The Gansbaai coast was once a popular spot for great white sharks, but sightings have declined significantly in recent years. The study used long-term observations and tag data to prove that orcas, sometimes known as killer whales, drove great whites away.
The researchers also analyzed five large white shark carcasses found submerged ashore, four of which had their nutrient-rich livers removed. And one with his heart as well. The researchers said they all sustained wounds from the same pair of orcas, which likely killed more bone eggs.
The study tracked 14 adult whites over a five-and-a-half year period and found that they fled the area when the orcas were there. Researchers believe that sharks’ feelings of fear lead to a rapid and long-term exodus when they know a predator is present.
Study lead author Alison Towner, chief white shark biologist at the Dyer Island Conservation Trust in New release.
Towner believes this is a “widespread avoidance,” similar to how Serengeti wild dogs avoid certain areas when lions are present.
“The more frequent orcas are to these sites, the longer the great white sharks can stay away,” she added.
A changing ecosystem
Before orcas started attacking the great whites, sharks were only absent from Gansai for one week in 2007 and three weeks in 2016.
This means that the extended periods of absence experienced by research are unprecedented, and the ecosystem in the region is changing.
Bronze whale sharks have emerged as new mid-rank predators in the area, Towner said.
“Bronze whalers are also being attacked by orca whales, which indicates a level of expertise and skill in hunting large sharks,” Towner said, adding that fur-head seals now feed on the endangered African penguins.
“This is a top-down effect, and we also have a ‘bottom-up’ trophic pressure from the extensive removal of abalone, which grazes the kelp forests to which these species are all related,” she added.
“In simple terms, although this is a hypothesis at the moment, there is significant stress that the ecosystem can withstand, and the effects of shark removal from orcas are likely to be broader.”
Towner also believes orcas are increasingly 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 each of the top predators could be related to the decline in prey numbers, including fish and sharks, causing changes in their distribution pattern,” she said.
Orcas are focusing on younger sharks, which can have a greater impact on large vulnerable white populations as sharks grow slowly and mature late in life, she said.
The researchers acknowledge that sea surface temperatures can also affect adult egg sightings, but “the immediate and sudden drop in sightings at the beginning of 2017 and the extended and increased absences cannot be explained.”
They added that other explanations could include direct hunting of large eggs or reduced prey numbers due to hunting, but that although this “may contribute to the overall decline in the numbers of large eggs in southern Africa, it is unlikely to explain the sudden local decline.”
Another 2016 study suggested that there are only a few hundred great white sharks left in South Africa, compared to previous estimates of a few thousand.
In addition, 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 change.
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