Search and Passage Are Decoupled for Migrating Trout Kelts at Multi‐Exit Dam

ABSTRACT Longitudinal and lateral river connectivity have been drastically reduced over time, with hydropower development as a major driver of fragmentation. While fishways are widely implemented to support upstream migration, downstream passage remains a major challenge. For salmonids, most downstream studies focus on juvenile fish, while post‐spawning adults (kelts) have received less attention. Here, we studied the forebay behaviour and downstream route choice of brown trout ( Salmo trutta ) kelts at a run‐of‐the‐river hydropower plant with multiple passage options using hydroacoustic telemetry. Kelts spent an average of 9 days in the forebay, with strongly right‐skewed residence times. Activity was approximately uniform across the diel period. Swimming depth was generally mid‐water to deep (median depth 3.9 m), and kelts were deeper during daytime, during upstream‐directed movement and at higher river discharge. Kelts switched between shallow and deep positions more often during night than day, and much more during up‐ and downstream movement than during episodes when horizontal movement was limited. Encounters with potential passage routes were not proportional to discharge distribution: spillways were underrepresented, whereas the fishway and ice gate were overrepresented. Once a kelt encountered a route, passage probability was higher at night than during the day, increased with higher mean opening velocity, and was higher for openings closer to the surface. Inter‐individual variation was substantial across analyses. Our results provide information for the design and operation of downstream‐passage measures for large‐bodied post‐spawners.