The impacts which beaver could potentially have on fish stocks, and especially salmon populations, has become a growing concern for some anglers across the country. Although the Scottish Beaver Trial site at Knapdale, Mid-Argyll does not contain any salmon populations, it is important for the Scottish Beaver Trial partners to address this concern.
1. Do beavers eat fish?
No, this is a common myth. Beavers do not eat fish – they are completely vegetarian, preferring a diet of aquatic plants, grasses, herbs and shrubs in summer months with more trees and woody shrubs being taken in the winter.
2. What concerns exist about the potential impact that beavers could have on fish?
Some people are worried that beaver activities (i.e. the creation of dams) could have a negative impact on some wild migratory fish species, most notably Atlantic salmon, and that this may therefore have a damaging impact upon wild salmon populations in Scotland.
3. Why do beavers build dams?
The beaver is a semi-aquatic animal, spending the majority of their life in or close to the water. They prefer to inhabit still or slow-moving watercourses that are sufficiently deep to allow them to access to both their lodges, and burrows underwater, as well as escape predators by diving. Dams are usually only built when water levels are too low to allow safe access to lodges, burrows or feeding areas.
Dams are built from accumulated sediment and felled timber and on average are less than 10 metres long and 1.5 metres high. Water permeates through the dam and when deep enough behind the dam will also overtop and flow round the sides of the dam.
4. How often do beavers build dams?
On average only c.10% of beaver territories in comparable landscapes in Norway contain dams (Parker & Cock Rønning, 2007 and Duncan Halley pers. comm.) The trial site in Knapdale has many lochs, lochans and burns lochans with established still or slow-moving water flow. This is an ideal environment for the beaver.
5. Is Scotland is a special case in relation to beavers and salmon?
As a country which has successfully reintroduced beaver alongside existing salmon populations, Norway can act as a useful comparison. The beaver populations in eastern, mid, and northern Norway (i.e. the whole range outside the southeast) are descended entirely from reintroductions, and so comparable to reintroductions elsewhere. There are well-established beaver populations in five of the top ten (by catch weight) salmon fishing rivers in Norway (the Gaula, Orkla, Namsen, Numedalslågen and Drammenselva; six if the Tanaelva (partly in Finland) is included. Beavers have been on these rivers for decades and on many others where salmon and trout populations are found. Salmon can and do spawn in small tributary streams on many of these rivers and in Norway there is no perception of a problem caused by beavers. The lack of research on beaver damming in relation to salmonid stocks in Norway is a function of the general lack of perception of a problem among either researchers, the authorities, or indeed anglers.
6. Can beaver dams block salmon migration?
There is very little scientific research in this area. In their 2007 paper Mitchell and Cunjak suggested, but did not prove, that a group of beaver dams studied in Canada may have reduced salmon movements and spawning past them in some years but not in others.
The Scottish Beaver Trial partnership believes that had beaver activity been considered a detrimental impact on such an economically important species as Atlantic salmon, then this would have been known about for many years and subsequently there would be in existence a great deal of research and active management on the matter. Such research and management is lacking because this is not perceived as an issue in countries where beavers and salmon co-exist. Beaver have been reintroduced successfully in 24 other European countries and have co-existed with fish, including salmon, for millennia.
7. What impact do beaver activities have on fish populations?
In their 2001 review of the influence of beavers on fish Collen & Gibson produced a summary of the possible consequences to fish of beaver activities:
Possible positive effects:
- Habitat created for larger fish, providing angling opportunities.
- The debris cover provided by beaver lodges and food caches can attract some fish species (e.g. salmonids and perch)
- Hydrological effects are stabilised, so that bed scouring and bank erosion are decreased. More stable stream flows are beneficial to invertebrate and fish production.
- Water temperature stabilisation and warming could increase (fish) productivity in cold water streams.
- In streams with high sediment loads, sediment will be trapped in the impoundment (beaver pond).
- Coarse, particulate and dissolved organic matter is increased in the pond, providing food for invertebrates, through fungal and microbial pathways.
- Nutrients may be generated, increasing the fertility of the pond and downstream stretches.
- Acidity may be reduced, and aluminium may be immobilised.
- The dam collects organic detritus, and provides a substrate for lotic (flowing water) type invertebrates, providing food for fish downstream.
