The disappearance of great white sharks from a major hunting ground off the coast of South Africa has allowed a species of “living fossil” predators to re-emerge and take over the top of the food chain, scientists have said.
A two-decade shark monitoring project around Seal Island in False Bay, South Africa – where great whites are renowned for flying out of the water to catch fur seals – has recorded a mysterious collapse in great white shark sightings since 2015.
But the vacuum has resulted in a rise in sightings of ancient sevengill sharks, which are unique among sharks for retaining the seven gill slits seen on their prehistoric ancestors – rather than the five slits in modern species.
“In 18-plus years of working at Seal Island, we had never seen sevengill sharks in our surveys,” said Chris Fallows, co-author of the study published in the journal Scientific Reports on Wednesday.
“Following the disappearance of white sharks in 2017, sevengill began to show up for the first time and have been increasing in number ever since.”
Sevengills are on the International Union for Conservation of Nature’s Red List of threatened species, in large part because they are not seen regularly enough to monitor their numbers, though they are hunted for food and sport in a number of countries.
Despite growing as large as 3m long and feasting on smaller sharks, sevengills are opportunistic predators that hunt by stealth, often at night, and larger predators can be a threat.
When great white numbers in 2017 and 2018 reached an “all time low” for the waters around Seal Island, researchers went months without a great white sighting in some cases.
“While the reasons for their decline and disappearance remain unknown, it provided a truly unique opportunity for us to see what happens to an ocean ecosystem following the loss of an apex predator,” said the study’s lead author, Dr Neil Hammerschlag, a marine ecologist from Miami University.
Historically sevengills had only been spotted congregating near kelp beds 11 miles away from Seal Island but began to occupy new territory vacated by their old predators.
“In South African waters sevengills have no equal in the food web, with the exception of the great white shark,” the authors said, though orca whales have also been known to attack them in other regions.
“We hypothesise that the emergence of sevengill sharks at Seal Island is due to the loss of white sharks which would otherwise have predated on them, as well as competed for prey, like the seals.”
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