MIAMI — Sharks are "functionally extinct" in one-fifth of the world's coral reefs as destructive fishing practices have dramatically reduced the populations of predators that help keep marine ecosystems healthy, according to a study led by scientists at Florida International University.
The study, using data from coral reefs in 58 countries and more than 15,000 hours of underwater footage, found that sharks were so rare in some of the areas surveyed that they were no longer fulfilling their roles of maintaining the balance of species in those ecosystems.
"The problem is that reef sharks are being caught in too great a number that they can't replenish themselves," said Damian Chapman, the co-lead on the Global FinPrint study and an associate professor of Biological Sciences at FIU.
The survey, an unprecedented look at reef sharks around the world, began in 2015 and used underwater baited cameras to attract sharks in reefs located in four key geographical regions: the Indo-Pacific, Pacific, the Western Atlantic and the Western Indian Ocean. Scientists hope the results will help guide conservation policies for the threatened predators whose numbers have drastically declined over the past few decades.
These "chum cams" didn't find any sharks at all on any of the reefs in six countries: the Dominican Republic, the French West Indies, Kenya, Vietnam, Qatar and some of the Windward Islands. Among these, a total of only three sharks were observed on more than 800 survey hours.
Sharks are apex predators that serve as an indicator of ocean health. Reef sharks help regulate prey and balance the food chain, ultimately contributing to ensuring species diversity. But sharks are also an important food source, which has led to overfishing, especially in areas where the use of commercial longlines and gillnets is widespread, the study said.
Chapman said that the data also dispelled a common misconception that all shark fishing is done by big industrial vessels and companies interested only in cutting off the fins to sell them in Southeast Asia for shark fin soup.
"The reality is that in most parts of the world most fishing is done by local communities in small-scale fishing operations," he said. Because sharks are worth more than other types of fish, many countries rely on this fishery for their livelihood.
But in countries where sharks are worth more alive than dead, reef populations were healthy, according to the study. In the Bahamas and the Maldives, for example, shark-diving tourism creates opportunities for local communities to make a living without having to remove the animals.
The best performing nations compared to the average of their region included Australia, the Bahamas, the Federated States of Micronesia, French Polynesia, the Maldives and the United States.
Conservation measures limiting the number of sharks that can be removed have worked in these countries, while shark sanctuaries and marine reserves, where no fishing is allowed, have also helped populations recover, the scientists said.
Including local shark fishing communities in efforts to conserve these animals has also proved an effective strategy, said Michael Heithaus, dean of the College of Arts, Sciences & Education at FIU and a co-lead of the study.
"When local fishermen were involved in the conversation from the beginning, we saw that it was easier to come up with a plan to implement conservation measures that worked for that particular community," Heithaus said.
The scientists hope that their findings will help inform plans to improve populations in countries including Barbados, Bermuda and Madagascar, where reef shark numbers are dangerously low but still have a chance of recovery, he added.
Over the course of four years, the scientists recorded and analyzed more than 15,000 hours of video from surveys of 371 reefs in 58 countries, states and territories around the world. The work was funded by the Paul G. Allen Family Foundation and conducted by hundreds of scientists, researchers, and environmentalists organized by a network of contributors from Florida International University, the Australian Institute of Marine Science, Curtin University, Dalhousie University and James Cook University
Overall, conservation efforts like catch limits and regulation prohibiting the use of longlines and gillnets can greatly improve shark populations worldwide, the study said.