Comparing cost, effort, and performance of environmental DNA sampling and trapping for detecting an elusive freshwater turtle

Environmental DNA (eDNA) analysis is an effective and non-invasive technique for surveying and monitoring rare, threatened, or endangered (RTE) species. Compared to conventional capture-based sampling, eDNA analysis may offer a more cost-effective approach for surveying RTE species, yet few studies have compared their cost-efficiency—a critical consideration for conservation planning. We compared the costs, effort, and relative performance of aquatic eDNA sampling and conventional trapping for detecting the Alligator Snapping Turtle, Macrochelys temminckii Troost, 1835, in southwest Louisiana, United States. Environmental DNA was sampled quarterly over 1 year (2018–2019) at 19 streams, including three streams where M. temminckii presence had been previously confirmed via conventional trapping efforts (2012–2013). Water samples from each stream were analyzed using quantitative polymerase chain reaction (qPCR) to assess M. temminckii eDNA presence/absence. Time and costs (i.e., labor, travel, wages, and supplies) per detection via eDNA analysis and trapping were calculated and compared. Environmental DNA analysis documented the presence of M. temminckii DNA at two of the three streams where individuals had previously been trapped and yielded detections (qPCR amplifications) at 16 additional streams not previously sampled, expanding M. temminckii's documented distribution at our study sites by 84%. Environmental DNA analysis returned a detection rate (per site) 5.55 times higher than conventional trapping and was 18.7% less expensive. Our results provide evidence that strategically deployed eDNA surveys may be an effective and cost-efficient approach for detecting freshwater RTE species. With eDNA analysis, additional resources can be invested toward expanding survey coverage and increasing sampling frequency, allowing managers to more effectively target subsequent intensive monitoring efforts.