In recent years, the hydropower development of the lower Mekong River has accelerated, accompanied by habitat loss and fragmentation. We conducted two experiments using video recordings and traps to evaluate the effectiveness of a nature-like rock ramp bypass in the Sesan River in 2019 and 2021. The results show that the fishway provides both upstream and downstream passage for at least 24 non-salmonid species of fish. The vast majority of fish choose to ascend from July to September, especially in August, and hardly between October and November. The fish inside the fishway prefer to move during the daytime (6:00–18:00), especially during August and September. An excessive water depth at the entrance can lower the number of ascending fish, whereas a higher water depth at the exit can cause the opposite result. Nevertheless, the size of fish monitored exhibits a decreasing trend, suggesting the nature-like bypass cannot completely mitigate the impact caused by this impassable Sesan Ⅱ dam. Therefore, a quantitative assessment of the bypass is highly encouraged, whereas the selection of the tracked fish species and experimental period requires considerable deliberation. This study alleviates the dilemma of insufficient fishway evaluation in tropical countries, which can provide researchers with data support on future non-salmonid fishway designs.

Fish collection and transportation facilities have been widely constructed as man-made passages that allow fish to bypass dams. These facilities usually provide proper conditions that attract and gather fish inside, and then transport them upstream over the dam. A novel design that includes water temperature compensation was presented, and the velocity, temperature distribution, and turbulent kinetic energy inside the fish collecting channel were studied using numerical tools. The facility mixes the warm cooling water from the electrical transformer with the tailwater in order to reduce the negative ecological effect of the low-temperature discharge. It can operate under a 2 m water level range in the tailrace channel. The numerical results show that the temperature inside the fish collecting channel increased by about 2 °C and settled within the suitable range of the target fish species. The water body maintained a relatively uniform and steady temperature. The velocity and the turbulence kinetic energy (TKE) field near the fish entrance were distinct from those of the background and were beneficial for fish migration. This study could potentially motivate ecological engineers to mitigate the negative impacts of low-temperature tailwater from hydropower plants when designing fish collection and transportation facilities.