Foraging Energetics of Antartic Fur Seals in Relation to Changes in Prey Availability

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Abstract
This research examines the energy budget of breeding female Antarctic fur seals, both when food was plentiful and when it is scarce. The energy expenditure and change in body mass of lactating female Antartic fur seals, Arctocephalus gazella, foraging at sea was measured in two years using doubly labeled water at South Georgia Island. There was no difference between years in mass gain, water influx, mass—specific field metabolic rate (FMR), or absolute FMR. Mean at—sea FMR over both years was 9.52 ± 0.55 W/kg (n = 22), a value that is 6.7 times the predicted basal rate but only 1.9 times the FMR measured onshored. Comparable results have been reported for similar—sized northern fur seals. Krill, the nearly exclusive prey of breeding females, were scarce in 1984 at South Georgia. Fur seal foraging trips were twice as long in 1984 as in 1985 and total mass—specific energy expended by females during these trips was significantly greater. In addition, females were significantly lighter at parturition in 1984, and both pup mortality and the proportion of pups that died from starvation were double the 1985 values. Female condition at parturition and average foraging—trip duration (i.e., offspring—provisioning rate) appear to reflect prey availability. The similarity between years in mass increase suggests that females may have a limited ability to increase the relative time spent foraging because even in normal years only 5% of their time at sea is spent resting. This contrasts with northern fur seals, Callorhinus ursinus, which typically spend 17% of their time at sea inactive. Apparently these northern seals can increase their foraging effort by increasing the proportion of time spent foraging. This would account for the observed between—year difference in at—sea FMR of C. ursinus while foraging—trip duration remained fairly constant.

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