This study investigated the availability, abundance, and seasonal variations of fish in Msimbazi, Kizinga, and Mbezi Rivers, Tanzania. Nine species, Schilbe mystus, Megalops cyprinoides, Terapon jarbua, Liza sp., Tilapia sp., Gerres filamentosus, Lutjanus fulvus, Clarias gariepinus, and Periophthalmus argentilineatus, were identified, with varying abundance across rivers and seasons. Fluctuations of mean %TOC and %TN in fish tissues were observed across seasons, with mean %TOC values varying from 30.4 ± 0.5% (during the dry season) to 35.1 ± 2.1% (during the wet season) and average %TN ranging from 9.3 ± 0.1% (in the dry season) to 10.7 ± 1.3% (in the wet season). Stable isotope ratios (δ13C, δ15N) also exhibited seasonal variations, with δ13C values ranging from - 21.3 ± 0.6‰ to - 16.0 ± 0.1‰ (dry season) and - 21.3 ± 1.1‰ to - 13.7 ± 0.1‰ (wet season), and δ15N values ranging from 9.3 ± 0.0‰ to 15.9 ± 2.7‰ (dry season) and 10.0 ± 0.1‰ to 17.9 ± 0.6‰ (wet season). These variations suggest diverse feeding habits and potential movement patterns between freshwater and marine environments. This study emphasizes the significance of factoring in seasonal fluctuations and dietary origins when investigating the ecology of fish inhabiting coastal rivers. Future research should focus on the impact of environmental factors (temperature, pH, dissolved oxygen) and human activities (land-use changes, pollution) on these ecosystems, with long-term monitoring programs being crucial for the sustainable management of these vital resources.