6.7. RECAPTURE OF ESCAPED JUVENILE morhua) IN NORTHERN NORWAY
COD
(Gadus
Cite this article as: Serra-Llinares RM, Nilsen R, Bjørn PA, Noble C, Uglem I (2013) Recapture of escaped juvenile cod (Gadus morhua) in Northern Norway. In: PREVENT ESCAPE Project Compendium. Chapter 6.7. Commission of the European Communities, 7th Research Framework Program. www.preventescape.eu ISBN: 978-82-14-05565-8
Authors: Rosa Maria Serra-Llinares1, Rune Nilsen1, Pål Arne Bjørn1, Chris Noble1 & Ingebrigt Uglem2 1 2
Nofima Marine, 9291 Tromsø, Norway. Norwegian Institute of Nature Research, Tungasletta 2, 7485 Trondheim, Norway.
INTRODUCTION Atlantic cod (Gadus morhua L.) is an important species for the Norwegian fish farming industry. Due to its highly exploratory behavior, cod seem more prone to escape than other cultured species (Moe et al. 2007). Between 2005 and 2010, on average 1.3% of the live stock of cod kept in the sea escaped annually, adding up to more than 1 million individuals over those years (Norwegian Directorate of Fisheries 2010). Escapes from fish farms are regarded as an environmental problem, and interactions between farmed and wild cod are likely to occur. Effective and practical routines to recapture escapees are urgently needed to reduce the ecological impact of cod escapes, should they occur. In addition, live recapture of escapees could reduce economic losses associated with escape events. The recapture rates of adult cod escapees through local commercial fisheries can be high, even with a small recapture effort (up to 52 % according to Uglem et al. 2008). We aimed to evaluate the efficiency of a recapture program aimed at juvenile cod escapees. For this purpose, a wide range of recapture methods were tested both immediately after a simulated escape and over the longer-term in a surveillance fishery. We simulates a series of escape incidents of cod in Nordfjord (67°07'N, 14°17'E), Gildeskål, Norway (Figure 6.7.1) in 2010 and 2011. During the study period, two cod farming locations were operating in the area, as well as a local, semi-commercial fishing fleet of 5 to 6 boats. Three escapes of approximately 1000 externally tagged juvenile cod each were simulated during the study (Table 6.7.1). The fish were tagged with external T-bar tags before being released from one of the farming locations. Each tag had the words “DUSØR UTBETALES” (“REWARD”) and the necessary contact information printed on it to promote the reporting of recaptures from the local community. Intensive recapture efforts were implemented immediately after the second and third releases. Live recapture methods were tested in the immediate vicinity of the farm during the first week post-escape. Recapture through local fisheries was encouraged through a reward program that continued for over six months after the last escape was simulated.
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Group
N
1
Total length (cm)
Weight (g)
Release date
Mean
SD
Mean
SD
1033
24.8
1.8
171.74
50.86
16 Sept 2010
2
874
28.9
2.5
331.25
*
10 Oct 2010
3
870
36.37
3.22
691.37
198.93
20 March 2011
Table 6.7.1. Overview of the three deliberate escapes of externally tagged Atlantic cod. Figure 6.7.1.Study area and the location of study farms.
Live recapture methods included: s six cod pots in three different sizes (1.8, 8 and 14 m3) distributed evenly around the farm (Figure 6.7.2); s two fyke nets fixed directly onto the net wall of the commercial net pens (Figure 6.7.3); s a net pen provided with a gate that could be opened and closed from the surface (“smart pen�); and s a large crane-operated dip net (Figure 6.7.4).
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Figure 6.7.2. Fishing pots used for recapture of Atlantic cod (Gadus morhua) farm escapees.
Figure 6.7.4. The large crane operated dip net in use.
Figure 6.7.3. Fyke net used for recapture of Atlantic cod (Gadus morhua) farm escapees.
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The idea behind the last two methods was to attract the escaped fish into the traps by offering commercial fish feed into the sea. In addition, two local semi-professional fishing vessels were hired to attempt to recapture juvenile escapees with gill-nets. The mesh size of the nets was chosen in accordance with the size of the fish in each release group. The nets were deployed 2 km north and south of the release site at depths ranging from 10 to 45 m, perpendicular to the coast in a line from shallow to deep waters. The gill-net fishing lasted for six days after each simulated escape. Posters with information about the recapture reward were distributed in strategic points around the area of study (Fig. 6.7.5).
Figure 6.7.5. The poster made for the reward program.
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There was no recapture of tagged juvenile cod with any of the live recapture methods tested in the vicinity of the fish farm. However, large amounts of saithe (Pollachius virens) and cod (both wild and farmed from previous escape events) were caught. Large aggregations of piscivorous fish have been previously reported around fish farms all around the world (Boyra et al. 2004, Dempster et al. 2002, 2009, Uglem et al. 2009). Wild fish attracted to farms also tend to be large adults (Dempster et al. 2002). The lack of recapture of tagged juveniles in the immediate vicinity of the farm might have been a consequence of the predator-avoidance behavior of the escapees or that escapees were eaten by the large wild fish in the vicinity of farms after the escape. In agreement with this hypothesis, we detected high predation pressure of the tagged fish. More than 100 tags, most of which belonged to fish from the first release group, were recovered by examining the stomach of large cod and saithe caught close to the net pens. A total of 40 tagged escapees were recaptured in the gill-nets. However, the by-catch outnumbered the recapture: more than 500 individuals from non-targeted species were recorded, with crabs (Cancer pagurus), saithe and cod (wild and farmed from previous escape events) the most abundant species (Figure 6.7.6). The by-catch also included tusk (Brosme brosme), haddock (Melanogrammus aeglefinus), pollack (Pollachius pollachius) and mackerel (Scomber scombrus), among other species. Only eight tagged escapees were recaptured through the reward program by local fishermen. All these recaptures occurred within 2 km from the release farm, in a time span from six days to six months post release. There were no reported recaptures in the gill-nets used by the semicommercial fishermen operating in the fjord, probably due to a mismatch between the size of the standard cod nets used in the fjord and the small size of the tagged fish (<40 cm total length), which keeps the juvenile escapees out of the fishing window of the local fisheries. In summary, of a total of almost 3000 juvenile cod that were released for this study, only 48 (1.7%) were recaptured, another 110 (3.9%) escapees were found in the stomach of predators and the remaining 94.3% were unaccounted for one year after the first escape event was performed. In conclusion, the present study indicates that small juvenile farmed cod escapees might suffer a high predation pressure as soon as they enter the wild. Those that are able to escape from the predators aggregated around the farm will rapidly seek protection in shallow waters close to the coast. Recapture efforts should therefore not concentrate around the farm, but in shallow coastal waters. However, the small mesh size of the gill-nets needed for the recapture of juveniles seems to be significant drawback to the efficiency of this recapture method; the high by-catch rates and the resulting workload make this method non-operational at large scale. The potential benefits of a recapture program should be considered in relation to the potential for depleting local wild fish stocks and the costs of recapture. With the suite of recapture methods evaluated in this study, the recapture rates achieved were too low to defend the efforts devoted to recapture. This emphasizes the need to prevent escape incidents by improving farming technology and operational routines rather than relying on trying to recapture fish after they have escaped. In other words, the battle to avoid possible negative ecological effects due to escape of juvenile farmed Atlantic cod should take place before the cod escape from the sea-cages.
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Figure 6.7.6. Total catch in gill-nets after one week of recapture effort in both October 2010 and March 2011.
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CITED
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