Qualitative and quantitative effects of reintroduced beavers on stream fish

Reintroduction of beaver (Castor spp) may facilitate rehabilitation of freshwater habitats providing a cost-effective sustainable means of improving ecological conditions. Despite extensive research, debate and consultation, a general consensus on the impact of beaver on fishes has proven elusive because of variability in biological response. This paper provides a systematic review of the impacts of beaver dams on fishes and fish habitat based on a meta-analysis of the literature and expert opinion. Research is regionally biased to North America (88%). The most frequently cited benefits of beaver dams were increased habitat heterogeneity, rearing and overwintering habitat and flow refuge, and invertebrate production. Impeded fish movement because of dams, siltation of spawning habitat and low oxygen levels in ponds were the most often cited negative impacts. Benefits (184) were cited more frequently than costs (119). Impacts were spatially and temporally variable and differed with species. The majority of 49 North American and European experts considered beaver to have an overall positive impact on fish populations, through their influence on abundance and productivity. Perceived negative effects related to the movement of aquatic organisms in tributary streams, including upstream and downstream migrating salmonids, and the availability of suitable spawning habitat.