Study on biological characteristics and development of seed production technology for the blotched sankehead channa maculata (lacepède, 1801)

araffin, cut into slices then stained with Haematoxyline and Eosin. The Gonadal Somatic Index was determined by the formula of Biswas (1993). Fercunlity was determined based on the egg mass of a female with gonads in stage IV and the number of oocytes was calculated according to Banegal's formula (1967). 2.3.3. Seed production technology 2.3.3.1. Broodstock rearing a/ Effects of food types on fecundity and quality of eggs and larvae The brood fish were fed with trash fish at 5-7% body weight for 1 month prior being put into the experiment system. Brood fish with an average weight of 0.85kg (ranging 0.65 to 0.97 kg) are cultured in 3 cages (4 mx 3 mx 2m) put in a pond of 1000 m2 with a stocking density of 24 fish/ cage (equivalent to 2 fish/ m2, 1 fish/ m3), the male to female ratio was 1: 1.5. There are 3 broodstock rearing periods corresponding to 3 treatments: – Treatment 1 fed 100% trash fish – Treatment 2 fed 50% trash fish: 50% comercial feed pellets. – Treatment 3 fed 100% comercial feed pellets. b/ Effects of food components on fecundity and quality of eggs and larvae Culture conditions, number of fish per treatment, stocking density, methods for spawning and assessment criteria are similar to those in the experiment 1, excepting weight of brood fish is smaller, 0.65 kg (from 0.55 to 0.71). The experiment was repeated 3 times (30-35 days each). Brood fish were fed trash fish with the following diet: - Treatment 1: fed with a diet of 5% body weight - Treatment 2: fed with a diet of 7% of body weight - Treatment 3: fed with a diet of 9% body weight - Treatment 4: fed with a diet of 11% body weight 2.3.3.2. Spawning stimulating methods a. Experiment 1. Determine a suitalbe time for hormonal injection The experiment consists of 4 treatments, each treatment was replicated 3 times and designed in a completely random manner. + Treatment 1: male fish were injected at the same time of giving the decision dose to females (0h). + Treatment 2: male fish were injected 8 hours before the time of of giving the decision dose to females 6 + Treatment 3: male fish were injected 16h before the time of of giving the decision dose to females + Treatment 4: male fish were injected 24 hours before the time of of giving the decision dose to females b. Experiment 2. Determine a suitale time for injecting decision dose of female fish The experiment consists of 4 treatments, each treatment was replicated 3 times and designed in a completely random manner. + Treatment 1: Final dose from premilary dose 6h + Treatment 2: Final dose from premilary dose 12h + Treatment 3: Final dose from premilary dose 18h + Treatment 4: Final dose from premilary dose 24h c. Experiment 3. Spawning stimulation by pituitary The experiment consists of 5 treatments, each treatment was replicated 3 times and designed in a completely random manner. Each treatment has 3 pairs of brood fish. + Treatment 1: 9 mg/kg female + Treatment 2: 10 mg/kg female + Treatment 3: 11 mg/kg female + Treatment 4: 12 mg/kg female + Treatment 5: 13 mg/kg female d. Experiment 4: Effects of hormone types on quality of eggs and larvae The experiment consists of 4 treatments and designed in a completely random manner. Each treatment has 3 pairs of brood