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Selectivity experiments on sole with diamond and square mesh codends in the Belgian coastal beam trawl fishery
Fisheries Research, 13 (1992) 221-233
221
Elsevier Science Publishers B.V., Amsterdam
Selectivity experiments on sole with diamond and square mesh codends in the Belgian coastal beam trawl fishery R. Fonteyne and R. M'Rabet Fisheries Research Station, Ankerstraat !, B-8400 Oostende. Belgium
ABSTRACT Fonteyne, R. and M'Rabet, R., 1992. Selectivity experiments on sole with diamond and square mesh codends in the Belgian coastal beam trawl fishery. Fish. Res.. 13:221-233. The selectivity of diamond +rod square mesh codends in the Belgian coastal beam trawl fishery for sole were compared. Two experiments, involving two different codend lengths were carried out. Contrary to the results obtained with roundfish, no influence of the mesh shape on the codend selectivity could be shown for the flatfish species investigated. It is s, t e~R,,qed that the application of square meshes may reduce the catch of small roundfish while maintaining the catch level of the target flatfish species. The length of the codend did not influence the selectivity for sole, bm a reduction in benthos by-catch was observed. The selectivity for sole in the coastal beam trawl fishery seems to be lower than the selectivity of the beam trawls used by larger vessels operating in the open sea. This is in agreement with the results of similar experintents, but there is no evidence that the selection factor depends on the amount of bycatch.
INTRODUCTION
Selectivity experiments with square mesh codends have shown that square meshes are more selective for most roundfish species than conventional diamond meshes (Robertson, 1983; Isaksen and Valdemarsen, 1986; Robertson and Stewart, 1988, Cooper and Hickey, 1989). This is due to the fact that square meshes remain open all along the codend, whereas diamond meshes tend to close when the codend fills up and the tension on the mesh bars increases. Apart from the results of Canadian experiments on plaice (Cooper and Hickey, 1989; Millar and Walsh, 1992; Walsh et al., 1992) no data have been published so far on the selective properties of square mesh codends for flatfish species. Correspondence to: R. Fonteyne, Fisheries Research Station, Ankerstraat 1, 15-8400 Oostende, Belgium.
© 1992 Elsevier Science Publishers B.V. All rights reserved 0165-7836/92/505.00
222
R. FONTEYNE AND R. M'RABET
In April 1987 and April 1988 two series of experiments were carried out by the Fisheries Research Station to investigate the influence of mesh shape diamond or square - on the codend selectivity for sole in the Belgian coastal beam trawl fishery. These experiments were part of a larger research program to evaluate the influence of a number of codend characteristics (mesh size, mesh shape, codend dimensions and netting material) on the selectivity in this fishery. This paper reports on the effects of codend mesh shape on the selection of sole for two codend lengths. Th~ codend selectivity for sole of small beam trawlers is discussed in relation to the selectivity oflarger beamers operating in more open seas.
MATERIAL AND METHODS
Vessel and gears The Belgian coastal beam trawl fishery for sole is practised in spring and summer by small beam trawlers with an engine power of up to 300 hp. Beam trawlers tow two nets from either side of the ship by means of two derrick booms as shown in Fig. 1. The nets are rigged to beams 4-7 m long. TicHer chains are used to drive the fishes from the sea bed. The number of tickler chains used in the coastal beam trawl fishery varies between two and four according to the beam length and the nature of the sea bed. The present experiments took place on board a chartered beam trawler with an engine power of 250 hp and a length overall of 20.60 m. The nets for the 7-m beams were equipped with two tickler chains only. The characteristics of the nets are given in Fig. 2.
CrOS~ 5~C t I ~
~azy d~cky t l c k l e ~ e ~
;?--//- ....
Fig. !. General arrangement of double rig beam trawlin&
223
SELECTIVITY EXPERIMENTS ON SOLE
L
°
"r,
1
5
b
,.1
iiiii d
:
4.%0m
NIX l(,mrr
Fig. 2. Net plan.
Codends and covers Two pairs of double-braided polyethylene codends with a nominal mesh opeping of 75 mm were studied. The first pair of codends was based on the traditional diamond mesh codend with a circumference •f 100 meshes and a length of 35 meshes. The corresponding square mesh codend, made of the same material mounted with one AB direction parallel to the long axis of the codend and the other AB direction at right angles to it, was comprised of 50 meshes around and 70 meshes in length. The codends in the second pair were twice as long as those in the first pair, i.e., 70 meshes for the diamond mesh codend and 140 meshes for the square mesh codend. Apart from their length the two pairs of codends had the same characteristics. The square mesh codends were constructed as described by Robertson (1986). The selection parameters were obtained by the covered codend (full cover) method. The covers (diamond mesh) were 1.6 times as long and wide as the codends and were also made of polyethylene. This material was chosen because it is buoyant, so that obstruction of the codend meshes by the cover will be minimized. The nominal mesh opening of the cover was 55 mm. Details on the codends and covers used are given in Table 1.
Experimental procedure A first series of experiments, 15 hauls in two trips, with the short codends, was carried out in April 1987. In April 1988 a second series of experiments, 13 hauls in two trips, with the two long codends took place. As a reference, the same number of hauls was made with a short diamond mesh codend. This codend had the same characteristics as the short diamond mesh codend in the first series of experiments (Table I ).
