Migration and genetic structure of Salmo salar and Salmo trutta in northern Swedish rivers

This thesis focuses on the migration of Atlantic salmon Salmo salar L.) and sea trout (Salmo trutta L.) before, during and after spawning in two northern Swedish rivers and influence of habitats and environmental factors on these activities. In addition, genetic structure of brown trout and genetic impact of stocked salmon and sea trout juveniles from foreign populations on native stocks was studied. Migrations and genetics are related to anthropogenic factors as historical log-floating and hydropower production. On their spawning migration, radio-tagged salmon (n=128) and sea trout (n=86) showed that salmon mainly migrated directly to their spawning area, while sea trout exhibited several up- and downstream migrations. Migration speeds was lowered or delayed at steep sections and was higher (25 kmd-1) later in the migration period compared to early in the run (8 kmd-1) and negatively correlated with water flow. Salmon and sea trout spawned in the main stem in the river Vindelälven, 230 km and 60 km upstream, respectively, while sea trout in the river Piteälven spawned in various tributaries. A majority (90%) of the sea trout survived after spawning and overwintered in the rivers. These kelts migrated seaward in spring at water temperatures of 4–6°C whereafter 69% and 25% was lost at power-station passage in the rivers Vindelälven and Piteälven, respectively. Genetic differentiation among brown trout with different life histories was high (FST=0.09) while weak differentiation among sea migrating populations was revealed (maximum FST of 0.03). There was no correlation between genetic and geographic distance among all samples while among samples within the Vindelälven such relationship was significant. Within the Piteälven a subpopulation of sea trout was found in one tributary. Genetic assignment supported that tagged sea trout was homing to this stream. Despite longterm stocking of foreign salmon stocks in the river Sävarån mixed-stock analysis were unable to detect genetic introgression. Likewise, genetic impact of stocking sea trout in the Sävarån seemed low, however, with overlap in genotypes genetic introgression could not be ruled out. Restorations of connectivity and river habitats were hypothesized to positively influence salmon and sea trout production and gene flow among populations