This surveillance programme uses environmental DNA (eDNA) monitoring for species specific detection of Aphanomyces astaci spores directly from water filtrates. The presence/absence of eDNA from noble crayfish (Astacus astacus) and signal crayfish (Pacifastacus leniusculus) is also determined to supplement the results, and to evaluate the habitat status. Detection of noble crayfish eDNA, combined with the absence of eDNA from A. astaci and signal crayfish, substantiate the presence of non-infected noble crayfish which constitutes the desired habitat status. These analyses are part of the collaboration and coordination with the national surveillance programme for noble crayfish. The geographic focus of the surveillance programme in 2023 is the Halden watercourse and neighbouring risk areas; the Mosse watercourse, Glomma watercourse, River Hæra and areas in the Eidskog municipality including the Buåa watercourse and the rivers Vrangselva and Finnsrudelva.

• In the Halden watercourse, eDNA of A. astaci was detected in the southern part of Lake Rødnessjøen within the restriction zone. eDNA of signal crayfish was also detected in the south of Lake Rødnessjøen, where there is a known signal crayfish population. All water samples in the risk area of the Halden watercourse region were negative for eDNA from A. astaci and signal crayfish, while most samples were positive for noble crayfish eDNA.
• In the Mosse watercourse, no eDNA from A. astaci or signal crayfish was detected, while noble crayfish eDNA was detected upstream of Lake Langen.
• In the Glomma watercourse, no eDNA from A. astaci, signal crayfish or noble crayfish was detected at any of stations in Glomma.
• In River Hæra, eDNA from both A. astaci and noble crayfish were detected at one location in June. Additional sampling confirmed an active crayfish plague outbreak in the risk area, upstream the restriction zone in the river. No eDNA from signal crayfish was detected.
• In Eidskog, all samples were negative for signal crayfish and A. astaci, while several samples were positive for noble crayfish eDNA in the rivers Buåa, Vrangselva and Finnsrudelva.
• Illegally introduced signal crayfish was discovered in Lake Østersjøen, which is connected to Glomma, after tip from the locals. The signal crayfish was confirmed to host A. astaci.

In summary, eDNA from A. astaci was detected within the restriction zone in Lake Rødnessjøen in the Halden watercourse, and in River Hæra within the risk area. The detection of A. astaci eDNA in Hæra initiated further sampling and an active crayfish plague outbreak was confirmed. By using eDNA we were able to follow the outbreak front moving upstream. Frequent detections of noble crayfish eDNA within the regulated A. astaci restriction zones of the Halden watercourse, Mosse watercourse, and the rivers Vrangselva and Finnsrudelva in Eidskog, suggest the presence of vital noble crayfish populations within A. astaci restriction zones.