Patterns of Aggressive Attack in uvenile Stee head Trout

Nipping is a potentially damaging aggressive behavior of juvenile steelhead trout (SaBmo gairdneri). By analyzing videotaped interactions of pairs of fish we determined where nips were aimed and where they contacted the opponent's body. Patterns of fin damage were also assessed. Aims and nips (contacts) were distributed differently i n reciprocal and nonreciprocal aggressive bouts. In nonreciprocal bouts, aims were directed mainly at the dorsal fin, central body section, and caudal fin. Nip contact. was biased towards the caudal fin. I n reciprocal bouts, both aims and nips were concentrated o n the dorsal f in and anterior portions of the body. We suggest that juvenile steelhead adjust their fighting tactics in response to opponent behavior. Mouth fighting, a novel behavior in juvenile trout, was observed in some reciprocal bouts. The dorsal f in incurred the greatest damage. We conclude that aggression (nipping) is responsible for the dorsal f in damage commonly observed in hatchery-reared salmonids.