Fish Ecology and Sustainable Management Strategies in the Yom River Basin, Thailand

Freshwater biodiversity faces unprecedented pressures from habitat fragmentation, land-use intensification, and climate change, yet few tropical basins have long-term records to assess ecological trajectories. Here, we analyze fish assemblages in the Yom River, one of the last free-flowing tributaries of the Chao Phraya system in Southeast Asia, using standardized surveys from 2000, 2011, and 2023. Across three decades, 223 species representing 43 families were recorded, with Cyprinidae, Nemacheilidae, and Danionidae dominating. Multivariate analyses revealed significant temporal and spatial restructuring of communities: upland zones remained specialized but species-poor, foothill reaches supported intermediate diversity, floodplains harbored tolerant generalists, and the main channel maintained the highest richness, including large migratory taxa. Canonical correspondence analysis identified substrate composition, dissolved oxygen, temperature, and agricultural land cover as key drivers of diversity. Species richness peaked in mid-elevation transitional habitats but declined sharply under elevated ammonia and intensified land use, highlighting nonlinear thresholds. Spawning phenology was tightly coupled to hydrological regimes, with upland species reproducing before monsoon flows and floodplain taxa synchronized with flood pulses. These findings demonstrate that biodiversity in tropical rivers is structured by synergistic environmental stressors and connectivity loss, with clear implications for basin-wide management. The Yom River case emphasizes the global importance of conserving free-flowing tributaries and integrating long-term biodiversity monitoring into adaptive freshwater conservation strategies.