Pair of orcas caused great white sharks to disappear in South Africa, study says

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A pair of orcas chased great whites off a stretch of South African coast after killing five animals in just a few months in 2017, according to a new study.

Great whites used to dominate areas off the coast of Gansbaai, about 100 kilometers east of Cape Town, but have avoided them in recent years, according to an article published in the African Journal of Marine Science on Wednesday. ).

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The coast of Gansbaai was once a popular spot for great white sharks, but sightings have declined sharply in recent years. The study used long-term sightings and tagging data to demonstrate that great whites were driven off by orcas, sometimes known as killer whales.

The researchers also analyzed five great white shark carcasses found on the beach, four without the livers, and one without the heart as well. They all had wounds made by the same pair of orcas, which likely killed more sharks, the researchers say.

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The study tracked 14 great whites over five and a half years and found that they fled the area when the orcas 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 on Gansbaai, individual great white sharks did not show up for weeks or months,” said lead study author Alison Towner, senior white shark biologist at Dyer Island Conservation Trust, in a statement to the study. press.

Towner believes they are “avoiding on a grand scale,” similar to how wild dogs in the Serengeti avoid certain areas when lions are present.

“The more orcas frequent these places, the longer the great whites stay away,” he added.

changing ecosystem

Before orcas started preying on great whites, they were absent from Gansbaai for just one week in 2007 and three weeks in 2016.

This means that the prolonged absences witnessed by the surveys are unprecedented and are changing the region’s ecosystem.

Bronze whale sharks have emerged as new mid-level predators in the area, Towner said.

“These bronze whalers are also being attacked by the orcas, which indicates a level of experience and skill in hunting large sharks,” Towner said, adding that Cape seals are now preying on African penguins, which are endangered.

“This is a top-down impact, we also have ‘bottom-up’ trophic pressures from extensive abalone removal, which graze the kelp forests that these species are connected by,” he added.

“To put it simply, while this is a hypothesis for now, there is only a certain amount of pressure an ecosystem can withstand, and the impacts of removing sharks from orcas are likely to be much broader.”

“Abrupt decline”

Towner also believes that orcas are increasingly present off the coast of South Africa, and this particular pair may be part of a rare group of shark-eating sharks.

“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,” she said.

Orcas focus on younger sharks, she said, which could have a greater impact on vulnerable populations of great whites, as sharks grow slowly and mature later in life.

The researchers acknowledge that sea surface temperatures can also affect great 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 the decrease in prey numbers due to fishing, they add, but while this could “potentially contribute to an overall decline in the number of great whites in South Africa, it is unlikely to explain the sudden localized decline.”

Another 2016 study suggested that there were only a few hundred great white sharks in South Africa, compared to earlier estimates of a few thousand.

Source: CNN Brasil

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