Beavers are herbivores and pose no direct threat to fish stocks.
Positive impacts include the creation of new feeding pools and regulation of water flow, but beavers do not dam fast flowing or large rivers. Normally dams are built on small burns and streams. Dams can increase the production of invertebrate food (invertebrate biomass typically 2–5 times that of unmodified streams), provide drought refuge sites for fish and holding areas for migrating salmonids.
Changes to the fish population will vary according to the location of a dam, topography and local conditions. Beavers favour floodplain habitats as opposed to fast flowing high gradient streams suitable for spawning.
Won’t dams impede movement of fish?
The impact of beavers on fish migration depends on the conditions on site. Most available reports consider dams to be insufficient to inhibit migration during periods of high flow, but some migration may be impeded in low flow conditions, with the impediment being removed when flow increases. There is little evidence from Europe and North America to indicate significant detrimental impacts of beaver on salmonid fish.
In Norway, beavers are not perceived as a problem on salmon rivers and the only, albeit limited, research work available indicates that salmon can successfully spawn upstream of beaver dams. Moreover, as trial animals will be monitored closely, any animals straying beyond the agreed boundaries of the trial site will be rapidly returned.
