Final Report Water Quality and Aquatic Life Study Theun Hinboun Impact, Lao PDR

The present report summarises the results from the environmental impact assessment study on water quality and aquatic life in connection with the Theun Hinboun Hydropower Development Project, Lao PDR. The river Nam Theun, which is going to be dammed and partly diverted to Nam Hai/ Nam Hinboun, is a nutrient poor softwater river with low content of sediments. The proposed hydropower development will have considerable negative effects on the productivity of the Nam Theun and Nam Kading river, particularly the production of large edible fish species will decline. To avoid catastrophy in the downstream river fisheries a minimum release from the dam of 6-15 m³/s is neccessary. Likewise it is recommended to build a fish bypass through the dam to allow for fish migration. It is not recommended to establish a stocking programme introducing new fish species. In Nam Hai considerable erosion is expected to occur in the first period after the regulation. This may impose some smaller negative effects on the productivity of the receiving Nam Hinboun, which however, will be of transitional character. The new hydrological regime in Nam Hai can be positive to the aquatic life and fish production if the river is not allowed to be dry at any time after the regulation. A power house bypass should be built to allow for at least some flow during periods of power station maintenance. Some basins, or weirs should also be built in the tailrace system to reduce the fast water flow fluctuation in Nam Hai. This to prevent stranding of juvinile fishes

Diel migration patterns of Atlantic salmon smolts with particular reference to the absence of crepuscular migration

Abstract – The real-time diel pattern of Atlantic salmon smolt migration was observed for 8 years using automatic resistivity counters verified by video surveillance. A clear dominant nocturnal migration was demonstrated early in the migration period, later becoming increasingly diurnal, until rates became approximately equal at day and night. Migration patterns were related to water temperature, such that when mean daily temperatures were below 12 °C, hourly rates of migration were significantly lower during the day than at night. When daily mean temperatures exceeded 12 °C, there was no significant difference between diurnal and nocturnal migration rates. Migration patterns showed a distinct suppression of migration at dawn and dusk throughout the migration period. It is hypothesised that this behaviour is an active decision and/or an adaptive strategy either to take advantage of increased food in the form of invertebrate drift or to reduce predation risk from actively feeding piscivores or both