This was the best place on Earth to see great white sharks—then they vanished
A new study based on two decades of data shows what happens in an ocean ecosystem without great white sharks. Great white sharks, like this one pictured in 2014, were once common residents in the waters around Seal Island in False Bay, South Africa. Photograph by Nature Picture Library, Alamy Stock Photo
Built like a torpedo with rows of terrifying 2.5-inch teeth, the world's largest predatory shark is an intimidating sight. So intimidating that some might think an ocean without great white sharks (Carcharodon carcharias) would be a good thing. But their loss causes a ripple effect that transforms the entire ecosystem.
A new study in Frontiers in Marine Science shows what happens when sharks vanish. Seal Island, off South Africa's False Bay was once a great white hotspot—one of the few places on Earth where the sharks could be seen surging out of the water to capture prey.
'It was air Jaws,' says marine ecologist Neil Hammerschlag, executive director of the Shark Research Foundation Inc. and a coauthor on the new study. 'I don't think there's anything more remarkable in nature than seeing a 2,000-pound great white flying out of the air with a seal in its mouth.'
When the predators disappeared, researchers and conservationists pointed fingers at both encroaching orcas and humans as the culprit. But Hammerschlag and his colleagues, who began studying the ecosystem around the island in 2000 long before the vanishing, saw some surprising changes. The sharks of Seal Island
Twenty years ago, Seal Island was 'the greatest place on earth to see great whites,' recalls Hammerschlag, who also serves as president of Atlantic Shark Expeditions. Around 2010, white shark numbers around False Bay started dropping off, and the decline got steeper from 2015 on. By 2018, the great whites were gone.
Exactly why the sharks vanished remains a mystery. 'The departure of the white shark is really open to discussion,' says Greg Skomal, a shark biologist with Massachusetts Division of Marine Fisheries who wasn't involved in the study.
Some suggest the animals fled because killer whales moved in. Orcas can kill a shark in minutes by precisely carving out its nutrient-rich liver.
Hammerschlag thinks humans could also be to blame, as nearby shark nets kill 'somewhere between 25 and 30 great whites a year.' Even a small loss can drive the population into decline, he says, because great whites become sexually mature late in life—males in their 20s and females in their 30s—and have small litters of up to 12 pups.
(A baby great white shark led scientists to a huge nursery near NYC.) When the great whites vanished
Sharks are thought to keep marine habitats healthy by removing weak and sick animals from the food chain and keeping things in balance. But proving these impacts in a real-world habitat this is challenging.
'These kinds of ecosystem effects are very difficult for us to tease out because they require long-term data sets,' says Skomal.
In False Bay, Hammerschlag's and his colleagues partnered with an ecotourism company, allowing them to spend 'full days on the water, 200 days a year.' The team collected over 20 years of data, from before, during and after the great whites' disappearance.
'We saw things happen that we'd never expect,' Hammerschlag says.
Broadnose sevengill sharks (Notorynchus cepedianus) suddenly appeared—sometimes as many as 15 in one day. These animals usually hang out several kilometers away in kelp beds, which offer protection from great white attacks. 'From nothing to double digits. It's just mind blowing,' he says.
The Cape fur seal (Arctocephalus pusillus pusillus) population also increased. Safe from the threat of sharks, seals were rafting—floating in groups, like a living raft—and going after the cage divers' bait. 'That would have been suicide just a few years earlier,' he says.
Both seals and sevengills were on the menu for great whites, so it made sense to see their numbers rise. But Hammerschlag and his colleagues also wanted to find out if the populations of animals that seals and sevengills eat had changed, too.
Luckily, scientist Lauren De Vos had installed cameras in 2012 to get a snapshot of fish populations at the time for another study. Hammerschlag's team followed the same method—even down to borrowing the same equipment. As expected, the new data showed a decline in seal prey like anchovies and Cape horse mackerel and sevengill prey, such as smoothhound sharks and pyjama catsharks.
This zigzag of impacts—white sharks vanish, their prey increases and, in turn, the animals they eat declines—suggests this isn't caused by problems like pollution or development.
'If habitat destruction was occurring, you would think everything would go down,' rather than some species declining and some increasing, Hammerschlag says. But the study numbers don't reflect that.
The small area of False Bay, with relatively few species, made figuring out some of these food chain impacts easier. 'Hammerschlag has that connectivity mapped out because he knows what eats what,' says Skomal. The more species in an ecosystem, the more difficult it is to map the food chain and track the impacts when an animal is lost. Establishing the relationships between animals in a broader area, like the Gulf of Maine, which has hundreds of species, would be much harder, he says.
(Cape Cod may have the highest density of great white sharks in the world.) Losing a great white hotspot
One possible outcome of all of these shifts is that seals and sevengills might run out of animals to eat. Could the ecosystem collapse? It's too early to know. 'That would be the next question,' says Skomal. 'Is it too many seals?'
Today, great whites' incredible aerial displays are a thing of the past at Seal Island. 'You'd never know this was a great white hotspot,' Hammerschlag says.
For him, seeing how the whole ecosystem has changed shows the importance of enforcing shark protections and using non-lethal methods to protect beachgoers from shark bites.
'We can't change the orcas' behavior,' he says, 'But we can stop the netting program. That's kind of archaic.'
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