R. FONTEYNE AND R. M'RABIE'r
224
TABLE I
Codend construction Codend type
First experiment Short-diamond
Dimensions (meshes)
35 iongx 1O0 round
Short-square Second experiment Long-diamond Long-square Short-diamond Covers Cover (short) Cover (long)
Material
PE, twisted
Mesh opening ( mm ) n
Range
Mean
200
67-83
75.3
200
68-83
75.7
2 X R4130rex
70 long)<
PE, twisted
50 round
2)< R4 ! 30rex
70 long x I00 round 140 long)<
PE, twisted 2 x R4130rex
200
70-86
76.4
PE, twisted
200
73-82
77.2
50 round
2 X R4130lex 150
65-80
74.3
125
51-63
56.2
25
51-59
55.8
35 IongX
PE, twisted
100 round
2 x R4130rex
78 long x 224 round 156 IongX 224 round
PE, twisted R 1560rex PE, twisted R ! 560rex
Use was made of the twin beam trawl principle, which uses the diamond mesh and the square mesh codends under the same conditions. One type of codend was rigged to the starboard net, and the other to the port side net. To overcome small differences that might occur between the catching efficiency of the two gears, two trips were carried out on successive nights in each experiment. After each trip the codends were switched. Apart from the covers, the experimental hauls were performed under exactly the same conditions as in commercial fishing. Fishing took place at night only. The fishing area was located at a distance of 6-9 miles from the coast, between Oostende and Wenduine. The hauls had a mean duration of I h. After each haul all soles were measured to the nearest cm. The commercial catches were weighed. To estimate their weight, the non-commercial bycatches were boxed, the average weight of each box being 30 kg. The mesh sizes of the codends were measured with an ICES mesh gauge operated at 4-kg pretension. Both the diamond and the square meshes were measured in the F!-direction of the netting. The mean mesh sizes were calculated from the measurement of 25 top panel meshes every other haul. Data analysis The combined catches of all hauls in each experiment were used to calculate the length selection curve. The calculations were based on the assumption
225
SELECTIVITY EXPERIMENTSON SOLE
that the selection curve may be represented by the logistic curve (Pope et al., 1975 ). The parameters of the logistic function were estimated by the method of maximum likelihood. The fitting of the logistic curve to the observed retention percentages was tested by the chi-square test, also dc=cribed by Pope et al. (1975). The level of significance was set at 0.05. If the expected number of fish was less than five in any class, neighbouring classes were pooled and the number of degrees of freedom for the chi-square test was decreased accordingly. RESULTS
The length distributions of the codend sole catches in the first experiment are given in Fig. 3. The selection curves of both the diamond and the square mesh codend are shown in Fig. 4 and compared in Fig. 5. The chi-square values in Table 2 indicate that there were no significant differences between observed and expected retention percentages, probabilities being more than 0.05. The 50% retention lengths were 20.6 cm and 20.3 cm for the diamond and the square mesh codend respectively. The selection factor for both codends was 2.7. From these data it appears that the square mesh codend had a somewhat narrower selection range. The length distributions of tLe sole catches in the second experiment are given in Fig. 6, and the selection curves are shown in Fig. 7. The selection curves of the two long co
A
15
20 Q square meshes
25
i
J
~4--.~-
L
30
~
:
L
i
,
t
t
,
35 ~0 Ler,gth (cm)
O diamond meshes
Fig. 3. Length distributions o f the codend sole catches in the first experiment.
226
R. FONTEYNE AND R. M'ItABET
% Pe%i~ed +00 90
OO 70 60
IL
SO ~0 ]0 20 10 0 9
Length (~.m)
%
Refa~ned lOOF
(b)
//"
90F
8oF
::i
/
,oF 3o~-
./: /I
~°l
/
:
17 io 3'3 3~
Length(era) Fig. 4. Selection curves for the first experiment. (a) Diamond mesh short codend; (b) square mesh short codend. % Retained 100F
square mesh
'°80 f 50
//~/
diamond mesh
// // --
1,0 30
~t
20
iI
10
IT
0 ~
9
12
15
18
21
.........................
2&
27
30 33 36 Leith ( crn)
FiR, 5. Comparison between the selectioncurvesof the diamond and squaremesh short codends.
227
SELECTIVITY EXPERIMENTS ON SOLE TABLE 2
Selection parameters (combined hauls)
Codend type
Number Numberof of hauls soles
Mesh size
SF SR X2 (cm)
L~o
(ram)
P(X 2) df Mean by.catch (kg per haul )
Codend Cover First experiment Short-diamond 15 Short-square 15 Second experiment Long--diamond 13 Long-square 13 Short--diamond 13
1306 1087
805 533
75.3 75.7
20.6 2.7 5.6 20.3 2.7 4.8
19.1 0.323 25.9 0.076
17 105 17 101
390 396 410
110 !!4 126
76.4 77.2 74.3
22.9 3.0 5.3 22.4 2.9 6.5 22.0 3.0 5.4
8.1 0.884 13.1 0.519 19.1 0.180
14 140 14 147 15 183
Lso, 50°/o retention length; SF, selection factor;,SR, selection range; df, degrees of freedom; ,~'= parameter for chi.square test. Number 60 50
3°t lO
0
--
15
--
J
20
25
t3 square mesh_ long codend
I
i
t
30
.
,
,
.
,
,
I
t
t
I
35 40 Lengfh ( cm )
dlamor~mesh. Ic~g codend
/- diamondmesh_ short codend
Fig. 6. Length distribution of the codend sole catches in the second experiment.
observed and expected retention points (5% level) (Table 2). The 50% retention lengths for the diamond and square mesh long codends, 22.9 cm and 22.4 cm respectively, were 2.3 cm and 2.1 cm larger than for the short codends experiment. The selection factor was 3.0 for the diamond mesh codend and 2.9 for the square mesh codend. Contrary to the short co(lend experiment, the square meshes showed the widest selection range: 6.5 cm against 5.3 cm for the diamond meshes. To find out whether the differences in selective properties between the short and long co(lends might be attributed to their difference in length, an additional number of hauls with the short diamond mesh
228
R. FONTEYNE AND R. M'RABET
% ~etained 100 r
~a)
~.~..~s..~-~
7ci
~°F ~o~ . . . . . . . . . . . . . . .
