Throughout the world, emerging barriers in river systems block longitudinal connectivity for migrating fish, causing significant impacts by precluding them from carrying out vital life cycle activities. Fishways are still the main mitigation solution implemented, where barrier removal is not feasible. Within the multiple technical fish passage devices, the vertical slot fishway (VSF) is considered the most reliable. Early design guidelines, established for cyprinids, indicate that the volumetric dissipation power (
Pv) in the pools should be
Pv < 150 Wm
−3, while most frequent slope values range from 10 to 12%. In this study, an experimental approach was conducted to question and debate the validity of these recommendations. For this, the Iberian barbel (
Luciobarbus bocagei, Steindachner, 1864) passage performance was assessed in a full-scale fishway that exceeded
Pv design guidelines, under different configurations. These varied in discharge (Q) and mean pool water depth (h
m): VSF1 (Q = 81 Ls
−1; h
m = 0.55 m); VSF2 (Q = 110 Ls
−1; h
m = 0.80 m); and the same design was equipped with a deep notch: DN1 (Q = 71 Ls
−1; h
m = 0.55 m); DN2 (Q = 99 Ls
−1; h
m = 0.80 m). The slope was set to 15.2% while the head drop per pool was Δh = 0.28 m, which generated a
Pv that ranged from 222 in VSF1 to 187 Wm
−3 in DN2. Passage behaviour was assessed using PIT telemetry and time-to-event analysis to evaluate the barbel upstream passage using standardized metrics: (i) motivation (ii) ascend success, and (iii) transit time. The hydrodynamic scenarios experienced by fish were characterized through a numerical model using computational fluid dynamics (CFD). The results, contrary to what was expected, showed a higher performance in VSF1 confirmed by the ascent analysis and transit time. Although no differences were found in fish motivation, the results indicate that larger fish displayed lower times to perform the first passage attempt. The CFD results show that, although maximum velocities and turbulence (turbulent kinetic energy (TKE) and Reynolds shear stress (RSS)) do not change significantly between configurations, their distribution in the pools is quite different. Regarding TKE, larger volumes with magnitudes higher than 0.05 m
2s
−2 were notorious in both DN1 and DN2 configurations compared to VSF1, influencing passage efficiency which is in line with the ascent and transit time metrics results. Overall, the present research undeniably debates the literature design guidelines and reinforces the need to jointly assess species-specific fish passage criteria and fishway hydrodynamics, whereas precaution should be taken when using very general recommendations.
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