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The effect of cover mesh size and cod-end catch size on cod-end selectivity
ELSEVIER
Fisheries Research 28 (1996) 291-303
The effect of cover mesh size and cod-end catch size on cod-end selectivity F.G. O’Neill
*,
R.J. Kynoch
SOAEFD Marine Laboratory, P.O. Box 101, Victoria Road, Aberdeen, AB9 8DB, UK
Accepted 5 March 1996
Abstract Selectivity trials were carried out to examine the effect of catch size on selectivity and to test whether an increase of the cover mesh size would have any impact on the selection parameters of the cod-end fished. There was a significant increase of the 50% retention length for both haddock and whiting over the range of catch weights considered which was from 113407 kg. It is suggested that this tendency will not continue and that with increasing catch size the I,, will begin to level out or decrease. This explanation is consistent with the work of other authors, where catches are generally much larger. The increase of cover mesh size from 40 mm to 60 mm had no effect on the selection parameters of the 100 mm cod-end tested. Keywords:
Selectivity;
Catch size; Cover mesh size
1. Introduction In recent years the design of the small mesh cover used in covered cod-end selectivity trials has come under renewed investigation (Stewart and Robertson, 1985, Main et al., 1992, Robertson et al., 1995). Concern has been expressed as to the effect of the cover on the selectivity of the cod-end. One consequence has been the introduction of much bigger covers held open by large hoops which has helped prevent small mesh masking
* Corresponding
author.
0165-7836/96/$15.00 Crown copyright PII SO165-7836(96)00501-2
0 19% Published
by Elsevier Science B.V.
F.G. O’Neill, RJ. Kynoch/Fisheries
292
Research 28 (1996) 291-303
of the large mesh cod-end (Main et al., 1992, Robertson et al., 1995). An aspect of cover design that has not been considered, however, is its mesh size. An increase in the mesh size of the small mesh cover may permit the escape of very small fish which, if on the fishing ground, are at present retained in the cover. The consequent reduction in overall bulk may lead to better sampling of the larger fish from the cover and to more accurate selectivity data in the region of the selection range. There may be a reduction of the drag of the cover and a corresponding reduction of the strain on the hoops and their likelihood of collapsing. It is also possible that the flow through the cover will be enhanced and more like that of an uncovered cod-end. The minimum mesh size of the standard whitefish demersal trawl cod-end in use in Scottish waters has risen from 70 mm to 100 mm. However, the mesh size of the cod-end covers used by the Marine Laboratory, Aberdeen, in commercial selectivity trials has remained in the range 30-40 mm. There has been a reluctance to increase the cover mesh size as it is thought that it may result in an overestimate of the proportion of small fish retained in the cod-end by allowing the escape of large numbers of these fish from the cover. A separate issue is the effect of cod-end catch size on the selectivity parameters. Pope et al. (1975) identified the need for further study of this parameter in view of the application of the results of selectivity experiments to commercial fisheries where the
T
j+--
176-15 50mm x 2mm o IPA 46 rows
7l-
+-176 --fit 5Omm x 2mm B tPA 46 10~s
1.ir
II-146-11 _____________ T :
221 --+I7
I____________
T !
10
10
i k_-____
______ 3
x / II
16.1
0 _____________ 71+
221 -+i
6Omm 536 r0w.s 1.6mm B tPE
50mm 644 rows 1.6mm B tPE
40mm 805 rows 1.6mm B tPE
@ __________ I _______ 0
+--
1
175--+16
I+---
- ____.
146 -II
NOTES @
FORWARD HOOP POSITION FROM SKIRT/COVER JOIN 40mm = 32 rows / 50mm = 26 row / 60mm = 21 rows
AFT HOOP POSITION FROM SKIRT/COVER JOIN 226 rows / 5Omm = 182 rows / 60mm = 152 rows
03
0
ZIP IS POSITIONED 0.66M DOWN FROM SKIRT/COVER JOIN IN CENTRE OF PANELAND RUNS FOR 2 x 3.05m. THE SAME IN ALL CASES LIFTING BAG JOIN - 120mm x 5mm 0 BRAIDED NYLON (69 meshes round x 17 rows)
Fig. 1. Cod-end cover specifications.
F.G. O’Neill. R.J. Kynoch/Fisheries
Research 28 (1996) 291-303
293
catches are generally higher. Hodder and May (1964) found a negative correlation between selection factor and size of catch for haddock and cod using a diamond mesh cod-end. Madsen and Moth-Poulsen (1994) demonstrated a similar dependency for whiting in square mesh panel cod-ends. Dahm ( 199 1) and Suuronen et al. ( 199 11, on the other hand, were unable to establish a significant relationship between catch size and the 50% retention length for pelagic herring trawls. The range of catch sizes considered by each of the above authors is wide and may lead to some localised trends being overlooked. Furthermore, as pointed out by Pope et al. (1975), the reduction of the 50% retention length with catch size, detected in covered cod-end hauls, may be due to the presence of the cover masking the cod-end when the hauls are large. This paper aims to test whether an increase of the cover mesh size would have any impact on the selection parameters of the cod-end fished and to examine the effect of catch size on the selection parameters. 2. Materials and methods The commercial fishing vessel “Solstice” (BF56) was chartered for a 10 day period in July 1994 to carry out the selectivity trials. Three covers of 40, 50 and 60 mm
Test cod-end length 6 m _______
-I
I
I
1st hoop
2nd hoop =75m
Total cover length 2
Fig. 2. Rigging of cod-end covers and hoops.
F.G. O’Neill, RJ. Kymch/Fisheries
294
Research 28 (1996) 291-303
Table 1 General haul summary Haul No.
