ABSTRACT1. Man-made barriers such as dams affect the movement of aquatic species, reducing gene flow and genetic variability. Such encroachments may also lead to selective changes in life history and behaviour. Hydropower construction worldwide has fragmented many previously continuous fish habitats, leading to loss of populations and production. It is therefore important to assess potential impacts on habitats before such developments begin.2. Here, the potential ecological and evolutionary consequences of planned hydropower development on two migratory salmonid fishes -brown trout (Salmo trutta) and European grayling (Thymallus thymallus) -were assessed, combining telemetry with population genetics. Almost 200 fish were radio-tagged and tracked weekly between March and November. Using microsatellite markers, the genetic population structure was assessed and the number of migrants among different river sections identified for both species.3. Overall, both species displayed extensive within-and between-river movement, with larger home ranges in grayling than in trout. Regular movements between distinct spawning, feeding and wintering areas were common. These vital habitats were often located within areas of planned hydropower development.4. Both species exhibited significant population genetic structuring within the study area, with waterfalls acting as impassable barriers to upstream gene flow for grayling. The structuring was more developed for trout than for grayling. However, downstream gene flow was common, resulting in a highly admixed trout population below a waterfall.5. The large-scale movement patterns and extensive connectivity of the system indicate that habitat fragmentation and changes in water flow will adversely affect both species, but most strongly the trout. The reduction in water flow over large and productive stretches of the river might select for less migratory genotypes in both species. The loss of particular genotypes may reduce the biocomplexity of the system and overall population resilience.

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The movement of 34 large (39-73 cm standard length) brown trout Salmo trutta was monitored using radio telemetry for up to 74 days in Brumunda, a small Norwegian river (mean annual discharge 3Á3 m 3 s À1 ) flowing into the large Lake Mjøsa. The maximum range of movement in the river was 20 km. No clear relationships existed between individual movement and water discharge, temperature and barometric pressure. Brown trout migrated at all levels of water discharge. At low discharge (<2 m 3 s À1 ) movements were nocturnal. A weir 5Á3 km from the outlet restricted ascending brown trout at low (c. 6 C), but not at high (c. 8 C) water temperatures. Spawning occurred in September to October and tagged individuals spent 2-51 days at the spawning sites. Mean migration speed from tagging to when the fish reached the spawning area, and from when they left the spawning areas and reached the lake was 1Á0 and 2Á3 km day À1 , respectively. All tagged brown trout that survived spawning returned to the lake after spawning. # 2004 The Fisheries Society of the British Isles