We developed an individual-based model (IBM) to understand the effects of hydropeaking on growth, survival and distribution of age 0+ to 1+ juveniles for highconservation value populations of native brown trout (Salmo trutta) and Atlantic salmon (S. salar) in river Gullspång, Sweden. We parameterized and applied inSTREAM (7.2-SD) and calibrated the model by comparing predicted versus observed growth under the current hydropeaking regime (n=>1,200 model fish for 365 days). Our objective was to model growth, survival and distribution under flow scenarios with and without hydropeaking. We observed that hydropeaking generally resulted in modest (10%) negative effects on growth and survival of both species.Survival was more affected than was growth, smaller fish more affected than larger fish. On-peak (high) hydropeaking flows resulted in less profitable feeding conditions (less growth) and higher predation (lower survival). Thus, inSTREAM 7.2-SD appears to capture ecologically-relevant behavioral patterns under hydropeaking, for example, habitat selection, in response to rapid flow changes. Understanding such patterns for large rivers via manipulative field studies, even if possible, would be time-consuming and costly. Our study demonstrates the potential of IBMs as powerful tools for testing research questions and assessing and prioritizing alternative management strategies in regulated rivers.

The growing demand for fossil-free energy makes hydroelectric power generation an important technology for flexible and renewable energy (Ashraf et al., 2018). Hydropower plants alter the natural flow regime of up-and downstream river reaches, however, with potential negative effects on aquatic organisms (Kuriqi et al., 2021;Poff et al., 1997). Particularly when electricity is generated to meet sub-daily fluctuations in energy demand, rapid changes in discharge downstream of hydropower plants can be extreme (Carolli et al., 2015;Cushman, 1985); these hydropeaking flow regimes, which result in rapid changes in stream flow patterns, have been shown to severely affect riverine biota (Moreira et al., 2019;Smokorowski et al., 2011;Zimmerman et al., 2010). The ecological effects of hydropeaking on riverine organisms are linked to alterations in the natural river hydrology, morphology (e.g. river depth, width, velocity, riverbed material and grain size) and water quality (e.g. temperature and turbidity) (Charmasson & Zinke, 2011;Hauer et al., 2014). Specific to fish and their instream habitat, alterations in river depths, velocity and water temperature are the major sources