Foraging and provisioning in Antarctic fur seals: interannual variability in time-energy budgets

This study examined three competing hypotheses to explain how lactating Antarctic fur seals (Arctocephalus gazella) respond to changes in the level of resource availability. Antarctic fur seals have episodic bouts of suckling (1-3 days), alternating with foraging trips (3-10 days). Foraging time budgets varied significantly (p <.001) among 8 consecutive years at Bird Island, South Georgia. Foraging trip duration increased during periods of relative food shortage. Time spent ashore was more consistent among years than foraging trip duration but declined during a year of particularly low food availability. In 4 of the 8 years, there was a significant positive correlation between time spent ashore and foraging trip duration. In the other years, the relationship was close to statistical significance. Energy delivery to pops during suckling bouts followed an asymptotic power function. Energy gain during foraging trips was estimated from diving behavior, which suggested that the energy gain function was linear. Distance traveled during foraging trips was correlated with foraging trip duration, and long foraging trips were associated with reduced foraging intensity. There was support for the hypothesis that lactating Antarctic fur seals compensate for reduced resources by increasing the foraging trip duration rather than working harder and increasing their energy expenditure. However, there was most support for the hypothesis that lactating Antarctic fur seals adjust time spent ashore as well as foraging trip duration, possibly to maximize the delivery of food to their offspring. Lactation appears to impose constraints on provisioning of offspring that differ from those of seabirds foraging in the same environment and often on the same prey.