for National Geographic News
On Cambodia's Tonle Sap River, conservationist Zeb Hogan is hoping to save the giant catfish, the largest freshwater fish in the world, one fish at a time.
If fishers here catch the critically endangered species, which can grow up to 10 feet (3 meters) long and weigh 650 pounds (295 kilograms), they now must turn it over to Hogan, who heads the Mekong Fish Conservation Project.
Hogan's team then tags and releases the fish back in the river.
In the past, Hogan has paid market price for any endangered fish, including giant catfish, caught by the fishers who operate nets on the Tonle Sap.
But the Cambodian Department of Fisheries recently declared the row of nets, the largest commercial fishing operation on the river, a special research and conservation area.
"What that means is that the fishermen are obligated to provide endangered species that they catch to us free of charge for tagging and release," said Hogan, a biologist affiliated with the University of Wisconsin-Madison.
The giant catfish was once found throughout the Mekong River system, which runs through China, Laos, Thailand, Cambodia, and Vietnam.
But in the last century, the population has declined 95 to 99 percent. Hogan says there may only be a few hundred adult giant catfish left in the system today.
"These fish are on the verge of extinction, and that's a warning to us that there's something wrong with the river system," he said. "The largest migratory fish are usually the first to disappear, but they won't be the last."
There are 14 rows of nets along the Tonle Sap River, one every kilometer (0.6 mile).
The southernmost row, where Hogan is based, has four cone-shaped nets about 410 feet (125 meters) long. It is one of the most productive nets of its kind in Cambodia, and it catches most of the giant fish species.
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