Home Science Killer whales are killing white sharks in South Africa

Killer whales are killing white sharks in South Africa

Killer whales are killing white sharks in South Africa

Great white sharks are no longer alone at the top of the food chain off the South African coast, now they have to fight orcas to survive

Until 2017, great white sharks dominated the food chain off the coast of South Africa’s Gansbaai region, 100 kilometers from Cape Town, where many tourists go to see these impressive animals that reach six meters in length in their habitat.

Nothing seems to be able to oppose these great sharks in the water, but suddenly everything has changed: at least eight carcasses of great white sharks washed up on the beach, with bite wounds and most without livers or hearts.

A study by Alison Towner of the Dyer Island Conservation Trust, published in the African Journal of Marine Scienceand, confirms the worst fears: a single pair of orcas are responsible for these deaths, as evidenced by the bite marks.

The researchers verified, using geolocators, that when orcas arrive in an area, the great sharks flee en masse. At the same time, the number of sightings has dropped considerably, affecting the tourism industry focused on shark conservation.

Orcas probably belong to a morphotype that hunts sharks as its main food.

According to the researchers, the longer the pair of orcas remains in a given area, the longer the sharks will avoid those waters. That was enough to keep sharks out of Gansbaai’s waters for months. This is not an isolated case, but rather a large-scale evasion strategy, similar to what occurs in the Serengeti, Tanzania, when hyenas are displaced en masse by the increased presence of lions.

Since the beginning of records, and before the appearance of the orcas, white sharks had left the region only twice: one week in 2007 and three weeks in 2016. With the change in the food chain, the entire ecosystem of the bay is affected. Great white sharks, for example, preyed on bronze sharks and seals, or made both species more wary by their mere presence.

Now that the big sharks have had to flee, smaller sharks and seals can move even further into the region. The seals increase predatory pressure on local penguins, which are at risk of extinction. In addition, orcas directly attack seabirds and compete with them for fish.

It is still unclear what attracted the orcas to the region. However, orca sightings in South Africa are on the rise. And it is likely that they belong to a morphotype known to hunt sharks as its main food.

While changes at the top of the food chain are a natural phenomenon, the researchers warn that in this case, orcas are putting additional pressure on endangered white sharks. These large sharks are often killed in the shark nets that are supposed to protect tourist beaches in the KwaZulu Natal region, and they are also threatened by overfishing.

REFERENCE

Fear at the top: Killer whale predation leads to absence of great white sharks at South Africa’s largest aggregation site

Photograph: Bernard DUPONT

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