Genetic and behavioral evidence for restricted gene flow among coastal cutthroat trout populations

Using a genetic and tag-recapture study, we assessed the geographic scale of population structuring and dynamics of gene flow in collections of coastal cutthroat trout Oncorhynchus clarki clarki in Washington State. Tests for genotypic differentiation at 10 microsatellite loci were significant for all pairwise comparisons among 10 collections from Hood Canal creeks separated by about 2-100 km. Similarly, F-statistic analogues indicated that low but significant population structure existed among the nine anadromous collections (θ_ST = 0.030, ρ_ST = 0.029). Results from tests for isolation by distance revealed significant correlation between genetic and geographic distances. Previously published genetic data at six common loci from 14 coastal cutthroat trout collections across Washington were incorporated for some additional analyses. When analyzing all 22 anadromous collections at the six common loci, estimates of population structure increased to 0.088 for θ_ST and 0.081 for ρ_ST. All collections were genetically heterogeneous and tended to cluster with their nearest geographic neighbors. Direct observation of migration patterns over 3 years of tag-recapture study in four creeks confirmed that adult coastal cutthroat trout migrate among those creeks and provided estimates for the effective numbers of migrants per generation that were generally consistent with those estimated from the allele-frequency data. Our results suggest that independent creeks contain distinct populations that form fundamental units in the genetic structuring of coastal cutthroat trout.