Timing is everything: survival of Atlantic salmon (Salmo salar) postsmolts during events of high salmon lice densities

Atlantic salmon in aquaculture act as reservoir hosts and vectors of parasites like salmon lice and this parasite is shown to harm wild salmonid populations.
In the present study, n=29817 tagged Atlantic salmon were studied in four release trials. Half of the released fish were given prophylactic treatment against lice, the other half represented sham control fish. We used a nested design comparing years with low and high lice density and seasonal dynamics in infestation pressure. The released Atlantic salmon thus experienced highly variable lice infestation pressures, which we linked to survival and growth in returning fish. The fish were released in a protected “National Salmon Fjord“ and n=559 Atlantic salmon were recaptured after spending 1-4 years at sea.
In most experimental groups 1 – 2.5 % of the fish were recaptured at return. However, survival of unprotected fish was extremely low for the trial released at the highest density of lice: only 0.03 % of these Atlantic salmo...