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
salmon returned to the river, compared to 1.86 % in the protected group.
Synthesis and applications . We document that a high lice density can
cause a more than 50 times higher mortality risk in Atlantic salmon on
their sea migration, even in a fjord with protected status. Fine-tuned and
hard-to-predict year-to-year differences in timing, both for the wild
smolt migration and the population build-up of lice released from
aquaculture, means life or death to wild salmon. Management actions such
as spatial segregation of farmed fish and lice (e.g. closed farm pens),
and/or moving farms away from vulnerable habitats for wild salmonids
(fjords and coastal areas), may be needed to ensure sustainable
co-existence of wild and farmed Atlantic salmon.