WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2014 31 (45 mg AA/kg diet) (GouilloCoustan et al. 1998). Ren et al. (2005) reported the optimum dietary level of AA for Japanese eel juvenile growth to be more than 27 mg AA/kg diet without stating an upper limit. Although no significant differences were recorded above this minimum level, specific growth rate continued to increase up to the maximum supplementation level. Any differences in the minimum requirement can be attributed to differences in the vitamin C source. Furthermore, no vitamin C deficiency signs, such as anorexia, abnormal swimming, and hemorrhagic areas under the skin, could be observed in our study, in contrast to the study of Ren et al. (2005). Therefore, the dietary vitamin C requirement of juvenile eel is equal to or greater than 41.1 mg/kg diet. Vitamin E Requirement Vitamin E (tocopheryl) is a fat-soluble antioxidant that stops the production of reactive oxygen species that are formed when fat undergoes oxidation. It is an indispensable nutrient required to maintain flesh quality, immunity, normal resistance of red blood corpuscles to hemolysis, capillary permeability and heart muscle (Halver 2002). Vitamin E has several naturally occurring forms, with α-tocopherol having the highest vitamin E activity (NRC 1993). Tocopheryl acetates do not act as antioxidants but are hydrolyzed by digestive enzymes prior to absorption into the body (Hung et al. 1982, Sau et al. 2004). Vitamin E functions as a lipid-soluble antioxidant, protecting biological membranes and lipoproteins against oxidation; it is an essential dietary nutrient for all fish species studied (NRC 1993). Its main function is to protect unsaturated fatty acids against free radical-mediated oxidation (Hamre et al. 1998). The level and state of oxidation of polyunsaturated fatty acids in the diet and the presence of other antioxidants and selenium may affect the dietary vitamin E requirements of fish (Murai and Andrews 1974, Poston et al. 1976, Watanabe et al. 1977, Hung et al. 1981, Cowey et al. 1983, Lovell et al. 1984, Gatlin et al. 1986). Until recently, there was no quantitative estimation of the dietary vitamin E requirement for Japanese eel (Bae et. al. 2012). In our experiment, inclusion of vitamin E did not affect whole body composition of Japanese eel. Similar results were reported by Gatta et al. (2000) and Sau et al. (2004), who found no differences in lipid, ash or moisture contents after feeding graded levels of vitamin E to rohu fry. The growth performance of fish fed vitamin E-supplemented diets improved to a supplementation level of 16.5 mg TA/kg diet and then dropped at higher levels. Based on these observations, the dietary vitamin E requirement of the juvenile Japanese eel is >21.2 but <21.6 mg/kg diet, as assessed by broken-line regression analysis of weight gain (Fig. 4), specific growth rate, feed efficiency and protein efficiency ratio. DL-α-tocopheryl acetate was used as the dietary vitamin E source under the experimental conditions in our laboratory. Arachidonic Acid Requirement Among n-6 HUFA, arachidonic acid (ARA, 20:4n-6) is the main fatty acid precursor of eicosanoids in fish (Henderson and Sargent 1985, Henderson et al. 1985, Bell et al. 1994). Arachidonic acid in fish tissues is located almost exclusively in the 2-position of the glycerol of the inositiol phospholipids, which have critical roles in many areas of cellular signal transduction (Sargent et al. 1989). Arachidonic acid produces eicosanoids with high biological activity, namely 2-series prostanoids and 4-series leukotrienes, while eicosanoids derived from EPA, namely 3-series prostanoids and 5-series leukotrienes, are less biologically active (Tocher et al. 2003). The relative abundance of the two fatty acids, subsequently, determines eicosanoid potency and mode of action. In fishes, eicosanoids are responsible for a range of physiological functions, such as modulating immune and neural function and osmoregulation, and controlling the stress response (Mustafa and Srivastava 1989, Sargent et al. 1999, Koven et al. 2001b, Tocher et al. 2003). Elevated dietary ARA increases overall survival (Bessonart et al. 1999) and improves resistance to handling stress in larval gilthead seabream Sparus aurata (Koven et al. 2001). An optimal concentration of dietary ARA maximizes stress resistance to a hypersaline challenge in larval summer flounder TOP, FIGURE 4. Broken-line analysis of vitamin E requirement of Japanese eel based on weight gain. BOTTOM, FIGURE 5. Broken-line analysis of arachidonic acid requirement of Japanese eel based on weight gain. 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. (CONTINUED ON PAGE 32)
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