Pribilof fur seal
Pribilof fur seal. Source: US Fish and Wildlife Service
The Pribilof fur seal (also Northern fur seal and Alaska fur seal; Scientific name: Callorhinus ursinus) is one of 16 species of marine mammals in the family of Eared seals which include sea lions and fur seals. Together with the families of true seals and Walruses, Eared seals form the group of marine mammals known as pinnipeds.
 Pribilof fur seal. Source: Anne Morkill/BioLib/Encyclopedia of Life |
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Conservation Status  |
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Scientific Classification Kingdom: Animalia (Animals) |
| Common Names Alaska fur seal Pribilof fur seal Northern fur seal |
Eared seals differ from the true seals in having small external earflaps and hind flippers that can be turned to face forwards. Together with strong front flippers, this gives them extra mobility on land and an adult fur seal can move extremely fast across the beach if it has to. They also use their front flippers for swimming, whereas true seals use their hind flippers.
Like other Eared seals, the male Pribilof fur seal is considerably larger than the female.
Like other fur seals, the Pribilof fur seal was long hunted for its skin which almost led to the species extinction in the mid 1700s. All commercial hunting has ceased at sea and only limited numbers are now taken under license.
Contents
Physical Description
Pribilof fur seals are extremely sexually dimorphic. Male and female Pribilof fur seal vary both in size and color.
A bull Pribilof fur seal is a big heavy animal, measure 213 cm, and weigh between 181 and 272 kg. Their fur varies in colour, ranging from reddish to black, and they have thick necks and a mane.
The females (cows) are much smaller than males, measuring about 142 cm and weighing 43-50 kg. Their colouring differs, too, having a silvery-grey back, with reddish-brown at the front and a whitish-grey patch on the chest.
Young seal pups are black with paler markings around the nose and mouth.
Additional characteristics include vibrissae, which are white on the adult and black on the pup. Pups usually have lighter markings on the nose and underside. Adults, especially males, have a mane, which is similar to the mantle of some squirrels. The adults have small noses, thick underfur, and long pinnae. The fur on the front flippers stops at the wrist.
Reproduction
Pribilof fur seals are polygynous. Males compete for territories and successful males can mate with many females.
Males arrive at the mating island prior to the females, at which time they begin to claim a mating territory. The mating islands in the Pribilofs, St. George and St. Paul, the southern California island San Miguel, and additional islands in the Bering Sea are the most common mating grounds.
At the start of the breeding season, males begin arriving at their traditional sites about a month ahead of the females and start competing for breeding territories. These fights can be extremely violent, with each mature bull aiming to slash an opponent's neck with his sharp canine teeth. Only the largest and heaviest bulls can hope to claim the title of 'beachmaster'; the smaller younger males, who have no chance of competing with the fully-grown animals, occupy the fringes of the breeding territories.
Males compete to secure as many females as he can within his harem. It is thought that females are influenced by the presence of other females and the characteristics of the territory rather than the mere size and power of the male.
The beachmasters will continue to squabble and fight over females right through the breeding season, usually because these colonies are crowded, and wandering females sometimes stray into another male's territory. Occasionally, younger males will attempt to steal a mating with a female and, if spotted by one of the beachmasters, he will be chased off. In order to ensure that his females are not claimed by another male, bull Pribilof fur seals do not feed throughout the breeding period and may eventually loose 20% of their body weight.
While males will participate in aggressive behavior with other males it is usually not lethal. In fact, females are more likely than males to be injured during the mating selection process. Occasionally, two males with fight over a female if she attempts to leave a mating territory, which could result in a tug of war. While males are sparing they will make attempts not to injure any pups in the area.
Pribilof fur seals are extremely polygamous. In some cases a single male may mate with up to 50 females during one breeding season. However, more typical is 15 to 20 females. The number of females a male mates with is dependent on the number of available females and the male’s ability to control a mating territory.
The females arrive in mid-June and give birth to the pups, conceived the year before, some two days after their arrival. Within a week of the birth, the females will mate again.
The fertilized egg undergoes delayed implantation. After the blastocyst stage occurs, development halts for approximately four months. Implantantion of the embryo occurs four months after fertilization. This ensures that that the developing pup will be born at the right time the following year when the animals return to their breeding grounds.
