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Gear-dependent size selection of snapper, Pagrus auratus
ELSEVIER
Fisheries
Research28 ( 1996) 119- 132
Gear-dependent size selection of snapper, Pap-us auratus N.M. Otway *, J.R. Craig, J.M. Upston N.S.W.
Fisheries Research Institute, P.O. Box
2 I, Cronulla, N.S.W. 2230, Austrulia
Accepted24 February 1996
Abstract The size-compositions of snapper, Pagrus auratus, caught using demersal trawls and longlines were compared for six sites sampled off Sydney, N.S.W., Australia. We predicted that if gear-dependent selectivity occurred, it would be evident as changes to (1) the mean size of individuals, (2) the variance in size of individuals, or (3) both the mean and variance in size of individuals. Neither technique caught individuals less than 90mm fork length and catches exhibited temporal fluctuations during the 4years of sampling. Although trawls were more productive and provided almost 65% of the entire catch, snapper caught on longlines were significantly larger in size and covered a wider range of sizes. These results were also manifest across sites despite the significant spatial differences in the mean size of individuals. The differential gear selectivity also affected catches of undersized and legal-sized snapper. Trawls caught significantly smaller undersized and legal-sized snapper compared with longlines. Several hypotheses were proposed to account for the absence of juvenile snapper in the catches of both techniques. The results were consistent with our last prediction in that both the mean and variance in size differed between trawls and longlines. Finally, the results are discussed in terms of their implications for fishery-independent, standardized trawl surveys and the subsequent use of the resultant data in stock assessment models. Keywords: Longlines; Trawls; Size-selection; Snapper
1. Introduction Management of many important demersal fisheries is often reliant on fishery-independent trawl surveys and the results of stock assessment models (Doubleday and
* Corresponding author. 01657836/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. PII SO 165-7836(96)00500-O
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Rivard, 1981; Godo and Walsh, 1992). Ideally, the trawls used in these surveys should not be size-selective, but this is clearly not the case. Factors affecting trawl selectivity include the size of the codend (Millar and Walsh, 1992), the type and size of the codend mesh (McCracken, 1963; Walsh et al., 1992), the length of sweeps (Engas and Godo, 1989; Andrew et al., 1991), fish morphology (Fonteyne and M’Rabet, 1992), duration and speed of the tow (Isaksen et al., 1992; Kennelly et al., 1993), and the presence or absence of by-catch reducing devices (Isaksen et al., 1992). The selectivity of trawls, together with the often large inter-annual variability in abundance estimates has led several workers (e.g. Godo and Sunnana, 1992) to urge that survey data be critically examined before their use in stock assessment models. Whereas early studies (e.g. Gulland, 1969; Pope et al., 1975) provided the stepping stones, more recent research (e.g. Millar, 1992) has led to the development of better techniques for quantifying trawl selectivity. There are, however, still some concerns about the most recent approaches because, in isolation, they may still not indicate whether the particular trawl is sampling all or some fraction of the population actually living in a given area (Godo and Walsh, 1992). Doubts along these lines can be answered, to some extent, by comparing the catches of trawls with those of a vastly different technique such as longlines. However, such comparisons may still be problematic because longlines are also size-selective (see Lokkeborg and Bjordal (1992) for a review). Like that of trawls, longline selectivity is also affected by numerous factors; these include the type of substratum (Cross, 1988), the depth of water (Sakagawa et al., 1987; Gong et al., 1989), the type and size of the bait (Imai, 1972; Bjordal, 1983; Lokkeborg, 1991), hook-size and/or shape (McCracken, 1963; Saetersdal, 1963; Ralston, 1990; Otway and Craig, 1993), and whether the lines are set on or off the bottom (Arimoto, 1984; Russell et al., 1988). Although researchers conducting fishery-independent trawl surveys are obviously aware of gear selectivity, fewer than half actually document this for the gear used (Table 1). It is possible that such information is available, but has simply not been published. Alternatively, the selective nature of the gear used may not have been quantified. The latter scenario has obvious ramifications for stock assessment models and the use of the modelling results in fisheries management. Fishery-independent trawl surveys in New South Wales, Australia, have mainly
Table 1 Number of papers published Fisheries Management
data on the selectivity
in Fisheries Research, Fishery Bulletin and the North Americun Journal of describing fishery-independent trawl surveys that provided of the trawl used or cited a study that contained the relevant information over the period 1984-1993
Journal
Total number of papers on fishery-independent trawl surveys
Number (o/o) of papers with data on the selectivity of trawl or a citation to a relevant study
Fisheries Research
17
9 (52.9)
Fishery Bulletin
20
5 (25.0)
North American Journal of Fisheries Management
10 47
5 (50.0) 19 (40.4)
Total
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concentrated on estuaries (e.g. Gray et al., 1990, Ferrell and Bell, 1991) and thus little is known about the distributional patterns of demersal fish in coastal shelf waters. However, more recently the spatial and temporal patterns of abundance of demersal fish inhabiting the coastal waters off Sydney were documented (using trawls) as part of a large environmental study (see Otway et al. (1996) for details). Whereas the trawl survey (Gray and Otway, 1994) was based on a stratified sampling design and demonstrated clear depth-related patterns of abundance and species composition, little was known about the selectivity of the trawl used. However, the size-age structures of populations of commercially and/or recreationally important species were not analysed. In a contemporaneous study using longlines, Otway and Craig (1993) examined hook selectivity and its effects on estimating the population size-structure of snapper in Sydney’s coastal waters. Consequently, in this paper we look for evidence of gear-dependent selectivity by comparing the size-frequency distributions of snapper caught in trawls and on longlines. If gear-dependent selection is evident, we predict that the differences in size-structure will arise in at least one of three ways. First, the mean size of individuals caught by each technique will not be altered, but the variance in the size of fish caught will differ significantly (Fig. I(a)>. Second, the mean size of individuals caught by each technique will significantly differ, but the variance in the size of fish caught will not be altered (Fig. l(b)). Third, the mean size and variance in size of individuals caught by each technique will both differ (Fig. l(c)). Although this comparison will only provide information on the selectivity of the trawl relative to the longlines. it should help to identify any possible bias that would need to be recognised in a future analysis of the size-frequency data emanating from the trawl survey by Gray and Otway ( 1994).
2. Materials
and methods
2.1. Species studied The snapper Pagrus aurutus (Bloch & Schneider, 1801) is demersal and grows to the minimum legal size of 280mm total length (about 250mm fork length, FL) in about 3 years (Bell et al., 1991). The species is currently fished commercially off the entire coast of New South Wales using mainly baited traps, although significant numbers of fish are caught on longlines and handlines (Kailola et al., 19931, and as legal by-catch (sensu Saila, 1983) of the oceanic prawn-trawl fleet (KeMelly et al., 1993). Whereas the species is reported to reach a maximum FL of 1300mm (Hutchins and Swainston, 1986), commercial and recreational fishers rarely catch individuals exceeding 800 mm FL (D.J. Ferrell and A.S. Steffe, unpublished data, 1995). 2.2. Field sampling Sampling was done in the coastal shelf waters off Sydney (N.S.W., Australia) in waters less than 1OOm deep. Fish were caught with demersal otter trawls and longlines using a 13 m, 114kW trawler (F.R.V. ‘Kamala’) and a 7 m sharkcat (F.R.V. ‘Wobbe-
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a.
