| Summary: | The impact of watergate operation on hydrology and habitat, and their effects on fish migration, fish diversity, population structure, and recruitment of fisheries resources were studied in the Nam Kam River system, a tributary of the Mekong in Thailand. The factors that influence the migration of fish were identified and the effectiveness of fish passage facilities installed at watergates in the river system were investigated. The flow regime and flood cycle were modified by watergate operation, which is designed to control floods in the wet season and retain water for irrigation in the dry season. Fluctuations in flow and water level were created and varied along the river in the wet season. Timing, frequency and duration of floods in each habitat down the river were altered. Operations for irrigation removed low flows and created longer periods of no flow in the dry season. The river becomes stagnant and water levels in the floodplains above the watergates are higher than pre-construction. Many new nursery and feeding habitats were created after construction, but water abstraction also disconnected the floodplain below the watergate and the mainstem river. Flow modification driven by watergate operation for flood control is likely to delay water level rise at the onset of the flood in the downstream area that trigger upstream migration of fish into the Nam Kam River system, especially pangasids. The watergate operation limits the upstream migration of adult fish at the onset of the flood season and inhibits the upstream migration of late migrating species, the downstream migration of fish, and drifting of eggs, larval and juvenile fish since all sluice gates were closed at the end of the flood season. Longitudinal migrating species are more likely to be impacted than lateral migrating species and this will impact on the overall population structure of fishes. Several longitudinal migratory white fish species, main channel residents and main channel spawners declined in abundance or were absent in the upstream area above Thoranit Naruemit Watergate and it most likely resulted from hydrological alteration driven by watergate operation. Fish larvae and juvenile fish in this river system were dominated by resident grey and black fish, while recruitment of white fish species was limited as watergate operation obstructs the upstream migration of adult fish during the spawning season. Diversity of fish in the river decreased from downstream to upstream area, it shows a negative relationship with number of barriers. The relative abundance of white fish was significantly different between the floodplain above and below Thoranit Naruemit watergate during the study period. Seasonal distribution of fish in the Nam Kam River system is different from the free-flowing Songkhram River. At the end of flood season, diversity of fish in the regulated river was high since larvae and juvenile fish and many white and grey fish species are stranded in the floodplain above the watergate after sluice gates were completely closed. These fish have subsequently adapted to feed and grow in the poor habitat conditions during the dry season. Genetic study of two target species, Hemibagrus nemurus and Osteochilus hasselti, indicates high genetic diversity and big population sizes maintained by gene flow from the downstream populations and nearby populations when the watergates are opened. Populations of the two target species in the most upstream lake, Nong Han, are contributed by migrating fish from Mekong River and resident populations with in the river system. Rate of migrations, small genetic distances and genetic differentiations between subpopulations of the target species support the occurrence of gene flow in this river; many subpopulations have little genetic differentiation between samples although there is the series of barriers in the river system. By chance, population size of the two migratory species migrating through the Thoranit Naruemit and Suraswadi fish passes were relatively big and bigger than populations along the river suggesting that upstream migration of target species is only partially limited by watergate operation. This is probably because the Nam Kam River is a rather short river and gene flow in the river system was supported by the operation schedule that fully open the watergates in flood season and the fish pass operation at the onset and the end of flood season. Thus, the most important things that need to be addressed to maintain fisheries resources in this river system are watergate and fish passage operations. Fish passage facilities in the Nam Kam support migration of more than 135 fish species and mitigate the impact of delayed watergate opening on the recruitment of fish in this river system. However, the operational schedule needs to be adjusted since fish can only use the fish passes when the sluice gates are closed or partially opened dring high river discharge in the wet season. Upstream migration is only completely unobstructed when the watergates are open to maximum capacity. The watergate and fish passes management schemes in the river system are key important factors to mitigate the impact on hydrological changes and habitat changes, to enable free movement of fish through the river system and enhance the fisheries in the Nam Kam River system. Recommendations for watergate and fish pass operation based on the integrated knowledge gained from this study are provided.
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