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September 2013    Download the Entire Issue (PDF) Available to the Public Vol. 28, No. 9   RSS Feed for Undercurrent Issues
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Where Do Whale Sharks Spend the Winter?

from the September, 2013 issue of Undercurrent   Subscribe Now

The whale sharks off the Yucatan Peninsula apparently travel up to 5,000 miles annually, according to a new study of their migration. Whale sharks are known to gather at a dozen major feeding locations around the world, from Western Australia and Indonesia to Belize. But between May and September, the waters of Mexico's Quintana Roo state, on the northeastern Yucatan Peninsula, draw far more animals than other spots --an estimated 800 or more in a given season.

"From this one feeding area, these animals spread throughout the Gulf of Mexico, into the Caribbean, through the Straits of Florida up into the Atlantic Ocean," said study co-author Robert Hueter, director of the Center for Shark Research at Mote Marine Laboratory. "We found animals coming back for as many as six years at a time. Clearly they are returning to fuel up on the food that's there to carry them through much of the rest of the year."

In 2003, Heuter to begin accumulating tagging and satellite tracking data that formed the backbone of the recent study by the Mote Marine Lab and Mexico's National Commission of Natural Protected Areas. Mike Maslanka, of Smithsonian's National Zoo in Washington, DC., said, "These tagging efforts allow us to discover more about what happens when they aren't gathering to feed in the summer . . . These things are so big, to think that they 'disappear' is pretty amazing. It's the largest fish in the ocean, and we don't know where it goes for six months of the year."

Among more than 800 individuals studied, one animal stood out. A mature and presumably pregnant female called Rio Lady was tagged and then tracked along an odyssey of some 4,800 miles. Hueter said. "She swam out between Brazil and Africa until she passed the equator, and that's where her tag came off." But her journey, and other whale shark sightings in the remote region, could help answer a question that has plagued whale shark researchers for years: Where are all the females? The Quintana Roo gathering is more than 70 percent male, and other global aggregations show the same gender imbalance. Such imbalances don't happen in nature.

"The females have to be somewhere, and we hypothesize that mature, pregnant females undergo long migrations to the middle of the ocean, near seamounts or remote islands, and that's where they give birth," Hueter explained. "In coastal zones where the feeding aggregations are, their young -- which are less than two feet long at birth -- might be subject to higher predation."

Conservation of the far-roaming animals will take international cooperation because whales spotted in one area may depend, in other seasons, on resources located many hundreds of miles away. And while mating remains a mystery, whale shark genetics suggest that animals swap genes among far-flung geographic locales, and that only two large metapopulations exist -- one in the Atlantic and another in the Indo-Pacific.

Each population requires management on a broad scale. The species as a whole is currently listed as vulnerable, and is still hunted for oil in some Asian waters. Hueter said he's encouraged that whale sharks can be protected by the process that's already begun, notably in his study area, with the Mexican government's designation of a Whale Shark Biosphere Reserve in the feeding aggregation grounds. But there's more work to be done, he cautioned. "This is the largest fish that has ever lived, and it's charismatic. It might be the largest animal on the planet that you can be close to in its natural environment and not be in any danger whatsoever."

-- Brian Handwerk, National Geographic

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