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Beavers (Castor canadensis and C. fiber) are known to be engineers of aquatic habitats and have the ability to strongly modify the physical and biological components of freshwater streams that provide important rearing habitat for juvenile salmonids (Oncorhynchus spp. and Salmo spp.) The return and expansion, as well as additional planned reintroductions, of Eurasian beavers to rivers holding important anadromous salmon and sea trout populations is considering considerable concern in Atlantic and Baltic seaboard regions because the impact of beaver dams on these fishes is unknown.

Study Area and Study Design
I conducted this study in the Trøndelag province within the Mid-Norway region. Two paired sites were in the Stjørdal basin (Stjørdal North and South sites, both with and without beaver) and the last pair of sites were on tributaries of the the Orkla River (Orkla beaver and no beaver). Each tributary study site was sampled from July-October. In each site, I measured habitat characteristics and sampled fish communities to determine if the presence of beaver dams altered the habitat use, growth, and movement of juvenile salmon and trout. I also sampled prey avabilibility including benthos from riffle habitats, falling inputs of adult and terrestrial invertebrates into the streams, and fish diets. I electrofished or minnow trapped each habitat type at each site three times each month using a capture-mark-recapture study design to measure fish distributions, species composition, population dynamics, and to tag individual fish with PIT (passive integrative transponders) tags. Fish movement was documented using pass-through PIT tag antennae. Fish were either tagged upstream or downstream, allowing me to measure directional movement. Additionally, each stream habitat was sampled using depletion sampling to make estimates of fish densities.

Habitat: Beaver-influenced and control site lotic reaches had similar physical characteristics and beaver ponds ranged in area from 1044 - 2966 m2.
Prey availability and diet: Prey availability was similar for beaver-influenced and control sites, both for in-stream macroinvertebrate samples and for the amount of prey that fell into beaver ponds, beaver-influenced streams and control streams. Interestingly, the juvenile fish diet samples from the two beaver-influenced sites contained significantly more prey items than the fish diet samples from the control sites.
Fish metrics: Species Composition.— The presence of beaver dams altered fish species composition (Fig. 3). The biggest impact of the beaver dams on tributaries was that trout and salmon were not captured or observed in any of the beaver ponds. In contrast, trout (Salmo trutta) almost exclusively dominated beaver-free tributaries (79-99%). Trout were much less prevalent in the below pond and above pond lotic reaches of beaver tributaries (9-31%), except for one higher gradient site where they predominated both above (100%) and below the pond (98%). The greatest proportion of juvenile Atlantic salmon were found in the above pond stream reaches (0-57%) and a small proportion were present downstream of the ponds (0.1-34%).
Population metrics.— My goal was to estimate population survival rate and detection probabilities using the capture-mark-recapture data, however too few fish were captured and recaptured and survival rates could not be estimated.
Growth.— We did not measure salmonid growth in beaver ponds because none were present in the ponds. There were no trout recaptured in the below dam lotic reaches of two of the beaver-dammed sites, but trout growth rates in the third beaver-dammed site (Stjørdal N.) were similar to trout growth rates in all three beaver-free sites in all months (Fig. 4).
Movement.— Out of the 759 individuals tagged in total for the six sites, 492 (65%) were detected by the PIT-tag antennae. Detection rates ranged from 39-79% of the total fish tagged at each site (Stj. N. Control 70% (190/272), Stj. N. Beaver 48% (57/118), Stj. S. Control 65% (64/98), Stj. S. Beaver 39% (18/46), Orkla Control 65% (70/107), and Orkla Beaver 79% (93/118)). Overall, a significantly greater proportion of individuals crossed the mid-point of the study reach in control sites compared to the proportion of individuals that crossed beaver dams (Fig. 5A; t = -4.3518 P = 0.04896). Conversely, a significantly greater proportion of individuals approached, but did not cross, the dam in beaver-influenced sites, compared to the number of fish that approached mid-reach but did not move in control sites (Fig. 5A; t = 4.2320 P = 0.01335). A similar proportion of fish remained either above or below the study reach and did not attempt to move in both the beaver-influenced and control sites (Fig. 5A; t = -0.3669 P = 0.7324). Although we did not observe or capture any salmonids in the beaver ponds we know that some individuals were present in the ponds long enough to move from the above pond lotic reach to or past the beaver dam (25 ind.), or to move from the downstream lotic reach up into the pond (21 ind.).
Density.— The total number of fish sampled in depletion sampling events was much higher than in capture-mark-recapture sampling because young-of-the-year fish were included in the sample. Juvenile Atlantic salmon densities were generally very low in the lotic sections of both beaver-influenced and beaver-free sites and there was no overall significant difference in salmon density by habitat type (t4 = 0.4326 P = 0.6876).

