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 the results produced by new protocols are transferable within and across eDNA monitoring programs. To meet this need, we evaluated two filter preservation methods (self-preserving filter housing vs. ethanol) of the 5.0-μm polyethersulfone (PES) filter membrane in a field trial typical of stream fisheries eDNA sampling. We compared DNA detection 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 two environmental covariates: stream discharge and water temperature. As these streams were part of an ongoing fisheries eDNA monitoring program, we also compared these methods to the original protocol, which used a 0.45-μm cellulose nitrate (CN) filter membrane and ethanol preservative. We found that the self-preserving filter housings collected and preserved eDNA well and provided similar results to identical filters preserved in ethanol. The 5.0-μm PES filters preserved in ethanol significantly outperformed the original protocol in terms of both DNA detection and yield, highlighting the importance of calibration of eDNA results when changing sampling methods during an ongoing monitoring program.