5

Rapp. P.-v. Réun. Cons. int. Explor. Mer, 170: 5-6. Février 1977.

F O R E W O R D

A. R. M a r g e t t s Symposium Convenor and Volume Editor

After World War 2 echo-sounders and sonar were It was to provide a forum for the exchange of new developed rapidly and widely to become standard research results, experience and ideas in this field fishing vessel equipment for finding fish. Fisheries between scientists from all parts of the world; speci­ scientists, particularly in northern Europe, were soon fically it did not include consideration either of the looking into methods of using echo-sounders not only use and application of acoustic instruments in com­ to find fish but to estimate the quantities of fish in mercial fishing or of bioacoustics. FAO and the an area. Their ideas and methods were developed very International Commission for the Northwest Atlantic rapidly in the 1960s, so m uch so that by 1968 echo Fisheries (ICNAF) collaborated with ICES in this surveying with automated quantification of received Symposium and financially supported this publication. fish echoes was in use as a method of fish abundance Mr A. R. Margetts (Lowestoft, England), was con­ estimation. The methods were new, highly promising, venor for the Symposium. He was assisted in the but fraught with many sources of error. Scientific Planning Group by Mr L. A. Midttun (Ber­ Progress was reported regularly to the Gear and gen, Norway), Mr B. B. Parrish (Aberdeen, Scot­ Behaviour Committee of the International Council for land), Dr D. F. S. Raitt (FAO, Rome), Mr K. A. the Exploration of the Sea (ICES) at its annual Smith (Woods Hole, USA) and Mr H. Tambs-Lyche Statutory Meetings. Through ICES and FAO, in (ICES). March 1969 an Acoustic Training Course was held Authors were invited to submit to the Symposium at Svolvaer in Norway, at which also the apparatus papers on acoustic methods applied to the topics set and methods developed separately in Norway, Eng­ out as follows: land, Scotland and the Netherlands were demonstrated 1 Fishing gear and topographical studies and compared on research vessels at sea. The Svolvaer 2 Fish behaviour studies course was reported as FAO Fisheries Report No 78, 3 Fish abundance estimation 1969. 4 Identification and sizing of echo targets FAO Manuals in Fisheries Science No 5, Manual 5 Novel instrumentation. o f methods fo r fisheries resource survey and appraisal. Part 2. The use o f acoustic instruments fo r fish detection and abun­ The Symposium was attended by 125 experts from dance estimation, (Ed. Forbes, S. T. and Nakken, O.), 25 different countries throughout the world. An published in 1972, was a revision of the preliminary opening address was appropriately (because Norway manual of 1969 (FAO Tech. Pap. 83). was the host country and her scientists and technicians Acoustic assessment of fish stocks was just one, were amongst the pioneers of echo-sounder develop­ albeit then the outstandingly important one, of a ment) and kindly given by Mr K. Sunnanaa, Director wide range of uses of underwater acoustics being General of Fisheries for Norway. He and all partici­ researched and developed at widely separated centres pants were welcomed by the President of ICES, M. R. not only to improve detection of fish but also to Letaconnoux. Wherever possible, which, happily, was provide new tools for research scientists. Their use was in the great majority of cases, papers were presented making possible new methods for assessing abundance by their authors. Considerable and profitable discus­ of both adult and pre-recruit fish, for estimating the sion followed the presentation of papers, either indi­ sizes of individual fish, for studying fish migrations and vidually or grouped according to topics. Altogether behaviour, and for investigating the performance of 50 papers were presented. fishing gear and reactions of fish to gear. The first two programme sections, covering studies ICES initiated, convened and organised a Sympo­ of fishing gear, topography, fish behaviour and sium on Acoustic Methods in Fisheries Research, held in migrations, illustrated the wide range of novel uses Bergen at the invitation of Norway, 19-22 June 1973. of underwater acoustic apparatus. High-resolution 6 A. R. Margetts: Foreword sector scanning sonar had produced very impressive of readings were taken to reduce variance. One major results in both fish behaviour and fishing gear in­ source of error in acoustic assessment lies with the vestigations, particularly those concerning individual shoaling patterns and packing density within the fish in the open sea while, in contrast, very long range shoals of fish; much consideration was given to this low-frequency sonar provided a means of plotting fish and a number of ways of dealing with the problem, shoals at a distance of many miles. A combination of some very promising, were being developed though two such systems could be invaluable to fisheries none was yet entirely satisfactory. Again, the collection biologists. Acoustic transmitting and transponding of data at an enormous rate requires great attention tags were being used to follow fish movements and being paid to suitable computerisation. Throughout migrations, Doppler shift had been applied to the the discussions on this programme section it was study of the dynamics of fish shoals and, in enclosed repeatedly emphasised that an understanding of the waters, a laid out range with hydrophones was being biology of the fish in the sea area investigated was employed to track the detailed short-distance move­ fundamental; fish target echoes were not from ment of acoustically tagged fish and the positions of inanimate objects and allowance must be made for towed gear. Now that data are collected by these this between theory and practical results. methods at such an enormous rate, there is clearly a This volume has been edited by Mr A. R. Margetts requirement for computerised methods of processing with assistance from Mr J. E. Ehrenberg on several them: one computerized display of the direction and of the papers. While the authors themselves are speed of movement of both fish and fishing gear responsible for the technical content and presentation during a fishing operation was presented. of the papers, it is possible that, especially with papers The programme sections on fish abundance estim­ from USSR and Japan, some misrepresentation ation and on fish target strength measurement and arising from translation difficulties may still be in this sizing provided very clear examples of how carefully publication. Technical questions may be resolved di­ thought-out and planned surveys using correctly rectly with the authors. designed and calibrated apparatus can quickly and Of the 50 Symposium papers, several were either comprehensively describe not only the distribution but withdrawn from publication or omitted as being not also the quantity of pelagic fish in an area on a very properly within the stated scope of the Symposium. large scale. No other method of such instantaneous This volume contains 42 papers on subjects within the assessment is comparably effective. But, inherent in the programme framework outlined above. Of these, three method are many sources of error. Together the are concerned with fishing gear and topographic presented papers and ensuing discussion showed that studies, six with fish behaviour and migrations, 32 these were recognised and that various steps had with fish abundance, identification and sizing, and already been taken, with varying but encouraging one with very recent or future possible developments. degrees of success, to eliminate or minimize biasses and The papers are here arranged in an order broadly variances. The papers speak for themselves, but here conforming to the main topic headings as above. In it is to be noted firstly that, as yet, there is no acoustic two instances two papers presented separately to the method by which fish in the open sea or elsewhere Symposium have been combined as one paper. can be identified so fish from which echoes are The organizers of the Symposium herewith express recorded must be sampled by some method such as their gratitude to Norway for providing the venue and fishing or photography, and, secondly, that thorough excellent facilities for this Symposium at the Students and correct calibration of acoustic apparatus is of Centre, Bergen, and to the Director and staff of the paramount importance. Methods of calibration using Fiskeridirektoratets Havforskningsinstitutt, Bergen, live fish have proved reasonably successful ; an elabor­ for their invaluable assistance in staging it. Thanks ate experiment with simulated targets under large- are also recorded here to the Scientific Planning scale laboratory conditions showed particularly how Committee, to the Chairmen of the sessions at the many serious and difficult to interpret inaccuracies Symposium, to the Secretariat of ICES, to the authors can occur in an acoustic echo quantification system. of contributed papers and to the participants at the The best measurements of target strengths so far have Symposium; all of these together made for the ack­ been with free-swimming fish on which large numbers nowledged success of the Symposium. 45

