You know that old joke about how the only thing worse than finding a worm in your apple is finding half a worm in your apple? Here’s a question in the same vein: what’s the one thing scarier than having your seas filled with terrifying apex predators?
How about: those same terrifying apex predators suddenly and mysteriously all getting gruesomely murdered?
A marine murder mystery
That’s exactly what happened to South Africa’s populations of great white sharks back in 2017. Out of nowhere, their carcasses started washing up on beaches, various organs removed with what was described as “surgical precision”.
The culprit? Orcas – a relatively new resident of the waters, and intelligent enough to pull off the brutal vivisections of their selachian prey.
Case closed, you might think – but then things got weirder.
All of a sudden, the great whites seemed to just… disappear. “The decline of white sharks was so dramatic, so fast, so unheard of,” Michelle Jewell, an ecologist at the Michigan State University Museum, told Hakai magazine. “Lots of theories began to circulate.”
Were the sharks victims of overfishing? Some thought so – but the idea didn’t hold water. Perhaps it was more orca attacks – but then where were the bodies?
Then, last year, one theory seemed to have won out. The sharks hadn’t died, the data seemed to suggest – they had just moved house.
Sharks on the move
“As False Bay and Gansbaai had major declines, other places reported huge increases in white shark populations,” Alison Kock, a marine biologist with South African National Parks and a co-author of the 2023 study that first demonstrated the shift, told Hakai. “Too rapid to be related to reproduction, since they don’t reproduce that fast.”
“It had to be redistribution,” she said. “The white sharks moved east.”
While it may seem too neat a solution to be true, it makes sense – after all, if someone moved into your neighborhood and started stealing all your friends’ livers, you’d probably move too.
“We know that predators have a huge influence on the movement and habitat use of their prey, so this isn’t really surprising,” Jewell pointed out. “The issue is that lots of people weren’t used to thinking of great white sharks as prey.”
Drawing on data from a wide range of sources – ecotourism companies, local anglers, even the numbers of shark bites logged over the years – Kock and her colleagues determined that local sharks had moved away from the orcas, and the Cape Town economy and identity that depends on their presence, and towards a more easterly habitat.
Of course, such a wholesale move was bound to have knock-on effects for the ecosystem, and we’re yet to see what they’ll be – but it’s got to be a relief that the sharks weren’t just pushing up sea daisies all this time, right?
“This has been very worrying for me, and it was good to see evidence that they hadn’t all died,” Kock said. “But it’s still unbelievable to me that I can go to [False Bay’s] Seal Island and not see any white sharks. It’s something I never expected, and I miss them a lot.”
But then, the plot thickened even further. Less than six months later, in the exact same journal as the first paper, a rebuttal was issued.
A disagreement
In March 2024, five months after the paper by Kock and her colleagues was published, a group of marine biologists presented a counterargument. Comprising researchers from universities across the world – as well as a couple of local expedition companies – the group didn’t just take issue with their predecessors’ conclusions, but with the very techniques and inferences that led them there.
For example, while shark populations in Algoa Bay have increased, they argued, the numbers simply aren’t enough to make up for the losses elsewhere. It may have looked that way to the other team, they wrote, but that’s because they standardized datasets collected from different sources – something the second team said they probably shouldn’t have done.
“The issue of proportionality […] is well illustrated when comparing the actual number of white sharks caught in Algoa Bay,” they wrote. “During [the] period of increased captures in Algoa Bay […] anglers reported capturing a maximum number of 59 white sharks in a single year (some of which could have even been recaptures of the same individuals).”
“In contrast, the Western Cape experienced a relative decrease from an average presence of several hundreds of individual white sharks to less than 10 sightings per year,” they pointed out. “If the entire white shark population was indeed regionally stable and those sharks previously observed in the West had redistributed toward the Eastern Cape, one would have expected the numbers of white sharks in Algoa Bay to be about tenfold higher.”
Perhaps more damningly, the drop in shark numbers started years before the orcas’ arrival in Cape Town, the team argued. “We agree that orcas have likely influenced white shark numbers and behaviors, and at least temporarily displaced many from their historical aggregation sites,” they wrote, “[but] the data as currently presented do not suggest that orcas are the primary driver of the declines in white shark observed in the Western Cape.”
So, what’s driving the shark numbers down? Well, it seems it’s still a mystery – which is a problem.
Where are all the sharks going?
As seductive as the “moving east” theory was, its spread throughout the media serves to remind us of the importance of skepticism in science. After all, tell everyone that shark numbers are actually stable, and you might just talk them out of supporting vital conservation efforts for the species.
Indeed, “our concern is that unsupported claims of population stability could jeopardize conservation actions urgently needed for white sharks,” said Sara Andreotti, a marine biologist in the Department of Botany and Zoology at Stellenbosch University and one of the co-authors of the rebuttal paper, in a statement this year.
“There is no evidence of the hundreds of white sharks counted in False Bay, Gansbaai, and Mossel Bay ten years ago to be aggregating now somewhere else along the South African coastline,” she said.
The orcas may have been the most immediate and striking of the great whites’ enemies, but they’re far from the only challenges the fish face. From climate change, to overfishing, to long-term unsustainable hunting by the KwaZulu-Natal Sharks Board, and even more basic problems like low genetic diversity, the sharks are swimming against the current when it comes to their survival.
And while some people may welcome a world with fewer Jawses in it, the knock-on effects would be catastrophic.
“Our paper highlights the importance of robust, transparent scientific inquiry in guiding conservation efforts while taking a precautionary approach,” the team concluded. “It also serves as a critical checkpoint, urging us to re-evaluate and reinforce our commitment to preserving South Africa’s white shark population given the critical role these apex predators play in marine ecosystems and in the economy of South Africa.”
Source Link: Where Have All Of South Africa's Great White Sharks Gone?
