Beaver Library

Expansion of populations of beaver Castor canadensis in northern Wisconsin has raised concerns over warming of coldwater fish habitats as a result of impoundments created by the mammals. We examined temperature with a network of electronic thermographs that recorded hourly water, air, and soil temperatures on four headwater streams occupied by beaver during summer 1990 and 1991. Stream temperatures followed air temperatures, even near groundwater sources. There was no consistent relationship between size or number of beaver impoundments and the degree of downstream warming. Large impoundments, although often warming downstream temperatures slightly, dampened temperature fluctuations immediately downstream. Local groundwater inflow and vegetative and topographic shading also dampened warming by impoundments. Several beaver impoundments were removed to evaluate ensuing temperature changes. Removal of beaver dams did not generally reduce the difference between upstream and downstream temperatures; in some cases dam removal increased the warming rate. Direct thermal benefits of dam removal in headwater streams may be outweighed by the potentially disruptive effects on the composition of fish and invertebrate communities downstream. It is suggested that management focus on relating topographical and geographical attributes to the potential for substantial groundwater discharge and to suitable summer temperatures for coldwater species such as brook trout Salvelinus fontinalis.

In degraded river systems, beaver dam analogs (BDAs) are an increasingly popular low-tech treatment used to reduce water velocity, increase floodplain connectivity, activate secondary side channels, and thus increase juvenile salmonid rearing habitat. However, BDAs may benefit non-native species as well, posing a potential conservation risk. In the Lemhi River basin in Idaho, an Intensively Monitored Watershed program quantifies responses of salmonid populations to restoration actions intended to remediate the effects of agricultural development. In 2017, BDAs were installed in Hawley Creek, to improve habitat conditions for native Rainbow Trout (Oncorhynchus mykiss). Non-native Brook Trout (Salvelinus fontinalis) also reside in Hawley Creek. We evaluated native and non-native salmonid responses to BDAs to understand their implications for achieving restoration goals. A BACI analysis was used to evaluate the effects of BDAs on the intrinsic rate of population growth of Rainbow Trout and Brook Trout. Demographic analysis was used to estimate the effects of treatment (i.e., BDA or control) on abundance and demographic rates of Rainbow Trout and Brook Trout. Our results suggested that Brook Trout did not displace Rainbow Trout in sites with BDAs, indicating that BDAs may not greatly change conservation risk to native salmonids. Rainbow Trout abundance and apparent survival in Hawley Creek post-treatment were typically higher than for Brook Trout. Our study suggests that BDAs in degraded western streams did not favor Brook Trout over Rainbow Trout.