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Keywords:

  • Orcinus orca;
  • top-down control;
  • predator–prey interactions;
  • elephant seal;
  • fur seal;
  • penguin;
  • apex predator

Abstract

Killer whales are the oceans' apex predators and their potential effects on ecosystems have been demonstrated. In the Southern Ocean, the role of killer whale predation in population declines of southern elephant seals remains largely speculative. We aimed to assess whether top-down control of pinniped and penguin populations at the Subantarctic Prince Edward Islands (PEIs) is generally plausible using a simple process of elimination. Based on published data, we predicted the energetic ingestion requirements of adult male and female killer whales as 1394 and 1028 MJ day−1, respectively. Expanding these requirements to the 37 killer whales photographically identified at the PEIs, the population requires 40 600 MJ day−1. Based on the energy density and mass data available, we predicted the energy content of available pinniped and penguin prey and calculated the rates at which killer whales would consume these prey in various scenarios. Penguins and Subantarctic fur seals are relatively insensitive to killer whale predation owing to their large population sizes (10 000–100 000 s). Conversely, the smaller populations (100 – 1000 s) of Antarctic fur seals and southern elephant seals are sensitive to predation, particularly the latter, as they have a high energy content (c. 2000–9000 MJ). Populations of these seals are currently increasing or stable and we conclude that presently killer whale predation is not driving population declines, although they clearly have the potential for the regulation of these smaller populations. Thus, if population sizes were reduced by bottom-up processes, if killer whale diet shifted or if prey availability changed, top-down predation by killer whales could become significant. By eliminating the possibility of some predation scenarios, we are better able to concentrate future efforts on plausible predation effects.