Freshwater prawn culture: a review

Aquaculture, 88 (1990) 99-143 Elsevier Science Publishers B.V., Amsterdam

99 -

Printed

in The Netherlands

Freshwater prawn culture: a review Michael B. New ASEAN-EEC Aquaculture Development and Coordination Programme (AADCP), P.O. Box 1006, Bangkok 10903 (Thailand) (Accepted

12 January 1990)

ABSTRACT New, M.B., 1990. Freshwater prawn culture: a review. Aquaculture, 88: 99-143. Although freshwater prawn farming forms only about 5% of the global aquaculture production of shrimps and prawns it is of considerable local importance, particularly in S.E. Asia. Recent entry into global markets is likely to stimulate renewed interest in Macrobruchium culture. This review summarizes the status of freshwater prawn culture globally in terms of research, rearing technology, marketing and economics.

INTRODUCTION

While freshwater prawns have been collected from the wild and on-grown to market size in impoundments for some considerable time, true culture systems began to be developed only after the life cycle of Macrobruchium rusenbergii was closed in a Malaysian laboratory (Ling, 1977). This stimulated widespread interest in the culture of this and other palaemonids. Following the importation of a few Macrobrachiumrosenbergiibroodstock in 1965, the Anuenue Fisheries Research Centre in Hawaii developed mass rearing techniques for larval production (Fujimura and Okamoto, 1972 ) . Successful growout experiments spawned commercial farms in Hawaii and elsewhere throughout the 1970s. In Asia, Thailand and Taiwan (Chen, 1976) were initiating what has become a significant industry in both countries. At first the stimulus was to supply domestic demand for freshwater prawns but, in Thailand at least, an export market has since developed. By 1988 annual Thai production of farmed Macrobrachium rosenbergii was about three times greater than that of Taiwan (M.B. New, unpublished data, 1989 ) . Early enthusiasm for Macrobrachium culture in the western hemisphere waned when it was realized that a developed global market for freshwater prawns did not yet exist. However, freshwater prawn farming continued to expand elsewhere, notably in Asia and, later, in Latin America. Global pro0044-8486/90/$03.50

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TABLE 1 Countries where production Country

Africa La Reunion Malawi Mauritius Zimbabwe Total Africa

of Macrobrachium rosenbergii is reported

Annual production (metric tonnes)

7 8

35 12

Year

Information source

1987 1987 1987 1987

A A A A

Notes

62

(R + D activity also reported in Cote d’Ivoire, Sierra Leone and South Africa )

+ +

Production data not separated from other crustacea

Asia Bangladesh India Israel Japan Malaysia Taiwan Thailand Vietnam

10 55 5 4500 11839 8600

Total Asia

25 009

(R + D or pilot-scale activity also reported in China (P.R.C.), Indonesia, Myanmar (Burma), Nepal, Pakistan, Philippines, Saudi Arabia, and Sri Lanka)

Nil

(R+ D activities reported in Germany (F.R.G. ) , Greece, Iceland, Italy, Sweden and U.S.S.R.)

1987 1987 1987 1988 1987 1987

A A A F A D

5000 t expected in 1989 underestimate?

Europe Total Europe

Latin America and the Caribbean Brazil 1000 Colombia 1 6 Costa Rica 186 Dominican Republic 1 El Salvador 52 Guadaloupe 13 Guatemala 63 Guyanne 10 Honduras 20 Jamaica 52 Martinique 361 Mexico

1987 1987 1987 1987 1987 1988 1987 1988 1987 1987 1988. 1987

A A A B B c B c A A C A

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Panama Peru Puerto Rico Venezuela

Annual production (metric tonnes) I 10 93 1

Total L.A. + Carib.

1876

North America United States (including Hawaii)

91

Total North America

91

Oceania and Pacific Hawaii (see U.S.A.) New Caledonia Polynesia Solomon Islands

+ 1 20 6

Total Oceania/Pacific

27

Global total

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A REVIEW

27 065

Year

Information source

1987 1987 1987 1987

A A E A

Notes

1988 expected to be 136 t

(R + D activities also reported in Dominica, Ecuador, Grenada, Suriname, and Uruguay )

1987

A

(R + D activity also reported in Canada)

1988 1988 1987

C C A ( R + D activity also reported in Australia, Fiji, Guam, Micronesia, New Zealand, Palau, Vanuatu and Western Samoa) ( 1989 global shrimp production= 565 000 t)

Sources: A=FAO, 1989; B=U.N. Wijkstrom, personal communication, 1989; C=IFREMER, 1989; D = Bui Dinh Chung, 1988; E = Aquaculture Digest (see Acknowledgements); F= I-Chiu Liao, personal communication, 1989.

duction of Macrobruchium rosen@rgii is now about 27 000 metric tonnes (t ) / year (Table 1) . Thailand (44%)) Vietnam (32%) and Taiwan ( 17%) are the main producers. Apart from these three countries only Brazil, the Dominican Republic, Mexico, the French Overseas Territories, and perhaps Puerto Rico currently produce more than 100 t annually. Neither Malaysia, where the life cycle of freshwater prawns was first closed, nor Hawaii, which pioneered mass larval rearing techniques, has become a major production centre for farmed Macrobrachium.

Research and development on the culture of other species of Mucrobrachium has been reported (New, 1988 ) in Cote d’Ivoire, Ghana and Nigeria (M.

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vollenhovenii); India, Malaysia and the Philippines (M. lanchesteri); India and Pakistan (M. malcolmsonii); Brazil, Colombia, Mexico and Venezuela (M. acanthurus); Brazil, Colombia, and Mexico (M. americanurn); Brazil, Mexico and Venezuela (M. carcinus); and Australia, El Salvador, India, Japan, Mexico, Venezuela and the U.S.A. (miscellaneous species). This review provides a summary of freshwater prawn farming and research in 1990 by supplementing information presented in earlier papers (New, 1980a, 1982; Malecha, 1983a,b; New and Singholka, 1985) which are not cited individually throughout the text. Except where otherwise stated, this review refers only to Macrobrachium rosenbergii (also known as the giant long-legged Malaysian prawn). Using FA0 terminology, Macrobrachium spp. are referred to as “prawns” throughout the review. Current global status Thai prawn production increased from 3 t in 1976 (Boonyaratpalin and Vorasayan, 1983) to 400 t in 198 1 through a joint Department of Fisheries and FAO/UNDP programme (New et al., 1982). Following this public-sector stimulation the commercial sector has since flourished. Official statistics show that 4508 t were produced in 1986 and 11 839 t in 1987. In 1987 there were 6048 farms with a total area of 11 770 ha. Freshwater prawn exports were 7 171 t in 1987 and an estimated 6860 t in 1988. Because much of the farmed product goes live to restaurants, official statistics underestimate true production. Unofficial estimates are that about 15 000 t and 18 000 t of farmed prawns were produced in Thailand in 1986 and 1987, respectively (M.B. New, unpublished data, 1989). Production in 1988 was undoubtedly less than in 1987, due mainly to better prices for Thai rice paddy, an alternative use for the same land. Official figures are expected to show 1988 Thai production of about 10 000 t. In Taiwan, marine shrimp farming expanded rapidly in response to global demand but freshwater prawn farming is also a significant and expanding industry. From only 65 t in 1979 (Liao and Chao, 1982)) annual production had increased to 250 t in 1982 (Liao and Chao, 1983), 3500 t in 1986 (I-C. Liao, personal communication, 1987) and reached 4500 t in 1988 (Hsieh et al., 1989); over 5000 t are expected in 1989. In 1989 freshwater prawn farming occupied 3500 ha in Taiwan. Vietnam’s farmed production (8600 t in 1987 ) is almost entirely from extensive, wild-stocked ponds (Bui Dinh Chung, 1988). The prawn farming industry in Hawaii, as in Thailand, was stimulated by the free government distribution of postlarvae. In 1980 there were at least 24 prawn farms, of which the largest was reported to be harvesting at the rate of 83 t/year in 1981 but never exceeded that output. Although forecasts were for 1000 t/year by 198 1, actual Hawaiian prawn production was only 118 t/ year in that year. Unlike Thailand, Hawaii did not have abundant sites, cheap

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labour, nor a large domestic market accustomed to freshwater prawn consumption, and output declined throughout the 1980s. Many southern states of the U.S.A. practise seasonal prawn culture in monoculture or polyculture. Geothermal, thermal and solar heated recirculating systems for prawn farming have been proposed but total commercial production of freshwater prawns in the United States, including Hawaii, remains less than 100 t/year. Research and foreign investment (in the Dominican Republic, Ecuador, Honduras, Jamaica, the Philippines and Puerto Rico, for example) have been the principal contributions of the U.S.A. to global prawn farming. In the Americas, Brazil ( 1000 t ), Mexico (36 1 t ), the Dominican Republic ( 186 t ), and Puerto Rico (93 t ) each exceed the total prawn production of the U.S.A. The French territories of Martinique, Guadeloupe and Guyanne produced a total of 167 t of farmed prawns in 1988 (IFREMER, 1989). Japan ( 55 t ), Mauritius ( 35 t ) , Jamaica (20 t ) and Polynesia (20 t ) also produce significant quantities of farmed freshwater prawns annually while many other countries produce less than 20 t/year (Table 1) . Commercial development of prawn farming is now widespread but piecemeal and, with the exceptions noted above, national production is insignificant in terms of volume. Research and development activities remain strong but the large-scale expansion of freshwater prawn farming which was expected in the late 1970s has not yet materialized. This is due to a combination of factors which include marketing and technical difliculties such as long larval life and territorialism during grow-out. A sound global commodity market for freshwater prawns has not yet been developed but increasing exports by Thailand may herald some changes in this respect. PRODUCTION

TECHNOLOGY

AND PRACTICES

Biology with special reference to broodstock Freshwater prawns are well distributed throughout the tropical and subtropical zones, there being over 100 species of the genus Macrobrachium. This genus typically requires estuarine conditions during the larval stages though some species, for example M. amazonicum and M. dayanum, complete their life cycle in inland saline and freshwater lakes. Juveniles of M. rosenbergii migrate upstream towards freshwater. Large adults are found in rivers and, often at considerable distances from the sea, in lakes, swamps, and irrigation canals. Larvae can survive in freshwater for the first 5 days but must then reach brackishwater (Ling, 1977). Prawns can surmount weirs and waterfalls and can migrate short distances over land where the vegetation is moist. M. rosenbergii appears to favour turbid conditions but some other species, such as M. americanum, are generally found in clear-water rivers. Of the three largest species known, M. americanurn, M carcinus, and A4. rosenbergii, the latter

