181 currents status of freshwater

FISHERIES SCIENCE 2006; 72: 1–12 Review Article Current status of freshwater prawn culture in Vietnam and the development and transfer of seed production technology Nguyen Thanh PHUONG,1 Tran Ngoc HAI,1 Tran Thi Thanh HIEN,1 Tran Van BUI,1 Do Thi Thanh HUONG,1 Vu Nam SON,1 Yoshinori MOROOKA,2a Yutaka FUKUDA2b AND Marcy N WILDER2* College of Aquaculture and Fisheries, Cantho University, Can Tho City, Vietnam, and 2Japan International Research Center for Agricultural Sciences, Tsukuba, Ibaraki 305-8686, Japan ABSTRACT: In Vietnam, the giant freshwater prawn Macrobrachium rosenbergii is becoming an increasingly important targeted species, as its culture, especially in rice fields, is considered to have the potential to raise income among impoverished farmers The production of M rosenbergii based on aquaculture reached over 10 000 tons per year in 2002, having increased from about 2500 tons since the 1990s Until recently, lack of a stable supply of seed had been an important obstacle to the further expansion and development of M rosenbergii culture, but cumulative research on larval rearing, especially in the 1990s, has led to the development of new seed production technology based on the ‘modified stagnant green water system’ Following its dissemination by the efforts of provincial authorities, hatchery operators, and farmers, the freshwater prawn seed production industry developed rapidly in the Mekong Delta with over 90 hatcheries producing 76.5 million postlarvae in 2003 This is considered to have affected the expansion of rice–prawn farming in the Mekong Delta, leading to increased aquacultural production in the region This paper reviews the current status of freshwater prawn culture in Vietnam and background history, and presents a socioeconomic evaluation of seed production technology implementation KEY WORDS: freshwater prawns, green water, Mekong Delta, rice–prawn farming, seed production AQUACULTURE INDUSTRY IN VIETNAM AND GENERAL STATUS OF FRESHWATER PRAWN CULTURE Vietnam has a high potential for aquaculture development due to the country’s favorable conditions in terms of natural habitats such as ponds, rice fields, rivers, lakes, estuaries, and coastal areas The total area of water bodies that may be *Corresponding author: Tel: 81-298-38-6630 Fax: 81-298-38-6316 Email: marwil@jircas.affrc.go.jp a Present address: Graduate School of Kuroshio Science, Kochi University, Kochi 783-0093, Japan b Present address: National Research Institute of Fisheries Science, Yokohama, Kanagawa 236-8648, Japan Received April 2005 Accepted 26 August 2005 targeted for aquaculture is estimated to be 1.6m by the Vietnamese government.1 Since the beginning of the 1990s, the aquaculture sector in Vietnam has been expanding in terms of culture area, production, targeted species and degree of management intensity Production based on aquaculture has been increasing rapidly in comparison with that from capture fisheries, particularly recently (Fig 1) However, in 2003, the total area of water including freshwater, brackish water and saline areas used in aquaculture was only about 1m ha, or under 60% of the total potential area Recently, some species, and certain culture system types, have contributed significantly to the growth of the aquaculture sector in terms of production and export values The black tiger prawn FISHERIES SCIENCE 1200 Production (capture) Production (aquaculture) Culture area 2000 1000 Ar e a ( x 0 h a ) Pro d u ctio n ( x 0 m t.) 2500 800 1500 600 1000 400 500 200 0 91 92 93 94 95 96 97 98 99 2000 2001 2002 2003 2004 Time (year) Fig Changes in capture fisheries and aquaculture production, and area under culture in Vietnam from 1991-2003.1 Other commodities Tilapia Catfishes Mollusks Freshwater prawn Marine shrimp 100 200 300 400 500 600 700 Production (x 000 m.t.) Fig Production levels of major farmed commodities in Vietnam in 2003.2,3 350 000 Shrimp 300 000 Production (m.t.) Catfishes 250 000 200 000 150 000 100 000 50 000 1994 1997 1999 2000 2001 2002 2003 2004 Time (year) Fig Production and growth of the saltwater–brackishwater shrimp and catfish industries from 1991– 2003.2,4 Peneaus monodon cultured in saltwater–brackishwater areas and Pangasius catfishes Pangasius hypophthalmus and P bocourti cultured in freshwater areas are most important, showing the highest production levels and constant growth of the industry (Figs 2,3).2–4 Other important aquaculture targets include the giant freshwater prawn Macrobrachium rosenbergii, tilapia Oreochromis niloticus, mollusks Anadara granosa and Meretrix meretrix, and indigenous fishes such as the climb- NT Phuong et al ing perch Anabas testudineus and snakehead fish Chana striata and C micropeltes There are many types of production systems observed in Vietnamese aquaculture; not only are intensive and semiintensive forms of aquaculture practiced, but also extensive systems where rice cultivation is integrated with fish or prawn culture Much of the black tiger prawn and Pangasius catfish culture is highly intensive, while other species are cultured under semi-intensive or extensive systems Total aquaculture production in 2004 slightly exceeded 150 000 tons, of which brackishwater aquaculture constituted 510 400 tons (mainly saltwater-brackishwater shrimp at 290 000 tons), while freshwater aquaculture constituted 639 700 tons (mainly catfish at 315 000 tons) The export values of saltwater-brackishwater shrimp (∼$US1.2bn) and catfish ($US300m) comprised the largest proportion of the total value of the fisheries sector.2,5 No figures corresponding to the export of freshwater prawn has been reported, as this product is destined mostly to local markets (only a very small quantity is exported) Currently, although the freshwater prawn culture industry is not equal to the scale of the saltwaterbrackishwater prawn culture industry (Fig 2), the giant freshwater prawn Macrobrachium rosenbergii, which is indigenous to the Mekong Delta, is becoming an increasingly important target species The culture of this species, especially in rice fields, has been a traditional activity, which was based mainly on wild seed collected from rivers and other freshwater bodies However, lack of a stable seed supply was a significant obstacle to the further expansion and development of rice–prawn farming systems Because the giant