Variation of mitochondrial control region sequences of Steller sea lions: the three-stock hypothesis

Sequence variation of a 238 bp segment of the mitochondrial control region was analyzed for 1,568 Steller sea lions (2.8% of the estimated species population) sampled from 50 rookeries representing nearly every locality at which Steller sea lions are known to breed in significant numbers. Haplotype diversity (H = 0.9164 ? 0.0035) was high and nucleotide diversity (? = 0.00967 ? 0.00586) was moderate. No evidence was observed for significant genetic bottleneck effects. Rookeries were grouped into regions and stocks to examine structure at different spatial scales. F- and ?-statistics were computed for all pairwise comparisons of rookeries, regions and stocks. Significant (P<0.05) divergence of eastern stock (southeastern Alaska to California) animals from western stock animals was supported in analyses at all spatial scales. Likewise, rookeries and regions from Asia were found to be significantly different from all other western stock rookeries. This was most clearly demonstrated using ?-statistics at the regional level. The Commander Islands clearly associate with Alaskan western stock rookeries, not with the Asian rookeries. Within each of the three stocks there is significant isolation by distance among rookeries. This relationship does not hold for inter-stock comparisons indicating that there are important barriers to gene flow among stocks. Mitochondrial DNA analysis supports the recognition of three stocks for appropriate conservation of the species. The currently recognized eastern stock is unaffected, but the western stock is now partitioned west of the Commander Islands yielding a western stock which ranges from Prince William Sound west to the Commander Islands, and an Asian stock including rookeries from the Kamchatka Peninsula, Kuril Islands, and Sea of Okhtosk.