Characteristics of diurnal and seasonal changes in fish detection patterns using environmental DNA metabarcoding in a mountain stream

Environmental DNA (eDNA) techniques can be used to help monitor the presence of aquatic organisms without causing a disturbance and are expected to become useful tools in aquatic environmental monitoring. The results of eDNA techniques are largely affected by the sampling method used. Few studies have investigated the effects of diurnal and seasonal changes on the detection of fish species using eDNA metabarcoding. In this study, diurnal (i.e., day, sunset, night, and sunrise) and seasonal (i.e., June, September, and December 2017 and April 2018) eDNA samplings were collected from a mountain stream, the Akabira River. The relationship between the number of analysed samples and the number of detected fish species, which was calculated using fish incidence data derived from eDNA metabarcoding, exhibited diurnal changes. In particular, a higher number of fish species were detected across all the seasons in the samples collected at night. Species with a low relative sequencing read abundance may be sensitive to diurnal changes. The detection frequencies of fish species and the total number of detected species in each season also demonstrated seasonal changes. In September and December, the number of detected fish species was higher than that in June and April. The community composition also showed seasonal changes based on detection frequencies, as determined by eDNA metabarcoding. The river environment and the spawning season possibly contribute to the success or failure of species detection using eDNA metabarcoding. Therefore, it is important to consider seasonal and diurnal changes when performing eDNA metabarcoding in mountain streams.