Abstract
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 3years, 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.
Highlights
In a variable and unpredictable environment, individuals that can adapt their behaviour to current conditions have a greater chance to maximize their long-term fitness
To evaluate how lice abundance varied among years, we modelled the lice abundance, intensity and prevalence on all sampled fish (n = 3,716) fitting a generalized linear mixed model (GLMM) using the “glmer” function of the “lme4′′ package of R (Bates, Maechler, Bolker, & Walker, 2015)
We used a GLMM from the binomial family with a logit link to assess the influence of treatment and year, as well as standardized covariates, on the weekly proportion of time spent in the delousing areas
Summary
In a variable and unpredictable environment, individuals that can adapt their behaviour to current conditions have a greater chance to maximize their long-term fitness (bet-hedging theory, e.g., Philippi & Seger, 1989). Even though sea migration both delays maturation and lowers an individual0s survival probability through increased predation risk and infestation risk by parasites and pathogens, it typically results in a much larger body size and greater reproductive success for the surviving individuals (Fleming & Reynolds, 2004; Roff, 2002; Stearns, 1992). This behaviour, will persist only if the benefits exceed the costs (Sandlund & Jonsson, 2016; Werner & Gilliam, 1984). We simulated sea trout growth based on different migratory scenarios to assess growth effects of sea trout behavioural adaptations in response to salmon lice infestations
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