Cover size
Warpaft (m)
Water depth
Start time
Haul duration
Distance covered
Mean
Headline height (m)
Wingend spread
(h)
(m)
speed (kts)
3 4 6 7 8 9
50 50 60 60 60 60
320 320 320 320 320 320
110 110 110 110 110 110
0730 1050 0715 1050 1220 1450
1.75 2.00 1.50 0.50 1.33 1.50
9668 11885 9888 2973 6892 8205
2.78 2.95 3.23 3.01 2.65 2.75
4.7 4.6 3.9 4.2 4.3 4.5
14.0 13.5 13.9 13.2 14.2 14.3
10 11 12 13 15 16 18 19 20 21 22 23 24 25
40 40 40 40 50 50 60 60 60 60 40 40 40 40
320 366 320 320 320 320 320 274 274 320 320 274 274 274
110 128 110 110 110 110 110 91 91 110 110 91 91 82
0720 0950 1240 1455 1015 1250 0720 0950 1220 1430 0725 1025 1350 1545
1.50 1.00 0.75 1.00 1.50 1.17 1.50 1.50 1.00 2.00 2.00 2.00 1.08 1.50
N/R N/R N/R N/R 8736 6026 947 1 7716 5363 12490 12477 10103 5733 9211
N/R N/R N/R N/R 2.95 2.64 3.28 2.65 2.89 3.19 3.19 2.49 2.78 3.13
N/R N/R N/R N/R 4.2 4.4 4.3 4.3 4.4 4.4 4.7 4.6 4.4 N/R
N/R N/R N/R N/R 13.9 14.1 13.9 13.5 14.5 13.7 13.4 13.5 14.2 13.5
26 27 28
40 60 60
274 274 274
91 82 91
0735 1012 1206
1.00 1.00 2.00
N/R 5907 10279
N/R 3.11 2.63
N/R 4.5 4.5
N/R 14.3 13.3
(mm)
(m)
Cm)
(nominal) netting made from 1.6 mm single twisted polyethylene (PE) twine were used, the large scale dimensions of each being the same (17.2 m stretched length and 17.6 m stretched circumference). A 100 mm (nominal) mesh cod-end of 3.5 mm double braided PE, 100 open meshes around and 6 m stretched length was used throughout the trials. Two hoops of 2.1 and 2.5 m diameter, made from 50 mm diameter alkathene piping, were employed to prevent the small mesh covers masking the cod-end. The trawl used was a Sinclair white fish rockhopper trawl with a 78.1 m fishing circle and was towed using 54.9 m single sweeps, 36.6 m double bridles and Perfect K200 spherical doors. Full details of the cover designs can be found in Figs. 1 and 2. The fishing grounds were in an area approximately 10 nautical miles north east of Fraserburgh. Scanmar acoustic instrumentation measured net geometry and a Garmin 75 GPS unit recorded navigational parameters (Table 1). Twenty-nine hauls were carried out, all during daylight hours, of which 23 were considered valid. Haddock were found in sufficient quantities in all hauls and whiting in most. After each haul the cod-end catch was sorted by species (haddock, whiting and the rest) into baskets and weighed. Subsequently, the lengths of a sample of haddock and whiting were measured. Owing to their much greater size the catches in the cover were not initially sorted but put into baskets and weighed. Baskets were then sampled and the fish lengths measured. The catch weights and sampling fractions are given in Table 2.
295
F.G. O’Neill, R.J. Kymch / Fisheries Research 28 (1996) 291-303 Table 2 Catch summary Haul
Cod-end catch weight (kg)
No.
3 4 6 7 8 9 IO 11 12 13 15 16 18 19 20 21 22 23 24 25 26 21 28
Cod-end sampling fractions
Cover catch weight (kg)
Cover sampling fractions
Haddock
Whiting
Mixed
Total
Haddock
Whiting
Total
Haddock
Whiting
238
15 24 19 27 14 25 17 96 10 15 13 10 25 50 23 27 23 22 9 16 14 27 8
106 56 21 51 117 79 68 71 34 43 95 73 100 97 73 85 75 38 32 40 57 59 81
3.59 342 191 270 245 294 275 396 377 254 171 137 217 205 113 221 280 219 122 237 407 247 302
0.42 0.36 0.56 0.52 1.00 0.49 0.48 0.41 0.28 0.53 1.00 1.00 1.00 I .OO 1.00 1.oO 0.49 0.46 I.00 0.50 0.38 0.63 0.43
1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.OO 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00
707 1863 464 1086 962 614 601 2259 1136 1146 451 504 359 733 319 629 1570 1988 758 1322 1166 532 951
0.19 0.14 0.29 0.16 0.27 0.24 0.26 0.06 0.13 0.18 0.5 1 0.68 0.37 0.53 1.oo 0.22 0.23 0.09 0.32 0.23 0.15 0.34 0.28
0.30 0.08 0.41 0.16 0.2 1 0.24 0.88 0.05
262 151 192 114 190 190 229 333 196 63 54 92 58 17 109 182 219 81 181 336 161 213
N/R 0.11 1.oo 1.oo 0.37 0.08 0.27 0.22 0.04 0.09 N/R 0.08 N/R 0.97 N/R
The weather conditions were consistent throughout the period of the trials and the wind, though variable in direction, was always in the range 2-4 on the Beaufort scale.
3. Statistical
analysis
and results
For each haul i the logistic curve P
( 1
log -
=vi,
+v;,1
1-p
was fitted to the data, where p is the proportion of fish at length I retained in the cod-end and ui, and ui2 are the parameters to be estimated. Having defined the following quantities, y, and yz, the number of fish of length 1 measured in the cod-end and cover respectively, y, = y, + y2, the total number of fish of length I measured, r, the probcfish of length 1 sampled in cod-end I fish of length I sampled), q, and q2, the proportions of fish of length I measured in the cod-end and cover respectively and Cc,= log(q,/q,), it can be shown (Milk, 1994) that
log($-)=lo@--) +IcI
F.G. O’Neill, RJ. Kyrwch/
296 Table 3 Estimation Haul
Fisheries Research
28 (1996) 291-303
of vi and R, for each haul “il
"i2
3 4 6 7 8 9 10 11 12 13 15 16 18 19 20 21 22 23 24 25 26 27 28
- 11.28 - 13.33 - 14.22 - 15.67 - 15.17 - 14.10 - 12.44 - 12.44 - 14.05 - 15.59 - 12.87 - 11.83 - 12.28 - 9.80 - 7.92 - 12.38 - 10.38 - 14.90 - 8.78 -11.90 - 11.85 - 10.17 - 10.71
0.372 0.455 0.505 0.533 0.537 0.513 0.461 0.430 0.465 0.526 0.463 0.441 0.45 1 0.340 0.28 1 0.434 0.365 0.508 0.298 0.416 0.395 0.349 0.372
3 4 6 7 8 9 10 11 13 15 16 18 19 20 21 22 23 25 27
- 18.26 - 27.59 - 11.97 - 13.59 - 18.15 - 14.14 - 9.66 - 12.72 - 17.85 - 15.87 - 18.47 - 20.07 - 16.73 - 14.19 - 11.93 -22.16 - 12.54 - 14.18 - 17.23
0.503 0.838 0.341 0.370 0.523 0.420 0.286 0.358 0.522 0.495 0.603 0.607 0.514 0.452 0.342 0.753 0.33 1 0.43 1 0.520
Ri,,
Haddock 0.906 0.858 0.832 1.273 0.929 0.820 0.780 2.961 1.100 1.353 0.756 0.727 0.78 1 0.863 2.436 0.778 0.673 1.017 0.473 0.632 0.818 0.711 0.986 Whiting 5.005 6.285 1.61 1.416 3.284 1.543 2.392 0.590 2.314 5.537 8.379 4.887 1.564 1.832 1.027 3.536 0.633 1.435 5.120
Ril2
Ri22
- 0.0324 -0.0318 -0.0315 - 0.0464 - 0.035 1 -0.0311 - 0.0303 -0.1068 - 0.0391 - 0.0487 - 0.0289 - 0.0290 - 0.0307 - 0.0322 - 0.0888 - 0.0299 - 0.025 1 - 0.038 1 -0.0184 - 0.0239 - 0.0293 - 0.0258 - 0.0355
0.00116 0.00119 0.00120 0.00170 0.00134 0.00119 0.00119 0.00388 0.00140 0.00176 O.cOlll 0.00117 0.00122 0.00121 0.00327 0.00116 0.00095 0.00144 0.00073 0.00092 0.00105 0.00095 0.00129
-0.1548 -0.2162 - 0.0567 - 0.0480 -0.1119 - 0.0549 - 0.0773 -0.0218 - 0.0832 -0.1831 - 0.2840 -0.1587 - 0.0574 - 0.0680 - 0.0367 -0.1452 - 0.0239 -0.0581 -0.1595