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has become the most favoured for aquaculture. It is indigenous to the whole of South and South-East Asia, together with northern Australia and the western Pacific islands. Most hatcheries capture berried (egg-carrying) females from farm ponds rather than the wild. Separate prawn broodstocks are rarely maintained except in Taiwan (Hsieh et al., 1989). Mature male prawns are considerably larger than females and the claws (second periopods ) much larger and thicker. In both sexes of Macrobruchium rosenbergii these second periopods are clawed and of equal length. The predominant adult colour is blue, sometimes brown. The male “head” (cephalothorax) is larger than the female. Male genital pores are between the bases of the fifth periopods while those of the female are at the base of the third periopods. The pleura of the female are longer; this and its wider abdomen form a brood chamber. The egg mass is orange-coloured, becoming brown and then grey-black as the eggs mature. Mating takes place between hard-shelled males and soft-shelled (recently moulted) ovigorous females. A gelatinous sperm mass is deposited near the gonopores between the walking legs of the female and external fertilization occurs as the eggs are spawned a few hours later. Eggs are transferred to a ventral brood chamber, held in place by a thin membrane between the abdominal pleura and periodically ventilated by vigorous movement of the pleopods. In the wild, mating takes place at all times of the year with some environmentally related peaks, e.g., the onset of the rainy season. Large M. rosenbergii females lay 80 000-100 000 eggs per clutch but first broods are often only 5000-20 000 eggs. Egg incubation time at 28°C averages about 20 days (range 18-23 days). Pre-mate intermoult periods may be only 23 days and ovaries can ripen while the brood chamber still contains eggs. Unilateral eyestalk ablation has been shown to significantly advance sexual maturation and increase the number of moults, clutches and eggs of Macrobrachium nobilii (Kumari and Pandian, 1987 ) but this technique is not used in commercial practice with A4 rosenbergii. Large, healthy, well-pigmented females are selected as broodstock, normally when the egg mass is brown. Brown eggs take from 6 to 12 days to hatch, depending on temperature. Recently, Ganeswaran ( 1989) has developed a key based on various visual and microscopic characters which enables the date of hatching to be predicted with an accuracy of ? 1 “C. This will assist hatchery managers in ensuring the synchronous hatching of each production batch. In Taiwan, brown-egged females are sold at U.S.$ 11.30-l 9.OO/kg, depending on season (Hsieh, 1989). Broodstock females are transferred in buckets, aerated tanks or, if the rostrum is blunted, in inflated plastic bags to separate tanks, or tanks mounted above hatchery tanks, or directly into hatchery tanks. Berried females are disinfected for 30 min in aerated water containing 0.20.5 ppm copper or 15-20 ppm formalin. They are then placed in water rang-

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ing from O%oto 12%0without acclimatization; hatchability is higher in brackishwater. Adult prawns are not only euryhaline but are tolerant of a wide temperature range ( 18-34’ C ) . Increased temperature ( 32 ‘C ) and photoperiod 12L: 12D) have been shown to significantly enhance reproductive capability (Chavez Justo et al., 1989). Optimum pH is 7.0-8.0. Dissolved oxygen level should preferably be 70% saturation but levels as low as 1 ppm are tolerated (Avault, 1987). Low-density larval rearing requires about three lo-12-cm (rostrum to telson) females (each carrying 10 000-30 000 eggs) to stock each cubic metre of tank volume. Allowing for physical losses, cannibalism and hatching rate, this results in a stocking density of 30-50 larvae/l. To maintain good larval water quality berried females are often not fed. However, broodstock may be maintained on grain and other seeds, algae, small molluscs and crustaceans, fish flesh, or on pelleted feeds. After the eggs hatch, females are returned to ponds or marketed. Hatching and larval rearing Macrobrachium rosenbergii eggs are slightly elliptical

(long axis 0.6-0.7 mm) and become grey-black 2-3 days before hatching. Eggs hatch within 3 weeks (usually 19 days) of being laid. Larvae hatched from eggs incubated at 25 “C show a wider range of tolerance to salinity and temperature than those incubated at 29°C or 3 1 “C (Gomez Diaz, 1987). Gomez Diaz and Ohno ( 1986) reported that the optimal temperature for survival rate to metamorphosis was 28°C (range tested 22-34”(Z). Within each brood most larvae hatch within 48 h, mainly at night, but brood females are often left in the incubation or larval rearing tanks for 4 days. The pleopods of the brooding female maintain high dissolved oxygen levels around the eggs and also disperse newly hatched larvae. First-stage larvae are planktonic, active tail-first swimmers, about 2 mm long. Ten more stages are generally recognized before metamorphosis, which occurs 16-28 days after hatching. However, Gomez Diaz and Kasahara ( 1987) have reported observing an additional six zoeal instars, making 17 in total. Many Hawaiian hatcheries stock first-stage larvae at 60/l (although some practise two-stage rearing with an initial stocking density of 160/l). Thai hatchery management results in a stocking density of 30-50/l and aims to achieve 100 000-200 000 postlarvae from each 1O-m3 larval tank ( 1O-20/1). Acceptance of low survival levels is linked to the ready availability of berried females. Hawaiian hatcheries expect 50-70% survival to achieve a postlarval production of 30/l. Intensive prawn hatcheries both in French Overseas Territories and in French-inspired hatcheries elsewhere (Palanisamy, 1989) stock at 100/l. Taiwanese .inland hatcheries often use a multistage hatchery system with initial stocking rates of 300-1000/l in 500-l tanks, with transfers to larger tanks as the larvae grow (Hsieh et al., 1989). Larvae are generally reared at lo- 15%0. The optimum water temperature

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range is 26-3 1 “C; temperatures below 24’ C or above 35 ‘C cause retarded development or mortality. However, Taiwanese hatcheries are reported to rear larvae at 30-33 ‘C (Hsieh et al., 1989). Other desirable water quality criteria include pH 7.0-8.5, maximum nitrite (NO,-N) and nitrate (NO,-N) levels of 0.1 ppm and 20 ppm, respectively, maximum total hardness of 100 ppm ( CaCO, ) and low iron and manganese content. Coastal hatcheries usually mix filtered seawater with well water or dechlorinated tap water. Inland hatcheries in Thailand operate by trucking seawater, brine or rock salt to the site and diluting this with freshwater (Tunsutapanich, 1981). Yambol and Cruz (1986) also reported that brine solution can be effectively used in prawn larviculture. The use of brackishwater reconstituted from sea salt also showed economic promise in the Philippines. Toxic levels of ammonia in the larval rearing water are normally avoided by water exchange (up to 50% per day) and sometimes also by the use of biological filters or “green water”. Recirculation systems for prawn hatcheries have also been devised (Singholka and Sukapunt, 1982; Chavez Justo and Ramirez Ochoa, 1983; Ong, 1983; Khin Maung Soe, 1989; Vu Zung Tien et al., 1989) but are only normally found in commercial form in French-owned or designed hatcheries, which employ coral, preactivated (Griessinger et al., 1989) biolilters at 5% of the larval rearing volume, high aeration, 30-3 1 ‘C, and 37 m3 conical or “bath-shaped” tanks (IFREMER, 1989 ) . Photoperiodicity is not normally controlled but there is some evidence that spectral quality is important and should resemble sunlight (M.B. New, unpublished data, 1970). Where “clear-water” systems are used tanks are normally 90% covered or housed indoors to prevent the “sun-cancer” effect (Fujimura and Okamoto, 1972). Most commercial hatcheries combine feeding with a “green-water” or a “clear-water” rearing system. In the former, fertilization (e.g., 4 parts urea: I part 15 : 15 : 15 NPK inorganic fertilizer ) produces a bloom of phytoplankton (mostly Chlorella spp.) in water used during part or all of the rearing cycle for the replacement of that lost during tank cleaning and water-exchange routines. The “green-water” technique has been claimed to increase postlarval production by lo-20% over clear-water systems because it maintains good water quality. However, mortalities are often greater in green water due to high pH and algal blooms (Hummel et al., 1987; Hudon et al., 1989). Few Thai or French hatcheries use this system now but Taiwanese hatcheries maintain a balanced ecosystem of culture water which includes algae (Hsieh et al., 1989 ) . Mass mortalities occur at pH 9.5 and above and in “green-water” systems pH can rise to pH 9.9-10.5. Stocking “green-water” systems at 18.30 or 00.30 h (high but decreasing pH) gives better survival than at 06.30 or 12.30 h. Prawn larvae must eat continuously to survive but do not actively search for food (Moller, 1978 ). Food density is therefore critically important and

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must be maintained regardless of the density of prawn larvae. However, the influence of food density decreases and larval population density increases as larvae grow (Lin and Uno, 1987 ) . Various live and inert feeds are used in different combinations. Newly hatched brine shrimp nauplii (Artemia sp.), fish flesh and egg custard are frequently used. Artemia nauplii are normally provided on the first day after hatching although most prawn larvae do not feed until the second day. Artemia are maintained at l/ml and increased to about 5/ml at feeding times in Thailand but Hawaiian hatcheries tend to use densities of 5-15 nauplii/ ml. Artemia are provided twice a day until about day 5. Brachionus plicatilis is neither a good substitute nor supplement for Artemia (Lovett and Felder, 1988 ). Prepared feed is also essential for success. Most Hawaiian hatcheries hand-feed fish flesh several times daily. Cooked egg custard, which often incorporates mussel flesh in Thailand, is frequently used as an additional larval food. Feeding of prepared feed commences on day 3 and increases as live feed decreases. By day 5 it is given four to live times during daylight hours, Artemia being reserved for the final evening feed. In French hatcheries a highprotein compound feed containing squid is used with Artemia. The particle size of prepared feeds varies from 0.3 to 1.0 mm, depending on larval size. Feed is kept suspended by vigorous aeration. Recently a number of live-feed substitutes and enhancers for crustacean larval rearing, some based on the pioneering work of Jones et al. ( 1974, 1979) on microencapsulation, have been marketed commercially but have not yet become standard in prawn hatcheries. Generally, Artemia nauplii and egg-fish diets prepared on-site are preferred, with or without “green water”. Modifying the HUFA content of Artemia nauplii has proved effective (Sorgeloos et al., 1986) and has led to Artemia-enrichment diets. Postlarval prawn production rates have been reported as 10-20/l in Thailand, 30/l in Hawaii, and up to 50/l, using highly intensive antibiotic-aided techniques, in French hatcheries (AQUACOP, 1977). The use of antibiotics, and sometimes sulpha drugs, in larval rearing is commonplace though usually not publicized. Massive blooms of Zuothamnium, EpisfyZis and hydroids, which are harmful to larvae, occur if hatchery hygiene is inadequate or incoming water is poorly treated. Water treatments and tank disinfection include the use of formalin, chlorine, potassium permanganate, malachite green and copper sulphate. Handling postlarvae

Newly metamorphosed postlarval prawns are about 7 mm long, begin to swim and crawl like adult animals and are salinity tolerant. Once about 90% have metamorphosed they can be acclimatized to freshwater over 2-3 h, harvested by dip nets and at the tank drain, and transferred to holding tanks before distribution to nursery or rearing ponds. In holding tanks densities of

