Visual cues are critical for orientation and migration in riverine fishes, providing potential mechanisms for behavioral guidance. This study investigated how light spectrum and intensity interact to modulate phototactic responses in two schizothoracine fishes from the upper Yalong River. Results showed Schizopygopsis malacanthus preferred blue light and avoided red light, with preferences shifting with flow intensity; Schizothorax kozlovi favored green light and avoided light-red light, with minimal flow impact. We propose that engineers build fishway entrances or ideal habitat attractors that prioritize low-intensity blue light (10 lx) and medium-intensity green light (50 lx), supplemented by medium-intensity blue light (50 lx). This study provides scientific evidence and application value for restoring fish habitats, fish passages, and fisheries.

Understanding the upstream-attempt behavior of fish in high-flow environments is important for refining behavioral simulations, optimizing fish-passage design, and protecting aquatic ecosystems. This study investigates Schizothorax oconnori, an endemic fish in the Yarlung Tsangpo River on the Qinghai–Tibet Plateau, using a multi-model analytical framework integrating a generalized linear mixed-effects model, Random Forest, SHapley Additive exPlanations, and GeoDetector. Results showed that remaining endurance and nominal flow velocity were positively associated with fish upstream-attempt initiation. Attempt termination reflected endurance depletion, initial endurance, and hydraulic variability. Model-predicted termination probability increased markedly when (i) cumulative endurance consumption during the current attempt exceeded ~11% or (ii) fish began an attempt with an initial endurance status below ~95%. These model-derived change points represent within-attempt endurance depletion and reduced endurance available at attempt initiation, respectively. Across attempt sequences, the dominant explanatory factors showed an apparent shift from physiological condition during the first attempt to hydraulic variability during subsequent attempts. This sequence-related pattern is consistent with behavioral adjustment based on repeated experience in the flow environment, although cumulative fatigue, stress, individual differences, and experimental procedures may also contribute. Hydraulic-variability metrics along the trajectory showed greater explanatory importance than several instantaneous hydraulic variables, and physiological condition and hydraulic variability together enhanced explanatory power. This study provides a quantitative multi-model framework for analyzing upstream-attempt behavior and offers a scientific basis for refining behavioral simulations and optimizing fish-passage facilities.

Fish can perceive and respond to local flow structures when selecting microhabitats, yet how scale-dependent hydrodynamic heterogeneity shapes behavioral habitat choice remains poorly understood. In this study, a multi-scale rock-array flow field was constructed using flume experiments, and the volitional swimming behavior of Schizothorax grahami was observed. Hydrodynamic parameters including velocity magnitude (U_mag), turbulent kinetic energy (TKE), and kinetic energy gradient (KEG) were analyzed. The results indicated that fish distribution changed with rock size: at a rock-element diameter of 0.12 m, fish were dispersed without forming stable high-utilization core areas; at 0.24 m, fish aggregated in the lee-side regions behind the rock elements; and at 0.50 m, fish shifted toward the upstream faces. The hydrodynamic variables significantly associated with fish microhabitat selection varied with rock-array scale: in the 0.24 m rock arrays, KEG differed significantly between high- and low-use units under all flow conditions; in the 0.50 m rock arrays, U_mag and TKE showed consistent and significant between-group differences across flow conditions. Moreover, cross-scale overlap analysis revealed that the utilization range of velocity magnitude had the highest overlap, followed by TKE, with KEG exhibiting the lowest overlap. Collectively, these findings support a hierarchical interpretation of fish microhabitat selection characterized by “velocity rigidity–turbulence elasticity,” providing a potential perspective for understanding differences in the hydrodynamic variables associated with fish microhabitat selection across studies. These results indicate that both mean velocity constraints and local turbulence-creating structures should be considered in river habitat restoration and roughness element configuration.

