Feeding preference of recaptured atlantic salmon salmo salar following simulated escape from fish pens during autumn

Abstract

Escapes of farmed fish from coastal farms around the world are believed to have genetic and ecological consequencesfor wild fish populations. Each year there are numerous escapes of adult farmed Atlantic salmon Salmo salar. However, survival until maturity appears to be low. One possible explanation is that many of these salmon have difficulties switching to live prey. To address this possibility, growth rate and dietary preference were recorded in recaptured salmon released in 3 groups from August to October 2008 within a small fjord in southéwestern Norway (Masfjord). The fish were primed with a commercial diet containing 11% of the 'vegetable oil marker' 18:2né6 before release. In the 14.5to 35.1% of fish recaptured, growth rates were generally good (0.21-1.23%d-1 of fish recaptured in main fjord >2 mo after release),with body weight doubled or more for fish recaptured after 19 wk and thereafter. However, fish recaptured in a small arm of Masfjord (Hopsvåg) had a growth rate close to zero and were in poor condition. While stomachs of recaptured fish weregenerally empty in the first weeks after release, pellets from fish farms were found in more than 80% of the stomachsafter 12 wk and thereafter. Additionally, the level of 18:2né6 in depot lipids (11% of fatty acids) of the muscle indicatedthat none of the fish recaptured the following autumn and winter in the vicinity of the releasesite had switched to wild prey diets. Salmon that escape during autumn are unlikely to compete for prey with wild fish in the vicinity of the release site.