Environmental DNA-based monitoring of the fish ladder in Dessau-Rosslau: a comparison with fisheries-based methods

Fast, reliable, comprehensive, and cost-efficient biodiversity data are needed for environmental assessment and decision making in water management. In this study, we investigated whether eDNA metabarcoding approaches can fulfil these requirements for fish monitoring. We collected 18 water samples of 1 l along a 2 km stretch of the river Mulde (Germany) on a single day in April 2019, in parallel with comprehensive fisheries monitoring conducted over 17 weeks at the fish ladder in Dessau-Rosslau. Additionally, data from Water Framework Directive (WFD) fish monitoring for the years 2017-19 of the same site were available. A total of 33 fish species were detected with all three sampling strategies. The highest number of species was detected with eDNA metabarcoding (28 species), followed by the fish ladder monitoring (27) and the WFD monitoring (22). In direct comparison with the eDNA-based approach, the fish ladder assessment had a higher overlap of detected species (23 of 32 species detected by both methods) than with the WFD monitoring data (19 of 31). While eDNA-based methods are currently limited to assessing species composition, data on abundance and even age composition could also be generated in future. Here, the smart combination of fisheries-based methods with a high spatial and temporal resolution of eDNA metabarcoding can contribute to a better understanding of changes in the ecosystem and thus improve water management.