To help fish to bypass dams and other human-made barriers, some fishways have ingeniously incorporated extended tunnel sections. This innovative design not only optimizes the overall structure of the fishway but also significantly reduces disturbances to the surrounding ecosystem. However, the potential challenges posed by long tunnel sections to fish upstream migration remain insufficiently studied and poorly understood. This study conducted in situ experiments utilizing a passive-integrated-transponder (PIT) system to quantitatively assess the effects of dark and natural light environments on the upstream migration behavior of plateau-endemic fishes (Schizothorax macropogon, Schizothorax waltoni, and Schizothorax oconnori) in a vertical-slot fishway. A 655 m section of the fishway was selected for the experiment, with shading cloth used to simulate the dark environment (DE) of tunnel sections, and its removal serving as the natural light environment (NE). The results showed that in the DE, the upstream behaviors of S. macropogon, S. waltoni, and S. oconnori were not hindered. The entry efficiency at the experimental segment (Ee) of all three species exceeded 65% in the DE, which was higher than that in the NE. The passage efficiency (Ep) of S. macropogon and S. waltoni showed no significant difference between the DE and NE, whereas S. oconnori exhibited a significant difference, with an overall Ep of 0% in the NE and 75.0% in the DE. Additionally, the DE caused a temporary disruption to the diel migration rhythms of the three species. The transit speeds (St) of S. macropogon and S. waltoni were both elevated in the DE, with S. waltoni showing a particularly significant increase; its average St in the DE (0.080 m/s) was much higher than in the NE (0.021 m/s). Ridge regression analysis further indicated that the DE was the primary factor influencing the St and had a positive effect on upstream behavior. Moreover, differences in the upstream migration performances of different species under varying light conditions highlighted species-specific sensitivity to light. This study offers key insights for fish passage design in canyon hydropower projects and highlights the potential of tunnel-type fishways in restoring river connectivity.

In pipeline-replenished fish lifts, the replenishment flow enters the collection system at a high velocity and must first be dissipated in a stilling basin, but a conventional deep basin requires a large excavation volume and a high construction cost; whether an adjustable-resistance boundary at the basin top can replace part of the basin depth still requires systematic quantitative assessment. Here, a permeable cover plate at the basin top is compared quantitatively with two conventional measures, a deeper basin and internal baffles, under identical inflow, using a unified set of fish-related indices. A three-dimensional computational fluid dynamics (CFD) model, validated against a 1:5.33 physical model of the Fengman Hydropower Station fish lift, was used to examine three energy-dissipation configurations: stilling-basin depth (2–5 m), cover-plate permeability (72–50%), and baffle height (1–4 m). Performance was assessed by the coefficient of variation of velocity (CV) at the control section upstream of the fish grid (X = 14 m) and by the streamwise turbulent kinetic energy (TKE) in the fish bucket and residence pool. The results show that increasing the basin depth from 2 to 5 m reduced CV from 0.31 to 0.23. Reducing the cover-plate permeability from 72% to 50% reduced CV to 0.12 and the streamwise TKE by about 87%, confining the high-velocity and high-turbulence regions to the stilling basin, whereas baffle height acted non-monotonically with an optimum at 2 m (CV = 0.20). Overall, a 3 m deep basin with a 50% permeability cover plate gave the most uniform approach flow and the lowest turbulence (maximum TKE about 0.003 m²/s²), providing more favourable conditions in the collection zone for fish to hold position and migrate upstream, while requiring 2 m less excavation than the reference scheme; an adjustable-resistance boundary can thus replace part of the basin depth without sacrificing hydraulic performance, offering a new design route for high-head fish lifts with constrained abutment excavation.

Closed-flume spatial constraints may bias swimming-performance data used in fish-passage design. Two independent experiments were conducted using different size classes of juvenile grass carp (Ctenopharyngodon idella). Fish with a mean body length (BL) of 7.92 ± 0.61 cm were used to assess five test-section lengths (8, 6, 4, 3.5, and 3 BL), whereas fish with a mean BL of 15.76 ± 0.97 cm were used to assess three blockage ratios (1.88%, 9.26%, and 15.31%). Incremental-velocity tests quantified critical swimming speed and behavior, and three-dimensional simulations examined the local flow field under the 9.26% blockage. Relative critical swimming speed was similar at 4–8 BL (9.71–9.95 BL/s) but decreased by 10.96% from 4 to 3.5 BL. Increasing blockage from 1.88% to 9.26% and 15.31% reduced it from 6.99 to 5.34 and 5.24 BL/s, respectively. Behavioral responses also depended on spatial constraint: shorter sections reduced the space available for forward movement and postural adjustment, whereas higher blockage restricted lateral body and tail movements. As velocity increased, fish intensified tail beating; under higher blockage, compensation relied increasingly on tail-beat frequency because amplitude adjustment was constrained, with greater variation among individuals. Simulations showed that local streamwise velocity near the maximum fish cross-section was 20.8–25.0% above inlet velocity. At least 4 BL is recommended for fish comparable to those in the length experiment. Below 5% is suggested as a precautionary blockage target.