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Impacts of salmon lice on mortality, marine migration distance and premature return in sea trout

R. M. Serra-Llinares*, T. Bøhn, Ø. Karlsen, R. Nilsen, C. Freitas, J. Albretsen, T. Haraldstad, E. B. Thorstad, K. M. S. Elvik, P. A. Bjørn

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ABSTRACT: Brown trout (Salmo trutta L.) is a facultative anadromous species, where a portion of individuals in populations with access to the sea perform migrations to utilize the richer feeding resources. We investigated the effect of salmon lice Lepeophtheirus salmonis (Krøyer 1837) infestation on the survival and behaviour of wild trout post-smolts during their marine migration. Comparisons of the marine migratory behaviour were made between an artificially infested group (n = 74, average fork length 180 ± 14 mm; range 155-226 mm) and a control group (n = 71, average fork length 181 ± 14 mm; range 156-220 mm) in an area with low natural lice infestation pressure. Artificial infestation was estimated to cause a 100% prevalence and a mean intensity of 65 lice per fish (mean relative intensity of 2.4 lice per g fish). Survival analysis showed limited statistical power but revealed lice-induced mortality, with an estimated hazard ratio of 2.73 (95% CI = 1.04-7.13) compared to the control group, when data from a previous pilot study were included. Surviving individuals in the infested group additionally responded by residing closer to freshwater while at sea, and by prematurely returning to freshwater. On average, infested fish returned to freshwater after only 18 days at sea, while control fish spent on average 100 days at sea. The residency in the inner part of the fjord and the premature return to freshwater represent an adaptive behavioural response to survive the infestation, at the probable cost of reduced growth opportunities and compromised future fitness.