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up to 5000 postlarvae/m2 can be successfully maintained for 1 week or 1OOO2000/m2 for 1 month. Postlarvae will quickly accept grow-out feeds. Floating catfish feeds or expanded pet feeds are sometimes used at first while most of the animals still live close to the water surface. Postlarvae may be transported short distances (up to one hour’s journey) in aerated tanks at 750/l. For journeys of up to 12 h inflated plastic bags in insulated containers stocked at 300/l are satisfactory, while density should be reduced to 100/l for journeys of 24-36 h (Alias and Siraj, 1988). Reduced temperature, slightly brackishwater and using postlarvae at least 7 days old increase survival (Harrison and Lutz, 1980). Experimentally, 17-g prawns have been successfully shipped at 19-20’ C in oxygenated water for 42 h at densities equivalent to 12-15 g/l (Smith and Wannamaker, 1983); 6-g juveniles could be shipped at 18 g/l for 24 h or 9- 11 g/l for 48 h. In this case neither brackishwater (2-8%0) nor increased substrate benefited survival. Adults packed in an unrestricted manner, rather than those immobilized in mesh, had substantially higher survival rates. Survival rate is more closely related to dissolved oxygen level than to any other water quality parameter. Sometimes tris (hydroxymethyl aminomethane) or other compounds are added to buffer the water, and clinoptilolite may be added to freshwater shipments to absorb ammonia. The sale price of hatchery-reared postlarvae varies according to local production costs and the state of the market. A 1976 cost study in Tahiti (AQUACOP, 1979) showed production costs to be U.S.$ 12/ 1000 but today, prawn postlarvae are commonly sold at U.S.$ 25/1000 in the U.S.A. In Taiwan (Hsieh et al., 1989) newly hatched larvae are sold at US$ 0.06-0.48/ 1000 while postlarvae are available at U.S.$ 9.40-22.60/1000. In Thailand postlarvae are only U.S.$ 1.4-4.0/ 1000. Many Thai hatcheries produce penaeid shrimp postlarvae as well as freshwater prawns in response to fluctuating demand. Recently production costs in a private hatchery in French Guiana were reported to be U.S.$ 17.6/ 1000 (IFREMER, 1989). Thailand seems to have a clear advantage in this respect, largely because of its many simple and versatile “back-yard” hatcheries_ Nursery and grow-out technology

Many farmers stock I-4-week-old postlarvae directly into rearing ponds while others use an intermediate l-3-month nursery phase with stocking densities from 350/m2 (Alston, 1989) to 1500/m2. Nursery-reared juveniles (0.5-2 g) are then stocked into rearing ponds. Artificial habitats may be used to increase stocking density in intensive prawn nursery systems (Smith and Sandifer, 1979 ) . Nursery rearing is particularly useful where climatic conditions or intermittent water availability prevent continuous culture to market size. Isosmotic conditions are not required for optimal growth (Singh, 1980)

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and postlarval prawns are tolerant of wide ranges of salinity and temperature. Salinities up to lO%o seem as good as freshwater (Smith et al., 1982) and 12%0-25%0 had been used experimentally (Popper and Davidson, 1982); despite this, nearly all commercial prawn farming takes place in freshwater. Temperatures below 14 ’ C or above 35 oC are generally lethal, 29-3 lo C being optimal. Rearing temperatures of 18-22 oC have been reported to markedly stunt growth and predispose prawns to idiopathic muscle necrosis ( Akiyama et al., 1982). Pond DO, levels down to 1 ppm can be tolerated (Avault, 1987) but levels below 25-30% saturation cause visible distress; pond managers prefer to maintain 6-8 ppm DOZ. Optimal water hardness appears to be in the 20-200 ppm range (Vasquez et al., 1989). pH 7.0-8.5 is reported to be optimal for prawn culture. High unionized ammonia nitrogen (NH,-N ) and high pH have a synergistic toxic effect (Strauss et al., 1989); juveniles are more tolerant of high NH,-N/pH than postlarvae. Higher NH3-N can be tolerated at lower pH levels. Normally prawns are reared to market-size in earthen ponds but commercial pen culture is also practised in Thailand (Kulkeo et al., 1985 ), yielding an average 1075 kg/ha ( 1985 ). Thai pond production averaged more than 1006 kg/ha in 1987 (Department of Fisheries, Thailand, personal communication, 1989); average unit productivity is greater in Hawaii at 1449 kg/ha in 1984 (Lam and Wang, 1986). The growth rate of individual Macrobrachium rosenbergii is rapid under tropical conditions. Increases of 100 g in stagnant earthen ponds in 9 months and 120 g in 7 months in running-water cement ponds in India (Panicker and Kadri, 198 1) and average weights of 142 g after 1 year have been reported in Bangladeshi ponds (Khan et al., 1980). In seasonal culture 43-54-g average weights have been achieved in Israel in 120 days from 0.5-g juveniles stocked at 3.5/m2. Experiments in the U.S.A. (Willis and Berrigan, 1977) showed that, in earthen ponds at 27 ‘C (range 20.5-30.5 “C), 0.8-g juveniles reached 43 g in 167 days while 0.06-g postlarvae reached 28 g in 170 days at 79% and 88% survival rates, respectively; 50% survival is normal in commercial farming. Prawns are sexually dimorphic, mature males being larger and having longer and thicker second periopods and more substantial cephalothoraxes than females. In a mature population three male morphotypes can be distinguished by claw colour, relative size within the population and claw length to body length ratio (Cohen et al., 198 1). Early maturing blue claw (BC) males cease to grow and lose their dominance to the rapidly growing orange claw (OC) males which will eventually transform to form new, larger BC males, following a “leap-frogging” pattern. Runts, or small males (SM), are also present. This process causes increasing size variation in the male population (Ra’anan, 1987). Conversely, early maturing females retain their position as the largest female animals. Early maturity of fast growing females coupled with their

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reduced growth rate results in an increasing size homogeneity in the female population. The growth rate of individual prawns is extremely variable and exhibits a bull-runt phenomenon. Removal of faster growing individuals permits the slower growing prawns to develop faster. This is the basis for the continuous cull-harvesting technique employed on many farms with year-round water availability. In this technique, large prawns are regularly harvested by seining and the ponds are rarely drained. Batch culture is practised where water availability is seasonal and continuous culture cannot be employed. In batch culture all prawns are harvested when the ponds are drained. In Thailand initial stocking rates vary from 5- 1O/m2 for ponds to be batchharvested within 8 months to 16--22/m2 for ponds employing a continuous cull-harvesting system. Similar stocking rates for continuous culture in Martinique (IFREMER, 1989) have been reported, where an 1S/m2 stocking rate was split into 9 : 4: 5 four-monthly batches in the first and 6 : 6 : 6 in the second year. D’Abramo et al. ( 1989) state that stocking rates of <4/m’ are the most economically attractive in temperate zones, giving greater mean harvest weight while reducing stocking and feeding costs. Some Thai farms use a 2.5-3-month nursery phase, stocked at 20-25/m2 followed by re-stocking at 3-5/m2 in a 3-month grow-out phase. Cull-harvesting starts 5.5-6 months after initial stocking and is repeated every 2-4 weeks in continuous systems. Re-stocking occurs once per year in Thailand; only one large farm is reported to stock more frequently. The number re-stocked is normally twice the number harvested. Many farms practise batch culture because of seasonal water shortages; continuous culture systems may exchange 10% of pond water daily or practise intermittent exchange. In Hawaii environmental conditions are ideal for continuous culture and initial stocking rates are similar to those in Thailand. Immediate post-stocking mortality, due to poor acclimatization and predation, is often severe. Detailed stocking strategies for the continuous culture management system have been derived (Malecha, 1983b; New and Singholka, 198 5 ) . Three- and four-phase systems, in which grow-out ponds are drain harvested and the stock split into two before final grow-out, have also been reported (Malecha, 1983b). PROCESSING,

MARKETING

AND ECONOMICS

Harvesting Prawns are harvested once at the end of the growing season, regularly (as in the continuous stocking/culling management system), or by some combination of the two techniques. Harvesting may be by drain-down, by gravity or pumping, or by seining. Both batch and continuous management systems are capable of producing up to 2500-5000 kg/ha of prawns annually but national average productivity is generally lower. For example, while well-man-

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aged farms in Taiwan are capable of producing 4000-5000 kg ha-’ year-‘, the national average productivity was less than 1500 kg ha- ’ year- 1 in 1989 (Hsieh et al., 1989). Culling is usually practised to harvest prawns of more than 45 g; batch culture yields a wide range of prawn sizes and types with differing values. Seining yield can be maximized by quite small improvements in husbandry. Pond productivity in Hawaiian earthen ponds increases with time of use, at least up to 3 years (Sarver et al., 1982), partly due to the development of a rich bottom layer and a grassy bank (providing increased shelter and nutrition). Yields of 1790, 2580 and over 2900 kg/ha, for the first, second and third year of operation, respectively, were quoted. However, cutting the grass planted to control pond bank erosion has been shown to increase yield by up to 20% annually (Wang and Losordo, 1985 ) . Total harvest weight increases with increasing stocking density, to an optimum level, but average size is inversely related. The percentage of the harvest which is marketable is best at low stocking density. In batch culture, prawns are harvested by a combination of seining and collection by hand and sometimes from a harvesting sump, when sufficient prawns are of marketable size or when environmental conditions necessitate it. Where continuous culture is practical, ponds are rarely drained. Marketsize animals are removed by regular seining (2-4-week intervals) with a 25mm mesh size net. Prawns of 40-45 g and above are normally culled out in Thailand, and in the French Caribbean (IFREMER, 1989 ) . In Tahiti the average harvest size from seasonal batch culture is only 20 g, for which a net mesh size of 18 mm is used. In Hawaii, seine harvesting retains prawns over 30 g. The “commercial” size in Taiwan is 25-30 g (Hsieh et al., 1989). Aids to improved management of prawn ponds have been suggested, including mechanical harvesting (Williamson and Wang, 1982; Losordo et al., 1986) and grading (Arndt et al., 1984). Canal-type ponds, not more than 30 m wide, assist seining operations (Lam and Wang, 1986). Seining is typically slow, labour-intensive and inefficient, and often exacerbates harvesting losses. Manual and mechanical seining are less efficient than drain harvesting (Peterson, 1982). However, much labour is necessary for post-harvest hand sorting in the latter technique. Manual seining will continue to be used on small farms and its efficiency can be improved by minimizing muddy and uneven pond bottoms, removing obstacles such as sticks or stones and controlling below-water-level vegetation. Investigating the suppression of growth of small prawns caused by the presence of large prawns, Lam and Wang ( 1987 ) showed that although harvesting efficiency could be increased by harvesting continuous-cultured ponds more frequently, it also increased stress and mortality and adversely affected growth. The composition and appearance of harvestable shrimp and prawns can be modified by dietary manipulation. For example, Sandifer and Joseph ( 1976) showed that shrimp fed a diet augmented with shrimp head oil have markedly