~°i
~ /
7 r
~0i-
/
~0~ 9
12
°/6 P~_tamed 100
15
18
/
I
I
i
21
2z,
27
(bl
.~
30
33 3'6 Length (cm)
S
~
~ ~
90
80
Xx ~
70
60
. . . . . . . . . .
50 z.O, 30 2O
I 1 9
% Retained 100 9O
12
15
18
21
2z,
21'
30 33 JO Len:~th. ~m !
(cl
00 70 60 50
--
4O 3O
,o y ' , 10 0
~
12
15
18
I
21
.
I
I
2Z,
2?
I
I
---J
30 33 3(. Length(tin)
FiB. 7. Selection curves for the second experiment. (a) Diamond mesh long codend; (b) square mesh long coclend; (c) diamond mesh short codend.
229
SELECTIVITY EXPERIMENTS ON SOLE % Retained 00
diamond mesh
90
80 7O 60
SO
/AI
4o
J',i
I! 9
12
15
18
21
2/,
27
30 33 36 Lengthtcm)
Fig. 8. Comparison between the selection curves of the diamond and square mesh long c~;lends.
codend was made. The selection curve is given in Fig. 7c. As can be seen in Table 2, the selection factor and the selection range of the short codend in this second expe~ment are 3.0 and 5.4 cm respectively. These parameters are almost identical to those for the long diamond mesh codend. The mean weight per haul of the non-commercial by-catches and debris is given in Table 2. These by-catches mainly consisted of starfish, but contained also crabs and sand. No clear differences could be observed between the diamond mesh and square mesh codends, but the by-catches were lower with both long codends than with the traditional short codend, as could be noticed in the second experiment. DISCUSSION
The mesh size of the cover may have been sufficiently near the mesh size of the test codends to enable some fish within the selection range of the test codends to escape from the cover. This may have reduced the expected 50% lengths and ;.1creased the selection ranges. To check this, expected retention points for the cover were calculated for each length class represented. To deduce the parameters of the equation of the (logistic) selection curve of the cover it was assumed that the selection factors of the test codends (2.7 for the first and 3.0 for the second experiment) do not change between 56 and 75 mm. The selection range was chosen as being 5 cm. The numbers of fish retained by the cover were corrected and these new numbers were used to recalculate the selection parameters of the test codends. The difference between the corrected and non-corrected 50% retention lengths was 0.2 cm in the first set of experiments and 0.4 cm in the second set, the corrected values being the larger ones. Only for the short diamond mesh codend in the first experiment and for the long square mesh codend, was the selection factor noticeably
230
R. FONTEYNE AND R. M'ilABET
affected. In both cases the increase wa.~ 0.1. The corrected selection ranges tend to be no more than 0.3 cm narrower than those obtained with the original data for the first experiment. In the second experiment the corrected selection ranges were 0.5 cm, 0.6 cm and 0.4 cm narrower for the long diamond mesh, the long square mesh and the short diamond mesh codends, respectively.
Mesh shape No important differences in the selective properties of the diamond mesh codends and the square mesh codends were discovered. This conclusion can be drawn from the calculated selection parameters (Table 2) and is also illustrated by the graphical comparison of the length distributions (Figs. 3 and 6 ) and the selection curves (Figs. 5 and 8 ). The difference between the selection factors of the diamond and the square mesh codends is negligible for both the short and long codends, but a difference of 0.2-0.3 was noted between the first and the second series of experiments. It is difficult to draw any conclusions from the differences in the selection range. A selection range between 5.3 cm and 5.6 cm was noted for the diamond mesh codends. The selection range for the square mesh codend was narrower (4.8 cm) in the case of the short codend, but wider (6.5 cm) in the case of the longer one. Soles do not seem to benefit from the fact that no stretching and closing of the meshes occurs in a square mesh codend and that all meshes remain equally open. It seems reasonable to assume that, owing to their flat body shape and their flexibility, soles can escape as easily, or even better, through stretched diamond meshes than through square meshes. Canadian researchers (Cooper and Hickey, 1989; Millar and Walsh, 1992; Walsh et al., 1992) found that diamond mesh codends are more effective in releasing small plaice than square mesh codends. As for sole, the escape of plaice is related to its width rather than to its girth which is the important parameter in the case of roundfish. The small differences in size selection between sole and plaice may be attributed to the more slender morphology of sole as compared to the more or less square shape of plaice. Owing to its flexibility, the escapement of sole will undoubtedly be less dependent on the codend mesh shape. Based on these results it can be suggested that the use of square mesh netting in flatfish trawls may thus reduce the roundfish by-catch while maintaining the catch of the target flatfish species at the same level. Robertson (1983) reported smaller quantities of debris and other by-catch when fishing with otter trawls with square mesh codends. This seems not to be the case in the beam trawl fishery. Neither the short nor the long codends showed marked differences in the weight of the by-catch between diamond and square meshes (Table 2).
SELECTIVITY EXPERIMENTS ON SOLE
231
Codend length The selection factors of the long codends in the second experiment were 0.3 (diamond meshes) and 0.2 (square meshes) larger than those obtained with the short codends in the first experiment. However, these differences did not show up in the control hauls carried out with the short diamond codend. Therefore the hypothesis of a codend length dependency of the selectivity in this fishery can be rejected. From the length distributions presented in Fig. 6 it appears that few small fish below the 50% retention length were caught. This will result in less accurate retention points for the lower length classes than for the larger ones. As weights are attributed to the samples, the position of the curve will be determined by the larger ones, i.e., by the upper length classes. This may have influenced the 50% retention length and could partly explain why larger 50% lengths, and hence larger selection factors, were obtained in the second experiment. In general, the by-catch volumes were smaller with the longer codends. This is reflected by the mean by-catch weights in the second series of experiments (Table 2 ).
