Until 2017, great white sharks dominated the food chain off the coast of South Africa’s Gansbaai region. But then a new top predator appeared. And everything changed completely: Since then, at least eight carcasses of the big sharks have 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 in the African Journal of Marine Science confirms a long-held suspicion. Killer whales are responsible for the victims – always the same pair, as evidenced by typical bite marks.
The killer whales even ensure that the sharks flee en masse when the marine mammals patrol the area, as Tower and Co have observed with the help of geolocators. The stretch of coast was long known as a concentration area for great white sharks. Tourists came to the region 100 kilometers east of Cape Town to dive in shark-proof cages and watch the fish.
Since then, the number of sightings has decreased considerably. And the longer the killer whale pair stays in a certain area, the longer the sharks then avoid these waters. »After an orca attack in Gansbaai, no individual great white sharks appeared for weeks or months. However, what we seem to be observing is a large-scale avoidance strategy, and not just a small-scale one. It resembles the behavior of wild dogs in the Serengeti of Tanzania, which respond to the increased presence of lions. The more orcas visit these places, the longer the great whites stay away,” says Tower.
Since records began and before the killer whales appeared, only twice have great white sharks stayed away from the region a little longer: a week in 2007 and three weeks in 2016. However, the shift at the top of the food chain is affecting the entire ecosystem. The great white sharks, for example, hunted bronze sharks and fur seals, and their mere presence made both species more cautious. With the escape of the larger sharks, the smaller shark and seals venture further into the region. However, while the bronze sharks are also being killed by the killer whales, the seals are now putting increased pressure on the region’s endangered African penguins. The marine mammals prey on the birds directly and compete with them for fish.