Spatiotemporal changes in environmental DNA concentrations caused by fish spawning activity

Determining the timing and location of spawning is critical for the conservation and management of aquatic species. Environmental DNA (eDNA) analysis can be used to monitor fish spawning activity by detecting peaks in eDNA concentrations and nuclear DNA (nuDNA)/mitochondrial DNA (mtDNA) ratios; however, the duration and diffusion distance of high concentrations of eDNA produced by spawning activities in water bodies are still unknown, preventing us from making accurate and effective survey plans to monitor spawning activities. Using common carp as a target species, we conducted artificial spawning experiments to investigate the spatiotemporal changes in eDNA concentrations during spawning. The results showed that the nuDNA concentration, mtDNA concentration, and nuDNA/mtDNA ratio that were increased by spawning activity decreased at rates of 7.78 %, 5.22 %, and 2.56 %, respectively, per 0.1 h after reaching the peak. Data simulations were performed to estimate the probability of successful monitoring of carp spawning activity during repeated sampling at different times and distance intervals. The results showed that carp spawning activity over the past 24 h could be monitored by eDNA analysis. Additionally, when carp spawning activity occurs once, spawning activity can be successfully monitored by measuring the nuDNA concentration or the nuDNA/mtDNA ratio with a probability of approximately 50–75 % based on a sampling plan of sample collection every 100 m and every 24 h. Thus, the spawning activity of aquatic species can be estimated with high spatial and temporal accuracy using eDNA analysis.