Growth performance and immune response of snakehead, Channa striata (Bloch, 1793) Fed Soy Diets with Supplementation of Mannan Oligosaccharides

A feeding trial, designed as a two-way factorial experiment, was conducted to evaluate survival, growth, feed efficiency, and immune response parameters. The first factor included three feed groups, one using only FM as the protein source and the second and third using SBM and SPC, respectively, to replace 40% of the FM. The second factor was the addition of MO either 0.0% (control), 0.2%, or 0.4% MO (Bio-Mos®, Alltech, USA, a commercial product that contains 25% glucomannoprotein) to each of the feed treatments. Each combination of diet and MO supplementation consisted of 3 replicates. Experimental diets were all formulated to be 45% protein, 9% lipid, and 4.2 Kcal.g-1 energy. All ingredients were mixed mechanically with water for 30 min (including MO as appropriate) and the dough was then passed through an extruder to obtain pellets of 2-mm diameter. Diets were dried in direct sunlight for 6 h, then cooled at room temperature for ½ h, and finally stored in airtight plastic bags in cool temperature until use throughout the experiment (i.e., all diets were made at one time only). Fingerlings used in the experiment were transferred from a hatchery in An Giang province to Can Tho University, acclimated in a 2,000-L circular tank, and fed the control (FM) diet prior to the experiment. Average initial weight per fish was 7.05 g. At the start of the experiment, fingerlings were randomly distributed into 27 composite tanks (500-L capacity, filled with 300 L of water) at a stocking density of 80 fish.tank-1. Each tank was provided with continuous aeration and flow-through water supply with 30% water exchange.d-1. Fish were fed twice a day (0900 and 1500) to satiation. The amount of consumed feed and uneaten feed in each tank was recorded daily (uneaten feed was siphoned out 30 min after feeding began, dried and weighed). At the end of the experiment, all fish in each tank were counted and weighed for calculation of growth rate and survival. Any fish mortality was recorded daily and dead fish were removed and weighed immediately. Experimental period was 8 weeks. Temperature ranged from 27.5-30.1 ºC, dissolved oxygen from 5.22 to 5.42 mg.L-1, pH from 7.53 to 8.01, NO2- from 0.62 to 0.69 mg.L-1 and NH3<0.1 mg.L-1, so the water quality parameters in all treatments were in a suitable range for the normal growth and development of this species. After 8 weeks, three fish from each tank were randomly collected and blood withdrawn for analysis of erythrocytes (RBC), leukocytes (WBC), lysozymes and total immunoglobulin (Ig). The remainder of the fish were then tranferred to the bacterial challenge experiment. RBCs were counted by the usual method using the Neubauer chamber and Natt – Hedrick solution (Natt and Hedrick 1952). WBCs were stained by Wright & Giemsa solution and counted (Hang et al., 2013). Lysozyme was analyzed by the method of Ellis et al. (1990). Total Ig was analyzed by the method of Siwicki and Anderson (1993), modified by Milla et al. (2010).

本研究采用双向析因试验设计开展饲喂试验，以评估受试对象的存活率、生长性能、饲料利用效率及免疫相关参数。试验设置两个因子：第一个因子为3组饲料，其中一组仅以鱼粉（Fish Meal, FM）作为蛋白质源，第二组与第三组分别以大豆粕（Soybean Meal, SBM）和大豆浓缩蛋白（Soy Protein Concentrate, SPC）替代40%的鱼粉；第二个因子为甘露寡糖（Mannan Oligosaccharide, MO）的添加水平，各饲料组分别添加0.0%（对照组）、0.2%或0.4%的MO（Bio-Mos®，美国奥特奇（Alltech, USA）生产的商业产品，含25%甘露糖蛋白）。每个饲料组合与MO添加水平的处理组均设置3个重复。所有试验饲料均配制为粗蛋白45%、粗脂肪9%、总能4.2 kcal·g⁻¹。将所有原料与水经机械混合30 min（视试验设计添加MO），随后将面团通过挤出机制备为直径2 mm的颗粒饲料。饲料经直射日光晾晒6 h，室温冷却30 min后，密封于气密塑料袋中低温储存，直至试验全程使用（所有饲料一次性制备完成）。试验所用鱼种取自安江省某孵化场，转运至芹苴大学后，先在2000 L圆形水槽中暂养，饲喂FM基础饲料直至试验开始。试验初始单鱼平均体重为7.05 g。试验开始时，将鱼种随机分配至27个容积为500 L的水槽（每槽注水300 L），放养密度为80尾·槽⁻¹。每个水槽均配备持续增氧装置与流水系统，日换水量为30%。每日投喂2次（09:00与15:00），投喂至饱食状态。每日记录各水槽的摄食量与残饵量：投喂后30 min虹吸收集残饵，经干燥称重后计算实际摄食量。试验结束时，统计各水槽内所有试验鱼的数量与体重，用于计算生长率与存活率；每日记录鱼只死亡情况，及时捞出死鱼并称重。试验周期为8周。试验期间水温范围为27.5~30.1 ℃，溶解氧为5.22~5.42 mg·L⁻¹，pH为7.53~8.01，亚硝酸盐氮（NO₂⁻）为0.62~0.69 mg·L⁻¹，氨氮（NH₃）<0.1 mg·L⁻¹，所有处理组的水质参数均符合该物种正常生长发育的适宜范围。试验开展8周后，从每个水槽随机选取3尾鱼，采血用于检测红细胞（Red Blood Cell, RBC）、白细胞（White Blood Cell, WBC）、溶菌酶活性与总免疫球蛋白（Immunoglobulin, Ig）；剩余试验鱼随后被转移至细菌攻毒试验组。红细胞计数采用牛鲍氏计数板（Neubauer Chamber）与Natt-Hedrick溶液法（Natt & Hedrick, 1952）；白细胞经瑞氏-姬姆萨染液（Wright & Giemsa Solution）染色后计数（Hang et al., 2013）；溶菌酶活性检测参照Ellis等（1990）建立的方法；总免疫球蛋白含量检测参照Siwicki与Anderson（1993）的方法，并经Milla等（2010）修正。