Sea trout face growth–mortality trade-offs when entering the sea to feed. Salmon lice
epizootics resulting from aquaculture have shifted these trade-offs, as salmon lice might
both increase mortality and reduce growth of sea trout. We studied mortality and behavioural
adaptations of wild sea trout in a large-scale experiment with acoustic telemetry
in an aquaculture intensive area that was fallowed (emptied of fish) synchronically biannually,
creating large variations in salmon lice concentrations. We tagged 310 wild sea
trout during 3 years, and gave half of the individuals a prophylaxis against further salmon
lice infestation. There was no difference in survival among years or between treatments.
In years of high infestation pressure, however, sea trout remained closer to the
river outlet, used freshwater (FW) habitats for longer periods and returned earlier to the
river than in the low infestation year. This indicates that sea trout adapt their migratory
behaviour by actively choosing FW refuges from salmon lice to escape from immediate
mortality risk. Nevertheless, simulations show that these adaptations can lead to lost
growth opportunities. Reduced growth can increase long-term mortality of sea trout
due to prolonged exposure to size-dependent predation risk, lead to lower fecundity
and, ultimately, reduce the likelihood of sea migration.