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The effect of twine thickness on cod-end selectivity of trawls for haddock in the North Sea
ELSEVIE,R
Fisheries Research26 ( 1996) 353-363
The effect of twine thickness on cod-end selectivity of trawls for haddock in the North Sea N. Lowry *, J.H.B. Robertson SOAFD Murk
Labomtory.
’
PO Box 101. Victoria Road, Aberdeen,
UK
Accepted 8 May 1995
Abstract Since June 1992, when the minimum mesh size for the North Sea demersal fisheries was raised to IO0 mm, there has been a trend towards the use of cod-ends made of much thicker twine. The effect of this increase in twine thickness on cod-end selectivity for haddock was investigated in June 1993 on a Scottish commercial vessel. Cod-end selectivity was measured using a cod-end cover with supporting flexible hoops. The results were analysed taking account of between haul variation in selectivity, and indicate a significant reduction in selectivity with increased twine size. The change from 3.5 mm double twine to 6 mm double twine reduced the selection factor from 2.64 to 2.38. The reduction in selectivity is thought to be due mainly to reduced mesh opening and increased twine stiffness which both inhibit escape, but behavioural effects may also be important. Kr~~t/.s:
Selectivity;
Cod-ends;
Twine thickness;
Haddock; Trawl
1. Introduction
Mesh size regulations for towed fishing gears aim to reduce fishing mortality by allowing small fish to escape from the nets. In the North Sea, minimum mesh sizes have been increased many times but catch and discard data collected by the Marine Laboratory sampling programme suggest that these have had little effect (Stewart and Newton, 1993). There has been a trend over the last 20 years in the whitefish fishery towards nets with longer extensions and sometirnes more meshes round the cod-end. This trend was associated with increases in engine power and changes in the design of fishing boats. The changes had the effect of reducing
the selectivity
of the nets (Robertson
and Ferro,
1988; Reeves
et al., 19921, and
” Present address: DIFTA, North Sea Centre, PO Box 93, DK-9850, Hirtshals, Denmark.
’ Present address: University of Aberdeen, College. Aberdeen, AB9 2UF, UK.
Department
of Land Economy,
0165.7836/96/915.00 0 1996 Elsevier Science B.V. All rights reserved SSD/Ol6S-7r376(9S)00418-1
St. Marys, Elphinstone
Road, Kings
354
N. Lowry, J.H.B. Robertson/Fisheries
Research 26 (1996) 353-363
negating the benefits of increasing minimum mesh sizes. When the minimum mesh size for fisheries in ICES areas III, IV, VI, VII and XII was increased to 100 mm (EC Council Regulation 3094/86 as amended by Reg 345/92) many fishermen replaced their 90 mm cod-ends with 100 mm cod-ends made of thicker double braided twines. This paper describes an investigation into the effect of using thicker twines on cod-end selectivity for the Scottish North Sea haddock trawl fishery. Previous experiments relating twine thickness to selectivity have been presented by Treshev and Shevtsov, 1975, Shevtsov (1979) and Isaksen et al. (1990). Although none of these were on haddock fisheries and none used polyethylene nets, the results all agreed that increasing twine thickness decreases the selectivity of the cod-end.
2. Materials and methods Experiments were carried out between 7-18 June 1993 on board the “Golden West” (FR363), a 19 m, 550 hp commercial trawler based in Fraserburgh, Scotland. Trawling was carried out in the Moray Firth sea area (ICES area IVa) approximately 10-20 miles northwest of Fraserburgh, in depths between 91-100 m (50-55 fathoms). The net was a Faithlie trawl used commercially by the “Golden West”, with a 10 m extension to which the experimental cod-ends were attached. Two hauls, each of two hours duration, were conducted during daylight each day. Towing speed was approximately 2.5 knots. Two experimental cod-ends were used and were interchanged at intervals during the trip. Both were double braided green polyethylene 120 meshes round ( 100 open meshes + selvedges) and 100 meshes in length and of nominal mesh size 100 mm. The cod-ends were made from twines of nominal size 3.5 mm (thin twine) and 6.0 mm (thick twine). Both of these twine sizes are used in haddock fisheries in the North Sea. The mesh size was measured during the trip, whilst the netting was wet, using an ICES gauge with 4 kg tension. Mesh size refers to the inside distance between two opposite knots when fully extended in the N direction. The actual mesh sizes were 95.1 mm for the thin twine cod-end and 100.0 mm for the thick twine cod-end. Twine thickness was measured using an optical technique as described by Ferro ( 1989). The actual thicknesses of the single twines were 3.5 and 5.2 mm for thin and thick twines respectively (Table 1) . Table I Twine thickness and mesh size measurements for the experimental twine but the thickness of a single twine is quoted here) Twine thickness
Thin twine Thick twine
(mm)
cod-ends. (Both cod-ends were made of double
Mesh size (mm)
Nominal
Actual
SE
Nominal
Actual
SE
3.5 6.0
3.5 5.2
0.1 I 0.13
100 100
95.1 100.0
0.10 0.14
SE, standard error. Number of mesh measurements ments for each cod-end = 40.
for each cod-end = 140. Number of twine diameter measure-
N. Lowry, J. H.B. Robermn / Fisheries Research 26 (I 996) 353-363
355
I
I I I I I I I I I
/
Fig.
