Fish Assemblage Diversity of Dharla River, Bangladesh, With Relation to Environmental Variables: A Multivariate Analysis

ABSTRACT The Dharla River is a significant freshwater system that runs through Bangladesh, India, and Bhutan, eventually meeting the Brahmaputra River and harboring numerous fish species. The study aimed to investigate the fish assemblage structures of the Dharla River in relation to the seasons and environmental variables. Monthly data on fish assemblages and environmental variables were collected from five sites along the Dharla River between July 2022 and June 2023. One‐way ANOVA was conducted at a 5% significance level to assess seasonal fluctuations in environmental variables, fish abundance, and diversity indices. Various multivariate analyses, including canonical correspondence analysis (CCA), principal component analysis (PCA), and NMDS plot, were used to examine the relationships between fish species abundances, environmental variables, and their seasonal variations. Furthermore, analysis of similarity (ANOSIM) and similarity percentage analysis (SIMPER) were applied to evaluate the diversity of fish communities throughout the seasons and identify the key fish species that cause the diversity. The environmental parameters of the Dharla River showed distinct differences during the monsoon (July–October) compared to pre‐monsoon (March–June) and post‐monsoon (November–February). Seventy‐nine fish species were reported from five selected sites over 12 months, representing eight orders, 25 families, and 57 genera. Fish abundance peaked in the post‐monsoon season, while the lowest number of fish species was observed during the pre‐monsoon. ANOSIM revealed a statistically significant variation ( p < 0.05) in fish species abundance across the seasons. Additionally, SIMPER indicated an average dissimilarity of 43.35% among three seasons, with two species ( Esomus danricus and Cirrhinus reba ) contributing the most to this dissimilarity. Based on an NMDS plot, fish samples from all seasons and sites were found to be divided into three distinct groups. The diversity indices, including Shannon‐Weaver diversity ( H ), Pielou's evenness ( e ), and Margalef richness ( D ), exhibited variations across the seasons, ranging from 3.53 to 4.05, 0.60 to 0.82, and 9.31 to 10.66, respectively. Furthermore, the study identified that temperature, pH, nitrite, and dissolved oxygen were positively correlated with the fish assemblage structure in the Dharla River. This study provides the baseline information on the seasonal fish assemblage structures of the Dharla River and the relationship between fish abundance and environmental variables, which is crucial for the sustainable management and conservation of the important trans‐boundary Rivers like Dharla and other open‐water fishery resources in Bangladesh.