The freshwater prawn, also known as the giant river or Malaysian prawn, is native to the tropical, Indo-Pacific region. Freshwater prawns belong to the family Palaemonidae which includes the brackish and freshwater grass shrimp and the larger river shrimp. Most species which comprise this family require brackish water (dilute seawater) to complete the early stages of their life cycle. Due to its large size, relative to other freshwater shrimp, the Malaysian prawn has received much attention from farmers, investors and researchers as a prospective culture animal in the United States and other countries. Although prawns have been grown to an individual weight of 0.5 lb, the weight of farm raised prawns is usually 2.5 oz, or less. In the United States, commercial prawn culture began in Florida in the 1970's. During this time, attempts were also made to start a prawn farming industry in other tropical and subtropical regions of the world.

Life History

Freshwater prawns breed and spawn in warm freshwater. Breeding takes place between a female which has molted her shell (softshell) and a hardshell male. Using its long claws, the male protects the female until her shell hardens. The male deposits sperm contained in a gelatinous mass between the walking legs of the female. A few hours after mating, the eggs are laid and fertilized. The female attaches the eggs to the underside of her abdomen where they are incubated. Pleopods (paddle-like abdominal appendages) circulate water over the egg mass to provide oxygenated water and to remove debris. At first, the eggmass or "sponge" is a bright yellow-orange color, but turns brown and then gray near the time of hatching. Females may lay 10,000 to 50,000 eggs per spawn, four to five times a year. Mating typically will occur throughout the year when water temperatures are above 70 degrees F. Hatching occurs in approximately three weeks when the water temperature is 82 degrees F. The entire brood hatches in one or two nights and the larvae are dispersed by the female.

Newly hatched larvae must reach brackish water with salinities of 10 to 14 parts per thousand (ppt) within two days or they will not survive. At this stage, larvae swim upside down and tail first. They feed on zooplankton, worms and the larvae of other aquatic organisms. To reach the postlarval stage, the larvae must undergo 11 molts in approximately 35 days. Postlarvae resemble adult prawns and are about 0.28 to 0.39 of an inch in length. They have translucent bodies and may have orange to pink colored heads. At this stage, postlarvae typically crawl along the bottom but can also swim in a forward direction with their dorsal side nearest the surface. Rapid backward movement is made by contracting the abdominal (tail) muscles. Postlarvae begin migrating into freshwater one to two weeks following metamorphosis. They are able to swim against currents, climb vertical surfaces and migrate over land when adequate moisture is present.

Postlarvae are cannibalistic and omnivorous. Insect, animal and plant material and even fecal matter from other organisms may be included in their diet. Freshwater and a wide range of salinities can be tolerated. As postlarvae reach the juvenile stage, their bodies become a blue or brown color similar to adults. Juvenile prawns weigh approximately 284 to 95 individuals per oz.

As freshwater prawns mature, they develop into one of three male or female morphotypes. Factors which influence the distribution of morphotypes among farm raised prawns include stocking density and probably water temperature. Large and small males form two size groups. Some mature males have long blue claws (BC) which may be 1.4 to 2.0 times their body length. Usually, BC males are the largest and most dominant prawns in the population. Orange claw (OC) males have claws which are 0.8 to 1.4 times their body length, and may reach the size of the BC males or larger. Small males (SM) have short clear claws 0.4 to 0.9 times their body length, or have no claws. Within a population, the approximate proportion of BC: OC: SM males is 1:4:5. BC males are territorial and mate with groups of females which they protect during the molting process. OC and SM males represent the males in the secondary and primary stages of sexual development, respectively. Removal of the BC males from the population will allow some OC and SM males to reach the next stage of sexual development. The male and female populations are nearly equal. Ovigerous females (OV) or those carrying eggs; open females (OP), those which had eggs; and virgin females (V), those which have not had eggs, display an even size distribution within the population.

Larvae Culture

Brackish water (10 to 14 ppt) in coastal tropical and subtropical regions is required to produce prawn postlarvae in outdoor ponds. Freshwater prawns will not tolerate water temperatures much below 59 degrees F. Cold weather restricts the outdoor growing season to four to seven months in temperate climates. In these climates, indoor hatchery facilities would be required. Inland production of postlarvae is restricted to brackish water recirculating systems which maintain water temperatures of 81 to 85 degrees F. In the past, most U.S. producers purchased juvenile prawns from tropical hatcheries. Currently, indoor, recirculating hatcheries supply much of the seed stock.

