Two killer whales are believed to have killed at least eight great white sharks off the coast South Africa since 2017, and managed to scare away many more.
Researchers have found that Great Whites avoid certain areas of the Gansbai coast for fear of persecution by killer whales.
Many of the shark carcasses were washed ashore without a liver and heart or with other injuries typical of a pair of killer whales.
Shark experts at the Dyer Island Conservation Trust say this suggests marine predators trigger a “flight” response in sharks when they are nearby.
This, in turn, leads to their rapid and long-term emigration from the area, creating an opportunity for the influx of new predators in order to deplete other species.
Alison Towner, senior white shark biologist, said: “The study is particularly important because 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 interactions between competitors or predator-prey relationships within a guild.”
Since 2017, at least seven great white shark carcasses have washed ashore in False Bay, showing tooth marks that indicate they were attacked by killer whales. Researchers say great white whales that encounter killer whales immediately leave their usual hunting grounds for up to a year.
Experts at the Dyer Island Conservation Foundation say the decline in great white sharks suggests killer whales are triggering their “fly” response to fear when they are nearby.
Between 2010 and 2016, shark watchers recorded over 200 great white shark sightings per year in False Bay, near Seal Island (pictured). In a study published today, biologist Alison Towner says she tracked 14 sharks escaping offshore Gansbaai when killer whales are present.
WHY DO RICKS HUNTER GREAT WHITE SHARKS?
Killer whales are the only natural predators of the great white shark.
Scientists have found evidence that they cut open sharks and eat their fatty livers.
Scientists suggest that this behavior may be responsible for the disappearance of large whites from the waters of False Bay off the coast of Cape Town.
The Great Whites frequented the area from June to October each year as part of their annual winter hunting season.
They were attracted to this region by the presence of the so-called Seal Island, a rock where a huge colony of seals lives.
However, they themselves fell to pray to killer whales – and retreat.
Once upon a time, Gansbaai was a world famous place to see the legendary Great White and tourists from all over the world visited and participated in cage diving.
In a study published today in the journal African Journal of Marine ScienceTowner reports that in five and a half years she has tracked 14 sharks escaping from areas where killer whales are present.
This was accompanied by a sharp decline in visual sightings in some bays of the Western Cape, where they dominated for many years.
The lead author of the study said: “Initially, after killer whale attacks in Gansbaai, individual great white sharks did not appear for several weeks or months.
“What we seem to be seeing is a large-scale avoidance strategy reflecting what we are seeing being used by wild dogs in the Serengeti in Tanzania in response to increased lion presence.
“The more killer whales that visit these places, the longer great white sharks stay away.”
Prior to these attacks on great white sharks, there have been only two instances since data collection began at Gansbae where they were absent for a week or more: one week in 2007 and three weeks in 2016.
The reduction of sharks in these areas has an impact on the marine ecosystem, for example, the introduction of a new predator in the area of the bronze whaling shark.
Towner added: “The bronze whaling shark, known to have been eaten by the great white shark, is also being attacked by killer whales, which indicates a level of experience and skill in hunting large sharks.”
“However, balance is critical in marine ecosystems, for example, since great white sharks do not limit the behavior of fur seals, seals may predate endangered African penguins or compete for the small pelagic fish they eat.
“It’s a top-down impact, we also have bottom-up trophic pressure due to the extensive removal of abalone that grazes in the kelp forests through which all these species are associated.
“To put it simply, although it is a hypothesis at the moment, the ecosystem can only withstand limited pressure, and the consequences of shark removal by killer whales are likely much wider.”
Great white sharks are a big tourist attraction in South Africa, and adventurous visitors are dropped into shark cages for a closer look. But shark sightings near Cape Town have dropped sharply since 2017, threatening ecotourism in the region.
HOW DO WHALES HUNTER SHARKS?
Great white shark liver can weigh over 60 kg (130 lb) and its rich organic chemicals make it an ideal food source for killer whales.
Dr. Ingrid Visser, who has studied killer whales for nearly 20 years, says killer whales use their powerful tails to create strong undercurrents that push sharks to the surface.
“As soon as the shark is on the surface, the killer whale turns around, lifts its tail out of the water and descends on it, like in karate,” she said.
This stuns the sharks, leaving them still a target for liver removal by killer whales.
The researchers believe that the behavior of a destructive killer whale pair may be indicative of a general increase in the number of species in the area.
In addition, the pair may be members of a rare shark-eating morphotype known to prey on at least three species of sharks as a primary food source in South Africa.
The prevalence of killer whales may indicate that declining populations of prey, including fish and sharks, are causing changes in their distribution patterns.
Other explanations for the decline in great whites in the area include shark fishing, fishing-induced reductions in prey, or rising sea surface temperatures.
However, while they may have a partial effect, they are unlikely to be the only factor behind such a sudden local population decline from 2017.
Towner said: “Orcas are targeting immature great white sharks, which could further impact an already vulnerable shark population due to their slow growth and late maturation strategy.”
“Increasing vigilance using citizen science, such as reports from fishermen and cruise ships, as well as ongoing tracking research, will help gather more information about how these predators may affect the long-term ecological balance in these challenging coastal seascapes.
“We know that great white sharks face the highest target mortality in KwaZulu-Natal’s basking shark protection nets, they simply cannot afford the added pressure from killer whales, killer whale predators.”