Diel patterns and migration behaviour of sympatric Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) smolts during seaward migration

Migrations between different habitats to maximise fitness are well documented in many organisms, although such migrations imply a potentially great risk when moving between habitats. For anadromous salmonids, these migrations occur between the freshwater stage in their natal rivers to coastal waters during spring/summer, where they can potentially experience high mortality rates. This study investigated the riverine migration dynamics of Atlantic salmon ( Salmo salar ) and sea trout ( Salmo trutta ) smolts with a multiple camera array over three consecutive years (2021–2023). A total of 19,238 individuals were recorded, and the dynamics of schooling behaviour and diel activity patterns were examined with respect to both intra- and interspecific traits associated with migratory behaviour. The observed pattern describes the migration dynamics for sympatric natural stocks of Atlantic salmon and sea trout over a 3-year period with little-to-no interferences with natural behaviour. Results indicate a clear diel pattern in schooling behaviour, governed by the influx of natural light, with clear species-specific differences in migratory behaviour. The observed diel pattern was less dominant later in the migration period, revealing daylight length as a factor altering migratory behaviour during the smolt run. The observed changes in migration dynamics could possibly be adapted as an anti-predator behaviour during river migration, where night/dark offers protection from visual predators, whereas safety in numbers aids predator detection and avoidance during daytime.