| Abstract | Crabs are considered one of the most valuable seafood items whose supply is still limited to catch from wild. Low survival of crab lets during nursing is the main barrier for crab farming. A series of four experiments (Exp) were conducted to test compare nursing methods and the potential applications of astaxanthin and commercial probiotics in blue swimming crab (Portunus pelagicus) with the objective of increasing survival and growth. Exp 1 was conducted for 35 days to compare the effects of nursing system i.e. with Cup and Substrate methods and to investigate the effects of natural astaxanthin supplementation. Exp 2 was to compare three strains of commercial probiotics, and the Exps 3 and 4 were to determine the optimum doses of the selected probiotics and the synthetic astaxanthin respectively. In the Exp 1, survival of crablets was three times higher in cup method; therefore, all the other experiments were conducted by using Cup method. In all the experiments, approx. 1 em (carapace width) size and 10-day old metamorphosed crab from megalopa were stocked and with pellet feed containing 43% crude protein, 4% crude lipid, 4% crude fiber, 3% vitamin mixture and 12% moisture (Kung Best Company). Feeding was done twice daily at 30% of biomass. Effects of treatments on water quality parameters; namely, temperature, pH, NH3, N02 and N03 were also assessed and their relationships with the growth and survival were also determined. Results of the Exp 1 showed that survival of crablets was three times higher (p<0.05) in Cup method (63.0±5.6%) than in substrate method (21.3±4.6%). The dose of 600 mg astaxanthinlkg feed used in Exp 1 showed no effects in the survival of crablets in neither of the nursing methods. Survival of crablets increased with the increase in salinity and dissolved oxygen but decreased with the increase in temperature, pH, nitrite and nitrate. Exp 2 was to select the strains of commercial probiotics with 107 CFU/mL was diluted with water then sprayed on to the shrimp pellets and coated with 5% squid oil. Results showed no significant difference on the survival, which was very high in all the treatments (range 95-100%). However, growth parameters obtained from Bacillus licheniformis showed significantly higher (p<0.05) than other two probiotics in terms of carapace gain (CG), relative weight gain (RWG) and specific growth rate (SGR) suggesting that B. licheniformis was promising probiotic. iv Exp 3 was conducted to determine an optimum dose of B. licheniformis selected from Exp 2. The probiotic dose had significant (p<O.Ol) linear relationships with carapace length gain (CG) and FCR. For every mLlkg diet of the probiotic supplementation, there was an increase of carapace length by 0.OS8 em and decrease in FCR by 0.12. The probiotic dose showed significant quadratic relations with weight gain (WG) (y = 0.0087x2 - 0.0274x + 0.S83S, R2 = 0.S9, n=12, p<O.OS), indicating higher the dose higher would be the WG but it did not allow to determine the optimum dose. Quadratic relationship of probiotic dose with moulting percentage (MP) (y = -0.2806x2 + 3.86x + 7S.722, R2 = 0.72, n=12, p<O.Ol) helped to determine the dose of probiotics, based on which the highest estimated moulting percentage (89%) to be achieved at 6.9 mL of probiotic/kg diet. In addition, it showed that there were significant quadratic relationship (p<O.OS) between the dose and th Vibrio count in both green or yellow colonies represented by the equations, y = 39.68Sx2 - 2S8.91x + 417.49 (R2 = 0.77, N=16, p<O.Ol) and y = 29.341x2 - 199.92x + 363.49 (R2 = 0.73, N = 16, p<O.Ol). Based on these quadratic equations, the lowest green and yellow colonies of Vibrio were estimated at 3.3 mL probiotic/kg diet. Exp 4 was conducted to determine the optimum dose of synthetic astaxanthin. Regression analysis showed that an increase in astaxanthin (AX) dose decreased in FCR linearly (y = -0.0732x + 2.9107, R2 = 0.14, n=36, p<O.OS). It indicated that for every g/kg feed of the AX dose there was a decrease of FCR by 0.07. Likewise, the colour appearance after 4S days showed significant difference (p<O.OS). On CIE Lab scale a* and b*, colour appearance for 3 g astaxanthin/kg feed was 3 times higher than control and AX extraction was S times higher than from the control. Quadratic regression (p<O.OS) the highest dose for a* and b* values were estimated to be 12 and 10 g/kg feed respectively. |
