Fine-scale sampling reveals distinct isolation by distance patterns in chum salmon (Oncorhynchus keta) populations occupying a glacially dynamic environment

Abstract

Populations with spatially restricted gene flow are characterized by genetic differentiation that may be positively correlated with the geographic distance separating populations, a pattern known as isolation by distance (IBD). Here we examined the fine-scale genetic structure of 66 chum salmon (Oncorhynchus keta) populations spawning in Alaska waterways and explored patterns of IBD using 90 nuclear and 3 mitochondrial single nucleotide polymorphisms. Estimating population structure of chum salmon in Alaska is of increasing concern because of fluctuating census sizes and the uncertain effects of harvest on specific populations. We hypothesized that IBD would be present because chum salmon spawn in coastal rivers that are distributed along a linear array and gene flow is spatially restricted due to homing. Evidence of very weak IBD was found throughout the region (R2 = 0.06, p < 0.0001) but the strength of the IBD relationship varied greatly over different spatial scales and geographic regions. Decomposed pairwise regression analyses identified nine outlier populations to regional IBD patterns, suggesting that geographic distance is not the only factor influencing genetic differentiation in the region. Instead, population structure appears to be heavily influenced by glacial history of the region and the presence of a glacial refugium on Kodiak Island.