There is no river in Vietnam which has as many giant fish like the Mekong River. There are fresh-water fish species of up to 300 or even 500kg in weight in this river.
The largest species is the Mekong
giant catfish (Pangasianodon gigas). This is perhaps the most interesting and
most threatened species in the Mekong
River. For this reason
conservationists have chosen it as a sort of “flagship” species to promote
conservation on the Mekong. 
With recorded sizes of up to 3.2m and 300kg, the Mekong giant catfish currently holds the Guinness Book of World Records' position for the world’s largest freshwater fish. A fish of up to 500kg, 5m and 60-year-old was netted in the river.
Endemic to the lower half of the Mekong River, this catfish is in danger of extinction due to overfishing, as well as the decrease in water quality due to development and upstream damming. The current IUCN Red List for fishes classes the species as Critically Endangered; the number living in the wild is unknown, but catch data indicate the population has fallen by 80 percent in the last 14 years. It is also listed in Appendix I of CITES, banning international trade.
Grey to white in color and lacking stripes, the Mekong giant catfish is distinguished by the near-total lack of barbs and the absence of teeth.
Giant fresh-water stingray (Himantura chaophraya) is also a special species of the Mekong River. It is a species of stingray in the family Dasyatidae, native to large rivers and estuaries of Southeast Asia. It is one of the largest freshwater fishes in the world, with reports from the Chao Phraya and Mekong Rivers of individuals weighing 500–600kg.
Its numbers are dwindling due to overfishing and habitat loss, and it is in danger of extinction. The World Conservation Union has assessed the giant freshwater stingray as endangered over most of its range.
However, the fish species that used to be called “the King fish” of Mekong River is Mekong giant carp or Siamese giant carp (Catlocarpio siamensis), the largest species of cyprinid in the world. It has declined drastically due to habitat loss and overfishing, and it is now considered Critically Endangered.
The giant barb ranks among the largest freshwater fish in the world, and is probably the largest fish in the family Cyprinidae. It may reach 3m and weigh up to 300kg. Few large individuals are caught today.
Today few barbs live to maturity. The main threats are from habitat loss (e.g., pollution and dams) and overfishing. The sharp population decline is well illustrated by catch data from Cambodia, where 200 tons of giant barbs were caught in 1964. By 1980, only about 50 fish were caught and by 2000, only 10. The giant barb is listed as Critically Endangered on the IUCN Red List.
(Vietnamnet)
