Managing water gate operation and fish passage facilities in irrigated systems of the Nam Kam River, Thailand

The migration of fishes through the Thoranit Naruemit water gate on the Nam Kam River, a tributary of the Mekong River in Thailand, was studied to understand the hydrological factors and improve the operational regime of the fish pass at the structure. Migration of fish from the Mekong River started in May, at the onset of the rainy season. During 60 days of operation, the fish pass supported the upstream migration of 83 species and ~440,000 fish, equivalent to about 11 tonnes of fish. The fish pass supported migration of a range of fish sizes between 45 and 700 mm, and most fish were larger than the size of first maturity for the species in question. The factors influencing migration into the Nam Kam included the lunar cycle, flow in the fish pass, water level in the river and flow variability. Migration tended to increase in the 3–4‐day period before or after a full moon. Discharge and water level changes associated with rising water levels in the Mekong River synchronize with the spawning season of most Mekong fish and trigger upstream migration. Many fish migrated through the pass at discharges through sluice gates between 8 and 323 m3/s and with a flow through the fish pass between 0.95 and 3.08 m/s. Restrictions to fish migration were found and recommendations for water gate and fish pass operation are provided.