Life history parameters and regulation of the southern elephant seal population at Marion Island

Abstract

Pup mortality of southern elephant seals at Marion Island from 1990 through 1999 ranged from 1.6 to 7.3%, averaging 3.8%. Pup mortality was density independent and dismissed as being a major population regulating agent at Marion Island. Juvenile survival over two periods, one during the population decline and the other after the population had stabilised, was assessed and compared using mark-recapture models. Survival was age- but not sex-related and on average, the probability of survival was 59.5%,81.4% and 78.1% for the first, second and third year respectively. There was no significant difference in survival over the two periods and juvenile survival does not appear to be an important population regulating component in the southern elephant seal population. Mark-recapture data for females from the Marion Island population since the population stabilisation (1994) was analysed and compared with similar data collected during the decline and with data collected from the increasing population in southern Argentina. Survival of prime-age adult females increased by 8% concurrent with the stabilisation at Marion Island, which ultimately compensated for an annual rate of decline of the population of 5.85% until 1994. Survival of adult females at the colony in southern Argentina was 6% greater than at Marion Island after the stabilisation. Adult female survival is concluded to predominantly regulate the population. Changes in adult female survival and breeding probabilities with age were quantified in order to measure the prevalence of senescence in the population. Less than a 1 % difference between prime-age and post prime-age survival was found over eight cohorts of marked females and no evidence of reproductive senescence in terms of reduced breeding probability with age was detected. Five percent of southern elephant seal females survived to age ten and 0.5 % to the age of seventeen. Pubescent male survival increased concordant with the population stabilisation. These animals have high energetic demands due to a secondary growth spurt during which growth is accelerated and this increase lends support to the hypothesis that food has been limiting the population at Marion Island. Future survival and breeding probabilities of females primiparous at different ages were estimated in order to investigate potential future fitness costs associated with early breeding. Annual future survival of females breeding at age three was the same as that for females primiparous at four and slightly higher than that for females primiparous at five and six. Future breeding probabilities of females primiparous at three were similar to that for females primiparous at four and older. I found no evidence for future fitness costs incurred through reproductive expenditure and conclude that age of primiparity IS determined by acquisition of a critical body mass, after which no future delay in reproduction takes place. I estimated future survival and capture probabilities of southern elephant seals that winter at Marion Island during their first three years and compared these values to similar estimates from individuals that were not observed to winter over the same time periods. There were no significant differences in survival between wintering and non-wintering elephant seals suggesting that survival is not a fitness correlate of wintering behaviour. I provide unique evidence of differential site fidelity within age- and sex classes, which has implications for population modelling.