Posted: August 21, 2013
A nine-year study tracked more than 800 of the massive and largely mysterious whale sharks. For the first time, researchers have tracked the sharks' far-flung migration and where they may go to give birth.
A whale shark dives near the surface in waters off the coast of Mexico. Marj Awai
Robert Hueter tags a whale shark off the coast of Mexico. The tags measure location, depth and water temperature, then relay the data back to a lab. John Tyminski
Of all the creatures in the sea, one of the most majestic and mysterious is the whale shark. It's the biggest shark there is, 30 feet or more in length and weighing in at around 10 tons.
Among the mysteries is where this mighty fish migrates and where it gives birth. Now scientists have completed the biggest study ever of whale sharks, and they think they have some answers to those questions.
The study was conceived by Robert Hueter, a marine biologist at Mote Marine Laboratory in Sarasota, Fla. Hueter swims with sharks, and has done so for 40 years, but he has a special fondness for whale sharks. "This is the largest fish as far as we know that's ever existed — there's nothing bigger in the fossil record," he explains. "But it's a very unusual kind of shark in that it's not a top predator; it feeds on plankton."
Plankton are the flotsam of tiny plants and shrimplike animals that float in the ocean. The whale shark just opens its mouth — about twice the size of a manhole cover — and sucks them in. It filters out the plankton much the way a baleen whale filter-feeds.
Hueter was studying some regular meat-eating sharks off the Yucatan Peninsula of Mexico when he came across whale sharks feeding. It turned out to be the world's largest concentration of whale sharks, over 400 of them. Hueter was smitten.
"It's polka-dotted," he says, "and it lets people swim with it. When you go down and see them at these feed aggregation sites, it's a spectacle of nature. It's unlike anything else that you'll ever experience."
Hueter organized a project to tag whale sharks. The tags record where a shark goes, how deep it dives, as well as the water temperature, and relay the data back to his lab via satellite.
Hueter attached many of the tags himself — by hand. "Your heart starts racing every single time because you're just next to something that's just so huge," he says, something that could easily toss you off like a mosquito.
After tagging more than 800 whale sharks over nine years, the team discovered that after feeding, the sharks head off in seemingly random directions. Some travel thousands of miles, and they can dive a mile deep.
One female in particular — they called her Rio Lady — swam to the middle of the Atlantic, between Brazil and Africa, and just hung out. So what was she doing there? Hueter suspects that she was giving birth — to pups, in the shark vernacular — out in the open ocean, which he surmises might be a safer place for young pups than close in to shore. "We think that Rio Lady has led us to the place where this particular species gives birth."
Demian Chapman studies sharks at Stony Brook University in New York. "For most sharks, that's oftentimes the million-dollar question ... where they give birth," he says. "Nursery areas are places usually close to the coast, so giving birth in the middle of the ocean is a fairly unusual thing for sharks to do."
Hueter describes the research in the journal PLOS One. He says the whale sharks' far-flung travels demonstrate that protecting this rare species will require international collaboration, not unlike the protection given migrating bluefin tuna. Already, the research has led Mexico's government to protect the sharks' big feeding grounds there.
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