Comparative Study of Fish Diversity and Water Quality in Two Contrasting Ecological Habitats of the AC Canal, Kuttanad, Kerala, India

Introduction: Aquatic vegetation is a major determinant of freshwater fish community structure, and invasive floating macrophytes such as water hyacinth (Eichhornia crassipes) are widely reported to alter habitat quality and reduce faunal diversity. Objectives: This study compared fish diversity and water quality between two ecologically contrasting stations of the Alappuzha–Changanacherry (AC) Canal, part of the Pamba River system in the Kuttanad wetland of Kerala, India: Ramankary, a relatively open, clear water habitat, and Kidangara, a habitat extensively covered by water hyacinth. Methods: Monthly fish sampling using traditional gears (cast nets, gill nets, hook and line, drag nets, and seine nets, operated from a country canoe) was carried out from February to April 2026, together with physicochemical analysis of water samples at the Kerala Water Authority laboratory, Thiruvalla. Results: Twenty three fish species belonging to fifteen families were recorded from Ramankary (442 individuals), compared with only thirteen species from Kidangara (230 individuals). Diversity indices computed from the catch data showed higher species richness, Shannon–Wiener diversity (H′ = 2.590) and Margalef’s richness (d = 3.612) at Ramankary than at Kidangara (H′ = 2.368, d = 2.207); however, Pielou’s evenness was higher at Kidangara (J′ = 0.923) than at Ramankary (J′ = 0.826), indicating that the smaller Kidangara assemblage was more evenly distributed among fewer, hardier species, whereas Ramankary’s richer assemblage was numerically dominated by a small number of highly abundant taxa. Dissolved oxygen was markedly lower at Kidangara (1.6 mg/L) than at Ramankary (5.2 mg/L), while turbidity, colour, acidity, sulphate, total dissolved solids and total hardness were all higher at Kidangara, which also showed acidic pH (5.11) against near neutral pH at Ramankary (6.94). Independent samples t tests indicated that these differences were statistically significant for 11 of 13 testable parameters (p < 0.05), with dissolved oxygen, pH, sulphate and turbidity showing the largest effect sizes. The surviving assemblage at Kidangara was dominated by air breathing, hypoxia tolerant taxa such as Channa striata, Channa marulius, Heteropneustes fossilis and Anabas testudineus. Conclusions: These findings indicate that water hyacinth infestation is associated with reduced fish diversity and significantly altered water quality in Kuttanad canals, underlining the need for weed management and improved water circulation to conserve fish biodiversity in this ecologically important wetland.