I. Position of cover and supporting hoops used for “Golden West” trials.
Gear geometry (wing spread, headline height) and towing speed were monitored using Scanmar instrumentation. The average headline height was 7.1 m (range 6.7-7.4 m) and average wing spread 19.3 m (range 18.0-20.2 m) Selectivity was measured using the covered cod-end method, using a cover of nominal mesh size 30 mm with supporting alkathene hoops to hold the cover clear of the cod-end (Main and Sangster, 199 1) (Fig. 1) .The cover rigging was observed on two hauls during the trip with a remote controlled underwater television observation system (see Priestley et al.. 19x5 for description of the system). The cod-end and cover were observed to be towing correctly with no problems. The entire catch in the cod-end and the cover was divided into species components and weighed. Al I the haddock caught were measured (to the nearest centimetre below) to avoid subsampling errors. Selection curves were fitted to the data from each haul using a linear logistic function (Reeves et al.. 1992 gives a summary of the probability expression). The 50% retention length (L.50) is the length of fish at which 50% are retained and 50% are released. The selection range (SR) is a measure of the slope of the selection curve and is defined by the difference between the 75 and 25% lengths. These were calculated for each haul for the thick twine (Fig. 2 and Table 2) and for thin twine (Fig. 3 and Table 3). The two sets of selection curves were analysed and compared using the model developed by Fryer ( 1991) which takes into account the between-haul variation in selectivity by allowing the selectivity
N. Lowry, J. H.B. Robertson /Fisheries Research 26 (1996) 353-363
356
1 .o $C .(d z s .r
0.8
;
0.4
0.6
f? 0.2
20
40
30 Length (cm)
50
Fig. 2. Haddock: thick twine cod-end.
curves to vary randomly about a mean selectivity curve according to a multivariate normal distribution. A summary of the modelling process is given in the appendix and a more complex approach is described in Reeves et al. ( 1992). The model parameters were estimated using a residual maximum likelihood estimation method (REML) to overcome problems of underestimation of variance associated with maximum likelihood (ML) methods used with small sample sizes (Fryer, 1991).
3. Results Haddock (Melanogrummus aeglejkus) was the major component by weight of the catch in each tow. The size range was from 12 to 45 cm, having a bimodal distribution with a large peak at 20 cm and a small one at 28 cm, representing 1+ and 2 + year classes. The length frequency distributions, normalised by total catch numbers for the catch in the codend and cover for all valid hauls were similar for each twine size (Fig. 4). In addition to Table 2 Thick twine cod-end selectivity Haul
5 6 8 9 16 17
parameters Total number of haddock
CE mesh size
50% length
Selection
(mm)
(cm)
Factor
Range (cm)
100 100 100 100 100 100
23.9kO.11 24.5 +0.14 24.4kO.13 23.2*0.12 22.3 kO.12 23.9 +_O.16
2.39 2.45 2.44 2.32 2.23 2.39
3.1 4. I 4.0 4.0 4.7 6.0
Number of haddock in SR
Cod-end
Cover
Cod-end
810 910 817 836 979 1087
2619 4114 3097 1905 1737 2633
233 271 232 361 566 584
Cover 399 484 459 663 963 999
N. Lowry, J. H. B. Robertson /Fisheries Research 26 (I 996) 353-363
357
1.o-0 .-E 0.8 s g! ‘o 0.6 .r ;
Haddockhauls 0 2 0 3 . 10 12 A 13 .? 14 15 l
0.4 -
a
l
0.2 -
20
30 Length (cm)
40
50
Fig. 3. Haddock: thin twine cod-end.
haddock, approximately equal quantities of the following species were captured: whiting (Merlangius merlangus) , cod (Gadus morhua), anglerfish (Lophius piscatorius) , plaice (Pleuronectesplatessa) and Norway pout ( Trisoperus esmarkii). Similar quantities of this by-catch were caught each haul (Table 4). Five hauls of the total of 18 were excluded from analysis at an early stage because it was considered that the trawl was not fishing properly due to gear damage or foul rigging. Selection curves were fitted to haddock data for each haul for the thin twine (Fig. 2) and thick twine (Fig. 3). L5Os and selection ranges were calculated. The two sets of curves were analysed according to Fryer ( 1991) . The results suggest that there are two competing models which can be fitted to the data, the first indicating a difference in the position of the two sets of curves, hence a difference in L50 and the second indicating a difference in slope and hence in SR. There is no way to differentiate between these two models statistically but the significance levels are greater
Table 3 Thick twine cod-end selectivity Haul
2 3 IO 12 13 14 15
parameters
CE mesh size
50% length
Selection ~
( mm )
(cm)
Factor
95.1 95.1 9.5.1 95.1 95.1 95.1 95.1
24.9rtO.16 25.2kO.14 25.81bO.13 24.8kO.16 25.2f0.15 25.210.19 24.7kO.16
2.37 2.40 2.45 2.36 2.40 2.40 2.35
Total number of haddock
Number of haddock in SR
Range (cm)
Cod-end
Cover
Cod-end
4.7 3.5 3.4 4.1 3.7 4.9 4.1
769 687 597 623 654 676 575
2142 3646 4260 2927 4174 3062 2180
188 109 126 179 106 184 143
Cover 342 168 184 296 165 342 274
358
N. Lowry, J. H.B. Robertson /Fisheries Research 26 (I 9%) 353-363
@Thin twine cod-end + cover A Thick twine cod-end + cover
30
20
40
50
Fish length (cm)
Fig. 4. Haddock normalised length frequency distributions.
Table 4 Catch weights (kg) Haul No.
