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Social interactions in small groups of juvenile rainbow trout (Oncorhynchus mykiss) lead to the formation of dominance hierarchies. Dominant fish hold better positions in the environment, gain a larger share of the available food and exhibit aggression towards fish lower in the hierarchy. By contrast, subordinate fish exhibit behavioural inhibition, including reduced activity and feeding. The behavioural characteristics associated with social status are likely the result of changes in brain monoamines resulting from social interactions. Whereas substantial physiological benefits, including higher growth rates and condition factor, are experienced by dominant trout, low social status appears to be a chronic stress, as indicated by sustained elevation of circulating cortisol concentrations in subordinate fish. High cortisol levels, in turn, may be responsible for many of the deleterious physiological consequences of low social status, including lower growth rates and condition factor, immunosuppression and increased mortality. Circulating cortisol levels may also be a factor in determining the outcome of social interactions in pairs of rainbow trout, and hence in determining social status. Rainbow trout treated with cortisol were significantly more likely to become subordinate in paired encounters with smaller untreated conspecifics.

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Two related experiments examined the relationship between plasma cortisol concentrations and the development of social hierarchies in fish. In the first, rainbow trout, Oncorhynchus mykiss, and brown trout, Salmo trutta, were observed for dominance interactions when confined within single-species pairs for 4, 48, or 168 h. Subordinate members of a pair exhibited significantly higher cortisol concentrations than dominant and single fish, but the pattern of cortisol elevation differed between the two species, being quicker to rise and increasing to a higher level in rainbow trout. Cortisol concentrations were correlated with behavioural measurements; the more subordinate 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 concentrations of plasma cortisol both before and after social stress. It therefore appears that in addition to cortisol being elevated during periods of social stress, an association may exist between initial cortisol levels and the likelihood of a fish becoming subordinate.

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