Selectivity experiments with escape windows in the North

Sea Nephrops (Nephrops norvegicus) trawl ®shery

N. Madsena,*, T. Moth-Poulsena, R. Holstb, D. Wilemana

a DIFTA, Danish Institute for Fisheries Technology and Aquaculture, The North Sea Centre, DK-9850 Hirtshals, Denmarkb ConStat, The North Sea Centre, DK-9850 Hirtshals, Denmark

Received 21 April 1998; received in revised form 17 February 1999; accepted 2 March 1999

Abstract

Selectivity experiments were carried out in the northern North Sea at the Fladen Ground and the East Ground from a Danish

commercial vessel. A conventional 70 mm Nephrops codend and a similar codend with a 90 mm square mesh panel window

inserted in the top panel were tested simultaneously in a twin trawl rig. Selectivity parameters and differences in catches of the

target species Nephrops and the by-catch species whiting, haddock and cod were estimated. The logistic function was found to

give the best description of ®sh selection ogives, whereas the asymmetric complementary loglog function described Nephrops

selection best. The Fryer model was used to describe and compare the selection ogives for haddock, whiting, cod and

Nephrops in the two codends. Due to low catches of cod and Nephrops only four out of 11 hauls could be used for modelling

the selection of these two species. Selection factor and selection range were larger for the window codend for all ®sh species.

The codend catch size had a signi®cant negatively correlated in¯uence on l50 and selection range for whiting in the window

codend. There were large signi®cant reductions in numbers of cod and haddock under the Danish MLS with the window

codend. Catches of small cod, however, were low. Very few whiting under MLS were retained in either codend but there was a

large signi®cant reduction in individuals over MLS with the window codend. A large proportion of the Nephrops in the

codends were under MLS. A reduction in catch weight of Nephrops over MLS was found for the window codend, but catches

of Nephrops were low for several hauls. # 1999 Elsevier Science B.V. All rights reserved.

Keywords: Selectivity; By-catch; North sea; Trawl; Windows; Nephrops norvegicus

1. Introduction

As the Nephrops ®shery has not been in¯uenced by

quotas the Danish nominal catches of Nephrops in the

North Sea increased from 478 tonnes in 1987 to reach

1.315 tonnes in 1996 (Anon, 1997). The Nephrops

®shery in the North Sea, especially in the northern

part, is more of a mixed species ®shery than the

Nephrops ®shery in the Kattegat-Skagerrak area.

The area around Fladen Ground is especially impor-

tant for the Danish Nephrops ®shery in the northern

North Sea.

There is a considerable difference in the minimum

allowed mesh size in the North Sea for the Nephrops

and demersal round®sh ®sheries, 70 and 100 mm,

respectively. This gives a considerable catch of under-

sized round®sh in the Nephrops ®shery. Because of

Fisheries Research 42 (1999) 167±181

*Corresponding author: Tel.: +45-3396-3200; fax:+45-3396-

3260; e-mail: mm@dfu.min.dk

0165-7836/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 5 - 7 8 3 6 ( 9 9 ) 0 0 0 2 4 - 7

decompression it must be assumed that physoclistous

round®sh have a low probability of surviving discard-

ing, whereas round®sh escaping through the codend

meshes during trawling have chances of survival

(DeAlteris and Reifsteck, 1993; Soldal et al., 1993;

Sangster et al., 1996). In view of the low state of

economically important round®sh stocks such as cod

and haddock in the North Sea (Anon, 1996), an

improved escape of undersized ®sh from Nephrops

trawls should bene®t these stocks and contribute to a

better utilisation of the resources.

A square mesh panel (henceforth window) placed in

the top panel of the trawl is a simple and cheap method

to achieve this. Use of these windows is already

stipulated for UK vessels according to their national

legislation, and is likely to be incorporated in future

legislation for the North Sea area for all European

Union ¯eets (Council Regulation 850/98). The win-

dow is today used on a voluntary basis by several

Danish vessels ®shing Nephrops in the North Sea.

Experiments with windows carried out in Icelandic

waters (Thorsteinsson, 1992), the Irish Sea (Briggs,

1992), the North Sea (Arkley, 1990) and the Kattegat-

Skagerrak (Ulmestrand and Larsson, 1991) have all

shown an encouraging release of smaller demersal

round®sh. These studies were made by comparing

codend catches of a window codend with those of a

conventional Nephrops codend. No codend selectivity

parameters were estimated. For an optimal under-

standing of the selectivity process, and to enable

®sheries managers to assess the overall bene®ts of

using selective ®shing gears, the selectivity para-

meters of window codends and corresponding con-

ventional codends are required.

The aim of this experiment was, therefore, to

estimate the selectivity parameters of a conventional

and a window codend ®shed under commercial con-

ditions in the northern North Sea Nephrops ®shery.

2. Methods

2.1. Vessel, gear and fishing grounds

It was decided that the covered codend method,

(Wileman et al., 1996), should be used to measure

codend selectivity and that the two codends should be

compared simultaneously using a twin trawl rig. In

order to handle two sets of hooped, covered codends it

was found to be most convenient to use a stern trawler

with a ramp. The 26 m long commercial vessel Tan-

nisbugt built in 1979, with a gross register tonnage of

148.6 tonnes and an engine power of 775 HP, was

chartered. This vessel is typical in size of the ¯eet of

larger vessels carrying out a mixed ®shery on

Nephrops, demersal round®sh and ¯at®sh with twin

trawl rigs but is the only one ®tted with a stern ramp.

