Wade, P.R. 1998. Calculating limits to the allowable human-caused mortality of cetaceans and pinnipeds. Mar. Mamm. Sci. 14:1-.

The focus of this paper was on the rate of recovery of the Antarctic fur seal (Arctocephalus gazella) population in the Elephant Island Area. Elephant Island is part of the South Shetland Islands north of the Antarctic Peninsula and is the largest island near the Seal Islands Archipelego. Two main study sites were examined on Seal Island: North annex (NA), and North cove (NC). Many factors contribute to the slow recovery of this fur seal population, but this study focused mainly on predation of fur seal pups by leopard seals (Hydrurga leptonyx). Several lines of evidence are consistent with the hypothesis that leopard seal predators limit the growth of the fur seal population in the Elephant Island area and perhaps in the broader population in the South Shetland Islands (Boveng, et. al. 1998).

Antarctic fur seals are found on the beaches of many sub-Antarctic islands. Hunting of these seals by man nearly drove the species to extinction during the 19^th century. In the South Shetland Islands they are recovering from exploitation more slowly than the main population at South Georgia which is located approximately 1300 km ENE of Elephant Island. Fur seals feed mainly on krill and various fish species and while they may be considered a top predator, they are also preyed upon by sharks, killer whales, and other species of seals. Male and female Antarctic fur seals exhibit differences in their foraging patterns during the post-breeding season. Females tend to remain close to the breeding colony for two years after breeding while males tend to migrate further away from the breeding colony (Boyd, 1998). Historical activities of the sealing and whaling industries could partly be responsible for the current inrease in fur seal populations in the Antarctic region (Hodgson, et. al. 1998). Currently, no fur seal populations are decreasing, however the population of Antarctic fur seals at Elephant Island is experiencing a slower rate of growth when compared to other populations in the South Shetlands. This rate of growth is directly related to the survival of Antarctic fur seal pups (Boveng, et. al. 1998).

There were four main objectives of this study. (1) Describe patterns in fur seal pup production across nine breeding seasons at Seal Island. This includes breeding chronology, total number of births, mortality rate of pups, and effects of predation by leopard seals. (2) Document abundance and trends of the Antarctic fur seal population breeding on or near Elephant Island. (3) Interpret these trends in fur seal abundance in the vicinity of Elephant Island and relate those trends to fur seal populations of the South Shetland Islands and South Georgia. (4) Assess the importance of predation by leopard seals in limitation or regulation of these populations (Boveng, et. al. 1998).

In order to describe patterns in pup production and mortality at Seal Island, the research team visually counted live and dead fur seal pups during the breeding season from 1986/87 to 1994/95. The pups were usually counted between 10am and 4pm during low tide, if possible. Observers counted from vantage points that allowed them views of both north cove and north annex colonies. These breeding colonies, which are 25 meters apart at their nearest point, both have shallow lagoons that connect to the sea by two channels. At north cove, the channels are flooded most of the time, but at north annex the channels only flood at the highest tides. These two sites were compared because there was an apparent difference in predation between them. Leopard seals were never seen at North Annex, but were seen at North Cove, sometimes taking fur seal pups. Sightings of leopard seals and incidences of predation on fur seal pups were recorded during daily research at NA and NC (Boveng, et. al. 1998).

While pup mortality is often estimated by counting the number of dead pups present, this may be an inaccurate estimate as some carcasses are scavenged, washed out to sea, or removed by predators. It is also difficult to distinguish a pup carcass observed one day from one observed on another day without marking or removing the carcass. Because such efforts may disturb the seal colony, no pup carcasses were marked or removed from North Cove or North Annex. As a result of this, estimates of the number of dead pups is partially subjective and may not accurately estimate the total number of births or the pup mortality rate (Boveng, et. al. 1998).

Leopard seals appear to be a ferocious predator in the Antarctic region. A study conducted at Bird Island in South Georgia reported that Antarctic fur seals accounted for 58% of the diet of leopard seals (Walker, et. al. 199). Thus, they were considered a main prey item. An additional study at Seal Island reported on the hunting behavior of the leopard seals in that area. Hiruki, et. al. found that leopard seals use several different methods to caputure their prey (1999). On a yearly basis, 60% of observed Antarctic fur seal pups captured were done so by one or two leopard seals (Hiruki, et. al. 1999).

In order to estimate those quantities using only live pup counts, a model was developed. This model assumed that (1) Birth dates of pups are normally distributed, (2) There is no leopard seal predation of fur seal pups at North Annex, and (3) Daily rate of pup mortality from non-predatory causes is the same at North Cove and North Annex and is constant throughout the breeding season. Non-predatory causes of death include starvation, storms, and crushing of pups by adult males. Fur seals at Large Leap Island and at Elephant Island were also counted in addition to the counts at North Cove and North Annex (Boveng, et. al. 1998).