Codend selectivity of small beam trawlers Previous selectivity experiments (De Clerck et al., 1981 ) on three Belgian beam trawlers of 285, 420 and 1320 hp, gave selection factors between 3.1 and 3.4. Dutch experiments (Van Beeket al., 1981, 1983) on beam trawlers of 1235, 1015 and 1310 hp also yielded selection factors ranging between 3.0 and 3.6. From published data on codend selectivity in beam trawling, covering a total of 676 hauls, Wileman (1988) calculated the average selection factor for sole as being 3.2. All the~e values are well above the selection factors found in this study. Bohl and Rauck (1982) also investigated the selection of sole by low powered (245 and 250 hp) beam trawlers in coastal waters. They too noted selection factors varying between 2.89 and 3.15, lower than the values usually obtained with the larger vessels fishing in more open seas. The German researchers suggested that the high by-catches, mainly consisting of epibenthos, in the coastal beam trawl fishery might be responsible for the poor selection, but no evidence for this could be given. From their experiments, De Clerck et al. ( 1981 ) could not show any connection between the nature of the bottom and the selection factor. Van Beek et al. (1983), on the other hand, found that an increase in the total amount ofcatch in the codend results in significantly lower selection factors and wider selection ranges. Burd and Vince (1979) also stated that the selectivity decreases with increasing catch volumes. In our experiments no such relationship between the selection factor and
232
R. F'ONTEYNEAND R. M ' R A B E T
the catch size could be found. On the contrary, the relatively low mean bycatches in the first experiment yielded lower selection factors (Table 2 ). The larger by-catches in the short codend control experiment apparently had no influence on the selective properties of the codend in the second series of experiments. A linear regression analysis between by-catch weight (60-150 kg) and selection factor (2.4-3.1) was made for individual hauls in the first experiment. Although there was some indication of a decrease in the selection factor with increasing quantities of by-catch, the regression was not significant, as shown by the analysis of variance. The catches in the second experiment were too low to allow a haul-by-haul analysis. Further research is required to examine whether the amount of by-catch can explain the difference in selective properties between the small coastal beamers and the larger beamers fishing in the open sea. These investigations should focus on the effects of both by-catch volume and composition on size selection. ACKNOWLEDGEMENTS
We are grateful to R. Moermans and two anonymous reviewers for constructive comments on an earlier draft. We thank R. Moermans and P. Demuynck for computer support in analysing the data and in drawing the selection curves. Thanks are also due to F. Daems, skipper of the 0.225 'Norman Kim', and his crew for their cooperation and assistance during the experimental trips, and to R. Focke and E. Buyvoets of the Fisheries Research Station for their assistance during field work.
REFERENCES Bohl, H. and Rauck. G., 1982. Selection of sole by beam trawls in the inshore waters of the German Bight in 1981 and 1982. Int. Coun. Explor. Sea, CM 1982/B:3, i8 pp. (mimeo). Burd, A.C. and Vince, M.R., 1979. Experiments with beam trawls. Int. Coun. Explor. Sea, CM 1979/B:9, 16 pp. (mimeo). Cooper, C. and Hickey, W., 1989. Selectivity experiments with square mesh codends of 130, 140 and 155 mm. In: S.G. Fox and J. Huntington (Editors), Proceedings of the World Symposium on Fishing Gear and Fishing Vessel Design. The Newfoundland and Labrador Institute of Fisheries and Marine Technology, St. John's, Newfoundland, Canada, g,p. 52-57. De Clerck, R., Vanden Broucke, G., Fonteyne, R. and Cloct, N., 198 I. Further results of selectivity experiments with beam trawls. Int. Coun. Explor. Sea, CM 1981/B: 19, 12 pp. (mimeo). Isaksen, B. and Valdemarsen, J.W., 1986. Selectivity experiments with square mesh codends in bottom trawl. Int. Coun. Explor. Sea, CM 1986/B:28, 18 pp. (mimeo). Millar, R.B. and Walsh, S.J., 1992. Analysis of trawl selectivity studies with an application to trouser trawls. Fish. Res., 13: 205-220. Pope, J.A., Margetts, A.M., Hamley, LM. and Akyz, E.K., 1975. Manual of methods for fish stock assessment. Part III. Selectivity of fishing gear. FAO Fish. Tech. Paper No. 41, Rev. I. Robertson, J.H.B., 1983. Square mesh cod-end selectivity experiments on whiting (Merlangius
SELECTIVITYEXPERIMENTSON SOLE
233
merlangus (L.) ) and haddock (Melanogrammus aeglefinus (L.) ). Int. Coun. Explor. Sea, CM 1983/B:25, 13 pp. (mimeo). Robertson, J.H.B., ! 986. The design and construction of square mesh codends. Scottish Fish. Inf. Pamph., No. 12, 10 pp. Robertson, J.H.B. and Stewart, P.A.M., 1988. A comparison of size selection of haddock and whiting by square and diamond mesh codends. J. Cons., 44: 148-161. Van Beek, F., Rijnsdorp, A.D. and Van Lceuwen, P.I., 198 !. Results of the mesh selection experiments on sole with commercial beam trawl vessels in North Sea and Irish Sea in 1979 and 1980. Int. Coun. Exp!or. Sea, CM 1981/B:31, 19 pp. (mimeo). Van Beek, E, Rijnsdorp, A.D. and Van Leeuwen, P.I., 1983. Results of the mesh selection experiments on sole and plaice with commercial bcamtrawl vessels in the North Sea in 1981. Int. Court. Explor. Sea, CM 1983/B: 16, 24 pp. (mimeo). Walsh, S.J., Millar, R.B., Cooper, C.G. and Hickey, W.M., 1992. Codend selection in American plaice: diamond versus square mesh. Fish. Res., 13: 235-254. Wileman, D., 1988. Codend Selectivity: A Review of Available Data. Appendix 1: Codend Selectivity Measurements for North Sea Species. Danish Fisheries Technology Institute, 1988.
