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Population growth of Antarctic fur seals: limitation by a top predator, the leopard seal?
Ecology, Dec, 1998 by Peter L. Boveng, Lisa M. Hiruki, Michael K. Schwartz, John L. Bengtson
Evidence for predation-limited growth in the South Shetland Islands. - The long-term rates of recovery from commercial extinction are consistent with a hypothesis that leopard seals have had much less impact on the fur seal population of South Georgia than on the population of the South Shetlands. Recovery at South Georgia from commercial extinction in 1907 (assuming arbitrarily that 100 fur seals remained) to a population of [approximately]28000 in 1958 (Payne 1977, 1979) would have required an average annual increase of [approximately]12%, a rather high, sustained growth rate (Wickens and York 1997). In contrast, fur seal recovery in the South Shetlands from commercial extinction in 1888 (Bonner 1968) to a population of [approximately]30000 in 1995 represents only [approximately]5% annual growth.
Recent observations of leopard seal abundance and growth rates of local fur seal populations are consistent with the long-term trends. McCann and Doidge (1987) remarked that predation by leopard seals (and killer whales) seemed insignificant in regulating the fur seal population at South Georgia; leopard seals are generally absent during the summer fur seal breeding season, though they do take juvenile fur seals there in winter (Bonner 1958, Walker et al. 1998). Fur seal researchers at Livingston Island observed a maximum of two leopard seals each austral summer from 1990/1991 to 1994/1995 (D. Torres, personal communication). No predation on fur seal pups was observed there until 1995/1996, when eight leopard seals were sighted. As we documented in the present study, leopard seals have preyed heavily on fur seal pups at Seal Island and perhaps other colonies in the Elephant Island area at least since 1986/1987 when observations began. Thus, abundance of leopard seals and rate of predation on fur seals seem to have been low at South Georgia (where the fur seal population growth rate has been the highest) and higher in the South Shetland Islands, especially in the Elephant Island area, where growth of fur seal populations has been slowest.
The relative abundance of leopard seals in summer at South Georgia and the South Shetland Islands may be a function of proximity to sea ice (Bonner and Laws 1964); most leopard seals are found in the pack-ice zone (Gilbert and Erickson 1977). This is consistent with the absence of leopard seals in the summer at South Georgia, where the nearest sea ice in January is typically 700-900 km south, and the common presence of leopard seals in the summer at the South Shetland Islands (Aguayo 1970, Rakusa-Suszczewski and Sierakowski 1993), where the ice is usually within 200 km in January.
Another factor, in addition to local abundance of leopard seals, that could lead to differences in predation rates among fur seal colonies at South Georgia, Livingston Island, and the Elephant Island area is the topography of fur seal colonies and surroundings. At South Georgia, fur seals prefer breeding sites that are backed by vegetated slopes (Boyd 1993). Unattended pups commonly roam or rest in the tussock grass on the slopes and join their mothers there to nurse. These pups may spend less time in the water- exposed to leopard seals - than pups in the Elephant Island area where colonies are backed by steep, rocky slopes. The terrain surrounding colonies at Livingston Island (Cape Shirreff) is gently sloping, open, and accessible to pups but has no tussock grass for cover; pups roam inland, but may spend less time there than pups at South Georgia because they would be more exposed to harassment by giant petrels (Macronectes giganteus), skuas (Catharacta spp.), and subadult male fur seals. Whether these differences in topography and cover actually correspond to differences in fur seal pup behavior could be tested by relatively simple measures of pup activity budgets (i.e., proportion of time spent in the water) at the three locations.
