The formation of dominance hierarchies within groups of salmonid fish is well documented and stream tanks are often used to create environmentally relevant conditions in which to study this aspect of fish behaviour. Although stream tanks simulate the natural environment of the fish in many ways, they have limitations in that they provide the fish with a rather predictable and constant habitat. The present study illustrates that under these constant conditions, the behaviour of fish in their natural environment may not be truly represented. Under constant conditions hierarchies were formed among groups of four brown trout, Salmo trutta, and the dominant fish displayed physiological advantages. However, when an environmental perturbation of increased water flow, simulating a spate, was imposed, the social behaviour of the fish was altered and the physiological advantages of dominance were lost. Clearly, environmental changes affect the behaviour, and consequently the physiology, of salmonid fish, therefore the importance of taking environmental disturbances into consideration in studies of salmonid behaviour should not be underestimated.