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 g raded amounts of ascorbyl polyphosphate equivalent to 0, 25, 50, 100 a nd 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 20 0 mg AA/kg, when compared with the group fed 0 mg AA/kg. An increase o f the ascorbate concentration within the liver and spleen occurred wit h the increasing vitamin supplementation. After 12 weeks, the fish wer e subjected to hypoxia for 24 h to determine the influence of AA suppl ementation on the physiological response to this stressor. A significa nt hyperglycaemia occurred in fish fed all the diets 3 h after the ons et of stress, although a significantly higher resting plasma glucose w as observed in fish fed the AA free diet. No significant difference wa s found in plasma cortisol concentration with stress, with the excepti on of fish fed the 100 mg of AA/kg diet, where a significantly lower c ortisol level was found after 9 h of hypoxia. Fish fed the non-supplem ented diet showed wider variation and a tendency to increase this vari able, having significantly higher levels at 9 h and 24 h of stress the n all the other groups and than fish fed the 200 mg AA/kg diet, respec tively. Stress had no detectable effect on liver AA concentration in a ll groups. However, spleen AA showed significantly increased levels be tween 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. Th ese results suggest that the ascorbic acid requirements for seabream i s 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 deficie ncy can be observed. Also, the fact that no variation in liver vitamin C concentration was be detected as a response to stress, suggests tha t this kind of stress does not significantly increase the utilization of this vitamin. However, a possible relation between dietary ascorbat e and the physiological response to hypoxia was found, since the fish receiving the non-supplemented diet showed increased plasma glucose an d a tendency to have wider plasma cortisol variations than the fish fe d the supplemented diets. (C) 1998 Elsevier Science B.V.