Effects of dietary vitamin C levels on tissue ascorbic acid concentration, hematology, non-specific immune response and gonad histology in broodstock Japanese eel, Anguilla japonica

• Supplemental effect of vitamin C was investigated in diet for broodstock Japanese eel.
• Dietary vitamin C supplementation augmented fish non-specific immune response.
• Increment of dietary vitamin C level reduced lipid peroxidation.
• Tissue vitamin C concentration reflected the dietary vitamin C levels.
• Gonad histology showed the enhancement of collagen synthesis by vitamin C increment.

A feeding trial was conducted to evaluate the effects of dietary vitamin C (L-ascorbic acid, AA) levels on tissue AA concentration, hematology, non-specific immune response and gonad histology in male broodstock Japanese eel. A basal commercial diet was used as a control, and four other diets were prepared by supplementing 200, 400, 800 or 1600 mg AA kg− 1 diet in the form of L-ascorbyl-2-monophosphate (AMP). The analyzed AA concentrations of the diets were 32, 206, 423, 840 and 1686 mg kg− 1 diet, respectively. Triplicate groups of fish (initial body weight, 360 g) were fed one of the test diets at a ratio of 2% body weight for 16 weeks. At the end of the feeding trial, AA concentration in liver, kidney, muscle and testes was increased significantly in a dose dependent manner by increment of dietary AA level (P < 0.05). Significantly higher hematocrit value was found in fish fed 1686 mg AA kg− 1 diet compared to 32 and 423 mg AA kg− 1 diets. White blood cells count of fish fed 1686 mg AA kg− 1 diet was significantly higher than those of fish fed 32–206 mg AA kg− 1 diet. Plasma glucose concentration was increased significantly with increasing dietary AA levels up to 840 mg AA kg− 1 diet; also significant enhancements in aspartate aminotransferase and alanine aminotransferase values were obtained at 1686 mg AA kg− 1 diet. Significantly higher superoxide dismutase activity was recorded in fish fed 840–1686 mg AA kg− 1 diet compared to 32–206 mg AA kg− 1 diet. The results of gonad histology showed an enhanced number of spermatogonia by dietary AA increment. The optimal dietary AA requirement level was estimated at approximately 410.8 and 911.8 mg AA kg− 1 diet by broken-line regression analysis based on liver and testes AA concentration, respectively.