Fork length Fig. 1. Predicted population size-structure arising through (a) significant differences in the variance in the size of fish caught, (b) significant differences in the mean size of individuals caught, (c) significant differences in the mean size and variance in size of individuals if gear-dependent selectivity occurs. Hypothetical population size-structure of P. aurarus caught using trawls (continuous line) and longlines (broken line).
gong’), respectively. Trawling and longlining were done at the same six sites at about 3 month intervals from autumn 1989 to autumn 1993 (Fig. 2). All sampling was restricted to daylight hours and was done on consecutive days, seas and weather permitting. The fork lengths of all fish caught were measured to the nearest millimetre. 2.3. Demersal otter trawls Demersal trawling was done using a 32m headline ‘Kent Bollinger’ wing trawl with 72m sweeps and 22m bridles. The net had 229mm mesh in the wings, a body with 229 mm mesh gradually reducing to 57 mm, and a cod-end of 42 mm diamond-shaped mesh. The net was towed at 2.5 knots and with a ratio of length of warp to depth of 3: 1 (for further details see Gray and Otway (1994) and Otway et al. (1996)). At each site, three replicate trawls, each of 20min bottom-time, were done over non-overlapping patches of ground.
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0
123
1Okm
I
I
Scale /;;
151020’
151 O30’E
--I
Fig. 2. Map showing the location of the six sites sampled in the coastal waters off Sydney, New South Wales, Australia.
2.4. Demersal longlines The demersal longlines used have been described in detail by Otway and Craig (1993) and only a brief account is given below. Each longline consisted of a 76m ground-line weighted at both ends; a 90- 120 m buoy-line and a danbuoy. Thirty-three hooks, attached to 50 cm snoods of monofilament line, were clipped onto the ground-line at 2 m intervals as the lines were set. The hooks were baited with uniformly sized pieces of squid and stored in a freezer until the day of sampling. At each site, 12 replicate longlines (n = 396 hooks in total) were set on the bottom and parallel to the depth contours. Each longline was allowed to fish for an average of 2.5 h and then hauled. Tuna-circle hooks of three different sizes (‘Mustad’ Nos. 12, 10 and 8-see Fig. 1 and Table 1 of Otway and Craig (1993)) were used; however, data for the individual hook-sizes were pooled for the purposes of this experiment. 2.5. Statistical analyses Size-frequency distributions were compared using Kolmogorov-Smimov tests (HOIlander and Wolfe, 1973). Differences attributable to the fishing gear used and/or spatial variation in population size-structure of snapper within the sampling region were examined using analyses of variance after preliminary tests for heteroscedasticity
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(Cochran’s test, Winer, 1971). When variances were heterogeneous, data were transformed using the procedures outlined by Scheffe (1959) and Winer (1971). When heteroscedasticity persisted, the analysis was done using a more conservative Type I error-rate of (Y= 0.01 (Underwood, 1981). Significant differences among means were identified by Student-Newman-Keuls (SNK) tests (Snedecor and Cochran, 1980).
3. Results A total of 774 snapper was caught over the entire sampling period, 64.6% in trawls and 35.4% on longlines. The numbers caught by both techniques exhibited substantial temporal variability and thus it was not possible to incorporate time as a factor in the analyses. Consequently, all analyses were done by pooling across times for individual sites within the sampling region. It was possible, however, to include ‘sites’ as a factor in some of the analyses, but only at Sites 2, 3 and 4 because of reduced catches at the remaining sites. 3.1. Mean size of snapper The size-frequency distributions of P. auratus pooled across all sites and at Sites 2, 3 and 4 (Fig. 3) showed that neither trawling nor longlining caught individuals under 90mm fork length (FL). Fig. 3 also indicated that there were gear-dependent and spatial differences in the population size-structure of snapper. Comparisons of the sizefrequency distributions of P. aurafus caught in trawls and on longlines at Sites 2, 3 and 4, and across all six sites were all significantly different (Table 2, Kolmogorov-Smimov tests, P < 0.05 in all tests). This confirmed the existence of gear-dependent selectivity, but did not indicate how it was manifest. The mean (SE) fork lengths of P. auratus (calculated from the length-frequency distributions) caught in trawls at Sites 2, 3 and 4 were, respectively, 261.7 (8.3)mm, 197.5 (4.5)mm and 191.3 (1.4)mm, whereas those caught on longlines were, respectively, 314.9 (7.0)mm, 248.2 (7.8)mm and 293.0 (9.5)mm. When pooled across sites, the mean fork lengths of P. auratus caught in trawls and on longlines were respectively, 209.8 mm and 295.7 mm. This 85.9 mm difference in the mean fork lengths suggested that the snapper caught on longlines were larger than those caught in trawls. This was investigated further by comparing the mean fork lengths of fish caught in trawls and on
Table 2 Results of Kolmogorov-Smimov tests of size-frequency distributions trawls and longlines at Sites 2, 3 and 4, and pooled across all six sites
of P. auratus
Site
x2 (2dfI
P
2 3 4 All
26.5 21.1 137.2 331.0
0.0100 0.0215 O.oool 0.0001
caught
by demersal
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126 Table 3 Analyses
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of the mean size of P. auratus caught in demersal
trawls and on longlines
at Sites 2, 3 and 4
(A) Analysis of variance Source of variation
df
MS
F
P
Site Gear Site X Gear Residual Total
2 1 2 198 203
0.2329 0.6902 0.045 1 0.0064
36.49 108.14 1.01
(B) SNK tests: between techniques
at each site
Outcome Longlines > Trawls Longlines > Trawls Longlmes > Trawls
Site 2 Site 3 Site 4
SNK tests: among sites for each technique Outcome Site 4 = Site3 < Site 2 Site 3 < Site 4 = Site 2
Trawls Longlines
longlines at Sites 2, 3 and 4 using a balanced analysis of variance with n = 34 individuals selected at random from the total numbers of fish caught by each technique at each site. This analysis showed that snapper caught on longlines were always significantly larger than those trawled (Table 3; analysis of variance and SNK test of means in Fig. 4, P < 0.05). The analysis also confirmed the presence of spatial differences in the mean size of snapper suggested by the size-frequency distributions. First, significantly larger P. aurutus were trawled at Site 2 compared with those at Sites 3 and 4, which did not differ, and second, the snapper caught on longlines were q Longlining n Trawling
4*l
Site Fig. 4. Mean ( + SE) fork length of P. auratus caught using trawls and longlines at Sites 2, 3 and 4.
