Consistency in the stress response of individuals of two strains of rainbow trout, Oncorhynchus mykiss

Post-stress plasma cortisol levels, following a standardized confinement stress, were monitored on a monthly basis for 5 months in 240 individually marked rainbow trout from each of two strains. At the end of this period individuals showing a consistent high or low response to stress were segregated. Post-stress plasma cortisol levels in selected high- and low-responding fish were similar for each strain (high 148.6 ± 3.2, 144.7 ± 2.4 ng/ml; low 80.3 ± 2.3, 72.9 ± 2.7 ng/ml). The difference in magnitude of response between the two selected groups was maintained for at least 28 months, post-stress plasma cortisol values in strain 1 at this point being, high responders 404.1 ± 28.2 ng/ml, low responders 172.8 ± 20.0 ng/ml. Despite this stability of response within the selected fish, analysis of the complete data set (480 fish) failed to demonstrate unequivocally whether a stable response level to stress is characteristic of all individuals. The implications of these results are discussed with reference to the potential benefits of producing strains of fish with a reduced sensitivity or responsiveness to common forms of aquacultural stress.