The presence of beaver dams had an effect on the habitat use and distribution of juvenile salmon and trout, but the dams did not block juvenile fish movement. No salmon or trout were captured in beaver ponds, but both species used lotic reaches above and below the ponds. Despite the apparent lack of use of beaver ponds by juvenile salmonids in Norway, we know that at least some individuals were present in the ponds long enough to move from the above pond lotic reach to or past the beaver dam, and that a few individuals were detected in the ponds for multiple days. Trout were generally less prevalent in beaver-influenced tributaries and salmon were most abundant in the above pond reaches. Despite differences in habitat use, the presence of beavers did not influence the growth or condition of juvenile trout and salmon in the lotic reaches of beaver-influenced tributaries. Beaver dams limited repeated movements by juvenile salmonids up and down the study reach, but a similar proportion of individuals moved up or down the study reach at least one time.
The beaver dams did not block the movement of juvenile fish in either the upstream or downstream direction. Rather, the dams limited the degree of repeated movements that individual fish made. Significantly more fish were able to repeatedly move up and down the study reach in control sites than in beaver-influenced sites. However, a similar number of fish moved upstream or downstream once, and a similar number of individuals stayed in the upstream or downstream reaches without attempting movement. Furthermore, because individuals were able to move past the dams, the dams did not keep juvenile salmonids from using lotic stream reaches upstream of the dams and ponds. Because habitats both below and above the ponds were still used by juvenile trout and salmon the overall impact of these small ponds is likely quite small. Although the beaver dams look like impassable barriers, the stream systems and dams are constantly changing and many opportunities for fish movement occur, especially following heavy rain events. The presence of beaver dams on the landscape in central Norway has a very low potential to negatively impact stream salmonid populations because the beaver dams don’t limit overall movement and use of other habitats downstream and upstream of the ponds. Furthermore, the actual area inundated by the ponds is very small compared to large pond complexes in North America and the presence of the dams and ponds on the landscape is quite rare.

Socio-economic impact of the project
Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) are an important economic, recreational, and ecological resource in rural areas of the European Atlantic and Baltic seaboard, from northern Norway to Spain. Radford et al. (1991) evaluated the salmon fisheries of Great Britain and, with capital asset values included, assessed each salmon to be worth €4273 in economic terms. Elliott (1989) considered the value of a single sea-trout to be €593. Additionally, the presence of salmon and sea trout stimulates the economy by providing jobs. For example, in a single Scottish River, the Spey, the recreational rod fishery for Atlantic salmon, sea trout and other fish species is estimated to create 401 full time equivalent jobs in the catchment (Butler et al. 2009), making it one of the main direct local employers. The Spey salmon and sea trout fishery alone contributes €13.5 million to household incomes annually. In addition to their social and economic importance, Atlantic salmon and sea trout play an important ecological role by providing important marine derived nutrients to freshwater systems when adults return from the sea to breed. The Atlantic salmon is listed in annexes II and V of the European Union’s Habitats Directive as a species of European importance. It is important to understand the different factors that can influence the production of salmon because salmon have experienced population declines over much of their range in recent years.
Eurasian beavers have the potential to strongly influence the significant economic and social resource that Atlantic salmon and sea trout populations represent because by damming rivers beavers alter the quality of freshwater spawning and rearing habitats. However, the influence of beavers on salmon and sea trout has been largely unstudied in Europe, which leaves a knowledge gap for management of growing beaver populations. This study shows that beavers have a very low potential to negatievly impact Atlantic salmon and sea trout populations in Norway.