Northern Fur Seal
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 Northern Fur Seal Bull, St Paul Island, 1992
photo by Rolf Ream, NMML. |
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| Callorhinus ursinus Linnaeus, 1758 |
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 Range map
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The Northern Fur Seal, Callorhinus ursinus, is an eared seal found along the north Pacific Ocean, the Bering Sea and the Sea of Okhotsk. It is the the largest member of the fur seal subfamily (Arctocephalinae) and the only species in the genus Callorhinus.
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[edit] Physical description
The northern fur seal has substantial physical differences compared to other otariids: its head is smaller, snout shorter and the hindflippers are proportionately the largest of any eared seal. The "fingers" on the hindflippers are conspicous by their length. Males are substantially larger (2m, 270kg) and darker-coloured (the pelage is dark brown or black) than the females (1.5m, 60kg, light brown to grey). In fact, northern fur seals probably exhibit the greatest sexual dimorphism among any mammal. Northern fur seal pups have black pelage. Pups usually have lighter markings on the nose and underside. Males live for up to 20 years, and females 25.
The most conspicuous physical feature of the fur seal is the fur itself; indeed, its genus name comes from the Greek for "beautiful hide". It consists of longer lighter guard hairs and a dense waterproof underfur of about 46,500 hairs per square centimeter, near the record for mammals.
[edit] Range
The northern fur seal is found in the north Pacific – its southernmost reach is a line that runs roughly from the southern tip of Japan to the southern tip of the Baja California peninsula, the Sea of Okhotsk and the Bering Sea. There are estimated to be around 1.1m Northern Fur Seals across the range, of which roughly one half breeds on the Pribilof Islands in the east Bering Sea. Another 200-250,000 breed on the Commander Islands in the west Bering Sea and some 100,000 breed on Tyuleni Island off the coast of Sakhalin in the southwest Sea of Okhotsk and another 60-70,000 in the central Kuril Islands in Russia. Smaller rookeries (around 5,000 animals) are found on Bogoslof Island in the Aleutian Chain and San Miguel Island in the Channel Island group off the coast of California [1]. During the winter months, northern fur seals display a net movement southward, with animals from Russian rookeries regularly entering Japanese and Korean waters in the Sea of Japan and Alaskan animals moving along the central and eastern Pacific as far as Baja California.
The northern fur seal's range overlaps almost exactly with that of Steller sea lions, with which they occasionally cohabit reproductive rookeries, notably in the Kurils, the Commander Islands and Tyuleni Island. The only other fur seal found in the northern hemisphere is the Guadalupe Fur Seal which overlaps slighly with the northern fur seal's range in California.
[edit] Ecology
Fur seals are opportunistic feeders, primarily feeding on pelagic fish and squid depending on local availability. Identified fish prey include hake, anchovy, herring, sand lance, capelin, pollock, mackerel and smelt. Their feeding behavior is primarily solitary.
Northern fur seals are preyed upon primarily by shark and orcas. Occasionally, very young animals will be eaten by Steller sea lions. Occasional predation on live pups by arctic foxes has also been observed.
Due to very high densities of pups on reproductive rookeries and the early age at which mothers begin their foraging trips, mortality can be relatively high. Consequently, pup carcasses are important in enriching the diet of many scavengers, in particular gulls and arctic foxes.
[edit] Reproductive behavior
Animals aggregate on traditional breeding grounds (rookeries) in May. Generally older males (10 years ald older) return first and compete for prime breeding spots on the rookeries. They will remain on the rookery fasting throughout the duration of the breeding season. The females come somewhat later and give birth shortly thereafter. Like all other otariids, northern fur seals are polygynous, with some males breeding with up to 50 females in a single breeding season. Unlike Steller sea lions, with whom they share habitat and some breeding sites, Northern fur seals are possessive of individual females in their harem, often aggressively competing with neighboring males for females. Deaths of females as a consequence of 'tug-of-war's have been recorded, though the males themselves are rarely seriously injured. Young males unable to acquire and maintain a territory of a harem typically aggregate in neighboring "haulouts" occasionally making incursions into the reproductive sections of the rookery in an attempt to displace an older male.
After remaining with their pups for the first eight to ten days of their life, females begin foraging trips lasting up to a week. These trips last for about four months before weaning, which happens abruptly, typically in October. Most of the animals on a rookery enter the water and disperse towards the end of November, typically migrating southward. Breeding site fidelity is generally high for fur seals females, though young males might disperse to other existing rookeries, or occasionally found new haulouts.
Peak mating occurs somewhat later than peak birthing from late June to late July. As with many other otariids, the fertilized egg undergoes delayed implantation: after the blastocyst stage occurs, development halts and implantantion occurs four months after fertilization. In total, gestation lasts for approximately one year, such that the pups born in a given summer are the product of the previous year's breeding cycle.
[edit] Status
Recently there has been some concern about the status of fur seal populations, particularly in the Pribilof Islands, where there has been a roughly 50% decrease in pup-production since the 1970's. This has caused them to be listed as "vulnerable" under the U.S. Endangered Species Act and has led to an intensified research program into their behavioral and foraging ecology. To date, there is no concensus on the possible causes of this decline.
[edit] Fur trade
Northern fur seals have been a staple food of native northeast Asian and Alaskan Inuit peoples for thousands of years. The arrival of Europeans to Kamchatka and Alaska in the 17th and 18th centuries, first from Russia and later from North America, was followed by a highly extractive commercial fur trade. An estimated 2.5 million seals were killed from 1786 to 1867. This trade led to a decline in fur seal numbers. Restrictions were first placed on fur seal harvest on the Pribilof Islands by the Russians in 1834. Shortly after the United States purchased Alaska from Russia in 1867, the U.S. Treasury was authorized to lease sealing privileges on the Pribilofs, which were granted somewhat liberally to the Alaska Commercial Company. From 1870 to 1909, pelagic sealing proceeded to take a significant toll on the fur seal population, such that the Pribilof population, historically numbering on the order of millions of individuals, reached a low of 216,000 animals in 1912.
Significant harvest was more or less arrested with the signing of the North Pacific Fur Seal Convention of 1911 by Canada, Japan, Russia and the United States. The Convention of 1911 remained in force until 1941. A successive convention was signed in 1957 and amended by a protocol in 1963. The international convention was put into effect domestically by The Fur Seal Act of 1966 (Baker et al., 1970). Currently, there is a subsistence hunt by the residents of St. Paul Island and an insignificant harvest in Russia.
[edit] See also
[edit] References
- R. Gentry: Behavior and Ecology of the Northern Fur Seal. Princeton University Press, 1998 ISBN 0-691-03345-5
- R. Nowak: Walker's Marine Mammals of The World. Johns Hopkins University Press, 1999 ISBN 0-8018-7343-6
- J. E. King: Seals of the world. Cornell University Press, 1983 ISBN 0-8014-1568-3
- Seal Specialist Group (1996). Callorhinus ursinus. 2006 IUCN Red List of Threatened Species. IUCN 2006. Retrieved on 11 May 2006. Listed as Vulnerable (VU A1b v2.3)
- Randall R. Reeves, Brent S. Stewart, Phillip J. Clapham and James A. Powell (2002). National Audubon Society Guide to Marine Mammals of the World. Alfred A. Knopf, Inc. ISBN 0375411410.
[edit] External links
- ARKive - images and movies of the northern fur seal (Callorhinus ursinus)
- NOAA's National Marine Mammal Laboratory northern fur seal research program
- Georg Steller's original description of the "Sea Bear" in De Bestiis Marinis (1751)