freshwater prawn is a high-value species, its culture by impoverished farmers is considered to have the potential to raise income and contribute to enhanced rural development Therefore, Vietnam’s Ministry of Fisheries has put forth that the annual production of M rosenbergii must reach 60 000 tons using 32 000 by 2010.6 The Vietnamese Government has also implemented a policy called ‘restructuring of agricultural production and its products consumption’.7 In order to address the needs of this burgeoning industry, basic and applied research has been conducted in order to develop appropriate seed production technology and verify the use of artificial seed in rice–prawn farming in the Mekong Delta Initial research on the larval rearing of M rosenbergii commenced in the early 1980s and was undertaken by Vietnam’s Cantho University and the Research Institute of Aquaculture no 2, although the first hatchery was established outside Ho Chi Minh City in Vung Tau in 1975.8 Basic Freshwater prawn culture in Vietnam FISHERIES SCIENCE research using rearing systems such as the open clearwater system, the closed clearwater system and the green water system was conducted However, at that time, it appeared that better results could be achieved with the open clearwater system in comparison to other systems, and it was therefore decided to use this system in commercial production Several hatcheries were then established near Ho Chi Minh City and in the Mekong Delta However, these hatcheries faced technical and management-related difficulties that inhibited them from maintaining operations, leaving only one state-owned hatchery located in Can Tho Province in the 1990s Most notably, the open clearwater system hatcheries were built at very large scale (greater than 100 m3 capacity), making it difficult for farmers to adopt, and requiring large amounts of sea water for the daily exchange of water For such reasons, at least until 1998, freshwater prawn hatcheries in the Mekong Delta ceased to function, and further development of the rice–prawn farming industry was held back by a shortage of seed.9 However, after a period of active research, a new rearing system referred to as ‘the modified stagnant green water system’ has been in place since 1998 This method of rearing is especially suitable for the household scale, and Cantho University has conducted a series of training courses targeting provincial authorities and private individuals, allowing this technology to be transferred widely through the Mekong Delta and to other parts of Vietnam The concept of this system arises from work10 in which water quality is maintained by natural microalgae (particularly Chlorella spp.) in the rearing water This seed production technology makes use of this concept, and has been adapted for use according to the environmental and social needs of the Mekong Delta This review focuses on the status of freshwater prawn farming in the Mekong Delta especially in the context of rice–prawn farming, and also traces the development of the seed production industry, with an overall discussion of the socioeconomic effect of technology development GENERAL MODES OF FRESHWATER PRAWN CULTURE IN VIETNAM The history and global status of M rosenbergii farming is reviewed by New and Valenti.11 In the early 1980s, global production was recorded as just above 5000 tons, but reached more than 17 000 tons by the end of the decade, plateauing to about 20 000 tons in the early 1990s In the late 1990s, China began to contribute greatly to M rosenbergii farming production, along with Bangladesh and India, culminating in a global production volume of nearly 130 000 tons in 1998 In the FAO’s 2002 statistics, world production of M rosenbergii farming is given as nearly 200 000 tons with a market value of over $US600m.12 In Vietnam, production of M rosenbergii based on aquaculture was estimated to be less than 3000 tons per year throughout the 1990s, with an additional 3000 tons of production being contributed by conventional fishing activity.9 The production of M rosenbergii based on aquaculture alone exceeded 10 000 tons in 2002.1 In this way, the freshwater prawn culture industry is small compared to Vietnam’s saltwater-brackishwater prawn industry and the freshwater prawn culture industry in other countries, but is showing continuous progress in its development and has already become a means of allowing impoverished farmers to raise their income In Vietnam, M rosenbergii is cultured in many ways, but rice–prawn farming and fence culture are the most important production models The total area of rice–prawn culture was estimated to be around 750 in 200313 and fence culture consisted of 1516 units in 2002 (pers comm., Department of Agriculture of Dong Thap Province 2002) Other culture modes are practiced in limited areas The culture of prawns in rice paddy fields is a traditional practice that farmers have developed using their own resources, and has received attention from the Vietnamese government as a means of helping to raise the standard of living of impoverished farmers Original methods are based on the recruitment of seed into farms by sluice gates with the changing of tides; seed would then be trapped and allowed to grow to full size Since early 1980s, some farmers began to stock prawn seed collected from wild sources in their rice paddies, and at present, the culture of freshwater prawns in rice fields is common Of note, areas where rice–prawn farming have taken root are located inland where water is completely fresh; however, since the Mekong Delta is about m above sea level, inland water levels are greatly affected by the change of tides With the recent success of artificial seed production technology in hatcheries, rice–prawn farming has intensified, and management practices have become very diverse More in-depth studies are required to fully categorize the ways in which this form of aquaculture is carried out in the Mekong Delta, but there are two general types of culture The first type is an integrated culture where prawn farming and rice cultivation are carried out simultaneously as a form of mixed farming, and the second type is an alternate culture where prawn culture and rice cultivation are carried out on an FISHERIES SCIENCE alternate basis as a form of ‘relay cropping’ Alternate culture consists of two types: (i) type 1, where