzforschung, Hamburg. The vessels
Continued from page 252
Such telemeters can also be used to ascertain the actual
depth of purse seines. A possibility of application at the
leadline is shown in Fig. 8. As it would be difficult to
direct the sound beam accurately enough, it is desirable
to have a wide beam with circular cross-section and the
transmitter should, therefore, be circular.
Acknowledgmaiti
All these instruments were produced by the Furuno Electric
Co., Ltd., and we are indebted to Mr. Junichi Fukiwara of this
company for his kind co-operation.
253
Fig. P. Dense schools of spawning herring, "towers" on Aberdeen
Bank (I) are now frightened by midwater trawl gear. The footrope
cuts clear through the schools (II). The two small traces on the "netz-
sonde echogram represent a catch of four tons.
Ffc. 10. A "tower"9 of spawning herring near the English east coast (I).
The strong trace on the "netzsonde" echogram represents a 20-ton
catch.
20 tons. The total towing time of this haul was 8 min.
The whole catch was collected in only 2 min. The two
small traces in Fig. 9b represent a catch of four tons. These
catches consisted entirely of fully mature herring with
eggs and sperm flowing (August 1962). Both hauls were
made with the small type one-boat midwater gear which
had given very poor results with non-spawning herring.
It may well be that these pelagic schools constituted a
preliminary phase of spawning, whereas the proper
spawning takes place close to the bottom.
Fig. 11 shows a "netzsonde" echogram of herring,
12
Fig. 11. Spawning and partly spent herring in the English Channel
showed no reaction to midwater trawl gear.
Fig. 12. "Plume" and "steeple" traces of spent herring on the feeding
grounds (Swallow Hole) (I). While the small schools escape, part
of the larger school is caught by a one-boat midwater trawl (II).
both spawning and already partly spent, from the
English Channel (December 1961). The small but dense
groups are not scared by the approach of the footrope
though it is quite possible that these small packs are
not the normal type of spawning schools as numerous
boats were fishing in a very small area.
(c) With herring at the feeding grounds the catches were
very heterogeneous, and consisted of all maturity stages
ranging from virgin herring to spents forming different
schools in different layers.
During day-time in the area around Swallow Hole
(western North Sea) in October 1962 mainly two types
of herring schools could be distinguished: (i) small
separate groups, "plumes"5, forming a layer in a depth
of 50 to 60 m in midwater; (ii) "towers" and "steeples"
sometimes beginning 40m below the surface and reaching
down to the bottom (70 to 80 m) and (iii) greyish layers
close to the bottom (70 to 80 m) resembling the schools
at the Norwegian Deep but smaller in extent.
Catches of (i) and (ii) consisted of a high percentage of
256
spent herring and recovering spents. Hauls of two-boat
trawlers were generally successful (Fig. 12). The fish
usually kept to a distance of 2 to 4 m from the net mouth.
One-boat trawlers with smaller gear failed to make good
catches in the layers of small schools which normally
disappeared entirely before the net reached them;
also when fishing for big "steeples" only a few hauls
were satisfactory.
The greyish layers close to the bottom3 consist mostly
of pre-spawners which reacted like the non-spawners at
the Norwegian Trench (Fig. 13).
Fig. 13
Fig. 13. Pre-spawning herring near the bottom of Swallow Hole (I)
show similar reactions to midwater trawls (II) as the hibernating
pre-spawners at the Norwegian Trench.
Echoes of other fish species
Other pelagic species were caught accidentally or by
design. In Autumn 1962 in the western North Sea and
Irish Sea traces of sprat on the ship's echo sounder
were indistinguishable from those of spent herring.
(Figs. 14 and 16.) On the "netzsonde" echogram,
Fig. 14. Sprat (Clupea Sprattus) schools in the Irish Sea give similar
traces to spent herring. They also descend with increasing daylight.
Fig. 15. During ascent at dusk, the small sprat schools in the Irish
Sea scatter widely before forming a layer near the surface.
Pi*. 18
Fi*. 16
Fig. 16. Unlike pre-spawninr herring schools of similar type (Fig. 12),
sprat in the Irish Sea could easily be caught with a small one-boat
midwater trawl.
Fig. 18. Widely spread single dogfish (Acanthias vulgaris) at the
Norwegian Trench showed no reaction to midwater trawl gear.
however, where the traces appeared in the same shape
because they were not frightened by the net mouth,
they were quite recognisable as those of sprat (Fig. 16).
Sprat was found to make daily vertical movements
similar to the herring. The descents were very regular but
during ascent the schools scattered widely.
Mackerel were sometimes mixed with herring. Off
the Norwegian coast they occasionally formed separate
schools of a shape similar to that of the "towers" of
herring non-spawners (Fig. 17). The schools were obvious -
Fig. J7. Mackerel (Scomber scombrus) schools at the Norwegian
Trench had similar shape (/) and showed similar reaction to midwater
trawl gear (II) as non-spawning herring.
]y compact and thoroughly scared by the headline and
footrope4 whereas dogfish showed no reaction to the
net mouth even when in very loose schools (Fig. 1 8).
References
1 Balls, R.: Fish capture. London. 1961.
2 v. Brandt, A. : Fischfanggerate und Fangmethoden, ein
Beitrag zu ihrer Systematik. Protokolle zur Fischereitechnik 5,
H. 22/23, p. 127-149. 1957.
3 v. Brandt, A. : Einschiff-Schwimmschleppnetzversuche mit
dem FD "Rendsburg" in der Zeit vom 28.10. bis 12.11.1959 in
der irischen See und im Kanal. ibid. 6, H.27/28, pp. 190-213. 1959.
4 v. Brandt, A. and R. Steinberg: Schwimmschleppnetzversuche
im Gcbiet vor Egersund. ibid. 7, H.33, pp. 256-328. 1961, 1962.
5 Gushing, D. H. : The interpretation of echo traces. Fishery
Investigations Series VI, V. 21, No. 3.
6 Hodgson, W. C. : Echo sounding and the pelagic fisheries,
ibid. V. 17 No. 4.
7 Sch£rfe, J. : A new method for "aimed" one-boat trawling in
midwater and on the bottom. Stud. Rev. Gen. Fish. Coun. Medit.
(13). 1960.
8 Scharfe, J. and Steinberg, R: Neue deutsche Erfahrungen mit
Schwimmschleppnetzen. Protokolle zur Fischereitechnik (in press).
9 Steinberg, R. and v. Seydlitz, H. : Schwimmschleppnetzver-
suche im Gebiet von Egersund (1-111 1962), ibid. 8, H. 35, pp.48-84.
1962.
Discussion on
Midwater Trawling
Dr. J. Scharfe (Germany) Rapporteur: Attempts to
develop a one boat midwater trawl started about 10 years
ago after the pair trawling technique had proved that fish can
be caught with trawls in mid-water. Due to the hydro-
dynamic problems involved in the design and rig of otter
boards for efficient and stable operations off the bottom, the
development of one boat midwater trawling was mainly
undertaken by or under the guidance of private engineers or
governmental institutes and almost simultaneously in several
countries. The problem was therefore tackled in a more
technical and even scientific way, employing rational means
like precalculation, model and full scale tests and a sizeable
variety of measuring and observing techniques and instru-
ments. As a result a number of one-boat midwater trawls
have been developed. All these gears are suitable for opera-
tion from conventional side or stern trawlers of the respective
country, employing as little additional equipment or skill as
possible. The trawl winches and warps are taken over
unchanged from bottom trawling.
For the otter boards a number of different versions are
used. Okonski describes a slightly modified conventional
rectangular fiat otterboard. Such boards have also been
used in various rigs by other workers. They have the
advantage that they are most easily accepted by fishermen
but hydrodynamically they are inferior to the more generally
employed hydrofoils which are certainly indicated for con-
ditions where towing power is to be utilised to the best
advantage. The full profiles of Larsson and of other designs
described by McNeely are perhaps slightly more efficient
but on the other hand also more costly and less rugged than
the well known Suberkrub design. Most designs employ one
pair of otter boards rigged with G-hook and Kelly's eye as
habitual in bottom trawling. The boards may be adjusted
for some downward sheer, as described by Okonski, or with a
speed dependent component for upwards sheer (Sueberkrub
design). They may also be rigged partly for an upward
and partly for a downward sheer as described by McNeely
for gear with two pairs of otter boards. Stable action of the
boards is sought either by designed combination of adjust-
ment and ballast at the lower edge only, or by additional
flotation.
The legs connecting the net with the otter boards are mostly
steel wire cables or combination manila wire rope with
lengths between 60 and 150 m. They are normally hauled
on the trawl winch. To avoid twisting together, the legs of
each side of the gear should be of opposite twist. The head-
line and footrope legs can be of equal or of different lengths
depending on the position of the otter boards desired in
regard to the net. If the headline legs are longer, as described
by Okonski, the otter boards will touch bottom earlier than
the net and the warps will sweep more of the water in front
of the net than when the headline legs are shorter than the
footrope legs. This adjustment can be significant for trawling
off or on the bottom at will.
In the nets themselves the four-side seam type consisting
of four equal panels has been taken over for most designs
from pair trawling. An exception was the trawl net described
in his own (Dr. ScMrfe's) paper. There, the two-seam type,
or a completely different four-seam type with one pair of
equal and one of unequal panels were used. The two-seam
type is somewhat like a modified Vinge trawl. Although this
type was found to perform technically to full satisfaction the
catching efficiency for non-spawning herring in the North
Sea did not meet expectations. This was probably due to
insufficient opening size of the net mouth and unfavourable
mesh shape. Since two-seam trawlnets can be tapered only
along four edges, they are rather long in relation to the
circumference of the opening and the size to which this type
can practically be built is therefore limited. The four-seam
type with eight edges to taper, has so far been made up to
1,400 meshes circumference and about 100 m in length and
proved more satisfactory in trials.
Unless four otter boards are employed, the net opening
in vertical direction must be secured by special shearing
devices or by weights and floats.
257
Most mid-water trawls are handled in practically the same
way as bottom-trawls, that is on sidctrawlers the usual
auxiliary ropes as quarter ropes and poke lines are employed
and large catches are taken on board by splitting.
Midwater trawling operations, however, require some
additional skill and means for efficiently coping with the
third dimension. Accurate fish detection by echo sounding
is, of course, a conditio sine qua non, and echo ranging is
highly desirable. The net must be brought into and towed
in the depth of the fish schools, which means it must not only
be easily manoeuvrable but its position or movements must
be observable, preferably continuously. Most commercial
midwater pair trawling for herring in northern European
waters is still done without any depth meters but the limita-
tions of this practice need not be stressed. It may suffice for
relatively small vessels fishing under favourable conditions
but for costly vessels and varying fishing conditions more
accurate net depth telemeters are essential.
The echo sounding Nctzsonde technique gives not only
the net depth but also the opening height of the net mouth
and showed the fish in and under the net opening and is
therefore considered particularly suitable, in spite of the
problems connected with the separate cable. The depth of
the midwater trawl is regulated by warp length and towing
speed. Quick manoeuvrability requires, therefore, a power-
ful trawlwinch, preferably with centralised control for
veering and hauling, and an as efficient and simple propulsion
control as possible, preferably directly from the wheelhouse,
as can be obtained for instance with variable-pitch propeller
or diesel-electrical drive. The lack of these facilities of
course does not prevent midwater trawling, but it makes
operation a bit slow and less convenient. For one-boat
midwater trawling, stern trawling technique appears to be
superior to side trawling. Apart from the operational
advantages, the distance between the gallows at the stern
is added to the opening width of the trawl gear and the warps
are kept more apart, which should both favour catching
efficiency.
In spite of the reasonably good technical performance
already achieved by one-boat midwater trawls, it is still felt
that the basis on which development work of this kind has to
rely is not yet sound enough. There is need for more Inter-
national co-operation and reports about active attempts to
fill this gap in knowledge by co-ordinated basic research.
Such research is needed perhaps more in regard to the inter-
action between fishing gear and the fish. In spite of technic-
ally satisfactory performances the catching results of one-boat
midwater trawls under many conditions are rather disappoint-
ing, particularly in comparison with pair trawlers. This
illustrates clearly that the gear is still being developed.
There is no doubt that good progress has been made since
the first Fishing Gear Congress. This however is unfortun-
ately not well enough shown by the papers submitted to this
Congress. There are, for instance no records on the situation
in Norway and France, where commercial one-boat midwater
trawling has developed.
Midwater trawling is supposed to fill the gap between the
working range of conventional near-surface and bottom fish-
ing gear, for better exploiting known fish stocks and for
opening up so far untapped resources. Although many high
expectations in general have not been fulfilled yet, the present
situation and the future development so far as it can be
foreseen from the current efforts, are promising enough, and
interesting results can be expected in the near future.
Mr. K. H. Larsaon (Sweden): Twenty years ago I got
258
interested in trawl gear from a technical point of view and
started my private research work. Having thoroughly studied
available papers dealing with midwater trawling, I would like
to give my impressions of the improvements in the techniques
of pelagic trawling made in these two decades.
It is overwhelming to note how much work has been done
around the world especially during recent years. Possibly
much money could be saved by some international co-
operation. Research work is rather expensive so it is satis-
factory that a degree of International team work has started.
1 would, however, recommend research organisations not to
forget those funny people called inventors. They very often
have good ideas and like most other people, they want to be
paid for their work.
I quite agree with Messrs. Dale and Moller in saying that
"the ideas arising from technical research seemed too
revolutionary and upsetting to many people". I would add,
however, that to base design of midwater trawl gear on
technical theories and experience will be the only way of
reaching good results.
A common trend seems to be the desire that trawl gears
should be used on the bottom, just off the sea bed, and in
midwater. Perhaps the English word "hovercraft", a sort of
floating craft moving just above the surface of the sea, can be
applied so that an all-round trawl gear will have to be a
"hover trawl".
In 1944 at Gothenburg I tested a model of an ordinary
bottom trawl to study its resistance. Then man-made fibres
were hardly known so it was not thought possible to decrease
resistance. Thus I concentrated on the design of the trawl
boards and found that if the trawl boards were lifted from the
bottom of the tank their sheering ability increased by about
32 per cent.
On behalf of the Swedish Navy I continued in 1945 testing
hovering trawl boards in order to design mine-sweeping gear
for picking up mines, torpedoes, aircraft and so on from the
sea bed in harbour entrances. In sheltered sea waters
fifteen different types of trawl boards were tested. A hovering
trawl board must have perfect stability and good sheering
capacity. I soon found that by increasing height in relation
to breadth, sheering power was increased. Definitely the
best type of trawl board was the wing board which looks like
an airplane wing standing upright in the water. This type
of hovering board has a sheering power just twice that of the
ordinary fiat board on the sea bed. Its angle against the
towing direction was only 13 to 14 degrees compared with
30 to 35 for the flat trawl board. This gave very low resistance
and a perfect and steady movement straight ahead.
In his paper Mr. McNeely describes a hydro-foil otter
board with a height to breadth ratio of little more than 1 1.
From my report to the Swedish Navy I could give Mr.
McNeely full information of the sheering ability of his board
as I tested a board almost exactly of that design. A model
like the Suberkrub trawl board was also on my test list in
three different makes with height to breadth ratios of 1-5,
2 and 4.
It is encouraging that hydro-foil trawl boards are coming
more and more into use. It would be reasonable to say that
all testing of midwater trawls with low flat rectangular trawl
boards is a waste of time.
To the merits of the hover trawl I would add the experience
that very little spawn and small fish is caught by a trawl which
moves just off the sea bed. And a trawl of that type would
not harm underwater cables. Other points in designing a
good midwater trawl are, trawling speed, size of net meshes,
frightening of fish in front of trawl mouth, and pressure in
the water in front of the trawl mouth.
The two-boat trawl has the great advantage of the warps
frightening the fish in towards the trawl net whereas the warps
in front of a one boat trawl will to a certain extent frighten fish
away from the trawl mouth. This can be overcome in differ-
ent ways. One is for a certain trawl net to increase the size
of the trawl boards, whereby the distance athwartships
between the boards would increase. Dr. Sch&rfe in his paper
says this has been done with good results. Lengthening of
the pulling line between the trawl boards and the trawl net
will also help. Another way, is to arrange so that the trawl
net moves lower down in the water than the trawl boards do.
In my experience such an arrangement is good. Regarding
suitable trawling speed, very different ideas are found in the
papers. McNeely describes a trawl with a very big net
calculated for a speed of only 24 knots. 1 would not be quite
happy if I had to work with the trawl he describes. It must
be very expensive and the handling of the total gear with four
trawl boards on two warps would mean much heavy hand-
work for the men on deck.
European tests show high trawl speed is essential for good
catches. Dr. Scharfe, in one report, says that for summer
fishing the highest possible speed is necessary and that it
should be investigated whether it would be possible with
higher speeds and new design of gear, to avoid frightening fish
in front of the net mouth.
When balls are used as floats on the headline, the speed is
definitely limited. If one passes that limit the resistance of the
balls will press the headline backwards and thus decrease the
height of the net opening. Dr. von Brandt establishes this
and urgently recommends the use of special types of kites
instead of balls.
Russian tests have given the same experience and they
suggest a profiled kite of improved design for direct and simple
attachment to the trawl.
As early as the Autumn of 1945 1 had made the same
observations. After tank tests in April 1946 and practical
tests up till 1949 the so-called trawl toad was born. It is a
self-stabilising trawl paravane of advanced design which can
be attached to the headline by a single rope. Its lifting
capacity increases with the square of the speed. Two different
types are made, one pulling up and the other pressing down,
so they can be attached to the headline as well as to the foot-
rope. German tests reported by Dr. Steinberg in April 1960
demonstrated that with trawl paravanes on headlines and
footrope, the net opening will be the same regardless of
trawling speed.
Mainly on account of manufacturing difficulties trawl
paravanes of my design have been sold mostly for research
purposes. The plastic industry, however, now offers great
possibilities for making them on a large scale. Once they are
on the market I am sure they will demonstrate their capacity
and make it possible to design an effective high-speed hover
trawl.
For such a trawl design I propose: (1) the trawl net should
have a rather small area of mouth — about half of an ordinary
trawl and be made of net with big meshes. This means less
resistance and decreased pressure in the water in front of the
net mouth. (2) There should be only very few floats and
weights on the headlines and footropes as at high speed the
pressure in the trawl net would give an almost ideal shape,
that is, with circular sections. (3) The trawl net should have
two seams and be shaped like a parachute. (4) In order to
give good catching ability two or more frightening lines are to
be arranged athwartships at certain distances in front of the
trawl mouth, each one with a number of self-stabilising
paravanes to lift the upper lines and pull down the lower ones.
These lines will frighten the fish in towards the centre lines
of the net. Trawl boards of perfect shape should be used.
This arrangement of a trawl has been patented.
Hamuro and Ishii's description of a new instrument for
giving the depth of the midwater trawl is most interesting.
As it works without direct cable connection between trawl and
ship it will be of very great value for promoting midwater
trawling.
Mr. Jakob Jakobsson (Iceland): As early as 1950 pelagic
trawling began in Iceland for spawning cod but in sp'te of
extensive trials in 1957 and 1958 we could not use this gear
for catching herring. In 1959 we carried out extensive trials
with an ordinary four-sided trawl with a head line of 20
metres. We used three sweeplines and followed the ideas of
Dr. Scharfe of putting the real weight of hauling on the centre
and side pieces through the middle line, having upper and
lower sweeplines a little longer so as to give the traw) full
opening. These three sweeplines were then connected to
Larsson's phantom wing trawl boards and we always found
them sturdy enough and they never failed to give good results.
Lacking a netzsonde equipment, we found it difficult to judge
the distance from the bottom and we concentrated on catching
herring near the surface. In December 1959, and January
1 960 we had good catches of herring during the night when the
shoals were approximately 10 to 30 fathoms from the surface.
We often had catches of 20 tons in a haul of 10 to 1 5 minutes.
On two or three occasions we had catches of up to 40 tons,
still working quite near the surface. At these depths we
found that the Larsson wing trawl boards were good; we
paid out five times the length of warps in relation to the
estimated depth of the trawl.
In the following year commercial trawling was unsuccessful
and the only reason that could be assigned was the different
shoaling behaviour of the herring. Later, however fantastic
success on these grounds was achieved by purse seiners with
Asdic guided search and power hauling. They got higher
catches — even as high as 150 to 300 tons per shot — and of
course it was quite out of the question for midwater trawling
to compete with that commercially. Looking back on the
situation, however, what he found most disturbing was that
when he calculated the density of the concentrations in
relation to the volume of the sea water that the seines sieved,
he found that with the midwater trawl they certainly got less
than 10 and probably only 5 per cent of the herring they
encountered. On those facts he thought they were certainly
a very long way short of perfection with this gear, that is,
provided those calculations were anywhere near to the truth.
Dr. A. T. Treschev (USSR): When fishing in midwater
we have to deal with three dimensions and operate in space
where the fish have more possibility of avoiding the trawl,
that is why we have smaller success in midwater fishing. In
fact in midwater trawling we have one problem: we must
either have a big trawl and small speed or a small trawl and
big speed. In catching each species of fish we must know
the optimal of this parameter. An increase in vertical and
horizontal scope can be obtained by hydrodynamic methods
but the real problem was the behaviour of the fish. They had
different types of trawlers which worked different trawls but
they had more successful fishing with their light trawls on
herring but it was not a constant success. One time they
would have good fishing and another time bad fishing — it
259
depended on the concentration of the fish. That was why
implements for measuring the parameter of the trawl and the
fishing were very important.
Dr. F. McCracken (Canada): Experience with midwater
trawling in Canada had been rather sporadic. A trawl had
been developed on the West Coast of Canada and used occa-
sionally more as a trawl just off bottom than as a midwater
trawl. It did not compete effectively with the purse seine
operation in that area. There had been some use of it as a
trawl just off the bottom in the freshwater fisheries of the
Great Lakes for smelt. There was however still some doubt
as to how much better it was than the ordinary high-rising
bottom trawl in that area. There were great difficulties in
operating a midwater trawl and there must be a strong
motivation to engage in it and their experience had been more
or less in abeyance over the past 18 months.
Mr. J. A. Tvedt (UK): We have been pondering this
problem and our solution is that we prefer to use a bottom
weight and to try a four-cornered selvedge. We are trying
to make the nets as small as possible and to use otter boards
that are self-stabilising and automatic and we fee) that by
trying to increase the speed (with such records as we have of
knowledge of the fish) we may be able to increase the catching
rate to a comparatively good figure. From the discussions
it is apparent that there is very little real knowledge about the
actual behaviour of the fish so that we do not know what we
are trying to do and whether it will lead to the result we
want. There must be more knowledge of fish behaviour—
that is the real answer to our problems.
Mr. D. L. Alveraon (USA): Mr. McNeely's big net was
designed to work in very transparent waters where fast
swimming sub-tropical fish lived. We wanted some of them
for biological purposes and the net was primarily aimed to
catch them as well as with an eye on commercial practice.
The philosophy behind it was that if we got a big enough net
and if the fish did not react until he was well back in the net,
there was a good chance of capturing it. From the point of
view of the biologist the net was an evident success. They
got a tremendous variety of fish including most of the high-
speed ones. He acknowledged it had shortcomings in
commercial application. There were difficulties in handling
a four-door arrangement. But they should not preclude a
net because of its difficulty. If a gear could catch fish,
fishermen would learn how to handle it. Its success was
dependent on the fish and it came back to the behaviour of
fish to the gear and the geographical location in which they
worked. There was a tendency to assume that because
something was quite successful in one region it was applicable
to another but that was not necessarily true.
Mr. Harper Gow: Thank you for that last remark. We
have seen repeatedly that successful methods in one part of
the world do not produce the same results in other parts.
Dr. Schftrfe, commenting on Mr. Larsson's references to
high-speed trawling, said high speed was not necessarily
essential as recent experiences demonstrated. Actual com-
mercial trawling on non-spawning herring was being carried
out in winter as well as summer in the North Sea both
by pair trawls and one boat trawls and they operated at
speeds 3*5 and 4*2 knots. Practical experience showed
that higher speeds did not necessarily increase catches— on
the contrary, but all experience must be restricted to the
260
conditions appertaining. In connection with floats, some of the
sophisticated types of equipment for midwater trawls caused
the nets to act something like parachutes so that weights and
floats were mainly needed for keeping the gear clear when
shooting. There was no doubt that Mr. Larsson's devices
performed well. Experience showed, however, that these
devices were somewhat difficult to handle over the rail,
particularly the lower devices attached to the footrope. In
connection with Mr. Larsson's proposal for new gear with
a small opening and large meshes and part of the net being
replaced by guide lines kept in shape by paravanes— that was
theoretically interesting and he expected it would work under
certain conditions. That they would have to find out. He
was not certain it would work for herring and there might
also be operational difficulties when shooting and hauling
As to the netzsonde he personally believed that the advantages
provided were such that the difficulties of handling the cable
were more than compensated for.
Mr. S. O'Meallain (Eire) asked for information concerning
the hauling of the cable on the netzsonde with which they had
had difficulties.
Dr. Scharfe: You have put your finger on the weak spot in
the system. The way of handling the cable depends on its
length and the size of the boat. On small boats cables up to
300 metres in length are handled by hand winches. On the
larger ships they handle the cables with a special electric winch
with pushbutton control. There might be four different
stages of power operation of the winch depending on whether
they were shooting, hauling or towing. These winches had
reached a stage of development where they worked very
satisfactorily. Special cables are used, having a breaking
strength of about 1 to 1 -5 tons. There are two types — one
has a separate steel wire cable to carry the load and two
copper cores and this type is not so advantageous because it
is difficult to repair and the copper cores may also break
under stress due to twisting. The better type is the co-axial
type where the central steel wire carrier at the the same time is
used as one of the conductors. This is very well insulated and
covered by a layer of braided copper wire as second conductor,
which again is well insulated. This complete cable has a
diameter of 12 mm and the breaking strength is quite
sufficient for one boat trawling. We have never experienced
any breaks during actual trawling. Occasional breaks during
shooting operations have been due to lack of experience.
For pair trawling higher breaking strength is needed because
of the higher towing resistance of the cable, particularly when
changing course. The combination drifter-trawlers who
operate midwater pair trawling all have this equipment.
The skippers definitely prefer this equipment for successful
fishing although they grumble at the difficulties. An import-
ant point for the successful operation of the cable was the
position of the winch and particularly of the small davit or
gallows leading the cable overboard, so that the chafing of the
cable on the ship's side is prevented. It was best to have these
davits as far aft as possible. The connection of the cores of
the cable to the recorder unit goes through rings with brushes
on the axis of the winch. Repairing of the cable was very
easy as there were good types of insulation tape available
which were absolutely waterproof and could be put on very
quickly.
Mr. McCracken (Canada): Our best success with midwater
trawling is on the East Coast working in the winter months,
for small herring of sardine size. Part of that success is due
to the transducer on the headline. On the West Coast there
are about five smaller vessels working and they achieve their
best success when the herring are close to the bottom. They
do not use a headline transducer. Recently, they have been
using some monofilaments in the forward pan of the trawl
and although not thoroughly tested yet this did offer some in-
creased success. In Canada they were designing a small
research vessel to investigate the feeding of salmon in the high
seas and they were contemplating designing a triangular
mouth trawl but no details of this were available yet.
Mr. J. Verhoest (Belgium): We started one-boat midwater
trawling in 1959 fishing with a two-panel net for herring on
the Sandettie grounds. In 1961 and 1962 those experiments
were continued on the Smalls— grounds between England
and Ireland. Further experiments were carried out with a
Netherlands research vessel. For the latter experiments a
Grouselle net was used for the Dogger Bank. During last
winter different types of one boat midwater trawls were tried
out along the Belgian Coast for commercial sprat fishing.
The main results Df their investigations could be summarised
thus: on the Smalls, using an Engel net, the results depended
on finding high swimming herring schools which was helped
by sonar and information from other vessels. In one haul
they took 15 tons and in two days fishing they got 55 tons.
Their opinion however, was that midwater trawls could only
be regarded as complementary to a bottom trawl. Both
gears should be carried. In the English Channel the catches
were very poor in relation to the two-boat midwater trawl.
On the Dogger Bank trip with the Grouselle net they got
one bucket of herring. This net had very small meshes for
herring and was constructed of heavy twines. Otter boards
were used which lifted the net off the bottom without changing
the position of the point of attachment to the warps. The
handling of this gear was more complicated than that of the
Engel net especially in shallow water.
In the case of sprat fishing two-boat midwater trawling was
more successful than one boat operation. They found that
with their Engel net it was possible to catch herring in deep
water but that the catches in shallow water with the same net
were rather poor in relation to those of two-boat midwater
trawls. To solve this problem the two research ships
mentioned would also be equipped with a two-boat midwater
trawl. This could easily be done on a side trawler. They
would do it in the following way: on the starboard side they
would have a herring bottom trawl, on the port side a one-
boat midwater trawl and on the stern a two-boat midwater
trawl. Much more needed to be found out about fish be-
haviour but their investigations into one-boat midwater
trawling would continue.
Dr. Heinsohn (Germany): When conditions require it, it
can be advantageous to change quickly from bottom trawling
to midwater trawling. Then the connection of the cable to
the transducer creates problems particularly on the bigger
sterntrawlers when the net is drawn up on the slip deck. In
that case the cable has to be run in some way from aft to
forward on the slip deck. On a few boats we have found that
the cable could be led from the aft bipod mast and that caused
us to abandon the idea of having the two gallows connected
by a bridge. Now we favour just two gallows posts to enable
us to have a better run with the cable forward when the net is
on the deck. Another point is that the change between
bottom trawling and midwater trawling can occur quite often
and should be easy to do. Therefore we experimented in
adapting our aft gallows so that the boards should not be
changed because changing of the boards is a major operation.
The idea we are trying at the moment is that the boards for
the bottom trawl in stern trawling should be on the outside
of the vessel. The first experiments in this direction were
quite successful and we think we can improve the whole
thing. But on most vessels the winch with the cable auto-
matic tension device should not be too far away from the
bridge or should be in view of the skipper so that he can act
quickly if anything goes wrong.
Mr. D. S. Howard (UK): We have made special mid-water
nets for sprats. One little net used on a 45 ft boat (80 horse
power) caught as much sprats as caught by two boats. With
the same net he himself tried bottom fishing from Ffoetwood
and got good results. It was made of two panels with wedges
at the sides. Since then, he had made 1 2 nets of that kind for
Scotland where they were being used in seiners. The net was
also to be used at Killibegs.
Mr. M. S. Parsey (UK) asked for information as to the
type of material used in constructing the midwater trawls.
Dr. Sch&rfe: In Germany for midwater trawls so far nylon
knotted netting is being used. We use normal nylon 6,
because in midwater trawls where twine is made as thin as
possible a rather high amount of elasticity is needed to
equalise the differences in the stress in the netting which may
lead to tears. To attain this elasticity even spun nylon is
used for attaching the webbing to the ropes. Twines of
Td 210/60 in the front pan decreasing to Td 210/33 in the
aft belly have given satisfactory results. Knotless netting
had been tried but results are not fully known yet.
Mr. L. Libert (France): On the subject of high and mid-
water trawling, using a single trawler, the Scientific and
Technical Institute of Sea Fishing Laboratory at Boulogne-
sur-Mer carried out a series of trials on small scale models
following a definite technique outlined in bulletins Nos. 79
and 92 in Science et Peche. These began in 1959. For
trawling, both on the bottom and midwater, these trials were
carried out with a Vinge type trawl (two unequal panels) and
with both two and four panels (equal) trawls.
These truly extensive trials were controlled by netzsonde
and carried out both by boats belonging to the Institute
(fisheries research vessels) and other boats in collaboration
with the Institute. In the course of these trials many different
kinds of equipment were tried out: (a) a system for purely
pelagic fishing with long legs and boards of the ordinary
type with modifications or special boards; and (b) two systems
for midwater trawling. In the latter case, first, with ordinary
boards with long legs for trawling with nets of two unequal
panels and, secondly, with ordinary boards with arms above
grafted to the trawl-cables and in front of the boards— this
system being nearly similar to the 'Icelandic Breidfjord
mounting".
The observations made during the course of these trials
lead to the identification of several conditions favourable to
this type of fishing: (a) detection of schools of fish away from
the sea bed; (b) the minimum speed of trawling necessary for
catching certain species of fish; and (c) trawling above
certain minimum depth of water to avoid the dispersal of
schools caused by the noise of the propeller. In every
case where these conditions have all been followed the results
obtained during these trials were very satisfactory.
As to the fish caught in the course of these trials: in 1961,
off the coast of the Vendee, the Roselys, a fisheries research
261
vessel belonging to the Institute, of 120 hp motor, when
midwatcr fishing with a trawl with two equal panels— secured
an average hourly catch of 450 kgs (sprat and anchovies) and
a best catch of 3 tons.
Again in 1961, off Boulogne, the Notre Dame du Carmel, an
Etaples trawler of 250 hp: when semi-pelagic trawling
using a trawl with two unequal panels and under normal
fishing conditions-— secured an average daily catch of 1-2
tons (sprat, whiting and cod). Under the best conditions
the catch's weight and value were 50 to 100 per cent above
that of trawlers working the same grounds with an ordinary
trawl. By the term semi-pelagic is meant midwater trawling
close to the bottom and by pelagic, trawling in midwater.
In 1962, off Lorient, using the occanographic vessel,
Thalassa, and operating midwater trawling with trawls of
2 and 4 panels (equal) an average hourly catch of 600-1,500
kgs (sprat and sardine) was obtained. The results were less
good in depths of less than 30 metres.
In 1962, the same vessel, Thalassa, using a midwater trawl
with two equal panels in the Gulf of Maine, got a maximum
hourly catch of 7 tons of herring. In 1962, in the Mediterr-
anean, the trawler Sainte Madeleine of 300 hp when motor
fishing from La Ciotat, and midwater trawling with a trawl
with 2 unequal panels got a very useful catch. The best
result obtained was 5 tons in less than half an hour (sardines
anchovies and sprat).
In the case of commercial fishing the records of usage and
catch of midwater trawling show that about 20 French trawlers
of from 150 to 1,300 hp during favourable periods have
used midwater trawling methods with a single boat. At
present this method of trawling is practically wholly confined
to semi-pelagic trawling with the methods indicated above:
long legs or the Breidfjord mounting, and with ordinary
boards in the two cases.
As to the catches made in 1962, four trawlers of 1,300-
1,400 hp from a Boulogne fishing company, equipped with
a trawl with 4 panels obtained useful results in fishing for
herring in the Smalls region during the period between August
1 and September 15. The catches recorded were two to
three times better than those of boats using the ordinary
bottom trawl. The best catch recorded was about 20 tons.
About a dozen trawlers from Boulogne of between 500
and 1,400 hp are now equipped sometimes with a trawl
with 4 equal panels, while others have a 2-pancl (unequal)
trawl. Many of these vessels have obtained good catches
of herring and whiting.
In Brittany the first trials of semi-pelagic trawling with a
single boat took place in 1962 at the commencement of the
mackerel season. Many vessels, equipped with a 2-panel
(unequal) trawl, obtained during the first week results twice
or three times those of trawlers using bottom trawls.
The results obtained in both pelagic and semi-pelagic
trawling in the course of the trials conducted by the Institute
have thus demonstrated that there are possibilities of using
different types of midwater trawls from a single boat of a type
adapted to the fishing conditions.
The results obtained by commercial fishing boats, even
under the present limited availability of these trawls, justifies
the further use of this equipment (two bulletins [Nos. 95 and
110J have been issued by the Institute on this subject.)
Mr. H. KristfoBMon (FAO): In South Japan they have
developed especially over the last few years successful
commercial midwater trawling for big-sized shrimp in the
East China Sea. Fishing is done on the bottom, just off the
bottom, and in midwater in depths of up to 70 metres on
most of the grounds and the shrimp schools rise from 10 to
40 metres off the bottom in winter months, and are detected
by high frequency (200 kc) echo sounders, which are standard
equipment for this fishery. One echogram from a high
frequency sounder showed one school of big shrimp (Penaeus
orientals Kishinorsye) with an estimated thickness of 35
metres and a length of about 480 metres.
The trawlers worked from December to March and they
generally used nets fitted with cable netzsonde. When he
was there six or seven months previously he made a special
point of asking the fleet managers and the men in the repair
shops what trouble they had in handling the cable. Their
answer was that there was some trouble but it was well worth
it. Some of the echo sounding companies in Japan were
also selling cableless depth indicators which, however, did
not do the same job as a netzsonde. They only gave the
depth of the headline and did not show anything else. Only
the cable units gave the full information. * The nets were very
big with headlines of 40 to 45 metres and footropes of 50 to
55 metres. The boats doing one-boat trawling were 300 to
400 tons with engines of 750-850 horse power. Similar nets,
but bigger, were used by two-boat trawlers of some 350 horse
power each. In this case the headline length would be up to
80 metres. The nets were invariably made of polyethylene
knotless of the Japanese two-strand twisted type and were
coloured black. The bigger companies had standardised on
that type. One-boat trawlers all used high curved otter
boards of the Suberkrub type. The boards were 3-2 metres
high and 1 -6 metres broad. They were now built of steel but
formerly had been of steel and wood construction. They
had gone completely over to high curved otter boards. They
had given up the conventional flat rectangular ones. Also on
the big Japanese sterntrawlcrs they used curved otter boards
of the Suberkrub type.
Dr. Scharfe, in summing up, made these points: Under
certain economic conditions there might still be scope for
midwater trawling even where purse seining was pract sed.
Mr. Verhocst's Belgian experiments showed that lighter types
of vessel could operate successfully but catches were not
always reliable. In shallow water pair trawling was normally
considerably more successful than one-boat trawling. This
was to be expected because a single boat had to go over the
school and its wake might disturb the fish. One possibility
of avoiding this, namely to have one otter board considerably
larger in order to get the trawl outside the wake of the prop-
peller was tested with unsatisfactory results. Dr. Heinsohn's
comments on the attempts of fitting big stern trawlers for
both bottom and midwater trawling indicate the growing
interest of trawler owners and suggest that interesting results
might be forthcoming next summer. His own remarks
about fitting netzsonde cable etc., related only to side trawlers.
On the siting of the cable winch, he could say that on stern
trawlers they had already centralised winch control with
success so that the man in charge of the other winches should
also operate the cable winch. On stern trawlers the winch
for hauling the cable must not necessarily be at the stern.
On the new German research vessel it was being installed
very close aft of the centralised winch control.
262
Part 2 Bulk Fish Catching
Section 8 GUlnetting, Longlining and Traps
King Crab Pot Fishing in Alaska
Abstract
The king crab Paralithodes camtchatica fishery of Alaska has
developed spectacularly during the last five years. In 1962 United
States fishermen, operating a fleet of over 200 vessels, produced
50,000,000 Ib of king crab valued, to them, at $5 million. King
crabs vary in commercial size from 7 to 25 Ib, with an average
weight of 10 Ib and a spread of 3J ft between leg tips. The whole
legs are frozen in the shell or the meat is extracted and canned or
frozen. Although various types of gear, such as tanglenets, trawls
and pots have been used since the establishment of a sustained
United States fishery in 1947, nearly all king crabs are now
caught in pots. Pots vary in size from six ft diameter round
pots to 7x7x2£-ft square pots, weighing 250 Ib each; the latter
type is now used on most of the Urge king crab vessels. The seven-
foot pots are constructed of welded mild steel round bar, usually
covered with 0-048-inch diameter soft stainless steel wire woven in
five-inch bar mesh. Nylon netting is now being used by some
fishermen in place of stainless steel. A tunnel at each end slopes up
to 40 x7£-inch opening. The pots are baited with frozen herring.
Pot lines are of f-inch diameter nylon and polypropylene, in 30 fm
sections, to fish in depths from 20 to 150 fm. The line is buoyed with
one or two 20-inch pneumatic plastic bags. The pots are set indivi-
dually, usually within 10 miles off the coast in a straight line or
"string" to facilitate location. Depending on the area, from 30 to
80 pots per vessel are used and hauled each day, weather permitting.
An average of 50 crabs per pot is considered good fishing, and 100
crabs per pot is considered an excellent catch. Vessels range in size
from 40 to 150 ft and the majority were built originally for halibut
and salmon fishing. All of the larger vessels are equipped with
seawater tanks in the hold since crabs must be alive up to the time
of processing. The fishing area of the smaller vessels is limited as
seawater tanks are not feasible for them and the crabs will live only
about 12 hrs out of water, even if kept wet with salt-water spray.
Pots are hauled on winches with warping heads at speeds up to
300 ft/min. A hydraulic drive is desirable since maximum line
tension must be limited to prevent breaking pot lines when sudden
surges occur when hauling in heavy seas. Even though pots can be
hauled and reset at the rate of 6 to ten per hour, ways are continually
being sought to speed-up the operation. The most recent device is a
hydraulically-driyen V-grooved sheave mounted on a davit or
boom. Speed, direction and maximum line tension of the crab
pot and topping and swinging to bring the pot aboard are controlled
by a console located adjacent to the bulwarks.
La pdche a la nasse du crabe royal en Alaska
Resume
Au cours de ces cinq dernieres annexes, la peche du crabe-royal
Paralithodes camtchatica s'est developpee d'une facon spectaculaire
en Alaska. En 1962, aux Etats-Unis, une flotte de 200 bateaux de
pfcche a produit environ 23,000 tonnes de ces crabes, d'une valuer
de cinq millions de dollars. Le poids de ces crustac&s varie de trois
a 1 1 kilos, avec une moyenne de 4-5 kilos et la distance entre les
extremites des pattes est d'un metre environ. Les panes sont
congelecs dans la carapace ou bien, la chair en est extraite et mise
en bolte ou conge!6e. Bien que divers types d'engins tels que filets
a crustaces, chaluts, aient etc utilises, depuis I'ttablissement aux
Etats-Unis d'une peche organised, presque tous les crabes sont
captures a la nasse. La taille des nasses varie de 1-80 m de diametre
pour les rondes a 2m x 2m x 0-75 m pour les carries, chacune
d'elles pesant 110 kilos. Les nasses carrees sont plus particuliere-
ment utilisccs par les grands bateaux. Elks sont construttes de
barres d'acier 16ger, recouvertes d'un grillage d'acier inoxydable
de 1-2 mm de diametre formant des mailles de 12 cm. Quelques
pficheurs utilisent maintenant, & la place du grillage d'acier, du
filet de nylon. Le tunnel construit a chaque extrcmitt se retrecit
jusqu'a une ouvcrturc de 120 cm x 18 cm. Les nasses sont appatees
avec du hareng congefe. Les lignes des nasses sont en nylon et
polypropylene de 16 mm de diametre, par sections de 55 m et
pgchent a des profondeurs de 37 a 270 m. Les lignes sont alldgees
avec deux sacs pneumatiques en plastique, de 50 cm de diametre.
Les nasses sont placees individuellement, generalement a une
by
R. F. Allen
Marine Construction & Design
Co., Seattle
distance de 10 milles de la cdte, en droite ligne pour faciliter la
localisation. Suivant la region, de 30 a 80 nasses sont utilisees par
chaque bateau et elles sont halves tous les jours si le temps le
permet. On considere que 50 crabes par nasse reprcsentent une
bonne peche; 100 crabes, une peche excellente. Les bateaux ont de
12 a 45 m de long et la plupart ont 6t6 construits a 1'origine pour
la pdche au saumon et au fletan. Tous les grands bateaux sont
equipes avec des reservoirs a eau de mer, les crabes devant fttre
vivants au moment de leur traitement. Pour cette raison, les lieux
de peche pour les petits bateaux sont tres limited car ceuxci n'ont
pas de reservoirs et les crabes ne peuvent vivre au-dela de 12 heures
hors d Peau, meme s'ils sont airosts. Les nasses sont halves par
des treuils, & une vitesse de 90 m/min. Pour eviter que sous 1'effet de
la houle, les lignes se cassent, il est desirable d'avoir un entralnc-
ment hydraulique pour les treuils. Bien que les nasses puissent
£tre halves et remises a 1'eau & une cadence de six a 10 par heure,
on recherche toujours des moyens pour acc£16rer les operations.
Un de ces moyens consiste en un rouleau de coupe en V, actionn£
hydrauliqucment et montd sur un bossoir. Les vitesses, direction
et tensions maximum sur les lignes des nasses aussi bien que le halagc
et le pivotement pour prendre la nasse & bord, sont contrdtes par
un table de manoeuvre situ£ dans le pavois.
Pesca del centollo de Alaska con nasas
Extracto
La pesca del centollo de Alaska Paralithodes camtchatica se ha
desarrollado espectacularmente en los ultimos cinco aftos. En 1962
Pescadores de los Estados Unidos a bordo de mas de 200 embarca-
ciones capturaron 50 millones de libras de cangrejo, que les rcpre-
sent6 un ingreso de cinco millones de dblares. El centollo comercial
pesa de 7 a 25 Ibs con una media de 10 Ibs y tiene una envergadura
de 3*5 pies entre los extremes de las patas. Estas se congelan enteras
o se les extrae la came que se enlata y congela. Aunque desde el
establecimiento en 1947 de una pesquera constante de los EUA
se ban empleado varias clases de artes como el de enredo y el de
arrastre, y nasas, casi todo el cangrejo se pesca en la actualidad con
nasas. Las dimensiones de £stas varian; las redondas tienen un
diametro de seis pies y las cuadrades son de 7x7x2-5 pies y un
peso de 250 Ibs. Las ultimas se emplean actualmente en casi todos los
barcos cangrejeros grandes. Las de ocho pies son de barra de
acero suave soldado y normalmente se cubren con una tela metalica
de acero inoxidable flexible de 0-048 pulgadas de diametro, con
mallas de cinco pulgadas entre nudos. En la actualidad algunos
Pescadores usan red de nylon en vez de acero inoxidable. Una
abertura en cada extremo conduce hacia arriba hasta entradas de
40x7-5 pulgadas. Las nasas se ceban con arenque congelado. Los
cables son de nylon o polipropileno de 5/8 pulgadas de diametro
en secciones de 30 brazas. Se da flotabilidad al cable con uno o dos
neumaticos de material plastico. Las nasas se calan individualmente
por lo general a menos de 10 millas de la costa y en Ifnea recta para
faciKtar la Iocalizaci6n. Segun el lugar, cada banco usa de 30 a 80
nasas que levanta a diario si lo pcrmite el tiempo. Un promedio
de 50 centollos por nasa se considera una buena pesca y de 100,
excelente. La eslora de los barcos varia de 40 a 150 pies y la mayoria
se construy6 originalmente para pescar hipogloso y salm6n. Todos
263
los barcos grandes estdn equipados con tanques de agua de mar
para almacenar los centollos, que ticncn quc estar vivos hasta el
momenta de la elaboraci6n. La actividad pesquera de los barcos
mfts pequefios esti limitada porque no pueden equiparse de tanques
dc agua de mar y los cangrejos s61o viven unas 12 horas fuera del
mar aim si se mantiencn hiirnedos con pulverizaciones de agua.
Las nasas se izan con maquinillas de tambores a velpcidades hasta
de 300 pics/min. Conviene cmplear una transmisi6n hidr&ulica
porque la tensi6n maxima del cable hay que limitarla para impedir
que se rompa cuando el barco se mueve violentamente en mar
gruesa. Aunque las nasas se levantan y se calan de nuevo a raz6n
de 6 a 10 por hora, continuamente se estudia la manera de acelerar
la maniobra. El dispositive m£s moderno es una roldana con
garganta en V accionada hidrfiulicamente y montada en una pluma
o botavara. La velocidad, direcci6n y tensi6n maxima del cable de
la nasa y los movimientos de la pluma para materla a bordo se
regulan desde un cuadro colocado junto a la regala.
IN Alaska the king crab fishery is the fastest growing
commercial fishing industry. In terms of value the
catch taken on the grounds from Prince William Sound
to the Aleutian Islands is second only to salmon for
the state of Alaska. The industry has developed spectac-
ularly in the last five years to an annual catch of
50,000,000 Ib (with a value of $5 million to the fishermen)
in 1962. A fleet of over 200 vessels is presently engaged
in this fishery.
The king crab (Paralithodes camtchaticd) varies in
commercial size from seven to 25 Ib, with an average size
of 10 Ib and a spread of 3£ ft between leg tips. Only the
meat in the legs is recovered; the whole legs are frozen
in the shell or the meat is extracted and canned or frozen.
Although commercial harvest of king crab by United
States fishermen began in the 1930's on a sporadic
basis with pots, a sustained fishery did not get underway
until 1947, when tanglenets, trawls and pots were used.
At that time trawling predominated. The first experi-
ments with pots did not produce promising results, and
tanglenets required considerable labour. Subsequently
pot designs have been improved to the extent that virtually
all king crabs are now caught in pots. The success of
this fishery can be attributed to the efficiency of modern
king crab pots and to the methods of handling pots with
relatively few men.
Handling crab pots
The crab fishermen's experience in the location of setting
pots has been by trial and error. Electronic detection
equipment is not presently in use for locating crabs prior
to setting the pots. Recording type fish finders do not
detect king crabs on the bottom except in moulting
season when crabs bunch together. Crab pots are set in-
dividually with a line and buoy in coastal waters, usually
within 10 miles of the coast. In order to facilitate the
location of the pots they are generally set in a straight
line or "string". Considerable care is taken to fix the
position of the string of pots. As the buoys are sometimes
out of sight of land, a radar reflector buoy is anchored at
the end of the string. Each buoy is numbered and its
location recorded when set. State of Alaska fishery
regulations limit vessels to 30 pots in the Kodiak and
Cook Inlet areas, but in more remote areas where no
legal limit is in force 50 to 80 pots per boat are often
fished.
264
Frozen herring is used as bait. It is placed in a per-
forated metal container or bag attached in the centre of
the pot. The pots are launched off the side of the vessel
in water of from 20 to 150 fm in depth. Since the tidal
currents are quite strong and the drag on the pot warp
is considerable, the buoys must be large enough to
remain above the surface. Fishermen are using two
20-inch diameter pneumatic plastic buoys similar to
Scandinavian longline fishing floats (Fig. 1). Weather
permitting, the pots are hauled every day.
Fig. L Retrieving a crab pot buoy. Small auxiliary buoy facilitates
recovery.
Several methods are employed in hauling the pots
aboard. Inasmuch as many of the crab boats are also
used for salmon seining during the summer, they are
generally equipped with gypsy winches. The pot line
is led over a davit mounted snatch block to the gypsy
head and is tailed and coiled on deck by the fishermen.
When the pot reaches the surface, a heavy fall from the
main boom is hooked to the rope bridle and the pot is
then swung on board and set across the bulwark and
the hatch coaming (Fig. 2).
A number of vessels are now equipped with a V-grooved
hydraulic pot hauler which is mounted on a davit or
boom (Fig. 3). Where a power boom is used it is possible
to lift the pots on the deck with the pot warp; however,
some fishermen prefer to use a safety line from the end
of the boom to minimise the risk of breaking the pot
warp when the pots are loaded (Fig. 4). The top of the
pot hinges open and the crabs are dumped onto the
F/£. 2. P0/J ore se/ between hatch and bulwark. Fishermen using
gypsy on longline-type gurdy, as this vessel also fishes for halibut.
deck; then the pot is rebaited and launched overboard.
The crabs are sorted and male crabs with a shell width
greater than seven inches are put into the circulating water
tank in the hold of the vessel. Small males and all female
crabs are returned to the water. In a good day's fishing
pots can be hauled at the rate of 6 to 10 an hour, depend-
ing on water depth and sea conditions. An average of
50 crabs per pot is considered good fishing, and 100
crabs per pot is excellent. A 7 x 7-ft pot containing
294 8- to 10-lb crabs has been reported.
The crabs are held in circulating water tanks and
delivered to processing vessels or shore plants for
freezing or canning of the leg meat. Many of the smaller
vessels have not been equipped with seawater tanks and
are therefore restricted in their range. King crabs can
Fig. 3, Hydraulic crab pot hauler on boom for positioning pots on
deck.
Fig. 4. 7 x 7 ft pots brought on board with a chain fixed to boom
end after being raised to water surface with hydraulic pot hauler.
live for about 12 hrs out of water if they are kept wet
with salt water spray. Crabs must be alive at the time of
processing; therefore circulating water tanks are becom-
ing a necessity as the vessels venture farther from the
plants.
Vessel types
Vessels employed range in size from 40 to ISO ft. No
new vessels as yet have been constructed specifically
for king crab pot fishing; the majority have been adapted
from salmon and halibut fisheries of the north Pacific.
The general trend in recent years has been toward
larger vessels that can operate in more remote areas,
transporting all their pots on board at one time. The
hazards of winter fishing in severe weather conditions,
with vessels loaded to capacity with water in the tanks,
has resulted in some loss of life and vessels, and has
hastened the trend toward larger boats.
A great deal of experimentation in equipping vessels
of all types has centred around obtaining a balance of
the following desirable characteristics :
(a) Pilot house located so that the helmsmen can
readily observe the fishermen and manoeuvre
the vessel with fingertip engine and steering con-
trols to each pot buoy.
(b) Large deck space for storing the pots. Outside
of the areas where state regulations permit only
30 pots, the larger vessels have some advantage
265
in their ability to carry more pots and travel into
relatively unfished areas. The small boat, which
must make several trips to a new fishing ground in
order to transfer the pots, loses considerable
fishing time.
(c) A stable working platform. The ability of the larger
vessels to haul crab pots in rough weather is a
most important factor in this area where heavy
seas are experienced.
(d) Ability to handle heavy loads of seawater in
tanks. A detailed study of stability and freeboard
is most important in view of the steep seas, high
wind and icing conditions encountered during the
winter king crab fishing. Adequate stability must
exist in both light and loaded conditions and during
flooding of the tanks.
The fishing vessel Jeanette F (Fig. 5) is typical of the
majority of recent additions to the king crab fleet. This
vessel is equipped with a hydraulically actuated boom
Fig. 5. Typical crab boat. 72 x 18 ft 6 in. Carries trawl gear as
well as 30 to 40 pots.
and pot hauler. Controls for speed and direction of the
pot hauler and for topping and slewing the boom are
located adjacent to the bulwark amidships. The vessel
is also equipped with trawling gear; however, this is
seldom used for king crab.
Most of these vessels are equipped with outrigger
fin stabilisers for rough weather fishing. The 20-ft
poles mounted port and starboard hinge outboard to
266
an angle of approximately 35° above horizontal. Fin
stabilisers hang from the ends of these poles approxi-
mately 20 ft below the surface. They are heavily counter-
weighted so that they have a continuous tendency to
dive. This type of stabiliser is quite effective in heavy
weather. (Figs. 5 and 6 show the poles extended and
stowed.)
Typical of the smaller type of inshore crab fishing
boats is the 46-ft steel combination salmon purse seiner
(Fig. 6). The hold consists of a single tank of 870 ft3
Fig. 6. Small steel crab and salmon seining boat. 46 x 75 ft with
stabiliser poles and fins used in rough weather.
in volume. It has a capacity of approximately 1 ,760 10-lb
crabs. The deck arrangement is typical of the Alaska
purse seiners, with a two-gypsy winch used to haul pots.
In the latest steel crab vessel designs the hold tank does
not extend the full beam of the vessel but is limited in
width by outboard voids or fuel oil tanks on either side
so as to reduce the loss in stability due to free surface in
the tank, should it be necessary to fill or empty the tank
at sea. During normal operations these tanks are com-
pletely full into the hatch trunk. The water level is 1 ft
below the bolted down hatch cover flowing overboard
through a screened discharge pipe. A small 2 x 2-ft
hatch for loading crabs is located in the starboard side
of the hatch cover (Fig. 3). When unloading the crabs
from the vessel with buckets, the entire hatch cover is
removed.
Fig. 7 shows a tender type vessel, twin screw, which
was converted for king crab fishing. The conversion
included two large steel tanks of 1 ,280 ft3 each in volume,
steel bulwarks port and starboard, a circulating seawater
system for the tanks, and pot hauling equipment. This
vessel has a capacity of 5,000 king crabs and, because of
its extreme beam, is ideal for carrying pots on deck. This
type of vessel is remarkably well suited to this service.
The most recent and largest king crab fishing vessel
in Alaska (Fig. 8) was converted from a naval landing
craft 159 ft in length. This large vessel operates with a
total crew of four men. The two tanks have a total
capacity of 6,780 ft3. She is reported to carry 15,000
. 7. Alaska salmon tender converted to a crab fishing vessel.
86 x 28ft.
8-lb crabs. The pots are hauled forward on the starboard
side, with a line led over a block on the end of a short
davit. A gypsy winch with one large diameter warping
head for high speed and one of smaller diameter for
slower speed and heavy lift is mounted on the starboard
side forward (Fig. 9). The crab pot is brought to the
surface and a fall from the main boom is attached to a
bridle on the pot for hoisting it aboard. Each crab tank
is equipped with an elevator consisting of a steel frame
with netting covering the entire area of the tank and
resting on the bottom. Cables from the four corners of
the frame are led to the main boom cargo gear. When
the crabs are being unloaded into containers on the
deck, this elevator is hoisted so as to keep a layer of
crabs at the level of the hatch coaming. This permits the
Fig. 9. Hydraulic gurdy with 18 in diameter chrome-plated gypsy
and auxiliary gypsy. The pot line Is led over a davit-mounted block.
crew to load crabs into hopper baskets directly at deck
level without pumping down or entering the tanks. In
using this system to raise the crabs in the tanks, the
top opening must be the full width and length of the
tank so that crabs are not crushed against the tank
overhead.
Fig. 8. Largest crab fishing vessel. 159 x 23 ft 3 In. Fishes 80 pots with a crew of four men.
267
Ttaks
Crab fishing vessels are equipped with tanks consistent
with the freeboard and stability requirement. Steel is
the most common tank lining. As the fish holds of most
existing vessels are sized for lower density fish and ice,
the volume is larger than can be safely considered for the
crab tank. Considerable care must be taken to analyse
the effect of the weight of the tank and water on the
trim and freeboard of the vessel. An inclining experiment
is suggested to determine the stability of the existing
vessel and to provide a basis for estimating the stability
during the period when the tanks are slack and under
full load conditions. Table I describes typical king crab
vessels of various sizes and their respective tank volumes.
The carrying capacity is approximately two 10-lb king
crabs per cubic foot.
Table I. —Tank capacities of various king crab fishing vessels
Gross Length
Tonnage Overall Beam Cu ft
Sunrise
Pacific Fisher
Falcon ..
Selief . .
Nordic
Pacific Pearl
North Beach
129
113
43
178
45
61
108
86
81
46
86
61
68
75-8
22
21
15
28
15
17-4
21-5
2,760
2,300
900
2,400
1,050
1,430
2,400
Consideration should be given, when designing crab
tanks in wooden vessels, to providing adequate ventila-
tion between the steel tank lining and the sides of the
vessel. Rot in the wood is a constant problem and it is
therefore suggested that the tanks be covered on the
outside surfaces with an insulating sweat-resistant coating.
The inside of the tanks are often coated with a coal tar
water-base coating.
Water is circulated through a perforated pipe manifold
located in the bottom of the tank and discharged
overboard at a level 1 ft below the hatch coaming.
The pumping capacity should be adequate to change the
water in the tank each 20 to 30 minutes. Fig 10 shows the
arrangement of a typical pumping system which is
driven by a small auxiliary 8 hp diesel engine suitable
for a small vessel.
Crab pot designs
The present trend is toward the 7 x 7 x 2^-ft square pot,
which is now used on most of the large king crab vessels
(Figs. 4 and 11). The smaller vessels are using the 6 x
6 x 2£-ft square pot, and six-ft diameter round pot
(Figs. 2 and 12). The square pot is more popular because
it can be stored more easily on deck and has a greater
capacity for the same overall dimensions. A seven-foot
pot weighs approximately 250 Ib, has a life expectancy
of five years, and has an average value with line and
buoy of $250. A 7 x 7 x 2£-ft Pot has an internal
volume of 104 ft3. Some of these pots have contained
over 200 crabs with a total weight of 2,200 Ib.
These pots are constructed of welded mild steel round
bar, one-inch diameter for the bottom bars with three-
quarter-inch diameter vertical and top frame bars. The
tunnel and the door are framed with half-inch diameter
round bar. The majority of the pots are covered with
0-048-inch diameter soft stainless steel wire, woven in
five-inch bar length mesh. On each end a tunnel slopes
up to the horizontal opening at an angle of 35 to 40°
CIRCULATIMQ WATCtt.
OVeBJbOAftD DISCHARGE
MAIM DECK
LOADING HATCH
'COAMING
CRAB TANK
OV60SOAQD
PUMP
SKIQIKIE Oft MOTQg OQI V1KL
CgNTRlFUQAL PUMP
^l ZED TO PILL TAMH
INJ 70 TO SO MINUTES
CIRCULATING WATEft
DISTRIBUTION HftAOCC
CQA6 TANK SUCTION
Fig. 10, Typical crab tank sea-water circulating system.
fit A WATBft SUCTIOKI
THAU 91A CH*»r
268
Fig. 11. 7 x 7 ft pot coming aboard. Note roller on gun whale.
This section of the pot is woven with two-and-a-half-inch
bar length wire mesh. Some fishermen are now using
nylon netting. This requires a double frame designed
pot, with the netting hung on inside bars which are
protected from chafe of the hanging (see Figs. 4, 1 1, and
13). The nylon netting covering the top, sides and
bottom is nine-and-a-half-inch stretched mesh, 10/72
thread. The tunnel is three-and-a-half-inch, 10/72 thread
netting.
The tunnel design is a compromise between the
Fig. 12. 6 x 6 ft pots hoisted on board with falls from the main boom
attached to a bridle on the pot.
Fig. 13. Tunnel design on? x 7 ft pot.
reduction in volume of the pot and an attempt to reduce
the slope of the ramp and provide an opening which will
discourage the crabs' escape. The opening is almost
horizontal, 40 x 1\ in. In areas of heavy fishing the
pot openings are often reduced to 30 in in width to
increase the internal volume. No trip mechanism or
triggers are used to prevent escape.
Other configurations have been introduced which
enable the pots to be stacked one inside the other, but
to date none of these has come into popular use. It is
possible that, as practicable mechanical means become
available to handle larger pots, the size will increase.
Lilies and buoys
Pot lines generally consist of 30-fm lengths of f -inch
diameter nylon and polypropylene. As the depths vary,
these lines are tied together to lengths sometimes greater
than 150 fm. Since polypropylene line will float, a 30-fm
section is generally attached directly to the pot to mini-
mise the possibility of the line becoming wrapped round
the pot. Nylon is used at the surface since it is desirable
to have the line sink vertically from the buoy to minimise
the chance of its being cut by the propellers of passing
vessels.
The typical buoy attachments consist of one or two
20-inch diameter pneumatic plastic bags, the same as
those used in longline fishing. A two-fm line with a light
float is attached to the buoy to facilitate retrieving (see
Fig. 1).
269
Hurting devices
The most common pot hauler is a gypsy winch, already
present on most crab vessels because they have been
converted from salmon purse seining. A modified purse
winch is shown in Fig. 9. This 18-inch gypsy produces a
speed of 300 fpm and a line pull of 2,500 Ib. It is hydrauli-
cally driven and has a maximum of 23 hp. Hydraulic
drive is desirable so that the maximum line tension can
be limited. This is almost a necessity when hauling in
rough seas so that a sudden surge of the vessel will not
break the pot lines (Lerch, 1963).
The most recent device introduced for crab pot hauling
is a hydraulically-driven V-grooved sheave which has a
hauling speed of 300 fpm and a line pull of 1,600 Ib
(Figs. 3, 4, 5, and 7). A stripping mechanism in the
form of a wedge laid in the bottom of the crab block
sheave prevents backlash. Small fairlead blocks guide
the line onto the sheave. The latest design has an addi-
tional fairlead sheave so that the line will remain on the
sheave when the knots ride over the groove. The deep
wedging action of the sheave grips the line so that it is
not necessary for the fishermen to put tension on the
line coming from the block to prevent slipping. The
slack line is merely coiled on the deck as it comes from
the crab block. Speed, direction and maximum line
tension of the crab block and topping and swinging of
the boom are controlled from a console located adjacent
to the bulwarks. At the beginning of the operation, the
boom is lowered to a position that enables a man to
insert the line over the sheave. Throughout the hauling,
the boom remains in the lowered position until the pot
comes to the surface. A hook fixed to the end of the boom
by a short chain is attached to the bridle or pot frame
so that the pot can safely be brought aboard by topping
and swinging the boom.
Future developments
With the increased emphasis on king crab fishing, further
developments in gear and methods can be expected in
the following areas:
Buoys — The existing floats ride low in the water and
are often difficult to locate in bad weather. A new design
incorporating radar reflectors and balloons that can be
Fig. 14. Typical details of Alaska king crab pot.
Fig. 15. King crab (Paralithodes Camtchatica).
270
Les Madraques Atlantique et Sicilienne
Rfeurnt
Le mou donn6 aux filets constitue la principale difference de con-
struction des madragues atlantique et sicilienne. Dans la madrague
atlantique, la profondeur des filets est de tres peu supericure a
a profondeur de 1'eau au site tandis dans la madrague sicilienne
les filets ont normalement un tiers de plus que la profondeur du
site. Ceci a une repercussion sur la comportement de la madrague
sous 1'effet des courants. Ainsi, les filets de la madrague sicilienne,
sous de forts courants, seront tires au-dessous de la surface et
deplaceront 6ventuellement les pierres qui les maintiennent en
place alors que dans la madrague atlantique qui a beaucoup plus
de flotteurs amarres, le lest sera simplement deplace et se remettra
en position correcte des que le courant diminuera. L'auteur fait
remarquer la difference de construction des deux types et signale les
a vantages et d6savantages de chaque madrague. Le Centre Exp6ri-
mental de Peche de Sicile a mis au point un type modifid de madra-
gue a thon qui combine les avantages des deux types. Par sa con-
struction la madrague modifiee ressemble beaucoup a la madrague
atlantique mais possede une chambre supptementaire entre la
chambre de mort et le corps de la madrague qui est ferme eet ouverte
par une porte du type sicilien.
The Mediterranean and Atlantic tuna traps
Abstract
The main difference in the construction of the Atlantic tuna traps
compared with the Sicilian traps lies in the looseness of the netting.
The depth of netting in the Atlantic trap is very little more than
the depth of water at the site, whereas in the Sicilian trap the netting
is usually one-third more in depth than the water depth at the site.
This has a repercussion on the behaviour of the trap in respect of
currents. The netting of the Sicilian trap will, at excessive currents,
drag the floats beneath the surface, and eventually displace
its sinkers. The Atlantic trap, which is more heavily floated,
will, at excessive currents, lift its sinkers which will reset
themselves in the correct position when the current decreases. The
author discusses the construction details of both types and points
to other advantages and disadvantages of both the Atlantic and
Sicilian traps. The Sicilian Experimental Fisheries Centre has
evolved a modified tuna trap which combines the advantages of
both. Constructionally, it is largely based on the Atlantic type but
has an additional chamber between the death chamber and the
body of the trap and includes also an additional collapsible trap-
door system between such chamber and the body of the trap.
Las almadrabas del atlantico y del mediterraneo
Extracto
La principal diferencia entre las almadrabas del Atldntico y las
sicilianas es que estas ultimas tienen los paftos muy flojos. La pro-
fundidad de la red en la almadraba del Atl&ntico es muy poco
mayor que la del agua en el lugar en tan to que las sicilianas son un
30 ppr ciento mas profundas que el agua. Esto repercute en sus
movimientos en las corrientes; los paftos de las almadrabas sicilianas
si la corriente es muy fuerte, arrastran los flotadores debajo de la
superficie y llegan a desplazar el lastre. La almadraba del Atldntico
que tiene mucnos mas flotadores, levanta el lastre si la corriente es
by
Vito Fodera
Fishery Adviser, Tunis
muy fuerte, pero lo deja caer en el mismo lugar cuando disminuye su
intensidad. El autor explica los detalles de la construccibn de
ambas clases y cita sus ventajas e inconvenientes. El Ccntro de
Pesca Experimental de Sicilia ha preparado una almadraba
modificada que combina las ventajas de ambas. Desde el punto
de vista de la construccidn, se basa principalmente en la del Atlan-
tico, pero entre la camara de la muerte y el cuerpo tiene una camara
mas y un doble sistema de trampas plegables.
UNE madrague est contjue surtout pour la peche des
thons rouges, mais elle peut aussi bien pecher
d'autres poissons migrateurs: germon, bonite, espadon.
Requins de diffSrentes esp&ces, marsouins et slrioles
sont aussi fr^qu eminent pris dans les madragues, tout
comme certains poissons assez rares en M£diterran6e
tels que les esturgeons. Dans certains endroits, les
madragues peuvent capturer des quantitfe parfois
importantes de sardines, anchois, maquereaux, saurels.
Principes generaux
La madrague est un engin de peche compris dans la
grande cat£gorie des trappes et se base sur deux principes
de capture, Pinterception des poissons et leur concen-
tration dans une zone restreinte ou la capture est possible.
Une madrague se compose done d'au moins deux
parties :
(1) un barrage plus ou moins perpendiculaire au
rivage dit "queue" qui sert £ 1 Interception et au
d£routement du poisson.
(2) deux ou plusiers chambres qui forment le corpe
proprement dit de la madrague (Isola), ou Is
Continued from page 270
seen at greater distances would reduce the time spent in
locating pots.
Line coiling mechanism — The present rate of hauling
(300 fpm) is limited by the ability of a man to hand coil
the line on deck. An automatic coiler, which would
operate regardless of knots, would speed up the hauling
and also free a man to sort crabs.
Rigging to handle pots on deck— The 7 x 7-ft pots
weigh 250 Ib and require several men to move them around
the deck of larger vessels to a stowed position. A rotating
crane or highlinc arrangement would be helpful (Fig. 14).
New types of pots — Further research in pot design is
indicated by the various changes which are taking place
throughout the fleet. A pot is needed which can be folded
or stacked one in another so as to allow more efficient
use of deck space.
Mechanised unloading — A mechanical method of
unloading to replace the manhandling of every crab would
increase efficiency considerably. (Fig, IS.)
Detection equipment — A means of locating crabs prior
to setting pots would greatly increase productivity.
271
poisson se rassemblc; c'est dans la derniire cham-
bre que s'effectue la capture.
On congoit facilement que, thgoriquement, la capa-
citi d'interception et aussi de production de la madrague
soit en rapport avec la longueur du barrage. Cette
longueur est elle-meme conditionnle par la profondeur
qui, comme on va le voir, ne peut exc&ler certaines
limites et pourtant, dans de nombreux cas, la madrague
est complete d'une queue supptementaire qui s'dtend
du corps vers le large (codardo).
Les figures 1 et 2 donnent les plans sch&natiques des
madragucs sicilienne et at Ian ti que.
Fig. 1. Plan d" ensemble de la madrague sicilienne.
Fig. 2. Plan d* ensemble de la madrague atlantique.
La madrague atlantique intercepte les poissons non
sculement au moyen de la queue et £ventuellement du
codardo, mais aussi par une mdthode de barrages sue-
cessifs se terminant en crochets, orient^es perpendiculaire-
ment & la queue, du cdt£ ou Ton attend le poisson.
Bien que plusiers madragues soient explores le long
de la c6t6 Atlantique d'Espagne et du Maroc, cet engin
est surtout employ^ en M6ditcrran6e ou les mantes
sont faibles. Dans les r£giones & marges fortes, le probl&me
de la flottabiliti pr£sente des difficult^ presque insurmon-
tables.
Bien que la question des migrations des thons soit
toujours ouverte et que les theories les plus disparates
et contradictoires aient £t£ inonc&s, depuis Aristote
& nos jours, il faut convenir que les thons, en M£diterra-
n& semblent avoir tendance & se diplacer dans la direc-
tion g£n£rale du courant. Les madragues en consequence
ont leur bouche orient£e face au courant dominant
(& l'6poque de la peche). On sait que le courant dominant
en Miditerrante se dirige vers Test le long des cotes
d'Afrique et en sens contraire le long des cotes Europ£en-
nes, ce qui fait que les madragues de 1'Afrique du Nord
ont la bouche vers 1'ouest et celles de la c6te nord de la
Sicile, vers Test.
La limite technique de la madrague reside dans la
possibility d'en assurer le flottement, r aplomb, le calage
et le maintien de 1'installation en itat de fonction-
272
nement dans des conditions variables, souvent par mers
et courants tris forts.
Le corps d'une madrague se situe par 50 ou 60 mitres
de fond et les filets vont de la surface jusqu'au fond.
En plus du poids de 1'engin meme, PefFet des courants
sur des nappes de filets d'une telle hauteur et d'une
longueur pouvant atteindre plusieurs milles, mettent
en jeu des forces considerables et posent des problemes
de flottaison.
La chambre de capture ou chambre de la mort qui
est la seule partie mobile de la madrague, doit
pouvoir Stre relevte du fond et de la rapidit6 de PopSration
depend le succis de la peche. Son poids propre et les forces
excretes sur elle par les courants, ne peuvent d^passer
certaines valeurs qui limitent les dimensions de chaque
installation, suivant les conditions bathymftriques de
la zone.
En ce qui concerne le choix des zones, il faut pr&iser
que s'offrent deux possibilites:
(a) placer la madrague sur la route de migration
principale des thons.
(b) intercepter les thons i I'int6rieur des golfes lorsqu'-
ils approchent de la cote pour frayer.1
Ceci ne veut pas dire que le thon g6n6tique ou de
course, ne puisse se pecher qu'avec des madragues de
golfe, car les madragues places sur le prolongement des
caps en interceptent de grandes quantit^s au cours de
leurs diplacements vers les frayfcres.
Le thon dit de retour au contraire, ayant d6j4 fraye,
ne s'engage plus dans les golfes et ne peut se pecher
qu'avec des madragues de pointe (de caps) lors de leur
migration trophique, des frayeres aux lieux d'hivernage.
Cette deuxieme migration s'effectue d'est en ouest et
la route empruntle, par suite de la propension du thon
& se dSplacer avec le courant, longe les c6tes sud de
1'Europe et particulterement la cote sud-ouest de la
Sicile.
Les madragues de course siciliennes sont tradition-
nellement des madragues de golfe et leur technique a
£volu£ selon les conditions hydrologiques des zones
exploit£es. En Atlantique et en Espagne ou les courants
sont plus forts on a diveloppi des techniques particu-
liires, ce qui fait que la madrague de type atlantique
diff&re sensiblement du type sicilien traditionnel.
Le developpement de la motorisation des bateaux de
peche ou de plaisance, la peche au feu, les residus des
usines, etc., d£rangent les thons lors de leur passage &
proximiti de la cdtes, et ont provoqu6 leur rarefaction
& Tinterieur ces golfes et a eu pour consequence la
suppression de la plupart des madragues siciliennes.
D'ou la n£cessit£ de r^examiner la thdorie de la madrague
sicilienne afin de choisir de nouveaux emplacements sur
le prolongement des caps ou se tient le thon & present
Depuis quelques ann&s d^j^ le Centre experimental
pour la Peche de la Region Sicilienne porte toute son
1 Le thon effectue au printemps une migration ginetique de ses
lieux d^hivernage (quels qu'ils soient) vers les frayeres; celles-ci
ne sont pas toutes connucs, mais on peut citer parmi elks, la basse
Tyrrheniennc, les deux Syttes, r ouest de la Sardaigne, etc.
attention sur ce point et propose maintenant un troisiime
type de madrague qui r^unit en thtorie les avantages des
madragues atlantiques et siciliennes, mais dont les rfsul-
tats pratiques sont encore & verifier pendant plusieurs
saisons de pcche.
Quel que soit le type de la madrague, le point essential
est que Tensemble doit maintcnir sa forme et sa place
durant la saison de p&he puisqu'il s'agit d'une installa-
tion fixe.
Dans la madrague sicilienne le has des filets est forte-
ment leste au moyen de grosses pierres de forme irr^gu-
liire qui s'enfoncent dans la vase du fond et en assurent
1'inamovibilite. La ralingue superieure formic d'un
cable en acier de 24 mm ^ est tenue en place par un
systime de cables aboutissant & des ancres de plusieurs
centaines de kilos chacune. Ces amarres sont couptees, £
des intervalles variables, sur la ralingue de surface & un
intervalle moyen de 20 brasses. Les cables d'amarre sont
longs d'au moins trois fois la profondeur et la chute des
filets est au moins J plus longue que la profondeur $e
qui laisse 33 pour cent de mou. Le flottement est assure
par des bouses mdtalliques ou en plastique ou par des
paquets de lifege ou de polystirol, places & meme le
cable de la ralingue superieure. II doit etre6quilibr6de telle
fagon que, sous Faction d'un courant d'environ 1 noeud/h
tout 1'ensemble de 1'installation commence & sombrer.
Les figures 3 et 4 montrent sch6matiquement, en-
Fig. 3. Madrague sicilienne.
Fig. 4. Madrague at/antique.
coupe verticale, la position que prend le filet des madra-
gues sicilienne et atlantique respectivement, sous Faction
de courants de diflKrentes forces.
Madrague sicilienne
Le bas du filet est fortement leste et inamovible sur le
fond. A faible courant, le mou du filet se rassemble vers
le bas, les cables d'amarre dterivent une courbe en S,
leur partie inflrieure gisant sur le fond pour environ
i de la longueur totale. Avec un courant allant jusqu'i
1 noeud/h environ, les cables d'amarre ne bougent pas
et la ralingue de surface est toujours sur le meme aplomb
que la ralingue inferieure qui est inamovible, mais le
mou du filet est progressivement engage. Si la force du
courant depasse 1 noeud/h, les cables d'amarre du
cdte de la provenance du courant se tendent tandis que
ceux du cdte oppose se reiachent ; la ralingue superieure se
deplace en surface et ensuite commence & s'immerger;
la courbe form6e par le mou du filet a tendance &
s'aplatir. Avec un courant de 1-8 noeuds/h, la ralingue
de surface est entrainde & environ 10-12 mitres sous 1'eau
et derive d'autant que le permet le raidissement du
cable d'amarre. Avec des courants encore plus forts,
les ancres des cables d amarre chassent ou bien les cables
cassent. II se peut encore que le filet se d6chire, ou enfin
que la ralingue de fond et son lest se cteplacent. Dans
chacun de pes derniers cas le d£gat est irreparable.
La limite de s£curit£ de la madrague sicilienne se
situe done & une force de courant d'environ 1*8 noeud/h.
En-de$d, de cette limite de securite, tout 1'ensemble
reprend progressivement sa position de depart, dis que
le courant diminue d'intensite.
Madrague atlantique
Dans la madrague atlantique, c'est la ralingue de surface
qui est inamovible et ne peut sombrer. Ceci s'obtient
en renforsant le nombre des cables d'amarre et le poids
des ancres et en multipliant le nombre et les dimensions
des flotteurs. Par contre, la chute du filet est sensiblement
egale & la profondeur (mou, environ trois pour cent) et
le lest compost de chaines est tris liger afin de pouvoir se
deplacer faiblement sur le fond sans s'accrocher et
s'enraciner. Sous Faction des courants, le filet execute
un mouvement pendulaire et reprend automatiquement
son aplomb quand le courant cesse. La securite de
1'emplacement de la madrague atlantique n'est done
jamais compromise & cause des courants, quelle que soit
leur force. Le pire qui puisse arriver est de ne pas p£cher
normalement pendant un certain nombre de jours et
de perdre par le fond, les poissons deji enfermis dans
la chambre puisqu'apres les forts courants Installation
se remet en place d'elle-meme. En outre, la madrague
atlantique 6tant construite avec un materiel de meilleure
qualiti que la madrague sicilienne, les filets peuvent se
faire plus elairs etoffrent ainsi une moindre resistance
aux courants.
Le filet de la madrague sicilienne, enracin£ au fond
ne peut se rfcupirer en fin de saison. II doit done 6tre fait
de matiire bon march6 (Ampelodesmus ou, au plus,
coco) qui n'a qu'une faible resistance & la rupture. C'est
pour cette raison qu'il est forml de mailles plus petites
et monte sur la ralingue 4 intervalles plus rapprochds
qu'il ne serait n&essaire pour empficher les poissons de
s'fehapper.
Dans la madrague atlantique, la Ugtretl du lest permet
la rfcup^ration des filets qui, en consequence, se font
en sisal ou en manille voire meme en nylon. Ce materiel
plus fort permet d'avoir des mailles plus grandes et plus
ouvertes avec une diminution correspondante de poids
et de resistance aux courants; de plus, la suppression
presque totale du mou amine une diminution de 30 pour
cent de la superflcie de filet que le courant doit traverser.
273
Plus importante encore est peut-£tre la construction
beaucoup plus simple du corps de la madraguc qui se
rdduit & une seule chambre de rassemblement du poisson
et & une chambre de mort. Cest de cette deuxi&me
caractdristique principale de la madrague atlantique
que dteoule la suppression du mou des filets; seuls les
filets ayant une chute Ug&rement supirieure & la profon-
deur permettent de rtaliser une bouche en souriciire qui,
en m£me temps qu'elle empcche les poissons d£j& pris
de s'&happer, laisse Fentrfe libre pour ceux venant de
(voir Fig. 5).
- - >- -- .:•.„
qu'une petite ouvcrture d'entrfe qui sc boucherait
complement avec le moindrc courant. Empccher le
poisson d6ji engagd dans la madrague dfen sortir tout
en laissant la bouche ouverte pour faciliter Tentr6e
des poissons encore & l'ext£rieur, telle est Texplication du
grand nombre de chambres de la madrague sicilicnne.
Les diffiirentes chambres sont mimics de portes form&s
de nappes de filet dont le bas est enracin£ au fond et dont
les deux cdt£s sont cousus aux parois de la madrague tout
en formant, de chaque cdt£, un gousset qui donne 1'ampli-
tude ndcessaire pour permettre a la nappe de filet con-
stituant la porte, de descendre jusqu'au fond. L'effet des
courants sur les portes provoque une deformation et une
Fig. 5. Madrague atlantique. Detail de la bouche.
Dans la madrague sicilienne, le mou empeche de
r^aliser une telle bouche en souriciire, puisque le filet
se disposerait comme les pans d'un rideau ne laissant
Fig. 6. Madrague sicilienne.
1 r.«.7 I
KADUAttUJC blCILXBNMC
274
dirive bcaucoup plus grande que celles subies par les
parois de la madrague et les filets de la queue puisqu'il
faut ajouter au mou normal de 33 pour cent, 1'amplitude
des deux goussets. (Voir Fig. 6.)
Dans les deux types de madragues la construction en
mer qui prend plusieurs jours, commence par Pinstallation
et Tamarrage du cable de surface. La madrague sicilienne
exige que les alignements, croisements, insertions des
portes et autres parties de 1'ensemble, soient tres exacts
et les cables de surface mesurts avec precision. La pose
des ancres et cables doit se faire selon une procedure
compliqu£e, deux ancres & la fois et au meme moment,
des deux c6t£s du cable de surface. Le cable de surface,
utilise pendant Installation des amarres n'est que
provisoire et est remplace par le vrai auquel est attachie
la ralingue superieure des filets, au moment oil ceux-ci
se mouillent.
Dans la madrague atlantique cette precision du dessin
n'est pas indispensable, les cables de amarre se font en
acier d'un bout £ Fautre, et le cable de surface definitif
est mis en place en meme temps. On peut indifKrem-
ment mouiller les amarres d'abord d'un cote et puis de
Tautre ou computer tout un cdte et terminer par 1'autre.
La figure 7 donne des details de 1'amarrague du corps et
de la partie superieure de la queue d'une madrague
sicilienne.
Le calage des filets
Une fois le lest pos6 sur le fond, son emplacement est
fixe pour toute la saison, et il est essential que la ralingue
du fond soit parfaitement & plomb avec la ralingue
superieure. Le calage des filets est done Toperation la
plus delicate de la construction de la madrague sicilienne.
Les deux moities des portes cousues au filet, chacune par
son gousset, sont reunies entre elles au moment du
calage et se posent sur le fond au meme instant, avec
leur lest. Les bateaux porteurs des deux moities de la
madrague sont hates sur un alignement rigoureux le
long du faux cable de surface, qui est remplace par le
vrai cable de surface au fur et a mesure que Fop6ration
progresse et que les filets sont mouilles.
Ces operations qui prennent de 6 & 8 heures exigent
des conditions hydrologiques et m£t£orologiques tres
favorables puisque le calage, une fois comment, ne
peut etre interrompu.
La madrague atlantique qui elle, n'a pas de portes,
se cale beaucoup plus facilement. On commence par
Tun des cotds de la sourici&re et on continue le calage
en contournant le perimfetre de la madrague (Fig. 8). A
X '
*v x '
*/
f . 0 SoteM inditntlf «•• Aiff«rat««
tout moment on peut interrompre reparation; il suffit
de dfcoudre le filet non encore cali, au point ou on est
parvenu. Pour reprendre le calage, il suffit de baler la
ralingue du fond du filet d£j& en mer et recoudre le
filet pour continuer. Le cable de surface ne pose aucun
problime car il est mis en place inddpcndamment des
filets. Pour les deux types de madrague la queue et les
barrages accessoires externes se calent successivement.
Cependant, si relier le filet de queue au corps d£j& en
mer ne prfsente aucune difficult^ pour la madrague
atlantique, ('operation pose un sdrieux problime pour
la madrague sicilienne dont la ralingue de fond ne peut
plus se haler une fois en mer.
Dans les deux types de madrague, le calage de la
chambre de la mort, complete les operations de con-
struction de la trappe. La chambre de la mort prolonge
la madrague vers 1'ouest et se situe sur le meme aligne-
ment que le corps de celle-ci dont elle est slparde par la
porte dite "de chanvre". Schdmatiquement, elle prend
en mer la forme d'un demi tronc de cone: la semi-
circonference majeure rattachee au corps de la madrague,
les deux cotes longs, months sur le prolongement des
cables de surface des filets, la semi-circonf&rence mineure
aboutissant sur le bord d'un bateau de longueur pro-
portionnee, place en travers del'axe de la madrague.
La semi-circonference majeure est cousue & la partie
inferieure de la porte de chanvre et, en meme temps &
une pitee de conjonction avec les filets du corps. Cctte
piece, dans la madrague atlantique est constitute simple-
ment par une paire de goussets, tandis que dans la
madrague sicilienne elle comporte plusieurs elements
(sous-fonds, pieces de 1'arriere, etc.) tris £tudi£s et
astucieusement congus, et qui rendent infiniment plus
efficaces les operations de halage.
La composition de la chambre de mort peut varier &
rinfini; le principe essentiel est qu'elle doit etre assez
resistante pour supporter le halage qui se fait parfois
plusieurs fois par jours, et sa partie terminale (vers la
semi-circonftrence mineure) doit pouvoir contenir
plusieurs tonnes de gros poissons tir6s presque & sec sans
etre trop lourde afin que le halage ne pr&ente pas de
diificult£s insurmontables.
La figure 9 donne le detail de la composition d'une
chambre de la mort de madrague sicilienne qui pourrait
tout aussi bien s'employer dans une madrague atlantique.
Les operations de pfche
Dans la madrague atlantique le rassemblement du
poisson ne demande aucune intervention de la part de
1 homme. Les thons ou autres poissons recontrant le
barrage de la queue, la longent. S'ils font route vers le
large, ils recontrent la bouche en souriciire, s'engagent
dans la madrague et ne peuvent en ressortir; s'ils font
route vers la cote, ils rencontrent aussitdt un crochet qui
les fait rebrousser chemin et les achemine vers la bouche;
s'ils s'61oignent de la queue en direction du large, ils
recontrent les barrages extdrieurs de la Ugitima et
contralegitima, avec leurs crochets qui, eux aussi, les
acheminent vers la bouche ou de nouveau vers la queue.
275
•Fascina"
Scale: 3 mm - 1 fathom
mm 3 m Brasse 1
"Scappolaro" M. 7. Pieces * y BR * 3 y Brasses
•Caduta" M.2 Pieces x-j-BR » 1 \ Brasses
"Spisselli" M.4 Pieces x 1 % BR - 7 y Brasses
'Gradotti11 M.4 Pieces x 1 y BR - 7 y Brasses
"Utimo" M.5 Pieces x 7
37 2 Brasses
/
Piece de 1'arriere
= 2 1 Brasses
. 9. Composition de la chambre de la mart de madrague sicilienne.
II est tres improbable que des poissons tomb£s dans la
zone de peche d'une madrague puissent eviter de finir
tdt ou tard dans la chambre.
Dans la madrague sicilienne les choses se passent de
la meme fa$on jusqu'au moment ou le poisson entre par
la bouche; celle-ci n'6tant pas en souriciere, permet
la sortie aussi bien que I'entrde et il faut prendre des
mesures pour empecher que les poissons ne s'£vadent.
Plusieurs bateaux guetteurs sont places sur le travers de la
madrague au-dessus des portes; les guetteurs montent
la garde soit & la vue, soit avec des lignes de touche en
nylon se terminant par un lest en plomb; les groupes
de thons en passant frolent ces lignes qui transmettent
une vibration avertissant le guetteur. Une longue
experience permet d'interprfter ces faibles indices et de
se rendre compte du norabre et de la taille des poissons
nageant en profondeur. Aussitot que les poissons se
sont engages dans une chambre, on relive la porte et on
repite reparation pour la chambre suivante jusqu'i
regrouper tous les poissons dans la dernieire chambre
qui prfe&de celle de la mort. A ce point-ci la situation se
pr&ente de la m£me fapon dans les madragues des deux
types c'est-&-dire, que les poissons sont regroup£s dans
une enceinte dont its ne peuvent sortir et qui est s£parde
de la chambre de la mort par une seule porte, la porte de
chanvre. A partir de ce moment, le systime sicilien est
incomparablement plus avantageux. Dans la madrague
sicilienne les poissons qui se trouvent assez & 1'ftroit dans
276
la chambre du pichou se ruent vers la chambre de la
mort, dont ils ne peuvent apercevoir I'extr6mit6 oppos£e
et qui leur apparait alors comme mer libre. Dans la
madrague atlantique ou 1'unique chambre de rassemble-
ment est immense, ils n'ont pas la meme impression et le
passage & la chambre de la mort se fait avec beaucoup plus
de difficultds, le plus souvent par petits groupes s£par£s
ce qui fait qu'un groupe entre, un autre sort et quelquefois
des heures passent avant de pouvoir relever la porte de
chanvre et commencer le halage de la chambre de la
mort qui se termine par la capture ou mattanza.
II est & remarquer que les capitaines espagnols ne
connaissent pas Fusage des lignes de touche et ne se
servant que de leurs yeux pour reconnaitre la presence
des thons dans la chambre de la mort. Or si le poisson
nage en profondeur, hors de la portfe de la vue, il se
peut qu'il entre dans la chambre de la mort et en ressorte
sans que personne ne s'en aper$6ive; le filet qui forme le
fond de la chambre s'61feve graduellement en transversale
de Test vers 1'ouest; quand le poisson recontre ce fond,
il en est facilement effrayl et rebrousse chemin. Le
maitre de peche sicilien au contraire, place son bateau
avec sept autres guetteurs & moiti£ de la chambre de la
mort et un deuxiime bateau en parall&le, un peu plus
vers 1'ouest; le guet se fait soit visuellement, soit avec les
lignes de touche. Le poisson qui entre est d£tect£ quand
il n'est qu'& mi-chemin vers 1'ouest et la porte de chanvre
est lev£e avant qu'il ne rebrousse chemin, la porte de
Eel Traps Made of Plastic
Abstract
Much of the eel catch in European countries, from rivers and estu-
aries, is made with traps of willow, rattan or other wooden lattice
work lasting in most cases no more than one year.
The Institute has, since 1961, carried out experiments with traps
made of plastic material. The trials showed that polyethylene
became too flexible during the warm season so that eels escaped
by squeezing between the laths; its low density of 0-94 on the other
hand caused no problems. Polyamide was more stable but much
too expensive for commercial application. The best material proved
to be polyvinyl chloride, but as PVC alone was found to be rather
brittle at low temperatures a special mixture, found to be available
in the chemical industry, was used.
The first plastic traps used in 1961 were of the same design as the
traditional types and on average caught 20 per cent less. The experi-
ments did, however, show that the material itself did not affect the
catch and that the decrease in catch was due to faulty construction.
In 1962 further experiments were carried out with more tightly
woven plastic traps, which resulted in catches which were on a par
with those of the traditional types. The production cost of these
plastic traps is three times higher than for willow traps, but as
their useful life is many times greater, they should be more
economical.
by
H.Mohr
Institut f ur Nctz- und Materialforschung,
Hamburg.
A solution to the time-consuming task of making traps could be
ound by casting them from suitable moulds such as were produced
fn Germany.
Casiers a anguilles fabriques en matiere ptastique
Resumt
Une grande partie des anguilles capturees dans les pays Europeans,
principalement dans les rivieres et les estuaires, est obtenue i
1'aide de casiers en saule, rotin ou lattis, bien que toutes ces fabrica-
tions durent rarement plus d'un an.
continued from page 276
chanvre pouvant etre relevte en 3, 4 minutes. Dans
ce but la connection entre les filets du corps et la chambre
de la mort est plus compliqute que celle de la madrague
atlantique. Pour haler la porte de chanvre il faut lever
toute la porte avec ses connections sur le corps et £
Tavant, c'est-&-dire la semi-circonfe'rence majeure de
la chambre de la mort. Les pieces de connection de la
madrague sicilienne sont ainsi faites qu'aucune rdsistance
ne vient du filet du corps car la pi&ce de 1'arrifere et les
sous-fonds qui, normalement gisent au fond, se ddploient
en Sventail et donnent tout le mou ndcessaire pour lever.
Dans la madrague atlantique il faut vaincre la resistance
des filets du corps qui sont en partie soulev6s avec leur
lest, ce qui pr&ente le double inconvenient d'offrir aux
poissons qui gventuellement sont restds dans la chambre,
une voie de liberte" vers le bas et de rendre le lever de
la porte beaucoup plus difficile. Une fois la porte de
chanvre levee, les operations se poursuivent de mani&re &
peu pr&s semblable dans les deux types d'engins: au fur
et 4 mesure que le bateau d'oii Ton fait la Iev6e, avance
vers I'extr6mit6 ouest, la chambre de la mort se r£trecit
et se soul&ve jusqu'£ se r£duire & un carr£ d'une quinzaine
de metres de c6t6 et de deux ou trois metres de profon-
deur oil la tuerie a lieu.
Type de madrague modifi6e
La pr&ente communication avait pour but, sans entrer
dans tous les details, de faire ressortir la grande superior-
ite technique de la madrague atlantique par rapport £
la sicilienne quant & sa conception g£n£rale, et de montrer
quels sont les details ou le systeme sicilien est plus
avantageux. Nous pouvons maintenant presenter le
type modifid qui est propose" par le Centre Experimental
pour la Peche de la Region Sicilienne et dont le plan
schimatique est represents. (Fig. 10.) Ce nouveau type
adopte la bouche en souriciire, le systime de Icstage, de
Fie. 10 7Un <U Mftracu* atlaatittM
flottaison, d'amarrage, la longueur et la qualit6 des
filets de la madrague atlantique mais interpose une
chambre supptementaire entre celle de la mort et le
reste du corps; ceci a pour but de rassembler le poisson
dans un espace plus restreint pour faciliter son passage
dans la chambre de la mort. La seconde modification
structural porte sur r adoption du systfcme sicilien pour la
connexion entre corps et chambre de la mort. Le principe
des lignes de touche et des bateaux de guet doit 6tre
retenu puisque les conditions de peche en M 6diterran6e
sont differentes de celles de 1' Atlantique et requiirent
d'autres techniques telles que celles employees par les
pechcurs siciliens qui, & cause de la plus petite quantiti
de poisson qu'intercepte une madrague de golfe, sont
obligds de recourir & toutes les ruses et i perfectionner
une "finesse" du mftier qui se transmet d'une gdniration
i 1'autre.
277
A I'lnstitut, depuis 1961, des experiences ont ct£ faites avec des
casters fabriques en matiere plastique, Ces experiences ont prouvd
que le polyethylene devenait trop flexible pendant la saison chaude
pennettant ainsi aux anguilles de s'evader en se glissant entre les
lattes. Par coatre, sa density de 0-94 ne posait aucun probleme.
Le polyamide plus stable, s'averait trop coftteux pour une applica-
tion commerciak. Cest en definitive le polyvinyl chloride qui a
paru etre la roeilieure matiere bien que celle-ci soil encore trop
fragile a basse temperature. Un melange special a 6t6 trouv6 et
experiment^ dans 1'industrie et s*est revete de meilleure qualite.
Les premiers casters en plastique, fabriques en 1961, dtaicnt du
meme modele que ceux de type traditionnel mats capturaient
environ 20 pour cent de moins que ces dcrniers. Les experiences
cependant ont montrt que la matiere, elle-meme n'avait aucun effet
sur la capture et que cette diminution etait due a une fabrication
defectueuse.
De nouvelles experiences conduites en 1962, avec des casiers en
matiere plastique, plus serres que les precedents, ont permis d'
obtenir des captures 6gales a celtes enectufes avec des casiers
traditionnels. Ces casiers sont a peu pres trois fois plus chers que
les paniers en saule mais, etant donnc qu'ils durcnt beaucoup de
fois plus longtemps, ils sont finalement plus economiques. 11 est
propose de reduire le temps de fabrication de ces casiers, actuelle-
ment assez long, par 1'emploi de moules.
Trampas de material plastko para la pesca de la anguila
Extracto
La mayor parte de las anguilas pescados en los rios y estuarios
Europeos, se capturan con trampas hechas de ramas de sauce,
bejucos y otras maderas, la mayor parte de las cuales dura menos de
un afto.
£1 Institute realiza desde 1961 experimentos con trampas
hechas de material plastico. En los ensayos se observd que el
polictileno adquiere demasiada flexibilidad durante el tiempo
calido y que las anguilas escapaban pasando por el entramado;
por otro lado, su baja densidad de 0-94 no planteaba problemas.
El poliamido era mas estable pero mucho mas caro y por ello menos
conveniente para su empleo en la practica. El mejor material result6
ser el cloruro de polivimlo, pero como por si solo resultaba bastante
quebradizo a temperaturas bajas se empleb una mezcla especial
que la suministra normal mento la industria quimica.
Las primeras trampas de material plastico empleadas en 1961
eran de la misma forma que las tradicionales y por termino medio
pescaban un 20 por ciento menos. Sin embargo, los experimentos
demostraron que el material en si mismo no influia en la captura y
que su disminucibn se debia a una construccibm defectuosa.
En 1962 se realizaron otros experimentos con trampas de material
plastico de entramado mas apretado con las que se lograron capturas
iguales a las obtenidas con las trampas adicionales. El costo de
producci6n de estas trampas de material plastico es tres veces
mayor que el de las trampas de sauce, pero duran muchos veces
mas, por lo que dcbcrian de resultar mas econ6micas.
La larga tarea de fabricar las trampas puede reducirse al minimo
hacicndolas en los moldes convenientes, como se practica ya en
Alemania.
IN many European countries eel traps of basketwork
(willow, rattan or wood) are still used in rivers and
estuaries. After one year they are often in need of
repair and after two years they have to be replaced.
Furthermore, they must be dried from time to time,
otherwise they smell foul and repulse the eels.
These drawbacks could be avoided by using non-
rotting synthetics. Since 1961, the Institut fur Netz-
und Matcrialforschung, Hamburg, has carried out
investigations with traps made of polyamide, poly-
ethylene and polyvinyl chloride (PVC). These tests were
made in areas affected by tides. In the first year the
trials were not successful as some materials proved
unsuitable, or the traps were not properly built.
Polyethylene (density 0*94) became too flexible during
the warm season. A large number of the eels already
278
Figs. 1-4. Method of building a plastic eel trap.
caught escaped through the bars. Also pure polyethy-
lene tends to float.
Polyamide-sticks (density H4) were more stable;
but too expensive.
Polyvinyl chloride (PVC) (density 1'38) proved most
suitable. In low temperature pure PVC is admittedly
short and brittle. But a special mixture, resistant to
cold and breaking, is now available.
In 1961 the plastic traps were still copies of the willow
Fig. 5. Building of the long funnel.
traps and therefore some small eels could escape.
Checks made in the estuary of the river Ems in Northern
Germany showed that the catch of these first traps was
20 per cent less than that of the traditional ones. But
it was evident that plastics were, in principle, suitable.
For the season of 1962 the PVC traps were plaited more
closely, so that the intervals between the rings in the
continued at foot of page 279
Types of Philippine Fish Corrals (Traps)
Abstract
One of the most important fishing gears operated in sheltered waters
of the Philippines, by both sustenance and commercial fishermen,
is the fish corral. It is set across the migration routes of fishes,
which affords them easy entrance to one or more compounds where
they are impounded. The walls of the fish corral are made of
bamboo splits which are laced together and held in position by
means of stakes or posts. Because the materials used in the con-
struction are abundantly found in the Philippines, the use of this
gear is widely spread. They also vary in forms and sizes, depending
upon the conditions obtained in the fishing ground and the capital
of the operator. This paper describes, with illustrations, the different
types of fish corrals in the Philippines.
Types de barrages dans les Philippines
Resume
Un des plus importants cngins de peche utilises dans les eaux
abritees des Philippines, mfcme a I'echelle commerciale, est le
barrage. II est construit sur la route de migration des poissons de
telle facon que ceux-ci puissent y entrer facilement, mais ne puissent
en ressortir. Le barrage est construit de bambous fendus, nattes
et tenus en position verticale a Taide de poteaux. Les materiaux
necessaires £ la construction de ces barrages abondent aux Philip-
pines c'est pourquoi ce type d'engin est utilise sur une grande echelle
dans cStte region. Les barrages sont de formes et de dimensions difffc-
rentes suivant le lieu de pSche et les moyens financiers de 1'operateur.
l/6tude decrit avec illustrations, les differents types de barrages de
poissons utilises aux Philippines.
Closes de corrales de pesca de las FiUpinas
Extracto
El corral es uno de los mas importantes artes de pesca empleados
by
Arsenio N. Roldan Jr.
and
Santos B. Rasalan
Philippine Fisheries Commission
Santos B. Randan
en aguas protegidas de las Filipinas por Pescadores de subsistencia
e industrials. Colocado a traves de las rutas de emigraci6n, ttene
entradas ftciles a uno o mas corrales de los que no pueden saiir
los peces. Las paredes del corral son de bambu hendido, trenzado
y mantenido en position por medio de postes. Como el bambu
abunda mucho en las Filipinas el empleo de estos corrales esta muy
extendido. Las formas y dimensiones dependen de las condiciones
reinantes en los caladeros y del capital del pcscador. Describe con
ilustraciones esta poncncia las diferentes clases de corrales de
pesca cmple ulos en las Filipinas.
THERE is hardly any sheltered water in the Philippines
where fish corrals are not used for either subsistence
or commercial fishing. The fact that the materials for
continued from page 278
basketwork did not exceed five cm (as against 8-10 cm
in the traditional traps). Trials were made in the mouths
of the Ems and Weser. Checks proved that in the Ems
catches were as high as those in willow traps. In the
Weser they were even five per cent better.
No colour effect was noticed, but whereas in the estua-
Fig. 6. Building of the short funnel.
ries the water is always muddy and turbid, in clear water
the visibility may be of some importance.
These results showed that plastic eel traps are at least
as efficient as traditional ones. They need less care in
handling and time, and do not rot. Their cost is about
three times that of willow traps. But they will probably
have a useful life of much more than five years and,
therefore, will be more economical.
Last year plastic traps made of mould-cast lattice
work were produced by a German plastics manufacturer
{Fig. 7). In some respects, this would be a further
Fig. 7. Eel-trap of cast plastic.
improvement because every year fewer people are able
or willing to make traps by hand. But there is one
difficulty: the size of the eels and the size and kind of
the bait change with place and season. Because of this
the fishermen generally use various types of traps, which
differ from 2-8 mm in the space between the sticks.
With hand-made traps, it is easy to meet these require-
ments but manufacture may be uneconomical as the
construction of moulds for casting plastics is very
expensive.
References
Kdhler, E., Schiemenz, K., Bugow, K., W. Rettig. Aalfang und
Aalwirtschaft. Sammlung fischereilteher Zeitfragen, Heft 23. 1934.
Schulz, O., Die Kunststoffe. MUncnen. 1959.
v. Brandt, A., Fischercigerftte der Bhinenfischerei. Arbeiten des
Deutschen Fischereiverbandes, Heft 6. 1956.
279
their construction may be found almost everywhere at
any time of the year at low cost, explains the widespread
use of this fishing gear.
Fish corrals catch fish by providing a barrier across
known or suspected paths of migration of fish and guid-
ing the fish to a one-way enclosure or series of enclosures ;
hence, the term "guiding barriers".
These fish corrals take various forms, from the small
movable weir installed in shallow water to the highly,
elaborate affairs operated in deeper waters. The varia-
tions in size and form are the result of continuous
adaptation by Filipino fish corral operators to varying
fishing conditions and the amount of capital available.
The various types are classified as aguila or pahubas,
inangcla, paugmad, bwgsod, zamboanga, and hasang.
AguDa or pahnbas
This type of fish corral is constructed on tidal flats
partly or entirely exposed at low tide. It usually consists
of one or more enclosures with a small collecting crib,
a gate, two wings and generally with a leader. The bamboo
screens are finer and woven closer together than those
used in deeper waters. It is set during the early part
of the season and transferred from one place to another
for better catches. The gear is usually removed once a
month, especially when set in sheltered waters. It catches
littoral fishes including crabs, shrimps, squids and mullets.
Based on the shape of the enclosures, aguila fish
corrals may be divided into baklad ordinario and tulfs.
Bakltd Ordinario
These are shallow water fish corrals (Figs. 1 to 12) with
two or more heart-shaped enclosures, one or two or no
wings, with or without a leader. The leader (when present)
and wings may be straight or curved at their distal ends.
In Busuanga, Palawan Province, the wings are sometimes
made of piles of stones. The collecting crib may be
terminal in position (located at the end of the heart-
shaped enclosure or series of enclosures) or lateral (on
left or right arm of the heart-shaped enclosure). The
positions of the lateral type of collecting cribs are
extremely varied but all are laid out in such a way as to
prevent escape of the fish once it enters the first enclosure.
Tulfc
Figs. 13 to 18 show the various forms of tulis fish
corrals. The term "tulis", which means pointed, is
derived from the arrowhead appearance of the fish corral.
At least three forms of tulis are recognised; namely
pangalato, baenakan, and tulis proper. Fig. 13 represents
the pangalato in Ragay Gulf, the simplest form of tulis.
Fig. 15 is a baenakan commonly used near the mouth
of rivers in Capiz Province to catch mullets and bangos.
The bamboo platform on the sides of the second compart-
ment is where the fish lands when it attempts to jump
over the enclosure. Figs. 16 to 18 are the intricate varia-
tions of the tulis type of fish corrals.
The catch of tulis fish corrals comprises shrimps,
crabs and other littoral species that come in with the
water at high tide.
280
o
Figs. 1 to 9. Various types of Aguila or Pahubas. (a) leader, (b) wing,
(c) gate or entrance, (d) first enclosure, (e) second enclosure, ( f) third
enclosure, (g) fourth enclosure.
Figs. JO to 18. Various types of Aguila or Pahubas. (a) leader, (b) wing,
(c) gate or entrance, (d) first enclosure, (e) second enclosure, (/) third
enclosure, (g) four thenclosure, (H) collecting crib, (sp) special platform.
Inangclt
This type of fish corral (Figs. 19 to 24) derives its name
from the word "angkla", Spanish for anchor, as the
general form is that of an anchor. The main features are a
Figs. 19 to 24. Various forms of Inangcla. Figs. 25 to 27. Various
forms ofPaugmad. (a) leader, (b) wings, (c) gate or entrance, (d) first
enclosure, (e) second enclosure, (/) third enclosure (g) collecting crib
(sp) special platform, (h) hut.
single semi-circular compartment and a leader which
corresponds to the shank of the anchor. The variations
in inangcla fish corrals are in the shape, size and position
of the collecting crib. They are operated both in shallow
and deep waters. The corral shown in Fig. 19 is operated
in Ragay Gulf by fishermen from Cavite Province white
that illustrated in Fig. 20 is widely used in the Philippines,
in waters ranging from 2 to 8 fm deep. Fig. 21 shows
an inangcla type with a series of small heart-shaped
compounds used in Cebu Province in 1 to 5 fm of water.
The trap illustrated in Fig. 22 is constructed with small
heart-shaped compartments on the tips of both arms,
topped by a semi-circular collecting crib, while that in
Fig. 23 features a special platform and a wing extension
at the tip of one arm, and a diamond-shaped collecting
crib on the other.
This type (Figs. 25 to 27 and 45) represents the simplest
fish corral in the Philippines. The gear is usually con-
structed in 7 to 12 fm of water. The special features of
this type are a landing platform and a small hut on one
or both sides of the enclosure where hauling of the fish
is done. Pelagic as well as demersal fish are caught.
Figs. 28 to 35. Various types ofBungsodZamboanga. Fig. 36. Hasang.
(a) leader, (b) wing, (c) gate, (d) first enclosure, (e) second enclosure*
(g) collect ing crib, (sp) special platform.
fig*. 37 to 44. Various types of Hasang. Fig. 45. Paugmad. (a) leader r
) wing, (c) gate or entrance, (d) first enclosure, (e) second enclosure,
c) forechamber, (/) third enclosure, (g) collecting crib, (sp) special
platform.
281
(b) wi
(/c)
A New Fish Trap Used in Philippine Waters
Abstract
Of the many types of fish traps used for catching fish in sheltered
waters of the Philippines, the new hasang moderno is considered
to be the most effective. Like the other fish traps, this gear consists
of a series of enclosures with a leader, four wings, and three entran-
ces or gates. It differs, however, from its prototype (hasang moderno)
in that the terminal pond is located at the same axis with the leader
on the outer side of the semi-circular enclosure which it super-
imposes. The multiple entrances, together with the leader and the
series of enclosures, effectively prevent the escape of the fish which
are intercepted. The method of construction and operation of the
new hasang moderno is described in this paper and is fully illus-
trated. Complete specifications of this gear are also discussed.
Une nouYdle trappe utilise* dans les eaux des Philippines
Ream*
Le nouveau hasang moderno est maintcnant consid&g comme cngin
tres efficace parmi tous les autres types de trappes a poissons utilise
dans les eaux abritees des Philippines. Tout comme les autres trappes
& poissons, 1'engin consiste en une serie de chambres, une queue,
quatre ailes et trois entrees. II differe cependant de son prototype
(hasang moderno) par le fait que le corps extcrieur est sur le mdme
axe que la queue, a 1'cxtericur de la chambre semi-circulaire sur
laquclle il est superposed Les portes multiples avec la queue et la
se>ie des chambres, emp&hent les poissons de s'6chapper une fois
intercepted. La communication decrit la methode de construction
et d'operation du nouveau hasang moderno et foumit aussi des
details tres precis de 1'engin.
Nueva trampa de pesca empteada en aguas de las Filipinas
Extracto
De las muchas trampas de pesca empleadas en aguas protegidas de
las Filipinas, el nuevo hasang moderno se considera como la mas
eficaz. Como las dcmas trampas, esta consiste en una serie de
corrales como una rabera, cuatro pernadas y tres entradas, pero
difiere del prototipo (hasang moderno) en que el corral terminal
esta superimpuesto. Las diversas entradas, junto con la rabera y
la serie de camaras, impiden que escapen los peces que se inter-
ceptan En esta comunicaci6n se dan detalles completes e ilustra-
ciones de los materiales, construccidn y funcionamiento del nuevo
hasang modeino.
PRIOR to the development of the new hasang moderno,
the hasang moderno (Fig. 1) had been considered
the most effective. This consists of three gates, m, se, and
by
Santos B. Rasalan
Philippine Fisheries Commission
s'e'\ four wings, w; a leader, /; a series of four enclosures,
a heart-shaped terminal pound, tp9 constructed at the
end of one of the arms of the semi-circular enclosure, si,
which is superimposed by a triangular compartment,
tc, and a triangular forechamber, fc. It has, however,
been improved through simpler design and construction,
giving rise to the new hasang moderno. This new type
was first operated in Kalibo, Capiz, from 1 April 1950
to 15 October 1950. The experiment was so successful
that many other parts of the Philippines adopted its use.
General description
Fig. 2 shows its plan. Like its prototype, it consists of
a leader, AB; two lower wings, CE and XZ; two upper
wings, JH and TV; three gates, EX, GH, G'T; a fore-
chamber, DFGG'WY; a semi-circular enclosure,
IKSRK'UT; and a terminal pound, LMNOPQ. The
terminal pound, however, is located at the same axis
with the leader at the outer side of the semi-circular
enclosure which it directly superimposes.
The leader of a typical new hasang moderno is 240 m
long and the lower and upper wings are 44 m and 42 • 50 m
long, respectively. The forechamber is 44-60 m at
its greatest width, FW, and 31-55 m long, measured
from the lower gate to the entrance of the semi-circular
enclosure. The latter is also 44-50 m wide, KK', and
26-90 m long, measured from midpoints of GG' and
SR, while the terminal pound is 8 -90 m, MP, and 8 -20 m
continued from page 281
Bungsod Zamboanga
Figs. 28 to 34 show the various forms of fish corrals
called bungsod, operated in Samboanga waters 5 to 7 fm
deep. The common characteristics are the triangular or
quadrangular enclosures, a semi-circular or heart-shaped
collecting crib, wings, leader and special platform, all
irregularly arranged. The catch comprises both demersal
and pelagic fish especially tuna, sardines and mackerels.
Hasang
This type of fish corral (Figs. 37 to 44) is installed in
waters 3 to 1 5 fm deep. Its characteristic feature is a fore-
chamber apart from the regular one or several enclosures,
a collecting crib, two or more wings, one or more gates,
a leader and one or more landing platforms. The fore-
chamber is either rectangular, triangular or diamond in
shape and is called hasang in the Visayas, meaning "gill",
282
from which the name of this type of fish corral was
derived.
Fig. 36, the bolonan of Capiz, is the simplest form of
hasang. Fig. 37, the hasang antiguo, has a rectangular
forechamber, one gate, a leader, two wings, a semi-
circular enclosure, a triangular compound and a heart-
shaped collecting crib.
Fig. 41 represents a bolonan ordinario, an improvement
of the bolonan, showing two collecting cribs. Fig. 42
illustrates the hasang moderno^ an improved hasang an-
tiguo with three gates and three wings that increase the
intercepting capacity of the gear. Fig. 43 represents the
hasang moderno as developed in Kalibo, Capiz. This
type has been found to be the most effective fish corral in
the Philippines, It differs from its prototype in that it has
four wings, a terminal type of collecting crib on a straight
line with the leader.
Fig. 1. Hasang moderno (J932). (g) entrance, (e) leader, (m) gate,
(w) wing, (el) elbow,(fc)forechamber, (se) side gate, (si) semi-circular
compartment, (tp.) terminal pound.
long measured from the mouth of the semi-circular
enclosure SR to Side NO.
The fences of the fish trap consist of frames and bam-
boo screens. The frames (Fig. 3) consist of well-seasoned
bamboo posts and braces. The bases of the posts are
pointed and triangular holes are bored just below the
nodes for the water to fill in the internodes, thus making
it easy to plant them at the muddy bottom.
The bamboo screens (Figs. 4 and 5) consist of bamboo
splits laced together at regular intervals with either
threaded coconut inflorescence bud scales, rattan
(Calamus spp.) or hagnaya (Polypodium spp.). Table I
gives their specifications. As the depths of the bamboo
screens are shallower than the depth of the water, two
or more are joined perpendicularly end to end to fence
the entire column of water. The screens for the leader,
wings and forechamber have slats coarser than those for
the semi-circular enclosure and terminal pound.
The screens are grouped according to their placings.
Those that go together are joined lengthwise, and then
rolled like mats to facilitate handling.
Before setting the ground will have been carefully
mapped out and all materials prepared.
Construction and setting
The gear is invariably set in sheltered waters with sandy
or muddy bottoms free from coral reefs and from 5 to
Fig. 2. The new hasang moderno: AB leader, CEand XZ lower wings,
JH and TV upper wings, EX, CH and GT gate, DGGG'WY fore-
chamber, IKSRK'UT semi-circular enclosure, LMNOPQ terminal
pound.
12 fm deep. Usually, waters shallower than their sur-
roundings are preferred.
The outline-plan of the fish trap (Fig. 2) is carefully
plotted by imaginary lines and temporary stakes. The
leader is laid perpendicularly to the direction of
the current or the shoreline; its length depends upon
the distance of the corral to the shore. Generally, the end
Fig. 3. The uprights or frames, (a) uprights, (b) braces, (c) a bundle
of two bamboos.
283
Fig. 4. Portion of the terminal pound and semi-circular enclosure
snowing lacing of the slats, (a) uprights, (b) brace, (c) bamboo screen,
(d) lacing.
Fig. 5. Portion of the bamboo matting used at the wings, for cchamber
and leader, showing how the slats are laced, (a) bamboo post, (b)
brace, (c) bamboo screen, (d) lacing.
of the leader reaches very near the shoreline. In this
setting the master fisherman directs several good divers
and other helpers.
The posts are planted at low tide— guide posts first,
next anterior and posterior ends of the leader and wings,
then the ends and corners of the different chambers.
The planting of the other posts follows by setting them
at 2 m apart at the semi-circular enclosure and terminal
pound; 3 m intervals at the forechamber; and 5 m
between them at the wings and leader. Care is taken to
make them stand perpendicularly to the water. To
strengthen the hold at the bottom and prevent the posts
from reclining due to wind and wave, bundles of bamboos
(Fig. 3c) or long coconut trunks are used for every
fourth post. Two to four lines of bamboos are braced
to connect each pole just above the bamboo screens.
The whole set of the bamboo poles constitute the
framework of the fish corral.
The setting of the bamboo screens follows the com-
pletion of the frame. A stone weight of about 15 kg
is tied at the lower end of every rolled screen to keep
it standing erect. The screen is unrolled little by little
284
and tied with the rattan splits, hagnaya, or threaded
coconut inflorescence bud scales to the framework.
The setting of the screens is usually begun with the
terminal pound followed by the semi-circular enclosure,
then the upper wings, forechamber, lower wings, and
lastly, the leader. This order is followed because fish
often begin to enter the enclosures as soon as they are set.
Unless destroyed by gales or typhoons, the fish trap
may last from six to ten months. Its upkeep, however,
requires periodical changing of the screens whenever
they become weak or when they need cleaning and dry-
ing. Hence, there must be a reserve of at least one-half
of the total number of screens. When the season is
over, all screens are removed, leaving the framework for
the next season. The screens are scrubbed, dried and
repaired for next season.
The nets
Two nets are used to catch the fish — the scoop seine
(sigin) for the semi-circular enclosure, and the shrimp
net at the terminal pound.
A diagram of the scoop seine appears in Fig. 6.
Tables II and HI give its specifications. It consists of a
bunt (Fig. 6A, B) flanked by two wings (Fig. 6C, D)
Fig. 6. Structural plan of the sigin. A, B, C and D wing; E, F, G
and H selvages; i breastline; j secondary leadline; m bridle; n ring;
o secondary float line; p primary floatline; g pursing rope; r eye splice.
hung on a headrope 27 • 10 m; a footrope, 22 • 50 m and a
breastline 12-20 m. Each wing is made of 9-thread
cotton twine netting of 2-5 cm mesh-stretched, 573
meshes wide and 800 meshes deep. The bunt is composed
of two nettings, the upper portion of which is 312 meshes
wide and 500 meshes deep, 2*5 cm mesh-stretched and
made of 12- thread cotton twine; while the lower portion
is a 9-thread cotton twine netting, 2-5 cm mesh-stretched
312 meshes wide and 300 meshes deep. These are joined
mesh to mesh.
Fig. 8 is a diagram of the shrimp net. It is in the form
of an isosceles trapezoid and made of several pieces of
sinamay cloth sewn together. This is hung to a manila
rope, 1 • 3 cm in diameter, with 40 per cent slack, by a
9-thread cotton twine. The finished shrimp net measures
7*70 m and 3*40 m on its parallel sides, while each of
the nonparallel sides is 8*40 m.
Fig. 7. Portion of the headline, (a) eye splice \ (b) float, (c) primary
floatline, (d) secondary floatline, (e) lacing line, (/) breast line.
Like the scoop seine, each end of the hanging line is an
eye splice (Fig. 8b) where the pull rope is tied during the
operation. On the midpoints of each of the nonparallel
sides and at the ends of the longer parallel side GI rings
(Fig. 8c) are tied and threaded into the gliding rope
when the net is shot and hauled in at the terminal pound.
The following materials are employed in the construc-
tion of the shrimp net:
Sinamay cloth: 87-40 mxl m.
Manila rope: 30 mx 1-5 cm diam.
GI rings: 4, 0-8x7-5 cm diam.
Cotton twine: 50 m, 9-thread medium laid.
The boats
Two boats are used— the bigger (Fig. 9) for transporting
the fishermen and nets to and fro, and the smaller one to
carry the catch ashore.
The bigger boat is an ordinary dugout, 14 m long,
1 m wide and 0-50 m deep, with one wooden plank
each side. An outrigger on the port side enables it to
be brought close to the fence of the corral.
The small boat (Fig. 10) is flat-bottomed.
Fig. 8. Structural plan of the shrimp net (not drawn to scale
(a) sinamay cloth, (b) sling, (c) GI rings, (d) hanging line, (e) lacing
line.
•fop View
Fig. 10. Lateral and side views of the small boat, (a) bamboo car*
locks, (b) planking.
Fig. 9. Top view of the main boat. A. hollow keel, B. bow, D wooden planking. F bamboo for car locks.
285
The crew Mi thdr fetfes
A complement of 16 fishermen operates the fish corral:
one master, two assistants, four divers and nine ordinary
fishermen. The master fisherman directs all the work,
including construction, maintenance and fishing opera-
tions, and the marketing. The divers construct and repair
the gear, and the ordinary fishermen are general utility
men*
The splitting of the bamboos, cleaning, lacing or
weaving of the bamboo screens are done by contract
labour. These expenses are deducted from gross sales
before a profit is declared.
The crew is paid on a profit-sharing basis. All the
expenses are deducted from gross sales and the remainder
is divided into two equal parts. One part goes to the
owner of the corral while the other part is divided into
19 shares distributed as follows:
1 master fisherman, 2 shares, 2 assistant masters,
1£ shares each, 4 divers 1£ shares each, 9 ordinary
fishermen 1 share each.
Fbhfag operation
The catching of the fish is done during early dawn to be
sold that morning.
A seven-man crew — one assistant master fisherman
and six ordinary fishermen, operates the shrimp net
in the terminal pound. Four stone weights of 2 kg each
are tied to the four corners of the net. Then gliding ropes
are tied at points e and /of sides AC and BD (Fig. 1 1)
11 &• t«rain*l pound, showing
how th« •hrimp n*t it laid out
At B. C, D, the ntt
respectively, and perpendicularly down to the base of
the frame. To this, each ring (Fig. 8c) is threaded by the
corresponding gliding rope, thus allowing the net to
glide up and down when setting and hauling. They also
286
serve as supports of the net when set, and keep it from
drifting.
In setting the net the eye splices (Fig. 8b) are each tied
down with a pull rope while the rings are threaded by
the respective gliding ropes. The corners are tied with
a stone weight of 2 kg each. Corners C and D (Fig. 1 1 ) are
brought near points e and /and dropped to the bottom.
The other portion of the net (Fig. 11, AB fe) is also
dropped. This is done to prevent fish inside the pound
being covered by the net. Then, the pull ropes at C and
D are pulled to spread the net so that the bottom of the
terminal pound is covered entirely. After a while, it is
raised to scoop the catch.
Fig. 12 shows the setting and hauling of the net at the
Fig. 12. Setting and hauling of the net at the semi-circular enclosure.
semi-circular enclosure. Nine fishermen, including the
master fisherman and one assistant master fisherman,
are needed. The net is dropped at the bamboo bridge
(Fig. 12W) at X, and then extended to Y, thus transform-
ing it into a curtain closing the entrance and gates of the
fish corral. The end, at Y, is dragged close to the fence
following the direction as indicated by arrow A of Fig. 12
until X2 is reached. When this has been carried out
the four divers see that the leadline touches the sea-bed
and the fence of the enclosure. Three men on the bam-
boo bridge hold the floatline while the leadline is brought
close to the fence below the net platform and the bottom
of the net is raised by pulling the pursed line, thus form-
ing a sort of hammock. Here the fishes are concentrated
at the bunt where they are scooped and placed on the
waiting banca. This is repeated several times.
The fish caught comprise most of the pelagic species
which are fished on a commercial basis. Shrimps are
also caught during the dark phase of the moon, particu-
larly when attracted by a lamp. Catches are sold to
fresh fish venders, or to wholesale buyers for salting
Dropline Fishing in Deep Water
Abstract
The development of fishing in the Pacific Islands is very restricted
owing to the particular fishing conditions. Many of the islands
are coral formations rising almost sheer out of depths of 2,000
fathoms so that most of the fishing gears which operate on the
world's continental shelves cannot be used. Tuna longlining can
only be done from the few islands which can offer adequate harbour
facilities. The paper describes the mechanization of handlining
in up to 500 fathoms. The A -inch steel wire lines carry 8 to 10
branchlines with hooks, and are wound on wooden drums. These
drums are slipped over a driving shaft which is connected by a
friction clutch to a small gasoline motor. The hauling speed is
adjusted to about 27 fathoms/min at slow speed, but can be
increased to more than double this speed with the engine at full
throttle. Up to 12 such drums of lines are set out, anchored by a
short length of chain. Five different types of hooks were tried
and it was found that the Polynesian hook with its incurving point
was superior for this type of handlining as they could catch and
hold a 3-lb fish as well as 500-lb sharks. The entire catch was
made on the lower three hooks spaced 4J ft apart while the bait on
the upper hooks was never touched, showing that at these depths
the fish stay close to the bottom. The experiment has proved that
at these great depths there are valuable food fish as Ruvettus,
Gymno thorax and Polymixidae as well as large sharks so that there
may be a future in this type of deep fishing, operational from small
canoes.
Peche a la ligne dans les eaux profondes
Resume
Le ctevcloppement de la peche dans les lies du Pacifique est tres
restraint par suite des conditions particulieres de la peche. Les
lies ctant en grande partie constitutes par des coraux 6mergeant
de profondeurs de 3750 m, la plupart des engins de peche utilises
sur les plateaux continentaux du monde ne peuvent etre employes
ici. La peche du thon a longue ligne ne peut etre pratiquee que
dans quelques Sles offrant les facilites de port necessaires. La
communication decrit la m£canisation de la pSche a la ligne
jusqu'£ des profondeurs de 900 m. Les lignes en fil d'acier de
1,5 mm diametre comportent de 8 a 10 branchons avecdes hamecons
et sont montdes sur des tambours. Les tambours sont glisses
sur un arbre de traction qui est coupte a un moteur a p&role par
un embrayage a friction. La vitesse de halage est regime a
50 m/min pour un regime lent mais peut 6tre doublec quand le
moteur tourne a plein. Jusqu'a 12 de ces tambours sont mis a
J'eau, ancres par de courtes longueurs de chaine. Cinq types
difffcrents d'hamegons ont M essayed et on a trouv£ que 1'hamecon
polyn&ien avec sa pointe rccourb£e 6tait sup£rieur pour cette
m&thode de pSche a la ligge puisqu'il pouvait capturer et retenir
des poissons de 1 ,5 kg aussi bien que des requins de plus de 200 kg.
Toute la capture a etc produite par les trois hamegons les plus
profonds, ceux-ci 6tant espaces de 1,30 m tandis que les amorces
des hamecx>ns supfrieurs n'&aient jamais touchers. Ceci prouve
qu'a ces profondeurs il existe beaucoup de poissons comestibles
de valeur tels que les Ruvettus, Cymnothorax et Polymixidae de
m&me que des requins et permet de penser que dans 1'avenir, ce
type de p£che i la ligne pourra £tre applique a la pgche commerciale
pratiquee avec de petits canots.
Pesca con palangres en aguas profundas
Extracto
El desarrollo de la pesca en las islas del Pacffico esta muy restringido
e causa de condiciones especiales. Muchas de las islas son for-
macipnes coralinas que se elevan casi verticalmentc desde pro-
fundidades de 2.000 brazas, por lo que no se pueden emplear la
mayor parte de los artcs de pesca usados en las plataformas
continentales del mundo. La pesca de atun con palangres
solameritc se puede realizar desde pocas islas que ofrecen buenos
puertos. Describe la ponencia la mecanizacibn del halado en
profundidades hasta de 500 brazas. La linea madre, de acero
de 1/16 pulg de diametro con 8 6 10 brazoladas, se arrolla en
carreteles de madera que se ponen en el arbol de una transmisi6n
conectada mediante un embrague de friccibn a un motor de
gasolina pequefio. La velocidad de halado se ajusta a 27 brazas/min
con el motor a poca marcha, pero es factible de duplicarse con el
motor a toda marcha. Se calan hasta 12 palangres que se fondean
by
Ronald Powell
Fisheries Officer, Rartonga
con cadenas cortas. Se ban ensayado 5 clases distintas de
anzuelos y se ha observado que el de la Polinesia con la jpunta
curvada hacia adentro es el mejor porque sujeta peces de 3 fb, asi
como tiburones de 500 Ib. Toda la capture se logra en los 5
anzuelos inferiores, distanciados 465 pies, en Unto que el cebo
de los superiores esta siempre intacto, lo que demuestra que a esas
profundidades los peces estan my cerca del fondo. La experiencia
ha demostrado que en aguas muy profundas se encuentran peace
comestibles como Ruvettus, Gymnothorax y Polymixidae, asi
como grandes tiburones, por lo que puede ofrecer buenas pos*-
bilidades esta clase de pesca practicada desde canoas pequeflas.
TPHE Pacific Ocean covers half the world's surface. It
A has an average depth of about 2 miles and less is
known about the biology of this vast inner space than
is known already about the moon's surface. Until com-
paratively recent years oceanographers stated that very
little life was possible below the level where visible
light could penetrate. The islands which dot this vast
stretch of Pacific Ocean inside the tropics are generally
small and the hundreds of coral atolls of the tropical
Pacific rise almost sheer from depths of over 2,000
fathoms. Few areas of shallow water are found where
the conventional fishing gear of the world's continental
shelves can be used.
While it has long been thought desirable to improve
the fishing industries of these island groups the methods
of improving subsistence fishing and developing limited
commercial fishing pose very special problems not
encountered in areas with a continental shelf.
Since the last war the expansion of the Japanese
pelagic fishing fleet has left no shadow of doubt that
almost unlimited quantities of large fish can be caught
by tuna longlines anywhere close to the tropics. This
type of operation, however, requires specialized vessels
and heavy commercial fishing material beyond the
financial means of most island communities, quite
apart from the lack of docking, ice making, transport
and communication facilities. Any methods of improving
the catch rates in shallow lagoons threaten the extinction
of local stocks in comparatively short time.
Development must then be directed towards deep-
water using a suitable method of fishing which would
be within the reach of the financial resources of the local
fishermen; operational from the presently used type of
boat as well as such types which could in ths immediate
future be operated from the Pacific islands; operational
with the type and supply of bait available in the islands.
While trolling does occasionally produce good catches,
it could not be developed into a regular fishery due to
the short season during whi h fish are available. Drift-
netting has been tried but damage to the nets has proved
it to be uneconomical. Longlining has a future in the
287
island fisheries but as it requires a minimum of 20 to 30
baskets for commercial operation it is doubtful whether
it would be economic with the present size of boats.
This leaves only handlining which is a well known
fishing method in the islands of the Pacific.
Traditional handlining
The Polynesians learned to catch fish in the deep waters
with handlines ages ago and their wooden Ruvettus
hooks which are quite part of their culture gradually
developed from the peculiar fishing conditions, and the
local means available to meet them. Operating handlines
down to 500 fathoms depth on or near to the bottom, as
practised around the Pacific islands, raises many
different problems. The boat generally rises and falls
over a long ocean swell which means that in fine weather
the line is tightening and slackening as each wave passes,
so that it is most difficult to feel a fish bite even if this
could be done over such a long line. With cotton lines
it is nearly impossible to jerk or strike the hook when a
fish bites— not only because of its elasticity but also
because of the curve the line normally takes in rising to
the boat. This can be partly overcome by attaching a
sinker to the line but this then causes the problem of
lifting such a sinker which has to be done many times
in a night's fishing. To overcome this the Polynesian
fishermen have developed several ways of attaching
stones of about 14 Ib which can be released when the
hook touches the bottom or when a fish strikes. Under
these conditions the Polynesian fish hook with its
incurving point was developed, but only produced good
results when used with the technique for which it was
designed.
The hook depends on rolling through part of a circle
when pressure is applied to the point and operates so
that when the fish seizes the bait the hook is swallowed
to the back end of the mouth while the fish moves off
with a trailing line. If the line is struck at this time, the
hook will be pulled out of the mouth easily. If, however,
the fish is allowed to move off with the leader trails
behind, the fish will at a certain moment pull against
the tightened line, pulling the hook down into the
corner of its mouth, or the top edge of the jaw is forced
between the hook's shank and its point. If the line is
kept steadily taut at this position the fish will change
direction so that it swims at right angles to the line. In
this position all the pressure bears on the hook's point
and it rolls through a part circle coming out very often
through the top of the head or the side of the jaw. This
is, in fact, the only disadvantage of this type of hook in
that very often it is necessary to cut the hook out as there
is no other means of disengaging it. Once a fish is taken
on these hooks it is rare that they can disengage them-
selves. On the other hand, they would be inefficient
where speed of unhooking is an important factor in the
operation.
The Polynesians operate two to three lines simulta-
neously from their canoes and while fishing they paddle
to keep the canoe as much as possible over the lines
288
which is a difficult and back-breaking job.
Each cotton handline is tapered towards the hooks by
using a decreasing number of threads in making up the
line to ensure that if the hooks snag in the coral bottom
the line will break well down. The lines carry two to
five hooks and this is a constant danger as a large fish on
the bottom hook may drag out the already hauled snoods
causing serious accidents when the outrunning hooks
catch any part of the fisherman's body or even his
clothing.
New method
It was clear that any development of this fishery had to
be adjusted to the boats available and within the financial
means of the fishermen. This already restricted the
scope considerably but it was decided to try mechanizing
the operations by developing a simple power gurdy
which would permit the operation of several lines at
the same time.
As handlining from the canoes themselves would
restrict the number of lines that could be operated to
two or three, and to avoid the danger of entangling lines
when hauling a large thrashing fish, it was decided to
anchor the lines out individually. This could be done by
having them coiled on a floating drum which could itself
be slipped on the power gurdy shaft for hauling.
Power gurdy
A gurdy was designed that could be constructed from
locally available material and driven by a small gasoline
engine. The drive was made through a worm wheel
and pinion.
Tests had shown that for hauling large fish a hauling
speed of 8-9 fathoms per minute would be the most
appropriate, and the drive was calculated to obtain this
speed with the engine running at 450 rpm. With the
engine running at 1500 revolutions at full throttle the
hauling speed was nearly 30 fathoms per minute. The
gurdy shaft included a spring loaded friction clutch
which could be adjusted so that it would slip at an
overload, to avoid breaking the lines on struggling fish.
This system (illustrated in Fig 1) worked so well that
several sharks of over 500 Ib were hauled on wire snoods
having a breaking strength of only 185 Ib. This was
possible by setting the clutch so that it reversed when the
fish brought on too much tension. The load spring was
set in a square hardwood block which pressed the
block against the shaft shoulder providing the required
friction.
Drums
The drums were constructed as shown in Fig 1. They
were of marine plywood with a soft wooden core, 16 in
in diameter and capable of holding 600 fathoms of
iV in wire. The drum core had a 2} in square hole in
the centre which could be slipped over the square block
of the gurdy. During the first few trials a revolution
counter was mounted on the gurdy to assess the length
of line out as each turn on the drum was on average 3 ft
Method of leading wire over
the Bow to Curdy drum
lf Hole
Marine Plywood
soft wood oore
glued together
4 oopper rivets.
Dimensions to take
- fathom per turn.
2 7 Square Hardwood Block
r
Bearing
Shaft of Winch
Steel Shoulder faced of
Spring for block to slip against.
long, but this was later disregarded as it was found
easy to judge the length of line left on the drum.
The gear
The lines were iV in steel wire having a breaking strength
of about 200 Ib; however, it was found during the
experiments that a line of about 350 Ib breaking strength
would be easier and safer to work the larger fish. The
lines were marked at several pkces with pieces of white
cotton to give warning, especially at night, of the
approach of the snoods to the fairlead.
Eight to ten snoods of 14 in length were used with
spreaders and swivels attached, as illustrated in Fig 2.
Swivels were furthermore inserted at every 100 fathoms
in the main line to avoid spinning of the baits as the line
is lowered to the fishing depth.
As the lines were to be operated individually, and to
avoid them drifting from the bottom slope into deep
water, they were anchored and this was at first done by
IS Ib stones. These stones, however, were found to slip
down the steep bottom slope, so they were later replaced
by about 2 fathoms of f in chain, which gave excellent
anchoring results.
Hooks of five different sizes and varieties were tried,
including three sizes of incurving tuna hooks of a
special design and of these, sizes 1, 3 and 5 gave the best
results. Japanese tuna longline hooks size 2/0 were also
used. Fish as small as 3 to 4 Ib were caught on the same
hooks which hauled 500 Ib sharks, especially with the
Polynesian type of hooks.
Results obtained
The gear was completed in March 1961 and the first
trials were made off Black Rock, to the north-west of
Rarotonga, during daylight. The area is on the leeside
away from the prevailing south-east trade wind, and
depths of 1,000 fathoms are reached at less than 1 mile
offshore. Twelve drums complete with lines, snoods and
hooks were carried, each drum having two yellow
plastic floats to assist in buoyancy, and also to act as a
damper to the wave-surge. It was found that the yellow
plastic floats were well visible during daytime. At night
the floats carried a small battery-operated light so that
no problems of finding the gear were experienced
except during a few heavy rain squalls when the visibility
dropped to nil.
The lines were set out in a V-pattern covering a depth
of 100 to 500 fathoms, and as the anchoring stones
resulted in some of the lines drifting, 2 fathom lengths
of chain were substituted and no further trouble was
experienced.
In spite of the forebodings of the local fishermen no
trouble was experienced in operating the drum-wound
steel wires, picking up the gear, mounting it on the
gurdy, and hauling; even the big 500 Ib sharks raised no
problems whatsoever.
289
In setting, the wire was allowed to run out from the
drum until the chain reached bottom, the wire remaining
on the drum was then stoppered and the drum simply
cast over-board with its floats attached. To avoid the
danger of hooks running out when a large fish is on one
of the bottom hooks, a fork was installed at the stem
consisting of a bronze rod lined with hardwood through
which a narrow sawcut had been made; as each snood
was hauled aboard it was dropped into the sawcut so
tha if the line should run out, it would stop when the
first hook reached the fork. (Fig 1).
On hauling large fish, such as sharks, as soon as they
come near to the surface, the friction on the clutch was
reduced to well below the breaking strength of the snood
and the line carefully hauled until the fish came alongside.
Once the fish was killed it was taken aboard, or if too
large for this, towed astern by a rope attached to the tail.
While good results were obtained with the hooks
described above, it was clear that the use of more
Ruvettus hooks would probably have given even better
resultes However, the construction of such hooks
requires suitable natural-grown crooks of hardwood
which are difficult to find. The peculiar conditions of
such deep water handlining require a different function
from that of hooks used in shallow water. When a
large predatory fish is hooked in the soft part of the jaw,
they break away easily. The Ruvettus hook is normally
swallowed to well back in the mouth where it strikes a
region normally including some part of the bone
structure; fish are therefore seldom lost with these
hooks. It would be interesting to see whether metal
hooks constructed on the same principle would be an
improvement for bottom-set longlines, especially in
fisheries where relatively long snoods are used and
where the same conditions of slack lines and slow
hooking exist.
The experiment brought to light some interesting
biological matters. The catch of Polymixidae shows that
there must be other fish feeding on bottom as its two
long barbels show that it is fully adapted to bottom
feeding. Unfortunately, none of the fish yielded any
stomach contents other than the bait used. Large
sharks were caught which are seldom seen near the
surface off Rarotonga. For many years the fishermen of
Rarotonga had wondered what kind of fish fed on the
bait thrown out during fishing for yellow-fin tuna with a
fleet of sometimes 50 canoes; the experiment indicates
that a sizeable fish population on the bottom takes
care of it.
Several species of valuable food fishes were brought
up which had never been caught before in these islands
and are, in fact, rare in other parts of the world. The
presence of Etelis marshii and Etelis carbunculus, which
are two of the most beautiful fish in the oceans, was a
pleasant discovery, as these are apparently only caught
on rare occasions. On one occasion a silver eel (Anguill-
dae) was caught and it was rather surprising to find that
it swims and feeds as deep down as 500 fathoms.
Gymnothorax spp was a common part of most catches.
These fish are normally associated with particular types
Box to hold
ton Sets of
ton Hook gear
GalY Pipes
Screwed into
bast of box above.
* Metal Spacer
between sets
In Box above.
Polynesian
Ruvettus Hook.
Spreader made from
Galv wire.
Hook siies
290
of coral near which they live and feed, and it would be
interesting to know whether such an environment is
present at such great depths or whether they change their
pattern of living.
Ruvettus has, of course, been caught on these islands
for centuries and was a common catch. It appears to
rise for feeding to about 100 fathoms on dark nights but
certainly seems to feed lower than this at other times.
This would seem indicated by its peculiar luminous
eyes which are obviously adapted to live at an extremely
low light level. Many of the fish caught were preceded
to the surface by bursts of air or gas from the swim-
bladders and usually offered little resistance. Sharks,
however, and Ruvettus still fought hard even when the
snood had reached the boat.
Practical fishing with this gear has shown that there is
more fish available between 350 fathoms and 500
fathoms depth than in the shallow regions. A further
remarkable fact is that nearly all catches were made on
the bottom three hooks while the baits on the top five
hooks were never touched; this clearly indicates that
the fish at this depth are mainly bottom feeding species.
Further trials with slightly adapted gear will be
carried out in deeper water.
Discussion on
Gillnetting
Longlining, etc.
J.Frechet
Mr. Jean Frechet (Canada) Rapporteur: Drift and gill-
netting activities are vastly important the world over. Masa-
take Neo describes how 12,000 Japanese fishermen worked
for two months in a mothership operation in the North-
West Pacific harvesting salmon. Under the Russo-Japanese
convention, driftnets totalling nearly 10,000 km can be set
and 369 driftnetters were so employed in 1962. After nine
hours* fishing at night the catches were delivered daily to the
mothership. One-third of the nets was made of monofilament
polyethylene while the remainder was of multifilament nylon.
The hanging ratio of the nets does not appear to be regulated
and I would appreciate knowing why the Japanese fishermen
hung their nets with a ratio of 45 per cent when it is under-
stood that much more efficiency would be derived from a
hanging ratio slightly under half the measure of the stretched
length. This paper is highly informative on the technique of
transferring catches.
The haulers in the driftnct salmon operation are described
by Miyazaki. Three main types of net haulers have been
developed and use of them has. more than doubled the net
hauling capacity of the fishermen. Many net haulers have been
developed all over the world to reduce hand labour, but that
hand labour still remains the most widely used source of
fishing energy. However, great progress is being made in
mechanisation. Kuraptscv of Russia particularly described
that country's progress. Many hundreds of vessels have been
equipped with a complete set of net or fish handling machinery.
Their range of mechanisation includes a net driving roller on
the bulwark which can be tilted inboard when necessary, a
net hauler with separate vertical gurdies working on the North
American Crossley principle of spring activated teeth gripping
the lines over half the circumference of the head, a warp
shock absorber protecting against excessive strains in bad
weather, a machine to shake the fish out of the nets, a fish and
salt mixing conveyor and vibrating barrel platform capable of
packing three to four tons an hour, a 2,000-fathom warp
stowing machine coupled to a warp storing drum, some over-
board net and warp leaders to shoot the nets well out from
the vessel's side and to prevent fouling the propeller, and,
finally, a retention guide to ensure a regular flow of webbing.
There is also a remote reading sea water thermometer to
make sure that the nets are set in an adequate habitat. Such
mechanisation gives easier and speedier fishing.
An interesting Canadian experiment with Japanese mono-
filament gillnets in inshore waters showed that when cod-
fishing at 20 to 50 fathoms in both clear and dirty waters,
results are always better than with multifilament. Their catch
was from three to ten fold greater. Comparative results from
other fisheries would be interesting.
In relation to longline Hirano details the measures developed
by the Japanese for learning the configuration of the lines at
certain depths related to the presence of fish. Echo sounders
using the high frequency of 200 kc have been found most
effective and gave better results than pressure gauges.
Technical developments which permitted the use of poly-
ethylene or 'Kuralon' longlines of 750 kg tensile strength are
described by Tominaga in his paper on mothership longline
fishing in the Bering Sea. In this connection Canada had
developed a braided longline and gangings to meet success-
fully the needs of fishermen for an improved longline.
In the Philippines fish corrals or traps were much used and
Roldan and Rasalan outlined a great number of types under
seven distinct headings while Rasalan gave a more elaborate
description of the most effective of these fishing corrals— the
hasang moderno. This trap consists of a series of enclosures
with a leader, four wings and three gates.
In describing the Mediterranean and Atlantic tuna traps
Fodera tells how the Sicilians had introduced a new chamber
between the death chamber and the body of the trap so as to
combine the advantages of both types.
Although tanglenets and trawlers are used in catching crabs
in Alaskan waters, the most successful modern method
described by Allen is to use big pots, if round with a diameter
of six feet, or square with seven-foot sides and two and a half
feet high. These last weigh 250 pounds each, were covered
with soft stainless steel wire in squares and have tunnels on
opposite sides. A good catch was 50 crabs per pot but they
sometimes got 100. On these craft, 40 to 150 ft long, hydraulic
winches are necessary. New requirements listed by Allen
constituted a challenge to technologists and engineers; a
new design of buoy, line-coiling machines, mechanical un-
loading devices and means of scouting for crabs.
Dr. Miyazaki (Japan): On the hanging ratio question said:
For merely getting the fish into the meshes about 30 per cent
hanging-in is quite adequate, but if you want to entangle
them, the hanging-in should be between 40 and 50 per cent or
even more than that; therefore, if you want both gilling and
291
at the same time a 40 per cent ratio would be
appropriate. This is based on my experience in using drift-
Mr. R. D. Leakey (U.K.): Many new inventions are
needed to facilitate longlining, gillnetting and trapping.
As regards longlining, very little research is being done to
develop a scoop for bringing the fish from the water to the
boat rapidly without having to gaff them when you are getting
large catches. There is also the question of bait protection.
There has been developed in this country a very fine polythene
net in the form of a sausage which protects the bait from
predators and vermin, and once bitten by fish they cannot
spit it out as the fine meshes entangle in their teeth. Quickly
detachable snoods could save time wasted on removing the
fish from the hooks in heavy fishing and also allow stowing of
the line in such a way that the hooks could be easily rebaited
on board. Folded traps for lobsters and crabs had been
available in Britain for six years but were not much used
because of price. They could, however, certainly give Canadian
and others useful points in relation to them once they had
been scaled-up to their requirements.
Mr. Davidson Thomas (India): Gillnetting is very important
to us in India. Our experiments with bottom-set nets and drift-
nets, showed that gilling must be partly combined with entang-
ling. This is because of the varying size and velocity of the
tropical fish. In bottom-set gillnets we use a hanging ratio of
about 50 per cent which gives scope for entangling as well as
gilling. We get the best results if three or four meshes are
allowed to ride loose on the framing lines between points of
attachment. In sardine fishery we hang the webbing mesh by
mesh hung in a little less than 50 per cent for gilling rather than
tangling. For materials they had tried polyester (Terylene'),
polyamide under various trade names, and 'Kuralon'. They
found that Terylene* had less resistance to abrasion, particu-
larly where sharks abounded as they damaged the nets much
more than was the case with polyamide. They had also tried
monofilament nets from Germany but they found that crabs
cut the monofilament so that it was not economical.
Mr. T. Paramanantharajah (Ceylon): After expressing
gratitude to FAO and its fishery officials for the assistance
given in developing their fisheries techniques by designing
new boats to reach distant grounds and introducing suitable
gear, asked that FAO should provide a glossary of technical
terms which would enable them to keep abreast of develop-
ments and commercial usage. Their development had reached
a stage where they were looking forward to the possible use
of small sterntrawlers and longlining techniques to work more
distant grounds, and he asked that businessmen abroad
should remember that there was the opportunity in Ceylon
for much development.
Mr. Lowell WakefleM (U.S.A.): Our experience is that
trawling is actually the most efficient way of catching king
crabs, but having regard to overall efficiency, the quality of the
output and the need for conservation, other methods of
catching are used such as the entangling net and the crab pot.
Japan used the tangling nets and the Americans used the pots.
On the question of selectivity and the possibility of releasing
female crabs and immature crabs he thought the pots were
superior. He mentioned that the formerly used standard
crabpot had been displaced in the last three years by the newly
designed large pot, invented by a man who came in from the
salmon gillnet fishery and knew nothing about catching crabs
292
at that time. This showed how new ideas came in from new
sources.
Mr. H. Kristjonsson (FAO): Gillnetting has proved most
effective in tropical areas and in Ceylon 20 to 30 thousand
gillnets were now being used since their introduction by FAO.
It was, however, also a quantity fishery in developed countries.
For instance, Iceland was an example where it had large-
scale application.
Fishing had become too complex and too highly capitalised
to rely on flair to the same extent as in the past for the selec-
tion of the gear, methods and materials. By techno-economic
studies we must evaluate more exactly the costs and earning
factors, and select the most economic tools. Taking the Ice-
landic cod fishery with gillnets as an example, each boat
fishes about 100 nets per day. These nets last sometimes only
a few weeks and each boat consumes 300 to 400 nets in a
four-month season. For each net the netting alone costs
about £7 to £8 sterling. In its lifetime of, say, a month and a
half, each net will catch on the average H to 2 tons of fish
worth £40 to £50 sterling. Thus the netting alone (the framing
lines, floats and stones are used again) costs 20 per cent of the
value of the catch taken in that net as long as it lasts. This is an
appreciable cost factor, and much stands to be gained if you
can shave a little off it, or if you can increase the value of
catch by selecting a more efficient net.
The first cost of the netting varies with the diameter of the
twine, somewhat like curve A in the Figure below. The dura-
bility is haphazard in Iceland owing to bad weather and heavy
fishing, but in other countries where the climate is more
500 R tex
500 Rfex
200 300 400
Twine size
500 R tex
clement durability would vary with twine size, something like
curve B.
Catchability of fine nets is normally much better than that
of the heavy nets, so the catchability might vary like curve C.
Now our problem is to determine line D, i.e., the optimum
twine size. It will vary from country to country, from fishery
to fishery, but can be calculated wherever the fishery is on a
big enough scale to give statistically valid numbers. This
brief statement serves only to provoke thought on this matter
both among the economists and the gear technologists who
will have to collaborate, since the economic considerations
are inevitably limited by technical factors as is so often the
case in engineering.
Dr. Miyazaki (Japan): One of the features of the 1-type net
hauler described in the paper on drift net haulers is that the
head section is interchangeable with another for tuna long-
line work. The SU-type, by fitting another head section on
its base, has a special groove arrangement different from that
of the YM-type. In the SU-type the two wheels of the drum are
each fitted on a different shaft, the inner wheel at a right angle,
while the outer wheel is a little off the right angle with the
horizontal axis of the drum. But the way the drum takes hold
of the net and releases it down is essentially the same as with
the YM-type.
