Visitor pressure in protected areas: Interactions between river tourists and freshwater pearl mussels in the Central European national park Šumava

The freshwater pearl mussel Margaritifera margaritifera (FPM) is an endangered bivalve species occurring in oligotrophic rivers. FPMs can be found in the Upper Vltava River in the Šumava National Park (Czech Republic, Central Europe), where suitable conditions exist for both adult and juvenile mussels. Non-consumptive human leisure activities are known to negatively affect wildlife in fragile aquatic ecosystems, and in the Vltava River boating tourism in particular may be a threat to local mussel populations. This study focused on an assessment of interactions between river tourists and the FPM, using “pseudo-mussels” in both field and lab experiments. In the field, fake concrete mussels were exposed at three rest sites for river tourists, and in the lab experiment glued shells were placed at the water-sediment interface in an aquarium tested for mechanical impacts of paddles (dislodgement and crushing). Interactions of river tourists with fake mussels were most frequent at low water levels (10–20 cm), and within 2 m from the nearest river bank. Mussel visibility and the presence/absence of a guide played an important role in people´s reactions, but site-specific effects were also found. Unintentional interactions (60–69%) were mostly observed at less-attractive sites (with a limited area of shallow water where people spent most time outside the river channel), whereas visual and manipulative interactions (76%) dominated at a more-attractive site (the confluence of two rivers where people move across and inspect the river channel). Crash tests revealed that 8.03 ± 1.37 (mean±SD) and 7.88 ± 1.13 (mean±SD) hits by paddles were needed for dislodgment and crushing, respectively. Those findings indicate that the direct effects of recreational boating might be less detrimental than those of accompanying activities (such as wading, bathing, and swimming). The role of trampling (and handling) disturbances should be investigated in more detail to help conserve FPM populations in protected areas.