Acclimating salmon as juveniles prepares them for a farmed life in sea-cages

Abstract

Parasitic salmon lice (Lepeophtheirus salmonis) are a severe problem for the Atlantic salmon (Salmo salar) farming industry. To reduce infestations, sea-cages can be submerged, below the depth at which lice are most common, with an underwater air dome being provided to allow salmon to refill their swim bladder and maintain buoyancy. When first transferred to a submerged sea-cage from freshwater tanks, salmon must adapt quickly to refill at the air dome or else become negatively buoyant, which has adverse consequences for fish welfare. Here, we tested if salmon that were habituated to surface-based air domes in freshwater tanks (dome-experienced; n = 3 cages) would use a submerged dome at sea more frequently than salmon that were reared in standard tanks (dome-naïve; n = 3 cages). PIT tagging revealed that dome-experienced individuals used domes 2.5× more and refilled their swim bladders 3× more often than dome-naïve individuals. Relative echo strength, which is an indicator of swim bladder volume, was on average 22% higher for dome-experienced fish compared to dome-naïve fish. Here, we demonstrate how training farmed salmon can improve behavioural and welfare outcomes for animals under industrial production. These findings show that salmon can learn a behaviour in the freshwater phase of production and apply this learning after being transferred to the marine environment.