Mr. C. E. P. Watson (U.K.): In Kenya they had not found
monofilaments to give any great superiority over any other
materials and the majority of nets continued to be made of
ordinary white nylon. Following earlier reports regarding the
great superiority of certain Japanese coloured nets, they had
tested as many different colours as they could and had not
found any advantage in Japanese salmon pink. Tests made in
Lake Victoria in both clear and dirty water, showed that colour
made no difference whatsoever to the catching power of
different nets. Much the same experience was obtained with
monofilaments. The most important factors to determine the
relative economy of gillnets were the strength of the material
and its resistance to deterioration. One big disadvantage of
monofilament net was its tendency towards permanent dis-
tortion of the mesh bars after fish were caught. This increased
even more the tendency of the monofilament nets to be very
bulky in stowage with the result that canoes could carry
fewer nets than otherwise would be the case. In trap fishing
they had replaced the traditional form of stationary traps
called pata and ozea traps made of fence sections of woven
coconut fibre, with an identical form of trap using poly-
ethylene net in place of the coconut fibre. This, tested along-
side the traditional trap, had showed a great increase in
catching power with the result that many had been sold and
were showing good commercial results.
293
Part 2 Bulk Fish Catching
Section 9: Purse Seining
Recent Developments in Icelandic Herring Purse Seining
Abstract
Since 1958 the annual yield of the Icelandic herring fishery has
increased rapidly after 14 very lean years. Apart from natural
causes, such as fluctuation in stock strength, this increased catch
is partially attributable to tye very drastic changes in Icelandic
herring fishing techniques, particularly to organized fish searching
with asdic and a new technique in asdic-guided purse seining for
deep-swimming schools, mechanized handling of the nets by power
blocks, and the use of deeper and stronger seines of nylon netting
hung to Terylene ropes. The author describes the technological
developments in the Icelandic purse seine fishery over the last two
decades, from the two-dory system with carrier boat, through an
intermediary stage when the net was operated from a single dory
towed by the carrier vessel, to the present system whereby the
net is operated from the main boat, usually stacked at the stern
after the wheelhouse on vessels up to 85 ft, or on the boat deck
on larger vessels. The asdic fish searching is discussed and a
detailed description given of the tactics of encircling submerged
schools by careful asdic plotting, taking full account of the inter-
action of wind, current and swimming direction of the school.
Normally the boat starts shooting into the wind. The purse line
gallows are placed far up forward. Although a power skiff is not
normally needed, most boats carry one for use in adverse conditions
to either tow the boat out of the net or the net away from the boat.
Catches of 100 or even 200 metric tons do not present serious
difficulties. Using nets 700 to 240 fathoms long floatline and 60 to
70 fathoms deep, stretched, at the centre but tapered sharply at the
bunt end, several top boats (90 to 110 ft) have an annual average
catch of 6,000 to 8,000 tons each. This method has made possible
year-round herring purse seining in Iceland, which previously was
limited to three summer months off the North Coast for surface
schooling herring. This has contributed to bolstering the Icelandic
herring catch to a record 478,000 tons in 1962.
Recent Devdoppement des operations
a la WMf couUstante en islande
Depuis 1958, la production annuelle de la p&che au hareng, en
Islande, s'est accrue rapidement aprfes 14 ann&s de maigre produc-
tion. En dehors des causes naturelles tcllcs que les fluctuations du
stock, cet accroissement est attribu£ particllemcnt au changement
radical de la technique islandaise de p&che au hareng et plus
particulifcrement au nouveau systfeme de localisation du poisson
au moyen de Tasdic, a une nouvelle technique de detection des
banes profonds, toujours au moyen de 1'asdic, a la manipulation
m&amque des filets par des power-blocks et a 1' utilisation de filets
plus profonds et plus forts, assembles sur des ralingues de tirylfcne.
L'auteur d£crit les d£veloppements technologiques dans la p6che
islandaise & la senne trainante, pendant les deux dernfcres decades,
du syst&me & deux canots avec bateau-porteur, en passant par un
stade hitermtdiaire oii le filet £tait op£r£ par un seul canot tir£ par
le bateau-porteur jusqu'au present systime dans lequel le filet est
op&6 par le senneur lui-rn&me, le filet itant rang* a I'arriere du
bateau deni&re la timoneric sur des bateaux de 26 m pu sur le
pont a canots sur les grands bateaux. La communication traite
de la localisation du poisson a 1'ascid et donne une description
d£tail!6e des tactiques utilises pour encercler les banes de harengs
submerges, ft partir des indications fournies par Tasdic, en tenant
compte du vent, du courant et du emplacement du bane. Normale-
ment le bateau commence la mise a 1'eau du filet, dans le vent. Les
potences pour la ligne coulissante sont places a 1'avant. Bien que
le canot motorist ne soit pas toujours utilist, la majority des
bateaux en emporte un pour touer le bateau hors du filet on pour
lloigner le filet du bateau selon la direction du vent. Des captures
de 100 jusqu'a 200 1 ne pr&entent pas de s6rieuses difficult^. Des
bateaux de 27,5 m a 33,5 m, utilisant des filets de 370 & 440 m de
ligne de ftotte et de 1 10 & 130 m de profondeur, 6tir£s au centre mais
effiKs au sac, ont obtenu une capture annuelle d'environ 6.000 a
8.000 t. Grace ft cctte m&hode, il est maintenant possible de
ptcher le hareng, en Islande, toute I'annfe alors qu'auparavant
cette p6chc ttait Iimit6e ft trois mois pendant l'6t£, sur la Cote
294
by
Jakob Jakobsson
Reykjavik
Nord et a des banes de surface. Ceci a contribu^ a porter la
capture de 1962 a un poids record by 478.000 t.
arenqoe con redes de cerco en Islaniia
Extracto
A partir de 1958 y tras 14 aflos de escasez, la captura anual de
arenque en Islandia ha aumcntado rapidamente. Excluidas las
causas naturales, como fluctuaci6n de las ppblacioncs, el incre-
mento se puede atribuir en parte a cambios radicales de las t&nicas
de la captura, particularmcntc la busqueda organizada con asdic v
un nuevD sistema de pcsca al cerco di cardumenes prof undo*
guiado por asdic, empleo de motones mecanicos en la maniobra
de la red y uso de paftos de nylon mas altos y robustos con relingas
de terilene. £1 autor describe los adclantos tdcnicos de la pesca
al cerco en Islandia en las dos d6cadas pasadas, desde los dias
de los dos "dories" con una embarcaci6n de transporte, a traves
de la fase intermedia en que la red la calaba un solo "dory*"
remolcado por el transportador, al sistema actual en el que el arte
lo manipula la embarcacibn principal y normalmentc lo estiba a
popa, detr&s de la caseta del timdn, en barcos hasta de 85 pies
o en la cubierta de botes en los de mayor eslora. Examine el autor
la Iocalizaci6n de peces con et asdic y da detalles completes de la
manera de cercar cardumenes sumergidos sigutendolos con el
asdic y teniendo en cuenta el efccto del viento, de la corriente y la
dirccci6n en que se mueven los peces. Normalmente el calamento
comienza con el barco proa al viento. Los pescantes de la jareta
estan muy a proa. Aunque por lo general no se necesita un bote
do motor auxiliar, casi todos los barcos lo llevan para sacar a
remolque el barco fuera de la red o separar 6sta de aqu61 so las
condiciones son adversas. Capturas de 100 y hasta de 200 tons
no presentan dificultades graves. Con redes de 200 a 240 brazas
en la relinga de corchos, 60 6 70 brazas-- estiradas— en el centre,
pero reducidndose considerablemcnte hacia el copo, varias em-
barcaciones muy pescadoras, de 90 a 110 pies de eslora, ban
logrado captures anuales de 6.000 a 8.000 tons cada una. Este
m&odo ha hccho factible pescar al cerco arenque en Islandia
durante todo el afto, en tanto que anteriormente s61o se pescaba
el que se agrupaba en la superficie en los tres meses del verano en
la costa norte. De esta manera la pesca al arenque en Islandia
en 1962 alcanzo la cantidad sin precedente de 478.000 tons.
DURING the last few years the annual yield of the
Icelandic herring fishery has increased rapidly after
14 years of very low yield, 1945-1958. Generally such
large fluctuations are at least partly related to natural
causes. While the greatly increased herring catch in
Iceland seems no exception to this, the past few years
have seen very drastic changes in port fishing technique
which have contributed greatly to the increased yield.
This paper will describe and discuss these technical
changes and the resulting changes in fleet fishing power
with regard to fish finding, fishing tactics, gear handling
and gear changes. To assist understanding of these
revolutionary changes, a brief historical and general
sketch of the fishery follows.
Historical and general sketch
The history of the Icelandic herring fishery has been
described by Th6rdarson (1930) and clearly outlined by
Fridriksson (1944). Only a few points of technical
interest will be mentioned here.
During the first four decades of purse seine herring
fishing (1904-1944) the Icelandic fishery was a typical
inshore and near-water one. The main fishing grounds
remained within or just outside the north coast bays
and fiords throughout this period. The traditional
American two-dory purse seine system suited these
conditions very well and proved most successful. Her-
ring were located most frequently by sight since hand
sounding equipment used in the Norwegian winter
fishery could be used only infrequently, owing to special
characteristics in the schooling behaviour of the north
coast summer herring.
Until the forties the 30 ft-long dories were propelled by
oars and shooting, pursing and hauling were manual.
During the late forties small diesel or petrol engines were
installed in the dories, saving the crew some labour.
Nets were of cotton and measured approximately
120 to 180 x 30 fathoms with a centre bag and wings on
either side.
Fishing boat sizes varied greatly; from 45-ft wooden
craft to 140-ft steam trawlers with a crew of 16 to over
20 men. Smaller boats towed their dories. Larger
vessels (above 70 ft) carried the dories in large davits.
Clearly this method is mainly suited for fishing in
sheltered waters since it makes the boats much more
vulnerable in heavy seas when steaming. Lowering
and lifting the dories can also be quite hazardous in the
open sea.
The migration and behaviour pattern of the north
coast herring changed drastically after 1944. The herring
no longer entered the fiords and seldom were good schools
found in the near waters. The fishery collapsed. The
annual yield of the purse seine fishery fell from the level
of 150,000-200,000 tons in 1944 to 10,000-40,000 tons
in 1945-1958 (Jakobson 1963).
Low catches forced a change from the two-dory
18-man system to a one-boat system needing only
10 to 1 1 men. Differences include:
(a) The bag is at one end of the'net with only one wing.
(b) When shooting, the dory is towed alongside the
vessel, which then makes a whole circle, instead of the
two dories each making a semicircle.
(c) Shooting depends on power from the ship's main
engine instead of the less dependable dory engines.
(d) Pursing is performed on board the main vessel
and the purse line is hauled by using the ship's trawl or
deck winch. The net itself was hauled manually from
one end only. This was often heavy work caused by the
faster drift of the larger vessel.
The main advantage of this one-dory system, apart
from reduction in manpower, was that, once the dory
had been tied alongside the vessel, the shooting of the
net needed no extra preparation such as lowering the
dories from the main vessel, starting dory engines and
sailing the dories toward the school. This difference
often proved valuable in keen competition. Most of the
Icelandic fleet changed to this one-dory system from
1946 to 1960. The changeover enabled experimentation
with asdic-guided shooting, described in Section 4 of
this paper.
Location of herring in Icelandic waters
Herring detection in the north coast purse seine fishery
from 1904 to 1953, depended almost entirely on visually
sighting surfacing schools. As a result, aerial scouting
began as early as 1928 and is still used. While herring
entered the coastal waters their surfacing was suffi-
ciently regular to show the great merit of aerial scouting,
i.e. the large area that can be covered per unit time in
good flying weather. But if schools stay far off shore,
surfacing occurs much less frequently and more irregu-
larly (Jakobsson 1961) and aerial scouting becomes less
dependable.
Contrary to many other fisheries, the introduction of
vertical echo sounding did not cause any great changes
in the location techniques in the Icelandic north coast
fishery. Schools were occasionally spotted on the echo
sounder while cruising, but usually it proved very difficult
to relocate them, owing to their very limited vertical
spread (Jakobsson 1959).
It was not until horizontal echo ranging equipment
(asdic), installed in a Norwegian research ship, was used
on Icelandic grounds in the early fifties by Finn Devoid
and his technical staff that acoustic devices proved
successful for locating herring in this area (Devoid 1951).
Asdic was installed in the Icelandic inspection and re-
search vessel Aegir in 1953. Since then horizontal
acoustic ranging has played an increasingly large part
in locating herring concentrations off Iceland's north
and east coasts. From 1953 to 1958, the main value of
such surveys was to locate probable fishing grounds.
Actual fishing did not generally start until the schools
surfaced. Information on these probable areas became
valuable for subsequent aerial scouting.
Asdic units were first installed in Icelandic fishing
boats in 1954 and since then more and more Icelandic
fishermen have learned to shoot purse seines round
submerged schools. After 1958 this method became
fairly widespread. During the last four years the value
of asdic surveys has greatly increased, since concentra-
tions of good schools, submerged or not, can now be
fished by the new methods. Figs 1, 2 and 3 show a few
examples of concentrations of herring schools located
by the research vessel Aegir that proved very profitable
for the fishing fleet. During the summer of 1962, three
Icelandic ships, under the leadership of the scientist in
charge on board the Aegir, continually searched the
295
whole fishing area, tracked down herring concentrations,
followed the migrations and reported on schooling
behaviour, etc. The value of thus dividing fish location
and actual fishing may be difficult to determine, but all
fishermen agreed that over SO per cent of the 1962
herring catch stemmed directly from the three survey
vessels' work.
Fig 1. Recordings from the K.H. whale finder asdic (upper fig) on
board the Aegir and the Simrad Echo Sounders (lower fig). Range
0 to 2,000 yards. Two schools are contacted at the maximum range
and one is picked up on the echo sounder, the vertical echo being from
IS to 22 m. While waiting stationary until the fishing fleet comes,
schools are contacted in all directions (see asdic (upper) echogram).
Fig 2. See explanation to tig L Herring concentrations were
located on the asdic at approximately 0500 hours, as shown on the
echogram, and one of the schools is as usual picked up on the echo
sounder that shows the vertical echo trace to be from 30 to 88 m.
The fleet arrived between 0730 and 0800 hours when the schools had
started to descend due to their diurnal vertical migrations. Yet the
Vidir II reported at 0800 a shot of 130 tons fine quality herring from
the school shown on the lower echogram.
296
Fig 3. See explanations of Fig L Late summer echoes in areas of
considerable thermocline9 showing how only faint echoes are received
in the longer range due to the fact that in the distance of 700 to 2,000
yards most of the beam passes underneath the schools. This area of
herring concentrations yielded about 20 thousand tons in the following
two to three days.
If these three vessels had been engaged in actual
fishing, one could reasonably expect that their share in
the catch would be proportional to the average catches,
i.e. the increase in the total yield would have been only
of the order of 1 to 2 per cent instead of about 50 per
cent. The success of these search vessels results partly
from their using, as far as possible, all previous environ-
mental research results. The rapidly increasing know-
ledge of the herring's relation to various environmental
factors is thus used and tested in daily work during the
season. This has resulted in many tentative scientific
suggestions being strengthened or discarded.
As a result of a close correlation between dense
herring concentrations and sharp temperature gradients,
many fishing boats are equipped with thermometers.
Before the 1963 season a few boats will install continual
registration type thermometers.
Some other factors in herring detection are various
navigational aids, such as radar, semi-automatic radio
direction finders, and radio communication. The
latter is of great value since information regarding con-
centrations located by individual boats is immediately
passed by radio to the research or search vessels, which
forward such information along with their own findings,
to the fleet.
Vestnes (1962 and earlier mimeographed paper), has
proposed an organized searching method in which a
number of fishing boats at short distances abreast,
would cruise parallel courses, thus thoroughly searching
a large area in a short time. The Icelandic fleet has tried
this on a few occasions, especially during the new winter
herring fishery off the south west coast, where long spells
of fierce gales prevent search boats from keeping con-
tinuous contact with the herring concentrations. On
these occasions when the weather becomes better the
research vessel usually cruises along one of the edges
of an area of probable concentration (as judged by
hydrographic conditions) and the fishing boats then
voluntarily arrange themselves at intervals of one-half to
one mile. Thus 50 to 80 boats have covered the whole
probable area in a few hours and invariably positive
results have been obtained. Regarding the method of the
actual searching technique, both Vestnes (1962) and
Juliusson (1961) have described in detail how the asdic
beam can be best utilized, e.g. by searching in steps from
75° on one side toward the course line and then going
quickly to the other side and searching forward again,
always allowing enough time between steps for an echo
to be received. All manufacturers of echo ranging
equipment supply detailed information on this point.
Use of asdic for netting submerged schools
Small horizontal transducers were first installed in
Icelandic herring boats in 1954-55. They were connected
with ordinary echo sounders normally used for vertical
sounding. Their very limited range could not locate new
herring areas. However, in an area of rich concentra-
tions, schools were frequently located and experiments
in shooting the purse seine net around such schools
began as early as 1954. Captain Eggert Gislason of the
Vioir II, after innumerable failures, obtained the first
positive results. Since then this method of shooting by
asdic alone has become more and more widespread,
and it can be stated without any reservation that only
those captains that have gained experience and skill in
using this method have been successful during the last
five years. Those who insist on shooting only when the
schools surface, or, passing over the schools, trying to use
vertical sounding, have not proved competent in this
highly skilled fishery.
Thorsteinn Oislason (1961) and Juliusson (1963)
have described this method to some extent but operations
will be outlined here in somewhat more detail and from a
slightly different angle.
In principle the method is quite simple, since the net
is simply to form a circle with the middle of a school
centred. Assuming the school is circular, the net and the
edges of the school should form two concentric circles.
The distance between these concentric circles is the
difference between their radii, i.e. L — D, where L
Wind direction
Wind direction
A Current direction and opposite
T relative direction of school
Fig. 4
I Wind direction
, Current direction and opposite
relative direction of school
I Wind direction
Current direction and opposite
relative direction of school
Fig. 6
Wind direction
Very strong current
and opposite relative
movem. of school
75m
Fig. 7
Fig. 5
Fig. 6
Fig 4. Starting position and the subsequent course of asdic-guided shooting. The dimensions refer to a 90 to 100ft boat, 400 m long purse seine
net and a school of 40m diameter.
Fig 5, 6 7, 8. Starting positions under varying conditions of current and relative movements of school*. The dimensions are the same as in Fig 4,
and in all cases the school is expected to be in the position shown in
.
in Fig 4 when the circle has been completed.
297
is the length of the net and D is the diameter of the school,
both of which are known. If this worked strictly in
practice the method would be to get the school 90° to
starboard in the distance L — 1 and then manoeuvre
*r i D
the ship through this distance with the school always 90°
to starboard until the ring was closed. Suppose the
net were 400 m long and the school 40 m in diameter,
the distance of the boat from the school while shooting
should be 400
-=- — 20 = 45 m.
2*
Owing to various reasons, maximum results are not
obtained by shooting strictly by this formula. The bag
end of the net is, for instance, not as deep as the rest of the
net, and as a result the purse seine will not prevent the
escape of a school in that area as efficiently as elsewhere.
Then it is clear that during the pursing operation the
last opening to be closed is where the ends meet, i.e. at
the boat. It has therefore proved advantageous to shoot
not in an exact circle, but more in an ellipsoid manner,
making the edge of the "ring" egg-shaped and starting
and finishing shooting at the "pointed" section. This
infers that in the example above, instead of starting to
shoot approximately 45 m from the school (or 64 m from
its centre) and keeping that distance constant throughout
the operation, one starts at a greater distance with the
centre of the school considerably less than 90° to star-
board. Still using 400-m net it has been found best to have
the centre of the school approximately 45 to 50° on. the
starboard and the distance about 75 to 85 m. At that
distance a school of 40 m diameter will give echoes of
more than 20° width and should disappear in just over
30° from the starting course. Then the net is shot in only
a very slight curve until the school is approaching 90°
to starboard, and then it should be at a distance of not
more than 40, or less than 60 m, from its centre. Assum-
ing no significant current or movement of the school, this
distance is kept as constant as possible until the boat
has made the turn and is heading for the buoy again,
i.e. the bunt end (see Fig 4).
Experience has shown that in most cases it is advan-
tageous to start shooting with the wind straight ahead.
Then the boat is also in that position while pursing takes
place and in bad weather this is the best position for
manoeuvring. As the net is hauled, the boat slowly
makes almost a whole circle, finishing with the stern or
even the port side, into the wind when brailing starts.
It should be noted that those boats that are heavy on
the rudder (longer than 100 ft) start with the wind
10 to 15° on the starboard side.
Figs 5 and 6 show schematically how the starting
position changes when the ship is heading into and going
with the current, respectively. These figures explain
equally well the case of no current and the school
swimming up or downwind. In fact, the two cases appear
identical on the asdic, i.e. Fig 5 thus shows the starting
position when the school moves away from the sta-
tionary ship, whereas Fig 7 shows the starting position
when the school approaches a stationary ship that in
298
both cases is heading into the wind. Shooting in calm
weather, but in areas of strong currents, can be very
tricky. If one starts shooting with a following current
(school approaching on the asdic) there is a good chance
that the school will dive through the opening that in-
evitably is formed under the ship. This opening cannot
be closed until pursing is completed.
This method of shooting has the advantage that the
net stays clear of the ship during hauling. There is
therefore no danger of it becoming entangled on the ship.
When starting shooting in calm weather against the
current, the conditions are reversed. Then there is a
much better chance of catching the school, but at the
same time the net then tends to drift onto the ship and a
big loop may be formed on the port side of the ship
during pursing and hauling. When the net has been
shot in this way, great hauling speed is essential to avoid
entangling with the ship. In very strong head current
(Fig 5), cases are known where shooting has been
successful only if the school is already more than 90°
to starboard when the operation begins. Similarly, if
there is a strong side current going at almost right angles
from right to left when facing the wind direction,
successful shooting can sometimes be accomplished only
if shooting starts with the school almost straight ahead
(into the wind) or even slightly to port (Fig 7).
During six years of practice the most skilled Icelandic
fishermen have accomplished successful shooting in a
variety of adverse conditions without needing any extra
powered skiff or help from other vessels. This is largely
due to a complete understanding of the interaction of
wind and current on both the ship and net. Another
important feature of the method, i.e. the position of the
hauling gear, is discussed later.
This discussion shows hows how important it is when '
shooting by asdic on submerged invisible schools that
the captain be absolutely sure of the relative movements
of the current, the school and the drift of the boat. The
best method is perhaps when approaching the school
(at, for example 150 to 250 m) to go at a dead slow speed,
or even stop with the school slightly to starboard and
heading into the wind. At such a distance the relative
movements of the boat and school can be detected;
yet there is enough space to manoeuvre into the best
starting position. Table I shows in the first column some
features that the captain is likely to be able to determine.
The second column shows starting positions as shown
in Figs 4-8 that are likely to give the best results.
In order to determine the movement of the school some
captains prefer, however, to approach from the wind
direction and pass it at close quarters before turning
into the wind and manoeuvring into a starting position.
Once the shooting position has been chosen the boat
must be manoeuvred into it. To determine the exact
bearing of the centre of the school at the small ranges
just prior to shooting, it has been found most advan-
tageous to trail the edges of the school very carefully
and take the centre as the average of the two bearings
(see Figs 9 and 10). The captain must then determine
Fig 9. An echogram from M.V. Gudrun Thorkelsddtlir, 92
skipper Thorsteinn Gislason. A school is picked up on the 0 to 200 m
scale using horizontal transducer connected with a Simrad echo-
sounder. The ship approaches slowly while the skipper is training
the beam at the outer edges (2) and the centre (3) of the school^ thus
finding its exact position as well as its relative movement to the ship.
In position 4 the shooting of the purse seine starts, and as the starboard
turn is made, the echo of the ship's wake (5) appears. Result: 40 tons
of herring. See reference in text.
whether the school stands high enough in the sea for his
particular net. This is done by previously relating the
distance at which the echoes of the school disappear from
a given horizontal beam. Optimum results of searching
by acoustic ranging is usually obtained with the beam
narrow and almost horizontal. Since this beam tends
to pass over schools at close ranges (Fig 10), most of the
Icelandic purse seine boats are equipped with a trans-
ducer that transmits another beam at 10 to 25° angle to
the ordinary search beam. This slanting beam is mainly
used at very close ranges and especially during the
shooting operation. Before gaining experience, many
fishermen circled the school before shooting. They
discovered that the wake acted as an airbubble curtain
that blocked all echoes from the school. As a result,
some fishermen had only their own ship's wake as a
target during the first shot or two.
An experienced captain, Mr. Th. Gislason, has com-
pared (Gislason 1961) shooting by asdic with learning to
ride a bicycle. As skill increases with practice, soon the
balance is so secure that one finds it difficult to imagine
ever shooting a purse seine without the aid of asdic.
In fact, many Icelandic fishermen today would rather set
purse seines around submerged schools than around
surfacing ones.
Power blocks and new deck arrangements
Schmidt (1959) described how the Puretic power
block was introduced in the American Pacific purse seine
fisheries in 1954 and 1955. During the winter of 1956
an Icelandic herring captain, Mr. Ingvar P&lmason, went
to the Pacific coast to study this instrument in action.
His report (P&lmason 1956) launched experimentation
with the Puretic power block in the Icelandic herring
fishery in 1957.
All P£lmason's experiments (1957) showed that the
Puretic power block was well suited for hauling the
Icelandic type of purse seine nets previously used in the
one dory system, but the deck arrangement of the par-
ticular boat used did not prove convenient. This boat
was of the conventional type with the wheelhouse very
far aft and there was not enough space for the bulky
net on the stern. Hence it had to be hauled midships
and most of the deck space was thus occupied by the net.
It was not until the summer of 1959 that a real success
was made in this respect. Then the captain of the 95 ft
vessel Gubmundur Thdrdarson, Mr. Haraldur Agustsson,
curried out successful experiments by shooting and
Fig 10. Echo picked up on a Simrad herring asdic at a distance of
approximately 1,300 m. At a distance of 200m, the skipper switches
over to the shooting scale and starts shooting at the solid black line.
The school is at such a depth that only diffuse echoes are received
during the operation. Yet the results were 130 tons.
299
Fig 1L The old "Vidir" — the asdic pioneer — coming to port from
one of Mr E. Gislason's successful early experiments with asdic —
guided shooting of purse seine. To the extreme left the tow line
to the dory can be seen.
hauling the net from the upper boat deck aft of the wheel-
house. Since then this arrangement has been used
throughout the Icelandic fleet with the exception of the
smaller boats (less than 85 ft) that have no upper deck,
instead, enough space has been provided aft of the
wheelhouse and in the starboard passageway leading aft.
(Figs 12 and 13).
Fig 12. M.V. Stapafell at the quay. The powerblock is supported
by a davit attached to the wheelhouse and the purse seine is stored
on the stern and in the starboard passage. See text.
Figs 14 and 15 show the power block arrangement of an
Icelandic herring boat. On top of the net is a ready
buoy (Fig 13). There are two lines shackled to this buoy.
One is a rope that is a continuation of the upper edge
of the net; the other is the purse line to which a float
is shackled (Fig 13). This float is intended to take the
300
Fig 13. The purse seine net ready for shooting on the stern with the
buoy (7) on top as well as a part of the purse line. Note the loop just
aft of the purse rings (3) and the large float (2) that is shackled to
the loop (4).
Fig 14. Shooting of the purse seine net from the upper deck of
P.M. Gudmundur Th6rdarson, the first Icelandic vessel to use the
powerblock successfully.
main weight of the heavy purse line off the buoy to make
it easier for the crew to retrieve the buoy when shooting
has been completed. Fig 16 also shows how the rings
are stored on a metal rod with the purse line running
through them. The purse line leads further forward
through a gallows block towards the pursing winch that
is most frequently situated just aft of the foremast
(Fig 18). The mechanism of shooting, pursing, hauling
and brailing is further illustrated in Figs 16-21.
As soon as the end-buoy has been retrieved, the
pursing line is unshackled and then reshackled to a short
line running through the pursing gallows to the winch
(Figs 17 and 18). Usually the whole net is shot, as well as
several tens of fathoms of ropes. These ropes are usually
Fig 15. Hauling the purse seine on board P.M. Gudmundcr
Th6rdarson.
/'
Fig 16. An illustration of the snooting operation showing (1) the
buoy. (2) the upper edge rope shackled to the buoy as well as the
purse line (3) running through the rings (4) and the net flowing off the
upper dory deck.
Fig 17. The pursing operations showing (1) the retrieving ropes
of the wing and (2) the pursing gallowt and (3) the purse line.
Fig 18. The hauling operation showing (1) the pursing gallow,
(2) the pursing winch, and (3) the rings unshackled from the purse
line and reshackled on a leading line towards the powerblock.
Fig 19A. The brailing operation showing (1) the boom on which
the breast is hooked and general arrangement of brailing.
301
Purae line removed
from rings when
only part
of net
ahot
Pulling
with the
power
— •-• block
PAULING W:B WITH POWJR BLOCK
Fig 19B. Further illustrations of the shooting, hauling and broiling operations.
.X Pole hooked
v*-*"" .^x" to breast of
WILING net
Fig 20. Vioir II broiling a good catch.
hauled during pursing by a special vertical winch
(normally used for longlining) on the main deck. When
pursing is completed the ropes and the wing end of the
net are taken through the power block and the net is
hauled continuously until drying-up is sufficient for
brailing. When handling large catches it is of great
importance that the net be hauled evenly— i.e. both
302
Fig 21. Brailing is continued as long as the upper edge of the
gunwale is not submerged.
float and leadlines as well as the webbing should come in
at the same rate. It is therefore common practice on
Icelandic herring boats to lift one edge of the net out of
the block and then only haul the other edge and adjacent
webbing until the net becomes even again. Although a
powered skiff is not normally needed, most boats carry
one. In adverse conditions it can tow either the boat
out of the purse seine or the net away from the boat,
especially if a port side loop is formed in a strong head
current.
The position of the pursing gallows as well as that of
the power block is chosen as far forward as possible to
avoid fouling the propeller. The power block system
has various advantages over the older dory system.
Some are: boats can travel faster and in worse weather
without the dories; shooting can take place without the
delay of lowering dories or skiffs; powered hauling saves
crew time and labour; handling of very large catches
over 100 or even 200 metric tons, no longer presents
serious difficulties or requires extra help from other
crews. Finally, the power block has enabled Icelandic
Table I. Boat heading into wind almost stationery, with school at
150 to 200 m distance, bearing slightly to starboard.
CUraoteristioa
of Mtiio
l»<»Unc« and
learinj itable
TJietanoe increasing,
tearing oli«htly
.a stance decreaeine,
tearing eli^htly
increasing
Diotanoe almoet con-
stant o tar board,
i earing increasing
lu stance alooat oou-
cuiit starboard,
bearing decreasing
kethod of
•hooting
Shoot ly
710. A
Shoot by
Fie,* A-a
Lhoot by
?ig. At-2
Shoot by
fig. A*3
Shoot by
71 «. A-»4
Stationary or moving in
ea»e direction its current
IkOviMfl opposite to wma
uiroation relative to the
current
Loving with the wind
relative to the current
having to starboard
relative to the current
Loving to port relative
to the current
)to current or in •*»
direction ab ochool
Currwni wit:, tua nntf
relative to tli« rcu<*oi
Current in opposite
direction of »iwi,
relative to
Current at rirfht anb!.ua
to the wind from star-
board to jjurt
Current ut ri«ht un^lea
to the wind froui jiort
to etoruct-rd
fishermen to increase the size (especially the depth) of
their nets beyond the effective limit of manual hauling.
Successful sets have thus often been accomplished when
the top edge of the school is at a depth of 35 to 40
fathoms, using a net 65 to 75 fathoms deep, stretched
mesh.
The net
The Icelandic purse seine nets with the bag in one end
have been developing over the last 20 years; first to
serve the one-dory system and later for powered hauling
without a dory. This second function has only developed
since 1958 and the exact shape and trimmings of the
Icelandic net have not been stabilized. Each net rigger
has his own formulas for details, often according to the
captains* special ideas. For illustration the author has
chosen two nets from different riggers (Figs 22 and 23).
Both nets have approximately the same over-all
dimensions and construction is the same in principle.
The main difference is that one rigger uses the same
webbing right through the depth of the net, while the
other increases the meshsize towards the lower edge. It
will be seen that the trimmings are different at the bunt
end ; one increasing the depth of the net at a higher rate
than the other. Both net types have proved very SUCCeSS-
28
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56
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56
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56
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FEET
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8 /is
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8 /I2
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500
Table II. Specifications of purse seine net shown in Figure 22. (Mesh
size equals 40 rows per alen throughout or 1 S.7 sq. m. Note the Scandinavian
term alen equals 24 in.)
*ii
Leoti Mincer of Twine number
Weahea
KeuheB Length in ft
irked pieoe*
deep
long upper edge
1 210U/42
000
500 28
3
/*>
14OJ
C
/JO
1910
i>
/24
2400
£
/a
3000
7
As
jy»
6
/12-15
•J5BO
H
/9-12-15
3640
I
3700
J
3760
K
3620
1
3S80
I
3940
•8
4000
0
4120
2000 112
P
3580
1UOO 56
4
3520
M N
R " /12-15
3450
500 21
Lof*th in ft
lower edge
Fig. 22. This successful Icelandic purse seine net for power hauling
was designed by Kristinn O. Karlsson, HafnarfJVrdur, Iceland. The
length of each panel on the upper and lower edge rope is shown in
feet. Throughout the mesh of the net is 15.7 mm bar (40 rows per
alen). (Note: the Scandinavian alen = 24 inches) the lower edge
is heavily weighted with lead. In the wing half the weight is 3 kg
per ft, but in the bunt half it ranges from 7,5 kg— 2.8 kg per ft.
Further specifications of this net are shown in Table II. The following
selvages are to be included in the total number of meshes: Outermost
selvages from A-Rt top and bottom and on one side of pieces A and
R, are 15 meshes in total with the outermost mesh all around of twine
210d/120 and with suitable decline in strength inwards to produce the
best results as regards the strength of the main net in the various
pieces concerned. As the drawing indicates there are 50 meshes of
twine 210dl/5 at the top of all pieces from G-R and 100 meshes
from H-Q of twine 2JOdll2—this is to come in between the main net
and the outermost selvage. At the bottom of pieces H-Q there ere
50 meshes likewise of twine 210d/12 to be ptaced between the main
net and outermost selvage. All outermost hatf-meshes throughout
the net, top, bottom and sides (Le. that of twine 210d/120) are 35 mm
bar.
303
52 32 92 32 31 51 31 31 31 31 31 61
61
60 60
36 36 FEET
34
[I!!::!!:
40 omf per alen 3^
32 » » »•
42 FEET
69
II
70
'if. 23
Table III. Specifications of purse seine net shown in Fiaure 23. (Mesh size
equals 40 rows per alen throughout with the exception of 1 and 2. Note the
Scandinavian term alen equals 24 in.)
Fieoet i'ucber of ?*ine number fceihw Veihec Length in ft Length in ft
Barked pieces deep long upper edge lower edge,
1.100/45
/42
/39
/30
/24
/21
/15
/9-12
500
35
/122>
1 piece 300 tee* deep, 32
pieoev, one •• in 1), t
1100
1400
2100
2350
2600
3000
3050
3150
3250
3300
3350
3400
3450
3500
3550 2000
3150 1000
3050
3000
2350 500 36 42
2250 mm N
2100 .. H »
n per ell (e*eJwise) of 2100/15 tvine
the other 75 &eeh deep, 11 rove par ell of 2104/39 twine
59
57
106
61
06
126
69
70
ful in recent season. Boats using either type were among
the top five vessels during the record 1962 summer
season. The increased meshsize at the lower edge is
expected to make the pursing and closing of the net
easier as a result of reduced resistance. The high rate of
increasing depth from the bunt end is expected to have
the same effect, i.e. easier pursing and better prevention
of escape at the bunt end. On the other hand, the lower
rate of depth increase (or decrease towards the bunt bag)
is said to lessen the danger of large schools causing the
net to burst as they are squeezed into the bunt when the
body of the net is hauled.
These arguments are not based on controlled experi-
mentation and must be regarded as somewhat speculative.
The hanging-in percentage has varied greatly during
recent years, but there has been a tendency in the
Icelandic type of rigging to hang the nets with less slack.
To increase sinking rate, there has also been a very
304
Fig. 23. Another Icelandic purse seine net used for power hauling,
as designed by Mr. Johann Klausen, Eskifjordir, denotation as in
Fig. 22. Mesh size "40 rows per alen" (the Scandinavian alen^24
inches) with the exception of pieces J-V where two pieces of larger
mesh 32 and 11 rows per alen are added at the bottom of the net, each
300 and 75 meshes deep, respectively. Weight same as in Fig. 22.
Total length: upper edge 243 fathoms, lower edge 275 fathoms.
Selvaging corresponding to that of the net shown in Fig. 22. Depth
of centre panel (R) 70 fathoms stretched mesh. See also Table III
and text. Note the difference in shape and hanging-in rates of the
two nets shown in Figs. 22 and 23.
marked increase in the weights on the lower edge. Net
specifications show the weight is now approximately
1-1 \ Ibs. per foot, depending on the purse winch power.
During the last few years of powered hauling, the
strength of the framing lines has been increased, and now
the floatline assembly consists of three to four lines (each
12 mm diam, Terylene), not counting the line going
through the plastic floats. Similarly, the lower edge
consists of two to three lines of the same thickness.
For the last three or four years practically all Icelandic
nets have been made of nylon twine hung to Terylene
lines.
Changes in fishing power in recent years
Changes in the fishing power of various fishing fleets
have been estimated by calculating power factors
between ships fishing on the same grounds at the same
time (Beverton and Holt 1957). Often it has been
possible to correlate the differences in power factors to
some measurable features of the ships, such as size or
engine power. This has given positive results, especially
where the gear is a townet of some kind. It is now the
common practice to express the effort statistics of the
great trawler fleets in standard units where such power
factors are taken into consideration.
For the purse seine fishery it is immediately clear that
one of its main features is the importance offish finding.
This may vary greatly from year to year, depending on
availability of the fish concentrations. During the early
years of Icelandic purse seining there were frequently
such large concentrations of herring surfacing in the
coastal waters that modern fish finding devices would
probably have been of little or no importance. In such
circumstances of high availability, herring boats function
as cargo vessels and fishing power is closely correlated to
the ship size and unloading speed.
Taking the other extreme, e.g. herring fishing out in
the ocean off the north coast of Iceland where schools
seldom surface, the probable area is very large compared
with the area occupied by visible schools. In such
circumstances the boat spends almost all the time
searching, and differences in searching power would
therefore obscure any correlation with other features
of the ship.
The increased division of work between searching and
catching in the Icelandic fleet and the close radio com-
munication has again decreased the importance of
differences in each ship's fish finding power. During the
1962 season, search ships were in contact with workable
schools throughout the season, thus greatly decreasing
searching time of individual boats.
It has been estimated that at least two-thirds of the
total yield of the season was caught by asdic-guided
shooting round invisible schools. In this case, fishing
power must depend very much on the acoustic ranging
instruments and especially the captain's skill in their
use. Admittedly this method requires considerable
practice and great understanding of wind and current
interactions on the ship and net. Thus the "captain
effect" is so great that it obscures any relation with the
various features of the ship. There have, for instance,
been several cases where a change of captain, with no
change in instruments or gear has made a particular
boat's fishing power rise or fall 10 to 20 times. The
calculation of unbiased power factors and the subsequent
correlation of these with measurable features of the
fishing vessels is thus a very complicated and difficult
process. Consider then the very sudden rise of the
Icelandic herring yield during the last few years, reaching
a record 478,000 metric tons in 1962. Compare this
with the well-known decline of corresponding fisheries
that are partly based on the same stocks. The comparison
shows clearly that fish finding, asdic-guided shooting,
powered hauling technique and the resulting increase in
the depths of the nets used on the boats in this fishery
have brought about enormous changes in the fishing
power of the fleet. Apart from that, these techniques
and instruments have made it possible to instigate new
successful winter and spring seasons off the south coast
of Iceland where purse seining is performed under
adverse conditions up to windforce 6 and in quite heavy
winter swell.
References
Beverton and Holt. On the Dynamics of Exploited Fish Popula-
tions. Fish. Inv. Series II Vol- XIX London 1957
Devoid, F. Pi jakt etter storsilden i Norskehavet. Fskets Gang
Vol. 20, pp 21 7-222, 1951.
Fridriksson, Dr. A. Nordurlandssildin (The Herring of the
North Coast of Iceland). Rit Fiskideildar, Vol. I, No. 1, Reykjavik,
1944.
Gislason, Th. Asdic og fistilcitarktacki. /tgir, Vol. 54, No 9,
Reykjavik, 1961.
Jakobsson, J. Locating Herring schools on the Icelandic North
Coast Fishing Grounds, 1959. (Introduced at the FAO Gear
Congress 1957.)
Jakobsson, J . Remarks on the distribution and availability of the
North Coast herring of Iceland. Herring symposim ICES in
print: Rapports et Proems- Verbaux, 1964.
Julfusson, K. Asdic og fiskileitartaeki. ^gir, Vol. 54 Nos. 5-8,
Reykjavik, 1961.
Julliusson, K. Modern equipment extends Icelandic herring
fishing. Fish. News International, Vo. 2, No. 1, 1963.
Palmason, I. Ny flotvarpa til sildvcida. ^Cgir, Vol. 49, No. 8,
Reykjavik, 1956.
Palmason, I. Sildveiditilraunir med hringnot med aostod
dyptarmaelis vid Sudvesturland i mai. &gir, Vol. 50, No. 11,
Reykjavik, 1957.
Schmidt, P. G. The Puretic power block and its effect on modern
purse seining. Modern Fishing Gear of the World, London, 1959.
Vestnes, G. Fish detection and echo sounder. Modern Fishing
Gear of the World, London 1959.
Vestnes, G. Fiskeletning med Sonar, Chapter 6 and 7, p 81,
Oslo, 1962.
305
u
Sonar Instruction Courses for Fishermen
Since World War II the use of acoustical instruments in fisheries
hat become more and more important and the stage has been
readied where purse seiners without modern electronic equipment
installed fail to find crews. In the Norwegian fisheries the need for
training in sonar operation became an urgent matter some years
ago and, at the request of fishermen, suitable instruction courses
were organized by tine Institute of Marine Research in co-operation
with Simonsen Radio A/S, Oslo. The courses given include the
theoretical elements of wave propagation and basic electronic
functions of ultrasonic equipment, as well as instruction on the
location and catching techniques with the use of sonar equipment.
The men were thoroughly trained in the three main sonar search
patterns. In sonar operation, as soon as contact is made with a
concentration of fish, the vessel is aimed direct at the school to a
distance of between 40 and 70 m from it. The seine is then shot
while the vessel keeps at the same distance from the school, steering
round it until fully surrounded. The paper contains definitions
for the various types of echoes received on the sonar which generally
have a disturbing effect in recognizing the real fish echo traces. The
course included practical training in searching and keeping contact
with a school of fish and consisted, in the first instance, of locating
an artificial target and later on of real fish schools. Eight courses
have been given up to the present and much experience has been
gained. It would appear that slightly more instruction should be
given in fundamental principles but that the training in classification
and identification of echo traces has, in general, produced the
results envisaged.
Com d'tatractioii de Sonar poor les pecfaeurs
Depuis la deuxieme Guerre Mondiale, 1'utilisation des instruments
acoustiques dans la peche est devenu de plus en plus importante a
tel point que, maintenant, ks senneurs non equipes avec des instru-
ments tlectroniques modernes ne trouvent plus d*6quipage, Depuis
quelques annees dans les peches norvegiennes, la n&essite d*un
entratnement pour utilisation du sonar devenait urgente et, a la
demande des pecheurs, des cours ^'instruction ont M organises
par Tlnstitut de Recherche Maritime, en cooperation avec la
:*Simonson Radio A/S" Oslo. Les cours donnes comprenaient
1'ftude des &6ments theoriques de la propagation des ondes et les
fractions dlectroniques de base d'un 6quiepment ultra-sonique,
aussi bien que 1'ttude des localisations et techniques de peche a
Taide de r£quipement sonar. Les trois principaux syst&mes de
balayage ont ete inculques aux 6fcves. Dans Toperation "sonar",
des que le contact a ete 6tabli avec une concentration de poisson,
le bateau est dirigg sur le bane jusqu'a une distance de 0 a 70
metres. La senne est alors mise a I'eau et le bateau, tout en con-
servant toujours la meme distance du bane, tourne autour de
celui-ci jusqu'a ce que le bane soit complement encercte. La
communication comporte des definitions de diffferents types d'fehos
recus par le sonar et sur lesquels il est parfois difficile de recon-
naltre les traces r6elles des poissons. Le cours comprenait aussi
un entrainement pratique dans le recherche des banes et la facon
de garder le contact avec ces banes. Les premieres lecons ont ete
faites sur des cibles artificielles puis plus tard, sur les banes
vdritabtes. Huit cours ont ete donnes jusqu'a present et on en a
tire beaucoup d'exptrience. II sembte que 1'ttude des principes
fondamentaux doive tire augmentee mais par centre, 1'entraine-
roent dans la classification et Identification des traces d'echos a,
en general donn£ les resultats esperes.
Corsos de capacitadoo de Pescadores en d empteo de sonar
Extracto
Desde que se acab6 la segunda guerra mundial el empleo de
jnstnimentos acusticos en la pcsca ha adquirido cada vez mas
importancia y ha Itegado el momento en que los barcos que
emplean redes de cerco de jareta y que no estan dotados de equipo
electrinico no encuentran tripulacion. En Noruega, la necesidad
de ensefiar el funcionamiento del sonar adquirio caracter urgente
hace varios aflos y, a peticidn de los Pescadores, se organizaron
cursos de capatitation por el Institute de Investigaciones Marinas
en cooperation con la Simonsen Radio A/S de Oslo. La instrucci6n
dada oomprende elementos teoricos de propagacidn de las sondas,
fundones etoctronicas fundamentals del equipo ultrasonoro y
306
by
G. Vestnes
Institute of Marine Research,
Bergen
tecnicas de localizacion y captura con el auxilio de sonar. Los
Pescadores recibieron una capadtaci6n muy completa en las tres
modalidades principals de la busqueda con sonar. Cuando se
cmplea este sistema, el barco, tan pronto como se ha estableddo
contacto con una concentracidn de peces, se dirige directamente
al cardumen hasta llegar a una distanda de 40 a 60 m, momento
en que large el artc raanteniendose a la misma distancia del cardu-
men y dandole la vuelta hasta que esta completamente cercado.
Da el autor definiciones de la diversas clases de ecos redbidos en
el sonar, que generalmente tienen un efecto perturbador en el
reconodmiento de los trazos reales del ceo de los peces. Los
cursos de capacitacibn comprenden instrucci6n practica en la
busqueda y mantenimiento del contacto con un cardomen y
consisteti en primcra instancia en localizar un objetivo artificial y,
mas adclantc, los cardumcnes verdaderos. Hasta el momento se
han dado ocho cursos y se ha adquirido muchisima experiencia.
Parece ser que deberfa darse mas instrucridn en los principles
fundamentles, pero la capacitaci6n en clasificacidn e idcntificaci6n
de los trazos de los ecos ha producido, en general, los resultados
previitos.
DURING the post-war period acoustical instruments
(echo sounders and sonars) were used more and
more by the Norwegian fishing fleets. In 1949/1950 the
echo sounder had its break-through as a fish detecting
instrument; ten years later, sonar instruments as well
were considered a necessity on a well equipped herring
boat. Today we have reached the stage where purse
seiners without a modern echo sounder and sonar have
difficulty in getting a qualified crew.
When the sonar equipped research vessel G.O. Sars
was commissioned in 1950, very few people believed that
this type of instrument would ever become widely
used on commercial vessels. However, the fishermen
soon realized that their boats, equipped only with echo
sounders, could not compare with the G.O. Sars in
ability to locate fishable concentrations of herring.
Consequently, the most far-sighted vessel owners
started to provide their vessels with sonar instruments.
The need for instruction
The first sonar vessels did not have immediate success,
probably because the skippers at first failed to master
the proper operational technique. This shortcoming was
soon realized by the fishermen and the fishing authorities
were requested to arrange suitable instruction courses in
the use of sonar. The Institute of Marine Research in
Bergen, in co-operation with Simonsen Radio A/S, Oslo
was given the task of developing a general plan for such a
course and preparing suitable instruction lectures.
The organization of the courses was conducted by the
Office of Education and Training of the Directorate of
Fisheries. The courses were published through news-
paper advertisements. Participants had to pay their own
fare and subsistence, but the course was otherwise free
of charge.
The expenses for the classroom and training vessel
were covered by the Directorate of Fisheries. Simonsen
Radio A/S installed complete sonar and echo-sounding
equipment in the classroom and provided experts from
their sonar factory to lecture on the construction of the
instruments. The author was chief instructor and in
charge of the courses.
General pedagogic viewpoints
The average age of the fishermen participating in the
courses was 40 years and, for most of them 5 to 45 years
had elapsed since their last formal school education. It
was therefore evident that the methods of instruction
had to differ considerably from those applicable in a
class of younger students.
Each participant was issued with a written compendium
covering all lectures but was not expected to read and
learn the contents in the fashion usually demanded in a
normal school. The compendium was merely regarded
as an aid to the memory and the main emphasis was laid
on oral lectures in all subjects, with extensive use of slides,
plates and blackboard. A strictly "correct" technical-
scientific explanation was purposely avoided in all
technical subjects for the benefit of a more easily under-
stood popular description. In fact, technical subjects
were only treated to the extent that they were regarded
as strictly necessary for the operational use of sonar
equipment and the courses were not aimed at training in
maintenance and servicing of the instruments.
For instruction in the main subjects great emphasis was
laid on frequent repetition of basic theorems and main
viewpoints. Thus, each morning the first hour was used
to repeat, discuss and sum up the lectures of the previous
day.
It should also be mentioned that, again for pedagogical
reasons, the lectures were purposely illustrated and
"spiced" with references to factual episodes from the
practical fisheries, and sometime also with stories from
other fields (see detailed course timetable).
Detailed Timetable
First Day. 9-9.45 a.m. : Introduction ; basic principles of sound ;
10-10.45 a.m.: Sound propagation spreading, attenuation, refrac-
tion, reflection; dopplcr effect; 11-11.45 a.m.: Film; Principles of
Sonar; 1-1.45 p.m.: Transducers for echo sounding and Sonar;
2-2.45 p.m.: Some basic principles of electronics; 3.30 p.m.:
Principle of electronic valves.
Second Day. 9-9.45 a.m.: Repetition of yesterday's lessons;
discussion; 10-10.45 a.m.: Valve-transmitters and receivers;
11-11.45 a.m.: Description of different types of echo-sounders;
1-1.45 p.m.: Classification of echo-sounding recordings (slides);
2-2.45 p.m.: Echo-sounding transducers; installation; 3.30 p.m.:
Sonar transducers; installation.
Third day. 9-9.45 a.m. : Short discussion on yesterday's lessons;
10-10.45 a.m.: The importance of good maintenance on ship's
electric power system; 11-11.45 a.m.: Using Sonar for fish finding
and three other short spells.
Fourth and Fifth Days* Practical training with Sonar gear in
classroom.
Sixth, Seventh and Eighth Days. Practical training at sea.
Bade principles of sound propagation In sea water
The velocity of sound in sea water, normally approxi-
mately 1,500 m/scc, is dependent on temperature,
salinity and pressure. These factors vary but for echo
sounders the effect of such variations is relatively small
and the error introduced by assuming a constant sound
velocity is hardly of any significance for practical fishing
purposes.
The sound velocity increases with increasing
temperature and with increasing salinity. Thus, for
example, under conditions of decreasing temperature
with depth, the top of the sonar beam will attain a
greater velocity than that of the lower part. This results
in a downward deflection of the beam and, consequently,
a reduction in horizontal range. Such conditions are
quite normal during summer in the Norwegian Sea. At
other times of the year, and in other areas, conditions
are different and it is easily understood that sonar
conditions may vary greatly.
A more comprehensive treatment of this topic here is
not necessary but it should be emphasized that sonar
range, as a result of differences in hydrographic conditions,
may be quite variable. Some knowledge about sound
propagation in the sea is therefore essential in using
sonar in the most efficient searching programme.
Search procedures
Location technique — Sonar search is carried out
according to certain patterns. A first type of searching
pattern is called "side to bow" (Fig 1 A). The transducer
is turned to between 70° or 90°, port or starboard. A
sound pulse is transmitted and the transducer quickly
turned 5° forward and kept in this position to allow
sufficient time for possible echoes to come in. The length
of time the transducer must stop depends on the scale
range used. With a scale range of 3,000 m, the transducer
must be kept pointed in the same position for two
seconds for the soundwave to travel 3,000 m, and another
two seconds for the echo to return. In addition, a dead
period of approximately 0.5 sec is required before the
next transmission. At a range of 3,000 m the sonar,
therefore, will need 19 X 4.5 sec, or 85.5 sec, to search
from 90° to directly ahead. If the ship is doing 10 knots
(5.1 m/sec), it will travel 436 m forward while the sonar
is searching one side. The transducer is then quickly
turned to the opposite side and the programme is
repeated.
A very common searching pattern is the so-called
"side to side" method (Fig IB). This pattern also
starts with the tranduccrs at between 70°-90° on the
bow. and the sweep alternates from one side to the other.
Fig 1C shows a searching pattern called "side to side
and back". This method differs from "side to side91 only
in that the transducer is quickly turned back to the
307
starting position as soon as both sides have been searched.
Figs 2A, B and C, which were obtained at vessel
speeds of 5, 10 and IS knots respectively, show situations
where the searching pattern is "side to bow". The range
scale is 3,000 m and the sonar range is 500 m. The
figures show that the search leaves pockets of the area
uncovered, either because the range scale was too large
or, alternatively, because the speed of the ship was too
great.
From Figs 1A, B and C it can be seen that the various
searching methods differ in efficiency in obtaining
complete coverage.
The three search patterns mentioned are those most
commonly used. In localities where many vessels are
operating simultaneously, the skippers will have to
modify the searching methods to minimize the effects of
disturbance from other vessels.
Catching technique — The sonar catching technique
varies with the type of gear the vessel is using.
For pelagic trawl fishing, the technique is to point the
ship towards the fish concentration. When the con-
centration is below the ship the echo sounder will show
the depth and the trawl depth can then be adjusted
accordingly.
A dory-equipped purse seiner uses the sonar to direct
the fishing master to the fish concentration. When he
has found the concentration he directs the dories with
the aid of his own small sonar (Basdic) installed in the
MO.2A
nan
FIG. 21
3000 2000
FIG. 1C
NG.2C
308
dory, in the catching procedure. •
The most advanced sonar catching technique is that
of a stern purse seiner. When the fish concentration has
been located by the searching technique, the ship moves
towards the concentration. The skipper, who is now
alone in the wheelhouse, keeps contact with the con-
centration. When the range has decreased to between
40 and 70 m he gives the order to start setting the seine.
This is a very critical moment and the skipper has to be
skilled if the operation is to be successful. With the aid
of sonar he keeps the same distance to the concentration
while it is being surrounded.
This technique is widely used in the fisheries for herring
near Iceland and also in the coalfish fisheries in north
Norway.
Classification and identification of echo traces
For the fishermen, the transition from interpretation of
echo sounder recordings to sonar traces is difficult
because in sonar the third dimension is being dealt with.
The ocean contains a great number of objects which
present themselves as good sonar targets but only very
few of these are concentrations of fish. A number of
these have been classified and given special names in
the "sonar language":
(a) Bottom echoes — Echoes from peaks and other
topographical irregularities. They often interfere with
the echoes from fish in shallow waters (50-100 m).
(b) Reverberation — Echoes caused by tiny pelagic
plants, animals and air bubbles in the sea. In the open
sea (deep water) the rule is that the greater the distance
from which the reverberation is recorded, the better are
the sonar conditions.
(c) Stratal Echoes— Stratal echoes associated with
large vertical or horizontal temperature gradients in the
sea. Great attention should be paid to the existence of
such conditions as they cause the sound beam to be
refracted and thereby limit the sonar range.
(d) Surface echoes— This kind of echo varies with the
seasons. It is less pronounced in the summer and most
apparent in the fall. The surface echoes appear at
distances from zero to a few hundred metres.
(e) Shore echoes — Shore echoes can be classified in
the same manner as bottom echoes.
(f) Wake echoes— Wake echoes can be a considerable
nuisance where many boats are operating in the same
small area.
(g) Wave echoes— In rough sea with big breakers air
will be "beaten" into the sea and cause the sonar beam
to be reflected. If the sea is very rough, wave echoes
may render the sonar useless.
(h) All fish give echoes, but to distinguish between
echoes received from different species is a very complex
matter. However, from the knowledge of differences in
behaviour between the various kinds and sizes of fish in
an area, it is sometimes possible to identify them by their
echo traces. Until now, sonar has most frequently been
used to detect herring, a typical schooling fish. In the
future sonar will, no doubt, be used for other species as
well. Herring schools vary in size and density but they
all yield very good and distinct echoes. When herring
appear on the spawning grounds, the fish usually spread
out during the night but accumulate in dense schools
during the day. At night, therefore, sonar is not a highly
efficient search instrument on the spawning grounds,
whereas in daytime it is often quite effective.
Practical instruction at sea
For practical sea training, an ordinary Norwegian
herring purse seiner of approximately 140 ft (approxi-
mately 300 tons), equipped with echo sounder and
SIMRAD herring asdic, was used. Because of limited
space, it was necessary to keep a strict timetable and
good discipline to give each man personal attention.
The training area chosen was an inshore locality with
little traffic and where temperature conditions permitted
good sonar conditions to be obtained with a range of
approximately 1,500 m.
Measurements of temperature gradients and judging
the resultant sonar conditions were part of the training
programme.
The training was carried out on artificial targets,
using a 1 m triplane as the target object. The depth of
the target was adjusted according to sonar conditions,
but in most cases the target was placed at a depth of
approximately 1 5 m with a marking buoy on the surface.
All participants had to serve in turn as skipper, helmsman
and sonar operator. This training was considered
essential to enable the trainees to fully appreciate the
necessity of close co-operation and to see to what extent
success is dependent on the skill and experience of these
three. During the first day of sea training, moderate
speed (4-5 knots) and helm were used in order to let the
participants practise methods and co-ordination in
slow motion.
All runs were commenced at a distance of about
1,000 m from the target and the first exercise was to
locate the target applying the standard searching methods
discussed in the classroom. Thereafter the vessel was
manoeuvred towards the target following the information
obtained by sonar. Each complete run, including a
subsequent discussion and criticism, had a duration of
about 20 min. On the second day at sea the speed was
increased to about 8 knots and the exercise consisted of
locating a herring school (i.e. the target) but without the
procedures necessary for the actual catching.
On the third and last day variable speed (10-0 knots)
was used and the complete sonar procedure for locating
and catching with purse seine was carried out.
The last phase of this procedure, which may be
termed the "attack" phase, differs according to whether
the seine is shot from the main vessel or from two domes,
and training was given in both methods.
General experiences from courses
The number of pupils varied but classes of approximately
20 pupils were most suitable.
It was generally agreed that an introduction to the
309
basic principles of underwater ranging and sounding was
absolutely necessary in order to make full use of the
possibilities of the sonar equipment. In addition, the
limitation caused by the varying environmental conditions
must be fully understood. In future courses the lessons
on fundamental theories should be given a broader
foundation.
The lessons given on classification and identification
of echo traces were evidently very useful. For efficient
instruction in this difficult field it is necessary that a good
supply of verified echo traces be available.
Some fishermen have, without receiving any instruction,
arrived at successful methods of search procedure and
sonar catching technique. However, the trial-and-error
procedure necessarily takes a long time before efficiency
—one to two years.
The importance of quick and precise handling of the
sonar controls should be stressed. A standardized
procedure of communication between the skipper,
helmsman and sonar operator is also important. Rapid
manoeuvring is absolutely necessary because of current,
wind, movements of the fish school and also because of
competition on the fishing ground. Moreover, in the
catching phase all relative movements are very fast since
the ship is close to the school. %
It is not possible in a short course to educate skilled
sonar operators but, with some training under sea
conditions and with experienced instructors, a foundation
is laid which will be useful to the fishermen in the
continued training aboard their own ships.
Discussion on
Purse Seining
Mr. £. A. Schaefers (U.S. A.) Rapporteur: Purse seines
have played an important role in the fish production of
various nations throughout the world for many years. Recent
developments, such as those described in Paper No. 79 by
Jakobsson on purse seining for herring in Iceland, and the
oral comments on anchovetta in Peru by Valdez during
earlier sessions, as well as developments elsewhere, indicate
this method may play an increasingly important role in the
ftiture.
The Icelandic fishery was a typical inshore one utilising the
traditional Norwegian two-boat dory system up to 1944.
Then, however, migration and behaviour patterns of north
coast herring changed drastically and the fish were not avail-
able to the traditional gear. The catch from purse seines
dropped from about 200,000 tons in 1944 to 10-40,000 tons
310
af^rward. That collapse forced a change from the two-dory
system to a one-dory system, which was replaced by the pre-
sent system utilising only the main boat.
The present method as described by Jakobsson is the result
of many inter-related factors, including organised fish search-
ing with horizontal echo-ranging equipment (asdic or sonar),
a new technique in asdic-guided purse seining for deep swim-
ming schools, mechanised handling of the nets by power
blocks and the use of deeper and stronger seines made of
nylon netting hung to 'Terylene* ropes. Asdic was first used on
Icelandic grounds by a Norwegian research ship in the early
1950*8 and was first installed in Icelandic commercial fishing
boats in 1954. Since then, many fishermen have learned to
shoot purse seines around deep swimming schools and after
1958 the method became fairly widespread. In 1962, three
vessels using asdic searched the fishing area continuously
and reported the location of fish to the fleet. It is agreed by
fishermen that over 50 per cent of the 1962 record herring
purse seine catch of 482,000 tons stemmed directly from this
work.
The methods now in use did not happen overnight. Con-
siderable effort was expended in adapting the power block to
the typical Icelandic vessel which has the wheelhouse very far
aft. Experiments in 1959 concerning shooting and hauling
the net from the upper deck aft of the wheelhouse were
successful and that arrangement has been used throughout
the Icelandic fleet, with the exception of boats less than 85
feet long which have no upper deck. On these, enough space
was provided aft of the wheelhouse and in the starboard
passageway.
The potential value of sonar for locating fishable concentra-
tions of herring was also realised in the early 1950's by
Norwegian fishermen following experiments aboard the
research vessel G. O. Sars. Vestnes indicates that although
vessel owners soon began to equip their vessels with sonar,
these vessels were not immediately successful. It was felt
this was probably because the skippers at first failed to master
the proper operational techniques. Accordingly, a course of
instruction in the use of sonar was organised by the Institute
of Fisheries Research in Bergen, in co-operation with a
commercial firm. This eight-day course, followed by additional
experience aboard ship, has resulted in the development of
skilled sonar operators in considerably less time than the one
to two years required through the trial and error procedure.
I am certain that much more will be heard concerning
developments in purse seine fishing throughout the world.
I know research and developmental efforts on this type of
gear, for various species of fish, have been underway off the
United States east coast, Chile, Peru and other places. These
efforts have, in many cases, gone beyond the experimental
stage and have resulted in totally new fisheries or in increasing
the efficiency of the gear in existing fisheries. Along the east
and Gulf coasts of the United States the menhaden industry
has adopted several new developments which have improved
the efficiency of this fishing method. Two of these were the
power block for retrieving the net and the mechanical net
lift from the topping boom to dry out the fish in the centre
or bag section of the net so that they could be pumped into
the carrier vessel more economically. Some additional
developments adopted by the industry which have contributed
to a more efficient operation include the automatic pilot,
two-way radio-telephones for communication between the
spotter aeroplane, carrier vessel and purse boats, and hydrau-
lic-ram operated boat falls for raising the purse boats.
The conversion, and subsequent success, of the California
based "clipper" fleet of pole and line tuna vessels to purse
seiners has been well documented. We should like to hear
more concerning these and other efforts from the various
experts assembled here.
Mr. R. F. Allen (U.S.A.): I think the Icelandic fishing
methods described in Mr. Jakobsson's paper might bo im-
proved by use of a heavy weight on the bottom purse line to
enable the net to be pursed at a lower depth than can be
accomplished when the weight is evenly distributed along
the net. In the menhaden fishery, as carried out by two boats,
they have a torn weight of approximately 700 pounds, which
slides down the purse line from the purse davit after the seine
is set and the two purse lines are hooked to the winch. The
weight enables the net to be pursed at a lower depth which
would be of considerable advantage when the herring purse
seines go down to 30 to 35 fathoms. This system was used and
abandoned in Iceland about 20 years ago but recent changes
indicate it should again be considered. Purse seining for
menhaden has been highly mechanised with two-boat opera-
tions and there has been little attempt to use the one-boat
system, at least to date. In South American waters a big
Norwegian vessel, of 350-ton capacity, recently operated
with two purse boats and caught some 32,000 tons of ancho
vetta in 8 to 10 months. Because of the different labour
conditions and other factors it is possible the operation was
not a financial success. Subsequent vessels used a system
similar to the Icelandic purse-seining methods but they had
not proved completely applicable. The anchovetta were very
difficult fish to bring up to the surface to concentrate for
brailing — more difficult than herring or South African pilchard.
That put a great deal of strain on the nets which made it
difficult to use the power block as a method of concentrating
the fish in the net. In the typical South American craft winches
were used and the strain was terrific during their operation.
He thought a number of additional refinements could be
made in purse seining, particularly in relation to tuna seiners
which handle nets that weighed up to 20 tons. Purse seining
for tuna is also being attempted in African Atlantic waters.
A number of changes are taking place in present techniques
to adjust to the different fishing conditions found in that
area. He would like to know more about those changes
because the American vessels fishing in those waters had not
been eminently successful.
Mr. B. Petrich (U.S.A.): While the principles of purse
seining are more or less the same the world over, conditions
vary. For example, he had been asked whether the Icelandic
net would be good for tuna fishing. He thought it could be
in principle but various problems must first be overcome,
particularly with regard to drying up the fish. Several sugges-
tions have been made for solving these problems, but they
have not been tried under actual fishing conditions. One
suggestion he mentioned was concentrating the fish by using
lines working as an endless belt through the power block.
When the weight of the fish becomes excessive, attach a
hook to the endless line, and with straps attached to the net,
bring it into the winch. It might be necessary to use the West
Coast type of strap to get the fish close enough to the surface
for brailing. He pointed out that tuna purse seines of very
large size are now being made. One, which he had just helped
make, weighed 30 tons before treating the net. Vessels
operating these are from 1,100 to 1,200 tons which usually
have a seine skiff at least 30 feet long. The use of such a
large net is possible only because of the advances over the
years in the strength of winches, nets, cables and, of course,
introduction of the power block. He told of one haul of 350
tons of tuna which was handled with little difficulty. Purse
seining for tuna is now more successful by fishing deeper,
and they are now developing even deeper but much lighter
nets for use in African waters. They had been working at
depths of 35 fathoms but were now getting down to 50 fathoms.
He thought purse seiners needed heavier winches and that
this was perhaps one area where they had fatten behind.
Although some winches were 50, 75 or even 100 hp
they could be made much larger and more powerful. With
larger winches it would be possible to use half-inch cable in-
stead of three-eighths-inch cable. This would give extra
strength and additional weight to help take the net down to
maximum depth. Successful purse seining under today's
conditions for big fish such as tuna means they must have very
strong boats, strong equipment and all necessary supplemen-
tary power.
Mr. Jakob Jakobsson (Iceland): I want to emphasise that
special asdic research in Iceland was begun in 1950 by the
Norwegian research vessel, and was first used by our com-
mercial fleet in 1954. With reference to the power block not
being successful because of the heavy weight in the net, he
would like to ask why could it not be used to bring the net
up a certain distance and then turn to the old methods for the
last part of the net?
Mr. R. Lenicr (France): He described purse seining exper-
iences for tuna in Atlantic tropical waters. One French vessel
had caught 500 tons and he emphasised that for fishing this
area they must get the right type of boat for the job. Speaking
of one model for a tuna vessel he had seen, he considered it
would never be able to do purse seining because the decks
were cluttered up with wells for live bait.
Mr. Mareschal (France): You cannot judge a boat from a
scale model. The model to which Mr. Lenier referred was a
clipper built craft and it could easily be changed into a purse
seiner. All that needed to be done was to remove the live
wells and put up a platform and place a power block in the
proper spot.
Mr. R. K. N. Ocran (Ghana): In our experience all nets
need to be adjusted to meet local conditions. One expert
who had criticised their own practice of 30 per cent slackness
in the web found that the heavy net he recommended got
stuck on their rough grounds. Similarly, others who had
originally criticised their nets changed to the Ghana system
after gaining some local experience. In Ghana they were now
using a Black Sea type of purse seine. It was smaller, with
heavy purse rings but no leadline and the damage to their
nets on rough grounds had been reduced considerably. They
used a helper boat — an 18-foot motorboat with a 22-hp
engine which cost about £2,000. Sometimes water currents
in the fishing area ran to the east and the fish went to the
west, with the wind yet in another direction. Because of the
nature of these conditions, unless they had the helper boat,
they would find fishing too difficult to be practicable. Those
who were going to develop and build equipment for use in
Ghana should first study conditions there.
Dr. Haim Levy (Morocco): One problem that we have in
purse seining for tuna is the difficulty of pulling in dead fish
lying at the bottom of the net. Sometimes we get a catch of
20 to 25 tons. We have then been compelled to call other
boats to help. I would like information on how best to transfer
tuna from the net to the deck of the boat when you have dead
tuna lying at the bottom of the net.
Mr. Keotaro Hannahlm* (Japan): We are greatly concerned
with the possibilities of mechanisation. In 1955 we discovered
311
a fishing ground in the East China Sea suitable for purse
seining. This was an offshore operation which required a
voyage of 20 to 25 days. The catch had to be delivered directly
from the fishing grounds to market by means of carriers. We
had to reduce the number of the crew and provide them with
adequate accommodations on the boat. We considered the
possibility of introducing the power block system. However,
due to peculiarities of our net the power block did not suit
our conditions, and we had to develop our own system in its
place. The problem of hauling the net was solved by fixing a
big V-grooved net roller onto the stern. The roller was driven
hydraulically. In our first experiment, the roller was not
properly made and the net was entangled on one side when
hauled up. This problem was cured when we fixed rubber
bands radially on the surface of the roller. By these methods
we succeeded in reducing the number of crew from 30 to less
than 17.
Subsequently an improvement was made by the addition
of a horizontal net roller athwartship riding on raised rails
on either side of the net bin at the stern of the vessel. This
roller aids the flaking down of the heavy and very big net
—up to 1000 fm long by 100 fm deep. With this latest method
the net is still hauled aboard by means of the V-grooved
sheave, and then passed over the power driven horizontal
roller for stacking. (See Fig).
But we have two problems yet unsolved. In our experiments
the total weight of the apparatus could not be reduced below
four tons. As this affects the stability of the boat, there were
both direct and indirect limitations to offshore operations.
We are considering further possibilities of reducing the weight
in the future. In the second place, a further reduction of man-
power depends on how we can mechanically move our catch
from our boat to the carriers. To do this we are considering
the use of a fish pump.
Mr. S. Remoy (Norway): He pointed out the difficulty of
the boat setting the purse seine net keeping out of the bay or
bight of the net. In experimental purse seining in South-West
Africa he had encountered the problem and it also existed in
Norway. He had suggested that an air propeller should be
mounted on top of the wheclhouse to provide the power
to keep the vessel out of trouble and his inquiries showed
it would take a 15 to 20-hp engine to provide the 1,800 or
2,000 revolutions per minute necessary. As it was, some
Norwegian vessels were employing a second vessel to assist.
This increased the cost of the operation.
Mr. Jem Frecbet (Canada): To prevent vessels drifting
towards the centre of the seine, we, in Canada, introduced
312
water-jet nozzles. They are fitted beneath a vessel with pumps
working from independent energy or from an electric motor.
Such water jets develop from 500 to 2,000-lb thrust so
by the addition of a number of them it has been possible to
forget the use of tow boats normally used in this technique
in order to take vessels out of the seine circle. Canadian vessels
purse seining for herring had completely abandoned the use
of turntables when they found that with an open deck they
could handle a high purse net. This gave much more flexibility
to the boat and better stability. He would like to know why,
in many countries, turntables were still in use.
Mr. E. A. Schaefers (U.S.A.): Even some of the most
modern and largest United States tuna seiners use turntables.
We should not overlook the fact that there is considerable
variation in the design of tuna seiners and, on some, the use
of a turntable is necessary for efficient operation. Some tuna
seiners even use two power blocks, one forward and one aft.
Some seine fishermen probably still use turntables out of
tradition.
Mr. Petrich (U.S.A.): In the United States, boats on the
west coast use power skiffs to keep the big boat out of the net.
These boats are equipped with 300 or even 400-hp engines.
The skiff is also used in the brailing operation which makes
the job much easier and safer. Turntables are used by some
craft and not by others; there are reasons for both points of
view. He personally thought there were advantages with the
turntable — the structure of the boat and the room on deck
all came into it. As regards the transfer of dead fish from the
tuna net, he would be glad to personally explain the details
of the procedure to Mr. Levy. There is no real problem pro-
vided fishermen have the experience and the proper equip-
ment. As compared with the Moroccan catch of 10 to 25 tons,
he had brought up 1 50 tons of dead fish in one shot.
Dr. Schirfe (Germany): We are interested in this method
of fishing, although in Germany we have not had much
experience in purse seining. Our new research vessel which
will be basically a large sterntrawler will therefore also be
equipped for both purse seining with a large purse seine from
the big boat itself and with smaller purse seines according to
the dory system. This may be an example where in my opinion
the two-boat system is unavoidable. With a boat about 83-m
long and a high superstructure it would be very difficult
to purse seine a small and light net if there is any wind and/or
currents. German experiments near Iceland with vessels of
the small trawler size, show that this is already a problem
and experiments along these lines have not been too promising.
There seems to be a limit with regard to the size of the vessel
up to which the one-boat technique can be successfully used.
To cope with the problem of keeping a big vessel out of the
centre of the purse seine they intend to use power dories as
helper boats.
Their new research vessel, however, would be equipped
with a Pleuger active rudder of 250 hp as well as a bow
stream rudder of 400 hp that would give something
along the lines of the Canadian device for keeping the vessel
out of the circle. These devices would provide 650 hp
to push the boat from the net. He had heard the Norwegians
had used a new-style rudder and had found the net was fouled.
He was therefore relieved to hear that the water-jet system
worked in Canada. What size were the Canadian vessels
that were kept out of the net by the 2,000-pound water jets?
Mr. Frecbet (Canada): The system was originally used on
very small boats, mainly 45 to 55 ft in length, but recently it
had been installed on a 91 -ft steel tuna purse seiner which is
still under construction. We hope good results will be
obtained.
Mr. H. Kristjonsson (FAO): Referring to drawings of the
new Icelandic purse nets shown in Jakobsson's paper, he
called attention to the fact that the net tapered sharply
towards the bunt end. The reason for this was that while the
power block could pull in the net lengthwise (i.e., pull on
board the wing and main body), it could not be used for
"drying-up" the net under the bunt. On conventional rect-
angular nets this required hand labour and was time consum-
ing. In Iceland a rational solution was found in simply elimina-
ting the net below the bunt, which served no useful purpose
anyway. Consequently the power block could be used to
crowd the fish in the suitably shallow bunt end so it could be
brailed with little or no manual "drying-up". This saved not
only labour, but also time which was of even greater impor-
tance when handling big catches in difficult sea conditions,
and helped to avoid trouble from the fish dying and sinking
to the bottom of the net. He thought this feature could be
adopted advantageously in other purse seine fisheries where
power blocks were used.
Mr. J. Jakobsson (Iceland): Under normal conditions and
even in considerable current, the Icelandic fishermen managed
quite well without any help from other vessels. 1 think this
is due primarily to their skill in ship handling and their
complete understanding of wind, current and sea. There may
be conditions when their chance of a catch is much greater
if they shoot in such a way that there is danger of the boat
becoming entangled and a skiff might be necessary to prevent
this. A film which he could show revealed this very clearly
but in those cases a small skiff was usually quite sufficient
to pull the ship away from the net or the net away from the
ship. He was, however, very interested in the use of water
jets and air propellers, and any other devices which might
be considered.
It would be much easier for the captain to press a button
and get things going than to have to lower a skiff. Jn one-boat
power block fishing there was an enormous difference in
relation to individual nets. Some nets tended to fall into a
bad shape whereas other nets were much easier to purse.
Mr. Valdez (FAO): In Portugal they use auxiliary boats to
avoid the boat becoming involved with the net. Another reason
is that whenever big catches are caught the small boats can
help with the catch and, further, those small boats act as
carriers of the fish to port while the main boat stays on the
fishing ground. The use of the second boat is really very econo-
mical and is generally practised wherever there is Portuguese
influence. In Peru they were using only United States gear
but in Chile they had the Icelandic type, the Norwegian
type and the United States type. The Norwegian vessel with
two dories had done very well lately, averaging 350 tons per
day, while the local boats had averaged 60 tons. The most
common boat in Peruvian fishing is 65 ft long. They also have
the problem of fishing in shallow water. Fishing goes on all
the year but there are two big seasons, one in December/
January and the other in June/July. A new, promising fishing
ground had been discovered but it had a rough bottom and
the fishermen were afraid to go there for fear of damaging
their nets. He was interested in discovering a gear which could
be used for fishing such rough ground.
Mr. Schaefers, Rapporteur, summing up: When I noticed
that there were only two papers on purse seining when there
were evidently so many developments going on throughout
the world, 1 was apprehensive that this important method of
fishing would not be thoroughly discussed. My apprehensions
now appear to have been groundless and we are thankful to
all of you who have enlightened us as to the various methods
used throughout the world.
After recapitulating the points made by each speaker, Mr.
Schaefers gave a rgsumg of events that led to the development
of a purse seine fishery for tuna in New England. Sporadic
attempts began in the late 1930's but a variety of conditions
interfered with the operations. Such things as hurricanes,
differences of the schooling habits of the fish from year to
year — all these factors led to unsuccessful initial attempts.
In 1958, however, a 62-ft Gulf of Mexico shrimp trawler was
rigged to tuna purse seining. Conditions that year were favour-
able and the fish appeared in abundant schools so that a
successful season was experienced by this particular vessel.
Since that time other vessels have entered this fishery and
although it is still small in relation to the total world tuna
catch, more vessels are entering the fishery each year. It was
quite apparent that the development of purse seining was still
taking place throughout the world. It must be adapted to
local conditions; a fact which cannot be over-emphasised.
313
Put 2 Bulk Fish Catching
Section 10: Deck Machinery
The Application of Hydraulic Power to Fishing Gear
Abstract
Hydraulic systems for fishing vessels may be classified as low pres-
sure (200 to 550 psig), medium pressure (1,000 to 2,000 psig) and
high pressure (3,000 to 5,000 psig). Of these, medium-pressure
systems are generally favoured in the United States because of
equipment availability. Of the two basic types of systems, the open
loop system is favoured over the dosed loop system because it is
more adaptable to driving a number of different devices. The prin-
cipal advantages of medium pressure hydraulic power for fishing
vessels over other types of power are: small size, light weight,
reliability, performance under varying conditions and the wide
degree of control available. Pumps convert rotary mechanical
power from electric motors or engines to flowing fluid under pres-
sure. Hydraulic motors convert the fluid power to rotary mechanical
power; hydraulic cylinders convert the fluid power to linear mecha-
nical power. The front power take-off on the main engine is the
most common method of driving hydraulic pumps in small fishing
vessels, although on some of them, such as gillnetters or seine skiifs,
automotive power steering pumps are directly connected to the
engine to drive reels or rollers. Larger vessels that have sufficient
generating power to drive electric motors often use electro-hydraulic
power units. The oil reservoir or tank usually has a capacity in
gallons equal to the gallons per minute flow in the system. Double
bottom tanks should not be used as reservoirs because they are
impossible to clean adequately. The most important factor for the
protection and long life of hydraulic pumps, motors and control
valves is the maintaining of clean oil at all times. Hydraulic power
installations vary from simple single pump systems to compound
pump systems, depending on the type of vessel. Illustrated examples
are given of hydraulic gear handling machinery, such as gillnet reels
and longline; a number of power units on a modern purse seiner
including a power block, purseline winch, corkline winch, anchor
winch, braihng boom winch, boom vang winches, etc., many of
which are remotely operated from a control console; also crab pot
haulers on hydraulically activated boom; salmon purse seine reel;
as well as deck machinery on a dmmtrawler.
Apptications de to puissance hy drauliqoe dans to p&fae
Les systemes hydrauliques employes sur les bateaux de peche
peuvent 6tre classes en basse pression (13,6 a 37 atm), moyenne
pression (68 a 1 36 atm) et haute pression (210 a 340 atm). Le systeme
a moyenne pression est gtatralement utilise* aux Etats-Unis parce
que ftquipement est disponibte. Des deux principes de base, le
circuit ouvert est pr6f6r6 au circuit ferra& parce qu'il est plus facile
a adapter pour faire fonctionner en m6me temps, plusieurs appareils.
Les principaux avantages de la puissance hydraulique & pression
«, pour les bateaux de peche sont: petites dimensions,
, stoete* , bons r6sultats sous des conditions varites et grandes
ites de contr61e. Les pompes convertissent la puissance
infcanique rotatrice, venant des machines ou moteurs dectriques,
en puissance fluide sous pression. Les moteurs hydrauliques
transformed la puissance fluide en puissance mfeanique rotatrice;
des cylindres hydrauliques transforment la puissance fluide en
puissance rafeanique lintaire. Dans les petits bateaux on prend
gfofratement la puissance directement de la machine principalc
pour actionner les pompes hydrauliques, bien que sur certains,
comme les bateaux pftchant au filet maillant et les skiffs de senne,
les pompes de commande automatique sont reuses directemente a
la machine pour actionner les tambours et les rouleaux. Les
grands bateaux qul ont assez de puissance g6n6ratrice pour faire
fonctionner les moteurs electriques utilisent souvent des unites de
puissance 61ectrp-hydraulique. Les reservoirs d'huile ont normale-
ment une capacite en litres 6gale a r&oulement en litres par minute
du systdme. Les reservoirs a double fond ne devraient pas toe
utilises parce qu'il est impossible de les nettoyer complement.
Le facteur k plus important pour la protection et la duree des
pompes hydrauliques, des moteurs et soupapes de contrdle est
de maintenir rhuife constamment propre. Selon le type de bateau,
les installations de puissance hydrauliques varient du systeme a
une seule pompe, simple, a des systemes de pompes complexes.
Des illustrations donnent des exemples de machines hydrauliques
314
by
D. W. Lerch
Marine Construction & Design
Co., Seattle
pour manipuler les engins comme les tambours & filets mailla;its el
palangres, les unites de puissance d'un senneur moderne comprenant
un power-block, un treuil pour lignc coulissante, un treuil pour
ralingues a liege, un treuil, d'ancre, un treuil pour actionner les
bigues, etc., et dont plusieurs sont actionnes a distance depuis un
tableau de contrdle ainsi que les Icvcurs de nasses montes sur les
biques actionnes hydrauliquement, les tambours pour les sennes &
saumon et les machineries de pont sur un chalutier a tambour.
La apUcacion de la fuerza hidraulica a los equipos de pesca
Extracto
Los sistemas hidraulicos para embarcaciones pesqueras pueden
clasificarse como de baja (200 a 550 lbs/pulgadaa), mediana
(1.000 a 2.000 Ibs/pulgada2) y alta presi6n (3.000 a 5.000 Ibs/
pulgada2). De estos, los sistemas de mediana presi6n se prefieren
en los Estados Unidos ppr la facilidad con que se encuentran en el
comercio. De los dos tipos basicos, el de derivaci6n abierta se
prefiere al de derivaci6n cerrada porque es m£s a proposito para
accionar diversos dispositivos. En el caso de los barcos de pesca,
las ventajas principales de la fuerza hidraulica de presi6n mediana
sobre otras clases de fuerza son: pequeflas dimensiones, poco peso,
seguridad, rendimiento constante en condiciones variables y
amplitud de regulation. Las bombas transforman la fuerza mecanica
circular de los motores eldctricos en un fluido que se mueve a
presi6n. Los motores hidraulicos transforman la fuerza del fluido
en fuerza mecanica circular y los cilindros hidraulicos la convierten
en fuerza mecanica rectilinea. Una toma de fuerza del motor prin-
cipal es la manera mas comente de accionar las bombas hidraulicas
de pesqueros pequefios, aunque en algunos de ellos, como los que
emplean redes de enmalle o los auxiliares de los cerqueros, las
bombas automotrices se concctan directamente al motor principal
para mover rodillos o carreteles. Las embarcaciones mayores que
general fuerza suficiente para mover motores eletricos emplean con
frecuencia grupos electrohidaulicos. La capacidad del deposito de
aceite es por regla general igual al flujo en litros por minuto en el
sistema. No deberian emplearse depositos de doble fondo porque
es imposible limpiarlos bien. El factor mas important^ para prote-
ger bombas. motores y valvulas de regulacion hidraulicos es
mantener el aceite siempre limpio. Las instalaciones hidraulicas
varian desde la bomba unica sencilk a sistemas compuestos de
varias bombas, segun la clase de barcos. Se dan ilustraciones de
dispositivos para manipular sistemas hidrdulicos como los emplea-
dos en los carreteles de las redes de enmalle y de los palangres, y de
varios motores hidraulicos usados en un cerquero moderno que
comprenden el motbn mecanico, la maquinilla de la jareta, la de
la reUnoa de corchos, el molinete, la maquinilla de la botavara de
los salabres, la maquinilla de las ostas de la botavara, etc., muchas
de las cuales tienen telemando en un cuadro central; asimismo se
dan ilustraciones de haladores de nasas en una botavara hidraulica,
el carretel de un arte de cerco para la pesca del salmdn y a Imaqui-
naria de cubierta de un arrastrero.
1. INTRODUCTION
The history of industrial revolution in all extractive
industries such as mining, fanning and lumbering has
shown a transformation from manual labour to mecha-
nisation. Machines, such as the wheat combine or the
mining machine that can dig a hole 20 feet in diameter
through the side of a mountain, have been developed
to permit fewer men to produce more for the greater
good of all. The fishing industry, basically an extractive
industry for the harvest of a natural resource, is one of
the last industries to mechanise. It is only natural that
we today see a tremendous expansion in fisheries tech-
nology and the development of improved boats, mecha-
nised gear and better tools for search, harvesting,
preparation and marketing.
Progress in fishing gear development, as in all other
marine industries, is slow because of the conservatism
of seafaring men. Nevertheless great strides have been
made in the application of power and mechanisation
on board fishing vessels (McNeeley, 1961). These
advancements, together with the fruits of other techno-
logies such as more reliable engines, remote controls,
alarms and warning devices, electronic gear and so
forth, enable the unit effort per crew man to reap richer
harvests from the sea. Higher productivity, lower costs
and greater profits lead to expansion, increasing economy,
higher standards of living and more jobs.
Physiological studies have shown that a man can
develop approximately one-half horse-power for a 30-
minute duration. Peak bursts of energy for extremely
short periods in a well-conditioned athlete perform
work at the rate of two to three horse-power. Let us
assume our man weighs 160 pounds .and occupies at
least 12 cubic feet aboard a vessel. If this one man is
replaced with hydraulic power, we can supply as much
as 12 horse-power — or the equivalent of 24 men as far as
work being accomplished is concerned. Of even greater
importance is the fact that hydraulic horse-power is
available continuously, day and night, not subjected
to periodic fatigue and exhaustion.
This paper will deal with a small facet of the general
industrial revolution sweeping the fishing industry;
that is, the application of medium pressure hydraulic
power for fishing gear.
2. CLASSIFICATIONS OF HYDRAULIC SYSTEMS
For the purposes of this paper hydraulic systems will
be classified as follows:
2-1 Low pressure— 200 to 550 psig (28 kg/cm2 to
39 kg/cm2). This type of equipment is characterised by
large pumps and motors, direct coupled and turning at
slow speeds. The pumps, normally direct driven by a
slow speed engine or through gear reducers, operate
at between 200 and 400 revolutions per minute. The
pumps weigh about eight to 15 pounds per horse-power.
Due to the low pressure a considerable volume of oil
must be circulated in order to produce the required
horse-power, consequently the piping lines are large in
diameter in order to minimise the pressure loss caused
by high oil velocities. A total cumulative pressure drop
in the system of 50 to 100 pounds per square inch can
rob the transmission system of 25 to 30 per cent of the
total horse-power. One of the most desirable features of
the low pressure system is the quietness of operation
because the motors are low in speed and no gears are
required between the motor and the winch. These low
pressure systems have proven very reliable and satis-
factory, particularly in the high horse-power ranges
(Anon., 1962). Normally the winch and pump are made
by one manufacturer and designed for a particular job.
When more than one winch is used on a vessel, the winches
are piped up in series so that the oil flows first from one
winch, then to the next winch, etc., and back to be
recirculated by the pump. The large size piping and
control valves mounted directly on each winch preclude,
to a great extent, the ability to provide simple remote
controls.
Figure 1 illustrates the relationship between pressure,
volume and horse-power.
2*2 Medium pressure— 1,000 to 2,000 psig (70 to
140 kg/cm8). This type of equipment is generally
favoured in the United States due to the commercial
availability of motors, pumps, control valves, cylinders
and associated equipment. The equipment used for
the powering of fishing gear falls generally into three
classifications— farm, mobile, and industrial. The "farm"
type equipment is limited to valves and cylinders.
Cylinders are available with stainless steel rods for cor-
rosion protection but otherwise there is little attempt
to produce corrosion-resisting equipment. The "mobile"
classification includes pumps, motors, valves, power
boost steering cylinders, filters, etc. Mobile valves are
available with a limited number of spool configurations;
however, they have exceptionally good throttling charac-
teristics and are very widely used in marine applications.
The "industrial" classification of equipment includes
pumps, motors, cylinders and all of the more sophisti-
cated valves. With pressures four to five times greater
than the low pressure system the oil volumes required
are from 20 to 25 per cent as great, consequently smaller
pipe lines may be used. For instance, up to 100 horse-
power can be handled with one and one-half inch pipe
and hydraulic hose.
2-3 High pressure— 3,000 to 5,000 psig (210 to 350
kg/cm2). This type of equipment is normally used in
the United States for high horse-power cargo winches,
steering gears, windlasses, etc. In order to utilise such
high pressures, expensive piston type pumps and motors
are necessary. These pumps and motors have extremely
fine tolerances and consequently are extremely suscept-
ible to any dirt in the system. The manufacturers of this
type of equipment require full filtering of the oil to
10 microns (0*0004 inches). This requirement for such
fine filtration produces extreme problems on a fishing
vessel, and indeed, sometimes for the manufacturers
themselves; consequently the most successful systems
have been the "containerised" or packaged units (Short,
1959). These units utilise an electric motor driving a
variable displacement pump in a dosed loop transmis-
sion system to a hydraulic motor. The pump motor,
valves, controls and tank are all mounted on a common
315
500
400
100
200
100
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o
70
40
10
10
10
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Fig. 1. Graph of hydraulic horse-power.
1400 1000
sub base with the winch, tested and filled with oil, thus
ready to be placed on board ship needing only the elec-
trical connections to the motor to be ready to operate.
The manufacturer in this manner has complete control
and responsibility for any dirt in the hydraulic system.
It is only the very occasional fishing vessel tnai is
equipped with piston type equipment. Some large
heavy duty seine or trawl winches are driven by low
speed, high torque, large displacement, radial piston
motors to reduce the amount of reduction gear required.
Piston pumps and motors weigh between three and 10
pounds per horse-power and cost roughly two to six
times as much as medium pressure vane type pumps and
motors of comparable power.
3. TYPES OF HYDRAULIC SYSTEMS
Before progressing further with the discussion of hydrau-
lic circuits, it is desirable to refer to Appendix 1, Standard
Graphical Symbols. These symbols, called JIC
symbols, were developed by the Joint Industry Confer-
ence, a group of American industries that have banded
together to establish specifications and standards.
Most hydraulic valves are spool type valves — that
316
is, a spool is moved within the housing to provide the
various functions, such as directional change, response
to pressure, relief valve discharge, etc. The rectangular
block in the JIC diagram represents the spool. Appendix
2 compares some of the most useful JIC valve symbols,
with schematic drawings of actual valves.
3*1 Closed loop system
There are two basic types of circuits, the first of which
is the closed loop system (Figure 2). This is most com-
mon for fixed transmissions where the pump is a variable
volume pump and the stroke is controlled over centre
to reverse the motor. A small supercharging pump keeps
a positive pressure on the pump suction. A relief valve
normally set between 50 and 100 psi maintains a constant
pressure which is fed to the suction side of the pump
through the crossed check valves. An overload relief
valve, again supplied by crossed check valves, protects
the system from overload.
3-2 Open loop system
Opposed to the closed loop system is the open loop
system (Figure 3). Here the pump takes suction from
the tank, the discharge is directed to the motor through
a directional control valve and the oil returns directly
CLOSED LOOP
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317
to tank from the control valve. Again, the pump is
protected by an overload relief valve.
The open loop system is by far and large the most
practical and the most used for driving fish-boat machi-
nery, since it is very adaptable to driving a number of
different devices.
(a) Open centre, series cfrorit. The open loop system
can be further divided into two types, the first of
which is the open centre, series circuit (Figure 4).
The pump circulates oil continually through any
number of control valves, all piped up in series.
The pressure in the system is held to a minimum,
being only that required to circulate the oil through
the piping and valves. When a control valve is
shifted, oil is directed to the motor or cylinder.
The pressure in the system is dependent upon
the load on the motor or cylinder. This is a
constant volume, variable pressure system.
(b) Closed centre, parallel circuit. Figure 5 illustrates
the closed centre, parallel circuit. Here the pump
maintains a constant pressure in a header. Any
number of valves are connected in parallel off
the pressure header. The flow is blocked when
the valve is in neutral. When the valve is shifted,
oil at a constant pressure is directed to the motor
or cylinder. A pressure compensated flow control
valve is usually required ahead of each device to
control the amount of oil that can be delivered to
the motor or cylinder in order to control the speed
with which the device is actuated. The flow of oil
in the header is dependent on the load. This is a
constant pressure, variable volume system.
This type of system generates a good deal of heat when
a device is actuated under very little load. For instance,
let us assume that the motor shown is operating a winch
being run under no load. Assume that the system pressure
is 1,000 psig (pounds per square inch gauge) and the
flow control valve is set for 30 gallons per minute. Allow
100 psi to idle the motor, then 1,000 psi minus 100 psi
times 30 gallons-— or approximately 16 horse-power
(see Figure 1)— will be converted into heat as the oil is
throttled through the flow control valve. Closed centre
systems invariably require heat exchangers to control
the oil temperature.
The main advantage of a closed centre system is the
ability to power a number of devices with widely differing
oil volume requirements. The disadvantages include
excessive heating, as described above, and the necessity
for either a pressure compensated, variable volume
pump to maintain the pressure in the header or a system
of accumulators and unloading valve to maintain the
pressure in the header. Figure 6 illustrates a typical
accumulator-unloading valve system. Here the large pump
pressurises the circuit and the accumulator (or accumula-
tors). An accumulator is a container in which the hydrau-
lic oil is stored under pressure by compressing a gas or
spring. When a preselected pressure is reached, the
pressure in the pilot line opens the unloading valve and
the discharge from the large pump is bypassed directly
318
back to the reservoir. The small pump maintains the
pressure in the system and makes up for any leakage
through the control valves. The excess flow from the
small pump discharges back to tank over the relief
valve. Since only a small flow at high pressure is going
across the relief valve the horse-power lost, or heat
generated, is kept to a minimum. The large pump is
operating under no pressure and the system horse-power
is kept to a minimum when at idle.
Henceforth all circuits discussed will be of the open
loop system, open centre, series type circuit unless speci-
fically defined as a closed centre system.
4. ADVANTAGES
The principal advantages of medium pressure hydraulic
power, particularly for fishing vessels, are :
4-1 Size
Hydraulic motors are extremely compact in size and
light in weight for horse-power delivered. As an example,
440-volt, three-phase, 60-cycle electric motors rated
between 1,200 and 1,800 rpm weigh between 20 and
30 pounds per horse-power in the range between 10 and
100 horse-power. High speed diesel engines (1,800 rpm)
also fall in this range of 20 to 30 pounds per horse-power.
Hydraulic pumps and motors such as are being discussed
in this paper weigh between one and two pounds per
horse-power. These are 1,500 to 2,000 psi, 1,800 rpm
motors of between 10 and 70 horse-power.
This small size and light weight makes possible the
application of power wherever it is required and permits
the development of such devices as the power block,
boom mounted winches, pot line haulers, etc. Countless
devices undreamed of today are feasible with such light
weight small prime movers.
As with all rotating machinery, for a given horse-power,
size and weight varies inversely as the function of the
rotational speed, or, stated more simply, the faster the
motor or pump rotates and the higher the pressure,
the greater the horse-power transmitted for an equal
size and weight.
4-2 Reliability and performance
Hydraulic pumps, motors and control valves have all
the major working parts fully encased and working in
an inhibited lubricant, thus providing extremely long
life and reliability. Certain common sense safeguards
must be observed for the protection and long life of the
components. The most important and critical of these is
the necessity for maintaining clean oil at all times. More
damage can be done to the finely machined polished
surfaces of the hydraulic components in the first hour of
operation after starting up an improperly cleaned system
than 4,000 or more hours of proper, clean operation.
Normal production hydraulic equipment can, and
often does, operate fully submerged in salt water. Most
inexpensive, commercially available, mass-produced
components are constructed out of cast iron or steel
and hence are extremely susceptible to rust and cor-
rosion on board ship. However, due to the excellent
seals required to contain the oil, we very seldom find
any damage inside motors or other components that can
be directly attributed to salt water or to the atmosphere
in which they are operating. It is common to find, on
poorly maintained vessels, what appear to be piles of
rusty scrap iron still performing as well as the day they
were installed.
It is extremely difficult to give concrete appraisals of
the life of hydraulic components. The service, or duty
cycle, is always intermittent on a fishing vessel and most
devices, even though they may run many hours a day,
are only operating at maximum capacity for short periods
of time. Most of the relatively inexpensive mass-produced
pumps and motors commonly used with fishing gear are
designed to operate between 3,000 and 6,000 hours at
full load. The more sophisticated piston type motors
often have a full load design life of between 7,000 and
15,000 hours.
4-3 Controls
Perhaps of even greater importance to the fishermen is
the wide degree of control available with hydraulic
systems. Stepless speed control from stop to full speed
with full torque is acknowledged as one of the prime
advantages. Properly designed control valves that have
good metering or throttling characteristics — that is, the
ability to divert differing amounts of oil to the actuator
and bypass the remainder — give this precise control of
speed. A great variety of spool types and shapes and
valve configurations afford the designer and operator
a wide choice of performance, function and cost. For
instance, f-inch control valves suitable for 20 to 25
horse-power vary in price from $20 to over $100, depend-
ing on style and function.
By controlling the maximum pressure in a system,
relief valves effectively limit the maximum torque or
force than can be developed. In a winch circuit the relief
valve can absorb sudden loads placed on the gear due to
the ship's rolling or heaving, thus protecting the winch and
the cables. Winches or other hauling devices can be set to
either exert a constant force that will pay in when the load
is reduced, or pay out when excessive loads are encountered.
Electric motors are capable of up to three times the
normal torque and diesel engine drives will produce
increasing torque until the engine is stalled or slowed
down. Neither can be properly controlled on a fishing
vessel.
This ability to exert a constant force is particularly
advantageous to net and pot haulers where excessive
loads suddenly applied due to the ship's motions could
easily cause the failure of gear or the loss of the net or
pot.
Other control valves useful in certain applications
include:
(a) Pressure reducing valve— Maintains a constant
reduced pressure downstream from the valve
regardless of the upstream pressure. These valves
are used primarily for control circuits or for control
of torque or cylinder force where it is not desirable
to use a relief valve.
(b)
maintains constant flow (within approximately
three to ten per cent regulation depending upon
the valve) downstream regardless of fluctuations
in pressure upstream. These valves are used in the
closed centre parallel type of circuit to control
the maximum amount of oil that can be delivered
to a motor or cylinder in order to control the speed.
They are useful as a bypass valve where a fixed
quantity of oil is bled out of the system. (Note
flow control bypass valve in Figure 22.)
(c) Needle valves — Variable orifices provide a res*
triction for bypass or control purposes where a
constant flow is not required. Unlike the pressure
compensated flow control valve, the needle will pass
a quantity of oil proportionate to the pressure.
(d) Sequence valves — Normally closed valves that
open when a pilot signal reaches a predetermined
value. The unloading valve in Figure 6 is an
adaptation of a sequence valve.
(e) Counterbalance valve — A modification of an
externally piloted sequence valve that permits the
lowering of a load by a winch or cylinder with full
control, or, in other words, to provide "power
down" operation. This is a normally closed valve
that opens a small amount to permit the oil to
throttle through so that an overhauling load on a
winch or cylinder does not "run away". Counter-
balance valves are necessary in all open loop type
winch circuits where the winch is reversible and
must lower the load.
S.JTHE TRANSMISSION OF HYDRAULIC POWER
Hydraulic oil flowing under pressure represents horse-
power. Pumps convert rotary mechanical power from
electric motors or engines to flowing fluid underpressure.
Motors convert the fluid power to rotary mechanical
power; cylinders convert the fluid power to linear
mechanical power. Figure 1, mentioned earlier, solves
the basic equation of horse-power-fluid flow-pressure.
The following formulas are useful in analysing fluid
power applications of pumps and motors:
D x P
(c)HP
63,025
Where: T=Theoretical torque, inch pounds for
either a pump or motor.
D~ Displacement, cubic inches per revolution.
P= Pressure, pounds per square inch, gauge.
G=Flow, gallons (at 231 cubic inches per
gallon) per minute.
n —Theoretical pump or motor speed, revo-
lutions per minute.
HP= Horse-power.
319
The above formulas arc all based on theoretical values.
In practice, the efficiency of vane type pumps varies
between 75 and 85 per cent. Figure 7 is a typical set
M
Fig. 7. Typical performance curves for vane pumps.
of performance curves for a vane pump. The efficiency
of vane-type motors varies between 75 and 90 per cent.
Fig. 8 illustrates typical performance curves for a vane
motor.
2«0o
TO*Q
IM.. ZOJ
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n«
fct>
J
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-i
o
r~>
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JO
f
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i ^
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400
tooo »»»
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Fig. 8. Typical performance curves for vane motors.
Further inefficiency in the transmission system is
caused by pressure drop through the hydraulic line.
It is common to assume the overall efficiency between
pump input power and motor output power of 50 to 60
per cent to allow for parasitic pressure drop through the
entire system. Figure 9 solves the relationship between
flow, pipe size and velocity. As a general rule of thumb,
velocities in pressure supply lines should not exceed 15
feet per second. Ten feet per second is recommended as a
maximum for return lines and two to four feet per second
for pump suction lines. Higher velocities may be per-
mitted in large size pipes compared with small size
since there is proportionately less oil in contact with the
walls of the pipe and tubing. Other factors such as the
smoothness of the pipe tubing or hose are to be taken into
consideration when sizing the transmission lines.
Figure 10, Recommended Pipe Sizes, is a guide for sizing
hydraulic lines based upon actual hydraulic pressure
loss data of oils with viscosities usually encountered
and allows for an increasing total pressure drop with
increasing length of total transmission system.
All pressure drops in the system that do not do useful
320
10000
8000*
7000*
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2000-
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700
600—
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200—
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NOMOQRAPHIC CHART INDICATING
FLOW CAPACITY Of WCt IT
RECOMMENDED FLOW VELOCITIES
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* RECOMMENDATIONS ARE POM OILS
M NAVINO A MAXIMUM VISCOSITY Of
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Fig. 9. Flow capacity of pipes.
work must be counted as horse-power lost that shows up
as heat. One horse-power is equal to 42*42 BTUs per
minute (0-178 calories per second). The heat generated
by the pressure drop in the piping of most hydraulic
systems is radiated to the surrounding atmosphere
at roughly the same rate that it is generated. The hydrau-
lic lines and the reservoir are the two most valuable
sources of heat rejection in the system, and are usually
sufficient to handle the heat load in a simple, well-designed
open centre series circuit with intermittent duty. A
workable "rule of thumb" for heat dissipation is to allow
two BTUs per hour per square foot per degree Fahren-
heit temperature difference.
Heat is added to the oil per the formula:
q =
G x AP
40-4
Where:
q=BTU/minute.
G= Gallons per minute.
AP= Pressure drop, psig.
Standard weight steel tubing and tube fittings, extra
heavy and double extra heavy pipe and various grades
KECQMMEKJDED PIPE
FIG 10
FOfc. ESTIMATING f MULTIPLY ACTUAL TOTAL
LENGTH OF P\P1NIC| RUM BV 1.5 TO ALLOW
FOR FITTINGS, BENDS ANJD VALVES.
PIPE
t V HOSE
25 30 40 fiO *O *9 80 90 \OO 200 9OO
EQUIVALENT LENGTH OF PIPING - FEET
>0 ffo 73 MS »4« I7S 20O
APPROXIMATE TOTAL PRESSURE PROP - PSI
of neoprene covered wire wound hose and hose fittings
are available for transmission systems. Satisfactory
installations can use any of these separately or in com-
binations. Obviously where flexibility is required hose is
essential. It is not the purpose of this paper to describe
in detail all installation materials. Let it suffice to say
they are available and the installer should follow the
manufacturer's instructions. (See Appendix 3, Hydraulic
System Installation Instructions.)
The front power take-off on the main engine is the
most common method of driving hydraulic pumps in
small fishing vessels. It is safe to assume that all fishing
vessels capable of handling mechanised fishing gear are
engine powered and thus have at least one large source
of rotating power. These vessels seldom have sufficient
auxiliary generating power to drive the hydraulic pumps
electrically. Figure 11 shows an hydraulic pump and
tank installation on a 46-ft steel combination seiner and
crab boat. The diesel engine drives the winch mechani-
cally by the roller chain and sprocket shown in the upper
right-hand part of the picture. The hydraulic pump
transmission system, which powers the Puretic power
block, transmits approximately 7| horse-power.
On some small fishing vessels such as gillnetters or
seine skiffs automotive power steering pumps are
directly connected to the engine to drive reels or rollers.
These pumps can be operated continuously, just as the
Fig. 11. Typical pump and tank installation. The 7±-hp hydraulic pump
is driven from the front power take-off of the main engine on a 46-foot
steel fishing vessel.
power steering pump in an automobile runs continuously
when the engine is running, and are designed for high
speeds of up to 3,000 or 4,000 rpm. Figure 12 shows this
type of pump power pack installed at the front end of a
gasoline engine in a 32-ft all-aluminium gillnetter. This
pump is equipped with a shaft mounted clutch. Figure
Fig. 12. "Power Pack" installation in small boat. A self-contained com-
bination hydraulic pump and oil reservoir as installed in a 32-foot alum-
inium gillnetter.
321
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Fig. 13. Hydraulic gillnet reel installation.
13 shows a typical installation of this type of pump
driving a stern mounted reel for hauling gillnets.
Larger vessels that have sufficient generating power to
drive electric motors often use electro-hydraulic power
units. The advantages of the electro-hydraulic power
unit include compactness, ease of starting and stopping,
ability to be located outside of the engine room, if
desired, and a constant speed drive for the pumps. An
auxiliary drive, either by electric motor or diesel engine,
is of course necessary with a direct reversing engine that
is stopped during the fishing operations. Most of the
California tuna vessel conversions from bait boats to
seiners use electro-hydraulic power units. These vessels
all have had sufficient generating capacity to handle the
refrigeration and bait circulating pumps. Frequently
these electro-hydraulic power units are installed on
either the main deck or the raised deck aft near the
equipment in order to minimise the amount of piping
and protected from the weather by plywood enclosures.
Figure 14 shows a typical power unit. The relief
valves are mounted directly on the tank and are piped
up to the pump discharges. The electric motor drives a
flange mounted double pump. The reservoir contains
SO gallons. This is the same type of power unit that is
utilised in Figure 23, Typical Double Pump System for
Tuna Vessel.
It is possible to include multiple pumps and motors on a
common sub base with the oil tank. Figure 1 5 illustrates
a double electro-hydraulic power unit where each
motor drives a flange mounted double pump. Thus there
are four separate pump discharges available. Figure
322
16 illustrates three 40-hp motors mounted on a common
sub base with a 250-gallon capacity oil tank and a total
of 10 separate pumping cartridges. This power unit
operates 12 separate winches on a recently commis-
sioned large fisheries research vessel in the United States.
This power unit was designed to accommodate the
owner's requirements for full power operation of any
Fig. 14. Electro-hydraulic power unit. A 50-gallon oil reservoir and a
30 hp electric motor driving a double hydraulic pump.
Fig. 15. Double power unit. Two 30-hp electric motors driving double
hydraulic pumps connected to a 100-gallon oil reservoir.
The pump would be driven between 1,200 and 1,800 rpm
by step-up chain reduction from the front end of the main
engine. The pump takes suction from the oil tank and
discharges to a control panel that has a pressure gauge,
built-in relief valve and by-pass speed control consisting
of a needle valve or ball valve so that part of the oil can
be bypassed for speed adjustments. The control valve
starts, stops and reverses the direction of the hydraulic
motor. This type of system is suitable for powering
small power blocks, gillnet rollers or reels, pot pullers
or other small simple devices. The basic arrangement
with the addition of larger pumps and motors, tank and
valves is often used for single or multiple drives up to
HYDRAULIC MOTOR
( A«OUT 5 W.R OUTPUT)
Fig. 16. Triple power unit. Three 40-hp electric motors driving the total of 10 pump cartridges mounted on a common base with a 250-gallon oil
reservoir.
pair of winches at any one time. Flange connections
on each of the discharge manifold blocks permit emer-
gency cross connections through use of portable hose in
the event of a motor or pump failure.
The oil reservoir is a very important item of equipment
in the hydraulic circuit. It is common practice to use a
reservoir with a capacity in gallons equal to the gallons
per minute in the system. Experience has shown that
this provides adequate reserve oil and proper operating
characteristics. The reservoir functions: (1) to permit the
oil to slow down and thus drop the heavy particles being
carried along with it; (2) to permit the escapement of
entrained air; (3) to serve as a heat sink during peak
demands of the system ; (4) To serve as a radiator to
dissipate the heat in the oil ; (5) by being located above
the pump suction, to provide a positive suction head to
the pump to prevent cavitation. The reservoir should be
pickled and cleaned after construction to remove all mill
scale, rust and loose weld spatter.
It is always tempting to consider the use of a double-
bottom tank in a vessel for a hydraulic oil reservoir.
With sea water on one side the tank would make an
excellent heat exchanger to keep the oil cool. However,
it is impossible to adequately clean a double-bottom
tank; consequently at some time or another dirt would
be picked up in hydraulic oil that may damage the pumps,
motors and valves. Double bottom tanks are not recom-
mended as hydraulic oil reservoirs.
A.PPROX. WEIGHT OF ^
»V1TKM IKICUUDINJ6 T~ T
PIPIM« AKSD OIL.—
ZOO POUMOft
-BrPASS SPEED CONTROL
6. SOME HYDRAULIC POWER INSTALLATIONS
6*1 Simple single pump systems
A typical simple hydraulic circuit is shown in Figure 1 7.
8 6PM PUWP
ABOOT Si M.P INPUT'
FIGURE 17
TYPICAL SIMPLE CIRCUr
323
f. 18. Giltoetter with hydraulic reel and block. Thirty~twc~Joot fibre-
reinforced plastic gillnetter with hydraulically driven power
blockgillnet reel. Only one man operates this vessel.
about 80 horse-power.
Figure 18 shows a fibreglass reinforced plastic gill-
netter equipped with a gillnet power block and a gillnet
reel. Both the reel and the power block are driven by a
circuit similar to that shown in Figure 17; however, only
one at a time is used, depending on the season and gear
used.
A pair of menhaden seine skiffs are shown in Figure
19. The hydraulic system on each of these skiffs operates
Fig. 19 Hydraulic cranes on purse seine skiffs, hydraulically operated
cranes and power blocks on a pair of menhaden all-aluminium purse
seine skiffs fishing off the E. and Gulf Coast of the U.S.
the power block, power block crane, both topping and
slewing, and the purse line storage drum. These skiffs
average about 36 ft in length by 9 ft to 10 ft in the beam.
Hydraulic power is the only practical solution for power-
ing the various items of equipment shown on these
boats. It would be impractical to provide an electrical,
mechanical or pneumatic power system to power the
four different devices.
6*2 Compound pump systems
The rapidly developing Alaskan fishery for King crab
has shown fantastic growth in the last few years (Allen,
324
1963). Here, as in the Dungeness crab fishery along the
Washington and Oregon coasts, relatively large vessels
with small crews have achieved exceptional productivity
through the use of hydraulically powered gear. Figure
20 shows a typical combination fishing vessel rigged for
fishing crab. The crab pot hauler is suspended from the
end of a special boom. The boom is topped by means of
a hydraulic cylinder mounted on the top of the boom. As
the cylinder retracts, the boom is raised. The boom is
vanged by a separate hydraulic cylinder that mounts on
the starboard after corner of the deckhouse. The control
valves for the crab block and the boom are mounted
just forward of the side trawl winch. (The side winches
and the main purse winch on this vessel are driven
mechanically from the front power take-off on the main
engine. The side winches are used for trawling and not
used when fishing crab.) When operating, the fisherman
lowers the boom and vangs it over the side. He then
retrieves the pot warp and passes it over the pot hauler.
The pot hauler pulls the crab pot at approximately
300 feet per minute. As the pot is being raised, the opera-
tor will raise the boom with the topping cylinder. When
the pot is pulled out of the water and suspended by the
crab block, the operator swings the boom inboard with
the vanging cylinder, reverses the crab block and drops
the pot on the deck.
Figure 21 illustrates an hydraulic system similar to
that shown in the photograph, Figure 20. This installa-
tion was on a slightly larger boat that also included an
hydraulically driven purse winch and anchor winch.
Here, a double pump is driven off the front power take-
off of the main engine. The discharge of 28 gallons per
minute from the large end of the pump is directed through
the relief valve and then to a selector valve that selects
either the fishing gear circuit or the anchor winch drive
circuit. From the selector valve, the 28 gallons per min-
ute passes through the directional control valve for the
crab block and then goes to the purse winch. The 12
gallons per minute circuit from the small end of the
double pump passes through a relief valve and then to
the two control valves in series for the topping and vang
cylinders, and then joins the 28 gallons per minute flow
from the crab block. This combined flow of 40 gallons
per minute is available to the purse winch. The purse
winch control consists of a bypass valve which, when
closed, forces the oil to circulate through the motor,
thus powering the purse winch. To stop the purse winch,
the bypass valve is opened. This circuit illustrates the
versatility gained by the use of a double pump with two
differing oil flows. Here we have the power equivalent
of 12 gallons per minute available to the vang cylinders,
28 gallons per minute for the crab block and anchor
winch and a total of 40 gallons per minute for the purse
winch drive.
Where differing flow requirements are involved the
double pump arrangement is by far the most preferable,
particularly where the device is to be given fairly severe
duty. For very intermittent duty, where a device will be
operated for only a short time, we occasionally use a
Fig. 20. Crabpot hauler with active boom. The boom supporting the crabpot hauler is topped and vanged by use of hydraulic cylinders on this
combination crab boat and trawler.
single pump and a bypass flow control. Such a system is
shown in Figure 22 — a typical fishing vessel suitable for
purse seining (as shown) or for stern trawling. The main
winch is mechanically driven through chain, sprockets
and shafting from the front power take-off on the main
engine. An hydraulic system is installed primarily to
drive the power block. This particular system is set up
for 28 gallons per minute to the power block. After the
hydraulic oil passes through the power block control
valve, part of the oil is bypassed to tank through the
flow control valve. The flow control valve is set for
approximately 12 gallons per minute. This valve is
pressure compensated; in other words, regardless of the
pressure upstream from the valve, the valve will adjust
itself so that it will pass 12 gallons only back to tank.
This forces the remainder of 16 gallons to circulate
through the topping winch control valve and the anchor
winch control valve. Thus the topping winch and the
anchor winch have 16 gallons per minute available to
them, although the hydraulic pump is delivering 28
gallons per minute. This is called a flow control bypass
circuit and should only be used where the service is of
very light duty. It must be remembered, as described
earlier, that any oil that passes over a flow control valve
under pressure will create heat. If the oil is subjected
to this heat producing operation for too long a time it
will become too hot and severe damage may result. If
the oil gets too hot, it will damage the seals in the valves,
motors and pumps and it may lose its lubricating charac-
teristics, which will cause physical damage to the pumps
and motors.
The larger the vessel and the greater the number of
devices powered, the more complex becomes the hydrau-
lic system. A power system for a tuna seiner conversion
is illustrated in Figure 23. Here a single electric motor
drives a double pump delivering 35 and 16 gallons per
minute. The total horse-power input required by this
double pump if both are delivering oil at rated speed at relief
valve setting would be approximately 50 horse-power;
however, when in actual use this condition would not
normally be encountered, therefore a 30 horse-power
motor can be used to drive the pump. This will provide
325
jl CAAS mu
\\"*»
\v AT \%OO F^
V~K
FIGURE Zl
TYPICAL DOUBLE PUMP SVSTEM
eooc^a* FOR, CRAB FISMIMQ VESSEL
-DOUBLE: PUMP - ZB 4 \z GPM
4O HP. MAXIMUM IWPUV
sufficient power for the power block, which is operated
a greater length of time than the other winches. In this
circuit we see that each section of the double pump has
one particular service and the discharges are not com-
bined. The larger section drives the power block and
the smaller section drives the vanging and topping
winches, the boom winch and the cork line winch.
A good deal of progress in the application of hydraulics
to fishing gear has developed among the purse seiners.
Before the advent of the power block many purse seiners
converted their turntables to purse seine reels. The reels
were mounted on the existing turntable rollers and pow-
ered hydraulically. Figure 24 illustrates a typical purse
seine reel mounted on such a conversion. The seine is set
with the reel in the position shown. To retrieve the net
the reel is turned approximately 90 degrees so it faces
over the side of the vessel. Pursing is done on the forward
section of the net only, and the after section of the net
is hauled in on the drum at the same time the net is being
pursed. An hydraulically powered fairlead runs back
and forth over a track and guides the net on to the drum
so that it may be spooled evenly. The rotation of the
turntable, the powering of the drum, and the powering of
326
the level wind are all done hydraulically. This is one of
the most efficient methods yet devised for the handling
of purse seines; the fewest number of men can set and
retrieve the net. Today only a very few new boats are
constructed with the purse seine reel due primarily to
its high cost and weight.
6-3 Remote controls on a purse seiner
It was mentioned in an earlier section of this paper that
one of the benefits from a hydraulic transmission was
the ability to provide remote control of winches and
other gear. Figure 25 shows a six-drum purse seine winch
and the winch control console. The six drums are:
(1) main purse line; (2) forward purse line; (3) tow line;
(4) auxiliary back drum for the heavy lift; (5) auxiliary
back drum for strapping and (6) auxiliary back drum for
either choking or brailing. All drums have hydraulically
actuated brakes and clutches that are controlled remotely
from the control console. The brakes, clutches and all
gearing for this winch are totally enclosed in a double
wall case and operate in oil, thus providing full splash
lubrication at all times and protecting the machined
surfaces from the weather. The control valves are
HBPPIMfl WINQ4
WITM A OOUA4.I «OW H'*ICM HOLI.I* CMAM).
TO TIMM AT l»ftO
l« ttTATAflAftr Iphfl.
UYDRADLIC INSTALLATION
DIAGRAM
. 22. Typical deck machinery and piping layout.
designed so that the operator can feel the load: in other
words, as he applies hydraulic pressure to the brake or
clutch by manipulating the valve the handle provides
increased resistance with increasing load. The winch
drive incorporates two motors that can be operated either
in series or in parallel. Both motors are permanently
geared (not clutched) to the winch drive. When the
hydraulic oil flows through both motors in series (see
Figure 28) the winch is driven at the maximum speed
and only one motor does useful work. When the flow
is divided so that half flows through each motor the speed
of the winch is cut in half and the torque is doubled
since each motor has full pressure available. All controls
for the drive, as well as the brake and clutches for the
drums, are located in the control console. Figure 26
shows this winch and control console installed on a recent
tuna vessel conversion. A separate control console
mounted to the right of the winch console controls
the power block, vang winches and topping winch. The
hydraulic topping winch can be seen mounted on the
mast to the left of the control console.
Figure 27 represents typical line pull requirements
encountered by a purse seine winch of this type. Data
for the curve are based upon research by Ishii and
Konagaya (1961) and modified by our observations
of hydraulic winch drives on tuna vessels. It will be
noted that the series-parallel drive provides high line
speed when the line pull is low at the beginning of the
pursing operation. As the net is pursed up the line pull
increases until the winch stalls. At this point (or slightly
before, depending upon the operator) the winch is
shifted into parallel drive which doubles the line pull
at one-half the speed. As the rings are picked out of the
water and are pulled up the side of the boat to the purse
davit, the operator has full control of speed and can
stop the winch and hold the rings with complete control
and maximum safety. This winch drive provides approxi-
mately 35 horse-power which, although not large, is
applied in the proper speeds and pull to do the most
efficient job.
Figure 28 illustrates a typical series-parallel winch
drive circuit similar to that utilised on the winch shown
in Figure 25. Here the winch is driven by two pumps,
the large pump providing two-thirds the total flow and
the small pump providing one-third the total flow. In
practice, each of these pumps could be expected to
327
DO&BLC RUMP - 35" ^ /6 6PM
SO UCt&POVCR. MAXIMUM MFt/T
. 25. Typical double pump system for tuna vessel.
Fig. 24. Salmon purse seine reel. The salmon purse seine is shown wound
on an hydraulically powered reel. The reel is mounted on an hydraulic-
ally powered turntable. An hydraulically powered fairlead roller (not
shown) spools the net evenly.
power other items of equipment such as power block,
topping winch, etc., on the vessel, as well as the winch
drive. The pilot-operated, remote-controlled relief
valves, when vented (that is, an internal compartment
in the valve is vented to tank, thus opening the valve
fully) open to allow oil to bypass directly from the high
side of the circuit to the tank return line. These valves
are controlled independently by the small, manually
operated bypass valves (note the small black knobs
extending through the top of the control console in
Figure 25). This arrangement permits bypassing either
328
Fig. 25. Hydraulic seine winch. A modern tuna seine winch with hy-
draulically operated brakes and clutches and drive. The winch is con-
trolled entirely from a remote location by the control panel shown.
the large or the small pump to provide one-third speed,
two-thirds speed or full speed in either torque range.
The manually operated series-parallel selector valve
directs the oil flow so that the oil must pass through the
two motors (series), or is divided and half flows to each
motor (parallel). The remote relief valve provides
partial venting of the two pilot operated, remote-con-
trolled relief valves, thus effecting fine speed adjustment.
The "panic button" controls the starting and stopping
of the winch. A second optional winch stop valve can
be located on or near the winch. This "panic button"
30 H.P. OUTPUT
ACTUAL POK.se:
LIMC LOAO\
AI LA OLE LtfiJC
PULL — SCR.ICS
300 200 700 I O
PUfZSE LftJE OUT -FATHOMS
(EACH
- 5 10
T/M£> MIMUTES
Fig. 26. (left} Tuna seine winch installation. The seine winch, its control
console and the power block and vang and topping winch control
sole are shown installed on a recently converted European tuna vessel.
Fig. 27. (above) Typical performance curves for series parallel seine
winch hydraulic drive.
Fig. 28. (below) Series-parallel winch drive circuit.
REMOTE RELIEF
PILOT OPERATED >
COVTKOLL&D
MALL PVMP QVPASS ,
PUMP- § TOTAL
SMALL PUMP-J TOTAL
329
Powcft SLOCK
ftRAlLlNQ BOOM WINCH
MAIN ftOOAA WINCHES
AAAIN VANd WIMCH(5)
CHOKtR WINCH
Fig. 31.
trawl w
MAIM VAN* WINCH (P>-
WINCM CONTROL CONSOLE -
PUQSft SOINfc WIMCH
Fig. 29. Deck machinery arrangement on tuna vessel.
r P on stern trawler. Two side
i each driven by a separate hydraulic pump, handle the
trtwl warps. The net reel is powered off one of the trawl winch pumps.
is one of the most valuable features of this type of hy-
draulic drive. It permits the operator to stop the winch
immediately by merely hitting the button with his hand,
thus venting both pilot-operated remote-controlled
relief valves, which react very rapidly to bypass the oil,
stopping the winch. Recently many winch drives have
been installed using a diesel engine and a torque con-
verter, a very effective and powerful drive that provides
usually both more line speed at the beginning of pursing
and more available line pull at a much reduced speed at
the end of pursing. However, due to the nature of the
torque converter the speed varies with the load as well as
the speed of the diesel engine prime mover. The main
disadvantage as far as the fishermen are concerned is the
inability to stop the winch immediately in the event of a
crisis.
A modern, fully equipped tuna purse seiner is illus-
trated in Figure 29. All major items of powered fishing
gear arc^ shown. All winches except the cork line winch
and the anchor winch, which have controls locally moun-
ted, are remotely operated from the winch control
console mounted on the raised deck. Here the operator
330
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fc
i
%
^
A.
-K
>
i
ISaUlG
in
1 !{
IL
K T
ii
ii
t
^
«
o
Vi 5
Ir-Sl
\J
t
> *
iv
y
&
vf
«N
,*«
«
331
has an unobstructed view of the whole fishing operation
and has fingertip control of all powered winches. His
hydraulically actuated brakes on the main purse winch
give him complete control of the purse line when making
a set He has complete control of lifting the rings or
skiff, strapping, choking and brailing, thus freeing the
rest of the crew to handle the net and the lines.
All winches are powered by a central electro-hydraulic
power unit. The circuit is illustrated in Figure 30 and
consists of a 125-gallon oil tank and two double pumps
driven by 60 and 40 horse-power electric motors. All
shipyard-installed piping is illustrated as connections
between the blocks representing the various items of
equipment. Unquestionably a great deal of piping is
involved for such a system; hence the initial installation
is expensive. However, once the system is installed and
properly set it can be expected to operate with a very
minimum of maintenance and expensive down-time.
6*4 Stem trawler installation
Although most of the discussion heretofore has con-
cerned either small vessels or purse seiners, hydraulically
powered equipment plays comparable roles on trawlers,
longliners and other types of vessels. Figure 31 shows
the installation of gear on a typical stern trawler as used
in the Pacific Northwest. This vessel is approximately
65 ft in length. The trawlwarps are carried on indepen-
dently powered side trawl winches. Each winch has its
own pump and motor — driven from the front end of the
main engine. The reel for retrieving and stowing the
trawl is powered by one of the pumps. Separate smaller
hydraulic pumps power two two-ton winches mounted
on the main boom. The control valves for the trawl
winches are mounted on each winch. The control valves
for the boom mounted winches are located on the back
of the deckhouse. Of particular interest are the clear,
unobstructed decks, made possible by the use of hydraulic
drives.
The use of hydraulic transmission systems, pumps and
motors is playing an ever-increasing role in the rapidly
advancing technology of modern fisheries. Hydraulically
powered fishing gear similar in design and operation to
that illustrated in this paper is in use in many different
fisheries of the world. Approximately 2,500 systems are
making fishing easier, safer and more productive. The
operation is basically simple and easily understood by the
fishermen. The components, nearly all of which are mass-
produced, are relatively inexpensive and very reliable.
Satisfactory oil for hydraulic service is available wherever
there is a major marketing programme for petroleum
products. Besides the so-called "hydraulic oils'9, prem-
ium heavy-duty diesel engine motor oil will give perfectly
satisfactory service.
Finally, it cannot be too strongly stressed that when
hydraulic systems are being installed, the manufacturers'
detailed instructions should be followed most closely in
order to avoid risk of leaks, dirt and circulation
troubles. Such care will be well repaid by trouble-free
operation.
Acknowledgments
The author would like to acknowledge the assistance
during the preparation of this paper of members of the
staff of Marine Construction & Design Co. — Mr. Robert
F. Allen, Mr. Jack L. Wilskey, Mr. Boyd P. Milbwrn
and others who have contributed suggestions, information
data and editorial comment; Vickers, Inc., for permission
to reproduce certain of the diagrams and nomographs ; The
Industrial Publishing Co., publishers of the Fluid Power
Handbook and Directory; and H. L. Peace Publications,
publishers of Fish Boat magazine, for permission to
reproduce diagrams and photographs previously pub-
lished.
References
Allen, R. F. : King crab pot fishing in Alaska. Second World
Fishing Gear Congress, FAO, London. 1963.
Ishii, Kazumi and Konagaya, Tuneo: On the form and tension
of purse line of a purse seine. Bulletin of the Japanese Society of
Scientific Fisheries, Vol. 27, No. 9, 1. 1961.
McNeeley, R. L. : Purse seine revolution in tuna fishing. Pacific
Fisherman, Vol. 59, No. 7, pp. 27-58. 1961.
Schmidt, Peter G., Jr. : Purse seining: Deck design and equipment.
Fishing Boats of The World: 2, Fishing News (Books) Ltd. 1959.
Short, John and Hawley, R. T.: Hydraulic design for deck
machinery. Vickers Marine and Naval Hydraulics Conference,
Nov. 10-11, 1959.
Anonymous: Norwegian hydraulic winches. World Fishing,
October, 1962.
(See Important Appendices in following pages)
332
APPENDIX 1
STANDARD GRAPHICAL SYMBOLS
(fromASA-732.10)
Basic symbols can be combined
in any form desired. No attempt
is made to show all combinations.
LINES AND LINE FUNCTIONS
LINE. WORKING
LINE, PILOT (L>20W)
— — — —
LINE. DRAIN (L<5W)
CONNECTOR
(DOT TO BE SX
WIDTH OF LINES)
•
LINE. FLEXIBLE
\j
LINE. JOINING
1
LINE. PASSING
-Jf-
DIRECTON OF FLOW
LINE TO RESERVOIR
ABOVE FLUID LEVEL
BELOW FLUID LEVEL
"1
LINE TO VENTED
MANIFOLD
-3
ill
PLUG OR PLUGGED
CONNECTION
X
TESTING STATION
(GAGE CONNECTION)
#
#
POWER TAKEOFF (HYD.)
— #
RESTRICTION. FIXED
v
XN
RESTRICTION, VARIABLE
op/
s*\
PUMPS
PUMP. SINGLE.
FIXED DISPLACEMENT
V^
PUMP, SINGLE,
VARIABLE, DISPLACEMENT
(W)
MOTORS AND CYLINDERS
MOTOR, ROTARY,
FIXED DISPLACEMENT
GO
MOTOR, ROTARY,
VARIABLE DISPLACEMENT
GO
MOTOR, OSCILLATING
GO
CYLINDER, SINGLE ACTING
t=.
CYINDER, DOUBLE ACTING
SINGLE END ROD
DOUBLE END ROD
^
M
i i i
MISCELLANEOUS UNITS
ROTATING SHAFT
(ARROW IN FRONT OF SHAFT)
-{-
COMPONENT ENCLOSURE
t"H
RESERVOIR
1 1
PRESSURE GAGE
(S>
OTHER
* IftMrt appropriate fetter combinations
and add appropriate symbols to indi-
cate shafts or connecting flow linas.
ACC ACCUMULATOR
ELEC MOT ELECTRIC MOTOR
ENG ENGINE
FLT FILTER
FM FLOW METER
HE HEAT EXCHANGER
INT INTENSIFIER
PS PRESSURE SWITCH
STR STRAINER
TACH TACHOMETER
G*)
333
APPENDIX 1
VALVES AND BASIC SYMBOLS
VALVE EXAMPLES (CONT.)
VALVE, CHECK
— <£
VALVE, COUNTERBALANCE
WITH INTEGRAL CHECK
1 -T-J
VALVE, MANUAL SHUTOFF
— B —
VALVE, MAXIMUM
PRESSURE (RELIEF)
1 UA/
F*s^ fyV
VALVE, FLOW-RATE
CONTROL, VARIABLE,
PRESSURE COMPENSATED
VALVE, BASIC SYMBOL
SINGLE FLOW PATH IS
MODIFIED.
D
s*^
^*r
VALVE, BASIC SYMBOL
MULTIPLE FLOW PATHS
ARE CHANGED
•-#-
i i i
i
VALVE, SINGLE FLOW
PATH, NORMALLY CLOSED
[h
VALVE, DIRECTIONAL,
2 POSITION
3 CONNECTION
rrrM
VALVE, SINGLE FLOW
PATH. NORMALLY OPEN
$
1
VALVE, DIRECTIONAL,
3 POSITION
4 CONNECTION
OPEN CENTER
1 i
II HfjIXI
VALVE, MULTIPLE FLOW
PATHS, BLOCKED
I i
IT?
VALVE, DIRECTIONAL,
3 POSITION
4 CONNECTION
CLOSED CENTER
i i
TT
It Itr jlXI
VALVE, MULTIPLE FLOW
PATHS, OPEN
(ARROWS DENOTE DIREC-
TION OF FLOW)
II
1 1
If IKI
METHODS OF CONTROL
1 1
SPRING
/W
VALVE. SPECIAL
(IDENTIFY AND CONNECT
ALL LINES)
i i
PILOT OPERATED
VALVE EXAMPLES
PILOT OPERATED,
DIFFERENTIAL AREA
1
IL
VALVE. RELIEF
REMOTELY OPERATED
(UNLOADING VALVE)
— 1^>
VALVE. DECELERATION
NORMALLY OPEN
mtf^
OTHER | * |
•Insert appropriate letter combinations
and add connecting flow lines.
CENT CENTRIFUGAL
COMP COMPENSATOR
CYL CYLINDER
DET DETENT
ELEC MOT ELECTRIC MOTOR
HYD MOT HYDRAULIC MOTOR
MAN MANUAL
MECH MECHANICAL
SERV SERVO
SQL SOLENOID
SOL PLT SOLENOID CONTROLLED,
PILOT OPERATED
THRM THtRMAL
VALVE. SEQUENCE.
DIRECTLY OPERATED
ilr
VALVE. PRESSURE
REDUCING
Prfc
i9lj
334
APPENDIX 1
SAMPLE COMPOSITE SYMBOLS
PUMP, DOUBLE, WITH ELECTRIC MOTOR
ONE FIXED DISPLACEMENT
ONE VARIABLE DISPLACEMENT WITH
COMPENSATOR CONTROL
PUMP, DOUBLE. WITH IN-BUILT CHECK,
RELIEF AND UNLOADING VALVES.
VALVE, PRESSURE COMPENSATED VARIABLE
FLOW-RATE CONTROL WITH
MAXIMUM PRESSURE CONTROL
VALVE, RELIEF AND REPLENISHING
VALVE, 4-WAY, MANUALLY CONTROLLED,
SPRING CENTERED.
(PORTING AT CENTER IS f TO T
WITH CYLINDER PORTS BLOCKED)
SIMPLIFIED
SYMBOL
VALVE, 4-WAY
SOLENOID CONTROLLED,
PILOT OPERATED,
SPRING OFFSET
COMPOUND
SYMBOL
VALVE. 4-WAY, SOLENOID CONTROLLED,
PILOT OPERATED, NO SPRING
(ONE TORT PLUGGED)
iJj
335
APPENDIX 2
Directional Controls
2*Wayf normally closod
4-Way, 3-posttion, closad-cantor
2vWayv normally opan
4-Way, 3*posHionv tandtm ctnter
t 3*positioni pump port blocked NI
yHndtr ports connocttd to tank
4-Way, 3-position, tank pert Mected in iMulral
cylinder ports connected to pump
4-Way, S^posftion, ona cyttiKlw portbtockadtn
336
APPENDIX 2
Pressure Controls
Flow Controls
Pressure reducing
Fixed flow,
337
w
Advances in Centralised Control and Automation
Attract
The rationalisation and integration of equipment in the wheelhotuc
of fishing vessels was discussed at the Second World Fishing Boat
Congress held in Rome in 1959. At that time it was suggested that
the use of centralised control in the bridge structure could add to
the efficiency of fishing operations. This paper discusses and gives
examples of how the integration of steering and engine controls in
the wheelhouse has progressed during the past four years. Future
trends are discussed.
i raatDmation et cortrok centralise
La rationalisation et I'integration de I'tquipement dans la timonerie
des bateaux de ptehe ont etc discutees au Second Congres Mondial
des Bateaux de Pftche, tenu & Rome en 1959. II fut alors suggfct
que rutilisation du contrdle centralise dans la timonerie pourrait
augmenter I'cfficacitd des operations de ptehe. La communication
explique et cite des exemple sur la facon dont a progress*, pendant
les quatre denudes annees, I'inttgration des contrdles du gouvernail
et de la machine dans la timonerie. Les perspectives futures sont
aussi discut6es dans cette communication .
ea la centralizackm de
y antrautizacioB
Extracto
La radonalizad6n e integraci6n del material auxiliar en los puentes
de mando de los barcos de pesca se examin6 en el Segundo Congreso
Mundial de Embarcaciones Pesqueras, eetebrado en Roma en
1959. Se sugirid entonces que el empleo de mandos centralizados
en la estructura del puente podrta incrementar la eficacia de las
faenas de pesca. Esta ponencia examina y da ejemplos de c6mo
fa integracidn de los mandos de govtemo y del motor en el puente
ha avanzado en los cuatro anos pasados. Se discuten las tendencias
futures.
IN 1939 a paper was presented to the Second World
Fishing Boats Congress in Rome entitled "Centralised
Control in Trawlers". At that time, although a great
deal had been talked and written about the subject,
no general effort had been made to rationalise and inte-
grate the large number of navigational and operational
services required in the wheelhouse, particularly of
modern deep-water fishing and control vessels. This
was due not so much to lack of knowledge as to the fact
that owners and operators had really not been able to
make a definite selection from the multitude of instru-
ments and "gadgets" that were becoming available to
assist in fish-finding. It may have been that, because of
the wide variety and number of types of equipment,
owners and operators had found it difficult to select
which were desirable and which necessary for their
purpose. Far from the market being oversold, rather
it can be said that the market had not yet determined
what it wanted.
Another aspect of the problem, which has had some
influence on the various ideas put forward, has been the
inclination towards stern-trawling instead of side-trawl-
ing. This relates not only to deep-water, ocean-going
vessels but also to smaller craft.
If such schemes are practicable, there are obvious
advantages in consolidating on "one class" type vessels.
In the same way and if a single class can be determined,
then it is of advantage to consolidate on the same type
338
by
H, £. H. Pain
S. G. Brown Ltd.
of equipment, although not necessarily of the same
manufacture, in each vessel of that class.
It is now the purpose to illustrate some of the advances
made in navigation and operational control in fishing
vessels down to and including middle-water types and to
postulate ideas and schemes suitable for the smaller
craft. The selection has been made at random and the
coverage is by no means comprehensive.
The subject matter is broadly divided into three sec-
tions. The first having emphasis, deals with basic prob-
lems concerning compasses and steering control, the
second concerning remote control of main and auxiliary
machinery. Finally, an attempt is made to forecast
probable trends and tendencies and to look towards a
future which is perhaps not very far distant.
Towards integration
In the last four years a great deal has been done in a num-
ber of countries towards rationalising the wheelhouse.
This rationalisation had and has the primary objective of
reducing fatigue and running costs and of increasing
operational efficiency, in a number of instances enabling
one man, the skipper, to control all the operations of
fishing from the wheelhouse. Examples of how this has
been done in some British fishing vessels range from a
very simple form of gyro automatic steering installed in
addition to the normal telemotor in the Prince Charles
Fig. 1. A simple form of gyro controlled automatic helmsman.
(Fig. 1), to the complete replacement of the telemotor,
illustrated in the auto-electric steering control system
installed in the Ross Renown (Fig. 2). The wheelhouse of
the Ross Renown also shows, as a separate feature, a
main engine control console sited at the starboard
wing fishing position alongside a remote steering control.
The photograph of the wheelhouse of the deep water
trawler D. B. Finn (Fig. 3) shows a complete control
trols include one-man operation of V.P. propeller,
ahead and astern and revolution control.
In all these examples, built-in safety factors are provi-
ded by visual and aural automatic alarms.
?i g^&jftjHf&j&siijtijffljiF
Fig. 3. Complete "one man" centralised control.
console. This includes not only the auto-electric steering
control system but also radars, echo sounders, ship shore
and close-range R.T. and main engine controls. Provision
is additionally made for normal engine room operation
of main machinery via engine room order telegraphs.
The illustration of the wheelhouse of the near-water
trawler Hazelhead (Fig. 4) shows an all-electric steering
control system controlling a standard electro-hydraulic
steering engine combined in a single console with full
main machinery controls. Because of the short-range
type of operation of this vessel, automatic steering and
gyro compass are not included. Main machinery con-
Fig. 4. Integrated electric steering and full engine control.
Fig. 2. Auto-electric steering control to replace telemotor steering with separate engine control console.
339
Despite such accurate navigational aids as Decca and,
in more extended areas Loran or similar types, the
requirement for a north-seeking compass remains and
will continue. The modern magnetic compass is a very
accurate instrument. While it has the advantage of low
cost by comparison with other types of north-seeking
compass, it has peculiar limitations in respect of accurate
plotting and navigation especially within a defined area.
On the other hand, the gyro compass, although more
expensive, can provide the essential accuracy required.
Apart from the inherent accuracy of an electrically-
driven north-seeking compass and its built-in ability to
transmit heading reference to remote positions, the
particular advantage, because of its transmitting facility,
has been to act as datum for automatic steering. In this
connection it must be emphasized that there are no
problems in hand or automatic steering control that are
essentially peculiar to fishing vessels. The problems of
navigation at sea are very much the same, whether the
vessel be very large or very small, and are a function of
the distance it is required to travel between two points
and of travel within an area. In the latter case, trawlers
and fishing vessels do have a special problem. To achieve
their aim of catching fish, it frequently is necessary to
maintain position within certain fairly well defined
limits. Echo sounding and plotting devices, the Decca
navigator and plotter, together with perhaps plotting
types of radar are very useful and the gyro compass
proves its advantage over the magnetic.
Here let it be said that one of the disadvantages of the
gyro compass has been its size and this has largely
been due to sensitivity to ship movement and vibration
and the necessity for rather elaborate means of protection
against extraneous influences. Modern techniques enable
a great reduction in size to be made in the gyro compass.
Automatic steering
There are many advantages in automatic steering,
especially those concerned with saving manpower.
The efficiency compared with manual steering increases
as weather conditions deteriorate and in any situation it
maintains a straighter course with quantitatively less
rudder application. This clearly is seen in Figs. 5A and
SB which show the automatically recorded comparison of
course and rudder quantity under manual and automatic
steering control. Sea conditions moderate, choppy in
Force 6 wind. The two records were taken in consecutive
periods on the same course.
To try and achieve the best of both worlds, much
thought has therefore been given to the problem of
providing a transmitting magnetic compass to act as
datum for automatic steering in small vessels. This work
has been based particularly on the need to reduce the
prime and running costs of an automatic helmsman.
For a number of years a wide variety of automatic helms-
tl 10
- ? ! * h'^lliiiiF; —
— U aisui&Y d/uwt i ~'-tg?»-~-^
- . -i. . . — »_,, i . . . 4* j . . ,« f~\. , .. ... ... . i
, •,
.......... l^£
•™f~f^fc
i . . .!• ... II.. ....
~*» i* i» m "
Fig. 5a. Comparison of hand and auto steering.
Fig. 5b. Comparison of hand and auto steering.
340
men using a magnetic datum has been offered but none
successfully solve the problem of universal application
to all classes of vessel, remembering that the requirement
is automatically to steer the ship and not necessarily to
provide heading reference to repeaters, radar and other
services.
A compromise had therefore to be made between low
cost by comparison with gyro controlled automatic
helmsmen and efficiency in dealing with the strenuous
conditions of fishing.
The problem
It is useful here to review the problem. The magnetic
compass should not only be able to act as datum for
automatic steering but should be suitable for use as the
main and perhaps only compass. It should be suitable
for installation as a deckhead compass, for incorporation
in a standard binnacle or for installation by itself without
a binnacle.
Because the system should have the widest possible
application, including very small vessels with perhaps
only a one-man crew, it should include some means of
indicating that the vessel has gone off the pre-determined
course, in fact an "off course" alarm. It should be able
to operate from any type of voltage supply AC or DC
including 12V, 24V or 32V. It should have the lowest
possible power drain, and be suitable for installation
in vessels of wood or metal construction. In view of the
very small size of many wheelhouses, its operation should
not cause magnetic interference.
It should be capable of operating with and controlling
any type of power assisted or non-power assisted steering.
It should include some means of disconnecting the main
steering wheel when the automatic helmsman is in
operation and yet provide a rapid means of reverting
to hand steering if required.
It is desirable that it should provide some alternative
means of hand steering, in or remote from the wheel-
house.
It should be completely weatherproof and have built-in
"fail safe" characteristics. As it should require virtually
no maintenance at sea, it should be rugged, reliable and
capable of operation continuously for long periods in
all types of sea and weather.
Its operation should be simple and within the capa-
bilities of the non-technical.
To these major points of the operations problem must
be added those of the manufacturer, especially as regards
cost. These include that the system should be capable
of using a gyro, a magnetic compass or both, in a single
installation, as alternative data. The basic system
should be capable of controlling any type of vessel from a
30-ft fishing boat to the very largest type of ocean-going
vessel. It should be capable of being engineered in a form
suitable to meet the space limitations present in many
vessels. It should contain as many universally standard
components as possible so as to permit easy maintenance
anywhere in the world.
There are, of course, other points which will be easily
recognised by all those connected with engineering and
the operation of vessels, but these are the broad essentials.
A solution
Taking advantage of the advance in automatic control
techniques and in the light of the long experience of a
number of companies throughout the world, solutions
have been found and equipments are now available that
incorporate virtually all the features listed.
Within the limitations of this paper it is not possible
to describe all the systems but an example has been taken
to illustrate the case. In essence it is generally descriptive.
The system is engineered on the "building brick"
principle. Thus, the simple requirement for automatic
steering can be built up into a sophisticated scheme to
cover every requirement. Fig, 6 shows the system in
Fig. 6. Automatic helmsman for small vessels, using magnetic datum.
multi-unit form, including the transmitting magnetic
compass. The same system is shown built as a single
console, including a gyro compass, in Fig. 7.
It is designed to operate with a fully certificated,
transmitting magnetic compass, with any type of gyro
compass or with a dual gyro-magnetic installation using
either compass as datum. It has a built-in "off course"
alarm system to indicate a deviation from the set course
by a predetermined amount, thus covering any type of
sea and weather condition. It can operate with and
control hand or power assisted steering of any type.
It can operate in conjunction with telemotor or electric
steering control or can replace both to provide a complete
steering control system. It can provide complete sophisti-
cation, including compensation for yaw and weather
and for the load and trim of the vessel, or, in its simplest
form, provide just automatic steering. Lever type hand
steering or remote steering by push buttons on an exten-
ded lead can be included.
It incorporates a special feature that has become extre-
mely useful in controlling a ship when in automatic
steering. This enables an alteration of course to be made
without reverting to hand steering, a feature particularly
valuable in fishing operations. The course can be altered
341
7. Integrated electronic steering control console incorporating
gyro compass.
round a datum or along a series of tracks with the vessel
constantly in automatic control and because of the sys-
tem's ability to apply "meeting" helm it can come to
each new course without overswing.
To illustrate the simplicity of the basic system, Fig. 8
shows in schematic form its application to the very
smallest type of fishing vessel with hand steering. Auto-
matic steering is integrated with remote engine control,
and it* will be appreciated that in the smaller craft this
can be extremely valuable in terms of safety and saving
in cost and space.
It is very significant that these systems have been
developed directly in response to the demands of oper-
ators and owners.
Automation of propulsion and auxiliary diesel engines
In the installations shown in Figs. 1 to 4 the main engines
are started manually although, in some, remote opera-
342
Fig. 8. Schematic layout of integrated automatic helmsman and engine
control in small fishing vessels.
tion is provided in the wheelhouse. In addition to new
electro-mechanical methods of remote manoeuvring
controls for single or twin-engine installations as illus-
trated in Figs. 9A and 9B, it is now possible for systems
to give fully automated remote starting for both simple
and complex starting procedures as well as full engine
control in the wheelhouse. The media for such fully
automated systems may be electric, hydraulic, pneumatic
or in combination. As with the latest types of automatic
steering these new systems are designed on the "building
brick" principle, each type of installation giving full
protection throughout the starting and stopping proce-
dure.
Typical operations that can now be automated or
remotely controlled in trawlers or, with less complexity,
in smaller fishing vessels are illustrated in Fig. 10. It will
be seen that these functions can include:
(a) Start and stop of propulsion and auxiliary diesel
engines.
(b) Ahead, neutral and astern operation of gearboxes.
(c) Engagement and disengagement of clutches and
propeller shaft brakes.
(d) Increase or decrease in engine rpm.
(e) Monitoring temperatures and pressures of engines
and associated machinery.
(f) Open and close seacocks.
(g) Control pitch mechanisms on variable pitch pro-
pellers.
(h) Malfunction protection and warning equipment,
(i) Main auxiliary service pumps for all duties, for
example lubricating oil, fuel oil, cooling water,
fire and bilge.
Figs. 9a and b. Electro-mechanical remote manoeuvring control systems for single and twin-engined installations.
( j) Operation of starting air compressors and selec-
tion of air bottles.
(k) Fuel and lubricating oil filtering and centrifuging.
(1) Automatic load-sharing and synchronisation of
auxiliary electric generators.
In any installation, it can be arranged that no operation
commences until the previous processes are working
correctly and should any not be completed successfully,
provision for more than one attempt can be made. This
applies particularly to engine starting and alternator
paralleling.
By the selective use of standard equipments, fully auto-
matic multi-engine installations can be operated with
fault protection, without human supervision. All
controls external to the control system can be interlocked
to prevent accidental misuse and visual indicators can be
343
Fig. JO. Typical operations that can be automated or remotely controlled in trawlers and vessels of all sizes. Manoeuvring: (a) Steering motor*
(b) Propeller pitch, (c) Reverse/ reduction gear control unit, (d) Governor (engine rpm), (e) Propeller shaft brake operation: Starting: (a) Main
engineCs), (b) Auxiliary engine(s), (c) Compressors), (d) Gearbox lubricating oil pressure pump, (e) engine lubricating oil priming pump, ( f) Air
bottk selection, (g) Air bottle charging, (h) Air starter valves: Operating: (a) Washing pumps, (b) Bilge pumps, (c) Sea cocks, (d) Fuel oil transfer
pumps, (e) Fire-fighting pumps.
LCVffP CONTROL
SYSTEM TO
AAttOUAOD TRANSMISSION
rr*rr/079P COTTONS
. 11. Examples of remote manoeuvring controls using pneumatic
or electro-hydraulic media.
included. All controls can be fitted with manual overrides
for emergency use. Provision can be made for the opera-
tion ef automatic firefighting equipment.