- Refugia can be provided in the pond at certain times.
Possible negative effects:
- Upstream migration may be impeded (larger dams; dams above culverts which were partial barriers).
Scottish Beaver Trial comment on the above effect. Note however that:
(i) Nearly all research currently available is from N. America and most suggests that both adult and juvenile salmonids are found both above and below beaver dams.
(ii) In their Norwegian study Parker and Rønning (2007) concluded that the presence of beaver is likely to have an insignificant negative impact on the reproduction of sea trout and salmon. Beaver dams considered by the landowner to be a problem for any reason are generally removed; on the Numedalslågen 5 of 14 landowners had removed dams at least once in the 46 years since beaver re-establishment, though never because they were considered to be hindering salmonid movements. This practice is not considered to pose a problem for beaver populations.
(iii) The Salmon & Trout Association (2008) stated “Research suggests the view that beaver dams are routinely impassable for anadromous species to cross is now untenable.”
- Warming of water temperatures may be detrimental in some marginal habitats for cold water fish.
- Spawning sites may be inundated and silted.
Scottish Beaver Trial comment on the above effect. Note however that:
(i) The only research currently available is all from N. America and only a potential issue in areas with limited spawning facilities and high beaver densities.
(ii) Conversely beaver damming effects may create additional spawning areas in streams with existing high silt loads.
- Warming water temperatures may be detrimental to cold water fish.
- The fish composition and interactions may change, so that less desired species for angling predominate.
- Habitat may be created for avian, mammalian or piscine predators, with negative effects on desired fish species.
- There is little scientific evidence to suggest that beaver dams significantly effect Atlantic salmon populations
- In small easily dammed streams, beaver activity can alter habitat features and may alter fish communities present (Rosell et al, 2005).
- Beaver activity has the potential to offer both positive and negative impacts for fish species, including salmonids (Collen & Gibson, 2001).
- It is entirely appropriate to compare beaver fish interactions in Norway with those that may be seen in Scotland.
- If beaver activity had a detrimental impact on such an economically important species as Atlantic salmon, then this would have been known about for many years and subsequently there would be in existence a great deal of research and active management on the matter. Such research and management is lacking because this is not perceived as an issue in countries where beavers and salmon co-exist.
- Under their statutory powers (Salmon and Freshwater Fisheries (Consolidation) (Scotland) Act 2003), Salmon boards have the power to keep salmon migratory routes open by “.the removal of nuisances and obstructions”. This means they will be able to manage or remove any beaver dams that they consider to be a hindrance to salmon migration. Such actions would be unlikely to have any impact on beaver populations.
- The Scottish Beaver Trial partners will continue to positively liaise with all stakeholders concerning the Knapdale trial, including representatives of the angling community.
Paper prepared by
Scottish Beaver Trial Project Manager
Scottish Wildlife Trust
Units 5 – 7 Napier Way
Cumbernauld G68 0EH
Tel: 01236 617113
Mobile: 07920 468556
Acknowledgements: Dr Duncan Halley
Norwegian Institute for Nature Research
Tel: +47 73 80 14 49
Parker, H. and Rønning, Ø, C. (2007) Low potential for restraint of anadromous salmonid reproduction by beaver Castor fiber in the Numedalslågen river catchment, Norway. River Research and Applications 23: 752-762
Mitchell, S.C. and Cunjak, R.A. (2007) Stream flow, salmon and beaver dams: roles in the construction of stream fish communities within an anadromous salmon dominated stream. Journal of Animal Ecology 76: 1062-1074.
Collen, P. and Gibson, R.J (2001) The general ecology of beavers (Castor spp.), as related to their influence on stream ecosystems and riparian habitats, and the subsequent effects on fish – a review. Reviews in Fish Biology and Fisheries 10: 439-461.
Gray, J. (2008) Briefing Paper – reintroducing Beavers into the UK. Salmon & Trout Association http://www.salmon-trout.org/issues_new_briefing_papers.asp
Rosell, F., Bozser, O., Collen, P., and Parker, H. (2005) Ecological impact of beavers Castor fiber and Castor canadensis and their ability to modify ecosystems. Mammal Review 2005, No. 3&4, 248-276