A single pair of killer whales have been attacking and killing great white sharks off the coast of South Africa in order to remove and eat their livers and hearts.
According to research published in the African Journal of Marine Science, the sharks have been avoiding sites where they have aggregated for many years in order to not bump into the two deadly orcas.
Researchers found that since 2017, eight white shark carcasses washed up on beaches in the Western Cape, near Gansbaai. Seven of the eight had their livers removed, and some their hearts as well. The wounds on the bodies were distinctively orca-caused.
“There were killer whale tooth impressions on the pectoral fins of the sharks, [and] their livers were removed so neatly—it would take coordination of large and sophisticated animals to tear a white shark open and do this,” Alison Towner, lead author and senior white shark biologist, told Newsweek. “Also each time a dead shark washed out, there had been sightings of killer whales in the area, so all the evidence pointed to killer whale predation, and was also confirmed by killer whale biologists.”
Orcas regularly feed on shark livers, as they are large and fatty, a favorite food of killer whales: a 2006 study of Canadian orcas found that they selectively targeted the most lipid-rich species of salmon in their feeding grounds, despite there being higher numbers of other species present. In some cases, the orca also ate the shark hearts.
“The orcas are targeting subadult great white sharks, which can further impact an already vulnerable shark population owing to their slow growth and late-maturing life-history strategy,” said Towner in a statement.
Gansbaai, located around 60 miles east of Cape Town, was previously a great white hotspot, attracting tourists for boat tours and cage diving. In the aftermath of these discoveries, the researchers noticed tagging data indicating that the great white sharks were leaving this region, swimming away from Gansbaai.
Since the deaths of their fellow sharks, sightings of great whites in this usually regular site dropped hugely over the next few years.
“Change-point analysis on both datasets confirmed these departures coincided with killer whale presence and shark carcasses washing out,” said the authors in the paper.
It seems that the sharks, upon the deaths of the others and the arrival of the two responsible orcas, left the territory that they had usually gathered in.
“Initially, following an orca attack in Gansbaai, individual great white sharks did not appear for weeks or months,” Towner said. “What we seem to be witnessing though is a large-scale avoidance—rather than a fine-scale—strategy, mirroring what we see used by wild dogs in the Serengeti in Tanzania, in response to increased lion presence. The more the orcas frequent these sites, the longer the great white sharks stay away.”
The orcas are thought to have newly arrived in the Gansbaai region as a result of a decline in their own prey populations where they usually hunt. Killer whales, living up to their names, often target the livers of great white sharks using a special feeding strategy to access the fatty organ from the shark’s torso, breaking the pectoral girdle and removing the liver through a rupture in the abdominal wall.
In some other cases of orca predation, the sharks have also appeared to leave the area rapidly.
This predation and subsequent fleeing of the sharks from their usual territories is a blow on the already International Union for Conservation of Nature (IUCN)-classified vulnerable species. Great white populations are declining across the globe, and in the Western Cape region specifically, they are under pressure from other factors including direct fishing, and the indirect effect of fishery-induced declines in prey species. According to Towner, these factors aren’t enough to explain the decline in sightings of the sharks, but may exacerbate the damage to the species and the surrounding ecosystem.
“Increased vigilance using citizen science (e.g. fishers’ reports, tourism vessels), as well as continued tracking studies, will aid in collecting more information on how these predations may impact the long-term ecological balance in these complex coastal seascapes,” she said.
In their absence, the hierarchy of the local ecosystem has changed, seeing the arrival of another predator called the bronze whaler shark.
“These bronze whalers are also being attacked by the orcas too, who are indicating a level of experience and skill in hunting large sharks,” said Towner. “However, balance is crucial in marine ecosystems, for example, with no great white sharks restricting cape fur seal behavior, the seals can predate on critically endangered African penguins, or compete for the small pelagic fish they eat.”
“To put it simply, although this is a hypothesis for now, there is only so much pressure an ecosystem can take, and the impacts of orcas removing sharks, are likely far wider-reaching.”