The trawl design used was a 544 � 110 mm TV-

trawl, a combined ®sh and Nephrops trawl, often used

in the North Sea Nephrops ®shery when ®sh by-

catches are important. Fishing height was about 4 m

and wing-end spread about 23 m, two identical trawls

were used in a three towing warp twin trawl system.

Warp length used was 503 m on all hauls, back strops

7 m, sweeps 157 m and bridles 13 m. Cambered vee

otterboards of approximately 5 m2 were used and a

centre warp clump weight of approximately 700 kg. A

sketch of the trawl is shown in Fig. 1.

Two identical 4 m long Nephrops codends, made in

green double 2.5 mm braided polyethylene, with a

nominal inside mesh size of 70 mm and a circumfer-

ence of 92 open meshes were used. A 2 m long (39

open meshes) and 1.15 m wide (23 open meshes)

square mesh panel window was inserted in one trawl

at the aft end of the 8 m long 80 mm full mesh

extension piece (which had a rather low taper and

was joined to the relatively short codend), 2 m in front

of the codend lifting strop and 4 m in front of the

codline (see Fig. 1). It was made of nominal 90 mm

black knotless 4.9 mm Ultra Cross polyethylene net-

ting. The window was placed between the selvedges.

One square mesh was attached to two diamond meshes

at the ends of the window. Directions for inserting

windows are given by Robertson (1993).

Covers with a nominal mesh size of 35 mm were

made of green polyamide, were 11 m in length,

attached 1 m in front of the window and extended

4 m beyond the codend as shown in Fig. 1. A 3 m long

zip was inserted in the upper part of the cover, making

it possible to take the codend out of the cover. Covers

were held from the codend by means of two blue

plastic hoops (Wileman et al., 1996). The front hoop

had a circumference of 5.28 m and the second of

6.28 m. The hoops could be separated into two halves

to avoid damage when hauling the catch on board.

Attached strops made it possible to open the hoops and

168 N. Madsen et al. / Fisheries Research 42 (1999) 167±181

release them from the cover on hauling, just before

they entered the stern ramp of the ship, using two

specially designed boat hooks.

Mesh sizes were measured with an ICES spring

loaded gauge with 5 kg tension as detailed in Table 1.

Codend and cover meshes were measured at three

different places in the upper and lower panel. Some

shrinkage of the codend meshes was observed from

the ®rst to the second sea trials. Mesh shrinkage is an

often observed phenomena when using new codends.

The mean of all measurements was 72.8 mm for the

conventional codend and 72.9 mm for the window

Fig. 1. The trawl with the inserted window covered by a hooped cover.

Table 1

ICES gauge mesh measurements for the two trials. Meshes were measured dry before sea trials and wet after the last haul. Number of

measurements (No.) and standard deviations (SD) are given.

Mesh Condition No. Mean mesh size (mm) � SD

First sea trials Second sea trials

Conventional codend Dry 96 74.2 � 1.71 70.9 � 2.26

Wet 96 74.6 � 1.53 71.3 � 2.57

Window codend Dry 96 74.1 � 2.01 71.8 � 2.27

Wet 96 74.0 � 1.68 71.7 � 2.27

Window Dry 80 91.5 � 1.53 91.0 � 1.05

Wet 80 ± 89.3 � 1.11

Conventional cover Dry 96 35.7 � 0.95 36.3 � 0.93

Wet 96 37.3 � 0.84 37.1 � 0.95

Window cover Dry 96 35.7 � 0.67 36.2 � 0.85

Wet 96 36.6 � 0.90 37.0 � 1.05

N. Madsen et al. / Fisheries Research 42 (1999) 167±181 169

codend. Equivalent measurements with the ICES

gauge set at 4 kg tension, which is the standard ICES

gauge setting (Wileman et al., 1996), were estimated

by measuring the same 100 meshes of a codend and of

a window with 4 kg tension and then 5 kg tension. The

reductions in mesh size were 1.3 mm and 1.1 mm for

the codend and window respectively. Estimated

equivalent measurements for the EU legal device, a

wedge gauge with a 5 kg hanging weight (as speci®ed

in the Fisheries Regulations), would be approximately

4% higher (Ferro and Xu, 1996). This leads to a

codend mesh size of 74.4 mm for the conventional

codend and 74.5 mm for the window codend.

Data were gathered on two cruises (Table 2) in the

northern North Sea (ICES division IVa). The ®rst sea

trials, hauls 1±5, were carried out at Fladen Ground

which is one of the main grounds for the Danish North

Sea Nephrops ®shery. The second sea trials, hauls 6±

11, were carried out at East Ground (east of Shetland).

This bank periodically provides a good Nephrops

®shery and it is often ®shed by larger boats from

northern Danish harbours. Under normal commercial

conditions two hauls are made every 24 h when ®sh

are the main target species. If catches of Nephrops rise,

three or four hauls are made per day. At the Fladen

ground where Norway pout (Trisopterus esmarkii)

were numerous it was necessary to reduce haul length

because of very large cover catches (see Table 3). In

most hauls all turns were made to starboard as space

on the ®shing grounds was restricted. The extension

pieces, codends and covers were, therefore, inter-

changed between the starboard and port trawls half

way through each cruise, Table 2.

2.2. Catch measurement procedure

On deck the two cover and two codend catches were

each weighed, while hanging from the lifting crane,

using an electric load cell. The weight was recorded

when there was no signi®cant water discharge from

the codend. The weight of the netting was ®nally

subtracted.

