Maintenance of a small anadromous subpopulation of brown trout (Salmo trutta L.) by straying

Summary 1. Microsatellite and isozyme loci variation were used to study structure and dynamics of a brown trout ( Salmo trutta ) population heavily affected by damming. The downstream area accessible for spawning was drastically reduced to a stream 1 km long influenced by regulated discharge. 2. Stocking of hatchery‐reared juveniles failed and the population is entirely supported by anadromous adults from neighbouring populations. 3. Temporal genetic stability is reported here. Some punctual between‐river genetic differences are likely because of different contribution from each neighbouring river through years. 4. High anadromy‐mediated gene flow produces a lack of genetic substructure in the region. The role of anadromous brown trout on maintenance of endangered small populations is emphasised.