Hierarchical Phylogeographic Structure of Coho Salmon in California

Evaluation of population genetic structure and variation is an important part of planning for the recovery and management of protected species. Data from 18 polymorphic microsatellite DNA markers were used to analyze the phylogeographic structure of protected Coho Salmon Oncorhynchus kisutch from populations throughout California. Fish from 30 locations in two evolutionarily significant units (ESUs) representing most of the extant populations in the state were studied. Multiple analyses indicated a hierarchical pattern of population structure: the greatest divergence was found at the broadest geographic scale (ESU), followed by the divergences between basins and populations within basins. The populations of the large Klamath River basin were consistently identified as a distinct phylogenetic group, nearly as divergent from all other populations as the two ESUs were from each other. All populations in different basins were differentiated from each other and a pattern of isolation by distance was found at a California-wide scale, but not at smaller spatial scales. Similarly, most individuals were accurately assigned to their population of origin, and almost all misassignments were to an adjacent or geographically proximal basin, indicating that there is substantial gene flow within each region but much less between regions. The number of parents contributing to each population was highly variable and reflected larger patterns of genetic variation, which was found to be generally higher in the southerly, low-elevation coastal populations than in the northern, interior, higher-elevation populations. The results strongly support the current boundary between the two ESU regions, and the detailed understanding of phylogeographic structure provided here will help to guide the management and recovery of Coho Salmon at the southern end of their geographic range.

Received August 25, 2015; accepted May 4, 2016 Published online August 12, 2016