Modelling the effects of capture and sea lice [Lepeophtheirus salmonis (Krøyer)] infestation on the cortisol stress response in trout

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Abstract
Measuring baseline cortisol in wild salmonids is problematic because of the stress induced by most capture methods. The cortisol response to different periods of confinement stress was modelled in a laboratory population of rainbow trout [Oncorhynchus mykiss (Walbaum)]. A quadratic model was used to predict baseline (precapture) cortisol levels in these fish. A similar quadratic response pattern of cortisol was observed in wild sea trout (Salmo trutta L.). These were captured in timed sequences on four dates by seine netting in Clew Bay, west coast of Ireland. The estimated precapture cortisol for these fish indicated higher levels soon after migration to sea and a positive correlation with the level of infestation by Lepeophtheirus salmonis (Krøyer). Curvilinearity of the quadratic model was correlated with levels of lice infestation, indicating an enhanced cortisol response to confinement stress during capture when parasitic abundance was high. Estimated precapture cortisol for individual sea trout was positively correlated with numbers of lice although the linear regression had low predictive power. The method provides, for the first time, a means to estimate baseline cortisol levels in wild salmonids, and addresses one of the most difficult problems in working with wild fish. Keywords: Cortisol, trout, Lepeophtheirus salmonis (Krøyer), capture, quadratic model