Environmental DNA can inform the trade-off between reactive and proactive strategies for crayfish conservation

1.       Species colonising environments outside of their native ranges can cause widespread and far-reaching damage to ecosystems. The introduction of the signal crayfish Pacifastacus leniusculus to British rivers has led to considerable ecological degradation and contributes to 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 programmes, and reactive invasive crayfish control and eradication programmes.

2.       Environmental DNA (eDNA)-based surveys are increasingly being used to detect and map distributions of invasive species in freshwaters. We used eDNA to assess the populations of native and invasive crayfish species across 50 sites in 10 river catchments in Norfolk 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.

3.       We detected both P. leniusculus and A. pallipes at more study sites using eDNA-based methods (12 and three study sites, respectively) than net searches (five and two study sites, respectively). In total, P. leniusculus eDNA was detected at five out of 10 river catchments, and A. pallipes eDNA in two river catchments. Neither eDNA or net searches detected A. pallipes at sites where A. pallipes had been reintroduced.

4.       Synthesis and applications: 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 organisations 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.