Over the past few decades, National Geographic Explorer and biologist Zeb Hogan has travelled far and wide in search of theworld’s biggest fish. The search has focused on rivers that are both understudied and at high risk of ecological damage from dams, overfishing, mines, and other manmade threats.

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“The race to study and save these megafish has never been more urgent,” says Hogan. More than 70 percent of the world’s freshwater megafish are at risk of extinction, he notes. TheChinese paddlefish may already be gone.

Hogan points to a study from Science which found that plans for recent dam projects around the world underestimated the damage to fish habitats and overstated the environmental benefits of the power.

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“It’s the most significant paper that’s been published in my field in the last several years,” says Hogan, who is also a professor at the University of Nevada, Reno and host of the Nat Geo WILD series MonsterFish, which returns for its sixth season Friday evening.

“Large dams often have catastrophic impacts on river life,” he says.

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As an example of what’s at stake, the massive Mekong giant catfish in Southeast Asia—which can grow to be as big as a grizzly bear—is threatened by a series of new dams, Hogan says.

“If all their spawning grounds are above the dams, they may be driven to extinction,” says Hogan, who has studied the fish for 20 years. “We just don’t know.”

“The story of the giant fish underlines the environmental crisis that many rivers and lakes around the world face today, with their biodiversity declining at a rate greater than the rate seen in the oceans or on land,” says Hogan.