Identifying the hydrological requirements for fish spawning and reproduction has become a fundamental prerequisite for environmental flow practices aiming at promoting fish reproductive habitats both domestically and internationally.Existing research on the relationship between fish reproduction and hydrology emphasizes the significant role of the flow increase process in the variation of fish egg quanti-ties.However,there is still insufficient research on the hydrological requirements for spawning and the role of recession characteristics during the flood peak process in the variation of fish egg density.In this study,we analyzed the relationship of spawning occurrence of fishes with drifting eggs and the size of fish egg density with the hydrological conditions of the reduced flow reach in the lower reaches of the Yalong River,and identified key influencing factors and explored the role of the recession phase during the flood peak process.The study was based on the daily monitoring of early stage fish resources at the Lizhuang or Mofanggou transects of the research river sections during the spring and summer seasons in 2008,2013,2020,2023 and 2024,and the hydrological data at Luning station,and a random forest model was employed for the relationship analysis.Results show that the decrease in transparency during the ris-ing phase of the flood peak process and the achievement of a suitable water temperature range were key conditions for triggering the spawning of fishes with drifting eggs in the reduced flow reach.Meanwhile,the flow and water level changes during the recession phase were key conditions affecting the variation of fish egg density.Specifically,when the daily average water temperature was in the range of 17.6－20.1℃and the daily average transparency was between 3－30 cm,the probability of spawning occurrence was 0.75 or higher;when the daily flow increase rate was below-4 m³/(s·d),the flow increase amount was below-121 m³/s,and the daily water level increase rate was between-1.59 m/d and-0.20 m/d,the daily fish egg density could reach 75%or more of the maximum value.It is recommended that ecological regulation for drift-spawning fish in the reduced flow reach should be carried out in conjunction with the basin rainfall process during ecological regulation,taking into account the amplitude and rate of flow and water level changes during the recession phase of the flood peak process.