2 3 5 6 8 9 10 12 13 14 15 16 17
Twine
Thin Thin Thick Thick Thick Thick Thin Thin Thin Thin Thin Thick Thick
Cover
Cod-end Haddock
Other species
Haddock
Norway pout and whiting
158.5 169.6 166 157 171 137 141 105.5 140.5 124.5 117.5 145.5 172
31.1 43 45 49.5 40.5 34.5 41 24.8 36.2 24.8 31 26 19.7
259 342.5 239.5 360 290 174 396 259.7 349.7 270.5 193.5 141.5 235
150 259.5 255 516.5 344 246 323 168.4 609.5 87.7 95 87.2 95.5
Note: Hauls 1,4,7, 11 and 18 excluded. “Other species” means approximately equal quantities of whiting, cod, anglerfish and plaice. “Norway pout and whiting” means approximately equal weights of these species.
N. Lowry, J.H.B. Roberrson / Fisheries Research 26 (19%) 353-363
359
-0
al
.c a 5 c .g
0 2Q
. Thin twine 0 Thick twine
0.6-
0.4 -
a
10
20
30 Length (cm)
40
50
Fig. 5. Haddock selection curves. (Curves are average selection ogives calculated according to Fryer, 1991. Points are combined data.) Table 5 Parameter estmates
comparing
Parameter ffl
a? CYI
the two cod-ends Estimate
Standard error
T value (a/s.e.)
- 13.5364 0.5390 0.6902
0.64160 0.02442 0.16966
4.06096
Table 6 Selectivity parameters Twine type
Thick Thin
Mesh size
L50
(mm) (ICES)
(cm)
100.0
23.8 25.1
95.1
se. of LSO
Selection range
s.e. of SR
Selection factor
0.18 0.18
2.38 2.64
(cm)
0.25 0.25
4.07 4.07
s.e.. standard error: SR. selection range.
the model in which L50 varies. From visual comparison of the selection curves it seems reasonable to conclude that there is a difference in their positions but not in their slopes. It has been shown that there is little evidence for systematic variation in selection range with net parameters (Reeves, 1989; Reeves et al., 1992). This finding also suggests that a model in which L.50 varies and SR is held constant may be appropriate. The model chosen is: for
360
N. Lowry,J.H.B. Robertson/Fisheries Research26 (1996) 353-363
Net 1 (thin)
Net 2 (thick)
E( oi) = (y’ 0ff2 E(q)
=
where the parameter (a$ represents the difference in L50 between the two cod-ends, associated with the difference in twine size. This interpretation indicates that the thick twine cod-end has a significantly lower average L50 than the thin twine cod-end (Fig. 5). This difference in the L50 would be expected to be accentuated if the two cod-ends used had been exactly the same mesh size. Parameter estimates comparing the two cod-ends are given in Table 5. The estimated average selectivity parameters for these cod-ends are given in Table 6.
4. Discussion Fishing with a cod-end made of thicker twine reduced the selectivity for haddock. The reasons for this are unclear but the main factors are probably the changes in the mesh opening and stiffness of the netting. Effects on the behaviour of fish and on water flow rate may contribute. 4. I. Mesh opening It is known that the width of the cod-end has an effect on selectivity (Reeves et al., 1992). Both cod-ends were of 100 open meshes around. If fitted to the same circumference, the meshes of the thick twine cod-end would be more closed than those of the thin twine codend because of the greater amount of material in the thick twine cod-end. The width at the forward end of the cod-end will be determined by the size of the extension to which it is attached. At the aft end the width will be determined mainly by the size of the catch. 4.2. Stiffness The two different twine sizes used in this experiment are of the same material. As the thicker twine consists of more filaments it is correspondingly stiffer. It is thought that the flexibility of the material used for the cod-end has an effect on the selectivity (Holden, 197 1) . Stiffer nets will have a greater resistance to deformation when fish attempt to escape through partly open meshes. This may cause fish to be retained in the cod-end which would otherwise escape. The fish which do escape will probably suffer more scale damage and be less likely to survive. However, the stiffness of the twine may also have an effect on mesh opening in that the meshes of stiffer netting will open less for the same circumferential force caused by water pressure on the catch. 4.3. Visual effect The thicker twine presents a greater visual barrier to fish which may discourage them from making escape attempts. Glass et al. (1993) observed that fish were more likely to approach and penetrate netting if it contrasted weakly with the background.
N. Lowry, J. H.B. Robertson /Fisheries Research 26 (I 996) 353-363
361
4.4. Waterjow The thick twine presents a greater physical barrier which reduces water flow through the meshes, and this reduced flow may also inhibit fish escape. It also creates more drag in the early stages of the tow and keeps the meshes closed, reducing the escape area for the fish.
This experiment was carried out from a commercial boat with a commercial net, fishing on grounds where there were large numbers of fish present. There is always some concern that the small mesh cod-end cover, despite being of the best design available, may have some adverse effect on the way in which the cod-end fishes. For example the cod-end meshes may have been masked by the cover. The underwater observations suggested a separation between the two of 30-45 cm in steady state conditions and with an average catch but during pulsing and turning the separation sometimes reduced considerably. Nevertheless, conditions were similar for both cod-ends and the conclusion that the use of thick twines will decrease the selectivity of a 100 mm cod-end is considered valid. The selection factor for the 3.5 mm double twine is similar to the average value for haddock quoted by Wileman ( 1991). The selection factors measured for the trial cod-ends indicate that the increase in twine thickness from 3.5 cm to 6.0 cm reduced selectivity by about IO%, equivalent to a reduction in mesh size of about 10 mm in a 100 mm cod-end. The recent trend towards using thicker twine in the North Sea whitefish fishery will negate the effect of mesh size increases and the high rate of discarding of small haddock will continue. The existing mathematical models of cod-end selectivity (Reeves et al., 1992; Galbraith et al., 1994) should be extended to take account of the effect of twine thickness. It has been demonstrated that cod-end twine thickness has a significant affect on selectivity, so it is essential to include this factor in calculation of fishing mortality for stock assessment purposes.
