Effects of Hydraulics on Fish Movement Behavior and Energy Expenditure in a Combined Technical Fishway

River fragmentation caused by hydropower development disrupts fish migration and threatens freshwater biodiversity worldwide. Fishways are widely used to restore connectivity, yet their effectiveness often varies among species due to differences in swimming ability and behavior responses to hydraulic conditions. This study evaluated a combined vertical slot–submerged notch with bottom orifice (VS–SNBO) fishway using laboratory experiments with two cyprinid species (Ctenopharyngodon idella and Opsariichthys bidens). We quantified hydraulic characteristics (velocity, turbulent kinetic energy, and strain rate), fish movement behavior, route selection, passage success, and energy expenditure. The VS–SNBO fishway achieved a overall passage success rate of 76.3% for both two cyprinid species. Among the successful upstream movement routes, 70.5% followed the vertical slot (VSL–VSL) route, despite higher velocities (0.60–0.83 m/s) and energy expenditure. Fish selectively utilized low-velocity zones near the orifice for recovery, indicating behavioral adaptation to heterogeneous flow conditions. Energetic analysis revealed trade-offs between passage efficiency and energy expenditure, with continuous movement routes reducing transit time but increasing metabolic demand. The combined fishway design provided multiple kinds of movement routes for different species to enable flexible movement strategies. These findings highlight that the VS–SNBO fishway can offer diverse hydraulics for multiple species and provide insights into how fish adjust their movement strategies in response to variable hydraulic conditions, and provide practical guidance for improving multi-species fish passage in regulated river systems.