Hydropeaking strands and displaces larval and juvenile fish across species

Hydropeaking, the intermittent operation of hydropower, generates rapid flow fluctuations that disrupt river ecosystems. However, the mechanisms driving stranding and displacement of young fish under varying hydropeaking conditions remain poorly understood. Using a nature-like experimental facility, we conducted ∼1,000 hydropeaking trials involving >120,000 larval and juvenile fish from four species to assess how hydropeaking intensity, environmental conditions, and biotic factors influence fish behavior. Stranding and downstream displacement increased with hydropeaking intensity, particularly at night, in smaller fish, and lower water temperatures. Flow peaks with cold-water releases (thermopeaking) further amplify displacement. Fish demonstrated behavioral adaptation, reducing displacement over successive peaks, suggesting learning effects. The patterns were largely consistent across species. By identifying critical ecological thresholds, our findings inform hydropower strategies that minimize ecological harm while supporting renewable energy production.