fish. + Treatment 1: 3500 IU HCG/kg female. + Treatment 2: 60µg LRHa + 15mg DOM/kg female). + Treatment 3: 12 µg Pituitary /kg female. + Treatment 4: Injection of saline solution at a dose of 0,5 mL/kg 2.3.3.3. Methods of nursing a. Effects of food types on growth, survival rate and coefficient of variance of larvae to fry stage The experiment consists of 4 treatments and designed in a completely random manner. Each treatment was replicated 3 times. Treatment 1: Moina + commercial food Treatment 2: Moina + Filariae Treatment 3: Moina + marine shrimp b) Effect of feeding conversion regime on growth, survival and coefficient of variance of fry stage The experiment was designed completely randomly with 6 treatments of feeding practices from trash feed to comercial feed at different time points, each treatment was replicated 3 times. Treatment 1: Changing to processed food from day 7; Treatment 2: Changing to processed food from day 9; Treatment 3: Changing to processed food from day 11; Treatment 4: Changing to processed food from day 13; Treatment 5: Changing to processed food from day 15; 7 Treatment 6: Changing to processed food from day 17. c. Effect of stoking density on growth, survival rate and coefficient of variance of fry to fingerlings stage. The experiment was designed completely randomly and each treatment was replicated 3 times, including 4 nursery treatments at the density of 1 fish/L; 1.5 fish /L, 2 fish /L and 2.5 fish/L. d. Effect of diets on growth, survival and coefficient of variance of fry to fingerlings stage. The experiment was designed completely randomly with 5 treatments corresponding to the diets of 3, 6, 9, 12 and 15% body weight/day. Each treatment was replicated 3 times. 2.4. Data analysis All collected data were analyzed and graphed on excell software v2010. Analysis of variance was performed on SPSS 16.0. One-way- ANOVA and Ducan test were then used to verify the statistically significant difference (P <0.05) among the treatments in each experiment. Chapter 3. RESULTS AND DISCUSSION 3.1. Biological characteristics 3.1.1. Nutritional biology 3.1.1.1. Morphology of digestion system a/ Mouth, teeth and gills The blotched snakehead C. malacuta has a large mouth, front facing and a protruding lower jaw with a row of sharp teeth. There are many small and pointed teeth in the mouth, growing in rows on the jaws and vomer. The upper jaw is longer than the lower jaw. There are several sharp teeth on two jaws. Tongue is long and sharp. Upper lip is thick. The jaw anterior teeth and vomer are are continuous arc shaped. The large mouth and well- developed teeth indicate that this is a predatory species (Figure 3.1). Gill rakers are arranged in two rows on gill arch and with hard spines. The first gill arch has an average of 22.33 ± 1.01 rakers (ranges from 19-24 gill rakers) (Figure 3.2). Figure 3.1. Mouth and teech Figure 3.2. Gill rakers b/ Esophagus The esophagus of the blotched snakehead is short, but its wall is thick. There are many white folds located behind the oral sinus inside the esophagus, which indicates that the esophagus has a high elasticity and can store a lot of food and catch large preys (Figure 3.3). 