A massive survey of the world's coral reefs has turned up a shocking decline in the populations of reef sharks. Over a three year period, almost no sharks at all were recorded at 19 percent of the reefs being monitored, meaning the predators are "functionally extinct" in up to eight nations.

The cause is likely overfishing, exacerbated by dense human coastal populations and poor governance; but it's not all bad news. The team's findings suggest that conservation policies could be set in place to help restore and manage reef shark populations.

Most of what we know about shark populations over the last few decades has been gleaned from an imperfect data source - the catch records of industrial fisheries. This doesn't include sharks in areas that aren't fished, such as shallow coastal waters, which include reef ecosystems.

The Global FinPrint survey was set up to address this gap in the data. Institutions in Australia, Canada and the US worked together to deploy 15,165 underwater video stations at 371 reefs in the waters of 58 nations.

Over three years, the researchers obtained over 18,000 hours of video, which they painstakingly trawled to quantify reef shark activity.

They assumed that reef sharks would be present on all reefs - but, horrifyingly, almost no sharks at all were observed on 69 of the reefs, across six nations: the Dominican Republic, the French West Indies, Kenya, Vietnam, the Windward Dutch Antilles and Qatar.

"In these countries, only three sharks were observed during more than 800 survey hours," said marine biologist Colin Simpfendorfer of James Cook University in Australia.

"This doesn't mean there are never any sharks on these reefs, but what it does mean is that they are 'functionally extinct' - they are not playing their normal role in the ecosystem."

Sharks perform an extremely important role in the world's oceans. They help keep populations of their prey species healthy by devouring weaker individuals, and they maintain biodiversity by keeping populations of other species in check.

But they also grow very slowly, and produce relatively few offspring. If they just had to contend with other marine life, this would be fine, maintaining a delicate balance. But such slow-growing, slowly reproducing species are extremely vulnerable to human overfishing, and it takes a long time for populations to recover.

When the team compared their data to human activity, they found that little to no shark activity was closely correlated with several problems.

"Although our study shows substantial negative human impacts on reef shark populations, it's clear the central problem exists in the intersection between high human population densities, destructive fishing practices, and poor governance," explained ecologist and Global FinPrint co-lead Demian Chapman of Florida International University.

But that also means that the inverse is true. "We found that robust shark populations can exist alongside people when those people have the will, the means, and a plan to take conservation action," he added.

Countries with higher reef shark activity than the average included Australia, the Bahamas, the Federated States of Micronesia, the Solomon Islands, French Polynesia, the Maldives and the United States. These countries, the researchers noted, tend to be well governed, sometimes remote, and have strong management of shark fisheries or shark sanctuaries.

This can include the outright banning of shark fishing, limits on how many sharks can be caught, and even restricting the use of equipment, such as gillnets and longlines, that increase shark bycatch, thereby having a negative impact on reef shark populations.

"These nations are seeing more sharks in their waters because they have demonstrated good governance on this issue," said marine biologist Aaron MacNeil of Dalhousie University, lead author on the paper.

"From restricting certain gear types and setting catch limits, to national-scale bans on catches and trade, we now have a clear picture of what can be done to limit catches of reef sharks throughout the tropics."

It won't be a universal approach, however. In some regions, overfishing is driven by the growing demand for shark fin, a delicacy consumed by those who can afford to buy it. In others, humans living in poverty fish what's available, and food availability is a harder problem to solve. And some regions could improve shark populations simply by using different equipment to reduce bycatch.

But the research offers a place to start finding those solutions. Now that we have a clearer picture of where the sharks are and are not, we can start studying the differences between those reefs to find out how, exactly, sharks fit into the bigger picture.

"Now that the survey is complete, we are also investigating how the loss of sharks can destabilise reef ecosystems," said marine biologist and Global FinPrint co-lead Mike Heithaus of Florida International University.

"At a time when corals are struggling to survive in a changing climate, losing reef sharks could have dire long-term consequences for entire reef systems."

The research has been published in Nature.