Fish Biodiversity in a Tropical Seasonal River: Taxonomic Composition, Trophic Guilds, and Conservation Status in the Vaigai River System, Southern India

Understanding fish assemblages and ecological drivers across four key Vaigai River habitats, informs tropical river biodiversity management through analysis of taxonomic diversity, trophic structure, and seasonal patterns. Over a one-year period (June 2015–May 2016), 94,474 fish specimens were sampled, representing 147 species (139 native, 8 exotic) from 99 genera, 58 families, and 21 orders, with Perciformes (25 species), Cypriniformes (24), and Siluriformes (18) dominating the community. The Athankarai estuary emerged as a biodiversity hotspot (87 species), while midstream sites showed reduced diversity likely due to anthropogenic pressures. Notably, invasive tilapias ( Oreochromis spp.) constituted 35% of total abundance, highlighting ecological disruption, and threatening native species like Pseudetroplus maculatus and Etroplus suratensis . Trophic analysis revealed omnivores preferring animal material (75 species = 51%) as the most common feeding group, followed by carnivores (36 species = 24%), omnivores with plant preference (17%), and herbivores (8%), reflecting adaptive strategies to seasonal flow variability. Predatory taxa (e.g., Carangiformes, Elopiformes) exhibited higher trophic levels (≥ 3.8), indicating functional stratification within the food web. Diversity indices such as Shannon-Wiener (H'), Fisher’s alpha, and Pielou’s evenness (J') indicated stable yet seasonally modulated species diversity, while functional diversity metrics (Delta+, Lambda+) pointed to evolutionary stability and habitat-driven community differentiation. Seasonal analyses revealed bimodal abundance peaks during the Southwest Monsoon (46.6%) and Pre-Monsoon (41.6%), driven by hydrological connectivity. Cluster and multivariate ordination (nMDS, MDS) analyses revealed significant seasonal turnover in fish communities (46–96% similarity), highlighting the influence of hydrological variability on biodiversity patterns. Principal Component Analysis (PCA) revealed hydrological seasonality as the dominant habitat specialized key driver of fish assemblage patterns, with PC1 (98.8%) separating high-flow (Southwest and Pre-Monsoon) periods dominated by Oreochromis spp, Cirrhinus mrigala , and Glossogobius giuris , from low-flow (Post- and Northeast Monsoon) periods. PC2 (0.9%) distinguished estuarine species ( Megalops cyprinoides , Nematalosa nasus ) from pelagics ( Thryssa vitrirostris , Stolephorus commersonnii ), while PC3 (0.3%) revealed trophic differentiation between detritivores and piscivorous small pelagics. Vaigai River's ecological complexity demands flow-dependent, site-specific conservation controlling invasives, preserving habitats, and maintaining hydrological connectivity amid growing anthropogenic and climate pressures.