Spatio-temporal distribution patterns of three stream-dwelling freshwater mussel species: towards a strategy for representative surveys

Conservation efforts for freshwater mussels (Unionoida) require a priori assessments of their populations' status quo. Unfortunately, collection of representative census and other data for most European species is currently hampered by an insufficient understanding of inter- and intra-specific variation in distribution patterns. We assessed distribution and movement in three Unio crassus populations and one sympatric population of Anodonta anatina and Unio pictorum, respectively. We surveyed vertical movement across four sediment depths, and horizontal movement by mark-recapture technique in 3-month intervals. For all the populations, movement and the proportion inhabiting surface layers increased considerably from winter to spring/summer. Spatial aggregation levels remained stable for some populations, while others became increasingly randomly distributed during the study. One U. crassus population exhibited elevated mortality and displayed movement rates exceeding twice those of conspecific populations. The visible proportion of U. crassus populations differed by up to 69% between sites. Current monitoring guidelines in Europe often insufficiently account for the extensive inter- and intra-specific differences in spatio-temporal distribution patterns observed. We suggest developing internationally standardised protocols that specify sampling season and methodology. In particular, U. crassus surveys should be restricted to summer months, and hand-sampling is crucial for some populations.