CONSISTENCY IN THE STRESS RESPONSE OF INDIVIDUALS OF 2 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-1; low 80.3+/-2.3, 72.9+/-2.7 ng ml-1). 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-1, low responders 172.8+/-20.0 ng ml-1. 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.