Estimation of dietary magnesium requirements of juvenile tilapia, Oreochromis niloticus x Oreochromis aureus, reared in freshwater and seawater

A growth trial was conducted to quantify the dietary magnesium (Mg) requirement for hybrid tilapia, Oreochromis niloticus x Oreochromis aureus, reared in freshwater and seawater. A semi-purified basal diet was supplemented with eight levels of magnesium sulfate (0.025, 0.061, 0.082, 0.15, 0.23, 0.30, 0.36 and 0.57 g Mg kg(-1) diet), each was fed to triplicate groups of tilapia (initial body weight: 0.88 +/- 0.01 g) reared either in the freshwater or seawater for 8 weeks. The Mg concentration in freshwater and seawater was approximately 4 and 1,400 mg Mg L-1, respectively. Tilapia reared in freshwater, weight gain (WG) was highest (P<0.05) in fish fed diets supplemented with >= 0.15 g Mg kg(-1) diet, followed by fish fed the diet with 0.061 g Mg kg(-1) diet, and lowest in fish fed the diet with 0.025 g Mg kg(-1) diet. Plasma alkaline phosphatase (ALP) activity was higher in fish fed diets with 0.23 and 0.30 g Mg kg(-1) diet than that in fish fed the unsupplemented diet. The concentration of Mg in whole body, scale, muscle and plasma increased when dietary Mg concentration increase from 0.025 to 0.23 g Mg kg(-1) diet and then stabilized after further increase in dietary Mg. In the seawater reared group, WG, plasma ALP activity, whole body, scale, muscle and plasma Mg concentration of tilapia were similar (P>0.05) among all dietary treatments. Analysis of WG, ALP activity, whole body, scale, muscle and plasma Mg concentration by broken-line model and whole body Mg retention by linear regression of the fish indicated the optimum dietary Mg requirement for tilapia reared in freshwater is about 0.2 g Mg kg(-1) diet. The result also suggests that dietary Mg beyond 0.025 g Mg kg(-1) is required for tilapia reared in seawater. (C) 2012 Elsevier B. V. All rights reserved.