Reef sharks in remote Chagos archipelago hit by big population decline

When swimming in the waters of the isolated Chagos archipelago in the Indian Ocean, a half-dozen sharks would sometimes surround Francesco Ferretti. 

“You are in another world,” said Ferretti, a marine ecologist at Hopkins Marine Station of Stanford University. “You feel like you’re in one of the last remaining paradises of the planet.”

The medium-sized reef sharks here are harmless (to humans). The coral is vibrantly colored. The water pure.  

And to a large degree, it’s an illusion.

“When you see these places you think there’s less human impact,” said Ferretti in an interview. “However, things are not so simple.”

In a study published Wednesday in Science Advances, Ferretti and his research team found that this remote group of islands — which were established as a regulated marine conservation zone in 1991 — were not as untouched and pristine as scientists previously thought. The dominant sharks here — gray reef and silvertip sharks — had populations dramatically lower than what would be expected (called “baseline levels”) in a truly untrammeled environment. 

Silvertip sharks had populations that were 93 percent below predicted baseline levels. Gray reef populations were more resilient, but still more than 20 percent under expected levels. 

“This is one of the places you would least expect for there to be problems,” said Henry Bart, a fisheries ecologist and director of the Tulane University Biodiversity Research Institute who took no part in the study, in an interview. 

Unlike many remote island places, the Chagos archipelago has a well-documented history, allowing the team to create models that reconstructed what the shark populations likely looked like before the region began to be impacted by exploitative and unregulated fishing. The first scientific surveys in the area were carried out in 1948, but in the 1970s the data became richer – that’s when scuba diving surveys began in the Chagos. This, combined with a long and incrementally improving history of catch numbers spanning back over 50 years, allowed the team to predict what the shark populations should have been without intensive fishing pressure.

Overfishing isn’t good for any ecosystem, but it affects sharks much worse than their counterparts, bony fish.  

Sharks are often the top predators in their environments. Removing them can bring lasting alterations to an ecosystem. “You can have a change in the composition of the reef,” said Bart.

“Most sharks don’t produce large numbers of young and they take a while to mature,” said Bart. “This means they have a difficult time recovering when their populations are depleted.”

This may have happened in Chagos, said Ferretti. Sure, there are still lots of gray reef sharks swimming around (though fewer than before), but the entire shark community could have once been dramatically different. The reef, for example, lacks some of the biggest sharks — like Mako and Tigers — which very well could have been done in by overfishing, even in the open sea, farther from the Chagos islands.

“The biggest threat [to sharks] is definitely overfishing,” said Diego Cardeñosa, who studies the international shark fin trade at Stony Brook University’s School of Marine and Atmospheric Sciences, in an interview. Cardeñosa had no involvement in the study. 

The world’s biggest sharks are hunted for both their fins — to satisfy the ravenous Asian market demand for shark fin soup — and their meat, said Cardeñosa. And even before the shark fin soup craze picked up, miles-long nets fitted with thousands of hooks to catch massive tuna inadvertently caught the big sharks. 

“In the 50s and 60s, for every tuna you could catch five or six sharks,” said Ferretti.

Like Chagos, there may be a number of other places around the world whose natural amazement may belie the truth: That these ecosystems aren’t nearly pristine. But unlike Chagos, other remote locations lack critical records of fishing surveys and human activity. 

Cardeñosa cites Malpelo Island, 300 miles off the coast of Colombia in the Pacific Ocean. “It’s a super remote rock in the middle of the ocean,” he said. “It’s one of the most well-known places in the world to see hammerhead sharks.”

But the veteran, sea-hardened captains say, antidotally, that there used to be vastly more hammerheads there.

“You can see [hammerhead sharks] there by the hundreds,” marveled Cardeñosa. “But, I talked to a captain of a boat that has been going to the region for a long time, and he said, ‘You should have seen this place in the 70s — you would have seen a thousand’.”

“So it’s very easy to say something is pristine, but that doesn’t mean that in the past it wasn’t much better,” said Cardeñosa.

This underscores the need to enforce conservation and fishing bans in protected areas like Chagos, said Ferretti. This natural “paradise” may seem unexploited — but it appears likely this is a deception. 

Although, if one looks closely at the reef around Chagos, human influence can be found without having to pour over decades of history and plug old survey data into modern, sophisticated computer models. 

“You see garbage — plastic bottles — even in these remote places,” said Ferretti.