EVALUATING GROWTH PERFORMANCE AND IMMUNE RESPONSES OF SNAKEHEAD (Channa striata) BY FEEDING PLANT PROTEIN DIETS SUPPLEMENTED WITH MANNAN OLIGOSACCHARIDE  

The objective of this study was to evaluate the effectiveness on growth performance and immune responses of mannan oligosaccharide (MO) supplementation in both soybean meal (SBM) and soy protein concentrate (SPC) formulated feeds for snakehead (C. striata). The experiment included three feed groups, one using fishmeal (FM) as the only protein source, the second replacing 40% of the FM with SBM, and the third replacing 40% of FM with SPC. Each feed group was then divided into three feed treatments which added 0%, 0.2%, and 0.4% MO. The diets were 44.3-45% protein and 19.17-19.69 KJ/g energy. Eighty snakehead fingerlings (initial mean weight 7.05±0.08 g/fish) were assigned randomly to each of twenty-seven 500-L composite tanks with continuous aeration and 30% daily water exchange. The experiment period was 8 weeks.

Both final weight and weight gain were significantly affected by diet and MO supplementation, as well as the interaction between the two. In general (with some exceptions), growth performance of fish was significantly better when they were fed SPC than when they were fed SBM or FM, and MO supplementation generally improved growth of the fish. Feed conversion ratio (FCR), protein efficiency ratio (PER) and survival of fisht were significantly affected by diet, but only survival was significantly affected by MO supplementation and in no case were the interactions significant. FCR was significantly improved (i.e., lower) when fish were fed the SPC diet compared to the SBM diet, but neither was significantly different from fish fed the FM diet. PER for fish fed the FM and SPC diets was significantly greater than that for fish fed the SBM diet. Survival of fish fed SPC diet was significantly lower than that of fish fed the FM and SBM diets, but supplementation with MO, especially at the level of 0.2%, significantly improved survival. Red blood cell (RBC) counts were not significantly affected by either diet, MO supplementation, or the interaction of the two, but white blood cell (WBC) counts were significantly affected by both diet and MO supplementation (although not the interaction). Fish fed the SPC diet had significantly higher WBC counts than did fish fed the FM diet, but neither group was significantly different from fish fed the SBM diet. MO supplementation at both 0.2% and 0.4% levels significantly increased WBC counts compared to the unsupplemented diets. Immunoglobulin (Ig) levels were significantly increased by MO supplementation and the interaction of MO and diet, but diet did not affect Ig levels. At the end of the feeding trial but prior to a bacterial challenge, lysozyme levels were significantly affected by diet, MO supplementation and the interaction between the two. For each diet, the greater the level of MO supplementation, the greater the level of lysozymes. Reduction of fish production costs per kg fish produced, compared to fish fed the FM unsupplemented diet as the standard, ranged from 8.7 - 15.1 % for the various other diets tested. Following a 15-d bacterial challenge with Aeromonas hydrophila, fish lysozyme levels were significantly increased by MO supplementation and the interaction between MO and diet, but not by the diets themselves.  Again, the greater the level of MO supplementation, the greater the lysozyme level. After the 15-d challenge, cumulative mortality was lower for fish given MO supplementation than it was for fish fed the unsupplemented diets. In general, fish fed the SBM diet, supplemented or not, had lower cumulative mortality than fish fed the FM or SPC diets.