Effects of an environmental perturbation on the social behaviour and physiological function of brown trout

We investigated the effect of an environmental perturbation on brown trout, Salmo trutta, dominance hierarchies. Hierarchies were established over a 1 -week period under constant simulated natural conditions in artificial stre am tanks. In the perturbation treatment water levels were then lowered for a week to simulate a drought, whereas conditions remained the same in the c ontrol tanks. We recorded behavioural interactions before and after the env ironmental perturbation. After the 2-week experiment, we killed the fish an d measured growth rate, plasma cortisol, hepatic glycogen content, hepatoso matic index, gill epithelial chloride cell densities and interrenal cell nu clear areas. Aggression showed a nonsignificant increase in the drought tan ks when the water level was lowered, and behaviour and social ranking of th e fish were significantly affected by the environmental perturbation with a general breakdown in the social hierarchy. The pronounced benefits of domi nance in terms of growth rate observed in the control tanks were not appare nt in the drought tanks. However, the cortisol concentrations of the drough t fish were not significantly higher than those of control fish at the end of the experiment, suggesting that the environmental change itself was not physiologically stressful in the long term. Neither were any other physiolo gical parameters measured significantly different to those of the control t anks. Given that a stable social system land its physiological consequences ) was observed only in a constant environment, misleading conclusions may b e drawn if environmental perturbations are not incorporated into experiment s studying the behaviour of stream-living fish in simulated natural conditi ons. (C) 2001 The Association for the Study of Animal Behaviour.