Are penguins and seals in competition for Antarctic krill at South Georgia?

The Antarctic fur seal and macaroni penguin are sympatric top predators that occur in the Southern Ocean around South Georgia where they are, respectively, the main mammal and bird consumers of Antarctic krill. In recent years the population of fur seals has increased whereas that of macaroni penguins has declined. Both species feed on krill of similar size ranges, dive to similar depths and are restricted in their foraging range at least while provisioning their offspring. In this study we test the hypothesis that the expanded fur seal population at South Georgia may have resulted in greater competition for the prey of macaroni penguins leading to the decline in their population. We use a) satellite-tracking data to investigate the spatial separation of these two species whilst at sea during the breeding seasons of 1999 and 2000; b) diet data to assess potential changes in their trophic niches between 1989 and 2000. Foraging ranges of the two species showed considerable overlap in both years but the concentrations of foraging activity were significantly spatially segregated. Over the last 12 years the prevalence of krill in the diet of the two predators differed with less krill in the diet of macaroni penguins than Antarctic fur seals in recent years. Krill in the diet of macaroni penguins was significantly correlated with estimated krill abundance between 1994 and 2000. We found little evidence for direct competition between fur seals and macaroni penguins as, although very similar in trophic niches, they showed significant segregation in their spatial resource use.