Until 2017, great white sharks dominated the food chain off the coast of the Gansbaai region of South Africa. But then a new top robber appeared. And everything became completely different: since then, at least eight carcasses of the big sharks have been washed up on the shore, with typical bite wounds and mostly without liver and heart. A study by Alison Towner from the Dyer Island Conservation Trust and her team confirms a long-held suspicion in the "African Journal of Marine Science". Killer whales are responsible for the victims – and always the same pair, as evidenced by typical bite marks.
The sword whales even ensure that the sharks are looking for the width when the marine mammals in the area patrol how tower and co have observed with the help of geolocators. The section of the coast was long known as a collection area of the White Haie. Tourists came to the region 100 kilometers east of Cape Town to dive there in hairproof cages and watch the fish.
Meanwhile, the number of sightings has decreased considerably. And the longer the killer whale pair stays in a certain area, the longer the sharks subsequently avoid these waters. "After an orca attack in Gansbaai, no individual white sharks appeared for weeks or months at first. However, what we seem to be observing is a large–scale avoidance strategy - and not just a small-scale one. It is similar to the behavior of wild dogs in the Serengeti in Tanzania, which react to the increased presence of lions. The more often the orcas visit these places, the longer the great white sharks stay away," says Tower.
Since the start of recording and before the sword whales appeared, white sharks of the region have only stayed away twice a little longer: a week in 2007 and three weeks in 2016. However, the change at the top of the food chain has an impact on the entire ecosystem. The white sharks, for example, chased bronze shape and fur seals or their mere presence ensured that both types became more careful. Due to the escape of the large sharks, the smaller shark and the seals continue to go to the region. However, while the bronze shape is also killed by the sword whales, the seals now exert increased pressure on the endangered glasses penguins in the region. The marine mammals captured the birds directly and compete with them for fish.
It is still unclear what attracted the killer whales to the region. In principle, however, the sightings of the species off South Africa are increasing. These also probably belong to a morphotype that is known to hunt sharks as the main food. While the change at the top of the food chain is natural, Tower and Co point out that the hunting of killer whales puts additional pressure on the endangered great white sharks. The fish regularly die in shark nets, which are supposed to protect beaches in KwaZulu Natal. Overfishing is also a problem for the animals, according to the scientists.