one prawn crop and one rice crop are carried out in a year; and (ii) type 2, where one prawn crop and two rice crops are carried out per year The type of culture that is practiced generally depends on NT Phuong et al region of the Mekong Delta, and farmers’ preferences A general description of each type of culture is described below, and a diagram is shown in Figure A map of the Mekong Delta indicates locations of hatcheries targeted in the socioeconomic survey described later (Fig 5) Integrated rice–prawn culture model Month 10 Summer-autumn rice 11 12 Winter-spring rice Integrated culture Alternate Culture: Type1 Alternate Culture: Type Fig Diagram of the major types of rice–prawn farming Periods of prawn culture are indicated by solid arrows, periods of rice cultivation are indicated by dotted arrows The short, heavy dotted arrow ( ) indicates the period of nursing culture in integrated culture The cropping pattern of this model consists of two rice crops, as summer–autumn rice from March to May/June and winter–spring rice from December to March One prawn crop is carried out between April and December This farming model is practiced in the Vinh Long and Tra Vinh Provinces of the Mekong Delta where farmers still produce two rice crops per year and where there are low levels of flooding (Fig 5) Farms are designed with a surrounding trench comprising 20–25% of total rice paddy area Mostly, trenches are either fenced with fine nylon netting or are built with simple earthen dikes for nursing prawns during the first period of stocking After 3–4 weeks, prawns reach 2–3 cm in (a) (b) Major areas of prawnrice farming Integrated culture Alternate culture, Type Alternate culture, Type Vinh Long Tra Vinh Hatchery survey Total no hatcheries Targeted no hatcheries Can Tho 41 10 Provinces Can Tho An Giang Provinces An Giang 10 Dong Thap Dong Thap Fig Mekong Delta hatcheries (a) map showing important areas of the three main forms of rice– prawn farming, and locations of hatcheries targeted in the socioeconomic survey detailed in ‘Socioeconomic evaluation of hatchery technology’, and (b) hatcheries selected from districts in each province where the highest numbers of hatcheries were located Ring-shaped symbols on the map show locations of sample clusters Freshwater prawn culture in Vietnam FISHERIES SCIENCE body length, and are ready to be released to the farm At this point, rice has already been sown or transplanted The production cycle of prawn is about 6–8 months, but prawns are cultured together with summer–autumn rice crop for only about 2–2.5 months Stocking density ranges from 1.5 to individuals/m2 depending on seed size Water is exchanged every weeks in order to maintain water quality levels suitable for aquaculture Typically, a metal pipe connecting the rice field with a nearby canal is filled with mud; because of the topography of the Mekong Delta, even inland areas are affected by changing tides, and water can either be let into, or discharged from, the field at high and low tides, respectively This method is used in alternate culture models as well The productivity of this model varies widely according to stocking density and stocking size of seed, and farm management regimes (e.g use of commercial feeds vs fresh feeds such as snail and trash fish) Productivity is low, ranging from about 40 kg/ha to over 500 kg/ha because of the small size range of prawns at harvest and low survival rates Alternate rice–prawn culture models As discussed, there are two types of alternate culture In type 1, prawns are stocked in April and harvested in December before the winter–spring rice crop (December–March) In type 2, prawn culture occurs from July to December between two rice crops (summer–autumn, March–May/June and winter–spring, December–March) Type has been developing rapidly in flooded areas, especially An Giang and Can Tho Provinces (Fig 5), where more than 350 operated in 2002 Hatchery-reared postlarvae are stocked directly into the rice fields or in small nursing ponds for month Stocking densities vary from to 12 postlarvae/m2 Prawns are fed commercial pellets and fresh feeds Fresh feeds such as golden snail, trash fish and crab are cheap and abundant during the flooding period During this time (July–October), water usually flows over the surrounding dykes, and farms are considered to be in a state of overflow After 6– months of stocking, prawns reach 50–110 g body weight and are ready to be harvested Prawn productivity is about 900 kg/ha In type 2, prawns are stocked into rice paddy fields when the summer– autumn rice is harvested This model is suitable for areas of high flooding such as in An Giang, Dong Thap, and Can Tho Provinces, (Fig 5) because there are very favorable natural conditions during the culture period, such as high water level, abundance of natural feeds and ready availability of fresh feed components (e.g snails, trash fish, and Table Simple cost-benefit analysis for type alternate rice–prawn farming (1:2 prawn–rice crops) Item Prawn Total cost ($US/ha) Gross return ($US/ha) Income ($US/ha) Rice Total cost ($US/ha) Gross return ($US/ha) Income ($US/ha) Grand total income (rice and prawn) Mean ± SD 907 ± 507 2093 ± 1187 1187 ± 840 379 ± 191 1187 ± 840 733 ± 273 1920 ± 940 $US1 = Vietnamese Dong (VND) 15 000; SD, standard deviation small freshwater crabs) This model operated for about 5–6 months; therefore, stocking of largesized seeds (average 15 g/individual) is required in order to ensure that the harvested prawns reach a marketable size The productivity of prawns in type (about 360 kg/ha) is much lower than in type (about 900 kg/ha) Table shows a simple costbenefit analysis for type Net income levels based on both rice and prawn production reached nearly $US2000, showing that prawn culture in rice fields can greatly supplement income obtained from rice cultivation only Data has been obtained for type alternate rice–prawn farming and integrated farming; however, in these two models, large variations in costs and income were observed, indicating some instability in these types of farming systems Therefore, the proper selection of culture model that considers the natural circumstances of each area is crucial for successful aquaculture However, for all models of aquaculture, it is essential to provide a stable supply of larvae to aquaculturists Through the 1990s, only