Genetic diversity from pre-bottleneck to recovery in two sympatric pinniped species in the Northwest Atlantic

Conservation successes of the past several decades provide natural settings to study post-bottleneck evolutionary processes in species undergoing recovery. Here, we study the impact of demographic change on genetic diversity in 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 concolor). We compare genetic diversity at the mitochondrial control region today to diversity in archaeological specimens, which represent the populations prior to the regional bounties of the late 1800s to mid-1900s that drastically reduced population sizes and led to local extirpations. We further assess genetic diversity throughout recovery, using biological collections from ongoing long-term studies of both species. Overall, the genetic data are consistent with the historical presence of large, genetically diverse populations of pinnipeds prior to human exploitation, and suggest that gray seals were more dramatically impacted by historical bottlenecks than harbor seals in the Northwest Atlantic. Current mitochondrial diversity in both species is relatively high, and we observe little change over the past several decades during a period of roughly parallel rapid population increases. However, there remain large differences in haplotype composition between pinniped populations of pre-exploitation and today, a lasting genetic signature of historical exploitation that is likely to persist into the future.