To address the question of population structure and provide much needed data for contemporary stock assessments, we used both mitochondrial (mtDNA) control region (CR) sequences and nuclear (nDNA) microsatellite genotypes to examine the genetic structure of both resident and Bigg’s killer whales in the northern North Pacific and to test hypotheses based on mtDNA phylogeny and behavioral data. Molecular genetic analyses were applied to 462 killer whale biopsy samples collected between the northern Gulf of Alaska and the Sea of Okhotsk (Fig. 2). Estimates of genetic distance between the two predominant North Pacific ecotypes indicated negligible levels of gene flow between ecotypes, highlighting the genetic and demographic isolation of these two divergent evolutionary lineages in the North Pacific. A recent paper using mitogenome sequences estimated that transient killer whales diverged from all other killer whale lineages some 700,000 years ago (Morin et al. 2010, Genome Research); our recent finding of negligible gene flow in nuclear DNA further emphasizes a lack of contemporary male-mediated gene flow between ecotypes. Analysis of molecular genetic data also revealed significant levels of population genetic subdivision within the two Orcinus ecotypes using both mitochondrial control region sequences and nuclear microsatellite (26 loci) genotypes. Strong evidence of geographic patterns of genetic differentiation was supported by significant regions of genetic discontinuity, providing evidence of multiple subpopulations within the currently recognized stocks for both resident and transient killer whales. Interestingly, the resolved patterns of population genetic subdivision suggested some notable differences in the geographic structuring of subpopulations between the two ecotypes.
Click maps to enlarge.
In the Aleutian Islands, subpopulations, or groups with significantly different mtDNA and microsatellite allele frequencies, were largely delimited by boundaries that coincided with major passes in the Aleutian Islands for the fish-eating resident-type killer whales (Fig. 3). A population subdivision for resident killer whales at Samalga Pass was supported by both a Bayesian cluster analysis of nDNA genotypic data and a striking shift in the frequency of mtDNA haplotypes. Samalga Pass has also been recognized as a physical and biogeographic boundary between the eastern and central Aleutians (Ladd et al. 2005, Fisheries Oceanography). While Amchitka Pass represented a major subdivision for Bigg’s killer whales between the central and western Aleutian Islands, whales sampled around the eastern Aleutians appeared to be genetically differentiated from those sampled near the Pribilof Islands in the Bering Sea (Fig. 4).
There was also significant support for a smaller sympatric subpopulation (UI) around Unimak Island that distinguished a small number of killer whales that have been observed intercepting and preying on northward-migrating gray whales in the waters around Unimak Island during the late spring and early summer (Barrett-Lennard et al. 2011, Marine Ecology Progress Series). In the western North Pacific, data for both nDNA and mtDNA suggested that killer whales in the western Aleutians are part of a population that includes Russia, with the boundaries between subpopulations occurring at Buldir Pass (residents) and Amchitka Pass (Bigg’s killer whale).
The patterns of genetic structure resolved by the current study provide strong evidence for the existence of multiple subpopulations of killer whales across the northern North Pacific, highlighting the need to revisit current stock designations. This species is impacted through both direct and indirect interactions with commercial fisheries. Evidence of population differentiation in this highly mobile species is a critical component for evaluating the impacts of incidental bycatch and estimating predator-prey relationships for species such as the endangered Western stock of Steller sea lions (Eumetopias jubatus).
