Population Structure as Revealed by mtDNA and Microsatellites in Northern Fur Seals, Callorhinus ursinus, throughout Their Range

Background: The northern fur seal (Callorhinus ursinus; NFS) is a widely distributed pinniped that has been shown to exhibit a high degree of philopatry to islands, breeding areas on an island, and even to specific segments of breeding areas. This level of philopatry could conceivably lead to highly genetically divergent populations. However, northern fur seals have the potential for dispersal across large distances and have experienced repeated rapid population expansions following glacial retreat and the more recent cessation of intensive harvest pressure. Methodology/Principal Findings: Using microsatellite and mitochondrial loci, we examined population structure in NFS throughout their range. We found only weak population genetic structure among breeding islands including significant FST and W ST values between eastern and western Pacific islands. Conclusions: We conclude that insufficient time since rapid population expansion events (both post glacial and following the cessation of intense harvest pressure) mixed with low levels of contemporary migration have resulted in an absence of genetic structure across the entire northern fur seal range

Bayesian skyline plot of historical female effective population size, light lines represent the 95% highest posterior probability density around the estimate.

<p>Bayesian skyline plot of historical female effective population size, light lines represent the 95% highest posterior probability density around the estimate.</p

Summary of mtDNA diversity in 9 populations of Northern fur seal.

<p>*haplotypic diversity (h), % nucleotide diversity (Π). Standard deviations in parentheses.</p

Distribution of northern fur seal breeding sites.

<p>Distribution of northern fur seal breeding sites.</p

Minimum spanning network of 112 core mitochondrial DNA haplotypes of northern fur seals.

<p>Branch lengths are the minimum number of steps between haplotypes. The size of the circle representing the individual haplotypes corresponds to the abundance of that haplotype. Numbers identify the most abundant haplotypes. Dashed lines represent alternative groupings.</p

Mitochondrial DNA based population differentiation for population pairs (estimates of F_ST above diagonal and Φ_ST below diagonal).

<p>*Bold indicates significant <i>P</i>-values (<i>P</i><0.05).</p

The observed pairwise mismatch distribution of mtDNA in northern fur seals as compared to the expected distribution based upon a model of sudden population expansion.

<p>The observed pairwise mismatch distribution of mtDNA in northern fur seals as compared to the expected distribution based upon a model of sudden population expansion.</p

Isolation by distance based on mitochondrial DNA analysis in northern fur seals including the relationship between genetic distance, pairwise comparisons of rookeries (Φ_ST ) and the natural log of the geographic distance between the rookery pairs.

<p>Isolation by distance based on mitochondrial DNA analysis in northern fur seals including the relationship between genetic distance, pairwise comparisons of rookeries (Φ_ST ) and the natural log of the geographic distance between the rookery pairs.</p

Extreme weather events influence dispersal of naive northern fur seals

Since 1975, northern fur seal (Callorhinus ursinus) numbers at the Pribilof Islands (PI) in the Bering Sea have declined rapidly for unknown reasons. Migratory dispersal and habitat choice may affect first-year survivorship, thereby contributing to this decline. We compared migratory behaviour of 166 naive pups during 2 years from islands with disparate population trends (increasing: Bogoslof and San Miguel Islands; declining: PI), hypothesizing that climatic conditions at weaning may differentially affect dispersal and survival. Atmospheric conditions (Bering Sea) in autumn 2005–2006 were anomalously cold, while 2006–2007 was considerably warmer and less stormy. In 2005, pups departed earlier at all sites, and the majority of PI pups (68–85%) departed within 1 day of Arctic storms and dispersed quickly, travelling southwards through the Aleutian Islands. Tailwinds enabled faster rates of travel than headwinds, a trend not previously shown for marine mammals. Weather effects were less pronounced at Bogoslof Island (approx. 400 km further south), and, at San Miguel Island, (California) departures were more gradual, and only influenced by wind and air pressure in 2005. We suggest that increasingly variable climatic conditions at weaning, particularly timing, frequency and intensity of autumnal storms in the Bering Sea, may alter timing, direction of dispersal and potentially survival of pups