Beavers and Fish
Although beavers are vegetarians and never eat fish there are many ways in which they interact. The positive benefits of beaver activity on fish populations largely derive from coppicing and habitat creation. Both of these functions create a much greater diversity of riverine habitat which fish can exploit. For example the submerged canopies of felled trees provide refugia from predators for fry and lays for large fish; the felling of large trees into water courses can create riffle systems which provide future spawning habitat while the diversity of pools and canals of varying depths and temperatures provides a broad habitat range for a diversity of fish species. Beaver activity boosts the habitat suitability for many invertebrate species leading to a significant rise in both species variety and biomass. Many of these provide a food source for fish.
The most significant potential negative impact of beavers on fish populations is that their dam complexes may have negative effects on fish migration and dispersal. They may also impact upon spawning gravels and water temperature. These potentially negative impacts could however be prevented through appropriate management. A wide range of evidence from Scandinavia and North America suggests that most beaver dams are passable to salmonids most of the time and that there is no clear evidence of a negative relationship between beaver activity and salmonids at a catchment level. Without appropriate management there could be negative effects on salmonid migration during dry autumns on smaller side streams with active beaver dams though this effect is likely to be highly site-specific. Most of the potential negative effects of beavers on fish are related to dam construction. While it is not likely that this activity will have significant effects on fish populations on a catchment level it is the leading issue about which concern has been expressed in Britain. If legally appropriate management strategies including culling are available where necessary to deter conflict then beavers are likely to provide a net beneficial impact for most fish species including those of commercial and recreational importance.
Kemp, P.S., Worthington, T.A., Langford, T.E.L., Tree, A.R.J., Gaywood, M.J., 2011. Qualitative and quantitative effects of reintroduced beavers on stream fish. Fish and Fisheries 13, 158-181.