In just half a century, humans have caused a staggering decline in the number of sharks and rays swimming in the open oceans worldwide. This is what scientists found in the first global assessment of its kind, published Wednesday in the journal Nature.

Ocean sharks and rays have declined 71 percent since 1970, mainly due to overfishing. The collapse is likely to get worse, the authors say, because of incomplete data from some of the hardest hit regions and the fact that fishing fleets were expanding in the decades before their analysis began.

“There’s a very small window to saving these iconic creatures,” said Nathan Pacoureau, marine biologist at Simon Fraser University in Canada and lead author on the study. More than three quarters of oceanic shark and ray species are threatened with extinction, which threatens marine ecosystems and the food security of people in many countries.

The research provides the latest data point on a grim path for Earth’s biodiversity. From butterflies to elephants, wildlife populations have plummeted in the past few decades, and up to a million species of animals and plants are threatened with extinction.

However, scientists stress that conservation works when done correctly, and the study urges governments to take action, such as setting scientifically based limits on the number of shark and stingray fishermen that fishermen are allowed to catch and keep.

“Action is required immediately,” the authors wrote.

Sharks and rays are taken for their meat, fins, gill plates, and liver oil. They are also commonly caught by fishermen who use nets or long lines with thousands of bait hooks to attract tuna or swordfish. Such an accidental catch isn’t the main target, but it’s often welcome when it happens.

This is one of the reasons sharks are particularly at risk, scientists say. Even if commercial shark fishing is no longer profitable due to falling numbers, accidental catches could keep the numbers down.

However, high levels of occasional catch are not inevitable, said Sonja Fordham, author of the study and president of Shark Advocates International, a nonprofit group devoted to protecting sharks.

“We now have a lot of scientific studies on how to avoid catching sharks and certainly a lot on best practices to safely release the shark and make sure it survives,” said Ms. Fordham. For example, how long a shark fights on a line is important, so fishermen should regularly monitor their lines. You should avoid shark hotspots and use shark-friendly equipment that will allow the creatures to break free while keeping tuna and swordfish on a leash.

Many fishermen don’t take these measures because they often have financial incentives to keep the sharks, she said. Governments often allow fishermen to keep them even when populations decline. For example, while short-fin mako sharks are classified as globally endangered, the United States, the European Union, and many other governments continue to allow the species to be caught.

For the study, scientists scoured the world for all available data on each species and combined numbers from fisheries and scientific surveys with information on reproduction rates, which tend to be slow. Scientists already knew sharks were in trouble, but there was no comparable global analysis. At a workshop in 2018, when the authors gathered to review the data of each type, they saw one catastrophic decline after another pop up on one screen. A grim silence filled the room, recalled Dr. Pacoureau. He himself was shocked at the magnitude of the decline and hopes that their work will help save sharks.

“The advancement here is the very elegant statistical analysis that sums it all up and puts a very solid, very well-reasoned number on it,” said Demian Chapman, a marine biologist and professor at Florida International University who studies sharks and was not involved in the Study. “It really helps to communicate the scope of the problem to policy makers. It’s a number that they can very easily grasp and see how bad it is. “

With these sharks and rays stretching across the open ocean and ignorant of national borders, reversing these declines will require international cooperation. A global movement to preserve 30 percent of the planet’s land and oceans by 2030 is gaining momentum, but Ms. Fordham said such commitments to help sharks, conservationists and scientists better attend fishermen’s meetings.

“We have this problematic separation between fisheries and environmental authorities, I would say in almost every country in the world,” said Ms. Fordham. “They make promises in one arena that are not fulfilled in another.”