Earlier this year Forbes covered how great white sharks (Carcharodon carcharias) had disappeared from False Bay, South Africa and made way for another shark in the area to become ‘top shark.’ The study found that the disappearance of great whites led to the emergence of sevengill sharks (Notorynchus cepedianus) and now they rule this area of South Africa.
Great white shark numbers began to drop in 2015, with numbers reaching an all-time low in 2017 to 2018. In fact, the great whites sometimes completely disappeared from ongoing surveys for weeks and months at a time. In 2018, the shark safety program Shark Spotters only recorded 50 sharks, a steep decline from the average 205 they saw from 2010 to 2016 in the same region according to Bloomberg. These numbers have not improved this year, even during the summer months where numbers can spike due to fish coming closer to shore.
In fact, the program has seen none this year. The waters around the famous Seal Island, known to be a popular feeding ground for these massive predators, have not seen any ‘Air Jaws’ acrobatics. And what about the many sharks tagged along the South African coast? None have “pinged” on the False Bay receivers.
“Further supporting evidence of the absence of these large apex predators is the lack of any feeding or bite marks on whale carcasses the city has removed from False Bay this year,” Cape Town’s municipality said in a statement on Wednesday. “We do not know how their absence from False Bay would affect the ecosystem. Neither do we know the causes for their disappearance.”
The great white shark (Carcharodon carcharias) was thrust into the spotlight by the movie Jaws, which starred a “killer” great white that is out for blood. These mackerel sharks (family Lamnidae) can reach total lengths of up to around 20 ft (6.1 m) and weigh up to 3,800 lbs (1.9 tons). These animals are characterized by a K-selected life history (slow-growing, late-maturing, and long-lived species with low fecundity) and have a worldwide range. They can eat different prey based on their age, size, and location but are known to gorge themselves on marine mammals and large fish like tuna. Although great white sharks have garnered the interest of both the scientific and public realms, large gaps still exist in our understanding of these sharks. Great white sharks are listed as Vulnerable by the IUCN.
While many find these animals to be fearsome, they bring a lot of money to the country. Documentary film crews, celebrities, and thousands of tourists have helped create a vibrant industry. In the last decade, shark cage diving has grown into one of South Africa’s top wildlife tourism activities, with various operators in Gansbaai alone. One of them is Marine Dynamics Tours, who says it has had some white shark sightings this year and that there has been “much confusion for shark cage diving tourists to South Africa and the tourism industry at large.”
Local media reports have suggested that orcas in the bay may be a reason for departure of the great whites, as they are known to predate on these animals. Both local and international shark and marine mammal scientists in South Africa are working together to understand this relationship between sharks and orcas. Since 2009 there has been an increase in the frequency of killer whale sightings in False Bay. Research from Monterey Bay Aquarium and their partners published in Nature’s Scientific Reports provides proof that great white sharks flee their feeding areas when orcas are present and do not return until the following season. “When confronted by orcas, white sharks will immediately vacate their preferred hunting ground and will not return for up to a year, even though the orcas are only passing through,” said Dr. Salvador Jorgensen, senior research scientist at Monterey Bay Aquarium and lead author of the study in a press release.
Could this be why they have said ‘sayonara’ to False Bay? Not all researchers are convinced the orcas are solely to blame. Sara Andreotti, a marine biologist at Stellenbosch University in South Africa told Science Magazine: “My worry is that the focus on the orcas is distracting the attention from problems that humans could solve.” Some scientists believe this could be a population-level decline due to overfishing, habitat loss, pollution, or maybe even a shift in environmental conditions or prey distribution.
“While the reasons for their decline and disappearance remains unknown, it provided a truly unique opportunity for us to see what happens to an ocean ecosystem following the loss of an apex predator,” said Neil Hammerschlag, a research associate professor at the University of Miami (UM) Rosenstiel School and Abess Center for Ecosystem Science & Policy in a press statement earlier this year.
It’s unclear why exactly the great white sharks left. But False Bay hopes they come back… or who knows what this ecosystem without their charismatic top-level predator will become.