The Scottish Beaver Trial – Survey of fish populations 2008–2013Alan Kettle-White, Argyll Fisheries Trust

This study aimed to determine whether beaver activity has affected fish populations in the SBT trial site through impacts on migration, spawning or habitat

The full fish monitoring report will be made available at www.snh.gov.uk/scottishbeavertrial

Other Scottish Beaver Trial monitoring reports can be viewed atwww.snh.gov.uk/scottishbeavertrial

ResultsNo change to fish community composition was observed between surveys of fish populations prior to (2008-2011) and after (2011-2013) beaver felling and dam construction took place. Despite the limited time series of data available, and the unpredictability of dam construction activity, the study also showed that trout had spawned both above and below dams. Trout recruitment can exhibit considerable inter-annual variability and differences in fish densities between years cannot be attributed to the presence or absence of dams using limited datasets.

Over the timescale of the trial, no observable changes in the quality of in-stream fish habitat occurred in areas where beaver dams were present and where they were absent. Many of the streams surveyed during the trial were, however, already modified by existing land use.

The density of trout redds in the streams flowing into Loch Linne and Loch Fidhle did not appear to be affected by beaver activity. Natural flow conditions may also have affected the number and distribution of redds within the survey area. Nordic survey gill netting indicates that larger lochs in the trial area support brown trout populations, but a lack of suitable spawning habitat means that smaller lochs may not. Hydroacoustic survey was unable to quantify the abundance of fish present in the Knapdale lochs, due to their relatively small size, abundance of aquatic vegetation and shallow depth. The fish species found by all methods combined were: brown trout, European eel, three-spine stickleback, European minnow and rainbow trout.

MethodsActivity of beavers with the potential to affect fish habitat and fish distribution was recorded in two streams between 2011 and 2013; one flowing from Loch Losgunn into Loch Fidhle and one flowing from Loch Linne. This followed the collection of baseline fish data from 2008–2011. Electrofishing, which involves temporarily stunning fish with an electric current so that the species, number and size can be taken, was used in streams to assess the status of these populations. Walk-over redd (salmonid spawning site) count surveys were used to monitor habitat utilisation for recruitment by brown trout. The location of active spawning sites and number and size of redds was recorded. Stream surveys were carried out annually. Nordic survey gill netting was used to sample fish in a small number of lochs.

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Discussion– It is important to note that five

years is a short time period from the perspective of assessing the impact of beaver on fish populations, and that the unpredictability of beaver activity meant that investigations into beaver-fish interactions at dams in the current study only covered the period 2011–2013. Some fish species vary greatly in the number of young produced from year to year, for example, so changes seen in these studies may be a result of the fish’s natural biological cycles rather than a response to beaver-induced environmental change. This means that more data would need to be collected over a longer time period before definitive statements could be made.

– Where tree felling and dam building had taken place, the surveys undertaken during the trial period found that the fish community did not change.

– On streams where no beaver activity was found, fish population and redd counts showed similar fish distribution and abundance in 2012 and 2013 to the baseline and follow-up surveys in 2008 and 2011.

– The data collected by fish and redd surveys from within and outside the trial area (2008-2011) has established a baseline of information that may be used to assess future changes within the beaver trial site.

– Two small dams on the burn between Loch Losgunn and Loch Fidhle do not appear to have affected fish spawning or juvenile recruitment upstream. Dam building by beaver at the outflow of Loch Linne has been managed to maintain existing water levels for the benefit of aquatic plants. This activity does not appear to have affected the movement of brown trout from Loch Linne to their spawning and nursery habitat downstream so far.

– The survey data gathered to date at Knapdale indicate that fish utilised a wide range of habitats, some of which appear to have been heavily modified by land use. Fish populations may benefit from habitat improvement in streams which can influence the recruitment levels of young fish, but more data is required to determine whether beaver activity would deliver this.

– If beaver are retained after the trial period at Knapdale, future monitoring may be necessary to assess potential interactions, such as barriers to fish movement and loss of spawning habitat, and to inform management and mitigation.

This project was designed, funded and supported as a partnership between SNH and the Argyll Fisheries Trust. We are grateful to staff at SNH, SWT, RZSS and FCS for their help and cooperation during the project. SWT and RZSS also contributed funds to the Monitoring Programme.

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1. Adult brown trout Photo: Neil Mitchell/SNH

2. A typical red at a spawning site Photo: Argyll Fisheries Trust

3. Map of loch locations©Scottish Natural Heritage. ©Crown Copyright and database rights 2015 Ordnance Survey 100017908. ©Forest Enterprise, ©Forestry Commission. © The James Hutton Institute Ordnance Survey All rights reserved 10017908 2015

4. Juvenile brown trout Photo: Colin Bean/SNH