0.00483 0.00749 0.00200 0.00165 0.00385 0.00198 0.00253 0.00081 0.00303 0.00611 0.00969 0.00519 0.00213 0.00255 0.00134 0.00602 0.00093 0.00238 0.00500
It is then possible to estimate uil and ui2 by fitting the general linearised model y, - Binomial( y, ,I)
291
F.G. O’Neill, R.J. Kynoch/ Fisheries Research 28 (1996) 291-303
where r
( 1
log l-r
= Vi, + ui*l + 4
Table 3 contains, for each haul, the estimates of the parameters ui = (ui,, ui2) and the elements of their covariance matrix, R,, for haddock and whiting. The Ri values and an inspection of the residual plots indicate that in each case the model adequately describes the data (the R,,l and the Riz2 elements are respectively the variance estimates of ui,, and ui2). It is shown in Table 2 that the cod-end catches of whiting were generally very small, a fact that is reflected in the larger values of the covariance matrix elements in comparison to those for haddock. The model of Fryer (1991), used subsequently by Reeves et al. (1992) and Galbraith et al. (19941, was employed to investigate the between-haul variation of the selectivity parameters, ui, and ui2. and their dependence on the explanatory variables of cover mesh size, m,, weight of catch in the cod-end, ci and tow duration, di. All possible linear and a number of non-linear expressions of the selectivity parameters as functions of these three variables were tested of which only two were found to describe the data at the 95% level (i.e. each of the terms of the design matrix were significant at the 95% level, see Fryer, 1991, Reeves et al., 1992). Neither demonstrated a dependency on either the cover mesh size l?zi or the tow duration d,. Employing the formulation of Fryer (1991) these two models can be expressed as follows: ModelA: lZ(~~~) = (“I Lr3ci)
IvIodelB: #::)
Table 4 Parameter
= (a2 +ab,,i)
estimates Parameter
Model A:
Model B:
Model A:
Model B:
Estimate
Standard error
- 11.52 0.429 -3.06~ - 12.30 0.456 - 1.03x Whiting - 13.95 0.463 -6.58X - 15.60 0.518 -2.17X
0.47 0.015 0.94x 0.42 0.018 0.34x
10-j
1o-4
1O-3
1O-4
lo-)
1o-4
0.98 0.028 2.01 x 1o-3 0.84 0.033 0.67x 1O-4
F.G. O’Neill, R J. Kyrwch / Fisheries Research 28 (1996) 291-303
298
.g 0.08 0 t?
n'
0.06 0.04
16.5 19.5 22.5 25.5 28.5 31.5'34.5 Length (cm) Fig. 3. Haddock retained in the cod-end covers as a proportion
of the total haddock catch of each cover.
They are valid for both haddock and whiting. The estimates of these parameters and their standard errors are in Table 4 from which it is clear they are all significant to at least the 99% level. The respective expressions for 50% retention length and selection factor are Model A:
Model B :
40= -(
aI +
a3ci)/a*
SR = log( 9) /cy2 ISO=
-aI/(a2+a3ci)
SR = log(9)/(
CQ+ qci)
Statistically there is little to differentiate between these two models and, as shown in Figs. 3-7, little difference in their predicted values over the range of weights considered here. Model A, however, may be preferred due to the more straightforward linear interpretation of its selection parameters.
4. Discussion 4.1. Cover mesh size The fact that it was not possible to show that the selection parameters were dependent on the cover mesh size suggests that the cod-end fished in the same manner for each of the covers. An unlikely possibility is that a change of cover mesh size modifies escape from both the cod-end and the cover in such a way that the proportion of fish retained by the cod-end remains unaltered. In particular, this would imply that haddock in the range 25-35 cm and whiting in the range 28-38 cm could escape through the small
F.G. O’NeiN, R-I. Kymch/Fisheries
Haddock:
50% Retention
Length
vs Catch
. .
30 1
100
150
299
Reseurch 28 (1996) 291-303
200 250 300 350 Codend Catch (kg)
.
400
450
Fig. 4. Haddock I,, values plotted against catch size for each haul and the estimation models A and B.
of this relationship
by
meshes of the cover in appreciable numbers. Margetts (1954) shows that the constricted girth of both a 25 cm haddock and a 28 cm whiting, each of average condition, is about 12 cm. Accordingly, given that the maximum cover mesh size is 60 mm (giving a maximum opening perimeter of 12 cm), it is very unlikely that any fish above these lengths will escape (indeed, it is unlikely that fish a few centimetres smaller will escape either). This conclusion is substantiated by Fig. 7 which plots, at each length, the catch of haddock in the cover as a proportion of the total cover catch of haddock. It is not possible to distinguish between these three curves implying that fish retained by the 40 mm mesh cover are also retained by the 50 and 60 mm mesh covers. It is noteworthy that this also applies to the smaller fish (15-20 cm) which, with the exception of one or
Haddock:
Selection
. 4 100
150
-,--,
Range
?? ? ?
vs Catch
)
200 250 300 350 Codend Catch (kg)
Fig. 5. Haddock selection range values plotted against relationship by models A and B.
;,+ 400
450
catch size for each haul and the estimation
of this
300
F.G. O’Neill. R.J. Kymch/Fisheries
Research 28 (1996) 291-303
Whiting: 50% Retention
T
38
284
Length vs Catch
.
I
:
100
150
300 200 250 Codend Catch (kg)
350
400
Fig. 6. Whiting I,, values plotted against catch size for each haul and the estimation models A and B.
of this relationship
by
two fish, invariably escaped from the cod-end and had similar proportions retained by each of the covers. Hence, it can be assumed that each of the three covers fished in a similar manner and that the increase of cover mesh size had no significant effect on the selectivity of the cod-end. This also implies that any modification of the flow arising from the decrease in cover solidity had negligible impact. Thus, given that there is no consequential change in the fishing performance of either cover or cod-end, it would seem that, for the type of cod-end used here, an increase of cover mesh size from 40 mm to 60 mm is justified. As there were very few small fish (1< 15 cm) on the grounds it is not known whether an increase in mesh size of the cover would permit their escape. In a number of hauls large quantities of herring were caught in the cover (usually those with a catch weight greater than 1000 kg, see Table 2). From a practical point of
Whiting: Selection
“1
_...