NM. Otway er d/Fisheries Table 4 Comparison 2,3 and 4
of the mean variance
Research 28 (1996) 119-132
in length of P. aururus
Mean (SE) variance
caught in demersal
12-l
trawls and on longlines at Sites
Homogeneity of variances (2-tailed F, df = 2.2)
Comparison of means (2-tailed I, df = 4)
Trawls
Longlines
F
P
f
P
I347 (660)
4845 (993)
0.44
0.61
2.93
0.04
significantly larger at Sites 2 and 4 compared with those at Site 3 (Fig. 4 and Table 3; analysis of variance and SNK tests of means, P < 0.05). 3.2. Variance in size of snapper The size-frequency distributions of P. aurutus pooled across all sites and at Sites 2, 3 and 4 (Fig. 3) also suggested gear-dependent differences in the variances of the distributions. This was examined further by calculating the variance of each sizefrequency distribution for snapper caught by trawling and longlining. The respective variances at each site were then used as independent (replicate) estimates of the true variance in size for snapper caught in trawls and on longlines. This allowed a comparison of the means of variance in the length of snapper caught in trawls and on longlines. Table 4 shows that the mean variance in the length of snapper was significantly greater for individuals caught on longlines compared with those trawled. 3.3. Legal vs. illegal snapper The data from longlining and trawling were examined further to address concerns over the capture of individuals under the minimum legal size of 280mm total length (about 250mm FL). Trawling caught a total of 500 individuals from all sites over the entire sampling period and of these, 89% were under the minimum legal size (Fig. 3). By comparison, only 26% of the 274 snapper caught on longlines were under the minimum legal size (Fig. 3). More detailed analysis of legal-sized snapper (250mm or more FL) showed that a significantly larger proportion were caught on longlines compared with trawls (Table 5, x2 = 319.25, P < 0.0001). In addition, snapper caught on longlines were significantly larger than those caught in trawls (Fig. 5 and Table 6; analysis of variance, P < 0.0001). Analysis of snapper under the legal size (i.e. less than
Table 5 Numbers longlines Technique
Trawls Longlines
and percentages of legal (L) and illegal (I) sized pooled across sites Numbers
P. auratus
% of method
caught
in demersal
trawls and on
% of total
L
I
L
I
L
I
53 202
447 72
10.6 73.7
89.4 26.3
6.8 26.1
57.8 9.3
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El
Longlining
n
Trawling
Illegal
Size Fig. 5. Mean ( + SE) fork length of legal-sized pooled across all sites.
and undersized
P.
uuratuscaught using trawls and longlines
Table 6 Analysis of the mean size of legal and illegal P. auratus caught in demersal
trawls and on longlines
Source of variation
df
MS
F
P
L vs. I Method L vs. IX Method Residual Total
1 1 1 208 211
1.8166 0.1394 0.0064 0.0063
287.36 22.04 1.02
< O.OWl < 0.0001 0.3149
A Type I error-rate at P = 0.05.
of Q = 0.01 was used because Cochran’s
test of the transformed
data was still significant
250 mm FL) indicated that a significantly larger proportion of illegal-sized snapper were caught in trawls compared with on longlines (Table 5, x2 = 319.25, P < 0.0001). Furthermore, the mean size of snapper caught in trawls was significantly smaller than the mean size of individuals caught on longlines (Fig. 5 and Table 6; analysis of variance, P < 0.0001). 4. Discussion Neither fishing technique caught P. aurutus under 90mm FL (i.e. under 1 year old), but this result was not unexpected as several previous studies (e.g. Otway and Craig, 1993; Gray and Otway, 1994; Otway et al., 1996) have not caught juvenile snapper in the inshore coastal waters of central N.S.W. These results are also consistent with those of a demersal fish survey using a purpose-built Stebbenhauser trawl with 25mm mesh throughout (M.P. Lincoln Smith, personal communication, 1995). The latter study was done in the same general area off Sydney and only caught snapper exceeding 110 mm FL. However, juvenile snapper (20-170mm FL) are caught in significant numbers as by-catch of the estuarine prawn-trawl fishery (Gray et al., 1990; Kennelly et al., 1992). These observations suggest that the absence of juvenile snapper in the trawls and
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longlines may result from a preference for the large, open estuaries over the inshore coastal waters along this part of the N.S.W. coast. Alternatively, juvenile snapper may indeed frequent inshore coastal waters, but their capture is prevented in other ways. First, the selectivity of the trawl and longlines may have prevented the capture of juveniles. Although this may indeed be true for the longlines (e.g. Otway and Craig, 1993) this is probably not so for the trawls, because the catches also included large numbers of Trachurus nouaezelandiae, Zeus faber, Anomalus inermis and Lepidotrigla argus with minimum fork-lengths of 65 mm, 75 mm, 60mm and 70mm, respectively. This suggests that juvenile snapper would also have been caught had they been present at the sites sampled. Second, juveniles may have avoided capture through ontogenetic changes in habitat usage in coastal waters. For example, juvenile snapper may congregate on or near rocky reefs and only venture out over trawlable grounds when older. Such behaviour would, however, be at odds with the juveniles trawled in estuaries. Clearly, more detailed sampling will be needed to differentiate between these hypotheses. The longlines caught snapper to a maximum size of 620mm FL compared with 450mm FL in the trawls. The absence of large snapper (FL greater than 620mm) on longlines was probably due to their rarity at the sites sampled. This is further supported by the size of individuals caught by recreational anglers in Sydney’s coastal waters (see Section 2). In contrast, the absence of individuals larger than 450mm FL in the trawls was probably due to the 20min tow duration. Studies (e.g. Isaksen et al., 1992) have shown that trawls of longer duration catch significantly larger individuals. Furthermore, the amount of time taken to exhaust a fish when herded by a trawl is most probably size dependent, and this has been postulated as the most likely reason for the reduced capture of larger individuals in trawls of limited tow duration (Wardle, 1983; Main and Sangster, 1983). Comparisons of the size-frequency distributions of P. auratus caught on longlines and in trawls demonstrated marked gear-dependent selectivity. The pattern of selection was consistent across sites, with significantly larger snapper covering a wider range of sizes being caught on longlines. These results are consistent with previous studies of cod, off Norway (Saetersdal, 1963) the Gulf of St. Lawrence (McCracken, 1963) and Greenland (Hovgard and Riget, 1992), which showed that longlines caught larger individuals covering a wider range of sizes. The gear-dependent selection resulted in a 85.9mm difference in the mean fork lengths of snapper caught on longlines and in trawls. However, in a previous study on longline selectivity, Otway and Craig (1993) showed that significantly larger P. auratus (mean FL 323.2mm) were caught on Number 8 compared with Number 10 and 12 Mustad tuna-circle hooks (mean FL 305.8 mm and 277.3 mm, respectively). Consequently, if the individuals caught on the Number 10 and 12 hooks were eliminated from the data examined here, the magnitude of the difference in the mean fork lengths of snapper caught by trawling and longlining, pooled across all sites, would be increased by a further 27.5 mm (i.e. from 85.9 to 113.4mm). This is equivalent to over half the annual growth increment for snapper with a mean size of approximately 210 mm FL (Francis and Winstanley, 1989). Gear-dependent selection was also evident when undersized and legal-sized P. auratus were examined. Longlines caught significantly larger undersized and legal-sized
