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 need to make different responses to different water flow conditions, as a variety of fish utilize or overcome the water flow while fish are in the water. This study aims to analyze the response of two types of carp (black carp and silver carp) to water flow in different body lengths (2.0–10.0 cm) in early developmental stages in an open channel. By analyzing the water velocity and fish swimming behavior, swimming against the water flow was the most frequent swimming behavior, and the frequencies of this behavior increased with the increased body length of fish. The results demonstrated that the detectable water velocity of two carp species of different body lengths remained in a certain range (0.020–0.060 m/s). Black carp and silver carp can swim against the water flow of 0.295–0.790 m/s and 0.245–0.825 m/s, respectively. The results of this study can provide invaluable data for engineers and biologists to protect fish in the early developmental stage and restore the ecological habitats in the regulated river systems.

The hydrodynamics at the fishway entrance play an important role in attracting fish into a fishway. Adjusting the entrance angle of the fishway to allow suitable water flow patterns at the entrance is an effective measure that can be used to improve the attraction efficiency. In this study, we analyzed the movement behavior of grass carp (Ctenopharyngodon idella) in a river channel at a fishway entrance with different fishway entrance angles (30°, 45°, and 60°) and different replenishment velocities (0.1 m/s, 0.2 m/s, and 0.3 m/s). The flow velocity was 0.32–0.50 m/s when the fish head deflected into the entrance under different entrance angles for grass carp. As the entrance angle of the fishway increased, the fish energy consumption increased. The range of energy consumption for grass carp increased from 1.26–3.59 × 10⁻³ J to 3.32–7.33 × 10⁻³ J when the entrance angle was increased from 30° to 60°. There was a negative correlation between the entrance angle of the fishway and the deflection angle of the tested fish’s head. This research presents a reference that combines fish swimming behavior and hydraulics to optimize the design of fishway entrances.

This study explored the hydraulic characteristics and the adaptability to water-level fluctuations of the vertical-slot fishway. The maximum allowable water depth difference between the entrance and exit was calculated for a one-entrance fishway and two-entrance fishways with different entrance distances (100 m, 200 m, 300 m) under insufficient entrance water depth, with a fishway slope of 2% and an exit water depth of 2.5 m. It was found that the maximum allowable water depth difference between the 1# entrance and exit of the two-entrance fishways (0.71 m, 0.85 m, 0.93 m) was greatly larger than that of the one-entrance fishway (0.48 m). Additionally, the maximum allowable water depth difference in the two-entrance fishway increased with the increased distance between the two entrances. The relationship between the maximum allowable water depth difference and the distance of the two entrances followed a logarithmic function. We suggested that the 2# entrance should be at least 1.6 m when the water depth of 1# entrance was decreased to 1.8 m. When the water depth of the 1# entrance was gradually decreased to 1.6 m, the water depth of the 2# entrance also gradually decreased to 1.2 m. The distance between the 1# entrance and 2# entrance subsequently changed. It was noteworthy that the conclusions proposed in this study were strictly limited to vertical-slot fishways with a slope of 2%, exit water depth of 2.5 m, similar geometric parameters, and target cyprinid species. Furthermore, different slopes or exit water depths should be studied to extend the relationship by introducing correction coefficients from subsequent studies. This study can provide references for the design and optimization of future fishway projects.