Typical examples of remote manoeuvring controls
using pneumatic or electro-hydraulic media are shown
in Figs. 11, 12 and 13. A more complex control console
designed for installation in a trawler is illustrated in
Fig. 14.
Hie future
With the advent of an electronic automatic helmsman
using computer techniques and the practicability of
344
Fig. 12. Examples of remote manoeuvring controls using pneumatic
or electro-hydraulic media.
fully automated main and auxiliary machinery control,
the way is open to more radical advances. It is perhaps
at this stage that we should pause for little while to
allow the human operators to assimilate and align them-
selves with all these new devices.
Attention was drawn by a number of speakers at the
Second World Fishing Boat Congress to the fact that
the human problem was becoming of paramount concern
in many sea-going operations and this applied particularly
to such complex operations as fishing.
While the "little" man concerned in comparatively
local fishing activity must remain an important factor in
achieving an increased haul from the sea, a greater and
CONTROL CAiJNtT
0* OQUtLt
OOVfRNOR WITH
jPseDcdN-ntOLum
. 7J. Examples of remote manoeuvring controls using pneumatic
or electro-hydraulic media.
increasing worldwide demand can only be met by the
operation of the bigger type vessels combined perhaps in
fleet formation. This inevitably must lead to further
requirements for increased automation, designed not
only to reduce the human factor and fatigue but also to
keep down cost to the consumer.
If fishing vessels and fleets are to operate at long dis-
tances from base and for extended periods, some form
of precise navigation must be evolved that can be opera-
tive equally in clear weather and in the densest overcast.
To operate a fleet in any weather condition an essential
feature is accurate position fixing. Advanced forms of
navigation using inertial techniques already make this
possible, albeit at high cost. Further development will in
due course reduce this to a more commercial common
denominator. Automatic position-fixing systems using
terrestrial satellites are already in experimental form.
There is reason to suppose that in the not too distant
future it will be possible automatically to resolve a satel-
lite data position fix that will use perhaps Doppler tech-
niques and atomic clocks for time data.
It will be necessary to evolve means in the vessel auto-
matically to evaluate and plot these position fixes and
subsequently transfer them in terms of steering control.
Fig. 14. Trawler type main engine andauxiliary remote-control console. Fig. 16. Design for navigating control centre showing key to equipment.
345
. 15. "Interim" scheme of semi-automatic ship control.
This is possible within the limits of present knowledge
and technique although here again the holding factor is
cost. Nevertheless, in view of the wide application it
wfll surely be possible for the fishing industry to take
advantage of such systems when they are available.
Clearly this is a sphere in which we must "make haste
slowly", not discarding old methods until new ones are
proved. It would be prudent to introduce a combination
system as a first step. Such an "interim" system, using
present methods for emergency and close range work
and fully automatic means for normal ocean navigation,
is postulated schematically in Fig. 15. This diagram
deliberately does not attempt to define the actual shape
of the wheelhouse itself. This is suggested in the further
Fig. 16 which abandons the conventional wheelhouse in
Continued at foot of page 347.
fig. 17. Example of integrated control console.
346
Driftnet Hauler for Salmon Fishing
In the Japanese North Pacific salmon fishery each driftnetter of
about 85 GT carries 260-300 nets, each SO m long by 7-8 m deep.
Thus the combined length of the nets is less than 1 5 km. In the early
years nets were hauled by hand at a rate of about 30 nets (1 ,500 m)
per hour. However, since 1954 mechanical nethaulers have been
adopted, with which as many as 70 nets (3,500 m) can be hauled
per hour under normal conditions. Only the leadline of the nets
is hauled mechanically. Various models have been produced but
all the variants can be grouped under three main types:
(a) I-type, where the leadline passes around three pulleys (Figs.
1-3).
(b) S-type, with a centre sheave which consists of two wheels
with a V-shaped groove between them (sometimes of variable-
gap) in which the leadline is wedged while passing over the
upper half of the sheave (Figs. 4-7) (The 1- and S-type are
mechanically driven).
(c) Hydraulic nethaulers are exemplified by the SU-type with a
variable-gap sheave on a semi-rotating head (Figs. 9-10).
Mechanically-driven haulers need 7-5 hp but the hydraulic
type 10 hp. The leadline is hauled at a rate of 40-80 m per minute,
depending on sea conditions and catch. The 1-type hauler needs a
railroller but can be converted into a longlinc hauler. Nethaulers
discussed in this paper cost from 250,000 to 300,000 yen (U.S.
$700- $830).
by
Chihiro Miyazaki
Regional Fisheries Research
Laboratory, Tokyo
montecs sur une tele semi-rotative (voir Fig. 9 a 10).
Les treuils mecaniques fonctionncnt avec 7-5 cv tandis que
pour le type hydraulique une puissance de 10 cv est nccessaire.
La vitesse de halage varie de 40 a 80 m par minute scion les con-
ditions de mer et Pimportance de la capture. L'emploi du type "I"
neccssite un rouleau de plat-bord mais il peut 6tre utilise aussi
comme treuil de longue ligne. Les treuils a filets maillants cites
dans cette etude coOtent de 250,000 a 300,000 yen (U.S.$: 700 &
800).
Treuils de filets maillants pour la ptche au
Resumt
Dans la peche iaponaise du Pacifique Nord, les bateaux sont de
85 tonnes et utilisent de 260 a 300 filets, chaque filet ayant 50 m
de long et 7 a 8 m de chute. La longueur totale de ces filets est
done moins de 15 kilometres. Autrefois les filets eiaient remont6s
& la main, & une vitesse de 30 filets par heure (1,500 m). Depuis
1954, des treuils mecaniques permettent de remonter, dans des
conditions normales, jusqu'a 70 filets par heure. Pendant I'op6ra-
tion la ralingue infeiieure seule est enroulee par le treuil.
II existe differents modeles de treuils mecaniques que Ton peut
classer en trois categories principals:
(a) Type "1" actionng mecaniquement ou les lignes a plombs
passent entre trois poulies.
(b) Type "S" actionnd mecaniquement dont la poulie au centre
est composee de deux pieces formant un V et dont 1'ecarte-
ment variable saisit la corde pendant son passage (voir
Fig. 4 a 7).
(c) Type "SU" actionnt hydrauliquement et dont les poulies
composees de deux pieces a ecartement variable, sont
Haladores de redes de deriva pom to pern dd
Extracto
En el Pacifico septentrional los japoneses pescan salm6n con
redes de deriva desde embarcaciones de unas 85 toneladas, cada
una de las cuales lleva de 260 a 300 secciones de red de 50 metres de
longitud por 7 a 8 de altura con las que forman andanas de menos de
15 km. Las redes se solian izar a mano a razon de unas 30 secciones
(1,500 m) pof hora, pero desde 1954 se emplean haladores mccani-
cos con los que se recogen hasta 70 secciones (3,500 m) por hora
en condiciones normales. Solamente la relinga de plomos se iza
mecanicamente. Se han construido varios modelos de haladores
todos los cuales pueden agruparse dentro de tres principales
(2 mecanicos y 1 hidraulico):
(a) modelo "I", en el que la relinga de plomos pasa por tres
poleas (Fig. 1-3).
(b) Modelo "S" con una roldana central formada por dos
ruedas con un espacio en V entre eltas (algunas veces el espacio
es variable) en el que la relinga baja se acufta al pasar por su
mitad superior (Fig. 4-7);
(c) Los haladores hidraulicos estan representados por el modelo
Continued from page 346.
favour of a "control centre" which might form the
upper compartment in the bridge structure.
These two suggestions are admittedly sophisticated and
applicable to the larger type of fishing control vessel,
but they are useful to illustrate the philosophy which
has application to smaller classes of vessels using far
simpler systems.
Fig. 17 illustrates that some forms of these systems have
already been introduced. The main console includes
auto-electric steering control.
Conclusion
It has to be accepted that automation will take its place
in every sphere of fishing and general ship operation.
The only matter of controversy is the degree and speed
with which it can safely be introduced. Whatever the
speed of advance, it must be agreed that no system will
be able completely to dispense with the obligation to
maintain an adequate visual "lookout" and that no
automatic device will replace the human element in
taking the final decision and action in emergency,
In the sphere of fishing operations it is pertinent to
recall the thoughts shared by the Chairman of the Second
World Fishing Boat Congress held in Rome, the late
Cecil Hardy:
"An early decision by the owner to take advantage of
centralised control would enable the naval architect to
prepare the most economical wheelhouse design, leading
to a change in the size and shape of the bridge and a
saving in weight and cost. Installation problems would be
modified and streamlined. An economy in manpower
would be achieved with, at the same time, reduction in
fatigue and increase in operating efficiency/*
347
"SU" con una roldana de garganta variable en una cabeza
scmigiratoria (Fig. 9-10).
Los haladores mecanicos tienen motores de 7*5 hp y los hidrau-
licot de 10 hp La relinga se iza a raz6n dc 40 a 80 metres por
minuto segiin las condkiones del mar y la cuantia de la captura.
El halador *T* ncccsita un rodillo en el cairel, pero este puede
aprovechane para izar palangres. Los haladores mencionados
en esta poncncia cucstan de 250,000 a 300,000 yen (700 a 830
d61ara EUA).
AFTER a long suspension during World War II, the
international conventions on fishing on the high
seas between the Governments of Canada, Japan and
the U.S.A., signed in 1953, made it possible for Japanese
fishermen to catch salmon in the vicinity of the Aleutian
Islands. Since 1956, salmon fishing by Japanese boats
off the Kamchatka Peninsula (U.S.S.R.) has been placed
under agreements between the countries concerned.
Almost all the fishermen who fish for salmon in these
areas, as well as in their home waters, have been employ-
ing driftnets.
Generally speaking, one net section of a fleet of drift-
nets used for salmon measures 50 m in length and 7 to
8 m in depth; an average driftnetter of about 85 GT
carries 260 to 300 such nets on board. On the fishing
grounds a number of nets are connected end to end and
set out for a distance less than 1 5 km. In the early years,
the nets were hauled by hand at a rate of about 30 nets
(a total of 1,500 m) per hour. This took long hours
since the combined length of the nets set by one boat
would cover several miles. In 1954 a mechanical
hauling device much improved the operation, as it
enabled the fishermen to haul as many as 70 nets (3,500
m) per hour under normal conditions.
The first model of net haulers introduced was an engine-
driven three-pulley system, called an I-type. Some of the
modifications that followed were the use of a sheave with
a deep V-shaped groove; hydraulic drive, etc. Various
models have been produced by the same or different
makers and brought into use in dose succession, but
these are all variants in their working principle on one
or other of three main types. For this reason, it seems
best to give a brief description of each one of these types
in regard to their construction, performance, relative
advantages, and other pertinent information about them.
I-type nethauler
Figs. 1, 2 and 3 illustrate the I-type, model 63-B, net-
hauler, as viewed from the bridge and the centreline of a
fishing boat. The body is made of cast steel. The lower
part houses the power transmission and clutch, etc.
A speed governor in the middle part responds to any
excessive strain on the net from heavy seas or from an
unusually large catch. The aft side of the head of the
Fig. /. Operation of I-type nethauler on board.
Fig. 2 Construction of I-type nethauler as viewed from bridge. A.
Guiding roller. B. Driving pulley. C. Pressing wheel. D. Gear levers.
E. Hand clutch. F. Clutch pedal. G. Driving shaft.
hauler has three pulleys with which the leadline of the
net is hauled. These consist of a guiding wheel, a driving
wheel and a pressing wheel (A, B, and C in Fig. 2), each
made of gunmetal and rubber-covered. The grooved
wheels are so arranged that the leadline forms a smooth
348
Fig. 3. I-type net hauler viewed from ship's centreline.
S curve passing over a semi-circle of each of the wheels.
Only the leadline is hauled by the grooved wheels while
the slack net and floatline is hauled by hand. The outer
sides of the wheels are partly covered with shields which
keep the net from chafing or tangling on the rotating
wheels.
When hauling the net, it first passes over a railroller
fitted on the bulwark. Then the leadline is passed under
the guiding pulley and over the driving sheave for about
half the perimeter. The spring-loaded pressing wheel on
the inboard side presses the leadline into the groove of
the driving sheave so as to secure a suitable traction.
Once fitted into the wheels, the line is automatically
hauled without the intervention of a deck hand. By
manipulating a clutch the hauling can be stopped at
intervals for removing fish from the net.
With the I-type nethauler a railroller is used for
guiding the net over the bulwark. This is an upright
wheel, mounted on a semi-rotary base, fitted on the
railing abeam of the hauler. As the base can turn hori-
zontally within an angle of 90°, the roller leads the lead-
line properly to the hauler regardless of the direction
at which the line comes to the boat.
S-type nethiuter
Figs. 4 and S show the S-type, model 38, nethauler. The
Fig. 4. Construction of S-type nethauler as viewed from bridge. A. Guide
roller. B. Drivintfjiheave. C. Press-wheel. D. Clutch pedal. E. Gear lever.
Fig. 6. Single drum nethauler working.
349
. S-type nethauler from centreline.
main difference of this type from the one described
above lies in its centre sheave, which consists of two
wheels with a deep V-shaped groove between them.
Passing over the sheave the leadline is wedged into the
groove. The nethauler is mounted on a pivoting base
so the sheave can be horizontally turned to any direction
within an angle of about 130?.
Fig. 6 shows a single-sheave nethauler of NT-type in
operation.
Another model of the sheave nethauler, called
YM-type, has a special spring arrangement which
always forces the upper half of the outer wheel to incline
a little toward the inner wheel, as seen in Fig. 7. Thus,
whil^ the upper sector of the sheave grips and pulls the
leadline, the lower half of the sheave readily releases the
line to coil on the deck.
350
U
/I
\
Fig. 7. (left) Drum of YM-type nethauler.
Fig. 8. Deck arrangement for an engine-driven net hauler. A. Nethauler.
B. Gear box. C. Counter shaft. D. Engine room. E. Belt conveyor for
transporting nets to the stern.
Fig. 8 is a schematic drawing of the deck arrangement
for an engine-driven nethauler.
Fig. 9 shows a more recent model of a driftnet hauler
which is driven by a hydraulic motor. It is called the
Fig. 9. Construction of SU-type hydraulic nethauler as viewed from
ship's centreline. A. Hauling sheave. B. Hydraulic motor. C. Speed
regulation lever.
SU-type. The head (containing the hydraulic motor)
pivots on the base. The spring-loaded variable-gap
arrangement of the upper half of the sheave is similar
to that of the YM-type.
The principle of the hydraulic driving arrangement is
schematically drawn in Fig. 10. A hydraulic pump is
driven by a belt from t£e auxiliary engine. The pump
forces oil to flow through a pipe to the hydraulic motor
of the hauler. A control valve regulates the quantity of
the oil to the motor, to give a maximum pull of up to
one metric ton. The motor is normally built for oil
pressure up to 20 kg per sq cm. The nethauling sheave
is driven by a chain from the hydraulic motor. The hauler
A-Jfi 1
l\
t
Fig. JO. Deck arrangement for a hydraulic nethauler. A. Nethauler. B.
Hydraulic motor. C. Hydraulic pump. D. Oil tank.
can turn from the athwartship line to any required haul-
ing direction within a horizontal angle of 60° towards
the bow and 70° towards the stern.
Table I lists the performance characteristics of the
various types of driftnet haulers described. An engine out-
put of about 7*5 hp suffices for driving most of them,
although a 10 hp engine is needed for the hydraulic
nethauler. The leadline is hauled at a rate of 40 to 80 m
per minute, depending on the sea and the catch.
Selection of one or the other of these types seems to
depend upon the particular duties of the fishing boat
concerned, as every type of nethauler has its specific
advantage and disadvantage, one setting off the other.
For instance, the I-type hauler must be used with a
railroller, but it can readily be converted into a longline
hauler by interchanging its head section with one for
hauling tuna longlines. So far, the other types are not
provided with this convenient feature, but they have
other advantages, such as the variable-gap sheave and a
wide horizontal turning arch to secure a normal hauling of
the leadline. The hydraulic nethauler generally performs
most smoothly and efficiently. The cost of installation is
higher than a mechanically-driven nethauler — about the
same as its purchase price. Current domestic prices
of driftnet haulers here discussed range from 250,000 to
300,000 yen (i.e., $700 to $830) approximately.
1-981
I-6JI
160
160
196
173
600
990
400
399
399
300
BtTOlUtiOM Of
-sur*
840 to MO
•40
ItO
200 to 300
200
JbullMMDffd
^Gt TSS
(V-in) (•/**)
71-81 97-67
71-81 97-67
98-67 90-98
40-94
79 *o 9 *• 0
TS
7*9
7.9
7.9
7.9
7.9
10
351
Mechanisation of Driftnet Fishing Operations
AMract
The mechanisation of driftnet fishing operations has lagged behind
when compared to other commercial fishing methods . In view of the
importance of herring drifting, the Soviet gear technologists and
engineers have co-operated in mechanising such operations as far
as possible. Hundreds of medium-size vessels have now been
equipped with net haulers and net shaking machines, special mecha-
nisms for hauling the bridles and for stowing the warp. The vessels
also have machines installed with the necessary conveyor systems
for salting the herring. They are, furthermore, fitted out with a
warp leader and a net guide which allows the vessel to shoot the
nets safely while it is steaming slowly ahead. To avoid snapping
of the warp under surge-loads when the vessels operate during
poor weather, the warp is led over a shock absorber which can
take up a load of up to 3} tons. The introduction of the mechanisa-
tion has increased the rate of hauling and handling the nets during
operations and reduced the number of crew required for operating
the gear.
Mecanlsatloii des operations de filet a la derive
La mtomisation de la peche a la derive ne s'est pas dtveloppee aussi
rapidement que les autres mtthodes de peche commercial. Etant
donne 1'importance de la peche a la derive pour les harengs, les
technologists d'engins de peche* et les ing£nieurs Soviitiques ont
coopere pour m6caniser ces operations autant que possible. Des
centaines de bateaux de peche moyens, sont maintenant equipes
avec des trcuils de filet, des machines a secouer les filets, des m6can-
ismes spttiaux pour haler les brides et ranger les funes. Les bateaux
ont aussi des machines pour saler les harengs avec systemes de
bandes convoycuses. Us sont £quip6s avec des conductcurs de
Ames et un guide de filet permettant de mettre les filets a l*eau en
toute s6curit£, pendant que le bateau avance au ralenti. Pour
6viter que les funes ne se cassent sous I'effet des vagues par mauvais
temps, la func est conduitc par un amortisseur de chocs qui peut
amortir une force de 3-5 tonnes. L'introduction de cette mteanisa-
tion des operations de peche a accru la vitesse de halage et r6duit
l'6quipage ntaessaire pour 1'op^ration de 1'engin.
Mecanizaddn de la pcsca con redes de deriva
Extracto
La mecanizaci6n de la pesca con redes de deriva ha quedado
atrasada con respecto a otros mttodos de pesca industrial. Dada la
importancia que tiene la pesca del arenque con redes de deriva,
los tecndlogos e ingenieros Sovteticos especializados en equipo
de pesca han cooperado en la mecanizaci6n, en todo lo factible,
de tal pesca. Cientos de embarcaciones de dimensiones medianas
se han dotado de maquinas haladoras y sacudidoras de redes, y de
mecanismos especiales para izar las bridas y adujar el cable;
llevan tambien maquinas con sus transportadores necesarios para
salar el arenque y cuentan asimismo con guias para los cables y la
red que permiten calar los artes con seguridad mientras se navega
tentamente. Para evitar que el cable se rompa por cfecto de esfuerzos
viokntos cuando la mar es gruesa, 6ste se pasa por un amortiguador
con resistencia hasta de 3-5 tons. La mecanizaci6n de las maniobras
ha incrementado la velocidad de halado y reducido el numero de
marineros necesarios para manipular el arte.
DRIFTNET fishing operations have lagged behind
other commercial fishing methods as regards
mechanisation. But Soviet gear technologists and engin-
eers have co-operated in developing different kinds of
mechanical deck equipment to reduce some of the time-
consuming operations.
Hundreds of medium-sized vessels are now equipped
with such equipment which includes net hauling machines,
bridle hauling mechanisms, warp stowing machines,
net shaking machines, herring salting machines and
mechanisation for protecting the warp during very high
352
by
P. A. Kuraptsev
Institute of Marine Fisheries
& Oceanography, Moscow
strain in bad weather; deck gear for shooting the net and
instruments for quick measurement of water temperature.
The introduction of these new techniques has increased
the rate of hauling and handling the nets, has cut labour
for catching and processing and has increased the yield
of the drifters.
Net chute installation
The best way of setting a herring net of which the warp
is underslung, is to shoot from a moving vessel; to
prevent fouling by the propeller special gear was devel-
oped consisting of a net leader and a warp leader
designed to shoot the nets and warp well out from the
vessel's side clear of the propeller. The deck arrangement
of this for a medium-sized vessel is in Fig. 1. The
warp leader consists of a horizontal guide roller, a
vertical roller and a retention guide. The vertical roller
is mounted on a bracket hinged to the vessel's side which
allows the roller to be turned inboard when not in use.
The net leader shown in Fig. 3 is constructed from
pipes welded together. It is 6 m in length and 1.5 m in
height. The after-ends of the pipes are smoothly bent
and connected to the cross-piece so that they lead the
net well outboard. A vertical pipe, 800 mm high, prevents
the nets leaving the leader. The entire assembly can be
easily removed.
Shock absorber
To protect the main warp from overload during bad
weather a special shock absorber was developed— Fig. 4.
The compression of springs releases a certain length
of warp and as soon as the surge-load is removed the
springs relax.
The shock absorber consists of a welded housing,
energy-absorbing unit and a stress-indicating device.
It can withstand a load of up to 3£ tons and pays out
six m of warp.
Net hauler
Several machines have been developed for hauling and
handling the nets. The hauling machinery shown in
Figs. 1 and 5 consists of a driving roller placed on the
ship's bulwark, two grooved hauling heads and an
Fig. 1. Deck arrangement of hauling machinery.
'fUs jM
. 2. Warip
Fig. 5. Model of herring driftnet hauling machinery. The float lines
and sinkerlines are hauled by the vertical gurdies gripping the
lines with spring-activated fingers over half the circumference of the
head.
353
x
apparatus for hauling the bridles which attach the nets
to the warp. It is driven electrically by one motor
located underdcck. The driving roller can be tilted
inboard.
The driftnets are hauled by two hauling heads which
grip the lines (floatline and sinkerline) with spring-
actuated fingers in the appropriate grooves. With this
machinery the hauling rate of the nets is from 15 to
35 m per min. The engine power is 3*7 kw, while the
machinery weighs 980 kg.
Fig. 6. Machine for shaking fish out of the net.
Drifteet shaker
Shaking the herring from the driftnets is a very time-
consuming operation requiring much labour, and
machines were introduced which mechanised this opera-
tion— see Fig. 6. This machine reduces shaking time
to about 1 to 3*5 min per net. It consists of two posts
carrying the shakers and two driving rollers for hauling
the nets up and taking them to the place of storage. It
is installed about amidships (Fig. 1) and is driven by
an electric motor of three kw, placed in one of the sup-
porting posts. The shaker vibrates from 165 to 168
shakes per min and weighs 750 kg.
Salting machine
The machine for salting herring (Fig. 7), comprises a
conveyor for transporting the fish from the deck, a salt-
hopper, a screw conveyor which feeds suitable quantities
of salt, and a barrel for mixing the salt with the fish.
This mechanisation has considerably raised productivity.
The capacity of the salting machine is from 3£ to 4 tons
per hour. The machine is driven by a 3'8 kw electric
motor and weighs 960 kg.
Warp stowing machine
A special machine has been developed for stowing the
net warp in the hold of the vessel (Fig. 8). Installed in
II j f* I ";«**'•*•""»• «"• •• rufrwtrns ptutjurm JUT compacting ine nerr
the need for laying them down by hand, head to tail. H. Kristjonsson photo,
The Complex Mechanisation of Beach Seining
Abstract
Although in recent years commercial fisheries on the high seas
have developed extensively in the U.S.S.R., the use of beach seines
on shallow coasts and in inland waters is still of importance. Before
the Revolution seines provided about half of the total catch and,
in spite of the large development in other types of fishing, beach
seining still ranks high in the general fish production. The method
has now been largely mechanised by fisheries research workers and
engineers and, while it was once one of the most laborious fishing
methods, the use of new hauling appliances has made possible large
savings of manpower. Depending on fishing conditions, the beach
seines used in the U.S.S.R. vary from 150 to 2,000 m in length with a
height of up to 40 m while the hauling lines may be up to 6,000 m
long. The type of hauling appliances used depend on whether the
fishing grounds are operated continuously, temporarily or at
irregular intervals. In inland waters the seines are operated from
self-setting boats on which the nets and lines are flaked down and
from which the gear runs out on its own during shooting. Such boats
are normally towed by 8-12 m motor boats powered by 6-15 hp
engines which develop a speed of up to seven knots. The seines are
hauled by winches installed ashore which have a traction force of
1-3 tons and a hauling speed of 6 to 80 m per min. Such winches can
be permanently installed or mounted on tractors. Net stowing
machines have also been developed so that the net is now reloaded
onto the boat mechanically. For marine operations a new type of
craft has been developed which combines the function of tug and
setting boat. These vessels are 13-5 m long, have a displacement of
10 tons for a draught of 0-75 mand are powered by a 15-hp engine
which gives a speed of up to five knots. They are provided with a
winch with a hauling power of one ton and a hauling speed of up
to 40 m per min. When fully mechanised, these vessels may also
carry a seine-stowing machine.
La mecanisation complexe des sennes de rivage
Resume
Bien que la pfcche commerciale hauturiere se soit developpee ces
dernieres annees de facon considerable en U.R.S.S., I'utilisation des
sennes de rivage sur les cdtes peu profondes et dans les eaux
continentales a encore une certaine importance. Avant la Revolu-
tion, ces sennes produisaient environ la moitie de la capture totale
et, malgre de grands developpements dans les autres methodes
de pdche, la senne de rivage a encore un rang important dans la
by
S. S. Torban
Institute of Marine Fisheries
and Oceanography, Moscow
production generate de poisson. Cette methode qui demandait
beaucoup de main-d'oeuvre dans le passe a M largement mecanisee
par lestechnologistes et les ingtnieurset le halage mecanique a permis
de realiser une economic de main-d'oeuvre. Selon les conditions
d' operations, les sennes utilisees en U.R.S.S. ont de 50 a 2000 m de
long et jusqu'a 40 m de haut tandis que les funes mesurent jusqu'a
6000 m. Suivant les lieux de pdche, les dispositifs de halage peuyent
fctre installed definitivement, temporairement ou a intervalles irre-
guliers. Pour la peche dans les eaux continentales, les sennes et
les funes sont rangees sur un bateau qui les met a 1'eau automattque-
ment; ces bateaux sont toues par d'autres bateaux motorists de
8 a 12 m, ayant des machines d'une puissance de 6 a 15 c.v. qui
d&veloppent une vitesse de sept noeuds. Les sennes sont halees par
des treuils installes a terre d'une force de traction de 1 a 3 tonnes et
d'une vitesse de halage de 6 a 80 m/min. Ces treuils peuvent etre
fixes au sol ou montes sur des tracteurs. Des machines pour ranger
les filets ont etc aussi dtveloppees de sorte que maintenant les
filets sont remis sur le bateau mecaniquement. Pour les operations
maritimes, un nouveau type de bateau qui combine les deux
fonctions (remorquage et mise a 1'eau automatique du filet) a ete
ddveloppe. II a une longueur de 13-5 m, un d£placeraent d'eau de
10 tonnes, un tirant d'eau de 0-75 m et des machines de 15 c.v.
lui assurant une vitesse de cinq noeuds. Ce bateau est 6quip6
d'un treuil ayant une puissance de halage d'une tonne et une vitesse
de halage de 40 m/min. Lorsqu'il est entteremcnt m6canisc, ce
bateau peut aussi porter une machine a ranger les filets.
Continued from page 354.
Fig. S. Machine for stowing the net warp in the hold of the vessel
the warp compartment of the forehold, it is operated
from the deck through stiff rods and pedal gear. It
stows the warp at a rate of up to 35 m per minute. When
using a 22 mm warp the capacity of its drum is up to
4,000 m. This machine is driven by a 2*8 kw electric
motor and it weighs, complete with drive arrangement,
800 kg.
Thermometer
A semi-conductor resistance thermometer which allows
remote readings of the sea temperature ensures a constant
control of the temperature within the ranges of — 2°C
to + 12°C. The accuracy of the instrument is 0*1 °C and
it is designed to work from the vessel's direct current
electrical system.
Results
Mechanisation increases the rate of hauling and the
speed of handling driftnets with a smaller crew. During
the last fishing trip of SRT/4256, which is fully equipped
with the machines and mechanisms described above,
its catch of 62 tons of herring was fully processed and
turned over to the mother ship within 24 hours.
355
•ecanizadon d* lot artw de playa
Extracto
Aunque en los tiltimos aftos la pesca de altura se ha desarrollado
extraordinariamente en la Uni6n Soyidtica, el empleo de artes de
playa en costas someras y aguas continentales todavia tiene impor-
tancia. Antes de la Revoluci6n las redes de oerco suministraban
ccrca de la mitad de la captura total y, a pesar de los muchos
addantos de otras clases de pesca, la que se practica en las playas
sigue desempeftando un importante papel en la producci6n pes-
quera total. Esta pesca se ha mecanizado mucho por los invest!-
gadores pesqueros y los ingenicros y mientras una vez fue uno de los
procedimientos mas laboriosos, el empleo de la mecanica facilita las
maniobras y ahorra mano de obra. Segun las condiciones locales,
los artes de playa empleados en la U.R.S.S. varian en longitud de
150 hasta 2,000 m y pueden tener una altura hasta de 40 m, en
tanto que los cables necesarios Uegan a ser hasta de 6,000 m. La
clase de dispositivos de halar depende de que los bancos de pesca
se exptoten continua, temporal o irregularmente. En las aguas
continentales la maniobra de los artes se hace desde embarcaciones
que calan, recogen y arrollan redes y lineas automaticamente.
Normahnente estas embarcaciones las remolcan botes de 8 a 12 m
con motores de 6 a 15 hp y con velocidad hasta de seite nudos.
Cuando se pesca desde la playa las redes las halan maquinillas
instaladas permanentemente en clla o montadas en tractores.
Actuahnente tambidn se fabrican maquinas estibadoras que meten
a bordo la red mecanicamente. Para la pesca maritima se ha
proyectado una nueva clase de embarcaci6n en la que se combinan
las funciones de remolcador y bote de calamento; tiene 13*5 m de
eslora, desplaza 10 tons, cala 0-75 m y lleva motor de 15 hp que le
da una velocidad de cinco nudos. Esta dotada de una maquinilla
con una fuerza de tracci6n de una ton y una velocidad de 40
m/min. Cuando se mecanizan totalmente estas embarcaciones
tambten pueden llevar una maquina para estibar la red de cerco.
BEACH seining on shallow coasts, in lakes, rivers and
artificial reservoirs is still of importance. It is especi-
ally popular in the Caspian Sea and in the delta of the
Volga River, in the richest inland fishing areas of the
Soviet Union.
Beach seining in the past was laborious but with the
development of the fisheries industry research officers
and engineers developed mechanisation.
The beach seine (Fig. 1) belongs to the filtering type
Fig. L Beach seines of the river type. (1) Headline (2} Floats. (3)
Groundrope. (4) Sinkers. (5) Bag. (6) Shore wing. (7) Tow wing.
(8-9) Spreaders. (JO) Bridles. (11) Shore line (hook line).
of fishing gears. It consists of a long sheet of webbing
hung on the headline and footrope with a centre bag.
Two kinds of beach seines can be distinguished: sym-
metrical seines with the bag in the centre and asym-
metrical seines with the bag located close to either end.
As a rule seines in lakes or reservoirs are symmetrical
while the wings of seines in rivers differ in length.
Length may vary from ISO to 2,000 m, height from
5 to 30-40 m and the total length of hauling lines from
150 m up to 5,000 - 6,000 m.
Fishing grounds may be permanent, temporary or
irregular. Permanent grounds are fished all the year
round, (except close seasons) with from 20 to 30 sets per
day. Temporary grounds are fished during 1-1 5 months.
Permanent and temporary grounds are equipped as a
356
rule with appropriate means of mechanisation. The
area has houses for fishermen, shops, canteens, dispen-
saries, recreation and reading rooms and schools for
children.
After stowing the net and lines on board, the seine
boat is taken in tow by a towing cutter, with a skiff
loaded with signal lanterns or flags. At the place of
operation a seine is set across a river for about two-thirds
of its width, as shown in Fig. 2, leaving a third of the
Fig. 2. Position of a b> ach seine at different stages of towing. (A)
Shooting commences. (B-C) Positions as tow wing is towed to the bank.
width free. The cutter than makes for the beaching
place, paying out a towline on its way.
When hauling commences, the line is first hauled at
70-80 m per min but, as the wing approaches, speed is
reduced to 10-12 m per min. The shore bridle is attached
to a block which travels on a wire as the net is carried
downstream to the hauling point. There both wings are
hauled together. The complete net can be hauled by the
winch. The fish are removed through a special slit, on
one side of the bag.
During hauling the towline and wing are stowed on
the seine boat ready for a new operation.
The tow cutters are 8-12-5 m long — have draught of
0*5 to 0'8 m — and are powered with 6 to 15-hp engines.
On the lakes the horse-power is increased to 20-30 hp
(Fig. 3).
Fig. 3. Small-sized towing cutter. (1) Internal combustion engine. (2)
Fly-wheel. (3) Reversing clutch. (4)Suctionpump. (5) Discharging pump.
(6) Muffler, (7) Exhaust pipe. (8) Lubrication. (9) Kerosine tank. (JO)
Screw shaft (11) Propeller. (12) Deadwoodpipe. (13) Metal "ski" d4)
rudder. (15) Tiller. (16) Tow hook. (17) Watertight bulkhead. (18) Eng-
gine room.
To set the net, a self-setting seine boat (Fig. 4) is
used on which the deck platform is slightly inclined
- Stiff
\* 9900
Fig. 4. Self-setting seine boat.
towards the stern. By flaking down the seine to a special
pattern, it slips easily into the water while the boat is
being towed (Fig. 5 and 6). A horizontal roller at the
stern minimises wear and tear.
Fig. 5. Shooting a seine from a self-setting seine boat.
Fig. 6. Paying out the towline.
The first beach seine winch in the U.S.S.R. was inven-
ted in 1907 by a self-taught carpenter, Guzhayev, for
hauling seines on the rivers of Siberia. The wide applica-
tion for seine hauling started in 1923-1924. These winches
did not differ much from those used in the building
trade (Fig. 7).
New developments brought electric seine winches of
LNR-1 type for river operation, and LNM-1 type for
sea operation. The only difference is their traction force
and range of hauling speeds which are 1,000 kg and 10-80
Fig. 8. Seine winch designed on tractor basis.
m per min for LNR-1 and 3,000 kg and 6-36 m per min
for LMN-1 respectively.
The LNR-1 winch is shown in Fig. 9. Specific charac-
teristics are (a) a wide range of hauling speeds by using
Fig. 9. Beach seine winch on the lower reaches of the Volga. This Is a
permanent installation.
a stepless frictional transmission, (b) two pairs of grooved
warping heads to make the hauling process safe; (c) an
automatic electric overload release control, and (d)
indicators of hauling force and speed for greater efficiency.
Apart from the lines, the wings of seines are hauled
ashore mechanically. An additional working line is
attached to the footrope of the wing and is hauled while
the wing is gathered on the deck of the seine boat.
Whereas previously from 100 to 120 people were
engaged in shooting and hauling marine beach seines
by hand, four or six men now do the same work.
In 1960-61 a special machine was developed to haul
the wings of seines operated on sea beaches, The machine
(Fig. 10) is a self-propelled tractor bogie on whose plat-
form two vertical frictional driving drums are driven by
357
flepedodnuft
Fig. 7. Seine winch ofMatveefs design. (1-2) Legs. (3-5) Stretcher bolts. (6) Driving shaft. (7) Intermediate shaft. (&) Working shaft. (9-14)
Toothed wheel. (15) Change gear. (76-77) Tractor drums.
Fig. 10. Mobile seine hauler.
the tractor engine and the drive runs underneath the
platform. Hauling can be done with the machine at
standstill as well as while moving along the shore.
In river seining a special arrangement is made for
releasing and hauling the shore wing. A steel rope is
stretched between the initial point of setting and the
stopping place ashore. A special snatch block with a
hook runs along this steel rope. A shore-bridle called the
hook-on line is fastened to the hook (Fig. 11). The
seine moves with the current towards the stopper and it
is necessary to retain it by making use of a brake arrange-
358
Fig. H. Scheme of transferring a shore line (hook-on line). (1) Steel
wire, (2-3) Anchoring points. (4) Metal block. (5-6) Cramp frame. (7)
Hook. (8) Hook-on line. (9) Bridle. (10) Spare hook-on line. (77) Fixing
peg.
ment or a fixing peg. The shore wing is then hauled from
the stopping place to the landing spot by means of an
endless rope, the arrangement of which is illustrated in
Fig. 12.
Flaking and stowing of the seine on the seine boat is
now done by special machines and devices. The NM-1
machine consists of three profiling drums and is installed
**»***&
Fig. 12. Scheme of rope-way for drawing a shore line from the stopper
to the place of stranding. (7) Endless- rope. (2-5) Guiding rollers. (6)
Supporting rollers. (7) Seine winch. (A) Stopper. (B) Start.
on a self-setting seine boat and, by guide rails, stows the
seine along the deck platform. The main features are
shown in Fig. 1 3 a and b. For stowing marine type beach
seines other machines are applied, which usually consist
of two vertical drums installed on the stern platform of
the seine boat. The drums are driven electrically with
the driving unit installed under the deck.
The above mechanisation is quite effective at stationary
fishing grounds but on temporary and irregularly-
Fig. 13b. Machine for automatically stacking seine out of water on to
boat ready for setting.
operated grounds would be less efficient. On such grounds
the gear must be mobile and simple, cheap and easily
manoeuvred, to ensure quick switch from one ground to
another.
Two types of gear are applied at occasionally-fished
grounds. These are : a hauling cutter in combination with
Fig. 13a. Seine-stowing machine. (1-3) Profiling
drums. (4) Frame. (5) Electro-motor. (6) Reduc-
tor. (7-70) Star-wheels of the chain, gear. (II).
Chain. (12) Tappet sleeve. (13) Adjusting handle
of the tappet sleeve. (14) Gutter. (15) Gipsy head.
(16) Roller. (17)Platform (18-19) Running wheels.
(20-21) Guiding rails. (22) Bulwark rail. (23)
Steering wheel. (24) Electric bulb. (25) Rejector.
(26-27) Legs. (28) Pipe (29-30) Arcs. (31) Elec-
tric cable.
359
a self-setting seine boat and a motor seine boat. The
winch cutter is a modified kind of 6-10 hp towing cutter
with a winch installed at the stern and driven by the
cutter's engine (Fig* 14). The hauling power of the winch
Fig. 14. Winch cutter.
is 750 kg; hauling speed 25 m per min. The draught of
the cutter is only 0*5 m which allows the vessel to pass
shallow rivers, streams and lakes. The helper boat is a
small-sized self-setting seine boat for stowing, transport-
ing and shooting the seine. This makes is possible to
Fig. 15. Scheme of operation of a seine on an occasional fishing
ground. (1) Seine. (2) Towline. (3) Fixing peg. (4) Snatch block.
(6) Winch cutter. (7) Mooring anchor. (8) Self-setting seine boat. (9)
Anchor for fixing a seine boat. (JO) Wing of a seine which is stowed.
(77) Lantern boat.
transfer fishermen from one ground to another quickly.
The team operates as follows: before shooting, the
cutter lands two men (with snatch block gear) and they
secure the necessary shore installation with anchors.
When the net has been run out, the towline is passed
through the snatch block to the winch drum of the cutter
which is anchored in the position shown in Fig. 15, and
hauling begins. Then they go to the next operating ground.
A team can operate six to eight sets per day, whereas
without such mechanisation 9-10 fishermen make only
one or two sets per day.
12 11 10 9 8 7 Q. S (4 3 . 2
Fig. 16. Motor seine boat.
360
Around 1940 it was suggested that it would be more
practical to combine on board one vessel the advantages
of the tow cutter and the self-setting seine boat as well as
the hauling devices. As a result, a motor seine boat was
designed as shown in Fig. 16. The vessel has a displace-
ment of 10 tons for a length of 1 3*5 m and 0*75 m draught.
It is powered by a 15-hp engine and develops a speed of
eight to nine km/hr (4-5-5 knots). The 1,000 kg at a
Fig. 17. Motor seine boat with a seine-stowing machine of 1953 model.
hauling speed ranging from 15 to 40 per min (50-130
ft/min). Apart from the winch they are also equipped with
a seine stowing machine of NM-1 type (Fig. 17). Such
vessels allow full mechanisation of seining and complete
mobility from one fishing ground to another.
Discussion on
Deck Machinery
and Control
Mr. E. A. Schaefers (U.S.A.) Rapporteur: Lerch (Paper
No. 64) indicates that progress in fishing gear development
has been slow. Great strides have been made, however, in the
application of power and mechanisation on board fishing
vessels. Mechanisation for handling king crab pots in Alaska,
as described by Allen, has been largely responsible for the
economic success of a fishery which utilises a passive type of
fishing gear. King crab fishing is so highly mechanised that
the most recent and largest king crab fishing vessel in Alaska,
which is 1 59 ft in length, operates with a crew of only four men.
Kuraptsev indicates that although driftnet fishing operations
for herring have lagged behind other fishing methods with
regard to mechanisation, Soviet gear technologists have
recently developed mechanical deck equipment which has
allowed for reducing the number of crew members while
increasing the hauling and handling speed. This equipment
includes net haulers, net shaking machines and special
mechanisms for hauling the bridles and stowing the warp.
The high degree of automation and extensive use of central-
ised controls will enable the 99-ft English sterntrawler Ross
Daring, when launched, to operate with a total of five men,
whereas a conventional sidetrawler of comparable size would
need ten men on deck alone. The recently launched highly
mechanised 83-ft United States combination sterntrawler-
purse seiner Narragansett, is already operating in the trawl
fishery with a total crew of only four men. New hauling appli-
ances for beach seines, as described by Torban, have replaced
the manual labour which formerly performed these operations.
Consequently, this is no longer one of the most laborious
types of fishing methods. Lerch also indicates that hydraulic
power has played an important part in increasing fishing
efficiency, especially in the United States fisheries. He divides
hydraulic systems for fishing vessels into low pressure,
200-550 psig, medium pressure, 1,000-2,000 psig and high
pressure, 2,000-5,000 psig. He outlines the various systems
and gives particular attention to controls. In the United
States the medium pressure system is favoured. These hydrau-
lic systems make possible multiple and remote controls and
the advantages of these are clearly pointed out, with examples
of hydraulic gear handling machinery such as gillnet reels,
crab pot haulers, and various power units on modern tuna
and salmon purse seiners. He emphasises that the most
important factor for the protection of long life of hydraulic
pumps, motors and control valves, is maintaining clean oil
at all times. It has been my experience that it is well worthwhile
to assign one individual the specific task of ensuring that no
dirt enters the system during installation and that no dirt
enters the system when oil is added later. If a hydraulic system
starts out with dirty oil it will never function properly. If it
starts out with clean oil, there is no reason to expect mal-
functioning.
1 am sure we are aware that increased mechanisation js the
key to the survival of commercial fishing as a business propo-
sition. But we must ever be mindful that we do not get techno-
logically ahead of the capabilities of man to direct the wonder-
ful advances made possible through technology. There
appears to be little danger of this at the present time. Pain's
paper on centralised control in trawlers covers such items
as rationalising the wheelhouse, navigational controls,
automatic steering and the automation of propulsion and
auxiliary diesel engines. This subject was discussed at the
World Fishing Boat Congress in 1959 and it would be
interesting if we can now have some reports of firsthand
experiences regarding the use of much of this equipment.
Dr. Schfirfe (Germany): The whole objective of centralised
control and improved deck machinery is to make fishing pay
better. Therefore, we must keep a sharp eye on economic
factors. At the last Fishing Boat Congress the late Commander
Hardy suggested that the cockpit of an airliner might be a
good example of how to design the ideal wheelhouse for a
trawler. There is, however, a danger that the skipper might
be overworked if asked to do too much in the way of control.
It might therefore be better to leave a part of the tasks with the
engineers, the radio operator and an eventual sonar operator.
There was also the danger that the centralised control panel
might take space in the wheelhouse which might be used more
profitably in other ways. A compromise for the large trawler
would be to distribute the tasks. Centralised winch control
appeared to be a very sound idea but the factors of cost and
the working power of the skipper should also here be taken
into consideration. The basically good idea of centralised
control should not be overdone — they should centralise with
sense.
Commander H. Pain (U.K.): "Centralisation with sense1*
is a very good catch phrase. What his company meant by
centralisation was only the gathering together of those instru-
ments and operations in the most convenient place for the
best operation by the right number of men to achieve the
prime objective. This applies to the biggest ship in the world
as well as to the smallest. His plea was that it was useless for
the automation companies to be given the task of automating
a ship when the ship's design has been already finalised. They
had to do this on a number of occasions and the result has
361
not been very happy. He recommended the company come
in for consultation with the owners, the yard and the naval
architect at a very early stage when the ship's specifications
were being drawn up. The Narragansett, the Ross Daring and
other ships had proved that if this concept was applied they
would get a more efficient and more economical class of vessel.
A very important factor was the human engineering problem.
Hiey might have to produce a newer type of skipper; "fishery
control officer9* might be the better term. Skippers were
being faced more and more with a mass of highly scientific
"black boxes" and many of those he had met were rather
frightened of them. He thought, however, that "Big Brothers'9
as he termed them, could help the fishing industry consider-
ably. I suggest that most owners, builders and architects know
some of the answers some of the time but no one class of
people know all the answers even half the time. If you can go
to the big organisations and are prepared to consider the
ship as a piece of system engineering then you can secure
advantages. If it is considered from the beginning as an
engineering problem, the systems engineer can give results.
Mr. Lusyne (FAO): Apart from asking too much from the
skipper in this mechanising and controlling I see another
difficulty, which is the problem of the skipper obtaining the
information necessary to exercise control. The skipper must
consistently be informed of what is going on. At the present
time, without mechanisation, the skipper sees the warps
closing and opening and that gives him vital information.
If he is locked up in the wheclhouse, however, he must
depend entirely on machines for the incoming information
he needs. If these fail to function properly, as machines
sometimes do, he will cease to get the information he wants.
Dr. Heinsohn (Germany): I agree with other speakers in
respect of not overdoing centralised controls. On deck machin-
ery they had heard much about hydraulics but he would like
to mention electricity. In many cases it did not matter whether
hydraulics or electrics were used. Many people had a horror
of electricity — he had too — but when he thought of high
pressure hydraulics it was just as bad. The stress put on
absolute cleanliness meant that they had to have experts
on that as well as with electricity. Where you can use your
prime source of energy for different purposes it may be more
favourable to use electricity rather than hydraulics. On big
bottom trawlers you use your winch power also on propulsion
and if you come to very modern achievement you use your
winch braking power to feed it back into the propeller.
Shooting a big trawl in deep water consumes a lot of energy
which is wasted on the brakes which have constantly to be
replaced at heavy cost. It is therefore, quite useful to look for
devices which feed this otherwise useless power into the
propeller. This is possible where prime movers for winch
power are derived from the propeller shaft which is the normal
case today. It is not possible to do this with high pressure
hydraulics because they are sensitive to over-speeding.
With electric power this is quite easily possible provided
you have the correct gear on the winch. I learned the other
day that it is also possible with low-pressure hydraulics and
that is a fact that should be taken into consideration. As
regards centralised control, it looks impressive and does save
a good deal of space. But with echo sounders and radar
features developing as they do, it might be desirable after
a few years, to install new fittings. That may then be difficult.
Therefore, it might be better to accumulate all your "black
boxes'* around the bridge and in the long run it would be more
practical.
362
Mr. J. G. Fontaine (France): In view of the complexity
of automation and the need for the skipper to supervise so
many things at the same time, guards should be provided
against possible damage. Automation is really an industrial
question. It should be possible then, in view of the automatic
control of engines, to provide some equipment which would
indicate any dangerous rise in temperature and would also
cut the engines out. In that way accidents might be avoided
and the damage, if any, repaired.
Mr. R. F. Allen (U.S.A.) : The original reason for the remote
controlling of various winches on big tuna vessels was for
swinging and lifting the main boom, for pursing the net and
for brailing. We, therefore, planned to locate the operator
in the position where he could see all phases of what was
going on, yet not be working on the deck itself. The safety
feature of this is most important and by using hydraulic
drives on some of these winches we have been able to obtain
instantaneous stopping and starting of them from at least
one position. In other cases we have achieved the same
objective with duplicate control points. This is most important
where gypsies are used and where men have to face the hazard
of getting their hands in the winch or being pulled up into
the power block in the case of seine haulers. Experience thus
far shows we have had no maintenance difficulties or failures of
equipment because of the remote location of the controls.
On some of our modern tuna seine winches the controls
evolved for clutch and brake engagements are of such a
nature that the operator from a distance of five metres can
feel the pressure applied to the brake which is accomplished
by getting the resistance off the valve in the same way as is
done with aeroplane controls. This is a very desirable charac-
teristic particularly where you are using a friction device as
a means of accelerating a load or stopping a load. Hydraulic
drives on these vessels have proved to be very satisfactory for
minimising horsepower requirements on deck gear because
all operations are performed in such a manner that maximum
pull does not come at the time of maximum speed require-
ments. The system of shifting the speed through hydraulics,
therefore, is limited to the horsepower requirements of the
gear. Remote control can be carried round the deck to activate
the power block. In the Pacific Northwest, salmon seiners,
operating with seven men, have been able to eliminate one
man because they have electrical controls on the hydraulic
system. This enables the captain of the boat to handle the
stacking of the net and eliminates having a man on deck at
the time. This is a simple device and it has eliminated about
1 5 per cent of the manpower on the vessel.
Mr. W. W. Carlson (U.S.A.): When we built the Narra-
gansett we did so not only for the benefit of the developed
countries but with an eye on the developing countries where
trained personnel is presently a problem. We felt that one
reasonably well-trained man could run the whole thing.
Mr. Schaefers, in summing up, after mentioning the points
of each speaker, said that the general tone brought out during
the discussions was that many developments contributed to
and led up to centralised control and automation which in
turn led to increased efficiency. This has been particularly
so in the United States where operations, in many fisheries,
are now carried out with fewer people and even includes those
trawl fisheries where it is not normal practice to process the
catch on board. He also stressed that more effort should be
devoted to automation of the entire fishing and ship operation,
rather than just getting the fish aboard.
Part 2 Bulk Fish Catching
Section 11 Fish Detection
A Comprehensive Echo Sounder for Distant-
Water Trawlers
Abstract
Operation of conventional fish-detection echo sounders in bottom
trawling has indicated some shortcomings, for example, the in-
adequate range of detection of single fish, the missing of echoes
due to aeration of the transducers, the strain on the operator
resulting from the transient nature of the cathode-ray tube expanded
display, the lack of memory in this display over the period of a tow
and the movement of displayed echoes due to the motion of the
ship. The improved techniques of the Humber Fish Detection
System largely overcome these difficulties. It embodies a high-power
transmitter (8 kw), a narrow-beam transducer lowered beneath the
vessel on a shaft and two sea-bed-locked scale-expanded displays
which indicate fish close to the sea bed. The transducer gives an
acoustic be*m 13° wide (between 3dB points) and 8° wide (in the
fore/aft direction). One display employs a cathode-ray tube with
a continuously-bright trace and the other is a triggered mechanical
recorder which gives a permanent record on dry paper. The trans-
mission interval is determined by the range required, for example,
0-6 sec for 240 fm. Echoes from fish near the sea bed are stored on a
magnetic drum recorder and are replayed many times on to the
cathode-ray tube whilst awaiting the next set of echoes, The stylus
on the triggered recorder is initiated by the sea-bed echo and crosses
the paper 6 in wide in 0-01 sec (equivalent to 4 fm range).
Un sondeur par 6cho comprehensif pour les chalutiers de haute mer
Resume
Des imperfections se sont revgjees dans Ic fonctionnement des
sondeurs par echo pour la detection pour le poisson seul, I'absence
d'echos causee par Iteration des transducteurs, la tension chez
I'operateur resultant de la nature transitoire de Hmage dilatee du
tube a rayons cathodiques, le manque de memoire de ce systeme
pendant une periode de remorquage et le mouvement de ces images
d'echos du au ddplacemcnt du bateau. Les techniques ameliorees
du systeme de detection de poissons Humber surmonte en grande
partie ces difficultes. II comprend un emetteur a haute puissance
(8 kw), un transducteur a rayon &troit, abaisse sous le bailment
sur une perche, et deux images a echelle ctendue et fixees vers le
fond qui indiquent le poisson proche du fond. Le transducteur
donne un rayon acoustique large de 13° (entre les points 3 dB)
et de 8° (dans les directions avant/arriere). Une image emploie un
tube £ rayons cathodiques avec une trace continuellement lumineuse,
et 1'autreest un enregistreur declenche mecaniquement qui donne un
rapport permanent sur papier sec. L'intervalle d'emission est
determine par la distance ctesiree par exemple, 0,6 seconde pour
440 m. Les 6chos des poissons se trouvant pres du fond sont
deposes sur un enregistreur cylindrique magnetique et sont rejoues
de nombreuscs fois sur le tube a cathodes en attendant le prochain
lot d'echos. Le style, sur I'enregistreur est amorc6 par le fond de
mer et traverse le papier au rythme d'une larjeur de 15 cm en 0,01
seconde (6gal & une gtendue de 8 m).
Un ecosonda complete para los barcos de pesca de altura
Extracto
El empleo de los ecosondas de tipo conventional para detcctar
la pesca en el fondo del mar ha revelado ciertas deficiencias, como
es por ejemplo el inadecuado poder de detecci6n con respecto a los
peces individualmente, la falla de los ecos a causa de la aereaci6n de los
transductores, el cansancio que padece el operario debido al car&cter
transitorio de la presentation ampliada del tubo de rayos catodicos,
la falta de memotia en esta unidad de presentation durante el
rastreo, y el desplazamiento de los ecos presentados por raz6n
del movimiento del barco. Merced a las ticnicas mas avanzadas
del Sistema Detector de Pesca Humber se nan logrado veneer
estas dificultades. Esta provisto de un transmisor de gran potcncia
(8 kw), un transductor de haz estrecho que se hace descender por
detauo <W barco en un tubo y dos umdades de presentaci6n a
escala ampliada dirigidas al fondo del mar, que indican cuAndo la
pesca se encuentra proxima al lecho del mar. El transductor emite
un haz acustico de 13° de ancho (entre puntos de 3 dB) y 8° de
by
G. H. Ellis,
P. R. Hopkin and
R. W. G. Haslett
Kelvin Hughes Division, S. Smith
& Sons (England) Ltd.
G. H. Ellis
P. R. Hopkin
R* W. G. Haslett
ancho (en la direccibn de proa y popa). En una de las unidades de
presentaci6n se emplea un tubo de rayos catddicos con un trazo
continuamente brillante, y la otra unidad es un aparato registrador
mecanico de maniobra independiente que da una indicaci6n
permancnte sobre papel seco. El intcrvalo de transmisi6n depende
del alcance requerido, por ejemplo, 0,6 segundo para 440 metres.
Los ecos causados por los peces cerca del fondo del mar son
memorizados en un registrador de tambor magnetico y son
reporducidos muchas veces en el tubo de rayos catodicos mientras
se espera que llegue la prdxima serie de ecos. La aguja del regis-
trador de maniobra independiente empieza a funcionar por el
efecto del fondo del mar y atraviesa el papel de 1 5 cm de ancho en
0,01 segundo (equivalente a un alcance de 8 metros).
1HXPERIENCE in the Arctic cod and haddock fisheries
Cj aboard Humber trawlers fitted with earlier types of
echo sounder for fish detection, has emphasised some
shortcomings in the performance of these equipments.
For example, when used in conjunction with a conven-
tional recorder, the cathode-ray tube display had to be
adjusted manually to indicate echoes from fish near the
sea bed. In order to obtain adequate information from
the equipment, the cathode-ray tube must be observed
for periods of hours, whether searching for fish over a
wide area or determining the most profitable section of
an actual tow. There was considerable strain on the
operator resulting from the intermittent nature of the
display (which is, in fact, a single flash on an otherwise
blank screen), and the necessity of continuously manipu-
lating the depth control when operating the equipment
over a sea bed of widely varying depth.
To make a reasonably accurate assessment of the poten-
tialities of a particular fishing ground, the echo sounder
363
OH MIME
STANDARD
ECHO-SOUNDER
RECORDER
RECORDING
SCALE-EXPANDER
SIGNAL
SELECTION
AND
STORAGE UNIT:
POWER UNIT
BETWEEN DECKS
HIGH- POWER
TRANSMITTER:
MOTOR
ALTERNATORS
AND
REGULATORS
CATHODE -RAY
TUBE
SCALE-EXP
TRANSDUCER
CHANGE-OVER
SWITCH
ALTERNATIVE
TRANSDUCER
MOUNTED ON HULL:
Hull
Transducer
Fig. L The complete Number Fish Detection System.
must be capable of detecting a single fish of minimum
catchable size at the trawling depth. Catching rates in
excess of 150 baskets of cod per hour have occurred
when the fish have been spread out singly, close to the
sea bed throughout the whole of the tow. The range of
detection of a single fish on existing echo sounders
aboard Humber trawlers falls well short of the maximum
trawling depth of 280 fm; in some cases the limit of
detection was about 12S fm for the smallest cod which
does not pass through the net.
In rough seas which may still permit trawling, the
operation of any echo-sounding equipment using hull-
mounted transducers is seriously impaired by aeration.
The Humber Fish Detection System has been designed
especially for use on distant-water trawlers to give
improved performance in the above respects.
364
The Humber Fish Detection System
In the new equipment (Fig. 1), a high-power transmitter
operating at 30 kc/s delivers a peak power of 8 kw in
pulses of |, 1 or 2 millisecond duration to a 6A by 4A
transducer which is mounted 2 ft below the skin of the
vessel on a remotely-controlled retractable shaft. In
addition, a similar transducer is mounted on the hull
in the normal way for use when steaming.
Received echoes may be observed on three different
types of display, one of which is a conventional echo-
sounder recorder with "white-line" facility. The other
two displays (Fig. 2) are locked to the sea-bed echo
irrespective of the vertical motion of the ship or changes
in depth and, in consequence, may be greatly expanded
to show echoes from fish near the sea bed. The required
echoes are selected by a time-gating circuit and stored
on a magnetic recording drum rotating at 3,000 rpm
(Hopkin 1963). The system is switched to display the
echoes by the arrival of the sea-bed signal.
!
Fig. 2. Block diagram showing the receiver and sea-bed locked displays.
The stored signals are used in two ways:
First, the stored echoes are replayed many times to
give a continuously bright picture on a seven-inch
diameter cathode-ray tube scale-expander (Hopkin 1963).
With the usual transmission interval of 0*6 second,
about 27 repetitions are possible before the arrival of
the next set of echoes. The time base may be set to
display echoes from the bottom four- or two-fathom
section of water.
Secondly, the stored signals are fed to a recording
scale-expander (Haslett 1962). This is essentially a
triggered rotating-arm recorder of high precision,
producing a permanent record of the stored signals on
dry paper 6 in wide. The leading edge of the sea-bed
echo appears as a thin straight line on the right-hand
side of the record. The stylus, which is triggered once
per transmission period, crosses the paper in O'Ol
second, producing a record of echoes from the bottom
four-fathom section of water.
Performance at sea
Prior to the manufacture of the Humber Fish Detection
System, a prototype model was used on board the Hull
trawler Westella for a period of two years during fishing
voyages to the Barents Sea, and off Iceland and Norway.
With this equipment it was found that the maximum
detection range of a single cod is increased to 250 fm.
When towing in a heavy swell, the "missing" due to
aeration is noticably reduced by the use of the lowered
transducer.
The sea-bed locked displays operate reliably over the
complete range of trawling depths on various types of
sea bed. With the increased time of display of each set
of echoes, and improved signal/noise ratio (Fig. 3), the
cathode-ray tube scale-expander traces are much easier
to interpret. The large diameter tube, the increased
brilliance obtained by the repeated replay system, and the
sea-bed locked display, allow the screen to be observed
from a distance without strain.
Fig. 3. With increased time of display of each set of echoes and
improved signal I noise ratio, the cathode-fay tube scale-expander traces
are much easier to interpret.
The record of the horizontal and vertical distribution
of fish close to the sea bed, provided by the recording
scale-expander (Fig. 4), permits a considered assessment
of the most profitable section of a tow. Large increases
in the catching rate have been obtained when tows were
modified to trawl more frequently through the positions
of maximum density of fish below headline height, as
shown by the recording scale-expander chart of the
previous tow. This chart also allows the skipper to use
his own judgment in the assessment of the best sections
of tows made whilst he is not on the bridge.
After some experience had been obtained with the
interpretation of the recording scale-expander chart,
an observer (other than the skipper) was asked to make
estimates of the expected catch by the examination of the
chart at the end of each tow, before the net was hauled.
One hundred and fifty estimates were made in depths
of water between 50 and 200 fm, during six fishing
voyages over a period of one year ; and 70 per cent of the
actual catches were within 40 per cent of the estimate,
the mean error being 38 per cent. The most accurate
estimates were made when all recorded fish echoes were
below headline height. There was a tendency to over-
estimate at short range.
After a total period representing over 50 trips and
involving several sets, it can be said that this equipment
is proving very successful, as an aid to trawling.
The extensive facilities made available by J. Mart and Sons Ltd.,
Hull, are gratefully acknowledged.
This paper is published by permission of the Directors, Kelvin
Hughes Division, S. Smith and Sons (England) Ltd.
References
Hopkin, P. R. "Cathode-ray tube displays for fish detection on
trawlers'9, J. Brit. I.R.E., 25, January 1963, p. 73.
Haslett, R. W. O. "A high-speed echo-sounder recorder having
sea-bed lock**, J. Brit. I.R.E, 24, December 1962, p. 441.
365
(I)
60
fltboftl
Mtftlrfstof
Stt-fcftf tCfcO
«bltt Hit
Fig. 4. Typical charts obtained with the recording scale-expander, compared with corresponding normal echo-sounder charts taken at same time.
Numerous fish-echoes are seen above the sea-bed echo; some points corresponding in time are indicated:
(a) scale-expander chart 1
^ depth 100 fathoms; fish— cod and haddock; 30 baskets/hour.
(b) normal chart J
(c) scale-expander chart 1
^ depth 70 fathoms; fish-shaddock; 70 baskets/hour.
(d) normal chart j
(M.T. Westclla trawling at 4 knots, July 1960)
366
Sector-Scanning Sonar for Fisheries Purposes
Abstract
Research at the Electrical Engineering Department of the Univer-
sity of Birmingham has led to the development of electronic sector-
scanning sonar systems which can be valuable in fisheries work.
Early experiments at sea demonstrated that such a system was very
effective in delineating schools of fish either ahead of the ship or
below it. The latest development in the equipment itself, its perform-
ance (particularly with respect to angular resolution), and its man-
ner of use have led to the possibility of the accurate detection and
location of fish very near to the sea bottom. Moreover, the receiving
part of the equipment has been developed in a universal form which
can be used with any acoustic frequency within a wide range, and
thus without difficulty and with great saving of cost can provide
a general-purpose sonar suitable for high resolution at short
ranges or lower resolution at longer ranges, using different trans-
ducers but the same electronic system. It seems clear that the equip-
ment is now ready for commercial exploitation in fisheries.
Balayage de secteur en sonar pour la peche
Rfeum*
La recherche dans le DSpartement Technique Electrique de TUni-
versitt de Birmingham a mene au developpement de systemes
sonar de balayage electronique de secteur qui peuvent fctre valables
dans la technique des peches. DCS experiences conduites en mer
ont dimontre* que ces systemes sont tres efficaces pour tracer les
banes de poissons reperes en avant ou sous le bateau. Les derniers
deVeloppements dans Tequipement luim£me, son fonctionnement
(respectant particulierement la resolution angulaire) et les modes
d'emploi ont mend & la possibility de detecter et de localiser pr6-
cis£ment les poissons se trouvant pres du fond. Le recepteur de
1'equipement a 6t6 cteveloppe sous une forme universelle de telle sorte
qu'il peut dtre utilise dans une large gamme de frequences acous-
tiques. De cette fagon on peut facilement et a peu de frais construire
un sonar a but general qui convient pour une haute resolution a
courte portee ou une basse resolution a longue portee, utilisant des
emetteurs differents dans le m&me systeme electronique. 11 semble
que 1'gquipement est maintenant prfct pour 1'exploitation com-
mercialc dans les peches.
£1 sonar de exploration por sectores en la pesca
Extracto
Las investigaciones realizadas en la Facultad de Ingenieria Etectrica
de la Universidad de Birmingham ban culminado en la fabricaci6n
de aparatos de sonar electrdnicos de explotaci6n por sectores que
pueden ser muy valiosos en la pesca. Los primeros experimentos
realizados en la mar demostraron que tal sistema es muy eficaz
en la Iocalizaci6n de cardumenes delante o debajp del barcp. Los
m£s recientes adelantos en el material, su rendimiento (particular-
mente con respecto a la resoluci6n angular) y la manera de usarlo
nan creado la posibilidad de encontrar y localizar con exactitud
cardumenes muy cerca del fondo. Ademas, la parte receptora de los
aparatos se ha perfeccionado de forma universal que puede emple-
arse con cualquier frecuencia acustica en una gama muy arnplia
y con ello, sin dificultad y con gran economia de coste, const ituye
un sonar de multiples usos a proposito para lograr gran resolution
a poca distancia o resoluci6n mas baja a distancias mayores emplea-
ndo diferentes transductores pero el mismo sistema electrdnico.
El material esta ya listo para su explotaci6n en la pesca industrial.
"EXISTING sonar, asdic, or echo-sounding equipments
L-J which are commercially available always use a single
fixed beam, usually several (or many) degrees in width.
In the case of equipments designed to use the beam in
an approximately horizontal position, provision is made
for swinging the beam by rotating the transducer bodily.
If it is desired to scan a sector, then it is necessary to
allow time for a pulse to travel out to the maximum
range and for echoes to return to the transducer before
the beam is rotated to its next position. Since the speed
D. G. Tucker V. G. Webby
University of Birmingham, U.K.
of sound in water is very low, about a mile a second,
it clearly takes a great deal of time to search a large
sector effectively. A compromise has to be made between
beam width and speed of search. Generally the beamwidth
is of the order of 5°, which not only gives a rather slow
rate of search but is quite unsuitable for detecting
fish near the sea bottom because of the high background
of reverberation which such a large beamwidth produces.
Electronic sector-scanning sonar (Figs. I, 2, 3, 4)
overcomes these difficulties. The way in which it works
is shown in Fig. 1. A relatively wide sector, say about
30°, is illuminated by a pulse transmitted from a transdu-
cer having a wide beam. The receiving beam is made very
narrow, say 1°, and by electronic means is swept rapidly
from one side to the other of the sector, the process
being continuous and repetitive; the time required for
one complete sweep is equal to the duration of the trans-
mitted pulse. This means that all directions within
the sector are sampled before the pulse has moved its
own length through the water. In other words, the whole
width of the sector is virtually simultaneously examined;
and by the time the pulse has reached its maximum
range, information has been received from all points
in the sector. It is thus possible to have the receiving
beam as narrow as may be desired without prejudice to
the rate of search. The method of displaying the informa-
tion is also indicated in Fig. 1 . The most effective display
seems to be the B-scan on a cathode-ray oscilloscope;
this is a display in which range and bearing are used as
rectangular co-ordinates, the range time base being
synchronised with the movement of the pulse as it
passes through the water, and the bearing time base
being synchronised with the electronic sweep of the
receiving beam. Any echoes received from targets
within the sector of search are thus displayed at the
correct range and bearing. There is of course a distortion
in this kind of display, and from some points of view a
triangular display, which would more nearly resemble a
P.P.L sector, should be preferable. Experience, however,
shows that this is not satisfactory owing to the crowding
of echoes at short ranges.
The width of the transmitted and receiving beam in
the plane perpendicular to that of the sector of search
367
may be as large or small as is desired or necessitated by
particular applications. In typical experimental equip-
ments this bcamwidth has been 12°, but it is now thought
that for many applications a width of perhaps 5° would
be better.
NAftiOW ftfCIJVIMO MAM
swt PT RAPIDLY oven SECTOR
SECTOR ILLUMINATED SY
TRANSMtTTfD PULSE
TRANSMITTED PULSE
TRAVELLING OUT
THROUGH THE WATER
•TRANSDUCERS
RANGE
(TIME-iASE SYNCHRONIZED
WITH PULSE TRAVELLING | |
OUT THROUGH WATER)
CATHODE RAY
DISPLAY
(••SCAN)
-ECHOES
BEARING
(TIME-BASE SYNCHRONIZED
WITH SWEEP OF RECEIVING
iEAM)
Fig. 1. Schematic diagram of sector-scanning sonar.
In order to scan a sector with a receiving beam which
is one nth the width of the sector, it is necessary to have
the receiving transducer divided into n sections and to
have n electronic channels connecting these sections to
the later parts of the receiving electronic system. The
expense of the system is therefore to some extent
increased by having a larger number of beam widths with-
in the sector, in other words by increasing the resolution
of the system. The order of resolution which it is thought
would be readily feasible and commercially worthwhile
is perhaps 1° angular resolution1 and one m range
resolution in a system operating around 50 to 100 kc/s
and giving ranges between 500 and 1,000 m on fish
schools. Very much higher resolution is worthwhile in
applications where shorter ranges are acceptable, and a
recent equipment has given a resolution of half a degree
368
in angle and about 10 cm in range up to a maximum
range of about 100 m. The former system would be
suitable for detecting large fish such as cod, or small
schools of fish, at reasonable distances. The latter
system is intended for the observation of fish movements
and perhaps the identification of individual fish in more
limited situations, notably in enclosed waters, and in
positions of special significance such as the intake to
the turbines of hydro-electric power stations and to the
cooling water system of thermal power stations, the
observations of fish entering nets, and so on.
Observation of fish in midwater
One of the difficulties in sonar work when using beams
which are very nearly horizontal is that the scattering
of sound from the sea bottom (also from the sea surface)
produces a background against which it may be difficult
to detect fish. If the water is fairly deep, it is possible to
have the beam clear of both the surface and the bottom
up to quite reasonable ranges, and in these circumstances
midwater fish may be detected without difficulty. The
parameters of the system are then not very critical, and
success has been obtained with ordinary single-beam
sonars having beamwidths of as much as 5°. Of course
the problem of search still remains, but perhaps with 5°
beams the time required to search a sector is not prohi-
bitive. Nevertheless a simple electronic-scanning
system can give a very considerable advantage, and with
a resolution of 5°, not many channels are needed to
cover a big sector. Thus the equipment need not be
very expensive. But obviously if a narrower beam is
used, say 1° in plan (although it may be say 10° vertically),
much better resolution is obtained and the details of
the fish school are more clearly determined.
600
RANGE t
(METRES) '
BEARING
Fig. 2. Typical 8-channel sector-scanning display showing small fish
schools. (Horizontal beam.)
Echo sounders, i.e. sonars with their beam axes vertical,
are frequently used for detecting midwater fish. They
are very successful in this role in spite of having very
wide beams, often over 30°, because there is no possibility
of bottom or surface reverberation forming a background
at the same range as the fish. The only background is
volume reverberation, i.e. scattering from particles and
bubbles, etc. in the water, and this reverberation is gener-
ally of very low level. But even with such a system as
this, although detection is very good, resolution is poor
and the details of schools cannot be determined. So
here again the use of electronic sector scanning could be
an asset, and only a relatively small number of channels
would be required to give a very big improvement in
the information obtained.
Searing
Pith
2$
fathoms
Fig. 3. Typical 8-channel display with sector axis vertically below ship.
Some examples of results obtained in a relatively
simple eight-channel electronic sector scanning sonar
are shown in Figs. 2 and 3. In this system the acoustic
frequency was 37 kc/s, the horizontal beamwidth was
1*5°, the pulse duration was 1*0 millisec, and the vertical
beamwidth was 12°. Since there were eight channels, the
width of the scanned sector was about 12°. Fig. 2 shows
some results obtained with an almost horizontal beam,
scanning in the horizontal direction. In interpreting
this display it must of course be realised that the range and
bearing scales are such that a true plan view is not presen-
ted. But it can clearly be seen how the small schools of
fish are distributed and it is not difficult to interpret these
in strict geometric terms if required. Since all this informa-
tion is obtained on one single transmitted pulse, it is
clear that there is no confusion due to movement of the
fish or of the boat while the search is being made.
Fig. 3 shows a result obtained with the axis of the
scanned sector vertically below the ship. The parameters
of the system were the same as in Fig. 2 but, owing to the
short range at which the fish are now being detected (the
depth of the water was only about 40 m), it is likely that
at least some of the echoes shown are due to individual
fish (probably herring).
It will be seen that, even with such a comparatively
crude system, the improvement in the information
obtained over that obtained with an ordinary sonar is
very marked.
Detection of fish very dose to the sea bottom
An important problem in fisheries is the detection of
fish near the sea bottom because this is where a very great
deal of trawling has to be done. If the beam is directed
at a shallow angle to the bottom, it is clear that fish near
the bottom have to be detected against a background
of the reverberation, or backscattering from the bottom
itself. Because of this factor most commercial single-beam
sonars have not been very successful in this role. With
a beamwidth as wide as 5° horizontally, too much of the
bottom is taken in by the pulse and the reverberation
from this is ample to drown the echoes from fish.
The use of an electronic sector-scanning sonar using
horizontal scanning, as in the example shown in Fig. 2,
can effect a very considerable improvement because it
permits the use of very much narrower beams, thus
greatly improving the ratio of fish echo to reverberation
background. The typical result shown in Fig. 2, indeed,
was taken under conditions where bottom reverberation
was of importance because the depth of water was only
about 40 m and, with the vertical beamwidth of 12°,
bottom reverberation was, in fact, being received over
most of the range. It will nevertheless be seen that the
fish schools show up quite clearly in spite of this. There
is no doubt, however, that bottom reverberation must
be always a limiting factor in the detection of fish when
the sonar system is used in this way.
Recently a completely novel way of utilising a sector-
scanning sonar for the detection of fish very near the
bottom has been proposed by Dr. F. R. Harden Jones
of the Lowestoft Fisheries Laboratory. The principle of
this new system is shown diagrammatically in Fig. 4.
Instead of the beam scanning horizontally, it now scans
vertically as indicated by the arrows in the diagram.
The beamwidth in the direction at right angles to the
plane of the diagram may be quite considerable, and
something between 5° and 12° is thought to be most
suitable. As the receiving beam (narrow in the vertical
plane) swings up and down as indicated, it returns echoes
from the sea bottom varying from comparatively short
range in the lowest position to a comparatively long
range in the highest position; the sea bottom returns,
369
therefore, appear as a curved line on the B-scan display
as shown in Fig. 5. Any fish near the bottom, such as
those indicated by numbered dots in Fig. 4, return echoes
which are received before the bottom echo for the
particular beam position along which they are received.
The fish echoes therefore appear clear of the sea bottom
echo, as shown by the numbered echoes in Fig. 5.
This, therefore, is a very effective system of obtaining
echoes from fish very near the bottom without inter-
ference from the bottom returns themselves. In fact the
bottom returns are made use of to provide a reference
line on the display. In a way this new system is a com-
promise between the ordinary conception of scanning
ahead of the ship and the conception of the echo sounder.
BEAM SCANNED
AMD-DOWN
//// / S/SSS SSsS / fS SsS / S f / S S / / / S S S / >
Fig. 4. Application of sector-scanning sonar for the detection offish
near the sea bottom.
•EAAING
SEA iOTTOM
ECHO
RANGE ^
Fig. 5. Display obtained by method of Fig. 4.
POINT
-TO
'4 I 4 i * fc V
*ANGE (METRES)
•-is*
But scanning in this particular way clearly gives some
very considerable advantages.
Although no experiments have yet been carried out at
sea using this method of search, some very extensive
trials of it have been made in a large seawater tank at
Cairn Ryan. The experimental electronic sector-scanning
sonar used for these trials was a high-resolution equip-
ment having 30 channels, so that 30 bcamwidths are
scanned within the sector. Moreover, the system was
provided with a special kind of signal processing which
enables double the angular resolution of a normal
acoustic system to be obtained with the same size of
transducer. In this experimental equipment the acoustic
frequency was 500 kc/s, and the pulse duration was 0*1
millisec, giving a resolution of about 10 cm in range.
The sector width was 30°, and the resolution was about
0*5°. The sea bottom was simulated by the concrete
wall of the tank or by a corrugated sheet, and fish targets
close to the bottom were simulated by artificial small
targets or by actual fish suspended in position near the
wall. Some typical displays obtained in this trial are
shown in Fig. 6. It will be seen that they reproduce the
theoretically expected picture exactly, and promise
great success for Dr. Jones's system. It is intended to
carry out operational sea trials of this method next win-
ter, using an acoustic frequency of 100 kc/s, with an
angular resolution of 1°. It seems probable that success
will be obtained, and that this will prove a most impor-
tant development in the use of sonar in fisheries.
1 By "angular resolution** is meant the smallest angular step
over which a change in target strength can be detected. The problem
of separate detection of two fish with a clear patch between them,
or of one fish clear of the bottom, involves two such steps. Thus
continued on page 371
FttM
•IAJUNC
O
7 f
16
6CANNCD S8C10*
Fig. 6. (a) Typical results with 30-channel system.
Fig. 6. (b) Typical results with 30-channel system.
370
A New Sonar System for Marine Research
Purposes
Abstract
A complete "search installation" comprises two combined sounder/
sonar sets working on 1 1 kc/s and 30 kc/s and two additional
sounders on 38 kc/s and 18 kc/s. The remote controlled sonar
transducers are housed in a retractable 18 knots dome. The
transmitters have variable pulsclcngth and an output power of
8-3 kw. The receiving equipment is calibrated and comprises
special circuits for TVG, RCG and White Line features. Display
and recording equipment includes range/depth recorders, tape
recorder, loudspeaker and an oscilloscope for detailed echo studies
and play back of tape recordings. The whole installation, including
two additional sounders, is manned by a single operator.
Un nouveau systente sonar pour la recherche maritime
Resume
Cette "installation de Recherche" complete comprcnd 2 sondeurs/
sonar fonctionnant a 1 1 kc/s et 30 kc/s et 2 sondeurs supple-
mentaires, fonctionnant sur 38 kc/s et 18 kc/s. Les emetteurs
de sonar sont enfermes sous un dome escamotable, capable de
resister a une vitcssc du bateau de 18 noeuds. Les dmetteurs ont
des pulsations de longueur variable et leur puissance demission
est de 8,5 kw. L'equipement de reception est calibrt et comprend
des circuits speciaux pour TVG, RCG et des caracteristiques de
"ligne blanche". L'equipemcnt d'etalage visuel et d'enregistre-
ment comprend enregistreur de portee/profondeur, enregistreur
sur bande, haut-parleur et un oscilloscope pour les Etudes
dftaillees des echos et le renvoi des enregistrements a bande. Toute
I'installation, y compris les deux sondeurs supplementaires, est
manipulee par un seul operateur.
Nuevo sistema de sonar para investigariones marinas
Extracto
Una "instalaci6n de busqueda" complete consta de dos grupos
combinados de sonda y sonar que functionan a 1 1 kc y 30 kc/seg
y 2 sondadores adicionales a 38 kc y 18 kc/seg. Los transductores
del sonar con sus telemandos estan montados en una cupula
retractable capaz de rcsistir velocidades hasta de 18 nudos y las
presiones que las acorn pa flan. La longitud de los impulses de
los transmisores son variables y la potencia es de 8,5 kv. £1
equipo receptor esta calibrado y comprende circuitos especiales
para "TVG", "RCG" y linea blanca. El material de presentacibn
visual y de rcgistro comprende registradores del alcance y la
profundidad, un registrador de cinta, un altavoz y un osciloscopio
para estudiar los ecos con detalle y rcpetir los registros de las
cintas. De toda la instalaci6n, inclufdos dos sondadores adicion-
ales, se encarga un solo operador.
MARINE biologists were the first to prove that sonar
had a practical use in locating schools of fish.
Naval sonar sets were first used but were in many ways
quite unsuitable. In spite of this lack of satisfactory
equipment, sonar soon became an indispensable tool
by
T. S. Gerhardsen
Simonsen & Mustad A/S, Morten,
Norway
in needed marine research. During the last decade sonar
has been installed in many research vessels, but until
recently scientists have been forced to use either rela-
tively simple equipment designed for fishing, or modified
and even obsolete naval equipment.
This paper gives a short description of a new sonar
system specially designed for research purposes.
By 1959 a great deal of experience was available
about the use of sonar for locating fish. One research
vessel alone had used sonar for over 30,000 hrs and a
large number of active fishermen had also been using
sonar for several years.
Purpose
The purpose of the new system can thus be outlined:
(a) Ocean-wide and coastal search for fishable concen-
trations in direct support to the fishing fleet.
(b) Perfection of the sonar and sounder search pro-
grammes and doctrines, to improve own methods
and procedures and as a part of a training pro-
gramme for fishermen.
(:) Behaviour studies and study of the level and fre-
quency dependence of the "target strength" of
various sizes of fish.
Other aims and tasks are for example the study of
dispersion, the correlation of recording to catch and the
counting of single fish.
After close study the main technical specifications were
formulated:
(a) The installation should comprise two separate
continued from page 370
when the angular resolution is stated to be 1°, this means that a
fish can be observed as a separate entity, provided its angular
separation from another fish or from the bottom is at least 2°.
The interpretation of range resolution is similar.
Acknowledgements
The work done at the University of Birmingham in this subject
has been greatly assisted by the co-operation of, and facilities
provided by, several organizations concerned with fisheries research
notably the National Institute of Oceanography, the Fisheries
Laboratory at Lowestoft and the Marine Laboratory at Aberdeen.
References
D. G. Tucker, V. G. Welsby and R. Kendell, "Electronic Sector
Scanning", J.BritJnst. Radio Engrs. 18, p. 465. 1958.
D. G. Tucker, V. G. Welsby, L. Kay, M. J. Tucker, A. R. Stubbs
and J. G. Henderson, "Under-water Echo Ranging with Electronic
Sector Scanning: Sea Trials on R.R.S. Discovery //", J.Brit.Inst.
Radio Engrs. 19, p. 681 . 1959.
V. G. Welsby, "Multiplicative Receiving Arrays: The Angular
Resolution of Targets in a Sonar System with Electronic Scanning",
J.Brit.Inst. Radio Engrs. 22, p. 5. 1961.
B. S. McCartney, "An Improved Electronic Sector-Scanning
Sonar Receiver'*, J.Brit.Inst. Radio Engrs. 22, p. 481. 1961.
371
Fig 1. System pictorial diagram of SIM RAD research sonar model
No. 580-10.
single beam searchlight sonar systems with both
of the sonar transducers in the same dome.
(b) The transducers and transmitters should be de-
signed to give the highest possible "source level"
(of the order of 125 db and 115 db//l(* Bar,
ref 1 m for 30 kc/s and 11 kc/s respectively),
within the limited space available for the dome
and the hull unit.
(c) The display should comprise range-depth recorder,
oscilloscope, tape recorder, loudspeaker and head-
phones.
(d) The entire installation should be calibrated to
facilitate routine measurement of echo level. The
receiver in particular must have the correct
characteristics (dynamic range, RCG, TVG and
"white line*' feature) and a good long-time
stability.
(e) The Operator's Display Console should be de-
signed in such a way that the whole installation
plus two separate echo sounders can be supervised
by one operator.
372
(0 The installation should, as far as possible, be
composed of self-contained units which can be
inspected and tested during operation.
(g) All units of the system should be designed for
continuous service (the estimated number of
working hours per annum will be approximately
3,000).
The system described
The system comprises three main units (Fig. 1): hull
unit, transmitting cabinet and operator's display console.
A complete installation also includes transducers for
echo sounding, a stabilised converter and a number of
junction and distribution boxes. The mechanical
construction of the chassis, cabinets and similar items is
mainly conventional.
Hull unit— The hull unit (Fig. 2) has two lifting columns
by which a streamlined dome can be lowered into its
working position underneath the ship and retracted
into an inboard trunk when the system is not in use.
Fig 2. Hull unit. Complete hull unit with dome In working position
(lowered) assembled at the factory for final tests. Total height from
underside dome to top of lifting column: 3,950 mm (158 in). Length:
1,750 mm (70 in). Width 840 mm (33 in). Depth below the keel
with dome in working position: 1,100 mm (44 in). Total weight:
1,300 kg (2,800 to).
The lifting columns and trunk are maintained from
earlier naval equipment. The dome is 60 in long, 24 in
wide and 52 in high. It is constructed entirely of stainless
steel and designed for 18 knots operational speed. The
dome houses two transducers which are mounted so that
they do not interfere with each other. The rear trans-
ducer (11 kc/s) is the lower and is trainable only,
Fig 3. Sonar transducers. Close-up view of sonar transducers (dome
removed). The 30 kc/sec transducer is til table and trainable. The
11 kc/scc transducer is trainable only. With the dome in the hoisted
position, the transducers may be removed while the ship is afloat.
Fig 4. Tilt and training mechanisms. Close-up view of tilt and
training mechanisms with covers removed. The positioning of the
transducers to any desired bearing is by servo systems controlled
from the operator's display console.
while the forward transducer (30 kc/s) is both trainable
and tillable down to 90°. The training and tilt mechanisms
with the servomotors, gear, synchros and sliprings
(Fig. 4) are mounted at the upper end of the transducer
shafts, which go through packing glands in the top of the
trunk. When the dome is raised the transducers can be
removed without docking the ship.
Transmitting cabinet— This unit (Fig. 5) contains two
complete transmitters and all the servo-amplifiers.
Each of the two identical transmitters occupies 2|
drawers in the cabinet and the sixth drawer houses five
servo-amplifiers with the test unit and power pack. All
Fig 5. Transmitting cabinet. This cabinet contains two complete
high power transmitters (30 kc/s and 11 kc/s) and the servo-
amplifiers. The components are mounted on six drawers running
on telescopic mountings allowing easy withdrawal for service and
maintenance. The outside dimensions are: height. 1,860 mm (73 in),
width, 1,050 mm (44 in), depth, 655 mm (26 in). The total weight it
800 kg (1, 700 Ib).
373
the drawers glide on telescopic runners and are fitted
with door switches and door stops. The output stage
of the transmitter consists of four 7094 valves in a push-
pull parallel coupling. With an anode voltage of 3,000 V
and a reservoir condenser of ISO jxF, 3,000 V, the
transmitter gives a pulse effect of approximately 8*5 kw,
measured at the beginning of the pulse and a pulse
energy of 300 w sec with a pulse-length of 80 ms. The
transmitter and the servo-amplifier are controlled from
the operator's display console.
Operator's display console— To meet the demand
that the system (Fig. 6) can be operated by one man,
all the control and display instruments are arranged in a
piano-like cabinet. Definite maximum measurements for
the cabinet's height and depth were laid down. The
width of the cabinet was decided by the fact that the
operator had to be able to reach all the knobs and switches
and, at the same time, see and operate two separate
echo sounders, which were to be mounted outside the
cabinet, one on each side.
Fig 6. Operator's display console. All receiving and display units
for both sets are arranged in this console in a manner to facilitate
the supervision by one operator. The top section contains the
receivers, master keying circuits, tape recorder and main starting
board. *&te graphical recorders, selection panes, bearing indicator
and CAT display are all arranged on the sloping front of the console.
Two additional echo sounders are to be mounted outside the console
in a position where they can be watched by the operator.
In constructing the operator's display console,
effort was made to place die most-used control panel
and display instruments in the most convenient position,
i.e. as near the centre of the console as possible.
The console is constructed with a middle section
which contains, at the top, the start panel for the whole
system, with voltmeters, frequency meter, clock and
elapsed time meter. A commercial tape recorder is
mounted underneath these instruments and, in the sloping
374
front, right in front of the operator, are the tilt and
bearing indicators and the oscilloscope. All these units
are common for both the sonar sets.
The two sections, one on each side of the middle panel,
are identical. The receiver is mounted in the vertical
part and the selection panel and recorder in the sloping
front. All these units are mounted on telescopic runners
and can be pulled out while the system is in use. The
terminal strips for the entire system are in the base of the
console.
In commercial equipment of a similar type which is
to be operated by unskilled or semi-skilled personnel,
it is common practice to restrict the number of controls as
much as possible. In this special system for scientific
use, the exact opposite has been decided upon and a
number of the parameters in the system are kept variable
so that an experienced operator has an opportunity of
choosing "optimum9' characteristics under different
conditions. As a result, each of the two sonar systems
has 40 control switches.
Important control functions
A detailed technical description of all units is outside the
scope of this paper and only the more important control
functions are therefore mentioned:
The polar diagram of the transducers can be varied
in one plane with the switches on the selection panels.
The output effect of the transmitter can be regulated
in three stages and the pulselength in six stages.
The training programme is either manual or automatic
with a variable stepping angle. The transducer's move-
ments are governed by a conventional servo-system which
uses two inch synchros, split-field motors and tran-
sistorised amplifiers.
The receiver has variable band width, a specially
large dynamic range, "time variable gain49 with a
selectable function, automatic gain control, special
circuits for white line effect and calibrated gain control.
The range-depth recorder has four basic ranges, 16
phasing ranges in all, variable paper speed and a high
pulse rate. '&
The oscilloscope is designed for three different pur-
poses:
(i) A PPI indicator for sonar.
(ii) A calibrated nonius for measuring the echo level.
(iii) A bottom lock in connection with the tape
recorder.
The oscilloscope is supplied with a built-in delay line
which makes it possible for the sweep to be triggered by
the echo. The oscilloscope has a 7 in CRT and P7
screen.
A somewhat modified commercial stereo tape recorder
has been used in which one of the channels can be used
for simultaneous commentary. During recording the
tape can be supplied with sweep trigger pulses for later
play-backs and display on the oscilloscope. Recording
on the tape recorder can take place simultaneously
with, and independently of, the registration on the
range-depth recorder and display on the oscilloscope.
The operator's display console is connected to the
ship's intercommunication system, so that in certain
situations the operator can give orders straight to the
helmsman.
In addition to the three main units, a complete
installation also includes junction and distribution
boxes and a frequency stabilised converter which sup-
plied 220 V, 50 c/s ±0-1 per cent. This converter is
only used for the recorder motors. The ship's 220 V,
50 c/s network supplies the power for all the other units.
The echo sounders
Both of the two sonar systems (1 1 and 30 kc/s) of
which the installation consists, can also be used as echo
sounders. When used for scientific work, an additional
two echo sounders are necessary for some investigations.
These special sounders work on 18 and 38 kc/s, and both
are equipped with variable polar diagrams, several depth
scales and special white line properties. The echo
sounders are placed so that they are easily observed from
the operator's seat.
In addition a 50 kc/s echo sounder is mounted on
the bridge for navigational purposes.
Operational use of the system
A detailed procedure for the operational use of the
system will not be laid down until some of the special
features have been more fully explored. The following
notes are merely an indication of the utilisation of the
system under various modes of operation.
Sonar search — The aim is to find as many schools of
fish as possible, either as a phase in a scientific investiga-
tion, or in direct support to the fishermen. As the
greatest possible detection range in this case is of interest,
both the systems are used on full power concurrently,
with a long pulse, a narrow bandwidth and an automatic
search programme which gives a comprehensive coverage.
The operator can listen to one or both of the systems and
record on the tape recorder any items he thinks might
be of special interest. These tape recordings can be
studied in detail later. The echo traces from the range
recorders are also preserved for later examination.
Sonar close contact — A range of less than 500 m is
called close contact. Under certain conditions it is of
interest, for example in connection with experimental
fishing and during study of fish migration, to keep
contact with one particular school. The operator then
directs (tilt and training) the 30 kc/s system and gets a
PPI picture of the school on the oscilloscope. During
this operation (particularly in shallow water) reduced
pulse effect and short pulse may give increased resolution.
On the oscilloscope it is possible to compare the form of
the transmitted pulse with the returned echo in the hope
of finding a correlation between the pulse deformation
and the species and configuration of the fish in the school.
While the operator thus diverts most of his attention
to the study of one particular school, the low frequency
set can be kept running on an automatic long range
search programme, i.e. on the look out for other schools.
General echo-somding survey— Since many types of
fish appear both pelagic and close to the sea bottom, it
is necessary in a comprehensive survey to cover the whole
depth range with high resolution recordings.
The usual procedure is to use one system as an echo
sounder on a survey range of, for example, 500 m. The
other system and the two additional sounders can then
cover different "phasing ranges" of the total depth
range and record the echoes with high resolution.
With the wide choice of frequencies and polar diagrams
at one's disposal, it will always be possible to get a
good coverage.
The tape recorder can also be used during echo-
sounding surveys and it has already proved to be of
special value to compare the echo recordings on the
paper with echo signals on the tape recorder when they
are later displayed on the oscilloscope and studied at
leisure. This procedure is of great interest since other-
wise valuable information would be lost due to lengthy
and intense observation of the oscilloscope being
extremely tiring.
Measurement of the echo level— Both systems are
calibrated so that quantitative measurements of the
echo strength from single fish can be obtained. This
means that the "source level" during transmission
and the total amplification of the receiving equipment is
known. The gain controls on both receiver and oscillo-
scope are calibrated in db and by noting a certain reading
on the oscilloscope one can, by simple addition and sub-
traction, read the echo level in db's referred to lp Bar.
The main difficulty, of course, is that the fish is not
stationary and, as a rule, one knows neither the size
nor species of the fish which gives the echo. Preliminary
experiments with live fish hooked on to a short line
fixed to a vertical wire have, however, given promising
results.
It is expected that proposed investigations with an
underwater camera and TV will give even more valuable
data. The aim is partly to be able to judge the size of the
fish from measurement of the echo level.
Fish counting — In certain investigations, the task is to
estimate the amount of fish in a given area. It has
already become apparent that the high "source level"
and good resolution of these systems has considerably
eased this problem even though the manual counting
of the recorded echoes is still extremely tedious. With
the high signal-to-noise ratio obtained on echoes from
single fish, even at the greatest trawl depths, it might
be a practical proposition to supplement the installation
with an automatic fish counter.
References
Gerhardsen, T. S. Location of herring by means of asdic and
echo sounder, Teknisk Ukeblad, Oslo, Norway, 51, pp 3-7. 1946.
Devoid, F. In pursuit of the winter herring in the Norwegian
Sea. Fiskets Gang, Bergen, Norway, 20, pp 217-222. 1951.
Jacobsson, J. Location of herring schools on the Icelandic
North Coast fishing grounds. Modern Fishing Gear of the World
(Fishing News (Books) Limited, London). 1959.
Vestnes. Fish detection by asdic and echo sounder. Modern
Fishing Gear of the World (Fishing News (Books) Limited, Lon-
don). 1959.
Gerhardsen, T. S. Use of electronic echo-ranging gear in
Norway herring fishery. Pacific Fisherman, Seattle, U.S.A. 1959.
375
Detection et Localisation des Banes de Poissons
L'&ude pasae en revue quelques signet de nature biologique,
oc&mographique et saisonnifcre sur tesquels, depuis des siecles,
les pecheurs §e sent basis pour pr6dire et d&ecter la presence de
bum de poissom. L'application de methodes modernes de detection
et localisation a transform* oes predictions (qui etaient toutes
twees sur des deductions d*informations sccondaires) en detection
r6eUe baaee sur des fehos venant des banes ou par 1'obscrvation
directe des poissons eux-mtoes. U est montrt que les m6thodes
teltes que 1'enregistreur japonais de sons de poissons "Sonobuoy"
et la "prpspection adrienne" sont tres peu employees pour des
raisons diverse*. Quant au "thermo-indicateur" utilise* dans la
peche du thon, il ne peut 6tre conside>e commc ddtecteur de
poisson, bien qu'il soit a la base de la ptehe a la longue ligne
du thon; il donne des indications sur les conditions octano-
graphiques plutot que sur les poissons eux-m&mes. Les techniques
actueltes d*echo-sondage se sont beaucoup d£velopp6es depuis le
premier Congres Mondial des Engins de Pdche en 1957, alors que
la detection etait encore le probleme principal. Aujourd'hui les
instruments Atectroniques ont tellement 6volu6 que la detection est
admise par tous et que les Etudes spnt maintenant dirigtes vers la
reconnaissance des especes de poissons par I'intetpi&ation des
traces obtenues sur les fehogrammes. Les traces caract&istiques
des diflfaents poissons sont aujourd'hui reconnaissables grace
aux ingfoieurs qui ont quitte* Icurs laboratoircs et font d&prmais
leun essais en mer, a bord de bateaux de recherche, 6quip£s dans
ce but. II est regrettable que beaucoup de pecheurs utilisent
encore lew 6cho-sondeur pour mesurcr la piofondeur des fonds
et non pour dftecter tes poissons; il est ntaessairt que des cours
soient organises dans les 6cotes de pfcchc pour leur apprendre a
interpreter tes traces d*6chos. La communication est illustrfe de
10 tehogrammes typiques de diffferentes especes de poissons, avec
commentaires sur tes caracteres individuels des traces.
The detection and location of fish schools
Abstract
The paper reviews some of the traditional biological, oceano-
graphical and seasonal signs which fishermen have always depended
on to predict and detect the presence of fish schools. The applica-
tion or modern detection and location methods has changed such
predictions (which are all based on the deduction from ancillary
information) to real detection based on echoes from, or direct
observation of, the fish themselves. It is pointed out that methods
such as the Japanese "Sonobuoy" fish noise recorder and aerial
spotting are either very restricted in their application or very
expensive and therefore only usable under specific conditions.
The various thermo-mdicators, as used for instance in tuna fishing,
can hardly be classed as fish detection devices, although they are
perhaps at the base of the tuna longline fishing. They provide
indications of oceanographical conditions rather than of the fish
themselves. The present echo-sounding technique has developed
fast since the time of the First Fishing Gear Congress in 1957, when
detection itself was still the main problem. Today electronic
devices have been so much developed that detection itself is taken
for granted and studies are now directed more towards recognising
individual fish from the traces obtained on the cchograms. The
characteristic traces of many fish species are now recognisable,
largely due to the fact that the engineers havs left their laboratories
and are now making their tests at sea on board vessels equipped for
this purpose. As many fishermen still use their echo sounder as a
depth recorder rather than a fish finder, it seems highly necessary
that courses on the recognition of echogram traces should be
given in fishery schools. The paper, which includes ten typical
echograms of different fish species, discusses each of these and
points to the characteristic individual difference of the traces.
Detection y localization de cardunenes
Extracto
Resefta el autor algunas de las scftales biol6gicas, oceanogr&ficas y
estacionates de las que los Pescadores se sirven desde tiempo
inmemorial para predecir y localizar la prescncia de cardumenes.
La aplicacion de los m&odos modernos ha transformado tales
predkciones (basadas todas en deducciones de datos auxiliares)
en una verdadera Iocalizaci6n fundamcntada en los ecos de los
376
by
Robert Lenier
Commission Marine Marchande
et Peche
peces o en la observacibn directa de £stos. Se pone de relieve qus
m&odos como el registrador japon6s de sonidos emitidos por los
peces "Sonobuoy" y la Iocalizaci6n a£rea son de aplicacion muy
restringida o muy costosos y, por tanto, factibles de usarlos sola-
mente en condiciones especiales. Los diversos termoindicadores,
como los empteados, por ejemplp, en la pesca del atun, apenas
pueden clasificarse como dispositivos de Iocalizaci6n de peces,
aunque probablcmente constituycn la base de la pesca del atun
con palangrcs, porque dan informaci6n de condiciones oceano-
grdficas mds bien que de los peces. La actual t6cnica de ecosondaje
ha evolucionado r^pidamcntc desde el Primer Congrcso de Artes
de Pesca de 1957, fecha en la que la Iocalizaci6n misma era todavia
el problema principal. Actualmente los dispositivos electr6nicos
ban mejorado de tal manera que la Iocalizaci6n se acepta como
normal y los estudips se dirigen hacia el reconocimiento de peces
individuates a partir de los trazos pbtenidos en los ecogramas.
Se reconocen los trazos caracteristicps de muchas especics de
peces debido principalmente a que los ingenieros han salido de los
labpratorios y realizan los ensayos a bordo de embarcaciones
equipadas para ello. Como muchos pescadores siguen empleando
sus ecosondas para registrar la profundidad mas bien que para
localizar peces, parece ser muy necesario que en las escuelas de
pesca se den cursos sobre el reconocimiento de los trazod en los
ecogramas. Esta comunicacibn, que da 10 ecogramas tipicos de
diferentes especies de peces, dtscute cada una e indica las diferencias
individuates caracteristicas de los trazos.
"T\EPUIS que la peche existe, les hommes ont toujours
JL/ essayd de local i ser les points possibles ou ils avaient
le plus de chance de jeter leurs lignes, leurs pieges ou
leurs filets.
L'observation de la mer, du temps, et des oiseaux
etaient restes la base de leurs deductions jusqu'&
I'apparition rtcente des ultra-sons.
Les Basques, par exemple, estimaient que pour la
capture du thon dans le Golfe de Gascogne, le temps
devait 6tre beau, la mer calme, avec risees de vent
d'Ouest, Sud-Ouest, Nord-Ouest. Les vents contraires
etaient ceux venant de terre soit Nord-Est et Sud-Est.
Ils ont utilis^ pendant longtemps des methodes
visuelles de localisation des thons. Par mer faiblement
agit^e, les Basques savent reconnaitre ce qu'ils qualifient
de "legouna", qui est un freinage de la houle de vent par
la masse de poissons progressant dans le meme sens que
la houle.
Si la houle est forte, ils savent distinguer dans
r^paisseur de la vague, sous une forme d' ombre couleur
chocolat la presence de thons. Cette observation
porte le nom de "gorrian-gory" (rouge dans le rouge).
Pour les Basques, le comportement de certains oiseaux
est une indication de la presence de banes de poissons
(Fous de Bassan, Sternes et Laridis).
Mais pour le then, ils s'en rapportent & certains
puffins, particuliirement le "Martina" et le "Carcoulos"
qui est un macareux. Us se trompent rarement, le
"Martina" est incapable de plonger, mais son vol
rapide lui permet de vivre des petits poissons (anchois)
que les thons chassent vers la surface en les pour-
suivant.
Si au cours de la nuits les banes de germon se sont
deplaces de 15 a 20 milles, les Basques les retrouvent,
gr&ce & 1'observation d'un ratissage de la mer opere par
les "Martina" volant en explorateurs, en ligne paraltele,
qui tournent alors en groupe aussitot qu'ils soup^onnent
la presence d'un bane de thons. Ce vol tournant,
denomme "Chobaya" signale de loin aux p&heurs que
les thons viennent de monter en surface.
Les pecheurs espagnols et portugais de la cote
atlantique, utilisent les memes methodes basees sur
r observation des oiseaux de mer.
La pfiche devait commencer suivant la precocite du
printemps, c'est-i-dire, suivant le rdchauffement des
eaux et seulement aux points de rencontres de courants
contraries.
Cette theorie s'est virifiie expirimentalement lors des
campagnes oceanographiques effectuees par les bfitiments
de recherches. II a ete remarque en effet que le germon
apparaissait dans les zones de contraste entre des eaux
d'origines differentes.
Les zones de premieres captures se situent Tune dans
le Nord-Est des Azores et 1'autre au large des cdtes
espagnoles et portugaises (60 a 100 milles du littoral).
Entre ces deux zones, malgre une temperature d'eau
favorable, on ne trouve pratiquement pas de thons. La
distribution des eaux y est reguliere, alors qu'elle est
perturbee dans les zones fecondes par des eaux entre
15 et 16° remontant vers le Nord et qui se heurtent & un
contre courant plus frais qui descend du Cap Finist&re
aux Berlingues. Dans le Golfe de Gascogne c'est aux
points de rencontre d'eaux de temperatures diflterentes
que se peche le germon.
Cette theorfe de la rencontre d'eaux differentes dans
toutes les mers justifie la presence de beaucoup de
migrateurs et de poissons voyageurs, notamment les
monies en Islande et Terre Neuve, et dans le Pacifique
le grouillement de faune aux points de rencontre des
grands courants equatoriaux. C'est incontestablement
la fusion d'eaux differentes qui modifie le caractire
des facteurs physico-chimiques qui conditionne la
repartition, la nature et 1'abondance du plancton.
En m6diterran6e on consid&re favorables pour la
pfiche du gros thon rouge par les madragues: une eau
bleue, une transparence de Teau permettant de voir le
disque de Secchi & 17/18 metres, un vent d'Est assez
fort.
Pour les bonites (Sarda sarda) les pficheurs recherchent
une eau bleue, les vols et plongeons d'oiseaux de mer
(Petrels, sternes) qui signalent les banes d'anchois ou
de sardinelles susceptibles d'attirer les pr6dateurs. La
presence et la direction des banes de dauphins sont
une indication, pour la meme raison.
Au Maroc, on situc la sardine en eau vcrte, le d£gage-
ment de bulles d'air provoqui parfois par les grosses
sardines nageant en profondcur est une indication, ainsi
que le vol des oiseaux et leurs plongies.
II serait fastidicux de traiter ici de tous les proc&tes
empiriques de la localisation des esptees dans le monde,
d'ailleurs influences par les phases de la lune qui restent
un element dominant pour les ptehes littorales.
Nous devons cependant mentionner pour mdmoire
"la prospection aerienne" au nombre des moyens
modernes propres & faciliter la localisation des banes
de thons. C'est une histoire enterr6c, bien qu'elle
trouve encore de temps en temps un certain regain
d'actualite. Les Am6ricains Putilisent en Californie
avec des avions speciaux pour rechercher la sardine.
Cette prospection, pour 8tre valable, cxige des moyens
financiers puissants. Elle ne nous paraft possible que
dans certaines mers, dans des regions calmes, et &
condition d'ftre appliquee sur un plan communautaire
de flottilles.
11 existe cgalement une methode de reperage du
poisson par "Sonobuoy", procede japonais base sur le
bruit produit par le deplacement sous 1'eau des banes.
Le dispositif utilise la modulation de frequence & la
tres haute frequence de 42 megacycles. Mais il semble
avoir seulement ete utilise pour determiner les quantites
de poissons heurtant ou se maillant dans des filets
coders. II transmet des indications graphiques &
environ 10 kilometres. Ce dispositif est inconnu en
Europe et nous ne le citons que pour memoire.
Enfin nous devons mentionner que la methode du
"Thermo-Enregistreur", bien que ne rempla^ant nulle-
ment le sondage par echo pour le reperage du poisson, peut
tout de m£me fctre consider commc un auxiliaire pour
le pficheur de thon. Cet instrument qui enregistre d'une
mani&re continue tout changement de temperature
associe & une remontce d'eau, variation de couleur,
peut etre interpret^ comme propice & la presence de
thons. Les points ou se produisent de brusques change-
ments de temperature des eaux de surfaces, ainsi qu'un
changement de couleur d'eau sont propices & cette
presence. Le thon & une repugnance marquee & passer
de 1'eau chaude (14°5) & une eau moins chaude. C'est
ce qui, vraisemblablement, am&ne les thons eparpilies
^ former des banes dans un m€me milieu isotherme
dans lequel ils evoluent. II devient gr6gaire et suit, 4
travers les modifications de temperature des eaux de
surfaces, la zone ou il se plait.
Sondeur A echo
Le sondeur & echo est & 1'origine d'une veritable
revolution dans la recherche et la localisation des
esp&es.
La Technique eiectronique, en constante evolution,
autorise aujourd'hui tous les espoirs. L'identification
des espices peiagiques & nage rapide et & grandc migra-
tion, comme le thon, est entree dans la pratique courante
des p£ches, alors qu'& 1'epoque du precedent Congr&s de
Hambourg, on la considirait comme une performance,
377
ct quc U suyoriti des p&heurs la jugeait mtoie
impossible.
Sur le plan technique de la recherche et des amiliora-
tions & apporter au sondeur, un H&nent capital de
stiocfes, inside dans le fait, que les Inglnieurs, qui se
contentaient d'exfeuter des mesures en laboratoirc, &
1'aide d'une cuve d'eau & 6chos & plan riflecteur mobile,
ont substitui & cctte m&hode les essais riels & bord
d'un navire en mer 6quip£ pour cette fin.
II faut regretter qu'une importante partie des pficheurs
font encore fonctionner leurs echomitres en sondeurs
de fond, et non en d&ecteurs de poissons.
II apparatt actuellement indispensable, que dans les
Ecoles de Pfiche, 1'enseignement comporte des exercices
r6els de localisation du poisson par les ultra-sons, et
que cet enseigncmcnt pour fitrc valable, ne peut avoir
lieu qu'A bord d'un navire.
M<
La morue (Gadus cdllarias) doit etre recherch6e dans
des eaux comprises entre 2 et 4 degr£s C., aussi bien en
hiver qu'en 6t&
L'utilisation du thermomftre de profondeur doit aller
de pair avec la recherche au sondeur. II est parfaitement
inutile de rechercher la morue ailleurs. L'iglefin, par
contre affectionne les eaux entre 5° et 6°C.
La morue s'est depuis 20 ans, considfrablement
rar£fi6e. Ceci tient au fait que la flotte de peche Russe
pr6sente une expansion gigantesque de ses activites,
soit sur les grands Banes, soit au large du Greenland,
que ses roithodes sont massives et organisees avec la
precision d'escadres de combat, et que ses navires
rcspectcnt en pfiche une stricte discipline dans tous les
r61es qui leurs sont assumes, et surtout dans la diffusion
de renseignements aux navires du meme pavilion. On
ne peut en dire autant des pficheurs fran$ais, farouche-
ment individualistes et qui cachent & leurs partenaires
Timportance des banes rencontres.
Les &hos de morue sont facilement identifiables. Us
se pr£sentent sous la forme successive de taches en
forme de delta.
La figure 1 vous donne rimage de monies par un
fond de ISO mitres. Elles sont extraites d'une bande
continue de poissons qui s'&endait sur 42 milles marins.
On voit le grouillement incalculable de monies que cela
peut reprdsenter.
II faut constater que le chalut, avec sa modeste
hauteur d'ouverture en p&che, qui est de 4 & 6 mitres,
est un engin de capture dont le rendement est plus que
mediocre, car le navire qui a rencontr£ ce bane & fait
des prises de 7 tonnes pour 2 Heures de drague, soit sur
une distance parcourue de 7 milles marins sur le fond,
alors qu'en realite, il a evolue parmi des milliers de
tonnes de poisson, ce qui tend & appuyer la thise que
nous avons toujours soutenue, que les chaluts sont trop
lestes, qu'ils raclent le fond, alors que la morue se
tient au-dessus du fond et echappe en partie au chalut.
Thons
La figure 2 nous donne une detection de germon (Germo
alalunga) presqu'en surface, entre 15 et 20 mitres. On
constate la forme lineaire des echos du thon, comparables
^ des traces de fusses rapides. Ces echos sont facilement
identifiables.
Lorsqu'un navire, recherchant le thon voit apparaltre
une image de thon isole sur la bande de son sondeur, il
doit immddiatement stopper et jeter Tappat vivant. II
• | **?* * : w-j**t* * - » . .,. i .
\
•r
; t f
*i*
Fig 2a. Bane de thons (tuna).
w?m-'jimm
Fig 1. Monies (cod).
fig 2b. Thons en bandes, icartts (tuna).
378
voit alors apparaltre sur la bande le gros du bane qui
monte eu surface et s'inscrit en quantite sous forme de
fusees.
Sardines
Nous devons rappeler le role que joue 1'influence du
milieu sur le comportement des groupements de sardines.
En premier lieu, I'hydrologie generate et ensuite
Thydrologie des zones de peche, la temperature et la
salinite.
Les pecheurs, par des observations ataviques, sont au
courant des zones littorales dans lesquelles ils ont
1'habitude de rechercher la sardine (conditions m^teoro-
logiques locales, vents qui ont une repercussion sur le
niveau et la temperature des eaux cotiires, phases
lunaires, etc. . . .).
Mais avant les constatations permises par le sondeur
& echo, ils etaient dans 1'ignorance de certains facteurs
sur les migrations et les mouvements quotidiens des
banes de sardines adultes.
Le sondeur nous a montre que les deplacements
quotidiens se manifestent dans un espace restreint. Ce
sont des mouvements de dispersion et de concentration.
Les migrations, qui entrainent les banes d'une zone
vers une autre n'excluent pas les phenomines de dis-
persion et de concentration qui semblent donner i
certains banes le caractire d'un vol d'etourneaux.
Ces mouvements desordonnes des banes de sardines
ont accredite dans 1'esprit des pScheurs la croyance i un
simple caprice des sardines, qui semblent obeir & un
instinct balladeur, alors qu'en realite, 1'action des
facteurs exterieurs reste dominante.
Les remarques sur le mouvement quotidien des
sardines dont les oceanographes et biologistes de
Tlnstitut des PSches frangais nous ont rfveie le caractire,
sont dties uniquement aux observations que leur a
permis le sondeur ultra-sonore.
Gr&ce 4 leurs experiences, on a pu creer, chez les
pScheurs restes longtemps incredules et sceptiques, un
mouvement d'opinion favorable £ la recherche des
banes de sardines par ultra-sons. 11 est admis
aujourd'hui en France, ainsi d'ailleurs, qu'au Portugal
et au Maroc, que la detection par sondeur reste le seul
procede efficace pour Amelioration de la pfiche i la
sardine.
Le sondeur & echo en vue de la recherche des banes de
sardines a ete utilise depuis 1951, d'abord au Maroc,
sous 1'impulsion de FURNESTIN, ensuite sur la cdte
Basco-landaise et les cdtes Vendeennes par PERCIER,
FAURE, KURC, de la TOURASSE, LETACON-
NOUX, MAUR1N, NEDELEC et THIERRY. Nous
ne revicndrons pas sur les r6sultats largement diffuses et
qui ont donnd les images des echos de sardines, d'anchois,
de sprats, ete. . . .
Nous donnons ici, figures 3 et 4, des images de
sardines mdditerran&nncs obtenues de nuit en Mai
1961 pris de Collioure (fonds de 40 mitres) (Institut
Scientifique des Pdches).
Fig 3. Sardines.
Fig 4. Ctlans, sondcr CRM.
Harengs
Le sondeur 4 6cho permet seul la preuve d'une dis-
tribution extensive des grands banes. Les recherches
avec le sondeur & 6cho ont permis de faire des estimations
de la densit6 minimum des harengs (Clupea harengus)
pour une pfiche donnie et dvaluie avec Tengin capteur
utilise.
On peut citer un bateau qui a p6ch6 80.000 Kgs de
harengs dans le Seal Cowe, apris avoir detect^ la
presence d'un bane de 9 mitres d'6paisseur ^ une pro*
fondeur permettant Temploi de la senne. La senne
ayant 60 mitres de diamitre, les harengs captures
occupaient 26,700 mitres cubes, avec une densite de
0,45 kilos pour 0,15 mitre cube d'eau. Ceci, devant
fitre considire comme une densite minimum, etant
donne que les poissons qui se sont evades avant la
fermeture complite du filet ne sont pas compris dans
revaluation.
Avec ce facteur de densite, il a ete possible d'estimer
la quantite de harengs enrcgistr6e par le sondeur & echo.
Le navire a pu determiner qu'il franchissait des banes de
379
hwengs de 22 miBkms dc Ink*. Ce navirc a enregistrt
sur son sondeur 40 banes somlaires, souvcnt prcsquc
continue ce qtri donnait, dftecti par Ic sondeur, unc
presence de «» millions dc kilos dc harengs.
Les banes de hareng soul trig faciles i identifier, 6tant
donnl fimage trts visible de tour concentration sur les
bandcs du sondeur. Cettc image se caract&ise par son
impression sur Ic papier en forme d'arbres, plus par-
ticuliirement de peupliers.
La figure 5, montrc des banes de harengs sur le fond,
et Ton remarque en surface la presence du plancton qui
conditionne les mouvements descendants diurnes ou
nocturnes des banes.
Merlans
Les banes de merlans (Gadus merlangus) ne donnent
jamais damages imposantes et ils sont constitu& par
des petites taches £loign6es les unes des autres situees &
quelques mitres du fond.
La figure 6, montrc des taches de merlans. II arrive
que la bande du sondeur pr&ente quelquefois des
Fig 5. Banes de harengs (herring). Foods de 35 a 40 metres—
Sondeur SCAM 610 Cie Radio- Maritime.
images plus denses, mais on remarque alors facilement
qu'il s'agit de banes de merlans m61ang£s 4 des harengs
ou autres esptees nageant Tun pris de 1'autre, et qu'en
examinant la bande, Pimage de leurs 6chos n'est pas la
mime. Le merlan scul, donne des images peu fournies.
La figure 7, repr&ente des maquereaux (Scomber
scombrus). Cette esp&e se pr&ente toujours en quantit6
importante. Son image affecte la forme d'un dessin
d*une for£t dense et continue dont quelques arbres
dominent les autres. Le sommet des arbres est toujours
plus noir, d'une touche plus dure. On note toujours la
presence du plancton en masse au-dessus des banes de
maquereaux. Ces derniers peuvent s'itendre avec la
meme densit6 sur plusieurs milles de longueur.
Fig 6. Merlans (whiting)— Fonda de 100 A 120 m*fte>-Chalutier
Notre Dame de Boulogne.
380
Fig 7. Maquereaux (mackerel)— Sondeur 610 CRM.
IVfcrhis
Ce poisson est rarement d&ect& On sait qu'en France,
il est considere comme une esp&ce noble et d'ailleurs
chire i la vente. Le Port de La Rochelle est axi vers
la peche du merlus (Merluccius merluccius) et les bateaux
rochelais le pourchasse inlassablement dans le Golfe de
Gascogne. Remarquons que ces navires p6chent en
moyenne 100 Kgs. au maximum de merlus par trait de
chalut de 4 heures, soit 100 Kgs pour 14 4 16 milles
parcourus par le filet sur le fond. Quand un chalutier
ramine 2 tonnes de merlus au port apris unc sortie de
14 jours en mer (soit une pSche effective de 10 4 1 1 jours)
on estimate cette quantity une trts bonne pSche.
La raret£ de ce poisson est la clef du myst&re de sa
detection difficile, puisqu'il nage sur le fond, solitaire, &
des distances fort 61oign6es les unes des autres.
Autres traces d'echogrammes
Les traces d'echos des figures 8, 9 et 10 montrent 1'image
caract&istique produite par les anchois (Engraulus
encrasecholus), les chiens de mer et de gros poissons sur
le fond.
Ligne blanche
Lorsque le poisson se trouve & proximite immediate du
fond de la mer et qu'il epouse les sinuosit6s de ce fond,
il est souvent difficile de 1'identifier avec certitude*
I/Industrie met maintcnant & la disposition du pficheur
Anchois (anchovy)— Bande de Sondeur Cie Radio-Maritime.
Fig 9. Chiens de Mer (dogfish}— Sondeur CRM.
Fig 10. Gros poissons sur le fond et entre deux eaux — Fonds de
110 a 150 metres Plancton en surface (big fish on bottom, plankton
on surface).
un dispositif qui live ce doute. Celt en rialiti un
"separateur d'fchos de fond", & qui on a donni la
denomination de "ligne blanche*9.
n permet la discrimination graphiquc entre le profit
du fond lui-mgme et les banes de poissons qui ivoluent
ou sont statiques & son voisinage.
On obtient cette discrimination par le d&oublement de
1'echo de fond & 1'aide d'une ligne blanche separatrice,
qui epouse toutes les sinuositis du fond et permet alors
au p&cheur d'avoir la certitude de la pr6sence rdelle du
poisson. On voit done combien ce perfectionnement
peut lui apporter de precisions comp!6mentaires.
Detection horizontale
II s'agit de nouvelle possibilit6s offertcs au pteheur pour
la recherche des banes. Le navire n'a pas besoin d'etre
& la verticale du bane pour constater la presence du
poisson, mais il peut le d&ecter & distance et determiner
le site du bane, sa hauteur de nage et la direction qu'il
suit. L'Industrie fran^aise vient de mettre au point un
appareil de ce genre qui trouve d£j& de nombreuses
applications a bord des navires de recherches et les
grands chalutiers de tous pavilions.
La television sous-marine
Alors que la television proprement dite, connait un
developpement mondial et nous donne m&me des images
du Cosmos, les utilisations de la television sous-marine
sont extremement limitees pour des raisons d'absorption
du milieu aquatique marin. En plus de cette absorption,
un halo dQ & Teffet Tyndall dans les eaux troubles ou
gluantes freine considerablement la portee utile des
appareils.
Pour le moment, les appareils dont nous disposons
permettent des recherches d'ordre biologique, des
travaux de coques et de ports. II est possible d'observer
les engins de peche en fonctionnement, on peut espirer
obtenir avec la television des renseignements pour un
perfectionnement des engins de ptehe, contrdler le
comportement du poisson en presence d'un art trainant
et ce meme comportement dans le cul du chalut. La
television sous-marine & un avenir incontestable dans les
peches maritimes, mais nous estimons que pour le
moment, elle reste au stade experimental pour la pAchc
courante et pratique. II est cependant probable que
dans les anndes qui vont suivre, nous cnregistrerons de
nouvelles possibilites offertes & 1'industrie des pfches par
la television.
Perspective d'avenir des sondeurs
Sur le plan industriel international, le marche des
sondeurs ultra-sonores est assez etoffe pour permettre
des etudes et des recherches de 1'Industrie eiectronique,
et tous les techniciens s'ingenient i apporter & ces
appareils des ameliorations constantes. Cette pers-
pective est dfle surtout aux navires de pdche, dont le
nombre tris important autorise les efforts des chercheurs
et des savants, puisqu'ils y trouvent une clientele eiargie.
continued on page 382
381
Echo-Detection of Tuna
Atetact
To fish for tuna scientifically, one must know tuna behaviour,
swimming depth, speed and school density. Many methods have
been used to father these data, but have not offered sufficient
conclusive evidence. In this paper, the author proposes use of an
echo sounder. After many experiments, it has been established that
an echo sounder can effectively detect tuna schools and contribute
to tuna fishing. First, the author discusses the properties of an echo
sounder able to detect tuna at a depth of 200 m, and concludes that
the instrument must have a sounding depth of bottom of more
than 3,000 m to operate efficiently. Secondly, data are mentioned
on swimming depth, speed and school density, procured by echo
sounders in the seas around Japan and in the South Pacific Ocean.
These data establish that the swimming depth depends on the degree
of illumination of the sky. The relationship between the shape of the
Finally, some examples of swimming speed and density of school
are mentioned.
Detection da ton pur
utara-sonore
La pftche scientifique du thon requtert des connaissances sur le
ontitwedfrom page 381
Cette Emulation entre les industries £lectroniques
Internationales nous permet d'envisager des progrcs et
des possibility de detection des poissons qui iront en se
divcloppant, surtout dans la d6tection horizontal.
II cxiste £galement un precede" de detection acoustique.
Dans ce domaine, non seulement les r&ultats acquis
sent souvent dccevants, mais ils exigent, pour poursuivre
des etudes, des moyens financiers hors de proportion
avec ccux dont dispose 1'industrie privee. Les ex-
p£riences poursuivics restent gene>alemcnt de caractcre
confidentiel, etant executes & 1'echelle des grandes
puissances militaires navales. La menace que represente
le sous-marin atomique est si grave qu'un effort con-
siderable est actuellement fait pour accroitre les moyens
de detection actuels.
Les conditions de detection acoustiques sont intime-
ment lices & la repartition des facteurs physiques
caractfrisant 1'ftat du milieu marin, alors que la
detection ultra-sonore s'en affranchit en partie.
Les techniques d'ex&ution des mesures qui affectent
les propagations acoustiques sont encore & un stade
rudimentaire, et elles echappent & 1'industrie privee qui
ne dispose pas d'un navire adapt£ et equipe pour de
tels travaux. Outre I'iquipement d'un tel bfttiment, il
faut tabler pour assurer 1'exploitation et Farchivage des
donnfes oceaniques qui doivent fctre pratiquees sur une
longue pcYiode, 1'utitisation de ces donnces n&essitera
un laboratoire & terre des plus complexes et des plus
on&eux.
On peut done, sans risque d'erreur, assurer les pecheurs,
que le sondeur ultra-sonore dont ils disposent, restera
certamement 1'outil le plus precicux et le plus efficace
de la rentabilitc* de leurs navires pendant longtemps.
Nous avons donnc" les identifications des principales
espcces commercials pratiquement ptehees dans les
382
by
Minoru Nishimura
Fishing Boat Laboratory, Tokyo
comportement du poisson, la profondcur a laquelle il se deplace,
sa Vitesse de nage et la densite des banes. Parmi les nombreuses
methodes employees pour rassembler ces donntes aucunc nfa
donnt de resultats suffisamment concluants. L'autcur de la prtaente
note propose d'utitiser a ces fins un apparcil a reperer les poissons.
De nombrcux essais ont permis d'etablir qu'un tel appareil peut
effectivement detectcr les banes de thons et contribuer a la capture
Mers europe*ennes. II est entendu que ces types d'echo
ne se retrouvent pas avec une netteti identique sur toutes
les bandes de sondeurs d'origine differente. Le trac^
pour un sondeur donn£ depend de facteurs divers:
tension electrique appliquee
cadence de sondage du d6tecteur
vitesse de deroulement du papier enregistreur
sensibilit6 de 1'appareil
densiti et multiplicity des couches d'eau traversees
vitesse du navire.
Les images principales exposes ici, ont 6te obtenues
avec un sondeur tres sensible, mais il se peut que des
appareils moins sensibles et moins modernes ne donnent
pas des precisions aussi remarquables.
Nous ajouterons meme que les experiences faites en
1954, par des occanographes, avec la plus scrupuleuse
attention et probite, donnaient des images d'echos
n'ayant plus de rapport avec celles obtenues aujourd'hui.
Ceci tient uniquement au fait que la technique des
sondeurs a ete am^lioree. On identifiait alors du gros
thon rouge sous forme de points & peine visibles, alors
qu'aujourd'hui le germon, plus petit, donne des images
en fusses, facilement identifiables.
Les classifications des 6chos enregistrds & cette epoque
ne nous paraissent plus valables, avec les sondeurs
actuels.
Sources
Les bandes de Sondeur ont M mises gracieusement a ma dis-
position par la Compagnie Radio-Maritime, Plnstitut Scientifique
des Ptehes et la Soci& SCAM.
J'ai consult^ mcs confreres suivants: M. Von Brandt (Alle-
magne); M. J. Rawlings (Colombie); Mm. G. Kurc, Maurin, de
Meglio, Pcrcier (France); M. Hashimoto (Japon); Mm. de Espada,
Manuel Rubio (Espagnc); M. Aloncle (Maroc); M.Tibbo (USA)
et jfai eu aussi des contacts directs avec les Capitaines de pdche.
de ces poissons. L'auteur examine tout d'abord les caractfcristiques
d'un appareil capable de dttecter tes thons 4 une profondeur de
200 metres, et conclut gue, pour donner satisfaction, le dispositif
doit pouvoir sonder & plus de 3,000 metres. II cite ensuitc des don-
nees sur la profondeur de nagc, la vitesse de nage et la denstte des
banes, obtenues & 1'aide d'appareils a dttecter les poissons au large
du Japon et dans le Pacifique Sud. Ces rensciencments ttablissent
que la profondeur de nage depend du degr6 d'intensite lumineuse
du del Le document fait tgalement etat du rapport entrc la con-
figuration des banes se trouvant sur les fonds de pftche et la riparti-
tion des thons. Enfin, Fauteur donnc quelques cxemples de vitesse
et de densiti des banes.
Fig. 1 shows the relationship of ultrasonic reflection loss
of tuna versus frequency through 10 to 400 kc.
Extracto
Para la pesca con caracter cicntifico del atun es necesario conoccr
el comportamiento de cste animal, la profundidad y la velocidad a
que nada y la densidad de los cardumenes. Para recoger tales datos,
se ha recurrido a muchos m6todos, pero 6stos no ban ofrecido una
prueba bastante concluyentc. En este trabajo, el autor propone el
emplco de un instrumento localizador de peces para realizar tales
observations. Desputs de muchos experimented, se ha compro-
bado que un localizador puede detectar cficazmente los cardumenes
de atun y efectuar investigaciones sobre estc. En primer lugar, el
autor examina las propiedades de un aparato localizador que sirva
para la detccci6n de atunes a una profundidad de 200 m, y llega a
la conclusidn de que este instrumento, para funcionar eficazmente,
debe tener una profundidad de sondeo de mas de 3,000 m. En segun-
do lugar, se indican los datps sobre profundidad y velocidad a
que nadan y sobre la densidad de los cardumenes, obtenidos
mediante fonolocalizadores en los mares que rodean al Jap6n y
en el sur del Oc6ano Pacifico. Estos datos muestran que la profundi-
dad a que nadan depende del grado de iluminaci6n del cielo.
Igualmente se explica la relacibn entre la configuraci6n de los bancos
en las zqnas de pesca y la distribuci6n de los atunes. Por ultimo,
se mencionan algunos ejemplos de velocidad y de densidad de
cardumen.
TUNA schools are generally less dense than other
schools. Consequently, to locate tuna, the acoustic
power, pulse transmission, beam angle and sounding
range must be properly selected. Also the observation
of d.s.L is important for forecasting tuna catch. The
echo sounder must therefore have the capacity for d.s.l.
detection.
The ultrasonic reflection loss of the fish body and of
d.s.l. are known; the values are about 15 decibel at
28 kc, 8 db at 200 kc for yellowfin, and 20 db at 28 kc,
12 db at 200 kc for albacore. The reflection loss of d.s.l.
is about 40 to 60 db within the range of 20 to 200 kc.
10
Fig. 1. Relation between frequency and reflection loss of tuna.
For tuna detection it is necessary to transmit a stron-
ger output than for bottom or fish-school detection.
For example, an echo sounder with bottom-sounding
range of more than 3,000 m is required to detect tuna
at a depth of 200 m.
The echo sounder used in the experiment was designed
as follows:
Frequency 28 kc
Ranges for recorder
(Oto 100m)x5phases
(Oto 120m)x5phases
(0 to 600 m) x 5 phases
33-3/40 m
valve oscillation
3kW
90/75/15 per minute
21 /1 7-5/3-5 mm/min
AF alloy
163x270 mm
1 1-5° x 8-2°
Maximum sounding capacity 6000 m
Maximum tuna finding capacity 500 m
Identification of fish appearing on the record of the
echo sounder is important but difficult. An acoustic
technique measuring the sound pressure of reflected
wave from tuna, offers some solution. The kind and
size of tuna can be roughly identified by measuring the
sound pressure of reflected wave on the amplifier of the
echo sounder, when the output acoustic pressure and
also the reflection loss of tuna is known.
for CRT
Transmitting system
Output power
Pulse transmission
Paper speed
Transducer
size
beam angle
Fig. 2. Echo trace of tuna as recorded in New Zealand waters. Each triangle indicates a tuna.
383
Fig. 2 shows a record obtained on the fishing grounds
north-east of New Zealand; each small triangle-like
mark indicates a fish. Fig. 3 shows measured values of
north-east of New Zealand; each small triangle-like
intensity of fish which appeared on this echo chart.
The abscissa indicates depth in metres, co-ordinate
sound pressure in db. Mark ( . ) shows measured value
for fish swimming in a deep zone of 20 to 100 m and which
are recorded intensively on the paper and mark (x)
indicates fish in the shallow zone. As the transmitted
acoustic pressure of the echo sounder used in this measure-
ment was known, drawing the curves of attentuation
for different reflection loss from 15 to 60 db, these
measured values show that they are closely within the
range from 15 to 60 db. In other words, some of the
echoes marked ( . ) with a value of 15 to 20 db seem to
be albacore or yellowfin. The mark (x) seems to indicate
small fish about the size of sardine or mackerel, as the
value was not intense.
120
100
60
20
I 'II
I Ml
Jtaf.1960. 26»0,176»V
28 ko
FISH OP WAI uruc-
TIQV RAB 8UBPACI
Tim OF nmai n-
FL1CTIOI XI MID VATBT
8 10
20
(M
40 60
100
200
)
Fig. 3. Identification of tuna by acoustic measurement. Each curve
indicates intensity of reflected wave from tuna or various reflection
loss, and (.) or (x) show measured value for fish echoes.
Detecting tuna
Using the new type echo sounder, useful results were
obtained in regard to tuna behaviour.
Fig. 4 shows the change of swimming depth of tuna.
During the night, the recording paper of the echo sounder
was filled with hazy echo-like d.s.l. and the trace of fish
could hardly be seen in the deep zone. But in the morning
about sunrise, the fish layer went slowly to 40 to 80 m.
Towards evening the fish layer began to rise slowly at the
speed of 20 or 30 m per hour, and fish were only recorded
near the surface after sunset. During 17 days of experi-
ment, this pattern of movement was found unchanged.
Similar findings have been recorded in other areas.
The rising and sinking of tuna in the morning and in
the evening respectively seems to be related to the illu-
mination of the sky. The measured value for this illu-
mination was noted in Fig. 4.
384
20
100
120
« jm-^m+y+Xj****.
-—m^Mf~ma^^m^m~a—~~*~~*'f*f~'~*^—^' t. , rT»»^^»^t» • • t >f> •"•
-S;fJpaffi^fi^H^-
*" f i ^A ^ i ^ ; \ ft*/' £%c\
4 56 7 8 9 10 11 12 13 14 13 16 17 18 19 20
TIM!
Fig. 4. Dally change in swimming depth of tuna, as recorded in New
Zealand waters. Change of illumination degree versus time is shown.
Fig. 5. Distribution of yellowfin around school of small fish. This
record was obtained in the sea east of the Solomon Islands.
The echo-chart in Fig. 5 shows the relationship
between the depth of the tuna and the bait fish school.
The small triangle in the deep 60-120 m zone indicates
yellowfin (many yellowfin were caught in this area).
The continuous black traces show dense schools of
small bait fish.
Fig. 6. Echo-chart of blue fin tuna in Tsugaru Strait. Each triangle is a
tuna-echo.
**;;<o.ooa> j*
A A fAA^
*»3&
(O.OOOD
mum a rau/ij
8 Jwly 19U "«L
li.30- 14. JO fc». Wtt
I . I I
900 1000 1)00 1000 1900 1000 3900 400 4900 9000
OltTAXll ( M )
Fig. 7. Distribution of bluefin tuna around bank, obtained by
continuous echo-chart.
In the fishing grounds of the bluefin tuna, in Tsugaru
Strait, in the northern offshore area of Shimokita
Peninsula, Aomori Prefecture, Japan, tuna gather
continued on page 385
Echo-Sounder Measurement of Tuna Longline
Depth
Abstract
Efficient tuna longline operation requires a thorough knowledge
of the behaviour of tuna and the ability to gauge the depth at which
the hooks operate. The depth of individual pans of a longline can
be obtained by pressure gauge recording, but this does not enable
the operator to form a clear idea of the shape of the longline as a
whole.
The authors compared the echo-sounder recordings of longlines
in operation with the data simultaneously obtained by pressure
gauges and ascertained that a true configuration of the shape of the
longline can be obtained by high-frequency echo sounding. Three
types of echo sounder were used having frequencies of 28 kc, 50 kc
and 200 kc. All parts of the gear were clearly recognisable on the
records, as well as the difference in shape due to the use of different
material in longline construction. The experiment further showed
that a high frequency (200 kc) gives much better i
low frequency (28 kc).
recordings than
Observation de la configuration de la longue-ligne du thon par le
sondage a echo
Resurn*
Pour 1'utilisation efficace de la longue-ligne, une connaissance
approfondie du comportement des thons est necessaire, mais il
faut aussi avoir la possibility de mesurer la profondeur & laquelle
operent les hame$ons. Or, si Ton peut actuellement mesurer en
certains endroits, la profondeur de la longue-ligne, la methode
employee ne permet pas de se faire une idee precise sur la configura-
tion de la ligne submergee.
Les auteurs ont compare les sondages de la longue-ligne en opera-
tion, enregistres par echo-sondeur, avec les renseignements obtenus
simultan6ment par des indicateurs & pression. Cette comparaison
a confirm^ qu'il est possible de connaftre, par Tenregistrement
d'echo-sondage, la forme cxacte prise par la longue-ligne en opera-
tion. Les experiences effectuecs a 1'aide de trois frequences differ-
entes: 28 kc, 50 kc et 200 kc, ont permis d'identifier aisement toutes
les parties de 1'engin et d' observer la configuration de ces di verses
parties construites de matieres differentes.
L'exp6rience a aussi ddmontre que, pour ce travail, recho-sondeur
£ haute frequence (200 kc) donne de meilleurs resultats que celui a
basse frequence.
Empleo de la ecosonda para determinar la profundidad a que estan
los palangres atuneros
Extracto
Para que los palangres atuneros den buenos resultados es necesario
by
Kyotaro Kawaguchi
Fisheries Experimental Station,
Miura City
M asakatsu Hirano
Sanken Electronics Co.,
Namazu City
Minoru Nishimura
Fishing Boat Laboratory, Tokyo
conocer a fondo el comportamiento del atun y poder determinar
a qu6 profundidad se encuentran los anzuelos. La profundidad de
partes individuates de un palangre puede determinarse rcgistrando
la presibn en un manbmctro, pero con esto el pescador no se puede
formar una idea clara de la forma que tiene el palangre en su
conjunto.
Los autores compararon registros obtcnidos con ecosondas de
palangres calados con datos recogidos simultaneamente con mano-
metros de presi6n y determinaron que puede conoccrse la forma
exacta del palangre mediante ecosondajes de alta frecuencia.
Emplearon tres tipos de ecosondas con frecuencias de 28, 50 y 200
kc. Todas las partes del aparejo se reconocian facilmente en los
registros y tambien se observaban las diferencias presentadas por
el palangre a causa de los diversos materiales empleados en su
fabricaci6n. £1 experimento tambien demostr6 que una frecuencia
alta (200 kc) da registros mucho mas claros que una frecuencia baja
(28 kc).
HITHERTO, water pressure gauges have generally
been used to measure the depth of the longline;
but shape of the line cannot be so determined during
fishing operations.
The echo sounder used in this experiment operated
with 28, 50 and 200 kc ultra-sound; some of the main
continued from page 384
around the bank of Oma. Fig. 6 shows an echo chart
obtained by using at 28 kc echo sounder, and each
triangular mark indicates bluefin tuna. The depth of
the sea bed changes steeply from 200 m to 60 m as shown
in Fig. 7. Tuna were distributed in a wide area, the
distribution being especially dense at the steep incline.
The depth of the tuna was measured at about 100 m
in the deep zone and at a few metres in the shallow zone.
Swimming speed of tuna
The swimming speed of fish may be calculated from the
echo trace appearing on the recording chart of the echo
sounder. When the ship speed is known, the swimming
speed can be calculated from the height and width of
the fish echo.
It was thus calculated that the maximum speed in
normal conditions was within the range of two to three
knots for yellowfin, albacore and bluefin respectively.
Density of the school can also be calculated from the
echo-chart. The calculation was based on the supposition
that the beam of sound was a circular cone, and the
beam angle a half-power beam angle for the combined
directivity of transmitting and receiving transducer.
The calculated values show the density of tuna schools
was as follows: for albacore in the Southern Pacific
Ocean, 0-0002 to 0-0003 tuna/m3, for yellowfin, 0-0001
tuna/m3, and for bluefin tuna along the Japanese coast
0*0002 tuna/m3 (maximum 0*002 tuna/m3).
Because of the echo sounder's value in tuna detection
new efficient fishing methods are to be developed.
References
1. Hashimoto, T. and Y. Maniwa, Technical examination and
tentative making of fish finder for tuna and experiment on it on sea.
Tech. Rep. Fish. Boat., 13:103-111. 1959.
2. Nishimuia M., Study on echo sounders for tuna fishing in the
north-eastern New Zealand Sea. 7>cA.-R^.F/5A.J5M/. 15:78-& 1961.
385
features are shown in Table I. In this experiment,
depth-recorders based on water pressure4 were also
used at the same time for comparative data. The initial
experiments were made at Towada Lake (depth 340 m)
because of the relative absence of waves, currents and
wind; but the operational experiment was made at
Sagami Bay under normal ocean conditions.
50
Utoatoofc reflection of
A knowledge of die longline's ultrasonic reflection loss
is necessary to adjust the echo sounder for longline
detection. The longline used in the test was slightly
longer than the diameter of the ultrasonic beam as shown
in Fig. 1, and it was suspended horizontally in the water.
The first experiments were made to measure the echo
margin of the rope at three different frequencies, and the
reflection loss was calculated from the echo-intensity
of a standard reflector with a constant reflection loss.
The longline was made of polyvinyl No. 20 x 55 x 3 x 3,
5-7 mm diam, coated with coal tar.
LONOLIBB
FOR TEST
IRON
PIPE
Fig. L Measuring method of reflection loss.
Table I
Frequency (kc)
Sounding range (m)
Beam angle
Sounding capacity (m)
NTL 500 Type
28
(0— 50)x5
(0— 75) x 5
(0— 100) x 5
13°
4,000
NTB 500 Type
50/200
(0-50) x 5
(0— 75) x 5
(0—100) x 5
13°/3-8°
1,000/500
The reflection losses measured were 46 db at 28 kc,
44 db at 50 kc and 41 db at 200 kc. Fig. 2 shows the
relationship between the frequency and reflection loss;
it seems that the reflection loss decreases when the
frequency increases for the range of 28 to 200 kc. The
ftt^jmum longline detection capacity based on the
values given above was calculated for the apparatus
(Table I) as 250 m for 28 kc, 90 m for 50 kc and 200 m
for 200 kc.'
386
10
20
30 50
FREQUENCY
100
( KC)
200
Fig. 2. Relationship between reflection loss and frequency.
The reflection loss tends to increase under low-
frequency, when the inclination of the longline increases.
Comparing recordings obtained by 28 kc and 200 kc
sounders, (Fig. 3) it is apparent that longline detection
is better with high-frequency than with low-frequency.
Fig. 3. Comparison of echo chart of longline obtained by 28 kc and
200 kc.
Reliability of longline echo recordings
The shape of the sea bottom can be accurately recorded
by echo sounder, when the beam of the ultrasound
transmitted by an apparatus is extremely narrow. 6 Accor-
dingly, 3*8° was taken as the beam angle of the 200 kc
echo sounder used in this experiment, resulting in the
shape of the longline bring recorded without error. To
verify this further experiments compared the depth of
the longline as obtained by echo sounder with that
obtained by pressure depth recorders attached to the
first third and fifth junctions of branchlincs.
( ) INDICATES DEPTH IN M
Fig. 4. Method for testing the reliability of depth measurement.
)
40
10*44'
t#t. 8. 11
ioV tint «"»*
F/£. 5. Depth indicated on the depth recorder.
Fig. 6. Depth indicated on the echo chart.
One basket of longline was set in a fixed position as
shown in Fig. 4. A recording by echo sounder was
obtained by running over the longline at constant speed.
Fig. 5 shows the record of the pressure depth recorder,
while Fig. 6 gives the recording of the echo sounder.
As the scale ratio of the ordinate (depth) and the abscissa
(distance) of these recordings are not proportionate,
the readings were converted and are given in Fig. 7 on
an identical scale basis. In this figure, the points marked
with an x give the depth at the junctions of snoods No. 1,
3 and 5 with the mainline as measured by the pressure
depth gauges and it can be seen that these depths almost
coincide with those obtained by the echo sounder.
Based on this it may be safely assumed that when a
sufficiently narrow acoustic beam is used, the operational
shape of the longline can be measured accurately.
DXSTANCI ( M )
10 100.196*
OOTTOB
BOOTS DXmiNOl Uta
IT! WO 20010 ^
Fig. 7. Shape of longline obtained by echo sounder and depth obtained
by depth recorder.
Fig. 8. Construction of longline used for detection.
Shape of longline of differing material
To investigate the effect on the operational shape of the
longline when made of materials of different density,
a section of longline was assembled as shown in Fig. 8.
continued on page 383
387
A 200 kc/28 kc Dual Frequency Echo Sounder
for Aimed Midwater Shrimp Trawling
Afattact
A remarkable development in midwater shrimp trawling has taken
place since the winter of 1959 in the East China* Sea. By the
application of a dual frequency echo sounder, with 28 and 200 kc/s
ultrasound, connected to two transducers on the headline and by
using specially designed trawl gear, it was possible to operate
successfully on compact shrimp schools swimming in midwater.
The ship's transducer records the depth at which the schools swim
at the time the ship passes over, while the net transducers give
accurate information on the position of the net in relation to the
bottom and surface. This makes it possible to lower or raise the
net into the water layer of the school and to conduct "aimed"
fishing. The trawl nets used by 700-800 hp sidetrawlers are of the
four-scam type with a 44 m headline and 55 m footrope, made of
black polyethylene knotless netting. Curved otter boards of the
Suberkriib type 3-2 m high and 1-6 m broad are used with these
nets. Full specifications and design drawings are given of the nets
and the otter boards. A successful shrimp fishery of about 200
trawls is presently using this gear.
Un echo-flondeur a deux frequences (200kc/28kc) pour la peche
pelagiqoe aux crevettes
Rfeumt
Au cours de 1'hiver 1959, la peche pglagique des crevettes s'est
Cteveloppee de fa$on remarquable dans la Mer de Chine orientate.
Par 1'application d'un £cho-sondeur a deux frequences, avec des
ultra-sons de 28 et 200 kc, relte a deux 6metteurs months sur la
ralingue superieure et par 1'utilisation de chaluts sp£ciaux, il a et£
possible d'operer avec succes sur des banes compacts de crevettes,
entre deux eaux. L'6metteur du bateau cnregistre la profondeur
du bane de crevettes au moment ou le bateau passe au-dessus,
tandis que les 6metteurs de la ralingue superieure donnent directe-
ment la position du filet par rapport a la surface et au fond. De
cette maniere, il a M possible de lever ou d'abaisser le filet dans la
couche d'eau et de pratiquer la peche "visee". Les chaluts utilises
par les chalutiers pcchant par le cot6 de 700-800 cv sont du type a
4 coutures, avec des ralingues sup^rieures de 44 m et ralingues
inferieures de 55 m, construits enticement de polyethylene noir,
by
Masakatsu Hirano
Sanken Electronics Co.,
Namazu City
and
Takashi Noda
Shibiara Technical Institute,
Tokyo
en filet sans noeuds. Des panneaux courb£s du type "Suberkriib**
de 3,2 m de hauteur et 1,6 m de largeur sont utilises avec ces filets.
Les caract^ristiques et dessins de ces engins sont donnas dans la
communication. A present, 200 chaluts environ, utilisent cen engins
avec succes.
Una esconda de frecuenda doble de 28 y 200kc para la pesca dirigida
del camarfn con artes de aurastre flotantes
Extracto
Una notabilisima innovaci6n en la pesca del camar6n con artes de
arrastre flotantes se puso en pi&ctica en el invierno de 1959 en el
mar del Este de la China. Aplicando una ecosonda de frecuencia
doble de 28 y 200 kc/seg conectada a 2 transductores en la relihga
alta y empleando un arte de arrastre construido especialmentc, se
pudieron explotar con gran exito cardumenes compactos de
camardn que nadaba entre dos aguas. El transductor del barco
registra la profundidad a que se encuentra el cardumen en el
momento en que el barco pasa por encima, en tanto que los trans-
ductores de la red indican exactamente cual es su posicidn con
respecto al fondo y a la superficie, lo que hace factible elevarla o
continued from page 387
Each unit was alternatively made of cotton, vinylon
and polyethylene. The line was first operated at Towada
Lake for observation and a recording was taken IS
minutes after setting.
This shows that all parts of the gear, mainline, branch-
line, sekiyama and hooks, were clearly recorded. As
this section of six units was set isolated, the outer ends,
units one and six closed in under their own weight and
the hook and bait of the centre branchline sank to a
depth of 143 m.
Unit three was made of polyethylene which has 0-95
density and was therefore only submerged by the weight
of the sekiyama and hook. The centre hook of this unit
only sank to 77 m, while the distance between the floats
remained much greater. The hooks of unit two which
was of vinylon material, attained a maximum depth of
115 m.
The experiment was repeated at sea during average
weather and recording was obtained IS minutes after
sotting. The gear again consisted of six units rigged as
before and the different parts of the longline are well
discernible in spite of rough seas.
388
The test showed that a clear record of longlines and
all connections can be obtained by high-frequency
(200 kc) echo sounders.
Such measurements are very much the same as those
obtained by pressure gauges, but an echo sounder records
more accurately the true shape of the longline.
References
1 . Nishimura, M. : Study on echo sounder for tuna fishing in the
north-eastern New Zealand sea. Tech. Rep. Fish. Boat 15:79-89.
1961.
2. Wathne, F.: Summary report of exploratory longline fishing
for tuna in Gulf of Mexico and Caribbean Sea, 1954-1957. Com.
Fish. Rev. 21-4:7. 1959.
3. Shibata, K. : Analysis of the fish-finder records—I. Bui. Faculty
Fish. Nagasaki Univ. 13:10-17. 1962.
4. Hamuro, C. and K. Ishii: Analysis of tuna longline by auto-
matic depth meters. Tech. Rep. Fish. Boat., 11 :42-52. 1958.
5. Hashimoto, T. and Y. Maniwa: Technical examination and
tentative making of fish-finder for tuna and experimenting with it at
sea. Tech. Rep. Fish. Boat, 13:103-111. 1959.
6. Hashimoto, T. and M. Nishimura: Reliability of bottom
topography obtained by ultrasonic echo sounder. Inter. Hydro-
graphic Rev. XXXVM:43-50. 1959.
bajarla quc este* en la capa de agua donde se encuentra el cardumen
y practical dcesamancra la pescai4dirigidaM. Los artes usados por
los arrastreros con motores de 700 a 800 hp, quc pescan por el
costado, son de 4 relingas, con la alta dc 44 m, la baja de 55 m y se
hacen de polietilcno negro sin nudos. Con &tos se emplean
puertas curvadas del tipo Suberkrttb, de 3,2 m de altura y 1,6 m de
longitud. Se dan detalles completes e ilustraciones de las redes y
las puertas, empleadas actualmente por 200 arrastreros dedicados,
con excelentes resultados, a la pesca del camar6n.
IN the winter of 1959 the development of a dual-
frequency echo sounder for trawl fishing in the East
China Sea remarkably increased the catches of prawn
(Penaeus orientatis Kishinouye). Subsequent exploratory
fishing using it provided information on the behaviour
and reactions of bottom fish to a net, the diurnal vertical
movement of the dsl and the close relations between
the dsl and the bottom fish which had been reported.
In early observations of the trawl net by echo sounder, a
second boat equipped with an echo sounder had to be
used which would follow the trawler, just above the net
to observe the performance of the net towed by the first
vessel. This often resulted in erroneous measurements
when the echo-sounding vessel was slightly out of line as
she would often obtain no echo at all or receive echoes
of parts of the net other than the actual headline bosom,
resulting in higher or lower readings.
