USING AN EXPERIMENTAL IN SITU FISHWAY TO PROVIDE KEY DESIGN CRITERIA FOR LATERAL FISH PASSAGE IN TROPICAL RIVERS: A CASE STUDY FROM THE MEKONG RIVER, CENTRAL LAO PDR

Abstract

Fish passage through an experimental vertical-slot fishway was assessed at a floodplain regulator on the Mekong River in Central Laos between April and July 2009. Experiments were conducted to investigate the influence of fishway floor slope (1v:15h or 1v:7.5h) on fish passage success with a view to developing a series of optimal design criteria for the construction of vertical-slot fishways at other barriers to fish passage in the Lower Mekong Basin. A total of 14 661 fish from 73 species were captured during the experiments. Catches were dominated by riverine (white) (n = 51; 69% of total) and floodplain (black) species (n = 15; 20%) which represented 19 families in total. The work demonstrated that fish were actively attempting upstream passage from the Mekong River to an adjacent floodplain and displayed strong migratory behaviour during river level rises. Migratory activity was greatest during sharp rises in water level but reduced substantially when river level fell. Fish community composition varied greatly among the two fishway floor slopes and the control group. More fish species were collected from control samples, but the most fish and species were collected when the fishway was configured on a moderate hydraulic slope (1v:15h). A range of size classes were also collected from control and moderate-slope groups, but steeper-gradient catches were dominated by larger fish. This study demonstrated that vertical-slot fishways could provide passage for a biodiverse fish community where fish move laterally onto floodplains. The construction of fishways which consider the local fish ecology and hydrology may therefore represent a valuable management tool to help restore important movement pathways for tropical freshwater fish.

The future of fish passage science, engineering, and practice

“…Fifty‐five per cent of 181 fish species in Canadian freshwaters have been described as migratory (38% diadromous, 62% potamodromous, Lucas & Baras, ); however, a detailed understanding of the migration behaviour and capacity is known for less than a third of these species. Knowledge concerning the importance of migration and dispersal phases in the life histories of tropical and subtropical freshwater fishes is far lower (Baras & Lucas, ) due to the extraordinary diversity that is present in those riverine systems, although rapid progress is being made in some regions (Araújo‐Lima & Ruffino, ; Baumgartner et al., ; Makrakis et al., ). Such information is urgently needed, as river obstruction is the single most pervasive threat to the functionality of freshwater systems worldwide.…”

“…For example, Baumgartner et al. () reported capture of 73 species during targeted fishway design research on the Mekong River in Laos, including a number of undescribed species for which limited biological knowledge exists. Effective fish passage for entire fish communities, rather than target species, is challenging and is a major barrier to progress.…”

“…Concepts developed at Bonneville Dam have been directly applied to projects in Brazil and South‐East Asia (Baumann & Stevanella, ). Further, the design of the Ben Anderson Barrage fishway (Burnett River, Queensland) was directly applied at Stung Chinit Irrigation district in Cambodia (Baumgartner et al., ). It is clear from these examples that, in the absence of suitable local solutions, there is a strong trend to adopt and apply existing solutions from elsewhere.…”

Perspective: Towards environmentally acceptable criteria for downstream fish passage through mini hydro and irrigation infrastructure in the Lower Mekong River Basin

“…26 However, most upstream moving fish that may be migrating to spawn, will subsequently produce passively drifting larvae or juveniles. These fish, in addition to the adult migrants, will also need to move downstream at some stage.…”

Effects of Slotted Water Control Structures on Nekton Movement within Salt Marshes

“…Importantly, because slots extend the full height of the WCS and permit flow throughout the water column (when open), they may facilitate passage of the entire nekton assemblage rather than only some species. In riverine systems, vertical slots incorporated into dam fishways were found to improve passage (Stuart and Mallen-Cooper 1999;Stuart and Berghuis 2002) and provided access for a large portion of the riverine nekton community (Baumgartner et al 2010(Baumgartner et al , 2012Thiem et al 2013).…”