Grey seals are found in temperate waters on both sides of the northern Atlantic Ocean, in eastern Canada, northwestern Europe and Norway, as well as in the Baltic Sea. These large marine mammals congregate mainly on sandy and rocky islands, but also haulout along the coast. There are three breeding areas for the grey seal population in Canada. The largest colony occurs on Sable Island where over 50,000 pups are born, however pups are also born on the ice in the southern Gulf of St. Lawrence and on coastal island along Nova Scotia. Smaller groups exist farther south, off the coast of New England. Grey seals are not migratory but do exhibit some large scale (1000 km) seasonal movements to feeding areas across the contentinal shelves off eastern Canada where they feed on sandlance, redfish, and a variety of other groundfish.

Grey seals have a short coat of fur covering their body, which varies from silver, grey to black brown, and is usually covered with irregular dark spots. Adult males have a horse-shaped head, a long muzzle and small, widely separated eyes, whereas females have a more dog-like head. They can be distinguished from harbour seals by their nostrils which are slanted. Newly born pups have long, soft white coats for the first few weeks of their lives. As they mature, males and females develop differently: males become darker while females develop a lighter, silvery grey coat with irregular dark spots. Adult males are Canada's largest seal -they can grow more than 2.3 metres long and weigh up to 400 kg, whereas females grow to 1.8 metres long and up to 250 kg. Grey seals are the longest lived pinniped, with females living as long as 45 years. Pups are born in January and are weaned after only 16-18 days of care, having tripled their body weight by feeding on milk with 60% fat.