7 t-_-+
100
Range vs Catch .
.
150
.
200 250 300 Codend Catch (kg)
Fig. 7. Whiting selection range values plotted against relationship by models A and B.
-
.
350
400
catch size for each haul and the estimation
of this
F.G. O’Neill,
R.J. Kynoch/
Fisheries
Research 28 (1996) 291-303
301
view, this may lead to some operational difficulties employing the larger mesh cover, for there was often meshing of herring in the 60 mm cover which, prior to the following haul. had to be cleared. 4.2. Cod-end catch size Over the range of cod-end catch sizes considered above there is a notable increase of the predicted I,, values. Such a rise cannot continue unabated and it is likely that as catch size increases the 50% retention length will approach some upper bound after which it will either level out or decrease. The negative correlation between selection factor and catch size found by Hodder and May (1964) for haddock and whiting and the similar relationship identified by Madsen and Moth-Poulsen (1994) for whiting and square mesh panel cod-ends suggests that the latter is the case. These trials, however, employed the covered cod-end method (though Hodder and May also used the alternate haul method) and as put forward by Pope et al. (197.51, there may be masking of the cod-end by the small mesh cover for large hauls resulting in the decrease in selection experienced by these authors. On the other hand, the studies of Dahm (1991) and Suuronen et al. (1991) support the theory that the I,, levels out. They employed the trouser trawl method and were unable to demonstrate a significant relationship between catch size and the 50% retention length. A simple mechanical explanation could be that the cod-end diameter (and hence mesh opening and consequently selectivity) increases with catch size until the point of maximum diameter is reached, any further increase of catch size having no effect on selectivity. A reassessment of the methodology and the subsequent analysis of commercial cod-end selectivity trials will be required if either of these scenarios proves an accurate description of cod-end selectivity. Given the available information it is not possible to predict, for catches greater than 407 kg, to what values the 50% retention lengths of haddock and whiting will tend and whether they will be greater or less than 30 and 36 cm respectively. It is these limiting values, however, that are important when investigating the selectivity of a commercial cod-end as the sizes of cod-end catches above, though typical of many experimental trials, are much smaller than those that would usually be obtained from commercial tows using the same cod-end. Furthermore how these values are reached may be of little significance as it is feasible that two cod-ends, with different design characteristics, could, for small cod-end catches, have different selectivities, yet the 50% retention lengths tend to the same value as the catch size increases. Given a limiting value for the I,, it is important to be able to determine how soon it is reached and the consequences this has for assessing the selectivity of a cod-end. Experimental trials and the statistical analysis would have to be designed so as to extract this information from the data. Although the relationship between I,, and catch weight is significant, a large amount of unresolved variability remains (Figs. 3-6). This variability will, at least in part, be due to different behavioural and environmental conditions between hauls. However, it is possible that the dependence of the I,, on catch weight may itself provide a further explanation and perhaps help identify some of the means by which factors such as the
302
F.G. O’Neill, R.J. Kyrwch/Fisheries
Research 28 (1996) 291-303
spatial distribution, the length-frequency distribution and the multispecies make-up of the fish population can influence the selection parameters. Consider, for example, a haul where all the large fish or, in the case of a multispecies fishery, all the fish of one species are captured during the initial stages of the tow when the catch weight and mesh opening are small. The chance these fish have of escaping will be reduced and their I,, values probably smaller than would have been expected if they were caught throughout the whole of the tow. Alternatively, if these fish are caught at the end of a tow, when the meshes are more open, the 1,, may be overestimated. These examples, though simplistic, demonstrate that the relationship of I,, and catch weight may have an inherent variability dependent on the biological make-up and spatial distribution of the fishery which may account for much of the observed scatter of Figs. 4-7.
5. Conclusion
The increase of cover mesh size from 40 mm to 60 mm had no significant effect on the selection parameters of the cod-end tested. Over the range of catch weights considered here there is a significant increase of the 50% retention length with catch weight. Given the strengths of the data set presented in this report, namely (i> the large number of hauls for the cod-end, (ii> the favourable weather conditions throughout and (iii) that the models describing the selection parameters are applicable to both whiting and haddock, it is clear that the relationship of catch weight and 50% retention length requires further attention. In particular how this relationship changes with increased catch and whether the explanations offered above accurately describes the selection process needs to investigated.
Acknowledgements
We are very grateful to Skipper Ian Gatt and the crew of “Solstice” for their advice and cooperation and to our colleague Mike Breen for his invaluable assistance.
References Dahm, E., 1991. Doubtful improvement of the selectivity of herring mid-water trawls by means of square mesh cod-ends and constructional modifications of diamond mesh cod-ends. ICES CM 1991/B:2. Fryer, R.J., 1991. 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: 13-27. Hodder, V.M. and May, A.W., 1964. The effect of catch size on the selectivity of otter trawls. ICNAF Research Bulletin No. 1. Madsen, N. and Moth-Paulsen, T., 1994. Measurement of the selectivity of Nephrops and demersal roundfish species in conventional and square mesh panel cod-ends in the northern North Sea. ICES CM 1994/B: 14. Main, J., Sangster, G.I., Kynoch, R.J. and Ferro, R.S.T., 1992. An experiment to measure the selectivity of cod-ends using two designs of cover. Scottish Fisheries Working Paper No. 2/92.
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Research 28 (1996) 291-303
303
Margetts, A.R., 1954. The length girth relationships in haddock and whiting and their application to mesh selection. J. Cons., 20: 56-61. Millar, R.B., 1994. Sampling from trawl gears used in size selectivity experiments. ICES J. Mar. Sci., 51: 293-298. Pope, J.A., Margetts, A.R., Hamley, J.M. and Akyuz, E.F., 1975. Manual of methods for fish stock assessment. FAO Fisheries Technical Paper No. 41. 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 ans seines. ICES J. Mar. Sci., 49: 279-288. Robertson, J.H.B., Lowry, N., Kynoch R. and Gzbilgin H., 1995. Improvements in designs of cod-end covers. ICES CM 1995/B:35. Stewart, P.A.M. and Robertson, J.H.B., 1985. Small mesh cod-end covers. Scottish Fisheries Research Report No. 32. Suuronen, P., Milk, R.B. and Jarvik, A., 1991. Selectivity of diamond and hexagonal mesh cot-ends in pelagic herring trawls: evidence of a catch size effect. Finn. Fish. Res., 12: 143-156.