Acknowledgements We are most grateful to Peter and Colin West and the crew of “Golden West” and to Bob Kynoch, Peter Barkel and Huseyin Ozbilgin who assisted us on the cruise and to two anonymous referees for helpful comments on an earlier draft of this paper.
Appendix
A
The selectivity of each haul is described by the linear logistic function, for a fish of length 1 and retention probability p: log f-_ (
= u,, + ui,l 1
362
N. Lowry, J.H.B. Robertson/Fisheries
Research 26 (19%) 353-363
It is convenient to represent the parameters Uii,Ui2by the vector: ui
=
0 “il
*i2
and let Dibe the maximum likelihood estimator of Di.Diis approximately normally distributed with mean Uiand variance matrix Ri. For multiple hauls with different nets there are two elements which combine to make up the variation between hauls. The first is the variation due to gear type which can be described by a combination of q variables, and the second variation due to “uncontrolled” variables which cannot be modelled explicitly (Fryer, 1991). The between haul variability can be modelled generally by assuming that the parameters Vivary between hauls according to a normal distribution with mean Xia and constant variance D. Where Xi is a 2 * q matrix of constants describing the differences between the gears (related to the twine thickness of the cod-ends in this case) and (Yis a 1 * q vector of parameters to be estimated. In reality, u, are not known exactly, only estimated as Oiwith variance Ri, i.e.: ^Ui= iV(Xia,D + R,) The expected selectivity parameters Uifor haul i are given by: E(ui) = Xi(Y To compare the two data sets with different twine thicknesses, initially a model was chosen with design matrix:
where t = 0 for the thin twine cod-end and t = 1 for the thick twine cod-end. This gives the model: Net 1 (thin) E(Di) = z: = Ly’ 00 Net 2 (thick) E(ai) =(~~)=~~~~:~) where cr3indicates the effect on L50 of the change in twine and cu, indicates the effect on SR. It was found that neither c+ or cu, are significantly different from zero at the 5% significance level but when either CQor cu, is dropped from the model the other becomes significant. As the variation is considered to be in the L50 and not SR, CC,is dropped from the net 2 model. The chosen model is therefore: Net 1 (thin) E( Ui) = Net 2 (thick) E( ui) =
a’ 0a2
N. Lowry. J. H.B. Robertson /Fisheries Research 26 (1996) 353-363
363
Estimates of parameters and variance matrices for these terms are obtained by REML using the EM algorithm, according to Fryer ( 1991).
References Ferro, R.S.T., 1989. Objective measurement of the thickness of netting twine used in the fishing indushy. Fish. Res., 8: 103-I 12. Fryer, R.J., 199I. A model of between haul variation in selectivity. ICES J. Mar. Sci., 48: 281-290. Galbraith, R.D.. Fryer, R.J., and Maitland, K.M.S., 1994. Demersal pair trawl cod-end selectivity models. Fish. Res., 20: I .3-27. Glass, C.W., Wardle, C.S. and Gosden, S.J., 1993. Behavioural studies of the principles underlying mesh penetration by fish. ICES Mar. Sci. Symp., 196: 92-97. Holden, M. ( ed. ) ,197 1. Report of the ICES/ICNAF working groups on selectivity analysis. Cooperative Research Report Series A, No. 25. Isaksen, B., Lisovsky, S. and Sakhno, V.A., 1990. A comparison of the selectivity in cod-ends used by the Soviet and Norwegian trawler fleet in the Barents Sea. ICES CM 1990/B:51. Main, J. And Sangster, G.1.. 1991. A different approach to covered cod-end selection experiments. Scot. Fish. Work. Paper No. 4/91. Priestley, R., Wardle, C.S. and Hall, C.D., 1985. The Marine Laboratoryremotecontrolled fishinggearobservation vehicle. ICES CM 1985/B:lO. Reeves, S.A., 1989. The variation in selection range with net parameters. Scot. Fish. Work. Paper No. 8/89. Reeves, S.A., Armstrong, D.W., Fryer R.J. and Coull K.A., 1992. The effects of mesh size, cod-end extension length and cod-end diameter on the selectivity of Scottish trawls and seines. ICES J. Mar. Sci., 49: 279-288. Robertson, J.H.B. and Ferro, R.S.T., 1988. Mesh selection within the cod-ends of trawls. The effects of narrowing the cod-end and shortening the extension. Scot. Fish. Res. Rep. No. 39, 11 pp. Shevtsov, S.E.. 1979. The effect of twine thickness and size of catch on the selectivity of trawl cod-ends. Rybnokhoz. lssled. Bass. Bait. Morya, 14, 140-154 (in translation). Stewart, P.A.M. and Newton, A.W., 1993. Observations on the size composition of haddock and whiting catches taken by the different fishing methods used in the Scottish North Sea demersal fisheries. NAFO SCR Dot 931 99, Serial No N229 I. Treshev, A.1. and Shevtsov, S.E., 1975. Selectivity of trawl cod-ends in fishing for hening in the Baltic Sea. ICES CM 1975/8:8. Wileman, D.. I99 I. Cod-end selectivity: An updated review of available data. EC DirectorateGeneral for Fisheries, 199l/lS.
