Zeb Hogan holds a taimen caught from a Mongolian river in this undated photo.

Hogan leads National Geographic Society's Megafishes Project, which documents the 20-some species of giant freshwater fish found around the world.

The largest recorded taimen, reportedly caught in the Kotuy River in Russia, reached a length of 83 inches (210 centimeters) and a weight of 231 pounds (105 kilograms).

When Hogan came to Mongolia to study taimen four years ago, he said he didn't fully understand the threats facing the fish.

"I had no idea how long it takes for them to grow large and how vulnerable these ecosystems are to overexploitation," he said.

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