Catches were normally large and had to be sub-

sampled. A sub-sample of around eight 30 kg baskets

(�2 baskets) was taken from each codend. Fish were

divided into cod, haddock, whiting and other species.

The remaining part of the sub-sample was divided into

Nephrops and debris and the total weight of each of

these components measured. The ®sh and Nephrops

were measured for length.

A sub-sample of ®ve baskets was taken from each

cover and all whiting, haddock, cod and Nephrops

were measured. Small whiting and haddock in the

lower part of the selection curve were then adequately

sampled due to the large numbers of small ®sh. It was

felt, however, that it was necessary to obtain further

measurements of larger ®sh within and above the

selection range to get suf®cient data for estimating

the selection ogive and to avoid statistical bias by

taking about the same proportion sub-sample from the

cover and codend. Therefore an additional sub-sample

of around 10 baskets (�5 baskets) was taken from the

cover and sorted for all Nephrops, all cod and by eye

for all whiting over 20 cm and all haddock over 15 cm.

To eliminate any errors only whiting and haddock

5 cm over this limit were then used in further calcula-

tions. In the last four hauls catches of Nephrops were

Table 2

Fishing log, information on operational conditions

Haul no. Date Start position Start (utc) Duration (h) Depth (m) Speed (kt) Wind (m/s) Window codend

1 28 May 1993 N58822,E001815 08.30 5.0 124 2.8 0±5 Starboard

2 28 May 1993 N58808,E001815 16.00 5.0 132 2.8 0±5 Starboard

3 29 May 1993 N58811,E001817 09.20 5.2 124 2.8 0±5 Starboard

4 31 May 1993 N58854,E000844 05.00 1.6 137 3.0 8±10 Port

5 31 May 1993 N59801,E001826 21.00 13.4 123 3.0 10±12 Port

6 20 June 1993 N60823,E001803 20.15 4.0 143 2.8 0±5 Port

7 21 June 1993 N60806,E000809 14.00 5.0 152 2.0 5 Port

8 22 June 1993 N60804,E000807 07.45 8.0 152 2.3 5 Port

9 22 June 1993 N60804,E000805 18.00 12.0 150 2.1 8±10 Port

10 23 June 1993 N60804,E000805 10.00 10.0 156 2.2 10 Starboard

11 23 June 1993 N60805,E000813 22.15 10.8 156 2.2 10±13 Starboard

170 N. Madsen et al. / Fisheries Research 42 (1999) 167±181

high and were further sub-sampled such that around

one basket of Nephrops from each codend and each

cover sub-sample were measured for length. No

Nephrops were measured in haul 7. The sampling

fractions for the codend and cover are shown in

Appendix A. The total catch numbers were estimated

from the numbers measured in the samples and these

fractions.

Lengths of cod, haddock and whiting were mea-

sured to the nearest cm below. Carapace length of

Nephrops was measured with an electronic gauge to

the nearest mm below. The middle of the one cm or

mm length interval was used in all further calculations.

2.3. Codend selectivity analysis

The selectivity analysis was conducted in four

steps. The ®rst step was to ®t selection curves to

individual hauls. The procedure described by Millar

(1994) was used where the selection curve is deter-

mined by ®tting to the raw data (unscaled). This

approach provides more reliable variance estimates

and a more reliable model deviance in cases where

subsampling has been necessary. Four different func-

tional forms for the selectivity curve were then ®tted

by haul to the catch data for each species. Examination

of the deviance residuals indicated that the logit

function described the ®sh data sets best:

r�l� � exp�v1 � v2l�1� exp�v1 � v2l�

where r(l) is the probability that a ®sh of length l is

retained in the codend. v1 and v2, which are the two

parameters to be estimated, represent the intercept and

slope, respectively after a logit transformation.

The complementary loglog function was found to

give the best ®t to the Nephrops catch data as the

deviance residuals showed systematic deviations at

large carapace length when ®tting a symmetrical

selection ogive (logit or probit function)

r�l� � 1ÿ exp�ÿexp�v1 � v2l��The function is asymmetrical with a steeper gradient

in the upper half. The ®tting of the models was

Table 3

Catch weights for conventional (con) and window (win) codend, cover and total (codend � cover)

Haul No. Codend Codend catch (kg) Cover catch (kg) Total catch (kg)

Whiting Haddock Cod Other fish Nephrops Debris Total

1 Con 311 363 133 103 7.3 27.3 945 1870 2815

Win 122 213 192 54 2.7 16.7 600 1470 2070

2 Con 200 185 239 152 14.5 49.9 840 1410 2250

Win 65 121 203 219 10.0 22.0 640 980 1620

3 Con 51 164 61 134 15.8 39.3 465 1440 1905

Win 14 39 77 119 5.4 15.5 270 1430 1700

4 Con 32 31 40 92 4.5 16.0 215 860 1075

Win 9 12 37 125 2.2 44.4 230 910 1140

5 Con 30 48 164 260 5.7 22.2 530 995 1525

Win 32 37 90 400 7.4 48.6 615 670 1285

6 Con 102 49 118 79 2.6 39.8 390 370 760

Win 55 17 66 185 2.9 29.3 355 470 825

7 Con 63 41 24 116 56.2 4.5 305 320 625

Win 31 22 32 65 34.0 6.0 190 280 470

8 Con 57 73 72 99 65.3 13.3 380 585 965

Win 25 30 51 92 26.4 5.7 230 680 910

9 Con 48 44 65 196 133.1 9.0 495 1065 1560

Win 46 49 56 211 141.3 26.5 530 980 1510

10 Con 133 52 127 130 43.4 34.7 520 980 1500

Win 59 33 82 146 35.4 19.8 375 880 1255

11 Con 76 63 49 132 86.0 24.3 430 1050 1480

Win 33 54 43 82 75.4 12.3 300 1005 1305

N. Madsen et al. / Fisheries Research 42 (1999) 167±181 171

optimised by maximum likelihood procedure (S-Plus

software).

