The effect of hook- and bait sizes on size selection of Cod, Haddock and Wolffish in longline fisheries

Transcription

1 Not to be cited without prior reference to the authors ICES CM 2009/M:18 The effect of hook- and bait sizes on size selection of Cod, Haddock and Wolffish in longline fisheries Ólafur Arnar Ingólfsson and Haraldur Arnar Einarsson Abstract This study demonstrates the effects of hook- and bait sizes on size selection of three commercial species, thereby bycatch of juvenile fish. The effects of hook and bait sizes on fishing efficiency and size composition of cod (Gadus morhua), haddock (Melanogrammus aeglefinus) and wolffish (Anarhichas lupus) were investigated in a designed experiment in the Icelandic longline fishery. Five hook sizes, EZ 10-14, and two bait sizes, ~10 and 30g, were compared. The bait was Saury (Cololabis saira). Bait sizes affect catch rates and size selection in longline fishery for haddock, cod and wolffish. Higher catch rates of haddock were obtained with small bait, but large bait caught cod and wolffish more effectively. The proportion of fish caught with large v.s. small bait increased significantly with fish size for all three species, and the relationship can be described by a sigmoid shaped curve. Smaller hooks result generally in higher catches, but there were interactions between bait- and hook sizes for cod and haddock; hook sizes affected size selection of cod when small baits were used and haddock selection when large baits were used. Proportions of catches of juvenile fish appear to be inversely related to fish abundance. Keywords: longline, hook size, bait size, cod, haddock, wolffish, selectivity Ólafur Arnar Ingólfsson, Marine Research Institute. Árnagata 2-4, 400 Ísafjörður, Iceland, olafur@hafro.is, tel Haraldur Arnar Einarsson, Marine Research Institute, P.O. box 1390, 121 Reykjavík, Iceland, haraldur@hafro.is. Tel , fax Page 1

2 Introduction Longline fishery is an important fishing method in Iceland and the catch proportion taken by longliners has gradually increased the past several years (Anon 2009). The targeted species are mainly cod, haddock and wolffish. In spite of the common belief that longline is more environmental friendly than most other commercial gears, the closure of fishing areas in Icelandic waters is most often due to high proportion of fish below minimum legal size (MLS) in longline catches (Anon 2008). Discarding of undersized fish is illegal in Icelandic waters, but discrepancies in length distributions from landed catches and catches measured by inspectors at sea have shown that discarding can be considerable (Pálsson 2003). Mortality experiments on cod and other species released from hooks have shown that mortality rates can be considerable and often inversely related to fish length (Pálsson et al. 2003). Mortality of fish escaping off hooks during the hauling phase has to the best of our knowledge not been investigated. The Icelandic longline fishery is quota regulated with Individual Transferable Quotas (ITQ). Fishermen may be in the position of having quota for one species but not the other. This also stresses the need to be able to select species. Knowledge of species and size selectivity and efficiency in longline fisheries is thus important to both fisheries managers for regulating the fishery and the fishermen to optimize their revenues. Experiments have shown that bait size affect size selection of fish in longline fisheries (Løkkeborg 1995), but larger bait increases the cost, hence tendency for using smaller bait. Species selectivity can be influenced by using different bait types (Løkkeborg and Bjordal 1992, Pol 2008). Halliday (2001) showed effect of hook size on size selection of cod and haddock, but the results were confounded with bait size as he increased bait size with hook size. In this study we conducted repeated experiments in fisheries relevant to our circumstances. Icelandic fishermen mostly use EZ hooks of sizes 11 and 12 or J- hooks of comparable sizes. Bait sizes used by fishermen vary from approximately 10 to 30g. In addition to these commercial hook sizes, we tested smaller and larger hooks. In addition we tested small (10g) and large bait (30g) on all sizes of hooks. Page 2

