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This is the fourth story in a continuing series on the Megafishes Project. Join National Geographic News on the trail with project leader Zeb Hogan as he tracks down the world's largest freshwater fishes.

Hucho taimen may be known to the Chinese as the river god's daughter, but in this remote corner of northern Mongolia, the world's largest trout has long been king.

A ferocious and even cannibalistic predator, taimen can grow more than six feet (up to two meters) in length and can weigh up to 200 pounds (91 kilograms).

But like many other freshwater giants around the world, the taimen is now threatened with extinction.

Scientists last month wrapped up a four-year study of the fish in a 60-mile (100-kilometer) stretch of the Eg and Uur rivers.

Their results show that the taimen, also known as the giant Eurasian trout, is now too rare to support sustained commercial or recreational harvest.

"This fish is not like other trout and salmon species," said Zeb Hogan, a fisheries biologist with the University of Reno in Nevada.

Waist-deep in the clear, fast-moving waters of the Eg, Hogan prepared to release a newly tagged, 44-inch-long (112-centimeter-long) taimen back into the river.

The fish, which takes up to nine years to fully mature, can live for 50 years, said Hogan, who is a National Geographic Society Emerging Explorer. (National Geographic News is owned by the National Geographic Society.)

"We only see a few animals this size per mile in the river, so if you remove one of them, it's going to take a long time before it can be replaced," he said.