Culture of freshwater prawns in rice fields and an orchard canal in Tan Phu Thanh Village, Chau Thanh A district, Can Tho province

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1 Culture of freshwater prawns in rice fields and an orchard canal in Tan Phu Thanh Village, Chau Thanh A district, Can Tho province Nguyen Thanh Phuong (1), Vu Nam Son (1), Vo Thanh Toan (1), Tran Thi Thanh Hien (1) and Pham Minh Duc (1) and Marcy N. Wilder () 1 College of Aquaculture and Fisheries, Can Tho University Japan International Research Center for Agricultural Sciences, Tsukuba, Ibaraki 3-66 Japan, Abstract Trials of freshwater prawn (Macrobrachium rosenbergii) farming in rice field and garden ditch is being conducted at the JIRCAS project research site, Tan Phu Thanh Village, Chau Thanh A, Can Tho Province. It includes three farms of integrated culture of prawn with summer-autumn rice and one farm of prawn culture in a garden canal. Juvenile prawns of.4 g/prawn in average were stocked at a density of prawn/m in riceprawn farms, and 4 prawn/m in garden ditch. In rice-prawn farms, juveniles were nursed in a small netting compartment for 1 month to reach bigger size before releasing into the whole rice field. Prawns was fed commercial pellet ( % protein) and supplemented daily by trash fish (fish, snail, paddy crab, etc.) from the third month. Water quality including temperature, ph, transparency, dissolved oxygen, etc. and prawn growth were monitored monthly. Water quality of all farms was suitable for the growth of prawns accepting some day of high water turbidity. Prawns in garden canal grew faster than that of prawn in rice-field. After 6 months, the prawn weight reached the size 4.g and.-31.g in garden canal and rice field, respectively. The production of prawn in garden canal and rice field was 6 kg/ha and 1- kg/ha, respectively. However, the economic return from both farming types was negative. Other issues will be further discussed in this paper. Introduction Freshwater prawn (Macrobrachium rosenbergii) is high commercial value, and is cultured in various water bodies such as pond, garden ditch, rice field, river, etc. According to Phuong (1), prawn can be farmed in rice fields by integrating with summer-autumn rice or replacing summer-autumn rice or autumnwinter rice. Fortunately, since, the Vietnamese government has issued a national degree to allow the restructuring of agriculture, in which un-productive rice land can be converted to aquaculture or combined with aquaculture. Freshwater prawn has been chosen as the most promising species to farm in rice fields. Phuong () reported that farming of prawn in rice field brings in high incomes to farmers. According Dung (), the replacement of summer-autumn rice by prawn farming could achieve 7 kg/ha/crop, and VND 34.9 million /ha net benefit. Production of prawn integrated with summer-autumn rice varies from -66 kg/ha/crop (Hai et al. 1). The productivity of the system depends mostly on stocking sizes and densities. In, aquaculture production component of the JIRCAS project started first study on integrated prawn-rice in Tam Binh District, Vinh Long Province, the study was successfully done using of prawn postlarvae. This study is the follow-up of the previous activities of the JIRCAS project, which focuses on stocking of juvenile in riceprawn farming and culture of prawn in garden canal. Research materials and methods Study site and farm selection: the study was conducted in Tan Phu Thanh Village (TPTV), Chau Thanh A, Can Tho province. It is a research site of JIRCAS project. The agro-hydroligical conditions of TPTV are favorable for agriculture, in which most of soil is alluvial, even the sulfidic materials very deep (Minh et al. ). Rice production is dominant. Prawn culture had not been introduced into this area until this study started. Two collaborative farmers were selected for conducting two types of farming models including 3 farms of integrated prawn and summer-autumn rice (called rice-prawn farm) and 1 monoculture of prawn in garden canal (called garden canal prawn farm). Farm design: the principle of prawn-rice farm is that the farm must has -3% ditch area, which 1

