Population Structure and History of Steelhead Trout in California

Steelhead trout (Oncorhynchus mykiss) are the most widespread of the anadromous salmonids. In the western continental United States, 14 Evolutionarily Significant Units (ESUs) have been delineated on the basis of genetic, geographic and ecological variation. Of these 14 western ESUs, 11 have been listed as protected under the US Endangered Species Act (ESA). In California, there are six ESUs, 5 on the coast and 1 in the Central Valley. All but the Klamath Mountain Province ESU is listed, with the Southern California ESU classified as “Endangered” and the others “Threatened”. Here, we investigate genetic population structure and demographic history of steelhead trout in coastal California using multilocus genetic data. We use size variation at highly variable microsatellite loci from populations at 62 sites from 41 basins, covering almost the entire range of the species in coastal California. The list of sites is found in Table 1. Results of phylogeographic analyses and assignment tests are described and the partitioning of variation at the tributary, river basin and ESU level is examined.