Early use of the netzsonde improved measurements of
the net height to some extent. With this method, a
transducer mounted on the headline emits and receives
ultrasound from sea surface and th^ bottom; the trans-
ducer itself is connected through a cable to the recorder
installed on the fishing boat. Under normal operation the
fisherman on board can clearly detect the height of net
mouth, the distance from the bottom and fish entering
the net. If the sea bed is rather flat, the net can be towed
near to the bottom, but on uneven bottoms, the record
becomes too complicated to measure the height of net.
The authors found that by combining the netzsonde
with another echo sounder on board it was possible to
adjust the towing depth to the swimming layer of fish.
Many of more or less similar attempts proposed in
various countries employed only one transducer on the
net.
The echo sounding records taken in the East China
Sea in depths of 60 m indicated that the prawn schools
usually swim about 10 m above the bottom, but that this
swimming layer often rises to 30 or 40 m from the bottom
(Fig. 1). This movement of the prawn in relation to the
bottom makes it necessary to have a device on the net
to accurately indicate its depth. As a result the Net-
Vision (Japanese patent pending) was designed, tested,
and has now come into service on more than 100 fishing
boats operating midwater at night, as well as in the
daytime. Much assistance was obtained from Hamuro
and his colleagues who designed the midwater trawls
(Hamuro and Ishii, 1960) on which the experiments with
the present device were carried out. Japanese and
Canadian patents are pending.
Construction and operation of Net-Vision
The apparatus consists of a dual-frequency echo sounder
with a transducer installed on the fishing vessel, an
electrical connection cable and two transducers for
mounting on the headline (Fig. 2). The net transducers
are housed, water and pressure tight, in a metal box of
140 X 330 X 125 mm in size of which the weight in
water is 9 kg.
Fig ]. Echo chart showing a large school of the prawn, Penatus
orientals Kishinouye, that rose from the bottom of the Yellow Sea
at dusk on January 9, 1962.
Depth of waters 75 m, the highest layer of the school 44 m and the
lowest layer 7 m, above the bottom.
Fig 2. Component parts of the Net-Vision, model NV 4-500, under
report. A. Switch board; B. main body housing the paper indicator,
oscillograph, and amplifier; C. pulse transmitter; D. spare box and
recording paper containers; E. invertor; F. net-side transducer;
G. boat-side transducer; H. pulley; I. oil pressure gauge; J. flow
control valve; K. hydraulic pump; L. oil tank; M. cable reel with a
motor for the pump.
The two transducers are installed on the headline in
such a way that the one faces up towards the surface and
the other towards the bottom. The surface transducer
emits an ultrasound beam of 28 kc and 75 kc, or 200 kc
towards the sea bottom.
The upper transducer has a beam angle of at least 1 10°.
The box containing the transducers is fastened on to the
headline and connected to a five-core electric cable made
in lengths of 800 m, which extends along the wings of
the net to a suitable reel at the stern of the vessel.
(The length of the cable veered out is about 5 per cent
longer than the warp length normally used.)
The terminal of the electric cable is electrically con-
nected to a slip ring mounted on one side of the reel;
a water-tight brush connector runs over the slip ring and
389
connects the transducers to the transmitter receiver and
indicator of the echo sounder* With the circuit closed
the cable resistance is constant and the cable can be
freely paid out or hauled in during the operation.
The echo sounder on board has two frequencies, SO kc
and 200 kc and is capable of sounding up to 500 m with
a maximum electric power for the transducer at a level
of about 3 kW. The indicator of the echo sounder
records two series of information at the same time;
i.e. the echoes received direct from the bottom giving the
water depth under the boat, the other giving the position
of the net in relation to the bottom and the surface;
these two indications can be received either independently
of each other or simultaneously. The instrument
furthermore provides for obtaining echoes from the
net transducers on the screen of the cathode ray oscillo-
graph.
Laboratory tests on the pressure range which the
net transducers can sustain have proved that they can
endure a pressure up to SO atmosphere corresponding to
an underwater pressure of 500 m depth. Detailed
specifications of the apparatus are given in Table I.
Table X Major •pacification* of th« Xet-Vieion, Model NV»T500,
OTB-500 (On boat •Ad») KVfr-500 (On net aide)
1X5
Soundinff rang* of
paper recorder (•)
•hallow 50
Middle 75
Deep iOO
Maximum depth teeted 250m
Cable length tested 1200*
*a»«* of cathode Shallow 1?
ray oiellloffraph (m) Middle *5
(tube diamttor 1*7»») »««P 33
dan he e witched to and
from boat trmneducer
trenueney of
mtraeou*d (He)
90 or 200 at ohoioe
28 for upward trenednoer
75 for downward transducer
13°(
«6°(
°(*«ok and forth}
°l.ft and ri«ht)
110°for S8ke, flat projection
47 for 75kc,eeniaal projeo-
rnlae tranemieoioM, (per mjmtte)| Shallow range 900, Middle rang* 600, deep
Keeordin/eyetemi Straight lino indication.
Beeordln* paper i Vet type, length 15 m, width 150 mm.
Vapor epeed (mm/win) i Shallow ranga *2, middle ren«e 28, deep
Time notch interval* i 1 minute.
Power eupply (V), D.C. 3k t A.C. 100 or 200.
Performance
Three transmissions of ultrasound follow each other
closely from the headline and ship's transducers. The
moment the ship's transducer has transmitted towards
the bottom, the upward transducer on the headline
transmits towards the surface. When the echo of the
upward headline transducer returns, the downward
transducer on the headline transmits towards the bottom.
With the vessel operating in 75 m depth and the head-
line situated at 25 m above the bottom the sonic func-
tions will be as follows: as the sound velocity in sea
water is about 1,500 m/sec the first pulse of ultrasound
emitted from the ship's transducer to the bottom returns
in one-tenth of a second covering the total distance of
ISO m for both directions. The first emission from the
net transducer to the surface returns in one-fifteenth of a
second having covered a distance of 100 m. The second
wave from the net transducer towards the bottom
returns in one-thirtieth of a second covering a total
distance of 50 m, so that the total time required for the
net transducer's emission and registration of echoes is
one-fifteenth plus one-thirtieth is one-tenth of a second,
390
which is the same as that needed for the emission and
return of the ship's wave.
The same principle of synchronisation in performance
is applicable to any depth of water within the range of the
echo sounder. The echo sounder therefore constantly
indicates a true position of the movements of the net in
relation to the bottom. The two patterns made by the
bottom echoes, the one received from the ship's trans-
ducer and the other from the downward directed headline
transducer can be recorded on paper of the same pro-
portional scale, showing corresponding positions to
each other. In the East China Sea operating with 800 m
of warp out, it was found that the patterns of the bottom
recording obtained by the ship's transducer and the
downward directed headline transducer agreed to within
one per cent. Where the bottom is rough the two indica-
tions of the bottom can be obtained separately by switch-
ing over from one recording to the other.
Observations of gear behaviour
A number of experiments using the Net- Vision, model
NV/4-500, were conducted in 1960 and 1961 in co-
operation with several leading fisheries firms in Japan.
For studying the behaviour of a trawl in operation the
essential factors include the size of the net, the warp
length and the speed of towing or propeller rpm in
addition to many other pertinent conditions. Where it
concerns the behaviour of a pair-trawl, the distance
between the two vessels is one of the main factors.
Much, however, can be observed in regard to the
behaviour of trawls at various depths by the study of echo-
sounder records, and the following discusses the records
obtained during one of the above mentioned trial
experiments carried out in co-operation with the Yama-
daya Fishing Company, with Yamada Maru No 12 in the
East China Sea during August 1962.
The net used had extra large bouyancy and was
operated with only about half the warp length used for
the traditional trawls.
Towing dose to the bottom
Fig. 3 shows an example of the echo record obtained when
towing the trawl close to the bottom in 61 m depth of
water. On the left of the echogram the opening height
of the net is 8-5 m (from footrope to headline), while the
distance between the footrope and the bottom is 9 m.
This distance kept constant as long as the engine revolu-
tions were kept at 300 rpm. During the experiment
the engines were stopped for about two minutes and this
can clearly be seen towards the middle of the echogram
where the headline rises about 3 m while the footrope
gradually sinks to the bottom.
With the vessel stopped the vertical opening of the net
reached 20 m with the footrope on the bottom. As the
vessel picked up speed again the headline was suddenly
pulled downwards while the footrope was lifted and was
then towed at about SO cm above the bottom.
Fig 3. Traces of net towed a little above the bottom with the propeller
rpm at 200 to 210 rpm. Depth of waters: 61 m.
Midwater trawling
Fig. 4 shows the vertical movement of the gear as re-
corded on the echogram when the rpm of the engine
were slowly changed. Looking at the traces of the
net from left to right on the echogram, it can be noticed
that the vertical net opening was 6*5 m with the engine
working at 280 rpm. At 300 rpm the vertical opening
becomes 5*5 m while the headline rises to 37 m above the
bottom. The vessel was then stopped and as can be
seen towards the right of the echogram, there is a sudden
rise of the headline followed closely by a gradual lowering
as the net sinks to the bottom. It can also be observed
that the footrope sinks much faster than the headline.
This echogram shows how clearly Net-Vision indicates
the various movements of the gear in operation and
points to its many advantages for fishing in midwater.
Fig 4. Traces of the net towed at 280 to 300 rpm. Panel A shows
reflections received only from the net transducer. Panel B shows
reflections received from both the boat and the net transducers.
The headline when rising to its highest is 37 m above the bottom.
Bottom trawling
Experiments were undertaken with the same gear used
during the above experiments, to ascertain whether such
gear, using a much shorter warp than the traditional
trawl, could also be used for bottom trawling. In Fig. 5
the echogram shows that when the trawl operated with a
footrope 4 m above the bottom the vertical opening
was 9 m, with the engine running at 220 rpm. As the
rpm of the engine is decreased to 180 rpm it can be
seen that the footrope gradually approaches the bottom
while the vertical opening increased to 11 m.
Fig 5. Traces of the net towed along the bottom at 220 to 180 rpm.
Observations of gear and fish
The name Net-Vision was given to the apparatus because
the entrance of fish to the net can clearly be detected on
the screen of the cathode-ray tube, as well as the changes
in the vertical opening of the net.
The height of the net mouth can also be seen precisely
and continuously with reference to the scale printed on
the screen itself. Fig. 6 gives an example of the cathode
ray flashes as seen on the screen and the wide flashes in
the upper (A) and lower (C) parts of the screen indicate
the position of the head and foot ropes respectively.
Fig 6. Tracts of a school of the prawn entering the net as seen an
the screen of the cathode-ray tube. Height of the net mouth: 7-5 m.
A. Headline; B. Prawn entering the net; C Footrope.
391
The opening of the net mouth in the illustration is 7*5 m
aad the broadening flashes at the centre represent a
school of fish passing into the net mouth; the intensity
and amplitude of the flash gives an indication of the
density of the school.
Hie sensitivity of the cathode ray oscillograph is
higher than that of the paper recording system and the
ultrasonic reflections on the screen are more reliable for
indicating fish schools than the traces on the recording
paper. With more dense schools of fish, for example, the
flashes may fill the full width of the screen which mea-
sures 127 mm from side to side. With very dispersed
schools on the other hand, the echoes may narrow down
to only 1 mm across the centre line of the screen, so
that between these two extremities there is a range of
1 to 64 for estimating the density of the school. The
flashes on the screen have a rate of 40 decibels between
their maximum and minimum amplitude.
The cathode ray oscillograph, furthermore, gives more
detailed and accurate information concerning the move-
ment of the footrope in respect of the bottom contour.
The Net-Vision can be of much assistance to the
fishermen in ascertaining the abnormal conditions in the
operation of the gear such as entanglements (Fig. 7).
By installing transducers to transmit ultrasound hori-
zontally at appropriate points of the net mouth, the
opening width of the gear can be measured in the same
way as was described for the height.
With special reference to the use of the instrument in
surveys, the senior author could observe that in the East
China Sea the diurnal migration of the dsl is closely
related to the changes in luminosity of the deeper water
layers as they occur at various times of the day. By
holding a photometer northward, he found that during
the daytime the luminosity was 7,000 Lux. At sunset
the luminosity suddenly decreased from 1,600 to 250 Lux
at which time the echo sounder indicated the dsl
emerging fast to the deeper water layers and dispersing.
This phenomena was often followed by indications of
big quantities of fish.
In the last few years, the application of a dual fre-
quency echo sounder with 200 kc ultrasound (wave length
7*5 mm) has corrected the long-held but misguided view
of the Japanese fishermen that fish schools scatter at
night.
The above has led to "aimed" fishing, by which is
meant that the gear is manoeuvred into the layer of
water containing the fish which have been detected by the
traces on the echogram from the ship's transducer.
This can be done either by veering or hauling the warps
or by changing the engine speed, or by a combination of
these methods. As alterations of the warps normally
take longer to raise or lower the net, preference is now
generally given to changing the speed of the engine
(Fig. 8).
Midwater trawling for shrimp
A number of years ago the catches diminished of the
two-boat trawlers operating on the bottom in the
East China Sea. The vessels then started fishing off
the bottom and it was soon found that large and compact
schools of shrimp were to be found in midwater. Since
then a successful fishery with some 200 trawlers, based
on these high-swimming schools, has developed, which
uses specially adapted trawls and techniques.
This method requires not only clear and accurate
detection of the shrimp schools under the vessel, but also
constant and accurate information as to the position of
Fig 7. Example of a net entangled and subsequently adjusted while towing. Depth of water; 55 m.
392
Fig 8. Example of aimed fishing Jjr the prawn in midwater. A.
School of the prawn before entering the net; B. School of the prawn
after entering the net. Depth of water: 59 m.
Fig 9. Shrimp midwatcr trawl.
the net so that it can be raised or lowered to intercept
the schools recorded on the vessel's echo sounder, The
above described Net-Vision technique, using 200 kc/s as
"1*1*1-
__! ' < L .«*
Detail - Scalt 1-6
Perspective
A- View
B-B Section
C-C Section
D-D Section
Fig. JOb. Perspective view and sections shown in lOa.
393
well as 28 or 50 kc/s is now in general use in this fishery.
Hie one-teat trawlers are commonly of about 44 m
length with a displacement of 370 GT, powered by
700-800 hp engines. They normally carry a crew of 24
and are fined with electric winches as well as all modern
electronic appliances.
B
Fig Ida. High concave oner board for tMmp mldwaier trawl. Site 1,400 x 2,300 mm. Total weight 7,040 kg.
394
Two-boat trawlers are also used in this fishery; the
vessels are smaller with engines of 350 hp each.
The gear
The trawlnets are of the 4-seam type and a common size
is 44 m headline and a 55 m footrope. They are made
entirely of black polyethylene knotless netting and rigged
in such a way that in operation the main strain on the
body is taken by the top sidelines which are appreciably
shorter than the lower ones. The nets are made very
long to improve water filtering to avoid the formation of
adverse water pressure at the net mouth. Full details
of construction are given in Fig. 9.
The nets are used with curved otter boards generally
3,200 mm high and 1,600 mm broad; they can be of
steel-wood construction as shown in Fig. lOa and lOb as
well as of all-steel construction.
The net is attached to the boards by three legs, of which
the upper and lower legs are 60 m, while the middle leg is
only 10 m long and attached to the upper leg.
The whole gear is rigged so that it can be operated on
the bottom, just off the bottom, or in midwater, by
altering the length of the upper and lower strops of the
otter boards, and the length of the warps veered out.
Fig. 1 1 shows the rig of the trawls for a one-boat
trawler. A similar net is used by two bull-trawlers of
about 350 hp although of a somewhat bigger size with a
headline of up to 80 m.
Conclusions
At the time of the first World Fishing Gsar Congress in
1957, the studies and experiments on midwater trawling
had advanced to where they showed high promise.
All endeavours have been directed towards pelagic fish,
and although many more experiments were carried out
and more headway has been made since then, one-boat
midwater trawling has, to the best of our knowledge,
nowhere reached the commercial fishing stage on the
same scale as in the fishery described above.
It may be that the particular fishery conditions in
general and the reactions and compact school formation
of the shrimp in the East China Sea are particularly
suitable for this method of operation. On the other hand,
these schools were only operated on with continued
success after a proper type of net had been developed,
giving a low resistance to the waterflow for very large
opening dimensions, and a netzsonde system had been
developed which allowed detection of the schools and at
the same time accurate positioning of the net.
The next step is to adjust this gear to the catch of
pelagic fish, and there seems to be no reason why it
should not be successful there also.
The possibilities of application of the Net- Vision are
not confined to commercial trawling alone and it is
expected that this new device will also be useful for
observing and measuring various phenomena, perti-
nent to fishing, in the course of fisheries and oceano-
graphic investigations.
References
Hamuro, C, and K. Ishii. The measurement of shapes of one-
boat trawl nets operated in midwater layer and the results with the
aid of the depth telemeter trially manufactured. Tech. Rep. Fisk.
Boat., 14:57-206, 1960.
Nishimura, M. Study on fish-finder for ground fish on the East
China Sea. Tech. Rep. Fish. Boat., 12:115-183, 1958.
Rothjen, F. W., and L. A, Fahlen. Progress report on midwater
trawling studies carries out off the New England coast in 1961 by
M.V. Delaware, Com. Fish. Review, 24 (11):1-11, 1962.
Sch&rfe, J. The use of echo sounding as a means of observing
the performance of trawling gear, Modern Fishing Gear of the
World, Fishing News (Books) Ltd., London, 241-244, 1959.
Takayama, S., and T. Koyama. Studies on midwater trawling-H
Field experiments of a net sonde. Bull. Tokal Reg. Fish. Res. Lab~
29:47-54, 1961.
Worp ond bridle orrongemant of 650 hp
midwater trawl for shrimp
Q *«'*
0 Cciwov. OHtr bear* - fttibtrkrilb
(I) UM«r brldl« •Om , I4*im wira
0 Lovtr brUI* «0m , 15 mm wlr«
0 l«0s lOm , llmm wire
0 MtMMt c«bl«
Fig. 11 Warp and bridle arrangement of shrimp mldwoter trawl.
395
Detecteur de Poisson "Explorator'
Dcpuis line quinzaine d'annees Jes navires de pdche modernes ont
un 6quipement 61ectronique que pcuvcnt leur envier de nombreux
navires de commerce. L'"Explorator" a M congu pour assurer aux
pfccheurs tea moyens de d&ecter le gtsement et la profondeur des
banes de poissons. Un tel instrument devait repondre aux exigences
suivantes:
(a) Frequence assez dlevee pour obtenir un bon pouvoir de
definition sans cependant dormer lieu & une trop grande
absorption qui nuirait & la portee.
(b) Faisoeau tres directif, done angle au sommet trts faible
dans le plan vertical et une vitesse de balayage dvitant des
"trous" sur Tindicatcur.
(c) Possibility de varier la frequence des impulsions selon
la distance du bane de poissons.
Entre ces exigences plus ou moins contradictoires, il a fallu
choisir des solutions de compromis qui ont amcne a ('adoption des
caracteristiques suivantes:
Frequence, 61 KHz.
Ouverture de faisceau dans le plan vertical, 2°
dans le plan horizontal, 10°
Vitesse de balayage, 45° en 14 secondcs.
Duree des impulsions: variable entre 1 et 30 millisecondes.
Le projecteur de l'"Explorator" montt dans un double fond
est prot6g<6 par un carenage.
Les premiers appareils de serie sont sortis a la fin du premier
semestre 1962 et & ce jour, 10 "Explorators" fonctionnent, la
plupart sur des chalutiers hauturiers. Les rcsultats obtenus ont
6te excellents et les patrons peuvent d&ecter le poisson pres du
fond a des profondeurs de 60 a 100 brasses ayec un site de 20°.
L'6tude souligne 1'importance de I'expdrience du patron dans
1'emploi de ce type d'instrument. Comme exemple est cit6 le cas
ou un patron de pfcche, pare une combinaison judicieuse de reglage,
a obtenu des r£sultats excellents a une distance de 1,200 m.
Echo ranger for fish schools
Extract
For several years, commercial fishing vessels have been equipped
with electronic gear at least as elaborate as that carried by ocean-
going cargo vessels. The echo ranger "Explorator" was developed
to provide. fishermen with a means of ascertaining the depth of
fish schools as well as the horizontal distance and their direction
from the vessel. The depth of schools outside the ship's course
can be read off directly on a special indicator.
The special requirements of such an instrument are:
(a) A high enough frequency to obtain clear indication with a
relatively low absorption loss.
(b) The beam must have a very small vertical angle and a sweep
of such a speed that the vessel's speed does not create voids
on the indicator.
(c) The possibility of changing the impulse frequency according
to the school's distance.
These requirements are contradictory and for the "Explorator"
a final choice was made on the following specifications: frequency,
61 KHz, vertical beam angle 2°; horizontal angle 10°; sweep speed
45° in 14 seconds; impulse from 1-30 ms. The instrument incor-
porates a retractable transducer mounted in the ship's hull.
The first instruments were installed in the fall of 1962 and at the
time of writing, 10 "Explorators" are in operation, mainly on
ocean-going trawlers. The results obtained have been excellent
m that the skippers currently obtain clear detection of fish near
the bottom in depths of 60-100 fm at an angle of 20°. The paper
further points out the importance of the skipper's experience in
manipulating the instrument to obtain proper results from it.
As an example, the case is cited where a skipper obtained excellent
results at more than 1,200 m distance.
i la tocmUzadon de cardumenes
Extracto
Dcsde hace varies afios los barcos de pesca estan dotados de mate-
rial electr6nico tan complete como el que llevan los mcrcantes
de gran altura. Se construy6 el ecotel&netro "Explorator9* para
facilitar a los Pescadores un medio de determinar la profundidad
y distancia horizontal a que se encuentran los cardumenes, asi
como la directidn en que se mueven con respecto al barco. La
by
J. Fontaine
Compagnie G6n6rale de
graphic Sans Fils (CSF)
profundidad a que se hallan los cardumenes que no estan en linea
con el rumbo del barco la da un indicador especial.
El aparato tiene que reunir las caracteristicas especiales siguientes :
(a) Una frecuencia lo bastante alta para dar indicaciones claras
con pfrdidas por absorci6n relativamente bajas.
(b) El haz debera tener un £ngulo vertical muy pequefto y tal
velocidad de desplazamiento horizontal que la propia del
barco no cree vacios en el indicador.
(c) Debera ser posible cambiar la frecuencia de los impulses
segun la distancia a que se encuentre el cardumen.
Estas necesidades son antagbnicas y para el "Explorator" se
hizo una selecci6n final de las caracteristicas siguientes: frecuencia,
61 KHz, dngulo vertical del haz, 2°; angulo horizontal del haz,
10°; velocidad de desplazamiento horizontal, 45° en 14 segundos;
impulse, de 1 a 30 ms. El aparato cuenta con un transductor
rebatible montado en el cascp del barco.
Los primeros aparatos se instalaron en el ptofto de 1962 y en el
momento de preparar esta comunicaci6n, existen 10 "Explorator"
instalados casi todos en arrastreros de altura. Los resultados
cpnseguidos han sido excelentes y los patronos obtienen localiza-
ciones claras de peces cerca del donfo en profundidades de 60 a
100 brazas y con un Angulo de 20°. El autor menciona que el patr6n
tiene que adquirir experiencia en el manejo del aparato para alcan-
zar buenos resultados y cita el case de uno que los Iogr6 excelentes
en la Iocalizaci6n de un cardumen a mas de 1,200 metres de
distancia.
T 'EXPLORATOR est le fruit d'une etroite collabora-
l^j tion entre des patrons de peche fran^ais eprouv^s et
les ing£nieurs de la C.S.F. (Compagnie G6n£rale de
T616graphie Sans Fil). Toujours soucieux d'augmenter
la rentabilitd de leurs navires, les pecheurs ont demand^
un appareil de detection des banes de poissons qui, ^
Tencontre des asdics de peche d£riv£s des asdics militaires,
permette de lire imm^diatement sur la bande enregis-
treuse:
la profondeur du bane de poissons ou sa hauteur sur
le fond,
la direction ou gisement du bane.
La distance qui n'intervient que si le capitaine decide
de capturer le poisson d£tect£, peut etre lue sur un indi-
cateur special.
L'Explorator a 6t£ con^u pour satisfairc & ces exigences.
Carmcteristiques techniques
L'Explorator est une application particuliere de la
technique bien connue des ultra-sons. Ses caractiris-
tiques originales resident dans la maniere dont les risul-
tats sont pr6sent6s pour satisfaire aux impdratifs ci-
dessus.
La representation giographique "en coupe" de la
zone explor^e, analogue & celle des sondeurs enregistrcurs
verticaux classiques, exige en ce qui concerne la faisceau
ultrasonore:
(a) Une frequence assez devfe pour obtenir un bon
pouvoir de definition sans cependant donner lieu
& une trop grande absorption qui nuirait & la
portte.
(b) Un faisceau trfes directif, done angle au sommet
tr&s faible dans le plan vertical.
(c) Une vitesse de balayage telle que Tavance du navire
entre deux passages du faisceau ne cr£e pas de
"trous" dans la zone explore.
(d) Une dur£e et une frequence des impulsions vari-
ables selon la distance d 'exploration choisie.
Entre ces exigences plus ou moins contradictoires, il
a fallu choisir des solutions de compromis qui ont
amend & 1'adoption des chiffres suivants:
Frequence: 61 KHz.
Ouverture du faisceau :
dans le plan vertical ±2°.
dans le plan horizontal ±10°.
Vitesse de balayage: ±45° en 14 secondes.
Dur6e des impulsions: variable & volonte entre 1 et
30 millisecondes.
Frequence des impulsions: Ii6e au site et comprise
entre 0-5 et 16-6 Hz.
La reflexion des ultra-sons sur le fond et les banes de
poissons permet de mesurer la distance de ces obstacles
au navire par mesure du temps slparant remission de
Timpulsion de la reception de son 6cho. Pour obtenir
la representation "en coupe", il faut convertir cette
donnie en profondeur en faisant intervenir les lignes
trigonomftriques du site et du gisement. II faut, en
outre, inclure dans 1'appareil des dispositifs d'asservisse-
ment et de teiiaffichage. Enfin, les variations du regime
des cadences et des longueurs d'impulsions compliquent
singuliirement la realisation des circuits de ddclenche-
ment et de marquage; ici une solution ingdnieuse
(couverte par des brevets), baste sur les phcnomenes
d'induction, a £t£ adoptee.
Tout ceci montre que, si le principe de 1'Explorator
reste simple, la realisation mdcanique et eiectronique
se rfvile d'autant plus complexe que Ton a voulu faire
un appareil robuste et ais&nent exploitable.
Description sommaire
L'Explorator se compose essentiellement de trois unites:
(a) Le meuble passerelle (Fig. 1) comprenant les
chassis.
— commande des buties de fin de course du
projecteur.
— g6n6rateur de tops.
— asservissements.
— riception.
—premier etage de remctteur.
Fig. ]. Ensemble enregistreur-receptew sur la passerelle <Tun navire.
A sa partie superieure, on trouve (Fig. 2) la table
d'enregistrement sur laquelle se d^roule le papier,
Tindicateur site et Tindicateur gisement. Toutes
les commandes sont rassemblees sur ce meuble.
(b) Le meuble emission-alimentation qui, ne com-
Fig. 3. Le projecteur en course d' Installation sous la coque du navire.
397
Fig. 2. Table d'enregistrement.
portant aucune commande, peut etre placi n'importe
ou & bord et comprend:
— une alimentation rfgulie— 150 V, +250 V.
— une alimentation r£gul£e + 450 V.
— remission.
— les relais de commutation des moteurs du
projccteur.
— les fusibles et relais de s£curit£.
— les barrettes (^interconnexion des meubles.
(c) Le projecteur et son bloc mfcanisme compose
du "m£canisme de mouvement du projecteur"
en site gisement. L'ensemble est mont6 dans un
doublefond, le projecteur en saillie sous la coque
et protigi par un carfnage (Fig. 3 et 4).
Resuttate Sexploitation
La maquette de 1'Explorator mise en experimentation
& bord du chalutier Saint-Louis, a donnl presqu'aussitdt
des rtsultats assez remarquables pour que le patron,
M. Delamer, et son armateur, M. Huret, demandent
son maintien & bord jusqu'i la sortie des appareils de
sine.
Un deuxiime prototype, mis en service & bord du navire
de recherchcs fan^ais Thalassa en 1961, est consid6r6 par
le Commandant comme un des instruments de base
pour les recherches de fonds de p&he et l'oc£anographie,
en particulier pour la navigation sur des lignes isobathes.
Les premiers appareils de sine ont comment &
sortir d'usine i la fim du premier semestre 1962. Au ler
mai 1963, les navires suivants itaient 6quip6s:
France:
Thalassa— mvirt de recherches— Institut Scientifiquc
des Pfcfaes.
Saint-Louis )
Saint- Jean [chalutiers— Pecheries de la Morinie,
Saint-Luc ) Boulogne.
ZMcmdc— chalutier de Grande Pfche— Pteheries de
Bordeaux-Bassens.
Fig. 4. Le projecteur install*, avec son coinage de protection.
Cabellou — thonnier coque bois de 120 tonneaux —
M. Dhellemmes, Concarneau.
Royamne-Uoi:
D. B. Finn— chalutier— St. Andrew's Steam Fishing
Co. Ltd.
Norvcge:
Longva— chalutier-usine— Longvadal, Aalesund.
Plusieurs autres navires fran^ais et Strangers doivent
6tre prochainement 6quip6s.
II est difficile, pour plusieurs raisons, de donner d&s
398
Figs. 5 and 6. Photos de bandes cnregistrfes d bord du chalutier
Saint-Louis.
maintenant des rfsultats statistiques d'exploitation. 11
conviendrait d'abord d'avoir une experience de plusieurs
ann6es sur de nombreux navires. Ensuite, selon les
ports, meme & Tintirieur d'un pays donnS, les esptees
recherchees, done les fonds fr6quent6s et les m&hodes
de pechc varient considirablement, ce qui compliquerait
Fig. 9. Photo de hmdt enrigislrie d borddu chalutier Saint-Louis.
le dipouillemcnt des r£sultats* Enfin, s'il est reiativement
aisi de rclever les quantit£s et prix des poissons dibarqute,
il est beaucoup plus dilicat d'obtenir des patrons de
pechc des informations sur leurs mdthodes de ptehe,
leurs engins, les fonds qu'ils frtquentent, etc. A cet
£gard la ptehe est un art; chaque artiste a ses secrets
et une personnaliti qu'il doit d£fendrt, ce que Ton
comprend ais£ment.
Monsieur Maurice Delamer, qui commande le chalu-
tier Saint-Louis, a bien voulu nous faire un commentaire
gtairal bas£ sur deux annfes d'exp&riences et que nous
r£sumons ci-dessous.
La mise en oeuvre de 1'Explorator, pour aisie qu'elle
soit, n'en demande pas moins un certain apprentissagc
pour acquirir le doigtd ntoessaire au rendement optimal
de 1'appareil. Cet apprentissage ne peut fitre le fait que
du patron de peche lui-meme qui, lorsqu'il aura acquis
1' experience suffisante, sera pcut-6tre ameni & modifier
ses m6thodes en fonction des informations dispenses par
TExplorator.
II est certain que le mode opiratoire est notablement
different selon qu'il s'agit de p£che p^lagique ou de
peche sur le fond.
Par cxemple, en essayant un chalut p^lagique, Mon-
sieur Delamer a obtenu avec un site de 7% des port6es
horizontals supirieures & 1,200 mitres donnant &
I'dcoute des 6chos caract6ristiques.
Jusqu^ present, 1'Explorator a 6t6 surtout utilisi en
chalutage sur le fond. On lui demande alors de d&eler
des concentrations de poissons (meme assez faibles ou
diffuses), se trouvant & une hauteur au-dessus du fond
inferieure & celle de la corde de dos du chalut, c'est-&-dire
trois ou quatre brasses, dans les limites de portde de
1'appareil. Si la trace reconnue parait intlressante,
il faudra la suivre et manoeuvrer pour amener le chalut
sur le poisson. Ceci n&essite de la part du patron une
bonne connaissance des boutons de contrdle, de Pincidence
des rdglages sur la detection et de Tinterpr6tation des
enregistrements. On ne peut & priori, ddfinir des valeurs
moyennes des rdglages convenant & tel poisson par tel
fond. C'est une affaire d'£xp£rience et de doigt£ du
patron qui doit tenir compte des conditions atmosphl-
riques, de la nature et de la profondeur du fond, des
mouvements de plateforme du navire. De m£me, il
faut savoir tenir compte des lobes secondaires inevitables
& tout projecteur d'ultrasons.
Par exemple, le skipper B. Warham du D. B. Finn,
en pfiche sur les cdtes norvdgiennes en ftvrier 1963, a
pu d&ouvrir un large bane de poissons en se guidant
sur de faibles detections et ramener plusieurs fois des
prises de 150 paniers. Suivant les mouvements du bane,
il a ramen6 au port, en 19 jours de voyage, 2,400 kits
vendus plus de 12,000 livres sterling, soit une prise sup6-
rieure de 40 pour cent & celles des autres navires pechant
dans les mfimes eaux. En appliquant la m£me m6thode
(nouvcllc pour lui), il a ramend & un rtcent voyage 2,300
kits et, le 8 mai, il a d£charg6 23,930 stones pour £13,070
(Yorkshire Post du 9 mai).
Le skipper B. Wharam met au crfdit de 1'Explorator
399
Bio-Acoustical Detection of Fish Possibilities and Future Aspects
Abstract
The possibility of using acoustical indications for locating fish
has only been investigated during recent years. The conductivity
of sound in water is about 3,200 times higher than in air and sound
waves propagate 4.5 times faster. Modern hydrophones connected
to recorders are now used to study fish sounds. Listening-in is
done from boats or from permanent stations ashore connected to
outlying devices such as the Japanese "Sonobuoy". The paper
then discusses the sounds produced by sea animals and their
sound mechanisms, such as those made by the air bladder, stridu-
latory and swimming movements. The intensity of fish sounds
can be correlated to the internal motivation state of the animals;
in pro-spawning and spawning periods the animals produce much
stronger sounds than at other periods. Experiments to date have
obtained some results in attracting and repelling fish and investiga-
tions in this direction are continuing, although little is known as
to the effective range of fish sound patterns. The main present
objective of sea acoustical studies is to establish standard sea
animal sounds, leading to full recognition of the species producing
such sounds.
Detection de pofeson par des methodes bio-acoustiques possibility
et aspects future*
Resume
Ce n'est qu'au cours de ces dcrnifcres anntes que Ton a comment
by
G. Freytag
Institut fur Netz- und Material-
forschung, Hamburg
& rechercher la possibility d'utiliser des indications acoustiques
pour la lacalisation de poissons. Les sons sont conduits dans
Feau & une vitesses 3.200 fois plus grande que dans 1'air et les ondes
soniqucs se propagent 4,5 fois plus vite. DCS hydrophones
modemes reltts a des indicateurs sont maintenant utihsfe pour
1'etude des sons de poissons. L'6coute a lieu sur des bateaux ou
des stations a terre permanentes, reltes a des instruments ancrfs
Continued from page 399
Figs, 7. and 8. Photos de bandes enrigistre*es a bord du chalutier Saint-Louis.
9 pour cent du total de ses peches.
Signalons enfin les exccllcnts r6sultats obtenus par le
chalutier Saint-Jean sur des banes de colins noirs.
En ce qui conccrne plus sp&ialement la pfche au
then, des experiences vont 6trc faites sur le Cabcllou, et
pcut-fitre d'autres navires, qui seront suivies avec int^ret
400
par les milieux scientifiques et les armateurs.
Enfin, le Commandant Brenot, du navire de recherche
Thalassa nous a signald 1'aide efficace que lui apporte
PExplorator en oc&nographie, en particulier dans la
navigation sur une ligne isobathc, en rinformant &
1'avance des variations du fond.
en mer tels que le Sonobuoy japanai. La communication traite
des sons produits par les animaux dc mer et de teurs m&anismes
sonorcs tels que les yessies natatoires, ler mouvements de stridula-
tion et de nage. L'intensit£ des sons de poissons peut avoir un
rapport avec des causes internes: par exemple pendant la pcrfbde
precidant le frai et pendant le frai, les animaux produisent des
sons beaucoup plus forts qu'a d'autres periodes. Jusqu'a present
les experiences ont obtenu un certain itsultat concernant Fattraction
et la repulsion des poissons et les observations dans cc sens con-
tinuent bien que tres peut soient connues sur la portee effective
des sons caracteristiques de poissons. A present, 1'objectif principal
des etudes acoustiques en mer est d'etablir des sons standardises
des animaux, diriges sur la reconnaisance des especes produisant
ces sons.
Localization bioacustica de los pecesposibilidades y aspector futures
Extractp
La posibilidad de emplear indicaciones acusticas para localizar
peces s61o se investiga desde hace pocos aftos. La conductibilidad
del sonido en el agua es unas 3,200 veces mayor que en el aire y
las ondas sonoras se propagan 4,5 veces mas rapidamente. Se
emplean actualmente hidr6fonos modernos conectados a regis-
tradores para estudiar los sonidos que emiten los peces. Se
cscuchan desde embaraciones o desde estaciones fijas en tierra
en comunicaci6n con dispositivos distantes como la "sonobuoy"
japonesa. £1 autor comenta los mecanismos que tienen los peces
para emitir conidos, como los de la vejiga natatoria, chirridos y
movimientos natatorios. La intensidad de los sonidos puede
relacionarse con el estado interno que los motivan ; antes y durante
el desove los peces producen sonidos mucho mas fuertes que en
otros mementos. Experimented realizados hasta la fecha ban
logrado ciertos resultados en la atracci6n y repulsibn de los
peces y continuan las investigaciones en esta direcci6n, aunque
todavia se sabe muy poco del alcance efectivo de las modalidadcs
de conidos de los peces. La finalidad principal de los actuales
estudios acusticos marines consiste en establecer patrones de
conidos emitidos por animales marines que culminen en el
reconocinvento inequivoco de las especies que los producen.
TO localize fish or shrimp, etc., by listening to the
sounds they make is an idea that has proved of
interest to fisheries in recent years but the practical
application of biological sound to fishing was started
only recently.
There are two main methods for detecting under-
water objects, echo-detection and acoustic-detection.
Acoustic detection is a passive method. Through
watertight microphones (so-called hydrophones),under-
water sound can be received and recorded. Different
types of sound can be distinguished as:
(a) Ambient sea-noises: — caused by waves, current,
and objects moving on the ground.
(b) Technical noises: — noises of ships, noise of the
hearing device itself.
(c) Biological noise: — caused by marine animals,
i.e. mammals, fish, shrimps, etc.
The noise-sources mentioned under (a) and (b) often
limit reception of animal sound. Quantitative measure-
ments of the level of the ambient sea-noise have shown
that in every octave-range a sound pressure of 1 dyn/cm2
is common and seldom exceeded. Biological noise level,
however, under special circumstances, can reach 200-
300 dyn/cm*.
Sound signals spread in water about 4 to 5 times
faster than in air. Much more important is the fact that
the sound conductivity in water is about 3,200 times
higher than in air.
Marine organisms have no genuine organs for sound
production such as are found in terrestrial species.
This is surprising in view of the reduced visual range
under water and the poor optical properties of most
marine animals.
During the last war, underwater microphones were
developed which were specifically adapted to the re-
ception of the pressure of sound-waves under water. The
low voltage produced by the hydrophones is amplified
by a preamplifier, and then recorded by a tape recorder.
Preamplifier and tape recorder are self-contained so
that they can be used anywhere.
Recording of underwater noise
Recent research shows that the continental shelfs are
much more noisy than the open sea and so are better
suited for investigations.
Reception of animal underwater sound is limited by
the level of interfering noise. To avoid such interference
the author has found an outboard-motor equipped
rubber raft excellent for his purpose. The additional use
of a transportable echo sounder made it possible to
find fish concentrations and to lower the hydrophone to
the proper level. The main factors which may still
interfere with such investigations are sea and weather
conditions and the sound level of concentrations of
fishing vessels which are often found near the fish schools
to be studied.
Listening from boats or vessels is naturally limited in
time. Therefore permanent stations are preferably
established ashore. They are connected by cables or
wireless communication ("Sonobuoy") with one or
several hydrophones which may be installed in free water
or on the bottom. Their location requires careful
selection.
In some instances, such listening stations have already
found a certain commercial application. For instance,
in the Japanese set net fishery, "Sonobuoys" which have a
wireless transmission range of up to about 10 km, are
placed in the anti-chamber of the trap. Since there is a
close relationship between the intensity of biological noise
and the amount of catch in the trap, it is possible to
estimate the number of impounded fish14.
According to Steinberg et aP*, 25 different categories of
sounds were recorded over several months and it can be
assumed that by permanent listening over long periods
better results can be obtained.
A basic difficulty is that most of the underwater noise
makers are invisible. The possibilities of using under*
water-television or divers are limited. Monitoring fish in
aquaria, therefore, is a useful method for establishing
definite relations between noise and noisemaker, but
noise-making fish have been found to behave in quite
different ways when confined in aquaria for instance:
(a) Completely silent: Opsanus spec., Aplodinotus
(freshwater drummer).
(b) Sounding with less spirit and vehemence: croakers.
(c) Sounding when seeing another individual of the
same species: gurnard (Trigla gurnardus).
(d) Sounding after electrical stimulation: tt\(AnguilIa
vulg.).
401
A1
(t) Sounding when sounds of the same species are
played back into the aquarium: Prionotus, Trigla.
Sound producing marine animals definitely established
are:—
(a) Shells: — Some shells produce noise by clapping
their shell-parts rapidly together or by pushing against
each other in great colonies. The respiratory murmur
of these animals lies in the ultra-sound region.
(b) Crustaceans: — shrimps and larger crustacean
species make noise by moving their antennae, claws,
etc. These sounds resemble the fire of burning twigs.
Noise of this type can be heard very often in all parts
of the oceans and even in small ponds as a steady
background noise which often interferes with under-
water listening.
(c) Squid: — the jet principle of their movements
results in a typical noise. Furthermore these animals
are able to produce an additional high pitched whistling
sound by pressing out the water with violent force when
attacked by predators.
(d) Fish: — approximately 400 fish species were acous-
tically monitored to find out the potential soundmakers.
Fish sounds represent the main part of underwater
animal soniferous activity.
(e) Marine mammals:— whales and dolphins have a
very extensive catalogue of sounds. Until recently,
it was not possible to find out the biological significance
of all these different noises. Some are obviously used
for echo-location in the water.
Sound-mechanisms in fish
Without a single exception, fish have neither lungs nor
an air-reservoir in connection with the respiratory app-
aratus. The sounds created by fish, therefore, cannot be
called 'Voices.*' Fish produce sounds by a variety of
different physical mechanisms, and the following,
sound-producing mechanisms can be distinguished: (a)
Air bladder-mechanisms, (b) Stridulatory-mechanisms,
(c) Swimming sounds: by either single fish or fish
schools.
Air bladder mechanisms— Well developed sound-
mechanisms are found in those species which are able
to set the air content of the air bladder into vibration by
rhythmic contractions of the lateral body muscles or
by the activity of special drum-muscles. In the latter case
the innervation is provided by a ventral rampus of the
II. spine-nerve. These contractions can be performed at
will by the animals. These air Madder mechanisms are
operated by an astonishing variety of constructions.
Sounds so created arc in all cases low pitched grunts
with the greatest pressure values in the region of 50 to
400 cycles per sec (cps).
Stridulatory-mf>chanisnw-Thcsc are anatomically much
simpler than air bladder mechanisms. Analogous to
insects, sound is produced by rubbing certain parts of
the skeleton against each other. Generally these moving
parts are teeth (eating sounds), fin-parts, etc. Sound so
produced is much higher pitched than air bladder
sound and lies generally between 200 and 5,500 cps.
With many particularly noisy fish species, sound-
402
production is more complicated because several mech-
anisms are engaged simultaneously, such as the com-
bination of teeth-grinding with air bladder resonance
effecting an increase of sound level.
Swimming somri— the swimming noise of single fish
normally does not exceed the ubiquitous background
noise of the sea. A slight rushing can be recognized if
the fish moves in an aquarium very close to the hydro-
phone. When it darts or touches the surface of the water
with the tail-fin, a drum-like noise will be caused.
Sounds of this kind of origin are called ''mechanical or
hydrodynamical." Moulton (1960) proposes the ex-
pression "swimming sound*' as more purposeful regard-
less of whether they are created by the movement of
bones, by contractions of the swimming muscles or of
hydrodynamic origin.
Fish schools at rest create no audible sound but if the
school moves or veers a considerable noise results —
the more the deviation or velocity, the greater the noise
level. Swimming sounds of moving fish schools are
not permanent. This demonstrates that neither con-
tractions of muscles nor hydrodynamical effects can be
responsible for these swimming sounds. Specific differ-
ences of noise exist according to the schooling species,
and the size of the school. Swimming sounds of larger
fish are lower pitched than those of smaller fish.
Veering offish schools can create additional acoustical
phenomena. The origin of this sound-type is not quite
clear. Such veering sounds of herring schools resemble
a high whistling sound reaching 6 to 7 kcps. (Murray20,
Freytag10). Harris18 heard a similar noise when schools
of menhaden crossed under vessels. They described that
noise as similar to the high pitched squeaking of field-
mice.
Listening to schools of mackerel and tuna have given
no satisfactory results. Investigations in aquaria by
establishing schools of more than 1,000 individuals of
genus Anchoviella and Atherina also gave no useful
sound production. In all cases only a slight rushing was
to be heard19. It is possible, however, that some of the
sound accompanying swimming activity lies in a higher
frequency range for which normal underwater hearing
devices are not receptive.
Future aspects: Current status of bio-acoustical methods
Through motor driven vessels in European fisheries
knowledge about hydrodynamical and biological noises
in the sea decreased steadily. Much more experience on
this subject can be found with fishermen off the coasts of
Africa, Asia and the Pacific who fish with small, un-
mechanized and therefore "noiseless" boats. They
succeed in locating and estimating the magnitude
of fish schools by eavesdropping at the hull or by using
special hearing instruments like paddles, etc., which are
dipped into the water. The success of these techniques
depends mainly on the personal experience of the listening
fishermen and on the quality of the hearing instruments.
It is not yet possible to predict which bio-acoustical
methods may be used in future in practical fishing, but
suggestions can be made.
Diurnal rhythms of sound production are well known
from mammals, birds and amphibious animals. They
are also known from many marine animals. The sound
activity of certain shrimps, for instance, shows a two-fold
increase at sunset with highest activity just before sunrise
and after darkness falls. Sea Catfish (Galeichthys sp)
and the fish of the genus Opsanus produce their numerous
sounds only at night. The freshwater drum (Aplodinotus)
starts to sound around 10.00 h, reaches a peak in the
afternoon and stops at sunset23.
Only theories apply to the frequency with which fish
can produce sound in active periods. Since fish may use
one and the same sound type under several different
biological conditions (spawning, attack, fright) it can
be concluded that the sound mechanism is not closely
coupled with one definite instinctive behaviour. It is
rather assumed that sound mechanisms in fish are of a
reflex like nature so that a longer lasting functioning
of a soniferous activity can be expected. With gurnard
the author recorded 4,600 drumming sounds within six
hours.
The sound intensity is correlated to the internal
motivation state of the animals. During courtship and
the following spawning period the same sounds are
produced but with more vehemence in comparison with
other seasons. Especially when fish concentrate for
spawning, the single sounds as well as the resulting
chorus reach a considerable amount of sound pressure
by "social infection." The drumming concerts of the
sciaenid fish are a typical example. These temporary
concentrations of fish during spawning provide favour-
able conditions for typical fishing seasons during which
in some cases, passive listening is being used for fish
detection8.
Passive listening
Much more attention should be paid to performances of
the fish brain which are connected with experience and
learning. There are well-known cases of self-training,
e.g. in connection with the breeding territory, or the
reaction of sharks to moving ships, etc. It is possible
that technical noises connected with fishing such as echo
sounding, noise of engines and propellers could be
associated with the following disturbances by the fishing
gear, so that fishes would start to flee from the vessel
long before the fishing gear and its noise becomes
recognizable.
Investigations regarding the noise level of fishing must
try to distinguish between the ship's noise and the noise
created by the gear22. Some results in this field had been
obtained by a working group in Bergen, Norway, in
connection with the development work on one-boat mid-
water trawl12.
The use of bandpass filters is not possible because
the frequency range of these technical ship noises (10 to
5,000 cps) covers the whole range of biological sound.
The sound pressure of a trawler's noise which depends to
some extent on the motor revolution, is in a distance of
200 m, still more than Ip bar.
Attracting and repelling
The counteracting effects of influencing fish by sound
are treated together because it can only rarely be decided
whether an effect is attracting or repelling and which or
both prevails.
Some knowledge is available on influencing fish by
playing back their own species' specific sounds19. The
author's own investigations with gurnards showed that
it is possible to attract the fish to the noise-source
(loudspeaker). But the "interest" decreased and they soon
turned away because of the missing optical stimuli of the
"expected" species partner.
Compared with this possibility of influencing fish with
their own species specific sound, influences based on the
preyfish predator relation seem to be more promising.
Schooling fish are always frightened by the feeding noise
of their predators19, while predatory fish are attracted
by the swimming noise of the schooling preyfish. If
those predator species are subjected to their own feeding
sounds, they show catching reactions and increase their
swimming activity in search of prey.
Little is known to date on the range in which reactions
to acoustical patterns can occur. It is known, however,
that actual biological sounds and artificial sound products
can influence animal behaviour. This is, however,
true only for "noise" and single distinct tones produced
by frequency generators have been found inefficient.
The application of sound attraction or repelling may
be valuable in connection with traps, seines, gillnets,
etc., or in areas where a rough seabed hampers fishing.
Fishing in connection with noise for attracting or repell-
ing is mentioned in literature in several cases6, l6. One
type of German fishery in the Baltic ("Klapperfischerei")
is performed by using striking noises to scare the fishes
into the nets under the ice.
Outlook
Animal underwater sounds have been a special subject of
research for over 20 years. It is well known, that bio-
logical noise can be expected everywhere in all oceans.
The continental shelf zones are much more noisy than
the open oceans. Many countries have started to study
their shelf areas in regard to biological sound production,
with a view to utilization for fishing. The "fishermen
of tomorrow" will have to rely much more on electro-
acoustical devices than on his eyes or ears for measuring
the depth and other distances, estimating the amount
of catch and for transmission of all sorts of values and
observations by underwater sound.
All these techniques require a good knowledge of all
sorts of underwater sound signals and, in view of the
abundance of animal underwater sound production,
also the capability to recognise and distinguish between
the different sounds and the background noise level.
As the sound producers are usually invisible and often
far away from the hydrophone, they have to be identified
also by laboratory tests in aquaria. In the Nanagansett
Marine laboratory, Rhode Island, all bioacoustical data
from field and laboratory tests are collected in a sound
403
Study of Acoustical Characteristics of Fish
Abstract
For designing hydro-acoustical instruments useful for commercial
fisheries, as well as for surveys aimed at assessing fish populations,
more information is required regarding the attenuation of the
strength of signals between emission and reception of the echo.
The Research Institute of Marine Fisheries and Oceanography of
the USSR has conducted a study of the acoustic characteristics of
fish and fish schools, and this paper gives in some detail the method
of approach to the problem, the mathematical treatment and
some of the results obtained. It is stated that while the individual
echo signals of a school offish are strengthened by reflections from
the fish, the total echo strength is nevertheless attenuated by ab-
sorption of the ultrasonic energy by the fish bodies. The formulae
expressing the attenuation of the echo signals are provided. The
reflection capacity differs of course between fishes and depends not
only on the form of the fish body but also on the presence, or
absence, of a swim bladder. The author found that the coefficient
of reflection depends on the frequency of the ultrasonic sound.
The investigations showed that the intensity of the echo signal is
proportional to the pulse length so that echo signals returned by fish
schools were not only increased in amplitude but also prolonged.
This results in records of fish schools being over-estimated in the
vertical direction and may even some time show an extension to
beyond the bottom line. A diagram based on acoustical measure-
ment during the investigations shows that the coefficient of
attenuation of the echo signal increases with an increasing con-
centration of fish. A considerable pan of the energy of a sonic
wave reaching the fish school is scattered so that only a small part
of the energy transmitted returns to the receiver. This loss of
propagation increases with the frequency.
by
E. V. Shishkova
Institute of Marine Fisheries,
Moscow
Etude des caracteristiques acoustiques des poissons
Resume
Pour dessiner des instruments hydro-acoustiques pour la peche
commercial, il est necessaire d'avoir davantage d'informations
concemant Faffaiblissement de la force des signaux entre 1'emission
et la reception des echos. L'lnstitut de recherche des peches
maritimes et de 1'Oceanographie de I'URSS a conduit une &tude
des caracteristiques acoustiques des poissons et des banes de
poissons et la communication donne quelques details de la m£thode
employee, la solution mathgmatique et quelques uns des r&ultats
obtenus. 11 y est dit que, bien que les signaux individuels d'un
bane de poissons soient amplifies par la reflexion venant du poisson,
Continued from page 403
archive7. Based on this material it will one day be
possible to make predictions regarding the general
sound conditions in the oceans.
Progress in this field of bioacoustics could be strength-
ened if scientists could be supported by observations of
sailors on an international scale. Acoustical data from
ships should be supplemented by log-books, notes, and
echograms.
One final aim is to establish standard animal sound-
types, e.g. for shrimps, fish, etc., which are to be sub-
divided according to frequencies and the different sound
mechanism. While today the special knowledge needed
for this subject depends mainly on the personal experi-
ence of specialists, it will then be possible to spread this
knowledge to every interested fisherman.
References
>v. Brandt, A. Unterwasserger&usche und Fischerei.
Fischereiwelt 4, 61-62, 1952.
•Burner, C. J., and H. L. Moore. Attempts to guide small
fishes with underwater sound. Spec. Scientific Report: Fisheries
No. Ill, Washington, 1953.
1 Chu, C. Y, The yellow Croaker Fishery of Hong Kong and
preliminary Notes of the Biology of Pscudosciacna crocea. Hong
Kong University Fisheries Journal, III, IV/1960.
4 Dobrin, M. B. Measurements of underwater noise produced
by marine life. Science, 105, 19*23, 1947.
'Fischer, Ch. Seefischerei und Seefischzucht in Nieder-
Iandisch4ndien. Mitt, dtsch. Sect Ver., 1927.
• Fish, M. P. The character and significance of sound produc-
tion among fishes of the Western North Atlantic. Bull. Bingham
Oceanographic Collection, 14; Art. 3; 1-109, 1954.
7 Fish, M. P. The basic approach to location of biological
underwater sound. Ausschuss fur Funkortung, Dusseldorf,
pp 73-77, 1957.
404
8 Freytag, G. Die bisherigen Kenntnisse uber tierische Lauter-
zeugung im Unterwasserraum. Protokolle zur Fischereitechnik,
Bd. 7, 1960.
9 Freytag, G. Aal erzeugt Unterwassergerausche. Infor-
mationen ftir die Fischwirtschaft 8, Nr. 2/3, 64/65, 1961.
10 Freytag, G. Heringsschwarmgera'usche. Informationen fur
die Fischwirtschaft 8, Nr. 5/6, 139-140, 1961.
11 Freytag, G. Laute und Lauterzeugung bei den Trigliden.
I. Trigla gurnadus L. Unpublished.
12 Fricdrich, Tom. Acoustic measurements in connection with
development of a pelagic one-Boat trawl. Ccntralinstitutt for
industriell Forskning, Blindern (Oslo), 1962.
18 Harris, V. E. Gear development progress in underwater
listening experiments and television. Atlantic States Marine
Fisheries Commission XII. Annual Meeting, New York, 1953.
14 Hashimoto, T, Nishimura, M., and Maniwa, Y. Detection of
fish by sonobuoy. Bull. Jap. Soc. Sci. Fish., 26, No. 3,)245-249.
"Ihering, R. v. Fischereiliche Erfahrungen in Nordost-
Brasilien. Ztschr. Fisch. Nof. 34, 549-559, 1936.
16 Irvine, F. R. The Fish and Fisheries of the Gold Coast. The
Crown Agents for the Colonies, London, 1947.
17 Mohr, H. Zum Verhalten von Fischschwarmen beim Fang
mit pelagischen Schlcppnetzen. Protokolle zur Fischereitechnik
Bd. 8f Heft 36, 1962.
18 Moulton, James M. Influencing the calling of sea robins
(Prionotus spp) with sound. Biological Bulletin, Vo. Ill, No. 3,
1956.
19 Moulton, James M. Swimming sounds and the Schooling of
Fishes. Biological Bulletin 119/2, October, p 210-223, 1960.
10 Murray, J. Herring making sounds. Mag. Nat. Hist.
1831:148.
11 Nishimoura, M. Frequency characteristics of sea noise and
fish sound. Technical Report of Fishing Boat No. 15, p 111-118,
1961.
"Scharfe, J. Echolotbeobachtungen uber die Gerftuscher-
zeugung durch das Rollergrundtau. Die Fischwirtschaft, 9, 250.
11 Schneider, H., and A. D. Hasten Laute and Lauterzeugung
beim Susswassertrommler Aplodinotus grunniens Rafinesque
(Sciaenidae, Pisces). Ztschr. vergl Physiol!, 43, 499-517,1960.
M Steinberg, John C., Morton Kronengold, William C.
Cummings. Hydrophone Installation for the Study of soniferous
marine animals. The Journal of the Acoustic Society of America,
6Vol. N34, o. 8, 1090-1095, 192.
la force totale de 1'dcho est nianmoins affaible par suite de
I'absorption de l'6nergie ultra-sonique par le corps des poissons.
DCS formulcs exprimant cet affaiblissement sont fournies. La
capaciti de reflexion differe entre les poissons et depend de la
forme du corps mais aussi de la presence ou de 1'absence de vessies
natatoires. L'auteur a trouvt que le coefficient de reflexion depend
de la frequence du son ultra-sonique. Les experiences ont montre
que rintensit6 de 1'echo etait proportionnelle a la longueur de
pulsation de sorte que les 6chos renvoySs du bane de poissons
etaient non seulement amplifies mais aussi prolonged. De ce fait
les enregistrements de banes de poissons sont surestimes dans la
direction verticale et peuvent parfois mfcme s'6tendre jusqu'au-dela
du fond. Un diagramme bas£ sur les mesures acoustiques prises
pendant les investigations montre que le coefficient d'affaiblisse-
mcnts des echos augmente avec raccroiseement de concentration
du poisson. Une quantit& considerable d'energie d'une onde
sonique aboutissant au bane de poissons est diffuses de telle
maniere qu'une petite quantitS seulement de l'£nergie retourne au
r£cepteur. Cette perte de propagation grandit avec la frequence.
Estudio de las caracteristicas acusticas de los peces
Extracto
Para proyectar instrumentos hidroacusticos de utilidad en la pesca
industrial y en los reconocimientos destinados a evaluar las
poblaciones icticas, se necesita mas informaci6n respecto a la
atenuacibn de la fuerza de las seflales desde la emisi6n hasta la
recepci6n del eco. £1 Institute de Investigaciones de Pesca
Maritima y Oceanografia de la URSS ha estudiado las carac-
teristicas acusticas de peces individuates y de cardumenes y en
esta ponencia se dan detalles de la manera de enfocar el problema,
su tratamiento matematico y algunos result ados obtenidos. Se
afirma que aunque los ecos individuates de un cardumen se
refuerzan por reflexiones de los peces, la luerza total del eco se
atenua por absorci6n de la energia ultrasonora por los cuerpos de
los peces. Se dan f6rmulas que expresan la atenuaci6n de los ecos.
La capacidad de reflexi6n varia entre peces y depende no s61o de
la forma del cuerpo sino tambien de la presencia o falta de una
vejiga natatoria. El autpr observd que el coeficiente de reflexibn
depende de la frecuencia del ultrasonido. Las investigaciones
dempstraron que la intensidad del eco es prpporcional a la longitud
del impulse, de modo que los ecos obtenidos de cardumenes no
s61o aumentaron en amplitud sino que tambi&i se prolpngaron.
Esto da por resultado una sobrestimaci6n de los registros de
cardumenes en direcci6n vertical que en ocasiones pueden indicar
una extensi6n mas alia de la linea del fondo. Un esquema basado
en medidas acusticas realizadas durante las investigaciones de-
muestra que el coeficiente de atenuacion de la serial del eco se
incrementa con la concentiaci6n de peces. Una gran parte de la
energia de una onda sonora que llega a un cardumen se dispersa
de modo que s61o una pequefta parte de la energia transmitida
regresa al receptor. Esta perdida de propagacidn aumenta con la
frecuencia.
HPHE study of the acoustic characteristics of fish is of
A great importance for designing hydroacoustic in-
struments used in fishing as well as for developing
methods of estimating the fish abundance, based on
data obtained with such instruments.
It is known that the radius of the equivalent sphere
of a flat-shaped fish school depends on the number of
fish in the school:
Re ^R! VrT
where
Re — radius of the equivalent sphere of school
n — number of fish in the school
Rx — coefficient in function of frequency, size and
species of fish. Numerically the coefficient is
equal to the radius of the equivalent sphere
of a single fish at a given frequency.
In actually existing volumetric schools the echo
signal is built up by reflections created by all the fishes
receiving not only the direct pulse emitted by the trans-
ducer, but also the additional ultrasonic energy reflected
by the fish within the scattering space element. This
useful reverberation of a volumetric fish school accounts
for the increased strength of the produced signal as
compared with signals returned by plane schools. A
quantitative estimate of the effect of multiple reflection
may be obtained from Fig 1, the dotted curve given for a
Fig I. Relation of radius of equivalent sphere to number offish in
a school.
school of large horse mackerel is derived from the
equation given above. The radius of the equivalent
sphere of a single horse mackerel 46 cm long is equal
to 0.022 m. On the same figure is given a curve:
AR=9(n) where
A. U - *D "O
volumetric flat
school school
It shows that AR is linear and dependent on the
number of fish above a certain minimum, guaranteeing
a multiplicity of reflections. In our experiments this
minimum was provided by four fishes. The increase of
the radius of the equivalent sphere of a fish school
accounted for by multiple reflection is obtained from
the equation:
AR=b (n— 4) where
n — number of fish
b — experimentally determined coefficient char-
acterising the ultrasound scattered by a
single fish at a given frequency (in our case a
horse mackerel 46 cm long, at a frequency of
S0kc/secb=0.0025).
405
In actually existing fish schools, apart from the
increase in signal strength induced by repeated reflections,
there is also a partial decrease in strength, owing to the
absorption of ultrasonic energy by the fish bodies.
The attenuation of ultrasound was measured for
different numbers of fish at a frequency of 50 kc/sec. It
may be assumed with a reasonable degree of accuracy
that A*fish=3 db/km for horse mackerel of 23 cm
and A* fish=6 db/km for horse mackerel of 46 cm.
Hence we may determine how many times the intensity
of sound will be changed when passing through a fish
school extending over 1 km.
20 Ig
consequently
In our case the pulse length (1ms) corresponded to a
thickness of the scattering space element of 0.75 m; the
coefficient characterising the changes of intensity of the
ultrasound caused by absorption by the bodies of fish
is then:
_ _
y ~r~iooo:
42-0.00,5
Therefore, taking into account the useful reverberation
in the school and the attenuation of ultrasound within it,
the equation for calculating the radius of the equivalent
sphere of a school of horse mackerel of 46 cm, at a
frequency of 50 kc/sec becomes:
R.=0.022VT
I + 0.0015 n
This equation gives sufficiently exact results provided
the fish in the school are distributed so as not to screen
one another. In the contrary case the coefficient will be
considerably lower. Generally this equation can be
represented in the following form:
The diagram of relation of the equivalent sphere to
the length and weight of fish is based on the results of
acoustical measurements carried out over three years
tan
Sflfcte
2M«c
300*c
300M
0 SO - 30 40 SO 60 Cc«
a.
Fig 2. Relation of radius of equivalent sphere to (a) length and (b) weight offish.
406
(Fig 2a). It shows that the radius of the equivalent
sphere is considered to be directly proportional to the
length of the fish. The angle of slope of the direct line
varies with varying frequencies. If R, is expressed in
meters and the length of fish in meters too, the angular
coefficient for different frequencies will have the values
shown in Table I:
Frequency
kc/sec
R.
Re
Table I
20 50
18
187
15
157
200
11
117
300
8.2
8.27
Fig 2b shows that the relation of the radius of the
equivalent sphere to the weight of fish is expressed by
the parabola:
R. - k,VG
where
G— weight of fish
k, — proportion coefficient related to frequency
The coefficient k, has a certain definite value at each
frequency (Table II):
Frequency
20
50
200
300
Table 11
k,
0.84
1.04
1.38
1.85
/=/kf IV G
0.84 G
1.04 G
1.38G
1.85 G
The reflecting ability of different pelagic fishes can be
obtained from the diagrams in Fig 2, if, as a first approxi-
mation, the shape of their bodies is assumed to be
similar to that of the horse mackerel. Using the data
given above, the author was able to calculate the radius
of the equivalent sphere of a whale on the assumption
that wave resistance of the whale is near to that of fish.
It was assumed too that the body of the whale has the
same form as a pelagic fish. Subsequently the radius of
the equivalent sphere of the sperm-whale was determined
by measurement and it was found that the difference
between the computed and measured values did not
exceed 10 per cent. This means that the diagrams in
Fig 2a can be used for theoretical calculation of the
reflective ability of fish of any size from sprat to whale
size, i.e. from 10 cm to 10 m, with an accuracy sufficient
for practical purposes.
The reflective ability of fish depends also on the form
of the body and presence or absence of a swimbladder.
If the cross-section of the fish is elliptical with the major
axis directed vertically, a smaller surface is involved in
the reflection of ultrasound than in flat fishes. Also fish
without swimbladders may sometimes give stronger
echo signals than fish of similar weight with swim-
bladders. Using the diagrams in Fig 2a and 2b we can
determine the echo signals given by fish without swim-
bladders with a body form like that of the horse mackerel.
In this case one must make corrections for the ratios of
coefficients of reflection of fish with and without swim-
bladders:
• without — • . . , _
swimbladder swimbladder 0 §
According to results obtained during our experiments,
Pw/s
the value depends on the frequency (Table III):
Ps
Frequency
kc/sec
|3w/s
Table III
30 50 100 200
300
0.6 0.63 0.69 0.77 0.79
(1
w/t
)100% 40
37
31
23
21
It can be seen that the swimbladder contributes
greatly toward the reflection of ultrasound at low
frequencies and to a lesser degree at higher frequencies.
In the latter case the major part is played by tiny air
bubbles contained in the body of the fish.
Comparing our results with the data of other authors,
we find that Gushing and Richardson have concluded
that from 40 to 80 per cent of a signal received from a
live cod came from the swimbladder. Harden Jones
and Pears who experimented with perch found that
SO per cent of the signal is produced by the swimbladder
which in this fish accounts for about 7 per cent of its
volume.
The results of Beverton & Blacker are in close agree-
ment with those of Harden Jones and Pears at a frequency
of 14.5 kc/sec. Harden Jones found that the relationship
between the strength of the echo signal and the length
of the fish (in this case cod 20 to 80 cm) is approximately
linear. The data of these authors are not in agreement
with our results.
In horizontal echo-sounding of homogeneous objects
of restricted extension a decrease of the pulse length will
reduce the level of reverberation proportionally without
altering the strength of the echo signal. But a fish school
can not be likened to a solid massive homogeneous
object. It is a cumulation of many small reflecting
objects (targets), so that in the echo-sounding of fish
schools the relationship between reverberation and
intensity of pulse length will bring about a considerable
decrease of the intensity of the echo signal.
The elucidation of the relation between intensity of
echo signal given by a fish school and length pulse is
therefore a very actual problem. The intensity of echo
signals from submerged objects is determined by the
following known equation:
P> R2 10-°-2ar
where
Pft — acoustical power
Y — coefficient of concentration of the transducer
Re — radius of the equivalent sphere of the reflecting
object (target)
407
r —distance from transducer to object
ft — coefficient of spatial attenuation
Yet this equation does give the relation of intensity of
the echo signal to the pulse length, if it is not presumed
to be concealed in the value of the radius of equivalent
sphere. In a series of experiments carried out at sea on
live horse mackerel, an attempt was made to find out
ths relationship between the signal given by fish and the
length of fish.
Adequate statistics permitted to achieve a high degree
of precision of our acoustical measurements during
which 740 cycles of measurements (recorded on oscillo-
grams) were carried out on three fish schools of different
numerical strength. All the measurements were made at
a frequency of 94 kc/sec. The diagram in Fig 4 based on
our acoustical investigations shows that the radius of the
equivalent sphere of a fish school Re is proportional to
the square root of the value of the pulse length T .
R. = Rx VT
The coefficient of proportionality Rx is numerically
equal to the radius of the equivalent sphere of a fish
school at a pulse length of 1ms. With increasing pulse
length the scattering space element increases and when
sounding a fish school in a vertical direction, increases the
strength of signal. By substituting the obtained values
from the equation given above:
In this case, as in the formula for the intensity of
reverberation, the intensity of the echo signal is pro-
portional to the pulse length. Physically this is quite
comprehensible as a fish school is a cumulation of small
scatterers, generating useful reverberation; echo signals
returned by fish schools are therefore not only increased
in amplitude, but obviously prolonged too. Records of
fish schools on echograms are therefore overestimated in
the vertical direction, and may even sometimes extend
to beyond the bottom line (Fig 3). The diagram in
Fig 4 illustrates the increase of Re with increasing T .
This increase is especially pronounced in huge concen-
trations of fish. The apparatus used in our experiments
allowed us to carry out measurements at pulse length
ranging from 0.1 ms to 2 ms and it may be inferred
that the regularity will be apparent at higher values too.
Our results can be used in the designing of hydro-
acoustical fishfinders:
(a) For selecting optimum pulse length according to
special requirements. Thus for echo sounders,
employed for fishfinding in pelagic fisheries
greater pulse lengths may be used; this will permit
economies on the power Pm, and hence on the
size of the apparatus; the optimum pulse lengths
C T
will bs that at which — j- is equal of the thick-
ness of the fish school in the direction of location.
(b) As initial data for the calculation of the radius of
equivalent spheres of schools in conformity with
408
t I * f f * , f f
4 I I * t ' I f It t 4 t
Fig 3. Record offish schools producing a strong reverberation (echo
recorded under the ground).
other pulse length; this is necessary for calcula-
tions of the range of fish finding equipment.
During the passage of ultrasound pulse through fish
schools the decrease of energy, characterized by the
coefficient of attenuation, will be greater than in a
relatively homogeneous marine medium, due to losses
caused by the fish bodies. An ultrasound pulse passing
through a fish school may be entirely attenuated and the
echo recorder will not record the bottom beneath the
school even at relatively shallow depths. In horizontal
echo-sounding the ultrasound pulse may encounter
simultaneously several fish schools of great extension ; if
the pulse is not strong enough it will be attenuated before
135
Fig 4. Relation of radius of equivalent sphere to duration of pulse
observed on three fish schools of different numerical strength.
giving an echo from each individual school. This may
occur also for a single but very compact school widely
extended in the direction of location.
Records of kilka (sprat) schools shown in Fig 5,
Fig 5. Record offish schools by a hydro-locator at different values
of emitted power.
were taken in the Black Sea at different values of trans-
mitting power. With a maximum power the echo sounder
recorded five schools disposed one behind the other. At a
ten times lower power the sounder recorded only four
schools and the last of them only slightly and partly
(about half of the actual thickness of the school). Even
at maximum power the fifth school was only faintly
marked. However, when the vessel approached nearer
this last school was fully and clearly recorded even at a
five times lower power.
Special experimental investigations were carried out
aimed at a quantitative assessment of the attenuation of
ultra sound in fish schools of different degrees of concen-
trations.The diagram in Fig 6 based on acoustical measure-
ments, shows the relation of attenuation of ultrasound to
frequency for four different concentrations offish schools.
We see that the coefficient of attenuation increases with
increasing concentration, i.e. with an increase of biomass
per cent of water volume.
The Japanese scientist, Hashimoto, suggests that since
the losses of ultrasonic waves reflected by fish schools
decrease with increasing frequencies, it bs inferred that
loss of propagation will increase with increasing fre-
quency. This assumption has been confirmed by our
results, though the values recorded by Hashimoto
are significantly higher and the coefficients of attenuation
are not physically equivalent and consequently not
directly comparable. Losses of propagations consist of
two components: attenuation of ultrasound in the fish
60
70
60
30
40
Jff
fff 30 30 4ff SO 60 70 Kfo
Fig 6. Extinction of ultrasound in fish schools of different concen-
tration in relation to frequency.
and water medium and scattering. An important part
of the energy of a sonic wave reaching a fish school is
scattered and lost uselessly, and only a small part of the
energy of a transmitted pulse return to the receiver.
Consequently an ultrasonic pulse passing through a fish
school will be weakened not so much by attenuation in
the medium, but rather by scattering by the fish bodies.
At a certain concentration of the school the losses of
propagations may become so high that the ultrasonic
pulse will be completely attenuated.
409
Frequency Analysis of Marine Sounds
As fandamental data preliminary to the development of methods
for delecting, luring or driving away fish schools with sound,
sounds of marine animals and ambient noises in the sea have been
studied and analyzed. The frequencies of sounds made by whale,
dolphin, dnimfish, spotfish, croaker, gurnard, lobster and shellfish,
and swimming sounds of yellowtail, squid, surf-perch a»d fitefish
were analyzed. Each sound showed a spectrum characteristic of
the species. Recorded sounds of risso's dolphin are in the range of
150-5,000 c/sec; drumfish gave a maximum pip at 500 c/sec;
spotfish 650 c/sec; croaker 250-800 c/sec; and gurnard 100-400
c/sec. Drumfish. spotfish, croaker and gurnard, etc., produce their
sounds with their air bladders. Yellowtail gave pips in the range
of 150-3.000 c/sec. The swimming sound of squid schools was at
1,000-2,000 c/sec. Lobster produces sounds at the base of its
antenna with maxima at 180*4,000 c/sec, with intensive com-
ponents in the ultrasonic range. Ambient sea noises of Aburat-
subo Bay and Kurihama Bay, Kanagawa Prefecture, Izu Ajiro
and Shunoda Shizuoka Prefecture, Choshi Kurohae, Chiba
Prefecture and others were analyzed. The spectra differed with the
areas and depth of water.
by
Tomlju Hashimoto Yoshinobu Maniwa
Fishing Boat Laboratory, Tokyo
Au cours d*une c*tude effectu£e en vue de de* velopper des mc"thodes
pour dttecter, attirer et effrayer des banes de poissons par le son,
les sons d'animaux marins et bruits ambiants de la mer ont tt&
ttudjes et analyses, Les frequences des sons produits par les
bafeine, dauphin, "drumfish", "spotfish", maigre, grondin,
hofnardetcrustacesont6t^analys6es. A chaque espece correspond
un spectre caract&istique de sons. Les sons 6mis par les squales
ont une poit6e de 150 a 5000 c/sec; par le "drumfish** un maximum
de 500 c/sec; le "spotfish**, 650 c/sec; le maigre, 250-800 c/sec
et le grondin, 100-400 c/sec. Les "drumfish'*, "spotfish**, maigre
et grondin produisent lews sons avec leurs vessies natatoires. Les
•Moles donnent des sons cntre 150 et 3000 c/sec. Le bruit de nage
des banes de calmars 6tait de 1000 a 2000 c/sec. Les crustacfc
produisent des sons a la base de leurs antennes avec un maximum
de 180-4000 c/sec avec des composants complexes intensifs, a la
frequence ultrasonique. Les bruits ambiants dans les bates de
Aburatsubo, Kurihama, Kanagawa Prefecture, Izu Ajiro Shimoda
et Shizuoka Prefecture, Choshi Kurohae, Chiba Prefecture et dans
d'autres eaux du Japon ont M analysts et Ton a trouve* que les
spectres de sons difffcraient scion le lieu et la profondeur de la mer.
Extracto
Por ser dates basicos preliminares para la formulaci6n de m£todos
para localizar, atraer o rechazar cardumenes con sonidos, se nan
estudiado y analizado los que emiten los animales maronis y los
ambientales en le mar. So analizaron las frccucncias de los sonidos
emitidos por ballenas, delfines, "drumfish**, "spotfish**, roncador,
perttn, bogavante y mariscos, y los sonidos que hacen al nadar el
jurel amarillo, calamar, "surf-perch** y "fitefish". Cada sonido
tenia un espectro caracteristico de la especie. Los sonidos regis-
trados de las ballenas estan en la gama de 150 a 5.000 c/seg; el
"drumfish" dio una sefial maxima de 500 c/seg "Spotfish" 650c/scg;
roncador de 250 a 800 c/seg ;yper!6nde 100 a 400 c/seg. Estospeces
producen sus sonidos con la vtjiga natatoria. El jurel amarillo
dio sonidos en la gama de 150 a 3.000 c/seg. Los sonidos produci-
dos nor cardumenes de calamares en movimiento eran de 1.000
a 2.000 c/seg. El bogavante produce sonidos en la base de sus
antenas en la gama de 180 a 4.000 c/seg, con componentes intensivos
en la gama ultrasonora. Tamibon se registraron y analizaron
ruidos marinos ambientales en las bahias de Aburatsubo y Kuri-
hama, en la Prefectura de Kanagawa, en las Prefecturas de Izu
Ajiro y Shimoda Shizuoka, Choshi Kurohae, Prefectura de Chinba
y otros lugares. Los espectros variaban con el lugar y
proftindidad del agua.
METHODS of detecting, luring or driving away fish
schools by sound are being investigated by the
authors. To obtain fundamental data, the intensity and
the frequency spectra of sounds produced by marine
410
animals and ambient noises in the sea were studied,
using a hydrophone.
A special hydrophone was manufactured. Its char-
acteristics are flat with a deviation less than 3 db in the
frequency range from SO to 35,000 c/sec as shown in
"-no
.05 ,1 £ .4 .6 1 2 4 6 10 8> 40
fr^qutncy (kc)
Odb
Fig 1. Sensitivity-frequency characteristics of hydrophone.
Fig 1. This hydrophone is made of barium titanate. It
is cylindrical with a diameter of 5.5 cm and is enclosed in
an acoustic rubber case filled with castor oil.
The tape recorder used was Sony 552-type, the
frequency characteristics of which are flat in the range
from 50 to 10,000 c/sec. The Panoramic Analyser used
was a super-heterodyne type with an analysing range
of 40 c to 20,000 c/sec.
The endless tape was made by joining several pieces
on which a single sound was recorded at the same level
with Ampex 307-type recorder (the frequency char-
acteristics are flat in the range of 150 c to 35,000 c/sec).
A photograph of several sweeps on the cathode ray tube
of the analyser was taken, by playing back the endless
tape. Sea noises were analysed by playing back the tape
recorded in the sea.
Analysis of sounds of sea animals
Fig 2 to Fig 13 show results of analysis of sounds made
by sea animals. Fig 2 shows the spectrum of low-pitch
sound of Risso's dolphin. The maximum pip is nearly at
2,500 c/sec. The second pips, which are lower than the
Fig 2. (Left) Low-pitch sound ofRisso's dolphin; 3. (right) High-pitch
sound of Risso's dolphin.
Fig 8. (Left) Swimming sound of yellowtail; 9. (right) Swimming
sound of squid school.
former by 17 db, are at 200 c and 250 c/sec. Fig 3 shows
the spectrum of high-pitch sound of Risso's dolphin which
is different from Fig 2. The maximum pip is between
3,000 c and 5,000 c/sec; and the second maximum, which
is lower than the former by 9 db, is at about 250 c/sec.
These sounds are intensive even in the range of ultrasonic
waves1. Fig 4 shows the spectrum of drumfish with the
maximum pip at 500 c/sec. Fig 5 is for spotfish. The
Fig 4. (Left) Spectrum of drumfish and 5 (right) Spectrum of
spotfish.
maximum pip is nearly at 650 c/sec. The second maximum,
less than the former by about 4 db, is at about 150 c/sec.
Fig 6 is for croaker. The maximum is nearly at 250 c/sec.
The second maximum, less than the former by 14 db, is
nearly at 800 c/sec. Fig 7 is gurnard and shows the
maximum pip between 100 c and 400 c/sec.
Fig 6. (Left) Spectrum of croaker; 7. (right) Spectrum of gurnard.
Drumfish, spotfish, croaker, gurnard etc., produce
their sounds with their air bladders. These sounds differ
with species. Their maximum pips are at frequency
ranges lower than 1,000 c/sec.
In the spectrum of swimming sound of yellowtail
(Fig 8), the maximum pips are at 150-600 c/sec, and at
about 1,200 c/sec, respectively, and the second maximum,
less by about 4 db, is at 2,000 c/sec, and the third maxi-
mum, less than the second by 10 db, is at about 3,000
c/sec. In the spectrum of swimming sound of squid
schools (Fig 9), the maximum is at 1,000-2,000 c/sec.
Fig 10 shows the swimming sound of surf-perch which has
the maximum at about 300 c/sec and the second maxi-
mum, less by about 14 db, is at 1,500 c/sec. Fig 1 1 shows
Fig JO. (Left) Swimming sound of a surf-perch school; 11. (right)
Swimming sound offllefish school.
the swimming sound of filefish which has the maximum
at 1,000 c/sec and its second maximum, less by 8 db, is
at 2,000 c/sec. ,
These swimming sounds have spectra characteristic of
the species of fish, and all of them have rather high-
frequency components.
Lobster produces sounds at the base of its antenna.
The spectrum is shown in Fig 12, which has its m«cji»M|
at about 180, 700 and 1,500 c/sec, respectively, and the
second maximum, less by 6 db, is at 4,000 c/sec. These
sounds have intensive components in the ultrasonic
range.
Fig 13 shows the spectrum of sounds of shellfish
Fig 12. (Left) Sound produced by antenna of lobster; 13. (right)
Sound of bubbles blown by shellfish.
produced when it blows bubbles; this has its maxima at
1 ,500-1 ,700 c/sec and 3,500 c/sec, respectively. Frequency
components in the ultrasonic range are also noted.
Frequencies at which pips are maximum in these
spectra are shown in Table I. The order of magnitude of
pips is marked by (1), (2), (3).
411
Table I—Frequency at which pip is maximum
Specks offish and kind of sound
Riiso's dolphin low-pitch
high-pitch
Drumfish
Gurnard
Swimming sound
Yellowtail
Squid
Surf-perch
Filefish
Sound produced by antenna of
lobster
Sound of bubbles blown by
shellfish
Frequency (c/s)
(1)2,500(2)200-250
(1)3,000-5,000(2)250
500
(1) 650 (2) 150
(1) 250 (2) 800
J 00-400
(1) 150-600, 1,200 (2) 2,000
(3)3,000
1,000-2,000
(1) 3CO (2) 1,500
(1)1,000 (2)2,000
(1)180,700,1,500 (2)4,000
1,500-1,700, 3,500
Spectra of sea noises of Aburatsubo Bay and Kurihama
Bay (Kanagawa Prefecture), Izu Ajiro and Shimoda
(Shizuoka Prefecture), and Choshi (Chiba Prefecture)
are shown respectively in Fig 14 to Fig 18. It will be
Fig 14. (Left) Sea noises of Aburatsubo Bay, Kanagawa Prefecture
15. (right) Kirihama Bay, Kanagawa Prefecture.
Fig 16. (Left) Ajiro, Shizuoka Prefecture; 17. (right) Choshi
Chiba Prefecture.
Fig 18. (Left) Entrance of Nabeta Bay, Shimoda, Shizuoka Prefec-
ture; 19. (right) Shimoda Kojirahama (sea depth is 3 m) Intensity
is 12.3 x 10U W\cm*
seen from these figures that spectra differ with water
areas. Measured values of sound intensity also differ
with water areas as shown in Table II. Frequency
components in the ultrasonic range are also noted.
Effects of depth on ambient noises in the sea were
measured at Shimoda. Fig 19 is the spectrum at Kojira-
hama (sea depth is 3 m), while Fig 20 is the spectrum
1,500 m away from the shore (sea depth is 20 m). Sound
intensity is greater near the coast.
Fig 20. Offing 1,5000 m away from Kojirahama (sea depth is 20 m).
Intensity is J.80 x /018
Table H— Pressure and intensity of sea areas
Areas Date
Aburatsubo Bay, 10.55 hrs
Kanagawa Prefecture (1 7/12/59)
Ajiro, Shizuoka Pre- 08.20 hrs
fecture (4/2/60)
Shimoda Nabeta Bay, 15.50 hrs
Shizuoka Prefecture (2/8/59)
Entrance of Nabeta 16.30 hrs
Bay (2/8/59)
Shimoda Kojirahama 17.30 hrs
(2/8/59)
Sound pressure Sound intensity
(10-"W/Om2)
14.10-21.0
4.60-5.60
0.50-1.00
3.63
3.13
4.30
0.17- 0.67
8.80
6.50
12.3
Conclusions
(a) The spectrum of sound produced by a marine
animal is characteristic of the species, so these
sounds can be used for detecting, attracting or
repelling fish schools. The authors are now
studying such methods.
(b) Spectra of ambient sea noises differed with the
areas and depth of water where the analyses were
made.
The authors acknowledge assistance received from M. Nishimura,
Fishing Boat Laboratory, Fisheries Agency, Tokyo, and Assistant
Professor K. Kusaka of Tokyo University.
Reference
Hashimoto, T., and Maniwa, Y., Noise of creatures in sea in
range of ultrasound. Technical Report of Fishing Boat Laboratory
No. 12. 1958.
412
Identifying Pacific Coast Fishes From Echo-Sounder Recordings
Abstract
Echo-sounder sensitivity has been developed more rapidly than the
capabilities of the average operator to use these improvements
to identify the fish species pictured by them. It is time the fishing
industry made better use of the tools now available for more econo-
mical fishing operations. Constant use of echo sounders and careful
observations of the traces they make can provide valuable informa-
tion on the length and density of specific fish concentrations below
the vessel. Examples are given of echograms showing characteristic
traces of hake, rockfish and anchovy.
Identification de poisson par des enregistrements d'echo-sondeur
sur les cotes du Paciflque
Rfeum*
La sensibilit^ des 6chos-sondeurs s'est developp6e plus rapidement
que les capacitds des op6rateurs & 1'emploi des ces instruments
pcrfectionnes pour 1'identification des especes de poissons enregis-
trces sur les graphiques. II est necessaire que 1' Industrie de peche fasse
un meilleur usage de ces instruments, pour obtenir des operations
plus econpmiques. L'utilisation continue des 6chos-sondeurs et
1'observation minutieuse des traces graphiques peuvent en efTet,
dormer des informations importances sur la longueur et la densitg
de certaines concentrations de poissons, sous le bateau. Comme
exemples, 1'auteur communique des 6chogrammes montrant les
traces caracteristiques de merlus, rascasses de fond et anchois.
Identificadon de peces de la costa del Padfico con registros de la
ecosonda
Extracto
La sensibilidad de las ecosondas ha aumentado mas rapidamcnte
que la capacidad del operador corriente de emplear estas mejoras
para identificar las especies de peccs que seflalan. Ha llegado el
momenta de que la industria pesquera emplee mejor los aparatos
de que dispone para pescar mas econ6micamente. El uso constante
de las ecosondas y la observaci6n cuidadosa de los trazos pueden
facilitar valiosa informaci6n de la longitud y densidad de con-
centraciones de peces especificos debajo del barco. Se dan ejemplos
de ecogramas que muestran trazos caracterfsticos de merluza,
gallineta y anchoa.
A LTHOUGH echo-sounder sensitivity has been greatly
Ji\ improved in recent years, little progress has been
made at identifying the traces recorded on echo-sounding
equipment.
On Pacific coast trawlers recently there has been a
trend to replace old equipment with newer, more sensi-
tive units.
Refinements in recording-type echo sounders have
prompted wider use of these instruments to find fish
and, in some instances, to identify these fish by the
characteristic traces made by different species on the
recording paper. Several trawler captains use echo soun-
ders to provide permanent records of their fishing trips.
Echograms provided by Mr. Harry Harrington, master
of the trawler Christine, illustrate their use for identifying
fish schools to species. Mr. Harrington uses a Kelvin
Hughes model MS 29 Echo Sounder while fishing pri-
marily for rockfish (Fam. Scorpaenidae) out of Santa
Barbara, California. Since rockfishes have well-devel-
oped swim bladders and school near the bottom, they
make excellent targets for detection with echo sounders.
The traces were made during otter trawling operations
and species determinations were made by examining
the catch upon completion of the haul.
by
E. A. Best
Marine Research, Californian
Department of Fish & Game
Two rockfishes produced a characteristic trace:
chilipepper, Sebastodes goodei, and stripetail rockfish,
Sebastodes saxicola (Figs. 1 and 2). Chilipeppers are
L
Fig. I. Echo-sounder tracings of chilipepper, Sebastodes goodei.
Fig. 2. Echo-sounder tracings of stripetail rockfish. Sebastodes saxi-
cola.
desirable market fishes and make up nearly 40 per cent
of the rockfish landings at Santa Barbara. Stripetail
rockfish rarely exceed 10 inches in length and are used
primarily as animal food.
Pacific hake, Merluccius productus, are often abundant
and characteristically show up on echograms as narrow
schools, often extending several fathoms above the bot-
tom (Figs. 3 and 4). Mr. Harrington's hake traces are
similar to those reported for hake off the Washington
coast during daylight hours (Schaefers and Powell
1959). After dark, hake schools apparently break up
and the fish become scattered in the mid-depths.
413
Fig. 3. Echo-sounder tracings of Pacific hake, Merluccius productus.
Fig. 4. Echo-sounder tracings of Pacific hake, Merluccius productus.
Pelagic fish are often abundant over the trawling
grounds. One echogram showed a dense concentration
of anchovies, Engraulis mordax, at mid-depth with
schools of hake near the bottom (Fig. 5).
Some recent electronic echo-sounder installations
have included instruments using a cathode ray tube
(C.R.T.) to present visually the returning echoes. The
success of these units depends to a great extent upon the
Fig. 5. Echo-sounder tracings of anchovy, Engraulis mordax at mid-
depth with hake showing near the bottom.
operator's ability to interpret and remember the images
produced on the C.R.T. These units have been used to
locate and determine the extent of bottomfish concentra-
tions, particularly rockfish, Sebastodes spp.
By determining the extent of a fish concentration and
adjusting individual trawl hauls to the length of the
school, rather than some arbitrarily chosen time unit,
more economical use of fishing time has been realised.
There has been little success in detecting flatfish
concentrations with cathode ray tube sounders, but
refinements may someday soon make such a task possible.
References
Schaefers, E. A. and Powell, D. £.: Correlation of midwater
trawl catches with echo recordings from the northeastern Pacific.
In Modern fishing gear of the world, edited by Hilmar Kristjonsson,
pp. 517-522. London, Fishing News (Books) Ltd., 607 pp. 1959.
414
Echo-Sounding Through Ice
Abstract
The echo traces of the bottom of, and fish schools in, a frozen lake
could be obtained with ease and speed by determining positions on
and by sounding through the surface ice. The transducers were
t into good contact with the ice by removing the snow and
[a cup full of water. Using 200 kc and 400 kc transistorised
ng echo sounders, it was possible to obtain echo traces of
pond-smelt, a very slender fish about 1 1 cm long, through surface
ice. The transmission loss of ultra-sound in ice was almost linear
with the thickness of ice. In artificial ice it varied from 0-25 db/cm
at 20 kc to 0-60 db/cm at 200 and at 400 kc, while in natural ice on
the surface of the frozen lake studied it was 0-40 db/cm at both
200 and 400 kc. The sound velocity through ice was found to be
3,120 m/sec. The results for natural ice are claimed to be valid
only under the conditions of the experiments reported.
Echo sondage a trtvers la glace
Resume
Les indications de fond et des banes de poissons dans un lac glace ont
etd obtenues facilement et rapidcmcnt, en des points predetermines
par echo-sondage a travers la glace. Un bpn contact avec la glace
fut assure par 1'enfcvement de la neige suivi d'un rincage avec un
peu d'eau. Office & des echo-sondeurs a transistors de 200 & 400 kc,
il a 6t£ possible d'obtenir des echos de sauclet, un petit poisson
mince, de 1 1 cm de long. Les pertes de transmission en ultra-sons
dans la glace etaient presque lineaircs avec 1'epaisseur de la glace.
A travers la glace artificielle les pertes de transmission vanaicnt
entrc 0-25 db/cm a 20 kc et 0-60 db/cm & 200-400 kc tandis qu'a
travers la glace naturelle du lac glace etudie, les pertes de transmis-
sions ont ete de 1'ordne de 0-40 db/cm a 200 et & 400 kc. La vitesse du
son a travers la glace atteignait 3-120 m/sec. Les resultats obtenus
pour la glace naturelle, cites plus haut ne sont valables que dans les
conditions de rexperience,
Ecosondajes a travfe del hielo
Extracto
Los trazos de los ccos de los cardumenes y del fondo de un lago
helado se obtuvieron con facilidad y rapidez determinando sus
posiciones con respecto a la superficie del hielo y a trav6s de clla.
Los transductores se pusieron en contacto con el hielo quitando
la capa de nieve y vertiendo un poco de agua para nivelar la super-
ficie. Empleando ecosondas registradoras transistorizadas de 200 kc
y 400 kc se pudieron obtenur ecos de "pond-smelt", que es un pez
muy delgado de unos 11 cm. de longitud, a traves del hielo.
La perdida de ultrasonidos ppr transmisi6n a traves del hielo estaba
casi en proporci6n aritmetica con el espesor de 6ste; en hielo
artificial varlaba de 0-25 db/cm a 20 kc hasta 0-60 db/cm a 200 y a
400 kc, en tanto que en hielo natural en la superficie del lago estudia-
do era de 0-40 db/cm a 200 y 400 kc. La velocidad de transmisi6n del
sonido por el hielo es de 3,120 m/seg. Los resultados para el hielo
natural se afirma que solo son validos en las condiciones de los
experiments de que se da cucnta.
by
Tomiju Hashimoto
Fishing Boat Laboratory, Tokyo
and
Yoshinobu Maniwa
and
Osamu Omoto
and
Hidekuni Noda
Shibara Technical Institute, Tokyo
Fig. L Top. The small transistorised 400-kc echo sounder and below
the 200-kc echo sounder (Fish camera).
ACCURATE soundings of water depths needed for
bottom surveys and any changes in the bottom
profile of water reservoirs may be difficult and requires
specific efforts because of the drifts of boats due to wind
and current. It may therefore be more convenient to take
soundings when the water body is covered with ice.
The common method had so far been to break holes into
the ice on each spot, but it would be a great simplification
if the soundings could be taken through the ice.
The authors, therefore, conducted experiments in
echo sounding from the surface ice of Lake Haruna,
Gumma Prefecture, Japan, in January and February
1962 to find out if echoes of the bottom and of fish can
be obtained and to furnish some of the necessary data.
Two very small and compact transistorised echo
sounders were used. One was a 400-kc type with a
sounding range of 75 m, manufactured by Furno Electric
Co.; the other was a 200-kc type with a sounding range
of 65 m manufactured by Nippon Electric Co., Ltd.
At regular points, the transducers of the echo
sounders were brought into good contact with the ice by
pouring about a cup full of water (Fig. 2). (Water was
found better for contact than castor oil.) Fig 3 shows
echo traces obtained from the lake bottom. The traces
for each point are horizontal lines as the transducers
were stationary. The transverse section of the lake is
drawn according to the echo soundings. The surface ice
was 25 cm thick and it would have been difficult to make
415
Fig. 2. Application of the echo-sounder transducers to the ice surface.
Proper acoustical contact is secured by bringing water between the
surfaces of the transducers and the ice.
Fig. 5. Echo traces of pond-smelt (Hypomesus olidus) in deeper water
obtained through 31 cm thick ice.
receiving transducers were put opposite each other and
ice plates of varying thickness were placed between them.
The sound pressure of the pulses received through the
ice was measured. The graph (Fig. 6) shows the relation
between the thickness of the ice plates and the transmis-
sion loss; the measured values are nearly on a straight
line.
100 800 800 400 600 600 TOO
Fig. 3. Bottom traces obtained at some of the sounding positions (A)
and the bottom profile under the surveyed line drawn according to the
echo sounder (B).
holes through it. By sounding through the ice this
difficulty was eliminated. The time necessary to determine
about 24 positions and to take the soundings was only
1 hr 25 min. With the same method, also echo traces of
pond-smelt were obtained with the 400-kc fish finder
(Fig. 4 and Fig. 5). The ice was 25 cm and 31 cm thick
respectively. Schools of pond-smelt are shown close to
the bottom (Fig. 4) or scattered from the surface to the
bottom (Fig. 5). Some of the pond-smelt were caught
and found to be about 1 1 cm long.
Fig. 4. Echo traces of pond-smelt (Hypomesus olidus) in shallow water
near the bottom obtained through 25 cm thick ice.
The transmission loss in artificial ice was measured
for 28, 100, 200, and 400 kc. In a 25-m long water tank
with a cross-section of 50 x 50 cm, the transmitting and
416
J
x*s
€40
s— *
!»
!»
1 w
trtifleial ic«
MMeurml ralu* • 400ke o 2001o
A lOOke A 28ko
B
200,400kc
0.60db/cm
lOOke
0«55db/cm
28kc
O.eSdb/cm
^
^
^.
"J
*^'
^r
r —
<^ t
<f*
m'***\
^
k
+*~
0 10 20 50 40 50 60
Thloknsss of ics (on)
Fig. 6. Transmission loss in artificial ice of ultrasound of 28, 100,
200 and 400 kc versus thickness of ice.
loo of Lake Haruna
Msasurod Talus
* ZOOko (measuring point i)
400kc
•8
Point 2)
au
20
10
°e
^
200,400kc
0«40db/om
^-*
J*
^
•
*>*
^
O
> 10 20 50 40 SO 60
Thicknsaa of ios (cm)
Fig. 7. Transmission loss in the natural ice of Lake Haruna of 200 and
400 kc versus thickness of ice.
The transmission loss in natural ice was measured for
various thickness of ice. It is emphasised that these values
are valid only under the conditions of the experiments
reported here; they may well be different under other
conditions of snowfall and freezing.
To determine the velocity of sound in ice, the trans-
ducers of the 400-kc echo sounder were placed on the
surface of an ice block S3 cm high, and the time taken
for the echoes to return from the opposite surfaces was
measured. By this method, the sound velocity was found
to be 3,120 m/sec.
Discussion on
Fish Detection
R. E. Craig
Mr. R. E. Craig (U.K.) Rapporteur: Direct acoustic
methods seem to offer the most immediate aid, but we must
not be blind to other possibilities. Alverson and Wilimovsky
call attention to the possibilities of indirect methods of using
acoustic techniques, by automatic stations linked by radio
to ship or shore. These authors also mention the possibility
of developing optical methods of detection. 1 think it is
important to see how optical pulse methods might compare
with acoustic ones.
I would like to mention the relation between range and
frequency in ordinary sonar. Signal strength from a single
target decreases by about 12 decibels for each doubling of
the range, due to the geometrical spreading of the sound waves.
So, for a perfect medium, we can show the relationship by a
simple curve like (a) in Fig. 1 .
36
SIGNAL $T«Ef4GTH(db)
/ HIGH FREQUENCY
LOW PRENQUENCY
i _ r I a 3
I I G. 1 RAN€ EC ARBITRARY UN IT
In addition to this reduction of signal by spreading we have
a loss due to absorption, which is directly proportional to
the range, as curve (c). So if the minimum detectable signal
is, on this scale, zero, the range is given by the distance X.
For higher frequencies the absorption loss is greater, and so
the range is less. Because of the dependence of absorption
on frequency, it turns out that the range obtainable with
any given frequency is limited. I illustrate this in a very simple
and approximate graph.
IQOOfr
Fie. 2
1000
IOO
MOO
It is in the region of high frequency and short range, that
optical methods might have value. Absorption of light in
the sea is very variable, but is of the same order as for mega-
cycle acoustic frequencies. Because of the very pure spectrum
that could be used optically, ranges of one or two hundred
metres or so seem indeed possible, and the great advantage
is that light can pass easily through the air-water interface
and so optical methods could be used from aircraft. Also,
the rate of transmission of light is so high that vastly more
information per second could in principle be gathered.
On the other hand it does not seem likely that range resolu-
tion could be as good as is given by acoustic methods.
In matters of more immediate importance to us today,
there has been progress since the last Gear Congress. First,
there has been a considerable extension of echo sounding to
smaller and cheaper boats, encouraged by the production of
small, relatively cheap echo sounders. Secondly, there have
been considerable advances in the tactical use of sonar in
deep sea pelagic fisheries and in trawl fisheries.
The first of these, unfortunately, is not mentioned in any
papers, and 1 would be happy if FAO would produce a brief
survey on this topic when the papers are published.
On the use of sonar, Scharfe, Steinberg, and Mohr show
that the problems of handling a headline transducer or
netzsonde can be solved. They show how fish behaviour can
be studied by this method, in intimate relation to the move-
ments of the trawl, and how dependent the development of
pelagic trawling and pelagic trawl design has been on the use
of this technique. These papers are the beautiful result of
instrumentation well used, bringing out from comparatively
simple techniques, information of vital importance in fish
behaviour and gear design.
Again, Jakobsson describes the successful use of sonar
in purse seining. The emphasis here is really on fishing tactics,
and this brings in Vestnes on training in the use of sonar.
These two papers are most timely in bringing out the gap that
can exist between development of equipment, and its success-
ful use. As techniques develop, so the need for the right kind
of training will develop also.
There are three papers on identification of echo traces.
These are the papers by Lenier, Nishimura and Best, and they
include excellent reproductions of various echo traces of
known origin. Nishimura also considers the desirable charac-
417
teriitics of a sounder for tuna fishing, and comes to the
conclusion that a very powerful but otherwise conventional
machine is suitable. To complete the topic of applications of
sounding, Kawaguchi, Hirano and Nishimura show that it is
possible to study the behaviour of longlines by this means.
The other papers in fish detection deal with new equip-
ments on the one hand, and fundamental acoustic studies on
the other. Taking the fundamental studies first, we have in
this group an important paper by Hashimoto and Maniwa
on the frequency analysis of marine sounds. This begins to
lay a serious foundation for fish detection by passive listening,
either directly or via a radio link. The authors give the fre-
quency spectrum from a number of living things in the sea,
and some detail of the background against which such sounds
would have to be detected.
The same authors, in collaboration with Omoto and Noda,
give details of the acoustic parameters of ice, and show the
practicability of echo sounding through ice, a technique
that could, on occasion, have practical value.
Finally, Shishkova treats the relationship between indi-
vidual fish echoes, and the echoes from shoals of such fish.
Of the four papers on new equipment two — those by
Gerhardsen on the research sonar, and by Ellis, Hopkins and
Haslett on the Humber echo sounder — are in some ways simi-
lar. Each presents a serious attempt to build the most efficient
possible machine on essentially conventional lines.
Both machines are characterised by very high transmission
power (about eight kW peak) and by the provision of the best
display methods available. To pick just one special feature,
I expect most engineers will feel that the production of a
satisfactory bottom-locked display on a recorder is a most
commendable technical achievement.
The paper by Fontaine on the "Explorator" equipment
describes an attempt to break new ground by searching ahead
for demersal fish, in addition this is an interesting sonar
equipment for general fishing purposes.
The paper by Tucker and Welsby is important in that the
equipment described is more advanced in design than any
other civil sonar equipment. By utilising the principle of
scanning in angle within one pulse duration, it becomes
possible to increase the information per transmitted pulse
many times. This experimental equipment seems to provide
the only means available for determining the speed and direc-
tion of targets. It has many research applications and promises
to provide a basis for a future commercial sonar.
Finally, 1 summarise by saying that the major advances
have been in tactical handling, that some interesting new
equipment is appearing, and suggest (along with Alverson and
Wilimovsky) that we should face the future with minds very
wide open to technical possibilities that seem to exist on a
very wide front.
Mr. Mross (Germany): The paper on sector scanning
sonar by Tucker and Welsby represents a real advance in the
problem of detecting fish on the bottom, from the scientific
point of view, and Mr. Fontaine's paper also represents a
practical attempt to overcome the shortcomings in that field.
There was no simple formula for the horizontal detection of
scattered fish ahead. Definite training of fishermen was
required to enable them to use sonar to advantage in respect
of the presence ahead of herring, mackerel and pilchard.
Future trawling might be carried out at depths of 800 to
1,000 ft and echo sounders using ultra-sonic intensity such
as was described by Ellis, Hopkins and Haslett for deep-sea
work represented an advance. His own firm had installed
a special echo sounder for deep sea research on the new
41?
German research vessel. High frequency sounders as proposed
by the Japanese were interesting and he agreed with their
arguments, but the maximum depth at which they could
give results was about 100 m. The well-known German
nctzsonde system allowed continuous study offish behaviour.
This was one of its main advantages. Intensive work had been
carried out to develop the cable which represented the best
compromise between the different electrical and handling
requirements. The solution was a co-axial cable with a steel
core well insulated. He personally had been surprised at the
very good results obtained with this cable in various countries.
It was very necessary to have an unobstructed run for the
cable and this could easily be arranged on stcrntrawlcrs.
One problem in handling the gear was the strangeness of the
equipment over the first three days to the crew. Once they
were familiar with it, however, the system did give valuable
information even in heavy weather.
Dr. Schftrfe (Germany) : In order to avoid misunderstanding
I would like to point out that netzsonde is not a German
invention. This system was mentioned at the first Gear Con-
gress as having been experimented with by Lowestoft scien-
tists in co-operation with the Pye organisation. The German
echo-sounding companies, however, had certainly achieved
the final success in working out this netzsonde telemeter
technique and applying it to commercial fishing.
Mr. J. Fontaine (France): The Explorator equipment
has been installed on the British trawler D. B. Finn, and I
would like to give results of its use. It was installed early
November, 1962, but there were initial difficulties and tests
could only be started in January, 1963. This vessel makes
voyages of from 19 to 20 days* duration, and after its last
voyage it was publicly stated that for the fifth consecutive
time it had landed a catch exceeding £10,000 in value. He
had discussed the position with the skipper and the skipper
had said that during February he had been fishing off the
Norwegian shore and his catches were 50 per cent higher than
those of his colleagues, and that over a period of a month he
could credit to the operation of the Explorator equipment
at least 10 per cent of the total catches obtained. The equip-
ment had been developed specifically to meet the require-
ments of skippers. They could never find two skippers who
were of the same opinion, and a detailed survey had to be
made before they could really find out what were the operating
characteristics needed. The conclusion they reached was that
although the Boulogne skippers were interested in midwater
trawling there was yet a real feeling that bottom trawling
would continue for a long time. The skippers wanted to know
where shoals of fish were and whether they could be caught
with a trawl or not. They wanted to know whether the school
was swimming close to the sea bed, and so they had to find
an equipment which could distinguish between echoes from
the school and any from the sea bed. The skippers wanted the
recordings to be read very easily like those from a normal
sounder. So they had to adopt the special recording tech-
niques and introduce a computer which calculated the distance
from the sea bed and from the shoal in terms of depth. Other
information which the fishermen wanted was whether the
school was going this way or that, and the Explorator
provided for this too. It was not a scientist's equipment but a
practical equipment developed to meet the needs of the working
skipper. The working skipper had no special interest in science.
All he wanted was results. Obviously, like other simple
pieces of equipment, it was rather complicated in its circuits.
On the other hand the set was built into boats which took
rather long voyages, and so it had to be sturdy and easy to
maintain. Instead of a normal projector they gave it an ellip-
tical shape so that in the vertical plane the beam is as narrow
as possible so that they could separate the shoals of fish from
other items on the sea bed. It was also necessary when the
shoal had been detected to change from automatic scanning
to manual control over a narrow sector. To test it out the
laboratory had to fit it into a boat and a boat was lent them
for some actual operating tests. They installed it on the bridge,
but when they wanted to get it back from the skipper he
wouldn't let them have it because he found it useful. So they
actually had to send technicians down to the boat to measure
it in order to develop a new prototype. Another machine was
installed in another boat, and although the skipper understood
quite clearly how it should operate, he operated it in a different
way and got good results. The two skippers then compared
results and found that although they differed they had both
got good results. The skippers evolved their own technique in
accordance with the results they obtained.
Mr. R. Lenter (France) : In electronics considerable advances
had been made throughout the world but they always
applied those advances for strictly economic reasons in order
to bring them into line with practical possibilities. In the
French industry they endeavoured to provide the fishing
boats with the best facilities taking into account all the reali-
ties of the case. He was privileged in having two allegiances
because he was a half-time technician and a half-time fisher-
man, having spent many years fishing round Newfoundland.
He was thus able to compare the differences of techniques as
they had developed. While congratulating all the international
industries on their progress he felt that so far as applications
were concerned they should operate on the principle of more
haste less speed.
Mr. Craig: Training is essential in respect of all aspects of
fish-finding techniques. You can have formal training by
instruction or use, and whole techniques of fishing may alter
with the introduction of new fish-finding equipment so that
equipment and the methods of fishing developed together.
It is frequently some time after the introduction of an equip-
ment before everybody is able to use it to the best advantage.
Paper recording was of the highest value in echo sounding
and in sonar. Other methods had their value, but recording
continued to be the principal method of displaying information.
This was clearly because of its integrating properties, and
because it did not require anybody to be tied to the machine.
He emphasised the great difficulty of detecting fish close
to the sea bed. In part, this was because the bottom was not
a flat surface but more or less irregular. When fish were one
or two feet off the bottom, their echoes might be confused
with those of rocks. When sonar or any sort of horizontal
equipment was used they must obviously use a comparatively
narrow beam. And he showed by blackboard demonstration
the difficulties when such beams were directed at an angle to
the sea bottom. It was difficult to detect fish in the centre of
the beam against reflections from perhaps 50 m x 50 m of sea
bed. Fontaine had said this could be done, and they must
agree that it could be done, but he thought it could only be
done when the fish were very heavily concentrated indeed.
Dr. R. W. G. Hastett (U.K.): I would like to comment
on Shishkova's paper on the study of acoustical characteristics
of fish. This paper gives very many useful pieces of informa-
tion regarding the assessment of signals from large shoals
of fish and single individual fish. This is very important in
assessing the size of fish shoals and likely catch. It would be
very interesting if more information had been given about
the technique for making these measurements of shoals
because measuring at sea is very difficult to do with accuracy
because of the variations hi the position and the aspect of the
fish. It is somewhat easier to make measurements by using
the scale model technique, in which case you had to have
carefully controlled conditions, and you must use the scale
factor of 50 : 1 down, in which case small fish may be used,
it is then possible to investigate the individual effect of inter-
ference between various factors. The results given in this
paper must be taken as an average, and probably are very
useful under practical seagoing conditions. When these pro-
perties are investigated in small-scale model techniques,
however, great complications of signal strength versus fish
length are found. In Fig. 2a in the paper an average has
presumably been taken disregarding the interference effects
which may be found in detail. The signal reflected by fish
depends very greatly on the size of the fish in relation to the
wave length of the acoustic sound. In the case of bottom trawl*
ing it may be that individual fish have to be detected as com-
pared with midwatcr trawling where there may be large shoals
to be detected. This explains the concentration for bottom
trawling on individual fish echoes and the use of the cathode
ray beam types of recorder displays which are locked to the
sea bed thus enabling a larger amount of expansion and obser-
vation of individual echoes. In one trawling experiment,
over a two-hour period on a single day, it had been found
that on an average one fish was passed over every 10 yards,
and that meant you had one fish every 10 yards over the
volume of 60 ft wide by 8 ft high multiplied by the length of
8 miles. Figs. 3 and 4 of the paper indicated that it was
hoped that the combination of the rather theoretical observa-
tions of fish echoes with the measurement of the signal
strength and the size of the echoes from these displays may
eventually lead to new designs of equipment which will be of
benefit in assessing the size of the catch and the size of indivi-
dual fish.
Dr. D. H. Gushing (U.K.): demonstrated on the blackboard
the information that Dr. Haslctt sought in connection with
the Russian paper by sketching the results of certain tests.
Some were made on perch in a small tank, some on cod and
herring in the dock at Lowestoft, and others were made in
Norwegian fjords. Some fish were rigidly suspended and some
not, but in both methods maximum signals were taken, and
they showed a clear relationship between signal strength and
the length of the fish. That was the basic information that
was required in order to analyse fish schools. The measure-
ments had been rather difficult and complicated to secure for
physical reasons but from data of this kind they could go on
to make analyses of the signal strength of fish schools and
eventually, one hoped, be able to count the fish.
Mr. P. G. Schmidt (U.S. A.): The use of aeroplanes has
been left out in the treatment of fish detection. The tuna,
the sardine industry and the menhaden people on the east
coast of U.S.A. all make extensive use of aeroplanes for
locating fish. We are working off the north of Chile where
conditions are somewhat similar to those in Peru. During
last year we used aeroplanes more and more, both in long-
range and dose-range activities, to support our operations
in locating and actually setting the boats on the fish. At the
same time a considerable quantity of echo-sounder equipment
has come into use and about 100 per cent of the boats have
some echo-sounder equipment. About 30 or more asdic type
419
units have been installed. But wchavc found that we wouldprcfcr
to spend on aeroplanes the IS cents a ton that it is estimated
is the cost of operating one aeroplane to every 10 boats, rather
than go into the asdic type of echo sounder.
When we started the survey we used Chilean operators or
pilots who had no previous experience, and we thought any-
body could learn the art of estimating the quantity of fish and
helping the boats. We had a very good pilot, and after three
or four months he had the confidence of the factories that
employed him, and also had obtained the confidence of the
captains.
About February, 1963, we brought down a foremost fish-
spotting pilot from the menhaden grounds, and in a very few
weeks* time the whole industry found that fish spotting was a
science. This man, in a few days, showed us that we knew
nothing about the subject, and that he could successfully
estimate, within a very few tons, the fish in large shoals and
could actually set the boats onto the fish far better than the
fish captains.
From this experience all operators are now going further
into the aeroplane spotting business at the expense of the
asdic type of echo sounders. There are two types of spotting —
long-range spotting to determine where the fish may lie, and,
secondly, after bringing the boats there, staying with the shoal
and actually setting the boats on the fish. In a way the captains
resented this because they felt they were being replaced by
one who was not a fisherman. But after a while they depended
more and more upon the man, and when he is not flying they
almost feel helpless.
It was interesting to see this evolution. Fish captains did
not necessarily make good aeroplane spotters. It took an
intensive training for years and actual experience to determine
how to set the nets and estimate the wind and currents, etc.
from the aeroplanes. It would be interesting if, in future
publications of the FAO, some more information could be
made available on this subject because they looked at this
now as an extension of their fleet. They looked at the aero-
planes as if they were buying another boat. It is just as compli-
cated to select an aeroplane and to pilot an aeroplane, and to
develop the pilot's side of the business, as to develop a captain
and the fishing operation itself. In the menhaden industry of
the U.S.A. they had used aeroplanes extensively for over 10
years and they were now employed at the ratio of about one
aeroplane for about every 10 boats.
Dr. Cote (U.K.): We are in some doubt as to whether Mr.
Fontaine with his apparatus can, in fact, detect fish very close
to the sea bottom. Mr. Craig referred to the interference
between bottom echoes and the echoes of fish very close to the
sea floor, and, having regard to the fact that the headline of
the trawl is only 2 or 3 m above the sea bed, we really doubt
whether the skippers can actually see the fish close to the
bottom. We are wondering whether skipper Warham was in
fact seeing the fish that he took. Might he not be seeing fish
that were somewhat higher above the sea bottom and was
inferring from those and, successfully inferring, that there
were also fish in close proximity to the sea bottom, and within
the level that he was actually fishing with his trawl?
Mr. Fontaine gave his answer in the form of a demonstration
on the blackboard aimed at showing how the energy emitted
by the Explorator device was directed towards the bottom of
the sea ahead of the ship. That beam could be directed
towards the sea bed at an angle of approximately 30 deg. but
that was variable and the opening of the beam could be plus
or minus two deg. Only a small portion of the energy reaching
420
the sea bed would be reflected back to the receiver; the greater
pan would be reflected upward and away from the ship like
light striking a mirror. A school of fish would return greater
energy to the receiver than the sea bottom so that they gut
bade echoes in a relatively thick trace. The sea bottom would
come out in a greyish form on the recorder, but the fish trace
would come out darker. The trace of the school of fish would
eat into the trace of the sea bed which showed that the fish
were very close to the sea bed. Slides reproduced in his paper
showed some of these schools which came up precisely in
that way. Skipper Warham of the D. B. Finn had carried out
this system, and on seeing the traces had had the inspiration
to deduce the presence of a large school of fish. He really
did a marvellous piece of fishing over a two-day period.
Mr. V. Valdez (Peru): In Peru our big fleet of 1,000 fishing
vessels has only one-fifth of its number using echo sounders.
They operated along the coast and ventured sometimes 20 or
30 miles offshore only in spring and in autumn. In summer they
have a difficult problem because the fish are too close to the
surface, and as the boats progressed they cut into the shoals
and cannot get traces on their recorders. They have tried to
overcome this by means of asdic, and in that way completed
their echo survey. In general, however, the fishing was so good
that the men did not need overmuch help in getting more fish.
Lately, however, with the high concentration of fishing boats
it was becoming necessary to find some new areas for fishing.
When working close to the coast it was difficult sometimes
for the fisherman to get good catches, and when the fish were
far from the coast the fisherman could not get there because
he did not have the skill or training to go those distances.
The problem was how to locate the schools and tell the fisher-
men where the schools were. He wanted to find a transducer
that would locate the fish in summer and avoid splitting the
schools.
Mr. V. G. Welsby (U.K.): I would like to add a few
remarks to the paper presented by Prof. Tucker and myself.
Two of these points arise from what we have heard about
the Explorator. One thing is that although we discussed rapid
electronic sector scanning, that is scanning of a complete
sector within one pulse, it is worth remembering that elec-
tronic scanning technique can be used for slow scanning at
any speed you like, and this provides us with a very convenient
method of doing something much the same as slow mechanical
scanning and of doing it by electronic means with a transducer
which is fixed, thereby removing the necessity for heavy,
expensive, complicated devices to move the transducer.
The other point is that which has been just discussed. If
indeed the reflection from the bottom of the sea is of the very
low level suggested, then 1 suggest that this shows very clearly
that the technique which we have put forward in our paper of
rapidly scanning a narrow sonar beam will definitely work,
because if, in fact, the back scatter from the bottom is
sufficiently low in level to allow this relatively wide beam
with its four deg beam angle to operate, then the system which
we have suggested of using a considerably narrower beam,
ought to work even better. That will be seen in the future.
My last point is that although in our paper we have talked
about the possibility of using electronic sector scanning —
looking forward from the ship to see fish near the bottom— of
course, this is not the only possible use. It can be used to look
astern or, in midwater trawling, to enable an immediate
continuous picture of the fish shoal to be seen during the
actual catching of the fish, and the net, too, can be watched
and its movement as well as the fish school itself.
Mr. Halliday (U.K.): I rather agree with Dr. Cole in doubt-
ing the ability of the Explorator to detect fish very close to
the bottom unless the fish happens to be there in very large
schools. 1 think Dr. Haslett mentioned earlier our experience
in that what is regarded as good commercial fishing is
fishing where the fish very often occur singly and have to be
detected singly. We carried out some experiments quite
recently with the object of finding out how the reflection
coefficient of the bottom does in fact change with the angle.
This was done with a very simple apparatus which consisted
of a transducer with a fan-shaped beam, the wide angie being
in the vertical plane, and very short pulse. By this method we
hoped to show that as the sound progresses farther, obviously
the angle at which it is arriving at the bottom becomes more
and more acute, and that at some particular distance the echo-
strength would fall sufficiently to allow fish to be detected
against the bottom echo. But although the experiments are
very preliminary we did not find any trace of fish echo occur-
ring at the same time as the bottom echo.
In connection with Shishkova's paper it might be of interest
to mention a fact which we have noticed which shows the
difference between vertical sounding of strong schools and
horizontal sonar fish detection. It is, of course, well known
that with vertical sounding the echo from the shoal is very
much drawn out and can give a false impression — and this
is really what one might expect if one considers that the sound
probably reverberates in the school from fish to fish. This
would obviously give the effect of a drawn-out echo, but,
curiously enough, in sonar work on pelagic schools this does
not seem to occur. In other words the traces of the recorder
are quite sharp at the farther edge. I cannot explain this and, so
far, we are content to make use of it, because obviously it is
a very neat way of measuring the extent of the shoal in range.
Mr. Lusyne (FAO): No answer has been given to Mr.
Valdez's question about a transducer which will enable
him to pick up schools on the surface and avoid splitting them.
I can, however, relate some experiments which were carried out
on the Sandettie grounds on spent herring last year by the
Belgians. They had noticed that whereas a school of fish was
clearly visible on the recorder they caught none. They tried to
ascertain why the fish had disappeared. They towed a trans-
ducer astern and they noticed that the school seen on the
ship's transducer was not picked up by the towed transducer;
this indicated that the school had in fact disappeared. They
conducted these experiments again on the North-West
Rough, and in the same depth. The result was different.
The fish schools picked up on the ship's transducer were found
on the towed transducer. They also noticed that in fishing at a
depth of 50 to 60 fathoms this disappearance of the school did
not happen. They have so far concluded — and this applies
particularly to spent herring — that the disappearance only
happens in up to 25 fathoms of water. The experiments have
been few but they do suggest something. A second indication
is that there is a vast difference between the movements of
the schools. In the sprat season on the French Coast the same
method was applied, but they found there that every single
school picked up from the ship's transducer was also picked
up on the towed recorder.
Mr. Alverson (U.S.A.): asked if Mr. Fontaine could put
into units of measure the terms "close" and "very close**
as they related to the sea bed.
Mr. J. Fontaine (France): I shall try to give a clear reply
to Mr. Alverson's question. Everything depends on the beam
angle of the projector. With a normal sounder the beam angle
is in the region of 25 deg to 30 deg and the power of definition
is not great. Our purpose with the Explorator is to locate fish
shoals some distance ahead of the ship. To do that we have
been using not a normal transmission beam but one in which
the narrow angle of opening is in the vertical plane. It is the
angle in this plane which determines the power of definition
in depth, since it is the vertical distribution of fish that we
want. Accordingly we have brought the vertical half-angle
down to two deg, more or less giving a total angle of opening of
four deg. This is a compromise between the angle of the beam,
the size of the transducer, the ultrasonic frequency and the
rolling and pitching of the ship. If a fish shoal, having a
thickness of about 5 m to 10 m and a normal density, is on
the ground, its echo will be partly confused by the trace of the
sea bed, but as it reflects a greater energy it gives a darker
trace in the greyish one of the sea bed. It will be for the skipper,
from his experience, to deduce whether that darker trace is a
fish shoal, a wreck or a rock.
Mr. Craig, summing up, said many useful points had been
brought out very clearly. One that should be emphasised
very strongly was that the detection of fish schools on the
bottom was very difficult. There seemed to be some division
of opinion about the amount of echo that would be returned
from the sea bed at various angles. This directly concerned
this particular problem. Papers published by the National
Institute of Oceanography describing the use of narrow beam
sonar in surveys of sea bed made it clear that signals that
came at an angle from the sea bed were very different, depend-
ing on the nature of the bottom. They were, in fact, so different
that the bottom could be quite adequately charted by the use
of narrow beamed sonar. He thought the nearest to a simple
summing up of this problem was that given by Mr. Halliday;
namely, that on smooth sea beds when dealing with fish
in considerable shoals it was very likely that sonar impinging
at a narrow angle would show the fish up. If conditions were
not so good it was not easy to see the fish unless they were
fairly well clear of the bottom. That was his feeling from all
the different points of view expressed — namely, that when
conditions were good this detection could be quite possible,
but when conditions were bad it would be very difficult indeed.
Another point was that made by Dr. Welsby that the scan-
ning technique described by him had a number of applica-
tions quite apart from direct fish finding. It could be used to
look at the trawl, to look at the behaviour of the fish in rela-
tion to the trawl. This point had much appeal. But the difficul-
ties were very similar to that of handling the netzsonde.
Cables would have to be provided and transducers would
have to be put in the right place — possibly on the bottom of
the ship. Another point of value was the importance of search-
ing for fish by vessels other than fishing research vessels. In
some fisheries where ships are not adequately equipped this
was necessary — and that was in fact the start of fisheries
sonar. Norwegian research vessels searched for the schools
and directed the fishing fleet.
As regards aeroplanes their use had, in fact, been covered
at the last gear Congress by a fairly considerable paper, but
certainly more information would be welcomed. It could
only be provided by people describing the conditions in their
own fisheries because only certain fisheries had the possibility
of effective air search. Clearly the fish must be very close to
the surface and they must be in compact schools. This was a
method of great value in suitable areas.
Mr. Valdez (Peru): inquired if instead of aeroplanes
421
helicopters could be used for flying more slowly and detecting Mr. Ahmon (U.S.A.): This is being done already very
patches of plankton or possibly towing a transducer at slow extensively by the military, but I do not think it is fish they
speed in the water to detect the fish. are looking for!
422
Part 2 Bulk Fish Catching
Section 12 Fleet Operation
Japanese Mothership and Fleet Operations for Salmon, Crab,
Longlining, and Tuna
Abstract
This is a composite article, describing in five sections by different
authors the organisation and operation of Japan's specialised
fishing fleets, which began activities in this form in 1940 by trawling
for flatfish and crab, but now take in other species such as cod,
pollock, shrimp and halibut. In addition to these fleet operations
covering trawling and purse seining, specialised craft and activities
now cover salmon gilmetting, crab fishing, longlining and tuna
longlining. The general composition and organisation of these
fleets, together with the operational techniques that have been
developed, are fully described, as well as the specialised gears that
have been evolved for securing the best results. The motherships
are attended by specific numbers of catchers in proportion to their
size and needs, and are serviced by fuel and supply vessels, as well as
reduction and transport craft. In the various sections are detailed
the procedures through which the fleets work as units covering
the assigned areas with maximum efficiency.
Les flottilles de chalutiers Japonais travaillant avec on bateau-mere:
lews methodes et leur equipement
Cet expos6 d£crit en cinq sections redig&es par dififerents auteurs
1'organisation et 1'exploitation des flottilles Japonaises de ptehe
specialises, qui entreprirent ce type d'activitfc en 1940, par la
p6che au chalut des poissons plats et des crabes, mais qui actuelle-
ment p&hent d'autres espdces telles que la morue, le colin, les
crevettes et le fldtan. II faut ajouter a la flotte p&chant au chalut
et a la senne tournante de nouveaux bateaux specialises ptehant
le saumon au filet maillant, les crabes, et prat i quant la p&che a la
palangre du thon et d'autres scombrid£s. Les auteurs decrivent dans
les details la composition et 1'organisation g6n6rale de ces flottilles,
ainsi que les equipements speciaux perfectionn6s en vue d'obtenir
les meilleurs resultats. Les bateaux-meres sont accompagnds d'un
nombre donne de bateaux de peche, proportionnel a leurs dimensions
et a leurs exigences, et sont desservis par des unites auxiliaires
assurant leur approyisionnement en combustible et en vivres,
ainsj que par des unites de traitement et de transport. Les differentes
sections decrivent dans les details les operations effectuees par ies
flottilles dans les zones de p6che qui leur sont assignees, qu'elles
exploitent avec le maximum d'efficacite.
Flotillas de arrastreros Japoneses que trabajan eon un buque madre;
sus metodos y equipo
Extracto
Esta ponencia describe en cinco secciones escritas por varios
autores, la organizacidn y explotaci6n de las flotas pesqueras
Mothership Fleets,
their Methods and
Fishing Gear
H. Tominaga
JAPANESE mothership trawling operations started in
J 1940. World War II interrupted the operations but
they were resumed afterwards, and have since grown year
by
Hiroshi Tominaga
Taiyo Fishery Co., Tokyo
Masataki Neo
Nichivo Syogyo Kaisha Ltd., Tokyo
Nippon Suisan Kaisha Ltd.
Tokyo
and
Goro Okabe
Taiyo Fishery Co., Tokyo
especializadas del Jap6n, que iniciaron esta clase de actividad en
1940 pescando al arrastre pcccs pianos y cangrejos de mar y que
actualmente tambten capturan especies como bacalao, abadejo,
camardn e hipogloso. A las flotillas dedicadas a la pesca al arrastre
y al cerco, hay que aftadir embarcaciones especializadas en la
captura de salm6n con redes de enmalle, cangrejos de mar y atun
y otros esc6mbridos con palangrcs. Describen los autores con
pormenores la composicidn y organizaci6n generates de estas
flotillas y los equipos especiales que se ban ido perfeccionando
para obtener los mejores resultados. Los buques madre van
acompaftados de un niimero dcterminado de pesqueros, que esti
en proporci6n con sus dimensiones y nccesidades y estaii atendidos
de barcos de suministro (combustible, vivercs, etc.), de reduction
y de transporte. En las divcrsas secciones se describe el trabajo
de conjunto de las flotillas, que explotan los caladcros que se les
asignan con la maxima cficacia.
by year. At first, the catch was limited to certain species
of flatfish and crabs but various fish-processing vessels
are now being used.
The fishing grounds most favoured are found along
the continental shelf at depths of 50 to 600 m, which
stretches from Bristol Bay in the east as far as the Oliu-
torsk area in the west, or from Cape Oliutorsk as far as
Unimak Island. The type offish caught varies according
to the depth fished and comprises species of flatfish
(Lepidopsetta mochigarei, Limanda aspera, Liopsetta
obscura, Pleuronectes pallasi), cod, Alaska pollock,
sablcfish, long-jawed rockfish, Pacific pink shrimp,
halibut, arrow-toothed halibut, etc.
The type and number of catcher boats in a fleet are
determined by the processing capacity of the mothership
423
and the individual fishing capacity of the catcher boats.
A reduction ship normally uses 23-25 Danish seiners
or a group composed of 18-20 bull trawlers, plus eight
to ten Danish seiners. A shrimp cannery ship or a
refrigerating ship has 10-12 bull trawlers or five to eight
otter trawlers.
Fishing operations last for about five months, from
the latter part of April to the latter part of September.
During this season the sea and weather conditions are
favourable. During the rest of the year, although fish
are present in abundance, the sea is rough and the weather
so bad that it is difficult for small vessels to fish.
Motherships
Reduction ships have a fish meal and oil plant as well as
freezing equipment and Baader filleting machines.
Besides fish meal and oil, they produce liver oil,
salted cod, frozen fillets, etc.
Refrigerator ships produce mainly salted cod and
frozen products of flatfish, sablefish and rockfish, etc.
Shrimp cannery ships produce both canned and frozen
shrimp or only canned shrimp.
Supply vessels provide the fleets with fuel oil, fresh
water, provisions and fishing gear. The supply ships
usually form a team and consist of 1,000 to 1,500 GT
refrigerated ships for the transport of the frozen products
and of 2,000 to 3,000 GT freighters for the transport of
fish meal and canned shrimp.
Trawling operations
Two vessels of about 75 to 100 GT, 250 hp of identical
design and engines, form a bull-trawling pair. In design
these vessels are a compromise between the Japanese
and European trawler types and have been developed
over many years. The bridge is approximately amidships,
leaving a free foredeck up to the forecastle. The mast
carries no derrick but has a gilson for working the fishing
gear. Two wire-reels are installed on either side just
aft of the forecastle for reeling the warps. One warping
head is provided on each side of the bridge for hauling
the gear. At the stern, both to port and starboard, a
roller is installed for leading the warps to the warping
heads.
The engine is normally installed a little aft of midships.
The vessels are fitted with the usual navigational instru-
ments as well as with echo sounders, radar, Loran, direc-
tion finders and radio telephones.
The crew normally numbers 12 men. For general
layout of a bull trawler see Fig. 1 .
c
b
o
n
Fig. L Deck arrangement of bull trawler, (a) bridge, (b) engine room,
(c) galley ; (d) stern roller, (e) stopper, (/) warping head, (g) quarter
'" " (k) bow roller,(l) forecastle.
Most Japanese Danish seiners are wooden vessels of
60 to 80 GT, powered by 250-270 hp engines. Deck
arrangements (Fig. 2) and appliances are more or less
the same as for the bull trawlers, except that the foredeck
is comparatively smaller, leaving a larger working space
aft. Crew numbers 18 usually.
,-,.-. , > W) *'«"« roller, (e) stopper, (/) nwp//v
roOer,(fi)hatch,(t)foremast,U)
424
Fig. 2. Deck arrangement of Danish seiner, (a) bridge, (b) galley,
(c) engine room, (d) warping head, (e) foremast, (/) derrick, (g) bow
roller, (h) hatch, (/) stern roller, (j) buoy, (k) gangway, (I) forecastle.
Fishing gear and methods
Design of bull trawlnets is mainly based on that of the
otter trawl. For details of net construction see Fig. 3
for shrimp and Fig. 4 for flatfish. The sweepline is made
of a combination rope, 45 mm diameter, 200-250 m long,
for operations at depths of 80-100 m. The warp is usually
17-18 mm diameter wire, 400-500 m long and is used
at depths of 80-100 m. To keep the warps close to the
bottom, a length of chain is sometimes inserted between
the warp proper and the combination sweepline. The
net is constructed of polyethylene twine (400 denier
filaments), or of 'Kuralon' twine (20's yarn). (In Figures
3 and 4 for upper bony read bating).
In shooting the gear both vessels take up position at
about 60° on opposite sides of the towing course. The
net boat, with the gear stacked ready for shooting, takes
over from the other boat the warp-end which is fastened
to the butterfly of the aftmost wing. The net is then shot
over the stern and, when both wings are in the water, both
ships steam out on opposite courses until the sweeplines
plus 200-300 m of warp have been payed out. The vessels
then manoeuvre until they are about 400-500 m apart,
set parallel courses and commence dragging. The general
course is normally with wind and/or current and the
tow varies from 15 to 80 min, depending on the catch.
Operated by two vessels, the net obtains a larger
opening than the otter trawl which is a main advantage of
bull trawling. (Fig. 5.)
To haul, the two vessels converge and begin hauling
so that by the time the sweeplines come in they are close
enough for the net boat to take over the sweepline from
the other boat. The warps are hauled in over the warping
heads and wound on to the reels forward. The combina-
tion ropes are coiled on the foredeck ready for shooting
again. As soon as the danlenos reach the rollers on the
stern the one wing is brought round to the bow and the
net is further hauled on the side.
Specifications of the gear used by the Danish seiners
is given in Fig. 6. The seine warps are of manila rope,
27-36 mm diameter, and made up of 12 to 13 coils for
fishing at depths of 300-600 m, or seven to eight coils
if the depth is less than 200 m.
Fig. 3 BULL TRAWL NET FOR SHRIMP
Notes K-Kuralon; P-Polyethylene;
24T- total number of yarns; 240 n/m-
mesh size m/m? H.L.-Head line;
P. R. -foot rope; B.L.-Balch line;
L.L« -lacing line.
BJL F.R
Codend
Not drawn to scale
425
Headlino and
main rope.
Side Panel
Upper Body
Cod-end
not drawn to scale
Cod-end (double)
outside K xkoOT x 10(5 mm.
inside K x300T x 85 mm.
(Cod-end nesh sisse larger
than belly, tvine is larger)*
Fig. 4. Bull trawlnet for flatfish.
426
X1 •
Fig. 5. Outline of bull trawling operation.
With Danish seines the catch is taken on board and
stowed in the hold. Before trans-shipment, about 10 tons
of fish are packed in baskets which are suspended over
the side and held by a strop which is fastened to the
foot of the mast.
With bull trawlers and otter trawlers the codend is
detached from the net as soon as the net is hauled in.
The codend is then kept suspended over the side in the
same way as in the Danish seiners.
In both cases the catcher boats approach to within
about 30 m or 40 m of the mothership. Wire ropes are
veered out from the mothership on to which are fastened
the basket or codend. The catches are then hauled aboard
the mothership. The connecting wire ropes between
the mothership and catcher are normally handed over
by a small boat called "Daihatsu", which also transfers
?ig. 6 DANISH SEINE, Bade of Xuralon (Motet 45T-nuaber of
yarns (yarn units British cotton no. 204) I 75 V»- Mih §!••
»/•» B.L.-Balch linoi H.L.-Head line; L.It.-Laoing ropt
(lacing rope length au« at net length).
HL
J.OOm ' I.SOffi fc<0m
Specifications for Figure 6
Position
Extension wing
Wing
Side panel
Belly
Belly
Square
Square
Baiting
Triangular upper
Triangular lower
Lengthener
Side panel
Belly and baiting
Lengthener
Lengthener
Codend
Codend bottom
Triangle net
Name
Twine size
Mesh size
mm
A
45
120
B
30
75
C
30
75
Di
60
75
30
75
Ej
45
75
Eg
30
75
p
30
75
O
30
75
H
30
75
Ij
45
75
Ig
45
75
j
60
75
K
75
75
L
90
75
M
120
75
N
45
75
No. o
mesh
60
175
175
75
75
100
75
75
75
35
75
110
75
75
75
75
10
Net
length
m
23-00
16-40
13-90
1-20
15-45
0-40
2-40
13-90
25-20
15-60
3-00
3-00
1-50
1-50
6-70
5-70
0-75
No. of
piece
2
2
2
Reference
Flat Knot
2
2
4
4
4
1
2
„ Cut down
H Triangular 2
Sheet bend
Cut down Triangular
Net 4
427
SptdflcatioM for Figure 6
Sinker & Float
(Sinker) China bobbin
Extension wing
Wing
Bosom
(Float) Glass ball
Extension wing
Wing
Bosom
Weight of
one piece Fitting
or Glass pitch
balldia
Total
190 gr 1 by 0-36 m 112
HOgr lbyO-18m 160
110 gr 1 by 2-40 m 20
120mm Ibyl-OOm 40
150mm 1 by 0-75 m 40
150mm 1 by 0-30 m 10
Composition Rope (Material: Manila)
Size
Position dia mm length m
Extension \ving Headline 21 mm 20*00 x 2
Balch line 28 mm 20-00 x 2
Sinker line 15 mm 20-00 x 2
(Groundrope)
Wing Lacing rope 21 mm 11 -20 x 2
Headline 21mm 11-60x2
Balch line 21mm 13-90x2
Groundrope 15 mm 13-90 x 2
Codend Lacing rope 21 mm 6-70 x 4
and Belly Lacing rope 21 mm 6-00 x 4
and Baiting Man rope 18 mm 4-00 x 2
fresh stores, etc. For refuelling the catchers go alongside
the mothership.
For cannery ships, catcher boats normally go alongside
the cannery mothership with careful manoeuvring.
The mothership prepares beforehand the mooring
ropes at bow and stern on her leeward side.
How the Salmon
Fleets Operate
M.Neo
TAPANESE salmon mothership fleets operate in the
J waters around the Aleutian Islands and off the Kam-
chatka Peninsula.
Each mothership has 30 to 36 driftnetters as catchers.
These driftnetters shoot their nets towards evening and
retrieve them the next morning. On board the mothership
catches are sorted by species and processed to canned,
frozen or salted products. Wastes from the plants are
reduced to meal and oil.
The fleets' home ports are in Hokkaido, the northern-
most island of Japan. They attain their allocated catch
quota in about 70 days from early May and return to
their bases late July or early August.
In 1962, 11 motherships operated in the Northwest
Pacific, with 369 driftnetters.
These catcher boats are steel or wooden boats of 75
to 85 GT, and are powered by 280 to 400 hp engines. They
428
are manned by 20 to 22 crew members and equipped
with nethaulers, either mechanically or hydraulically
driven. Navigation instruments consist of radar, Loran,
direction finder, compact gyro-compass and micro-wave
radiophone.
A licence to engage in mothership fishery is given to
applicants on joint applications by the owner of the
mothership and the owners of the catchers to organise
a fleet. The 11 motherships operated in 1962 are owned
by eight different fishing companies. To keep mothership
operations in order, an agreement is made between the
eight owning companies. This is designed to co-ordinate
activities and prevent crowding ; it divides the whole
area into 169 districts, (longitudinally and latitudinally)
each wide enough to accommodate about 40 driftnetters.
No mothership is permitted in more than one district
at a time. The area of these districts varies slightly
between east and west of the 170°25'E longitude
or so-called "Bulganin Line", because of the regulations
specified in the Annexe of the Russo-Japanese Conven-
tion. (Fig. 7.) The right to occupy any district is given
Fig. 7. Fishing area open to mothership operation.
to the fleet which first declares its intention to use it,
prior to its entry. Once declared, that particular district
is closed to fishing by, or passage of, other fleets. The
fleet which intends to transfer to another district must
announce its desire to leave its present district and to
occupy a new district.
The daily fishing pattern of each fleet is decided by the
mothership. This prepares each day a chart indicating
the spots to set nets, and allocates one of these spots to
each catcher boat at its option according to the order of
its arrival for the delivery of catches. (Much of the
success of fishing depends upon the ability of the staff
of the mothership to determine the productive spots,
because salmon are not distributed equally over the
entire district.)
The fishing grounds of catcher boats are very often
far away from the motherships, so that they may have
to go SO miles in extreme cases. Catcher boats usually
start shooting nets around 16.00 hrs and begin retrieving
around 01.00 hrs next morning. By Government
regulations they are required to deliver their catches
aboard their mothership at least once every day.
When vessels find their catch is tapering off, they leave
their territory and transfer to a more promising district.
In deciding their destination, they depend upon the
following information: (a) Catches of other fleets judged
by daily catch reports, which are exchanged mutually.
(b) Reports of scouting boats, of which it is permitted
for each fleet to have three or four to prospect outside
the territory, where other fleets are not operating.
They can operate as many days as they want, (c) Past
experience. The probable routes of the major salmon
runs and the timing of their arrival at different districts
can be more or less predicted from previous experience.
This knowledge is also used by scouter boats.
Some fleets often occupy in advance a favourite dis-
trict in order to forestall others.
Catcher boats and operation
To the east of the "Bulganin Line" a total length of net of
330 nets (15 km long) can be operated by each catcher
boat, while to the west of the same line only 264 nets
(12 km long) can be used. The mesh size of these nets
must be 130 mm and 121 mm respectively; the total
fleet of nets being made up of an equal number of nets
of each mesh size. Before the Russo-Japanese Conven-
tion was concluded in 1956, three different mesh sizes
were used; 118 mm, 121 mm and 124 mm. The 118
Table 1.— Specifications of salmon driftnet (Mesh size: 130 mm)
Webbing
1 . Main webbing
Nylon multifilament 210d/15.
Trawler knot 61 meshes deep and 700 meshes long.
2. Selvage strips and floatline and leadline.
Nylon multifilament top or bottom half-mesh: 210d/15;
second half-mesh: 210d/27; third half-mesh: 210d/45 (mesh
size: 85 mm).
3. End selvage.
Nylon multifilament.
Half-mesh: 210d/39.
Attached to the main web by 210/21 twine.
Ropes
1. Floatline.
Polyethylene monofilament 400 d; 3 x 3 strands, and 13 gm.
Stabilised.
Total length: 51-90 m.
Length of connecting ends: 0-45 m each.
Length on net: 50-00 m.
2. Leadline.
Manila hemp, 2x3 strands, 45 gm, and treated with 'Life
dye.
Coil proof.
Total length: 50-84 m.
Length of connecting ends: 0*42 m each.
Length on net: 50-00 m.
Floats
55 floats, each with 225 g of buoyancy; of synthetic rubber.
Leads
57 pieces of lead, each weighing 75 g.
Hanging Materials
To fasten floats to floatline: 'Kitmona' twine (20 S, 3 x4S).
To fasten leads to leadline: Spun nylon twine (6 S, 3 x 6).
To hang web to floatline : 'Kremona' twine (20 S, 3 x 27).
To hang web to leadline: 55 pieces of spun nylon twine (3 x 24),
each measuring 2-42 m and folded in two.
To connect webs: 'Kremona' twine (20 S, 3 x 54).
Hanging-in ratio
91 m stretched webbing hung to 50 m line.
mesh size was suspended in 1960 and 130 mm mesh size
was introduced to replace the 124 mm mesh size nets.
Whereas the proportion of 130 mm mesh size nets was
initially 25 per cent of the full number of nets to be used,
this proportion was raised to 50 per cent in 1 96 1 . The change
in the legal mesh size was designed to give the smaller
fish a better chance to escape; however, some Japanese
fishermen believe that 121 mm mesh is the most effective,
both to prevent netted fish from dropping out and for
gilling mature fish. For specifications of driftnets see
Table I and Fig. 8a, b and c.
Fig. 8a. Construction of drift nets.
Most driftnets now in use are of nylon multifilament,
but Teteron' (polyester) nets are also in use. Nylon
monofilament nets have been found to be very efficient
but, because of the higher cost and technical difficulties
involved in handling, their use is limited to about 50
to 100 nets per boat. They are interspaced among the
multifilament nets so that the fish are led to the more or
less transparent monofilament nets.
Auxiliary gear
Each catcher boat is fitted out with a nethauler (Fig. 9)
installed on the port side near the bow. In hauling,
only the leadline, which carries most of the weight is
pulled in by the nethauler while the floatline and the loose
net are hauled in by hand. The haulers are mechanically
or hydraulically driven and the hauling speed ranges
from 50 to 70 nets per hour.
429
jRfr. £6. Construction of float and sinker. Float: Material: synthetic
rubber. Main dimensions, depth 38 mm, wW/A (JO mm, length 200 mm.
Volume: 297 cc, Weight: 67 gr, Buoyancy: 234 gr. Sinker: Material:
lead. Main dimensions, length 30 mm, inside dia. 14 mm, outside
dia. 23 mm. Weight 75 gr.
»»tail«d drawing of A
l*ft lay
ri^ht lay
Kuralon 20/27
nylon nultifilaaont 210d/45
nylon multifi lament 21M/27
nylon fflaltifilaacnt 210d/21
210d/15
X
i ." •". I
. & * A j-
v ' \/ »w* *
of B
Nylon multifila»«it 2lOd /21 ) «id*
luralon 20/3/54 - Joining tim»
OetaiUd drawing of c
Kylon »ultifila«mt 210d /15 body
210d /2?
210d /45
•Inktr
Spun nylon 210/24 l«nf th
8.42 • (donblt)
l«ft lay
right lay
Fig. Be. Construction of drlftnets, detailed drawings of sections
marked A,B,andC in figure 8a.
430
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oapaoity 80-120
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There is a net conveyor on the starboard side to
facilitate moving nets from the foredeck to the stern,
where they are stacked ready for the next shooting.
These belt conveyors are mechanically or hydraulically
driven (see Fig. 10).
Although each catcher vessel has an echo sounder,
little use is made of it in fishing since regulations prohibit
the fleet from fishing while moving to new areas.
Operation
The best time to set the nets is immediately before dark.
Boats arriving early usually wait for sunset before shoot-
ing their gear.
When the nets are set much before sunset the current
adversely affects the shape of the nets, while if the nets
are set after sunset the best fishing time is lost. In both
cases, the catch efficiency of the nets decreases. In shoot-
ing, all the nets of each boat are set in one direction.
This is partly to facilitate observation of the regulations
regarding the total length of nets allowed per boat,
as well as the minimum distance between adjacent fleets
of nets. The direction of the set varies according to the
area and season and it is determined in such a way as
will best intercept the run of salmon under normal sea
conditions. When the wind exceeds seven to eight m per
second the nets are shot into the wind irrespective of the
direction of the salmon migration.
During shooting, the nets are led over the roller on
the stern (Fig. 1 1), while the boat has a speed of around
six knots. The roller tapers towards the ends.
Fig. 11. Setting the net using net roller.
To facilitate location of nets, radio buoys and light
buoys are attached to both ends (Fig. 12). They assist in
revealing the general direction of drift as well as in
recovering lost nets.
Hauling normally starts about 01.00 hrs. The boat is
placed so that the nets come in on the port bow while
moving dead slow ahead into the wind. The nethauler
pulls on the leadline while the other parts of the net are
hand-hauled by the fishermen (Fig. 13). After the nets
are cleared of fish they are conveyed to the stern by the
net conveyor.
.*a
Fig. 13. Hauling the net, using the nethauler.
Transfer of catches
While catcher boats are out fishing, the mothership
drifts around the area with current and wind but is
careful not to interfere with the fishing operations. The
exact position of the mothership and catcher boats is
always determined on board the mothership and com-
municated to the catcher boats. Early each morning the
mothership moves to the point where it is most convenient
to receive catches. The new position of the mothership
is communicated to the catcher boats in exchange for
the latter's catch reports. Catcher boats, after they have
completed hauling their nets, return to the mothership
by a direction finder bearing.
Three Methods Used
in Catching Crab
by Nippon Suisan Kaisha Ltd.
IN fleet operations the crabs sought are the schools
moving inshore during spawning migration (chiefly
representing vertical movement) from April through
May, and during feeding migration (chiefly representing
horizontal movement) from late July through autumn.
The search for schools of crab is conducted according
431
Total length 15 km (330 net*)
every 10 nets
Detailed drawing
of A and C
Target
Bamboo
3«6 m
alas s float
(buoyancy 10 kg)
Big flag
(60 om x 45 om)
Southend 2 pcs, Buoy lanp
Northend 1 pc.
North en<
Orange lei
Radio buoy
Power drain 3 W
Call sign at option
Power
0 V
Southend white lamp
about 10 m
about 10m
Stone
(200 gp)
fig. 12. Arrangement o/to/oyj on a fleet of nets.
432
to plans mapped out on the basis of such factors as the
special characteristics of the waters involved, season,
depth of sea, water temperature, etc. Such surveys are
usually conducted at depths of 50-150 m in the Bristol
Bay area, and in the fishing grounds off West Kamchatka ;
but Soviet factoryships operate closer inshore. The
method is to lay short strips of nets at one to three-mile
intervals and check at 12-24-hour spells.
In such surveys, watch is kept on the following points:
(a) distribution of bottom temperature, (b) composition
of currents and tide; (c) depth of water; (d) composition
of school; (e) size of school; (f) composition of legal-size
crabs; (g) quality of legal-size crabs.
Hanging-in: —
On floatlinc: 120m/47-20 m.
On sinkerline: 120m/42-60 m.
Same for a spun nylon net.
Treatment: —
Netting: Resin-t retted.
Float and sinkerlines: Dyed in Canadium.
For spun nylon: —
Netting: Hairs burnt, resin-treated.
Float and sinkerlines: Dyed in Canadium.
Remarks: —
Polypropylene is now regarded as a promising fibre for crab
gillnets because of its light weight and water-repellent property
but it is still in the experimental stage.
For sinkerlines, manila hemp is gradually being replaced by
synthetic fibre, such as vinylon (45-50 gm) and spun nylon
(45-50 gm).
Fishing equipment and method
There are three kinds of crab fishing equipment: (a)
bottom gillnets, or rather tanglenets; (b) trawlnets and
(c) crab pots. Trawling was used on the fishing grounds
of Bristol Bay during 1953/54 but has since been aban-
doned because of the low yield resulting from the intru-
sion of fine-grained sand in the shoulder-meat of the
crabs caught by this method. In Japanese crab fisheries
pots are employed only in a sector of Hokkaido waters
for catching Korean crab.
In the bottom gillnet vinylon is the principal kind of
twine used, with spun nylon also used to some extent.
(For specifications see Table II and Fig. 14.)
Table II.— Specifications of crab gillnet in vinylon and spun nylon.
For a vinylon net, the netting: —
6 meshes: Vinylon 20s/21 z3.
half-mesh top and bottom :
Vinylon 20s/24 z3.
