ANIMAL BEHAVIOUR, 2001, 61, 325–333 doi:10.1006/anbe.2000.1567, available online at http://www.idealibrary.com on Effects of an environmental perturbation on the social behaviour and physiological function of brown trout KATHERINE A. SLOMAN, ALAN C. TAYLOR, NEIL B. METCALFE & KATHLEEN M. GILMOUR Institute of Biomedical and Life Sciences, University of Glasgow (Received 26 April 2000; initial acceptance 12 June 2000; final acceptance 5 September 2000; MS. number: 6562) 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 stream tanks. In the perturbation treatment water levels were then lowered for a week to simulate a drought, whereas conditions remained the same in the control tanks. We recorded behavioural interactions before and after the environmental perturbation. After the 2-week experiment, we killed the fish and measured growth rate, plasma cortisol, hepatic glycogen content, hepatosomatic index, gill epithelial chloride cell densities and interrenal cell nuclear areas. Aggression showed a nonsignificant increase in the drought tanks when the water level was lowered, and behaviour and social ranking of the fish were significantly affected by the environmental perturbation with a general breakdown in the social hierarchy. The pronounced benefits of dominance in terms of growth rate observed in the control tanks were not apparent in the drought tanks. However, the cortisol concen- trations of the drought 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 physiological parameters measured significantly different to those of the control tanks. Given that a stable social system (and its physiological consequences) was observed only in a constant environment, misleading conclusions may be drawn if environmental perturbations are not incorporated into experiments studying the behaviour of stream-living fish in simulated natural conditions.  2001 The Association for the Study of Animal Behaviour Simulated natural environments have been widely used in the study of the behaviour of freshwater fish (Kalleberg 1958; Mason & Chapman 1965; Li & Brocksen 1977). The use of stream tanks provides an intermediate approach between studies carried out in the natural environment, which is not always possible, and those conducted under artificial laboratory conditions where behaviour may be affected by the unnatural holding environment. Species such as salmonids, however, will establish apparently natural dominance hierarchies in artificial stream tanks (Kalleberg 1958; Mason & Chapman 1965; Noakes & Leatherland 1977; Fausch 1984). The primary response to stress in teleosts is the release of ‘stress hormones’: adrenaline, noradrenaline and cor- tisol (Sumpter 1997). Adrenaline and noradrenaline, both catecholamines, are released in what is commonly termed the adrenergic response (Mazeaud & Mazeaud 1981) and cortisol is secreted in the hypothalamo-pituitary- interrenal (HPI) response (Donaldson 1981). These stress hormones, in turn, produce the secondary responses to stress. Elevated plasma cortisol concentrations elicit physiological changes including increased oxygen con- sumption rate and a depression of the respiratory quo- tient (Chan & Woo 1978), an increase in interrenal cell activity (Noakes & Leatherland 1977), and changes in carbohydrate metabolism (Pickering & Pottinger 1995). Tertiary physiological changes may also occur, including decreases in growth and condition (Redding et al. 1986). Using simulations designed to resemble the natural environment of the fish as much as possible, Sloman et al. (2000b) found that there are physiological consequences of being a subordinate fish. While dominant fish tend to acquire the most profitable position in streams in terms of energy intake (Fausch 1984) and so may grow fastest (Sloman et al. 2000b), subordinate fish may experience high levels of stress. Noakes & Leatherland (1977) showed that interrenal activity was inversely correlated with dominance rank, except in the most dominant fish, and Correspondence: N. B. Metcalfe, Fish Biology Group, Division of Environmental and Evolutionary Biology, IBLS, Graham Kerr Build- ing, University of Glasgow, Glasgow G12 8QQ, U.K. (email: n.metcalfe@bio.gla.ac.uk). K. M. Gilmour is now at the Department of Biology, Carleton University, 1125 Colonel By Drive, Ottowa, Ontario K1S 5B6, Canada. 0003–3472/01/020325+09 $35.00/0  2001 The Association for the Study of Animal Behaviour 325