Gestation lasts for approximately one year. Pups are born between June and July, and remain on land for 60-70 days. Peak pupping season is mid-June to mid-July, and peak mating occurs just after this, from late-June to late July.
Females typically become sexually mature between 3 and 6 years of age. Males reach sexually maturity between the same ages, but typically will not participate in mating until three years later.
The same group of males and females tends to return to the same mating island every year. However, male pups born on a particular island will go to a different island when they reach reproductive age. This can be considered dispersal of males.
Pups are born unable to swim or to move around much. Mothers suckle their pups for up to 10 days before returning to feed at sea, usually during the night. She will stay at sea from between four and ten days feeding, returning to feed her pup for one or two days. Weaning occurs between three and four months of age, and is apparently begun by the pup. After the pup is weaned, the mother migrates south (usually in late October). There are no long-term social bonds in this species.
The pups will spend as long as 22 months at sea before returning to the beach where they were born. Fur seals mature between the ages of three and six, but males will probably not begin to breed for an additional three years.
Lifespan/Longevity
Actual lifespans of up to 26 years have been measured, although it is thought that the actual maximum may be around 30 years of age.
Behavior
Pribilof fur seals are described as gregarious, but only during mating and feeding. At all other times males are solitary. Females are also solitary except while they are nursing young or mating.
Pribilof fur seals tend to be most active during the early morning and evening or night. The middle of the day is spent sleeping, usually floating on their sides.
Vocal communication is important twice during the lifetime of a Pribilof fur seals. Vocalizations are crucial to the mother and pup relationship. While the mother and pup are on the mating island the mother frequently returns to the ocean to hunt. When the mother returns vocalizations are used to find the young. If the young becomes lost the mother is able to locate her young by distinctive individual vocalizations. This type of vocalization is described as being loud, repetitive, and low or deep. Once the mother and pup have located each other the mother uses scent to verify the pup is her young.
Verbal communication also becomes important when males are competing for territory. Males will bark at each other during competition.
These animals are also reported to use scent marks.
Distribution
Distribution of the Pribilof fur seal. Dark blue dots indicate prime breeding locations. Source: Mirko Thiessen/Wikipedia Pribilof fur seals range over most of the northern Pacific Ocean, as far south as southern California in the east and to central Japan in the west. Northwards, their range extends to the Bering Sea, north of the Aleutian Islands, and the Sea of Okhotsk, west of the Kamchatka Peninsula. Their main breeding grounds are on the Pribilof Islands of St. George and St. Paul in the southern Bering Sea. They also breed on a number of other islands scattered throughout their range, principally the central Kuril Islands and Tyuleniy Island in the Sea of Okhotsk, the Commander Islands off the coast of Alaska, Bogoslof Island in the Aleutians and San Miguel Island off the coast of California.
Habitat
Although Pribilof fur seals spend most of the year at sea in the cold northern waters of the Pacific Ocean, their breeding grounds are on rocky coastlines close to the edge of the continental slope. Pribilof fur seals spend the majority of their lives in the cool water of the North Pacific Ocean. During the reproductive season Pribilof fur seals arrive at island breeding grounds. Male Pribilof fur seals remain on land. Females will remain on land during mating but will re-enter the ocean to hunt while nursing a pup.
Since the main predators of Pribilof fur seals are marine mammals, the main strategy of escape is to emerge onto land. Because of their feeding habits, these seals play an important role in food webs, affecting the population of prey fishes. They also may have an effect on predator populations.
Food Habits
Pribilof fur seals are described as opportunistic feeders. The main components of the diet are fish such as pollack, herring, lantern fish, cod, and rockfish. Pribilof fur seals also eat squid and, occasionally will feed on birds such as loons and petrels. The diet is mainly dependent on the availability of prey due to season and location.
The principal natural predators of fur seals are orca (killer whales), great white sharks and the much larger Steller sea lion Eumetopias jubatus. On land, pups can fall prey to foxes.
Conservation Status
Harvesting of the seal for its coat almost led to the species extinction in the mid 1700s. At the time of the Alaskan Treaty provisions where put into place in order to maintain population sizes. The treaty stated that only juvenile male seals, found on land, could be killed.
