SPAWNING HABITAT POTENTIAL EVALUATION FOR AYU BASED ON AN ENVIRONMENTAL DNA ANALYSIS AND FIELD WORKS OF PHYSICAL ENVIRONMENT FACTORS IN THE LOWER ASAHI RIVER

The spawning bed for Ayu in rivers has been conventionally inspected by human eyes. Consequently, the previous method cannot be applied for consideration of the potential of a limited spawning habitat, especially in rivers with larger targeted areas, steep bed sections, and environmentally degraded sections. This study investigated the potential of the spawning habitat for Ayu at three riffle regions of the lower Asahi River where the river forestation has prevented the renewal of bed materials under flooding. For that purpose, we examined the relation among the spawning behavior, the hydraulic characteristics at riffle, and the environmental DNA (eDNA) concentration of Ayu in water at both day and night, in conformity to the downstream migration of Ayu in autumn. Results demonstrated that we can observe the spawning bed at the riffle where the characteristics are preferable and the concentration drastically increases from day to night. Results also showed that we might locally find the spawning bed even at the riffle where the eDNA concentration increases from day to night, though the average characteristics are not suitable for spawning.