SBR- Content.pmd - INBO

State of the Basin Report - 20032.7 Factors impacting on fisheries resources2.7.1 Dams and water extractionDevelopment of water resources, particularly dam and weir construction for hydroelectric power,irrigated agriculture, water supply and flood control has led to water abstraction, water diversion,water retention, and increased water evaporation. At impoundment sites and at locations downstreamfrom them, aquatic habitats have been lost or modified. Dams for reservoirs, headponds, and weirschange rivers and streams and also change the timing, duration and quantity of flooding furtherdownstream. 74To date, the only dams that have been built across the mainstream Mekong are the Manwan andDachaoshan Dams in Yunnan Province of China. 75 The 300 metre high Xiaowan Dam is underconstruction and China has proposals to construct more mainstream dams on the Lancang JiangRiver, as the Mekong River is known in Yunnan. 76Dams are of concern because they cause fragmentation of aquatic habitats, block fish migrationroutes, change water flow patterns and water levels through water abstraction and water retention,and increase water loss by evaporation. 77The construction of a diversionary weir at Nam Song in Lao PDR seriously disrupted the naturalriver flow patterns of the Nam Song River, which is part of the Nam Ngum sub-catchment. Followingcompletion of the Nam Song weir in 1996, 40 fish species disappeared and 20 trans-boundarymigratory fish species were lost from catches in neighbouring countries (Table 5). 78 Of these, 20species were trans-boundary migratory (TBM) or long distance migratory species (LDM). 79 (SeeTable 4)Prior to the construction of a dam in Northeast Thailand, there was a large-scale fishery for thetrans-boundary long distance migrating Pangasius macronema at Pak Mun. After the constructionof the dam, a series of more than 50 rapids, which were important spawning grounds for P.macronema, were flooded. This has caused a significant decrease in the size of the P. macronemafishery. 80Before filling the Pak Mun Dam, which has a catchment area of 117,000 km 2 , 265 fish species wererecorded in its diverse variety of aquatic habitats, and the fish spawning grounds were accessed byfish from the Mekong mainstream. Following the construction of the dam, only 96 fish species havebeen recorded upstream of the dam, and of those, 51 fish species have declined in abundance. Theconstruction of the dam has caused the upstream extinction of long distance trans-boundary migratoryspecies, which previously returned annually to spawn in the rapids. 81A variety of fish have evolved to fill the specialised feeding niches, which are available in theMekong River Basin. Many Mekong River fish species in the Cyprinidae and Gyrinocheilidaefamilies feed primarily on periphyton (algae attached to substrate). Changes in light intensity,increased water levels, increased turbidity, and reduction in nutrient levels caused by constructionof dams and weirs can wipe out the periphyton populations on which specialised feeding fishdepend. Increased water levels caused by the Nam Hinboun and the Theun-Hinboun hydropowerprojects in Lao PDR wiped out periphyton communities and with them, the periphyton-feedingfish species. 82112