Environmental DNA facilitates accurate, inexpensive, and multiyear population estimates of millions of anadromous fish.

Although environmental DNA shed from an organism is now widely used for species detection in a wide variety of contexts, mobilizing environmental DNA for management requires estimation of population size and trends in addition to assessing presence or absence. However, the efficacy of environmental-DNA-based indices of abundance for long-term population monitoring have not yet been assessed. Here we report on the relationship between six years of mark-recapture population estimates for eulachon (Thaleichthys pacificus) and "eDNA rates" which are calculated from the product of stream flow and DNA concentration.