Influence of supplementation of practical diets with vitamin C on growth and response to hypoxic stress of seabream, Sparus aurata

Gilthead seabream were fed a fish meal based diet, supplemented with graded amounts of ascorbyl polyphosphate equivalent to 0, 25, 50, 100 and 200 mg of l-ascorbate (AA)/kg, for 12 weeks. Although there were no significant differences between growth rates of each group, the feed gain ratio and voluntary feed intake were significantly lower, and the protein efficiency ratio was significantly higher for the fish fed 200 mg AA/kg, when compared with the group fed 0 mg AA/kg. An increase of the ascorbate concentration within the liver and spleen occurred with the increasing vitamin supplementation. After 12 weeks, the fish were subjected to hypoxia for 24 h to determine the influence of AA supplementation on the physiological response to this stressor. A significant hyperglycaemia occurred in fish fed all the diets 3 h after the onset of stress, although a significantly higher resting plasma glucose was observed in fish fed the AA free diet. No significant difference was found in plasma cortisol concentration with stress, with the exception of fish fed the 100 mg of AA/kg diet, where a significantly lower cortisol level was found after 9 h of hypoxia. Fish fed the non-supplemented diet showed wider variation and a tendency to increase this variable, having significantly higher levels at 9 h and 24 h of stress then all the other groups and than fish fed the 200 mg AA/kg diet, respectively. Stress had no detectable effect on liver AA concentration in all groups. However, spleen AA showed significantly increased levels between 3–6 h of hypoxic stress in fish fed 25 and 200 mg AA/kg diet and a further increase after 9 h in fish fed the 200 mg of AA/kg diet. These results suggest that the ascorbic acid requirements for seabream is less than 25 mg/kg diet based on a 12-week growth study and that it requires about a four-field increase in weight before signs of deficiency can be observed. Also, the fact that no variation in liver vitamin C concentration was be detected as a response to stress, suggests that this kind of stress does not significantly increase the utilization of this vitamin. However, a possible relation between dietary ascorbate and the physiological response to hypoxia was found, since the fish receiving the non-supplemented diet showed increased plasma glucose and a tendency to have wider plasma cortisol variations than the fish fed the supplemented diets.