The Gainsby Coast was once a popular spot for great white shark spotting, but visibility has decreased significantly in recent years. The study used long-term sighting and tagging data to demonstrate that great whites have been driven away by orcas, sometimes known as killer whales.
The researchers also analyzed five great white shark carcasses that were washed ashore, four with their nutrient-rich livers removed. And the heart of one has also been taken out. Researchers say they all had wounds from the same pair of orcas, which may have killed more great whites.
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 when sharks sense that a predator is present, the shark’s sense of fear sets off rapid, long-term mass migrations.
“Initially, after an orca attack in Gansbaai, individual great white sharks did not appear for weeks or months,” said study lead author Alison Towner, a senior white shark biologist at the Dyer Island Conservation Trust. News release.
Towner believes this is “collective avoidance”, in the same way that wild dogs in the Serengeti avoid certain areas when lions are present.
“The more the orca frequents these sites, the longer white sharks stay away,” she said.
a changing ecosystem
Before orcas began attacking whites, the sharks were only absent from Gansby for one week in 2007 and three weeks in 2016.
This means the extended absence seen by the research is unprecedented, and is changing the ecosystem in the region.
Towner said bronze whaler sharks have emerged as the new medium-range predators in the region.
“These bronze whalers are also being attacked by orcas, which shows the level of experience and skill in hunting large sharks,” Towner said, adding that Cape fur seals are now preying on African penguins. , which are in danger.
“It’s a top-down effect, we also have ‘bottom up’ trophic pressure from widespread removal of abalone, which grazes kelp forests, these species are all linked.”
“To put it simply, although this is a hypothesis for now, an ecosystem can only take so much pressure, and the effects of orca removing sharks are likely to be much wider.”
a ‘sudden fall’
Towner also believes that orcas are increasingly prevalent off the coast of South Africa, and that this particular pair may be part of a rare group of shark-eaters.
“This change in the behavior of both apex predators may be related to a decline in prey populations, including fish and sharks, leading to a change in their distribution patterns,” she said.
That said, orcas tend to focus on young sharks, which could have a major impact on vulnerable great white populations as sharks grow more slowly and mature late in life.
The researchers acknowledge that sea surface temperatures may also affect the vision of great whites, but “the immediate and sudden decline in vision in early 2017 and the extended and increasing period of absence cannot be explained.”
Other explanations may include a reduction in prey numbers due to direct fishing or over-fishing of great whites, but although this “could potentially contribute to the overall decline in great white numbers in South Africa, they are suddenly are unlikely to explain the local decline.”
Another 2016 study suggested that there were a few hundred great white sharks left in South Africa, compared with earlier estimates of a few thousand.
Furthermore, DNA analysis of shark tissue has shown that the genetic diversity of South African whites is exceptionally low, making them more vulnerable to external shocks such as disease or environmental changes.