Challenges for Fish Pass Solutions in the Mekong River

While state-of-the-art technologies for fish passage at hydropower dams for small and medium-sized rivers or for single species such as salmon are available nowadays, solutions for large rivers with multi-species such as the Mekong River are widely missing. Objective of this review is to elucidate migratory particularities of large rivers inhabiting multi-species fish assemblages, analyse advantages and disadvantages of existing fish pass solutions, discuss the potential applicability to the Mekong River and identify research gaps. Large warm-water rivers inhabit a wide range of different species encompassing migratory fish ranging from 20 to >200 cm, migration distances >1000 km and iteroparous life histories. Migrations may take place throughout the year due to species-specific spawning seasons and other migratory needs. A key challenge for fish pass solutions at large rivers is to attract fish to fish pass entries. As a solution, multiple fish passes and/or entries are recommended including options for bottom, surface and open water orientated species. Large fish and large number of fish during migration peaks require dimensions of fish passes going far beyond the size of conventional fish passes. A further challenge is that solutions or even experiences for downstream migration in large, multi-species rivers are widely lacking.