2 functions as a nursing area for the first month and as a sheltering area during rice harvesting, planting or seeding, and high temperature period. All three farms have surrounding ditches of 3-4 m wide and.-1 m deep, covering -3% of total area. On the top of dike, 1 mm mesh size net fence was installed to avoid fish and snakes coming to the farm and also preventing the loose of shrimp during high water level (the flood). The garden canal is an existing canal of m long, 3 m wide and 1. m deep. Ten percent of the ditch area of prawn-rice farm was used as a nursery compartment by net fencing (Table 1). Table 1: Description for experimental farms Farmers name Farming model Total area (m ) Nguyen Hong Dan (rice-prawn farm 1) Nguyen Hong Dan (rice-prawn farm ) Dinh Cong Doi (rice-prawn farm 3) Dinh Cong Doi (garden canal prawn farm) Prawn-rice Prawn-rice Prawn-rice,,, 6 Ditch area (m ) 1, - Ratio (ditch per total area) (%) 3 - Farm preparation: rice-prawn farms were washed up for times by water then applied 3 kg CaO/ha. Farms were filled-up with water and fertilized at the rates of 1 kg urea + 4 kg DAP/ha and 1 kg fish meal/ha. Farms were ready for stocking after four days. Juvenile, stocking and nursing: hatchery reared juveniles of 1.- cm in length and.4-.6 g in weight were bought from a nursery farm in Can Tho City. Juveniles were transported to the farms in the morning in oxygenated plastic bags. Bags were floated on water of the farm for 1- minutes to acclimate temperature. Stocking densities were juveniles/m and 4 juveniles/m for prawn-rice farms and garden canal prawn farm, respectively. Juveniles of rice-prawn farms were nursed for 1 month using high quality commercial feed to enhance fast growth. Feed and feeding: prawns were fed commercial pellet (KP) for the first three months. From month fourth onward, commercial pellet and daily supplementary live feed (trash fish, paddy crab, snail) were employed in order to reduce feed costs. Feeding rates were according to the growth of prawn, and adjusted monthly. Feeding rates for prawn were 3% body weight during the first month, and reduced according to body weight from the nd month (Table 1). Prawns were fed 3-4 times per days. Table 1: Feeding rate for prawn during culture period Age (month) >6 Feeding rates (% BW) Water quality management: water quality of farms was managed according to the bi-weekly observation of project staff and monthly sample analysis. Liming is often used to control water quality, especially after heavy rain to stabilize ph and reduce water turbidity. Sample collection: water quality parameter such as transparency of water, temperature and ph, NO 3 - ; NH 4 + ; Chlorophyll-a, dissolved oxygen and prawn growth were monitored monthly. Samples were collected from -9 am, and analyzed at the laboratory of the Department of Hydrobiology of the College Aquaculture and Fisheries, Can Tho University. Thirty prawns were sampling for length and weight measurement, and sex and orange/blue claw male distinguishing. Productivity (yield), survival rate, average body weight and size distribution were determined at harvest. Cost and benefit analysis was computed based on total costs and gross income.

