In some places, whale sharks routinely swim between countries, even on a daily basis. But at Mafia Island—a New York City-sized atoll just off the coast of Tanzania—the world’s largest fish are such regulars that scientists started to name them.

“There aren't too many places where you repeatedly see the same whale sharks,” says Simon Pierce, principal scientist and researcher for the Marine Megafauna Foundation. “These days, it's almost like visiting old friends.”

Yet his old friends are dangerously close to extinction.

In July 2016, Pierce advised the International Union for the Conservation of Nature (IUCN) to reclassify these gentle giants as an endangered species on the group’s Red List. There is no good estimation of the number of whale