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).