Rapp. P.-v. Réun. Cons. int. Explor. Mer, 170: 45-51. Février 1977.

THE EFFICIENCY OF THE GRANTON OTTER TRAWL DETERMINED BY SECTOR- SCANNING SONAR AND ACOUSTIC TRANSPONDING TAGS

F. R. H a r d e n J o n e s, A . R. M a r g e t t s , M . G r e e r W a l k e r a n d G . P. A r n o l d Fisheries Laboratory, Lowestoft, Suffolk, United Kingdom

The efficiency of a 24 m Granton otter trawl, rigged with bunt and midwing tickler chains, was de­ termined directly using sector-scanning sonar and plaice fitted with acoustic transponding tags. The work was carried out on the Black Bank - Brown Bank grounds in the southern North Sea in day­ light. From 1971 to 1974, 153 plaice (size range 30-50 cm) were released and kept under surveil­ lance from RV ‘Clione’; RV ‘Corella’ made 343 attacks on 124 of these fish. From 166 valid attacks made on fish lying between the otter boards, the overall efficiency of the gear was 44 °/o ± 8 °/0. For fish lying between the boards and wing ends the efficiency was 22 °/0 ± 10% and for those in the path of the net 61 °/0 ± 10 °/0. The gear was not rigged with a door-to-door tickler chain. The addition of this chain could lead to a marked increase in the efficiency in respect of fish lying between the otter boards and the wing ends of the trawl. Observations were also made on the behaviour of plaice in relation to the gear and some preliminary results are described.

