Plasma cortisol concentrations before and after social stress in rainbow trout and brown trout

Two related experiments examined the relationship between plasma cortisol c oncentrations and the development of social hierarchies in fish. In the fir st, rainbow trout, Oncorhynchus mykiss, and brown trout, Salmo trutta, were observed for dominance interactions when confined within single-species pa irs for 4, 48, or 168 h. Subordinate members of a pair exhibited significan tly higher cortisol concentrations than dominant and single fish, but the p attern of cortisol elevation differed between the two species, being quicke r to rise and increasing to a higher level in rainbow trout. Cortisol conce ntrations were correlated with behavioural measurements; the more subordina te the behaviour exhibited by a fish, the higher its cortisol concentration . Social stress was a chronic stressor, and no acclimation to social status occurred during the week. In the second experiment, measurements of plasma cortisol were made before pairing of rainbow trout and then after 48 h of confinement in pairs. Subordinate fish demonstrated significantly higher co ncentrations of plasma cortisol both before and after social stress. It the refore appears that in addition to cortisol being elevated during periods o f social stress, an association may exist between initial cortisol levels a nd the likelihood of a fish becoming subordinate.