The Gansbaai coast was once a popular spot for great white shark spotting, but sightings have declined significantly in recent years. The study used long-term views and labeling data to show that the great whites were driven out by orcas, sometimes known as killer whales. The researchers also analyzed five carcasses of large white sharks found washed ashore, four with their nutrient-rich livers removed and one with its heart also removed. They all had wounds from the same pair of orcas, which probably killed more large whites, the researchers say. The study followed 14 large whites for five and a half years and found that they left the area when the orcas were there. Researchers believe that 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, isolated large white sharks did not appear for weeks or months,” study lead author Alison Towner, a senior white shark biologist at the Dyer Island Conservation Trust, told a news release. Towner believes this is a “large-scale avoidance”, similar to how wild dogs in the Serengeti avoid certain areas when there are lions. “The more orcs frequent these locations, the longer the great white sharks stay away,” he added.
An ecosystem that is changing
Before orcas began attacking the great whites, sharks were missing from Gansbaai for only one week in 2007 and three weeks in 2016. This means that the extensive absences observed by surveys are unprecedented and are changing the ecosystem in the area. Bronze whales have emerged as new middle-class predators in the area, Towner said. “These bronze whalers are also attacked by orcas, which indicate a level of experience and skill in hunting large sharks,” said Towner, who added that cape fur seals are now looting endangered African penguins. “This is a top-down impact. We also have bottom-up food pressures from the extensive removal of zucchini, which grazes the algal forests that connect all these species,” he added. “To put it simply, although this is a hypothesis at the moment, there is only so much pressure that an ecosystem can accept, and the effects of shark-removing vows are probably much broader.”
A “sharp drop”
Towner also believes that orcas are increasingly prevalent off the coast of South Africa and that this particular pair may belong to a rare group of shark eaters. “This change in the behavior of both top predators could be related to a reduction in the game population, including fish and sharks, causing changes in their distribution pattern,” he said. Orcas focus on younger sharks, he said, which could have a greater impact on vulnerable large white populations as sharks grow slowly and mature late in life. The researchers acknowledge that sea surface temperatures could also affect the observations of large whites, but “the immediate and sharp drop in views in early 2017 and the prolonged and increasing periods of absence can not be explained.” Other explanations could include direct whitefish fishing or reduced prey due to fishing, they added, but while this may “potentially contribute to the overall decline in whitefish population in South Africa, they are unlikely to explain the sudden local decline.” “ Another 2016 study showed that there were only a few hundred 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 extremely low, making them more prone to external shocks such as disease or environmental change.
