The research on the impacts of beaver dams on fish is somewhat limited (certainly in the UK) and the results do not give a clear picture of ‘beavers are good’ or ‘beavers are bad’ for fish. Much of the research is from the USA and focuses on their native fish, principally different forms of Pacific salmon which do not have the same habitat requirements as our native fish in Europe, such as Atlantic salmon or brown trout.
It may well be that, where beavers build dams, the habitat diversity they create is good for some (maybe many) fish species.
Some research talks about ‘fish’ biomass increasing with beaver dams without acknowledging that the fish species above and below the dam are different – a shoal of minnows or sticklebacks in a beaver pond is not the same as a salmon in a river.
Much of the research and debate on fish and beaver interaction tends to focus on the impact of dams on the upstream migration of salmon (principally) and trout. This is important, but not the whole story. Young salmon and sea trout migrate downstream as smolts in the late spring, and they do so in shoals, tail first. Young resident brown trout also move downstream as they grow.
This downstream migration of trout and salmon could also be also be negatively impacted by beaver dams delaying migration and leaving young trout, salmon and sea trout smolts vulnerable to predation in the ponds above the dam. There is very little research on beaver dams and smolt migration, but data from the River Tweed on salmon smolts and a man-made weir showed that 80% of the smolts were preyed upon whilst held up above a weir.
Whether trout and salmon populations are negatively impacted by beaver dams depends on the size of the dam and the water levels. Small leaky dams with good consistent flow through, over and around the dam are likely to be passable by both adults moving upstream and juveniles moving downstream, but high, solid dams in low flows may be completely impassable both up- and down-stream.
If the river has sufficient energy, it may create a by-pass channel around the dam which fish can use for upstream and downstream migration given sufficient water level. However, that is assuming that there is sufficient space and that the river is not already incised or otherwise constrained by stone revetment or flood bunds.
There is little or no evidence of the impact of beaver dams on many other fish species – grayling (which don’t tend to jump barriers), eels, lamprey and many other species need connected rivers rather than rivers fragmented by barriers. With between 10,000 and 40,000 man-made barriers in rivers in England, our fish populations are already struggling, so we must be careful about further endangering our fish when we re-introduce beavers.
Further reading:
Professor Jonathan Grey of the WTT and Lancaster University has reviewed the scientific literature and numerous reports from other organisations. Click on the links below to read the summary paper or full paper:
Wild Trout Trust beaver-trout interactions: summary
Wild Trout Trust beaver-trout interactions: full report
Professor Ian Cowx was commissioned to carry out a review for the Angling Trust and other groups in December 2020. Review of Evidence of Interactions between beavers and fish and fisheries in England and Wales.