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.
The Mekong River is also—depending on whom you ask—either the second or third most biodiverse river on Earth (in terms of freshwater fish) and it’s logical that a river with so many species of fish would also support several species of giants.
Not only is the diversity of large fishes found in the Mekong amazing, so is these fishes’ persistence, given the number of people who live on the river and the level of fisheries’ exploitation. It just goes to show that fish populations can be remarkably resilient: It’s not typically overfishing that drives species to extinction. Usually, it’s habitat degradation or invasive species.