Can laboratory studies on dominance predict fitness of young brown trout in the wild?

Laboratory studies suggest that dominance and aggression increase fitness, but this hypothesis has rarely been tested under natural conditions. We therefore designed a combined laboratory-field experiment to detect how social status and aggression relate to growth rate, movement and habitat choice in a natural stream. In 1998 and 1999, juvenile brown trout were caught in the wild and paired in staged dyadic contests in the laboratory where relative dominance rank was determined. Three categories of fish could be distinguished: dominants, subordinates and non-aggressive individuals of indeterminate status. All tested fish were released back into the stream and recaptured after 3 and 8 weeks. Dominant fish grew faster than subordinates, but non-aggressive fish grew as fast as dominants. Social status had no significant effect on recapture rates. Movement was not significantly related to status, but smaller individuals were more mobile and preferred faster-flowing habitats closer to the shore than larger fish. The utilisation of pool and riffle habitats varied among status categories, but this relationship was not consistent between years. These results support the hypothesis that dominance increases fitness in the wild. However, our findings also indicate that less aggressive individuals can be successful in heterogeneous natural habitats. Thus, studies performed under laboratory conditions may overestimate the fitness advantage of aggressive behaviour.