In just the past half-century, humans have caused a staggering worldwide drop in the number of sharks and rays that swim the open oceans, scientists have found in the first global assessment of its kind, published in the journal Nature.

Oceanic sharks and rays have declined by 71% since 1970, mainly because of overfishing.

The collapse is probably even more stark, the authors point out, because of incomplete data from some of the worst-hit regions and because fishing fleets were expanding before they started their analysis.

"There is a very small window to save these iconic creatures," said Nathan Pacoureau, a marine biologist and the study's lead author. More than three-quarters of oceanic shark and ray species are now threatened with extinction, jeopardizing marine ecosystems and the food security of people in many nations.

Sharks and rays are taken for their meat, fins, gill plates and liver oil. They are also frequently caught incidentally. That is one reason sharks are especially vulnerable, scientists say. Even if commercial shark fishing stops being viable, incidental catches could continue to drive down numbers.

But high levels of incidental catch are not inevitable, said Sonja Fordham, an author of the study and the president of Shark Advocates International, a nonprofit.

The study calls on governments to adopt measures such as setting science-based fishing limits. Since these sharks and rays range across the open ocean, oblivious to national borders, reversing these declines will require international cooperation.