Northern fur seals have extreme sexual dimorphism, with males being 30-40% longer and more than 4.5 times heavier than adult females. The head is foreshortened in both sexes because of the very short down-curved muzzle, and small nose, which extends slightly beyond the mouth in females and moderately in males. The pelage is thick and luxuriant, with a dense underfur that is a creamy color. The underfur is obscured by the longer guard hairs, although it is partially-visible when the animals are wet. Features of both fore and hind flippers are unique and diagnostic of the species. Fur is absent on the top of the fore flippers and there is an abrupt ‘clean shaven line’ across the wrist where the fur ends. The hind flippers are proportionately the longest in any otariid because of extremely long, cartilaginous extensions on all of the toes. There are small claws on digits 2-4, well back from the flap-like end of each digit. The ear pinnae are long and conspicuous, and naked of dark fur at the tips in older animals. The mystacial vibrissae can be very long, and regularly extend beyond the ears. Adults have all white vibrissae, juveniles and subadults have a mixture of white and black vibrissae, including some that have dark bases and white ends, and pups and yearlings have all-black vibrissae. The eyes are proportionately large and conspicuous, especially on females, subadults, and juveniles.
Adult males are stocky in build, and have an enlarged neck that is thick and wide. A mane of coarse longer guard hairs extends from the top of the head to the shoulders and covers the nape, neck, chest, and upper back. While the skull of adult males is large and robust for their overall size, the head appears short because of the combination of a short muzzle, and the back of the head behind the ear pinnae being obscured by the enlarged neck. Adult males have an abrupt forehead formed by the elevation of the crown from development of the sagittal crest, and thicker fur of the mane on the top of the head. Canine teeth are much longer and have a greater diameter in adult males than those found on adult females, and this relationship holds to a lesser extent at all ages.
Adult females, subadults and juveniles, are moderate in build. It is difficult to distinguish the sexes until about age 5. The body is modest in size and the neck, chest, and shoulders are sized in proportion with the torso. Adult females and subadults have more complex and variable coloration than adult males. They are dark-silver-gray to charcoal above. The flanks, chest, sides, and underside of the neck, often forming a chevron pattern in this area, are cream to tan with rusty tones. There are variable cream to rust-colored areas on the sides and top of the muzzle, chin, and as a ‘brush stroke’ running backwards under the eye. In contrast, adult males are medium gray to black, or reddish to dark brown all over. The mane can have variable amounts of silver-gray or yellowish tinting on the guard hairs. Pups are blackish at birth, with variable oval areas of buff on the sides, in the axillary area, and on the chin and sides of the muzzle. After 3-4 months, pups molt to the color of adult females and subadults.
Males can be as large as 2.1 m and 270 kg. Females can be up to 1.5 m and 50 kg or more. Newborns weigh 5.4-6 kg, and are 60-65 cm long.
The dental formula is I 3/2, C 1/1, PC 6/5.
Can be Confused With
Northern fur seals can be confused with three other otariid species in their range: the Guadalupe fur seal, and California and Steller sea lions. Coloration of Guadalupe and northern fur seals is similar. Guadalupe fur seals have fur on the dorsum of the fore flipper beyond the wrist, proportionately shorter rear flippers that only have moderate length cartilaginous extensions, and a long tapering muzzle with a bulbous nose that makes the muzzle seem slightly-upturned on the end. Distinguishing females and young can be difficult if not seen well.
At sea, both species of fur seal can be seen actively grooming while at the surface. Guadalupe fur seals will rest in a posture characteristic of fur seals of the genus Arctocephalus with the head down and both rear flippers held in the air and apart forming a ‘Y’ shape. Northern fur seals do not use this posture and routinely sleep in a ‘jug handle’ position with palm of one fore flipper draped over the soles of the hind flippers which are rotated forward to meet the fore flipper. Other fur seal species of the genus Arctocephalus use this posture, so it is possible that Guadalupe fur seals use it as well, although it has not been described for the species. The head is often held up at approximately a 45° angle while the seal is in this position.
Northern fur seals can be readily separated from both sea lions, based on differences in pelage density and coloration, overall size, length and lack of fur on the top of the fore flippers beyond the wrist, head and muzzle shape, and relative size and prominence of the ear pinnae. In California sea lions the muzzle tapers to a blunt end, and the first toe or hallux is larger and longer than all of the other toes. Adult female, subadult, and juvenile California sea lions are tan to pale brown and much lighter in coloration than northern fur seals. Only adult and subadult male California sea lions become dark brown and are similar in color to northern fur seal bulls. However, dark male sea lions have a sagittal crest in contrast to the head of northern fur seal bulls that is slightly domed and lacks a conspicuous sagittal crest. California sea lion bulls lack the light tipped hairs in their much shorter mane. The loud repetitive bark of male California sea lions is distinctive and different from all northern fur seal vocalizations. Steller sea lions are very large with a massive head, blunt thick muzzle, stocky body, pale color, and short fur, and should never present an identification problem with northern fur seals.
