Stress and stress coping in fish, behavioural correlates and neuroendocrine mechanisms

The development of a dominance based social hierarchy is common among teleost fish and individual social history greatly affects physiology and behaviour of the fish. Long-term social subordination inhibits aggressive behaviour and predisposes the fish for losing subsequent fights for social dominance, whereas experience of being dominant, has the opposite effect, making the fish more likely to win future encounters. Subordinate individuals are exposed to chronic stress and show a chronic activation of the hypothalamic-pituitary-interrenal (HPI) axis and the brain serotonergic system. Elevated brain serotonergic activity is important for mediating the behavioural inhibition observed in subordinate fish. However, cortisol and neuropeptides involved in the control of the HPI axis also seem to play important roles in mediating behavioural effects of social experience. Moreover, in rainbow trout (Oncorhynchus mykiss), competitive ability is linked to suite of other behavioural traits and to the neuroendocrine stress response profile. Trout responding to stress with a low increase in plasma cortisol have an advantage in dyadic fights for social dominance with a conspecific responding to the same stressor with a larger increase in plasma cortisol. Moreover, rainbow trout selectively bred for low post-stress plasma cortisol (LR trout) have an advantage over trout selected for high post-stress cortisol (HR trout) in dyadic encounters. Thus, in addition to the epigenetic effects of social history there are also genetic factors predisposing individuals for dominant and subordinate social positions.