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 Gildeskül area in county Nordland, while adult cod were tagged and released at two farms in the Trondheimsfjord area in county Trøndelag. 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
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