What controls migratory fish spawning habitat distribution and quality? A novel hydrological-topographic framework for identifying essential factors.

The distribution and quality of fish spawning habitats are jointly associated with basin topography and hydrological conditions, whose spatial and temporal variability creates heterogeneous environmental settings among river reaches and may influence both habitat occurrence and reproductive suitability. Using Coreius guichenoti in the Jinsha River Basin, China, as a case study, we proposed a novel Stratified Identification Framework (SIF) using 72 reach-year observations for habitat-distribution analyses and 23 surveyed reach-year observations for habitat-quality analyses during 2007-2018. Results showed that streamflow had a weak direct effect on habitat distribution (0.08) but a strong indirect effect (0.71), resulting in a total effect of 0.79, whereas water temperature showed the strongest positive effect on habitat distribution (0.80); river gradient and sinuosity further contributed to habitat availability. In contrast, increased streamflow was positively associated with habitat quality (0.66), although sediment flux weakened this association (-0.32), and water temperature showed no significant association with habitat quality (-0.05). Topographic characteristics exerted a considerably stronger influence on habitat quality (0.63) than on habitat distribution (0.19). These findings underscore the necessity of integrated management strategies to facilitate the recovery of migratory fish populations and sustain river ecosystem integrity.