Presented by Brandy Potter and Beth Sharrer

Antarctic Fur Seals, Arctocephalus gazella, occupy an upper trophic level at Seal Island, consuming mostly krill and various fish species (Boveng et. al., 1998). Fur seals feed mostly at night, adjusting their diving depths with the nightly rise of krill (Gentry and Kooyman, 1986). In an experiment by McCafferty et. al., the abundance of krill was associated with the amount of time pups were left alone on land while the mothers foraged for food(McCafferty, et. al., 1998). In 1996, krill was in high abundance. Females were able to take fewer shorter foraging trips, therefore providing more protection to the pups. It was also found that females base the amount of feeding in the sea to the amount of energy needed to the pup. Krill was also found to have an influence on the reproductive performance of the seals(Croxall, et. al., 1999). Large groups of fur seal pups have been observed foraging for food. It is believed that pups stay in large groups to guard against predation. Even though they are considered a top predator, many are preyed upon by other seals, killer whales, or sharks (Boveng, et. al., 1998). Antarctic fur seals inhabit the Elephant, King George, Livingston, and South Shetland Islands, which circle the Antarctic waters. The population of Antarctic fur seals is estimated at 1,200,000 and is considered to still be rising (Riedman, 1990).

Fur seals form colonial social organizations (Riedman, 1990). During seasonal migration, females leave the breeding rookeries in the winter and migrate to unknown areas while the males remain in the rookeries. Breeding occurs south of the Antarctic convergence in the Atlantic and Indian Oceans (Boveng, et. al., 1998). The pupping season lasts from early December to early March. Fur seals give birth to live young on land, with a long gestation period (Riedman, 1990). Female fur seals maintain a strategy of pumping up their young with fat rich milk. This maternal care lasts from a few weeks to even a few days. Most females give birth to single young on a yearly or biennially basis with each birth being up to two years apart. There are cases of females giving births to twins. This is a somewhat frequent event, but it is rare that the twins survive. The mother usually doesn’t have enough milk to wean both pups (Riedman, 1990).

Europeans first discovered fur seals on February 3, 1488 during a voyage by Bartolomeu Dias (Bruemmer, 1988). Eleven years later, Vasco de Gama counted 3,000 seals in Mossel Bay. In the late 1700’s, breeding colonies were discovered in the Antarctic region. Dutch sailors hunted 45,000 seals near the Cape of Good Hope in 1610. In 1889 seal numbers were reduced to fewer than 100,000 and were eliminated from 23 to 47 islands of Southern African. It only took 2 years for the species to be commercially exterminated from the South Shetland Islands. In these islands, 50 sealing vessels took 250,000 sealskins. By the late 1800’s, the species was thought to be extinct.

These fur seals have experienced the most spectacular recovery from exploitation and near extinction of any fur seal (Boveng, et. al., 1998). Fur seals fell prey to commercial hunting in the 19^th century. Southern Africa ended the free-for-all hunting in 1893 and began a careful management of seal harvesting (Bruemmer, 1988). A full recovery began in 1915. One juvenile male found on South Georgia Island, which started a serious search for other survivors on the island in 1933. This search revealed a breeding group of 38 animals at Bird Island near South Georgia. By 1959, nearly 8,000 seals inhabited South Georgia and the surrounding islands. In 1959, there were only 32 seals counted in the South Shetland Islands (Boveng, et. al., 1998). By 1987, 19,000 seals were found on these islands. From 1963 to 1973, South Georgia’s population grew at a rate of 16.8% per year.

Leopard seals and Killer whales are thought to be two of the most common predators to the Antarctic fur seal (Riedman, 1990). One of the main predators of the fur seal at Seal Island is the Leopard seal, Hydrurga leponyx (Boveng, et. al., 1998). The composition of the diet of the leopard seal varies geographically, but mainly consists of birds, krill, cephalopods, and other invertebrates. In certain areas of their range, pinnipeds are an important part of their diet. Pinnipeds are thought to consist 15% of the leopard seal’s diet (Riedman, 1990). At South Georgia Island, they feed upon various fur seals, especially juvenile Antarctic fur seals (Boveng, et. al., 1998). The main predators of the leopard seal are killer whales and sharks (Riedman, 1990). Leopard seals are the largest of the four Antarctic phocids. They have powerful jaws with long sinuous necks. They can coil their neck back and strike at their prey like a snake. This adaptation makes them a very efficient predator in the Antarctic region.