INTRODUCTION the gear may become less efficient and fishing power The bottom otter trawl is probably the most impor­ and catch-per-effort could decrease. One suspects - tant fishing gear in the world. In the United Kingdom with the advantage of hindsight — that this is what oc­ it accounts for over 70 °/o by value of all fish landed curred in the so-called ‘SARO project’ which was a by British vessels. Modifications have been made to recent United Kingdom attempt to improve the otter Robert Scott’s (1894) original design of an otter trawl, trawl, sponsored by the British Government, the Bri­ the most important of which have been the shaping of tish Trawlers Federation, and the White Fish Autho­ the wings of the net, the use of headline floats, and the rity. The project had as its ultimate objective the de­ insertion of bridles between the otter boards and the sign of a trawl with greater spread and headline height wings of the net. These modifications arose from pa­ which would require no more power than that nor­ tents taken out by Alphonse Vigneron (1920) and mally used in trawling operations (White Fish Autho­ Vigneron, Dahl et Cie (1922; 1923 a, b, c, 1924 a, b, rity, 1960). It was clearly anticipated that the larger c). The introduction of bridles — the so-called V-D net would catch more fish. The engineering objective gear - led to a marked increase in the catch-rate, par­ was achieved, the new trawl having a mouth area some ticularly of cod and haddock (Bagenal, 1958). 2/2 times that of the standard Granton trawl. But The fishing power of the gear can be assessed in trials with the new gear gave no evidence for any sub­ terms of the catch taken from a given density of fish stantial increase in fishing power (White Fish Autho­ per unit of fishing time and Gulland (1969) argues rity, 1963). that it can be considered in two parts. Firstly, there is Little is known of the efficiency of the otter trawl in the area or volume over which the influence of the terms of the proportion of the fish within its influence gear extends and within which the fish are liable to be that are caught. For example, does the trawl catch 50, caught; secondly, there is the proportion of the fish 70 or 90 °/o of the fish that pass between the otter within the influence of the gear that are actually caught. boards? This simple but intractable question has been The distinction is between fishing intensity and gear impossible to answer until recently and the change in efficiency. Fishing intensity is measured in terms of circumstances has arisen with the availability of high- area or volume swept per unit time and can be in­ resolution sector-scanning sonar and acoustic trans­ creased by towing the same trawl faster or by towing a ponding fish tags. larger trawl at the same speed. But the efficiency of the The Fisheries Laboratory, Lowestoft has been asso­ trawl must also be considered. If the fishing intensity is ciated with the application of sector-scanning sonar to increased by faster towing, or making the trawl larger, fisheries problems for over 10 years (Harden Jones and 46 F. R. Harden Jones - A. R. Margetts - M. Greer Walker - G. P. Arnold