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increased carotenoid content. Levels of n-3 fatty acids in shrimp mirror those in their diet. Prawn flesh is sometimes said to be bland but taste panels have rated prawns with lobster over marine shrimp. Free amino acids and chloride ions have a generally flavour-enhancing effect on aquatic foods. Elevating rearing salinity several days before harvest may enhance the flavour of prawns (Fox et al., 1985), and starving prawns 3 days before harvesting is said to obviate the need for de-veining (Lee, 198 1) . Harvesting techniques need modification for prawn-fish polyculture. Channel catfish ponds, normally rarely drained, may need to be drained if prawns are co-stocked (Pave1 et al., 1985). Methods for sorting mixed harvests will also need development. Grow-out research

Although early research work with prawns concentrated on the larval stages, once hatchery production became routine, attention moved to the grow-out stages. Methods of increasing the revenue from prawn ponds including polyculture, repeated grading, monosex culture and improved feeding strategies are being sought through research, particularly in Israel and the U.S.A. Cage experiments have shown that an all-male prawn population yielded 473 g/m2 compared to 260 g/m2 and 248 g/m2 for mixed and all-female populations, respectively, after 115 days (Sagi et al., 1986); 80% of the all-male population was of marketable size, twice that of the other two populations. Female growth rate is inhibited in the presence of males but not vice versa. In earthen ponds all-male populations have been reported to yield nearly 8% more and all-female nearly 2 1% less than mixed populations (Cohen et al., 1988 ). Because the yield of larger prawns was greater in all-male populations, revenue was 24.5% better than the mixed population and 85.7% better than all-female. While sexing large juvenile or adult prawns is easy, it has not proved possible to identify males under 3.5 cm total length (Janssen, 1987). Handsexing would be commercially impractical but research into methods of producing all-male populations is on-going (Sagi et al., 1986). A management system which involves mechanically grading nursed juveniles, stocking them separately and alternating partial (seine) harvests and complete harvests by draining until the remaining biomass is small enough to consolidate with that of other ponds, has been described (Malecha et al., 1987a). It remains to be demonstrated whether such complex systems are practicable or economic for commercial prawn culture. However, hand-seine grading is practised by some farmers in Taiwan (Hsieh et al., 1989) to transfer prawns of similar size from one pond to another. Computer-assisted pond sampling for inventory control allows predictions on the cost and time necessary to produce animals of selected market value (Malecha et al., 1987a). One of the basic difficulties in prawn culture is the accurate estimation of the

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numbers of animals in enclosures. Research on sampling techniques is being carried out by IFREMER (Falguiere et al., 1989 ) . The effects of size-grading of juvenile prawns on yield characteristics is under intensive study (Karplus et al., 1986, 1987). Juvenile prawns (ca. 1.1 g) graded into upper (32%)) middle (45%) and lower (23%) fractions and recombined fractions as a control differed in their male and female morphotypes after 97 days in polyculture with tilapia and common, silver and grass carps. Small male (SM) frequencies were 50%, 32%, and 8% in the lower, medium and upper fractions, respectively, while blue claw males (BC) showed the reverse trend (3, 10 and 22%). Orange claw males (OC) followed the same trend, 42% in the lower and 63% in the upper fractions. Virgin female (V) frequencies were 94%, 78% and 37% in the lower, middle and upper fractions, respectively. Mature females showed the reverse trend (3-43% for berried females (BE) and 3-20% for previously spawned females (OP ) from the lower to the upper fraction). Mean weight ranged from 32 g in the upper to 21 g in the lower fraction; since survival rates were similar, yields showed a similar differential. Net income (from prawns alone) was almost nine times greater from the upper than the lower fraction and nearly twice that of the middle fraction. The yield of fish was relatively unaffected. Net income from the reconstituted catch was similar to the middle fraction but much less than the upper fraction. However, since the weighted mean yield of the three fractions is also almost the same as the control, stocking graded weight classes did not increase overall net income. Discarding the lower fraction would waste the nursery investment in these prawns. Methods of discarding low growth potential prawns sooner by tracing the earliest stage of male morphotype differentiation (embryo, larvae or postlarvae) may be necessary before profitability can be increased. All-male populations have more OC males, which are fast growing. High density speeds up maturity and the early appearance of BC males. SM and BC males are more sexually active than OC males but the latter grow more rapidly (Sagi and Ra’anan, 1988 ). Females in a mixed population may induce a larger fraction of males to enter into the more reproductively active but slower growing BC and SM types (Cohen et al., 1988 ) , SM males become OC males through a “weak OC intermediate” phase but OC males become BC males at a single metamorphic moult (Kuris et al., 1987). The golden short-claw prawn type which is almost’ exclusively male (OC ) is less aggressive and has 5-8% more tail meat than the blue long-claw (BC) form (Sandifer and Smith, 1977). Genetic or environmental manipulation might increase the marketable yield. Variability in larval development time, postlarval temperature tolerance, and salinity and temperature-related growth have been detected in stocks derived from different geographic locations (Sarver et al., 1979). Intraspecific hybrids of Macrobrachium rosenbergii races have demonstrated im-

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proved growth rates (Dobkin and Bailey, 1979). Collecting broodstock as early as possible in the production cycle would be a simple but effective change in prawn culture management (Doyle et al., 1983). The earliest maturing females should be selected as broodstock for a potential improvement of the genetic base because they are also the fastest growing females (Ra’anan, 1987 ) . The bull-runt phenomenon in male prawns seems to be non-genetic (Malecha et al., 1984)) being controlled by intrapopulation environmental-social factors. Male size may be a sex-linked secondary sexual characteristic under natural sexual selection, having high genetic fitness and little genetic variance. Karplus et al. ( 1989) have shown that ablating the second pair of claws of prawns increases survival and biomass production and decreases the variability of individual prawn weight. However, the economic viability of routine claw ablation is doubtful. The current practice of selecting many small ( < 40 g) females from ponds for hatchery use (because of their ready availability) does not take advantage of the natural fecundity of the species and exerts no control on the genetic worth of the parents used. Simple control of broodstock age, which could lead to genetic progress (Doyle et al., 1983), is difficult in continuously stocked and harvested ponds. Doyle et al. ( 1983 ) proposed that separate broodstock ponds could be kept, with a sex ratio of one male to four or five females and that females could be reared to 100 g to increase larval production per animal. This ratio is sometimes maintained in broodstock ponds in Taiwan (Hsieh et al., 1989). External examination of gonadal development and observation of the reproductive behaviour of dominant males can be used to detect the onset of female maturity as early as 7 days before the pre-mate moult (Sagi and Ra’anan, 1985). Such techniques assist studies on breeding, artificial insemination and genetics. Other aids developed to further prawn genetic studies include methods for increasing hatchability and spawning frequency through artificial incubation (Balasundaram and Pandian, 198 1; Pandian and Balasundaram, 1982)) artificial insemination (Sandifer and Smith, 1979; Chow, 1982 ) and spermatophore cryopreservation (Chow et al., 198 5 ) . Progeny have resulted from matings of surgically sex-reversed prawns and F2 generations created (Malecha et al., 1987b). Interspecific hybrids have been reported in the literature, including M. asperulum xM. shokitai (Shokita, 1978), whose offspring were sterile, M. rosenbergii xM. malcolmsonii (Sankolli et al., 1982)) and M. nipponensexM. formosense (Wickins, 1976). Electrophoretic studies of gene-enzyme variation in natural prawn populations from 11 locations, from Sri Lanka to New Guinea, Palau and the Hawaiian cultured stocks, have supported the hypothesis (Hedgecock et al., 1979) that the species has undergone considerable racial divergence and that natural populations represent a diverse genetic resource for prawn aquaculture. However, no useful allometric variation in these genetic stocks and their

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hybrids was found (Malecha et al., 1980). The “Anuenue” stock had diverged little from its wild relatives. The fact that the two principal size classes, sometimes referred to as “jumpers” (exceptionally fast growing individuals) and “laggards” (severely growth repressed prawns) appeared only in communal groups as opposed to individually raised prawns indicates that variance is more affected by interactions within the group than by genetic difference in the growth potential of individuals (Ra’anan and Cohen, 1984). Artificial submerged substrates (made from plastic net and pipes) in ponds have been shown to increase marketable yield and average marketable weight, increasing the efficiency of an early selective harvest (Cohen et al., 1983 ) . Although total marketable yield was increased by 14%, survival and the marketable proportion of the total harvest were unaffected. In temperate zones the economic feasibility of prawn culture depends on maximizing the percentage of prawns that attain minimum marketable size (25 g) in 6-7 months. Interestingly, in contrast to the work of Cohen et al. ( 1983), net substrates were found by Ra’anan et al. (1984) to markedly increase both overall survival and the percentage of marketable prawns where paddlewheel aeration was also used in an intensive system. Mulla and Rouse (1985) reported that additional substrate (Spanish moss/plastic-coated wire mesh) gave some improvements in average size and size distribution of juveniles produced from nursery ponds but overall production was not improved. Further work on this topic seems necessary to elucidate the value of artificial substrates at various stages of the rearing cycle. Research or pilot-scale efforts to rear prawns in thermal effluents or geothermal water have also been conducted, notably in the U.S.A. (Rhodes et al., 1984; Smith and Wannamaker, 1984)) Italy (M.B. New, unpublished data, 1984), New Zealand (De Zylva, 1988) and the U.S.S.R. (Khemeleva et al., 1989). Many geothermal waters have an unsuitable ionic content for freshwater prawn farming; heat-exchange systems would be required to utilize such sources for this purpose. Survival and growth in thermal water is profoundly influenced by changes in temperature and dissolved oxygen (Farmanfarmaian and Moore, 1978 ). Dissolved oxygen is not a critical factor at low temperatures but low DO2 reduces upper temperature tolerance. Growth nearly doubled for every 5°C increase in the 20-30°C range, with no appreciable change in FCR. Cannibalism and predation are problems in high-density culture. Although the technical feasibility of rearing prawns in outdoor raceways supplied by geothermal water in ambient air temperatures as low as - 20 oC has been demonstrated (Johnson, 1979), venture analysis suggests that highdensity culture of prawns would not be profitable (Guerra et al., 1979 ) . Studies in the U.S.S.R. have shown that prawns can be raised up to 60 g and an extrapolated productivity of 1 t/ha on natural food alone in cages in a watercooling reservoir or in earthen ponds at water temperatures of 25-33 “C (Khmeleva et al., 1989).

