Evaluating the Efficiency of a Fishway Installed Near a High, Artificially Created Waterfall

ABSTRACT The installation of fishways is the most common method to restore connectivity and allow fish to carry out their life cycle. However, the performance and efficiency of fishways are still highly variable, particularly for freshwater potamodromous species. We aimed to determine the passage efficiency of a fishway installed in 2021 downstream of the Coo waterfall in Belgium to allow upstream migration and crossing of the 11.8 m height. We double‐marked 38 individuals (RFID tag and radio transmitter) belonging to three species ( Barbus barbus , Salmo trutta and Thymallus thymallus ) from upstream and then released them 1.2 km downstream of the waterfall. A total of five automatic detection antennas were installed downstream of the waterfall and within the fishway, and the individuals were tracked with manual radio telemetry. We used several behavioural metrics to assess efficiency and attractiveness. The results indicate a lack of attractiveness of the fishway (overall rate of attraction < 25%). There was a higher detections at the waterfall (26 detections) than at the restitution channel (12 detections), where the entrance of the fishway was located. For individuals that reached the fishway entrance, the fishway efficiency was 12.5% for barbel and 6.3% for trout, with an average fishway entrance searching delay of 25 days for barbel. The lack of attractiveness led to numerous back‐and‐forth movements by individuals to find the entrance and the search for a substitute spawning habitat downstream. Our results indicate the need to improve the attractiveness of the fishway, in particular by improving the attraction flow.