Abstract
Abstract The European noble crayfish Astacus astacus is threatened by crayfish plague caused by the oomycete Aphanomyces astaci, which is spread by the invasive North American crayfish (e.g. signal crayfish Pacifastacus leniusculus). Surveillance of crayfish plague status in Norway has traditionally relied on the monitoring survival of cage‐held noble crayfish, a method of ethical concern. Additionally, trapping is used in crayfish population surveillance. Here, we test whether environmental DNA (eDNA) monitoring could provide a suitable alternative to the cage method, and a supplement to trapping. We took advantage of an emerging crayfish plague outbreak in a Norwegian watercourse following illegal introduction of disease‐carrying signal crayfish, and initiated simultaneous eDNA monitoring and cage‐based surveillance, supplemented with trapping. A total of 304 water samples were filtered from several sampling stations over a 4‐year period. eDNA data (species‐specific quantitative real‐time PCR [qPCR]) for the presence of A. astaci, noble and signal crayfish within the water samples were compared to cage mortality and trapping. This is the first study comparing eDNA monitoring and cage surveillance during a natural crayfish plague outbreak. We show that eDNA monitoring corresponds well with the biological status measured in terms of crayfish mortality and trapping results. eDNA analysis also reveals the presence of A. astaci in the water up to 2.5 weeks in advance of the cage method. Estimates of A. astaci and noble crayfish eDNA concentrations increased markedly during mortality and vanished quickly thereafter. eDNA provides 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. Synthesis and applications. Simultaneous eDNA monitoring of Aphanomyces astaci (crayfish plague) 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. This non‐invasive, animal welfare friendly method excludes the need for cage‐held susceptible crayfish in disease monitoring. Furthermore, eDNA monitoring is less likely to spread A. astaci than traditional methods. This study resulted in the implementation of eDNA monitoring for Norwegian crayfish plague and crayfish surveillance programmes, and we believe other countries could improve management strategies for freshwater crayfish using a similar approach.
Highlights
Environmental DNA monitoring of aquatic systems is a rapidly advancing research field that promises improvements, to aquatic species conservation, and for early detection of invasive species and harmful pathogens at low densities and at any life stage or season (Bohmann et al, 2014; Kelly et al, 2014; Strand et al, 2014)
The European noble crayfish Astacus astacus is threatened by crayfish plague caused by the oomycete Aphanomyces astaci, which is spread by the invasive North American crayfish
During the predicted freshwater crayfish disaster in the Norwegian Halden watercourse, we demonstrated that environmental DNA (eDNA) monitoring can reveal the invasion of signal crayfish at low densities, as well as low numbers of waterborne infectious A. astaci spores 2–3 weeks prior to observation of mortality in cage‐held susceptible crayfish
Summary
Environmental DNA (eDNA) monitoring of aquatic systems is a rapidly advancing research field that promises improvements, to aquatic species conservation, and for early detection of invasive species and harmful pathogens at low densities and at any life stage or season (Bohmann et al, 2014; Kelly et al, 2014; Strand et al, 2014). Aphanomyces astaci infection is a notifiable disease both nationally in Norway (list 3, national disease; Vrålstad et al, 2017) and internationally (OiE, 2017) It causes a rapid decline in European crayfish populations, and is spread and maintained by invasive non‐indigenous North American carrier crayfish that have rapidly established themselves in Europe (Holdich, Reynolds, Souty‐Grosset, & Sibley, 2009). We took advantage of an emerging crayfish plague outbreak and compared traditional cage surveillance with eDNA monitoring using species‐specific qPCR assays for targeted detection and quantification of A. astaci (Strand et al, 2014), noble crayfish and signal crayfish (Agersnap et al, 2017), from the same water samples. We propose that eDNA monitoring of the three species will prove a suitable, non‐invasive and animal welfare friendly alternative to the traditional cage method
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