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

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 r ainbow trout [Oncorhynchus mykiss (Walbaum)]. A quadratic model was used to predict baseline (precapture) cortisol levels in these fish. A similar qua dratic response pattern of cortisol was observed in wild sea trout (Salmo t rutta L.). These were captured in timed sequences on four dates by seine ne tting in Clew Bay, west coast of Ireland. The estimated precapture cortisol for these fish indicated higher levels soon after migration to sea and a p ositive correlation with the level of infestation by Lepeophtheirus salmoni s (Kroyer). Curvilinearity of the quadratic model was correlated with level s of lice infestation, indicating an enhanced cortisol response to confinem ent stress during capture when parasitic abundance was high. Estimated prec apture cortisol for individual sea trout was positively correlated with num bers of lice although the linear regression had low predictive power. The m ethod provides, for the first time, a means to estimate baseline cortisol l evels in wild salmonids, and addresses one of the most difficult problems i n working with wild fish.