Northern Fur Seal

Northern fur seals are members of the "eared seal" family (Otariidae). They spend most of the year in the ocean. Weaned pups typically spend nearly 2 years away before returning to their breeding colonies. Northern fur seals primarily use open ocean for foraging and rocky beaches for resting, molting, and reproduction.  

Historically, northern fur seals were hunted for their fur on land and at sea. In 1911, the Fur Seal Treaty created an international prohibition on hunting fur seals at sea. In 1984, the United States ended commercial harvest of northern fur seals on the Pribilof Islands. A subsistence harvest is currently co-managed by NOAA Fisheries and the tribal governments of St. Paul and St. George.

Northern fur seals, like all marine mammals, are protected under the Marine Mammal Protection Act. The Pribilof Islands/eastern Pacific stock is listed as depleted under the MMPA. 

NOAA Fisheries is committed to conserving and protecting northern fur seals. Our scientists and partners use a variety of innovative techniques to study, learn more about, and protect this species.

Northern fur seals have a stocky body, small head, very short snout, and extremely dense fur (46,500 fibers/cm²) that ends at the wrist lines of their flippers. Their flippers are the longest in the Otariidae family. Their hind flippers can measure up to one-fourth of their total body length. Their fore flippers are incredibly strong allowing them to walk or run on all fours. They can outrun a human on slippery rocks and can climb nearly vertical cliffs. Northern fur seals exhibit sexual dimorphism, which means that adult males are much larger (up to 370 percent) than adult females. Males can grow to 7 feet and weigh up to 600 pounds, while females can grow to 5 feet and weigh more than 120 pounds. Pups are uniformly black until they molt when they are around 3 months old.

Adult males are dark brown to black, and adult females are dark gray or brown on their backs and light gray, silver, or cream on their throat, chest, and stomach.The adults also have white whiskers, known as vibrissae, while the juveniles and pups have black vibrissae. 

Northern fur seals are a highly pelagic (live in the open ocean), are thought to be mostly solitary at sea, nocturnal species. They are known to be aggressive on land, especially during the breeding season. Adult males forcefully defend their breeding territory site. Male to male aggression is most frequently used in defense of territories and consists of pushing and biting, sometimes to the death. Female to female aggression is frequent but mild (open mouth threat). Intense female to female aggression (biting and pushing) is rare.   

Northern fur seals are described as generalist or opportunistic foragers, consuming a wide variety of midwater shelf fish and squid species. Walleye pollock is the predominant prey of northern fur seals that forage over the Bering Sea shelf. They consume greater amounts of oceanic fish and squid species when they forage over the slope and in off-shelf waters. Other primary prey include Pacific sand lance, Pacific herring, Northern smoothtongue, Atka mackerel, and Pacific salmon. The northern fur seal diet differs depending on geographic area and time of year. Northern fur seal diet and tracking (telemetry) studies indicate they forage in colony-specific areas, while they use their preferred land sites for resting, reproduction, and rearing their young. 

Northern fur seals primarily inhabit two types of habitat: open ocean and rocky or sandy beaches on islands for resting, reproduction, and molting. 

Northern fur seals seasonally breed on six islands in the eastern North Pacific Ocean and Bering Sea in the United States—St. Paul, Bogoslof, St. George, Sea Lion Rock, San Miguel, and South Farallon. They also breed on the Commander Islands, Kuril Island, and Robben Island.

The Pribilof Islands support the largest aggregation of northern fur seals, which breed on St. Paul, St. George, and Sea Lion Rock (about half of the world's northern fur seal population). Non-breeding northern fur seals haul-out on Walrus Island and Otter Island which are also part of the Pribilof Islands.

Adult northern fur seals spend more than 300 days per year (about 80 percent of their time) at sea. During the summer and autumn they intermittently fast while on land and feed at sea. During the winter and spring they are pelagic, occupying the North Pacific Ocean as well as the Bering and Okhotsk Seas. In the open ocean, concentrations of northern fur seals may occur around oceanographic features—such as eddies, convergence-divergence zones, and frontal boundaries—because of the availability of prey in those places. In the winter, the southern boundary of the northern fur seal range extends across the Pacific Ocean, between southern California and Honshu Island, Japan, but they are found as far north as the Bering Sea. In the spring, most northern fur seals migrate north to breeding colonies in the Bering Sea. Territorial adult male northern fur seals leave their breeding colonies in August and are thought to spend most of their time in the Bering Sea and North Pacific Ocean along the Aleutian Islands. Pregnant adult females begin their winter migration in November and generally travel to either the central North Pacific Ocean or to offshore areas along the west coast of North America to feed. Some northern fur seals may spend all year in the waters around San Miguel Island, California.