five hatcheries existed in the Mekong Delta region and the Ho Chi Minh City vicinity (with only one of these fully operating); their production levels were not sufficient to meet the needs of the industry, and there was thus a dependence on the use of seed obtained from natural sources.9 This was an obstacle to the further development of the industry, until new seed production technology could be developed and disseminated Recirculating systems are commonly used in large-scale hatcheries in most countries with M rosenbergii culture industries.14 However, the need to develop small-scale hatcheries that can be implemented on a backyard basis was recognized, and during the late 1990s and early 2000s, the modified stagnant green water system was refined and adapted to the needs of Vietnam’s industry as detailed below FISHERIES SCIENCE STUDIES ON THE DEVELOPMENT OF TECHNOLOGY FOR PRAWN SEED PRODUCTION BASED ON THE ‘MODIFIED STAGNANT GREEN WATER SYSTEM’ Experiments on larval rearing were conducted during 1998–2002 at Cantho University’s minihatchery.15 These experiments included the effects of type of rearing system, rearing density, feed content and feeding regime, algal densities, source of saline water, use of probiotics, and size of rearing tanks on the development and survival of freshwater prawn larvae At the outset, in order to evaluate the feasibility of different larval rearing systems, as well as appropriate rearing densities for each system, a small-scale experiment was conducted comparing two water maintenance systems, the recirculating clear-water system and the modified stagnant green water system, under different stocking densities of 30, 60, 90 and 120 larvae/L for each system Fiberglass tanks of 100 L were placed under an opaque roof For the recirculating system, water was recycled at rate of 100–200% tank volume daily through a biofilter For the modified stagnant green water system, green water from tilapia tank culture was added to the prawn larval rearing tanks at a density of 0.5 × 106 cells of algae/mL before stocking of prawn larvae No water was added or exchanged during the larval rearing cycle Larvae in both systems were fed with Artemia nauplii (twice/day, Artemia/mL) alone for the first four stages From stage four, larvae were fed with custard (4 times/ day) in the daytime and Artemia nauplii (once/day, Artemia/mL) in the evening No siphoning was conducted during larval rearing in either system Table shows the results length of rearing cycle and survival rate results The modified stagnant green water system yielded significantly better survival rates (32.3–92.3%) compared to the recirculat- Table Larval development under the recirculating and modified stagnant green water systems15,16 Treatment Rearing cycle (days) Recirculating system 30 larvae/L 34 60 larvae/L 34 90 larvae/L 34 120 larvae/L 34 Modified stagnant green water system 30 larvae/L 30 60 larvae/L 30 90 larvae/L 30 120 larvae/L 30 Survival rate (%) 52.5 28.8 31.7 27.4 92.3 46.3 46.4 32.3 NT Phuong et al ing systems (27.4–52.5%) The presence of algae in the rearing water may help to stabilize water quality and enhance the nutritional effects from the feeding of Artemia, hence shortening the time required to complete the rearing cycle This in turn may lead to decreased cannibalism of larvae and postlarvae, thereby promoting higher survival rates Based on the survival rates given in Table 2, although lower stocking densities yield higher survival rates, it can be calculated that mid-level densities provide greater numbers of postlarvae upon completion of the rearing cycle Higher stocking levels lead to decreased survival rates and thus decreased efficiency For these reasons, and due to its simple management needs and promising results, the modified stagnant green water system was then selected for further development using suggested stocking densities of 60–90 larvae/L.15,16 In order to develop appropriate larval feeds for use in the green water system, a series of experiments was conducted comparing feeding treatments including regimens of Artemia in combination with commercial feeds and handmade custards containing chicken egg, powdered milk and fish oil only, or these three components with other ingredients such as shrimp flesh, pork liver or cockle meat Custard in combination with Artemia yielded high survival rates, but the use of Artemia alone or in combination with commercial feed yielded very poor results The simple custard containing egg, powdered milk, and fish oil was then selected for application to subsequent experiments In follow-up studies by Hien et al.,17 the lipid component of the custard was modified, and the effects of differing sources of lipids in combination with lecithin on larval growth and survival were examined As a result, it was found that a basic recipe containing Anlene Gold powdered milk (New Zealand Milk, Wellington, New Zealand; 53.8% w/w), chicken egg yolk (41.7% w/w), 3% squid oil and 1.5% lecithin yielded satisfactory survival rates and body length of postlarvae Representative data from this study are shown in Table Increasing dietary lecithin from 1.5% to 3.0% did not increase larval growth rates; for this reason, supplementation at 1.5% was considered most effective in terms of cost It was also found that a feeding regime using 1– Artemia nauplii/mL/day with custard feeding (ad libitum) four times daily was most suitable Higher densities of Artemia nauplii or increased frequency of daily feeding times did not improve the survival of postlarvae In production terms, this result means that 2.1–2.4 kg of Artemia cysts and 20– 24 kg of custard are needed to produce × 106 postlarvae The optimal density of algae was determined as × 106 cells of algae/mL in terms of sur- Freshwater prawn culture in Vietnam FISHERIES SCIENCE Table Production of M rosenbergii postlarvae reared with six experimental diets containing differing lipid sources and levels Diet Soybean oil 4.5% Soybean oil 3.0%, lecithin 1.5% Soybean oil 1.5%, lecithin 3.0% Squid oil 4.5% Squid oil 3.0%, lecithin 1.5% Squid oil 1.5%, lecithin 3.0% Survival rate (%) PL body length (mm) Rearing cycle (days) 22.0 ± 8.2a 32.0 ± 11.9a 48.7 ± 1.9b 51.1 ± 7.2b 65.0 ± 5.4c 68.1 ± 8.1c 6.29 ± 0.04a 6.45 ± 0.39a,b 6.75 ± 0.13b 7.14 ± 0.05c 7.26 ± 0.17c 7.43 ± 0.11c 33 34 34 31 30 30 Differing letters indicate significant