Aggression and growth depression in juvenile Atlantic salmon: the consequences of individual variation in standard metabolic rate

In aquaculture, competitive interactions for food are a major source of growth rate variation, since they result in aggressive individuals acquiring a disproportionate amount of food and growing faster. Consequently, such competition increases the variance and skew of the size distribution of fish. In Atlantic salmon Salmo salar acquaculture, rearing is often initiated with juveniles of uniform size. However, the initial factors allowing fish to out-compete others of the same size is unclear. This study shows that individual differences in standard metabolic rate (SMR) may contribute to differences in aggression between juvenile Atlantic salmon. Fish were segregated into three holding tanks on the basis of differences in relative SMR aggression was highest in the tank containing high SMR fish, and lowest in a low SMR group. However, there were no significant differences in mean growth between the three treatments, although the skew of the size distribution was much less in the low SMR group than in the high SMR group. Therefore, although mean growth was not improved in the low SMR group by excluding high SMR fish, their growth subsequently showed less variability, possibly as a consequence of fewer despotic individuals gaining disproportionate access to food.