On the high seas, far from any continent, sharks and rays were once abundant. Shortfin makos, the fastest sharks on the planet, chased after their prey at speeds of over 20 miles an hour. Scalloped hammerheads plied the waters, scanning the ocean expanse for food with their wide-set eyes and other specialized sensory organs.

These animals traveled widely across open waters so vast and inaccessible that many fishermen, and even some biologists, found it hard to believe that overfishing would ever endanger them.

“A decade ago,” recalls Nicholas Dulvy, co-chair of the International Union for Conservation of Nature’s Shark Specialist Group, “we would have extremely heated debates about listing an oceanic shark as threatened.”

Now, a sweeping analysis of