Predation Risk and Depth Use During Migration of Atlantic Salmon Smolts Through a Fjord Impacted by Hydropower Discharge

ABSTRACT Atlantic salmon ( Salmo must survive the transition from river to fjord in order to migrate to northern latitudes where they feed and grow at sea. In the River Suldalslågen in southern Norway, hydropower regulated water discharge determines the timing and extent of river flows and fjord currents. To better understand how hydropower discharge influences smolt migration, we investigated early marine migration using acoustic telemetry and sensor tags in the fjord during 2 years. Smolts were tagged from the wild and a hatchery facility in 2023 and from the wild in 2024 using ID tags, depth sensor tags, and predation sensor tags and released in the river or towed to the fjord. Once smolts entered the fjord, they moved out to sea relatively quickly, and the route selected was influenced by the river flow from River Suldalslågen such that the northern route was infrequently used during periods of low river flow. Mortalities were concentrated near the river mouth and at one physical bottleneck in the fjord, which coincided with a bridge constructed over the fjord. Predation sensors helped resolve that a share of this mortality was attributable to aquatic predators, likely brown trout ( Salmo and a mix of other marine teleosts. Smolts navigated the fjord mostly close to the water surface, with minimal spatial variation in depth use but some clear diel patterns of moving deeper during daytime. Hydropower regulators should consider how artificial freshets in rivers influence downstream behaviour of the smolts even after exiting the river to the fjord.