8 Figure 3.3. Esophagus a/ Outside of the esophagus; b/ Inside of the esophagus Figure 3.4. Transversal slices f esophagus (a: Sphincter layer, b: Vertical muscle layer, c: Mucosa, d: adipose tissue) c/ Stomash The stomach of the blotched snakehead is U-shaped, short, large in size and with thick walls. The inner surface of stomach has many folds making it easy to be expanded and to have a large contraction force to accommodate large-sized preys (Figure 3.5). Cross section of the stomach shows that its wall has 3 layers: outer membrane, mucous membrane, smooth muscle layer (Figure 3.6). Figure 3.5. The blotched snakehead stomash a/ Outside of the stomash; b/ Inside of the stomash Figure 3.6. Stomash structure a. Outer membrane b. Sphincter layer c. Vertical layer of muscle d. The submucosa e. Lining f. Crease e/ Intestine The intestine is a continuation after the stomach. The intestine receives digestive enzymes from the pancreas and bile fluid from the liver to digest food and absorb nutrients a b a b 9 through the intestinal wall into the bloodstream to serves organs, organizations, tissues in the body. Therefore, the intestine is considered an important digestive organ. The blotched snakeheadinstestine is straight, short, but its wall is thick (Figure 3.7). Similar to other parts of the digestive tract, the wall of the intestine is composed of the mucosa, submucosa, muscle and outer membrane (Figure 3.8, Figure 3.9) Morphological characteristics and structure of digestive tract such as the position of the mouth, teeth, gill rakers, esophagus, size, structure of the stomach and intestines indicate that the blotched snakehead is a preadtory species. 3.1.1.2. The relative length of gut (RLG) The result of determining RLG is presented in Table 3.1. Table 3.1. RLG in different sizes. Body length Mean Min Max Sample size (mm) (Li/Lt) (Li/Lt) (Li/Lt) (n=344) <50 0.51±0.02 0.47 0.55 34 50 – 99 0.52±0.07 0.38 0.61 22 100 – 149 0.56±0.03 0.49 0.61 23 150 – 199 0.57±0.06 0.40 0.71 59 200 – 249 0.59±0.06 0.46 0.74 79 250 – 299 0.60±0.05 0.45 0.79 63 ≥ 300 0.60±0.02 0.57 0.65 64 Mean 0.58 ± 0.06 0.38 0.79 Figure 3.7. Digestive track Figure 3.8. Transverse slice of intestine a. Wall; b. Below submucosa; c. submucosa Hình 3.9. Intestine structure a. submucosa; b. below submucosa c. Smooth muscle layer; d. Outer membrane; e. Folds Stomash Instestine Manh tràng 10 3.1.1.3. Natural food components Figure 3.11. Occurence frequency of food for fish smaller than 100 g (n=120) and bigger than 100 g (n=148). Figure 3.13. Food spectrum of blotched snakehead 3.1.2. Reproductive biology 3.1.2.1. Female fonadal development stages Figure 3.18. The histolorical image of gonadal development at stage I (40X) 63.33 83.33 61.67 19.17 0.00 0.00 35.83 78.38 0.00 15.54 81.08 56.08 14.19 10.14 0 20 40 60 80 100 O cc u re n ce f re q u e n cy (% ) Food types W <100 g fish 57% Crustatio ns 18% Worm 11% Reptile 8% Spills 5% Others 1% Figure 3.22. The histolorical image of female gonadal development at stage III (10X) Figure 3.24. The histolorical image of female gonadal development at stage IV(4X) Figure 3.20. The histolorical image of gonadal development at stage II (40X) 11 b/ Male gonadal development satges Figure 3.18. The histological image of male gonadal development at stage III (40X) Figure 3.19. The histological image of male gonadal development at stage VI (40X) 3.1.2.2. Gonado somatic index Table 3.3. Gonado somatic indexof C. maculate during study time Month /Year Female Male W(g) Wg(g) GSI (%) W(g) Wg(g) GSI (%) 1/2017 83483±105.77 3.41±1.02 