Comparison of visual observation and excavation to quantify density of the endangered bivalve Unio crassus in rivers of north-eastern France

In the context of a rapid decline in several protected unionid species, government agencies urgently require a reliable method to estimate population size for the most endangered species. We used a dataset collected from 16 river stations in north-eastern France to compare the efficiency of visual estimation (bathyscope) and a manual excavation to estimate numbers of the endangered bivalve Unio crassus . Our investigations indicated that, whereas a visual approach was sufficient to detect unionid presence, only 10% of all individuals were registered compared with manual excavation at the same site. In order to obtain an accurate density estimate (especially as regards the juvenile population), sediment excavation is necessary, despite it being time consuming and damaging to the mussel’s habitat.