Field Test of the Self-Preserving eDNA Filter and the Importance of Calibration When Changing Methods During Long-Term Monitoring

Filtering water is currently the primary field method used for collecting aquatic environmental DNA (eDNA). One of the drawbacks of filtering is the need to transfer the filter from the filter housing to a preservative-filled container in the field. New products are being developed to avoid this handling step, but comparative studies are needed to ensure that new protocols are transferable within and across eDNA monitoring programs with requisite calibration. To meet this need, we evaluated two filter preservation methods (self-preserving vs. ethanol) of the 5.0 μm polyethersulfone (PES) filter membrane in a field trial typical of stream fisheries eDNA sampling. We compared eDNA detection rates and yield for free-swimming rainbow trout, Oncorhynchus mykiss (Walbaum, 1792), from streams in Washington, United States, and British Columbia, Canada, while accounting for the effects of three environmental covariates: stream discharge, water temperature, and target species abundance. Given these streams were part of an ongoing fisheries eDNA monitoring program, we also compared results against those generated from the original protocol, which used a 0.45-μm cellulose nitrate (CN) filter membrane and ethanol preservative. This secondary comparison allowed us to demonstrate the importance of calibration when changing sampling methods. We found no significant difference in rainbow trout detection rate and limited evidence for a difference in DNA yield between the self-preserving and ethanol-preserved 5.0 μm PES filters. DNA yield was higher in samples collected using the original protocol, possibly because of the smaller pore size or CN membrane material. Detection rate was not influenced by environmental covariates; however, DNA yield increased with increases in trout abundance and declined with increases in discharge and water temperature. These results could help inform fisheries eDNA monitoring programs that are considering switching to self-preserving filters.