Mesh size: 50 cm.
Net length: 240 meshes.
Depth: 7 meshes.
Knot: English.
For Spun nylon net: —
6 meshes: Vinylon 20s/21 z3.
Half-mesh top and bottom:
Vinylon 20s/24 z3.
Mesh Size:
Net length: (Same as above).
Depth:
Knot:
Hanging Twine:—
Spun nylon 20s/36 z3.
Floats and sinkers are fastened tightly to the float and sinkerlines
at intervals of every third and fourth mesh, respectively, with
hanging twines using the double clove-hitch tying method.
Distance between fastening points: On floatlinc: 59 cm.
On sinkerline: 71 cm.
The same for spun nylon net.
Floatline:—
Manila twine, thickness: 5-7 gm.
Lay: three strands, right twist.
Length of netting in hung measurement: 47*20 m.
End rope on both ends: 67 cm.
Total length: 48-54 m.
The same for a spun nylon net.
Sinkerline:
Manila rope.
Strands: 2x3.
Thickness: 45 gm right twist.
Length in hung measurement: 42*60 m.
End rope on both ends: MO m.
Total length: 44-80 m.
The same for a spun nylon net.
Length of net {
240 •••he»
SiAk«rlin« -*-
Fig. 14. Specifications of crab gillnet.
Other gear components are small glass float spheres
of 8*5 cm diameter, each enveloped in a five gm palm
coir netting to which is attached a four gm manila twine
of 54 cm length. These floats are tied to the fishing nets
at the rate of about 10-12 per net. These floats together
with other auxiliary equipment are shown in Fig. IS.
The equipment required for a set of 200 nets is: 2,400
small glass floats, 1,200 concrete sinkers, eight large
glass floats, two net anchors, four stone sinkers, six
weights for six bamboo poles, two large flags, seven
small flags and 16 lines.
Concrete sinkers.— each weighs one kg, measures
10 cm in diameter and is 7*5 cm thick; to it is attached a
five gm manila twine 60 cm long tied to an imbedded
galvanised wire. Six or seven are used per net.
Glass floats (large). — Each float 36 cm in diameter,
433
43 cm X 43 cm indicate each group of nets in various
colours.
Wooden tags, buoylines, joining lines and anchor
lines complete the equipment associated with the nets.
A nethauler (Fig. 16) is installed on the port foredeck
Fig. 15. (a) Class float (small}, (b) glass float (large), (c) bamboo
pple, (d) weight for bamboo pole, (e) concrete sinker, (/) stone sinker.
is enveloped with a netting of used 45 gm manila rope
or 10 mm diameter straw rope. A loop is formed at
both ends and one is connected to the rope at the middle
of the bamboo pole and the other to the buoyline.
Weights for bamboo poles. — A naturally rounded
river stone of eight to nine kg in netting of used 45 gm
manila twine is attached at the lower end of the bamboo
pole to make it stand upright.
Stone sinkers are attached to the middle part of the
joining lines connecting one group ("ma") of nets (each
"ma" consists of 35-40 nets) with another.
Bamboo poles of 10 cm circumference and approxi-
mately six m length are used.
* Iron stock anchors of 10 kg weight are used for anchor-
ing the ends of nets to prevent drifting.
Flags (large) of either cotton or nylon and measuring
63 cm x 96 cm and of various colours, are attached to
the poles at both ends of each set of nets as fleet identi-
fication markers.
Small flags of cotton or nylon material and measuring
434
Fig. 16. Nethauler.
of both the clippers and Kawasaki's for hauling in the
nets by means of a drum powered by a connecting rod
from the main engine. Usually capable of handling
about 20 nets per hour, the hauler is operated by extend-
ing it over the side at almost right angles to the ship's
centre line and running the net through the groove in
the middle of the drum.
A grapnel anchor, weighing 8-10 kg, is used for drag-
ging the sea bottom for hooking the sinker lines and
nettings with its claws when breakage occurs.
Operation of net
Nets are set when a favourable fishing ground has
been found by a surveying ship. The nets are mainly
set by the clippers, although Kawasaki boats are some-
times used at the outset. After the Kawasaki's have left
early in thfe morning to haul in the nets already laid,
work is commenced on loading the clippers with fishing
gear. Clippers, of 80-ton class, take aboard a day's
supply of nets (approximately 1,400-1,600 nets) and
auxiliary equipment, while the 100-ton class carry a
two days' supply (approximately 2,600 nets). En route
to the net-laying grounds, the nets are stacked on the
poop deck in groups of 35-40 nets each, with the float-
line aft and the sinkerline foreward; while on the
foredeck due preparations are made on the lines.
Upon arriving at the spot where the nets are to be set,
the vessel stops to windward and uptide. Laying speed
will vary with conditions but is usually about four knots.
The time required for laying one set ("ha") of nets (e.g.,
five groups of 40 nets each, or a total of 200 nets) is
between SO and 60 minutes.
After completing one set of nets, the clipper returns
to the starting point along the chain of nets just laid,
which takes about 30-40 minutes, and proceeds to lay
the second set of nets parallel to the first set, keeping a
distance of 250-350 m. For setting and operation of crab
gillnets see Fig. 17.
JUrtor Buoy
Buoj-
•Mtor BUQ/
••t aneter
Fig, 17. Sketch of operation of crab gillnets (example of one set
of 200 nets comprising five groups of 40 nets each).
Nethauling
The nets which have been laid are hauled in after a lapse
of three to four days. While heading for the nets, a
wire basket, 2*10 x 2-40 m (7x8 ft), is spread out in
each of the fish hatches and a table for removing the
crabs from the nets is set up over the hatches. The
hauling operation is usually begun at the uptide and
windward end of the nets. The crabs are removed and
dropped down the hatches. The female and undersized
(less than legal size) crabs are carefully removed and
returned to the water. The Kawasaki boat then returns
to the ship (Fig. 18). Capacity per trip is about 2,000-
Fig. 18. "Kawasaki" boat returning to factoryship with full load of
crabs.
2,400 crabs of 145 mm size. The time extended per trip
is generally about six to eight hours.
Lo&glining
is new and
successful
by Hiroshi Tominaga
Taiyo Fishery Co., Tokyo
MOTHERSHIP bottom longline fish ing has developed
to its present form since 1960 as a result of experi-
mental fishing carried out during the period 1957-59.
The fishing is mainly conducted on the continental slope
in depths ranging from 150-800 m which forms an inter-
mediate zone between the continental shelf and the deep-
sea areas of the Bering Sea eastward of 170°E.
The first operations were mainly carried out in depths
of 150-350 m off the coast between Cape Oliutorsk and
Cape Navarin. Technical improvement to the fishing
gear, especially the mainlines, but also in the operational
technique, has extended fishing to a depth of 800 m and
sometimes even more, so that the operational area has now
been extended as far as the Pribilof and Aleutian Islands,
covering more or less the whole Bering Sea. The species
caught are mainly cod, sablefish, halibut, red rockfish,
arrow-toothed halibut, etc., but the catch composition
of these species depends on the operational depth. Cod
are normally found in depths of 100-200 m, while
sablefish are abundant in depths ranging from 450-800 m
and constitute the largest catch. Operations conducted
in 300-500 m normally result in a mixture of the above
species.
Weather conditions restrict the fishing season to about
five months, from April to the latter part of September.
A fleet normally consists of a mothership, several
catchers and one or two supply-transport ships. The
mothership can be of several types, ranging from 500
GT to 10,000 CRT, and will have a number of operational
catcher boats, according to her size, as follows: 500 GT
mothership— 2-3 catchers; 2,000 GT mothership— 7-8
catchers; 4,000 GT mothership— 15-16 catchers; 10,000
GT mothership— 25-30 catchers.
The mothership vessels are all equipped with freezing
equipment and refrigerated holds, the major part of the
catch being processed into frozen products.
Some of the larger motherships are also equipped with
fish reduction plants and produce fish meal, fish oil and
liver oil from offal as well as from the cheaper fish species
caught.
Supply-transport ships are normally of about 500-1,500
tons. They have large cold storage space for the frozen
products and commute between the home ports at
about 11-13 knots.
The catchers are mostly wooden vessels of 50-100 tons,
with an average of about 85 tons. An 85 GT vessel has
an overall length of 26 m, a beam of 5*50 m and a depth
of 2*50 m, and is powered by an engine of 270-310 hp,
the crew normally consisting of 18*20 men. The vessels
are equipped with wireless, radio telephone, direction
435
finder and echo sounders; in recent years several catchers
have been fitted with radar and Loran.
For operating they have a linehauler on the main deck
and storage space for stacking the longline gear at the
stern. The vessels are necessarily very seaworthy to
stand up to the heavy weather.
Longline fishing gear consists of mainlines, branch-
lines and hooks see Figs. 19 and 20.
Fig. 19. Bottom longline layout.
Fig. 20. Bottom longline assembly.
Owing to the depth at which fishing is carried out and to
guard against chafing on the bottom, the mainlines
normally have a tensile strength of 750 kg. Formerly
made of manila or 'Kuralon' (vinylon) the lines are
presently a mixture of polyethylene and 'Kuralon9
with a weight of about 33 g/m and a diameter of eight mm.
The branchlines are normally made of 'Kuralon9
No. 5/15: A skate of longline is 75-100 m long and the
number of branchlines varies according to the method
of operation. When the hooks are baited while the line is
being shot, a section has 27-30 branchlines; when the
hooks are baited beforehand a section carries 35-40
branchlines. Baiting the hooks while shooting has the
advantage that less bait falls off the hooks during shoot-
ing. The branchlines are 1-20-2*00 m.
A complete longline consists of 250-300 sections of a
436
length of 20-25 km. On hauling, each section is coiled
in a bamboo basket ready for further operations.
The longline gear includes buoys and buoylines,
anchors, bamboo spars, light buoys, radio buoys, etc.
The buoylines are adjusted in length so that the line can
settle on the sea bottom and usually are from 1*3-1-5
times the sea depth, according to tide and/or current.
Marker buoys, carrying lamps and set at appropriate
distances assist in retrieving gear in darkness and when
lines snap. A set of longline normally carries a radio
buoy at each end to locate the gear during fog.
Lines are baited before shooting with defrosted cuttle
fish, or else baits are attached as the line is shot. Shooting
is over the stern and starts about 18.00 or 19.00 hrs.
A stone of about one kg is attached at the junction of
each section. A buoy is attached at every 30-40 sections
and a weight of 10-15 kgs is hung on to the tie-up of the
buoyline to the mainlines. During shooting the vessel
keeps a speed of about seven knots so that a shooting
speed of 200-230 sections per hour is achieved. The vessel
then drifts until 03.00 or 04.00 hrs when hauling starts.
The main marker buoy is picked up and the line led in
over the side roller on the gunwhale to the linehauler.
As each section of line comes in it is disconnected and
coiled in its basket, and carried to the stern for stacking.
Under favourable conditions, the line is hauled at an
average of 27-30 sections per hour. The fish are de-
hooked, sorted by species and stowed in large poly-
ethylene baskets for trans-shipment to the mothership.
Up to the present, echo sounding has not been very
effective in locating fish on the bottom, and fishing
grounds are mainly determined according to the depth,
and past experience. Depth contour lines and the nature
of the bottom serve as additional indicators.
The mothership normally positions herself more or
less centrally to the location of her catchers, with her
port side to windward and pneumatic fenders hung out
ready to receive the catchers alongside. Transfer is
made every day as the catchers moor to the mothership.
During the trans-shipment fuel oil, fresh water, provisions,
bait, fishing gear, etc., are replenished.
Catching Tuna
by Longline
is Efficient
Ooro Okabe
HPHE Japanese tuna longline fishing operations are
JL conducted in three different ways using three distinct
types of vessels.
Individual longlincrs are based on the Japanese home*
land, certain islands in the South Pacific, or operate in
the Atlantic Ocean. These vessels return to their base
at the end of each trip to land and market their catch.
Mothcrship carrier vessels carry up to eight catcher
boats and operate during three to six months in the
Atlantic, East Pacific or Indian Ocean, or conduct
continuous operations in the Atlantic over a period of
about two years.
Mothership fleet vessels are of about 3,000 to 10,000
GT and operate continuously for about six months with
40 to 50 subordinate catcher vessels of 100-200 GT.
These fleets are at present operating mostly in the South
Pacific. The main advantage of this type of operation is
the gain in fishing time. While in the past the catch was
iced and brought to Japan by individual fishing vessels
of 100-200 GT, mothership operation saves the time
spent in steaming to and from the fishing grounds, by
refrigerating the catch on board.
Since the first typical mothership fleet started opera-
tions in 1950, two or three new fleets have been organised
every year, making a total of 45 fleets today. Because
of the decreasing catch both in inshore and offshore
fishing in Japan, the operations are usually conducted
about 4,000 miles from the base.
The law requires that every catcher boat should be a
longline fishing vessel of 40-300 GT. The most convenient
type is of 100-150 GT. Such a vessel can operate for
50-80 days continuously.
A complete longline has about 2,000 hooks divided
into about 400 baskets. The line is shot with the vessel
steaming at nearly full speed and the setting operation
usually begins at 03.00 hours, lasting for about five to
six hours. As soon as the lines are set, the crew go off
duty by shifts. At noon, hauling of the line begins and
this lasts about 10 to 12 hours depending on the catch.
The catcher vessels work this daily routine around their
chosen fishing area. They rarely operate for more than
10 days in one area, nor do they work at more than
1,000 miles away from the mothership.
Fishing gear
Each catcher boat has 350 to 400 baskets of longlines,
all lines being constructed as in Fig. 21 . The total length
of the line is about 130 km, but, owing to the slack given
during setting, the actual distance from end to end while
fishing is only 60-70 km.
Each basket is made to the following specifications:
Length Gouge Material Quantity
50 ply 'Kremona'
50 ply
40 ply
No. 28 Steel
No. 28
Buoyline . .
Mainlines
Branchlines
Sekiyama
Snood wire
Hook ..
23m
47m
11 m
8m
2-5 m
10cm
1
6-7
5-6
5-6
5-6
5-6
Every basket has one 33 cm diameter float to hold up
the lines. Three radio buoys and 13 lightbuoys are
normally used to mark the gear.
A complete set of gear can be used for about 350
operations or about two to two and a half years and
must then be renewed. The vessels are fitted with line
haulers and rollers appropriate to their size.
2
2| 21 21
2 21
7
7 I 7
7 7
' 7
<». *Q1«M float
5. Polo
6. Flat
7* Hook
Mainlino
Sckiyra*
Fig. 21. Details of tuna longline construction.
Bait
Bait is stored in wooden or cardboard cases, each con-
taining 110 pieces. Frozen mackerel pikes of 100-140 g
each are used as bait, the 100 g bait being suitable for
albacore fishing. Freshness is important to prevent it
falling off the hooks.
Concluded on page 438.
Fig. 22. Catcher boat unloading.
437
Las Pesquerias Espanolas Austro-Atlanticas
Exiracto
Todos los precedentes europeos del experimento cspaflol ocurrieron
en el Atlintko norte y aun los mts perfectionados tecnicamente no
salicrofl de los limltes feogiificos cUsicos. En el tiltimo scmestre
de 1961 las dos primeras unidades totalmente congeladoras de la
flota espafiola rcalizaron los viajcs initiates. Se trata de dos barcos
iguatos, de eslora total de 52 m y desplazamiento 880 tons. Uno
se dirigi6 al cstc de la Patagonia y el otto al suroeste de Africa;
una distancia de 5,000 y 4,500 millas respectivamente. Obtuvieron
resuhados espttndidos y en tres meses regresaban a Vigo con una
carga completa de cxceknte pescado congelado. En 1962 se agre-
garon cuatro unidades congeladoras nuevas, algunas de 56-20 m
de eslora y 960 tons de desptazamiento, con motor de 1 ,250 hp, y en
1963 el numero se ha elevado a ocho, estos dos ultimos arrastreros
con rampa a popa. El vi^je redondo es de 90 u 80 dias. Esto supone
la posibuidad teorica de realizar cuatro expcdiciones anualcs pero
en la pr&ctica no scria asequible ni aun si la velocidad se clevara de
los 11 6 12 nudos actuates a 13 6 14 y la base se desplazara al sur
de Espafia en el puerto de Cadiz, Se decidid por ello para aprovechar
hasta el mAximo la capacidad de los barcos, o sea, pescando con
el mismo numero de barcos mis dias, recurrir al mercante frigo-
rifico qua recoge la pesca de los arrafttreros congeladores y la lleva
a la base micntras 6stos contintian la faena o recurriendo al buque-
madre. En el experimento espaflol se ban cmpleado ambas solucio-
nes. El buquc-madre accptara las captures de 10 arrastreros que
lievara el pescado en agua de mar refrigerada a 2°C. Cuatro de estos
10 Iran dotados de motones motriccs Puretic para dedicarlos a las
captures de superficic, aunque tambien llevaran artcs flotantes y de
fondo para usarlos cuando convenga. En los buques mercantes se
instalara equipo de congelacidn para congelar la capture de los
arrastreros que no lo Iteven o que prefieran entregar sin congelar
el producto de los ultimos lances. El buque-madre tendrl una
capacidad de 7,000 tons de pescado congelado, podrf congelar
filctcs y fabricar 1,500 tons de harina y como se confia que pueda
hacer por lo mcnos dos viajcs al afio, sus descargas totales de
producto de primerisima calidad scran considerables e indudable-
mente influiran en los precios de venta al publico de las especies
particularcs de que se trata, que son merluza capensis y hubbsi tan
apreciadas en el mercado cspaflol. Representan estas dos merluzas
el 90 por cicnto de la produccidn. El 10 por ciento restante lo forman
el congrio Colorado, el jurel de gran tamaflo y otras pocas especies
aprovechables mis. De la operacidn austral se esperan enseftanzas
titiles sobre el comportamiento de los artcs de pesca.
The Spanish Sooth Atlantic Fisheries
Abstract
Prior to the Spanish experiment described here, European distant-
by
V. Paz-Andrade
Vigo, Spain
water fishing had been mainly in the North Atlantic, and even the
most technically advanced kept within the old geographic limits.
In the latter half of 1961 two freezer trawlers of the Spanish fishing
fleet left port and set course, one for the grounds of the Patagonian
Shelf in the South-Western Atlantic and the other for South-West
Africa, a distance of 5,000 and 4,500 miles respectively. Both of
them were side trawlers of 52 m total length and 880 tons displace-
ment. About three months later, after a successful trip, they
returned to their base in Vigo with a full load (about 250 tons)
of excellent frozen fish. In 1962 four new freezer side trawlers of
56-20 m total length, 960 tons displacement and main engine of
1,250 hp, were added to the fleet; and in 1963 two more units,
both sterntrawlers of over 1,000 tons displacement, have gone out
to those distant grounds. The round trip takes from 80 to 90 days.
Theoretically four annual trips are possible but in practice this is not
attainable even if the present speed of 1 1-12 knots were increased to
13-14 knots and the base were transferred to the southern Spanish
port of Cadiz. In view of that it was decided to increase as much as
possible the production of the vessels by keeping them permanently
on the fishing grounds and transferring the catch to refrigerated
merchant ships or to a mothership. Both methods are being used in
the Spanish experiment. In the refrigerated merchantmen, to which
the catch will be trans-shipped in the nearest convenient port,
freezing equipment has also been installed. The mothership, a
converted passenger ship, will take the catch of ten trawlers which
will be fitted with seawater tanks, refrigerated at— 2°C. Four of these
trawlers will carry midwater and bottom trawls as well as Puretic
power blocks for surface fishing. The capacity of the mothership
will be 7,000 tons of frozen fish plus 1,500 tons offish meal which
will be processed on board. She will also be equipped with filleting
machinery. As it is hoped that she will be able to do two round
trips per year, well over 15,000 tons of quality products will be
unloaded in Vigo. About 90 per cent of the catch is hake (Merluccius
capensis and M. hubbsi). The other 10 per cent is made up by king-
klip (Genypterus capensis), very large horse mackerel, smooth
(Macruroplus nigromaculatus) seasonly, and a few other commercial
Continued from page 437
Mothership and catcher boats communicate with
each other three times a day on special wave-lengths.
Wireless operators are on duty in shifts all round the
clock.
Every catcher boat (Fig. 22) reports on the following
subjects twice a day:
(a) Before dawn: Number and weight of catch
previous day, classified by species; operating
position, weather, wind, temperature, water tem-
perature, programme (bound to fishing grounds;
to mothership; shifting fishing grounds; setting
lines; hauling lines; ceasing, etc.).
(b) lo the evening: Noon position by observation,
weather, wind temperature, water temperature,
hour and course of setting line^ number of hooks
used, number of operations and direction and
velocity of current.
438
Catcher boats are generally equipped with thermo-
meters and electric thermographs for surface water,
water colorimeters, echo sounders, etc. Very few are
equipped with bathythermographs, hydraulic barometers
and thermometers.
Catcher boats pay great attention to the movements
and operations of other boats fishing in the adjacent
areas and inform each other of catch rates by radio.
Although echo sounders detect the "deep scattering
layer'*, tidal shifting and even single bodies of tuna at
times, their usefulness has not been finally decided as
yet
The mothership usually appoints two or three catcher
boats as research boats to find richer fishing grounds.
They survey the surface and deep-water temperatures,
etc., while conducting operations for themselves.
The mothership decides the adequacy of fishing grounds
and gives instructions after corrrfatinf the information
received from research boats aad catchers.
species. It is expected that, from these southern operations, some
usefUl experiences on the performance and effectiveness of fishing
gear may be gained.
La peche dans to Md Athurtkpie Etpagnoi
Avant 1'expdriencc espagnole decrite ici, la peche hauturiere
europeenne elait principalement pratiquee dans 1'Atlantique du
Nord et mfcme les plus avances techniquement se tenaient dans les
ancicnnes limites geographiques. Au cours du deuxieme semestre de
1961, deux chalutiers fngonfiques de la flotte espagnole quittaient
Vigo, Tun se dirigeait vers les fonds du plateau Patagonicn dans
1'Atlantique du sud-ouest, 1'autre vers 1'Afrique sud-ouest, £
une distance de 5,000 milles et de 4,500 milles, respectivement.
Les deux chalutiers itaient de 52 metres de long, d'cnviron 880
tonnes et operaient par le cote. Trois mois plus tard ils rentraient a
Icur base avec 250 tonnes de poisson congele excellent. En 1962,
quatre nouveaux chalutiers frigorifiques, chacun de 56-2 metres de
long, 960 tonnes et ayant des machines de 1,250 cv furent ajoutes
£ la flottc et en 1963, deux autres unites de plus de 1,000 tonnes, ces
dernieres op6rant par 1'arriere, sont parties pour ces lieux de peche
lointains. Le voyage aller-retour durant de 80 a 90 jours, theorique-
ment, quatre voyages par an sont possibles mais dans la pratique
cela est impossible, meme si la vitesse de 1 1 a 12 noeuds des bateaux
est augmented jusqu'a 13 ou 14 noeuds et la base transferee a
Cadix, au sud de 1'Espagne. On a done decidd d'augmenter la
production des bateaux autant que possible, en les laissant constam-
ment sur les lieux de peche et en transbordant la capture sur des
bateaux de commerce frigorifiques ou sur un bateau-mere. Les
deux mdthodes sont utihsecs dans cette experience espagnole.
Les bateaux frigorifiques de commerce ont ei& pourvus de Tequipe-
ment necessaire et les captures seront transporter au port le plus
proche. Le bateau-mere un paqucbot converti, prendra la capture
de 10 chalutiers equipes avec des reservoirs a eau de mcr refroidie a
— 2°C. Quatre de ces chalutiers seront equipes avec des chaluts
peiagiques et des chaluts de fond aussi bien qu'avec des poulies
"power blocks Puretic" pour la peche a la surface. La capacit6
du bateau-mere est de 7,000 tonnes de poisson congele et de 1,500
tonnes de farine de poisson produite a bord. Le bateau aura aussi
des machines a fileter. On espere que le bateau-mere pourra faire
deux voyages par an pendant lesquels 15,000 tonnes de produits de
premiere qualitd seront decharg&s a Vigo. A peu pres 90 pour cent
de la capture se compose de merlus (Merluccius capensis et M.
hubbsi). Les autres 10 pour cent des kingklip (Genypterus capensis},
tres grands saurels saisonniers (Macruroplus nigromaculatus) et
quelques autres especes commerciales. On espere que de ces opera-
tions dans le sud on obtiendra une experience pratique concernant
la performance et I'efficacite des engins de peche.
TIL fen6meno de mayor calado, que esta produciendose
JQ en la economia pesquera mundial, parece identi-
ficarse con el cambio de perspectiva y dimension en el
uso de los recursos del mar. Las ideas de corto vuelo
espacial sobre la accesibilidad a la apropiacidn estan
perdiendo vigencia. Simultdneamente, ciertas posiciones
consagradas sobre determinados modelos de explotacidn
comienzan a quedar desplazadas por otras mucho mis
dinamicas.
Hasta hace pocos aflos este proceso revestia la forma
de mero alargamiento lineal de la 6rbita de pesca.
Siempre con las limitaciones propias de las unidades
navales de tipo tradicional. Tal es la frontera quebrada
por el impacto de la tecnologia modcrna. La proyeccion
que ista adquiere puede constituir una respuesta con-
vincente a las penurias de la "overfishing" y una contri-
buci6n positiva a la lucha universal contra el hambre.
Para cllo sent necesario que al incrcmento de la
potencialidad de captura y consiguiente movilidad de los
factores se afiada otro elemcnto. El de una mejor redis-
tribucidn de los recursos sin mengua de su conservacidn.
Dentro de la nueva tendencia ha despertado interes en
la esfera international la expansi6n hacia el hemisferio
sur emprendida desde base espaftola. El desarrollo de
tal experiencia no ha llegado al apogeo, pero ya permite
obtener algunas deducciones de naturaleza t&nico-
econ6mica con significado orientador para la evolucidn
futura de la pesca a larga distancia.
Reladon entre factor natural y factor ttoiko-ecooomico
Comencemos por analizar el papel de las fuerzas produc-
tivas dentro de la modcrna dindmica de las pesquerias.
La relaci6n existente entre el factor bio!6gico y el factor
tecnico, como aferentes a la formation del producto
bruto, viene ahora evolucionando hacia el predominio
del segundo.
Antes eran las condiciones naturales favorables las
que predcterminaban el mayor rendimiento. La anchura
de la plataforma arrastrable, su inmediacidn a las bases,
la concentraci6n permanente o estacional de biomasas, la
adhesi6n litoral de ciertas corrientes profundas y en
todo caso la distancia a las dreas mas ftrtiles, ban jugado
papel decisivo en el crccimiento de las grandes potencias
pesqueras.
Copiosos ejemplos podrian patentizar la prioridad
asumida por los favores de la naturaleza en el desarrollo
de las mas prestigiosas estruc:uras industriales cimentadas
sobre las riquezas del mar. Algunos como el Jap6n,
Peru, la U.R.S.S., Noruega, Suddfrica, Canada, Islandia,
etc., parecen ejemplos tipicos. El primero, el tercero y
otros han dejado de serlo.
Sin que el factor bioldgico perdiera su fuerza como
clave del rendimiento, el factor t£cnico-econdmico
asume cada dia nueva preponderancia. Incluso los paises
que encabezan la escala mundial de los grandes produc-
tores no apoyan todos su jerarquia en la vecindad de las
fuentes. En cambio, siempre gana terreno la inversidn en
equipos dotados de movilidad y capacidad suficientes
para tornar accesibles grandes masas de recursos que se
tenian como situ ados fuera de la drbita de una explota-
ci6n rentable. La relaci6n equipo-rendimicnto acentua
cada vez mas su prioridad.
El lanzamiento de nuevas flotas sobre el mapa de los
mares responde ahora al principio de la dispersi6n
contrapuesto al de la concentraci6n operacional. Se
dirige hacia fondos densamente poblados, aunque
resulten remotos. Norteamirica puede considerarse
precursora de este despliegue al establecer una base
atunera en Samoa y al enviar "tuna clippers'* hasta el
archipelago de Galdpagos recorriendo mds de 3,000
milias. Despu6s, pasando del Pacifico al Atldntico,
establece bases industriales en Puerto Rico y Africa
ecuatorial. Desde el mar Amarillo el Jap6n se proyecta
sobre el Pacifico americano y australiano primero, y
despuis se introduce en los hemisferios norte y sur del
Atldntico. La U.R.S.S. canaliza su expansion hacia las
mismas latitudes llegando hasta los bancos de Terranova.
Y Espafia, al armar su primera flota congeladora, la
destina a los fondos remotos del Atldntico austral1.
CompmAla armadora: Peicanova S.A.
439
El fciwSmcno cuya noct6n acabamos de esbozar presu-
ponc la adopcufa de en&gicos cambios en el sistema
pesquero tradicional. Cambios que afectan a la estructura
tanto como al modus operandi. Aunque tengan su raiz
en la fase de extraccidn, sus cfectos extravasan el marco
primario. Se propagan a los ciclos siguientes del proceso
productive sea el de transformacidn sea el de comerciali-
zacidn.
Un enfoque que restringiera los tirminos del problema
a recoger una mutaci6n estructural de los cquipos de
captura, resultaria incompleto. Cierto que en la concep-
cidn constructiva y funcional del buque de pesca y del
aparejo acoplado al raismo, asi como de la maniobra
de tales elementos mecinicos, es donde se hace mas
visible el impacto de la innovaci6n tdcnica. Pero esta
innovaci6n reclama otras. Al implantarse en el sistema
introducicndo una dimensidn nueva, los demis compo-
nentes de aqud ban de someterse a un proceso paralelo de
adaptation.
La nccesidad de poner en claro esta relaci6n interna,
en la actualidad debe considerarse perentoria. Un
respetable y copioso conjunto de intereses industrials se
vcrk directa o indirectamente implicado en el viraje.
Es de esperar una reaccidn de tales intereses ante los
incentives que el fendmeno ofrece. Pudiera resultar
desafortunada si se apoyara s61o en el deslumbramiento
que origina el avance tccno!6gico olvidando los demis
aspectos, especialmente los ccon6micos.
Ni la posibilidad de incorporarse a nuevos modelos de
cxplotacidn, ni la complcjidad que revisten cuando situan
su base operational sobrc los recursos de mayor lejania,
son privativas de determinado pais. Siempre las ventajas
de la Iocalizaci6n jugarin su papel con mayor o menor
incidencia segun los casos. Pero la barrera del radio
de acci6n del buque impucsta por la capacidad de los
tanques de combustible, o la velocidad de crucero, ya no
constituyen freno reductor de su 6rbita de trabajo.
Dentro del orden de ideas que comicnza a moldear
en el mundo un nuevo pensamicnto sobre la producci6n
alimenticia de la mar, las cnsefianzas de la experiencia
espaftola pueden resultar orientadoras. Se trata del
testimonio mis reciente sin que haya contribuido a
forjarlo la ortodoxia tradicional. Tiene el valor de un
ensayo deliberadamente concebido para no reincidir
en las posiciones heredadas. Responde, por el contrario,
al afin de superarlas, mediante la apertura de nuevos
horizontes para la produccidn pesquera.
Antes de ponerse en ejecucidn el experimento espaflol
otros paises del mundo pesquero nordatlintico como el
Reino Unido y Alemania Occidental, habian introducido
en las estructuras clisicas injertos renovadores. Como
tales hay que citar siempre a los buques-factoria, los
trawlers congcladores, la incorporaci6n a unos y otros
del arrastre y maniobra del arte por la popa, la propul-
si6n diesel-etectrica y otros.
Estas innovationcs a pesar de su audacia t&nica no
habian alterado sustantialmcntc la geografia pesquera
del continentc. En £ste no fueron aprovechadas antes de
440
que Espafia las adoptara para asegurar la accesibilidad a
nuevas fuentes de recursos cuya plenitud bio!6gica
prometiera altos niveles de producci6n, aun a expenses
de costos multiplicados. Esta doble condici6n valoriza y
singulariza la aportaci6n de Galicia a la evoluci6n del
sistema pesquero occidental.
Compatlbilidad eotre disdntos tamafios de empresa
El trinsito del patr6n clisico de explotaci6n al modelo
altamente evolucionado comienza por alterar la estruc-
tura de la empresa. Asi como del artesanado adherido
a la pesca litoral se efectu6 el despliegue hacia la unidad
industrial capitalizada de tamaflo pequefio o medio
preferentemente, otro ciclo de la evoluci6n esti en
marcha. El nuevo se caracteriza por una combinaci6n
a escala mucho mayor de los factores productivos
comenzando por el capital fijo.
La tendencia cuya proyecci6n describimos no supone
ni mucho menos que la empresa armadora pequeiia y
media haya agotado su misidn en la economia pesquera.
Asi como el artesanado precapitalista ha sobrevivido
hasta hoy incluso como vivero humano vocacional del
desarrollo industrial, las formas menores de la empresa
pesquera — individuates, cooperativas o sociedades mer-
cantiles — mantendran su actividad sin limite visible en
el tiempo.
L6gicamente, con mayor horizonte de prosperidad
sobre el futuro. Primero, porque en proporci6n tambten
aprovecharin los frutos del avance tecno!6gico general.
Despuds, porque seguirin operando sobre recursos
localizados en mayor proximidad a las bases con bajos
costos de transferencia. Y, finalmente, porque la dis-
ponibilidad de los mismos debe resultar espontinea-
mente acrecentada al disminuir la presidn extractiva
que sobre las mismas areas habrfan de ejercer las unidades
atraidas hacia caladeros lejanos.
Se trata de esferas distintas del desarrollo pesquero
perfectamente compatibles. El crecimiento de una de
ellas no debe repercutir en contracci6n o encogimiento de
las otras. Debe, por el contrario, favorecer su vitalidad
dentro de las lindes correspondientes al modelo de
explotaci6n elegido.
Incluso desde el punto de vista del mercado donde la
evoluci6n hacia la macroempresa conduciri a un
aumento copioso de la oferta. Sea 6sta, o no sea, creadora
de su propia demanda, el deficit de alimentos proteinicos
que la humanidad aun padece, absorbera cada dia
mayor proporci6n de los que gratuitamente entrega el
mar a diario.
Dimensiones principales de la macroempresa
En la configuraci6n de la empresa que esta surgiendo
predestinada a proyectarse mis alii de las fronteras
que se habia impuesto la industria tradicional de las
pesquerias, domina sobre los demis caracteres el del
crecimiento de sus dimensiones. Tanto el de las dimen-
siones econ6micas como el de las tdcnicas. Y asi con
referenda al volumen como a la complejidad de las
estructuras. Para ofrecer una imagen mis demostrativa
del fendmeno, conviene analizar, aunque sea sumaria-
mente, sus elementos cscnciales:
(•) Ainnento de kM cotfos de flnandacUn.— Pant
emprender la construcci6n de unidades especiales, cuyo
calculo, disefio y equipamiento respondan a prototipos
tdcnicamente avanzados, la primera barrera a veneer
surge de la magnitud de la inversi6n. Serd necesario
realizarla en proporci6n que duplique o triplique el nivel
ordinario del costo de financiaciOn. Esta proporci6n
aproximada puede admitirse como valedera en la relaci6n
entre el moto-trawler tradicional y el moto-trawler con-
gelador, y entre el primero y el "stern freezer trawler",
con aquellas demasias de capacidad en los segundos que
reclaman el prototipo al cual se ajusten y la tasa de
rentabilidad esperada.
Sobra decir que en el calculo de rendimientos las
deducciones de costos para retribuir el capital de pris-
tamo, adquieren mayor incidencia. La que corresponde
al interns y al plazo de reembolso cuya doble gravitaci6n
debiera producirse en relaci6n inversa y directa, respec-
tivamente, al volumen de la inversi6n y la vida probable
de la unidad financiada.
(b) Aumento del nivcl de capadtaddn ttarfca.— Dentro
de los prototipos modernos el elemento maximo de
diferenciaci6n radica en el uso del frio industrial. El
montaje a bordo de la congelaci6n rdpida, sea para
parte de la carga, sea para la totalidad, bien mediante
tuneles circulados por salmuera o chorros de aire helado,
bien utilizando armarios divididos con placas para enfriar
por contacto, determina la necesidad de aptitudes
profesionales especificas. Tanto para el entretenimiento
del equipo frigorifico, su manejo y reparation, como para
el proceso de los productos capturados que supone la
creaci6n inmediata de una mercancia distinta mucho
mis resistente y de mas intacta composici6n que la
almacenada entre capas de hielo.
La especializaci6n tambidn recae en la maniobra del
arte. Tanto por cambio de la forma y materiales con
que est£ confeccionado, como porque intervenga mayor
automatismo para largar o izar, ya sea usando rampa
trasera o pdrtico basculante, ya sea utilizando "power-
block" para aparejos de superficie u otro dispositivo
mecanico destinado a la misma maniobra.
En un tercer aspecto, los elementos destinados a la
orientation ndutica, la telecomunicaci6n y la detection
de bancos, deben entrar en este recuento. Aunque se
trate de dispositivos de utilizaci6n general necesitan
extremar su eficiencia y afinamiento cuando se instalen
en buques llamados a grandes recorridos donde los
errores de enfilaciOn, los retardos en la localization de
los bancos, las colisiones previsibles, etc., pueden irrogar
considerables pgrdidas.
(c) Asunddn del proceso de commerdalizaddiu— La
funci6n de la empresa armadora traditional puede
darse por terminada al entregar sus productos al licitador
que los remata en la subasta. Cuando la oferta se corn-
pone de mercancias no sujetas a descomposici6n acelera-
da, la situaci6n es distinta. Uno y otro tipo de oferta
se destinan al mismo consumidor, pero el sistema de
distribution forjado para la comertializaciOn del
pescado fresco no siempre resultard utilizable para los
productos congelados.
Primero, por resistencia al cambio de la estructura
comertial preexistente. Despu6s, porque al proceso de
fijaciOn que deriva de la congelation rdpida, tambife
contribuye a la estabilidad de precios, asimilando los
productos, espetialmente si se lanzan con presentaciOn
homologada, a los manufacturados del ramo de la
alimentation.
Para que la empresa armadora pueda independizarse
de la comercializaciOn directa, seria necesario disponer
de organizationes generates de distribution apoyadas
en la cadena nacional del frio. Donde no existan atin,
tambiin el tamafto de la empresa que desarrolla la
production primaria habrd de ensancharse hacia el
mercado, si quiere defender con eficacia el ingreso
derivado de la venta de sus productos, y contribuir al
equilibrio del sistema general de precios.
La necesidad de que sea asumida la distribution por
la unidad productora, diferird de un pais a otro. La
circunstancia de que en Espafia se haya producido tal
fenOmeno obliga a pensar que, con mayor o menor
agudizaciOn, podrd repetirse alii donde las posiciones
adquiridas a favor de la especulaciOn con el pescado
fresco hayan hecho inviable por el momento la creation de
alguna organizaciOn distribuidora especializada y autO-
noma.
LA opdOn cntrc don hemkferiof
Todos los prcccdentes europeos del experimento espafiol
tuvieron como escenario el Atldntico norte. Aun los
que alcanzaron mayor dimension t6cnica no ampliaron
la dimension geogrdfica dentro de la cual venia operando
la flota clasica. Las unidades mds modernas — buques-
factoria, semicongeladoras, congeladoras, "stern traw-
lers", etc. — volvieron a calar sus aparejos en los fondos
sometidos desde hace siglos a la mayor intensidad de
captura.
Esta evoluciOn se iniciO por el Reino Unido al mediar
la ddcada de los aflos cincuenta. Fue pronto secundada
por Alemania Occidental. No obstante, cuando en
1960 se concibiO la empresa y se planeO la flota, que
desde un puerto de Galicia habia de introducir cambios
radicales en el modelo de explotaciOn hasta entonces
vigente, la experiencia centroeuropea sobre buques
congeladores era corta y mds timida que alentadora.
Pronto se echO de ver que en la flota europea de factura
novisima, al gran adelanto t£cnico y a la magnitud de la
inversion, no correspondfa un aumento espectacular
de la productividad marginal. Parccia claro que la
causa del fenOmeno debia atribuirse exclusivamente a
la baja concentration de la biomasa demersal en los
lugares explotados.
Asi se perfilO con claridad la optiOn entre los hemis-
ferios norte y sur. Entre el norte cercano, con sus
dreas biolOgicamente fatigadas, y el sur lejano, mucho
mds lejano, pero con una disponibilidad de recursos
superdensa y partialmente ociosa. El dilema se resolviO
en favor de la solution mis arriesgada en cuanto suponia
una initial desorbitatiOn de los costos operatives.
En el ultimo scmestre de 1961 las dos primeras unidades
441
totalmcntc congcladoras de la flota espafiola realizaron
los viajes inicialcs. Uno a los fondos de la plataforma
preantirtica que se extiende al este de la Patagonia.1
Otro a los fondos de la plataforma del surocstc africano.3
Durante el afio siguiente cuatro nucvas unidades,4
tambiin congeladoras, y algunas de mayor capacidad,
se sumaron a las primeras. En 1963 el numero se ha
etevado a ocho, las dos ultimas con rampa para el
arrastre per popa.* Esta reincidcncia en la Iocalizaci6n
ultralejana de las opcraciones, desarrollada por la
misma empresa armadora, constituye un tcstimonio
convincente de que la experiencia austral desde bases
europeas puede considerarse consolidada.
Este acontecimiento inicia un cuarto camino. Las
rutas pesqueras de Europa ya no terminan en el mar de
Barentz o en Jan Mayen al norte, en Terranova al oeste
y en Dakar o Freetown al sur. Mas alia de este
limite se abrieron nuevas fuentes para la despensa
occidental. No es posible prever las dimensiones que
podra alcanzar en el future la expansidn basada en las
reservas de cnergia biologica del Atlantico austral. De
cualquier modo, el estadio en que la experiencia se halla,
aun siendo inicial, pcrmite obtcner, en terminos econo-
micos, algunas consccuencias orientadoras.
Reducdta de UMI eortot de operacton
Las principales son dos. Consiste la primera en la necesi-
dad de reducir al limite las "external diseconomies'9.6
Desde el paralelo 43 N-Vigo al 40 S o a 33 S, con escala
Motopesquero congdador 44Lcmos".
Motopesquero congelador "Andrtde".
"PtrobnT, "Doncos", "Soutomaior" y "Sobroso".
"Vfflalba" y "Vitmiwizo".
C J. Bottemanne D.: "Principles of Fisheries Development,**
en Dakar, cada viaje suponc un rccorrido de 5,000 6
4,500 millas. Sumando ida y vuclta, ticmpo de opcraci6n
sobrc los cakderos, recaladas en algiin pucrto pr6ximo
y dcmoras imprevisibles, la duraci6n media de cada
expedici6n no debe calcularsc en menos de 90 dias u 80,
segun se trate del destine mis o menos lejano. (Fig. 1.)
Esta media supone tedricamente la posibilidad de
realizar cuatro expedicioncs por afio. Tal objetivo no
serfa alcanzado en la prdctica aunque la base se desplazara
al paralelo de Cddiz. Tampoco seria asequible probable-
mente aunque la velocidad de crucero de las unidades
se elevara de 11 6 12 nudos a 13 6 14. Los tiempos
perdidos en la descarga del pescado, restauraci6n de
desperfectos del buque y afinaci6n de su maquinaria,
aprovisionamientos, etc., suponen al afio aproximade-
mente la mitad de la duraci6n atribuida a cada viaja
redondo.
Comparando la operaci6n ejecutable en el escenario
austral y la misma en cualquiera de los escenarios
nordicos, se descubre inmediatamente la forma dispar en
que una y otra funcionan. Nos referimos a operaciones
emprendidas desde la misma base europea.
No es ncccsario entrar a discriminar los factores que
juegan en cada una, ni su diferente gravitaci6n en los
resultados. Bastard destacar que en la primera debe con-
cederse prioridad al volumen de los costos unitarios
mientras que en la segunda asume papel principal el
volumen de la producci6n bruta por viaje. Ambos
factores estdn sujetos a fuerte fluctuaci6n, agudizada por
el lado de los costos totales.
En la rcduccidn de los mismos reside la clave del
problema. Asi para moderar los riesgos como para
elevar la tasa de productividad marginal. Tal objetivo
en la operaci6n de base austral no resultaba atacable
ni por compresidn de los suministros, ni por supresi6n de
eventuales arribadas, ni por minoraci6n de capitulo de
salaries y participaciones.
La solucidn mas acorde a la ortodoxia econdmica
y el interns social era otra. Podia hacerse consistir en
el aumento sustancial de la producci6n unitaria logrado
sin acudir al empleo directo de mayores dosis de capital
fijo. O sea, pescando con los mismos buques durante
un numero mayor de dias, entre el viaje de la base al
caladero y el de vuclta. No aplicando a la flota conge-
ladora el esquema dinamico adoptado por la fresquera.
Hacicndo, por el contrario, uso ma's id6neo y prolongado
de la superioridad de mcdios comenzando por dar a su
empleo la maxima continuidad operativa en la mar.
Para desplegar tal estrategia era indispensable conceder
entrada a un nuevo elemento en el modelo de explotacidn.
Asf, el circuito que nacid homog6nco se hizo mixto.
El modus operand! amplia su desarrollo incluyendo el
trasbordo in situ, o en pucrto inmcdiato. A la flota
pescadora se asocia el mercante frigorffico. Este zarpa
para la base con la carga congelada mientras aquilla
continua la faena.
Dcntro de la organizacidn armadora que protagoniza la
experiencia europea en las regiones austro~atlinticas, la
442
introducci6n de los trasbordos en la rotaci6n de los
"trawler's'*, pudo suponcr la rcducci6n del cincuenta por
cicnto, o poco menos, del numero de transferencias
directas por aflo. No quicrc esto decir, ni mucho menos,
que la misma proporcidn valga para medir el incrcmento
de productividad resultants. Ni siquiera por tosca
aproximaci6n. Lo que proporciona es una positiva
contraccidn de las deseconomias propias de la pesca a
largufsima distancia, permitiendo diluir el costo total
forzosamente elevado, en un numero mayor de unidades
producidas.
Al mismo tiempo, la f6rmula acrecienta la regularidad
de la explotaci6n en cualquiera de sus posibles versiones.
Tanto puede ser realizada por la simple conexidn
sincronizada de pesqueros y mercantes, como por
vinculacidn de aquillos a alguna unidad madre. En el
ejemplo espaflol que sirve de eje al presente estudio,
ambas soluciones fueron puestas en prdctica si bien con
independencia entre una y otra.
La flota de "freezer trawler's" habia de tener a su
servicio aquel numero de cargueros frigorfficos que su
nivel de actividad exija. Alguno de £stos sera congelador
para admitir cuando sea necesario carga de pesquero
que no lo sea.
El buque madre centralizara las descargas de una
flotilla de diez "trawler's" ambivalentes con tanques
para conducir el pescado en su medio natural, refrigerado
a— 2°C. Cuatro unidades iran dotadas de "power-block
puretic" para dedicarse a las capturas de superficie,
aunque todas resultardn aptas para alternar aqu£llas con
las de arrastre, demersal o a medias aguas.
No hay experiencia evaluable, hasta el momento,
para calcular el indice de productividad relativa de
una y otra combinaci6n. Ambas responden a las mismas
premisas economicas y completan, por ahora, la estruc-
tura del modelo de explotaci6n desarrollado desde base
espaflola en el hemisferio antdrtico. El factor velocidad
a plena carga — 14-5 nudos en los mercantes y 17 en el
buque madre — seri preponderante en una y otra opera-
ci6n.7
Calidad, aceptabilidad y rentabilidad
La segunda de las deducciones a que antes apuntamos
se obtiene en relacidn a la morfologia de la producci6n
fisica y su aliciente en el mercado. No cabe subestimar
este aspecto no operacional del problema si se quiere
adquirir del mismo una visi6n rclativamente completa.
Poco importaria descubrir en otro hemisferio reservas
-copiosas y casi intactas de especies marinas accesibles
desde bases europeas si transferidas al mercado de
destino despertaran insuficiente aceptabilidad.
Para montar un modelo de explotaci6n a base de
caladeros ultralejanos, ademds del indice elevado de
concentraci6n es necesario encontrar recursos relativa-
mente valiosos. La rentabilidad depende tanto de la
7 Se esta transfonnando en buque madre congelador el ex-
trasatlantico de 16,000 tons. "Habana". Tendra capacidad para
congelar y almacenar 4,500 tons, de pescado, transformar una
parte en fitetcs y produtir en planta reductora propia 1,500 tons,
de harina de pescado por expediddn.
cantidad como de la calidad, pero la apreciacidn de la
ultima difiere de mercado a mercado. Este principio
explica que los langosteros de Bretafia se desplacen hasta
las aguas prdximas al Nordcste brasiteao- No justificarfa
en cambio, que los "freezer trawler's" de Galitia se
desplazasen a Mar del Plata para pescar anchoita o jurel.
Es de presumir que el catdlogo de recursos sufitiente-
mente concentrados en el habitat de las regiones austro-
atldnticas, sea por ahora incomplete. La variedad real
ird mostrando sus valores efectivos, a medida que la
explotaci6n se intensifique y ensanche. Probablemente,
bajo la cortina del desconocimiento eventual, queden
grandes masas de peces y crusticeos fines que algun dfa
localizard la investigaci6n empirica o cientifica.
Mientras tanto hay algunos motives para sospechar
que la expansi6n pesquera de Europa hacia los anti-
podas pueda resultar frenada por cortedad de la gama
faunistica, Nos referimos a especies de alto valor
apreciable en este viejo mercado.
Tanto en la regi6n sudeste como en la sudocste del
Atlantico, el recurso ictioldgico de mayor masividad efc
las merluza (sockfish). La capensis a un lado, a otro la
hubbsi. Cubren el noventa por ciento del volumen de
las copadas en la latitud respectiva. En la composition
del diez restante suelen entrar permanentemente el
kinglip — "congrio Colorado" — en Sudam£rica, el jurel
de gran tamafio, estacionalmcnte el smooth (macruroplus
nigromaculatus) y pocas especies aprovechables mis.
Esta reducida variedad es menos acentuada en las
especies de superficie — albacora, otros esc6mbridos,
sardina, anchoa . . . — pero tiene innegable importancia
para las pesquerias de arrastre. El area del consumo
preferencial de merluza es mis restringida que su aerea
bio!6gica. En el Japdn se desecha. En Norteamerica se
subestima. En la Gran Bretafia no tiene especial aprecio.
Su campo favorito es Europa meridional y Espafta
especialmente.
He ahi otra razon que contribuye a explicar la vincu-
Iaci6n a este pais de la experiencia austral y su impaciencia
por iniciarla. Es notorio que los demds paises del mundo
pesquero occidental tendrian que partir de otras premisas
tanto de naturaleza operacional como de indole mercato-
16gica.
£1 aparejo como factor ccondraico
De la operaci6n austral en su desarrollo sucesivo se
esperan ensefianzas utiles sobre el comportamiento de
los artes de pesca. Algunas pueden referirse a la dimen-
si6n de la malla en los de arrastre. Otras al uso de los
dispositivos que facilitan o accionan la maniobra. Bajo
ambos aspectos pueden acusarse en el sur diferencias
ticnicamente valorables respecto a los mismos procesos
en el escenario n6rdico.
Volviendo al primer extrcmo convicnc advertir que la
dimensi6n de la malla en una y otra latitud se ajusta a las
medidas aprobadas por la Convention de Londres.
Han side calculadas, como es sabido, para proteger la
auto-renovaci6n de los recursos bent6nicos o filo-
bentdnicos, permitiendo la evasi6n de los ejemplares
inmaturos. Esta garantia, intcrnacionalmente respetada,
443
asegura la compatibilidad de la explotaci6n mis intensa
y la den&idad de todos los "stocks'* vivientes. Incluso
ppdri cstimular el incremento de este ultimo indice, al
disminuir la concurrcncia tr6fica de ejemplarcs tan
talludos como voraces, pcro ya est&iles.
Con independencia del aspecto aqui considerado,
donde la concentraci6n es grande y el valor comercial
esti en relari6n directa con el tamafio— como en el caso
de la merluza — pudiera resultar escasa en el sur la
dimensi6n del mallaje establecido como minima para
el norte. La raz6n econ6mica, en este caso, puede ir
mis alii de la raz6n bio!6gica ya que la ampliacidn del
rombo liberatorio del pez constituyc un medio anticipado
de selecci6n comercial.
Las mismas circunstancias pueden influir en los resulta-
dos de la maniobra del arte por la popa. Es Idgico
deducir que alii donde la concentraci6n sea baja, el
coeficiente de productividad logrado con la rampa o el
pdrtico basculante rcsulte relativamente superior al
acostumbrado. En tal supuesto, el mayor niimcro de
lances por Jornada proporcionado por la mecanizaci6n
de la maniobra puede compensar la eventual flojedad de
cada uno.
No ocurriri lo mismo cuando el volumen de la cosecha
sobre la cubierta y su tasa de aprovechamiento para
consumo humano, no dependa del numero mayor o
menor de copadas. Queda excluida la hip6tesis del
suministro para fabricaci6n de subproductos, ora sea
realizado a bordo, ora sea rcalizado en tierra.
En el primer supuesto, si se trabaja sobre bancos bien
poblados y circunstancias atmosftricas normales, el
numero de lances por dia no determinari mayor produc-
tividad. Entonces la actividad desplegada por la mari-
ncria en la selecc!6n y preparaci6n de las piczas, y el
ajuste entre este factor y la capacidad de congelaci6n
instalada en el buque, parecen ser los elementos condi-
cionantes del nivel de rendimiento efectivo. Donde no
operen estos factores, especialmente el ultimo, la renta-
bilidad de la inversi6n representada por los dispositivos
de maniobra del arte, sera sin duda mis elevada. Sin
embargo, tal apreciaci6n s61o debe considerarse vilida
cuando la captura se ejecuta con Cotter trawl". No en
el supuesto de operar en superficie utilizando mecanismos
semiautomaticos para halar y largar el aparejo.
Tambiin puede la utilidad de aquellos dispositivos
resultar realzada al permitir trabajar con mal tiempo como
es frecuente en aquellas rcgiones.
fi) Sin que el factor bio!6gico deje de mantener su
prioridad, en el desarrollo modcrno de las pesquerias
el factor ticnico adquiere papel predominante dando
origen a modelos de explotaci6n mas arriesgados, en los
cuales el principio de la dispersi6n espacial se contrapone
al de la concentraci6n del esfuerzo en las ireas ya intensa-
mente explotadas.
(ii) Los cambios que tal evoluci6n comporta no se
limitan a la estructura de los equipos si bien en el buque
de pesca, sus artes y la maniobra de ambos, se haga mis
visible el impacto de la innovaci6n tecno!6gica. Las
aplicaciones de la misma precedieron en algunos paises
centro-curopcos al experimento cspaftol que inicid la
explotacidn de mares australes, pero s61o el ultimo puso
en evidencia la accesibilidad a nuevas fuentes de recursos
en plcnitud de producci6n.
(iii) El modelo evolucionado de explotacion presupone
la organizaci6n de la gran empresa, pero su compatibili-
dad con la de tipo pequefio y medio se veri favorecida no
s61o por la menor gravitacidn de los costos, especial-
mente de transfcrcncia, sino por mayor reponibilidad de
las poblaciones explotables en proximidad al desviarse
una parte del poder de captura hacia caladeros muy
lejanos.
(iv) La estructura de la macroempresa responde al
incremento de sus dimensiones principals. Este fen6-
meno engloba el aumento de los costos de financiaci6n,
el del nivel de capacitacidn tlcnica en el factor humano
y el de la asunci6n del proceso de comercializaci6n en
los paises no dotados de algiin sistema id6nco de dis-
tribuci6n de productos congelados.
(v) Para responder a la magnitud de la inversi6n y
el aumento en los demis factores es necesario cuidar la
estrategia de la explotacidn. Dentro de ella en el experi-
mento espafiol se planted la opci6n entre el hemisferio
norte y sur como espacio operacional resolviindose a
favor de la soluci6n mis arriesgada. Esta decisi6n supuso
la apertura de las pesquerias austro-atlinticas desde base
europea utilizando flota congeladora para la totalidad de
la carga.
(vi) Dada la excepcional longitud de los viajes y sus
elevados costos, se advirti6 pronto que en la reducci6n
de estos residia la clave del aumento de productividad.
En consecuencia la operaci6n debia oricntarse a eliminar
dfas inactivos haciendo uso mas prolongado de la
superioridad especifica del buque congelador sobre el
fresquero.
(vii) Tal necesidad econ6mica determine la introduc-
ci6n de nuevos elementos en el modelo de explotacion
ligando la flota pescadora, mediante trasbordos, al
servicio de buques de transporte frigorificos. Con
fundamento en el mismo principio econ6mico se puso
en ejecuci6n un proyecto de buque madre, con flotilla
auxiliar de diez unidades pescadoras polivalentes para
arrastre y pesca de superficie, dotadas de tanques de
agua de mar refrigerada a — 2°C. para conducir la pesca*
(viii) Aunque la gama austral de especies marinas no
esti totalmente conocida, los recursos de relativa con-
centraci6n en cuanto a especies de fondo se reducen a
poco mis de la merluza hubbsi y la capensis cuya acepta-
bilidad en el mercado espafiol favorecid el experimento.
El resultado serfa distinto si se destinara a mercados
indiferentes o subestimatorios de dicha especie.
(ix) Sobre el comportamiento de los artes de pesca,
la operacidn austral pudiera permitir con fin selective
la ampliacidn de las dimensiones de mallaje autorizadas
por la Convcncidn de Londres. En cuanto a los disposi-
tivos para facilitar la maniobra del arte, dada la densidad
de la biomasa, las limitacioncs de rendimiento mis
pueden proceder de la manipulacidn previa a la congela-
ci6n y la capacidad de tratamiento del pescado, que del
aumento en el numero de copadas en circunstancias
normales.
Discussion on
Fleet Operations
Mr. G. C. Eddie (U.K.) Rapporteur: Fleet operations
are not new, Portugal, Canada and the United Kingdom
having in their different ways practised the technique for over
300 years. But under modern conditions it has revived and
developed to meet special needs with particular concentration
on motherships although in some instances bigger trawlers
with enlarged storage accommodation would seem to meet
the same needs.
While the Japanese in some cases are using merchantmen
for transporting fish from distant bases, that system has not
found favour in British thinking for three reasons — the
availability of cargo space in the right place at the right time,
the cost of transfer, and the small amount of low temperature
refrigeration space available.
While mothership operations give greater fishing time, this
was partly offset by the cost of the mothership unless ade-
quately compensated for by smaller and simpler catchers.
This called for study.
The requirement for daily delivery to the mothership of
some species was dictated by the need for quality, thus lifting
the subject into the realm of fish processing technology,
rather than of gear. In the transfer of catches, the Germans
found that redfish could be transferred successfully through
detachable codends even after several hours' immersion, but
the British found that the quality of cod was noticeably
poorer even after half an hour's immersion and of gutted
cod, after a few minutes.
The British, in their experiments, found no difficulty in
transferring catches between trawlers by various methods in
at least up to force six winds. The Japanese papers describe
their transfer procedure in both "fine" weather and "rough"
but it would help understanding if the definition of these
terms could be given in the Beaufort scale of wind strength
and sea state. Pictures of Japanese "rough" conditions did
not seem very rough by North Atlantic standards.
Their papers showed that successful fleet operations required
a complete exchange of information between the catchers so
that each was working for all. I think the Western Europeans
have something to learn here and I congratulate our Japanese
friends for the considerable contribution they have made to
the Congress by their papers.
While the motherships have their place it is noteworthy
that European nations, including the Russians, have tended to
use independent freezing trawlers and factory trawlers rather
than motherships. It would be interesting to have more detail
of economic information in justification of motherships and
in particular as to whether with modern propulsion equipment,
much large, independent vessels freezing their own catch were
not possible. Directions from motherships lead to greater
efficiency in many cases; since the best British distant water
trawlers caught up to three times as much as the poorest
improvement by direction might be as effective and easier to
achieve than improvements in the gear itself.
Mr. V. Pmz-Andrade (Spain): The Spanish experiment in
conducting fleet operations in South American and South
African waters represents a technological effort within a
framework of costs. The provision of all refinements had to
be reflected in increased production. We also had to consider
size in relation to economy. We first considered the construc-
tion of our freezer ships with a capacity of 250 to 300 tons
and two further ships with a load capacity of 3 50 to 400 tons and
two further sterntrawlcrs with a capacity of 1,000 tons
each. For 20 months the eight ships have been in commission;
and we have experience of the operation of the first six of
these units. But as these experiments have developed, in spite
of the short term concerned, we have seen that the ships have
to move over distances of 5,000 miles and we have found that
it would be more profitable to introduce freezer transport
under the conditions described by the rapporteur rather than
receive the catches direct at home ports from the fishing ships.
Thereby it was possible to increase the time spent by the ships
on the grounds and that will increase the flow of fish.
The second part of the programme consisted in the building
of a factory and mothership. This is planned by the conversion
of an ex-passenger liner of 1 5,000 tons. This ship in association
with the fleet of 10 auxiliary ships will be versatile and highly
developed. Naturally, their operations will include the lessons
which have been learned from the Japanese experiences with
motherships. The factoryship will have capacity for 5,000 tons
of frozen fish, 2,000 tons of fish meal (processed aboard)
and about 500 tons of frozen fillets. The fleet will be made up
by 10 auxiliary ships which can be used for trawling and either
line fishing or seining. Four of these ships will be equipped for
handling gear and almost all of them will have refrigerated
salt-water tanks. Naturally we do not have the experience yet
concerning these items which are still under construction but
we think this unit made up of a mothership and 10 fishing
ships will provide us with a very profitable operation in spite
of the very high investment. This whole venture is a result
of private enterprise and is operated without any subsidy.
The ships are fitted with comfortable accommodation suited
for all problems that may arise. The accommodation gives
the crew, in addition to high wages, all the necessary comforts
they need and which it is possible for the firm to provide.
The Spanish experiment may provide one or two lessons
on the possibility of opening up these new grounds for provid-
ing more vital food. The species actually caught so far consist
mainly of hake and whiting for which there is much demand
in Spain. That could be a limiting factor from the marketing
point of view for countries which do not like that type of
fish. Generally speaking they felt that in this long-range
fishing enterprise they were exploring an attractive field
which made a dual contribution to the fishing economy of
the world. It would relieve those fishing grounds which have
been worked over, possibly to exhaustion, and would enrich
those sources which are available by increasing the profit-
ability of new grounds. They felt that they were helping
Europe in this way.
Dr. Miyazaki (Japan) : I can give a general idea of the Japan-
ese fleet operations. The first is that because of the long dis-
tances that have to be covered in our fishing operations for
tuna, salmon and crab, the industry has to rely considerably
upon motherships and the concentration of the catch from
each catcher and by processing it immediately on the mother-
ship. By this method, the total tonnage of the catch during
the season has been considerably increased: in the case of
salmon to 53,000, with tuna to 221,000 and even in crab
trawling they have achieved a total tonnage of something
like 3,000 in a season. A second point of value is that
445
very good directives can be given by the mothcrship to
minimise inefficiency that might be caused by disorderly
fishing. During the off-season the mothcrships are normally
used as cargo carriers while the catchers are used in other
operations. The tonnage of motherships in tuna fishing is
something like 4,000 and there are normally five catchers of
something like 100 tons each.
Mr. Eddie, summing up, said the supplementary contribu-
tion to Spanish thinking was important and it emphasises
how fishing was governed by economics and that they varied
between country and country. The additional information
from Japan about fleet operations was valuable as justification
for the use of motherships and their contribution to the greater
efficiency of the fleet
M. B. F. Ranken (U.K.): wrote that he was
concerned at the lack of accurate information on average
and maximum catching rates by different methods of fishing
and on different fishing grounds throughout the world. Most
information given by vessel owners or skippers was based on
hearsay or on very good days and tendcJ to be grossly in-
flated—real fishermen's tall tales! Also lacking was informa-
tion on average and maximum fish sizes, weights and thick-
nesses on the various grounds; in many cases the exact
varieties caught were uncertain due to the wide variation in
names for identical or similar fish over quite a small area.
Cases were known where the names varied from village to
village.
When planning the size of fishing vessels and the equipment
to be provided for catching and processing, it was essential
to have accurate information on catching rates as well as
on fish sizes and variety. This information was even more
essential when planning fleet operations since they affected
not only the factory/mothership but also the number and
type of catchers, the logistic support required and many
other aspects, technical, commercial and economic.
A Fishing Gear Congress seemed to him to be a logical
occasion on which to make a plea for accurate statistical
information on these and related subjects to be collected and
correlated under the auspices of FAO. He hoped that this
work could be put in hand with the help of delegates to the
Congress and others. It was also important that it should be
kept up to date as conditions or methods changed in particular
fisheries both large and small.
446
INDEX TO ADVERTISEMENTS
SECTION 2
Page
ASSOCIATED ELECTRICAL INDUSTRIES LTD 457
ATLAS WERKE A. G, 451
BENN, CHAS., & SONS LTD. 453
BERGENS, A/S, MEKANISKE VERKSTEDER 465
BLOCTUBE CONTROLS LTD. 454
BLOUNT MARINE CORPORATION 461
BROWN, S. G., LTD. 450
DECCA NAVIGATOR Co. LTD., THE 468
DECCA RADAR LTD. 458
DUBIGEON, SOCIETE, ANONYME DES ANCIENS CHANTIERS 452
ELECTROACUSTIC GMBH 465
FAO FISHERIES STUDIES 459
FAO YEARBOOK OF FISHERIES STATISTICS . . . . 458
FISHING NEWS (BOOKS) LTD. 462 and 464
FURUNO ELECTRIC Co. LTD. 460
HEIGHWAY, ARTHUR J., PUBLICATIONS LTD. 466
HUNDESTED, A/S., MOTOR & PROPELLER FABRIK . . 467
KELVIN HUGHES Div. OF S. SMITH & SONS (ENGLAND) LTD. 449
LISTER BLACKSTONE MARINE LTD 467
MACTAGGART SCOTT & Co. LTD. 466
MARINE CONSTRUCTION & DESIGN Co. . . . . 463
MIRRLEES NATIONAL LTD 452
MUSTAD, O., & SON 462
NETZFABRIK AHLERS GMBH 466
NORTHERN RADIO Co. 462
POLYFORM 456
S.C.A.M 460
SIMONSEN RADIO A/S 455
447
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452
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454
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FAQ
YEARBOOK OF FISHERY STATISTICS
Trilingual. First published in 1948
The most important internationally-published source of world-wide statistical
tabulations covering, on a calendar year basis, catches and landings, fishing craft,
disposition of catches, production of preserved and otherwise processed fishery
commodities and imports and exports of fishery commodities. Special efforts are
made to publish within approximately 10 months of the dose of the last calendar
year covered. The YEARBOOK data for many countries appear months before the
data are released nationally.
Until the end of 1963 the Production and Fishing Craft volume appeared annually
and the International Trade volume biennially. The latest volumes are:
Volume 13
Volume 14
Volume 15
International Trade
1960-61 data (published 1962)
Production
1961 data (published 1962)
Production and Fishing Craft
1962 data (published 1963)
In 1964 and subsequent years two volumes will appear annually. Volumes scheduled
for publication in 1964 and 1965 are:
Volume 16 Catches and Landings 1963 data
Volume 17 Fishery Commodities 1963 data
Volume 18 Catches and Landings 1964 data
Volume 19 Fishery Commodities 1964 data
The YEARBOOK is supplemented by a trilingual series of Bulletins of Fishery
Statistics. Numbers are issued from time to time and are available in a very limited
number of copies from the STATISTICS SECTION, FAO FISHERIES DIVISION,
VIA DELLE TERME DI CARACALLA, ROME, ITALY.
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WORLD FISHERIES ABSTRACTS
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value and quality control. A representative coverage of the world literature is achieved
through a world-wide system of editors and/or abstractors. More than 600 periodicals
in 26 languages are regularly searched for each issue. The average issue contains about
140 references, abstracted on 69 cards, convenient for filing (3 x 5 in. or 76 x 127 mm.),
plus two pages of technological notes. Abstracts can be filed alphabetically by author,
subject, UDC and FWS code systems. To assist the reader to obtain copies of references,
a List of Periodicals Searched is published from time to time. For the same purpose,
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FISHERIES ^ 1 l. STUDIES
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in English, French and Spanish editions.
FAO FISHERIES STUDIES No. n
Financial Assistance Policies and Administration for Fishery Industries
by E. S. Holliman, 1962, 121 pp. Price: Si.oo or 58. or FF3,5O
This is the third FAO publication on financial aid schemes Drawing on his experience as Assistant Chief Executive of
in fishery development. The study is based on discussions the White Fish Authority, in London, Mr. Holliman
and papers contributed to the FAO Technical Meeting on covers many of the finer points of administrative decision-
Credit for Fishery Industries, held in Paris in 1960. making in fishery industries.
FAO FISHERIES STUDIES No. 10
Costs and Earnings Investigations of Primary Fishing Enterprises
by A. E. Ovenden, 1961, 60 pp. Price: $0.75 or 35. 9d. or FFa,70
A study of the main concepts and definitions of the Meeting on Costs and Earnings of Fishing Enterprises,
principal elements of primary fishing economies, with held in London in 1958, which noted that lack of uniformity
definition of terms commonly used in costs and earnings in fisheries organization seems to act as a barrier to agree-
studies and with the application of those definitions to ment on many fishery concepts. Mr. Ovenden was formerly
practical work. The book resulted from the FAO Technical Statistician with the White Fish Authority, London.
FAO FISHERIES STUDIES No. 9
Financial Assistance Schemes for the Acquisition or Improvement of Fishing Craft
by C. Beever and K. Ruud, 1960, 89 pp., 3 appendices. Price: $1.00 or 55. or FF3,50
A compendium of selected schemes in 13 countries with field. Mr. Ruud is a former FAO staff member; Mr.
developed fisheries, preceded by a discussion of their Beever is Acting Chief, Economics Branch, FAO Fisheries
principal features; this is the first study published in this Division.
A limited number of the following earlier FAO Fisheries Studies is still available:
No. 8 La Science Appliqu£e aux PSches Int£rieures
1959 Aplicadon de la Ciencia a la Pesca Continental (Out of print in English) $0.50 or 2s. 6d. or FFi,75
No. 7 La PSche & I'aectricit*
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1957 French and Spanish editions) $1*50 or 7*. 6d. or FF5,25
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1957 (English, French and Spanish editions) $1*25 or 6s. 3d. or FF^o
Information on specific world areas:
The Proceedings and Technical Papers of the General Fisheries Council for the Mediterranean have been published
biennially since 1952 in bilingual English-French editions. The proceedings of the sixth session (1961) are available at
$4.00 or 2os. or FFi4,oo. The seventh session proceedings are in press.
The Proceedings and Technical Papers of the Indo-Pacific Fishenes Council have been published in English since 1949.
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and III) are in press. Complete sets of proceedings are still available for the sixth, seventh, eighth and ninth sessions.
Both the GFCM and IPFC have additional documentation for specific fisheries problems in their areas.
Aside from its publications, the FAO Fisheries Division also produces documents in the following series: Fisheries Papers;
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TERME DI CARACALLA, ROME, ITALY.
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Japan's World Success in Fishing by Georg Borgstrom
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Most complete compendium of information yet published on
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STERN TRAWLING
This book resulted from a symposium organised by the White Fish
Authority (U.K.) held at Grimsby, 1963. The important papers given
and discussions thereon, together with plans and illustrations and
other supplementary material, are included to make the volume the
most complete single survey yet published on this subject.
Stem Trawling Design by H. Heinsohn of Rickmers Experiences with the Fairtrys by B. C. Sealey, B.A.
Werft, Shipbuilders, Bremerhaven. (Cantab.), Fairtry Manager, Chr. Salvesen and Co.
Ltd., Leith.
Data of stern trawlers of 175 ft. to 225 ft. illustrate desirable
features of deck layout and general arrangement. The
economic limit of centralised control, in reducing manpower,
is set by the manpower required to mend nets. Stability
and values of reserve buoyancy discussed for various types
of vessels.
Based on a decade of operation, the functional efficiency
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made for the future.
Future Design of Small Vessels for Fuller Development
Norwegian Experience with Stern Trawlers by S. M oiler, of the Fishing Industry by E. C. B. Corlett Ph.D., Burness,
A. S. Bergens Mekaniske Verksteder, Shipbuilders, Corlett and Partners, Marine Consultants, Basingstoke
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Details of design and performance of two stern trawlers built A thoughtful, far-seeing, vision of the future, based on practical
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How to Make and Set Nets by John Garner.
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Introduction to Trawling by A. Hodson
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PART 3
TECHNICAL RESEARCH
Section 13 — Gear Research
The Theory of Designing and Testing Fishing Nets in Model Tasae Kawakami 471
The Development of a Midwater Trawl P. Dale and S. Moller 482
Fishing Methods and Gear Research Institutes : Their Organisation and Scope . . A. von Brandt 489
Discussion on Gear Research 493
Section 14 — Instruments for Testing Gear
Trawl Gear Instrumentation and Full-Scale Testing J. Nicholls 497
Some Japanese Instruments for Measuring Fishing Gear Performance Chikamasa Hamuro and
KenjilsMi 513
Trawl Studies and Currents J. N. Carruthers 518
Performance of the Granton Trawl W. Dickson 521
Discussion on Instruments for Testing Gear 525
Section 15 — Fish Behaviour
The Importance of Vision in the Reaction of Fish to Driftnets
and Trawls J. H. S. Blaxter, B. B. Parrish, and W. Dickson 529
Importance of Mechanical Stimuli in Fish Behaviour, Especially to Trawls . . C. J. Chapman 537
The Use of Air-Bubble Curtains as an Aid to Fishing Keith A. Smith 540
Utilisation of Fish Reactions to Electricity in Sea Fishing Conradin O. Kreutzer 545
Problems of Electro-Fishing and Their Solution Jurgen Dethloff 551
Notes on the Importance of Biological Factors in Fishing Operations B. B. Parrish and J. H. S. Blaxter 557
Tuna Behaviour Research Programme at Honolulu . . John J. Magnuson 560
Shrimp Behaviour as Related to Gear Research and Development . . ..Charles M. Fuss, Jr.,
andF. Wathne 563
Experimental Dispersion of Chum Kentaro Hamashima 570
Evolution de la Peche a la Lumiere dans les Lacs Africains A. Collart 573
Pump Fishing with Light and Electric Current /. K. Nikonorov 577
Discussion on Fish Behaviour . . 579
Section 16 — Science and the Future
Prospective Developments in the Harvesting of Marine Fishes Dayton L. Alverson and Norman J.
Wilimovsky 583
Automatic Data Processing and Computer Use in Fisheries . . .. Benjamin F. Leeper 590
Discussion on Advance in Science and the Future 591
469
470
Part 3 Technical Research
Section 13 Gear Research
The Theory of Designing and Testing Fishing
Nets in Model
Abstract
The paper describes the theory of model experiments and gear
construction and is meant for those fisheries specialists who are not
familiar with the mechanical and mathematical theories involved.
Once the basic fishing method has been decided on, in accordance
with the commercial scale of the industry, the fishing ground and
other physical and economical factors, the design of the gear can
be undertaken. Based on knowledge of these factors the net design,
the shape and tension distribution can be predicted by analytical
methods. However, due to the difficulty of obtaining exact informa-
tion on the operational conditions, the net designer normally works
from rough estimations and it is therefore often an advantage to test
the design in model form. Such model tests can only be successful
in so far as the conditions assumed when applying the modelling
laws were correctly estimated.
The paper describes in an easily understood manner the calcula-
tion of the weight, resistance and shape of netting in water as a
first step in analytical gear design. The mechanical similarity
between full scale and model nets are then explained according to
the laws developed by Prof. M. Tauti (1934).
Tauti's laws, however, do not include suitable modelling laws
for ropes and certain auxiliary gear. The author worked out
mechanical similarity rules covering ropes, floats and sinkers used
in fishing nets, and provides mathematical explanations of these
rules.
The paper furthermore leads on to equations for calculating the
parameters of large nets from smaller prototypes.
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 mathe-
matical calculations. As yet, we have only a very incomplete
knowledge of many of the complex factors involved. However,
thanks to recent technological advances, we are learning more about
the behaviour of fishing gear underwater and the reaction of fish
to it, and the challenge is to continue our research towards develop-
ing the general principles of analytical gear design.
Dessins de filets et epreuves de leurs modeles rtduits
Resume
La presente etude qui decrit la theorie des modeles r&duits et la
construction d'engins de peche est destinee aux techniciens qui
n'ont pas de connaissances mathematiques et mecaniques appro-
fondies.
Pour projeter le dessin d'un engin de peche il faut, partant de la
methode de base qui sera appliquee, tenir compte de Fimportance
de Findustrie, des lieux de peche et autrcs facteurs physiques et
economiques. Connaissant ces facteurs, il est possible de prevoir
par des methodes analytiques, le dessin du filet, ses formes et la
distribution des tensions. Cependant, les conditions-mfemcs de
reparation 6tant difficilement previsibles, il est toujours preferable
de faire des 6preuves avec des modeles r&duits. Ces Epreuves seront
d'autant plus satisfaisantes que les conditions d'operations auront
M estimdes justement.
L'etude decrit clairement la facon de calculer, d'apres les moyens
analytiques, le poids, la resistance et la forme que le filet prend dans
Feau. La similitude mecanique entre I'engin en vraie grandeur et
son modele est expliquee suivant les lois deVeloppees par le Pro-
fesseur M. Tauti (1934). Mais ces lois ne s'appliquant pas aux
cordages ou aux engins auxiliaires, I'auteur a 61abore des lois de
similarity mecanique pour les cordes, flotteurs, plombs utilises
dans la construction d'engins de peche et donne ^explication de
ces lois.
Dans cette 6tude egalement sont donnees les formules necessaires
pour calculer les parametres d'un grand filet en partant des mesures
de la maquette.
L'ancienne methode de tatonnements utilisee pour la fabrication
des filets a recemment fait place a une mfthode plus mathematique
bien que certains facteurs tres complexes de ce probleme soient
encore mal connus. Plus recemmcnt encore, les rccherchcsconccrnant
by
Tasae Kawakami
Department of Fisheries, Kyoto
University
le comportement d'engins de peche en action et la reaction des pois-
sons vis-a-vis de ces engins ont fait de grands progres et nous nous
sommes rapproches du but qui consiste a deVelopper les principes
gene>aux concernant le dessin analytique des engins de peche.
La teoria del proyecto de artes de pesca y del enrayo de ms modek*
Extracto
Describe el autor la teoria de la construcci6n y los experimentos con
modelos de artes de pesca ; se dirige a los especialistas pesqueros que
no cstan familiarizados con las teorias mecanicas y matematicas
que entran en juego. Una vez que se ha decidido el metodo de
pesca fundamental de acuerdo con la importancia comercial de la
mdustria, el caladero y otros factores fisicos y econ6micos, se
comienza el proyecto de los artes. Basandose en el conocimiento de
dichos factores, la forma y distribuci6n de la tcnsidn de la red puede
predccirsc por m&odos analiticos, aunque debido a la dificultad de
obtener informaci6n exacta sobre las cpndiciones de trabajo, el
proycctista parte de estimaciones aproximadas por lo que puede
convenir ensayar un modclo del proyecto. Estos ensayos solamente
pueden dar resultados satisfactorios si al aplicar las leyes de model-
ismo se acierta en las suposiciones de las condiciones reales.
El autor describe de manera facil de comprcndcr el calculo del
peso, de la resistencia y de la forma de la red en el agua como
un primer paso en el proyecto de redes analitico. A continuaci6n
la similitud mecanica entre los modelos y la red a toda escala se
explican segun las leyes del Prof. M. Tauti (1934). Sin embargo,
estas leyes no comprenden las relatives a cables y cierto material
accesorio. El autor formulb leyes de similitud mecanica que com-
prenden cables, flotadores y plomps empleados en las redes de
pesca y da explicaciones matematicas de estas leyes; menciona
tambi^n ecuaciones para calcular los pardmetros de redes grandes
a partir de prototipps pequeftos.
En el pasado casi todo el material de pesca se ha construido
siguiendo m6todos de tanteo, pero en los ultimos aftos se ha comen-
zado a complemcntar el enfpque empirico con calculos matemiticos.
Todavia tenemos conocimientos muy incpmpletos de los muchos
y muy complejos factores que entran en juego, pero gracias a los
recientes adelantps t6cnicos se va conociendo mejor el comporta-
miento del material de pesca en el agua v las reacciones de los peces
ante 61, lo que estimula a continuar las investiyaciones para formu-
lar los principios generales del proyecto analitico de los artes.
1. INTRODUCTION
HPHIS paper aims to describe the theory of model experi-
JL ments for fishing gear specialists who may not be
completely familiar with mechanical and mathematical
theory. A special attempt will be made to discuss these
theories on the basis of elementary mechanics.
To devise new fishing methods or improve old ones,
one must first study fish behaviour and environment.
471
Despite modern echo sounders, underwater cameras
and other aids, these studies are still relatively new and
difficult and the net designer must often work from
incomplete biological data.
Next compare fishing methods and choose the basic
method which best fits commercial factors, type of
fleet operation, fishing 'ground location and other
essential factors.
Gear designing begins only when this fundamental
method has been chosen. It begins with accumulation
and analysis of previous data. Dr. Terada and co-
workers were among the first to test the resistance of
plane netting towed through water. Since then, there
has been much experimental and theoretical work on
fishing gear mechanics.
Tauti's law on the hydrodynamic resistance of netting
and his model theory are based on these assumptions :
(a) that the elongation of the netting twine is negligible9
(b) that the netting twines are perfectly flexible,
(c) that Newton's law of hydrodynamic resistance is
valid for every portion of the net, irrespective
of its Reynolds' number, and
(d) that any 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.
Recently the writer added some extensions to the
modelling law for ropes in the nets and for cases in
which inertia cannot be neglected because of accelerated
motion caused by surface waves.
If net design details and operating conditions are
given, the shape, attitude, and tension distribution
can be predicted by analytical methods, with no need for
physical testing. The author and co-workers have found
this analytical treatment of some practical value for
trawl nets. The difficulty is that, because we must find a
set of solutions by solving simultaneous equations involv-
ing transcendental functions, the numerical computations
needed to solve the equations for the analysis are long
and tedious. A further difficulty is that the exact opera-
tion conditions of the fishing ground can seldom be
determined. This means that the net designer must
work from a rough estimation of the total situation.
Because of this, it is an advantage to be able to test
designs in model.
Some doubts have been expressed on the practical
value of testing fishing nets in model. These model tests
depend on the accuracy of the modelling law and will be
successful only in so far as conditions assumed in the
modelling law are fulfilled. Test effectiveness also
depends on strict observance of certain rules.
One of these is to ensure, as far as possible, mechanical
and geometrical similarity between the full-size net and
its model. Even if all conditions are properly satisfied,
the mechanics and geometry of the full-size net may not
be identical. These differences must be expected and
allowances made for them.
Because the modelling law itself is imperfect and its
conditions are difficult to comply with exactly, the
472
fidelity of model experiments is limited. Thus, the next
steps are to correct design faults noticed in the model
tests, build the gear to full scale and test it under opera-
ting conditions. This final test is necessary, not only
because of model limitations, but also because we may
be ignorant of true operating conditions such as waves,
sea bottom or underwater currents. It is often impossible
to duplicate such conditions in model tests.
Modern gear tests demand sturdy and reliable under-
water instruments with self-recording or telemetry
appliances.
2. NET STRUCTURE
The mesh is the smallest unit of a fishing net. A mesh
is a rhombic opening enclosed by four bars of twine of
equal length firmly knotted at the four corners. Fig. 1
shows how one bar is related to two meshes, how one
knot is related to four meshes and how two bars and
one knot are the basic elements of mesh structure. This
last point is seen clearly if one considers that in weaving
a net by hand, one mesh is completed by making two
bars and one knot.
Although a good net is flexible, it resists over-stretching.
The net can be made to take any form but, in doing so,
cannot exceed the total length of the individual bars.
Thus the net shape changes in accordance with the
distribution of external forces acting upon it. This
makes accurate knowledge of these forces imperative.
The area of a single mesh can be changed by hanging
or mounting the webbing to the frame line, i.e., by
changing the angle between two adjacent bars to achieve
a more "open" or more "closed" mesh. The mesh
opening can be expressed by this angle, 7, as shown in
Fig. 1.
Bar
Mesh
Mtsh
Fig . L Construction of mesh.
If L is the length of a bar, the mesh area A is given
by:
A = L2 sin 2<p (I)
A stretched piece of netting may be considered as a
continuous and homogeneous membrane (through
which water can pass like a sieve) if the differences in
physical state between neighbouring meshes are negli-
gible. In actual conditions, this membrane of netting
will be subjected to two kinds of external forces, i.e.,
the hydrodynamic force due to the current and the
apparent weight in water. When the change of motion
of the net occurs so slowly that the inertia force can be
neglected, these two kinds of external force will be in a
balanced state against the tension in the netting. In
the following two sections we shall be concerned with
these two kinds of external forces.
3. APPARENT WEIGHT OF THE NET IN WATER
(litaka 1958)
The apparent weight of a body in water is given by the
difference between gravity and buoyancy, and is obtained
by multiplying the density difference between the body
and the volume of the water the body displaces. Let D
be the diameter of twine, L be the length of the bar of
mesh, (/—/>) be the porosity of twine. With regard to
the knot, let / be the length of twine allotted to one knot.
(As the twine is tightened in some measure at the knot,
the diameter, D*, and the porosity, (/—/>*), of the twine
may be somewhat smaller at the knot than at the bar.)
Then the apparent weight, W, of one mesh (i.e., two
bars and one knot) is given by:
Dividing this expression by the area of mesh, we obtain
the apparent weight of netting in water per unit area:
uo = — — .
This equation can be expressed, by uniting constants, in
the form:
w =
P)
where/ (<p) =^xp/(2 sin 2<p) is a constant depending jpn
the degree of slackness of the netting, and a~(k/2)\/p/p*
is also a constant depending on the type of knot and its
tightness.
4. THE RESISTANCE OF THE NET IN A CURRENT
When any solid body of a given shape is placed in a
certain position relative to a uniform current of any
fluid, a hydrodynamic force is exerted upon that body.
The magnitude of this force depends mainly on the
density pw of the fluid, the relative velocity, v9 of the
body to the fluid, and the projected area, 5, of the body to
the plane normal to the current. These quantities can
be combined in a unique form pwv2S to give the dimen-
sions of a force. The parallel component of this force to
the current is commonly called resistance and is expres-
sed as:
(8)
W = [2* Lp + n ] (P -
(2)
where p and pw are the densities of the fibre and water,
respectively. Since the density of the fibre or filament of
the twine itself is considered to be unaltered by tightening
the knot, then:
where CD, and thus 6, is a constant coefficient of resis-
tance depending mainly on the shape and attitude of
the body, if the fluid is water. Strictly speaking, the
value of CD is not constant, but varies depending on
the Reynolds' number, /?«,, of the flow. Reynolds'
number is a function of the form:
- 0\/£- (3)
A knot is simply two pieces of twine twisted and tied
together. Therefore, if the knots are the same type,
the length / may be directly proportional to the circum-
ference or to the diameter of the twine:
/ - kD* -
(4)
where k is a constant depending upon the type of knot.
Thus we have:
W =
(5)
where / is the typical length of the body and v is the
kinematic viscosity of the fluid. However, according to
experimental results on fishing nets, it has been found
that we can put C/>, and thus 6, as constant for a rather
wide range of the Reynolds1 number, i.e., we can assume
that the resistance varies proportionally with the square
of current velocity for practical purposes. This resistance
law was first proposed by Newton, and holds pretty
well for motions where the resistance is due to current
inertia. Thus, hereafter, we shall use the equation (8)
as the resistance law of the fishing net.
Now we proceed to estimate theoretically the resistance
of a mesh of netting set perpendicularly to the current.
To this end it is sufficient to calculate the resistances
of two bars and one knot.
473
Since the projected area of two bars is 2LD, their
resistance will be of the form:
b(2LD)v2.
(10)
The projected area of a knot is considered to be propor-
tional to two-thirds power of its volume. On the other
hand, the volume of a knot must be proportional to the
volume of twine used to tie the knot, and this is given by :
Thus the resistance of a knot will be of the form :
(12)
where c is a proportional constant depending upon the
type of knot and b* is also a constant depending not
only upon the type of knot, but also its tightness. Thus
we can obtain the resistance per unit area of webbing
by dividing the sum of these resistances by the area of
one mesh as follows:
_ [2bLD + b*D2]v2
"""" L'sinfy
=j^[ffl+K(Tr\*. <13)
nn2?LW »WJ
Next, in the case where the netting is supported parallel
to the current, the projected area of the bars may be
reduced to 2LD sin^, and the constant b* in the
above expression may change to some other analogous
value &**, according to the type of knot. Then the resis-
tance acting per unit area of webbing may be given by:
— 2^8in 99
°~° sin 2y>
When the netting is inclined to the current at an angle 0,
it may be clear, from the analogical inference, that the
resistance may be represented by the form :
where g and h denote definite functions of angles
0,
and
5. MECHANICAL SIMILARITY OF FISHING NETS
(Taoti 1934)
We now consider the static situation, in which the
forces acting on a segment of netting are in equilibrium,
when the net segment moves through water at a constant
velocity or, conversely, the net remains at rest and current
474
of a constant velocity flows through it. This situation
is said to be in equilibrium under the actions of the three
forces: apparent gravity, current resistance, and the
tension in the webbing. Owing to the fundamental law
of mechanics, if a particle is in equilibrium, the resultant
force acting upon the particle must be zero, compensating
mutually. Let A be the area of an element of the net and
S its circumference, and T be the tension in the webbing.
Then the vectors of these forces, wA, rA and 75 must
complete a closed polygon, in this case a triangle, as
shown in Fig. 2. This condition will hold approximately
ftttuH0*t ftf
ttfltion
Wftir
•!
•ittrntl ftrett
Pig. 2. Equilibrium of forces in the netting.
true for the case in which the motion of netting has
some acceleration, because the mass of netting is usually
very small, so that the inertia force cannot grow as
large as the other forces. This is called quasi-equilibrium.
Consider two nets of different size but of similar
design, of which one is the full-scale net and the other
is its model, and distinguish the notations of model net
from that of full scale by putting a prime sign (') after
the corresponding notations. Under what conditions
will these nets have a similar geometric configuration
and a similar attitude? The configuration and attitude
of a net must be completely determined merely by the
external forces acting upon them, if the net materials
are perfectly flexible and the state of the net remains
unchanged. Therefore, the answer to the above question
evidently is that forces acting on corresponding parts
of the two nets bear the same ratio to each. The relations
which satisfy these conditions make up the law of
mechanical similarity, and are indeed the modelling
law which we are seeking, i.e., the two nets should be
geometrically similar.
To begin with, suppose both nets are divided into
components in such a manner that corresponding to
any part of the full-scale net there is a geometrically
similar part in the model net. If we continue this equal
division until we have a large number of very small
parts, any single divided part can be considered as a
flat plane or an element of netting membrane. We now
focus attention on a set of these small parts in a given
area of one net and locate corresponding elements in
the other net.
If the webbings of the two nets are hung to the framing
with an equal ratio of shortening, it is clear, for a geo-
metrically similar configuration, that the meshes of the
corresponding parts will open similarly (i.e., to the same
degree) irrespective of the mesh size, i.e.,
9? *= <p' . (16)
i
Since we assumed that the corresponding parts of both
nets take similar attitudes against the current, we can put :
6 = 0' .
(17)
When the model net takes the same shape as the full-
scale net, the triangles of force vectors must be geometri-
cally similar for corresponding parts of both nets. That
is, the corresponding angles must be equal and the three
sides of the triangle are in proportion (Fig. 3).
/\ y
\ / \
Circumference \ / \ /
s / \ *.
xAr.o> / NX
i -i
r A
Fig. 3. Mechanical and geometrical similarity between full scale and
model.
Then the condition mentioned above is expressed by:
w'A'
wA
r'A'
T'S'
TS
(18)
where F denotes the total force exerted by or acting on
the net. It may be needless to say that A and 5 are
proportional to the total area and the linear size of the
net, respectively, and the total area is proportional to
the square of the linear size, A, of the net; then we may
write:
4L
A
A2 ' 5
(19)
When both nets are the same shape, i.e., <p =9',
6=0', and if the knots of both nets are the same type,
it follows that a=a', /=/', g=g', and h=h't where a
is a constant depending upon the type of knot and /,
g and h are definite functions of <p and 8 as appeared
in equations (7) and (IS). Thus we have from equation
(18):
a(D'/LJ]D'(P' - Plt.)
A2[(Z)/L) + a(D/LY]D(p -
AT'
A2[(Z)/L)
F
F
Now, if the netting of the model has been woven so as
to satisfy the next relationship:
D
L
D'
T"
(21)
the factors in the bracket in equation (20) would be
reduced, with this result:
A'*P V - ft,) A'V* AT' F
- Ptt) A «o 2 AT F '
Then if we arbitrarily chose the following two ratios,
A and K2:
A=-
I/2 _ D'(p' - P«)
~D(p -ft,)'
we have, from equation (22),
7"
T
F
F
(23)
(24)
475
The equations (16), (17), (21), (23) and (24) are the
fundamental criteria for mechanical similarity of the
fishing net. Expression (24) should be satisfied through-
out the whole net according to equation (18); while
equation (21) holds adequately only for similarly situated
portions in both nets, because this condition has been
introduced to reduce the fraction in equation (20).
Notice that equations (21) and (23) give the conditions
to be satisfied by the webbing; equations (16) and (23)
give the rule to be obeyed in designing and tailoring
the model net; equation (24) shows the relations which
will be established in the operation; and equation (17)
with equation (23), is a mathematical representation
of geometrical similarity between the full-scale net and
its model.
The above results can be summarised, in order of
practical procedure, as follows:
(a) First, define the reduction ratio, A, of the model
as large as the circumstances permit; i.e., put:
(25)
Each part of the webbings and frame lines should be
reduced in length according to this ratio. It is desirable
that this ratio be larger than 1/20.
(b) Next, determine the twine diameter, D, and its
density, />, such that the ratio:
(26)
has the same value for all sets of corresponding parts
throughout the whole net, pw being the density of water,
(c) Then the length, L, of the bar of a mesh should
be determined so that this ratio:
TV T '
tL ^tL ^- M
D L
(27)
has an arbitrary constant value for sets of corresponding
parts. Notice that this value can be chosen indepen-
dently of the reduction ratio A.
(d) Assemble the webbings to the frame line or seam
the webbings to each other with the same ratio of take-up
for corresponding parts of both nets. This means:
V = V
(28)
when both nets assume similar shapes.
(e) For the model thus made up, the ratio of velocity,
K, between model and full-scale net is given by:
l-F.
(29)
(f ) The ratio, T, of tension in the webbing (the force
acting per unit length of webbing) and the ratio of force,
476
F, at any fixed point in the net are given, respectively,
by:
(30)
F'
—
(31)
The force that acts uniformly upon the edge of the' netting
(i.e., the buoyancy of floats per unit length of corkline,
or apparent weight of sinkers per unit length of leadline)
should be determined by means of equation (30).
Equation (31) describes the relation of forces that act
on a definite point in the net, such as the towing force
of a trawl net, or the holding power of the mooring
rope in a fixed net.
(g) Sometimes the net changes its form in operation,
as in the case of the purse seine. Then the distance which
a definite point in the net must travel to complete a
definite stage of process is proportional to the net's
size A, and the time, /, required to complete a definite
process will be proportional to its linear size, and in-
versely proportional to its velocity, v. Thus we have:
A
V
(32)
6. MODEL LAW FOR ROPES IN FISHING NETS
(Kawakami 1959)
Ropes of various sizes are indispensable in building
fishing nets. There are two kinds of ropes, viz., those
whose length must be proportional to the size of the
net itself (e.g.. the framing lines) and, on the other hand,
those whose length can be varied irrespective of the
size of the main body of the net, as is the case of towing
warp of a drag-net.
Suppose there are two ropes; one for the model, the
other for the full-scale net. Then consider the conditions
to be fulfilled to make them both similar in shape. As
with netting, there are three forces that act on part of a
rope in a current:
(a) the hydrodynamic force that arises from the flow,
(b) the weight of the rope in water, and
(c) the tension in the rope at the segment.
In order to get a geometrical and mechanical similarity
for both ropes, it is necessary that the ratio between these
forces be the same at each similar point on both ropes.
Experiments have shown that the force acting on a rope
in a uniform current of velocity, v, is practically normal
to the rope, and that its magnitude varies proportionately
with the square of sine of the angle, 0, between the rope
and the current, and also with the square of the current
velocity, v. Therefore, the hydrodynamic resistance,
r,, acting per unit length of the rope due to the current
is given by the form:
rr = rro sin20
(33)
where rro is a constant and Dr is the diameter of the
rope.
The apparent weight of rope of unit length in water
is simply given by:
D
(34)
where pr denotes the density of the rope.
On the other hand, the tension, Fr, in a rope must be
proportional to the forces that act upon the point at
which the rope is attached to the net:
F, ocF
(35)
In order to maintain mechanical similarity, these three
forces must be in the same ratio with one another for
two ropes, i.e., those of the full-scale net and its model.
Hence, denoting the length of the rope by /, from the
equation (31), we have:
/v
F ' F'
__ J r _ r .„ J2T/2
_ _ _-.l V
_
In making the model, the mean density of a rope can
be adjusted by inserting short strips of rope of different
materials at regular intervals in proportion of (a : 1).
For a small model, this can easily be done by using
modern chemical adhesives. Let pr* and Dr* be the
density and the diameter, respectively, of the inserted
material, which may even be a metal filament. Then,
if the two ropes maintain a geometrical similarity,
i.e., 6'=^0, the equation given above becomes:
/'[(I ~~ a)D,' + aD*]v'2
lDrT2
/-[(I - a)D/2(p/ - ft,) -f flPr*
(37)
Pw)]
lDr\Pr -
F '
rr „
_-
(38)
This is the most general expression of the model law for
ropes. Several cases will be deduced from this:
(a) The first is:
When the rope is used as a main part of the net, the
reduction ratio should be equal to that of the main
body of the net, i.e.:
a * 0, J- = A, - A,
the equation (36) becomes :
(1 - a)pr> + aD* _
A
- d)Dr'2(Pr' - fc) +
•ftr)
(39)
The density and the size of the rope should be chosen
so as to satisfy simultaneously the above two relations,
(b) The second case is:
Usually where
a = 0, and - = A9
the relations are simplified to:
(40)
Pr
Pr ~ PTV
A
(c) The third case is:
If
we have:
a — 0, and - = Ar ^ A,
Dr' _ A1
(41)
Pr ~
A2
This is the case where the rope length is independent
of the size of the main body of the net, as in the case of
the towing warp of a trawlnet.
In usual cases, since the ropes in a fishing net have less
influence on its mechanical properties than its webbing,
rigorous observations for this law will be needless,
except in the case of the towing warps of trawlnets.
If a net is worked in a relatively weak current, e.g.,
with a fixed net set in a cove, the resistance of the rope
(due to the current) is negligible in comparison with its
apparent weight in water. We can then ignore the
expression (37); then for case (b) mentioned above, the
values of Dr and pr can be chosen so as to satisfy only
the next relation:
Pu __
"
-' ft.
(42)
If a net is worked in a relatively strong current, as in the
case of high speed trawling, the weight of the rope is
negligible compared with its resistance; then the rela-
tions (38) are simplified to:
(43)
7. MODEL LAW FOR FLOATS AND SINKERS
Floats and sinkers are used to impart a constant
buoyancy or gravity to a point or along an edge of
477
fishing nets. Therefore, these two forces, together with
the force or tension in the webbing, must keep the same
ratio between similar locations in the full scale and its
model.
Denote by n the number of these accessories
attached per unit length of the webbing and put:
(44)
Then if Da and pa are the size and density, and if the
shape of these accessories is geometrically similar,
the buoyancy or gravity per unit length, wa, is represen-
ted by:
from which it follows that:
(45)
(46)
where kw and kr are constants depending upon the
form of the accessories.
Therefore the necessary condition to be fulfilled is :
ID, = kj)a\pa - />„>,
and resistance ra by:
r =
T
T
(47)
(*'>
From'these'relations it follows that:
Pa - Pu
Da
(48)
Strictly speaking, floats or sinkers of the model should
satisfy these two relations simultaneously.
It occurs frequently that the model accessories cannot
be made up in strict sense with the real materials due to
the extraordinary denseness of these materials. This
difficulty may be overcome by neglecting the hydro-
dynamic resistance acting on the accessories in compari-
son with their buoyancy or gravity. This may be allow-
able if the current velocity is not too high. Thus the
second term of equation (47) is ignored and we have:
- ft.)
D.*(Pa - ft.)
N
(49)
in place of the relations in (48).
When these accessories are attached to a fixed point
in a gear, as is the case with the king buoy of a pound
net or the torn weight of a purse seine, similar considera-
tions, from equation (31), give:
(so)
(51)
When the accessory causes a hydrodynamic force, as in
the case of otter boards in a trawlnet, its size and material
should satisfy these relationships simultaneously. If,
however, its apparent weight in water is negligible when
compared with its hydrodynamic resistance, it may be
sufficient if only the first relationship in expression (51)
is fulfilled.
When the net is fixed to the sea bottom by sand bags,
as in the case of a fixed trap net, the weight, W, of the
bag in water should be chosen so as to satisfy the relation :
W k'
1L_ 2,
W k
(52)
where k is the holding coefficient of the bag to the sea
bottom.
8. THE CASE IN WHICH THE RATIO OF SCALE
REDUCTION MODEL IS DIFFERENT IN THE
HORIZONTAL AND VERTICAL DIRECTIONS
(Kawakami).
It is sometimes requested that a model net should be
made up in such a manner that the horizontal and
vertical reduction ratios are different from each other.
For instance, imagine a long, narrow gillnet several
hundred metres long, several metres deep. In this case,
if we intend to construct an ordinary geometrically
similar model, the depth of the model would be extremely
small. In this section, we shall consider the case in which
the reduction ratio in horizontal direction:
a*
= A
differs from that in the vertical direction :
The gravity and buoyancy do not act in the horizontal
direction, but they do act in the vertical direction.
Therefore, corresponding with equation (20), we have
next two relations:
V VKD'/L') +
lvTh
a(D'/L')*\D'(p' -
A* A, ((D/L) + a(DIL)*\D(p -
478
r = r0 exp( -
(63)
where subscripts h and v denote horizontal and vertical.
Similar treatment as in Section 5 would result as follows:
If we chose the netting of the model so as to satisfy
the relation:
D D'
L~ L"
the above relations will be simplified to
(55)
'2 T1 ' F '
A A — — A. il— — *
* "^- ^n FA'
(56)
-A A v - A* -"
-A"A^~ A"l\. -ir-
Then putting:
we have:
D'(P' - ftr) -
T '
±
* I •
(57)
(58)
(59)
(60)
(61)
9. THE CASE IN WHICH ACCELERATION DUE
TO WAVE MOTION CANNOT BE NEGLECTED
(Kawakami, 1961)
When gear which operates at or near the surface, such
as the driftnet, is subjected to the effect of wave motion,
and when the mass of the accessory is large enough, the
force due to acceleration cannot be ignored. Here some
further considerations will be presented.
According to the theory of "surface waves", the
period, tp, of the wave motion is given as a function of
the wave length, /, as follows: —
(62)
VT
where g denotes the gravity acceleration. If the height
of the wave at the surface be denoted by 2r0 the radius
of orbit of the water motion at a depth z is given by:
Hence at geometrically similar points, z'/r =/'//, we
have:
(64)
Therefore, if the reduction rate, A% of the model is equal
to that of the wave length and if both waves are geo-
metrically similar,
I
(65)
This means that at corresponding positions of both nets
the reduction rate of the orbit of wave motion is equal
to that of the model.
The velocity, t.v, of the water motion at a correspond-
ing position is given by:
(66)
Remembering equation (29), this gives:
F2- A.
This is the conclusion drawn from the fact that the
velocities of both the net and the water are determined
by the wave length.
The acceleration, a, of the sinker under considera-
tion is proportional to r//p2, then we have:
«; = rW2 = rl 1 =
a rlt.,2 r /'
(67)
i.e., the magnitude of acceleration of both nets is equal,
irrespective of their sizes.
Now we consider a leadline to which n sinkers per
unit length are attached; and let the mass of the sinker
and its acceleration be so large that the hydrodynamic
resistance can be neglected in comparison with the
apparent weight and the inertia. Let ps be the density
of the material and D5 be its size. Then the apparent
weight per unit length of the leadline will be propor-
tional to nDs*(ps — pw).
In order to accelerate a solid body in a fluid, it is not
only necessary to exert a force equal to the product
of the mass of the body and its acceleration, but an
additional force to accelerate the mass of the fluid
substance set in motion by it. This additional force for
various bodies depends on its shape and on the direction
of motion, and is represented by supposing that the mass
of the body is increased by a certain amount proportional
to the product of the density of the fluid and the volume
of the body. Therefore, this apparent inertia force per
unit length of leadline will be represented by nD*,
479
(p,+ppv)a, where p is a constant depending on the
shape of the sinker and on the direction of motion.
Therefore the necessary condition to be fulfilled is
written in the same manner as expressed by equation
(47):
Dt>(p, + pPn)an
(68)
Hence if we could choose the material of the sinker so
as to satisfy the relation:
PI' — Pu _ PI' +
P« — Pw P* + PP,»
P«-
taking the relations (66) and (67) into consideration and
writing n'/nsN as before, it follows that:
IV'
/IF2
'N
-
N'
(70)
Then if the total number of sinkers is equal in both
nets, i.e., NA=19 we have:
A
(71)
These results lead to the conclusion that, if the model
is to be built of the same material as the full-scale gear,
the reduction of size must be the same throughout every
part of the gear.
10. LAW OF ENLARGEMENT OF FISHING NET
(Kawakami, 1961)
In the preceding sections we have not been concerned
with the tension acting in the bars of mesh. This tension
becomes highly significant in connection with the tensile
strength of netting twine, at times when we wish to
enlarge a given full-scale gear, without changing its
design, to a proper size for the special case which con-
fronts us.
It seems to leave no margin for adjusting the breaking
strength of the twine, except for choosing the material,
in the preceding theory of mechanical similarity. Here
we must make some compromise. For this purpose, the
writer has made an assumption that the resistance and
apparent weight of a knot are much smaller than those
of two bars. This may be allowable for ordinary gear
where the value of (D/L) is less than 1/10. Under this
assumption, we can neglect the term (D/L)2 as compared
with the term (D/L) and equations (7) and (15) will be
simplified, respectively, to:
(72)
r -
(73)
Next, consider the tension, r, in a netting twine of a
gear in action. Let n be the number of meshes in
unit length. Then n is inversely proportional to
(L sin p), or strictly, as shown in Fig. 4:
T> t»r cM
^ UAH Ltnjth
I • 2 M L tin y
Fig. 4. Tension in the netting end in the twine.
= *
2L sin 9;'
and the tension in a bar is given by:
T
r —
2n cos
^ TL tan
(74)
(75)
On the other hand, the tension which a twine can bear,
or breaking strength, is clearly the product of the tensile
strength, or, of the material and its cross-sectional area
(proportional to Z>2), and r must not exceed this breaking
strength, i.e.:
n("\ a> T = jTLtan^'.
Thus for the critical condition we have:
(76)
(77)
where the coefficient q is determined by the openness
of the mesh or angle <p. Then equation (20) may be
transformed into:
(78)
This is analogous as treated in Section 5, where we had
three ratios which we could determine arbitrarily according
to the circumstances. But in this case one more degree
- Pu:)
480
of freedom is increased due to the relaxation of the restric-
tion on the forces acting upon the knot. Hence if we
choose four arbitrary ratios as follows :
P ~ Pw
the equation (78) is written:
'2
From these relations it follows that:
"- = AP,
a
LJ V
F'
F
AV*
PAT
AW*
PM'
V A a' _ F'
Ma ~ F'
(79)
(80)
(81)
(82)
(83)
This is the relation which determines the tensile strength
and diameter of the twine, amount of sinkers or floats per
unit length of webbing, and the force which should be
applied to the manipulating or mooring rope.
There is, however, another point of view. The strength
of webbing should not only be determined by its tolerance
against the tension in its underwater operation, but also
by its endurance against the tension exerted by the brailing
of catch on to the vessel. Let us focus our attention on
this point.
We assume at first that the amount of catch is pro-
portional to the volume of the net, i.e., the weight which
the webbing must sustain is proportional to the cube of
the linear size of the net. This tension due to the weight
of catch is quite different in its nature from that due to
the underwater operation, but the equation (77) is valid,
irrespective of the origin of the tension. Hence, if we
denote the diameter of the twine determined from this
point of view by /)*, we have:
(84)
or
AT1 A(D2/L)o
D* AM
(85)
The value of D* obtained by this way must be compared
with that of D' obtained previously. If the value of D*
is noticeably larger than that of D', the material of the
twine should be replaced by a stronger material; other-
wise the use of some rib-lines, for reinforcement, is
advisable.
Here, in order to avoid the apparent contradiction
in the diameters of the twine obtained by the working
condition and by the brailing operation, equate equation
(81) with equation (85) and take expression (80) into
account. We get:
or
= AMP
Yl
M'
a
a
(86)
(87)
The problem with which we shall be confronted will
depend on whether we can find the material that satisfies
this condition.
It may be worthwhile to notice that fishing nets can
their mechanical behaviour. Every fishing net takes its
proper form during its operation by being subjected
to the influences of gravity and current, as explained
before. However, there is one type of net which takes
its proper form mainly by the action of gravity and is
this is the case with the bag of a pound net. Another type,
which takes its ideal form from the current and is un-
desirably deformed by the action of gravity is the case
with the towed net. We tentatively call the former the
"gravity type", and the latter the "resistance type".
Now, for instance, let us consider the resistance type
in which the tension in the webbing will be caused mainly
by the resistance due to the current, and the form of
the net is hardly influenced by the gravity. In this case,
the first term of equation (78) is dropped, and one more
degree of freedom may appear. Thus we are allowed to
choose the ratio of tensile strength as the factor which
can be determined freely, i.e., put:
the analogous treatment as before gives:
£.'
D
II
T
S '
MS
MS'
and as to /)*, we have:
M
(89)
(90)
(91)
(92)
continued on page 482
481
The Development of a Midwater Trawl
The paper outlines the objectives, organisation and methods of
the Norwegian Government-sponsored APE group (Working
Group for the Development of the One-boat Midwater Trawl). It
is pointed out that ideas arising from technical research will often
appear upsetting and that their adoption requires not only con-
fidence in, but an understanding of, science and scientific method.
A strong plea is made for unfettered international co-operation
in research. A step in this direction was the establishment of the
International Group for Pelagic Fishing Methods and Gear (IF),
in which six countries now co-operate in a programme comprising
standardisation of descriptive methods and technological gear
research.
Le devetoppentent d'un chalut pelagique
by
La communication decrit les objectifs, 1'organisation et les
roethodes utilisees par TAPE (Groupe de Travail pour le Developpe-
mcnt du chalut pelagique a un bateau), groupe finance par le
Gouverncment Norvegien. Les auteurs de cet article attirent
Fattention sur le fait que certaines idees provenant de la recherche
technique paraissent souvent incroyables et que leur adoption
requiert non seulement une certaine confiance mais aussi une
comprehension de la science et de ses mtthodes et plaident en
faveur d*une cooperation Internationale libre dans la recherche.
L'ctablissement d'un Groupe International pour les methodes et
engins de pechc pelagique (I.F.) constitute un pas dans cette voie
et six pays cooperent actuellcment a un programme comprenant
la standardisation des methodes descriptives et la recherche tech-
nologique sur les engins.
El peif ecdooamiento del barco de arrastre flotante
Extracto
Da a conocer la ponencia las finalidades, organizacibn y mctodos
del grupo APE (Comit6 para el Perfeccionamiento de un Arte
flotante remolcado por una Embarcaci6n) patrocinado por el
Gobierno noruego. Se hace observar que los resultados obtenidos
por la investigation tecnica con frecuencia parecen ser descon-
certantes y su aplicacidn no s61o nccesita confianza sino tambien
compression de la ciencia y los m&odos cientfficos. Se aboga
vigorosamente por la cooperacibn internacional sin trabas en
a investigaci6n. Un paso en esta direcci6n fue la creaci6n de un
P. Dale
S. Moller
Norwegian Group. Special working group for
one-boat midwater trawl
grupo internacional para metodos y artes de pesca pelagica (IF)
en el que 6 paises cooperan en un programa que comprende la
normalizacibn de metodos descriptivos y tecnologia de la inves-
tigacibn de artes.
FOR detailed investigation of fishing gear problems,
the Norwegian Government sponsored the formation
of a special study group, the working group for the
development of a one-boat midwater trawl (APE).
The work thereby initiated has been comprehensive and
seems likely now to be of a permanent character with far
more facets than can be covered in this report.
It is, therefore, necessary at this stage to select those
angles of special interest, namely the tests that have so
far been conducted in relation to midwater trawling and
the development of testing gear.
continued from page 481
It is to be noticed that the density of the material dis-
appears in these relations. This is natural because the
action of gravity has been ignored.
11. CONCLUSION
Our present fishing gear is the result of trial-and-error
experience over centuries. Fishermen long ago discovered
that the simple device is not always the successful one.
While respecting these traditions, we must make con-
tinuing efforts to improve obvious faults in present gear.
Our modern advantage lies in great and continuing
improvements in net material, such as the use of syn-
thetic fibres. Meanwhile, the fundamentals of how to
catch fish remain unchanged and we may assume that
fish behaviour has also followed constant principles over
the centuries. These simple facts mean that our potential
to harvest the sea has greatly expanded.
The sea remains largely unknown and its changing
faces must remind us that no single method or gear will
be the best for all circumstances. Thus the thoughts
482
expressed in this paper do not comprise a body of strict
rules. Rather, these laws must be applied to circumstances
as they arise.
Despite the changeability of the factors with which
we must work, we have no doubt that there must be a
general rule for gear design and operation. The challenge
is to continue our research toward these general prin-
ciples. As bigger boats fish more and more distant
grounds with larger-scale gear, the challenge will require
the net designer to devise mechanical solutions for many
new problems.
References
Tauti, M. A relation between experiments on model and full scale
of fishing net. Bull. Jap. Soc. Scient. Fish., 3 (4), 1. 1934.
litaka, Y. On the weight of webbing in water. Bull. Jap. Soc.
Scient. Fish., 24 (8), 620. 1958.
Kawakami, T. On the law of mechanical similarity for ropes of
fishing nets. Bull. Jap. Soc. Scient. Fish., 24 (10), 795. 1959.
Kawakami, T. Unpublished.
Kawakami, T. On the law of mechanical similarity for drift gill-
net. Bull. Jap, Soc. Scient. Fish., 27 (2), 124. 1961.
Kawakami. T. Rule of enlarging or reduction of the scale of fishing
net (in Japanese). Suisan Kagaku (Fishery Science), 3 (8), 12. 1961.
The basic research programme is overwhelming and
far too wide to be done by one party in a reasonable time.
It is a task for international co-operation where all
interested nations should contribute without national
restrictions. Certain developments along those lines
have some promising significance.
The ideas arising from technical research seem too
revolutionary and upsetting to many and the adoption
of them certainly needs a good deal of confidence in,
and understanding of, science and scientific methods.
When trying to analyse the problems there is, however,
no other safe way to go. Reviewing the time and money
spent hitherto on many projects within the pelagic fishery
in relation to the results achieved, there is no doubt that,
in the long run, the rational approach is the economical
way also.
The first step in this direction has already been taken
by the establishment of the IF group (International
Group for Pelagic Fishing Methods and Gear) wherein
six nations share the results from research work done by
each in the field of midwater trawling. This group
thought these themes were suitable for wide co-operation,
namely :
(a) Standardisation : Methods and scaling of draw-
ings, terms and definitions, testing methods, test pro-
cedure and testing conditions, terminology and specifi-
cations of materials.
(b) Technologic Gear Research: Hydrodynamics,
patterns offish behaviour, instrumentation, test, installa-
tions.
The objective is to simplify comparison of one design
with another, to make drawings understandable to
anybody, to show characteristics of constructions in a
synonymous way; to standardise terminology; to avoid
the numerous codes used for specifying materials; to
avoid cumbersome and often misleading recalculations;
to ease the repair of gear; to realise an effective spare-part
service, etc.
There have been numerous objections to this way of
tackling the problems, mainly originating in the view
that the programme is too big and all-embracing for a
restricted tool like a midwater trawl. The APE group
is, however, of the opinion that:
(a) This type of fishing gear is one with a great future
for many fishing purposes.
(b) It might very well form the basis for new fisheries.
(c) This method of solving the problems has value
not only for this specific class of gear but will help the
understanding and construction of any gear type, thus
also providing possibilities for the improvement of other
existing ones.
Viewed from this angle the chosen procedure may
not seem so overwhelming but rather the only reasonable
one.
The APE group does noK think it possible to arrive
at formulae covering all phenomena connected with
fishing gear or otherwise establish theoretical control
that can give an answer to any question that may arise.
However, it is thought that a combination of formulae,
models and full-scale tests with suitable equipment, test
installations and test procedures framed by a pattern
made up from empirical data will give answers to most
questions of importance. This should also help to
further the general understanding of the underwater
behaviour of gear in relation to fish, and will strengthen
the foundation for further development.
An example of the work so far done is available in a
report on model tests made with three bottom trawl
boaids, by H. A. A. Walderhaug and A. Akre. We quote
extensively from that report which was published in
the Norwegian Fishing and Maritime News, No. 1, 1963,
thus:
In this report are described some results of otterboard
hydrodynamical tests carried out at the Noiwegian
Ship Model Experiment Tank.
The otterboards tested are all designed for bottom
trawls, and following notation is used :
B 1: Rectangular otterboard of orthodox design
(Fig. 1).
Fig. L Otterboard B I (Rectangular otterboard).
Weight: 1100 kilogram. Area: 456m2. t/1: 0-0355.
Reynolds' number at towing speed 3 -5 knot: 5-5 X JO6.
B 2 : Oval otterboard (Matrossow board) with one
vertical slot at the centre of the board (Fig. 2).
B 3 : Oval otterboard with 3 slots (Fig. 3).
B 2a: As B 2, but with 2 slots, one at the centre and
one at the leading edge (Fig. 4).
B 2b : As B 2, but with a streamlined leading edge,
NACA wing section (Fig. 4).
483
Diagram no. 2. Freestream tests, ottcrboard B /, B 2, and B 3.
Force coefficient and lift-drag ratio of otterboard as function of
angle of attack.
Diagram no. 3. Freestream tests otterboard B 2. Force co-
efficients and lift-drag ratio of otterboard as function of angle of
attack.
oj
0*'
O.T
0*
CM
Diagram no. 4. Freestream tests, otterboard B 3. Force co-
efficients and lift-drag ratio of otterboard as function of angle of attack.
484
Concerning the modified ottcrboards, it is shown in
Diagram 3 that B 2a as well as B 2b has a 12-13%
higher lift-drag ratio than B 2.
In Diagram 4 is shown the effect of varying the slot
angle for B 3. The effect is quite marked with a maxi-
mum lift-drag ratio at a slot angle between 42° and 45°.
Bottom tests
During these tests the otterboards were towed very
close to the bottom (maximum distance about 5 mm)
but without actually touching the bottom. Mechanical
friction was therefore not involved, and the law of
similarity between model and prototype mentioned
earlier is still valid.
. Ttttttf at onglt of attack 30*
. _ _ " M * u it 28°
I,'
1,0
0,*
o,s
07
0,5-
0,5
0*
0,1
Co
C0
fit I09
0,4 0,3 0,6 0,7 0,0 0,9 1,0
Diagram no. 5. Bottom test, otterboard B /, B 2. B 2. Force co-
efficient of otterboard as function of Reynolds' number.
In Diagram 5 is shown the effect of Reynolds9 number
variations on the force coefficients. It will be observed
that the critical Re are higher than those for the open
water tests. This effect is more pronounced for B 1
than for B 2, which might have been expected since B 1
has a horizontal bottom line. The critical Re for B 1
and B 2 will be approximately 0*8 x 106.
The effect of variations in angle of attack is shown
in the Diagrams 6, 7, 8, 9 and 10. It will be observed
that the maximum lift for the three parent otterboards
(Diagram 6) will be reached at a smaller angle of attack
than during the open water tests. This effect might
have been expected since stalling is delayed by 3-
dimensional effects. ,
As for the open water tests the maximum lift-drag
ratio will occur at an angle of attack less than 20°, and
the rectangular otterboard is still inferior to the other
two.
JUUL
Fig. 4. Otterboard B 2a, B 2b. Modifications of otterboard B 2.
Reynolds' number is defined as
Vc
Re =
where
c = length of otterboard (cord)
v = kinematic viscosity.
Scale effects— Model size
The first problem in model testing is to find a relation
between model and prototype as regards forces and
moments. The forces involved when an otterboard is
towed deeply submerged in water are factional forces
and inertia forces, and the streamlines around the model
and prototype are geometrically similar if Reynolds'
number is the same for model and prototype. This can
not easily be realised in a model tank, but fortunately
the force and moment coefficients are practically
independent of Reynolds' number variations for Re
larger than 0-5 x 106— 1 x 10*. As model scale for B 1
and B 2 were chosen 1 :5, and for B 3, which is a smaller
otterboard, were chosen the scale 1 :4. With these models
the critical Reynolds' number mentioned above is
reached in the model tank.
1,0
0,9-
0,7
0,8-
0,4-
0,3-
0,1
• it
Otttr boor* BI , onglt of attack 30°
0,4 Q£ 06 0,7 0,8 0,7 1,0 1,1
Diagram no. 1. Freestream tests, otterboard B 1 and B 2. Force
coefficient as function of Reynolds* number.
Free stream (open water) tests
In Diagram 1 are shown the force coefficients of
otterboards B 1 and B 2 as a function of Reynolds'
number. B 2 is tested in this series at two angles of
attack, i.e. 28° and 38°, and for this board the force
coefficients will be approximately independent of Re
for Re larger than 0-65 x 10«. The critical Re for the
rectangular otterboard B 1 is lower, approximately
0-5 x 106. The higher critical speed of B 2 than that
of B 1 may be due to the shape of the otterboard as
well as the effect of the slot.
Based on this test series and on results of airfoil
tests (see f ex the British Shipbuilding Research
Association, Reports No. 70 and 142) it is expected
that full-scale behaviour of otterboard in open water
may be predicted from model test carried out at a
Re of at least 0-65 x 10*.
The force coefficients and lift-drag ratios as a function
of angle of attack is given in Diagrams 2, 3 and 4.
Only the angles of attack between 20° and 45° have
been considered of practical interest ; however, the
efficiency or lift-drag ratio of the otterboards will have
a maximum at an angle of attack less than 20°.
It may be observed from the curves that otterboard
B 3 has the highest L-D ratio, whereas B 2 has the
highest lift-coefficient.
485
Fig. 2. Qtterboard B 2. Oval otterboard with one slot.
Weight: 950 kilogram. Area: 4-55 m*. til: 0-0555.
Reynolds' number at towing speed 3- 5 knot: 5J6 x 10*.
B 2c : As B 2, but without slot.
B la and B 2d : As B 1 and Ba respectively, but without
fittings such as brackets, strengthening strips and
edges, hooks, links for back strops, bolts, pints, etc.
B 3a, b, c : As B3, but with slot angles equal to 40°—
45° and 50° respectively.
The slot angle for B 3 is 42°, where the slot angle is
defined as the angk between the after slot cord line and
the otterboard cord line (Fig. 3).
The following definitions were made in order to
relate otterboard model tests to tests with the complete
trawl gear:
The tangent plane is defined as the plane tangent to
the pressure side of the otterboard. Some examples of
tangent planes are shown with dotted lines in the sketch.
486
Suction
Fig. 3. Otterboard B 3. Oval otterboard with three slots.
Weight: 290 kilogram. Area: 2035m2. t/1: 0-0498.
Reynolds' number at towing speed 3- 5 knot 3* 6 x JO6.
For the otterboards described in this report the tangent
planes and pressure sides are synonymous.
The reference plane is defined as a vertical plane
parallel to the speed direction (speed vector).
During the tests described the angle of inclination and
the angle of tilt were both zero, so that the tangent plane
was always vertical.
The angle of attack, a is measured in a plane through
the speed vector and perpendicular to the tangent plane.
The following nondimensional coefficients are used:
CL =
CD
==- = lift coefficient
- = drag coefficient
L/D= lift/drag ratio
where L = lift force in the plane of the angle of attack
and perpendicular to the speed vector.
During the tests the lift and spread forces are synony-
mous.
D = drag force in the speed direction
Q = mass density
A = projected area of otterboard in the tangent
plane
V = velocity.
Streamlining the leading edge of the otterboard will
increase the lift-drag ratio about 6 % at the stalling point
(Diagram 8). Further the stalling angle is increased
about 4°.
Totted at Rt • O.iOS . 10*
* .. Rt • 0,81 . 10*
.. i. R* * 0.664 . I0f
Diagram no. 6. Bottom test, otterboard B 7, B 2 and B 3. Force co-
efficients and lift-drag ratio of otterboard as function of angle of attack.
The effect of the slot in B 2 is comparatively small as
shown in Diagram 9. At the point of maximum lift,
the slot seems to increase the lift-drag ratio about 5%,
Removing the otterboard fittings has a very marked
effect on the lift characteristics as shown in Diagrams
7 and 10. The lift is increased by 10 %-l 5 % and the lift-
drag ratio is increased by 5 %-7 %. Further the stalling
angle is reduced 3° for B 2. The increase in drag may
be the net result of a reduced frictional resistance and
an increased induced resistance.
Comments on the test results
First consider Diagram 2. It may be observed from
these curves, valid in open water, that otterboard B 3
has the highest L-D ratio, whereas B 2 has the highest
lift-coefficient. Which is the best otterboard is not
obvious. However, we may compare the boards on a
basis of constant area or constant lift-coefficient when the
speed is regarded as constant. In that case we draw a
horizontal line through f ex the lift-coefficient of B 2
at a = 30°. Where this line crosses the CL curves of
B 3 and B 1 we proceed vertically downwards to the
50,1
0,
0,*
0,4
0,1
Original otter board B, ,
Fitting of
otttr board retf^^d t|o
Anglo of attack
Diagram no. 7. Bottom tests, otterboard B L Force coefficient
and lift-drag ratio of otterboard as function of angle of attack.
1,1
1,0'
o,t-
07'
0*
OA
0,4-
\
6* - Original of tor board
Btb- At Bt but with a ttroom-
Itnod loading tdgt.
I of Ro « 041 . 10*
Anglo of aftaek
90
40
Diagram no. 8. Bottom tests, otterboard B 2. Force coefficient
and lift-drag ratio of otterboard as function of angle of attack.
•t- Original otttr board
Diagram no. 9. Bottom tests, otterboard B 2. Force coefficient
and lift-drag ratio of otterboard as function of angle of attack.
Original attar board Bc —
Otttr board without tlott ft^a
Toctod at tto • 0.81 . 10*
Diagram no. 10. Bottom tests, otterboard B 2. Force coefficient
and lift-draff ratio of otterboard as function of angle of attack.
487
respective L-D ratio curves. We then find that B 2 is
approximately 9 % better than B 3 and approximately 40 %
better than B 1. The corresponding angle of attack of
B 3 and B 1 is approximately 35° and 36*5° respectively.
We may also compare B 2 and B 3 on a basis of
constant angle of attack, f ex 30°. If the two boards
shall have the same lift at this angle of attack, we find
by considering the definition of lift-coefficients, that the
areas of the two boards must be inversely proportional
to the lift-coefficient, i.e.
A, CLf
A,
CL,
At a = 30° the area of B 3 must be approximately 12 %
greater than the area of B 2, but at the same time its
L-D ratio is approximately 7% better. During this
comparison the speed is of course kept constant.
Proceeding to Diagram 3, we find that based on con-
stant lift-coefficient (or constant area) the otterboard B 2a
with a leading edge slot is approximately 25% better
than B 2 at a = 30°. Comparing B 2 a with the rectang-
ular otterboard B 1 of Diagram 2 we find that on a
constant area basis, the board B 2a at a=30° is approxi-
mately 80% better than B 1. The corresponding
angle of attack of B 1 is approximately 39°.
In connection with Diagram 6 it may be observed that
the maximum lift-coefficients are smaller than those of
the open water tests, and further that maximum lift is
reached at a smaller angle of attack. Both these results
are in agreement with the observations of Dickson on
tests by Gawn and Yakovlev.
Comparing with Diagram 2 we find that at an angle
of attack of 30°, the L-D ratio of B 2 is slightly better
under bottom conditions than under open water con-
dition. The same CL can of course be obtained at a
lower angle of attack, i.e. 27°, and at this point the L-D
ratio is still much better.
The APE-II measurement system
Devices for measuring were also evolved and the
latest specifications for those devices are here given:
The present measurement arrangement — the APE-II —
is based upon a system used for experiments in the
autumn 1962. Both systems were developed at the
Norwegian Central Institute for Industrial Research.
The first system was based upon the use of one standard
10 mV multipoint recorder and a 3-single-wire connection
between ship and trawl. The experiences with this system
showed that such a system can be used when handled
with care. It is however very sensitive to water leakage.
(See diagram.)
1. All transducers at the trawl had their separate
stabilised power supply built into the transducer.
For battery economy these supplies can be turned on/off
electronically by switching pulses from the ship.
2. The selector-box contains two alternative systems:
(a) straight DC, as original
(b) a digital system, using pulse frequency modulation.
488
Fig. 5. APE-telemeter towing block.
T owing tlork
tvp« No.l,with-*.|
••t.r-e«unt«r
Fig. 5a. APE detail of telemeter towing block.
3. The three separate steel wires combined to one
3-wire PVC-coated cable.
4. Because of the pulse frequency modulation and the
inclusion of a few other features such as audible signal
indicator a separate signal control box has been inserted
before the multipoint recorder.
5. Provision was made for recording the pulse
frequency modulated signal on magnetic tape to follow
the rapid oscillations or changes of variables, which the
multipoint-recorder could not trace. The second channel
of the recorder for oral indications or comments.
6. Measurements which are performed on board
the ship by-pass the control-box and goes directly to the
mV recorder.
continued on page 489
Fishing Methods and Gear Research Institutes:
Their Organisation and Scope
Abftract
Until recently the development of fishing gear and methods was
done solely by the fishing industry. The increasing complexity of
fishing, which is due to changes in fishing conditions and the general
technical development, is making such development more and
more difficult, expensive and risky for most fisheries to carry on
on their own. Many governments have therefore started or are
considering supporting development in fishing gear and methods
by financial subsidies and through the services of specialised
research institutes. The paper gives detailed recommendations for
the scope of work and the organisation and set-up of a Research
Institute for Fishing Gear and Methods. These recommendations
are based on several decades of practical experience in the Federal
Republic of Germany. The recommendations are meant to cover
the subject comprehensively. They include the required facilities
and the problems of staffing. The main task of a Research Institute
for Fishing Gear and Methods is considered to be the improvement
of existing fishing gear and methods, and the development and
introduction of new ones. The growing need for international
collaboration and co-ordination of efforts is emphasised.
Institut de recherche d'engins et mtthodes de pkbe: Organization et
portee
Rfeurn*
Jusqu'& ces temps derniers, le d6veloppement des m&hodes et
engins de peche a toujours 6t6 men6 par 1'industrie de peche elle-
m£me. Le changement survcnu dans les conditions de peche et le
d£veloppement technique g6n6ral ont accru la complcxitd des
operations de telle sorte qu'il devient de plus en plus difficile,
coOteux et risqu6 pour 1'industrie de continuer a le faire. Certains
gouvernements participant d6ja au cteveloppement des engins de
pfcche— d'autres envisagent de le faire — par une aide financiers
et par 1'organisation d'Instituts de recherche specialises en la
matiere. L'6tude communique des recpmmandations d6taillees
sur 1'importance du travail et ^organisation d'un Institut de
recherche sur les m&hodes et engins de peche. Ces recommenda-
tions sont bastes sur plusieurs d6cades d'exp&iences pratiques
effectu6es en R6publique F6d£rale d'Allemagne; elles couvrent
entterement le sujet et traitent 6galement des installations et
probtemes de personnel. Le but principal d'un Institut de recherche
de ce genre est I'am&ioration des m&hodes et engins de peche
existants et le d6veloppement et 1'intrpduction de nouveaux engins
et de nouvelles m&hodes. La n£cessit6 d'une collaboration Inter-
nationale et d'une coordination des efforts y est soulignee.
Institutes de Investigation de m&odos y artes de pesca: Su organi-
zaci6n y flnalidades
Extracto
Hasta recicntementc el pcrfeccionamiento de los m&odos y artes
de pesca lo realizaba exclusivamente la industria pesquera. La
creciente complejidad de la pesca, debida a cambios en las con-
diciones y adelantos ttancos generales hace cada vez mas dificil,
costoso y arriesgado para armadores individuates introducir las
propias. Debido a ello, muchos gobiernos han iniciado o examinan
la posibilidad de facilitar la mejora de los artes y m&odos de pesca
mediante subsidios en met&lico y la prestacion de scrvicios de
by
A. von Brandt
Institut fUr Netz und Material-
forschung, Hamburg
institutes especializados en la investigaci6n. El autor hace recpmen-
daciones referentes a las finalidades de la labor y organizacidn de
un institute de investigacioncs de mdtodos y artes de pesca. Se
basan en la experiencia prictica adquirida durante varias decadas
en la Repiiblica Federal Alemana y su finalidad es abarcar toda
la materia, inclusive los medios y personal necesarios. Se considera
que la principal labor de tal instituto consiste en mejorar los
mdtodos y artes de pesca en existencia y perfeccionar e introducir
otros nuevos. Se pone de relieve la necesidad creciente de colaborar
sobre el piano internacional y de coordinar los esfuerzos.
UNTIL recently the development of fishing gear and
methods was based solely on the experience and
efforts of practical fishing. The aim of these efforts
was always to improve the economy by increasing catches
while reducing costs and labour. Most fishermen are still
trying to improve their gear and methods by applying
modifications. Due to this simple empirical and unco-
ordinated approach of many groups and individuals,
technical development has been rather slow.
Nowadays the fisheries of many countries are faced
with the growing difficulties of longer distances to the
fishing grounds, decreasing fish stocks and higher
demand for quality products. More and more this
limits the fisherman's possibilities — and those of the
fishing companies — of introducing innovations without
taking too serious economic risks. Consequently many
countries have started to support their fishing industries
by subsidising technical development. Furthermore, it
has been found that it often is difficult for a fishing
industry to conduct and evaluate trials and experiments
on its own, even when the costs are borne by the Govern-
ment. Further difficulty is met in recording and evaluat-
ing results obtained elsewhere for industry use, as well
as distributing these results for the benefit of others. In
view of this situation, the Governments of several coun-
tries have established governmental institutions for
continued from page 488
The variables included in the system are:
1-3 three angles on the otterboard,
3-4 two forces at the otterboard,
5 depth,
6 height of net opening, measured by differential
pressure,
7 distance between otterboards,
8 relative velocity at the otterboard,
9-10 two warp-angles at deck,
11 total towing force,
12 torque on the propeller shaft.
The towing blocks (see Figs S and Sa) were designed
to give information concerning the load in warps and the
length of warps. They were dimensioned like ordinary
trawl blocks and were of very robust construction and
water and shock proof.
489
fishing methods and gear research. These are either
individual institutes, or branches or sections of existing
fisheries institutes.
2. Talks and duties
In the following compilation of the proposed tasks
and duties of such an institute, an attempt has been
made to take into account everything related to catching
collecting fish and other sea and freshwater products.
This ranges from detecting and locating fish, to landing
it. It is suggested that such an institute should put
main emphasis on investigations of fishing techniques,
and it will depend upon the particular conditions as to
what extent the field of work of a certain institute will
have to be devoted to the other subjects. In general, the
work of a Research Institute for Fishing Gear and
Methods should be concerned with the following main
subjects:
(a) Textile materials for the production of fishing gear
(b) Manufacture of netting
(c) Non-textile materials and equipment for the
manufacture of fishing gear
(d) Treatment and maintenance of fishing gear
(c) Design and construction of fishing gear
(f) Fishing techniques
(g) Location and detection of fish
(h) Biological problems connected with catching fish
(i) Mechanisation of fishing gear and methods
(j) Correlation between fishing bo?** and fishing
techniques.
The above sequence of subjects was made up according
to practical considerations and should not be taken as
being in order of value.
2*1 Textile materials
This item includes all materials used for the manu-
facture of twines, netting, lines and ropes. In modern
fisheries these can be natural or synthetic fibres, or metal
wires. The Institute would not be concerned with basic
investigations of the fibres, but would restrict its work to
the products made from fibres, i.e. twines, cores, lines
and ropes. The determination of the property of net
materials is meant to enable the best choice of material
for specific purposes as well as for the appropriate
substitution of one material for another, e.g. natural
fibres versus synthetic fibres. For such investigations
the following properties will be of main importance.
They all should be determined with the material in wet
condition.
(a) Physical tests
(1) Breaking strength
(2) Extension
(3) Stiffness
(4) Abrasion resistance
(5) Weight and sinking
speed
(b) Chemical tests (11)
These tests would mainly be concerned with the
490
(6) Diameter
(7) Surface roughness
(8) Variations in length
(9) Reaction to tempera-
ture
(10) Reaction to light
resistance against tar products, oils, fuels, etc., parti-
cularly of synthetic fibres.
(c) Biological tests
(12) Resistance to rotting
(13) Resistance to macro-organisms (particularly
crustaceans and larvae of insects)
(14) Resistance against fouling (particularly im-
portant for gear left in the water for long periods)
(d) Workability tests
(15) Processability of net materials
(16) Ability to absorb colour and other treatments
(17) Storability.
Some of these tests are habitually conducted by special
institutes for textile research or by official material-
testing institutes. They do not make these tests from a
fisheries point of view, so the results are often of limited
value for fisheries purposes.
2*2 Netting
(a) Manufacturing techniques for knotted netting
(1) Net braiding by hand
(2) Half-mechanised netmaking
(3) Fully mechanised netmaking
(b) Manufacturing techniques for knotless netting
(1) Twine-twisting technique
(2) Raschel technique
(3) Other techniques
(c) Testing of netting
(1) Mesh size
(2) Strength and extensibility of netting
(3) Knot firmness and stability of mesh size
(4) Weight of netting
(5) Susceptibility to fouling or contamination by
natural or synthetic materials found in water
(coalescing of meshes).
(6) Visibility of netting in water
(7) Hydrodynamic resistance of netting.
Apart from the tests under "c", testing of netting may
also include some of the tests listed under 'Textile
materials" (2'1).
2-3 Non-textile materials
Apart from netting, etc. made of textile materials, for
the manufacture of most fishing gear, other implements
are needed which often consist of other materials, the
testing of which may be of value. They are for instance :
(a) Floats, buoys and sheering devices
(b) Sinkers and other weights
(c) Hooks and containers for bait
(d) Surface and underwater lamps
This list could be extended considerably, depending
on the respective fishing technique (see 2*6).
2*4 Treatment and maintenance
Very often the netting used to construct fishing gear
has to be treated specifically. These treatments may
have the following purposes:
(a) Prevention of rot (net preservation in the original
sense)
(b) Prevention of fouling by vegetable or zoological
water organisms
(c) Stiffening of netting or twines to improve the
workability or the catching efficiency
(d) Prevention of destruction by sunlight
(e) Colouring to improve catching efficiency
(f) Prevention of net destruction by other water
organisms or during storage ashore.
2'5 Design and construction
The design and construction of fishing gear has so
far been predominantly based on practical experience.
To what extent engineering methods can be used for this
purpose is still under discussion. The following means
are available for the development, testing and evaluation
of new fishing gear and methods:
(a) Model tests with
(1) Small-scale model (e.g. 1 :30) in model tanks
(2) Large-scale models (e.g. 1 :2) in open water
(b) Comparative fishing trials under normal com-
mercial conditions with new or improved gear
versus conventional gear
(c) Observation of the properties and behaviour of
fishing gear in action
(1) Direct observation by:
(i) divers, diving chambers or submarines
(ii) underwater photography
(iii) underwater television
(2) Indirect observation by:
(i) Echo sounding
(ii) Measuring instruments.
2-6 Fishing techniques
The main task of a Research Institute for Fishing Gear
and Methods would probably always be the improvement
of existing and the development of new fishing techniques.
This would also include handling and operation tech-
niques.
(a) Classification of fishing methods
(1) Fishing without specific gear (e.g. collecting of
shellfish, crabs, etc.)
(2) Wounding methods (shooting of fish, har-
pooning)
(3) Stupefying methods (fishing with poison, elec-
trical fishing)
(4) Hook and line methods
(5) Trapping methods (traps, fyke, and stake nets,
barriers)
(6) Methods for jumping fish (particularly for
mullet, salmon)
(7) Bagnet methods (scoop, stow and gape nets in
rivers and estuaries)
(8) Dragging methods (dredges, beam trawls, otter
trawls on the bottom and in midwater)
(9) Seining methods operated from the shore or from
boats
(10) Surrounding and encircling methods (purse
seines, lampara seines, etc.)
(11) Dip or liftnet methods (including water wheels)
(12) Falling net methods (castnets, cover pots, etc.)
(13) Gillnet methods
(14) Tangle net methods (with one or several net
walls, e.g. trammel nets)
(b) Applicability of fishing techniques
(1) Hydrographical conditions (water depth, cur
rent)
(2) Metcrological conditions (wind, waves)
(3) Human prerequisites (number and quality of
workers, co-operation in pair trawling or fleet
operation)
(c) Influence of the fishing techniques on the quality of
the catch.
2-7 Location and detection of fish
A prerequisite for commercial fishing is naturally the
availability of fish in such quantities or concentrations
that fishing will pay. Apart from the location of fish,
i.e. the determination of areas where commercial fish
schools are likely to be present, the Institute suggested
here should mainly be concerned with the detection of
fish, namely the spot plotting of profitable grounds or
fish schools. On the accurate detection of these fish
concentrations or schools, the rational selection of the
fishing gear and mode of operation can be based. For
fish detection the following ways and means are available:
(a) Observation of certain phenomena from the fishing
boat (direct observation of fish and fish schools,
behaviour of birds or other animals)
(b) Sensing lines and other implements derived from
these observations
(c) Observation from aircraft with communication to
fishing boats or fleets.
(d) Observation of typical fish smell (probably com-
mercially utilised only in the sardine fishery in the
Mediterranean off the African coast)
(e) Observation of biological underwater sound (many
fish and shellfish produce specific noises)
(f) Echo sounding and echo ranging
(g) Underwater photography and television.
2-8 Biological problems
As was already mentioned under 2*5 (c), the direct or
indirect observation of fishing gear can help to determine
the effects of modifications. Direct observation is also
particularly suitable for observing the reaction of the fish
towards the fishing gear and thus to find means for
improving the catching efficiency.
(a) Behaviour of fish
(1) Behaviour of fish schools
(2) Development of means for attraction and
repulsion (particularly optical, acoustical and
chemical baits, use of electricity).
Furthermore the suggested Research Institute for
Fishing Gear and Methods will have to be concerned
491
with the conservation of fish stocks, in particular as
regards the influence of the different fishing techniques
on the fish stocks.
(b) Population dynamics
(1) Influence of fishing on the fish stock
(2) Fishing methods and selectivity of fishing gear
and methods*
M
The growing difficulties in many countries of obtaining
enough suitable fishermen make it increasingly necessary
to mechanise fishing operations. For this development,
no general scheme can be given. The development of
the North-west European trawl fishery from sidetrawling
to sterntrawling with a chute is one illustration that
basic improvements and developments can be achieved
by the team work of fishermen, fishery technologists,
mechanical engineers, and shipbuilders. Another
example is the conversion of purse seining from the two-
boat technique to the one-boat technique with powered
block.
2*10 Correlation between fishing boats and fishing tech-
A fishing boat should be more than just a platform
from which any fishing method can be applied somehow.
Intensive commercial fishing requires specialised boats
which preferably should be equipped for the efficient
operation of more than one fishing method. This
demand leads to the necessity of good co-operation
between fishermen, fishery technologists and shipbuilders.
There are two main items which require attention:
(a) Best adaptation of the fishing boat to the intended
fishing methods
(b) Best adaptation of the deck layout and auxiliary
gear to the respective fishing methods
These last items are already near the border of the
suggested scope of work for a Research Institute of
Fishing Gear and Methods. However, all fishery
research institutes and the Institute for Fishing Gear
and Methods would have the strongest need for good
co-operation and co-ordination with naval architects
and shipbuilders.
One of the main problems connected with the estab-
lishment of a Research Institute for Fishing Gear and
Methods will be to find qualified professionals, because
in most countries there is, so far, no special education
for this type of work. According to experience so far,
it appears advantageous to train fishery biologists who
are interested in technical problems rather than mechan-
ical engineers, textile engineers, shipbuilding engineers
or chemists for the specific tasks of research in fishing
gear and methods. There are fewer difficulties in finding
suitable assistants because the education and training of
specialised textile, biological and chemical assistants is
more or less adequate. The Institute will further be in
492
need of skilled labourers for metal and woodwork,
because of a variety of measuring and other instruments
which would have to be constructed and maintained.
Since an important part of the Institute's work will be
devoted to practical fishing trials, assistants trained as
fishermen and netmakers will be needed.
All this personnel must be able to work efficiently at
sea under difficult conditions. The ability of smooth
collaboration with fishermen is indispensable.
Naturally, not all the subjects listed above will need
specific personnel, but the same scientist could be in
charge of several. There are many ways of arranging
the distribution of work in such an Institute as regards
staff and facilities, and the actual set-up will largely
depend on the requirements of the specific fishing industry
which this Institute is meant to serve.
4. Buildings and equipment
It was tried to make the following layout of the
working facilities comprehensive. This suggestion should,
however, not be taken as binding. There are naturally
other possibilities of co-ordination and collaboration
which may be equally efficient and would require a
modified set-up.
4'1 Textile laboratories
(a) Climate conditioned testing laboratory for net
twines, with breaking strength testers, abrasion
testers, flexibility testers, etc.
(b) Test laboratory for lines and ropes, with breaking
strength tester having a measuring range of up to
about 20 tons or more.
(c) Laboratory for biological tests, (with installations
for aquaria and a waterproof floor with drainage)
(d) Laboratory for mould and soil rotting tests. In
this room there must be provision for temperature
and humidity control (29°C±1°, humidity 60-
100%)
(e) Netmaking workshop for the construction of
fishing gear
(f) Workshop with various types of net-making
machines, rope-making machines, plaiting and
braiding machines, etc.
(g) Storeroom for fishing gear and net materials. This
room must be dry and have facilities to keep out
light.
In addition to these laboratories two outposted
stations are needed.
(h) Station for testing resistance of different materials
against weather influences. (This station could
possibly be placed on the roof of the Institute)
(i) Station for testing against rotting and fouling.
(This station could be at a suitable spot on the sea
coast, or on a river or lake.)
4*2 Chemical laboratories
(j) Analytic laboratory to analyse tanning substances,
metal compounds, oil, etc.
(k) Chemical laboratory for general purposes such as
experiments on net preservation
(1) Conservation and preservation plant for the
treatment of netting and fishing gear on a larger
scale. This room must be equipped with large
heated drums and other containers for tanning,
colouring and dyeing
(m) Storeroom for preservation materials and chemicals.
In addition to these indoor laboratories, outside
facilities are needed, namely:
(n) Covered drying space for nets and ropes. This
should preferably be close to the conservation and
preservation plant.
4*3 Physical laboratories
(o) Laboratory for precise mechanical work concerned
with observation and measuring equipment
(p) Mechanical workshop with tools and equipment for
metal and woodwork
(q) Test-stand and workshop for marine engines and
other motors
(r) Tank for model tests
(s) Echo-sounding laboratory
(t) Storeroom for measuring instruments and other
auxiliary equipment such as rubber rafts, out-
board engines, etc. needed for outdoor trials.
4*4 Biological laboratories
(u) Laboratory with aquaria for the investigation of
fish behaviour
(v) Physiological laboratory specially for electrical
fishing
(w) Large aquarium (round basin) for investigations of
the behaviour offish schools.
Apart from all theso laboratories, the usual auxiliary
rooms will be needed; such as, room for analytical
balances, drawing room, photographic laboratory,
library, registry, administration and conference room
with facilities for showing slides and films. Furthermore
each scientist should be provided with a separate small
room near his laboratory.
In this context the need for a research vessel must be
mentioned. Experience so far has shown that investiga-
tions in fishing gear and methods at sea are difficult to
combine with biological and oceanographical investi-
gations while it is comparatively easy to have a combina-
tion with fish-processing investigations. The research
vessel should be basically a fishing vessel of the type and
size of the commercial fishing boats whose needs must
be served. For carrying special measuring equipment,
motorised rubber boats have been found most suitable.
5. Co-operation with other institutions
The Research Institute for Fishing Gear and Methods
will have to collaborate closely with other scientific
institutions. The scope of work of fishing gear and
methods research is however already so widespread and
the work involved has become so expensive that many
problems cannot efficiently be tackled any more on a
national basis. Consequently, the collaboration with
fisheries institutions of other countries becomes more and
more important. To facilitate this collaboration and also
the co-ordination of efforts, a number of agencies,
societies and working groups are available such as the
International Council for the Exploration of the Sea
(ICES), the International Council for the North-west
Atlantic Fisheries (ICNAF), the Food and Agriculture
Organisation of the United Nations (FAO), The
International Organisation for Standardisation (ISO),
International Working Group for One-boat Midwater
Trawling (APE or IF).
Discussion on
Gear Research
Mr. W. Dickson (U.K.) Rapporteur: Kawakamc's paper
sets out pretty clearly the Japanese ideas on model testing.
In this many rules are set up to obtain similarity between the
model and the full-scale net. It is not always possible to obey
all these rules and that is where difficulties start, particularly
in relation to a desirable speed of towing the model. Dale
and M oiler give an interesting report on the testing of model
trawl boards, oval and slotted.
Crewe's paper on the big comprehensive tests by Saundcrs
Roe starts at the ship and works towards the trawl. His
Figs. 8, 9 and 10 are very useful because they show how the
drag of the gear matches the propeller thrust for a range of
rpm as calculated from thrust curves. A heavier gear will
have a steeper slope across these curves. If you know a net has
been towed by another ship at four knots and gave a drag of
seven tons you can tell right away what rpm's you need to
drag that gear at four knots. Your own propeller characteristics
and engine power curves will show whether your boat is
capable of doing it.
By measuring the warp tension at the surface and at the
otter boards and by measuring the warp declination at the
surface and at the otter boards it becomes possible to build
up a theory for warp shape and drag. The general finding
was that the actual drag of the warps was 50 per cent greater
than the drag as it would be calculated from the warp imagined
simply as two cylinders running between boards and ship.
Armed with that theory of warp shape it can be decided
how much of the otter-board weight to lift off the bottom
and how much to let rest on it. That is determined by the
tension in the warp and the angle at which it approaches the
bottom. The warp length is chosen to get these two factors
correct in relation to the weight of the otter board and that
is the significance of Figs. 13 and 14. .
It is a question of how much of the otter board's weight
ought to rest on the bottom for it may either profit from ground
shear or lose by it. It depends on whether the ground is hard
or soft. Up to 25 per cent of the shearing force on the board
can be due to ground shear.
