The Gansbai coast was once a popular spot for great white shark spots, but has declined significantly in recent years. The study used long-term observation and tagging data to show that great whites were driven out by orcas, sometimes known as killer whales.
Researchers have analyzed that the carcasses of five great white sharks have been washed ashore, four of which have had their nutritious livers removed. And one with his heart out too. All of them had wounds made by the same pair of deadly whales, which probably killed more great whites, the researchers say.
The study tracked 14 great whites over five and a half years and found that they fled the area while the orcas were there. Researchers believe that the sense of fear of sharks begins with rapid, long-term mass migration when they know that predators are present.
“At first, after the orca attack in Gansby, individual great white sharks were not seen for weeks or months,” said Alison Touner, a senior white shark biologist at the Dyer Island Conservation Trust. News release.
Touner believes this is a “large-scale avoidance”, with wild dogs in Serengeti avoiding certain areas when lions are present.
“The more frequent orcs come to these sites, the longer the great white shark stays away,” he added.
A changing ecosystem
Before the orcs started attacking the great whites, the sharks were absent from Gansby for one week in 2007 and three weeks in 2016.
This means that the increased absence observed by research is unprecedented, and it is changing the ecosystem of the area.
The bronze whale shark has emerged as the new mid-ranking hunter in the area, Touner said.
“These bronze whales are also being attacked by orcas, indicating the level of experience and skill in large shark hunting,” said Touner, who added that Cape Far seals are now preying on African penguins, which are endangered.
“This is a top-down effect, we have massive removal of abalone resulting in‘ bottom to top ’trophic pressure, which is connected through all of these species.
“Simply put, although this is a guess at the moment, there is so much pressure on an ecosystem and the effects of the shark-removing orcas could probably be even wider.”
A ‘sudden fall’
Touner also believes that killer whales are on the rise off the coast of South Africa and that this particular pair could be part of a rare group of shark eaters.
“This change in the behavior of both top predators may be related to the decline in hunting populations, including fish and sharks, which in turn changes the way they are distributed,” he said.
Orcas focuses on younger sharks, he said, which could have a major impact on the weaker white population as sharks grow slowly and mature later in life.
Researchers acknowledge that sea surface temperatures can also affect the vision of great whites, but “the immediate and abrupt decrease in vision in early 2017 and the extended and increasing period of absence cannot be explained.”
Other explanations include a reduction in the number of prey caused by the direct whites fishing or fishing, but although it “could potentially contribute to the overall decline in the number of great whites in South Africa, they are less likely to explain the sudden decline locally.”
Another 2016 study suggested that only a few hundred great white sharks remained in South Africa, compared to an earlier estimate of a few thousand.
Also, DNA analysis of shark tissue has shown that the genetic diversity of whites in South Africa is exceptionally low, which makes them more susceptible to external shocks such as disease or environmental changes.