A great white Shark near the water surface, six nautical miles from the coast of Gansbaai.
Photo: Education Images/Universal Images Group via Getty Images
In 1996, shark specialist Chris Fallows was the first to discover and photograph South Africa’s world famous “flying” sharks: the great whites of Seal Island in False Bay, which breached the ocean’s surface to hunt Cape fur seals.
“When we discovered the breaching great whites, it was the most spectacular behaviour ever seen by humanity in the 50 million years that these animals have been around,” said Fallows, a cage-diving operator and great white shark expert.
Now, great whites are “incredibly rare” in False Bay and Gansbaai, world-renowned hotspots for the apex predators.
“In just the 25 years from when we discovered them [breaching], until when they left, it’s an evolutionary blink of an eye to take an animal that essentially has been around for 50 million years – and we’ve managed to destroy that in less than a quarter of a century.”
There are two leading schools of thought for why they have vanished from these waters. One is that a pair of male orcas, or killer whales, with a predilection for shark livers, is causing them to flee.
The other, which Fallows believes is the main culprit, is the targeting of their food sources – the endangered smoothhound sharks and critically endangered soupfin sharks – by the demersal shark longline industry, which “sends them to Australia for fish and chips”.
Orcas deterring great white sharks by ‘ripping their torsos open’ for livers
According to a new peer-reviewed study, the pair of orca, dubbed Port and Starboard, that have been terrorising and killing great whites in Gansbaai since 2017, has managed to drive large numbers of the sharks from their natural aggregation site.
The study uses long-term sightings and tagging data to show that the great whites have been avoiding certain regions of the Gansbaai coast – territories which they have dominated for many years – for fear of becoming an orca meal. The study was led by Alison Towner, senior white shark biologist at the Dyer Island Conservation Trust and a PhD candidate at Rhodes University.
Since 2017, eight great whites have washed up on shore after orca attacks. Seven had their livers removed and some their hearts too. According to the researchers, their wounds were made by the same pair of orcas.
The researchers suggest the orcas might be members of a rare shark-eating group in South Africa known to hunt at least three shark species as a prime source of nutrition.
The scientists tracked 14 great whites over 5.5 years, finding they fled when the orcas were around, with sightings dropping dramatically in certain Western Cape bays. “We provide evidence that killer whale predation on white sharks directly induced emigration of sharks from Gansbaai and individual white sharks did not return for weeks or months,” they said.
The findings add further weight to an argument that suggests sharks use their “flight” sense of fear to trigger a rapid, long-term emigration en masse when a marine predator is nearby.
“White sharks appear to use a strategy of large-scale avoidance in response to the threat of predation … This resembles strategies used by wild dogs in the Serengeti in Tanzania, which, in response to increased lion presence, abandon areas for the long term.”
Unprecedented absences
Before the orcas started attacking great whites, the sharks had only been absent from Gansbaai for a week in 2007 and three weeks in 2016. The new absence is “unprecedented” for the area and is changing the ecosystem, said Towner. It is triggering the emergence of a new mid-ranking predator to the area, the bronze whaler shark, which great whites eat, and that are also being attacked by the orcas.
“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,” Towner said, which upsets the marine balance.
“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.”
That the orcas are targeting sub-adult great whites can further affect an already vulnerable shark population owing to their slow growth and late-maturing life-history strategy, she said.
Alternative explanations
While sea surface temperature could have an impact on the great white’s recent absence, “the immediate and abrupt decline in sightings at the beginning of 2017 and the extended and increasing periods of absence cannot be explained” by this, according to the study.
It describes how other potential explanations for a decline in white shark observations at Gansbaai could be direct fishing of white sharks or the indirect effect of fishery-induced declines in potential prey.
“For example, the bather protection nets and drumlines maintained by the KwaZulu-Natal Sharks Board … on the east coast may catch up to 32 white sharks per year.
“Although located farther east than our study site, white shark movements can easily extend into KwaZulu-Natal. Whereas there is debate about the white shark population size in South Africa, all studies agree that the low numbers of white sharks are of concern.”
Demersal longline fisheries target one of the prey types of white sharks: small shark species. “For example, there are well-documented declines in populations of the smoothhound shark and the soupfin shark. Hence, declines in white shark observations hypothetically could be because of a decrease in white shark population size or emigration caused by declines in prey abundance.”
While these factors could contribute to an overall decline in numbers of white sharks in South Africa, “they are unlikely to explain the sudden localised decline in observations at Gansbaai”, it said, noting how the population sizes of soupfin and smoothhound sharks have been in steady decline since the 1990s but the sudden decline in white shark observations started in 2016.
“Furthermore, the fishing areas that demersal longliners operate in are closer to the eastern part of the Western Cape coast and to the western part of the Eastern Cape coast (i.e. well to the east of Gansbaai), where areas such as Mossel Bay and Algoa Bay have not experienced the rapidly vanishing and extended absence of white sharks since 2017.
The demersal shark longline fishery generally uses relatively small, baited hooks with nylon traces – no steel – that most white sharks could bite through, which suggests that the fishery is likely to catch very few white sharks, “although entanglement in longlines is possible”.
Gansbaai has a large and permanent breeding colony of Cape fur seals, numbering about 32 000 individuals, which provides a “predictable and stable food source in time and space”.
Human-caused threats
In 2016, the results of a seven-year study by Dr Sara Andreotti, a marine biologist at Stellenbosch University, revealed that only 353 to 522 white sharks remained on the country’s coastline. This comprised one single population with low genetic diversity, being affected by depleted food resources, fishing bycatch, poaching and legal shark nets and drum lines in KwaZulu-Natal.
Andreotti suspects the orcas are not the main driver for the dwindling of the great whites.
“That event that has been happening of orca bite marks on some white sharks that were found on the shore, I still don’t think that is the main cause of white shark decline, based on our genetic data, photo identification data and sightings around the coast.
“It’s good that there is now a publication explaining exactly what happened, and the bite marks on the sharks, but it is my hope that this won’t change the attention to the anthropogenic (human-caused) threats that are still a real problem for South Africa,” she said.
Fallows concurred. “There certainly were incidents where orcas were involved in killing them, but as in many locations around the world where a similar thing has happened, it leads to short to medium displacement of the great whites and, invariably, they do come back.”
In the case of False Bay, he said, the disappearance of the great whites “happened long before the two implicated orcas were even present in the bay … three years before they were first seen”.
Great whites face huge pressures in South Africa’s waters, he said. “We still have the Natal Sharks Board, which is the world’s largest sanctioned culling machine of great whites. We have poaching, we have bycatch, we have the mass eradication of their primary prey species, which are actually smaller sharks, and which I believe is the primary cause of these animals disappearing from our coastline, and then add to that the orcas, which certainly have a localised effect.”
For a small population, with low genetic diversity, “it’s all too much”, Fallows said. “This sadly reflects on our government’s lack of ability to actually address the real issues that face these animals.”
Over 100 globally famous documentaries have been made about the breaching great white sharks of False Bay and the sharks of Gansbaai.
“They were getting South Africa unbelievable global free exposure, more so than any wildlife species, attracting over 100 000 tourists to our shores every year in a country that is critically in need of employment and positive brand exposure … and yet to tick a few boxes for job creation by creating a fishery like the demersal shark longline fishery, we have not only killed the golden goose, we’ve effectively strangled it.”
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