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Great whites used to dominate areas of the Gansbaai coast, around 100 kilometers (62 miles) east of Cape Town, but have been avoiding them in recent years, according to to an article published in the African Journal of Marine Sciences on Wednesday.
The Gansbaai coastline was once a popular spot for great white shark viewing, but sightings have noticeably declined in recent years. The study used long-term sightings and tagging data to show that orcas, sometimes known as killer whales, have chased away great whites.
The researchers also analyzed five great white shark carcasses found on shore, four of which had their nutrient-rich liver removed. and one with the heart removed as well. All had wounds made by the same pair of killer whales, which are likely to have killed more great whites, the researchers say.
The study tracked 14 great white sharks for five and a half years and found that they fled the area when killer whales were there. The researchers believe that the sharks’ sense of fear triggers a rapid, long-term mass migration when they know the predator is present.
“Initially, after an orca attack in Gansbaai, individual great white sharks didn’t turn up for weeks or months,” study lead author Alison Towner, senior white shark biologist at the Dyer Island Conservation Trust, said in an email. Press release.
Towner believes this is “large-scale evasion,” similar to how wild dogs in the Serengeti avoid certain areas when lions are present.
“The more orcas frequent these sites, the longer great white sharks stay away,” he added.
Before killer whales started attacking great whites, the sharks had only been absent from Gansbaai for one week in 2007 and three weeks in 2016.
This means that the prolonged absences attested by investigations are unprecedented and the ecosystem in the area is changing.
Bronze whaler sharks have emerged as new mid-range predators in the area, Towner said.
“These bronze whalers are also being targeted by killer whales, indicating a level of experience and skill in hunting large sharks,” Towner said, adding that Cape fur seals now prey on African penguins, which are in danger of extinction.
“That’s a top-down impact, we also have ‘bottom-up’ trophic pressures from the extensive removal of abalone, which graze on the kelp forests through which these species are connected,” he added.
“Bottom line, while this is a hypothesis for now, there is a lot of pressure an ecosystem can withstand, and the impacts of killer whales wiping out sharks are likely to be much more far-reaching.”
Towner also believes killer whales are becoming more prevalent off the coast of South Africa, and this particular pair could be part of a rare group of shark-eaters.
“This change in the behavior of both top predators could be related to a decline in prey populations, including fish and sharks, causing changes in their distribution pattern,” he said.
Killer whales target younger sharks, he said, which could have a greater impact on vulnerable white shark populations, since sharks grow slowly and mature late in life.
The researchers acknowledge that sea surface temperatures could also affect white shark sightings, but “the immediate and abrupt decline in sightings in early 2017 and the prolonged and increasing periods of absence cannot be explained.”
Other explanations could include direct fishing of great whites or declines in prey numbers due to fishing, they add, but while this may “potentially contribute to an overall decline in great white numbers in South Africa, it is unlikely to explain the sudden localized decrease”. . . ”
Another 2016 study suggested there were only a few hundred great white sharks left in South Africa, compared to earlier estimates of a few thousand.
Furthermore, DNA analysis of shark tissue showed that the genetic diversity of South African whites is exceptionally low, making them more susceptible to external impacts such as disease or environmental change.