Environmental DNA (e-DNA) is an emerging tool for species detection in natural waters. This method has been utilized to identify rare species of fish, amphibians, etc. For understanding fish distribution and their biomass in natural environments, however, there are many uncertainties around the new method yet. In this presentation, I will briefly summarize the current status of the e-DNA method, and discuss its limitations. I also introduce our own studies for detecting salmonid species in Japan as case studies, arguing that the spatio-temporal variations of fish abundance would be captured by this method at the specific ranges. Limiting factors for the e-DNA detection range will be discussed.