Competitive asymmetries in territorial juvenile Atlantic salmon, Salmo salar

Many animals may face brief periods of extreme intraspecific competition for resources, an example being when populations of territorial species first migrate into a habitat. The factors that determine an individual's success in obtaining a territory will then depend on its relative competitive ability, which in turn is influenced by many potential asymmetries between contestants. Here we compare the relative importance to competing Atlantic salmon fry of several such asymmetries (size, prior residence and relative Standard Metabolic Rate (rSMR)) that have previously been suggested to be important when examined in isolation. Concurring with earlier studies, SMR was higher in the earliest fry From a nest to begin feeding and was a better predictor of dominance than body size in pairwise contests for territories. However, prior residence (where one fish was resident on a territory for 48 h before the arrival of the other) was found to be the best predictor of dominance, masking any effect of relative SMR. Moreover, body size also played a role in encounters involving a prior resident, as an intruder was more likely to displace a resident territory holder if it was larger. The outcome of competition for territories among dispersing salmon fry is therefore likely to be a complex interaction between these asymmetries.