INFLUENCE OF DIETARY VITAMIN-C DOSAGE ON TURBOT (SCOPHTHALMUS-MAXIMUS) AND EUROPEAN SEA BASS (DICENTRARCHUS-LABRAX) NURSERY STAGES

Stable forms of vitamin C were verified as dietary sources of ascorbic acid (AA) for the nursery stages of European sea bass and turbot. In a first experiment, various concentrations of ascorbyl palmitate (AP) and one level of AA 2-polyphosphate (ApP) were evaluated in a semi-pur ified diet for European sea bass, Dicentrarchus labrax. A significantl y lower AA concentration was detected in the fry fed the AP-supplement ed diets compared to the ApP-fed group, providing evidence that AP is an inferior source of vitamin C for young stages of sea bass. ApP was subsequently selected for the determination of the AA requirements of both sea bass and turbot (Scophthalmus maximus) fry. The AA levels inc orporated in the body tissues reflected the levels of ApP in the diet, providing evidence of the bioavailability of this stable phosphate es ter. Reduced production results were only obtained in the AA-free trea tment, indicating that a level of 20 mg AA/kg is sufficient for sea ba ss and turbot during nursery rearing. This was supported with data fro m the ultrastructural evaluation of the hepatocytes. Results of challe nge tests with pathogenic Vibrio strains remained inconclusive and did not provide evidence for an immunostimulative effect of high AA level s during the early post-weaning phase of these species.