International Council for C.M.1994/B:15 the Exploration of the Sea Capture Committee

Comparative Fishing with FRVs "Walther Herwig" and "Walther Herwig 111"

by

Siegfried Ehrich*, Erdmann Dahm**, Holger Dornheim*, Klaus Lange**, Nils Mergardt*** & Manfred Stein*

* Sundesforschungsanstalt für Fischerei, Institut für • Pa~maille Seefischerei, 9, 22767 Hamburg, Germany

** Bundesforschungsanstalt für Fischerei, Institut für Fischereitechnik, Palmaille 9, 22767 Hamburg, Germany

*** Institut für Hydrobiologie und Fischereiwissenschaft, Universität Hamburg, Olbersweg 24, 22767 Hamburg, Germany

Abstract

Comparative fishing experiments were carried out in two small areas of 100nm 2 each in the German Bight and in the northern North Sea to estimate possible differences in the relative fishing power of "Walther Herwig 111" and "Walther Herwig", which has been replaced by· the former. Both vessels were equipped with the standard GOV-trawl and the standard ground gear, as used in the former 'International Young Fish Survey' and the present ' International Sottom Trawl Survey' in the North Sea. In each area both vessels independently made two sampie series of hauls, each at randomly selected stations with random towing directions to smooth out effects such as currents and.tides. No differences were found in the catch in number for the age-groups 1 to 4 for whiting and haddock and for the total catch in number for dab.

1. Introduction

During the conduct of the lCES coordinated ' International Sottom Trawl Survey' (former lYFS and IYHS) in the North Sea a large amount of standardization work became necessary to make the survey results more reliable. A standard method and standard gear were introduced, at least for the survey in the first quarter. However the variability of the catch data remained very high and possible differences in the catching . , t

power of the partizipating research vessels were regardedto be partly responsible for this variability (SPARHOLT 1990).

Due to the replacement of the German research vessel 'Walther Herwig' at the end of 1993 it became necessary to conduct a comparative fishing experiment to determine conversion factors for correcting possible breaks in the time series as a result of differences in the fishing power of the vessels. As the report of the 'Workshop on the Analysis of Travl Survey Data' (ANON. 1992a) gives no proposals for a practicable. comparative fishing experiment, the same method as in 1986, when the FRV 'Anton Dohrn' was replaced by the 'Walther Herwig', was therefore used (EHRICH 1991).

2. Material and Method

2.1. Description of the vessels

"Walther Herwig", designed as a stern trawler with ramp, was constructed at the "Schlichting-Werft" shipyard in Travemlinde and was delivered in 1972.

The main dimensions are:

Length over all L o • a • 77.45 m Length between perpendiculars L pp 69.00 m Length of waterline at design draught LCWL = 73.30 m Beam moulded B 14.90 m Depth to main deck Hr 7.40 m Depth to shelterdeck Hrr 9.90 m Design draught TOKK 5.90 m Displacement D 3200 t Tonnage 2250 BRT Main engine power P 2 x 1725 kW Engine type MAN V8V22/30 ATL • Bollard pull Z = 450 kN Propeller type controllable pitch (CP)

The design of the vessel corresponded to the standard of modern freezer trawlers of 1972.

The main travl winch, with all auxiliary drums, was placed at the fore end of the working deck, a netdrum for midwater trawls was also available. There were laboratories for fishery biology, microbiology, chemical investigations and general purposes.

The "Walther Herwig 111" was built at the shipyards "Peehewerft, Wolgast" and "Rolandwerft, Berne", both members of the "Hegemann-Group". This new vessel replaces the research ve sse 1 "Wal ther He n-lig", decommiss ioned in January 1994. ·"

The main dimensions of "Walther Herwig III" are:

Length over all La • a . 64.'50 m Length between perpendiculars Lpp = 54.50 m Length of waterline at design draught LCWL 57.50 m Beam moulded B = 14.80 m Depth to main deck HI 6.00 m Depth to shelterdeck HII 8.90 m Design draught TORR 5.20 m Maximum draught Tmax 5.96 m Displacement D 2485 t Hain engines power P 1800 kW Main engine type HAK 6M ·453 Additional DC-propulsion motor PE = 1100 kW Propeller type controllable pitch propeller, fixed Kort nozzle Bollard pull Z 500 kN / .....

