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Biologist Zeb Hogan is getting a close-up look at one of the world's largest freshwater fishes: the Chinese sturgeon. \
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The sturgeons can grow up to 16 feet or 5 meters long and weigh almost 1,000 pounds.\
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HOGAN: It's a big fish ... strong ... tail free ... can really whip around.\
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The Chinese sturgeon may be powerful, but this giant species is also in big trouble.\
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Its home, the Yangtze River in China, Asia's longest river, is increasingly threatened by heavy boat traffic and pollution.\
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Hogan is leading the National Geographic Society's Megafishes Project, a three-year study to document the status of the 20 or so species of giant fish found in rivers and lakes around the world. \
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More than a fifth of the world's known freshwater fish species have become extinct or gravely threatened in recent years.\
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Megafishes are often the first ones to suffer from overfishing or habitat loss.\
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Today there may be less than 1,000 adult sturgeons left in the Yangtze River.\
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The construction of the Ghezouba Dam in the early 1980s blocked the migration route to the spawning grounds of the sturgeon.\
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The new Three Gorges Dam, the largest hydroelectric dam in the world, will present another threat to these giant fish.\
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HOGAN (at Dam): From the fish's perspective, nothing worse than a dam ... no spawning ground left for the sturgeon.\
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Each year at least 10 sturgeons are also killed after being hit by boat propellers.\
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Researchers like Wei Qiwei of the Yangtze River Fisheries Institute are locked in a race against time to save the giant fish.\
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Here at a hatchery outside Jingzhou, Wei and his colleagues are running a breeding program to enhance the sturgeon stock.\
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WEI AND HOGAN: 1/2 million larvae producing fingerlings.\
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Researchers may yet save the Chinese sturgeon, but for an even larger Yangtze giant, the Chinese paddlefish, it may be too late. \
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In late 2002, Dr. Wei and his team attempted to rehabilitate one of the last paddlefishes captured in the wild.\
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It had been injured by a boat, and died in captivity less than 30 days after its capture.\
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The paddlefish can reportedly grow up to 23 feet or 7 meters long and weigh more than half a ton.\
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But no paddlefish has been seen since 2003.\
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Wei believes there may still be a dozen paddlefish in the Yangtze, perhaps hiding in caves and holes in the upper parts of the river.\
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WEI: No fish (yet), but we use a fish finder to find what could be a Chinese paddlefish. We found some signals.\
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His 60-foot research vessel is equipped with a tank and a crane ready to rescue any giant fish that he might find.\
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If the search fails, this may be the closest we'll ever get to see the Chinese paddlefish again.\
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