Fish community structure in accordance with environmental signatures in a tropical river ecosystem in the Eastern Himalayan ecoregion

Abstract

Environmental characteristics significantly influence the distribution of fish communities in aquatic ecosystems. This study examined the relationship between fish community structure and ecological characteristics in the Dhansiri River, a tropical river within the Eastern Himalayan ecoregion. Sampling was conducted across three seasons (monsoon, premonsoon, and postmonsoon) at four stations representative of the whole river stretch. The highest number of species was recorded along the upper stretch S1 (54), followed by S2 (45), S3 (41), and S4 (37). Seasonally, the number of species peaked during monsoon (64), decreased in postmonsoon (59), and was lowest in premonsoon (54). The Shannon diversity index (H′) ranged from 3.327 to 3.750, with higher values upstream and gradually declining downstream. Species diversity was lowest at S4, likely owing to poor environmental conditions and high anthropogenic pressure. Cyprinids emerged as the most dominant fish group, with relative family abundance varying from 0% to 5.63%. Nonmetric dimensional scaling indicated a distinct separation of S4 from S1, S2, and S3. Analysis of water quality revealed a pristine nature at S1, with gradual deterioration downstream. Significant relationships were identified between most water quality variables and fish community structure. Principal component analysis showed that pH (20.96%), total alkalinity (13.80%), specific conductivity (9.92%), nitrite (NO₂; 12.19%), and total dissolved solids (TDS; 6.22%) contributed significantly to the first principal component (Dim1), while nitrate (NO₃; 53.43%) and water temperature (6.05%) influenced the second principal component (Dim2). Biota and/or environment matching (BIO-ENV) analysis reflected that ammonia (NH₃), NO₃, carbon dioxide (CO₂), TDS, total alkalinity, pH, specific conductivity, dissolved oxygen (DO), and water temperature significantly correlated with fish abundance and community composition. This study provides critical insights into the role of environmental parameters in shaping the fish community structure in a less-explored tropical river of the Eastern Himalayas and offers valuable information for the sustainable management of riverine fish diversity.