Fisheries Research26 ( 1996) 353-363
The effect of twine thickness on cod-end selectivity of trawls for haddock in the North Sea N. Lowry *, J.H.B. Robertson SOAFD Murk
Labomtory.
’
PO Box 101. Victoria Road, Aberdeen,
UK
Accepted 8 May 1995
Abstract Since June 1992, when the minimum mesh size for the North Sea demersal fisheries was raised to IO0 mm, there has been a trend towards the use of cod-ends made of much thicker twine. The effect of this increase in twine thickness on cod-end selectivity for haddock was investigated in June 1993 on a Scottish commercial vessel. Cod-end selectivity was measured using a cod-end cover with supporting flexible hoops. The results were analysed taking account of between haul variation in selectivity, and indicate a significant reduction in selectivity with increased twine size. The change from 3.5 mm double twine to 6 mm double twine reduced the selection factor from 2.64 to 2.38. The reduction in selectivity is thought to be due mainly to reduced mesh opening and increased twine stiffness which both inhibit escape, but behavioural effects may also be important. Kr~~t/.s:
Selectivity;
Cod-ends;
Twine thickness;
Haddock; Trawl
1. Introduction
Mesh size regulations for towed fishing gears aim to reduce fishing mortality by allowing small fish to escape from the nets. In the North Sea, minimum mesh sizes have been increased many times but catch and discard data collected by the Marine Laboratory sampling programme suggest that these have had little effect (Stewart and Newton, 1993). There has been a trend over the last 20 years in the whitefish fishery towards nets with longer extensions and sometirnes more meshes round the cod-end. This trend was associated with increases in engine power and changes in the design of fishing boats. The changes had the effect of reducing
the selectivity
of the nets (Robertson
and Ferro,
1988; Reeves
et al., 19921, and
” Present address: DIFTA, North Sea Centre, PO Box 93, DK-9850, Hirtshals, Denmark.
’ Present address: University of Aberdeen, College. Aberdeen, AB9 2UF, UK.
Department
of Land Economy,
0165.7836/96/915.00 0 1996 Elsevier Science B.V. All rights reserved SSD/Ol6S-7r376(9S)00418-1
St. Marys, Elphinstone
Road, Kings
354
N. Lowry, J.H.B. Robertson/Fisheries
Research 26 (1996) 353-363
negating the benefits of increasing minimum mesh sizes. When the minimum mesh size for fisheries in ICES areas III, IV, VI, VII and XII was increased to 100 mm (EC Council Regulation 3094/86 as amended by Reg 345/92) many fishermen replaced their 90 mm cod-ends with 100 mm cod-ends made of thicker double braided twines. This paper describes an investigation into the effect of using thicker twines on cod-end selectivity for the Scottish North Sea haddock trawl fishery. Previous experiments relating twine thickness to selectivity have been presented by Treshev and Shevtsov, 1975, Shevtsov (1979) and Isaksen et al. (1990). Although none of these were on haddock fisheries and none used polyethylene nets, the results all agreed that increasing twine thickness decreases the selectivity of the cod-end.
2. Materials and methods Experiments were carried out between 7-18 June 1993 on board the “Golden West” (FR363), a 19 m, 550 hp commercial trawler based in Fraserburgh, Scotland. Trawling was carried out in the Moray Firth sea area (ICES area IVa) approximately 10-20 miles northwest of Fraserburgh, in depths between 91-100 m (50-55 fathoms). The net was a Faithlie trawl used commercially by the “Golden West”, with a 10 m extension to which the experimental cod-ends were attached. Two hauls, each of two hours duration, were conducted during daylight each day. Towing speed was approximately 2.5 knots. Two experimental cod-ends were used and were interchanged at intervals during the trip. Both were double braided green polyethylene 120 meshes round ( 100 open meshes + selvedges) and 100 meshes in length and of nominal mesh size 100 mm. The cod-ends were made from twines of nominal size 3.5 mm (thin twine) and 6.0 mm (thick twine). Both of these twine sizes are used in haddock fisheries in the North Sea. The mesh size was measured during the trip, whilst the netting was wet, using an ICES gauge with 4 kg tension. Mesh size refers to the inside distance between two opposite knots when fully extended in the N direction. The actual mesh sizes were 95.1 mm for the thin twine cod-end and 100.0 mm for the thick twine cod-end. Twine thickness was measured using an optical technique as described by Ferro ( 1989). The actual thicknesses of the single twines were 3.5 and 5.2 mm for thin and thick twines respectively (Table 1) . Table I Twine thickness and mesh size measurements for the experimental twine but the thickness of a single twine is quoted here) Twine thickness
Thin twine Thick twine
(mm)
cod-ends. (Both cod-ends were made of double
Mesh size (mm)
Nominal
Actual
SE
Nominal
Actual
SE
3.5 6.0
3.5 5.2
0.1 I 0.13
100 100
95.1 100.0
0.10 0.14
SE, standard error. Number of mesh measurements ments for each cod-end = 40.
for each cod-end = 140. Number of twine diameter measure-
N. Lowry, J. H.B. Robermn / Fisheries Research 26 (I 996) 353-363
355
I
I I I I I I I I I
/
Fig.
