EXAMINATION OF DRIFTING OF THE LARVAE OF AYU, PLECOGLOSSUS ALTIVELIS ALTIVELIS, IN THE GONOKAWA RIVER

Abstract

 We aimed to grasp the relationship between saline-intrusion behavior and the drifting dynamics of ayu larvae in the Gonokawa River using a three-dimensional fluid simulator. Both measured and computed results revealed that the salinity ranged from 1 to 7 PSU on the surface of the estuary, while it was about 30 PSU near the bottom of the estuary. Therefore, we inferred that the bottom of the estuary is not suitable for the habitat of the larvae soon after hatching. Particle-tracking simulations showed that about 90% of the larvae were suspended near the surface and drifted to the sea in one night. On the other hand, the remaining 10% of the larvae were trapped in the saline layer near the estuary bed and transported back upstream by a tidal excursion in the model simulating the behavior of larvae in the daytime. They could not reach the sea, so about 10% of the larvae were expected to grow in the brackish water in the early stage, where feed plankton are less abundant than in the sea. These results showed that this method could be applied to other rivers to identify if the flow blockage is a cause of the depletion of drifting ayu larvae.