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7 - FisheriesTonle SapThe Tonle Sap Great Lake area varies dramatically in area from 2,500 - 3,000 km 2 in thedry season to up to 13,000 square km in thewet season, when the water level rises eightmetres or more above the average dry seasonlevel. 12 This provides fish with temporaryaccess to enormous quantities of food,especially in the flooded forest areas. Manyfish species release eggs just before or duringthe floods and their seed are thereby widelydispersed across the floodplains.Rising waters, turbidity and/or the first rains trigger adults of many Mekong fish species to spawn. 9Long distance migratory fish species or “white” fish have adapted to spawn at the onset of themonsoon season (May-July), so that their fry and juveniles are ready to enter and feed when theplains become flooded from July to September. 10 Short distance migratory or “black” fish speciesalso migrate to spawn and feed in the inundated floodplains. 11Deep pools and channels in the mainstream of the Mekong near Kratie in Cambodia, in the NamTheun and Nam Hinboun in Lao PDR, and in the Se San River in Cambodia are widely acknowledgedas dry season refuges for fish, which re-colonise flooded areas during the following monsoon. 13Important spawning areas are located in the Mekong in Kratie and Stung Treng Provinces. The longdistance fish migrations within main river channels and their main tributaries are referred to as“longitudinal migrations”. Such long distance migrations can cover distances of hundreds ofkilometres from the Mekong Delta in Viet Nam, through Cambodia, to (i) Thailand or Lao PDRthrough the Mekong River mainstream, (ii)Lao PDR through the Se Kong River, or(iii) the Central Highlands in Viet Namthrough the Se San and Sre Pok Rivers. 14A small section of the Sambor Rapids, northern CambodiaAnother type of fish movement in rivers isthe passive downstream movement by fishlarvae, or “larval drift”. During the floodseason, larvae of some species may driftseveral hundred kilometres from upstreamspawning areas to downstream nurseryareas in the floodplains. 15The most comprehensive study to date on fish migrations in the Mekong River was produced byPoulsen and his co-authors in 2000. 16 Migration studies confirm the critical importance of migrationevents in the biological functioning of the fish resource. Some 63 percent of the total catch taken bylarge and middle scale gears in the Tonle Sap fisheries constitutes longitudinally migrating fish. 17Other important migration patterns include lateral migration from the mainstream to the floodplains,breeding migrations of adult broodstock, migration of eggs and juveniles with water currents, feedingmigration and migration to dry season refuge habitats with water currents as water levels decline.All these migrations are essential features of the Mekong fish resource.103