Fisheries Research, Z(1983) 29-46 EIsevier Science Publishers B.V., Amsterdam - Pyinted in The Netherlands

29

NET SELECTIVITY STUDIES IN THE NORTHERN IRELAND NEPHROPS FISHERY

R.P. BRIGGS

Department of Agriculture for Northern Ireland. Fisheries Research Laboratory,

Coleraine, Co. Londonderry. N. bebnd (U.K.)

(Accepted 3 February 1983)

ABSTRACT

Briggs. R.P., 1933. Net selectivity studies in the Northern Ireland Nephrops fishery. Fish. Res., 2: 29-46.

The effect on catch composition of a large mesh lifter net fitted to a trawl cod-end was investigated in the Northern Ireland Nephrops fishery in the north west Irish Sea (ICES area VIIa). Two experiments were conducted using chartered commercial fish- ing vessels. In the first experiment, parallel hauls showed there to be no significant difference in the Nsphrops size distribution caught by nets with and without a lifter net. The result OP the second experiment, in which the codend was fItted with a small mesh cover to retain escaping Nephrops, did not suggest that the cod-end meshes were blinded by the lifter. Field data are contrasted with those from iheorrztical studies of

INTdODUCTION

The Northern Ireland fishery for Nephrops norvegicus (L) (Dublin Bay prawn, scampi or Norway lobster) is the most vaitiable single %hery to the Province (Briggs, 1981a). Nephrops are caught in otter trawls fished by vessels ranging from 14 to 24 m in length. It is usual commercial prac-

tice to use a large meshed net cover or lifting bag over the trawl cod-end (Fig. 1). The purpose of this cover is to protect the cod-end from chafing on the sea bed and to act as a safety factor in lifting the catch on board. Current Irish Sea (ICES area VIIa) legislation (EEC regulation 2527/80) pro- hibits any device which obstructs or diminishes the meshas of the net. Recent recommendations based upon empirical studies, propose that the meshes of cod-end lifting bags should exceed those of the cod-end by a factor of 5. Although such empirical studies (STCF - %x/80) demonstrate a possible “blinding effect” of the cover upon cod-end selection, there is no documented evidence on the actual effect of a cover on catches under commercial conditions. Net selectivity studies in Nephrops fisheries have resulted in valuable contributions to the knowledge of net design (Symonds

0165.7836/83/$03.00 0 1983 Elsevier Science Publishers B.V.

Fig. 1. Cod-end of a typical Nephrops trawl with lifter net attached.

and Simpson, 1971; Warren, 1974), the choice of twine (Pope and Thomas, 1965) and the effect of different mesh sizes on selectivity (Thomas, 1965; Pope and Thomas, 1965; Cole and Simpson, 1965; Ancellin, 1965). More recent studies have assessed the effect of a minimum mesh size of 70 mm for Nephrops fishing in the Firth of Clyde (Pope and Thomas, 1975), Firth of Forth (Jones, 1979), Irish Sea (Brander, 1975; Briggs and Watson, 1980) and Celtic Sea (Conan and Morizur, 1979).

In this paper, the results of two sea trials are presented in an attempt to assess the effect of a cod-end lifter upon Nephrops catch. Observations made during these studies art contrasted with those from a theoretical assessment of net obstruction.

METHODS

This investigation was conducted in two phases involving a parallel haul experiment (Phase I) and an experiment in which a fine mesh cover was used to retain escapes from the cod-end (Phase II). The methods employed during these two phases are described separately.

2 x 253 MESIES

300 MESHES

ZOVER 140mm

Fig. 2. Mesh plan of 25 fathom Nephrops trawl as used in Phase I experiment by MFV Lomita and Olive Tree.

32

Phase I

During January 1981 two similar vessels, MFV Lomita and MFV Olive Tree, were chartered from the Co. Down port of Kilkeel. The two vessels carried identical, 25 fathom prawn trawl nets with interchangeable cod- ends of nominal mesh size - 70 mm (mean 71.2 and 70.6 mm). One of the cod-ends was fitted with a detachable lifter net of nominal mesh size - 140 mm (mean 139.8 mm) and circumference similar to the cod-end. Details of net dimensions are given in Fig. 2. Apart from the first day’s fishing when the Lomita used a double-twine, 70 mm (mean 70.0) cod-end without a cov- er, single-twine cod-ends were used. Cod-ends were interchanged between

IRISH SEA

lllllllll COmmelClal fIStmy Study area

Fig. 3. Map of study mea off Co. Down coast, N. Ireland. Inset shows the rest of the Irish Sea (ICES area VII&.

