Fish diversity in reservoirs is vital for sustaining food security and livelihoods in rural communities. Wang Nong Reservoir, in Nakhon Phanom Province, northeastern Thailand, supports local fisheries but has remained scientifically unexplored until now. This study presents the first comprehensive assessment of fish community diversity and structure from January to December 2024. Monthly samples were collected at four stations using gill nets and a beach seine. A total of 8,294 specimens representing 47 species from 14 families were recorded. Dominant families included Cyprinidae (51.06%), Bagridae (10.64%), Cobitidae (6.38%), and Osphronemidae (6.38%). The most abundant species were Rasbora borapetensis (17.36%), Esomus metallicus (11.35%), Thynnichthys thynnoides (5.57%), Cyclocheilichthys repasson (4.77%), and Puntius brevis (4.46%), with twelve species occurring in all samples. Most species were classified as Least Concern on the IUCN Red List, while two were Vulnerable and one was Data Deficient. Seasonal variation was evident, with peak abundance and species richness in summer. The average Shannon-Wiener diversity (H’), richness (R), and evenness (E) indices were 3.12, 4.46, and 0.87, respectively. Water temperature and dissolved oxygen were positively correlated with H’, R, and E (p < 0.01), whereas transparency showed a negative correlation; other parameters (TDS, pH, NH₃, NO₂, and NO₃) were not significantly related. These findings provide the first baseline dataset on fish diversity in Wang Nong Reservoir, filling a critical knowledge gap for inland fisheries in northeastern Thailand and offering a robust scientific foundation for site-specific conservation planning and evidence-based sustainable fisheries management.

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