Social interactions between 0+ brown and rainbow trout in experimental stream troughs

Social interactions between 0+ brown trout (Salmo trutta L.) and rainbow trout (S. gairdneri Richardson) were examined in a pair of stream observation troughs. The two species exhibited similar agonistic behaviour and interacted socially. As fry (< 35 mm), rainbow trout were dominant over brown trout in the shallow, faster water (< 15 cm deep, < 15 cm s ‐1 ) nearest the incoming food supply. In fingerlings (c. 43–67 mm), social status was reversed throughout the troughs with brown trout dominant over rainbow trout. A size ratio of 1.7 between early‐emerged brown and rainbow fingerlings and late‐emerged rainbow fry reduced aggression between these fish. Prior residence had only a minorpositive influence on social dominance.