Causes of offspring mortality in the Antarctic fur seal, Arctocephalus gazella:: the interaction of density dependence and ecosystem variability

Rates of pup production and causes of pup mortality, recorded in a designated study colony on Bird Island, South Georgia, from 1989 to 2003, were used to evaluate the factors influencing the growth of the population of Antarctic fur seals, Arctocephalus gazella (Peters, 1875). The mean number of pups produced per year was 680 (range 444-822) with a mean survival rate of 77.6% (range 52.6%-92.8%). Starvation, arising from reduced food availability within the mothers' foraging area, was the most frequently recorded cause of death and was positively correlated with the overall rate of pup survival, although it showed no relationship with the number of pups produced. However, traumatic injury showed a local relationship with seal density, increasing significantly with increasing numbers of seals born. This suggests that environmental processes that reduce the availability of prey to lactating mothers, rather than space limitation within colonies, are the limiting factor in the population increase of Antarctic fur seals at South Georgia. The spatial scales over which such processes operate, relative to the local-scale effects of densities of animals within colonies, have important implications for the future expansion of the population and the resultant trophodynamic interactions.