The effects of stream hydromorphology on fish communities in the central European Alps

Many freshwater fish species rely on specific hydrological and morphological stream characteristics and are sensitive to their changes. However, the incomplete understanding of hydromorphological processes responsible for changes and their effect on fish populations creates further difficulties in conserving and managing these ecosystems. In this study, we analysed the effects of river hydromorphology on fish communities by using the recently introduced Hydromorphology Index (IHM). The IHM was developed based on European standards in a study area composed of 80 representative stream sections in a transect of conditions from nearly natural to highly anthropogenically influenced in the Alps of Italy and Austria. The IHM is a combination of the hydrological index (based on three indicators) and the morphological index (a set of eight indicators). Fish diversity, abundance, and biomass were highly correlated with riverbed and riverbank vegetation. The IHM provided strong evidence for understanding fish communities and thus, gives more information about stream conditions. This study shows that the IHM provides a standardised methodology to collect data on historical and future changes in river morphology and hydrology in a uniform manner and subsequently relates these to changes in the fish fauna. The results of this study establish an important foundation in the form of a summarising index to enable decision-makers to prioritise measures for improving fish habitats.