Ascorbic acid presents a challenge. The vitamin's importance for good health and performance in farmed fish is well known, but researchers have yet to determine the dietary supplemental levels that permit optimum health and performance in various species. Further complicating the issue are the poor stability of most vitamin C products and the fact that supplemental levels which are sufficient under laboratory conditions may prove inadequate for intensive commercial operations.
The most clearly established functions of vitamin C involve synthesis of collagen, the fibrous protein constituent of bone, cartilage, tendon and other connective tissue. Other studies have shown a role for vitamin C in immune response, in lessening the effects of toxic chemicals in water, and in helping prevent negative effects of water temperature fluctuations.
Research suggests that the effects of ascorbic acid deficiencies or inadequacies vary with the age of the fish. For example, Gabaudan et al. (1990) reported significant differences in weight gains between rainbow trout fed a vitamin C deficient diet and those receiving 181 g/ton (200 ppm) ascorbic acid, beginning at 10 months of age. After 15 months, the vitamin C deficient fish had only half the weight gain of those receiving normal supplementation.
Moreover, the researchers note that growth is an incomplete indicator of ascorbic acid status. A low dietary intake can support normal growth rates despite the occurrence of both long-term clinical and subclinical deficiencies. Just five months after the study began, the first clinical and macroscopic signs appeared: fish lay on their side, exhibited abnormal swimming behavior and (or) had shortened gill covers and abnormal curvature of the spine (scoliosis).
The most notable pathological changes with vitamin C deficiency occurred in supportive tissue (collagen, bone and cartilage), muscle development and blood-forming organs. Similar deformities--especially scoliosis--have been observed in studies of vitamin C deficient catfish (Lim and Lovell, 1978).
To determine optimal levels of dietary supplementation, Ikeda (1990) studied young rainbow trout in a normal state of health, beginning four weeks after hatching and continuing for 20 weeks. Although there were no visible signs of deficiency with vitamin C supplementation at even 18 g/ton (20 ppm) of dry diet, Ikeda reported that collagen synthesis was depressed when the fish received 45 g/ton (50 ppm) of vitamin C or less. At 90 g/ton (100 ppm), collagen synthesis stabilized.
Liver tissue analysis showed that body stores of vitamin C did not peak until dietary intake reached 454 mg of vitamin C per ton (500 ppm) of dry diet. As a result, Ikeda recommends this level to help maintain good health for young rainbow trout. In 10-month-old fish, Ikeda reported different effects with vitamin C deficiencies or inadequacies. There were no deformities in skin collagen, suggesting that the fish could biosynthesize vitamin C up to the minimum amount necessary to maintain life. However, when fish suffered from skin wounds, there was a significant difference in regenerative capacity between fish receiving no dietary vitamin C and those receiving 1,814 g/ton (2,000 ppm). Thirty days after wounding, supplemented fish had almost six times more regenerated tissue than the deficient fish.
Ikeda says these results suggest an increased demand for vitamin C when collagen synthesis is urgently needed to promote healing--for instance, in net cage rearing when fish are affected by bacterial diseases caused by "rub."
Ikeda also reported that rainbow trout fed increased dietary vitamin C showed enhanced tolerance of environmental stresses, such as reduced ambient oxygen. When water oxygen level was maintained below 0.9 ml/l for four hours, fish survivability increased in proportion to dietary vitamin C levels. Livability declined to 50 percent in the fish receiving no dietary vitamin C, compared with 80 percent for fish receiving either 272 g/ton (300 ppm) or 907 g/ton (1,000 ppm) and 90 percent for those receiving 2,722 g/ton (3,000 ppm).
Less clear but nonetheless supported by a number of studies has been the relationship between stress, immune response and vitamin C intake. The intensive nature of aquaculture itself increases the occurrence of stressful situations, which have been shown to increase susceptibility to disease.
Various studies have shown that vitamin C-deprived fish are more susceptible to stress and seem to develop impaired immune functions. Conversely, in a summary of studies with both rainbow trout and channel catfish, Gabaudan et al. (1990) noted increased survivability to bacterial, parasitic and viral infections with increased dietary levels of vitamin C.
Although the reasons for this relationship remain unclear, various studies suggest it may be due to specific antibody response and (or) nonspecific reactions such as improved phagocytosis and lymphocyte stimulation. Li and Lovell (1985), for instance, measured specific antibody titers and serum complement (the proteins found in blood serum that combine with antibodies to destroy pathogenic bacteria and other foreign cells). Both were significantly enhanced when channel catfish received 2,722 g/ton (3,000 ppm) of dietary vitamin C--or 10 times the amount required to maintain growth and bone structure (Figure 1).
