Abstract A pan-European, classification of the extent of environmental degradation from chemical, physical and biological pressures on fish communities as a precursor to assess the ecological status of running waters based on fish is proposed. Twenty-four potential pressures acting on fish communities at three different spatial scales (river basin, segment and site) were identified and class boundaries for high, good, moderate, poor and bad status, based on existing data and/or expert judgement, were defined. Four pressures (hydrological regime, morphological conditions, toxic or acid conditions, nutrients and organic load) were found to describe the majority of degradation at a specific site and these were combined into a single pressure variable to describe impact at each location. Principal Component Analysis showed that the four variables were correlated with other physical and chemical variables not included in the combined pressure variable. However, biological pressures, e.g. introduction of fish, and longitudinal connectivity were not well correlated, suggesting that two dimensions of human impact on stream fish were poorly accounted for. Low-resolution Geographical Information Systems (GIS) data (1 km grid) on land use and population density correlated well with the four chosen pressures, suggesting it is possible to use standardised GIS data to aid pre-classification of stream degradation.
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