Energetic and behavioural metrics for evaluating multi-slot fishway performance

Fishways are structures built across dams to restore river connectivity and allow fish to pass through. Their effectiveness depends strongly on fish swimming behaviour and interaction with the flow hydrodynamics within the fishway. However, most fish-passage efficiency is still measured by counting the number of fish that successfully pass the structure. This metric alone does not capture the behavioural difficulty or the energetic cost experienced by individual fish. Therefore, in this study, we investigated the effort required for fish to pass a multi-slot fishway (MSF) using fish-tracking experiments. Ten Labeo rohita (rohu) were tested in a multi-slot fishway operated at two discharges (15 and 25 L/s). Individual fish trajectories were used to quantify passage success, the number of attempts, and swimming speed. Passage efficiency was similar at both discharges; however, the effort required to pass through the fishway differed. Fish at 15 L/s made more repeated approaches before passing, whereas fish at 25 L/s typically passed in fewer attempts. To quantify energetic cost, we calculated an effort index based on the swimming speed, representing the mechanical power used during passage. This index was higher at 15 L/s, indicating greater energy use due to repeated searching and failed approaches. Fish trajectory showed that fish at lower discharge spent more time in low-velocity and recirculating flow regions, while higher discharge produced a clearer attraction flow that guided fish directly to the slot. This study introduces a behavioural and energetic metric that complements traditional passage efficiency and provides a more informative measure of fishway performance. These results suggest an important role for hydraulic conditions in promoting guidance and movement, and highlight the importance of including behavioural and energetic considerations when evaluating passage performance.

Keywords: Fish passage, Fish behaviour, multi-slot fishway, swimming energetics, river connectivity