Northern fur seals are known for their thick fur, which gives them their name. There are about 300,000 hairs on each square inch of their bodies. Adult males grow short, bushy manes with lighter-colored fur around their neck and shoulders, occasionally seen on females. The color of their fur shows their gender, age, and activities. Females and young males at sea typically have gray coats. On land, while breeding, the fur tends to become yellowish-brown due to the mud and excrement found on the rocks. Males usually have a brownish-black in color when they are older, but this could be reddish brown or dark gray. Pups are black, the color changing to dark brown, with a lighter color on the belly and chest.
Northern fur seals inhabit the Northern Pacific Ocean from southern California to Japan and as far as the Bering Sea. They spend much of their time at sea, usually coming to land just for the summer breeding season, males spending only about 45 days each year on land, and females about 35 days.
Northern fur seals are carnivores hunting at night, being primarily nocturnal. They are relatively social, tending to be solitary during the feeding months of winter. They sometimes swim about in larger groups. Males show some aggressive territorial behavior towards other males when their territory is entered. Females do not develop any social bond with any other seals. A characteristic of these seals is returning to where they were born (called "philopatry"), males returning to stake out territory and females to give birth.
These seals are carnivorous, and eat mainly fish and cephalopods, mostly small fish from schools, such as anchovy, capelin and herring. They also eat squid and sometimes hake, rockfish, saury, and salmon.
These seals are polygynous and in one season a male can mate with as many as 50 females. Females come to shore from late June for a month. The males will have already staked out territories. Females usually give birth to just one pup each season, after a gestation of 51 weeks. A few days after giving birth, the female goes to sea for 8 - 14 days to feed, then returning to nurse her pup, who must consume enough milk to last for these absences. Pups are weaned when about 4 months old, at which time their mothers leave to migrate south for winter. Females are sexually mature between 4 and 6 years, males between 8 and 10.
Historically, humans are the main threat to this seal. They have been hunted mainly for their pelts. Commercial hunting continued till 1984. Alaska native people are still allowed to hunt for this seal and at least 200-500 seals are taken every year. Other threats are bycatch in fishing gear, entanglement in marine debris, disturbance from vessels, habitat loss and climate change, the availability of prey and environmental pollutants.
According to IUCN, as of 2014, the Northern fur seal population have been estimated at 1 - 1.3 million, the majority breeding on the Pribilof Islands located in the southern Bering Sea. The Commander Islands is the second largest breeding site for the northern fur seal, where about 225,000 go to breed. Smaller sites include Tyuleniy Island in the Okhotsk Sea (55,000 - 65,000 seals), the central Kuril Islands (50,000 - 55,000), Bogoslof Island in the Aleutian Islands (5,000), and San Miguel Island off southern California (4,300). The ICUN classifies the Northern fur seal as "Vulnerable", their population is decreasing now.
Northern fur seals are predators of schooled fish and squid and are prey of several larger marine species.