Temporal variations in fish diversity, guild dynamics, and their association with environmental variables in a tropical estuary

Agniyar estuary in Palk Bay on the southeast coast of India, is an important estuary bordering Point Calimere Key Biodiversity Area. Information on the fish diversity and guild dynamics of this typical tropical estuary is lacking. Hence, the present study provided an account of fish diversity and guild dynamics. The study collected 2342 fish specimens from 18 orders, representing 76 species from 32 families. The predominant order and family were Eupercaria and Sciaenidae, respectively. Thryssa mystax (Bloch & Schneider, 1801), T. malabarica (Bloch, 1795), and Setipinna taty (Valenciennes, 1848) were the most dominant species in terms of abundance. The study recorded seasonal variations in temperature, salinity, pH, and dissolved oxygen, with the summer exhibiting the highest temperature and salinity. The Kruskal-Wallis test followed by Dunn's post hoc analysis revealed significant differences in the nutrient concentrations (nitrite, nitrate, and phosphate). The redundancy analysis and Pearson correlation explained the significant relationship between species abundance and environmental conditions. Diversity indices and dominance/diversity patterns during the pre-monsoon were more dominant and diverse, whereas the summer recorded the lowest diversity. Cluster analysis and similarity profile (SIMPROF) revealed a high level of similarity and a lack of distinct differentiation among the seasonal clusters. The analysis of guild dynamics indicated seasonal fluctuations in ecological, feeding, vertical distributional, and reproductive guilds. The seasonal occurrence of the IUCN Red List category of fishes was presented, and their vulnerability to seasonal changes was elucidated. Significant information on the fish community structure, dynamics, and conservation measures was provided to protect the Agniyar estuary.