Abstract: Distribution and migration of fishes in Nam Kam River, one of the Mekong tributary in Thailand, were conducted during the migration period in 2013. Nam Kam River consists of four cascade watergates, all of which equipped with fish passage. Migration of fish from the Mekong River into the Nam Kam river system started at the onset of the rainy season around the end of May to early October. It was found that 92 species from 21 families were observed in the fish passages. The highest number (440,015 fish) was found at the most downstream passage consisting of 83 species. Maximum number of fish was found in early rainy season with averaging 46,831 individuals per day. Fish migrated during daytime more than nighttime in term of abundance (93.03-95.80% vs 4.18- 6.97%) while number of species was not significantly difference. Cyprinids was the dominant group migrated during daytime while Bagrids and Silurids were mostly migrated at nighttime. Majority of fishes were mature and ready to spawn, indicated the reproductive migration. Distribution after migrating through fish passage using physical and genetic (microsatellites) tags were studied in two economically important species (Hemibagrus nemurus and Osteochilus hasselti). The recapture data revealed target species could migrate through a few watergates upstream at the farthest distance of 72.2 km from most downstream watergate. While the microsatellite analysis indicated that fishes could migrate, although at limited number, almost 100 km upstream. Tag recovery from physical and genetic tagging technique were very low (0.62- 10.05% and 0.95% in H. nemurus and 0.75- 0.92% and 0.58% in O. hasselti) as well as the effective numbers of migrants (2.7426 in H. nemurus and 8.2676 in O. hasselti). The result indicated that a series of watergates might, to some extent, obstruct the migration of fishes in Nam Kam River. Management of watergates based on the migrating period and spawning season could increase the rate of fish migration in the river.
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