May 8, 2009 -- Whale sharks live throughout the world's tropics -- from the Indian Ocean to the Pacific to the Caribbean -- but these giant fish are surprisingly similar from population to population, according to a new study.
VIDEO: Learn some interesting facts about the gentle giants of the sea.
The finding suggests that even far-flung populations of whale sharks intermix and breed -- an important insight for conservation efforts. The work also adds to our basic understanding of a magnificent and elusive giant that can grow up to 60 feet long and weigh more than 40,000 pounds. Its fin alone can be 5 feet tall.
"To see one in the ocean is amazing," said Jennifer Schmidt, a developmental biologist at the University of Illinois at Chicago. "Their sheer size is overwhelming. The first time I ever got into the water near a whale shark, you're next to something that makes you feel tiny. It's hard to describe."
Whale sharks are the biggest fish in the sea, but scientists know remarkably little about them. That's partly because the sharks swim deep underwater, where they're hard to find. They may migrate long distances. And they appear to spend most of their time alone.
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Until the mid-1980s, there were only 320 confirmed sightings of whale sharks, said Brad Norman, a marine biologist and whale shark expert in Perth, Australia. More recently, scientists have identified about a dozen hot spots around the world where large groups of whale sharks occasionally come together -- probably to coincide with plankton blooms, coral spawns, and other similar events. Today, scientists estimate the global population of whale sharks at between 100,000 and 200,000 individuals.
In an effort to learn more about the animals and where they go, some researchers have been tagging them with satellite trackers. The information the tags collect is useful, but the technology is expensive and inefficient. Most tags fall off. Batteries die in many others.
Schmidt and colleagues decided instead to take the genetic route. They collected DNA from 68 whale sharks at 11 sites around the world, including South Africa, Costa Rica, Australia and the Maldives. Comparing eight main regions of DNA among all of the sharks, the researchers found that genetically, animals in distant populations were very similar.
"That means these animals at a fairly good frequency move and interbreed between oceans," Schmidt said. "If they swam around the world and came to their home ocean to breed, the genetics would not normalize."
Whale sharks are currently listed as threatened by the International Union for Conservation of Nature. Some countries protect them, but in other places, they are actively and legally fished for their meat and fins.
The new findings suggest that more widespread protection might be necessary to save the fish from extinction since whale sharks in protected waters cannot be assumed to stay in those waters .
"I think the genetic study is a tremendous first breakthrough into our understanding of the movements and population associations between whale sharks in different parts of the world," Norman said. "All types of research add to our understanding of this cryptic species."
Still, Norman said, the work is in its early stages, and more needs to be done. His studies using photographs to identify and track hundreds of whale sharks based on their unique patterns of spots suggest that most whale sharks actually stay close to home most of the time. Many are spotted in the same places year after year.
As scientists work to learn more about whale sharks, it's worth making efforts to protect them now, Norman said. In a new study in Endangered Species Research, he and colleagues reported that a population of whale sharks that lives in Ningaloo Marine Park in Western Australia is maintaining its numbers and even increasing slightly, thanks to careful monitoring and management.
"Hopefully," he said, "We can take that message to other parts of the world."
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