Averting co‐extinction: Successful mussel translocation rescues an endangered population of the European bitterling, Rhodeus amarus

Abstract Local extinctions break species interactions and have cascading effects throughout ecosystems; parasites are often severely affected. The European bitterling, Rhodeus amarus, is a cyprinid fish that parasitizes unionid mussels by laying eggs into the mussel gill cavity, where embryos develop and emerge as active juveniles; this relationship is obligatory for the bitterling. This article describes a field experiment aimed at averting the secondary extinction of the European bitterling after a complete die‐off of a freshwater mussel community, as a result of habitat destruction. Approximately 5,000 unionid mussels were reintroduced within the short time frame in which the remnant bitterling population was still present at the site. Mussel survival was high, and bitterling resumed reproduction, with vigorous courtship observed within 24 hours of mussel release. Recruitment was successful, as evidenced by increased occupancy, densities, and relative frequencies in the fish assemblage. The frequency of sub‐adults and young‐of‐the‐year changed from 0% before mussel reintroduction to 80% a year later, and 50% 2 years later, when young‐of‐the‐year contributed to about half of the young fish. No bitterling were observed at two control sites where mussels were not reintroduced. This study exemplifies how the timely restitution of affiliate species can avert co‐extinction. It also shows how the conservation of the bitterling within its historical distribution range can serve mussel conservation, including species that although not legally protected, are important keystone species and ecosystem engineers, shaping the structure and function of a broad range of freshwater habitats.