At the southern most tip of Laos, a couple kilometres away from the Cambodian border, lies the region of Si Phan Don, literally in Laotian, « 4000 islands ». Here, the Mekong slows its flow and spreads out onto the South Laotian plains, depositing tons of sand during each rainy season, thus creating a labyrinth of islands and channels. The golden river then enters a succession of canyons to eventually feed the great fertile plains of Cambodia. There are beautiful landscapes, and water as far as the eye can see. At Si Phan Don, the Mekong, depending on the season, can become 14 km in width. The source of this Asian giant is

located in the heights of the Himalayas, whose muddy waters are the habitat of a great number of species.

Scientists agree to consider this the second largest river in terms of biodiversity, just behind the Amazon. An estimated 800 species live in this hydrosystem. By way of comparison, there are 60 species in the basin of the Rhone.

       The most emblamatic species of this ecosystem is the giant cat fish of the Mekong (Pangasianodon gigas). It is endemic to the Mekong, and its story summarises perfectly the dangers that they both face. The Pa Beuk, as the Laotians call it, has been venerated since the dawn of time ; it was caught « …once a year, the 12th day of the 3rd buddhist month, when the moon had disappeared behind the mountain… » this annual fish catch lasted 3 days and 3 nights. Presents were offered, and the river spirits were summoned to guarantee the success of the catch, and thereby making them bring out those giants from their water-caves, some reaching 3 metres in length and weighing 350 kg. This great catch was celebrated annually up until the fifties, but the demand for the fish sky-rocketed along with the price that customers were willing to pay, especially in neighbouring Thailand. Fishing became daily and intensive. Adding to this, the destruction of habitat, pollution, migratory routes altered because of human activity, all of these factors have helped put Pangasianodon gigas on the ugly red list of threatened species.

       There are other lesser known giants of the Mekong. Himantura chaophraya, is worth mentioning, a freshwater ray with a fin-span of 2.4 metres, and can weigh more than 500 kg. There is also the giant carp of Siam, Catlocarpio siamenis, 3 metres for 300 kg, which places them among the largest freshwater fish known. There are also many species kept in aquariums. The family most commonly seen is that of the cyprinidae : Barbus, Rasboras, Danio, Labeo, Balantiocheilos, Epalzeorhynchos, Garra, Puntius…that is to say, more than 280 species in total. The Cobitids (Acanthopsis, Cobitis, Pangio, Botia), the Balitorids (Schistura, Sewellia, Nemacheilus) and the Gobiids (Brachygobius, Rhinogogius, Awaous) are found in great numbers as well. One can also find Betta, Trichogaster, Mastacembelus, Toxotes, Tetraodon, Chanda, Notopterus, Datnoides, Channa, Kryptopterus, Badis, Cynoglossus… it is a long list, which leads us to believe that this river has not yet revealed all its secrets.

       What is most interesting about the region of Si Phan Don, is that it offers an array of water environments : the current, the depth, the relief or the type of substrate varies greatly from one point to the other, affecting thus flora and fauna. In calm water, lotus display their flowers at the surface, whereas vallisnerias thrive where the current is at its most powerful. In shallow water, among intertwined roots and dead-wood, bunches of Ceratophyllum and Potamogeton grow, and in the sandy zones cryptocorynes flower at the surface. Along the banks, riparian forest drink up the sun, and its roots get lost in the river water.

       Underwater visibility is quite average, but during the dry season, one manages to see some species. At first sight, barbels of all types dart about in little groups, one can glimpse their little silver silhouettes in the dark depths of the river. Balitora, Shistura, and other gobies keep watch, on roots or on rocks, ready to hide in a crevice at the first warning.

In a branch where the current is stronger, many Mastacembelus, slip between rocks and vallisnerias, a Tetraodon abei tries to camouflage itself by staying still in the middle of the substrate. In the shallow sandy zones, Puntius and Rasboras feed in little groups. Close to the banks, among roots and rocks, archer fish (Toxotes) watch the surface waiting for an insect to be within firing range.

It would take hours of research and identification to know and to understand such a vast territory. On returning to the bank, by talking with local fishermen, more is learnt on the region and on the fauna. A couple kilometers downstream, after the Khone falls, the last populations of Irrawaddy Dolphins live (Orcaella brevirostris). These freshwater dolphins are in danger of extinction, their population is estimated at a couple hundred in the whole of East Asia (Myanmar, Laos/Cambodia, Indonesia). The population of the Mekong is one of the most threatened, and the WWF has recently called for the implementation of a conservation programme. Let us hope that they will be luckier than their cousins of the Yangtze River (Lipotes vexillifer) who died out amongst general indifference in 2007.