Fisheries Research 28 (1996) 291-303
The effect of cover mesh size and cod-end catch size on cod-end selectivity F.G. O’Neill
*,
R.J. Kynoch
SOAEFD Marine Laboratory, P.O. Box 101, Victoria Road, Aberdeen, AB9 8DB, UK
Accepted 5 March 1996
Abstract Selectivity trials were carried out to examine the effect of catch size on selectivity and to test whether an increase of the cover mesh size would have any impact on the selection parameters of the cod-end fished. There was a significant increase of the 50% retention length for both haddock and whiting over the range of catch weights considered which was from 113407 kg. It is suggested that this tendency will not continue and that with increasing catch size the I,, will begin to level out or decrease. This explanation is consistent with the work of other authors, where catches are generally much larger. The increase of cover mesh size from 40 mm to 60 mm had no effect on the selection parameters of the 100 mm cod-end tested. Keywords:
Selectivity;
Catch size; Cover mesh size
1. Introduction In recent years the design of the small mesh cover used in covered cod-end selectivity trials has come under renewed investigation (Stewart and Robertson, 1985, Main et al., 1992, Robertson et al., 1995). Concern has been expressed as to the effect of the cover on the selectivity of the cod-end. One consequence has been the introduction of much bigger covers held open by large hoops which has helped prevent small mesh masking
* Corresponding
author.
0165-7836/96/$15.00 Crown copyright PII SO165-7836(96)00501-2
0 19% Published
by Elsevier Science B.V.
F.G. O’Neill, RJ. Kynoch/Fisheries
292
Research 28 (1996) 291-303
of the large mesh cod-end (Main et al., 1992, Robertson et al., 1995). An aspect of cover design that has not been considered, however, is its mesh size. An increase in the mesh size of the small mesh cover may permit the escape of very small fish which, if on the fishing ground, are at present retained in the cover. The consequent reduction in overall bulk may lead to better sampling of the larger fish from the cover and to more accurate selectivity data in the region of the selection range. There may be a reduction of the drag of the cover and a corresponding reduction of the strain on the hoops and their likelihood of collapsing. It is also possible that the flow through the cover will be enhanced and more like that of an uncovered cod-end. The minimum mesh size of the standard whitefish demersal trawl cod-end in use in Scottish waters has risen from 70 mm to 100 mm. However, the mesh size of the cod-end covers used by the Marine Laboratory, Aberdeen, in commercial selectivity trials has remained in the range 30-40 mm. There has been a reluctance to increase the cover mesh size as it is thought that it may result in an overestimate of the proportion of small fish retained in the cod-end by allowing the escape of large numbers of these fish from the cover. A separate issue is the effect of cod-end catch size on the selectivity parameters. Pope et al. (1975) identified the need for further study of this parameter in view of the application of the results of selectivity experiments to commercial fisheries where the
T
j+--
176-15 50mm x 2mm o IPA 46 rows
7l-
+-176 --fit 5Omm x 2mm B tPA 46 10~s
1.ir
II-146-11 _____________ T :
221 --+I7
I____________
T !
10
10
i k_-____
______ 3
x / II
16.1
0 _____________ 71+
221 -+i
6Omm 536 r0w.s 1.6mm B tPE
50mm 644 rows 1.6mm B tPE
40mm 805 rows 1.6mm B tPE
@ __________ I _______ 0
+--
1
175--+16
I+---
- ____.
146 -II
NOTES @
FORWARD HOOP POSITION FROM SKIRT/COVER JOIN 40mm = 32 rows / 50mm = 26 row / 60mm = 21 rows
AFT HOOP POSITION FROM SKIRT/COVER JOIN 226 rows / 5Omm = 182 rows / 60mm = 152 rows
03
0
ZIP IS POSITIONED 0.66M DOWN FROM SKIRT/COVER JOIN IN CENTRE OF PANELAND RUNS FOR 2 x 3.05m. THE SAME IN ALL CASES LIFTING BAG JOIN - 120mm x 5mm 0 BRAIDED NYLON (69 meshes round x 17 rows)
Fig. 1. Cod-end cover specifications.
F.G. O’Neill. R.J. Kynoch/Fisheries
Research 28 (1996) 291-303
293
catches are generally higher. Hodder and May (1964) found a negative correlation between selection factor and size of catch for haddock and cod using a diamond mesh cod-end. Madsen and Moth-Poulsen (1994) demonstrated a similar dependency for whiting in square mesh panel cod-ends. Dahm ( 199 1) and Suuronen et al. ( 199 11, on the other hand, were unable to establish a significant relationship between catch size and the 50% retention length for pelagic herring trawls. The range of catch sizes considered by each of the above authors is wide and may lead to some localised trends being overlooked. Furthermore, as pointed out by Pope et al. (1975), the reduction of the 50% retention length with catch size, detected in covered cod-end hauls, may be due to the presence of the cover masking the cod-end when the hauls are large. This paper aims to test whether an increase of the cover mesh size would have any impact on the selection parameters of the cod-end fished and to examine the effect of catch size on the selection parameters. 2. Materials and methods The commercial fishing vessel “Solstice” (BF56) was chartered for a 10 day period in July 1994 to carry out the selectivity trials. Three covers of 40, 50 and 60 mm
Test cod-end length 6 m _______
-I
I
I
1st hoop
2nd hoop =75m
Total cover length 2
Fig. 2. Rigging of cod-end covers and hoops.
F.G. O’Neill, RJ. Kymch/Fisheries
294
Research 28 (1996) 291-303
Table 1 General haul summary Haul No.
