REGULATION OF SATURATION AND DEPLETION OF ASCORBIC-ACID IN RAINBOW-TROUT

Rainbow trout (Oncorhynchus mykiss) juveniles were fed fish meal-based diets supplemented with 0, 30, 60, 120, 240, and 480 mg ascorbic acid kg-1 dry diet or 15, 60, and 240 mg ascorbate monophosphate (expresse d as ascorbate equivalents) kg-1. Significant differences in both grow th and instantaneous mortality rate between two compared strains (Lond on, OH USA and Mount Shasta, CA USA) were found in fish fed the vitami n C-free diet. Deficiency symptoms included anorexia and lethargy, but no acute signs of lordosis or scoliosis were observed before heavy mo rtality after 9 weeks. Diets with supplemental ascorbate monophosphate supported significantly better growth and resulted in lower mortality than diets supplemented with equivalent amounts of ascorbic acid. Asc orbate concentrations in tissues (liver kidney, and intestine) during the saturation period were higher in the groups fed 60 and 240 mg asco rbate monophosphate/kg than in groups supplemented with corresponding or higher levels of ascorbic acid. Significant differences in mortalit ies were experienced in fish with a different ascorbate status during the depletion phase. Kaplan-Meier analysis indicates that initial asco rbate level during the depletion phase can influence survival of fish. Analyses of ascorbate concentration during depletion revealed that th e half-life (t1/2) amounted to 42.8 to 50.2 days for the liver and 47. 8 to 79.6 days for the kidney. These data imply that a conservation o f tissue ascorbate is not operative in juvenile rainbow trout. We prop ose a two-compartmental model as describing ascorbate depletion patter n in juvenile rainbow trout.