In the past half-century, humans have caused a staggering global drop in the number of sharks and rays swimming in open oceans, scientists found in the first such global assessment, published in the journal Nature on Wednesday. .
Sharks and ocean rays have declined 71 percent since 1970, mainly due to overfishing. The collapse is likely even more abrupt, the authors point out, due to incomplete data from some of the worst-affected regions and because fishing fleets were already expanding in the decades before their analysis began.
“There is a very small window to saving these iconic creatures,” said Nathan Pacoureau, marine biologist at Simon Fraser University in Canada and lead author of the study. More than three quarters of shark and ocean ray species are now threatened with extinction, endangering marine ecosystems and the food security of populations in many countries.
The research offers the latest data point on what is a dismal trajectory for terrestrial biodiversity. From butterflies to elephants, populations of wildlife have collapsed in recent decades and as many as one million species of animals and plants are threatened with extinction.
But scientists stress that conservation works when done properly, and the study calls on governments to adopt measures such as setting scientific limits on the number of sharks and rays that fishermen can catch and keep.
“Action is needed immediately,” the authors wrote.
Sharks and rays are mistaken for their meat, fins, gill plates, and liver oil. They are also frequently caught accidentally by fishermen using nets or longlines with thousands of baited hooks to attract tuna or swordfish. Such accidental capture is not the primary focus, but it is often welcome when it does occur.
This is one of the reasons sharks are particularly vulnerable, scientists say. Even if the commercial shark fishery ceases to be viable due to declining numbers, bycatch could continue to depress numbers.
But high levels of bycatch aren’t inevitable, said Sonja Fordham, study author and chair of Shark Advocates International, a nonprofit group dedicated to shark conservation.
“We now have volumes of scientific studies on how you might avoid catching sharks to begin with, and certainly a lot on best practices for releasing sharks safely and ensuring they survive,” he said. Mrs. Fordham said. For example, it is important to know how long a shark wrestles on the line, so anglers should monitor their lines regularly. They must avoid shark hotspots and use shark-friendly gear that allows the creatures to break free while keeping the tuna and swordfish on the line.
Many fishermen do not take these measures because they often have financial incentives to keep sharks, she said. Governments often allow fishermen to keep them, even as populations collapse. For example, while the shortfin mako shark is listed as an endangered species globally, the United States, the European Union, and many other governments still allow fishing for the species.
For the study, scientists scoured the world looking for all available data on each species, combining figures from fisheries and scientific surveys with information on reproductive rates, which tend to be slow. Scientists already knew the sharks were in trouble, but there was no comparable overall analysis. At a workshop in 2018, as the authors gathered to examine the data for each species, they saw one catastrophic decline after another appear on one screen. A dark silence fills the room, remembers Dr Pacoureau. He himself has felt shocked by the extent of the declines and hopes their work will help save the sharks.
“The advancement here is the very elegant statistical analysis that brings it all together and puts a very firm and very well-justified figure,” said Demian Chapman, a marine biologist and professor at Florida International University who studies sharks and does not been involved. in the study. “It really helps communicate the scale of the problem to decision makers. This is a number that they can pick up very easily and realize how bad it is.
As these sharks and rays cross the open ocean, oblivious to national borders, reversing these declines will require international cooperation. A global movement to conserve 30% of the world’s land and oceans by 2030 is gaining momentum, but Ms Fordham said for such commitments to help sharks, conservationists and scientists should participate better in fisheries meetings.
“We have this problematic disconnect between fisheries and environment agencies, I would say in almost every country in the world,” Ms. Fordham said. “They make promises in one arena that aren’t kept in another.”