181 currents status of freshwater

rosenbergii culture, but cumulative research on larval rearing, espe-cially in the 1990s, has led to the development of new seed production technology based on the ‘mod-ified stagnant g

*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 8 April 2005 Accepted 26 August 2005.

Review Article

Current status of freshwater prawn culture in

Vietnam and the development and transfer of seed

production technology

1College of Aquaculture and Fisheries, Cantho University, Can Tho City, Vietnam, and 2Japan International Research Center for Agricultural Sciences, Tsukuba, Ibaraki 305-8686, Japan

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,

espe-cially in the 1990s, has led to the development of new seed production technology based on the ‘mod-ified 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 consid-ered 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 cul-ture in Vietnam and background history, and presents a socioeconomic evaluation of seed production technology implementation

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

condi-tions 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

targeted for aquaculture is estimated to be 1.6m ha

by the Vietnamese government.1 Since the begin-ning 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 com-parison with that from capture fisheries, particu-larly 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 sys-tem types, have contributed significantly to the growth of the aquaculture sector in terms of pro-duction and export values The black tiger prawn

Peneaus monodon cultured in

saltwater–brackish-water areas and Pangasius catfishes Pangasius

hypophthalmus and P bocourti cultured in

fresh-water areas are most important, showing the

high-est production levels and constant growth of the

industry (Figs 2,3).2–4 Other important aquaculture

targets include the giant freshwater prawn

Macro-brachium rosenbergii, tilapia Oreochromis

niloti-cus, mollusks Anadara granosa and Meretrix

meretrix, and indigenous fishes such as the

climb-ing perch Anabas testudineus and snakehead fish Chana striata and C micropeltes There are many

types of production systems observed in Vietnam-ese aquaculture; not only are intensive and semi-intensive forms of aquaculture practiced, but also extensive systems where rice cultivation is inte-grated 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 1 150 000 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 val-ues of saltwater-brackishwater shrimp (∼$US1.2bn) and catfish ($US300m) comprised the largest pro-portion 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 saltwater-brackishwater prawn culture industry (Fig 2), the

giant freshwater prawn Macrobrachium rosen-bergii, which is indigenous to the Mekong Delta, is

becoming an increasingly important target spe-cies 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 sta-ble 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 impov-erished 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 ha by 2010.6 The Vietnamese Government has also implemented a policy called ‘restructuring

of agricultural production and its products con-sumption’.7 In order to address the needs of this burgeoning industry, basic and applied research has been conducted in order to develop appropri-ate 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

Fig 1 Changes in capture fisheries and aquaculture

production, and area under culture in Vietnam from

1991-2003 1

0

500

1000

1500

2000

2500

91 92 93 94 95 96 97 98 99 2000 2001 2002 2003 2004

Time (year)

0 200 400 600 800 1000 1200

Production (capture)

Production (aquaculture)

Culture area

Fig 2 Production levels of major farmed commodities

in Vietnam in 2003 2,3

Marine shrimp

Freshwater prawn

Mollusks

Catfishes

Tilapia

Other commodities

Production (x 1 000 m.t.)

Fig 3 Production and growth of the

saltwater–brack-ishwater shrimp and catfish industries from 1991–

2003 2,4

0

50 000

100 000

150 000

200 000

250 000

300 000

350 000

1994 1997 1999 2000 2001 2002 2003 2004

Time (year)

Shrimp

Catfishes

research using rearing systems such as the open

clearwater system, the closed clearwater system

and the green water system was conducted

How-ever, at that time, it appeared that better results

could be achieved with the open clearwater system

in comparison to other systems, and it was

there-fore decided to use this system in commercial

pro-duction 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,

fresh-water 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

stag-nant 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

nat-ural 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

farm-ing 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

aquac-ulture 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 contin-uous 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 ha in 200313 and fence culture con-sisted of 1516 units in 2002 (pers comm., Depart-ment 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 atten-tion from the Vietnamese government as a means

of helping to raise the standard of living of impov-erished 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 1 m above sea level, inland water levels are greatly affected by the change of tides

With the recent success of artificial seed produc-tion 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 simul-taneously as a form of mixed farming, and the sec-ond type is an alternate culture where prawn culture and rice cultivation are carried out on an

alternate basis as a form of ‘relay cropping’

Alter-nate 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

region of the Mekong Delta, and farmers’ prefer-ences A general description of each type of culture

is described below, and a diagram is shown in Figure 4 A map of the Mekong Delta indicates locations of hatcheries targeted in the socioeco-nomic survey described later (Fig 5)

Integrated rice–prawn culture model

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 prac-ticed 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 sur-rounding 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

Fig 4 Diagram of the major types of rice–prawn

farm-ing Periods of prawn culture are indicated by solid

arrows, periods of rice cultivation are indicated by

dotted arrows The short, heavy dotted arrow ( )

indi-cates the period of nursing culture in integrated culture.

