Depth use of wild Atlantic salmon post-smolts migrating through fjords

Abstract Juvenile Atlantic salmon ( Salmo salar ), known as post-smolt as they enter the sea, undergo an arduous migration from their natal rivers to their feeding grounds in the North Atlantic Ocean. It has become increasingly important to understand how post-smolts use the fjord environment as they migrate in order to properly assess the potential threats posed by large-scale salmon farming. Chief among these threats is the increased numbers of ectoparasitic salmon lice ( Lepeophtheirus salmonis ) inhabiting the water column, which are known to have specific depth preferences based on light, temperature, and salinity. Here, we present data on the depth use of wild Atlantic salmon post-smolts migrating through fjords. We aimed to investigate to what extent depth use varied throughout the fjord and from year to year. Using acoustic telemetry, tagged fish from four rivers in two fjords in western Norway were monitored as they migrated towards the open sea during two consecutive years. We found that post-smolts predominantly migrated in the top three meters of the water column throughout the length of both fjord systems. Among 61 successful migrants and 7013 detections, 98% of detections were in the top three meters of the water column. This corresponds well with past findings showing similar depth use in hatchery-reared smolt and in adult Atlantic salmon kelts returning to their feeding grounds after spawning. We found little evidence of a consistent diel pattern in depth use. Our results support assumptions of representative sampling when trawling the upper portion of the water column for post-smolts in order to estimate sea lice infection rates and may improve the precision of efforts to model salmon lice infection risk. The results may also be valuable in evaluating other threats to wild salmon.