The second step was to combine the selection

curves of single hauls. The ®xed and random effects

selectivity model of Fryer (1991) was used. A model

was formulated (EC software), where parameters were

estimated as a ®xed effect, measuring the difference

between the estimated mean value of v1 and v2 for the

two codends used. This procedure is described in

Fryer (1991) and Lowry and Robertson (1996). Resi-

dual maximum likelihood ®tting procedure was used

in all calculations (Fryer, 1991).

Step 3 was to investigate if the two estimated

parameters describing the difference of v1 and v2 of

the two codends mean selection curves were signi®-

cant at the 95% con®dence level. The covariance

matrix of the two parameters describing the difference

was then transformed to be expressed in terms of the

50% retention length (l50) and the selection range

(SR). It was then possible to investigate if the para-

meter expressing the difference in l50 and SR was

signi®cant.

The last step (Step 4) was to investigate if expla-

natory variables had any signi®cant effect on the

selectivity parameters estimated for whiting and had-

dock. Initial analyses indicated the catch size had an

effect on the selectivity parameters. Separately models

were formulated for the conventional and the window

codend with the catch size as a ®xed effect. The

covariance matrix of the selectivity parameter l50

and SR was computed from the generic data (v1

and v2) by transforming the covariance matrix of each

haul. This transformation made it possible to investi-

gate catch size effect directly on l50 and SR.

2.4. Catch difference between the codends

For whiting, haddock, cod and Nephrops the num-

bers retained in the different length classes were

converted to weight by means of the formula

W � alb

Condition factor is likely to vary according to year,

area and stock so it was decided to use mean annual

values of a and b for the ®sh obtained from Coull et al.

(1989) rather than speci®c monthly values. The values

of a and b were 0.0157 and 2.8268 for haddock,

0.0093 and 2.9456 for whiting and 0.0175 and

2.8571 for cod. The catch weights of these species

in Table 3 were calculated using the above formulae.

Nephrops weights were estimated from carapace

length using the same formula with a set to 0.0003 and

b to 3.25 for males, and 0.00074 and 2.91, respectively

for females (Anon, 1995). Sex as well as the length

was recorded for the sub-samples from the conven-

tional codend and its cover for hauls 8, 9 and 11. These

data were used to estimate the proportion of a given

length class that were females. The overall proportion

that were females was 13% but only 5% of those over

the Danish MLS of 4.0 cm carapace length were

females.

The twin trawl system has major advantages in

analysing catch differences between the two codends

as it is possible to make paired comparisons.This was

done using the paired t-test (two-tailed, S-Plus soft-

ware) comparing codend catches for each haul and

species: (1) numbers caught below the minimum

landing size MLS; (2) numbers caught above the

MLS and (3) total catch weight of those above MLS.

The test was also used to test for differences in the

total catch (codend � cover) by comparing the num-

bers under and above MLS. The assumptions for the

test are that each pair of measurements is independent

of other pairs and that differences are normally dis-

tributed. The test is, however, parametric and is fairly

robust to deviations from the normal distribution.

3. Results

3.1. Catches

Fig. 2 gives two sets of plots and curves for each

species: (1) selection ogives for the two codends

together with the total catch numbers, codend and

cover catch combined, by length; (2) codend catch

numbers by length for the two codends. In all the

®gures the Danish MLS is indicated which is 23 cm

for whiting, 35 cm for haddock, 40 cm for cod and

4.0 cm carapace length for Nephrops.

The total catch length distributions for whiting

(Fig. 2) were bimodal with two approximately equal

high peaks, the ®rst around 15 cm and the second

around 27±30 cm. Very few whiting were larger than

40 cm. The total catch length distributions for had-

dock (Fig. 2) consisted of a very high mode around

172 N. Madsen et al. / Fisheries Research 42 (1999) 167±181

15 cm and a second smaller mode around 22±26 cm.

Few haddock were larger than 30 cm. Cod total catch

length distributions were unimodal at about 35 cm.

Compared to whiting and haddock, numbers of cod

caught were low. The codend catch weight of the cod

(Table 3) was relatively high, however, due to the

Fig. 2. Catch numbers and selection ogives by species and length class. The selection ogives for each codend are shown on the left together

with the total combined catch numbers (codend � cover). The codend catch numbers are shown on the right. The solid line indicates the

conventional codend, the dotted line the window codend and the vertical line the Danish minimum landing size MLS.

N. Madsen et al. / Fisheries Research 42 (1999) 167±181 173

larger size of the specimens caught. Nephrops total

catch length distributions were unimodal at around

3.5 cm. Very few Nephrops less than 2.0 cm were

caught.

For all the species (Fig. 2) it is evident that the

combined catches of the codend and cover were not

identical for the conventional and window codends.

For length classes subjected to selection this is parti-

cularly evident for whiting larger than the second

modal length, haddock around the second modal

length and cod and Nephrops around the modal length.

In all cases the total catch for the conventional codend

tended to be larger. Differences are also evident

around the ®rst modal length for whiting and haddock,

but these lengths lie outside those subjected to selec-

tion. More small haddock entered the window codend.

