This study quantitatively analyzes fish community responses to environmental gradients in Myanmar’s Irrawaddy Delta. Integrating beta-diversity partitioning, Threshold Indicator Taxa Analysis (TITAN), single-season occupancy modeling, and Structural Equation Modeling (SEM), and species co-occurrence network analysis, we identified primary environmental filters shaping ichthyofaunal structure. Spatial comparison between Bogale and Pyapon ecosystems revealed fundamentally distinct communities driven predominantly by species turnover (87.1%). Network topologies further demonstrated a significant spatial restructuring of biological interactions, with the primary network hub role shifting from the highly sensitive Tenualosa ilisha in the upper estuary to the highly adaptable Macrognathus zebrinus in the lower delta. Furthermore, SEM established a substantial structural connection between environmental stress and biological assemblage response (β = 0.99), suggesting water quality as the ecosystem’s master driver. TITAN and occupancy models demonstrated an “estuarine enrichment” effect, where primary network hubs (Tenualosa ilisha, Coilia neglecta) reached peak occupancies only beyond high salinity thresholds (>18.16 ppt). However, escalating water temperatures act as a critical limiting factor, with a strict thermal boundary identified at 27.6 °C, beyond which sensitive taxa populations rapidly decline. These findings provide direct implications for adaptive fisheries management, underscoring the necessity of monitoring osmotic and thermal change-points to protect vital fisheries from compounded climate change impacts.