I. Position of cover and supporting hoops used for “Golden West” trials.
Gear geometry (wing spread, headline height) and towing speed were monitored using Scanmar instrumentation. The average headline height was 7.1 m (range 6.7-7.4 m) and average wing spread 19.3 m (range 18.0-20.2 m) Selectivity was measured using the covered cod-end method, using a cover of nominal mesh size 30 mm with supporting alkathene hoops to hold the cover clear of the cod-end (Main and Sangster, 199 1) (Fig. 1) .The cover rigging was observed on two hauls during the trip with a remote controlled underwater television observation system (see Priestley et al.. 19x5 for description of the system). The cod-end and cover were observed to be towing correctly with no problems. The entire catch in the cod-end and the cover was divided into species components and weighed. Al I the haddock caught were measured (to the nearest centimetre below) to avoid subsampling errors. Selection curves were fitted to the data from each haul using a linear logistic function (Reeves et al.. 1992 gives a summary of the probability expression). The 50% retention length (L.50) is the length of fish at which 50% are retained and 50% are released. The selection range (SR) is a measure of the slope of the selection curve and is defined by the difference between the 75 and 25% lengths. These were calculated for each haul for the thick twine (Fig. 2 and Table 2) and for thin twine (Fig. 3 and Table 3). The two sets of selection curves were analysed and compared using the model developed by Fryer ( 1991) which takes into account the between-haul variation in selectivity by allowing the selectivity
N. Lowry, J. H.B. Robertson /Fisheries Research 26 (1996) 353-363
356
1 .o $C .(d z s .r
0.8
;
0.4
0.6
f? 0.2
20
40
30 Length (cm)
50
Fig. 2. Haddock: thick twine cod-end.
curves to vary randomly about a mean selectivity curve according to a multivariate normal distribution. A summary of the modelling process is given in the appendix and a more complex approach is described in Reeves et al. ( 1992). The model parameters were estimated using a residual maximum likelihood estimation method (REML) to overcome problems of underestimation of variance associated with maximum likelihood (ML) methods used with small sample sizes (Fryer, 1991).
3. Results Haddock (Melanogrummus aeglejkus) was the major component by weight of the catch in each tow. The size range was from 12 to 45 cm, having a bimodal distribution with a large peak at 20 cm and a small one at 28 cm, representing 1+ and 2 + year classes. The length frequency distributions, normalised by total catch numbers for the catch in the codend and cover for all valid hauls were similar for each twine size (Fig. 4). In addition to Table 2 Thick twine cod-end selectivity Haul
5 6 8 9 16 17
parameters Total number of haddock
CE mesh size
50% length
Selection
(mm)
(cm)
Factor
Range (cm)
100 100 100 100 100 100
23.9kO.11 24.5 +0.14 24.4kO.13 23.2*0.12 22.3 kO.12 23.9 +_O.16
2.39 2.45 2.44 2.32 2.23 2.39
3.1 4. I 4.0 4.0 4.7 6.0
Number of haddock in SR
Cod-end
Cover
Cod-end
810 910 817 836 979 1087
2619 4114 3097 1905 1737 2633
233 271 232 361 566 584
Cover 399 484 459 663 963 999
N. Lowry, J. H. B. Robertson /Fisheries Research 26 (I 996) 353-363
357
1.o-0 .-E 0.8 s g! ‘o 0.6 .r ;
Haddockhauls 0 2 0 3 . 10 12 A 13 .? 14 15 l
0.4 -
a
l
0.2 -
20
30 Length (cm)
40
50
Fig. 3. Haddock: thin twine cod-end.
haddock, approximately equal quantities of the following species were captured: whiting (Merlangius merlangus) , cod (Gadus morhua), anglerfish (Lophius piscatorius) , plaice (Pleuronectesplatessa) and Norway pout ( Trisoperus esmarkii). Similar quantities of this by-catch were caught each haul (Table 4). Five hauls of the total of 18 were excluded from analysis at an early stage because it was considered that the trawl was not fishing properly due to gear damage or foul rigging. Selection curves were fitted to haddock data for each haul for the thin twine (Fig. 2) and thick twine (Fig. 3). L5Os and selection ranges were calculated. The two sets of curves were analysed according to Fryer ( 1991) . The results suggest that there are two competing models which can be fitted to the data, the first indicating a difference in the position of the two sets of curves, hence a difference in L50 and the second indicating a difference in slope and hence in SR. There is no way to differentiate between these two models statistically but the significance levels are greater
Table 3 Thick twine cod-end selectivity Haul
2 3 IO 12 13 14 15
parameters
CE mesh size
50% length
Selection ~
( mm )
(cm)
Factor
95.1 95.1 9.5.1 95.1 95.1 95.1 95.1
24.9rtO.16 25.2kO.14 25.81bO.13 24.8kO.16 25.2f0.15 25.210.19 24.7kO.16
2.37 2.40 2.45 2.36 2.40 2.40 2.35
Total number of haddock
Number of haddock in SR
Range (cm)
Cod-end
Cover
Cod-end
4.7 3.5 3.4 4.1 3.7 4.9 4.1
769 687 597 623 654 676 575
2142 3646 4260 2927 4174 3062 2180
188 109 126 179 106 184 143
Cover 342 168 184 296 165 342 274
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@Thin twine cod-end + cover A Thick twine cod-end + cover
30
20
40
50
Fish length (cm)
Fig. 4. Haddock normalised length frequency distributions.
Table 4 Catch weights (kg) Haul No.