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snapper compared with trawls. This result was also consistent across all sites despite spatial differences in the mean fork length of local populations of P. auratus. Again, if the individuals caught on the Number 10 and 12 hooks were eliminated from the data, then the difference in the numbers of undersized individuals caught by trawling and longlining, pooled across all sites, would be increased by a further 16%. By comparing the catches of snapper in trawls and on longlines this study has shown that the population size-structure can be significantly modified by the fishing gear used, a fact that was well documented in earlier studies (e.g. Allen, 1963; Gulland, 1969; Pope et al., 1975). These modifications follow the model illustrated in Fig. l(c) in that the mean size and variance in size of individuals caught by each technique differed significantly. The trawl clearly underestimated the number of larger individuals and range of sizes in the population. We feel that this particular limitation can be overcome by increasing the trawl duration and/or the towing speed, but this will need to be tested experimentally. Moreover, the results of this study should permit a more informed analysis and interpretation of the size-frequency information obtained in the trawl survey by Gray and Otway (1994) of demersal fish communities off Sydney. For example, a high proportion of juveniles in the catch might have suggested that the site was a nursery habitat for the particular species. However, this study has shown that the trawl selectively captured proportionally more small individuals, and this demands that such a conclusion be modified. Although a range of explanations could be proposed to account for the possible patterns observed, it is clear that data on gear selectivity will allow one possible source of confounding to be eliminated. 4.1. Implications for future research and management It is obvious that the trawl used in this study could also be used in more widespread, fishery-independent, standardised trawl surveys documenting the abundances and sizeage structures of commercially important species along the N.S.W. coast. If such an approach is used, it will be extremely important to ensure that a ‘standard survey’ can be repeated through time at a number of sites and that the resulting data are free from bias. To this end, the quality of standardised trawl surveys would be greatly improved if more is known about the selectivity of the fishing gear used. This could be achieved by ensuring that appropriate field experiments are used to identify the selective nature of the gear. Obviously, such experiments would need to be done at several sites and repeated through time to quantify the degree of spatial and temporal variation in selectivity of the gear. Not only would these experiments identify the potential biases in demographic studies, but their results could also be incorporated into cost-benefit analyses for optimising the design of fishery-independent surveys. However, the results of cost-benefit analyses for multi-species fisheries are less definitive because levels of replication may be optimal for some species and sub-optimal for others (Leaman, 1981). This was clearly the case for several species of fish caught as by-catch in the oceanic prawn-trawl fishery off New South Wales (Kennelly et al., 1993). Furthermore, as trawl selectivity can bias the size-composition of the catch it will be necessary to ensure that tow durations and levels of replication, as determined by cost-benefit analyses, also provide unbiased estimates of population size-structure. Only then will standardised surveys provide robust inputs into stock assessment models.
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Acknowledgements
We are grateful to our colleagues in the Institute’s Deepwater Ocean Outfalls Monitoring lab. for their efforts. We also wish to thank J. McClenaughan and R. Norington (the masters of F.R.V. ‘Kamala’) and the crews of F.R.V. ‘Wobbegong’ for their help with sampling. Funds for this study were provided by Sydney Water and the N.S.W. Environment Protection Authority as part of the Sydney Deepwater Outfalls Environmental Monitoring Programme. C. Gray, M. Broadhurst and two anonymous reviewers provided helpful comments on the draft manuscript. Finally, we thank K.R. Allen, who provided some useful suggestions that assisted in revising the paper. References Allen, K.R., 1963. The influence of behaviour on the capture of fish with baits. Int. Comm. Northwest Atl. Fish. Spec. Publ., 5: 5-7. Andrew, N.L., Graham, K.J., Kennelly, S.J. and Broadhurst, M.K., 1991. The effects of trawl configuration on the size and composition of catches using benthic prawn trawls off the coast of New South Wales, Australia. ICES J. Mar. Sci., 48: 201-209. Arimoto, T., 1984. Interspecific competition among demersal fishes in catch distribution of coastal set-line. Bull. Jpn. Sot. Sci. Fish., SO: 205-210. Bell, J.D., Quartararo, N. and Henry, G.W., 1991. Growth of snapper, Pagrus auratus, from south-eastern Australia in captivity. N.Z. J. Mar. Freshwat. Res., 2.5: 117- 121. Bjordal, A., 1983. Effect of different long-line baits (mackerel, squid) on catch rates and selectivity for tusk and ling. ICES CM/B, 31: 9. Cross, J.N., 1988. Aspects of the biology of two scyliorhinid sharks, Aprisfurus bruntzeus and Parmarurus xaniurus, from the upper continental slope off Southern California. Fish. Bull., 86: 691-702. Doubleday, W.G. and Rivard, D., 1981. Bottom trawl surveys. Can. Spec. Publ. Fish. Aquat. Sci., 58: 273. Engas, A. and Godo, O.R., 1989. The effect of different sweeps on length composition of bottom sampling trawl catches. J. Cons. Int. Explor. Mer, 45: 263-268. Ferrell, D.J. and Bell, J.D., 1991. Differences among assemblages of fish associated with Zosreru capricorni and bare sand over a large spatial scale. Mar. Ecol. Prog. Ser., 72: 15-24. Fonteyne, R., M’Rabet and R.M., 1992. Selectivity experiments on sole with diamond and square mesh codends in the Belgian coastal beam trawl fishery. Fish. Res., 13: 221-233. Francis, R.I.C.C. and Winstanley, R.H., 1989. Differences in growth rates between habitats of southeast Australian snapper (Chrysophrys aurafus). Aust. J. Mar. Freshwater Res., 40: 703-710. Godo, O.R. and Sunnana, K., 1992. Size selection during trawl sampling of cod and haddock and its effect on abundance indices at age. Fish. Res., 13: 293-310. Godo, O.R. and Walsh, S.J., 1992. Escapement of fish during bottom trawl sampling: implications for resource assessment. Fish. Res., 13: 281-292. Gong, Y., Lee, J.U., Kim, Y.S. and Yang, W.S., 1989. Fishing efficiency of Korean regular and deep longline gears and vertical distribution of tunas in the Indian Ocean. Bull. Korean Fish. Sot., 22: 86-94. Gray, C.A. and Otway, N.M., 1994. Spatial and temporal differences in assemblages of demersal fishes on the inner continental shelf off Sydney, south-eastern Australia. Aust. J. Mar. Freshwater Res., 45: 665-676. Gray, C.A., McDonall, V.C. and Reid, D.D., 19910. By-catch from prawn trawling in the Hawkesbury River, New South Wales: species composition, distribution and abundance. Aust. J. Mar. Freshwater Res., 41: 13-26. Gulland, J.A., 1969. Manual of methods for fish stock assessment. Part I. Fish population analysis. FAO Man. Fish. Sci., 4: 1-154. Hollander, M. and Wolfe, D.A., 1973. Nonparametrical Statistical Methods. Wiley, New York. Hovgard, H. and Riget, F.F., 1992. Comparison of long-line and trawl selectivity in cod surveys off West Greenland. Fish. Res., 13: 323-333.
