Environmental and behavioural stressors : effects on physiological function in salmonid fish
Social interactions and aggression among salmonid fish are known to induce stress responses, particularly in subordinate fish, and the physiological effects of these stress responses have implications for fish in both aquaculture and natural environments. The physiological responses exhibited by subordinate fish (i.e. those fish that are low ranking in a social hierarchy) vary depending on the nature and extent of the social interaction and can also be influenced by environmental factors. Artificial environments - typically aquaria - generally elicit a larger stress response in the subordinate fish due to the fish being held in close confinement. The present study has clearly demonstrated that the physiological responses to social interaction and the formation of dominance hierarchies in rainbow trout and brown trough are affected by the environment of the fish e.g. whether the environment is artificial or natural, and stable or subject to fluctuations. In an artificial environment, the present study has confirmed that the effects of the social stress encountered by subordinate fish include decreases in growth rate and condition and increases in plasma cortisol but has also demonstrated decreases in the ability to secrete further cortisol and increases in the standard metabolic rate of subordinates. In a semi-natural environment these physiological consequences of subordinance were seen to be reduced; indeed with the presence of environmental perturbations no physiological differences were noted between dominant and subordinate fish. Finally the present study also investigated the role of cortisol in chloride cell proliferation and the determination of social status and concluded that cortisol appears to play a mineralocorticoid role in the proliferation of chloride cells and may also influence the outcome of social interactions.