New research: Steep decline for shark attack rate in California
It’s summer beachgoing season and with the recent spate of shark bite reports in the Carolinas, sharks are more top of mind than ever. But are shark attacks really on the rise?
Research published today by Stanford University’s Hopkins Marine Station and the Monterey Bay Aquarium shows that, indeed, the overall number of shark bites on the California coast is climbing gradually every year. But there’s a catch. Since 1950, the annual rate of shark attacks has actually decreased – and fairly dramatically.
Using data from the Global Shark Attack File, Aquarium shark biologist Sal Jorgensen and his Stanford colleagues discovered a surprising story. The research team noticed that even though the number of attacks was rising, they weren’t keeping pace with the tripling of California’s coastal population – from 7 million people in 1950 to 21 million coastal residents by 2013.
The numbers of surfers, scuba divers and swimmers rose at much faster rates than the overall population. So when the team weighted their data to reflect the number of ocean users, they found that the likelihood or rate of an individual being bit by a white shark dropped substantially – by 91 percent between 1959 and 2013.
“This shows that the goals of public safety and conserving the ocean wilderness, intact with top predators, are actually compatible.” Sal says. “Our results also suggest that attacks could be further reduced by modifying when and where we get in the ocean.”
Using a statistical model, the team was able to determine the most and least risky times and locations for shark attacks. According to their data, October through November – when sharks are feeding on seals along the coast – are the most likely times for attacks to occur. March through May are relatively safer times to be in the water – when most white sharks are far offshore at the mysterious White Shark Café.
The authors noted that beachgoers and water enthusiasts face many greater perils than a shark attack. Hospitalizations from drowning and scuba-related decompression sickness occur at much higher rates than those from shark bites.
“Our disproportionate fear of shark attacks is amplified by a lack of having control when we enter the ocean wilderness,” notes Sal, an avid surfer himself. “This type of data can give people the ability to have more control and minimize their risk.”
For instance, the results showed it’s 1,566 times safer to surf between San Diego and Los Angeles in March, compared with surfing between October and November in Mendocino County.
Making these types of informed choices would be far more effective at increasing public safety than culling, the research finds. In Australia, officials have tried to reduce public risk by killing white sharks in a large culling program – a tragic and uninformed approach. In fact, culling sharks is ineffective.
“These programs often serve more to reassure people rather than effectively increase beach safety,” says Francesco Ferretti, a postdoctoral research fellow at Hopkins Marine Station and the study’s lead author.
Though culls are meant to target white sharks, other shark species are often killed as well. Because of the importance of all shark species to maintaining the balance of the food web, culls can dramatically disrupt the ecosystem. And they’re extremely costly. The cull in western Australia is slated to cost $22 million.
Francesco says the money could be more wisely used to promote research and awareness of sharks, and to come up with more effective solutions to keep people from encountering sharks.
In California, white shark attack rates have declined so much that the researchers wonder if perhaps the numbers reflect a decrease in the shark population over the last half decade. An alternative possibility is that as populations of marine mammals – adult white sharks’ favorite prey – have bounced back, the sharks have relocated closer to their rookeries.
Marine mammals, especially elephant seals, tend to congregate on island beaches far away from those used by people. The Aquarium’s ongoing research with Hopkins colleagues may provide some answers in the near future.
Feretti, F., Jorgensen, S. Chapple, T.,K, De Leo, G., Micheli, F. 2015. Reconciling predator conservation with public safety Frontiers in Ecology and the Environment. (Vol. 13, Issue 6)