Table 6. The parameters of the ARL sector-scanning nique has been applied to a direct measurement of the sonar (from Voglis and Cook, 1966) efficiency af the otter trawl and give the results ob­ tained. Operating frequency 300 kHz METHODS Scanning of receiver beam Scans per second 10 000 The objective was to determine the efficiency of the Scanning sector 30° Granton trawl for plaice (Pleuronectes platessa L.) in Independent channels 75 quantitative terms with an error limit of ± 10 °/o. It Transmitter characteristics was hoped that the results would show the extent, if Array form Curved any, to which the efficiency of the trawl could be im­ Horizontal coverage 30° proved. In principle the method was simple. A single Vertical beam width 5° Nominal range resolution 8 cm plaice carrying an acoustic transponding tag was re­ leased from RV “Clione” and kept under surveillance, Receiver array characteristics at a range of 150-250 m, for at 2-4 h settling-down Array form Linear Horizontal beam width 0-33° period. Then RV “Corella”, towing an otter trawl, was Vertical beam width 10° directed by radio link to catch this particular fish. The Number of elements 75 whole fish-capture process was followed by sector-scan­ ning sonar on RV “Clione” from the moment when McCartney, 1962; Cushing and Harden Jones, 1966). RV “Corella” passed near or over the plaice until The Admiralty Research Laboratory’s 300 kHz sector- either the fish was seen to be caught in the trawl or scanning sonar was fitted in RV “Clione” in 1969. was left behind in the event of an unsuccessful at­ Voglis and Cook (1966) have described the parameters tack. The behaviour of the fish in relation to the gear of the equipment which are summarized in Table 6. was observed and subsequently analysed from film and Mitson and Cook (1971) have described the installa­ video records of the sonar display. Thus the efficiency tion and the stabilization system which allows the of the trawl was determined directly. transducer to be used in the horizontal or vertical mode to present a plan or elevation picture of the target un­ THE SCALE OF THE WORK der surveillance. The facility for horizontal and verti­ The number of attacks required to determine the ef­ cal scanning allows the range, bearing and depth of a ficiency of the trawl at a 95 °/o confidence level can be target to be determined, and the high resolution of the estimated as system gives a very detailed picture on a B-scan dis­ n = 4 pq/L2 play. Thus an otter trawl can be observed at ranges of 100-180 m, and the main warps, otter boards, eddy w here p and q are the binomial proportions or percent­ trails, bridles, dan lenos, headline, footrope, selvedges ages and L is the limit of error (Snedecor and Coch­ and cod-end of the net are clearly visible. An estimate ran, 1967, p. 517). As shown in Table 7, with an error can also be made of the height of the headline above limit of ± 10 °/o, and with an efficiency within the the bottom. range of 40-60 °/o, the number of attacks required The ARL sector scanner can detect 30-40 cm would be about 100. It was clear that the objective roundfish out to ranges of 150 m and it was thought could only be achieved with a considerable research that it might be possible to detect individual fish in effort which would have to be spread over a number front of the trawl and to determine what proportion of of cruises within a 2-3 year period. The dates of the them was caught. But it was difficult to recognize and joint RV “Clione”- “Corella” cruises used for the pro­ follow the signal from a single fish within 0-5 m of the ject are summarized in Table 8. bottom against the reverberation and this led to the development of an acoustic transponding tag which THE FISH returned a powerful and obvious signal when insoni­ The plaice used in the work were caught by otter fied by the pulse transmitted by the ARL scanner. The trawl in the southern and middle North Sea and tag has been described by Mitson and Storeton-West brought back to the Lowestoft Laboratory, where the (1971). Greer Walker, Mitson and Storeton-West best fish were held in large reception tanks for 4-6 (1971) have given a preliminary account of the results weeks. After 4-6 weeks in the laboratory, fish selected obtained when it has been used for tracking plaice, and for the work (size range 30-50 cm) were tagged with Harden Jones, Greer Walker and Arnold (1973) have Petersen discs or button tags. The fish for a particular applied the technique to following Woodhead seabed cruise were taken on board RV “Clione” shortly be­ drifters. In this paper we shall describe how the tech­ fore sailing and kept in large aged galvanized iron deck The efficiency of the Granton otter trawl determined by sector-scanning sonar and acoustic transponding tags 47