Over the course of study, numbers of Antarctic fur seal pups at NC and NA increased each year as pups were born but then leveled off or declined due to various sources of mortality with in each colony. Pup counts at NC experienced a much greater rate in decline than NA. Between 1987/88 and 1994/95 annual peak counts of pups at North Annex increased by about 25% per year. This was due to a near tripling of counts between 1987/88 and 1989/90 suggesting the colony was growing primarily by immigration. Pup counts at North Cove decreased by about 4.2% per year from 1987/88 to 1994/95. From 1986/87 to 1994/95, leopard seals were seen 244 times at different locations around Seal Island. 63% of these observations occurred when counts at NC were declining. Leopard seals were never seen in the lagoon at NA, probably due to poor access. However, they were observed on three occasions outside the lagoon. Leopard seals were frequently observed at NC and on 27 occasions they were seen capturing or consuming fur seal pups. Periods when sightings of leopard seals and predation on fur seals were most frequent at NC corresponded to rapid declines in fur seal pup numbers. Estimates of fur seal pups taken by leopard seals ranged from 32 to 38%. Total pup mortality ranged from 51 to 66%. During 1989/90 the estimated number of pups taken by leopard seals were twice as high as other years (Boveng, et. al. 1998).

This study assumed that birth dates of Antarctic fur seals are normally distributed. Previous studies report that individual females tend to give birth at the same time each year, however, a particular study reported on pre-mature pupping on Antarctic fur seals. These events undoubtedly have a negative affect on population growth. Pregnancy loss in a large number of females are correlated with pup growth in previous seasons, suggesting that poor feeding conditions in one season led to lower production in the next (McCafferty, 1999). A second assumption in the study was that leopard seals did not prey on pups at NA. This was based on the fact that leopard seals were never observed there even though they were seen frequently at NC. Since the counts were conducted during day light hours, it’s possible that leopard seal may be feeding at night at the NA. Other predatory species in the area are known to be nocturnal feeders. An example of this, is the Hooker sea lion. A recent study shows that during 1996-1997 breeding season, 43% of fur seal pup mortality was caused by one sub-adult hooker sea lion alone (Robinson, et. al. 1999). A sub-adult was actually observed thrashing a fur seal pup from side to side, causing the pups skin to be peeled back from an opening in the belly, and the sea lion was feeding on torn strips of flesh. The remains of the pups carcass was retrieved prior to scavenging allowing for direct evidence of sea lion predation to be recorded. A final assumption was that pup mortality from non-predatory causes was equal at both sites. This seems reasonable as both colonies are close to one another and experience the same environmental conditions. Both colonies are low density allowing less competition for food (Boveng, et. al. 1998).

While recovery from near extinction of the fur seal population in the Antarctic region is good for species diversity, it is necessary that controlling factors remain in place. Leopard seals and Hooker’s sea lions appear to be key to controlling the growth of fur seal populations. It is essential that neither of these two species comes under harm as it may cause the fur seal population to grow out of control. An explosion of any population can be detrimental to the fragile ecosystems on surrounding islands. It is important to keep in mind how all species co-exist when making management plans for any area.

Boveng, P.L., et. al. 1998. Population growth of Antarctic fur seals: Limitation by a top predator, the Leopard seal? Ecology. 79(8): 2863-2877.

Boyd, I.L., et. al. 1998. Dispersal of male and female Antarctic fur seals. (Arctocephalus gazella). Can. J. Fish. Aquat. Sci. 55: 845-852.

Hiruki, L.M., et. al. 1999. Hunting and social behaviour of leopard seals (Hydrurga leptonyx) at Seal Island, South Shetland Islands, Antarctica. J. Zool. 249(1): 97-109.

Hodgson, D.A., et. al. 1998. Paleolimnology of Antarctic fur seal Arctocephalus gazella populations and implications for Antarctic management. Biol. Conserv. 83(2): 145-154.

McCafferty, D.J. 1999. Premature pupping in Antarctic fur seals (Arctocephalus gazella). Mar. Mamm. Sci. 15(3): 882-885.

Robinson, S., et. al. 1999. Predation by a Hooker’s sea lion (Phocarctos hookeri) on a small population of fur seals (Arctocephalus spp.) at Macquerie Island. Mar. Mamm. Sci. 15(3): 888-893.

Walker, T.R., et. al. 1998. Seasonal occurrence and diet of leopard seals (Hydrurga leptonyx) at Bird Island, South Georgia. Antarct. Sci. 10(1): 75-81.