Fishers at Khone Falls had long known that the giant catfish traveled through their area.
But in 2007 Thai photographer Suthep Kritsanavarin for the first time photographed a giant catfish caught at Khone Falls.
(See his photos of Khone Falls and the giant fish.)
Giant catfish were once plentiful throughout the Mekong River basin, but in the last century the population has declined 95 to 99 percent, according to Hogan.
The massive fish is not targeted by fishers, but it is sometimes caught as bycatch. In Cambodia, where the largest population of giant catfish is found, eight of the giant fish were caught last year.
Although fishing is the biggest immediate threat to the giant catfish in the Mekong, dams and habitat fragmentation could disrupt the animal's ability to reproduce, Hogan said.
"There is only one known spawning ground for Mekong giant catfish, and that spawning ground is in northern Thailand," he said.
"Until we know better, we have to assume that fish from Cambodia may migrate to Thailand to spawn."
The construction of the Don Sahong dam, which is slated for completion in 2010, would make that migration impossible, Hogan said.
Advocates of the dams point to the enormous hydropower potential that the Mekong offers to an energy-hungry region. With oil prices at record levels, the projects have taken on added urgency, they say.
So far the Mekong runs almost uninterrupted; only China has dammed the river at two locations in Yunnan Province.
One of the dams that China is now building will be second in size only to the Yangtze River's Three Gorges Dam.
"China, as an upstream country, will never do anything that will harm the interests of downstream countries," He Yafei, China's assistant foreign minister, told reporters before last week's regional summit.
But officials in Vietnam, where the Mekong empties into the South China Sea, say water extraction for farm irrigation upstream has already caused ocean salt water to move inland, destroying rice fields.
Conservationists likewise warn that the new dams could harm fish stocks that millions of people throughout the Mekong depend on.
"Fisheries resources are essential to the population of the Mekong Basin," said Eric Baran of the nonprofit World Fish Center in Phnom Penh, Cambodia.
"In Cambodia, for instance, the fish catch contributes 65 to 75 percent of the animal protein of households."
More than a thousand fish species live in the Mekong River system, a biodiversity second only to the Amazon in South America, Baran said.
In a report last year, the World Fish Center warned that the economic benefits of the U.S. $300-million Don Sahong dam may be outweighed by losses from local fisheries, whose production has been estimated at over U.S. $2 billion a year.
At Khone Falls the Mekong River drops up to 100 feet (30 meters) from the Khorat plateau to the Mekong plain, forming a complex network of narrow channels, called hoo in Lao.
Scientists have found that the area supports at least 201 fish species, including several endemic or endangered species.
The area is also home to one of the few remaining concentrations of freshwater dolphins in the Mekong.
(Read related story: "River Dolphin Closer to Extinction Despite Reports, Experts Say" [March 21, 2007].)
The dam will block Hoo Sahong, the deepest channel and the only one that migratory fish can pass through at the peak of the dry season, in April and May, when the Mekong is at its lowest.
Nongovernmental organizations in Cambodia called on their government this month to ask Laos for a construction moratorium on the Don Sahong dam to allow for more environmental assessments.
Hogan admits that dams provide a number of benefits, including flood control, water for irrigation, and electricity.
"But it is also important to consider the costs," he said, "especially in an area where a large part of the population is dependent on fish for food."
"From a migratory fish's perspective, there is nothing worse than a dam."
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