3 Data analysis: normal software such as excel, statistica were used to analyze the collected data. Results and discussions Water quality of experimental farms: generally, the water quality of all experimental farms was in the suitable range for the normal growth of prawn, acceptable at critical points of time (Fig. 1,, 3 and 4). The temperature of all farms was in the suitable range for the normal growth of prawn. The temperature in the garden canal was usually lower than in the rice fields as it has deeper water level. The temperature in the rice field varied according to the management of water for rice cultivation. During the land preparation phase for winter-spring rice, the temperature increased as no water on the rice surface. This is a critical phase for prawn, which needs an attention. The ph of water was close to the lower limit for the normal of prawn, and varied during the culture period. Water ph usually dropped after heavy rain; therefore lime was applied regularly to stabilize the ph (Fig. 1) Temperature (oc) ph Fig. 1: Variation of temperature (left) and ph (right) during culture period The dissolved oxygen of water was in the suitable range for the growth of prawn. But there were few periods that dissolved oxygen dropped, especially in the garden canal. The oxygen drop in the garden canal was due to the increase of shading of plum trees; however it was improved by thinning the plum trees. In the rice field, it was a critical period of low oxygen when the rice field in the preparatory phase of winter-spring rice cultivation (November-December or 3 months after stocking). Prawns were gathered in surrounding ditches of farm, which increased density. This creates an attention that the stocking of prawn much be chosen to avoid such happen. The turbidity of water in rice-prawn farms caused by high suspended solids, and highly increased after heavy rain causing low oxygen dissolved. It is due to mud/dust particles from newly built dike washed into water. Removal of grasses on the flat form also created high water turbidity in some days. 1 4 DO (mg/l) Turbidity (cm) Fig. : Variation of dissolved oxygen (left) and turbidity (right) during culture period 3

4 The nitrate and ammonia in water fluctuated highly during the culture period. They increased after the fertilization for rice. In November and December, these two parameters of rice-prawn farms increased significantly due to the application of inorganic fertilizers for rice after seedling. However, they had not reached a dangerous level for prawn (Fig. ). Hai et al. (1) found similar measurements in his rice-prawn study. Nitrate (mg/l) Total ammonia (mg/l) Fig. 3: Variation of dissolved nitrate (left) and total ammonia (right) during culture period The chlorophyll-a concentrations of water in rice-prawn farms and garden canal were different. The results showed that the garden canal water was more productive than that of rice-prawn farms water. Lower concentration of chlorophyll-a in rice-prawn farms can be explained by high level of suspended solids in water due to rain and flood water. This showed that natural food in rice-prawn farms were low and feeding contributed mainly to the growth of prawns. However, rice-prawn and garden canal prawn farms (especially, the shaded garden canal) are not ideal for the growth of algae due to the limit of light penetration into water. Feeding for cultured species (fish, prawn.) are important for these environments. 14 Chlorophyll-a (ug/l) Fig. 4: Variation of chlorophyll-a during culture period Growth of prawn: the growth of prawn in rice-prawn farms and in garden canal prawn farm was different during the culture period. Prawns grew rather fast during the first month that their weight increased about 1 folds. From th month onward, the growth of prawn in garden canal farm and in rice farms was distinguished. At harvest, the average weight of prawn in garden canal prawn farm was 4.g compared to.-31.g in rice-prawn farms (Fig. ). These results is close to the findings of Hai et al. (1) that the prawn cultured in rice field reached the weight of 3.±14.6g after 6 months. The growth of male and female prawn was found to be significantly different (Fig. 6). Male prawns grew much faster compared to the female ones toward the end of the culture cycle. The males in rice-prawn farms obtained the body weight of 41.g compared to 3.6g of females after 6 months of culture. Same figures were also recorded for prawns in the 4

