In most cases, conservation of aquatic species is conducted through complex, multiple activities such as habitat mitigation and release of seedlings. When a migration species is a target, we have to assess the contribution of each conservation activity for the successful migration and reproduction for effective management. In the present study, we used environmental (eDNA) analysis to illustrate a year-round history of an amphidromous fish to assess the successfulness of the migration.
Aquatic organisms shed DNA into water, and, in recent years, it has been used as genetic markers for species identification without catching the organisms directly. We conducted a year-round eDNA monitoring of ayu sweetfish Plecoglossus altivelis altivelis in the Yahagi river, central Japan, from the headwater to the river mouth. Spatial and seasonal distributions of ayu DNA were monitored to estimate the seasonal migration of ayu between the river and sea and within the river.
Results/Conclusions
Overall results of eDNA analysis illustrated seasonal upstream migration of ayu from the sea in Spring, downstream migration from upper reaches to lower reaches for spawning in Fall, and decreasing of adult individuals due to mortality after spawning. Moreover, ayu DNA were detected in some sites which could be due to the release of seedlings of ayu for stock enhancement by local fisheries associations. These interpretations could be reasonably made based on eDNA analysis referring to previous knowledge about the migration history of ayu.
Each site was PCR tested with four replications and the total number of PCR with positive results in all sites decreased after the beginning of spawning season in Fall. The lowering detection frequency would correspond to the reduction of ayu biomass due to spawning mortality. The concentration of eDNA from ayu also fluctuated spatially and temporally within the river. Furthermore, the sites with high eDNA concentrations seemed to be biased to the downstream side of a large dam located in the middle reach of the Yahagi River in the season of upstream migration. It implies that the dam may restrict the fish migration for some extent, even though ayu actually can swim up to the upstream side via a fish ladder. Those analysis results would be used to find inhibition factors on fish migration or to evaluate the effectiveness of seedling release for stock enhancement.