Rheotactic response of fry from beach-spawning populations of sockeye salmon: evolution after selection is relaxed

Rheotactic response was quantified for newly emerged sockeye salmon (Oncorh ynchus nerka) fry from a beach population (Pleasure Point) and from an adja cent inlet population (Cedar River). The Pleasure Point population was smal l in number and had been established by straying from the much larger Cedar River population no more than 13 generations previously. When tested in la boratory raceways, fry from the Cedar River population were displaced downs tream in the dark but not in the light. Such behavior is typical of inlet p opulations and presumably reflects selection for rapid movement to rearing lakes with minimal losses to predation. Fry from the Pleasure Point populat ion showed greater downstream displacement than the Cedar River fry. Behavi oral divergence of the Pleasure Point population could not be explained by selection, because water movement was absent from the beach environment. Ge netic drift appeared to be a more plausible divergence mechanism. Wt: sugge st that the rheotactic response of beach populations should reflect the fou nding genotypes, gene flow from other populations, and random genetic drift . The results of previously published studies on the rheotactic response of beach fry in two other lake systems qualitatively support our hypothesis.