Are sharks being attacked by killer whales off Cape Town's coast?

February 4, 2019 by Alison Kock And Tamlyn Engelbrecht, The Conversation

Large, predatory sharks occupy the top of ocean food chains, where they play important roles in maintaining diverse and healthy ecosystems. The loss of these predators can therefore have significant impacts on ecosystems.

For a long time broadnose sevengill sharks have occupied the apex of the food chain alongside the more famous great white sharks in False Bay on the southern tip of South Africa. Both species feed on seals, dolphins, other sharks and fish.

However, the structure of the False Bay food chain began to change significantly in 2015 with the appearance of a "new" predator, shark-eating killer whales.

The change was noted with the discovery of several dead sevengill sharks by scuba divers from a popular dive site inside the Table Mountain National Park marine protected area. This site was home to an exceptionally large group of sevengill sharks. Divers could dive with up to 70 sharks on a single hour-long dive – no other place in the world had this many broadnose sevengill sharks in one place.

Initially, the cause of death remained a mystery because no dead sharks were recovered for examination. Initially fingers were pointed to humans, great white sharks or killer whales. It was only months later following the discovery of more dead sharks and examination of the carcasses by scientists that the fingers pointed straight at killer whales.

With this information in hand we set about reviewing the literature on killer whale behaviour, dietary specialisation, and population delineation globally and locally. Based on the review we hypothesised that the attacks on broadnose sevengill sharks in False Bay were possibly indicative of the arrival of a different sub‐group – or ecotype – of killer whale in the bay that feeds on sharks.

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