4.4. POST-ESCAPE DISPERSAL OF ATLANTIC COD (Gadus morhua L.) AND JUVENILE ATLANTIC SALMON (Salmo salar L.) Cite this article as: Uglem I, Llinares Serra RM, Mork J, Nilsen R, Økland F, Varne R, Rikardsen A, Bjørn PA (2013) Post-escape dispersal of Atlantic cod (Gadus morhua L.) and juvenile Atlantic salmon (Salmo salar L.). In: PREVENT ESCAPE Project Compendium. Chapter 4.4. Commission of the European Communities, 7th Research Framework Program. www.preventescape.eu ISBN: 978-82-14-05565-8
authors: Ingebrigt Uglem1, Rosa Maria Llinares Serra2, Jarle Mork3, Rune Nilsen2, Finn Økland1, Rebekka Varne3, Audun Rikardsen4 & Pål Arne Bjørn2 Norwegian Institute of Nature Research, Tungasletta 2, No-7485 Trondheim, Norway Nofima Marin, Muninbakken 9-13, P. box 6122, No-9291 Tromsø, Norway 3 Trondhjem Biological Station, Department of Biology, Norwegian University of Science and Technology, No-7491 Trondheim, Norway 4 University of Tromsø, No-9037 Tromsø 1 2
INTRODUCTION Understanding the post-escape dispersal pattern of fish farm escapees would be useful for predicting their possible ecological impacts as well as for improving recapture efficiency. While a range of studies have been carried out to assess the post-escape behaviour of adult Atlantic cod, Gadhus morhua (Uglem et al. 2008, 2010) and adult Atlantic salmon, Salmo salar (Skilbrei 2010a, b, et al. 2010), little is known about the post-escape behaviour of juvenile farmed cod and salmon. Therefore, we focused mainly on the behaviour and movements of juvenile cod and salmon following experimental escape events. Post-escape behaviour and dispersal were studied by tracking fish in their natural environment, using acoustic transmitters and traditional mark-recapture methodologies. For the latter, fish were marked with external, visible tags, released from participating fish farms, and their movements were assessed on the basis of recapture event, either in organised recapture efforts, or following capture within commercial and recreational fisheries.
The Atlantic cod is an economically important species marketed in fresh, dried, salted, smoked and frozen form. It is widely distributed in a variety of habitats, from the shoreline to the continental shelf, along the North American coast, around Greenland and Iceland, and along the Europe coastline from the Bay of Biscay to the Barents Sea. The most important stocks are the Norwegian Arctic stock, in the Barents Sea, and the Icelandic stock. The populations around Greenland and Newfoundland have declined dramatically, while the Barents Sea stock remains healthy. It is an epibenthic-pelagic species that may grow up to 1.5 m in length and weigh more than 50 kg, although fish of this size are rarely encountered today. Cod caught within commercial fisheries typically range in weight from a couple of kg to around 20 kg. They are omnivorous; feeding at dawn or dusk on invertebrates and fish, including young cod. They spawn in batches once a year, usually during late winter and early spring, and have pelagic eggs. A 5 kg wild female may spawn around 2.5 million eggs during a spawning season. The Atlantic salmon is another important commercial species and is a target for both recreational and commercial fisheries. However, wild stocks have declined significantly during the last few decades, mainly due to anthropogenic impacts. Salmon is now one of the most important aquaculture species in the world, but salmon aquaculture is thought to have serious negative impacts on wild salmon stocks. The Atlantic salmon is an anadromous species; i.e. it reproduces in rivers while the juveniles migrate to the ocean for feeding before they return to the rivers as adults. Before the juvenile salmon migrate from freshwater to sea water they undergo a physiological transition that enables them to survive in high salinity. Juvenile salmon are referred to as “parr” before this transition and “smolt” afterwards. Salmon are pelagic in the ocean and have a similar geographical distribution to cod. They can grow to more than 1.2 m in length and weigh more than 30 kg.
OBJECTIVES We examined the spatio-temporal distribution of juvenile farmed Atlantic cod and Atlantic salmon after simulated escape incidents using acoustic telemetry, and the long-term dispersal and movements of both adult and juvenile cod using mark-recapture techniques.
METHODS Acoustic telemetry Small acoustic transmitters were implanted into the abdominal cavity of juvenile cod (n=24) and salmon (n=97) prior to the simulation of an escape incident. Cod were implanted and then released from commercial cod farms in GildeskĂĽl in county Nordland and juvenile salmon from a land-based commercial smolt farm in the Trondheimsfjord in county TrĂ¸ndelag. Two groups of salmon were examined; pre-smolt fish i.e. parr (n=50), which have not developed complete sea water tolerance, and smolts (n=47), which are adapted to saltwater. The shortterm dispersal behaviour of the fish was tracked by an array of automatic receivers positioned at the farms and also throughout the fjord system (see Figure 4.4.1 for the receiver array for cod). Receivers deployed in the sea were attached to ropes at 3 m depth, while receivers in rivers were deployed on the river bed. All receivers recorded the transmitter identification code, and the date and time of detection when a tagged fish was within the receiver range. Range tests showed that the receiver ranges varied between 200 and 600 m in radius. Figure 4.4.1. Study area and configuration of the telemetry receiver array. The positions of the receivers are indicated with black circles.
Capture-mark-recapture The long-term behaviour and distribution of escaped juvenile and adult cod was studied by tagging 3377 farmed cod with external spaghetti tags and subsequently releasing them from three large-scale commercial farms to simulate escape incidents. Juvenile cod were tagged and released at one farm in the GildeskĂĽl area in county Nordland, while adult cod were tagged and released at two farms in the Trondheimsfjord area in county TrĂ¸ndelag. Three groups of juvenile cod were tagged and released from the same farming location in Nordland. The first group was released for the purpose of studying long-term dispersal, and was deliberately released 3 weeks prior to initiation of an organized recapture program (gill nets and commercial cod pots). The two other groups were released only 24 hours before the organized recapture programs started. Recaptures were also recorded in both target recapture fisheries, as well as recreational and commercial fisheries.