3 Materials and methods Five fishing experiments were conducted from November 2008 to April 2009 in the fishery for cod, haddock and wolffish in the coastal banks off NW Iceland at depths of 50 to 140 m. The experiments were conducted on a 13.4 m commercial longliner, Ramona IS-190. A 9 mm swivel longline was used with EZ type (semi-circle) hooks, sizes From end of the line to first hook was a 10 m space, then came 20 hooks of one size and then 20 hooks of the next size etc. The first 100 hooks were baited with 10g bait (saury), after series of 100 hooks there was 10m spacing and then another series with the same hook sizes but 30g baits. At the beginning and end of 100 hook series, there were tags with information on bait and hook sizes. At the beginning and end of every 20 hooks of same size, twines of different colours (yellow, red, green, blue, grey) were tied to secure that hook sizes were identified during hauling. The lines were coiled in plastic stamps, 300 hooks pr. stamp. Except for the first trip, the total number of hooks in a trip was The bait was cut in a cutting machine, i.e. series of knives on an axle, and bait sizes determined by adjusting the gap between the knives. Head and tail peaces of the saury were discarded, the foremost bait being cut at the pectoral fin. The 10 g bait were cut using 15-17mm knife gaps, but the 30g baits were cut by increasing the gaps to 42-47mm. The baits were then sorted by hand, and a sample of >30 pieces weighted. After setting the line it was set for approx 1h before hauling at speed of hooks h -1. All fish were measured and recorded. Size distribution of fish for all hook and bait sizes was compared to investigate size selectivity and catch efficiency. Proportion of fish below minimum legal size (MLS) was calculated (45 cm for haddock, 50 cm for cod). Relationship between fish length, hook- and bait sizes is calculated as a linear function with mixed-effects model, allowing for analyzing the data in one run with trips as random effects (Pinheiro and Bates 2000). We used the function lme in R (R Development Core Team 2007). Number of fish caught with 30g bait was also calculated as proportion of fish calculated with 10 and 30g bait with GLM, logit link. In addition, differences in proportion of fish below MLS were tested with t- test. Page 3

4 Figure 1 Setup for the longline experiment showing the bait- and hook sizes. Twenty hooks of the same size in a row, one series consists of 100 hooks with the same bait size. The series are identical except that bait sizes are swapped. Space between hooks is 1.2m. Figure 2 Hooks and baits used in the experiments. EZ hooks no. 10 to 14 (from left to right), 10 and 30g saury baits. Page 4

5 Results Five trips have been conducted, one in November 2008, in January, February and two in April On the first trip, 2400 hooks were set out, but 4800 in the other trips. Table 1 Summary table showing the actual weight of baits (weight of <30 pieces along with s.d.) and catc in numbers for all trips. Catch in numbers Trip no 10g bait weight in g (s.d.) 30g bait weight in g (s.d.) Haddock Cod Wolffish Other (1.4) 28.0 (4.8) (2.0) 28.7 (4.8) (1.8) 32.1 (3.6) (2.3) 33.4 (4.5) (1.7) 32.8 (4.5) Haddock In November, 262 haddock were caught, 196 in January and 118 in February (Table 1). The Haddock catches were higher on the smaller bait, but the proportion caught on the larger bait increased with fish size. At 63cm fish length, catches were about equal for both bait sizes (Figure 3). Most haddock was caught on the smallest hook with the smallest bait (Table 3). Total weight of large haddock with the smaller bait was also higher than with the larger bait (Table 3). The haddock on the smaller bait was on average smaller than on the large one (t-test, p<0.05). Proportion of fish below MLS, caught on the smaller bait was 50% on average but 26% on the larger one (t-test, p<0.05). There were interactions between bait size and hook size (Table 2); when small bait was used, hook size did not affect average length of haddock (on average 44.6cm), but when large bait was used, the average length was 45.0cm on hook no. 10, increasing of 0.9cm for every increase in hook size. Table 2 Results from a linear mixed effect model for Haddock, fish size as response variable Explanatory variable Estimate Std.error DF p-value Intercept <0.001 Hook Factor (bait 30g) Hook:factor (bait 30g) σ Intercept 2.64 Page 5

6 Table 3 Haddock results from three of the first trips where haddock catches were more than 50 specimens. Hook size Bait size (g) No. fish Mean length % Below MLS Weight pr 500 hooks (all sizes) Weight pr 500 hooks (large fish) Trip 1 Trip 2 Trip Cod In November, 254 cod were caught, in January 416 and 83 in February (Table 1). The cod catches were higher with the larger bait and highest with the smallest hook and the large bait (Table 5). Proportion caught on the larger bait increases with fish length, and at 50 cm fish length, catches of cod were about equal for both bait sizes (Figure 3). The cod caught on the smaller bait was on average smaller than on the larger one (t-test, p<0.05) and the proportion of fish below MLS thus higher on the larger bait (72% on average) than on the larger one (39%) (t-test, p<0.05). When large bait was used, the average size of cod was 41.1 cm on hook no. 10 and increased by 1.6cm for every increase in hook size. When large bait was used, average size of cod was 55.0cm, independent of hook size (Table 4). Total weight of cod above MLS was Page 6