Newly hatched prawn larvae are fed live brine shrimp nauplii (the free swimming, first life stage of the brine shrimp) at a density of not less than 28 nauplii per oz of water. The live nauplii are expensive. Ground fish and eggs, or a commercial diet is gradually introduced between 25 and 45 days following hatching. Food particle size ranges from 0.01 to 0.04 of an inch depending on the size of the larvae. These particles are kept suspended in the water column by vigorous aeration.

Following metamorphosis from the larval stage, a floating catfish diet may be used to feed postlarvae while they remain near the water's surface. Successful hatchery culture yields 121 - 140 postlarvae per gal. When they begin to crawl on the tank bottom, postlarvae may be stocked into freshwater growout ponds and fed a sinking (winter) catfish diet. In temperate climates, stocking juvenile prawns 60 days past the postlarvae stage, which weigh approximately 113 individuals per oz or greater, is a common practice (Table 1).

Ponds and Water Supply

Levee style, earthen ponds, similar to those used to grow channel catfish, are well suited for freshwater prawn production. Rectangular ponds with a minimum and maximum depth of 3 and 6 feet, respectively will help discourage unwanted aquatic plant and algae growth. Shallow ponds will store less oxygen depleted water than deeper ponds since their water volume per unit of surface area is less. With shallow ponds, there is less risk of a crop loss due to dissolved oxygen depletion. Prawns are harvested from these ponds by draining the water into a catch basin or large sump, and removing them with a small seine. Ponds with surface areas between 0.5 and 4.0 acres are considered the easiest to manage.

The use of well water reduces the likelihood of introducing pathogens and unwanted fish and insects which will prey upon newly stocked prawns. Compared to surface water, ground water is less likely to contain pollutants. Well water sources which provide 10 to 80 gallons of water per minute for each acre of pond surface can fill or flush ponds rapidly. The ponds should be filled 3 days prior to stocking juveniles to inhibit the establishment of predacious, aquatic insect populations. Surface water from rivers and streams may be used to fill the pond two weeks prior to stocking prawns, but must be screened to prevent unwanted fish and insects from entering the pond.

Watershed ponds could be used for prawn production provided aquatic insect populations can be controlled before juveniles are stocked and the pond can be drained and is no greater than 8 feet deep. Most watershed ponds do not have additional ground or surface water sources to quickly fill them. If the pond is not filled within a few days of stocking, populations of aquatic insects may become established. Predatory fish and aquatic insects must be eradicated shortly before juvenile prawns are stocked. Fish may be removed by applying 1 to 2 ppm solution of 5% rotenone when water temperatures reach 59-70 degrees F. At these water temperatures, juvenile prawns may be stocked 10 to 30 days following application. Approximately 3 days before stocking prawns, motor oil (1 quart) is often mixed with diesel fuel (3 gal per acre) and applied to the pond's surface when no wind is present. This mixture and other oils are used to suffocate air-breathing aquatic insects. However, these practices and other off-label uses of chemicals as pesticides are illegal.

The water in most prawn ponds is kept mixed by continuous or nightly aeration or with the use of paddlewheel circulation devices. If the pond waters are not mixed, large volumes of oxygen depleted water may accumulate near the pond bottom due to thermal stratification. Stratification occurs during the summer when warmer, less dense water remains near the pond's surface. Sunlight penetrates these waters and photosynthetic activity from phytoplankton (microscopic algae) combine to release oxygen into the water. Deeper waters which may receive little sunlight may become cooler and deficient in dissolved oxygen. Cool water is denser than warm water and will remain near the pond bottom. Without adequate water circulation, much of the bottom habitat available to the prawns could be eliminated due to the absence of dissolved oxygen. Prawns are territorial and stocking density (prawns/yd²) influences pond yield and individual size.

Dissolved oxygen (D.O.) should not be allowed to drop below 3 ppm at any time. Low D.O. may cause prawns to crawl out of the ponds or congregate at the pond edge during daylight. This will increase their vulnerability to predation. Prawns can tolerate temporary low D.O. conditions; however exposure should be avoided if possible. Dissolved oxygen meters should be used to monitor D.O. in the early evening hours and before dawn. Fulltime aeration (0.5 to 0.75 hp per acre) will reduce the risk of crop loss and keep oxygen rich water circulated throughout the pond. Since prawns are grown at lower stocking densities and are not as mobile as finfishes, water circulation may be as beneficial as aeration. Juvenile prawn growth has been reported to decline in waters with calcium hardness greater than 53 ppm CaCo₃ which may interfere with the molting process. Optimal postlarvae growth ranges between 20 and 200 ppm total hardness. Total alkalinity of 180 ppm or less, has been show to have little effect on prawn growth. Optimum pH ranges from 7.0 to 8.5, while pH in excess of 9.5 may be lethal.

