Beaver are exclusively herbivorous, so do not directly impact on fish stocks.
Studies in Litlelva River, Norway of the effects of dam creation found no impediment to salmon and trout [Reference Halley and Lamberg 2001].
A recent study of the entire Numesdalen watershed, an important salmon and sea trout river in southern Norway with a capacity beaver population, found no evidence of any negative effect; indeed, salmon and sea trout catches increased over the period of beaver recolonisation. [ Reference Duncan Halley ]
More generally, fisheries authorities in Norway, where there is a valuable salmon sport fishing industry, consider that there is insignificant impact by beaver – and thus no need to fund research.
Siltation of beaver ponds may on occasion cover spawning sites, but this occurs on a very small scale and can be readily managed – indeed dams are often responsible for reducing water turbidity. Such ponds themselves are regarded as good fishing sites.
Beaver cannot carry Gyrodactylus salaris , a small (0.4mm) freshwater obligate ectoparasite of salmonids found in parts of Europe. This can only live for a short time away from a fish host, and cannot swim. It is normally spread by direct contact between fish. There are no known cases of any non-fish species acting as vectors, apart from humans.
There are, by contrast, beneficial effects for fish from the presence of beaver.
Beaver activity, by coppicing bank-side trees and creating pools, increases food supply by allowing more abundant growth of aquatic plants and invertebrate food supply – measurements of the latter suggesting 2-5 fold increases.
Oxygenation of water flowing over dams and retention of polluted silt also improves water quality, which again leads generates increase in invertebrate life forms.
Creation of deeper pools in a water course can stabilize water temperatures during extremes of weather. There are several recorded instances of beaver dams enabling fish stocks to survive during periods of sustained drought.
“Salmon, trout and beavers have lived in harmony together for millions of years. Those with experience of ‘living with beavers’ confirm that there is very little conflict with angling”. Welsh Salmon and Trout Angling Association website (www.wstaa.org). August 2006.
Halley, D.J. & Lamberg, A. (2001). Populations of juvenile salmon and trout in relation to beaver damming of a spawning stream. Pp. 122-127 in: Schwab, G. (eds.): The European beaver in a new millennium: Proceedings of the 2nd European Beaver Symposium, Bialowieza, Poland 27-30 September 2000.