Abstract:
The Southern Ocean plays a critical role in global climate and the demography and population abundances of apex predators are directly influenced by the availability of prey. To elucidate effects of climate change on sub-Antarctic ecosystems we investigated the population dynamics1 2 and foraging ecology3 5 of Antarctic and Subantarctic fur seals and southern elephant seals at the Prince Edward Islands, in
comparison with other island ecosystems6. The declining southern elephant seal population stabilised in the mid-1990s concomitant with an increase in pup weaning mass which suggests improved food availability to their mothers6. As a result age at first reproduction decreased and age specific fecundity and adult female survival increased1. Both the fur seal populations increased exponentially, the Subantarctic fur seal population approaching carrying capacity2, and they feed almost exclusively on the same species of myctophid fish in similar proportions, with little annual variation in pup growth rates, feeding trip durations and onshore attendance patterns of their mothers4 5. Satellite tracked immature southern elephant seals spend most of their time upstream of Marion Island during winter. Post-moulting adult females range some 2122-3133 km to the APF, to inter-frontal zones south of it and to the Antarctic Continental Shelf. Post-breeding adult females remain within about 1500 km from Marion Island at the outer edge of their feeding range, largely within inter-frontal zones south of the APF, and between the STC and the SAF. Post-breeding adult males behave similarly but during the post-moulting period males travel far and wide within a latitudinal belt of approximately 15° (40°S- 55°S) primarily westward. Adults occasionally dive to depths in excess of 1900 m and primarily execute foraging dives between 300-600 m presumably following vertically migrating prey. Future work should continue tracking the demography and condition of the southern elephant seal population and their killer whale predators7 through direct counts, mark-recapture and photogrammetry; monitoring the population growth, attendance patterns, pup growth rates and diet of the fur seal populations; satellite track both elephant seals and fur seals at sea to reconcile their foraging areas with oceanographic and bathymetric characteristics in a changing environment, and in the context of conservation 8.
