EFFECTS OF DIFFERENT LEVELS OF DIETARY ZINC ON THE GILTHEAD, SPARUS-AURATA DURING THE GROWING-SEASON

Gilthead were fed three diets. Diet A was the control diet, whereas di ets B and C were supplemented with 300 and 900 mg Zn/kg, respectively. Fish fed with diet C, at the end of the experiment, showed the lowest weight. Zinc concentrations presented the higher values in gills, liv er, and kidney. Muscle and brain had the lower mean values and showed a tight control of zinc levels. These results reinforce the hypothesis that zinc in the CNS should be strictly controlled in order to mainta in the functional role of the metal. Significant differences in tissue zinc concentrations were obtained between fish fed different amounts of zinc, the metal concentrations being higher in tissues of fish fed diet C. The tissue decrease of zinc, found at the end of the experimen t, may depend on a lower feed consumption or on different zinc require ments during the cold season. These changes, even if not univocal amon g the three diets, may be associated with the life cycle of fish. Furt hermore, copper concentrations were little affected by the different c oncentrations of zinc in the three diets; liver and kidney presented t he highest concentrations; liver showed a significant decrease in copp er content at the end of the experiment. We conclude that: zinc concen trations of the diet may affect the gilthead weights and the tissual m etal content; and zinc concentrations in the diets, depending on the g rowth rate, may be varied depending on the season.