The basic design of the vessel corresponds to that of "Walther Herwig". Technical details have been improved on according to the developments in the field of distant water fishery and tishery research since 1972.

Especially the deck layout has been changed. completely compared with "Walther Herwig". The trawl winch is split into two separate drums mounted on the port and starboard side of the rear part of the working deck. A net drum 1s positioned at boat deck level. Two deck cranes of 3.5 t/12 m and 3 t/10.4 mare available for handling fishing gear and other equipment on deck. Laboratories for general purposes, hydrography, fishery biology and biochemistry are situated at working and catch processing deck level.

2.2 Gear

The gear chosen for the comparison was the 36/47 GOV standard trawl. This weIl documented fishing gear is used now for over ten years in international fishing surveys' of ICES. Soon after its introduction in 1976 the need was feIt to standardize the gear, and an optimum rigging plan was set up after extensive flume tank trials (WILEMAN 1984). The manual prepared for this purpose was repeatedly revised (1981,1986). The present rig-up was published in a very detailed paper in 1992 (ANON. 1992b)

Compliance with this actual reference concerning rigging and operation of the GOV as sampling trawl was examined on both ships prior to the fishing trials. Deviations, found on "Walther Herwig" in the number of floats on the he~dline (56 instead of 60) and in the length of the upper bridles (20.2 m instead of 20 m), were made good by adding the missing number of floats and by adding extensions to the corresponding bridles at lestridge and lower wing. •.

Gear technological measurements were carried out during all hauls. On "Walther Herwig" the net opening height was measured continuously and recorded by a conventional cable netsonde. Spread at the wingtips was measured by SCANMAR distance measuring sensors. However, technical difficultie s wi th the loading of the sensors prevented their operation during every haui.· As can be seen from Tab. 1 synchronous measurements of height and spread were only obtained on 6 of 26 hauls at Box A and on 9 of 27 hauls at Box D.

Due to a failure of the computer program installed for recording the SCANMAR data, recording had to be done by noting per hand the actual value of spread giving a set of 15 data per haul. The echograms of the net opening were evaluated later by taking at 15 different points in equal distance measurements of the continuous record .

Then, mean and variation coefficient were calculated for every set of 15 data ·and f illed into Tab. l. Except 0 f the few set of data indicated, variation kept in reasonable limits of 3 to 4 percent.

Measurements on " Walther Herwig III" were made Hith a new purchased ITI-trawl measuring system (Tab. 2).

2.3. Sampling

The comparative fishing experiments took place from the 28 of December 1993 to the 4 of January 1994 in two small limited areas (boxes) of 100nm 2 each, one in the German Bight (64°17'N to 54°27'Ni 06°58'E. to 07°15'E) and the second off the Scottish east coast (57°48'N to 58°58'Ni 00044'W to 01°04'W).

In each of the two boxes a minimum of 25 hauls per vessel was planned, assuming a relatively homogeneous distribution of haddock and whiting in both areas, comparable with the­ situation in 1986 (EHRICH, 1991). This target number could not be reached due to some technical problems aboard the new 'Walther Herwig III'. In Tab. 5 the number of hauls per area and vessel is listed.

The vessels were operating simultaneously but independently in the boxes. Hithin a box the shooting positions and the tOHing directions ~lere randomly selected for each of the 3 days in each area t.O avoid systematic errors. Fig. 1 shot'1s the distributions of hauls per vessel and area. The towing duration Has 30 min. Hith a constant tOHing speed of nearly 4 knots over ground. During sampling in anyone area there Has no changes in the gear and ship parameters.

For each species the catch in numbers, the weight and the length Here determined. Otoliths Here taken from haddock and Hhiting to split the total number into age-groups, this being the base of the calculations. Oceanographic profiles of temperature and salinity were obtained by means 0 f a CTD 1 rosette system Hi th reve rsing thermometers for temperature calibration. 14 profiles \lere taken in Box A (29./30.12.93) and 0 (2.-4.1.94), with 43 water sampies taken at different depths for calibration of the salinity profiles.