Crewe also discusses the merits of cambered doors verms
oval doors and settles for the cambered ones. Here it is notice-
able what advance has been made since the last Congress.
Hydrodynamic facts on otter-board performance have been
produced from several sources so that calculations on net
spread can now be made. As the warp is curved in the hori-
zontal plane it means that the real otter-board spread is some
493
IS per cent up on the spread as measured by the warp diver*
fences at the ship. Forevcry value of spread of a headline of
flawd length there is an appropriate lead-in angle which the
wing and Us spreading wire must follow without discontinuity*
Thus with the otter-board spread established, it follows that
the net spread can be established, Roughly speaking that is
the significance of Crewe's graphs 28 to 30.
Crewe also gives the drag of sheet netting at angles of
attack of 90 deg down to zero with a range of drag coefficient
of from 1*25 down to the residual value of 0*16. May I appeal
to other workers measuring the drag of sheet netting to give
their results in this dimensionless form. 1 believe that this
diagram No. 36, taken in conjunction with the ideas of
M. Verhoest concerning drawing the developed area of a net
and so knowing the angle of attack of each bit of it will, within
the next few years, lead to a significant step forward in the
calculation of trawl drag.
But it may not be quite so simple because Hamuro's paper
gives different water speeds for inside and outside the net.
In one case he gives 3*5 knots for the towing speed and a water
speed of only 2-7 knots a couple of metres behind the footrope.
That seems a very big difference to me — has anybody any
confirmatory results? By improved net design he apparently cuts
down the difference to less than 10 per cent at which figure
it seems fair to say that there is not a big spitl or form drag,
if you like to call it that, and the bulk of the drag then arises
locally at the netting. Farther aft in the region of the codend
it is fairly certain that the waterflow through the meshes is
at considerably less than the towing speed. The American
underwater television film and our cine films show fish swim-
ming along rather lazily in the codend when the towing speeds
were such that in a full flow their swimming speeds would
most likely have been exceeded. Permanganate crystals
dropped in a towing tank show the tendency of water outside
the net in the region of the model codend to swirl and follow
the codend.
Crewe also shows in Fig. 37 that by doubling the headline
height of a net its drag rose from 4*1 to 4*9 tons — a rise of
20 per cent — not big, but very significant. Now, can the neces-
sary power be better spent on that extra headline height or on
achieving extra spread, extra net size, or even speed or some
combination of these, possibly even at reduced speed? Those
ait practical questions.
Von Brandt's paper on fishing methods and gear research
institutes is set out with his usual thoroughness. In it and in
the Norwegian organisational plan of Dale and Moller there
might be more emphasis on what I would call technological
economics — the sort of thing the Poles have been doing in their
choice of boat types and fishing methods, but carried over
instead to fishing gear and methods. Von Brandt says : "Accor-
-ding to experience so far it appears advantageous to train
fishery biologists who are interested in technical problems,
rather than mechanical engineers, textile engineers, ship-
building engineers or chemists, for the specific tasks of research
in fishing gear and methods/' This may be the experience in
Germany but one American paper gives a hint of a different
approach. I know that it has been for me a most exhilarating
experience to work with a team comprising biologists,
mathematicians, hydrodynamicists, physicists, engineers and
statisticians and not only these but with some of the Humber
skippers and ex-skippers who have made a real contribution
to the work of the team.
Mr. Dtaid* Roberts (U.K.): I think the world has opened
up into two communities in fishing. The first comprises those
who want to catch more fish to provide more food for their
people and the other— the community to which I belong—
494
has to provide commercial fish. My problem is not to catch
more fish but to catch more fish more cheaply and in a com-
mercial way. To do that we have planned a mechanised ship,
the Ross Daring, and with the increased efficiency we conceive
will be achieved, we have been able to reduce crew. Some new
and advanced instruments such as the fish-finding devices
mentioned yesterday may help us catch more fish but it
won't help us catch more fish more cheaply. It represents an
additional expense and my problem is to achieve economy
in operation.
Mr. P. R. Crewe (U.K.): I will contribute a few remarks
on our broad objectives. Our contract with the White Fish
Authority required us to make a general engineering design
investigation of Arctic bottom trawl gear, including a mathe-
matical analysis of its performance characteristics, leading
to the design and development of a gear having a headline
height and mouth width greater than that of the Granton
type of trawl generally used by British distant-water trawlers.
The specified objectives could be achieved without reference
to problems of fish behaviour, and we were not required to
concern ourselves with the latter. However, we naturally
considered that it was most important to bear the available
information on fish behaviour ii mind. This, in the main,
comprised two observations: first, the practical experience
of the industry that catch is increased as the otter boards are
moved farther and farther ahead of the net, and, second,
indications of fisheries* laboratory experiments that fish do
not appear to react to gear until it is extremely close to them,
unless they can see it.
Within this scope, we first studied the problem of headline
height, and related it to the characteristics of the leg and
sweepwire system, the headline floats and the design of the
net wings. Mr. Denis Roberts has explained that a major
problem for the British fishing industry is to catch a given
amount of fish more cheaply. In view of this and, in particular,
because of the importance placed by the industry on keeping
the area of net to be mended as small as possible, and the
water resistance of the gear low enough for most existing
sidetrawlers to tow it, we did not especially favour solutions
of the type described by Mr. Hamuro in Paper 2, where a net
having a surface area about 2*5 times that of the Granton
trawl, requires over 50 per cent more horse-power to tow it
at the same speed.
Instead we first considered how to achieve a given headline
height and mouth width by using a relatively moderate area
of net, with wings and bridles spread to a larger angle than
the value of about 16 deg appropriate to the Granton trawl.
In order to keep resistance as low as possible, methods of
refining the otter boards were studied, and model tests were
made of many different shapes. These involved measurements
of sideforce and drag with the boards both upright and heeled
over, and also when tilted so as to bring the fore or aft end
of the sole plate off the ground. In addition towing tank tests
of the stability of boards during shooting were made.
In view of the broad scope of our studies it was necessary
to establish mathematical methods for predicting gear per-
formance over a wide range of angles between the wings and
sweepwires, and the direction of motion. In fact some of the
graphs in the paper cover the overall academically possible
range from 0 to 90 deg.
Furthermore, the stability equations for the otter boards
had to be sufficiently general to cover aspect ratios from below
one-half to above two. It was also necessary to consider the
effect of the ground forces that act on an otter board that is not
free flying, and also to study the behaviour when the board is
lifted off the bottom, and the ground forces are no longer there.
Detailed studies of the behaviour of warps were made so
that the maximum information could be deduced from meas-
urements of warp load and angles, made by instruments
attached to the warps in the neighbourhood of the block.
The mathematical performance prediction methods have
been developed to the point where deductions of sea-bed
behaviour, based on the readings of the instruments at the
block, are in gratifying agreement with records provided by
self-contained recording instruments, mounted on the sea-bed
gear.
Furthermore, satisfactory comparisons have been made
between towing pulls measured at the block, and values
estimated from the resistance and propulsion characteristics
of the trawler hull, the known engine rpm, and the ship's
log speed. This can greatly reduce the necessity of measuring
towing pull directly, since, of course, a single family of curves
exists for a given ship, irrespective of the gear being towed.
Allowance must, however, be made for windage on the ship's
superstructure.
The estimated lengths of warp, of any chosen diameter,
required for a gear of a given drag, at any towing speed or
depth of water, agree very well with the rule of thumb practice
of commercial skippers.
I have mentioned some of our methods for designing
gears to any given engineering specification. The problem
is to decide what the details of the specification should be.
Information on fish behaviour in relation to the catching
power of trawl gear, given in various papers, seems to me to
show that increasing net mouth area, and the distance between
the otter boards, does not in itself guarantee anything like
a proportional increase in catching ability. Whether fish
behaviour characteristics favour a large net, with a small
angle of the sweepwires, or the reverse, or some compromise,
does not appear to have been established by these papers.
However, irrespective of what the answer is, the general
methods of design analysis, and hydrodynamic test results,
of the type referred to in my paper, are proving to be very
useful in the design and development of trawl gears to meet
various specified engineering requirements.
It may well be that the best way to solve fish behaviour
problems is to design by these methods a range of gears so
chosen that comparative fishing trials with them will have a
sporting chance of disclosing what features will give us the
greatest advantage over the fish.
Dr. J. N. Carruthers (U.K.): What I don't understand is
why when nets are tested, care is not taken to study the
variation of the currents on the bottom. They can be enor-
mously different from hour to hour. The skippers who are
fishing just do not know how these currents are setting and
the variations can be quite considerable. It puzzles me to
understand how the hydrodynamicists disentangle the influence
of the sea from the other influences that they notice and which
they are investigating. This is a matter which is particularly
important when you come to midwater trawling. What we
have to know is what the sea is doing to the net before we
can make inferences regarding the actual behaviour of the net.
Mr. Crewe said that when trawling at four knots he thought
that the bottom current would have relatively little importance.
He hoped, however, that it would be possible in future to
give more attention to the point made. They had put speed-
meters on the trawl nets but had not so far got much informa-
tion. The point would be pursued.
Dr. Miyazaki (Japan): When a fishing net has considerable
dimensions it is impossible to observe simultaneously the
configuration and performance of every part under water.
We therefore often undertake model net experiments to
determine the hydraulic functions of a net that can be made
equivalent with those of a full-scale fishing net. These model
nets are made on principles laid down by Tauti in 1934. His
essential hypotheses for establishing the hydraulic relationship
between two nets were: (1) that the elongation of twines of
both nets is negligibly small, (2) netting twines of both nets
are perfectly flexible, (3) that change in the form of the net
shall take place slowly enough to keep quasi-equilibrium of a
net under study among external forces acting on every portion
of the net. Models were therefore built to satisfy those require-
ments. Numerous experiments showed that model nets so
built enabled gear technologists to obtain results which were
well applicable to a full-scale net. Such models were particu-
larly useful for observing the effects of currents on the setting
of the net and gave a general idea of how the net actually
behaves under the water.
Dr. Bosch (Netherlands): From Bombeke's paper one gets
the impression that braided nylon is superior as regards drag
to other nylon twines of the same diameter. Could this be
explained by the -difference in the actual diameter? Was that
confirmed by the actual handling of the net at sea? In connec-
tion with such tests he would recommend they be carried out
in water after prolonged immersion of the twines and,
secondly, that the dimensions should be measured under
test conditions especially with regard to tension and water
absorption before and after the tests. These results should be
presented in the form of drag coefficients.
Dr. Sch&rfe (Germany), agreed thoroughly with Dr.
Carruthers that the currents encountered in trawling and
testing were most complicated. In their experiments they tried
to find areas where the currents were as small as possible
such as in the Baltic. The fact that the currents were in different
layers and also changed quickly made it practically impossible
to work in certain conditions. They had to find gear which
would operate efficiently in a great many different conditions.
To do that, they would have to avoid aiming at the last few
percentages of efficiency. They had to have gear that would
adapt itself to change and be reasonably good for overall fishing.
Dr. W. M. Chapman (U.S.A.): Our view is that we have to
catch fish for profit. If we can do this by increasing the volume
of fish or by decreasing the unit cost it is all the same to us.
What we are discussing comes back to economics. We look
to technical advances and stimulate research in various fields
and then management must grasp ideas from those advances
and research which can be put into the economic context
and work. That is not always left to management. In our
tuna fisheries it has been the adaptation made by adventurous
boat owners and fishermen that has been the more important
factor than the adventurousness of management. The adoption
of tuna purse seining in the Eastern Pacific was stimulated by
the absolute economic desperation of the fishermen. They were
going broke under the former cost conditions and they under-
took adaptation to the power block plus the nylon purse
seine as an act of absolute desperation. The man chiefly
responsible reckoned that if he did not take this gamble,
within three years he would be bankrupt; so he raised 250,000
dollars and put it in a purse seiner and converted his boat to
purse seining. He was lucky and the method worked.
On Dr. Carruthers' point about the temperature and condi-
tion of the sea he admitted there was a condition in the West
African waters which affected the success of purse seining.
Here was a case where an extremely efficient technological
method must await further advances in oceanographic research
in that area before it would be economically possible to apply
it there. They had to investigate the influence of the thermo-
clime in West African waters. There was a condition in the
495
tropical seas where directly below the thermoclimc mere was
not only a layer of lower temperature water but also of lower
oxygen content water. The theory that was being investigated
& retpect of the use of purse seining on tuna in tropical
waters was that the tuna will not dive through a sharp
thermodime into this colder and oxygen-deficient water for
simple physiological reasons. Therefore if the net goes down
to the thermoclime you had a floor under the net as well as
having a wall net round the fish. This looked as if it was
proving to be the case. They were having difficulty in getting
information on this question because there were very few
tuna vessels which carried both a thermograph and a scientist
capable of interpreting the results. They did, however, have
some such work going on in the Eastern Pacific and it was
possible to say that the theory appeared to be generally correct.
Where this was of importance in the West African Area was
that they had had extremely varying results in the use of purse
seines for tuna, probably because of the depth of the mixed
layer and as a result they had for the time being given up the
attempt to use this highly efficient gear of purse seining in
that area and had gone back to the relatively low type of
efficiency gear of using live bait, because in the conditions it
was the most economical fishing method for the time being.
His point in making these comments was that they required
to examine the full range of technological and scientific data
as well as oceanographical information in order to make
managerial decisions as how to catch both fish and dollars.
Mr. Dfckson, summing up, referred to Mr. Hamuro's large
Japanese net which was three times the area of the Granton
trawl, and the sort of difficulties that the adoption of such
a trawl would encounter in the Arctic where there were bad
grounds. The alternative was adopt a net that was not very
much bigger than the Granton but push up the angle at which
the wings and the sweep wires presented themselves to the
water. One of the difficulties of designing new otter boards
with better lift to drag ratio was that it was not only a case of
getting something to have a good lift once it was on the bot-
tom. Such a design can lead to difficulties in shooting the net,
and they had to resolve these two things — design a new oner
board that would give a good lift and one that could be shot
with some facility. The engineering side said that they could
now design a net according to the specifications required.
The real trouble was what should the specifications be. As
to currents affecting the behaviour of gear, Mr. Crewe was
right in saying that with the normal speed of trawling approxi-
mating four knots they would be less affected by tides than
when towing speeds were slightly slower. Experimental testing
of nets should certainly be done where possible where currents
were small. They had noticed when comparing the results of
tests taken on the Fladen ground, where the bottom was
smooth, they got a much closer scatter in their speed and drag
curves than they did in the Faroes where the tide was pretty
fierce. The point made by Dr. Miyazaki as to the value of
model testing of nets was good and here it had to be borne in
mind that models of nets should be made flexible and simple
so as to be identical with the principal net to which it related.
Many people used twine that was far too heavy for model
making and it gave a false picture of what was happening
in the net. This was another case where they had to reach
a reasonable compromise. Dr. Bosch's point as to the tensions
of twine in sheet netting in relation to drag was sound in that
twine tension could influence the drag of the netting. In tests
on the drag of sheet netting it seemed to be agreed that results
should be quoted in terms of drag coefficient. As to the point
raised by Mr. Roberts we seem to agree that whether we want
to catch more fish or the same amount at less cost, we should
aim to do so with the smallest crews.
496
Part 3 Technical Research
Section 14 Instruments for Testing Gear
Trawl Gear Instrumentation and Full-Scale
Testing
Abstract
This paper describes the instruments which were used in the
trawl gear studies reported on in papers by Crewe (No. 65) and
Dickson (No. 59). A market survey indicated that very few of the
necessary instruments were readily obtainable and this work
concerned the design, development and construction of suitable
instruments. Instruments considered necessary to measure and
if possible record continuously for proper understanding of trawl
performance, configuration and behaviour to be obtained, are
divided by the author into two groups, deck type instruments and
underwater instruments. The instruments are fully described in
the text and some 35 photographs and detailed engineering drawings
are included. Deck type instruments included warp load meters,
divergence meter, warp declination meter, warp heel meter, ship's
speed log and control panel and recorders. Underwater instru-
ments were designed for operation at all depths down to 300 fm.
They included warp load cells, warp declination meter, otter board
angle of heel meter, otter board angle of attack meter, board pitch
meter, electronic spread meters Mark 1A and Mark 2, headline
height manometer, trawl speed meters, and net twine load cells.
In carrying out full-scale trials, the number and type of instruments
used in any given haul depends on the particular aspects of the
trawling behaviour or performance being studied. In general,
however, the deck type instruments are used and complete records
made for all hauls together with loading cells, in the warps im-
mediately forward of the boards and in backstrops, and the headline
height manometer. In many hauls the whole range of instruments
with up to 10 load cells inserted in the cables were used. The
author believes that a full range of instruments is now available,
the data from which in full-scale tests of various trawls has proved
invaluable in the development of basic theory relating to trawl
gear design, and will in the future give further data of importance
in the confirmation and extension of the theory.
Utilisation d 'instruments dans les essais sur des chaluts de grandeur
nature
Resume
La communication decrit les instruments qui ont gte* utilises au
cours des Etudes de chalut rapportees par M. Crewe (No. 65) et
M. Dickson (No. 59). Une enqueue avait rev616 que, tres peu
des instruments ndcessaires a ce travail etaient disponibles sur le
march£ et ce document concerne le dessin, le d£veloppement et la
construction d'instruments appropri£s. La parfaite connaissance
du fonctionnement, de la configuration et du comportement du
chalut necessite des instruments pouvant mesurer et si possible
enregistrer continuellcment. Ces instruments, d'apres les auteurs,
peuvent 6tre classes en instruments de pont et instruments sous-
marins; ils sont complement decrits dans le texte et illustre's de
35 photographies et dessins d6tail!6s. ^instrumentation de pont
comprend des instruments pour mesurer la tension, la divergence,
la declinaison des funes, les locks, panneaux de contrdle et les
enregistreurs. Les instruments sous-marins dessin6s pour operer
& des profondeurs allant jusqu'fc 300 brasses, comprennent: des
cellules de tension, de declinaison pour les funes, des instruments
pour mesurer la bande, Tangle d'attaque, le tangage des panneaux,
des instruments tlectroniques pour mesurer 1'ouverture horizontal,
des manometres pour mesurer Touverture verticale et la vitesse du
chalut, et un tensiometre pour fils du filet. Au cours des essais,
le nombre et le type des instruments utilises dependaient des
aspects particuliers du comportement et de la performance du
chalut, et un tensionmetre pour les fils du filet. Au cours des essais,
pont sont employes. Pour chaque operation, des enregistrements
complets ont M faits avec des cellules de tension inserees dans les
funes au-devant des panneaux, sur les branchons et sur le mano-
metre mesurant la hauteur de la ralingue suptrieure. Dans de
nombreuses operations, tous tea instruments avaient jusqu'i 10
cellule* de tension inserees dans tes cables. L'auteur croit que
toute une gamme d'instruments existe maintenant et les renscigne-
by
J. Nicholls
Westland Aircraft Ltd.
(Saundcrs-Roc Division)
ments obtenus au cours d'essais sur des chaluts "grandeur nature*'
ont etc" d'importance capitate pour le developpement d*une thforie
de base concernant le dessin des chaluts, et que dans l*avenir, ces
instruments donneront d'autres informations importantes qui
confirmeront et ftendront cette thdorie.
Instrumentos para ensayar a toda escala artes de arraitre
Extracto
Describe la ponencia los instrumentos empleados en los estudios
de artes de pesca de que se da cuenta en las ponencias de Crewe
(No. 65) y Dickson (No. 59). Un exarnen del mercado indico
quernuy pocos de los instrumentos nccesarios se podian obtenercon
facilidad. Esta comunicacibn da a conocer el proyecto, construc-
ci6n y perfeccionamicnto de instrumentos adecuados. Los
instrumentos considerados necesarios para medir y, de ser factible,
registrar contimiamente para obtener una comprension clara del
rendimiento, forma y comportamiento del arte, los divide el autor
en dos grupos: los de cubierta y los submarines. Se describen con
toda clase de pormenores y se dan 35 fotografias y algunos
esquemas muy detallados. Entre los instrumentos de cubierta
estan los medidores de la carga, dcclinacidn, inclinacidn y
divergencia del cable, correderas, cuadros de mandos y registra-
dores; los instrumentos submarines se proyectaron para funcionar
a todas las profundidades hasta 300 brazas y comprenden c61ulas
de carga y medidores de la declinaci6n del cable, medidores del
angulo de inclinaci6n, del dngulo de ataque y del cabeceo de las
pucrtas, medidores clectrdnicos de la abertura Mark 1 A y Marck 2,
manbmetro de la altura de la relinga de corchos, registradores de
la velocidad del arte, y celulas registradoras de la carga a que
estan sometidos los hilos. Al realizar los ensayos a toda escala,
el numero y clase de instrumentos empleados en cada lance dado
depende de aspectos especiales del comportamiento o rendimiento
del arte qut se estudia. En general, en todos los lances se emplcan
los instrumentos de cubierta para obtener registros completes y
tambien se usan c&ulas de carga en los cables immediatamente
delante y detras de las puertas y el mandmctro de altura de la relinga
de corchos. En algunos lances se emplearon todos los instrumentos,
insertandose en los cables hasta 10 c61ulas de carga. Cree el autor
que actualmente se encuentran instrumentos de todas clases, con
los que en ensayos a toda escala de diyersos artes de arrastre se
han obtenido datos que ban resultado valiosisimos en la formulacion
de teorias fundamen tales rclativas a las formas de los artes, y que
en el futuro daran otros datos de importancia que serviran para
confirmar y ampliar las teorias.
1. T ATE in 1958 the White Fish Authority with the
JL/ financial backing of the British Trawlers'
Federation and H.M. Government placed a
Research and Development contract with Saunders-Roe
Ltd (now the Saunders-Roe Division of Westland
Aircraft Ltd), the ultimate object of which was the
497
production of a new trawl with increased headline height
and spread to replace the Small Granton trawl as used
by the distant-water fishing fleet.
It was obvious that for the efficient carrying out of such
a programme a substantial range of special instruments
was required. A market survey showed that very few of
these instruments were readily obtainable, and hence the
contract included the design, development and supply of
suitable instruments.
This paper describes some of the instruments and their
mode of operation and use in full-scale testing of trawl
gear. It should be noted that patents have been taken
out on some of the instruments.
2, RANGE OF INSTRUMENTS
The following parameters are those which it was
considered essential to measure and if possible record
continuously, to enable a full understanding of the trawl
performance, configuration and behaviour to be obtained.
Although some are of greater importance than others
they are not given in order of importance but in order of
location from the winch to the codend.
(a) Warp tensile loads at ship
(b) Warp divergence at towing block
(c) Warp declination at towing block
(d) Warp heel at towing block
(e) Ship's speed
(f ) Loads in cables, i.e. warp forward of otter boards,
bridles, headline, groundrope, etc.
(g) Warp declination at otter board
(h) Otter board angle of heel
(j) Otter board angle of attack
(k) Otter board angle of pitch
(1) Board spread, i.e. distance between the boards
(m) Netmouth spread
(n) Headline height
(p) Loads in the twines of the net
(q) Trawl speed
It is noted that the instruments divide into two main
groups each with two sub groups as follows:
Group 1. Deck type instruments. (This designation
simply means that they are used on the deck of the ship
and not under water.)
1*1 Load measuring instruments.
1*2 Configuration (angle or distance) measuring
instruments.
Group 2* Underwater instruments.
2*1 Load measuring instruments.
2*2 Configuration instruments.
3. BASIC DESIGN OF INSTRUMENTS
3*1 Deck type
It was decided that these should be of electro-mechanical
type, with remote visual recording in the ship's laboratory.
Based on the experience of the Aberdeen Fisheries
Laboratory, these instruments were designed to be used
with Leeds and Northrup Type H Model S 0-12 mv
498
servo operated strip chart recorders, these having a res*
ponse of 1 sec for full-scale deflection and a chart
speed of 0 • 5 inch per min. Of course any other recorder
of suitable characteristics can be used.
3-2 Underwater instruments
It was decided that all the instruments should be designed
for operation at all depths down to 300 fathoms.
While the very desirable ideal was to transmit all the
information to the ship for visual recording so that at all
times a complete running picture of what was happening
to the trawl could be seen, it was concluded from a study
of the state of the development of underwater telemetry
that this was not practicable within the time and money
available. The use of warps with multi-core electric
conductors was also considered impracticable. The
underwater instruments therefore had to be of a com-
pletely self-contained recording type.
Investigations were made as to which type of instru-
ment—electric, electronic, or mechanical— would be
most suitable and it was concluded that, apart from the
electronic spread meters, mechanical type instruments
offered the best solution.
As regards the recording, three types of chart were
considered: circular, continuous strip, and drum charts.
For reasons of both cost and space economy, drum
clocks and charts were decided upon.
Of the many methods available for marking the trace
on the chart, a heated stylus operating on heat sensitive
paper was thought best. This method had the advantage
of minimum stylus pressure combined with the fact that
the recording was completely unaffected by the attitude of
the instrument. One disadvantage was that an electric
battery was required for stylus heating and this proved
to be a difficulty in some instruments from a space point
of view.
4. DECK TYPE INSTRUMENTS
4*1 Warp load meters
The complete equipment consists of the meter for
attaching to the warp, a multi channel control panel or
an individual control box, an inter-connection lead, a
6 V DC power supply, and a recorder.
The meter is designed to measure the load, up to a
maximum of 10 to 11 tons, in a continuous warp of
which the largest size allowed for is 3 J-inch circumference
and is of the well-known 3-wheel centre deflected
principle.
Many instruments of this type are made for a fixed
cable diameter or are adjustable in discrete steps for
other cable diameters and hence considerable errors in the
indicated loads are induced by variations in the nominal
diameter of the cables and by the additional changes due
to wear and tear. These troubles have been overcome
by incorporating a screw adjustment on the centre wheel
which permits the meter to bib used on cables ranging in
size from If-inch circumference or less to 3J-inch cir-
cumference and, what is very important, allows a zero
adjustment to suit the exact si^c of the cable being tested.
Fig. 1. Warp load meter (deck type).
The meter shown in Fig. 1 in use on a trawl warp,
consists of a rigid beam with two fixed wheels, one each
end, which rest on the warp, and a centre wheel on the
opposite side of the warp. This centre wheel is remov-
able to allow fitting the meter to the warp and is adjust-
able for the cable size and zero setting, and a definite
deflection (i inch) to the warp between the two outer
wheels is applied through a lever-operated eccentric.
The centre wheel adjustable and sliding mounting
incorporates a strain gauged tensile link for measuring
the lateral load, which has a definite relationship to the
warp tensile load. The electrical strain gauges are
standard Saunders-Roe J-inch foil gauges and have given
complete reliability and trouble-free service over a period
of four years with some 250 to 300 hrs recording.
The strain gauge bridge is designed for a 5 V DC
power supply and as the output of the bridge is dependent
on the voltage applied it is essential that this is kept
constant. For this reason a nominal 6 V battery is used
with a potentiometer and voltmeter in the circuit so that
the bridge voltage can be set and maintained at the
required 5 V.
The electrical system is shown in Fig. 2.
The output, approximately 11 mv for 10 tons warp
load, is not linear with the warp load, due to the small
deflections of the beam and crushing of the warp and
varies slightly with warp size. A typical calibration
curve using a 3J-inch circumference warp is shown in
Fig. 3. The reason for the scatter of the results was at
first obscure but was found by careful and detailed tests.
On individual calibration tests a complete cycle of loading
gives a hysteresis loop of approximately ± 1 per cent
about the mean, and this is also shown in Fig. 3; pro-
viding the meter is located in precisely the same position
on the warp this is repeatable within about ±\ per cent
change in the mean line position. However, as the meter
location on the warp relative to the spiral lay of the warp
is altered so the slope of the mean line alters. The mean
curve given in Fig. 3 is from 8 cycles of loading at differing
• At, I I V I
GAUGE
STRAIN GAUGE
TENSILE LINK
-ACTIVE
GAUGE
•INACTIVE
...
w**mmm
FOUR CORE
CONNECTING
LEAD
RECORDER
1
_l
POTENTIOMETER
Fig. 2. Warp load meter (deck type) electrical system.
9
1-
''"
/
8
$/
/
7
6
TONS
5
4
3
2
1
ft,
'
/
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tfjf)
/
r'l
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0 2 4 6 8 IO 12
MV.
Fig 3. Deck warp meter calibration. Typical.
positions along the spiral and the upper and lower
boundary points show limits of ± 6 per cent relative to
the mean line. This variation is presumably due to the
499
Fig. 4. Diagram of contact of warp on pulley, showing effect of
spiral lay.
differing crushing deflections of the warp, see Fig. 4,
altering the total warp deflections. However, the
accuracy of ± 6 per cent is considered very reasonable
in view of the fact that a large part of the variation is
due to the inherent characteristics of the warp, although
an improvement might possibly be obtained if separate
sets of wheels, with grooves machined to fit snugly round
the warp, were provided for each nominal size of warp.
It is noted that in full-scale tests the analysis of a
number of hauls where the ship speed, tide, trawls, and
bottom conditions were as nearly identical as possible,
showed scatter of loads in the order of only ± 3 per cent.
A typical recording is shown in Fig. 5.
Fig. 5. Typical warp meter recording (deck type).
The meter, which weighs 44 Ib, is very easy and simple
to fit to the warp by two men and takes only 2 min or
so even in fairly rough weather.
The rigid beam is of welded channel section in corro-
sion-resistant aluminium alloy. No trouble from
corrosion has been found in the four years the instru-
ments have been in use.
4*2 Divergence meter
This instrument is designed to measure the divergence
angle of the warps at the towing block, but for conven-
ience in interpreting the results is arranged to indicate
the angle in terms of the spread in feet per fathom of
warp aft of the block.
The meter, shown fitted to the warps in Fig. 6, consists
of two tubes, nominally parallel to each other, and
500
Fig. 6. Divergence meter with declination and heel meters.
referred to hereafter as the warp tubes, each with two
hooks for locating the meter on the warps. Their ends
are connected by two spring loaded telescopic struts, the
forward one of which is fixed rigidly at 90° to one of the
warp tubes and is pivoted to the other warp tube. At this
pivot a toroidal potentiometer is incorporated in such a
manner that any angular movement of the warp tubes
relative to each other causes a corresponding displacement
of the potentiometer. The telescopic strut at the aft end
is pivoted to both warp tubes. The telescopic struts and
their pivots are arranged so that warping of the whole
assembly, to allow for differential declinations of the
two warps, can take place. In use the meter, as shown
in Fig. 6, is placed on the warps, just aft of the towing
block, with the hooks between the warps, the spring
loading of the telescopic struts holding the warp tubes
against the warps, thus displacing the potentiometer to
the angle corresponding to that of the warps.
It should be noted that the instrument shown is made
for starboard sidetrawling, and for port sidetrawling a
meter of opposite hand is required to avoid the telescopic
struts fouling the ship's side.
On the present instruments the maximum divergence
angle of the warps which can be measured is 1*2 ft
per fathom of warp (approximately 11J°) but could be
modified if required, to measure larger divergencies.
The electrical system shown in Fig. 7 incorporates the
measuring potentiometer and a zero setting potentio-
meter in the meter itself and the meter is connected by a
plug-in 4-core cable to a control panel carrying a fixed
resistance and a helical setting potentiometer arranged so
that the same 6 V power supply as used for the warp load
meters can be used and this gives an output of 1 mv for
each -1 ft per fathom of warp spread.
A setting and checking jig is supplied with the meter.
For all hauls, immediately prior to fitting to the warp
and after connecting to the control panel, the meter is
METER.
1950X150° WINDING
MEASURING
POTENTIOMETER
APPROX 12° USED,
OOOAZERO
SETTING TOROIDAL
POTENTIOMETER.
FOUR CORE
CONNECTING
LEAD.
,.30000 A
HELICAL
POTENTIOMETER
CONTROL
PANEL
6.V.
BATTERY
'FIXED RESISTANCE
APPROX IOO,OOOn
Fig. 7. Divergence meter electrical system.
set by placing it in the jig with the bars, representing the
warps, parallel and in this condition the recorder should
indicate zero. If this is not so, the zero setting potentio-
meter, access to which is given by removing the water-
tight cover, should be adjusted. It is noted that the zero
setting potentiometer is fitted for two reasons, first as
without it a true zero cannot be obtained on the meas-
uring potentiometer due to the resistance of the end
connection and secondly it enables any error caused by
wear of the hooks due to warp vibration, to be corrected.
The jig bars are then opened to the 1-0 ft/fm position
and the control panel potentiometer is set so that 10 mv
is indicated on the recorder. The meter is now set and
ready for use.
At the end of each haul the meter is checked in the
same way as above to confirm the satisfactory functioning
of the instrument during the haul.
As regards accuracy, on basic calibration its error is
not more than ± •! mv — equivalent to '01 ft/fm of
warp. Fishing with 400 fathoms of warp aft of the block
this means an error of ±4 ft in the spread between the
boards of say 200 ft. In practice even though the error
may be slightly higher than indicated above because of
vibration of the warps, this accuracy is considered very
reasonable and acceptable.
A typical recording is shown in Fig. 8.
In fitting the meter to the warps, two pairs of hands
are necessary. First, a retaining rope on the meter is
securely tied to the ship and then if the towing block
is on the level of, or only slightly below, the gunwale
the meter can be closed by hand and placed on the warpi
with the hooks between the warps and then released so
that the hooks engage underneath the warps. For use on
ships where the towing block is well below the gunwale
special tongs are provided which with the aid of an
overhead block and tackle attached to the tongs to take
the weight, lower the meter onto the warps and as the
tongs are then released the meter opens and the hooks
engage under the warps. Even in rough weather and
using the tongs the meter can be fitted to the warps in a
matter of 12 to IS sec.
The meter is made throughout of stainless steel and,
complete with the declination meter (qv), weighs 20 Ib.
Fig. 8. Typical divergence and declination recordings.
4-3 Warp declination meter
The meter, designed to measure angles of declination
from 0 to 45°, is set so that 1 mv output =5° declination.
It consists of a simple pendulum the pivot of which is the
spindle of a low friction toroidal potentiometer. The
whole assembly is in a case filled with transformer oil to
provide damping of the pendulum. For convenience of
fitting to the warps, the case is bolted to the top of the
divergence meter as shown in Fig. 6.
The electrical system shown in Fig. 9 incorporates the
measuring potentiometer and a zero setting potentio-
meter on the meter these being connected by a 4-core
cable to a control panel carrying a fixed resistance and a
setting potentiometer arranged so that the same 6 V DC
power supply as for the warp load meters can be used.
Two accurately set stops are fitted, one at 0° and the
other at 45° declination.
After bolting to the divergence meter and connecting
to the control panel, the meter is set by first tipping the
meter so that it is at a negative angle with the pendulum
501
oa hi 0* stop and checking that zero mv is indicated on
Hie recorder. If it is necessary to adjust the zero, access
to the zero set potentiometer by a screwdriver is by
removing the plug in the top of the case. The meter is
then tipped to a positive angle greater than 45° so that
the pendulum is on the 45° stop and the setting potentio-
meter cm the control panel is adjusted so that 9 mv is
indicated on the recorder.
MEASURING
POTENTIOMETER
(45° USED)
POUR CORE
COHNECTING
LEAD.
IOOA ZERO
SITTING TOROIDAL
POTENTIOMETER.
•" 1
nfti.iv.Ak
POTENTIOMETER
I
•N
[-1 1
1
-<J
I)-
CONTROL
1
V.
-X
PANEL
6.V.
BATTERY
-IXED RESISTANCE
APPROX ISjOOOA
rig. 9. Declination meter electrical system.
The accuracy of the meter in calibration checks is well
within ± -25° although in practice with severe warp
vibration present, interpretation of the records to better
than ± -5° would not be claimed.
A typical recording is shown in Fig. 8.
4*4 Warp heel meter
It is noted that in all cases where the trawl is properly
set, the heel if any is such that the outer warp is high and
therefore it is not necessary to have a meter reading both
sides of zero. This meter is therefore exactly the same
as the declination meter described in paragraph 4-3 but
is mounted transversely on the divergence meter as
shown in Fig. 6.
It is noted that as the heel angle is of secondary
importance, the heel meter is in effect a spare for the
declination meter.
4*5 Ship's speed log
A Kelvin Hughes Standard Direct Reading Current
Meter (range 0 to 6 knots) has been adapted for this
purpose. The meter is towed off the ship's side at a
502
distance of about IS ft and a depth of about 20 ft and
the electrical lead run to the indicator unit located adja-
cent to the main control panel and recorder rack. A
modification to the standard indicator unit enables the
signal to be taken to one of the recorders for continuous
recording. The calibration, made in our own water test
tank, is of log scale type and a setting potentiometer
added to the indicator unit is set so that the output to
the recorder at 6 knots is 12 mv.
The meter and indicator unit is shown in Fig. 10.
rig. 10. Kelvin Hughes Direct Reading Meter used for ship's
speed recording.
4*6 Control panel and recorders
The individual control circuits and recorders referred
to in paragraph 4-1 to 4-5 have in practice been incor-
porated in a single control panel mounted in a rack with
six recorders. This is shown in Fig. 1 1 . Wander leads
with Jack plug connectors are used for the recorder
signals and hence any one of the six recorders can be used
with any one of the meters and in the event of a recorder
failure, the least important meter can be cut out with only
a minimum of loss of recording of the more important data.
Incorporated in the panel is a recorder test point
whereby a known signal of say 5 mv can be fed into the
recorders to check that they are functioning correctly.
As previously mentioned in paragraph 3* 1 , the recorders
are Leeds and Northrup Type H model S strip chart 0 to
12 mv. The power supply required for these is 240 V
AC and as it is necessary that a zero potential neutral
wire system is used, an isolating transformer is included
in the rack to suit the usual ship's supply system. Power
supply for the meters is from a nominal 6 V 90 amp hour
accumulator mounted on the rack. The high capacity
is provided so that even with all the meters in use at the
same time, the voltage change in the course of a haul is
completely insignificant. The setting potentiometers on
the panel are such that a voltage range of approximately
5-8 to 6-3 V can be covered and hence it is usually
unnecessary to require a recharge of the accumulator
during the course of a three weeks' test cruise.
Fig. 11. Control panel and recorder rack.
5. UNDERWATER INSTRUMENTS
5*1 Pressure strength factors
As stated in paragraph 3*2, all the underwater instru-
ments were designed for operation at all depths down to
300 fathoms and the strength of the casings of the
instruments was based on the following design and test
load factors and pressures:
Static head pressure at 300 fm
Design ultimate factor 2-0
Proof test factor 1 • 5
Water tightness pressure test
factor 1-25
800 lb/in2
pressure 1,600 lb/in2
„ 1,200 lb/in2
„ 1,000 lb/in2
Stress calculations were also made to check that the
design ultimate and proof factors were achieved with the
pressure loading in combination with other appropriate
loading, eg warp tension loading in the case of the warp
load cells. In addition, particular attention was paid to
the general robustness of the instruments to withstand,
again in combination with the pressure loading, the hard
knocking about to be expected with the trawl in use on
rough ground. In consequence of this general robust-
ness, together with practical engineering aspects, some
of the instruments have pressure strengths in excess of
the design ultimate quoted above. One example of this
is the larger size load cell which has been by accident
tested to 2,500 lb/in8 without damage, distortion, or
leakage occurring.
5*2 Warp load cells
These cells of a completely self-contained recording
type are designed to be inserted in cables by means of
shackles.
Cells with three differing load ranges are in use:
(a) recording up to 12£ tons load
(b) „ „ 5 tons load
(c) „ „ 3J tons load
Except that bourdon tubes of different stiffnesses are
fitted, cells (a) and (b) are identical to each other. Cell
(c) is of the same basic design as (a) but much smaller
in overall size and weight.
To provide general robustness, allow for wear and tear,
and to ensure that structural failure did not occur prior
to a warp breakage, design ultimate and proof or yield
factors of 2-0 and 1-5 respectively on the breaking
strength of the warps was used for the design of the cells.
For cells (a) a warp size of 3J-inch circumference was
assumed with a breaking load of 26 tons, this including
an efficiency factor of -8 on the nominal strength of the
wire of 32-5 tons to allow for the reduction in strength
at the splice.
Thus
Design ultimate load = 26 x 2 =52 tons
Design proof load = 26 x 1 -5 = 39 tons
Load cells (c) were designed for use in the lighter
headline and legs and a cable breaking load of 8 tons was
assumed giving :
Design ultimate load =8x2 =16 tons
Design proof load = 8x1-5 = 12 tons-
5-2-1 Cells (a) and (b)
Fig. 12 shows an opened cell, and Fig. 13 illustrates
cells fitted to the trawl prior to shooting.
Fig. 14 shows a longitudinal section through the cell
from which it is seen that it works on the hydrostatic
principle, with the applied load giving a proportional
pressure in an oil filled cylinder, the pressure being
measured by a bourdon tube and recorded.
An oil filled thin walled synthetic rubber bag is housed
in a cylinder and fixed to the underside of a piston on the
top of which are mounted the bourdon tube and drum
Fig. 12. 121-ton underwater hod cell
503
Fig. 13. Underwater load cells fitted in trawl wires prior to
shooting.
clock and the battery for the stylus heating power. The
oil connection from the bag to the bourdon tube is
through one of the fixing studs of the bag to the piston.
The piston rod passes through the bag and the cylinder
end to form the eye for one of the attachment shackles.
Water-tightness at the piston rod entry through the
cylinder end is achieved by a diaphragm seal, the seal
clamping ring on the cylinder end also being used as the
stop for the torque pin which prevents rotation of the
piston in the cylinder under the action of the twisting
loads in the warp.
The pressure-tight case, which at its other end has an
eye for the shackle attachment, is screwed onto the
cylinder and locked by two screws. It is this joint which
is broken for access to the recording charts, etc. each time
the cell is used. This joint is sealed with a synthetic
O-ring seal. Fitted to the top of the piston by four
screws is a removable extension tube which terminates in
a flat-ended spigot of the same diameter as the piston rod.
The spigot locates in a bearing in the end of the case and
the flat end butts against a synthetic rubber diaphragm,
the outside of which is open to the water. Thus both
the piston and cylinder have balanced longitudinal loads
due to the static head pressure of the water and the
recorded cable loads are completely independent of
depth of water.
The cells are made mainly of mild steel with a few
highly stressed parts, such as the piston rod, of high
tensile alloy steel, and with cleaning and suitable main-
tenance after each cruise, corrosion has not been a
serious problem. A i-inch thick rubber cover is fitted
around the cell to act as a shock absorber and protect
the clock and bourdon tube movement.
The length between the shackle pin holes is 16*5 in
and the outside diameter of the case is 5-6 in.
The cells weigh 71 Ib in air and 53 Ib in water.
The effective piston area is approximately 10 in8 and
thus the oil pressure at the maximum recording load of
12i tons is 2,830 lb/in2.
The bourdon tube, of stainless steel, is of the normal
circular type with a simple lever movement. The stylus
end is formed by a loop of nickel-chrome resistance wire
and the heating power is from a standard rechargeable
lead acid dry accumulator which will give approximately
3\ hrs continuous recording.
Starting the clock and switching on the stylus heating,
is effected immediately prior to shooting by inserting a
screw in the meter case which acts on a simple combina-
tion switch.
The spring driven drum clock has a chart of heat-
sensitive paper of 2-5 inch effective width and 10 inch
length which with a drum speed of 1 revolution per 200
minutes gives a paper speed of -05 inch/min. Note that
this is referred to in subsequent paragraphs as the
"standard" drum clock. The load scale is 5 tons/inch
for cell (a) and 2 tons/inch for cell (b). These load and
time scales give adequate accuracy and discrimination,
and correlation of the results from numerous instruments
used at the same time has been very good.
Typical recordings are shown in Fig. 15.
A typical static calibration curve, Fig. 16, shows a
hysteresis loop of about ± 6 per cent which is mainly
. 14. 121 and 5-ton had cells.
504
from the bourdon tube itself. Limited response tests
with a superimposed sinusoidal vibrating load for cell (a)
showed, at a typical loading of about 3 tons ± -4 tons,
results very close to those for the static loading up to a
frequency of about 50 cycles/minute, and at 60 cycles
some slight fall-off was noticeable. For cell (b) a similar
result was found at a loading of 2- 15 ± -65 tons.
JfcWtCMOCCU
Fig. 15. Typical had cell recordings.
Thus taking the mean calibration line and the mean
amplitude of typical recordings the error should be very
small but taking into account all the possible variations
a general accuracy of less than about ± 4 per cent is not
claimed. For general research this has been found to be
adequate. It should be noted that for individual high
peak loads — such as in shooting loads — the increasing
load calibration line should be used.
10
TONS
a
•5 tO 14 2O
SCALE READING INS.
2*5
•9 W 14 20
SCALE READING INS.
Fig. 16. (Left ) 12-5-ton load cell calibration. Typical.
Fig. 17. (Right) 3-5-ton load cell calibration. Typical.
5-2-2 Cells (c)
These cells are in their general operation and con-
struction the same as cells (a) and (b).
The effective piston area is approximately 5 -9 in1 and
thus the oil pressure at the maximum recording load of
3i tons is 1,330 Ib/in1.
A smaller size drum clock is fitted and this has a chart
of effective width of 1 -75 in and length 7 in which with
a drum speed of 1 revolution/200 min gives a paper
speed of -035 in/min. The load scale is 2 tons/in.
A typical recording is shown in Fig. 15 and a typical
calibration curve in Fig. 1 7. The accuracy is of the same
order as the larger cells (a) and (b).
The length between the shackle pins is 14 • 6 in and the
outside diameter of the case is 4*25 in.
The cells weigh 26 Ib in air and 21 Ib in water.
5*3 Warp declination meter
This meter, shown in Fig. 18, has a simple pendulum,
the pivot shaft of which is carried in two miniature low
friction ball bearings, with damping by an air cylinder
and piston made from a standard glass hypodermic
syringe. A small needle valve is fitted to the cylinder
end for damping adjustment. This damper has proved
to be very cheap and effective. A stylus fitted on the
end of the pendulum shaft traces its movements onto a
standard drum clock chart.
Fig. 18. Warp declination meter.
The instrument, including the stowage of the battery
for the stylus heating, is assembled on a base plate,
which is bolted into a spherical pressure case. On the
top of the case two projecting lugs, with split bearings,
form a hinge with the warp as the hinge pin, thus the
whole instrument hangs as a pendulum below the warp
which can rotate without affecting the instrument but
the instrument is constrained to take up the declination
attitude of the warp and this is recorded.
The zero angle datum is the centre-line of the chart, the
stylus being carefully set, after assembly of the instrument
in the case, with the warp at zero angle.
The recorded angle is simply determined by
Angle = Sin
_l ordinatc from centre line of chart
Sin
— 1 ordinate*
2-75*
length of stylus
The maximum angles which can be recorded are
±27°
It is obvious, however, that the water flow past the
instrument mounted in the manner described above
505
would cause instability and rotation of the instrument
around the warp. To obviate this, a completely in-
dependent spherical shell is fitted on the warp around the
instrument case. This shell is water flooded and thus
provides dead water around the instrument and allows it
to hang steady and stable. Fig. 19 shows a diagrammatic
arrangement of the assembly and Fig. 20 the meter fitted
to the fore warp ready for shooting. This has proved
very satisfactory and good recordings have been obtained,
a typical one being shown in Fig. 21.
TIR taorecriVE
SHELL
MAIN WARP
PENDULUM
ft STYLUS
CENTRE
RECORDING DRUM
PJENJHJLUM
CAiLE CLAMPS
CASTING
DECLINATION
UNIT
FORWARD
Fig. 19. Warp declination meter.
The spherical instrument case, 8 j-inch outside diameter,
is made of two corrosion resistant aluminium alloy
castings, bolted together by three swing bolts on a
diametric joint incorporating an O-ring seal.
No corrosion or leakage problems have arisen.
The spherical shell 22-inch diameter and split into two
halves on the diameter containing the warp, is made of
10 swg corrosion resistant aluminium alloy. The two
halves are held together and onto the warp by only two
clamps around the bearings on the warp and the instru-
ment and shell are located by four stops clamped on the
warp.
The weight of the meter excluding the warp stops and
shell is 19 Ib in air and 3 Ib approximately in water.
5*4 Otter board angle of bed meter
The meter itself is exactly the same as the warp declina-
tion meter described in paragraph 5*3. It is also
assembled in a cast aluminium alloy spherical case of
the same size as that of the warp meter but has four lugs,
in the shape of a cruciform, on each half casting for a four
stud attachment to the otter board. The cruciform
arrangement of the lugs is so that the meter can be turned
through 90° and used as a pitch meter. Fig. 22 shows
the meter mounted on an otter board, and Fig. 21 a
typiqal recording.
The meter weighs 18 Ib in air and 2 Ib approximately
in water.
506
5*5 Otter board angle of attack meter
This instrument was not designed by Wcstland Aircraft
but was "bought-out" as a fully developed working
meter. It was designed on the principle that a rod
hinged on the board at one end on both the vertical
and horizontal axis and the other end trailing on the
ground would take up the direction of the otter board
over the ground and thus the rotation around the vertical
axis with respect to the board would give the angle of
attack and could be recorded. The instrument consists
of a vertical shaft operating, through a gear, a stylus
Fig. 20. Warp declination meter at board ready for shooting.
v
Fig. 21. Typical recordings.
which records on a circular chart driven by a spring clock.
The instrument is housed hi a flat cylindrical phosphor
bronze case, with the shaft entry sealed with rubber "O"
rings and the case cover fixed by eight studs and sealed
with a square section seal ring.
The meter weighs 45 Ib in air and 34 Ib in water.
In practice satisfactory records could not be obtained,
as on a good smooth ground the impossibly large angle
of 60°, the limiting stop angle of the instrument, was
indicated, whilst on rougher ground random fluctuating
angles of large amplitude intermittent with the limiting
stop angle were indicated. In addition repeated failures
either by complete breaking or bending of the shaft
occurred.
A theoretical study of the otter board motions and of
the meter operation indicated that both the unsatisfactory
records and the shaft failures were probably from the
same cause. Normally the board is heeled outwards
10-20° and in this condition the weight of the trailing
rod taken at the board end biased the reading to a much
higher apparent angle of attack and this combined with
the oscillatory heeling motion of the board induced by
the varying ground side forces, particularly on rough
ground, gave the random fluctuating angles. Although
attempts were made to use a trailing wire with floats to
act as a drogue these were not successful for various
reasons and it was decided that this method of measuring
the angle of attack of the boards should be abandoned.
Fig. 22. Otter board heel, pitch and angle of attack meters fitted
to board.
However, it was decided to modify the meter and use it
to record the board angle of attack relative to the bridles,
and thus from the angle of the bridal relative to the
direction of motion found by spread measurements, etc.
the true angle of attack of the boards could be deduced.
This was achieved by attaching the upper backstrop to
the board via a vertical shaft which was caused to rotate
with any change of angle of the backstrop to the board
and this rotation was transmitted to the meter by a
system of levers and rods. Fig. 22 shows the meter
mounted on the board.
By this means the board angles of attack have been
determined to a probable accuracy of ± 3°.
5*6 Board pitch meter
This instrument was not designed by Westland Aircraft
but "bought-out" as a developed working instrument.
The instrument, shown, mounted on the board, in
Fig. 22, consists of a pendulum on pivot bearings carrying
a pin stylus recording on a circular wax faced paper chart.
The chart is on a disc driven by a spring operated clock
at a speed of 1 revolution/4 hrs, and of course the paper
speed varies with the amplitude of the angle.
When first used the results were unsatisfactory as the
only damping of the pendulum was by adjustment of the
stylus pressure, which proved difficult to set just right;
either the pendulum thrashed about at random or the
stylus "dug in" and tore the surface of the paper.
An air cylinder and piston damper similar to that
described in paragraph 5*3 was fitted and this improved
the performance considerably and satisfactory recordings
were obtained.
The instrument is housed in a flat cylindrical phosphor
bronze case, and is attached to the board by four studs
and with a special bracket can be mounted to measure the
heel angles of the board.
The meter weighs 35 Ib in air and 26 Ib in water.
The very high weight of this instrument as compared
with the heel or pitch meter described in paragraph
5-3 is to be noted.
5-7 Electronic spread meters Mk 1A and Mk 2
These electronic instruments were designed by the
Electronics Department of Saunders-Roc. AS a Struc-
tural Engineer, the author limits himself to describing the
structural and operational aspects*
5*7*1 Board spread meters (Mk 2)
The functioning of this spread meter depends on basic
principles similar to those relating to conventional echo
sounders. It consists of two units referred to as the
Recorder and Responder unit respectively, these being
fixed at the two points between which it is required to
measure the distance. On each of these units two
piezoelectric transducers are fitted, one transmitting and
the other receiving. A signal is transmitted from the
Recorder unit to be received by the Responder unit which
re-transmits it, at a different frequency, back to the
Recorder unit where it is recorded on "teledeltos" paper
507
together with the original transmitted signal. The
recorder itself is a standard instrument using standard
echo sounder chart paper marked in fathoms. The
electronic units are housed in identical cases made up
of aluminium alloy thick walled tubes with bolted on
end covers. One end cover is semi-permanent and has
built in an on-off switch whilst the other is removable for
normal access to the units. Synthetic rubber O-rings are
used for sealing to ensure water-tightness. The trans-
ducers are mounted in the wall of the tube and once
again O-ring seals are used.
Direct mounting on the boards proved to be un-
satisfactory because of ground interference with the
signal and the changing heeling angles causing misalign-
ment of the units. A satisfactory method of operation
was found to be by towing the units well clear of the
ground and for this purpose a conical tail cone of the
type developed by Messrs. Kelvin Hughes for the Current
Meters was made and fitted to each unit. The unit is
pivoted on a horizontal axis at its centre of gravity to
extension arms of a towing hanger or frame which lies
above the unit. The units complete with tail cones and
hangers are shown in Fig. 23.
Fig. 23. Otter board electronic spread meters (Mk 2).
The units are towed via the hangers, one from each
warp, at a distance of 20 to 25 fathoms forward of the
otter boards by cables, approximately 12 ft long, with
freely rotating attachments to the warps. A clamp stop
on the warp prevents the meter sliding down towards the
board. A wire about 18 ft long is attached to the aft
end of the frame and at the other end deep-sea floats are
fixed to act both as a drogue to give towing stability and
also to lift the units clear of the warps. The floats have
508
a buoyancy of about 60 Ib so that if the unit breaks
away from the warp it will be floated to the surface, the
weight of the unit in water being about 45 Ib (90 Ib in air).
The arrangement is shown diagrammatically in
Fig. 24.
It is to be noted that care must be taken to see that
the units are attached to the appropriate warps with the
transducers facing each other.
tOTATlNO
ATTACHMfNT
Fig. 24. Towing arrangement ofMk 2 Spread Meter.
5*7*2 Netmouth spread meter (Mk 1A)
In this spread meter a single unit is used with two
remote transducers — one transmitting, the other receiving
—connected to the unit by electric cables.
The unit is shown in Fig. 25.
Fig. 25. Net mouth. Electronic Spread Meter Mk 1A.
The electronic and recording systems are almost
identical to those of the Mk 2 unit of paragraph 5*7*1
and are housed in a spherical case, 13 in outside diameter,
of cast aluminium alloy, made in two halves bolted
together with an O-ring seal in the joint. The joint
bolts also fix a square mounting plate to the case and
this plate is attached to the headline at about the quarter
point by two shackles. A number of deep-sea floats
are attached to the opposite edge of the plate to give
buoyancy to the meter to lift it clear of the headline and
float it to the surface if it breaks away from the trawl.
The meter weighs 14 Ib in water and 77 Ib in air.
The electric cables, one to each wing end, are lashed
to the headline and the transducers are fixed to wooden
boards with wedge shaped tails, towed from the wing end
on short lines. Tail lines with three deep-sea floats
attached act as drogues and lift the transducers clear of
the net.
5*8 Headline height manometer
This meter measuring the headline height relative to
the groundrope is a differential manometer fully enclosed
in a pressure-resistant case so that it is completely
surrounded by air at ambient pressure, and thus the
instrument operation and recording is independent of
the depth of water in which the trawl is being used. A
diagrammatic arrangement is shown in Fig. 26.
VERY FLEXIBLE
RUBBER BAG,
CASTING UNIT
SPPING
PISTON FLUID CYLINDER
Fig. 26. Diagrammatic arrangement of manometer.
Fig. 27 shows the manometer with the recorder case
open.
The pressure measuring unit consists of a cylinder
filled with fluid with a Bellofram rolling seal to transmit
the fluid pressure to a piston, whose movement, pro-
portional to the pressure, is controlled by a helical
compression spring. The piston is connected by a
simple rod and lever system to a heated stylus recording
the piston movement on a "standard" clock drum chart.
The cylinder is connected by a nylon tube of 4*3 mm
O/D x 2-9 mm (-17 in x -115 in) bore to a head
capsule which consists of an open ended cylinder
enclosing a very thin and very flexible rubber bag.
The system is filled with a low viscosity silicone fluid.
The pressure recording unit is housed in a spherical
case of cast aluminium alloy of the same diameter,
thickness, etc. as for the declination meter of paragraph
5*3 but on the lower half an integral skirt is provided
for the attachment to a base plate. The spherical
case is connected to the small case containing the
head capsule by a hose which contains the nylon tube.
The hose is a standard braided, rubber, high pressure
hydraulic hose with a steel wire helically wound flexible
conduit inserted through the bore, to give rigidity
of the hose under the external water pressure from the
static head. The bore of the conduit is -25 inch and
thus gives an annular air space around the nylon tube
and the outside diameter of the hose is approximately
•8 inch. The hose is jointed with standard re-usable
connectors.
Two channels are provided m the instrument although
of course one can easily be removed if required. A
version with four channels has been used but handling
on the trawl proved difficult and overlapping of the traces
caused difficulties in interpretation of the records and
hence two-channel instruments are to be recommended.
To prevent stretching of the hose, which would cause
damage to the nylon tube, it is marled along a {-inch
circumference flexible wire cable the ends of which are
secured to the recorder and head cases respectively.
The installation on the trawl is shown diagrammatically
in Fig. 28.
HEADLINE HEIGHT
CAPSULE
WING END HEIGHT
CAPSULE
DAN LENO
•OIBIN
GROUND ROPE
=«
LASHED
TO HEADLINE
•OlflN AT
•OSSOM REMOVED
MANOMETER RECORDING
UNIT
•HACKLED TO QROUND
ROPE 4 FISHING LINE
LIGHT LASHING TO
/NET4-i'AFTOF
FISHING LINE
FISHING LINE
Fig. 27. Headline height manometer.
Fig. 28. Headline height manometer diagrammatic
arrangement in trawl.
509
Hie head capsules are lashed to the headline, generally
at the centreline and the wing end, with Jhe wing end
capsule hose lashed along the inside of the headline to
the quarter point. The hoses then flow in a large bight
inside the net to the recording case on the centre of the
groundrope. The recording case base plate is fixed to
the groundrope by two large bow shackles and by a
farther two shackles to the fishingline. This method
of operation has proved to be very satisfactory and very
few difficulties have arisen even with hoses of up to
80 ft in length.
The height recording range of the instruments in use
is 0 to 25 ft and this is essentially for net openings of 10
to 20 ft but the range can easily be changed by fitting
springs of different stiffnesses.
A typical static calibration curve, Fig. 29 shows a
hysteresis loop of about ± 9 inch.
25
20
HEIGHT
FEET
15
HO 1*5 2O 2<6
SCALE READINGS INS.
Fig. 29. Manometer calibration. Typical.
In practice continuous oscillations of the trawl,
particularly of the groundrope occur, and using the mean
calibration curve and the mean indicated ordinate, is
considered to give the headline height with an accuracy
of within ± 6 in. It is noted that the datum of the
instrument is 6 in above the base plate and this 6 in
plus the height of the groundrope must be added to the
indicated height to obtain the true headline height
above the ground.
A typical record is shown in Fig. 21 .
The weight of the complete meter is broken down as
below :
Case, recorder and
mounting plate
Hose, 50 ft length
excluding wire
Head capsule and
case
28 Ib in air, 8 Ib in water
28 Ib in air, 18 Ib in water
5 Ib in air, 3 Ib in water
5*9 Trawl speed meters
To make an accurate analysis of a trawl test, it is nec-
essary to know the trawl speed through the water as
this is not necessarily (due to tides and currents) the same
as the ship's speed and it is also desirable to know the
510
pressure or speed of the water in the trawl mouth. A
Kelvin Hughes Current Meter has been adapted for use
in determining these speeds.
The standard underwater meter has been modified
so that the electrical impulses from the cam contacts
are at a frequency of about 35/min at 5 knots, the
frequency being proportional to the speed. The electrical
lead is taken to a spherical cast aluminium case contain-
ing the recording unit and power supplies. The power
supply consists of four 4} V dry flat type torch batteries
A spring clock drives a drum with a plain heat sensitive
chart and also rotates a lead screw which moves its
cross head, carrying a heated stylus across the chart
and thus a helical trace is recorded. The electrical
impulses operate, through a relay and condenser circuit,
a solenoid which moves the lead screw, and hence the
stylus endways about -03 in and produces a series of
blips on the helical trace. The number of blips per unit
of time or from the paper speed, the number per unit
of length of trace is proportional to the water speed and
can be counted. This is a very laborious process and
in general only a few short periods of a few minutes
each in a full haul are analysed. The drum timing is
5 minutes/revolution and the paper speed approximately
2 in/min and thus at 5 knots there are about 17 blips/in
of trace.
The recorder case is of cast aluminium of the same
dimensions as that of the declination meter of paragraph
5-3. An integral lug on the centre of each half of the
case is used for a shackle attachment. A general view
is shown in Fig. 30.
Fig. 30. Trawl speed meter and recorder.
For the general trawl speed the meter and recorder
are assembled as shown in Fig. 31 and attached to the
aft warp about 25 fathoms forward of the otter board
and this method has been found simple and4 effective in
operation. For the trawl mouth speed the recorder case
is attached directly to the head line and the meter, with
a ballast weight hung below it, is streamed on an appro-
priate length of wire. Only limited tests for the mouth
speed have been made to date and, as can be imagined,
difficulties in preventing damage to the meter arise in
all but good weather conditions.
Hf-OWlA
MOOHDMG CICCTRIC
UNIT
Ffc. 57. Towing arrangement of trawl speed meter.
5-10 Net twine load cells
In the past, numerous attempts have been made to
determine the loads in the twines of the net and as far as
can be ascertained these have all been on the principle
of replacing the twines by wires of known strength and
seeing which ones break. This system in fact only
indicates that a certain load has been exceeded.
With the load cells used in the present project a new
approach has been made to determine the maximum
load occurring in any selected twine during the course of
a haul.
The principle of the design is based on the well-
proven Brinell Hardness test, a hardened steel ball
being used to give an indentation in a relatively soft
metal plate. The depth of the indentation, and hence
the diameter which can easily be measured, given in a
uniform plate by a given diameter of steel ball, has a
regular relationship with the load applied to impress
the ball in the plate.
The load cell is shown in Fig. 32 and consists of two
phosphor bronze U fittings— one sliding inside the
other and held together by a light spring — one having
a stainless steel ball fixed to the inside of the base and
the other having a recess to hold an indentation disc.
KNOT IU KNOT
ASSEMBLE OF LOAD CELL.
I STEEL SPRING INDENTATION
DISC
LOAD CE LLS
POSITIONED IN NET
EXPLODED VIEW OP LOAD CELL
Fig. 32. Arrangement of net twine load cells.
The size of ball and indentation material can be selec-
ted to suit the load range. On the ones in use a Vis-m
diameter ball with aluminium alloy plates of the order
of 140 Brinell Hardness give a load range of 10-300 Ib.
A measuring micrometer microscope is used to obtain
the impression diameter. In the development stage,
tests were made with the load maintained for various
periods from 5 sec to 2 hrs and also ones when the load
was varied up and down within the maximum during
a period of 1 hr and the results all fell within the normal
scatter range.
Calibration tests are carried out on each batch of
discs and show an accuracy of better than 5 per cent but
in practice on the trawl it is probable that a somewhat
lower degree of accuracy is obtained although consider-
ably better results can be claimed as compared with
those from methods used hitherto.
In practice it is desirable, in fact necessary, to use a
number of cells, say not less than 12, in a row across
the net in adjacent twines. The simplest and quickest
way to insert the cells in the net is shown in Fig. 32. The
cells are knotted onto a single length of twine, the
distance between the centres of the cells being equal to
the mesh-bar length (plus allowance for the knot).
Three rows of the net are cut out over the required
length and then re-braided using the twine containing
the cells for the centre row.
The length of the cells is only 1 in between the pins
and thus can be fitted fairly easily in about 4-in mesh
netting.
6. FULL-SCALE TRAWL TESTS
In carrying out full-scale trials, the number and type of
instruments used in any given haul depends on the
particular aspects of the trawl behaviour or performances
being studied. In general, however, the deck type
instruments are used and complete records made for
all hauls together with load cells, in the warps immediately
forward of the boards and in the backstrops, and the
headline height manometer. From this limited in-
strumentation a very good indication of the trawl
behaviour can be obtained.
In many hauls the whole range of instruments with
up to ten load cells inserted in the cables are used, and
with these the trawl must be about the most expensive
of its type ever put over the side.
Many of the instruments, such as the underwater
load cells, can be used in almost any weather but it has
been found that, in general, force 5 is the limiting
weather for most research work. This is because of
the difficulty in operating the gear with instruments and
the possibility of instrument damage or loss and injury
to personnel and also, with worsening weather, the
oscillation amplitudes become so large that the records
become unsatisfactory and correlation of the results
from the various instruments is virtually impossible.
Considering the very severe conditions under which
the instruments have to work, the reliability and service-
ability of the instruments has been found, in general,
to be very good although, it must be admitted, some do
511
require improvement in this respect. All the deck
instruments give almost 100 per cent reliability, occasion-
al trouble being given, mainly due to vibration, by the
divergence meter potentiometer, replacement of which
can be made without difficulty on board the ship. In
the underwater mechanical instruments the troubles
are practically wholly confined to failures of the clocks,
styluses and the wiring all of which can be repaired or
replaced on board the ship. Very little trouble from
water leakage has been experienced. In about one in 10
to 15 hauls the manometer hoses get tangled around
the groundrope and are broken, causing flooding of the
whole instrument. Repairs can be carried out on board
by washing out with fresh water and drying and fitting
a new clock and hoses. In recent cruises with up to
ISO individual records per cruise the reliability of the
load cells has been 95 per cent and the other instruments
are only slightly lower.
The speed meters have given trouble mainly through
water leakage and consequent short circuiting of the
joints of the electric cables to the meters and recorder
cases and similar problems have arisen on the spread
meters but these have now been overcome and it is
thought that a high degree of reliability will be obtained
in future on all these instruments.
As an indication of the degree of correlation of
results which can be obtained from various instruments
and also to show the interdependence of the various
paramenters Fig. 33 shows portions of the echo sounder
depth, the warp load, divergence, declination and
manometer records of one haul The depth records
shows a valley, over which the trawl took about 20 min
to cross, and the other records show the corresponding
behaviour of the trawl very clearly. As to be expected,
as the trawl passed down the valley side, the trawl
mouth closed, the declination of the warps increased,
the warp load decreased, and the headline height in-
creased, and then as it climbed the other side the reverse
happened and the trawl regained its normal configuration
when it had passed completely over the valley.
It is interesting to note that analysis of the trawl in the
two widely differing configurations, i.e. normal and at
the bottom of the valley, by the methods and equation
continued on page 513
Fig. 34. Otter Boards instrumented to determine ground reactions.
LOAD CELLS.
RECORDING >-
'A* DRAG.
B* SIDE LOAD.
kC* UP LOAD.
Fig. 33. Illustration of correlated recordings.
OTTER BOARD
GROUND REACTION TESTS?'
DIAGRAM OF LEVER SYSTEM.
FIG. 35.
512
Some Japanese Instruments for Measuring
Fishing Gear Performance
Abstract
Measurements and observations of the main characteristics of
full-scale fishing gear in operation under actual fishing conditions
are indispensable for rational studies aiming at development of
new, or improvement of existing, fishing gear and methods.
Since 1957, the authors have been engaged in designing measuring
instruments adapted for this purpose. A short description of the
principle, construction and performance is given of the following
measuring instruments presently used in Japan: recording net
height meter, recording depth meter, combined recording depth
and temperature meter, recording current meter, recording under-
water dynamometer, dynamometer for trawl warps, recording
groundrope indicator, low-speed log for trawlers, recording depth
meter for gillnets. All these instruments have a surprisingly high
grade of accuracy and are, at the same time, extremely small and
light and thus very handy. Their main characteristics are given
in Table I.
Quelques instruments Japonais pour mesurer le comportement des
engin de pgche
Resum^
Les measures et observations des principales caract&istiques des
engins de peche op£rant actuellement sont n&essaires pour conduire
rationellement des Etudes visant a perfectionner les engins existants
ou en ddvelopper de nouveaux. Dans ce but, depuis 1957 les
auteurs ont travail 16 sur les dessins d'instruments de mesures.
Cette communication comprend une courte description des
principes de consttuction et du comportement d'enregistreur de
hauteur de corde de dos, enregistreur-sondeur, enregistreur-sondeur
et thermometre combines, enregistreur de courant, dynamomfctre,
by
Chikamasa Hamuro
and
Kenji Ishii
Fishing Boat Laboratory, Tokyo
enregistreur sous-marin, dvnamometre de fune, enregistreur de
ligne de fond, loch de chalutage, enregistreur-sondeur pour filets
maillants, tous ces instruments £tant actuellement utilises au Japon.
11s sont d'une haute precision et en mdme temps petits, 16gers et
faciles a manipuler. Les caracteristiques de ces instruments sont
donnees au Tableau I.
Instrumentos Japoneaes para medir el rendimiento de loa artes de
pesca
Extracto
Medir y observar las caracteristicas principales de los artcs de pesca
funcionando en condiciones reales son indispensables para efectuar
estudios racionales encaminados a preparar nuevos materiales y
mdtodos de pesca o a mejorar los actuates. Desde 1957 los autpres
proyectan aparatos medidores especialmente para &llo. Se describen
continued from page 512
developed, shows complete balance of forces and this is,
of course, what must be so in the trawl,
These results give us confidence in both the instruments
and in the theoretical work.
A test arrangement worthy of note is that made to
determine the ground loads applied to the boards. The
special board is of the same size and very nearly the
same weight as the standard board. The keel bar or
shoe is attached to the board at only two points through a
system of levers and links incorporating five 3J-ton load
cells to measure the upward, side and drag ground
reaction forces. Fig. 34 shows the door with the -covers
and guards removed to expose the lever systems. The
drag load on the keel bar is transferred to the board and
is measured only at the forward end but the vertical and
side loads are transferred and measured at both the
forward and aft ends of the board, and thus the centres
of these loads can be determined. Fig. 35 shows a
diagrammatic arrangement of the lever systems at the
forward end. The system at the aft end is very similar
but the horizontal arm of the bell crank lever and the
attached load cell for measuring the drag forces are
deleted, leaving the vertical arm as a plain link unable
to transfer drag forces. This test arrangement worked
very well and some very useful information was obtained.
7. Conclusions
In this paper it has only been possible to present broad
descriptions of the various instruments, and many of the
finer points in the design, manufacture and operation
have had to be omitted.
We claim that we have now full range of instruments,
the data from which in full-scale tests of various trawls
has proved invaluable in the development of basic
theory relating to trawl gear design, and will in future
give further data of importance in the confirmation and
extensions of the theory.
This paper is published with the permission of the
Westland Aircraft Ltd., and of the White Fish Authority
and British Trawlers' Federation for whom the work was
undertaken. The responsibility for any statement of
fact or opinion is, however, solely the author's.
Acknowledgements
The design and development of these instruments has presented
many difficult problems, and full acknowledgement for the valuable
advice, information and assistance is given to many colleagues in
the author's Company, representatives of the White Fish Authority,
many persons in the fishing industry, and finally to the Scientific
staff of the Fishing Laboratories of Aberdeen and Lowestoft (on
whose Research ships Explorer and Ernest Holt practically all the
full-scale testing was carried out), for their additional help in
developing the handling techniques.
Footnote
The instruments described in this paper were used in the trawl gear
studies reported on in papers by Crewe and Dickson.
513
HI
Japfa: me
lot prindpioi, conftnicd6n y fcndfanlento de tot
medidom cmptoadoi ictualmente en cl
de la mltura de la red; medidor ngia-
trador de )a profundidad; medidor registrmdor de la proftindiid
y la temperatura combinadat; regittrador de la corrknte; dtoa-
m6metro regirtrador submarine; dinamometro para los cables
de k» artee; indicadom leytetradora de loe burlonet; correderas
tent** para arrastreros; medkbres regbtradores de la profundidad
de toe artes de enroalk. Es sorprendente la exactitud de todos
eetoi inttniraentos que son, a la vez, muy pequefios y ligeros y,
POT tanto, iidles de mancjar. Sus caractedsticas principals se
dan en el Cuadro I.
IN the authors* opinion, work aiming at the develop-
ment or improvement of fishing gear has to include
the quantitative determination of the main character-
istics of the gear and their variations by taking measure-
ments of full-scale fishing gear under actual fishing
conditions. Consequently, for such studies a number of
suitable measuring instruments is needed which, to a
certain extent, have to be newly designed by modifying
and adapting existing measuring principles and instru-
ments to the specific purpose. Since 1951, the authors
have been active in the development of such instruments
for measuring the forces acting on and for observing the
behaviour of fishing gear during operation. Here a
short description is given of such instruments as presently
used in Japan. Their main characteristics are in Table I.
Recording net height meter
This instrument (Fig. 1) is meant for measuring the
vertical distances between different parts of fishing gear,
such as groundrope and headline (opening height),
wing tips, sweeplines, etc. of trawlnets and Danish
seines. In principle, it is a recording differential mono-
meter to which two pressure sensing parts are connected,
one directly and one by a plastic (vinyl) hose. Thus the
difference in depth, i.e. hydrostatic pressure (Fig. 1, I,
hrhf), between the two points to which the pressure
sensing parts are attached is measured and recorded
continually. In the schematic drawing (Fig. 1, I) one
pressure sensing part (B), which is attached to a higher
point, contains a polyethylene bag (D) on which the
water pressure acts. This pressure is transferred to the
measuring and recording part (A), which contains the
other pressure sensing part, through the plastic hose and
acts on one side of a diaphragm (K2). Plastic bag and
hose contain air. The measuring and recording part
(A), which is fixed to a lower part of the fishing gear,
receives with its pressure sensing part the higher hydro-
static pressure according to its greater depth over the
diaphragm (K) and this pressure acts on the other side of
the diaphragm K8. In order to avoid extensive defo? ma-
tion of both diaphragms, the room between them is
filled with liquid silicone. In this room a set of bellows
(C) is installed which measures the pressure difference
between the parts A and B. The respective deformation
of the bellows is mechanically amplified and transmitted
to a pen (H) which records the pressure difference, con-
verted into a metric depth scale, on a clockwork-driven
paper recorder (F).
lloaaurlag
ranga
Aoouraoy,
8«a»itiYitr
Maori ami operating
dopth or rang*
Woight in air
Woight in wator
ttpood of rooording
papor. Working tiat
Booording nat-
haight aotar
0 to 6 a
0 to 8 a
0 to 14 M
0 to 18 •
1 1 to 2 oa
500 a
3*2 kg
2.5kg
1 aa/ain
24 hr»
laoordiag
depth aotar
0 to 180 a
0 to 240 m
+ 0.5 to 1.0 m
300 a
, 32 gr.
(in tho wator)
- do -
Booording
depth and
taaparatura
aatar
A Tra« 0 to 190 a
-10°C to + 30° 0
B Typo. 40 to 100m
0°0 toHO° C
+ 0.5 •
+ 0.1° 0
7 0.05 m
I 0.03° o
200 a
A 2.10 kg
1 1.15 1«
B 3.95 kg
3*10 kg
A (1.5 an/Win
( 24 hra.
B ( 5.0 am/ain
( 24 hra
Booording
• ourraat aotar
0*3 to 2.0 V"°
± 0.025 a/ioo
7 0.01 B/IOO
300 m
0.415kg
(in tho wator)
Baoording
underwater
dynaaoaatar
0 to 1000 kff
0 to 2000 kg
±5kg
±10 kg
500 a
700 a
3.5kg
2.1 kg
1 na/ain
24 hra
Bgraaaoaatar
, for trial
warps
0 to 2700 kg
±10 kg
10 ton*
-
-
•fttfoadiag
grouadropo
indicator
0 to 100 <U*r***
(for loft *id«
and right site
footro'oo)
±0.5 dftgrooa
150 a
9*7 kg
(in tho wator)
8 aa/aia
6 hra
Yew wpaad
lee for
trawlara
0:3 to 7*0 a/"*
+ 0.05 a/»«o
50a
4.2kg
3.1fcg
-
Booording
d«p*h attar
for gilln*ta
(4 •l«n«nti)
0 to 3«
2 to 5 •
4 to 7m
6.5 to 10 m
±l.to 2 oa
-
-
1 aa/aia
24 hra
514
Fig. L General structure (/) and photo (//) of the recording net height meter.
Recording depth meter
This instrument (Fig. 2), which is meant for measuring
the water depth in which fishing gear, or parts of it,
are operating, is actually a simplified version of the
recording net height meter described before. It consists
of only one pressure sensing part with paper recorder.
The pressure is measured by a membrane (Fig. 2, 1, A),
acting on a calibrated spring (B). The displacement of
the membrane according to pressure is mechanically
amplified and converted into the movement of a pen (C)
recording on a clockwork-driven paper recorder (D).
i
fig. 2. Construction
Ing (/) and photo (//) of the recording
meter.
Combined recording depth and temperature ;
This instrument (Fig. 3), which basically is very much
like a "Bathythermograph", is intended to measure the
water temperature at the depths where fishing gear is
being operated. By investigating the amount of catch
versus temperature and depth, correlations can be found
which may be very useful as a guide for locating profitable
fish concentrations. As usual, the depth is determined by
measuring and recording the hydrostatic pressure. For
the temperature, a liquid thermometer is used. In order
to increase accuracy, it has a particularly long, and there-
fore coiled, tube filled with a temperature sensitive liquid.
The variations of the volume of the temperature sensitive
liquid in the capillary tube are measured by bellows and
recorded by a mechanism similar to the depth meters.
Fig. 4 gives an example of a combined record obtained
with this instrument.
Fig. 3. Construction drawing (I) and photo (II) of the recording
depth and temperature meter.
" I •• fc • 4 • 4r '
ffc. 4. Example of a record of the depth and temperature meter.
515
Fig. 5. Schematic principle (I) and photo (II) of the recording current meter.
Recording current meter
This instrument (Fig. 5) is mainly used for measuring
the actual speed of a trawl through water. It may also
be utilised to determine the difference of water flow
inside and outside a trawlnet. In principle, this instru-
ment is a resistance log, i.e. the towing speed is deduced
from the towing resistance of a specific body, in this case
a sphere of 10 cm diameter. Before choosing this
principle, the envisaged range of speed and water
temperature variation in regard to Reynold's number
was considered, to make sure that the coefficient of
resistance of the sphere would be constant. It was found
to be 0*45. The sphere is made of brass (Fig. 5, I, 3)
and has a buoyancy of 360 g. It is connected with the
recording balance, measuring its towing resistance by a
0*3 mm diameter not too rigid piano wire (2). The
indicating mechanism of the spring balance (4) is
specifically designed so that the resistance values are
converted into indications of a pointer (5) with linear
correlation to the speed. The pointer records on a
clockwork-driven paper recorder (6). The whole balance
and recording mechanism is installed in a lightly built
container made of "Hydronarium" and the movements
of the piano wire, in connection with the lever of the
spring balance, are transferred from the outside through
an elastic diaphragm (1). The part of the container
separated from the outside by this diaphragm is filled
with silicone oil to provide resistance against corrosion
and water pressure.
Recording underwater dynamometer
With such dynamometers, the tension on various lines
and ropes of fishing gear can be measured underwater.
By combining the measurements of such instruments at
several points underwater, and also on board the trawler,
the relative share of different parts of the gear in the total
towing resistance can be determined. In principle, the
instrument is a hydraulic dynamometer (Fig. 6) in which
the pressure created by the pull on the rings A and B
(Fig. 6, 1) on the piston (C) in a cylinder is measured by
a bourdon tube (D) and, after mechanical amplification,
recorded by the pen (E) on a clockwork-driven paper
recorder (F).
Dynamometer for ti"*wl waips
This instrument is a combination of a spring-type
516
dynamometer with electrical transmission from meter to
indicator (Fig. 7). The indicator is a type of ammeter.
I
Fig. 6. Construction drawing (I) and photo (II) of the recording
underwater dynamometer.
Record groundrope indicator
This instrument is meant for measuring the curvature
of the groundrope of bottom trawls. For this, several
such instruments have to be distributed over the ground-
rope for simultaneous measurement. In principle, the
instrument (Fig. 8) measures and records the angle
between a free movable external lever (A) and the axis
of the instrument which represents the direction of the
groundrope at the point of attachment. The recording
part is installed in a water- and pressure-tight casing.
Fig. 7. Dynamometer for trawl warps in measuring position.
Fig. 8. Recording groundrope indicator.
x>w-speed log for trawlers
ror research on trawl gear, the towing speed must be
.nown accurately. Since ordinary logs are not accurate
nough at the low speeds normally used in trawling, the
uthors designed and constructed a new type of log for
rawlers. In principle, this log measures the water speed
>y the revolutions per unit of time induced by the water
low on a calibrated propeller which is towed by the
rawler. In order to maintain the propeller in a stable
position, it is installed in an aeroplane-like stabiliser
Fig. 9). The propeller revolutions are transmitted to the
tidicating unit on board the trawler through an electric
able. For measurements, the propeller is towed in 10 m
vater depth, which requires different lengths of towrope
or different speeds.
continued on page 518
Fig. 9. Low-speed log for trawlers.
Top Right) Fig. 10. Schematic principle (1) and photo (II) of the
ecording depth meter for gillnets. Example for the application of
this instrument to a gittnet (HI).
Bottom Right) Fig. //. Example for the determination of some
nain characteristics of a trawl by applying several of the described
measuring instruments simultaneously.
PTM«U1*
••atiaff
P«rU
— n«t
> for Tv«rl Van*
517
Trawl Studies and Currents
Trawl instrumentation can be very sophisticated and expensive
so there is a lot to be said for the instrument that is simple in prin-
ciple. Two such instruments are the Jelly-Filled Directional Rolling
Inclinometer and the Pisa Current Indicator, both very simple in
construction and operation. They are based on the solidifying of a
jelly in a container, thereby freezing indicators in operational
position. The jelly sets after half an hour or so, giving readings in
the vertical and horizontal planes of the angle of the wire to which
it is attached. Various uses are discussed; the indication of trawl
warp curvature, the indication of trawl headline height and measur-
ing the bends in a line of driftaets under the action of the tide.
It is pointed out that bottom and surface currents may be quite
different and this can often cast doubt on the reliability of instru-
mented gear testing results. In particular, the query is raised as to
just what is meant by the water speed of a trawl. Carrying a small
circular compass, the Pisa tube is a 'Pyrex* bottle filled half with jelly
and half with castor oil; it is used for measuring the direction and
rate of bottom currents. It can be used down to 750 fm (1,370 m).
Other than its obvious use in gear testing, it is suggested that it may
give useful dues on the orientation of fish schools at the bottom
where they often lie strung along the tide.
Les instruments utilises dans les ftudes de chalut peuvent toe tres
comptiques et ontaeux et il serait int6ressant d' avoir des instruments
de construction simple. Le "Jelly-Filled Directional Rolling
Inclinometer" (inclinomtoe dircctioncl) et le "Pisa Current Indi-
cator" (indicateur de courant) sont deux instruments de construc-
tion et ^utilisation simples. Ils sont base* stir la solidification d'une
gette. Les instruments places dans un flaconcontenant cette gelee
sont ftote dans leur position d'op6ration lorsqu'au bout d'une
demi-heure environ, la ge!6e se solidifie. Le premier permet de
lire les angles horizontaux et verticaux pris par la partie de 1'engin
sur laquefle a etc place 1'indicateur. Diffdrentes utilisations sont
examinees: 1'indication de la courbure des funes; Indication de la
hauteur de la ligne de tfcte; la mesure des courbes prises par les
filets maillants sous Faction des courants. II est signale que les
oourants de fond et de surface peuvent etre tres different ce qui
peut faire douter de la vtracite des resultats obtenus pendant les
essais instruments d'engins de peche. La question est aussi
potee de savoir ce qu'on veut reellement dire par vitesse de l*eau
d'un chalut Le "Pisa Current Indicator'* est un flacon en 'Pyrex'
rempli moitie avec de la gelce, moitie ayec de Fhuile de ricin et ou
flotte un petit compas tirculaire. II est utilise pour mesurer la direc-
tion et la vitesse des courants defond, jusqtfa des profondeurs de
1370 mfttres (750 brasses). II est sugger* en .outre que cet instrument
peut donner des indications utites concemant 1'orientation des
Danes de poissons pres du fond ou ces banes sont tres souvent
alignea avec te courant.
i eon artes de arrastre
Extmcto
Los instrumentos empteados para los experimentos con artes de
arrastre pueden ser muy costosos y complies, por lo que tienen
grandes ventajas los sentillos y baratos. Dos de 6stos son el Jelly-
rilled Directional Rolling Inclinometer (un inclindmetro lleno de
by
J. N. Carnithers
National Institute of Ocean-
ography, Wormley, Surrey
jalea) y un corrcnt6metro marca Pisa. Se basan en la solidificacidn
dc una jalea al cabo de unos 30 min. quc inmoviliza unos indica-
dores dc posicidn y scflala los angulos vertical y horizontal del
cable al que se siyetan. Se explica su uso para determinar la cur-
vatura del cable, la altura de la relinga de corchos del arte y los senos
que por efecjo de la corricntc forma una andana de redes de deriva.
Se mcnciona que las corrientes del fondo y de la superficie pueden
ser totalmente distintas, lo que con frecuencia hace dudar de la
seguridad de los resultados de ensayos con instrumentos en las
redes. Pregunta el autor que se da a entender cuando se habla de la
velocidad de un arte de arrastre en el agua. El corrent6metro
Pisa es una bptella de 'Pyrex' la mitad llena de jalea y la otra mitad
de aceite de ricino, con una pequefta brujula circular, que se emplea
para medir la velocidad u dircccidn de la corriente en el fondo y
puede usarse a profundidades hasta de 750 brazas (1.370 m).
Ademas de su uso directo en las pruebas de artes, puede dar infor-
maci6n de utilidad respecto a la orientaci6n de cardumenes de
peces en el fondo, en el que con frecuencia se encuentran situados
en la direcci6n de la corriente.
IT is of course an inevitable feature of studies made on
the behaviour of commercial fishing gear that inves-
tigations can be carried out only by the relatively small
number of persons who possess the essential facilities.
Apart from the major expense (the ship), large sums of
money are represented by the trawls, whether these
be for bottom or midwater fishing. Furthermore, if
studies of the geometry of towed trawls are to be made
whilst they are actually fishing, there is the added expense
of electronic telemetering gear and that represented by
the salaries of the specialists who are needed to use it.
All in all, there is so much both of expense and complexity
that the studies of trawl behaviour have necessarily
become very much of a closed shop — which surely
is not all to the good! Outsiders, who may incline to
credit themselves with a few ideas thought to be poten-
tially useful, must needs tread very warily when airing
views on such an important subject of which they can
have had little if any experience.
It would be quite wrong, however, to suppress such
ideas when, given a little courage, the chance offers to
continued from page 517
Recording depth for gfflnets
The principle of this instrument is a combination of the
principles used for the recording net height meter and
depth meter described above. It is designed for simul-
taneous measurement at four points. Four pressure
sensing parts with plastic bags are connected with
separate plastic hoses, each to its own bellows and pointer
indicating system for recording on one common, larger
518
sized, clockwork-driven paper recorder. The measuring
and recording unit is installed in a watertight metal case
which floats at the surface. Fig. 10 shows a schematic
drawing of the measuring and recording unit with one
pressure sensing part (I), a photo of the whole instrument
(II) and an example for application at a gillnet (HI).
A wireless net depth telemeter is also used and is
described elsewhere.
advance them— as it now does at this present congress.
Presumably, the search for the best trawl requires
successive trials as different components within the
total assemblage of towed gear are modified and as
various fitments are incorporated. Granted that the
sophisticated instrumentation possessed by the specialist
investigators can telemeter back to them aboard ship
all the parameters of interest as the trawl is towed through
the water at varying speeds and with different lengths
of warp veered, there nevertheless remains one dis-
quieting consideration.
If all the studies designed to promote the progressive
improvement of a trawl's aperture or towing characteris-
tics were made in a large body of motionless water, then
it would be admissible that all inferences made as to the
influence of introduced modifications would be justified.
As regards net studies carried out in tidal seas, it is a
matter for surprise not to have come across any reports
wherein adequate attention has been paid to the possible
effects of intrinsic water movements. If, for instance,
between two successive tests the angle of door attack had
been altered, how can it be safe to assume that a discov-
ered change in the net's shape or resistance is attributable
solely to that alteration unless it be known that the water
on the bottom was moving exactly the same (speed and
direction) during the two experimental tows? In saying
this we are assuming that the work is being carried out in
the open sea where tidal streams may reign or where
at Isast there may be notable currents — perhaps even a
stratification of currents differing in speed and direction
at one and the same time. It has, of course, been fully
recognised that "conditions are never exactly repeatable
from one tow to the next as regards ground and tide",
and the need has been expressed for "a more accurate
instrument to measure water speed at the trawl".
The word "accurate" is a tricky one indeed when
applied to current measuring at sea, as is well realised
by all who have spent long years on that job, and what is
meant by the expression "water speed at the trawl" also
gives one to think.
There are two hard facts about water speed as related
to trawling and both could be catered for. One is the
actual movement of the water itself over the ground,
which ought to be (and could be) known during trials
in terms both of speed and direction. The other is the
(so to speak) negative or "manufactured" current due to
the travel of the ship and therefore of the trawl which
follows it. The former can be measured vectorially by
simple instrumental means and the latter can also be
very acceptably assessed vectorially. One way for the
latter is to follow the method so much in vogue nowa-
days with oceanographers who take constant radar
bearings upon an anchored buoy — or better on a pair
of anchored buoys. The naval surveyors' taut wire
method would be ruled out because of the presence of the
trawl behind the ship. We are here thinking of course
of work in the open sea away from the convenience of
shore sights which might offer in certain regions. What
would be the meaning (and therefore the permissible
application) of the showings of a current-measuring
instrument mounted on or within a trawl is difficult to
appreciate and it would require much space to discuss
the point fully.
As regards the actual movement of the water itself,
a good deal can of course be done by making comparative
tows when the surface water would expectably have the
same movement as judged from tidal stream atlases.
Even for the surface water the matter is fraught with
difficulty despite the recent appearance of excellent new
presentations of the tidal streams around Britain. The
ratios of stream speeds at syzygies, quadratures, and
intervening times have been regionally plotted on charts,
and so too have the angular spreads of the streams in
various areas. It would, therefore, be possible to work
in a certain area and to assume that at "x" hours away
from reference high water time on one day, the stream
on the surface would be the same as at "y" hours away
from the same reference tidal state on another. To a
specialist in current measuring, however, there would
seem to be no escape from the need to do trawl tests
in still (or virtually still) water if certainty has to attach
to inferences made regarding the influence of incorpora-
ted modifications.
If, as may well be the case, there is a stratification of
currents different in speed or direction (or in both at
the same time) the situation will be further bedevilled.
This might well apply also in a region where the tidal
streams are almost always in the same direction all
down through the column if they nevertheless change
direction out of phase somewhat at the different levels.
The worst situation of all, however, with stratified
currents heading differently at differing speeds, will
arise from depth considerations. The longer the towing
warps, the greater will be the difficulties introduced.
The point of the remarks so far made is merely to
emphasise the need to take the environment into proper
consideration when tests are made of trawl behaviour
and geometry aimed at gear improvement.
The writer has no experience whatsoever of, and ne;ct
to no knowledge of, the sophisticated electronic tele-
metering gear used by the ingenious specialists whose
charge it is to make the studies aimed at the production
of the best trawl. However, because of the troubles
and doubts involved for the reasons sufficiently touched
upon already, he wonders whether his own very simple
device which does not telemeter but merely reveals
after recovery of the gear how a trawl was shaped, and
how its warps, etc., were sloped (azimuth and obliquity)
during a settled-down tow, might not be of more use
than hitherto considered. The device in question — the
"Jelly-Filled Directional Rolling Inclinometer"— has now
been even further simplified (see Appendix). Besides
being able to reveal what had been the headline shape
in the vertical plane, it can also reveal the same in the
horizontal plane. Also, besides being able to reveal
divergence and slope along the warps at as many points
as instruments might suffice for, the simple tool could
undoubtedly be used to reveal headline height as well-
all this of course a posteriori.
Tests once made with a former version of it on a
519
mkiwatcr trawl towed through the virtually still waters of
Loch Ness, revealed that the net had towed away from
dead astern with one door somewhat higher than the
other and with the net not "square" to the direction
of tow.
Though we are here dealing with trawls, it is perhaps
permissible to give a passing thought to certain recent
work of great interest on the driftnetting of herring.
The bare bones of this important work has been the
discovery of the significance attaching to whether the
driftnets lie streamed true "in the tide9' or are disposed to
some degree athwart it owing to windage on the vessel.
Because the reported research attested considerable
hard work, it is perhaps worth stating here that it would
be very simple indeed to measure the lie of driftnets
(not in sight) with regard to the magnetic meridian and
at the same time to reveal whether there had been an
urge of water through them from one side or the other
— and, if so, at what angle.
This could be done with no difficulty at all using a
couple of Pisa Current Indicators (see Appendix).
We have so far said nothing whatsoever about fish
behaviour. This is a very big subject indeed and one
which is wisely left to specialists in keeping with the
old adage that fools rush in where angels fear to tread.
However, one or two relevant remarks can be made here
without the assumption of any worthwhile knowledge of
the subject.
The first is to refer to the full recognition on the part
of the expert gear specialists that, no matter what they
come up with by way of a "best trawl", whether or not
it makes the hoped-for big catches will depend very
markedly upon the behaviour of the fish it aims to catch
— fish which may have been "seen" on the echo sounder
or asdic set before shooting and which also may still
be "seen" as the net is conned to take them. When
speaking above of trawl trials it was not necessary to
advance hesitant opinions about the importance of
currents; assertions could safely be made. Though it
is not possible to speak nearly so positively concerning
any connections between currents and fish behaviour,
enough is known about bottom currents in some regions
to make it seem at least likely that they will influence
fish behaviour on certain grounds. It has been written
of herring (by F. R. Harden Jones) that "shoals which
appeared to hold their position in the daytime must
have been stemming the tide, and the sensory stimulus
involved was probably a tactile or visual clue from the
bottom to the fish nearest to it. Fish out of touch and
out of sight of the bottom might use the first fish as
markers and maintain their position relative to them."
This provokes various thoughts. The first is that for
all his wisdom and lore there is one thing which the
most experienced skipper does not know and that is
the speed and direction of the bed current at the time
he shoots. If he knew this he would presumably know
the direction in which the on-bottom fish would be
heading and there might well be profit in trying the
results of approaching them along different courses as
between different tows. The writer is surprised that in
520
the long records of fisheries research he can turn up no
records of experimental trawl hauls having been carried
out with a knowledge of on-bottom current. It is easy
to investigate the latter and it would seem at least worth
a trial to discover whether benefit might not accrue from
approaching the fish at various angles to their lie as this
might be dictated by the direction of the bed current
Recent work devoted to the observation of sea bed
currents has shown that, in areas where the ground is
highly convoluted, the current may give rise to consider-
able turbulence when on certain headings. In such cases
it seems reasonable to expect that on-bottom fish might
well mill about under conditions of impaired vision and
be more prone to capture in the resulting silty water.
Perhaps the day may come when, besides having atlases
of surface tidal streams presenting the water movements
at hourly intervals from a stated reference hour, we may
seek to produce the same for the bottom water move-
ments as well. If it ever does, a start could be made most
easily with the North Sea.
APPENDIX
(1) The Jelly-Filled Directional Rolling Inclinometer
This simple device, illustrated by Figs. 1 and 2, remains very much
what it was when described in the Fishing News (issue of 23rd
September, 1960). Since that time, however, the contained compass
has been much improved and is now mounted centrally within the
cylinder. A new method of attachment has been devised which,
apart from being easier, affords a longer setting time of the initially-
hot jelly and provides a protective cover to the Terspex' cylinder
continued on page 521
Fig. 1. The Jelly-Filled Directional Rolling Inclinometer empty and
without casing.
Performance of the Granton Trawl
AMnct
This paper describes instrumented gear testing of the heavy 78-ft
headline, 120-ft groundropc, Granton trawl used by the Humber
Fleet. The first drawing shows details of the trawl. It is rigged with
iron bobbins for half the length of the groundropc. The second
drawing shows where the deck instruments, the warp tension
meters, warp divergence, decimation and heel meter are situated.
All these, plus the speed log over the side, the ship's Chcrnikccf
log and engine rpm are recorded electrically on a panel situated
in the research ship's laboratory. A propeller thrust meter is also
installed on the ship. Underwater loads are measured in front of
and behind the otter boards, also in the headline and toe legs.
Headline height is measured on a manometer; headline spread by
an electro-acoustic instrument. The true spread between otter
boards is measured on an electronic-acoustic instrument. There
follows a general picture of the shape of the trawling gear as found
by these instruments showing how the warp declination was 25°
at the surface and only 11° at the otter boards. The true spread
between the otter boards is 218 ft; a 15 per cent augmentation over
the spread as obtained by surface measurement. The main harness
lines of the net lying in the shape of a catenary give a headline
spread of 49} ft, with an overhang between headline and ground-
rope of 24 ft. The load measuring instruments show how the drag
of the gear can be broken down from the seven-ton total into 58
per cent for the net and appendages, three per cent for sweeplines
and danlenos, 29 per cent for the otter boards and 10 per cent for
the warps. Values as high as 25 per cent of the total shearing
force of the otter boards have been measured as arising from ground
shear rather than hydrodynamic force. The ship's thrust curves
against towing speed over a range of propeller rpm, with the
by
W. Dickson
Marine Laboratory, Aberdeen
trawl drag plotted across them, is illustrated and described as a
powerful tool in gear research.
Performance du chalut Granton
La communication decrit les tpreuyes instrumcntees du cnalu
Granton de 24 m de ralingue superieure et trois m de ralinfue
inferieure utilise par la flotte Humber. Les details de la construction
sont donnes sur le premier dessin. La ralingue inftrieure du chalut
est gree en son milieu, avec des rouleaux sur la mo i tie de la longueur.
Le second dessin indique la position des instruments de pont,
des tensiometres de funes ainsi que des instruments servant a
mesurer les angles d'inclinaison et de divergence des funes. Les
indications donndes par ces instruments, par le loch de cdt£, le
Chernikeef loch et, le nombre de t.p.m. de la machine, sont en-
registres dlectriquement sur un tableau situ6 dans le laboratoire du
bateau de recherche. Un indicateur de la puissance de pouss6e
de I'h61ice est aussi install* sur le bateau. Les forces sousmarines
exercees devant et derriere les panneaux sur la ralingue superieure
continued from page 520
to minimise risk of scoring. The 'Perspex' cylinder, 8J in long and
4 in wide, is now pushed into a tube of common yulcathene
(J-inch thick) which is just a trifle too small to admit it until slit
full length (16 in) by one cut. Using a couple of transfixing bolts
seated in large washers it is arranged that the inclinometer can be
speedily bound on to a cable by means of two separate short lengths
of trawl twine as shown in the second photograph. Within reason,
the vulcathene tube can be as long as wished in the interests of
directional indications.
Naturally, when the attachment is to a "slippery" warp, it is
wise to secure a bulldog on the latter between the points where the
two lashings are to be made. The filled device, which weighs only
2f Ib in water, allows ample time for a shot trawl to settle down
because, if attached on deck whilst very hot to the headline (or
other component of the gear), everything will still be free after
half an hour's submergence even in water as cold as 0-7°C. Under
the most usual conditions the inclinometer jelly will have set firmly
before the end of a normal tow. After recovery the inclination
can be read off directly from the protractor within the tube and
the azimuth of the slope got from the "frozen-in" compass.
Fig. 2. The Jelly-Filled Directional Rolling Inclinometer encased
and rigged.
(2) The Pisa Current Indicator
This is a well-tried device for extending the depth of use of the
simple "Current Measuring Bottle for Fishermen" described and
figured in the Fishing News (issue of 1 2th May, 1961). Its indicating
unit is a 'Pyrex' bottle filled half with jelly and half with coloured
castor oil which, not being buoyant, is used fixed within a 40-inch
long tube of low density polythene. With the filled bottle within it,
this latter stands erect in motionless water when submerged whilst
fastened at its lower end to a weight with a limp tether consisting
of six inches of thin strong twine. Because the bottle has to be hot
when initially sent down into the sea and has to cool down under
pressure when anchored on bottom under a considerable head of
water, it has a flexible rubber diaphragm in its cap to accommodate
for changing volume of contents. There is a strong circular com-
pass within the bottle. This hangs on a thin Terylene' thread from
the tip of a rod which runs axially down the bottle from a seating in
the underside of the screwed-on brass cap.
It is arranged that the device goes down into the sea with not
only the bottle hot (and the jelly therefore liquid), but with the
entire polythene tube filled with hot water as well. This provision
greatly extends the depth of use because it prolongs the setting
time of the jelly. The instrument is used whilst the ship is adrift and,
when bottom is reached, the sudden removal of a thin rubber
"hat" from the top of the tube voids the warm water envelope to
expose the bottle to the cold water down below. There is a provision
whereby the tube remains at anchor undragged by the drifting ship
until enough time has elapsed for the jelly to set. This is achieved by
means of a pay-out line which is an adequate length of strong nylon
braid spooled within a box of hard plastic carried by the bottoming
gear.
On recovery, the speed of the bottom current is got by referring
to a calibration curve the degree of slope of the interface between
the set jelly and the coloured oil. This latter is easily measured
through the glass by means of a protractor or special angle-
measurer. The direction of the current is got by referring the "up-
hill" direction of the set jelly slope (as the cold bottle stands on its
base) to the "frozen-in" hanging compass.
Whereas the original (and still much-used) simple current-
measuring bottle for fishermen can be used down no farther than
about 60 fm, the Pisa Current Indicator just described in brief
can operate to well beyond 750 fm for the measurement of bottom
currents.
A new variant of it, of no interest here, can be used down to all
depths.
521
S.'
jMUi&li d'attacfae dec entremises toot 6gaWnfint mesurees,
tnomtoe hxfo^lahauteur <te la ralmgue sup6rieure ct
i Hfrpafaq •oooctkioc eJectronique pei'niet dc mesurer r ouverturc
horte»t«te rt Tcmverture rtdk cntre tet panoetux. Lacommuni-
cttiOTdtoHcniuitel'id6Bafcn6r^
d*«iKBS to donates dc ces instruments, montrant que I'inclinaison
detfuDCia«it<k250AUiiurfaccct8Cukmcntdell5auxpanncaux.
Lft v6ritabte ouverturc entre to panneaux ett de 67 m, soit en
augmentation de 15 pour cent sur 1'ouvertuie cakutee 4 partir de
Tangle de surface des tones. Les ralingues prament une tonne de
catenate et donnent une ouvcrture de 15 m. Le dtpassement du
dot sur la ralingue inf6rieure mesurait 7,5 m. Les tensiomfttres
monfrent que la resistance globak de sept tonnes de 1'engin peut
toe dtcompotee en 58 pour cent pour le fuet et ses accessoires, trois
pour cent pour to bras et to danknos, 29 pour cent pour to
panneaux et 10 pour cent pour to fanes. De la force lat&ale totale
des panneaux, 25 pour cent provenaient de la rdsistancc du fond
et 75 pour cent de forces hydrpdynamiqucs. Les courbcs represent-
ant la poussee de 1'helice par rapport a la vitesse de touagc pour
difftonts t.p.m. avcc la resistance de rengin en resultant, sent
donnees graphiquement et constituent un instrument precieux
pour la recherche d'engins de pftche.
F^MCioMBlento del arte Gnutoo
Extracto
Describe el autor los ensayos realizados con auxilio de instrumentos,
del pesado arte de arrastre Granton, de 78 pies de relinga alta y
120 pto de burldn, emptodo por la flota pesquera del rio Humber.
El primer esquema muestra los detalles del arte con las bobinas de
hierro que lleva la mitad del bur!6n; el segundo esquema da la
position que ocupan en cubierta los diyersos instrumentos, com-
prendidos los medidores de la tensi6n, divergencia e inclinacidn de
los cables. La informaci6n dada por los tres instrumentos, asi
como la de la corredera normal y la corredera Chernikeef y las
revohiciones del motor se indican e!6ctricamente en un cuadro
situado en el laboratorio de investigaciones del barco, el que
tambien cuenta con un medidor del empujc de la hdlice. Las cargas
submarinas se miden delante y detras de las puertas del arte, en la
relinga de corchos y en los extremos inferiores delanteros de las
pernadas. La altura de la relinga alta se mide con un manometro y
la abertura con un instrumento electroaciistko. La verdadera
separation entre las puertas de arrastre se mide con un aparato
ad&stico ckctronico. Sigue una description general de la forma
del arte determinada por estos instrumentos, que demuestra que
el fagulo que forman con la horizontal los cables de arrastre es
de25*enlasuperficieydesolamente 11° en [las puertas* La verdadera
separation entre las puertas es de 218 pies, distancia superior en
un 15 por ciento a la lograda con medidas tomadas desde la super-
fitie. Las relingas principato de la red, en forma de curva catenaria,
dan a la de corchos 49,5 pies, quedando esta de 24 pies por delante
del burlon. Los instrumentos para determinar la caga muestran
que la resistentia que ofrece todo el arte, con un total de septo
tons, se descompone en un 58 por ciento para laredy susap6ndices,
un ties por ciento to malletas y los calones, un 29 por ciento para
to puertas yun 10 por ciento para los cables. Secalculaquehasta un
25 por ciento de la fuerza de separaci6n de las puertas la causa la
resistentia del fondo mas bien que la fuerza hidrodin&mica. Las
curvas de empire del barco con respecto a la velocidad de remolque
a diversas r.p.m. del propulsor, con la resistentia del arte trazada
a trav6s de ellas, se ilustra y describe como un arma poderosa en la
investifatidn de los artes.
HPHE instruments described in Mr. NicholFs paper
JL "Trawl Gear Instrumentation and Full-Scale
Testing'9, have been used to measure the operating
dimensions of several different kinds of trawl, but
chief among those was the Granton trawl. The study of
the recorded dimensions led to theories relating the
operating dimensions one with another and from there
on to the design of new trawling gear. This paper is
largely confined to describing how the performance of
the well-known Granton trawl (Fig. 1) was measured
and what was learned from it.
The positions of the instruments on the ship and
trawling gear are shown schematically in Fig. 2. Not all
S22
the instruments were mounted on the trawl every time,
but always enough to investigate the particular aspect
of trawl behaviour being studied during that haul.
The instruments used fall under four headings: (1) those
pertaining to the ship; (2) the deck instruments; (3)
underwater electro-mechanical instruments and (4) under-
water electronic-acoustic instruments.
Fig. 1. Granton trawl.
The ship instruments comprised: echo sounder, Decca
Navigator, Chernikeef log and rpm counter. Later a
Michell thrust meter was fitted to the thrust block.
The deck instruments comprised: the three-wheel
strain gauge tension meters fitted to each warp, the
combined warp divergence, declination and angle of
heel meter, as well as the Kelvin Hughes speed log
towed over the side. All these instruments recorded their
readings on a centralised control panel and later it was
arranged so that engine rpm and the speed by the ship's
Chernikeef log could also be recorded on this control
panel.
Underwater electro-mechanical instruments comprised :
the large and small load cells for tension measuring and
for measuring the otter-board forces, the otter-board
angle of attack and angle of heel meters, the headline
height manometer, the underwater speed log and an
angle meter used for measuring warp declination angles
near the bottom and also at times used to measure the
angle of pitch of the otter boards.
Electronic-acoustic instruments — two spread meters
were used, one to measure directly the distance between
523
the otter boards and the other the spread between the
headline ends. It was later found more practical to
measure the spread 25 fm up the warp from the otter
boards, and to adjust this measurement to give the true
spread between the otter boards.
Drag breakdown
The measurements of tension at the various points
indicated in Fig. 2 give the following breakdown for
the load in the various parts of the trawl gear. It is given
for a towing speed of 3£ knots, at a depth of 100 fm
and using 32S fm of 3^ in circumference warp.
Netting and net appendages 4*1 ton 58%
Ground cables (sweeplines) and
danlenos 0*2
Otter boards
Warps
4-3 ton
2-0
6-3 ton
0-7
3%
29%
7-0 ton 100%
The 58 per cent due to the drag of netting and net
appendages can be further split up into 48 per cent due
to the netting and 10 per cent due to the appendages,
i.e., bobbins, groundropes, headline and the 60 floats.
This further subdivision was deduced more from the
results of tank testing than from measurements at sea
and it also provided an estimate of the friction of the
net appendages as three per cent out of the 10 per cent.
The breakdown gives a lead on where to set about
attempting improvements; the effort should be directed
to cutting down the percentage of the drag that goes into
warps, otter boards, net appendages and friction and
so pushing up the percentage that goes into the fishing
parts, the ground cables (sweeplines) and the net. The
breakdown also helps in setting a reasonable overall
limit to the possibility of drag improvement with any
gear that is at all similar. This is done by setting realistic
targets for reduction in drag of each of the unwanted
items and then re-summing and re-allocating the per-
centages.
Angular measurements and planfonn
The measurement of angles together with the known
lengths of the various parts of the gear, give the broad
picture of the gear in the fishing position as shown in
Fig. 3. From this, and corrected for the true spread as
given by the acoustic spread meters, the planform of the
main harness lines can be drawn up as in Fig. 4. The
shape of the headline and the bobbin part of the ground-
rope agree fairly well with the shape of a catenary, which
makes the planform susceptible to mathematical treat-
ment. It was from the drag breakdown and a knowledge
of the angles, that tables could be prepared showing
the performance characteristics of warps when trawling
and related to this was the question of otter board beha-
viour.
Otter board behaviour
It was only over a relatively narrow range of speed,
around 3f knots, that the otter boards were substantially
upright. At 2J knots they were heeled 15° top inwards
and when speed was raised to 4J knots they were heeled
15° top outwards. It is perhaps chiefly for this reason
that the Granton trawl has been reckoned by fishermen
to fish the best at about 3$ knots. The upward compo-
nent of the pull in the trawl warps eases some of the
weight of the otter board off the sea bed. The weight of
an otter board in water is about J ton and at 3| knots
the ground reaction is cut down to about £ ton, i.e.,
two-thirds of the otter board weight is eased off the
bottom; even so a considerable proportion of the total
shearing force of the otter board can result from ground
shear. Values as high as 25 per cent of the total shearing
Fig. 3. Principal angular measurements of Granton trawl.
524
force have been measured as arising from ground shear
but this figure varies with the nature of the bottom, being
lower on hard ground. It follows that the lift and drag co-
efficients of model otter boards, as measured in towing
tanks or wind tunnels, require some review under
operational conditions. The emphasis in the design of
new otter boards is on improving their lift to drag ratio,
keeping the boards upright over a greater speed range
and keeping the ground reaction large enough to profit
from the ground shear, yet not so large that the boards
just sink too far into soft mud.
Fig. 4. Planjorm of harness lines.
Headline height
The headline height of the Granton trawl is poor, being
only about 5 ft at 3 £ knots. Headline height is very
much dependent on towing speed, falling from 8£ ft
at two knots, sometimes to as low as 4 ft at four knots.
The volume of water filtered in unit time is just about
the same at two knots as at four knots and for any
reasonable towing speed the frontal area presented by
the net is poor for the amount of netting that the net
comprises.
It was not found possible to increase the headline
height of the Granton trawl greatly and still keep it
fishing. The short spreading wires restrict the height but
lengthening these does not in itself bring much gain
because of the constraint in the netting due to lack of
width at the lower wing end. Extra flotation serves mainly
to increase the constraint in the netting between headline
the groundrope, easing the groundrope off the bottom
without adding much to the headline height. Increasing
the headline height up to some IS ft requires a three-
pronged attack, such as that described by Dickson
(I960) but briefly involving:
(a) Altering the wire rig considerably to give very
much longer spreading wires.
(b) Relaxing the constraint in the wings by reshaping
them and adding more net.
(c) Increasing the flotation.
Ship thrust and trawl drag
Curves for the propeller thrust against speed of advance
over a range of engine rpm can be drawn up without
taking either the ship or trawling gear to sea but, while
such curves are a good guide as to what to expect, it
usually turns out that they are only approximate.
Once a considerable number of instrumented hauls have
been made with trawling gear it becomes possible to
establish what the actual curves for that ship are.
The experimentally derived curves for F.R.S. Explorer
with the drag of the Granton gear superimposed, are
shown in Fig. 5. Such curves, once established, are a
powerful tool in fishing gear design and the matching
of trawling gear to ship.
Fig. 5. Granton trawl drag — speed and rpm curves (for F.RS.
Explorer).
Discussion on Instruments
for Testing Gear
Mr. W. Dickson (Rapporteur): Hamuro and Ishii describe
a neat series of instruments for measuring headline height,
trawl depths, speed of the trawl and an underwater dynamo-
meter for measuring tension in front of and behind the otter
boards. There is also a Recording Groundrope Indicator,
several of which are, 1 gather) secured to the groundrope at
various places. A rudder trails behind in the sand or mud;
the rudder works a pen which records the angle of the ground-
rope at the point where the instrument was fixed. Given, say,
four of these instruments on the groundrope and knowing
its length you would have a good idea of its shape and spread.
Does the groundrope indicator work well, I wonder? They
also have a low speed log for trawlers. It is an instrument of
the propeller type a measure of whose rpm is transmitted to
an indicating unit aboard via an electric cable.
525
Speed measurement is, in fact* still one of our greatest
difficulties and it can very nearly be said that you will get
as many answers as the number of speed togs you like to use.
It is the conditions under which they are used rather than the
logs themselves which produce the variety of answers.
J. Nkfaolls describes a similar set of instruments, an impres-
sive number having been evolved in the research work on
trawls at Saunders-Roe's testing tank.
The measurements of otter-board shearing and drag forces
just did not tie up with the otter board model tank results
until a fully instrumented otter board was made. Several
load cells were used to measure the ground reaction of the
otter board and by a system of levers to measure also
the ground forces normal to the board and in the plane of
the keel.
All the deck instruments recorded electrically on a panel
of six recorders. It was found that for measuring things like
warp tension, which varies rapidly, even a high-speed six
point recorder was not good enough. Now it is only used for
measuring diesel engine temperatures, rpm and other things
that don't fluctuate rapidly.
Dale and M oiler show similar deck recording instruments
for measuring the length of warp paid out and the warp
tension of a sterntrawler.
Hamuro measured warp tension, the vessel's speed, the
torque at the propeller shaft, the exhaust temperature of the
main engine and the mean effective pressure of each cylinder
and the fuel consumption, in order to match a new design
of bull trawler to the power absorbed by controllable pitch
propellers newly installed on a pair of bull trawlers.
Camithers poses, a sly question; what do we mean by
"water speed at the trawl'9 and the supplementary question is,
how do you measure it? In bottom trawling the gear is drag-
ging on the bottom. My view of what is meant by water
speed at the trawl is this: all tensions and dimensions are
measured with respect to the centreline of the trawl gear,
therefore velocities ought to be referred to that same direction.
Because the gear is dragging like an anchor this is not neces-
sarily the direction of motion of the trawl with respect to the
bottom nor yet the direction of the ship's headings. The water
speed of a trawl is, then, the component of its ground speed
which lies along the gear centreline plus the component of
bottom current velocity which lies along the gear centreline.
This is not the same thing as what is often taken as the water
speed of the trawl, by floating a current meter on a line above
the trawl headline. Such an instrument adds the ground speed
of the trawl and the bottom current. What I am calling the
water speed of the trawl would be more truly measured by
fixing the current meter rigidly along the centreline of the gear
but at some distance ahead of the net. But how does one do
that and even if one did, the objection could be raised that a
cross-current at the bottom made the gear slightly assymetrical
about the gear centreline anyway. We might argue about this
all day and still leave Dr. Carnithers with the last laugh.
Two other instruments are the netzsonde and the depth
telemeter. Dr. Schfirfe has discussed the former and the latter,
without any wires, is described by Hamuro and Ishii. Water
pressure on a pressure sensitive element is used to change the
frequency of the transmitted signal which is picked up by a
receiving transducer slid a little way down the warp on a short
cable, far enough down to take it below the propeller wake.
The National Institute of Oceanography in this country have
produceda similar instrument. No wit is true that at the moment
the netzsonde maybe has the best of the argument, because it
gives information on fish behaviour in the net mouth and
below it as well. In the long run, however, it wouldn't seem
526
too difficult to effect a frequency change and relay inform^
tion from an underwater echo sounder up to thie surface.
Every winch for a netzsonde cm a big trawler costs severa
thousand pounds so if, say, 100 trawlers were going to tx
fitted with netzsonde equipment it would look well worthwhile
to spend a bit of money on telemeter development instead.
It certainly won't be paid for with peanuts.
Once a sonic link is established for telemetering information
to the surface all sorts of things begin to look possible
and if information can be sent up, command signals can also
be sent down. What sort of information could be sent up?
In the first place perhaps research information and a likely
first choice is the water speed at various places inside and
outside the net. It would help the commercial fisherman if
the net could be monitored to see if it was towing level, that
it was still free from major splits and how full was the codend.
Think how that information would help a trawler skipper in
his choice of tactics!
Dr. Schftrfe (Germany): In some of our tests we used the
jellybottle principle which Dr. Camithers introduced and
we found it most convenient for measuring the angle of attack
of kites as used in the German herring bottom trawling indus-
try. This helped us to confirm what we had expected — that
those kites were often working at a very inefficient angle of
attack. We used the jellybottle in its simplest form namely,
without compass and other equipment. We had one bottle
attached to the board and when the jelly was set you could
determine the angles quite easily. They had also experimented
with a wireless telemeter in trawl tests because they had had
bad experience with cables in the beginning. They found,
however, especially with large vessels considerable inter-
ference from the sound waves created by the propellers and
the engines. Because of that experience he was very interested
to learn that other workers had been successful in establishing
an acoustic link between the net and the ship and over consider-
able distances like 1,000 to 2,000 m. He understood this was
partly possible by the use of much higher frequencies than
they had used. It remained true, however, that with the simple
kind of instrument the scope of information you could trans-
mit was limited. Another thing was the question of ruggedness
of such instruments. If anything went wrong with the under-
water instrument they might get a measurement on which
they could not possibly check. That was the great difference
of this principle compared with the netzsonde. Through its
echo-sounding principle, they knew netzsonde measurements
to be correct as long as the recording instrument worked at
the right speed. It would be too much to ask a trawler skipper
to check up and calibrate repeatedly such a complicated
instrument as a wireless telemeter.
Mr. D. L. Alverson (U.S.A.): We had six or eight years
experience in using various types of cable which carry a
conductor within the towing rope itself. This offers a promising
solution if and when the cable producers can provide a suit-
able cable with sufficient durability. One of the basic problems
in using a so-called armoured cable is the flexibility and break-
age of the conductors within the cable. We have not been
satisfied to date that we have a sufficiently good cable to
apply to the heavy work of commercial trawling. It has, how-
ever, been very profitable in research as you can convey a
number of parameters along those cables. There is some pro-
mise along the lines of durability. Because of the great in-
crease in interest in oceanographic research some of the wire
companies are actively engaged in investigating the possibility
of producing a trawl cable which would carry a number of
conductors from which by means of slip rings you could
have information from measuring devices on the trawls.
Dr. Ode (U.K.): We have produced a different type of
instrument which is quite useful: a small thermograph for
recording the bottom temperature which may be attached to
the trawl and gives a continuous record of the temperature
of the water through which you have towed it. This might
be important in the future, inasmuch as the bottom tempera-
ture governs the distribution of the fish.
Dr. Schftrfe (Germany): Referring to model testing,
commented that Mr. Dickson said how research teams should
be built up. I think we all agree you need the right people
for the right job. You need specialists in this field. We, being
biologists interested in techniques, do not try and interfere
with engineers. We don't engage in model testing with small-
scale models and we try to get the answers to our questions
mainly by working with the actual gear. One problem is why
does one type of gear catch fish better than another. Among
other things this may depend on the shape and behaviour
of the trawl net and the shape of the meshes in the net. This
is difficult to observe in the full-scale gear.
To secure an answer on that point we therefore intend to
carry out tests with models in the scale of 1 :4 to see why the
two-sided seam midwater trawl is so much inferior to the
four-sided seam midwater trawl. These tests will be made
in the Baltic in 1 0 to 1 5 m depth of water over a sandy bottom
and observations will be made by skin divers in the usual way.
Mr. J. A. Tvedt (U.K.): Sixteen years ago we used a
kind of measuring device that was connected from the net to
the ship much as the netzsonde is. We found however that,
when hauling and shooting, the connection was an encum-
brance. With the Japanese system you are still using a short
length of trailer over the stern and we feel we would rather do
without that as well. We feel the fewer things that you have,
the better, so I want to see if anybody has a device where a
link can be established between the hull and the net without
having to use a winch or a cable. There was a case for consider-
ing the ship itself as part of the fishing gear. In this connection
there was a strong case for building research vessels of two
different sizes through which they could investigate the hand-
ling characteristics of gear and at the same time those con-
cerned with hydrodynamics could consider the ship as a whole
and investigate various means of handling gear on the ship as
well as the efficiency of the propulsive devices, steering, etc.,
as related to the trawls. Naval architects as well as other
technicians would find much useful information from such a
test. Naval architects were just as keen to know about fish
behaviour as the biologists.
Mr. T. V. Hinds (U.K.): In our work at Aden we use an
instrument for measuring water depths and temperatures.
The instrument is called a Thermistor thermometer. It is
contained in a small rugged box which can be used on various
sizes of boats like the 20- to 30-footers that we run.
The thermostat head is on a strong cable which we can lower
to over 100 fm. The thermocline measurement is recorded
directly on a recording machine in the boat. This is manually
operated and does not require any attention. The instrument
gives the actual depth and we can read the actual temperature.
Some of our Somali fishermen who are longlining for tuna,
after they arrive on the grounds in the morning, discover
from this instrument, that the fish are being caught on a
pattern at a certain depth say about 30 fm and the word is
passed round to the others. This has been very valuable and
interesting to us. As a result of it we have found, on the
Japanese longlining we are using, that the catch rate has
increased from 16 per cent to 90 per cent over a period of
four months. An Italian vessel caught 240 yellowfin on 270
hooks. This was through setting the line at the right depth.
The Arab fishermen can use this instrument without much
trouble. They also use a longline hauler.
Mr. EJdon Nichols (U.S.A.): Dr. Schfirfe was quite right
in saying that the capabilities of the wire telemeter system
applied to trawls are very great compared with the sonic or
other systems. Someone should go into the possibilities of
developing a system of using a coaxial line for applying to
the centre of the trawl wire. This coaxial line would have to
have a flexible centre conductor insulated by polyethylene
and outer tapes or braid. It would have to be a very tough
cable which would stand a lot of use. They would need an
array of electronic or vacuum tubes at the bottom. They could
do this with solid transducers and transistors and so on.
These require very little power which can be furnished by
means of batteries at the bottom or the power could be sent
down through the centre conductor and filtered out without
much difficulty. That is what they did in their transoceanic
submarine cables and they had hundreds of these devices on
the bottom. They had been there for 10 or 12 years without
giving trouble so it was entirely a matter of devoting some
ingenuity and development effort to this problem. A good
idea of the angle of attack would be got by observing the
scratches on the bottom of the shoe of the otter board. Fisher-
men did this quite regularly. In connection with otter boards
there was always a difference between the theoretical and
actual performance of any device. He had wondered some-
times at the extreme roughness of the ordinary commercial
trawl boards and he thought they might consider the use of
'Fibreglass' with foam plastic interiors. They could get suitable
densities and abrasion stresses that were entirely compatible
with the use of such boards up to 2,000 fm.
Mr. J. NichoUs (U.K.): 1 am very interested in the tele-
metering system. We considered that method in the first place
but with the time and money we had available we could not
see it being done. I am therefore hoping that in future we will
get more details from the Japanese in relation to the paper by
Hamuro and Ishii. I was rather surprised to see the accuracies
given as one per cent. That is considerably better than one
can normally obtain in laboratory machines. As a simple
engineer I would like to know how they get those accuracies.
They did use the method of interpreting the scratches in
relation to the angle of attack and thereby had obtained good
co-ordination with other measurements they had made.
Dr. Miyazaki (Japan) : The author of the paper is not present
and the question is too specialised for me to attempt an answer.
After returning to Japan and checking with the author I will
answer the question.
Mr. D. L. Alverson (U.S.A.): We have encouraged people
to look into the production of the type of cable you have
mentioned and studies are going on along those lines. If they
could develop a cable of this nature which precludes the need
for having a third wire they would have overcome many of
the problems facing them. Some of the instrumentation they
had used gave them good results. There was an on-bottom
indicator which signalled the bridge when the boards were on
the bottom. This was important when working along the
Continental Shelf. They also took off temperature readings
from a depth telemeter unit and they also used a load indica-
tor. This did not correlate out as well as they would like it to
do. They were not getting more than 50 or 60 drags out of a
cable before they were being plagued with conductor break-
ages.
Mr. J. O. Trmung (F.A.O.): No research is better than its
measurements. Speed measurements are especially important
for gear research. During the first Gear Congress it was
pointed out that fishing gear resistance increased by the square
527
ONLY with some knowledge of the behaviour of fish
in the immediate vicinity or path of the gear can
the gear technologist approach objectively the problem
of improving the design, rig and/or operation of existing
gears or of developing new ones. Relatively few
studies have been made of these characteristics of
behaviour in exploited fish species, due mainly to
difficulties of making underwater observations and
measurements in the vicinity of fishing gears, especially
in deep water. Recently, however, this type of work
has been stimulated by activity in the gear technology
field, and by the development of new underwater
observation techniques, which have made it possible,
for the first time, to obtain detailed information on the
orientation, movements and responses of fish in the
vicinity of different fishing gears. Developments in
the application of echo sounding,8 of underwater photo-
graphy and television,0 of direct observation in shallow
water by frogmen and from underwater vehicles8 and
in the production of light meters and other instruments
for measuring the stimuli produced by fishing gears and
by the fish themselves have been of major significance.
At the same time there has been an increase in experi-
mental work in aquaria on the behaviour offish to differ-
ent types of known artificial stimuli, with a view to
determining their range of perception and responses.7
The importance of visual stimuli and vision in the
behaviour of fish in their natural environment is well
established and has been mentioned for herring.10
Therefore, as a first step in investigating responses of
fish to stimuli produced by fishing gear, the study of
visual stimuli seemed appropriate. Such experiments
were started in aquarium tanks at the Marine Laboratory,
Aberdeen, in 1958, and have been continued in aquaria
and at sea since then. Results of studies concerning
the reactions of commercially important marine species,
especially herring, to both stationary and moving nets
and to other types of visual stimuli are presented in this
paper. Particular attention is paid to the differences
in the responses of the fish to these stimuli in daylight
and darkness.
STATIONARY NETS
Daylight experiments
An initial report of these experiments was given by
Blaxter, Holliday and Parrish (1958).8 Aquarium
observations were made first in tanks measuring 3*7 m
X l*8m x 1 • 1 m on two groups of herring, the fish
in one group being about 25 cm long, and in the other
about 12 cm long. The herring usually swam in a
shoal round the entire tank, but when they were confined
to one end and a panel of netting or other barrier
placed across the centre of the tank, their behaviour
changed. If the netting or barrier presented a strong
enough effect, the herring avoided it and circled the
half of the tank they were in. Where the barrier was
less effective some or all of the herring would swim
through. To test the effectiveness of the barriers the
number of herring swimming through in a given time
was counted.
The following aspects of netting were studied:
(a) Type, colour and thickness of material.
(b) Mesh size, or distance between horizontal or
vertical strands where these alone were used. Distances
between strands from 2 • 5 to 30 cm were used. Meshes,
when used, were not knotted (to avoid gilling the fish).
Table I— The "effectiveness" of different barriers in daylight
12 x 20-25 a
Distance
between
strands
cm
7-5
15-0
22-5
30-0
Number of fish crossing in
30 minutes
Vertical Horizontal
Mesh strands strands
only only
Trawl twine (white) 0-3 cm dia
0 12 5
8 23 422
— 600 —
450 648 420
Cotton driftnet twine (B-black, G-Green, W-white). 0-05 cm dia
B
3-75
7-5
3-75
7-5
0
w
75
0
7-5
103
148
344
Polyfilament nylon driftnet twine (white). 0-05 cm dia
3-75
5-0
7-5
0
9
0
Monofilament nylon
7-5 80
Household cotton white.
•75 6
clear
9
312
311
0-02
cm dia
0
0
2
0-02 cm dia
7-5
15-0
Frame
alone
14
24
750 crossings in 30 minutes
17 x 12 cm herring
Polyfilament nylon driftnet twine (white). 0-05 cm dia
2-5 33 30 114
3-75 183 120 213
5-0 210 78 348
7-5 144 351 387
Frame
alone 678 crossings in 30 minutes
For some results of these experiments see Table I.
They all point to the importance of vision in avoiding
these barriers. In the first place the reaction distance
was J-l m with the thicker more conspicuous materials,
whereas with less conspicuous ones it was £ m or less.
The other main results were:
(a) Effectiveness of barriers was independent of
material used, e.g. nylon and cotton driftnet twine of
the same colour and diameter were equal in effect.
(b) Thickness of material was very important, e.g.
trawl twine with strands 7-5 cm apart was a complete
barrier; driftnet twine strands had to be placed 4 cm
apart before they became as effective.
(c) Colour was also important. Using different
colours of driftnet twine and the same mesh size, it was
530
apparent that the greater barriers were those colours
(black and green) that made the greater contrast with the
grey, end walls of the tank. Monofilament nylon was
almost completely ineffective and herring swam straight
into it.
(d) A mesh of netting was usually a more effective
barrier than horizontal or vertical strands alone. Results
obtained using horizontal or vertical strands alone were
quite variable with no evidence for either being more
effective.
(e) Effectiveness of barriers depended on the size of
the herring. For example, using driftnet twine, a distance
of 4 cm between strands was completely effective for
herring 25 cm long, but the strands had to be less than
2*5 cm apart to be completely effective for herring
12 cm long.
To test the importance of vision two confirmatory
experiments were carried out in daylight. In one a sheet
of transparent plastic was placed across the tank. The
herring swam "headlong" into it and had apparently no
other sense to determine its presence. In the other, a
herring was "blinded" by placing opaque plastic over
the eyes; it swam into the netting and seemed unable to
perceive it. A control fish with transparent plastic over the
eyes behaved normally and avoided the netting.
Experiments at low light intensities and in darkness
A further test was to experiment at dusk and in the
dark. The group of larger herring (25 cm) were first
used and barriers which had proved effective in daylight
(driftnet twine of different colours with strands 4 cm
apart), were placed across the tank with the fish confined
at one end. The tank was covered and the light intensity
was gradually reduced. The movements of the herring
were observed by taking a sequence of flash photographs
at one-minute intervals with an underwater camera or
by switching on an artificial light for a few seconds so
that an observer could see the herring. As expected,
the netting ceased to be an effective barrier, the fish
passing through it, when the light reached a certain low
value (this was called the "threshold value", measured
by an underwater light meter6). Fig. 1 shows herring on
both sides of the barrier.
Fig. 1. Herring passing through a stationary net (horizontal strands
of white drtftnet twine 4 cm apart) in darkness.
In further experiments in a much larger tank measuring
13 x 7 -5 x 1 m somewhat larger herring (25-30 cm) were
used and either black or white driftnet twine barriers
(strands 4 cm apart) were placed across the tank against
a white or black background. The herring were observed
by flashing a light during the dusk-dark period.
For the threshold light intensities (when fish started to
pass through the netting) see below. These intensities
varied somewhat, depending on the barrier and the back-
ground, the herring avoiding the netting for longer as the
light intensity dropped (giving a lower threshold) when
the netting was more conspicuous, either because it
contrasted well with the background or reflected light
from above.
Small tank 0*001—0*0001 metre candles.
Large tank 0*01—0*001 metre candles.
The reason for the threshold being lower in the small
tank was probably that the fish, being more confined,
had a better knowledge of the topography of the tank
and netting. They were thus able to avoid the netting
at lower light intensities.
As a further test of the possible importance of vision,
a curtain of air bubbles was set up across the smaller
tank by laying a weighted plastic pipe drilled with small
holes across the bottom and pumping air through it.
The herring would not pass through this by day but did
pass through it in darkness, indicating that the sound or
pressure stimuli given off by the air bubbles were less
important than the visual ones.
Discussion
Presumably driftnets which are difficult to see, i.e.
provide the least barrier effect, will catch the most
herring. Thus nets with a high threshold light intensity
for avoidance are required and the herring will then
start to pass through, or into, them at higher light
intensities. Of all the materials used monofilament
nylon was definitely most difficult for the herring to see
and they swam into it even in daylight. It is of interest
that v. Brandt4 found such nets effective for herring in
the Baltic.
Measurements of light intensity at different depths
were made on the Scottish east coast driftnet grounds
during summer fishing. The values were compared with
the threshold found for net avoidance in the large tank
(thought to resemble sea conditions best). It was found
that the light intensity at the depth of the driftnets was
quite high compared with the "threshold" for avoiding
nets during a considerable part of the "night". This
suggests that capture of herring at these times of year
may be relatively inefficient. Of course capture is not
only dependent on the light intensity, it may also depend
on the luminosity in the water, the number of fish
already caught (i.e. net saturation), the angle of approach
of the fish, the schooling "pressure", and the activity of
fish due perhaps to panic or other factors, as well as the
tide.
531
MOVING NETS
Further experiments were done in daylight and dark-
ness on moving nets, parts of nets and other devices,
in large tanks measuring 16mxl3mx2m. The
species used included herring from 25-30 cm long, cod
50-80 cm, whiting 15-35 cm, haddock 30-50 cm and
flatfish 15-40 cm. Various numbers were used, but most
experiments were done using about 100 herring or 20-30
roundfish or flatfish. After the fish had been established
in the tanks the devices mentioned below were pulled
through as shown in Fig. 2. Different hauling speeds
were used, ranging from 0*25-2-0 m/sec, the usual
speed being just under 1 m/sec.
l__ . IC..
r^ loin— — - - - - — — — — . •—• .. — ..
Fig. 2. Diagram showing experimental technique for studying
moving nets.
(a) Various types of "groundrope": manila, grass and
hemp ropes ranging in diameter from 1-4 cm, with and
without bobbins. Trawl twine, polythene tubing, which
was nearly invisible, and a bamboo pole (giving a device
without a bight) were also used.
(b) Ropes with the following attachments: chain,
floats of different colours (12-5 or 22-5 cm dia), under-
water lights (40 W, 100 W).
(c) A pierced polythene tube through which air could
be pumped, giving a "curtain" of air bubbles.
(d) A rope frame with wooden or wire danlenos,
simulating the mouth of a trawl (without the netting).
This was used with and without model otter boards and
herding devices such as floats on double or single
swccplines.
(e) Panels of netting of different colours ranging in
mesh size from 15 cm (6 in) to 120 cm (48 in) and in
diameter from 0-1 cm to 0-4 cm.
(f ) Model nets
Daylight Experiments
An observer in a moving bosun's chair above the tank
noted the reaction distance of the fish, the distance they
swam in front and the number of fish passed by the gear
(i.e. failed to be herded). A well-defined type of response
was revealed by each species. They reacted by turning
away from, and swimming in front of, the advancing
532
"groundrope" or other device. The general pattern of
reaction was similar to that observed by frogmen of fish
in front of trawls and seine nets in shallow water (see
films "Trawls in Action", "Fish and the Seine Net").
Table II— Response of different fish to towed devices in daylight.
Average range Average
Experiment of reaction swimming Average %
distance (m) distance (m) herded*
Herring
To "Groundropes", etc.
0-3-1-9
0-5-2-0
60-70
To Panels of
0-7-3-3
0-7-2-5
95-100
netting, model nets
(6-5
shoal)
Cod
To "Groundropes", etc.
0-1-2
0-1-6
40-50
To Panels of
netting, model nets
0-1-2
0-1*6
90-100
Haddock
To "Groundropes", etc.
0-1-2
0-3-1-6
45
To Panels of
netting, model nets
0-1-2
0-3-1-6
70-80
Whiting
To "Groundropes", etc.
0-3-1-0
0-3-1-2
20
To Panels of
netting, model nets
0-3-1-0
0-3-1-2
75
Flat Fish
To "Groundropes'*, etc.
0-3-0-6
0-3-1-0
55
To Panels of
netting, model nets
0-3-0-6
0-3-1-0
100
* Herring usually herded the full
length of the
tank. All the
other fish usually herded
only a few
metres. Here
herding means
for more than 5 metres.
Results in Table II show:
(i) Reaction distances. When the devices were pulled
along the tanks, the fish were herded inwards by the
sides (i.e. warps of the model nets) and forwards by the
centre, as shown in Fig. 2. The reaction distances
ranged up to 3 m, this depending on how conspicuous
and how deep the device was, the reaction distance being
greater with conspicuous deep devices, e.g. net mouths
or panels of netting. The distance was usually about
1 m.
(ii) Reaction to warps. These were usually of 1 -5 cm
dia rope. The usual reaction distance was 0-5-1 m,
the fish swimming inwards at an angle depending on the
relationship between the warp and the direction of the
haul. The warps were effective in herding in water
depths of up to 1 m. Where the water was 1 -5 m or
more the herding was much reduced.
(iii) Herding by the device. As the fish came under the
influence of the device itself in the centre of the tank, they
turned away from it (see Fig. 2), and towards the middle
of the tow were swimming in front of it. The swimming
distance was usually 0-5-1 -0 m in front of the device.
In the case of herring some fish might be up to 7 m in
front, the fish swimming as a school. With some
inconspicuous devices the fish were touched by them
before being herded.
The percentage of fish herded varied greatly. In
general, herring were herded better and for longer than
roundfish and flatfish. The roundfish and flatfish were
often herded for a distance of 5 m and then fell back
again over the device; the herring were usually herded
the length of the tank. It was apparent that percentage
herding was greatly increased by conspicuous herding
devices, especially those occupying the whole water
column. In general netting, even of mesh size up to
120 cm (60 cm x 60 cm), was the best herding device,
especially when made of conspicuous material such as
0-4 cm dia white nylon. Air bubble curtains were
ineffective as the bubbles streamed back at a high towing
speed. Underwater lights were quite effective and
tended to give higher reaction distances and swimming
distances than other devices. The usual reaction when a
fish ceased to be herded was that it was overtaken by the
gear passing beneath it.
(iv) Speed of tow. At towing speeds above 1 -3 m/sec
the device sometimes tended to lift off the bottom. It
was apparent though at these speeds that fish of all
species were not keeping in front of it. The percentage
herding dropped, as shown in Table III. The total
number of fish herded also decreased as there was less
time for the fish to react to the warps and many were
passed by the warps before they could be herded
inwards.
Table III— Effect of towing speed on percentage herding.
Towing speed
Average percentage herding
(all gears)
Below 1-3 m/sec
Above 1 • 3 m/sec
Cod
38
31
Haddock
49
34
Herring
84
53
(v) Depth. It was obvious that for good herding the
device (and also the warps) needed to be at the same
depth as the fish (compare "groundropes" alone and
nets in Table II). For fish distributed throughout the
depth of the tank coarse mesh netting of conspicuous
material produced the best herding.
(vi) Differences between species. Herring were most
active and were usually herded as a group over the length
of the tank, whereas roundfish, and especially flatfish,
reacted individually and were more often herded a few
metres before "falling back". This depended, however,
to a large extent on the device used. Skate and dogfish
usually failed to react at all (see Mohr, this Congress).
The results of these daylight experiments confirm the
importance of vision in the response of fish to moving
devices. In one experiment a number of herring were
"blinded" by placing opaque plastic over the eyes; they
failed to respond but tended to swim near the surface.
Experiments at low light intensities and in darkness
The same types of experiments were done in the evening
through dusk to darkness. Observations were made as
before until it became too dark to see the fish. Artificial
lights were then flashed during the haul to assess herding,
or photographs were taken with a Polaroid-land camera,
using electronic flash. In a few experiments observa-
tions were also made with a 500 kc/s sector scanning
echo sounder.
The experiments showed that the extent of herding
gradually decreased as the light intensity dropped. School-
ing in herring ceased at about the same time. The reduc-
tion in herding appeared in all species and was shown
by the reduced reaction and swimming distances, by the
low percentage herded (of the total fish in the tank) the
disorientation of fish one to another and the low number
of fish in the bight of the gear at the end of the haul.
It was possible by continuous measurement of the light
intensity to estimate the threshold light intensity for
herding (taken to be the intensity when herding had
dropped to half what it was in full daylight = 50 per cent
herded). This value lay usually between .0-5 and 0*05
metre candles. It tended to be lower for roundfish than
for herring and was lower with the more conspicuous
devices. Photographs of herring and cod reacting to
netting and a "groundrope", at light intensities above
and below the threshold, are shown in Figs. 3-6. Figure 7
shows a generalised relationship between light intensity
and herding. A surprising result was that herding did
not take place in darkness when underwater lights were
used; despite the importance of vision the underwater
lights were ineffective, though they gave the lowest
threshold of all. Figs. 8 and 9 show herring reacting to
underwater lights by day but not in darkness. It seems
that the activity of the fish drops at night and the under-
water lights (up to 100 W) are not bright enough to
stimulate them.
Experiments at sea
While the importance of vision was demonstrated in
all the experiments it was realised that the tank conditions
were artificial due to their relatively small size, giving
limited space and reflections from the walls. Nor could
the other stimuli produced by fishing gear (high and low
frequency sounds, damming phenomena) be simulated.
The fish had also been caught and therefore might behave
abnormally. To obtain further evidence of the import-
ance of vision in herding by gear it was decided to photo-
graph fish in the vicinity of fishing gear at sea in different
Fig. 3
Fig. 4
Fig. 5
Fig. 6]
Fig. 3. Cod herded at above threshold light Intensity (45 cm mesh net).
Fig. 4. Cod below threshold haul (45 cm mesh net).
Fig. 5. Herring herded at above threshold light intensity (45 cm mesh net).
Fig. 6. Herring below threshold haul (45 cm mesh net).
533
DARK AT SURFACE
DUSK AT SURFACE
0-1 K)
LIGHT NTENSTTY Cmctrt condteg
10-0
Generalised graph showing the relationship between light
intensity and percentage herding.
Fig. 8. Herring herded by underwater lights (40 W) in daylight.
Fig. 9. Herring not reacting to underwater lights in darkness.
light conditions. To do this an automatic underwater
camera6 fitted with electronic flash, was attached,
pointing downwards, just behind the headline of various
trawls, thus giving photographs of fish in the centre of
the mouth of the net and in front of the groundrope. An
exposure was made each minute throughout a haul.
534
According to the results of the tank experiments most
species of fish would be expected to be oriented and
swimming in front of the groundrope in good light
conditions and disoriented and poorly herded at low light
intensities. The light intensity was therefore measured
on the sea bed for each haul.
The results of the analysis of many photographs taken
in hauls at different light intensities in different parts of
the North Sea tend to confirm the results obtained in the
tank experiments. Thus under visual conditions (i.e.
at light intensities above the visual threshold measured
in tanks) the fish appeared to be oriented with respect to
the groundrope and swimming away from it. Below
the visual threshold the fish appeared to be less active,
and less well oriented, even when in close proximity to
the groundrope. Some specimen photographs of herring
in front of the groundrope of a trawl are shown in
Figs. 10 and 11.
Fig. 10. Herring in front of a trawl groundrope — above threshold.
Discussion
It seems that, under conditions of reasonably good
visibility (e.g. in depths up to about 50 fathoms in the
open sea under average daylight conditions), fish at the
same depth as (within £-1 m) and in the path of an
approaching trawl will react to it at a distance of about
1-2 m. The fish encountering the warps, trawl boards,
sweeps or bridles will tend to be herded forwards and
inwards towards the path of the net, at an angle depend-
ing on the angle the particular part of the gear makes
with the direction of tow. The fish encountering the net
itself will be fyerded forward. During this process some
fish will "fall back" over the warps, sweeps or bridles
and be lost, or over the groundrope and be caught.
The rate of "falling back" depends on the nature and
conspicuousness of the gear, the speed of tow and the
• distance already herded. For the species which we have
Fig. 11. Herring in front of a trawl groundrope— below threshold.
investigated herding will tend to persist for only a short
distance when the towing speed exceeds about 3 knots.
Therefore, at the relatively low towing speeds encountered
in many demersal trawl fisheries, and under visual
conditions, the warps, trawl boards, sweeps and bridles
probably play a positive part in the capture mechanism.
This is in conformity with experimental results and
fishermen's experience, showing an increase in fishing
power of trawls with the introduction of the Vignernon-
Dahl sweep gear1. The results of our experiments also
indicate that the efficiency of these components of the
gear will probably depend on their conspicuousness and
their "depth" (i.e. the vertical distance over which they
are effective). Thus, it seems that, for maximum
efficiency, warps, sweeps and bridles should be as
conspicuous, and have an influence over as great a
vertical range, as possible.
These considerations suggest:
(a) That the use of two or three bridles for spreading
the net, as now commonly employed with some high-
headline trawls, is probably more efficient for herding fish
than a single sweep (for a given bridle and sweep angle
and towing speed).
(b) The use of paints and other measures to increase
the conspicuousness of the bridles might increase their
herding efficiency.
(c) Herding efficiency with double or triple bridle gears,
might be increased by inserting coarse, large-meshed
netting between the bridles, especially near the wing ends
of the net, where the distance between the bridles is
greatest. However, this will increase the total drag of
the gear and introduce additional handling problems.
(d) The production of an air-bubble curtain from the
sweeps or bridles would not seem to be an effective
measure.
The conspicuousness of the trawl itself may also play
an important part in determining the efficiency of capture.
As our observations show, fish are herded forward by
the mouth of the net so that if it is too conspicuous fish
may not be caught because they swim away from it
throughout the haul. Reports by Cdr. J. Hodges, during
the filming of "Fish and the Seine Net'*, confirmed that
large numbers offish (principally plaice) avoided capture
in this way. In this series of experiments mackerel have
been photographed in front of the groundrope and none
found in the catch. On the other hand if the netting in
the mouth of the trawl is inconspicuous, herding into
the centre will be small, and fish might be lost through
the large meshes of the wings. Further investigations of
the behaviour offish inside the net (in front of the codend)
are needed before the relative importance of visual and
other stimuli at this stage of the capture process can be
properly judged, but it seems that increasing the con-
spicuousness of the extremities of the mouth of the trawl
(wing ends, bridles, etc.) and reducing it at the centre
might be advantageous.
TOWING SPEED I 1-5 m/sec (3knofs)
Fig. 12. Diagram showing direction of escape
offish from the centre of the bight of a trawl (see text).
In relation to the herding properties of the net itself
and of the consequent loss of fish, it is interesting to
consider the chances of escape of a fish and to see how
these vary with trawl width and towing speed. A very
simplified example will be taken. In Fig. 12 a fish at
point £, in the centre of the path of a net, reacts 2 m
from the groundrope of a trawl 16 m wide towed at 1 -5
m/sec (3 knots). It swims at an angle 0. It is assumed
that a cod 75 cm long can swim at a sustained speed of
2 m/sec for 100 body lengths (75 m) and a herring 30 cm
long at 1 -75 m/sec for 1,000 body lengths (300 m) before
becoming tired (Blaxter and Dickson 1959*). It can be
shown that the swimming speed required to escape
depends on 6 and the towing speed, i.e. swimming speed
1-5 .
Sine 6*
towing speed
to escape = — ~e *~
Sine B
Clearly this must not exceed 2-0 m/sec for the cod and
1 • 75 m/sec for herring. This means these fish can avoid
capture if they swim far enough at the following angles:
cod— from 40 to 90°; herring— from 58 to 90°.
However, the swimming endurance must also be
considered. If the fish swims at 90°, for example, it will
535
only escape if it can swim ahead of the trawl up to the
end of the haul* The distance swum to escape depends
on the width of the trawl and, in the special instance
taken with a trawl of 16 m spread, is
8
Cosine 0
For cod, this distance must not exceed 75 m (100 body
lengths) and in the herring must not exceed 300 m
(1,000 body lengths). This means that a cod in the centre
of the path of a trawl must swim at its maximum sustained
speed at an angle 0 between 49° and 84° and a herring
between 58° and 88° in order to escape before becoming
tired.
These may be called the angles of escape. Such angles
for trawls of greater dimensions are given in Table IV.
Table IV— Angles between which fish must swim to escape.
Trawl width Angles for escape
(metres) (0)
16
20
24
32
Cod
49-84°
49-82°
49-81°
49-78°
Herring
58-88°
58-88°
58-88°
58-87°
It will be seen that the angles do not change very greatly
showing that widening a trawl will not tend to decrease
much the chances of escape. If, however, trawling
speed is increased — in the case of cod to 2 m/sec (4 knots)
or for herring 1-75 m/sec (3J knots) — no fish at any
point within the bight of the groundrope can escape
sideways without having to pass through the net. So
from the escape point of view increasing trawling speed
should be far more effective than increasing trawl width.
Other factors are involved, however. The greater area
fished by wider nets should be of importance, while
increasing speeds of tow may reduce herding. Where
herding is an important factor in capture, the speed of tow
would need to be adjusted to reduce escapes, but not be so
fast that fish had no time to be herded.
So far only the question of increasing the effective
fishing width of bottom trawls has been discussed. The
question of the advantage of increasing headline height
will depend to a large extent on the distribution of fish
near the bottom. These experiments suggest that the
usual response of fish to bottom trawls is to escape
sideways rather than upwards.
In marked contrast to the situation in daylight it would
appear that herding by different parts of a trawl becomes
much reduced at low light intensities. Some reactions
will be caused by tactile stimuli and by pressure dis-
turbances but the effect of the latter, at any rate, will tend
to be less directional than visual stimulation. It would
seem therefore that the effective "swept area" of a trawl
or seine net will be lower at night or in very deep water
in daylight where the light intensity would be below
threshold.
On the other hand the escape of fish in the path of the
net may be lower at night due to lower activity of the
fish and of their inability to orientate to the net. It may
536
be that the ineffectiveness of Danish seining at night in
some localities may be, in part, due to lack of herding by
the ropes. The possibility of increasing the herding power
of ropes or warps by night requires consideration. It
seems from the experiments that low power lights would
not be beneficial.
It is of interest that Bagenal (1958)1 found that the use
of V.-D. sweeps increased catches by day and night. He
considered this was due to the mechanical stimuli pro-
duced by the sweeps affecting fish by day and night. This is
not confirmed by the experiments described here, which
suggest that the mechanical stimuli provided by sweeps
are far less important than the visual ones, which would
not be operating at night. Sweeps also change the spread
or headline height of the trawl, but clearly, further study
is required before these results can be finally evaluated,
for V.-D. gear may have an effect unconnected with
visual or mechanical herding.