Summer-autumn rice Winter-spring rice

Integrated

culture

Alternate

Culture:

Type1

Alternate

Culture:

Type 2

Fig 5 Mekong Delta hatcheries (a) map showing important areas

of the three main forms of rice– prawn farming, and locations of hatcheries targeted in the socio-economic survey detailed in

‘Socioeconomic evaluation of hatchery technology’, and (b) hatcheries selected from districts

in each province where the high-est numbers of hatcheries were located Ring-shaped symbols on the map show locations of sample clusters.

Provinces Major areas of prawn-

(a)

(b)

rice farming Vinh Long Tra Vinh Can Tho An Giang Dong Thap

Integrated culture

Alternate culture, Type 1

Alternate culture, Type 2

Provinces Hatchery survey

Can Tho An Giang Dong Thap

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 5 individuals/m2 depending on seed size

Water is exchanged every 2 weeks in order to

main-tain 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

pro-ductivity 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

cul-ture In type 1, prawns are stocked in April and

har-vested 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 1 has been

developing rapidly in flooded areas, especially An

Giang and Can Tho Provinces (Fig 5), where more

than 350 ha operated in 2002 Hatchery-reared

postlarvae are stocked directly into the rice fields

or in small nursing ponds for 1 month Stocking

densities vary from 3 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–

8 months of stocking, prawns reach 50–110 g body

weight and are ready to be harvested Prawn

pro-ductivity 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,

abun-dance of natural feeds and ready availability of

fresh feed components (e.g snails, trash fish, and

small freshwater crabs) This model operated for about 5–6 months; therefore, stocking of large-sized 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

2 (about 360 kg/ha) is much lower than in type 1 (about 900 kg/ha) Table 1 shows a simple cost-benefit analysis for type 2 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 1 alternate rice–prawn farming and integrated farm-ing; 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 pro-vide 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 nat-ural sources.9 This was an obstacle to the further development of the industry, until new seed pro-duction technology could be developed and dis-seminated 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 recog-nized, 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

Table 1 Simple cost-benefit analysis for type 2 alter-nate rice–prawn farming (1:2 prawn–rice crops)

Prawn

Gross return ($US/ha) 2093 ± 1187

Rice

Gross return ($US/ha) 1187 ± 840

Grand total income (rice and prawn) 1920 ± 940

$US1 = Vietnamese Dong (VND) 15 000; SD, standard deviation.

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

mini-hatchery.15 These experiments included the effects

of type of rearing system, rearing density, feed

con-tent and feeding regime, algal densities, source of

saline water, use of probiotics, and size of rearing

tanks on the development and survival of

fresh-water 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

con-ducted comparing two water maintenance

sys-tems, 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,

2 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,

2 Artemia/mL) in the evening No siphoning was

conducted during larval rearing in either system

Table 2 shows the results length of rearing cycle

and survival rate results The modified stagnant

green water system yielded significantly better

sur-vival rates (32.3–92.3%) compared to the

recirculat-ing systems (27.4–52.5%) The presence of algae in the rearing water may help to stabilize water qual-ity 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 sur-vival rates, it can be calculated that mid-level den-sities 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 ex-periments was conducted comparing feeding

treatments including regimens of Artemia in

combination with commercial feeds and hand-made 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

exper-iments 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 combi-nation with lecithin on larval growth and survival were examined As a result, it was found that a ba-sic 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 sur-vival rates and body length of postlarvae Repre-sentative data from this study are shown in Table 3 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–

2 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 1 × 106

postlarvae The optimal density of algae was deter-mined as 1 × 106 cells of algae/mL in terms of

sur-Table 2 Larval development under the recirculating

and modified stagnant green water systems 15,16

Treatment Rearing cycle (days) Survival rate (%)