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Hutchins, B. and Swainston, R., 1986. Sea Fishes of Southern Australia. Swainston, Perth, W.A.. Imai, T., 1972. Studies on the several raw fish baits in a tuna long-line. Fishing-I. Mem. Fat. Fish. Kagoshima Univ., 21: 45-50. Isaksen, B., Valdemarsen, J.W., Larsen, R.B. and Karlsen, L., 1992. Reduction of by-catch in shrimp trawl using a rigid separator grid in the aft belly. Fish. Res., 13: 335-352. Kailola, P.J., Williams, M.J., Stewart, P.C., Reichelt, R.E., McNee, A. and Grieve, C., 1993. Australian Fisheries Resources. Inprint, Brisbane, Qld. Kennelly, S.J., Keamey, R.E., Liggins, G.W. and Broadhurst, M.K., 1992. The effect of shrimp trawling bycatch on other commercial and recreational fisheries-an Australian perspective. Proc. Int. Conf. on Shrimp Bycatch, Lake Buena Vista, Florida. Southeastern Fisheries Association, Tallahassee, FL, pp. 97-l 13. Kennelly, S.J., Graham, K.J., Montgomery, S.S., Andrew, N.L. and Brett, P.A., 1993. Variance and cost-benefit analyses to determine optimal duration of tows and levels of replication for sampling relative abundances of species using demersal trawling. Fish. Res., 16: 5 l-67. Leaman, B.M., 1981. A brief review of survey methodology with regard to groundfish stock assessment. Can. Spec. Publ. Fish. Aquat. Sci., 58: 113- 123. Lokkeborg, S., 1991. Fishing experiments with an alternative longline bait using surplus fish products. Fish. Res., 12: 43-56. Lokkeborg, S. and Bjordal, A., 1992. Species and size selectivity in longline fishing: a review. Fish. Res., 13; 31 l-322. Main, J. and Sangster, G.I., 1983. Fish reactions to trawl gear-a study comparing light and heavy ground gear. Scot. Fish. Res. Rep. 27: l-17. McCracken, F.D., 1963. Selection by codend meshes and hooks on cod, haddock, flattish and redtish. Int. Comm. Northwest Atl. Fish. Spec. Publ., 5: 131-155. Millar, R.B., 1992. Estimating the size-selectivity of fishing gear by conditioning on the total catch. J. Am. Stat. Assoc., 87: 962-968. Millar, R.B. and Walsh, S.J., 1992. Analysis of trawl selectivity studies with an application to trouser trawls. Fish. Res., 13: 205-220. Otway, N.M. and Craig, J.R., 1993. Effects of hook size on the catches of undersized snapper Pagrus auratus. Mar. Ecol. Prog. Ser., 93: 9-15. Otway, N.M., Gray, C.A., Craig, J.R., McVea, T.A. and Ling, J.E., 1996. Assessing the impacts of deepwater sewage outfalls on spatially and temporally variable marine communities. Mar. Environ. Res., 41: 45-71. Pope, J.A., Margetts, A.R., Hamley, J.M. and Akyuz, E.F., 1975. Manual of methods for fish stock assessment. Part III. Selectivity of fishing gear. FAO Fish. Tech. Pap., 41(Rev. 1): l-46. Ralston, S., 1990. Size selection of snappers (Lutjanidae) by hook and line gear. Can. J. Fish. Aquat. Sci., 47: 696-700. Russell, G.M., Gutherez, E.J. and Barans, C.A., 1988. Evaluation of demersal longline gear off South Carolina and Puerto Rico with emphasis on deep-water reef stocks. Mar. Fish. Rev., 50: 26-31. Saetersdal, G., 1963. Selectivity of longlines. Int. Comm. Northwest Atl. Fish. Spec. Publ., 5: 189-192. Saila, S.B., 1983. Importance and assessment of discards in commercial fisheries. FAO Fish. Circ., 765.62 pp. Sakagawa, G.T., Coan, A.L. and Bartoo, N.W., 1987. Patterns in longline fishery data and catches of Bigeye Tuna Thunnus obesus. Mar. Fish. Rev., 49: 57-66. Scheffe, H., 1959. The Analysis of Variance. Wiley, New York. Snedecor, W.G. and Cochran, G.W., 1980. Statistical Methods, 7th edn. Iowa State University Press, Ames, IA. Underwood, A.J., 1981. Techniques of analysis of variance in experimental marine biology and ecology. Oceanogr. Mar. Biol. Annu. Rev., 19: 513-605. 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. Wardle, C.S., 1983. Fish reactions to towed fishing gears. Experimental Biology at Sea. Academic Press, New York, pp. 167-195. Winer, B.J., 1971. Statistical Principles in Experimental Design. Vol. 2. McGraw-Hill, Kogakusha, Tokyo.
Fisheries
Research28 ( 1996) 119- 132
Gear-dependent size selection of snapper, Pap-us auratus N.M. Otway *, J.R. Craig, J.M. Upston N.S.W.
Fisheries Research Institute, P.O. Box
2 I, Cronulla, N.S.W. 2230, Austrulia
Accepted24 February 1996
Abstract The size-compositions of snapper, Pagrus auratus, caught using demersal trawls and longlines were compared for six sites sampled off Sydney, N.S.W., Australia. We predicted that if gear-dependent selectivity occurred, it would be evident as changes to (1) the mean size of individuals, (2) the variance in size of individuals, or (3) both the mean and variance in size of individuals. Neither technique caught individuals less than 90mm fork length and catches exhibited temporal fluctuations during the 4years of sampling. Although trawls were more productive and provided almost 65% of the entire catch, snapper caught on longlines were significantly larger in size and covered a wider range of sizes. These results were also manifest across sites despite the significant spatial differences in the mean size of individuals. The differential gear selectivity also affected catches of undersized and legal-sized snapper. Trawls caught significantly smaller undersized and legal-sized snapper compared with longlines. Several hypotheses were proposed to account for the absence of juvenile snapper in the catches of both techniques. The results were consistent with our last prediction in that both the mean and variance in size differed between trawls and longlines. Finally, the results are discussed in terms of their implications for fishery-independent, standardized trawl surveys and the subsequent use of the resultant data in stock assessment models. Keywords: Longlines; Trawls; Size-selection; Snapper
1. Introduction Management of many important demersal fisheries is often reliant on fishery-independent trawl surveys and the results of stock assessment models (Doubleday and
* Corresponding author. 01657836/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. PII SO 165-7836(96)00500-O
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Rivard, 1981; Godo and Walsh, 1992). Ideally, the trawls used in these surveys should not be size-selective, but this is clearly not the case. Factors affecting trawl selectivity include the size of the codend (Millar and Walsh, 1992), the type and size of the codend mesh (McCracken, 1963; Walsh et al., 1992), the length of sweeps (Engas and Godo, 1989; Andrew et al., 1991), fish morphology (Fonteyne and M’Rabet, 1992), duration and speed of the tow (Isaksen et al., 1992; Kennelly et al., 1993), and the presence or absence of by-catch reducing devices (Isaksen et al., 1992). The selectivity of trawls, together with the often large inter-annual variability in abundance estimates has led several workers (e.g. Godo and Sunnana, 1992) to urge that survey data be critically examined before their use in stock assessment models. Whereas early studies (e.g. Gulland, 1969; Pope et al., 1975) provided the stepping stones, more recent research (e.g. Millar, 1992) has led to the development of better techniques for quantifying trawl selectivity. There are, however, still some concerns about the most recent approaches because, in isolation, they may still not indicate whether the particular trawl is sampling all or some fraction of the population actually living in a given area (Godo and Walsh, 1992). Doubts along these lines can be answered, to some extent, by comparing the catches of trawls with those of a vastly different technique such as longlines. However, such comparisons may still be problematic because longlines are also size-selective (see Lokkeborg and Bjordal (1992) for a review). Like that of trawls, longline selectivity is also affected by numerous factors; these include the type of substratum (Cross, 1988), the depth of water (Sakagawa et al., 1987; Gong et al., 1989), the type and size of the bait (Imai, 1972; Bjordal, 1983; Lokkeborg, 1991), hook-size and/or shape (McCracken, 1963; Saetersdal, 1963; Ralston, 1990; Otway and Craig, 1993), and whether the lines are set on or off the bottom (Arimoto, 1984; Russell et al., 1988). Although researchers conducting fishery-independent trawl surveys are obviously aware of gear selectivity, fewer than half actually document this for the gear used (Table 1). It is possible that such information is available, but has simply not been published. Alternatively, the selective nature of the gear used may not have been quantified. The latter scenario has obvious ramifications for stock assessment models and the use of the modelling results in fisheries management. Fishery-independent trawl surveys in New South Wales, Australia, have mainly