221
Elsevier Science Publishers B.V., Amsterdam
Selectivity experiments on sole with diamond and square mesh codends in the Belgian coastal beam trawl fishery R. Fonteyne and R. M'Rabet Fisheries Research Station, Ankerstraat !, B-8400 Oostende. Belgium
ABSTRACT Fonteyne, R. and M'Rabet, R., 1992. Selectivity experiments on sole with diamond and square mesh codends in the Belgian coastal beam trawl fishery. Fish. Res.. 13:221-233. The selectivity of diamond +rod square mesh codends in the Belgian coastal beam trawl fishery for sole were compared. Two experiments, involving two different codend lengths were carried out. Contrary to the results obtained with roundfish, no influence of the mesh shape on the codend selectivity could be shown for the flatfish species investigated. It is s, t e~R,,qed that the application of square meshes may reduce the catch of small roundfish while maintaining the catch level of the target flatfish species. The length of the codend did not influence the selectivity for sole, bm a reduction in benthos by-catch was observed. The selectivity for sole in the coastal beam trawl fishery seems to be lower than the selectivity of the beam trawls used by larger vessels operating in the open sea. This is in agreement with the results of similar experintents, but there is no evidence that the selection factor depends on the amount of bycatch.
INTRODUCTION
Selectivity experiments with square mesh codends have shown that square meshes are more selective for most roundfish species than conventional diamond meshes (Robertson, 1983; Isaksen and Valdemarsen, 1986; Robertson and Stewart, 1988, Cooper and Hickey, 1989). This is due to the fact that square meshes remain open all along the codend, whereas diamond meshes tend to close when the codend fills up and the tension on the mesh bars increases. Apart from the results of Canadian experiments on plaice (Cooper and Hickey, 1989; Millar and Walsh, 1992; Walsh et al., 1992) no data have been published so far on the selective properties of square mesh codends for flatfish species. Correspondence to: R. Fonteyne, Fisheries Research Station, Ankerstraat 1, 15-8400 Oostende, Belgium.
© 1992 Elsevier Science Publishers B.V. All rights reserved 0165-7836/92/505.00
222
R. FONTEYNE AND R. M'RABET
In April 1987 and April 1988 two series of experiments were carried out by the Fisheries Research Station to investigate the influence of mesh shape diamond or square - on the codend selectivity for sole in the Belgian coastal beam trawl fishery. These experiments were part of a larger research program to evaluate the influence of a number of codend characteristics (mesh size, mesh shape, codend dimensions and netting material) on the selectivity in this fishery. This paper reports on the effects of codend mesh shape on the selection of sole for two codend lengths. Th~ codend selectivity for sole of small beam trawlers is discussed in relation to the selectivity oflarger beamers operating in more open seas.
MATERIAL AND METHODS
Vessel and gears The Belgian coastal beam trawl fishery for sole is practised in spring and summer by small beam trawlers with an engine power of up to 300 hp. Beam trawlers tow two nets from either side of the ship by means of two derrick booms as shown in Fig. 1. The nets are rigged to beams 4-7 m long. TicHer chains are used to drive the fishes from the sea bed. The number of tickler chains used in the coastal beam trawl fishery varies between two and four according to the beam length and the nature of the sea bed. The present experiments took place on board a chartered beam trawler with an engine power of 250 hp and a length overall of 20.60 m. The nets for the 7-m beams were equipped with two tickler chains only. The characteristics of the nets are given in Fig. 2.
CrOS~ 5~C t I ~
~azy d~cky t l c k l e ~ e ~
;?--//- ....
Fig. !. General arrangement of double rig beam trawlin&
223
SELECTIVITY EXPERIMENTS ON SOLE
L
°
"r,
1
5
b
,.1
iiiii d
:
4.%0m
NIX l(,mrr
Fig. 2. Net plan.
Codends and covers Two pairs of double-braided polyethylene codends with a nominal mesh opeping of 75 mm were studied. The first pair of codends was based on the traditional diamond mesh codend with a circumference •f 100 meshes and a length of 35 meshes. The corresponding square mesh codend, made of the same material mounted with one AB direction parallel to the long axis of the codend and the other AB direction at right angles to it, was comprised of 50 meshes around and 70 meshes in length. The codends in the second pair were twice as long as those in the first pair, i.e., 70 meshes for the diamond mesh codend and 140 meshes for the square mesh codend. Apart from their length the two pairs of codends had the same characteristics. The square mesh codends were constructed as described by Robertson (1986). The selection parameters were obtained by the covered codend (full cover) method. The covers (diamond mesh) were 1.6 times as long and wide as the codends and were also made of polyethylene. This material was chosen because it is buoyant, so that obstruction of the codend meshes by the cover will be minimized. The nominal mesh opening of the cover was 55 mm. Details on the codends and covers used are given in Table 1.
Experimental procedure A first series of experiments, 15 hauls in two trips, with the short codends, was carried out in April 1987. In April 1988 a second series of experiments, 13 hauls in two trips, with the two long codends took place. As a reference, the same number of hauls was made with a short diamond mesh codend. This codend had the same characteristics as the short diamond mesh codend in the first series of experiments (Table I ).
