The influence of hydrographic structure and seasonal run timing on genetic diversity and isolation-by-distance in chum salmon (Oncorhynchus keta)

We used 20 microsatellite loci to compare genetic diversity and patterns of isolation-by-distance among three groups of chum salmon (Oncorhynchus keta) from two physically distinct watersheds in western Alaska, USA. The results were consistent with the hypothesis that gene flow decreases as the complexity of the hydrographic system increases. Specifically, higher gene flow was inferred among 11 populations from a nonhierarchical collection of short coastal rivers in Norton Sound compared with 29 populations from a complex hierarchical network of inland tributaries of the Yukon River. Within the Yukon river, inferred gene flow was highest among 15 summer-run populations that spawn in the lower drainage, compared with 14 fall-run populations that spawn in the upper drainage. The results suggest that the complexity of the hydrographic system may influence population connectivity and hence the level of genetic diversity of western Alaska chum salmon. Finally, evidence of isolation-by-time, when controlling for geographic distance, supported the hypothesis that genetic divergence in Yukon River chum salmon is influenced by seasonal run timing. However, evidence of isolation-by-distance, when controlling for season run timing, indicated the populations are not sufficiently isolated, spatially or temporally, to prevent gene flow. Dispersal among summer- and fall-run populations may play a role in maintaining genetic diversity.