Dietary zinc requirement of adult Nile tilapia (Oreochromis niloticus) fed semi-purified diets, and effects on tissue mineral composition and antioxidant responses

The present study was conducted to determine the dietary zinc (Zn) requirement of adult Nile tilapia (Oreochromis niloticus), and to evaluate its effects on growth performance, tissue mineral composition, and antioxidant responses for this fish species. Seven experimental diets were formulated with semi-purified ingredients and supplemented with ZnSO4·7H2O to provide total Zn concentrations of 15.9, 28.0, 36.4, 53.5, 99.7, 176.7, and 334.0mg/kg diet which were determined by analysis. Each diet was assigned to three replicate groups of 15 experimental tilapia (initial body weight=166.9±1.3g) for 12weeks. The percent of weight gain of tilapia increased with increasing dietary Zn levels from 15.9 to 53.5mg/kg over the 12-week period, and then decreased above these levels. The Zn content of the liver, muscle, bone, and scales of fish showed a similar tendency as weight gain with changes in dietary Zn. The Zn requirement was estimated to be 37.2mg/kg based on the weight gain, and 52.1mg/kg based on the bone Zn content. The Fe and Mg contents of fish liver, muscle, bone, and scales were significantly affected by dietary Zn level, with the exception of scale Mg content. The Ca content of the liver and muscle decreased significantly, while Ca content of fish bone and scales did not change significantly with increases in dietary Zn supplementation. Blood chemistry analysis revealed that the activities of serum glutathione peroxidase, total superoxide dismutase, and serum glucose content were significantly increased, while activities of catalase, alanine aminotransferase, and aspartate aminotransferase were significantly decreased with increasing dietary Zn level. Considerable nuclear migration was observed in hepatocytes of fish fed the non-supplemented Zn diet and the highest Zn diet (334.0mg/kg).