The reintroduction of the Eurasian beaver (Castor fiber) represents a major success story for European conservation, but concern exists over their impact on the production of Atlantic salmon (Salmo salar) and sea trout (Salmo trutta). These two species of fish are an important economic, recreational and ecological resource, with a single salmon and a single sea trout estimated at EUR 4 273 and EUR 593, respectively including capital asset values. Beavers have the potential to affect Atlantic salmon and sea trout populations by damming rivers and altering the quality of freshwater spawning and rearing habitats. However, little research has been conducted in this area, resulting in a knowledge gap with regard to how best to manage growing beaver populations. This information deficit was addressed by the EU-funded SALMONDAM project, which studied paired sites (with beavers and with no beavers) located on Norwegian rivers. Researchers studied the habit and sampled fish communities and prey availability to determine if the presence of beaver dams altered the habitat use, growth and movement of juvenile salmon and trout. Electrofishing and minnow trapping was carried out as part of a capture-mark-recapture study designed to measure fish distribution, species composition and population dynamics. In addition, individual fish were tagged with transponders in order to monitor their movements into and out of beaver ponds. Results indicated that despite differences in habitat use, the presence of beavers did not influence the growth or condition of juvenile trout and salmon. Furthermore, beaver dams and ponds did not block the movement of juvenile fish in either the upstream or downstream direction. Although the dams look like impassable barriers, the stream systems and dams are constantly changing and many opportunities for fish movement occur, especially following heavy rain. Since beaver dams did not limit overall movement and use of other habitats downstream and upstream of the ponds, SALMONDAM found that their presence within the landscape of central Norway has a low potential for negatively impacting salmonid populations.
Dams, Eurasian beaver, Atlantic salmon, sea trout, SALMONDAM