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Biologist Zeb Hogan holds a taimen, a giant member of the salmon family, while searching for "megafishes" in Mongolia.

Hogan leads the newly launched Megafishes Project, the first major effort to document, study, and protect the world's largest freshwater fish.

The project aims to better conservation of megafishes—species that grow to at least six feet (two meters) and weigh more than 200 pounds (90 kilograms). They include the mammoth taimen, catfish the size of bears, and half-ton river stingrays.

Hogan, a National Geographic Emerging Explorer. calls the giant fish "the real-life Loch Ness monsters and Bigfoots of the aquatic world." (The Megafishes Project is funded by the National Geographic Society, which operates National Geographic News.)

Many megafish species are endangered due to a variety of threats, including overfishing, pollution, habitat destruction, and dams, which interrupt their migration routes, Hogan said.

The largest migratory fish are usually the first to disappear from an ecosystem, Hogan added.

"Freshwater biodiversity conservation—including protection of animals like trout, catfish, sturgeon—is every bit as important as protection of animals like tigers and whales—perhaps more so," Hogan said.

Hogan's survey will cover 14 lake and river systems on six continents over the next three years.

Photograph by Brant Allen