PATTERNS OF AGGRESSIVE ATTACK IN JUVENILE STEELHEAD TROUT (SALMO-GAIRDNERI)

Nipping is a potentially damaging aggressive behavior of juvenile steelhead trout (Salmo 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 in 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. In reciprocal bouts, both aims and nips were concentrated on the dorsal fin 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 fin incurred the greatest damage. We conclude that aggression (nipping) is responsible for the dorsal fin damge commonly observed in hatchery-reared salmonids.