Fish species diversity and richness are influenced by ecological niches, environmental stress, and the tolerance range of species traits. The Baya River, which flows between the Ganga River basin and the Burhi Gandak River basin, has been a significant site for ichthyofaunal studies. A total of 56 fish species were previously reported near Koiria Nizamat, representing 39 genera, 22 families, and 9 different orders. However, during the pre-monsoon period, only 34 fish species were collected from the same site. These 34 species were classified under 24 genera, 13 families, and 6 orders. The family Cyprinidae was found to be the most diverse, comprising 15 species. Most of the identified species fall under the Least Concern (LC) category of the IUCN Red List. The observed decline in fish diversity and related ecological concerns are discussed in this study, highlighting the need for conservation efforts to protect and preserve native fish germplasm.