Population Growth of Antarctic Fur Seals: Limitation by a Top Predator, the Leopard Seal?

Abstract

Abstract. Antarctic fur seals (Arctocephalus gazella) in the South Shetland Islands are recovering from 19th-century exploitation more slowly than the main population at South Georgia. To document demographic changes associated with the recovery in the South Shetlands, we monitored fur seal abundance and reproduction in the vicinity of Elephant Island during austral summers from 1986/1987 through 1994/1995. Total births, mean and variance of birth dates, and 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 predation on fur seal pups were recorded opportunistically during daily fur seal research at both sites. High mortality of fur seal pups, attributed to predation by leopard seals frequently observed at NC, caused pup numbers to decline 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 colony, 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 locations 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 Shetland Islands. Several lines of circumstantial 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. The sustained growth of this fur seal population over many decades rules out certain predator-prey models, allowing inference about the interaction between leopard seals and fur seals even though 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 the fur seal population in the South Shetland Islands.

Metapopulation consequences of site fidelity for colonially breeding mammals and birds

“…The possible cost of migration can be manifested either as lower survival or lower fecundity. Survival costs are usually due to predation (Boveng et al . 1998).…”

The Behavioral Basis for Nonlinear Functional Responses and Optimal Foraging in Antarctic Fur Seals

“…Although the pup and the juvenile are attacked by predators such as leopard seals (Hydrurga leptonyx; Boveng et al 1998, the adult is under lower predation risk. This is a challenging species in which to study foraging because foraging itself cannot be observed directly, but this is compensated by other features of its biology.…”

Early post-release survival of stranded Cape fur seal pups at Black Rocks, Algoa Bay, South Africa

“…Mortality of seal pups due to catastrophic meteorological events varies with the topographic characteristics of the haulout area (Le Boeuf and Briggs 1977, Riedman and Le Boeuf 1982, Doidge et al 1984, Boveng et al 1998, Baker et al 2006; some rookeries experience very little mortality during storms because the topography of the haulout area provides adequate protection (de Villiers and Roux 1992, Pemberton and Kirkwood 1994, Boveng et al 1998, Gucu et al 2004. By contrast, Black Rocks is low-lying and offers very little protection during storms.…”