The Harbor Seal are the most aquatic member of the species. Males are six feet long, up to 300 pounds (rarely 550 pounds) white the females are five feet long, 150 pounds. There muzzles are short, head noticeably round, flippers well-haired, and there colors are extremely variable; from pale silver gray spotted with black to dark brown or almost black with spots hardly discernible. Females are darker than the males. The white fluffy fur of the young are highly valued in many regions.

The feeding habits of Harbor Seals have been studied closely in many parts of their range; they are known to prey primarily upon fish such as menhaden, anchovy, sea bass, herring, cod, whiting and flatfish, and occasionally upon shrimp, shellfish and squid. Harbor Seals are able to dive for up to ten minutes, reaching depths of 50 meters or more, but average dives may be three minutes long at depths of about 20 meters. In the winter they come out on ice. In the summer they come out on the shore.

Birthing of pups occurs annually on shore, beginning in February for populations in lower latitudes, and as late as July in the subarctic zone. The mothers are the sole providers of care with lactation lasting four to six weeks; males occupy themselves with fights between other males. The pups are born singly and well developed, capable of swimming and diving within hours. Suckling for three to four weeks, pups feed on the mother's rich, fatty milk and grow rapidly; born weighing up to 16 kilograms, the pups may double their weight by the time of weaning.

This type of seal is considered to be solitary, but can be very aggressive amongst several hundred. The aggressive type of Fighting involves biting, head butting, flipper waving snorting and growling. Males are thought to be polygamous and may fight for mating privileges.

The species is divided into three main populations, in the Arctic Ocean, in Greenland, and in Newfoundland. Their habitat is in Arctic waters and ice floes or along the coastlines. Atlantic harbor seals migrate to southeastern New England from Maine and Canada during the winter months (November - April). In April, these seals return to the Gulf of Maine to give birth to young pups. During low tides, harbor seals bask on exposed rocks off the shores in New England. Sometimes, they will bask on the beaches that are not populated with people during the winter.

They can be found along the coastal regions of Canada, China (south toKiangsu), Denmark, Germany, Great Britain, Greenland, Iceland, Ireland, Japan (Hokkaido), Mexico (Baja californai, Isla Guadalupe-vagrant), Netherlands, Norway, Portugal, Rusia (Kurile Isls and Kamchatka), Sweden, USA (Atlantic coast: Maine, Massachussetts, New Hampshire, vagrants: New York, Florida, Vermont, Pacific Coast: Alaska, Washingon, Oregon, California).