Density-dependent pup mortality in the Antarctic fur seal Arctocephalus gazellu at South Georgia

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Abstract

The extent and causes of pup mortality in the Antarctic fur seal, Arctocephalus gazella, were investigated at sites of high and low density at South Georgia. Mortality was greater at the high density site, (17—31% of annual pup production) than at the low density site (36%). The main causes of death, starvation and skull injury, occur more frequently at the high density site. Most starvation was caused by failure of the mother-pup bond to form, often caused by disturbance induced by the activities of breeding bulls. In some seasons starvation of older pups may be influenced by reduced food availability but this was usually of minor importance. Injury to the skull resulted from bites inflicted by females either accidently during birth or when pups tried to suck from females other than their mother. Trampling of young pups by bulls was probably responsible for the appreciable incidence of ruptured livers. Infectious disease and drowning played minor roles in pup mortality. Pups born late in the season suffered disproportionately greater mortality which may relate to female age and condition. Food availability (both during and prior to the breeding season) and weather are likely to account for year to year variation in pup mortality rates but the basic rate is primarily determined by breeding density. Further population increase and colonization of new beaches is expected until food resources during the summer, or more probably the winter, become limiting.

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