R. FONTEYNE AND R. M'RABIE'r
224
TABLE I
Codend construction Codend type
First experiment Short-diamond
Dimensions (meshes)
35 iongx 1O0 round
Short-square Second experiment Long-diamond Long-square Short-diamond Covers Cover (short) Cover (long)
Material
PE, twisted
Mesh opening ( mm ) n
Range
Mean
200
67-83
75.3
200
68-83
75.7
2 X R4130rex
70 long)<
PE, twisted
50 round
2)< R4 ! 30rex
70 long x I00 round 140 long)<
PE, twisted 2 x R4130rex
200
70-86
76.4
PE, twisted
200
73-82
77.2
50 round
2 X R4130lex 150
65-80
74.3
125
51-63
56.2
25
51-59
55.8
35 IongX
PE, twisted
100 round
2 x R4130rex
78 long x 224 round 156 IongX 224 round
PE, twisted R 1560rex PE, twisted R ! 560rex
Use was made of the twin beam trawl principle, which uses the diamond mesh and the square mesh codends under the same conditions. One type of codend was rigged to the starboard net, and the other to the port side net. To overcome small differences that might occur between the catching efficiency of the two gears, two trips were carried out on successive nights in each experiment. After each trip the codends were switched. Apart from the covers, the experimental hauls were performed under exactly the same conditions as in commercial fishing. Fishing took place at night only. The fishing area was located at a distance of 6-9 miles from the coast, between Oostende and Wenduine. The hauls had a mean duration of I h. After each haul all soles were measured to the nearest cm. The commercial catches were weighed. To estimate their weight, the non-commercial bycatches were boxed, the average weight of each box being 30 kg. The mesh sizes of the codends were measured with an ICES mesh gauge operated at 4-kg pretension. Both the diamond and the square meshes were measured in the F!-direction of the netting. The mean mesh sizes were calculated from the measurement of 25 top panel meshes every other haul. Data analysis The combined catches of all hauls in each experiment were used to calculate the length selection curve. The calculations were based on the assumption
225
SELECTIVITY EXPERIMENTSON SOLE
that the selection curve may be represented by the logistic curve (Pope et al., 1975 ). The parameters of the logistic function were estimated by the method of maximum likelihood. The fitting of the logistic curve to the observed retention percentages was tested by the chi-square test, also dc=cribed by Pope et al. (1975). The level of significance was set at 0.05. If the expected number of fish was less than five in any class, neighbouring classes were pooled and the number of degrees of freedom for the chi-square test was decreased accordingly. RESULTS
The length distributions of the codend sole catches in the first experiment are given in Fig. 3. The selection curves of both the diamond and the square mesh codend are shown in Fig. 4 and compared in Fig. 5. The chi-square values in Table 2 indicate that there were no significant differences between observed and expected retention percentages, probabilities being more than 0.05. The 50% retention lengths were 20.6 cm and 20.3 cm for the diamond and the square mesh codend respectively. The selection factor for both codends was 2.7. From these data it appears that the square mesh codend had a somewhat narrower selection range. The length distributions of tLe sole catches in the second experiment are given in Fig. 6, and the selection curves are shown in Fig. 7. The selection curves of the two long co
A
15
20 Q square meshes
25
i
J
~4--.~-
L
30
~
:
L
i
,
t
t
,
35 ~0 Ler,gth (cm)
O diamond meshes
Fig. 3. Length distributions o f the codend sole catches in the first experiment.
226
R. FONTEYNE AND R. M'ItABET
% Pe%i~ed +00 90
OO 70 60
IL
SO ~0 ]0 20 10 0 9
Length (~.m)
%
Refa~ned lOOF
(b)
//"
90F
8oF
::i
/
,oF 3o~-
./: /I
~°l
/
:
17 io 3'3 3~
Length(era) Fig. 4. Selection curves for the first experiment. (a) Diamond mesh short codend; (b) square mesh short codend. % Retained 100F
square mesh
'°80 f 50
//~/
diamond mesh
// // --
1,0 30
~t
20
iI
10
IT
0 ~
9
12
15
18
21
.........................
2&
27
30 33 36 Leith ( crn)
FiR, 5. Comparison between the selectioncurvesof the diamond and squaremesh short codends.
227
SELECTIVITY EXPERIMENTS ON SOLE TABLE 2
Selection parameters (combined hauls)
Codend type
Number Numberof of hauls soles
Mesh size
SF SR X2 (cm)
L~o
(ram)
P(X 2) df Mean by.catch (kg per haul )
Codend Cover First experiment Short-diamond 15 Short-square 15 Second experiment Long--diamond 13 Long-square 13 Short--diamond 13
1306 1087
805 533
75.3 75.7
20.6 2.7 5.6 20.3 2.7 4.8
19.1 0.323 25.9 0.076
17 105 17 101
390 396 410
110 !!4 126
76.4 77.2 74.3
22.9 3.0 5.3 22.4 2.9 6.5 22.0 3.0 5.4
8.1 0.884 13.1 0.519 19.1 0.180
14 140 14 147 15 183
Lso, 50°/o retention length; SF, selection factor;,SR, selection range; df, degrees of freedom; ,~'= parameter for chi.square test. Number 60 50
3°t lO
0
--
15
--
J
20
25
t3 square mesh_ long codend
I
i
t
30
.
,
,
.
,
,
I
t
t
I
35 40 Lengfh ( cm )
dlamor~mesh. Ic~g codend
/- diamondmesh_ short codend
Fig. 6. Length distribution of the codend sole catches in the second experiment.
observed and expected retention points (5% level) (Table 2). The 50% retention lengths for the diamond and square mesh long codends, 22.9 cm and 22.4 cm respectively, were 2.3 cm and 2.1 cm larger than for the short codends experiment. The selection factor was 3.0 for the diamond mesh codend and 2.9 for the square mesh codend. Contrary to the short co(lend experiment, the square meshes showed the widest selection range: 6.5 cm against 5.3 cm for the diamond meshes. To find out whether the differences in selective properties between the short and long co(lends might be attributed to their difference in length, an additional number of hauls with the short diamond mesh
228
R. FONTEYNE AND R. M'RABET
% ~etained 100 r
~a)
~.~..~s..~-~
7ci
~°F ~o~ . . . . . . . . . . . . . . .
