Predatory impact of killer whales on pinniped and penguin populations at the Subantarctic Prince Edward Islands : fact and fiction

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 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 1 394 MJ.day-1 and 1 028 MJ.day-1, respectively. Expanding these requirements to the 37 killer whales photographically identified at the Prince Edward Islands, the population requires 40 600MJ.day-1. Based on available energy density and mass data, 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 000s – 100 000s). Conversely, the smaller populations (100s – 1 000s) of Antarctic fur seals and southern elephant seals are sensitive to predation, particularly the latter as they have a high energy content (ca. 2 000 – 9 000 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 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.