When comparing the total catch numbers with the

paired t-test there was found to be only signi®cant

difference for whiting above MLS, where 15% more in

numbers (p < 0.05) were caught in the conventional

codend.

Comparisons of codend catch numbers and weights

above MLS are presented in Table 4 together with the

results of the paired t-tests. Only 1% of the whiting

numbers in the conventional codend were under MLS.

The 57% reduction of individuals under MLS when

the window codend was used was therefore based on

very few individuals and was not signi®cant. There

was a highly signi®cant 59% reduction in numbers of

whiting caught over MLS. The reduction would be

53% if adjustment is made for the signi®cant 15%

difference in total catches, using the fact that 19% of

the total whiting catch over MLS was retained in the

window codend. This rough estimate indicated that the

total catch difference did not have a dramatic in¯uence

on the codend catch difference. Ninety-six percent of

the haddock in the conventional codend were under

MLS. A highly signi®cant 50% reduction in numbers

under MLS was found for the window codend. There

was no difference between the two codends (1%, NS)

in catch numbers of haddock over MLS. It was

estimated that 67% of the cod caught in the conven-

tional codend were under MLS and that the 33%

reduction in numbers under MLS caught by the win-

dow codend was highly signi®cant. A small 4%

reduction in cod numbers above MLS was not sig-

ni®cant. For Nephrops 49% were under MLS in the

conventional codend, a 11% reduction in numbers

below MLS in the window codend was not signi®cant,

whereas a 14% reduction of individuals over MLS was

signi®cant. The losses of ®sh and Nephrops above

MLS when converted to weight are also presented in

Table 4. There are only relatively small differences in

the estimates of the catch reductions for ®sh and

Nephrops if they are expressed in terms of weight

as opposed to numbers.

3.2. Roundfish selection

Estimates of the parameters v1, v2 and their covar-

iance matrix are provided for all single hauls in

Appendix B.

The model deviances and degrees of freedom indi-

cated good model ®ts for all hauls for whiting and cod.

The deviances were however relatively high for had-

dock in several hauls. A goodness of ®t test (Wileman

et al., 1996) indicated a lack of ®t (p < 0.05) for two

hauls with the conventional codend and eight hauls

Table 4

Comparisons of codend catch numbers (No.) and weights (kg) above minimum landing size (MLS) for the conventional (con) and window

(win) codends. The result of the paired t-test for the significance of the differences between the two codends is given

No. < MLS No. �MLS Kg �MLS

Con Win Difference (%) Con Win Difference (%) Con Win Difference (%)

Haddock 6360 3207 50c 284 282 1NS 133 123 8NS

Whiting 37 16 57NS 4074 1671 59c 1100 491 55c

Cod 976 650 33c 485 464 4NS 707 666 6NS

Nephrops 4940 4386 11NS 4769 4103 14a 359 302 16b

NS±non significant.a p < 0.05.b p < 0.01.c p < 0.001.

174 N. Madsen et al. / Fisheries Research 42 (1999) 167±181

with the window codend. As the residuals showed no

clear structure, it appeared that the assumption using

the binominal distribution, that ®sh should behave

independently, had been violated causing overdisper-

sion (Wileman et al., 1996).

Estimated selectivity curve parameter values are

listed in Table 5 together with their associated stan-

dard errors or con®dence limits. For each ®sh species

the resultant mean selection ogives for the two

codends (Fig. 2) originate from similar length classes.

With increasing length the difference between the

ogives increases, such that the selection curve of

the window codend is to the right of that of the

conventional codend. Consequently the estimated

selection factors (SF) and selection ranges (SR), pre-

sented in Table 5, are larger for the window codend.

SF increases for haddock from 3.4 to 3.8, for whiting

from 4.2 to 4.7 and for cod from 3.5 to 4.0. SR

increases from 4.3 to 5.9 cm for haddock, from 4.5

to 6.8 cm for whiting and from 5.0 to 6.3 cm for cod.

When comparing the selectivity parameters, using

Fryer's model, v1 (p < 0.05), v2 (p < 0.01), l50

(p < 0.001) and SR (p < 0.01) were all signi®cantly

different for haddock between the two codends. Also

all parameters for whiting, v1 (p < 0.05), v2 (p < 0.05),

l50 (p < 0.001) and SR (p < 0.05), were signi®cantly

different. The parameters for cod were not signi®-

cantly different (p > 0.05).

It was found that increasing codend catch weight

had a signi®cant effect on l50 and SR for whiting but in

thewindow codend only. A four parameter model where

l50 and SR were made linearly dependant upon the

catch weight in the window codend was formulated:

l50 � �1 � �2ci SR � �3 � �4ci

The parameters (and standard errors) where estimated

to be �1 � 40.23 (1.2), �2 � ÿ0.01151 (0.0027),

�3 � 11.86 (1.2), �4 � ÿ0.01204 (0.0026) and ci is

the total weight of the catch in the codend in kg. The

resulting in¯uences on l50 and SR of catch weight as

estimated by the models are shown in Fig. 3. The

effect of the cover catch was also investigated and

found to be not signi®cant.

