Water quality drivers modulate the biology of Cirrhinus reba (Hamilton, 1822) and validate its role as a bioindicator in the tropical river Burhi Gandak, India.

Freshwater ecosystems are under severe threat, necessitating the use of bioindicator species to monitor ecological health. This study provides a holistic assessment of the spatio-temporal linkages between water quality and the biological responses of the minor carp, Cirrhinus reba, in the midstream stretch of the River Burhi Gandak (RBG), India, from July 2020 to June 2021. Monthly assessments of key water parameters and 220 specimens (90 juveniles, 55 males, 75 females) were conducted to study morphometry, growth, feeding, and reproduction. Marked seasonal variations were observed in water temperature (18.0-30.5 °C), conductivity (250-405 µS/cm), pH (7.4-7.8), dissolved oxygen (6.3-8.3 mg/L), Biochemical Oxygen Demand (BOD) (1.5-3.4 mg/L), and nitrate (0.31-0.50 mg/L). The length-weight relationship indicated negative allometric growth (b < 3) across all groups. The condition factor peaked during pre-spawning (March-April) and declined post-spawning. Gut content analysis confirmed an omnivorous, detritivorous diet, dominated by plant material (24.75%) and debris (22.92%), with a distinct seasonal shift to zooplanktivory (up to 8.69% cladocerans) in winter. The annual mean trophic level, derived from diet composition, was 2.06 ± 0.07, confirming a primary consumer status. Reproductive biology revealed a female-biased sex ratio (1:1.3), five gonadal maturation stages, and peak spawning during June-July, with fecundity ranging from 20,722 to 102,230 eggs. Principal component analysis (PCA) explained 76.8% of the total variance, demonstrating strong synergistic linkages between water quality variables (temperature, conductivity, nitrate, BOD) and biological indices (Gonado-Somatic Index, GSI; gastrosomatic index, GaSI; relative gut length, RGL), with clear seasonal clustering. The findings underscore that C. reba's life-history strategies are tightly synchronized with the river's physico-chemical regime and validate its utility as a sensitive bioindicator for informing conservation and sustainable fisheries management in the RBG and similar tropical river systems.