Environmental Stress-Driven Trophic Associations Modulate Fish Biogeography in a Multi-Tributary River Basin

Mountain fish biodiversity is increasingly threatened by global environmental change and anthropogenic pressures, yet the mechanisms linking abiotic factors to fish biogeography through trophic associations remain unclear. In this study, we explored these ecological linkages in the upper Hanjiang River basin (China) using environmental DNA (eDNA) metabarcoding, remote sensing, water quality analysis and hydrological monitoring at 187 seasonal sampling sites. We identified 140 fish species from 79 genera and observed a shift from Leuciscidae-dominated communities upstream to Xenocyprididae-dominated communities downstream. Cumulative dendritic distance upstream (CDDU) was identified as a key spatial driver of fish communities. Three patterns emerged: (1) the species richness of fish taxonomic and functional feeding groups exhibited hump-shaped patterns along the CDDU gradient, whereas food resources exhibited group-specific distribution patterns; (2) the local contribution to beta diversity (LCBD) and stability of fish communities showed U-shaped and hump-shaped patterns, respectively; (3) deterministic processes dominated fish community assembly. Water quality (43.6%) was the primary determinant of fish α-diversity, followed by biotic factors (32.3%) and land use (7.4%). Fish stability was influenced by water quality (32.3%), hydro-morphology (26.8%) and land use (17.0%). Moving window analysis showed that biotic factors (food resources) peaked in their impact on fish α-diversity at intermediate CDDU before declining, with their influence on stability increasing nonlinearly along CDDU gradients. Trophic associations followed U-shaped patterns along environmental stress gradients, peaking at moderate stress. High stress regimes triggered trophic decoupling, reducing fish diversity and stability. These findings highlight how anthropogenic pressures disrupt fish biogeography through altered trophic associations.