Management of invasive, plague-carrying signal crayfish by physical exclusion barriers

Invasive, plague-carrying signal crayfish represent a significant threat to imperiled European crayfishes. In the absence of a feasible eradication technique, physical barriers that separate invasive from native crayfish populations have been suggested as management strategy. Here, we investigated the effectiveness of three serial barriers with different functionality (flow-based vs . waterfall-based) in a headwater stream in southwestern Germany on containment of signal crayfish and crayfish plague. Crayfish distribution was surveyed three and six years after the construction of the barriers using manual search, trapping, and eDNA detection, whereby a tributary stream without barriers served as a form of control for a “do-nothing” scenario. The efficacy of the barriers was also empirically assessed by stocking marked crayfish downstream of the barriers and tracking their nocturnal behavior. After six years, native crayfish were still present upstream of the barriers but went extinct in the control stream due to signal crayfish invasion. Following two years of extreme drought after the first survey, signal crayfish were able to overcome the flow-based barrier but were never detected upstream of the waterfall-based barriers. Overall, this case study provides evidence for the effectiveness of intentional stream fragmentation as management strategy against invasive aquatic species.