~°i
~ /
7 r
~0i-
/
~0~ 9
12
°/6 P~_tamed 100
15
18
/
I
I
i
21
2z,
27
(bl
.~
30
33 3'6 Length (cm)
S
~
~ ~
90
80
Xx ~
70
60
. . . . . . . . . .
50 z.O, 30 2O
I 1 9
% Retained 100 9O
12
15
18
21
2z,
21'
30 33 JO Len:~th. ~m !
(cl
00 70 60 50
--
4O 3O
,o y ' , 10 0
~
12
15
18
I
21
.
I
I
2Z,
2?
I
I
---J
30 33 3(. Length(tin)
FiB. 7. Selection curves for the second experiment. (a) Diamond mesh long codend; (b) square mesh long coclend; (c) diamond mesh short codend.
229
SELECTIVITY EXPERIMENTS ON SOLE % Retained 00
diamond mesh
90
80 7O 60
SO
/AI
4o
J',i
I! 9
12
15
18
21
2/,
27
30 33 36 Lengthtcm)
Fig. 8. Comparison between the selection curves of the diamond and square mesh long c~;lends.
codend was made. The selection curve is given in Fig. 7c. As can be seen in Table 2, the selection factor and the selection range of the short codend in this second expe~ment are 3.0 and 5.4 cm respectively. These parameters are almost identical to those for the long diamond mesh codend. The mean weight per haul of the non-commercial by-catches and debris is given in Table 2. These by-catches mainly consisted of starfish, but contained also crabs and sand. No clear differences could be observed between the diamond mesh and square mesh codends, but the by-catches were lower with both long codends than with the traditional short codend, as could be noticed in the second experiment. DISCUSSION
The mesh size of the cover may have been sufficiently near the mesh size of the test codends to enable some fish within the selection range of the test codends to escape from the cover. This may have reduced the expected 50% lengths and ;.1creased the selection ranges. To check this, expected retention points for the cover were calculated for each length class represented. To deduce the parameters of the equation of the (logistic) selection curve of the cover it was assumed that the selection factors of the test codends (2.7 for the first and 3.0 for the second experiment) do not change between 56 and 75 mm. The selection range was chosen as being 5 cm. The numbers of fish retained by the cover were corrected and these new numbers were used to recalculate the selection parameters of the test codends. The difference between the corrected and non-corrected 50% retention lengths was 0.2 cm in the first set of experiments and 0.4 cm in the second set, the corrected values being the larger ones. Only for the short diamond mesh codend in the first experiment and for the long square mesh codend, was the selection factor noticeably
230
R. FONTEYNE AND R. M'ilABET
affected. In both cases the increase wa.~ 0.1. The corrected selection ranges tend to be no more than 0.3 cm narrower than those obtained with the original data for the first experiment. In the second experiment the corrected selection ranges were 0.5 cm, 0.6 cm and 0.4 cm narrower for the long diamond mesh, the long square mesh and the short diamond mesh codends, respectively.
Mesh shape No important differences in the selective properties of the diamond mesh codends and the square mesh codends were discovered. This conclusion can be drawn from the calculated selection parameters (Table 2) and is also illustrated by the graphical comparison of the length distributions (Figs. 3 and 6 ) and the selection curves (Figs. 5 and 8 ). The difference between the selection factors of the diamond and the square mesh codends is negligible for both the short and long codends, but a difference of 0.2-0.3 was noted between the first and the second series of experiments. It is difficult to draw any conclusions from the differences in the selection range. A selection range between 5.3 cm and 5.6 cm was noted for the diamond mesh codends. The selection range for the square mesh codend was narrower (4.8 cm) in the case of the short codend, but wider (6.5 cm) in the case of the longer one. Soles do not seem to benefit from the fact that no stretching and closing of the meshes occurs in a square mesh codend and that all meshes remain equally open. It seems reasonable to assume that, owing to their flat body shape and their flexibility, soles can escape as easily, or even better, through stretched diamond meshes than through square meshes. Canadian researchers (Cooper and Hickey, 1989; Millar and Walsh, 1992; Walsh et al., 1992) found that diamond mesh codends are more effective in releasing small plaice than square mesh codends. As for sole, the escape of plaice is related to its width rather than to its girth which is the important parameter in the case of roundfish. The small differences in size selection between sole and plaice may be attributed to the more slender morphology of sole as compared to the more or less square shape of plaice. Owing to its flexibility, the escapement of sole will undoubtedly be less dependent on the codend mesh shape. Based on these results it can be suggested that the use of square mesh netting in flatfish trawls may thus reduce the roundfish by-catch while maintaining the catch of the target flatfish species at the same level. Robertson (1983) reported smaller quantities of debris and other by-catch when fishing with otter trawls with square mesh codends. This seems not to be the case in the beam trawl fishery. Neither the short nor the long codends showed marked differences in the weight of the by-catch between diamond and square meshes (Table 2).
SELECTIVITY EXPERIMENTS ON SOLE
231
Codend length The selection factors of the long codends in the second experiment were 0.3 (diamond meshes) and 0.2 (square meshes) larger than those obtained with the short codends in the first experiment. However, these differences did not show up in the control hauls carried out with the short diamond codend. Therefore the hypothesis of a codend length dependency of the selectivity in this fishery can be rejected. From the length distributions presented in Fig. 6 it appears that few small fish below the 50% retention length were caught. This will result in less accurate retention points for the lower length classes than for the larger ones. As weights are attributed to the samples, the position of the curve will be determined by the larger ones, i.e., by the upper length classes. This may have influenced the 50% retention length and could partly explain why larger 50% lengths, and hence larger selection factors, were obtained in the second experiment. In general, the by-catch volumes were smaller with the longer codends. This is reflected by the mean by-catch weights in the second series of experiments (Table 2 ).
