Influence of hydrological environments on the spawning behavior of landlocked amago salmon in the headwater streams of the Tendani River, Japan

The behavioral activity of freshwater fishes is highly influenced by the hydrological regime. However, the relationship between the hydrological environment and the spawning behavior of landlocked amago salmon Oncorhynchus masou ishikawae in headwater streams remains unclear. We investigated the effects of rainfall-induced changes in the hydrological environment on the spawning behavior of the landlocked amago salmon. We recorded digging activities and hydraulic parameters in an 1,800-m reach in the Tendani River, Okayama Prefecture, Japan, in their spawning season. Our results revealed that increased turbidity and water velocity over the redds enhanced digging behavior. This may be attributed to increased water discharge, creating a more suitable hydraulic environment for spawning, and decreasing the risk of egg predation due to increased turbidity. Our results indicate that the timing selection of the spawning behavior in amago salmon could be influenced by the hydrological environment.