Understanding detection probability in a low-density population to inform optimal eDNA sampling for freshwater mussels

Environmental DNA (eDNA) offers a sensitive tool for detecting aquatic species, including those at low population densities. However, effective use of eDNA in conservation and monitoring efforts requires an understanding of detection probabilities and optimal replication effort necessary to minimize false negatives. This study assessed the probability of detecting Anodonta nuttalliana, a native freshwater mussel, in two Utah populations with contrasting densities (medium and low). We conducted traditional visual sampling alongside highly replicated eDNA sampling at both sites. In the medium-density population, eDNA provided high detection probabilities with minimal replication. However, in the low-density population, substantially greater replication was needed to achieve similar detection probabilities. Despite the increased eDNA replication effort needed to detect the low-density population, eDNA outperformed traditional visual sampling, which failed to detect live mussels at the low-density site. Our findings highlight the critical balance between sampling effort and detection success, offering guidance for optimizing eDNA sampling strategies in monitoring efforts, especially for species which occur at low densities.