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Vitamin C (VC) plays an essential role in fish physiological function and normal growth. However, its effects and requirement of coho salmon Oncorhynchus kisutch (Walbaum, 1792) are still unknown. Based on the influences on growth, serum biochemical parameters, and antioxidative ability, an assessment of dietary VC requirement for coho salmon postsmolts (183.19 ± 1.91 g) was conducted with a ten-week feeding trial. Seven isonitrogenous (45.66% protein) and isolipidic (10.76% lipid) diets were formulated to include graded VC concentrations of 1.8, 10.9, 50.8, 100.5, 197.3, 293.8, and 586.7 mg/kg, respectively. Results showed that VC markedly improved the growth performance indexes and liver VC concentration, enhanced the hepatic and serum antioxidant activities, and increased the contents of serum alkaline phosphatase (AKP) activity, low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), and total cholesterol (TC) whereas decreased the serum aspartate aminotransferase (AST), alanine aminotransferase (ALT) activities, and triglyceride (TG) level. Polynomial analysis showed that the optimal VC levels in the diet of coho salmon postsmolts were 188.10, 190.68, 224.68, 132.83, 156.57, 170.12, 171.00, 185.50, 142.77, and 93.08 mg/kg on the basis of specific growth rate (SGR), feed conversion ratio (FCR), liver VC concentration, catalase (CAT), hepatic superoxide dismutase (SOD) activities, malondialdehyde (MDA) content, and serum total antioxidative capacity (T-AOC), AKP, AST, and ALT activities, respectively. The dietary VC requirement was in the range of 93.08–224.68 mg/kg for optimum growth performance, serum enzyme activities, and antioxidant capacity of coho salmon postsmolts. [ABSTRACT FROM AUTHOR]

Vitamin C or ascorbic acid (ASA) is an essential micronutrient for normal growth, antioxidant capacity and immunity of fish. However, appropriate requirements of ASA for Ctenopharyngodon idella, grass carp juveniles (8.46 ± 0.19), remain unknown. For this purpose, a feeding trial of 60 days was conducted in triplicates. Six isolipidic and isoproteic diets containing 0, 25, 50, 100, 200 and 400 mg/kg of ASA were prepared. Results showed a linear increase in growth as ASA supplementation was increased from 0 to 100 mg/kg. However, a further increase in dietary ASA supplementation from 200 to 400 mg/kg resulted in a slight decrease in growth parameters. A similar trend was also recorded for the whole body, proximate composition, antioxidant enzymes and organ indices against ASA supplementation. On the contrary, fish fed with basal diet had higher thiobarbituric acid reactive substances (TBARS) content in the liver and muscles than those fed with ASA supplemented diets. ASA concentration in muscles increased in a dose‐dependent manner, while ASA value was the highest in the fish fed with 100 mg/kg ASA concentration. The weight gain (WG%) data were subjected to broken‐line regression analysis, and the optimal ASA requirement of grass carp juveniles was estimated to be 67.1667 mg/kg. Taken together, the inclusion of 67.1667 mg/kg of ASA is recommended in the feed of grass carp juveniles for optimum growth performance. We also recommend analysing the effect of ASA in combination with vitamin A and E. [ABSTRACT FROM AUTHOR]