There is no other sea creature that instills fear – rightly or wrongly – like the white shark does.
With its streamlined body perfectly adapted for hunting, sharp teeth, and (somewhat undeserved) reputation for having a taste for human flesh, the ‘great’ white (Carcharodon carcharias) is one of the top predators in the ocean.
And it is, but there is something that even the great white fears.
Since 2017, researchers have noted that the sharks have all but disappeared from their regular haunt, the waters off the coast of South Africa, where they have met, gone out for a swim up the bay, and immediately returned, making the circumvention incredibly odd. The loss of sharks was first attributed to human activity like over-fishing.
Then in 2022, researchers went into detail to describe the actual threat to great whites: a pair of orca whales (Orcinus orca), dubbed Port and Starboard, after the recognizable kinks in their dorsal fin, were hunting white sharks and slurping their tasty, nutritious, vitamin-rich livers out.
The fishing town of Gansbaai is, or at least was, a bit of a mecca for shark spotters – the sharks were so numerous that the small island nearby, Dyer Island, is a ‘great white shark capital’, and the white sharks were everywhere. Over the past couple of years, however, sharks the sight of sharks was decreasing.
Further, since 2017, at least nine dead great white sharks have washed ashore at Gansbaai, some with missing livers (and some hearts too) – the classic signs that an orca had attacked it. Of course, great whites are not the only victims.
Port and Starboard have been implicated in a broadnose sevengill shark killing spree, killing at least 17 in one day.
The wounds on these sharks are characteristic and have been attributed to the same orca pair. Scientists think the pair is likely responsible for many more great white deaths that did not wash ashore.
Studies show that when orcas are present, they can “drive” great white sharks away quite effectively. One study from 2020 demonstrated that great whites would depart without fail from preferred hunting waters off San Francisco if an orca was in the area.
In a study published in 2022 by a group led by marine biologist Alison Towner of the Dyer Island Conservation Trust using long-term sighting and tracking data from both tagged sharks, they demonstrated both orcas were to blame for sharks starting to avoid habitats that they formerly used.
“Initially, after an orca attack in Gansbaai, individual great white sharks did not appear for weeks or months,” Towner explained.
“What we seem to be seeing, though, is a broader scale avoidance (as opposed to specific scale) strategy, like what we see used by wild dogs in the Serengeti in Tanzania, in reaction to increased lion presence. The more orcas return to these sites, the longer great white sharks are gone.”
The team tracked 14 sharks over five years; all GPS tagged. Great whites left the area on every occasion that orcas were present. Sightings of great white sharks have also decreased significantly in the several bays of Gansbaai.
This is an enormous trend. Since record-keeping began in Gansbaai, there have only been two instances where great white sharks were absent for a week or more: one week in 2007, and three weeks in 2017.
The new absences, the researchers said, are unprecedented. They are also still occurring. In a paper published earlier this year, Towner and her co-authors described two sightings of Port and Starboard attacking sharks and eating their livers.
Worse, these attacks are changing the ecosystem.
Copper sharks (Carcharhinus brachyurus) are now migrating to occupy the ecological opportunity which has arisen from the absence of great white sharks. Great white sharks prey on copper sharks; with no great, the orcas are hunting coppers instead.
Importantly, they are hunting them as predators with experience in hunting larger sharks, the researchers said.
“One example of that balance has been related to marine ecosystems, where we lose top predators like the great white shark so that the Cape fur seal can predate on the critically endangered African penguin, or they can compete with the same small pelagic fish, which is a top-down pressure, however, we also have ‘bottom up’ trophic pressures, just with the extensive removal of abalone that graze these kelp forests, the remaining species, and have been impacted by everything we are seeing,” Towner said.
“To simplify it, this is just a hypothesis; however, there is only so much pressure an ecosystem can take, and the effects of orcas removing sharks, is likely to be of substantially further reach.”
We can also consider, what are the reasons why the orca hunt sharks? Surely sharks’ livers which are a high nutrition-density (huge, plump, fats and oil – that sharks need to fuel their epic migratory journeys) are a good reason to target sharks as prey.
However, it is unclear how orcas figured this out, or why they may chose the shark livers as an preferred source of nutrition.
It is possible that some orca species are adapting to preferentially hunting sharks, perhaps due to a decline in overall numbers of their preferred preys. An un-connected pod of orcas in the Gulf of California has developed their own hunting techniques for whale sharks!
Of course, considering the global declines of great whites, it’s troubling to add the pressure of an efficiently evolving predator.
“The orcas are targeting subadult great white sharks, which presents more issues to this already vulnerable shark population of many unlikely reasons to add an efficient predator based on slow growth and long life-history maturation strategy,” Towner said.
With increased vigilance using citizen science, such as fisher reports, tourism vessels, and continued tracking studies, it is adding more information from which we can understand how these predations may cause to long-term ecological balance in coastal seascapes that are complex.
The teams data has been published in the African Journal of Marine Science, and African Journal of Marine Science,
An earlier version of this story was published in July 2022.