Grow out Culture

Optimum water temperatures have been reported to range from 84 to 88 degrees F. Prawns can survive water temperatures between 57 to 95 degrees F. When stocking, juveniles must be temperature acclimated from the transport water to that of the receiving pond. There should be no more than a 5 to 10 degree F difference in water temperatures. Prawns can be acclimated by replacing the transport water with that from the pond in a period of about one hour. Before stocking, the pond water temperature must be at least 68 degrees F. In Kentucky, approximately 16,200 juvenile prawns are stocked per acre (3.34/yd²) in late May or early June to obtain 1 to 1.5 oz prawns. Individual size decreases as stocking density increases. Field trials are currently being conducted to determine if the addition of artificial pond substrate is economically feasible to increase per acre prawn yields.

Prawns are harvested in early to middle October, before low water temperatures cause mortality. Survival may vary between 50 to 90%. Yields for mono-cultured prawns range from 500 to 1800 lbs/acre-yr throughout most of the world. Multiple prawn crops in Taiwan are estimated to yield averages of 2220 to 2670 lbs/acre-year, while single crop production in the U.S. averages approximately 800 1000 1bs/acre-yr.

Research conducted in South Carolina, Mississippi and Kentucky has shown juvenile prawns will grow to a harvestable size of 0.92-2.0 oz in 100 to 168 days. Research in Kentucky has indicated cooler water temperatures of 77 to 82 degrees F, may delay maturity in the female population which apparently determines the rate of maturation of the male prawns. At lower water temperatures, dominant males mature more slowly which reduces their ability to suppress the growth of smaller males in the pond. As a result, higher yields of prawns may result from cooler pond water temperatures which have traditionally been considered sub-optimal.

Newly stocked juveniles will feed on natural food organisms which occur in the pond. Supplemental feeds, such as distillery by-products should be added to prawn ponds at the rate of 20 lbs per acre per day following stocking. Prawns should be fed a prepared feed once they reach an average weight of approximately 91 individuals per lb. A 32% protein, sinking catfish, or formulated freshwater prawn feed should be fed twice daily. The feed should be distributed uniformly over the pond's surface. A feeding table is provided in Table 1. The following assumptions are used to calculate the rates of feeding: prawns convert feed at a ratio of 2.5:1; one percent mortality occurs each week; and average individual weight is determined by sampling every three weeks.

TABLE 1.

Weight-dependent feeding rates for semi-intensive pond grow out of freshwater prawns
NO. OF INDIVIDUALS PER POUND	DAILY FEEDING RATE (% of body weight)^a
>91	0
91 to 30	7
30 to 18	5
<18	3

Weight-dependent feeding rates for semi-intensive pond grow out of freshwater prawns

NO. OF INDIVIDUALS

PER POUND

DAILY FEEDING RATE

(% of body weight)^a

>91

91 to 30

30 to 18

<18

^aAs-fed weight of diet/wet biomass of prawns x 100

Freshwater prawns are harvested at specific sizes based on market demand. Large prawns (1.0 - 1.5 oz) are completely harvested in fall by draining and seining the pond’s catch basin. "Cull" harvesting adults and replacement stocking of postlarvae may be practiced in sub-tropical and tropical climates which have a 5 to 12 month growing season. Harvesting with 0.71, 0.87 and 1.0 inch bar-mesh seines will select 0.71, 1.0 and 1.5 oz prawns, respectively. For a selective harvest, larger prawns can be removed with a 1 to 2 inch bar-mesh seine, four to six weeks before the final harvest. Small numbers of prawns may also be trapped with baited crawfish traps.

The removal of large, dominant males can increase the growth of smaller prawns, thus increasing potential pond yields. Additionally, smaller quantities of larger prawns would be available over a longer marketing period. Two batches of smaller prawns (approximately 0.71 oz) may be grown provided markets are available. However, production costs may increase since juvenile prawns would be stocked twice during the season.

In temperate climates, unless a greenhouse or other warmwater holding facility is available, the prawns must be marketed quickly before the onset of cold weather. Substantial cannibalism may occur in densely stocked holding facilities.

