Presented by Stacia Weaver and Chris Prosser

Antarctic fur seals (Arctocephalus gazella) are a species of Pinniped that lives in primarily cold water has recently undergone vast population expansion in the sub-arctic habitat. Males are on generally much larger than the females in this species. The males are known to weigh up to 200 kg, while the females weigh up to 40 kg. This follows along with many different pinniped species as well. Antarctic fur seals also rely on krill for a large part of their diet. Boyd (1996) also agrees with this fact that Antarctic fur seals feed primarily on krill; it is also stated in that paper that the krill can be very patchy in distribution. Breeding colonies of the Antarctic fur seal also pup from November to January in many sub-arctic islands around Antarctica. Boyd (1996) states that pregnancy and birth dates are associated with food availability in the region.

The site of study in this paper was Signy Island. This island is located in the South Orkney Islands. It is in the maritime Antarctic and is a small (8 x 5 km) mountainous island. This island also centers the Center of the British Antarctic Surveys Terrestrial and Freshwater Ecology Research Programme. This center has provided an unrivaled natural laboratory in which to study the sub-Antarctic ecosystem. The ecosystem includes a diverse flora and fauna. Some of which is found nowhere else in the Antarctic bio-dome. A rich microbial and invertebrate fauna is also associated with the plentiful vegetation.

The main objective of this paper was to assess the effect of the increasing Antarctic fur seal populations upon Signy Island, and more importantly on; it’s rich, diverse, and unique flora and fauna. The Antarctic fur seal population on Signy Island has increased drastically since 1976. In 1976, the numbers of the animals were in mere dozens, and by 1988, they had reached 14,000. This demonstrates one of the most rapid population increases of any marine mammal. In this paper, it was noted that there was an increase in population of about 15-17%. This population peaks in late February after which it declines rapidly. The peaks in late February and the declines illustrate a migratory species; this idea is further supported by American Zoologist (1999). This population explosion is due to conservation measures as well as an increase in food source. This recovery has been documented in the South Shetland Islands following the 19^th century exploitation (Boveng et al, 1998). The numbers of Antarctic fur seals have also been increasing because of a direct connection between seal populations and trends in sealing and whaling (Hodgson & Johnston, 1997).

Not only has the overall number of animals increased, but so has their ranging over the island. An increase of 740% of ranging was recorded between 1980-1987. This increase in ranging and social behavior of the animals has resulted in a devastating effect on the vegetation on the island. The northeastern part of the island, 75% of the vegetation has been destroyed. Calcicolous cryptogans, a unique species to the Antarctic bio-dome had been virtually eradicated. A removal of most of the dominant mosses is a direct result of seal influence. The green alga, Prasiola, has recolonized areas formerly held by moss turf. These effects are likely to increase if a breeding colony is established on this island. With the changing distributions of krill, the fur seals may be moving to Signy Island to follow the krill patterns. Lunn et al (1993) states that the growth of pups is directly related to the foraging efficiency of the mothers. Therefore, if the mothers can have better foraging efficiency at Signy Island then a breeding colony may be established. A subsequent acidification of the substrata has resulted from seal impacts. This acidification has lowered the pH from anywhere between ½ to 1 unit.

The problem does not end on Signy Island; South Georgia is beginning to show effects of increasing fur seal populations as well. This problem is likely to worsen before it gets better within the next few years. The recovery of the flora is unlikely to be achieved within the next century. The increasing fur seal population may also have a detrimental effect on bird populations due to the change in the island’s environment. The flora and fauna have no resilience of perturbation on either of the two islands, and the lowland environment is undergoing a possibly irreversible biotically induced transformation.

The situation is not without hope however. Some attempts have been made to rectify or slow the destruction of the vegetation. However, no attempts are being made to reduce the seal populations directly. Enclosures of chain link fence measuring 5m x 5m or 10m x 5m have been constructed in the attempt to protect the fragile vegetation. Positive effects have also resulted in the use of electric fences to surround these enclosures as well.

So, the question still remains, what should be sacrificed. Should an ever-increasing fur seal population or an Antarctic ecosystem that is found nowhere else in the world be sacrificed?

1999. Antarctic fur seals are a sub-polar migratory Otariid that are an increasingly dominant APEX predator in the South Shetland Islands region. American Zoologist, 39: 128.

Boveng, Peter L., Lisa M. Hiruki, Michael K. Schwartz, John L. Bengtson. 1998. Population growth of Antarctic fur seals: limitation by a top predator, the leopard seal? Ecology, 79: 2863-2871.

Boyd, I.L. 1996. Individual variation in the duration of pregnancy and birth date in Antarctic fur seals: The role of the environment, age, and sex of fetus. Journal of Mammalogy, 77: 124-134.

Boyd, I.L. 1996. Temporal scales of foraging in a marine predator. Ecology, 77: 426-429.

Hodgson, Dominic A., Nadine M. Johnston. 1997. Inferring seal population from lake sediments. Nature, 386: 31-32.

Lunn, N.J, I.L. Boyd, T. Batron, J.P. Croxall. 1993. Factors affecting the growth rate and mass at weaning of Antarctic fur seals at Bird Island, South Georgia. Journal of Mammalogy, 74: 908-912.

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