Table 7. Number of attacks required to establish the sen for two reasons. Firstly, plaice often come off the efficiency of the gear at a 95 °/0 confidence level bottom at night, and they are then not accesible to the with an error limit of ± 10 °/0 gear. Secondly, although two ships can work close to­ gether at night under good conditions, it seemed pru­ Percentage of fish caught Number of attacks dent — in the first instance - to master the technique by day. 50 100 60 96 70 84 RV “ c o r e l l a ”’s OTTER TRAWL 80 64 90 36 RV “Corella” used a small Granton otter trawl of 95 19 23-8 m headline length. The mesh sizes of the net were 98 8 normal for North Sea trawling, ranging from 140 mm in the forward parts to 80 mm in the cod-end. The bridles were 18-3 m long and the otter board back- Table 8. Dates of joint cruises of RV “Clione” and strops 7-3 m, giving a total distance from otter board RV “Corella” in the Granton trawl programme to dan leno af 25-6 m. The trawl was rigged for general 1971-1974 purpose use and not specially for plaice fishing. Some details of RV “Corella” and her trawl are given in RV “Clione” cruise number Dates Table 9. Compared with the most efficient commer­ 9/71 11-25 Aug cial rig for plaice-catching on good ground, RV “Co- 13/71 4-17 Dec rella”s trawl had longer bridles, fewer and shorter tick­ 13/72 16-27 Oct ler chains, less groundrope chain, and more floats, and 14/72 1- 8 Nov was also fitted with dan leno scuttle bobbins and bo­ 15/72 17-30 Nov 16/72 5-20 Dec som bobbins. The omission of the door-to-door tickler 3/72 31-13 Jan/F eb chain was to enable the possible herding effect of the 12/73 16-24 Aug bridles to be determined. Careful examination of the 16/73 6-19 Nov gear after hauling showed that the backstrop-bridle 17/73 26- 5 Nov/Dec 18/73 11-20 Dec complex never dragged on the bottom. 9/74 1-10 Jul 10/74 15-24 Ju l Table 9. Some details of RV “Corella” and her otter trawl tanks provided with a copious flow of sea water and RV “Corella” Stem trawler 10-15 cm of clean sand. The tanks were covered by netting to prevent fish from leaping out and screened Fishing Registration No. LT 767 by canvas from full daylight or ship’s lighting. The Gross register tonnage 459 plaice buried deep into the sand and gave no indica­ tions of damage due to abrasions by being held in tanks Length overall 41"5 m on deck. The fish were not fed at sea. Brake horse power 1 040 Otter boards 3-05 x 1-27 m, 700 kg THE WORKING AREA Backstrop 7-3 m So far as weather allowed, the work was restricted Bridle 18-3 m to one area in the Southern Bight of the North Sea. Dan leno Scuttle bobbin with butterfly The choice fell on the Black Bank - Brown Bank Legs 3-0 m (lower of 25 mm grounds, 140 km NE of Lowestoft, where the bottom, chain) at a depth of about 26 m, is smooth and relatively free Headline 23-8 m from noise and fasteners, and there is little interference Floats 25 x20 cm diam. from other shipping. Groundrope 36-6 m (6 m bosom with 14x20 cm diam. bobbins EFFICIENCY OF THE TRAWL BY DAY AND BY NIGHT and 2 x 15'3 m wing pieces with 9-0 cm diam. rubber It was thought that the response of the fish to the discs on 22 mm diam. wire, trawl might be different by day and by night which loosely wound with 6 mm could lead to differences in the efficiency of the gear. chain) It seemed best to restrict the effort to day or night Tickler chains Bunt 13*7 m, midwing 22 m, work rather than to attempt both. Day work was cho­ both 16 mm chain 48 F. R. Harden Jones - A. R. Margetts - M. Greer Walker - G. P. Arnold

/.Om Otter the tag transmits a 3 ms pulse at 300 kHz which is very board easily recognized on the sector-scanner display. The tag Backstrop weighs 3-4 g in sea water and exerts a drag force of approximately 0-01 N at a steady flow of 50 cm/s. The battery life is ample for the present requirements and the tags were activated immediately before use at sea by a simple soldering and sealing operation. After the ■Bridle electrical characteristics had been checked, the tag was ready for use.

THE RELEASE OF A TAGGED PLAICE ■11m Dan Leno An activated acoustic tag was attached by a fine nylon braided cord to the eye of the Petersen disc tag- Headline retaining pin immediately before the plaice was re­ leased. As the acoustic tag was being attached, RV Floats Wing “Clione” went slowly astern so that when the fish Groundrope was ready to be released the ship was gliding back­ ■Tickler chains wards with engines stopped. The plaice was care­ fully dropped into the water on the starboard side Bobbins and was usually located by the sector-scanner within a few seconds. As a general rule plaice released in the Black Bank - Brown Bank area immediately went Belly to the bottom. Many plaice moved over the ground during the settling-down period and data relating to the movements of the fish released on one cruise are summarized in Table 10. On other cruises some fish showed little or no movement and the weak signal Cod-end returned by the tag suggested that both fish and tag were partly buried. M e tre s MAINTAINING SURVEILLANCE AND MAKING AN ATTACK Figure 35. Diagram to show, in plan view, the parameters of the otter trawl towed by RV “Corella”. When the fish was released its range and bearing was regularly passed by the sector-scanner operator, not less than once a minute, to the Officer-of-the- A plan view of the fishing geometry of the trawl, as Watch who positioned the RV “Clione” to keep the determined from the sector-scanner display, is shown in Figure 35. The maximum headline height was esti­ fish under surveillance. When the fish had settled on the bottom it was most convenient to maintain po­ mated as 1-5-2 m. Most attacks were made in 26 m of water when the length of warp out (water surface to sition head to tide with the fish on the bow at ranges otter board) was 160 m, and the towing speed through between 150 and 250 m. Station keeping was made the water l-8 m/s (3'5 knots). Under these conditions easier with the use of the Pleuger active rudder. After the spread between the otter boards was 35-40 m, of 2-4 h, RV “Corella” prepared to ‘attack’ the fish which the middle 18 m was covered by the path of the and RV “Clione” changed her station to position the net. target at a bearing of 90° on her starboard beam and at a range of 125-150 m. RV “Corella” with her gear in the water, usually THE TRANSPONDING ACOUSTIC TAG approached the fish from astern of RV “Clione”, but Details of the Mitson-West tags have already been occasionally from ahead. During RV “Corella”s ‘run- published (Mitson and Storeton-West 1971) and only in’, the range and bearing of the fish from RV a few details will be given here. The complete tag, “Clione” was continually relayed by VHF radio link. comprising an electro-acoustic transducer, an electro­ The attack course was based on this information and nic transmitter-receiver, and a Hg battery power sup­ that derived from RV “Corella”s radar using a spe­ ply, is housed in an oil filled polyethylene cylinder ap­ cial plotting board with inscribed lines of range and proximately l'O cm in diameter X 5'0 cm long. When bearing relative to RV “Clione”s heading. The plot­ insonified by the pulse from the ARL sector scanner ting board and its accompanying table of relative ran- The efficiency of the Granton otter trawl determined by sector-scanning sonar and acoustic transponding tags 49

