Genetic Structure and Migratory Behavior in Heavily Managed Partially Migratory Atlantic Trout (Salmo trutta)

Historical and contemporary complex stocking practices and management of salmonid species may result in complex genetic population structures within drainages and unforeseen effects on migration patterns. In order to examine such potential effects, we combined population genetics with passive telemetry in a heavily managed tributary to a large perialpine lake over seven years. Microsatellite analysis of 1,426 PIT-tagged partially lake-migratory Atlantic trout Salmo trutta suggested that fish belonged to seven different genetic clusters. The relative abundance of individuals belonging to the different clusters fluctuated highly across years. Migratory propensity varied significantly among genetic clusters, with certain groups exhibiting substantially higher rates of downstream migration to the lake than others, demonstrating that they cannot be regarded as a single partially migratory population. Larger individuals generally migrated earlier in the season, but the relationship between body size and timing differed among clusters, indicating size-dependent variation in migration strategies. Return migration from the lake to natal streams was low, with only individuals from a single cluster being recorded to complete the full migration cycle. This indicates that artificial stocking, while intended to support populations, may inadvertently compromise the persistence of migratory phenotypes in partially migratory species, demonstrating the need for genetics-informed management and conservation strategies.