A key  issue from  the point of view of fisheries management is that large, spawning, Salmon and trout adults can have considerable difficulty in entering smaller spawning tributaries and burns when Autumn rainfall is low. This has been shown, for Scotland, by the long-term study on Salmon spawning in the Girnock Burn on the Aberdeenshire Dee. The first information relating the numbers of Salmon managing to enter this tributary to the amount and pattern of water flow was given in Hay (1989). Recently this has been updated and extended by Tetzlaff et al. (2008), demonstrating the same point in more detail – that unless flows of the right quantity occur at the right time, entry of spawning Salmon can be restricted. As fish delayed by low flows become more “desperate” to enter the Girnock Burn, they will utilize smaller flows than they would have done earlier in the season. Instream structures such as Beaver dams would effectively remove this option to utilize less than ideal flows. Data on the relationship between water levels and the numbers of spawning Sea-trout running a small stream in the Tweed catchment is given in Appendix 2.

It was found that in the 2002/3 drought in Estonia, migrating trout could not pass downstream of Beaver dams and, along with other fish, became stranded in the small reservoirs above the dams and that most stranded fish did not survive. Those that did manage to migrate downstream were subsequently unable to migrate back upstream through the dams when flows came back up to normal. It was concluded that “the beaver dams may seriously inhibit the restoration of riverine fish fauna after it has become extinct due to extreme climatic conditions (eg. drought) or other factors”. (Tambets M, et al. (2005). Amplification of negative impact of Beaver dams on fish habitats of rivers in extreme climatic conditions. Journal of Fish Biology, 67 (Supplement B) 275-276).

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.