In an experiment by Boveng and other researchers, patterns in fur seal pup production at Seal Island was evaluated (Boveng, et. al., 1998). The evaluation included the total number of births, mortality rates of pups, and the effects of predation by leopard seal at two locations on Seal Island, North Cove and North Annex. Both locations are within 25 meters. Both locations experience the same conditions such as temperature, tides, and weather. Each location is surrounded by shallow lagoons that keep the seals relatively protected. North Cove, on the other hand, is more susceptible to predation by leopard seals. This area remains completely flooded expect for very low tides, allowing leopard seals a passage into the cove. North Annex only floods during the highest of high tides. Leopard seals have never been observed at North Annex, but have frequently been observed at North Cove. In this experiment, the number of dead pups were counted every other day between the hours of 1000 and 16000 during low tide if possible. Only one observer was used, as to not distract the population. The number of dead pups counted may have been an under representation of the actual mortality. Some pups may have washed away, been scavenged or removed, or have been counted more than once. None of the pups were removed so as not to disturb the environment.

Many assumptions had to be made in order to carry out the experiment (Boveng, et. al., 1998). First, the proportion of pups born each day corresponded to a natural distribution of birth dates. Studies have shown that females tend to give birth around the same time, which shows a normal distribution. Second, there was no leopard seal predation at North Annex. Leopard seals were never observed at North Annex. The shallowness of the channels at North Annex prevented leopard seals from preying on the pups. Lastly, a constant fraction of live pups died each day due to causes other than leopard seal predation. Since both locations were very similar, mortality from environmental causes would be the same for both locations.

The presence of leopard seals corresponded to the decrease in pup numbers at North Cove (Boveng, et. al., 1998). Leopard seals were found capturing and killing pups on 27 separate occasions. Leopard seals were not seen at North Cove after pup numbers stabilized. Pup counts at North Annex decreased by less than 20% while pup counts decreased by more than 50%. These results suggested significant predation at North Cove, causing a decrease in pup counts.

Very little is known about the nature of the relationship between the fur seal and the leopard seal (Boveng, et. al., 1998). Predation by the leopard seal may not be a dependent variable on the fur seal population. This is believed because the fur seals are only at Seal Island for four months out of the year. Leopard seals also have other food sources. Leopard seals definitely have an impact on the fur seal population at North Cove. One reason for the decreased predation in other areas of the Seal Islands is the topography of the breeding areas. In other areas, there is plentiful vegetation growing on the slopes and banks. For this reason, pups spend less time in the water, which decreases chances of predation by leopard seals. Leopard seals are not as efficient on land as predators as they are in the water.

Literature Cited

Boveng, Peter; Hiruki, Lisa; Schwartz, Michael; and Bengtson, John. 1998. Population Growth of Antarctic Fur seals: Limitation by a top predator, the Leopard Seal? Ecology. 79:2863-2877.

Bruemmer, Fred. 1988. A Fate Unsealed. Natural History. 11:59-65.

Croxall, J.P., Reid, K., and Prince, P.A. 1999. Diet, provisioning and productivity responses of marine predators to differences in availability of Antarctic krill. Marine Ecology Progress series. 177:115-131.

Gentry, Roger and Kooyman, Gerald. Fur Seals. New Jersey: Princeton Universtiy Press, 1986.

McCafferty, Dominic; Boyd, Ian; Walker, Tony; and Taylor, Robert. 1998. Foraging responses of Antarctic fur seals to changes in the marine environment. Marine ecology progress series. 166:285-299.

Riedman, Marianne. The Pinnipeds. Los Angeles: University of California Press, 1990.

[Course Homepage] [Syllabus] [Lecture Schedule] [Lab Schedule] [Student Presentations] [Marine Mammal Links]