Integrated Hydraulic and Biological Monitoring Reveals Stage-Specific Passage Performance in a Vertical-Slot Fishway in the Upper Yangtze River

Dams fragment river networks, but post-construction evaluations of fishways often treat hydraulic suitability and biological performance separately. We assessed a 1486-m-long vertical-slot fishway comprising 326 pools at the Jinsha Hydropower Station in the upper Yangtze River during its first operating season. The assessment integrated upstream and downstream fish-assemblage surveys, as-built geometric inspection, field hydraulic measurements, diagnostic three-dimensional computational fluid dynamics (CFD), underwater video, block-net sampling, and passive integrated transponder (PIT) telemetry of 95 fish representing 12 species. Pre-commissioning inspection revealed substantial deviations in vertical-slot width, which were corrected before operation. Mean measured entrance velocity was approximately 0.40 m/s and representative slot velocities were 0.90–1.08 m/s; diagnostic CFD reproduced slot-jet velocity magnitudes of approximately 0.9–1.1 m/s and indicated localized flow-acceleration magnitudes of approximately 1.9 m/s2 near slot transitions. PIT detections declined from 57 fish at the downstream monitoring checkpoint (T1) to 34 at an intermediate dam-site checkpoint (T2) and 12 at the upstream-exit checkpoint (T3). Accordingly, 60.0% of released fish were detected at T1, 21.1% of fish detected at T1 subsequently reached T3, and 12.6% of all released fish reached T3 from the release location within the monitoring period. The combination of broadly acceptable representative velocities with a low observed route-completion proportion within the PIT-tagged experimental group indicates that compliance with nominal hydraulic criteria alone is insufficient for evaluating long fishways. The progressive decline in PIT detections, local hydraulic heterogeneity, as-built geometric variability, and unoptimized operation emerged as priority hypotheses for adaptive testing. Post-construction evaluation should therefore couple structural quality control and stage-specific biological monitoring with controlled operational trials of attraction flow, gate settings, and water levels.