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.

The results, from over 370 reefs in nearly 60 countries, are alarming, said lead author Aaron MacNeil.

"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.

At reefs surveyed in eight countries, including Qatar, India, Vietnam and Kenya, no sharks were detected at all.

The findings do not mean sharks do not exist in the waters of these countries, but are evidence that their numbers on reefs are now critically low.

"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.

'Transformational'

The study, published Wednesday in the journal Nature, says 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 says.

Gillnets use a wall of netting, while long line fishing involves a single line strung with multiple baited hooks. Both methods have been criticized 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.

The new study relied on more than 15,000 hours of video from the underwater cams, analyzed 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 told a press briefing.

Banning harmful fishing practices, imposing catch limits, closing areas to fishing and creating shark sanctuaries could all help restore shark populations, the authors said.

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.

Modelling 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."