It is not yet clear if the Canadian population is simply growing and moving south, or if there are other, environmental factors at work. Highly gregarious, gray seals are often found in large groups hauled out on quiet sand or rock beaches for rest and breeding. Females in this area, such as Monomoy Island in Nantucket Sound, give birth to one, white-coated pup from late December to mid February. The pup is nursed intensively for about 15 to 20 days on an increasingly fatty milk. Females come into estrus about 2 weeks after weaning their pups.
Males are highly competitive over access to groups of females on shore.
Usually they have light gray or white coats marked with dark brown patches about the head and back. These dark markings may form a U on the back, much like the base of a hand held, musical harp. Despite the fact that harp seals are considered a northern species, dependent upon pack ice habitats for pupping, sightings of juveniles in the southern Gulf of Maine have increased in the last decade, during the winter months. Sightings of adults are less common but increasing. . When inflated, it forms a black, oval hood.