In early May, great white sharks began washing up dead on beaches in South Africa. All of them had massive tears beneath one of their pectoral fins, and all of them were missing their livers. Researchers now believe it’s a clear sign of orca predation–an odd thing, considering that orcas aren’t exactly common in the area.
“From a scientific perspective in South Africa, it’s absolutely unprecedented,” said Alison Towner, a white shark biologist at the Dyer Island Conservation Trust in Gansbaai, South Africa. “I’ve seen some incredible things working with the Dyer Island Conservation Trust, but one thing I never ever expected to see was predation pressure from killer whales. We had never even seen an orca here prior to 2011.”
In the days prior to the dead sharks, scientists at Marine Dynamics saw two orcas off the southwestern coast of South Africa. Soon after, three dead sharks washed up within days of each other, leaving researchers baffled. When they performed the autopsies, though, it quickly became clear that killers whales were responsible. All the sharks were missing their livers, and one was missing its heart, which as odd as it sounds, is something that orcas do.
Killers whales have been known to hunt great whites, but only on very rare occasions. They normally eat seals, other whales, and some smaller sharks. Towner, though, thinks that now that these ones have learned to kill great whites, they’ll keep on doing it. “Now they have learned how to kill white sharks,” she said, “it’s probably not going to stop anytime soon.”
While it’s not known exactly how orcas might kill a great white, there are a few theories. When a shark is upside down, it goes into a comatose-like state called tonic immobility. It’s possible that an orca might flip the shark over, then drag it backwards until it drowns. In the South African cases, the orcas then removed the livers, the largest and most nutrient-rich organs in a great white.
Experts suspect that the great whites that washed up in South Africa are just a fraction of the ones that may have been killed. If that’s the case, the others likely sank to the seafloor.
This is only the third time that orcas have been recorded hunting great whites, once off California and once of Australia. Scientists are more than a little worried about this new threat to the shark population in the area.
“Sharks face many threats in Southern Africa” Towner explained. “Fisheries, bather protection nets, poaching, pollutants, coastal development, global warming impacts. Now they have predation pressure from another predator, the odds are just not stacked up in their favor.”