Estimation of dietary copper requirements of juvenile tilapia, Oreochromis niloticus x O-aureus

An 8-week feeding trial was conducted to determine the dietary copper (Cu) requirement of juvenile hybrid tilapia, Oreochromis niloticus x O. aureus. Purified diets with eight levels ( 0, 1, 2, 3, 4, 8, 12 or 20 mg Cu per kg diet) of supplemental Cu were given to tilapia ( mean initial weight 0.79 +/- 0.03 g). Each diet was given to three replicate groups of fish. The rearing water contained 0.98 mug Cu per l. Weight gain was highest in fish given diets supplemented with 2 mg Cu per kg diet, followed by the group given 1 mg Cu per kg diet, then the unsupplemented control group, and was lowest in the 20 mg Cu per kg diet group. The differences between each of these four groups were significant (P < 0.05). Food efficiency (FE) of fish showed a similar trend to that of weight gain. Blood haemoglobin (Hb) and haematocrit (Hct) concentrations were lowest in fish given >= 12 mg Cu per kg diet, followed by fish given the unsupplemented control diet and highest in fish given 2 mg Cu per kg diet ( Hb) and 2 and 3 mg Cu per kg diet (Hct). Plasma ceruloplasmin activity was higher in fish given 1 and 2 mg Cu per kg diet, followed by 3 to 12 mg Cu per kg diet and the control group, and lowest in fish given 20 mg Cu per kg diet. The body Cu content in fish generally increased as the dietary Cu supplementation level increased. Analysis by broken-line regression of weight gain percent and by linear regression of the whole-body Cu retention of fish indicated that the adequate dietary Cu concentration in growing tilapia is about 4 mg Cu per kg diet.