The genetics of recolonization: an analysis of the stock structure of grey seals (Halichoerus grypus) in the northwest Atlantic

Although historically distributed along the northeast coast of the United States (US), grey seals (Halichoerus gry- pus (Fabricius, 1791)) were considered locally extinct until the late 1980s when three naturally re-established pupping colo- nies were discovered. Two large populations in Canada, the Gulf of St. Lawrence (GSL) and Sable Island (SI) seals, are possible sources of immigrants for the recovering US population. To assess the stock structure of grey seals in the northwest Atlantic, tissue samples were collected from Canadian and US populations for genetic analyses. We examined nine highly variable microsatellite loci (n = 158; mean number of alleles per locus = 7.22). When population differentiation was as- sessed without a priori inference of potential subpopulations, all individuals were placed into one population. Pairwise FST values showed little difference in allele frequencies between the SI and the GSL or the Canadian and the US samples. We sequenced a 319 bp segment of the mitochondrial control region and identified 25 haplotypes (n = 163). Nucleotide diver- sity was similar at SI, GSL, and the US sites. Based on mtDNA haplotypes, no significant difference was found between the SI and GSL populations or the Canadian and the US populations. Although grey seals are philopatric, our study demon- strated that the genetic structure of the northwest Atlantic grey seal population is not different from the null hypothesis of panmixia.