2 3 5 6 8 9 10 12 13 14 15 16 17
Twine
Thin Thin Thick Thick Thick Thick Thin Thin Thin Thin Thin Thick Thick
Cover
Cod-end Haddock
Other species
Haddock
Norway pout and whiting
158.5 169.6 166 157 171 137 141 105.5 140.5 124.5 117.5 145.5 172
31.1 43 45 49.5 40.5 34.5 41 24.8 36.2 24.8 31 26 19.7
259 342.5 239.5 360 290 174 396 259.7 349.7 270.5 193.5 141.5 235
150 259.5 255 516.5 344 246 323 168.4 609.5 87.7 95 87.2 95.5
Note: Hauls 1,4,7, 11 and 18 excluded. “Other species” means approximately equal quantities of whiting, cod, anglerfish and plaice. “Norway pout and whiting” means approximately equal weights of these species.
N. Lowry, J.H.B. Roberrson / Fisheries Research 26 (19%) 353-363
359
-0
al
.c a 5 c .g
0 2Q
. Thin twine 0 Thick twine
0.6-
0.4 -
a
10
20
30 Length (cm)
40
50
Fig. 5. Haddock selection curves. (Curves are average selection ogives calculated according to Fryer, 1991. Points are combined data.) Table 5 Parameter estmates
comparing
Parameter ffl
a? CYI
the two cod-ends Estimate
Standard error
T value (a/s.e.)
- 13.5364 0.5390 0.6902
0.64160 0.02442 0.16966
4.06096
Table 6 Selectivity parameters Twine type
Thick Thin
Mesh size
L50
(mm) (ICES)
(cm)
100.0
23.8 25.1
95.1
se. of LSO
Selection range
s.e. of SR
Selection factor
0.18 0.18
2.38 2.64
(cm)
0.25 0.25
4.07 4.07
s.e.. standard error: SR. selection range.
the model in which L50 varies. From visual comparison of the selection curves it seems reasonable to conclude that there is a difference in their positions but not in their slopes. It has been shown that there is little evidence for systematic variation in selection range with net parameters (Reeves, 1989; Reeves et al., 1992). This finding also suggests that a model in which L.50 varies and SR is held constant may be appropriate. The model chosen is: for
360
N. Lowry,J.H.B. Robertson/Fisheries Research26 (1996) 353-363
Net 1 (thin)
Net 2 (thick)
E( oi) = (y’ 0ff2 E(q)
=
where the parameter (a$ represents the difference in L50 between the two cod-ends, associated with the difference in twine size. This interpretation indicates that the thick twine cod-end has a significantly lower average L50 than the thin twine cod-end (Fig. 5). This difference in the L50 would be expected to be accentuated if the two cod-ends used had been exactly the same mesh size. Parameter estimates comparing the two cod-ends are given in Table 5. The estimated average selectivity parameters for these cod-ends are given in Table 6.
4. Discussion Fishing with a cod-end made of thicker twine reduced the selectivity for haddock. The reasons for this are unclear but the main factors are probably the changes in the mesh opening and stiffness of the netting. Effects on the behaviour of fish and on water flow rate may contribute. 4. I. Mesh opening It is known that the width of the cod-end has an effect on selectivity (Reeves et al., 1992). Both cod-ends were of 100 open meshes around. If fitted to the same circumference, the meshes of the thick twine cod-end would be more closed than those of the thin twine codend because of the greater amount of material in the thick twine cod-end. The width at the forward end of the cod-end will be determined by the size of the extension to which it is attached. At the aft end the width will be determined mainly by the size of the catch. 4.2. Stiffness The two different twine sizes used in this experiment are of the same material. As the thicker twine consists of more filaments it is correspondingly stiffer. It is thought that the flexibility of the material used for the cod-end has an effect on the selectivity (Holden, 197 1) . Stiffer nets will have a greater resistance to deformation when fish attempt to escape through partly open meshes. This may cause fish to be retained in the cod-end which would otherwise escape. The fish which do escape will probably suffer more scale damage and be less likely to survive. However, the stiffness of the twine may also have an effect on mesh opening in that the meshes of stiffer netting will open less for the same circumferential force caused by water pressure on the catch. 4.3. Visual effect The thicker twine presents a greater visual barrier to fish which may discourage them from making escape attempts. Glass et al. (1993) observed that fish were more likely to approach and penetrate netting if it contrasted weakly with the background.
N. Lowry, J. H.B. Robertson /Fisheries Research 26 (I 996) 353-363
361
4.4. Waterjow The thick twine presents a greater physical barrier which reduces water flow through the meshes, and this reduced flow may also inhibit fish escape. It also creates more drag in the early stages of the tow and keeps the meshes closed, reducing the escape area for the fish.
This experiment was carried out from a commercial boat with a commercial net, fishing on grounds where there were large numbers of fish present. There is always some concern that the small mesh cod-end cover, despite being of the best design available, may have some adverse effect on the way in which the cod-end fishes. For example the cod-end meshes may have been masked by the cover. The underwater observations suggested a separation between the two of 30-45 cm in steady state conditions and with an average catch but during pulsing and turning the separation sometimes reduced considerably. Nevertheless, conditions were similar for both cod-ends and the conclusion that the use of thick twines will decrease the selectivity of a 100 mm cod-end is considered valid. The selection factor for the 3.5 mm double twine is similar to the average value for haddock quoted by Wileman ( 1991). The selection factors measured for the trial cod-ends indicate that the increase in twine thickness from 3.5 cm to 6.0 cm reduced selectivity by about IO%, equivalent to a reduction in mesh size of about 10 mm in a 100 mm cod-end. The recent trend towards using thicker twine in the North Sea whitefish fishery will negate the effect of mesh size increases and the high rate of discarding of small haddock will continue. The existing mathematical models of cod-end selectivity (Reeves et al., 1992; Galbraith et al., 1994) should be extended to take account of the effect of twine thickness. It has been demonstrated that cod-end twine thickness has a significant affect on selectivity, so it is essential to include this factor in calculation of fishing mortality for stock assessment purposes.
