Parker & Ronning (2007) actually found that Beavers on smaller streams preferentially chose Salmon and Sea-trout spawning areas to set up colonies on, i.e. there were more often colonies in such areas than would be expected by random – and, obviously, when Beaver build dams it is in the areas that they colonise.

 European Beavers build dams on streams with gradients of 1 to 2.5% (Hartmann & Tornlov, 2006) and Atlantic Salmon spawn in areas with gradients of 3% or less (Hendry & Cragg-Hine, 2000). Canadian Beavers can build on gradients up to 4% (Hartmann & Tornlov, 2006).

In lowland river catchments spawning gravels are not confined to “fast flowing upland streams”, they are very widespread. Hartmann & Tornlov (2006) quote evidence that (Canadian) Beaver dams can affect 30-50% of the total length of 2nd to 4th order streams in Quebec. Fish have to be able to reach spawning gravels, so a dam anywhere on a stream, whether near gravels or not, is a potential problem. A dam on a gentler, more downstream section would have to be passed by fish heading for upstream sections.

Preferred spawning velocities for salmon are, in fact, 0.25 to 0.90 metres per second (Hendry & Cragg-Hine, 2000). Beavers and spawning Salmon both like and dislike the same water velocities. Beavers prefer the tail ends of pools in which to build dams (Parker & Ronning, 2007) and these are exactly the sites that Salmon and trout prefer to spawn at. 

The seriousness of the problems that can be caused by Beaver dams is highlighted by Beaver “activity” being listed as a threat to Sea-trout stocks in Lithuania (Kesminas, Leliuan & Rymantus, 2006). Sea-trout are considerably more agile than Salmon in reaching spawning grounds so any barrier that is a problem for Sea-trout is certainly a problem for Salmon.
 

Whilst this paper has been prepared by The Tweed Foundation on the basis of information that it believes is accurate, any party seeking to implement or otherwise act on any part or parts of this paper is recommended to obtain specialist advice. The Tweed Foundation does not accept responsibility under any circumstances for the actions or omissions of other parties occasioned by their reading of this paper.