Multiple fish diversity facets in the Nu-Salween basin: Unexpected bimodal patterns, environmental drivers, threats, and conservation implications.

The Nu-Salween River represents the second-longest river and one of the last major free-flowing systems in Southeast Asia, constituting a globally significant reservoir of freshwater biodiversity. However, basin-wide patterns of fish diversity across taxonomic, phylogenetic, and functional dimensions, together with weighted endemism, remain poorly understood, and the environmental determinants shaping these patterns are not well resolved. To address this limitation, a basin-scale analysis was conducted using 26 998 georeferenced occurrence records encompassing 445 native freshwater fish species distributed across 382 subdrainages. Species richness, phylogenetic diversity, functional diversity, and weighted endemicity exhibited unexpected bimodal distributions along the longitudinal gradient, revealing two distinct peaks from headwaters to downstream reaches. Climatic and geographic gradients emerged as dominant drivers of spatial structure. Mean annual air temperature, distance to river mouth, elevation, and net primary productivity exerted strong influences on both species richness and phylogenetic diversity. Functional diversity increased primarily with mean annual air temperature and annual total precipitation, whereas weighted endemicity declined with increasing subdrainage area. Although the conflicts between anthropogenic disturbances and fish diversity patterns were weak, protection coverage across priority areas identified for different diversity facets remained critically low, ranging from 0.1% to 15.9%. Divergent spatial configurations among diversity dimensions, combined with inadequate representation within existing protected-area networks, highlight an urgent need for more targeted and context-specific conservation strategies. Effective management strategies must consider the atypical bimodal longitudinal structure of fish diversity and the multidimensional relationships among diversity facets to sustain ecosystem function and resilience under intensifying anthropogenic pressure.