Codend selectivity of small beam trawlers Previous selectivity experiments (De Clerck et al., 1981 ) on three Belgian beam trawlers of 285, 420 and 1320 hp, gave selection factors between 3.1 and 3.4. Dutch experiments (Van Beeket al., 1981, 1983) on beam trawlers of 1235, 1015 and 1310 hp also yielded selection factors ranging between 3.0 and 3.6. From published data on codend selectivity in beam trawling, covering a total of 676 hauls, Wileman (1988) calculated the average selection factor for sole as being 3.2. All the~e values are well above the selection factors found in this study. Bohl and Rauck (1982) also investigated the selection of sole by low powered (245 and 250 hp) beam trawlers in coastal waters. They too noted selection factors varying between 2.89 and 3.15, lower than the values usually obtained with the larger vessels fishing in more open seas. The German researchers suggested that the high by-catches, mainly consisting of epibenthos, in the coastal beam trawl fishery might be responsible for the poor selection, but no evidence for this could be given. From their experiments, De Clerck et al. ( 1981 ) could not show any connection between the nature of the bottom and the selection factor. Van Beek et al. (1983), on the other hand, found that an increase in the total amount ofcatch in the codend results in significantly lower selection factors and wider selection ranges. Burd and Vince (1979) also stated that the selectivity decreases with increasing catch volumes. In our experiments no such relationship between the selection factor and
232
R. F'ONTEYNEAND R. M ' R A B E T
the catch size could be found. On the contrary, the relatively low mean bycatches in the first experiment yielded lower selection factors (Table 2 ). The larger by-catches in the short codend control experiment apparently had no influence on the selective properties of the codend in the second series of experiments. A linear regression analysis between by-catch weight (60-150 kg) and selection factor (2.4-3.1) was made for individual hauls in the first experiment. Although there was some indication of a decrease in the selection factor with increasing quantities of by-catch, the regression was not significant, as shown by the analysis of variance. The catches in the second experiment were too low to allow a haul-by-haul analysis. Further research is required to examine whether the amount of by-catch can explain the difference in selective properties between the small coastal beamers and the larger beamers fishing in the open sea. These investigations should focus on the effects of both by-catch volume and composition on size selection. ACKNOWLEDGEMENTS
We are grateful to R. Moermans and two anonymous reviewers for constructive comments on an earlier draft. We thank R. Moermans and P. Demuynck for computer support in analysing the data and in drawing the selection curves. Thanks are also due to F. Daems, skipper of the 0.225 'Norman Kim', and his crew for their cooperation and assistance during the experimental trips, and to R. Focke and E. Buyvoets of the Fisheries Research Station for their assistance during field work.
REFERENCES Bohl, H. and Rauck. G., 1982. Selection of sole by beam trawls in the inshore waters of the German Bight in 1981 and 1982. Int. Coun. Explor. Sea, CM 1982/B:3, i8 pp. (mimeo). Burd, A.C. and Vince, M.R., 1979. Experiments with beam trawls. Int. Coun. Explor. Sea, CM 1979/B:9, 16 pp. (mimeo). Cooper, C. and Hickey, W., 1989. Selectivity experiments with square mesh codends of 130, 140 and 155 mm. In: S.G. Fox and J. Huntington (Editors), Proceedings of the World Symposium on Fishing Gear and Fishing Vessel Design. The Newfoundland and Labrador Institute of Fisheries and Marine Technology, St. John's, Newfoundland, Canada, g,p. 52-57. De Clerck, R., Vanden Broucke, G., Fonteyne, R. and Cloct, N., 198 I. Further results of selectivity experiments with beam trawls. Int. Coun. Explor. Sea, CM 1981/B: 19, 12 pp. (mimeo). Isaksen, B. and Valdemarsen, J.W., 1986. Selectivity experiments with square mesh codends in bottom trawl. Int. Coun. Explor. Sea, CM 1986/B:28, 18 pp. (mimeo). Millar, R.B. and Walsh, S.J., 1992. Analysis of trawl selectivity studies with an application to trouser trawls. Fish. Res., 13: 205-220. Pope, J.A., Margetts, A.M., Hamley, LM. and Akyz, E.K., 1975. Manual of methods for fish stock assessment. Part III. Selectivity of fishing gear. FAO Fish. Tech. Paper No. 41, Rev. I. Robertson, J.H.B., 1983. Square mesh cod-end selectivity experiments on whiting (Merlangius
SELECTIVITYEXPERIMENTSON SOLE
233
merlangus (L.) ) and haddock (Melanogrammus aeglefinus (L.) ). Int. Coun. Explor. Sea, CM 1983/B:25, 13 pp. (mimeo). Robertson, J.H.B., ! 986. The design and construction of square mesh codends. Scottish Fish. Inf. Pamph., No. 12, 10 pp. Robertson, J.H.B. and Stewart, P.A.M., 1988. A comparison of size selection of haddock and whiting by square and diamond mesh codends. J. Cons., 44: 148-161. Van Beek, F., Rijnsdorp, A.D. and Van Lceuwen, P.I., 198 !. Results of the mesh selection experiments on sole with commercial beam trawl vessels in North Sea and Irish Sea in 1979 and 1980. Int. Coun. Exp!or. Sea, CM 1981/B:31, 19 pp. (mimeo). Van Beek, E, Rijnsdorp, A.D. and Van Leeuwen, P.I., 1983. Results of the mesh selection experiments on sole and plaice with commercial bcamtrawl vessels in the North Sea in 1981. Int. Court. Explor. Sea, CM 1983/B: 16, 24 pp. (mimeo). Walsh, S.J., Millar, R.B., Cooper, C.G. and Hickey, W.M., 1992. Codend selection in American plaice: diamond versus square mesh. Fish. Res., 13: 235-254. Wileman, D., 1988. Codend Selectivity: A Review of Available Data. Appendix 1: Codend Selectivity Measurements for North Sea Species. Danish Fisheries Technology Institute, 1988.