Northern fur seal lactation, attendance and reproductive success in two years of contrasting oceanography

The fitness of any individual, population or species in the evolutionary sense is ultimately determined by its reproductive success. To maximize reproductive success and fitness organisms must respond and adapt to biotic and abiotic factors in the environment. The Bering Sea ecosystem, in recent years, has undergone changes that appear to be in response to atmospheric anomalies. Some of these can influence hydrographic structure of the Bering Sea on very short time scales; others, such as winter temperatures, the extent of ice cover and the timing of its retreat in the spring have lasting effects on hydrography and productivity through the summer months when migrating marine mammals and seabirds rely on marine resources for reproduction. This study took place in 1995 and 1996, two years of very different physical oceanography on the eastern Bering Sea continental shelf throughout the range of foraging northern fur seals. Fatty acid signature analysis (FASA) was used to examine dietary shifts related to changes in physical oceanography, dive pattern and foraging location. I use female fur seal attendance patterns as a broad measure of offspring provisioning and measured pup growth rates, length of lactation, and mass at weaning. In 1995, a year in which the continental shelf was strongly stratified throughout most of the summer with a large "cold pool" below a relatively shallow thermocline, fur seals had longer trips to sea, shorter visits to shore, weaned their pups later at a lower mass. Trip durations in 1995 were also more variable and were longer in early lactation (July and August) for both St. Paul and St. George islands. The contrasting hydrography of the region in 1995 and 1996 affected prey distribution and availability to apex predators of the region and resulted in altered patterns of offspring provisioning and reproductive success.