Ecological risk assessment of exotic fish species on native fish diversity in the Ganga River

Abstract

The Ganga River supports a rich assemblage of native freshwater fishes; however, the increasing establishment of exotic species has emerged as a major ecological threat. This study presents an ecological risk assessment of exotic fishes in the middle stretch of the Ganga River (Sarai Mohana-Ramnagar, Varanasi), integrating species composition surveys, morphometric analysis, and ecological indicators. A total of 18 fish species were recorded, including 13 indigenous and 5 exotic species, with an exotic-to-indigenous catch ratio of approximately 40:60. Exotic fishes such as Cyprinus carpio, Oreochromis mossambicus, and Ctenopharyngodon idella were abundant, particularly at downstream nutrient-rich sites. Morphometric analysis of three native and three exotic species demonstrated strong positive correlations (r=0.80-0.99) between total length and other body traits. Indigenous species exhibited more uniform growth patterns, whereas exotic fishes displayed higher variability and adaptive plasticity. Length-weight relationships revealed negative algometric growth in most native species (b<3), while C. idella showed positive algometric growth (b=3.616), indicating enhanced adaptability and competitive advantage. Condition factor values further suggested that exotic species can thrive under fluctuating habitat conditions. Water quality parameters such as slightly elevated BOD and reduced DO in downstream stretches favored resilient exotic fishes, enhancing their capacity to outcompete native fauna through resource competition, habitat alteration, and rapid reproduction. Overall, the results indicate that exotic fishes pose significant ecological risks to native fish diversity in the Ganga River. The study emphasizes the need for continuous monitoring, regulatory control of species introduction, habitat restoration, and awareness-based management strategies to protect native fish populations.