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A Fisherman's Tale: Monitoring fish-beaver interaction in Cornwall

Posted: Tuesday 6th June 2017 by CornwallBeaverProject

One hot topic surrounding beaver reintroduction is the effect the animals may have on fish. Recently, Peter Cooper went along to the first fish survey at the Cornish release site, which is hoped to start a long-term investigation into this relationship.

From an anecdotal point of view, I would conclude that beaver dams are very good for wildlife indeed. Certainly those I’ve visited are cornucopias of revitalised nature to my eyes. Where a sullen, stagnant stream barely flowed before, the sudden appearance of deep ponds that spread over rushes and onto willows has summoned forth the silver sparks of damselfly wings in the air all around you. Each footstep produces living firecrackers from the wet grass as hordes of tiny froglets & toadlets hop away, and the bones of these same animals are found within the otter spraint deposited not far from the beaver’s lodge.

Interestingly, the benefits of beavers to other wildlife has a sound scientific backing. A review of 63 studies into the effects of beaver activity on various species groups came up with an overwhelmingly positive effect, with 73% of the studied taxa being ‘winners’; of the remainder, 10% were ‘losers’ and 17% saw no change (Stringer & Gaywood 2016). These studies covered aforementioned taxa such as the insects, amphibians and (signs of) mammals I saw, but also plants, reptiles and birds.

One tricky group to assess however has been one with perhaps the greatest economic benefit- fish. Here the results tend to be a lot more variable, and much of the work has based in North America, which hosts a different species of beaver Castor canadensis with its own unique traits compared to our native C. fiber. Nonetheless, another review looking specifically at fish concluded that while beavers still had an overwhelmingly positive effect (184 studies), such as through providing new spawning, cover and foraging grounds, there was still a fair number of negative trade-offs through occurrences such as blocked migration routes and siltation (119 studies). (Kemp et al 2012).

To try and fill some gaps in our understanding, we’re making fish surveys one of our priority actions within our project at Woodland Valley Farm. We’re very happy to be working with several academic partners, and among them is the lead author of the aforementioned review, Dr Paul Kemp of the University of Southampton. Along with other beaver projects in Britain we’re looking to see what effect our Cornish beavers might have on fish abundance, and recently we were delighted to host one of Paul’s PhD students, Dr Rob Needham, on our first survey prior to the beaver’s release.

This involved a method called electrofishing, whereby an electrical current strong enough to stun but not kill the fish is delivered into the water, and the resulting fish found are measured and then released after they come round. The beaver release site consists of a stream that flows into a large pond along its route, and it was these two stretches either side of the pond that we surveyed.
Interestingly, by far the richest stretch was the one below the pond that contained many trout fry and several bullheads. The total catch was lower above the pond, but there were far more bullheads, and the few trout caught were much larger and more mature than the fry we saw downstream.

We’re hoping to continue this study annually over at least the next five years, so we can hopefully start to see patterns and ‘before-and-after’ effects emerging. This is just one of several studies we are working on with our partner universities, and in the next Cornish beaver blog I’ll be looking at one of the most important areas we’re looking at. For clues, think of Cornish weather…


Stringer, A. P. & Gaywood, M. J. (2016) The impacts of beavers Castor spp. on biodiversity and the ecological basis for their reintroduction to Scotland, UK. Mam. Rev. 46, 270-283

Kemp, P. S. et al. (2012) Qualitative and quantitative effects of reintroduced beavers on stream fish. Fish & Fisheries. 13, 158-181.

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