Multi-year Assessment of Smolt Migrations in a Large, Fragmented River : Hydropower and the Remnant Wild Atlantic Salmon (Salmo salar) of the River Dalälven

Anthropogenic river modifications can impede diadromous fish migration and passage success. We investigated downstream passage probabilities, migration speeds, and route selection of wild Atlantic salmon smolts in the lower 28 km of the river Dalälven, Sweden, where two hydroelectric power dams lack downstream passage structures. We also assessed how fish length and condition, diel period, water temperature, passage year, river discharge, and turbine operation influenced passage probability and the instantaneous rate of downstream passage. Over three years (2019–2021), 260 smolts were acoustically tagged and released either 28 km upstream of both dams or downstream as a control. The study area was divided into five sections delineated by acoustic receivers. Passage success was 53-61% among years. Section passage probabilities were lowest (71-95%) in lentic and hydropower-affected sections and highest (96-98%) in free-flowing sections. Migration speeds were also lowest through a hydropower-affected section (0.76 km/day) and highest in free-flowing habitat (18.5 km/day). Passage rate was positively associated with water temperature, fish length, and diel period. Our results identify lentic and hydropower sections as bottlenecks to downstream migration and demonstrate the value of telemetry for quantifying spatial variation in passage and migration speed.