The law in Scotland requires “free” access for spawning salmon and sea-trout at instream structures at the times that they require it. Restricted access – i.e. fewer juveniles upstream than downstream – does not meet this requirement. While man-made weirs with fewer salmon fry upstream than downstream would therefore be illegal and would be required to  be dealt with by Fishery Boards , Beaver dams causing similar problems would be protected by European law (and by a substantial section of British public opinion)

The effect of all types of instream structure also depends greatly on the amount of rainfall in Autumn, when Salmon and Sea-trout are migrating upstream into the headwaters. In a wet season, obstacles can be easy to pass, in a dry season, very difficult or impossible. Beaver dams, too, are not necessarily barriers in all years and in all places – it depends very much on the amount of rainfall during the short, final, period of fish migration when they are trying to enter smaller streams.

However on many rivers, especially the East Coast Rivers of Scotland, Autumn water flows are not reliable and migrating fish often have trouble reaching spawning grounds in smaller streams, even without any obstacles. Beaver dams would make such situations even worse. There is, however, very little information relating water flows to the degree of  upstream penetration achieved by spawning fish –most comes from the Girnock Burn trap, run by Fisheries Research Services, Faskally, since the 1960’s (also, see 10 below).

All instream structures (bridge foundations, weirs, fords, fish passes, etc.)  can be barriers to fish movement especially in dry Autumns when these are more likely to be total rather than partial. The only way to know if they are impediments or not, and to what degree, is quantitative sampling of salmonid juveniles upstream and downstream of them. Beaver dams are, from a fisheries management point of view, simply another class of instream structure and need to be subjected to the same examination. If this upstream / downstream quantitative sampling is not done, then it cannot be said if an instream structure is a total or partial obstacle or no obstacle at all. Such sampling has to be done at a range of water flows, in wet and dry Autumns. Without such sampling* no valid statements can be made as to whether Beaver dams are obstacles or not. Beaver dams can be the same size as man-made weirs or fords. It cannot be seriously suggested (though it has been) that while the latter two are often problems for fish migration, the former never are.

The only quantitative study of this sort concerning Beaver dams on a small tributary (Mitchell & Cunjak, 2007) found that the (Canadian) Beaver dams were obstacles to some degree in all but 2 out of 14 years, with dry Autumns making them total barriers.

While Scottish fisheries managers have, obviously, no experience of Beaver dams, they do have considerable experience of the other classes of instream structures and, in fact, spend a significant amount of their time and resources assessing the impacts of these on spawning runs and removing or easing those that are problems. In the absence of any evidence that Beaver dams are somehow completely different from other types of instream structure, the experience of Scottish fisheries managers with such structures must be considered relevant to their assessment of the situations that will be created by Beaver dams.

*See Appendix 2 for an example of this sort of assessment from the Teviot Water, a tributary of the Tweed.
 

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.