Feeds and Feeding Management Nursery Culture of Mud Crab Scylla serrata Using Different Feeding Rates Veronica R. Alava*, Jony D. Sumile and Fe D. Parado-Estepa Aquaculture Department Southeast Asian Fisheries Development Center Tigbauan 5021, Iloilo *vralava@seafdec.org.ph Abstract The effect of different feeding rates on the production and profitability of Phases 1 and 2 (3-week each) nursery culture of hatchery-produced crab Scylla serrata was determined. Minced mussel meat and formulated diet (at a ratio of 30:70) were fed to crabs. The crabs were stocked randomly in 12-m2 net cages installed in the nursery earthen pond at stocking densities of 50 m-2 for Phase 1 and 10 m-2 for Phase 2. Crabs were fed three times daily at 0830, 1300 and 1630h h. In Phase 1, feed conversion ratio (FCR) at a feeding rate of 100% of initial crab biomass day-1 for the entire three weeks was the lowest (p<0.05) while survival, body weight (BW), carapace width (CW) and carapace length (CL) were not different (p>0.05) among crabs given different feeding rates. For Phase 2, the feeding rate of 40-30-20% of crab biomass day-1 (week 1-2-3) resulted in lowest (p<0.05) FCR that was not significantly different from FCRs of crabs fed 50-40-30% and 60-5040% of BW. Crab BW, CW and CL were not different (p>0.05) among feeding rate treatments. Profitability was better when feeding rate used was 100% of initial crab biomass day-1 for the entire Phase 1 or 100-50-40% of crab biomass day-1 (for week 1-2-3). A feeding rate of 50-40-30 % of crab biomass day-1 (week 1-2-3) was more profitable in Phase 2. Keywords: Mud crab, Scylla serrata, nursery culture, feeding rates Introduction SEAFDEC/AQD has developed the crab hatchery technology (Quinitio et al., 1999 and 2001; Quinitio and ParadoEstepa, 2008) with megalopae and early juvenile crabs produced to support the crab industry. The megalopae or early juvenile crabs need to be grown further to bigger sizes before stocking in growout ponds. The pond nursery system using net cages was developed for easier retrieval (Rodriguez et al., 2001; 2007; SEAFDEC et al., 2010). Cannibalism is the main cause of mortality during the nursery phase and several strategies are used to reduce cannibalism thereby increasing crab survival and improving the economic viability. Juvenile crabs are reared in net cages in nursery pond for 3-4 weeks (Phase 1) and crabs are harvested for selling or reared for another 3-4 weeks (Phase 2). One of the strategies to improve survival is 46 Philippines: In the forefront of the mud crab industry development the use of suitable feed and feeding management. It has been shown that providing food consisting of minced mussel meat (M) in combination with a complete formulated diet (FD) at 30:70 ratio was adequate to support good survival and growth of juvenile crabs (Rodriguez et al., 2007). Various feeding ratios of M and FD were equally effective in supporting good survival and growth of mud crab in the nursery (Alava et al., 2017). The present experiment was conducted to determine the effect of different feeding rates on survival, growth, feed conversion ratio (FCR), and profitability for both Phase 1 and 2 of the nursery culture of mud crab. Materials and Methods Pond preparation was done following procedures described in SEAFDEC/AQD et al. (2010). Rectangular net cages (4 m long x 3 m wide x 2 m height, 12 m2 bottom surface area) made of green polyethylene net (1 mm mesh size) were installed in nursery earthen ponds. Bamboo poles support the cages and the base of the nets were buried 3-5 cm deep into the soil. At least 30% of pond water volume was drained and replenished every spring tide. Water depth was maintained at least 80 cm during the culture period. Water temperature was 26 to 31°C while salinity ranged from 25 to 30 ppt. Pond water transparency was 20 to 30 cm and pH values ranged from 7.5 to 8.5. Crabs were obtained from the Crab Hatchery, Tigbauan Main Station of SEAFDEC/AQD, Iloilo. Eight pieces of framed ribbon net shelters were used per net cage (Parado-Estepa