Distribution characteristics of hydraulic habitat indicators for fish in typical compound channels in the upper reaches of the Yangtze River

Compound channels consisting of obvious riverbank beaches and deep grooves are widely found in the upper reaches of the Yangtze River, and the special and complex flow structure of compound channels has an important impact on fish habitats. Currently, there are insufficient studies on how the flow characteristics of compound channels affect the suitability of spawning habitats for fish. In this study, two different spawning types of fishes, namely, the Four major Chinese carps and the Schizothorax prenanti, were taken as representatives, and two different hydrodynamic effects, namely, the characteristic flow and the typical flow pulse process, were considered. Using the 3D hydrodynamic model and physical habitat suitability model, we investigated the spatiotemporal distribution characteristics of the habitat suitability indicator for representative fishes in a compound channel, and analyzed the impact of hydraulic characteristics of the compound channel on the suitability of spawning habitats for representative fish species. The results show that the suitable areas for spawning and habitat of the Four major Chinese carps are concentrated in the middle and upper layers of the flow field, with the distribution characteristics of locating in deep grooves under low flow and locating in riverbank beaches under high flow; the spawning and habitat suitability area for the Schizothorax prenanti is located mainly in the near-bottom flow field of the riverbank beach of the compound channel, and with the increase of the flow, the size of the suitable area in the riverbank beach decreases and migrates to the bank gradually. Under the effect of a high-flow pulse process in a certain flow interval, the ideal weighted usable area (IWUA) of habitat suitability indicator greater than 0.8 (HSI >= 0.8) for the Four major Chinese carps and Schizothorax prenanti will increase with the increase of flow, and the connectivity of ideal habitat patches will also be enhanced. The results of this study are of great significance for the conservation of river fish resources and ecological engineering construction, etc.