Dietary calcium requirements of juvenile tilapia, Oreochromis niloticus x O-aureus, reared in fresh water

A feeding experiment was conducted to determine the dietary calcium (Ca) requirement for juvenile hybrid tilapia, Oreochromis niloticus x O. aureus reared in nature water. Purified diet supplemented with 0, 1, 2, 3, 4, 5, 7 and 10 g Ca kg(-1) diet providing of 0.6, 1.6, 2.6, 3.7, 4.7, 5.5, 7.5 and 10.7 g Ca kg(-1) diet, respectively, were fed to tilapia (mean initial weight: 0.52 +/- 0.01 g, n = 3) for 8 weeks. Each diet was fed to three replicate groups of fish in a closed, recirculating fresh water rearing system. The rearing water contained 27.1-33.3 mg L-1 Ca. The tilapia fed the diets supplemented with 3.7 g Ca kg(-1) had significantly (P < 0.05) higher weight gain, when compared with fish fed the diet with <= 1.6 g Ca kg(-1). Fish fed the unsupplemented control showed significantly lower weight gain when compared with the other groups (P < 0.05). Bone Ca concentration was highest in fish fed the diets with >= 4.7 g Ca kg(-1), intermediate in fish fed the diet with 2.6 g Ca kg(-1) and lowest in fish fed the control diet. Scale Ca concentration was higher in fish fed the diets with 3.7 g Ca kg(-1) than in fish fed the diets with <= 2.6 g Ca kg(-1). Serum alkaline phosphatase activity was 36% increased in fish fed the diets with 2.6 g Ca kg) 1 than fish fed the diets with <1.6 g Ca kg(-1). Analysis by broken-line regression of weight gain, bone and scale Ca concentrations indicated that the adequate dietary Ca concentration for tilapia in water containing 27.1-33.3 mg Ca L-1 was 3.5, 4.3 and 4.2 g Ca kg(-1) diet, respectively, supplied as Ca-lactate.