33

TABLE I

Cod-end net combination used Phase I trial

Day Lomita Olive Tree No. of comparable tows

1 70 mm double twine 70 mm + cover 2 2 70 mm IO mm + cover 2 3 70 mm + c.xer 70 mm 1 4 70 mm + cover 70 mm 2

vessels as indicated in Table I. Simultaneous trawls were made in parallel over Nephrops grounds off the Co. Down coast in ICES area VIIa (Fig. 3).

Throughout the study, every effort was made to simulate commercial conditions with trawls of 34 h duration. The catch from each trawl was sampled by filling a 50 kg capacity fish-box shortly after the catch had been deposited on the deck. The Nephrops component of the catch, after separating from the fin-fish was subsampled to 200-300 individuals and carapace length measured to the nearest millimetre below.

Analysts of length distribution data

Since the carapace length distributions of samples were skewed towards large Nephrops, the data were normalised by logarithmic transformation to enable mean values to be compared.

The size classes found to be most sensitive to mesh changes (<20 mm CL) were analysed by performing a binomial comparison of proportion, in which the accumulative proportion of occurrences at each length class for each comparison were subjected to a significance test by applying the equation

(P,- P,)

t= p, u- PI) pz u- P2) -+

N, NZ

where PI and P1 = accumulative proportions occurring in a particular length class being compared. N, and N, = sizes of the respective samples being compared.

Phase II

During March 1982, MFV Conquest D was chartered from Kilkeel for a five-day period. A Cosalt 23 fathom prawn trawl of nominal mesh size 65 mm (2 = 64.3) throughout (Fig. 4) w,as fitted with a mesh cover (mesh

\a6 \

CODEND I I I I

Fig, 4. Mesh plan of 23 fathom Nephrops trawl is used in Phase II experiment by MFV conquest D.

size 25-30 mm) over the last 2 m of its cod-end. A lifter net of nominal mesh size 130 mm (z? = 133.8) was attached between the cod-end and the cover and rigged according to normal commercial practice for the first half of the experiment (tows l-5). The lifter net was removed for the second half of the experiment (tows 6-10). A cover of mesh size 25-30 mm was selected because this was considered the most suitable size in order to reduce the excessive “drag” that a very small mesh (< 20 mm) would cause, and still prevent the loss of small Nephrops that a larger mesh would permit. Using this gear, tows of 2 h duration were possible,

Total catch samples from both cover and cod-end were weighed sepa- rately to the nearest kg and subsamples (200-300 individuals) of Nephrops

35

were taken for measurement of carapace length frequency distribution. From these data, the proportion of available catch retained by the cod-end was determined for hauls with and without a cod-end lifter net.

In both phases of the study the mean cod-end and lifter mesh size of the nets used were determined by measuring 25 randomly selected meshes in each with a standard, spring loaded, ICES mesh guage set to give a tension of 5.0 kg. Throughout these experiments, towing speeds were carefully regulated because changes in towing speed can effect catch composition (R.P. Briggs, 1981, unpublished data). A fast tow can favour fish catch while Nephrops are more readily caught at slower speeds.

TABLE II

Estimates of total weight (kg) of fish and Nephrops caught with and without a li.ter net Phase I(3.5 h)

JJaY Lifter No lifter No. tows -

Wt. Nephrops Wt. fish (kg) Wt. Nephrops Wt. fish (kg)

(kg) _ (kg)

1 46.4 187.6 fi6.7 279.4 2 2 163.3 187.2 255.1 1578.6 2 3 43.4 214.2 93.6 463.7 1 4 71.4 124.9 129.4 246.1 2

Total 332.5 713.9 544.8 2568.0 7 ______

RESULTS

Phhase I

Table II shows the weights of fish and Nephrops caught by the two vessels during this phase of the study. Nephrops carapace length distribu- tions are compared graphically in Fig. 5 for catches with and without lifter nets for the four days of the experiment (‘7 tows). Analysis of Nephrops length frequencies by binomial comparison of proportion indicated sig- nificant differences (Table III) in the proportion of Nephrops sizes caught in certain tows. These differences, however, did not correspond to the presence or absence of a lifter, suggesting that the lifter had no effect upon the size distribution of Nephrops catch. Estimates of the number of Neph- raps caught using methods similar to those described by Jones (1979) and by Briggs (1981b) indicated that the total number of Nephrops caught was greatest when a lifter was not used (Table IV).