Cover size
Warpaft (m)
Water depth
Start time
Haul duration
Distance covered
Mean
Headline height (m)
Wingend spread
(h)
(m)
speed (kts)
3 4 6 7 8 9
50 50 60 60 60 60
320 320 320 320 320 320
110 110 110 110 110 110
0730 1050 0715 1050 1220 1450
1.75 2.00 1.50 0.50 1.33 1.50
9668 11885 9888 2973 6892 8205
2.78 2.95 3.23 3.01 2.65 2.75
4.7 4.6 3.9 4.2 4.3 4.5
14.0 13.5 13.9 13.2 14.2 14.3
10 11 12 13 15 16 18 19 20 21 22 23 24 25
40 40 40 40 50 50 60 60 60 60 40 40 40 40
320 366 320 320 320 320 320 274 274 320 320 274 274 274
110 128 110 110 110 110 110 91 91 110 110 91 91 82
0720 0950 1240 1455 1015 1250 0720 0950 1220 1430 0725 1025 1350 1545
1.50 1.00 0.75 1.00 1.50 1.17 1.50 1.50 1.00 2.00 2.00 2.00 1.08 1.50
N/R N/R N/R N/R 8736 6026 947 1 7716 5363 12490 12477 10103 5733 9211
N/R N/R N/R N/R 2.95 2.64 3.28 2.65 2.89 3.19 3.19 2.49 2.78 3.13
N/R N/R N/R N/R 4.2 4.4 4.3 4.3 4.4 4.4 4.7 4.6 4.4 N/R
N/R N/R N/R N/R 13.9 14.1 13.9 13.5 14.5 13.7 13.4 13.5 14.2 13.5
26 27 28
40 60 60
274 274 274
91 82 91
0735 1012 1206
1.00 1.00 2.00
N/R 5907 10279
N/R 3.11 2.63
N/R 4.5 4.5
N/R 14.3 13.3
(mm)
(m)
Cm)
(nominal) netting made from 1.6 mm single twisted polyethylene (PE) twine were used, the large scale dimensions of each being the same (17.2 m stretched length and 17.6 m stretched circumference). A 100 mm (nominal) mesh cod-end of 3.5 mm double braided PE, 100 open meshes around and 6 m stretched length was used throughout the trials. Two hoops of 2.1 and 2.5 m diameter, made from 50 mm diameter alkathene piping, were employed to prevent the small mesh covers masking the cod-end. The trawl used was a Sinclair white fish rockhopper trawl with a 78.1 m fishing circle and was towed using 54.9 m single sweeps, 36.6 m double bridles and Perfect K200 spherical doors. Full details of the cover designs can be found in Figs. 1 and 2. The fishing grounds were in an area approximately 10 nautical miles north east of Fraserburgh. Scanmar acoustic instrumentation measured net geometry and a Garmin 75 GPS unit recorded navigational parameters (Table 1). Twenty-nine hauls were carried out, all during daylight hours, of which 23 were considered valid. Haddock were found in sufficient quantities in all hauls and whiting in most. After each haul the cod-end catch was sorted by species (haddock, whiting and the rest) into baskets and weighed. Subsequently, the lengths of a sample of haddock and whiting were measured. Owing to their much greater size the catches in the cover were not initially sorted but put into baskets and weighed. Baskets were then sampled and the fish lengths measured. The catch weights and sampling fractions are given in Table 2.
295
F.G. O’Neill, R.J. Kymch / Fisheries Research 28 (1996) 291-303 Table 2 Catch summary Haul
Cod-end catch weight (kg)
No.
3 4 6 7 8 9 IO 11 12 13 15 16 18 19 20 21 22 23 24 25 26 21 28
Cod-end sampling fractions
Cover catch weight (kg)
Cover sampling fractions
Haddock
Whiting
Mixed
Total
Haddock
Whiting
Total
Haddock
Whiting
238
15 24 19 27 14 25 17 96 10 15 13 10 25 50 23 27 23 22 9 16 14 27 8
106 56 21 51 117 79 68 71 34 43 95 73 100 97 73 85 75 38 32 40 57 59 81
3.59 342 191 270 245 294 275 396 377 254 171 137 217 205 113 221 280 219 122 237 407 247 302
0.42 0.36 0.56 0.52 1.00 0.49 0.48 0.41 0.28 0.53 1.00 1.00 1.00 I .OO 1.00 1.oO 0.49 0.46 I.00 0.50 0.38 0.63 0.43
1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.OO 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00
707 1863 464 1086 962 614 601 2259 1136 1146 451 504 359 733 319 629 1570 1988 758 1322 1166 532 951
0.19 0.14 0.29 0.16 0.27 0.24 0.26 0.06 0.13 0.18 0.5 1 0.68 0.37 0.53 1.oo 0.22 0.23 0.09 0.32 0.23 0.15 0.34 0.28
0.30 0.08 0.41 0.16 0.2 1 0.24 0.88 0.05
262 151 192 114 190 190 229 333 196 63 54 92 58 17 109 182 219 81 181 336 161 213
N/R 0.11 1.oo 1.oo 0.37 0.08 0.27 0.22 0.04 0.09 N/R 0.08 N/R 0.97 N/R
The weather conditions were consistent throughout the period of the trials and the wind, though variable in direction, was always in the range 2-4 on the Beaufort scale.
3. Statistical
analysis
and results
For each haul i the logistic curve P
( 1
log -
=vi,
+v;,1
1-p
was fitted to the data, where p is the proportion of fish at length I retained in the cod-end and ui, and ui2 are the parameters to be estimated. Having defined the following quantities, y, and yz, the number of fish of length 1 measured in the cod-end and cover respectively, y, = y, + y2, the total number of fish of length I measured, r, the probcfish of length 1 sampled in cod-end I fish of length I sampled), q, and q2, the proportions of fish of length I measured in the cod-end and cover respectively and Cc,= log(q,/q,), it can be shown (Milk, 1994) that
log($-)=lo@--) +IcI
F.G. O’Neill, RJ. Kyrwch/
296 Table 3 Estimation Haul
Fisheries Research
28 (1996) 291-303
of vi and R, for each haul “il
"i2
3 4 6 7 8 9 10 11 12 13 15 16 18 19 20 21 22 23 24 25 26 27 28
- 11.28 - 13.33 - 14.22 - 15.67 - 15.17 - 14.10 - 12.44 - 12.44 - 14.05 - 15.59 - 12.87 - 11.83 - 12.28 - 9.80 - 7.92 - 12.38 - 10.38 - 14.90 - 8.78 -11.90 - 11.85 - 10.17 - 10.71
0.372 0.455 0.505 0.533 0.537 0.513 0.461 0.430 0.465 0.526 0.463 0.441 0.45 1 0.340 0.28 1 0.434 0.365 0.508 0.298 0.416 0.395 0.349 0.372
3 4 6 7 8 9 10 11 13 15 16 18 19 20 21 22 23 25 27
- 18.26 - 27.59 - 11.97 - 13.59 - 18.15 - 14.14 - 9.66 - 12.72 - 17.85 - 15.87 - 18.47 - 20.07 - 16.73 - 14.19 - 11.93 -22.16 - 12.54 - 14.18 - 17.23
0.503 0.838 0.341 0.370 0.523 0.420 0.286 0.358 0.522 0.495 0.603 0.607 0.514 0.452 0.342 0.753 0.33 1 0.43 1 0.520
Ri,,
Haddock 0.906 0.858 0.832 1.273 0.929 0.820 0.780 2.961 1.100 1.353 0.756 0.727 0.78 1 0.863 2.436 0.778 0.673 1.017 0.473 0.632 0.818 0.711 0.986 Whiting 5.005 6.285 1.61 1.416 3.284 1.543 2.392 0.590 2.314 5.537 8.379 4.887 1.564 1.832 1.027 3.536 0.633 1.435 5.120
Ril2
Ri22
- 0.0324 -0.0318 -0.0315 - 0.0464 - 0.035 1 -0.0311 - 0.0303 -0.1068 - 0.0391 - 0.0487 - 0.0289 - 0.0290 - 0.0307 - 0.0322 - 0.0888 - 0.0299 - 0.025 1 - 0.038 1 -0.0184 - 0.0239 - 0.0293 - 0.0258 - 0.0355
0.00116 0.00119 0.00120 0.00170 0.00134 0.00119 0.00119 0.00388 0.00140 0.00176 O.cOlll 0.00117 0.00122 0.00121 0.00327 0.00116 0.00095 0.00144 0.00073 0.00092 0.00105 0.00095 0.00129
-0.1548 -0.2162 - 0.0567 - 0.0480 -0.1119 - 0.0549 - 0.0773 -0.0218 - 0.0832 -0.1831 - 0.2840 -0.1587 - 0.0574 - 0.0680 - 0.0367 -0.1452 - 0.0239 -0.0581 -0.1595