Polyculture

A detailed account of the species and techniques involved in freshwater prawn polyculture is beyond the scope of this paper; however, some general considerations should be mentioned. Dissolved oxygen concentrations of less than 5 ppm is considered unhealthy for most warmwater fish species. Approximately 1000 to 1500 lbs/acre of fish or prawns may be supported in a pond without aeration. At a stocking density of 4 juvenile prawns/y², a harvest yield of 900 lbs/acre may be expected. The addition of another fish species will raise the pond biomass. As biomass increases, the risk of crop loss due to low dissolved oxygen and other water quality problems also increases. Increased costs associated with water quality management, feeding and labor must be compared to the economic benefits of rearing the additional species. Fish must not be stocked until the prawns are too large to be preyed upon by the fish. Similarly, freshwater prawns are predacious and aggressive, and losses of some fish fingerlings can be expected. Harvests of polycultured fish crops may be more labor intensive than from monocultured ponds since species are usually hand separated. Following harvest, it will be necessary to market, process, transport or hold the different species simultaneously.

Freshwater prawns have been reared in ponds with tilapias, chinese carps, catfish and crawfish. In Puerto Rico, polyculture of prawns in tilapia ponds and tilapia in prawn ponds increased economic returns by approximately 112% and 21%, respectively. Catfish polyculture and crawfish intercropping has been demonstrated in Mississippi.

Marketing and Economics: Opportunities and Constraints

By 1994, farm raised shrimp and prawns represented approximately 25% of the world's marketed shrimp since 1988. In recent years, there has been a renewed interest and some new developments in freshwater prawn farming.

Urban areas of the US and Canada could provide the best niche markets for U.S. grown, freshwater prawns. The sale of live prawns in areas with large Asian or other ethnic populations may have the most profit potential. Live prawns could be sold at prices ranging from $4.00 to 8.00/lb, while prices of $10.00 to 22.00/lb have also been reported.

Adequate processing facilities must be available if fresh, whole prawns or tail meat are to be sold. Without proper handling, prawn flesh will break down quickly after death due to enzymatic activity (autolysis) which destroys the tissue. Prawn tail meat which can develop a "mush-like" consistency has created marketing problems in the past. Following harvest, the prawns must be transported and sold live in aerated tanks or immediately chill-killed in iced water for ten minutes. Following chilling the prawns may be sold fresh packed in ice, or frozen individually or in blocks. Prawns may be stored up to one year provided they are glazed with sprayed cold water (35.6 degrees F) and stored at 0 degrees F. The cost of equipment and labor to process freshwater prawns may be prohibitive to all but large scale prawn farmers. Retail processing facilities must comply with local health department regulations while wholesale processors must file a food and Drug Administration Hazard Analysis Critical Control Point (HAACP) plan and follow its guidelines.

Commercially caught and farm raised, marine shrimp may compete with freshwater prawns particularly in coastal areas. Workers in Alabama reported that freshwater prawns would reach marketable size (0.81 oz) during late summer and could not compete with lower cost, gulf-coast marine shrimp of similar size and appearance. Gulf shrimp of this size often sell for $3.00/lb, a price near the cost of production for freshwater prawns. Particularly in coastal areas, live freshwater prawns may receive market competition from other live crustaceans, such as blue crab, crawfish and lobster. However, others have stated that freshwater prawns have a distinct market and do not generally compete with marine shrimp. Marine shrimp are not typically sold live, whole or before freezing.

To assure profitability, the sale of whole as opposed to de-headed prawns has been recommended by researchers in Mississippi. Where headless shrimp or prawns are sold, the freshwater prawn can be distinguished from that of the marine shrimp (family Penaeidea). The freshwater prawn's second tail or abdominal segment (which would be closest to the head) overlaps the first and third segments while in marine shrimp the second abdominal segment is overlapped by the first.

