Adult females have a straight profile to the dorsal surface of the head. The massive skull of Halichoerus shows the elevated frontonasal area and large narial openings also found in Cystophora, but the cheek teeth are larger and stronger. The gray seal generally breeds on rocky or cliff-backed coasts or on small offshore islands and is most often observed foraging in waters adjacent to such areas. It is also seen frequently in estuaries, particularly when salmon are present. Certain breeding colonies are present on open sandy beaches, most notably on Sable Island, a sand bar about 40 km long and 1.5 km wide east of Nova Scotia.
Some breeding also occurs in sea caves.
In the Baltic Sea and the Gulf of St. Lawrence breeding usually takes place on drifting or shorefast ice. There are no regular migrations, but young animals disperse widely. Pups from the Farne Islands sometimes appear in Norway, Denmark, and the Netherlands. Such individuals probably return to their natal beaches during the breeding season. Adults may forage in coastal waters after breeding but then come ashore again to molt, generally not at the same sites used for breeding.