Downstream migration of juvenile fish in the Krasnaya River (Vietnam)

The first material is presented on downstream migration of fish juveniles in the Krasnaya River, one of the largest rivers in South-East Asia. The spatial-temporal structure of distribution of downstream-migrating fish juveniles in the Krasnaya River has traits usually characteristic of migration in rovers with turbid water. Concentration of fish was extremely variable by the time of the day, river length, and depth. In length-wise distribution, there was a tendency to increasing concentration of downstream migrants from the upper stretches to the downstream stretch. In the upper reaches, Perciformes prevailed, in the middle part, Cypriniformes prevailed, and in the lower reaches of the river Clupeiformes prevailed. During 24 h, Clupeiformes and Perciformes migrated more intensively at night and Cypriniformes migrated more in the daytime. By depth horizons, Clupeiformes made downstream migration predominantly at the surface and Cypriniformes and Perciformes did at the bottom. These differences may be caused by species differences in fish biology, local specific traits of stream hydraulics, anthropogenous factors, etc.