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Sharks have disappeared from many coral reefs around the world due to destructive and unsustainable fishing, according to a new study published in the science journal Nature.
Using a network of underwater cameras across 58 countries, the researchers found sharks were "functionally extinct" at 20% of the 371 reefs they surveyed for four years.
Sharks are crucial to the coral reef systems as they pick off sick and weakfish, allowing the stronger ones to reproduce, thus helping to maintain the health and vitality of the marine ecosystem.
The scientists said that the loss of sharks is putting further pressure on coral reefs that were already under threat from global warming.
The number of sharks was lowest on 69 reefs surveyed in the Dominican Republic, the French West Indies, Kenya, Vietnam, the Windward Dutch Antilles and Qatar, where during the 800 hours of footage, only three sharks were seen.
"The worst-ranked nations for reef sharks included Qatar, the Dominican Republic, continental Colombia, Sri Lanka and Guam, which have suffered from varying levels of poor governance and extreme overfishing," the study said.
The nations with the highest numbers of sharks across four regions – the Indo-Pacific, Pacific, the western Atlantic and the western Indian Ocean - included Australia, the Bahamas, the Federated States of Micronesia, French Polynesia, the Maldives and the United States.
The shark population was relatively high on the Great Barrier Reef - the world's largest coral reef system - off the coast of Australia. Here, 1,178 cameras were used across 11 reefs for the survey.
While the worst affected reefs were close to human populations and in countries with poor governance, 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.
"Reef sharks were almost completely absent from reefs in several nations, and shark depletion was strongly related to socio-economic conditions such as the size and proximity of the nearest market, poor governance and the density of the human population," the study said
According to the Australian Institute of Marine Science (AIMS), a partner in the research, the study revealed a previously undocumented global decline in sharks on reefs.
"Stopping destructive fishing practices and getting some good governance into these fisheries could change the situation almost overnight. These are very doable things," Dr Mark Meekan, of AIMS, was quoted as saying by The Guardian.
"Sharks are important for the ecology of coral reefs, particularly at a time when they are facing so many other threats from climate change," he added.
Dr Meekan added that sharks were also important to many economies across the globe. In the case of Palau, 8 per cent of the country's GDP came from shark tourism.
"Many people are scared of sharks but, in fact, there's a whole slew of people who dive just to see them. On the Great Barrier Reef, divers say they want to see sharks," Dr Meekan said.
According to Science Magazine, sharks are vulnerable to overfishing like other large animals as they grow slowly and do not have many offsprings. The consumption of these large mammals and the demand for shark fins has also grown. In addition, in some fishing communities, the consumption of sharks have grown because other species have declined.
A large collaboration was started about five years ago by marine biologists Mike Heithaus and Demian Chapman of Florida International University called Global FinPrint. Through the project, the researchers aimed to survey all the world's reef shark species, such as tiger sharks and hammerheads.
As reef sharks are easier to spot than those that roam the high seas, the researchers focused on them. To conduct the study, the researchers deployed underwater cameras on 371 reefs across 58 countries. Six researchers coordinated surveys of over 120 scientists on coral reefs in various parts of the world.
The researchers lowered video cameras attached to 1.5-meter-long poles with shark bait at the far end at dozens of places on each reef. The cameras recorded over 15,000 hours of footage and captured 59 different shark species after three years, between July 2015 and June 2018.
Over 90 per cent of the species sighted were those that frequented the reefs or were residents.
"Overall, 59% of nations (34 out of 58) had abundance scores below 50% of their regional expectation, suggesting that loss of reef sharks is pervasive among reefs globally," the study said.
Highlighting measures for the conservation of shark reefs, the study said, "However, opportunities for the conservation of reef sharks remain: shark sanctuaries, closed areas, catch limits and an absence of gillnets and longlines were associated with a substantially higher relative abundance of reef sharks."
"These results reveal several policy pathways for the restoration and management of reef shark populations, from direct top-down management of fishing to indirect improvement of governance conditions. Reef shark populations will only have a high chance of recovery by engaging key socio-economic aspects of tropical fisheries," it added.
Dr Aaron MacNeil of Dalhousie University in Canada and lead author of the study told the media, "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."
According to the study, nearly no sharks were spotted in 19 per cent of the reefs.
"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," said Prof Colin Simpfendorfer, a co-author of the study from James Cook University.
Meanwhile, Dr Mike Heithaus, of Florida International University, and a leader of the Global FinPrint project said that they are also looking at how the loss of sharks can destabilise the reef ecosystems.
"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," Dr Heithaus said.Also Read: Nearly Three Billion Animals Affected By Australian Bushfires: WWF Study
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