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Considerable government (Forster and Wickins, 1972; Sandifer and Smith, 1975; Wickins and Beard, 1978) and commercially funded research has been conducted into the feasibility of intensively culturing prawns in re-circulation systems in temperate countries. The common goal of this research was to culture prawns close to consumer markets but it has not yet proved economically viable. The difficulties in maximizing the stocking density of a substratedwelling, territorial species and the high cost of heat have so far been insuperable. Prawns are suitable candidates for commercial polyculture. They have been reared with many species of fish including mullet (using Mucrobrachium acanthurus) in Colombia (Martinez-Silva et al., 198 1); catfish, tilapia and/ or Chinese carps in the U.S.A. (Stickney, 1980; Buck et al., 1979, 198 1; Behrends et al., 1986; Valentin, 1988 ), grass, silver and bighead carp in Thailand (Tunsutapanich et al., 1982); silver carp in Hawaii (Weisburd et al., 1987); Chinese carp in Guam (Fitzgerald and Nelson, 1979); bighead and grass carp in Taiwan (Liao and Chao, 1982); golden shiners (Notemigonus crysoleucas) in Louisiana (Perry and Tarver, 1987 ) ; channel catfish in Louisiana (Lamon and Avault, 1987) and Mississippi (Heinen et al., 1989); and Nile tilapia in Israel (Mires, 1987). In one experiment (Brick and Stickney, 1979) monosex Tilapia aurea stocked at 2000/ha in ponds stocked with prawns at 11 OOO/ha gave excellent survival of both fish and shrimp over a 4.5-month period; prawn growth rate was not depressed much by the presence of tilapia. Behavioural experiments with prawns, Tilapia mossambica and the red swamp crawfish (Procambarus clarkii) showed that tilapia had no apparent negative impact on crustacean survival or growth (Martin0 and Wilson, 1986). Adult prawns were, however, predacious on tilapia fry; crawfish, though aggressive, were unable to capture the fry. However, Rouse et al. ( 1987 ) reported that tilapia reproduction had an adverse effect on prawn production. Despite this, prawn-tilapia polyculture has a potentially higher net return than prawn monoculture (Rouse and Stickney, 1982). Polyculture experiments with common carp, tilapias and various Chinese carps, at various fish and prawn stocking densities, indicate that the growth and survival of fish and prawns are independent ( Wohlfarth et al., 1985 ) . Prawns were influenced only by their stocking rate which correlated positively with yield and negatively with individual growth. The species of polycultured fish used, fish stocking rate, or the differences in feeding and manuring regimes used did not influence prawn production. However, Cohen and Ra’anan ( 1983 ) had found that carp and tilapia growth was strongly affected by the number of tilapia stocked and by the feedingmanuring strategy used in polyculture with prawns, so the picture is confused. Prawn growth, survival and yield have been shown to be unaffected by the presence of golden shiners (Notemigonus crysoleucas) . Although the presence of prawns in minnow ponds decreased survival, the total yield of shiners was

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not affected (Scott et al., 1988). Polyculture of prawns with silver carp, grass carp and mullet increases total yield but competition for feed resources has been noted (Costa-Pierce et al., 1987). The feasibility of using wastewater from this type of polyculture to irrigate tree crops such as banana, papaya and avocado is being studied (Costa-Pierce, 1987). Crayfish (Procambarus spp. ) have been harvested in channel catfish (Ictalurus punctatus) ponds in Louisiana (Huner et al., 1983a) where a marginal crop ( 146 kg/ha) of prawns could also be raised (Cohen, 1984). The presence of prawns did not depress catfish yields but significantly increased net income. However, lowered survival and growth rates for prawns in ponds also stocked with channel catfish fry and in which resident crayfish (Procambarus spp.) were harvested were reported by Huner et al. ( 1983b). Prawns need to be at least 1 g in size before being stocked in catfish ponds to prevent excessive predation by fish fry. This would necessitate on-site prawn nursery ponds for the commercial success of this polyculture system. After initial enthusiasm for commercial polyculture of prawns with carps, tilapia and cattish, Taiwanese farmers have returned mainly to prawn monoculture because of predation problems and the high profitability of intensive prawn monoculture (Hsieh et al., 1989). However, some farmers are obtaining promising results with prawn-aquarium fish polyculture. Biological control of phytoplankton dynamics and maintenance of satisfactory DOz concentrations at high phytoplankton standing crops by Chinese carps appears to be a viable method of water quality control in semi-intensive prawn aquaculture (Costa-Pierce et al., 1985). A two-stage polyculture pilotplant system, using water hyacinth (Eichhornia crassipes) in the first stage and a combination of azolla and prawns in the second stage to provide tertiary treatment to municipal wastewater has been described by Zachritz ( 1986). Polyculture of microphagous tilapia species combined with bottomfeeding prawns is proving very productive. This species combination, which is non-conflicting in terms of feed and living requirements (Little and Muir, 1987), would be well suited to a “Taiwanese” system for intensive tilapia production. Tilapia have also been grown in cages in prawn ponds. Prawns have been cultured in rice fields in Puerto Rico without depressing rice production (Cepeda, 1982). This practice is also reported from Japan (Anonymous, 1980b). A pilot study (Cange, 1984) has been conducted on a channel catfish/prawn rotation with red swamp crayfish (Procambarus spp. ) which showed a return of U.S.$ 1092/ha (annual rate of return 23.5%) in rice fields. The yield did not include rice income. Feeds and feeding Early work on the nutritional requirements of prawns has been reviewed and examples of diets quoted by New (1976, 1980b, 1987), Biddle (1977) and Sick and Millikin ( 1983 ) . The field observation that Mucrobrachium spp.

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are omnivorous has been confirmed by enzyme studies (Lee et al., 1980). Their diet includes aquatic insects and larvae, algae, nuts, grains, seeds, fruits, small molluscs and crustaceans, fish flesh and offal of fish and other animals. They may also be cannibalistic and will readily accept compound pelleted feeds. Prawns have been shown to depend mainly on natural feeds independently of the presence or absence of feed pellets by Schroeder ( 1983) in dC studies; this confirmed earlier observations by Weidenbach ( 1982) of the stomach contents of prawns. In the latter study, prawns were shown to increase consumption of natural organisms to compensate when compound feed pellets were absent. Fertilization is therefore an alternative to supplementary feeding in semi-intensive prawn culture. In channel catfish-prawn polyculture, seston and macrophytes were found to contribute 18-75% of growth and pelleted feeds the remainder (Lilystrom and Romaire, 1987) while 68-99% of catfish growth originated from the compounded diet. Prawns fed more on seston when under 7 g in weight, moving increasingly to macrophytes and pelleted catfish feeds later. Prawn feeds are cheaper than marine shrimp feeds because the former require less dietary protein. Prawn feeds were U.S.$ 360-720/t (ASEAN/ UNDP/FAO, 1988) in Thailand, U.S.$ 650/t in French Guiana (IFREMER, 1989) and U.S.$ 700-1500/t in Taiwan (Hsieh et al., 1989). Many farms use chicken starter feeds. Some farms feed according to biomass but others use pond turbidity and colour to determine feeding rate. Feeding is normally once daily in practice though multiple feeding may (Taechanuruk and Stickney, 1982; Corbin et al., 1983) or may not (Mensi and Heinen, 1988) have advantages. Food conversion ratios (FCR) of 1.7: 1 to 3: 1 are obtained with dry diets while farm-made moist diets, containing waste prawn heads, trash fish, etc., achieve an FCR of 3 : 1 to 5 : 1. Costa-Pierce and Pullin ( 1989), commenting on the gross inefficiencies of nutrient conversion in aquaculture generally, cite the report of Weisburd and Laws ( 1986) that as little as 6.5% of the organic carbon in prawn feeds added to Hawaiian ponds is harvested as prawn biomass. Lipid levels of 6-9% in commercial prawn feeds in Thailand (ASEAN/ UNDP/FAO, 1988)) 5-8% in French Guiana (IFREMER, 1989) and 2-4% in Taiwan (Hsieh et al., 1989) have been reported. Fatty acid composition is more important than total lipid content. Using a 2: 1 cod liver oil/corn oil mixture, Sheen and D’Abramo ( 1989) found that a 6% inclusion rate was optimal; 0%, 10% and 12% levels depressed growth. Response to the dietary inclusion of both PUFAs and HUFAs has been demonstrated (D’Abramo and Sheen, 1989). Postlarval prawns can convert 18 : 2n-6 to 20: 2n-6 and desaturate 20 : 2n-6 to 20 : 3n-6 (Reigh and Stickney, 1989). No evidence that prawns could synthesize 18: 2n-6 or 18: 3n-3 was found but the ability of prawns to elongate or desaturate both apparently did occur. Dietary n-3 : n-6 ratios affect tissue ratios and may affect growth; the optimum dietary ratio is

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as yet unknown. Feeding linolenate (n-3 ) as the sole lipid source has been shown to depress growth while linoleate (n-6) did not. In marked contrast to marine crustaceans, no advantage has been demonstrated in supplementing a semi-purified diet (0.12% cholesterol and 0.048% lecithin) with 0.5 or 1% cholesterol or 5% lecithin (Briggs et al., 1988). Commercial prawn feed protein levels have been reported as 23.8-38.5% in Hawaii (Corbin et al., 1983)) 22-30% in Thailand (ASEAN/UNDP/FAO, 1988), 28-36% in Taiwan (Hsieh et al., 1989) and 25-30% in French Guiana (IFREMER, 1989). However, studies in concrete ponds (Boonyaratpalin and New, 1982), asbestos asphalt or earthen bottom ponds (Bartlett and Enkerlin, 1983 ) and aquaria ( Antiporda, 1986 ) have produced satisfactory growth at protein levels as low as 14%, emphasizing the importance of natural feed. The earthen substrate in ponds appears to supply a major growth factor (Stahl, 1979). Clifford and Brick (1979) concluded that optimum conditions for prawn growth were achieved with a 25% protein diet and a I:4 lipid : carbohydrate ratio. However, under laboratory conditions, optimum dietary protein levels of 30-35% have been demonstrated (D’Abramo and Reed, 1988; Freuchtenicht et al., 1988). A method of determining crude protein digestibilities in prawns, devised by Ellis et al. (1987), gave results of 93.5% and 96.2% respectively for 40% and 30% protein commericial feeds. Although the quantitative amino acid requirements of prawns have not yet been defined, improvements due to amino acid supplementation have been demonstrated (Farmanfarmaian and Lauterio, 1980). Alanine is thought to have an osmoregulatory function while taurine is involved in cardiac and neural processes (Smith et al., 1987 ). In addition to conventional feedstuff ingredients, the suitability of several less conventional ingredients for prawn feeds has been demonstrated, including corn silage (Moore and Stanley, 1982 ) , fresh leaves (Harpaz and Schmalbath, 1986)) and moist pressed brewers grains (Kohler and Kreuger, 1985 ) . Garces and Heinen ( 1989) have shown beef liver, orange flesh, peeled sweet potatoes, frozen peeled bananas, turnip greens and carrot tops to be useful supplements to a commercial trout chow for feeding prawns. Shrimp and prawns appear to be able to utilize complex carbohydrates better than simple ones like glucose (New, 1976 ) . Prawns have cellulases (Noborikawa, 1978 ) and chitinases, and carbohydrate can be used to spare protein. The lower gelatinization temperature of barley seems to make it superior to wheat as a carbohydrate source (Ashmore et al., 198 5 ) . Dietary fibre levels of up to 30% do not appear to suppress growth experimentally; if commercially applied this would be cost-effective (Fair et al., 1980). Astaxanthin and its esters are the primary pigments in prawns (Maugle et al., 1980). Heinen ( 1984, 1988) has reported vitamin C deficiency symptoms in prawns, one of which was failure to moult. Dietary fat-soluble vitamins and trace minerals seem to be less important than water-soluble vita-