36

1

1

%

%

%

%

net with lifter ‘-. no llflc?l

. _ _ _ _ _ _

Fig, 5, Comparison of carapace length frequency distribution of Nephrogs caught by parallel tows (Phase 1) in trawls with and without lifter nets.

TA

BL

E

III

Cu

mu

lati

ve

perc

enta

ge

of

tota

l ob

serv

atio

ns

per

len

gth

cl

ass

and

the

leve

l of

di

ffer

ence

be

twee

n

com

para

ble

tow

s (f

rom

bi

nom

ial

com

pari

son

of

pr

opor

tion

an

alys

is)

Day

T

ow

and

gear

N

4

16m

m

s17m

m

S

18ll

Xll

c

1Sm

m

GZ

Om

m

%

Sig

. di

ff.

%

Sig

. di

ff.

%

Sig

. di

ff.

%

Sig

. di

ff.

%

Sig

. di

ff.

1 T

ow

lL(7

0 D

T)

299

1.67

7 ,/

lOT

(70

C)

+

771

1.42

7 *,

S.

;:;;;

N.S

. ;;‘

;;;

N.S

. ;;:

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N.S

. ;;:

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N.S

.

,. 2L

(70

DT

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4 0.

675

20T

(70

+ C

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7 0.

660

N,S

, ;:;

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ow

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96

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621

0.93

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866

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9.70

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0.68

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2.56

4 e

8.04

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94

” 50

T(7

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701

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563

4.09

9 11

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.063

M

4 T

ow

6L(7

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C)

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(70)

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= c

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T

= d

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7.96

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18.1

94

11.0

67

17.6

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NS

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25.6

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38

TABLE IV

< 19 743 ,146 3009 5303 674 1000 1309 2127 20-24 1498 2306 6945 11624 1583 3865 3388 5774 25-29 1426 ,581 4723 7446 1185 2697 2622 3930 30-a 538 718 1380 1998 540 863 565 1011 35-39 176 330 463 616 150 312 155 392 40-44 25 32 93 128 14 74 33 42 45-49 10 16 - _ 14 _

250 _ _ 5 -

TOhI 1406 6112 16623 27135 4146 8831 8091 13276 ___-

Phase II

Figure 6 shows a selectivity graph for the combined data for all ten tows, in which the percentage of Nephrops at each length retained by the cod-end is plotted against the carapace length. From this, it appears that more Nephrops are retained when a lifter net is not used (Fig. 6) agree- ing with Phase I results. Further analysis using regression methods indicated a significant correlation (P Q 0.02) between the amount of bulk (fish, Nephrops and other invertebrates) in the cod-end and the proportion of

, 20

carapace lk”,IP”,.~ 40 50

Fig. 6. Selectivity graph in which the percentage of available Nephrops, in l-mm length classes, retained by the cod-end is plotted against carapace length for combined catches using a cod-end with and without B lifter net during the Phase II experiment.

available Nephrops retained by the cod-end (Fig. 7, Table V). Therefore the proportion of available Nephrops retained by the cod-end ranged from 40% when the net was partially empty to 81% when there was a large catch. Significant correlation (P < 0.01) was also present between the propor- tion of Nephrops retained and the “potential” catch bulk (Table V) i.e., the bulk in the cod-end combined with that in the fine mesh cover.

3-

‘5 -

50

i

L// .

.

100 200 300 400 500

total combined catch of cod-end kg

Fig. 7. Regression analysis of bulk in the cod-end and the proportion of available Neph- raps retained by the cad-end. Combined data from all ten tows of the Phase II experi- merit..

Selectivity plots (Fig. 8) of Nephrops retained by the cod-end for tows of comparable catch bulk (fish and other benthic organisms including Nephrops) indicated (though to a much lesser extent than in Fig. 6) a slight increase in the proportion of smaller Nephrops when the lifter is not used.

