Abstract — Rapid bioassessment was carried out to examine the ecological condition of an aquatic ecosystem of the three major rivers of Calabanga. The site was analyzed
using indicators such as fish and significant invertebrate populations. Fish profiling and invertebrate composition, abundance, diversity, and distribution were analyzed. Fish samples were collected using a fine mesh net, trowel method, and traps for mollusk and crustacean samples. Nonparametric multivariate statistical analyses were used to analyze the data. Fifteen (15) mollusk genera were observed and identified in the study, followed by fish with ten (10) genera and crustaceans (shrimp and crabs) with eight (8) genera. Tigman River showed the highest number of species 32 identified on site, followed by the Hinagyanan River and Inarihan river with 25 and 21 species, respectively. Using the Shannon Weiner Index of Diversity, Tigman river showed a high value of diversity index of 3.00 while Hinagyanan and Inarihan with 2.40 and 2.31, respectively. Water quality was analyzed and exhibited direct and indirect impacts on the species composition. The downstream part of Tigman River also observed a highly diverse area, specifically in fish and mollusk species while species composition and diversity of the Hinagyanan and Inarihan rivers were similar values. This observed trend was affected by several anthropogenic impacts than environmental stress. Anthropogenic activities in the area can explain the existence of introduced species in the river. For instance, in O. niloticus, some local residences in Inarihan river (Tawang station) have tilapia aquaculture near the river. Once their population has been established, invasive species are difficult to eliminate. Moreover, introduced fish species are more likely to occur in anthropogenically impacted environments.