1.20±0.28 446.08±29.19 1.60±0.16 0.45±0.04 2/2017 398.69±144.16 4.01±1.34 1.30±0.47 361±116.18 1.58±0.23 0.56±0.09 3/2017 441.76±184.21 5.17±1.47 1.45±0.46 394.23±125.22 1.69±0.44 0.69±0.11 4/2017 436±132.57 5.44±0.45 2.02±0.56 457±164.88 2.84±0.26 1.10±0.42 5/2017 496.94±113.07 8.57±0.44 2.71±0.56 423.67±79.91 3.52±0.30 1.32±0.16 6/2017 441.76±130.27 6.95±0.35 2.54±0.68 399±104.09 3.12±0.31 1.24±0.27 7/2017 422.71±128.23 5.88±0.33 2.31±0.23 432.38±85.72 3.29±0.32 1.20±0.13 8/2017 436±102.61 4.53±0.32 1.44±0.25 412.33±68.44 2.59±0.32 0.85±0.09 9/2017 410±91.19 4.66±0.29 1.35±0.24 431.5±105.61 2.40±0.31 0.65±0.09 10/2017 450.94±89.07 3.02±0.45 0.90±0.07 422.79±106.96 1.67±0.37 0.53±0.06 11/2017 352.73±93.02 2.86±0.47 0.77±0.12 448±101.12 1.20±0.33 0.36±0.04 12/2017 386.14±86.82 1.63±0.37 0.57±0.06 392.5±106.97 0.69±0.41 0.22±0.09 Figure 3.25. The image of ovary at stage V Figure 3.26. The histological image of ovary cells at stage V (4X) 12 3.1.2.3. Condition factor (CF) Figure 3.32. The condition factor of C. maculata following investigated months 3.1.2.4. Spawning season Figure 3.23. Fluctuation of gonado somatic index and fatness Figure 3.34. The maturation period of C. malacuta in the months 13 Thus, within the scope of this study, we found that the spawning season of C. malacuta occurs between March to October, mainly focuses from May to June. Therefore, it is can be predicted the main spawning sason of C. malacuta is from April to June. 3.1.2.5. Fecundity Table 3.4. Fecundity of C. malacuta in diffennt groups Body weight (g) Samples Size (n) Individual weight (g/fish) Absolute fecundity (egges/fish) Relative fecundity (eggs/g fish) < 300 15 262.33 ± 36.0 3.985 ± 777 15.262 ± 2.2249 301 - 400 15 360.60± 29.63 5.288 ± 813 14.697 ± 2.132 401-500 15 455.67 ± 27.50 5.918 ± 1.004 12.973 ± 1.947 501- 600 15 563.07 ± 27.03 6.631 ± 1.004 11.759 ± 1.528 > 600 15 746.00 ± 79.59 7.283 ± 654 9.799± 701 3.1.2.6. Egg diameter Table 3.5. Egg diameter in different female groups Body weight (g) Samples size (n) Individual weight (g/fish) Egg diameter (mm) < 300 12 252.50 ± 35.33 1.17 ± 0.01 301 – 400 10 347.20± 22.44 1.19 ± 0.01 401-500 15 454.80 ± 26.78 1.23 ± 0.02 501- 600 14 555.07 ± 22.46 1.24 ± 0.01 > 600 7 755.20 ± 80.48 1.25 ± 0.01 3.2. Development of seed production techniques for C. malacuta 3.2.1. Broodstock rearing 3.2.1.1. Effects of food types on fecundity, quality of eggs and fish arvae Table 3.7. The maturation rate, fecundity, quality of eggs and larvae of C. malacuta fed by different foods. Reproductive performance Treatment 1 (Trash fish food) Treatment 2 (Trash fish food+ comcercial feed pellets) Treatment 3 (Comcercial feed pellets) Maturation rate (%) 82.26 ± 5.23 a 81.76 ± 6.35 a 65.76 ± 3.97 b Fecundity (eggs/kg female) 45.346 ± 5.009 b 46.776 ± 5.526 b 32.645 ± 3.821 a Egg size (mm) 1.21 ± 0.006 1.23 ± 0.015 1.22± 0.010 Oil droplet size (mm) 0.27 ± 0.00 0.28 ± 0.006 0.27 ± 0.006 Yolk-sac size (mm) 1.13 ± 0.015 b 1.16 ± 0.015 b 1.08 ± 0.020 a Fertilization rate (%) 80.43 ± 2.32 b 82.54 ± 3.21 b 72.51 ± 3.12 a Hatching rate %) 81.87± 1.49 b 83.54 ± 1.46 b 78.30 ± 1.18 a Larvae size (mm) 2.57± 0.025 b 2.61 ± 0.030 b 2.42 ± 0.030 a Abnormality rate (%) 2.54± 0.04 a 2.73 ± 0.12 a 4.34 ± 0.66 b Survival rate (%) 62.50± 3.69 a 65.7 ± 5.45 a 66.61 ± 2.70 a Note: different letters in the same row represents a statistically significant difference (P <0.05). 