Acknowledgements We are most grateful to Peter and Colin West and the crew of “Golden West” and to Bob Kynoch, Peter Barkel and Huseyin Ozbilgin who assisted us on the cruise and to two anonymous referees for helpful comments on an earlier draft of this paper.
Appendix
A
The selectivity of each haul is described by the linear logistic function, for a fish of length 1 and retention probability p: log f-_ (
= u,, + ui,l 1
362
N. Lowry, J.H.B. Robertson/Fisheries
Research 26 (19%) 353-363
It is convenient to represent the parameters Uii,Ui2by the vector: ui
=
0 “il
*i2
and let Dibe the maximum likelihood estimator of Di.Diis approximately normally distributed with mean Uiand variance matrix Ri. For multiple hauls with different nets there are two elements which combine to make up the variation between hauls. The first is the variation due to gear type which can be described by a combination of q variables, and the second variation due to “uncontrolled” variables which cannot be modelled explicitly (Fryer, 1991). The between haul variability can be modelled generally by assuming that the parameters Vivary between hauls according to a normal distribution with mean Xia and constant variance D. Where Xi is a 2 * q matrix of constants describing the differences between the gears (related to the twine thickness of the cod-ends in this case) and (Yis a 1 * q vector of parameters to be estimated. In reality, u, are not known exactly, only estimated as Oiwith variance Ri, i.e.: ^Ui= iV(Xia,D + R,) The expected selectivity parameters Uifor haul i are given by: E(ui) = Xi(Y To compare the two data sets with different twine thicknesses, initially a model was chosen with design matrix:
where t = 0 for the thin twine cod-end and t = 1 for the thick twine cod-end. This gives the model: Net 1 (thin) E(Di) = z: = Ly’ 00 Net 2 (thick) E(ai) =(~~)=~~~~:~) where cr3indicates the effect on L50 of the change in twine and cu, indicates the effect on SR. It was found that neither c+ or cu, are significantly different from zero at the 5% significance level but when either CQor cu, is dropped from the model the other becomes significant. As the variation is considered to be in the L50 and not SR, CC,is dropped from the net 2 model. The chosen model is therefore: Net 1 (thin) E( Ui) = Net 2 (thick) E( ui) =
a’ 0a2
N. Lowry. J. H.B. Robertson /Fisheries Research 26 (1996) 353-363
363
Estimates of parameters and variance matrices for these terms are obtained by REML using the EM algorithm, according to Fryer ( 1991).
References Ferro, R.S.T., 1989. Objective measurement of the thickness of netting twine used in the fishing indushy. Fish. Res., 8: 103-I 12. Fryer, R.J., 199I. A model of between haul variation in selectivity. ICES J. Mar. Sci., 48: 281-290. Galbraith, R.D.. Fryer, R.J., and Maitland, K.M.S., 1994. Demersal pair trawl cod-end selectivity models. Fish. Res., 20: I .3-27. Glass, C.W., Wardle, C.S. and Gosden, S.J., 1993. Behavioural studies of the principles underlying mesh penetration by fish. ICES Mar. Sci. Symp., 196: 92-97. Holden, M. ( ed. ) ,197 1. Report of the ICES/ICNAF working groups on selectivity analysis. Cooperative Research Report Series A, No. 25. Isaksen, B., Lisovsky, S. and Sakhno, V.A., 1990. A comparison of the selectivity in cod-ends used by the Soviet and Norwegian trawler fleet in the Barents Sea. ICES CM 1990/B:51. Main, J. And Sangster, G.1.. 1991. A different approach to covered cod-end selection experiments. Scot. Fish. Work. Paper No. 4/91. Priestley, R., Wardle, C.S. and Hall, C.D., 1985. The Marine Laboratoryremotecontrolled fishinggearobservation vehicle. ICES CM 1985/B:lO. Reeves, S.A., 1989. The variation in selection range with net parameters. Scot. Fish. Work. Paper No. 8/89. Reeves, S.A., Armstrong, D.W., Fryer R.J. and Coull K.A., 1992. The effects of mesh size, cod-end extension length and cod-end diameter on the selectivity of Scottish trawls and seines. ICES J. Mar. Sci., 49: 279-288. Robertson, J.H.B. and Ferro, R.S.T., 1988. Mesh selection within the cod-ends of trawls. The effects of narrowing the cod-end and shortening the extension. Scot. Fish. Res. Rep. No. 39, 11 pp. Shevtsov, S.E.. 1979. The effect of twine thickness and size of catch on the selectivity of trawl cod-ends. Rybnokhoz. lssled. Bass. Bait. Morya, 14, 140-154 (in translation). Stewart, P.A.M. and Newton, A.W., 1993. Observations on the size composition of haddock and whiting catches taken by the different fishing methods used in the Scottish North Sea demersal fisheries. NAFO SCR Dot 931 99, Serial No N229 I. Treshev, A.1. and Shevtsov, S.E., 1975. Selectivity of trawl cod-ends in fishing for hening in the Baltic Sea. ICES CM 1975/8:8. Wileman, D.. I99 I. Cod-end selectivity: An updated review of available data. EC DirectorateGeneral for Fisheries, 199l/lS.