The hydromorphology index IHM based on fish requirements

Stream ecosystems are diverse and dynamic habitats that are strongly influenced by the direct and indirect consequences of human interventions. Several initiatives have been started all over Europe to satisfy the European guidelines for the protection of local water bodies, but a standardised procedure across national borders fulfilling all relevant aspects and parameters of the Water Framework Directive (WFD) does not exist. In this context, we propose a new methodical approach based on salmonid fish populations to assess stream quality to increase the comparability of individual assessments due to standardized survey and evaluation methods for hydromorphology. Our approach is based on the Austrian and Italian methods, which account for the European standards, and it has been tested in 81 stream sections representative of different site and anthropogenic use conditions in the European Alps. The assessment procedure is composed of a set of 11 indicators, which were selected to evaluate longitudinal and lateral morphological and temporal dynamics of morphological and hydrological conditions. The indicators were combined into three indices, the morphology (I_M), hydrology (I_H), and hydromorphology indices (I_HM), to obtain a holistic picture with as few parameters as possible. The definition of indicators and indices, their methodical implementation and practical testing in the field, were carried out by an interdisciplinary team of international experts. The resulting and tested field manual is included with this publication as Supplementary material. The applicability, strengths, and weaknesses of the approach are discussed in the present paper.