Scientists say the four-year study – in over 370 reefs in nearly 60 countries – used more than 15,000 baited and remotely operated cameras to yield "alarming" results.
An unprecedented global survey has revealed a shocking decline in the number of reef sharks, with the predators "functionally extinct" in nearly 20 percent of sites studied.
The results, from over 370 reefs in nearly 60 countries, are alarming, said lead author Aaron MacNeil on Wednesday.
"We expect ... that there should be sharks on every reef in the world and to find 20 percent of the reefs we surveyed didn't have any sharks on is very concerning," he told a press briefing.
The four-year study used more than 15,000 baited and remotely operated cameras, so-called "chum cams," to obtain the first comprehensive picture of where reef sharks are thriving and where they are virtually non-existent.
At 69 reefs – or about 19 percent of the reefs sampled – no sharks were caught on video.
At reefs surveyed in eight countries, including Qatar, India, Vietnam, and Kenya, no sharks were detected at all.
"These nations are places where we're saying that reef sharks ... play no role in the ecosystem there and they're functionally extinct," MacNeil, an associate professor at Dalhousie University said.
The bad news?— Global FinPrint (@globalfinprint) July 22, 2020
➪ Reef sharks were not observed on 20% of coral reefs sampled, highlighting the negative effect of fishing.
➪ Being close to large human populations with weak governance is bad for sharks.
Dearth of big-picture information
The study, published in the journal Nature, said destructive fishing practices are the most likely culprit for the losses.
"The use of gillnets and longlines had the strongest negative influence on the relative abundance of reef sharks," the study said.
Gillnets use a wall of netting, while longline fishing involves a single line strung with multiple baited hooks.
Both methods have been criticised for high levels of bycatch, snaring marine life indiscriminately, including endangered animals.
The study backed by the Global FinPrint project was motivated by the dearth of big-picture information about shark populations in areas near coastlines.
In the past, researchers relied either on examining catch records or underwater visual surveys by divers, both of which have shortcomings and produce results that are difficult to compare, MacNeil told AFP news agency.
The new study relied on more than 15,000 hours of video from the underwater cams, analysed by a team of volunteers and researchers.
That method has given the team "a baseline against which we can both predict and gauge the success of future conservation actions for reef sharks," he added.
"It is transformational."
And while the results might appear disheartening, the researchers said there are were some bright spots.
'Reservoirs of hope'
"There are reservoirs of hope," said Mike Heithaus, co-author of the study and dean of the College of Arts, Sciences and Education at Florida International University.
"There are places where reef sharks are doing well that could repopulate and rebuild in these areas that are degraded," he said.
Banning harmful fishing practices, imposing catch limits, closing areas to fishing, and creating shark sanctuaries could all help restore shark populations, the authors said.
'Need to figure out solutions'
But they emphasised the need for solutions that fit particular circumstances, for example where fishing communities rely on shark fishing to survive.
"They have no alternative ... So we really need to figure out solutions that can work with those communities to still protect reef sharks effectively," said co-author Demian Chapman, an associate professor at Florida International University.
The study also warns that policies focused on protecting reef sharks may not be enough, given the predators rely on a healthy reef and abundant prey to survive.
Modeling done by the team suggests "focusing on reef sharks alone can only restore about 35 percent of their abundance, relative to restoration of the wider ecosystem," MacNeil told AFP.
"Results like these demonstrate that conservation of any group of animals must be embedded within a wider ecosystem."