CONCLUSIONS
The results of direct observation in tanks and photo-
graphy offish near the mouth of trawls at sea suggest that,
of all the types of stimuli produced by stationary and
moving fishing gears, the visual ones are the most
important in determining the efficiency of fish capture.
At light intensities above threshold the fish will tend to
avoid stationary nets or be herded by moving ones. At
low light intensities the reactions will be minimal.
The implication of these factors in fish capture are
discussed with special reference to the herding properties
of warps and the relative importance of trawl width and
towing speed. It must be emphasised, however, that
much remains to be learned of the behaviour offish in the
vicinity of fishing gear. Although in these experiments
other types of stimuli have been tested, as well as visual
ones (see contribution by Chapman in this Congress), it
has not been possible to simulate in tanks the full range of
stimuli produced by fishing gear at sea. Further work is
required especially to measure the extent of these stimuli
and observations of fish in relation to different parts of
the gear need to be extended at sea both using cameras
and by diving.
Acknowledgements
Thanks are due to Mr. R. G. Lawrie for electronic equipment
and light meters and Mr. R, Priestley for the photography.
References
1 Bagenal, T. B. J. Cons. Int. Explor. Mer, Vol. 24, pp. 62-79.
1958.
2 Blaxtcr, J. H. S. and Dickson, W. J. Cons. Int. Explor.
Mer, Vol. 24, pp. 472-479. 1959.
3 Blaxtcr, £H.S.,Hofflday,F.G.T. and Parrish,B.B. Proc.
Indo-Pacific Fish. Coun. Vol. 3, pp. 46-49. 1958.
4 v. Brandt, A. Der Fischwirt, Vol. 4, pp. 295-300. 1954.
9 Craig, R. E. and Lawrie, R. G. Limnol. Occanogr., Vol. 7.
pp. 259-261. 1962.
• Craig, R. E. and Priestley, R. Mar. Res. Scot. No. 1, 24pp.
1963,
7 Mohr, H. Prot. Fischercitechnik. part 6, pp. 296-526. 1960.
• Radakov, D. V. Byull. Okeanogr. Kom. AJcad. Nauk.
S.S.S.R., Vol. 6, pp. 39-40. 1960.
' Schftrfe, J. Stud. Rev, Gen. Fish. Coun. Medit., No. 13, 38
pp. 1960.
" Verheyen, F. J. Experientia, Vol. 9, pp. 193-197. 1953.
Importance of Mechanical Stimuli in Fish
Behaviour, Especially to Trawls
Abstract
This paper concerns the sensitivity of fish to mechanical stimuli
and the importance of mechanical stimuli in the behaviour of fish.
Mechanical disturbances may be conveniently divided into three
types: (a) sounds (b) "damming phenomena** which are produced
a short distance in front of an object moving through the medium
with constant velocity and (c) tactile (touch) stimuli. Sound
transmission in sea water 1,500 m/s — almost five times the speed
in air could provide a useful early warning system for marine
animals. Some marine mammals (whales, porpoises) do employ
an echo-location system. Marine fish, in general, however,
appear not to have made use of this as they are characterised
by relatively poor hearing ability (hear mainly low frequencies
— up to 1 ,000 c/s). The author believes that the dimensions and
acoustic properties of the fish head and the very nature of under-
water sound within the low range of frequencies to which fish
respond would seem to prevent directional hearing based on
binaural differences of time, intensity and phase. It seems,
however, that the organs of the lateral-line enable a fish to locate
moving objects by virtue of the damming phenomena. It seems
that complex stimuli such as might be produced by turbulent flow
in a net, and very low-frequency vibrations of large displacement
amplitude, would stimulate the lateral-line organs. Several
hypotheses are drawn by the author concerning mechanical stimuli
influencing the behaviour of fish in relation to trawls. The
reactions of fish to various mechanical stimuli have been tested in
large aquaria. They include sounds from underwater loudspeakers
(pure notes and noise), displacements and pressure changes from
low frequency vibrations of a trawl warp, rubber diaphragm and
a "vibrating door". From these experiments predictions can be
made concerning the behaviour of fish towards commercial size
trawls. This will enable more reliable interpretations to be formu-
lated from direct field observations on the gear in action and the
results of comparative fishing experiments.
L 'importance des stimulations mecaniques dans le comportement
des poissons, specialement envers le chalut.
La communication traite de la sensibility des poissons envers
des stimuli mecaniques et de 1' importance des stimuli mecaniques
dans le comportement du poisson. Les troubles mecaniques
peuvent £tre di vises en trois types: (a) les sons; (b) les ph6nomenes
de blocage qui sont produits a une courte distance au-devant d'un
objet se d6placant dans un milieu a une vitesse constante; (c) les
stimuli tactiles. Le son est transmis dans 1'eau de mer a une vitesse de
1500 m/sec presque cinq fois plus vite que dans Pair et peut fournir
un systeme d'alarme utilisable pour les animaux marins. Certains
mammifcres marins (baleine, marsouin) emplpient un systeme
de localisation par echo. Par cpntre, les poissons marins, en
general, semblent ne pas avoir utilise ce phenomena puisqu'ils sont
caracterises par une faible capacite auditive (seulement les basses
frequences jusqu'a 1000 c/sec). L'auteur croit que les dimensions
et les proprietes acoustiques de la tdte des poissons et la nature
mdme des sons sous-marins, dans les basses frequences auxquelles
les poissons reagissent, semblent leur interdire de situer les sons
bases sur des differences bi-auditives de temps, intensite et phase.
D'un autre cdt6 il semble que les organes de la ligne laterale
permettent a tout poisson de localiser des objects mouvants en
vertu du phenomenc de blocage; de m&me les stimuli complexes
qui peuvent 6tre produits par le flot turbulent dans un filet et la
tres basse frequence des vibrations d'une grande amplitude de
defacement pourront stimuler les organes de la ligne laterale.
L'auteur en deduit diffdrentes hypotheses concemant les stimuli
mecaniques qui influent sur le comportement du poisson vis 4
vis du chalut. Les reactions despoissons envers des stimuli
mecaniques diverses ont 6t6 contrdlces dans de grands aquariums.
Ccs stimuli comprenaient les sons emis par un naut-parleur sous-
marin (notes et bruits purs), defacements et variations de pression
par vibrations de basse frequence d'une fune de chalut, diaphragme
en caoutchouc, et par une "porte vibrante". Ces essais permettent
de faire certaines predictions conccrnant le comportement du
poisson envers les chaluts de dimensions commerciales et per-
mettront de formuler des interpretations plus sures, a partir
d*observations directes de 1'engin en action et de resultats d'exper-
icnces de peche comparative.
by
C. J. Chapman
Marine Laboratory, Torry,
Aberdeen
Importanda de los estimulos mecftnicos en d comportamiento de
los peces, particularmente ante los artes de amutre.
Extracto
Trata esta ponencia de la sensibilidad de los peces ante los
estimulos mecanicos y de la importancia de estos estimulos en el
comportamiento de aqueilos. Las perturbaciones mecanicas
pueden dividirse en tres clases: (a) sonoras, (b) "fen6meno de
barrera" que se produce a corta distancia delante de un objeto
que se mueve por un medio a velpcidad constante y (c) estimulos
tangibles. Los sonidos se.transmiten por el agua de mar a 1.500
m/seg., que es casi cinco veces mas que en el aire, y pueden constituir
un valioso sistema de alarma para los animales marinos. Algunos
mamiferps marinos (ballenas y marsopas) emplean un sistema de
ecolocalizaci6n, pero los peces marinos en general no parecen
poseerlo y se caractcrizan por oir mal (oyen principalmente
frecuencias bajas hasta de 1.000 c/s.) El autor cree que las dimen-
siones y propiedades acusticas de la cabeza del pez y la naturaleza
de los sonidos submarines dentro de gamas de frecuencia muy bajas
ante las que reaccionan los peces, parecen impedir la percepci6n
de sonidos dirigidos, debido a diferencias bi-auriculares de tiempo,
intensidad y fase. Empero, parece ser que los 6rganos de la linea
lateral permiten a los peces localizar objetos en movimiento por
efecto del fenbmeno de barrera. Tambien parece ser que estfmulos
complejos como los que podria producir el flujo turbulento de
una red y yibraciones de frecuencia muy bajas y gran amplitud de
desplazamiento, estimulan los 6rganos de la linea lateral. £1
autor formula varias hip6tesis rclativas a los estimulos mecanicos
que influyen en el comportamiento de los peces con respecto a
los artes de arrastre. Las reacciones de los peces a varies estfmulos
mecanicos se han determinado en acuarios grandes y comprenden
sonidos de altavoces submarines (notas puras y ruidos), desplaza-
mientos y cambios de la presi6n de vibraciones de baja frecuencia
del cable de un arte, un diafragma de caucho y una "puerta
vibrante". De estos experimentos se hacen deducciones relatives
al comportamiento de los peces ante artes de arrastre de dimen-
siones normales, que permjtiran interpretar con mis seguridad
las observations hechas directamente en la practica sobre los
artes en movimiento y los resultados de ios experimentos de pesca
comparativa.
VERY little is known of the relative importance of
different stimuli in determining the behaviour of
Ash in the vicinity of trawls. In daylight, vision is
undoubtedly important (see paper by Blaxter, Parrish and
Dickson in this Congress) but what happens during
darkness? The movements of trawls create a variety of
mechanical disturbances in the surrounding water to
which fish might respond, particularly when vision is
restricted.
Mechanical disturbances are produced by the move-
ments of objects in a medium (water) and it is convenient
to divide these into three types:
(a) sounds which are propagated waves produced by
a vibrating body.
537
(b) *4daminingphenomcna)>(Dijkgraaff 1963)whicharc
produced a short distance in front of an object
moving through the medium with constant velocity
(Fig. 1) and
(c) tactile (touch) stimuli.
These disturbances stimulate the mechanical sense
organs (ear, lateral-line and touch receptors) by producing
displacement or deformation of the receptor.
Soundwaves
Sound waves are produced by the vibration of an
object whose movements are imparted to the surrounding
medium as alternate phases of compression and rare-
faction in the form of a wave.
The velocity of sound in sea water is approximately
1,500 m/s— almost five times its speed in air and the
rate of attenuation of underwater sound is much less
than in air. Sound transmission in sea water could
therefore provide a useful early warning system for
marine animals. The potentialities of such a system
have been utilised by some marine mammals (whales,
porpoises) which employ an echo-location system.
Marine fish in general appear pot to have made use
of this and are characterised by:
(a) Relatively poor hearing ability restricted to
frequencies below about 1000 c/s.
(b) Relative inability to locate sound sources except
at very short range.
The propagation of sound waves is characterised by
vibratory displacements of the particles of the medium
and rhythmical pressure changes. Due to the low
compressibility of water the displacement amplitude is
small for a given pressure amplitude with respect to air
and since the ear in most marine fish is believed to be a
displacement detector this may partially account for the
relatively poor hearing in these fish. An auditory
mechanism which would detect rhythmical pressure
changes would be much more efficient. Such a mechan-
ism requires a compressible component connected to the
ear such as a gas filled vesicle. This sort of arrange-
ment is in fact found in some freshwater fish and a few
marine fish such as herring.
Hearing in marine fish appears then to be restricted
to relatively low frequencies and it is unlikely that fish
could detect and react to the high frequencies emitted by
echo sounders. It is well known however that some
echo pulses are accompanied by an audible "ping" of
fairly low frequency to which fish in range might respond.
•»«>»i»a>B»*t«»B«
percepuon
At the present time there is no evidence that fish are
able to detect the direction of a vibrating sound source
by means of the ear. At very close range orientation to
intense sound sources by means of the skin tactile
receptors (v. Frisch and Dijkgraaf 1935) and possibly
the lateral-line has been demonstrated (Harris and v.
Bergeijk 1962). Under these conditions the stimulus
would appear to be large displacements produced by
538
"damming" movements of the vibrating sound source
("near-field effect" (Harris and v. Bergeijk 1962)) rather
than the small displacements of the propagated sound
wave.
Fish could only, detect the direction of sound by
comparing it in some way between their two ears, but
clearly the dimensions and acoustic properties of the
fish head and the very nature of underwater sound
within the low range of frequencies to which fish respond
would seem to prevent directional hearing based on
binaural differences of time, intensity and phase. The
lateral-line, on the other hand, presents a different story.
It seems that these organs enable a fish to locate moving
objects by virtue of the displacements associated with the
damming phenomena.
The lateral-line organs
The term "damming phenomena" (Fig. 1) has been used
to describe the local pressure changes and water dis-
placements which occur in front of moving objects1.
These disturbances are not propagated and are detected
at short range by the lateral-line organs. Displacements
resulting from small objects, are detected at very short
range. Large objects will produce larger displacements
which will stimulate more of the lateral-line receptors
and these objects will be located by the fish at greater
distances. It seems that complex stimuli such as might be
produced by turbulent flow in a net, and very low
frequency vibrations of large displacement amplitude,
would stimulate the lateral-line organs.
direction of y/
Fig. 1. Damming phenomena in front of a flat moving object showing
local pressure increase (broken line) and water displacement (con-
tinuous line).
Importance of "whanfart stimuli
Marine fish hear and many of them produce sounds of
frequencies up to about 1,000 c/s. These sounds are
usually produced during social behaviour in relation to
territory and breeding. No behaviour in relation to
sound depends on localisation of the source except in
conjunction with visual and possibly lateral-line stimuli.
"Strange" sounds sometimes induce "fright" responses
in schooling fish but the orientation of the school in
these reactions is often pre-detennined (pelagic herring,
for example, will tend to dive into deeper, darker water
when disturbed) and again does not depend on locating
the source. These reactions are observed at the com-
mencement of the sound. If the sound continues the
fish often appear to get used to it and "normal" behaviour
returns. In view of the volume of background noise in
the sea it is likely that sounds, more often than not,
have no lasting effect on the behaviour of marine fish.
Stimuli to the lateral-line are very important in co-
ordinating the behaviour of fish in all its aspects partic-
ularly when vision is restricted. Large disturbances
usually result in avoidance reactions whereas small
displacements give rise to food seeking reactions resulting
in an approach towards the source. Although vision is
the most important sense involved in schooling behaviour,
a limited role may be attributed to the lateral-line organs.
For example, individual fish which are disturbed and
increase their swimming activity might transmit the
"fright" response to the rest of the school through the
lateral-line organs.
Fish behaviour in relation to trawls
A few points can be mentioned : There is evidence,
mainly from echo sounding, that noises associated with
fishing vessels and pelagic nets frighten clupeoids and
cause them to dive into deeper water. It is also known
from comparative fishing experiments that the use of
sweeps increases the catch both in daylight and at night.
How far this is due to the herding effect of the sweeps
and how far to a change in the shape of the mouth of
the net is not known. Any herding effect might be
attributed to orientation away from the otter boards
and the warps as a result of visual (in daylight) and
mechanical stimuli (daylight and at night) produced by
them. Possibly the otter boards would have the greater
effect as they would undoubtedly be detected at a greater
distance than the warps.
A greater difference in the behaviour of fish between
daylight and darkness would probably occur in the net.
Daylight observations in the mouth of the net have
shown that fish are orientated and swimming away from
the approaching groundrope. This reaction will not
occur at night unless the fish can detect, and react in a
directional way to, mechanical disturbances produced
by the groundrope or the headline or a damming effect
of the net. During darkness it is very probable that fish
will pass back into the net more quickly and any orienta-
tion away from the net will not be as persistent or
accurate as it would be in daylight (Parrish, Blaxter and
Dickson 1962).
The onset of escape behaviour will depend on a number
of different factors. The method, direction, speed and
persistence of escape attempts will obviously vary
between species and with physiological condition. The
magnitude of the shoaling drive will also be important
and variation in this factor will partially account for
differences of escape between daylight and darkness.
There may also be changes in the general level of activity
of the fish.
It has been observed sometimes that escape of fish
occurs where the net narrows in front of the codend.
Several factors might be operating here. A likely
hypothesis is that due to the tapering of the net, the fish
are herded closer together thus upsetting the balance of
stimuli maintaining the school. There will then be a
tendency for the fish to spread out and escape* At night,
when the schooling drive is reduced, it is still conceivable
that multiple disturbances produced by the swimming
movements of the fish, and in addition tactile stimuli, will
initiate escape responses in the individual fish. In the
absence of any detailed knowledge on the distribution of
pressure gradients and turbulent flow in the net, it is also
tempting to suppose that in front of the codend the fish
are confronted with a resistance ("stow") or zone of
high pressure. This is merely speculative, however.
Reactions in aquaria to mechanical stimuli
Experiments to test the reactions of fish to various
mechanical stimuli have been started. Stimuli used
have included sounds from underwater loudspeakers
(pure notes and noise), displacements and pressure
changes from low frequency vibrations of a trawl warp,
rubber diaphragm and a "vibrating door". In addition,
observations on the herding of fish by different towed
obstacles at different light intensities were made and the
results of some of these are given in the paper presented
by Blaxter, Parrish and Dickson at this Congress.
The experiments were done mainly on herring and
cod in large tanks measuring approximately 15m x 13m
in a depth of water between 0*6 and 1 -6 m.
Pure notes from an audio-oscillator, ranging from 100
to 15,000 c/s, and recorded noises of various sorts and
the noise from a trawl, when introduced into the water
through a loudspeaker, did not initiate any consistent
reactions in herring or cod. Occasionally cod were
observed to turn away from the loudspeaker at the begin-
ning of low frequency sounds.
Reactions of fish to a trawl warp (c 1-25 cm diam)
which could be vibrated at different amplitudes and
frequencies were observed in different light intensities.
The warp which was anchored by a ring on the bottom
at one end of the tank, ran obliquely out of the water
and was vibrated by an eccentric wheel attachment
worked from an electric motor. Frequencies of 2 to
4 c/s were produced with an amplitude of about 1 cm.
In daylight the fish appeared to avoid the warp when it
was vibrating although the reaction distance was quite
small and of the order of 1 m or less. At night, however,
the fish appeared to ignore the vibrating warp for the
most part although some cod were seen to turn away
from it when about 0 • 5 m away.
Both cod and herring showed a tendency to avoid the
area of the tank in which a rubber diaphragm and a
wooden "door" were vibrating— both by day and by
night. These reactions were particularly noticeable with
herring although they were spawning fish and relatively
sluggish. It is difficult to say exactly what the stimulus
was that produced these avoidance reactions. The
diaphragm (area 2 sq m) and the door (area 0-3 sq m)
continued an page 540
539
The use of air-bubble curtains as an aid to fishing
Abrtrmct
The use of air-bubble curtains for guiding herring schools to
point of capture was first tried by the U.S. Pish and Wildlife
Service hi Maine, U.S.A., in the fall of 1957, and has been reported
on in the Commercial Fisheries Review of the U.S. Bureau of
Commercial Fisheries. The air-bubble curtain was used to inter-
cept the movement of young herring (Clupea hareyus) and lead
them to stationary weirs, traps or seines fencing off a bay. The
most satisfactory air-compressing unit consisted of a three-cylinder
single-stage compressor, 196 ft3/min at 80 psi, driven by a 52-hp
dieael engine. The required power input is about 25-30 hp. The
air compressor has a 30-gallon air receiver and a sea water-cooled
compressed air aftercooler. To lay out the air curtain, a polyethy-
lene pipe was used. Being flexible, it can be wound on to a reel
and minutely small holes can be easily drilled in the wall of the
pipe to provide air outlets; furthermore it is inexpensive, lightweight
and easy to handle. The disadvantages are that it is buoyant and
must be weighted to sink, melts at relatively low temperature so
that an aftercooler is required to remove heat of compression from
the air. The above compressor could supply air for 2,400 ft of air
curtain, emanating from 0* 016 inch holes in the pipe, or else 600 ft
by
Keith A. Smith
Fish and Wildlife Service,
Gloucester, Mass.
of a heavier curtain achieved with 0-0312 inch holes, drilled in the
plastic pipe at 1-ft intervals. A galvanised wire rope was attached
along the pipe to sink it. In commercial fishing trials the air
curtain was looped around the school of herring and the pipe was
then towed slowly inshore by boats on each end, thus driving the
school to within range of the stop-seine. At other times the pipe
continued from page 539
had an amplitude of 2 to 4 cm and at a working fre-
quency of between 1 and 2 c/s large displacements were
produced that might be detected by means of the lateral
line. Herring did appear to give sudden orientated
movements away from the door by day when they swam
into what would probably be the zone of greatest
influence in front of the door. These reactions occurred
at a distance of 3 m from the source. It was not possible
to observe such reactions at night.
Conclusions
Mechanical stimuli are important in fish behaviour
particularly when vision is restricted and an attempt
has been made to see to what extent these stimuli may
affect the behaviour of fish towards trawls.
In view of the limitations in the hearing ability of
most marine fish, it seems reasonable to attribute a
relatively minor role to the detection of sound waves as
a way of avoiding capture, although the success of
midwater trawling operations for herring and other
clupeoids may be influenced by the reactions of these fish
to the noise of the ship and the net in some circumstances.
Clupeoids probably have a better hearing sense than
most marine fish of commercial importance.
Damming phenomena, associated with the movements
of the net, and which may cause avoidance reactions in
fish could be important in determining their behaviour
to the approaching trawl and in bringing about escape
once the fish are in the net. Tactile stimuli may also be
important in causing escape attempts.
Clearly a great deal of research is needed before a full
appraisal of the relative importance of different stimuli
in the reactions of fish to trawls can be made. So far
very few suitable observations have been made in
darkness when mechanical stimuli may be expected to
affect the behaviour of the fish. Echo sounding in
connection with midwater trawls has given useful
information on the behaviour of pelagic species and
540
recently flash photography, by means of a camera
attached to the square of a bottom trawl, have shown
different patterns of orientation of fish in front of the
groundrope between daylight and darkness.5
Comparative fishing experiments, although difficult to
interpret can give valuable information if carefully
planned. Emphasis should be laid on experiments in
which the catching efficiency and selectivity of different
gears between daylight and darkness are compared.
Interpretation of the results of such experiments would
be easier if more basic knowledge about the mechanical
disturbances associated with trawls was available. Very
little is known, for example, about the damming pheno-
mena in front of otter boards or in front of the net.
One would also like to know more about the pressure
gradients and turbulent water flow within the net and
the effect these stimuli may have on the pattern of escape.
A much simpler approach is to observe the behaviour
of fish in relation to various mechanical stimuli in large
aquaria. This method can give valuable information
on reactions to stimuli likely to be produced by trawls.
From this, predictions can be made about the behaviour
of fish towards real trawls which will enable more
reliable interpretations to be formulated from direct
field observations on the gear in action and the results
of comparative fishing experiments.
References
1 Dijkgraaf, S., 1963: The Functioning and Significance of the
lateral-line organs. Biol. Rev. 38, 51-105.
2 Frisch, K. von and Dijkgraaf, S., 1935 : Konnen Fische die
Schallrichtung wahrnchmen? Z. vergl. Physiol. 22, 641-55.
8 Harris, G. G. and van Bergeijk, W. A., 1962 : Evidence that
the lateral-line organ responds to near-field displacements
of sound sources in water. J. acoust. Soc. Amer. 34,
1831-41.
4 Mohr, H., 1960: Zum Verhaltcn dcr Fische gcgenUbcr
Fanggerftten. Prot. Fischerei tech nick, Bd. 6, Heft 29, 296-
326.
5 Parrish, B. B., Blaxter, J. H. S. and Dickson, W., 1962 :
Photography offish behaviour in relation to trawls. I.C.E.S.
Comp. Fish. Comm. Paper No. 77.
was left idle on the bottom and the compressed air started at ebb
tide to stop the retreating herring schools and lead them to a herring
weir. The air curtain has also been tried on menhaden which in
clear water reacted much the same as the herring, but trials were
unsuccessful in turbid waters and in strong currents of 3 knots or
more. The air-bubole curtain has proved ineffective as a barrier
to sharks. The reaction of herring to the air curtain appears to be
primarily a response to visual stimulus. During the 1959 and 1960
seasons 12 units of air curtains gear were assembled for commercial
use but the 1961 Maine herring season almost completely failed
while during the 1962 season there was a glut and the air curtain
was not utilised by the industry, so the future use of this gear on
the Maine herring is still subject to question.
L'UtUisation du rideau de bulles d'air comrae aide dans la peche
Resumt
L'emploi du rideau de bulles d'air pour guider les banes de
harcngs au point de capture fut experiments la premiere fois par
1'U.S. Fish and Wildlife Service, Maine, Etats Unis, en automne
1957 et fut rapport^ dans la Commercial Fisheries Review de
1'U.S. Bureau of Commercial Fisheries. Le rideau fut utilise pour
interceptor le mouvement des jeunes harengs (Clupea harengus)
et les guider vers les barrages, trappes ou seines fermant la baie.
L*unit£ de compression d'air la plus efficace consistait en un
compresseur a stade unique de trois cylindres, de 5,54 m3/min a
une pression de 5,6 kg/cm2 actionne* par un moteur Diesel de 52 cv.
La puissance d'alimentation itait de 25 a 30 cv. Le compresseur
comportait un collecteur d'air de 113,5 litres et un refroidisseur a
1'eau de mer. Pour produire le rideau d'air on a utilise* un tube de
polyethylene, flexible, pouvant etre enroule sur un tambour et
dans lequel des trous minuscules ont et£ perces pour assurer
l'6vacuation de l'air. Ce tube est le*ger, facile d manipuler et par
cher; il a cependant deux inconv&iients: il flotte et on doit 1'alourdir
pour le faire couler; il fond a des temperatures rtlativement basses
de sorte qu'un refroidisseur d'air comprime" venant du compresseur
est ndcessaire. Le compresseur decrit plus haut produisait assez
pour 731 m de rideau d'air dmanant de trous de 0,3 mm ou bien
pour 183 m d'un rideau plus lourd, form6 par des trous de 0,79
mm perfores dans le tube & des intervalles de 0,305 m. Pour faire
couler le tube, un fil d'acier *alv anise* a M attache tout au long.
Dans les essais de peche commerciale le rideau d'air e"tait pormale-
ment mis autour d'un bane de harengs et le tube e"tait alors tire"
lentement vers la terre par un bateau de chaque cdt£ de maniere a
chasser le bane dans la portee de la seine. En d'autres occasions
le tube fut install^ sur le fond et remission d'air comprimd commen-
gait au leflux pour 6viter que les banes de harengs ne se retirent
et les guider vers le barrage. Le rideau de bulles d'air a 6te* essay6
aussi sur le menhaden qui, dans les eaux claires rdagit de la mdme
facon que les harengs mais les essais n'ont pas M satisfaisants
dans les eaux troubles et dans les courants de plus de 3 noeuds.
Le rideau de bulles d'air e"tait inefficace comme barriere pour les
requins. La reaction des harengs envers le rideau de bulles d'air
semble £tre essentiellement une response a une stimulation visuelle.
Pendant les saisons de 1959 et 1960, 12 unites de rideaux de bulles
d'air ont tit utilis&s commercialement mais en 1961 les harengs
gtaient presque inexistants tandis qu'en 1962 ils 6taient en surabon-
dance ce qui fait que le rideau de bulles d'air n'a pas tit utilisl par
Tindustrie. L'utilisation future de cet engin pour le hareng du Maine
est done toujours en question.
El empleo de cortinas de burbujas de aire como auxiliares de la pesca
Extracto
El empleo de continas de burbujas para guiar los cardumenes
de arcnques al lugar de su captura lo ensay6 por primera vez el
Servicio de Pesca y Caza de los E.U.A. en Maine en el otofio de
1957 y del experimento se ha dado cuenta en la Commercial
Fisheries Review de la Oficina de Pesca Industrial de ese pais,
La cortina se emple6 para interceptar los movimientos de arenques
jdvenes (Clupea harengus) y dirigirlos hacia estacadas, trampas o
redes de cerco que cierran una ensenada. El grupo mas satisfactorio
consiste en un compresor de 3 cilindros, 196 pie$3/min a 80 lb./
pulg.2 movido por un motor diesel de 52 hp. La fuerza motriz
necesaria es de 25 a 30 hp. El compresor tiene una botella de
aire de 30 gal y un enfriador de aire comprimidp refrigerado
por agua de mar. Para crear la cortina de burbujas se empleo
un tubo depolietileno, que por ser flexible puede arrollarse en un
carretel. La descarga de aire se hace por agujeros diminutos perfor-
ados en el tubo, el que es barato, ligero y facil de manipular. Los
incon venienties son que flota y hay que lastrarlo para que vaya al
fondo; se derrite a temperaturas relativamente bajas y se necesita un
enfriador para extraer el calor de la compresi6n del aire. £1 com-
presor citado puede suministnur aire para crear una cortina
de 2.400 pies de iongitud, con las burbiuas saliendo de agiyeros de
0,016 pulg o una mas espesa de 600 pies con agiycros de 0,0312
pulg hechos a intervalos de un pie. Para hundirlo el tubo ae
lastra con alambre galvanizado. En los ensayos de pesca industrial
con la cortina de aire se rodeo al cardumcn de arcnque y el tubo
se remolco lentamente hacia tierra por ombarcaciones, dirigieado
de esta manera a los peces a la red de parada. En otras ocasiones
el tubo se deja en el fondo y se inytcta aire comprimidp al comenzar
a bajar la marea para parar al arenque que se retira y dirigirlo
hacio la estacada. La cortina de burbujas tambien se ha ensa\ ado
con menhaden, que, cuando el agua esta clara, reacciona igual que
el arenque, pero no se nan obtenido resultados positives en aguas
turbias y en corrientes fuertes de 3 nudos o mas. La cortina de
burbitfas ha resultado ser inutil como ban-era al paso de los
tiburones. La reacci6n del arenque a las burbujas parece ser
principalmente un cstimulo visual. Durante las campafias de
1956 y 1960 se emplearon 12 grupos de cortinas de burbujas, pero
la campafta de arenque de 1961 en Maine fue un fracaso casi
completo en tanto que en 1962 hubo tal abundancia que no se
tuvo que recurrir a la cortina, de modo que el empleo futuro de este
sistema para la pesca del arenque del Maine es todavia objeto de
controversia.
THE use of air curtains for guiding herring schools
to points of capture was first tried in the territorial
waters of Maine, U.S.A., in the fall of 1957. Positive
results of these trials were reported to American fishermen
by a series of informal progress reports and finally in
Commercial Fisheries Review of the U.S. Bureau of
Commercial Fisheries.
In view of the wide interest in the method and its
potentialities (some realised and some still unrealised),
description of the original trials and a r£sum6 of develop-
ments to date is warranted.
Use of this gear grew out of a need to extend Maine
sardine fishing activities beyond the limits of depth and
distance from shore imposed by the conventional passive
methods of fishing with stop seines and weirs. The young
two- and three-year-old herring (Clupea harengus) that
are canned as Maine sardines are taken in stationary
weirs or traps and in "stop seines" which are used to
"stop off" a cove or section of beach after herring have
moved into it.
More active fishing was needed. The herring schools
frequently failed to migrate close inshore causing
intermittent supply with periodic shortages. Even during
periods of shortage, however, schools of small herring
were frequently found near weirs and seining sites. (Fig, 1.)
Since herring schools in inshore waters can be led
effectively by brush and stake weir leads, it was concluded
that a visual barrier of air bubbles might also be effective.
(It is necessary to catch and hold the herring in conven-
tional seine gear alive for a period of eight hours or
longer to clear their intestines of food before they are
transferred to the cannery.)
Fish-guiding trials
The basic units for setting-up an air curtain are (1)
a vessel for carrying the gear and personnel to the fishing
location, (2) an air-compressing unit, (3) air transport
and discharge lines, and (4) a reel or spooling device to
wind up and carry the air lines.
Two vessels were used at various times in the Bureau
experiments: the 35 ft Clupea and the 38 ft Blueback.
541
Both vessels were diesel powered. They were somewhat
smaller than typical sardine fishing boats which are
generally in the 38 to 50 ft class.
Fig. J.
View of a school of herring "stopped off"
An empty herring weir is at the right.
with a seine.
The most satisfactory, and most used, air-compressing
unit consisted of a three-cylinder single-stage compressor
driven by a 52 hp diesel engine. The compressor had
an air-volume rating of 196 ft3 of air per minute and a
pressure rating of 80 Ib per square inch. The required
power input for such a machine is approximately 25 to
30 hp. The air-compressing unit also included a 30
gallon air receiver and a sea water-cooled compressed
air aftercooler.
The air lines used for carrying and discharge of the
air was polyethylene flexible pipe. This material was
chosen because (1) it is flexible and can be wound up
on to a reel. (2) holes can be easily bored in the wall of
the pipe to provide air outlets of a particular size, (3)
it is inexpensive, and (4) it is lightweight and easy to
handle. Disadvantages of polyethylene pipe are (1)
it is buoyant in water and must be weighted to sink it
to the bottom, (2) it melts at relatively low temperature
(an aftercooler was required to remove the heat of
compression from the air before being piped), and (3)
very small holes (approximately 0-02 inch and smaller)
after being drilled in the pipe will apparently "flow"
closed in time.
Air-discharge holes from 0 -03 1 to 0 -01 35 inch diameter
were tried at various times. The larger air holes to
produced a heavier, more active air curtain which was
most successful. Naturally, more air is required to
supply them. Where one 196 ft8 per minute, maximum
free-air rated compressor could supply air for 2,400 ft
of air curtain emanating from l/64th inch (0-016 in)
holes in the plastic pipe, the same compressor could
supply only 600 ft of the heavier curtain achieved with
the larger 1/32 inch (0-0312 in) holes. (Fig 2.)
Holes were drilled in the plastic pipe at 1 ft spacings
and '/i< inch lead wire was wrapped on the pipe
542
(originally to make it sink). It was later found that a
galvanised-wire rope attached along the plastic pipe was
superior, while it made the entire assembly very strong.
IfiHf dtam.
pir foot of pip»
Fig. 2. Diagram of the air-bubble curtain as set from the boat.
Trials on Maine herring
Trials were first attempted in the autumn of 1957 on
herring impounded in a rectangular shaped seine "hold-
ing pocket". Partial success was indicated when all of
the herring were crowded into one-half the pocket
before passing through the air discharge. The gear was
next tried on schools of herring found migrating through
a group of islands in West Penobscot Bay near Rockland,
Maine. The first trial indicated that they could be
diverted from their migration route. Another showed
that, if an air curtain was set up directly in their path, the
schools would stop and then run along the curtain
parallel to it much as they do when confronted with a
seine. (Fig. 3.)
Herring, Sipt. 9
Fig. 3. Initial herring-guiding trials. Herring schools passing
between the islands stopped and moved around the air curtain rather
than passing through it.
The first commercial fishing trials were made the
following season in Casco Bay near Portland, Maine.
Herring schools were located moving between Great
Diamond Island and Peakes Island each evening. For
many days the schools had remained in the centre of
the channel beyond the reach of the stop-seine gear.
The air curtain was set up in several different positions,
on several occasions, as diagrammed in Fig. 4. Each
time that schools were moving down the channel, it was
successful in guiding them over the seining sites where
they were impounded.
Fig. 4. Diagram of 1958 air-curtain trials in Casco Bay, Maine.
In the seasons following these demonstrations, several
sardine fishermen installed air-curtain gear of their own.
Figs. 5 and 6 show two applications of one fishermen's
air-curtain gear. When herring schools were beyond
the reach of the stop-seine twine, an air curtain was used
to drive the herring school inshore where it could be
impounded. The air curtain was looped around the
school of fish (see Fig. 5). The plastic pipe was then
towed slowly inshore by boats on each end thus driving
the herring school to within range of the stop-seine gear.
The other diagram (Fig. 6) shows the same unit of
gear applied as a weir-lead in Cutler Bay, near Lubcc,
Maine. In this application, the polyethylene pipe was
laid on the bottom and left idle until herring schools,
moving in on the flood tide, had progressed beyond the
weir and the pipe. The air-compressing gear was then
started, setting up the air-bubble curtain. At ebb tide,
the retreating herring schools encountered the air curtain
and were guided along it to the herring wire.
Trials on menhaden
The air curtain has also been tried on menhaden schools,
both along the Maine and Delaware shoreline. The
Fig. 5. Method of driving herring schools to fishing site.
findings generally are: in clear water, and in the absence
of strong currents, the menhaden react approximately
the same as herring. Trial* made in the relatively clear
waters of Casco Bay, Maine, on large adult menhaden,
were effective in guiding and confining the schools.
Similar results were obtained in attempts made by a
private company in the State of Delaware. These latter
trials were successful in holding menhaden schools in
clear shallow waters along the shore but unsuccessful
in turbid waters inside of Delaware Bay.
Also, strong currents of 3 knots or more, disrupted the
curtain and rendered it ineffective.
H earn NO SCHOOL*
HCTRCAT TO OECPCH
WATCH ON CM TlOf
Fig. 6. An air-bubble curtain used as an ultra-long weir lead.
Air curtains ?. sharks
Reports of the Bureau's trials led some seaside resort
operators to believe that an air-bubble curtain might be
543
an effective barrier to sharks . Exhaustive tests however,
brought the report that "the bubble curtain is ineffective
as a barrier to tiger sharks".
Reaction of herring
The reaction of herring to the air curtain appears to
be primarily a response to visual stimulus. While some
sound and lower frequency piessure oscillations are
undoubtedly produced, which may augment the visual
stimulus, the similarity of the herring's reaction to that
produced by stationary silent objects in the water is quite
apparent. The fish follow the line of bubbles just as
they follow a line of brush and stakes, rope leads, or
seine netting. Although most of the trials were made at
the time of evening twilight and continued until after
darkness, the rising bubbles could always be seen from
the surface; at times the curtain appeared as a bright
white sheet due to illumination by disturbed phosphores-
cent organisms. On other occasions, the residual light
from the moon or stars was enough to make the rising
bubbles visible; they could, at least, always be seen at
the surface. The conclusion that herring react primarily
to the visual perception of the air-curtain barrier,
therefore, seems best warranted.
Current utilisation in Maine
After trials and demonstrations of the air-curtain
gear in 1958 and 1959, several units were assembled and
operated commercially during the 1959 and 1960
herring seasons. Approximately twelve units were
installed although some of them were quite small,
cheaply constructed and of questionable utility. It was
anticipated that use of the method might increase with
successive seasons (Fig. 7). However, the 1961 Maine
herring season was an almost complete failure. The usual
inshore runs of herring did not materialise and little
fishing activity was carried on. Under these circum-
stances, air-curtain units were not used to any appreciable
extent. During the 1962 season, the opposite situation
prevailed and herring were abundant; the schools were
caught in great quantities by conventional weirs and
seines and no need for additional herring (as could have
been made available by air-bubble gear) developed.
There was a glut of herring and the air curtain was not
utilised by the industry.
Fig. 7. An air-bubble curtain set in Boothbay Harbour, Maine.
After two years of disuse, the first because of failure
of the herring runs and the unavailability of fish in the
inshore areas, and the second because of an over-
abundance of fish which allowed the conventional gear
to catch more than an adequate supply of herring, the
future use of the air curtain in the Maine herring is
subject to question. However, when another season
arrives during which the demand for herring is good and
the supply is light, the air-curtain equipment will
probably be put into use again.
References
1 Blaxter, J. H. S., Holliday, F. G. T., and Parrish, B. B.
Some preliminary observations on the avoidance of obstacles by
herring. Proceedings Indo-Pacific Fisheries Council (III): 46-49.
1958.
2 Blaxter, J. H. S., and Parrish, B. B. Observations on the
avoidance of netting by herring in aquaria Int. Counc. Expl. Sea,
CM 1959 No. 18, 5 p. 1959.
3 Enami, Sumio. Studies on the bubble net I. Kagoshima
University, Faculty of Fisheries* Bull. Vol. 8. 1960.
4 Enami, Sumio. Studies of the bubble net II. Bulletin of the
Japanese Society of Scientific Fisheries, Vol. 26, No. 3, pp. 269-272.
1960.
5 Kobayashi, Kirchiro, Shugio Igarashi, Yaji Abika and Koje
Hagashi. Studies on the air screen in water. 1— Preliminary
observations of behaviour of a fish school in relation to an air
screen. 1959.
6 Mohr, H. On the behaviour of fishes in relation to fishing
gear. Protokalle Zeir Fischer istechnik, Bd. 6, Heft 29, pp. 296-329.
(English language translation prepared on behalf of: Fisheries
Laboratory, Lowestoft, Suffolk, England). 1960.
544
Utilisation of Fish Reactions to Electricity in
Sea Fishing
Abstract
Electrical fishing with continuous direct current is a well-
established technique for freshwater fishing in small brooks or
shallow lakes, particularly in Northern Europe. The application
of similar techniques in sea fishing would be uneconomic because
the much higher conductivity of sea water would require too large
and costly power plants. Only after it had been found that pulsed
direct current of 20 to 80 impulses per second of about 0- 15
millisecond half-value period has the same physiological effect on
fish (electrotaxis, electronarcosis), the utilisation of electricity
as an adjunct to sea fishing gear could be considered. The author
has developed two electrical techniques, one for facilitating men-
haden purse seining and one for improving the catching efficiency
of trawls. By fitting the anode of a pulsed direct current circuit
to the nozzle of the conventional fish pump hose, the fish are
strongly attracted to the nozzle and the efficiency of pumping
the catch from the bunt of the purse seine on board the
carrier vessel is highly improved. This does away with the
need of drying up the net and lifting the catch to the same extent
as was necessary before, which means a considerable saving of
labour and time. Under the attracting influence of the electric
field, the fish do not dive down so strongly, Consequently the risk
of capsizing the dories and of damage to the net, with loss of fish
and gear, when handling large catches is very much reduced. This,
together with other rationalisation measures, has made it possible
to reduce the crews from 22 to 10 men per boat, and at the same
time the fishing power has increased. Since 1958/59 all of the
Smith Company menhaden purse seiners have been equipped with
this electrical device which requires only the comparatively low
peak current of 2,000 to 3,000 amp. Conventional trawls catch
only a part (perhaps about 10 to 60 per cent) of the fish in the path
of the gear, because the fish avoid the net or escape from it. By
having an electrical field of pulsed direct current close in front of
the trawl net mouth, all fish in this area can be attracted, stunned
and then collected in the net. To this end high voltage pulsed
direct current produced on board the trawler is conducted to step-
down transformers at the trawlnet through conducters inside the
special trawl warps. The stepped-down current is led to four pairs
of electrodes distributed around the net mouth between which a
practically homogeneous electrical field is built up. By under-
water television it was confirmed that this arrangement works
according to expectation. By comparative fishing tests the rate of
improvement of the catching efficiency was determined to be from
100 to 500 per cent depending on fish species and fishing conditions.
Pulsed direct current from 15,000 to 40,000 amp peak current
and an impulse half-value period of 1-2 to 1-5 millisec is used
according to size and species of fish to be caught. Apart from
normal bottom trawling, this technique might also be promising
for trawling close over rough bottom (attraction of fish out of
shelters) and particularly for midwater trawling.
LOJtilisation des reactions du poisson a L'filectricitt dans la
ptche Commerciale
Resum*
La pgche & 1'electricite— A courant continu— est bien connue,
particulierement dans le Nord de TEurope, pour la pSche en eau
douce, dans les ruisseaux et les lacs peu profonds, mais ces techni-
ques ne pouvaient fctre appliqu£es & la peche maritime £ cause de la
grande conductibilit* de 1'eau de mer qui necessitait des installations
eJectriques trop importantes et trop oneYeuses. Ce n'est qu'apres
avoir constat6 quo le courant continu puls6, de 20 a 80 pulsations
par seconde et d'une durie, par dcmiperiode d'environ 0,15
milliseconde, avait le mdme effet psychologique sur le poisson
(61ectrotaxis et electronarcosis) que 1'utilisation de r£lectricit6 en
mer put toe envisages. L'auteur a d£velopp£ deux techniques:
la pompe a poissons augmente Pefficacitd de celle-ci. Par ailleurs,
attires dans le champ tiectrique les poissons ne plongent plus avec
autant de vigueur et, de ce fait, on 4vite les risques de chavirement,
de d6gats au filet avcc perte de poisson et de 1'engin— surtout
lorauron manipule de grosses captures— et puisqu'il n'est plus
by
Conradin O. Kreutzer
Smith Research and Development
Co., Inc.
ndcessaire de lever le filet plein aussi haut qu'auparavant, on
realise en mdme temps une Economic de travail. Cette technique
et d'autres mesures de rationalisation ont permis de r6duire les
6quipages de 22 & 10 homines et d'augmenter les possibility de
plche. Depuis 1958/59, eet 6quipement 61ectrique qui n'utiliae
qu'un courant maximum de 2.000 & 3.000 amperes a 6t6 install*
sur tous les seineurs de la Cie Smith. Les chaluts traditionnels ne
prennent normalement qu'une partie (pcut-dtre 10 a 60%) des
poissons se trouvant dans le champ de 1'engin parce que ceux-ci
peuvent s'dvader du filet. Par contre, un courant 61ectrique
continu puls6, situ£ pr&s de la bouche du chalet, attire et dtourdit
tous les poissons se trouvant & proximitt et les rassemble dans le
filet. Dans ce but, un haut voltage produit a bord du bateau est
conduit au transformateur -r£ducteur du chalut par I'interm6diaire
de fils conducteurs situ6s & I'interieur de fumes sp6ciales. Quatre
pairs d'61ectrodes reparties autour de Touverture du chalut assurent
un champ tlectrique homogene sur toute la bouche du chalut.
Des observations sous-marines tel£vis6es ont confirm* que cet
6quipement donnait les r&ultats preyus. Des essais de pdche
comparative ont montrd que I'amtlioration de I'efficacit* de
capture 6tait de 100 a 500% selon les esptees de poisson
et les conditions de p&che. Suivant la taille et I'espdce de poisson
& capturer, on utilise un courant continu puls£ d'une intensity
maximum de 15.000 & 40.000 amperes et d'une dur6e, par demi-
periode, de 1,2 a 1,5 milliseconde. Ind6pendamment du chalutage
de fond normal, cette technique peut aussi gtre d6velppp6e pour le
chalutage sur des fonds accidents (attraction des poissons hors de
leurs refuges) et particulierement pour le chalutage entre deux eaux.
Reaccion de los peces a la electricidad y su aprovechamiento en la
pesca maritima industrial
Extracto
El empleo de corrientes el6ctricas continuas es una t6cnica bien
establecida para pescar en rios pequefios y lagos someros, part-
icularmente en Europa septentrional. Su aplicacidn en el mar
seria antiecon6mica por que la conductividad mucho mayor del
agua salada exigirfa instalaciones e!6ctricas mas potentes y costosas.
Hasta que se descubri6 que la corriente continua de 20 a 80 impulses
por segundo con semiperiodos de 0,15 milisegs. tiene el mismo
efecto fisio!6gico en los peces (electrotaxia y electronarcosis) no
se pudo pensar en aprovechar la electricidad comp auxiliar del
equipo de pesca marino. £1 autor ha ideado dos t&nicas el^ctricas,
una para facilitar la pesca al cerco de menhaden y otra para mejorar
el rendimiento de los artes de arrastre. Instalando el anodo de una
corriente continua pulsante en el extreme de la manguera de una
bomba para peces normal, se ejerce una fuerte atraccidn y se
mejora considerablemente el bombeo de la captura desde el saco
del arte de cerco al barco transportador. De este manera se reduce
considerablemente la necesidad de secar la red y elevar la captura,
con el consiguiente ahorro de mano de obra y tiempo. Bajo el efecto
atrayente del campo eldctrico los peces no nadan con tanto vigor
por lo que el peligro de que vuelquen los botes o de que se averien
los artes con la p6rdida consigtente de peces y material cuando se
manipulan capturas grandes dismmtryc mucho. Esto, junto con
otras medidas de racionalizacibn, ha hecho factibte reducir las
tripulaciones de 22 a 10 hombres por embarcaci6n a la vez que ha
aumentado la capacidad de pesca. Desde 1958 todas las embarcac-
iones de Smith Co. que se dedican a la pesca del menhaden con
545
K1
redes dc oerco ban sido dotadas de cstc dispositive eldctrico que
s61o neoesita una corricntc maxima de 2.000 a 3.000 amp. Los
artes de arrastre normales s61o capturan una partc (quiz* del 10
al 60 por ciento) de los peccs que encuentran en su camino, porque,
fetos los eluden o escapan de ellos. Dtspontendo un campo c!6ctrico
de corricnte continua de impulses dclantc de la boca del arte, se
pueden atraer todos los peccs que haya en sus proximidades, los
que quedan aturdidos y entran en la red. Para cllo, la corricntc
continua de impulses de mucho voltaje producida en el arrastrero
pasa a transformadores-reductores en el arte por conductores
montados dentro de cables de arrastre especiales. La corricnte
reducida va a 4 pares de electrodes distribuidos alrededor de la
boca del arte, entre los que se crea un campo clectrico prdcticamente
homogeneo. Mediante el empleo de la tclevisi6n subacuatica se
confirm6 que esta disposition funciona comp se esperaba. Mediante
ensayos de pesca comparatives se determind que el rendimientp
de pesca mejoraba de 100 a 500% segun las especies y las condici-
ones. Se emplea corriente continua de impulses con un mdximo
de 15.000 a 40.000 amps, y un semiperiodo de 1, 2 a 1,5 milisegs.
segun la talla y especies de peccs que se van a pescar. Ademas
de la pesca al arrastre normal en el fondo, esta tecnica tambien
podria dar buenos rcsultados cuando se arrastra cerca de fondos
sucios (para obligar a los peccs a salir de los refugios) y, en part-
icular, para al arrastre entre dos aguas.
TJLECTRICAL fishing in freshwater, which was
JZj developed at the beginning of the century, has
become established in Central Europe.
If the negative pole (cathode) of a direct current
source of about 220 V is grounded or placed in a body
of water, and the positive pole (anode), connected to a
metal plate about 30 x 30 cm in size (catching electrode),
is dipped into the same water, body at some distance
from the cathode, the fish in the immediate vicinity will
swim towards the anode in a very short time. Shortly
before reaching the electrode, most fish stop swimming,
turn over and go into a state of narcosis. They can then
easily be taken from the water with a dip-net.
Physiologists have determined that the voltage over
the fish body, between head and tail, causes three
reactions, i.e. attraction (electrotaxis), stunning (electro-
narcosis) and electrocution, and that the requited voltage
differs according to the species.
Since in a given electrical field the voltage over large
fish is greater than over small fish, large fish of the same
species will be more affected; electrical fishing is therefore
selective.
The effective range of the electric field is, however,
rather small. As can be seen from the following formula,
the effective range increases, with the square of the
current and the power requirements even with the fourth
power.
G 2 n
R — effective range in metres
J — current supplied by the electrode into the water
L — length of fish
r — specific conductivity of water in ohm x metre
C — body voltage of the fish species
Note : In the above formula the factor 2 is for surface operation
and increases up to 4 when the electrode is submerged deeper.
Increase of effective range is therefore practically
limited, and commercial electrical fishing in freshwater
remains most effective in small or shallow waters like
brooks or certain lakes.
In view of the limitations of electrical fishing there
appeared to be little promise in applying the technique
546
to sea fishing as the high conductivity of sea water
presents additional serious difficulties. A current of
10 amp, which would be sufficient in freshwater, would
for the same efficient range have to be increased to
about 10,000 amp in sea water. This indicated that
continuous direct current in sea water would be un-
economical, but a solution to this problem was found
when it was discovered that fish have the same reactions to
pulsed direct current as they do to continuous direct
current, provided that the shape, duration and rate of
the impulses is suitable. Turning on the electric field
for only a short time and then turning it off for a relatively
longer period has an effect similar to that in motion
pictures, where the eye gets the impression that the
screen is continuously lighted. Tank experiments have
shown that from 20 to 80 impulses per sec are sufficient.
The half-value periods of these impulses need to be only
of the order of 0/15 milliseconds and for technical
reasons are produced in practice by condenser discharges.
The exact relation between the maximum voltage and the
half-value period required are given for instance by
Muralt2 (1945). For physical reasons electrical fishing
in sea water must be done with currents having impulses
of longer duration.
In sea fishing the simple method of electrode and
dip-net cannot be applied, but electrotaxis and electro-
narcosis can be utilised in certain cases as a valuable
auxiliary means for improving purse seining and trawling.
This paper describes a method of using electricity for
simplifying and improving the discharge of big quantities
of fish from purse seines, and a method of electrically
attracting and stunning fish as an adjunct to trawling.
Both methods have been developed over the past six
years by the Smith Research and Development Company
in Lewes, Delaware, U.S.A. The first method was
introduced five years ago and is operated with good
results on 80 purse seiners. The second method is
about three years old and has so far been tested on several
research vessels and is now in use on a commercial
trawler, while equipping of other trawlers is planned.
Menhaden purse seining
On the east coast of U.S.A., menhaden is fished by
purse seining with two dories which set and close the net
around the fish schools (Fig. 1). When the seine is
pursed, the net is hauled and the fish concentrated in the
bunt— about 30 fathoms deep and 30 fathoms in diameter.
The fish are then pumped into the carrier boat. Efficient
pumping is only possible if the fish are so concentrated or
"dried up" that the nozzlehead is continuously surrounded
by fish. They can only be pumped if crowded so densely
that they cannot escape the suction of the pump.
Good sets may result in the catch of 100,000 to 300,000
fish of 300 g each. Lifting such large catches by hand-
hauling is extremely difficult and drying up can be danger-
ous as the fish tend to dive. The pump hose and pump
intake have an internal diameter from 25 to 40 cm and
can pump from 7,000 to 18,000 fish per minute. It
occasionally happens that the weight of the fish over-
comes the buoyance of the floats or the breaking strength
of the nets so that the nets are torn and the catch lost.
Sometimes the sets are too large to lift and the nets
have to be cut and the fish released.
case the fish would sink out of suction range. The
current needed is adjusted according to the actual size
of the fish.
Fig. L Aerial view of the operation of a menhaden purse seine with
two dories. The inner ring is the float line, the outer ring the fish
which push the netting outwards in their attempt to escape.
Electricity has been introduced to strengthen this weak
link of menhaden purse seining. The nozzle of the pump
hose was fitted with the anode of an electrical power
source producing pulsed direct current. The cathode is
placed in the water but at some distance from the
nozzle. When the fish are concentrated in the bunt of
the seine and the electricity is switched on, they are
electrically attracted to the nozzle so that the pump
continuously works at full capacity. In this way the
bunt does not have to be lifted ("dried-up") against the
weight of the fish trying to go down, at the risk of
capsizing the dories, and the danger of damage to the
net is considerably decreased. The fish actually form a
dense spherical concentration of about 4 to 6 ft dia round
the anode at the nozzle and this concentration is under
such a strong attracting force that it moves up and down
with the nozzle when the carrier boat rolls in the sea.
Part of the fish may even be lifted out of the water
(Fig. 2). After this dense concentration has formed, the
peak electric field around the nozzle decreases as there
is a lower current flow due to the lower conductivity of
the fish compared with that of the sea water, and less
fish are attracted from a distance. When most of the
fish from the concentration are pumped off, the electric
field increases in strength again and more fish are
attracted so that a balance is established between fish
attracted and those pumped away, which gives continuous
and efficient pumping.
With this electrical technique, it is no longer necessary
to lift the bunt by hand or mechanically to concentrate
the fish for brailing. With large catches this brailing
may easily take more than an hour. The pump nozzle is
simply lowered and forms its own concentration, this
means a considerable saving of time.
For this practical application of electrotaxis a rather
low current of 2,000 to 3,000 amp is sufficient. The
electric field is made just sufficiently strong to attract
the fish but not high enough for electronarcosis, in which
Fig. 2. Menhaden being attracted to the electrified nozzle of the
pump hose. The nozzle is near the surface so that some of the fish
are actually being lifted above the surface by the fish from below.
Schools of commercial fish are often accompanied or
chased by large predatory fish which thus also get in
the purse seine. When using the conventional pumping
method without electricity, these large fish (mainly
shark) often block the hose on the pump, causing con-
siderable inconvenience and loss of time. Due to their
greater length, these fish take a much higher voltage when
they enter the electrical field around the anode at the
nozzle. They are consequently stunned and sink down
out of the suction range so that there is no blocking of
the pump.
This type of electrified pumping equipment was installed
during the years 1958 and 1959 on all menhaden purse
seiners operated by the Smith Company off the U.S.
east coast. It has been responsible, together with other
improvements, for a more rational use of labour on board
and has resulted in a reduction in crew members from
22 to 10, while at the same time the catching efficiency
per boat has increased due to the gain in fishing time and
the avoidance of catch losses.
Trawling experiments
Even with modern trawling techniques using echo
sounding, there is no guarantee that all fish detected or
available in the path of the trawl gear will really be caught.
This is because fish may be frightened by the trawl gear
547
to such an extent that they either avoid the gear com-
pletely or escape through large meshes in the front part
of the net. The estimates of fishery experts regarding
the percentage offish actually caught from that available,
varies widely between about 10 and 60 per cent. There
are reports of midwater trawling trials with underwater
television in which an estimated 30,000 baskets of fish
entered through the net mouth but only four baskets
were found in the codend.
In recent years underwater television has increasingly
been utilised for observing the reaction of fish towards
fishing gear. Similarly, as was done by other researchers
before, the author has also used underwater television
in connection with trawls (Fig. 3). To allow these
observations to be conducted without artificial light they
had to be made in about 30 m water depth. Unfortun-
ately on these grounds only few fish were available. The
conditions were therefore not the same as in commercial
trawling which is done in depths ranging from about 60
to 600 m. Nevertheless, the results give some indication
regarding the reactions of fish which may also be valid
for greater depths. It was found that the fish always
swain in the towing direction some metres ahead of the
net, adjusting their swimming speed to the towing speed.
They always seemed to keep a certain distance from the
net which may be in relation to their swimming capacity.
Only very rarely could fish be seen entering the net and
sometimes it was further observed that fish, which bad
entered the net, would leave again through the net mouth.
After some time of swimming in the towing direction in
front of the net, the fish normally move over to one side
and then let the net pass.
tmmw **•*»** A,U
Fig. 3. Schematic vitw of a conventional otter trawl equipped with
underwater television for the observation of the behaviour of fish
towards the net.
Commercial trawling depends therefore on fish
which are almost incidentally caught. Although catches
have been increased by introducing bigger and more
powerful trawlers with improved echo sounders, nets
and winches, this situation has not been changed basically.
These observations indicate that there is a possibility of
utilising the known effects of electricity on fish. Since
the fish seem to keep in a rather small zone in front of
548
the net, it was suggested that, even with an electric
field covering only about the height and the width of the
net opening, it should already be possible to increase the
catching efficiency very considerably. By anodes attached
to the net, the fish would first be attracted into the net
and then stunned to preclude further escape. Even if
they should recover in the bag of the net and try to swim
out through the net mouth, they would again meet the
"electric wall" which thus acts as a valve letting the fish
pass only in one direction, i.e. into the net.
The instrumentation for this mode of applying electri-
city as an adjunct to trawling was developed step by step
and throughly tested in practical fishing. Underwater
television was used for finding the most suitable position
for the electrodes and to observe the effect of electricity
on the fish. It could thus be confirmed that, with the
electricity switched on, no more fish could be seen
swimming ahead of the net. Instead, stunned fish could
be observed suspended upside-down in the water; after
having passed by the camera, they were collected in the
trawlnet.
In order to accurately determine the advantage of this
electrical technique, comparative fishing trials were
conducted using the same trawlnet alternatively with and
without current. Since in conventional bottom trawling
on one and the same ground, catches may also vary
widely, a great number of comparative tows have to be
made to secure statistical significance. These tests
which must also be made in such a way that external
conditions such as time of day, current, weather, etc.
are as uniform as possible for each pair of alternative
tows, should also be extended to commercial fishing
grounds and greater depths.
To achieve some statistical evaluation, 170 test tows
were made, alternately with electrified and non-electrified
trawls. The tows were deliberately short so catches
could be kept small enough for convenient counting, and
so that the shorter interval between electrified and
non-electrified tows would minimise variations in general
conditions.
Electro -Trawl Unit
Fig. 4. Block diagram of the electrical equipment used in the trials
to improve the catching efficiency of trawl gear.
Fig. 5. The electronic convenor which transforms the continuous
DC into pulsed DC of the desired characteristics.
Fig. 4 shows the arrangement of the equipment
schematically. A diesel-driven electric generator on
board, the trawler produces the electrical power which
in the electronic converter (Fig. 5) is converted into
impulses, i.e. pulsed direct current. By means of a small
control board (Fig. 6) the skipper is able to vary the
power output and the impulse rate per sec within a wide
range. By means of a transformer the impulse voltage
is stepped up to high voltage which is conducted through
a concentric conductor in the warp (Fig. 7) to the net
which may be at a distance of up to 2,000 m. By
underwater impulse transformers attached to the net
(Fig. 8) the voltage is again stepped down to the level of
low voltage and high current suitable for fishing purposes.
To obtain an electric field as homogeneous as possible
over the whole area of the net opening, four pairs of
electrodes were distributed around the net mouth.
Furthermore, the primary coils of the step-down trans-
formers were connected in series to have almost equal
current flowing between each of the four pairs of elec-
trodes. The peak current in water was varied
between 15,000 and 40,000 amp according to the
size of the fish to be caught. The half-value
time of the impulse was chosen between 1/2 and
1/5 milliscc. This is about ten times the value
found necessary for fish reaction in the tank tests
mentioned above. This longer impulse period is needed
because, due to the propagation conditions of electrical
current of such high frequency in sea water, the current
would otherwise concentrate around the cables connect-
ing the two electrodes with the step-down transformer
rather than build up a field in the water between the two
electrodes.
Fig. 6. The control board by which the output and the Impulse
repetition rate can be carried over a wide range according to the size
and species of the fish to be caught.
Part of the catching results obtained in a series of 82
comparative tows in 60 to 100 m depth off the east coast of
the U.S.A. are shown in Fig. 9. Out of the great number
of species caught, the figure shows the catch results for
one typical flatfish species, grey sole, and one typical
roundfish species, whiting Am. The increase in catches
when applying electricity was of a similar range for
the other fish species.
The results show that the increase in catch was larger
for the roundfish which, because of their better swimming
capacity, can escape the net more easily when there is
no electric field. The difference was not so pronounced
for flatfish which are poor swimmers. In the few cases
when larger quantities offish were available, the difference
Fig. 7. The special trawl warp with insulated conductor core for transferring the impulse current from the trawler to the trawlnet.
549
Fig. 8. The underwater step-down transformer (above) which is
attached to the groundropes of the trawl net (below).
between electrified and non-electrified tows was partic-
ularly pronounced.
On average the rate of improvement of the catch
efficiency by means of this electrical device was between
100 and 500 per cent, depending on fish species and
fishing conditions. The graphs in Fig. 10 show the amount
of catch by the number of fish taken but fail to show
another advantage of the electrified trawl, namely, its
selectivity. It was found that the catches made with the
electrified gear were always composed of larger fish
than the normal catches. This is, of course, due to the
fact that larger fish are more affected by a given electric
field strength than smaller fish. Apart from the voltage,
the selectivity of electric fishing can be improved by
choosing the optimal impulse output and impulse
repetition rate for the desired size and species of fish.
Much more experimentation will however be required
before these valuable possibilities offered by electrifica-
tion of trawls can be utilised to full advantage and
further.
Discussion
Since the electric trawl showed its greatest catching superior-
ity when large schools were encountered, one possible theory
is that most fish not only try to avoid the net mouth but also
try to avoid being crowded by their own or other species.
Thus, with this double reaction towards avoiding the net, the
conventional trawl would be at a disadvantage but electrifica-
tion would prevent the escape.
The catch increases for the electric nets do not show the
further advantage that the electric net always caught more of
the larger fish which have higher market value. The catch
of larger fish concurs with freshwater electrical fishing results
which indicate that larger fish are especially responsive to
electric attraction.
continued on page 551
Fig. 9. Graphical presentation of the comparative trawl catches, obtained without (hashed) and
with (black) electric current, of whiting and grey sole.
550
Problems of Electro-Fishing and Their Solution
Abstract
The paper starts with a brief outline of the physical and physio-
logical theory of electric fishing, then goes on to describe the
equipment installed in 120-ft seiner. The electrical plant is powered
by a 100-kw generator and the anode of the electrode system is
formed round the end of a hose connected to a fish pump. The
hose is sufficiently long to pump from 150 ft. Underwater lights
are also used. The correct choice of the various ranges of the
equipment is discussed. The visual effect of the pump hose, the
suction effect and the electric field all have ranges at which the fish
are scared away. The underwater lights and the electric field also
have ranges over which they are effective in attracting the fish
within the suction range of the pump. Lastly there is the electrical
stunning range which must be less than the suction range. The
paper is well illustrated with cchograms showing the effect of lower-
ing the equipment into fish traces. From one of these traces 12
tons of herring were pumped during trials on Newfoundland
grounds in December 1958. The damage to the fish coming
through the pump is given as less than 3 per cent.
Problemes de la pfehe electrique et leurs solutions
La communication contient une description generate des theories
physiques et physio logiques dc la peche 6lectrique et decrit 1'equipe-
ment ins tall 6 sur un senneur de 37 m. L 'ensemble electrique est
alimentg par un geneiateur de 100 kw et l'£lectrode de Panode est
placee autour de I'orifice du boyau de la pompe a poissons. Le
boyau est d'une longueur suffisante pour pomper d'une distance
de 45 m. Des lampes sous-marines sont aussi utilisees. L'etudc
traite du choix correct des diffeYentes portees de Fequipement et
il est signal^ que reflet visuel du boyau, l'effet de la succion et la
champ electrique ont tous des portees qui effraient les poissons.
Les lampes sous-marines, le champ electrique ont des portees
efficaces pour 1'at tract ion du poisspn dans le champ de succion
de la pompe. Enfin, le champ Electrique peut assommer le poisson
mais la portee de cat effet ne doit pas etre inferieure & celle de la
succion. La communication est abondamment illustree avec des
echogrammes signalant Pemplacement de requirement dans la
couche d'eau, en m&me temps que les traces de poissons. D'apres
un de ces echogrammes, 12 tonnes de harengs ont et6 pompees
pendant les essais de peche sur les fonds de Terre-Neuve, en
decembre 1958. Moins de 3% des poissons ont ete* endommages
en passant par la pompe.
Probletnas de la electropesca y sus solucioaes
Extracto
Comienza la comunicaci6n con un breve bosquejo de la tepria
fisica y fisio!6gica de la electropesca y pasa a describir el equipo
instalado en un barco para le pesca al cerco, de 120 pies de eslora.
La instalaci6n electrica cuenta con un generador de 100 kw. El
by
Jurgen Dethloff
INTELECTRON
International Electronics,
GMBH & Co., Hamburg
anodo del sistema de electrodes se forma alrededor del extreme
de la manguera de la bomba que eleva el pcscado. La longitud
de la manguera es de 150 pies. Se emplean luces submarinas.
Se discute la seleccibn exacta de los diversos alcances del material.
Los efectos visual y de succi6n de la manguera y del campo electrico
tienen limites traspasados los cuales los peces se asustan y huyen.
Las luces submarinas tienen tambidn un alcance dentro del cual
atraen a los peces al sector de succibn de la bomba. Finalmente,
hay un alcance de aturdimiento que tiene que ser inferior al de la
succi6n. La ponencia esta bien ilustrada con ecogramas que
muestran el efecto de arriar el material hasta que llega a los trazos
de los peces. De uno de estos trazos se elevaron 12 tons de
arenque durante los ensayos de diciembre de 1958 en los bancos
de Terranova. Sufre danos menos del 3% de los peces que
pasan por la bomba.
THIS is a brief outline of the physical and physiological
theory of electro-fishing:
When fish are influenced by direct current or pulsed
DC electric fields, three reactions can be observed at
certain values of field strength :
(a) The first reaction. This is caused by weak electric
fields. It is indicated by the fish jerking slightly and
moving out of the field.
(b) Electrotaxis. This occurs in strong electric fields
of direct current or of pulsed direct current. The fish
turns its head towards the anode (positive electrode) and
swims straight to it.
(c) Elect ronar costs. This occurs in very strong fields.
The fish becomes unable to move, either due to narcosis
or even to electrocution.
continued from page 550
Since the electrode pairs were evenly spaced over the net
mouth, a homogeneous field resulted in the area in which the
fish accumulate. The term "homogeneous" is not used in
its strict physical sense but indicates that the field varies only
within the limits of optimum voltage to affect certain sizes
and species.
There is no quantitative proof that sizes or species can be
selected by electrical fishing but sufficient staff was not
available in these experiments to record and analyse these
factors. This will be done in future. It was obvious that
larger fish were taken in greater quantity by electrification and,
to a certain degree, there seems reason to believe that selection
can be made for both size and species. Traditional methods
permit only one practical method of size selection, i.e.
through net mesh size. The electrical method influences fish
over a certain size, since with the right pulse rate per sec,
fish under a certain size are not as greatly influenced. The
fact that different species gather at different distances from the
net gives a further possibility of species selection through
adjustment to the extension of the electrical field.
The electrical method is not limited to the bottom-trawl
industry. In experiments over rocky grounds where normal
bottom trawls could not be used, electrodes operated off
the bottom, drew the fish from the rocks up into the area
where net operation is possible. Future research will
concentrate on midwater trawling.
References
1 Rtidenberg, R. Elektrische Schaltvorgange, Springer Verlag,
Berlin. 1953.
3 v. Muralt, A. Die Signaltlbermittlung im Nerven, Verlag
Birkhftuser, Basel. 1945.
551
For electro-fishing procedures in general, electrotaxis
is of most importance.
The value of the field strength at which these three
reactions occur is determined by the corresponding body
voltage. The voltage drop of the fish from mouth to
tail in an electric field is dependent on the following
factors :
(a) Strength of field in the surrounding water.
(b) Length of fish.
(c) Relation of specific resistance of the fish body to
specific resistance of the water surrounding the fish.
A large fish of a certain species picks up a higher
tension with its body than a smaller one. As soon as
this voltage drop is equivalent to a body voltage, it
causes the reaction corresponding to the body threshold
voltage in question. Each of the three reactions corres-
ponds to a different body voltage. Within one species
offish these body voltages are the same and independent
of the length of fish.
Since the field strength varies inversely as the square
of the distance, adult fish are attracted on account of
these correlations by the electrotaxis effect from distances
where immature fish are not yet subject to any reaction.
This selectivity provides a very necessary and most
efficient method of conserving immature fish. By
conventional fishing methods, this cannot always be
controlled satisfactorily.
The electric power required for a certain electrotaxis
range is dependent on the species offish to be caught and
the average length of the fish. In practice spherical
electrotaxis ranges with diameters of 6 m (20 ft) to
20 m (65 ft), dependent on species and length of fish,
are obtained with powers considerably below 50 kw.
Electrotaxis, fish heading towards the anode in DC
or pulsed DC field, is the main effect used to catch fish
electrically.
The problem of actually catching fish electrically is
how to combine this effect of electrotaxis with suitable
mechanical means to get the fish aboard. A series of
parameters have to be harmonised with each other in
order to avoid disturbances. The problem is explained
below using the most difficult example of pumping
electrically concentrated fish without nets.
A shoal of herring is to be pumped from depths be-
tween 5 and 40 fathoms by means of an electro-fishing
device producing DC pulses, a fish pump and a hose.
Combining the effects of electrotaxis and a fish pump
is a very difficult task. Other techniques, for example
catching tuna by means of an electrified stick-held
dip-net, would not raise so many problems.
For a fishing test a 100 kw 3-phasc diesel generator and
a main switchboard were installed in the engine room
(Fig* 1) whilst the fish pump was fixed on deck (Fig. 2a).
The pump hose was lead over the rail in a steel chute
(Fig. 2b). The pump hose was sufficient for pumping
from a depth of 150 ft with an allowance of 50 ft for
pump discharge to reach the hold or other carriers.
552
Fig. 1. Pulse generator with instrument and control panel on top
installed in engine room. Some covers removed.
Note: On the right below — ignitrons in gimbles measuring
cords andoscillope (right).
The hose was in 18 ft lengths, easily coupled together
by means of three hinged bolts. The anode (positive
electrode) was attached to the end of the pump hose
(Fig. 3). The cathode (negative electrode) can be seen
in the centre of Fig. 2b (triangular pipe construction).
Fig. 2a. Fish pump on deck — hose to fish hold going from centre
of the photograph to the right.
Fig. 2b. Fish pump (right) and pump hose ready to be lowered for
netless pumping offish from the open sea.
IN AREA r 4 • r 5
"FIRST REACTION*
NO ABSOLUTE MEASURES OF RADIUSES
NOR RELATIVE DISTANCES FROM RADIUS TO
RADIUS MAY^E TAKEN FROM THIS DRAWING
SINCE IT STATE^EXAMPLES ONLY.
FIG 3 ANODE ( POSITIVE ELECTRODE) ATTACHED
TO PUMP HOSE END
THE ACTUAL RELATION
BETWEEN r S AND r 2
IS APPROX. I : 1
r8
\ ___
NO ABSOLUTE MEASURES OF RADIUSES
NOR RELATIVE DISTANCES FROM RADIUS
TO RADIUS MAY IMC TAKEN FROM THIS DRAWING
SINCE IT STAtCS EXAMPLES ONLY.
FIG.5
IN AREA r 4 - r B
fFIRST REACTION'
FIG.*
1 MAIN PULSE
2 AMPLITUDE OF NEGATIVE PEAK
3 ZERO PASSAGE AND INSTANT
FOR FIRING THE
COMPENSATING PULSE
For operation it is submerged close to the pump hose.
The pump hose is lowered or lifted according to the depth
of the fish shoals by means of a derrick.
Occurrences around the anode
A number of ranges around the anode influence the
fish as shown in:
Fig. 4 — r.l The pump hose end with the anode has a
visual chasing range.
r.2 The pump hose opening has a mechanical
suction chasing range.
r.3 Within the suction range fish are sucked up; the
radius of the suction range is approximately one-third
of the radius of the suction-chasing range.
r.4 and 5 The electric field around the anode has
in its "outer space" (outside the electrotaxis range) a
chasing area (hatched in Figs. 4 and 5) of the first
reaction (see Introduction), limited by range 4.
r.5 Then there is the electrotaxis range: within this
range all fish head towards the anode and swim straight
towards this electrode,
r.6 Inside the range of electrotaxis exists the range,
as it might be called, of "indifferent reaction94. The
reason is that the pulse cables plus sea water have a
certain inductance. This inductance affects the pulse,
which originally has DC character, causing it to pass
through zero and then deadbeat (Fig. 7). If the negative
amplitude after the first zero passage reaches or surpasses
the threshold voltage for electrotaxis (body voltage) the
anode at this instant becomes a cathode to the fish.
At the following pulse the electrode is again positive,
then passing through zero becomes negative again, and
so on. Thus the fish reacts indifferently, i.e. it stops
heading and swimming towards the anode.
At a given pulse voltage, the radius of this range of
553
"indifferent reaction" is influenced by the size of the sur-
face of the anode. Therefore, to reach a large range of
electrotaxis, the anode should have a maximum size.
On the other hand, the voltage gradient in the water
depends on the effective surface of the electrode; the
bigger the electrode, the less sharp is the voltage gradient
and vice versa. Therefore it happens that pulses with
zero passages having negative peak amplitudes large
enough to cause this "indifferent reaction" are outside
the suction range (see r.3).
T.I Another and most important radius is the range
of electronarcosis.
r.8 The phototaxis range also has influential effects
under certain conditions.
All the ranges but the suction range and the suction-
chasing range can be assumed to be almost spherical in
shape. The shape of the suction range and the suction-
chasing range is assumed to have the form of a pear
(Fig. 4a) in cross-section and circular in plan.
The problem is to harmonise all these ranges in the
following manner:
Fig. 5 — r.5 and 2 The electrotaxis range must be
larger than the suction-chasing range. The electrotaxis
range should enclose as many fish as can be sucked up
by the pump after concentration around the hose mouth,
but this is limited by the period during which the fish can
be kept in the stage of electrotaxis before they sink down
or rise up fatigued or stunned by muscular stress. In
principle, this depends on the electric power output, the
capacity of the pump and the density of the shoal. The
capacity of the pump is given within small tolerances.
The density of the shoal is taken from the echo recording
and according to this the pulse voltage is regulated.
r.9 The electronarcosis range is to be kept smaller
than the suction range. If the narcosis range is larger
than the suction range, some fish are not able to entei the
suction range but get narcotised before they enter the
suction range and accordingly they sink down or
rise up, thus being lost. If at a given electric power
output a great electrotaxis range is to be obtained, which
means using an anode with a large surface, the range of
electronarcosis is also relatively large and usually larger
than the suction range as already mentioned above.
Theoretically this could be prevented by decreasing the
surface of the anode but, by doing so, the electrotaxis
range is also decreased. Also, as already mentioned
above, the voltage gradient of a smaller anode is much
sharper and the voltage drop in the vicinity of the anode
is bigger.
Fig. 4 — r.6 Further, the range of "indifferent reaction"
must be prevented.
Independently from the question of ranges, there has
to be a good timing of the pulse duty cycle.
This problem can be reduced to two main items : to
prevent indifferent reaction and electronarcosis. The
solution is the so-called Pulse Compensation Method
(patented).
According to tht principle of the Pulse Compensation
Method (Fig. 6), a small compensating pulse is fired at
the very instant of zero passage of the main pulse. The
554
width and amplitude of this compensating pulse are
similar to that of the negative wave of the main pulse (see
also Figs. 7 and 8). Physiologically the fish are no
longer prevented from swimming farther towards the
anode. But there still remains the range of electro-
narcosis.
Fig. 7. (left) Oscillogram of pulse with zero passage and con-
siderable negative peak.
Fig. 8. (right) Oscillogram of pulse with overcompensated negative
peak by means of positive auxiliary pulse.
When the fish are just entering the electronarcosis
range the main pulse is automatically switched off, the
compensating pulse still being "on". Because the
amplitude of the compensating pulse is much smaller
than that of the main pulse, it cannot create narcosis.
But it is still big enough to cause electrotaxis, since the
fish are now already in the vicinity of the electrode (in
other words, the voltage drop close to the anode is small
enough).
Various experimental proceedings to optimise some of
the parameters are demonstrated in the following para-
graphs. Reference is also made to the echograms.
The visual chasing range of the pump hose end is
shown in Fig. 9.
Fig. 9. No pump or pulse operating. The track of the hose shows
clearly and the drop in the middle shows where it was lowered.
The suction-chasing range is shown in Fig. 10. It
can clearly be seen how suction-chasing starts when the
pump is switched on, how it stops when switched-off,
and that fish follow the pump hose end when being
lowered or raised.
Fig. 10. Echo-sounding diagram with pump on and off. The clear
patch was where the propeller disturbed the echo sounding.
This figure illustrates suction chasing.
In Fig. 1 1 phototaxis and some suction chasing can be
seen. Phototaxis is a good means to pre-concentrate
fish if shoals are not dense enough or, due to the in-
sufficient length of the hose, it is necessary to draw a
deep-lying shoal towards the surface and within the
vicinity of the hose mouth. In certain cases it can be
used as a "light magnet" to pull fish upwards. Phototaxis
is also the way to re-concentrate fish after the electrotaxis
area has been swept empty before another series of
pulses is "shot" according to the pulse duty cycle.
Electrotaxis and ekctronareosb
During this trial, the suction-chasing range was made
smaller by reducing the pump efficiency by two 10 per
cent steps. Electrotaxis was now stronger than the
chasing effect of pump suction. Herring were pumped
aboard (Fig. 14).
Electronarcosis range was decreased (Fig. 15). These
echograms demonstrate another way to overcome the
narcosis range. This time the possibility of switching
off the main pulse just before the bulk of fish enters the
narcosis range was ignored and a lower pulse voltage
and a smaller anode was used.
Fig. 11. Light 65 ft down* see track of the lamp: concentrations
above and below. Illustrates Phototaxis.
Electrotaxis and re-concentration by light (phototaxis)
are shown in Fig. 12. These echograms represent the
tremendous effect of electrotaxis and immediate re-con-
centration when light is switched on again after pump
and pulse have been stopped. As long as pulses are
"on" herring are pumped aboard.
Fig. 12. Ad. C. E. electrotaxis and re-concentration by light
(phototaxis).
The echogram in Fig. 13 shows perfect electrotaxis
and herring being pumped (8 tons).
Fig. 15. Pumping herring with lower pulse power and smaller
anode — still some narcosis.
The echogram in Fig. 16 records the state where
optimum combination of parameters has been achieved
(see also Fig. 5).
(a)
range
(b)
(c)
(d)
(e)
Electrotaxis range larger than suction-chasing
Phototaxis compensating visual chasing.
Electronarcosis range smaller than suction range.
Range of "indifferent reaction" eliminated.
Correct timing for switching off main pulse and
only running compensating pulse for itself before fish
get narcotised by fatigue.
(f) Right duty cycle for pulses: main pulse and/or
compensating pulse switched on and off with re-concen-
tration during the intermissions, e.g. by phototaxis. The
operation of the duty cycle: main pulse plus compensating
pulse "on", main pulse "off", compensating pulse
"off", intermission for re-concentration or drifting into
"new" part of shoal, main pulse plus compensating pulse
Fig 13 Ad C F. electrotaxis and electronarcosis. On the left electrotaxis enables 8 tons of herring to be pumped, when associated with some
electro-narcosis. The hose track is visible on the right.
Fig. 14. Electrotaxis Range larger than suction chasing range.
555
Fig. 16. Ad. D.C. optimum ranges and perfect pumping with almost no narcosis.
Fig. 17. Pumped fish — having passed the fish pump — arc led to
the hold. The Water of the fish/water mixture (approx. 40%
fish; 60% water) is separated by a provisional fish/ water screen.
Fig. 18. Pumped herring leaving the fish/ water screen into fish hold.
556
Fig. 19. First herring electrically concentrated and pumped from
sea without nets (rate of damaged fish less than 3%).
"on", and so forth, takes altogether less than three
seconds per cycle. This results in a continuous flow of
fish aboard.
Note: the echograms shown do not represent a
consecutive series of recordings of the step-by-step
trials. They are just taken from hundreds of examples
of this measuring and trial-and-error development work
(see Figs. 17, 18 and 19).
It is actually possible to optimise the various para-
meters so that they do not work against each other but
co-operate in the light manner. Further, it is likely
that after this most difficult task (pumping of electrically
concentrated fish) has been solved, other electro-fishing
problems can be solved more easily if mechanical means
other than a fish pump are applied to take the fish aboard.
However, it must be stressed that for every species of
fish, and also with regard to the intended electro-
mechanical gear combination, a series of tests has to be
carried out in order to establish the optimum electrical
and mechanical parameters.
Notes on the Importance of Biological Factors
in Fishing Operations
Abstract
The paper outlines the importance of the distributional and
behavioural properties of the exploited fish stock when designing
fishing gears and deciding upon fishing tactics. The horizontal
and vertical distribution, density of concentration, response to
stimuli, general school behaviour, etc., are important factors
which must be carefully considered in the construction and oper-
ation of fishing gears. The behaviour of fish is furthermore
closely associated with light intensity, water temperature, salinity
and currents. The authors next indicate the general range of
vertical distribution of different exploited species, and discuss the
behavioural responses to fishing gear of certain species emerging
from recent experiments.
by
B. B. Fairish
and
J. H. S. Blaxter
Marine Laboratory, Torry,
Aberdeen
Notes sur I'importance des facteurs biologiques dans la p&he
Ces notes traitent de 1'importance des caracteristiques de com-
pottcmcnt et de distribution du poisson dans le dessin des engins-
de peche et des tactiques & employer. La distribution horizontal
et verticale, la densite" de concentration, la r6ponse aux stimuli,
le comportement general des banes de poissons, etc, sont des
facteurs tres importants et doivent etre considered dans la con-
struction et reparation des engins de peche. Le comportement
des poissons depend de I'intensite de lumiere, de la temperature
de 1'eau, de la salinit6 ainsi que des courants. Ces notes nous
donnent ensuite des indications sur la portee de la distribution
verticale des differentes especes et traitent des reactions du poisson
et du comportement de certaines especes envers les engins de
pfcche, indications basees sur de recentes experiences.
Notes sobre la importancia de factores biologicos en las activldades
de pesca
Extracto
Subraya la ppnencia la importancia de conocer la distribuci6n
y comportamiento de poblaciones explotadas cuando se disefian
materiales y se decidcn los metodos de pesca. La distribuci6n
horizontal y vertical, densidad de las concentraciones, reacci6n a
los estimulos y comportamiento general de agrupacibn son factores
de importancia que se tienen que tener muy presentes en la con-
strucci6n y uso del material de pesca. £1 comportamiento de los
peces esta estrechamente relacionado con la intensidad de la luz
y la corriente, temperatura y salinidad del agua. Los autores
pasan a indicar el grado de distribucibn vertical de diversas especies
explotadas y examinan las respuestas a estimulos producidos en
ciertas especies por los artes de pesca, examinadas en experimentos
recientes.
IT is important for fishermen to know how fish behave
and in this paper the subject is dealt with under three
main headings as follows :
' (a) Horizontal distribution
(b) Vertical distribution
(c) Response of fish to gear.
Horizontal distribution
Horizontal distribution of an exploited fish stock is
usually uneven. The fish tend to form "concentrations"
(or aggregations) of varying size, density and permanence.
The extent of these differs between species and they often
change suddenly and unpredictably. Clearly such
behaviour is of great importance to the fisherman in his
choice of fish locality, tactics and gear.
Although the distribution of fish and their degree of
concentration is always changing, the types of concen-
tration formed by different species fall, in general, into
two main categories:
(a) "organised" (Polarised) shoals or schools, the
members of which are kept together by mutually attract-
ing forces. This is generally characteristic of the pelagic
species (e.g. sardines, herrings, mackerels, tunas and
anchovies). Shoals (schools) may vary widely in size,
density and permanence but in the shoal the members
general y react as a unit. The density of fish in a shoal
is usually high.
(b) "loose" (unpolarised) concentrations of individuals
(or groups of individuals) brought together and main-
tained in a reasonably well defined locality by a common
external or physiological stimulus, for example, accumula-
tion on a rich feeding ground or in a spawning area. In
general, density is less than in shoals, and they tend to
cover a wider area.
The distinction between these types of aggregations is
not sharp. However, the broad, general distinction
between them is important in that the choice of fishing
method for exploiting a particular species in an area is
often critically governed by these properties. Thus,
encircling gears (e.g. ring nets, purse seines, etc.) are
particularly appropriate for catching species which form
tight shoals, whereas towed or drifting gears (e.g. trawls,
driftnets, bottom and pelagic lines, etc.) are more suitable
for species forming looser, more diffuse concentrations.
It is often possible from a general knowledge of how the
species aggregate to make a first choice of fishing gear
in a new, unfished area.
Although the horizontal distribution and degree of
concentration of most species vary widely (both long and
short term) they tend to follow a regular, seasonal
pattern especially during spawning and/or feeding
seasons. Knowledge of these seasons is, therefore, of
major importance. Examples of their importance are
found in the herring and cod fisheries off the west coast
of Norway. Both of these fisheries are centred on dense
seasonal aggregations of fish during spawning. Other
fisheries are also based on these stocks in other seasons,
but in quite different localities.
557
Other regular features of horizontal distribution and
aggregation relate to the distribution of different sizes and
age-groups. In many species the young, unmarketable
members of the stock occupy, or concentrate in, different
localities than the older, marketable ones. For example
"nursery areas" of plaice are usually situated in shallow
water close to the coast, but the fish gradually move into
deeper water with age; similarly, the young stages of
most clupeoid species are mostly located in reasonably
well defined areas. Therefore, information on the
whereabouts of the "nursery areas" and seasonal main
aggregation centres of older, marketable fish can be
important to fishermen.
Superimposed on these "regular" patterns of behaviour
are local, short-term variations in distribution and
aggregation within any area, resulting from variations in
physical, chemical or biological factors. In the main
these variations are "irregular" and they cannot usually,
as yet, be predicted in advance. They contribute largely
to the uncertainty and fluctuating fortunes of fishing from
day to day.
Such variations may be caused by many different
factors which may differ from place to place, time to time
and between species, thus making their identification
difficult. Much useful information on these variations
can be obtained by detailed studies on:
(a) food and feeding habits (including the presence of
principal food items).
(b) predator species.
(c) depth of water; type and topography of sea bed.
(d) temperature, salinity and current systems.
(e) local weather conditions.
The investigations of cod in the Barents Sea6'3 and
of herring in the Norwegian Sea2 have shown close
associations between the distribution of local aggrega-
tions of these species and particular temperature con-
ditions. Such results show clearly the potential value
of temperature records as indicators of likely centres of
aggregation of cod and herring respectively.
Many examples of the use of environmental informa-
tion in fish location are to be found in the fisheries
literature, covering a large number of species in different
parts of the world, and they clearly illustrate its potential
value1*5*4. In the course of fishing, too, fishermen
acquire an extensive working knowledge of the local
"haunts" of the main commercial species and of the
signs of their presence. While such information is
gathered by fishermen and is used in their choice of
fishing grounds, more detailed information demands
painstaking long-term scientific studies of the physical,
chemical and biological properties of the environment in
relation to the distribution of fish. Such investigations
are being carried out by fishery scientists all over the
world, with a view to predicting both general and more
detailed aggregations of exploited fish stocks.
Vertical distribution
As well as having uneven horizontal distribution, fish
are usually also unevenly distributed in depth. Clearly,
558
this is also of major importance in day to day fishing.
It is also important to the gear technologist in assessing
possible improvements in gear design, rigging and
operation for different species. For example, the question
of the importance of headline height in bottom trawls
is critically governed by this factor.
Echo sounders have greatly increased knowledge of
vertical distribution and its variations. In pelagic
trawling for herring the depth at which the net is towed
is determined from echo-sounder observations — indeed
they are now considered to be indispensable for this
method. Although for most demersal species precise
information about vertical distribution is still limited,
because of the greater difficulty of detecting fish by echo
sounder on or near the sea bed, the introduction of
narrow beam, high frequency sounders and improved
display techniques have greatly improved the effectiveness
of echo sounding and led to better understanding of the
depth ranges of the main commercial species and the
ways in which these vary. Clearly variations in vertical
distribution may greatly affect the efficiency of particular
fishing gears. This is especially so for bottom and pelagic
trawls and driftnets whose catching surface covers a
very small part of the total water column. Of particular
importance are the diurnal changes in vertical distribu-
tions. For some species these are of sufficient magnitude
to seriously reduce the effectiveness of bottom fishing
gears (e.g. trawls) in darkness and of pelagic gears (e.g.
driftnet), in daylight.
The general features of vertical distribution of fish
are closely associated with their feeding habits. Thus
species (e.g. most flatfish) which feed mostly on bottom-
living organisms during their adult life are adapted to a
demersal life and spend most time on or close to the
sea bed. Similarly, most of the clupeoids, which are
dominantly plankton feeders spend a large part of their
life away from the sea bed. Others (e.g. cod) have a
more varied diet, and so have a more variable vertical
distribution. In addition to this general association
with feeding habits, the vertical distribution, especially of
pelagic species, is also associated with other environ-
mental factors, such as light, temperature and salinity.
Light — The activity and behaviour of many species
of fish vary markedly with the light intensity and the
diurnal variations in vertical distribution of species such
as herring, sardine and pilchard are closely associated
with the diurnal light cycle. Most fish seem to have an
optimum light intensity, which they will normally seek
in accordance with light penetration affected by climatic
factors (cloud cover, etc.) and the turbidity of the water.
It is well known that, in some areas, the level to which
herring rise at night is often deeper on bright moonlit
nights than on dark ones, thus markedly affecting the
efficiency of driftnet fishing.
Temperature and Salinity — The vertical temperature
structure may also influence vertical distribution, either
directly or indirectly, through its effect on the distribution
of food organisms. In the northern North Sea in
summer, the upward migration of herring at night often
appears to be restricted by the strength of the temperature
discontinuity layer, the thcrmocline. This may influence
their accessibility to driftnets or ring nets. In regions of
mixing of water masses of different physical characteristics
(e.g. oceanic and polar water) marked temperature and/or
salinity stratification is also often present, and this again
influences the vertical distribution of fish, which may
only be found within narrow depth limits, characterised
by particular temperatures and/or salinities. Other
factors which may also affect vertical distribution include
the oxygen content of the water, the distribution of
predators and climatic factors (weather, moon).
Behavioural responses to fishing gear
How fish behave to fishing gear also affects their
catchability and such knowledge is of great importance
to gear technologists in designing and operating different
types of gear. The recent application of underwater
photographic and television techniques, the use of echo
sounders, frogmen, underwater observation chambers
including a submarine with direct observation windows,
experimental work in tanks and the results of compara-
tive fishing experiments have provided some important
pointers to the principal factors involved. The results
of some of these studies can be summarised as follows:
(a) In conditions where vision is possible, fish in the
vicinity of towed gears (e.g. trawls, Danish seines) on
the sea bed respond by being herded inwards by the
sweeps and bridles and forward by the net, at about the
same vertical level. There is no clear evidence of fish
avoiding the gear by moving upwards, out of its sphere
of operation. However, in mid water avoidance of
pelagic trawls does sometimes appear to take place by
fish moving downwards. The distance from the gear at
which these responses take place differs between species
(it seems from tank observations that it is greatest in
the active shoaling species) and varies according to the
visibility of the gear. The effectiveness of the fishing
gear in defeating this behavioural response is therefore
a function of its visibility and the speed with which it is
towed.
(b) In darkness (light intensities less than about 0-5-
0-05 lux), these responses do not appear to take place,
and the orientation and movement away from the gear
are much less pronounced. Observations in tanks imply
that the generation of noise (both high and low fre-
quencies) and of pressure gradients by the towed gears
does not appear to produce marked directional behaviour
responses in darkness (Chapman, this Congress). Further,
the level of activity of fish is generally lower and their
"organisation" is less in darkness than in daylight, thus
reducing their ability to escape from towed gears.
(c) The entry of fish into traps or other stationary or
drifting gears (driftnets, trammel nets, etc.) often appears
to be critically governed by their visibility, which is a
function of their conspicuousness, the incident light
intensity and the clarity of the water. Similarly, the
taking of bait on lines is sometimes partly governed by its
visibility. These results point to vision as one of the
most important factors governing the responses offish to
fishing gear. Although it is well known that fish are
sensitive to sound stimuli over a wide range of frequencies
(they emit sounds themselves), the evidence available at
present suggests that sounds play a relatively small role
in governing the reactions offish to gear or to the fishing
unit as a whole. Thus, the very high frequencies pro-
duced by echo sounders do not seem to produce marked
flight or other responses in herring shoals or the concen-
trations of other species; similarly, there is no substantial
body of evidence to support the view that the engine
and propeller noises of ships cause major flight reactions
except when the fish are close to the ship, although it is
often claimed that the combined noises of large numbers
of fishing vessels cause fish in an area to "take off".
A possible feature of fish behaviour about which
little is yet known is the ability of fish to learn to avoid
fishing gear when they are continually exposed to it.
Laboratory experiments have shown that fish can be
taught to behave conditionally (e.g. to attempt to take
food in response to a stimulus other than food) itself so
that some element of "conditioning" or learning might
occur among exploited fish concentrations. More inform-
ation is undoubtedly needed on this.
Conclusions
Success of fishing operations is clearly governed by
many complex factors concerned with the distribution
and behaviour of the exploited fish stocks, and these
may change rapidly and sometimes unpredictably.
Although this uncertainty is perhaps most marked in
vertical distribution the main elements are now clear
enough to be useful. Pelagic feeding species such as the
herring may be distributed at almost all levels, according
to environmental conditions (especially the distribution
of food and light), while others such as hake and cod,
although mainly bottom living species may frequently
be found well above the sea bed. However, with the
exception of oceanic pelagic species, such as tuna, almost
all species are found frequently on the sea bed. Of these,
only the typical flatfish are normally found only within
a few feet of it; most others may at times be found near
to and even extending well above the bottom, and it is
these species for which the facility to adjust trawls either
to give more height or more spread would be invaluable.
In general, the most modern types of echometer will show
when either is desirable — as they will show when a
species is (and the trawl should, therefore, be) in mid-
water.
The principal stimulus governing the behaviour of
fish to many gears is light, and on incident light intensity,
the clarity of the water and the visible properties of the
gear (e.g. colour and thickness of twine; size of mesh,
etc.), depend the type and magnitude of the fish's
responses. (Blaxter, Parrish and Dickson, this Congress).
This conforms with the marked responses made by
many species of fish to artificial light stimuli and in many
fisheries lights are used for aggregating or guiding fish
before capture.
continued on page 560
559
Tuna Behaviour Research Programme at
Honolulu
Abstract
The behaviour research programme of the Biological Laboratory
of the U.S. Bureau of Commercial Fisheries at Honolulu is con-
ducting studies of the sensory abilities and the behaviour of tuna.
The methodologies of the comparative psychologist and ethologist
are being utilised to gain a basic understanding of the ways in which
a tuna responds to its environment. At present, detection of
underwater sound, feeding behaviour, social behaviour, vision, and
the changes in behaviour throughout the day are being investigated.
These studies are conducted at sea using underwater viewing ports
in the research vessel Charles H. Gilbert and in a floating raft,
Nenue /, and in eight shoreside tanks at Kewalo Basin, Honolulu.
de recherche, * Honolulu, sur le comportenMOt du
Dans le cadre du programme de recherche sur le comportemcnt
du poisson, le Laboratoirc biologiquc de 1'U.S. Bureau of Com-
mercial Fisheries & Honolulu, conduit actucllement des Etudes sur
les facult£s sensorielles et le comportemcnt des thons. Les
methodologies de psychologic et d'dthologie comparatives ont 6t£
utilisees pour arriver & une base de compr6hcnsion de la facon dont
rdagit le thon & son environnement. Jusqu'& ce jour, la vision, la
detection des sons sous-marins, 1'habitude alimentaire, le com-
portemcnt social et les changements de comportcment durant la
journcc ont 6t6 observes. Ces 6tudes ont 6t£ conduites en mer, &
travers des hublots sous-marins montds dans la coquc du bateau
de recherche Charles H. Gilbert, sur un radeau flottant, le Nenue /,
et dans hit bassins a Kewalo Basin, Honolulu.
Programa de investigaciones del comportamiento del atun en
Extracto
Como parte del programa de investigaciones del comportamiento
del atun, el Laboratorio dc Biologia de la Oficina de Pesca Industrial
de los E.U.A. en Honolulu, estudia la capacidad scnsorial con
relacion al comportamiento. Se emplea la metodologia de la
sicologia y etiologia comparativas para tener un conocimiento
basico de la manera en que el atun reacciona ante su ambiente.
Actualmente se investigan la vista, la determinaci6n de los sonidos
subacu&ticos, el comportamiento durante la alirnentacidn y en sus
relaciones con otros miembros del cardumen y los cambios del
comportamiento durante el dia. Estps estudios se realizan en la
mar empleando los portillos submarinos del buque investigador
Charles H. Gilbert, en una balsa flotante Nenue I y en ocho estanques
en la costa en Kewalo Basin, Honolulu.
by
John J. Magnuson
Bureau of Commercial Fisheries,
Hawaii
TUNA behaviour studies are one of eight major
research endeavours of the Biological Laboratory of
the U.S. Bureau of Commercial Fisheries at Honolulu.
The objective of the behaviour research is to gain a basic
understanding of the behaviour and the sensory abilities
of tuna. An understanding of tuna behaviour is expected
to be useful in the future design of fishing gear and
fishing methods and in locating tuna.
The research programme was originally conceived in
1957 as a study of stimuli which attracted or repelled
tuna to or from pole-and-line tuna fishing boats. Between
1 958 and the present the orientation of the research has
slowly changed toward a more general query with the
potential of more generalised application.
Both a sea phase and a shore phase were begun in 1958.
The first problem of each was to develop methods of
studying the fast swimming pelagic tunas. Progress in
these efforts was reported for the sea phase by Strasburg
and Yuen (I960)4 and for the shore phase by Nakamura
(1962)8. By 1960 the oceanographic research vessel
Charles H. Gilbert had been equipped with stern and
bow underwater viewing ports, and feeding behaviour of
tuna had been observed and photographed at sea. By
1961 the skipjack, Euthynnus pelamis, had successfully
been kept and studied in shoreside tanks at Kewalo
Basin, Honolulu, for almost six months. In 1962 a
12 ft by 12 ft raft with a viewing chamber beneath it was
continued from page 559
While a large body of information has been accumu-
lated there are still many unknowns concerning the
detailed behaviour responses of fish, both to natural
environmental, and to artificial influences. This is largely
due to the difficulties of observing fish in the sea directly.
The recent development of high resolution echo sounders
and techniques of underwater observation, and the growth
of experimental aquarium work have, however, provided
basic information. Work involving personal observa-
tion of fish in the sea, under varying environmental
conditions and throughout the diurnal light cycle, is
needed in order to confirm the pointers obtained from
aquarium studies and by applying less direct methods.
560 t
The need for appropriate underwater vehicles or cham-
bers, for conveying observers to depths up to 80-100
fathoms, cannot be too strongly stressed. The use in
the U.S.S.R. of a submarine with observation windows
for this purpose is especially interesting.7
Reft
1 Awerinzew, S. 1935. J. Cons. int. Explor. Mer, 10, 66-74.
2 Devoid, F. (In Press). Rapp. Cons. Explor. Mer, 154.
' Dietrich, G., Sahrhage, D. and Schubert, K. 1959. Modern
Fishing Gear of the World 453-61.
4 Fleming, R. H. and Laevastu, T. 1956. F.A.O. Fish Bull.,
9, 181-96.
9 Hela, I. and Laevastu, T. 1962. Fisheries Hydrography.
• Lee, A. J. 1952. Rapp. Cons. Explor. Mer, 131. 74-102.
' Zaitsev, V. R 1959. Nature, Lond., 187, 559.
designed and built by R. Gooding and was successfully
used as a passive observation platform to observe and
photograph dolphin fish, Coryphaena hippurus, and other
large predators at sea. By the end of 1962, a total of
eight tanks had been designed and erected for specific
investigations at Kewalo Basin. In 1963 these facilities
are being used by six biologists and aides engaged in
analysing the enigmas of tuna's behaviour.
Two types of study are being conducted, those of
sensory ability and those of behaviour. The methodolo-
gies of the comparative psychologist and the ethologist
are being applied to species whose ways have long been
observed by fishermen. Application of these methods
is increasing the rate at which understanding of tuna
behaviour increases.
Both visual and hearing abilities are being quantified
with captive tuna at Kewalo Basin. The tuna are taught
to associate an underwater sound or image with food or,
in some cases, an electric shock. After training is
completed, the tuna reacts to the visual or sound stimulus
in the absence of the food or electric shock. The
conditioned response indicates that the sound or image
was perceived by the tuna's sensory system. A careful
choice of stimuli make it possible to describe the ability
of the sensory system to perceive the environment in
standardised and generalised terms.
More specifically, the visual acuity of little tuna,
Euthynnus yaito, and skipjack is presently being measured.
Such data provide information on the ability of these
tuna to see an object of a certain size at various light
intensities and distances underwater. In the sound
studies the range of frequencies tuna can perceive and
the minimum intensity that can be perceived at each
frequency are being described.
Behaviour research both at sea and in shoreside tanks
is concentrated upon feeding behaviour, social behaviour,
and changes in behaviour throughout the day and night.
Most sea work is directed at skipjack and yellowfin,
Thunnus albacares, whereas most shore work is on skip-
jack, little tuna and bonito, Sarda chiliensis. The work
on bonito is in co-operation with J. Prescott at Marine-
land of the Pacific in California.
At sea the stern underwater ports on the Charles H.
Gilbert have provided an opportunity to observe feeding
behaviour of tuna. Experiments were conducted in the
waters of Hawaii and the Line Islands to determine the
influence of different species of bait and artificial feeding
stimuli on the feeding behaviour of skipjack (Yuen 1962)7.
The fishing techniques of the Hawaiian sampan fleet
were used: skipjack found by looking for bird flocks, were
attracted to the vessel by chumming live bait into the
sea, and were then caught on pole-and-line. Measure-
ments were made of changes in catch rate and in the
feeding rate (number of feeding attacks per fish per
minute), which resulted from changing from live to dead
bait, from one species of bait to another, or from turning
off and on a water spray over the surface of the water.
Catch rate increased when water sprays were turned on
and decreased when they were turned off, when any one
of six different species of chum was used. Both
catch rate and feeding rate increased when living
bait was chummed and decreased when dead bait was
chummed. Catch rate is higher with the live bait,
because movement is one of the stimuli inducing feeding
behaviour of skipjack, and because dead bait tends to
drift away behind the vessel and take the skipjack away
with it. Experiments using different species of bait are
still in progress.
While being fished by the pole-and-line techniques,
surface schools of skipjack characteristically dive or
sound, returning to the surface seconds or as long as
28 min later (Strasburg 1961)6. Zero to eight dives
were observed per school in Hawaiian waters and the
frequency of diving was found to be associated with the
occurrence of a lizard-fish, Synodus variegatus, and a
squirrelfish, Holocentrus lacteoguttatus, in the stomach
contents of the skipjack. It was hypothesised that these
two species were more attractive than the live bait used
by the fishermen and that the skipjack dived in pursuit
of them.
Tuna schools are usually composed of fish of the same
species and approximate size. Occasionally, pole-and-line
or purse seine tuna boats catch both skipjack and yellow-
fin in a single fishing operation. Observations from the
stern chamber of the Charles H. Gilbert have made it
possible to study the structure of the school or schools
from which such mixed catches are made (Yuen in
1963)8. When underwater movie sequences of such
schools were analysed, it was obvious that each species
tended to school with its own kind even though fish
of the other species were near. A higher frequency of
groups of a single species was observed than would
have been expected if the fish schooled indiscriminately
with the other species.
The floating observation raft is being used to discover
the ecological significance of floating logs or flotsam to
pelagic fishes. The reason that large numbers of dolphin
fish and occasionally yellowfin and skipjack are found in
the immediate vicinity of floating objects has been a
debated question among fishermen for years. This
study should provide some insight into the problem.
The three underwater viewing ports in the bow of the
Charles H. Gilbert have been used to observe tuna,
porpoise, flying fishes, and large plankton organisms.
Most small fish dart rapidly away from the moving ship,
but tuna and porpoise swim within view of the bow
windows for prolonged periods of time. The bow
viewing ports made it possible to observe in detail the
behaviour of porpoise when riding the bow wave (Yuen
1961).
Very loose schools of skipjack have been observed in
the central Pacific, with 7 to 1 8 yards between fish. These
schools, 3 to 6 ft beneath the surface, were only one fish
deep. During periods of observation when small
numbers of natural prey were sighted by the skipjack, a
vertical barred colour pattern appeared on the skipjack,
and two or three of the fish nearest the prey organisms
converged on them and fed. We have not yet succeeded
in observing tuna in a feeding frenzy on natural prey
concentrations, although we have attempted to do so by
561
heading the ship through areas marked by the surface
splashes of feeding fish. Although the tuna were sighted
by the underwater observer, they dived immediately and
were not visible for prolonged periods.
Observations from the bow chamber on large plankton
organisms in daylight and, by their luminescence, at night
were interesting but difficult to record in numerical terms.
It is hoped that the bow chamber can be used to estimate
the abundance of surface fishes, but perhaps its greatest
value will continue to be as a tool for making observa-
tions on marine mammals and fishes as the ship is
actually moving with and within surface schools.
Direct observations at sea can be made on tuna that
are either attracted near the observer by food stimuli,
floating objects, or some other attractant, or that can be
successfully followed at distances within 100 ft. This
limits the spectrum of behaviour which can be observed
at sea to feeding and schooling. In the shoreside tanks
other aspects of behaviour can be examined, and feeding
and schooling behaviour can be given closer scrutiny.
On shore the detailed patterns of tuna behaviour are
first described. Whenever possible the function of the
behaviour is determined. If the function is not obvious,
experiments are devised to define the utility of the be-
haviour to the tuna. The functions of most of the
observed patterns of behaviour have not yet been recog-
nised. At present certain mouth movements, believed
to be important in olfaction and food searching behaviour,
are being investigated. The possibility of an olfactory
function to the tuna's mouth movements was suggested
by an anatomical study of the olfactory capsule of
skipjack by Gooding (in 1 963)1 . An accessory sac of the
olfactory capsule is compressed when the mouth is
closing and inflated while the mouth is opening. This
pumping seems to suck water into the naris and over the
olfactory sense organ with each opening of the mouth.
Another interesting behaviour of tuna is the change
from the typical striped coloration to a vertically barred
coloration in skipjack (Nakamura 1962)8. This change
results when a food stimulus is presented to a skipjack
that has not eaten recently. A skipjack that has just
eaten its fill does not take on the vertical pattern. The
significance of this colour change is not yet known.
In addition to describing behaviour patterns and
attempting to determine their function, the changes in
behaviour, especially feeding motivation, occurring
throughout the day are also being studied. The time of
day when a tuna contacts prey and the amount of prey
contacted can be expected to greatly influence the res-
ponse of tuna to food stimuli during successive hours. For
example, if skipjack are fed to excess all day long, they eat
over half the food they will take during the day in the
first three hours of daylight (Nakamura 1962)8.
Presently, the variations in response to food are being
associated with the previous prey-contact history of a
tuna.
The present state of knowledge on tuna behaviour was
recently summarised by Magnuson (in 1963)2. The
inability to observe tuna behaviour directly at sea prior
to 19S8 greatly restricted the accumulation of knowledge.
Now the tuna, still evasive and difficult to work with, is
subject to the close scrutiny of the ethologist and com-
parative psychologist both in the field and in the labora-
tory. The understanding that these studies are providing
should give man an additional advantage over the tuna
in future exploitation.
References
1 Gooding, Reginald M . The olfactory organ of the skipjack,
Katsuwonus pelamis. Proceedings of the World Scientific Meeting
on the Biology of Tunas and Related Species, La Jolla, California,
2-14 My 1962 (Vol 3, pp. 1621-31. Experience paper No. 36).
2 Magnuson, John J. Tuna behaviour and physiology, a review.
Proceedings of the World Scientific Meeting on the Biology of Tunas
and Related Species, La Jolla, California, 2-14 July 1962 Vol 3 pp.
1057-66 (Methodological paper No. 5.)
3 Nakamura, Eugene L. Observations on the behaviour of
skipjack tuna, Euthynnus pelamis, in captivity. Copeia (3):
pp. 499-505. 1962.
4 Strasburg, Donald W. and Yuen Heeny S. H. Progress in
observing tuna underwater at sea. J. Cons. 26 (1): pp. 80-93. 1960.
3 Strasburg, Donald W. Diving behaviour of Hawaiian skipjack
tuna. J. Cons. 26 (2): pp. 223-9. 1961.
6 Yuen, Heeny S. H. Bow wave riding of dolphins. Science 134,
No. 3484. pp. 1011-1012. 1961.
7 Yuen, Heeny S. H. Experiments on the feeding behaviour of
skipjack at sea. In Pacific Tuna Biology Conference (Edited by
J. C. Marr). U.S. Fish and Wildlife Service, Spec. Sci. Rept.-
Fish No. 415. 1962.
8 Yuen, Heeny S. H. Schooling behaviour within aggregations
composed of yellowfin and skipjack tuna. Proceedings of the
World Scientific Meeting on the Biology of Tunas and Related
Species, La Jolla, California, 2-14 July 1962 vol 3 pp. 1419-29
(Experience paper No. 25.)
562
Shrimp Behaviour as Related to Gear Research
and Development
1. Burrowing Behaviour and Responses to Mechanical Stimulus
AMract
The data presented in this paper on the burrowing behaviour of
pink shrimp (Penaeus duorarum) are part of a current study on
general shrimp behaviour by the U.S. Bureau of Commercial
Fisheries, Gear Research Station, Panama City, Florida. Shrimp
were released on the bottom or placed in bottomless cages and
their activity was observed over 24 hr periods by divers or with
closed circuit television* All shrimp studied burrowed during
daylight hours and showed varying degrees of nocturnal activity.
Activity was observed between 19.00 hours and 04.39 hours with a
maximum occurring between 19.00 hours and 23.15 hours. A
correlation between burrowing behaviour and moon phase or light
level is indicated. Two general methods of burrowing were ob-
served that allowed all animals to penetrate the bottom to just below
the surface of the substrate. A water circulation system was noted
in burrowed shrimp, and they were observed to draw deeper into the
bottom under certain conditions of mechanical stimulation. Some
of the data and illustrations given in this paper are being published
elsewhere as observations on the burrowing behaviour of pink
shrimp, Penaeus duorarum Burkenroad, C. M. Fuss, 1963 (in
manuscript). Supplementary data and observations by Frederick
Wathne outline the reaction of shrimp to electrical gear devised
to facilitate catching.
Le comportement des crevettes envers les engins de p&che.
Rfeurn*
Les donnles presentees dans cette communication sui 1'havitude
qu'ont les crevettes roses (Penaeus duorarum) de se terrer, font
partie d'une £tude g£n£rale sur le comportement des crevettes
mcnde par 1'U.S. Bureau of Commercial Fisheries, Gear Research
Station, Panama City, Florida. Des crevettes ont eV mises en
liberty sur le fond ou plac&s dans des nasses sans fond pour
observer leur activity par periode de 24 heures en scaphandriers
et aussi ayec la television en circuit ferme. Les crevettes etudiees
manifestaient divers degrfs d'activite* nocturne et se terraient
surtout pendant le jour; on a observ6 que 1'activite durait de 18 h
& 4.*9 h avec maximum entre 19 et 23.15 h. Les observations
indiquent qu'il existe une relation entre 1'habitude de se terror
et les phases lunaires ou 1'intensite de la lumicre. Deux methodes
generalcs ont £t6 observees qui permettaient aux animaux de
penetrer le fond, au dessous du substrat. II a 6t6 observe que
les crevettes enfouies avaient un systeme de circulation d'eau,
et qu'elles se retiraient plus profondement dans le fond sous
certaines conditions de stimulation mfeanique. Quelques vines
des illustrations et informations de cette etude ont etc publiees
ailleurs, comme "Observations sur le compoitement des crevettes
roses" (Penaeus duorarum) Burkenroad, C. M. Fuss, 1963 (en
manuscrit). Des informations supplerncntaire des observations
par Frederick Wathne resument la reaction des crevettes a
I'equipment 61ectrique invent^ pour faciliter la prise.
Comportamiento del camarfo con relacion a las inrettigadones
para perfeccionar el material de pesca
Extracto
Los datos que conticne esta ponencia, relatives a los habitos
excavadores del camar6n rosado (Penaeus duorarum), son parte
de un estudio de actualidad sobro el comportamiento general de
estos crustaceos que se realiza en la Estaci6n de Investigaciones
de Material de Pesca de la Oficina de Pesca Industrial de los E.U.A.
en Panama City, Florida. El camardn se solt6 en el fondo de
cajas o se coloco en cajas sin fondo y su actividad se observ6
durante periodos de 24 horas por buzos o con televisidn de circuito
cerrado. Todo el camar6n estudiado se enterraba de dia y dts-
plegaba actividad mas o menos intensa de noche. Se observ6
actividad entre las 19.00 y las 04.39 horas con un maximo entre
las 19.00 y las 23.15 horas. Se indica la existencia de una relaci6n
entre la actividad excavadora y la fase de la luna o la intensidad
de la luz. Se observaron dos maneras generales de hacer galerias
by
Charles M . Fuss, Jr.
Gear Research Station,
Bureau of Commercial Fisheries,
Panama City, Fla, U.S.A.
que permitian a los animates penetrar en el fondo o quedarse
inmediatamente debajo de la superficie del substrate. Se observ6
que el camar6n enterrado mantenia un sistema de circulaci6n
de agua y que bajo ciertos estimulos medmicos seenterraba
hasta el fondo. Algunos de los datos e ilustraciones que se dan en
esta ponencia se van a publicar en otro lugar como observaciones
de los habitos excavadores del camar6n rosado, Penaeus duorarum
Burkenroad, C. M. Fuss, 1963 (en manuscrito). Adicional informa-
ci6n y observaciones de Frederick Wathne delinea brevemente
las reaccibnes de los camerones a equipaje electrico inventado
para facilitar la captura.
THE Gear Research Station, U.S. Bureau of Commer-
cial Fisheries, Panama City, Florida, recently
initiated a shrimp behaviour study as part of the gear
research project. Hitherto little effort has been devoted
to studies of their behaviour. As the effects of certain
aspects of behaviour on the efficiency of fishing gear are
profound, gear development is directly related to animal
behaviour.
Instead of, as in the past, relying on laboratory
observations the plan was to study the animals in their
natural habitat by using diving equipment, underwater
motion pictures and closed circuit television.
Methods
Pink shrimp (Penaeus duorarum) were obtained by night
trawling with the vessel George M. Bowers in St, Andrew
Bay, Florida. They were removed to an observation
area immediately adjacent to the Panama City, Florida,
marina, approximately 1 mile from the capture area.
Shrimp were separated into 10 mm total length size
groups and held in live tanks during transfer. While
under observation, they were placed in 3 ft square
bottomless metal frame cages covered with small mesh
trawl webbing. The cages were secured to the sea
bottom by pushing their legs into the sediment.
Observations were made at half-hour intervals during
24 hr periods by divers using self-contained underwater
breathing apparatus. Underwater flashlights were used
at night, with the illumination interval kept at a minimum.
Diver observation proved more efficient than closed
circuit television observations, as a diver can position
his face mask within a few inches of the animal and
563
Fig. 1. Stages of burrowing.
view from various angles. Five measured shrimp were
placed in the cage at approximately 13.00 hrs on the
first day and at the end of the period the cage was
removed and the depth of burrowing measured with a
millimetre ruler. The shrimp were gently probed until
they vacated their burrows and the depths of the de-
pressions were measured. Fresh shrimp were used in
each series.
For detailed observations of the mechanics of burrow-
ing, individual shrimp were released on the bottom and
their activity was recorded with a 16 mm Bolex H16-R
underwater motion picture camera. Vertical views of
the animals in the sediment were obtained by using
plexiglass containers measuring 12 in square and 1 in
wide. The containers were filled with about 6 in of
sediment and were either placed on the bottom or held
on the surface while individual shrimp were allowed to
burrow.
To determine responses to mechanical stimulation,
burrowed shrimp were probed with a diving knife or
disturbed by lengths of chain that were dragged over
them. These responses were also recorded by under-
water motion pictures.
Results
Shrimp released on the bottom during daylight usually
burrow immediately using one of two general methods.
In one approach the animal grasps the substrate with
the walking legs, thereby holding itself in position while
setting up a water current with the swimming legs. The
induced current, flowing beneath the body, scours an
initial furrow into which the animal settles. The shrimp
then ploughs ahead into the anterior end of the depres-
sion, using the walking legs to force itself farther into the
sediment. At the end of the ploughing phase of the
burrowing process, the shrimp settles vertically into
the substrate using its walking legs to force the bottom
material laterally and upward around its body. Imme-
diately after the shrimp burrows, there is a slight furrow
between two small mounds marking the anterior-
posterior plane of the shrimp body. Within a short time
564
all external signs of the burrowed shrimp have usually
disappeared and visual detection is very difficult.
The second observed method of burrowing differs
from the first by the elimination of the scouring phase,
with the animal ploughing immediately into the bottom
and then settling. (Fig. I)8.
The measurements given in Table I for the depth of
burrow indicate a correlation between shrimp size and
degree of penetration into the sediment. This correlation
apparently depends on body depth of the individual
animal since all shrimp were observed to burrow to
just below the surface of the substrate. In most cases
even the antennae and eyestalks were completely
concealed.
To determine the validity of depth of burrowing
measurements in the observation area, five 130-140 mm
shrimp were placed in a cage in the capture area and
their burrow measurements were recorded. Depth of
burrow for these individuals ranged from 38 to 45 mm.
No significant changes in burrow depth were observed
with differences in bottom material, except that a number
of various-sized shrimp, released on hard sand bottom
just offshore of St. Andrew Bay, experienced difficulty
in penetrating the bottom.
Diurnal cycles
The pink shrimp remained burrowed during daylight and
showed varying degrees of nocturnal activity. (Tables II
and III). To the nearest half hour, the mean time of
emergence for all animals studied was 19.00 hrs and
the mean time of burrowing was 04.30 hrs. Activity
periods ranged in length from 3'5 hrs to 10'5 hrs with
a mean of 9*3 hrs.
Fig. 2 shows the degree of activity in each series of
observations. Times of maximum activity ranged from
19.00 hrs to 23.15 hrs.
From the data shown a correlation between moon
phase or light level and activity can be derived. Activity
is generally more restricted during full moon than during
new moon.
Fig. 2. Activity periods of three size groups of pink shrimp.
Table HI— Time but ihrimp burrowed (to aeftrert hfttf boor)
110-120 mm— Aug. 2, 05.00, Aua. 4, 05.00. ^A ^
120-130 mm— June 23, 04.30, July 31, 05.00, August 28, 04.00
130-140 mm— June 19, 03.30, July 29, 05.00, August 29, 04.30,
Initial stimulation to the dorsal body surface of
burrowed pink shrimp caused the animal to withdraw
immediately into the bottom sediment. If the stimulation
was continued for a few seconds, or applied to the side
of the burrowed shrimp, an escape reaction in the form
of a vertical hop would usually take place.
A water circulation mechanism was observed. Two
small holes in the substrate were usually visible (one near
the tip of the rostrum, the other usually to the side of
the rostrum) but occasionally at other positions. Dye
injected into the water above the burrowed animal, to
determine which opening served as the inhalant syphon,
showed that the hole near the tip of the rostrum is
generally the inhalant opening, but the flow is reversible,
apparently for clearing the channel of debris.
It was observed that shrimp will occasionally use one
of the first walking legs to help form the lateral opening
of the circulation system. There also seems to be a
cavity below burrowed animals which serves some func-
tion in the system. Dye injected into the sediment just
below the abdominal segments would be ejected from
one of the system openings.
Table I— Depth of burrow measurements (millimetres)
110-120 mm
38
31
30
30
30
29
28
28
28
27
27
25
25
25
25
20
20
Mean 27-4
Length groups
120-1 30 mm
38
38
32
32
28
28
25
25
25
25
20
Mean 28-7
Table H-Time first shrimp emerged from bi
r(to
Date (1962)
15 June
16 June
17 June
18 June
19 June
22 June
28 July
30 July
1 August
2 August
2 August
3 August
27 August
28 August
Length groups
110-120 mm 120-130 mm
19.30
19.30 19.30
19.30
19.00
19.00
— 19.00
19.00
19.00
19.00
19.00
130-140 mm
50
50
50
45
45
45
45
45
38
38
38
35
33
32
32
30
25
Mean 39-7
half hour)
130-140 mm
19.30
19.30
19JO
19XX)
19.00
18JO
Discussion
The i/i 51/n approach to the study of shrimp behaviour was
chosen to eliminate, as far as possible, the undesirable effects
of habitat modification. A compromise was necessary,
however, to facilitate continuous observations. Tidal cur-
rents, and resulting poor visibility in the capture area,
prevented the work from being done on the shrimp grounds.
Consequently, an observation area with required conditions
was selected as close as possible to the capture area.
Underwater television was initially considered but for the
study of microfeatures in shallow water, diver observations
proved best in this investigation.
Strong tidal currents were not present but there were
currents of i knot in the capture area.6. The observation
area10 was located in a sand regime (greater than 80 per cent
sand) whereas the capture area was in a silt clay regime
(greater than 50 per cent clay, less than 50 per cent silt).
The variations noted must be considered in attempting to
estimate the burrowing behaviour of pink shrimp from the
data obtained. Another factor to be considered is that most
large concentrations of pink shrimp are found in calcareous
sand4 or sand-shell bottoms5 whereas most of these observa-
tions and measurement were obtained on sand-silt-clay
bottom.
The only previous description of burrowing methods1 for
Metapenaeus masterii generally agrees with the first method
described in this paper.
All the burrow measurements here made show that pink
shrimp penetrate the bottom so that their dorsal surfaces are
just below the surface of the substrate. Williams (1958)12
found that pink shrimp tend to burrow deepest in sand-shell
type bottoms. His laboratory experiments showed that
some animals often buried as deep as the bottom of the
container (2 in) and he suggests that bottom type may
influence burrowing depth.
565
The nocturnal activity of pink shrimp is generally accep-
ted7' tt»*'1 and the fishery is principally a night fishery.
Hie results of this study show that activity may be limited
even during hours of darkness and indicate that this limited
activity may be interrupted. As a generalisation, it appears
that adult P. duorarum is primarily a fossorial animal.
There are a number of records of pink shrimp being taken
in trawls during daylight hours. Eldred (1961)* reports
personal communications with commercial fishermen that
indicate daytime pink shrimp captured under muddy water
conditions and on cloudy overcast days. Hiidebrand ( 1 955)*
states that on one occasion daytime fishing produced more
pink shrimp than either the preceding or following nights.
Similar situations were found by the Fishing Gear Research
Unit in Mississippi Sound in spring 1962. Various reasons
have been proposed for the phenomenon, including ground
swell and low light intensity.
Burrowing as an effective means of trawl escapement has
been mentioned11 and results in this investigation substantiate
such statements and may explain spotty catches on the
shrimping grounds if burrowing periodicity is intermittent.
The use of tickler chains on conventional shrimping gear
apparently has little effect on burrowed individuals.
The water circulation system mentioned 12f *• 8 has been
previously noted. There is a possibility that pleopod activity
in a cavity below burrowed animals may assist the scaphog-
nathite as a gill pump in establishing the waterflow. The
cavity may also provide a reserve space for deeper withdrawal
into the burrow under certain conditions of stimulation.
References
1 Dall, W., Observations on the Biology of the Oreentail Prawn
Metapenaeus matter// (Haswcll) (Crustacea Decapoda: Penacidae).
Australian J. Mar. Freshw. Res. 9:111-134. 1958.
2 Eldred, B., R. M. Ingto, K. D. Woodburn, R. F. Mutton and
H. Jones. Biological Observations on the Commercial Shrimp,
Penatus duorarum Burkenroad, in Florida Waters. Fla. St. Bd.
Conserv. Prof. Ser. No. 3:1-139. 1961.
3 Egusa and T. Yamamoto. Studies on the Respiration of the
"Kununa" Prawn, Penaeus japonicus Bate— T. Burrowing Behaviour
with Special Reference to its Relation to Environmental Oxygen
Concentration. Bull. Jap. Soc. Sci. Fisheries. 27:22-26. i960.
4 Gunter, G. Principles of Shrimp Fishery Management, Proc.
Gulf and Carib. Fish. Inst. Eighth Annual Session 1955:99-106.
1956.
3 Hiidebrand, H. H. A study of the Fauna of the Pink Shrimp
(Penaeus duorarum Burkenroad) Grounds in the Gulf of Campeche.
Publ. Inst. Marine Sci. 4:169-232. 1955.
6 Ichiye, T. and M. L. Jones, On the Hydrography of the St.
Andrew Bay System, Florida Limmol. Oceanogr., 6: 302-31 1 . 1961 .
7 Iversen, E. S. and C. P. Idyll. The Tortugas Shrimp Fishery:
The Fishing Fleet and its Method of Operation. Fla. St. Bd.
Conserv. Tech. Ser. No. 29:1-35. 1959.
I Idyll, C. P. The Commercial Shrimp Industry of Florida.
Fla. St. Bd. Conserv. Educ. Ser. No. 6:1-33. Reissued, 1957. 1950.
9 Viosca, P. The Louisiana Shrimp Story. Seventh Bienn.
Rep. La. Wildl. Fish. Comm., pp. 102-105. 1957.
10 Waller, R. A. Ostracods of the St. Andrew Bay System.
Unpublished. 1961.
II Williams, A. B. A Contribution to the Life Histories of Com-
mercial Shrimps (Penaeidae) in North Carolina. Bull. Mar. Sci.
Gulf and Carib., 5:116-146. 1955.
12 Williams. Substrates as a Factor in Shrimp Distribution.
Limnol. Oceangr. 3:283-290. 1958.
2. Shrimp Reaction to Electrical Stimulus
IN association with this research into shrimp behav-
iour, investigations were made to determine the
possibility of using electrical aids in harvesting shrimp.
The preliminary experiments were designed to provide
basic information for use in more precise and detailed
future tests directed toward the development of electrical
shrimp trawl gear. They are restricted to electrically
stimulated responses of animals burrowed in the sediment.
No attempt was made to obtain either galvanotaxis
or galvanonarcosis.
The information sought involves two general categories
—first the response of shrimp to electrical stimulus, and
second, factors associated with an electrical field between
electrodes in sea water. In the first category, information
required included: (a) Power requirements for positive
responses (b) effect of animal-electrode orientation, (c)
effect of pulse rate on response.
In the second category, necessary information included
the shape, magnitude and characteristics of the electrical
field. The data presented here cannot be taken as
indicative of absolute threshold values in securing
"gear" response but they are considered adequate for
determining electrical power requirements for developing
shrimp harvesting gear.
Water temperature and salinity in the holding area
were approximately equal to that in the capture area.
The temperature ranged from 27 °C to 31°C and the
salinity from 33 to 35 parts per thousand
An experiment as to the effect of alternating and direct
current in producing responses indicated that AC was
566
by
Frederick Wathne
Gear Research Station,
Bureau of Commercial Fisheries,
Panama City, Fla, U.S.A.
approximately 75 per cent more effective than DC in
water of 33-35 parts per thousand salinity. For this
reason all experiments of the preliminary work were
conducted with 60-cycle AC current, except in one
where AC currents of varying frequencies were compared.
The term "pulses" as used here refers to short periods
of AC current.
Electrodes used were brass rods, J-inch in diameter
and 24 in long. They were mounted in plastic separa-
tors 24 in apart to form a square frame, two sides of
which were formed by the electrodes and two sides by
the separators (Fig. 3). The electrodes rested on the
bottom.
Electrical pulses were delivered to the electrodes
through a mechanical circuit breaker (Fig. 4). Pulse
rate and input voltage were variable. Voltage drop
recorded was that between the electrodes, i.e. a vacuum
tube voltmeter was connected directly to the electrodes.
Distance between electrodes and power supply was
approximately 30 ft. In varying the power used, the
input voltage was selected as the parameter and the
Ffc. 3. Electrode frame used in shrimp response and field strength
experiments.
current as the dependent variable. This was necessary
because salinity and temperature fluctuated through the
day, owing to tidal action.
Individual shrimp from the desired size group were
removed from the holding tank and allowed to burrow.
A diver placed the electrode frame around the burrowed
animal in a pre-determined position and signalled the
surface operator who activated the pulser. The diver
observed the response and reported it to the surface
where it was recorded. A positive response was defined
as any visible reaction of the animal either weak or
strong.
Fig. 4. Mechanical circuit breaker used in preliminary shrimp
behaviour experiments.
Electrode orientation
From the first it was apparent that with relatively
low power levels, orientation of the shrimp to the elec-
trical field is a very important factor in producing a
positive response. The animals responded much more
readily if the body axis was perpendicular to the electrodes
than if it was parallel to them.
Fig. 5 depicts the method of describing orientation.
Table IV is a tabulation of typical responses observed at
various shrimp positions in 3 V, 6 V, and 10 V fields.
The results indicate that response is directly related to
the magnitude of voltage drop across the animal's body.
24"
12"
0°
24"
POWER SUPPLY CLECTMOOt
Fig. 5. Method used in identifying shrimp — electrode orientation.
Power requirements
In tests for power requirements the voltage drop
across the shrimp body was measured with separate
probes. If the animal did not respond the circuit was
closed and the voltage drop was measured between head
and tail. If the animal responded and consequently
vacated its burrow, the circuit was closed and the voltage
drop was measured across the empty burrow. In these
experiments, in addition to varying the position of the
shrimp, its proximity to the electrodes was also varied.
Table V summarises results obtained. An approximate
threshold level appears to be near 40 millivolts for shrimp
in the 120-130 mm size group.
The input power at the electrodes necessary to achieve
a voltage drop of 40 millivolts across a distance of 4-5 in
in the field is dependent on the orientation of the
measurement to the electrodes. Experience to date indi-
cates that 40 millivolts at the electrodes (24 in long and
spaced 24 in) will achieve the desired field strength.
These experiments will be continued to determine the
optimum power level-electrode characteristics for maxi-
mum effectiveness.
Effect of pube rate
Knowledge of maximum effective pulse rate is necessary
because the proposed application is with mobile gear.
567
After tests with pulse rates varying from 1 to 12 per
sec, results indicated that a pulse rate of 4 per sec for
shrimp in the 120-130 mm size group is near maximum.
Faster pulsing produces more rapid, but weaker, con-
tractions. This is apparently because the animal does not
have time to relax completely between pulses, which, if
true, indicates that a somewhat faster rate should be
effective for smaller shrimp and a somewhat slower rate
necessary for larger ones. The size-maximum pulse
rate relationship will be examined in future experiments.
General observations
The results reported suggest that shrimp response
was always either positive or negative. As might be
expected, this is not the case. Animals under identical
experimental conditions occasionally displayed different
reactions. The most common and significant variation
was that of magnitude of response. Occasionally very
weak responses were observed under experimental
conditions that produced strong reactions in the majority
of the animals. Also, it was observed that three pulses
were usually required to move a shrimp out of its burrow
up into the water a distance of 6 to 12 in. However,
this response varied from one to six pulses. The apparent
progression of response strength exhibited by the atypical
animals was independent of field strength (above the
threshold level). Because of its progressive nature it is
possible that this is due to a physiological condition of
the muscle resulting from the moulting process.
Electrical field strength in sea water
The objective of these experiments was to establish,
within general limits, the nature of a relatively low power
electrical field between electrodes in sea water. As in the
shrimp response experiments, information was desired
Table V
the body (i
M^f— ft ,-
rewuve
Power Position
volts degrees
to electrodes.
Shrimp
Number
Response
1
X
0°
2
X
3
X
1
X
3
45°
2
X
3
X
1
X
90°
2
x
3
X
1
X
0°
2
X
3
X
1
X
6
45°
2
X
3
X
1
X
90°
2
X
3
X
1
X
0°
2
X
3
X
1
X
10
45°
2
X
3
X
1
X
90°
2
X
3
*
Voltage
Drop
•018
•019
•024
•027
of shrimp responses to voltage drop across
bead to tafl). Shrimp size 120-130 mm.
Response
•039
•042
•050
•057
•059
•064
•072
•078
•095
•110
•133
•138
Fig. 6. Probes used for measuring field strength.
o
B
Fig. 7. Pattern used in determining field strength. (A) Measurements
in horizontal plane. (B) Measurements In vertical plane. Circles on
(A) indicate location of vertical measurements.
568
for the design of future more precise and specific tests.
Factors examined included: determination of the field
strength and configuration, the effect of electrode length
on field strength and the influence of electrical frequency.
The input voltage at the electrode frame was again
selected as the parameter. Field strength measurements
were made with a pair of brass probes, J inch in diameter
and 6 in long (insulated except for end i inch)
Fig. 8. The general configuration of an electrical field between two
electrodes in sea water.
I 4 | 10 ,
1 io '20'
7 , 19
16 '28
, 14 , 6 i 5 , 4 , 4
1 27 ' 1 1 ' 10 ' 8 ' 8
, 16 , 10 , 8 , 7 , 8 ,
1 36 '20' 16 '14 '14'
16 10
9 16 ,
Fig. 9. The relationship between input voltage and field strength
(millivolts). Top figure of each pair is with 3-F input and bottom
figure with 6- V input.
18"
36"
spaced 2 in apart (Fig. 6) and connected to a vacuum
tube voltmeter. When measurements were made the
probes were always aligned to maximum potential
difference at the position being tested. Fig. 7 describes
the pattern used for field strength measurements.
Configuration of electrical field
Fig. 8 depicts the shape of a typical electrical field
between electrodes. The field strength is approximately
the same below the electrodes (into the sediment) as
it is above them (into the water), and a significant field
exists immediately ahead of, behind, and outside each
electrode; also the magnitude of the field at the power
input end is slightly greater than at the opposite end.
It is shown in Fig. 9 that the field strength is directly
related to the input voltage, i.e. doubling the input
voltage doubles the field strength. It should be noted,
however, that the total power requirement for doubling
field strength was on the order of four to five times.
For example, to produce a 3-V input required 2-4 amps
(7-2 W), whereas a level of 6 V required 6 amps (36 W)
and a 12-V input required 10 amps (120 W).
continued on page 570
26 '82'
II 21
'zo'-w1
,22,40,
'48'iO1
,37.20, 16, 14, »,
'eo'sr'so'ee'zo'
.30.20.16
'57 '40 '31
14.10.
29 'at1
,40,22, !• 1 14, II ,
I72140'34I3I 'SO1
12" •
24"
J
50%
100%
Fig. 10. The relationship between electrode length and field strength
(millivolts). Top figure of each pair it with 6 ft electrodes, bottom
figure with 3ft electrodes.
569
Experimental Dispersion of Chum
Abstract
Most of the studies and experiments with chum have been
concerned with the reactions of the fish towards the chum; this
paper deals with the movement in the water of the chum itself
alter spreading. In the "Kaitsuke" fishery in Japan about 100
to 200 kg of whole sardine is scattered twice a day and as this
bait disperses horizontally according to the direction and rate of
the current, and sinks in relation to its own weight, the handling
of bait is an important factor. The experiments conducted showed
that the whole sardines sink erratically for the first 5 m depth layer
but thereafter have a more regular sinking speed of about 11
cm/sec. Minced sardine sinks at an almost constant rate of
11*5 cm/sec, as compared with sawdust which sinks at 5 • 7 cm/sec.
By repeated scattering the sinking accumulated chum takes a
conical shape with an angle at the top of 20-25°. The sinking speed
at the top is about 9 cm/sec with a spreading velocity of about
1 m/sec.
Une experience war la dispersion de la bo*tte
Rfen*
Dans plusieurs dtudcs et experiences il a 6t£ question des reactions
des poissons envers la bo£tte; la prdsente communication traite
des mouvements dans 1'eau de la bodtte m£me apres sa diffusion.
Dans la pdche "Kaitsuke** au Japon, & peu pres 100 a 200 kilos
de sardines hachecs sont repandus deux fois par jour et la Iboe'tte
s'ttale horizontalcmcnt suivant la direction et la Vitesse du courant,
puis coule selon son propre poids. La manipulation de la boette
est done un facteur important dans cette ptehe. Les experiences
conduites ont montrt que les sardines entieres coulaient de fa$on
erratique pendant les cinq premiers mitres de profondeur, plus
rtgul&rement ensuite & une vitesse de 5*7 cm/sec. En r£p6tant la
diffusion, la bogtte accumulee prend, en coulant une forme conique
avec un angle au sommet de 20 a 25°. La vitesse de coulage au
sommet est de 9 cm/sec avec une velocitd d'&alage d'environ un
metre par seconde.
by
Kentaro Hamashima
Nagasaki Prefectural Fisheries
Station
Un experimento sobre la dispersion de raba
Extracto
Casi todos los estudios y experimentos realizados con raba se
ban limitado a observar las rcacciones de los peces hacia ella;
esta comunicaci6n trata de su movimiento en el agua despu6s
que se ha esparcido. En la pesquera "Kaitsuke*' del Jap6n se
cmplean dos veces al dia de 100 a 200 kg de sardina mqlida que se
dispersa horizontalmente segun la dirccci6n y velocidad de la
corriente y se hunde segun su propio peso. Debido a ello su
manipulaci6n reviste importancia. Los experimentos demostraron
que las sardinas enteras se hunden caprichpsamcnte los 5 primeros
metros, pero despu6s adquleren una velocidad regular de unos 1 1
cm/seg: las sardinas picadas se hunden a la velocidad casi constante
de 1 1 • 5 cm/seg en tanto que el aserrin lo hace a 5* 7 cm/seg. La raba
acumulada adquiere al hundirse una forma cbnica con un &nfulo
en la parte superior de 20° a 25°. La velocidad de hundimiento
en la parte alta es de unos 9 cm/seg y la de esparcimiento de 1 m/seg.
continued from page 569
Electrode length and frequency
To determine the relationship of field strength to
electrode length, a series of measurements were made
utilising 3-ft and 6-ft electrodes (£ inch dia). Fig 10
shows the location of each measurement and the value
obtained with each electrode length. It is evident that
the voltage field strength is inversely proportional to
electrode length.
Another factor investigated was the effect of electrical
frequency on the magnitude of the voltage field. For this
experiment the test probes were positioned, the input
voltage was held constant and the frequency was pro-
gressively increased using a test oscillator. No variation
in field strength due to frequency was found.
Discussion and
Fast experiments directed toward influencing marine
animals with electrical fields have usually had an objective
of either gal vanotaxis or galvanonarcosis. 1 7 Also, projections
of experimental data to possible commercial scale application
have apparently been based on calculations for generating an
effective Add between a single pair of widely spaced electrodes
(Harris 1 952; Higman 1956; and others18'16), These project ions
invariably yield power requirements which are economically
unfeasible. The results of experiments reported here, however,
indicate that the use of multiple pairs of closely spaced elec-
trodes and achieving a blocking response only affords a
potential method of practical application of electricity to
570
shrimp harvesting. In this way, it appears that relatively
low levels of electrical power may be utilised to harvest
shrimp on a commercial scale.
It is anticipated that successful development of this tech-
nique would have application in the present commercial
shrimp fishery of the Gulf of Mexico by permitting 24 hr
operation. Currently the fishery for pink and brown shrimp
is prosecuted largely during hours of darkness, because the
shrimp are burrowed during daylight hours. Also it is
expected that areas which are presently untrawlable can be
made productive through use of electrical power and "off-
bottom" trawls. Another possible application is on potential
deep-water (200 fathoms) shrimp grounds where irregular but
trawlable bottom conditions may significantly limit the
effectiveness of conventional gear.
References
" Dickson, W. Marine Electrical Fishing.
4:148:151. 1954.
World Fishing.
14 Harris, V. F. Some Practical Aspects of Electrical Fishing in
the Sea. Proc. Gulf and Carib. Fish. Inst., Fifth Annual Session,
pp. 38-41. 1952.
19 Higman, J. B. The Behaviour of Pink Grooved Shrimp,
Penaeus duorarum Burkenroad, in a Direct Current Electrical
Field. Florida State Board of Conservation Technical Series
16:25 pp. 1956.
10 Meyer- Waarden, P. F. Electrical Fishing. Food and Agri-
culture Organisation of the United Nations. 78 pp. 1957.
17 Schwartz, F. J. A Bibliography— Effects of External Forces
on Aquatic Organisms. Contribution No. 168, Chesapeake
Biological Laboratory. 85 pp. 1961.
A S with the visual sense, knowledge of the response
jfV offish to taste and odour stimuli is obtained through
study of the organs and of experiments in tanks. Fish
become immune to the continued stimuli of a certain
odour while they do not lose response to a different kind
of smell. Observations and experiments show that fish
are highly sensitive to taste and smell.
During experiments to ascertain the response, learning
and reaction movements, Dr. Uchida of Japan found
that some species of minnow responded to a sugar
solution of as low as 1 /5000th mol, which shows that
the sensitivity offish is 500 times as strong as that of man.
Dr. A. Tester of Hawaii, on the other hand, found that
yellowfin tuna kept in tanks responded to a solution
containing extracts from tuna meat but not to the
solution containing extract from anchovy, in spite of
the fact that it is known that the tuna feed on anchovy.
Though the chemical components of the various
chums used at present to attract fish are well known, it
is still not clear which component or combination of
components is the active attracting agent. In practice,
the materials to be used in fish attraction experiments
are extracts of the materials normally used by the
fishermen, such as raw meat prepared into juice or
powder. Normally some chemicals are added which
cloud the water and attract the fish. In mixing specific
gravity of the materials must be considered to prevent the
material sinking too fast and taking the fish down
beyond reach. The materials are therefore usually
processed into an emulsion which disperses the fatty
substance in the water. Much more study is needed of
the known fish-attracting materials which tend to stay
suspended for some time and thus keep the fish at the
desired depth.
The following materials are used rather extensively in
different countries for chumming: (a) fish meal, fish
solubles, (b) baked rice, bran, wheat noodles, etc., (c)
fish oil, fish extracts, (d) minced fish meat, meat solution,
(e) fermented fish and squid, (f) cooked fish offal,
(g) fish roe, (h) insect meal.
To these are normally added chemicals such as idol,
skatol, aromatic compound, anise oil, sweet wine, etc., to
stimulate the olfactory sense of the fish. This type of
bait, furthermore, often contains bentnite or rhodamine
which accelerates turbidity of the water.
Chumming with chemicals or organic extracts may
fall into three distinct physico-chemical kinds: (a) soap,
"soapless soap" (ABS) and powder, (b) emulsion, (c)
extract solution. To assist in dispersing the material, gas
is often used and may include pressurised butane gas or
air mixed with water.
Chumming with ground minced fish is practised to a
considerable extent in Japan in fishing for mackerel by
means of bouke-ami stick-held dip-nets and also with a
handline fishery for yellowtail and seabream. The method
is called "Kaitsuke" and is widely used near the reefs off
the coast.
"Kaitsuke" fishery
About 20 days before actual fishing starts, a bamboo
raft is anchored up-tide of the fishing area. The fishing
grounds are near the reefs which rise 1 5-25 m in depths of
45-75 m. About 100-200 kg of sardine bait is scattered
twice daily throughout the season which lasts 2 to 2\
months, resulting in from 5 to 7 tons of bait being
dispersed. The sardine is minced before chumming.
As the bait disperses according to the direction and
rate of tide and current, the handling of the bait is an
important factor.
The gear for "Kaitsuke" fishing consists of a handline,
lead weight, balance wire, bait bag, snood and hook:
Handline— 150-200 m cotton or hemp stiffened by
shibu (persimmon juice) or egg white.
Sinkers— 500-1,000 g lead.
Balance wire— No. 12-6 brass wire of about 50 cm in
length, curved in an arc, one extremity carrying the
sinker while the snood is attached to the other end.
The hand-line is fastened at about 10 cm from the sinker
end.
Bait bag — A square cotton cloth of about 30 cm
fastened about 1 m above the balance wire.
Snood — 1-1 J m nylon gut (sometimes two snoods are
used).
Hook — Various sizes, one per snood.
200-300 g of minced sardine is wrapped in the bait bag.
The handline is knotted with a slipknot around the bag.
The hook is baited with a sardine and the gear is lowered
to near the bottom. By giving the line a smart snap the
knot is released; the bag opens and the groundbait
disperses. The hook is kept, as far as possible, in the
middle of the slowly sinking bait.
Experimental method
Very little is known as to the precise nature of the
dispersion of the bait once it has been thrown out so
the dispersion and sinking rate of the chum were watched.
The research vessel was anchored where no current
occurred at a depth of about 25 m. Four different
chumming methods were used and for each type of chum
the horizontal dispersion was measured by reference to
floats while the relative sinking speed was observed by
reference to a Secchi disc or by echo sounding.
The bait and methods used were:
(a) Whole sardine of about 12 cm body length. About
120 fish were thrown at intervals and the dispersion and
sinking speeds were recorded by an echo sounder at
10-second intervals so that the dispersion could be
accurately measured.
(b) A volume of about 2 litres of minced sardine meat,
mixed with an appropriate quantity of sea water, was
scattered overboard and its dispersion observed by the
same methods.
(c) About 13 kg of wet sawdust was scattered overboard
and its dispersion observed for comparison purposes.
(d) At intervals of about 5 sec, half-litre volumes of
minced sardine meat were thrown over and the dispersion,
especially in midwater, was observed.
During the latter experiments water samples were
taken to ascertain its albumin content.
571
Whole fish— The data obtained by echo sounder is
plotted in Fig. 1. The curves obtained are rather
irregular, probably due to the shearing motion of the
bait while sinking; however, all lines show a more or less
identical inclination below the 5 m line which shows that
the sinking speed below 5 m was about 1 1 cm/sec. For
the water layer from the surface to 5 m depth the sinking
speed would appear to have been as low as 7 cm/sec,
which may have been due to the shearing motion on
entering the water.
25
Fig. 1.
The lines of upper and lower edge of sinking ground bait by
echo-sounding record.
The numerical data of the experiment is given in
Tables I and II and shows the sinking speed for every
3 m in the water layer 5 to 23 m. The depth at which the
fish arrived at 30-second intervals is given in Table III.
Minced sardine meat— The data observed on the dis-
persion of minced sardine meat is illustrated in Fig. 1
by the curves marked H and G. A comparison of these
tables with the previous ones appears to show the slightly
higher sinking speed of 1 1 -5 cm/sec of the minced sardine
meat.
40* l
U. I.
0 5 10 MtTM
Fig. 2a, bt c. The shape of horizontal and vertical dispersion of
sawdust.
572
Sawdust dispersion — A graphical illustration of the dis-
persion is given in Fig. 2 (a, b and c). Although the
pattern of dispersion changes somewhat according to the
condition of scattering, generally the sawdust spread
out to a fixed pattern formed about 40 sec after throwing.
The sawdust appeared to sink at about 5-7 cm/sec and
to spread horizontally at 2-3 cm/sec.
Repetition of scattering — About half a litre of minced
sardine meat was scattered every 5 sec. The resultant
shape of the sinking mass had a conical form of which the
top angle was about 20 to 25°. The sinking speed was
about 9 cm/sec at the top while the horizontal spreading
velocity was about 1 m/sec.
During these experiments the concentration of albumin
in water samples was examined by Ninhydrin colour
reaction. The highest concentration was found at 5 m
depth at about 10 m distance in down-current direction;
this point coincides with the centre of dispersion of the
chum at 2 min after scattering (Fig. 3).
W.L.
metres
. . . . 10 . metres
Fig. 3. The shape of horizontal and vertical dispersion of sawdust
scattered at 5-sec intervals.
Table I— Sinking time of whole sardine per 3-m
layer between 5-23 m depth at upper edge
5-8 m
8-11 m 1
n 14-17 m
17-20m
20-23 m
Sec
Sec
Sec
Sec
Sec
Sec
A
40
45
45
40
—
—
B
30
50
50
40
50
—
C
40
40
40
—
—
30
D
32
20
45
—
—
—
E
40
45
45
40
50
—
F
40
—
—
—
—
—
Avge
time
37
40
45
40
50
30
Table II— Sinking time of whole sardine per 3-ra
layer between 5-23 m depth at lower edge
5-8 m
8-11 m 1
m 14-17 m
17-20 m
20-23 m
Sec
Sec
Sec
Sec
Sec
Sec
A
18
27
35
30
30
10
B
23
20
20
35 ^
30
30
C
20
20
—
...
—
—
D
15
15
15
20
—
—
E
29
25
30
25
—.
Avge
time
21
21
25
28
30
30
continued on page 573
Evolution de la Pfcche a la Lumiere dans Les
Lacs Africains
La pfcche & la lumiere dtait pratiquee dans les Lacs africains
depuis bien des generations notamment dans le Lac Tanganyika.
Cettc pfiche a toiyours 6te effectuec avec un "carrelet" conique,
fait de tulle moustiquaire mais 1'introduction de fils de nylon et
1'utilisation de lampes a k&osene de 250 bougies a am61ior£ les
captures. En 1954, des armateurs grecs ont introduit la senne
tournante mediterraneenne "grigri" dans les Lacs afiicains. Ces
sennes ont des ralingues de liege de 300 m, unc chute de 80 m. Les
ailes sont construites en nylon de 210 d/9 a 210 d/12, et en mailles
de 32 mm tandis que le sac est fait de nylon de 210 d/12 avec
mailles de 12 mm (etir6es). Les bateaux posent les lumieres normal-
ement dans la soiree et au bout de cinq heures, les poissons concen-
tres dans la zone de lumiere sont suffisaiment nombreux pour
permettre une operation de la senne. La composition des captures
depend de l'intensit£ de la lumie>e et on a constate qu'en utilisant
une intensity de 2000 & 4000 bougies, 70 & 80 pour cent de la capture
eiaient composes de stolothrissa tandis qu'avec 6,000 a 8,000
bougies seulement 50 a 70 pour cent 6taient des stolothrissa, les
autres 30 a 50 pour cent de la capture 6tant composes de lates et
luciolates.
