Fear at the top: killer whale predation drives white shark absence at South Africa’s largest aggregation site

Abstract

Risk-induced fear effects exerted by top predators are pervasive in terrestrial and marine systems, with lasting impacts on ecosystem structure and  function. The loss of top predators can disrupt ecosystems and trigger trophic cascades, but the introduction of novel apex predators into  ecosystems is not well understood. We documented the emigration of white sharks Carcharodon carcharias in response to the presence of a pair of  killer whales, Orcinus orca, at a large white shark aggregation site in South Africa. Between February and June in 2017, five white shark carcasses  washed up on beaches in Gansbaai, Western Cape Province, four of which had their livers removed. Sightings per unit effort (sea days) and telemetry  data demonstrated that white sharks emigrated from Gansbaai following these predation events, and in response to further sightings of this pair  and other killer whale pods in the vicinity. Tagging data demonstrated the immediate departure of white sharks from Gansbaai, and some sharks  were subsequently moving east. Contrary to expected and well-documented patterns of white shark occurrence at this site, their sightings dropped  throughout the following 2.5 years; change-point analysis on both datasets confirmed these departures coincided with killer whale presence and  shark carcasses washing out. These findings suggest that white sharks respond rapidly to risk from a novel predator, and that their absence  triggered the emergence of another predator, the bronze whaler Carcharhinus brachyurus. Predator–prey interactions between white sharks, other  coastal sharks, and killer whales are increasing in South Africa and are expected to have pronounced impacts on the ecosystem.