Hybridization between Atlantic salmon (Salmo salar) and brown trout (S. trutta) in a restored section of the River Dälalven, Sweden

Enzyme electrophoresis revealed a significant increase of Atlantic salmon (Salmo salar) times brown trout (S. trutta) hybrids among ascending spawners in the River Dalälven. The hybrid frequency increased from 0.1% in 1989 to 3.1% in 1995. Electrophoresis also showed a high hybrid frequency (41.5%) among parr from a river section restored for natural reproduction in 1989. Restriction fragment analysis of a species-specific region of the mitochondrial cytochrome b gene showed unidirectional hybridization as all hybrids had brown trout mitochondrial DNA genotypes. The increased incidence of hybrids was evidently a result of natural spawning in the restored river section. Massive stockings of hatchery-reared fish and environmental constraint have forced Atlantic salmon and brown trout to common spawning grounds leading to a high level of hybridization. The unidirectional hybridization may be explained by the involvement of sexually mature Atlantic salmon parr acting as sneakers.