White-clawed crayfish were once found in most of Norfolk’s rivers. Invasive signal crayfish have since wiped them out across most of their range, and the three remaining populations are threatened by signal crayfish nearby.

As part of its work to protect and restore Norfolk’s internationally rare chalk streams, Norfolk Rivers Trust (NRT) used aquatic eDNA to track both native and invasive crayfish. Its aim was to understand where each species was present, so it could protect surviving white-clawed crayfish populations and check that potential reintroduction sites were free from signal crayfish.

The surveys revealed a previously undiscovered white-clawed crayfish population and signal crayfish at a site thought to be safe. Both findings gave NRT clear evidence for its management decisions.

Remnant white-clawed crayfish populations are very difficult to locate with conventional trapping and hand-searching. With traditional methods, it took at least two people a full day to hand-search around 500 m of river.

NRT needed a faster, more scalable way to map both crayfish species accurately. It had to find white-clawed populations to protect and confirm that potential reintroduction sites were free from invasive signal crayfish.

NatureMetrics worked with the NRT team to deliver an aquatic eDNA survey targeting white-clawed and signal crayfish. We advised on sampling strategy, provided the eDNA sampling kits and processed the samples in our laboratories. We then supported NRT with data interpretation so the team could get the most from the results.

Using eDNA, one surveyor covered multiple sites in just three days. With traditional methods, that work would probably have taken weeks and a larger team.

The eDNA surveys detected populations of both crayfish species that traditional monitoring had not identified. Figure 1 shows the proportion of DNA sequences assigned to each species across ten sampling sites.

Evidence to protect and restore native crayfish

• A new population to protect: eDNA detected a white-clawed crayfish population that was suspected but had not been found with traditional surveys. It can now be actively protected.
• A costly mistake avoided: Signal crayfish were detected at a site previously considered safe. That ruled it out as a reintroduction site and saved precious conservation resources.
• Early signs of recovery: White-clawed crayfish were confirmed at two release sites, an encouraging sign that NRT’s reintroduction work is succeeding.
• Faster, more scalable monitoring: A single surveyor covered multiple sites in three days, which made wider and more frequent monitoring realistic.
• A long-term partnership: NatureMetrics and NRT have since set up a long-term collaboration to monitor crayfish at key sites throughout the year. The work is also improving understanding of how crayfish eDNA varies across the seasons in chalk streams.