Wildlife management and conservation requires knowledge about a species’ population structure, diversity, demographic history and adaptive potential. However, often such information is lacking, or based on insufficient and sometimes contrasting data. This is the case for the grey seal (Halichoerus grypus), for which there remain uncertainties regarding subspecies and population delineations, diversity and recent evolutionary history, despite numerous genetic and non-genetic studies. Here, we present the first range- and genome-wide population genomic analysis of grey seals based on 3,812 nuclear SNP markers genotyped in 188 samples from 17 distinct localities. Our analyses support the existence of three main grey seal populations centred in the NW Atlantic, NE Atlantic and Baltic Sea, but also point to the existence of previously unrecognised substructure within the NE Atlantic, in particular separating grey seals sampled in Iceland, Norway and Russia from the core NE Atlantic population inhabiting the wider North Sea region. We detected remarkably low levels of genetic diversity in NW Atlantic grey seals, which may be the result of evolutionary founder effects, as well as more recent historic hunting and culling. We also found some localities that deviate from the general isolation by distance pattern, likely reflecting wide-scale metapopulation dynamics associated with recolonisation and recovery of grey seals in regions where they were historically extirpated. Our genetic results allow us to identify at least six grey seal management units across the species’ Atlantic range, but also highlights knowledge gaps that should be addressed in future research into this species.

Genetic analyses of host-specific parasites can elucidate the evolutionary histories and biological features of their hosts. Here, we used population-genomic analyses of ectoparasitic seal lice (Echinophthirius horridus) to shed light on the postglacial history of seals in the Arctic Ocean and the Baltic Sea region. One key question was the enigmatic origin of relict landlocked ringed seal populations in lakes Saimaa and Ladoga in northern Europe. We found that that lice of four postglacially diverged subspecies of the ringed seal (Pusa hispida) and Baltic gray seal (Halichoerus grypus), like their hosts, form genetically differentiated entities. Using coalescent-based demographic inference, we show that the sequence of divergences of the louse populations is consistent with the geological history of lake formation. In addition, local effective population sizes of the lice are generally proportional to the census sizes of their respective seal host populations. Genome-based reconstructions of long-term effective population sizes revealed clear differences among louse populations associated with gray vs. ringed seals, with apparent links to Pleistocene and Holocene climatic variation as well as to the isolation histories of ringed seal subspecies. Interestingly, our analyses also revealed ancient gene flow between the lice of Baltic gray and ringed seals, suggesting that the distributions of Baltic seals overlapped to a greater extent in the past than is the case today. Taken together, our results demonstrate how genomic information from specialized parasites with higher mutation and substitution rates than their hosts can potentially illuminate finer scale population genetic patterns than similar data from their hosts.