Fish ecology and sediment conditions in freshwater pearl mussel (M. margaritifera) habitats

Stable, healthy populations of locally adapted brown trout (Salmo trutta) are crucial for the reproduction of the freshwater pearl mussel (Margaritifera margaritifera), as they serve as host fish in its life cycle. In the Interreg project Malšemuschel at the river Maltsch/Malše the hydro-morphology and sediment conditions were analysed regarding their influence on the biocoenoses of rhithral waters on granitic bedrock. In this paper, the fish fauna is highlighted in particular. It could be shown that sand on the one hand and finer fractions on the other impair watercourses and their fish fauna in different ways. While an increase in fine sediments (clay, silt) causes shifts in species compositions and leads to a steady decrease in abundances, sand rather influences the hydromorphology by reducing pools, flattening out the river bed and covering structures. Thus, sand seems to more directly influence the biomass and to cause a sharp deterioration of the fish fauna when a certain threshold is exceeded. Furthermore, it could be shown that brown trout populations are strongly related to the presence of natural structures e.g. large boulders or riparian vegetation.