In a sweeping survey of 371 reefs across 58 countries, from the Central Pacific to the Bahamas, scientists discovered that about 20 percent were devoid of sharks. As essential apex predators, sharks help keep fish populations healthy by eating sick individuals and preventing prey numbers from exploding.
Some of the reefs with the most depleted shark numbers were closer to human populations, such as Qatar, the Dominican Republic, Colombia, Sri Lanka, and Guam.
The research, published today in Nature and part of the Global FinPrint Project, “is the largest reef shark study ever,” says study co-author Enric Sala, a National Geographic Society Explorer-in-Residence.
Aaron MacNeil, a biologist at Dalhousie University, and colleagues set out more than 15,000 baited camera traps, and the local snapshots provided by each revealed that species such as grey reef sharks, blacktip reef sharks, and Caribbean reef sharks were often missing from reefs in which they historically lived.
“Having dived in hundreds of places around the world, from pristine to degraded, it was no surprise that a fifth of the reefs surveyed had no sharks,” says Sala, who founded the Pristine Seas initiative in 2008 to conserve the world’s oceans. (Read how the world’s coral reefs are dying.)
And even in places where reef sharks can still be found, Sala adds, their numbers have been so reduced that they’re no longer playing the same ecological roles as predators.
Nick Graham, a marine biologist at Lancaster University in the U.K. who was not involved with the research, notes that while the camera-trapping approach “is tried and tested at local scales, this study has effectively coordinated a global assessment.”
“Sharks are easily overfished,” Graham agrees, calling them “a rare occurrence when diving in many nations.”
'You can only sell a dead shark once'
Two-thirds of the world’s 500 shark species are threatened by overfishing, often to meet demand for shark meat and fins, while nets and longline fishing equipment that unintentionally trap sharks have also severely diminished their numbers.
But there is still some hope for sharks. “The good news is that if we fully protect areas from fishing, marine life and sharks can bounce back,” Sala says. Protecting the waters around Cabo Pulmo, Mexico, for instance, has restored rich underwater communities, complete with sharks. (Read more about sharks, lords of the sea.)
Bringing back healthy shark populations isn’t just about creating space for the animals to recover—fisheries management is also key, such as imposing catch limits and limiting fishing gear that harms sharks. The new study, Graham says, “highlights the importance” of these approaches.
By more carefully regulating how sharks are fished and reducing the number of sharks caught accidentally as bycatch, populations will have more of a chance to recover, he adds.
Communication and outreach also has a critical role to play, says Nova Southeastern University shark researcher Carlee Jackson. “Many countries do partake in shark meat extremely regularly, but it’s never ok to tell those countries they’re doing something 'wrong'", Jackson says.
Instead, helping people around the world understand how important sharks are to the health of the oceans is a critical step. And part of that process may involve switching from fishing to ecotourism centered around sharks and the reefs they call home.
“This is good for local people too because they can sell sharks to diving tourists many times over,” Sala says, “whereas you can only sell a dead shark once.”