Table 10. Movements of plaice fitted with acoustic corded using a video tape recorder (Sony CV 2100 transponding tags. RV “Clione” cruise 12, 14-24 CE) and black and white polaroid photographs were August 1973. Black Bank/Brown Bank; sea temper­ taken as required. The video facility was very useful ature 16°C and allowed the details of an attack to be repeatedly examined on the ship so that complete notes could Fish Time Time of Distance Speed over the be made to help in the interpretation of the 16 mm number released first attack moved ground film during its subsequent analysis on shore. GMT GMT (m) cm/s Fish lengths/s ANALYSIS AND INTERPRETATION OF DATA 1 ...... 1329 1530 165 2-3 0-06 The attacks provided data on the behaviour of the 2...... 0521 0715 1 188 17-4 0-39 fish in response to the gear and on the efficiency of 3 ...... 0858 1100 363 5-0 0-14 4 ...... 1424 1600 198 3-4 0-09 the gear in terms of the proportion of the fish that 5...... 0546 0730 330 5-3 0-14 came within its influence that were caught. 6 ...... 1049 1230 2 970 49-0 1-40 In the laboratory the 16 mm films were examined 7 ...... 1445 1545 396 11-0 0-28 8 ...... 0542 0730 594 9-2 0-24 with a Specto 16 mm motion film analyser to obtain 9 ...... 0508 0715 594 7-8 0-21 detailed information on the position of the fish in 10 ...... 0900 1330 3 300 20-4 0-41 relation to the trawl and, for those fish that respond­ 11 ...... 1522 1630 462 11-3 0-23 ed to the gear, a frame-by-frame analysis of their movements. When a true picture was needed, select­ ed frames were projected and traced and the original ges and angles between the fish and the two ships rectilinear display was converted to a polar coordi­ was used to determine the range at which RV “Co­ nate system. rella” should maintain a straight course to pass abeam of RV “Clione” and over the target. This estimate was continuously revised and set on the variable range CALCULATION OF THE EFFICIENCY OF THE GEAR marker ring on RV “Corella”s radar. Fine adjust­ The efficiency of the gear was based on the valid ments of course were made to bring the ring exactly attacks as defined by the following criteria: on RV “Clione”s radar echo. RV “Corella” was required to maintain a straight 1 the fish was on the bottom and inside the area swept course when approaching within 50 m of the fish. At by the otter boards; this stage checks were always made in vertical scan 2 there was no sudden change of course by RV “Co­ to make sure that the plaice was on the bottom and rella” as the gear approached the fish; accessible to the gear. 3 the attacks were independent. It will be appreciated that the highest standards of seamanship are required in handling vessels safely at The chance of capture would appear to depend on these short ranges and it is appropriate to acknow­ the position of the fish in relation to the gear, and ledge the skill and care of the Masters and Officers a distinction was made between four positions inside of both research vessels, without whose effort the the area swept by the otter boards: work could not have been carried out. After a successful attack, RV “Corella” was direct­ i. within 3 m of the boards; ed to haul her gear. If the fish was not caught, fur­ ii. between 3 m from the boards and the line of the ther attacks were made until the fish was either dan lenos (backstrop-bridle) ; caught or abandoned. Turning between a series of iii. between the dan lenos and the line of the wing- attacks was speeded up by RV “Corella” partially ends of the net and thus very close to but out­ hauling her gear so that the otter boards were lifted side the path of the net; out of the water. The trawl was shot away at the iv. within the line of the wing-ends and thus directly start of the next attack. In an extended series the in the path of the net. attack rate usually averaged 2'5 attacks per hour. As regards the third criterion, the results would be biased by including repeated attacks on a fish which RECORDING OF DATA could not be caught, for example, because it was The B-scan of the sonar display was recorded on buried deep in the sand. In such a case only the first 16 mm black and white (Ilford FP4) or colour (Ko­ attack in which the fish was in any one of the four dak Ektachrome 7241) film during each attack. The positions was used for determining the efficiency of display and the operator’s comments were also re­ the gear and one fish could therefore contribute a

