Dietary calcium requirements of juvenile tilapia, Oreochromis niloticus × O. aureus , reared in fresh water

A feeding experiment was conducted to determine the dietary calcium (Ca) requirement for juvenile hybrid tilapia, Oreochromis niloticus  ×  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.