Spatiotemporal response of pelagic fish aggregations in their spawning grounds of middle Yangtze to the flood process optimized by the Three Gorges Reservoir operation

sFlood patterns altered by regulation and operation of the Three Gorges Reservoir (TGR) on the Yangtze has negatively affected fish spawning downstream, particularly results in the decline of spawning sizes. The TGR release was optimized by reoperation for enhancing fish reproduction downstream since 2011. Response of spawning sizes to the flood alteration was documented, yet litter is known concerning the adult fish. Accurate estimate of spatiotemporal changes and abundance of adults in the spawning grounds is critical for quantifying the contribution of flood changes to the spawning. This study used hydroacoustic surveys to document the response of fish aggregations to the changes of flood process (i.e., deployed before-, during- and after flood) on the Jingjiang- and Yichang spawning ground. Acoustic signals from fish tracking were allocated into two groups according to frequency distributions of Target strength (TS), defined as small fish (−60 dB < TS < −35 dB) and large fish (TS > −35 dB). Sizes of small fish acquired before- and during flood were significant larger than those acquired after flood whereas no significant different observed for large fish. Both small and large fish aggregated in both spawning grounds during flood and disappeared after flood. Besides, fish numbers presence in both spawning grounds during flood was greater than those before and after flood. This study suggested spatial distributions of fish aggregations with different sizes in both spawning grounds shifted with the flood changes. Fish aggregated into the spawning grounds for spawning during the flood and departed after the flood, resulting in large fish numbers presence during the flood period. Further research is required to reveal relationships between the hydrological and hydraulic changes results from flow alteration and fish responses to realize their precise requirements and behaviors for spawning.