SBR- Content.pmd - INBO

State of the Basin Report - 20032.7.2 Barriers to fish migrationDams and other structures across streams are barriers to migrating fish. A mainstream dam, onthe Mekong below the Khone Falls, for instance, would prevent the migration of adult white fishfrom the floodplains and others to their spawning grounds upstream in northeast Cambodia. 84 Attimes of peak migration from the Tonle Sap to the Mekong River, the movement of fish species isconcentrated in time and abundance. At these times at least 50,000 fish per minute are swimmingpast a given point in the Tonle Sap River. 85 No existing fish ladder design could cope with thesenumbers of fish. 86Catching fish in area with rapids orwater fallFloodplains can be altered through land use and modifiedby development of water resources, resulting in changes inwater flow and the timing, frequency, duration, and heightof flooding. Other important aquatic habitats such as rapidsand riffles, backwaters, and deep pools can also be impactedby changes in water flow regime. Fish migration is closelyrelated to annual fluctuations in water flow and water levels.If natural water flow fluctuation patterns change drastically,fish will be unable to complete their migrations, andspawning and recruitment will be reduced, and hence fishresources will be reduced.While the spawning of the majority of Mekong fish speciesis triggered by monsoon rains, rising water levels, andturbidity, other species such as members of the genusProbarbus, spawn during low flow periods. 87 All threespecies in this genus are extremely popular as food fishand are all included on the IUCN list of endangeredspecies. 88 Dry season spawning species are likely to beespecially susceptible to water released from dams during the dry season because they may beunable to find suitable spawning sites or their eggs may be washed away or smothered by sedimentdeposits.If an integrated approach to fish and water resource management is to be undertaken, methodologiesare needed that can quantify the importance of migrations in the production cycle and quantify theimpact of dams on migrations, fish yield, and the livelihoods of fisher communities.2.7.3 Habitat lossFresh waters account for less than 1 percent of the world’s surface area, but they provide at least 8percent of global fisheries yield. These productive ecosystems are, however, under pressure from agrowing human population, conversion and draining of wetlands, construction of dams, waterdiversions, canals, increased water consumption and pollution. 89Fisheries productivity and diversity varies with habitat type. Productivity is often considered to behighest in flooded forests and lowest in rice fields, but there is no literature and no studies from theMekong region to support this opinion.In the Mekong, destruction of flooded forest for fuel, or conversion to rice fields or other uses hasa negative impact on recruitment to the wild fishery and fish growth. 90114