Although historically distributed along the northeast coast of the United States (US), grey seals ( Halichoerus grypus (Fabricius, 1791)) were considered locally extinct until the late 1980s when three naturally re-established pupping colonies 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 assessed 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 diversity 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 demonstrated that the genetic structure of the northwest Atlantic grey seal population is not different from the null hypothesis of panmixia.
Gray seals were historically distributed along the northeastern coast of the United States, but bounties and lack of protection reduced numbers and they were rarely observed for most of the 20th century. Once protections were enacted, the population started to rebound. Here, we describe the recolonization and recovery of gray seals in the United States, focusing on the re-establishment of pupping sites. We fit individual generalized linear models to various time series (1988–2019) to estimate rates of increase in observed pup counts at four of the more data-rich sites. Annual rate of increase at individual sites ranged from −0.2% (95% CI: −2.3–1.9%) to 26.3% (95% CI: 21.6–31.4%). The increase in sites and number of pups born in the United States is driven by population growth and immigration from Canadian colonies and is part of a larger recovery of the Northwest Atlantic population. Wildlife protection, a healthy source population, habitat availability, and species traits that allow for dispersal and high productivity were all important factors in this recovery.
The northwest Atlantic subspecies of gray seal (Halicheorus grypus grypus) has been increasing for more than a half century and has reestablished breeding colonies in Canadian and US waters. In 2016, visual, oblique, and vertical large-format digital photographic surveys were conducted at all known breeding colonies in the northwest Atlantic. Total pup production in the northwest Atlantic was estimated to be 109,000 (SE = 17,500) pups. At 87,500 (SE = 15,100) pups, Sable Island accounts for 80% of total pup production. Regional differences in pup production trends are evident. Pup production in the Gulf of St. Lawrence and along the eastern shore of Nova Scotia has been relatively stable. Since 2004, the rate of increase in pup production at Sable Island has slowed to about 5%-7% per
Population increases over the past several decades provide natural settings in which to study the evolutionary processes that occur during bottleneck, growth, and spatial expansion. We used parallel natural experiments of historical decline and subsequent recovery in two sympatric pinniped species in the Northwest Atlantic, the gray seal (Halichoerus grypus atlantica) and harbor seal (Phoca vitulina vitulina), to study the impact of recent demographic change in genomic diversity. Using restriction site‐associated DNA sequencing, we assessed genomic diversity at over 8,700 polymorphic gray seal loci and 3,700 polymorphic harbor seal loci in samples from multiple cohorts collected throughout recovery over the past half‐century. Despite significant differences in the degree of genetic diversity assessed in the two species, we found signatures of historical bottlenecks in the contemporary genomes of both gray and harbor seals. We evaluated temporal trends in diversity across cohorts, as well as compared samples from sites at both the center and edge of a recent gray seal range expansion, but found no significant change in genomewide diversity following recovery. We did, however, find that the variance and degree of allele frequency change measured over the past several decades were significantly different from neutral expectations of drift under population growth. These two cases of well‐described demographic history provide opportunities for critical evaluation of current approaches to simulating and understanding the genetic effects of historical demographic change in natural populations.
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