Predation on endangered mussels by invasive raccoons: a case study from western Poland

The raccoon Procyon lotor is an invasive alien carnivorous mammal whose impact on populations of native species in Europe is poorly understood. Raccoon populations are rising both in range and density, which increases predation pressure. The raccoon is a food opportunist and its impact on local animal populations depends on the availability of food, especially during the critical winter period. In the winter of 2020–2021, we observed an extreme accumulation of empty shells of native mussels (Bivalvia, Unionidae) foraged by raccoons in the Lubuskie Lakeland in western Poland. The aim of this study was to determine whether mussels could be an important food source for raccoons during the difficult winter period in areas where they have been introduced. We hypothesised that raccoons would prefer thin-shelled mussels over thick-shelled species as their food. We identified the mussel species and estimated their numbers in the piles of shells found on the small river banks. The raccoons’ prey consisted of 2,340 mussels belonging to three genera ( Anodonta , Pseudanodonta , Unio ) and five species ( Anodonta cygnea , Anodonta anatina , Unio pictorum , Unio tumidus , Pseudanodonta complanata ). Two of them, the Anodonta cygnea and Pseudanodonta complanata , have the status of endangered species in Poland and are red listed by IUCN respectively as Vulnerable (VU) in Europe and Endangered (EN) globally.