7 significantly higher on the large bait compared to small bait (Table 5). Large bait caught on average 30 kg more of fish above 50cm pr 500 hooks than small bait. Table 4 Results from a linear mixed effect model for Cod, fish size as response variable Explanatory variable Estimate Std.error DF p-value Intercept <0.001 Hook Factor (bait 30g) <0.001 Hook:factor (bait 30g) σ Intercept 2.96 Table 5 Cod results from three of the first trips where cod catches were over 50 of in total. Weight pr 500 Hook size Bait size (g) No. fish Mean length % Below MLS hooks (all sizes) Trip 1 Weight pr 500 hooks (large fish) Trip 2 Trip Page 7

8 Wolffish In the first wolffish trip (fourth trip, April 2009) 810 Kg of wolffish were caught but significantly less in the later trip or 190 Kg. Both bait- and hook sizes affected size selection (Table 6). On average, wolffish caught on 30g bait was 7cm larger (0.6 Kg heavier) than wolffish caught on 10g bait. Bait size also affected catch quantity. The catches were higher on the larger bait by 69% in the fourth trip, but 21% in the fifth. Hook size also affected average size, which increased by 1.27cm or 0.1 Kg by every increase in hook size. When the catches are investigated by length, independent of hook size, the larger bait caught relatively less proportion of the smallest fish, but the proportion increases with fish length. The bait sizes caught fish size of 57cm equally (Figure 4). Table 6 Results from a linear model for Wolffish, fish size as response variable Explanatory variable Estimate Std.error DF p-value Intercept <0.001 Hook <0.001 Factor (bait 30g) <0.001 Table 7 Wolffish results from three of the first trips where wolffish catches were more then 50 specimens. Trip 4 Trip 5 Hook size Bait size (g) No fish Mean length (cm) Mean weight (Kg) Weight pr 500 hooks Total: Total: 153 Page 8

9 Proportion of cod caught with 30g bait by length 1.0 Proportion caught with 30g bait Fish length (cm) Proportion of haddock caught with 30g bait by length 1.0 Proportion caught with 30g bait Fish length (cm) Figure 3 Proportion of cod and haddock caught with 30g bait compared to that of 10g (all hook sizes pooled). The broken lines show where 10g and 30g baits catch equally related to best fitted (logistic) curve shown on the figures. Page 9

10 Proportion of wolffish caught with 30g bait by length 1.0 Proportion caught with 30g bait Fish length (cm) Figure 4 Proportion of wolffish caught with 30g bait compared to that of 10g (all hook sizes pooled). The broken lines show where 10g and 30g baits catch equally related to best fitted (logistic) curve shown on the figures. Discussion These investigations show that hook- and bait sizes affect efficiency and size selectivity in the longline fisheries for cod, haddock and wolffish. Higher catches are in general obtained with smaller hooks, and there seem also to be interactions between bait sizes and hook sizes. Hook size affected size selection of cod when small bait was used and size selection of haddock when large bait was used. Bait size has considerable effect on both total catch and size selectivity as there was a significant relationship between bait sizes and sizes of fish caught and fish quantity. It is tempting to believe that for a given size of fish there is a profitability threshold (Stephens and Krebs 1986), and that with increasing fish size, small food becomes less profitable. The acceptance for small food particles however, is likely to be dependent on food availability (food encounter rate). The distance between large and small bait was 10m and fish approaching a given bait size thus unlikely to be aware of the choice between bait sizes. The fish therefore does not choose between Page 10