et al., 2017). The procedures for sorting the crab size per week and harvest were similar to those described by Alava et al. (2017). Mussel meat was chopped into small pieces while the dry pellets were cut into small pieces appropriate to size of the crabs. The pellets were given to crabs in the morning and late afternoon while mussel was provided at mid-day. Feed was distributed in the net cages, with a portion placed on a feeding tray that was placed inside each net cage to monitor feed consumption and condition of crabs. This experiment followed a completely randomized design comparing seven feeding rates, each using three replicate cages. The feeding rates (% of crab biomass day-1) used for week 1-2-3, respectively were: (1) 100-100100%, (2) 100-90-80%, (3) 100-80-70%, (4) 100-70-60%, (5) 100-60-50%, (6) 10050-40%; and (7) control, 100% of initial crab biomass day-1 for the entire three weeks of culture. In all treatments except for control, the basis for feed weight adjustment was the mean weight for that sampling week. The crabs were stocked randomly in 21 units of 12 m2 net cages installed in a nursery pond at 50 m-2 for Phase 1 and 10 m-2 for Phase 2. The crabs were fed three times (0830, 1300 and 1630 h) daily for three weeks per phase with combination of mussel and formulated diet at 30:70 ratio. Survival, growth and feed conversion ratio (FCR) were compared using ANOVA at 5% level of significance and Bonferroni post-hoc separation of treatment means (SPSS Statistics 17.0, WinWrap Basic, Polar Engineering and Consulting, USA). The cost and return analysis of nursery culture methods using the different Philippines: In the forefront of the mud crab industry development 47 feeding rates was determined. Pond development cost was not included in the analysis as it was assumed that the pond was available and ready for use. Results and Discussion In Phase 1, survival rates of 56.9 to 62.8% were obtained and these were not significantly different (p>0.05) among the seven feeding rates tested (Table 1). The FCR was lowest (0.72, p<0.05) in the feeding rate using 100% of the initial crab biomass day-1 for the entire 3-week experiment. The feeding rate of 100% of crab biomass day-1 (1st week) - 50% (2nd week) - 40% (3rd week) also gave low FCR (0.96, p<0.05). Body weight (BW) ranged from 0.89 to 1.05 g and mean body weight were not different (p>0.05) among various treatments. Carapace width (CW) and length (CL) ranged from 1.59 to 1.76 cm and 1.23 to 1.32 cm, respectively. Both feeding rates of 100% of initial crab biomass day-1 for the entire three weeks and feeding rate of 100% of the crab biomass (1st week) - 50% (2nd week) - 40% (3rd week) resulted in the highest return of investment (ROI, 101.5% and 101.7%, respectively) and payback period (0.53 year) (Table 2). The other five feeding rates also gave good ROI of 78.55% to 91.50% with payback period of 0.56 – 0.61 year. In Phase 2, highest survival of 94.4% (p<0.05) was obtained from feeding rate of 100% (1st week) - 80% (2nd week) - 70% (3rd week) of crab biomass day-1 (Table 3). Lowest FCR (0.97, p<0.05) was obtained in the feeding rate of 40-30-20% of the crab biomass day-1 (week 1-2-3), but not different from those fed 50-40-30% (1.28) and 60-50-40% (1.84). Other feeding rates tested resulted in FCR of 2.24 to 2.61. There was no significant difference (p>0.05) in BW, CW and CL of crabs fed the different feeding rates. The BW was 1.21 to 1.57 g, CW was 1.84 to 1.96 cm, and CL was 1.36 to 1.44 cm. Cost and return analysis indicated that the 5040-30% of crab biomass day-1 (week 1-23) gave the highest ROI of 53.16% with payback period of 0.75 year (Table 4). The other feeding rates also gave good ROI of 50.56% to 52.49% with payback period of 0.75 to 0.76 year whereas feeding rate using 100% of initial crab biomass day-1 for three weeks had lower ROI of 47.37% with payback period of 0.78 year. To improve the overall performance of mud crab in the nursery and to support the industry, strategies have been developed in reducing mortalities