0.00483 0.00749 0.00200 0.00165 0.00385 0.00198 0.00253 0.00081 0.00303 0.00611 0.00969 0.00519 0.00213 0.00255 0.00134 0.00602 0.00093 0.00238 0.00500
It is then possible to estimate uil and ui2 by fitting the general linearised model y, - Binomial( y, ,I)
291
F.G. O’Neill, R.J. Kynoch/ Fisheries Research 28 (1996) 291-303
where r
( 1
log l-r
= Vi, + ui*l + 4
Table 3 contains, for each haul, the estimates of the parameters ui = (ui,, ui2) and the elements of their covariance matrix, R,, for haddock and whiting. The Ri values and an inspection of the residual plots indicate that in each case the model adequately describes the data (the R,,l and the Riz2 elements are respectively the variance estimates of ui,, and ui2). It is shown in Table 2 that the cod-end catches of whiting were generally very small, a fact that is reflected in the larger values of the covariance matrix elements in comparison to those for haddock. The model of Fryer (1991), used subsequently by Reeves et al. (1992) and Galbraith et al. (19941, was employed to investigate the between-haul variation of the selectivity parameters, ui, and ui2. and their dependence on the explanatory variables of cover mesh size, m,, weight of catch in the cod-end, ci and tow duration, di. All possible linear and a number of non-linear expressions of the selectivity parameters as functions of these three variables were tested of which only two were found to describe the data at the 95% level (i.e. each of the terms of the design matrix were significant at the 95% level, see Fryer, 1991, Reeves et al., 1992). Neither demonstrated a dependency on either the cover mesh size l?zi or the tow duration d,. Employing the formulation of Fryer (1991) these two models can be expressed as follows: ModelA: lZ(~~~) = (“I Lr3ci)
IvIodelB: #::)
Table 4 Parameter
= (a2 +ab,,i)
estimates Parameter
Model A:
Model B:
Model A:
Model B:
Estimate
Standard error
- 11.52 0.429 -3.06~ - 12.30 0.456 - 1.03x Whiting - 13.95 0.463 -6.58X - 15.60 0.518 -2.17X
0.47 0.015 0.94x 0.42 0.018 0.34x
10-j
1o-4
1O-3
1O-4
lo-)
1o-4
0.98 0.028 2.01 x 1o-3 0.84 0.033 0.67x 1O-4
F.G. O’Neill, R J. Kyrwch / Fisheries Research 28 (1996) 291-303
298
.g 0.08 0 t?
n'
0.06 0.04
16.5 19.5 22.5 25.5 28.5 31.5'34.5 Length (cm) Fig. 3. Haddock retained in the cod-end covers as a proportion
of the total haddock catch of each cover.
They are valid for both haddock and whiting. The estimates of these parameters and their standard errors are in Table 4 from which it is clear they are all significant to at least the 99% level. The respective expressions for 50% retention length and selection factor are Model A:
Model B :
40= -(
aI +
a3ci)/a*
SR = log( 9) /cy2 ISO=
-aI/(a2+a3ci)
SR = log(9)/(
CQ+ qci)
Statistically there is little to differentiate between these two models and, as shown in Figs. 3-7, little difference in their predicted values over the range of weights considered here. Model A, however, may be preferred due to the more straightforward linear interpretation of its selection parameters.
4. Discussion 4.1. Cover mesh size The fact that it was not possible to show that the selection parameters were dependent on the cover mesh size suggests that the cod-end fished in the same manner for each of the covers. An unlikely possibility is that a change of cover mesh size modifies escape from both the cod-end and the cover in such a way that the proportion of fish retained by the cod-end remains unaltered. In particular, this would imply that haddock in the range 25-35 cm and whiting in the range 28-38 cm could escape through the small
F.G. O’NeiN, R-I. Kymch/Fisheries
Haddock:
50% Retention
Length
vs Catch
. .
30 1
100
150
299
Reseurch 28 (1996) 291-303
200 250 300 350 Codend Catch (kg)
.
400
450
Fig. 4. Haddock I,, values plotted against catch size for each haul and the estimation models A and B.
of this relationship
by
meshes of the cover in appreciable numbers. Margetts (1954) shows that the constricted girth of both a 25 cm haddock and a 28 cm whiting, each of average condition, is about 12 cm. Accordingly, given that the maximum cover mesh size is 60 mm (giving a maximum opening perimeter of 12 cm), it is very unlikely that any fish above these lengths will escape (indeed, it is unlikely that fish a few centimetres smaller will escape either). This conclusion is substantiated by Fig. 7 which plots, at each length, the catch of haddock in the cover as a proportion of the total cover catch of haddock. It is not possible to distinguish between these three curves implying that fish retained by the 40 mm mesh cover are also retained by the 50 and 60 mm mesh covers. It is noteworthy that this also applies to the smaller fish (15-20 cm) which, with the exception of one or
Haddock:
Selection
. 4 100
150
-,--,
Range
?? ? ?
vs Catch
)
200 250 300 350 Codend Catch (kg)
Fig. 5. Haddock selection range values plotted against relationship by models A and B.
;,+ 400
450
catch size for each haul and the estimation
of this
300
F.G. O’Neill. R.J. Kymch/Fisheries
Research 28 (1996) 291-303
Whiting: 50% Retention
T
38
284
Length vs Catch
.
I
:
100
150
300 200 250 Codend Catch (kg)
350
400
Fig. 6. Whiting I,, values plotted against catch size for each haul and the estimation models A and B.
of this relationship
by
two fish, invariably escaped from the cod-end and had similar proportions retained by each of the covers. Hence, it can be assumed that each of the three covers fished in a similar manner and that the increase of cover mesh size had no significant effect on the selectivity of the cod-end. This also implies that any modification of the flow arising from the decrease in cover solidity had negligible impact. Thus, given that there is no consequential change in the fishing performance of either cover or cod-end, it would seem that, for the type of cod-end used here, an increase of cover mesh size from 40 mm to 60 mm is justified. As there were very few small fish (1< 15 cm) on the grounds it is not known whether an increase in mesh size of the cover would permit their escape. In a number of hauls large quantities of herring were caught in the cover (usually those with a catch weight greater than 1000 kg, see Table 2). From a practical point of
Whiting: Selection
“1
_...
