How Many Seals Were There? The Global Shelf Loss during the Last Glacial Maximum and Its Effect on the Size and Distribution of Grey Seal Populations

Abstract

Predicting how marine mammal populations respond to habitat changes will be essential for developing conservation management strategies in the 21st century. Responses to previous environmental change may be informative in the development of predictive models. Here we describe the likely effects of the last ice age on grey seal population size and distribution. We use satellite telemetry data to define grey seal foraging habitat in terms of the temperature and depth ranges exploited by the contemporary populations. We estimate the available extent of such habitat in the North Atlantic at present (between 1.42·106 km2 and 2.07·106 km2) and at the last glacial maximum (between 4.74·104 km2 and 2.11·105 km2); taking account of glacial and seasonal sea-ice coverage, estimated reductions of sea-level (123 m) and sea surface temperature hind-casts. Most of the extensive continental shelf waters (North Sea, Baltic Sea and Scotian Shelf), currently supporting >95% of grey seals, were unavailable during the last glacial maximum. A combination of lower sea-level and extensive ice-sheets, massively increased seasonal sea-ice coverage and southerly extent of cold water would have pushed grey seals into areas with no significant shelf waters. The habitat during the last glacial maximum might have been as small as 3% of today's extent and grey seal populations may have fallen to similarly low numbers. An alternative scenario involving a major change to a pelagic or bathy-pelagic foraging niche cannot be discounted. However, hooded seals currently dominate that niche and may have excluded grey seals from such habitat. If as seems likely, the grey seal population fell to very low levels it would have remained low for several thousand years before expanding into current habitats over the past 12,000 years or so.

Biogeography and temporal progression during the evolution of striped dolphin population structure in European waters

“…According to Boehme et al. () investigating habitat suitability for grey seals ( Halichoerus grypus ), during the LGM the higher latitudes of the North Atlantic Ocean were isolated, and only the Biscay Gulf and Iberian Peninsula were suitable for habitation. Our data suggest that population differentiation was generated in this region following the LGM, and a post‐glacial expansion associated with population structuring in the eastern North Atlantic has been discussed for other marine mammals (Banguera‐Hinestroza et al., , ; Fontaine et al., ).…”

It’s about time: A synthesis of changing phenology in the Gulf of Maine ecosystem

“…Although gray seals use coastal mainland habitats for pupping, these areas are associated with increased disturbance from human activities (Lesage & Hammill, ). In contrast, preference for ice‐free waters drives harbor seals ( Phoca vitulina concolor ) to move into southern New England during the winter and spring (Baechler, Beck, & Bowen, ; Boehme et al, ; Burns, ). A portion of the North Atlantic right whale population and some seabirds continue to utilize habitats within the GoM during winter (Cole et al, ; Fayet et al, ), although their activities during this time are still largely unknown.…”

“…Many benthic and pelagic species that inhabit the continental shelf such as American lobster (Homarus americanus), Atlantic mackerel (Scomber scombrus), and longfin squid (Doryteuthis pealeii) move from their deeper offshore overwintering grounds in late spring as water temperatures and prey concentrations rise in shallow coastal areas (Black, Rowell, & Dawe, 1987;Collette & Klein-MacPhee, 2002;Cooper & Uzmann, 1971;Ennis, 1984;Lange & Sissenwine, 1980;Methratta & Link, 2007;Mills et al, 2013). Conversely, spiny dogfish (Squalus acanthias) migrate offshore to deeper, relatively warmer waters for parturition (Methratta & Link, 2007;Sagarese et al, 2014).…”

“…to move into southern New England during the winter and spring (Baechler, Beck, & Bowen, 2002;Boehme et al, 2012;Burns, 2009).…”

Genomic signatures of population bottleneck and recovery in Northwest Atlantic pinnipeds

“…This demographic history is described in climatic, archeological, and historical records. Low availability of continental shelf habitat in the North Atlantic during the last glacial maxima suggests gray and harbor seal populations may have been small (~15,000–21,000 gray seals) until approximately 12,000 years ago when habitats expanded following glacial retreat and the populations accordingly increased (Boehme et al., 2012). Historical records thereafter suggest an abundance of seals in the Northwest Atlantic in the 16th century when European explorers arrived (reviewed in Lavigueur & Hammill, 1993), but by the mid‐20th century, subsistence hunting, government‐sponsored bounties (Bowen & Lidgard, 2013; Lelli, Harris, & Aboueissa, 2009), and commercial exploitation (Mowat, 1984) had drastically reduced both seal populations.…”