Comparing classic crayfish cage surveillance with eDNA water monitoring during an on-going crayfish plague outbreak in Norway

The use of environmental DNA (eDNA) for detection and uantification of aquatic organisms is a rapidly growing field with a great potential for streamlined inventory- and monitoring purposes. The TARGET-project aims at implementing eDNA approaches for monitoring the red-listed Astacus astacus and its threats. In 2014-2016, we followed an outbreak of crayfish plague in the Norwegian Halden watercourse which had resulted from illegal transfer of Pacifastacus leniusculus. A surveillance program based on cages containing noble crayfish was already established, thus the spread of disease was monitored by observing and diagnosing mortalities in the cages, and water samples were collected regularly. Water (~5 L) was filtered on-site through glass fiber filters, and each sample was analysed using species specific qPCR assays for the crayfish plague pathogen Aphanomyces astaci, and the crayfish A. astacus and P. leniusculus. eDNA from all three species was successfully detected in the water samples. Crayfish plague spores was detectable in the water before the caged crayfish succumbed to the disease. The infection source (signal crayfish), representing a scarce P. leniusculus population (0.11 CPUE) in the southern part of the lake, was detected at trace levels. Furthermore, eDNA from noble crayfish was readily detected and increased in quantity during the mortalities, before decreasing to trace levels about 4-8 weeks after the outbreak. Our study demonstrates an efficient and non-invasive approach for combined eDNA monitoring of native crayfish and its threats (invasive crayfish and crayfish plague) from the same water samples.