On Cambodia's Tonle Sap River, local men help conservationists gather data on a rare Mekong giant catfish caught and released on November 13, 2007. The incident was part of a wider project to monitor the endangered fish in the Mekong River Basin.

In 2003 the Mekong giant catfish was listed as critically endangered by the World Conservation Union after research showed the fish's numbers had fallen by at least 80 percent since 1990.

The fish shown in this exclusive photo was likely moving from the Tonle Sap Lake to the Mekong River via the Tonle Sap River—a classic migration pattern for many species of Mekong fish.

"For the Mekong giant catfish, northern Thailand is a spawning ground, whereas the Tonle Sap Lake in Cambodia is a rearing area," said U.S. biologist Zeb Hogan, who studied the fish pictured for his Megafishes Project, which is documenting the world's giant freshwater fish.

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