Evaluation of Three Monitoring Methods for the Native European Crayfish Austropotamobius torrentium

For the stone crayfish, a threatened species listed in the Habitats Directive in Annex II and V, three monitoring methods were evaluated: hand capture by day, hand capture by night and eDNA sampling respectively. The lack of a standardized sampling method in Austria and the simultaneous obligation to monitor and report on the status of protected species requires therefore an evaluation of the different techniques. However, data show that day sampling massively favored the observation of juveniles whereas night sampling obtained higher shares of adults and males. Moreover, organic layer primarily determines the applicability of hand capturing methods. If the share of organic layer exceeded 40%, night sampling was more effective. In brooks with low amount of organic layer, no notable divergences were found. eDNA sampling provided reliable presence-absence data but cannot fully replace hand capturing methods. Data indicate that eDNA detectability depends probably on the discharge level. Below 1.5 L·s-1 eDNA detection failed but provided reliable results in sections with a higher discharge regime. In summary, all three methods are appropriate for crayfish surveillance programs, but must be applied according to the sampling design appropriate for the research questions and with an awareness of their strengths and weaknesses.