Dietary sodium requirement determined for juvenile hybrid tilapia (Oreochromis niloticus * O. aureus) reared in fresh water and seawater

Two 8-week feeding trials were conducted to determine the dietary Na requirement for juvenile hybrid tilapia (Oreochromis niloticus * O. aureus) reared in fresh water and seawater. In each experiment, NaCl was added to the basal diet at 0, 0·5, 1, 2, 3, 5, or 7 g Na/kg diet (fresh water) and at 0, 0·2, 0·5, 0·8, 1·2, 1·5, 2, or 3 g Na/kg diet (seawater). Each diet was fed to three replicate groups of fish, individual fish initially weighing 0·69 (se 0·01) g, in a closed, recirculating rearing system. In fresh water, the tilapia fed the diet supplemented with 2 g Na/kg diet had significantly (P<0·05) greater weight gain than the fish fed the diets supplemented with =>3 and <=0·5 g Na/kg diet. Feed efficiency (FE) in fish generally followed the weight-gain pattern. Gill Na+–K+ ATPase activity was highest in the fish fed the diets supplemented with 1-3 g Na/kg diet, followed by the fish fed the diet with 7 g Na/kg diet and lowest in the fish fed the unsupplemented control diet. In seawater, the weight gain, FE and gill Na+-K+ ATPase activity in fish were not affected by the dietary treatment. Analysis by polynomial regression of weight gain, by broken-line regression of gill Na+-K+ ATPase activity and by linear regression of whole-body Na retention of the fish reared in fresh water, indicated that the adequate dietary Na concentration for tilapia is about 1·5 g/kg diet. The present study also suggests that no dietary Na is required for tilapia reared in seawater.