A river of giants
The Mekong River near Luang Prabang, Laos, is the last confirmed spawning area for the world’s largest freshwater fish, the Mekong giant catfish (Pangasianodon gigas). This species, which can reach up to 3 meters (9.8 feet) in length and weigh 300 kilograms (660 pounds), undertakes a massive migration to reproduce here. Historically, their range spanned nearly the entire 4,800-kilometer length of the Mekong, from Vietnam to China. Today, the wild population has collapsed by more than 90% in recent decades, and this stretch of river in northern Laos is the final refuge for their spawning cycle.
Genetic analysis of the remaining catfish reveals a perilous situation. Studies of their mitochondrial DNA show that the fish throughout the Mekong basin likely belong to a single, panmictic population. This means there is little genetic differentiation between catfish found in Cambodia and those found upstream in Laos and Thailand. The entire species is effectively one interbreeding family. This lack of genetic diversity shows a severe population bottleneck, making the species extremely vulnerable to disease and environmental changes. The few hundred adults thought to remain are the last hope for the species' survival in the wild.