Northern fur seals are a widely-distributed pelagic species in the waters of the North Pacific Ocean and the adjacent, Bering Sea, Sea of Okhotsk, and Sea of Japan. They range from Northern Baja California, Mexico north and offshore across the North Pacific to Northern Honshu, Japan. The southern limit of their distribution at sea across the North Pacific is approximately 35° N. Vagrants reach the Yellow Sea in the west and eastern Beaufort Sea in the Arctic. The vast majority of the population breeds on the Pribilof Islands, with substantial numbers on the Commander Islands as well. Still other sites are used, including San Miguel Island in California, Bogoslof Island in the Bering Sea, and Robben Island off Sakhalin Island in Russia. Numerous other sites were formerly occupied and may still be visited. Rookeries are inhabited in summer and fall. These oceanic pinnipeds spend most of the year at sea, rarely (if ever) returning to land until the beginning of the next breeding season. Many animals, especially juveniles, migrate from the Bering Sea south to southern California or the waters off Japan, to spend the winter feeding.
Ecology and Behavior
This is a highly polygynous species. Males arrive at the rookeries up to one month before females and vocalize, display and fight to establish and maintain territories. Breeding on the Pribilof Islands occurs from mid-June through August, with a peak in early July (the median date in southern California is approximately 2 weeks earlier than at the Pribilofs). Northern fur seals become sexually mature at 3-5 years old, at which time females usually produce one pup a year for most of the rest of their lives. Males do not become physically mature, and large enough to compete for a territory that will be used by females, until they are 8-9.
Northern fur seals usually give birth a day after arrival at the rookery. Mean time from birth to estrous is 5.3 days, and 8.3 days for departure on the first feeding trip. Females breed at the Pribilof Islands relatively far from the foraging areas at the edge of the continental shelf and consistently make longer foraging trips than most other female otariids, with a mean trip length of 6.9 days. Once foraging begins the mean depth of dives is 68 m and duration is 2.2 minutes with maximum depth recorded of 207 m, and duration of 7.6 minutes. Pups are visited 8-12 times and attended for a mean of 2.1 days before being abruptly weaned at 4 months old.
Northern fur seals are one of the most pelagic pinnipeds. Adults are at sea most of the year, only coming ashore for the breeding season for 35 and 45 days (on average) for adult females and males respectively. They do not haul-out between breeding seasons, and once weaned, juveniles go to sea and do not haul-out until they return, usually to the island of their birth, 2-3 years later. At sea, northern fur seals are most likely to be encountered alone or in pairs, with groups of 3 or more being uncommon. They forage relatively far from shore, over the edge of the continental shelf and slope. Diving is very active at dawn and dusk. Northern fur seals spend quite a bit of time rafting at the surface, either asleep or grooming. They employ a wide variety of resting postures, including raising one or more flippers into the air, and draping one of their fore flippers over both of the rear flippers to form a posture known as the ‘jug handle’ position. Predators include killer whales, sharks, and Steller sea lions.
Feeding and Prey
The diet varies by location and season, and includes many varieties of epipelagic and vertically-migrating mesopelagic schooling and non-schooling fish and squid. Prey species of importance in the waters off California and Washington include anchovy, hake, saury, several species of squid and rockfish, and salmon. In Alaskan waters, walleye pollock, capelin, sand lance, herring, Atka mackerel, and several species of squid are important prey.
Threats and Status
Northern fur seals have one of the longest and most complex histories of commercial harvesting, which began when the main breeding colonies were discovered in the late 18th century, and lasted until 1984. Numerous international treaties and agreements were put in force in efforts to manage this species. There were many periods of decline and recovery over this long period. It is estimated that the population numbered up to 2.5 million animals in the 1950s. They may have been considerably more numerous than this recent high level, when there were many more active rookeries before the onset of exploitation by Europeans and Americans. The current population is estimated at 1.3-1.5 million and is declining.
Entanglements in commercial fisheries, and in derelict and discarded fishing gear and marine debris, have caused significant annual mortality in the past. This mortality was highest during the period of active high seas drift net fishing in the North Pacific in the 1980s. Entanglement in debris is ongoing and affects juveniles and subadults more than adults. Northern fur seals compete for walleye pollock with one of the largest commercial fisheries world. Mortality from interactions with numerous fisheries and entanglement in debris, large annual harvests of prey species in commercial fisheries, long-term ecosystem regime change in the North Pacific, and possible changes in the foraging patterns of a key predator (the killer whale), may all be working synergistically to cause the current population decline.
Fowler, C.W. 1987. Marine debris and northern fur seals: A case study. Marine Pollution Bulletin 18(6B):326-335.
Gentry, R.L. 1998. Behavior and ecology of the northern fur seal. Princeton University Press. 392 pp.
Gentry, R.L. 2002. Northern fur seal Callorhinus ursinus. pp. 813-817 in W.F. Perrin, B. Würsig and J.G.M. Thiewissen, eds. Encyclopedia of marine mammals. Academic Press.
Kurle, C.M. and A.J. Worthy. 2001. Stable isotope assessment of temporal and geographic differences in feeding ecology of northern fur seals (Callorhinus ursinus) and their prey. Oecologia 126:254-265.
National Marine Fisheries Service. 1993. Final conservation plan for the northern fur seal (Callorhinus ursinus). Prepared by the National Marine Mammal Laboratory/Alaska Fisheries Science Center, Seattle, Washington, and the Office of Protected Resources/National Marine Fisheries Service, Silver Spring, Maryland. 80 pp.
York, A.E. 1987. Northern fur seal, Callorhinus ursinus, eastern Pacific population (Pribilof Islands, Alaska, and San Miguel Island, California). pp. 9-21 in J.P. Croxall and R.L. Gentry eds. Status, biology, and ecology of fur seals proceedings of an international workshop Cambridge, England, 23-27 April 1984. National Oceanic and Atmospheric Administration Technical Report National Marine Fisheries Service 51.