Table 1 Number of papers published Fisheries Management
data on the selectivity
in Fisheries Research, Fishery Bulletin and the North Americun Journal of describing fishery-independent trawl surveys that provided of the trawl used or cited a study that contained the relevant information over the period 1984-1993
Journal
Total number of papers on fishery-independent trawl surveys
Number (o/o) of papers with data on the selectivity of trawl or a citation to a relevant study
Fisheries Research
17
9 (52.9)
Fishery Bulletin
20
5 (25.0)
North American Journal of Fisheries Management
10 47
5 (50.0) 19 (40.4)
Total
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I32
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concentrated on estuaries (e.g. Gray et al., 1990, Ferrell and Bell, 1991) and thus little is known about the distributional patterns of demersal fish in coastal shelf waters. However, more recently the spatial and temporal patterns of abundance of demersal fish inhabiting the coastal waters off Sydney were documented (using trawls) as part of a large environmental study (see Otway et al. (1996) for details). Whereas the trawl survey (Gray and Otway, 1994) was based on a stratified sampling design and demonstrated clear depth-related patterns of abundance and species composition, little was known about the selectivity of the trawl used. However, the size-age structures of populations of commercially and/or recreationally important species were not analysed. In a contemporaneous study using longlines, Otway and Craig (1993) examined hook selectivity and its effects on estimating the population size-structure of snapper in Sydney’s coastal waters. Consequently, in this paper we look for evidence of gear-dependent selectivity by comparing the size-frequency distributions of snapper caught in trawls and on longlines. If gear-dependent selection is evident, we predict that the differences in size-structure will arise in at least one of three ways. First, the mean size of individuals caught by each technique will not be altered, but the variance in the size of fish caught will differ significantly (Fig. I(a)>. Second, the mean size of individuals caught by each technique will significantly differ, but the variance in the size of fish caught will not be altered (Fig. l(b)). Third, the mean size and variance in size of individuals caught by each technique will both differ (Fig. l(c)). Although this comparison will only provide information on the selectivity of the trawl relative to the longlines. it should help to identify any possible bias that would need to be recognised in a future analysis of the size-frequency data emanating from the trawl survey by Gray and Otway ( 1994).
2. Materials
and methods
2.1. Species studied The snapper Pagrus aurutus (Bloch & Schneider, 1801) is demersal and grows to the minimum legal size of 280mm total length (about 250mm fork length, FL) in about 3 years (Bell et al., 1991). The species is currently fished commercially off the entire coast of New South Wales using mainly baited traps, although significant numbers of fish are caught on longlines and handlines (Kailola et al., 19931, and as legal by-catch (sensu Saila, 1983) of the oceanic prawn-trawl fleet (KeMelly et al., 1993). Whereas the species is reported to reach a maximum FL of 1300mm (Hutchins and Swainston, 1986), commercial and recreational fishers rarely catch individuals exceeding 800 mm FL (D.J. Ferrell and A.S. Steffe, unpublished data, 1995). 2.2. Field sampling Sampling was done in the coastal shelf waters off Sydney (N.S.W., Australia) in waters less than 1OOm deep. Fish were caught with demersal otter trawls and longlines using a 13 m, 114kW trawler (F.R.V. ‘Kamala’) and a 7 m sharkcat (F.R.V. ‘Wobbe-
122
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a.
Fork length Fig. 1. Predicted population size-structure arising through (a) significant differences in the variance in the size of fish caught, (b) significant differences in the mean size of individuals caught, (c) significant differences in the mean size and variance in size of individuals if gear-dependent selectivity occurs. Hypothetical population size-structure of P. aurarus caught using trawls (continuous line) and longlines (broken line).
gong’), respectively. Trawling and longlining were done at the same six sites at about 3 month intervals from autumn 1989 to autumn 1993 (Fig. 2). All sampling was restricted to daylight hours and was done on consecutive days, seas and weather permitting. The fork lengths of all fish caught were measured to the nearest millimetre. 2.3. Demersal otter trawls Demersal trawling was done using a 32m headline ‘Kent Bollinger’ wing trawl with 72m sweeps and 22m bridles. The net had 229mm mesh in the wings, a body with 229 mm mesh gradually reducing to 57 mm, and a cod-end of 42 mm diamond-shaped mesh. The net was towed at 2.5 knots and with a ratio of length of warp to depth of 3: 1 (for further details see Gray and Otway (1994) and Otway et al. (1996)). At each site, three replicate trawls, each of 20min bottom-time, were done over non-overlapping patches of ground.
NM.
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et al./
Fisheries
Research
28 (1996)
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0
123
1Okm
I
I
Scale /;;
151020’
151 O30’E
--I
Fig. 2. Map showing the location of the six sites sampled in the coastal waters off Sydney, New South Wales, Australia.
2.4. Demersal longlines The demersal longlines used have been described in detail by Otway and Craig (1993) and only a brief account is given below. Each longline consisted of a 76m ground-line weighted at both ends; a 90- 120 m buoy-line and a danbuoy. Thirty-three hooks, attached to 50 cm snoods of monofilament line, were clipped onto the ground-line at 2 m intervals as the lines were set. The hooks were baited with uniformly sized pieces of squid and stored in a freezer until the day of sampling. At each site, 12 replicate longlines (n = 396 hooks in total) were set on the bottom and parallel to the depth contours. Each longline was allowed to fish for an average of 2.5 h and then hauled. Tuna-circle hooks of three different sizes (‘Mustad’ Nos. 12, 10 and 8-see Fig. 1 and Table 1 of Otway and Craig (1993)) were used; however, data for the individual hook-sizes were pooled for the purposes of this experiment. 2.5. Statistical analyses Size-frequency distributions were compared using Kolmogorov-Smimov tests (HOIlander and Wolfe, 1973). Differences attributable to the fishing gear used and/or spatial variation in population size-structure of snapper within the sampling region were examined using analyses of variance after preliminary tests for heteroscedasticity
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(Cochran’s test, Winer, 1971). When variances were heterogeneous, data were transformed using the procedures outlined by Scheffe (1959) and Winer (1971). When heteroscedasticity persisted, the analysis was done using a more conservative Type I error-rate of (Y= 0.01 (Underwood, 1981). Significant differences among means were identified by Student-Newman-Keuls (SNK) tests (Snedecor and Cochran, 1980).
