Impact of Water Quality and Pollution Diversity on Fishes and Macrofauna in the Koshi River, Madhepura District, Bihar

The current research paper looks at how the deterioration and diversity of the water quality and pollution have affected fish and the aquatic macrofaunal fauna of the Koshi River around the Madhepura district in Bihar. The Koshi River is a very dynamic river-floodplain system which lies in the North Bihar region and has a crucial ecological and socio economic application because it has supported fisheries, agriculture and livelihoods of the locals. Nevertheless, the growing human activities like agricultural runoff, household sewage effluent, sand mining, and heavy application of fertilizers and pesticides have caused dramatic harm to water quality especially in tributaries and abandoned channels (Dhars). Through the analysis of various physico-chemical parameters measured during the period of 2020 to 2025, it is possible to discuss high turbidity, high biochemical oxygen demand, variable levels of dissolved oxygen, high levels of nutrients and new microplastic pollution. These have negatively impacted aquatic biodiversity because they have changed the conditions of the habitat, food webs, and also caused physiological stress in aquatic organisms. Ecological homogenization is shown by ichthyofaunal evaluation of a gradual displacement of sensitive, economically valuable species such as Catla catla, Labeo rohita, Cirrhinus mrigala, and Wallago attu by small, pollution-resistant species, including Puntius sophore and Mystus vittatus. Siltation, organic pollution, and metal toxicity significantly negatively affect aquatic macrofauna, including mollusks and bivalves, as well as insect larvae, which are indeed sensitive bioindicators of the well-being of the ecosystem. Seasonal changes also indicate that there are increased biodiversity in winter when water conditions are relatively constant.