The fishes of the Nam Theun and Xe Bangfai drainages, Laos

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The fish diversity of the Nam Theun and Xe Bangfai drainages was surveyed between 1996 and 2012. 74 species are now recorded from the Nam Theun and 178 from the Xe Bangfai drainages. Twenty-one (28%) of the Nam Theun species are endemic and 9 (5%) of the Xe Bangfai species. At the time of the respective surveys, 54 species were new records for Laos, and 25 species were new to science. While the fish fauna of Xe Bangfai is similar to that of the other main tributaries of the Mekong in Laos, the fish fauna of the Nam Theun is characterised by a great proportion of endemics, several of which have affinities with species known from the opposite slope of the Annamite Range, in Vietnam. Besides, work conducted in relation with aquatic biodiversity surveys for the Nam Theun 2 project resulted in an increase of the fish fauna of Laos from 210 to 481 in 3 years between March 1996 and June 1999 and the discovery of 128 species new to science and lead to the publication of an identification guide. Specific studies have been conducted to assess the status of some of the endemic species that were potentially threatened by the Nam Theun 2 scheme: <i>Scaphognathops theunensis<i/> (still observed in 2012 downstream of Nakai Dam), <i>Tor ater<i/> (mainly restricted to streams upstream of the reservoir), and <i>Oryzias<i/> sp. 'swamp' (along the fringe of the reservoir). The status of the bitterling <i>Rhodeus laoensis<i/> and its associated mussel is still not clear. Most of the species endemic to the Nam Theun drainage are known upstream of the reservoir, which is a protected area. The endemic species known on Nakai Plateau and downstream of the dam are more at risk and require closer monitoring.