Evolution of light fishing in the African lakes
Abstract
Light fishing has been conducted on African lakes for many
generations, but confined to certain localities. The fishing was
formerly only carried out with small dip-nets made of mosquito
netting, but the use of nylon netting and kerosene lamps of about
250 candlepower have greatly improved the catches. In 1954
Greek fishing operators introduced the Mediterranean "grigri"
purse seine in the lakes. The purse seines are about 300 m long
on the floatline by 80 m deep (stretched netting). The wings are
constructed of 210 d/9 to 210 d/12, 32 mm mesh, while the bunt is
of 210 d/12, 12 mm mesh. The boats normally set the lights out
in the evening, and after about five hours the fish is sufficiently
attracted for encircling and pursing. The catch composition depends
on the intensity of light used and it was found that when using from
2,000 to 4,000 candlepower, 70 to 80 per cent of the catch consisted
of stolothrissa, while 6,000 to 8,000 candlepower resulted in only
50 to 70 per cent of stolothrissa, the other 30 to 50 per cent of the
catch being lates (perch) and luciolates.
Evolution de la pesca con luces en los lagos Africanos
Extracto
En algunos lugares de los lagos africanos se practica la pesca
con luces desde hace muchas generaciones. Antiguamentc se
empleaban solamente pequeflos salabres hechos de red para
mosquiteros, pero actualmente se emplean redes de nailon y
l&mparas de keioseno de 250 bujias y gracias a ello ha mejorado
considerablemente la pesca. En 1954 los Pescadores griegos
introduieron en los lagos el arte de cerco "grigri" del MediterrAneo.
Las redes de cerco tienen unos 300 m de longitud en la relinga de
corchos y 80 m de altura (malla estirada). Las pernadas son de
by
A. Collar!
FAO/EPTA Economiste
des Peches
210 d/9 a 210 d/12, malla de 32 mm, en tanto que el saco es de
210 d/12, malla de 12 mm. Normalmente las embarcaciones encien-
den las luces a la caida de la tardc y unas cinco horas despues nan
atraido peces suficientes para calar los artes. La composici6n
de la captura depende de la intensidad de la luz: cuando se emplean
de 2,000 a 4,000 bujias, del 70 al 80 por ciento consiste en stoloth-
rissa, con 6,000 a 8,000 bujias su numero se reduce a entre el 50 y
el 70 por ciento, formdndolo el resto lates (percas) y luciolates.
Ep6che nocturne aux feux, ou & la lumiire artificielle,
n'a pas 6chapp6 & 1'astuce des pScheurs lacustres
africains qui la pratiquent depuis des temps imm6mo-
riaux. Cette technique particuliire est rest6e toutefois
tr6s localisfe, car ce genre de pfiche n'a pu se giniraliser
4 1'ensemble des grands lacs africains pour deux raisons
principales :
(a) la plupart des grands lacs sont eutrophes et
la turbidit6 trop 61evte de lews eaux empfiche la p6n6tra-
tion de la lumiire qui reste dans ce cas inopSrante.
(b) la faune £conomique de ces lacs peu profonds
est surtout repr&entte par des Cichlidae (Tilapia),
absolument indiflterents & la lumifere.
De ce fait, la peche & la lumiere est reside le privilege
pratiquement exclusif du Lac Tanganyika, immense mer
interieure de prfcs de 35,000 km2, aux eaux transparentes
et profondes colonis6es par une faune enctemique
particuliirement phototrope; il s'agit d'esptees pilag-
iques reprtsent&s par :
— Stolothrissa Tanganikae, Clup6ide dulcicole
(planctonophage), mesurant & la taille adulte 8 cm de
long et pesant & l'£tat frais, 8 grammes environ. Ce
continued from page 572
Table in—Vertical dispersion of whole sardine per 30 sec throw out after 1 min.
]
I min 1 min 30 sec
2 min
2 min 30 sec
Uadd L
U*
L*
D*
U
L
D
U L
D
U
L
D
A
5
9
4
7
12
5
9 15
6
12
18
16
B
5
8-5
3-5
7-4
13
5-6
10 16
6
11-2
19
7-8
C
5
11
6
8
—
9-5—
—
12-2
—
—
D
4
11
7
7
16
9
11-0—
—
_
—
—
E
5
8
3
7
12
5
9-5 15
5-5
- 12-5 —
Avge
4-7
6-2
5-8
6-9
Dis
0-5
0-8
0-2
0*6
u
and L
A
B
C
D
E
Avge
Dis
U
13
13-
3
*
5
min
L*
20
19
3 min 30 sec
D* U L D
7 16 24 8
5-516 — —
4
U
18-4
17-0
9fi»7
min
L D U
L D
13
— ~ •
6-2
0-5
15-5
•—
8
0
18-0
—
"™~ ~—
•U«upper edge. L— lower edge D -difference
573
minuscule poisson constitue 80 & 90 pour cent de
1'ensemble des productions du Lac.
Lates microlepis, L. Mariae et L. Angustifrons
(Capitaines d'eau douce), gros voraces dont le poids
varie suivant les esptees, de 3 & prfcs de 100 kilos.
— Une sous-esptee, Luciolates stappersii, pesant de
150 & 500 grammes.
Ces voraces se capturent en quantitls considerables,
depuis rintroduction de la peche industrielle.
La pecbe tnditioniielk
Jusqu'en 1954, les pechcurs africains ont utilisi comme
mati&re fclairante, du bois et des roseaux sees. La
pechc au Stolothrissa est simple; un feu placi & 1'avant
des pirogues attire et concentre le poisson qui est captur6
au moyen de vastes epuisettes. Cette peche n'est
possible que par nuit noire, sans lune.
dependant, dis 1953-54, une Evolution des mithodes
ancestrales s'annonce :
— les filets de nylon & mailles de 6 mm de cdt£,
remplacent le tulle moustiquaire utilise jusqu'alors
pour la fabrication des Epuisettes;
— les feux de bois ou de roseaux sees font rapide-
ment place aux lampes & pitrole sous pression; il s'agit
de lampes du type Coleman, P&romax ou Tilly, d'une
puissance de 250 & 500 bougies. Toutefois, ces lampes
dont le rfservoir a pftrole (kerosene) se trouve sous
le systeme lumineux, projettent sur Feau un cone d' ombre
entour£ d'un large cercle £clair£, contrariant la bonne
concentration du poisson. Pour parer & cet incon-
v6nient, on a utilis£ avec un succ&s dclatant, des lampes
Standard (Suisse), de 250 bougies seulemcnt et chez
lesquelles les rayons lumineux sont projetis directement
sur Feau. La puissance Eclairante de ces petites lampes
dqutvaut & celle fournie par une ampoule ilectrique de
100 watts.
Ces ameliorations ont fait passer la production
annuelle de la pirogue de 1 • 5 1 & pris de 4 1 de Stolothrissa
umquement. Malgri ces r£sultats remarquables, on
s'apercut rapidementque la peche traditionnelleamdiorde
ne pourrait suffire, & elle seule, & assurer Fexploitation
rationnelle des eaux et des poissons tanganyikais, et
Ton imagina, dis 1954, rintroduction de peches indust-
riclles & la senne tournante.
La peche industrielle
Introduite en 1954 dans la partie Nord du Lac, la
pSche industrielle a rapidement fait son chemin; elle
est connue aujourd'hui sur 1'ensemble du Lac Tangan-
yika (700 km de long) qui groupe actuellement 15 unites
(Usumbura, Uvira, Albertville, Kigoma, M'Pulungu).
Elle est le monopole d'armateurs Grecs qui ont reproduit
localement leurs unites de peche m6diterran6ennes au
grigri ou senne tournante. Un maitre de peche europien
(Hellene) commande chaque unite", servie en outre par
30 hommes d'equipage recrut^s parmi les pecheurs
locaux.
Chaque unit6 de peche se compose d'un bateau
principal & moteur et de plusieurs annexes non motor-
is6es. Dans cette flottille d'acier, toutes les pieces sont
soud£es; on n'utilise pas les rivets.
Le bateau senneur varie de 12 £ 15 m de long sur
4 & 5 m de large et 1 -75 a 2 m de creux. II est entiere-
ment ponti et 6quip6 d'un moteur diesel marin d'une
puissance de 90 £ 120 c.v. Un treuil £ double poupee
est actionn£ par prise de force sur le moteur.
Le senneur remorque son bateau d'accompagnement
(aidant & la relive du filet), de 8 4 9 m de long sur 3 m de
— DOODG-
It
E
IB. MO
tio/st
2400
5000
5000
50OO
2*00
52 mm
12mm
12 mm
12 mm
32 mm
C
B
A
e
210/12
210/9
210/12
210 /t
210/12
E
10000
82mm
D
2IO/ 15
E
\ II 1 1 1
1.
2.
3.
4.
5.
Fig. 1. Schema type d'une senne tournante montie et armte pour la lac Tanganyika.
Montage: 20 pour cent de mou dans les filets A et B, 10 pour cent dans C, D et £.
Ralingues de flotte et de plombs: 2 x 350 m. Tresse nylon 0 8 mm.
Flottcurs: sphtriques, en plastiquc, 0 3', & raison de 8 par metre dans les ailes et 12 par metre dans la poche.
Plombs: olives de 80 gr; 12 par metre dans les 30 premiers metres, & chaque extr&nite* , et 8 par metre entre ces 2 cxtr6mit6s.
Anneaux: en acier 0 4", suspendus & des brides de 75 cm, accrochtes & la ralingue des plombs. Un anneau tous les 3 • 50 m, sauf dans la
partie centrale, aux points de depart des 2 cables de fermeture fixes & 2 dmerillons espaccs de 6 & 8m.
Cables de fermeture: cables en acier, 0 9 mm. 2 longueurs de 200m, fixees aux emerillons centraux.
574
large et 1 m de creux; de plus, il traine 3 ou 4 barques
porte-lampes. Ces barques pont&s mesurent 4-20 m
de long sur 1 -45 m de large et 0-65 m de creux: chacune
supporte 2 grosses lampes jumetees, 4 gaz de pdtrole
(k6rosine), dc 4,000 4 8,000 bougies, suspendues en
porte-4-faux, de fa$on 4 surplomber d'un metre environ,
la surface des eaux. L'6clairage de ces lampes £quivaut
4 celui d'ampoules ilectriques de 500 4 1,000 watts.
Les filets utilises sont des sennes flottantes, tournantes
et coulissantes que Ton emploie en mer, pour la pfiche
au poisson bleu (Fig. 1).
Ces engins constituis par une dnorme nappe rectang-
ulaire en filet, mesurent en moyenne 300 m de long sur
80 & 100 m de chute 6tir£e. La ralingue supirieure
porte des flotteurs plus nombreux 4 hauteur du sac
central. La ralingue infgrieure lest£e de 800 grammes
4 1 kilo par metre, permet Petalement vertical tris
rapide du filet. De plus, cette ralingue est garnie d'
anneaux m&alliques suspendus 4 1 m, par des brides
espacfes de 5 m. Un long cable de fermeture coulisse
4 travers les anneaux et serre le filet par le bas.
La poche centrale de 50 x 50 m, en fil de nylon n°
9 a 12 et a maille de 6 mm de cdt£, est prolongee par
des ailes de 50 x 50 m, 4 maille identique en fil No 210
d/6 4 9. Ces ailes sont 4 leur tour prolong6es par des
nappes de grandeur variable (50 a 100 m), & maille de
16 mm de c6t6 et en fil de nylon No 210 d/12 4 15. Des
nappes semblables a ces dernieres, prolongent la chute
du filet dont le pirimfctre est consolid6 par une laize 4
maille de 2 cm, en fil de nylon No 210 d/35.
Le filet se trouve range sur la plage arriere du senneur
et on le cale en virant par tribord.
A noter que les filets de nylon, sans noeuds, ont iti
utilises avec grand succes et sont tres appreci6s.
Par nuit sans lune, les barques 6quipees de leurs
feux sont ancrdes au large, vers 20 h. Apres 4 ou 5
heures d'attente, le poisson s'est concentri sous les
lampes et la p6che commence. L'ancre est d'abord
retiree et la barque est maintenue en place a la rame.
Le bateau senneur se rapproche et, tenant compte du
vent, mouille rapidement la senne en ddcrivant un cercle
complet autour de la barque porte-lampes, ce qui
prend 243 minutes. Le filet est totalement d^ployd,
en cercle ferm6, les deux extr£mit6s du cable de traction
coulissant dans les anneaux, sont enroultes autour des
pouptes du treuil qui entre alors en action; 1'opdration
de coulissage se termine en 10 4 15 minutes, par la
fermeture complete du filet par le bas. En fin de
manoeuvre, les anneaux et la ralingue plomWe se
trouvent rassembtes en paquets, contre le bordS, sous
les deux chiens de fune (1 babord, 1 tribord). II ne
reste plus qu'4 baler le filet par les deux bouts, 4 bord
du senneur et de son annexe, pour arriver enfin 4 la
poche et aux poissons qui 1'emplissent; ceux-ci sont
retirfs 4 1'aide d'6puisettes et mis en caisses sur le port.
La vindage de la poche terming la partie~du filet
embarqu£e sur 1'annexe repasse en bon ordre sur le
senneur, et Ton se trouve pr£t 4 rfpfcter reparation de
pfiche autour d'une scconde lampc et ainsi de suite,
jusqu'au lever du jour. En g6n£ral, on arrive fi caler
trois fois la senne au cours de la mcme nuit.
Dans la partie Nord du Lac, les captures par nuit
de peche atteignent en moyenne 1 -5 tonne de poissons;
cependant, dans la partie Sud du Lac, oil la couverture
biologiquc est beaucoup plus importante, les productions
journalises enregistrfes sont de 1'ordre de 3 tonnes.
En ce qui concerne les feux, on a constat6 que le
pourcentage de voraces (gros poissons) augmentait
avec I'intensitS lumineuse. Des 6clairages de 2,000 4
4,000 bougies donnent des prises comprenant 70 4 80
pour cent de Stolothrissa et 20 4 30 pour cent de Lates
et Luciolates, tandis que des 6clairages de 6,000 4 8,000
bougies assurent des captures de 50 4 70 pour cent de
Stolothrissa et de 30 4 50 pour cent de Lates et Luciolates.
La pfiche artisanale
Parallilement au developpement de la peche indust-
rielle, on a recherchi le moyen de promouvoir la peche
coutumiire traditionnelle, par Introduction de tech-
niques de peche plus modernes. Dans la partie Nord
du Lac, oil les eaux sont g£n£ralement calmes, on a
mis au point un systeme de peche artisanale simple et
tr&s avantageux.
L' unit£ de peche artisanale se compose de 2 canots
m£talliques assembles en catamaran; ces embarcations
munies de caissons Stanches mesurent 6 m de long sur
85 cm de large et 50 cm de creux; la tdle d'acier a 2 mm
cTepaisseur. Ces canots sont accouptes par des fers
U boulonnes, les maintenant parallilement 42m
d'teartement. Aux quatre coins externes des canots,
sont fix6s des chiens de funes, largement courWs ext6rieu-
rement, de fa^on 4 atteindre 7 m d'fcartement ce qui
assure 1'ouverture maximum du filet. Get ensemble
est remorqu6 par un 3ime canot iquipi d'un moteur
marin interne de 2 cv (Penta-Marinett), ce qui permet
d'atteindre de nouvelles zones de peche situ£es plus
au large (345 km).
Le catamaran est arm6 de trois lampes Standard de
500 bougies, ainsi reparties: une lampe au centre de
I1 ensemble catamaran et une lampe par canot, 4 Tune
des extr6mit6s, de maniere 4 obtenir une large repartition
des lumi&res.
L'unite comprend cinq hommes d Equipage; on
compte actuellement 60 de ces unites en activite dans
le Nord du Lac.
Le filet-carrelet ou "lift-net", figure une longue poche
conique de 5 m de cdt6 comme ouverture, sur 8 m de
profondeur, terminde par une poche (sac) de 1 m de
cdtg garnie d'un cadre plomb£. La partie conique du
filet est en nylon 210 d/6, 4 maille de 12 mm, 6tir£e
(filet sans noeuds) et de 8 mm dans la poche (Fig. 2). II
comprend \
—quatre panneaux droits, rectangulaires de 5 x
3 m, soit 625 mailles sur 375, compte tenu d'un mou de
33 pour cent s'exer^ant sur la dimension des mailles
6tir£es. Ces panneaux sont d<coup£s, Tun suivant Tautre,
dans une nappe comptant 625 mailles en largeur. Comme
ces pitees doivent recevon tout le poids de la traction,
575
CHlf N Df
CAILi
SCHEMA
dcL'UNITEdePECHE
ARTISANALE
IROGOt
* metres
.FILET c«rr* emtfas^
Fig. 2. Schema de FuniM de ptche artisanale.
il est recommandl de les disposer de telle sorte que la
traction s'exerce dans le sens du tissage des mailles.
Le danger de rupture, provenant des noeuds du filet,
est ainsi minimise.
— quatre panneaux en forme de trapezes isociles
foSm&lvi grande base (625 mailles), 1 m & la pet;te base
(125 mailles) et 4 m de hauteur, filet arm* (500 mailles).
Ces trapezes sont d£coup£s dans une nappe de 500
mailles de largeur, soit la hauteur recherchte. Partant
de la grande base (625 mailles), on coupe la nappe en
biais, en diminuant de chaque c6t6 d'une maillc toutes
les deux rangdes. On arrive ainsi & laisser 125 mailles
pour la petite base.
— cinq panneaux carris, d'un mitre de c6t6, & mailles
de 4 mm de cot6, assemblis de fa$on & former un sac
cubique d'un mitre cube.
Les quatre panneaux rectangulaires qui forment la
"gueute" du lift-net sont simplement census, cot6 375
mailles centre cotd 375 mailles. Quatre coutures sont
done ndcessaires. Le bord supfrieur de la "gueule"
est ensuite rabattu et cousu solidement sur une ralingue
en nylon c&td£ de 4 mm; 20 m de ralingue suffisent pour
ce travail et Ton coud sur 1 m de ralingue la valeur de
1 -50 m de filet £tir6 ce qui donne le mou d<sir£.
Les grandes bases des panneaux trap6ziformes sont
ensuite cousues aux longueurs inftrieures des panneaux
576
rectangulaires (625 mailles contre 625 mailles). Les
cdt£s sont £galement assembles maille contre maille et
figurent ainsi une pyramide tronqute.
Enfin, la poche terminate cubique, compos6e de cinq
panneaux de 1 m2, vient s'ajouter & 1'ensemble.
La ralingue de nylon bordant, au sommet, la gueule
carr£e du lift-net revolt & chacun de ses quatre coins une
patte d'oie qui, en action de pfiche, maintient le filet
largement ouvert. Chaque patte d'oie est compos£e de
trois brides en nylon cabl£ de 4 mm, soit une bride
verticale de 2-50 m fix£e exactement & Tangle de la
gueule et deux brides de 3 • 50 m fixees de part et d'autre de
la bride centrale & 2-50 m de distance. Les trois brides
se rejoignent au sommet dans un 6merillon d'acier,
tres resistant. Aux quatre gmerillons sont attaches les
quatre funes de traction, ou cordes de rappel, longues de
50 m, sur lesquelles sont fix^s, au prealable, des points de
repire tous les 5 ou 10 m, de fa^on & synchroniser les
mouvements lors de la pose et du relevage.
Le fond du lift-net, soit un panneau de 1 m sur 1 m,
est leste extirieurement par un cadre carr6 en fer & b&on,
sur lequel on a enfite 10 kg d'olives de plomb. Ce
cadre est reli£ aux quatre coins exterieurs du fond du
sac par des brides en nylon de 50 cm; il assure le
mouillage rapide du filet, evite sa torsion et le
maintient en position verticale.
La nuit venue, Funite motorisee gagne le lieu de
peche. Une fois sur place, les lampes sont allum^es et
le filet immerge directement, le cadre plomb6 Tentrainant
automatiquement en profondeur, jusqu'& bout de
course des cordes de traction (30 m environ), Fembarca-
tion £tant £ la derive. A signaler ici que le lift-net
immerg£ freine fortement la derive du catamaran et
constitue, en raison de ses mailles fines, une veritable
ancre flottante. Dans des conditions de vent assez
dures, la derive n'a jamaJs exc6d6 2 ou 3 km. Ceci
n'exclut pas que, dans certains cas, vent particuliirement
violent et zone tr&s d&ouverte, il faille augmenter le
poids du lest et mfeme recourir i une ancre flottante
ordinaire qui maintiendra les embarcations dans le vent.
D^s que la concentration du poisson sous les lampes
est suffisante (apres 3 ou 4 heures), les 2 lampes ext^r-
ieures sont occult^es tandis que la lampe centrale est
continued on page 577
SCHEMA OU LIFT— NET OE 5M DE COTE
EN NYLON tlO tf/t
4- 12* MAILLES
D
PAMNCAUX OU SAC
Fig. 3. PSche artisanale ou "lift net".
Pump Fishing with Light and Electric Current
Abstract
The development by Russian fisheries experts and the success
of a new, netless method of kilka fishing in the Caspian Sea,
using lights and pumps, indicated such a method might be adaptable
to the saury (Cololobis saira) fishery. Saury form the basis for
an extensive stick-held dip-net fishery, utilising lights to attract the
fish, by the Japanese and Russian fleets in the Sea of Okhotsk and
adjacent areas. Preliminary experiments commenced in 1957,
using the same type of equipment as employed in the Caspian kilka
fishery. Although dense concentrations of saury were present
during experimental fishing efforts, only a few fish were captured
as they easily avoided the suction intake. Then an electro-fishing
unit and associated equipment to provide an electrical field near
the suction nozzle was constructed. Initial electro-fishing opera-
tions were carried out from 28 August to 2 September, 1962, on
sparse concentrations of fish. A total of 4-3 tons of saury was
caught and the experiments provided an opportunity to make
modifications to the equipment and mode of operation. Subse-
quently, on the night of 2 September, experimental operations
were undertaken in dense concentrations of fish. During 1 hour
and 25 min of pumping, 4-5 tons of saury were pumped aboard
the vessel. The maximum catch rate was 1 • 5 tons in 9 min of
pump operation. This work has indicated the practicability of
utilising electricity, in conjunction with lights and pumps, for
catching saury. It is quite possible this method may also prove
practicable in commercial operations for other species.
Attraction de la lumiere et du courant electrique pour le pompage
du saury
Resume
Le succes d'une nouvelle mdthode de pSche sans filet pour les
kilka (clupeonella spp) dans la Mer Caspienne, developpee par des
technologistes de peche russes et dans laquelle on utilise des
lampes et des pompes, a indiqu6 que cette methode pouvait dire
adaptee a la peche du saury (Cololobis saira). Le saury forme la
base d'une peche extensive au carrelet a perche, avec attraction du
poisson par la lumiere, pratiquee par les flottes japonaises et russes
dans la mer de Okhotsk et les eaux adjacentes. Des experiences
preliminaires ont eu lieu en 1957 a 1'aide du meme type d'equipe-
ment que celui employe dans la peche au kilka (Caspienne). Malgr6
des concentrations de saury assez denses, pendant la peche expe>i-
mentale peu de poissons furent captures parce qu'ils s'evadaient
aisement de Torifice de succion. On a alors construit une unite
de peche a FelectricitS avec equipement auxiliaire pour produire
by
I. V. Nikonorov
Caspian Institute of Marine
Fisheries
un champ Electrique pres de I'orifice ce succion. Les premieres
operations de peche electrique ont et£ executees du 28 aoOt au
2 septembre 196? sur des concentrations assez claires. Un total
de 4,3 t de saury ont et£ captures et les experiences ont montre
qu'il etait opportun de modifier requipcment et la maniere d'ope>er.
Plus tard, pendant la nuit du 2 septembre, des operations experi-
mentales ont ete entreprises sur des concentrations denses et
pendant 1 h 25 minutes de pompage, 4,5 t de saury furent pompes
a bord d'un bateau. Le taux de capture maximum etait done de
1 ,5 t en une minute d'operation. Ce travail a indique la possibilite
d'utiliser reiectricite associee a la lumiere et aux pompes pour
la capture des saury. II est fort possible que cette methode se
reVele aussi efficace pour la capture d'autres especes.
Pesca con bombas de la paparda atraida con luces y corrientes
electricas
Extracto
£1 exito obtenido por los especialistas rusos en el perfccciona-
miento de un m&odo para pescar "kilka91 con luces y bombas, en
vez de redes, en el mar Caspio, hizo creer que podria adaptarse a
la pesca de paparda de la especie Cololobis saira, que constituye
la base de una importante actividad pesquera, con salaores y luces
para atraer a los peces, practicada por las flotas japonesa y rusa
en el mar de Okhotsk y aguas adyacentes. Los experimentos se
iniciaron en 1957 con el mismo material que el empleado en la
pesca de kilka en el Caspio. Aunque se observaron densas
cpncentraciones de paparda, solo se capturaron unos pocos
ejemplares, porque evitaban facilmente el extremo de succi6n.
Se construyo entonces un grupo de electropcsca y equipo rela-
cionado, pa. a crear un campo electrico junto a la entrada de
continued from page 576
coiffee de maniere & n'avoir qu'un jet de lumifcre plongeant
dans 1'axe du filet autour duquel se regroupe le poisson
en bane compact, au bout de quelques secondes. A ce
moment, quatre pecheurs empoignent chacun leur corde
de rappel et halent le filet aussi rapidement que possible.
Dans sa remontte, le lift-net dont la gueule est largement
ouverte, capture le poisson concent^ sous le jet lumineux.
D6s que 1'engin Emerge, il est ramene par dessous les
canots, dans 1'intervalle libre s^parant les deux embarca-
tions, et le lift-net est hisse 4 bord, ou le sac est vid6 de
son contenu. Mis en caisses, le poisson passe dans le
canot & moteur.
Le lift-net est remis imm6diatement & 1'eau, et la p6che
continue, & raison d'un halage tous les quarts d'heure
environ; on donne par nuit, de 5 & 10 coups de filet,
parfois davantage. A noter que 1'occultation des lampes
ne dure que le temps de la remontSe du filet.
La production atteinte par ces unites artisanales est
d'environ 10 fois sup&ieure & celle des pirogues coutu-
mi&res, soit de 15 & 40 tonnes de Stolothrissa, par an et
par unite, suivant la valeur des secteurs de peche.
Dans le Sud de Lac Tanganyika, les conditions
d'exploitation different; les vents nocturnes y constituent
notamment un facteur Ires limitant, et ces petites unites
artisanales ne sont pas suffisamment armies pour faire
face & des eaux trop agit£es. Aussi, compte tenu de ces
conditions particulteres, envisage-t-on d'y promouvoir
la peche africaine, par 1'introduction d'unitds r^duites de
peche 4 la senne tournante. La vulgarisation de cette
technique de predilection, combinte & 1'emploi judicieux
des lumieres conduira i Texploitation rationnelle du
Lac Tanganyika.
Bibliographic
Collart, A La peche au Ndagala, au Lac Tanganyika. Bulletin
Agricole du Congo Beige, Vol XLV, No. 3. 1954.
Collart, A. Note sur la peche au Ndagala au Lac Tanganyika.
Bulletin Agricole du Congo Beige, Vol. XLVII, No. 4. 1956.
Collart, A. Peche artisanale et peche industrielle au Lac Tangan-
yika. Publication de la Direction de F Agriculture, des Fordts et
de 1'Elevage, 7, Place Royale, Bruxelles. 1958.
577
sucridn. Las actividades initiates dc ckctropcsca se rcalizaron
del 78 de agosto al 2 de septicmbrc dc 1962 en conccntracioncs
poco dcnsas de peccs. Se captur6 un total de 4,3 tons, de
paparda y el experimento fad)it6 una oportunidad para hacer
modiflcaciones en el material y en la manera de emplearlo. Pos-
teriormente, en la noche del 2 de septiembre, ae realizaron
experimented en concentraciones muy dcnsas de pcces. En 1 hora
y 25 min. se bombearon 4,5 tons, de paparda. La velocidad
maxima de capture fue de 1,5 tons, en 9 nun. Estos rcsultados
indican quc es practice utilizar la eiectricidad, en combinati6n
con luces y bombas, para pescar paparda. Tambttn es posible
quc estc mttodo resulte ser factiblc en la pesca industrial de otras
especies.
A NEW method of fishing for Caspian kilka by pumps
/\ and underwater lights, was established in 1954.
By 1956, eleven ships were equipped with pumps and
their catch totalled 4,500 tons. Now a growing fleet
of pump-fishing vessels catch over 100,000 tons of kilka.
The success of this new method led to experiments for
determining its efficiency in the saury (Cololobis saird)
fishery in the Sea of Okhotsk and adjacent areas. Saury
form the basis of a large fishery undertaken by Japanese
and U.S.S.R. fishermen using stick-held dip-nets together
with electric lamps.
Experimental fishing was initiated in 1957. Although
dense concentrations of saury were observed, these active
fish easily avoided the suction of the nozzle, and only
an occasional fish was captured.
It was decided that an electrical field would be required
in addition to the pumps and lights. Accordingly a low
voltage direct current unit was constructed and installed
aboard the medium-size trawler (SRT) Izwnrud (Fig. 1 ).
The unit consisted of an electric-drive fish pump
(RB-150), suction equipment, hoses, non-return valve
and water separator, together with DC generator. The
cathodes consisted of two steel pipes suspended on booms
near the bow and stern of the vessel; the suction nozzle,
insulated on the outside, served as the anode. Electrical
current to the electrodes was supplied through a rubber-
insulated cable. The vessel was also equipped with the
same type of lighting system for attracting saury that is
commonly used in the stick-held dip-net fishery.
Saury were attracted to the lights in the usual way,
except that the customary red lamps were replaced with a
500 W focusing red light source in a diffusion dome lamp.
This lamp was suspended at a height of 0-5 — 1 m above
the water surface, directly over the suction nozzle. Saury,
which had gathered in the large zone illuminated by the
customary blue lamps, were directed to the zone under
the red light by switching off the blue lights consecutively
till all other lights were off. The pump was started, and
5 or 10 sec after the red light had been switched on,
current was supplied to the electrodes to make the
concentration of saury denser and to direct the fish to
the suction nozzle. When the current was switched on
the saury got extremely excited, rushed swiftly to the
suction nozzle (anode) and were caught.
During these experiments, various modifications were
necessary before the gear functioned properly. Due to
rough seas, the suction nozzle was frequently lifted out
of the water, but this was remedied by suspending it on
floats, and a weight attached to the lifting cable to help
Fig. L Medium fixe trawler (SRT) Izumrud used in electro-pumping experiments L Water separator. 2. Pump RB-150. 3. Suction hoses.
4. Non-return valve. 5. Nozzle (anode). 6. Buoy. 7. Focusing lifht source (red). 8. Balance weight. 9. Lifting cable. JO. Cathodes.
11. Electrical cable.
578
keep the equipment % stable and submerged. It was
noticed that saury, rushing towards the suction nozzle
under the influence of the electric current, sometimes
passed by the nozzle and out of the electric field, thus
eluding capture. This was remedied by connecting a
metal pin to the anode in such a way that it was fixed
over the centre of the plane of the suction nozzle.
From 28 August to 2 September, 1962, only sparse
concentrations of saury were found. The main purpose
then, however, was to improve fishing techniques.
Even so, 4,300 kg of saury were captured. On the night
of 2 September, dense concentrations of saury were
located, and a total of 4-5 tons was taken during the
pumping operation which lasted only 1 hr 25 min.
The maximum catch rate was 1 '5 tons in 9 min of opera-
tion. The entire catch was graded first-quality by the
cannery.
These initial experiments indicate that netless fishing
for saury with combined pumps, lights and electrical
current is practicable. Future efforts should be directed
at increasing catch rates by extending the zone of the
attracting effect of the electric current with minimum
expenditure of energy through use of impulses of short
duration and optimum frequency. It is also likely that
this type of netless fishing with combined light attraction
and electrotaxis may prove to be practicable for other
species of fish.
Discussion on
Fish Behaviour
Mr. Blaxter (U.K.) Rapporteur: In the last Congress
discussions on fish behaviour followed three distinct lines;
( 1 ) distribution — the factors in environment which concentrate
fish ; (2) use of artificial lights for attracting fish, and the vision
offish, and (3) electrical fishing. In that 1957 Congress two
rapporteurs emphasised the need for much more work on
behaviour, and this has been done. Work on artificial light
stimulae, especially combined with electrical fishing, has
continued, and there have been interesting developments in
electrical fishing in connection with trawling. New develop-
ments concern the use of an air bubble "curtain" at sea,
aquarium experiments on fish behaviour and new techniques
and improved old ones for observing fish in the sea. Unlike
in 1957 there is no detailed report of either the horizontal or
vertical distribution of fish. However, by diving and under-
water television of shrimps by day and night, Fuss found he
could correlate burrowing habits by day and greater activity
by night with poor and good catches respectively. Nishimura's
paper shows that changes in vertical distribution of tuna dur-
ing day and night were related to light intensity and the pre-
sence of prey.
Three levels of research are dealt with in the papers:
(1) Basic experimental work in aquaria; (2) Experimental
work at sea; (3) Comparative fishing experiments to test
ideas on behaviour and new fishing techniques.
(1) On the first point two authors mention the disadvan-
tages of tank experiments— the effect of capture on the fish
and the confined surroundings, etc. These tank experiments
seem of great value when part of a wider programme, but
their expense can be a great drawback.
Blaxter, Parrish and Dickson, after experiments on herring,
ground fish and other species to test their reactions to drift-
nets and trawls, reached the main conclusion in relation to
driftnets that vision was very important to herring in avoiding
such nets, and it was possible to measure the light intensity
when herring started to pass into nets which they would
avoid in daylight. Light measurements at sea during the
Scottish summer herring fishery showed that the light was
above the threshold during a greater part of the fishing period,
thus suggesting that driftnet fishing is then relatively inefficient.
Of all nets tested, monofilament nylon had the highest thres-
hold, aquarium herring swimming into it in full daylight.
This relative invisibility of polyamide monofilament fibre,
especially of small diameter, was confirmed by Steinberg
and also by Tran-Van-Tri and Ha-Khac-Chu and by Shimo-
zaki. In contrast Mr. Watson from Kenya found that low
visibility of driftnets was not of importance. Presumably
this factor will vary in importance in different species, and
with their physiological state as well as other factors.
As to the herding properties of different parts of towed fishing
gear — reaction distance being usually less than 2 m — the speed
of tow is of great importance — over about three knots the herd-
ing drops markedly. While trawling speed may need to be
increased to prevent fish escaping, if it is increased too far
any value of herding by the peripheral points of the gear may
be lost, because the fish have insufficient time to react. As
with stationary nets, visibility is important and the herding
reaction is reduced to a very low level at night. Underwater
lights, however, although providing visual stimulae, were
found to be ineffective at night in herding herring or cod,
these fish being very inactive at night in these particular
experiments.
Chapman's experiments with sound sources of low fre-
quency showed that herring and cod could keep away from
such sources, but probably not by direct avoidance.
(2) Recent techniques for observing fish behaviour at sea
are underwater television for observing prawns (Fuss),
television on trawls (Kreutzer), cameras (Blaxter, Parrish and
Dickson), diving (Fuss), underwater viewing ports (Magnuson)
and submarines.
Of greatest interest is the development of echo sounders,
especially of the netzsonde described by Mohr and of the
high resolution Sector Scanner by Tucker and Welsby. Mohr
shows the value of the netzsonde for observing herring and
other species in the mouth of a midwater trawl. His main
conclusions are important and worthy of study in the original
paper. The disadvantage of the netzsonde is presumably its
inability to assess movement of fish over the headline. This
suggests the need for netzsonde in tandem — one pointing
down and one pointing up — when observing fish behaviour.
The fact that towing speed appeared optimal for herring
between 3*5 and 4*2 knots as suggested by Sch&rfe is interest-
ing in relation to the observation that herding became reduced
above about three knots in tanks.
Tucker and Welsby have pointed the way to a new and valu-
able development of echo sounding for tank and sea work —
the high resolution and PPI presentation of their scanning
sounder permitting the following of the movements of indivi-
dual fish. This might well provide a breakthrough in behaviour
studies where fish must be observed at a distance or in darkness.
However, Nishimura, using a more conventional sounder,
was able to follow the diurnal distribution of tuna and to
estimate their "maximum speed under normal conditions*'.
Lenier's report on species recognition by echo sounder —
579
and important aid to fishermen— was based, at least to some
extent, on behaviour.
Cameras equipped with flash on trawls, and television on
trawls in shallow water without artificial lights have shown
how fish remain in front of a groundrope during towing.
Some may swim forwards and out of the net depending,
presumably, on the speed of the tow. While fish are swimming
in an orientated manner in front of groundropes in good
light conditions, at night this orientation becomes reduced or
lost as the use of vision decreases. This confirms the impor-
tance of vision in reactions to nets found in the tank experi-
ments. Clearly it is not satisfactory to use underwater
television at night because of the artificial lighting required
but day and night use of nctzsonde would be valuable.
(3) Observation by comparative fishing demands much
time and patience and may give very little certainty in results
as pointed out by Dickson. They can, however, be planned in
conjunction with more basic experimental work so that
worthwhile results may be obtained. An open question is
how the Vigneron-Dahl sweep gear works. Further trials
by day and night, with and without sweeps, especially using
a net of fixed mouth aperture, are needed to assess the sweep
effect. It is not at all clear at present how the stimuli produced
by the sweeps might be effective by night and how important
is the length of the sweeps. How do sweeps alter the shape of
the mouth of the net? Do long sweeps permit fish to settle after
the boards have passed, and what is the sphere of influence
and effect of the boards themselves? These are serious gaps
in our present knowledge.
Variations of mesh size and shape as mentioned by Scharfe
suggested that nets with four seams gave a shape of mesh
which may have prevented escape of fish within the net.
Dickson used small otter trawls with different mesh sizes in
the front, and found that a small mesh produced better catches
and seemed to prevent the escape of larger fish. Perhaps these
were held within the net after making a spurt from farther
back. This raises the question of the crowding of fish within
and in front of the codend. What influence do fish have on
each other at this point — as they become more and more
crowded? Do they suddenly "burst" outwards? The need for a
small mesh net at this position, which would vary with fish
density, species and size is essential. Equally, the importance
of the taper of the net both on fish behaviour and pressure
stimuli produced by the net is obvious.
Perhaps the highlight of this section, and even of
Congress as a whole, is Kreutzer's paper in which he describes
the use of pulsed DC electric fields around the mouth of a
trawl. He estimates that only 10-60 per cent of fish in the path
of a trawl are normally caught by conventional gear. By the
use of electrodes on the mouth of a trawl and extensive com-
parative fishing trials on mv Delaware, it was shown that
catches of fish might be increased from 100-500 per cent
depending on the species and fishing conditions. Fish near
the mouth were prevented from escape around the periphery
and through the meshes, as a result of electronarcosis. This
technique would seem to have a bright future for both bottom
and midwatcr trawls, and, as suggested by Kreutzer, for
enticing fish from rough ground.
But his technique, if developed, will not be the panacea for
all fish behaviour problems. The study of behaviour in relation
to the peripheral parts of the gear, in the leading and herding
into the net mouth and the prevention of premature escape
reactions and early warning of the nets approach, will be still
necessary.
One advantage of electrical fishing is its selective effect on
large fish, but the danger in this new method of catching
580
many undersized fish may be serious 'from the conservation
point of view. Dethloff has made a very detailed study of the
effects of lights, electrical fields and pumping on Newfound-
land herring, which should lead to developments in that area.
Other methods described are by Nikonorov of light attrac-
tion electrical field pumping technique for saury fishing and
by Wathne .whereby prawns may be made to leave their
burrows in daylight through the use of low power electric
fields. Collart reports methods of encircling fish by a purse
seine after they have been attracted to lights.
A quite novel method described by Smith is the use of
air-bubble "curtains" by pumping air through a perforated
pipe dragged along the bottom. This guides herring towards
stake nets and weirs and seems to have been highly successful.
In summary, it is valuable to consider how the main
senses are involved in the reaction offish to gear. In this way,
lines of future work become clearer. Vision has been especi-
ally emphasised and confirmed in tank and sea experiments.
The low visibility of monofilament fibres in driftnets is also
stressed and vision is further involved also in the reaction
to air-bubble curtains and in the reaction of tuna to bait.
Further work on catches of trawls using different types and
lengths of sweeps and different boards and herding devices,
by day and night, is badly needed. Observations by netzsonde,
camera, and by diving, or the use of towed bodies or submarine
need to be developed.
More information is required on the importance of sound
and other mechanical stimuli in reaction to gear. While
Chapman has presented some theoretical evidence that fish
are unlikely to perceive, with any exactness, the direction of
low frequency sound sources, Freytag mentions experiments
where gurnards seem to have been attracted by sounds from
a loudspeaker. Freytag also describes the mechanism of
sound production in fish and its importance. A wide field of
investigation is open here. For instance, herring seem to
produce a whistling noise of 6-7 kcs as a school veers. The
possible value of playing back sounds to fish in order to
attract them has obvious possibilities.
The identification of fish species by their sound spectra,
described by Hashimoto and Maniwa, could be of the greatest
value.
Finally, chemical or olfactory stimulation is of special
importance in bait fishing. Magnuson shows that the smell,
as well as the sight of bait in tuna fishing, needs consideration.
Problems of behaviour necessitate the development of new
techniques, and more manpower and facilities may become
allotted to this work as advocated by von Brandt. I suggest
the following developments for early prosecution in order of
importance: —
(1) Development of electrical fishing in association with
trawls; (2) development of a monofilament gillnet of low
visibility which can be easily stowed (not bulky), does not
damage fish and is easily handled; (3) the use in more basic
behaviour studies of trawls with a fixed mouth opening and
the comparison of day and night fishing with different types
of herding device.
Dr. Cole (U.K.) : I would like to make some general remarks
to supplement a very able summary. Fish behaviour is an
extremely complex subject and requires years and years of
study before we can even attempt to provide full knowledge
of particular species that will enable gear to be devised for
satisfactorily catching them. The influences that affect fish
behaviour must be considered species by species, and when
we have settled on one particular species for study its be-
haviour will be found to differ with age, physiological condi-
tion, nutritional state, stage of maturity, hormone balance,
and so on. There is temperature, and we have an indication
last winter of its importance in affecting fish behaviour. A
great deal of work had been done on cod in the Arctic in
relation to its reaction to temperature. This is very important,
especially in areas where there is a strong flow of cold water.
It is important to the fisherman so that he can work more
efficiently. The thcrmocline is important in relation to the tuna.
Also important are salinity, oxygen content, currents and their
direction — and this is not a simple matter because there
are internal waves, eddies and tidal cycles. Turbidity and light
and bottom topography may be related to currents— all these
have to be considered. There is the association of species—
what species are found together — whether they are predators.
Noise — that produced by gear or the ship, and probably even
the noise of the fish themselves— all these can affect behaviour.
Pressure, feeding, trace elements, smell— this catalogue is not
even a complete one, and finally the rapporteur had suggested
that fish might even learn how to avoid gear! I cite these
points not to show that progress is impossible but to indicate
that we cannot make any simple statement about any partic-
ular species.
Having mentioned these points J would like to comment
on the methods by which progress may be'obtained. First, I
would like to make a strong plea for precise laboratory
experiments isolating a single stimuli and studying its effect.
It seems to me that work of this kind is basic and essential.
Of course it is not easy to organise because it requires con-
siderable facilities in the way of tanks and environmental
control and instrumentation, and these are somewhat expen-
sive. In the second place there are methods of direct and
indirect observation. Most of these have been mentioned. I
feel that after our experience with frogmen these methods
have distinct limitations — limitations of depth of working —
speed at which one wishes to tow the trawl and, further,
most frogmen observations of trawling gear have taken place
in situations that are not particularly fishing situations.
Bodies which can be mounted, such as television cameras,
and echo sounders, undoubtedly have a large place in investi-
gation. We at Lowestoft are very interested in the possibilities
of using echo sounders on the gear — not merely netzsondes.
We are thinking in terms of multiple transducers put inside
the gear in various places observing in several directions and
bringing in results over a number of recorders or in a series on
the same recorder. Sector scanning, too, can become important.
Comparative fishing experiments, 1 myself view with some
caution because it seems to me that to provide significant
answers it is necessary to conduct very strong controls in
general, because the normal variation that one notices from
consecutive hauls on the same ground are considerable. Jt
is very difficult indeed in comparative fishing experiments
to isolate a single variable and subject it to tests. Most of the
alterations that are made in the gear, the bridles and the doors
are likely to have other and multiple effects, and then it is
very difficult to know what you are examining and then to
assess your results. After mentioning all these difficulties,
however, I hope that in future years we shall be able to make
substantial progress along a large number of lines.
Mr. Dennis Roberts (U.K.): I would like to stress that fish
behaviour is one of the most important things for the trawler-
man to know about. There are two things we can do— go
after fish, or attract the fish to ourselves. In relation to the
diurnal movement of the fish, can anybody devise some way of
attracting the fish down to the trawl instead of working to
get the trawl up after the fish?
Mr. V. T. Hinds (U.K.): Our problem at Aden is with
phosphorescence when fishing on moonless nights. After we
have detected a school of fish and endeavoured to encircle it
with a purse seine, the net itself disturbs the phosphorescence
so that the fish make for the gap where there is no phosphor*
escence and escape before the ring can be closed. In midwater
trawling at night the gear disturbs the phosphorescence and
scares the fish away. What is the remedy?
Mr. B. Petrich (U.S.A.): When we fish at night time we
use a flashing submarine light and that keeps the fish away
from the opening. We also, some years ago, used air bubbles
with partial success.
Prof. Diaz de Espada (Spain): In tuna fishing one problem
is that the fish escape from the lower part of the net before
we can close the purse seine. I understand this is similar to
what happens in whaling. I understand that whales dive to
great depths and the practice has been developed of using a
special noise to bring them back to the surface. Can any
similar practice be evolved to frighten the fish away from the
bottom?
Mr. Harper-Cow: My understanding is that the whale
frightener was devised not to bring the whale up, but to stop
it from going down by frightening it and making it run.
When a whale runs it doesn't go very deep and has to come
up to breathe.
Dr. W. M. Chapman (U.S.A.): I alluded this morning to
the effect of the thermocline on tuna behaviour, and it might
be useful to comment at greater length now. There has been
one series of observations made with adequate scientific gear
in the Eastern Pacific during the past six months, and thip
series of data, accumulated from three vessels on which there
were thermographs and adequate operators to interpret the
results, has proved interesting. The data cover about 225 sets
of nets. Of the sets which were in water where the thermo-
cline was not more than 20 fm deep between 60 and 80 per
cent were successful. Of those sets where the thermocline
was deeper than 20 fm but less than 25 fm, the number of
successful sets was 37 per cent — if my memory is correct.
Where the thermocline was between 30 and 35 fm the percent-
age of success fell to 26 per cent. There was a sharp drop in
the success of the sets after a rather small difference in the
depth of the thermocline. There has not been any more posi-
tive data on this subject as yet. We have also quite a good
record of the variation of the thermocline depth in the area
directly off the Californian Coast where the Navy, for their
own purposes, keep a record. There also exist in that area and
in the season an active purse seining fishery for bluefin tuna.
When the scientists were working over this data from last
year's catch they found that 80 per cent of the successful sets
for bluefin in the Californian waters had been made in an
area about three miles long each side and within an area where
there was a persistent shallow thermocline during that period.
There is a great deal of purse seining in adjacent areas extend-
ing over 40 or 50 miles, but the percentage of successful sets
is very small.
Turning to the influence of sound on fish they do use sound
to scare tuna in the course of setting the net. They use what
they call cherry bombs — a small explosive that the men threw
into the water when the net is being closed, and during the
pursing period. This is done for the purpose of frightening
the tuna away from the opening, and it is highly successful—
although sometimes they will come right through the sound.
Experimentation has been done in Hawaii with a view to
obtaining a pure record of the sounds made by skipjack tuna
during schooling and feeding with a view to playing them back
so as to attract the skipjack. But this so far has not been suc-
cessful. The skipjack tuna is very difficult to purse seine
because they move very actively, and their first instinct is to
581
go right down quickly. This is in general true of tuna of all
varieties— they are very quick to sound.
Mr. B. Petrich (U.S.A.): A recent development has been
to improve those cherry bombs by giving them a delayed
action so that they sink and explode lower down. The purpose
of this is to make them explode below the tuna and keep them
up and stop them diving. This has been a complete success.
Mr. J. Detfaloff (Germany): Is it known when the tuna try
to escape what is the diameter of the net just being pursed,
or at what diameter does the tuna start to escape? Secondly,
will they start diving vertically or round the edges of the net?
I know tunny are affected by an electrical field of an insulated
cable at a distance of about 30 ft.
Dr. W. M. Chapman (U.S.A.) : The tuna net is about 600 fm
long and the net is set in a wide circle so that with extension
lines it will run more than 900 to 1,000 fm. The tuna are
perfectly capable of escaping from that maximum entrance
until the ring is hauled. My own observations are that the
tuna dive vertically and instantaneously. The difficulty of
Mr. Dethloff's method is that the length of reach of the pulsed
DC current in salt water is such that it will not reach far
enough out in such a large diameter as the net was enclosing.
The method would not much affect purse seining.
Mr. R. Balls (U.K.): How much has the change in the
abundance of North Sea herring affected its behaviour, and,
secondly, how is the pressure of new methods of catching and
the interference of trawls affecting the herring's behaviour?
Dr. G. Freytag (Germany): to supplement his paper on the
bioacoustical detection of fish and the paper by Hashimoto
and Maniwa on the frequency analysis of marine sounds,
played recordings made from fish and other marine noises as
referred to in those papers. They revealed, he said, possibili-
ties of influencing fish by attraction or repulsion as well as
by operating against predators. Congress listened to the tapes
with much interest because of their practical demonstration
of the points made.
Dr. H. Levy (Morocco): In Morocco we experimented
with colour in our net material. We found the fish behaviour
quite marked. With nylon you can choose your own colour,
and in Morocco we tried out brown, then blue, and then pink,
and the fishermen confirmed that pink does not frighten the
fish. Some people used green and white, but brown, blue and
black proved to be failures. Has any detailed study been done
on the point of fish behaviour when faced with colour?
Mr. Harper-Cow (U.K.): In relation to electrical fishing,
I am under the impression that the difficulty of using electrical
fishing in a moving trawl is that whilst the effect on the fish
actually in the field can be determined, fish on the extremes of
the field are inclined to be frightened away.
Dr. C. O. Kreutzer (U.S.A.): We tried to find out whether
fish were scared away at the fringe of the effect. For this pur-
pose we interrupted the experiments. We left the field on for
two seconds and then cut it off for 10 seconds so that the
fish could assemble. We then compared this with continuous
current and there was no difference whatsoever. We think we
have an explanation for this but it is difficult to explain.
Mr. A. W. Anderson (U.S. A.): Can you say anything about
the directional perception of sound in fish? Can fish detect
where the sound comes from and swim towards it?
Dr. Freytag (Germany): We experimented on fish at
Hawaii in a tank with a loudspeaker at about 50 cm or so
from the fish. The fish immediately turned towards the sound
and then turned left or right, and then began to lose
interest. We think that their lack of interest was due to the
lade of any optical stimuli associated with the sound and
which they expected to find in association with that sound,
582
as indicating the piessnce of another of their species.
Mr. Anderson: Can Mr. Welsby say anything about the
effect of sector scanning for use in fish behaviour studies?
Mr. V. G. Welsby (U.K.): Very briefly I can say we have
carried out experiments in tanks which have shown that
experimental sector scanning equipment of this type is capable
of giving information concerning the behaviour of fish which
would be very difficult to obtain in any other way. I am
quite convinced that this pulse electronic scanning technique
which we have developed is something which will in future
provide an extremely valuable research instrument for
studying fish behaviour. One thing that comes to my mind
is that I have seen a demonstration in which it was required
to find how fish behaved in the dark when an experimental
net was being towed. Visual methods cannot be used in the
dark and any attempt to introduce light into the vicinity of
the net will alter the surroundings and the fish behaviour,
and as far as 1 know, the use of ultrasonic, and particularly
of an electronic scanning device of sufficiently high resolution
required to watch the movements of individual fish is the only
way in which the behaviour of individual fish can be watched
in complete darkness.
Mr. E. Bayagbona (Nigeria): The sending of electrical
impulses through the footrope of the trawl — would not that
run foul of fishery regulations, and also create a lot of in-
balance to the fauna of the benthos?
Dr. Kreutzer (U.S.A.): We have studied this problem very
thoroughly. We think that with an electrical trawl we are
capable of chasing away all the small fish and catching only
the large fish, and we even hope that we can select the species.
That sounds like science fiction, but we have every good
reason to believe so, and we have scheduled some new tests
on the Fish and Wild Life Service vessel, Delaware, for
September to prove these ideas. On our first test we saw it
quite clearly that we had more large fish, but we did not have
enough personnel to evaluate the knowledge and measure
each individual fish, but the fact that only large fish are in-
fluenced by the current has been known since the early 1920's.
Mr. K. C. Rowe (U.K.): While the biologists say it may
take many years before any progress is made in scientific
research into fish behaviour, 1 would remind them that over
90 per cent of our nation's fish is landed by deep-sea otter
trawling. If they intend to play any real part in the progress
of our fishing industry in its present form 1 would suggest they
consider a greater degree of specialisation in their work and
the spending of a proper percentage of their time on otter
trawl gear and the main species of fish that are caught by
this method. Progress in trawling gear through scientific
research has been practically nil. Advance has only been made
by comparative fishing experiments by commercial trawlers.
Last year some British trawlers working the Faroe grounds for
haddocks found that if they raised the headline at the expanse
of a little spread, and if they let their wings fly as is the custom
over rough ground, they caught more haddocks and fewer
codlings. These trawls were rigged somewhat similar to the
French type of trawl used experimentally at Aberdeen. Those
boats which later fished for whiting in other areas adopted a
similar rig butkept to theold style of smaller mesh in thesquares
and baitings, and thus provided a fairly successful year-round
trawl. I suggest co-operation between the scientists and the
practical fishing interests to determine: (1) the right distance
between the otter boards in ratio to the right cable length,
having regard to the power available and to the species sought ;
(2) to solve the maximum headline height required, having
regard to the density height of various species of fish when
meeting the trawl.
Part 3 Technical Research
Section 16 Science and the Future
Prospective Developments in the Harvesting of
Marine Fishes
Abstract
To utilise fully the protein resources of the oceans, man must
devise better methods to harvest the great diversity of marine life
dispersed throughout the seas, by devising extremely efficient
detection and straining systems or artificial methods to cause fish
to aggregate where they can be easily caught. Below are listed
some likely means of improving harvesting efficiency. Determina-
tion of watermass boundaries and surface temperature by infra-red
ray procedures. Single-wavelength light beams might facilitate
visual fish detection down to depths of 100-200 m, or even deeper.
There is scope for refinements in sonar equipment for detecting
the direction and speed of fishes by use of doppler sonar, and in
high-resolution short-ranged equipment for identification of targets
by studying three-dimensional patterns of fish school shapes
characteristic of species. It might be possible to develop recording
spectrophotometers for tracking organic odours in the sea and thus
identify fish schools. This may lead to use of artificial odours to
force or guide fish along a given path. Introducing water soluble
chemicals into air-bubble curtain might enhance its effectiveness
as a fish barrier. Reproducing sounds of prey or predator might
be used to attract or herd fish. Electrical fields may be used in
conjunction with light, sound or chemical stimuli to aggregate
and lead fish. Retrievable floats with built-in detection systems
could automatically signal to catcher vessels the presence of fish.
A network of unmanned buoys will detect fish and transmit data
through satellitic telemetering to a shore-based "hydro-central"
for computer analysis and transmission of data summaries by
facsimile technique to fishing centres. Motorised units could replace
otter boards and even lead to remote-controlled self-propelled
trawls. Manned underwater stations for research and harvesting
are entirely conceivable. The environment may be improved by
fertilising lagoons and by installing nuclear readers at the sea
bottom to create thermal upwelling. Plastics and lightweight
metals will be increasingly used in vessels and gear. Irradiation and
other innovations in preserving seafood may affect the pattern of
fishing operations.
Perspectives de developpement dans la recolte d'animaux marins
Rfeuin*
Pour utiliser pleinement les resso urges de proteines des oceans, il
faut trouver de meilleures methodes pour recolter la grande
diversity d'animaux marins disperses dans les mers, par exemple,
des systemes efficaces de detection et de filtrage ou des procedes
artificiels obligeant les poissions a se rassembler dans des lieux ou
il serait facile de les trier. Quelques uns de ces r oyens pourraient
£tre: la determination des limites de masses d'eau et de la tempera-
ture de surface par des rayons infra-rouges; des faisceaux de
lumiere a onde unique pouvant faciliter la detection visuelle du
poisson jusqu'a des profondeurs au-dela de 100 et 200 m. II est
certainement possible aussi d'apporter des raffinements a l'6quipe-
ment sonar, pour d6tecter la direction et la vitesse des poissons par
1'emploi du doppler sonar, et aussi dans requirement a portee
reduite a haute resolution pour ridentification des objectifs par
1'etude de modeles tri-dimensionnels caracteristiques des differents
banes de poissons. D'autre part, on pourra pcut-etre developpcr
des spectrophotometres enregistreurs pour deceler des odeurs
organiqucs dans la mer et identifier ainsi les banes de poissons ce
qui conduirait a f utilisation d'odeurs artificielles pour diriger les
poissons par une route determinee. L'introduction dans les rideaux
d'air, d'dlements chimiques solubles dans I'eau, pourra peut-^tre
renforcer FerTet de barriere de ccuxci. La reproduction des sons
emis par les proies et les prfidateurs pourra dtre utilisjfee en con-
jonction avec la lumiere, le son ou des stimuli chimiques pour
guider les poissons. Des flotteurs recuperables contenant des
systemes de detection pourraient signaler automatiquement la
presence de poisson aux bateaux de peche. Un reseau de telles
bouees pourrait d&ecter le poisson et transmcttre 1'information par
t£temctrage satellitique a r'hydro-centre" install a terre pour
by
Dayton L. Alverson
Exploratory Fishing and Gear Research Base,
U.S. Bureau of Commercial Fisheries,
Seattle, U.S.A.
and
Norman J. Wilimovsky
Institute of Fisheries, University of British
Columbia
Dayton L. Alverson
Norman J. Wilimovsky
analyse par des calculateurs electroniqucs et transmission de
rinformation aux centres de peche. Des unites motorisees pourront
replacer les panneaux de chalut et meme mener au chalut auto-
propuls6 commande a distance. 11 est a present possible de concevoir,
pour la recolte des mers, des stations sous-marines avec Equipage.
L'environnement mSme pourra 6tre amcliore par la fertilisation des
lagunes ou par Installation de niacteurs nuclcaires au fond de la
mer pour crier des courants thermiques, verticaux. Les metaux 16gers
et les plastics seront davantage utilises dans la construction des
tateaux et des engins de peche. I/irradiation et autres innovations
dans la preservation des produits de mer pourront affectcr les
m&hodes actuelles de peche.
Evoludon futura de la cosecha de peces marinos
Extracto
Para aprovechar plenamente los recursos de proteinas que existen
en los oc&mos, el hombre tiene que idear metodos mejorados
para recoger la gran cantidad y variedad de vidas marinas desper-
digadas por las aguas, que pueden consistir en sistemas eficaces
de Iocalizaci6n y filtrado o dispositivos que les obliguen a congre-
garse donde pueden capturarse fdcilmcntc. Entre los proccdimicn-
tos que probablemente mejorarian el rendimiento de las captures
estan los siguientes: determinacibn de los limites de las masas de
agua y de las temperatures de la superficie con rayos infrarrojos;
haces de luz de una sola longitud de onda podrian factlitar la
Iocalizaci6n visual de los peces hasta profundidades de 100 a 200
metres y aun may ores; son factibles de afinamicnto los aparatos
sOnar para dcterminar la direccidn y vclocidad de los peces median-
te el uso del doppler y los de poco alcance pero mucha resoluci6n
para identificar objetos estudiando patrones de tres dimensiones
583
dc formas de cardiiroenes caractoisticas dc las especics; dcbcria
act* poaible fabricar espectrofot6metros registradores para encontrar
olores organicos en el mar e identificar mediantc ellos lew cardu-
mcncs. Esto pucdc llevar al empleo de olores artificiaks para hacer
que los peccs se diryan por una direccidn dctcrminada ; el empleo de
compucstos qulmicos solubles en agua en las cortinas de burbiyas
de aire podria reforzar su eficacia como un obstaculo al paso de
los pcces. La reproduced de los sonidos emitidos por los preda-
dores o sus presas podria emplearse para atraer o congregar peces ;
los campos electncos podrlan emplearse en combinaci6n con
estfmulos luminosos, sonoros o quimicos para conpregar y dirigir
peces; flotadorcs recuperables en los que se hubieran instalado
aparatos localizadores podrfan seftalar automaticamente a los
pesqueros la presencia de peces; una red de boyas podria localizar
los peces y a traves de telemetros satelites transmitir datos a esta-
cioncs costcras donde se analizarian con maquinas calculadoras y
desde las que se transmitirian resumenes de los datos a los centres
de pesca; grupos motorizados podrian sustituir las puertas de los
artes e incluso permitir la construcci6n de artes de arrastre auto-
propulsados y con tclemandos; es perfectamente factible la
construcci6n de estaciones submarines cuya tripulaci6n humana
haria investigaciones y captures de peces; el ambiente puede
mejorarse fertilizando las lagunas e instalando en el fondo del mar
rcactores nucleates que crearan afloramientos termicos; los mate-
riales plasttcos y los metales ligeros se usaran cada vez con mas
frecuencia en barcos y material de pesca; la irradiaci6n y otras
innovaciones en la preservaci6n de alimentos de origen marino
pueden influir en la modalidad de las actividades pesqueras.
1. INTRODUCTION
THE most striking progress in man's efforts to harvest
proteins from the oceans has occurred since the
termination of World War II. Rapid strides made in
development of electronics and acoustical devices have
given fishermen better navigational and fish detectional
methods. These systems have gone far towards providing
the precision navigation needed. Radar increased the
capability of inshore navigation and allowed greater
safety for around-the-clock fishing operations. Adoption
of acoustical devices and sonar for fisheries has yielded
a wide array of echo sounders, as well as "fish finders"
which are gradually coming into general use.
Interest in a more scientific approach to fish harvest
has been demonstrated by the large number of countries
which have initiated research into the investigational
area now called gear research. Most major fishing
nations of the world are now active and some important
advances have occurred in recent years. Synthetics have
been adapted for twine and net construction and have
had a tremendous impact. In mechanics the power
block for hauling large nets from the sea has provided
fishermen with a more effective method of handling
nets. Electro-fishing and mechanisation of the "kilka"
fisheries in the Caspian Sea by lights and pumps, indicate
that progress is being made. Progress in utilisation of
ocean resources, however, is in an embryonic state as
compared with ideas which have swept manufacture
and agriculture.
Our purpose here is to explore the engineering techno-
logies of the industrial, military and space fields as to
their application and use for improving existing or
developing new concepts for harvesting marine life.
We have limited our considerations to the next three
decades. The rapidity of scientific development and th$
fruits of discovery will, even within this time period,
probably outdistance the imagination. In this vein we
584
must admit that science fiction has perhaps already
anticipated the greater part of our text.
2. DETECTION AND LOCATION
The location or detection of biological populations can
be carried out from above the water or in the water.
Much military research has been directed to search and
recognition of underwater targets. In specific application
military engineering need is diametrically opposed to the
fishery need. Whereas great effort has been expended to
reduce background "noise" and "clutter" from military
devices, the interest to the fisherman is often the source of
the clutter and noise. The development problem is the
reduction of noise and clutter into its physical and
biological components and the amplification of the latter.
2*1 Aerial surveys
The state of the air-sea interface as well as its physical
nature limit the utility of the aerial surveys, particularly
as to depth. Objects which break surface can be located
through high resolution radar. But often the "target"
is ephemeral such as the leaping of fish or the surfacing
of a whale. These features make target identification
extremely difficult even under ideal sea conditions. When
the water surface is rough, background scatter is such
to make most radar techniques ineffectual for loca-
tion of fish populations.
2-2 Infra-red detection
Determination of water mass boundaii^^ and surface
temperature by infra-red procedures has been conducted
on an exploratory basis. Tidal streams can be located
with accuracy and temperatures of the surface layer
measured. It is not known whether turbulence caused
by fish schools swimming near the surface can be detected
by infra-red techniques. With suitable sensitivities,
theoretically, this anomaly would provide a means of
detecting and plotting movements of surface schooling
fishes.
2-3 Light
Penetration of light through the air-water interface is
slight, except in the blue-green region of the electro-
magnetic spectrum. Laser1 (light amplification by the
stimulated emission of radiation) offers a means of
determination whether a "window" exists in this spectral
region. Heretofore studies of electro-magnetic propaga-
tion through the air-water interface have been hampered
by the lack of pure single wavelength sources. Laser
techniques provide the narrow band widths necessary for
such a study. If such a "window" exists, some theoretical
studies imply that the use of Laser beams would allow
reconnaissance of the sea surface to depths of 100 to
200 m, while others forecast much greater ranges.
Aerial survey techniques from manned or unmanned
vehicles would permit the plotting of aquatic resources
in both space and time at much lower cost than is pos-
sible today.
1 A device which produces light as a pure coherent wave in a
highly columnated pencil-like beam.
2-4 Underwater detection systems
Subsurface methods of detecting biological resources
fall into two general categories: sound techniques and
non-sound methods (the latter being facetiously referred
to as "unsound" by non-acoustic workers). Both methods
can be employed in the active or passive sense. The tech-
nological development of sonar has apparently reached an
asymptote and it appears unlikely that any major break-
through will permit detection of biological populations
at ranges of greater than 15 nautical miles. Pulse coding
and doppler techniques allow some refinements ; for exam-
ple, movement of fishes can be detected both as to direction
and speed through the use of doppler sonar. Range of
reception (passive) of noises produced by animals is
dependent upon the frequency of the production and
will probably result in a range limit of the aforementioned
magnitude. The only major development likely in sonic
fields is for high-resolution short-ranged equipment
which will allow identification of targets with relative
ease once the population is detected.
Implied along with fish detection is the all important
item of identification. The range at which this is possible
is considerably smaller than the range of detection. Biolo-
gical and physical data on the patterns (in three dimen-
sions) of fish school shapes should greatly facilitate
remote identification. Just as the field ornithologist can
recognise, within a given number of types, the species of
bird by the precise manner in which it flies, so fish schools
also exemplify patterns in swimming and schooling. In the
few instances where school pattern has been studied in
sufficient detail, evidence for distinguishing species is
promising. Remote techniques of identifying patterns are
within the limits of present-day technology. An unman-
ned active sonar interrogator (high speed submersible or
fixed buoy) could compare the pattern of a passing fish
school with an "electronic library" of likely species types
stored in its memory. Identification of sound producing
fishes by the nature of sound characteristics is likewise
feasible.
Non-sound methods of location and identification of
biological resources are still largely speculative. A
possible visual means of searching surface waters has
been referred to — Laser. There is biological evidence for
believing that fish can detect extremely small quantities
of substances in their environment. Much more research
is required but, if the basis for these olfactory sensory
patterns can be detected, it would be technically possible
to develop recording gaseous spectrophotometers for
tracking organic odours in the sea. With the accrual of
information on fish odour, it would be possible to iden-
tify fish schools by spectrophotometric means. An
obvious ramification would be the dispersal of a dis-
tasteful odour to force or guide fish along a given path.
2-5 Artificial logs
A possible scheme for bringing about aggregations of
some of the larger pelagic species, such as tuna, would
be the use of artificial log stations (Fig. 1). It is well known
that schools of tuna are often found associated with
logs or other floating debris.
Fig. 1. An artificial log both attracts and detects schools of pelagic fish
and relays to a catcher vessel information on concentration offish in
vicinity of the log.
The artificial log could have a built-in detection
system, either active or passive, which automatically
would signal catcher vessels when concentrations of
fish were near. The detection and communication unit
would be capable of indicating not only the presence
but also the size of fish schools. By using a number of
such logs the opportunity of locating and capturing tuna
schools would be increased. The logs, constructed of
lightweight durable plastics, would be easily retrievable
by the catcher vessel.
3. HARVEST
Most existing world fisheries rely on environmental
systems or inherent biological characteristics of fish
to provide concentrations or aggregations necessary for
fishing. Thus the majority of fisheries are in restricted
oceanographic areas where relatively high productivity has
provided the environment suitable for sustaining concen-
trations of fishes. If man is to realise the full protein
harvest from the seas, he must devise methods to utilise
the great diversity of forms throughout a large geographic
segment of the oceans. Many of the latent resources may
not form natural aggregations but may be widely dis-
persed throughout the seas. Maximising the harvest
of the oceans thus confronts man with several alterna-
tives: (a) he must devise extremely efficient straining
systems, or (b) he must devise artificial methods which
will cause fish to aggregate where they can be easily
captured.
3-1 Modification of conventional gears
Within the realm of contemporary fishing gear, more
effective harvest of mid-depth forms has been progressing
by developments in midwater trawling techniques.
Current investigations into improved electrical conduc-
ting cables could provide fishermen with the means of
monitoring the rates of fish capture in either bottom or
midwater trawls, and through shocking systems to prevent
fish escaping from nets after once entered.
585
Within the realm of existing technology, it is feasible
to consider motorised units to replace otter boards and
other net-spreading devices which depend upon hydro-
dynamic characteristics for operation (Fig. 2). The
powered unit to replace the conventional otter boards
could control the depth of the midwater trawl and its
gape as well as the attitude of the net.
Fig. 2. Conventional offer trawlboards have been replaced with powered
spreading devices. Cables with electric conductors relay information to
ship's bridge concerning the concentrations offish in front of the trawl,
the behaviour of the gear on the bottom, and quantities offish entering
the mouth of the net. A remotely-controlled midwater trawl is deployed
from the catcher vessel.
Research currently devoted to guiding unmanned
submersibles may in future allow development of remote-
controlled trawls. Such trawls could be directed from
aboard ship and would be of particular value in exploiting
herrings, sardines and other densely schooling fish.
The motorised trawl perhaps would provide the straining
capability which may one day be needed to harvest the
lower trophic forms (zooplankton, etc.).
A development likely to occur soon is remote controlled
motorised submersibles which could be employed for
aiding in setting and closing of large purse seines or
gillnets. Military needs are likely to control the rate
at which manned underwater vehicles will be developed.
This is primarily because of the great expense in develop-
ing such a chamber with suitable safety factors. At
least one firm has stated, however, that once the design
was frozen, the construction could be completed in
about a year. Their use would accelerate and parallel
development of unmanned vehicles.
3*2 Air bubbles and chemical curtains
A barrier to fish movement which shows some promise
is the bubble curtain. This technique involves the pump-
ing of air through a hose laid on the sea bottom and
the resulting bubbles form a barrier to the movement of
some species. The introduction of a water soluble
chemical into the bubble curtain has been proposed.
These chemicals would serve to enhance the effectiveness
of the barrier.
586
3*3 Odours, light and sound
Perhaps some of the most interesting by-product
developments of space and military research which
may have application to fish capture are those previously
mentioned as techniques for fish detection (light and
odours). The possibility that fish detect their prey or
predators through olfactory stimuli and thus track down
prey or avoid predators, presents man with the opportun-
ity of duplicating the odours which either attract or repel
fish. Possible artificial aggregation can be achieved by
introducing a scent or trail which duplicates that of prey.
Fishing vessels could then proceed to areas where the
artificial odours had concentrated schools of tuna, herring
mackerels, or other species. Likewise, odours may be
used to repel and thus direct certain fish to areas where
they may be captured. Aircraft or underwater vehicles
could lay out systematic patterns (Fig. 3) of odour-
producing chemicals for attracting particular species.
Fig. 3. Helicopter deploys chemicals which concentrate fish. The
catcher vessel will subsequently proceed to the area of concentration
and harvest schools of pelagic fish.
A similar approach might either attract or herd fish
by reproducing the sounds of prey or predators. The use
of electricity or electrotaxic methods for capturing fish
is definitely feasible for emptying nets or for bringing
fish aboard a catcher vessel. However, because of power
losses imposed by physical limitations, we do not believe
these methods will attract fish from any great distances.
Use of electrical fields, however, in conjunction with
light, sound, or chemical stimuli to help aggregate and
lead fish is likely. All of these means are dependent on
determining the basic biological and behaviour charac-
teristics of various species to these various stimuli.
3-4 Underwater harvest
Not to be overshadowed by advances in fishery is the
development of techniques for studying the environment.
Our French colleagues have already shown the real
possibility of establishing research colonies at depths for
extended periods. It is entirely conceivable that, in the
distant future, some types of harvest and production
could be accomplished at fixed locations under the sea
and the end produce moved by freight submarines to
consumer ports.
4. MODIFICATION OF THE ENVIRONMENT
When meteorological and oceanographic models have
been tested and integrated, it is probable that the en-
vironment can be modified, at least on a limited scale.
Certainly weather conditions will be changeable, and
even a small change projected over several months can
have an influence on the aquatic environments. Small
term shifts and currents could be used to direct movement
of fishes to more desirable or accessible locations.
Calculations have been performed which suggest the
installation of a nuclear reactor (Fig. 4) at the bottom of
the sea could bring about thermal upwelling on a limited
basis. Where nutrient production is critical, for example
in a nursery ground or area, such a scheme might find
utility. On a broader range the possibilities are endless
but it is hoped that such manipulations of the future
will take into consideration the fact that, while it might
be possible to modify the environment, we should
assess the change critically as the same techniques might
not readily return the environment to its near initial state.
The future of fish culture looks brighter. Fertilising
and farming the seas is not remote when considered in
Fig. 4. Nuclear reactor on ocean floor provides heal necessary to bring
about upwelling process. The deeper waters (rich in nutrients) provide
continued high productivity when reaching the upper photic zone.
terms of chemical fertilisation of restricted access areas
so as to provide nutrients for essential food items.
Studies of such artificial enrichment of lagoons and other
relatively confined bodies of water capable of handling
large numbers of fishes have indicated this to be com-
mercially feasible.
5. COMMUNICATIONS
Weather data now being gathered from satellites (and
soon to be obtained from a net of meteorological buoys
established on the ocean surface) will allow the develop-
ment of more accurate and longer range meteorological
predictions. The forecasting of the events which bring
about El Nino conditions and prolonged rough-sea
states will allow better deployment of fishing fleets up
to three months in advance. Surface communication
and navigation through satellites could provide for the
rapid transmission of fishery intelligence on an auto-
matic basis. These same developments will allow the
preparation of more accurate charts. Associated navi-
gational schemes (available today for limited areas)
will allow fishing fleets to operate in waters considered
inaccessible by contemporary standards. Fishable
grounds will be pinpointed with accuracy and auto-
pilot vessel operation will allow large fishing vessels
to return to trawlable grounds within rough territory
with repetitive accuracy in all types of weather.
However, there are problems "of space" in this area.
Available transmission frequencies are rapidly being
exhausted for other scientific and commercial purposes.
It behoves the authorities of all nations to consider
suitable transmission frequencies for the fishing industry.
6. MATERIALS
The demands which space vehicles have made on indus-
tries for development of lightweight, high-strength
metals will certainly provide side-benefits to the fishing
industry of the future. Pure and alloyed light metals
such as aluminum, titanium and beryllium, may well
offer construction materials for vessels, vessel machinery
and cables, which should decrease the deadload and
consequently increase the payload of fishing craft.
Aluminum in fishing craft is already widely accepted
and titanium is currently commercially available.
Such materials are not always as reliable as their pre-
cursors; but only use will provide such data. Perhaps
the most challenging of the lightweight metals is beryllium.
This material has a high weight to strength ratio, with a
density of only 1-8 g/cm3. This may be compared with
iron which has a density of 7 g/cm3, titanium with
4*5 g/cm3, and aluminum which has a density of 2*7 g/cm3.
Use of beryllium is currently limited because of high
cost and difficulties in fabrication. Small particles of
this element (chips, etc.) have proven toxic to industrial
workers. It seems likely that these problems will be
resolved and satisfactory systems can be developed
for processing and working these materials.
Engineering technology has already demonstrated
that use of fine filament wires greatly increases the
587
strength of a material per cross-sectional area. One can
therefore anticipate the development of a cable composed
of fine filament beryllium which would have extremely
low weight and high strength. ' Such a development
would reduce the required diameter of cable needed and
greatly reduce the weight of comparable cable used in
deep-sea trawling.
In plastics, float materials are already available which
will withstand great depths and provide considerable
buoyancy. If materials as mentioned should be avail-
able commercially in the next several years, they would
overcome the problems involved in deep-sea operation.
The use of both plastics and lightweight metals in vessel
construction seems a certainty.
7. PROPULSION
We look with some anticipation to the development of
smaller and lighter weight propulsion units. Gas tur-
bines, which are currently being tested aboard smaller
naval craft, could find wide use in fishing industries
when fuel consumption and reliability problems are
solved. Although there is considerable debate as to
when small atomic units might be available for fishing
craft, there seems little doubt that technically it would
be feasible to employ such units in larger distant-water
factory or support vessels. Within the next several deca-
des, smaller atomic units could well be available for
smaller fishing craft. It is even conceivable that other
sources of energy might be employed which would
allow reduction in current space devoted to engine-
rooms while increasing reliability.
8. PROCESSING OF FISHERY PRODUCTS
Revolutionary innovations in preserving and processing
seafood products undoubtedly will occur within the
next few decades. Two of the most exciting preservation
developments expected to occur are irradiation-pas-
teurisation and freeze-drying.
8*1 Irradiation and pasteurisation
The normal storage life of iced fish and shellfish is now
from one to two weeks. By pasteurising seafood by
irradiation prior to placing it in ice, the storage life
probably can be extended to a month or longer. Within
30 years irradiation-pasteurisation should be in general
use for those products which can be brought to central
irradiation plants. By then experimental mobile irradia-
tors also should be available. Eventual adaptation of
mobile irradiators for use aboard motherships or even
larger catcher vessels would reduce (but not replace)
dependence upon freezing catches at sea when frequent-
ing grounds distant from port. Irradiation-pasteurisation
also will make seafoods more available to people living
in inland areas where cold storage facilities are lacking.
8*2 Freeze-drying
Freeze-dried seafoods should be commonplace within
the next 30 years. This process, which removes water
from frozen foods at relatively low temperatures,
588
provides a superior dried product. By adding water
to the dried product it is restored to an essentially fresh
state in less than one minute. Species processed by
this method will necessarily be those with a relatively
low oil content. Shellfish, such as shrimp and oysters,
would be particularly suited to freeze-drying. For some
species, such as shrimp, freeze-drying may in part
replace canning. Advantages of freeze-dried over canned
products include greater resemblance to the fresh items,
especially with regard to texture, and greater convenience
for use. Once opened, canned goods have to be used
within a fairly short time. By contrast, water could be
added to all or any portion of freeze-dried seafood
upon removal from its container. A disadvantage of
freeze-dried seafood would be its somewhat shorter
storage life than canned products.
8*3 Fish protein concentrates
Hope for more than one-half of the world's population
who suffer from an inadequate diet is offered by current
research to develop a satisfactory fish protein concen-
trate (fish flour). Within the next few years technical
processing problems should be solved which will result
in the large-scale manufacture of fish protein concen-
trates suitable for world-wide incorporation into human
diets. Many species of fish now discarded at sea for
lack of markets could be converted into fish protein
concentrate. This would, of course, provide a tremen-
dous stimulus to the fishing industry.
9. FISHING IN THE FUTURE
A fictional picture of fishing in the future might run
along the following lines:
A net of unmanned buoys has been established for
several years in the sea and the patterns of occurrence
and distribution of natural resources have been deter-
mined and plotted (Fig. 5). The buoys are interrogated
at regular intervals through satellite telemetering and
from their surface transmitters by pulse-coded sonic
means to instrument heads at various depths in the sea.
Transmission redundancy is reduced to a minimum as
only points of parameter change are telemetered. As
the data come in to "hydro-central" (Fig. 6), computers
reduce the mass of informational bits to contoured
plots of biological oceanographic and meteorological
parameters. By facsimile techniques, these data sum-
maries are transmitted to the research laboratories and
fishing centres of the world. When a biological parameter
anomaly occurs, the nearest buoy would automatically
be instructed to assess the nature of the instance with
high-resolution sonar and auto-spectrophotometric
methods. These data would be transmitted back to
hydro-central for computer and human interpretation.
The movements of the identified resources would be
plotted.
In some instances it might be necessary to verify the
nature of the resource or an anomaly by an on-the-spot
check using aircraft perhaps equipped with Laser-
scopes or hydrofoil research craft equipped with high-
speed self-propelled submersible television vehicles.
Short Hydro-ctntrol
Stoflon
Fig, 5. A net of unmanned huoys assesses the distribution of natural
resources in the sea. Information collected concerning water properties,
which are important factors in fish distribution, is relayed via a satellite
to a shore station (" hydro-central^).
Depending on the species, the main fishing fleet could
be deployed into the path of the fish, or conversely,
suitable deterrents could be placed in the sea to guide
the fish to the catcher. Aircraft could disperse the neces-
sary chemical pellets to olfactorily guide the fish, or
remote-controlled underwater vehicles would produce
the necessary electrical -sonic or bubble barrier to
perform the same function. Depending on the depth of
harvest, catches would be performed by catcher boats
assigned to permanently anchored factory ships or by
automated underwater vehicles operated from ship or
shore stations.
Surveillance of the main plotting board in hydro-central
would allow detection of weather conditions and pre-
cursors of El Nino type shifts in advance. Similarly,
areas of high or low basic nutrient production could be
watched and, with broad environmental limits, spawning
populations deflected accordingly.
Although this picture of the future may appear revo-
lutionary to the point of distastefulness, one can always
find refuge in the fact that the conservative nature of
fishermen will tend to stabilise the road to change and
progress. That such changes will eventually occur seems
undebatable. There appears to be little difference
whether the developments arc sponsored by individual
firms and companies or as part of the national economy.
The increasing population growth is already far out-
distancing our protein production. Clearly, the social
and economic factors must parallel technological
developments; otherwise, local over-fishing, market
development, over-regulation and duplication of effort will
bring about the destruction of the very goal being sought.
continued on page 590
Fig. 6. A data collection centre ("hydro-central") analyses incoming information and quickly transmits data to research laboratories and fishing craft
concerning weather, water properties, and indicated distribution patterns of resources.
589
Automatic Data Processing and Computer use
in Fisheries
AMraet
This paper indicates some of the possible applications of the high
speed computer and associated elements of automatic data proces-
sing (ADP) to the fishing industry. The author feels that the fishing
industry cannot afford to overlook the advantages and possibilities
of the incorporation of effective ADP techniques into its operations
any more than they could disregard the advantages of modem
engines, mechanical refrigeration, electronic instruments and other
modern assistance both afloat and ashore.
La communication indique quelques applications possibles des
machines a calculer electronique et des elements associes de
traitement automatique de donnees (ADP), a 1'industric des peches.
L'auteur pense que cette Industrie ne peut ignorer les avantages et
possibility de 1'utilisation de techniques ADP, pas plus qu'elle ne
peut ignorer les avantages des machines modernes de refrigeration
mecanique, des instruments electroniques et d'autres aides modernes
a terre ou en mer.
Extracto
Indica la ponencia las posibles aplicaciones de las maquinas
calculadoras, de gran velocidad y elcmentos asociados de
preparaci6n automatica de datos, a la industria pesquera.
El autor cree que la industria pesquera no puede hacer caso omiso
de las ventajas y posibilidades del cmpleo de tecnicas eficaces de
preparaci6n automatica de datos en us actividades, como tampoco
podria hacerlo de las ventagas de los motores, refrigeracibn
mccanica, aparatos clectronicos y otros auxiliares modernos en
tierra y a bordo.
WHEN a few years ago, the first sequence-controlled
computer was built, it was regarded more as a
curiosity than as a practical tool. And when the first
electronic digital computer was installed in the United
States Bureau of the Census, the estimated potential
market for the entire United States of America was
set at 17 machines; yet, in June 1962, the number of
computers in the United States was 9,500 and growing
rapidly.
Automatic data processing cannot supply information,
but can only process it.
Automatic data processing involves the use of simpler
equipment for handling bulk data than computers,
but it is to the computer that we must turn for solution
to complex problems and to really obtain full value
from accumulated records.
by
Benjamin F. Leeper
Sperry Rand Corporation,
Baton Rouge
The number of tasks to which a computer could be
put in fishery research and practice is infinite, but it
must be remembered that the basic requirement is
reliable data which can be fed into the machine. One
very simple example, however, may be given of possible
use. In any endeavour, the man who knows what his
chances are of winning is the man who will most likely
succeed. If adequate records of voyages and catches
together with all seasonal and weather data could be
fed into a computer, it would undoubtedly increase the
chances of a skipper securing good catches.
Whereas by hand one could only work with small
samples and a limited number of variables (species,
areas, seasons), with a computer such factors as bottom
type, dragging with or against the current, sea state, net
size and type, and others can be added to define the
limits more concisely. Still the operation will be a
relatively simple one for a computer.
Basic statistics have become an important part of the
computer load in most installations, and regression,
correlation, and matrix algebra programmes are im-
portant parts of the library of routines.
Fortunately many of the more useful of these methods
are fairly standard and are available for most computers.
Any researcher would shudder at having great masses
of material flooded to him for evacuation but scanners
have been developed that can read several types of
continuous charts and reduce the readings to digital
values at predetermined increments. It is not impossible
that a computer might be programmed to abstract
continued from page 589
If the fishing industry is to provide the raw materials
required for large-scale production of ocean resources,
rapid progress will be required in harvest and processing
methods. Advances in fishery harvest are not limited
only to existing technology but by major gaps in basic
biology. Much more effort must be devoted to gathering
fundamental knowledge on fish of all species before
improvement can be had in harvesting marine resources.
Acknowledgments
The authors wish to acknowledge the following organisations for
suggestions and information received from members of their
staffs:
590
Boeing Company
Fisheries Research Board of Canada
General Motors Corporation Defense Research Laboratories
Inter- American Tropical Tuna Commission
Marine Construction and Design Company
National Aeronautics and Space Administration
Oceanic Instruments, Inc.
Scripps Institution of Oceanography
Stanford University
University of British Columbia
University of Washington
U.S. Bureau of Commercial Fisheries
U.S. Navy Electronics Laboratory
U.S. Weather Bureau
Van Camp Sea Food Company
individual voyage records with a minimum of hand
operation.
There can be little doubt that as our students of basic
science become familiar with the ability to make rapid
determinations between the effects of large numbers of
variables and to filter out promising clues without being
faced with months and years of hand labour to have
their ideas validated, or discarded, recognisable factors
of cause and effect will provide inestimable benefits,
and it is doubtful that there will be a more fruitful
media for these studies than is to be found in the sea.
Discussion on
Advance in Science
and the Future
Mr. D. L. Alverson (U.S.A.) Rapporteur: Mr. Leeper's
paper reviews some of the general applications of automatic
data processing to fisheries and provides information con-
cerning assessment and preparation of information to be
presented to an automatic data processing system. In essence,
ADP is the application of rather simple equipment to handling
and recording bulk data which can be subsequently summari-
sed and complicated problems resolved by computer analysis.
Automatic data processing has, of course, been applied to a
variety of purposes in both the scientific and business fields.
Mr. Leeper notes, in introducing one of science's brightest
proteges (ADP) to one of the world's oldest industries, that
the most brilliant and beneficial accomplishments must come
from those concerned with the harvesting and processing of
ocean resources. It is noted that ADP cannot supply informa-
tion, but coupled with technical knowledge and mechanisation,
can improve efficiency. Mr. Leeper refers here to efficiency
of processing and evaluating data concerned with fisheries
science and/or the business end of fisheries. The author points
out that in fisheries operations both afloat and ashore we may
well borrow from the experiences of other industries in adopt-
ing ADP for the processing and recording of data. It is sug-
gested that ADP may be suitably adopted for basic research
activities, and that computers have been developed that can
read several types of continuous charts and reduce them to
digital values and predetermined increments. Thus, the com-
puters may be programmed to extract individual voyage
records with a minimum of hand operations.
A case for increasing the fishing efficiency is provided by
proposing ADP, as a method for programming statistical
opportunity of probability of catch. His analogy is this: "In
any endeavour, the man who knows what his chances are of
winning is the man who will most likely succeed. Would it not
be helpful if we could say, go to a specified area in a specified
season and obtain from (for instance) 1,500 to 2,000 tons
of cod in a certain amount of fishing time? By accumulating
careful records over the year, converting them to computer
language, and programming a very simple statistical treatment,
we can determine with reasonable surety the most probable
catch to be derived from an area." A variety of other uses for
automatic data processing are proposed, including statistical
analysis of scientific data, forecasting the distribution and
abundance of fish patterns, market analysis, routine business
operations, and information retrieval. Can we afford to over-
look the advantages and possibilities of effective ADP tech-
niques any more than we might disregard the advantages of
diesel engines, radar, sonar and other modern aids? At the
same time each potential user must examine and weigh
carefully the advantages of ADP before embarking on it.
Concerning the paper on the future aspect by himself and
Mr. Wilimovsky, this, although some might have considered
it fanciful and imaginative, was meant to be constructive
in its opening-up prospects of securing information through
the very advanced fields of space technology and military
research. He had had the opportunity of visiting all the
organisations mentioned at the back of this paper, and talking
to prominent people in all branches of the industry. A lot
of thought by many important scientists had gone into the
paper and all the points mentioned were definitely feasible
and possible of attainment provided the finance for achieving
them was provided. As in industry, we cannot afford to ignore
the possibilities open to us by the avenues mentioned. Having
regard to the population explosion and the future world food
need we will be required to move ahead and develop new
techniques for securing greater resources from the ocean. We
will have to discover means of utilising stocks and resources
that are not now being utilised and to do so we must evolve
new techniques to achieve our purpose. The purpose of the
paper was to start people thinking along those lines.
Mr. D. J. Doust (U.K.): As a naval architect I would like
to speak about some of the applications of computers in
our field which is allied to your own. With the increasing
numbers of second generation computers able to tackle large-
scale analytical studies many previously insoluble problems
in fisheries research are now within sight of solution. For
example, in fishing vessel design we are now able to determine
the optimum parameters of the hull form to specified design
conditions using high-speed digital electronic computers to
evaluate the performance. The results of all previous tank
tests conducted under laboratory trial conditions are analysed
using multilinear regression techniques which are particularly
suitable to some types of computer. (Incidentally some forms
of computer were very much not suited to this type of research
study.) The Fishing Boat section of FAO was currently enga-
ged on large-scale analysis of this type which should prove
of great value to owners and builders of smaller fishing vessels.
The fabrication of the ships' hull and of internal structures is
clearly possible for defining the lines of the vessel in mathe-
matical terms and using computer controlled machines to cut
the various plates and components which go to make up the
final hull form. These applications are, of course, particularly
valuable in the production of fishing vessels which are so
often built in considerable numbers to the same design per-
mitting the cost of computer technique per vessel to be
considerably reduced. Many types of management problem
in the fishing industry and in many others had already been
solved using the mathematical aids available in operational
research most of which rely on a mechanical solution using
the aid of the computer. There is every indication that tech-
niques of this sort will apply in an ever-increasing role in the
fishing industry since it is evident that the fast-moving develop-
ments of the day had in some cases overtaken the previous
lack of experience to such an extent that decision taking has
been a somewhat risky business. In those circumstances,
decisions are, in fact, deferred until the issues become clearer.
This is where, 1 think,-we have a big opportunity of using the
computer to help us by studying our past background of
experience to foretell the future. Typical examples of the need
for decision taking are provided by some of the subjects such
591
as the most economic size of fishing vessel for particular
fishing areas, whether to adopt a large mothership supplied
by a number of small catchers or to increase the radius of
action and size of the distant-water trawler fleet, to select the
most profitable speed of operation of a fully refrigerated
trawler and many other associated economic and technical
problems, can all be tackled by these techniques. In the fields
of the gear technologist with the hydrodynamic research now
being undertaken, both model and full-scale tests, so well
illustrated in Mr. Crewe's paper where accurate records are
becoming available, lend themselves to better methods of
performance. A good example here is provided by the orienta-
tion of the trawl boards and net surfaces when the vessel is
engaged in trawling operations by formulating the general
equation of motion of the various components, hydrodynamic
and gravity forces in the overall term, and, knowing the
external forces acting on the deck which are transmitted
back to the towing points of the vessel, it should not be im-
possible to derive the net configurations for particular towing
speeds, warp lengths and mesh sizes under seagoing condi-
tions. In such cases where a particular problem of fishery
research can be expressed in equational form and where
adequate precision of input data can be supplied, the com-
puter can be used to determine the required solution without
difficulty.
Dr. Cole (U.K.): I do not believe it is just sufficient to
think about long-range lines of work that might result finally
in very substantial improvement in the amount of food we
can get from the sea. It is necessary to act and to begin,
wherever possible, work of a forward-looking nature. Old-
established fishing institutions should be able to employ a
few men along these lines. My own special interest is the
augmentation of the stocks of fish. In that line we have had
three men at work on the artificial production of plaice and
we propose to extend this as soon as possible to cover com-
mercially valuable species of fish such as crustaceans and
molluscs. That has nothing to do with gear research, but
Mr. Alverson has thrown enough ideas into the pool to enable
communities that had a substantial number of people engaged
in fishery research, to select topics from which they can make
a start. Techniques of aggregation are going to be of very
great importance in the future. They are important not only
in the case of grounds that cannot be commercially fished but
also in heavily fished areas because if we can improve the
catch per unit effort by concentrating the fish closer together
so that they can be swept up at lesser cost we will be doing a
very successful job.
I would also like to refer to Dr. von Brandt's paper on the
organisation of gear institutes and research as a whole. He
has very courageously put his ideas on paper but I am not
at all sure that gear research and technology should be
organised in separate institutions apart from biologists and
so on. I do not like the idea of making a biologist into a
hydrodynamist. I much prefer a team of full, qualified
specialists. Why can't we have them all working together in
some institution and in that way we will make sounder and
more rapid progress.
Dr. W. M. Chapman (U.S.A.): I would like to extend some
remarks by Dr. Cole and describe some of the things that are
going on at the present time which I am sure will have quite a
startling effect on the fishing industry in the coming years. I
think that perhaps people dealing with fishery research in the
ocean are often a little too close to the work they are doing to
see how well the general subject is developing. One of the
principal objects of fishery research is the production of events
in the ocean and, in respect of the resources of the sea, of
592
events of such a nature that the fisherman can take advantage
of the products of their work and reduce the cost per ton of
production. The basic problem of fishing is to strain the fish
out of the water. And in that connection aggregation is a
prime factor so that the fisherman can get a big catch in a
small amount of time. So one of the problems involved is the
aggregation of the fish. Useful work on this is already being
done. This is work which enables one to estimate and perhaps
predict the aggregative effects of various ocean parameters
so that the instrument can take advantage of them. We have
several of these factors of ocean conditions upon which some
definite progress is being made. One is that of the surface
temperatures. This is not, at the moment, of tremendous im-
portance to trawlermen but when the Lowestoft people
get their thermograph working on trawl boards and footropes
the trawlers will find out that the same thing I am referring
to, does affect them as well as the tuna people in the Eastern
Pacific. We are now regularly charting the surface tempera-
tures of the Eastern North Pacific and publishing them at
intervals quite regularly. This is being done by the Bureau
of Commercial Fisheries, Los Angeles. These temperatures
are compiled from the records that the merchant ship send
in for the weather bureaus. These are all automated, tapped
out by machines, and punched onto cards according to a
programme. Once every month a bundle of cards is taken
down to the computer and in the matter of an hour or two
the man is back with the information. One afternoon's work
enables us to split not only the surface temperatures for the
whole of the North-eastern Pacific (as for the middle of that
period) onto an average but also chart the deviations of the
isotherms over that period from the same situation a year
previously and, in the second place, to relate that to a 1 5-year
mean. This has become of enormous importance to the tuna
fishermen of the Eastern Pacific — in particular to the albacore
catchers and also of other tropical tuna as well.
In Tokai University in Japan they are doing this on a differ-
ent basis with respect to the North-west Pacific Ocean and are
extending their work in respect of some parameters to cover
most of the world's ocean. At any one period of time, they
have about 600 tuna fishing vessels scattered over the tropical
and sub-tropical oceans, most of whom are in touch with the
station at Tokai daily or at least weekly. They relay the meas-
urements of the parameters they are concerned with to those
stations and they are correlated in Japan and are relayed out
to the ships at five- or ten-day intervals. This relaying is now
being done by facsimile television and these facsimile methods
have been worked out quite rapidly. A year ago the dispatch
did not reach more than 500 miles from Tokai but when one
of their big vessels was in San Diego recently she was receiving
facsimile reports from Tokyo quite regularly and that is a
distance of a few thousand miles. Surface temperatures are
also being handled in the North Atlantic by the U.S.A.
hydrographic bureau and the same charting is being done now
in the Indian Ocean area and the Intergovernmental Oceano-
graphic Commission now has under consideration two things
— first the compilation of surface temperature charts on a
world-wide basis, keeping in mind that some fishing countries
are now interested in all parts of the world ocean so that they
would like to know the position over the whole area. There
are certain phenomena that repeat themselves and a great deal
of use can be made of them. We will find in a period of just a
few years a widespread gathering of information from vessels
at sea. This will be correlated at short notice by computer
assistance and then immediately relayed out to the vessels at
sea for their use. This will require a great deal of information
to be gathered and some understanding from the fishermen at
sea. At present the information mainly comes from commercial
merchant vessels but as they do not cover all areas, the co-
operation of the fishermen themselves on their vessels at sea
will be required. Fishermen will have to be convinced that it is
money in their pockets to co-operate. There are two ways of
getting this problem solved in respect of regular collaborative
distribution. Once you have the charts of the parameters
produced by the computer they can be distributed by the
facsimile methods such as Japan is using, but another method
may be to shoot one of these charts up to one of the communi-
cation satellites at regular time intervals so that it can be
received on board vessels on television sets. This is being
handled and programmed by people now in an experimental
way. It may prove to be quite useful to have such information
easily available to men at sea.
On the computer side my company, the Van Camp Organi-
sation which operates in nine countries, sells in 38, harvests
800,000 tons of fish of various kinds, now makes use of com-
puters. We bought one five years ago. Two years later it was
too small for our use. We bought another one, a much larger
one and more complicated. It was supposed to satisfy our
needs for 1 5 years. But at the present time we are now building
a house to contain a bigger one and have hired two more
biologists to try and interpret what the computer is doing.
The computer technique as applied to the fishing business as
a whole is being utilised by our hard-nosed board of directors.
Mr. Alverson, summing up, added the comment that the
salmon fishermen in the Pacific north-west were also keenly
interested in watching the temperatures data distributed by
the Bureau of Commercial Fisheries as well as the tuna men.
A New Fish Trap Used
in Philippine Waters
Continued from page 286.
The tables mentioned are given on this and the
following page. Table I, because of its size is on the
next page.
Table II appears below.
Table III— dealing with the ropes and other accessories
is now conveniently summarised thus:
Primary float line (manila rope as are all the ropes),
diameter in cm 0.9, length 27.1 m, 70 floats tied at
3.5 cm distances.
Secondary float line, 1.5 cm, 27.10 m, where the lacing
line of selvage is hung.
Breastline 1.5 cm, 12,20 m, where the lacing line or
width is hung.
Primary leadline 0.9 cm, 22.60 m where the lacing line
of cotton selvage is hung.
Secondary leadline 1.5 cm, 22.60 m, lead weights are
tied at 35.5 cm distances.
Pursing rope, 1.3 cm, 50.0 m,. each end is tied to the
end of the primary leadline.
Bridle line, 0.9 cm, 0.50 m, 62 pieces, distributed along
primary leadline 45.5 cm apart.
Floats are of soft wood 4 cm in diameter, 7.6 m long,
70 pieces, equally distributed along the primary corkline
30.5 cm apart.
Sinkers, lead, 4 oz., 62 in number, equally distributed
along the primary leadline 35.5 cm apart.
Table //. Specification of the seine (sigin)
Materials
Sice of
Parts
Symbols
Kind Twine
nesties
Horizontal
Meshes Strip
Remarks
Number
stretched
deep
A
oo t ton
12
2.24
312
500 1
Het tings joined mesh
Bunt
twine
to mesh
B
ootton
9
2.54
300
300 1
twine
¥ings
C
ootton
9
2.54
573
800 1
lettings joined mesh
to mesh. Outer edges
D
it
9
2.54
573
800 1
of bunt and wings are
Joined to the selvages
S
E
H
36
5.72
7
466 1
(4 meshes of the
former to every mesh
E
F
H
36
5-72
7
407 1
of the latter) end
then laoed by a 26-
L
a
M
36
5-72
7
220 1
thread ootton twine
to the hanging line.
V
H
N
36
5-72
7
220 1
593
Table I. Specifications of the new hasang moderno operated at Kalibo, Capiz
Parts
Bamboo soreens
Hame
Symbols
Length
in
metres
No.
of
posts
So.
of
soreens
Site of
soreens
in metres
depth x
length
in metres . *J?/? Remarks
width z «plits
L
E
A
P
B
P
AB1
AB2
AB3
AB4
AB5
AB6
AB7
30
35
35
35
35
35
35
6
7
7
7
7
7
7
12
14
14
14
14
14
14
4.2
4.0
3.8
3.5
3.2
3.2
3.0
x5
x5
x5
x 5
x5
x 5
0.02 z 4.2
0.02 z 4.0
0.02 z 3.8
0.02 z 3.5
0.02 z 3.2
0.02 z 3.0
0.02 z 3.0
0.12
0.12
0.12
0.12
0.12
0.12
0.12
Except for
Al, two rows
of bamboo
screens are
set on the
framework
overlapping
eaoh other
by 10 om
Total «
240
28
¥
DC
29
6
12
4.2
x5
0.013
z
4.2
0.83
Two rows of
DB
15
3
6
4-5
x 5
0.013
z
4-5
0.83
bamboo soreens
I
TZ
29
6
12
4.2
x'5
0.013
z
4.2
0.83
are set on
V
XT
15
3
6
4-5
x5
0.013
z
4.5
0.83
the framework
JI
28.50
6
11
4.7
x 5
0.013
z
4.7
0.12
overlapping
0
su
14
3
6
5-4
x5
0.012
z
5.4
0.42
eaoh other
s
uv
28.50
6
11
4.7
x+5
0,013
z
4.7
0.12
by 10 em
TO
14
3
6
5.4
x5
0.012
z
5.4
0.42
Total «
171
40
70
Pore-
DP
12.50
4
4
4.5
z 6.25
0.013
z
4.5
0.83
oham—
fO
30.25
10
10
4«9
z 6.15
0.013
z
4-9
0.83
ber
T¥
12.50
4
4
4-5
z 6.25
0.013
z
4-5
0.83
H n
¥0'
30.25
10
10
4.9
z 6.15
0.013
z
4.9
0.83
To tali
85.50
28
28
Semi-
IE
10
3
4
6
x 5
0.012
z
6
0.42
oir-
KL
36.50
12
14
6
x5-30
0.012
z
6
0.42
oular
LS
4
2
2
6
x 4
0.012
z
6
0.01
M n
enclo-
HQ
4
2
2
6
x 4
0.012
z
6
0.01
sure
QD
46.50
15
18
6
x 5.30
0.021
z
6
0.42
Totals
98
34
440
Ter-
IX
2.60
2
3
6
x 2.50
0.012
z
6
0.01
minal
US
8.50
4
6
6
x4.30
0.012
z
6
0.01
pound
BO
3.50
2
3
6
x 3.70
0.012
z
6
0.01
n n
OP
8.50
4
6
6
x 4-30
0.012
z
6
0.01
PQ
2.80
2
3
6
.z 2.50
0.012
z
6
0.01
Total i
26.1
14
21
Gates
SB
6.50
and
BX
6.50
en-
KG
3.20
A bamboo bridge is
constructed above each
gate
and entrance.
trances or
3.20
00'
4-25
BS
0.50
594
DETAILED INDEX
595
596
Abrasion resistance, of monofilaments, 106
— of netting twines, 21-22
— of polyamides, 53
— of polypropylene, 52, 53, 56
— of synthetic twines, 52, 80, 116-118
Acoustic detection, of fish, 401 et seq., 419
— References, 404
Acoustical characteristics offish, 404 et seq.
Acoustical instruments for fish detection see
Echo Sounding; Sonar Equipment
Aerial surveys, for fish detection, 584
Aeroplanes, use of in fish detection, 419, 584
Air curtains, use of, 541-544, 586
— References, 544
Aluminium, use of in fishing craft, 589
Aluminium hydrofoil, use of in otter board
design, 245 et seq., 258
Amilan, catchability of, 56-57
— heat treatment effects, 72-73
— properties of, 55-56
— use of in gillnets, 73
Artificial logs, use of in fish detection, 585
Artificial stimuli see Mechanical stimuli
Asdic, guiding for purse seiners, 259
— installation in Icelandic purse seiners,
295, et seq.
— use of for netting submerged schools, 297
Automatic controls in wheelhouse, 338
et seq.
Automatic data processing, use of computer
in, 590-593
Automatic steering, 339, 340-342
Automatic trawling, 151, 159 et seq., 340,
346
Bait, dispersion of, 571-573
— fish behaviour to, 570-573
— for crab catching, 264
— for tuna catching, 437
— protection of, 292
Ballast capacity of sterntrawlers, 156
Beach seining, 356 et seq.
— construction, 356
— crew, 360
— gear, 359
— operation, 356-360
— winch, 357 et seq.
Beam trawling, 209 et seq.
— crew, 213
Beamnet, construction of, 211-212
Behaviour of fish, 253 et seq. 530-536,
537-540, 546-551, 551-556, 557-560,560-
562, 567-570, 580-582
— acoustical, 404 et. seq.
— escape, 539
— feeding habits, 558
— herding, 532
— herring, 254
— in horizontal distribution, 557-558
— in North Sea, 534
— in vertical distribution, 558
— psychological, 254-255
— recorded by echo sounding devices,
253 et seq., 407 et seq.. 539
— reflective, 407
— shrimp, 563-570
— burroughing mechanism of, 564-565
— day activity of, 564
— night activity of, 564
— swimming endurance, 535
— swimming force, 113
— swimming sound, 411 et seq.
— swimming speed, 241
— to air bubble curtain, 540-544
— to bait, 570-573
— to chum, 570-573
— to colour, 112, 530-532, 582
- to dark, 531, 533, 559
— to driftnets, 530 et seq.
— to electro fishing, 546-551, 551-556
- to fishing gear, 253 et seq., 491-492, 559
— to light, 255, 531-533, 558, 561
— to mechanical stimuli, 537-540, 559
— to mesh size, 189, 580
— to moving nets, 532-536
— to netting materials, 113, 389
— to salinity, 558
— to sound, 401 et seq., 561, 581-582
— to stationary nets, 530
— to temperature, 558
— to trawl warp, 539
— to trawls, 241, 537, 539, 547-548
— tuna, 383-385, 385-388, 560-562
— use of sector scanning in determination
of, 582
— use of sonar equipment in determination
of, 306, et seq., 582
— References, 536, 540, 544, 560, 562, 566,
570
— See also Fish Echo traces
Bending of netting twines, 19
— apparatus for, 19-20
Bending resistance of netting twine, 19
Bibliography see References
Biological tests on synthetic fibres, 27
Board pitch meter, 507
Board spread meter, 507-508
Bottom tests on trawl boards, 486-487
Bottom trawls, advantages of, 261
— damage caused by, 219
- design of, 166 et seq., 192, 204
— discussion on, 218-220
— fishing performance of, 61
— gear for, 204 et seq., 207
— Norwegian, 62
— use of echo sounding, in, 391
— use of synthetic fibres in, 61-62
— U.S.S.R. design and usage, 207
— Warp behaviour in, 166 et seq.
Braided twines, 4, 7
— testing, 12
- synthetic, 54, 115
Brailing operations in purse seines, 301-303
Breaking strength, definition, 12
— of monofilaments, 85, 1 14
— of netting twines, 14-15
— of polyamide twines, 52
— of iwlyethylene, 85
— of polypropylene twines, 52
— wet knot, 15
Breaking tenacity of monofilaments, 83
Bridles, 192
Bull trawling, 199 et seq.
Bull trawlnets, 424-426
Buoys, design and detection of, 270-271
— for crab pots, 269
Bursting strength, see Netting, strength of
Cabled yarn, 4
Cables, breaks in due to trawling, 219
— electrical conductive, 585
— handling of, 260
Camber see curvative effects
Catch composition, 188
Catch, histograms, 185, 188, 190
Catcher boats, communication with
Mothership, 429, 438
— equipment for, 429
Catching capacity of sterntrawlers, 159
Cathode ray oscilloscopes, 374, 392
Cells, net twine load, 511
— warp load, 503-505
Characteristics offish see Behaviour offish
Chemical resistance of netting twine, 27
Chumming, reactions of fish towards,
570-573
Clamps, design of in testing machines, 13-14
Classification of netting twine, 27
Coastal trawlers, 150
Cold reactions of netting materials, 13-14
Colour, reactions of fish to, 112, 530-532,
582
— usage of in trawlnets, 293
Comparative fishing* 182 et seq.
Computers, use of, 169, 344, 590-591
see also Electronic devices
Concentration of fishery products, 588
Conductive cables, electrical, 585
Continuous fibres, 3
Control panels and recorders, 502
Cord, definition of, 12
Corrals, crews of, 286
— traps, 279-282, 282-286
Cotton, comparison with synthetics, 76,
80, 115-118
— netting weight of, 46
— testing of, 11
Crab fishing vessels, bait requirements, 264
— crews for, 267
— design of, 264 et seq.
— equipment for, 431 et seq.
— hydraulic equipment for, 435
— longlining, 435
— tank requirements of, 268
— use of nylon in, 433
— use of vinylon in, 433
Crab pots, buoys for, 269
— design, 268-269
— handling, 264 et seq.
— hauling devices, 270
— hydraulic power for, 325
— gillnets for, 292, 433
— synthetic lines in, 269
Crew training in use of sonar instruments,
307 et stq.
Crew work, amount according to materials,
78
— in beach seines, 360
— in beam trawlers, 213
— in crab fishing vessels, 267
— in fish corrals, 286
— in stern trawlers, 155
Current effects on driftnets, 520
Current indicators, 520-521
Current measurement, 519
Current meter. 516
Curvature, effects of on trawl boards, 1 75
Damage to fish, by electro fishing, 556
— by synthetic fibres, 76-77, 109
— in Pelagic trawls, 247
— in stern trawling, 155-156
Damage to trawls, 187
Danish seine nets, monofilaments in, 107
— use of Ulstron in, 63
Danish seining, deck arrangement, 424
— gear specification, 424
— Japanese methods of, 424 et seq.
Dark, reactions of fish to, 531, 533, 539
Data collection, 589
— by computer, 590-591
Deck layout, in Danish seiners, 424
— in Icelandic purse seiners, 300-303
— in stern trawlers, 154-155
— trawls, 148-149
— Unigan arrangement, 155, 157, 160
— with hydraulic equipment 327, 352
Deck-type trawl instruments, 498-502, 522
— control panel and recorders, 502
— design, 498
— divergence meters, 500-501
— ship's speed log, 502
— warp declination meters, 501-502
— warp heel meter, 502
— warp load meters, 498-500
Dederon, 75
Deep-sea trawls, design of, 149
— suitability of monofilaments for, 107-108
Density, of fish in shoals, 557, 558
— of yarns, 5-6
Depth measurement net, 249 et seq.
— tuna longlines, 385-388
Depth meters, net, 249 et seq., 526]
597
— recording, 515
— for gillnets, 518
— with temperature, 515-516
Designation of netting twines, 6-7
Detection offish see Fish detection
Diameter of netting twines, measurement,
24-27
— of synthetic twines, measurement, 52,
113
Dimensions of netting, 34
Direction finders, in Icelandic purse seiners,
296
— radio, 2%
Discontinuous fibres, 3
Distant-water trawlers, echo sounding
devices in, 295 et seq.
— freezing of fish in, 438 et seq.
Divergence meters, 500-501
Doors, trawl see Trawl doors
Double-rig beam otter trawls, 210 et seq.
Drag, breakdown measurement, 524
— measurement of, 171, 178
— resistance of monofilaments, 107, 116-
117
— trawl measurement, 525
Draw ratio, 83
Drifting of purse seines, 312
Driftnet fishing, by mechanical means, 352-
355
— Japanese methods of, 428 et seq.
Driftnet shaker, 345
Driftnets, automatic handling of, 348 et seq.
-— construction of, 429
— current effects on, 520
— discussion on 291
— fish reaction to, 530 et seq.
— knotted and knotless, 94
— mechanisation of, 352 et seq.
— mesh arrangement, 77-78
— synthetics in, 57, 74, et seq., 291 , 429, 495
— usage in Pacific ocean, 287
Dropline fishing, hook size for, 290
— in Pacific Ocean ,291
Dry testing of nets, 1 1, 22
Dual frequency echo sounding devices,
388 et seq.
Durability of synthetic fibres, 80
Dyeability, of netting twines, 32
— of synthetic twines, 73
Dynamometers, for trawl warps, 516
— recording underwater, 516
— strength testing, 13-14
Echo level, measurement, 375
Echo ranger, for commercial fishing vessels,
396 et seq.
Echo signals, intensity of, 407
Echo sounding devices, 221, 234, 252, 254
et seq., 258, 306 et seq., 363 et seq., 375,
376 et seq., 396 et seq., 407 et seq., 413 et
seq*, 522
— development of, 240, 241
— dual frequency, 388 et seq.
— for dog-fish detection, 257
— for distant-water trawlers, 295 et seq.
— for herring detection, 255-256
— for longline operations, 385-388
— for mackerel detection, 257
— for midwater trawls, 249 et seq., 388 et
seq.
— for recording fish characteristics, 407 et
seq.
— for recording noise, 401
— for sprat detection, 256
— for tuna detection, 382 et seq.
— frequency of, 396
— German, 238
— high frequency, 262, 291
— horizontal, 407
— Humber fish detection system, 364 et seq
— in bottom trawls, 391
598
— in Icelandic herring purse seiners, 295
et seq.
— in midwater fishing, 369
— operation of, 367
— through ice, 415-417
— to determine bait dispersion 572
— to determine fish behaviour, 539
— to determine fish density, 558
— transistorised, 415
— References, 257, 365, 375, 388
see also Netzonde
Echo traces see Fish echo traces
Echograms see Fish echo traces
Eel traps, 277-279
Elasticity, calculation of, 17, 51
— of gillnets, 113
-— of monofilaments, 105, 113-114
— of netting materials, 17-18
— of synthetic twines, 53-54, 105
Electrical conductive cables, 585
Electrical fishing, 546-551, 551-556, 578-579
— compared with non-electro methods,
548-551
— damage to fish, 551
— effect on fish, 546-551, 551-556
— efficiency of, 550
— range of, 546
— results, of 546-547
— selectivity of, 580
— steering, 339, 340-342
— theory of, 551
Electrical stimuli, effect on shrimps, 566-570
see also Mechanical stimuli
Electro-pump fishing, 547, 552-555
Electronarcosis, occurence of, 551, 554, 555
Electronic devices, for fish detection, 367
et seq., 419
— for measuring spread, 167, 507
— for measuring warp length, 169
— References, 371
see also Sonar equipment
Electronic recording instruments see
Dynamometers
Electrotaxis, occurrence of, 551-555
Elongation of monofilaments, 83
Engines, automation of, 342-344
Escape of fish, 539
— from synthetic nets, 61, 76
— from wingtrawls, 182
Extensibility, comparison of different net-
ting materials, 1 7
— definition for netting twines, 16-17
— of synthetic twines, 51, 53
Factory trawlers, 148
Feeding habits of fish, 558
Fibres, continuous, 3
— discontinuous, 3
— terminology of, 3-4
see also netting twines; synthetic fibres;
twine; yarn; monofilaments
Filleting of fish, in trawlers, 438 et seq.
Filtering, 197
Fish behaviour see Behaviour of Fish
Fish counting, 375
Fish damage see Damage to Fish
Fish detection, 364-366, 376 et seq., 417 et
seq., 491, 584 et seq.
— by acoustical methods, 400 et seq., 419
— by aerial surveys, 584
— by asdic, 295
— by echo ranger, 396 et seq.
— by echo sounding devices, 257, 363, et seq.
— by electronic devices, 367 et seq., 419
— by frequency analysis 410-412
— by infra-red procedures, 584
— by light, 584
— by noise, 401 et seq.
— by odour, 585
— by reflectivity, 407 et seq.
— by sonobuoy, 376 et seq.
—• by sound frequency, 401 et seq.
— dog fish, 257
— herring, 255-256, 294 et seq.
— in midwater, 368-369
— location, 307 et seq.
— mackerel, 257
— near sea bottom, 182, 369-370
— pelagic, 241
— sprat, 256
— tuna, 382 et seq.
— through ice, 415 et seq.
— underwater, 585
— use of aeroplanes in, 419
— use of artificial logs in, 585
— use of sonar in, 306, et seq., 367 et seq.
371 et seq., 582
Fish echo traces, 254, 366, 376-382
— anchovy, 381, 414
— bottom, 390-391
— classification of, 309
— cod, 378
— dogfish, 256, 381
— hake, 414
— herring, 255, 256, 299, 380
— identification of, 309, 376, et seq.
383, 413-414
— interpretation of, 365
— mackerel, 257, 380
— prawn, 391
— rockfish, 413
— sardines, 379
— searching patterns, 307-308
— tuna, 378, 384
- whiting, 380
Fish environment see Behaviour of Fish
Fish noise detection, 401 et seq.
Fish sound, analysis of, 410 et seq.
— mechanism of, 402
Fishing gear research, future problems for
investigation, 489-493
Fixed nets, synthetic fibres for, 67
Fleet operation, 423-438, 438-446
— communication with catcher boats, 429,
438
— cost of, 445
— for bottom longlining, 435-437
— for crab catching, 431-434
— for salmon fishing, 428-43 1
— for tuna fishing, 436-437
— gear, 424, 429 e t seq.
— Japanese methods, 423 et seq., 438-445
— processing of fish, 438-446
— searching, 431-432
— Spanish methods, 438-445
— trans-shipment, 427-428
Floating ability of netting twines, 23,
477-478
Floats, 144, 430
Flexibility, of monofilaments, 106
— of nets, 19, 104, 106,472
Flexural stiffness, of monofilaments, 106
— of netting twines, 19
Folded yarn, 4
Fouling resistance of fishing gear, 27, 31-32
Four-door otter trawls, 241 et seq.
Four-seam trawl nets, 193 et seq., 235-239
Freeze-drying of fishery products, 588
Freezing of fish, in distant trawlers, 438 et
seq.
Freshwater fishing, use of monofilament
gillnets in, 111-115
Frequency analysis offish sounds, 410 et seq.
Frequency of echo sounding, 396
Frictional resistance measurements of trawl
boards, 213
Frictional strength, of netting materials, 40
— of trawl boards, 213
Future trends of fishing, 489-493, 584 et seq.
Gantry, design of, 155
Gauges, mesh, 35
— pressure, 291
Gillnets, Amilan in, 73
— breaking strength of, 114
— catch efficiency of, 111, 114-115
— depth recording of, 518
— diameter of, 113
— discussion on, 291
— elasticity of, 113
— floats and sinkers, 1 14
— for crab eatching, 292, 433
— hanging, 114
— height of, 115
— in fresh water fishing, 1 12
— in Indian waters, 292
— Kuralon in, 75-78
— mesh arrangement in, 77
— mesh measurement in, 36, 114
— mesh size, 114
— monofilaments in freshwater, 111-115
— nylon in, 433
— recording depth for, 518
— softness of, 113
— use of polypropylene, 56-57
— use of synthetic materials, 54, 56, 62, 67,
74, 292
— use of Ulstron in, 62-63
— visibility of, 48, 112
— wearing rate of, 77
— References, 115
Granlon trawls, 522 et seq.
Groundrope indicator, 516, 525
Guiding of fish, by air bubble curtain,
540-544
Gurdy, power, 288
Hailing, 238
Hanging of nets, 114
Hanging ratio, 291-292
Hasong modern fish trap, 232-286
Hauling devices, for crab pots, 270
Hauling machines, net, 352 et seq.
Hauling-up ramp, 148, 155
Headline, depth, 230
— height, 232, 509-510, 525
— measurements in trawlboards, 177, 195
— transducers, 224
Heat resistance,
— of monofilaments, 105
— of netting twines, 26, 239
— of ropes, 72-73
— of synthetics, 72-73
Heavy trawling, 204 et seq,
Height of netting, 1 14
Hemp yarn, identification of, 5
Herding behaviour of fish, 532 et seq., 539
High frequency echo sounding devices, 262,
291
High speed trawling, 260
Hook size, dropline, 290
Humber fish detection system, 364-366
Hydraulic circuits, closed loop system, 316
— open loop system, 316-318
Hydraulic power installations, 314 et seq.
— compound pump, 324-326
— disadvantages of, 362
— in purse seiners, 301-303
— single pump, 323-324
— transmission of, 319 et seq.
— winches, 328 et seq., 362
— References, 332
Hydraulic systems, advantages, 318-319
— controls of, 319
— deck layout for, 327, 352
— high pressure, 315-316
— low pressure, 315
— medium pressure, 315
— nethaulers, 351
Hydrodynamic forces on gear, 167 et seq.
— References, 180
Hydrodynamic resistance of netting twines,
48-49,473-474
Hydrofoil, 205
— use in trawl boards, 243 et seq., 258
Hydrophones, for measuring marine
sounds, 401, 410 et seq.
Ice, echo sounding devices for, 415-417
Identification offish see Fish detection
Identification of netting twines, 3-5
Impact strength of synthetic yarns, 106
Inclinometer, Jelly-filled directional rolling
519-521
Infra-red procedures for fish detection, 584
Instruments, trawl gear, 497-513, 514-518,
522 et seq.
— deck type, 498-502, 522
— control panel and recorders, 502
— design, 498
— divergence meters, 500-501
— ship's speed log, 502
— warp declination meters, 501-502
— warp, heel meters, 502
— warp load meters, 498-500
— ship, 522
— underwater, 498, 503-511, 522
— board pitch meter, 507
— manometer headline height, 509-510
— net twine load cells, 511
— otter board angle of attack meter, 506
— otter board angle of heel meter, 506
- pressure strength factors, 503
— spread meters, board, 507-508
— electronic, 507
— faults in, 512
— netmouth, 508-509
— trawl speed meters, 510
— warp declination meters, 505-506
- warp load cells, 503-505
Intensity of echo signals, 407
Interpretation of fish echo traces, 365
Inversion resistance of knots, 42
Irradiation of fishing products, 588
Jelly-filled directional rolling inclinometer,
519-521
Kaitsuke fishery, 571
Kempf log, 528
Knotted netting, exchangeability of, 90-91
Knot less netting, advantages of, 90, 101-102
— catching efficiency of, 94
— in Icelandic fisheries, 98-100
— in Italy, 97-100
- in Norwegian fisheries, 96-97
— looms, 98
-- manufacture of, 89-90
— mesh size constancy, 93-94
— Raschel technique, 89 et seq.
— strength testing of, 38, 91-92, 118
— synthetic fibres in, 67, 88-95
— towing resistance of, 92-93
— weight of, 92
— References, 95
Knots, in synthetic fibres, 51, 52, 83-84
— inversion resistance of, 42
— loosening resistance of, 42-43
— slip resistance of, 42
— stability of, 20, 41-43
— strength of, 15-16
— tenacity of monofilaments in, 105, 106
— testing of, 15, 51
Knotting efficiency, 41 et seq.
Knotting machines, operation, 89
Kuralon, 57
— usage in driftnets, 75-78, 291
— usage in gilhiets, 75-78
— usage in longlines, 436
Lengthening of netting twines, 25-26
Light, reaction of fish towards, 531, 533,
558, 561
— use of in fish detection, 584
Light fishing, 573-577, 577-579
— in Caspian Sea, 578-579
— in Mediterranean, 573-577
Livlon synthetic fibre, 7
Load cells, net twine, 51 1
— warp, 503-505
Load-elongation, calculation for netting
twines, 16-18
— of monofilaments, 105
Loading, testing of, 208
Lobster pots, use of synthetic materials in,
108
Location techniques, 307 et seq.
see also Fish detection
Longlining, echo sounding for tuna, 385
et seq.
— for crab fishing, 435
— for tuna fishing, 70, 385-388, 436
— of different materials, 387-388
— research into, 292
— ultrasonic reflection of, 386
— use of, 287-288
— use of synthetic materials in, 68, 108,
291, 436
Looms, for knotless netting, 98
— netting, 60
Looseness of knots, 42-43
Luggers, trawl gear for, 236
Macro organisms, resistance of netting
materials to, 31
Magnetic equipment in steering, 341-342
Manila twine, use of in crab gillnets, 433
Manometer headline height, 509
Mar 'on synthetic fibre, 7
Mechanical stimuli of fish. 537-540, 559
— by electricity, 566-570
— by pressure changes, 538
— by sound waves, 538
— to the lateral line, 539
Mesh arrangement in driftnets, 77-78
Mesh construction, 472
Mesh gauges, 35
Mesh size, behaviour offish to, 189, 580
— distribution and grading, 189
— in gillnets, 36, 114
- in knotless netting, 93-94
— in relation to synthetics, 79
— in wing trawls, 189-190
— measurement of, 34-37, 183
— selectivity, 34, 77 et seq., 114
— stability, 79
— standardisation of, 429
— strength, 37-38, 91-93
Mesh strength see Netting, strength of
Meters, board pitch, 507
— divergence, 500-501
— dynamometer for trawl warps, 5 1 6
- otter board angle of attack, 506
— otter board angle of heel, 506
- record groundrope indicator, 516
— recording current, 516
— recording depth, 515
— recording depth and temperature, 515-
516
— recording net height, 514
— recording underwater dynamometer, 5 1 6
— speed measuring, 528
— spread, 507-509
— trawl speed, 510
— warp declination (deck type), 501-502
— warp declination (underwater) 505-506
— warp heel, 502
— warp load, 498-500
Midwater trawling, echo sounding devices
for, 249 et seq., 388 et seq.
— gear testing, 482-489
— net depth measurement, 249 et seq.
— nets for, 54, 62, 107
— one boat. 221-228, 254 et seq., 257
599
— advantages of, 261, 262
— catching efficiency of, 227
— design of, 223, 225
— depth regulation, 254 et seq.
— Dogger Bank, 261
— English Channel, 222
— for cod catching, 222, 228
— for redfish, 228
— for saithe catches, 227
— Icelandic coast, 224, 225
- Irish sea, 222
— North Atlantic, 227
- North Sea, 222
— Norwegian coast, 224, 227
— off Belgian coast, 261
— off Greenland, 222
— pelagic, 240 et seq. 247-248, 262
— Polish experiments in, 229
— U.S.S.R. experiments in, 207 et seq.
— References, 228, 248
— pair, 227
— semi-pelagic, 262
— shrimp, 388 et seq.
— two-boat, 235-239
— advantages of, 259
— for herring catching, 254
— German developments, 235-239
— North sea, 235-239
— towing of, 200-203
— use of nylon in, 238
— versus otter trawling, 196
— References, 395
Minnow netting, strength testing of, 40
Moisture content of fibres see Wet testing
of fibres
Monofilaments, abrasion resistance of, 106
— advantages of, 109-110
— breaking strength of, 85, 1 14
— breaking tenacity of, 83
— definition of, 3-4, 103-104
— derniers, 104
— disadvantages of, 110-111
— drag resistance of, 107, 116-117
— draw ratio, 83
— effect of heat on, 105
— elasticity of, 105, 113-114
— elongation of, 83
— flexibility, 106
— for freshwater fishing, 111-115
— heat effects on, 105
— in Danish seines, 107
— in deep-sea trawls, 107-108
— - in freshwater gillnets, 111-115
— in gillnets, 56, 109
— knot stability of, 84-85, 105, 106
— load elongation, 105
— nylon, 104, 108-111, 429
— polyethylene, 81-88
— properties, 104-107, 118-119
— rotting resistance of, 107
— stiffness of, 106
— storage of, 104-105
— strength of, 104-105
— suitability for netting, 107
— tenacity, 107
— weather resistance, 107
see also Synthetic fibres
Mothership operations, 423 et seq.
set also Fleet operations
Mounting requirements for synthetic nets,
60-61
Navigational control automatic, 340
Nearwater trawling, 150
Net depth telemeters, 234, 238, 249 ft seq.
Net height meter, 514
Net shaking devices, 352 et seq.
Net shooting by asdic, 297
Net twine load cells, 511
Net-Vision, construction and operation of,
389-391
600
Nethauters, 348 et seq., 352 et seq., 429
— hydraulic, 351
— I-type, 348-349
— in crab fishing, 435
— in purse seiners, 298
— performance of, 351
— S-type, 349-350
— YM-type, 350-351
Netless fishing, 579
Netmouth spread meters, 508-509
Netting, classification of, 33
— comparisons of different materials, 182
et seq.
— comparisons of knotless and knotted,
94-95
— depth measurement of, 249 et seq.
— dimensions of. 34
— dyeability of, 32
— flexibility of, 472
— hanging of, 114
— height of, 114
— looms, 60
— minnow, 40
— mounting requirements for, 60-61
— roughness of, 25
— stiffness of, 19
— strength of, 37, 39, 40, 91-93
— suitability of monofilaments for, 107
— thick, 6
— visibility of, 46, 48, 112
— waterflow through, 197
— wearing rate, 77
see also Knotless netting'. Trawlnets
Netting, testing of, 10 et seq., 32
— directional differences, 478-479
— dry, 11, 22
— effect of wave motion, 479
— enlargement effects, 480
— flexibility, 19
— floating and sinking, 23-24, 477-478
— heat resistance, 26, 239
— hydrodynamic resistance, 48-49, 473-474
— physical tests, 12
— rending strength, 40
— resistance, 473-474
— rope properties, 476-477
— similarity of, 474-476
— weight in water, 473
— References, 482
see also Mesh strength
Netting twine, abrasion resistance of, 21-22
— bending, 19-20
— braided, 4, 7, 12, 54, 115
— definition of, 4-6
— designation of, 6-7
— diameter measurement of, 24-27
— dyeability of, 32
— dissimilar component, 6-7
— elasticity of, 17-18
— extensibility of, 16-17
— fish behaviour towards, 113
— floating ability, 23, 477-478
— fouling resistance of, 27, 31-32
resistance of, 48-49,
- hydrodynamic
— identification of, 3-5
— lengthening of, 25-26
— load elongation, 16-18
— numbering systems for, 4 et seq.
— pollution of, 46
— processability of, 32
— reaction to cold, 26
— resistance to bending, 19
— resistance to macro-organisms, 31
— resistance to chemicals, 27
— resistance to oils, 27
— resistance to rot, 28-31
— rolling resistance, 28
— roughness of, 25
— shrinkage of, 25, 26, 32
— standardisation, 3-8, 429
— storage of, 33
— sinking speed of, 23-24, 477-478
— suitability tests for, 32
— surface roughness of, 25-26
— thermal reactions on, 26
— visibility of, 46, 48, 112
— weather resistance of, 26-27
— weight of, 6, 11, 22-23, 43-46
— wet breaking strength, 14-15
— References, 8
see also Synthetic fibres, Trawlnets
Netzsonde, definition of, 221
— hauling of cables on, 260
— use of, 219, 222 et seq., 238-239, 254
etseq., 258, 389, 418, 526, 579
Noise, detection of fish by, 401 et seq.
— recording by echo sounding, 401
Numbering systems for netting twines,
4 et seq.
Nylon, in crab gillnets, 433
— in driftnets, 291, 429, 495
— in gillnets, 54, 62, 1 14 et seq.
•—• in Viet-Nam fisheries, 108-111
— monofilaments, 104, 108-111, 429
— properties compared with other syn-
thetics, 59-60,80, 116-118
— use of in two-boat midtrawlers, 218
Odour, fish detection by, 585
Oils, netting materials resistance to, 27
Oscilloscopes, cathode ray, 392
— design of, 374
Otter board angle of attack meters, 506
Otter board angle of heel meters, 506
Otter boards see Trawl boards
Otter trawls, defects of, 191-192
— design and performance of, 184 et seq.,
191 et seq.
— double-rig beam, 210 et seq.
— four-door, 241 et seq.
— four seam, 195
— gear, 191 et seq.
One boat midwater trawls, advantages of,
261, 262
— catching efficiency of, 227
— design of, 223, 2i5
— depth regulation for, 254 et seq.
— Dogger Bank, 261
— English Channel, 222
— for cod catching, 222, 228
— for redfish, 228
— for saithe catches, 227
— gear, 229 et seq.
— Icelandic coast, 224, 225
— Irish sea, 222
— North Atlantic, 227
— North sea, 222
— Norwegian coast, 224, 227
— off Belgian coast, 261
— off Greenland, 222
— pelagic, 240 et seq. 247-248, 262
— Polish experiments, 229
— References, 228, 248
Pair trawling, 227
Pareja trawls, 151
Pasteurisation of fishery products, 588
Pendulum testing machines, 13-14
Perlon, 93, 238
Phototaxis, occurrence of, 555
Phosphorescence, 581
Physical tests on nets, 12
Pisa current indicators, 520-521
Pitch meters, 507
Pollution of nets, 46
Polyamides, abrasion resistance, 53
— breaking strength, 52
— diameter of, 52
— elasticity of, 53-54
— extensibility of, 53
— testing of, 52
— use in eel traps, 278
— use in driftnets, 75-78
— wet testing of, 1 1
Polyesters, wet testing of, 1 1
Polyethylene, breaking strength of, 85
— monofilaments, 81*88
— production of, 82-83
— properties of, 83-84
— ropes, 81 et seq.
— testing of, 12
— usage for eel traps, 278
— usage for longltnes, 291
Polyolcfin monofilaments, 104
Polypropylene, abrasion resistance, 52, 53,
56
— breaking strength of, 52
— catchability, 56-57
— diameter of, 53, 56
— elasticity of, 53-54
— extensibility of, 51, 53
— heat treatment of, 72-73
— knot breaking strength of , 5 1 , 52
— production of in Japan, 72
— properties of, 55-57, 79
— testing of, 51 et seq.
— Ulstron, 57-64, 79
— use of in crab gillnets, 433
— use in gillnets, 56-57
Polyvinyl alcohol fibres, wet testing of, 1 1
Polyvinyl chloride, use of for eel traps,
278-279
Pot hauling, 264
Power blocks in purse seiners, 299-303
Power gurdy, 288
Preservation of netting materials, 30
Pressure changes, effect of on fish, 538
Pressure effects on trawl gear, 1 74
Pressure gauges, 291
Pressure strength factors, 503
Processability of netting materials, 32
Processing of fishery products, by con-
centration, 588
— by freeze-drying, 588
— by irradiation, 588
— by pasteurisation, 588
— in stern trawlers, 1 60
— salting machines, 354
Processing of nets, 32
Propulsion, research into, 588
Pump fishing, 577-579
— electro, 547, 552-555
Pumping electrical, advantages, 547, 552-
555
Purse seines, Asdic guiding for, 259
— automatic handling of, 151
— depth telemeters for, 248 ct seq.
— development of, 259
drifting of, 312
— electro, 544-551
— hydraulic power in, 314 et seq.
— light, 573-577
— nets for, 303-304
— remote controls in, 326 et seq.
— sonar equipment for, 308-309
— synthetic fibres for, 67
Purse seining, 294-305, 310-313
— Icelandic, 294-305
— asdic installations, 295 et seq.
— brailing operations, 301-303
— deck arrangement, 303-303
— direction finding equipment, 296
— echo sounding devices, 295 et seq.
— • hauling operation, 301-303
— herring fishing, 295-305
— net hauling, 298
— power blocks, 299-303
— pursing operation, 301-303
— shooting operations, 301-303
— refinements of, 311
— tuna, 271, 311
— References, 205
Ramp trawls see stern trawlers
Raschel nets see Knotless netting
Record groundrope indicator, 516
Recording current meters, 516
Recording depth and temperature meter, 5 1 5
Recording depth for gillnets, 518
Recording depth meter, 515
Recording net height meter, 196, 514
Recording underwater dynamometer, 516
References
— acoustic detection of fish, 404
— behaviour of fish, 526, 540, 544, 560,
562, 566, 570
— double-rig beam trawls, 217
— echo sounding devices, 257, 365, 375,
388
— eel traps, 279
— electronic sonar scanning, 371, 375
— hydraulic power installations, 332
— hydrodynamics of otter boards, 180
— Icelandic purse seining, 305
— knotless netting, 95
— midwater trawling, 228, 248, 375
— monofilament gillnets, 115
— netting twines, standardisation of term-
inology, 8
— testing, 482
— synthetic fibres, 54
Reflection of longlines, 386
Reflective ability of fish, measurement,
407 et seq.
Remote control of purse seines, 326 et seq.
Rending strength of netting, testing, 40
Research into fishing gear, future pro-
gramme, 489-493, 584, et seq.
Resistance, knot, 42-43
— of netting, 473-474
— to bending. 19
— to chemicals, 27
— to macro-organisms, 31
— to rolling, 28
— to oils, 27
— to rot, 28-31
to tow, 92-93
— of trawl boards, 213
see also Heat resistance
Rigging, 210
River seining, 258
Rolling resistance, of netting twines, 28
Ropes, polyethylene, 86 et seq.
— properties of, for netting, 476-477
- synthetic fibres in, 63-64, 68-70, 86, 108
— tests on, 116-118
— U.S.S.R. investigations into, 208
Rotting resistance, of monofilaments, 107
— of netting twines. 28-31
Roughness of netting material, 25
Rupture of synthetic yams, 106
Safety arrangements, in automatically con-
trolled wheel houses, 339
— in double-rig beam trawlers, 214
Salinity, fish reactions to, 558
Salting machines, 354
Scoop seines, 284
Sea water resistance see Rotting resistance
Searching patterns, sonar techniques for,
307-309
Sector scanning, 367-371, 418 et seq.
— use in fish behaviour determination, 582
Seine-stowing machiniery, 359
Seining see Beach Seines; Danish seining',
Purse seining; River seining; Scoop seines;
Tuna seines
Semi-pelagic midwater trawling, 262
Ship's speed log, 502
Shrimp nets, design, 284
— synthetic materials in, 108
Shrimp trawling, 214^17, 388 et seq.
Shrinkage of netting twines, 25-26, 32
— of synthetic fabrics, 79-80
Sidetrawls, 147, 149, 159
— manoeuvrability of, 161
Single yarn, definition, 4
Sinkers see weights
Sinking speed of netting twine, calculation
of, 23-24, 477-478
Slackness in trawlnets, overcoming, 243
Slip resistance of knots, 42
Sliphook, construction of, 214-215
Softness of gillnets, 113
— of netting materials, 32
Sonar equipment, for fish behaviour
determination, 306 et seq., 367 et seq., 582
— for research purposes, 371-375
— in purse seining, 308-309
— sector-scanning, 367-371
— training crew in use of, 307 et seq.
— transmitting cabinets for, 373-374
see also Echo sounding devices
Sonobuoy, fish detection methods, 376
et seq. 401
Sound, fish reaction towards, 401, 538, 561,
581-582
— marine, measurement of, 410 et seq.
Sound frequency, use of in fish detection,
401 et seq.
Sound propagation, analysis of, 410
— measurement of, 307
Specific strength, of fibres, 15
Specific weight, of fibres, 15-16
Speed, measurement of, 528
— of towing, 172, 200-203, 232
Speed log, ship's, 502
Spread factors, 171
— of trawl boards, 176, 178
Spread meters, board, 507-508
— electronic, 167, 507
— faults in, 512
— netmouth, 508-509
Stability, of knots, 20, 41-43
— of trawl boards, 176, 219, 262
Standardisation of netting twines, 3-8, 429
Staple fibres, 63 et seq.
Steelon, testing of, 76
Steering, auto-electric, 339, 340-342
— comparison with hand, 340
Sterntrawls, 147 et seq. 154 et seq., 158
et seq.
— automatic, 151, 152, 159, et seq., 340, 346
— ballast capacity of, 156
— catching capacity of, 159
— coastal water, 150
— crew requirements, 155
— deck layout, 154-155
— deep-sea, 149
— design of, 150, 156, 159, 160
— development of, 148
— double-rig beam, 219
— economics of, 160
— gear, 148-152, 154, 160
— integrated control in, 159
— mid- water, 149
— near- water 150
— nets for, 155, 163-164, 149, 151
— Paieja, 151
— processing offish in, 160
— ramp, 161
— small, 154-158
Sterntrawling, automation in, 338 et seq.
— damage to fish in, 155-156
— development of, 148, 163
— machinery for, 158 et seq.
Stiffness, of twine, 19
— of fibres, 33
Storage, of monofilaments, 104-105
— of netting twine, 33
Strength of knotless netting, 38, 91-92, 1 18
— of monofilaments, 104-105
— of netting, 37-40, 9 1-93
Strength testing, machines, 13-14
— of knots, 15-16
— of meshes, 37 et seq.
601
— of minnow netting, 40
Stress and strain, in nets, 17 et seq.
— in ropes, 208
String, definition of, 12
Structure of netting 472
Sttberkrub trawl boards, 223, 262
Sweep lines, 220
Swimming endurance of fish, 535
Swimming force of fish, 113
Swimming sound of fish, 41 1 et seq.
Swimming speed of fish, 241
Synthetic fibres, abrasion resistance of, 52,
80,116-118
— biological tests on, 27
— compared with cotton, 76, 80, 115-118
— cost of, 67
— damage to fish by, 76-77
— diameter of, 52, 113
— durability of, 80
— dyeabihty of, 73
— elasticity of, 53-54, 105
— extensibility of, 53
— fishing efficiency of, 76
— for fixed nets, 67
— for ropes, 63-64, 68-70, 108
— heat treatment of, 72-73
— impact strength of, 106
— in bottom trawls, 61-62
— in crab pots, 269
— in driftnets, 74 et seq. 429
— in gillnets, 54, 56, 62, 67, 74, 292
— in Jcnotless netting, 67, 88-95
— in knots, 52
— in longlines, 68, 108, 291, 436
— in purse seines, 67
— in trawlnets, 68, 85
— mesh size, 79
— polypropylene, 50 et seq.
— production in Japan, 65-67
— rupture of, 106
— shrinkage of, 79-80
— standardisation of terms, 8
— tensile strength of, 69
— testing of, 11 et seq.
— trademarks, 64
— treatment with bitumens, 80
— - usage, 205
— usage in Japan, 55-57, 64 et seq.
— usage in traps, 63
— visibility of, 112
— References, 54
see also Monofilaments; and under names of
individual fibres
Tanglenets, 291, 433
Tank requirements of crab fishing vessels,
268
Telemeters, 234, 2^8
—APE, 488
— for depth measurement, 249 et seq.
— net, 234
Television, underwater, use of for observing
fish reactions, 548, 580
Temperature, fish reactions towards, 558
Temperature recording meters, 516
Tenacity of monofilaments, 105
Tensile strength of synthetics, 69
Tension, measurement of, 524
Tension of warps, 168, 200, 232, 493
Terminology 01 twines, 3-8 12
Terylene twines, properties, 58-59
Testing of netting see Netting, testing of
Tetoron, properties of, 57
— use of in driftnets, 429
Tex system, 3
Thermocline, effect of, 581
Tilt, effect of on trawl boards, 174
Titanium, use of in fishing craft, 587
Thermal reactions of netting materials, 26
see also Heat resistance
Towing pull, measurement of, 168
602
Towing resistance, of knotlcss netting, 92-93
Towing speed, 172, 200-203, 232, 517
Trademarks of synthetic fibres, 64
Training crew in use of sonar instruments,
307 et seq.
Transducers, for headline height, 224
— in Icelandic purse seiners, 297, 299
— sonar, 373
— use of, 261, 307, 390
Transmitting cabinets, design of in sonar
equipment, 373-374
Trapnets, use of svnthetics in, 108
Traps, corral, 279-282
— in Philippines, 279-281, 282-286
— crew requirements for, 286
— eel, 277-279
— References, 279
— tuna, Atlantic, 271-277
— Sicilian, 271-277
— use of synthetics in, 63
Trawl boards, 167 et seq., 193 et seq.,
223, 230
— behaviour of, 149, 524-525
— bottom tests on, 486-487
— curvature effects on, 175
— design of, 192 et seq., 229 et seq., 258, 262
— developments, 257
— effects of heel on, 172
— forces on, 176, 485
— fouling of ocean cables, 219
— friction resistance, 213
-— hydrodynamic forces on, 171 et *eq.
— hydrofoil, 245 et seq., 258
— improvement of, 205
— in U.S.S.R., 207
— in wing trawlers, 189
— influence of camber, 175
— measurement of headline height, 177,
1%
— net connection to, 194
— performance of, 171
— resistance of, 213
— sheering effect of, 175
— spread, 176, 178
— stability of, 176, 219, 262
— Siiberkrub, 223, 262
— testing of, 483-485
— tilt effects on, 174
— variations of tilt with speed, 172
— warp force, 176
— water effects on, 485-486
— wind tunnel tests on, 167 et seq.
— References, 180
Trawl doors, design, 243 et seq., 493
Trawl drag measurement, 525
Trawl instruments see Instruments
Trawl mouth, area measurement, 233
Trawl speed meters, 510
Trawl warp dynamometer, 516
Trawling, automatic, 151, 159 et seq,. 346
— beam, 209 et seq.
— Belgian practice, 210 et seq.
— electro, 545-551
— for crab catching, 292
— heavy, 204 et seq.
— high speed, 260
— in deep water, 149
— Japanese practice, 423 et seq.
— near-watei, 150
— pair, 262
— power requirements, 199
— two-boat, 191 et seq.
— U.S.S.R., practice 206-208
— water speed in relation to, 519
see also Midwater trawling; Bull (rawing
Trawlnets, behaviour, 392
— bull, 424-426
— changes in design of, 191 et seq., 257
— cost of, 292
— depth measurement of, 249 et seq.
— depth regulation of, 239
— for crab fishing, 433
— four-seam, 193 et seq., 235-239
— overcoming slackness in, 243
— use of colour in, 293
— - use of synthetic fibes for, 68, 85
— waterflow in, 197
Trawls, behaviour offish to, 241, 537, 539,
547-548
— centralised control in, 338 et seq.
— coastal, 150
— damage to, 187
— deck arrangement, 148-149
— design and construction, 181 et seq.
— double-rig shrimp beam, 209-217, 219
— References, 217
— factory, 445
-- filleting of fish in, 438 et seq.
— freezer, 438
- floats, 259
— gear, 181 et seq., 200 et seq.
— Granton, 522 et seq.
~ headline height, 232
— heavy, 204 et seq.
— pelagic, fish searching, 308
— shrimp, 214-217, 388, et seq.
— towing speed of, 172, 200-203, 232
see also: Bottom trawls; Deep-sea trawls;
Distant-water trawls, Otter trawls; One-
boat midwater trawls; Side trawls, Stern-
trawls, Two-boat midwater trawling; Wing
trawls
Trolling, disadvantages of, 287
Trunkdeck ship, 148
Tuna, bait for, 437
— behaviour of, 383-385, 385-388, 560-562
— detection of, 381 ft seq.
— longlining of, 436
— reaction of, 560-562
- traps for, 271-277
Tuna longlines, 70, 385-388
— depth measurement, 385-388
Tuna mainlines, synthetics for, 68
Tuna purse seines, 271, 31 1
Two-boat midwater trawling, advantages of,
259
— for herring catching, 254
— German developments in, 235-239
— nets for, 193
— North sea. 235-239
— nylon in, 238
— towing of, 200-203
— versus otter trawling, 196
Twine, braided, 4, 7, 12, 54, 115
— coefficients of, 78
— definition of, 3-8, 12
— designation of, 5
— dimensions of, 182
— manila, 433
— size of, 182 et seq.
— terminology of, 3 et seq.
— weight of, 6
see also Netting, testing of; Synthetic fibres
Twist, 6, 8
Ulstron, uses of, 57-64, 79
— catching efficiency, 61
— in gillnets, 62-63
Ultrasonic reflection, longline, 386
Underwater dynamometer, 516
Underwater methods of fish detection, 585
— by artificial logs 585
Underwater observation of gear, 246
Underwater trawl instruments, 243, 498,
503-511, 522
— board pitch meter, 507
— manometer headline height, 509-510
— net twine load cells, 511
— otter board angle of attack meter, 506
— otter board angle of heel meter, 506
— pressure strength factors, 503
— spread meters, board, 507-508
— electronic, 507
— trawl speed meters, 510
— warp declination meters, 505-506
— warp load cells, 503-505
Unigan gantry, 155, 157, 160
Vibration, measurement of, 167
— warp, 166
Vinyl on, use of in crab fishing, 433
Visibility of netting, 46, 48, 112
Visual, sense of fish, 530 et seq.
— to colour, 530-531, 532, 582
— to dark, 531-533, 539
— to light, 531, 533, 558
— to nets, 530 et seq.
— to trawls, 534
Warp declination meters, deck type, 501-502
— underwater, 505-506
Warp effects on trawlboards, 176
Warp heel meters, 502
Warp load cells, 168, 503-505
Warp load meters, 498-500
Warp stowing machines, 354-355
Warp trawl dynamometers. 516
Warps, behaviour of, 166 et seq.
-— curvature of, 166
— fish reaction towards, 539
— improvements for, 2C4
— length of, 169, 232
— protection of, 352 et seq.
— tension of, 168, 200, 232, 493
— vibration of, 166
Water, depth effect of, 170
— effect on netting materials, 27
— effect on trawl boards, 485-486
— flow of, 197-198
— force of, 170
— speed of in relation to trawling, 170, 519
Wave motion, effect on netting, 479
Wearing rate of netting, 77
Weather resistance, of monofilaments, 107
— - of netting materials, 26-27
— of synthetic materials, 56
Webbing twines, synthetic, 67
Weight of netting twines, 6, 1 1, 22-24, 43-46
— dry, 11, 22
- measurement of, 23
— of knotless netting, 92
— of synthetic fibres, 54
— wet, 473
Weights (sinkers), 114, 430
Wet knot breaking strength, 15
Wet testing of fibres, 11 et seq., 493
Wheelhouses, automation in, 338 et seq.
Winches, for beach seines, 357 et seq.
— gipsy, 270
— hydraulic, 328 et seq., 362
— trawl, 159, 210
Wind tests on trawl boards, 167 et seq.
Wing trawls, catching performance, 182,
184, 186
— design of, 183 et seq.
— limitations of, 189
— mesh size for, 189-190
— trawl boards, 189
— usage, 182
— use of synthetic fibres in, 107
Woven netting, 32
Yarn, cabled, 4
— definition of, 3-4, 12
— density of, 5-6
— designation of, 5
— folded, 4
— hemp, 5
see also Netting
Yarn count, 4
603
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