Sharks are missing from 19% of the world’s coral reefs, the greatest decline of reef sharks ever recorded, according to a new analysis. The study suggests overfishing, driven largely by dense human populations and poor governance, has made the ocean’s top predators “functionally extinct” in the waters of eight countries. But some reefs elsewhere had abundant sharks, suggesting conservation measures can work.
“This study is a tour de force,” says Nick Dulvy, a conservation biologist at Simon Fraser University. “It’s the most comprehensive study that’s ever been done to look at shark abundance,” adds Ellen Pikitch, a marine biologist at Stony Brook University. Both say the findings bolster the conclusion that fishing has profoundly depleted reef shark populations in many places. (Neither was involved in the project.)
Like other large animals, sharks are vulnerable to overfishing because they grow slowly and don’t have many offspring. Demand for shark fins has grown, along with consumption, by a burgeoning Asian middle class. In other places, fishing communities are eating more shark meat as other species have declined. Researchers know shark populations have dropped in many places, but these studies are difficult to compare.
So about 5 years ago, marine biologists Mike Heithaus and Demian Chapman of Florida International University began a large collaboration called Global FinPrint. The project’s aim was to survey, in a standardized way, all the world’s reef shark species, such as tiger sharks and hammerheads. The group focused on reef sharks because they are easier to spot than those—such as blue sharks and whale sharks—that roam the high seas. Six researchers coordinated surveys of coral reefs in various parts of the world by more than 120 scientists. At dozens of places on each reef, researchers lowered video cameras attached to 1.5-meter-long poles with shark bait at the far end (see video).
After 3 years, the team reviewed about 18,000 hours of video from 371 tropical reefs. More than 700 volunteers, many of them university students studying marine science, helped. Heithaus’s mother reviewed the most footage—1721 hours.
At 69 reefs—or about 19% of the reefs sampled—no sharks were caught on video, the team reports today in Nature. “It’s pretty grim, but not completely unexpected,” says Sonja Fordham, president of Shark Advocates International, who was not involved in the project. There may have been a few sharks that didn’t take the bait, but the overall low numbers suggest sharks no longer perform an ecological role in these reefs, says Aaron MacNeil, a reef ecologist at Dalhousie University who led the design of the sampling.
Not all reefs were in dire straits. Sharks were plentiful in remote French Polynesia, for example. “It’s just this gorgeous utopia, especially if you’re into reef sharks,” MacNeil says. Worldwide, the Bahamas came out on top for shark abundance, whereas Guam ranked last.
To find out what kinds of conservation actions might be helping sharks, MacNeil created a computer model that compared the relative abundance of reef sharks and factored in potential threats, such as the number of people living nearby and the distance to markets where shark fins might be sold. The countries with the most abundant sharks tended to have declared protected areas. The Bahamas has banned shark fisheries for 30 years, MacNeil says, and it has done extremely well in maintaining a reef shark population.
Another effective measure, the team found, is to regulate fishing so sharks are caught less often, whether intentionally or by accident. The places that are doing the worst have few or poorly enforced fishing regulations and higher levels of poverty, which can force fishing communities to exploit declining populations.
Shark conservation is not one-size-fits-all, the researchers say, and their analysis suggests some management measures have more potential in certain places. In the British West Indies, for example, sharks would especially benefit from replacing long-line fishing gear, in which many hooks are left in the water for a day or so, with gear that is better at targeting particular fish. Large nets that catch fish by their gills are also especially dangerous, because they are nearly invisible to marine life and catch almost everything, including sharks.
Although the Global FinPrint project is over, the researchers plan to use their data to study the ecological role of sharks and examine what happens to the reef ecosystem when they are extirpated. The data are already being used to review the conservation status of various shark species. “We really need to substantively move toward conservation and recovery in the next decade,” Dulvy says, “or else we’re going to be in real trouble.”