Population demography of Antarctic fur seals: the costs of reproduction and implications for life-histories

1. This study examined the costs of reproduction in terms of future survival and reproduction in female Antarctic fur seals from Bird Island, South Georgia. It used mark-recapture data from 11 consecutive years, including 3 years when several indices showed that food availability was well below average.
2. Population age structures were used, in conjuction with the measured age-specific survival rates, to estimate the rate of increase of the population as 10•7% per annum.
3. The average annual survival rate was 0•83 (SD = 0•10) with a range from 0•65 to 0•93. Survival rate showed no trend through time but was weakly correlated with pup growth rate, suggesting that it may be influenced by availability of food. Survival rate was unrelated to any other environmental or demographic parameter including population size.
4. There was no evidence of senescence. Survival rate was not related to year of birth or age, after accounting for variation due to pregnancy and calendar year. Survival was reduced as a result of pregnancy which accounted for 40–50% of adult female mortality. This effect was greatest in the age classes with the highest reproductive output (ages 5-8 years).
5. Mean pregnancy rate was 0•70 (SD = 0•11) with an interannual range of 0•59–0•88. Although females normally produced their first pups at age 3–4 years, pregnancy rate peaked at age 8 years and declined thereafter. Otherwise pregnancy rate was unrelated to the environmental or demographic variables we tested. Food availability during the pup-rearing period had no effect on pregnancy rate. 40–50% of failures to become pregnant related to animals having been pregnant in the previous year.
6. Reproduction incurs costs to females, in terms of reduced survival and future fecundity, and consequently, on average, females which survive longest tend to do so because they have lower fecundity.