They are 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.
Researchers used a photo-identification library called Wildbook for Whale Sharks to catalog 20 hot spots around the world, including just off the coasts of Ningaloo in Western Australia; Cancun, Mexico; Mozambique; the Philippines; and the Maldives, a group of atolls south of India.
The study is a collaboration of three dozen of the world's top whale shark experts and thousands of citizen-scientists — mostly snorkeling ecotourists who snap photographs and video as they swim alongside the behemoths.
Giving tourists the opportunity to participate in science is fun and empowering, and deepens their commitment to conservation, said Darcy Bradley, a postdoctoral researcher at University of California, Santa Barbara.
"The fact that we can all be citizen scientists is a really important and special thing," said Bradley, who was not involved in the paper, but gets regular emailed updates when whale sharks she has submitted to the database get spotted again. "We need more stewards for the environment."
The animals that have been spotted are mostly juvenile males, measuring about 12-21 feet, although a site in the Galápagos recorded mostly females. It is not clear where the adults live, though it is presumed that they are in the open ocean, said Dr. Brad Norman, the paper's first author.
Whale sharks are covered with spots and can be individually identified by the pattern of spots on their sides, just behind the gills, above the pectoral fin, said Norman, also founder and principal research scientist with Ecocean, a nonprofit based in Western Australia.
A scientist or tourist can take a picture or video of a whale shark — ideally from its left side — then upload the image to www.whaleshark.org. There it is analyzed and compared to the 30,000 other images and either matched or confirmed as a new individual.
"The ability to photo-identify species — and indeed the whale shark was the first shark we could do this for — has transformed our ability to understand how many individuals are out there, who is moving where, how far do they go, and how many are staying around," said Nicholas Dulvy, a professor of marine biodiversity and conservation at Simon Fraser University in British Columbia, Canada, who was not involved in the research.
Dulvy is also co-chairman of the shark specialist group at the International Union for the Conservation of Nature, which develops environmental management policies and best practices.
Whale sharks were declared endangered last year after their numbers apparently dwindled. They are often illegally fished or caught as by-catch along with tuna, Dove said. Many are also struck by ships. Unlike whales, their bodies sink, so it is hard to know how many are hit, he said, describing the animals as gentle, polka-dotted and slow-moving.
"This is a shark that lives in first gear," he said.
The juvenile males do not appear to migrate much, Norman said, with many seen repeatedly in the same region, and not much mixing across the hot spots.
The lack of sightings of fully mature adults means scientists do not have a good idea of what the animals are doing, where they are living or their mating habits. Whale sharks are unlikely to have specific grounds where they give birth, because of long gestation periods, and the fact that no more than one or two pups have been found in the same place and time, the study said.
Even their numbers are a mystery. Worldwide, estimates of the population range from about 20,000 to about 200,000 whale sharks, Dulvy said.
In captivity, where scientists see the animals over time, it becomes clear that they have distinctive personalities, Dove said. "We get to know the bossy one, the shy guy."
Though less familiar, some of the wild animals have been seen enough to earn nicknames, he said, citing two in Mexico: Rio Lady, and Keyhole, who has a distinctive hole on one fin. "They begin to have stories and narratives and lives that you can follow along with," he said. "That's one of the things that adds a layer of fun for the citizen-scientist."
Norman, who is in the Maldives studying whale sharks, said there is nothing like the joy of being near the animals — or watching first timers do it. "You're swimming with the biggest fish in the sea. Sometimes you're in water hundreds of meters deep," he said.
As long as a person stays 10-15 feet away from the animal's path and does not try to touch it, swimming alongside the animals should be safe.
"They're so graceful, they're so beautiful," he said. "All fear or trepidation goes and you're just in awe."