TA

BL

E

V

Rel

atio

n be

twee

n ca

tch

bulk

(k

g)

and

Nep

hrop

s re

tent

ion

(bul

k =

fish

and

inve

rteb

rate

s ot

her

than

N

ephr

ops)

TO

W

Bul

k in

B

ulk

and

Nep

hrop

s B

ulk

in

Bul

k an

d N

ephr

ops

%N

ephr

ops

cod-

ad

in

cod-

end

cove

r an

d co

d-en

d in

EO

WI

and

cod-

end

in c

od-e

nd

1 12

6

: 63

90

4

82

5 56

6

191

7 24

7 a

371

9 16

4 10

14

9

192

259

185 89

59

24

2 33

0 50

3 46

1 36

3

636

203

128

143 97

24

4 46

3 74

3 47

0 30

6

713

472

255

171

105

310

632

912

339

573

Cor

rela

tion

(r

) w

ith

% N

ephr

ops

reta

ined

0.

504

0.10

66

0.71

92

0.81

13

Pro

babi

lity

leve

l c

0.1

< 0.

02

< 0.

02

< 0.

01

81

60

68

49

40

59

75

74

74

31

41

W--I t 30 40 50

carapace IenEfh mm

Tie. 8. Selectivity graph of tows of comparable catcn bulk (T2: T6, T3: T9, Tl: TlO). The .wight of fish caught (kg: in the tows is shown next to the tow number.

42

Data presented in STCF report No SEC/SO demonstrate theoretically that when a large mesh is superimposed over a smaller mesh, a number of the latter meshes are obstructed. When this is applied to a number of different mesh sizes and the percentage of the meshes obstructed is plot- ted against the relative mesh sizes superimposed, a curvilinear relation- ship develops (Fig. 9). It is evident from this curve that to guarantee that more than 50% of the meshes are unobstructed the covering mesh size must exceed that being covered by a factor of 4. If an example is consid- ered in which a mesh of x mm is covered by one of 2.v mm, 75% of meshes would be obstructed. In this example, the maximum degree of ohstruetion would be 50% of meshes halved, 25% quartered and the remainder (25%) clear. If this example is applied to a net of 70 mm with a 140 mm cover, the theoretical mean mesh size can be calculated by considering each mesh to be a square of sides 35 mm in length. The total area of mesh per 100 meshes of such a net may be calculated by multiplication (a) for 60 halved meshes 50 X 35 X 17.5 = 30625.00 mm2 (b) for 25 quartered meshes 25 X 17.52 = 7656.25 mm* (c) for 25 clear meshes 25 X 352 = 30625.00 mm’ :. total area mesh = 68906.26 mm* :. mean mesh side length = .,/68906.25/100’ = 26.25 mm :. mean mesh size = 2 X 26.25 = 52.50 mm

The factor by which the theoretical effective mesh size is reduced by a cover of mesh size twice that of the cod-end may therefore range from 0.75 to 0 when the meshes are exactly superimposed.

These trials suggest that in practice a mesh “blinding” effect of cod-end lifter does not occur. Discrepancies between theoretical and field studies imply that other factors play a more important part in determining catch composition than the presence or absence of a cod-end lifter net. For ex- ample, the density of fish and overall catch bulk appears to have a signif- icant effect on the proportion of potential Nephrops catch retained by the trawl net. A factor affecting catch bulk that was not varied during this study is towing time. Short tows usually have lower catch bulk than long tows over the same ground. Figure 10a illustrates how a near full cod-end may have a “choking” effect on more meshes of the distal cod- end than one that is empty or partially filled. Such a situation could help to explain why no significant reduction in Nephrops catch could be attrl- buted to the lifter net in the experiments described here, where catch bulk in several of the tows would have filled the distal cod-end normally covered by the lifter net.

Another important factor likely to effect net selectivity studies of this type is the relative circumference of the cod-end and lifter net used, The data presented were obtained using cod-ends fitted with lifters of similar

i I

Xl x2 x3 x4 x5 x6 x7 x6 x9 x10 RELATI li: MESH SIZE

Fig. 9. Graphical representation of a theoretical study of mesh obstruction by differ- ent sizes of mesh cover (modi ‘ied from STCF report SEC/SO).

44

circumference to the cod-ends. Such a situation allows the cod-end meshes to open freely as in nets without lifters. A lifter net of smaller circum- ference than the cod-end, however, would have a restricting effect upon the opening of cod-end meshes (Fig. lob) preventing escapes.

a.

short tow

smal;Jrcatctl

b. I

I I

%

I I I I I

tight loose lifter lifter

Fig. 10. (a) Diagrammatic representation of cod-end after a short tow or with a small catch and after B long tow or large catch showiag available escape routes for Nephropp. (b) Diagrammatic representation of cod-end (ce) with tight and loose lifter net (Ii),

Despite efforts to control variables some, such as weather, tides and the distribution of fish and Nephrops, could not be regulated.

