We compared Antarctic fur seals Arctocephalus gazella breeding at 2 contrasting sites on South Georgia: one high density colony at Bird Island and one lower density colony at Cooper Bay. The population at Cooper Bay was considerably smaller than that at Bird Island despite ample suitable breeding area being available. At Cooper Bay, female seals were longer but weighed less than those breeding at Bird Island and, whilst both maintained the same rate of female pup growth, male pups grew faster at Cooper Bay. Although Bird Island seals dived deeper, they dived less often than Cooper Bay seals so that both populations spent comparable amounts of time in the bottom phase of dives actively foraging. Longer distance oceanic foraging trips that were observed at Bird Island were almost entirely absent from Cooper Bay. Both populations fed on Antarctic krill, but there was an absence of myctophid prey in the diet of seals at Cooper Bay. Evidence suggests that the favoured myctophid prey of fur seals at South Georgia, Protomyctophum choriodon, are absent from the colder waters around the south-east of the island. We propose that, if these energy-rich prey are unavailable in this region then seals at Cooper Bay may find it hard to offset the increased costs of foraging trips with longer duration and distance. This potentially reduced niche width means that the Cooper Bay population may be less buffered against environmental variability. Although food resources appeared to be sufficient during the period of our study, the south-eastern region of South Georgia has increased variability in food resources that, coupled with a smaller area in which to forage, might explain the reduced population size compared to the north-eastern end of the island
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