3.3. Nephrops selection

The model deviances indicated a good model ®t for

all hauls. It can be seen from Table 5 and Fig. 2 that

the mean selection ogives for Nephrops for the two

codends are virtually coincident with no signi®cant

Table 5

Estimated selectivity parameters of conventional (con) and window (win) codend. Standard error (SE) is given for estimates of v and 95%

confidence intervals (CI) for l50 and SR. Selection factor (SF) based on an EEC wedge gauge measurement of the mesh size

Haddock Whiting Cod Nephrops

Con Win Con Win Con Win Con Win

No. hauls 11 11 11 11 4 4 4 4

No. caught 57656 57371 15111 13262 962 813 16339 15605

v1 ÿ12.74 ÿ10.49 ÿ15.33 ÿ11.37 ÿ11.65 ÿ10.36 ÿ4.02 ÿ404

SE v1 0.64 0.63 1.4 1.3 1.9 1.7 0.52 0.52

v2 0.505 0.374 0.489 0.324 0.442 0.348 0.097 0.096

SE v2 0.028 0.027 0.044 0.043 0.067 0.059 0.0097 0.0098

l25 (cm) 23.0 25.1 29.1 31.7 23.9 26.6 2.86 2.92

CI l25 (cm) 22.3±23.8 24.0±26.2 28.3±29.9 30.6±32.9 20.2±27.6 23.3±30.0 2.28±3.43 2.35±3.48

l50 (cm) 25.2 28.1 31.4 35.1 26.3 29.8 3.76 3.84

CI l50 (cm) 24.4±26.0 26.7±29.4 30.6±32.1 33.5±36.7 22.7±29.9 26.3±33.3 3.38±4.14 3.46±4.21

l75 (cm) 27.4 31.0 33.6 38.5 28.8 33.0 4.47 4.56

CI l75 (cm) 24.7±30.1 26.9±35.1 28.0±39.2 29.2±47.8 20.0±37.6 21.7±44.3 4.22±4.72 4.31±4.81

SR (cm) 4.3 5.9 4.5 6.8 5.0 6.3 1.62 1.64

CI SR (cm) 3.9±4.8 5.0±6.8 3.7±5.3 5.0±8.6 3.3±6.7 3.9±8.7 1.25±1.98 1.26±2.02

SF (EU wedge) 3.4 3.8 4.2 4.7 3.5 4.0 0.51 0.52

N. Madsen et al. / Fisheries Research 42 (1999) 167±181 175

difference between their selectivity parameters. When

all 8 hauls were combined in the analysis l50 was

estimated to 3.80 cm (3.56±4.04 cm) and SR to

1.63 cm (1.39±1.87 cm).

As suggested elsewhere (Larsvik and Ulmestrand,

1991; Polet and Redant, 1994) it is likely that covers of

the mesh size used in this experiment do not retain all

small Nephrops. The selection ogives of Nephrops

(Fig. 2) do not reach the point of no retention at the

abscissa. The absence of small individuals and the low

con®dence around l25 (Table 5) make it impossible to

investigate further. Attempts were made to correct for

a loss through the cover by establishing a selection

curve (SF � 0.5, SR � 1.6 cm) of the 37 mm mesh

size cover. This correction of data did not change the

choice of function to describe the selection ogive but

increased l50 to 3.81 cm (3.45±4.17 cm) and decreased

SR to 1.52 cm (1.18±1.85 cm) for the conventional

codend, 3.88 cm (3.55±4.22 cm) and 1.51 cm (1.18±

1.84 cm) for the window codend and 3.85 cm (3.62±

4.08 cm) and 1.51 cm (1.30±1.73 cm) when all the 8

hauls were combined.

4. Discussion

4.1. Fish behaviour in relation to the window

The selectivity curves for the window codend for all

the ®sh species investigated in this experiment are not

coincident with those of the conventional codend. It

was found that the larger ®sh within the range sub-

jected to selection have better possibilities of escaping

through the window. Similar tendencies for selection

ogives were found by Polet and Redant (1994) for

whiting in a Nephrops codend with a window. Ulmes-

trand and Larsson (1991) noted that small whiting

about 10±15 cm were not released through a window

in a Nephrops codend. Underwater observations

clearly show that escape through a window situated

well in front of the codend catch is given by an active

escape behaviour (Main and Sangster, 1991; Briggs,

1992). As swimming performance (He, 1993) and

likely visual acuity (Douglas and Hawryshyn, 1990)

increase with size, larger ®sh might have better pos-

sibilities to escape through the window. Another pos-

sible explanation can be that smaller ®sh stick together

in schools, while larger ®sh behave as individuals.

The relationship between codend catch and selec-

tivity parameters was signi®cant for whiting but not

haddock. Whiting seem to be very keen to escape

through a window and often do so at the ®rst attempt

when encountering the window section (Briggs,

1992). Main and Sangster (1991) observed that had-

dock, unlike whiting, did not appear to make active

escape attempts through a window. Moth-Poulsen

(1994) utilised this behaviour difference when devel-

oping a species selective trawl that could separate

haddock from whiting. There was no catch size effect

in the conventional codend, which indicates that the

accumulating catch in¯uenced the escape through the

window. In the present experiment the window was

located a relatively short distance (4 m) in front of the

codline. It is, therefore, possible that the normal active

escape pattern of whiting was disturbed or the geo-

metry of the square window meshes adversely affected

Fig. 3. Correlation between codend catch weight and l50 and SR for whiting. Squares indicate the window codend and crosses the standard

codend. The lines show the correlation for the window codend predicted by the model.

176 N. Madsen et al. / Fisheries Research 42 (1999) 167±181

when large numbers of ®sh passed down the trawl and

large catches built up in the codend. The differences

found in this study should be further assessed by the

use of underwater observations. From a management

point of view the catch size effect makes the selectiv-

ity of the more selective ®shing gear less well de®ned.

