An assessment of the relationship between habitat controls and Atlantic salmon and brown trout abundance using remote sensing and GIS in the river Eden catchment

Abstract

A new approach to the assessment of relationships between habitat controlling processes and salmon and trout abundance is presented and applied to the River Eden, Cumbria, UK. The potential of Geographical Information Systems (GIS), remote sensing, aerial photography, risk- based environmental modelling and electrofishing is demonstrated for the collection and integration of habitat and species abundance data at the scale of large catchments (˃1000km2). Based on this data, a key output of the research is the development of a spatially-structured, hierarchical database that allows hypotheses regarding the relationship between habitat controls and salmon and trout abundance to be tested at multiple scales. In particular, assessment has been made at the whole catchment-scale (2,300km2) and then at a series of sub-catchment scales (10-100s km2). Analyses at these two scales revealed contrasting results, emphasising that the scale of observation and analysis is crucial in determining the relationships identified. In the catchment-scale analysis, salmon and trout abundance were significantly correlated with the catchment-scale process of surface hydrological connectivity, both weighted and un-weighted by land cover. However, as the scale of analysis contracted, the spatial variance exhibited by catchment-scale processes declined and more local-scale riparian and in-stream habitat controlling processes such as cover and bank erosion became important. These results provide evidence in support of theories which suggest a hierarchical structuring of catchments where large scale processes provide the structure within which riparian and in-stream habitat controls operate. Results are also presented showing that fish abundance responds and maps onto to this hierarchical structuring in different ways depending on the potential for mobility at different life- stages and the location of habitat utilised within the landscape. Based on these results it is concluded that effective habitat restoration strategies must adopt a multi-scale approach in which in-stream and riparian scale actions are situated within the context of their controlling catchment-scale processes. The concept of hydrological connectivity is also recommended as an effective tool by which to assess the influence of landscape factors such as land cover on in-stream condition and salmon and trout abundance.