Are there physiological correlates of dominance in natural trout populations?

Competition over limited resources can lead to serious injury and may be minimized by the formation of social hierarchies. However, there are often physiological consequences associated with social status which can affect both dominant and subordinate animals. In salmonid fish, at least under laboratory conditions, physiological costs are mainly associated with subordinance. The structure of hierarchies formed among salmonids in the laboratory is likely to be different from those formed in complex natural environments, and yet little is known about the physiological consequences of dominance in the field. We tested the hypothesis that there are specific physiological correlates associated with specific social behaviours among natural populations of juvenile salmonid fish by observing brown trout, Salmo trutta, in small streams. Fish were tagged and their behaviour observed by video recording over several weeks at three sites along Devonport Leat (Devon, U.K.). Although diet and tissue metal concentrations differed between sites, the behaviour of the fish at the three sites was very similar. At the end of the observation period, we sampled fish for parameters including specific growth rate, plasma cortisol and osmolality, brain monoamines and gut contents. There was no relationship between social status and growth rates but, contrary to laboratory predictions, dominant fish had higher plasma cortisol. We conclude that physiological correlates of dominance do exist among these natural fish populations but they may differ to those found in the laboratory. Further research is now required to test a wider range of physiologies in the field.