3. Results A total of 774 snapper was caught over the entire sampling period, 64.6% in trawls and 35.4% on longlines. The numbers caught by both techniques exhibited substantial temporal variability and thus it was not possible to incorporate time as a factor in the analyses. Consequently, all analyses were done by pooling across times for individual sites within the sampling region. It was possible, however, to include ‘sites’ as a factor in some of the analyses, but only at Sites 2, 3 and 4 because of reduced catches at the remaining sites. 3.1. Mean size of snapper The size-frequency distributions of P. auratus pooled across all sites and at Sites 2, 3 and 4 (Fig. 3) showed that neither trawling nor longlining caught individuals under 90mm fork length (FL). Fig. 3 also indicated that there were gear-dependent and spatial differences in the population size-structure of snapper. Comparisons of the sizefrequency distributions of P. aurafus caught in trawls and on longlines at Sites 2, 3 and 4, and across all six sites were all significantly different (Table 2, Kolmogorov-Smimov tests, P < 0.05 in all tests). This confirmed the existence of gear-dependent selectivity, but did not indicate how it was manifest. The mean (SE) fork lengths of P. auratus (calculated from the length-frequency distributions) caught in trawls at Sites 2, 3 and 4 were, respectively, 261.7 (8.3)mm, 197.5 (4.5)mm and 191.3 (1.4)mm, whereas those caught on longlines were, respectively, 314.9 (7.0)mm, 248.2 (7.8)mm and 293.0 (9.5)mm. When pooled across sites, the mean fork lengths of P. auratus caught in trawls and on longlines were respectively, 209.8 mm and 295.7 mm. This 85.9 mm difference in the mean fork lengths suggested that the snapper caught on longlines were larger than those caught in trawls. This was investigated further by comparing the mean fork lengths of fish caught in trawls and on
Table 2 Results of Kolmogorov-Smimov tests of size-frequency distributions trawls and longlines at Sites 2, 3 and 4, and pooled across all six sites
of P. auratus
Site
x2 (2dfI
P
2 3 4 All
26.5 21.1 137.2 331.0
0.0100 0.0215 O.oool 0.0001
caught
by demersal
N.M. Otwuy et al./Fiskeries
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4 ._ CA
N.M. Otway et al./Fisheries
126 Table 3 Analyses
Research 28 (1996) 119-132
of the mean size of P. auratus caught in demersal
trawls and on longlines
at Sites 2, 3 and 4
(A) Analysis of variance Source of variation
df
MS
F
P
Site Gear Site X Gear Residual Total
2 1 2 198 203
0.2329 0.6902 0.045 1 0.0064
36.49 108.14 1.01
(B) SNK tests: between techniques
at each site
Outcome Longlines > Trawls Longlines > Trawls Longlmes > Trawls
Site 2 Site 3 Site 4
SNK tests: among sites for each technique Outcome Site 4 = Site3 < Site 2 Site 3 < Site 4 = Site 2
Trawls Longlines
longlines at Sites 2, 3 and 4 using a balanced analysis of variance with n = 34 individuals selected at random from the total numbers of fish caught by each technique at each site. This analysis showed that snapper caught on longlines were always significantly larger than those trawled (Table 3; analysis of variance and SNK test of means in Fig. 4, P < 0.05). The analysis also confirmed the presence of spatial differences in the mean size of snapper suggested by the size-frequency distributions. First, significantly larger P. aurutus were trawled at Site 2 compared with those at Sites 3 and 4, which did not differ, and second, the snapper caught on longlines were q Longlining n Trawling
4*l
Site Fig. 4. Mean ( + SE) fork length of P. auratus caught using trawls and longlines at Sites 2, 3 and 4.
NM. Otway er d/Fisheries Table 4 Comparison 2,3 and 4
of the mean variance
Research 28 (1996) 119-132
in length of P. aururus
Mean (SE) variance
caught in demersal
12-l
trawls and on longlines at Sites
Homogeneity of variances (2-tailed F, df = 2.2)
Comparison of means (2-tailed I, df = 4)
Trawls
Longlines
F
P
f
P
I347 (660)
4845 (993)
0.44
0.61
2.93
0.04
significantly larger at Sites 2 and 4 compared with those at Site 3 (Fig. 4 and Table 3; analysis of variance and SNK tests of means, P < 0.05). 3.2. Variance in size of snapper The size-frequency distributions of P. aurutus pooled across all sites and at Sites 2, 3 and 4 (Fig. 3) also suggested gear-dependent differences in the variances of the distributions. This was examined further by calculating the variance of each sizefrequency distribution for snapper caught by trawling and longlining. The respective variances at each site were then used as independent (replicate) estimates of the true variance in size for snapper caught in trawls and on longlines. This allowed a comparison of the means of variance in the length of snapper caught in trawls and on longlines. Table 4 shows that the mean variance in the length of snapper was significantly greater for individuals caught on longlines compared with those trawled. 3.3. Legal vs. illegal snapper The data from longlining and trawling were examined further to address concerns over the capture of individuals under the minimum legal size of 280mm total length (about 250mm FL). Trawling caught a total of 500 individuals from all sites over the entire sampling period and of these, 89% were under the minimum legal size (Fig. 3). By comparison, only 26% of the 274 snapper caught on longlines were under the minimum legal size (Fig. 3). More detailed analysis of legal-sized snapper (250mm or more FL) showed that a significantly larger proportion were caught on longlines compared with trawls (Table 5, x2 = 319.25, P < 0.0001). In addition, snapper caught on longlines were significantly larger than those caught in trawls (Fig. 5 and Table 6; analysis of variance, P < 0.0001). Analysis of snapper under the legal size (i.e. less than
Table 5 Numbers longlines Technique
Trawls Longlines
and percentages of legal (L) and illegal (I) sized pooled across sites Numbers
P. auratus
% of method
caught
in demersal
trawls and on
% of total
L
I
L
I
L
I
53 202
447 72
10.6 73.7
89.4 26.3
6.8 26.1
57.8 9.3
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El
Longlining
n
Trawling
Illegal
Size Fig. 5. Mean ( + SE) fork length of legal-sized pooled across all sites.
and undersized
P.
uuratuscaught using trawls and longlines
Table 6 Analysis of the mean size of legal and illegal P. auratus caught in demersal
trawls and on longlines
Source of variation
df
MS
F
P
L vs. I Method L vs. IX Method Residual Total
1 1 1 208 211
1.8166 0.1394 0.0064 0.0063
287.36 22.04 1.02
< O.OWl < 0.0001 0.3149
A Type I error-rate at P = 0.05.
