Atlantic salmon smolts were tracked using acoustic telemetry in the River Deveron, Scotland, and adjacent coastal area. Higher rates of mortality were observed in the river than the early marine stage of migration. Mortality likely resulted from predation. Higher swim speeds were recorded in the early marine stage compared with the river, a potential predator avoidance behaviour. Overall river and marine migration success were linked to nights of lower lunar brightness. These findings highlight factors that influence smolt migration survival and behaviour