Grey Seals are large sexually dimorphic phocids. In the United Kingdom, adult males are on average 2.07 m long and weigh an average 233 kg, with a maximum of 310 kg. Adult females average 1.8 m in length and have a postpartum body mass of 185 kg (Pomeroy et al. 1999). At birth, pups are 90-105 cm, with male pups averaging 15.8 kg and female pups averaging 14.8 kg in weight (Bonner 1981). Pup weaning weights average 45 kg for males and 41 kg for females. Pupping occurs from September to March with peak dates differing significantly between populations: in Eastern Atlantic, September-December; and in the Baltic Sea, late February to early March, which probably reproductively isolates each group (Bonner 1981). Pups are born on land in the Eastern Atlantic and ice in the Baltic Sea (Bonner 1981). Pups are born with a woolly lanugo that is molted several weeks after weaning. Lactation lasts 17 days on average and weaning is abrupt (Pomeroy et al. 1999). Pups undergo a post-weaning fast on land of about three weeks during which they lose 24% of their body weight (Reilly 1991, Noren et al. 2008). Males do not defend a fixed territory, rather they defend individual or small groups of females until they mate with those females and then move on to other females (Anderson et al. 1975). Larger males generally stay longer in the colony and have higher reproductive success than smaller males (Anderson and Fedak 1985). Survival is age dependent in males and females, but throughout life adult females have higher survival than adult males (0.988 vs 0.904, den Heyer and Bowen 2017). Survival in the first year of life and to recruitment is positively related to pup size at weaning (Hall et al. 2001; Bowen et al. 2015).

Grey Seals do not undertake long-distance migrations but do exhibit seasonal dispersal throughout their range. In one study, they spent 43% of their time within 10 km of frequently used haul-out sites, traveling to sandy areas, or gravel sea-bed sediments, the preferred habitat of sandeels, an important part of the Grey Seal’s diet. Short trips away averaged 2.3 days (McConnell et al. 1999). More recent research indicates that Grey Seals often forage in the same areas within 100km of a haulout site although more distance foraging also occurs (Russell et al. 2013, 2015) with occasional long trips of up to 2,100 km, crossing from northern England to Shetland or Orkney, and from the Faroes into the North Sea. In the Baltic Sea, Grey Seals make short trips, spending roughly 75% of their time within 50 km of haul-out sites (Sjoberg and Ball 2000).

Grey Seals are generalist predators feeding mainly on a variety of pelagic and benthic fish species at depths of up to 100m, although they can feed at all the depths found across the UK continental shelf (Smout et al. 2014; SCOS 2020). They consume a wide variety of prey including sandeels (usually the dominate prey), gadoids (cod, whiting, haddock, ling), and flatfish (plaice, sole, flounder, dab) (Prime and Hammond 1990). Diet varies seasonally and from region to region (Wilson and Hammond 2019).

In the Eastern Atlantic, Grey Seals are abundant upper-trophic level consumers. They are potential competitors with harbour seals as their diets overlap (Wilson and Hammond 2019). In some areas, at sea movement data show that Grey Seals use offshore areas connected to their haul-out sites by prominent corridors, whereas harbour seals primarily stay within 50 km of the coastline. Both species show fine-scale offshore spatial segregation off the east coast of Britain and broad-scale partitioning off western Scotland (Jones et al. 2015). Grey Seals also interact with fisheries. In the West of Scotland, estimates of cod consumption by Grey Seals have exceeded reported catches and spawning biomass. A simple population model was used to identify the likely direction of cod population change at recent mortality rates (Cook et al. 2016). The analysis suggests that predation by grey seals can be an important component of the total stock mortality. Increasing seal populations in central and southern North Sea are likely to increase levels of interactions between seals and fisheries in the region (SCOS 2020). Depredation of salmon by Grey Seals from coastal static net fisheries represented a significant economic loss to the fisheries concerned and an additional source of mortality for salmonids, a source of mortality that is probably largely dependent upon the presence of the net fishery. Grey Seals have few natural predators but are known to be taken by killer whales (Wier 2002) and presumably large sharks as reported for the northwest Atlantic subspecies (Bowen 2011).

Grey Seals have a cold temperate to sub-Arctic distribution in North Atlantic waters over the continental shelf (Hall 2002). There are three populations isolated both geographically and by the timing of reproduction (Bonner 1981); mtDNA differences are large between these three breeding areas, though the Baltic and Northeast Atlantic populations are much closer to one another than to the Northwest Atlantic population (Boskovic et al. 1996). In the western Atlantic, the population is centred on the eastern Scotian Shelf off northeastern North America, but the Grey Seal ranges from the Gulf of Maine to southern Labrador, including the Gulf of St Lawrence (Lesage and Hammill 2001). The northeast Atlantic population is concentrated around the UK and Ireland but is also found around Iceland, the Faroe Islands, and along the European mainland coast from the Kola Peninsula (Russia) south to southern Norway, and from Denmark to Brittany in France. The Baltic Sea subpopulation is confined to the Baltic Sea (Bonner 1981, Hall 2002). Vagrants are known from as far south as New Jersey in the western Atlantic and Portugal in the eastern Atlantic (Rice 1998).

Numerous countries have invoked protective measures to limit Grey Seal harvests, culls, disturbance, and by-catch (Bonner 1981, ICES 2005). In the UK, Grey Seals are protected under the Conservation of Seals Act 1970 (England, and Wales), the Marine (Scotland) Act 2010 and The Wildlife (Northern Ireland) Order 1985. Until recently permits to protect fishing and aquaculture operations from grey seals were required, however, the lethal removal of grey seals in defence of gear damage this provision has been removed from legislation (SCOS 2020). In the UK such shooting was already restricted (in numbers and/or location) and it was unlikely to have had impacts on population trends.

Special Areas of Conservation (SAC) set out under the European Union's Habitats Directive (92/43/EEC) have been established to support the conservation of grey seals throughout its range. Pollutant loads in Baltic grey seals have declined in step with regulations banning the use and/or discharge of toxic pollutants such as DDT and PCBs beginning in the 1970s, and the reproductive health of female Grey Seals has improved as has the population level in the Baltic (Bergman et al. 2001). The establishment of coastal marine reserves for seals in Norway has been more effective in protecting harbour seals than Grey Seals because the latter are more likely to travel outside the areas closed to fisheries and become entangled in nets (Bjore et al. 2002).