Reproductive performance of female Steller sea lions: an energetics-based reproductive strategy?

We examined the reproductive performance of female Steller sea lions (Eumetopias jubatus) in order to evaluate the hypothesis that low pup production was associated with nutritional stress and to assess whether reduced birth rates could have been a factor in a recent large-scale decline in numbers. Nearly all (97%) sexually mature females were pregnant during early gestation. However, by late gestation, pregnancy rates had declined to 67 and 55% in the 1970s and 1980s, respectively, owing to reproductive failures. We found that body condition, as depicted by mass index and blubber index, had a positive effect on the probability that a female would be pregnant during late gestation. Age, age2, and lactation were also associated with pregnancy status during late gestation. These findings support the hypotheses that reproductive failures were associated with lowered nutritional status and that the resulting low birth rates were a proximate factor in the decline. We speculate that abortion is a part of the reproductive strategy of the female Steller sea lion that enhances overall reproductive performance during times of suboptimal nutrition. A major shift in oceanic regime occurred in the Gulf of Alaska during the late 1970s that resulted in a reduction of about 50% in the overall biomass of fishes and a shift in species composition. Prey resources may not have been adequate to successfully support the Steller sea lions' "energetically expensive" reproductive/foraging strategy during the period of our study.