14 The results show that feeding 50% of trash fish combined with 50% of comercial feed pellets not only improves the reproductive performance of the brood fish, but also significantly improves the quality of eggs and larvae. 3.2.1.2. Effect of diets on fecundity, quality of eggs and fish larvae Table 3.8. Weight, maturation rate, fecundity, quality of eggs and fish larvae of brood fish fed with different diets Reproductive performance Diets 5%BW 7%BW 9%BW 11%BW Weight before experiment (g) 643 ± 37 653 ± 61 658 ± 15 643 ± 47 Weight after experiment (g) 857 ± 19 a 952 ± 54 b 954 ± 46 b 973 ± 80 b Weight gained (g) 214 ± 19 a 299 ± 54 b 313 ± 46 b 330 ± 80 b Maturation rate (%) 81,23 ± 3,05 b 82.48 ± 1.47 b 89.56 ± 2.07 c 71.43±4.49 a Fecundity (eggs/kg female) 45.321 ± 3.582 a 47.886 ± 8.175 a 65.325 ± 6.842 b 51.637± 5.442 a Egg size (mm) 1.19 ± 0.015 a 1.21 ± 0.017 b 1.22± 0.058 b 1.23±0.058 b Oil droplet size (mm) 0.27 ± 0.058 0.28 ± 0.058 0.27 ± 0.058 0.28± 0.00 Fertilization rate (%) 75.54± 4.99 81.56 ± 5.17 79.24 ± 3.66 77.35± 2.33 Hatching rate (%) 81.61± 3.17 85.52 ± 1.54 85.31 ± 2.38 87.25± 3.26 Larvae size (mm) 2.51± 0.026 a 2.61 ± 0.026 b 2.62 ± 0.030 b 2.62± 0.030 b Abnormality rate (%) 3.71± 0.25 3.42 ± 0.16 3.88 ± 0.12 3.87± 0.41 Note: different letters in the same row represent a statistically significant difference (P <0.05). The results showed that the brood fish fed a diet of 9% BW not only improved their maturity, fecundity, and egg quality but also improved their growth rate. 3.2.2. Spawning stimulation 3.2.2.1. Determine a suitable time for hormones injection of male fish Figure 3.42. Male gametogensis rate Thus, injecting male fish 24h before the decision dose of the female is also a solution to overcome the asynchronous maturation of C. malacuta. 3.2.2.2. Determine a suitalbe time for injecting decision dose of female fish The results of determining a suitalbe time for injecting decision dose of female fish is showed in Figure 3.44 and Figure 3.46. 15 Figure 3.44. Spwaning rate Figure 3.46. Fertilization and hatching rate of C. malacuta at different premilary and decision doses The experimental results recomend that injections time of preliminarily doses is 18h from the decision dose of the female. 3.2.2.3. Spawning stimulatation of C. malacuta using pituitary Table 3.9. Spawning rate, fecundity and responding time of brood fish stimulated by pituitary Note: Different letters in the same column represent a statistically significant difference (P<0,05) Treatment Responding time (h) Spawning rate (%) Fecundity (Eggs/kg female) Saline solution - - - Pituitary 09 mg/kg - - - Pituitary 10 mg/kg - - - Pituitary 11 mg/kg 37.53 11.11 ± 9.24 a 20.476 ± 0000 a Pituitary 12 mg/kg 35.42 100.00 ± 0.00 b 27.580 ± 834 b Pituitary 13 mg/kg 36.43 100.00 ± 0.00 b 22.633 ± 2.055 a 16 Table 3.10. Hatching time, quality of eggs and larvae from brood fish stimulated by pituitary at different dosages. Reproductive performance Treatments Pituitary 11 mg/kg Pituitary 12 mg/kg Pituitary 13mg/kg Hatching time 45h 30 min 40 h15 min 43h 30 min Fertilization rate (%) 81.67 ± 1.47 a 91.47 ± 1.88 c 86.58 ± 0.83 b Floating rate of eggs (%) 66.70 ± 4.98 a 82.21 ± 1.58 b 76.21 ± 1.93 b Hatching rate (%) 75.55 ± 2.10 a 85.01 ± 1.62 b 81.87 ± 1.41 b Egg size (mm) 1.21 ± 0.000 a 1.23 ± 0.006 b 1.22 ± 0.006 b