Passage efficiency and behavioral performance of Schizothorax davidi through different sections of a long vertical slot fishway

Fish passage facilities are constructed worldwide to enable fish to pass anthropogenic obstacles (i.e., dams) and re-establish river connectivity. The construction of these facilities involves sophisticated engineering designed to attract fish and enable passage. The behavior of fish encountering these structures, particularly in long vertical slot fishways, has been poorly studied. This study was conducted on the Daduhe River in Southwest China to quantify the upstream passage efficiency and performance of tagged Schizothorax davidi in different sections of a long vertical slot fishway spanning 1228.3 ​m. The overall passage efficiency was 13% although the passage efficiency in seven sections (A0-A1, A1-A2, A2-A3, A3-A4, A4-A5, A5-A6, A6-A7) ranged from 43% to 100% reflecting differences in slope among sections. The highest passage efficiency was documented in rest pool sections with a slope of 0 (A3–A4, 100% and A6–A7, 100%) and during passage through the dam itself (A5–A6, 90.0%). The lowest passage efficiency was section A2–A3, where a garbage interception facility affected passage efficiency. Average transit time from A0 to A7 was 85.2 ​h and ranged from 8.8 to 237.6 ​h. Transit speed varied over various sections. The first section (i.e., A0–A1) had slow transit speeds which presumably was influenced by acclimation and recovery from tag implantation. After this, the transit speed had a slight increase but decreased again. Additionally, more than half (5 out of 9 fish) of the fish fell back after reaching the most upstream section and reascended the fishway which tripled the total passage time for those individuals. No significant diel activity rhythms were observed, but a strong bimodal distribution was noted in fish transit time when passing the first monitoring site. Cox-proportional hazards model showed that transit time was negatively correlated with body size but positively correlated with water level and water temperature. Our study provides valuable insights into activity when passing through a long fishway, which can inform design of fishway structures and operational patterns to improve upstream passage efficiency. Specifically, we recommend increasing the number of resting pools and maintaining appropriate water levels to reduce transit time and fallback frequency in long fishways.