Male northern fur seals can live up to 18 years, while females can live up to 27 years. Beginning in May, male seals start returning to the breeding islands. Older males, or bulls, arrive first to vie for prime breeding territories before the females arrive. A male’s ability to mate depends on several factors, such as body size, fighting ability,  size and location of the chosen breeding territory, and skill at interacting with females. Because males do not leave the breeding territory to feed, their ability to fast is critical. Males remain on their territory an average of 46 days, losing 32 percent of their body mass. Breeding males are typically 10 or more years old and maintain females within their territories. A small number of the total adult males in a territory accomplish most of the breeding. Adult males are counted annually during the peak of the breeding season as an index of the population size. They are categorized as territorial with females, territorial without females, or non-territorial. There is some turnover of territorial males in August, allowing non-territorial males to occupy sites abandoned by territorial males, but the vast majority of adult females have already mated at this point. Territorial males exclude juvenile males from the breeding areas. Juveniles instead congregate on land in areas called "haulouts" during the summer breeding season. The haulouts can be located inland, typically behind the breeding areas, or adjacent to the breeding areas. The typical structure of northern fur seal terrestrial habitat consists of the core group of breeding males with females, idle males without females on the fringe of the core area, and non-territorial males and juvenile males on haulouts outside the breeding areas.

Females generally have their first pup at 5 to 6 years of age and are in their reproductive prime between the ages of 8 and 13. They are not selective in their choice of mate, but they do show an affinity for a specific breeding site. Females typically start returning to the breeding islands in late June and give birth to a single pup 1.3 days after arrival on land. Mating occurs within 5.3 days of giving birth. Female northern fur seals undergo embryonic diapause, meaning the embryo does not immediately implant in the uterus and is thought to be delayed until after lactation, or weaning. Females suckle their newborn pups for 5 to 6 days and then go to sea to forage for 3.5 to 9.8 days. Females continue to cycle between land and sea for the remainder of the nursing period.  Their time on land declines to less than two days and their time a sea generally increases. Pups are nursed until weaning (about 4 months) and leave the breeding site before their mothers to forage independently for the first time.  

Entanglement in Marine Debris and Lost or Abandoned Fishing Gear

Northern fur seals can become entangled in lost or abandoned fishing gear such as trawl webbing, packing bands and monofilament nets, or other marine debris, either swimming off with the gear attached or becoming anchored. Northern fur seals encounter marine debris during their winter and spring migrations when they spend most of their time in the North Pacific foraging in the transition zone and eddies. They also encounter marine debris in the Bering Sea during their travels to and from their rookeries. Once entangled, seals may drag and swim with attached gear for long distances, ultimately resulting in fatigue, compromised feeding ability, or severe injury, which may lead to reduced reproductive success and death.

Changes in Available Food Due to Commercial Fishing

The type of available prey, access to prey, and distribution of prey can change for a variety of reasons. For example, regional and local prey can change because of changes in climate, oceanography, and the overall complexity of the oceans ecosystem. In addition, cumulative and annual commercial fisheries may result in reduced availability of northern fur seal prey. Fishery interactions can include fisheries bycatch (fur seals caught in nets) and indirect effects like competition for commercial fish species. 

Environmental Contaminants

Contaminants can enter the ocean and subsequently affect the food chain of the northern fur seal. Contaminant studies on northern fur seals have shown exposure to various toxic substances and evidence of accumulation in various tissues. Contamination sources can include oil and gas development, industrial runoff, vessel discharge, microplastics, vessel grounding, and oil spills. These contaminants have the potential to affect the immune, digestive, or endocrine systems of northern fur seals, leaving them more susceptible to disease and reducing their survival and reproduction.   

Predation

The killer whale is a primary predator of northern fur seals, but Steller sea lions are also known to prey on northern fur seals. Sharks may also prey on northern fur seals.

Other

Other threats include environmental changes, oil and gas exploration, habitat degradation, human presence, disturbance by marine vessels or aircraft, chronic pollution, and illegal harvests.