Oil droplet (mm) 0.27 ± 0.006 a 0.28 ± 0.006 a 0.28 ± 0.006 a Larvae size (mm) 2.40 ± 0.02 a 2.67 ± 0.02 c 2.57 ± 0.04 b Yolk-sac size (mm) 1.09 ± 0.02 a 1.16 ± 0.02 b 1.14 ± 0.02 b Abnormality rate (%) 4.55 ± 0.88 ab 3,.80 ± 0.66 a 5.74 ± 0.70 b Survival rate of 3-day old fish (%) 59.31 ± 2.87 a 67.99 ± 4.93 b 61.67 ± 2.40 ab Note: different letters in the same row represent a statistically significant difference (P <0.05). Thus, through reproductive performance such as spawning rate, responding time of hormones, actual fecundity, egg size, oil droplet, floating egg rate, fertilization rate, hatching rate, larva size, yolk-sac size, abnormality rate and survival rate of 3-day-old fish, it is recomended to use pituitary at a dose of 12mg/kg to stimulate the reproduction of C. malacuta. 3.2.2.4. Effects of hormone types on quality of eggs and larvae Table 3.11. Maturation rate, gametogenesis recurrence and fecundity of brood fish injected with different hormones. Treatment Maturation rate (%) Gametogenesis time (day) Spawn ig rate (%) Fecundity (eggs/kg female) Saline solution 56.85 ± 7.44 a 38.00 ± 3.61 b - - HCG 81.47 ± 6.96 b 30.67 ± 2.52 a 100 24.221 ± 3.315 LHRHa + DOM 77.36 ± 5.64 b 37.67 ± 2.88 b 100 27.404 ± 2.017 Pituitary 75.46 ± 9.58 b 33.67 ± 1.53 ab 100 25.779 ± 959 Note: Different letters in the same column represent a statistically significant difference (P<0,05). 17 Table 3.12. The effects of hormones types on responding time and reproductive performance of C. malacuta. Reproductive performance Hormone types HCG LHRHa + DOM Pituitary Responding time (h) 34.36 ± 1.49 a 40.79 ± 1.76 b 35.90 ± 0.65 a Fertilization rate (%) 82.31 ± 3.36 a 87.91 ± 1.38 b 83.66 ± 2.59 a Floating egg rate (%) 88.13 ± 2.14 91.31 ± 2.40 90.80 ± 2.47 Hatching time (h) 41.77 ± 3.66 44.61 ± 1.09 42.23 ± 1.34 Hatching rate (%) 79.80 ± 2.47 a 85.85 ± 3.45 b 79.40 ± 3.64 a Egg size (mm) 1.23 ± 0.015 1.23 ± 0.006 1.22 ± 0.006 Oil droplet (mm) 0.28 ± 0.006 0.27 ± 0.010 0.28 ± 0.006 Larvae size 2.65 ± 0.015 2.67 ± 0.020 2.65 ± 0.025 Yolk-sac size (mm) 1.16 ± 0.015 1.16 ± 0.020 1.17 ± 0.015 Larvae abnormal rate (%) 3.20 ± 0.28 ab 3.79 ± 0.60 b 3.03 ± 0.24 a Survival rate of 3-day old fish (%) 69.31 ± 2.84 68.95 ± 4.16 73.25 ± 2.14 Note: Different letters in the same row represent a statistically significant difference (P<0,05). Based on the results obtained, injection of LHRHa + DOM showed the highest fercundity (27,404 eggs/kg), fertilization rate (87.91%) and hatching rate (85.85%). Meanwhile, no significant difference was observed in other criteria such as the rate of floating eggs, deformities, survival rate of 3 days old larvae, egg size, oil droplets, yolk-sac and fry in 3 treatments LHRHa + DOM, HCG, pituitary. Therefore, it can be concluded that LHRHa + DOM is the most suitable hormone to stimulate the reproduction of the blotched snakeheadC. malacuta. 