2.4. Evaluation

The evaluation of the experiment was restricted to the most frequent and homogeniously distributed species or age-groups in an area. By means of the experimental design and the constant conditions a further standardisation and transformation of the original catch data was not necessary. In Tabs. 3 and 4 the catch data per area, species and age­ group are listed. For the direct comparison ·of t\;fO independent sampies a non-parametric test, the Mann-Whitney­ U-test, was chosen, believing that both curves of distribution are of the same type and that there is only a difference in the median values.

3. Results·

3.1. Environmental conditions

Wind conditions varied considerably during the periods of observation in Box A and D. Whereas southwesterly winds of Beaufort 7 prevailed in Box A, southeasterly winds of 6 were encountered in Box D.

The southern Box (A) showed a weak, thermohaline strati f ication , ranging from 5.41 °C at the sea surface to 6.68 ° C in the bottom layer and salinitie s ranging in the respective layers from 33.356 PSU (Practical Salinity Units) to 33.820 PSU. Largest gradients in the thermohaline stratification were observed in the southeastern corner of Box A. It would appear that the salinity gradient here reflectsthe influence of freshwater discharges due to rivers ( e . g. EIbe).

Completely different conditions \;fere obteüned in Box 0 off Northeast Scotland. Under the influence of the Atlantic water mass entering the North Sea, a homogenous water column was f~und at about 100m water depth. Temperature gradients at the individual stations were less than 0.05K, salinity differences between surface and bottom \'1aters amounted to about O.02PSU. The observed actual temperature/salinity value ranged from 7.60°C/35.157PSU at the surface to 7.70°C/35.088PSU in the bottom layer. 3.2. Gear measurements.

Results of gear measurements on a haul by haul basis are presented in Tabs. 1 and 2 Further evaluation of all data compiled per fishing area and ship yielded in average net opening and \vingspread values. The relevant coefficients of variation for both ships are presented in the following text table:

Ship Box A Box D

height v.c spread V.c. height v.c. spread .. V.c.

Walther 5.6 9.82 20.02 2.61 5.76 8.73 21.21 5.97 Herwig

Walther 5.4 5.79 19.53 5.15 5.56 10.3 20.1 3.22 Herwig III

3.3. Catches

The results of the comparison of both vessels related to dab (total catch) and 1 to 4 years old haddock and whiting are shown in Tab. 5. The non-parametric "Mann-Whitney V-test" determined no significant differences in the fishing power of both vessels, for the test value p in all cases is considerably higher than 0.05 .

• 4. Discussion Keeping in mind the experiences gained from the comparative fishing expe riment in 1986 (EHRICH 1991) and \-1 i th the f ish distribution patterns within small areas, a square area of only 10nm side length was chosen; this being considered large enough to avoid interaction between the vessels. As only 1% of the total working area was swept by the net, a 'thinning­ out' effect on the catch data can be neglected

A dependence of net opening measures on depth as illustrated by the data above has been mentioned already earlier (KOELLER 1990, DAHM 1991). GODe and ENGAS (1990) found a considerable depth dependency of the swept area. In their data sets of the Norwegian bottom trawl survey in the Barents Sea the depth varied between 50 and 600m and the wingspread between 11 and 19m.

The ditterences in the mean wingspread of the described experiment between Box A (40m depth) . and Box D (95m depth) are tor both vessels only 0.6 and 1.1m. Possible bigger .; ..

differences were probably smoothed by the fact that in agreement with the existing ICES directives in Box D (depths > 70m) 100 m bridles were used instead of 50 m as in Box A. However, wi thin the very small depth range of a box the wingspread and the vertical height of the net vary relatively to the current and wind directions (Tabs. 1 and 2).

EHRICH (1992) showed that in any. such experiment a standardization of mean towing distance, of mean swept area or mean swept volume does not decrease the variance of the catch data. This has also been affirmed for the present sets of data. This· means, that using a giyen standard gear and standard fishing procedure in a small area, and if the towing time is always the same and the towing speed over ground is nearly constant (re1. coef. 0 f variance less than 10%), the high variability of the catches due to the patchy distribution of the fish species even in small areas conceals the existing positive correlation between the catch and the swept area. The original raw data were therefore used as input data for the statistical analysis.

