They’re the largest fish in the sea, lumbering quietly in open ocean waters where little has been known about them.

The bus-sized whale sharks — docile creatures with a gaping mouth — have recently become endangered, as researchers have been enlisting tourists to help track and identify them.

A study published Wednesday in the journal BioScience catalogs human encounters with the whale shark in the last quarter-century. Scientists and tourists have recorded 30,000 encounters with 6,000 individual sharks in 54 countries around the world.

The research “has really advanced what we know about how whale sharks work and how they divide up the ocean,” said Alistair Dove, a co-author of the paper, and vice president of research and conservation at the Georgia Aquarium in Atlanta, which has four whale sharks.