3.2.3. Nursing of C. malacuta at the larvae to fry stage 3.2.3.1. Effects of different food types on growth, survival and coefficient of variance of C. malacuta at the larvae to fry stage a/ Survival rate Figure 3.47. Survival rate of C. malacuta at the larvae to fry stage fed by different foods 18 c/ Growth The nursing experiment was conducted for 28 days (4 weeks). The growth results of C. malacuta at the larvae to fry stage are presented in Table 3.14 Table 3.14. The growth of C. malacuta at the larvae to fry stage fed with different foods Moina – Pellets Moina – thread Worm Moina -Shrimp Release W0 (g) 0.0025±0.0003 a 0.0025±0.0003 a 0.0025±0.0003 a L0(cm) 0.89±0.03 a 0.89±0.03 a 0.89±0.03 a Harvest Wfl (g) 0.38±0.02 a 0.45±0.03 b 0.42±0.03 ab Lfl(cm) 3.10±0.10 a 3.47±0.15 b 3.37±0.06 b Daily growth rate DWG(g/day) 0.013±0.00 a 0.016±0,001 b 0.015±0.001 ab DLG(cm/day) 0.08±0.000 a 0.093±0.005 b 0.090±0.001 b Specific growth rate SGRW(%/day) 17.94±0.18 a 18.51±0.20 b 18.29±0.26 ab SGRL(%/day) 4.46±0.12 a 4.85±0.16 b 4.68±0.21 b Note: Different letters in the same row represent a statistically significant difference (P<0,05). The results showed that fry fed by moina - threadworms and moina - shrimp developed better and more uniformly than larvae fed by other foods. 3.2.3.2. Effects of procecced food regime on growth, survival and coefficient of variance of C. malacuta at the larvae to fry stage. a/ Survival rate Figure 3.53. Survival rate of larvae following feeding practice of processed food b / Growth The growth of length of the larvae to fry satge following feeding practices of processed food is shown in Table 3.16+3.17. Table 3.16+3.17. The growth rate of length of larvae following feeding practice of processed food during nursing time. Treatment Fish at the end of experiment Daily growth rate Specific growth rate Wfl(g) TLfl (cm) DWG (g/day) DLG (cm/day) SGRW (%/day) SGRL (%/day) Feeding at day 7 0.21 ± 0.020a 2.20 ± 0.10a 0007 ± 0.001a 0.043 ± 0.003a 14.62 ±0.32a 2.96 ±0.15a Feeding at day 9 0.24 ± 0.015a 2.40 ± 0.10ab 0.008 ± 0.0006a 0.050 ± 0.003ab 15.02 ±0.22a 3.25 ±0.14ab Feeding at day 11 0.22 ± 0.006a 2.53 ± 0.06b 0.007 ± 0.0006a 0.054 ± 0.002b 14.83 ± 0.09a 3.43 ± 0.10b Feeding at day 13 0.32 ± 0.02b 3.10 ± 0.27c 0.010 ± 0.0006b 0.073 ± 0.009c 15.99 ± 0.22b 4.10 ± 0.29c Feeding at day 15 0.34 ± 0.017b 3.33 ± 0.21c 0.011 ± 0.0006b 0.081 ± 0.007c 16.23 ± 0.17b 4.35 ± 0.21cd Feeding at day 17 0.41 ± 0.037c 3.64 ± 0.06d 0.014 ± 0.001c 0.091 ± 0.002d 16.82 ± 0.30c 4.64 ± 0.10d Note: Different letters in the same column represent a statistically significant difference (P<0,05). TLfl (cm): fish length at the end of the experiment; Wfl (g): fish weight at the end of the experiment; DLG (cm/day): daily length growth; DWG (g/day): daily weight growth; SGR(%/day): specific growth rate. 19 c/ Coefficient of variance and differientation of growth Figure 3.61. Coeeficient of variance and growth differientation of C. malacuta at the larvae to fry stage during nursing time Based on results of survival, growth and differientation during experimental time, it is suggested that processed food can be used for nursing C. malacuta at the larval to fry stage. This is the basis to open up the prospect of replacing trash foods with processed food in rearing C. malacuta. The appropriate time to start feeding processed foods is 15 days old with the method of gradually replacing trash foods by processed foods at the rate of 20% per day. 3.2.4. Nursing of C. malacuta at the fry to fingerling stage 3.2.4.1. Effects of nursing density on growth, survival rate and coeficiente of variance of C. malacuta at the fry to fingerling stage a/ Survival Figure 3.64: Survival rate of C. malacuta at the fry to fingerling stage at different nursing densities b/ Growth In addition to the increase in body weight, body length was also observed