Economic data from prawn farms is usually considered proprietary information and therefore is not published or available for examination. Prospective farmers must use cost and return estimates as guidelines which are based on economic feasibility studies prepared by universities or other groups. Profitability of a prawn operation will be affected by a number of factors such as: marketing season, competition from other seafood products, distance to markets, and the demand for a certain size, type, volume and quality of product. Local and site specific factors will determine the costs of land, pond and road construction, water supply, and installation of utilities. Operational considerations will include: a source of reliable and affordable seed stock, feed, equipment, labor, utilities, processing and technological services. Unlike marine shrimp, the production of freshwater prawns may be conducted in inland locations since only a small portion of their life cycle depends on the availability of saltwater. However, since freshwater prawns can only live in sub-tropical or tropical environments, outdoor production is limited to approximately 100 to 150 days in the southeastern United States. The hatchery period for freshwater prawns is approximately twice that for marine shrimp species. An economic model for a clearwater recirculating system in Mississippi estimated postlarvae production with 80% survival would be economically feasible at a selling price of $8.00/1000 postlarvae. Postlarvae imported to the United States cost $20.00 -50.00/1000, during this period. However, minor variations in survival substantially affected net returns when tested by a price/sensitivity analysis. Commercial-scale recirculating systems require high levels of technical ability and considerable investment.

Freshwater prawns can not be grown as intensively as marine shrimp. However, reduced risk of crop loss, lower capital investment, less labor and minimal effluent discharge from ponds, represents some of the advantages of more extensive aquaculture practices. Similarly, few diseases associated with freshwater prawn culture have been reported. Low incidence of prawn diseases may be the result of reduced stocking densities and feed inputs which generally improves water quality and minimizes stress. Freshwater prawn culture may be especially well suited to small-scale operations targeting niche markets.

Lack of reliable sources and the high cost of juvenile prawns have been considered a major industry constraint. The cost to stock 1 acre with 16,200 juvenile prawns which are 60 days past postlarvae is approximately $1,600. These costs may not include packaging or transportation. To compensate for the short grow-out season in temperate climates, advanced, juvenile prawns (approximately 57 individuals per oz) are stocked as opposed to the smaller, less expensive postlarvae. This practice generally reduces stocking mortality and increases yields. Stocking densities in excess of 16,200 juveniles per acre typically results in smaller prawns with less market value. Size grading juveniles into separate weight classes prior to stocking, has been shown to increase yields and reduce the number of smaller prawns of lesser value. Homogenous prawn growth would likely result in increased returns.

Based on prawn research data collected in Mississippi from 1984 to 1990, a simulated prawn farm model with 10 acres of ponds was tested with different variables affecting production. The simulated farm utilized watershed ponds constructed on 42 acres of land comprised of rolling hills. An 8 acre reservoir would supply by gravity 5, two acre production ponds. Where possible, proportional costs of equipment and facilities were divided among other commodities produced on the farm. The study concluded that the production and sale of headless freshwater prawns was not economically feasible. Headless prawns were sold at the 5 year averaged, ex-vessel price for headless marine shrimp, which ranged from $2.67 to $7.93 for 68 and 18 shrimp per lb, respectively. However, the data generated by the sale of whole, size-graded freshwater prawns at $3.50 to $4.00/lb displayed positive net revenues in most production scenarios. When Kentucky State University's production data was entered into the synthesized 10 acre farm model, estimated net returns were $7,857 and $52,909 for whole prawns sold at $4.00 and $8.00/lb, respectively. The study included complete ownership and operating costs.

Freshwater prawns are typically sold in local or domestic markets which have not been well developed. However, potential for export markets may also exist. The lack of a regular supply of freshwater prawns has limited export markets as opposed to poor demand or profitability. Fluctuations in supply created by a limited outdoor growing (June to October) and marketing (September-October) season may also impact local and domestic markets for prawns. Unless proper processing facilities are available, rapid enzymatic breakdown of the product remains a constraint to the industry. Sale of live prawns may yield higher prices, but may also require live transport over greater distances. Recent research using shelves stacked inside a live hauling tank has allowed the transport of 8 oz of prawns per gallon of water for at least 24 hours. Water temperatures were kept at 68-72 degrees F to minimize water quality problems and to reduce injury to the prawns. In many regions, holding facilities which will minimize cannibalism will be necessary to extend the marketing season for live prawns. Pond bank sales of live prawns may be appropriate for small scale operations located near their customers.

Acknowledgments

Without the efforts of the following authors, this article would not have been possible: L.R. D'Abramo, W.H. Daniels, J.H. Tidwell, C.D. Webster, J.L. Montanez, M. Brunson, M.F. Fuller, H.K. Dupree, J.V. Huner, J. Jensen, R. Kelly, M.B. New, S. Singholka, J.L. Silva, G.R. Ammerman, and T.L. Wellborn. The purpose of omitting the literature citations and references throughout this article was an effort to reduce its length and to allow for easier reading. A similar edition with metric measurements, citations and references is available from the author.