5 garden canal in that males and females reached the size of 47.4g and 3.g, respectively. The size variation of male and female prawns occurred since the 3 rd month, when the females reached maturation. This is a common phenomenon in prawn culture, which reported by a number of authors such as Hai et al. (1) and Thang (199). Male and female ratios were also different in rice-prawn and garden canal prawn trials. Male ratios were % (or 33.3% in average) and 9.% in rice-prawn and garden canal, respectively. Hai et al. (1) reported that the ratio of male prawns varied by trials of rice-prawn farming from -76.7%. Hai et al. () also reported that the ratio of male prawns cultured in tanks was very low. The differentiation of prawn sex ratio is therefore hard to explain. However, mortality of male and female could be variable according to culture condition or stocking densities. It was also observed that the male prawns with claw color varied during culture period. Prawns changed claw color from orange to blue over the culture period. The organ color of prawn decreased to less than 4% and 7% in rice-prawn and garden canal trials at the 6 th month. Organ claw color of prawn means that they are in the growing phase. 4 Weight (g) Culture period (month) 7 Fig. : Comparison of growth of prawn cultured in rice field and garden canal Weight (g/ind.) Male Female Culture period (month) Weight (g/ind.) Male Female Culture period (month) Fig. 6: Size variation of prawn cultured in rice field (left) and in garden canal (right)

6 Percentage (%) Male Female Percentage (%) Culture period (month) Fig. 7: Comparison of sex ratio of prawn cultured in rice fields and garden canal (right) Fig. : Variation of male prawn with organ claw during culture period in rice field and garden canal. The first maturation of female prawns (berried eggs) was observed since 3 rd month. High ratio of berried egg prawns was recorded from 4 th month. However, this ratio reduced to very low level at the th month, then increased at 6 th month. This showed that female prawns hatched the first batch at th month. Prawn cultured in rice-prawn farms had a high ratio of female maturation compared to prawn cultured in garden canal at 4 th month. This is properly due to lower water temperature of garden canal compared to rice field (Fig. 9). Percentage (%) Rice-prawn 1 Rice-prawn Rice-prawn Fig. 9: Comparison of growth of prawn cultured in rice field and garden canal Survival rates and productivity of prawn: the production of prawn depends on the survival rate and individual weight at harvest. Table showed that the survival rate of prawn varied from 31.-.%. The survival rate of prawn in garden canal (.%) higher than the others ( %); and these results closes what found by other studies. Hai et. al. (1) reported that the survival rates of prawn in rice-prawn culture varied.1-4.1%. These are even better than results found by Phuong et al. () that the survival rate of prawn in rice-prawn farming in Tra Vinh Province that varies from 7.-6.%. The yield of prawn in garden canal trial was 6 kg/ha, which was very good. This resulted from big size of prawn at harvest. The yield of prawn in rice-prawn trials varied from 1- kg/ha, which was lower that result found by Hai et al. (1) that from -66 kg/ha. However, these are comparable to studies reported by Phuong et al. () in Tra Vinh, which ranged from 4-37 kg/ha. Table : Yield of prawn from rice-prawn and garden canal trials Farmers Survival rate (%) Yield (kg/ha) Nguyen Hong Dan (R-farm 1) Nguyen Hong Dan (R-farm )

7 Dinh Cong Doi (R-farm 3) Dinh Cong Doi (GC-farm) Costs and benefit analysis: the economic return of the models depends very much on the yield, size distribution and market price of prawn. Table 3 showed that the net return from garden canal prawn model is negative. This was caused by the rather high FCR (.1) and low market price of prawn. The prawn price dropped down about % from,-6, VND per kg of mixed sizes in to 3,-3, VND in 1. If the price remained at the levels of, then the model would make great return (about 14 millions VND per ha). Table 3: cost and benefit analysis for prawn culture in garden canal (for 1 ha) Items Total costs Prawn seed Feed and fertilizers Pond preparation Harvesting cost Gross income Net return Unit: in million VND Tables 3 is the cost and benefit analysis for rice and prawn components. The income of prawn component is negative (-. millions VND), but rice is high due to high yielding rice variety was applied. The total income of the system was about.6 millions per ha. However, the reason for negative income of prawn component can be explained that firstly the price of prawn reduced shapely in 1 if compared to and secondly price of prawn seed and feed are high covering about 9% of total costs. If price of prawn remained un-change then the prawn component has profit. However, it is necessary to state that the reduction of seed cost and feed cost would improve the profit of the prawn culture. Table 4: cost and benefit analysis for prawn culture in garden canal (for 1 ha) Items Unit: million VND a) For prawn Prawn seed 4± Feed 4.69±.1 Pond preparation and others.3±.3 Total costs 9.