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mins but this observation may be masked by leaching of the latter from pelleted diets. Omission of pyridoxine caused significant growth rate reduction but deletion of riboflavin significantly increased growth rate, indicating that it is possible to have a deleteriously high level of dietary vitamin BZ. Prolonged water stability of crustacean feeds is now considered less important than originally thought (Bordner et al., 1986), though binders are commonly used. Regurgitation of food by prawns might have energetic benefits by decreasing the requirement for energy to transport unassimilable material through the entire digestive system (Newman et al., 1982). Like many fish (New, 1987 ), prawns are coprophagous and trimethylammonium hydrochloride (TMAH), which imparts a faecal odour to formulated feed, has been shown to increase pellet ingestion (Costa-Pierce and Laws, 1985 ) . Taurine, betaine, glycine and proline are effective chemoattractants (Harpaz et al., 1987). Colour also has a role to play in feed acceptability, it having been reported that light-coloured flaked diets are more readily taken by prawn larvae than dark ones (Meyers and Hagood, 1984). Lack of standardization in experimental design, culture conditions, and analytical techniques has limited the value of published information on shrimp and prawn nutrition (New, 1976) and made comparisons of the results difficult. However, a Nutrition Task Force, established by the World Aquaculture Society in 1977 (Caste11 et al., 198 1) to propose guidelines for standardizing aquaculture nutrition research methodology, has resulted in the development of standard reference diets for crustacean nutritional research (Caste11 et al., 1985; Caste11 and Kean, 1986; Caste11 et al., 1989). Reference diets developed by Caste11 et al. ( 1989) have been successfully evaluated with Mucrobruchium rosenbergii by Reed and D’Abramo ( 1989); their use as controls in future nutritional studies should be encouraged. Problems in rearing Prawn deaths may be caused by opportunist protozoans (such as Zoothamnium, Vorticellu and Epistylis), fungal pathogens (such as Lagenidium callinectes, Sirolpidium spp. and Fusarium solani), stress and neurotic symptoms, bacterial problems following injury to the exoskeleton, and moult arrest due to poor environmental conditions or nutritional deficiencies (Johnson, 1982). Zoothamnium and Vorticella settle on the more exposed and moving parts (uropods and pleopods ) of pond-reared adult prawns whereas Epistylis is non-selective (Carnacho, 1987 ). Zoothamnium on larval prawns (Macrobrachium acanthurus) has been controlled by 20 ppm formalin (Roegge et al., 1977). Antibiotics (such as chloramphenicol and tetracycline), furazolidone, formalin and iodine are among the chemicals used in larval rearing (Hsieh et al., 1989); their indiscriminate use is reported to be one of the problems in Taiwanese hatcheries. “Crowding diseases” are also caused by opportunist filamentous bacteria

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(such as Leucothrix and Flexibacter), blue-green algae, and diatoms (Nash, 1988 ). Idiopathic muscle necrosis, resulting in marked stunting and caused by suboptimal conditions (Akiyama et al., 1982 ) , has been reported to have caused sudden mortality of up to 60% in 28-day-old prawn postlarvae in intensive rearing systems in Thailand (Nash et al., 1987). The symptoms were a milky-white body opacity; recurrence was prevented by lower stocking densities and increased DOZ. Encrustation with Bryozoa sp. and Epistylis sp. is often associated with high water hardness and retards growth rate. Optimal hardness is 20-200 ppm CaC03 (Vasquez et al., 1989). Vitamin C deficiency has been shown to cause mortalities due to moult failure, together with subcuticular lesions and blotches in surviving prawns (Heinen, 1988 ) . Darkening of the gill cavity is considered to be due to precipitating chemicals and nitrogenous waste products (Johnson, 1982 ) . Disease management includes salinity modification for fungal problems, the use of antibacterials, improved tank cleaning and water exchange for epibionts, mechanical water Iiltration, biofiltration and ultraviolet light. Black burn spot exoskeletal lesions of bacterial aetiology, a frequent feature of prawns reared in intensive culture, have been treated by oxolinic acid (El-Gamal et al., 1986). One of the most common disease problems in prawn larval rearing is the appearance of pathological lesions of the bacterial necrosis type (Anderson et al., 1989). Vibrio anguillarum has been detected in larval prawns (Fujioka and Greta, 1984; Colorni, 198 5 ) which frequently exhibit low survival rates in the early stages. However, members of the Vibrio genus are thought to be part of the normal microflora and may not be primary pathogens for Macrobrachium rosenbergii. Virus diseases have also been reported in prawns (Nash, 1988). Certain types of muscle lesion are good indicators of postlarval fitness. Early stocking losses in grow-out ponds are often caused by exposure to low temperature or high pH (Sarver et al,, 1982). These mortalities have also been shown to be affected by genetic background and larval history. Seine harvesting during culling also affects stocking survival and subsequent yield. Midcycle disease (MCD ) , a toxic period of unknown aetiology beginning on the 10th day of larval prawn culture and resulting in poor survival rates, has been reported by Akita et al. ( 198 1) while haemocytic enteritis (HE) affects prawns as well as shrimp (Lightner, 1982). Water quality and contamination are also important in prawn culture. The 24-h L& value of mirex for prawns is 104 pg/l (Summer and Eversole, 1978 ), while lower levels cause moult failures and decreases in moult frequency. Prawns are susceptible to hydrogen sulphide toxicity (Jayamanne, 1986b), the 96-h LCsObeing 2.6 mg/l and 4.2 mg/l for 21-day and 2-month-old juveniles, respectively. Prawn postlarvae are more tolerant of mercury (lethal concentration 0.325 ppm Hg) than stage 1 larvae (0.04 1 ppm Hg ) (Piyan et al., 1985). Prawns are stressed by temperatures below 22°C and DOz levels

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below 2 ppm (Rogers and Fast, 1988 ) . Blooms of algae cause oxygen deficits at night while alive, and toxic ammonia levels following collapse (Jayamanne, 1986a); chemicals produced by the phytoplankton may also affect flavour. Blue-green algal blooms are a particular problem due to their indigestibility by prawns. Stocking small numbers of “sanitary” planktivorous fish, such as silver, bighead, and common carp, and tilapia, has been suggested (Harimurti, 1986) as a means of controlling algal blooms in prawn culture. Dense phytoplanktonic blooms can increase pond pH above 10.5 (Avault, 1979 ) . The algicide Clarosan has been found to lower pH levels rapidly and, at 0.02 ppm, not to be deleterious to prawns. A relationship between high bacterial contamination in rearing water and high haemolymph bacterial counts has been shown (Lasso de la Vega and Brady, 1989). Heavily contaminated water may thus endanger the health of prawns, transmit human pathogens and cause spoiling in poorly stored prawns. On the credit side, prawns have potential in human disease control. Lee et al. ( 1982) found that they would prey on the two major South American species of schistosome vector snails (Biomphalaria glabrata and B. tenagrophila >. This finding could be particularly useful if applied in Africa and South America, especially if prawns were cultured with tilapia. Processing

Processing yields of prawns are size and sex dependent (Smith et al., 1980). Tail weight percentage decreases with increasing prawn size and female prawns give consistently greater tail yields due to differences in claw size and weight. The eventual female contribution to pond yield can be predicted because the final B-month harvesting average weight of females is similar to that of the earliest maturing females at 3-4 months (Ra’anan, 1987). Processing yields of deheaded prawns (average 50%) are less than those of penaeid shrimp (Huner, 1980) but greater than those obtained from crayfish and lobsters. Many freshwater prawns are sold close to the farm site either on ice or alive. Ice-chilled uncooked prawns have a short shelf life, becoming mushy (Nip and Moy, 1988 ) after 3 days. Mushiness also occurs if prawns are cooked for more than a few minutes (Lee, 198 1) . Quality deterioration of raw prawns under iced or refrigerated storage may be caused by the activity of a collagenolytic enzyme released postmortem from the hepatopancreas. Some farms have their own on-site restaurants. Prawns can be shipped in aerated water to other restaurants and hotels if the extra value of live prawns warrants it. Some farms “kill-chill” by dipping the prawns in iced water followed by blanching at 65 ‘C for 15-20 s. The prawns are then iced and transported to market. In Thailand, most prawns are sold whole (head and shell on). The original quality of prawns, in terms of microbial count and hypoxanthine value, is important (Passy et al., 1983). Initial microbial load can be reduced significantly by heading and intensive washing. Packing in ice pre-

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vents desiccation and lengthens the microbial growth lag phase from l-2 days to 5. Flushing with CO2 and a storage temperature of 4’ C extends shelf life by 2-3 days (to 8-9 days) without affecting flavour. Cooking tends to remove off-flavours and odours and toughens up the texture of prawns. Prawns can be frozen for up to 6 months with almost no deterioration in flavour. Marketing The market value of cultured prawns is time, size and site specific (Table 2 ) . Farm-gate values have fallen considerably in Thailand since 1980, due to the increase in production, but farming remains economically viable. Where head-on prawns are long established as a desirable fisheries product (as in Thailand, Tahiti, Puerto Rico, La Reupion, and Vietnam, for example), marketing freshwater prawns is not a problem. Early attempts to market them where they were a novel product found comparisons with marine shrimp inevitable. Most South Carolina consumers are said to evaluate prawns as similar to marine shrimp (Liao and Smith, 198 1) but prefer them heads-off, retailers prefer a 35-50 tails/lb size category. Direct sales to consumers, as well as through seafood retailers and restaurants, were the most attractive marketing approaches there (Liao and Smith, 1982). One 1978 Hawaiian advertising campaign promoted heads-on prawns because it was thought that “heads-off” marketing might lead to “hepatopancreas contamination”. In 198 1 Boston chefs found fresh prawns more tasty than saltwater shrimp and that serving them shell-on helped to retain flavour. In north-east U.S.A., prawns are said to have been requested in preference to marine shrimp because the latter have an “iodine taste” and an objectionable cooking odour. In one South Carolina marketing test, restaurateurs found good acceptability for dishes involving either head-on or head-off prawns; microwaved tails stuffed with crab was the most preferred dish (Liao et al., 198 1) . A consumer survey in the same state in 1982 (Liao and Smith, 1983 ) indicated that the typical purchaser of freshwater prawns would be “a 44-year-old white-collar worker with an average yearly income of U.S.$23 182 and 12 years of formal education” which seems a little over-specific. Prawn farming was abandoned in one Texan farm because “prawn proved impractical to market”. In 1982 only 15% of Hawaiian farmed prawns were marketed chilled on ice in the U.S. mainland, the rest being sold live locally, mainly to the Filipino community. It was thought necessary to “educate Causasians” to consume prawns. The Hawaiian domestic market for prawns is a little over 100 t/year (Lee, 1984 ) . An inability to economically and regularly supply the quantities demanded by the continental U.S. market was one of the factors leading to the declitle in Hawaiian prawn farming. While the Amfac Food Group predicted in 1982 that the U.S. “heads-on” market for prawns was over 4500 t/year with avalue of U.S.$ 50 million (Anonymous,1 982) and test marketed

