Update on environmental DNA (eDNA) monitoring of Aphanomyces astaci and freshwater crayfish

Environmental DNA (eDNA) methodology has become an important non-invasive tool to monitor freshwater micro- and macro-organisms. From a single water sample, it is possible to detect several species of interest or even whole communities. eDNA can be used to reveal elusive targets, such as alien invasive species at an early stage or rare and endangered species. eDNA can also be used to investigate presence/absence and relative quantities of pathogens in water. Over years, we have developed and used eDNA monitoring tools for Aphanomyces astaci, the causative agent of crayfish plague, in combination with eDNA monitoring of susceptible- and tolerant freshwater crayfish. In a recently published study, we used eDNA monitoring to follow a crayfish plague outbreak in a large water course, and compared it to traditional crayfish plague monitoring that uses cages with live crayfish as “canaries in a coalmine”. We show that eDNA‐monitoring corresponds well with the biological status measured in terms of crayfish mortality and trapping results. It reveals the presence of A. astaci in the water up to 2.5 weeks in advance of the cage‐method. eDNA estimates of A. astaci concentration and noble crayfish numbers increased markedly during mortality, and vanished quickly thereafter, demonstrating the dynamics during an outbreak of crayfish plague. The eDNA monitoring also provided a snapshot of the presence, absence or disappearance of crayfish regardless of season, and constitutes a valuable supplement to the trapping‐method that relies on season and legislation. One of the main benefits of eDNA monitoring is the possibility for temporal and spatial monitoring of several organisms from the same water samples. The simultaneous eDNA monitoring of A. astaci and relevant native and invasive freshwater crayfish species is well-suited for early-warning of invasion or infection, risk assessments, habitat evaluation and surveillance regarding pathogen and invasive/native crayfish status. In Norway, eDNA monitoring have replaced the traditional cage dependent crayfish plague monitoring in the national surveillance program of crayfish plague, and was recently also implanted as a supplement method in the national surveillance program for noble crayfish and spread of signal crayfish.