Monitoring fish passage and fisheries at the Don Sahong dam site, Khone Falls, Mekong River, southern Laos

The Mekong River in southern Laos splits into seven anabranch channels which flow across Khone Falls, south-east Asia’s widest waterfall complex. Fish attempt to migrate upstream through all channels, and three (Xang Pheuak, Sahong and Sadam) were considered the most important for upstream fish passage by researchers who interviewed fishers in the 1990s. However, there are no actual data which show quantities or species of fish which migrate upstream through any of the channels. Sahong Channel was closed by coffer dams and dewatered in January 2016 to allow for the construction of the Don Sahong Hydropower Plant (DSHP), scheduled for completion in mid-2019. Fish passage improvements in the other channels are being carried out to mitigate the impact of closure of Sahong Channel. Ongoing monitoring is required to evaluate the success of these measures and the status of the fishery at Khone Falls to provide a basis for adaptive management during operation of the hydropower plant. Assessing fish passage directly at Khone Falls is potentially hazardous and technically challenging, as the channels are generally inaccessible, large and steep with fast and turbulent flow over a jagged substrate of fractured bedrock. Methods used include: 1) standardised sampling upstream and downstream of barriers, 2) monitoring daily catches of 60 households, 3) direct observation and counting of migrating fish, 4) interviews of fishers, 5) monitoring of catches and prices of fish in markets, and 6) downstream drift of larvae. Each of these methods produces useful information, but all have limitations. Taken together they provide multiple lines of evidence for a balanced assessment of fish passage and changes in the status of fisheries over time

Fish passage at the Don Sahong dam site, Khone Falls, Mekong River, southern Laos

The Mekong River in southern Laos splits into seven anabranch channels which flow across Khone Falls, south-east Asia’s widest waterfall complex. Fish attempt to migrate upstream through all channels, and three (Xang Pheuak, Sahong and Sadam) were considered the most important for upstream fish passage by researchers who interviewed fishers in the 1990s. However, there are no actual data which show quantities or species of fish which migrate upstream through any of the channels. Sahong Channel was closed by coffer dams and dewatered in January 2016 to allow for the construction of the Don Sahong Hydropower Plant (DSHP), scheduled for completion in mid-2019. Fish passage improvements in the other channels are being carried out to mitigate the impact of closure of Sahong Channel. Ongoing monitoring is required to evaluate the success of these measures and the status of the fishery at Khone Falls to provide a basis for adaptive management during operation of the hydropower plant. Assessing fish passage directly at Khone Falls is potentially hazardous and technically challenging, as the channels are generally inaccessible, large and steep with fast and turbulent flow over a jagged substrate of fractured bedrock. Methods used include: 1) standardised sampling upstream and downstream of barriers, 2) monitoring daily catches of 60 households, 3) direct observation and counting of migrating fish, 4) interviews of fishers, 5) monitoring of catches and prices of fish in markets, and 6) downstream drift of larvae. Each of these methods produces useful information, but all have limitations. Taken together they provide multiple lines of evidence for a balanced assessment of fish passage and changes in the status of fisheries over time