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TABLE 2 Examples of value of (head-on) Location

Date

Bangladesh

1981-1982 1982-1983 1983-1984 1985 1986

farmed freshwater prawns Notes

Price (U.S.$/kg) 1.82 2.27 2.42 2.73 3.48

1 Retail (1 U.S.$=Taka

I

August 1985 March 1987 May 1989

6.2 14.8 8.8

‘Caribbean”

1988

13.0-14.0

French Overseas Territories

1988

> 20.0

Honduras

1980

4.4

Farm-gate

Israel

1984

2.6 2.9 4.4 6.0 7.5

<2og 20-25 25-30 30-45 45-90

Malaysia

1989

4.50

Retail

Mauritius

1984 1985 1986

8.0 9.5 8.9-10.0

Farm-gate Farm-gate Farm-gate

Puerto Rico

1984 1986 1988

11.0 12.6 12.6

> 30 g farm-gate 30 g farm-gate Farm-gate

Taiwan

1988 1989

8.93-14.29 15.00

Wholesale Wholesale

Thailand

1980 1980 1986

7.5-10.0 10.0-12.5 3.0 2.9 5.7 4.9 4.2 3.4 3.2 3.2 7.80 6.80 5.40

Farm-gate Retail Farm-gate Farm-gate Farm-gate Farm-gate Farm-gate Farm-gate Farm-gate Farm-gate Ex-factory Ex-factory Ex-factory

1989

33)

C&F Europe 16/20 tails, frozen

35-37 g

Retail

farm-gate g farm-gate g farm-gate g farm-gate g farm-gate

females berried females # 1 size males # 2 size males # 3 size males “long claws” “soft shells” “unmarketable size” heads-on 4-6/lb” heads-on 6-S/lb heads-on 8-12/lb

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Location

U.S.A. California Hawaii New York S. Carolina

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A REVIEW

Date

Price (U.S.$/kg)

Notes

1989

5.0-6.0 3.2-3.6 18.0

Ex-factory heads-off 26-60/1b Farm-gate Supermarket, live

1984

19.8 13.2 8.8 9.4 12.65 8.8-l 1.0

Heads-off, retail, 8-12/lb Heads-off, retail 21-25/1b Farm-gate Farm-gate Wholesale 15 /lb Retail

1980 1982 1986 1980

al lb=approx. 454 g. Sources: INFOFISH (Globefish data bank); IPFC, 1988; Kwei Lin and Boonyaratpalin, 1988; Sastradiwirja, 1986; Aquaculture Digest; New et al., 1982; Shang and Mark, 1982; Wulff, 1982; Karplus et al., 1987; FAO, unpublished data; Glude, 1984, Anonymous, 1984; Altonn, 1982; Liao and Smith, 198 1; Palanisamy, 1989; Hsieh et al., 1989.

its prawns in a frozen, whole, size-graded form in Los Angeles and Chicago, the company abandoned its 2-year pilot project in Hawaii and returned the land to sugar production. Test marketing showed “weak consumer support for the product at cost-effective prices”. General Mills sold their Honduran prawn farm in 1983 to a local company in order to concentrate on its (Red Lobster Inn) retailing activities and to develop a technology transfer business in prawn culture. The Weyerhauser Company constructed a pilot prawn farm in Brazil in 198 1, intending to test market the product on the East Coast of the United States and in Europe. The company was expecting its prawns to end up in “the white table cloth class of restaurants” (Kadera, 198 1). In 1986, small quantities of Israeli-produced prawns were reported as being exported headon to Europe and the U.S.A. Potential was said to be for 30-g head-on animals ( Arieli et al., 198 1) . Again, regularity of supply was a problem and Israeli production has since declined. Although Taiwan, where there is strong local demand, was said to export small quantities of prawns to Japan in the early 198Os, where restaurants and processing firms also buy them from local rice paddy farmers (Anonymous, 1980b) no exports were reported in 1989 (Hsieh et al., 1989). The Taiwanese domestic market is reported to be showing increasing preference for prawns compared to shrimp, especially when sold live. During peak production, 12-20 t of live prawns are sold daily in Taiwan, being transported to market in specially designed vehicles. However, though previously thought to be a product for domestic consumption only, prawns are now increasingly entering export markets. Ja-

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maica and Puerto Rico are said to be exporting “large quantities into the U.S.“; 60% of the output of the largest Puerto Rican farm was being exported to the U.S. and other Caribbean countries, while the rest was equally divided between farm-gate sale and sales to supermarkets and restaurants. Thailand is currently exporting about 7000 t of prawns to Europe annually, mainly as tails. A “heads-on” market has also been located for Thai prawns in Italy and Spain. Malaysia exports small quantities of prawns to Singapore and Hong Kong (Palanisamy, 1989). Taiwan may develop export markets for freshwater prawns, notably in Japan, which is familiar with its farmed shrimp. Now that production of the latter has declined, Taiwan is well placed to exploit an export market for Macrobrachium but, in the long term, may lind it difficult to compete with other countries where production costs are lower. Exports of farmed prawns into the U.S.A. are probably constrained more by lack of regular supply than by poor demand or profitability. The same applies to Israeli exports to Europe. Thailand already exports about 26% of the global production of farmed prawns. While large male prawns have a poor head-to-tail ratio, the average for the total population is perhaps only lo- 15% less than that for marine shrimp. This would be a severe disadvantage in a “tails” market if the farm-gate prices of whole prawns were the same as shrimp. However, the current farm-gate value of marine shrimp in Thailand (for example), though considerably reduced in 1989, is still 40-70% higher than freshwater prawns. For a consumer market which does not differentiate between prawn and shrimp tails, prawns represent an attractive buy for the processor/exporter under these circumstances. If prawn farmers can keep their production costs lower than those for marine shrimp, the outlook for expansion should be favourable for those selling into the “tails” market as well as into the “heads-on” market. The developing export market may stimulate expansion in freshwater prawn farming globally but production is likely to remain a small proportion of the total farmed shrimp and prawn output. In summary it seems that, despite their traditional “domestic market image”, freshwater prawns are now showing considerable promise in export markets. Economics Few economic studies of prawn farming have been published and many were feasibility studies rather than case studies of existing farms. One study ( ADCP, 1983a) of the feasibility of farming prawns in Jamaica examined the potential economics of a 4-acrea farm (8 ponds x 0.5 acre) buying in its postlarvae and marketing its product fresh on ice domestically after 9 months of batch culture. The projected annual return, after interest and depreciation, was about U.S.$ 2580. A similar appraisal (ADCP, 1983b) of the potential “1 acre=approx.

0.4 ha.

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of prawn farming in the Dominican Republic calculated that the annual return on a 4-acre farm would be U.S.$ 15 000. A feasibility report on prawn farming in Dominica (New et al., 1978 ) concluded that 2-, 5- or 1O-acre farms would be economically viable as an alternative land use for existing private or government land owners. Based on a mixture of projected farm sizes and types with a total annual national output of 50 t of prawns per year, a positive cash flow would have appeared in year 5 and cumulative cash flow would have turned positive in year 9. A study of the economic status of prawn farming in Thailand in 1980 (Anonymous, 1980a) found that both hatcheries and grow-out units of all sizes were highly profitable. In most cases the initial investment could be paid off within the second year of operation. The initial investment for hatcheries ranged from U.S.$ 1000 for “back-yard” operations, to more than U.S.$ 50 000 for large units. Annual hatchery operating costs were 5.6 times the initial investment for small hatcheries ( < 2 million postlarvae/year) but only 1.8 times for medium and large hatcheries. Most small hatcheries were inland. Feed and water costs accounted for 42-67% of operating costs. Annual hatchery profits ranged from U.S.$2200 for small hatcheries to about U.S.$ 37 200 for large ones. The annual rates of return on investment were 236% for “back-yard” hatcheries, 154% for medium and 62% for large hatcheries. Costs of production were U.S.$ 11.5, U.S.$9.0 and U.S.$ 10.0 for 1000 postlarvae for small, medium and large hatcheries, respectively. Hatcheries integrated with grow-out units produced postlarvae at lower cost (U.S.$6.0/ 1000) and coastal hatcheries produced at lower costs than inland hatcheries trucking seawater or brine. Generally, labour is said to constitute 52% (Shang, 1974) to 77% (Fujimura and Okamoto, 1972) of postlarval prawn production costs. It is interesting to note that prawn postlarvae are currently being sold at U.S.$ 1.4-4.0/ 1000 in Thailand (Sombhong Suwannatous, personal communication, 1989) so production costs must have been significantly reduced. Investment in Thai grow-out farms ranged from U.S.$ 3125-6250/ha in 1980 (Anonymous, 1980a). Land costs were U.S.$ 3 125-5000/ha. Annual operating costs were U.S.$4396, U.S.$ 33 19 and U.S.$4225 for small ( < 1.6 ha), medium (2.4-6.4 ha), and large ( > 8 ha) farms, respectively. All farms studied were profitable, averaging U.S.$ 2754, U.S.$ 543 1 and U.S.$ 3275/ ha for small, medium and large farms, respectively. Corresponding rates of return on investment were 5-l%, 108% and 58%. For me’dium size farms the initial investment could be paid off within 1 year; for others it took 2 years. At the average market value of prawns in 1980 in Thailand (U.S.$ 7.5O/kg) a break-even annual productivity of 588 kg/ha, 444 kg/ha, and 563 kg/ha was calculated for small, medium and large farms, respectively. If 1986 prices had prevailed then the break-even annual productivities would have been

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much higher. However, prawn farming in Thailand was still rapidly expanding in 1987, despite the drop in market value. Another study (Shang, 1982) showed that the investment required for a Hawaiian prawn farm was much higher than in Thailand, due to higher pond construction costs (U.S.$ 20 500-43 OOO/ha) which decreased with increasing farm size due to economies of scale. IFREMER ( 1989) report that prawn farm investment costs in French Guiana varied from U.S.$ 26 OOO/ha for a 5-ha diversification farm to U.S.$43 300/ha for a 15-ha farm, where electricity, warehousing, transport, housing and processing costs were also included. Actual pond construction and hydraulic costs were U.S.$ 24 400/ha for the 5-ha and U.S.$ 23 533/ha for the 15-ha farm, similar to those reported by Shang ( 1982) for Hawaii. Hawaiian labour, land, interest, feed and depreciation costs were much higher than in Thailand (Shang, 1982); average productivity was also higher in Hawaii (2275 kg ha- ’ year- ’ compared to 1068 kg ha- ’ year- ’ in Thailand at that time). This was attributed to lower stocking rates and the shortage of water in certain seasons in Thailand. Postlarval costs per unit of marketable weight were lower in Hawaii at that time due to Government intervention and better survival rates. Total production costs were U.S.$8.6110.50/kg in Hawaii, compared to U.S.$3.00-4.5O/kg in Thailand. Farm-gate prices for prawns in Hawaii averaged a little higher (U.S.$ 8.8O/kg) than in Thailand (U.S.$ 7.3O/kg) at that time. While all sizes of Thai farms were profitable, only large (8 ha) Hawaiian farms were economically viable. Even at a wholesale price of U.S.$ 11 /kg, a conceptual model (Samples and Leung, 1985) showed that there was a 50% possibility that a representative 8-ha Hawaiian farm with ponds over 0.8 ha would incur a financial loss in any given year. The probability fell to 30% if ponds smaller than 0.4 ha were envisaged. Clear competitive advantages were seen for Thailand (Shang, 1982) when prawns became a global commodity. By 1987, accelerating Thai exports to Europe were demonstrating the truth of that forecast. The production costs of Taiwanese freshwater prawn farms have been estimated as U.S.$3.80-5.7O/kg in 1989, while revenue was U.S.$ 11.30-l 5.00/ kg (Hsieh et al., 1989). Taiwanese prawn farms are small and utilize family labour. Land cost is normally disregarded in these estimates as most of the farms have been transferred within the same family for generations. Rent for freshwater ponds, at U.S.$ 3200 ha-’ year- ’ in 1987, was only 25% of that for marine shrimp ponds. Now that marine shrimp farming has suffered a severe setback in Taiwan, interest in prawn (and marine fish) culture is expanding (L.A. de Oliveira Gomes, personal communication, 1989). However, prawn farming is subject to the same potential problems of disease and environmental pollution which have beset the Taiwanese shrimp farming industry; caution will be required to avoid a further blow to aquaculture there. D’Abramo et al. ( 1989) concluded that in temperate zones, stocking den-

