Relationship between eDNA concentration from metabarcoding method and stream fish density under field conditions

Estimating abundance or biomass using eDNA metabarcoding is a powerful emerging tool that may provide an alternative to conventional laborious methods for biological monitoring. However, inferring aquatic macroorganism abundance or biomass using eDNA concentrations remains challenging, especially in lotic environments, because of several potential confounding factors. In this study, we tested whether quantitative eDNA metabarcoding that uses internal standard DNA can be used to estimate the abundance of four fish species. We collected eDNA samples and concurrently estimated fish densities using the conventional removal method in small tributaries in four seasons during a year. The effects of potential confounding factors, including the body mass of the individuals, water temperature, and discharge volume, were assessed using an allometric scaling model. We found an increasing trend of eDNA concentration against the increase in abundance across all species. In the most abundant species, ...