In the Phase II experiments, for example, more fish were caught during the second half of the experiment when the lifter was not used. This was attributed tc a variation in fish abundance available to be caught, rather than to an r?;ect on fish selection by the lifter. In several tows few fish were caught % Ien a lifter was not used, e.g., tow 6 of Phase II.

CONCLUSION

The overall conclusion of this study is that a lifter, of the dimensions described in the experiments, does not have a blinding effect upon Nephrops catch under commercial conditions. In fact the contrary appears often to occur, with more Nephrops being caught without a lifter attachment. This is attributed to a “tea strainer” effect of the catch bulk already in the net and to the increased draught of water current through the net when the lifter is removed, flushing potential catch into the cod-end.

ACKNOWLEDGEMENTS

The author thanks Messrs. W.J. McCurdy and B.T. Hart for their tech- nical assistance and service at sea during this study. Thanks are also due to skippers C. Shields, R. Shields and J.H. Doyle for their co-operation. Dr. D.A. Stewart is thanked for his help and advice on the use of statis- tical analysis and Mr. A. Heaney for laboratory work.

REFERENCES

Ancellin, J., 1965. SelectivitO des chaluts P langoustines (Nephrops Norwegian (L.)). Rapp. P.V. Ran. Cons. Int. Explor. Mer, 156: 202.

Brander, K.M., 1975. A yield assessment OF Irish Sea Nephrops and the effect of a mesh change. Int. Count. Explor. Sea, CM 1975/K: 30,12 pp. (mimeo.).

Brig@, R.P., 1981a. Trends in the Northern Ireland Nephrops fishery. 1970-1980 Int. Count. Explor. Sea, CM 1931/K: 9.11 pp. (mimeo.).

Briggs, R.P., 1981b. Preliminary observations on the effect of a cod-end cover or lifter bag cn catch composition in the Northern Ireland Nephrops fishery. Int. Count. Erplor. Sea, C M 1981/B: 11.17 pp. (mimeo.).

Briggs, R.P. and Watson, P.S., 1980. A comparison of catch composition with differ- ent net mesh sizes using a parallel haul technique in the Northern Ireland Nephrops fishery. Int. Count. Explor. Sea, C M 1981/K: X3,26 pp. (mimeo.).

Cole, HA. and Simpson, A.C., 1965. Selection by trawl nets in the Nephrops fishery.

Rapp. P. V. Reun. Cons. ht. Explor. Mer, 156: 203-205. anan, G. and Morizur, Y., 1979. Long term impact of a change in mesh size from

45-50 to 70 mm on yield in weight and fecundity per recruit for Norway lobster populations; is there a simple solution to a complex problem? 4 simulation model. Int. Count. ExpIor. Sea, C M 1979/K: 43.50 pp. (mimeo.).

Jones, R., 19’79. An analysiis oi a Nephrops stock using length composition data. Rapp. P. V. Reun. Cons. ht. Ewplor. Mer, 175: 259 -269.

Pope, J.A. and Thomas, H.J., 1965. A sunnnary of Scottish comparative fshing ex- periments on Nephrops noruegicus (L.) Rapp. P. V. Ran. Cons. Int. Explor. Mer, 156: 190-201.

Pope, J.A. and Thomas, H.J., 1975. A comparison of the catch of Nephrops by trawls of 50 mm and 70 mm mesh size. Int. Count. Explot. Sea, C M 1975/K: 43, 6 pp. (minwo.).

Symonds, D.J. and Simpson, A.C.S., 1971. Preliminary report on a specially designed Nephrops trawl for releasing undersized roundfish. Int. Count. Explor. Sea, C M 1971/B: 6,s pp. (mimeo.).

46

Thomas, H.J., 1965. A comparison of the catch of Norway lobsters using trawls of 50 mm and 70 mm. respectively. Rapp. P. V. Reun. Cons. Int. Explor. Mer, 156: 155-160.

Warren, P., 1974. A consideration of two types of trawl in a mixed fishery for Nephrops, Pandalid shrimps and white fish. Int. Count. Explor. Sea, C M 1974/B: 6, 10 pp. (mimea).