WWW.WAS.ORG • WORLD AQUACULTURE • JUNE 2018 51 (CONTINUED ON PAGE 52) more expensive than soybean meal and is more difficult to obtain in Vietnam. Stachyose and raffinose are prohibitively expensive in Vietnam, even for small-scale experiments, so a commercial immunostimulant (containing mannan oligosaccharide) was tested as a dietary additive. After an eightweek feeding trial, fish fed diets containing the immunostimulant showed increased immune responses for factors such as immunoglobulins and lysozymes, but diet did not affect fish survival in a subsequent 15-d bacterial challenge with Aeromonas hydrophila, a known pathogen of snakehead. These experiments resulted in a decision not to focus on SPC in snakehead diet development in Vietnam or Cambodia. Investigation of Vitamin C Requirements of Snakehead in Vietnam As pelleted feed usage in Vietnam became widespread, some farmers reported abnormalities in the C. striata that they were producing, particularly lordosis and scoliosis of the vertebral column (Hien et al. 2016a). Such abnormalities are widely known in aquaculture worldwide and are caused by inadequate levels of dietary ascorbic acid (vitamin C). In the final phase of snakehead diet investigations in Vietnam, we investigated ascorbic acid requirements of C. striata. Two experiments were conducted, one each in hatchery and grow-out phases (Hien et al. submitted) (Fig. 4). In the hatchery phase, maximal growth was observed at 500 mg/kg vitamin C in the diet, although differences were not significantly different at 250, 500, 1000 and 2000 mg/kg. In the bacterial challenge following the feeding trial, mortality was significantly reduced from about 60 percent (0 mg/kg and 2000 mg/kg) to about 20 percent (500 mg/kg and 1000 mg/kg). Results of the grow-out experiment at a commercial facility confirmed that vitamin C should be added to snakehead diets in the 250-500 mg/kg range, substantially higher than levels previously used. Transfer of Technology to Cambodia Because snakehead culture was banned in Cambodia, diet and feeding strategy technology was transferred from CTU in Vietnam to the Inland Fishery Research and Development Institute (IFReDI), a government agency in Cambodia. First, CTU researchers transferred technology for controlled breeding and larval rearing to personnel at the IFReDI hatchery at Baty Hatchery Center, Prey Veng province, Cambodia (Fig. 5). In particular, IFReDI scientists wanted to use snakehead of Cambodian origin in their research program, so a major focus was on the use of Vietnamese diets and protocols developed for domesticated fish with wild Cambodian fish. Once IFReDI researchers were able to produce sufficient numbers of offspring from Cambodian fish, they compared hatchery (weaning) success of wild Cambodian fish with domesticated Vietnamese fish and followed that with a comparison of grow-out success (Nen et al., submitted). In the hatchery phase, fish of Vietnamese origin grew markedly and significantly better (10.9 g/fish) compared to all those of Cambodian origin: Tonle Sap (4.6 g/fish); Mekong (3.2 g/fish); and F1 from IFReDI broodstock (4.9 g/fish). In the grow-out phase, the greater weight gain of Vietnamese-origin fish (324 g/fish) was continued compared to Cambodian-origin fish: Tonle Sap (148 g/fish), Mekong (133 g/fish), and F1 (147 g/fish). This suggests that the roughly two decades of fish breeding for snakehead domestication in Vietnam have resulted in fish that grow better under controlled aquaculture conditions. Finally, a five-month feeding trial to confirm the Vietnamese vitamin C results was conducted in experimental ponds at IFReDI. Growth performance (final weight, daily weight gain and yield) of fish in the experimental diet plus 500 mg/kg vitamin C treatment was significantly greater than that in the commercial feed treatment. FIGURE 4. Snakehead hapas in pond vitamin C trial in 2017 in Vietnam. FIGURE 5. The Baty Hatchery Center, Prey Veng province, Cambodia. As part of a technology transfer program between Vietnam and Cambodia, fish of Vietnamese origin grew markedly and significantly better in hatchery and grow-out compared to all those of Cambodian origin. This suggests that the roughly two decades of fish breeding for snakehead domestication in Vietnam have resulted in fish that grow better under controlled aquaculture conditions.
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