Factors influencing detection of eDNA from a stream-dwelling amphibian

Environmental DNA (eDNA) methods for detecting and estimating abundance of aquatic species are emerging rapidly, but little is known about how processes such as secretion rate, environmental degradation, and time since colonization or extirpation from a given site affect eDNA measurements. Using stream-dwelling salamanders and quantitative PCR (qPCR) analysis, we conducted three experiments to assess eDNA: (i) production rate; (ii) persistence time under different temperature and light conditions; and (iii) detectability and concentration through time following experimental introduction and removal of salamanders into previously unoccupied streams. We found that 44-50g individuals held in aquaria produced 77ng eDNA/h for 2h, after which production either slowed considerably or began to equilibrate with degradation. eDNA in both full-sun and shaded treatments degraded exponentially to <1% of the original concentration after 3days. eDNA was no longer detectable in full-sun samples after 8days, whereas eDNA was detected in 20% of shaded samples after 11days and 100% of refrigerated control samples after 18days. When translocated into unoccupied streams, salamanders were detectable after 6h, but only when densities were relatively high (0.2481 individuals/m(2)) and when samples were collected within 5m of the animals. Concentrations of eDNA detected were very low and increased steadily from 6-24h after introduction, reaching 0.0022ng/L. Within 1h of removing salamanders from the stream, eDNA was no longer detectable. These results suggest that eDNA detectability and concentration depend on production rates of individuals, environmental conditions, density of animals, and their residence time.