Sharks are vanishing at some coral reefs around the world. What’s going on?

While sharks are abundant at some coral reefs, a four-year study reveals they are “functionally extinct” at one in five reefs across the globe, according to researchers.

Scientists placed 15,000 baited video cameras on 371 reefs in 58 nations to look for reef sharks, such as tiger sharks and hammerheads, in the worldwide project.

“We observed no sharks on almost 20% of the surveyed reefs,” says the study, published Wednesday in Nature. “Reef sharks were almost completely absent from reefs in several nations.”

Researchers noted, however, that sharks were spotted in abundance on some other reefs, suggesting that shark depletion was closely related to local economic conditions, the study says.

The results reveal the “profound effect” of overfishing prompted by economic desperation, according to the study.

“It’s pretty grim, but not completely unexpected,” said Sonja Fordham, president of Shark Advocates International, reported Science. Fordham was not involved in the study.

No sharks were seen in video at reefs in six nations — the Dominican Republic, the French West Indies, Kenya, Vietnam, the Windward Dutch Antilles and Qatar, said Philip Matich, a marine biologist at Texas A&M-Galveston, reported Texas A&M Today.

But researchers found plenty of sharks elsewhere, such as in the reefs of French Polynesia in the South Pacific Ocean, Science reported.

“It’s just this gorgeous utopia, especially if you’re into reef sharks,” said Aaron MacNeil, a reef ecologist at Dalhousie University who took party in the study, according to the publication.

The findings show that proper conservation and protection strategies can help the shark population rebound, Texas A&M Today reported.

“Change takes time, and like many other management and conservation issues that we currently face, it’s unclear if the number of nations without sharks inhabiting their coral reefs will increase, decrease, or remain stable,” Matich said, according to the publication.

“I’m personally optimistic based on my interactions with communities and the value they place on live sharks over dead sharks, but there is still a lot of change that needs to happen,” Matich said.