There is no sea animal whose name – rightly or wrongly – inspires as much terror as the great white shark.
With its slender, hunting-optimized body, sharp teeth and (somewhat undeserved) reputation for feasting on human flesh, the great white (Carcharodon carcharias) is widely regarded as one of the ocean’s largest predators.
And that’s true, it is – but there’s something even the great whites are afraid of.
Since 2017, scientists have documented that the sharks have made themselves extremely rare off the coast of South Africa, where they typically congregate. Originally, the strange disappearances were attributed to human activities such as overfishing.
But research has confirmed the real culprit in detail: a pair of orcas (Orcinus killer whale), hunt the sharks and slurp up their delicious, nutritious, vitamin-packed livers.
Once upon a time, the fishing village of Gansbaai on the South African coast was something of a mecca for shark-watchers — so densely populated with predators that nearby Dyer Island is considered to be just that Great White Shark Capital of the World.
In recent years, however, the presence of sharks has decreased.
In addition, at least eight great white sharks have washed ashore at Gansbaai since 2017, several of them missing their livers (and some without their hearts) – the Signs of an orca attack.
The wounds on these sharks are distinctive and have been traced to the same pair of orcas. Scientists believe the pair are responsible for many more major white deaths that did not wash ashore.
We know from other studies that the presence of orcas can be quite adept at repelling great whites. That’s what a 2020 study found great whites will sneak awaydefinitely from preferred hunting waters off the coast of San Francisco if an orca appears in the area.
In a 2022 study, a team of scientists led by Dyer Island Conservation Trust marine biologist Alison Towner, using long-term sighting and tracking data of tagged sharks, found that orcas are the reason sharks are starting to avoid them , what used to be some of her favorite haunts.
“Initially, after an orca attack in Gansbaai, individual great white sharks would not show up for weeks or months,” Towner explained.
“However, what we seem to be observing is a large-scale avoidance strategy (rather than a fine-scale one) that mirrors what we are seeing from wild dogs in the Serengeti of Tanzania in response to the increasing presence of lions. The more frequent the orcas are in these places, the longer the great whites stay away.”
Over the course of five years, the team tracked 14 GPS-tagged sharks as they fled the area when orcas were present. Great white shark sightings have also decreased significantly in several bays.
This is a huge deal. Since records began, only two great white sharks have been noted as absent for a week or more in Gansbaai: one week in 2007 and a three-week period in 2017.
The new absences, the researchers said, were unprecedented. They are also changing the ecosystem.
In the absence of great white sharks, copper sharks (Carcharhinus brachyurus) move in to fill the unoccupied ecological niche. These sharks are hunted by great whites; With no great whites around, the orcas hunt the bulls instead.
And most importantly, they do so with the ability of predators experienced in hunting large sharks, the researchers said.
“However, the balance in marine ecosystems is crucial, since, for example, no great white sharks restrict the behavior of Cape fur seals, the seals can eat endangered African penguins or compete for the small pelagic fish they eat. This is a top-down impact, we also have bottom-up trophic pressures from the extensive removal of abalone grazing on the kelp forests that all link these species together. said Towner.
“To put it simply, while this is a hypothesis for now, there are limited pressures that an ecosystem can withstand, and the impact of orcas removing sharks is likely more far-reaching.”
It’s also worth considering the reasons why orcas might prey on sharks. your livers are rich food sourceshuge, bulging, and full of the fats and oils that the sharks use to fuel their epic migrations across the ocean.
But it’s unclear how orcas got it or why they seek shark liver as their preferred food source.
It’s possible that some orcas are adapting to preferentially hunt sharks, perhaps in response to the declining numbers of their preferred prey species. However, given this large white population are declining worldwidethe added pressure of an efficient predator is a concern.
“The orcas are targeting subadult great white sharks, which due to their slow growth and late life history maturation strategy may have further impacts on an already vulnerable shark population,” said Towner.
“Increased vigilance using citizen science (e.g. fisherman reports, tourist vessels) as well as continued tracking studies will help gather more information on how these predators may affect the long-term ecological balance in these complex coastal seascapes.”
The team’s research was published in African Journal of Marine Sciences.
A previous version of this story was published in July 2022.
