Passive eDNA sampling as a method for freshwater crayfish monitoring

Freshwater crayfish are ecosystem engineers, yet native populations across Europe are severely threatened by the introduction of invasive species and the crayfish plague pathogen, Aphanomyces astaci . Reliable detection of crayfish and associated pathogens is therefore essential for conservation and management. Monitoring with passive eDNA sampling recently emerged as a promising alternative approach to traditional field surveillance or to the more labour-intensive active filtration eDNA-based monitoring. However, systematic evaluations of the passive eDNA sampling for monitoring crayfish remain limited. Here, we experimentally compared active water filtration (with glass microfiber filters) and passive eDNA sampling for detecting the European noble crayfish ( Astacus astacus ), the America-originated invasive signal crayfish ( Pacifastacus leniusculus ), and the crayfish plague pathogen ( Aphanomyces astaci ) in controlled indoor tanks and outdoor ponds. Passive samplers included glass microfiber, cellulose nitrate, and coffee filter paper substrates deployed for 12 hours. DNA concentration and detection probability were assessed using species-specific quantitative PCR assays. Active filtration generally yielded higher DNA concentrations in tanks, whereas passive samplers, particularly coffee filter paper, achieved comparable DNA concentrations in ponds. Detection accuracy in tanks was higher for active filtration compared to passive approaches (90%, versus 69–83%, respectively). Statistical modelling revealed that species detection success was not significantly differ between eDNA sampling approaches, whereas it was significantly influenced by qPCR assay type and crayfish density or species composition. Our results demonstrate that passive eDNA sampling can provide reliable detection of freshwater crayfish and their pathogen under controlled and open-air conditions. Given its low cost, minimal equipment requirements, replicates potential and logistical flexibility, passive sampling represents a valuable homologous tool to active filtration, particularly for large-scale, remote, or resource-limited monitoring programs.