Abstract:
Antarctic (Arctocephalus gazella) and Subantarctic (A. tropicalis) fur seals are important predators
in the Southern Ocean. Marion Island (southern Indian Ocean) hosts the largest
sympatric breeding populations of these two species. Environmental and population changes
here over two decades may have influenced their diet and trophic interactions. To quantify
diet, we analysed prey remains in scat samples from Antarctic (661 scats) and Subantarctic
(750 scats) fur seals collected at Marion Island (2006–2010). We assessed diet composition
over time and calculated dietary overlap. The diet of both species was dominated by fish prey
(98.2% and 99.4% of prey items), mainly myctophids. Antarctic fur seals consumed small
numbers of penguins, cephalopods and crustaceans. In Subantarctic fur seal scats, crustaceans
and cephalopods were rare and penguin remains were absent. The diets of the two
species overlapped substantially (Pianka’s index = 0.98), however, small but significant
differences in the relative proportions of prey were evident. Seasonal and annual diet
changes suggest that their diet is similarly influenced by patterns of local prey availability
and abundance. Despite substantial changes in the population size and trajectory of Antarctic
and Subantarctic fur seals – which would be expected to influence trophic interactions
between them – comparing our data to those from earlier studies (1989–2000) did not reveal
significant long-term dietary changes in either species.
