Impact of Elevated Aqueous Ammonia on Liver of Rabbitfish, Siganus rivulatus. Histopathological and Ultrastructure Studies

The world's population is increasing while levels of capture fisheries remain roughly stable with limited potential of increase. The only choice to meet the world's increasing demand of fish is by farming them. Aquaculture is a developing industry and a large proportion of the industry is using aquaculture technology. A consequence of high-density aquaculture is the increased probability to exposure to elevated concentrations of nitrogenous wastes; the principal end product of amino acids metabolism that tends to accumulate and negatively affect the fish. Yet, the effects of ammonia on economically important rabbitfish (Siganus rivulatus) are not well documented. The present study aims to investigated the hypothesis that chronic exposure of fish to increased aqueous ammonia would affect certain parameters including, the structural liver integrity. Fish were treated with total ammonia nitrogen (TA-N) using 0-12 mg/L. Results obtained indicated that fish reared in TAN-0 mg/ L had 100% survival and 50% weight increase in 50 days. All fish at TAN-2 and 4 mg/ L (low dose) died, whilst fish in 6 -12 mg/ L TA-N survived and grew albeit less than in treatment 0 mg/ L. After 20 days of the chronic exposure, fish was randomly taken from each tank in treatments 0, 2 and 10 mg/ L TA-N and their liver were fixed and stained by routine methods and examined using light and electron microscopy. Results indicated that high dose ammonia (TAN -10) is deleterious to marbled rabbitfish since histologically liver exhibited a disrupted architecture, necrosis, blood congestion and sever ultrastructural alterations in comparison to low dose exposed to TA-N -2 and those exposed to TA-N -0. Lipid droplets and glycogen content were highly accumulated in hepatocytes as a result of exposure to TA-N, this could be due to induced imbalance between fat production and utilization and disturbance in lipid metabolism. Results obtained suggests chronic structural and functional damage, as a result of exposure to ammonia. It is recommended that ammonia levels be measured regularly during production of rabbitfish and be maintained below the threshold concentration used in this study to maximize production.