Pinnipeds of the South Shetland Islands

The Antarctic Ecosystem Research Division conducts research primarily at the northern reaches of the Antarctic Peninsula, where the dynamic nature of the physical environment, its high productivity and key location of the South Shetland Islands gives rise to a pinniped diversity and abundance rare to most places in the world. The South Shetland Islands are considered a transitional environment because it is the site of mixing between the warmer Antarctic Circumpolar Current colder water from the Weddell Sea. Because of this it is where sub-polar species of Pinnipeds (Antarctic fur seals and Southern elephant seals) haul-out with more polar, pagophilic (ice-loving) species (Weddell, leopard, and crabeater seals).Antarctic pinnipeds are unusual among the world’s pinnipeds in their choice of habitat. They have evolved strategies to survive a frigid ecosystem that is replete with competitors and predators. The four species of seals that live on and around the South Shetland Islands – Antarctic fur seals, Weddell seals, elephant seals and leopard seals - all have strikingly different ways of coping with the harsh Antarctic environment. However, no matter the survival strategy employed by each species, they are all dependent upon the same krill that humans harvest commercially.

Each species has evolved different strategies to survive in this transitional and variable environment replete with competitors and predators. However, no matter the survival strategy employed by each species, they are all dependent, directly or indirectly, upon the same krill that humans harvest commercially.

AFS fur seal by Mike Goebel SES with pup by Mike Goebel

Antarctic fur seal

Lifespan:

Females - 20 years
Males - 10 years

Average size:

Females -
30-55 kg
Males - 120-200 kg

Favorite food:

Antarctic krill

Southern elephant seal

Lifespan:

Females - 23 years
Males - 20 years

Average size:

Females - 800 kg
Males - 3,000-4,000 kg

Favorite food:

Squid
Antarctic krill
LS by Mike Goebel Weddel with pup by Mike Goebel

Leopard seal

Lifespan:

Females - 25 years
Males - 27 years

Average size:

Females - 450 kg
Males - 400 kg

Favorite food:

Antarctic krill
Antarctic fur seals and penguins

Weddell seal

Lifespan:

Approximately 12-15 years

Average size:

400-460 kg

Favorite food:

Antarctic finfish
Antarctic krill

Antarctic fur sealAFS breeding beach by Mike Goebel

Antarctic fur seals (Arctocephalus gazella) are the most common pinniped found breeding around the South Shetland Islands. There are about 5,000 female fur seals that visit Cape Shirreff every year during the summer to feed and reproduce; those seals haul out at about 14 different breeding sites each year.

Female fur seals will get pregnant during one breeding season and carry their young until the next summer, when they return to their breeding beach to give birth to their pups. They have a very high degree of site fidelity and return to the same beach to give birth every year. After spending an entire winter foraging at sea, fur seals are known to go as long as seven days without any type of nourishment while they give birth and suckle their offspring. Once a fur seal has given birth, she will leave again to forage for food, returning as often as possible to feed her pup. The length of their foraging trips is one of the best indicators of how good food resources are offshore.

AFS trip length vs. krill length by Mike Goebel

Figure: Antarctic fur seals spend less time looking for food when krill are large.

Each austral summer, fur seals feed primarily on lipid-rich krill. Because fur seals rely on fur to insulate their bodies and not blubber, as most polar pinnipeds, they have relatively high metabolic rates and rates of energy acquisition. They are thus, excellent sentinels for measuring the health and sustainability of a marine ecosystem faced with climate change and the commercial exploitation of a keystone species such as krill. Antarctic fur seals are a sub-polar species and breeding populations in the South Shetland Islands are at the southern-most edge of their preferred habitat. At these latitudes fur seals must forage and rear their young in a colder more physiologically demanding environment than at the center of their breeding range, South Georgia, approximately a thousand miles northeast of the SSI, where hundreds of thousands of fur seal pups are born each year. Lipid-rich krill provide them with a slow-burning, long-lasting energy source. However, krill are not their only prey and in the SSI many fur seals switch from eating krill to eating fish by mid-January, about halfway through the breeding season. The reason for this mid-season switch is unknown.

Fur seals prefer larger, older krill, and will actively seek them out when they are foraging. In years when large krill are plentiful, female fur seals are able to feed themselves and make it back to their pups in a short period of time. In other years, they will spend more time foraging, and have to spend more nights away from their pups.

Population trendsAFS pup by Carolina Bonin

Soon after the discovery of the South Shetland Islands in 1819, fur seals were driven to local extinction by the lucrative fur trade of the 19th century. For over 100 years they were absent from the SSI until the first pup was observed in December 1959. Once they began to return to the SSI their populations expanded rapidly. The increases throughout the 1970s and 1980s were likely fueled by abundant krill after decades of commercial whaling and fin-fish exploitation of the 1970s.

Krill, however, are dependent upon ice for recruitment and high over-winter survival. Recent warming of the Antarctic peninsula and a southerly retreat of ice is believed to have led to less krill availability to penguins and seals. The rate of population growth for fur seals leveled off in the late-90s and populations have now begun to decline. The population decline, however, has not been as severe as for krill-dependent penguin species such as the Adelie and chinstrap penguins. This may be due to the fur seal's greater ability to switch to foraging on fish.

The recent decline in fur seals numbers, however, is not entirely due to a lack of food. U.S. AMLR studies at Cape Shirreff indicate a substantial number of fur seal pups each year are consumed by leopard seals (see leopard seal section). Though leopard seals are typically associated with ice environments and are likely to decline with decreasing ice, they have increased locally at Cape Shirreff in the summer. We are uncertain why leopard seals are increasing but perhaps, just as fur seals switch prey between krill and fish, leopard seals may also be switching from ice-inhabiting crabeater seals to the more abundant Antarctic fur seal in the SSI. Even a small number of large predators such as leopard seals can exert substantial top-down forces and have a dramatic effect on structuring communities and influencing marine ecosystems.

Southern elephant seal

More information coming soon!

Leopard seal

More information coming soon!

Weddell seal

More information coming soon!

Last modified: 12/24/2014