Robertson (1993) recommends placing the window

close to the codend in order to achieve the best escape

of round®sh. In a mixed ®shery with occasional bulk

catches, the conditions for this experiment, it might be

a good idea to place the window further in front of the

codend in order to avoid the catch size effect.

4.2. Effect of the window on the catch of roundfish

It was found that inserting a window in a Nephrops

trawl releases large proportions of smaller haddock

and whiting which is in agreement with the previous

experiments mentioned in the introduction. Only lim-

ited data sets for cod exist (Ulmestrand and Larsson,

1991; Thorsteinsson, 1992) and these also indicated

that there was some release of cod through the win-

dow. As placing, dimensions and mesh size of the

window, the populations ®shed and MLS all differ, it is

not possible to make direct comparisons with existing

experiments.

The differences found between the two trawls in the

combined cover and codend catches illustrate that

some care should be taken when comparing codend

catches in twin trawl rigs. In this experiment straight

towing was not possible and there were likely to have

been port-starboard differences in swept areas during

most hauls. Even though the codends extension pieces

and covers were interchanged between the two trawls,

the conventional codend tended to have more bulk

weight of ®sh and Nephrops entering the area enclosed

by the cover in most hauls.

If the results of this experiment are related to the

Danish MLS, the effects of introducing a window are

very encouraging for haddock and cod. A highly

signi®cant reduction in the large proportion of the

catch that is undersized and an insigni®cant loss of

individuals over MLS were obtained. Amounts of

smaller cod caught were, however, low during these

sea trials and thus there is less con®dence in the results

for cod. Cod have distinct nursery grounds and small

cod are not very abundant on offshore grounds in the

northern North Sea, (Daan et al., 1990), and thus, they

might not be a serious discard problem in the ®shery

investigated. As selection of whiting took place well

above MLS in the conventional codend there was a

considerable signi®cant loss of marketable whiting.

Whiting are of small economic importance in the

Danish Nephrops ®shery and often discarded even

above MLS. On the other hand whiting are heavy

predators of economically important ®sh species

(Hislop et al., 1991). The selectivity parameter esti-

mates, together data on the survival of codend esca-

pees, should be further assessed by including

ecological and ®nally economic interactions in order

to estimate the likely impact of introducing windows

in the Nephrops ®shery. Forecasts made using the

North Sea multispecies virtual population analysis

(MSVPA) indicate that making gears more selective,

by increasing the mesh size, will not necessarily

bene®t the landings in the ®shery (Pope, 1991). This

is due to the predator-prey interactions, between nine

species of major commercial importance, is being

taken into account in this model. The Fladen Ground

Nephrops ®shery could be considered as a restricted

area for such an impact analysis.

4.3. Nephrops selection

The asymmetric Complementary loglog function

has not been used before to describe Nephrops selec-

tion curves. After the onset of sexual maturity iso-

metric growth will be replaced by positive allometric

growth for the abdomen width of females and the

chela of males (Farmer, 1974). These growth phenom-

ena could be expected to increase mechanical contact

with the net thereby increasing retention rates for large

carapace lengths and increasing the slope of the

selection curve with increased carapace length.

Further supporting data are required.

It seems reasonable to believe that there was an

overlap between the cover and codend selectivity

curves. Selectivity parameters are, however, only

changed slightly when corrected for estimated loss

through the cover. Previous Danish experiments have

been carried out in the Kattegat-Skagerak where

Bagge (1982) obtained estimates for SF of 0.40 and

0.45 and SR of 0.6 and 1.1 cm for a nominal 70 mm

codend. Less con®dence can be placed in the results of

this early experiment as it was carried out from a

research vessel, selection ogives were hand ®tted and

N. Madsen et al. / Fisheries Research 42 (1999) 167±181 177

there are indications of masking of the codend by the

cover. Another Danish experiment (Kirkegaard et al.,

1989) was carried out comparing catches of nominal

60 and 70 mm mesh size codends ®shed in twin trawl

systems over a large number of hauls. Selection ogives

were derived by assuming that SF and SR were the

same for the two codends then ®tting a logistic

selection curve to the pooled catch data. SF was

estimated to be 0.48 and SR to be 1.4 cm only slightly

less than the estimates of this study. In a Belgium

selectivity experiment from the Central and Southern

North Sea SF varied from 0.37 to 0.57 according to the

sea state, whereas SR varied from 1.39 to 1.82 cm but

was not clearly in¯uenced by the sea state (Polet and

Redant, 1994). The SF of this study is comparable to

values presented in the review of Briggs (1986) for

experiments conducted in the Bay of Biscay, Galicia

and Iceland, whereas estimates from the Irish Sea in

general were lower.

More than half of the Nephrops catch was below the

Danish 4.0 cm MLS as found in other experiments

(Kirkegaard et al., 1989; Larsvik and Ulmestrand,

1991) when using a Nephrops codend with the mini-

mum legal mesh size of 70 mm. Considering that

Nephrops is the target species it is very obvious from

the selection ogives in this study that there is a bad

correlation between this MLS and the selectivity

properties of a 70 mm Nephrops codend. It is usually

recommended that the MLS and the l25 should corre-

spond to each other (see Reeves et al., 1992). The MLS

speci®ed in the European Union legislation for the

North Sea of 2.5 cm carapace length (EC council

regulation No. 894/97) seems to be more appropriate.

