Environmental <scp>DNA</scp> can inform the trade‐off between proactive and reactive strategies for crayfish conservation

Abstract The introduction of the signal crayfish Pacifastacus leniusculus to British rivers has led to ecological degradation and the decline of the native white‐clawed crayfish Austropotamobius pallipes . To manage and mitigate the impact of the signal crayfish, conservation agencies and government bodies employ multiple conservation strategies. These take the form of proactive native crayfish breeding and stocking programs and reactive invasive crayfish control programs. Here, we used eDNA to assess the populations of native and invasive crayfish species across 50 sites in 10 river catchments in Norfolk, United Kingdom (UK). The sites were chosen to enable assessment of the potential of eDNA to inform proactive and reactive crayfish conservation strategies. Three of the catchments sampled were selected to assess the success of recent A . pallipes reintroduction, whereas the remaining seven were selected to better understand the distribution of each species at the landscape scale. Combining results of eDNA‐based methods with net searches within an occupancy model enabled us to confidently determine the presence of P . leniusculus at eight sites, and A . pallipes at three sites, which was more than visual searches alone (five and two study sites, respectively). Neither eDNA nor net searches detected A . pallipes at sites where A . pallipes had been reintroduced. We recommend that practitioners using eDNA‐based surveys for management and conservation of crayfish should consider: (1) designing eDNA surveys with an emphasis on large spatial scales to comprehensively describe the distributions of native and invasive crayfish in a region of interest; (2) work with local conservation organizations and/or government bodies to inform the selection of study sites to generate results that are meaningful to real‐world conservation actions; and (3) use results from eDNA‐based crayfish surveys to target limited conservation resources to appropriate proactive and/or reactive conservation actions.