The Mekong is a trans-boundary river in Southeast Asia. It is the world's 12th-longest river and the 7th-longest in Asia.

It's unclear why so many species of giant fish occur in the Mekong River, the 2,700-mile (4,350-kilometer) river that runs from southern China to the delta south of Ho Chi Minh City, Vietnam. Certainly part of the answer is the river's size: Large rivers have more space and more food to accommodate larger fish.

Another part of the answer may lie in the productivity of the Mekong River Basin ecosystem, including the floodplains and flooded forests that provide an abundant source of food for many species of fish during the rainy season.