SALMONID FLEXIBILITY - RESPONSES TO ENVIRONMENTAL EXTREMES

The responses of salmonid fishes to the problems posed by marginal hab itats are genetic exercises in population insurance. The costs increas e as the risks increase, but the risks are met by a wide repertoire of biological capacities. The most general proximate response to adversi ty is behavioral: ontogenetic niche shifts are an acknowledgment that a series of environments becomes marginal for all salmonids during dev elopment. Physiological tolerances and developmental flexibility gover n the timing of these movements. Such shifts are the product of natura l selection in relatively predictable environments, but less predictab le or catastrophic events are accommodated at a different genetic leve l. Low spatial flexibility is counteracted by temporal insurance, and vice versa. Thus, high homing precision is coupled with complex multip le-age structures, and simple age structuring is coupled with relative ly high spatial straying