Scottish Marine and Freshwater Science Volume 6 Number 4: A Mapping Study of the Overlap of Potential Eurasian Beaver (Castor fiber) Habitat and Atlantic Salmon (Salmo salar) Distribution in Scotland

1. Introduction

Eurasian beaver, Castor fiber, are large semi-aquatic mammals that use trees and herbs as food and also material for maintaining lodges and damming rivers under certain circumstances (Kitchener, 2001). Atlantic salmon, Salmo salar L., use rivers for development of eggs and early growth before a majority of the population migrates to sea as smolts and subsequently returns to spawn (Mills, 1991). Salmon are an iconic component of Scotland's cultural and natural resources and have a high conservation status, being listed under the EC Habitat Directive, as well as supporting high value fisheries. Beavers occurred in Scotland's ancient landscape but have been extinct since about 1600 (Raye, 2014). In the intervening years, the nature of that landscape and the character of river ecosystems have changed substantially. Against this backdrop, the Scottish Government is currently assessing the likely consequences of allowing beavers to re-colonise (Gaywood, 2014). This assessment includes a consideration of potential interactions between beavers, fish and fisheries with particular focus on Atlantic salmon.

Beavers radically change the characteristics of riverine habitats and influence a range of physical and chemical processes, many of which may have either or both positive and negative effects on fish communities and populations (Collen & Gibson, 2001, Kemp et al., 2012). The overall net impact (positive or negative) can be expected to vary depending on the species of fish involved and the local environment in which they live. Atlantic salmon are highly migratory swift-water specialists (Armstrong, 2010). As such they could be expected to be more negatively affected than many other fish species by obstructions from damming activities and change from fast-flowing riffle to pool habitat. However, they may also benefit from local presence of beavers if these animals increase food supply, the availability of cover from predators, and/or quantity of suitable local habitat (Kemp et al., 2012). The potential extent of these possible positive factors depends largely on the overlap that can be expected between salmon and beavers. By contrast, where damming hinders fish migration, the negative effects of this obstruction may extend for the entire area upstream.

A basic requirement in determining where potential interactions may occur is to estimate the spatial overlap between the expected range of beavers and the distribution of salmon. This study uses a geographic information system ( GIS) approach to consider potential spatial overlap in six river catchments that cover much of Scotland's geographical and hydrological diversity: the Rivers Awe, Ayr, Conon, North Esk, Tay and Tweed (Fig.1). Salmon distribution is that recorded in recent years, whereas potential beaver distribution is based on predictions of suitable habitat, defined as existing broadleaf and mixed broadleaf woodland within 200m of freshwater habitats including rivers and wetlands, but excluding high gradient streams, tidal areas and land above 400m in elevation. The data are also considered as a comparison of larger and smaller river reaches, taking a 10-m width cut-off. This approach provides an estimate of how the area of overlap may differ between major rivers (predominantly main stems) and minor rivers (predominantly tributaries).