4 50 F. R. Harden Jones - A. R. Margetts - M. Greer Walker - G. P. Arnold

Table 11. Total number of attacks made on plaice path of the net; some fish stopped short of the posi­ carrying acoustic transponding tags, August 1971- tion in which they would have been directly in its July 1974 path, but none swam right across the net to avoid capture. The ‘herding’ movements did not appear Total Outside Inside otter boards to be at right angles to the line of the bridle and num ber otter Total Valid most plaice swam relatively slowly, at speeds up to of attacks boards attacks 3 fish lengths per second. 4. Fish lying directly in the path of the net were not n ...... 343 143 200 166 disturbed as the headline passed over them, but 100 41-6 58-2 48-3 •/0...... some responded just before the approach of the groundrope. This suggested that they were disturbed maximum of four valid attacks. Results from repeated by the tickler chains. There is no evidence to sug­ attacks on a fish in any of the four positions were gest that the plaice swam along with the ground­ only used for information on its response to the gear. rope, because it was impossible to make a distiction between fish which kept up with the gear by swim­ ming and those that would be carried along with it RESULTS if the tag became meshed in the net. Altogether 153 plaice were released and 343 attacks 5. A proportion of the plaice lying directly in the path made on 124 of these fish. As shown in Table 11, in 200 of the net were not caught. In some cases the poor attacks the fish was positioned between the otter signal from the acoustic tag suggested that the fish boards and the final number of valid attacks amounted was well buried in the bottom. In one attack a to 166. The results are summarized in Table 12. check in vertical scan showed that a plaice was still The detailed analyses of the movements of the on the bottom after being passed over by the net plaice in response to the gear have not yet been com­ but it came up off the bottom about 10 s later and pleted, but the following points may be made. stayed in midwater for 20 min before returning to the bottom.1 1. Although most fish moved about during the ‘sett­ ling-down’ period, there was no increased activity during RV “Corella”s ‘run-in’, even when the ves­ DISCUSSION sel passed directly overhead in a total depth of 25- Plaice caught in trawls and kept in the laboratory 30 m. cculd differ from wild fish in their response to stimuli. 2. Plaice positioned outside the otter boards were ne­ But it is not possible to predict how this might bias the ver caught and only three of these fish reacted to present estimates of gear efficiency. The observations the gear. An otter board passed within 1-2 m of these of ‘herding’ movements made in these experiments can fish which responded by swimming rapidly for 40- be compared with those made by scuba divers on the 50 m away from the line of tow. reactions of plaice and other species to the Danish 3. Some of the plaice positioned between the otter seine, as described by Hemmings (1969). Hemmings boards and the wing ends of the net moved directly (1969, p. 647) noted that “the most striking observa­ into the path of the net. The otter boards appeared tion was the absence of any behaviour which could be to be most effective in disturbing the fish and the regarded in any respect as ‘panic-stricken’. The swim­ bridles — which were off the bottom (see p. 47) - ming of fish of all species observed with the exception had little effect. All the plaice swam in towards the of mackerel appeared to be quite natural.” The reac­ tions of plaice to the approach of the otter trawl ap­ Table 12. Number of valid attacks made and number peared to be similar: the tape recordings made during of fish caught in trials to determine the efficiency of the attacks and notes made immediately after, refer to a Granton trawl the almost leisurely and casual movements of the plaice which should be reflected in the swimming Position N um ber N um ber Efficiency of speeds when the analyses are completed. of fish of valid of fish traw l Hemmings (1969) reported that flatfish usually attacks caught °/0 error limit swam at right angles to the advancing warps of the seine net so that in the early stages of hauling there is Within boards 166 73 43-9 ± 7-7 Between boards 1 A 16 mm film of the sector-scanner display showing RV and wing ends 72 16 22-2 ± 9-9 “Corella”s otter trawl, a plaice carrying an acoustic transponding tag, and an attack leading to a ‘herding’ response was shown at 57 60-6 ±10-1 In path of net 94 the meeting. The efficiency of the Granton otter trawl determined by sector-scanning sonar and acoustic transponding tags 51

