Deciphering the carbon and nitrogen cycling in selected tropical coastal fish: revelations from stable isotope fluctuations and feeding patterns

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 (delta 13C, delta 15N) also exhibited seasonal variations, with delta 13C values ranging from - 21.3 +/- 0.6 parts per thousand to - 16.0 +/- 0.1 parts per thousand (dry season) and - 21.3 +/- 1.1 parts per thousand to - 13.7 +/- 0.1 parts per thousand (wet season), and delta 15N values ranging from 9.3 +/- 0.0 parts per thousand to 15.9 +/- 2.7 parts per thousand (dry season) and 10.0 +/- 0.1 parts per thousand to 17.9 +/- 0.6 parts per thousand (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.