±.17 Gross income 6.7±.4 Net return -.±.4 b) For rice Rice seed.33±.4 Fertilizers.633±.47 Harvest.433±.11 Total costs 1.39±.173 Gross return.96±. Net return 7.6 Total income of systems (rice+prawn).6 Prawn disease: black gill disease was found at low ratio in four farms due to the high turbidity of water. Improvement of water quality through water exchange and liming showed good results. Conclusions The environmental conditions of rice field and garden canal are suitable for the normal growth prawn. However, the rice field has some critical period that water quality has low oxygen, high temperature and 7

8 turbidity. Prawns cultured in garden canal grows well and reach high yield of 6 kg/ha, while the yields of rice-prawn farms are at normal level of 1- kg/ha. This study shows that the culture of prawn in rice field and in garden canal did not improve the income of the farmers. However, if the price and feed costs can be reduced and the prawn price remained at normal level, the benefit of these systems would be high. Literature Cited Minh, V.Q., L.Q. Tri, and R. Yamada (). Classification of land mapping units based on soil and hydrological characteristics of Tan Phu Thanh village, Chau Thanh A district, Can Tho province. Proceedings of the annual workshop of JIRCAS Mekong delta project. Nov ,, Can Tho University. Hai, T.N., T.T.T. Hien, D.H. Tam, V.T. Toan and N.T. Phuong). Culture of freshwater prawn (Macrobrachium rosenbergii) in rice-fields using hatchery reared postlarvae in Tam Binh District, Vinh Long Province. (Paper to be presented in the 1 annual workshop of JIRCAS Mekong delta project. Nov. 7-3, 1, Cuu Long Rice Research Institute. Minh, T.Q and N.T. Dung (). Trials on aquaculture in flooded fields in An Giang province, Viet Nam. Proceedings of the International workshop on rice-fish farming, Can Tho University, December,. Phuong, N.T., B.M. Tam and N.T. Long (). Improvement of produced efficiency in mon-rice culture area by aquaculture. Proceedings of the International workshop on rice-fish farming, Can Tho University, December,. Thang, N.V. (199). Techniques of freshwater prawn culture. Agricultural Publishing Houses, 1p. Thực Nghiệm Nuôi Tôm Càng Xanh Trong Ruộng Lúa và Mương Vườn ở xã Tân Phú Thạnh, huyện Châu Thành A, tỉnh Cần Thơ Nguyễn Thanh Phương (1), Vũ Nam Sơn (1), Võ Thành Toàn (1), Trần Thị Thanh Hiền (1) and Phạm Minh Dức (1) and Marcy N. Wilder () 1 Khoa Thủy sản, Đại học Cần Thơ Trung tâm Nghiên cứu Khoa học Nông nghiệp Quốc Tế Nhật Bản, Tsukuba, Ibaraki 3-66 Japan, Tóm tắt Thí nghiệm nuôi tôm càng xanh (Macrobrachium rosenbergii) trên ruộng trồng lúa và trong mương vườn đang được thực hiện tại địa bàn nghiên cứu của dự án JIRCAS thuộc xã Tân Phú Thạnh, huyện Châu Thành A, tỉnh Cần Thơ. Thí nghiệm gồm 3 ruộng nuôi tôm kết hợp lúa hè thu và một mương vườn. Tôm giống có khối lượng trung bình.4 g/con được thả nuôi với mật độ con/m đối với ruộng và 4 con/m đối với mương vườn. Đối với tôm nuôi kết hợp với trồng lúa, mỗi ruộng có một mương ương nhỏ có lưới chắn xung quang và tôm giống được thả ương trong các mương nầy 1 tháng để đạt cở lớn trước khi thả ra cả ruộng nuôi. Tôm được cho ăn thức ăn công nghiệp % đạm và cho ăn bổ sung thêm thức ăn tươi sống (cá, ốc, cua, ) hàng ngày kể từ tháng thứ 3 trở đi. Các chỉ tiêu chất lượng nước như nhiệt độ, độ trong, oxy hòa tan, ph và tôm được theo dõi hàng tháng. Nhìn chung, chất lượng nước thích hợp cho sự sinh trưởng và phát triển bình thường của tôm ngoại trừ một vài ngày độ trong của nước giảm thấp. Tôm nuôi trong mương vườn tăng trưởng nhanh hơn tôm nuôi trong ruộng lúa. Sau 6 tháng nuôi, khối lượng tôm bình quân đạt 4. g và.-31. g/con đối với mương vườn và ruộng lúa. Năng suất tôm nuôi trong mương vườn đạt 6 kg/ha và trong ruộng lúa là 1- kg/ha. Tuy nhiên, về mặt hiệu quả kinh tế thì cả mô hình đều lỗ. Những vấn đề khác sẽ được bàn luận trong báo cáo.