differences at P < 0.05 Data presented as mean ± standard deviation Survival rate of larvae (%), body length of postlarvae (PL) and length of rearing cycle are shown.17 Table Effects of differing algal density on larval rearing Treatment No algae 0.25×106 cells/mL 1×106 cells/mL 4×106 cells/mL Rearing cycle (days) Survival rate (%) 38 36 35 36 11.7a 22.2b 41.4c 25.1b Differing letters indicate significant differences at P < 0.05 vival rates of postlarvae (Table 4) In order to produce green water, large tilapia (50 g) cultured in m3 tanks at density of 10 fish/m3 were fed with pelleted feed at a rate of 5% body weight per day keeping water salinity at 12 ppt After week of rearing, the culture was filtered through a μmmesh filter bag in order to selectively obtain microalgae consisting mainly of chlorella Finally, in order to apply the modified stagnant green water system to commercial production, it was necessary to evaluate the effects of different sizes of rearing tanks on the development and survival rates of postlarvae An experiment was conducted using differing tank volumes of 0.5, 1.0, 2.0 and 4.0 m3/tank, with three replications for each treatment Stocking density was 60 larvae/L Green water was also added to the tanks at a density of × 106 cell/mL before stocking larva Artemia (2 nauplii/mL), and custard were also fed to larvae It was found that the smaller tanks gave better survival rate and productivity; therefore, rearing periods of larvae and the tanks of 0.5–1 m3 were selected for use in commercial hatcheries Brine from saltpans in Bac Lieu Province was found to be suitable for larval rearing As salinity ranges 60– 140 ppt, only a small amount of seawater is necessary to be used in seed production operation These studies have provided concrete data for the establishment of a complete technical package for larval rearing for use in Vietnam, especially in the Mekong Delta.18 The modified stagnant green water system performs as well, or better than, previously existing larval rearing systems that include recirculating or open water models The survival rate of larvae until reaching the postlarvae-15 stage (15 days after metamorphosis) in the green water system averages about 40% (ranging 30–75%) compared to 35–50% for open systems, or 15.2–66.2% for recirculating systems.19 There have been other studies on larval rearing of giant freshwater prawn aiming to develop various technical packages by Vietnamese scientists Most of this research focused on the improvement of larvae survival rate using differing rearing methods, feeds, stocking density, and origin of prawn Thang19 focused his studies on the application of exchange and recirculating water methods and found that the survival of larvae until the postlarvae-15 stage could attain 35–50% for open systems, or 15.2–66.2% for recirculating systems Tinh20 studied the effects of stocking density on the survival rates of larvae until the postlarvae-15 stage using water exchange methods, finding that under a stocking density of 80–100 larvae/L a survival rate greater than 40% could be attained However, Hung and Phuc21 reported that rearing larvae using water exchange methods showed a variation of survival rates of larvae until postlarvae-15 from 24.0 to 53.2% Yen22 conducted a study on the effects of broodstock origin on the survival rates of larvae applying water exchange methods He found that the survival rate of larvae until the postlarvae-15 stage was 51.9% and 16.5% for broodstock of Thai and Vietnamese origin, respectively Phuc et al.23 reported that on the experimental scale of 100-L tanks using water exchange methods, survival rate of larvae until the postlarval stages varied depending on stocking density yielding survival rates of 12.0, 13.0 and 44.6% for stocking densities of 80, 100 and 200 larvae/L, FISHERIES SCIENCE TRANSFER OF FRESHWATER PRAWN SEED PRODUCTION TECHNOLOGY USING THE ‘MODIFIED STAGNANT GREEN WATER SYSTEM’ Based on the results of the green water technology development detailed above, Cantho University bid for funding from the relevant organizations and authorities in 2000 in order to transfer this technology throughout the Mekong Delta Provincial extension workers and technicians, and private persons were mainly targeted Trainees visited Cantho University to learn the techniques in a hands-on format As part of the training course, each person was responsible for his or her own hatchery-scale culture tank; Cantho University staff instructed the trainees in the full use of the technology, including larval collection, water quality maintenance, and feeding techniques By 2003, six new provincial hatcheries were established in the Mekong Delta, and 150 trainees representing the Mekong Delta provinces and other provinces throughout Vietnam, including Ho Chi Minh City, Dong Nai, Binh Thuan, Ninh Thuan, Bac Ninh Provinces, received training from Cantho University.24 A technical package detailing seed production methods is detailed in a manual with accompanying audio-visual media that has been distributed widely to users in Vietnam.18 This technology covers all aspects of the seed production process, including hatchery construction, water quality control, feed development, and end-marketing SOCIOECONOMIC EVALUATION OF HATCHERY TECHNOLOGY Following the rapid dissemination of freshwater prawn hatchery technology throughout the Mekong Delta in the early 2000s, a survey of the 100 90 80 70 60 50 40 30 20 10 NT Phuong et al 90 Number of hatcheries 80 Production of PL 70 60 50 40 30 20 10 Production of PL (m illions) respectively However, these systems were not accepted because each of the authors referred to above conducted only one or a few research projects, without implementing follow-up studies to confirm the results In our collaborative studies (Japan International Research Center for Agricultural Sciences and Cantho University), a series of research projects were undertaken until reproducible results were obtained that could be easily transferred to end-users Moreover, our model is based on simple techniques, and has been scaledup to meet the needs of commercial production The active transfer of this technology to end-users has further led to its well-grounded establishment in the Mekong Delta No hatcheries 1999 2000 2001 2002 2003 Year Fig Development of freshwater prawn hatcheries and annual production