For migratory fish, access is the priority. Even the poorest quality nursery area can produce at least some salmon and trout if spawning adults can get to it, but even the very best quality habitat cannot produce anything if spawners cannot reach it. Habitat “quantity” always has priority over habitat ”quality”.

A Beaver dam in Estonia. It is claimed that having such barriers across streams is “good” or “mildly beneficial” for migratory Salmon and Trout.  However, no similar claim would ever be made for a man-made weir of similar size.

Scotlands’s migratory trout and Salmon are by far and away the most economically important native fish. They breed throughout river systems including the smaller headwaters. The juveniles migrate downstream to the sea one to three years after hatching, returning as fully grown adults to migrate upstream to breed again. All Scotland’s native fish, in fact, are migratory (or are resident forms derived from migratory fish) because Scotland never had a land-link (and therefore, fresh-water link) with continental Europe.  As the ice retreated after the last Ice Age the only fish species that were able to colonise were those that could cross the sea. Anything that impedes this migration process affects the success of these species.

The pools created by Beaver dams may be good for larger Trout and other predators such as Pike (which is not native to Scotland). Beaver ponds will create habitat for these predators on Salmon and trout nursery streams which would otherwise be too shallow for them to live in. A series of Beaver ponds, each with large trout / Pike in them would be a series of  “ambushes” that Salmon and juvenile Trout migrating downstream would have to get through. If there was a large spate at the right time, the fish could get past quickly – without a spate, they would be very vulnerable.

The best account of the effect of such habitat changes on fish populations is given in a Swedish paper, Hagglund & Sjoberg, (1999). This showed that streams are changed by Beaver dams from being shallow waters dominated by small juvenile trout to deeper, slower, waters dominated by larger Trout and non-salmonids. Overall, Trout numbers in the stream sections with Beaver dams declined compared to sections without, due to this change-over from more numerous juvenile to less common adult Trout. Minnows (Phoxinus phoxinus) did the opposite, increasing in the sections with Beaver dams. There have been a number of papers in recent years from Norway, where Minnows have been recently spreading to coastal river systems, which show they compete with Trout juveniles and reduce their numbers and restrict their ranges. Spread of Minnows within trout spawning burns, such as is described in this Swedish paper as an effect of Beaver dams, is not therefore desirable.

In many river systems in Scotland, even large ones, it is becoming apparent that the resident Brown-trout fished for by anglers are largely produced by Sea-Trout females whose egg production dominates small streams. Spawning Brown-trout females also migrate up smaller streams from the deeper water in the main channels where they live. In such situations, habitat for larger trout on small streams is of no advantage to the population, since it is more important that the migratory females get as far and as fast upstream as possible to cover as much spawning area as possible.

The natural situation is for juvenile trout produced in the smaller streams of river catchments to drop down into the main channels, stocking the areas fished by anglers. Any reduction in the amount of habitat for juvenile trout in these smaller burns must have an impact on trout fisheries downstream. The replacement of juvenile trout by Minnows in such small streams is not an advantage to fisheries.

For enhanced growth to be an actual advantage to a fish population (as opposed to individuals), a significant proportion would have be living in Beaver dams, which would (on a stream of any length) require a lot of dams that would cause access problems. negating any benefits. There is, apparently, no study in the literature that shows what proportion of a total stream population lives in Beaver dams and benefits from the temporarily better growth to be found in them.
 

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.