Recirculating system

Modified stagnant green water system

vival rates of postlarvae (Table 4) In order to

produce green water, large tilapia (50 g) cultured in

1 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 1 week of

rearing, the culture was filtered through a 5

μm-mesh 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

sur-vival rates of postlarvae An experiment was

con-ducted 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

1 × 106cell/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

neces-sary 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, pre-viously 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%) com-pared 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 vari-ous technical packages by Vietnamese scientists Most of this research focused on the improvement

of larvae survival rate using differing rearing meth-ods, 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,

respec-tively Phuc et al.23 reported that on the experi-mental 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,

Table 3 Production of M rosenbergii postlarvae reared with six experimental diets containing differing lipid sources

and levels

Diet

Survival rate (%)

PL body length (mm)

Rearing cycle (days)

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 4 Effects of differing algal density on larval

rearing

Treatment

Rearing cycle (days)

Survival rate (%)

Differing letters indicate significant differences at P < 0.05.

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

Agricul-tural Sciences and Cantho University), a series of

research projects were undertaken until

reproduc-ible results were obtained that could be easily

transferred to end-users Moreover, our model is

based on simple techniques, and has been

scaled-up 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

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

Provin-cial extension workers and technicians, and

pri-vate 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

Univer-sity 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

estab-lished in the Mekong Delta, and 150 trainees

rep-resenting 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

produc-tion process, including hatchery construcproduc-tion,

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

status of new hatchery construction was con-ducted 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 under-line 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 technol-ogy implementation (Fig 8), revealing the effec-tiveness of technology transfer and confidence of end-users towards the implementation of this

Fig 6 Development of freshwater prawn hatcheries and annual production levels of postlarvae (PL) in the Mekong Delta.

0 10 20 30 40 50 60 70 80 90 100

1999 2000 2001 2002 2003

Year

0 10 20 30 40 50 60 70 80

90

Number of hatcheries Production of PL

Fig 7 Distribution of freshwater prawn hatcheries in the Mekong Delta in 2003.

Can Tho 45%

Ben Tre 13%

An Giang 11%

Dong Thap 10%

Tien Giang 8%

Long An 5%

Tra Vinh 5%

Vinh Long 2%

Ca Mau 1%

technology As a result, during the past several

years, there has been an increase in yearly

produc-tion 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

prov-inces, 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

prov-ince with the largest number of hatcheries

Hatch-ery operators were interviewed using a 10-page

survey containing questions on general

back-ground information, capacity of hatchery, stocking

density, broodstock treatment, type of rearing

sys-tem 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

Based on this survey, it has been revealed that the status of technology implementation is as fol-lows With the exception of some hatcheries apply-ing the recirculatapply-ing 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 prac-tices, 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 1 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) Aver-age capital investment was $US2500 and did not show a large deviation from the average, but oper-ational costs showed a very large range of varia-tion (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 facil-ities sufficient to meet high production levels are constructed, it is necessary to make full use of the

Fig 8 Hatcheries using the modified stagnant green

water system and open and recirculating systems in the

Mekong Delta.

0

10

20

30

40

50

60

70

80

90

100

Green water system Open and recirculating systems

Table 5 Characteristics of prawn hatcheries in surveyed provinces of the Mekong Delta

Provinces

Total volume of rearing tanks/hatchery (m 3 )

Stocking density (larvae/L)

Rearing cycle (days)

Survival rate (%)

facilities by maximizing the number of production

cycles implemented, as well as promoting other

measures that lead to a stabilization of

produc-tion In particular, data presented on operating

costs (average $US1457) and net income

($US7709) suggest that there still exist risks

associ-ated with running a hatchery as a business

enter-prise (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

associ-ated 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,

elec-tricity, 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

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 subse-quent surveys

Table 6 Economic parameters of prawn hatcheries in the Mekong Delta

Productivity

(thousand PL/hatchery/year)

Capital investment ($US/hatchery)

Operation costs ($US/hatchery/year)

Net income ($US/hatchery/year)

Table 7 Cost-benefit analysis for a standard prawn hatchery of 10 m 3 capacity with three operating cycles per year 25

Itemized operational costs

Production operation cycles per

Unit cost ($US)

Quantity/1 production cycle

Total (for 3 operation cycles) ($US)

Harvest (number of postlarvae at PL 15 stage per year = 1 034 700)

Gross income per year (assumption of

$US1 for 150 PL)

6898

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.

Fig 9 Operating costs in prawn hatcheries.

Feed 42%

Broodstock 8%

Labor 13%

Brinewater 13%

Other 24%