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

3. Results

The distribution patterns of potential beaver and known salmon habitat and the degree of overlap at whole-catchment and major and minor river-scales are summarised in Table 1. At the catchment scale in the six rivers (Awe, Ayr, Conon, North Esk, Tay and Tweed), the percentage wetted area of known salmon distribution likely to occur in proximity to potential beaver habitat (termed percentage overlap) ranged from 47-73%. In all six rivers, the degree of overlap in the habitat distributions was greatest in major rivers (predominantly main stems) >10m in width, ranging from 54 to 87%. The highest percentage overlap in the River North Esk reflected the extensive woodland close to major rivers in this catchment. In minor rivers <10m in width, that is predominantly tributaries, percentage overlap ranged from 15% to 59%. In each case the higher or lower percentage values reflect the extent to which both major and minor rivers are currently accessible to salmon, and feature broadleaf riparian woodland. For example, percentage overlap is particularly low on the River Conon minor rivers due in part to limited accessibility to salmon and in part to comparatively low levels of suitable beaver woodland. Conversely, minor rivers of the River Tay feature relatively more suitable beaver woodland compared to the other catchments studied, resulting in a high overlap percentage.

Details of the salmon and beaver distributions in the six catchments are illustrated in Figure 2(a) - 2(f). From examination of the distributions it is evident that suitable beaver woodland is generally well dispersed throughout the major river reaches, while patterns of distribution are more variable in minor rivers. In many cases the distribution of salmon extends above the treeline to open moorland areas with no woodland, though in some cases suitable beaver habitat exists.