of Q = 0.01 was used because Cochran’s
test of the transformed
data was still significant
250 mm FL) indicated that a significantly larger proportion of illegal-sized snapper were caught in trawls compared with on longlines (Table 5, x2 = 319.25, P < 0.0001). Furthermore, the mean size of snapper caught in trawls was significantly smaller than the mean size of individuals caught on longlines (Fig. 5 and Table 6; analysis of variance, P < 0.0001). 4. Discussion Neither fishing technique caught P. aurutus under 90mm FL (i.e. under 1 year old), but this result was not unexpected as several previous studies (e.g. Otway and Craig, 1993; Gray and Otway, 1994; Otway et al., 1996) have not caught juvenile snapper in the inshore coastal waters of central N.S.W. These results are also consistent with those of a demersal fish survey using a purpose-built Stebbenhauser trawl with 25mm mesh throughout (M.P. Lincoln Smith, personal communication, 1995). The latter study was done in the same general area off Sydney and only caught snapper exceeding 110 mm FL. However, juvenile snapper (20-170mm FL) are caught in significant numbers as by-catch of the estuarine prawn-trawl fishery (Gray et al., 1990; Kennelly et al., 1992). These observations suggest that the absence of juvenile snapper in the trawls and
NM. Otwuy et al./ Fisheries Reseurch 28 (1996) 119-132
129
longlines may result from a preference for the large, open estuaries over the inshore coastal waters along this part of the N.S.W. coast. Alternatively, juvenile snapper may indeed frequent inshore coastal waters, but their capture is prevented in other ways. First, the selectivity of the trawl and longlines may have prevented the capture of juveniles. Although this may indeed be true for the longlines (e.g. Otway and Craig, 1993) this is probably not so for the trawls, because the catches also included large numbers of Trachurus nouaezelandiae, Zeus faber, Anomalus inermis and Lepidotrigla argus with minimum fork-lengths of 65 mm, 75 mm, 60mm and 70mm, respectively. This suggests that juvenile snapper would also have been caught had they been present at the sites sampled. Second, juveniles may have avoided capture through ontogenetic changes in habitat usage in coastal waters. For example, juvenile snapper may congregate on or near rocky reefs and only venture out over trawlable grounds when older. Such behaviour would, however, be at odds with the juveniles trawled in estuaries. Clearly, more detailed sampling will be needed to differentiate between these hypotheses. The longlines caught snapper to a maximum size of 620mm FL compared with 450mm FL in the trawls. The absence of large snapper (FL greater than 620mm) on longlines was probably due to their rarity at the sites sampled. This is further supported by the size of individuals caught by recreational anglers in Sydney’s coastal waters (see Section 2). In contrast, the absence of individuals larger than 450mm FL in the trawls was probably due to the 20min tow duration. Studies (e.g. Isaksen et al., 1992) have shown that trawls of longer duration catch significantly larger individuals. Furthermore, the amount of time taken to exhaust a fish when herded by a trawl is most probably size dependent, and this has been postulated as the most likely reason for the reduced capture of larger individuals in trawls of limited tow duration (Wardle, 1983; Main and Sangster, 1983). Comparisons of the size-frequency distributions of P. auratus caught on longlines and in trawls demonstrated marked gear-dependent selectivity. The pattern of selection was consistent across sites, with significantly larger snapper covering a wider range of sizes being caught on longlines. These results are consistent with previous studies of cod, off Norway (Saetersdal, 1963) the Gulf of St. Lawrence (McCracken, 1963) and Greenland (Hovgard and Riget, 1992), which showed that longlines caught larger individuals covering a wider range of sizes. The gear-dependent selection resulted in a 85.9mm difference in the mean fork lengths of snapper caught on longlines and in trawls. However, in a previous study on longline selectivity, Otway and Craig (1993) showed that significantly larger P. auratus (mean FL 323.2mm) were caught on Number 8 compared with Number 10 and 12 Mustad tuna-circle hooks (mean FL 305.8 mm and 277.3 mm, respectively). Consequently, if the individuals caught on the Number 10 and 12 hooks were eliminated from the data examined here, the magnitude of the difference in the mean fork lengths of snapper caught by trawling and longlining, pooled across all sites, would be increased by a further 27.5 mm (i.e. from 85.9 to 113.4mm). This is equivalent to over half the annual growth increment for snapper with a mean size of approximately 210 mm FL (Francis and Winstanley, 1989). Gear-dependent selection was also evident when undersized and legal-sized P. auratus were examined. Longlines caught significantly larger undersized and legal-sized
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snapper compared with trawls. This result was also consistent across all sites despite spatial differences in the mean fork length of local populations of P. auratus. Again, if the individuals caught on the Number 10 and 12 hooks were eliminated from the data, then the difference in the numbers of undersized individuals caught by trawling and longlining, pooled across all sites, would be increased by a further 16%. By comparing the catches of snapper in trawls and on longlines this study has shown that the population size-structure can be significantly modified by the fishing gear used, a fact that was well documented in earlier studies (e.g. Allen, 1963; Gulland, 1969; Pope et al., 1975). These modifications follow the model illustrated in Fig. l(c) in that the mean size and variance in size of individuals caught by each technique differed significantly. The trawl clearly underestimated the number of larger individuals and range of sizes in the population. We feel that this particular limitation can be overcome by increasing the trawl duration and/or the towing speed, but this will need to be tested experimentally. Moreover, the results of this study should permit a more informed analysis and interpretation of the size-frequency information obtained in the trawl survey by Gray and Otway (1994) of demersal fish communities off Sydney. For example, a high proportion of juveniles in the catch might have suggested that the site was a nursery habitat for the particular species. However, this study has shown that the trawl selectively captured proportionally more small individuals, and this demands that such a conclusion be modified. Although a range of explanations could be proposed to account for the possible patterns observed, it is clear that data on gear selectivity will allow one possible source of confounding to be eliminated. 4.1. Implications for future research and management It is obvious that the trawl used in this study could also be used in more widespread, fishery-independent, standardised trawl surveys documenting the abundances and sizeage structures of commercially important species along the N.S.W. coast. If such an approach is used, it will be extremely important to ensure that a ‘standard survey’ can be repeated through time at a number of sites and that the resulting data are free from bias. To this end, the quality of standardised trawl surveys would be greatly improved if more is known about the selectivity of the fishing gear used. This could be achieved by ensuring that appropriate field experiments are used to identify the selective nature of the gear. Obviously, such experiments would need to be done at several sites and repeated through time to quantify the degree of spatial and temporal variation in selectivity of the gear. Not only would these experiments identify the potential biases in demographic studies, but their results could also be incorporated into cost-benefit analyses for optimising the design of fishery-independent surveys. However, the results of cost-benefit analyses for multi-species fisheries are less definitive because levels of replication may be optimal for some species and sub-optimal for others (Leaman, 1981). This was clearly the case for several species of fish caught as by-catch in the oceanic prawn-trawl fishery off New South Wales (Kennelly et al., 1993). Furthermore, as trawl selectivity can bias the size-composition of the catch it will be necessary to ensure that tow durations and levels of replication, as determined by cost-benefit analyses, also provide unbiased estimates of population size-structure. Only then will standardised surveys provide robust inputs into stock assessment models.
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Acknowledgements
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