Author(s): Makoto Hyodo; Shinji Takahashi; Hiromi Yamasaki; Yasuhiro Takemon; Tetsuya Sumi
Linked Author(s): Tetsuya Sumi
Keywords: Spawning redds; Hydro-geomorphological process; River channel configuration; Up-welling zone; Turbid water
Abstract: Plecoglossus altivelis altivelis (Ayu [in Japanese]) is anadromous fish well-known and important fishery resources in Japan. They spawn on the loose gravel-riverbed in riffles, like Salmon and Trout, in November to December in Japan. Although the Tenryu River was known for fairly high density of Ayu among Japanese fishermen, the fish has been reduced in population due to gravel mining and dam construction upper stream leading to reduced sediment supply, riverbed degradation and turbid flow discharge, negatively affecting environmental conditions of the spawning redds. Objective of this study is to analyze relationship between hydro-geomorphological formation processes and spawning sites for Ayu in the Tenryu River under the altered flow and sediment regimes and turbid water conditions. Firstly, we conducted a series of surveys on spawning sites in 2018 and 2019. We could not find any spawning redds in riffles of the main channels, but found in riffles of the spring channels in the gravel bars. Secondly, we analyzed relationship between changing patterns of riverbed by floods and distribution patterns of spawning sites, using aerial photos and satellite images. It was revealed that old channels deposited during floods play an important role in creation of the spring channels gathering the up-welling water from hyporheic zones in the gravel bars. Thirdly, we measured the turbidity in main channels and the secondary channels including the spawning redds of Ayu. Results showed that turbidity was distinctively lower at the spawning sites than the main channels. Based on these results, we discussed on characteristics of the river channel configuration maximizing spawning success of Ayu and on the geomorphological processes for creating the channel configuration.