Population growth of Antarctic fur seals: Limitation by a top predator, the leopard seal?

Antarctic fur seals (Arctocephalus gazella) in the South Shetland Islands a re recovering from 19th-century exploitation more slowly than the main popu lation at South Georgia. To document demographic changes associated with th e recovery in the South Shetlands, we monitored fur seal abundance and repr oduction in the vicinity of Elephant Island during austral summers from 198 6/1987 through 1994/1995. Total births, mean and variance of birth dates, a nd average daily mortality rates were estimated from daily live pup counts at North Cove (NC) and North Annex (NA) colonies on Seal Island. Sightings of leopard seals (Hydrurga leptonyx) and incidents of leopard seal predatio n on fur seal pups were recorded opportunistically during daily fur seal re search at both sites. High mortality of fur seal pups, attributed to predat ion by leopard seals frequently observed at NC, caused pup numbers to decli ne rapidly between January and March (i.e., prior to weaning) each year and probably caused a long-term decline in the size of that colony. The NA col ony, where leopard seals were never observed, increased in size during the study. Pup mortality from causes other than leopard seal predation appeared to be similar at the two sites. The number of pups counted at four locatio ns in the Elephant Island vicinity increased slowly, at an annual rate of 3 .8%, compared to rates as high as 11% at other locations in the South Shetl and Islands. Several lines of circumstantial evidence are consistent with t he hypothesis that leopard seal predators limit the growth of the fur seal population in the Elephant Island area and perhaps in the broader populatio n in the South Shetland Islands. The sustained growth of this fur seal popu lation over many decades rules out certain predator-prey models, allowing i nference about the interaction between leopard seals and fur seals even tho ugh it is less thoroughly studied than predator-prey systems of terrestrial vertebrates of the northern hemisphere. Top-down forces should be included in hypotheses for future research on the factors shaping the recovery of t he fur seal population in the South Shetland Islands.