Patterns of Temporal Genetic Variation in Coho Salmon: Estimates of the Effective Proportion of 2-Year-Olds in Natural and Hatchery Populations

Abstract

The majority of coho salmon Oncorhynchus kisutch south of central British Columbia spawn when they are 3 years old, so gene flow among brood years occurs predominately when precocious 2-year-old males (jacks) spawn successfully. To determine how much gene flow among brood years is occurring, we evaluated temporal differences in allozyme allele frequencies in four coho salmon populations in Washington State. Forty-three loci were polymorphic in the 26 samples, totaling 2,328 fish, that were collected from three naturally spawning populations and one hatchery population over 7 years. Our results indicate a greater degree of geographic variation among populations than temporal variation within populations, suggesting that population genetic analyses that depend on spatial variation can be done on natural coho salmon populations even in the absence of temporal sampling. Using a modification of the temporal method for estimating effective population size, we estimated the effective proportion of 2-year-olds to be 35% for each of the naturally spawning populations and 2% for the hatchery population. These results suggest that precocity is a viable and important life history strategy in the naturally spawning coho salmon populations we studied.