30 DECEMBER 2014 • WORLD AQUACULTURE • WWW.WAS.ORG increasing production mirrors regional skills in capturing glass eels and transporting them with minimum stress to aquaculture farms and in stocking and feeding for maximum yield, which has advanced greatly. A government policy to protect freshwater resources in 1997 directed the majority of inland farms to terminate eel aquaculture and subsequently production moved to indoor systems. A total of 236 farms covering about 133 ha comprising 202 flow-through systems and 34 recirculating system farms in the Jeon-nam area were exclusively designed and adopted for eel aquaculture (KSNO 2009). Indoor systems for eel farming have always been under scrutiny for improvement of quality, with the view to increase the overall efficiency of farms. Flow-through systems with concrete bottoms have been replaced by polypropylene polymer-bottom circular tanks. Circular tanks ranging from 30-50 m2 up to 200-300 m2 are used, based on eel life-stage. The Need to Establish Nutrient Requirements There are a number of constraints that need to be resolved to develop a complete technology package for eel aquaculture. Poor understanding of nutrient requirements and the availability of balanced diets are major barriers to further expansion of eel aquaculture. The greatest mortality at eel farms has been reported during the weaning period and the adaptation period to dry feed. Inasmuch as it is unlikely capture production will increase further, studies have been conducted to boost aquaculture production to meet the ever-increasing demand for this species (Lee and Bai 1997, Okorie et al. 2007, Bae et al. 2008). Our research center (FFNRC) has been conducting a series of experiments with different age groups of Japanese eel to reevaluate the nutrient requirements, the efficiency of various dietary ingredients and additives and the body composition of wild and cultured eel. Here we present some important micronutrient requirements, namely those for vitamin C, vitamin E and arachidonic acid, in Japanese eel. Vitamin C Requirement Most marine and freshwater teleosts are unable to synthesize vitamin C (ascorbic acid / AA) from D glucose because of the lack of an enzyme, L-gulonolactone oxidase, that is responsible for the synthesis of vitamin C de novo (Dabrowski 1990, Fracalossi et al. 2001, Wilson 1973). In general, marine and freshwater teleosts depend fully on a dietary supply of ascorbic acid. Many general physiological functions of L-ascorbic acid (AA) are well defined, the most important among them being its capacity to act as a co-factor in the hydroxylation of proline to hydroxyproline, critical for the helical structure of collagen. L-ascorbic acid is also the most powerful reducing agent available to cells, losing two hydrogen atoms to become dehydroascorbic acid, and is of general importance as an antioxidant because of its high reducing potential (Bai 2001). The standard reference for aquatic species nutrition, Nutrient Requirements of Fish and Shrimp (NRC 2011), has documented the studies devoted to evaluating AA requirements in economically important species. The previous edition (NRC 1993) listed only a few economically important species in the vitamin C section, but the current edition covers the majority of commercially important species. A number of symptoms linked to vitamin C deficiency, such as impaired collagen formation, spinal deformation, haemorrhaging, retarded growth and depressed immunity (Ai et al. 2006, Al-Amoudi et al. 1992, Gouillou-Coustans et al. 1998, Halver et al. 1969) were formerly common problems encountered at aquaculture farms. Knowledge of vitamin C requirements has advanced substantially. As a result, fish producers have been relieved of severe economic losses linked to high and frequent incidence of malformed fish and subsequent mortality. Large discrepancies in quantitative requirements of vitamin C in and between fish species are a result of differences in species, size and methodological approaches and experimental conditions (NRC 1993). The dietary source of AA used in different experiments is another major and fundamental difference, which makes it complex to oversimplify the quantitative requirement of vitamin C. L-ascorbic acid is the traditionally used vitamin C source in fish and shrimp feeds, but it is thermolabile, unstable and easily oxidized to an inactive form during feed processing and storage. Various derivatives of AA, including L-ascorbyl-2-sulfate (C2S), L-ascorbyl2-monophosphate-Mg (C2MP-Mg), L-ascorbyl-2-monophosphateCa (C2MP-Ca), L-ascorbyl-2-polyphosphate (C2PP) and ascorbate2-glucose (C2D), are more stable than the parent compound and provide antiscorbutic activity in fish and shrimp. The dietary vitamin C requirement of Japanese eel has been estimated using L-ascorbic acid Ca as the source of vitamin C (Ren et al. 2005). In an experiment to reevaluate the vitamin C requirement in juvenile eel using L-ascorbyl-2-monophosphate (AMP) as the vitamin C source, survival of eels fed the AMPsupplemented diets was significantly greater than those of fish that did not receive vitamin C supplementation. No vitamin C could be detected in the whole body of fish fed AMP0 diet. The vitamin C level in fish fed AMP108 diet was significantly greater than that of eels fed AMP24 and AMP52. The vitamin C level of fish fed the AMP1137 diet was significantly greater than those in fish fed all other diets. Dietary vitamin C in juvenile eel is essential. However, there seems to be no benefit of increasing vitamin C supplementation in diets beyond 24 mg AMP/kg diet, inasmuch as fish fed any of the vitamin C supplemented diets had similar growth performance. The requirement based on broken-line analysis of weight gain (Fig. 3) is comparable to values obtained in common carp Cyprinus carpio TOP, FIGURE 3. Broken-line analysis of vitamin C requirement of Japanese eel based on weight gain.
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