Sympatric association influences within-species dominance relations among juvenile Atlantic salmon and brown trout

Abstract

Size and aggressiveness are determinants of social dominance in many vertebrate species, including juvenile stream-dwelling salmonids. We used seminatural stream channels, landscaped to provide a range of depths, to test whether the factors influencing the formation of social hierarchies are similar in single- and mixed-species groups of Atlantic salmon,Salmo salar , and brown trout, Salmo trutta. We also tested whether these species have similar feeding rhythms during the day and whether dominant and subordinate individuals feed at different times, under both allopatric and sympatric conditions. Size appeared to be a good predictor of feeding success of both species in allopatry, but not when they were in direct competition. In contrast, rate of aggression was positively correlated with feeding success in both allopatry and sympatry. However, the timing and rate of feeding of dominants and subordinates differed significantly only in the allopatric trials, with subordinate individuals in the sympatric trials being able to adopt a nonaggressive alternative strategy and continue feeding. These results highlight the behavioural plasticity of juvenile salmonids, which can adapt their feeding behaviour to the social environment, and suggest that the presence of another species with similar, but not identical, ecological requirements may increase the opportunity for the expression of alternative behavioural strategies. We conclude that the advantages of social dominance may to a large extent be specific to the species assemblage. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.