RESULTS Acoustic telemetry Nearly all of the cod remained within the fjord where they had been released (Figure 4.4.1), but most of the juvenile cod moved away from the release site (the cod farm) within the first week (Figure 4.4.2); the majority left along the shore line (Figure 4.4.3). Some of the fish were continuously detected around the release site during this period, and two were observed at another cod farm located in the same fjord system. One third of the salmon pre-smolt, equipped with acoustic transmitters, died within the immediate vicinity of their release location, compared with only 8.5% mortality of the smolts. The surviving parr dispersed away from the release site, at the fish farm, after two to three days, and, as with the juvenile cod, they moved predominantly along the shore line (Figure 4.4.3). In contrast, most of the surviving smolts left the farm area during the first day and around half moved away from the shore, adopting a more pelagic distribution than the pre-smolt. Both pre-smolt and smolt appeared to have a similar movement pattern and speed following departure from the release site close to the smolt farm. The number of surviving fish recorded in the fjord decreased throughout the study period, possibly due to fish migrating out of the fjord (Figure 4.4.4), although none of the fish migrated up into freshwater. Compared to existing knowledge on movements of released hatchery-reared smolts during spring, our results indicate a less directional and slower migration pattern during autumn.
Capture-mark-recapture Recapture rates of the three groups of juvenile cod were low. No recaptures from the first group were reported. However, several of these tags were found in the stomach of a small sample of adult saithe (Pollachius virens) and cod caught in the proximity of the fish farm immediately after release. As a result of this observation, we caught 160 potential predators around the release site during the following 3-4 weeks and checked their stomachs for tagged fish. A total of 105 tags from the first group of tagged juveniles were recovered accounting
for 10.1% of the tagged fish. As the abundance of large predators around salmon farms may be very high (e.g. Dempster et al. 2009), it can be assumed that the first release group suffered high mortality due to predation during their first days after release. The other two release groups were larger in size than the first group and probably too large to be exposed to the same intense predation pressure. The recapture rate for the second release was 5% (44 fish), with 39 being caught in organized recapture efforts and the 5 by local fisheries. Five fish were found within the stomachs of captured predators. The third group of juvenile cod was released about 6 months after the first group and one fish from this group was recaptured in organized gillnet fishery, while 4 fish were reported in commercial fisheries during the following months. All of the juvenile cod were recaptured less than 3 kilometers from the release site (Figure 4.4.5). As the organized recapture fisheries accounted for the majority of the recaptures and since this fishery was carried out close to the farms immediately after release, it is difficult to generalize with respect to the long term dispersal pattern of juvenile cod after escape. The recapture of adult tagged cod varied between the two farms in TrĂ¸ndelag (Figure 4.4.6), with higher recapture rate for fish released from the farm located in the inner part of the fjord (N = 500, recapture rate: 4.5%) compared to the outermost farm (N = 250, recapture rate: 0.5%). Only one of the adult fish released at the farm located close to the coast was recaptured, three months after release. The adult fish from the innermost farm fish were recaptured throughout the entire fjord, up to 70 km away from the farm. The difference in recapture rates between these two locations may be because fish released closest to the coast rapidly dispersed to areas with a lower intensity of both commercial and recreational fishery. The recaptures of fish from the inner part of the fjord took place during a 7 month period, with the highest recapture rate from day 100 to day 160 after release. This period corresponds to July and August, or mid-summer in Norway with high water temperatures and a high fishing pressure due to recreational fishing during the summer holidays. Figure 4.4.2. Cumulative proportion of juvenile farmed cod departing from the release site at the fish farm within the first week following the simulated escape.
Figure 4.4.3. Direction for the first departure from the release location for Atlantic salmon pre-smolt/parr and smolt following simulated escape from a land based hatchery.
Figure 4.4.4. Proportions of surviving Atlantic salmon pre-smolt/parr and smolts detected in other areas than the hatchery area throughout the five week study period.
Figure 4.4.5. Study area for juvenile cod with release and recapture locations.
Figure 4.4.6. Study area for adult cod with release and recapture locations.
DISCUSSION Following a simulated escape incident from an Atlantic cod farm, juveniles initially showed rapid dispersal, however, the escapees did not appear to move far from the source farm. The long-term recapture of tagged juveniles was close to zero, and smaller juveniles experienced high predation pressure due to the abundance of large predatory fish in the vicinity of the farms, as evidenced by the presence of tags in captured predator stomachs. These results indicate that the immediate mortality for juvenile fish following escape may be high and that their escape from sea cages may be of a limited ecological importance compared to escape at late life stages. Previous research has shown that adult cod also disperse rapidly from farms, but that adult escapees may be recaptured relatively far from the farms (Uglem et al. 2008, 2010). However, both the results from Uglem et al. (2008; 2010) and our results on adult recaptures suggest that the recapture period is relatively limited. This may indicate that escaped adults also suffer substantial mortality following escape, albeit over a longer timeframe than the juveniles. The results were similar for juvenile Atlantic salmon, which showed considerable mortality of pre-smolt. However, the mortality of seawater-adapted smolts was significantly lower. Our study, which was conducted in autumn, indicates a less directional and slower migration pattern of released hatchery-reared smolts compared with other studies where the smolts were released during spring. In general, the rapid dispersal from farms for both cod and salmon indicate that the potential for recapture of escapees following an escape incident is limited.
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