Two killer whales are thought to have killed at least eight Great White Sharks off the coast of South Africa since 2017, and managed to scare off many more.
Researchers found that the Great Whites have been avoiding certain regions of the Gansbaai coast for fear of being hunted by orca.
Many of the shark carcasses washed up without their livers and hearts, or with other injuries distinctive to the orca pair.
Shark experts at the Dyer Island Conservation Trust claim this suggests the marine predators trigger the ‘flight’ response to fear in sharks when nearby.
This in turn results in their rapid, long-term emigration from the area, creating an opportunity for an influx of new predators to deplete other species.
Senior White Shark biologist Alison Towner said: ‘The research is particularly important, as by determining how large marine predators respond to risk, we can understand the dynamics of coexistence with other predator communities.
‘These dynamics may also dictate the interactions between competitors or intra-guild predator-prey relationship.’
At least seven great white shark carcasses have washed ashore in False Bay since 2017, with telltale teeth marks indicating they were savaged by orcas. Researchers say Great Whites that encounter killer whales will immediately abandon their usual hunting ground for up to a year
Experts at the Dyer Island Conservation Trust claim the decline in Great White Sharks suggests the orcas trigger their ‘flight’ response to fear when nearby
Between 2010 and 2016 shark spotters recorded more than 200 great white sightings a year at False Bay, near Seal Island (pictured). In a study published today, biologist Alison Towner reports that she has tracked 14 sharks fleeing the Gansbaai coast areas when orcas are present
WHY DO ORCAS HUNT GREAT WHITE SHARKS?
Orcas are the only natural predator of the Great White.
Scientists have found proof that they are gashing the sharks open and eating their fatty livers.
Scientists speculate this behaviour may be behind the disappearance of great whites from the waters of False Bay, off of the coast of Cape Town.
Great Whites frequented the area between the months of June to October every year as part of their annual winter hunting season.
They were drawn to the region by the presence of the so-called Seal Island, a rock home to a huge seal colony.
However, they have themselves fallen pray to orcas — and are on the retreat.
Gansbaai was once a world-renowned place for spotting the legendary Great White, with tourists across the globe visiting and partaking in cage diving.
In a study published today in the African Journal of Marine Science, Towner reports that she has tracked 14 sharks fleeing the areas when orcas are present over a five-and-a-half year period.
This accompanied a dramatic decrease in visual sightings in certain Western Cape Bays, where they have dominated over many years.
The study’s lead author said: ‘Initially, following an orca attack in Gansbaai, individual Great White Sharks did not appear for weeks or months.
‘What we seem to be witnessing though is a large-scale avoidance 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.’
Prior to these predations on the Great White Sharks, there were only two instances since data collection began in Gansbaai where they were absent for a week or more: one week in 2007, and three weeks in 2016.
The decline in sharks in these areas is having an effect on the sea’s ecosystem, such as the emergence of a new predator to the area of the Bronze Whaler Shark.
Towner added: ‘The Bronze Whaler Shark – which is known to be eaten by the Great White Shark – are also being attacked by the orcas, who are indicating a level of experience and skill in hunting large sharks.
‘However, balance is crucial in marine ecosystems, for example, with no Great White Sharks restricting Cape Fur seal behaviour, the seals can predate on critically endangered African Penguins, or compete for the small pelagic fish they eat.
‘That’s a top-down impact, we also have ‘bottom up’ trophic pressures from extensive removal of Abalone, which graze the kelp forests these species are all connected through.
‘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.’
Great White Sharks are a big tourism draw in South Africa, with daring visitors getting lowered into shark cages for an up-close visit. But the number of shark sightings near Cape Town has dropped precipitously since 2017, threatening eco-tourism in the region
HOW DO THE WHALES HUNT THE SHARKS?
A Great White Shark liver can weigh more than 60 kilograms (130lbs) and its rich organic chemicals make it a perfect food source for killer whales.
Dr Ingrid Visser, who has studied killer whales for nearly 20 years, said orcas use their powerful tails to create strong underwater currents that force the sharks to the surface.
‘Once the shark is at the surface, the killer whale pivots and lifts its tail out of the water and comes down on top of it like a karate chop,’ she said.
This stuns the sharks, leaving them as a still target target for the killer whales’ removal of the liver.
The researchers believe that the behaviour of the destructive orca pair could be an indication of a general increase of the species in the area.
In addition, the pair could be members of a rare shark-eating morphotype, known to hunt at least three shark species as a prime source of nutrition in South Africa.
The orcas’ prevalence may suggest that a decline in prey populations, including fishes and sharks, is causing changes in their distribution pattern.
Other explanations for the decline in Great Whites in the area include shark fishing, fishery-induced declines in prey or an increase in the sea surface temperature.
However, while these may have a partial effect, they are unlikely to the sole contributor to such a sudden, localised population decline from 2017.
Towner said: ‘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.
‘Increased vigilance using citizen science, for example fishers’ reports and 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.
‘We know that Great White sharks face their highest targeted mortality in the anti-shark bather protection nets in KwaZulu Natal, they simply cannot afford additional pressure now from Orca, killer whale predation.’