There may be no single fear as intense and as widespread as the one associated with sharks – they even inhabit the nightmares of people who don’t swim! But do sharks have a boogeyman they are afraid of? Well, it turns out they do. And they fear this black-and-white terror so much it’s driving them away from what used to be some of their favorite spots.
Great white sharks (Carcharodon carcharias) were once an icon off South African waters, giving it the title of great white shark capital of the world. The ocean here was full of sealife to let these predators flourish, giving people all around the world a show as they underwent stunning aerial acrobatics to catch prey. But a silent killer soon started picking these predators off one by one, leaving an unmistakable calling card on the eight victims that washed ashore: a distinctive wound and the vitamin-rich, oily liver of the great white being ripped out. In fact, sometimes they even took the heart!
While humans were blamed at first for the slow disapperance of one of nature’s greatest predators off South Africa, a recently published article in the peer-reviewed African Journal of Marine Science revealed the true culprit behind this whole situation: a pair of orcas. Also known as killer whales, Orcinus orca were given that moniker by ancient sailors’ observations of groups of orcas hunting and preying on larger whale species. And it seems they were living up to their names, quieting the waters around Gansbaai since 2017.
These aren’t just any orcas. They’re easily recognizable by their distinctive collapsed dorsal fins, and the scientists believe they’re responsible for many more great white shark deaths that haven’t washed ashore. “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 (rather than a fine-scale) 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,” lead author Alison Towner, a senior White Shark Biologist at the Dyer Island Conservation Trust, told the T&F Newsroom. “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; and these dynamics may also dictate the interactions between competitors or intra-guild predator/prey relationship.”
A PhD candidate at the Rhodes University in Makhanda, Eastern Cape, Towner has studied the great white sharks here for 15 years and says the great white shark’s absence is unprecedented. There were only two other times where the sharks were absent for a week or more (one week in 2007 and three weeks in 2016) and the team points out the worrying consequences this removal of great white sharks is having on the surrounding ecosystem. “It has triggered the emergence of a new mesopredator to the area, the Bronze Whaler Shark (Carcharhinus brachyurus)– which is known to be eaten by the Great White Shark – and these Bronze Whalers are also being attacked by the Orcas too, who are indicating a level of experience and skill in hunting large sharks,” explains Towner. “However, balance is crucial in marine ecosystems, for example, with no Great White Sharks restricting Cape Fur seal (Arctocephalus pusillus) behavior, the seals can predate on critically endangered African Penguins (Spheniscus demersus), 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.”
In other words? “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.”
This isn’t the first time orcas have driven great white shrks away. A 2019 study found that great white sharks would avoid their preferred hunting waters off the coast of San Francisco if an orca makes an appearance in the region, disrupting shark feeding behavior for extended periods at this aggregation site. Using long-term sighting and tracking data from 14 GPS-tagged sharks over the last five years, the South African team of scientists has demonstrated the same thing; even the sightings of great white sharks are down, quite significantly, in several bays.
It isn’t clear how these orcas came to figure out this area was ripe with sharks that have nutritious livers… or how they figured out the livers were such good eating in the first place. Yet-to-be-published data suggests that this pair might be members of a rare shark-eating morphotype who choose these top predators are their preferred meal due to the declining numbers of their preferred prey. Yet this adds even more pressure to the great white shark population, which isn’t exactly doing too well. “We know that Great White Sharks face their highest targeted mortality in the anti-shark bather protection nets in KwaZulu Natal, [and] they simply cannot afford additional pressure now from Orca, killer whale predation,” said Towner. “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.”
Towner suggests that increased vigilance using citizen science (e.g. fishers’ reports, 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. And while alternative explanations should be considered — such as sea surface temperature and fishery-induced declines — the scientists believe they do not explain the “immediate and abrupt decline in sightings at the beginning of 2017 and the extended and increasing periods of absence.”