associated with cannibalism. Intrinsic factors such as removal of chelipeds or trimming of dactylus and pollex in juvenile mud crabs (Quinitio et al., 2001; Quinitio and Estepa, 2011) were used. Extrinsic factors involved reduced stocking density (Rodriguez et al., 2001), improvement of culture systems (stocking in net cages in pond versus direct stocking in pond) (Rodriguez et al., 2007), provision of appropriately designed shelters, sorting by size, and shorter culture period (Parado-Estepa et al., 2017). One crucial strategy is the use of suitable feeds and feeding management. In the nursery, crabs were fed three times daily to minimize the risk of cannibalism which was considered more likely to happen if they were hungry. Since hatcherysourced crabs were used to frequent feeding, the feeding of crabs in net cages inside the pond was done in the morning, mid-day and late afternoon daily. Crabs were observed to be most active late in the afternoon and early evening, hence feeding was done in late afternoon. 48 Philippines: In the forefront of the mud crab industry development Table 1. Survival, size distribution, number of survivors, feed conversion ratio (FCR), body weight (BW), carapace width (CW), and carapace length (CL) of crabs given different feeding rates in Phase 1 nursery. Mean BW was 0.02, 0.05 and 0.1 g for week 1, 2 and 3, respectively Feeding rate (%BW day-1) Week 1-2-3 Survival rate (%) 100-100-100 61.6 ± 0.3 100-90-80 59.6 ± 1.5 100-80-70 61.3 ± 0.3 100-70-60 59.6 ± 2.6 100-60-50 56.9 ± 2.3 100-50-40 62.8 ± 3.0 100% of initial BW day-1 for 3 61.6 ± 2.8 weeks FCR * 2.12 ± 0.05e 1.94 ± 0.19e 1.65 ± 0.08d 1.38 ± 0.02c 1.33 ± 0.05c 0.96 ± 0.04b 0.72 ± 0.05a BW (g) 0.93 ± 0.02 0.89 ± 0.07 1.02 ± 0.04 0.99 ± 0.04 0.96 ± 0.04 1.05 ± 0.05 0.99 ± 0.02 CW (cm)* CL (cm)* 1.65 ± 0.02ab 1.59 ± 0.02a 1.69 ± 0.04ab 1.68 ± 0.03ab 1.66 ± 0.03ab 1.76 ± 0.04b 1.24 ± 0.01a 1.23 ± 0.03a 1.26 ± 0.01ab 1.27 ± 0.02ab 1.25 ± 0.02ab 1.32 ± 0.03b 1.68 ± 0.04ab 1.27 ± 0.02ab *Mean ± SEM with different superscripts are significantly different (p<0.05) Table 2. Cost and return analysis for crabs given various feeding rates in Phase 1 nursery. Initial total stocking density was 14,400 juvenile crabs per run* Technical information Feeding Rate (% BW day-1) Week 1-2-3 100-100- 100-90- 100-80- 100-70- 100-60100% 80% 70% 60% 50% 3 weeks 100-5040% 100% of initial BW day-1 No. of crabs harvested (pcs) 8,856 8,582 8,827 8,582 8,194 9,043 8,870 Revenue/run (PhP) 88,560 85,824 88,272 85,824 81,936 90,432 88,704 Variable cost/run Feeds 4,011 3,619 3,213 2,808 2,402 2,304 223 Fertilizers, lime, electricity & crab instar* 38,100 38,100 38,100 38,100 38,100 38,100 38,100 Fixed cost per run* 9,354 9,354 9,354 9,354 9,354 9,354 9,354 Total cost/run 53,134 52,741 52,336 51,931 51,525 51,120 49,346 Total cost/year (6 runs) 318,804 316,448 314,016 311,584 309,152 306,719 296,074 Net income/year (6 runs) 212,556 198,496 215,616 203,360 182,464 235,873 236,150 Payback period 0.56 0.58 0.56 0.58 0.61 0.53 0.53 ROI (%) 91.5 85.4 92.8 87.5 78.6 101.5 101.7 Break-even price 6.00 6.15 5.93 6.05 6.29 5.65 5.56 * Buying price per crab instar, PhP 2.50; selling price per crab, PhP 6.25/cm 1.6-cm crab, PhP 10.00; cost of feeds (dry weight): mussel, PhP 200/kg; FD, PhP 72.20/kg Philippines: In the forefront of the mud crab industry development 49 Table 3. Survival, feed conversion ratio (FCR), body weight (BW), carapace width (CW), and carapace length (CL) of crabs stocked at 10 m-2 and given various feeding rates in Phase 2 nursery. Mean BW was 0.15 g, 0.2 g, and 0.25 g for week 1, 2 and 3, respectively Feeding rate (%BW day-1) Week 1-2-3 Survival rate (%)* 100-80-70 94.4 ± 3.4a 80-70-60 92.8 ± 4.3b 70-60-50 91.7 ± 4.4b 60-50-40 91.7 ± 4.2b 50-40-30 93.9 ± 6.1b 40-30-20 92.8 ± 3.1b 100% of initial BW day-1 for 3 92.2 ± 6.5b weeks FCR * 2.30 ± 0.13c 2.40 ± 0.22c 2.24 ± 0.18c 1.84 ± 0.21ab 1.28 ± 0.19ab 0.97 ± 0.13a 2.61 ± 0.38c BW (g) 1.57 ± 0.05 1.31 ± 0.08 1.21 ± 0.06 1.23 ± 0.10 1.38 ± 0.12 1.36 ± 0.14 1.34 ± 0.13 CW (cm) CL (cm) 1.96 ± 0.02 