While a sample size of 25 hauls was sufficient in 1986 to get representative results for haddock and cod (EHRICH 1991), only 14 (Box A) and 12 (Box D) hauls could be carried out by the new 'W. Herwig III'in the present experiment. Compared to the usual statistical requirements for bottom trawl surveys, such as the "International Bottom Trawl Survey" in the North Sea, the number of hauls of the new "Wo Herwig III" in this experiment appears to be sufficiently high despite their comparatively low number, which nevertheless give relatively clear results to allow the use of the catch data of "Walther Herwig III" without the need 6f any conversions .

5. Literature • ANON. 1992a: Report of the Workshop on the Analysis of Trawl Survey Data. ICES C.H.1992/D:6

ANON. 1992b: Report from FTFB Subgroup: Evaluation of sources of variability in the fishing power of the GOV-Trawl. ICES C.H.1992/B:39

DAHH, E. 1992: Neue Erkenntnisse zum Fangvorgang in und vor Grundschleppnetzen, Teil II: Einfluß einiger externer Faktoren. Infn Fischw.38(1),20- 23

EHRICH, S. 1991: Comparative fishig experiments by research trawlers for cod and haddock in the North Sea. J.Cons.int.Explor.Mer, 47:275-283

EHRICH, S. 1992: Do different towing distances.influence the . var1ance of catch data, if the towing time 1s constant? ICES C.H.1992/B:24 • •

GODO, O.R. and Engas, A. 1989: Swept area variation with depth and its influence on abundance indices from trawl surveys. J. Northw. Atl. Fish. Sei., 9(2):133-139

KOELLER,P.A. 1990: Controlling the variability of survey gear performance. lCES C.M.1990/B:3,1-19

SPARHOLT, H. 1990: Using GLM analysis on the lYFS herring data for the North Sea. ICES C.M.1990/H:6

WILEMAN,D. 1984: Model testing of the 36/47 m GOV young fish sampling trawl. Dansk Fiskeritechnologisk Institut Report. March 1984. Paper referred to in ICES C.M.1984/B:3 Tablc 1: Trawl opcning mcasurcmcnts on FRV "Walthcr Hcrwig"

a: Box A

Haul Nr. opcnmg vanation ....QRcnmq vanatJon rcmml

n.d.= not determined ..•...•

b: Box 0

Haul Nr. opentnQ vanatJon opentnq vanatJon remarl

Haul Nr. openlnq variation openlnq variation remarks height-m- coefficient width-m- coefficient 3 5.5 1.27 19 0 4 5.5 1.27 19.5 3.62 '5 5.4 7.85 19 0 6 5.1 5.3 20.2 2.46 7 5.3 3.23 19.5 2.96 8 ·5.6 11.81 20.6 2.79 9 5.5 1.81 20.2 2.21 10 5.2 8.05 20 3.53 11 4.9 3.53 20 0 12 5.7 6.56 17.2 7.58 13 5.4 3.2 19.8 2.25 14 5.6 3.57 19.3 2.98

b: Box 0

Haul Nr. openlnq variation openlnq variation remarks heiqht-m- coefficient width-m- coefficient 18 5.4 3.3 20.4 2.68 19 5.1 3.17 21 0 20 5.3 11.47 no readings for width 21 4.2 4.87 17.6 3.26 22 6 9.77 18.6 8.74 23 5.9 6.81 19.6 . 2.93 24 5.6 6.21 20.6 2.65 25 5.7 7.6 19.5 4.29 26 5.9 2.99 20.8 2.15 27 5.8 0 19 0 28 5.6 1.78 21 . 0 29 5.4 3.94 21 0 30 5.6 10.4 20.6 . 2.79 ------1

