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

摘要

A growth trial was conducted to quantify the dietary magnesium (Mg) requirement for hybrid tilapia, Oreochromis niloticuscOreochromis 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 0.57gMgkgp# diet), each was fed to triplicate groups of tilapia (initial body weight: 0.88pl0.01g) reared either in the freshwater or seawater for 8weeks. The Mg concentration in freshwater and seawater was approximately 4 and 1,400mgMg Lp#, respectively. Tilapia reared in freshwater, weight gain (WG) was highest (P<0.05) in fish fed diets supplemented with E0.15gMg kgp# diet, followed by fish fed the diet with 0.061gMg kgp# diet, and lowest in fish fed the diet with 0.025gMg kgp# diet. Plasma alkaline phosphatase (ALP) activity was higher in fish fed diets with 0.23 and 0.30gMg kgp# 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 0.23gMg kgp# 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 0.2gMg kgp# diet. The result also suggests that dietary Mg beyond 0.025gMg kgp# is required for tilapia reared in seawater.