Metabolic cost of acute and chronic exposure of Nile tilapia (Oreochromis niloticus) to different levels of salinity

This study investigated metabolic responses of acute and chronic exposures of tilapia, Oreochromis niloticus, to different levels of salinity. Fish were transferred from freshwater to water with different salinity concentrations (0, 6, 12, 18 and 24 g/L), and oxygen consumption and ammonia excretion were measured at various exposure times (1, 3, 6, 12, 24, 48, 72, 96 h and 30 days). Estimates of metabolic oxygen consumption rate (MO2), metabolic ammonia excretion rate (T-amm), ammonia quotient (AQ) and oxygen-to-nitrogen ratio (O:N) were computed. The results showed that O. niloticus could not tolerate sudden transfer from freshwater to water with salinity concentration of 24 g/L. The best survival rate was recorded from fish cultured in 12 g/L salinity, while the best weight gain was recorded in fish cultured in 6 g/L and 12 g/L salinities. The study revealed that both salinity and time had significant effects (p < 0.050) on MO2, T-amm, AQ and O:N. MO2 was observed to decrease significantly (p < 0.050) with increasing salinity for almost all the exposure times, which suggested that less energy was spent by O. niloticus to maintain a balanced osmoregulation as salinity levels increased. It was also revealed that protein metabolism played a minor role in the maintenance of a balanced osmoregulation in the fish since its rate was seen to significantly decrease (p < 0.050) with increasing salinity and time. Protein metabolism was estimated to contribute not more than 14.74% of the total energy used for osmoregulation in O. niloticus.