1.90 ± 0.06 1.84 ± 0.03 1.5 ± 0.04 1.89 ± 0.05 1.89 ± 0.05 1.44 ± 0.01 1.41 ± 0.05 1.36 ± 0.02 1.38 ± 0.02 1.40 ± 0.03 1.40 ± 0.04 1.89 ± 0.04 1.40 ± 0.03 *Means ± SEM with different superscripts are significantly different (p<0.05) Table 4. Cost and return analysis for crabs stocked at 10 m-2 and given different feeding rates in Phase 2 nursery* Feeding Rate (% BW day-1) Technical information 100-8070% 80-7060% 70-6050% Week 1-2-3 60-5040% 50-4030% 40-3020% 3 weeks 100% of initial biomass day-1 Revenue/run (PhP) 32,625 32,072 31,692 31,692 32,452 32,072 31,864 Variable cost/run Feeds Fertilizers, lime, electricity & crab instar* Fixed cost per run Total cost/run Total cost/year (6 runs) Net income/year (6 runs) 1,139 1,018 1,008 886 765 643 2,100 2,100 2,100 2,100 2,100 2,100 9,354 12,246 73,476 9,354 12,125 72,747 9,354 12,114 72,687 9,354 11,993 71,958 9,354 11,871 71,229 9,354 11,750 70,499 122,272 119,683 117,462 118,191 123,482 121,931 2,419 2,100 9,354 13,526 81,156 110,030 Payback period 0.75 0.76 0.76 0.76 ROI (%) 52.64 51.52 50.26 50.88 Variable cost/unit 27.26 27.73 28.06 28.06 Break-even price 4.50 4.54 4.59 4.54 * Twenty-four 12 m2 net cages; selling price PhP 12.00 per cm CW 0.75 53.16 27.41 4.39 0.75 52.49 27.73 4.40 0.78 47.37 27.91 5.09 50 Philippines: In the forefront of the mud crab industry development The goal of feeding management is to make available to the animals the most cost-effective feed in the proper amounts and at the right time and locations. Feed costs are a daily expense for crab nursery culture system and knowledge of optimum feed ration help control expenses and improve the costbenefit ratio of husbandry operations. Juvenile crabs are opportunistic benthic scavengers and have welldeveloped feeding appendages adapted for processing both soft and hard food types. Good storage condition should be considered as the quality and nutritive value of natural food, such as mussel meat, is important. The use of complete formulated diet which is easier to store is promising and is likely to become increasingly important to reduce reliance on wet feeds. Acknowledgement This study was under the National Mud Crab Science and Technology Program supported by DOST-PCAARRD and SEAFDEC/AQD with Study Code TV-01-C2012D. The authors would like to thank Dr. Adelaida T. Calpe (ISP Manager of PCAARRD-DOST) and Dr. Emilia T. Quinitio (SEAFDEC/AQD overall Mud Crab Program Leader). We are also grateful to Esteban Garibay, Churchill Bejemino, Jose Masa, Ronnie Tenefrancia, Roderix Escosa, and Rey Salvador Olpenda, who provided technical assistance. C.P., Blackshaw, A. (Eds.). Mud Crab Aquaculture and Biology. Proceedings of an International Scientific Forum held in Darwin, Australia. ACIAR Proceedings No. 8, 125-130. Quinitio, E.T., Parado-Estepa, F.D., Millamena, O.M., Rodriguez, E., Borlongan, E., 2001. Seed production of mud crab Scylla serrata juveniles. Asian Fisheries Science 14, 161–174. Quinitio, E.T., Parado-Estepa, F.D., 2008. Biology and Hatchery of Mud Crabs Scylla spp. Aquaculture Extension Manual No. 34, 2nd edition. SEAFDEC Aquaculture, Iloilo, 48 pp. Quinitio, E.T., Parado-Estepa, F.D., 2011. Survival and growth of mud crab, Scylla serrata, juveniles subjected to removal or trimming of chilipeds. Aquaculture 318, 229234. Rodriguez, E.M., Quinitio, E.T., ParadoEstepa, F.D., Millamena, O.M., 2001. Culture of Scylla serrata megalops in brackishwater ponds. Asian Fisheries Science 14, 185-189. Rodriguez, E.M., Parado-Estepa, F.D., Quinitio, E.T., 2007. Extension of nursery culture of Scylla serrata (Forsskal) juveniles in net cages and ponds. Aquaculture Research 38, 1588-1592. References Quinitio E.T., Parado-Estepa F.D., Alava, V.R., 1999. Development of hatchery techniques for the mud crab Scylla serrata: 1. Comparison of feeding schemes. In: Keenan, SEAFDEC/AQD, ACE, MODE/SPPI, BVFMC, ACELT, BFAR, and ACIARICATP., 2010. Mud crab nursery in ponds. Aquaculture Extension Manual No. 47. SEAFDEC/AQD, Iloilo, 27 pp. Philippines: In the forefront of the mud crab industry development 51