The Gansbai Coast was once a popular spot for great white shark sightings, but sightings have dwindled in recent years. The study used long-term observational and tagging data to demonstrate that great white whales were driven out by killer whales, sometimes known as killer whales.
The researchers also analyzed five great white shark carcasses washed ashore, four of which had their nutrient-rich livers removed. and another with a ripped out heart. All had wounds inflicted by the same pair of killer whales, which likely killed more great whites, researchers say.
The study tracked 14 large whites for five and a half years and found that they had fled the area when killer whales were there. The researchers believe that sharks’ sense of fear triggers a rapid and long-term mass migration when they know a predator is present.
“Initially, after the killer whale attack in Gansbaai, individual great white sharks did not appear for several weeks or months,” said study lead author Alison Towner, senior white shark biologist at the Dyer Island Conservation Foundation. news release.
Towner believes this is “large scale avoidance” similar to how wild dogs in the Serengeti avoid certain areas when lions are present.
“The more killer whales visit these places, the longer the great white sharks stay away,” she added.
Before killer whales started attacking great whites, sharks were absent from Gansbae for only one week in 2007 and three weeks in 2016.
This means that the long absence witnessed by research is unprecedented and it is changing the ecosystem in the area.
Bronze whale sharks have become the new mid-ranking predators in the area, Towner said.
“These bronze whalers are also being attacked by killer whales, which indicates a level of experience and skill in hunting large sharks,” Towner said, adding that fur seals are currently preying on endangered African penguins.
“This is a top-down impact, we also have bottom-up trophic pressure due to the extensive removal of abalone that grazes in the kelp forests through which all these species are associated,” she added.
“To put it simply, although it’s just a hypothesis at the moment, the ecosystem can only withstand that much pressure, and the consequences of removing killer whales from sharks are likely to be much larger.”
Towner also believes killer whales are becoming more common off the coast of South Africa and this particular pair may be part of a rare group of shark eaters.
“This change in the behavior of both apex predators could be due to the decline in prey populations, including fish and sharks, causing changes in their distribution patterns,” she said.
Killer whales are focused on younger sharks, she said, which could have a greater impact on vulnerable great white shark populations as sharks grow slowly and mature late in life.
The researchers acknowledge that sea-surface temperatures may also influence white whites’ appearances, but “the immediate and dramatic decline in sightings in early 2017, as well as extended and increasing periods of absence, cannot be explained.”
Other explanations could include direct fishing of the great whites or a reduction in prey due to fishing, they add, but while this could “potentially contribute to the overall decline in the number of great whites in South Africa, they are unlikely to explain the sudden localized decline.” . “
Another 2016 study found that only a few hundred great white sharks remained in South Africa, compared to earlier estimates of several thousand.
In addition, DNA analysis of shark tissues has shown that the genetic diversity of South African whites is exceptionally low, making them more susceptible to external shocks such as disease or environmental change.