EFFECTS OF HATCHERY AND WILD ANCESTRY AND REARING ENVIRONMENTS ON THEDEVELOPMENT OF AGONISTIC BEHAVIOR IN STEELHEAD TROUT (ONCORHYNCHUS-MYKISS) FRY

At emergence, fry from a wild steelhead trout (Oncorhynchus mykiss) po pulation displayed higher levels of one type of mirror-elicited agonis tic behavior (swimming against a mirror) than did fry from a locally d erived hatchery (domesticated) population. Newly emerged wild fry also dominated hatchery fry in size-matched dyadic dominance challenges. H owever, given an approximately 3.0-4.5% size advantage, hatchery fry d ominated smaller wild competitors in 68% of encounters, indicating tha t small size differences at emergence can strongly influence dominance . Hatchery fry reared in a natural stream channel for 105 days were mo re aggressive than those reared in hatchery tanks and than wild fry re ared in either environment. In otherwise identical hatchery tanks, low density and low food rations were associated with elevated agonistic behavior by hatchery fry, but not by wild fry. This study suggests tha t four to seven generations of domestication has resulted in behaviora l divergence of the hatchery population from its wild donor population . The extent to which such differences determine the outcomes of inter actions between of spring of wild and hatchery steelhead spawning in s treams will depend on the size differences and emergence dates of the populations as well as genetic bases of aggression.