FRESHWATER

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sities of < 4/m2 were the most economically attractive, giving greater harvest mean weight while reducing stocking and feeding costs. Higher stocking rates increase yield but insufficient high-value larger prawns are produced in a limited growing season. Stocking larger juveniles alleviates this situation and increases total yield. In temperate zones, maximizing harvest weight and minimizing size variation is more important than total yield. Kwei Iin and Boonyaratpalin ( 1988) concluded that population structure was as important as total biomass production since different types of prawns fetched markedly different farm-gate prices. Males are generally larger than females but the proportion of tail meat in females is greater. Where tails only are consumed, female dominance in a mixed harvest would be desirable but where the product is sold whole (as in the Thai domestic market) the reverse is true. However, both farmers and consumers in Thailand find long claw (BC) males, though larger in body weight (80- 130 g) less desirable than short claw (OC) males. The claw waste in 18-28% in BCs and only 7- 13% in OCs. Cohen et al. ( 1988), working in a different market environment, believed that all-male populations would be the most profitable. Greater yield and larger prawns were both achieved in pond experiments. The revenue from an all-male population was nearly 25% more than that from a mixed and nearly 86% more than an all-female population. All-male populations have a greater percentage of fast growing orange claws (OCs) whereas in a mixed population a larger proportion of males are induced to enter into the reproductively active but slower growing BC and SM phases. High stocking density also speeds up maturity and the early appearance of BCs. Bauer et al. ( 1983), investigating the costs and returns for prawn farming as a supplemental enterprise in South Carolina, reported that potential profitability was particularly likely if existing pond facilities, already discounted into the value of the land or having been constructed during a period of lower investment costs, were available. Pashen ( 1984)) examining the feasibility of prawn farming in Queensland, Australia, estimated that an 8-ha prawn farm would require an initial investment of U.S.$210 000. After 2 or 3 years’ operation the break-even price of prawns would be U.S.$8.13/kg. Prawns would not have competed favourably with marine shrimp in Australia at this price. In Israel, 6-month seasonal prawn monoculture was said to raise a gross revenue of U.S.$ 14 250/ha. After postlarval costs of U.S.$ 5500/ha and feed costs of U.S.$ 1250, U.S.$ 7500/ha was left to cover other costs and profit. Many other site-specific reports on the economic feasibility of the hatchery and grow-out phases of prawn farming in various locations exist but are not reviewed here. These include studies in Martinique (Elizabeth-Mesnager, 1981), Samoa (Popper, 1982), Malaysia (Tiensongrusmee, 1983), the Indian subcontinent (Karim, 1982)) a geothermal farm in Oregon, U.S.A. (Oregon Institute of Technology, 198 1 ), South Carolina, U.S.A. (Smith et al., 1983), Panama (Engle, 1987), and Guam (Fitzgerald, 1988).

130

M.B. NEW

Not everybody makes money growing prawns. Some of the hazards that attend all aquaculture ventures have befallen prawn farming. Wulff ( 1982) recorded that all ponds were flooded and the prawns escaped from a farm in Honduras, following hurricane Fifi. Cook ( 1980) noted that a farmer in Louisiana, who stocked 252 000 young prawns costing U.S.$ 10 000, harvested only 28 000 harvest-sized prawns which were sold to Antoine’s Restaurant in New Orleans for U.S.$ 5 1OO!This brave farmer tried again, only to lose his second crop in an unexpected cold front. Grow-out strategy and the development of monosex populations are probably the most critical research areas for the technical advancement of freshwater prawn farming. The development of successful domestic and export market strategies is also imperative for further production expansion. Globally, the outlook for freshwater prawn farming is promising.

ACKNOWLEDGEMENTS

The author is most grateful to FA0 friends who have contributed to and encouraged the preparation of this review, including Imre Csavas, Claire Cuerden, Manuel Martinez, Michael Robinson, Michel Vincke and Ulf Wijkstrom. Particular thanks go to Chen Foo Yan, who supported an earlier paper which was published (New, 1988 ) within the project he coordinates (Network of Aquaculture Centers in Asia and the Pacific), on which the current review has been built. Many others provided data through personal communications, including I-Chiu Liao and IFREMER (Guyanne). The pages of Aquaculture Digest (9434 Kearny Mesa Road, San Diego, California 92 126, U.S.A.) during 1981-1989 provided many business and marketing items which are not individually referenced in the text. My gratitude also goes to the reviewers of this paper for their constructive criticism, to Gunhar Yoontrakul, who patiently typed the many drafts of the manuscript, and last (but not least) to my wife Janet, for her support and encouragement. REFERENCES ADCP, 1983a. A policy for development of aquaculture in Jamaica. Report No. ADCP/MR/ 83/22. Aquaculture Development and Coordination Programme. FAO, Rome, 115 pp. ADCP, 1983b. A plan for aquaculture development in the Dominican Republic. Unpublished report. Aquaculture Development and Coordination Programme. FAO, Rome, 236 pp. Akita, G., Nakamura, R., Brock, J., Miyamoto, G., Fujimoto, M., Oishi, F., Onizuka, D. and Sumikawa, D., 198 1. Epizootiologic study of mid-cycle disease of larval Mucrobruchium rosenbergii. J. World Maricult. Sot., 12(2): 223-230. Akiyama, D., Brock, J. and Haley, S., 1982. Idiopathic muscle necrosis in the cultured freshwater prawn (Mucrobruchium rosenbergii). Vet. Med. Small Anim. Clin., July, p. 1119.

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Alias, A.Z. and Siraj, S.S., 1988. The effect of packing density and habitat material on survival of Mucrobruchium rosenbergii post larvae. Aquacult. Fish Manage., 19 ( 1): 39-43. Alston, D.E., 1989. Mucrobrachium culture: a Caribbean perspective. World Aquaculture, 20( 1): 18-23. Altonn, H., 1982. Island prawn farmers delighted with quiet times. Star Bulletin (Honolulu, HI), 11 November. Anderson, LG., Shamsudin, M.N. and Nash, G., 1989. A preliminary study on the aerobic heterotrophic bacterial flora in giant freshwater prawn, Macrobrachium rosenbergii, hatcheries in Malaysia. Aquaculture, 8 1: 2 13-223. Anonymous, 1980a. Thai freshwater prawn and brine shrimp farming. Report on a study of economics, marketing and processing requirements. FAO, Rome, Report No. THA/75/008/ 8O/WP/17: 124 pp. Anonymous, 1980b. Language Service Bi-weekly. USDC/NOAA/NMFS, Office of International Fisheries Affairs, 80-22/ST, 22 November. Anonymous, 1982. Predicting prawn success at mana. The Garden Isle (Lihue, HI), 19 April. Anonymous, 1984. Advertisements in the Los Angeles Times, Part 8, 8 November, pp. 11,4 1, 55. Antiporda, J.L., 1986. Optimum dietary protein requirements for Macrobrachium rosenbergii juveniles. Network of Aquaculture Centres in Asia, Bangkok, Thailand, Working Paper NACA/WP/86/45: 20 pp. AQUACOP, 1977. Macrobrachium rosenbergii culture in Polynesia: progress in developing a mass intensive larval rearing technique in clear water. Proc. World Maricult. Sot., 8: 3 1 l326. AQUACOP, 1979. Intensive larval culture of Mucrobruchium rosenbergii: a cost study. Proc. World Maricult. Sot., 10: 429-434. Arieli, Y., Sarig, S. and Bejerana, Y., 1981. Observations on pond growth of Mucrobruchium rosenbergii at the Ginosar Fish Culture Station in 1978 and 1979. Bamidgeh, 33 (2): 57-68. Arndt, G., Gautz, L. and Wang, J.-K., 1984. Mechanical size grading of postharvest prawns. J. World Maricult. Sot., 15: 6 l-72. ASEAN/UNDP/FAO, 1988. Country review paper (Thailand) presented at ASEAN Workshop on Shrimp and Finfish Feed Development, 25-29 October, Johore Bahru, Malaysia. ASEAN/UNDP/FAO Regional Small-Scale Coastal Fisheries Development Project, Manila, Philippines, pp. 13-l 5 (unpubl. ms. ). Ashmore, S.B., Stanley, R.W., Moore, L.B. and Malecha, S.R., 1985. Effect on growth and apparent digestibility of diets varying in grain source and protein level in Macrobrachium rosenbergii. J. World Maricult. Sot., 16: 205-216. Avault, J.W., 1979. Mucrobruchium rosenbergii culture in Polynesia, pH control in experimental pond waters by phytoplankton limitation with an algicide. Proc. World Maricult. Sot., 10: 392-402. Avault, J.W., 1987. Species profile - freshwater prawns and marine shrimp. Aquaculture Mag., 13(3): 53-56. Balasundaram, C. and Pandian, T.J., 198 1. In vitro culture of Macrobrachium (nobilii) eggs. Hydrobiologia, 77 (3): 203-208. Bartlett, P. and Enkerlin, E., 1983. Growth of the prawn Macrobrachium rosenbergii in asbestos asphalt ponds in hard water on a low protein diet. Aquaculture, 30: 353-356. Bauer, L.L., Sandifer, P.A., Smith, T.I.J. and Jenkins, W.E., 1983. Economic feasibility of prawn Macrobrachium rosenbergii in South Carolina, U.S.A. Aquacultural Eng., 2: 18 l-20 1. Behrends, L.L., Kingsley, J.B. and Price, A.H., 1986. Polyculture of freshwater prawns, tilapia, channel catfish and Chinese carps. J. World Maricult. Sot., 16: 437-450.

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