Biber (Castor fiber) vs. Bachmuschel (Unio crassus) Ist eine Koexistenz ohne Management möglich? Ergebnisse aus dem Fall eines verlegten und umgestalteten Fließgewässers bei Ravensburg (Baden-Württemberg).

As part of the construction of a new section of the main road (B 30) north of Ravensburg (Baden-Württemberg), a segment of a stream containing a sub-population of the thick-shelled river mussel (Unio crassus) had to be relocated and remodelled. This project was planned and implemented at the end of the 1990s, a long time before any project-related relevance of strict species protection regulations became relevant. At that time, the species was examined as part of the impact mitigation regulations under the nature conservation law. This publication first briefly describes the project, as well as the assessment of the mussel population at the time and the measures taken. Following a phase of interim monitoring, in-depth investigations were carried out in 2013 and 2022. Hence, it could be determined how the population of the mussel species had developed in the relocated and redmodelled segment of the stream as well as in adjacent sections. The 2022 survey took place under the impression of an altered situation, as in the meantime the beaver (Castor fiber) has settled and taken up activities in the area. The results of these investigations are presented and discussed from the point of view of a nature conservation perspective. Stream remodelling had led to a significant improvement in the population situation of the thick-shelled river mussel. However, the habitat quality and population situation of the species deteriorated significantly after 2013 due to the activities of the beaver, possibly also in part due to the increase in water shortages or dry phases in some sections. Consistent management of beaver activities is essential for the medium and long-term coexistence of the two species in this stream.