ABASTRACT
We demonstrate that sockeye salmon populations can exchange many migrants each generation and yet remain genetically distinct, owing to reduced reproductive success in strays. We studied a small beach population that receives strays each generation from a much larger river population (both in Lake Washington, Washington). Site-specific otolith microstructure patterns were used to determine which beach spawners had been born at the beach (residents) and which had been born in the river (strays). In each of two years, about 1% of the river population strayed to the beach but these strays composed 35 - 44% of the beach spawners. If strays and residents had similar reproductive success, such levels of gene flow would prevent any neutral genetic divergence of the populations. However, allelic variation at microsatellite loci revealed that beach residents were distinct from the river population and from river fish that strayed to the beach. Strays were morphologically similar to river fish but quite different from beach fish, suggesting that local adaptation may play a role in their reduced success at the beach. Local adaptation of residents and declining success of strays can arise early in a population’s history (the beach site was colonized less than 14 generations ago).