Which river is home to the most species of very large fish?

The Mekong basin is one of the richest areas of biodiversity in the world. Only the Amazon boasts a higher level of biodiversity. Biota estimates for the Greater Mekong Subregion (GMS) include 20,000 plant species, 430 mammals, 1,200 birds, 800 reptiles and amphibians, and an estimated 850 freshwater fish species.

New species are regularly described from the Mekong. In 2009, 145 new species were described from the region, including 29 fish species previously unknown to science, two new bird species, 10 reptiles, five mammals, 96 plants, and six new amphibians. Between 1997 and 2015, an average of two new species per week were discovered in the region.

No other river is home to so many species of very large fish. The biggest include three species of Probarbus barbs, which can grow up to 1.5 m (4.9 ft) and weigh 70 kg (150 lb), the giant freshwater stingray which can reach at least 5 m (16 ft) in length and 1.9 m (6.2 ft) in width, the giant pangasius, giant barb and the endemic Mekong giant catfish. The last three can grow up to about 3 m (9.8 ft) in length and weigh 300 kg (660 lb). All of these have declined drastically because of dams, flood control, and overfishing.