7 t-_-+
100
Range vs Catch .
.
150
.
200 250 300 Codend Catch (kg)
Fig. 7. Whiting selection range values plotted against relationship by models A and B.
-
.
350
400
catch size for each haul and the estimation
of this
F.G. O’Neill,
R.J. Kynoch/
Fisheries
Research 28 (1996) 291-303
301
view, this may lead to some operational difficulties employing the larger mesh cover, for there was often meshing of herring in the 60 mm cover which, prior to the following haul. had to be cleared. 4.2. Cod-end catch size Over the range of cod-end catch sizes considered above there is a notable increase of the predicted I,, values. Such a rise cannot continue unabated and it is likely that as catch size increases the 50% retention length will approach some upper bound after which it will either level out or decrease. The negative correlation between selection factor and catch size found by Hodder and May (1964) for haddock and whiting and the similar relationship identified by Madsen and Moth-Poulsen (1994) for whiting and square mesh panel cod-ends suggests that the latter is the case. These trials, however, employed the covered cod-end method (though Hodder and May also used the alternate haul method) and as put forward by Pope et al. (197.51, there may be masking of the cod-end by the small mesh cover for large hauls resulting in the decrease in selection experienced by these authors. On the other hand, the studies of Dahm (1991) and Suuronen et al. (1991) support the theory that the I,, levels out. They employed the trouser trawl method and were unable to demonstrate a significant relationship between catch size and the 50% retention length. A simple mechanical explanation could be that the cod-end diameter (and hence mesh opening and consequently selectivity) increases with catch size until the point of maximum diameter is reached, any further increase of catch size having no effect on selectivity. A reassessment of the methodology and the subsequent analysis of commercial cod-end selectivity trials will be required if either of these scenarios proves an accurate description of cod-end selectivity. Given the available information it is not possible to predict, for catches greater than 407 kg, to what values the 50% retention lengths of haddock and whiting will tend and whether they will be greater or less than 30 and 36 cm respectively. It is these limiting values, however, that are important when investigating the selectivity of a commercial cod-end as the sizes of cod-end catches above, though typical of many experimental trials, are much smaller than those that would usually be obtained from commercial tows using the same cod-end. Furthermore how these values are reached may be of little significance as it is feasible that two cod-ends, with different design characteristics, could, for small cod-end catches, have different selectivities, yet the 50% retention lengths tend to the same value as the catch size increases. Given a limiting value for the I,, it is important to be able to determine how soon it is reached and the consequences this has for assessing the selectivity of a cod-end. Experimental trials and the statistical analysis would have to be designed so as to extract this information from the data. Although the relationship between I,, and catch weight is significant, a large amount of unresolved variability remains (Figs. 3-6). This variability will, at least in part, be due to different behavioural and environmental conditions between hauls. However, it is possible that the dependence of the I,, on catch weight may itself provide a further explanation and perhaps help identify some of the means by which factors such as the
302
F.G. O’Neill, R.J. Kyrwch/Fisheries
Research 28 (1996) 291-303
spatial distribution, the length-frequency distribution and the multispecies make-up of the fish population can influence the selection parameters. Consider, for example, a haul where all the large fish or, in the case of a multispecies fishery, all the fish of one species are captured during the initial stages of the tow when the catch weight and mesh opening are small. The chance these fish have of escaping will be reduced and their I,, values probably smaller than would have been expected if they were caught throughout the whole of the tow. Alternatively, if these fish are caught at the end of a tow, when the meshes are more open, the 1,, may be overestimated. These examples, though simplistic, demonstrate that the relationship of I,, and catch weight may have an inherent variability dependent on the biological make-up and spatial distribution of the fishery which may account for much of the observed scatter of Figs. 4-7.
5. Conclusion
The increase of cover mesh size from 40 mm to 60 mm had no significant effect on the selection parameters of the cod-end tested. Over the range of catch weights considered here there is a significant increase of the 50% retention length with catch weight. Given the strengths of the data set presented in this report, namely (i> the large number of hauls for the cod-end, (ii> the favourable weather conditions throughout and (iii) that the models describing the selection parameters are applicable to both whiting and haddock, it is clear that the relationship of catch weight and 50% retention length requires further attention. In particular how this relationship changes with increased catch and whether the explanations offered above accurately describes the selection process needs to investigated.
Acknowledgements
We are very grateful to Skipper Ian Gatt and the crew of “Solstice” for their advice and cooperation and to our colleague Mike Breen for his invaluable assistance.
References Dahm, E., 1991. Doubtful improvement of the selectivity of herring mid-water trawls by means of square mesh cod-ends and constructional modifications of diamond mesh cod-ends. ICES CM 1991/B:2. Fryer, R.J., 1991. 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: 13-27. Hodder, V.M. and May, A.W., 1964. The effect of catch size on the selectivity of otter trawls. ICNAF Research Bulletin No. 1. Madsen, N. and Moth-Paulsen, T., 1994. Measurement of the selectivity of Nephrops and demersal roundfish species in conventional and square mesh panel cod-ends in the northern North Sea. ICES CM 1994/B: 14. Main, J., Sangster, G.I., Kynoch, R.J. and Ferro, R.S.T., 1992. An experiment to measure the selectivity of cod-ends using two designs of cover. Scottish Fisheries Working Paper No. 2/92.
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Research 28 (1996) 291-303
303
Margetts, A.R., 1954. The length girth relationships in haddock and whiting and their application to mesh selection. J. Cons., 20: 56-61. Millar, R.B., 1994. Sampling from trawl gears used in size selectivity experiments. ICES J. Mar. Sci., 51: 293-298. Pope, J.A., Margetts, A.R., Hamley, J.M. and Akyuz, E.F., 1975. Manual of methods for fish stock assessment. FAO Fisheries Technical Paper No. 41. 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 ans seines. ICES J. Mar. Sci., 49: 279-288. Robertson, J.H.B., Lowry, N., Kynoch R. and Gzbilgin H., 1995. Improvements in designs of cod-end covers. ICES CM 1995/B:35. Stewart, P.A.M. and Robertson, J.H.B., 1985. Small mesh cod-end covers. Scottish Fisheries Research Report No. 32. Suuronen, P., Milk, R.B. and Jarvik, A., 1991. Selectivity of diamond and hexagonal mesh cot-ends in pelagic herring trawls: evidence of a catch size effect. Finn. Fish. Res., 12: 143-156.