Reef sharks are threatened with extinction around the world by overfishing, according to a study published Thursday, which underlines previously underestimated risks for these sharks.
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Such a disappearance would have a strong impact on humans, say the authors of the study, because of the role that the species plays in the regulation of marine ecosystems. Reef sharks indeed maintain a delicate balance in a food chain on which millions of human beings depend.
The study, published in the journal Science, is the result of a global collaboration started in 2015 within the Global FinPrint project, which collected more than 22,000 hours of video images of reefs in Africa, the Middle East , Asia, Oceania and the Americas.
Using a computer model, a team of around 100 scientists from around the world estimated that the populations of five of the most common species of reef sharks – gray reef shark, Atlantic nurse shark, Caribbean reef shark, blacktip, coral shark – were 60-70% lower than they could be without outside human pressures.
Photo: AFP
Additionally, sharks were absent from 14% of the reefs where they had previously been observed.
The researcher leading the work, Colin Simpfendorfer, of James Cook University and the University of Tasmania, Australia, told AFP that before the study, little concern existed around the fate of the coral shark, unlike that of its larger cousins.
“But when we sat down and looked at the overall results, it was pretty amazing,” said the scientist.
Ricochet effect
The findings of the study are expected to help update the International Union for Conservation of Nature (IUCN) Red List over the next year or so. New species should thus be classified as “endangered”, an important step towards taking measures to safeguard them.
According to Colin Simpfendorfer, the main factor in this decline is overfishing, both that specifically targeting sharks for their fins and meat, but also that accidental.
In terms of impact, the loss of these sharks would cause a ricochet effect in the food chain. The prey they feed on would increase in number, but the next level would decrease and so on, triggering unpredictable disruptions that would endanger global food security.
Photo: AFP
Especially since reef sharks help control herbivore populations, says the researcher. And when herbivores increase in number, they ingest more algae, which is an important factor in the absorption of CO2 by photosynthesis.
“Carbon sequestration on coral reefs without sharks is much lower than on those with sharks,” says Colin Simpfendorfer. With an impact on global warming.
“Places of Hope”
Funding for the project comes from the Paul G. Allen Family Foundation, which notably initiated the Large Elephant Census, an aerial survey across Africa.
In the reef shark study, scientists used stationary underwater cameras, with a small amount of oily fish placed on an arm, to attract and observe the sharks.
In total, they surveyed 391 coral reefs in 67 countries and territories, using 22,756 cameras, and generating three years of video images.
The reefs with the healthiest shark populations were in high-income countries with stricter regulations.
But the research team has also discovered some “places of hope” in developing countries, such as Sipadan Island in Malaysia or Lighthouse Reef in Belize.
Around it, shark populations have clearly declined, “but in those areas where you have important marine protected areas and good ways to control them, you have strong shark populations,” Michael Heithaus told AFP. from Florida International University and co-author of the study.
Such data offers grounds for hope that areas where reef shark populations have been severely depleted can be repopulated as long as a number remain there and a careful management program is followed.