Tab. 3: Catch in number per species ahd age-group in BOX A

A: W. Herwig B: W. Herwig 111

station Whltlng Idab statIon whltlng IdaD number aQ1 total n number aQ1 total n 96 3069 -'mnr 1 3tlf 331 97 1253 ·910 2 89 229 98 1474 839 3 55 236 99 1148 760 4 1093 392 100 358 240 5 1300 351 101 .360 129 6 270 234 102 197 265 7 711 180 103 77 278 8 1455 1753 104 839 374 9 147 .308 105 48 113 10 722 318 106 30 155 11 344 226 107 247 190 12 148 252 108 208 161 13 247 97 109 289 239 14 200 92 110 1793 553 111 877 464 112 236 212 113 121 86 114 108 46 115 63 136 116 106 81 117 215 117 118 197 121 119 206 93 120 957 445 121 745 995

• Tab. 5: Results of the comparison between 'Wo Herwig' and 'Wo Herwig 111' per box, spe~ies and age-group (Mann-Whitney-U-test)

variable number of hauls test values W.H.(n) W.H.lli(m) U p BoxA dab total 26 14 168 0.6914 whiting AG1 26 14 175 0.8316 Box D haddock AG1 27 12 160 0.9515 haddock AG2 27 12 156 0.8551 haddock AG3 27 12 131 0.3455 haddock AG4 27 12 123 0.2351 whiting AG1 27 12 147 0.6481 whiting AG2 27 12 155 0.8313 whiting AG3 27 12 144 0.5839 whiting AG4 27 12 145 0.6049 . Tab. 4 : Catch in number per species and age-group in Box 0

A: W. Herwig

station . whltln~ haddocK number aq1 aq2 ag3 aq4 aq1 ag2 ag3 ag4 122 129 793 494 11 165 684 66 14 123 248 1906 648 60 182 638 83 27 124 324 214 116 19 337 1117 135 43 125 1191 1565 445 45 1024 2068 252 43 126 .2655 2765 642 59 1431 2373 220 35 127 1283 3281 931 96 2810 2762 322 58 . 128 657 2245 1250 204 990 2263 606 166 129 1201 1716 561 30 485 2005 364 73 130 . 352 3797 2634 330 158 876 153 34 131 205 1381 726 53 141 630 103 22 132 1197 2835 970 110 593 2355 187 30 133 1407 2031 427 31 526 1294 138 19 134 1592 904 325 46 1253 2164 143 36 135 519 835 239 18 327 . 868 114 35 136 628 482 193 21 1069 1825 336 71 137 478 1711 874 276 106 1321 210 37 138 669 1097 360 38 531 974 72 14 139 258 568 153 16 156 404 45 4 140 447 1620 524 75 294 1056 185 49 141 940 1992 639 119 875 4829 514 71 142 873 1814 1145 258 3399 3518 423 50 143 964 1668 516 62 1421 2533 381 96 . 144 219 2010 980 100 463 1947 241 58 145 557 675 131 13 527 740 33 2 146 339 840 349 49 515 2276 186 47 147 119 722 415 78 486 1381 368 78 148 2230 3337 992 102 2946 2106 266 18

B: W. Herwig 111

~~~llon whitln~ naddocl< 11 ber ag1 ag2 ag3 ag4 ag1 ag2 ag3 ag4 1B 324 1B/1 93B 145 1B36 3580 1111 üb 19 296 1673 1406 442 806 1121 97 14 20 916 671 79 0 144 1095 189 46 22 560 2281 720 75 535 1387 188 40 23 1419 2104 715 88 1461 2401 470 143 24 3523 1998 0 0 3055 2360 213 37 25 369 1293 513 75 768 1285 312 55 26 92 597 198 9 196 927 235 58 27 373 127 22 1 104 240 55 22 28 840 1032 295 ·12 272 353 58 13 29 612 4485 1752 164 869 2610 619 112 30 303 1112 529 132 170 2660 1549 481 • •

a: Box A, "Walther Herwlg. 111"

---- '77 o15'Ec

b: Box A, .g" "Walther Herw__l ----:

~ %\ ------1-0-6--- c: Box D, "Walther Herwig IIl"

23

d: Box D, "Walther Herwig"

------~ 136

.. --=_~12...... 2:.....

Q044'W

Fig. 1 a-d: Distribution of hauls and towing direction per box and vessel