11 alternative bait sizes side by side, but evaluates cost and effort irrespective of other choices. That decision is species-specific, but factors such as hunger level, food availability and bait type possibly affect the choice, and thereby shift in the curves in figures 3 and 4. The curves have asymptotes of 0 and 1, i.e. assume that the largest fish will refuse to take small bait. This is though not necessarily true, the upper asymptote might be < 1, or perhaps a different model than the logistic curve may explain the behaviour better. Most haddock was caught with the smaller bait, but most of cod and wolffish with the big ones. The results concerning the effect of bait size on haddock are in agreement with the results of Johannessen (1983) who caught more than twice the haddock catches when using 10g bait compared to 30g. Our results also show relationship between cod size and proportion caught on the larger bait. In general, this is in agreement with the results of Johannessen (1983), who caught more small cod with small bait. But in contrary to our results, he concluded that at given fish length (~60 cm) 10 and 30 g baits caught cod equally, while we see a trend towards less acceptance for small bait with increasing fish size. The same pattern is also seen when catching wolffish, i.e. big bait big fish. It is, however, apparent that there are variations in results by species. For cod, equal catches were obtained at 50 cm, haddock 63 cm and wolffish 57. When small bait is used, hook size appears to matter for size selection of cod. The lowest average size was caught with the smallest hook. By using large bait, the hook size on the other hand appears not to affect selection in the same way, but most catches were in general taken on the larger bait and the smaller hook. Higher catching efficiency of the smaller hooks is in concordance with other published results (Johannessen 1983). Løkkeborg and Bjordal 1995 showed that by adding a plastic body on hooks to make the bait look bigger, the proportion of small ling and tusk were reduced. Similar experiments in haddock fisheries resulted in lower catches of both small and large haddock (Huse and Soldal 2000). Such comparisons in cod fisheries have, to the best of our knowledge, not been performed. To maximize cod- and wolffish catches, small hook and large bait seem to be the best composition. We must, however, add that other bait sizes, e.g. 20 g have not been tested and the increased catches must compromise increased bait cost. Page 11

12 On the other hand we may expect less haddock catches if bait size is increased. To maximize haddock catches a small hook and small bait would be the best composition. The dark side of that, however, is that bycatches of cod and haddock below MLS would be expected. The proportion of small fish appears to be higher when catches are low. This is in agreement with previously published results (Engås and Løkkeborg, 1994) and the most plausible explanation is that at lower densities there is less competition for food and/or lower proportion of large fish in the area. Acknowledgement The authors thank the AVS national research funding for their contribution to this research. We thank Friðrik Jóhannsson skipper on Ramóna ÍS and his crew for valuable help. Special thanks go to Bjarni Gestsson who has baited the longlines for the trip and the staff of MRI, namely Anna Ragnheiður, Jón Ólafur and Herdís for a job well done. References Anon, Starfsskýrsla Fiskistofu Fiskistofa 2009 (Directorate of Fisheries). Anon, Nytjastofnar sjávar 2008/2009. Aflahorfur fiskveiðiárið 2009/2010. State of Marine Stocks in Icelandic Waters 2008/2009. Prospects for the Quota Year 2009/2010. Hafrannsóknastofnunin, Hafrannsóknir nr. 146 (English summary) Engås, A. and Løkkeborg, S Abundance Estimation using Bottom Gillnet and Longline The Role of Fish Behaviour. In Marine Fish Behaviour. Anders Fernö and Steinar Olsen (Ed). pp Halliday, R.G A comparison of size selection of Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) by bottom longlines and otter trawls. Fisheries Research, 57, Huse, I. and Soldal, A.V An attempt to improve size selection in pelagic longline fisheries for haddock. Fisheries Research, 48, Page 12

13 Johannessen, T Betydning av krok- og agnstørrelse for fangsteffektivitet og lengdeseleksjon i linefisket etter torsk og hyse. Hovedoppgave i fiskeribiologi, Institutt for fiskeribiologi, Universitetet i Bergen. 109 pp. Løkkeborg, S. and Bjordal, Å Species and size selectivity in longline fishing: a review. Fisheries Research (3), 13, Løkkeborg, S. and Bjordal, Å Size-selectivity effects of increasing bait size by using an inedible body on longline hooks. Fisheries Research, 24, Pálsson, Ó.K A length-based analysis of haddock discards in Icelandic fisheries. Fisheries Research, 59, Pálsson, Ó.K., Einarsson, H. and Björnsson, H Survival experiments of undersized cod in a hand-line fishery at Iceland. Fisheries Research, 61, Pol, M.V., S.J. Correia, R. MacKinnon, J. Carver Longlining haddock with manufactured bait to reduce catch of Atlantic cod in a conservation zone. Fisheries Research, 94, Pinheiro, J.C. and Bates, D.M Mixed-effects models in S and S-PLUS. Springer Verlag USA. R Development Core Team (2007). R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. ISBN , URL Stephens, D.W. and Krebs, J.R Foraging Theory. Princeton University Press, UK. 247 pp. Page 13