Seasonal Fish Passage Patterns Through an Estuarine Fishway in Relation to Tidal Hydraulics

Fish passage through estuarine fishways varies with tides and seasons, but their associations remain poorly understood. At the Cao’e River Sluice fishway, sonar monitoring quantified the timing and intensity of crossing events at a section near the freshwater-side exit over 961 half-hour intervals on 30 monitoring days, while numerical simulations characterized hydraulic conditions. Lower seaward levels reduced entrance depth, with local connectivity loss simulated at a seaward level of 0.20 m. At a seaward water level of 1.16 m, entrance depth averaged 0.55 m, and this screening reference was met for 5.7–6.2 h per 12 h cycle. Total passage intensities reached 371 and 344 ind/h during late flood and early ebb in March–June, compared with 43 and 57 ind/h during late ebb and subsequent early flood in September–November. Adjusted upstream intensity was lower in September–November than in March–June across tidal stages (ratios 0.03–0.18, p < 0.001), while daytime upstream intensity was 3.0–5.5 times that during 00:00–06:00 (p < 0.001). Elevated upstream crossing intensity at the exit section showed season-specific, non-monotonic associations with concurrent head difference (ΔH), occurring at ΔH = 2.25–3.75 m during rapid ebb in March–June and under small positive to reversed heads during flood or large positive heads during ebb in September–November. Mean target lengths in selected subsamples were 17.6 ± 3.4 and 26.8 ± 6.7 cm, respectively. Upstream movement predominated in both periods, and upstream intensity was somewhat higher during spring tides. These findings support maintaining entrance connectivity and considering seasonal passage timing and tidal conditions in fishway operation.