Diversity of Upstream-Migrating Fish Passing Xayaburi Hydroelectric Power Plant in Northern Laos

The Mekong River is one of the most fish-species-rich rivers on Earth, yet most of our knowledge on fish diversity and migrations comes from fishery catch data, and fishery-independent, standard effort surveys are needed. Specifically, migratory fish data sets are heavily biased by the influence of fisher gear types and by fisher location, with most major Mekong fisheries, and consequently fishing effort, being in southern Laos, Cambodia and Vietnam. Fish using the Xayaburi Hydroelectric Power Plant (XHPP) fish pass have been monitored since commencing operations in 2019. This programme offers a unique, eventual long-term data set, with standard effort, aimed at migrating fish species, and capable of providing data on the scarcely studied fish migration patterns of northern Laos. Species that migrate from floodplain feeding and spawning habitats to the main channel were dominant in the dry season, but used the fish pass throughout the year. On the other hand, known long-distance migratory species were infrequently collected in the fish pass, but showed very strong affiliations with the flood season from May to July. We demonstrate that fish passage mitigation in tropical southeast Asia can, and should be, designed for use by a multitude of species, including resident species that are not classified as migratory. If changes in connectivity from future downstream barriers (i.e., scheduled hydropower dams) occur, then the monitoring program at XHPP will be able to detect the impact on migratory fish populations. We identify several species whose presence and relative abundance have potential to serve as indicators for future downstream connectivity issues.