levels of postlarvae (PL) in the Mekong Delta Tra Vinh 5% Long An 5% Vinh Long 2% Ca Mau 1% Tien Giang 8% Can Tho 45% Dong Thap 10% An Giang 11% Ben Tre 13% Fig Distribution of freshwater prawn hatcheries in the Mekong Delta in 2003 status of new hatchery construction was conducted in each province in mid-2003 It was revealed that there were 91 hatcheries in the region, belonging to both provincial authorities and private individuals (Fig 6) Nearly 80% of these hatcheries were located in Can Tho (45%), Ben Tre (13%), An Giang (11%) and Dong Thap Provinces (10%) (Fig 7) Whereas in 1999, there was only one hatchery in the Mekong Delta, these data underline the rapid increase in hatchery establishment during the past 3–4 years This increase in hatchery facilities has been accompanied by a proportional increase in the frequency of green water technology implementation (Fig 8), revealing the effectiveness of technology transfer and confidence of end-users towards the implementation of this FISHERIES SCIENCE Freshwater prawn culture in Vietnam technology As a result, during the past several years, there has been an increase in yearly production from 1×106 to 77.5×106 postlarvae/year (Fig 6) The general status of the hatcheries in Can Tho, An Giang, and Dong Thap Provinces was surveyed to obtain a representative sampling of Mekong Delta hatcheries (Fig 5) Hatcheries in Ben Tre Province were not targeted due to its geographic proximity to Can Tho Province In these three provinces, 60 hatcheries have been established, and rice–prawn farming activities are strong In order to achieve a survey rate of 30%, 17 hatcheries were selected at random from the districts of each province with the largest number of hatcheries Hatchery operators were interviewed using a 10-page survey containing questions on general background information, capacity of hatchery, stocking density, broodstock treatment, type of rearing system used, feeding practices, investment costs, and running costs, and were asked to report actual income levels Survey rates differ according to province; this is due to factors such as the absence of the hatchery operator at the time of the visit, or the inability of the operator to provide a full set of data, necessitating particular hatcheries to be excluded from the analysis 100 Green water system 90 Open and recirculating systems 80 No of hatcheries 70 60 50 40 30 20 10 1998 1999 2000 2001 2002 2003 Fig Hatcheries using the modified stagnant green water system and open and recirculating systems in the Mekong Delta Based on this survey, it has been revealed that the status of technology implementation is as follows With the exception of some hatcheries applying the recirculating system, almost all of the hatcheries (88.2% of the surveyed hatcheries) have been applying the modified stagnant green water system (Fig 8) The hatcheries were normally small-scale with total volume of rearing tanks of 10–40 m3, normally of 20–30 m3 (Table 5) Rearing tanks mostly consisted of composite with a volume of 0.5–1 m3/tank Regarding management practices, most of the hatcheries based their operations on technology transferred from Cantho University Some hatcheries used wild broodstock (52% of all hatcheries) or farmed broodstock (48%) Prawn broodstock showed a weight range of 20–160 g, averaging 50 g The number of larvae that could be obtained from kg of broodstock ranged from 150 000 to 500 000 larvae, averaging 322 850 larvae Larvae were stocked at a density of 40–60 larvae/L Green water from tilapia culture media was used to inoculate to rearing water, and Artemia nauplii and custard were the main feed used for larvae during rearing The rearing cycle normally lasted for 25– 45 days depending on season Most hatcheries achieved very good results with survival rates of 20–90% to postlarvae-15, averaging 45.3% in Can Tho, 36.2% in Dong Thap and 46.7% in An Giang Provinces (Table 5) In analysis of the green water technology, of particular interest is the initial investment required to set up a hatchery Costs cover a very large range according to the scale of each facility (e.g., total capacity, and number of aquaria) Average capital investment was $US2500 and did not show a large deviation from the average, but operational costs showed a very large range of variation (Table 6) In this way, a hatchery’s ability to produce postlarvae depends greatly on rearing methods and number of cycles There is not one way to characterize the economic character of the hatcheries, but an examination of the data of the 17 hatcheries showed an average net income of $US7709 was achieved As revealed from the range of variation, it can be considered that even if facilities sufficient to meet high production levels are constructed, it is necessary to make full use of the Table Characteristics of prawn hatcheries in surveyed provinces of the Mekong Delta Provinces Can Tho Dong Thap An Giang Total volume of rearing tanks/hatchery (m3) Stocking density (larvae/L) Rearing cycle (days) Survival rate (%) 28.8 ± 14.7 38.8 ± 31.5 16.7 ± 5.77 60 ± 12.4 55 ± 12.9 53.3 ± 5.77 25–45 30–45 30–45 45.3 ± 19.8 36.2 ± 9.4 46.7 ± 23.1 10 FISHERIES SCIENCE NT Phuong et al Table Economic parameters of prawn hatcheries in the Mekong Delta Productivity (thousand PL/hatchery/year) 1134 (180–5000) Capital investment ($US/hatchery) Operation costs ($US/hatchery/year) Net income ($US/hatchery/year) 2500 (2000–2667) 1457 (353–4733) 7709 (1200–33 333) Table Cost-benefit analysis for a standard prawn hatchery of 10 m3 capacity with three operating cycles per year25 Itemized operational costs Production operation cycles per year Unit Broodstock kg Artemia kg Custard kg Freshwater m3 Brine m3 Labor Man-months Electricity KW Gasoline L Other costs per cycle Cycle Depreciation for fixed costs (10 years) Cycle Total operating costs Harvest (number of postlarvae at PL 15 stage per year = 034 700) Gross income per year (assumption of $US1 for 150 PL) Net income per year ($US) Unit cost ($US) Quantity/1 production cycle 6.67 100 4.67 0.13 20 33.3 0.1 0.38 44.3 111 1.5 20 624 Total (for operation cycles) ($US) 40 450 98 120 400 187 133 333 1777 6898 5121 Fixed costs in $US: Total investment for hatchery housing materials ($US1000), assorted equipment ($US667), tanks ($US1333), materials ($US333) = $US3333 Depreciation for fixed costs (10 years) is calculated