Differences in behaviour between rainbow trout selected for high- and low-stress responsiveness.

Two F1 lines of rainbow trout Oncorhynchus mykiss, divergent for plasma cortisol responsiveness, were generated by individual selection for post-stress cortisol values within the F0 generation. Adult females of the F1 generation were transferred to rearing in social isolation in observation tanks. After 6 days, locomotor activity in high-responding (HR) and low-responding (LR) individuals was quantified as time spent moving during a 20 min observation period. Behavioural observations were repeated the next day with a smaller conspecific intruder present in each observation tank. Differential hypothalamus-pituitary-interrenal axis activity in the two lines was subsequently confirmed by a standardised confinement stress test, which resulted in significantly higher plasma cortisol concentrations in HR than LR fish. HR fish displayed higher levels of locomotor activity than LR fish in the presence of an intruder, but not when in isolation. Aggressive behaviour towards the intruder was not seen, suggesting either a state-dependent lack of territorial aggression, or chronic stress in the experimental fish. A significantly higher incidence of feed intake was seen in LR trout when held in observation tanks (40% versus 0% of the fish took food when in isolation), suggesting that these fish acclimated more successfully to the experimental conditions than HR fish did. These results suggest that selection for stress responsiveness in salmonid fish leads to behavioural alterations, which are of potential importance to the performance of these fish in aquaculture rearing operations.