The Dyer Island Nature Reserve, which lies roughly 8 kilometers off the South African coast near Gansbaai, has historically been
The largest predatory fish in the ocean, the species is already in steep decline: Its global population has dropped an estimated 30 to 49 percent since the late 1800s, earning the white shark a “vulnerable” designation on the International Union for the Conservation of Nature (IUCN) Red List.
The Nature Reserve includes Dyer Island and a rocky outcrop called Geyser Island to its southwest, separated by a sandy bottom channel known as Shark Alley. The reserve hosts a plethora of marine life, fed by the nutrient-rich Benguela Upwelling Ecosystem, one of the most productive areas of ocean in the world. The system nourishes microscopic phytoplankton and seaweeds, luring Cape anchovy and pilchards that in turn attract seabirds and seals.
There are about 60,000 Cape Fur Seals on Geyser Island. They attract what has arguably become Dyer’s most popular visitor: great white sharks. Tourism boomed on the back of these riches from the sea, and nearby Gansbaai became a shark-cage diving hotspot after the great white was declared a protected species in South Africa in 1991
Yet around 2016, white shark sightings began to decrease. The dominant culprit, according to Alison Towner of the Dyer Island Conservation Trust, was the arrival of two orcas. Why exactly they came to the South African coast is unclear, but one theory is that declines in pelagic fish and shark species due to overfishing may have caused a shift in the distribution of certain killer whales to coastal regions.
The two specific killer whales, known as Port and Starboard, predated on white sharks primarily for their nutrient-rich liver, says Towner, a white shark biologist who performed necropsies on the white shark carcasses to confirm the cause of death.
“There was a flight response and the white sharks moved out of the area,” Towner says, adding that media reports on the “disappearance” have caused public confusion. While great white sharks are commonly found in temperate continental shelf waters, they also undertake long-distance trans-oceanic movements, so it can be hard to determine if they’ve departed any stretch of ocean for good. Plus, the sharks are only monitored in areas where shark-cage diving occurs, Towner says.
In their wake, the great whites left shifting predator dynamics similar to those previously documented in the North Pacific. Along that coastline, when killer whales started hunting sea otters, the sea urchin population — now free from predation — exploded. In turn, the sea urchins decimated kelp forests. Previously, orcas also displaced white sharks from the Farallon Islands, decreasing pressure on seals, the white sharks’ main prey.
At Dyer Island Nature Reserve as well, the recent decline in sharks has affected the seals, which normally avoid deep, open water during times of high predation risk. Now free to roam, Towner says the seals have been predating on the island’s endangered African penguins.
On land, unpredictable shark sightings have impacted the tourism industry too. “Working at sea is full of challenges, including difficult sea and weather conditions, and coupled with unpredictable sightings this can affect guest satisfaction and overall tourism perceptions,” says Marine Dynamics CEO Wilfred Chivell. The company runs a shark-cage diving operation in Gansbaai and is heavily involved in white shark research. Data gathered during diving trips — including sex, size and markings — are used for the company’s own research, and forwarded to the Department of Environmental Affairs, Forestry, and Fisheries.
Research by Marine Dynamics supports earlier findings that humans can interact with sharks without long-term impact. “Our research from tagging data collected in Gansbaai further supports the observations that sharks spend less time at cage diving vessels over time and more experienced individual sharks choose not to respond at all,” says Marine Dynamics spokesperson, Brenda Walters. “We have been documenting the sharks since 2005 and have seen growth and maturity over the years so no long term impact, even if they may use some energy while visiting the boats,” she says.
Since the first orca predation was recorded in 2017, figures for shark-cage diving in Gansbaai steadily dropped from its 2016 peak of 85,000 shark-cage divers for the year to 54,000 in 2019, Chivell says. Aside from the impact on the company’s research and conservation efforts, Chivell says the consequences are felt much further along the chain, impacting employment, hotels and guest houses, restaurants, food and beverage purchases, transport, conservation projects, community education, and more.
Despite these challenges, Chivell says shark-cage diving operators have managed to sustain their business thanks to concerted educational marketing efforts, which emphasized the possibility of still seeing a white shark as well as another species, the bronze whaler shark. “The bronze whaler is a shark that is commercially fished and had only been recorded a few times before at the boat,” Chivell says, noting that the sightings, while rare, are a great thrill for tourists.
The sharp decline in white shark sightings has reminded locals to never take any species for granted, Chivell says. “Prior to this disappearance, we had very few days that one would not see a white shark in what is one of the most accessible populations in the world,” he says. “The absence of the white shark as an apex predator in our marine ecosystem is markedly prevalent, and we are concerned just how quickly other species, some critically endangered like the African penguin, are being impacted by increased predation.”
Whether white shark sightings will increase to numbers experienced before 2017 is “impossible to say at this stage,” Walters says, though she points out that the sightings have not stopped. Around 19 sharks were recently seen on a trip at Mossel Bay, she says. “So, we will have to wait and see.”
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