Summary
In this surveillance program, we used two methodological approaches. One is based on the traditional cage experiments, the other relies on eDNA monitoring of the water, where DNA from spores of A. astaci are detected directly from water filtrates. The main geographic focus of this surveillance program has been the Halden water course and neighbouring risk areas. Other geographic areas covered include the Glomma water course region and the Buåa water course region. In total, 76 and 38 water samples were collected from selected sites in the Halden- and Glomma water course regions, respectively. For the cage experiments, 10 cages with live noble crayfish were distributed in strategically selected sites primarily in the risk areas outside crayfish plague control zones. The exceptions include cages placed in the Buåa, which is in the process of being declared disease-free area, and in the bordering areas of Glomma, where extra effort to pinpoint the long hunted infection source was prioritized. For comparison and evaluation, the TARGET-project provided eDNA monitoring data to this report of susceptible native noble crayfish Astacus astacus, and alien signal crayfish Pacifastacus leniusculus that carry and transmit A. astaci. The presence/absence of all three target species has been screened simultaneously with species specific qPCR assays. No crayfish mortality was observed, that could be attributed to crayfish plague in any of the cages. In the control zone of the Halden water, A. astaci eDNA was detected in water samples in the stretch from Aremarksjøen (Skoteberg) to the Hølandselva outlet. In areas with known presence of P. leniusculus, this presence was confirmed by positive eDNA detection. No signs of crayfish plague were observed during the surveillance period from the northern part of Hølandselva up to the control zone border at Fosserdam. This result was supported by positive detections of A. astacus eDNA in all water samples from Hølandselva and upstream. All water samples in this risk area were negative for A. astaci and P. leniusculus eDNA, while positive for A. astacus eDNA. In Glomma, no sign of crayfish plague was found. The samples were negative for all screened targets apart from two weak A. astacus positives in Stortjennet and Opstadåa.