as (fixed costs × 1/10) = $US333 facilities by maximizing the number of production cycles implemented, as well as promoting other measures that lead to a stabilization of production In particular, data presented on operating costs (average $US1457) and net income ($US7709) suggest that there still exist risks associated with running a hatchery as a business enterprise (Table 6) The cost of larval feed including Artemia and custard comprised 42% of total costs, being the highest portion of total operating costs, followed by the costs of brine water and labor (Fig 9) In order to present a detailed case study for hatchery operations, an additional hatchery in Can Tho Province was selected, and the hatchery owner was given a record book to record all costs associated with three production cycles As shown by the results in Table 7, in a typical year in which three production cycles with a capacity of 10 m3 were performed, the cost of broodstock, larval feeds including custard and Artemia, rearing water, electricity, gasoline, labor and other operational costs are calculated, total operational costs were $US1777 After subtracting total costs from gross income, an average net income of more than $US5000 can be expected.25 However, these data Other 24% Feed 42% Brinewater 13% Labor 13% Fig Broodstock 8% Operating costs in prawn hatcheries are from only one case study; in this way, the scale of the facility, number of production cycles, and other operational factors greatly influence overall profits More detailed analysis is required in subsequent surveys Freshwater prawn culture in Vietnam FISHERIES SCIENCE GENERAL DISCUSSION AND FUTURE PROSPECTS Both experimental and practical seed production of Macrobrachium rosenbergii have been conducted in Vietnam, generally in the Mekong Delta, for several decades However, these activities have reached a very dynamic state only recently This is because the techniques required to implement the modified stagnant green water system are very simple and can be practiced by nearly any person with a minimum of training Investment and production costs are also low These conditions make the system appropriate for use in household-scale production Further, the need for only limited use of brackish water (obtained from saltpans), due to the fact that rearing water does not need to be exchanged, means that it is possible for hatcheries to be established in freshwater areas located far from coastal areas With the increasing demand on prawn postlarvae for grow-out, seed production of freshwater prawn harbors great advantages for development Support from central and local governments in relation to agricultural restructuring policies and support from research institutes and universities have also played an important role in the development of prawn seed production in the Mekong Delta and Vietnam However, the modified stagnant green water systems may incur problems especially during the rainy season Large-scale hatcheries or large rearing tanks are sometimes difficult to manage, for larval availability, feeding and water quality control (especially water temperature due to prolonged decrease in atmospheric temperature) There is advantage in using brine for larval rearing, especially when it is highly concentrated, as less volume would be required However, servicing, where an intermediary is employed to obtain seawater from saltpan areas and provide it to the hatchery operator, and water quality, need to be controlled In addition, often there is a very high demand for prawn seeds but during a short period of several months before and after the rainy season, which has started to put pressure on seed production activities At present, broodstock used in seed production is mainly collected from natural waters, so it is becoming increasingly urgent to develop a program of broodstock domestication These merits and risks relating to seed production in the Mekong Delta have been frequently observed by hatchery managers and technicians Prawn farming in the Mekong Delta had remained at low levels until hatchery-reared seed became widely available in the early 2000s Integrated and alternate rice–prawn cultures characterize the main rice–prawn farming practices in the 11 Mekong Delta The integrated rice–prawn culture model harbors large areas that have potential to be developed It is considered to be a subsistence activity, which brings income that supplements that obtained from rice production This type of culture is more suitable for non-flooding areas or in areas where two crops of rice per year are still carried out However, this model achieves low productivity, small prawn size at harvest, and low survival rates, which leads to low economic return compared to other models The two alternate types are more commercial, and have become a central activity of rice farmers These types are suited for flooded areas or in locations where summer–autumn rice cultivation is subjected to high flooding risk Many provincial governments in the Mekong River Delta plan to expand alternate rice–prawn culture in the future The advantages of these models include high economic return and suitability to rice paddy ecology However, these models require high investment and a high level of technology, which may not be suitable for poor and low-skilled farmers The economic return from these models is attractive, and is creating momentum toward further future development However, while alternate culture in which only prawns are cultured during a specific period offers higher economic returns, there is the danger that with increased production, unit price will decrease In addition, although environmental impact has not been a concern until now, it is possible that the high profits that are being attained will entice farmers to engage in more intensified forms of culture, which may have detrimental effects on the local environment In this regard, further on-site studies should be conducted to examine the relationship between prawn culture and water quality in both rivers and sources destined for human use ACKNOWLEDGMENTS Seed production technology development and socioeconomic evaluation of hatchery technology was conducted within an international collaborative program (‘Development of new technologies and their practice for sustainable farming systems in the Mekong Delta’: FY 1999–2003), between the