A massive new survey of nearly 400 coral reefs around the world reveals that sharks once common in those waters are disappearing, a worrying sign that the fearsome fish are at a much greater risk of extinction than previously thought.

Populations of five common reef sharks have experienced a staggering decline of between 60% and 73% over the past half-century, a team of 150 researchers reported in the journal Science on Thursday.

It’s absolutely astounding, said David Shiffman, a marine biologist at Arizona State University not involved in the survey work. And it’s telling that if you look at all the reef sharks in all the reefs everywhere, you get a very similar pattern.

The findings add to fears not only about the loss of these apex predators, but also about the effects on fish along the food chain and on coastal communities that depend on vibrant marine ecosystems for protein.

The ocean, simply put, isn’t as healthy without sharks.

The situation is really scary for us scientists who work with sharks, said Mario Espinoza, a marine biologist at the University of Costa Rica and co-author of the paper.

The culprit behind the decline, the researchers say, is rampant overfishing.

In much of Asia, shark fin soup is a dish served at big celebrations, with fishermen in some waters cutting off the sharks’ fins and dumping the rest of the body overboard. The United States banned the sale of shark fins last year, but the appendage still commands a high price in overseas markets. In other areas, fishing fleets inadvertently euthanize sharks by targeting their prey.

When he began the survey, Colin Simpfendorfer, a shark scientist at James Cook University in Australia, didn’t expect to see such severe declines, but the final measurements were disappointing.

All of them have dropped to levels we didn’t expect them to drop to, said Simpfendorfer, who led the research.

A herculean effort to find sharks

To conduct the shark census, researchers working around the world, from Indian Ocean reefs off East Africa to Australia’s Great Barrier Reef, to coral groups in the Caribbean, have dropped GoPro cameras under water along with bait bags.

Then they waited, recording more than 20,000 hours of footage. An army of volunteers spent multiple hours poring over the video to count the sharks that showed up for a snack. They focused on five key species: the Caribbean reef shark, nurse shark, gray reef shark, blacktip reef shark and whitetip reef shark.

It’s a herculean effort to watch so many videos, Simpfendorfer said. To repeat the study now, we’d probably look to use a lot more artificial intelligence. Much of the funding for that massive shark search came from the family foundation of Microsoft co-founder and ocean enthusiast Paul G. Allen.

The declines, which were calculated using models of what healthy coral reefs should look like, were sharpest in less affluent nations with lax regulations. Many more sharks swarmed in the waters off wealthier countries and in designated conservation areas.

Scientists have detected 5,000 sea creatures that no one knew existed. It’s a warning.

In places without many sharks, ray species have exploded, taking over as the top predator. But rays are not a true substitute for sharks. Without their top predator, oceanic food webs become unraveled. The carnivore promotes biodiversity in coral reefs by feeding on large sized prey populations, giving other fish a chance to thrive.

All told, a third of all sharks, rays and related species are at risk of extinction, researchers have previously warned.

Where are sharks better off?

Among the best-managed areas are the waters off Costa Rica, which is working with Ecuador, Colombia and Panama. The four countries are vying to expand and connect marine protected areas in the Pacific and facilitate the migration of sharks, whales and sea turtles between ecologically rich areas such as Costa Rica’s Cocos Island and Ecuador’s famed Galpagos.

We need to start improving the work we do with local governments, Espinoza said.

Off the coast of Jamaica, by contrast, the sharks are faring worse as local fishermen catch the grouper and red snapper that the sharks prey on.

One of the biggest obstacles to shark protection in Jamaica and elsewhere is their reputation. Every report of a shark biting a human, while such occurrences are rare, stokes fears among the public that it makes it more difficult to enact policies to protect them.

It’s very unlikely that shark conservation initiatives will be picked up, embraced, actually implemented, said Dayne Buddo, director of global ocean policy at the Georgia Aquarium, which has done much of the investigative work in Jamaica. They see sharks as ferocious creatures.

An encouraging finding: Many shark-free reefs are close to other shark-filled reefs. By reducing overfishing, those healthy populations could recolonize shark-depleted waters.

What it does say is that, for me, it’s not too late, Shiffman said.

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