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

The Nu-Salween stands as the second-longest and the last major free-flowing river in Southeast Asia. Despite its pivotal significance in global biodiversity conservation, patterns of fish diversity across multiple facets—including taxonomic, phylogenetic, functional diversities, and weighted endemicity—and the associated environmental mechanisms underlying these patterns remain poorly understood. We tackled this gap by compiling a comprehensive database documenting 26,998 occurrences of 445 native freshwater fish species across 382 subdrainages covering the entire basin. Fish species richness, phylogenetic, functional, and endemic diversity showed unexpected bimodal patterns along the upstream-downstream gradient, with two distinct peaks within the distributional spectra. These spatial patterns were primarily driven by climate (mean annual air temperature), geography (distance to river mouth, elevation), and net primary productivity, which strongly influenced both species richness and phylogenetic diversity. In addition, functional diversity increased mainly with mean annual air temperature and annual total precipitation, while endemicity decreased with subdrainage area. Although the conflicts between anthropogenic disturbances and fish diversity patterns were weak, the priority areas of different diversity facets remain limitedly protected (0.1% to 15.9%). The inconsistencies in fish diversity patterns combined with inadequate protected areas highlight an urgent need for more targeted and context-specific conservation strategies. Such strategies must consider the uncommon bimodal upstream-downstream patterns of fish diversity and the complex relationships between different fish diversity facets to safeguard ecosystem functionality and stability amidst escalating anthropogenic disturbances.