Current threats include loss of breeding habitat on islands.
The North Pacific Fur Seal Convention lapsed in 1984 after an extension was vetoed by the USA, but all commercial hunting has ceased at sea and only limited numbers are now taken under licence. It is believed the world population currently stands at about 1,350,000 animals, but with no current international agreement, some experts fear that commercial hunting could start again at some time in the future. There is also evidence that the El Nino event of 1997-8 affected the pup survival rate of the San Miguel Island breeding colony. A drop in the availability of fish led to 87% of young fur seals dying before they were weaned.
Classified as Vulnerable (VU A1b) on the IUCN Red List.
Threats
The fur that protects the Pribilof fur seal from the cold has led to the animals being hunted for centuries. Although early native peoples made little impact on their numbers, the 'discovery' of the species in the 18th century was followed by commercial hunting that nearly led to the seals' extinction by the end of the 19th century. In 1911 a treaty, the North Pacific Fur Seal Convention was signed by the USA, Japan, Russia and the UK (acting for the Dominion of Canada), limiting hunting to immature males on land and banning all sea hunts.
Although serious commercial hunting of the Pribilof fur seal has ended, the animals no longer enjoy the protection of the international treaty which lapsed in 1984. Neither is the animal currently protected under CITES (Convention on International Trade in Endangered Species).
There is still some hunting permitted under licence in Canadian waters by native peoples only, and a similar agreement exists for natives of the Aleutian Islands. But the main threats globally are now believed to be caused by entanglement in the nets of the Japanese squid fishing fleets and in the Bering Sea. Seals are also threatened by marine pollution such as plastic twine and waste packaging, as well as discarded trawl nets. The animals are very vulnerable to oil pollution and, with an increase in oil and gas exploration around several of their breeding grounds, there are fears that accidental oil spills and the inevitable industrial disturbance will affect seal populations.
Economic Importance for Humans
Pribilof fur seals have been harvested mostly for their fur, but are also used as a food source. This species has often been accused of having a negative impact on fisheries. However, analysis of stomach contents indicate that Pribilof fur seals do not feed on the same species that humans take. Most of the diet of these seals appears to be lanternfish, which are not harvested by people.
Further Reading
- Callorhinus ursinus (Linnaeus, 1758) Encyclopedia of Life (accessed April 7, 2009)
- Callorhinus ursinus, Holmes, J., 2001, Animal Diversity Web (accessed April 7, 2009)
- Northern Fur Seal, Seal Conservation Society (accessed April 7, 2009)
- Callorhinus ursinus, Hilton-Taylor, C. 2001. IUCN Redlist of Engangered Species (accessed April 7, 2009)
- The Pinnipeds: Seals, Sea Lions, and Walruses, Marianne Riedman, University of California Press, 1991 ISBN: 0520064984
- Encyclopedia of Marine Mammals, Bernd Wursig, Academic Press, 2002 ISBN: 0125513402
- Marine Mammal Research: Conservation beyond Crisis, edited by John E. Reynolds III, William F. Perrin, Randall R. Reeves, Suzanne Montgomery and Timothy J. Ragen, Johns Hopkins University Press, 2005 ISBN: 0801882559
- Walker's Mammals of the World, Ronald M. Nowak, Johns Hopkins University Press, 1999 ISBN: 0801857899
- Northern fur seal, MarineBio.org (accessed April 7, 2009)
- Gay, J. 1987. American Fur Seal Diplomacy . New York: Peter Lang.
- Fur seals: maternal strategies on land and at sea , Gentry, R., G. Kooyman, Princeton University Press, 1986 ISBN: 0691084009
- Behavior and Ecology of the Northern Fur Seal , Gentry, R., Princteon University Press, 1997 ISBN: 0691033455
- Insley, S. 2001. Mother-offspring vocal recognition in northern fur seals is mutual but asymmetrical. Animal Behavior , 61: 129-137.
- Mori, J., N. Baba, T. Kubodera. 2001. Squid in the diet of northern fur seals, *Callorhinus ursinus*, caught in the western and central North Pacific Ocean. Fisheries Research , 52: 91-97.