a forward component to the movement. Preliminary f fish shoals: sea trials on RRS “Discovery II”. J. Cons. int. analyses of the present observations suggest that when Explor. Mer, 27: 141-49. Harden Jones, F. R. & Scholes, P. S. 1974. The effect of the door- the fish moves there is little or no forward component, to-door tickler chain on the catch-rate of plaice (Pleuronectes and it swims more or less directly into the path of the platessa L.) caught by an otter trawl. J. Cons. int. Explor. Mer, net. 35: 210-12. The overall efficiency of this particular otter trawl, Harden Jones, F. R., Greer Walker, M. & Arnold, G. P. 1973. The movements of a Woodhead seabed drifter tracked by sector on the Black Bank — Brown Bank ground, is about scanning sonar. J. Cons. int. Explor. Mer, 35: 87-92. 44°/0 ± 8 °/0. The gear was not rigged with a door-to- Hemmings, C. C. 1969. Observations on the behaviour of fish door tickler chain. Haul-by-haul catch data from a during capture by the Danish seine net, and their relation to Lowestoft trawler suggests that the door-to-door tick­ herding by trawl bridles. Fish. Rep. FAO, (62) 3: 645-55. Mitson, R. B. & Cook, J. C. 1971. Shipboard installation and ler increases the efficiency of the gear by 2-3 times trials of an electronic sector-scanning sonar. Radio electron. (Harden Jones and Scholes, 1974). Table 12 suggests Engr, 41: 339-50. that it would be reasonable to expect a marked in­ Mitson, R. B. & Storeton-West, T .J . 1971. A transponding crease in the efficiency of the gear in respect of those acoustic fish tag. Radio electron. Engr. 41: 483-89. Scott, R. 1894. Improvements applicable to trawl nets. British fish lying between the otter boards and the wing ends patent (3187), 2 pp. of the net. Few attacks should be required to show Snedecor, G. W. & Cochran, W. G. 1967. Statistical methods. this and the first observations on the effect of the Iowa State Univ. Press, 6th edition, 593 pp. door-to-door tickler, using the sector-scanner, were Vigneron, J-B.J. A. 1920. Improvements in and relating to trawl­ ing gear for deep sea fishing. British patent (175,824), 5 pp. made in July 1974. Vigneron, Dahl et Cie. 1922. Improvements in trawling gear. British patent (189,750), 4 pp. Vigneron, Dahl et Cie. 1923a. Improvements in fishing nets. REFERENCES British patent (192,064), 2 pp. Vigneron, Dahl et Cie. 1923 b. Improvements in trawling gear. Bagenal, T. B. 1958. An analysis of the variability associated with British patent (209,032), 2 pp. the Vigneron-Dahl modification of the otter trawl by day and Vigneron, Dahl et Cie. 1923 c. Improvements in trawling gear. by night and a discussion of its action. J. Cons. int. Explor. Mer, British patent (210,396), 2 pp. 24: 62-79. Vigneron, Dahl et Cie. 1924a. Improvements in trawling nets. Cushing, D. H. & Harden Jones, F. R. 1966. Sea trials with modul­ British patent (211,165), 1 p. ation sector scanning sonar, with an appendix by J. A. Gulland. Vigneron, Dahl et Cie. 1924b. Improvements in trawling or the J . Cons. int. Explor. M er, 30: 324—45. like nets. British patent (213,906), 3 pp. Greer Walker, M., Mitson, R. B. & Storeton-West, T. 1971. Trials Vigneron, Dahl et Cie. 1924 c. Improvements in trawling gear. with a transponding acoustic fish tag tracked with an electronic British patent (232,914), 2 pp. sector scanning sonar. Nature, Lond., 229: 196-98. Voglis, G. M. & Cook, J. C. 1966. Underwater applications of an Gulland, J. A. 1969. Manual of methods for fish stock assessment. advanced acoustic scanning equipment. Ultrasonics, 4: 1-9. Part 1. Fish population analysis. FAO Man. Fish Sei., (4), White Fish Authority 1960. Annual Report and Accounts for year 154 pp. ended 31 M arch 1960, 50 pp. H arden Jones, F. R. & M cCartney, B. S. 1962. The use of elec­ White Fish Authority 1963. Annual Report and Accounts for year tronic sector-scanning sonar for following the movements of ended 31 M arch 1963, 55 pp.

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