Size heterogeneity can reduce aggression and promote growth in Atlantic salmon parr

Aggression in groups of 0+ Atlantic salmon (Salmo salar) was monitored at weekly intervals in two tanks containing 100 fish each. Three 1 + salmon parr were added to one of these. After 5 weeks, fish weights were measured in both tanks and the conditions reversed. At ten weeks, weights of fish in both tanks were measured again. In both populations, levels of aggression among the smaller fish were significantly lower and growth rates significantly higher when the large fish were present. Although the large fish attacked the small ones, the rate at which they did so was an order of magnitude lower that the rate at which small fish attacked each other in the absence of larger conspecifics. This raises the possibility that levels of aggression among farmed salmon might be reduced by the addition of a few large conspecifics.