Beaver slides provide reproduction habitat for the endangered thick-shelled river mussel (<i>Unio crassus</i>) in modified streams

Freshwater mussels are among the most threatened animal groups globally, with many species declining due to habitat degradation and lack of recruitment. The Unio crassus complex, like other unionids, depends on host fish for larval development, but requires access to shallow riverbanks where females release their glochidia larvae. This study describes beaver slides, which are sloped riverbank paths created by Castor fiber , as a spawning habitat for U. crassus in structurally modified stream systems. Mussel densities were surveyed across 159 quadrats placed directly at and near beaver slides in four streams in Bavaria, Germany. During the period of larval release, most mussels (80%) were found within 0.5 meters of the river bank, with peak densities occurring in direct vicinity to the beaver slides. These findings suggest that beaver slides can locally provide key habitats at least for U. crassus reproduction in the form of shallow near-bank areas at beaver slides, especially in highly structurally degraded systems such as regulated or channelized streams. While the ecological impact of beavers on mussels seems to be controversial and species-specific, our study supports that beaver activity can also create beneficial microhabitats for certain mussel species.