Japan International Research Center for Agricultural Sciences (JIRCAS) and Cantho University, Vietnam M.N Wilder was supported by the Research and Development Program for New Bioindustry Initiatives of the Bio-oriented Technology Research Advancement Institution (BRAIN) of Japan 12 FISHERIES SCIENCE REFERENCES Ministry of Fisheries Aquaculture Achievements in 2002 and Implementation Plan for 2003 Government of Vietnam Ministry of Fisheries, Hanoi 2003 (in Vietnamese) Ministry of Fisheries Aquaculture Achievements in 2004 and Implementation Plan for 2005 Government of Vietnam Ministry of Fisheries, Hanoi 2005 (in Vietnamese) Tiep NN Quality and solutions for sustainable development of catfish production National Conference on Quality and Trade Name for Catfishes of Vietnam Angiang Province, December 14–15, 2004 Government of Vietnam Ministry of Fisheries, Long Xuyen 2004 (in Vietnamese) Mieng TT Issues on planning and management for catfish production to the year 2010 National Conference on Quality and Trade Name for Catfishes of Vietnam Angiang Province, December 14–15, 2004 Government of Vietnam Ministry of Fisheries, Long Xuyen 2004 (in Vietnamese) Hanh DD Continuous growth of the Vietnamese fisheries sector in 2004 Fish Mag 2005; 1: 3–5 (in Vietnamese) Government of Vietnam Approval of Aquaculture Development Plan Period from 1999–2010, Decree no 224/1999/QDTTg Government of Vietnam, Hanoi 1999 (in Vietnamese) Government of Vietnam Restructuring of Agricultural Production and Its Products Consumption, Decree no 09/2000/NQ-CP Government of Vietnam, Hanoi 2000 (in Vietnamese) Thang NV Giant Freshwater Prawn Farming Agricultural Publishing House, Ho Chi Minh City 1995 (in Vietnamese) Hien TTT, Minh TH, Phuong NT, Wilder MN Current status of freshwater prawn culture in the Mekong River Delta of Vietnam JIRCAS J 1998; 6: 89–100 10 Ang KJ, Cheah SH Juvenile production of the Malaysian giant freshwater prawn (Macrobrachium rosenbergii de Man) using modified stagnant ‘green water’ system In: Chan HH (ed.) Development and Management of Tropical Living Aquatic Resources Universiti Pertanian Malaysia, Malaysia 1987; 141–144 11 New MB, Valenti WC Freshwater Prawn Farming: The Farming of Macrobrachium rosenbergii Blackwell Science, Oxford 2000 12 FAO World production of fish, crustaceans and mollusks, etc., by principal species FAO Fisheries Department Statistical Databases and Software, Yearbook of Fisheries Statistics; Summary Tables 2002 Available from URL: http:// www.fao.org/ 13 Phuong NT, Duc PM, Son VN, Bui TV, Nguyet ATA Application of high technologies for quality improvement of catfish, tilapia and giant freshwater prawn products and reduction of their production costs Report submitted to An Giang Department of Science and Technology, Long Xuyen, 2004 (in Vietnamese) 14 New MB Farming Freshwater Prawns: A Manual for the Culture of Giant Freshwater Prawn (Macrobrachium rosenbergii) Food and Agriculture Organization of the United Nations, Rome 2002 NT Phuong et al 15 Phuong NT, Tuan NA, Hien TTT, Hai TN, Wilder MN, Ogata HY, Sano M, Maeno Y Development of freshwater prawn (Macrobrachium rosenbergii) seed production and culture technology in the Mekong Delta of Vietnam: a review of the JIRCAS project at Cantho University JIRCAS Working Report 2003; 26: 39–47 16 Wilder MN, Fukuda Y Development of freshwater prawn seed production technology in the Mekong Delta region of Vietnam Yoshoku (Aquaculture Monthly) 2002; 39: 98–101 (in Japanese) 17 Hien TTT, Hai TN, Phuong NT, Ogata HY, Wilder MN The effects of dietary lipid sources and lecithin on the production of giant freshwater prawn (Macrobrachium rosenbergii) larvae in the Mekong Delta region of Vietnam Fish Sci 2005; 71: 279–286 18 Phuong NT, Hai TN, Hien TTT, Wilder MN Giant Freshwater Prawn (Macrobrachium rosenbergii) Seed Production: Principles and Practices Agricultural Publishing House, Hanoi 2003 (in Vietnamese) 19 Thang NV Studies on biological characteristics and seed production of giant freshwater prawn (Macrobrachium rosenbergii) in the southern part of Vietnam PhD Thesis Fisheries University, Nha Trang 1993 (in Vietnamese) 20 Tinh NV Status of prawn seed production of the Vung Tau Prawn Research Center during the past few years Conference on Development of Giant Freshwater Prawn Production in the Southern Part of Vietnam, An Giang Province, May 29, Long Xuyen, 1999 (in Vietnamese) 21 Hung NQ, Phuc TT Report of the giant freshwater prawn (Macrobrachium rosenbergii) technology transfer project for some provinces of the southern part of Vietnam Conference on Development of Giant Freshwater Prawn Production in the Southern Part of Vietnam, An Giang Province, May 29, Long Xuyen, 1999 (in Vietnamese) 22 Yen PT Macrobrachium Rosenbergii seed production in Vietnam: an examination of the performance of broodstock and larvae from sources in Thailand and Vietnam Msc Thesis, Asian Institute of Technology, Bangkok 1999 23 Phuc TT, Hung NQ, Thanh V, Van NTK, Quyen L, Nhut N, Lich HT, Thao NTT Technical improvement and production cost reduction for giant freshwater prawn (Macrobrachium rosenbergii) seed production in the Mekong River Delta Report submitted to Ministry of Fisheries, Vietnam, Hanoi 2001 (in Vietnamese) 24 Phuong NT, Hai TN, Hien TTT, Oanh DTH, Yen NLH, Viet TV Transfer of giant freshwater prawn (Macrobrachium rosenbergii) seed production technology Report submitted to the National Fisheries Extension Center, Hanoi 2003 (in Vietnamese) 25 Phuong NT, Hien TTT, Hai TN, Liem PT, Yen NLH, Viet HV, Bui TV, Hang BTB Studies to complete the technical package of giant freshwater prawn (Macrobrachium rosenbergii) seed production using the modified stagnant green water system Report submitted to the SUFA project, Hanoi 2002 (in Vietnamese)  ... needs of the Mekong Delta This review focuses on the status of freshwater prawn farming in the Mekong Delta especially in the context of rice–prawn farming, and also traces the development of the... with an overall discussion of the socioeconomic effect of technology development GENERAL MODES OF FRESHWATER PRAWN CULTURE IN VIETNAM The history and global status of M rosenbergii farming is... total volume of rearing tanks of 10–40 m3, normally of 20–30 m3 (Table 5) Rearing tanks mostly consisted of composite with a volume of 0.5–1 m3/tank Regarding management practices, most of the hatcheries