The great whites dominated areas of the Hansbay coast, about 100 kilometers (62 miles) east of Cape Town, but avoided them in recent years, according to an article published Wednesday in the African Journal of Marine Science.
The Hansbay coast was once a popular spot for great white sharks, but sightings have declined significantly in recent years. The study uses long-term observations and tagging data to demonstrate that large whites were chased away by killer whales, sometimes known as killer whales.
The researchers also analyzed five carcasses of a great white shark found ashore, four with a nutrient-rich liver removed and one with a heart removed. They all had wounds inflicted by the same pair of killer whales, which probably killed more large whites, researchers say.
The study tracked 14 large whites in five and a half years and found that they fled the area when the killer whales were there. Researchers believe that the fear of sharks causes rapid, long-term mass migration when they know the predator is present.
“Initially, after a killer whale attack in Hansbay, individual great white sharks did not appear for weeks or months,” lead author Alison Towner, a senior white shark biologist at the Dyer Island Conservation Trust, told the press.
Towner sees this as “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.
A changing ecosystem
Before killer whales began attacking great whites, sharks were absent from Hansbay for only one week in 2007 and three weeks in 2016.
This means that the long absences observed by the research are unprecedented and change the ecosystem in the area.
Bronze whale sharks have emerged as new medium-sized predators in the area, Towner said.
“These bronze whalers have also been attacked by killer whales, which show a level of experience and skills in hunting great sharks,” said Towner, who added that sea seals now hunt endangered African penguins.
“It’s a top-down impact, we also have a bottom-up trophic pressure from the extensive removal of the abalone that grazes the algae forests through which all these species are connected,” she added.
“Simply put, although this is a hypothesis for now, there is so much pressure that the ecosystem can withstand, and the impact of killer whales that kill sharks is probably much wider.”
“Sharp decline”
Towner also believes that killer whales are becoming more common off the coast of South Africa, and this particular pair may be part of a rare group of people who eat sharks.
“This change in the behavior of the two largest predators may be related to declining populations of prey, including fish and sharks, leading to changes in their distribution pattern,” she said.
Killer whales focus on younger sharks, she said, which could have a greater impact on vulnerable large white populations as sharks grow slowly and mature late in life.
Researchers acknowledge that sea surface temperatures may also affect observations of great whites, but “the immediate and sharp decline in observations in early 2017 and the prolonged and increasing periods of absence cannot be explained.”
Other explanations could include direct fishing for large whites or reduced numbers of prey due to fishing, they added, but while this could “potentially contribute to the overall decline in large whites in South Africa, they are unlikely to explain the sudden localized decline. “
Another study from 2016 suggests that only a few hundred great white sharks remain in South Africa, compared to earlier estimates of several thousand.
In addition, DNA analysis of shark tissue has shown that the genetic diversity of whites in South Africa is extremely low, making them more susceptible to external shocks such as disease or environmental changes.
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