Assessing Upstream Passage of Two Potamodromous Leuciscids in an Experimental Retrofitted Pool-Weir Fishway Physical...

The installation of hydraulic elements in existing pool-type fishways to improve performance for potamodromous species remains poorly tested, particularly for flexible substrates and non-salmonid species. We combined detailed hydraulic measurements with live-fish tests to quantify the effects of flow type (orifice flow [OF] vs. orifice-notch flow [ONF]), substrate condition (with vs. without brush blocks) and body size on the upstream passage success of two contrasting potamodromous leuciscids: Squalius cappadocicus and Chondrostoma colchicum. In a 1:2 scale pool-weir model, the presence of brush blocks reduced the extent of the main jet by 67% and lowered the maximum near-bed velocity by 79% downstream of the blocks, creating low-velocity refugia. Binomial GLMs revealed significant species- and size-specific responses. For Chondrostoma, ONF markedly reduced success relative to OF (odds ratio, OR = 0.16; 95% confidence interval, CI 0.06-0.40), and fish measuring less than 10.6 cm total length (TL) had much lower passage probabilities. For Squalius, success was primarily limited by substrate; brush blocks increased success by around fourfold (OR = 3.97; 95% CI 1.58-10.64), and there was a weaker length threshold of 18.5 cm TL for passage success. Model discrimination was fair to good (area under the curve (AUC) ≈ 0.77 for Chondrostoma and 0.71 for Squalius), and higher-order interactions did not improve predictive performance. This indicates that simple configurations capture the main ecological response. Overall, an orifice-only hydraulic condition combined with bottom brush blocks provided the most inclusive solution for both potamodromous leuciscids, particularly the smaller size classes. This configuration also offers a practical retrofitting option for existing pool-weir fishways in fragmented river networks, though periodic maintenance will be required to prevent clogging of orifices and brush elements by woody debris and sediment.