| Summary: | Diet seems to be a key factor driving diversity and isolation among killer whale populations. Killer whales at Marion Island, Southern Ocean, have been observed preying on seals and penguins but are also know to depredate Patagonian toothfish from longline fishing vessels. However, their diet is poorly known especially when they occur offshore. We analysed carbon and nitrogen stable isotope ratios in 32 skin samples collected from 24 killer whales belonging to 8 social units. Adult males showed higher δ 15 N values than adult females or subadults, indicating that they occupy a higher relative trophic level. There were no significant differences in δ 13 C among social units, but δ 15 N differed significantly and 2 individuals from social units which have been observed depredating Patagonian toothfish had higher δ 15 N values. The inshore presence of killer whales at Marion Island was a significant predictor of δ 13 C values, but not of δ 15 N values. This suggests some foraging north of Marion Island, potentially on lower trophic level prey. We also analysed tissue samples from seal, penguin and Patagonian toothfish prey and used available values for Antarctic fur seals. Results show that killer whales around Marion Island are apex predators, but that they do not feed exclusively on other high trophic level predators such as elephant seals, fur seals, and Patagonian toothfish. Killer whales had δ 15 N values similar to those of Pata-gonian toothfish and adult male elephant seals, implying that the diet of killer whales at Marion Island includes some lower trophic level prey such as cephalopods or fishes.
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