Hydropeaking-induced fish escape swimming behavior: Hydrodynamic mechanisms and conservation implications

The abnormal flow regimes caused by hydropeaking adversely affect freshwater ecosystems, which would be exacerbated in natural rivers by the compound channels, notably with rapid flow fluctuations posing survival challenges for fish. However, our knowledge of hydrodynamic mechanisms driving fish to deploy escape swimming to cope with hydropeaking remains sparse. This study investigated the fish escape swimming behavioral response to hydropeaking events with three ramping rates and two phases (rising or falling) in the nature-like compound channel combined with image-based individual tracking. Our results revealed that fish residence time coefficient peaked in the channel-floodplain slope area and decreased with increasing ramping rates. Fish preference for flow velocity, turbulent kinetic energy, turbulence intensity, and spatial velocity gradient showed similar bell-shaped distributions, each converging on distinct optimal ranges. Shorter rising or falling durations led to earlier initiation of fish channel-floodplain relocation, The critical hydrodynamic drivers of fish relocation were identified as the channel-floodplain mean velocity difference of 1.25∼1.34 BL/s (rising phase; BL = body length) and the drawdown rate of 1.96∼3.08 × 10^-2 BW/s (falling phase; BW = body width) respectively. By elucidating the mechanistic link between fish escape swimming and hydrodynamic-specific indicators, these findings offer a robust framework for ecologically informed fish conservation in hydropeaked rivers.