Destructive and unsustainable fishing has caused a crash in shark numbers across many of the world’s coral reefs, upsetting the ecological balance of the critical marine ecosystems, a major study has found.
A network of remote underwater cameras across 58 countries found sharks were “functionally extinct” at almost one in five of the 371 reefs studied over four years.
The loss of sharks was putting further pressure on coral reefs around the world that were already under threat from global heating, scientists said.
Shark numbers were lowest on 69 reefs surveyed in the Dominican Republic, the French West Indies, Kenya, Vietnam, the Windward Dutch Antilles and Qatar, where just three sharks were seen during 800 hours of footage.
Across four regions – the Indo-Pacific, Pacific, the western Atlantic and the western Indian Ocean – the nations with the highest numbers of sharks included Australia, the Bahamas, the Federated States of Micronesia, French Polynesia, the Maldives and the United States.
The Australian Institute of Marine Science (AIMS), a partner in the research published in Nature, said the study revealed a previously undocumented global decline in sharks on reefs.
Reefs close to human populations in countries with poor governance were the worst affected.
Sharks did best in places where the use of longlines and gillnets were controlled, catch limits on sharks were in place and marine sanctuaries had been created.
Dr Mark Meekan, of AIMS, said the study, part of a project called Global FinPrint, painted a bleak picture, but it also demonstrated the problem could be fixed.
“Stopping destructive fishing practices and getting some good governance into these fisheries could change the situation almost overnight,” Meekan said. “These are very doable things.”
Sharks play a critical role on coral reefs, keeping the balance of species across the marine habitats in check, he said. Losing sharks was impacting the health of coral reefs that many millions of people relied on for food.
“Sharks are important for the ecology of coral reefs, particularly at a time when they are facing so many other threats from climate change.”
To carry out the study, underwater cameras were deployed on 371 reefs across 58 countries. Bait was attached to a pole at the front of the cameras at regular intervals to attract any nearby sharks.
Between July 2015 and June 2018, the cameras recorded 15,000 hours of footage and captured 59 different shark species. More than 90% of the species sighted were species that frequently visited reefs or were residents.
Some 34 out of 58 nations had shark numbers that were half what was expected, “suggesting that loss of reef sharks is pervasive among reefs globally”, the study said.
The lead author of the study, Dr Aaron MacNeil of Dalhousie University in Canada, said: “From restricting certain [fishing] 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.”
On 19% of reefs, almost no sharks were seen. Prof Colin Simpfendorfer, a co-author of the study from James Cook University, said: “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.”
Shark numbers were comparatively high on the world’s largest coral reef system – the Great Barrier Reef – where 1,178 cameras were used across 11 reefs.
Dr Michelle Heupel, a marine ecologist at AIMS and the University of Tasmania, coordinated the study efforts across the western Pacific, including the Great Barrier Reef. She said shark numbers were highest in the more remote northern section of the reef.
Dr Mike Heithaus, of Florida International University, and a leader of the Global FinPrint project, said: “Now that the survey is complete, we are also investigating how the loss of sharks can destabilise reef ecosystems.
“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,” he said.
Meekan said it was “not all doom and gloom out there” and the plight of sharks was not irretrievable.
As well as sharks performing a critical ecological function, he said sharks were also important to many economies around the world, particularly those with strong diving tourism sectors. One example was Palau, where 8% of the nation’s GDP was generated through shark tourism.
“Many people are scared of sharks but, in fact, there’s a whole slew of people who dive just to see them,” he said. “On the Great Barrier Reef, divers say they want to see sharks.”
Global FinPrint said the study was the most comprehensive data-collection and analysis of the world’s populations of reef sharks and rays ever compiled.
Jody Allen, chair of the Global FinPrint funder the Paul G Allen Family Foundation – named after the late Microsoft co-founder – said the shark losses discovered in the study were tragic.
But, she added, “the data collected from the first-ever worldwide survey of sharks on coral reefs can guide meaningful, long-term conservation plans for protecting the reef sharks that remain”.
