Comparing fishway designs for application in a large tropical river system

River infrastructure poses a serious threat to diverse and productive fish stocks in many tropical river-floodplain systems; particularly the Lower Mekong River, where the fisheries are vital for food security. Dams and weirs block fish migration pathways and prevent access to feeding, spawning or nursery habitat. Fishways are becoming increasingly important for mitigating the effects of barriers; however, knowledge regarding their effectiveness for the biodiverse tropical river systems is still scant. This study examined the effectiveness of differing low-cost fishway designs for rehabilitating degraded floodplain fisheries in the Lower Mekong Basin (LMB) in Laos: (1) vertical slot; (2) submerged orifice - 150 mm square opening; and (3) submerged orifice - 300 mm square opening. Day and night in situ field experiments were undertaken to compare the abundance, biomass, species richness and size range of fish able to pass through each design with relatively low drops between pools (i.e. 150 mm each) and low water velocities (i.e. 1.71 ms(-1)). Passage of a total of 73 species was supported by the fishway designs at a similar abundance, biomass, species richness and size range of fish, during both the day and night; although, the vertical slot design supported a different suite of fish species to that of the other two designs during the day. This suggests that each of these fishway designs could be successfully used to support the rehabilitation of fisheries in the LMB and potentially other large tropical river systems with relatively diverse migratory fish communities and variable hydrological characteristics. However, the vertical slot provides greater design and operational flexibility over the submerged orifice designs particularly in tropical systems with inherently variable hydrology. The final fishway design choice ultimately depends on the fish species and size classes being prioritised for restoration and the unique hydrological characteristics of the site.