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    Spatial distribution of foraging by female Antarctic fur seals

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    I.L. Boyd, I.J. Staniland and A.R. Martin (United Kingdom)
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    The study tested the hypothesis that the distribution of critical habitat for foraging by female Antarctic fur seals breeding at the island of South Georgia was influenced by physical gradients in the oceans and also by the need to avoid local competition for food. It also tested the hypothesis that fur seal predation was capable of causing local depletion of prey species. When foraging in support of dependent offspring, fur seals travelled down physical gradients defined by the bathymetric features of the continental shelf around the island of South Georgia. Fur seals foraging from different sites followed the same pattern of travel. There was no detectable difference in this behavior among years when there were different amounts of prey available. Females were constrained to forage mainly within 100 km of the location at which the offspring was being raised. When this constraint was removed at the end of lactation, females foraged to much greater ranges and there was evidence that they dispersed to specific regions associated with the continental shelf east of Patagonia (>1000 km) and to the northern edge of the Antarctic pack ice (500 km). A model of the spatial distribution of foraging by lactating female fur seals predicted spatial distributions that were consistent with past observations from ship-based surveys. The model also allowed estimation of the spatial impact of fur seals on krill. This suggested that, in extreme cases and assuming that krill influx is limited, female fur seals could eat most of the krill present in some regions where they forage intensively. However, mean consumption was about one-tenth of the mean density of krill.