As evident in this study selection ogives of Nephrops

are less `knife-edge' than for ®sh species and an

increased mesh size will result in a considerable loss

of individuals over MLS.

Unlike ®sh there is little or no herding effect for

Nephrops produced by the sweeps and bridles (Main

and Sangster, 1985; Thorsteinsson, 1986; Newland

and Chapman, 1989). The swimming escape reaction

of Nephrops caused by contact with the ground-gear

(Newland and Chapman, 1989) is essential in the

capture process as it lifts them over ground-gear. It

is likely that between trawl differences in bottom

contact, or area swept by the ground-gear, occurred

during some hauls even though the difference in the

total catch was not found signi®cant. This could

explain why there were signi®cant differences in

the Nephrops catches for the window and conven-

tional codends, whereas the selection ogives were

very alike and not signi®cantly different. In under-

water observations reported by Briggs (1992) no

escape of Nephrops through the window was

observed. A loss might nevertheless occur if catches

in the codend and cover destabilise the codend causing

it twist around so that the window is no longer on

the top. It is believed that if the innovative codend

was implemented under commercial conditions any

possible loss of Nephrops through the window would

quickly be minimised.

Acknowledgements

The authors wish to thank DG XIVof the European

Commission for funding this study, the skipper and

crew of Tannisbugt for their advice, help and good co-

operation throughout the trials, the ICES secretary for

allowing us to cite an ICES assessment report and the

editor and two anonymous referees for useful com-

ments. This article does not necessarily re¯ect the

views of the European Commission and in no way

anticipates any future opinion of the Commission.

Appendix A

Subsampling proportions from codend and cover

Haul Codend Codend catch Cover catch

Haddock

Whiting Cod

Nephrops Haddock < 20 cm

Whiting < 25 cm

Haddock � 20 cm

Whiting Cod � 25 cm

Nephrops

1 Con 0.188 0.188 0.102 0.204 0.204Win 0.300 0.300 0.080 0.160 0.160

178 N. Madsen et al. / Fisheries Research 42 (1999) 167±181

(Continued )

Haul Codend Codend catch Cover catch

Haddock

Whiting Cod

Nephrops Haddock < 20 cm

Whiting < 25 cm

Haddock � 20 cm

Whiting Cod � 25 cm

Nephrops

2 Con 0.212 0.212 0.146 0.671 0.671Win 0.278 0.278 0.103 0.390 0.390

3 Con 0.379 0.379 0.101 0.325 0.325

Win 0.643 0.643 0.101 0.322 0.3224 Con 1.000 1.000 1.000 1.000 1.000

Win 1.000 1.000 1.000 1.000 1.0005 Con 1.000 1.000 0.144 1.000 1.000

Win 1.000 1.000 0.200 1.000 1.0006 Con 0.462 0.462 0.319 0.637 0.637

Win 0.509 0.509 0.407 0.813 0.8137 Con 1.000 1.000 0.467 1.000 1.000

Win 1.000 1.000 0.538 1.000 1.0008 Con 0.472 0.226 0.256 0.769 0.229

Win 0.779 0.538 0.235 0.704 0.3759 Con 0.364 0.093 0.141 0.423 0.282

Win 0.339 0.102 0.153 0.459 0.15310 Con 0.347 0.346 0.153 0.459 0.306

Win 0.480 0.361 0.171 0.512 0.51011 Con 0.420 0.137 0.143 0.429 0.143

Win 0.600 0.176 0.149 0.448 0.149

Appendix B

Estimates of v with covariance matrix (R).

Haul Conventional codend Window codend

v1 v2 R11 R22 R12 v1 v2 R11 R22 R12

Haddock1 ÿ10.12 0.385 0.168 0.00030 ÿ0.0070 ÿ14.59 0.564 0.609 0.00114 ÿ0.02592 ÿ12.49 0.560 0.498 0.00093 ÿ0.0213 ÿ10.77 0.401 0.370 0.00060 ÿ0.01483 ÿ12.96 0.528 0.303 0.00055 ÿ0.0127 ÿ9.35 0.297 0.118 0.00024 ÿ0.00524 ÿ11.28 0.457 0.207 0.00046 ÿ0.0095 ÿ5.89 0.173 0.102 0.00030 ÿ0.00535 ÿ10.86 0.388 0.157 0.00029 ÿ0.0066 ÿ10.18 0.348 0.164 0.00028 ÿ0.00666 ÿ14.70 0.573 0.798 0.00140 ÿ0.0331 ÿ12.64 0.432 0.881 0.00139 ÿ0.03457 ÿ13.35 0.553 0.519 0.00102 ÿ0.0227 ÿ7.89 0.294 0.282 0.00052 ÿ0.01198 ÿ13.31 0.526 0.598 0.00101 ÿ0.0243 ÿ10.08 0.345 0.259 0.00046 ÿ0.01079 ÿ14.35 0.581 1.380 0.00254 ÿ0.0585 ÿ10.29 0.410 0.643 0.00119 ÿ0.0272

10 ÿ12.99 0.489 0.922 0.00149 ÿ0.0366 ÿ13.92 0.507 1.417 0.00212 ÿ0.054111 ÿ15.23 0.578 1.272 0.00207 ÿ0.0508 ÿ11.54 0.408 0.573 0.00086 ÿ0.0219

N. Madsen et al. / Fisheries Research 42 (1999) 167±181 179

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11 ÿ4.62 0.108 0.157 0.00010 ÿ0.0039 ÿ5.18 0.119 0.144 0.00009 ÿ0.0036

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