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

Abstract

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 7g Na/kg diet (fresh water) and at 0, 0·2, 0·5, 0·8, 1·2, 1·5, 2, or 3g 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 2g Na/kg diet had significantly ( P <0·05) greater weight gain than the fish fed the diets supplemented with ≥3 and ≤0·5g 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 1–3g Na/kg diet, followed by the fish fed the diet with 7g 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 1·5g/kg diet. The present study also suggests that no dietary Na is required for tilapia reared in seawater.