Hydromorphological preferences of freshwater pearl mussel (Margaritifera margaritifera) in upland streams of the Bavarian Forest – A case study

The freshwater pearl mussel is a highly specialized freshwater bivalve. In four pearl mussel streams located in the Bavarian Forest, hydromorphological microhabitat conditions were investigated in mussel colonized and adjacent non-colonized river stretches to describe hydromorphologic preferences of freshwater pearl mussel. For that purpose, tachymetry, flow velocity and substratum composition were investigated. Hydraulic-numerical models were calculated to predict flow velocities, maximal shear stress and substratum stability under different discharge scenarios. Results indicate that pearl mussels prefer river stretches with certain minimal currents during low flow but at the same time stable substrata even during bankfull discharge. Implications for habitat restoration and release of juvenile mussels from breeding programs are discussed.