An Assessment of Population Genetic Variation in Chinook Salmon from Seven Michigan Rivers 30 Years after Introduction

Abstract

Chinook salmon Oncorhynchus tshawytscha were introduced into the Lake Michigan basin from 1966 to 1968 with the transfer of fertilized eggs from the Green River Hatchery, Washington. In the more than three decades since this founding, Chinook salmon occur and are managed throughout the Great Lakes basin as a mixture of feral and supplemented populations. To evaluate the extent of genetic diversity since initial founding, we examined the pattern of variation at 18 allozyme loci among feral Chinook salmon populations in seven Michigan watersheds. We observed little divergence and low heterogeneity among the seven populations, which was consistent with the pattern expected in a single, randomly mating gene pool. We believe that the two major mechanisms contributing to the lack of among-population heterogeneity are the relatively recent founding from a single source and the routine homogenization from annual releases of hatchery-propagated, common-brood-source juveniles. We compared genetic diversity in the founded population with that of the source population to assess impacts from management activities or other events that might have caused genetic bottlenecks. Six rare alleles observed in the source stock were absent in our combined Michigan samples (N = 256), and frequencies for two of these alleles were significantly different (P ≤ 0.01) between the source and Michigan samples. Several other allele frequencies also differed significantly between the source and founded populations, providing evidence that one or more sources of genetic drift have affected allelic variation in the Michigan population. Although conservation of native genetic diversity is not a prevailing issue for Chinook salmon in Michigan waters, management approaches aimed at maintaining overall compositional and structural diversity and any recently evolved but undetected river-specific diversity should avoid further depletion of genetic variation in this important Great Lakes recreational resource.