Fish‐Net: Probabilistic models for fishway planning, design and monitoring to support environmentally sustainable hydropower

26 27 The construction of fishways for upstream and downstream connectivity is the 28 preferred mitigation measure for hydropower dams and other riverine barriers. 29 Yet empirical evidence for effective design criteria for many species is missing. 30 We therefore assembled a group of international fishway designers and 31 combined their knowledge with available empirical data using a formal expert 32 elicitation protocol and Bayesian networks. The expert elicitation method we use 33 minimises biases typically associated with such approaches. Demonstrating our 34 application with a case study on the temperate Southern Hemisphere, we use the 35 resulting probabilistic models to predict the following, given alternative design 36 parameters: (i) the effectiveness of technical fishways for upstream movement of 37 migratory fish; (ii) habitat quality in nature-like bypasses for resident fish; and 38 (iii) rates of mortality during downstream passage of all fish through turbines 39 and spillways. 40 41 The Fish Passage Network (Fish-Net) predicts that fishways for native species 42 could be near 0% or near 100% efficient depending on their design, suggesting 43 great scope for adequate mitigation. Sensitivity analyses revealed the most 44 important parameters as: (i) design of attraction and entrance features of 45 technical fishways for upstream migration; (ii) habitat preferences of resident 46 fish in nature-like bypasses; and (iii) susceptibility of fish to barotrauma and 47 blade strike during turbine passage. Numerical modelling predicted that 48 mortality rates of small bodied fish (50-100 mm TL) due to blade-strike may be 49 higher for Kaplan than Francis turbines. Our findings can be used to support 50 environmentally sustainable decisions in the planning, design and monitoring 51 stages of hydropower development. 52 53